A new role for CD28 in the survival of autoreactive T cells in the periphery after chronic exposure to autoantigen
Jian-Xin Gao,
Xing Chang,
Xincheng Zheng,
Jing Wen,
Lijie Yin,
Peishuang Du,
Pan Zheng and
Yang Liu
Division of Cancer Immunology, Department of Pathology, Ohio State University Medical Center, Columbus, OH 43210, USA
Correspondence to: Y. Liu; E-mail: liu-3{at}medctr.osu.edu
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Abstract
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Recent work demonstrates that costimulatory molecules play a critical role for clonal deletion of autoreactive T cells in the thymus. The role of CD28 in the survival of autoreactive T cells in the periphery, however, has not been reported. Here we demonstrate that while mutation of the CD28 gene consistently increased the burden of autoreactive T cells in the thymus, such an increase was not always found in the periphery, as the CD28(/) autoreactive T cells disappeared in the spleen over a period between 4 and 10 weeks. The disappearance of autoreactive T cells associates with a diminished induction of Bcl-2 protein by the self antigen and an increased proportion of apoptotic cells in the periphery. Moreover, the elimination of autoreactive T cells in the periphery requires chronic stimulation by the self antigen, as adoptive transfer analysis revealed no enhancement of apoptosis in CD28(/) T cells in antigen-bearing hosts over a 3 day period. Thus, CD28 plays a significant role in both clonal deletion and survival of autoreactive T cells after chronic exposure to autoantigens, resulting in opposite effects on the burden of autoreactive T cells.
Keywords: cell-surface molecules, repertoire development, T lymphocyte
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Introduction
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The repertoire of autoreactive T cells is controlled by their clonal deletion and their survival in the periphery. Several lines of previous studies suggest that the costimulatory pathway is involved in T cell clonal deletion. First, in thymocyte suspension cultures, a role for B7-1/2 and CD28 has been well documented. Thus, anti-B7-1/2 antibodies and fusion proteins with similar activity block the death of antigen-induced thymocytes in vitro (13). Agonistic anti-CD28 mAb was also found to promote antigen-induced deletion of T cells (2,4,5). Correspondingly, transfection of B7-1 into antigen-bearing cells promotes antigen-induced death of immature thymocytes (6). Second, Page reported that anti-B7-1 and anti-B7-2 prevented clonal deletion of MHC class II-restricted transgenic T cells (AND), induced by either cognate antigenic ligand or allogeneic MHC (H-2s) (7), in the thymic organ culture. Third, several in vivo analyses suggest that in addition to TCR engagement, costimulatory molecules may contribute to the efficacy of clonal deletion. Thus, Sprent and colleagues showed that deletion caused by in vivo injection of low doses of antigen is affected by CD28 (8). Li and Page have reported that among the F2 mice of the BALB/cxCD28(/)B6 mice, the CD28(/) mice have significant, although small, defects in the deletion of Vß5+ or Vß11+ CD4 T cells (9). Interestingly, CD28(/) mice have almost twice the number of thymocytes, and the CD28(/) thymocytes appear more resistant to anti-CD3 induced cell death in vivo (10). Should the increase of thymocyte numbers be attributed to the decreased clonal deletion, one would expect a more substantial effect of CD28 deficiency on the accumulation of self-reactive T cells.
We have recently revisited the issue of whether costimulatory molecules B7-1 and B7-2 are involved in clonal deletion by perinatal blockade of B7-1 and B7-2 (11). We reported that this treatment can block clonal deletion of T cells specific for endogenous VSAgs. In addition, in mice transgenic for both the tumor antigen P1A and P1A-specific T cells, we found that anti-B7-1/2 mAbs resulted in drastic reductions of T cell clonal deletion. More recently, similar results were reported in mice with targeted mutations of B7-1/2, CD28 and CTLA-4 (12). Together, these results established a more general function of CD28 in clonal deletion of autoreactive cells than what was perceived.
