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Blood, 1 June 2002, Vol. 99, No. 11, pp. 3892-3904
REVIEW ARTICLE
Interleukin-7: from bench to clinic
Terry J. Fry and
Crystal L. Mackall
From the Immunology Section, Pediatric Branch, National
Cancer Institute, National Institutes of Health, Bethesda, MD.
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Introduction |
Interleukin-7 (IL-7) was initially isolated
more than 10 years ago.1-4 Nevertheless, the complete set
of physiologic roles for this cytokine, especially those involving
lymphocyte homeostasis, have only recently been elucidated. After the
initial descriptions of effects on B-cell precursors, recognition that
IL-7 also has marked activity on immature5-7 and
mature8 T cells soon followed. Information from
gene-deleted mice showed IL-7 is a nonredundant cytokine for murine T
and B lymphopoiesis.9,10 Mutations in the  chain of the
IL-7 receptor in patients with severe combined immunodeficiency (SCID)
confirmed that IL-7 is indispensable for T-cell development in humans.
However, the presence of B cells in these individuals suggests
important differences between the role of IL-7 in murine and human
lymphocyte development.11 IL-7 also has potent effects on
mature T cells. Recent work has shown that IL-7 is a critical modulator
of low-affinity peptide-induced proliferation, which is a central
feature of the homeostatic regulation of T-cell
populations.12,13 Furthermore, circulating levels of IL-7
increase in response to T-cell depletion, suggesting a role in T-cell
regeneration.14-16 Importantly, the primary sources of
IL-7 are non-marrow-derived stromal and epithelial cells. Thus, IL-7
is a pleiotropic cytokine with central roles in modulating T- and
B-cell development and T-cell homeostasis. The potency and breadth of
effects suggest that IL-7 administration or neutralization of IL-7 may
allow the modulation of immune function in patients with lymphocyte
depletion, vaccine administration, or autoimmunity.
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Genetics and structure |
The gene for human IL-7 is located on chromosome
8q12-13,17 spans 6 exons, and has open-reading frame of
534 base pairs (177 amino acids), including a 25-amino acid signal
peptide18 (Figure 1).
Homology between the human and the murine IL-7 sequence is 81% in the
coding regions and approximately 60% to 70% in the 5' and 3'
noncoding regions. Although human IL-7 has activity in murine cells,
murine IL-7 fails to stimulate human pre-B cells. The sequence of human
IL-7 predicts a molecular weight of 17.4 kd, but glycosylation results
in an active protein of 25 kd. IL-7 is classified as a type 1 short-chain cytokine of the hematopoietin family, a group that also
includes IL-2, IL-3, IL-4, IL-5, granulocyte macrophage-colony-stimulating factor (GM-CSF), IL-9, IL-13, IL-15, M-CSF, and stem cell factor (SCF).
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| Figure 1.
Structure of the human and murine IL-7 genes.
Human IL-7 locus consists of 6 exons and 9 introns with extensive 3'
and 5' untranslated regions. The gene is located on chromosome 8q12-13.
IL-7 4 variant, which lacks exon 4, has been found in multiple
tissues and lacks biologic activity. Whether this variant has any
biologic significance is unknown. The murine IL-7 gene has
approximately 80% homology to the human gene in the coding regions but
lacks the 54-base pair (bp) exon 5.
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Sites and regulation of production |
Production of IL-7 has been detected from multiple stromal
tissues, including epithelial cells in thymus and bone
marrow.19,20 Within the thymus, the predominant cell
responsible for IL-7 production appears to be a major
histocompatibility complex (MHC) class II+ epithelial cell that likely
represents a cortical epithelial cell.21 Additional sites
of IL-7 production include intestinal epithelium,22 keratinocytes,23 fetal liver,24 adult
liver,25 dendritic cells,26,27 and follicular
dendritic cells.28 Importantly, IL-7 mRNA has not been
detected in normal lymphocytes, though production by Epstein-Barr virus
(EBV)-transformed lymphocytes has been reported.29 Thus,
IL-7 is essentially a tissue-derived cytokine, with the primary sources
stromal and epithelial cells in various locations, whereas bone
marrow-derived dendritic cells appear to be relatively minor sources
of IL-7. IL-7 has been shown to bind extensively to the extracellular
matrix-associated glycosaminoglycan, heparan sulfate, and
fibronectin a feature that is likely to play an important role in the
regulation of local tissue availability and IL-7-induced signaling
within the microenvironment.30-32
Transforming growth factor- (TGF-) and IL-7 share a reciprocal
relationship wherein each is capable of down-regulating the expression
of the other. Indeed, the ability of TGF- to inhibit IL-7-induced
proliferation of pre-B cells was recognized soon after IL-7 was
identified.33 In addition, TGF- has been shown to
down-regulate IL-7 mRNA and protein secretion from human bone marrow
stromal cells.34 Interestingly, IL-7 has also been shown to down-regulate TGF- production.35,36 Thus IL-7 shares
an antagonistic relationship with TGF- wherein TGF- can
down-regulate IL-7 production by stromal cells and IL-7 can
down-regulate the production of TGF-. Although the mechanisms and
implications of this relationship are yet to be elucidated, the potency
of both agents on a breadth of immune populations suggests that this represents an important level of immune regulation.
