IL-7 Hergen Spits* Division of Immunology, Netherlands Cancer Institute, Plesmanlaan 121, 1066 CX Amsterdam, The Netherlands * corresponding author tel: 31-20-5122063, fax: 31-20-5122057, e-mail:
[email protected] DOI: 10.1006/rwcy.2000.03003.
SUMMARY Interleukin 7 (IL-7) was discovered in 1988 as a factor that promotes the growth of murine B cell precursors. IL-7 is a single-chain protein of 25 kDa produced predominantly by epithelial cells, especially keratinocytes and thymic epithelial cells. It plays an essential, nonredundant role in the development of T and B cells in the mouse and of T cells in humans. IL-7 promotes survival and proliferation of T cell precursors in humans and mice and of B cell precursors in mice. It may also play a role in differentiation of T cells (notably of TCR T cells) and B cells. In addition, IL-7 affects survival and proliferation of mature T cells but not of mature B cells. Some data suggest that IL-7 is important for the function of mature T cells in the periphery. There is no evidence that IL-7 is involved in development of cells other than T and B cells, but this cytokine may affect the function of mature NK cells and monocytes/macrophages. Data in the mouse suggest that IL-7 may be of clinical benefit by stimulating T cell development in recipients of hematopoietic stem cell transplantation, but so far no clinical trials have been performed.
BACKGROUND
Discovery Following the establishment of techniques for the study of murine B cell development in the early 1980s by C.A. Whitlock and O.N. Witte it was shown that in vitro B cell maturation was dependent on the presence of bone marrow stromal cells, raising the possibility of the involvement of a growth or differentiation-inducing cytokine. This notion was confirmed by Namen and coworkers, who demon-
strated that conditioned medium from stromal cells promoted the growth of B cell precursors (Namen et al., 1988b). A stromal cell line was established by immortalization of bone marrow stromal cells with a plasmid encoding the large and small T antigens of SV40. A clone of this cell line (IxN/A6) was found to produce a factor called lymphopoietin 1 (LP-1) that is responsible for growth of B cell precursors (Namen et al., 1988a). A protein of 25 kDa, isolated from supernatants of this cell line, was responsible for the biological activity of the IxN/A6 supernatant. The cloning of this factor, which was subsequently named interleukin 7 (IL-7), was reported in 1988. The cDNA encoding IL-7 was isolated from a library derived from the IxN/A6 line (Namen et al., 1988a). Pools of cDNA from this library were expressed in murine COS-7 cells and tested in a B cell differentiation assay. One positive pool was identified and further subdivided and tested. Eventually one clone (1046) was identified. The murine IL-7 cDNA was used to probe a cDNA library of a human hepatoma cell line SK-HEP-1. A highly homologous cDNA clone was found, and upon expression of this cDNA clone in COS-7 cells the supernatant was able to support the culture of human and mouse bone marrow cells (Goodwin et al., 1989).
Alternative names IL-7 is also known as lymphopoietin 1 (LP-1) and pre-B cell factor.
Structure IL-7 is a single-chain glycoprotein of 25 kDa that contains six cysteine residues. The disulfide bonds are essential for the biological activity of the protein.
138 Hergen Spits Human (152 amino acids: 17.4 kDa) and murine (127 amino acids) IL-7 show 60% sequence homology at the protein level. The human IL-7 gene contains an open reading frame of 534 nucleotides encoding a polypeptide of 177 amino acids with a predicted molecular weight of 17.4 kDa with three potential N-linked glycosylation sites and a signal peptide of 25 amino acids. The murine gene contains an open reading frame encoding a signal peptide of 25 amino acids and a functional protein of 127 amino acids with a predicted molecular weight of 14.9 kDa. The molecular weight of the expressed protein is 25 kDa, due to glycosylation of two potential N-linked glycosylation sites. Both human and mouse proteins contain three disulfide bonds which are indispensable for the biological activity. A cDNA coding for the mature bovine IL-7 was isolated from a bovine leukemia virus (BLV)-induced B cell lymphosarcoma cDNA library. Bovine IL-7 protein is 176 amino acids long and shows 75% and 65% homology to published sequences of human and murine IL-7, respectively.
Main activities and pathophysiological roles Gene targeting studies in the mouse have demonstrated that IL-7 is a major factor involved in development of T and B lymphocytes (von FreedenJeffry et al., 1995). In humans, IL-7 plays an essential role in development of T cells, but it appears that it is not essential for B cell development since patients who lack a functional IL-7R chain have normal numbers of B cells (Puel et al., 1998). In vitro studies indicate that IL-7 is a growth/survival factor for T and B cell progenitors. In addition, IL-7 promotes growth of mature murine and human T cells, but it is not a growth factor for mature B cells.
GENE AND GENE REGULATION
Accession numbers Human gene: M29048; cDNA: NM_000880 Mouse gene: AH_001973; cDNA: X07962 Bovine gene: X64540
Chromosome location The human IL-7 gene is located on chromosome 8 bands q12-13. It has a length of approximately 33 kb
and contains six exons. Murine IL-7 has a length of 56 kb and maps to chromosome 3.
Regulatory sites and corresponding transcription factors The murine IL-7 gene lacks a TATA box and uses multiple transcription initiation sites resulting in transcripts of different sizes (Lupton et al., 1990). A potential binding site for the basic helix loop helix (bHLH) transcription factor E12 is conserved in the 50 region of the human and mouse genes. In addition, five other potential binding sites for bHLH factors were conserved in both genes. It is, however, not known whether bHLH factors actually control IL-7 transcription. Keratinocytes are an important source of IL-7 and may play a role in homeostasis of T cell subsets in the skin. Interestingly, IFN upregulates expression of IL-7 in a keratinocyte cell line. Treatment of keratinocytes with IFN results in expression of 1.5 and 2.6 kb transcripts. The 2.6 and 1.5 kb mRNAs are produced through the use of alternative transcription initiation sites; these mRNAs are transcribed within 250 bp from the coding sequence, whereas 2.9 and 1.7 kb mRNAs contain > 400 bases in the 50 untranslated region. IFN induces conversion to the 2.5 and 1.5 mRNA species through the IFN-stimulated response element (ISRE) 270 bp upstream from the coding sequence. The ISRE is followed by a non-TATA-type transcription control element. IFN -dependent transcription was initiated from immediately downstream of this complex and its deletion resulted in an abrogated IFN responsiveness in transcriptional regulation.
