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Departments of Medicine and Immunology, University of Washington, School of Medicine, Seattle, WA 98195
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Abstract |
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 Top Abstract IntroductionMaterials and MethodsResultsDiscussionReferences |
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Introduction |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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c, which
is shared among receptors for IL-4,-7, -9, -13, and -15
(2, 3, 4, 5, 6, 7, 8, 9). The IL-2Rß and c chains are expressed on
resting T cells and together can bind IL-2 with low affinity. In T
cells activated by TCR ligation or nonspecific stimuli, such as IL-1 or
TNF-
, expression of the IL-2R chain (CD25) is induced
(9, 10, 11). Although this subunit has no known signaling
role, it confers the ability to bind IL-2 with high affinity, thus
regulating cellular sensitivity to this cytokine (1, 9, 10, 12). IL-2R signals are generally understood to affect T cell fate after antigenic encounter. IL-2 is a potent growth factor for T cells in vitro, in large part due to effects on promoting the transition from the G1-S phases of the cell cycle following TCR stimulation (1, 13, 14, 15, 16). In the setting of appropriate costimulation, TCR ligation also leads to IL-2 secretion, predominantly by the CD4+ T cell subset. Thus, under commonly used culture conditions, T cell proliferation is driven by an autocrine/paracrine hormonal loop (9, 17). These and other observations engendered the view that IL-2 played a key role in amplifying immune responses by mediating clonal expansion of Ag-reactive T cells. This view has become more complicated in recent years, based on the finding that the effects of IL-2 on cell cycle progression can confer sensitivity to cell death induced by further TCR stimulation (18).
The role of IL-2R signals in regulating the immune system has been explored using mice either deficient in IL-2 or lacking the ability to form high-affinity IL-2 receptors through targeted deletion of CD25 (19, 20). Both types of mice exhibit expansion of peripheral lymphoid tissues, hypersecretion of T cell-dependent Ig subclasses, and susceptibility to autoimmune disorders, demonstrating that IL-2R signals are required for tissue homeostasis and self-tolerance (19, 20, 21, 22). In both strains, defects in peripheral T cell deletion in response to superantigens has been reported, suggesting that IL-2R signals may be required for efficient activation-induced T cell death in vivo (20, 23). We and others have hypothesized that the lymphoid expansion and autoimmunity observed in mice with defective IL-2R signals is driven by a diverse array of environmental Ags, for which T cell activation is uncoupled from IL-2R-dependent deletion. In this paper, we have examined requirements for CD25 in the homeostatic regulation of several immune system components, including CD4+ and CD8+ T cell subsets, B cells, and CD4+ T cells populations with progressively restricted sets of specificities. The cell-autonomous regulation of T cell Ag responses by CD25 was also examined in the context of a normal lymphoid compartment. These studies indicate a limited role for CD25 in regulating Ag-induced proliferation and cell death in vivo, and suggest that homeostasis is achieved indirectly, possibly through regulation of bystander responses.
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Materials and Methods |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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CD25-/- mice on a mixed 129/C57BL/6 background (20) were crossed with DO11.10 TCR-transgenic mice (provided by Dr. Abul Abbas, Brigham and Women’s Hospital, Boston, MA) on a BALB/c background (24). Four to five backcrosses with the DO11.10 strain were performed, with offspring selected following flow cytometric analysis for H-2Kb vs H-2Kd expression (Abs from PharMingen, San Diego, CA) to ensure homozygosity for the H-2d haplotype. CD25 was genotyped using PCR with the following primers: 5'-GTAGTCAGTCTTCTCAGGCAATGT-3'; 5'-CTTGTAGGAGAGGGCTTTGAATGT-3'; and 5'-TCCTGCCGAGAAAGTATCCATCAT-3'. These are specific, respectively, for sequences 5' of the induced mutation, for the deleted segment, and for the inserted neomycin resistance gene (20). The DO11.10 phenotype was determined by staining peripheral blood cells using the KJ1-26 Ab (24). Rag-2-/- mice were obtained from Dr. F. W. Alt (Children’s Hospital, Boston, MA). After interbreeding with DO11.10 CD25-/- mice, animals were backcrossed to achieve homozygosity of H-2d. Iaß-/- mice (25) were crossed with CD25-mutant mice. Double heterozygotes were intercrossed, and Iaß-/- mice were selected based on flow cytometric analysis of peripheral blood for CD4+ cells. All mice were housed in microisolator cages under pathogen-free conditions.
