RIP

Members of the tumor necrosis factor receptor (TNFR) superfamily are potent regulators of apoptosis, a process that is important for the maintenance of immune homeostasis. Recent evidence suggests that TNFR-1, Fas and TRAIL receptors can also trigger an alternative form of cell death that is morphologically distinct from apoptosis. Because distinct molecular components including the protein serine/threonine kinase RIP are required, we have referred to this alternative form of cell death as programmed necrosis. We showed that TNFR-2 signaling could potentiate programmed necrosis via TNFR-1. When cells were pre-stimulated through TNFR-2 prior to subsequent activation of TNFR-1, enhanced cell death and recruitment of RIP to the TNFR-1 complex was observed. However, TNF-induced programmed necrosis was normally inhibited by caspase-8 cleavage of RIP. To ascertain the physiological significance of RIP and programmed necrosis, we infected Jurkat cells with vaccinia virus (VV) and found that VV-infected cells underwent programmed necrosis in response to TNF, but deficiency of RIP rescued the infected cells from TNF-induced cytotoxicity. Moreover, TNFR-2-/- mice exhibited reduced inflammation in the liver and defective viral clearance during VV infection. Interestingly, death effector domain (DED) containing proteins such as MC159, E8, K13 and cFLIP, but not the apoptosis inhibitors Bcl-XL, p35 or XIAP, potently suppressed programmed necrosis. Thus, TNF-induced programmed necrosis is facilitated by TNFR-2 signaling and caspase inhibition and may play a role in controlling virus infection.