Precursor B cells for autoantibody production in genomically Fas-intact autoimmune disease are not subject to Fas-mediated immune elimination

The Fas/Fas ligand (FasL) system participates in regulation of the immune system through the apoptotic process. However, the extent to which abnormalities in this system are involved in the loss of self-tolerance and development of autoimmune disease not associated with Fas/FasL mutations remains unknown. The present study addresses this issue in Fas/FasL-intact, systemic lupus erythematosus (SLE)-prone (NZB × NZW) (NZB/W) F1 mice. While splenic B cells from 2-month-old mice before overt SLE expressed Fas poorly, in vitro stimulation with an agonistic anti-CD40 mAb up-regulated their Fas expression, thus revealing the existence of two populations: one was Fashigh and highly susceptible to anti-Fas mAb-induced apoptosis, and the other was Faslow and apoptosis-resistant. The Faslow cells were included in the CD5+ B cell subpopulation and contained most of the cells that produced IgM anti-DNA antibodies. The isotype of anti-DNA antibodies switches from IgM to IgG in NZB/W F1 mice at ages beginning at about 6 months. These IgG anti-DNA antibodies were produced almost exclusively by a subpopulation of splenic B cells that spontaneously expressed low levels of Fas in vivo and were apoptosis-resistant. The findings indicate that precursor B cells for autoantibody production and presumably autoantibody-secreting cells in these mice are relatively resistant to Fas-mediated apoptosis, a finding supporting the concept that abnormalities of Fas-mediated apoptotic process are involved in the development of autoreactive B cells in Fas/FasL-intact autoimmune disease.

Activation of distinct caspase-like proteases by Fas and reaper in Drosophila cells

The cytoplasmic region of Fas, a mammalian death factor receptor, shares a limited homology with reaper, an apoptosis-inducing protein in Drosophila. Expression of either the Fas cytoplasmic region (FasC) or of reaper in Drosophila cells caused cell death. The death process induced by FasC or reaper was inhibited by crmA or p35, suggesting that its death process is mediated by caspase-like proteases. Both Ac-YVAD aldehyde and Ac-DEVD aldehyde, specific inhibitors of caspase 1- and caspase 3-like proteases, respectively, inhibited the FasC-induced death of Drosophila cells. However, the cell death induced by reaper was inhibited by Ac-DEVD aldehyde, but not by Ac-YVAD aldehyde. A caspase 1-like protease activity that preferentially recognizes the YVAD sequence gradually increased in the cytosolic fraction of the FasC-activated cells, whereas the caspase 3-like protease activity recognizing the DEVD sequence was observed in the reaper-activated cells. Partial purification and biochemical characterization of the proteases indicated that there are at least three distinct caspase-like proteases in Drosophila cells, which are differentially activated by FasC and reaper. The conservation of the Fas-death signaling pathway in Drosophila cells, which is distinct from that for reaper, may indicate that cell death in Drosophila is controlled not only by the reaper suicide gene, but also by a Fas-like killer gene.

Fas-induced apoptosis of T cells occurs independently of ceramide generation

The Fas receptor is one of a number of important physiological inducers of programmed cell death (apoptosis). Current models for regulation of this process involve rapid conversion of sphingomyelin to ceramide by cellular sphingomyelinases. Induced changes in cellular levels of such sphingosine-based ceramides are normally extrapolated from measurements of sphingomyelinase activity or following their conversion to ceramide phosphate by treatment of cellular lipid extracts with bacterial diacylglycerol kinase (DAGK). To allow direct study of cellular sphingosine- and sphinganine-based ceramide levels, we developed a mass spectrometric technique capable of determining inducible changes in both overall ceramide levels and species distribution in cellular lipid preparations. Contrary to current models, we detected no changes in cellular ceramide levels up to 2 hr poststimulation of Jurkat T cells with an anti-Fas IgM, although this treatment did induce apoptosis. We also determined in the same system that, when utilizing the DAGK assay, increased phosphorylation of substrates that comigrated with ceramide standards was apparent but that this effect was due to an enhancement of DAGK activity rather than increases in levels of cellular ceramides as substrates per se. Thus, the first direct measurement of ceramides present in cells undergoing apoptosis indicates that, insofar as it can be measured, the induction of apoptosis does not involve the generation of sphingosine-based ceramides, contrary to many published accounts.

