CD8+ memory T cells (CD44high, Ly-6C+) are more sensitive than naive cells to (CD44low, Ly-6C-) to TCR/CD8 signaling in response to antigen

Memory CD8+ T cells from mice previously primed with alloantigen (alloAg) can respond in vitro to IL-2 and purified class I alloAg presented on microspheres, while no response can be detected using cells from naive mice. Similar results have been obtained using cells from OT-1 mice expressing a transgenic TCR that is specific for OVA(257-264) (SIINFEKL) peptide bound to H-2Kb. A population of resting memory cells (defined on the basis of low forward scatter and CD44high, Ly-6C+, CD25-, CD69-surface phenotype) that is present in the OT-1 mice exhibits a substantially higher sensitivity to Ag-stimulation than do naive cells (CD44low, Ly-6C-) expressing the same TCR. CD44high cells respond vigorously to H-2Kb immobilized on microspheres and pulsed with peptide, while CD44low cells respond weakly and only at high class I density and peptide concentration. The Ag-presenting surface only has ligands for TCR and CD8 (class I and peptide), thus ruling out the possibility that differences are due to ligand binding by other adhesion or costimulatory receptors that are expressed at high levels on the memory cells. Experiments using anti-TCR mAb as the stimulus and coimmobilized non-Ag class I as a ligand for CD8 suggest that the difference between naive and memory cells may be at the level of stimulation through the TCR. Thus, in addition to expressing increased levels of adhesion receptors that may enhance responses to Ag on APCs, memory CD8+ T cells appear to be intrinsically more sensitive than naive cells to stimulation through the TCR/CD8 complex.     

Human effector memory T cells express CD86: a functional role in naive T cell priming

The glycoprotein CD86 expressed on APCs provides a costimulatory signal necessary for an efficient activation of naive T cells. In contrast, there is controversy about the condition of expression and the function of CD86 on T cells. In this study, we have analyzed the phenotype and the biological activity of CD86+ T cells generated from human PBMC. Results show that CD86 expression on T cells is induced by long term stimulation via CD3 and IL-2R and is down-regulated as the cells become quiescent. The CD86-expressing cells are memory effector T cells: 1) they express CD45RO and high levels of the activation markers CD25, CD54, and HLA-Dr; 2) they selectively express CD30, CD40-ligand, and CD70; and 3) in response to stimulation, most of them produce IFN-gamma before dying by apoptosis. We then analyzed whether CD86 expressed on T cells is functional. Results show that paraformaldehyde-fixed CD86+ T cells enhance the proliferation and production of IFN-gamma by anti-CD3 mAb-stimulated naive T cells and induce proliferation of resting allogenic T cells. All these effects are prevented by neutralizing anti-CD86 mAbs. In contrast, we report no autocrine effect of CD86 in CD86+ T cell activation. In conclusion, these data show that human memory effector T cells express a functional form of CD86 that can costimulate naive T cell responses.     

Imaging antigen recognition by naive CD4+ T cells: compulsory cytoskeletal alterations for the triggering of an intracellular calcium response

Antigen recognition was analyzed at the single-cell level by using for the first time T cells which were not altered by in vitro selection, transfection or immortalization. The first consequence of antigen recognition by ex vivo naive CD4+ T cells from T cell receptor (TCR)-transgenic mice is the formation of a "contact zone" with the B cell presenting the antigen. The T cell intracellular calcium (Ca2+) response begins after a delay of 30 s on average, following the formation of the contact zone. The T cell response is entirely inhibited by either protein tyrosine kinase or actin polymerization inhibitors but, surprisingly, it is insensitive to inhibitors of phosphoinositide 3-kinase. Moreover, inhibition of microtubule polymerization and use of Ca2+-free medium do not prevent the beginning of the T cell response, but do reduce the stability of the contact zone and/or the amplitude of the Ca2+ plateau. The critical involvement of the cytoskeleton in antigen recognition on B cells introduces a checkpoint in T cell activation: the initial TCR engagement triggers a Ca2+ response only after an amplification step corresponding to a cytoskeleton-controlled increase in the number of engaged TCR.     