Cells and tissues that express the gene The main cell types that express the gene are bone marrow stroma, thymic stroma (Wiles et al., 1992), intestinal epithelial cells (Watanabe et al., 1995), and keratinocytes (Heufler et al., 1993). IL-7 mRNA has furthermore been detected in mature but not immature dendritic cells (Sorg et al., 1998) and dendritic cells derived from CD34+ cord blood cells cultured with GM-CSF and TNF (de Saint-Vis et al., 1998), but it is not clear whether these cells secrete IL-7. In addition, the gene was expressed in platelets and a megakaryocyte cell line (Soslau et al., 1997). Various malignant T cell types express the gene (Table 1).
IL-7 139 Table 1
Cellular sources that produce IL-7
Tissue or cell type
mRNA (human, mouse)
Protein
Bone marrow stromal cells
+ (hu, mo)
+ (hu, mo)
Thymic stromal cells
+ (hu, mo)
+ (hu, mo)
Keratinocytes
+ (hu, mo)
+ (hu, mo)
Intestinal epithelial cells
+ (hu)
+ (hu)
Follicular dendritic cells
+ (hu)
+ (hu)
Dendritic cells derived from CD34+ cells cultured with GM-CSF and TNF
+ (hu)
nd
CMRF-44+CD14-CD19- Peripheral blood low-density dendritic cells, purified after overnight tissue culture
+ (hu)
+ (hu)
Adult liver
+ (rat)
nd
Uterus
+ (mo)
nd
Brain
+ (hu)
nd
Vascular endothelial cells
+ (hu)
+ (hu)
Fibroblasts
+ (hu)
nd
Oral mucosa
+ (hu)
+ (hu)
Psoriatic placques
+ (hu)
nd
Lesions from tuberculoid lepra
+ (hu)
+ (hu)
Colorectal cancer cells
+ (hu)
+ (hu)
Renal cell cancer tissues and cells
+ (hu)
+ (hu)
Bladder cancer
+ (hu)
nd
Burkitt's lymphoma (American)
+ (hu)
+ (hu)
Epstein-Barr virus B cell lines
+ (hu)
+ (hu)
Chronic B cell leukemia cells
+ (hu)
+ (hu)
Hepatocarcinoma
+ (hu)
nd
For references, see Maeurer et al. (1998). nd, not determined.
PROTEIN
Accession numbers Human IL-7: NP-000871 Murine IL-7: CAA30779 Bovine IL-7: CAA45838
Discussion of crystal structure The crystal structure of IL-7 has not been determined, but IL-7 is predicted to form a four helix structure.
Important homologies A factor called stromal cell-derived lymphopoietin 1 (TSLP-1), which was reported in 1994 (Friend et al., 1994), interacts with the IL-7R in a complex with an as yet to be described receptor distinct from c (Levin et al., 1999). This suggests an homology between IL-7 and TSLP-1. Although cDNA encoding this factor has been cloned, the sequence of TSLP-1 has, regretfully, not been published and therefore the exact homology between these two factors is unknown.
140 Hergen Spits
CELLULAR SOURCES AND TISSUE EXPRESSION
Cellular sources that produce See Table 1.
Eliciting and inhibitory stimuli, including exogenous and endogenous modulators IFN upregulates expression of IL-7 in keratinocytes via activation of an IFN -stimulated response element located in the 50 upstream region of the IL-7 gene (Ariizumi et al., 1995) (see Regulatory sites and corresponding transcription factors). TGF has been shown to downregulate IL-7 production by human stromal cells.
RECEPTOR UTILIZATION The IL-7 receptor consists of a complex of the IL7R chain and the c chain (Noguchi et al., 1993a). Both chains are required for proliferative responses to IL-7. Furthermore, the phenotype of the c- and IL7-deficient mice are the same with regard to T and B cell development. The phenotype of one IL-7R ÿ/ÿ mouse strain is similar but yet slightly different compared with that of IL-7ÿ/ÿ mice, while that of another IL-7R ÿ/ÿ strain is identical. Together these data indicate that both c and IL-7R are essential for the biological effects of IL-7.
IN VITRO ACTIVITIES
In vitro findings B cells IL-7 can act as a growth factor for murine B cell precursors. While early pro-B cells require IL-7 and a stroma cell support, late pro-B and early pre-B cells proliferate to IL-7 alone. Rearrangement of the genes for the light chain and expression of IgM correlates with downregulation of the IL-7R and with unresponsiveness to IL-7. Mature B cells do not express the IL-7R and are IL-7 unresponsive. These in vitro findings indicate a role for IL-7 in B cell development consistent with the phenotype of IL-7 ÿ/ÿ mice that show a developmental arrest at early
stages of B cell development and numerically reduced numbers of mature B cells (see section on Phenotype of IL-7ÿ/ÿ mice: T cell development). A combination of in vitro and in vivo experiments using monoclonal anti IL-7R antibodies suggests a role for IL-7 in receptor editing of germinal center B cells (Hikida et al., 1998). Receptor editing involves the occurrence of secondary V(D)J (V, variable; D, diversity; J, joining) recombination mediated by the reexpressed products of recombination-activating gene 1 (RAG1) and RAG2. IL-7 was shown to be effective in inducing functional RAG products in mouse IgD+ B cells activated via CD40 in vitro. Blocking of the IL-7 receptor by injecting an anti-IL-7R monoclonal antibody resulted in a marked suppression of the re-expression of RAG2 and subsequent V(D)J recombination in the draining lymph node of immunized mice. IL-7 also has effects on human B cell precursors. Human CD34+CD19+ pre-B cells proliferate modestly to IL-7 in the presence of stromal cells. In contrast, human CD34-CD10+cytoplasmic m+ pre-B cells did not proliferate to IL-7 despite expression of IL-7R (Dittel and LeBien, 1995). In another report, however, it was documented that IL-7 induces proliferation of pro-B cells in the absence of stromal support in serum-free conditions (Namikawa et al., 1996). In addition to its proliferation-inducing effect, IL-7 may also regulate differentiation of B cell precursors since signals generated through the IL-7 receptor and CD19 modulate immunoglobulin gene rearrangement during the early stages of human B cell differentiation (Billips et al., 1995). It was found that IL-7 enhances the expression of CD19 molecules on bone marrow progenitor B-lineage cells and downregulates the expression of terminal deoxynucleotidyltransferase (TdT) and RAG1 and RAG2. CD19 crosslinkage by monoclonal antibodies (mAbs) completely blocked the IL-7 downregulation of RAG expression without affecting the earlier TdT response but had no direct effect on TdT or RAG gene expression (Billips et al., 1995). Despite the clear effects of IL-7 on human B cell precursors in vitro, IL-7 is not essential for B cell development in humans. In a stromal cell assisted B cell differentiation system anti-IL-7 mAbs failed to affect human B cell development (Prieyl and LeBien, 1996). More importantly, two IL-7R -deficient severe combined immunodeficiency patients were found to have normal levels of B cells (Puel et al., 1998). Thymocytes and mature T cells IL-7 acts as survival factor for murine thymocyte precursors (Conlon et al., 1989). All subsets of early