A role for CD28 in the survival of autoreactive T cells has not been reported. Here we report a new role for CD28 in the survival of autoreactive T cells in the periphery after chronic exposure to self antigen. Our results demonstrate that the survival of autoreactive T cells in the periphery is a checkpoint regulated by the costimulatory pathway.
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Methods
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Experimental animals
CD28-deficient and wild-type BALB/c mice were purchased from the Jackson Laboratories (Bar Harbor, ME). Transgenic BALB/c mice expressing tumor antigen P1A under the Id2 promoter and Eµ enhancer and/or TCR specific for tumor antigen P1A (P1CTL) have been described elsewhere (13). The CD28(/)P1CTL+ mice have also been reported (14). These mice were all backcrossed to BALB/c background for more than 10 generations and were used to produce CD28(/)P1CTL+P1A+ mice. All mice were maintained in the University Laboratory Animal Research Facility at the Ohio State University under specific-pathogen-free conditions.
Flow cytometry
Single-cell suspensions of thymocytes and splenocytes were prepared and stained for expression of CD4 and CD8 in combination with cell maturation marker HSA (heat-stable antigen), P1CTL TCR marker V
8, and viral superantigen (VSAg)-reactive T cell markers Vß3, 5, 8, 11 and 12. Anti-Bcl-2 antibody was used for intracellular staining of Bcl-2 expression, while annexin V was used to determine cells undergoing programmed cell death. The annexin V and mAbs conjugated with various fluorescent dyes were purchased from BD-PharMingen (San Diego, CA). In some experiments, the stained cells were analyzed by three or four-color flow cytometry.
Adoptive transfer of P1CTL T cells into P1A transgenic mice
Purification of CD8 T cells from P1CTL transgenic mice and their labeling with CFSE have been described (15). The CFSE-labeled WT and CD28(/) P1CTL cells were injected intravenously into syngeneic mice that express P1A as a transgene. At 20, 42 and 66 h after adoptive transfer, the spleen cells were harvested and analyzed for the number of divisions based on their CFSE intensity and for other parameters by flow cytometry.
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Results
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CD28 is required for both clonal deletion and survival of antigen-specific T cells
We have previously produced mice that express tumor antigen P1A in the thymus (13). The over-expressed antigen causes clonal deletion of the T cells expressing the transgenic TCR (P1CTL) from a P1A-reactive CTL clone. The deletion can be inhibited by treatment with anti-B7-1 and B7-2 mAbs (11). We produced CD28(/) and CD28(+/+) mice that were transgenic for both P1A and P1CTL, and compared the numbers and subset distribution of the transgenic T cells. Among the CD28(+/+) mice, transgenic expression of the tumor antigen P1A resulted in about a 2-fold reduction in the number of thymocyte (Fig. 1), as we have reported (11). Targeted mutation of CD28 significantly increased the number of thymocytes in mice transgenic for both P1A antigen and its specific TCR (Fig. 1a). In the CD28(/) background, the number of thymocytes was also lower when the tumor antigen was expressed, although this reduction is not statistically significant (P = 0.07).

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Fig. 1. CD28 deficiency attenuates the clonal deletion of transgenic mice that express both P1A antigen and P1A-reactive TCR. P1ATg+P1CTL+ and P1ATgP1CTL+ mice with or without targeted mutation of CD28 were sacrificed at 47 weeks after birth and their thymocyte numbers of subset distribution were determined by counting and flow cytometry. Data shown are the results of 23 independent experiments with a combined 68 mice per group. (a) Numbers of thymocytes. The results are expressed as mean ± SEM. P-values were determined by student's t-test. (b) Effect of P1A and CD28 on the thymocyte subset. Data shown as dot plots that depict the distribution of thymocyte subsets. The percentage of each subset is marked in the quadrants. (c) Percentage of CD4+CD8+ T cells in the thymus of mice with different genotypes.