Pulmonary lymphocyte recruitment: depletion of CD8+ T cells does not impair the pulmonary immune response to intratracheal antigen

CD8+ T cells predominate in the lungs in hypersensitivity and human immunodeficiency virus-related lymphocytic pneumonitis, but their role in the immunopathogenesis of lung disease is unknown. We have shown that in immunized mice depleted of CD4+ T cells, CD8+ T cells are recruited into the lungs in response to intratracheal antigen challenge with sheep red blood cells (SRBC) (J. Clin. Invest. 1991; 88:1244-1254) or to pulmonary infection with Pneumocystis carinii (Am. J. Respir. Cell Mol. Biol. 1991; 5:186-197), suggesting that recruitment of CD8+ T cells does not depend on CD4+ T cell-derived signals. Because CD8+ T cells themselves produce a variety of chemotactic and immunoregulatory cytokines, CD8+ T cells may be important participants in, and modulators of, pulmonary immune responses. To test this hypothesis, we examined the effects of CD8+ T cell depletion on the generation of a pulmonary immune response in vivo. We monitored the recruitment of mononuclear cells into lungs in the absence of CD8-dependent signals and measured the duration of pulmonary inflammation in the absence of suppressor CD8+ T cells. Primed mice were treated with anti-CD8 monoclonal antibody to deplete CD8+ T cells and subsequently were challenged intratracheally with 5 x 10(8) SRBC. At various times after challenge, total and differential cell counts and lymphocyte phenotypes were measured in bronchoalveolar lavage fluid by flow cytometry and lungs were scored histologically. We found that depletion of CD8+ T cells neither decreased recruitment of immune and inflammatory cells nor prolonged the pulmonary immune response.(ABSTRACT TRUNCATED AT 250 WORDS)     

Partial activation of naive CD4 T cells and tolerance induction in response to peptide presented by resting B cells

Tolerance is thought to occur when Ag is presented to T cells in the absence of costimulatory interactions from APC accessory molecules. Of the professional APC, the resting B cell may be the main tolerizing cell in vivo. We have analyzed several aspects of activation of naive transgenic CD4 cells stimulated with resting or activated B cells presenting peptide Ag. Similar results were obtained with stimulation from peptide presenting fibroblast APC lacking or expressing B7-1 with intracellular adhesion molecule-1. TCR ligation with little or no accessory molecule coreceptor engagement induced efficient blastogenesis; up-regulation of CD25, CD44, CD69, CD95 and CD71; and down-regulation of CD62L over a 48-h period. Accessory molecule help enhanced the expression of CD25, CD44, CD69, and CD71, but to very modest degrees. Only two molecules, CD40 ligand and IL-2, were found to be extremely dependent on accessory molecule help, with little or no expression evident with peptide presented on resting B cells or class II-positive fibroblasts. T cells induced on resting B cells expanded minimally over 3 days, and this was followed by extensive cell death and hyporesponsiveness of the resulting cells. These studies suggest that under tolerizing conditions, such as Ag presentation by resting B cells, much of the naive CD4 response is induced efficiently. Partial activation, however, may be the overall result due to the lack of CD40 ligand expression, which may regulate costimulatory activity in APC and, in turn, may contribute to limiting the production of IL-2 required for T cell expansion and survival.     

In vitro responses of human CD45R0brightRA- and CD45R0-RAbright T cell subsets and their relationship to memory and naive T cells

The cellular basis of immunological memory, particularly with respect to T cells is not understood. In humans, monoclonal antibodies to CD45 have been used to identify memory (CD45R0) and naive (CD45RA) T cells. However, this identification has been called into question by various studies which suggest that high molecular weight CD45 isoforms may be re-expressed by previously activated cells. In the present study, using cultures which supported responses of naive T cells, we examined the responses of purified CD45R0brightRA- or CD45R0(-)-RAbright T cell subsets. The former subset was found to respond preferentially to recall antigens with minimal responses apparent to neo-(or non-recall)-antigens. The inverse pattern was found for CD45R0-RAbright T cells, which converted to CD45R0brightRA- after stimulation with a neo-antigen. Moreover, the two populations of T cells exhibited distinct response kinetics with a faster response evident from the CD45R0brightRA- T cells compared to the CD45R0-RAbright subset. The poor responses of CD45R0-RAbright T cells to recall antigens compared to neo-antigens suggests that this putative naive population is specifically depleted of reactive T cells following an encounter with antigen. We propose that T cell priming results in the stimulation of many CD45R0-RAbright T cells with various T cell receptor specificities from which memory T cells are selected for survival. If re-expression of higher molecular weight isoforms does occur in humans in vivo, our results suggest that R0 expression would be retained (CD45R0+RA+). Alternatively, if primed CD45R0-RAbright T cells exist, they are not prevalent in peripheral blood and thus may be sequestered within lymphoid tissues. Our data support the view that in human peripheral blood, CD45R0bright and CD45RAbright expression identify memory and naive CD4+ T cells, respectively.     