IL-7 141 CD4ÿ CD8ÿ (double negative, DN) thymocytes are responsive to IL-7 (Kim et al., 1998). Whether IL-7 is actually a growth factor for immature thymocytes is somewhat controversial. Earlier studies pointed in that direction but a more recent report strongly suggests that IL-7 does not directly promote cell cycle progression but needs SCF to do so (Kim et al., 1998). IL-7 has rather pronounced effects on mature T cells in vitro. IL-7 is a survival factor for resting and for activated murine T cells (Vella et al., 1998). It mediates a radioprotective effect on resting T cells (Boise et al., 1995) and induces proliferation of activated T cells (Grabstein et al., 1990). It has also been documented that IL-7 induces in vitro growth of murine antitumor cytotoxic T lymphocytes (CTLs) (Lynch and Miller, 1994). IL-7 also has strong effects on human T cells in vitro. It acts as a survival and growth factor for human T cells (Welch et al., 1989). Interestingly, IL-7 is directly mitogenic for CD45RA+ naõÈ ve human T cells and culture of these cells results in maintenance of the CD45RA phenotype (Soares et al., 1998). IL-7 is involved in induction of survival and expansion of T cells after their export from the thymus. Another report documented that IL-7 primes naõÈ ve T cells for IL-4 production (Webb et al., 1997), suggesting that IL-7 plays a role in induction of a TH2 response. However, the observation that skin lesions in patients suffering tuberculoid leprosy, characterized by a strong TH1 response, express IL-7 mRNA (Sieling et al., 1995) argues against a determining role of IL-7 in TH2 responses. IL-7 has the capacity to counteract apoptotic stimuli. For example, it rescues human activated T cells from apoptosis induced by glucocorticosteroids and regulates Bcl-2 and CD25 expression (Hernandez-Caselles et al., 1995). IL-7 was furthermore shown to upregulate expression of CD80 (B7.1) on T cells, resulting in enhancement of T cell stimulation by these cells (Yssel et al., 1993). IL-7 can costimulate TCR-mediated activation of T cells, resulting in enhanced IL-2 mRNA accumulation and IL-2 secretion. The effect of IL-7 is in part mediated at the transcriptional level and involves the upregulation of the DNA-binding activity of NF-AT and AP-1 (Gringhuis et al., 1997). Consistent with both a growth-promoting and a costimulatory activity of IL-7, this cytokine stimulated and expanded HIVspecific T cells in humans (Ferrari et al., 1995; Kim et al., 1997). In vitro studies have also established an essential role of IL-7 in development of human T cells. IL-7 acts as a growth/maturation factor for human thymocyte precursors (Fabbi et al., 1992). IL-7 may not only affect survival and growth of human thymocytes but also migration of these cells as it
upregulates expression of the chemokine receptors CXCR4, the receptor for SDF-1 (Pedrosa-Martins et al., 1998). Moreover, addition of anti-IL-7 and anti-IL-7R antibodies strongly inhibited human T cell development in a fetal thymic organ culture (FTOC) system (Plum et al., 1996; Pallard et al., 1999). Development was inhibited at an early stage of development. These in vitro findings are consistent with the finding that IL-7R -deficient severe combined immunodeficiency patients lack T cells (Puel et al., 1998). Natural killer cells IL-7 has also been reported directly to augment cytolytic activity in human NK cells but less efficiently than IL-2 (Naume and Espevik, 1991). Moreover, the NK cell proliferation-inducing effect of IL-2 was considerably higher than that of IL-7. However, it is unlikely that IL-7 is involved in generation of NK cells in humans, since IL-7R deficient severe combined immunodeficiency patients have normal numbers of NK cells (Puel et al., 1998). NK cell numbers are also not affected in IL-7ÿ/ÿ mice. Myeloid cells Several reports have documented effects of IL-7 on development and function of cells of the myeloid lineages. For example IL-7 has been shown to synergize with SCF in generation of myeloid cells, in particular of granulocytes from primitive murine bone marrow progenitors (Jacobsen et al., 1993; Fahlman et al., 1994). In the human system, IL-7 has also been shown to enhance myeloid colony formation from CD34+ cells in vitro (Jacobsen et al., 1994). Since myelopoiesis is reportedly normal in IL-7 and IL-7R ÿ/ÿ mice, the physiological relevance of in vitro studies suggesting a role of IL-7 in myeloid development is questionable. In vitro experiments suggest that IL-7 may play a role in the function of mature monocyte-macrophages. IL-7 is able to induce secretion of cytokines and tumoricidal activity by human monocytes (Alderson et al., 1991; Standiford et al., 1992), but rather high concentrations were necessary to induce this effect. In addition, IL-7 was shown to inhibit growth of intracellular bacteria in human macrophages (Tantawichien et al., 1996). This effect may be mediated by nitric oxide (NO) and superoxide radicals which are upregulated by IL-7 in murine macrophages (Gessner et al., 1993). Given these in vitro findings it would be of interest to investigate the function of mature myeloid cells in IL-7ÿ/ÿ mice.
142 Hergen Spits Leukemic cells Certain lymphoid leukemia cells were found to express IL-7 receptors and to respond to IL-7 in vitro (Smiers et al., 1995; Consolini et al., 1997). Cells from acute myeloid leukemia also proliferated in response to IL-7 (Digel et al., 1991). These data indicate that IL-7 can induce proliferation of relatively mature tumor cells, and that this effect is not restricted to the lymphoid lineage. IL-7 is a growth factor for Sezary syndrome cells (Dalloul et al., 1992). These cells respond by proliferation to IL-7 plus IL-2, and in some instances produce IL-7 (Foss et al., 1994).
