Bronchial Asthma: Principles of Diagnosis and Treatment

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Bronchial Asthma Principles of Diagnosis and Treatment Fourth Edition

Edited by

M. Eric Gershwin, MD Timothy E. Albertson, MD, PhD

Humana Press

BRONCHIAL ASTHMA

BRONCHIAL ASTHMA PRINCIPLES OF DIAGNOSIS AND TREATMENT Fourth Edition

Edited by

M. ERIC GERSHWIN, MD University of California at Davis, School of Medicine, Davis, CA

and

TIMOTHY E. ALBERTSON, MD, PHD University of California at Davis, School of Medicine, Davis, CA

HUMANA PRESS TOTOWA, NEW JERSEY

© 2001 Humana Press Inc. 999 Riverview Drive, Suite 208 Totowa, New Jersey 07512 For additional copies, pricing for bulk purchases, and/or information about other Humana titles, contact Humana at the above address or at any of the following numbers: Tel: 973-256-1699; Fax: 973-256-8341; E-mail: [email protected] All rights reserved. No part of this book may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, microfilming, recording, or otherwise without written permission from the Publisher. Due diligence has been taken by the publishers, editors, and authors of this book to assure the accuracy of the information published and to describe generally accepted practices. The contributors herein have carefully checked to ensure that the drug selections and dosages set forth in this text are accurate and in accord with the standards accepted at the time of publication. Notwithstanding, as new research, changes in government regulations, and knowledge from clinical experience relating to drug therapy and drug reactions constantly occurs, the reader is advised to check the product information provided by the manufacturer of each drug for any change in dosages or for additional warnings and contraindications. This is of utmost importance when the recommended drug herein is a new or infrequently used drug. It is the responsibility of the treating physician to determine dosages and treatment strategies for individual patients. Further it is the responsibility of the health care provider to ascertain the Food and Drug Administration status of each drug or device used in their clinical practice. The publisher, editors, and authors are not responsible for errors or omissions or for any consequences from the application of the information presented in this book and make no warranty, express or implied, with respect to the contents in this publication. Production Editor: Jason S. Runnion Cover design by Patricia F. Cleary This publication is printed on acid-free paper. ' ANSI Z39.48-1984 (American National Standards Institute) Permanence of Paper for Printed Library Materials. Photocopy Authorization Policy: Authorization to photocopy items for internal or personal use, or the internal or personal use of specific clients, is granted by Humana Press Inc., provided that the base fee of US $8.00 per copy, plus US $00.25 per page, is paid directly to the Copyright Clearance Center at 222 Rosewood Drive, Danvers, MA 01923. For those organizations that have been granted a photocopy license from the CCC, a separate system of payment has been arranged and is acceptable to Humana Press Inc. The fee code for users of the Transactional Reporting Service is: [0-89603-861-0/01 $8.00 + $00.25]. Printed in the United States of America. 10 9 8 7 6 5 4 3 2 1 Library of Congress Cataloging-in-Publication Data Bronchial asthma: principles of diagnosis and treatment: a guide for primary care physicians / edited by M. Eric Gershwin and Timothy E. Albertson.--[4th ed.] p. ; cm.-- (Current clinical practice) Includes bibliographical references and index. ISBN 0-89603-861-0 (alk. paper) 1. Asthma. I. Gershwin, M. Eric, 1946- II. Albertson, Timothy Eugene. III. Series. [DNLM: 1. Asthma--diagnosis. 2. Asthma--therapy. WF 553 B8693 2001] RC591 .B753 2001 616.2'38--dc21

00-054101 CIP

PREFACE “They asked if the sneezles came after the wheezles, or if the first sneezle came first.” It has been nearly 25 years since the first edition of this textbook was published. During that time, we have witnessed an enormous improvement in the understanding of the basic pathophysiology of asthma and, more importantly, better treatment options. However, and with regret, the incidence and prevalence of asthma during this 25 year period increased significantly. Recent studies from the NIH highlight this point and illustrate that despite improved care and diagnosis, mortality continues to rise. In fact, asthma remains the most common chronic childhood illness and is among the most common chronic adult diseases. Despite improved medications, increased awareness, and a better understanding of the pathophysiology of this disease, mortality and morbidity continue to rise. Both international and national consensus positions have been published that offer guidance on treatment approaches. The importance of the primary care physician and provider cannot be overestimated in the appropriate diagnosis and management of this disease. The management options in asthma are changing rapidly with the advent of new drugs and approaches. The recent introduction of the leukotriene inhibitors has added an entirely new class of anti-inflammatory agents in the treatment of asthma. The potential of even newer approaches, including cellular modulation of the asthma patient with specific anti-IgE antibodies, opens up exciting possible treatments. Bronchial Asthma: Principles of Diagnosis and Treatment is directed to primary care providers. As in earlier editions, we have tried to inform them of the changing approaches to diagnosis and management of asthma, and serve as a standard resource. Thus, we have provided a review of basic definitions, the mechanisms, and the medications of asthma. A definitive emphasis has been given to the management and recognition of asthma, and such special problems as asthma in the pregnant patient, in the pediatric patient, and the patient with exercise-induced asthma. Allergic broncho-pulmonary aspergillosis, alternative/herbal medications, environmental and occupational effects, food additives, recreational drug use, and other topics are reviewed with the primary care physician in mind. Although much emphasis is placed on diagnosis and treatment, the psychological, social, and legal aspects of asthma are also addressed. We also have not ignored the adult onset asthmatic, where the data suggest that the frequency of this problem is also increasing. We believe that an understanding of these areas are key for complete and comprehensive care of the asthma patient. We hope that this review of the spectrum of cellular to psychological/social aspects of asthma provides the primary care provider, in the same way it has provided us, with a framework or guide to the day to day interactions with asthma patients. In this volume, we hope we have prepared a practical, but spirited, review that will help patients, as well as provide algorithms for physicians. Patients need to be individualized, and the care v

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Preface

between people is rarely the same. However, our goal was to provide a basic framework upon which good treatment options will follow. We wish to thank the faculty at the University of California at Davis for reading and critiquing our manuscripts. As in any book, there will be errors and omissions; any faults will be our own. Finally, we especially wish to thank Nikki Phipps for her steadfast diligence in helping in the organization of Bronchial Asthma: Principles of Diagnosis and Treatment.

M. Eric Gershwin, MD Timothy E. Albertson, MD, PHD

CONTENTS Preface ...................................................................................................... v Contributors .............................................................................................. ix

PART I. DEFINITIONS AND HOST RESPONSES TO BRONCHOSPASM 1

Pathogenesis of Asthma: Genetics and Epidemiology..................... 1 Russell J. Hopp and Robert G. Townley

2

Pathogenesis of Asthma: Mediators and Mechanisms .................. 29 Maurice E. Hamilton and M. Eric Gershwin

PART II. PATIENT MANAGEMENT 3

Clinical and Allergic Evaluation of the Patient with Bronchial Asthma ............................................................. 75 Stephen M. Nagy, Jr.

4

The Role of the Pulmonary Function Laboratory in Patients with Bronchial Asthma .............................................................. 95 Richard E. Kanner and Theodore G. Liou

5

The Differential Diagnosis of Asthma in Childhood .................. 119 Gary A. Incaudo

6

Differential Diagnosis of Asthma in Adults: Asthma, Occult Asthma, and Pseudoasthma .......................... 137 Glen A. Lillington and John L. Faul

7

Treatment of Asthma in Children ................................................ 155 Christopher Chang

8

Treatment of Asthma in Adults ................................................... 201 Samuel Louie, Ken Y. Yoneda, and Nicholas J. Kenyon

9

Pregnancy, Lactation, and Asthma .............................................. 233 Arif M. Seyal

10

Alternative Therapies in Asthma ................................................. 255 Irwin Ziment

Part III. Special Clinical Problems 11

Asthma, Infection, and the Environment .................................... 279 Laurel J. Gershwin

12

Exercise Induced Asthma: Sports and Athletes .......................... 301 Rahmat Afrasiabi

13

Foods, Additives, and Nonsteroidal Anti-Inflammatory Drugs in Asthma ...................................... 315 Suzanne S. Teuber vii

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Contents

14

Allergic Bronchopulmonary Aspergillosis.................................. 343 Harold S. Novey

15

Occupational Asthma ................................................................... 365 Marc B. Schenker and Stanley Naguwa

16

Anesthesia for Asthmatic Patients ............................................... 383 Leland H. Hanowell and Dennis L. Fung

17

Recreational Drug Abuse and Asthma ........................................ 401 Nicholas J. Kenyon and Timothy E. Albertson

PART IV. LIVING WITH ASTHMA 18

Self-Management Programs for the Patient with Asthma: Empowering the Patient to Make Decisions that Will Improve Outcomes .................................................... 427 Joann Blessing-Moore

19

Psychological Considerations in Asthma: Implications for Treatment ...................................................... 445 Ed Klingelhofer

20

Asthma and the Law .................................................................... 459 Charles Bond

Index ..................................................................................................... 465

CONTRIBUTORS RAHMAT AFRASIABI, MD • Division of Rheumatology, Allergy, and Clinical Immunology, University of California at Davis, Davis, CA, and Allergy Associates, Chico, CA TIMOTHY E. ALBERTSON, MD, PHD • Division of Pulmonary and Critical Care Medicine, University of California at Davis, Davis, CA, and the VA Northern California Health Care System, Sacramento, CA JOANN BLESSING-MOORE, MD • Stanford University Medical Center, Palo Alto, CA CHARLES BOND, JD • Charles Bond & Associates, Berkeley, CA CHRISTOPHER CHANG, MD • Division of Rheumatology, Allergy, and Clinical Immunology, University of California at Davis, Davis, CA and Crescent City, CA JOHN L. FAUL, MD • Division of Pulmonary and Critical Care Medicine, Stanford University, Stanford, CA DENNIS L. FUNG, MD • Department of Anesthesiology, University of California at Davis, Sacramento, CA M. ERIC GERSHWIN, MD • Division of Rheumatology, Allergy, and Clinical Immunology, University of California at Davis, Davis, CA LAUREL J. GERSHWIN, DVM, PHD • Department of Pathology, Microbiology, and Immunology, University of California at Davis, Davis, CA MAURICE E. HAMILTON, MD • Division of Rheumatology, Allergy, and Clinical Immunology, University of California at Davis, Davis, CA LELAND H. HANOWELL, MD • Department of Anesthesiology, University of California at Davis, Sacramento, CA RUSSELL J. HOPP, DO • Department of Pediatrics, Creighton University School of Medicine, Omaha, NE GARY A. INCAUDO, MD • Division of Rheumatology, Allergy, and Clinical Immunology, University of California at Davis, Davis, CA, and Allergy Associates, Chico, CA RICHARD E. KANNER, MD • Division of Respiratory, Critical Care, and Occupational Medicine, University of Utah Health Sciences Center, Salt Lake City, UT NICHOLAS J. KENYON, MD • Division of Pulmonary and Critical Care Medicine, University of California at Davis, Sacramento, CA ED KLINGELHOFER, PHD • Department of Psychology, Sacramento State University, Sacramento, CA GLEN A. LILLINGTON, MD • Division of Pulmonary and Critical Care Medicine, Stanford University, Stanford, CA THEODORE G. LIOU, MD • Division of Respiratory, Critical Care, and Occupational Medicine, University of Utah Health Sciences Center, Salt Lake City, UT SAMUEL LOUIE, MD • Division of Pulmonary Medicine, University of California at Davis, Saramento, CA STANLEY NAGUWA, MD • Divisions of Rheumatology/Allergy, Clinical Immunology, Epidemiology, and Preventive Medicine, University of California at Davis, Davis, CA, and the VA Northern California Health Care System, Sacramento, CA ix

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Contributors

STEPHEN M. NAGY, JR., MD • Division of Rheumatology/Allergy, and Clinical Immunology, University of California at Davis, Sacramento, CA HAROLD S. NOVEY, MD • Division of Immunology, UCI Medical Center, Orange, CA MARC B. SCHENKER, MD • Department of Epidemiology and Preventive Medicine, University of California at Davis, Davis, CA ARIF M. SEYAL, MD • Division of Rheumatology, Allergy, and Clinical Immunology, University of California at Davis, Davis, CA, and Kaiser Permanente Medical Center, Rancho Cordova, CA SUZANNE S. TEUBER, MD • Division of Rheumatology/Allergy and Clinical Immunology, University of California at Davis, and the VA Northern California Health Care System, Sacramento, CA ROBERT G. TOWNLEY, MD • Department of Pediatrics, Creighton University School of Medicine, Omaha, NE KEN Y. YONEDA, MD • Division of Pulmonary and Critical Care Medicine, University of California at Davis, Davis, CA, and the VA Northern California Health Care System Sacramento, CA IRWIN ZIMENT, MD • Division of Pulmonary Medicine, Olive View UCLA Medical Center, Sylmar, CA

Short Chapter Title

PART I DEFINITIONS AND HOST RESPONSES TO BRONCHOSPASM

1

Genetics and Epidemiology

1

1

Pathogenesis of Asthma


RUSSELL J. HOPP, DO AND ROBERT G. TOWNLEY, MD Contents Key Points Introduction Pediatric-Onset Asthma Adult-Onset Asthma Wheezing

Key Points • The incidence of asthma has increased over the past twenty years, especially in children. • Genetic factors are critical for asthma development. • Asthma can develop at any age. • Children of minority races and children from lower socioeconomic backgrounds will be more likely to develop asthma. • Boys will be more likely to develop asthma prior to puberty. • Atopy is a strong risk factor for asthma in children and young adults. • Children who are exposed to smoke, regularly attend large day cares, and are not breast fed will have a better chance to develop asthma. • Bronchial hyperresponsiveness is intrinsic to the asthma, and likely has a genetic background. • Respiratory syncytial virus appears to have a unique infectious role in asthma development. • A modern, more hygienic, society may have an effect on the development of asthma, especially in children. • A reasonable percentage of children with asthma will enter their adult years free from asthma symptoms. From: Bronchial Asthma: Principles of Diagnosis and Treatment, 4th ed. M. E. Gershwin and T. E. Albertson, eds. © Humana Press, Totowa, NJ

1

2

Hopp and Townley

• Older adults who develop asthma demonstrate sputum and serum eosinophilia, but are less likely to have positive skin tests. • Adults tend not to have permanent asthma remission. • More children will have recurrent wheezing than will have eventual asthma. • Respiratory syncytial virus and passive smoke exposure are common backgrounds in young children with recurrent wheezing syndrome.

Introduction A common illness, asthma, is in many ways very complex. Despite its obvious familial background, the genetics of asthma is still under intense investigation. Ultimately, the genetic cause(s) of asthma may be multiple, and environment factors may have an important influence. Asthma has distinct occurrence patterns in prepubertal males and females, and in age of onset. Asthma also is influenced by another genetic disease, atopy. There are distinct risk factors for the development of asthma. One important factor is the role of certain viral infections, especially respiratory syncytial virus (RSV). Certain bacterial infections, such as tuberculosis and vaccination with bacille Calmette-Guérin (BCG), may stimulate the helper T-cell (Th1), and be protective. Finally, asthma may remit, and the natural history of the disease is critical for planning clinical strategies for therapy. This chapter reviews current information about the genetics of asthma, known and recognized epidemiological factors, and the trends in the natural history of the disease. Another important fact about asthma: There are probably few other diseases of its magnitude that carry such emotional and ambiguous trappings. This is driven, in part, by parental concern, insurance reimbursement issues, and the lack of a true test for its presence. The lack of a diagnostic litmus test, and these other idiosyncrasies, only amplify themselves in the design of all genetic, epidemiological, and outcome studies.

Genetic Studies Although a very common disease, asthma has defied a clear genetic explanation. Modern genetic techniques now allow evaluation of the cause(s) of a disease that probably have a genetic background. A review of the clinical presentation of asthma and methods available to investigate the genetics of asthma, suggest the difficulties encountered in the elucidation of the heritable component of the disease (1–4).

Clinical Presentations of Asthma Phenotype Asthma has frequently been referred to as a syndrome, rather than a single disease. In addition, the age of the patient often influences the clinical diagnosis. In adult-onset asthma, allergy is not as frequently a concern. These different presentations have influenced the potential for genetic analysis.


3

Table 1 Possible Ascertainment Criteria for Asthma Risk factors for atopy-related Asthma-related surrogates surrogates Serum IgE at a specific elevated level + Methacholine challenge + Skin test(s) to standardized allergens + Histamine challenge + RAST test(s) Clinical diagnosis of asthma Reversibility to a `-agonist Combinations of column A and column B

Infants and young children often have wheezing problems. Because a variety of clinical conditions can present with wheezing, this age group is not routinely studied in genetic evaluations. Children over 6 yr of age with asthma are routinely included in genetic studies of asthma. Atopy is an important contributing factor to asthma in the pediatric age group, and most genetic studies have used this associated feature in their definition of the asthma proband. The issue is whether it is the asthma condition or the atopic status that is identified by the analysis. Another clinical presentation of asthma is usually seen in adults, in whom a readily identifiable allergic trigger is often absent. To date, the so-called “intrinsic asthmatic” has not been routinely studied using modern genetic approaches (5,6). Asthma clearly has different clinical expressions, which requires the researchers involved in genetics studies of asthma to select an asthma type that is consistent for each person entered in a genetic study. A particular study must also use population, racial, and, possibly, environmental consistency.

Candidate Gene Studies (7–9) The most commonly performed genetic studies of asthma focus on nuclear families, sib pairs, or large extended families. In these evaluations, a representation of the asthmatic condition is used to define the disease (Table 1). Genetic studies have measured serum immunoglobulin E (IgE), skin test response to common allergens, radiosorbant immunoassay (RAST) results, measures of bronchial responsiveness using methacholine or histamine, reversibility to a `-agonist, and a clinical diagnosis of asthma. In most studies, a combination of factors is used to identify the proband (index subject) as asthmatic. In a candidate gene study, the search for the asthma gene is preselected, based on known biological information about asthma. This method of genetic analysis proceeds to identify whether the asthmatic proband, or surrogate, has a statistical association with a known gene or regions of specific chromosomes (10–21a). Some of these allergy-associated candidate genes have included interleukin 4 (IL-4), IL-5, granuloctye-macrophage colony-stimulating factor, interferon-a, tumor necrosis factor, IL-9, the promoter regions for IL-4 and IL-10, the high-affinity receptor for IgE, bronchial hyperresponsiveness (BHR), and the `-adrenergic receptor. If there is a significant association between the surrogate used for defining

4

Hopp and Townley

asthma (Table 1) and a known marker on a specific chromosome, by inference, the gene for the asthma condition, or the surrogate marker for the asthma condition, may be on that chromosome and near that repeat sequence loci. In essence, it is a process of town, neighborhood, street, and address identification. Currently, studies have identified a number of towns (chromosomes), and several neighborhoods in these towns (gene areas). Chromosomes 5 and 11 have gathered the most attention to date (14,22–33). Chromosome 5 is of particular importance, because the genes for IL-4, IL-5, IL-3, and IL-13 (allergy-associated genes) are located on this chromosome. Studies have linked this area to bronchial hyperresponsiveness to histamine, a surrogate of asthma. Chromosome 11 attracted the first attention as a location of an important atopy gene (high-affinity IgE receptor) in the studies of Cookson et al. (14,30,33). Unfortunately, other studies have not found the same linkage in their study population to the same areas of chromosome 5 and 11 (34–39), which underscores the complexity of the search for the asthma gene. Differences in populations or races, disease ascertainment, and definition of surrogate markers can all be considered as potential confounding variables.

Genome Search Studies (40–46) The genome search approach will allow for unsuspected genes to be identified. The Collaborative Study on the Genetics of Asthma has detected a number of potential candidate gene areas for further research. Using the entire genome, linkages to specific marker areas are sought. Genome studies have linked chr 2p, 3, 5p, 6, 7, 9, 11p, 12, 13, 16, 17, 19q, and 21q to various clinical parameters of asthma and/or atopy, and in different ethnic backgrounds. This further points to the daunting task ahead in the search for the responsible gene(s).

Structural Gene Variations If a candidate gene is the cause of asthma, a reasonable approach to elucidating the etiology of asthma is to study the various forms of a specific gene. To date, differences in the IL-4 _-subunit (47), the `-chain of the high-affinity receptor for IgE have been found (48), although the latter not without controversy (49). The `-receptor gene has several forms, and a difference has been seen in more severe asthma (21). The full question to be answered is whether these structural differences account for phenotypic expressions of asthma, or for different clinical expressions or severity of the disease.

Pediatric-Onset Asthma

Prevalence of Asthma Asthma is classically noted as being extrinsic or intrinsic. In many ways this terminology is limited. This chapter will refer to asthma as being pediatric-onset asthma or adult-onset asthma.


