Cutting edge: soluble HLA-G1 triggers CD95/CD95 ligand-mediated apoptosis in activated CD8+ cells by interacting with CD8

The nonpolymorphic soluble HLA-G1 (sHLA-G1) isoform has been reported to be secreted by trophoblast cells at the materno-fetal interface, suggesting that it may act as immunomodulator during pregnancy. In this paper, we report that affinity-purified beta2-microglobulin-associated sHLA-G1 triggered apoptosis in activated, but not resting CD8+ peripheral blood cells. We demonstrate by Western blotting that sHLA-G1 enhanced CD95 ligand expression in activated CD8+ cells. Cytotoxicity was inhibited by preincubation of the cells with a CD95 antagonist mAb (ZB4) or a soluble recombinant CD95-Fc, indicating that apoptosis is mediated through the CD95/CD95 ligand pathway. Finally, we show that such sHLA-G1-induced apoptosis depends on the interaction with CD8 molecules, with cell death being blocked by various CD8 mAbs.     

CD 95-independent mechanisms of IL-2 deprivation-induced apoptosis in activated human lymphocytes

Growth factor deprivation-induced apoptosis plays an important role in several cellular systems. However, knowledge of the molecular mechanisms involved are restricted to a few murine models or tumor cell lines. Therefore, we aimed studying signaling pathways leading to apoptosis in activated human peripheral T cells after IL-2 withdrawal. Lymphoblasts from patients with CD 95 (Fas/APO-1)-deficiency revealed that functional CD95 was not required to induce apoptosis after IL-2 withdrawal. Moreover, apoptosis induction in response to various cytotoxic stimuli was found to be mediated in the absence of functional CD95 but was affirmatorily influenced by IL-2 signaling. Immunoblots showed no downregulation of Bcl-2 or Bcl-xL and no upregulation of Bax, whereas decreased mitochondrial membrane potential was readily measurable 24 h after cytokine deprivation. Tetrapeptide inhibitors showed limited efficacy in preventing apoptosis whereas the caspase inhibitor zVAD-FMK potently blocked induction of apoptosis. Cleavage of different fluorogenic substrates revealed multiple caspase enzyme activities in lymphoblasts, which were not negatively affected by the fas mutation. Starting at 8 h after IL-2 withdrawal, upregulation of active caspase-3 but not of caspase-8 could be detected. Taken together, our data argue for molecular mechanisms of cytokine deprivation-induced apoptosis in activated human lymphocytes independent of CD95.     

Signalling by CD95 and TNF receptors: not only life and death

Members of the TNF family of receptors play important roles in normal physiology and in defence. The recent rapid progress in the understanding of the mechanisms of apoptosis has been accompanied by assumptions that TNF family receptors such as CD95(Fas/APO-1) only have a role in regulating cell survival. While regulation of cell death is one important function of TNF family receptors, they are capable of activating signal transduction pathways that have many other effects. The present review will focus on signalling of some TNF family receptors in the immune system, not only for apoptosis, but also for survival or activation.     

Mechanism of Staphylococcus aureus exotoxin A inhibition of Ig production by human B cells

