A Hematopoietic RIPK1 Deficiency Triggers IFNγ-Mediated Necroptosis and Bone Marrow Failure

RIPK1 has emerged as an important regulator of necroptosis and apoptosis through kinase dependent and independent functions. While complete RIPK1 deficiency results in cell death and widespread inflammation, tissue specific effects of RIPK1 deletion is context dependent, leading to apoptosis, necroptosis and/or systemic inflammation depending on the cell type. We have previously demonstrated that mice with a hematopoietic RIPK1 deficiency exhibit elevated serum TNFα and IFNγ levels, hematopoietic stem and progenitor cell (HSPC) loss, and ultimately succumb to bone marrow failure (BMF) (Roderick et al, PNAS, 2014). When mice with a hematopoietic RIPK1 deficiency were placed on a RIPK3-deficient background, plasma pro-inflammatory cytokine and chemokine levels were reduced, HSPC numbers increased and BMF was significantly delayed. These mouse genetic data demonstrate that in the hematopoietic system, RIPK1 mediates survival by preventing RIPK3-mediated necroptosis. To identify the receptor/ligands that trigger necroptosis, we generated mice with a hematopoietic RIPK1 deficiency on the Tnfr1^-/- , Tnfr1^-/-Tnfr2^-/- , or Ifngr1^{-/ -} genetic backgrounds. While an absence of TNF receptors failed to prevent necroptosis and all mice developed BMF, an absence of the IFNγ receptor significantly delayed BMF, revealing that HSPCs undergo IFNγ-mediated necroptosis. Unexpectedly, we discovered that vav-iCre Ripk1^f/fRipk3^-/- mice on the Tnfr1^-/- or Tnfr1/Tnfr2^-/- genetic backgrounds do not develop BMF but rather exhibit myeloproliferative disease (MPD), characterized by increases in neutrophils and inflammatory monocytes/macrophages in peripheral blood, spleen, bone marrow and liver. We detect statistically significant increases in the number of LT (long-term) and ST (short-term)-HSCs in the bone marrow of these mice, while the multi-potential progenitors (MPPs) remain unaffected. Competitive transplant studies reveal that HSCs from these mice fail to compete with wild type (WT) HSCs, suggesting that the inflammatory microenvironment results in HSC exhaustion. Collectively, these data reveal novel roles for TNFR1 signaling in MPD and raise the intriguing possibility that the cytokine hypersensitivity and progressive HSPC loss observed in BMF patients reflects in part, IFNγ-induced necroptotic death.