No abstract

Detection of microbial products by host inflammasomes is critical for innate immune surveillance. Inflammasomes activate the CASPASE-1 (CASP1) protease, which processes the cytokines interleukin(IL)-1β and -18, and initiates a lytic host cell death called pyroptosis1. To identify novel CASP1 functions in vivo, we devised a strategy for cytosolic delivery of bacterial flagellin, a specific ligand for the NAIP5 (NLR family, apoptosis inhibitory protein 5)/NLRC4 (NLR family, CARD domain containing 4) inflammasome2–4. Here we show that systemic inflammasome activation by flagellin leads to loss of vascular fluid into the intestine and peritoneal cavity, resulting in rapid (< 30 minutes) death in mice. This unexpected response depends on the inflammasome components NAIP5, NLRC4, and CASP1, but is independent of IL-1β/-18 production. Instead, inflammasome activation results, within minutes, in an ‘eicosanoid storm’ – a pathological release of signaling lipids that rapidly initiate inflammation and vascular fluid loss. Mice deficient in cyclooxygenase-1 (COX-1), a critical enzyme in prostaglandin biosynthesis, are resistant to these rapid pathological effects of systemic inflammasome activation by either flagellin or anthrax lethal toxin. Inflammasome-dependent biosynthesis of eicosanoids is mediated by activation of cPLA2 (cytosolic phospholipase A2) in resident peritoneal macrophages, which are specifically primed for production of eicosanoids by high expression of eicosanoid biosynthetic enzymes. Thus, our results identify eicosanoids as a novel cell type-specific signaling output of the inflammasome with dramatic physiological consequences in vivo.

NOD-like receptor (NLR) proteins (Nlrps) are cytosolic sensors responsible for detection of pathogen and danger-associated molecular patterns through unknown mechanisms. Their activation in response to a wide range of intracellular danger signals leads to formation of the inflammasome, caspase-1 activation, rapid programmed cell death (pyroptosis) and maturation of IL-1β and IL-18. Anthrax lethal toxin (LT) induces the caspase-1-dependent pyroptosis of mouse and rat macrophages isolated from certain inbred rodent strains through activation of the NOD-like receptor (NLR) Nlrp1 inflammasome. Here we show that LT cleaves rat Nlrp1 and this cleavage is required for toxin-induced inflammasome activation, IL-1 β release, and macrophage pyroptosis. These results identify both a previously unrecognized mechanism of activation of an NLR and a new, physiologically relevant protein substrate of LT.