The Nlrp3 inflammasome is critical for host immunity, but the mechanisms controlling its activation are enigmatic. Here, we show that loss of FADD or caspase-8 in a RIP3-deficient background - but not RIP3-deficiency alone - hampered transcriptional priming and post-translational activation of the canonical and non-canonical Nlrp3 inflammasome. Deletion of caspase-8 in the presence or absence of RIP3 inhibited caspase-1 and caspase-11 activation by Nlrp3 stimuli, but not the Nlrc4 inflammasome. FADD deletion in addition prevented caspase-8 maturation, positioning FADD upstream of caspase-8. Consequently, FADD- and caspase-8-deficient mice had impaired IL-1β production when challenged with LPS or infected with the enteropathogen C. rodentium. Thus, our results reveal FADD and caspase-8 as apical mediators of canonical and non-canonical Nlrp3 inflammasome priming and activation.

SUMMARY NLRP12 is a member of the intracellular Nod-like receptor (NLR) family that has been suggested to down-regulate the production of inflammatory cytokines, but its physiological role in regulating inflammation has not been characterized. We generated mice deficient in Nlrp12 and studied its role in inflammatory diseases such as colitis and colorectal tumorigenesis. We show that Nlrp12-deficient mice are highly susceptible to colon inflammation and tumorigenesis, which is associated with increased production of inflammatory cytokines, chemokines and tumorigenic factors. Enhanced colon inflammation and colorectal tumor development in Nlrp12-deficient mice are due to a failure to dampen NF-κB and ERK activation in macrophages. These results reveal a critical role for NLRP12 in maintaining intestinal homeostasis and providing protection against colorectal tumorigenesis.

Members of the intracellular nucleotide-binding and oligomerization domain (NOD)-like receptor (NLR) family contribute to immune responses through activation of NF-κB, type I interferon and inflammasome signaling1. Mice lacking the NLR family member NLRP6 were recently shown to be susceptible to colitis and colorectal tumorigenesis2-4, but the role of NLRP6 in microbial infections and the nature of the inflammatory signaling pathways regulated by NLRP6 remain unclear. Here, we show that Nlrp6-deficient mice were highly resistant to infection with the bacterial pathogens Listeria monocytogenes, Salmonella typhimurium and Escherichia coli. Infected Nlrp6-deficient mice had increased numbers of monocytes and neutrophils in circulation, and NLRP6 signaling in both hematopoietic and radio-resistant cells contributed to increased susceptibility. Nlrp6-deficiency enhanced activation of MAPK and canonical NF-κB upon TLR, but not cytosolic NOD1/2 ligation in vitro. Consequently, infected Nlrp6-deficient cells produced elevated levels of NF-κB- and MAPK-dependent cytokines and chemokines. Thus, our results reveal NLRP6 as a negative regulator of inflammatory signaling, and demonstrate a role for this NLR in impeding clearance of both Gram-positive and –negative bacterial pathogens.

NOD2 receptor and the cytosolic protein kinase RIPK2 regulate NF-κB and MAP kinase signaling during bacterial infections, but the role of this immune axis during viral infections has not been addressed. We demonstrate that Nod2−/− and Ripk2−/− mice are hypersusceptible to influenza A virus infection. Ripk2−/− cells displayed defective mitophagy leading to enhanced mitochondrial superoxide production and accumulation of damaged mitochondria resulting in increased NLRP3 inflammasome activation and IL-18 production. RIPK2 regulated mitophagy in a kinase-dependent manner by phosphorylating the mitophagy inducer ULK1. Accordingly, Ulk1−/− cells displayed enhanced mitochondrial superoxide production and caspase-1 activation. These results demonstrate a role for NOD2-RIPK2 signaling in protection against virally triggered immunopathology by negatively regulating NLRP3 inflammasome activation and IL-18 production via ULK1-dependent mitophagy.

Background: C. rodentium and E. coli induce noncanonical Nlrp3 inflammasome activation through caspase-11. Results: TLR4-TRIF are important for caspase-11 expression, caspase-1 activation, and downstream IL-1␤ and IL-18 production. Conclusion: TLR4-TRIF axis plays an important role in the up-regulation of caspase-11 and activation of noncanonical inflammasome. Significance: Our study identifies novel molecules upstream of caspase-11 that are involved in activation of noncanonical inflammasome.