Silencing Toll-like receptor-9 in Pseudomonas aeruginosa keratitis

PURPOSE. To determine the effects of silencing Toll-like receptor (TLR) 9 signaling in Pseudomonas aeruginosa keratitis. METHODS. Corneal TLR9 mRNA levels were tested by RT-PCR in C57BL/6 (B6, susceptible) and BALB/c ( resistant) mice and compared. The response of B6 mice to CpG DNA, which binds TLR9, was tested after subconjunctival injection of mice with control or CpG DNA; TLR9, IL-1 beta, macrophage inflammatory protein (MIP)-2, IL-4, IL-10, IL-12, IL-18, and IFN-gamma levels were measured by RT-PCR. Langerhans cells (LCs) were stimulated with CpG DNA and treated with TLR9 or control siRNA, and mRNA levels of TLR9, IL-1 beta, and MIP-2 were detected by RT-PCR. In addition, IL-1 beta levels were tested by ELISA. Then B6 mice were injected subconjunctivally with control or TLR9 siRNA before infection and treated topically afterward. Slit lamp, clinical score, RT-PCR, ELISA, myeloperoxidase assay, and plate counts were performed. RESULTS. TLR9 mRNA levels were sixfold higher in B6 than in BALB/c corneas the day after injection. B6 mice injected with CpG DNA exhibited an increase in corneal mRNA for TLR9, IL-1 beta, MIP-2, IL-12, and IFN-gamma over controls. LCs stimulated with CpG DNA and treated with TLR9 siRNA exhibited reduced TLR9, IL-1 beta, and MIP-2 levels compared with controls. Finally, B6 mice treated with TLR9 siRNA showed decreases in corneal opacity, polymorphonuclear leukocyte number, IL-12 and IFN-gamma mRNA, IL-1 beta, and MIP-2 protein compared with those treated with control siRNA. Fewer corneas perforated in these mice, but bacterial loads were higher than in controls. CONCLUSIONS. Signaling through TLR9 appears important in P. aeruginosa keratitis, and silencing TLR9 signaling reduces inflammation but likely contributes to decreased bacterial killing in the cornea.