Crosslinking of Fas/CD95 suppresses the CD3-mediated signaling events in Jurkat T cells by inhibiting the association of the T-cell receptor zeta chain with src-protein tyrosine kinases and ZAP70

On How Fas Apoptosis-Independent Pathways Drive T Cell Hyperproliferation and Lymphadenopathy in lpr Mice

Fas induces massive apoptosis in T cells after repeated in vitro T cell receptor (TCR) stimulation and is critical for lymphocyte homeostasis in Fas-deficient (lpr) mice. Although the in vitro Fas apoptotic mechanism has been defined, there is a large conceptual gap between this in vitro phenomenon and the pathway that leads to in vivo development of lymphadenopathy and autoimmunity. A striking abnormality in lpr mice is the excessive proliferation of CD4⁺ and CD8⁺ T cells, and more so of the double-negative TCR⁺CD4⁻CD8⁻B220⁺ T cells. The basis of lpr T cell hyperproliferation remains elusive, as it cannot be explained by Fas-deficient apoptosis. T cell-directed p21 overexpression reduces hyperactivation/hyperproliferation of all lpr T cell subtypes and lymphadenopathy in lpr mice. p21 controls expansion of repeatedly stimulated T cells without affecting apoptosis. These results confirm a direct link between hyperactivation/hyperproliferation, autoreactivity, and lymphadenopathy in lpr mice and, with earlier studies, suggest that Fas apoptosis-independent pathways control lpr T cell hyperproliferation. lpr T cell hyperproliferation could be an indirect result of the defective apoptosis of repeatedly stimulated lpr T cells. Nonetheless, in this perspective, we argue for an alternative setting, in which lack of Fas would directly cause lpr T cell hyperactivation/hyperproliferation in vivo. We propose that Fas/Fas ligand (FasL) acts as an activation inhibitor of recurrently stimulated T cells, and that its disruption causes overexpansion of T cells in lpr mice. Research to define the underlying mechanism of this Fas/FasL effect could resolve the phenotype of lpr mice and lead to therapeutics for related human syndromes.

HLA-DR alpha 2 mediates negative signalling via binding to Tirc7 leading to anti-inflammatory and apoptotic effects in lymphocytes in vitro and in vivo

Classically, HLA-DR expressed on antigen presenting cells (APC) initiates lymphocyte activation via presentation of peptides to TCR bearing CD4+ T-Cells. Here we demonstrate that HLA-DR alpha 2 domain (sHLA-DRα2) also induces negative signals by engaging TIRC7 on lymphocytes. This interaction inhibits proliferation and induces apoptosis in CD4+ and CD8+ T-cells via activation of the intrinsic pathway. Proliferation inhibition is associated with SHP-1 recruitment by TIRC7, decreased phosphorylation of STAT4, TCR-ζ chain & ZAP70, and inhibition of IFN-γ and FasL expression. HLA-DRα2 and TIRC7 co-localize at the APC-T cell interaction site. Triggering HLA-DR - TIRC7 pathway demonstrates that sHLA-DRα2 treatment inhibits proinflammatory-inflammatory cytokine expression in APC & T cells after lipopolysaccaride (LPS) stimulation in vitro and induces apoptosis in vivo. These results suggest a novel antiproliferative role for HLA-DR mediated via TIRC7, revise the notion of an exclusive stimulatory interaction of HLA-DR with CD4+ T cells and highlights a novel physiologically relevant regulatory pathway.

Apoptosis and antigen receptor function in T and B cells following exposure to herpes simplex virus

T cells are an essential component of the immune response against herpes simplex virus (HSV) infection. We previously reported that incubation of T cells with HSV-infected fibroblasts inhibits subsequent T cell antigen receptor signal transduction. In the current study, we found that incubation of T cells with HSV-infected fibroblasts also leads to apoptosis in exposed T cells. Apoptosis was observed in Jurkat cells, a T cell leukemia line, and also in CD4(+) cells isolated from human peripheral blood mononuclear cells. Direct infection of these cells with HSV also resulted in apoptosis. Clinical isolates of both HSV type 1 and 2 induced apoptosis in infected T cells at comparable levels to cells infected with laboratory strains of HSV, suggesting an immune evasion mechanism that may be clinically relevant. Further understanding of these viral immune evasion mechanisms could be exploited for better management of HSV infection.

Fas (CD95) ligation inhibits activation of NF-kappa B by targeting p65-Rel A in a caspase-dependent manner

Apoptosis is an important mechanism in T cell regulation. Initiation of apoptosis can be activated through two signaling pathways via proteins that bind the death domain, the MAPK-JNK pathway mediated by DAXX and the caspase pathway mediated by FADD. T cell proliferation is initiated by ligation of the T cell receptor (TCR) and activation of NF-kappaB, a transcription factor that has antiapoptotic functions. These pathways however are not isolated, and potential crosstalk between elements of the apoptotic pathway and growth pathway may be essential in determining cell survival. We studied the interaction between Fas- and the TCR-initiated pathways in Jurkat T cell as these pathways lead to opposing consequences. We show that Fas activation can inhibit TCR- and PMA/ionophore-initiated activation of NF-kappaB activity. The inhibition is caspase-dependent since an inhibitor of caspase activation, DEVD, can block the suppression of NF-kappaB activity following crosslinking of Fas. Analysis of the expression of the subunits of NF-kappaB revealed that the levels of p50 remained constant, whereas the levels of p65 were markedly decreased by crosslinking of Fas. These findings suggest that the Fas-ligation-mediated suppression preferentially targets p65 protein expression as a mechanism for suppression of antiapoptotic activities of NF-kappaB during apoptosis.