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RIP1 suppresses innate immune necrotic as well as apoptotic cell death during mammalian parturition

Significance The protein kinase receptor interacting protein 1 controls signaling via death receptors, Toll-like receptors, and retinoic acid-inducible gene 1-like receptors, dictating inflammatory outcomes as broad as cytokine activation and cell death. RIP1 makes a vital contribution during development, evident from the fact that RIP1-deficient mice die soon after birth. Here, we show that a kinase-independent function of RIP1 dampens the consequences of innate immune cell death. During parturition, RIP1 prevents the lethal consequences of RIP3-dependent necroptosis as well as caspase 8 (Casp8)-dependent apoptosis. In contrast to the RIP1-deficient phenotype, mice lacking a combination of RIP1, RIP3, and Casp8 are born and mature into viable, fertile, and immunocompetent adults. These results demonstrate the important protective role of RIP1 against physiologic and microbial death cues encountered at birth. The pronecrotic kinase, receptor interacting protein (RIP1, also called RIPK1) mediates programmed necrosis and, together with its partner, RIP3 (RIPK3), drives midgestational death of caspase 8 (Casp8)-deficient embryos. RIP1 controls a second vital step in mammalian development immediately after birth, the mechanism of which remains unresolved. Rip1−/− mice display perinatal lethality, accompanied by gross immune system abnormalities. Here we show that RIP1 K45A (kinase dead) knockin mice develop normally into adulthood, indicating that development does not require RIP1 kinase activity. In the face of complete RIP1 deficiency, cells develop sensitivity to RIP3-mixed lineage kinase domain-like–mediated necroptosis as well as to Casp8-mediated apoptosis activated by diverse innate immune stimuli (e.g., TNF, IFN, double-stranded RNA). When either RIP3 or Casp8 is disrupted in combination with RIP1, the resulting double knockout mice exhibit slightly prolonged survival over RIP1-deficient animals. Surprisingly, triple knockout mice with combined RIP1, RIP3, and Casp8 deficiency develop into viable and fertile adults, with the capacity to produce normal levels of myeloid and lymphoid lineage cells. Despite the combined deficiency, these mice sustain a functional immune system that responds robustly to viral challenge. A single allele of Rip3 is tolerated in Rip1−/−Casp8−/−Rip3+/− mice, contrasting the need to eliminate both alleles of either Rip1 or Rip3 to rescue midgestational death of Casp8-deficient mice. These observations reveal a vital kinase-independent role for RIP1 in preventing pronecrotic as well as proapoptotic signaling events associated with life-threatening innate immune activation at the time of mammalian parturition.