Toll-like receptor-deficient mice reveal how innate immune signaling influences Salmonella virulence strategies.

Pathogens utilize features of the host response as cues to regulate virulence gene expression. Salmonella enterica serovar Typhimurium (ST) sense Toll-like receptor (TLR)-dependent signals to induce Salmonella Pathogenicity Island 2 (SPI2), a locus required for intracellular replication. To examine pathogenicity in the absence of such cues, we evaluated ST virulence in mice lacking all TLR function (Tlr2(-/-)xTlr4(-/-)xUnc93b1(3d/3d)). When delivered systemically to TLR-deficient mice, ST do not require SPI2 and maintain virulence by replicating extracellularly. In contrast, SPI2 mutant ST are highly attenuated after oral infection of the same mice, revealing a role for SPI2 in the earliest stages of infection, even when intracellular replication is not required. This early requirement for SPI2 is abolished in MyD88(-/-)xTRIF(-/-) mice lacking both TLR- and other MyD88-dependent signaling pathways, a potential consequence of compromised intestinal permeability. These results demonstrate how pathogens use plasticity in virulence strategies to respond to different host immune environments.