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IL-7 receptor and signaling |
IL-7 is a member of the family of cytokines that signal through
the common cytokine gamma chain ( c) (Figure
2).37-39 A recent addition
to this family is IL-21.40-42 IL-7 also uses a second component, the IL-7 receptor alpha chain (IL-7R) (CD127).
Signaling through the IL-7R requires both IL-7R and the c
component. Because c is expressed ubiquitously on lymphoid cells,
the identification of IL-7R implies that IL-7 binding and subsequent
signaling could occur. In general, IL-7R can be identified on
immature B cells through the early pre-B stage, on thymocytes, and on
most mature T cells with transient down-regulation upon
activation.43,44
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| Figure 2.
IL-7 shares the common cytokine c with IL-2, IL-4, IL-9, IL-15, and
IL-21 and the IL-7R chain with TSLP.
Binding of IL-7 to the IL-7R chain and c leads to
heterodimerization of these components and to juxtaposition of the
intracellular signaling molecules Jak 3 and Jak 1. Phosphorylation of
tyrosine residues within the cytoplasmic domain of the IL-7R chain
and the Jak 1 molecule results in the activation of multiple downstream
signaling pathways, including STAT5a and STAT5b, PI3-kinase (PI3K), and
src kinases. In addition IL-7 signaling alters bcl-2 family member
expression and localization, resulting in cell survival
signals.
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IL-7R is also used by thymic stromal-derived lymphopoietin (TSLP) as
part of a complex that contains a second receptor chain that, thus far,
appears to be used solely by TSLP.45-49 Indeed, the myriad
subtle but significant differential effects observed in
IL-7 / versus IL-7R/ mice may be
attributed to the ablation of the effect of IL-7 alone in the
IL-7/ mice, whereas IL-7R/ mice are
deficient in IL-7 and in TSLP signals.
Like other members of the hematopoietin receptor family, IL-7R is a
type 1 membrane glycoprotein folded to accommodate the binding of alpha
helical cytokines. The 220-amino acid extracellular domain contains
major regions of homology with other members of this family. In
addition, there is a single 25-amino acid transmembrane region and a
195-amino acid cytoplasmic tail important in recruiting intracellular
signaling molecules (reviewed in He and Malek50). Recruitment of kinases is required for signal transduction because the
intracellular portion of IL-7R does not contain intrinsic tyrosine
kinase activity.
IL-7 signaling involves a number of nonreceptor tyrosine kinase
pathways that associate with the cytoplasmic tail of the receptor. These include the Janus kinase/signal transducer and activator of
transcription (Jak/STAT) pathway, phosphatidylinositol 3-kinase (PI3-kinase), and Src family tyrosine kinases. Details of IL-7 signaling have been comprehensively reviewed elsewhere.51
Of note, IL-7 shares intracellular signaling molecules with a number of
other cytokines, and the exact mechanisms responsible for signaling specificity remain unclear.
In humans, mutations in c52 and Jak353,54
result in a SCID syndrome with defective T- and natural killer
(NK)-cell generation similar to that observed in c-deficient mice.
Recently, patients with deficiencies in T cells, dysfunctional B cells,
and normal to increased NK cells were identified as having mutations in
IL-7R.11 Thus, whereas IL-7 is required for B-cell
development in mice, it is not absolutely required in humans. It should
be noted, however, that B-cell function remains severely impaired in
these patients, and the mechanisms responsible for this immune
dysfunction are not well understood. Table
1 shows the phenotype of relevant knockout mice and corresponding human correlates from spontaneous mutations.