Regulatory molecules: Inhibitors and enhancers There are many cytokines that have been reported either to enhance or to inhibit in vitro activities of IL-7 on a variety of cell types. However, the physiological relevance of those activities is often not clear. An exception is the ligand for the c-kit receptor (c-kit ligand or stem cell factor, SCF) which acts synergistically with IL-7 to promote the growth of early thymocytes. The physiological relevance of this in vitro finding is underscored by the phenotype of mice double deficient for c and c-kit. These mice have a much more reduced thymic cellularity than either c- or c-kit-deficient mice and completely lack pro-T cells (Rodewald et al., 1997). SCF synergizes with IL-7 in B cell development in vitro (Yasunaga et al., 1995) but the B cell defect in c-kit, c double deficient mice is not more pronounced than that of
cÿ/ÿ mice, indicating that IL-7 and SCF do not cooperate in B cell development in vivo (Rodewald et al., 1997). Other factors that potentiate IL-7-induced growth of B cell precursors are insulin-like growth factor 1 (Gibson et al., 1993), Flt-3L, and IL-3. These latter two factors have been reported to enhance the IL-7mediated growth of human CD34+CD19+ pro-B cells (Saeland et al., 1991; Namikawa et al., 1996). As IL-7 is not essential for human B cell development, the physiological relevance of these findings remains unknown. TGF 1 is a strong inhibitor of IL-7mediated proliferative effects, for example, in thymocytes (Chantry et al., 1989). TGF 1ÿ/ÿ mice do not seem to show developmental abnormalities, but they develop an excessive inflammatory response with massive infiltrations of leukocytes in several organs. It is possible that a release of negative control of IL-7-mediated T cell proliferation somehow contributes to this effect. Other potential negative
regulators of IL-7-mediated control of B lymphopoiesis include the interferons. Type 1 IFN and IFN were found to be potent inhibitors of IL-7-induced growth of early B cell precursors, without an effect on cell growth induced by IL-2, IL-3, or IL-4 (Wang et al., 1995). The combination of IL-7 and IFN or IFN induces Bcl-2 downregulation and cell death by apoptosis. As functional type 1 interferons are produced by resident bone marrow macrophages, they may cooperate with IL-7 to control B cell homeostasis. IFN abrogates IL-7-dependent proliferation in pre-B cells, coinciding with the onset of apoptosis (Garvy and Riley, 1994). Another factor that affects the function of IL-7 is IL-10, which has been shown to inhibit IL-7-mediated murine pre-B cell growth in vitro (Elia et al., 1995). Physiological concentrations of retinoic acid (RA) reduced the colony-forming ability of the IL-7stimulated Lin-B220(+)-containing cells. RA was not toxic to the cells, as the inhibition of colony formation after 24 hours was reversible at concentrations as high as 1 mM. The growth-inhibiting effect of RA was directly mediated, as revealed by single cell analysis of the Lin-B220(+)-containing cells (Fahlman et al., 1995).
Bioassays used The most straightforward bioassay for both human and mouse IL-7 is determining the proliferative response of murine pro-B cell or T cell lines to IL-7 (Namen et al., 1988a; Willcocks et al., 1993).
IN VIVO BIOLOGICAL ACTIVITIES OF LIGANDS IN ANIMAL MODELS
Normal physiological roles On the basis of the phenotype of IL-7- and IL-7Rdeficient mice and on that of IL-7R -deficient human severe combined immunodeficiency patients it can be concluded that IL-7 plays an essential nonredundant role in the development of T cells and B cells in the mouse and of T cells in humans. Various aspects of T cell development are controlled by IL-7. IL-7 affects B cell development by promoting both proliferation and differentiation of B cell precursors. There is no evidence for an in vivo role of IL-7 in development of myeloid and erythroid lineages. Studies in
IL-7 143 Figure 1 A model of cellular stages in T cell development. The model is adapted from Shortman and Wu (1996) and DiSanto and Rodewald (1998). The stages where IL-7 and IL-7R deficiencies affect T cell development are indicated. *One of the IL-7R ÿ/ÿ strains that was reported by Peschon et al. (1994) showed variability in their thymic phenotype. Sixty per cent had a block in the transition of CD44+CD25ÿ to CD44+CD25+ cells, whereas the other 40% had a phenotype comparable to that of the IL-7ÿ/ÿ and cÿ/ÿ mice. TCRγδ
IL-7Rα –/–*
CD44+ CD25 –
IL-7–/– IL-7Rα–/–* γ c –/–
CD44+ CD25+
CD44– CD25+
TCRα β + CD4+
CD44– CD25+ TCRβ + pTα +
γ c / IL-7Rα + Responsive to IL-7
IL-7R -deficient mice suggest that IL-7 plays a role in survival and proliferation of peripheral T cells.
Species differences There is a species difference in the requirement for IL-7 in B cell development. Severe combined immunodeficiency (SCID) patients lacking IL-7R or c have normal numbers of B cells. In contrast, mice deficient for IL-7 or components of its receptor have strong reductions in numbers of B cells.
Knockout mouse phenotypes Phenotype of IL-7ÿ/ÿ mice: T cell development All lymphocytes are derived from a common lymphoid precursor. Progeny of these cells migrate to the thymus where they develop into T cells. The earliest murine thymic precursors are characterized by expression of high levels of CD44, but lack the differentiation antigens CD3/TCR, CD4, and CD8 (Shortman and Wu, 1996). The CD4ÿ CD8ÿ compartment contains at least four cell populations that can be discriminated on the basis of expression of CD25 and CD44 (Figure 1). Development proceeds as follows: CD25ÿ CD44+ ! CD25+CD44+ ! CD25+CD44 !
