Extinction of IL-12 signaling promotes Fas-mediated apoptosis of antigen-specific T cells.

In previous studies we have shown that peripheral tolerance achieved by high dose feeding of OVA to intact OVA-TCR transgenic mice was enhanced when endogenous IL-12 was neutralized simultaneously. To generalize this phenomenon, in the present study we investigated the tolerogenic mechanisms underlying the blockade of IL-12 signaling following oral and systemic Ag delivery. We found that the numbers of Ag-specific T cells in several lymphoid organs were significantly reduced due to T cell apoptosis following oral OVA or systemic OVA administration when combined with anti-IL-12 injection, but there was no decrease in T cell numbers for OVA-fed, OVA-injected, or anti-IL-12 alone-treated mice compared with those in untreated control mice. In addition, mostly Fas+ T cells were subject to apoptotic deletion in the OVA- plus anti-IL-12-treated groups, and an enhanced cell death of T cells upon OVA restimulation in vitro could be partially reversed by blockade of the Fas/Fas ligand interaction. Finally, in a murine model of superantigen-driven T cell expansion and deletion, we observed no deletional effects of anti-IL-12 treatment on CD4+ cells in Fas-deficient (MRL/lpr) mice, but did find these effects in MRL wild-type mice. In summary, our data suggest that in the course of Ag-induced cell proliferation of Th1 cells, signaling through IL-12 is required to prevent an induction of Fas-mediated apoptosis. Thus, the use of anti-IL-12 may be potentially useful in modulating peripheral immune responses by promotion of Fas-mediated cell death.     

Dual upregulation of Fas and Bax promotes alloreactive T cell apoptosis in IL-10 gene targeting of cardiac allografts

Activation-induced cell death and cytokine deprivation are demonstrated by peripheral T cell populations at the conclusion of natural immune responses, and each of these processes is modulated by the immunosuppressive cytokine interleukin (IL)-10 in vitro. This study employs a clinically relevant in vivo model of IL-10 gene transfer with heterotopically transplanted cardiac allografts to determine the mechanisms of the effects of IL-10 on T cell survival. IL-10 protein overexpression within allografts 4-5 days after gene transfer augments apoptosis of CD4+ and CD8+ graft-infiltrating lymphocytes by 7.1-fold (P < 0.001) and 6.0-fold (P < 0.001), respectively. Graft-infiltrating T cells express 10-fold more proapoptotic Fas (P < 0.01) and 30-fold more Bax (P < 0.01) than controls. The fractions of activated caspase-8 (FADD-like IL-1beta-converting enzyme) and activated caspase-9 were increased 7- and 2.3-fold, respectively, in IL-10 gene-treated allografts at postoperative day 4-5. These changes in the Fas-Fas ligand pathway and Bcl-2 mitochondrial apoptosis regulation are enhanced by complete suppression of antiapoptotic FADD-like IL-1beta-converting enzyme inhibitory protein (FLIP) (from 30.5 to 0.0%, P < 0.01) and Bcl-xL (from 22.5 to 0.1%, P = 0.03) expression among these cells from the earliest days after gene transfer. Although changes in proteins of Fas- and Bcl-2-mediated apoptosis signaling occur, only the levels of Fas and FLIP correlate to the rate of apoptosis of graft-infiltrating CD3 lymphocytes and histological rejection scores. These results indicate that dichotomous apoptosis-regulatory pathways are affected by IL-10 gene therapy, but Fas-mediated mechanisms of activation-induced cell death more substantially contribute to the greater cell death of graft-infiltrating T cells after ex vivo IL-10 gene transfer.     