Based on the information available, the following model for
IL-7-mediated signaling can be put forth (Figure 2). First, IL-7 binds
to IL-7R, leading to dimerization with c, which also has binding
sites for IL-7.55 Jak3, associated with c,
phosphorylates tyrosine residues in the cytoplasmic portion of
IL-7R, leading to recruitment of Jak1 and of STAT molecules.
Although a number of cytokines use the components involved, specificity
in signaling may be achieved by specific docking sites for particular
STAT molecules or through the use of additional kinases as described above. It is important to point out that though the c component is
required for IL-7 signal transduction,39 this requirement appears to be based solely on the lack of intrinsic tyrosine kinase activity in IL-7R and the need for Jak3 to "trigger"
phosphorylation of IL-7R-associated proteins.56
Indeed, in a chimeric receptor system, an erythropoietin receptor
containing Jak2 can substitute for c-associated Jak3.57
Thus, IL-7R appears to function as the driver of signaling once
activated by dimerization with an appropriate receptor containing a
trigger, which in the native setting involves c.
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B cells |
B-cell development can be divided into distinct phases in mice
that can be characterized by surface phenotype (Figure
3).58 IL-7 was first
identified based on its capacity to induce the growth of immature B
lymphocytes.1-3 The generation of
IL-7-deficient10 and IL-7R-deficient
mice9 and monoclonal antibody blocking experiments59 confirmed the requirement of IL-7 for B-cell
development in mice. In IL-7/ mice, a block in B-cell
development occurs at the pro-B-cell to the pre-B-cell
transition.10 Interestingly, in IL-7R/
mice, the block in B-cell development occurs earlier, at the pre-pro-B-cell stage.9 This indicates the presence of a
second molecule (such as TSLP) that uses IL-7R and regulates B-cell development at the pre-pro-B-cell stage.49 Transgenic
IL-7 expression in lymphoid cells using an immunoglobulin promoter
results in the dramatic expansion of immature and mature B
cells.60 Mice that express IL-7 under an MHC class II
promoter develop an expansion of immature B cells in the spleen, lymph
nodes, and bone marrow with the eventual development of
lymphoproliferative disorders bearing immature B-cell
markers.61-63 Furthermore, the administration of exogenous
IL-7 to normal mice leads to significant expansion of pre-B cells and
mature B cells in normal and lymphocyte-depleted mice.64-66
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| Figure 3.
Schematic of B-cell development in relation to IL-7R expression and
IL-7 responsiveness.
B-cell development proceeds from a common lymphoid progenitor (not
shown) that is characterized by the expression of IL-7R and
c-kit but that lacks lineage-specific markers (eg,
B220). The first identifiable progenitor committed to the B lineage is
the pre-pro-B cell expressing B220 and low levels of heat stable
antigen. Transition to the pro-B-cell stage involves a period of
proliferation, probably in response to factors other than monomeric
IL-7, and the beginning of immunoglobulin heavy chain rearrangement.
Heavy chain rearrangement is completed at the early pre-B-cell stage.
This stage also involves the expansion of successfully rearranged cells
in response to IL-7 and other factors. By the late pre-B-cell stage,
IL-7R expression ceases. Fractions listed correspond to those
described by Hardy et al58 in the
mouse.
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Although there is no doubt that supraphysiologic levels of IL-7
potently expand B-cell progenitors in mice, leading to the expansion of
the entire B-cell compartment, several questions remain regarding the
exact physiologic role for IL-7 in regulating the proliferation,
survival, and differentiation of developing B cells in normal mice. In
particular, developing murine B cells show significant changes in the
capacity and threshold for IL-7-induced proliferation, depending on
the exact stage in B-cell development. Pre-pro-B cells display a high
threshold for IL-7-induced proliferation, followed by a diminished
threshold at the pro-B-cell stage with a return of a higher threshold
at the pre-B-cell stage.67 Furthermore, IL-7-induced
proliferation of pre-pro B cells (before immunoglobulin rearrangement)
requires stromal contact,58,68 whereas IL-7 induces the
proliferation of pro-B cells (D-J rearranged) in a contact-independent
manner.33,69 One potential explanation for the stromal
cell requirement in pre-pro-B cells was provided by the recent
description of a heterodimeric "hybrid cytokine" formed by IL-7 and
the chain of hepatocyte growth factor termed pre-pro-B-cell
growth-stimulating factor (PPBSF).70 PPBSF
stimulates the proliferation and differentiation of pre-pro B cells in
vitro, thus inducing receptivity to proliferation induced by monomeric IL-7 alone as the cells enter the pro-B-cell stage. Although a true
low-affinity receptor on pre-pro-B cells has not yet been defined, it
is postulated that PPBSF can signal through a low-affinity receptor on
pre-pro-B cells, thus leading to the up-regulation of a high-affinity
receptor and subsequent responsiveness to monomeric IL-7 on pro-B
cells. In addition, TSLP may suffice for pre-pro-B cell proliferation
because IL-7/ mice generate pro-B cells whereas
IL-7R/ mice do not.