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We have reported that clonal deletion of P1A-reactive T cells by the P1A antigen resulted in specific reduction of CD4+CD8+ T cells (11). As shown in Fig. 1(b and c), on average, CD28(+/+)P1ATg+P1CTL+ mice have
40% of CD4+CD8+ T cells, which is sharply reduced in comparison to
70% found in CD28(+/+)P1ATgP1CTL+ thymi. The difference between P1ATg+ and P1ATg mice very much disappeared in the CD28(/) mice. CD28(+/) mice had an intermediate level of clonal deletion and significant clonal deletion was observed in most but not all mice (Table 1). The function of CD28 in causing a reduction in the number of total thymocytes and in reduction of CD4+CD8+ transgenic T cells is consistent with the idea that CD28 may play a role in negative selection of T cells by the P1A antigen over-expressed in the thymus.
Interestingly, CD28-deficiency also resulted in a significant increase in the total number of thymocytes among mice transgenic for P1CTL but not P1A. Since the P1A is a self antigen with expression in testis and lymphoid organs, such as spleen (13) and thymus (16), it is likely that ablation of CD28 inhibited the negative selection mediated by the endogenous P1A, reportedly expressed in the peripheral antigen-expressing cells in the thymus (16).
Paradoxically, the rescue of T cells in the thymus from negative selection did not always result in an increased number of T cells in the periphery. A clear-cut example is presented in Fig. 2(a). Two 7-week-old littermates that differed in P1ATg had similar thymocyte subset distributions when CD4 and CD8 markers were used. Furthermore, among the CD4CD8+ T cells, the proportion of HSAlow cells was also comparable (Fig. 2b). Since expression of HSA is a marker for maturity of the single-positive thymocytes (17), the amount of mature T cells in the two thymi must be comparable. These results further underscore the notion that the deletion imposed by transgenic expression of P1A can sometimes be completely ablated by targeted mutation of the CD28. The only difference that we observed was the increased proportion of larger cells, perhaps reflecting the stimulation these cells received from the transgenic P1A antigen. Surprisingly, despite normal development of T cells in the thymus, CD28(/) mice had very few CD8 T cells in the spleen. Figure 2(c) depicts the proportion of T cells in the spleen of CD28(/) mice that were either P1A+P1CTL+ or P1AP1CTL+. Although the two mice have an almost identical proportion of CD4CD8+ thymocytes (Fig. 2a and b), there was an almost 7-fold difference in the proportion of mature V
8+ CD8 T cells in the spleens (Fig. 2c). The CD8 T cells were larger in the spleen of P1ATG+ mice.

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Fig. 2. Distinct roles for CD28 in clonal deletion and survival of autoreactive T cells. (a and b) CD8+CD4 thymocytes from CD28(/)P1CTL+P1ATg+ mice have similar maturity to those from the CD28(/)P1CTL+P1ATg mice. Thymocytes from two 7-week-old female littermates were analyzed by flow cytometry using anti-CD4, CD8 and HSA antibodies. The CD8+CD4 cells were further analyzed for their expression of HSA and the forward scatters. (a) Distribution of thymocyte subsets. (b) Cell size and HSA expression on the gated CD4CD8+ thymocytes. (c) The spleen cells from two female littermates (left panel, P1ATgP1CTL+ mice; right panel, P1ATg+P1CTL+ mice) were analyzed for the presence of CD4 and CD8 T cells in the spleen. (d) Size of the CD8 T cells in the spleen. The two groups of mice used were littermates of F1 from a CD28(/)P1ATg+xCD28(/)P1CTL+ breeding.
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A comparison of 4- and 7-week-old CD28(/) mice revealed that, even though the numbers of mature transgenic T cells were comparable among mice of the two different age groups, the number of transgenic T cells in the peripheral is reduced by >5-fold in 7-week-old mice (Table 2). These results support the notion that disappearance of autoreactive cells may require chronic exposure to autoantigens.
