Caspase-8 auto-cleavage regulates programmed cell death and collaborates with RIPK3/MLKL to prevent lymphopenia.

Caspase-8 is an initiator of death receptor-induced apoptosis and an inhibitor of RIPK3-MLKL-dependent necroptosis. In addition, caspase-8 has been implicated in diseases such as lymphoproliferation, immunodeficiency, and autoimmunity in humans. Although auto-cleavage is indispensable for caspase-8 activation, its physiological functions remain poorly understood. Here, we generated a caspase-8 mutant lacking E385 in auto-cleavage site knock-in mouse (Casp8). Casp8 cells were expectedly resistant to Fas-induced apoptosis, however, Casp8 cells could switch TNF-α-induced apoptosis to necroptosis by attenuating RIPK1 cleavage. More importantly, CASP8(ΔE385) sensitized cells to RIPK3-MLKL-dependent necroptosis through promoting complex II formation and RIPK1-RIPK3 activation. Notably, Casp8Ripk3 mice partially rescued the perinatal death of Ripk1 mice by blocking apoptosis and necroptosis. In contrast to the Casp8Ripk3 and Casp8Mlkl mice appearing autoimmune lymphoproliferative syndrome (ALPS), both Casp8Ripk3 and Casp8Mlkl mice developed transplantable lymphopenia that could be significantly reversed by RIPK1 heterozygosity, but not by RIPK1 kinase dead mutation. Collectively, these results demonstrate previously unappreciated roles for caspase-8 auto-cleavage in regulating necroptosis and maintaining lymphocytes homeostasis.