5

Centers for Disease Control and Prevention The Centers for Disease Control and Prevention (CDC) has surveyed and summarized asthma statistics in the United States from the National Ambulatory Medical Care Survey, National Hospital Discharge Survey, and the National Health Interview Survey (NHIS), and from the CDC’s National Center for Health Statistics multiple causes of death file. Since 1979, the Ninth International Classification of Diseases (ICD-9) has been used for defining asthma for mortality analysis, using codes 493–493.9. From 1980 until 1990, the prevalence of self-reported asthma increased from 31 to 43/1000 population (50). In 1990, individuals of African-American descent had an asthma prevalence of 52/1000 (51). In 1991, the age-adjusted asthma death rate was reported as 1.9/100,000 individuals (52). By 1993, the total number of asthmatics in the United States was estimated to be 14.5 million, up from 6.8 million in 1980, including 4.8 million below age 18 yr (52). Age-adjusted asthma deaths in 1993 were reported as 3.7/1,000,000 (52). Asthma death rates continued to be highest in black populations. A comprehensive report was released by CDC in 1998 (51), which documented a 75% increase in self-reported asthma from 1980 to 1994. This increase crossed all races, both genders, and all age groups. Children demonstrated remarkable increases: 160% in those 0–4 yr old, and 74% for yr 5–14. Using data generated by 1995 state-specific estimates of asthma (51), the CDC estimated that, in 1998, there were 17 million people in the United States with asthma (53). Asthma was defined, for this report, as having been physician-diagnosed and symptomatic within the past 12 mo. State-specific rates ranged from 5.8 to 7.2% (53). With the hesitancy of physicians to diagnosis asthma in children, and to report this diagnosis to parents, this estimate may be conservative.

NHIS: Child Health Supplements (54) A random sample of 15,224 children ages 0–17 yr in 1981, and of 17,110 in 1988, showed that 3.1% of children in 1981 and 4.3% in 1988 were identified by their parents as having had a physician diagnosis of asthma, and as having been symptomatic in the past year. Black children with asthma maintained at 5% at each survey, with white children increasing from 2.7 to 4.1%. Severity of asthma indices did not change between the two time periods (54).

International Study of Asthma and Allergies in Childhood (ISAAC) (55) Phase one of the ISAAC protocol was open to any collaborator in the world who agreed to follow the protocol. Children at age 6–7 yr, and adolescents between 13 and 14 yr were enrolled, and most questionnaires were administered in 1994– 1995. Most ISAAC centers were in urban areas. Sites in the United States included Chicago and Seattle. The remainder of the selection details have been published (56). A variety of questions were asked regarding to asthma and asthma symptoms, including “Have you ever had asthma?” for the 13–14-yr-olds, and “Has your child ever had asthma?” for the 6–7-yr-old children.

6

Hopp and Townley

In the older children, positive responses for asthma ranged from 1.4 to 28.2%, with a global total of 11.3%. The North American total was 16.5% (United States and Canada). The United Kingdom was consistently over 18%. In children 6–7 yr, the parents answered positively to the asthma question 10.2% globally, and 14.2% in North America (United States not surveyed). The Oceanic region, Australia, and New Zealand, was 26.8%. The steering committee clearly stated that the question “asthma ever” was “a less reliable measure for epidemiological purposes.” The questions about wheezing are discussed at the end of this chapter.

Risk Factors for Pediatric-onset Asthma The authors extensively reviewed Medline from 1966 through the present to provide a comprehensive perspective on the factors associated with the development of asthma. Many of these factors also contribute to the severity, or to exacerbations of asthma, but the authors attempted to isolate those factors that appear, epidemiologically, to be associated with its inception.

Gender An asthma diagnosis is made more often in male children less than age 10 yr, but, by age 21 yr, females are proportionally more represented (57). Since 1968, on an annual basis, there have been more females than males who die from asthma (51). Although it can be stated with some assurance, that asthma is not a X-linked disease, genetic factors have been found to locate to the pseudoautosomal portion of the XY chromosome (9). The preadolescent dominance of asthma in males suggests an innate difference in the young male that predisposes him to asthma. Possible factors include enhanced airway reactivity, or greater inflammatory responsiveness to viruses incriminated in asthma (e.g., RSV). It is also possible that the preasthmatic young male has a smaller airway size; In addition, there may be other factors, such as a greater propensity for allergic disease or atopy in preadolescent males. This may also suggest an enhanced immune response to allergens in preadolescent males. Strong support for allergic diathesis, in the male gender in the preadolescent years, to allergic diathesis was reported in a study of the development and prediction of atopy in high-risk children (58). In this randomized, controlled study evaluating maternal and infant food allergen avoidance, male children, at age 7 yr, had significantly higher levels of IgE, period and cumulative prevalence of allergic rhinitis, asthma, allergy skin tests, atopic dermatitis, and nasal eosinophilia than their female counterparts.

Age Asthma symptoms can begin at any age. A new asthma diagnosis is most commonly made between birth and age 20 yr. Bronchial hyperresponsiveness is seen in all asthmatics, and probably is present in all infants at birth. Nonspecific bronchial hyperresponsiveness, as measured by histamine or methacholine, is more commonly present in children of all ages, compared to adults (59). The increased presence of bronchial hyperresponsiveness in children may make the develop-


7

ment of asthma in this age group more permissive (see the subheading Bronchial Hyperresponsiveness). The authors have hypothesized that, because infants have heightened bronchial hyperresponsiveness, environmental and genetic factors may allow for its persistence, although most children outgrow this hyperresponsivness, and thus do not develop wheezing or asthma. Infants appear to have a Th2 (atopy-oriented) T-cell profile at birth. This appears to decrease, unless the child is genetically prone to atopy and becomes sensitized. A CDC report for asthma data, through 1995, showed a self-reported prevalence for asthma in children 0–4 yr to have increased 160% since 1980, and 74% in children 5–14 yr (51). Other factors that are associated with being a child, and thus may track with age as a risk factor for asthma, are the increased incidence of viral illnesses, unavoidable exposure to passive smoke, irritant and environmental factors, the propensity to develop allergic disease, allergen sensitization, and immune factors.

Atopy There is probably no factor more commonly seen with or before an asthma diagnosis in children than atopy. In the age groups in which new-onset asthma is most common (birth through age 21 yr), atopy is very common. A survey of allergy skin test reactivity in the United States, through the auspices of the National Health and Nutrition Examination Survey from 1976–1980 (NHANES II) showed that the rates of skin test response to one or more common allergens was greatest in adolescents and young adults (60,61). Males were more positive than females, and blacks more than whites. In the 6–11-yr age group, 18% of whites and 28% of blacks were positive. In the age group 12–17 yr, 23% of whites and 36% of blacks were positive. The atopic-immunologic milieu in young persons provides a fertile ground in which asthma may develop. There have been a large number of studies that have investigated the risk for developing asthma in relationship to allergens responsible for positive skin tests. Since certain allergens are more frequently represented, it would suggest that children (predominantly) who are regularly exposed to these allergens, and who have an atopic and/or asthmatic genetic predisposition, are at greater risk of developing asthma. What is not known, however, is if these allergens are more potent, or asthmatics are more frequently exposed. The more important allergens include housedust mites, cockroach, and alternaria. A recent report (62) suggests a dichotomy for alternaria sensitization and parental history of asthma. (Alternaria is the dominant allergen in arid desert environments.) Six-yr-old children with asthma in Tucson, AZ were skin tested for alternaria, a major antigen in that region. Children who were skin-test positive to alternaria had both maternal and paternal histories of asthma; alternaria-negative children were more likely to have a mother with asthma. The alternaria-negative children had lower IgE levels than their alternaria-positive asthmatic counterparts, and had other features of their asthma that were different than the alternaria-positive asthmatics. It will be important to confirm this type of result in areas with housedust mite or cockroach or more dominant allergens.

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Hopp and Townley

Epidemiological data are commonly expressed as an relative risk or odds ratio. A relative risk is used with cohort studies, and odds ratio with case–control studies. A ratio of 1.0 indicates no effect, a ratio >1.0 indicates an effect, and a ratio 5.0 for alternaria; 2.9 for housedust, 1.0 or greater for ryegrass, oak, bermuda grass, and dog; and less than 1.0 for ragweed and cat.

Passive Smoke Exposure A comprehensive analysis of the effect of passive smoke on asthma was recently published (63). The authors of that report reviewed all existing studies of the effect of parental smoking on asthma. Sixty studies through 1997 were selected from Medline and Embase database electronic searches, for the extractability of their data for determination of a pooled odds ratio. Most studies included the question, “Has this child ever had asthma?” The pooled odds ratio (OR) for the 60 studies was 1.21 (95% CI 1.10–1.34) for either parent smoking. The authors concluded that the evidence is very likely casual, given the statistical significance, and for evidence of a dose-response. The risk for developing asthma, if only the father smoked, was also significant, but not as much as mothers, and would suggest a postnatal effect. The finding was consistent among nations. The Third National Health and Nutrition Examination Survey, 1988–1994, stated that environmental tobacco smoke appears to increase the prevalence of asthma in children 2 mo–5 yr (OR=2.1), for asthma before 12 mo of age (OR = 2.6), or for asthma medication requirement (OR = 4.6) (64).

Bronchial Hyperresponsiveness (Fig. 1) Enhanced nonspecific bronchial hyperresponsiveness, as measured by histamine or methacholine, is an intrinsic component of every asthmatic’s constitution (65). In a limited number of studies, when BHR was followed longitudinally (66,67), and in some cases serially (67), a significant number of subjects subsequently developed clinical asthma. In nearly every subject, BHR was enhanced prior to the onset of asthma. Several studies have looked at BHR itself, independent of asthma, and there were familial (genetic?) tendencies explaining its presence (68,69). To date, however, the exact cause for BHR, with or without asthma, is unknown. Knowing the cause for BHR will greatly assist with understanding the cause of asthma.

Racial/Ethnic The NHIS, 1980–1994 (51) reported the average annual rate (per 1000 population) of self-reported asthma during the previous 12 mo by race, age, and sex (51). In all reporting periods (1980, 1981–1983, 1984–1986, 1987–1989, 1990–1992, 1993–1994), the rate of asthma among blacks outnumbers whites, except in 1984– 1986. In the final reported year, 1993–1994, the rate per 1000 reported individu-


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Fig. 1. Components of bronchial hyperresponsiveness.

als, was 57.8 among blacks and 50.8 whites, and 48.6 among those reporting “other” as their race (51). The 1988 Child Health Supplement to the NHIS (54) reported a prevalence of asthma of 4.1% for white children, 5.1% for black children, 3.5% for Hispanic children, and 4.3% for non-Hispanic children. In 1981, the same survey reported rates of 2.7% for whites, 5.3 for blacks, 2.8 for Hispanic, and 3.1 for non-Hispanic (54). These surveys asked parents whether their child had asthma, and whether it been present in the past 12 mo. An analysis of the 1981 Child Health Supplement to the NHIS data revealed significant differences in asthma prevalence, by age 3 yr, between black and white children. Furthermore, the poverty status, maternal cigarette smoking status, large family size, size of home, low birth weight, and maternal age of 20 yr or less were all significantly associated with increased rates of childhood asthma (70). A 1997 report of a telephone survey of middle-class children of all families of third -graders in Southfield, MI (71) found a lifetime prevalence of asthma of 12% for blacks and 6% for whites. The community is multiethnic, socioeconomically homogenous, with only 4% of blacks and 7% of whites having incomes below federal poverty levels. By third grade, 14% of boys and 5% of girls had reported asthma in their lifetime. A 1996 report of a survey of 998 fourth grade students, with a high percentage of inner-city Hispanic, Mexican-American children, in San Diego, found that 14.4% were categorized as having probable current asthma, and an additional 13.5% had respiratory symptoms indicating possible asthma (72). The lowest rate of insurance coverage existed in the Hispanic children. Among Latino children, Puerto Rican children have a higher prevalence of asthma than Mexican-American or Cuban American children (73). These differences in

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Latino children appear to be multifactorial, with family structure, education, and economic factors all playing a role. A survey of asthma among 475 non-Hispanic and 371 Hispanic pregnant women (Mexico, Puerto Rico, Central and South America), living in Boston (74) showed a 6% incidence of current asthma in Hispanic women and 12% in non-Hispanic women. Eighty-two percent of the Hispanic women came from outside the United States. If the rates for asthma in first-generation Hispanic-American children is greater than adult Hispanic women who have come from other countries, factors other than genetics are influencing the increase. A study of childhood asthma ($27,300, and 1.58 times more likely if living in a central portion of a city. Data from the 1981 and 1988 Child Health Supplements to the NHIS (54) reported, in 1981, an asthma prevalence of 4.4% in children from low-income families and 2.9% in children from higher-income families. By 1988, the rates were 5.4% for low income and 4.% for higher income (54). Selected studies of children 18 yr or younger, since 1991, from the cities of New York (Bronx County) (77), San Diego (impoverished area) (78), Chicago (random school sample) (79), and Detroit (school children, 98% black) (80), have identified asthma prevalence rates ranging from 13 to 16% depending on the criteria used for establishing an asthma diagnosis. The crucial question is, if asthma prevalence is increasing, and that increase is predominantly in inner-city children, often in minorities, what factor(s) is driving this phenomenon (81)? Speculation may lie in some of the following observations: National smoking rates are decreasing, but not among poorer individuals, and presumably not by poorer parents; daycare attendance has increased tremendously in the past decade, exposing children to increased infection, especially RSV and parainfluenza; a decrease in breast-feeding; tighter homes, with more indoor carpeting; smaller family size; increased exposure to petroleum distillate particulates, including diesel; and a decrease in serious intracellular infections. Although not an exhaustive list, any one or several may be contributing to the rise in asthma prevalence.

Prematurity The most reasonable estimate of the effect of prematurity on asthma prevalence should focus on studies after the widespread use of surfactant. In the past, premature infants who did not receive surfactant were more often subject to postnatal


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barotrauma and long-term oxygen therapy. Presurfactant infants were more likely to develop various degrees of bronchopulmonary dyplasia (BPD), a chronic lung disease with wheezing as a frequent symptom (82). It has been well recognized that asthma/reactive airway disease is a frequent complication of BPD (82). In addition, studies (83) have suggested that a family history of asthma contributes to the severity of lung disease of the premature infant. Published reports (82–85) have shown an enhanced prevalence of asthma in younger children who were born prematurely. Young children, less than 5 yr, are more likely to have wheezing-associated respiratory illnesses, and post-RSV wheezing. These illnesses are asthma-like, and may appear to be statistically associated with prematurity. A report published in 1996 (85) examined neonatal characteristics predictive of a physician’s asthma diagnosis during the ages 0–4 yr. Using logistic regression, the following neonatal factors favored preschool asthma: birthweight < 1500 g (OR = 1.61); respiratory distress syndrome with (OR = 2.95) or without (OR = 1.61) BPD; prematurity (OR = 1.34). Whether or not a preadolescent or adolescent, born prematurely, but treated with surfactant, has an increased prevalence of allergic asthma, has not yet been studied (82).

Respiratory Infections The role of respiratory infections in the inception of asthma is an important but confusing issue. There are a well recognized number of respiratory illnesses, especially in children, that can induce an acute onset of wheezing, which is also a feature of acute asthma. The issue, of course, is which episode of viral-associated wheezing to call asthma, and/or did the previous episodes of viral infection (and wheezing) cause the asthma. In addition, once an asthma diagnosis is established, there is probably no greater cause of asthma exacerbations than are infections. Of all the known viruses that have been associated with asthma, the most commonly recognized is RSV (86,87), which is unique in its association with IgE and leukotriene production, both features of known asthmatics (86). Furthermore, following a serious RSV illness, young children can subsequently present with recurrent wheezing (88–90). A recent epidemiological study (91) followed 888 children for the first 3 yr for lower respiratory illness (LRI). Those authors reported that, in those children with a LRI by age 3 yr (35.5% caused by RSV), there was a significantly greater OR for physician diagnosis for asthma at ages 6 and 11 yr, compared to all children without a LRI by age 3 yr. Significant differences in atopy, however, did not track with the asthma diagnosis. Children with LRI, but no pneumonia, also had lower pre-LRI lung function. Mycoplasma and Chlymadia infections can cause community-wide outbreaks of wheezing, and are commonly associated with new exacerbations of wheezing in established asthma. Their asthmagenic potential has been suggested (92–94). Several reports (95) have shown a statistical reduction in asthma in individuals with a marked immunological response to BCG (possibly tuberculosis also). A recent paper (95a) shows that newborns are very capable of mounting a Th1 response to BCG, which points to the possibility of using strategies to induce an early Th1 profile, which may reduce the potential for asthma development. Measles

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vaccination has been reported to reduce atopy (96). This line of investigation points to the role of certain infections to alter the allergic–immunologic pathway, and provides avenues for possible primary prevention.

Air Pollution Because air pollution, increased diesel particulates, and ozone are all common urban air pollution problems, it has been suggested that the increase in asthma incidence could be associated with these pollutants. The most convincing argument against this, however, is the data coming from surveys of asthma and allergy in reunified Germany. In several studies, the incidence of childhood asthma was not greater in East Germany, the more industrialized and polluted country, and atopy was more common in West Germany. Bronchitis, however, was more common in East Germany (97,98). The direct role of ozone and diesel particulates in the increase in asthma in industrialized nations has not been adequately examined, and speculation on their role persists (98a).

Familial Aspects A set of recent observations has suggested a inverse relationship between infections and asthma prevalence. A seminal article from Japan (95) revealed that children who had received BCG, and who had a positive TB skin test >10 mm, were significantly less likely to be asthmatic or atopic. This finding was followed by several reports (99,100) showing a decreasing possibility of asthma and hay fever in families with multiple children. A recent report from Germany (101) evaluated age of daycare entry, family size, and atopy at ages 5–14 yr in 2430 children. Children from small families entering daycare from 12 to 23 mo were more likely to be atopic than children entering day care at 6–11 mo. In children from large families, entering daycare at any age was not a risk factor for becoming atopic. The possible lesson from family size and daycare entry time focuses on the concept of Th1 and Th2 CD4+ T-cells. Th1 cells are responsible for thwarting serious infectious agents; Th2 cells are seemingly involved with atopic responses. If children are more vigorously using their Th1 T-cells, less stimulation of Th2 occurs. Thus, children from larger families and younger children entering daycare have more time with their Th1 T-cells in infection surveillance, and less time with Th2 activities. The balance between Th1 and Th2 is shown in Fig. 2. In a fascinating report (102), adopted young adults were evaluated for asthma and allergies, depending on the presence or absence of asthma and allergies in the parents who adopted them. If the adoptive mother had asthma or allergic rhinitis, there was a significant risk that the adoptee would have a diagnosis of asthma (OR = 3.2). If the adoptive father had asthma or allergic rhinitis, the risk for asthma in the adoptee was 1.9.

Daycare and School Environments The few studies that have examined this issue have found that young children who attend daycare are more likely to have a physician diagnosis of asthma 102q.


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Fig. 2. Balance between TYH1 + TH2 lymphocyte function.

Possible factors contributing to the suspected increased prevalence of asthma in children attending daycare are increased viral and allergen exposure, especially housedust mites and pet dander. A recent report from Oslo, Norway (103) found that 11.7% of children who attended daycare before age 2 yr were at risk to have a doctor diagnosis of asthma throughout life, compared to 8.8% of children who did not attend daycare before age 2 yr. The risk of early respiratory tract infection appears to be a possible explanation. A study of 762 young adolescents in Sweden (104) evaluated the effect of school environment. Controlling for atopy, food allergy, and daycare, there were more adolescents with asthma from larger schools, open shelves, lower classroom temperature, higher air humidity, viable molds, bacteria, or cats in settled dust in the school. A recent report showed a fivefold difference in cat dander in classrooms when many (>25%) and few (2% fat), had a significantly reduced rate of current asthma, when all other risk factors for asthma were controlled (OR = 0.26). High salt intake may increase bronchial hyperresponsiveness, a critical factor in asthma (110).

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Weight Several reports have examined the effect of obesity on asthma and asthma severity, and whether asthma is a risk factor for obesity. A preliminary study (111) has suggested that being overweight has a doubling risk for developing asthma. After controlling for Tanner stage, the boys (OR = 2.3) and girls (OR = 1.5) asthma diagnosis was increased in those in the highest quintile for body mass, compared to the lowest quintile. In examining inner-city minority asthmatics and their nonasthmatic controls, there were significantly more asthmatics who were >95th percentile for body mass index, and asthmatics were significantly more overweight than their nonasthmatic controls (112). Eighty-six percent of the study, children were Hispanic, 17% black. The weight differences were seen in both sexes and across all ages 4–16 yr. Another report (113) found a greater proportion of moderate-to-severe asthma in Hispanic and black children who were overweight. Type 2 diabetes in children has markedly increased, largely because of obesity. Are the lungs of obese children more prone to chronic hypoventilation, caused by lack of exercise? Would this increase their asthma risk?

Breast-feeding The role of breast-feeding in asthma prevention has been recently reviewed (114). In addition, a recent study of 2834 children in Australia (115), presented as a report at the 1999 American Thoracic Society meeting, showed children at age 6 yr were significantly less likely to have a doctor diagnosis of asthma, if they exclusively breast-feed for the first 4 mo. Those authors speculate that the decline in breast-feeding in developed and developing countries may be responsible for the increase in asthma. Breast-feeding has numerous beneficial effects for infants, and should remain the choice for nutrition for newborns. Its benefit as an asthma prophylaxis appears conservative.

Medications BENEFICIAL (PROPHYLACTIC?) A recent report (116) showed a protective benefit to cetirizine in reducing the onset of asthma in children with previously diagnosed atopic dermatitis, and who were sensitized to grass, mite, or cat. A possible advantage in asthma prevention is, therefore, speculative. In an interesting report (117), cromolyn sodium was administered to 24 nonasthmatic children who had asthmatic parents. These children had greater degrees of bronchial hyperresponsiveness than seven children from normal families. After 6 wk of therapy 8/24 children had less bronchial hyperresponsiveness. A long term study of the protective effects of cromolyn in preventing asthma would have been a natural outcome of this report, but was never done.