Minimal bystander activation of CD8 T cells during the virus-induced polyclonal T cell response

Acute infections with viruses such as lymphocytic choriomeningitis virus (LCMV) are associated with a massive polyclonal T cell response, but the specificities of only a small percentage of these activated T cells are known. To determine if bystander stimulation of T cells not specific to the virus plays a role in this T cell response, we examined two different systems, HY-specific T cell receptor (TCR)-transgenic mice, which have a restricted TCR repertoire, and LCMV-carrier mice, which are tolerant to LCMV. LCMV infection of HY-transgenic C57BL/6 mice induced antiviral CTLs that lysed target cells coated with two of the three immunodominant epitopes previously defined for LCMV (glycoprotein 33 and nucleoprotein 397). Although LCMV-induced cytotoxic T lymphocytes (CTLs) from C57BL/6 mice could lyse uninfected H-2(k) and H-2(d) allogeneic targets, LCMV-induced CTLs from HY mice lysed only the H-2(k)-expressing cells. The HY mice generated both anti-H-2(k) and anti-H-2(d) CTL in mixed leukocyte reactions, providing evidence that the generation of allospecific CTLs during acute LCMV infection is antigen specific. During the LCMV infection there was blastogenesis of the CD8+ T cell population, but the HY-specific T cells (as determined by expression of the TCR-alpha chain) remained small in size. To examine the potential for bystander stimulation under conditions of a very strong CTL response, T cell chimeras were made between normal and HY mice. Even in the context of a normal virus-induced CTL response, no stimulation of HY-specific T cells was observed, and HY-specific cells were diluted in number by day 9 after infection. In LCMV-carrier mice in which donor and host T cells could be distinguished by Thy1 allotypic markers, adoptive transfer of LCMV-immune T cells into LCMV-carrier mice, whose T cells were tolerant to LCMV, resulted in activation and proliferation of donor CD8 cells, but little or no activation of host CD8 cells. These results support the hypothesis that the massive polyclonal CTL response to LCMV infection is virus-specific and that bystander activation of non-virus-specific T cells is not a significant component of this response.     

Highly active antiretroviral therapy results in a decrease in CD8+ T cell activation and preferential reconstitution of the peripheral CD4+ T cell population with memory rather than naive cells

Criteria for detection of chromosome aberrations by Comparative Genomic Hybridization (CGH) are not standardized and improvement of this part of the analysis is of paramount importance to the applicability of the technique. The aim of this work was to suggest CGH detection criteria that increase the specificity and sensitivity and at the same time include chromosome regions previously excluded from CGH analysis. We analyzed 33 hybridizations with normal DNA and modified our CGH software in order to use a selection of these normal analyses as a model for interpretation of analyses of unknown samples. This approach was successfully tested on 14 samples with known aberrations.     

Semantic activation and episodic memory: Age similarities and differences.

In 2 experiments, 48 19–35 yr olds and 48 59–75 yr olds were engaged in semantic and nonsemantic orienting tasks and were subsequently given incidental or expected recall and recognition tasks. Reaction time (RT) patterns from the orienting tasks suggested that all Ss experienced similar semantic activation during encoding. Under incidental conditions, age differences in memory performance were minimal. When memory tests were expected, younger Ss recalled and recognized more items than did older Ss, suggesting that younger Ss were more effective in their deployment of mnemonic strategies. The age difference was particularly pronounced for unattended items, which suggests an age difference in the capacity to encode all of the episodic information. (31 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Hepatocytes induce functional activation of naive CD8+ T lymphocytes but fail to promote survival