CD44– CD25– (CD8+)
CD4+ CD8+
CD4+ CD8+ TCRα β ± CD69+
TCRα β + CD8+
γ c / IL-7Rα + Responsive to IL-7
CD25ÿ CD44ÿ. The latter cells acquire first CD8, followed by CD4 to attain the double positive (DP) phenotype. The thymus of IL-7ÿ/ÿ mice is reduced in size, but the percentages of CD4+CD8+ cells and mature CD4 or CD8 single positive T cells are the same as in the thymus of wild-type mice (von Freeden-Jeffry et al., 1995). The reduced thymic cellularity is caused by an inhibition of the transition of CD44+CD25+ to CD44-CD25+ cells (von Freeden-Jeffry et al., 1997) (reviewed in DiSanto and Rodewald, 1998). It is unlikely that IL-7 is required at earlier, pre-thymic stages in development of lymphoid cells. The fact that NK cell development is numerically only slightly affected in IL-7ÿ/ÿ mice (Moore et al., 1996) could imply that the precursor of these cells, common lymphoid stem cells (CLPs), which can produce T cells, NK cells, and B cells, are not affected by the IL-7 deficiency. Indeed a population phenotypically identical to CLPs was found in the IL-7ÿ/ÿ mice in proportions similar to those in wild-type mice (Kondo et al., 1997b). The mechanism by which IL-7 controls development of TCR , TCR , and NK-T cells remains to be fully defined. In general, cytokines such as IL-7 involved in hematopoiesis could promote survival and proliferation (trophic effect) and differentiation (mechanistic effect) (reviewed in Candeias et al., 1997). In the case of T and B lymphocytes differentiation-inducing signals are typically those that affect rearrangements of their receptor genes. The
144 Hergen Spits observation that TCR and NK-T cells are present in IL-7ÿ/ÿ mice indicates that IL-7 is not absolutely essential for differentiation of those cell populations, but the absence of TCR cells strongly suggests a mechanistic function of IL-7 in development of these cells. There is consensus that IL-7 regulates the size of the early thymic compartment by promoting survival and probably also proliferation of early thymocyte precursors (DiSanto and Rodewald, 1998). Earlier in vitro studies have shown that IL-7 acts as a survival factor for CD44+CD25+ thymocytes. Consistent with these results, IL-7ÿ/ÿ mice have an increased proportion of DN thymocytes binding annexin-V, a marker for cells undergoing apoptosis (von FreedenJeffry et al., 1997). The trophic action of IL-7 is independent of the Fas pathway or p53, since p53 and Fas-deficient T cell progenitors undergo apoptosis in the absence of IL-7 (Kim et al., 1998). DN thymocytes of IL-7ÿ/ÿ mice are compromised in expression of the antiapoptotic protein Bcl-2 (von Freeden-Jeffry et al., 1997). In a normal thymus Bcl-2 shows a biphasic expression pattern: expression is high in DN thymocytes and is downregulated in DP cells. Bcl-2 is re-expressed in mature single positive (SP) cells. Bcl-2 expression in the earliest CD44+ CD25ÿ thymocytes in IL-7ÿ/ÿ and wild-type mice is the same but IL-7ÿ/ÿ mice lose Bcl-2 upon progression of differentiation through the CD44+CD25+ and subsequent stages. The increase of apoptotic cells in IL-7ÿ/ÿ mice coincides with the decrease in expression of Bcl-2. Moreover, culturing thymocytes of IL-7ÿ/ÿ mice with IL-7 leads to restoration of Bcl-2 levels (von Freeden-Jeffry et al., 1997). Together, these findings strongly suggest that IL-7 mediates survival of early T cell precursors through upregulating Bcl-2. To address this more directly, Bcl2 has been expressed as a transgene in mice deficient for the IL-7 receptor components IL-7R or c. Transgenic expression of Bcl-2 increased the size of the thymus 2-fold in some studies and 4- to 10-fold in other studies compared to the IL-7R - or c-deficient mice without Bcl-2; however, the thymic cellularity remained less than that of wild-type mice (Akashi et al., 1997; Kondo et al., 1997a; Maraskovsky et al., 1997; DiSanto and Rodewald, 1998). The reasons for these discrepancies are not immediately clear but could be related to differences in expression levels of Bcl-2 in different transgenic animals. There are several possible explanations for the observation that transgenic expression of Bcl-2 only partially rescues T cell development. One is that Bcl-2 is not involved in the antiapoptotic effect of IL-7; transgenic expression of Bcl-2 could mimic another antiapoptotic protein. This notion is supported by the
observation that in contrast to IL-7ÿ/ÿ mice, T cell development in Bcl-2-deficient mice proceeds normally during embryonic life. Moreover, IL-7 can promote survival of Bcl-2-deficient T cells (Nakayama et al., 1995), indicating that the antiapoptotic effect of IL-7 can be Bcl-2 independent. It is also possible that Bcl-2 is one of several molecules involved in regulation of survival of T cell precursors. It is in this respect noteworthy that withdrawal of IL-7 not only decreases levels of Bcl-2 but also increases expression of Bax (Kim et al., 1998). A third explanation for the fact that overexpression of Bcl-2 only leads to partial reconstitution is that IL-7, in addition to its antiapoptotic effect, is also required for cell cycle progression and thus expansion of the pre-T cell pool. There is indeed evidence that IL-7 controls proliferation of early T cell precursors. The cell cycle status of the early stages of T cell development in IL-7ÿ/ÿ mice and their wild-type counterparts has been analyzed recently. These studies revealed that while the cell cycle status of CD44+ CD25ÿ of wild-type and IL-7ÿ/ÿ mice is comparable, IL-7ÿ/ÿ mice displayed a progressive reduction of cells in the S and G2/M phases of the cell cycle (von Freeden-Jeffry et al., 1997). These data indicate that IL-7 is important for normal cell cycle progression when cells differentiate from CD44+ CD25ÿ into CD44+CD25+ cells. It is, however, not clear whether IL-7 directly promotes growth of murine thymocyte precursors, since it has been reported that IL-7 induces survival but not cell cycle progression of T cell precursors in vitro (Kim et al., 1998). It is possible that another factor is required to drive IL-7-dependent cell cycle progression. A candidate factor is stem cell factor, which acts with IL-7 synergistically to support T cell development (Rodewald et al., 1997). Moreover, mice deficient for both stem cell factor and c, a component of the IL-7R complex, have a much stronger reduction of the size of the thymus than mice deficient for c only. The requirement of IL-7 for adult thymopoiesis may be much more stringent than for fetal thymopoiesis. It was observed that in contrast to the profound developmental arrest observed in the adult thymus, fetal thymocytes from IL-7R ÿ/ÿ mice have normal proportions of all of the major thymocyte subpopulations, including CD25+ thymocytes and the most mature SP subsets. Moreover, normal levels of RAG1 and RAG2 were observed. Total thymocyte numbers, however, remained reduced (Crompton et al., 1998). The fact that IL-7 is essential for survival of developing T cells has made it difficult to determine whether IL-7 is important for rearrangement at the TCR or TCR loci. One report documented the
IL-7 145 presence of TCR gene rearrangements in fetal thymocytes cultured in IL-7 but not in the same thymocytes cultured in other combinations of cytokines (Muegge et al., 1993). Another study used an in vitro reaggregate culture system with thymic stromal cells to address the role of IL-7 in induction of TCR gene rearrangements. While fetal liver precursor cells reaggregated with wild-type thymic stromal cells develop into T cells, thymic stromal cells of IL-7ÿ/ÿ mice failed to induce RAG upregulation and TCR gene rearrangements (Oosterwegel et al., 1997). These studies could not exclude that the absence TCR gene rearrangements was due to a failure of the pre-T cells to survive in the absence of IL-7. Moreover, while there is a delay in the onset of TCR rearrangement during embryonic life of IL-7ÿ/ÿ mice, TCR gene rearrangements are normal at birth (Haks et al., 1999). In addition, IL-7ÿ/ÿ mice have TCR cells, albeit in reduced numbers in the spleen. These findings together argue against an essential role of IL-7 in TCR (and ) gene rearrangements. DN cells in the thymus acquire CD4 and CD8 following stimulation via the pre-TCR. DP cells undergo rearrangements at the TCR locus and start to express low levels of the TCR . These cells are subjected to positive and negative selection through interactions of the TCR and MHC/peptide complexes. This results in the expression of the activation antigen CD69 but also of IL-7R and the receptor for stem cell factor. Evidence has been reported that IL-7 and SCF may cooperate in maintaining survival and inducing proliferation of TCR + cells that have been subjected to positive selection (Akashi et al., 1997, 1998). Phenotype of IL-7ÿ/ÿ mice: Development of TCR cells While the TCR cells are reduced in numbers but not completely absent, there are no TCR cells present in IL-7- (Laky et al., 1998) and IL-7R deficient (Maki et al., 1996) mice. Both intraepithelial lymphocyte (IEL) and thymic TCR cells are affected. The defect in TCR cells is not rescued by transgenic expression of Bcl-2. This raises the possibility that IL-7 might be important for differentiation, i.e. gene rearrangements of the and/or loci. Indeed Appasamy and colleagues have reported that incubation of fetal liver cells in IL-7 inducedTCR generearrangements(Appasamy,1992). It is well established that TCR gene rearrangements are present in TCR cells. Thus despite the virtual absence of TCR cells in IL-7R ÿ/ÿ mice, the residual TCR cells in IL-7R ÿ/ÿ mice could be
