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*
Immunobiology Section, Yale University School of Medicine, New Haven, CT 06510;
Department of Microbiology, University of Maryland, College Park, MD 20910
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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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Memory CD4 T cells develop during a primary infection and, like naive T cells, are resting, long-lived lymphocytes. However, unlike naive CD4 T cells, memory T cells are previously primed cells and have been reported to be more sensitive to triggering through the TCR than naive T cells (7, 8), to respond more vigorously to Ag in terms of proliferation (9) and cytokine production (10), and to exhibit altered coupling of proximal signaling events to the TCR (11). Furthermore, while naive T cells recirculate between blood and secondary lymphoid organs, memory CD4 T cells are capable of entering nonlymphoid tissue directly from the blood (12, 13). The altered recirculation pattern enhances the chance of contacting foreign Ag, but also exposes memory T cells to new self peptides for which they may not have been selected in the thymus. While the altered signaling and recirculation pathways are believed to contribute to a more vigorous secondary immune response, they also pose the question of how memory T cell responses are regulated and how proliferation due to recognition of self peptide is prevented.
While it is established that naive T cell responses to self are controlled by their requirement for costimulation, the role of costimulation in regulating memory T cell responses is still unclear. It has been shown that unlike naive T cells, resting, Ag-primed CD4 T cells respond to Ag presented by resting B cells that do not express costimulatory molecules (14). Furthermore, in vitro stimulated memory CD4 T cells, but not naive T cells, proliferate to plate-bound anti-CD3 in the absence of additional costimulatory molecules (15). Several in vivo studies using CTLA4-Ig to block costimulation delivered through B7 also indicate that memory T cells are costimulation independent (16, 17), while other studies suggest that a secondary response can be blocked by CTLA4-Ig in vivo (18). It is possible that the discrepancies in these studies resulted from the inability to deliver CTLA4-Ig efficiently in the former studies or that the requirement of memory CD4 T cells for costimulation depends on the circumstances of activation.
CTLA-4, a CD28 homologue that negatively regulates the proliferation of effector T cells (19, 20), has recently been shown to prevent costimulation through CD28 when APCs express limiting B7 molecules. The importance of CTLA-4 in regulating effector T cells is emphasized by the massive lymphoproliferation observed in mice that lack CTLA-4 (21, 22). A role for CTLA-4 in the regulation of memory CD4 T cell responses in mice has recently been suggested (23) but has not yet been demonstrated.
We have recently shown that CD4 ligation in memory, but not naive, CD4 T cells leads to unresponsiveness (24, 25). In these studies memory CD4 T cells failed to proliferate to soluble anti-CD3 presented by syngeneic MHC class II+ APCs. The response was restored when anti-CD3 was presented by APCs that did not express MHC class II or expressed a mutant MHC class II molecule unable to engage CD4. We hypothesized that memory, but not naive, CD4 T cells fail to proliferate under conditions that do not recruit CD4 into the TCR complex, but instead allow independent CD4-MHC class II interactions, which resemble the inhibition produced in response to Ab-mediated CD4 cross-linking. Such a mechanism may be important in preventing memory CD4 T cells from proliferating to self peptides that fail to recruit CD4 to the TCR interaction site.
To determine the role of costimulation in activation of resting memory CD4 T cells, we investigated the role of CD28/CTLA-4 in memory T cell responses. We show here that both naive and memory CD4 T cell proliferation in response to soluble anti-CD3 requires costimulation through CD28. We further show that under conditions where CD4-MHC class II interactions occur, memory CD4 T cells have a higher threshold for costimulation through CD28 than do naive T cells. Furthermore, the inability of memory CD4 T cells to proliferate in the presence of CD4-MHC class II interactions is dependent on CTLA-4-B7 interaction early after activation. CTLA-4 functions uniquely in memory CD4 T cells in that it continuously recycles from intracellular stores to the cell surface. We propose that the ability of CTLA-4 signaling to inhibit memory CD4 T cell responses is dependent on CD4 ligation. This mechanism ensures that low intensity TCR engagement, which mediates CD4-MHC class II engagement away from the TCR, does not cause memory T cell activation, but instead raises their threshold for costimulatory signals. This may extend the life span of memory CD4 T cells with reduced risk of inappropriate activation.
