Innate Immune Detection of Flagellin Positively and Negatively Regulates Salmonella Infection

Abstract

Salmonella enterica serovar Typhimurium is a flagellated bacterium and one of the leading causes of gastroenteritis in humans. Bacterial flagellin is required for motility and also a prime target of the innate immune system. Innate immune recognition of flagellin is mediated by at least two independent pathways, TLR5 and Naip5-Naip6/NlrC4/Caspase-1. The functional significance of each of the two independent flagellin recognition systems for host defense against wild type Salmonella infection is complex, and innate immune detection of flagellin contributes to both protection and susceptibility. We hypothesized that efficient modulation of flagellin expression in vivo permits Salmonella to evade innate immune detection and limit the functional role of flagellin-specific host innate defenses. To test this hypothesis, we used Salmonella deficient in the anti-sigma factor flgM, which overproduce flagella and are attenuated in vivo. In this study we demonstrate that flagellin recognition by the innate immune system is responsible for the attenuation of flgM- S. Typhimurium, and dissect the contribution of each flagellin recognition pathway to bacterial clearance and inflammation. We demonstrate that caspase-1 controls mucosal and systemic infection of flgM- S. Typhimurium, and also limits intestinal inflammation and injury. In contrast, TLR5 paradoxically promotes bacterial colonization in the cecum and systemic infection, but attenuates intestinal inflammation. Our results indicate that Salmonella evasion of caspase-1 dependent flagellin recognition is critical for establishing infection and that evasion of TLR5 and caspase-1 dependent flagellin recognition helps Salmonella induce intestinal inflammation and establish a niche in the inflamed gut. We also hypothesized that the Naip5/Nlrc4 inflammasome has distinct roles during systemic and mucosal detection of flagellin. To test this hypothesis, we used wildtype, flagellin deficient and flagellin overproducing Salmonella to assess the contribution of each inflammasome component during systemic and mucosal infection. We demonstrate that during systemic infection, Salmonella efficiently evades flagellin detection. During mucosal infections, Casp1 and Nlrc4 are required for flagellin detection and host protection. In contrast, Naip5 has a more complex function. Similar to Nlrc4 and Casp1, loss of Naip5 abrogates Salmonella flagellin detection in vivo, and increases host susceptibility to infection. In addition, loss of Naip5 leads to protection against Salmonella infection, which is independent of flagellin expression by Salmonella. Thus Naip5 detection of Salmonella's flagellin protects against infection, and Naip5 promotes Salmonella infection through an undefined mechanism, possibly involving interactions with the commensal microbiota. Our studies illuminate the complexity of mucosal infections, where the multitude of interactions between microbiota, pathogens and the innate immune system shape host defenses.