Innate Interferon Response in Macrophage and Epithelial Cells Infected with Wild-Type Compared to DNA Adenine Methylase and Flagellin Mutant Salmonella enterica Serovar Typhimurium

Salmonella enterica serovar Typhimurium is highly virulent and mediates robust interferon (IFN)-stimulated gene (ISG) induction, whereas bacterial mutants that lack the DNA adenine methylase (Dam) are attenuated, elicit a reduced ISG activation profile, and establish immunity to murine typhoid fever. We show here that in contrast to observations in mice, infection of macrophage cell cultures with either wild-type (WT) or dam(-) mutant Salmonella resulted in surprisingly similar kinetics and amplitudes of induction of IFN-beta, the type I IFN-alpha,beta beacon gene Mx, and the type II IFN-gamma beacon inducible nitric oxide synthase (iNOS). Likewise, activation of NF-kappaB-dependent gene expression in epithelial cells was comparable with WT and dam(-) mutant Salmonella. In contrast, the flagellin-deficient flhC(-) mutant did not activate NF-kappaB in epithelial cells but activated ISG expression comparable to that of WT Salmonella in macrophage cells. WT and dam(-) strains displayed a similar Toll-like receptor 5 (TLR5)-dependent NF-kappaB activation, whereas the flhC(-) mutant lacked this activity. UV-inactivated Salmonella elicited similar ISG induction to that of viable Salmonella in macrophages and mediated the establishment of a functional antiviral state but displayed decreased cytocidal activity. These results establish that the inherent IFN system-inducing capacities of dam(-) and WT Salmonella strains in cultured macrophage and epithelial cells, unlike the mouse, are indistinguishable.