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-β, the type I IFN-α,β beacon gene Mx, and the type II IFN-γ beacon inducible nitric oxide synthase (iNOS). Likewise, activation of NF-κB-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-κB 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-κB 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.