Fas-mediated Activation-induced Cell Death Research Articles

Graft-Derived IL-6 Amplifies Proliferation and Survival of Effector T Cells That Drive Alloimmune-Mediated Vascular Rejection.

IL-6 is an inflammatory cytokine that controls effector T cell responses but the mechanisms by which it controls allogeneic immune responses and vascular rejection that leads to transplant arteriosclerosis (TA) are poorly understood. We have examined the mechanism by which IL-6 contributes to the pathogenesis of vascular rejection and TA using a murine aortic interposition model of vascular rejection. The absence of IL-6 production from artery graft cells reduced the development of vascular rejection and arteriosclerotic thickening. There was no apparent effect of donor-derived IL-6 on endothelial cell integrity or on the intimal accumulation of smooth muscle cells, macrophages, and anti-donor antibodies. However, reduced vascular pathology in IL-6 artery grafts was accompanied by a significant reduction in the accumulation of CD4 and CD8 T cells. Further, the absence of graft-derived IL-6 resulted in a significant decrease in the activation and proliferation of alloreactive CD4 and CD8 T cells after transplantation as well as in a marked increase in cell death of effector T cells. Alloreactive effector T cells that expanded in the absence of IL-6 were also more susceptible to Fas-mediated activation-induced cell death in vitro. Finally, systemic neutralization of IL-6R did not reduce arteriosclerotic thickening but reduced endothelial integrity in allograft arteries, indicating differential effects of specific elimination of IL-6 in graft cells and systemic IL-6 neutralization. Donor-derived IL-6 amplifies the expansion of allogeneic T cell responses that cause vascular rejection and TA by increasing T cell proliferation and preventing Fas-mediated T cell death.

Inflammatory Cytokines Determine the Susceptibility of Human CD8 T Cells to Fas-mediated Activation-induced Cell Death through Modulation of FasL and c-FLIPS Expression

The nature of inflammatory signals determines the outcome of T cell responses. However, little is known about how inflammatory cytokines provided to human CD8 T cells during activation affects their susceptibility to post-activation cell death. We have examined and compared the effects of the inflammatory cytokine IL-12, as well as the combination of IL-1, IL-6, and IL-23 (IL-1/6/23) on the susceptibility of primary human CD8 T cells to post-activation cell death. Human CD8 T cells activated in the presence of IL-1/6/23 underwent significantly less cell death after activation as compared with those activated in IL-12. This was due to reduced susceptibility to Fas-mediated activation-induced cell death (AICD). Mechanistically, the reduced level of cell death in CD8 T cells activated in IL-1/6/23 was a result of a low level of FasL expression and high level of c-FLIP(S) expression. When the effect of IL-1, IL-6, and IL-23 individually was examined, IL-1 or IL-6 alone was sufficient to inhibit CD8 T cell death that occurs after activation in IL-12. IL-1, but not IL-6, inhibited expression of FasL, whereas IL-6, but not IL-1, increased c-FLIP(S) expression. Our findings show that the presence of IL-1 and/or IL-6 during activation of human CD8 T cells attenuates Fas-mediated AICD, whereas IL-12 increases the susceptibility of activated CD8 T cells to this form of cell death.

Inflammatory cytokines determine the susceptibility of human CD8 T cells to Fas-mediated activation-induced cell death through modulation of FasL and c-FLIP expression (46.4)

Abstract The nature of inflammatory signals determines the outcome of T cell responses. However, little is known about how inflammatory cytokines provided to human CD8 T cells during activation affect their susceptibility to post-activation cell death. We have examined and compared the effects of the inflammatory cytokines IL-12, as well as the combination of IL-1, IL-6, and IL-23 (IL-1/6/23) on the susceptibility of primary human CD8 T cells to post-activation cell death. Human CD8 T cells activated in the presence of IL-1/6/23 underwent significantly less cell death after activation as compared to those activated in IL-12. This was due to reduced susceptibility to Fas-mediated activation-induced cell death (AICD). CD8 T cells activated in IL-1/6/23 expressed less FasL and more c-FLIPs as compared to those activated in IL-12, and were also less sensitive to cell death induced by a Fas agonistic antibody. Inhibition of c-FLIP expression by siRNA increased cell death of CD8 T cells activated in both conditions. When the effect of IL-1, IL-6, and IL-23 individually was examined, IL-1 or IL-6 alone was sufficient to inhibit CD8 T cell death that occurs after activation in IL-12 while the effect of IL-23 was variable. Our findings show that the presence of IL-1 and/or IL-6 during activation of human CD8 T cells attenuates Fas-mediated AICD while IL-12 increases the susceptibility of activated CD8 T cells to this form of cell death.

