IntroductionActivation of naive peripheral T cells by their cognate antigen results in induction of their proliferation and differentiation into T effector cells. These comprise CD4 ϩ T-helper cells and CD8 ϩ cytotoxic T cells, which in turn can induce immune responses by the secretion of cytokines, cell-cell interactions, or their ability to directly kill target cells. Importantly, despite high expression of death receptors such as CD95, effector T cells display an antiapoptotic phenotype allowing immune defense function and clearance of invading antigens. 1,2 Once the infection has been efficiently contained, these T cells become apoptosis-sensitive, leading to deletion of most cells and survival of a few memory T cells. 3,4 Apoptosis thus represents a tightly regulated process by which lymphocyte homeostasis is controlled and the emergence of autoreactive cells is prevented.Upon activation, T cells show a rapid up-regulation of antiapoptotic proteins. In addition to Bcl-x L , a strong increase in FLICEinhibitory proteins (FLIPs) is most prominent. 5-7 FLIPs inhibit death-receptor-mediated apoptosis by preventing the cleavage of death-inducing signaling complex (DISC)-associated procaspase-8 and Ϫ10 to the mature active enzymes, thereby blocking initiation of the extrinsic apoptosis cascade. 2,8 Three different isoforms of the cellular FLIP (c-FLIP) have been reported so far, namely c-FLIP long , c-FLIP short , and c-FLIP R , which are generated by alternative splicing. 2,5,9 All 3 isoforms comprise a tandem death effector motif (DED) that is crucial for their recruitment into the DISC. 10 Additionally, c-FLIP long contains an inactive caspase-like domain, whereas each of the 2 short isoforms, c-FLIP short and c-FLIP R , possesses a unique truncated C-terminus. For the latter ones only antiapoptotic functions have been described so far, whereas the role of c-FLIP L remains controversial. Next to its antiapoptotic abilities, low levels of c-FLIP long have also been shown to promote cell death by increasing the enzymatic activity of caspase-8. [11][12][13] In vitro studies with primary human T cells have shown that FLIP proteins, especially c-FLIP short , are highly up-regulated upon T-cell activation, which correlates with a protection against CD95-mediated apoptosis. 7,14 Conversely, treatment of activated T cells with the protein translation inhibitor cycloheximide reduced c-FLIP expression, without affecting Bcl-x L expression, and rendered the cells apoptosis sensitive. 7 Therefore, c-FLIP proteins are thought to play a central role in the protection of activated T cells.How c-FLIP expression in T cells is regulated at the molecular level is a matter of active debate. Inducible c-FLIP expression in peripheral T cells has primarily been linked to the nuclear factor-B (NF-B) signaling pathway [15][16][17][18] ; however, other transcription factor-binding sites do exist in the FLIP promoter. In addition, studies in various tumor cell lines have indicated a regulation of c-FLIP expression by the PI3K/Akt and Er...