Purpose.: The purpose of this study was to investigate the protective mechanisms evoked by TLR2 and MyD88 signaling in bacterial endophthalmitis in vivo.

Methods.: Endophthalmitis was induced in wild-type (WT), TLR2−/−, MyD88−/−, and Cnlp−/− mice by intravitreal injections of a laboratory strain (RN6390) and two endophthalmitis isolates of Staphylococcus aureus. Disease progression was monitored by assessing corneal and vitreous haze, bacterial burden, and retinal tissue damage. Levels of inflammatory cytokines/chemokines were determined using quantitative RT-PCR (qRT-PCR) and ELISA. Flow cytometry was used to assess neutrophil infiltration. Cathelicidin-related antimicrobial peptide (CRAMP) expression was determined by immunostaining and dot blot.

Results.: Eyes infected with either laboratory or clinical isolates exhibited higher levels of inflammatory mediators at the early stages of infection (≤ 24 hours) in WT mice than in TLR2−/− or MyD88−/− mice. However, their levels surpassed that of WT mice at the later stages of infection (> 48 hours), coinciding with increased bacterial burden and retinal damage. Both TLR2−/− and MyD88−/− retinas produced reduced levels of CRAMP, and its deficiency (Cnlp−/−) rendered the mice susceptible to increased bacterial burden and retinal tissue damage as early as 1 day post infection. Analyses of inflammatory mediators and neutrophil levels in WT versus Cnlp−/− mice showed a trend similar to that observed in TLR2 and MyD88 KO mice. Furthermore, we observed that even a 10-fold lower infective dose of S. aureus was sufficient to cause endophthalmitis in TLR2−/− and MyD88−/− mice.

Conclusions.: TLR2 and MyD88 signaling plays an important role in protecting the retina from staphylococcal endophthalmitis by production of the antimicrobial peptide CRAMP.