Activation interferes with the APO-1 pathway in mature human T cells

A membrane-bound Fas decoy receptor expressed by human thymocytes

Human thymocytes at several stages of maturation express Fas, yet resist apoptosis induction through its ligation. A proximal step in apoptotic signaling through Fas is implicated in this resistance, as these cells undergo normal levels of apoptosis induction after exposure to tumor necrosis factor-alpha. We studied the Fas receptors expressed in human thymocytes to search for mechanisms of receptor-mediated inhibition of Fas signaling in these cells. We describe here a unique, membrane-bound form of Fas receptor that contained a complete extracellular domain of Fas but that lacked a death domain due to alternative splicing of exon 7. This Fas decoy receptor (FDR) was shown to have nearly wild-type ability to bind native human Fas ligand and was expressed predominantly at the plasma membrane. Unlike soluble forms of Fas receptor, FDR dominantly inhibited apoptosis induction by Fas ligand in transfected human embryonic kidney cells. Titration of FDR in Fas-expressing cells suggests that FDR may operate through the formation of mixed receptor complexes. FDR also dominantly inhibited Fas-induced apoptosis in Jurkat T cells. In mixing experiments with wild-type Fas, FDR was capable of inhibiting death signaling at molar ratios less than 0.5, and this relative level of FDR:wild type message was observed in at least some thymocytes tested. The data suggest that Fas signal pathways in primary human cells may be regulated by expression of a membrane-bound decoy receptor, analogous to the regulation of tumor necrosis factor-related apoptosis inducing ligand (TRAIL)-induced apoptosis by decoy receptors.

Apoptosis in human autoimmune diseases

The description of apoptosis or programmed cell death nearly thirty years ago did not initially stimulate a great deal of interest. However, the ways cells die is clearly an essential part of biological homeostasis and well worth of study in its own right as the enormous literature on the subject in the past 15 years confirms. In the past decade new avenues of apoptosis research have opened up as the relationship between this form of cell death and autoimmune disease has come under increasing scrutiny. Although most research to date has been in animal study models, there are a variety of studies which have begun to explore links between apoptosis and a wider range of human autoimmune conditions. In this review we analyse briefly the background to what is known about apoptosis and focus on the increasing likelihood that abnormalities in apoptosis are contributory factors in the development of human autoimmunity.

The topoisomerase inhibitors camptothecin and etoposide induce a CD95-independent apoptosis of activated peripheral lymphocytes

The effect of etoposide and camptothecin, two topoisomerase inhibitors directed against topoisomerases II and I, respectively, was evaluated on human peripheral blood lymphocytes. Etoposide and camptothecin induced apoptosis of mitogen-activated but not resting CD4+ and CD8+ T lymphocytes. Cell sensitivity to these agents required G1 to S-phase transition of the cell cycle. Conversely, daunorubicin, an intercalating agent and topoisomerase II inhibitor, induced apoptosis of both resting and activated lymphocytes. Although etoposide and camptothecin induced CD95-ligand mRNA expression, drug-induced apoptosis of activated human lymphocytes was not inhibited by CD95 antagonists. Drug-induced cell death was also not inhibited by p55 TNFR-Ig fusion protein. Activation of the caspases cascade was suggested by the partial inhibitory effect of the tripeptide zVAD-fmk and documented by activation of caspase 3. Finally etoposide and camptothecin induced a rapid production of ceramide in activated but not resting peripheral blood lymphocytes, suggesting that ceramide might initiate the signaling apoptotic cascade in sensitive cells.

Enhancement of susceptibility to Fas-mediated apoptosis of TH1 cells by nonmitogenic anti-CD3epsilon F(ab')2

BACKGROUND

Anti-CD3epsilon F(ab')2 are nonmitogenic for naive T cells but can induce apoptosis of antigen-activated T cells in vitro and in vivo. We studied the mechanisms by which nonmitogenic anti-CD3epsilon F(ab')2 antibodies induce T cell death.

METHODS

OVA-responsive T cell lines were generated by immunization in vivo and restimulation in vitro. Fas or Fas ligand (FasL) expression was tested by surface staining with specific mAbs. The apoptotic DNA and cell cycle phase were tested by staining DNA with propidium iodide. Interferon-gamma was measured by ELISA, whereas interleukin (IL)-2 and IL-4 were detected by bioassays.

RESULTS

Restimulation with anti-CD3epsilon F(ab')2 induced apoptosis of antigen-activated wild-type T cells, but not Fas or FasL-defective T cells. Anti-CD3epsilon F(ab')2 induced death of cells expressing high levels of Fas and FasL, and preferentially deleted T helper (Th)1 cells but spared Th2 cells. Soluble anti-CD3epsilon F(ab')2 did not regulate Fas or induce FasL expression, indicating that the ability of anti-CD3epsilon F(ab')2 to induce T cell apoptosis depends on a distinct mechanism. T cells in S/G2 were found relatively resistant to Fas-mediated apoptosis, but anti-CD3epsilon F(ab')2 rendered those T cells exquisitely sensitive to Fas-mediated apoptosis.

CONCLUSIONS

Anti-CD3epsilon F(ab')2 induces apoptosis of cycling CD4+ T cells through activation of the Fas/FasL pathway. Anti-CD3epsilon F(ab')2 does not regulate Fas or FasL expression but induces susceptibility to Fas-mediated apoptosis of cycling T cells. Anti-CD3epsilon F(ab')2 can induce death of polarized Th1 cells, but not Th2 cells, thus potentially skewing the repertoire of antigen-activated T cells toward the Th2 phenotype. These features predict that nonmitogenic anti-CD3epsilon F(ab')2-like antibodies can be useful to prevent or reverse pathogenic immune responses mediated by Th1 cells.

