Involvement of the CD95 (APO-1/Fas) receptor/ligand system in multiple sclerosis brain

Immunohistochemical methods were used to search for Fas receptor/Fas ligand system involvement in multiple sclerosis (MS) white matter brain lesions. We found large numbers of Fas ligand (Fas-L)-bearing cells present in two acute lesions and 12 of 16 chronic MS lesions, and very few positive cells in non-inflammatory controls. Four of six brains from non-MS neuropathologic conditions associated with inflammation and white matter disease were, however, also positive for Fas-L. Double staining with cell-specific markers revealed that the pattern of ligand- positive cells in chronic MS lesions was complex and composed of several different cell types which were primarily resident glial cells with a small overlay of macrophages. Fas/APO 1 (CD95) receptor expression in MS tissue was also evaluated and marked upregulation of the receptor was found. In addition, Fas receptor was induced, but to a lesser extent, in numerous control brains. The observations that TUNEL- positive dying cells were present in MS lesions and showed excellent co- localization with Fas-L, indicate that the Fas death system may contribute to plaque pathogenesis and could lead to the development of a new category of therapeutic agents for MS.     

Involvement of the CD95 (APO-1/Fas) receptor and ligand system in Helicobacter pylori-induced gastric epithelial apoptosis.

Helicobacter pylori infection is associated with chronic gastritis, peptic ulceration, and gastric carcinoma. The potential role of CD95-mediated apoptosis was investigated in a panel of gastric biopsies obtained from patients with H. pylori-associated chronic gastritis (n = 29) and with noninfected normal mucosa (n = 10). Immunohistochemistry revealed increased CD95 receptor expression in epithelial and lamina propria cells in chronic gastritis. By in situ hybridization, CD95 ligand mRNA was absent or low in normal mucosa but expressed at high levels in lamina propria lymphocytes and, unexpectedly, in epithelial cells in chronic gastritis. Apoptotic cells were rare in normal mucosa but were observed regularly in chronic gastritis in close proximity to CD95 ligand mRNA expression throughout the epithelial and lamina propria cells. In a functional analysis gastric epithelial cell lines were incubated with supernatants of H. pylori. Treatment with the cytotoxic isolate H. pylori 60190 but not with the noncytotoxic isolate Tx30a upregulated CD95 in up to 50% of gastric epithelial cells and induced apoptosis in these cells. H. pylori-induced apoptosis was partially prevented by blocking CD95, demonstrating the functional role of the CD95 system. These findings suggest that H. pylori-associated chronic gastritis involves apoptosis of gastric epithelial cells by activation of the CD95 receptor and ligand system.     

Regulation of Fas (APO-1, CD95) and Fas ligand expression in leukocytes during systemic inflammation in humans

A potential role of Fas/FasL in sepsis is suggested by recent clinical studies showing that Fas and FasL could serve as markers for severity of sepsis. We sought to determine the effect of endotoxin infusion on expression of Fas and FasL. Healthy volunteers (n = 30) received 2 ng/kg endotoxin i.v. Endotoxin infusion decreased Fas expression on neutrophils and monocytes by 15-20% at 2-4 h in vivo and also in vitro. A rebound increase in Fas (30%) was seen on neutrophils at 24 h, and soluble FasL levels increased by 100% at 24 h. Fas mRNA levels increased 6-fold 4-6 h after endotoxin infusion as measured by real-time polymerase chain reaction. In contrast, FasL-mRNA levels in circulating leukocytes decreased by >80% 2h after lipopolysaccharide infusion. In summary, low-grade endotoxemia induces early down-modulation of Fas on leukocytes, followed by a several-fold increase in Fas-mRNA expression leading to later Fas surface upregulation on neutrophils. The upregulation of Fas expression, Fas mRNA, and later in FasL and sFas levels in endotoxemia replicates the increased fas levels found in septic patients.     

Involvement of Fas (CD95/APO-1) and Fas ligand in apoptosis induced by ganciclovir treatment of tumor cells transduced with herpes simplex virus thymidine kinase.