CD28 promotes clonal deletion and the survival of VSAg-reactive T cells
BALB/c mice have insertion of mouse mammary tumor virus 6, 8 and 9 and normally delete T cells expressing Vß3, 5, 11 and 12 (18). We have shown that in BALB/c mice, perinatal blockade of anti-B7-1 and anti-B7-2 significantly inhibited clonal deletion of VSAg-reactive T cells. To study the contribution of CD28 in the survival of autoreactive T cells in the periphery, we compared the number of VSAg-reactive T cells in the CD28(+/+) and CD28(/) BALB/c mice at 3 and 10 weeks, respectively. As shown in Fig. 3, CD28 deficiency resulted in a significant increase of Vß3, 5, 11 and 12+ cells in CD4+CD8 thymocytes, regardless of the age of the mice. Among the 3-week-old mice, the proportions of Vß11 and Vß12+ cells in the spleen were similar to those in the thymus. The Vß5-expressing cells declined in the periphery regardless of the CD28 status, although there were still more Vß5+ cells in the CD28(/) mice. Nevertheless, for the Vß3+ CD4 T cells, the decline was significantly faster in the CD28(/) mice. By 10 weeks, although CD28 deficiency continued to increase the accumulation of autoreactive T cells in the thymus, it paradoxically reduced the number of VSAg-reactive T cells in the spleen. These results further support a general role for CD28 in the survival of autoreactive T cells.

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Fig. 3. Role for CD28 in both clonal deletion and survival of VSAg-reactive T cells in BALB/c mice. Thymocytes and spleen cells from wild-type and CD28(/) BALB/c mice were analyzed for CD4, CD8, and Vß3, 5, 11 and 12. Groups of three mice at 3 or 10 weeks of age were analyzed.
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CD28 deficiency resulted in accelerated cell death of autoreactive T cells in the spleen
CD28 can promote T cell survival by inducing an anti-apoptotic protein, particularly Bcl-XL (19). A more recent study, however, suggested that a natural CD28 ligand, CD80, induced expression of Bcl-2 rather than Bcl-XL (20). We compared the expression of Bcl-2 by intracellular staining of the protein with PE-conjugated anti-Bcl-2 antibody. The representative profiles of the gated CD8+ transgenic T cells are presented in Fig. 4(a), and the summary data of groups of three mice are presented in Fig. 4(b). The CD28(/) P1CTL from P1ATg mice expressed a low level of Bcl-2. As expected, the expression was elevated among the T cells from P1A-transgenic mice. However, among the P1A-transgenic mice, CD28(+/)P1CTL expressed higher levels of Bcl-2 than the CD28(/)P1CTL. Consistent with differential Bcl-2 levels among CD28(+/) and CD28(/)P1CTL mice that also had P1A-transgene, we also observed significantly higher proportion of cells that were undergoing apoptosis, as judged by annexin V staining (Fig. 4c and d). Our results suggest that CD28 deficiency resulted in increased activation-induced cell death, at least in part by diminished induction of Bcl-2 after stimulation by the model self antigen.

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Fig. 4. CD28 deficiency resulted in diminished induction of Bcl-2 (a and b) and increased apoptosis (c and d) of ex vivo transgenic T cells from P1A and P1CTL double transgenic mice. (a) Representative profile of Bcl-2 expression among gated CD8 T cells in the spleen of P1CTL transgenic mice. (b) Means and SEM of the mean fluorescence of anti-Bcl-2 antibody staining, with three mice per group. (c) Representative histograms of annexin V staining among gated CD8 T cells of the spleen P1CTL transgenic mice with different genotypes. (d) Mean and SEM of % of annexin V+ cells (n = 3). The P-values given were calculated by student's t-test.
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In short-term adoptive transfer experiments, CD28 promoted division but not survival of P1CTL in P1A-transgenic mice
We labeled transgenic T cells from CD28(+/+) and CD28(/)P1CTL and transferred them into P1A-transgenic mice, and monitored the rate of division and cell death of the transgenic T cells. The adoptively transferred T cells were identified by their expression of transgenic V
8 as well as their ability to bind to H-Ld:P1A complex as described (15,21). As shown in Fig. 5(a), at 20 h after adoptive transfer, no T cell division was observed, which is consistent with our previous studies using tumor-bearing mice (15). By 42 h, CD28(+/+) T cells had undergone 15 divisions. CD28 deficiency caused a substantial delay in the division of T cells, as the majority of T cells have undergone only one division. By 66 h, substantial division of T cells was observed in both groups, although the extent of CFSE dilution prevented an accurate determination of the numbers of cell divisions. To determine whether CD28 deficiency also caused death of dividing T cells, we analyzed the transgenic T cells for their expression of Bcl-2 and binding to annexin V. As shown in Fig. 5(b), although a significant proportion of transgenic T cells underwent apoptosis at 42 and 66 h, CD28 expression neither decreased apoptosis nor increased expression of Bcl-2.