Stimulation of CD95 (Fas) blocks T lymphocyte calcium channels through sphingomyelinase and sphingolipids

Calcium influx through store-operated calcium release-activated calcium channels (CRAC) is required for T cell activation, cytokine synthesis, and proliferation. The CD95 (Apo-1/Fas) receptor plays a role in self-tolerance and tumor immune escape, and it mediates apoptosis in activated T cells. In this paper we show that CD95-stimulation blocks CRAC and Ca2+ influx in lymphocytes through the activation of acidic sphingomyelinase (ASM) and ceramide release. The block of Ca2+ entry is lacking in CD95-defective lpr lymphocytes as well as in ASM-defective cells and can be restored by retransfection of ASM. C2 ceramide, C6 ceramide, and sphingosine block CRAC reversibly, whereas the inactive dihydroceramide has no effect. CD95-stimulation or the addition of ceramide prevents store-operated Ca2+ influx, activation of the transcriptional regulator NFAT, and IL-2 synthesis. The block of CRAC by sphingomyelinase metabolites adds a function to the repertoire of the CD95 receptor inhibiting T cell activation signals.

Activation of Fas by FasL induces apoptosis by a mechanism that cannot be blocked by Bcl-2 or Bcl-xL

Fas activation triggers apoptosis in many cell types. Studies with anti-Fas antibodies have produced conflicting results on Fas signaling, particularly the role of the Bcl-2 family in this process. Comparison between physiological ligand and anti-Fas antibodies revealed that only extensive Fas aggregation, by membrane bound FasL or aggregated soluble FasL consistently triggered apoptosis, whereas antibodies could act as death agonists or antagonists. Studies on Fas signaling in cell lines and primary cells from transgenic mice revealed that FADD/MORT1 and caspase-8 were required for apoptosis. In contrast, Bcl-2 or Bcl-xL did not block FasL-induced apoptosis in lymphocytes or hepatocytes, demonstrating that signaling for cell death induced by Fas and the pathways to apoptosis regulated by the Bcl-2 family are distinct.

Caspase-3 controls both cytoplasmic and nuclear events associated with Fas-mediated apoptosis in vivo

Both caspase-1- and caspase-3-like activities are required for Fas-mediated apoptosis. However, the role of caspase-1 and caspase-3 in mediating Fas-induced cell death is not clear. We assessed the contributions of these caspases to Fas signaling in hepatocyte cell death in vitro. Although wild-type, caspase-1−/−, and caspase-3−/− hepatocytes were killed at a similar rate when cocultured with FasL expressing NIH 3T3 cells, caspase-3−/− hepatocytes displayed drastically different morphological changes as well as significantly delayed DNA fragmentation. For both wild-type and caspase-1−/− apoptotic hepatocytes, typical apoptotic features such as cytoplasmic blebbing and nuclear fragmentation were seen within 6 hr, but neither event was observed for caspase-3−/− hepatocytes. We extended these studies to thymocytes and found that apoptotic caspase-3−/− thymocytes exhibited similar “abnormal” morphological changes and delayed DNA fragmentation observed in hepatocytes. Furthermore, the cleavage of various caspase substrates implicated in mediating apoptotic events, including gelsolin, fodrin, laminB, and DFF45/ICAD, was delayed or absent. The altered cleavage of these key substrates is likely responsible for the aberrant apoptosis observed in both hepatocytes and thymocytes deficient in caspase-3.

Molecular ordering of the Fas-apoptotic pathway: The Fas/APO-1 protease Mch5 is a CrmA-inhibitable protease that activates multiple Ced-3/ICE-like cysteine proteases

The Fas/APO-1-receptor associated cysteine protease Mch5 (MACH/FLICE) is believed to be the enzyme responsible for activating a protease cascade after Fas-receptor ligation, leading to cell death. The Fas-apoptotic pathway is potently inhibited by the cowpox serpin CrmA, suggesting that Mch5 could be the target of this serpin. Bacterial expression of proMch5 generated a mature enzyme composed of two subunits, which are derived from the precursor proenzyme by processing at Asp-227, Asp-233, Asp-391, and Asp-401. We demonstrate that recombinant Mch5 is able to process/activate all known ICE/Ced-3-like cysteine proteases and is potently inhibited by CrmA. This contrasts with the observation that Mch4, the second FADD-related cysteine protease that is also able to process/activate all known ICE/Ced-3-like cysteine proteases, is poorly inhibited by CrmA. These data suggest that Mch5 is the most upstream protease that receives the activation signal from the Fas-receptor to initiate the apoptotic protease cascade that leads to activation of ICE-like proteases (TX, ICE, and ICE-relIII), Ced-3-like proteases (CPP32, Mch2, Mch3, Mch4, and Mch6), and the ICH-1 protease. On the other hand, Mch4 could be a second upstream protease that is responsible for activation of the same protease cascade in CrmA-insensitive apoptotic pathways.

