A multi-component bacterial toxin incites host inflammation via the NLRP3 inflammasome

Abstract

Abstract Innate immune recognition of microbial components serves as a cornerstone in mediating an effective immune response. The inflammasome is an intracellular signaling complex comprising of a sensor, an adaptor protein ASC and the cysteine protease caspase-1. Inflammasomes regulate secretion of the pro-inflammatory cytokines IL-1β and IL-18 and induction of a cell death pathway known as pyroptosis. Certain intracellular bacteria require cytosolic access to activate the inflammasome, however, how extracellular bacteria are sensed by the cytoplasmic inflammasome remains unclear. To understand innate immune recognition of extracellular bacteria, we analyzed a panel of clinically important intracellular and extracellular bacteria and identified an unknown secreted factor from the foodborne bacterium Bacillus cereus that activates the inflammasome without gaining cytosolic access. The tripartite enterotoxin called haemolysin BL (HBL) was identified as the novel activator of the NLRP3 inflammasome. The multi-component toxin assembled in a linear order on the mammalian cell membrane to form a pore, which induces potassium efflux and activates the NLRP3 inflammasome. Furthermore, HBL-producing B. cereus induced rapid lethality in the host via activation of the NLRP3 inflammasome. B. cereus-induced lethality was completely abrogated with administration of the NLRP3 inhibitor, MCC950. Our unpublished results further show that the transmembrane region of the apical subunit of HBL is dispensable for membrane binding but essential for inflammasome activation. Overall, our results indicate that intracellular sensing of a toxin secreted by an extracellular bacterium is critical for the innate immune recognition of infection.