B-214. Coordinated Host Responses during Pyroptosis: Caspase-1-Dependent Lysosome Exocytosis and Inflammatory Cytokine Maturation
Salmonella typhimurium infection of macrophages leads to activation of the cysteine protease caspase-1 and induction of pyroptosis, which features both cell death and processing and release of the inflammatory cytokines IL-18 and IL-1β. Active caspase-1 and mature cytokines have been reported to reside within lysosomes, with lysosome fusion with the cell surface, or lysosome exocytosis, mediating cytokine secretion. However, we observed Salmonella infection of macrophages resulted in active caspase-1 that was not restricted to lysosomes. Additionally, Salmonella-infected macrophages had a reduction in the number of lysosomal compartments per cell, suggesting that lysosome exocytosis occurs during Salmonella infection. This was confirmed by monitoring secretion of the mature form of the lysosomal enzyme cathepsin D. Enzyme release was not due to cell lysis; the cytoprotective agent glycine did not prevent cathepsin D secretion. Loss of lysosomal compartments and cathepsin D secretion were not observed during infection with Salmonella mutants that do not induce pyroptosis, suggesting that although active caspase-1 does not localize to lysosomes, it may play a role in mediating exocytosis. Supporting this hypothesis, inhibition of caspase-1 activity prevented lysosome exocytosis during Salmonella infection. Treatment of macrophages with other caspase-1 activating stimuli, inclucing Bacillus anthracis lethal toxin and nigericin, also triggered caspase-1-dependent lysosome exocytosis. Finally, lysosome exocytosis has been proposed as a mechanism for secretion of the cytokines processed by caspase-1, IL-1β and IL-18. However, we determined that cytokines are secreted independently of lysosome exocytosis during Salmonella infection; IL-18 and IL-1β processing and release still occurred when lysosome exocytosis was blocked. These studies indicate lysosome exocytosis is a feature of pyroptosis; this process is not required for cell lysis or cytokine secretion, but secreted lysosomal components may mediate inflammation or exert antimicrobial functions.