Salmonella SPI2 evades detection by NAIP/NLRC4 despite utilization of a detectable needle

Salmonella enterica serovar Typhimurium uses two type 3 secretion (T3S) systems to reprogram host cells. Whereas activity of the SPI1 T3S can be detected by the NAIP/NLRC4 inflammasome, the SPI2 T3S is not detected. In this study, we demonstrate that the SPI2 needle protein, SsaG, can be detected by the NAIP-NLRC4 inflammasome when artificially delivered to the cytosol in mouse macrophages. Surprisingly, this detection occurs primarily through NAIP2, which normally detect rod proteins. However, we found that during infection with live S. Typhimurium, this detection fails to trigger the downstream pyroptotic pathway. We investigate the hypothesis that the SPI2 effector protein SpvC inhibits NLRC4 activation. In mouse macrophages, spvC mutants were detected under SPI2-inducing conditions in an NLRC4-dependent manner. However, this masking effect of SpvC is not seen in vivo, where SpvC contributes to S. Typhimurium virulence in mice independent from NLRC4. We also hypothesized that S. Typhimurium may minimize SsaG detection by limiting protein expression. In support of this, overexpressing SsaG causes limited NLRC4-dependent clearance in vivo. Therefore, the SPI2 may evade NAIP/NLRC4 inflammasome detection by precisely controlling the quantity of detectable SsaG needle protein expressed.