Flow cytometric analysis
Single-cell suspensions of lymphoid organs were prepared and incubated with the indicated Abs as described previously (20). mAbs conjugated to FITC, PE, biotin, or APC were obtained from PharMingen, except KJ1-26 (hybridoma cells provided by Dr. Abul Abbas), which was prepared in our laboratory and conjugated to FITC or biotin using standard procedures. Secondary staining of biotinylated Abs used streptavidin-CyChrome. For analysis of DO11.10+ cells transferred into BALB/c mice, cell suspensions were preincubated with anti-CD16/CD32 (PharMingen) before staining with fluorochrome-conjugated Abs. For analysis of apoptosis, lymph node cells were incubated with FITC-conjugated KJ1-26 and propidium iodide (125 µg/ml) on ice for 30 min. Live and apoptotic KJ1-26+ cells were identified using forward scatter vs propidium iodide fluorescence. Stained cell preparations were analyzed using a FACScalibur instrument equipped with dual lasers and CellQuest software (Becton Dickinson, San Jose, CA). Viable cells were gated using forward and side scatter.
Quantitative Southern blot analysis
CD4+ T cells were purified by incubating cell suspensions with biotin-conjugated Abs to B220 and CD8, followed by streptavidin-conjugated beads and passage over magnetic separation columns (Miltenyi Biotec, Auburn, CA). Purities of 94–96% as determined by flow cytometry were achieved for the indicated experiments. Quantitative Southern blot analysis was performed as described previously (26) using a BamHI digest of genomic DNA. Germline TCRß alleles were detected using a probe encompassing the Jß1 segment and lane loading controlled with a probe for the Bcl-2 family member A1 (27). Band intensity was quantified using a phosphor imager (Bio-Rad, Hercules, CA), and TCRß rearrangement was calculated as follows: (density of germline TCRß band)/(density of A1 band) x (density of kidney germline TCRß band/(density of kidney A1 band).
Immunization of mice
Mice were injected s.c. with synthetic
OVA323–339 peptide (600 µg) at two sites over
the upper backs on days 0 and 1. On the day of harvest, brachial,
axillary, inguinal, and mesenteric lymph nodes cells of individual mice
were dissected and analyzed by flow cytometry. Adoptive transfer of
DO11.10 T cells was performed as described elsewhere (28).
Lymph node cells were pooled from either DO11.10
CD25+/- or DO11.10
CD25-/- mice and CD4+
KJ1-26+ cells were quantitated using flow
cytometry. After normalization for equivalent numbers of these cells,
recipient BALB/c mice were injected i.v. with 
5 x
106 lymph node cells via the tail vein. Sixteen
hours after transfer (designated as day 0), mice were s.c. challenged
with 600 µg of OVA323–339 peptide at two sites
over the backs. Lymph node cells were harvested on the indicated days
for analysis.
