Role of endogenous interleukin-18 in resolving wild-type and attenuated Salmonella typhimurium infections

The stimulation of gamma interferon (IFN-gamma) has been shown to be essential in resolving infections by intracellular pathogens. As such, several different cytokines including, interleukin-12 (IL-12) and IL-18, can induce IFN-gamma. To resolve Salmonella infections, the stimulation of IL-12 and IFN-gamma are important for mediating its clearance. In this present study, the relevance of IL-18 in protection against oral challenge with Salmonella typhimurium was investigated to determine the role of this IFN-gamma-promoting cytokine. Rabbit anti-murine IL-18 antisera was generated and administered prior to the oral challenge of BALB/c and IL-12p40-deficient knockout (IL-12KO) mice with a wild-type S. typhimurium strain. The median survival time was reduced by 2 days for the anti-IL-18-treated BALB/c mice, while no significant reduction in survival rate for the anti-IL-18-treated IL-12KO mice was observed compared to vehicle-treated mice. To investigate the contribution of IL-18 to resolving Salmonella infections, an attenuated aro-negative mutant (H647) was orally administered to BALB/c mice. This Salmonella infection induced both IL-12 and IFN-gamma in both the Peyer's patches and the spleens. In vehicle-treated mice, Peyer's patch IL-12 peaked by 24 h, while IL-18 levels peaked at 3 days, suggesting sequential support by these cytokines for IFN-gamma. Anti-IL-18 treatment exerted its greatest effect upon the mucosal compartment, limiting early IFN-gamma production. However, anti-IL-18 treatment had little effect upon splenic IFN-gamma levels until late in the response. Infection of IL-12KO mice with H647 strain induced IFN-gamma, but it was not supported by IL-18, although IL-18 levels were reduced by this treatment. These results suggest that IL-18 does contribute to the clearance of S. typhimurium and that endogenously induced IL-18 could not substitute for IL-12.