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Materials and Methods |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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Female BALB/c ByJ mice were obtained from The Jackson Laboratory (Bar Harbor, ME) and were maintained at the animal facility at Yale University (New Haven, CT). All mice were used at 8–12 wk of age. MHC class II-negative mice were originally obtained from Dr. D. Mathis (Institut de Chimie Biologique, Strasbourg, France) (26) and were bred and maintained at the Yale animal facility.
Cell lines
DAP.3 fibroblasts and DAP.3 transfected with I-Ad
(line 44) were gifts from Dr. R. Germain (27). The fibroblasts were
cotransfected with Fc
RII by Dr. D. L. Farber as recently
described (24).
Antibodies
The following mAbs were purified from supernatants of hybridomas
maintained in this laboratory using standard protein A or protein G
affinity chromatography: anti-CD3
(rat IgG2c, C363.29B),
anti-CD4 (rat IgG2b, GK1.5), anti-CD8 (rat IgG2b, 2.43),
anti-CD8 (rat IgG2a, 53-6.72), anti-Thy-1 (rat IgG2c, Y19),
anti-I-Ad (mouse IgG2a, 212.A1), and anti-murine
FcRII (rat IgG2b, 2.4G2). Purified as well as labeled mAbs to
anti-CD45RB (rat IgG2a, C353.16A), CTLA-4 (Armenian hamster IgG,
UC10-4F10), isotype control (pooled Armenian hamster IgG, G94-56), CD28
(Syrian hamster IgG, 37.51), CD80 (rat IgG2a, 1G10), and CD86 (rat
IgG2a, GL1) were obtained from PharMingen (San Diego, CA). Anti-CTLA-4
(goat polyclonal, C19) for Western blotting and secondary
anti-goat IgG/horseradish peroxidase were purchased from
Santa Cruz Biotechnology (Santa Cruz, CA). Anti-CD4/Quantum red was
purchased from Sigma (St. Louis, MO), and CTLA4-Ig was provided by Dr.
P. S. Linsley (28).
Naive and memory CD4 T cell preparation
CD4-positive T cells were isolated from spleens and separated
into CD45RBhigh and CD45RBlow fractions by MACS
separation as recently described (15). In brief, CD4-positive T cells
were isolated from spleens of BALB/c ByJ mice by negative selection
using mAbs to CD8, I-Ad, and FcRII followed by
incubation with anti-rat IgG-, anti-mouse IgG-, and
anti-mouse IgM-coated magnetic beads (Collaborative Research,
Bedford, MA). CD4 T cells were collected by magnetic depletion of
Ab-labeled cells and centrifuged through a 85–62% Percoll gradient to
deplete activated T cells and contaminating accessory cells. For MACS
separation into CD45RBhigh and CD45RBlow
fractions, CD4 T cells were stained with biotinylated CD45RB mAb
followed by incubation with streptavidin-coated beads (Miltenyi Biotec,
Sunnyvale, CA). CD45RBlow cells were eluted within the MACS
column, while the retained CD45RBhigh fraction was
collected outside the magnetic field. The purity of the two fractions
was verified by FACS analysis (FACScan, using CellQuest software,
Becton and Dickinson, Mountain View, CA) after incubation of the cells
with FITC-avidin D. CD45RBlow cells were >95% CD4
positive, while CD45RBhigh cells contained 85–90%
CD4-positive T cells.
