Cellular FLICE-inhibitory protein is required for T cell survival and cycling

Fas-associated death domain (FADD) and caspase-8 are key signal transducers for death receptor–induced apoptosis, whereas cellular FLICE-inhibitory protein (cFLIP) antagonizes this process. Interestingly, FADD and caspase-8 also play a role in T cell development and T cell receptor (TCR)–mediated proliferative responses. To investigate the underlying mechanism, we generated cFLIP-deficient T cells by reconstituting Rag^−/− blastocysts with cFLIP-deficient embryonic stem cells. These Rag chimeric mutant mice (rcFLIP^−/−) had severely reduced numbers of T cells in the thymus, lymph nodes, and spleen, although mature T lymphocytes did develop. Similar to FADD- or caspase-8–deficient cells, rcFLIP^−/− T cells were impaired in proliferation in response to TCR stimulation. Further investigation revealed that cFLIP is required for T cell survival, as well as T cell cycling in response to TCR stimulation. Interestingly, some signaling pathways from the TCR complex appeared competent, as CD3 plus CD28 cross-linking was capable of activating the ERK pathway in rcFLIP^−/− T cells. We demonstrate an essential role for cFLIP in T cell function.