Caspase-8 and c-FLIPL associate in lipid rafts with NF-κB adaptors during T cell activation

Humans and mice lacking functional caspase-8 in T cells manifest a profound immunodeficiency syndrome due to defective T cell antigen receptor (TCR)-induced NF-kappa B signaling and proliferation. It is unknown how caspase-8 is activated following T cell stimulation, and what is the caspase-8 substrate(s) that is necessary to initiate T cell cycling. We observe that following TCR ligation, a small portion of total cellular caspase-8 and c-FLIPL rapidly migrate to lipid rafts where they associate in an active caspase complex. Activation of caspase-8 in lipid rafts is followed by rapid cleavage of c-FLIPL at a known caspase-8 cleavage site. The active caspase (.) c-FLIP complex forms in the absence of Fas(CD95/APO1) and associates with the NF-kappa B signaling molecules RIP1, TRAF2, and TRAF6, as well as upstream NF-kappa B regulators PKC theta, CARMA1, Bcl-10, and MALT1, which connect to the TCR. The lack of caspase-8 results in the absence of MALT1 and Bcl-10 in the active caspase complex. Consistent with this observation, inhibition of caspase activity attenuates NF-kappa B activation. The current findings define a link among TCR, caspases, and the NF-kappa B pathway that occurs in a sequestered lipid raft environment in T cells.