Specific targeting of EGP-2+ tumor cells by primary lymphocytes modified with chimeric T cell receptors

A promising strategy for cancer treatment is adoptive immunotherapy with gene-modified lymphocytes expressing a chimeric T cell receptor (cTCR) that directs tumor targeting and stimulates T cell effector functions. In this study, the activities of two novel cTCR molecules (GAgamma and GAHgamma) were investigated. Both encode a single-chain variable fragment (scFv) derived from the monoclonal antibody (MAb) GA733.2, which binds the epithelial glycoprotein 2 (EGP-2) overexpressed on a variety of human carcinomas. In the GAgamma cTCR, the scFv is directly fused to the transmembrane/cytoplasmic portions of the immunoglobulin Fc receptor (Ig FcRI) gamma subunit, which mediates T cell signaling. GAHgamma possesses an extracellular spacer composed of the CD8alpha immunoglobulin hingelike domain inserted between the scFv and gamma chain. Activated T cells (ATCs), stimulated ex vivo using anti-CD3 MAb, were derived from either healthy donors or patients and transduced with recombinant retrovirus encoding the respective GA cTCR molecules. After culture expansion for 14 days, GAgamma-modified ATCs demonstrated enhanced targeting and lysis of EGP-2+ colon cancer cells and increased cytokine secretion. Cells transduced with the GAHgamma cTCR displayed specific lytic activity that was about twofold greater than that of GAgamma-ATCs and produced significantly more cytokine. In addition, reactivation of GAHgamma-ATC with anti-CD3 MAb prior to addition to EGP-2+ tumor target induced a further increase in lytic activity. Because the activation status influences T cell antitumor functions, our data suggest that reactivation prior to adoptive transfer would improve the clinical efficacy of GAHgamma-modified ATCs.

Activation of Fas by FasL induces apoptosis by a mechanism that cannot be blocked by Bcl-2 or Bcl-x(L)

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-x(L) 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 activation is required for T cell proliferation

Triggering of Fas (CD95) by its ligand (FasL) rapidly induces cell death via recruitment of the adaptor protein Fas-associated death domain (FADD), resulting in activation of a caspase cascade. It was thus surprising that T lymphocytes deficient in FADD were reported recently to be not only resistant to FasL-mediated apoptosis, but also defective in their proliferative capacity. This finding suggested potentially dual roles of cell growth and death for Fas and possibly other death receptors. We report here that CD3-induced proliferation and interleukin 2 production by human T cells are blocked by inhibitors of caspase activity. This is paralleled by rapid cleavage of caspase-8 after CD3 stimulation, but no detectable processing of caspase-3 during the same interval. The caspase contribution to T cell activation may occur via TCR-mediated upregulation of FasL, as Fas-Fc blocked T cell proliferation, whereas soluble FasL augmented CD3-induced proliferation. These findings extend the role of death receptors to the promotion of T cell growth in a caspase-dependent manner.

Transcriptional regulation of Fas gene expression by GA-binding protein and AP-1 in T cell antigen receptor.CD3 complex-stimulated T cells

Fas (CD95 or APO-1), a transmembrane cell surface receptor of the tumor necrosis factor receptor family, is up-regulated in activated T lymphocytes. Our present study identified an upstream enhancer element (between nucleotide positions -862 and -682) containing a GA-binding protein (GABP) site and a low affinity activating protein-1 (AP-1)-binding site. T cell activation increased the DNA binding of GABP and AP-1 to this enhancer site. The specificity of GABP and AP-1 binding was demonstrated by competition electrophoretic mobility shift assay and supershift electrophoretic mobility shift assay with antibodies against GABP and AP-1, respectively. Mutational analysis of Fas promoter revealed that both GABP- and AP-1-binding sites were required for initiating Fas gene transcription. We further show that anti-CD3 mAb, phorbol 12-myristate 13-acetate, and phorbol 12-myristate 13-acetate/ionomycin strongly activated promoters carrying multiple copies of the Fas enhancer, and mutation of either the GABP or AP-1 binding site severely reduced transcriptional activity. Taken together, these results suggest that the transcription factors GABP and AP-1 play a critical role in the induction of Fas gene expression in T cell antigen receptor.CD3-stimulated Jurkat cells.

TGF-beta1: immunosuppressant and viability factor for T lymphocytes

Transforming growth factor-beta (TGF-beta) is a multifunctional cytokine with multiple roles in the immune system. To date, it has been difficult to develop a comprehensive picture of the effect of TGF-beta on T lymphocytes, because TGF-beta not only acts directly on T lymphocytes, but also acts indirectly by regulating the function of antigen-presenting cells. In early studies, it was mostly the inhibitory function of TGF-beta that was demonstrated; recently, however TGF-beta was recognized as an antiapoptotic survival factor for T lymphocytes. The outcome of the TGF-beta effect on T lymphocytes was shown to strongly depend on their stage of differentiation and on the cytokine milieu. TGF-beta cannot be classified as a classical Th1 or Th2 cytokine. However, recently the existence of the TGF-beta-producing Th3 subset was described which might play an important regulatory role during an immune response. A better understanding of the molecular mechanism of how TGF-beta inhibits or stimulates T lymphocytes will help to predict the complex functions of this cytokine.

T cell receptor-induced activation and apoptosis in cycling human T cells occur throughout the cell cycle

Previous studies have found conflicting associations between susceptibility to activation-induced cell death and the cell cycle in T cells. However, most of the studies used potentially toxic pharmacological agents for cell cycle synchronization. A panel of human melanoma tumor-reactive T cell lines, a CD8+ HER-2/neu-reactive T cell clone, and the leukemic T cell line Jurkat were separated by centrifugal elutriation. Fractions enriched for the G0-G1, S, and G2-M phases of the cell cycle were assayed for T cell receptor-mediated activation as measured by intracellular Ca(2+) flux, cytolytic recognition of tumor targets, and induction of Fas ligand mRNA. Susceptibility to apoptosis induced by recombinant Fas ligand and activation-induced cell death were also studied. None of the parameters studied was specific to a certain phase of the cell cycle, leading us to conclude that in nontransformed human T cells, both activation and apoptosis through T cell receptor activation can occur in all phases of the cell cycle.

Lack of activation induced cell death in human T blasts despite CD95L up-regulation: protection from apoptosis by MEK signalling

The generation of effective immunity requires that antigen-specific T cells are activated, clonally expanded and ultimately eliminated by apoptosis. The involvement of CD95-mediated apoptosis in T-cell elimination is well established, but the conditions which regulate the death pathway under normal circumstances are still emerging. Using superantigen-activated human T cells, we found that whilst T-cell receptor (TCR) signalling triggered up-regulation of CD95 ligand (CD95L), the majority of T cells were resistant to apoptosis induction, despite co-expressing high levels of CD95. Resistance was maintained following direct antibody-mediated cross-linking of CD95 and was not confined to early time periods following activation. Our data implicate TCR-derived signals in protection from apoptosis and reveal a role for the mitogen-activated protein (MAP) kinase pathway by use of a MAP kinase kinase (MEK) inhibitor. Collectively these data demonstrate that resistance to activation-induced cell death in human T cells is prolonged rather than transient, is not attributable to a lack of CD95L up-regulation and is due, at least in part, to signalling via the MEK pathway.