Fas-dependent CD4+ cytotoxic T-cell-mediated pathogenesis during virus infection

β2-Microglobulin-deficient (β2m−) mice generate a CD4+ major histocompatibility complex class II-restricted cytotoxic T-lymphocyte (CTL) response following infection with lymphocytic choriomeningitis (LCM) virus (LCMV). We have determined the cytotoxic mechanism used by these CD4+ CTLs and have examined the role of this cytotoxic activity in pathogenesis of LCM disease in β2m− mice. Lysis of LCMV-infected target cells by CTLs from β2m− mice is inhibited by addition of soluble Fas-Ig fusion proteins or by pretreatment of the CTLs with the protein synthesis inhibitor emetine. In addition, LCMV-infected cell lines that are resistant to anti-Fas-induced apoptosis are refractory to lysis by these virus-specific CD4+ CTLs. These data indicate that LCMV-specific CD4+ CTLs from β2m− mice use a Fas-dependent lytic mechanism. Intracranial (i.c.) infection of β2m− mice with LCMV results in loss of body weight. Fas-deficient β2m−.lpr mice develop a similar wasting disease following i.c. infection. This suggests that Fas-dependent cytotoxicity is not required for LCMV-induced weight loss. A potential mediator of this chronic wasting disease is tumor necrosis factor (TNF)-α, which is produced by LCMV-specific CD4+ CTLs. In contrast to LCMV-induced weight loss, lethal LCM disease in β2m− mice is dependent on Fas-mediated cytotoxicity. Transfer of immune splenocytes from LCMV-infected β2m− mice into irradiated infected β2m− mice results in death of recipient animals. In contrast, transfer of these splenocytes into irradiated infected β2m−.lpr mice does not cause death. Thus a role for CD4+ T-cell-mediated cytotoxicity in virus-induced immunopathology has now been demonstrated.

The dual functions of Fas ligand in the regulation of peripheral CD8+ and CD4+ T cells

Although Fas ligand (FasL) is well characterized for its capacity to deliver a death signal through its receptor Fas, recent work demonstrates that FasL also can receive signals facilitating antigen (Ag)-specific proliferation of CD8+ T cells. The fact that the gld mutation differentially influences the proliferative capacity of CD8+ and CD4+ T cells presented the intriguing possibility that a single molecule may play opposing roles in these two subpopulations. The present study focuses on how these positive and negative regulatory roles are balanced. We show that naive CD4+ T cells are responsive to FasL-mediated costimulation on encounter with Ag when Fas-mediated death is prevented. Thus, the machinery responsible for transducing the FasL positive reverse signal operates in both CD4+ and CD8+ T cells. Instead, differential control of FasL expression distinguishes the role of FasL in these two T cell subpopulations. FasL costimulation occurs immediately on T cell receptor ligation and correlates with the up-regulation of FasL expression on CD8+ and naive CD4+ T cells, both of which are sensitive to the FasL costimulatory signal. Conversely, FasL-initiated death occurs late in an immune response when high levels of FasL expression are maintained on CD4+ T cells that are sensitive to Fas-mediated death, but not on CD8+ T cells that are relatively insensitive to this signal. This careful orchestration of FasL expression during times of susceptibility to costimulation and conversely, to death, endows FasL with the capacity to both positively and negatively regulate the peripheral T cell compartment.

A mouse CD8 T cell-mediated acute autoimmune diabetes independent of the perforin and Fas cytotoxic pathways: Possible role of membrane TNF

Double transgenic mice [rat insulin promoter (RIP)-tumor necrosis factor (TNF) and RIP-CD80] whose pancreatic β cells release TNF and bear CD80 all develop an acute early (6 wk) and lethal diabetes mediated by CD8 T cells. The first ultrastructural changes observed in β cells, so far unreported, are focal lesions of endoplasmic reticulum swelling at the points of contact with islet-infiltrating lymphoblasts, followed by cytoplasmic, but not nuclear, apoptosis. Such double transgenic mice were made defective in either the perforin, Fas, or TNF pathways. Remarkably, diabetes was found to be totally independent of perforin and Fas. Mice lacking TNF receptor (TNFR) II had no or late diabetes, but only a minority had severe insulitis. Mice lacking the TNF-lymphotoxin (LTα) locus (whose sole source of TNF are the β cells) all had insulitis comparable to that of nondefective mice, but no diabetes or a retarded and milder form, with lesions suggesting different mechanisms of injury. Because both TNFR II and TNF-LTα mutations have complex effects on the immune system, these data do not formally incriminate membrane TNF as the major T cell mediator of this acute autoimmune diabetes; nevertheless, in the absence of involvement of the perforin or Fas cytotoxic pathways, membrane TNF appears to be the likeliest candidate.

