Salmonella pathogenicity island 2-dependent macrophage death is mediated in part by the host cysteine protease caspase-1

Salmonella typhimurium invades host macrophages and can either induce a rapid cell death or establish an intracellular niche within the phagocytic vacuole. Rapid cell death requires the Salmonella pathogenicity island (SPI)1 and the host protein caspase-1, a member of the pro-apoptotic caspase family of proteases. Salmonella that do not cause this rapid cell death and instead reside in the phagocytic vacuole can trigger macrophage death at a later time point. We show here that the human pathogen Salmonella typhi also triggers both rapid, caspase-1-dependent and delayed cell death in human monocytes. The delayed cell death has previously been shown with S. typhimurium to be dependent on SP12-encoded genes and ompR. Using caspase-1(-/-) bone marrow-derived macrophages and isogenic S. typhimurium mutant strains, we show that a large portion of the delayed, SP12-dependent death is mediated by caspase-1. The two known substrates of activated caspase-1 are the pro-inflammatory cytokines interleukin-lp (IL-1 beta) and IL-18, which are cleaved to produce bioactive cytokines. We show here that IL-1 beta is released during both SPI1- and SP12-dependent macrophage killing. Using bone marrow-derived macrophages and a neutralizing anti-IL-18 antibody, we show that neither IL-1 beta nor IL-18 is required for rapid or delayed macrophage death. Thus, both rapid, SPI1-mediated killing and delayed, SPI2-mediated killing require caspase-1 and result in the secretion of IL-1 beta, which promotes inflammation and may facilitate the spread of Salmonella beyond the gastrointestinal tract in systemic disease.