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.
Anthrax lethal toxin (LT) is a protease which can induce rapid death of macrophages accompanied by activation and release of pro-inflammatory cytokines. The previously identified cellular substrates for this toxin have not been shown to play a role in this rapid cell death. This report identifies a new substrate for LT, and demonstrates that its cleavage by the toxin is required for macrophage death. The substrate, Nlrp1, is a member of a large family of intracellular sensors of danger. These sensors, once activated, form a multiprotein complex called the inflammasome and are essential to the host innate immune response. The mechanism of activation for these sensors is not known. The demonstration of cleavage-mediated activation of Nlrp1 in this study represents the first report on a direct biochemical mechanism for inflammasome activation.