Staphylococcus enterotoxins and toxic shock syndrome toxin 1 are members of a family of exoproteins that are produced by staphylococci and bind specifically to MHC class II molecules. Upon binding to MHC class II molecules, these exoproteins are potent stimulators of T cell proliferation via interaction with specific TCR V-beta segments of both CD4+ and CD8+ T cells. These exoproteins also directly stimulate monocytes to secrete IL-1 and TNF-alpha. Furthermore, these exoproteins have a profound inhibitory effect on Ig production by PBMC. We examined the effects of Staphylococcus enterotoxin A (SEA) on proliferation and Ig production of highly purified human B cells. Our results demonstrated that the binding of SEA to MHC class II molecules on B cells does not alter their ability to proliferate in response to Staphylococcus aureus Cowan strain I (SAC) or to produce Ig in response to SAC plus rIL-2. In contrast, the anti-DR mAb L243 inhibited both B cell proliferation and Ig production. Unable to determine a direct effect of SEA on B cell function, we investigated whether the capacity of SEA to inhibit SAC-induced Ig production by PBMC was T cell-dependent. Our results demonstrated that in the presence of T cells, under appropriate conditions, SEA can either function as a nominal Ag for stimulation of B cell proliferation and Ig production or induce T cell-mediated suppression of Ig production. SEA-induced Ig production required T cell help, which was dependent on pretreatment of the T cells with irradiation or mitomycin C; Ig production was not induced by SEA in the absence of T cells or in the presence of untreated T cells. Furthermore, SEA inhibited Ig production in SAC-stimulated cultures of autologous B cells and untreated T cells; pretreatment of the T cells with irradiation or mitomycin C abrogated SEA-induced inhibition of Ig production. Thus, T cell suppression of SAC-induced Ig production was dependent on T cell proliferation. Similar results were observed with both SEA and toxic shock syndrome toxin 1.     

Induction of apoptosis by IFNgamma in human neuroblastoma cell lines through the CD95/CD95L autocrine circuit.

The CD95 (APO-1/Fas) system can mediate apoptosis in immune cells as well as in tumour cells, where it may contribute to tumour immune-escape. On the other hand, its induction by anticancer drugs may lead to tumour reduction. Interferongamma (IFNgamma) increases the sensitivity of tumour cell lines to anti-CD95 antibody-mediated apoptosis. We describe induction of apoptosis by IFNgamma through the expression of CD95 and its ligand (CD95L) in human neuroblastoma cell lines. Neuroblastoma cells showed low constitutive expression of CD95 and CD95L. Subsequent to IFNgamma-modulated increase in CD95 and CD95L mRNA as well as protein levels, apoptosis was observed. Our results demonstrated that cytokine-mediated apoptosis was mediated through the activation of the CD95/CD95L autocrine circuit since: (i) cell death occurred following CD95/CD95L expression and correlated with CD95 and CD95L expression levels, (ii) failed to occur in a clone which weakly upregulated CD95 and lacked CD95L induction after IFNgamma stimulation, (iii) was at least partially inhibited by using blocking F(ab')2 anti-CD95 antibody fragments and the recombinant Fas-Fc protein, that prevented the interaction between CD95 and CD95L. The intracellular molecular mechanisms elicited by IFNgamma are clearly highly complex, with several signalling pathways being activated, including the CD95 system. These findings suggest that IFNgamma may have a significant potential in the therapy of neuroblastoma in vivo.     

Induction of isotype switching and Ig production by CD5+ and CD10+ human fetal B cells.

In the present study the capacity of early fetal B cells to produce Ig was investigated. It is shown that B cells from fetal liver, spleen, and bone marrow (BM) can be induced to produce IgM, IgG, IgG4, and IgE, but not IgA, in response to IL-4 in the presence of anti-CD40 mAb or cloned CD4+ T cells. Even splenic B cells from a human fetus of only 12 wk of gestation produced these Ig isotypes. IFN-alpha, IFN-gamma, and transforming growth factor-beta inhibited IL-4-induced IgE production in fetal B cells, as described for mature B cells. The majority of B cells in fetal spleen expressed CD5 and CD10 and greater than 99% of B cells in fetal BM were CD10+. Highly purified CD10+, CD19+ immature B cells and CD5+, CD19+ B cells could be induced to produce Ig, including IgG4 and IgE, in similar amounts as unseparated CD19+ B cells. Virtually all CD19+ cells still expressed CD10 after 12 days of culture. However, the IgE-producing cells at the end of the culture period were found in the CD19-,CD10- cell population, suggesting differentiation of CD19+,CD10+ B cells into CD19-,CD10- plasma cells. Pre-B cells are characterized by their lack of expression of surface IgM (sIgM). Only 30 to 40% of BM B cells expressed sIgM. However, in contrast to sIgM+,CD10+,CD19+ immature B cells, sorted sIgM-,CD10+,CD19+ pre-B cells failed to differentiate into Ig-secreting cells under the present culture conditions. Addition of IL-6 to these cultures was ineffective. Taken together, these results indicate that fetal CD5+ and CD10+ B cells are mature in their capacity to be induced to Ig isotype switching in vitro as soon as they express sIgM.     