analyzed for the presence of TCR rearrangements and these were found to be absent in IL-7R(exon2) ÿ/ÿ mice with the exception of a few rearrangements involving V 5 (Maki et al., 1996). These data suggest that the absence of TCR cells in IL-7R ÿ/ÿ mice is due to an inability to rearrange the genes. Another group, however, demonstrated the presence of TCR gene rearrangements albeit at a reduced level in IL-7R(exon 3)ÿ/ÿ mice (Durum et al., 1998). The reduction in TCR rearrangements in IL-7R (exon 3)ÿ/ÿ mice was confirmed in a report in which it was shown that IL-7 controls TCR locus accessibility. The TCR locus was found to be methylated in IL-7R ÿ/ÿ thymocytes and the block of TCR gene rearrangement could be released by the histone deacetylase inhibitor trichostatin A (Durum et al., 1998). Reduction of TCR gene rearrangements was also observed in JAK3ÿ/ÿ and in cÿ/ÿ mice (Eynon et al., 1999), indicating that control of gene rearrangements requires the complex of IL-7R and
c relaying a signal through JAK3. It was also reported that transcription of the rearranged TCR was impeded in IL-7R ÿ/ÿ mice suggesting another level of control of TCR development by IL-7 (Perumal et al., 1997). However, inhibition of TCR gene rearrangements cannot be the only explanation why development of TCR cells is inhibited in IL-7R -, c- and JAK3-deficient mice. In the first place, sufficient TCR gene rearrangements were present for the generation of TCR cells and secondly introduction of a functionally rearranged TCR receptor into cÿ/ÿ (Malissen et al., 1997) or JAK3ÿ/ÿ (Eynon et al., 1999) mice did not restore TCR development. This suggests that IL-7 also controls TCR development at another level, most likely through regulation of survival and proliferation. In murine fetal development, TCR cells expressing V 3 are produced first. These cells populate the epidermis and represent the dendritic epidermal cells. Moore and coworkers reported that IL-7ÿ/ÿ mice lack mature V 3+ cells but exhibit the immature stage, expressing low levels of V 3 and high levels of the HSA antigen (CD24), indicating that IL-7 controls maturation of this particular TCR cell type (Moore et al., 1996). These data together suggest that IL-7 can control TCR development at several levels, namely of TCR gene rearrangements, maturation and survival/proliferation. A recent study documented differences in development of TCR intraepithelial lymphocytes in IL-7 and IL-7R ÿ/ÿ mice (Fujihashi et al., 1997). While the IL-7R deficiency resulted in a complete absence of the TCR cell lineage with lack of V 4and V 7-specific messages in the epithelium of the
146 Hergen Spits gastrointestinal (GI) tract, in IL-7ÿ/ÿ mice the depletion of TCR cells in IELs is severe but not complete (Fujihashi et al., 1997). These observations support the hypothesis that both IL-7 and another IL-7R -binding molecule, probably TSLP-1, can influence the development of TCR cells in the intestinal epithelium. Phenotype of IL-7ÿ/ÿ mice: Development of NK-T cells Subsets of T cells expressing the NK1.1 antigen have been proposed to play an immune regulatory role by producing cytokines, notably IL-4. NK1.1+ T cells (NK-T cells) can be readily stimulated by IL-7. The NK-T cells develop at normal relative frequencies in IL-7ÿ/ÿ mice (Vicari et al., 1996; Boesteanu et al., 1997) although there is a numerical reduction of these NK-T cells. They express a biased V TCR repertoire identical to the one observed in IL-7+/+ mice, indicating a normal selection of these cells. However, NK-T cells from IL-7ÿ/ÿ mice were found to be impaired in IL-4 and IFN production in vitro and in vivo which could be restored by IL-7. Interestingly, NK-T cells do not develop in c-deficient mice; they are absent in the thymus and peripheral lymphoid organs such as the liver and the spleen. Since NK-T cell ontogeny is not impaired in IL-2- or IL-4deficient mice, this suggests that IL-15 is required for development of NK-T cells. Indeed, NK-T cells are absent in IL-15R ÿ/ÿ mice (Lodolce et al., 1998). These findings together indicate that differentiation to the NK-T cell lineage requires signal transduction through the c and IL-15R , and once committed their expansion requires signals relayed through the IL-7R . (Boesteanu et al., 1997). Phenotype of IL-7ÿ/ÿ mice: Function of mature TCR cells It has been reported that mature TCR T cells are compromised in their functional capacities in the absence of the IL-7R (Maraskovsky et al., 1996). The majority of IL-7R ÿ/ÿ mice failed to reject an allogeneic tumor, due to a strongly diminished frequency of CTLs. IL-7R ÿ/ÿ T cells were hyporesponsive to TCR-independent stimuli, such as the combination of the phorbol ester PMA and ionomycin. In IL-7ÿ/ÿ mice, however, T cells responded normally to polyclonal stimuli such as ConA and PMA plus ionomycin on a per cell basis (von Freeden-Jeffry et al., 1998). The T cells of IL-7ÿ/ÿ mice have not been extensively analyzed for their responses to more physiological stimuli. It is
noteworthy that mice deficient for JAK3, an essential component of IL-7R signaling, also show defects in T cell function (Thomis et al., 1995). An important question is whether functional defects in JAK3ÿ/ÿ T cells are the consequence of aberrant development in the thymus or whether the T cells require a c ligand in the periphery for maintaining T cell function. By selectively expressing JAK3 in the thymus of JAK3ÿ/ÿ mice, Thomis and Berg found that JAK3 signaling in peripheral T cells is required to maintain T cell function (Thomis et al., 1995). These findings sup-port the notion of a role for IL-7 as the c ligand necessary for survival and maintenance function of mature T cells. However, this role for IL-7 has yet to be established. Similar experiments to those performed by Thomis and Berg have to be performed with IL-7R ÿ/ÿ mice unambiguously to determine the role of IL-7 in maintenance of T cell function. Phenotype of IL-7ÿ/ÿ mice: B cell development IL-7ÿ/ÿ mice show strong defects in the development of B cells (von Freeden-Jeffry et al., 1995, 1998). A phenotypic analysis of different stages in B cell development using a model of Hardy et al. (Li et al., 1996) (Figure 2) revealed that development of B cells in the bone marrow is arrested in the transition of pro-B cells (B220+ CD24+ CD43+) to pre-B cells (B220+ CD24+ CD43ÿ). Mature B cells that express both B220 and surface IgM were present in these mice but at strongly reduced numbers. The B cell phenotype of IL-7ÿ/ÿ mice is identical to that of cÿ/ÿ mice (von FreedenJeffry et al., 1998). However, IL-7R ÿ/ÿ mice have a block at an earlier stage of B cell development, namely the transition of pre-pro B to pro-B cells (Peschon et al., 1994, 1998; von Freeden-Jeffry et al., 1998). These observations suggest the involvement of TSLP-1. In contrast to the partial rescue of T cell development, transgenic expression of Bcl-2 does not reconstitute B cell development in IL-7R ÿ/ÿ and cÿ/ÿ mice, suggesting that IL-7 is required for differentiation of B cells. Indeed studies by Venkitaraman and colleagues indicate that IL-7 is required for IgH gene rearrangements (Corcoran et al., 1996, 1998). It is likely that IL-7 is also required for protection against apoptosis and for expansion of early B cell precursors.