Transduction of cancer cells with herpes simplex virus thymidine kinase gene (HSVtk) followed by prodrug ganciclovir (GCV) treatment has been shown to induce apoptosis. In this study, four murine tumors including B16F10 melanoma, NG4TL4 sarcoma, H6 hepatoma and 1MEA 7R.1 hepatoma were found to vary in sensitivity to this gene therapy strategy in vitro but, at effective doses of GCV, the HSVtk-transduced cells of all four tumors showed similar kinetics of early rise in p53 protein levels, then cell cycle S-/G2-phase arrest and finally signs of apoptosis. Immunoblot analyses revealed that Fas (CD95/APO-1), Fas ligand (FasL) and two downstream mediators, RIP and caspase-3, (CPP32, YAMA, Apopain) were increased in GCV-treated HSVtk-transduced tumor cells the cell cycle arrest and before apoptosis. Increased expression of FasL could also be observed in vivo in HSVtk-transduced tumors induced to regress by GCV treatment. Enzyme measurements using specific substrate showed that the caspase-3 activation followed kinetically the FasL expression. More than half of the HSVtk/GCV-induced cell death could be abrogated by addition to the cell culture medium of a specific antisense oligonucleotide to block FasL synthesis, a recombinant Fas/Fc chimeric protein to compete with Fas receptor for FasL binding, or cell-permeable specific tetrapeptide inhibitors of caspase-3 or caspase-8.     

Effect of endogenous interleukin-6 on Fas (APO-1/CD95) receptor expression in advanced melanoma patients

Interleukin-6 (IL-6) has been shown to support either autocrine or paracrine growth in melanoma, and may prevent programmed cell death in different cell types. We have previously demonstrated that the endogenous IL-6 level is significantly correlated with tumor burden and nonresponse to biochemotherapy in metastatic malignant melanoma patients. In the present study, we investigated the relationship between endogenous IL-6 and apoptosis signal through Fas (APO-1/CD95) receptor expression in 9 responder and 15 refractory patients with metastatic disease treated by biochemotherapy. Before any treatment, double immunostaining demonstrated that 61.5% of the tumor cells were HMB45+CD95+. At day 49 in refractory patients, a significant decrease (p = 0.04) of total Fas expression was observed. Furthermore, a significant reduction (p = 0.032) in the percentage of HMB45+CD95* cells occurred. An 11-fold increase in serum IL-6 level was detected (p < 0.002). This increase was negatively correlated (r = -0.2, p = 0.008) with the decrease in total Fas expression. However, in responding patients, no detectable decrease in Fas expression was observed, while a very low increase in serum IL-6 (2-fold) was detected. These results suggest that the increased endogenous IL-6 level in refractory patients may inhibit apoptosis via modulation of Fas expression. These preliminary results must be interpreted with caution, and further study with a greater number of patients is needed to understand the mechanism by which IL-6 inhibits apoptosis in melanoma.     

Drug-induced apoptosis in hepatoma cells is mediated by the CD95 (APO-1/Fas) receptor/ligand system and involves activation of wild-type p53.

Chemotherapeutic drugs are cytotoxic by induction of apoptosis in drug-sensitive cells. We investigated the mechanism of bleomycin-induced cytotoxicity in hepatoma cells. At concentrations present in the sera of patients during therapy, bleomycin induced transient accumulation of nuclear wild-type (wt) p53 and upregulated expression of cell surface CD95 (APO-1/Fas) receptor in hepatoma cells carrying wt p53 (HepG2). Bleomycin did not increase CD95 in hepatoma cells with mutated p53 (Huh7) or in hepatoma cells which were p53-/- (Hep3B). In addition, sensitivity towards CD95-mediated apoptosis was also increased in wt p53 positive HepG2 cells. Microinjection of wt p53 cDNA into HepG2 cells had the same effect. In contrast, bleomycin did not enhance susceptibility towards CD95-mediated apoptosis in Huh7 and in Hep3B cells. Furthermore, bleomycin treatment of HepG2 cells increased CD95 ligand (CD95L) mRNA expression. Most notably, bleomycin-induced apoptosis in HepG2 cells was almost completely inhibited by antibodies which interfere with CD95 receptor/ligand interaction. These data suggest that apoptosis induced by bleomycin is mediated, at least in part, by p53-dependent stimulation of the CD95 receptor/ligand system. The same applies to other anti-cancer drugs such as cisplatin and methotrexate. These data may have major consequences for drug treatment of cancer and the explanation of drug sensitivity and resistance.     

