Caspase-8 homo-oligomerization induces apoptosis and suppresses necroptosis to regulate tissue homeostasis

Caspase-8 is a central regulator of death receptor signalling. It induces apoptosis and suppresses necroptosis through its catalytic activity, while it serves as a scaffold promoting inflammasome activation and NF-κB-mediated gene expression. The currently prevailing model is that homo-oligomerization promotes full activation of caspase-8 to induce apoptosis, while heterodimerization with its catalytically inactive homologue cFLIP mediates limited activation of caspase-8 to suppress necroptosis. Here, we show that caspase-8 homo-oligomerisation not only initiates apoptosis but also plays a critical role in inhibiting necroptosis in susceptible cell types in vivo and in vitro. Inhibition of caspase-8 homo-oligomerisation by knock-in mutation of critical residues in its second DED (F122G/L123G; Casp8 ^FGLG/FGLG ) prevented hepatocyte apoptosis and the development of hepatitis and hepatocellular carcinoma in mice lacking NEMO specifically in liver parenchymal cells. In this model, inhibition of caspase-8 homo-oligomerisation did not sensitize hepatocytes to necroptosis because these cells do not express RIPK3. In contrast, in the Sharpin ^cpdm/cpdm mouse model of dermatitis induced by caspase-8-dependent keratinocyte apoptosis, Casp8 ^{FGLG/FGLG /FGLG} mutation suppressed apoptosis but did not prevent skin inflammation. Combined inhibition of caspase-8 homo-oligomerisation and necroptosis fully prevented dermatitis in Sharpin ^cpdm/cpdm mice, demonstrating that inhibition of caspase-8 homo-oligomerization sensitized keratinocytes to MLKL-dependent necroptosis. Consistently, primary cells from Casp8 ^FGLG/FGLG mice were protected from apoptosis but became highly sensitized to necroptosis in response to TNF and FasL stimulation. Mechanistically, caspase-8(FGLG) expression altered TNFR1 complex II and Fas DISC assembly to promote RIPK3 recruitment and activation. Taken together, our results challenge the prevailing model of necroptosis inhibition by caspase-8 and suggest that distinct mechanisms of caspase-8 activation cooperate to regulate apoptotic and necroptotic signalling and tissue homeostasis.