Unexpected overlapping roles of multiple caspases and programmed cell death pathways in the response to bacterial infection
Large multi-protein complexes known as inflammasomes control pathogen invasion and induce inflammatory cell death known as pyroptosis via the activation of caspases, aspartate-specific cysteinyl proteases.
Salmonella Typhimurium activates pyroptosis by activating caspases -1 and -11, in part via the NLRP3 and NLRC4 inflammasomes. Previous reports implicated also caspase-8 (and possibly other caspases) in the inflammatory response. Additionally, there is evidence for a high level of functional back-up between different cell death pathways in the cellular response to pathogens. We tested these hypotheses by generating mice deficient for multiple caspases and RIPK3 an essential mediator of necroptotic cell death.
Interestingly, primary macrophages lacking caspases-1/-11/12 and also caspase-8 as well as RIPK3 were highly resistant in vitro to infection with the Salmonella Typhimurium Sl1344 strain even at MOIs as high as 500. This was surprising, as previous reports have shown that caspase-8/RipK3 double-deficient macrophages showed only a minor delay in Salmonella induced cell death compared to WT cells.
Importantly, when infecting the Caspase-1/11/12/8/RipK3-/- mice with the vaccine strain BRD509, these mice were not able to clear the bacteria from their livers and spleens. This observation was accompanied by a lack of Caspase-1/11/12/8/RipK3-/- to shed these bacteria through their faeces. Interestingly, the lack of shedding of bacteria through their faeces was also observed in the Caspase1/11/12 triple knockout mice, but not in Caspase 1/11 double deficient animals, suggesting a role for Caspase-12 in shedding of bacteria.
Collectively, these findings provide clear evidence that multiple cell death pathways and hence the caspases involved in these processes show functional overlap in the response to infection with bacteria and possibly other pathogens. Moreover, our studies suggest a novel role for caspase-12 in clearing the infection through regulation of faecal shedding of bacteria.