It was recently shown that assembly of the B-cell antigen receptor
(BCR) complex regulates IL-7-induced proliferation because pro-B cells
from RAG2/ mice, which lack a pre-B-cell receptor,
have an increased threshold for IL-7 responsiveness at the pro-B-cell
stage and a failure to shut down IL-7 responsiveness at the pre-B cell
stage.67,71 Thus, developing B cells appear to become
transiently susceptible to IL-7-induced proliferative and trophic
effects at the pre-B-cell stage associated with BCR rearrangement.
However, subsequent to this point, tight control of IL-7-induced
effects occurs by increasing the IL-7 signaling threshold in the
presence of the BCR.
Whether IL-7 acts directly to induce BCR rearrangement or facilitates
antigen receptor rearrangement indirectly by acting as a trophic factor
that enhances the survival of cells undergoing BCR gene rearrangement
has been a controversial area in B-cell development. Corcoran et
al72 found impaired immunoglobulin gene rearrangements in
IL-7R/ mice, but D-J and V-D-J rearrangements of the
heavy chain locus were detectable in IL-7/
mice.73 However, the expression of cytoplasmic µ was
reduced in IL-7/, c/, and
Jak3/ mice, suggesting that IL-7 may be involved in
cytoplasmic µ expression after rearrangement. A possible explanation
for this discrepancy was that the lack of signaling of another putative
factor that uses IL-7R was responsible for the impaired gene
rearrangement in IL-7R/ mice. Indeed, using an in
vitro system, Corcoran et al74 demonstrated a direct role
for IL-7R in promoting immunoglobulin gene rearrangement. By
transferring mutated forms of the gene for IL-7R into
IL-7R/ mice, they identified a tyrosine residue on
the IL-7R cytoplasmic domain that is required for PI3-kinase
signaling. Mutations at this site abrogated proliferation but retained
the ability to mediate immunoglobulin gene rearrangement as measured by µ protein expression. Therefore, it appears that signaling through
IL-7R may play a mechanistic role in immunoglobulin rearrangement,
and it remains possible that TSLP or another yet to be identified molecule can induce these effects in IL-7/ mice.
Furthermore, these studies demonstrated that at least 2 distinct
IL-7R-mediated signaling pathways differentially regulate the
proliferation of developing B cells and the mechanistic effects on BCR rearrangement.
In a number of cell types, including developing B cells, IL-7 can act
as a trophic factor. Thus, in addition to the proliferative effect of
IL-7 on developing B cells, IL-7 can maintain developing B cells by
providing a survival signal. This effect appears to involve the
modulation of bcl-2 family members, a group of intracellular, membrane-associated proteins that includes both proapoptotic and antiapoptotic members.75 Although this mechanism has been
well established for T cells (as will be discussed later), the role of
this pathway in developing B cells remains less clear. Transgenic expression of the antiapoptotic molecule bcl-2 was unable to restore B
lymphopoiesis in c/ mice76 or
IL-7R/.77 However, analysis of
IL-7/ mice indicated that the absence of IL-7 results
in decreases in bcl-2, increases in bax, and increased apoptosis in
developing B cells.78 Thus, the physiologic role of IL-7
as a trophic factor for developing B cells through the modulation of
bcl-2 family members has not yet been determined definitively.