DETRIMENTAL The use of aspirin products in children was essentially ceased in the early 1980s secondary to their association with Reyes syndrome. Because aspirin blocks


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cyclooxygenase-2 enzyme activity, decreasing prostaglandin E2 activity, which PGE2 promotes Th2 CD4+ T-cells, which are atopy- and asthma-supporting. Acetaminophen, in large part the aspirin substitute used since the 1980s in children, has no effect on cyclooxygenase-2 activity. It has been hypothesized (118) that the increase in asthma, starting in the past 20 yr in the United States, could have origins in this shift in nonasthma therapy. A recent report on antibiotics and asthma (119) reported an association between a history of asthma among children 5–10 yr of age and the use of antibiotics (OR = 2.74). If antibiotics were used in the first year of life, the risk was greater (OR = 4.05) Our overzealous use of antibiotics in the United States may have a downside other than the emergence of bacterial resistance.

Indoor Environment Since the effect of specific indoor factors probably has direct bearing on allergen exposure, atopic development, and its asthmagenic potentiating effects, some of the studies reporting this are worth mentioning (63,64,70,72,120–124). Numerous studies have shown direct association between the level of dust mite in bedrooms and the presence of asthma, wheeze, and asthma severity. A major report showed cockroach dander was a risk factor. Other reports have found home dampness, humidifer use, and mold contamination to be risk factors. Most studies examining indoor ETS reported significant risks for asthma.

Natural History of Pediatric-Onset Asthma A frequently asked question in pediatrician’s offices in the United States is: “Will my child outgrow his/her asthma?” There are now available a reasonable number of longitudinal studies of the outcome of asthma in children. These studies have recently been summarized (125). Unfortunately, the studies have not used a consistent criteria for asthma. Some studies use symptoms, such as wheeze, or lung measurements of bronchial hyperresponsiveness, or forced expiratory volumes. A smaller group of studies used asthma as a criteria for enrollment and at followup showed at a range of the of 5–20 yr, 27–66% of subjects were asymptomatic or improved. The authors of a recent review of the natural history of asthma (125) summarized the above studies as showing a 50% chance for a child with asthma to not have asthma as an adult. Less encouraging are results of bronchial hyperresponsiveness over time. This characteristic of asthma tends to persist to the same degree in both childhood and adulthood, regardless of the presence of asthma symptoms (125,125a).

Adult-Onset Asthma Asthma can present in adults in a number of ways. Children with asthma can maintain their disease into their adult years; alternatively, an adult can reactivate their quiescent pediatric asthma. In part, the latter probably results from a persistence of bronchial hyperresponsiveness (125). Asthma can develop as a new diagnosis in an adult, either as a young adult, probably with risk factors similar to

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childhood asthma, or as a older adult. Finally, adult-onset asthma may be hidden with other respiratory illnesses, often associated with chronic smoking, and the symptoms of wheezing, shortness of breath, and chest tightness are attributed to chronic bronchitis or emphysema.

Prevalence of Adult-Onset Asthma Adult-onset asthma has not gained a national or international sense of urgency, as has pediatric-onset asthma. The available data, however, suggests increasing trends. The U.S. NHIS from 1980 through 1994, reported the average annual rate of self-reported asthma (51). Per 1000 individuals, rates were given for ages 15– 34, 35–64, and *65 yr. The results are presented in Table 2. The CDC has estimated that, based on the 1994 data, the adjusted asthma estimates for all ages in the United States in 1998 was 64/1000, which includes both adult and children with asthma (51). The cumulative prevalence of asthma in the U.S. population was ascertained in adults by the Second National Health and Nutrition Examination Survey, 1976– 1980 (126). Defined as ever being told by a physician that he/she had asthma, and/ or frequent problems with wheezing in the past 12 mo, the rates were 9.9% for 12–44 yr, 11.8% for 45–64 yr, and 12.4% for 65–74 yr (126). A report on Swedish adults (127) revealed a mean annual cumulative incidence of asthma of 0.5%. The European Commission Respiratory Health Study (128), a multicenter survey of the prevalence and determinants of asthma in adults 20–44 yr of age, is probably the most recent comprehensive study of the disease symptoms in young adults. In the 45–48 centers reporting data, the median 12-mo prevalence of asthma attacks was 3.1% and treatment for asthma was 3.5%; the maximum prevalence in any center was 9.7% for attacks, and 9.8% for treatment. In the United States, deaths with asthma as the underlying cause have increased in the adult population from 1960 through 1995. Using the results from the the ICD-9, 1979 and following, deaths in adults 65 yr of age or greater have increased from 1481 to 2972 from 1979 to 1995: a 200% increase (51). In adults 34–65 yr, an increase of 186% in asthma-associated deaths has occurred during the same years, although the total number of deaths is about 60% of the older adults (51).

Risk Factors for Adult-Onset Asthma Two unique reports from a prospective study of respiratory disease provide insight into antecedent factors that were predictive for the future development of asthma in adults. The first report (129) evaluated adults with asthma, diagnosed at ages 20–39 yr, and determined what factors were present at an earlier nonasthmatic age, 15–21 yr, which predicted the onset of asthma asthma. Compared to control subjects who did not develop asthma, the asthmatics were more likely to have had wheezing, shortness of breath with wheezing, rhinitis, and a positive allergy skin test at a preasthma visit; however, standard measures of pulmonary function were not predictive. Those authors suggest that young adult-onset asthma may have a long prodrome period.


Age (yr)

1980

15–34 27.7 35–64 28.1 *65 30.7

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Table 2 Average Annual Rate of Self-reported Asthma by Age 1981–1983 1983–1986 1987–1989 1990–1992 1993–1994 30.2 33.1 34.4

35.1 32.0 38.9

40.1 36.8 42.1

41.7 42.3 36.4

51.8 44.6 44.6

A similar report from the same center (130) examined a similar protocol for older adults (>60 yr) with new-onset asthma. A variety of symptoms were predictive, including wheezing, attacks of shortness of breath with wheezing, current allergic rhinitis, childhood respiratory trouble, and a chronic bronchitis diagnosis. Smoking was only predictive for 0 were all significant risk factors. The authors acknowledge the potential confusion between an asthma diagnosis and other chronic obstructive pulmonary diseases. Because wheezing is a major co-determinant for asthma in that report, it is also likely that asthma had a long prodrome, as with the younger adult asthma study. Evaluations of young adult asthmatics in other larger studies have shown maternal or paternal asthma, atopy, smoking, serious respiratory illnesses in early childhood, and low income to be risk factors (131). A recent report (131a) identified perennial rhinitis to be an independent risk factor for asthma in adults 20–44 yr, even without positive skin tests and with normal IgE levels. Risk factors for asthma in older adults include a high-fat diet, exogenous estrogen use in females, and smoking (132–136).

Natural History of Adult-Onset Asthma (137) Although it is not as common to develop asthma as an adult as it is to have asthma start during the pediatric years, the chance that the asthma will wane and/ or remit is less common in adults, compared to children. Without question, the adult asthma issue is confused with other adult-onset diseases with other similar symptoms, especially chronic bronchitis. Regardless of the true diagnosis, smoking will clearly worsen both asthma and mixed chronic respiratory obstructive disease. In general, a nonsmoking adult with adult-onset asthma needs aggressive treatment and longitudinal care, with only a small possibility for a eventual normal outcome for lung function and the total absence of symptoms.

Wheezing Prevalence Wheezing is a common clinical presentation of asthma. In the ISAAC survey, the question “Have you had wheezing or whistling in your chest in the past 12

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months” was asked of 13–14-yr-old adolescents, and of the parents of children 6–7 yr old (55,56). Older children in North America responded affirmatively 24.2% of the time, with a global response rate of 13.8%. In younger children, the Canadian rate was 17.6%, globally it was 11.8%.

Epidemiology A recent report (138), however, showed that the presence of recurrent wheezing in younger children is much higher than the current prevalence estimates for pediatric-onset asthma, and much higher than reported in older children in ISAAC. Published in 1995, the study of Martinez et al. (138) provides impressive epidemiological evidence about the patterns of recurrent wheezing in infants and young children. A group of 826 infants, enrolled in an HMO, were prospectively followed for 6 yr. Incredibly, over this period of time, 49% of the enrolled subjects had a wheezing episode. The authors retrospectively divided these wheezing children into three groups: transient early wheezers, late wheezers, or persistent wheezers. Using OR analysis, the characteristics of these three groups of wheezers were defined and compared to the 425 children who had not wheezed by age 6 yr. Transient early wheezers had wheezing within the first 3 yr, but not at 6 yr of age. In these children, maternal smoking was significantly associated with wheezing. These children also had lower length-adjusted pulmonary function, suggesting a deleterious effect of passive smoke exposure. The children who developed wheezing after age 3 yr (late-onset wheezers) were more likely to have mothers with asthma, to be male, and to have had rhinitis in the first year of life (although not stated, these would be common characteristics of young asthmatics) Children who wheezed throughout the 6 yr of the study (persistent wheezers) had a significant incidence of maternal asthma, wheezing often or very often, wheezing without colds, eczema, Hispanic background, and maternal smoking. Twenty-five percent of these children had been labeled asthmatic by age 6 yr.

Infection Associated Wheezing There is a general acceptance that an episode of bronchiolitis, generally caused by RSV, can be followed by recurrent wheezing episodes (87–90). There are a number of concerns with existing studies. Was the initial episode of RSV bronchiolitis diagnosed clinically or by laboratory techniques? Are the infants exposed to tobacco smoke at home? Did the infants with post-RSV pulmonary symptoms have pulmonary function disability before the RSV infection? Was the variable, studied and reported after the RSV infection, limited to bronchial hyperresponsiveness, wheezing episodes, or pulmonary function changes? Are there differences in these variables in young children? Are IgE, eosinophilic cationic protein, or eosinophilia generation during the RSV infection the critical element(s) for inducing post-RSV pulmonary problems? Is a family history of asthma or atopy the critical component for inducing a post-RSV change in pulmonary mechanics? These and other concerns limit a firm conclusion linking RSV infection and recurrent wheezing. Given these limitations, however, it is the popular opinion that infections with RSV, and other viruses capable of causing bronchiolitis, can result in recurrent wheezing (87–90).


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Small Airways An early study by Martinez et al. (139) examined pulmonary function in 124 children at a very early age and prior to any respiratory. Longitudinal observations of their wheezing patterns revealed that those infants with reduced pulmonary function were significantly more likely to have wheezing associated with a lower respiratory tract illness. In that study, maternal smoking did not further influence the predictability of wheezing. A study of 97 infants by Tager et al. (140) provides additional evidence of the effect of diminished lung function and wheezing in the first year. In the studied infants, the level of lung function measured before 6 mo predicted a wheezing episode.

Smoke Exposure The literature on passive smoke exposure and infantile lung function has been recently reviewed (141). In particular, a report by Young et al. (142) revealed, in 63 infants, that either a family history of asthma or parental smoking was more likely to result in increased bronchial hyperresponsiveness to inhaled histamine. Bronchial hyperresponsiveness is a hallmark of all asthmatics, and the presence of increased bronchial hyperresponsiveness at an early age may be to indicative for recurrent wheezing, especially at younger ages. This group subsequently reported (147) that in utero smoke exposure is associated with a significant reduction in pulmonary function in infants measured within 1 wk of birth. Infants with a family history of asthma also had significantly diminished lung function. Another group of investigators has reported (140,144) supporting evidence for the deleterious effect of intrauterine smoke (extrauterine also) exposure, postbirth pulmonary function, and the onset of wheezing in the first year of life. A recent report (145) documented diminished lung function in premature infants exposed to in utero smoke, but prior to hospital discharge, eliminating the effect of postuterine smoke exposure on lung mechanics.

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Mediators and Mechanisms

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2 Pathogenesis of Asthma Mediators and Mechanisms

MAURICE E. HAMILTON, MD AND M. ERIC GERSHWIN, MD Contents Key Points Introduction Mast Cells Eosinophils Basophils Neutrophils Mononuclear Phagocytes Lymphocytes Cytokines

Key Points • Asthma is characterized by reversible obstruction and hyperreactivity of the airways associated with an inflammatory infiltration by lymphocytes, macrophages, and eosinophils which may induce airway remodeling and permanent lung damage. • Mast cells are key players in the allergic response. Activated by antigens cross-linking surface IgE, mast cells release preformed mediators (including histamine and chemotactic factors) and synthesize newly-generated mediators (including LTC4 and PGD2) which induce an early-phase reaction associated with bronchospasm and a late-phase reaction characterized by inflammation. Mast cells also synthesize cytokines, low molecular weight peptides that regulate immune and inflammatory responses. • Eosinophils are major effectors of airway damage in asthma, primarily through the release of major basic protein and eosinophil cationic protein, both of which are toxic to airway epithelial cells. From: Bronchial Asthma: Principles of Diagnosis and Treatment, 4th ed. M. E. Gershwin and T. E. Albertson, eds. © Humana Press, Totowa, NJ

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• T lymphocytes are the primary regulators of the immune response. CD4+ helper T cells may differentiate into Th1 cells, which secrete predominantly IL-2 and IFN-g, or Th2 cells, which preferentially secrete IL-4, IL-5, IL-6, IL-10, and IL-13. Th1 cytokines promote cytotoxicity, delayed-type hypersensitivity reactions, and monocyte activation, whereas the Th2 cytokines IL-4 and IL-13 induce IgE synthesis and IL-5 activates eosinophils. • Monocytes are the principal source of IL-12, which induces IFN-g synthesis and promotes differentiation of Th1 cells, but also release IL-10, which inhibits Th1 development. In the lungs, monocytes differentiate into alveolar macrophages, which produce nitric oxide and may contribute to airway damage. • Airway epithelial and smooth muscle cells actively participate in the allergic response by releasing arachidonic acid metabolites, cytokines, and chemokines, thereby serving both as effector and target cells. The bronchial epithelium is the primary source of the potent bronchoconstrictor endothelin and, with macrophages, accounts for most nitric oxide synthesis in the airways. • Cellular adhesion molecules regulate both cell-cell and cell-extracellular matrix protein interactions and are essential for the recruitment of leukocytes to sites of inflammation. Proinflammatory cytokines upregulate the expression of these molecules and thus augment the inflammatory response. • Neural control of the airways is mediated through adrenergic (sympathetic), cholinergic (parasympathetic), and nonadrenergic, noncholinergic (NANC) systems. Stimulation of the cholinergic nervous system induces bronchoconstriction and mucus secretion. Neurotransmitters in the NANC system include the excitatory mediators substance P and neurokinin A, which induce bronchoconstriction, and the inhibitory mediators vasoactive intestinal peptide and nitric oxide, both potent bronchodilators.

Introduction Atopic asthma is a disease characterized by reversible airway obstruction and hyperreactivity to allergens and nonspecific stimuli. Associated with these phenomena are mucus hypersecretion, airway edema, and an inflammatory infiltrate of the bronchial mucosa composed of lymphocytes, macrophages, neutrophils, and eosinophils, which may induce desquamation of the epithelium and remodeling of the airways. The sequence of events leading to these changes is initiated when allergens encounter B-cells, which produce antigen-specific immunoglobulin E (IgE) following stimulation by T-helper (Th) cells. Binding of this IgE by its Fc portion to cells with high-affinity IgE receptors (mast cells and basophils) or cells with low-affinity IgE receptors (B-cells, monocytes, and some T-cells) sensitizes them for activation by surface IgE crosslinking during subsequent allergen exposure. Mast cells activated in this manner release histamine (Hi) and other biologically active molecules that induce bronchoconstriction and recruit inflammatory cells that mediate damage to the airways.


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Fig. 1. Mast cell with IgE FcR.

This chapter reviews the major participants in the pathogenesis of asthma, starting with two key players, mast cells and eosinophils, followed by basophils, neutrophils, mononuclear phagocytes, lymphocytes, and their immune response mediators, cytokines. Next, considered are the dual roles of airway epithelial and smooth muscle cells, which function both as effector and target cells. Then the innervation of the airways and the potential role of neuropeptides in asthma are described. Central to this discussion is current understanding of asthma as an inflammatory process induced by these cells and their products and regulated by lymphocytes.

Mast Cells Mast cells play a key role in the development of acute bronchoconstriction and initiate the events that lead to chronic inflammatory airway disease. The immediate hypersensitivity reaction (the hallmark of allergic disorders) is triggered by the binding and bridging of antigen-specific IgE molecules on the surface of mast cells, leading to the release of preformed and newly generated mediators (Fig. 1). These bioactive molecules act on target cells, such as eosinophils, neutrophils, vascular endothelial cells, airway smooth muscle cells, neurons, and mucous cells, to produce characteristic, and sometimes life-threatening, allergic reactions, including asthma and anaphylaxis.

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Described in 1878 by Paul Ehrlich, who, as a medical student, observed their characteristic metachromatic staining, mast cells are found throughout connective tissues, where they reside adjacent to blood vessels, lymphatic channels, and nerves, and beneath epithelial surfaces that interface with the external environment, such as the respiratory mucosa, conjunctivae, gastrointestinal tract, and skin. In the respiratory tract, mast cells are located beneath the basement membrane of the airways and within bronchial smooth muscle, the submucosa near glands and blood vessels, and the bronchial lumen. The concentration of mast cells varies, depending on anatomic site, but increases in areas of chronic inflammation. Lung tissue from patients who have died from asthma demonstrates decreased mast cell staining, presumably resulting from degranulation during the asthma attack. Analysis of bronchoalveolar lavage (BAL) fluid from patients with allergic asthma shows increased numbers of mast cells compared to normal subjects (1). Mast cells are derived from CD34+ hematopoietic progenitor cells. However, unlike blood cells, most of their maturation occurs in peripheral tissues. Thus, unipotential mast cell-committed progenitors migrate from the bone marrow to mucosal or connective tissue sites, where they expand and differentiate into mature mast cells, under the influence of stem cell factor (SCF) and perhaps other fibroblast-derived mediators (2). The surface of each mast cell contains 104–106 high-affinity Fc¡ receptors (Fc¡RI), each composed of one _ chain (which binds to the Fc portion of IgE), one ` chain (which functions as a signal enhancer), and two identical disulfide-linked a chains (which are the main intracellular signaling components of the receptors). The Fc¡ receptors on a given mast cell may bind IgE antibodies of different specificities, thereby sensitizing the cell to more than one antigen.

Mast Cell Mediators The cytoplasmic granules observed in mast cells by Ehrlich contain a crystalline complex of preformed mediators that are released and initiate the early-phase reaction upon activation of mast cells. These mediators include Hi, which is responsible for many of the phenomena associated with the early-phase reaction, and proteoglycans, serine proteases, carboxypeptidase A, sulfatases, exoglycosidases, and cytokines, including tumor necrosis factor-_ (TNF-_) and interleukin 4 (IL-4) (Table 1).

Histamine Hi is synthesized in the Golgi apparatus of mast cells and basophils by decarboxylation of histidine and associates with the acidic residues of the glycosaminoglycan (GAG) side chains of heparin and other proteoglycans (3). Human mast cells contain 3–6 pg Hi per cell and secrete Hi spontaneously at low levels, producing a normal plasma level of 0.5–2 nM. Hi is rapidly metabolized (usually within 1–2 min) following extracellular release by either of two mechanisms, methylation by histamine-Nmethyltransferase or oxidation by diamine oxidase (histaminase). The biologic effects of Hi are mediated by activation of specific cell surface receptors, of which three subtypes have been identified. Binding of H1-receptors


Preformed mediators

33 Table 1 MC Mediators Newly synthesized mediators

Biogenic amines Histamine Neutral proteases Tryptase Chymase Carboxypeptidase Cathepsin G Acid hydrolases Arylsulfatase `-Galactosidase `-Glucuronidase `-Hexosaminidase Proteoglycans Heparin Chondroitin sulfate Chemotactic factors Eosinophilic chemotactic factor of anaphylaxis Neutrophil chemotactic factors Cytokines IL-4 TNF-_

Cyclooxygenase products Prostaglandins Thromboxanes Lipoxygenase products Leukotriene B4 Leukotrienes C4, D4, E4 Platelet-activating factor Cytokines

by Hi causes contraction of airway and gastrointestinal smooth muscle, mucus secretion, and increased vascular permeability. Stimulation of H2-receptors inhibits T-cell cytotoxicty, interferon-a (IFN-a) production, and release of lysozymes, but increases suppressor T cell activity, neutrophil and eosinophil chemokinesis, and expression of complement receptors for C3b (CR1) on human eosinophils. H3-receptors are located presynaptically on histaminergic nerves and function as autoreceptors, whereby Hi controls its own synthesis and release from nerves (4,5).

Neutral Proteases Neutral proteases constitute the largest proportion of the protein in human mast cell secretory granules and include tryptase, chymases, carboxypeptidase, and cathepsin G. The variable distribution of these neutral proteases forms the basis of human mast cell classification as MCT, to denote mast cells containing tryptase, but not chymase, and MCTC, for cells containing both tryptase and chymase. In addition, MCTC cells contain carboxypeptidase and cathepsin G. MCT cells appear to play a primary role in host defenses and constitute >90% of the mast cells present in the alveoli, airway epithelium, and airway lumen; MCTC mast cells are located in the submucosa of the respiratory tract and appear to be primarily involved with angiogenesis and tissue remodeling (6). MCTC cells are also the predominant type in skin, synovium, and gastrointestinal submucosa. Nonetheless, both types of mast

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cells are present in most tissues, and the relative proportions may change during inflammation. Moreover, both MCT and MCTC cells express high-affinity receptors for the Fc portion of IgE (Fc¡RI), enabling them to participate actively in IgEdependent reactions.