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Fig. 5. Roles for CD28 in proliferation (a) and programmed cell death (b) of P1CTL. P1CTL (5 x 106/mouse) were labeled with CFSE and adoptively transferred into syngeneic P1ATg mice. The spleen cells were harvested at 20, 42 and 66 h after adoptive transfer and stained with anti-CD8 or V 8 in conjunction with P1A:H-2Ld dimer to identify transgenic T cells. The transgenic T cells were gated and the profiles of CFSE intensity, binding the annexin V or anti-Bcl-2 antibody are presented. Data shown are representative profiles of 23 mice in each group. Dotted lines in (b) represent staining with isotype controls.
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Discussion
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CD28 interacts with B7-1/2 and plays an important role in the induction of T-cell immunity (22,23). Recent studies by several groups, including ours, have established that CD28-B7-1/2 interaction plays a significant role in eliminating autoreactive T cells in the thymus (11,12). Our new data presented in the manuscript show that mice with targeted mutation of CD28 have a low number of periphery autoreactive T cells even though the number of autoreactive T cells in the thymus is drastically increased. These results reveal an unexpected new role for CD28 in the survival of autoreactive T cells in the periphery.
At least three mechanisms can be responsible for the disappearance of autoreactive T cells in the CD28-deficient mice. CD28(/) autoreactive T cells may die immediately prior to their migration into the periphery. Alternatively, the autoreactive T cells that do migrate into the periphery die after encountering antigen, due to the lack of CD28-mediated costimulation. Two lines of evidence presented here support the alternative interpretation. First, we have observed increased apoptotic cells in the peripheral of TCR transgenic mice that also express the autoantigen. Second, despite comparable numbers of autoreactive T cells in the thymus in 4- and 7-week-old mice, the older mice had 5-fold less autoreactive T cells in the spleen. Similar conclusions can be reached by studying the number of VSAg-reactive T cells. The disappearance of autoreactive T cells over the 4- to 7-week period suggests that elimination of autoreactive T cells in the absence of CD28 may require an extended period of stimulation, although the relative contribution of antigenic stimulation in the thymus and spleen has not been addressed. This requirement for chronic antigen stimulation is consistent with the results of the adoptive transfer study, which demonstrated that CD28 deficiency reduced the rate of T cell division, but did not promote apoptosis within 23 days of adoptive transfer. The requirement for chronic stimulation may explain the existence of autoreactive T cells in the periphery of the mice that received perinatal treatment with anti-B7 antibodies, as we have reported (11). It should be pointed out that we did note in our early studies that autoimmune inflammation in anti-B7-treated mice was resolved by 5 months, although it was unclear whether the disappearance of autoreactive T cells was responsible. Thirdly, since CD28 promote proliferation of T cells, it is formally possible that the differential accumulation of T cells is due to differences in expansion of autoreactive T cells in the periphery. We do not believe this is likely, as the frequencies of autoreactive T cells in the thymus are higher than those in the spleen. In addition, the number of autoreactive T cells diminished between the 3rd and 7th week of age. These data indicate minimal expansion of autoreactive T cells in both WT and CD28-deficient mice.
We have observed a correlation between the levels of Bcl-2 protein and CD28 expression, which may provide a molecular mechanism by which CD28 promotes survival of autoreactive T cells. Our results on the role of CD28 in reducing programmed cell death after engagement of TCR by antigen is consistent with previous reports that CD28 promotes antigen-driven proliferation and reduces activation-induced cell death (19). A recent report by Yu et al. (24) showed that CD28 exerts distinct functions depending on the avidity between TCR and its ligands. When high-avidity ligand is present, CD28 actually promotes death of T cells, although when low and medium-avidity TCR ligand is used, CD28 appears to promote T cell expansion. These results suggest that for different autoantigens, CD28 may play a different role for the activation and survival of T cells. Our systematic comparison of the number of self VSAg-reactive T cells in WT and CD28(/) mice suggests that in the absence of CD28, the disappearance of different subsets of VSAg-reactive T cells seems to follow different kinetics.