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Results |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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Mice lacking CD25 are unable to regulate the size of the peripheral lymphoid compartment, which expands 5–10-fold by 6 wk of age (20). We sought to investigate how high-affinity IL-2R signals controlled the size of peripheral lymphoid tissues through regulation of the CD4+ T cell subset before and after Ag administration. The CD25 mutation was bred along with the class II MHC-restricted DO11.10 TCR transgene, which is specific for OVA323–329 peptide (24). Mice were backcrossed to the DO11.10 (BALB/c) background for four to five generations, and homozygosity for H-2d was verified. In the studies reported here, CD25-/- mice were compared with CD25+/- littermates, which separate analyses have indicated are functionally and phenotypically equivalent to CD25+/+ mice. Thymus development was assessed using flow cytometry for CD4, CD8, and CD25 expression. Comparison of DO11.10 CD25+/- (n = 3) vs DO11.10 CD25-/- mice (n = 7) revealed equivalent thymus cellularity (2.5 ± 0.55 x 108 vs 2.8 ± 1.23 x 108 cells, mean ± SD), and no discernible effects on subset distribution. In particular, there were equivalent numbers of CD4+ single-positive thymocytes in DO11.10+ CD25+/- (2.4 ± 4.9 x 107) and DO11.10+ CD25-/- mice (2.8 ± 3.7 x 107).
Despite the vast reduction in TCR repertoire diversity engendered by
the DO11.10 transgene, we found that expansion of
CD4+ T cells in 5–12-wk-old adult
CD25-/- mice was unaffected (Fig. 1
). This phenotype was observed in some
mice as early as 4 wk of age. The peripheral homeostatic defect in
CD25-/- mice involves both CD4 and CD8
subsets. Since DO11.10-transgenic mice have a marked reduction in
peripheral CD8+ T cells (24), the
observed defect in CD4+ T cell regulation
suggests that this effect is not mediated through interaction with the
CD8+ subset. To test whether IL-2R-dependent
CD8+ T cell homeostasis requires
CD4+ T cells, we bred the CD25 mutation onto the
Iaß-/- background (25). We found
that the CD8+ T cell subset was expanded
equivalently in CD25-/- mice which were
Iaß+ or Iaß-/- (Fig. 1). These observations suggest that both the CD4+
and CD8+ T cell subsets autonomously require
IL-2R signals for homeostatic regulation in vivo.
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). As expected, CD25 was required to
regulate IgG1, but not IgM levels Iaß+ mice.
This effect was abolished in Iaß-/- mice.
Despite the limited TCR repertoire, DO11.10
CD25-/- mice had IgG1 levels 10-fold higher
than the CD25+/- littermates. Taken together,
these results indicate that hypersecretion of Igs in the absence of
CD25 occurs as a result of deregulated CD4+ T
cells. This effect may be nonspecific, inasmuch as a diverse TCR
repertoire was not required.
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Because suppression of endogenous TCR expression by the DO11.10
TCR transgene is incomplete, the lymphoid expansion observed in DO11.10
CD25-/- mice could reflect selective
proliferation of the subset of T cells with a more diverse TCR
repertoire, which would presumably retain some capacity for
high-affinity recognition of environmental Ags. Expression of the
DO11.10 TCR on peripheral T cells was assessed using either Abs to
Vß8, which recognize the transgenic TCRß chain, or the KJ1-26
anti-clonotypic Ab, which requires expression of both the
transgenic TCR and ß-chains (Table I). No differences were noted in the
proportions of CD4+ cells expressing these
markers in CD25+/- and
CD25-/- mice, demonstrating that the T cell
expansion in DO11.10 CD25-/- mice was not due
to selection of cells lacking OVA specificity. Peripheral T cells in
CD25-/- mice characteristically display an
activated phenotype, including a large proportion of cells expressing
high levels of CD44, low levels of CD62L, as well as up-regulation of
CD69 (20, 29). A similar phenotype was observed in the
CD4+ T cell population in DO11.10
CD25-/- mice (Fig. 3). Comparison of
CD4+ T cells expressing or lacking the transgenic
TCR clonotypic marker KJ1-26 revealed that both subsets displayed an
equivalent phenotype in CD25-/- mice,
suggesting that differences in TCR repertoire diversity between these
subsets were irrelevant to the regulatory role of CD25. We next
considered whether expansion of CD25-/- T cells
reflected escape from allelic exclusion of endogenous TCR.