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 Ca(2+) influx in lymphocytes through the activation of acidic sphingomyelinase (ASM) and ceramide release. The block of Ca(2+) 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 Ca(2+) 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-induced CD4+ T cell death in HIV-positive individuals correlates with Fas susceptibility, CD4+ T cell count, and HIV plasma viral copy number

The relevance of activation-induced cell death (AICD) of CD4+ T cells to AIDS pathogenesis is unknown. The present study investigates the relationship of AICD to a defined molecular mechanism regulating peripheral T cell homeostasis, Fas-mediated apoptosis, and clinical correlates of the pathogenesis of AIDS. Increased pokeweed mitogen (PWM)-induced AICD (22.8 versus 4.4%, p = 0.006) and Fas-mediated apoptosis (27.7 versus 12.0%, p = 0.002) of CD4+ T cells were observed in HIV+ versus HIV- individuals. Similarly, increased PWM-mediated AICD (16.2 versus 2.2%, p < 0.001) and Fas-mediated apoptosis (25.8 versus 7.6%, p = 0.005) were noted in CD8+ T cells from HIV+ versus HIV- individuals. PWM-induced AICD of CD4+ T cells was blocked (83% median specific inhibition) by Fas-blocking antibodies, whereas PWM-induced AICD of CD8+ T cells was Fas independent. Comparison between PWM- and anti-CD3-mediated AICD of CD4+ T cells indicated that PWM- and not CD3-induced AICD is Fas dependent. HIV+ individuals with an HIV RNA copy number of <30,000 copies/ml had lower PWM-induced AICD of CD4+ T cells than did those with an HIV RNA copy number of >30,000 copies/ml (5.7 versus 22.1%, p = 0.034), and PWM-induced AICD inversely correlated with CD4+ T cell count (R = -0.567, p = 0.043). Initiation of HAART decreased PWM-induced CD4+ T cell AICD from 24.4 to 9.4% posttreatment (p = 0.035). These results demonstrate that PWM-induced AICD of CD4+ T cells from HIV+ individuals is mediated by Fas/FasL, parallels the in vivo susceptibility of the CD4+ T cell to Fas-mediated apoptosis, and correlates with clinical markers of AIDS pathogenesis and response to HAART.

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-γ and/or TNF-α 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.

Protein Kinase C Inhibits CD95 (Fas/APO-1)-Mediated Apoptosis by at Least Two Different Mechanisms in Jurkat T Cells

We have recently reported that activation of protein kinase C (PKC) plays a negative role in CD95-mediated apoptosis in human T cell lines. Here we present data indicating that although the PKC-induced mitogen-activated protein kinase pathway could be partially implicated in the abrogation of CD95-mediated apoptosis by phorbol esters in Jurkat T cells, the major inhibitory effect is exerted through a PKC-dependent, mitogen-activated protein kinase-independent signaling pathway. Furthermore, we demonstrate that activation of PKC diminishes CD95 receptor aggregation elicited by agonistic CD95 Abs. On the other hand, it has been reported that UV radiation-induced apoptosis is mediated at least in part by the induction of CD95 oligomerization at the cell surface. Here we show that activation of PKC also inhibits UVB light-induced CD95 aggregation and apoptosis in Jurkat T cells. These results reveal a novel mechanism by which T cells may restrain their sensitivity to CD95-induced cell death through PKC-mediated regulation of CD95 receptor oligomerization at the cell membrane.

Membrane-Bound Fas (Apo-1/CD95) Ligand on Leukemic Cells: A Mechanism of Tumor Immune Escape in Leukemia Patients

There is evidence from bone marrow transplantation that T cells may be involved in the immunologic control of leukemia. But many patients relapse despite a potent graft-versus-leukemia effect mediated by allogeneic T cells. The expression of the FasL protein has been suggested as a mechanism of tumor immune escape. We, therefore, evaluated the capacity of leukemic cells from patients with acute or chronic myelogenous leukemia to escape the allogeneic or autologous immune response by bearing the FasL molecule. Although almost all leukemic cells express the 37-kD form of FasL, only 54% of acute myeloblastic leukemia and 27% of chronic myeloid leukemia (CML) cells bore a FasL with killing properties, as assessed by the ability of leukemic cells to cause the apoptosis of a Fas-sensitive target cell line or autologous activated T cells in 3 tested leukemic cases. Experiments with a recombinant Fas-Fc molecule confirmed the role of Fas/FasL in leukemic-mediated cell death. Only CML leukemic cells from certain individuals contained the 26-kD truncated form of FasL. Thus, myeloid leukemic cells from some, but not all patients can set up a mechanism of immune escape involving the Fas/FasL pathway. This leukemic escape may have implications for patients eligible for adoptive cellular immunotherapy.

Membrane-Bound Fas (Apo-1/CD95) Ligand on Leukemic Cells: A Mechanism of Tumor Immune Escape in Leukemia Patients

There is evidence from bone marrow transplantation that T cells may be involved in the immunologic control of leukemia. But many patients relapse despite a potent graft-versus-leukemia effect mediated by allogeneic T cells. The expression of the FasL protein has been suggested as a mechanism of tumor immune escape. We, therefore, evaluated the capacity of leukemic cells from patients with acute or chronic myelogenous leukemia to escape the allogeneic or autologous immune response by bearing the FasL molecule. Although almost all leukemic cells express the 37-kD form of FasL, only 54% of acute myeloblastic leukemia and 27% of chronic myeloid leukemia (CML) cells bore a FasL with killing properties, as assessed by the ability of leukemic cells to cause the apoptosis of a Fas-sensitive target cell line or autologous activated T cells in 3 tested leukemic cases. Experiments with a recombinant Fas-Fc molecule confirmed the role of Fas/FasL in leukemic-mediated cell death. Only CML leukemic cells from certain individuals contained the 26-kD truncated form of FasL. Thus, myeloid leukemic cells from some, but not all patients can set up a mechanism of immune escape involving the Fas/FasL pathway. This leukemic escape may have implications for patients eligible for adoptive cellular immunotherapy.