Susceptibility to Fas and tumor necrosis factor‐α receptor mediated apoptosis of anti‐CD3/anti‐CD28‐activated umbilical cord blood T cells

Decreased severity of graft-versus-host disease after mismatched umbilical cord blood (UCB) transplantation may be attributed in part to the increased propensity to apoptosis of UCB T cells following activation. Interleukin (IL)-15, a pleiotropic cytokine that is essential for T-cell proliferation and survival, may serve as promising immunomodulative therapy post-CB transplantation for its anti-apoptotic effect. This study aimed to determine the kinetics of Fas or tumor necrosis factor-alpha receptor (TNFR) mediated caspase-3 expression and apoptosis of anti-CD3/anti-CD28 activated UCB T cells in the influence of IL-15. Activated caspase-3 expression was analyzed by Western blotting and the percentage of apoptotic cells was determined by annexin-V/propidium iodide (PI) flow cytometric staining. Significant expression of Fas and TNFR2 was detected on anti-CD3/anti-CD28 pre-activated UCB T cells. These cells were susceptible to anti-Fas but not TNF-alpha-induced apoptosis. Kinetic study shows that caspase-3 expression became evident at 6th-8th h following anti-Fas stimulation, while early apoptotic cells with annexin-V(+)/PI(-) expression appeared at 12th-16th h. IL-15, though successful in decreasing apoptosis in pre-activated UCB T cells, failed to completely prevent Fas-mediated caspase-3 expression and apoptosis of CB T cells. The pre-activated UCB and adult peripheral blood T cells behaved similarly with regard to death receptor expression, caspase-3 expression and apoptosis upon Fas-engagement. Although IL-15 promotes overall activated UCB T-cell survival, it did not particularly prevent Fas-mediated activation-induced cell death.

Interleukin‐2 in the development and control of inflammatory disease

Interleukin-2 (IL-2) has multiple, sometimes opposing, functions during an inflammatory response. It is a potent inducer of T-cell proliferation and T-helper 1 (Th1) and Th2 effector T-cell differentiation and provides T cells with a long-lasting competitive advantage resulting in the optimal survival and function of memory cells. In a regulatory role, IL-2 is important for the development, survival, and function of regulatory T cells, it enhances Fas-mediated activation-induced cell death, and it inhibits the development of inflammatory Th17 cells. Thus, in its dual and contrasting functions, IL-2 contributes to both the induction and the termination of inflammatory immune responses.

Molecular profiling of LGL leukemia reveals role of sphingolipid signaling in survival of cytotoxic lymphocytes

AbstractT-cell large granular lymphocyte (LGL) leukemia is characterized by clonal expansion of CD3+CD8+ cells. Leukemic LGLs correspond to terminally differentiated effector-memory cytotoxic T lymphocytes (CTLs) that escape Fas-mediated activation-induced cell death (AICD) in vivo. The gene expression signature of peripheral blood mononuclear cells from 30 LGL leukemia patients showed profound dysregulation of expression of apoptotic genes and suggested uncoupling of activation and apoptotic pathways as a mechanism for failure of AICD in leukemic LGLs. Pathway-based microarray analysis indicated that balance of proapoptotic and antiapoptotic sphingolipid-mediated signaling was deregulated in leukemic LGLs. We further investigated sphingolipid pathways and found that acid ceramidase was constitutively overexpressed in leukemic LGLs and that its inhibition induced apoptosis of leukemic LGLs. We also showed that S1P5 is the predominant S1P receptor in leukemic LGLs, whereas S1P1 is down-regulated. FTY720, a functional antagonist of S1P-mediated signaling, induced apoptosis in leukemic LGLs and also sensitized leukemic LGLs to Fas-mediated death. Collectively, these results show a role for sphingolipid-mediated signaling as a mechanism for long-term survival of CTLs. Therapeutic targeting of this pathway, such as use of FTY720, may have efficacy in LGL leukemia.