Rearrangement of endogenous TCRß chains was compared in purified
CD4+ T cells from DO11.10
CD25+/- and DO11.10
CD25-/- mice. Genomic DNA was subjected to
quantitative Southern blot analysis using a probe specific for the
germline (unrearranged) TCRß locus (Fig. 4). Using a probe for a nonrearranging
gene to normalize for lane loading, retention of the germline TCRß
signal in T cells was compared with kidney DNA (26) and
quantitated in CD25+/- and
CD25-/- samples at 66 and 67%, respectively. A
smaller band seen in the T cell lanes is consistent with DJ rearranged
alleles, in agreement with previous studies (30). These
results show that expansion of CD25-/- T cells
was not selective for cells with productively rearranged endogenous
TCRß genes. Allelic exclusion of TCR chains is much less stringent
than for TCRß. In DO11.10 mice, expression of the KJ1-26 clonotype on
peripheral CD4+ cells is variable and is
down-regulated in cells with high expression of endogenous TCR
chains. Our observation that KJ1-26 expression is equivalently
distributed among CD4+ T cells in
CD25-/- mice (Table I) therefore suggests that
T cell expansion was not due to selection for cells expressing
endogenous TCR. We directly assessed allelic exclusion of V2 and
V8 in CD4+ T cells from DO11.10 mice,
comparing KJ1-26- and KJ1-26-positive subsets (Table II). In CD25+/-
mice, the percentage of DO11.10-
CD4+ T cells expressing V2 and V8 was
similar to that seen in BALB/c mice, whereas a marked reduction in
endogenous V expression was observed in the
CD4+KJ1-26+ T cell subset.
The degree of TCR allelic exclusion was essentially the same in
CD25-/- mice, indicating that T cell expansion
in CD25-deficient mice was not due to selection of cells expressing
these endogenous TCR. Taken together, these data indicate that
CD25-dependent regulation of CD4+ T cells is
equally important for T cell subsets with a highly restricted TCR
specificity and those with a semidiverse repertoire.
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chains
The observation that CD25 is needed for homeostasis in T cell
subsets across an increasingly restricted range of TCR diversity raises
the question of whether TCR specificity is at all relevant to the
expansion of T cells in CD25-/- mice. To
explore this question, we bred CD25-mutant DO11.10 mice onto the
Rag-2-deficient background to generate mice with only a single TCR
specificity. Rag-2-/- mice on a mixed C57BL/6
and 129 background were intercrossed and subsequently backcrossed for
three to four generations with DO11.10 CD25+/-
mice, selecting for animals which were homozygous for
H-2d. Rag-2-/- DO11.10
H-2d/d mice were then generated with either the
CD25+/- or CD25-/-
genotype. As expected, >90% of lymph node cells were
CD4+, and all had uniformly high KJ1-26
expression (Fig. 3
). No differences were observed in the number of
peripheral CD4+KJ1-26+ T
cells in CD25-/- vs
CD25+/- mice: the
CD25-/- group (n = 6) had
1.9 ± 0.41 x 106 cells (mean ±
SD), whereas CD25+/- littermates
(n = 5) had 2.8 ± 1.2 x
106 cells (differences not statistically
significant). Analysis of surface markers (Fig. 3) indicated that
CD4+KJ1-26+ T cells in both
the CD25+/- and CD25-/-
groups had a resting, naive phenotype. These results show that mutation
of Rag-2 suppressed the expanded and activated T cell phenotype in
CD25-/- mice, suggesting that a measure of TCR
diversity, presumably reflecting the capacity of some T cells to react
with environmental Ags, is a required element for the expanded T cell
phenotype.
Ag responses in DO11.10 CD25-mutant mice
Signals from the IL-2R have been implicated in both Ag-induced T
cell expansion and activation-induced cell death (1, 9, 16, 17, 18, 31). In DO11.10 mice, as with other TCR-transgenic mice,
immunization with antigenic peptide generates a biphasic response. T
cell proliferation and expansion is followed by activation-induced
apoptosis (28, 32, 33). We tested the role of the
high-affinity IL-2R in this model system, comparing DO11.10 mice which
were either heterozygous or homozygous for the CD25 null mutation.