Transgenic overexpression Several IL-7 transgenic mouse strains have been established. These strains differ with respect to the
IL-7 147 Figure 2 A model of early stages in murine B cell development. The model is based on observations made by Li et al. (1996). The effects of IL-7 on various B cell precursor populations are based on information that is summarized in the chapter and in a review by Maeurer et al. (1998). IL-7Rα –/–
IL-7–/–, γ c –/–
Pre-pro-B Fraction A
Early pro-B Fraction B
Late-pro-B Fraction C
Pre-B Fraction D
Immature-B
Mature B
CD43+
CD43+ CD24+ CD19+
CD43+ CD24+ CD19+ BP-1+
CD43– CD24+ CD19+ BP-1+
IgM+
IgM+ IgD+
Not IL-7 responsive
Responsive to IL-7 + stromal cells
IL-7-responsive. Late pre-B cells do not respond to IL-7
promotors that were used. Two of these strains developed cutaneous disorders. Transgenic mice constitutively expressing IL-7 under control of the SR promotor developed severe dermatitis with erythroderma and alopecia (Uehira et al., 1998). The skin lesions were characterized by massive infiltration of mononuclear cells. Most of the infiltrating cells were T cells with the majority bearing the TCR . A progressive cutaneous disorder with infiltrating T cells was also observed in another strain with IL-7 under control of the human L chain promoter and mouse H chain enhancer (EmPm promoter). The infiltrating T cells lacked CD4 and CD8 and a majority expressed the TCR , although TCR cells were also observed. Interestingly, dermal disorders with infiltrating T cells were also observed in EmPm-IL-7-transgenic nude mice, which lack a thymus, indicating a role of extrathymically developed T cells in the skin disease. Dermis infiltrating lymphocytes from the skin lesions of IL-7 transgenic mice showed a moderate response to mitogens, a poor response to alloantigens, and the absence of cytotoxic activities to several tumor cell lines and skin-derived cells (Uehira et al., 1998). EmPm-IL-7 transgenic mice also develop B and T cell lymphomas within the first 4 months of life, indicating that chronic exposure of T cells and B cells to IL-7 predisposes these cells to oncogenic transformation. B cell malignancies were also observed in another transgenic strain: transgenic mice carrying mouse IL7 cDNA under the control of MHC class II (E ) promoter develop B lymphoid tumors (Fisher et al., 1995). In bone marrow of these transgenic mice, the number and proliferative activity of pro-B cells and
Not IL-7 responsive
pre-B cells were markedly increased. Moreover, the bone marrow cavity was considerably expanded and cortical bone showed focal osteolysis. Immature lymphoid cells compressed the venous sinusoids and exuded through eroded bone. Apoptotic bodies, macrophages, and plasma cells were prominent. The number of B lymphocytes and B cell precursors were also increased in the spleen. These results demonstrate that overexpression of IL-7 causes excessive proliferation of a wide range of precursor B cells in bone marrow (Valenzona et al., 1996). Mice carrying the IL-7 transgene under control of the cytokeratin promoter express IL-7 in keratinocytes (Williams et al., 1997). The density of dendritic epithelial T cells (DECT) in the skin of these mice is increased considerably, consistent with the in vitro growth-promoting effect of IL-7 for DECT (Matsue et al., 1993). The observations with IL-7-transgenic mice indicate a role for IL-7 in immune reactions in the skin. The fact that IL-7 is produced by keratinocytes is consistent with this notion. Production of IL-7 by keratinocytes is upregulated by IFN , a product of T and NK cells (Ariizumi et al., 1995). Interestingly UV irradiation of the skin, which leads to immunosuppression, strongly downregulates IL-7 production by keratinocytes (Aragane et al., 1997).
