Inhibition of caspase-1 or gasdermin-D enable caspase-8 activation in the Naip5/NLRC4/ASC inflammasome

Legionella pneumophila is a gram-negative,flagellated bacterium that survives in phagocytes and causes legionnaires’ disease. upon infection of mammalian macrophages,cytosolic flagellin triggers the activation of naip/nlrc4 inflammasome,which culminates in pyroptosis and restriction of bacterial replication. although nlrc4 and caspase-1 participate in the same inflammasome,nlrc4-/-mice and their macrophages are more permissive to l. pneumophila replication compared with casp1/11-/-. this feature supports the existence of a pathway that is nlrc4-dependent and caspase-1/11-independent. here,we demonstrate that caspase-8 is recruited to the naip5/nlrc4/asc inflammasome in response to flagellin-positive bacteria. accordingly,caspase-8 is activated in casp1/11-/-macrophages in a process dependent on flagellin,naip5,nlrc4 and asc. silencing caspase-8 in casp1/11-/-cells culminated in macrophages that were as susceptible as nlrc4-/-for the restriction of l. pneumophila replication. accordingly,macrophages and mice deficient in asc/casp1/11-/-were more susceptible than casp1/11-/-and as susceptible as nlrc4-/-for the restriction of infection. mechanistically,we found that caspase-8 activation triggers gasdermin-d-independent pore formation and cell death. interestingly,caspase-8 is recruited to the naip5/nlrc4/asc inflammasome in wild-type macrophages,but it is only activated when caspase-1 or gasdermin-d is inhibited. our data suggest that caspase-8 activation in the naip5/nlrc4/asc inflammasome enable induction of cell death when caspase-1 or gasdermin-d is suppressed. © 2017 mascarenhas et al.