IL-18 Contributes to Host Resistance Against Infection with Pseudomonas aeruginosa Through Induction of IFN-γ Production

Abstract

Pseudomonas aeruginosa keratitis destroys the cornea in susceptible (B6), but not resistant (BALB/c) mice. To determine mechanisms mediating resistance, the role of IFN-γ, IL-12, and IL-18 was tested in BALB/c mice. RT-PCR analysis detected IFN-γ mRNA expression levels in cornea that were significantly increased at 1–7 days postinfection. IL-18 mRNA was detected constitutively in cornea and, at 1–7 days postinfection, levels were elevated significantly, while no IL-12 mRNA was similarly detected. To test whether IL-18 contributed to IFN-γ production, mice were treated with anti-IL-18 mAb. Treatment decreased corneal IFN-γ mRNA levels, and bacterial load and disease increased/worsened, compared with IgG-treated mice. To stringently examine the role of IFN-γ in bacterial killing, knockout (−/−) vs wild-type (wt) mice also were tested. All corneas perforated, and bacterial load was increased significantly in −/− vs wt mice. Because disease severity was increased in IFN-γ−/− vs IL-18-neutralized mice, and since IL-18 also induces production of TNF, we tested for TNF-α in both groups. ELISA analysis demonstrated significantly elevated corneal TNF-α protein levels in IFN-γ−/− vs wt mice after infection. In contrast, RT-PCR analysis of IL-18-neutralized vs IgG-treated infected mice revealed decreased corneal TNF-α mRNA expression. Next, to resolve whether TNF was required for bacterial killing, TNF-α was neutralized in BALB/c mice. No difference in corneal bacterial load was detected in neutralized vs IgG-treated mice. These data provide evidence that IL-18 contributes to the resistance response by induction of IFN-γ and that IFN-γ is required for bacterial killing.