A role for CD28 in the survival of autoreactive T cells illustrates that the survival of autoreactive T cells is a checkpoint that can be regulated by costimulation and may thus provide a new window of opportunity to eliminate autoreactive T cells. In this regard, it would be of interest to test whether blocking the costimulatory pathway, which is increasingly used for experimental therapy of autoimmune diseases (25,26), may have done so by reducing the burden of autoreactive T cells in the periphery.
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Acknowledgements
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We thank Lynde Shaw for editorial assistance in the preparation of the manuscript. This study was supported by grants from National Institute of Health AI51342, CA58033, CA82355 and CA69091.
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Abbreviations
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HSA | heat-stable antigen |
P1CTL | transgenic TCR specific for tumor antigen P1A |
VSAg | viral superantigen |
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Notes
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Transmitting editor: C. Terhorst
Received 4 May 2004,
accepted 9 July 2004.
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References
|
---|
- Amsen, D. and Kruisbeek, A. M. 1996. CD28-B7 interactions function to co-stimulate clonal deletion of double-positive thymocytes. Int. Immunol. 8:1927.[Abstract]
- Amsen, D., Revilla Calvo, C., Osborne, B. A. and Kruisbeek, A. M. 1999. Costimulatory signals are required for induction of transcription factor Nur77 during negative selection of CD4(+)CD8(+) thymocytes. Proc. Natl Acad. Sci. USA 96:622.[Abstract/Free Full Text]
- Carlow, D. A., van Oers, N. S., Teh, S. J. and Teh, H. S. 1992. Deletion of antigen-specific immature thymocytes by dendritic cells requires LFA-1/ICAM interactions. J. Immunol. 148:1595.[Abstract/Free Full Text]
- Punt, J. A., Havran, W., Abe, R., Sarin, A. and Singer, A. 1997. T cell receptor (TCR)-induced death of immature CD4+CD8+ thymocytes by two distinct mechanisms differing in their requirement for CD28 costimulation: implications for negative selection in the thymus. J. Exp. Med. 186:1911.[Abstract/Free Full Text]
- Punt, J. A., Osborne, B. A., Takahama, Y., Sharrow, S. O. and Singer, A. 1994. Negative selection of CD4+CD8+ thymocytes by T cell receptor-induced apoptosis requires a costimulatory signal that can be provided by CD28. J. Exp. Med. 179:709.[Abstract]
- Kishimoto, H., Cai, Z., Brunmark, A., Jackson, M. R., Peterson, P. A. and Sprent, J. 1996. Differing roles for B7 and intercellular adhesion molecule-1 in negative selection of thymocytes [see comments]. J. Exp. Med. 184:531.[Abstract]
- Page, D. M. 1999. Cutting edge: thymic selection and autoreactivity are regulated by multiple coreceptors involved in T cell activation [In process citation]. J. Immunol. 163:3577.[Abstract/Free Full Text]
- Kishimoto, H. and Sprent, J. 1999. Several different cell surface molecules control negative selection of medullary thymocytes. J. Exp. Med. 190:65.[Abstract/Free Full Text]
- Li, R. and Page, D. M. 2001. Requirement for a complex array of costimulators in the negative selection of autoreactive thymocytes in vivo. J. Immunol. 166:6050.[Abstract/Free Full Text]