APC preparation
Spleen cells from BALB/c and RHAßo/o mice were
depleted of T cells with complement after incubation with
anti-Thy.1, anti-CD8, and anti-CD4 mAbs, and were treated
with mitomycin (50 µg/ml; Boehringer Mannheim, Indianapolis, IN) for
30 min as previously described (15). Resultant APC preparations were
>95% pure as determined by FACS analysis of FcRII-stained cells.
In experiments in which fibroblasts were used as APCs, mitomycin
treatment (75 µg/ml) was conducted for 90 min.
Proliferation assays
CD45RBhigh and CD45RBlow CD4 T cells were cultured at 25,000 cells/well in 96-well flat-bottom tissue culture plates (Costar, Cambridge, MA) with soluble anti-CD3 (10 µg/ml final concentration) and 50,000 splenic APCs or 25,000 fibroblasts/well in Eagles’ high amino acid medium supplemented with 5% FCS, 50 U/ml penicillin, 50 µg/ml streptomycin sulfate, and 10 mM HEPES. Titration of APC numbers and the concentration of anti-CD3 have been published previously (15, 24). Where indicated, anti-CTLA-4 (5 µg/ml), anti-CTLA-4 Fab fragments (50 µg/ml), anti-CD28 (10 µg/ml), and/or CTLA4-Ig (8 µg/ml) were added to the cultures. To immobilize anti-CD3 and anti-CD28, the mAbs were diluted to 10 µg/ml in 50 mM Tris-HCl (pH 9.5), resuspended at 100 µl/well in 96-well culture plates, and incubated for 2–4 h at 37°C. The wells were washed twice in medium before culturing 25,000 T cells/well. For assessment of proliferation, cultures were pulsed with 1 µCi of [3H]dTdR (6.7 Ci/mmol) after 72 h of incubation at 37°C. Pulsed plates were harvested 18–24 h later on a Tomtec 96-well plate harvester (Wallac, Gaithersburg, MD). Radioactivity was quantitated by scintillation counting, and results are presented as the mean of triplicates ± SEM.
CTLA-4 immunoblots
Naive (CD45RBhigh) and memory (CD45RBlow) CD4 T cells were prepared as described above. As a positive control for CTLA-4 expression, naive and memory CD4 T cells were activated with plate-bound anti-CD3 and anti-CD28 as described above for 48 h. Cells were lysed in lysis buffer (20 mM HCl (pH 7.5), 150 mM NaCl, 1 mM MgCl2, 1 mM EGTA, containing 10 µg/ml leupeptin, 10 µg/ml aprotinin, 10 mM PMSF, 50 mM NaF, 10 mM sodium pyrophosphate, and 1 mM sodium orthovanadate) as recently described (24). Protein lysates were resolved on a 12% SDS gradient gel, transferred electrophoretically to nitrocellulose, and hybridized to anti-CTLA-4 Ab C19 (Santa Cruz Biotechnology) followed by horseradish peroxidase-coupled anti-goat secondary Ab (Santa Cruz Biotechnology). Bands were revealed by chemiluminescence using the enhanced chemiluminescence Western blotting detection reagents (Amersham, Arlington Heights, IL) according to the manufacturer’s instructions.
CTLA-4 staining
For the detection of intracellular CTLA-4 on resting CD45RB separated naive and memory CD4 T cells, an intracellular staining kit from Caltag Laboratories (Burlingame, CA) was used. The staining protocol was conducted according to the manufacturer’s recommendations. Cells were analyzed by flow cytometry and were gated on low angle and sideways light scatter to exclude large, activated lymphocytes. For CTLA-4 expression upon culture, naive and memory CD4 T cells were incubated with or without plate-bound anti-CD3 and anti-CD28 at 37°C in the presence of either phycoerythrin-labeled anti-CTLA-4 or an isotype control mAb for 3 h. To distinguish between CTLA-4 cell surface staining and staining resulting from the internalization of CTLA-4/PE3 mAb, parallel cultures were left unstained during the culture period and were cell surface labeled with anti-CTLA-4/PE or the isotype control for 30 min on ice after culture. Cells were analyzed by flow cytometry for the uptake of CTLA-4 mAbs.