TRYPTASE The major enzyme in the cytoplasmic granules is tryptase, a neutral protease stored in active form in association with heparin. Tryptase is present in all human mast cells but is lacking in other cell types, which forms the basis for a useful assay of mast cell activation. Two forms of tryptase have been identified in humans: _-tryptase and `-tryptase. _-Tryptase is released constitutively from mast cells and represents a measure of mast cell mass or hyperplasia; `-tryptase is stored in mast cell secretory granules and provides an indicator of MC activation. Tryptase is released from human mast cells in association with proteoglycans, forming macromolecular complexes that diffuse poorly and exert biological effects within the microenvironment of the mast cells. Like trypsin, tryptase digests peptide and ester bonds on basic amino acids. Substrates for tryptase include calcitonin gene-related peptide (CGRP), vasoactive intestinal peptide (VIP), fibrinogen, urokinase plasminogen activator, fibronectin, and type IV collagen. Tryptase accounts for the IgE-mediated kininogenase activity described in mast cells. Tryptase has also been shown to function as a growth factor for airway smooth muscle cells, epithelial cells, and fibroblasts. Through such actions, tryptase may contribute to neurogenic inflammation, kinin generation, localized anticoagulation, and tissue remodeling. Tryptase may also augment inflammatory responses by potentiating adhesion molecule expression and releasing chemotactic factors for neutrophils and eosinophils (7). In addition, tryptase can activate mast cells, providing positive feedback that amplifies the allergic response (8). Antigen challenge of allergic asthmatic subjects results in a significant increase in tryptase levels in BAL fluid, implying that mast cells are activated. However, tryptase levels in peripheral blood are usually normal, except in cases of systemic mastocytosis and anaphylaxis, which may be associated with elevated levels of _-tryptase and `-tryptase, respectively. CHYMASE Chymase, another serine protease stored in active form within mast cell granules, is localized to the MCTC cell subset in humans. Chymase possesses various biologic activities that may modulate vascular tone and permeability, inflammation, and tissue destruction and remodeling. In experimental models, human chymase converts angiotensin I to angiotensin II, increases vascular permeability, and induces chemotactic activity for neutrophils and eosinophils. Chymase also activates prostromelysin and procollagenase, permitting degradation of fibrotic tissue, and converts procollagen to collagen fibrils, which may enhance fibrosis (3,9). Although speculative, this combination of collagen degradation and synthesis may contribute to airway remodeling. Chymase is released from mast cells in association with carboxypeptidase and proteoglycans and is inhibited by _-antichymotrypsin and _1-antitrypsin.


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CARBOXYPEPTIDASE Carboxypeptidase, also localized to MCTC cells, cleaves carboxy terminal residues, thereby degrading angiotensin, leu-enkephalin, kinetensin, and neurotensin (10). It remains closely linked to heparin proteoglycan, which binds to the exterior surface of mast cells after release, suggesting that its action is limited to the cellular microenvironment.

Acid Hydrolases Mast cell granules also contain acid hydrolases, which express their optimal activity in an acidic environment such as occurs at sites of inflammation. These enzymes include arylsulfatase, `-galactosidase, `-glucuronidase, `-hexosaminidase, superoxide dismutase, and peroxidase, which inactivates cysteinylleukotrienes (cys-LTs) and may facilitate the synthesis of lipid mediators (11).

Proteoglycans Proteoglycans are macromolecules composed of GAG chains covalently linked to a protein core. The presence of acidic GAGs explains the affinity of mast cell and basophil granules for basic dyes, such as toluidine blue, which leads to the metachromasia that characterizes and identifies these cells. Mast cell granules contain two classes of proteoglycans, heparin and chondroitin sulfates. During mast cell development, the type of proteoglycan present in the mast cell granules varies, influenced in part by stem cell factor, which induces the synthesis of heparin. Within mature human pulmonary mast cells, the ratio of heparin to chondroitin is 2:1. Proteoglycans bind Hi, neutral proteases, and carboxypeptidases, and may enhance the packaging of these molecules within the secretory granules. Intracellularly, heparin facilitates the production of `-tryptase, which is localized to a complex different from chymase and carboxypeptidase A (12). During degranulation, the mediators associated with proteoglycans dissociate at varying rates, releasing Hi very quickly, but tryptase and chymase much more slowly. HEPARIN Heparin exerts a variety of biologic effects, including anticoagulant activity mediated by antithrombin III and fibrinolysis. Heparin also regulates mast cell proteases and other enzymes, neutralizes the cytotoxic activity of eosinophilderived basic proteins, inhibits eosinophil cationic proteins (ECP) and chemokines, and decreases synthesis of cytokines, activation of complement, binding of fibronectin to collagen, and migration of leukocytes from vessels. Thus, heparin may be uniquely positioned to modulate a range of proinflammatory effects mediated by mast cells.

Chemotactic Factors Preformed chemotactic factors in mast cell granules include high-mol-wt neutrophil chemotactic activity, which causes a transient leukocytosis and is inhibited by cromolyn; heat-labile neutrophil chemotactic factor; and eosinophil chemotactic factors of anaphylaxis, which induce eosinophil chemotaxis and increase the expression of eosinophil complement receptors. In addition, Hi exerts chemoattractant effects for inflammatory cells. Release of these factors by activated

36


mast cells initiates the development of the late-phase reaction and the cellular infiltration that characterizes allergic inflammation. Rather than existing in a preformed state, some mast cell mediators are synthesized following activation of the cell. These include prostaglandins (PGs), leukotrienes, cytokines, and platelet-activating factor. Synthesis of prostaglandins and leukotrienes involves signal transduction pathways that activate cytosolic phospholipase A2, which releases arachidonic acid from membrane phospholipids (Fig. 2).

Prostaglandins Prostaglandins are formed by the conversion of arachidonic acid to the unstable intermediates prostaglandin G 2 (PGG 2 ) and prostaglandin H 2 (PGH 2 ) by cyclooxygenase-1 (COX-1) or cyclooxygenase-2 (COX-2). COX-1, present in most types of cells, is a constitutive isoenzyme that exerts cytoprotective effects on the gastric mucosa, regulates renal blood flow, and decreases platelet aggregation. In contrast, COX-2, present in mast cells, macrophages, and leukocytes, is an inducible enzyme activated by proinflammatory mediators, including cytokines, growth factors, and endotoxin. PGH2 is converted to the biologically active PGs PGD2, PGE2, and PGF2_ by prostaglandin synthases and isomerases, to prostacyclin (PGI2) by prostacyclin synthase, or to thromboxane A2 (TXA2) by thromboxane synthase. PGD2, the major cyclooxygenase product generated by human pulmonary mast cells, exerts a variety of effects relevant to asthma and allergic diseases. As a bronchoconstrictor, PGD2 is 30× as potent as Hi in patients with mild allergic asthma (13). Other actions include pulmonary and coronary artery vasoconstriction, peripheral vasodilatation, neutrophil chemotaxis, and decreased platelet aggregation. PGF2_, a metabolite of PGD2, exerts similar effects on the airways and blood vessels. TXA2 may represent an even more potent bronchoconstrictor than PGD2 and also induces vasoconstriction and increases platelet aggregation. These prostanoids are believed to mediate bronchoconstriction by binding to a thromboxane receptor on airway smooth muscle cells. In addition, a vagal contribution to the bronchoconstrictor effects mediated by PGD2 is implied by the finding that ipratropium bromide decreases PGD2-induced airway narrowing. In contrast, PGE2 and prostacyclin induce bronchodilatation, and inhaled PGE2 has been reported to inhibit bronchoconstriction following allergen challenge (14). The potential role of PGs during allergic reactions is suggested by the finding of elevated serum levels of PGF2_ and PGE in asthmatic subjects, and increased levels of PGD2 in BAL fluid from patients with allergic asthma, following challenge with inhaled allergen (15,16).

Leukotrienes Metabolism of arachidonic acid by lipoxygenase enzymes yields unstable hydroperoxyeicosatetraenoic acids (HPETEs). 5-lipoxygenase converts arachidonic acid to 5-HPETE, then to 5-HETE or leukotriene A4 (LTA4). In turn, LTA4 may be metabolized by LTA4 hydrolase to LTB4 or by LTC4 synthase to LTC4. Sequential amino acid cleavage from LTC 4 yields LTD 4 and LTE 4. Collectively known as cys-LTs, LTC4, LTD4, and LTE4 constitute the major lipoxygenase products synthesized by mast cells. An alternative metabolic pathway catalyzed by


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Fig. 2. Arachidonic acid metabolism.

15-lipoxygenase produces 15-HETE from arachidonic acid, but appears to be of lesser importance in mediating allergic reactions. Cys-LTs exert potent bronchoconstrictor effects that are up to 1000× more potent than Hi and 100× more potent than PGs (17). In addition, cys-LTs increase postcapillary venule permeability, enhance bronchial mucus secretion, and attract eosinophils (6). Present in bronchoalveolar lavage fluid from asthmatic subjects following allergen inhalation during both the early and late phases, cys-LTs are significantly decreased after treatment with inhibitors of leukotriene synthesis. PGD2 and tryptase are absent from bronchoalveolar lavage fluid during the late phase, suggesting that the cys-LTs present at this stage are derived from eosinophils or basophils, rather than mast cells. LTB4, produced in small quantities by human mast cells, facilitates binding of neutrophils to vascular endothelial cells, and possesses potent chemotactic activ-

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ity for neutrophils, eosinophils, monocytes, lymphocytes, and fibroblasts, which may promote the late-phase response and tissue remodeling. LTB4 also increases vascular permeability, and may enhance production of cytokines and IgE (18).

Platelet-activating Factor (PAF) PAF is an ether-linked phospholipid (alkylacetyl-glycerylether-phosphorylcholine) produced in a two-stage reaction, during which phospholipase A 2 hydrolyzes membrane phospholipid to form lyso-PAF. Acetylation of lyso-PAF yields PAF, which is inactivated by conversion back to lyso-PAF. PAF, synthesized by activated human lung mast cells, eosinophils, neutrophils, mononuclear phagocytes, endothelial cells, and epithelial cells, manifests a variety of biologic effects, including platelet aggregation, bronchoconstriction, chemotaxis for eosinophils and neutrophils, and increased vascular permeability (19).

Cytokines Cytokines represent an important group of mediators synthesized and secreted by mast cells, lymphocytes, and other cells, following activation. This diverse group of glycoproteins can modulate both nonspecific inflammatory and specific immune effects on target cells and may contribute to the vascular and epithelial changes that induce tissue remodeling in chronic asthma. Mast cell activation may directly or indirectly facilitate the release of cytokines from other cells, including fibroblasts, nerves, vascular endothelial cells, bronchial epithelial cells, and alveolar macrophages. Among the cytokines produced by activated human lung mast cells are TNF-_, IL-3, IL-4, IL-5, IL-6, IL-8, IL-10, IL-13, granulocytemacrophage colony-stimulating factor (GM-CSF), macrophage inflammatory protein-1_ (MIP-1_), and basic fibroblast growth factor. These cytokines are discussed in more detail later in this chapter, but information of particular relevance to mast cells is presented in this Subheading. One of the most important cytokines in inflammatory responses is TNF-_. Upon activation, mast cells provide an initial source of TNF-_, which is stored preformed within mast cell granules. In contrast, macrophages and lymphocytes synthesize this cytokine following activation, but contain little or no preformed TNF-_. Within bronchial and nasal mucosa, TNF-_ is confined to MCT mast cells, but it is localized to MCTC cells in the skin (20). Other key cytokines in allergic responses include IL-4 and IL-5. IL-4 has been identified within secretory granules of human MCT and MCTC cells in bronchial and nasal mucosa. This cytokine is important for inducing IgE synthesis and Th2 cell proliferation. IL-5 has also been identified in human mast cells derived from bronchial and nasal mucosa but, unlike IL-4, appears to be confined to MCT cells. IL-5 is secreted within several hours of mast cell activation and persists for 2–3 d (21). This mediator is essential for the maturation, activation, and survival of eosinophils. IL-6, a cytokine that enhances mucus secretion, also appears to be localized to MCT cells within the bronchial and nasal tissues, especially in the submucosa adjacent to mucus glands. IL-8, another cytokine released by mast cells, represents a major neutrophil chemotactic factor in the lung.


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Glucocorticoids inhibit cytokine production by many types of cells, which probably represents one of the mechanisms by which these drugs decrease inflammation. Indeed, in a murine model, corticosteroids have been shown to suppress mast cell and TNF-_-dependent allergic inflammation (22).

Mast Cell Activation Activation of mast cells is triggered by linking of adjacent Fc¡RI receptor-bound IgE molecules by bivalent or multivalent antigens or by antibodies directed against either IgE or its receptor, resulting in the rapid release of preformed mediators and the synthesis of newly generated mediators. Cyclic adenosine monophosphate (cAMP) may represent an important second messenger in the coupled activation– secretion process, and Fc¡RI-mediated activation may induce transmembrane activation of adenylate cyclase (23). The level of Fc¡RI expression on mast cells is upregulated by IL-4. In addition, IgE increases Fc¡RI receptor levels on mast cells and basophils, potentially increasing mediator release following antigen challenge. Treatment of human lung mast cells with `-agonists inhibits Fc¡RI-associated mediator release, decreasing LTC4 and PGD2 compared to Hi (24). Mast cells may also be activated by a variety of biologic, chemical, and physical stimuli to produce clinical symptoms that may mimic Fc¡RI-dependent mast cell activation, even though the pattern of mediator release elicited by these stimuli may differ from an IgE-mediated response. For MCTC cells, these stimuli include complement fragments C3a and C5a (anaphylatoxins), neutrophil lysosomal proteins, basic polypeptides (polyarginine and polylysine), peptide hormones, substance P, radiocontrast media, melittin in bee venom, drugs such as opiates (morphine) and muscle relaxants, calcium ionophores, and cold. Among these nonimmunologic stimuli, mast cells from human lung are activated only by calcium ionophores. Whether these differences in mast cell response are the result of local environmental influences or cell lineage has not been fully explored, but some data suggest a role for the mast cell microenvironment (23). Evidence of mast cell activation in asthmatic subjects includes the finding of elevated levels of Hi, tryptase, and PGD2 in bronchoalveolar lavage fluid following challenge with inhaled allergen (15,16). Moreover, the demonstration of increased Hi and tryptase in BAL fluid, even during subclinical asthma, implies ongoing activation of mast cells in these patients (25).

Eosinophils Eosinophils are characterized by distinctive granules that stain red with acid dyes, such as eosin. They are probably derived from a common basophil–eosinophil progenitor cell that differs from neutrophils and monocytes. Within the bone marrow, proliferation and maturation of eosinophils are regulated by various cytokines, including IL-3, IL-5, and GM-CSF (26). IL-3 and GM-CSF stimulate eosinophils, basophils, and neutrophils; IL-5 exhibits more specificity for

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Hamilton and Gershwin Table 2 Eosinophil Mediators Major basic protein Eosinophil cationic protein Eosinophil-derived neurotoxin Eosinophil peroxidase Charcot-Leyden crystal protein (lysophospholipase) Acid phosphatase Arylsulfatase B `-Glucuronidase Collagenase 5-HETE LTC4 PGE1, PGE2, PGF1 Thromboxane B2 Platelet-activating factor Cytokines (IL-3, IL-5, GM-CSF, and others)

eosinophils, even inducing eosinophil–basophil progenitors to differentiate into eosinophils (27). In contrast, transforming growth factor-` (TGF-`) and IFN-_ inhibit eosinophil proliferation and differentiation. Most eosinophils reside in tissues, especially those exposed to the external environment, such as the skin, respiratory tract, and gastrointestinal tract. Various receptors on the surface of eosinophils regulate cellular activities relevant to allergy and inflammation. Human peripheral blood eosinophils express receptors for the Fc portion of IgG (FcaRII or CD32), IgA (Fc_), and IgE (Fc¡R). Increased numbers of Fc_ receptors have been reported on eosinophils from patients with atopic diseases, and Fc¡ receptors have been implicated in the killing of parasites such as schistosomula (28). Surface receptors for C3a, C5a, C3b (CR1), intercellular adhesion molecule-1 (ICAM-1), and C-C chemokines have also been identified on eosinophils (26).

Eosinophil Mediators Activated eosinophils release a variety of inflammatory mediators, including granule proteins, enzymes, lipid mediators, reactive oxygen intermediates, and cytokines. Specific secretory granules contain major basic protein (MBP), eosinophil cationic protein (ECP), eosinophil peroxidase (EPO), eosinophil-derived neurotoxin (EDN), and `-glucuronidase; small secretory granules contain acid phosphatase, arylsulfatase B, and other enzymes (Table 2).

Major Basic Protein Eosinophils represent the primary source of major basic protein. Localized to the crystalloid core of the secretory granules, MBP binds acid dyes, such as eosin, to produce the distinctive staining of these cells. To a lesser extent, MBP is also present in basophils.


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MBP neutralizes heparin and releases Hi from human basophils, lysozyme and superoxide from neutrophils, and 5-hydroxytryptamine from platelets. Exposure of airways to MBP induces bronchoconstriction and increased airway responsiveness. Moreover, MBP impairs ciliary function and causes exfoliation of epithelial cells in the respiratory tract. Because these findings are characteristic of the airways in asthmatic patients, MBP is believed to play a significant role in inducing airway damage in asthma (29).

Eosinophil Cationic Protein ECP is a basic peptide present in the granule matrix. Biological activities include neutralization of heparin, potentiation of kallikrein activity, inhibition of mitogeninduced proliferation of human lymphocytes, and killing of schistosomula and other parasites. Like MBP, this protein is toxic to airway epithelial cells and presumably contributes to airway damage.

Eosinophil-derived Neurotoxin EPO and hydrogen peroxide (H2O2), in the presence of halide, have been shown to kill a variety of microorganisms (including bacteria and protozoa), tumor cells, and mast cells. In addition, EPO with H2O2 and halide trigger mast cell degranulation in a murine model (27). Since eosinophils generate H2O2, the EPO–H2O2– halide complex could represent a means whereby eosinophils induce mast cell degranulation in humans. Other actions of EPO include inactivation of LTB4 and LTC4, and release of 5-hydroxytryptamine from platelets.

Eosinophil Peroxidase EDN and ECP contain similar amino acid sequences, and are homologous to pancreatic ribonuclease (30). EDN damages myelinated neurons, which may account for neurologic disease in patients with hypereosinophilic syndrome, and inhibits proliferation of lymphocytes. However, its antihelminthic effects are weak, and it has not been shown to represent an important mediator in the pathogenesis of asthma (27).

Charcot-Leyden Crystal Protein (Lysophospholipase) Estimated to constitute up to 10% of eosinophil protein, Charcot-Leyden crystal protein is a hexagonal bipyramidal crystal with lysophospholipase activity described in the sputum of patients with asthma (31). This protein is also produced by basophils. Although the function of this protein is unknown, it may exert a protective effect against lysophospholipids produced by inflammatory cells.

Other Enzymes Other enzymes identified in eosinophils include acid phosphatase, arylsulfatase B, `-glucuronidase, neutrophil elastase, and collagenase, which degrades types I and III collagen, both present in lung tissue.

Arachidonic Acid Metabolites In eosinophils, LTC4 is the most prevalent lipoxygenase metabolite (27). Only small amounts of LTB4 are produced by eosinophils. Eosinophils also produce the

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Adhesion molecule

Table 3 Cell Adhesion Molecules Major cell expression Major ligands

E-selectin L-selectin P-selectin

Endothelial cells Most leukocytes Endothelial cells, platelets

ICAM-1 ICAM-2 ICAM-3 VCAM-1 LFA-1 Mac-1 VLA-4

E-selectin ligand-1 CD34, GLYCAM-1 P-selectin glycoprotein ligand-1 Endothelial cells, PMN, LFA-1, Mac-1, hyaluronan, fibroblasts, ASM fibrinogen Endothelial cells, lymphocytes LFA-1 PMN, monocytes, lymphocytes LFA-1 Endothelial cells, ASM VLA-4 PMN, monocytes, lymphocytes, ICAM-1, 2, 3 eosinophils PMN, monocytes, eosinophils ICAM-1, Factor X, iC3b, fibrinogen Lymphocytes, monocytes, VCAM-1, fibronectin eosinophils, basophils

ASM, airway smooth muscle. PMN, polymorphonuclear cell.

5-lipoxygenase metabolite 5-HETE. These mediators promote a variety of biologic activities relevant to asthma, including bronchoconstriction, mucus secretion, increased vascular permeability, and infiltration by eosinophils and neutrophils. In addition, eosinophils synthesize PAF, which attracts and activates platelets and neutrophils, and induces contraction of airway smooth muscle.

Cytokines Eosinophils secrete a variety of cytokines, including autocrine cytokines, such as IL-3, IL-5, and GM-CSF, which act on eosinophils themselves. Eosinophils also have the capacity to produce IL-1, IL-4, IL-6, IL-8, IL-10, IL-16, regulated activation, normal T-cell expressed and secreted (RANTES), TNF-_, TGF-_, TGF-`1, and MIP-1_, although apparently in smaller quantities than lymphocytes (26). Although the relative importance in allergic reactions of cytokines from eosinophils vs. lymphocytes is uncertain, it is plausible that cytokines from eosinophils may play a significant role in the microenvironment of the cells.

Eosinophil Recruitment Migration of eosinophils and other leukocytes from the circulation through the vascular endothelium requires the expression of specific cell surface proteins, known as cell adhesion molecules (Table 3). Cell adhesion molecules are classified into discrete groups that include integrins, selectins, and members of the immunoglobulin superfamily. These proteins regulate both cell–cell and cell–extracellular matrix protein interactions, and are essential for the recruitment of eosinophils and other leukocytes to sites of inflammation. This process is finely regulated, and occurs in sequential stages: leukocyte rolling and attachment, activation, firm adhesion, and migration.