RIP-in CD95-induced cell death: The control of alternative death receptors pathways by cIAPs

Comment on: Geserick P, et al. J Cell Biol 2009; 187:1037-54.     

CD134L engagement enhances human B cell Ig production: CD154/CD40, CD70/CD27, and CD134/CD134L interactions coordinately regulate T cell-dependent B cell responses

CD134 is a member of the TNFR family expressed on activated T cells, whose ligand, CD134L, is found preferentially on activated B cells. We have previously reported that the CD70/CD27 interaction may be more important in the induction of plasma cell differentiation after the expansion phase induced by the CD154/CD40 interaction has occurred. When CD134-transfected cells were added to PBMCs stimulated with pokeweed mitogen, IgG production was enhanced in a dose-dependent fashion. Addition of CD134-transfected cells to B cells stimulated with Staphylococcus aureus Cowan I strain/IL-2 resulted in little if any enhancement of B cell IgG production and proliferation. We found that while CD134-transfected cells induced no IgG production by themselves, it greatly enhanced IgG production in the presence of CD40 stimulation or T cell cytokines such as IL-4 and IL-10. The addition of CD134-transfected cells showed only a slight increase in the number of plasma cells compared with that in the culture without them, indicating that an increased Ig production rate per cell is responsible for the observed enhancing effect of CD134L engagement rather than increase in plasma cell generation. These results strongly suggest different and sequential roles of the TNF/TNFR family molecules in human T cell-dependent B cell responses through cell-cell contacts and the cytokine network.     

Induction of proliferation and Ig production in human B leukemic cells by anti-immunoglobulins and T cell factors

The proliferation and differentiation of human leukemic B cells (B-CLL cells) with anti-Ig and T cell-derived helper factors are described. Stimulation of B-CLL cells with anti-Ig and T helper factors could induce proliferation as well as differentiation into IgM- and IgG-producing cells. Neither anti-Ig nor T helper factors alone could induce any proliferation and/or differentiation of B-CLL cells. Not only whole molecules of anti-Ig but also F(ab')2 fragments could induce proliferation and differentiation of B-CLL cells in the presence of T helper factors, but monovalent Fab' fragments were not effective. Induction of both IgM and IgG with the same idiotype was confirmed by immunofluorescent and SDS-PAGE analysis. By employing an IL 2-dependent cytotoxic T cell line and a TRF-responsive B cell line, T cell factors were separated into a fraction with IL2 activity but no TRF activity and a fraction with TRF activity but no IL 2 activity by chromatofocusing. Anti-Ig and IL 2 fraction could induce proliferation of B-CLL cells, but TRF fraction was not effective for the induction of proliferation in anti-IG-stimulated cells. For IgM and IgG production, anti-Ig and both IL 2 and TRF fractions were required. Depletion of IL 2 fraction in the first 2 days' culture inhibited Ig production, whereas the absence of TRF fraction in the first 2 days did not show any inhibitory effect on Ig production.     

Caspase-10-dependent cell death in Fas/CD95 signalling is not abrogated by caspase inhibitor zVAD-fmk

Background

Upon CD95/Fas ligation, the initiator caspase-8 is known to activate effector caspases leading to apoptosis. In the presence of zVAD-fmk, a broad-spectrum caspase inhibitor, Fas engagement can also trigger an alternative, non-apoptotic caspase-independent form of cell death, which is initiated by RIP1. Controversy exists as to the ability of caspase-10 to mediate cell death in response to FasL (CD95L or CD178). Herein, the role of caspase-10 in FasL-induced cell death has been re-evaluated.

