Impact of innate immune factors on the persistence of Staphylococcus aureus in the ocular environment

Microbial keratitis can be detrimental to vision, causing corneal damage and inflammation that can lead to painful epithelial defects and scarring. Staphylococcus aureus is frequently isolated from bacterial keratitis cases. This organism's virulome and propensity for antibiotic resistance make it a formidable ocular pathogen that is often difficult to eradicate. S. aureus is not typically isolated from an immunocompetent ocular environment. We therefore hypothesized that elements of innate immunity are important in protecting the cornea from S. aureus infection, so their absence would facilitate persistence that might lead to keratitis. In the current study, we used a corneal scratch and topical inoculation model with S. aureus strain 8325-4 to infect mouse eyes and assess infection and inflammation in wild type C57BL/6J mice and mice genetically deficient in TLR2 (i.e. TLR2-/- or TLR2/4-/-) or TLR2-associated proinflammatory mediators (CXCL1-/-, CXCL2-/-, CXCL10-/-, CCL2-/-, CCL3-/-, or TNFα-/-). We included males and females in these experiments to determine whether sex was a biological variable. We assessed staphylococcal burden and clearance by quantifying colony forming units (CFU)/eye, inflammation by quantifying myeloperoxidase (MPO) from infiltrating neutrophils, and ocular pathology each day for 3-6 days, depending on the assay. Our data shows that, in general, the absence of the TLR2 pathway and its downstream mediators facilitated persistence of S. aureus in the mouse eye, but did not lead to ulcerative keratitis. S. aureus was cleared from the C57BL/6J mouse cornea within 3 days, while the organism persisted in knockout mouse eyes through day 6. Although pockets of staphylococci and neutrophil influx at the sites of infection were observed in some eyes, there were surprisingly very few corneal epithelial defects noted irrespective of mouse strain. These results suggest that defects in innate immunity create an environment that can facilitate persistence of S. aureus at the ocular surface, an area typically devoid of harmful bacteria.