Inhibition of TCR-induced CD8 T cell death by IL-12: regulation of Fas ligand and cellular FLIP expression and caspase activation by IL-12

In this study we demonstrate the anti-apoptotic effect of IL-12 and its underlying mechanism in CD8 T cells. The prolonged stimulation of CD8 T cells with anti-CD3 alone caused apoptosis mediated by Fas and the caspase signaling pathway. However, costimulation with IL-12 significantly prevented anti-CD3-induced apoptosis of CD8 T cells. IL-12 decreased the number of Fas ligand-positive CD8 T cells and inhibited the activation of caspase-8 and caspase-3. In addition, IL-12 up-regulated cellular FLIPs but not Bcl-2 family proteins or cellular inhibitor of apoptosis proteins. These data suggest that IL-12 provides survival signals to CD8 T cells by down-regulating Fas ligand and up-regulating cellular FLIPs, followed by inhibiting caspase activation, which implies a role for IL-12 in peripheral responses of CD8 T cells in vivo.     

Chronic morphine treatment promotes specific Th2 cytokine production by murine T cells in vitro via a Fas/Fas ligand-dependent mechanism

Improper homeostasis of Th1 and Th2 cell differentiation can promote pathological immune responses such as autoimmunity and asthma. A number of factors govern the development of these cells including TCR ligation, costimulation, death effector expression, and activation-induced cell death (AICD). Although chronic morphine administration has been shown to selectively promote Th2 development in unpurified T cell populations, the direct effects of chronic morphine on Th cell skewing and cytokine production by CD4(+) T cells have not been elucidated. We previously showed that morphine enhances Fas death receptor expression in a T cell hybridoma and human PBL. In addition, we have demonstrated a role for Fas, Fas ligand (FasL), and TRAIL in promoting Th2 development via killing of Th1 cells. Therefore, we analyzed whether the ability of morphine to affect Th2 cytokine production was mediated by regulation of Fas, FasL, and TRAIL expression and AICD directly in purified Th cells. We found that morphine significantly promoted IL-4 and IL-13 production but did not alter IL-5 or IFN-gamma. Furthermore, morphine enhanced the mRNA expression of Fas, FasL and TRAIL and promoted Fas-mediated AICD of CD4(+) T cells. Additionally, blockade of Fas/FasL interaction by anti-FasL inhibited the morphine-induced production of IL-4 and IL-13 and AICD of CD4(+) T cells. These results suggest that morphine preferentially enhances Th2 cell differentiation via killing of Th1 cells in a Fas/FasL-dependent manner.     

Dendritic cells are resistant to apoptosis through the Fas (CD95/APO-1) pathway.

Immunoregulation of lymphocytes and macrophages in the peripheral immune system is achieved in part by activation-induced cell death. Members of the TNF receptor family including Fas (CD95) are involved in the regulation of activation-induced cell death. To determine whether activation-induced cell death plays a role in regulation of dendritic cells (DCs), we examined interactions between Ag-presenting murine DCs and Ag-specific Th1 CD4+ T cells. Whereas mature bone marrow- or spleen-derived DCs expressed high levels of Fas, these DCs were relatively insensitive to Fas-mediated killing by the agonist mAb, Jo-2, as well as authentic Fas ligand expressed on the CD4+ T cell line, A.E7. The insensitivity to Fas-mediated apoptosis was not affected by priming with IFN-gamma and/or TNF-alpha or by blocking the DC survival signals TNF-related activation-induced cytokine and CD40L. However, apoptosis could be induced with C2-ceramide, suggesting that signals proximal to the generation of ceramide might mediate resistance to Fas. Analysis of protein expression of several anti-apoptotic mediators revealed that expression of the intracellular inhibitor of apoptosis Fas-associated death domain-like IL-1-converting enzyme-inhibitory protein was significantly higher in Fas-resistant DCs than in Fas-sensitive macrophages, suggesting a possible role for Fas-associated death domain-like IL-1-converting enzyme-inhibitory protein in DC resistance to Fas-mediated apoptosis. Our results demonstrate that murine DCs differ significantly from other APC populations in susceptibility to Fas-mediated apoptosis during cognate presentation of Ag. Because DCs are most notable for initiation of an immune response, resistance to apoptosis may contribute to this function.     