Methodology and Principal Findings

The present study shows that FasL-induced cell death was completely impaired in caspase-8- and caspase-10-doubly deficient (I9-2e) Jurkat leukaemia T-cell lines. Over-expressing of either caspase-8 or caspase-10 in I9-2e cells triggered cell death and restored sensitivity to FasL, further arguing for a role of both initiator caspases in Fas apoptotic signalling. In the presence of zVAD-fmk, FasL triggered an alternative form of cell death similarly in wild-type (A3) and in caspase-8-deficient Jurkat cells expressing endogenous caspase-10 (clone I9-2d). Cell death initiated by Fas stimulation in the presence of zVAD-fmk was abrogated in I9-2e cells as well as in HeLa cells, which did not express endogenous caspase-10, indicating that caspase-10 somewhat participates in this alternative form of cell death. Noteworthy, ectopic expression of caspase-10 in I9-2e and HeLa cells restored the ability of FasL to trigger cell death in the presence of zVAD-fmk. As a matter of fact, FasL-triggered caspase-10 processing still occurred in the presence of zVAD-fmk.

Conclusions and Significance

Altogether, these data provide genetic evidence for the involvement of initiator caspase-10 in FasL-induced cell death and indicate that zVAD-fmk does not abrogate caspase-10 processing and cytotoxicity in Fas signalling. Our study also questions the existence of an alternative caspase-independent cell death pathway in Fas signalling.     

CD95 death receptor and epidermal growth factor receptor (EGFR) in liver cell apoptosis and regeneration

Recent evidence suggests that signaling pathways towards cell proliferation and cell death are much more interconnected than previously thought. Whereas not only death receptors such as CD95 (Fas, APO-1) can couple to both, cell death and proliferation, also growth factor receptors such as the epidermal growth factor receptor (EGFR) are involved in these opposing kinds of cell fate. EGFR is briefly discussed as a growth factor receptor involved in liver cell proliferation during liver regeneration. Then the role of EGFR in activating CD95 death receptor in liver parenchymal cells (PC) and hepatic stellate cells (HSC), which represent a liver stem/progenitor cell compartment, is described summarizing different ways of CD95- and EGFR-dependent signaling in the liver. Here, depending on the hepatic cell type (PC vs. HSC) and the respective signaling context (sustained vs. transient JNK activation) CD95-/EGFR-mediated signaling ends up in either liver cell apoptosis or cell proliferation.     

CD83 modulates B cell function in vitro: increased IL-10 and reduced Ig secretion by CD83Tg B cells

The murine transmembrane glycoprotein CD83 is an important regulator for both thymic T cell maturation and peripheral T cell responses. Recently, we reported that CD83 also has a function on B cells: Ubiquitous transgenic (Tg) expression of CD83 interfered with the immunoglobulin (Ig) response to infectious agents and to T cell dependent as well as T cell independent model antigen immunization. Here we compare the function of CD83Tg B cells that overexpress CD83 and CD83 mutant (CD83mu) B cells that display a drastically reduced CD83 expression. Correlating with CD83 expression, the basic as well as the lipopolysaccharide (LPS) induced expression of the activation markers CD86 and MHC-II are significantly increased in CD83Tg B cells and reciprocally decreased in CD83mu B cells. Wild-type B cells rapidly upregulate CD83 within three hours post BCR or TLR engagement by de novo protein synthesis. The forced premature overexpression of CD83 on the CD83Tg B cells results in reduced calcium signaling, reduced Ig secretion and a reciprocally increased IL-10 production upon in vitro activation. This altered phenotype is mediated by CD83 expressed on the B cells themselves, since it is observed in the absence of accessory cells. In line with this finding, purified CD83mu B cells displayed a reduced IL-10 production and slightly increased Ig secretion upon LPS stimulation in vitro. Taken together, our data strongly suggest that CD83 is expressed by B cells upon activation and contributes to the regulation of B cell function.     

