IL-33 shifts macrophage polarization, promoting resistance against Pseudomonas aeruginosa keratitis

Abstract

Purpose. To determine the role of IL-33 in resistance to Pseudomonas aeruginosa keratitis. Methods. Corneal IL-33 mRNA and protein levels were tested in susceptible C57BL/6 (B6) and resistant BALB/c mice. B6 mice were injected with recombinant mouse IL-33 (rmIL-33) and disease severity, bacterial load, polymorphonuclear neutrophils (PMN) infiltrate, gene expression of inflammatory, and T-helper (Th)1/Th2 cytokines were tested by RT-PCR. IL-33 signaling and macrophage (MΦ{phonetic}) polarization also were examined. Results. IL-33 mRNA and protein were expressed constitutively in the normal corneas of both groups and were significantly elevated at 1 to 5 days after infection in BALB/c over B6 mice. rmIL-33-treated B6 mice showed less severe disease than did PBS controls and exhibited decreased bacterial load, PMN infiltrate, and corneal mRNA levels for IL-1β, MIP-2, and TNF-α. Th2-type cytokines (IL-4, -5, -10) also were significantly upregulated, and protein levels for TNF-α and IL-10 confirmed the mRNA data. To further investigate IL-33 in corneal inflammation, it was overexpressed in MΦ{phonetic} (RAW264.7 cells). This significantly increased IL-5 and IL-10, while it decreased IFN-γ and other pro-inflammatory cytokines. The role of the MΦ{phonetic} was further tested in infected rmIL-33 compared with PBS-injected mice. Immunostaining showed that rmIL-33 injection shifted MΦ{phonetic} polarization from NO synthase 2 to arginase production. Furthermore, peritoneally elicited cells (B6 mice) treated with lipopolysaccharide and rmIL-33 exhibited elevated ST2 levels and a shift from IL-12 to IL-10 mRNA production. Conclusions. These data provide evidence that IL-33 promotes a Th2-type immune response and reduces inflammation by polarizing the MΦ{phonetic} production of anti-inflammatory mediators in the cornea. © Association for Research in Vision and Ophthalmology.