Potential role for IL-7 in Fas-mediated T cell apoptosis during HIV infection

IL-7 promotes survival of resting T lymphocytes and induces T cell proliferation in lymphopenic conditions. As elevated IL-7 levels occur in HIV-infected individuals in addition to high Fas expression on T cells and increased sensitivity to Fas-induced apoptosis, we analyzed whether IL-7 has a regulatory role in Fas-mediated T cell apoptosis. We show that IL-7 up-regulates Fas expression on naive and memory T cells through a mechanism that involves translocation of Fas molecules from intracellular compartments to the cell membrane. IL-7 induced the association of Fas with the cytoskeletal component ezrin and a polarized Fas expression on the cell surface. The potential role of IL-7 in Fas up-regulation in vivo was verified in IL-7-treated macaques and in HIV-infected or chemotherapy treated patients by the correlation between serum IL-7 levels and Fas expression on T cells. IL-7 treatment primed T cells for Fas-induced apoptosis in vitro and serum IL-7 levels correlated with the sensitivity of T cells to Fas-induced apoptosis in HIV-infected individuals. Our data suggest an important role for IL-7 in Fas-mediated regulation of T cell homeostasis. Elevated IL-7 levels associated with lymphopenic conditions, including HIV-infection, might participate in the increased sensitivity of T cells for activation-induced apoptosis.     

CD28 costimulation accelerates IL-4 receptor sensitivity and IL-4-mediated Th2 differentiation.

The development of Th1 and Th2 cells is determined by the type of antigenic stimulation involved in the initial cell activation step. Evidence indicates that costimulatory signals, such as those delivered by CD28, play an important role in Th2 development, but little is known about how CD28 costimulation contributes to Th2 development. In this study, TCR cross-linking was insufficient for Th2 development, while the addition of CD28 costimulation drastically increased Th2 generation through the IL-4-mediated pathway. Th2 generation following CD28 costimulation was not simply explained by the enhancement of IL-4 production in naive T cells. To generate Th2 cells after TCR cross-linking only, it was necessary to add a 20- to 200-fold excess of IL-4 generated after TCR and CD28 stimulation. TCR cross-linking increased the expression level and binding property of the IL-4R, but enhanced the sensitivity to IL-4 only slightly. In contrast, as evidenced by the enhanced phosphorylation of Jak3, the IL-4Ralpha-chain, and STAT6 following IL-4 stimulation, CD28 costimulation increased IL-4R sensitivity without affecting its expression and binding property. This evidence of the enhancement of IL-4R sensitivity increases our understanding of how CD28 costimulation accelerates Th2 development.     

OX40-mediated differentiation to effector function requires IL-2 receptor signaling but not CD28, CD40, IL-12Rbeta2, or T-bet

Ag-specific CD4 T cells transferred into unirradiated Ag-bearing recipients proliferate, but survival and accumulation of proliferating cells is not extensive and the donor cells do not acquire effector functions. We previously showed that a single costimulatory signal delivered by an agonist Ab to OX40 (CD134) promotes accumulation of proliferating cells and promotes differentiation to effector CD4 T cells capable of secreting IFN-gamma. In this study, we determined whether OX40 costimulation requires supporting costimulatory or differentiation signals to drive acquisition of effector T cell function. We report that OX40 engagement drives effector T cell differentiation in the absence of CD28 and CD40 signals. Two important regulators of Th1 differentiation, IL-12R and T-bet, also are not required for acquisition of effector function in CD4 T cells responsive to OX40 stimulation. Finally, we show that CD25-deficient CD4 T cells produce little IFN-gamma in the presence of OX40 costimulation compared with wild type, suggesting that IL-2R signaling is required for efficient OX40-mediated differentiation to IFN-gamma secretion.     