Three days after s.c. immunization with OVA peptide,
CD25+/- animals exhibited expansion of
peripheral transgenic T cells (Fig. 5).
In CD25-/- mice, a significant change in T cell
numbers was not observed, although the wide variation in the size of
expanded lymph nodes in CD25-/- mice limits
interpretation of this result. To circumvent this problem, and because
the hypertrophic lymph node tissue in CD25-deficient mice might
influence responses to Ag in vivo, we utilized the adoptive transfer
system described by Kearney et al. (28) to further
evaluate the responses of OVA-specific DO11.10 T cells in vivo in the
context of a normal lymphoid compartment. By ameliorating the
abnormally high percentage of T cells responding to a given Ag, T cell
responses appear to behave in a more physiologic manner using this
system. As shown in Fig. 6, T cell
expansion at day 3 after immunization was partially impaired in DO11.10
CD25-/- mice, suggesting that during the early
phases of Ag responses in vivo, IL-2R signals contribute to expansion
of CD4+ T cells.
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, peripheral T
cell deletion was efficient in both the CD25+/-
and CD25-/- backgrounds, despite the fact that
unimmunized CD25-/- mice had an expanded
lymphoid compartment. Deletion of transgenic T cells in response to OVA
was accompanied by an increase in apoptotic cells in the lymph node,
which was comparable in the CD25+/- and
CD25-/- backgrounds (Fig. 7). We observed a contraction of
CD4+KJ1-26+ T cells
adoptively transferred into BALB/c mice (Fig. 6), with similar numbers
of CD25+/- and CD25-/-
cells remaining 8 days after OVA immunization. However, given that the
maximal expansion of CD25-/- T cells was only
half that observed for CD25+/- controls,
CD25-/- T cells underwent a proportionately
lesser degree of clonal contraction. Taken together, these data suggest
that high-affinity IL-2R signals are not required for efficient
CD4+ T cell death in vivo following an encounter
with cognate Ag.
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Discussion |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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c
receptor chain (38). Genetic dissection of the phenotype
of mice defective in IL-2R signaling has demonstrated that peripheral
lymphoid homeostasis is mediated by effects on T cells. Expansion of
the lymphoid compartment and autoimmune disease are abrogated in
IL-2-/- nude mice but are unaffected in
IL-2-/- JH-/- mice
which lack B cells (22, 39). It has been postulated that
the major pathologic changes in IL-2-/- mice
are mediated by activated CD4+ T cells
(40). Our data clarify and extend these observations as
follows. High-affinity IL-2R signals appear to regulate the size of the
CD4+ and CD8+ T cell
compartments independently, since CD8+ T cell
expansion in CD25-/- mice was unaffected in the
Iaß-null background where CD4+ T cells are
deficient and because CD4+ T cell expansion was
intact in CD25-/- DO11.10 mice which have few
CD8+ T cells. In the B cell compartment,
defective IL-2R signals lead to marked elevations in serum quantities
of Ig isotypes characteristic of secondary immune responses. Our data
show that this phenotype is dependent on the CD4+
T cell subset and suggest that hypersecretion of certain Ig isotypes is
an indirect effect of expanded and activated CD4+
T cells, rather than reflecting an intrinsic abnormality of B cell
function. The fact that the CD4+ T cell
compartment in DO11.10 CD25-/- mice was
sufficient to drive Ig hypersecretion in vivo, despite a severely
limited TCR repertoire, further suggests that B cells were activated