Cutting edge: human B cell function is regulated by interaction with soluble CD14: opposite effects on IgG1 and IgE production

The mechanism(s) controlling activation of naive B cells, their proliferation, Ag receptor affinity maturation, isotype switching, and their fate as memory or plasma cells is not fully elucidated. Here we show that between 24 and 60% of CD19+ cells in PBMC bind soluble CD14 (sCD14). Tonsillar B cells also bind sCD14, but preferentially the CD38-ve/low cells. Interaction of sCD14 with B cells resulted in higher levels of IgG1 and marked inhibition of IgE production by activated tonsillar B cells and Ag-stimulated PBMC. We found that sCD14 interfered with CD40 signaling in B cells, inhibited IL-6 production by activated B cells, and increased the kinetics and magnitude of CD40 ligand expression on T cells. Together with the previously reported effects on T cells, these findings define sCD14 as a novel soluble regulatory factor capable of modulating cellular and humoral immune responses by interacting directly with T and B cells.     

Manganese induces apoptosis of human B cells: caspase-dependent cell death blocked by bcl-2.

Manganese ions block apoptosis of phagocytes induced by various agents. The prevention of apoptosis was attributed to the activation of manganous superoxide dismutase (Mn-SOD) and to the antioxidant function of free Mn2+ cations. However, the effect of Mn2+ on B cell apoptosis is not documented. In this study, we investigated the effects of Mn2+ on the apoptotic process in human B cells. We observed that Mn2+ but not Mg2+ or Ca2+, inhibited cell growth and induced apoptosis of activated tonsilar B cells, Epstein Barr virus (EBV)-negative Burkitt's lymphoma cell lines (BL-CL) and EBV-transformed B cell lines (EBV-BCL). In the same conditions, no apoptosis was observed in U937, a monoblastic cell line. Induction of B cell apoptosis by Mn2+ was time- and dose-dependent. The cell permeable tripeptide inhibitor of ICE family cysteine proteases, zVAD-fmk, suppressed Mn2+-induced apoptosis. Furthermore, Mn2+ triggered the activation of interleukin-1beta converting enzyme (ICE/caspase 1), followed by the activation of CPP32/Yama/Apopain/caspase-3. In addition, poly-(ADP-ribose) polymerase (PARP), a cellular substrate for CPP32 protease was degraded to generate apoptotic fragments in Mn2+-treated B cell lines. The inhibitor, zVAD-fmk suppressed Mn2+-triggered CPP32 activation and PARP cleavage and apoptosis. These results indicate that the activation of caspase family proteases is required for the apoptotic process induced by Mn2+ treatment of B cells. While the caspase-1 inhibitor YVAD was unable to block apoptosis, the caspase-3 specific inhibitor DEVD-cmk, partially inhibited Mn2+-induced CPP32 activation, PARP cleavage and apoptosis of cells. Moreover, Bcl-2 overexpression in BL-CL effectively protected cells from apoptosis and cell death induced by manganese. This is the first report showing the involvement of Mn2+ in the regulation of B lymphocyte death presumably via a caspase-dependent process with a death-protective effect of Bcl-2.     

Life and death in mammalian cell culture: strategies for apoptosis inhibition

Mammalian cell culture is widely used to produce valuable biotherapeutics including monoclonal antibodies, vaccines and growth factors. Industrial cell lines such as Chinese hamster ovary (CHO), mouse myeloma (NS0), baby hamster kidney (BHK) and human embryonic kidney (HEK)-293 retain many molecular components of the apoptosis cascade. Consequently, these cells often undergo programmed cell death upon exposure to stresses encountered in bioreactors. The implementation of strategies to control apoptosis and enhance culture productivities represents a major goal of biotechnologists. Fortunately, previous research has uncovered many intracellular proteins involved in activating and inhibiting apoptosis. Here, we summarize three apoptotic pathways and discuss different environmental and genetic methodologies implemented to limit cell death for biotechnology applications.     

Interferon alpha augments activation-induced T cell death by upregulation of Fas (CD95/APO-1) and Fas ligand expression.