IL-7 synergizes with stromal-derived factor 1 (SDF-1)79
and SCF80 in inducing the proliferation of developing B
cells, and the combination of IL-7 and flt3 ligand induces dramatic
expansions of B cells in vitro.81 Furthermore, IL-7 and
flt3 ligand can support B-cell generation within the
thymus.82 Thus, in the physiologic setting, it is likely
that factors such as SDF-1, SCF, and flt3 ligand work in concert with
IL-7 to regulate B-cell development. In summary, it appears as if the
proliferative effect of IL-7 on pro- and pre-B cells is tightly
regulated within the marrow environment by a complex interaction
between antigen receptor assembly, responsiveness to IL-7, and action
of other B-cell growth factors.
Mature B cells are generally incapable of responding to IL-7. However,
recent work has demonstrated that a least a subset of peripheral B
cells can become transiently IL-7 responsive. B-cell receptor antigen
diversity is generated during development by immunoglobulin gene
rearrangements mediated by recombinase-activating (RAG) genes. However,
secondary rearrangements, termed receptor editing, can occur in
IgM+IgD immature B cells in
vitro.83-85 Isotype switching, somatic hypermutation, and
affinity maturation occur within germinal centers and result in
modifications in antibody affinity, but because RAG expression was
thought to cease in peripheral B cells, it was originally assumed that
further immunoglobulin rearrangements did not occur. However, it is now
known that the immunization of mice results in the re-expression of
RAG genes in B cells in lymphoid germinal centers and can
result in functional V-D-J recombination in vitro and in
vivo.86,87 Hikida et al88 showed that that
IL-7R is also re-expressed in germinal center B cells and that IL-7 could induce RAG re-expression. Furthermore, the administration of an
IL-7R blocking antibody suppressed V-D-J recombination in the
germinal centers of immunized mice. Therefore, IL-7 can act to restore
the plasticity of B-cell antigen receptor specificity in mature B
cells, implying that IL-7 may play a direct role in immunoglobulin rearrangement.
As discussed earlier, humans with SCID caused by IL-7R mutations
show normal numbers of B cells.11 Furthermore, in an in vitro system, the generation of immature B cells from bone
marrow-derived stem cells did not require the presence of
IL-7.89 Nonetheless, there is abundant evidence that IL-7
can modulate B-cell development in humans. In the initial report on
human IL-7, effects on human B cells derived from normal marrow were
described.2 In one report, pro-B cells responded to IL-7
in the presence of stromal cells, whereas pre-B cells did not
proliferate despite comparable IL-7R expression.90
Thus, although human B-cell development does not appear to require
IL-7, immature human B cells do proliferate in response to IL-7, and
IL-7 treatment in mice leads to the expansion of immature B cells.
Thus, it appears likely that pharmacologic doses or increased
availability of endogenous IL-7 may affect B-cell generation in humans.
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Developing T cells |
The development of T cells within the thymus proceeds through a
complex series of stages (Figure 4). The
first stage is represented by
CD3CD4CD8 triple-negative
(TN) immature thymocytes, followed by a
CD4+CD8+ double-positive (DP) stage and,
finally a CD4+ or CD8+ single-positive (SP)
stage containing mature T cells. The TN, DP, and SP stages represent
5%, 80%, and 15% of the thymocyte pool, respectively. A complete
description of thymic T-cell development can be found
elsewhere.91 Because most (98%) T cells are lost to
apoptosis during positive and negative selection, substantial numbers
of T-cell progenitors are needed to generate a diverse repertoire of
T-cell receptor (TCR) specificities in sufficient quantities.
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| Figure 4.
Schematic of T-cell development.
The earliest T-lineage cell is the TN CD44+CD25 pro-T1
thymocyte. This cell can also give rise to B cells, NK cells, and
dendritic cells. The next stage is a CD44+CD25+
pro-T2 cell, which can give rise to T cells and probably dendritic
cells. This stage involves proliferation in response to IL-7 and SCF.
Rearrangement of the , , and TCR chains begins at the end of
this stage and is associated with diminished proliferation. The
CD44CD25+ and
CD44CD25 stages are characterized by the
completion of rearrangement and the death of thymocytes that fail to
undergo successful rearrangement, followed by a period of expansion.