Herpes simplex virus type 1 infection of activated cytotoxic T cells: Induction of fratricide as a mechanism of viral immune evasion

Herpes simplex virus type 1 (HSV1), a large DNA-containing virus, is endemic in all human populations investigated. After infection of mucocutaneous surfaces, HSV1 establishes a latent infection in nerve cells. Recently, it was demonstrated that HSV1 can also infect activated T lymphocytes. However, the consequences of T cell infection for viral pathogenesis and immunity are unknown. We have observed that in contrast to the situation in human fibroblasts, in human T cell lines antigen presentation by major histocompatibility complex class I molecules is not blocked after HSV1 infection. Moreover, HSV1 infection of T cells results in rapid elimination of antiviral T cells by fratricide. To dissect the underlying molecular events, we used a transgenic mouse model of HSV1 infection to demonstrate that CD95 (Apo-1, Fas)-triggered apoptosis is essential for HSV1-induced fratricide, whereas tumor necrosis factor (TNF) also contributes to this phenomenon but to a lesser extent. By contrast, neither TRAIL (TNF-related apoptosis-inducing ligand) nor perforin were involved. Finally, we defined two mechanisms associated with HSV1-associated fratricide of antiviral T cells: (a) T cell receptor-mediated upregulation of CD95 ligand and (b) a viral "competence-to-die" signal that renders activated T lymphocytes susceptible to CD95 signaling. We propose that induction of fratricide is an important immune evasion mechanism of HSV1, helping the virus to persist in the host organism throughout its lifetime.

Fas expression in non-small cell lung cancer: its prognostic effect in completely resected stage III patients

The aim of this study was to examine Fas expression in non-small cell lung cancer (NSCLC) and examine its correlation with clinicopathological features and prognosis. Fas expression was determined by an immunohistochemical analysis using the labelled streptavidin-biotin method from 220 paraffin specimens of completely resected primary stage I-III NSCLC. 80 (36%) of 220 cases were positive for Fas immunostaining. These 80 cases included 44 adenocarcinomas (33%) and 30 squamous cell carcinomas (40%). 33 stage I (33%) 13 (43%) stage II and 34 (37%) stage III tumours were Fas positive. No statistically significant differences were observed regarding the Fas status with respect to age, sex, histological type, or stage of disease. There was no significant difference in survival between early stage (stages I-II) disease patients with positive Fas expression and those with a negative expression (P = 0.719). However, for patients with completely resected stage III tumours, the patients with positive Fas staining were found to survive for a longer period than those with negative staining (P = 0.026).

Death and destruction of activated T lymphocytes

The massive clonal expansion that occurs during an antigen-specific immune response results in the flooding of immune organs with activated T lymphocytes. At the end of a specific response, the vast majority of these activated T cells are cleared from the immune system. The T cells receive signals through specific death receptors that are expressed as a result of activation. Death receptors transmit their apoptotic signals through the activation of caspases. Function of the death receptors is intimately linked to cell-cycle control, and many cell-cycle control proteins are caspase substrates. Among CD8+ T cells, apoptotic death occurs at a specific site, the sinusoids of the liver. The liver appears to contain a mechanism for the trapping and killing of activated T cells, rendering it an immunologically privileged site.

Novel Egr/NF-AT composite sites mediate activation of the CD95 (APO-1/Fas) ligand promoter in response to T cell stimulation

Expression of the CD95 (APO-1/Fas) ligand (CD95L) in activated T cells is a major cause of activation-induced T cell apoptosis. The transcription factors NF-AT and Egr-3 (a member of the immediate-early transcription factors involved in cellular growth and differentiation) have been implicated in activation of the CD95L promoter upon T cell activation. On the basis of DNase I footprinting, electrophoretic mobility shift assay, antibody supershift analysis and transfection studies, we have identified two novel Egr-binding sites 5' upstream of the previously identified Egr site. Mutation analysis of each Egr site shows that all three sites are important for full CD95L promoter activity. Strikingly, all Egr sites, including the previously identified Egr site, are adjacent to or overlap with DNA sequences homologous to NF-AT binding sites and confer T cell activation-induced, cyclosporin A-sensitive transcriptional activity. Antibody supershift analysis revealed that NF-AT and Egr proteins are the components of inducible DNA-binding complexes formed on the two novel Egr sites. Cotransfection experiments showed that Egr-1, Egr-3 and NF-AT display a cooperative and synergistic activation of transcription mediated by these three Egr/NF-AT composite regulatory elements. These findings provide further insight into the mechanisms involved in the regulation of the CD95L expression in response to T cell activation.

Induction of apoptosis and apoptotic mediators in Balb/C splenic lymphocytes by dietary n-3 and n-6 fatty acids

The present study was designed to investigate the effect of dietary n-6 and n-3 polyunsaturated fatty acids (PUFA) on anti-CD3 and anti-Fas antibody-induced apoptosis and its mediators in mouse spleen cells. Nutritionally adequate semipurified diets containing either 5% w/w corn oil (n-6 PUFA) or fish oil (n-3 PUFA) were fed to weanling female Balb/C mice, and 24 wk later mice were sacrificed. In n-3 PUFA-fed mice, serum and splenocyte lipid peroxides were increased by 20 and 28.3% respectively, compared to n-6 PUFA-fed mice. Further, serum vitamin E levels were decreased by 50% in the n-3 PUFA-fed group, whereas higher anti-Fas- and anti-CD3-induced apoptosis (65 and 66%) and necrosis (17 and 25%), compared to the n-6 PUFA-fed group, were found when measured with Annexin V and propidium iodide staining, respectively. In addition, decreased Bcl-2 and increased Fas-ligand (Fas-L) also were observed in the n-3 PUFA-fed group compared to the n-6 PUFA-fed group. No difference in the ratio of splenocyte subsets nor their Fas expression was observed between the n-3 PUFA-fed and n-6 PUFA-fed groups, whereas decreased proliferation of splenocytes was found in n-3 PUFA-fed mice compared to n-6 PUFA-fed mice. In conclusion, our results indicate that dietary n-3 PUFA induces higher apoptosis by increasing the generation of lipid peroxides and elevating Fas-L expression along with decreasing Bcl-2 expression. A reduced proliferative response of immune cells also was observed in n-3 PUFA-fed mice.

CD95/Fas-triggered apoptosis of activated T lymphocytes is prevented by dendritic cells through a CD58-dependent mechanism

T-cell apoptosis is a mechanism regulating T-cell homeostasis. Activation renders T cells susceptible to activation-induced cell death, a process mediated through CD95 ligand/CD95 (Apo-1/Fas) ligation. The aim of this study was to test whether antigen-presenting cells can inhibit CD95/Fas-triggered activation-induced cell death. Dendritic cells (DC), which are highly effective antigen-presenting cells, were generated in vitro from human peripheral blood monocytes by culture in granulocyte-macrophage colony-stimulating factor and interleukin 4. Subsequently, DC were cocultured with activated T cells and the effect of DC on CD95/Fas-mediated apoptosis was determined. Coculture with increasing amounts of DC prevented CD95/Fas-triggered apoptosis in a dose-dependent fashion by inhibiting activation of caspase 8 and caspase 3. This protective effect of the DC on T-cell death could be blocked by 50% by adding an anti-CD58 antibody, whereas further addition of anti-CD80 (B7.1) and anti-CD86 (B7.2) led to an even more pronounced effect. Our findings suggest that DC can protect T cells from activation-induced cell death, with CD58 ligation playing a key role.