Gene transfer of Fas ligand induces tumor regression in vivo

The Fas–Fas ligand (FasL) system plays an important role in the induction of lymphoid apoptosis and has been implicated in the suppression of immune responses. Herein, we report that gene transfer of FasL inhibits tumor cell growth in vivo. Although such inhibition is expected in Fas+ tumor cell lines, marked regression was unexpectedly observed after FasL gene transfer into the CT26 colon carcinoma that does not express Fas. Infection by an adenoviral vector encoding FasL rapidly eliminated tumor masses in the Fas+ Renca tumor by inducing cell death, whereas the elimination of Fas− CT26 cells was mediated by inflammatory cells. Analysis of human malignancies revealed Fas, but not FasL, expression in a majority of tumors and susceptibility to FasL in most Fas+ cell lines. These findings suggest that gene transfer of FasL generates apoptotic responses and induces potent inflammatory reactions that can be used to induce the regression of malignancies.

An NO derivative of ursodeoxycholic acid protects against Fas-mediated liver injury by inhibiting caspase activity

Caspases are key mediators in liver inflammation and apoptosis. In the present study we provide evidence that a nitric oxide (NO) derivative of ursodeoxycholic acid (UDCA), NCX-1000 ([2-(acetyloxy)benzoic acid 3-(nitrooxymethyl)phenyl ester]), protects against liver damage in murine models of autoimmune hepatitis induced by i.v. injection of Con A or a Fas agonistic antibody, Jo2. Con A administration causes CD4+ T lymphocytes to accumulate in the liver and up-regulates FasL expression, resulting in FasL-mediated cytotoxicity. Cotreating mice with NCX-1000, but not with UDCA, protected against liver damage induced by Con A and Jo2, inhibited IL-1β, IL-18, and IFN-γ release and caspase 3, 8, and 9 activation. Studies on HepG2 cells demonstrated that NCX-1000, but not UDCA, directly prevented multiple caspase activation induced by Jo2. Incubating HepG2 cells with NCX-1000 resulted in intracellular NO formation and a DTT-reversible inhibition of proapoptotic caspases, suggesting that cysteine S-nitrosylation was the main mechanism responsible for caspase inhibition. Collectively, these data suggest that NCX-1000 protects against T helper 1-mediated liver injury by inhibiting both the proapoptotic and the proinflammatory branches of the caspase superfamily.

CD95/Fas induces cleavage of the GrpL/Gads adaptor and desensitization of antigen receptor signaling

The balance between cell survival and cell death is critical for normal lymphoid development. This balance is maintained by signals through lymphocyte antigen receptors and death receptors such as CD95/Fas. In some cells, ligating the B cell antigen receptor can protect the cell from apoptosis induced by CD95. Here we report that ligation of CD95 inhibits antigen receptor-mediated signaling. Pretreating CD40-stimulated tonsillar B cells with anti-CD95 abolished B cell antigen receptor-mediated calcium mobilization. Furthermore, CD95 ligation led to the caspase-dependent inhibition of antigen receptor-induced calcium mobilization and to the activation of mitogen-activated protein kinase pathways in B and T cell lines. A target of CD95-mediated caspase 3-like activity early in the apoptotic process is the adaptor protein GrpL/Gads. GrpL constitutively interacts with SLP-76 via its C-terminal SH3 domain to regulate transcription factors such as NF-AT. Cleavage of GrpL removes the C-terminal SH3 domain so that it is no longer capable of recruiting SLP-76 to the membrane. Transfection of a truncated form of GrpL into Jurkat T cells blocked T cell antigen receptor-induced activation of NF-AT. These results suggest that CD95 signaling can desensitize antigen receptors, in part via cleavage of the GrpL adaptor.

Ubiquitin-protein ligase activity of X-linked inhibitor of apoptosis protein promotes proteasomal degradation of caspase-3 and enhances its anti-apoptotic effect in Fas-induced cell death

The inhibitor of apoptosis (IAP) family of anti-apoptotic proteins regulate programmed cell death and/or apoptosis. One such protein, X-linked IAP (XIAP), inhibits the activity of the cell death proteases, caspase-3, -7, and -9. In this study, using constitutively active mutants of caspase-3, we found that XIAP promotes the degradation of active-form caspase-3, but not procaspase-3, in living cells. The XIAP mutants, which cannot interact with caspase-3, had little or no activity of promoting the degradation of caspase-3. RING finger mutants of XIAP also could not promote the degradation of caspase-3. A proteasome inhibitor suppressed the degradation of caspase-3 by XIAP, suggesting the involvement of a ubiquitin-proteasome pathway in the degradation. An in vitro ubiquitination assay revealed that XIAP acts as a ubiquitin-protein ligase for caspase-3. Caspase-3 was ubiquitinated in the presence of XIAP in living cells. Both the association of XIAP with caspase-3 and the RING finger domain of XIAP were essential for ubiquitination. Finally, the RING finger mutants of XIAP were less effective than wild-type XIAP at preventing apoptosis induced by overexpression of either active-form caspase-3 or Fas. These results demonstrate that the ubiquitin-protein ligase activity of XIAP promotes the degradation of caspase-3, which enhances its anti-apoptotic effect.