The poststimulation program of CD4 versus CD8 T cells (death versus activation-induced nonresponsiveness)

Both CD8 and CD4 T cells undergo autocrine IL-2-induced proliferation and clonal expansion following stimulation with Ag and costimulation. The CD8 T cell response is transient because the cells rapidly become activation-induced nonresponsive (AINR) and exhibit split anergy. In these cells, the capacity for IL-2 production is lost, but TCR-mediated IFN-gamma production and cytotoxicity are maintained. At this point, the CTL become dependent on IL-2 provided by CD4 Th cells for continued expansion. If IL-2 is available to support expansion for a brief period, AINR is reversed and the cells regain the ability to produce IL-2. In this study, we show that CD4 T cells do not become AINR, but instead are rendered susceptible to Fas-mediated activation-induced cell death following stimulation through TCR and CD28. Using z-VAD-fmk or anti-Fas ligand mAb to inhibit cell death, we demonstrate that previously activated CD4 T cells retain the ability to up-regulate c-Jun N-terminal kinase activity and IL-2 mRNA levels upon TCR engagement and no longer require costimulation. This rewiring of signaling pathways is similar to that seen following reversal of AINR in CD8 T cells. Thus, CD8 and CD4 T cells appear to use distinct mechanisms, AINR and activation-induced cell death, respectively, to limit excessive clonal expansion following a productive response, while permitting important effector functions to be expressed.     

Intercellular adhesion molecule 1 is critical for activation of CD28-deficient T cells

Presentation of Ag to T lymphocytes in the absence of the requisite costimulatory signals leads to an Ag-specific unresponsiveness termed anergy, whereas Ag presentation in conjunction with costimulation leads to clonal expansion. B7/CD28 signaling has been shown to provide this critical costimulatory signal and blockade of this pathway may inhibit in vitro and in vivo immune responses. Although T cells from CD28-deficient mice are lacking in a variety of responses, they nonetheless are capable of various primary and secondary responses without the induction of anergy expected in the absence of costimulation. This suggests that there may be alternative costimulatory pathways that can replace CD28 signaling under certain circumstances. In this paper, we show that ICAM-1becomes a dominant costimulatory molecule for CD28-deficient T cells. ICAM-1 costimulates anti-CD3-mediated T cell proliferation and IL-2 secretion in CD28-deficient murine T cells. Furthermore, splenocytes from ICAM-1-deficient mice could not activate CD28-deficient T cells and splenocytes lacking both ICAM and CD28 fail to proliferate in response to anti-CD3-induced T cell signals. This confirms that not only can ICAM-1 act as a CD28-independent costimulator, but it is the dominant, requisite costimulatory molecule for the activation of T cells in the absence of B7/CD28 costimulation.     

Differential regulatory effects of intercellular adhesion molecule-1 on costimulation by the CD28 counter-receptor B7.

Although ligation of the CD3/TCR complex initiates an activation signal in T cells, additional costimulatory signals generated during cell-to-cell interactions with APC transduced via ligation of CD11a/CD18 and CD28 by their specific counter-receptor intercellular adhesion molecule (ICAM)-1 and B7, respectively, are required for optimal T cell proliferation and cytokine synthesis. Using soluble IgC gamma 1 fusion proteins of these costimulatory counter-receptors, we have recently shown that unactivated resting CD4+ T cells and Ag-primed CD4+ T cells differ in their response to the costimulation by ICAM-1 and B7. Preferential proliferative responses of resting T and Ag-primed T cells to ICAM-1 and B7, respectively, prompted us to speculate that ICAM-1-induced signals may regulate coupling of the CD28 signaling pathway. Furthermore, both B7 and ICAM-1 are co-expressed on APC and thus, may co-regulate activation-driven maturation of T cells. In this study, we have examined regulatory effects of IgC gamma 1 fusion proteins of B7, ICAM-1, and ICAM-2 (a homologue of ICAM-1) on each other's costimulation. We first demonstrate that TCR-directed costimulation of resting CD4+ T cells with ICAM-1 (ICAM-1 priming) but not ICAM-2 induces increased responsiveness to B7. Priming of CD4+ T cells with ICAM-1 induced higher expression of both CD18 and CD28 than that with either B7 or ICAM-2. Cross-linking of CD28 induced faster and significantly higher cytoplasmic free calcium mobilization response in ICAM-1-primed CD4+ T cells than in resting, B7-primed, or ICAM-2-primed CD4+ T cells. B7 synergized with ICAM-1 but not ICAM-2 to augment proliferative responses of not only resting CD4+ T cells but also those that had been primed with either ICAM. Unlike resting or ICAM-2-primed CD4+ T cells, ICAM-1-primed CD4+ T cells efficiently proliferated in response to the synergistic costimulation of B7 and ICAM-2. In contrast, both ICAM-1 and ICAM-2 inhibit B7-driven proliferation of Ag-primed CD4+ T cells. Thus, B7 and ICAM-1 exert contrasting regulatory effects on the proliferation of CD4+ T cells depending on their state of activation-induced maturation.     

