Modulation of CD4+ T-cell activation by CD95 co-stimulation

For 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 co-stimulation 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-κB- 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-κB by directly interacting with the IKK complex.8 So far, three cFLIP isoforms (cFLIPL, cFLIPS and cFLIPR) were identified, with cFLIPS/R mediating a block in apoptosis by inhibiting procaspase-8 at the death-inducing signaling complex (DISC). The role of cFLIPL regarding an inhibition at the DISC is still a matter of debate.8, 9 Further downstream, antiapoptotic proteins including Bcl-2/Bcl-XL and ‘X-linked inhibitor-of-apoptosis protein' (XIAP) may prevent apoptosis.9, 10 A decreased expression of antiapoptotic checkpoint proteins in CD95-sensitive cells could therefore shift NF-κB-associated proliferative signaling pathways to caspase-associated death signaling in the course of cellular activation.11 We now report that CD95 engagement modulates the activation of primary human CD4+ T cells in a dose-dependent manner. Whereas high concentrations of CD95 agonists result in the block of activation, low concentrations augment TCR-induced proliferation. As this differential signaling capacity was seen for different ligands, it points to a novel mechanism to regulate T-cell activation in a context-specific manner. In addition to the inhibitory effect (recently described by Strauss et al.12) we provide a detailed analysis of signaling events associated with the positive co-stimulatory function of CD95. We demonstrate that CD95 ligation with low doses of agonists promotes TCR-triggered MAPK phosphorylation, non-apoptotic caspase activation, and the upregulation of activation markers and antiapoptotic checkpoint proteins. As a consequence, enhanced cell-cycle progression, proliferation and cytokine production are associated with a generalized partial apoptosis resistance.