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Inhibition of caspase or FADD function blocks proliferation but not MAP kinase-activation and interleukin-2-production during primary stimulation of T cells.

Caspases are instrumental in the implementation of apoptotic cell death, and caspase activation is in most investigated cases closely linked to apoptosis. Recent data demonstrate, however, that caspases are also activated during primary T cell activation in the absence of apoptosis. Here we provide evidence that caspase activity is required for some but not all aspects of T cell activation. CD3-triggered proliferation of mouse T cells was impaired in the presence of the pan-caspase-inhibitor Z-Val-Ala-Asp-fluoromethylketone (Z-VAD-fmk) and the number of cells entering the cell cycle was reduced. Costimulation by CD28 or externally added interleukin-2 (IL-2) failed to rescue proliferation. Re-stimulation of pre-activated T cells, however, was not affected by Z-VAD-fmk. Intriguingly, CD3-induced production of IL-2 by primary T cells was not impaired in the presence of Z-VAD-fmk. Likewise, CD3-induced activation of mitogen-activated protein kinases was unaffected by Z-VAD-fmk and intracellular levels of inhibitory kappaBalpha were the same as in control cells. T cells transgenically expressing a dominant negative mutant of the caspase-adaptor Fas-associated molecule with death domain (FADD)/MORT1 displayed the same pattern of reaction, i.e. a reduced proliferative response but normal IL-2-production. These data show a distinct role of caspases during primaryT cell activation and provide evidence for a FADD-caspase-pathway not only in the induction of apoptosis but also of T cell proliferation.