Cholesteryl ester transfer protein deficiency causes slow egg embryonation of Schistosoma japonicum

It was investigated whether proteasome activity was implicated in susceptibility of human vascular smooth muscle cells (VSMCs) to Fas-mediated death. Human fetal aorta smooth muscle cells were treated with agonistic anti-Fas antibody (CH11) and proteasome inhibitors (MG115 or MG132) and then cell death was determined by morphology, viability, and DNA fragmentation. The present study reports that: (a) crosslinking of Fas receptor with anti-Fas antibody in the presence of proteasome inhibitor-induced death and DNA degradation in human VSMCs that were blocked by caspases inhibitor z-DEVD.fmk; (b) cotreatment with anti-Fas antibody and proteasome inhibitor activated caspase-3; (c) proteasome inhibitors did not influence expression of procaspase-8, procaspase-3, c-FLIP, and Bcl-2; and (d) proteasome inhibitors up-regulated Fas and FADD. The data indicate that proteasome activity is important in survival of VSMCs and provide the first evidence that proteasome is involved in Fas signal transduction. The present study proposes novel mechanism(s) by which VSMCs become susceptible to FasL.     

Induction of a Th1 response from Th2-polarized T cells by activated dendritic cells: dependence on TCR:peptide-MHC interaction, ICAM-1, IL-12, and IFN-gamma

Dendritic cells (DC) are important regulators of T cell immunity. The degree of stimulation, the pattern of costimulatory molecules expressed, and the cytokines secreted by DC dictate the nature of the effector and memory cells generated, particularly with respect to their Th1 or Th2 phenotypes. In this study, we demonstrate that the addition of activated DC to spleen cultures containing established Th2-polarized CD4(+) T cells was sufficient to suppress Th2 and induce Th1 cytokines in a recall response, a phenomenon referred to as phenotype reversal. The ability of activated DC to induce phenotype reversal displayed exquisite Ag specificity. The DC activator B7-DC cross-linking Ab (XAb) was >10,000-fold more efficient at inducing phenotype reversal than the TLR agonists CpG-oligodeoxynucleotide and Gardiquimod. Characterization of the mechanisms governing phenotype reversal revealed the requirement for cognate interaction between the TCR:peptide-MHC complex, the expression of the costimulation/adhesion molecule ICAM-1, and secretion of IL-12 and IFN-gamma by the activated DC. The requirement for the costimulation/adhesion molecule SLAM (signaling lymphocytic activation molecule) was found to be quantitative. Thus, activation of DC, particularly by crosslinking B7-DC, can modulate well-established Th2 T cell responses in an Ag-specific manner. Because the regulation of mouse and human DC by B7-DC XAb overlaps in several significant ways, immune modulation with B7-DC XAb is a potential strategy for treating Th2-mediated diseases.     