Selective cleavage of nuclear autoantigens during CD95 (Fas/APO-1)- mediated T cell apoptosis

Intracellular proteases appear to be important mediators of apoptosis. Substrates cleaved by proteases during apoptosis include nuclear autoantigens targeted in systemic autoimmune diseases. Using human autoantibodies as probes, we demonstrate here that T cell apoptosis mediated by CD95 (Fas/APO-1) is associated with substantial cleavage of a subset of nuclear autoantigens (7 of 33 examined). This subset included poly (ADP-ribose) polymerase, the 70-kD protein of the U1 small nuclear ribonucleoprotein particle, lamin B, the nuclear mitotic apparatus protein NuMA, DNA topoisomerases I and II, and the RNA polymerase I upstream binding factor UBF. Several of the cleaved autoantigens are involved in ensuring the integrity and proper conformation of DNA in the nucleus through interactions with the nuclear matrix, suggesting the possibility that their cleavage may contribute to the collapse of nuclear structure during apoptosis. The relative cleavage kinetics indicated that the autoantigens were targeted at various times after induction of apoptosis, suggesting either differential accessibility or activation of distinct proteases during the cell death process. These data reinforce the hypothesis that apoptosis is accompanied by selective cleavage of key substrates and not by a generalized degradation of intracellular material.     

Serum soluble Fas (CD95) and Fas ligand profiles in chronic kidney failure

Apoptosis, or programmed cell death, is an active form of cell death that is initiated by a number of stimuli and is intricately regulated. Apoptosis in both excessive and reduced amounts has pathophysiologic implications. Accelerated programmed cell death has been observed in leukocytes among patients with chronic renal failure (CRF). This has been ascribed in part to the retention of uremic toxins. The Fas/Fas ligand (FasL) system is a key regulatory apoptotic pathway. Membrane-bound Fas is a cell-surface receptor that transduces apoptosis after interaction with membrane-bound or soluble FasL (sFasL). By contrast, soluble Fas (sFas) binds sFasL and inhibits its activity. In an attempt to examine the balance between these soluble factors in uremia, we measured soluble sFas and sFasL levels in the serum of healthy control subjects and patients with various degrees of CRF and examined the distribution of the various molecular mass fractions of these proteins in uremic serum. In brief, serum was obtained from 15 healthy volunteers, 17 patients with CRF, 11 patients undergoing maintenance hemodialysis (HD), and 7 patients undergoing peritoneal dialysis (PD). Serum sFas and sFasL were measured by enzyme-linked immunosorbent assay, and their molecular distribution was determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis followed by immunoblot. Compared with results in healthy control subjects, sFas levels were significantly higher in patients with CRF and in patients undergoing dialysis. There was a significant inverse correlation between sFas levels and creatinine clearance. Serum sFasL levels were not different among the four groups. However, the sFas-to-sFasL ratio was significantly lower in healthy control subjects as compared with patients with CRF and patients undergoing dialysis. Immunoblots and densitometric analyses of sFas and sFasL depicted a known 48-kd sFas, a known 27-kd sFasL, and a 60-kd sFas-sFasL protein aggregate signal. In conclusion, serum sFas levels are increased in patients with various degrees of CRF and may bind circulating sFasL, thereby minimizing mediation of cellular apoptosis.     

Decreased Expression of CD95 (FAS/APO-1) on CD4+ T-lymphocytes from Participants with Autism

Although autism remains an enigmatic disease, there is mounting evidence that the immune system plays an important role in the pathogenesis. Immune system involvement is apparently widespread as numerous humoral and cellular abnormalities have been reported in both the innate and adaptive responses. Fas (CD95/APO-1) is a type I cell-surface protein from the TNF/NGF-R superfamily present on the surface of many immune related cells. Fas activation is instrumental in starting a complicated chain of events that results in programmed cell death (apoptosis) by DNA fragmentation. Preliminary data is presented, which indicate that subjects with austism have lower levels of Fas on their CD4⁺ helper T cells (p = .048) than have normal subjects. Data also indicates subjects with autism have significantly higher levels of soluble Fas (p = .01) than have normal subjects. A maturing individual must eliminate cells for proper morphogenesis to occur. Preliminary data suggest that faulty apoptosis may be involved in the pathogenesis of autism.     