Thymus-derived T cells arise from the
CD44CD25+ and possibly the
CD44CD25 stages. IL-7R chain is
expressed throughout the TN stage, contributing to proliferation,
survival, and rearrangement (at least for the locus) as described
in more detail in "Developing T cells." TN thymocytes comprise
approximately 5% of the thymocyte fraction. Positive selection occurs
during the DP stage, resulting in the death of most thymocytes and in
self-MHC restriction. IL-7R expression is down-regulated during this
stage. Rearrangement of the TCR- component takes place during the DP
stage. Clonal deletion of thymocytes expressing self-reactive TCRs
begins toward the end of the DP stage and probably continues
through the early SP stage. IL-7R chain is re-expressed at the SP
stage and remains, at some level, throughout the life of a mature T
cell. Eighty percent of thymocytes are DP, and 15% are
SP.
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The process of T-cell development occurs within a relatively poorly
understood microenvironment. Anatomically, TN precursors enter the
thymus in or migrate to the subcapsular zone, then, as they mature,
proceed centrally through the cortex to the medulla from which mature
SP T cells emigrate to the peripheral circulation. Figure
5 shows the anatomic locations within the
thymus in the context of T-cell developmental stages. The supporting
cells within the thymus include epithelial cells, dendritic cells,
fibroblasts, and a variety of other cell types. These cells provide a
network of growth factors and other molecules that are critical for
T-cell development. IL-7 can be identified within the thymus of 13-day murine embryos coincident with the first wave of thymocyte
expansion.19 The production of IL-7 has been identified in
a subset of MHC class II+ epithelial cells21 that also
express SCF, another important growth factor for early
thymocytes.92 Interestingly, SCF synergizes with IL-7 in
thymocyte proliferation, but it also acts at an earlier stage to
up-regulate CD25 followed by IL-7- and SCF-mediated
proliferation,93 suggestive of the massive proliferation
of CD44+CD25+ thymocytes that is known to occur
in vivo. Thus, the orderly development and selection of mature T cells
occurs within the complex thymic microenvironment containing a variety
of critical factors, including IL-7.
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| Figure 5.
T-cell development stages in relation to thymic architecture.
Differentiation of TN thymocytes occurs within the subcapsular zone of
the thymus. This region contains a network of epithelial reticular
cells. At the DP stage, thymocytes migrate to the cortex, where they
encounter cortical epithelial cells with long processes, fibroblasts,
and macrophages. These cells are important for MHC class restriction
and negative selection. Thymocytes then migrate to the medulla, where
CD4 or CD8 lineage commitment occurs. This region contains medullary
epithelial cells with shorter processes, dendritic cells, and
macrophages. Mature T cells exit the thymus from the medullary region
and enter the peripheral circulation.
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Soon after the identification of IL-7 as a growth factor for developing
B cells, it was recognized that IL-7 also could induce the survival and
proliferation of immature thymocytes in culture.94 As in
B-cell development, differences in T-cell phenotype between IL-7/ and IL-7R/ mice has
suggested that other molecules using the IL-7R chain are also
important for early T-cell development. In IL-7/ mice,
thymic cellularity is decreased 20-fold. Analysis of thymocyte subsets
showed a partial inhibition in TN differentiation with a relative
accumulation of TN thymocytes.95 In
IL-7R/ mice, thymic cellularity is reduced to 0.01%
to 10% of normal.9 Development of T cells occurs
in a subset of IL-7R/ mice, but these cells do not
function normally. Anti-IL-7 monoclonal antibody treatment
for 12 weeks resulted in a greater than 99% decrease in thymic
cellularity and an interruption before the CD44+CD25+ stage similar to that in
IL-7/ mice.96 Proportionally, there was an
increase in the CD4CD8 compartment because
of an accumulation of
CD3+CD4CD8+ cells.
Expression of IL-7 under an immunoglobulin  light chain promoter
resulted in increases in all mature T-cell subsets but no increase in
pro-T cells.60 In a second transgenic line with an IL-7
transgene fused to an immunoglobulin heavy chain enhancer and promoter,
there was a perturbation in thymic development with a profound decrease
in DP thymocytes but a marked increase in mature T
cells.97 Expression of the same IL-7 transgene in nude mice led to the restoration of mature T-cell numbers,98
though it was not possible to distinguish between enhanced T-cell
development and expansion of the small numbers of mature T cells in
these mice. Finally, the expression of IL-7 under an MHC class II
promoter resulted in a 30-fold increase in mature T cells, but thymic
development was intact.99 Despite these conflicting
reports, it appears that the overexpression of IL-7 results in
increases in T-cell numbers attributed, at least in part, to increased
thymic output.