Intraperitoneal peptide injection of TCR-transgenic mice or expression of antigen in hepatocytes leads to an accumulation in the liver of specific apoptotic CD8+ T cells expressing activation markers. To determine whether liver cells are capable of directly activating naive CD8+ T cells, we have studied the ability of purified hepatocytes to activate TCR-transgenic CD8+ T cells in vitro. We show that hepatocytes which do not express CD80 and CD86 co-stimulatory molecules are able to induce activation and effective proliferation of specific naive CD8+ T cells in the absence of exogenously added cytokines, a property only shared by professional antigen-presenting cells (APC). Specific T cell proliferation induced by hepatocytes was comparable in magnitude to that seen in response to dendritic cells and was independent of CD4+ T cell help or bystander professional APC co-stimulation. During the first 3 days, the same number of divisions was observed in co-cultures of CD8+ T cells with either hepatocytes or splenocytes. Both APC populations induced expression of early T cell activation markers and specific cytotoxic T lymphocyte (CTL) activity. However, in contrast to T cells activated by splenocytes, T cells activated by hepatocytes lost their cytolytic function after 3 days of co-culture. This correlated with death of activated T cells, suggesting that despite efficient activation, proliferation and transient CTL function, T cells activated by hepatocytes did not survive. Death could be prevented by adding antigen-expressing splenocytes or exogenous IL-2 to the co-culture, indicating that hepatocytes are not involved in direct killing of CD8+ T cells but rather fail to promote survival. Dying cells acquired a CD8(low) TCR(low) B220+ phenotype similar to the one described for apoptotic intrahepatic T cells, suggesting an alternative model to account for the origin of these cells in the liver. The importance of these findings for the understanding of peripheral tolerance and the ability of liver grafts to be accepted is discussed.     

Differential susceptibility of naive and memory CD4+ T cells to the cytopathic effects of infection with human immunodeficiency virus type 1 strain LAI

CD4+ T lymphocytes of individuals infected with human immunodeficiency virus type 1 (HIV-1) exhibit a qualitative defect in their ability to mount memory responses to previously encountered antigens although their responses to mitogens remain normal. T cells responsible for memory responses can be distinguished from naive T cells based on differential expression of isoforms of the tyrosine phosphatase CD45. It has been suggested that memory CD4+ T cells from infected individuals have a greater virus burden than naive CD4+ T cells and that this accounts for the loss of recall responses in infected individuals. However, it has been unclear whether naive and memory T cells are equally susceptible to infection and to the cytopathic effects of the virus. We therefore infected highly purified resting naive and memory CD4+ T cells from HIV-1-seronegative individuals with HIV-1(LAI). Infected cells were then stimulated with phytohemagglutinin to render them permissive for viral replication. Cell viability and growth rate were monitored for 8 to 10 days as indicators of cytopathic effects induced by HIV-1(LAI). Our results indicated that naive and memory CD4+ T cells display marked differences in susceptibility to the cytopathic effects induced by HIV-1(LAI), infection. The cytopathic effects induced by HIV-1(LAI) were much more severe in memory CD4+ T cells than in naive CD4+ T cells. Differential cytopathic effects in naive and memory T cells were not due to differences in virus entry into and replication in these cell populations. Rather, memory cells were more susceptible to cytopathic effects. Pronounced cytopathic effects in memory cells were clearly detectable at 7 day postinfection. Cell death occurred at the single-cell level and was not accompanied by syncytium formation. The growth rate of infected memory CD4+ T cells was also severely compromised compared to that of naive CD4+ T cells, whereas the growth rates of both uninfected naive and memory CD4+ T cells were approximately the same. At least a portion of the dying cells exhibited biochemical changes characteristic of apoptosis. These results suggest that the selective functional defects present in the memory CD4+ T-cell subset of HIV-1-infected individuals may in part be the result of the greater susceptibility of memory T cells to cytopathic effects induced by HIV-1.     

CD9-mediated costimulation of TCR-triggered naive T cells leads to activation followed by apoptosis

The induction of full activation or death in TCR-triggered T cells depends largely on whether appropriate costimulatory signals are provided. In this study, we show that the costimulation of CD9 on naive T cells during TCR stimulation results in transient, albeit potent, activation followed by apoptosis, rather than full activation. Anti-CD9 mAb synergized with suboptimal doses of anti-CD3 mAb in inducing T cell activation. [3H]TdR incorporation determined 2 days after CD9 costimulation was as potent as that induced by CD28 costimulation. In contrast to progressive T cell proliferation induced by CD28 costimulation, CD9 costimulation led to the induction of apoptosis of once-activated T cells. Although IL-2R expression was induced significantly earlier and to a greater degree after CD9 costimulation than after CD28 costimulation, CD9 costimulation only transiently produced a small amount of IL-2 and induced apparently low levels of bcl-xL compared with those observed in CD28 costimulation. Addition of rIL-2 to cultures of CD9 costimulation induced strikingly enhanced expression of bcl proteins, especially of bcl-xL, and protected TCR-stimulated T cells from apoptosis. These data indicate that CD9-mediated costimulation of TCR-triggered naive T cells leads to activation followed by apoptosis as the result of failure to generate a positive signal for sufficient levels of IL-2 production.     