Apoptotic signaling through CD95 (Fas/Apo-1) activates an acidic sphingomyelinase

Intracellular pathways leading from membrane receptor engagement to apoptotic cell death are still poorly characterized. We investigated the intracellular signaling generated after cross-linking of CD95 (Fas/Apo-1 antigen), a broadly expressed cell surface receptor whose engagement results in triggering of cellular apoptotic programs. DX2, a new functional anti-CD95 monoclonal antibody was produced by immunizing mice with human CD95-transfected L cells. Crosslinking of CD95 with DX2 resulted in the activation of a sphingomyelinase (SMase) in promyelocytic U937 cells, as well as in other human tumor cell lines and in CD95-transfected murine cells, as demonstrated by induction of in vivo sphingomyelin (SM) hydrolysis and generation of ceramide. Direct in vitro measurement of enzymatic activity within CD95- stimulated U937 cell extracts, using labeled SM vesicles as substrates, showed strong SMase activity, which required pH 5.0 for optimal substrate hydrolysis. Finally, all CD95-sensitive cell lines tested could be induced to undergo apoptosis after exposure to cell-permeant C2-ceramide. These data indicate that CD95 cross-linking induces SM breakdown and ceramide production through an acidic SMase, thus providing the first information regarding early signal generation from CD95, and may be relevant in defining the biochemical nature of intracellular messengers leading to apoptotic cell death.     

Essential Lymphocyte Function Associated 1 (LFA-1): Intercellular Adhesion Molecule Interactions for T Cell–mediated B Cell Apoptosis by Fas/APO-1/CD95

B cells are susceptible to Fas ligand (FasL)⁺ CD4⁺ Th1 cell–mediated apoptosis. We demonstrate that blocking the interactions between lymphocyte function associated (LFA)-1 and intercellular adhesion molecule(ICAM)-1 and ICAM-2 completely suppresses Fas-dependent B cell lysis. Antibodies to CD2 and CD48 partially suppress B cell apoptosis, whereas anti-B7.1 and anti-B7.2 antibodies have no effect. Also, B cells from ICAM-1–deficient mice are resistant to FasL⁺ T cell–mediated death. Our results suggest that LFA-1/ICAM interactions are crucial for Th1 cell–mediated B cell apoptosis and may contribute to the maintenance of B cell homeostasis in vivo.     

The Central Executioner of Apoptosis: Multiple Connections between Protease Activation and Mitochondria in Fas/APO-1/CD95- and Ceramide-induced Apoptosis

According to current understanding, cytoplasmic events including activation of protease cascades and mitochondrial permeability transition (PT) participate in the control of nuclear apoptosis. However, the relationship between protease activation and PT has remained elusive. When apoptosis is induced by cross-linking of the Fas/APO-1/CD95 receptor, activation of interleukin-1β converting enzyme (ICE; caspase 1) or ICE-like enzymes precedes the disruption of the mitochondrial inner transmembrane potential (ΔΨ_m). In contrast, cytosolic CPP32/ Yama/Apopain/caspase 3 activation, plasma membrane phosphatidyl serine exposure, and nuclear apoptosis only occur in cells in which the ΔΨ_m is fully disrupted. Transfection with the cowpox protease inhibitor crmA or culture in the presence of the synthetic ICE-specific inhibitor Ac-YVAD.cmk both prevent the ΔΨ_m collapse and subsequent apoptosis. Cytosols from anti-Fas–treated human lymphoma cells accumulate an activity that induces PT in isolated mitochondria in vitro and that is neutralized by crmA or Ac-YVAD.cmk. Recombinant purified ICE suffices to cause isolated mitochondria to undergo PT-like large amplitude swelling and to disrupt their ΔΨ_m. In addition, ICE-treated mitochondria release an apoptosis-inducing factor (AIF) that induces apoptotic changes (chromatin condensation and oligonucleosomal DNA fragmentation) in isolated nuclei in vitro. AIF is a protease (or protease activator) that can be inhibited by the broad spectrum apoptosis inhibitor Z-VAD.fmk and that causes the proteolytical activation of CPP32. Although Bcl-2 is a highly efficient inhibitor of mitochondrial alterations (large amplitude swelling + ΔΨ_m collapse + release of AIF) induced by prooxidants or cytosols from ceramide-treated cells, it has no effect on the ICE-induced mitochondrial PT and AIF release. These data connect a protease activation pathway with the mitochondrial phase of apoptosis regulation. In addition, they provide a plausible explanation of why Bcl-2 fails to interfere with Fas-triggered apoptosis in most cell types, yet prevents ceramide- and prooxidant-induced apoptosis.     

