Cytomegalovirus inhibition of extrinsic apoptosis determines fitness and resistance to cytotoxic CD8 T cells

Abstract

Viral immune evasion is currently understood to focus on deflect-ing CD8 T cell recognition of infected cells by disrupting antigen presentation pathways. We evaluated viral interference with the ultimate step in cytotoxic T cell function, the death of infected cells. The viral inhibitor of caspase-8 activation (vICA) conserved in human cytomegalovirus (HCMV) and murine CMV (MCMV) pre-vents the activation of caspase-8 and proapoptotic signaling. We demonstrate the key role of vICA from either virus, in deflecting antigen-specific CD8 T cell-killing of infected cells. vICA-deficient mutants, lacking either UL36 or M36, exhibit greater susceptibility to CD8 T cell control than mutants lacking the set of immunoeva-sins known to disrupt antigen presentation via MHC class I. This difference is evident during infection in the natural mouse host infected with MCMV, in settings where virus-specific CD8 T cells are adoptively transferred. Finally, we identify the molecular mech-anism through which vICA acts, demonstrating the central contribu-tion of caspase-8 signaling at a point of convergence of death receptor-induced apoptosis and perforin/granzyme-dependent cytotoxicity.