Differential modulation of apoptosis sensitivity in CD95 type I and type II cells

We have recently identified two different pathways of CD95-mediated apoptosis (Scaffidi, C., Fulda, S., Srinivasan, A., Feng, L., Friesen, C., Tomaselli, K. J., Debatin, K.-M., Krammer, P. H., and Peter, M. E. (1998) EMBO J. 17, 1675-1687). CD95-mediated apoptosis in type I cells is initiated by large amounts of active caspase-8 formed at the death-inducing signaling complex (DISC) followed by direct cleavage of caspase-3. In contrast, in type II cells very little DISC and small amounts of active caspase-8 sufficient to induce the apoptogenic activity of mitochondria are formed causing a profound activation of both caspase-8 and caspase-3. Only in type II cells can apoptosis be blocked by overexpressed Bcl-2 or Bcl-x(L). We now show that a number of apoptosis-inhibiting or -inducing stimuli only affect apoptosis in type II cells, indicating that they act on the mitochondrial branch of the CD95 pathway. These stimuli include the activation of protein kinase C, which inhibits CD95-mediated apoptosis resulting in a delayed cleavage of BID, and the induction of apoptosis by the ceramide analog C(2)-ceramide. In addition, we have identified the CD95 high expressing cell line Boe(R) as a CD95 apoptosis-resistant type II cell that can be sensitized by treatment with cycloheximide without affecting formation of the DISC. This also places the effects of cycloheximide in the mitochondrial branch of the type II CD95 pathway. In contrast, c-FLIP was found to block CD95-mediated apoptosis in both type I and type II cells, because it acts directly at the DISC of both types of cells.

Death by neglect as a deletional mechanism of peripheral tolerance

In contrast to most organs, the anatomy of the liver may allow naive CD8(+) T cells to make direct contact with liver parenchymal cells. We have previously shown, using a combination of TCR transgenic T cells specific for H-2 K(b) and hepatocytes expressing a transgenic H-2 K(b) molecule, that hepatocytes can induce antigen-specific activation and proliferation of naive CD8(+) T cells independently of CD28 co-stimulation. However, T cell activation by hepatocytes leads to premature T cell death and tolerance, both in vivo and in vitro. In this study, we investigated the mechanisms of T cell death induced by hepatocytes in vitro using primary hepatocytes to activate purified CD8(+) T cells. Neither Fas nor tumor necrosis factor receptor were involved, indicating that hepatocyte- induced death was distinct from activation-induced cell death. Before they started to divide, T cells activated by hepatocytes expressed lower levels of the bcl-x(L) survival gene and 30 times less IL-2 mRNA than CD8(+) cells activated by splenic antigen-presenting cells. Since CD28 co-stimulation increases both IL-2 and bcl-x(L) expression, this suggests that hepatocyte-activated T cells die by neglect because they fail to receive effective co-stimulatory signals. In agreement with this model, premature death promoted by hepatocytes could be prevented by cross-linking CD28. Survival after CD28 cross-linking correlated with increased IL-2 and bcl-x(L) expression, and sustained T cell proliferation, while cytotoxic T lymphocyte activity was prolonged as compared with cells stimulated without CD28 co-stimulation. This study confirms that high- affinity TCR transgenic antigen-specific CD8(+) T cells can be activated to proliferate and differentiate into cytotoxic effector cells. However, prolonged T cell survival and cytotoxicity required CD28 co-stimulation as well. To our knowledge, this is the first report suggesting that tolerance in the context of lack of CD28 co-stimulation can result from Fas-independent peripheral deletion rather than from anergy.

Fas Is Expressed Early in Human Thymocyte Development But Does Not Transmit an Apoptotic Signal

We investigated the expression and function of Fas on human thymocytes prepared from fetal and pediatric tissue specimens and from SCID-hu Thy/Liv grafts. Unlike mouse thymocytes, human thymocytes exhibited a pattern of Fas expression skewed to immature cells, in that the highest expression was seen on double negative thymocytes and on intrathymic T progenitor cells. Fas expression was intermediate on double positive human thymocytes, and low or negative on mature single positive CD4 and CD8 medullary thymocytes. In spite of this relatively abundant surface expression, cross-linking of Fas with agonist mAb was incapable of triggering an apoptotic signal in human thymocytes. Apoptotic signaling was not enhanced by treatment with cycloheximide, nor by restoring a cosignaling milieu by addition of thymic stromal cells. Mouse thymocytes were induced to apoptosis by cross-linked recombinant soluble human Fas ligand both in vitro and in vivo, though human thymocytes were also resistant to this mode of receptor ligation. Membrane-bound Fas ligand also induced apoptotic death in murine thymocytes but not in human thymocytes. Human thymocytes were as sensitive as Jurkat cells, however, to apoptosis induced by TNF-α, suggesting that these cells have a signaling defect before activation of the earliest caspases. These data demonstrate a durable and specific resistance of human thymocytes to apoptosis induced through Fas receptor engagement, and reveal significant species-specific differences in the biology of thymocyte-programmed cell death.

Aberrant expression and reverse signalling of CD70 on malignant B cells

In normal lymphoid tissues the tumour necrosis factor-receptor family member CD27 and its ligand CD70 have a restricted expression pattern. Previously, we reported that expression of CD27 is deregulated in B-cell leukaemias and lymphomas. Here we show that, although infrequently expressed by normal human B cells in vivo, CD70 is found on 50% of B-CLLs, 33% of follicle centre lymphomas, 71% of large B-cell lymphomas, and 25% of mantle cell lymphomas. Interestingly, in the majority of leukaemias and lymphomas examined, CD70 was found to have a capped appearance, a feature that coincided with co-expression of CD27. Functional analysis showed that a subset of B-CLLs could proliferate vigorously in response to CD70 mAb but not to CD27 mAb. This response was synergistically enhanced by ligation of CD40 but inhibited by the presence of IL-4. Additional experiments indicated that the proliferative response was due to an agonistic signal delivered via CD70, rather than blocking of negative signalling by CD27. Thus, next to its role as ligand, in a subset of malignant B cells CD70 can operate as receptor and as such might contribute to progression of these B-cell malignancies.