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Results |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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Memory CD4 T cells proliferate to plate-bound anti-CD3, while
naive T cells fail to do so, suggesting that memory T cells proliferate
in the absence of costimulation (15). However, both naive and memory
CD4 T cells respond equally well to soluble anti-CD3 when expressed
on fibroblasts (DAP.3) transfected with Fc- receptor II (FcRII).
In this instance naive CD4 T cells are capable of proliferating because
costimulation is provided by the fibroblasts in the form of B7.1. To
determine whether memory CD4 T cells can proliferate in the absence of
costimulation, proliferation assays were performed either in the
presence of CTLA4-Ig or in the presence of fibroblasts that had been
sorted for a lack of B7.1 expression. Interestingly, proliferation of
naive and memory CD4 T cells was equally blocked when stimulated in the
presence of B7.1+ fibroblasts and CTLA4-Ig (Fig. 1
A) or in the presence of
B7.1- fibroblasts (Fig. 1B). The importance of
costimulation was further accentuated by the fact that both T cell
populations proliferated when Abs to CD28 were added to cultures
containing B7.1- fibroblasts. These data clearly show that
naive and memory CD4 T cells are equally dependent on costimulation
when anti-CD3 is expressed on FcRs of APCs.
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Previous studies have shown that unlike naive CD4 T cells memory T
cells fail to proliferate to anti-CD3 in the presence of syngeneic
MHC class II+ APCs (APC/II+).
Hyporesponsiveness was associated with a lack of cytokine production
(24) and an inability to enter cell cycle (25). The incapacity of
memory CD4 T cells to proliferate was dependent on an interaction of
CD4-MHC class II, since responses were restored when APCs were derived
from MHC class II-negative mice or from fibroblasts transfected with
MHC class II that was mutated at the CD4 binding sites (29). It was
proposed that memory T cells fail to become activated upon CD4-MHC
class II interaction in situations where weak TCR stimulation occurs
and CD4 is not efficiently recruited to the TCR complex (25). Since the
costimulation requirements of memory CD4 T cells in the presence of CD4
ligation might differ, we investigated the potential role of CD28 in
this response. Naive and memory CD4 T cells were stimulated with
T-depleted splenocytes from MHC class II+ or class
IIo mice in the presence or the absence of anti-CD28.
Fig. 2 shows that both memory and naive
CD4 T cells proliferated in response to soluble anti-CD3 expressed
on APC/IIo in the presence or the absence of
anti-CD28 mAb (Fig. 2B). In contrast, naive, but
not memory, CD4 T cells proliferated in response to anti-CD3 in the
presence of resting APC/II+ (Fig. 2A).
mAb-mediated CD28 ligation restored proliferation of memory CD4 T cells
in the presence of APC/II+ and enhanced the proliferation
of naive T cells (Fig. 2A). The difference in the ability of
APC/II+ and APC/IIo to induce memory T cell
responses was not due to differences in B7 expression, since
APC/II+ and APC/IIo expressed similarly low
levels of B7.1 and B7.2 (Fig. 2C). Although B7 expression
was increased in both APC populations later in the response (data not
shown), late expression of high B7 on the APC is unlikely to be
involved in the initiation of the T cell response. We conclude that in
the presence of resting APC/II+, but not
APC/IIo, the proliferation of memory CD4 T cells in
response to anti-CD3 is more dependent on costimulation than is the
proliferation of naive T cells.
