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^{ΔE385/ΔE385}). Casp8^{ΔE385/ΔE385} cells were expectedly resistant to Fas-induced apoptosis, however, Casp8^{ΔE385/ΔE385} 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, Casp8^{ΔE385/ΔE385}Ripk3^-/- mice partially rescued the perinatal death of Ripk1^-/- mice by blocking apoptosis and necroptosis. In contrast to the Casp8^-/-Ripk3^-/- and Casp8^-/-Mlkl^-/- mice appearing autoimmune lymphoproliferative syndrome (ALPS), both Casp8^{ΔE385/ΔE385}Ripk3^-/- and Casp8^{ΔE385/ΔE385}Mlkl^-/- 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.