outside the context of Ag-specific immune responses. Lymphoid homeostasis involves a balance between cells generated within or entering the peripheral compartment and cell death. IL-2R signals promote cellular proliferation in vitro (9, 16, 18); however, experiments testing whether IL-2R signals play a central role in mediating T cell expansion following Ag stimulation in vivo have given mixed results. CD4+ T cell expansion in response to superantigens is intact in mice lacking either CD25 or IL-2 (20, 23). In young IL-2-deficient mice, immune responses, including antiviral CTL or T cell-dependent Ab responses, are normal or mildly impaired, although the CTL defect is severe in older animals (41). A more detailed accounting of CTL expansion in response to lymphocytic choriomeningitis infection found a substantial reduction in IL-2-deficient mice (31). Interpretation of these studies is complicated by the fact that the peripheral lymphoid compartment in mice with defective IL-2R signaling is markedly abnormal, and this factor is difficult to separate from cell-autonomous effects of IL-2R signals on T cells stimulated by Ag. Our data show that when placed in the context of a normal lymphoid compartment, CD25-deficient T cells exhibit a significant reduction in Ag-induced expansion, indicating that although IL-2R signals are not required for T cell proliferation in vivo, they do contribute to this process. In studies similar to those presented here, Khoruts et al. (42) reported that Ag-induced expansion of adoptively transferred DO11.10 IL-2-/- T cells was normal or higher than that of wild type. However, because IL-2-/- T cells are sensitive to IL-2 (19), a contribution of paracrine activity from IL-2 generated by recipient T cells could not be excluded. Taken together, the existing data suggest that T cell expansion during immune responses is mediated by multiple, partially redundant growth factors in vivo, and the relative contribution of IL-2R signals may vary with the particular conditions of individual responses.
It has been postulated that the homeostatic defect in mice lacking CD25
or IL-2 reflects a role for IL-2R signals in deletion of peripheral T
cells following activation by cognate Ags (9, 18, 20, 23).
Our data suggest that for Ag-specific T cell responses in vivo,
high-affinity IL-2R signals are not required for activation-induced
apoptosis in the periphery. These data appear to conflict with the
observation that peripheral deletion of T cells in response to
immunization with bacterial superantigens is impaired in mice lacking
CD25, IL-2, or c (20, 23, 43), as well as the finding
that Fas-dependent apoptosis, which is involved in a subset of
Ag-induced peripheral T cell deletion events, is impaired in activated
CD25-/- or IL-2-/- T
cells (23, 33, 44, 45). However, mice lacking IL-2Rß
have normal superantigen-mediated deletion of peripheral T cells,
suggesting that IL-2R signals are not fundamentally required for this
process. Like the situation with T cell expansion, the activity of
IL-2R signals in promoting Ag-induced T cell death appears to be
redundant with other cytokines that promote cell cycle progression,
including IL-4, IL-7, and IL-15 (35, 45, 46). The effects
observed may therefore vary depending on characteristics of the Ag and
dose used. Altogether, investigations addressing the consequences of
defective IL-2R signals for Ag-dependent T cell responses in vivo do
not explain the phenotype of lymphoid expansion in gene-deficient
mice.
Because the DO11.10 TCR transgene is specific for a high-affinity Ag
not normally encountered in the environment of laboratory mice, we
predicted that transgenic CD4+ T cells would be
unaffected by the absence of CD25 until Ag was administered
experimentally. Unexpectedly, we found that CD4+
T cells in DO11.10 CD25-/- mice exhibited
peripheral expansion which was equivalent to nontransgenic
CD25-/- mice, both in magnitude and in age of
onset. This finding suggested that the diversity of the TCR repertoire,
or perhaps TCR specificity itself, was unimportant in CD25-dependent
homeostasis. Because of leakiness in allelic exclusion, endogenous
chains are expressed in a subset of T cells in DO11.10-transgenic mice.