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While B7 expression on MHC class II+ and class
IIo APC was the same, CD28-B7 interactions were only
sufficient to activate memory CD4 T cells when APCs lacked MHC class
II. Yet the same CD28-B7 interactions induced naive CD4 T cell
responses when APCs expressed MHC class II. This suggested that memory
CD4 T cells needed more costimulation than naive T cells when MHC class
II was expressed on the APC. To test this hypothesis, we sorted MHC
class IIo and class II+ fibroblasts for low and
high expression of B7.1 and used these sublines to stimulate naive and
memory CD4 T cells. Both populations proliferated in response to
anti-CD3 in the presence of class II-negative fibroblasts
independent of the level of B7.1 expression (Fig. 3B). However, in the presence
of class II only naive CD4 T cells proliferated in response to
anti-CD3 at low levels of B7.1. In contrast to the responses
induced at low levels of B7.1, both naive and memory CD4 T cells
responded in the presence of high B7.1 (Fig. 3A).
Furthermore, the induction of unresponsiveness seen upon CD4/MHC class
II interactions was abrogated; therefore, CD4 ligation due to MHC class
II expression on APCs increases the threshold for costimulation on
memory, but not naive, CD4 T cells. We concluded that memory CD4 T
cells have a greater requirement for costimulation than naive T cells
to overcome the inhibitory signal delivered through CD4-MHC class II
interactions.
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It has recently been suggested that CTLA-4 signaling increases the
threshold for costimulation through B7 molecules in activated T cells
(20, 30). We sought to investigate the possibility that CTLA-4
signaling affected memory CD4 T cells at the initiation of a response
to soluble anti-CD3. CTLA-4 has not been detected on resting T
cells (31, 32). However, resting memory CD4 T cells represent
previously activated cells that may retain residual expression of
CTLA-4. We investigated this possibility by staining isolated resting
naive and memory CD4 T cells for CTLA-4 expression. No cell surface
expression of CTLA-4 was detected on naive or memory CD4 T cells (Fig. 4A). However, anti-CTLA-4
Western blots prepared from resting and 48-h activated naive and memory
CD4 T cell lysates showed several bands between 33–37 kDa in the
activated naive and memory T cell populations that were likely to
represent differentially glycosylated forms of CTLA-4, as has been
previously described (33). These bands were also visible in resting
memory, but not resting naive, CD4 T cells (Fig. 4B),
suggesting that resting memory CD4 T cells express intracellular stores
of CTLA-4. Despite a Percoll gradient separation, we cannot rule out
that some activated T cells contaminated the resting memory CD4 T cell
pool used for Western blotting. To exclude this possibility we stained
intracellularly for CTLA-4 and analyzed the data by flow cytometry,
which allowed us to gate on low angle and sideways light scatter to
exclude large activated T cells. Similar to the Western blot, resting
memory CD4 T cells revealed a small amount of intracellular CTLA-4
(Fig. 4C). While the levels indicated by staining are low,
no CTLA-4 is seen in naive CD4 T cells, and no signal is seen in memory
T cells with the isotype control Ab. The prevalence of intracellular
stores of CTLA-4 in memory T cells suggested that CTLA-4 may be
available to resting memory CD4 T cells.
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It has recently been shown that cell surface expression of
CTLA-4 is unstable on activated T cells, and that CTLA-4 continuously
recycles between endosomal compartments and the cell surface (32, 34).
To determine whether CTLA-4 is recycled in resting memory T cells or
whether TCR ligation induces CTLA-4 recycling, we incubated CD4 T cells
in the presence of anti-CTLA-4/PE or an isotype control/PE mAb for
3 h at 37°C. Cells were stimulated with plate-bound anti-CD3
and anti-CD28 or remained unstimulated. If CTLA-4 cycles between
the cell surface and endosomes, then labeled anti-CTLA-4 mAbs added
to the cultures will accumulate intracellularly, as has recently been
shown (31). To distinguish between reaccumulated CTLA-4 inside the
cells and stable cell surface expression, duplicate cultures were
labeled with anti-CTLA-4/PE or the isotype control after culture
for 3 h. Unlike resting naive T cells, a subpopulation of memory
CD4 T cells was CTLA-4/PE positive after incubation with the mAb for
3 h, whereas no staining was observed with the isotype control mAb
(Fig. 5A). Expression of
CTLA-4 was enhanced when memory T cells were stimulated with
plate-bound anti-CD3 and anti-CD28, while CTLA-4 expression was
not observed in similarly activated naive CD4 T cells (Fig. 5B). Naive and memory CD4 T cells that were stained with
anti-CTLA-4/PE after culture for 3 h did not stain for CTLA-4
(data not shown). The fact that memory CD4 T cells were positive for
CTLA-4 when cultured in the presence of the mAb but not when
anti-CTLA-4 was added after culture suggested that CTLA-4
continuously recycled between the cytoplasm and the cell surface, and
that cell surface expression of CTLA-4 was unstable.