Differential expression of Fas (CD95) and Fas ligand on normal human phagocytes: implications for the regulation of apoptosis in neutrophils

Human neutrophils, monocytes, and eosinophils are known to undergo apoptotic cell death. The Fas/Fas ligand pathway has been implicated as an important cellular pathway mediating apoptosis in diverse cell types. We conducted studies to examine the importance of the Fas/FasL system in normal human phagocytes. Although Fas expression was detected on neutrophils, monocytes, and eosinophils, constitutive expression of FasL was restricted to neutrophils. The three types of phagocytes demonstrated differential sensitivity to Fas-induced apoptosis. Only neutrophils were highly susceptible to rapid apoptosis in vitro after stimulation with activating anti-Fas IgM (mAb CH-11). Fas-mediated neutrophil apoptosis was suppressed by incubation with G-CSF, GM-CSF, IFN-gamma, TNF-alpha, or dexamethasone, as well as the selective tyrosine kinase inhibitors, herbimycin A and genistein. Spontaneous neutrophil death in vitro was partially suppressed by Fas-Ig fusion protein or antagonistic anti-Fas IgG1 (mAb ZB4). In coculture experiments, neutrophils released a soluble factor inducing death in Fas-susceptible Jurkat cells via a mechanism sensitive to the presence of Fas-Ig or anti-Fas IgG1. Immunoblot analysis using specific anti- human FasL IgG1 (mAb No. 33) identified a 37-kD protein in lysates of freshly isolated neutrophils and a 30-kD protein in the culture supernatant of neutrophils maintained in vitro. Our results suggest that mature neutrophils may be irrevocably committed to autocrine death by virtue of their constitutive coexpression of cell-surface Fas and FasL via a mechanism that is sensitive to proinflammatory cytokines, glucocorticoids, and inhibitors of tyrosine kinase activity. Furthermore, neutrophils can serve as a source of soluble FasL, which may function in a paracrine pathway to mediate cell death.     

Amelioration of collagen-induced arthritis by CD95 (Apo-1/Fas)-ligand gene transfer.

Both rheumatoid arthritis and animal models of autoimmune arthritis are characterized by hyperactivation of synovial cells and hyperplasia of the synovial membrane. The activated synovial cells produce inflammatory cytokines and degradative enzymes that lead to destruction of cartilage and bones. Effective treatment of arthritis may require elimination of most or all activated synovial cells. The death factor Fas/Apo-1 and its ligand (FasL) play pivotal roles in maintaining self-tolerance and immune privilege. Fas is expressed constitutively in most tissues, and is dramatically upregulated at the site of inflammation. In both rheumatoid arthritis and animal models of autoimmune arthritis, high levels of Fas are expressed on activated synovial cells and infiltrating leukocytes in the inflamed joints. Unlike Fas, however, the levels of FasL expressed in the arthritic joints are extremely low, and most activated synovial cells survive despite high levels of Fas expression. To upregulate FasL expression in the arthritic joints, we have generated a recombinant replication-defective adenovirus carrying FasL gene; injection of the FasL virus into inflamed joints conferred high levels of FasL expression, induced apoptosis of synovial cells, and ameliorated collagen-induced arthritis in DBA/1 mice. The Fas-ligand virus also inhibited production of interferon-gamma by collagen-specific T cells. Coadministration of Fas-immunoglobulin fusion protein with the Fas-ligand virus prevented these effects, demonstrating the specificity of the Fas-ligand virus. Thus, FasL gene transfer at the site of inflammation effectively ameliorates autoimmune disease.     

The molecular basis for apoptotic defects in patients with CD95 (Fas/Apo-1) mutations

Heterozygous mutations of the receptor CD95 (Fas/Apo-1) are associated with defective lymphocyte apoptosis and a clinical disease characterized by lymphadenopathy, splenomegaly, and systemic autoimmunity. From our cohort of 11 families, we studied eight patients to define the mechanisms responsible for defective CD95-mediated apoptosis. Mutations in and around the death domain of CD95 had a dominant–negative effect that was explained by interference with the recruitment of the signal adapter protein, FADD, to the death domain. The intracellular domain (ICD) mutations were associated with a highly penetrant Canale-Smith syndrome (CSS) phenotype and an autosomal dominant inheritance pattern. In contrast, mutations affecting the CD95 extracellular domain (ECD) resulted in failure of extracellular expression of the mutant protein or impaired binding to CD95 ligand. They did not have a dominant–negative effect. In each of the families with an ECD mutation, only a single individual was affected. These observations were consistent with differing mechanisms of action and modes of inheritance of ICD and ECD mutations, suggesting that individuals with an ECD mutation may require additional defect(s) for expression of CSS.     