The administration of IL-7 following T-cell depletion has been
evaluated in murine models as a potential modulator of immune reconstitution. In a report by Abdul-Hai et al,100 IL-7
administered after syngeneic bone marrow transplantation (BMT) resulted
in a 12-fold increase in thymic cellularity. In addition, RAG-1
expression and V-D-J recombination were increased in IL-7-treated
animals. Bolotin et al101 showed that the administration
of IL-7 after BMT resulted in a more rapid normalization in thymic
cellularity and thymic subsets. Furthermore, increased numbers of
thymus-derived mature T cells were seen following BMT with IL-7
treatment.102 Thus, exogenous IL-7 enhances thymopoiesis
after radiation-induced lymphopenia.
The effects of IL-7 on developing thymocytes are multiple. Initial
experiments using the fetal thymic organ culture system revealed that
IL-7 could enhance the viability of thymocytes independent of a
proliferative effect.103,104 von Freeden-Jeffry et
al105 showed that bcl-2 protein is markedly decreased in
CD44+CD25+ thymocytes from
IL-7/ mice, resulting in increased apoptosis. In
addition, bcl-2 transgene expression in IL-7R/ mice
increased thymocyte numbers with substantial increases in peripheral T
cells and restored mature T-cell function.77,106 The
antiapoptotic effects of IL-7 also involve bax, a proapoptotic family member.107 This is further supported by the
phenotype of bax-deficient mice that develop marked increases in
thymocyte numbers.108 These results suggest that a
substantial component of IL-7 action within the thymus involves a
modulation of apoptosis through alterations in bcl-2 family members.
Thus, IL-7 maintains the survival of early thymocytes during the TN
stage of development through the modulation of bcl-2 family members. In
addition, in concert with other growth factors such as SCF, IL-7
contributes to the expansion of T-cell precursors. The end result is
that sufficient numbers of T-cell precursors undergo TCR rearrangement
before the massive cell loss that occurs during positive and negative
selection. A lack of IL-7R signaling severely curtails this process,
leading to a subsequent reduction in T-cell export.
IL-7 is absolutely critical for the development of T
cells. In 2 separate strains of IL-7R/ mice,
T cells cannot be detected.109,110 Although the thymus is
the predominant site of T-cell development, it has been suggested that
T cells can also develop within extrathymic sites, most notably the
intestine.111 Expression of an IL-7 transgene in the
intestinal epithelium of IL-7/ mice using a
tissue-specific intestinal fatty acid-binding protein promoter rescued
extrathymic T-cell development.112 Therefore, IL-7 plays
an essential role in the generation and maintenance of thymus-derived
T cells and T cells derived from extrathymic pathways.
IL-7 also appears to be directly involved in the induction of TCR
rearrangement. It has been difficult to definitively show whether IL-7
directly contributes to the process of gene rearrangement or simply
maintains the survival of cells undergoing the rearrangement process
because these effects are occur simultaneously (reviewed in
113). Recently, it has been demonstrated that IL-7
regulates accessibility of the TCR locus by affecting histone
acetylation through STAT5.114,115 Thus, IL-7R-mediated
signals appear to be important for the rearrangement of the locus;
however, for the other TCR loci, a mechanistic role for IL-7 in gene
rearrangement is less clear.
Taken together, the information available suggests the following role
for IL-7 during T-cell development in the thymus. After the migration
of precursor cells to the thymic subcapsular zone, IL-7, in concert
with other factors such as SCF, drives the proliferation TN precursors.
The relative role of IL-7 in relation to other proliferative signals in
vivo remains unclear. With the loss of CD44, these cells begin to
undergo rearrangement of the TCR , , and genes. During this
phase, survival signals (bcl-2 family members) generated through
IL-7R appear to be important. However, except for the chain,
direct involvement of IL-7 in gene TCR rearrangement is less well
established. Decreased expression of the IL-7R chain on DP
thymocytes suggests that IL-7 may be less important at this
stage. At the SP phase, IL-7R is re-expressed and is maintained (at
least at some level) throughout the life of the T cell. The role of
IL-7 in mature T cells is discussed in subsequent sections.
