Gasdermin-D and caspase-7 are the key caspase-1/8 substrates downstream of the NAIP5/NLRC4 inflammasome required for restriction of legionella pneumophila

Abstract

Inflammasomes are cytosolic multi-protein complexes that detect infection or cellular damage and activate the Caspase-1 (CASP1) protease. The NAIP5/NLRC4 inflammasome detects bacterial flagellin and is essential for resistance to the flagellated intracellular bacterium Legionella pneumophila. The effectors required downstream of NAIP5/NLRC4 to restrict bacterial replication remain unclear. Upon NAIP5/NLRC4 activation, CASP1 cleaves and activates the pore-forming protein Gasdermin-D (GSDMD) and the effector caspase-7 (CASP7). However, Casp1–/–(and Casp1/11–/–) mice are only partially susceptible to L. pneumophila and do not phenocopy Nlrc4–/–mice, because NAIP5/NLRC4 also activates CASP8 for restriction of L. pneumophila infection. Here we show that CASP8 promotes the activation of CASP7 and that Casp7/1/11–/–and Casp8/1/11–/–mice recapitulate the full susceptibility of Nlrc4–/–mice. Gsdmd–/–mice exhibit only mild susceptibility to L. pneumophila, but Gsdmd–/–Casp7–/–mice are as susceptible as the Nlrc4–/–mice. These results demonstrate that GSDMD and CASP7 are the key substrates downstream of NAIP5/NLRC4/ CASP1/8 required for resistance to L. pneumophila.