CD4+ T cells orchestrate both amplification and deletion of CD8+ T cells

This review focuses on the role of CD4+ T cells in regulating immune responses, orchestrating both the amplification and deletion of immune cells, particularly CD8+ T cells. These two functions, which represent only an apparent contradiction, appear to be two faces of the same process of regulation. In fact, because the immune response, once activated, needs to be carefully controlled or switched off when the antigenic stimulus is eliminated, the immune system has developed several strategies either to regulate clonal amplification or to avoid useless expansion of activated cells. In particular, we have reported many data demonstrating that CD4+ T cells may be indicated as the regulatory element in the activation as well as the deletion of CD8+ T cells. New data are also reported on the ability of anergic CD4+ T cells to suppress CD8+ T-cell activation through induction of apoptosis, and on the need for CD8+ T cells for antigen recognition in inducing cell death in CD4+ T cells. Moreover, the central role of CD4+ T cells in the maintenance of peripheral tolerance has been widely described.     

Tumor-specific cytokine release by donor T cells induces an effective host anti-tumor response through recruitment of host naive antigen presenting cells

We recently reported that tumor eradication induced by immunotherapy (IT) in a congenic mouse model using tumor infiltrating lymphocytes (TIL) + recombinant interleukin-2 (rIL-2) is dependent on recruitment of naive host immune cells at the tumor sites. The recruitment of host immune cells was induced mainly through a local secretion of interferon-gamma (IFN-gamma) produced by donor T cells. We now further investigated how a non-specific inflammatory response progresses to a host T-cell-mediated tumor-specific response. In cross-over experiments using MCA-105 and MCA-205 sarcoma tumors, pulmonary metastatic disease was eradicated only in mice treated with tumor-matched TIL + rIL-2. In vitro, TIL stimulated with the tumor of origin secreted relatively high levels of IFN-gamma and granulocyte-macrophage colony stimulating factor (GM-CSF) compared to TIL stimulated with mismatched tumor cells. In lungs of tumor-bearing mice treated with matched TIL + rIL-2, significant increases in the percentages of IFN-gamma, GM-CSF and tumor necrosis factor-alpha (TNF-alpha) positive cells were detected, as well as of macrophages, natural killer (NK) cells and dendritic cells. Depletion of macrophages or NK cells did not inhibit the efficacy. In contrast, depletion of dendritic cells partially inhibited the efficacy of the treatment. Combined depletion of dendritic cells and macrophages abrogated more than 80% of the efficacy. Our data suggest that successful IT may require 3 steps: (1) release of inflammatory cytokines by donor TIL after restimulation by tumor cells; (2) infiltration of host immune cells in response to local cytokine production; and (3) activation of tumor-specific host immune cells by dendritic cells and to a lesser extent by macrophages.     

The nature of the signals regulating CD8 T cell proliferative responses to CD8alpha+ or CD8alpha- dendritic cells

The CD8alpha(-)-expressing dendritic cells (DC) of mouse spleen have been shown to be poor inducers of interleukin (IL)-2 production by CD8 T cells when compared to the CD8- DC. As a consequence, CD8 T cells give a more prolonged proliferative response to CD8- DC than to CD8+ DC. The possible mechanisms underlying these functional differences in DC subtype have been investigated. Inadequate co-stimulation did not underlie the poor T cell response to allogeneic CD8+ DC. Equivalent levels of B7-1 (CD80) and B7-2 (CD86) were found on the two DC subtypes and co-stimulator assays did not reveal any functional differences between them. Although CD8+ DC were found to die more rapidly in culture than CD8- DC, this did not explain their reduced stimulatory ability. Neither prolonging DC survival in culture nor renewing the stimulator cells by repeated addition of freshly isolated DC had any significant effect on the T cell responses. Furthermore, later addition to the cultures of DC of the opposite type to the initiating DC did not reverse or eliminate the differential response to the initiating DC. The role of DC-derived soluble factors was examined by addition to the cultures of supernatants derived from freshly isolated or stimulated DC of the opposite type. This neither enhanced the poor stimulatory capacity of CD8+ DC nor inhibited the stimulation by CD8- DC. Furthermore, addition of a series of cytokines that might have been produced by the DC did not eliminate the differences in T cell proliferation. Only the addition to the cultures of the growth factors IL-2 and IL-4 overcame the stimulatory difference between the two DC populations, confirming that the difference in T cell proliferative responses was a consequence of differences in induced cytokine production. The difference in the response of CD8 T cells to CD8+ and CD8- DC is therefore determined by direct DC-T cell contact during the earliest stages of the culture and involves an undetermined and possibly new signaling system.     

