Salmonella Enteritidis T1SS protein SiiD inhibits NLRP3 inflammasome activation via repressing the mtROS-ASC dependent pathway.

Inflammasome activation is an essential innate immune defense mechanism against Salmonella infections. Salmonella has developed multiple strategies to avoid or delay inflammasome activation, which may be required for long-term bacterial persistence. However, the mechanisms by which Salmonella evades host immune defenses are still not well understood. In this study, Salmonella Enteritidis (SE) random insertion transposon library was screened to identify the key factors that affect the inflammasome activation. The type I secretion system (T1SS) protein SiiD was demonstrated to repress the NLRP3 inflammasome activation during SE infection and was the first to reveal the antagonistic role of T1SS in the inflammasome pathway. SiiD was translocated into host cells and localized in the membrane fraction in a T1SS-dependent and partially T3SS-1-dependent way during SE infection. Subsequently, SiiD was demonstrated to significantly suppress the generation of mitochondrial reactive oxygen species (mtROS), thus repressing ASC oligomerization to form pyroptosomes, and impairing the NLRP3 dependent Caspase-1 activation and IL-1β secretion. Importantly, SiiD-deficient SE induced stronger gut inflammation in mice and displayed NLRP3-dependent attenuation of the virulence. SiiD-mediated inhibition of NLRP3 inflammasome activation significantly contributed to SE colonization in the infected mice. This study links bacterial T1SS regulation of mtROS-ASC signaling to NLRP3 inflammasome activation and reveals the essential role of T1SS in evading host immune responses. Author summary: Salmonella Enteritidis (SE) is a Gram-negative, non-host specific, facultative intracellular pathogen. SE infection is one of the most important global public health security problems. In recent years, SE has become the predominant serotype of Salmonella responsible for salmonellosis. Inflammasome activation plays an important role in the host immune defense against pathogen infections. However, the knowledge of the interactions between SE and inflammasomes is limited. How SE modulates inflammasome activation to evade host immune clearance and achieve persistent infection remains poorly understood. Our study demonstrates that bacterial T1SS protein SiiD is involved in the inhibition of NLRP3 inflammasome activation during SE infection. More importantly, we revealed the molecular mechanism by which SiiD depresses NLRP3 inflammasome signaling and subsequently promotes the virulence and colonization of SE in vivo. Our findings expand the understanding of the contribution of Salmonella T1SS in counteracting host immune responses mediated by the NLRP3 inflammasome. [ABSTRACT FROM AUTHOR]