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Adhesion Leukocyte rolling and the initial loose tethering to vascular endothelium is mediated primarily by selectins, a family of single-chain glycoproteins that have been divided into several subsets. L-selectin is constitutively expressed on the surface of most leukocytes; E-selectin is expressed on activated endothelial cells; and P-selectin is present on platelets and endothelial cells (32). Multiple selectin ligands have been identified, including CD34 for L-selectin. Firm adhesion and migration of eosinophils and other leukocytes are regulated by integrins and members of the immunoglobulin superfamily. Integrins are heterodimeric proteins composed of two noncovalently linked subunits, _ and `, which traverse the cell membrane, and are constitutively expressed on the surface of leukocytes, endothelial cells, and other cell types. The integrins may be subdivided into groups on the basis of `-subunit structure. `1-integrins are expressed on leukocytes and other cells; `2-integrins are only expressed on leukocytes. The expression of integrins on the surface of eosinophils is dependent on their level of activation. The ligands for these integrins include cell surface molecules that are members of the immunoglobulin supergene family, such as ICAM-1,-2, and -3 and vascular cell adhesion molecule-1 (VCAM-1) (33). These proteins are constitutively expressed on endothelial cells, neutrophils, lymphocytes, and other cells. VLA-4, a `1-integrin, binds to VCAM-1. Leukocyte function-associated antigen-1 (LFA-1) and Mac-1 are `2-integrins that bind to ICAM-1. Each of these integrins is important for the firm adhesion of eosinophils to endothelial cells and their subsequent migration from blood vessels into the tissues (34). Proinflammatory cytokines, such as IFN-a, IL-1`, and TNF-_ upregulate the expression of ICAM-1 and VCAM-1; IL-4 and IL-13 upregulate the expression of VCAM-1 on endothelial cells. This promotes VLA-4/VCAM-1-mediated adherence of eosinophils, but not neutrophils, which lack VLA-4 (35). In addition, IL-5 selectively augments adhesion of eosinophils to unstimulated endothelial cells. Thus, these cytokines may induce the selective recruitment of eosinophils into airways or other tissues. The potential significance of VCAM-1 and ICAM-1 in eosinophil mobilization is suggested by an experimental model in which mice lacking VCAM-1 and ICAM1 failed to develop pulmonary eosinophilia following antigen challenge (36).

Eosinophil Chemotaxis Migration of eosinophils from blood vessels into tissues is mediated by various chemoattractants. LTD4 is chemotactic for eosinophils, but not neutrophils, and thus may be important in allergic reactions. Platelet-activating factor, generated by eosinophils, mast cells, neutrophils, monocytes, macrophages, endothelial cells, and epithelial cells, is a more potent eosinophil chemotactic factor than Hi or LTD4, and displays selectivity for eosinophils over neutrophils (27). The anaphylatoxins C3a and C5a are both chemoattractants for eosinophils, although C5a is also chemotactic for neutrophils. T-lymphocytes, among other cells, produce various cytokines that are chemoattractants for eosinophils. These include GM-CSF, IL-2, IL-3, IL-4, and IL-5. IL-4 is chemotactic for eosinophils, but not neutrophils, from patients with

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atopy, but not from normal subjects (37). IL-5 is a selective eosinophil chemoattractant because eosinophils are the only peripheral blood leukocytes with receptors for IL-5. Inhalation of IL-5 by mice induces airway eosinophilia followed by hyperreactivity and mucus secretion (38). In addition, lymphocyte chemoattractant factor, or IL-16, induces eosinophil migration. Indeed, the importance of T-cells in mediating eosinophilia is demonstrated by its absence in athymic and T-cell-depleted animals (39). Eosinophil migration is also modulated by cytokines with chemotactic activity, known as chemokines. These proteins have been categorized, on the basis of the position of two cysteine residues as C-C (or `) and C-X-C (or _) chemokines. The C-C subfamily includes RANTES, MIP-1_, monocyte chemotactic protein-1 (mcp-1), and eotaxin, which are chemotactic for and activate eosinophils. C-C chemokines also target T-cells and monocytes. The C-X-C subfamily primarily targets neutrophils, although one member, IL-8, displays chemotactic activity for primed eosinophils (40).

Eosinophil Activation Degranulation of eosinophils may be triggered by IgG, IgE, IgA, secretory IgA (sIgA), RANTES, MIP-1_, platelet-activating factor, C3a, C5a, substance P, melittin, and `-integrin ligands. Among immunoglobulins, sIgA is the most potent mediator of degranulation. Combined with the observation that eosinophils reside at epithelial surfaces, this suggests an important physiologic role for sIgA in eosinophil degranulation. In addition, the eosinophil granule proteins, MBP and EPO, induce eosinophil degranulation, implying the presence of an autocrine pathway for the release of eosinophil granules. Eosinophils may also be activated by GM-CSF, IL-1, IL-3, IL-4, IL-5, TNF-_, and IFN-a. IL-5 stimulates eosinophil phagocytosis, degranulation, production of LTC4 and superoxides, and activation of kinases, such as mitogen-activated protein (MAP) kinase. IL-5 has been demonstrated in bronchoalveolar lavage fluid from subjects with allergic rhinitis following allergen challenge, and may represent the primary cytokine for eosinophils in late-phase reactions (41). In fact, inhibition of IL-5 prevents the pathologic changes induced by eosinophils (42). IL-3 and GM-CSF likewise enhance eosinophil phagocytosis, degranulation, and cytotoxicity. Moreover, IL-3, IL-5, and GM-CSF prolong eosinophil survival by inhibiting programmed cell death, or apoptosis. TNF-_ promotes eosinophil binding to endothelial cells, enhances cytotoxicity, increases synthesis of LTC4, and decreases eosinophil apoptosis. Eosinophils cultured in the presence of various cytokines, including GM-CSF alone, IL-3 plus IFN-a, or IL-5 plus TNF-_ express HLA-DR antigen, enabling them to present antigen to CD4+ T-cells. In addition, eosinophils isolated from bronchoalveolar lavage fluid of patients with allergic asthma express CD69, ICAM-1, and HLA-DR, none of which is present on peripheral blood eosinophils (27). These findings imply that eosinophils in the lungs of these patients are activated and may have the capacity to activate T cells via antigen presentation.


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Table 4 Mast Cells vs Basophils Mediator Mast cells Basophils Histamine Tryptase PGD2 LTB4 LTC4

+ + + + +

+ – – – +

The ability of eosinophils to release proinflammatory mediators, combined with the direct toxicity of eosinophil granule proteins on airway epithelium, implies that eosinophils represent major effector cells in the pathogenesis of allergic airway disease. To a significant degree, these activities are controlled by cytokines, which regulate release of eosinophils into the circulation, mediate adhesion of eosinophils to the endothelium, and direct eosinophils to sites of inflammation.

Basophils Basophils constitute the least common granulocytes in humans, and, unlike eosinophils, are not found in significant numbers in peripheral tissues. However, basophils infiltrate sites of inflammatory or immunologic reactions, often in conjunction with eosinophils. Like mast cells, basophils are derived from CD34+ progenitor cells and constitutively express surface receptors that bind the Fc portion of IgE with high affinity (Fc¡RI) (2). However, basophils differentiate and mature in the bone marrow under the influence of IL-3 and circulate in the blood, rather than residing in the tissues. Mature human basophils, isolated from peripheral blood, release IL-4 and IL-13 following Fc¡RI-dependent activation (43). Like mast cells, basophils store Hi, neutrophil chemotactic factor, and other preformed mediators in secretory granules. The predominant proteoglycan in human basophils is chondroitin sulfate A. Basophils also contain small amounts of Charcot-Leyden crystal protein and major basic protein. Following allergen stimulation, basophils generate LTC4, LTD4, LTE4, and PAF. In contrast to mast cells, basophils contain negligible or undetectable amounts of tryptase, chymase, carboxypeptidase, and cathepsin G, and do not produce LTB4 or PGD2 (Table 4). Thus, the finding of an elevated Hi level in conjunction with either tryptase or PGD2 implies mast cell activation; elevated Hi in the absence of tryptase and PGD2 suggests basophil activation (2). Basophils are recruited into tissues during the late-phase reaction that follows antigen challenge. They have been described in lung tissue from patients with asthma, based on the finding of cells staining for IgE, but not tryptase (44). However, eosinophils, dendritic cells, and monocytes also express Fc¡ receptors, so these criteria may not be specific for basophils.

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Neutrophils Although identified as participants in the inflammatory response in pulmonary diseases such as bronchitis and cystic fibrosis, the role of neutrophils in asthma has been less well defined. However, since asthma represents an inflammatory disease, it is plausible that neutrophils may contribute to its pathophysiology. After release from the bone marrow, neutrophils circulate in the blood stream for 6–8 h, and are then sequestered through the process of margination, primarily in the lung capillaries. Migration of neutrophils from blood vessels into tissues requires the expression of specific cell surface proteins, known as cell adhesion molecules, as discussed previously. Neutrophils express several adhesion proteins, including the integrins Mac-1 and LFA-1. Data from Mac-1-deficient mice demonstrate that Mac-1 plays a significant role in mediating binding of neutrophils to fibrinogen and neutrophil degranulation, but is not necessary for neutrophil emigration, which is more dependent on LFA-1 (45). Neutrophils recruited to sites of allergic inflammation may generate a number of products that induce tissue damage, including elastase, collagenase, toxic oxygen radicals, TXA2, PAF, and LTB4. This lipoxygenase profile differs from mast cells and eosinophils, which produce primarily LTC4. Both PAF and LTB4 have been identified in human airways following allergen challenge, consistent with neutrophil activation (46). In addition, increased neutrophils have been reported in sputum during exacerbations of asthma, and a correlation has been demonstrated between the number of neutrophils in bronchoalveolar lavage fluid and airway responsiveness in patients with asthma (47). Moreover, in patients who died from an acute asthma attack, neutrophils comprised the majority of cells infiltrating the airways; in patients who died from hours to days after an asthma flare, most infiltrating cells were eosinophils (48). These data suggest that neutrophils may play a significant role in the initial allergic inflammatory response, but that they are replaced by eosinophils as the reaction proceeds.

Mononuclear Phagocytes The monocyte–macrophage system, which consists of monocytes in the circulation and macrophages in tissues, plays a key role in generating and modulating the immune response by presenting antigen to lymphocytes and producing cytokines that are involved in the activation of T-cells. In addition, macrophages produce mediators that may directly damage airway tissues. In adults, monocyte production is stimulated by IL-3, GM-CSF, and macrophage-colony-stimulating factor (M-CSF), but is inhibited by PGE2 and IFN–_/` (49). Human monocytes secrete multiple cytokines relevant to infectious and allergic diseases, including IL-1, IL-6, IL-8, IL-10, IL-12, IL-15, IL-18, colonystimulating factors, and TNF-_. They also generate the chemokines RANTES, MIP-1_, and MCPs. Monocytes are the major source of IL-12, which acts on T-cells and natural killer (NK) cells to induce the production of IFN-a, thereby augmenting the Th1


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pattern of differentiation (see below). In contrast, IL-10 secreted by monocytes inhibits the synthesis of IFN-a, which may counterbalance the effects of IL-12. Activated mononuclear phagocytes also produce inflammatory mediators, such as PGD2, LTB4, LTC4, and PAF. Peripheral blood monocytes from patients with asthma manifest both phenotypic and functional differences from normals. Monocytes from atopic subjects express increased high- and low-affinity receptors for IgE, potentiating allergenspecific activation (50). In addition, monocyte production of the cytokines GM-CSF, TNF-_, and IL-6 is enhanced in asthmatic patients. Alveolar macrophages demonstrate potent phagocytosis and antimicrobial properties, but in normal subjects represent weak activators of T-cells, compared to peripheral blood monocytes from the same individuals. This failure of alveolar macrophages to activate T-cells contrasts with pulmonary dendritic cells, which are the primary antigen-presenting cells in the lung and are considerably more effective in stimulating T-cells. In fact, alveolar macrophages from normal individuals probably suppress dendritic cell antigen presentation and T-cell activation. However, this suppressive effect may be decreased in patients with asthma following antigen challenge, perhaps as a consequence of GM-CSF, which enhances mononuclear phagocyte antigen presenting function (51). Macrophages are also potential inducers of airway damage through production of nitric oxide (NO), which reacts with superoxide anions to form peroxynitrite and, subsequently, hydroxyl radicals. Synthesis of NO in macrophages is induced by IL-1_, IL-1`, IFN-a, TNF-_, bacteria, and bacterial products, such as endotoxin, and therefore may be triggered by either allergic or infectious processes. In addition, alveolar macrophages produce platelet-derived growth factor (PDGF), TGF-`, and fibroblast growth factors, which promote collagen secretion and may contribute to irreversible airway remodeling in asthma. Thus, mononuclear phagocytes play an important role in allergic reactions by activating T-cells and regulating Th cell responses. Although alveolar macrophages may function as suppressors, rather than inducers, of the immune response, they may also mediate tissue damage and airway remodeling.

Lymphocytes B-lymphocytes are essential to the immune response by virtue of their synthesis of specific IgE Ab following sensitization by antigen, whereas T lymphocytes regulate B-cells and play a significant role in producing the late-phase reaction and allergic inflammation through the release of cytokines and chemokines.

B-Lymphocytes B-cells utilize membrane-bound immunoglobulin as receptors for soluble antigens. Immature B-lymphocytes express surface receptors for IgM; most mature B-cells display both IgM and IgD surface receptors. Following antigenic stimulation, B-cells proliferate and differentiate into immunoglobulin-secreting

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cells with the capacity to express different immunoglobulin heavy-chain isotypes, resulting in the production of different classes of antibody with the same antigenic specificity. The principal antibody produced in response to a primary antigenic challenge is IgM, followed by IgG, IgA, and IgE. During the subsequent, or memory, response, IgG, IgA, and IgE account for most antibody produced. Among the factors that control immunoglobulin isotype switching are cytokines. IL-4 induces switching from IgM to IgG4 and IgE, perhaps sequentially. IL-13 also promotes isotype-switching to IgE production. IL-5, IL-6, and TNF-_ stimulate IL-4-mediated IgE synthesis; IFN-_, IFN-a, TGF-`, IL-2, IL-8, IL-10, and IL-12 inhibit IgE synthesis (52). In addition to IL-4 or IL-13, direct contact between B-lymphocytes and activated T-cells appears necessary for switching to IgE production. B cells present processed antigen bound to major histocompatibility complex class II (MHC-II) molecules to T-cell receptors (TCR). Other cellular surface interactions are also required. These may occur between the B-cell surface glycoprotein CD40 and the T cell receptor for CD40 ligand (CD40L), and between the B-cell protein CD23 (a low-affinity receptor for IgE) and CD21 on T-cells.

T-lymphocytes Classification T-lymphocytes express a number of unique cell surface molecules that have been utilized for classification and functional characterization. CD4+ T-cells, which constitute about 60% of circulating T-cells, provide help for B-cell differentiation and mediate delayed-type hypersensitivity (DTH) reactions. CD8+ T-cells participate in the host response to intracellular microorganisms and mediate cytotoxic and suppressor activities. On the basis of these functions, CD4+ T-cells have been categorized as helper-inducer cells and CD8+ T-cells as cytotoxic-suppressor cells. This functional characterization is now recognized as an oversimplification, because both CD4+ and CD8+ lymphocytes can act as helper-inducer and cytotoxic-suppressor cells (Table 5). Moreover, both CD4+ and CD8+ T-cell subsets proliferate and can yield similar cytokines following antigenic challenge. The major difference between CD8+ and CD4+ T-cells is that the former recognize antigens presented by MHC-I molecules (HLA-A, -B, and -C); the latter recognize antigens presented by class MHCII molecules (HLA-DR, -DP, and -DQ in humans). CD4+ T-cells have been divided into Th1 and Th2 subsets (Table 6). Th1 cells secrete primarily IL-2 and IFN-a, whereas Th2 cells preferentially secrete IL-4, IL-5, IL-6, IL-10, and IL-13 (53). Th1 cytokines promote cytotoxicity, DTH, and activation of monocytes, leading to a proinflammatory response; the Th2 cytokines IL-4 and IL-13 induce IgE production and inhibit monocytes, and IL-5 activates eosinophils (54). Th1 cytokines also regulate Th2 cytokines, and vice versa. Thus, IL-4 and IL-10 inhibit IFN-a production, and IFN-a inhibits IL-10 synthesis. Both types of Th cells secrete IL-3, TNF-_, and GM-CSF. CD4+ cells may also produce `-chemokines, such as eotaxin, RANTES, MIP-1_, and MCP-3. Similarly, CD8+ T-cells have been categorized into T cytotoxic 1 (Tc1) and T cytotoxic 2 (Tc2) subsets, based on their cytokine profiles. Thus, CD8+ T-cells


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Table 5 Comparison of CD4+ and CD8+ T-Cells Characteristic CD4+ cells CD8+ cells Antigen source Antigen presentation Cytokines Cytotoxic DTH

MHC-II Extracellular Th1 and Th2 MHC-II Yes

MHC-I Intracellular Tc1 and Tc2 MHC-I Yes

Table 6 Comparison of Th1 and Th2 Cells Characteristic Type of response Functions Activators Cytokine inducers Cytokine inhibitors Cytokines produced

Th1 Cell-mediated DTH, cytotoxicity Microbes IFN-a, IL-12 IL-4, IL-10 IFN-a, IL-2

Th2 Humoral-mediated B cell help, IgE synthesis Allergens, parasites IL-4, IL-5, IL-10 IFN-a, IL-12 IL-4, IL-5, IL-10, IL-13

DTH, delayed-type hypersensitivity.

that produce cytokines similar to CD4+ Th1 or Th2 cells have been designated Tc1 or Tc2 cells, respectively. Another type of Th cell, Th0, secretes cytokines characteristic of both Th1 and Th2 and has been considered a precursor to Th1 and Th2 cells, with the pattern of differentiation determined by cytokines and other factors (Fig. 3). Infection of monocytes by bacteria or viruses induces the secretion of IFN-_ and IL-12, which promote the formation of Th1 cells, and IFN-a, which inhibits the development of Th2 cells. In contrast, IL-4 from mast cells or other sources promotes the formation of Th2 cells, and allergen stimulation of T-cells induces the synthesis of Th2 cytokines (54).

Antigen Recognition The most characteristic T-cell surface markers are TCRs, through which T-cells recognize antigens. These receptors are heterodimers composed of two polypeptide chains (_ and ` or a and b), which are associated with the CD3 complex. Each T cell expresses a single type of TCR, either _` (>90%) or ab. In normal subjects, cells bearing ab TCR seldom express either CD4 or CD8. _` T cell receptors are members of the immunoglobulin supergene family and recognize peptide fragments that have been degraded, or processed, by antigenpresenting cells (APCs). Stimulation of T-cells by IL-1, which is secreted by macrophages and other APCs, and binding of the T-cell ligands CD28/CTLA-4 to the receptors CD80/CD86 on the surface of APCs, are also necessary for activation of _` T-cells.

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Fig. 3. Th0, Th1, Th2.

ab TCRs may recognize some antigens directly without processing, acting more like antibodies than _` receptors, and perhaps representing a more primitive immune system. In addition, ab T-cells may constitute the first line of defense at mucosal surfaces, and thus may be particularly relevant in respiratory and gastrointestinal diseases. During sensitization, ab T-cells produce IL-4, which is crucial for a Th2 immune response. They may also interact with epithelial cells


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through growth factors and cytokines to help repair damaged tissues. However, understanding of these cells is relatively limited, and most of the discussion that follows applies to _` T-cells (55). Antigens presented to CD4+ T-cells originate extracellularly and include allergens such as pollens, molds, and housedust mites, in addition to extracellular bacteria, bacterial toxins, fungi, and vaccines. These exogenous proteins are internalized into antigen-presenting cells such as macrophages, monocytes, and dendritic cells, by phagocytosis or pinocytosis. They are subsequently localized into endosomal vacuoles, which fuse with lysosomes, where the antigens are degraded into peptides. These peptides bind to MHC-II molecules and are transported to the APC surface, where the peptide–MHC- II molecule combination is recognized by specific _` TCRs on the surface of CD4+ T-cells (53). In contrast, antigens presented to CD8+ T-cells originate in the cytoplasm. These endogenous antigens may include intracellular infectious agents, such as viruses and bacteria, tumor-associated antigens, and transplantation antigens. Endogenous antigens are also processed by APCs into peptides, after which they preferentially bind to MHC-I molecules for transport to the cell surface, where the combination is recognized by specific _` TCRs on CD8+ T-cells. The relevance of T-cells to allergic asthma is suggested by studies of bronchial biopsies and peripheral blood from asthmatic patients, which reveal a correlation between the number of activated (CD25+) T-cells, the number of activated eosinophils, and asthma severity (56). The number of ab T-cells in the lungs of patients with asthma is also increased, and the majority of these cells express CD4. Treatment of asthma patients with glucocorticoids profoundly decreases pulmonary ab T-cells, presumably as a result of steroid-induced apoptosis (55). In addition, increased serum levels of IL-5 have been reported in patients with asthma. Glucocorticoid therapy of patients with allergic asthma decreases the proportion of bronchoalveolar lavage fluid cells expressing IL-4 and IL-5, and increases those expressing IFN-a, implying a shift toward a Th1 response (57). Among asthmatic patients in whom skin tests for aeroallergens are negative and serum levels of IgE are normal, representing so-called intrinsic asthma, inflammatory cells also infiltrate the bronchial mucosa. Moreover, the cellular infiltration and IL-4 and IL-5 expression in these patients is similar to allergic asthma (58). This suggests that IgE-mediated reactions may elicit pulmonary disease typical of allergic asthma, even in the absence of systemic atopy.