This creates subpopulations with TCR diversity ranging from semidiverse
(endogenous TCR paired with transgenic TCRß) to highly restricted
(transgenic TCRß only). This spectrum of diversity is paralleled
by expression of the KJ1-26 clonotype: endogenous -chains are
present in proportions similar to normal BALB/c mice in
CD4+ T cells with low or absent KJ1-26
expression, whereas they are markedly reduced within the
KJ1-26high T cell subset. If
CD4+ T cell expansion in DO11.10
CD25-/- mice was due to impaired cell death
following stimulation by a variety of environmental Ags, then
preferential expansion of cells with a broader T cell repertoire would
be predicted, leading to a skewing in favor of cells lacking KJ1-26, or
otherwise escaping allelic exclusion. Instead, we found that the
expanded T cell population had an identical composition with respect to
KJ1-26 expression, Vß8 expression, as well as allelic exclusion of
V3 and V8 within the KJ1-26high
subpopulation. In addition, we found no evidence for selection of cells
that had increased rearrangement of the endogenous TCRß locus. In
parallel with these results, the activated T cell phenotype which
accompanies lymphoid expansion in CD25-/- mice
was identical in KJ1-26+ and
KJ1-26- T cell subsets. The latter observation
differs from DO11.10 mice lacking c, where the activated phenotype
of peripheral T cells was restricted to the
KJ1-26- subset (47). These
differences presumably reflect the requirement for c in multiple
cytokine receptors, which may be involved at several levels in the
positive and negative regulation of the peripheral T cell
compartment.
The observation that CD25 is required for peripheral homeostasis of T
cells with restricted specificity suggests that regulatory signals may
operate independent of exposure to cognate Ag. However, there is an
apparent contradiction in that CD25 was not needed for T cell
homeostasis in DO11.10 Rag-2-/-
CD25-/- mice, which have unispecific TCR
repertoire. One potential explanation is that all of the expanded T
cells in DO11.10 CD25-/- mice express
endogenous TCR and were stimulated by cognate Ags from the
environment. Because we were not able to assess expression of all
possible TCR -chains, this explanation remains a formal possibility
but is strongly discounted by the observation that allelic exclusion of
V3 and V8 in DO11.10+
CD25-/- T cells was identical to that in
control mice. A more likely explanation is that many if not most of the
individual T cells were stimulated by signals other than cognate
interactions between TCR and high-affinity Ags. Such signals could be
generated indirectly as a result of Ag-specific activation of the
subset of T cells bearing a more diverse TCR repertoire, thus
representing a bystander effect. This interpretation may also apply to
CD25-dependent regulation of CD8+ T cells in
Iaß-/- mice, which are also severely
deficient in their ability to respond to exogenous Ags
(25). Following this argument, our data suggest that a
major function of high-affinity IL-2R signals in vivo may be to
suppress activation and expansion of T cells by signals of low
specificity, including bystander activation.
Immune stimuli efficiently activate T cells through high-specificity interactions. However, such responses likely include a spectrum T cell activation event at varying levels of TCR affinity, many of which are presumably nonproductive with respect to immune effector responses or possibly even detrimental. Production of IL-2 is limited to T cells activated under conditions of high specificity, in that proper costimulatory signals are required (48, 49). In contrast, CD25 expression, which represents a critical link in cellular responsiveness to IL-2, occurs in response to a much broader range of stimuli: including TCR signals outside the context of costimulation as well as non-TCR signals such as IL-1 (9, 11). IL-2R signals may promote cell cycle progression in both of these situations, leading to increased proliferation in the case of high-affinity TCR interactions and conditions of optimal costimulation and promoting apoptosis in the case of poorly constituted TCR-Ag-MHC interactions. In this manner, high-affinity IL-2R signals may assist in narrowing the specificity of T cell activation and controlling the overall magnitude of lymphoid expansion.
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Acknowledgments |
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Footnotes |
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2 Current address: Clinical Immunology Unit, Faculty of Medicine, Chinese University of Hong Kong, Prince of Wales Hospital, Shatin, Hong Kong.
3 Address correspondence and reprint requests to Dr. Dennis M. Willerford, Division of Hematology, University of Washington School of Medicine, Box 357710, 1959 North East Pacific Street, Seattle, WA 98195. E-mail address:
Received for publication August 23, 1999. Accepted for publication January 20, 2000.