Interferon alpha (IFN-alpha) plays a prominent role in the therapy of a variety of diseases. The Fas/FasL system is crucial for the cytotoxic function and the peripheral elimination of activated T lymphocytes (ATC) by a mechanism referred to as activation-induced cell death (AICD). Recent studies suggest a link between IFN-alpha, the 2', 5'- oligoadenylate system and apoptosis. We therefore asked whether IFN-alpha is able to regulate the Fas/FasL pathway and thereby affects AICD. Peripheral blood mononuclear cells (PBMC), purified T cells and ATC of healthy volunteers were stimulated with various agents and the influence of IFN-alpha on Fas/FasL was assessed by mRNA and protein studies. The proportion of ATC undergoing AICD or anti-Fas-induced apoptosis was determined by FITC-annexin V staining and propidium iodide uptake. IFN-alpha upregulated mRNA expression of Fas and FasL in activated PBMC. Furthermore the concentration of the soluble form of FasL (sFasL) was increased in PBMC and T cells co-stimulated with IFN-alpha and various agents, whereas Fas surface expression was enhanced by IFN-alpha alone. IFN-alpha enhanced apoptosis induced by anti-Fas antibody and augmented AICD via the Fas/FasL pathway. IFN-alpha-regulated AICD may contribute to lymphopenia observed during IFN-alpha therapy. Our data further support that IFN-alpha is a multifunctional cytokine with profound effects on the immune cascades.     

Erk-dependent cytosolic phospholipase A2 activity is induced by CD95 ligand cross-linking in the mouse derived Sertoli cell line TM4 and is required to trigger apoptosis in CD95 bearing cells

In the present study we demonstrated that CD95L cross-linking generated reverse signalling in the mouse derived Sertoli cell line TM4. Treatment of TM4 cells with mAb anti-CD95L induced activation of the cytosolic phospholipase A2 (cPLA2). Cytosolic PLA2 activation was controlled by the MAPK pathway as indicated by the ability of the specific MEK inhibitor, PD098059, to abolish cPLA2 activation. In addition, Western blot experiments showed a rapid increase in phosphorylated Erk1/2 following CD95L cross-linking, while no effect on the phosphorylation of other MAPK, p38 or JNK, was observed. CD95L cross-linking by mAb increased the levels of soluble CD95L and apoptotic activity of TM4 cell supernatants, which was blocked by co-incubation with the PLA2 inhibitor, AACOCF3 or PD098059. Finally, pre-treatment of TM4 cells with AACOCF3 or PD098059 completely abolished TM4-induced apoptosis of Jurkat T cells, thus indicating that the Erk/cPLA2 pathway is required for CD95L-induced apoptosis.     

Possible role of fas antigen (CD 95) in human amniotic epithelial cell death: an in vitro study

Treatment of human amniotic epithelial (HAE) cells with anti-Fas monoclonal antibody (CH 11) at 100 ng/ml or 1 microg/ml for 12 or 24 h increased necrotic cell death. Apoptotic cell death induced by this antibody was significantly increased, although far fewer cells underwent apoptosis, as determined by the TUNEL method. This study suggests that Fas antigen is an important mediator in HAE cell death.     

Separation of events mediating B cell proliferation and Ig production by using T cell membranes and lymphokines

The initiation by Th cells of B cell proliferation and differentiation to produce Ig involves both cell contact- and lymphokine-mediated signals. Plasma membrane-enriched fractions from stimulated, but not unstimulated, Th cells induced Ag nonspecific and MHC unrestricted proliferation of 60 to 70% of small dense B cells. Induction of stimulatory membrane activity was inhibited by cycloheximide, and the activity was eliminated by both protease and heat treatment of membranes. Membrane-stimulated B cells did not differentiate to secrete Ig; however, addition of a lymphokine-containing supernatant from activated Th cells or the combination of IL-4 and IL-5 resulted in substantial Ig production, predominantly of the IgM, IgG1, IgA, and IgE isotypes. The quantity and isotype distribution of the antibodies secreted were similar to those produced after B cell activation by the intact Th cells and Ag. Therefore, membranes from activated Th cells in combination with lymphokines normally secreted by such cells can replace intact Th cells and provide a defined system to identify molecular events important for B cell activation.