Costimulation of T cell receptor/CD3-mediated activation of resting human CD4+ T cells by leukocyte function-associated antigen-1 ligand intercellular cell adhesion molecule-1 involves prolonged inositol phospholipid hydrolysis and sustained increase of intracellular Ca2+ levels.

Activation of resting human CD4+ T cells mediated by mAb ligation of the TCR/CD3 complex requires costimulatory signals to result in proliferation; these can be provided by intercellular cell adhesion molecule-1 (ICAM-1, CD54) a natural ligand of leukocyte function-associated Ag-1 (LFA-1, CD11a/CD18). We analyzed early signaling events involved in T cell activation to determine the contribution by the LFA-1/ICAM-1 interaction. We studied in detail the hydrolysis of phosphatidylinositol(4,5)bisphosphate and intracellular levels of free Ca2+ during stimulation with beads coated with the CD3 mAb OKT3 alone or in combination with purified ICAM-1 protein. Our investigations show no response to LFA-1/ICAM-1 alone, but that costimulation by LFA-1/CAM-1 interaction induces prolonged inositol phospholipid hydrolysis (up to 4 h), resulting in generation of both inositol(1,4,5)phosphate3 and inositol(1,3,4,5)phosphate4 and their derivatives. Based on studies with cycloheximide, this costimulatory effect of prolonged inositol phospholipid hydrolysis appears dependent in part on de novo protein synthesis. A sustained increase in intracellular levels of free Ca2+ level is also observed after LFA-1/ICAM-1 costimulation, which is at least partly dependent on extracellular sources of Ca2+. Kinetic studies indicate that costimulation requires a minimal period of 4 h of LFA-1/ICAM-1 interaction to provide maximal costimulation for OKT3-mediated T cell proliferation. Thus, the necessary costimulation required for OKT3-mediated proliferation in this model system may be provided by an extended LFA-1/ICAM-1 interaction that in combination with OKT3 mAb leads to signal-transducing events, resulting in prolonged phospholipase C activation and phosphatidylinositol(4,5)bisphosphate hydrolysis, and a sustained increase in intracellular levels of free Ca2+.     

Characterization of endogenous Chinese hamster ovary cell surface molecules that mediate T cell costimulation

Chinese hamster ovary (CHO) cells are commonly used in the generation of transfectants for use in in vitro costimulation assays. However, we have noted that nontransfected CHO cells can themselves provide a low-level B7/CD28 independent costimulatory signal for CD3-mediated murine T cell activation and IL-2 production. This study set out to identify those molecules that contribute to this CHO-dependent costimulatory activity. We describe a CHO subline capable of delivering potent CD28-independent costimulation to murine T cells and the generation of monoclonal antibodies against these CHO cells that inhibited this costimulatory activity. These blocking antibodies do not affect CHO cell-independent costimulation or bind mouse cells, suggesting an effect mediated by their target molecules on the costimulatory competent CHO cells. Immunoprecipitation and expression cloning revealed that these antibodies bound the hamster homologues of Crry (CD21/35), CD44, CD54 (ICAM-1), CD63, CD87, CD147, and an 80- to 90-kDa protein which could not be cloned. Expression of these hamster genes on COS cells demonstrated that hamster CD54 was able to costimulate both CD3-mediated IL-2 secretion and T cell proliferation by naive murine T cells independent of the other molecules identified.     