Pharmacological effects Effects of in vivo administration of IL-7 The effects of administration of exogenous IL-7 has been studied in various animal models. A general
148 Hergen Spits observation in these experiments is a marked stimulation of B lymphocyte production but also increases in T cells, NK cells, and macrophages were observed. Exogenous IL-7 at optimal dose can markedly stimulate in vivo B lymphopoiesis from the earliest detectable TdT+ pro-B cell stage, eventually resulting in elevated levels of pre-B and B cells (Valenzona et al., 1998). Changes in myelopoiesis were observed after in vivo administration of recombinant human (rh)IL-7 to mice. These resulted mainly from the emigration of myeloid progenitors from the bone marrow through the blood to the spleen, liver, and, possibly, other peripheral organs (Grzegorzewski et al., 1994). This observation suggests that IL-7 modifies the expression of chemokine receptors on myeloid progenitors. It is, in this respect, noteworthy that IL-7 increases expression of CXCR4, the receptor for SDF-1, on thymocytes (Pedrosa-Martins et al., 1998). In another study rhIL-7 administration to normal mice caused a pronounced leukocytosis in the spleen and lymph nodes, with increases in B lineage cells, T cells, NK cells, and macrophages (Komschlies et al., 1994). The numerical increases of T cells after IL-7 treatment were primarily the result of an expansion of the peripheral T cell population. Interestingly, the T cells from rhIL-7-treated mice have enhanced proliferative responses to various stimuli in vitro and were able to potentiate an allogeneic CTL response in vivo. It was also documented that administration of rhIL-7 to tumor-bearing mice profoundly increased the number of B and T cells, and reduced the number of early renal adenocarcinoma pulmonary metastases. An adverse effect of IL-7 administration was reported by Miyaura et al. (1997), who observed not only increased B cell poiesis but also an increased bone resorption in female mice treated with IL-7. The effect of IL-7 was similar to that observed with oestrogen deficiency. No follow-up studies have been reported so far and this phenomenon has yet to be confirmed. IL-7 has also been administered by gene transfer (Fraser et al., 1993). Murine bone marrow infected with a helper-free recombinant retrovirus expressing the murine IL-7 gene was used to reconstitute lethally irradiated hosts. Twenty-three per cent of mIL-7 retrovirus-infected recipients became moribund within 4±16 weeks post transplant, with splenomegaly and enlarged lymph nodes. Marked changes in T cell subsets of spleen and lymph nodes were observed. The thymic size was not altered and the proportion of CD4+CD8+ cells was generally decreased, with corresponding increases in CD4+, CD8+, or CD4ÿ CD8ÿ cells.
PATHOPHYSIOLOGICAL ROLES IN NORMAL HUMANS AND DISEASE STATES AND DIAGNOSTIC UTILITY
Role in experiments of nature and disease states It has been shown that defects in the IL-7 system in humans cause severe combined immunodeficiency (SCID) syndromes. SCID patients with defects in the c, IL-7R , and JAK3 have been described. A complete c deficiency in these patients is characterized by an absence of T and NK cells. However, normal numbers of B cells are present in c-deficient patients (Noguchi et al., 1993b). The same characteristics are also observed in JAK3-deficient patients (Macchi et al., 1995). Two IL-7R -deficient patients were reported with normal numbers of NK and B cells but absence of T cells in their peripheral blood (Puel et al., 1998). These data indicate that IL-7 is required for T cell development, but dispensable for NK and B cell development in humans. On the basis of the phenotype of IL-15R ÿ/ÿ mice (Lodolce et al., 1998), it is plausible to assume that IL-15 is the
c ligand required for human NK development. However the cytokine responsible for B cell development in humans remains to be determined.
IN THERAPY
Preclinical ± How does it affect disease models in animals? Infectious diseases IL-7 has been tested in various models for its efficacy in treating infectious diseases. IL-7 augments protective immunity against Toxoplasma gondii in A/J mice (Kasper et al., 1995). Moreover, a combination of IL-1 and IL-7 augments immunity against Listeria monocytogenes, due to enhanced responsiveness of TCR cells to IL-7 (Skeen and Ziegler, 1995). These observations indicate that exogenous administration of human rIL-7 is able to protect mice against acute parasite challenge by stimulating IFN production and augmenting the response mediated by CD8+ CTLs (Kasper et al., 1995). IL-7 does not always confer a beneficial effect. Treatment of genetically susceptible Balb/c mice with IL-7 at the onset of the infection with Leishmania major led to enhanced
IL-7 149 lesion development and a significantly accelerated death of the animals. This was correlated with a 40-fold increased parasite burden in spleens and lymph nodes (Gessner et al., 1995). A strong increase in numbers of B cells was noted. It was suggested that an enhanced TH2 response was responsible for the aggravating effect of IL-7 in this model. Bone marrow transplantation Bone marrow transplantation (BMT) is increasingly used in the clinic to promote hematopoietic recovery. The activities of IL-7 on T cells have led to the proposal that IL-7 can be used to overcome immunodeficiencies in certain situations by accelerating T cell development, by expanding the pool of newly developed T cells, and by promoting functional maturation of T cells. Indeed, several preclinical studies have found that IL-7 could accelerate murine lymphocyte regeneration and acquisition of immune competence following chemotherapy and bone marrow transplantation. One group documented that IL-7 boosted survival after challenge with influenza virus following syngeneic BMT (Abdul-Hai et al., 1996). Both B and T cell responses were stimulated by IL-7 (Abdul-Hai et al., 1997). In another study it was reported that the BMT recipients had thymic hypoplasia and strongly reduced numbers of mature T cells 28 days after syngeneic BMT (Bolotin et al., 1996). When these mice were treated with IL-7, thymic cellularity was normalized. Moreover, the function of antigen-specific T and B cells was improved. Recombinant human IL-7 was found to accelerate bone marrow engraftment affecting both myeloid and lymphoid compartments (Boerman et al., 1995). These studies indicate a beneficial effect of IL-7 in autologous BMT. One study, however, suggested that in an allogeneic H2-matched BMT, IL-7 enhanced the responses of the allogeneic T cells posttransplant (Levy et al., 1995). This may result in a graft-versus-host disease. More preclinical studies are necessary to evaluate the effects of IL-7 on immune reconstitution following allogeneic BMT. IL-7 may also be used to mobilize long-term reconstituting hematopoietic stem cells by inducing migration of these cells from the bone marrow to the peripheral blood. (Grzegorzewski et al., 1995, 1996). Tumor rejections Most IL-7R ÿ/ÿ failed to reject an allogeneic tumor, due to a diminished frequency of CTLs, indicating that IL-7 is needed to enable the mature T cells to reject tumor cells (Maraskovsky et al., 1996). It has
also been documented that IL-7 induces in vitro growth of murine antitumor CTLs (Lynch and Miller, 1994). Several studies investigated the protective effects of immunization with tumor cells transfected with IL-7 DNA. A glioma cell line transfected with IL-7 reduced tumorigenicity in vivo dependent on the amount of IL-7 produced. This reduction in tumorigenicity could be reversed in a dose-dependent fashion by injection of anti-IL-7-neutralizing monoclonal antibody at the tumor site. The response was specific, as other tumors were not rejected and rejection was mediated by CD8+ T cells (Aoki et al., 1992). Coexpression of IL-7 and B7.1 in mammary adenocarcinoma and a plasmacytoma resulted in a stronger protective response than either IL-7 or B7.1 alone (Cayeux et al., 1995). A protective effect of immunization with IL-7-transfected tumor cells was observed with the plasmacytoma cell line J558L (Hock et al., 1991), but in that model CD4+ rather than CD8+ cells were responsible for the protection.
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