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To determine how CTLA-4 may function in the activation of memory
CD4 T cells, we investigated memory CD4 T cell proliferation in
response to soluble anti-CD3 in the presence of APC/II+
under conditions where CTLA-4-B7 interactions were blocked by Fab of
anti-CTLA-4. Blocking CTLA-4-B7 interactions had no effect on the
proliferation of naive CD4 T cells in response to anti-CD3 in the
presence of APC/IIo or APC/II+ (Fig. 6B). However, while memory CD4
T cell responses in the presence of APC/IIo were also
unaffected by CTLA-4 Fab fragments (Fig 6B), proliferation
in the presence of APC/II+ was reinstated when CTLA-4-B7
interactions were blocked by anti-CTLA-4 Fab (Fig. 6A).
These data suggest that at limiting amounts of B7.1, CTLA-4 signaling
prevents memory CD4 T cell proliferation in the presence of
APC/II+ but not APC/IIo. We conclude that
CD4/MHC class II interaction is essential for CTLA-4-induced
unresponsiveness.
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Discussion |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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It has been shown that the strength of the TCR signal and the need for costimulation are inversely correlated; the weaker the TCR signal, the more CD28 ligation is required (23). Early signaling events occurring after T cell stimulation with soluble anti-CD3 presented by APCs are identical with those seen upon antagonistic peptide stimulation (35). In this instance, CD28 ligation is probably required to enhance the strength of the TCR signal. In contrast, plate-bound anti-CD3 and soluble anti-CD3 presented by FcR-transfected CHO cells (15), which induce memory CD4 T cell proliferation in the absence of costimulation, are likely to act as agonists, leading to extensive cross-linking of the TCR, which overrides the need for costimulation in memory, but not naive, CD4 T cells. Therefore, the strength of the TCR signal is probably involved in determining whether memory CD4 T cells proliferate in the absence of costimulation. This is in contrast to naive CD4 T cells, where a strong TCR signal may lower the amount of costimulation that is required but does not allow proliferation in the absence of costimulation.