An in vitro model of allogeneic stimulation of cord blood: induction of Fas independent apoptosis

Cord blood is increasingly used in transplantation as it is a readily available source of progenitor cells and is reputed to generate less severe graft-versus-host disease (GVHD) than adult bone marrow. We have compared apoptosis of cord blood lymphocytes (CB) and adult lymphocytes (PBMC) after stimulation via HLA class I, HLA class II or CD3 in order to reproduce in vitro some of the stimuli occurring after allotransplantation. CB spontaneously apoptose more than PBMC ex vivo, stimulation via HLA class I dramatically increased CB apoptosis without altering viability of PBMC. Expression of Fas was markedly lower on CB than on PBMC and this difference was maintained even after activation. Fas ligand was expressed in CB and in PBMC. CB were activated via either HLA class I or class II molecules although proliferation was not observed. Only phorbol ester pre-activation allowed Fas to subsequently induce a death signal. Proliferation of PBMC via CD3 led to enhanced Fas signals. CB therefore differ from PBMC with regard to both spontaneous and activation induced apoptosis and either allo- or CD3 mediated stimulation. Finally, the apoptosis of CB via HLA-class I could have an important role in the moderation of graft-versus-host disease.

CD47 Signals T Cell Death

Activation-induced death of T cells regulates immune responses and is considered to involve apoptosis induced by ligation of Fas and TNF receptors. The role of other receptors in signaling T cell death is less clear. In this study we demonstrate that activation of specific epitopes on the Ig variable domain of CD47 rapidly induces apoptosis of T cells. A new mAb, Ad22, to this site induces apoptosis of Jurkat cells and CD3ε-stimulated PBMC, as determined by morphological changes, phosphatidylserine exposure on the cell surface, uptake of propidium iodide, and true counts by flow cytometry. In contrast, apoptosis was not observed following culture with anti-CD47 mAbs 2D3 or B6H12 directed to a distant or closely adjacent region, respectively. CD47-mediated cell death was independent of CD3, CD4, CD45, or p56lck involvement as demonstrated by studies with variant Jurkat cell lines deficient in these signaling pathways. However, coligation of CD3ε and CD47 enhanced phosphatidylserine externalization on Jurkat cells with functional CD3. Furthermore, normal T cells required preactivation to respond with CD47-induced apoptosis. CD47-mediated cell death appeared to proceed independent of Fas or TNF receptor signaling and did not involve characteristic DNA fragmentation or requirement for IL-1β-converting enzyme-like proteases or CPP32. Taken together, our data demonstrate that under appropriate conditions, CD47 activation results in very rapid T cell death, apparently mediated by a novel apoptotic pathway. Thus, CD47 may be critically involved in controlling the fate of activated T cells.

MEK1 activation rescues Jurkat T cells from Fas-induced apoptosis

Although the protease cascade initiated by Fas (CD95, Apo-1) is well characterized, there remains little known about how kinase pathways may impact on Fas-mediated apoptosis. We recently observed that in T lymphocytes Fas strongly induced activation of JNK (c-Jun N-terminal kinase) but not of second messengers leading to activation of ERK (extracellular regulated kinase). Additionally, Fas-mediated apoptosis was significantly inhibited with PMA, a potent activator of the ERK signaling pathway. This suggested a model whereby activation of the ERK pathway might attenuate Fas-mediated apoptosis. This was confirmed in the current study by showing that activation of MEK1, the upstream regulator of ERK, reduces Fas-mediated apoptosis, whereas inhibition of MEK1 augments apoptosis by Fas. Furthermore, Fas-mediated apoptosis of Jurkat T cells is not affected by constitutively active or dominant negative variants that modulate the JNK pathway. These results demonstrate that Fas-induced JNK activation is not required for apoptosis by Jurkat T cells, but rather is more likely secondary to cell stress during the early phases of apoptosis. This is supported by the ability of the caspase blocker zVAD to inhibit both apoptosis and JNK activation by Fas.

T cell signaling: a decision of life and death

T lymphocytes constitute an essential part of the immune system. Their generation, activation, proliferation but also survival is subject to tight regulation by several extracellular factors including cytokines, MHC-antigen complexes and co-stimulatory ligands. The balanced interplay between these factors determines the fate of the T cell. Both in thymic development and in a peripheral immune response, triggering of the T cell antigen receptor (TCR) through interaction with the MHC-antigen complex can result in T cell proliferation. However, in the absence of co-stimulatory signals from antigen-presenting cells a state of non-responsiveness is induced that is called anergy. In addition, stimulation of the TCR on activated T cells or thymocytes can lead to the induction of apoptosis. Here we will give an overview of the intracellular signal transduction pathways that are activated by the stimuli that dictate the fate of a T cell as they were presented at the International Symposium on soluble HLA antigens held in 1997 in Brussels.

Functional Fas Expression in Human Thymic Epithelial Cells

Fas, a cell surface receptor, can induce apoptosis after cross-linking with its ligand. We report that Fas antigen is constitutively expressed in medullary epithelial cells of the human thymus. Expression is decreased in cultured thymic epithelial cells (TEC), similarly to HLA-DR antigen. TEC are resistant to anti-Fas–induced apoptosis after 4 days of primary culture, and this resistance is reversed by concomitant addition of cycloheximide. Cycloheximide also downregulated the expression of Fas-associated phosphatase-1, which has been found to inhibit Fas-induced apoptosis. This phosphatase could be involved in the resistance to Fas-induced apoptosis observed on day 4 of TEC culture. When TEC were subcultured after 10 to 13 days of primary culture, exposure to interleukin-1-β, tumor necrosis factor-, and interferon-γ, alone or together, reinduced Fas mRNA and protein expression. In coculture with activated thymocytes, TEC also upregulated Fas protein expression. Cytokine-activated TEC became sensitive to apoptosis induced by an agonistic anti-Fas antibody. This apoptosis was inhibited by Z-VAD-fmk but not by Z-DEVD-fmk and DEVDase activity was slightly increased in Fas-stimulated TEC, suggesting that DEVDase activity is not sufficient to induce TEC apoptosis. Taken together, these data show that the Fas receptor is expressed in medullary epithelial cells of the human thymus and is able to induce apoptosis.

