Caspase-8-Dependent Inflammatory Responses Are Controlled by Its Adaptor, FADD, and Necroptosis.

Cell death pathways regulate various homeostatic processes. Autoimmune lymphoproliferative syndrome (ALPS) in humans and lymphoproliferative (LPR) disease in mice result from abrogated CD95-induced apoptosis. Because caspase-8 mediates CD95 signaling, we applied genetic approaches to dissect the roles of caspase-8 in cell death and inflammation. Here, we describe oligomerization-deficient Caspase-8 and non-cleavable Caspase-8 mutant mice with defective caspase-8-mediated apoptosis. Although neither mouse developed LPR disease, removal of the necroptosis effector Mlkl from Caspase-8 mice revealed an inflammatory role of caspase-8. Ablation of one allele of Fasl, Fadd, or Ripk1 prevented the pathology of Casp8Mlkl animals. Removing both Fadd alleles from these mice resulted in early lethality prior to post-natal day 15 (P15), which was prevented by co-ablation of either Ripk1 or Caspase-1. Our results suggest an in vivo role of the inflammatory RIPK1-caspase-8-FADD (FADDosome) complex and reveal a FADD-independent inflammatory role of caspase-8 that involves activation of an inflammasome.