The inhibitory signal delivered by CD4-MHC class II interaction to memory CD4 T cells (24) can be overcome by ligation of CD28 or by blocking CTLA-4-B7 interactions, suggesting that CTLA-4 signaling in memory CD4 T cells prevents costimulation through CD28 under conditions where B7 expression on the APC is limiting. Interestingly, CTLA-4 does not inhibit proliferation of memory CD4 T cells when CD4-MHC class II interactions are removed. This suggests that signaling through both CD4 and CTLA-4 is required to prevent memory CD4 T cell proliferation. Studies of CTLA-4 recycling in memory CD4 T cells in the presence or the absence of CD4 ligation do not suggest that CD4 influences CTLA-4 cell surface expression within the first few hours of activation (preliminary data, not shown); however, a small change in surface expression may have significant consequences. It is also possible that CTLA-4 activity is dependent on CD4 signaling, and that in the absence of CD4 ligation, CTLA-4 proceeds to recirculate but fails to become activated. Although little is known about CTLA-4 function, it has been shown that the cytoplasmic domain of CTLA-4 contains two SH2 binding domains that require phosphorylation for CTLA-4 to signal (19). It is feasible that CD4-associated p56lck phosphorylates CTLA-4-associated SH2 binding domains or that it is required for the activation of enzymes that associate with these domains upon phosphorylation. The alternative route, that CTLA-4 signaling is required for CD4 activation, seems unlikely because it has been shown that CD4-associated lck is constitutively active in the absence of CTLA-4 (36); therefore, lck activity cannot be dependent on CTLA-4 signaling. It is, however, conceivable that inhibition of proliferation induced by CTLA-4 and inhibition of that induced by CD4 occur independent from each other, but signaling through both is required to reach the threshold that inhibits memory T cell proliferation. Although CTLA-4 has a 10-fold higher binding affinity for B7 than CD28 (37), it is unlikely to bind all B7 even at limiting B7 expression because CTLA-4 cell surface expression at the peak time of expression (48 h after stimulation) is only 5% that of CD28 (38). Memory CD4 T cell proliferation may therefore occur at minimal CD28 ligation as long as inhibitory signals through CD4 are not delivered. However, this possibility seems unlikely because blocking CTLA-4-B7 interactions did not significantly increase the proliferative response of memory CD4 T cells in the absence of MHC class II on the APC.
It has recently been shown that the lymphoproliferative disorders seen in CTLA-4-negative mice are mediated by CD4-positive, not CD8-positive, T cells (30). While the role of CD4 in CTLA-4 signaling was not defined, the study suggested that CTLA-4 plays a different role in CD4 vs CD8 T cell homeostasis.
A role for CD4 in the maintenance of CD4 T cell homeostasis may lie in the fact that long term survival of mature peripheral CD4 T cells is dependent on the expression of MHC class II molecules but not on the presence of specific Ag (1, 35, 39). This implies that peripheral T cells require TCR engagement with MHC class II loaded with self peptide. Given that memory T cells have a lower threshold for proliferation than naive T cells (40), there is a danger that memory CD4 T cells might become activated by the signals for their survival. It is likely that weakly cross-reactive self peptide, similar to antagonistic peptide or soluble anti-CD3, does not recruit CD4 into the TCR-MHC class II interaction site. CD4 engagement away from the TCR complex may therefore induce CTLA-4-mediated unresponsiveness to prevent memory CD4 T cell proliferation to self peptide but allow memory T cell survival.
Furthermore, this model may have implications for HIV-infected memory T cells, where CD4 is similarly ligated outside the TCR complex by gp120. It has been shown that hyporesponsive gp120-positive CD4 T cells can be induced to proliferate when CD28 is ligated by mAb (41, 42). It would be of interest to see whether such T cells express CTLA-4.
Even though it is currently not clear how CD4 and CTLA-4 inhibit memory CD4 T cell responses, it is evident that the two mechanisms act in concert. We propose that CTLA-4 signaling is not just required for down-regulation of already activated T cells, but that it is a critical regulator of resting memory CD4 T cells. CTLA-4 inhibits memory CD4 T cell proliferation under conditions where CD4 is ligated outside the TCR complex.
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Acknowledgments |
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Footnotes |
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2 Address correspondence and reprint requests to Dr. K. Bottomly, 310 Cedar St., LH 405, New Haven, CT 06510.
3 Abbreviations used in this paper: PE, phycoerythrin; FcRII, Fc- receptor II.
Received for publication May 5, 1998. Accepted for publication July 23, 1998.
