CD95 is a dual-function receptor that exerts pro-or antiapoptotic effects depending on the cellular context, the state of activation, the signal threshold and the mode of ligation. In this study, we report that CD95 engagement modulates TCR/CD3-driven signaling pathways in resting T lymphocytes in a dose-dependent manner. While high doses of immobilized CD95 agonists silence T cells, lower concentrations augment activation and proliferation. We analyzed the co-stimulatory capacity of CD95 in detail in resting human CD4 þ T cells, and demonstrate that low-dose ligand-induced co-internalization of CD95 and TCR/CD3 complexes enables non-apoptotic caspase activation, the prolonged activation of MAP kinases, the upregulation of antiapoptotic proteins associated with apoptosis resistance, and the activation of transcription factors and cell-cycle regulators for the induction of proliferation and cytokine production. We propose that the levels of CD95L on antigen-presenting cells (APCs), neighboring T cells or epithelial cells regulate inhibitory or co-stimulatory CD95 signaling, which in turn is crucial for fine-tuning of primary T-cell activation. Cell Death and Differentiation (2011) 18, 619-631; doi:10.1038/cdd.2010.134; published online 5 November 2010For activation of resting T cells, two signals are required. The first signal emerges from an engagement of the T cell receptor (TCR)/CD3 complex, whereas the second signal is generated through the ligation of co-stimulatory receptors (i.e. CD28). Recently, 'tumor necrosis factor (TNF) receptor-associated factor' (TRAF) binding receptors were identified as a second class of co-stimulatory receptors. 1 As an example, Alderson et al. 2,3 provided first evidence for the role of CD95 (Fas, APO-1), the prototypic death receptor of the immune system, in the activation of human T cells. It was subsequently reported that CD3-crosslinking alone or CD3/CD95 costimulation induces the processing of caspase-8 and/or caspase-3 as a prerequisite for full T-cell proliferation. [4][5][6] CD95 co-ligation also influences several other routes of intracellular signal transduction. Kataoka et al. 7 reported the activation of NF-kB-and mitogen-activated protein kinase (MAPK)-related pathways following an interaction of CD95-recruited 'cellular FLICE-inhibitory protein' (cFLIP) with downstream signaling molecules. Apparently, this process required the cleavage of cFLIP into a p43 fragment. More recently, however, it was argued that p22-FLIP (but not p43) can activate NF-kB by directly interacting with the IKK complex. 8 So far, three cFLIP isoforms (cFLIP L , cFLIP S and cFLIP R ) were identified, with cFLIP S/R mediating a block in apoptosis by inhibiting procaspase-8 at the death-inducing signaling complex (DISC). The role of cFLIP L regarding an inhibition at the DISC is still a matter of debate. 8,9 Further downstream, antiapoptotic proteins including Bcl-2/Bcl-X L and 'X-linked inhibitor-of-apoptosis protein' (XIAP) may prevent apoptosis. 9,10 A decreased expression of antiapoptotic c...

Activation of resting T cells in vitro is triggered by combined TCR and CD28 engagement and can be modulated by simultaneous ligation of various other surface receptors. Although the Fas ligand (FasL) is best known for its capacity to initiate cell death in Fas-bearing cells, it has recently been implicated in the regulation of T cell activation. Thus, a cross-talk between the TCR and FasL is likely, but far from being biochemically elucidated. We now report that FasL engagement by immobilized but not soluble FasFc fusion protein and anti-FasL polyclonal antibody blocks the activation of human peripheral T cells even in the presence of CD28 co-stimulation. The data presented here stress the importance of the Fas/FasL system for signal initiation via the TCR-CD3 complex and provide further arguments for a retrograde signaling capacity of FasL or a crucial role of Fas as a co-stimulatory molecule.