Functional Fas Expression in Human Thymic Epithelial Cells

Fas, a cell surface receptor, can induce apoptosis after cross-linking with its ligand. We report that Fas antigen is constitutively expressed in medullary epithelial cells of the human thymus. Expression is decreased in cultured thymic epithelial cells (TEC), similarly to HLA-DR antigen. TEC are resistant to anti-Fas–induced apoptosis after 4 days of primary culture, and this resistance is reversed by concomitant addition of cycloheximide. Cycloheximide also downregulated the expression of Fas-associated phosphatase-1, which has been found to inhibit Fas-induced apoptosis. This phosphatase could be involved in the resistance to Fas-induced apoptosis observed on day 4 of TEC culture. When TEC were subcultured after 10 to 13 days of primary culture, exposure to interleukin-1-β, tumor necrosis factor-, and interferon-γ, alone or together, reinduced Fas mRNA and protein expression. In coculture with activated thymocytes, TEC also upregulated Fas protein expression. Cytokine-activated TEC became sensitive to apoptosis induced by an agonistic anti-Fas antibody. This apoptosis was inhibited by Z-VAD-fmk but not by Z-DEVD-fmk and DEVDase activity was slightly increased in Fas-stimulated TEC, suggesting that DEVDase activity is not sufficient to induce TEC apoptosis. Taken together, these data show that the Fas receptor is expressed in medullary epithelial cells of the human thymus and is able to induce apoptosis.

Apoptotic human lymphocytes have diminished CD4 and CD8 receptor expression

We used quantitative multiparameter flow cytometric assays to simultaneously detect viable, apoptotic, and necrotic human peripheral blood mononuclear cells (PBMC) and immunophenotyped lymphocyte subsets within the PBMC. Apoptosis was induced by a spectrum of treatments, including camptothecin, cisplatin, dexamethasone, hyperthermia, staurosporine, and etoposide in anti-CD3 mAb-stimulated cells and by cyclohexamide in both quiescent and stimulated cells; apoptosis in the latter was augmented by anti-fas mAb. We found that CD4(+) and CD8(+) cells were significantly underrepresented in the apoptotic PBMC and that the percentage of CD4(+) and CD8(+) PBMC each markedly decreased as apoptosis increased. This suggested that surface expression of these receptors was lessened on apoptotic CD4(+) and CD8(+) cells. This was directly confirmed by observation of sorted CD4(+) PBMC. This analysis of a wide variety of apoptotic stimuli demonstrates that diminished CD4 and CD8 surface receptor expression is a common feature of human T lymphocyte apoptosis.

TCR Engagement Regulates Differential Responsiveness of Human Memory T Cells to Fas (CD95)-Mediated Apoptosis

In this work, we have tried to establish whether human memory T cells may be protected from Fas (CD95)-induced apoptosis when correctly activated by Ag, and not protected when nonspecifically or incorrectly activated. In particular, we wanted to investigate the molecular mechanisms that regulate the fate of memory T cells following an antigenic challenge. To address this issue, we chose an experimental system that closely mimics physiological T cell activation such as human T cell lines and clones specific for viral peptides or alloantigens. We demonstrate that memory T cells acquire an activation-induced cell death (AICD)-resistant phenotype when TCRs are properly engaged by specific Ag bound to MHC molecules. Ag concentration and costimulation are critical parameters in regulating the protective effect. The analysis of the mechanisms involved in the block of CD95 signal transduction pathways revealed that the crucial events are the inhibition of CD95-associated IL-1β-converting enzyme (ICE)-like protease (FLICE) activation and poly(ADP)-ribose polymerase cleavage, and the mRNA expression of FLICE-like inhibitory protein. Furthermore, we have observed that TCR-mediated neosynthesis of FLICE-like inhibitory protein mRNA is suppressed either by protein tyrosine kinase inhibitors or cyclosporin A. In conclusion, the present analysis of the effects of TCR triggering on the regulation of AICD suggests that AICD could be inhibited in human memory T cells activated in vivo by a foreign Ag, but may become operative when the Ag has been cleared.

Functional expression of Fas and Fas ligand on human intestinal intraepithelial lymphocytes

Intestinal intraepithelial lymphocytes (IEL) constitute the first lymphoid compartment to encounter dietary antigens and intestinal pathogens. IEL are proposed to be involved in the defence against bacterial and viral invasion and to play an important role in mucosal immunity. Fas (CD95/APO-1) is a surface receptor that induces apoptotic cell death upon ligation with Fas ligand (FasL). The aim of this study was to examine the expression and function of Fas and FasL on freshly isolated normal human colonic IEL. The expression and function of Fas and FasL on IEL isolated from 40 normal colonic specimens were examined by flow cytometry, reverse transcriptase-polymerase chain reaction, immunohistochemistry, and DNA-release cytotoxicity assay. Virtually all CD3+ IEL (95.2 +/- 4.3%) expressed Fas and were sensitive to agonistic anti-Fas antibody, whereas only 56.6 +/- 8.4% of peripheral T lymphocytes expressed Fas and were resistant to the antibody. We also detected FasL mRNA and protein (40.1 +/- 4.2%) on IEL, and found that IEL exerted FasL-mediated cytotoxicity against Fas-expressing target cells. These findings suggest that human IEL are activated in situ but are tightly regulated by the constitutive expression of functional Fas and FasL to maintain homeostasis of the mucosal immune system.

Potent Inhibition of CD4/TCR-Mediated T Cell Apoptosis by a CD4-Binding Glycoprotein Secreted from Breast Tumor and Seminal Vesicle Cells

We previously isolated a CD4 ligand glycoprotein, gp17, from human seminal plasma; this glycoprotein is identical with gross cystic disease fluid protein-15 (GCDFP-15), a factor specifically secreted from primary and secondary breast tumors. The function of gp17/GCDFP-15 in physiological as well as in pathological conditions has remained elusive thus far. As a follow up to our previous findings that gp17 binds to CD4 with high affinity and interferes with both HIV-1 gp120 binding to CD4 and syncytium formation, we investigated whether gp17 could affect the T lymphocyte apoptosis induced by a separate ligation of CD4 and TCR. We show here that gp17/GCDFP-15 is in fact a strong and specific inhibitor of the T lymphocyte programmed cell death induced by CD4 cross-linking and subsequent TCR activation. The antiapoptotic effect observed in the presence of gp17 correlates with a moderate up-regulation of Bcl-2 expression in treated cells. The presence of gp17 also prevents the down-modulation of Bcl-2 expression in Bcl-2bright CD4+ T cells that is caused by the triggering of apoptosis. Our results suggest that gp17 may represent a new immunomodulatory CD4 binding factor playing a role in host defense against infections and tumors.