The critical role of protein kinase C-theta in Fas/Fas ligand-mediated apoptosis

A functional immune system not only requires rapid expansion of antigenic specific T cells, but also requires efficient deletion of clonally expanded T cells to avoid accumulation of T cells. Fas/Fas ligand (FasL)-mediated apoptosis plays a critical role in the deletion of activated peripheral T cells, which is clearly demonstrated by superantigen-induced expansion and subsequent deletion of T cells. In this study, we show that in the absence of protein kinase C-theta (PKC-theta), superantigen (staphylococcal enterotoxin B)-induced deletion of Vbeta8(+) CD4(+) T cells was defective in PKC-theta(-/-) mice. In response to staphylococcal enterotoxin B challenge, up-regulation of FasL, but not Fas, was significantly reduced in PKC-theta(-/-) mice. PKC-theta is thus required for maximum up-regulation of FasL in vivo. We further show that stimulation of FasL expression depends on PKC-theta-mediated activation of NF-AT pathway. In addition, PKC-theta(-/-) T cells displayed resistance to Fas-mediated apoptosis as well as activation-induced cell death (AICD). In the absence of PKC-theta, Fas-induced activation of apoptotic molecules such as caspase-8, caspase-3, and Bid was not efficient. However, AICD as well as Fas-mediated apoptosis of PKC-theta(-/-) T cells were restored in the presence of high concentration of IL-2, a critical factor required for potentiating T cells for AICD. PKC-theta is thus required for promoting FasL expression and for potentiating Fas-mediated apoptosis.     

c-FLICE inhibitory protein expression inhibits T-cell activation

Cellular FLICE-inhibitory protein (c-FLIP) inhibits death receptor-mediated apoptosis by specific interaction with FADD and procaspase-8, and may thus interfere with activation events mediated by FADD and caspase-8. Recent studies, however, suggest that c-FLIP also transmits activation signals. The role of c-FLIP on T-cell activation was examined here using several transgenic mice with variable c-FLIP expression. In all c-FLIP-transgenic mice, Fas-mediated apoptosis and in vitro activation-induced T-cell death were suppressed, and T-cell proliferation and IL-2 production were inhibited. c-FLIP transgene also promoted in vivo thymocyte death. Higher c-FLIP transgene expression was correlated with a more profound suppression of T-cell activation and a prominent disturbance in mature thymocyte development. There was no evidence of increased activation and proliferation in all c-FLIP-transgenic T cells examined. Instead, suppression of T-cell activation in c-FLIP-transgenic T cells could be a combinatory effect of FADD/caspase-8-dependent signals and c-FLIP-specific activities.     

Costimulatory Effect of Fas in Mouse T Lymphocytes

To induce proper immune responses, T lymphocytes require two types of stimuli, antigen-specific and costimulatory signals. Among costimulatory molecules, CD28-engagement promotes the survival and proliferation of both naive and memory T cells. In addition, it is now believed that Fas may play a role in T cell activation in the human system. It is, however, controversial whether Fas can act as a costimulatory signal in the murine system. Thus, we investigated fundamental differences in the capacity to induce proliferation of T cells between Fas and CD28 in mice. Fas-mediated T cell proliferation was observed only with a full mitogenic dose of anti-CD3 antibodies, whereas CD28 engagement was able to enhance T cell proliferation in the presence of a suboptimal level of anti-CD3 antibody. Furthermore, Fas-engaged T cells showed faster response in the upregulation of CD25 and CD69 expression than CD28-engaged ones. Here, we report that Fas might play a role in mature T cell activation in the mouse system through a different mechanism from that in CD28 costimulation.     

Costimulatory Effect of Fas in Mouse T Lymphocytes

To induce proper immune responses, T lymphocytes require two types of stimuli, antigen-specific and costimulatory signals. Among costimulatory molecules, CD28-engagement promotes the survival and proliferation of both naive and memory T cells. In addition, it is now believed that Fas may play a role in T cell activation in the human system. It is, however, controversial whether Fas can act as a costimulatory signal in the murine system. Thus, we investigated fundamental differences in the capacity to induce proliferation of T cells between Fas and CD28 in mice. Fas-mediated T cell proliferation was observed only with a full mitogenic dose of anti-CD3 antibodies, whereas CD28 engagement was able to enhance T cell proliferation in the presence of a suboptimal level of anti-CD3 antibody. Furthermore, Fas-engaged T cells showed faster response in the upregulation of CD25 and CD69 expression than CD28-engaged ones. Here, we report that Fas might play a role in mature T cell activation in the mouse system through a different mechanism from that in CD28 costimulation.