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 P. A. Antony, C. M. Paulos, M. Ahmadzadeh, A. Akpinarli, D. C. Palmer, N. Sato, A. Kaiser, C. Heinrichs, C. A. Klebanoff, Y. Tagaya, et al. Interleukin-2-Dependent Mechanisms of Tolerance and Immunity In Vivo J. Immunol., May 1, 2006; 176(9): 5255 - 5266. [Abstract] [Full Text] [PDF]
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H. Jin, D. Gong, D. Adeegbe, A. L. Bayer, C. Rolle, A. Yu, and T. R. Malek Quantitative assessment concerning the contribution of IL-2R{beta} for superantigen-mediated T cell responses in vivo Int. Immunol., April 1, 2006; 18(4): 565 - 572. [Abstract] [Full Text] [PDF]
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P. R. Burkett, R. Koka, M. Chien, S. Chai, D. L. Boone, and A. Ma Coordinate Expression and Trans Presentation of Interleukin (IL)-15R{alpha} and IL-15 Supports Natural Killer Cell and Memory CD8+ T Cell Homeostasis J. Exp. Med., October 4, 2004; 200(7): 825 - 834. [Abstract] [Full Text] [PDF]
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R. M. Teague, R. M. Tempero, S. Thomas, K. Murali-Krishna, and B. H. Nelson Proliferation and Differentiation of CD8+ T Cells in the Absence of IL-2/15 Receptor {beta}-Chain Expression or STAT5 Activation J. Immunol., September 1, 2004; 173(5): 3131 - 3139. [Abstract] [Full Text] [PDF]
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H. Dooms, E. Kahn, B. Knoechel, and A. K. Abbas IL-2 Induces a Competitive Survival Advantage in T Lymphocytes J. Immunol., May 15, 2004; 172(10): 5973 - 5979. [Abstract] [Full Text] [PDF]
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B. H. Nelson IL-2, Regulatory T Cells, and Tolerance J. Immunol., April 1, 2004; 172(7): 3983 - 3988. [Abstract] [Full Text] [PDF]
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W. N. D'Souza and L. Lefrancois IL-2 Is Not Required for the Initiation of CD8 T Cell Cycling but Sustains Expansion J. Immunol., December 1, 2003; 171(11): 5727 - 5735. [Abstract] [Full Text] [PDF]
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 A. L. Cornish, M. M. Chong, G. M. Davey, R. Darwiche, N. A. Nicola, D. J. Hilton, T. W. Kay, R. Starr, and W. S. Alexander Suppressor of Cytokine Signaling-1 Regulates Signaling in Response to Interleukin-2 and Other {gamma}c-dependent Cytokines in Peripheral T Cells J. Biol. Chem., June 13, 2003; 278(25): 22755 - 22761. [Abstract] [Full Text] [PDF]
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A. R. M. Almeida, N. Legrand, M. Papiernik, and A. A. Freitas Homeostasis of Peripheral CD4+ T Cells: IL-2R{alpha} and IL-2 Shape a Population of Regulatory Cells That Controls CD4+ T Cell Numbers J. Immunol., November 1, 2002; 169(9): 4850 - 4860. [Abstract] [Full Text] [PDF]
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W. N. D'Souza, K. S. Schluns, D. Masopust, and L. Lefrancois Essential Role for IL-2 in the Regulation of Antiviral Extralymphoid CD8 T Cell Responses J. Immunol., June 1, 2002; 168(11): 5566 - 5572. [Abstract] [Full Text] [PDF]
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P. I. Chuang, S. Morefield, C.-Y. Liu, S. Chen, J. M. Harlan, and D. M. Willerford Perturbation of B-cell development in mice overexpressing the Bcl-2 homolog A1 Blood, May 1, 2002; 99(9): 3350 - 3359. [Abstract] [Full Text] [PDF]
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)
and CD25-/- (
) mice were injected s.c. with 600 µg
of OVA323–339 peptide on days 0 and 1. Peripheral lymph
nodes were collected from uninjected control mice (day 0) or from
injected mice on day 3 or day 8 and analyzed by flow cytometry. Results
of one experiment showing mean and SE for three to seven animals in
each group are shown and are representative of five independent
experiments.