A Tailless Fas-FADD Death-Effector Domain Chimera Is Sufficient to Execute Fas Function in T Cells But Not B Cells of MRL-lpr/lpr Mice

The Fas receptor delivers signals crucial for lymphocyte apoptosis through its cytoplasmic death domain. Several Fas cytoplasmic-associated proteins have been reported and studied in cell lines. So far, only Fas-associated death domain protein (FADD), another death domain-containing molecule has been shown to be essential for Fas signals in vivo. FADD is thought to function by recruiting caspase-8 through its death-effector domain. To test whether FADD is sufficient to deliver Fas signals, we generated transgenic mice expressing a chimera comprised of the Fas extracellular domain and FADD death-effector domain. Expression of this protein in lymphocytes of Fas-deficient MRL-lpr/lpr mice completely diminishes their T cell but not their B cell abnormalities. These results suggest that FADD alone is sufficient for initiation of Fas signaling in primary T cells, but other pathways may operate in B cells.

Dendritic cells prevent CD95 mediated T lymphocyte death through costimulatory signals

T cell apoptosis is a mechanism regulating T cell homeostasis. Prolonged stimulation renders T cells susceptible to activation induced cell death (AICD), a process mediated through CD95 (Apo-1/Fas). While under some circumstances AICD can be prevented, little is known about molecules involved. Here, we wanted to assess whether dendritic cells (DC) have the capacity to prevent CD95-dependent AICD. T cells activated with PHA/PMA or anti-CD3 monoclonal antibody (mAb) were cocultured with increasing amounts of DC. While spontaneous T cell apoptosis amounted to 25%, the presence of an agonistic anti-CD95 antibody increased cell death to 64%. Addition of scalar amounts of DC prevented T cell apoptosis in a dose dependent fashion, where coculture of 10(5) DC/ml with 10(6) T cells/ml reduced apoptosis almost to baseline level (33%). Further addition of an anti-CD58 antibody partially abolished this protective effect. This was even more pronounced if anti-CD80 and anti-CD86 antibodies were added. Our findings suggest that dendritic cells are able to rescue T cells from AICD, with CD58 ligation playing a key role.

Resistance to Fas-Mediated T Cell Apoptosis in Asthma

Over activation of CD4+ T cells in the peripheral blood and airway tissues is characteristic of asthma; therefore, we investigated whether activated T cells from asthmatic subjects have altered apoptotic potential through the Fas death receptor. We found that mitogen-stimulated peripheral blood T cells of asthmatic subjects expressed cell surface Fas, but failed to undergo the normal degree of apoptosis after Fas receptor ligation. T cells from asthmatics exhibited normal apoptotic responses to γ-irradiation (dependent on IL-1 converting enzyme family proteases), ceramide, and mitogen challenge, suggesting functional integrity of the apoptotic pathway. Furthermore, the defect in Fas-dependent apoptosis was overcome by prestimulation with allogeneic accessory cells instead of mitogen. Taken together, the findings suggest that selective resistance to Fas-dependent apoptosis reflects altered Ag-driven, accessory cell-dependent signaling and that ineffective activation of Fas signal transduction may contribute to T cell-dependent immunoinflammation in asthma.

The role of c-FLIP in modulation of CD95-induced apoptosis

Upon stimulation, CD95 (APO-1/Fas) recruits the adapter molecule Fas-associated death domain protein (FADD)/MORT1 and caspase-8 (FADD-like interleukin-1beta-converting enzyme (FLICE)/MACH/MCH5) into the death-inducing signaling complex (DISC). Recently, a molecule with sequence homology to caspase-8 was identified, termed cellular FLICE-inhibitory protein (c-FLIP). c-FLIP has been controversially reported to possess apoptosis-promoting and -inhibiting functions. Using c-FLIP-specific monoclonal antibodies, we now show that c-FLIP is expressed in two isoforms, both of which, like FADD and caspase-8, are recruited to the CD95 DISC in a stimulation-dependent fashion. In stably transfected BJAB cells, c-FLIP blocks caspase-8 activation at the DISC and thereby inhibits CD95-mediated apoptosis. During this process, both caspase-8 and c-FLIP undergo cleavage between the p18 and p10 subunits, generating two stable intermediates of 43 kDa that stay bound to the DISC. c-FLIP has been suggested to play a role in protecting activated peripheral T cells from CD95-mediated apoptosis (Irmler, M., Thome, M., Hahne, M., Schneider, P., Hofmann, K., Steiner, V., Bodmer, J. L. , Schroter, M., Burns, K., Mattmann, C., Rimoldi, D., French, L. E., and Tschopp, J. (1997) Nature 388, 190-195). In contrast to this hypothesis, neither caspase-8 nor c-FLIP were cleaved in these cells, ruling out c-FLIP as the main factor regulating DISC activity. Moreover, recruitment of FADD, caspase-8, and c-FLIP to the DISC was strongly reduced in the apoptosis-resistant but readily detectable in the apoptosis-sensitive T cells.

Fas and Fas-ligand are expressed in the uteroplacental unit of first-trimester pregnancy

PROBLEM

Fas and Fas-ligand (FasL) are thought to provide a strategy for reducing graft rejection in immunologically 'privileged' tissues by controlling injurious lymphocyte reactions. As the uteroplacental unit is often defined as an immune-privileged site, we investigated the expression of Fas and FasL in this tissue in the first trimester of pregnancy.

METHOD OF STUDY

Western blotting, immunohistochemistry, and double immunofluorescence were used for this examination.

RESULTS

Western blotting with purified first-trimester trophoblast cells revealed one specific band for FasL. The presence of FasL on different trophoblast populations could be confirmed by immunohistochemistry and double immunofluorescence. In the villous part of the placenta, FasL is mostly located on cytotrophoblast cells with no access to maternal blood flow, whereas in trophoblast-invaded uterine tissue, interstitial trophoblast cells, which are in close contact with maternal leukocytes, revealed a strong signal for FasL, but no staining for Fas on these cells. However, Fas was found on CD45+ maternal leukocytes.

CONCLUSION

Based on our experimental findings, we speculate that the abundant presence of FasL on trophoblast cells within the maternal decidua may play an important role in the maintenance of immune privilege in the pregnant uterus by endowing fetal trophoblast cells with a defense mechanism against activated maternal leukocytes, whereas in the villous part of the placenta, the Fas FasL system seems to be involved in the regulation of placental growth.