Cytokines Cytokines, low-mol-wt proteins that regulate immune and inflammatory responses, are secreted by lymphocytes, mast cells, macrophages, and airway cells, among others. These mediators enhance IgE synthesis (IL-4 and IL-13), promote eosinophil development (IL-5, IL-3, and GM-CSF), and facilitate recruitment of eosinophils (IL-3, IL-5, IL-16, GM-CSF, and certain C-C chemokines), basophils (TNF-_ and IL-4), and monocytes and T-cells (IL-16 and certain C-C chemokines). This subheading summarizes relevant properties of cytokines identified as playing a significant role in the pathophysiology of asthma (Table 7).

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Cytokine GM-CSF IFN-a

IL-1

IL-3 IL-4

IL-5 IL-8 IL-9

IL-10

IL-11 IL-12 IL-13

IL-16 IL-18

Hamilton and Gershwin Table 7 Cytokines in Allergic Diseases Effects Promotes differentiation of macrophages Activates eosinophils Prolongs eosinophil survival Activates macrophages Stimulates B-cell proliferation Inhibits Th2 lymphocytes Inhibits IL-4-induced IgE synthesis Enhances MHC-I and MHC-II expression Increases ICAM-1 expression Increases proliferation of B cells and antibody synthesis Promotes growth of Th cells in response to APCs Stimulates production of T-cell cytokines and IL-2 receptors Induces fibroblast proliferation and synthesis of fibronectin and collagen Increases ICAM-1 and VCAM-1 expression Stimulates development of MCs, lymphocytes, macrophages Activates eosinophils Prolongs eosinophil survival Promotes growth of Th2 cells, MCs, eosinophils, basophils Induces IgE isotype switching Enhances MHC-I and MHC-II expression on APCs Increases VCAM-1 expression Activates eosinophils Prolongs eosinophil survival Attracts eosinophils Inhibits IL-4-mediated IgE synthesis Attracts primarily neutrophils, also activated eosinophils Promotes MC and T-cell proliferation Stimulates IgE synthesis Produces eosinophilia Induces bronchial hyperreactivity Promotes growth of B-cells, cytotoxic T cells, MCs Inhibits monocyte/macrophage function Induces tolerance in T helper lymphocytes Inhibits IL-4-induced IgE synthesis Decreases eosinophil survival Promotes generation of MCs and B-cells Induces bronchial hyperreactivity in response to viral infections Enhances activity of cytotoxic T cells and NK cells Promotes Th1 and inhibits Th2 cell development Inhibits IL-4-induced IgE synthesis Induces IgE isotype switching Increases VCAM-1 expression Suppresses production of proinflammatory cytokines and chemokines Decreases synthesis of nitric oxide Promotes growth and migration of CD4+ T-cells Induces IL-2 receptors and class MHC-II expression on CD4+ T-cells Stimulates synthesis of IFN-a and GM-CSF Decreases IL-10 synthesis Promotes Th1 responses

Mediators and Mechanisms Table 7 (cont.) Cytokines in Allergic Diseases Effects

Cytokine TGF-`

TNF-_

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Stimulates fibroblasts and epithelial cells Inhibits B-cells, T-helper cells, cytotoxic T-cells, MCs Inhibits airway smooth muscle cell proliferation Attracts macrophages, MCs, fibroblasts Activates neutrophils Enhances class I and II MHC expression Increases ELAM-1, ICAM-1, and VCAM-1 expression Stimulates cytokine production by monocytes and airway epithelial cells Induces COX-2 expression in airway smooth muscle Induces bronchial hyperreactivity

Chemokines Chemokines are chemoattractant peptides that are structurally related and distinguished by the presence of four cysteine residues (59). They are classified on the basis of the position of the first two of these cysteine residues as C-C (containing adjacent cysteines) or C-X-C (containing another amino acid positioned between cysteine residues) subfamilies. The C-C subset displays chemotactic activity for eosinophils, T-lymphocytes, and monocytes, but not neutrophils. Members of this group include eotaxin, RANTES, MCP-1, MCP-3, MCP-4, MIP1_, and MIP-1`. RANTES and eotaxin are especially potent inducers of eosinophil migration, and eotaxin appears to be specific for eosinophils. Bronchoalveolar lavage fluids from patients with allergic asthma demonstrate increased levels or bioactivity of MIP-1_, MCP-3, and RANTES (60). The C-X-C subfamily exerts chemotactic activity primarily toward neutrophils, although one member, IL-8, also expresses chemotactic activity toward activated eosinophils.

Granulocyte-macrophage Colony-stimulatory factor GM-CSF is secreted by activated macrophages and T-cells in addition to mast cells, eosinophils, endothelial cells, epithelial cells, smooth muscle, and fibroblasts. This molecule promotes the differentiation and activation of neutrophils and macrophages. GM-CSF also activates mature eosinophils and, like IL-3 and IL-5, prolongs eosinophil survival.

Interferon-a IFN-a, derived mainly from Th1 lymphocytes but also from cytotoxic T cells, ab T-cells, NK cells, and macrophages, represents the most important cytokine activator of macrophages. IFN-a stimulates expression of MHC class I and II antigens and B cell proliferation and differentiation. It also enhances the adherence of granulocytes to endothelial cells by inducing the expression of ICAM-1 and augments killing by neutrophils and NK cells. IFN-a downregulates allergic responses, in part by inhibiting the effects of IL-4 on B cells, thereby decreasing secretion of IgE.

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Interleukin-1 IL-1 is synthesized primarily by monocytes and macrophages but also by lymphocytes, neutrophils, endothelial cells, fibroblasts, and many other cell types. IL-1 secretion is induced by endotoxin, microorganisms, cytokines, and antigens. IL-1 functions as a growth factor for Th lymphocytes in response to antigen-primed APCs (40). In the absence of IL-1, tolerance or a decreased immune response develops. IL-1 stimulates the synthesis of T-cell cytokines (including TNF, IL-1, IL-2, IL-6, and GM-CSF) and IL-2 receptors, and induces the proliferation of Bcells and antibody synthesis (61). IL-1 also enhances fibroblast proliferation and the synthesis of fibronectin and types I, III, and IV collagen. In addition, IL-1 promotes the formation of arachidonic acid metabolites, including PGE2 and LTB4. The proinflammatory effects of cytokines such as IL-1 and TNF-_ in allergic and other disorders may be significantly related to the ability of these mediators to enhance the recruitment of leukocytes by inducing the formation of cell adhesion molecules such as VCAM-1, ICAM-1, E-selectin, and P-selectin on vascular endothelial cells. Of relevance is the observation that E- and P- selectins preferentially promote the migration of Th1 over Th2 cells (62). IL-1 receptor antagonist (IL-1ra), also a member of the IL-1 family, is produced during inflammatory processes and appears to antagonize the proinflammatory effects of IL-1. For example, IL-1ra inhibits the late-phase reaction and decreases airway inflammation in experimental models of asthma (63).

Interleukin-3 IL-3 is derived primarily from Th cells but can also be produced by mast cells and eosinophils. IL-3 promotes the development of various hematopoietic cells, including mast cells, lymphocytes, macrophages, granulocytes, and erythrocytes. It also activates eosinophils and prolongs their survival, analogous to IL-5 and GM-CSF.

Interleukin-4 IL-4 resides in mast cells and eosinophils as a preformed peptide which is rapidly released following IgE-antigen stimulation. IL-4 is also secreted by Th2 lymphocytes, cytotoxic T cells, and basophils. IL-4 promotes the growth of mast cells and, in conjunction with IL-3, enhances the growth of eosinophils and basophils. IL-4 enhances the ability of B cells to present antigen by stimulating the expression of MHC class II antigen, CD40, CD80, CD86, surface IgM, and low-affinity receptors for IgE (CD23) (64). IL-4 also initiates isotype switching from IgM to IgE and is essential for the synthesis of IgE. IL-4 serves as a growth factor for Th2 CD4+ cells and cytotoxic T-cells and induces the expression of MHC class I and II antigens and low-affinity receptors for IgE on macrophages. Moreover, by stimulating the expression of VCAM-1 on endothelial cells, IL-4 increases the adhesiveness of T lymphocytes, eosinophils, basophils, and monocytes, but not neutrophils. In addition, IL-4 decreases monocyte-mediated antibody-dependent cellular cytotoxicity (ADCC) and inhibits monocyte synthesis of IL-1, IL-6, TNF-_, and NO.


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Thus, IL-4 acts at multiple levels relevant to the allergic response, including IgE synthesis, T cell development, and migration of effector cells to sites of allergic inflammation.

Interleukin-5 IL-5 is produced by Th2 cells and mast cells and exerts multiple actions on eosinophils. These include stimulating eosinophil maturation and activation, acting as an eosinophil chemoattractant, and prolonging eosinophil survival (65).

Interleukin-6 Monocytes and macrophages represent the major sources of IL-6, but it is also synthesized by T and B lymphocytes, endothelial cells, epithelial cells, fibroblasts, synoviocytes, and other cell types. IL-6 induces B cells to differentiate into mature plasma cells and secrete antibodies. IL-6 also regulates growth, differentiation, and activation of T-cells and promotes the production of platelets from megakaryocytes. IL-6 inhibits TNF and IL-1 synthesis, thereby downregulating the proinflammatory cycle.

Interleukin-8 IL-8 is synthesized primarily by monocytes, phagocytes, and endothelial cells but also by eosinophils, neutrophils, T-cells, mast cells, and fibroblasts. IL-8 represents a potent chemotactic factor for neutrophils and is classified as a C-X-C chemokine. IL-8 also induces neutrophil degranulation and activation and inhibits IL-4-mediated IgE synthesis.

Interleukin-9 IL-9 is produced by Th2 cells and may play a central role in the pathogenesis of atopic asthma. Effects of IL-9 on mast cells include increased proliferation, IgE receptor expression, and IL-6 secretion. IL-9 also induces IgE synthesis by B cells and stimulates T-cell proliferation. Moreover, data from murine studies indicate that IL-9 induces bronchial hyperresponsiveness and eosinophilia (66). Whether the gene encoding IL-9 represents a susceptibility gene for asthma remains to be determined.

Interleukin-10 IL-10 is produced by Th1 and Th2 lymphocytes, cytotoxic T-cells, B lymphocytes, mast cells, monocytes, and macrophages (67). In humans, IL-10 is derived primarily from monocytes and B-cells. IL-10 is a potent inhibitor of mononuclear phagocyte function. It suppresses the ability of monocytes to express CD80 and CD86, which are necessary for Th activation, and inhibits the synthesis of superoxide anions and NO by activated mononuclear phagocytes. IL-10 induces permanent tolerance in Th lymphocytes and decreases synthesis of IFN-a and IL-2 by Th1 cells and IL-4 and IL-5 by Th2 cells. IL-10 also decreases production of GM-CSF and TNF-_ by eosinophils and shortens eosinophil survival. On the other hand, IL-10 stimulates the growth of mast cells, B cells, and

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cytotoxic T-cells and increases immunoglobulin secretion, although IL-4-induced IgE synthesis is decreased. The result is that IL-10 stimulates humoral and cytotoxic immune reactions but inhibits cellular immunity and allergic inflammation.

Interleukin 11 IL-11 exhibits synergistic activity with stem cell factor (SCF) to produce mast cells and promotes differentiation of lymphocytes. IL-11 is produced in response to viral infections such as respiratory syncytial virus, rhinovirus, and parainfluenza virus type 3, all of which exacerbate asthma. The finding that IL-11 levels are highest in patients with clinically detectable wheezing suggests that it induces airway hyperresponsiveness (68).

Interleukin 12 IL-12 is produced by monocytes, macrophages, dendritic cells, B-cells, neutrophils, and mast cells. IL-12 induces secretion of IFN-a and TNF-_ by Th1 cells and inhibits IL-4, IL-5, and IL-10 production by Th2 cells, thereby promoting development of Th1 cells and inhibiting Th2 cells. IL-12 also activates and stimulates proliferation and differentiation of NK cells and cytotoxic T-cells (49). Synthesis of IL-12 is enhanced by IFN-a, microorganisms, and binding of monocyte CD40 to T-cell CD40 ligand (CD40L) (69). Glucocorticoids inhibit IL-12 synthesis by human monocytes and facilitate their ability to induce IL-4 secretion by CD4+ T-cells, thereby shifting the balance toward Th2 cell production.

Interleukin-13 IL-13 is produced by Th1 and Th2 lymphocytes, mast cells, and dendritic cells. IL-13 exerts effects similar to IL-4 on monocytes, macrophages, and B cells. However, unlike IL-4, IL-13 does not affect T-cells, which lack IL-13 receptors (70). IL-13 induces IgE isotype switching and endothelial VCAM-1 expression but decreases monocyte ADCC, production of proinflammatory cytokines and chemokines, synthesis of NO, and glucocorticoid receptor-binding affinity. This last effect may be relevant to impaired glucocorticoid responsiveness in asthma (49).

Interleukin 16 Secreted by CD8+ T-cells, epithelial cells, eosinophils, and mast cells, IL-16 is the major source of CD4+ lymphocyte chemotactic activity shortly after antigen challenge in asthmatic subjects (71). IL-16 also promotes growth of CD4+ T-cells and induces IL-2 receptors and class II MHC molecules on these cells.

Interleukin-18 IL-18 stimulates secretion of IFN-a and GM-CSF, facilitates development of Th1 cells, and activates NK cells, effects similar to IL-12. IL-18 also induces synthesis of TNF, IL-1, Fas ligand, and several chemokines and decreases IL-10 production (72). Coadministration of IL-18 with IL-12 in a murine model of asthma produces a synergistic effect with inhibition of antigen-specific Th2 cell differen-


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tiation and lack of airway hyperresponsiveness (73). IL-18 is produced by lung, liver, and other tissues but not by lymphocytes.

Transforming Growth Factor TGF-_, synthesized primarily by macrophages and keratinocytes, stimulates the proliferation of fibroblasts, induces epithelial cell development, and promotes angiogenesis. Transforming growth factor-` (TGF-`), which exerts primarily antiinflammatory effects, is synthesized by many types of cells, including platelets, monocytes, some T-cells, and fibroblasts. TGF-` functions as a chemoattractant for mast cells, macrophages, and fibroblasts. This factor activates monocytes but inhibits mast cells, B-cells, T helper cells, and cytotoxic T-cells (74). TGF-` also induces differentiation of epithelial cells and stimulates fibroblasts but inhibits proliferation of airway smooth muscle cells. Its role in asthma may range from inhibiting IgE synthesis and mast cell proliferation through inducing fibrosis that leads to airway remodeling.

Tumor Necrosis Factor TNF is produced primarily by mononuclear phagocytes (TNF-_) and lymphocytes (TNF-`). TNF-_ is present as a preformed mediator in mast cells and is also synthesized by neutrophils, activated lymphocytes, NK cells, endothelial cells, and smooth muscle cells. Endotoxin is the most potent trigger for TNF synthesis by monocytes, although cytokines, including IL-1, IL-3, IFN-a, and GM-CSF, may also induce TNF secretion. TNF-_ activates neutrophils, enhances class I and II MHC molecule expression, stimulates cytokine production by monocytes and airway epithelial cells, and induces COX-2 in airway smooth muscle cells. TNF-_ also increases the expression of ICAM-1, VCAM-1, and endothelial-leukocyte adhesion molecule-1 (ELAM-1) on endothelial cells (75). TNF-_ is necessary for the activation of NF-gB, a transcription factor that enhances the expression of mRNA for TNF-_, GM-CSF, IL-2, IL-6, IL-8, and E-selectin in a number of cells, including endothelial, epithelial, and mast cells (76). Inhalation of TNF-_ by normal subjects induces bronchial hyperreactivity (40). This broad range of effects indicates that cytokines may represent one of the essential elements that link the early-phase reaction, the late-phase reaction, and the persistent inflammation associated with chronic allergic disorders. Moreover, cytokine production and arachidonic acid metabolites are interrelated, as demonstrated by stimulation of IL-5 synthesis by LTB4.

Airway Epithelial Cells Airway epithelial cells have been considered to constitute a relatively passive barrier against the external environment, but recent data indicate that they not only play an active role in preserving mucosal integrity, but also regulate inflammatory and immune responses through the generation of multiple biologically active mediators.

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The integrity of the airway epithelium, typically compromised by allergic inflammation, is maintained by airway epithelial cells through vigorous repair capabilities and secretion of matrix glycoproteins, including fibronectin, tenascin, and entactin (77). Fibronectin, a primary component of the extracellular matrix, acts as a chemoattractant for fibroblasts and epithelial cells and as a substrate for cell adhesion, thereby augmenting recruitment of epithelial cells and repair of damaged airways. Airway epithelial cells actively participate in inflammatory responses by secreting a number of cytokines, including colony-stimulating factors such as GM-CSF and the proinflammatory cytokines IL-6 and IL-11, in addition to low levels of IL-1, IL-10, and TNF-_ (77). Moreover, epithelial cells recruit leukocytes through the synthesis of IL-16, the C-X-C chemokine IL-8, and the C-C chemokines eotaxin, RANTES, MCP-1, MCP-4, and MIP-1_. The potential importance of such chemoattractants is demonstrated by the finding of increased concentrations of IL-8, RANTES, MCP-1, and MIP-1_ in BAL fluid from patients with asthma (60). Furthermore, epithelial cells may play a major role in airway remodeling by releasing the growth factors TGF-_, TGF-`, stem cell factor, and basic fibroblast factor. Arachidonic acid metabolites synthesized by airway epithelial cells may contribute to inflammation. The 15-lipoxygenase pathway predominates, leading to the synthesis of a variety of biologically active metabolites, including 15-hydroxyeicosatetraenoic acid, which activates the 5-lipoxygenase pathway in mast cells (77). Airway epithelial cells also generate the cyclooxygenase products PGF2_ and PGE2, which may exert a protective effect against bronchoconstriction. Another lipid mediator, platelet-activating factor, is produced in small amounts and may promote inflammatory responses by recruiting neutrophils and eosinophils and increasing vascular permeability. Among the peptide mediators released by epithelial cells, endothelins may be the most important. These molecules are potent bronchoconstrictors and vasoconstrictors and stimulate mucus secretion and smooth muscle proliferation. Although endothelins are produced by a number of cells within the airways, including endothelial cells, macrophages, and mast cells, the bronchial epithelium is the primary site of endothelin expression (78). Levels of endothelin-1 and endothelin3 are increased in bronchoalveolar lavage fluid from patients with asthma and correlate with the severity of symptoms, suggesting a possible role for these mediators in the pathogenesis of asthma (79). Epithelial cells also produce substance P, CGRP, hydrogen peroxide, and nitric oxide. NO is derived from the amino acid L-arginine by the enzyme nitric acid synthase (NOS), of which three isoforms have been identified (40). Two of these isoforms are constitutive (cNOS) and perform physiological functions in nerves and endothelial cells. The third isoform, inducible NOS (iNOS), is not expressed normally but is induced by proinflammatory cytokines, such as TNF-_, IL-1, and IFN-a, and endotoxin. Inducible NOS is associated with inflammatory diseases and host defenses against infectious agents and generates much larger quantities of NO than cNOS. Expression of iNOS, but not cNOS, is inhibited by corticosteroids. Epithelial cells and macrophages account for the synthesis of most pulmonary NO, which is produced via iNOS and may mediate respiratory epithelial pathol-


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ogy through the formation of peroxynitrite and hydroxyl radicals. In addition, nitric oxide selectively suppresses Th1 cells, leading to a predominantly Th2 response, characteristic of allergic reactions (80). Nonetheless, the effects of NO may not always be detrimental to the airways: NO is capable of inducing bronchodilatation and vasodilatation and may inhibit the production of proinflammatory cytokines, at least in viral infections (81). Expression of iNOS is increased in airway epithelial cells of patients with asthma (82). Exhaled NO is significantly increased in patients with asthma and other inflammatory airway diseases, and its measurement may provide a noninvasive means of assessing airway inflammation and the effect of therapeutic agents (83). Thus airway epithelial cells may synthesize proinflammatory factors that induce epithelial damage, as well as mediators that repair that damage, albeit with possible remodeling of the airways. In addition, epithelial cells secrete NO, which under various circumstances may be either beneficial or detrimental to the host.

Airway Smooth Muscle Cells Characteristic findings in asthma include hyperreactivity of airway smooth muscle (ASM) cells to bronchoconstrictor agents in association with inflammation and remodeling of the airways, which encompasses increased airway smooth muscle mass and alterations in the airway extracellular matrix. To a significant extent, these abnormalities are the result of mast cells, eosinophils, neutrophils, monocytes, lymphocytes, and epithelial cells, as previously described. Nevertheless, airway smooth muscle cells function not only as targets for mediators produced by these cells, but also as effector cells that play an active role in airway inflammation and the immune response. A broad range of biologically active molecules induces physiologic changes in ASM that are central to the pathogenesis of airway disease in asthma. Constriction of airway smooth muscle may be triggered by multiple agonists, including histamine, PGD 2 , PGF 2_ , and cys-leukotrienes (Table 8). In contrast, bronchodilatation is mediated by `-agonists, VIP, PGE2, prostacyclin, and nitric oxide (Table 9). Moreover, airway hyperresponsiveness may result from proliferation of airway smooth muscle cells, which is stimulated by contractile agonists, histamine, endothelin-1, LTD4, and thrombin, among other factors, and inhibited by PGE2, `-agonists, TGF-`, nitric oxide, and glucocorticoids (Table 10) (84). Airway smooth muscle cells may contribute to chronic inflammation in asthma by producing arachidonic acid metabolites, cytokines, and chemokines (Figure 4). In human airway smooth muscle cells, PGE2 and prostacyclin constitute the primary products of COX-2, which is induced by the cytokines IL-1, IFN-a, and TNF-_ and inhibited by dexamethasone (85). ASM cells also secrete GM-CSF upon stimulation by these cytokines, an effect also blocked by dexamethasone (86). Moreover, ASM cells produce the chemokines RANTES, IL-8, and IL-6, suggesting they may recruit eosinophils and other inflammatory cells into the airways (87).