Th17 Cells Undergo Fas-Mediated Activation-Induced Cell Death Independent of IFN-γ

IL-17-secreting CD4+ T cells (Th17 cells) play a critical role in immune responses to certain infections and in the development of many autoimmune disorders. The mechanisms controlling homeostasis in this cell population are largely unknown. In this study, we show that murine Th17 cells undergo rapid apoptosis in vitro upon restimulation through the TCR. This activation-induced cell death (AICD), a common mechanism for elimination of activated T cells, required the Fas and FasL interaction: Fas was stably expressed, while FasL was up-regulated upon TCR reactivation of Th17 cells; Ab ligation of Fas induced Th17 cell death; and AICD was completely absent in Th17 cells differentiated from gld/gld CD4+ T cells. Thus, the Fas/FasL pathway is essential in regulating the AICD of Th17 cells. Interestingly, IFN-gamma, a cytokine previously found to be important for the AICD of T cells, did not affect Th17 cell apoptosis. Furthermore, Th17 cells derived from mice deficient in IFN-gamma receptor 1 (IFN-gammaR1-/-) underwent AICD similar to wild-type cells. Thus, AICD of Th17 cells occurs via the Fas pathway, but is independent of IFN-gamma.

CD4 regulation of activation‐induced cell death

Defects in FAS‐mediated activation induced cell death (AICD) are associated with the development of autoimmunity in both mouse models and human disease. We have previously found that mature T cells with a mutant CD4 co‐receptor unable to associate with Lck are resistant to AICD. Since activated CD4 mutant T cells express similar levels of FasL, Fas, and FADD compared to wild‐type cells, we hypothesize that the CD4‐dependent signaling pathways provide a “competency” signal required by T cells to become susceptible to AICD. In support of this hypothesis, mixing experiments indicate that FasL expression by wild type CD4 T cell blasts could not rescue the defect in AICD seen in CD4 mutant T cells. Additionally, we find that the AICD resistant phenotype in CD4 mutant T cells is primarily observed following peptide restimulation, while wild‐type and mutant CD4 T cells are similarly sensitive to AICD upon re‐stimulation with plate bound anti‐CD3. These experiments suggest that death receptor signaling pathways are compromised in mutant CD4 T cells following physiologic stimulation with peptide that involves CD4 signaling. Since CD4 association with Lck alters the compartmentalization of cellular Lck pools, these data suggest that that distinct pools of Lck may be differentially involved in modulating sensitivity to Fas signaling.Supported in part by a grant from the American Cancer Society.

Familial NK Cell Deficiency Associated with Impaired IL-2- and IL-15-Dependent Survival of Lymphocytes

We previously reported the clinical phenotype of two siblings with a novel inherited developmental and immunodeficiency syndrome consisting of severe intrauterine growth retardation and the impaired development of specific lymphoid lineages, including transient CD8 alphabeta T lymphopenia and a persistent lack of blood NK cells. We describe here the elucidation of a plausible underlying pathogenic mechanism, with a cellular phenotype of impaired survival of both fresh and herpesvirus saimiri-transformed T cells, in the surviving child. Clearly, NK cells could not be studied. However, peripheral blood T lymphocytes displayed excessive apoptosis ex vivo. Moreover, the survival rates of CD4 and CD8 alphabeta T cell blasts generated in vitro, and herpesvirus saimiri-transformed T cells cultured in vitro, were low, but not nil, following treatment with IL-2 and IL-15. In contrast, Fas-mediated activation-induced cell death was not enhanced, indicating a selective excess of cytokine deprivation-mediated apoptosis. In keeping with the known roles of IL-2 and IL-15 in the development of NK and CD8 T cells in the mouse model, these data suggest that an impaired, but not abolished, survival response to IL-2 and IL-15 accounts for the persistent lack of NK cells and the transient CD8 alphabeta T lymphopenia documented in vivo. Impaired cytokine-mediated lymphocyte survival is likely to be the pathogenic mechanism underlying this novel form of inherited and selective NK deficiency in humans.

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.

