Cholesterol-dependent cytolysins induce rapid release of mature IL-1β from murine macrophages in a NLRP3 inflammasome and cathepsin B-dependent manner

CDC are exotoxins secreted by many Gram-positive bacteria that bind cholesterol and oligomerize to form pores in eukaryotic cell membranes. We demonstrate that CDC TLO induces caspase-1 cleavage and the rapid release of IL-1 beta from LPS-primed murine BMDM. IL-1 beta secretion depends on functional toxin pore formation, as free cholesterol, which prevents TLO binding to cell membranes, blocks the cytokine release. Secretion of the mature forms of IL-1 beta and caspase-1 occurs only at lower TLO doses, whereas at a higher concentration, cells release the biologically inactive proforms. IL-1 beta release at a low TLO dose requires potassium efflux, calcium influx, and the activities of calcium-independent PLA2, caspase-1, and cathepsin B. Additionally, mature IL-1 beta release induced by a low TLO dose is dependent on the NLRP3 inflammasome, and pro-IL-1 beta release induced by a high TLO dose occurs independently of NLRP3. These results further elucidate a mechanism of CDC-induced IL-1 beta release and suggest a novel, immune evasion strategy in which IL-1 beta-containing macrophages might release primarily inactive cytokine following exposure to high doses of these toxins. J. Leukoc. Biol. 86: 1227-1238; 2009.