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Cell death is achieved by two fundamentally different mechanisms: apoptosis and necrosis. Apoptosis is dependent on caspase activation, whereas the caspase-independent necrotic signaling pathway remains largely uncharacterized. We show here that Fas kills activated primary T cells efficiently in the absence of active caspases, which results in necrotic morphological changes and late mitochondrial damage but no cytochrome c release. This Fas ligand-induced caspase-independent death is absent in T cells that are deficient in either Fas-associated death domain (FADD) or receptor-interacting protein (RIP). RIP is also required for necrotic death induced by tumor necrosis factor (TNF) and TNF-related apoptosis-inducing ligand (TRAIL). In contrast to its role in nuclear factor kappa B activation, RIP requires its own kinase activity for death signaling. Thus, Fas, TRAIL and TNF receptors can initiate cell death by two alternative pathways, one relying on caspase-8 and the other dependent on the kinase RIP.

Viruses have evolved many distinct strategies to avoid the host's apoptotic response. Here we describe a new family of viral inhibitors (v-FLIPs) which interfere with apoptosis signalled through death receptors and which are present in several gamma-herpesviruses (including Kaposi's-sarcoma-associated human herpesvirus-8), as well as in the tumorigenic human molluscipoxvirus. v-FLIPs contain two death-effector domains which interact with the adaptor protein FADD, and this inhibits the recruitment and activation of the protease FLICE by the CD95 death receptor. Cells expressing v-FLIPs are protected against apoptosis induced by CD95 or by the related death receptors TRAMP and TRAIL-R. The herpesvirus saimiri FLIP is detected late during the lytic viral replication cycle, at a time when host cells are partially protected from CD95-ligand-mediated apoptosis. Protection of virus-infected cells against death-receptor-induced apoptosis may lead to higher virus production and contribute to the persistence and oncogenicity of several FLIP-encoding viruses.

We provide evidence that FLIP is not simply an inhibitor of death-receptor-induced apoptosis but that it also mediates the activation of NF-kappaB and Erk by virtue of its capacity to recruit adaptor proteins involved in these signaling pathways.

The paracaspase MALT1 is pivotal in antigen receptor-mediated lymphocyte activation and lymphomagenesis. MALT1 contains a caspase-like domain, but it is unknown whether this domain is proteolytically active. Here we report that MALT1 had arginine-directed proteolytic activity that was activated after T cell stimulation, and we identify the signaling protein Bcl-10 as a MALT1 substrate. Processing of Bcl-10 after Arg228 was required for T cell receptor-induced cell adhesion to fibronectin. In contrast, MALT1 activity but not Bcl-10 cleavage was essential for optimal activation of transcription factor NF-kappaB and production of interleukin 2. Thus, the proteolytic activity of MALT1 is central to T cell activation, which suggests a possible target for the development of immunomodulatory or anticancer drugs.

TRAIL induces apoptosis through two closely related receptors, TRAIL-R1 (DR4) and TRAIL-R2 (DR5). Here we show that TRAIL-R1 can associate with TRAIL-R2, suggesting that TRAIL may signal through heteroreceptor signaling complexes. Both TRAIL receptors bind the adaptor molecules FADD and TRADD, and both death signals are interrupted by a dominant negative form of FADD and by the FLICE-inhibitory protein FLIP. The recruitment of TRADD may explain the potent activation of NF-kappaB observed by TRAIL receptors. Thus, TRAIL receptors can signal both death and gene transcription, functions reminiscent of those of TNFR1 and TRAMP, two other members of the death receptor family.

Apoptosis is a vital mechanism in multicellular organisms to eliminate unwanted cells during development, tissue homeostasis, and immune system function (1). Initiation and regulation of apoptosis is highly controlled through specific proteinprotein interactions and by a family of proteolytic enzymes, the caspases (2, 3). One way to induce apoptosis is via death receptors, a subgroup of the tumor necrosis factor receptor superfamily (4). The death signal is transmitted through the binding of extracellular death ligands such as the Fas ligand (FasL) 1 to its receptor Fas resulting in conformational changes of preformed receptor clusters (5). Intracellularly this change leads to the recruitment of the adaptor protein FADD (6, 7) and of the initiator caspases, caspase-8 and -10 (8, 9). Fas and FADD interact via homophilic death domain interactions, whereas FADD and the pro-caspases interact through death effector domains (DED). Ligand, receptor, adaptor protein, and caspases form the death inducing signaling complex (DISC) (10). When recruited to the DISC, pro-caspase-8 or -10 is activated through a series of proteolytic cleavage steps. Activation of pro-caspases generally involves the cleavage within the proteolytic caspase domain, resulting in active caspase comprising a large (␣) and small (␤) subunit, as well as the removal of the N-terminal domain. Apoptosis by death receptors is regulated at different levels of the signaling pathway. The viral caspase inhibitors CrmA and p35 block caspase-8 once it is activated and released from the membrane-bound DISC (11). FLIP is a potent inhibitor of death receptor-mediated pro-apoptotic signals, blocking the signaling pathway more upstream, before caspase-8 activation and release (12)(13)(14)(15)(16)(17)(18)(19). Two forms, FLIP L (long form) and FLIP S (short form) have been characterized so far (20, 21), which correspond to FLIP splice variants at the mRNA level. FLIP S consists of two DEDs, whereas FLIP L has an additional C-terminal caspase domain and resembles caspase-8 in its overall structural organization. In the protease-like domain of FLIP L the catalytically active cysteine is replaced by a tyrosine rendering the molecule proteolytically inactive (20,21).Pro-caspase-8 and FLIP L are recruited to the DISC, where both molecules are partly processed and the cleaved intermediates remain bound to the DISC (12,22). In a recent paper, Krueger et al. (23) demonstrated that FLIP L but not FLIP S or a mutant lacking the small subunit of the protease domain contributes to the first cleavage step of caspase-8. It is assumed that, in both cases, caspase-8 activity is highly impaired, rendering cells resistant to death receptor-induced apoptosis (24).The precise physiological role of FLIP is still debated. Analysis of FLIP-deficient mice revealed not only its importance in the regulation of death receptor-induced apoptosis, but also in embryonic development (25). Cells deficient for FLIP are more susceptible to death receptor-mediated apoptosis (26), and this anti-apopto...

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