- Noel, P. J., Alegre, M. L., Reiner, S. L. and Thompson, C. B. 1998. Impaired negative selection in CD28-deficient mice. Cell Immunol. 187:131.[CrossRef][ISI][Medline]
- Gao, J.-X., Zhang, H., Bai, X. F., Wen, J., Zheng, X., Liu, J., Zheng, P. and Liu, Y. 2002. Peripheral blockade of B7-1 and B7-2 inhibits clonal deletion of highly pathogenic autoreactive T cells. J. Exp. Med. 195:959.[Abstract/Free Full Text]
- Buhlmann, J. E., Elkin, S. K. and Sharpe, A. H. 2003. A role for the B7-1/B7-2:CD28/CTLA-4 pathway during negative selection. J. Immunol. 170:5421.[Abstract/Free Full Text]
- Sarma, S., Guo, Y., Guilloux, Y., Lee, C., Bai, X.-F. and Liu, Y. 1999. Cytotoxic T lymphocytes to an unmutated tumor antigen P1A: normal development but restrained effector function. J. Exp. Med. 189:811.[Abstract/Free Full Text]
- Bai, X. F., Liu, J., May, K. F. Jr, Guo, Y., Zheng, P. and Liu, Y. 2002. B7-CTLA4 interaction promotes cognate destruction of tumor cells by cytotoxic T lymphocytes in vivo. Blood 99:2880.[Abstract/Free Full Text]
- Bai, X.-F., Gao, J.-X., Liu, Q., Wen, J., Zheng, P. and Liu, Y. 2001. On the site and mode of antigen presentation for the initiation of clonal expansion of CD8 T cells specific for a natural tumor antigen. Cancer Research 61:6860.[Abstract/Free Full Text]
- Derbinski, J., Schulte, A., Kyewski, B. and Klein, L. 2001. Promiscuous gene expression in medullary thymic epithelial cells mirrors the peripheral self. Nat. Immunol. 2:1032.[CrossRef][ISI][Medline]
- Crispe, I. N. and Bevan, M. J. 1987. Expression and functional significance of the J11d marker on mouse thymocytes. J. Immunol. 138:2013.[Abstract/Free Full Text]
- Abe, R., Foo-Phillips, M. and Hodes, R. J. 1991. Genetic analysis of the Mls system. Formal Mls typing of the commonly used inbred strains. Immunogenetics 33:62.[ISI][Medline]
- Boise, L. H., Minn, A. J., Noel, P. J., June, C. H., Accavitti, M. A., Lindsten, T. and Thompson, C. B. 1995. CD28 costimulation can promote T cell survival by enhancing the expression of Bcl-XL. Immunity 3:87.[ISI][Medline]
- Buggins, A. G., Lea, N., Gaken, J., Darling, D., Farzaneh, F., Mufti, G. J. and Hirst, W. J. 1999. Effect of costimulation and the microenvironment on antigen presentation by leukemic cells. Blood 94:3479.[Abstract/Free Full Text]
- Bai, X. F., Liu, J., Li, O., Zheng, P. and Liu, Y. 2003. Antigenic drift as a mechanism for tumor evasion of destruction by cytolytic T lymphocytes. J. Clin. Invest. 111:1487.[Abstract/Free Full Text]
- Linsley, P. S. and Ledbetter, J. A. 1993. The role of the CD28 receptor during T cell responses to antigen. Annu. Rev. Immunol. 11:191.[CrossRef][ISI][Medline]
- Liu, Y. and Linsley, P. S. 1992. Costimulation of T-cell growth. Curr. Opin. Immunol. 4:265.[CrossRef][ISI][Medline]
- Yu, X. Z., Martin, P. J. and Anasetti, C. 2003. CD28 signal enhances apoptosis of CD8 T cells after strong TCR ligation. J. Immunol. 170:3002.[Abstract/Free Full Text]
- Finck, B. K., Linsley, P. S. and Wofsy, D. 1994. Treatment of murine lupus with CTLA4Ig. Science 265:1225.[ISI][Medline]
- Abrams, J. R., Kelley, S. L., Hayes, E. et al. 2000. Blockade of T lymphocyte costimulation with cytotoxic T lymphocyte-associated antigen 4-immunoglobulin (CTLA4Ig) reverses the cellular pathology of psoriatic plaques, including the activation of keratinocytes, dendritic cells and endothelial cells. J. Exp. Med. 192:681.[Abstract/Free Full Text]