Mouse cytomegalovirus M36 and M45 death suppressors cooperate to prevent inflammation resulting from antiviral programmed cell death pathways

The complex interplay between Caspase-8 and receptor-interacting protein (RIP) kinase RIP 3 (RIPK3) driving extrinsic Apoptosis and Necroptosis is not fully understood. Murine cytomegalovirus triggers both Apoptosis and Necroptosis in infected cells; however, encoded inhibitors of Caspase-8 activity (M36) and RIP3 signaling (M45) suppress these Antiviral responses. Here, we report that this virus activates Caspase-8 in macrophages to trigger Apoptosis that gives rise to secondary Necroptosis. Infection with double-mutant ΔM36/M45mutRHIM virus reveals a signaling pattern in which Caspase-8 activates Caspase-3 to drive Apoptosis with subsequent RIP3-dependent activation of Mixed Lineage Kinase domain-like (MLKL) leading to Necroptosis. This combined cell death signaling is highly inflammatory, greater than either Apoptosis induced by ΔM36 or Necroptosis induced by M45mutRHIM virus. IL-6 production by macrophages is dramatically increased during double-mutant virus Infection and correlates with faster Antiviral responses in the host. Collaboratively, M36 and M45 target Caspase-8 and RIP3 pathways together to suppress this proinflammatory cell death. This study reveals the effect of Antiviral programmed cell death pathways on inflammation, shows that Caspase-8 activation may go hand-in-hand with Necroptosis in macrophages, and revises current understanding of independent and collaborative functions of M36 and M45 in blocking apoptotic and necroptotic cell death responses.