FLIP _L permits apoptotic and inflammatory signaling and inhibits necroptosis in mice without Caspase-8 oligomerization

Caspase-8 signaling has proapoptotic, antinecroptotic, and proinflammatory signaling roles dependent on interaction with the adapter molecule FADD, oligomerization, and autocleavage. Previously, a Caspase-8 binding partner cFLIP_L (FLIP, encoded by <i>Cflar</i>) was shown to prevent Caspase-8-dependent apoptosis, but permit Caspase-8-dependent inhibition of necroptosis. We sought to explore the role of FLIP in Caspase-8-dependent apoptosis induction, necroptosis inhibition, and inflammatory signaling inhibition in vitro and in vivo. We provide evidence that in mice with a mutation that prevents Caspase-8 oligomerization (<i>Casp8^FGLG/FGLG</i>), FLIP is necessary to inhibit necroptosis, promote apoptosis, regulate inflammation, and control lymphoproliferative disease. Unlike <i>Casp8^FGLG/FGLG mice, Casp8^FGLG/FGLG,Cflar^-/-</i> mice do not survive embryogenesis, but ablation of <i>Mlkl</i>, required for necroptosis, allows their survival to adulthood. Further, unlike <i>Casp8^FGLG/FGLG,Mlkl^-/-</i> mice, <i>Casp8^FGLG/FGLG,Cflar^-/-,Mlkl^-/-</i> mice display lymphoproliferative disease. We analyzed apoptosis, necroptosis, and inflammatory signaling in <i>Casp8^FGLG/FGLG</i> mice with or without FLIP, gaining insights into the functions of the Caspase-8-FLIP heterodimer in vitro and in vivo.