Exogenous IL-7 induces Fas-mediated human neuronal apoptosis: potential effects during human immunodeficiency virus type 1 infection

The use of exogenous cytokines is part of translational immune-antiretroviral approaches to induce immune reconstitution and possibly eliminate the persistence of human immunodeficiency virus type 1 (HIV-1) in virally suppressed infected individuals on highly active antiretroviral therapy (HAART). Recently, our laboratories demonstrated that interleukin-7 (IL-7) has significant efficiency in stimulating HIV-1 replication from proviral latency in CD4+ T lymphocytes of infected patients. The authors now investigated the possible role of IL-7 in HIV-1-associated dementia (HAD). The authors demonstrated that the IL-7 receptor is expressed on both human neurons (i.e., differentiated NT2 cells) and human astrocytes, with relatively higher mRNA levels in neurons. The translational protein levels of IL-7 receptor alpha were not proportional to those of the mRNA levels in these central nervous system (CNS)-based cell types. Exogenous IL-7 was observed to only slightly down-regulate IL-7 receptor alpha expression on both neurons and astrocytes, as assayed by Western blotting. Instead of promoting survival, surprisingly, exogenous IL-7 induced neuronal apoptosis, as detected by TUNEL assays. Furthermore, IL-7 augmented neuronal apoptosis induced by HIV-1 gp120. Human apoptosis genomic microarray analyses of IL-7-treated human neurons showed up-regulated expression of proapoptotic genes: protein kinases, caspase-10, FAST kinase, tumor necrosis factor (TNF) receptor, and BCL2-antagonist of cell death. These data suggest that IL-7 leads to neuronal apoptosis by a molecular mechanism(s) that occurs via Fas-mediated activation-induced cell death. These studies may therefore not only be key in evaluating the potential use of IL-7 in vivo as a therapeutic modality, but also suggest that IL-7, which is increased endogenously in HIV-1-infected individuals late in disease, may be involved in the neuronal apoptosis demonstrated during HAD.

The response of human T lymphocytes against alloantigens following Fas-mediated activation-induced cell death

We examined the response of T lymphocytes activated with specific alloantigens following Fas-mediated apoptosis; using a mixed lymphocyte culture (MLC) system. Cells obtained from an MLC after 6 or 7 days of culture were incubated for are additional 24 hours in the presence or absence of the agonistic monoclonal antibody (MoAb), 7C11, or the antagonistic MoAb, ZB4. We assessed DNA fragmentation/specific cytotoxiy of the MoAb-treated cells. Cells harvested after 4 days of culture were sensitive to apoptosis induced by 7C11 with maximum DNA fragmentation observed on day 6. ZB4 slightly inhibited apoptosis of the cells compared with controls. The simultaneous addition of recombinant interleukin-2 (rIL-2) with the MoAbs significantly inhibited DNA fragmentation in control and ZB4-treated cells, but had little effect on the 7C11-treated cells. Control and ZB4-treated MLC cells showed cytotoxic activities against specific target cells, namely >10%. In contrast, the 7C11-treated cells showed <5% cytotoxicity. Although the addition of rIL-2 increased specific percentage cytotoxicity of control and ZB4-treated cells, it had little effect on the specific cytotoxic activity of the 7C11-treated MLC cells. These results suggest that specific cytotoxic T lymphocytes may be eliminated via apoptosis mediated by the Fas/Fas ligand system.

Defective lymphocyte caspase-3 expression in type 1 diabetes mellitus

Activation-induced cell death (AICD) is a major mechanism in the regulation of peripheral tolerance and its impairment can determine the development of autoimmunity. In the present study, in order to evaluate the role of caspase-3 in type 1 diabetes mellitus (T1DM) AICD, caspase-3 expression was analyzed in peripheral blood lymphocytes from 37 new onset T1DM patients and from 36 normal control subjects (NC) in resting conditions and after anti-Fas-triggered apoptosis. Caspase-3 expression was determined by semiquantitative RT-PCR and Western blot. Apoptosis was induced in activated lymphocytes by anti-Fas monoclonal antibody and quantified by flow cytometry and morphological analysis. Caspase-3 mRNA expression was reduced in resting lymphocytes in 18/37 T1DM patients and in 1/36 NC (P < 0.01). Patients studied for both Fas-mediated AICD and caspase-3 mRNA expression revealed that a reduced caspase-3 mRNA expression in resting lymphocytes occurred in all patients showing resistance to Fas-mediated apoptosis (T1DM vs NC, P < 0.02) with the exception of 3 patients who exhibited normal caspase-3 expression levels. Caspase-3 protein analysis confirmed mRNA data and showed an impaired expression of caspase-3 active form in T1DM subjects compared with NC. Our data show that defective expression and function of caspase-3 in peripheral lymphocytes of T1DM patients may contribute to the development of AICD resistance in type 1 diabetes.

