Caspase-8 is the molecular switch for apoptosis, necroptosis and pyroptosis

Caspase-8 is the initiator Caspase of extrinsic Apoptosis^1,2 and inhibits Necroptosis mediated by RIPK3 and MLKL. Accordingly, Caspase-8 deficiency in mice causes embryonic lethality³, which can be rescued by deletion of either RIPK3 or Mlkl^4-6. Here we show that the expression of enzymatically inactive CASP8(C362S) causes embryonic lethality in mice by inducing Necroptosis and Pyroptosis. Similar to Casp8^-/- mice^3,7, Casp8^C362S/C362S mouse embryos died after endothelial cell Necroptosis leading to cardiovascular defects. MLKL deficiency rescued the cardiovascular phenotype but unexpectedly caused perinatal lethality in Casp8^C362S/C362S mice, indicating that CASP8(C362S) causes necroptosis-independent death at later stages of embryonic development. Specific loss of the catalytic activity of Caspase-8 in intestinal epithelial cells induced intestinal inflammation similar to intestinal epithelial cell-specific Casp8 knockout mice⁸. Inhibition of Necroptosis by additional deletion of Mlkl severely aggravated intestinal inflammation and caused premature lethality in Mlkl knockout mice with specific loss of Caspase-8 catalytic activity in intestinal epithelial cells. Expression of CASP8(C362S) triggered the formation of ASC specks, activation of Caspase-1 and secretion of IL-1β. Both embryonic lethality and premature death were completely rescued in Casp8^C362S/C362SMlkl^-/-Asc^-/- or Casp8^C362S/C362SMlkl^-/-Casp1^-/- mice, indicating that the activation of the inflammasome promotes CASP8(C362S)-mediated tissue pathology when Necroptosis is blocked. Therefore, Caspase-8 represents the molecular switch that controls Apoptosis, Necroptosis and Pyroptosis, and prevents tissue damage during embryonic development and adulthood.