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References |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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and ß chains in CD4 function indicates a role for ordered oligomerization in T cell activation. J. Exp. Med. 182:779.This article has been cited by other articles:
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C. Ariyan, P. Salvalaggio, S. Fecteau, S. Deng, L. Rogozinski, D. Mandelbrot, A. Sharpe, M. H. Sayegh, G. P. Basadonna, and D. M. Rothstein Cutting Edge: Transplantation Tolerance through Enhanced CTLA-4 Expression J. Immunol., December 1, 2003; 171(11): 5673 - 5677. [Abstract] [Full Text] [PDF]
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T. Keler, E. Halk, L. Vitale, T. O'Neill, D. Blanset, S. Lee, M. Srinivasan, R. F. Graziano, T. Davis, N. Lonberg, et al. Activity and Safety of CTLA-4 Blockade Combined with Vaccines in Cynomolgus Macaques J. Immunol., December 1, 2003; 171(11): 6251 - 6259. [Abstract] [Full Text] [PDF]
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S. Krishnan, D. L. Farber, and G. C. Tsokos T Cell Rewiring in Differentiation and Disease J. Immunol., October 1, 2003; 171(7): 3325 - 3331. [Full Text] [PDF]
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 F. S. Hodi, M. C. Mihm, R. J. Soiffer, F. G. Haluska, M. Butler, M. V. Seiden, T. Davis, R. Henry-Spires, S. MacRae, A. Willman, et al. Biologic activity of cytotoxic T lymphocyte-associated antigen 4 antibody blockade in previously vaccinated metastatic melanoma and ovarian carcinoma patients PNAS, April 15, 2003; 100(8): 4712 - 4717. [Abstract] [Full Text] [PDF]
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R. J. DiPaolo and E. R. Unanue Cutting Edge: The Relative Distribution of T Cells Responding to Chemically Dominant or Minor Epitopes of Lysozyme Is Not Affected by CD40-CD40 Ligand and B7-CD28-CTLA-4 Costimulatory Pathways J. Immunol., September 15, 2002; 169(6): 2832 - 2836. [Abstract] [Full Text] [PDF]
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 S. Chikuma and J. A. Bluestone CTLA-4: Acting at the Synapse Mol. Interv., July 1, 2002; 2(4): 205 - 208. [Abstract] [Full Text] [PDF]
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 S. Mirshahidi, C.-T. Huang, and S. Sadegh-Nasseri Anergy in Peripheral Memory Cd4+ T Cells Induced by Low Avidity Engagement of T Cell Receptor J. Exp. Med., September 17, 2001; 194(6): 719 - 732. [Abstract] [Full Text] [PDF]
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M. D. Griffin, D. K. Hong, P. O. Holman, K.-M. Lee, M. J. Whitters, S. M. O'Herrin, F. Fallarino, M. Collins, D. M. Segal, T. F. Gajewski, et al. Blockade of T Cell Activation Using a Surface-Linked Single-Chain Antibody to CTLA-4 (CD152) J. Immunol., May 1, 2000; 164(9): 4433 - 4442. [Abstract] [Full Text] [PDF]
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N. A. Gomes, C. R. Gattass, V. Barreto-de-Souza, M. E. Wilson, and G. A. DosReis TGF-{beta} Mediates CTLA-4 Suppression of Cellular Immunity in Murine Kalaazar J. Immunol., February 15, 2000; 164(4): 2001 - 2008. [Abstract] [Full Text] [PDF]
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C. A. Chambers, M. S. Kuhns, and J. P. Allison Cytotoxic T lymphocyte antigen-4 (CTLA-4) regulates primary and secondary peptide-specific CD4+ T cell responses PNAS, July 20, 1999; 96(15): 8603 - 8608. [Abstract] [Full Text] [PDF]
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 C.A. CHAMBERS and J.P. ALLISON CTLA-4 -- The Costimulatory Molecule That Doesn't: Regulation of T-cell Responses by Inhibition Cold Spring Harb Symp Quant Biol, January 1, 1999; 64(0): 303 - 312. [Abstract] [PDF]
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F. Luhder, C. Chambers, J. P. Allison, C. Benoist, and D. Mathis Pinpointing when T cell costimulatory receptor CTLA-4 must be engaged to dampen diabetogenic T cells PNAS, October 24, 2000; 97(22): 12204 - 12209. [Abstract] [Full Text] [PDF]
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