The Nlrp3 inflammasome is critical for host immunity, but the mechanisms controlling its activation are enigmatic. In this study, we show that loss of FADD or caspase-8 in a RIP3-deficient background, but not RIP3 deficiency alone, hampered transcriptional priming and posttranslational activation of the canonical and noncanonical 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. In addition, FADD deletion 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 Citrobacter rodentium. Thus, our results reveal FADD and caspase-8 as apical mediators of canonical and noncanonical Nlrp3 inflammasome priming and activation.

RIPK1 kinase activity has been shown to be essential to driving pyroptosis, apoptosis, and necroptosis. However, here we show a kinase activity–independent role for RIPK1 in these processes using a model of TLR priming in a TAK1-deficient setting to mimic pathogen-induced priming and inhibition. TLR priming of TAK1-deficient macrophages triggered inflammasome activation, including the activation of caspase-8 and gasdermin D, and the recruitment of NLRP3 and ASC into a novel RIPK1 kinase activity–independent cell death complex to drive pyroptosis and apoptosis. Furthermore, we found fully functional RIPK1 kinase activity–independent necroptosis driven by the RIPK3–MLKL pathway in TAK1-deficient macrophages. In vivo, TAK1 inactivation resulted in RIPK3–caspase-8 signaling axis–driven myeloid proliferation and a severe sepsis-like syndrome. Overall, our study highlights a previously unknown mechanism for RIPK1 kinase activity–independent inflammasome activation and pyroptosis, apoptosis, and necroptosis (PANoptosis) that could be targeted for treatment of TAK1-associated myeloid proliferation and sepsis.

Lipopolysaccharide (LPS), commonly known as endotoxin, is ubiquitous and the most-studied pathogen-associated molecular pattern. A component of Gram-negative bacteria, extracellular LPS is sensed by our immune system via the toll-like receptor (TLR)-4. Given that TLR4 is membrane bound, it recognizes LPS in the extracellular milieu or within endosomes. Whether additional sensors, if any, play a role in LPS recognition within the cytoplasm remained unknown until recently. The last decade has seen an unprecedented unfolding of TLR4-independent LPS sensing pathways. First, transient receptor potential (TRP) channels have been identified as non-TLR membrane-bound sensors of LPS and, second, caspase-4/5 (and caspase-11 in mice) have been established as the cytoplasmic sensors for LPS. Here in this review, we detail the brief history of LPS discovery, followed by the discovery of TLR4, TRP as the membrane-bound sensor, and our current understanding of caspase-4/5/11 as cytoplasmic sensors.