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Mediators

Table 8 Bronchoconstrictors Neurotransmitters

Histamine PGD2 PGF2_ Thromboxane Cys-LTs Platelet-activating factor Bradykinin Endothelin-1

Acetylcholine Neurokinin A Substance P Calcitonin gene-related protein

Table 9 Bronchodilators Epinephrine PGE2 Prostacyclin Vasoactive intestinal peptide Nitric oxide

Inducers

Table 10 Airway Smooth Muscle Proliferation Inhibitors

Histamine Platelet-derived growth factor Epithelial growth factor Basic fibroblast growth factor Insulin-derived growth factors Thromboxane LTD4 IL-1` TNF-_ Endothelin-1 Thrombin

PGE2 `-agonists Nitric oxide Glucocorticoids Transforming growth factor-`1 Heparin

TNF-_ and other proinflammatory cytokines upregulate the adhesion molecules ICAM-1 and VCAM-1 on ASM cells, inducing binding of inflammatory cells to the muscle cells. This effect is inhibited by PGE2 (87). TNF-_ also upregulates the adhesion molecule CD44 on ASM, thereby promoting T-cell binding (88). Adherence of activated T-cells to ASM cells induces the expression of MHC class II molecules on these cells, although ASM cells have not been shown to be capable of presenting antigen to T-cells (89).


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Fig. 4. Airway smooth muscle cell.

Fig. 5. Neural network.

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Consequently, airway smooth muscle cells are targets for multiple biologically active mediators, whose functions include bronchoconstriction, bronchodilatation, and cellular proliferation. Furthermore, these cells are capable of serving as effector cells, producing prostanoids, cytokines, chemokines, and adhesion molecules.

Neural Regulation of Airway Function Neural control of the airways is mediated through adrenergic (sympathetic), cholinergic (parasympathetic), and nonadrenergic, noncholinergic (NANC) systems (Figure 5). In addition, sensory nerves synapse with local axons, local parasympathetic ganglia, and the central nervous system to elicit local neuropeptide-mediated inflammation, local parasympathetic reactions, and systemic parasympathetic and sympathetic responses.

Adrenergic (Sympathetic) System Preganglionic sympathetic nerve fibers from the thoracic spinal cord synapse in the second through fourth thoracic ganglia to innervate the lung. Mediators from adrenergic nerves are either norepinephrine alone or norepinephrine in combination with neuropeptide Y. Stimulation of these nerves constricts mucosal vessels and may increase glandular secretion but does not affect airway smooth muscle, which lacks significant adrenergic nerve fibers. Although sympathetic innervation of airway smooth muscle is minimal, large numbers of _- and `-adrenergic receptors are present in the human lung. _-Adrenergic receptors include postsynaptic excitatory _1-receptors and presynaptic inhibitory _2-receptors. Stimulation of _1-receptors constricts bronchial blood vessels, weakly constricts airways, enhances mucus secretion, and promotes mast cell degranulation, whereas stimulation of _2-receptors inhibits cholinergic and noncholinergic excitatory transmission and therefore may exert a protective effect in asthma. Nonetheless, the paucity of _-receptors compared to `-receptors in lung tissue suggests that their effects may be relatively minor. Airway epithelial, smooth muscle, and vascular endothelial cells express `-receptors that mediate a variety of biologic effects. `2-receptors are several times as common as `1-receptors in pulmonary tissue and represent the only `-receptors present on airway smooth muscle, epithelial cells, and mast cells. Stimulation of `-receptors on airway smooth muscle cells causes bronchodilatation, explaining the efficacy of `-receptor agonists in the treatment of asthma. Binding of circulating catecholamines to `-receptors on airway epithelial cells increases mucus and water secretion, ciliary beating, and epithelial cell proliferation (90), while binding to `-receptors on vascular endothelium induces vasodilatation and decreases vascular permeability. Moreover, stimulation of pulmonary `-receptors decreases mast cell degranulation and inhibits both cholinergic and noncholinergic neurotransmission (91). Various studies have revealed abnormalities in `-receptors in asthmatic patients, suggesting these may influence disease susceptibility. Certain `2-receptor poly-


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morphisms have been linked to bronchial hyperreactivity and asthma severity (92). In addition, decreased `-adrenergic responsiveness has been linked to airway inflammation associated with viral respiratory infections (93). Furthermore, certain medications used to treat asthma affect `-receptor concentration: `-agonists decrease the density of `-receptors, whereas glucocorticoids increase `-receptor density. Consequently, genetic, environmental, and therapeutic factors may influence `-receptor function.

Cholinergic (Parasympathetic) System Stimulation of the parasympathetic nervous system is initiated by vagal afferent fibers that originate in the airways in addition to sensory nerve fibers that mediate parasympathetic reflexes. Parasympathetic efferent fibers from the sphenopalatine ganglion innervate the anterior nasopharynx, nasal mucosa, and ethmoid sinuses, whereas efferent fibers from the dorsal motor nucleus innervate glands and blood vessels in laryngeal, tracheal, and bronchial tissues. Preganglionic parasympathetic nerve fibers release acetylcholine, which binds to nicotinic receptors on postganglionic neurons in the airways. Postganglionic parasympathetic neurons release acetylcholine and vasoactive intestinal peptide (VIP), which bind to muscarinic receptors on target tissues. Stimulation of the cholinergic nervous system induces secretion of mucus from submucosal glands and goblet cells (94). Cholinergic innervation also maintains resting bronchial tone and, to a significant degree, mediates acute bronchoconstriction, which is induced by acetylcholine binding to muscarinic receptors in the airways. Psychogenic factors have been postulated to produce bronchospasm via this pathway (95). The importance of the cholinergic system in asthma is demonstrated by the efficacy of anticholinergic agents, such as atropine and ipratropium bromide, in relieving bronchoconstriction. Although five muscarinic receptor genes have been cloned, three receptor subtypes (M1, M2, M3) have been identified on the basis of pharmacologic binding studies. M1-receptors, located in submucosal glands and airway parasympathetic ganglia, facilitate vagal transmission. M 2 -receptors may represent presynaptic inhibitory receptors, stimulation of which decreases acetylcholine release. M 3-receptors, which constitute the muscarinic receptors on airway smooth muscle and the majority of those in bronchial submucosal glands, are the receptors primarily responsible for airway smooth muscle contraction, vasodilatation, and mucus secretion. Thus, increased M1- or M3-receptor stimulation may exacerbate asthma, whereas stimulation of M2 may improve asthma. Abnormal muscarinic receptor function represents one mechanism whereby the threshold for neurogenic bronchospasm may be modulated. Some evidence exists for M2-receptor dysfunction in patients with asthma. For example, pilocarpine (an M2-agonist) has been reported to inhibit cholinergic reflex bronchospasm induced by inhaled SO2 in normal subjects but not patients with asthma (96). One possible explanation for this finding is that eosinophil major basic protein or oxygen-derived free radicals may inactivate M2-receptors (97).

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Neuropeptide

Substance P Neurokinin Aa CGRP GRP VIP Nitric oxide aNeurokinin

Table 11 NANC Neuropeptides in Asthma Biologic effects BronchoBronchoVasodilatation Mucus dilatation constriction production + + +

+

+

+

+ +

Vascular permeability +

+ +

A is a more potent bronchoconstrictor than substance P or CGRP.

Other neurogenic abnormalities that may contribute to asthma include increased acetylcholine release at the level of preganglionic or postganglionic nerve endings and increased cholinergic reflex activity as a result of sensory fiber stimulation by inflammatory mediators, such as substance P, PGF2_, and thromboxane A2 (98).

Nonadrenergic Noncholinergic (NANC) System The neurotransmitters in this system include neuropeptides within neurons, neuroendocrine cells, and inflammatory cells. Of most relevance to asthma are substance P (SP), neurokinin A (NKA), calcitonin gene-related peptide (CGRP), vasoactive intestinal peptide, and nitric oxide (Table 11). SP and NKA are members of the peptide class called tachykinins because of their rapid-onset contractile effects on smooth muscle. This system is divided into excitatory and inhibitory subsets on the basis of whether the neuropeptides increase or decrease bronchoconstriction, respectively.

Nonadrenergic Noncholinergic Excitatory System (e-NANC) Nociceptive signals are transmitted through slow-conducting unmyelinated C fibers and thinly myelinated Ab fibers. The bare neural endings of C fibers are stimulated by inflammatory mediators, including bradykinin, histamine, and serotonin. Depolarization of these nerves results in the release of peptides associated with neurogenic inflammation, including SP, NKA, CGRP, and gastrin-releasing peptide (GRP).

Substance P Within the lungs, SP is present in nociceptive sensory nerve fibers near airway epithelium, blood vessels, and, to a lesser degree, airway smooth muscle. Stimulation of these nerves releases SP and other neuropeptides. Eosinophils also produce substance P. Substance P enhances vasodilatation, vascular permeability, epithelial goblet cell exocytosis, and glandular secretion in addition to regulating various cells involved in allergic and inflammatory reactions. SP induces proliferation of T cells


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and endothelial cells, activates macrophages and neutrophils, stimulates monocytes to secrete IL-1, IL-6, and TNF-_, and stimulates IgM and IgA synthesis by B cells. Although substance P degranulates certain human mast cells, it does not cause degranulation of human pulmonary mast cells or basophils (99). Administration of SP to patients with severe asthma has been reported to induce bronchospasm, but significant airway narrowing was not observed in normal subjects or patients with mild asthma (100). A pathophysiologic role for SP is suggested by the identification of SP-immunoreactive material in BAL from atopic patients following antigen challenge (101).

Neurokinin A NKA is expressed in nociceptive sensory nerve fibers in association with substance P. Both of these neuropeptides are released by capsaicin. Receptors for NKA are present on airway smooth muscle, cholinergic ganglia and nerves, and inflammatory cells. In contrast to SP, NKA administered intravenously or by inhalation induces bronchospasm even in patients with mild asthma, implying that NKA represents a more potent bronchoconstrictor.

Calcitonin Gene-Related Peptide CGRP is localized with substance P in sensory afferent nerves and is released in response to similar stimuli. A potent vasodilator, it is characterized by slow onset but prolonged duration of action. Other actions attributed to CGRP include bronchoconstriction, mucus secretion, and eosinophil chemotaxis. Elevated tissue levels of CGRP have been described following aeroallergen challenge and chronic exposure to smoke, suggesting a possible role for this peptide in mediating airway disease (90).

Gastrin-Releasing Peptide GRP binding sites are localized to the epithelial cells and submucosal glands in human nasal and tracheal mucosa (102). Studies utilizing human nasal mucosa have shown that GRP induces mucous and serous cell exocytosis.

Endothelins Endothelins are potent bronchoconstrictors produced by bronchial endothelial cells, epithelial cells, macrophages, mast cells, and neuroendocrine cells in patients with asthma. Endothelins also stimulate mucus production and proliferation of epithelial cells. In addition, endothelins are present in dorsal root ganglia, where they may function as neurotransmitters (103).

Bradykinin BK is generated from high molecular weight kininogens in the presence of inflammation. In asthmatic subjects, BK is a potent inducer of bronchoconstriction, which is probably mediated through stimulation of sensory nerve-parasympathetic bronchoconstrictor reflexes and release of neuropeptides from sensory nerves. This concept is supported by the finding that BK-induced bronchospasm is decreased by cholinergic and tachykinin antagonists (104).

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The activity of the NANC excitatory system may be enhanced in patients with atopic asthma through activation of nociceptive sensory nerves by mast cell products such as histamine and bradykinin. In addition, damage to airway epithelial cells induced by eosinophils and toxins may render sensory nerve endings more accessible to stimulation by allergens and inflammatory mediators. Moreover, decreased metabolism of substance P and NKA by neutral endopeptidase (NEP) may prolong their activity, thereby increasing noncholinergic excitatory effects. Cigarette smoke and viral infections have been reported to decrease NEP activity and increase airway responsiveness (105).

Nonadrenergic Noncholinergic Inhibitory System (i-NANC) This system is unique in that it is the only known bronchodilatory neural pathway in the human lung. The putative mediators of this inhibitory system are VIP and NO.

Vasoactive Intestinal Peptide VIP is a potent vasodilator and bronchodilator that has been identified in postganglionic parasympathetic nerves (106). In nasal and pulmonary tissue, VIP-binding sites are located on epithelial cells, submucosal glands, blood vessels, smooth muscle of the large airways, and alveolar cells. VIP is degraded by neutral endopeptidase, tryptase, and chymase. In vitro studies using bronchial airway smooth muscle indicate that VIP possesses bronchodilatory activity about 50× more powerful than isoproterenol. Furthermore, VIP-induced bronchodilatation is not affected by adrenergic blockage or indomethacin, a potent cyclooxygenase inhibitor, implying that this represents a direct effect of VIP on the muscle fibers. VIP also decreases mucus secretion and exhibits a number of antiinflammatory effects. These include inhibition of mast cell degranulation, lymphocyte proliferation, natural killer cell activity, and IL-2 secretion. In addition, VIP and related peptides stimulate adenylate cyclase to increase cAMP levels. Some investigators have reported decreased VIP in pulmonary nerve fibers from patients with asthma (107). Whether this represents a primary abnormality or the effect of the disease remains speculative.

Nitric Oxide Neuronal NO is synthesized by constitutive nitric oxide synthase (cNOS) and acts as a potent neurotransmitter mediating bronchodilatation. Inhibition of NO synthesis significantly decreases the i-NANC effect, implying that NO may be the primary mediator of this function, rather than other mediators such as VIP (108). Thus, sympathetic, parasympathetic, and nociceptive sensory nerves maintain respiratory homeostasis and mediate pathophysiologic changes in asthma, including bronchoconstriction, vasodilatation, increased vascular permeability, and mucus secretion. Modulation of neural receptors forms the basis for treatment of asthma with `-agonists and anticholinergic drugs.


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Summary Atopic asthma develops in individuals genetically predisposed to generate an IgE antibody response following exposure to aeroallergens. Bound to the surface of B-cells and to Fc receptors on mast cells, eosinophils, basophils, monocytes, and some T-cells, antigen-specific IgE is poised to induce a complex series of events following binding and cross-linking by antigen. Although of necessity discussed as discrete events in this chapter, in reality the processes involving these cells and their mediators occur along parallel and interacting pathways that may depend in part on genetic and environmental factors. The mast cell has traditionally been viewed as the key player in this drama and maintains a pivotal role. Following activation, mast cells release histamine and newlysynthesized mediators such as LTC4 and PGD2, which induce bronchospasm. Mast cells also produce interleukins that are essential for the allergic response, notably IL-4 and IL-5. Mast cells initiate the late-phase allergic reaction by releasing chemoattractant factors for eosinophils and neutrophils, thereby promoting the inflammatory response. Furthermore, TNF-_ and IL-4, both preformed in mast cells, may upregulate the expression of endothelial cell VCAM-1, augmenting adhesion and subsequent transendothelial migration of eosinophils, basophils, and mononuclear cells. Sharing certain characteristics with mast cells are basophils, which also express surface receptors for IgE and store histamine and other preformed mediators in secretory granules. In contrast to mast cells, basophils lack tryptase and chymase and do not produce LTB4 or PGD2. By virtue of their small numbers, these cells probably play a relatively small role in asthma, although they have been reported in lung tissue from patients with asthma. Eosinophils are now recognized as the most important effector cells mediating airway damage in asthma, primarily because of the release of major basic protein and eosinophil cationic protein, both of which are toxic to airway epithelial cells. Among the cytokines of particular relevance to eosinophil activation are IL-5, IL-3, and GM-CSF. GM-CSF is produced mainly by macrophages and T-cells, whereas IL-5 and IL-3 are produced primarily by T-cells. Monocytes are the major source of IL-12, which induces IFN-a synthesis and promotes Th1 differentiation, but also produce IL-10, which inhibits Th1 development. Monocytes also secrete the proinflammatory cytokines GM-CSF, TNF-_, and IL-6 in addition to chemokines. Macrophages produce NO, which may damage airways, and PDGF, TGF-`, and fibroblast growth factors, which may contribute to irreversible airway damage. Interestingly, alveolar macrophages represent inefficient antigen-presenting cells and appear to suppress the function of dendritic cells, the major APCs in the lungs. The primary regulators of the immune response are lymphocytes. Th1 helper cells secrete predominantly IL-2 and IFN-a, whereas Th2 cells preferentially secrete IL-4, IL-5, IL-6, IL-10, and IL-13, cytokine profiles that significantly affect the nature of the immune reaction. Th1 cytokines stimulate DTH, cytotoxicity, and mono-

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cyte activation, whereas the Th2 cytokines promote allergic reactions. For example, IL-4 is required for IgE synthesis and Th2 development and IL-5 activates eosinophils. Since Th2 cells represent the major source of IL-4 and IL-5, the Th1/Th2 balance is critically important in determining the degree of allergic response. Airway epithelial cells have recently been recognized as active participants in the inflammatory process, producing cytokines, chemokines, and arachidonic acid metabolites. Moreover, the bronchial epithelium is the primary source of the potent bronchoconstrictor endothelin and, with macrophages, accounts for most nitric oxide synthesis in the airways. Epithelial cells also maintain vigorous repair capabilities, which may contribute to airway remodeling. Similarly, airway smooth muscle cells play an active role in airway inflammation by secreting PGE 2 , prostacyclin, GM-CSF, and chemokines. In addition, proinflammatory cytokines such as TNF-_ upregulate the expression of adhesion molecules on ASM, promoting direct interaction with T cells and inflammatory cells. Interlaced through these tissues are neural networks that regulate airway function and produce neuropeptide-mediated inflammation. Stimulation of the cholinergic (parasympathetic) nervous system induces mucus secretion and bronchoconstriction, whereas activation of the adrenergic (sympathetic) system constricts mucosal vessels but does not affect airway smooth muscle. Neurotransmitters in the nonadrenergic, noncholinergic (NANC) system include the nociceptive sensory nerve neuropeptides substance P, neurokinin A, and calcitonin gene-related peptide, all of which induce bronchoconstriction, and vasoactive intestinal peptide and nitric oxide, both potent bronchodilators. Stimulation of airway smooth muscle adrenergic receptors with `-agonists and inhibition of the cholinergic system with anticholinergic agents form the basis of current bronchodilator therapy.

Conclusion The understanding of asthma has evolved from a process characterized by reversible airway obstruction to a complex inflammatory process that not only induces bronchial hyperresponsiveness, but may also cause remodeling of the airways, leading to permanent damage with impaired pulmonary function. The recognition that asthma is primarily an inflammatory process carries important therapeutic implications that constitute the basis for current treatment guidelines, which emphasize the use of antiinflammatory agents for persistent disease. The expectation is that by controlling airway inflammation, it may be possible to mitigate permanent lung disease, rather than providing merely symptomatic relief with bronchodilating agents. Moreover, increased understanding of these inflammatory pathways, combined with advances in biotechnology, should yield new therapeutic modalities.

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93. Busse WW. Respiratory infections: their role in airway responsiveness and the pathogenesis of asthma. J Allergy Clin Immunol 1990; 85: 671–683. 94. Lundgren JD, Shelhamer JH. Pathogenesis of airway mucus hypersecretion. J Allergy Clin Immunol 1990; 85: 399–417. 95. McFadden ER Jr, Luparello T, Lyons HA, Bleecker E. The mechanism of action of suggestion in the induction of acute asthma attacks. Psychosom Med 1969; 31: 134–143. 96. Minette PA, Lammers JW, Dixon CM, et al. A muscarinic agonist inhibits reflex bronchoconstriction in normal but not in asthmatic subjects. J Appl Physiol 1989; 67: 2461–2465. 97. Barnes PJ. Modulation of neurotransmission in airways. Physiol Rev 1992; 72: 699–729. 98. Daniel EE, O’Byrne P. Effect of inflammatory mediators on airway nerves and muscle. Am Rev Respir Dis 1991; 143: S3–S5. 99. Lawrence ID, Warner JA, Cohan VL, et al. Purification and characterization of human skin mast cells: Evidence for human mast cell heterogeneity. J Immunol 1987; 139: 3062–3069. 100. Joos G, Pauwels R, van der Straeten M. Effect of inhaled substance P and neurokinin A on the airways of normal and asthmatic subjects. Thorax 1987; 42: 779–783. 101. Nieber K, Baumgarten CR, Rathsack R, et al. Substance P and beta-endorphin-like immunoreactivity in lavage fluids of subjects with and without allergic asthma. J Allergy Clin Immunol 1992; 90: 646–652. 102. Baraniuk JN, Lundgren J, Shelhamer JH, Kaliner MA. Gastrin releasing peptide (GRP) binding sites in human bronchi. Neuropeptides 1992; 81–84 103. Giaid A, Gibson SJ, Ibrahim BN, et al. Endothelin 1, an endothelium-derived peptide, is expressed in neurons of the human spinal cord and dorsal root ganglia. Proc Natl Acad Sci USA 1989; 86: 7634–7638. 104. Barnes PJ, Baraniuk JN, Belvisi MG. Neuropeptides in the respiratory tract. Am Rev Respir Dis 1991; 144: 1391–1399. 105. Dusser DJ, Djokic TD, Borson DB, Nadel JA. Cigarette smoke induces bronchoconstrictor hyperresponsiveness to substance P and inactivates neutral endopeptidase in the guinea pig. Possible role of free radicals. Clin Invest 1989; 84: 900–906. 106. Said SI. VIP as a modulator of lung inflammation and airway constriction. Am Rev Respir Dis 1991; 143: S22–S24. 107. Ollerenshaw S, Jarvis D, Woolcock A, Sullivan C, Scheibner T. Absence of immunoreactive vasoactive intestinal polypeptide in tissue from the lungs of patients with asthma. N Engl J Med 1989; 320: 1244–1248. 108. Ward JK, Barnes PJ, Springall DR, et al. Distribution of human i-NANC bronchodilator and nitric oxide-immunoreactive nerves. Am J Respir Cell Mol Biol 1995; 13: 175–184.

