Interferons (IFNs) are critical determinants in immune-competence and autoimmunity, and are endogenously regulated by a low-level constitutive feedback loop. However, little is known about the functions and origins of constitutive IFN. Recently, lipopolysaccharide (LPS)-induced IFN was implicated as a driver of necroptosis, a necrotic form of cell death downstream of receptor-interacting protein (RIP) kinase activation and executed by mixed lineage kinase like-domain (MLKL) protein. We found that the pre-established IFN status of the cell, instead of LPS-induced IFN, is critical for the early initiation of necroptosis in macrophages. This pre-established IFN signature stems from cytosolic DNA sensing via cGAS/STING, and maintains the expression of MLKL and one or more unknown effectors above a critical threshold to allow for MLKL oligomerization and cell death. Finally, we found that elevated IFN-signaling in systemic lupus erythematosus (SLE) augments necroptosis, providing a link between pathological IFN and tissue damage during autoimmunity.

The innate immune response is a central element of the initial defense against bacterial and viral pathogens. Macrophages are key innate immune cells that upon encountering pathogen-associated molecular patterns respond by producing cytokines, including IFN-β. In this study, we identify a novel role for RIPK1 and RIPK3, a pair of homologous serine/threonine kinases previously implicated in the regulation of necroptosis and pathologic tissue injury, in directing IFN-β production in macrophages. Using genetic and pharmacologic tools, we show that catalytic activity of RIPK1 directs IFN-β synthesis induced by LPS in mice. Additionally, we report that RIPK1 kinase-dependent IFN-β production may be elicited in an analogous fashion using LPS in bone marrow-derived macrophages upon inhibition of caspases. Notably, this regulation requires kinase activities of both RIPK1 and RIPK3, but not the necroptosis effector protein, MLKL. Mechanistically, we provide evidence that necrosome-like RIPK1 and RIPK3 aggregates facilitate canonical TRIF-dependent IFN-β production downstream of the LPS receptor TLR4. Intriguingly, we also show that RIPK1 and RIPK3 kinase-dependent synthesis of IFN-β is markedly induced by avirulent strains of Gram-negative bacteria, Yersinia and Klebsiella, and less so by their wild-type counterparts. Overall, these observations identify unexpected roles for RIPK1 and RIPK3 kinases in the production of IFN-β during the host inflammatory responses to bacterial infection and suggest that the axis in which these kinases operate may represent a target for bacterial virulence factors.Copyright © 2017 by The American Association of Immunologists, Inc.