Rapid induction of inflammatory lipid mediators by the inflammasome in vivo

Abstract

Induction of an eicosanoid storm is shown to be an unexpected consequence of inflammasome activation in peritoneal macrophages, leading to vascular leakage and rapid death in mice. Inflammasomes are multiprotein complexes that initiate early cellular responses to cellular pathogens. The mechanisms of inflammasome activation have been the focus of intense research, but relatively little is known about what pathways are activated downstream of inflammasomes. This study shows that systemic activation of the inflammasome in vivo results in the rapid induction of potent signalling lipids called eicosanoids, which cause a catastrophic loss of fluid from the blood, contributing to the death of the animal within 30 minutes. When restricted to the site of infection, eicosanoids may have a beneficial role in host defence, for example by increasing local vascular permeability, allowing an influx of immune cells. Detection of microbial products by host inflammasomes is an important mechanism of innate immune surveillance. Inflammasomes activate the caspase-1 (CASP1) protease, which processes the cytokines interleukin (IL)-1β and IL-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) inflammasome2,3,4. Here we show that systemic inflammasome activation by flagellin leads to a loss of vascular fluid into the intestine and peritoneal cavity, resulting in rapid (less than 30 min) death in mice. This unexpected response depends on the inflammasome components NAIP5, NLRC4 and CASP1, but is independent of the production of IL-1β or IL-18. Instead, inflammasome activation results, within minutes, in an ‘eicosanoid storm’—a pathological release of signalling lipids, including prostaglandins and leukotrienes, that rapidly initiate inflammation and vascular fluid loss. Mice deficient in cyclooxygenase-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 the activation of cytosolic phospholipase A2 in resident peritoneal macrophages, which are specifically primed for the production of eicosanoids by high expression of eicosanoid biosynthetic enzymes. Our results therefore identify eicosanoids as a previously unrecognized cell-type-specific signalling output of the inflammasome with marked physiological consequences in vivo.