Short Chapter Title

PART II PATIENT MANAGEMENT

3

Evaluation of Bronchial Asthma Patient

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3 Clinical and Allergic Evaluation of the Patient with Bronchial Asthma STEPHEN M. NAGY, JR., MD Contents Key Points Introduction Social History Family History Past Medical History Pediatric Considerations Present Illness Physical Examination Diagnostic Studies Laboratory Studies Gastroesophageal Reflux and Asthma

Key Points • A diagnosis is usually in the details. • Asthma is primarily an inflammatory disease of the bronchi with a bronchospastic component. • The symptoms of bronchial asthma will suggest a wide variety of clinical conditions; the history is critical in defining etiology. • The history should focus on seasonality, associated factors, current medications and other illnesses under treatment. • Anosmia (loss of sense of smell) and/or hyposmia (a reduction in an ability to smell) are frequently symptoms of sinusitis with consequent asthma. • Aspirin/Nsaid sensitivity suggests triad asthma, i.e., nasal polyps/asthma/ Nsaid sensitivity. From: Bronchial Asthma: Principles of Diagnosis and Treatment, 4th ed. M. E. Gershwin and T. E. Albertson, eds. © Humana Press, Totowa, NJ

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• Physical exam focuses on the nasal airway, i.e., polyps, turbinate swelling, septal perforation, and the chest, i.e., wheezing and degree of expiratory obstruction. • Even in patients with a clear chest, a spirometric study is critical in assessing acute/chronic respiratory complaints to define the degree of obstruction. • Sinus radiographs, i.e., sinus CT and x-rays, are underutilized in defining sinusitis as an etiology for acute and chronic asthma. • An assessment of IgE mediated sensitivity, i.e., allergies, should be conducted in any asthmatic with a seasonal/exposure related history not only to confirm the diagnosis but also to initiate appropriate environmental control. • Food sensitivity is rarely a cause of bronchial asthma. • Recurrent cough/wheezing in an older/obese patient suggests G-E reflux even in the absence of upper GI complaints.

Introduction Few diseases have come under more scrutiny and redefinition as bronchial asthma. Originally conceived as a bronchospastic condition with psychiatric overtones, and treated primarily with `-agonists/xanthines/sedatives, bronchia asthma is now considered an inflammatory disease of the airways, with bronchospasm, i.e., reversibility, representing a secondary but identifying feature. This is not to suggest that inflammation had not been identified as an associated feature. Unfortunately, the disease came under the microscope only during autopsies, primarily in patients who had died of status asthmaticus. The inflammation defined both on gross and microscopic examination was felt to represent a terminal event. It is tempting to postulate that the genesis of the data, which holds that the inflammatory component represents a complex interaction between mediators, neurons, effector cells, and triggering proteins/haptens, may have been spurred by the response of the disease, 45 yr ago, to the newly discovered corticosteroids. Nonetheless, despite the obvious saluatory effect of an oral/inhaled anti-inflammatory agent it was difficult to envision that nonspecific inflammation, a response long deemed beneficial and healthful, especially in responding to bacterial/viral invasion, could be responsible not only for asthma, but for a host of other inflammatory conditions, now termed autoimmune/connective-tissue diseases. Research has now gone beyond the histopathology of the inflammatory response. Researchers find themselves in a molecular labyrinth of cytokines, interleukins/ chemokines, sophisticated physiology, and a hereditary predisposition, all of which are detailed in subsequent chapters. These findings have, furthermore, led not only to an overall dramatic change in the pharmacotherapy of the disorder, but to the development of specific drugs that go to the molecular basis of inflammation. The diagnosis of asthma in any given patient occurs after a thorough historical assessment of the patient’s complaints, directed physical examination, specific evaluative studies, and, possibly, their response to a variety of therapies. The patient in whom the diagnosis is confirmed will characteristically report a plethora of


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respiratory complaints, including cough, wheezing, shortness of breath, chest tightness/constriction, even a seemingly benign symptom such as chronic clearing of the throat. The disease exhibits a periodicity, with episodic exacerbations related to multiple factors, i.e., allergenic/irritant exposures, infections, medications; patients may remain asymptomatic for long periods, lending the impression that they are free of disease, when, in fact, the inflammatory process persists. It is incumbent upon the physician/physician extender presented with this constellation of complaints to rule out other treatable conditions that can mimic this disorder, i.e., pneumonia, cardiac failure, aspiration, reflux, neoplasm, or adverse drug reaction. Once the diagnosis is established and an etiology addressed, the physician outlines strategies to deal with the chronic inflammatory component, as well as with periodic exacerbations. The initial history should, of course, be directed by the clinical situation; clearly, one cannot review an extensive checklist in a patient who presents to the emergency room with acute shortness of breath/wheezing. Additionally, the patient with wellestablished asthma, who presents with an acute exacerbation, need not undergo an exhaustive reappraisal; nonetheless, physicians who evaluate a patient for acute/ chronic respiratory complaints for the first time should always maintain a healthy skepticism of those who iterate a current or prior history of asthma. The following is a discussion of a suggested interview in a nonacute patient with respiratory complaints. Traditionally, the interview is divided into five areas: social history, family history, past medical history, present illness, and review of systems. Tables 1 and 2 display a history form on which the ensuing discussion is based. It not only organizes and outlines the ascertained facts, especially in areas not customarily explored, but also allows for the rapid retrieval of specific points of information. The order in which this information is obtained can be very meaningful. It is unclear why most physicians begin their questioning with the most crucial aspect, i.e., present illness. Except to indicate an underlying impatience and adherence to traditional methods, the narrative of acute/chronic pulmonary complaints should unfold within the perspective of social, genealogical, and past medical relationships. Symptoms may then be plotted graphically and chronologically against exposures, drugs, travel, stress, and other factors that relate to alterations of environment.

Social History Respiratory histories deal, in the great majority, with a constellation of symptoms that span months and, more frequently, years. In order to properly evaluate the chronology of these complaints, and especially to relate them to specific exposures, it is important to know where the patient has resided and for what periods of time. Even moves within a state and, to a lesser extent, within a city may result in significant changes in environmental flora. Animal pedigree, number, degree, and length of exposure should all be carefully documented. Small, frequently shampooed poodles seldom produce the problems of a German shepherd that sleeps in the bedroom. Siamese cats are seldom outdoors, despite protestations to the

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Nagy Table 1 Patient History Checklist

History

Possible problem areas

SH-Birthplace Marital status Occupation(s) Military service Tobacco EtOh Narcotic abuse Pets Hobbies Home Bedroom FH-Mother Father Siblings Children PH-Operations Hospitalization Illnesses Injuries Medications

Residences

—Hypertension

—Asthma

—Allergy

—Sprays

—Aspirin

—Other

—Heart disease —Arthritis —Food allergies

—Vitamins

Past allergic history

—Hormones —Drug

reactions

—Urticaria —Food

sensitivities —Insect stings

—Hay

fever Eczema — —Migraine

—Antibiotics

—Asthma —Hay

fever

—Eczema —Otitis

contrary. Some observations should be made on the nature of the home; the duration of occupancy, degree of dampness, especially recent water damage, which could create a significant increase in mold exposure. Similarly, the occupation of a patient may be important. Gardeners and veterinarians undergo exposures easily recognized by medical observers; however, occupational asthma may be more subtle, and represents reversible obstructive lung disease defined more by a peak incidence within specific industries than by biochemical mechanisms. These include lumber mills (1), carpentry, electronics (2), bakeries (3), to name a few; a complete list is found in Chapter 15. Classically, symptoms remit on weekends or on vacations, then gradually increase with a return to work. The history should focus on the length of employment within the industry and the degree and type of exposure within the work environment. Rarely, the offending material is introduced into the home by an nonsensitized family member.


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Table 2 Present Condition Checklist Exacerbating factors

Present Illness Nasal

Travel Exercise Change in temperature Dust Animals Foods Infection Stress Drugs (ASA) Outside Irritants Smoke Seasonal Jan. Feb. Mar. Apr. May June Nasal Pulmonary Response to medications Nasal Sxs Pulmonary Sxs Asthma chronologya Jan. Feb. Mar. Apr. May June July Trees Olive Grasses Bermuda Weeds Molds Dust Animal dander aSeasonal

Pulmonary

July Aug. Sept. Oct. Nov. Dec.

Aug. Sept. Oct. Nov. Dec. Chinese elm

Peak Peak

patterns of inhalant-associated asthma in Sacramento, CA.

The deleterious effects of addictive habits, e.g., cigarette smoking, alcohol and narcotic abuse, require little emphasis. Recent studies (4), however, emphasize that asthmatic children of smoking parents present to emergency rooms more frequently than those children of nonsmokers. Alcohol use, even in moderate amounts, reduces ciliary function and, therefore, adversely effects pulmonary clearance mechanisms; in large amounts, chronic aspiration becomes more likely. Cocaine, used intranasally, produces not only septal perforation, but also osteal obstruction, leading to chronic sinusitis and consequent asthma (5). Obtaining accurate information will test a physician’s tact, especially in an upscale suburban practice.

Family History Both atopic disease and asthma demonstrate a definite genetic bias. On the other hand, office genealogic histories are almost always obtained secondhand, unless

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the entire family is under the same physician’s care. To establish such facts in family members is difficult. Is grandfather’s wheezing asthma or chronic bronchitis? The entire exercise is possibly meretricious, because it is still incumbent upon the practitioner to establish the diagnosis in the patient.

Past Medical History A rigorous outline of major medical landmarks, i.e., operations, hospitalizations, emergency room visits, and past and current illnesses, is important to obtain the proper prospective with which to view the primary complaints. Hypertension, coronary artery disease, valvular heart disease, arrhythmias, vascular headache, and arthropathies are treated with a wide variety of medications that may create de novo, or exacerbate, asthma symptoms; e.g., all `-blockers will exacerbate asthma to some degree, although the `1-selective agents are less likely to do so. Systemic absorption even occurs when used as an eye drop and may adversely effect pulmonary function. Nonsteroidal anti-inflammatory drugs, (NSAIDS), increase asthma by inhibiting the cyclooxygenase pathway; ASE inhibitors have been associated with a severe cough that appears to be dose related (6), but the mechanism is still unclear. In fact, new patients should be encouraged to assemble a list of all their medications; they frequently provide clues to illnesses poorly understood or simply forgotten. Concomitantly, prior illnesses with similar symptomatology, i.e., pulmonary emboli, cardiac failure, or respiratory infections, may have represented recurrent asthma, especially if the diagnosis was made by a different physician. The physician should specifically inquire about aspirin intolerance: Some patients are exquisitely sensitive, and have experienced a life-threatening reaction that mimics the allergic response (7). Frequently, these patients have nasal polyps (8) and associated sinusitis; others will remark a mild increase in cough/wheezing after taking the drug. Between 20 and 40% will demonstrate a decrease in flow rates, but may not be symptomatic (9). Prior surgeries, particularly those involving the nose/sinus area, can provide critical information; both a history of prior sinusitis severe enough to require surgery, and nasal surgery to remove polyps, suggest the possibility of triad asthma, particularly if there is also a history of aspirin/NSAID sensitivity. Even cosmetic nasal surgery can predispose to sinusitis, because it inevitably leads to a degree of osteal obstruction. A history of longstanding seasonal nasal/eye complaints, with exacerbations on specific exposures, and/or season, childhood eczema, and anaphylactic food sensitivity, suggest an atopic diathesis.

Pediatric Considerations Airway geometry is primarily responsible for the increased frequency of wheezing in infants/toddlers (10). As bronchi/bronchioles enlarge with age, children are less likely to experience lower respiratory complaints with infections; in fact, immunologic mechanisms are infrequently detected in children under the age of 4 yr. Between ages 6 and 25 yr, a much larger percentage of patients will fall into the latter atopic class of extrinsic asthma (11); with increasing age, the number steeply diminishes.


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With the rise of two-income households, the number of children spending time in daycare centers has markedly increased: This simultaneously increases their risk of infectious exposure. Furthermore, although such children may not be exposed to animals within their home, there may be animals within the center, or offending dander may be introduced by other children who have pets. Many schoolrooms have high levels of dander introduced in just such a manner (12).

Present Illness Asthmatic symptoms vary from mild cough (frequently nocturnal), to severe wheezing, chest tightness, and shortness of breath. It is important for the patient to realize that seemingly insignificant complaints, i.e., clearing of the throat, postnasal drainage, and occasional mild episodes of dyspnea, may all represent asthma. Once defined and understood, such symptoms undergo retrospective and chronologic analysis. A date of onset is approximated, preferably as to the month, but, at least, the season. The following questions must then be answered. 1. Has there been a gradual progression from mild cough to severe disabling disease, or have symptoms been stable, with occasional exacerbations? 2. Are the patient’s symptoms consistent with expiratory obstruction? Hoarseness, and especially inspiratory stridor, represent upper airway obstruction caused by tracheal collapse, neoplasm, or epiglottical swelling, and can represent an emergent problem. 3. Are symptoms strictly seasonal, with a large hiatus when the patient is well, or are they chronic and perennial? 4. Do symptoms remit on travel to climatically different areas, or are they altered with a change in local environment? 5. Are the asthmatic complaints associated with specific exposures, situations, or a change in medications? 6. Is there a history of concomitant or prior nasal/eye complaints? 7. Is there evidence of an infection/neoplastic etiology, i.e., fever/weight loss/ purulence/hemoptysis? 8. If the patient is of an appropriate age, is there a history of reflux esophagitis? Does the patient have symptoms thereof? Asthmatics have a predilection for nocturnal attacks, which, for many years, was never adequately explained; it invariably led to meticulous scrutiny of all items/exposures within the patient’s bedroom (13). Research in a new field, i.e., chronobiology, has made it clear that the human body does not remain static over a 24-h-period. Investigators found (14), even in healthy patients without asthma, an inherent circadian variation in airway resistance, the worst values occurring between 10:00 PM and 8:00 AM. The frequency and intensity of nocturnal awakening should be documented, because these represent a major indicator of severity. When attempting to evaluate the overall disease, frequency of office visits, emergency room visits, hospitalizations, school absences, and/or work disability can represent a more accurate index than the patient’s assessment of intensity. The patient’s requirement for, and response to, various medications may represent even

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more objective criteria. Seasonal asthma, adequately treated with an antihistamine, is clearly not as severe as that requiring systemic steroids. It is the unusual asthmatic whose sole manifestation is cough/wheezing: Upper respiratory tract complaints, i.e., nasal congestion/rhinorrhea/sinus tenderness/ headaches/ sneezing/hyposmia, or watery and itchy eyes, occur with varying intensity. The presence of these complaints provide important clues not only to establish the diagnosis, but also to provide an additional framework with which to evaluate the seasonal/exposure-related aspect. Of these, hyposmia or anosmia (a loss of the ability to smell) represents a major clue to significant sinusitis; in the opinion of the author it is a question rarely asked by the inquiring physician. Thus, as the history of this illness unfolds within the perspective of environmental factors/prior and current illnesses, the physician should develop some sense of etiology, to direct the evaluative studies.

Physical Examination An examination of the patient should actually begin during the interview. Patients often learn to disguise symptoms that are not socially acceptable; therefore, a chronic cough, or such habits as chronic clearing of the throat, may be handled very quietly and even denied, but are rather obvious on close observation. An accompanying family member may even attest to the severity and persistence of such symptoms. Children may be observed for the distressing habit of rubbing their nose, i.e., allergic salute, and sniffling (the voluntary inhalation of nasal secretions). Overall, one usually performs a directed physical examination, concentrating on specific areas, (e.g., vital signs, respiratory, cardiac, insegumentry systems; for example, pelvic, rectal, and extensive neurologic exams are usually not performed unless indicated by an abnormality in the history). Vital signs should include the patient’s weight, temperature, respiratory rate, cardiac rhythm/rate, and blood pressure. The state of the patient’s nutrition should be noted: A common observation is that children with chronic asthma appear somewhat undernourished and small for their age. In the acutely symptomatic patient, one should remark the presence of cyanosis, particularly of the lips/nail beds; similarly, one should also note the patient’s ability to speak without pausing for a breath; the pulsus paradoxus is an accentuation of the normal variation in cardiac output during the respiratory cycle, and is increased in several pathologic states (15). In moderately severe asthma, when, during inspiration, the decrease in systolic arterial pressure exceeds the normal of 10 mmHg, it indicates severe obstruction, and has been shown to have some predictive value in terms of asthma severity. In the nonsymptomatic patient, the exam can be more extensive. Significant obesity clearly predisposes to gastroesophageal reflux (GER). The skin should be examined for associated atopic disorders, particularly urticaria and atopic dermatitis; the latter condition is characterized by erythema, scaling, and thickening of the skin. In children, it characteristically develops in a flexural distribution involving the neck, antecubital and popliteal areas, eyelids, and behind the ears. Among older children and in teenagers, there is a distinct subset who develop


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extensor involvement, with the lesions occurring primarily on the anterior and lateral aspects of the thighs, upper arms, and forearms. The facies and upper respiratory tract should be examined in some detail. Pigmentation within the infraorbital area (shiners) is commonly seen in patients with allergic rhinitis or other diseases associated with chronic nasal congestion, i.e., sinusitis/adenoiditis, and is secondary to chronic lymphedema. A horizontal crease (allergic crease) across the bridge of the nose may be a sign of chronic nasal inflammation; it is secondary to the patient chronically rubbing the nose; simple, upward pressure on the tip of the nose will demonstrate how the crease was produced. Assessment of the tympanic membranes, particularly in children, will rule out purulent/serous otitis in addition to prior ear infections. The nasal membranes should be described: Pale boggy turbinates imply an allergic diathesis. At times, the engorgement/edema may be so severe that a distinction between severe allergic disease and true nasal polyps is impossible. Nevertheless, polyps are usually grayish-white, glistening excrescences that may even be mistaken by the untrained observer for nasal mucus. They are associated with aspirin sensitivity/infectious sinusitis, and, in children, with cystic fibrosis. The presence and nature of the nasal discharge should be described. Allergic disease produces a clear whitish discharge; purulence implies possible sinusitis. Septal perforations, especially small ones, are frequently missed by the untrained observer. Acute allergic conjunctivitis is characterized by tearing and either conjunctival hyperemia or a boggy pale conjunctiva. In its most severe form, the conjunctivae becomes markedly edematous, i.e., chemosis, and have a characteristic milky appearance. The disease is usually bilateral, although the exposure of one eye to an inordinate amount of antigen (Ag), usually pollen, may produce a unilateral response. Vernal conjunctivitis is a condition characterized by the presence of giant papillae on the upper tarsal conjunctivae (cobblestone appearance) (16). The presence of either type of conjunctivitis strongly suggests an allergic, i.e., atopic, component to the asthma. In the acute asthmatic, the neck should be palpated for the presence of subcutaneous emphysema, an indicator of pneumediastinum and/or pneumothorax, a complication of severe obstruction. Adenopathy may be prominent and a sign of chronic infectious sinusitis. The chest configuration should be noted, with attention given to the degree of hyperinflation, pectus deformity, and symmetry of expansion area. Chronic asthmatics commonly develop a kyphotic deformity. The use of accessory muscles of respiration should be noted in patients with severe asthma, because their use correlates with the severity of airway obstruction (17). The lungs should initially be auscultated during quiet respiration, because a major characteristic of asthma is the wheeze, or a high-pitched piping or whistling sound, resulting from partial airway obstruction. It has a musical quality, and, occurs during both inspiration and expiration, although it is usually louder during expiration. The degree and amount of wheezing should be noted, as well as the amount of expiratory prolongation (the inspiratory:expiratory ratio), the presence of adventitious sounds, and an overall assessment of the adequacy of air exchange. In many asymptomatic asthmatics, the chest exam is normal (see Table 3). In this case, several maneuvers

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Nagy Table 3 Confounding Conditions in Asthma Diagnosis Congestive heart failure Pulmonary emboli Emphysema Chronic bronchitis Pneumonia Neoplasm Bronchiectasis Aspiration Foreign body Laryngeal dysfunction Tracheal collapse Deconditioning Malingering Gastroesophageal reflux Sinusitis Adverse drug response

Fig. 1. Evaluation Positive IgE evaluation Chest film Exposure-related atopy Positive Spirometry History Upper GI symptoms Reflux

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