Salmonella enterica serovar Typhimurium invades host macrophages and induces a unique caspase-1-dependent pathway of cell death termed pyroptosis, which is activated during bacterial infection in vivo. We demonstrate DNA cleavage during pyroptosis results from caspase-1-stimulated nuclease activity. Although poly(ADP-ribose) polymerase (PARP) activation by fragmented DNA depletes cellular ATP to cause lysis during oncosis, the rapid lysis characteristic of Salmonella-infected macrophages does not require PARP activity or DNA fragmentation. Membrane pores between 1.1 and 2.4 nm in diameter form during pyroptosis of host cells and cause swelling and osmotic lysis. Pore formation requires host cell actin cytoskeleton rearrangements and caspase-1 activity, as well as the bacterial type III secretion system (TTSS); however, insertion of functional TTSS translocons into the host membrane is not sufficient to directly evoke pore formation. Concurrent with pore formation, inflammatory cytokines are released from infected macrophages. This mechanism of caspase-1-mediated cell death provides additional experimental evidence supporting pyroptosis as a novel pathway of inflammatory programmed cell death.

Caspase-1 cleaves the inactive IL-1β and IL-18 precursors into active inflammatory cytokines. In Salmonella -infected macrophages, caspase-1 also mediates a pathway of proinflammatory programmed cell death termed “pyroptosis.” We demonstrate active caspase-1 diffusely distributed in the cytoplasm and localized in discrete foci within macrophages responding to either Salmonella infection or intoxication by Bacillus anthracis lethal toxin (LT). Both stimuli triggered caspase-1-dependent lysis in macrophages and dendritic cells. Activation of caspase-1 by LT required binding, uptake, and endosome acidification to mediate translocation of lethal factor (LF) into the host cell cytosol. Catalytically active LF cleaved cytosolic substrates and activated caspase-1 by a mechanism involving proteasome activity and potassium efflux. LT activation of caspase-1 is known to require the inflammasome adapter Nalp1. In contrast, Salmonella infection activated caspase-1 through an independent pathway requiring the inflammasome adapter Ipaf. These distinct mechanisms of caspase-1 activation converged on a common pathway of caspase-1-dependent cell death featuring DNA cleavage, cytokine activation, and, ultimately, cell lysis resulting from the formation of membrane pores between 1.1 and 2.4 nm in diameter and pathological ion fluxes that can be blocked by glycine. These findings demonstrate that distinct activation pathways elicit the conserved cell death effector mechanism of caspase-1-mediated pyroptosis and support the notion that this pathway of proinflammatory programmed cell death is broadly relevant to cell death and inflammation invoked by diverse stimuli.

Salmonella enterica are facultatively intracellular pathogens causing diseases with markedly visible signs of inflammation. During infection, Salmonella interacts with various host cell types, often resulting in death of those cells. Salmonella induces intestinal epithelial cell death via apoptosis, a cell death programme with a notably non-inflammatory outcome. In contrast, macrophage infection triggers caspase-1-dependent proinflammatory programmed cell death, a recently recognized process termed pyroptosis, which is distinguished from other forms of cellular demise by its unique mechanism, features and inflammatory outcome. Rapid macrophage pyroptosis depends on the Salmonella pathogenicity island-1 type III secretion system (T3SS) and flagella. Salmonella dynamically modulates induction of macrophage pyroptosis, and regulation of T3SS systems permits bacterial replication in specialized intracellular niches within macrophages. However, these infected macrophages later undergo a delayed form of caspase-1-dependent pyroptosis. Caspase-1-deficient mice are more susceptible to a number of bacterial infections, including salmonellosis, and pyroptosis is therefore considered a generalized protective host response to infection. Thus, Salmonella-induced pyroptosis serves as a model to understand a broadly important pathway of proinflammatory programmed host cell death: examining this system affords insight into mechanisms of both beneficial and pathological cell death and strategies employed by pathogens to modulate host responses.