Two other reports are noteworthy regarding the role IL-7 plays in
T-cell development. In terms of IL-7 signaling in the thymus, it was
recently demonstrated that PIM1, a proto-oncogene that may be involved
in pre-T-cell differentiation, partially restores thymic cellularity
in IL-7/ mice.116 Brugnera et
al117 examined the role of IL-7 during the differentiation
from DP to SP thymocytes, a step critical for lineage commitment
because it involves the loss of expression of either the CD4 or the CD8
co-receptor. These authors suggest that IL-7 mediates the suppression
of CD4 transcription in thymocytes destined to become CD8 SP cells.
Thus, our understanding of the role of IL-7 in T-cell development,
particularly as it relates to other developmental signals, continues to
evolve and appears to extend beyond the TN stage.
| |
IL-7 and thymic aging |
Despite continued thymic T-cell development well into adulthood,
there is a marked age-related decline in thymic
function.118 The mechanisms underlying thymic atrophy
remain unclear.119 IL-7 has been investigated in this
regard because of its importance in T-cell development. Knowledge of
the relative availability of IL-7 during the thymic aging process is
critical to our general understanding of human T-cell development. In
mice, the action of IL-7 in fetal and adult thymi may be distinct.
Crompton et al120 demonstrated that the arrest in thymic
maturation in IL-7R/ mice was more marked in adult
than in fetal thymi, suggesting different requirements for IL-7R
signaling. Further analysis showed that IL-7R is required for
proliferation, survival, and RAG expression of adult thymocytes. In
contrast, in fetal thymocytes, IL-7R signals remain critical for the
proliferation of thymocytes, but RAG expression and, perhaps, survival
can occur in the absence of IL-7R signals. In a report by
Aspinall,121 some strains of TCR transgenic mice did not
develop age-associated thymic atrophy, suggesting that a diminished
rate of TCR rearrangement plays a role in the thymic aging process.
Based on the effects of IL-7 on TCR rearrangement, it was hypothesized
that IL-7 deficiency may contribute to age-associated thymic
involution. In recent studies, the treatment of aged mice with IL-7 led
to significant increases in TN thymocytes with no appreciable change in
the relative proportion of cells within in each subset.122
Treatment with SCF did not result in the same effect. However,
treatment of very aged mice with IL-7 does not lead to increases in
thymic output (C.L.M., unpublished observations, June 1996).
Furthermore, analysis of IL-7 mRNA expression in adult human thymi did
not show an age-associated decline.123 Thus, though IL-7
may be able to enhance thymic function during aging, it appears
unlikely that isolated IL-7 deficiency is the sole cause of thymic
involution associated with aging.
| |
IL-7 and mature T cells |
Although IL-7 is best known for its effects on developing B-cell
and T-cell populations, IL-7 also potently modulates mature T-cell
function.124 First, IL-7 costimulates for T-cell
activation by enhancing proliferation and cytokine production,
especially in the setting of suboptimal TCR triggering. Although some
of this effect is IL-2 dependent through the up-regulation of IL-2R by IL-7, murine and human studies have shown that at least some of the
costimulatory effects of IL-7 are IL-2 independent.125,126 Second, although IL-7 is not generally considered to play a central role in determining type 1 versus type 2 T-cell differentiation, IL-7
tends to induce type 1 immune responses because it potently up-regulates interferon- (IFN-) and IL-2 production, only weakly induces IL-4 production, and synergizes with IL-12 in inducing T-cell
proliferation and IFN- production, in part by up-regulating the
IL-12R on mature T cells.124,127,128 IL-7 also enhances
expression of the chemokine receptor CXCR4, which is expressed on a
subset of memory CD4+ T cells and may be important in
T-cell homing to lymphoid tissues because of its binding to
SDF-1.129
A third major effect of IL-7 on mature T cells is the inhibition
of programmed cell death. Thus, IL-7 acts as a trophic factor for
mature T cells, similar to the effects observed on developing B and T
lymphocytespartly through the up-regulation of bcl-2 family
molecules130-132 and potentially through the up-regulation of the T-cell survival factor, lung Kruppel-like
factor.133 Not surprisingly then, IL-7 enhances T-cell
survival in long-term cell cultures, and, in some studies, IL-7 was
shown to be superior to IL-2 in this regard.134 The
combination of enhanced costimulation and programmed cell death
inhibition by IL-7 is likely responsible for the role of IL-7 in
facilitating memory T-cell differentiation in vivo. Unlike IL-15, which
is absolutely required for the development of memory T-cell
populations, the absence of physiolo |
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Introduction
Genetics and structure