Overexpression of heme oxygenase (HO)-1 renders jurkat T cells resistant to Fas-mediated apoptosis: involvement of iron released by HO-1

We recently demonstrated that heme oxygenase (HO)-1 is constitutively expressed in human CD4 +CD25 + regulatory T cells and induced by anti-CD28 or anti-CD28/anti-CD3 stimulation, even in CD4 +CD25 − responder T cells. To study the effects of HO-1 expression on lymphocyte survival, we transfected the HO-1 gene or induced the gene to express HO-1 protein with cobalt protoporphyrin (CoPP) in Jurkat T cells. Consistently, anti-Fas antibody triggered apoptotic cell death in wild-type Jurkat T cells. Surprisingly, however, HO-1-overexpressing Jurkat T cells showed strong resistance to Fas-mediated apoptosis. In contrast, abrogation of HO-1 expression by antisense oligomer against HO-1 gene from CoPP-treated cells or depletion of iron by desferrioxamine from HO-1-transfected cells abolished the resistance. In addition, exogenously added iron rendered wild-type Jurkat T cells resistant. The resistance involved IκB kinase (IKK) activation via iron-induced reactive oxygen species formation, NF-κB activation by activated IKK, and c-FLIP expression by activated NF-κB. Primary CD4 + T cells induced by CoPP to express HO-1 also showed more resistance to Fas-mediated apoptosis than untreated cells. Our findings suggest that HO-1 plays a critical and nonredundant role in Fas-mediated activation-induced cell death of T lymphocytes.

Lack of Role for CsA‐Sensitive or Fas Pathways in the Tolerization of CD4 T Cells Via BMT and Anti‐CD40L

Anti-CD40L mAb plus bone marrow transplantation (BMT) and recipient CD8 T-cell depletion permits long-term mixed hematopoietic chimerism and systemic donor-specific tolerance to be achieved across full MHC barriers. Initial tolerance is characterized by peripheral deletion of donor-reactive CD4 cells. In regimens using costimulatory blockade without BMT to achieve allograft survival, cyclosporine inhibited graft survival, suggesting that the combination may not be clinically applicable. We assessed the role of cyclosporine-sensitive mechanisms and the mechanisms of T-cell apoptosis involved in the induction of early peripheral CD4+ T-cell tolerance by BMT with anti-CD40L. Neither a short course of cyclosporine (14 days) nor the absence of FAS-mediated activation-induced cell death (AICD) blocked the induction or maintenance of donor-specific tolerance. IL-2 production was not associated with tolerance induction, consistent with the lack of a role for Fas-mediated AICD. Mice in which passive T-cell death was impaired because of constitutive expression of a Bcl-xL transgene did not develop tolerance with this protocol. These data confirm that deletion of donor-reactive T cells is critical for the induction of mixed chimerism and tolerance. However, the mechanisms involved may differ from those involved in costimulatory blockade regimens that do not include BMT.

The effects of immunosuppressants on FAS-mediated activation-induced cell death in human T lymphocytes

Abstract The effects of cyclosporin A (CsA) and methylprednisolone (MP) on Fas-mediated activation-induced cell death (FMAICD) of T lymphocytes were examined. T lymphocytes were activated with the immobilized anti-CD 3 and CD 28 monoclonal antibodies (MoAbs) (activation phase) and incubated further with the agonistic MoAb against Fas (death phase). Cell proliferation and DNA fragmentation were measured by XTT and diphenylamine assay. CsA in the activation phase inhibited DNA fragmentation mediated by anti-Fas MoAb but not MP. The combination of CsA and MP at the lower concentrations had little effect on FMAICD, although they had similar degrees of suppression on T lymphocyte proliferation as the maximum obtained by CsA or MP alone. In the death phase, MP induced apoptosis without 7C11 and CsA had no effects. These results indicate that the combination of CsA and MP at low concentrations could maintain FMAICD with the suppression on T lymphocyte proliferation.