Neurokinin-1 receptor antagonists protect mice from CD95- and tumor necrosis factor-alpha-mediated apoptotic liver damage

Previously, we have shown that primary afferent neurons are necessary for disease activity in immune-mediated liver injury in mice. These nerve fibers are detectable by substance P (SP) immunocytochemistry in the portal tract of rodent liver. Antagonists of the neurokinin-1 receptor (NK-1R), which is the prime receptor of SP, prevented liver damage by suppressing the synthesis of proinflammatory cytokines. Here, we investigated the influence of primary afferent nerve fibers, SP, and NK-1 receptor antagonists on hepatocyte apoptosis in vivo induced by administration of activating anti-CD95 monoclonal antibody (mAb) to mice. Depletion of primary afferent nerve fibers by neonatal capsaicin treatment prevented CD95-mediated activation of caspase-3, measured as enzymatic activity in liver homogenates or by demonstration of hepatocellular immunoreactivity for active caspase-3 in liver slices, and liver damage. This effect was reversed by administration of SP to anti-CD95 mAb-treated mice depleted from primary afferent neurons. The presence of the NK-1R on mouse hepatocytes was demonstrated by immunocytochemistry and flow cytometry. Intraperitoneal pretreatment with the NK-1 receptor antagonists (2S,3S)-cis-2-(diphenylmethyl)-N-([2-methoxyphenyl]-methyl)-1-azabicyclo(2.2.2.)-octan-3-amine (CP-96,345) or (2S,3S)3-([3,5-bis(trifluoromethyl)phenyl]methoxy)-2-phenylpiperadine (L-733,060) dose dependently protected mice from CD95-mediated liver injury. Similar results were obtained when apoptotic liver damage was induced by administration of tumor necrosis factor-alpha to d-galactosamine-sensitized mice. In conclusion, SP, probably by binding to its receptor on hepatocytes, might aggravate apoptotic signals in these cells. Because NK-1 receptor antagonists not only suppress the proinflammatory cytokine response in the liver but also prevent liver cell apoptosis in vivo, they might be suitable drugs for treatment of immune-mediated liver disease.     

Expression of Fas (CD95) and Fas ligand on peripheral blood mononuclear cells in critical illness and association with multiorgan dysfunction severity and survival

OBJECTIVE: This was an exploratory study with three goals: a) to quantify the expression of the apoptotic receptor Fas and its ligand (FasL) on peripheral blood mononuclear cells (PBMCs) in patients with, or at risk for, multiple organ dysfunction syndrome (MODS); b) to compare this expression with the respective expression in matched controls; and c) to explore the association with MODS severity and survival. DESIGN: Repeated-measures correlational and cross-sectional design. SETTING: The surgical, medical, and the trauma/burn intensive care unit of an academic institution. PATIENTS: Thirty-five adult, critically ill patients meeting the diagnostic criteria for systemic inflammatory response syndrome (SIRS) with MODS, or at risk for MODS, were followed for 14 days. Thirty-five non-SIRS controls matched with patients for age, gender, and race comprised the control group. INTERVENTIONS: Peripheral blood sampling every 48 hrs. MEASUREMENTS/MAIN RESULTS: T cells were considerably depleted in SIRS/MODS patients (p <.001), and Fas and FasL expression on PBMCs (flow cytometric analysis) was elevated significantly compared with controls (p <.001). In contrast to controls, non-T cells were the major sources of Fas and FasL in SIRS/MODS patients (p <.01). Expression of Fas and FasL exhibited a bimodal correlation with severity (p <.03). High severity patients demonstrated increasing Fas and FasL expression with increasing severity in contrast to declining expression in moderately severe patients. Fas and FasL measurements were significantly and positively associated with the likelihood of survival (p <.05). CONCLUSIONS: Dysregulation in the expression of apoptotic receptors Fas and FasL, at least in PBMCs, may be involved in the pathophysiology of SIRS, the related lymphocytopenia, and the onset of MODS and the related morbidity and mortality rates.