Virus-activated CD8 T cells and lymphokine-activated NK cells express the mast cell function-associated antigen, an inhibitory C-type lectin

The mast cell function-associated Ag (MAFA) is an inhibitory C-type lectin that was originally identified on the cell surface of a rat mucosal mast cell line, RBL-2H3. We have cloned the mouse homologue of the rat MAFA gene, and Northern blot analysis revealed that mouse MAFA (mMAFA) gene expression was strongly induced in effector CD8 T cells and lymphokine-activated NK cells but not in effector CD4 T cells and in mouse mast cells. Moreover, mMAFA gene expression was only found in effector CD8 T cells that had been primed in vivo with live virus because in vitro activated CD8 T cells did not express mMAFA. Primary sequence comparison revealed a high degree of conservation (89% similarity) between rat MAFA and mMAFA. Thus, the MAFA molecule in the mouse is a putative inhibitory receptor on anti-viral CD8 T cells induced in vivo and on NK cells.     

B7.1 is a quantitatively stronger costimulus than B7.2 in the activation of naive CD8+ TCR-transgenic T cells

Using a TCR transgenic mouse bred onto a recombinase-activating gene-2-deficient background, we have examined the influence of B7.1 and B7.2 on activation of naive, CD8+ T cells in vitro. We found that B7.1 was a more potent costimulus than B7.2 for induction of proliferation and IL-2 production by naive CD8+ T cells. This difference appeared to be quantitative in nature, as determined using transfectants expressing various defined levels of B7.1 or B7.2, or using purified B7.1 or B7.2 fusion proteins. In contrast to the quantitative differences seen in stimulation of naive T cells, B7.1 and B7.2 were comparable in their ability to costimulate responses in T cells previously primed in vitro. In addition, primed, but not naive, T cells were capable of proliferating and producing IL-2 in response to a TCR stimulus alone, apparently in the absence of B7 costimulation. Lastly, we found that B7.1 and B7.2 were equivalently capable of driving differentiation of naive CD8+ T cells into an IL-4-producing phenotype when exogenous IL-4 was added to the primary culture or to an IFN-gamma-producing phenotype in the presence of IL-12. These results indicate that signals generated by B7.1 and B7.2 are qualitatively similar, but that B7.1 is quantitatively stronger than B7.2. Further, our results indicate that the activation state of the responding T cell may influence the efficiency with which the T cell can respond to a costimulatory signal provided by either B7.1 or B7.2.     

B cells directly tolerize CD8(+) T cells

This report investigates the response of CD8(+) T cells to antigens presented by B cells. When C57BL/6 mice were injected with syngeneic B cells coated with the Kb-restricted ovalbumin (OVA) determinant OVA257-264, OVA-specific cytotoxic T lymphocyte (CTL) tolerance was observed. To investigate the mechanism of tolerance induction, in vitro-activated CD8(+) T cells from the Kb-restricted, OVA-specific T cell receptor transgenic line OT-I (OT-I cells) were cultured for 15 h with antigen-bearing B cells, and their survival was determined. Antigen recognition led to the killing of the B cells and, surprisingly, to the death of a large proportion of the OT-I CTLs. T cell death involved Fas (CD95), since OT-I cells deficient in CD95 molecules showed preferential survival after recognition of antigen on B cells. To investigate the tolerance mechanism in vivo, naive OT-I T cells were adoptively transferred into normal mice, and these mice were coinjected with antigen-bearing B cells. In this case, OT-I cells proliferated transiently and were then lost from the secondary lymphoid compartment. These data provide the first demonstration that B cells can directly tolerize CD8(+) T cells, and suggest that this occurs via CD95-mediated, activation-induced deletion.