The effects of immunosuppressants on FAS-mediated activation-induced cell death in human T lymphocytes.

The effects of cyclosporin A (CsA) and methylprednisolone (MP) on Fas-mediated activation-induced cell death (FMAICD) of T lymphocytes were examined. T lymphocytes were activated with the immobilized anti-CD 3 and CD 28 monoclonal antibodies (MoAbs) (activation phase) and incubated further with the agonistic MoAb against Fas (death phase). Cell proliferation and DNA fragmentation were measured by XTT and diphenylamine assay. CsA in the activation phase inhibited DNA fragmentation mediated by anti-Fas MoAb but not MP. The combination of CsA and MP at the lower concentrations had little effect on FMAICD, although they had similar degrees of suppression on T lymphocyte proliferation as the maximum obtained by CsA or MP alone. In the death phase, MP induced apoptosis without 7C11 and CsA had no effects. These results indicate that the combination of CsA and MP at low concentrations could maintain FMAICD with the suppression on T lymphocyte proliferation.

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.

CD8+ tumor-infiltrating lymphocytes are primed for Fas-mediated activation-induced cell death but are not apoptotic in situ.

Induction of Fas-mediated activation-induced cell death in antitumor T cells has been hypothesized to permit tumor escape from immune destruction. Several laboratories have proposed that expression of Fas ligand (L) by tumor is the basis for this form of T cell tolerance. In this study, we characterized murine tumor-infiltrating lymphocytes (TIL) for activation status, cell cycle status, level of apoptosis, cytokine secretion, and proliferative capacity. TILs express multiple activation markers (circa CD69, CD95L, CD122, and LFA-1) and contain IL-2 and IFN-gamma mRNAs, but are neither cycling nor apoptotic in situ. In addition, TIL are dramatically suppressed in proliferative response and do not secrete IL-2 and IFN-gamma. However, upon purification and activation in vitro, TIL secrete high levels of IL-2 and IFN-gamma, enter S phase, and then die by Fas-mediated apoptosis. Activation by injection of anti-TCR Ab or IL-2 into tumor-bearing mice induced TIL entrance into S phase preceding apoptosis, showing that TIL have functional TCR-mediated signal transduction in situ. Our data demonstrate that TIL, not tumor, express both Fas and FasL, are arrested in G(1), do not secrete cytokine in situ, and, upon activation in vitro and in vivo, rapidly die by activation-induced cell death.

Early Cellular Events in Systemic Autoimmunity Driven by Chromatin-Reactive T Cells

In vivo exposure of the thymus of normal mice to procainamide–hydroxylamine, a lupus-inducing drug, causes development of chromatin-reactive T cells. Autoantibodies subsequently appear, but their origin and significance are unknown. The current studies were undertaken to determine the specificities of B cells that respond to chromatin-reactive T cells at the initiation of this autoimmune process. Three days after adoptive transfer of 6 × 106 chromatin-reactive T cells, B cells with the capacity to secrete IgM anti-chromatin antibodies were detected in 1/106 splenocytes, and these became 10- to 50-fold more numerous if either the donor T cells or the recipient had defective Fas due to the lpr allele. Five days later these mice developed IgG anti-chromatin-secreting B cells at a precursor frequency of 3–6 × 10−5. B cells with dDNA-binding activity isolated from mice primed in vivo to a complex of methylated pigeon cytochrome c and dDNA could stimulate naive, cytochrome c-reactive T cells in vitro, demonstrating that B cells can internalize dDNA-bound proteins through their dDNA immunoblobulin receptor and can functionally present a T cell epitope. However, no capacity of chromatin for binding anti-dDNA antibodies was detected, and IgM dDNA-specific B cells did not expand when challenged with chromatin-reactive T cells in vivo. The rapid and robust expansion of anti-chromatin-secreting B cells indicates that the normal immune repertoire includes nontolerant autoreactive B cells that respond to strong T cell drive and are readily manifested if Fas-mediated activation-induced cell death is inhibited.