Establishment of a Murine Model and Quantitation of the Host Response to S.aureus Endophthalmitis

Abstract

Abstract: : Purpose: To develop a murine model of bacterial endophthalmitis in order to quantitate the host response during intraocular infection with S.aureus. Method: Right eyes of 6 week old C57/BL6 mice received intravitreal injections of 500 or 5000 CFU S.aureus RN6390. Left eyes received mock injections and served as controls. Infections were allowed to progress for up to 96 hours, retinal function was assessed by electroretinography, and eyes afterwards collected for flow cytometric analysis and bacterial enumeration. Results: S.aureus rapidly grew from 500 CFU injected at the time of infection to 1x10₈ CFU/ml by 24 hours, but were largely cleared by 96 hours and retinal function only decreased transiently. In contrast, 5000 CFU injected at the time of infection resulted in a population of 1x10₉ CFU/ml by 24 hours with little reduction in numbers by 96 hours and led to complete loss of retinal function and organ destruction. The inflammatory response was quantified by flow cytometry, which showed that at 12 hours post infection equal amounts of granulocytes (Gr-1₊) and macrophages (F4/80₊) infiltrated the intraocular space irrespective of the inoculum. By 48 hours, 60 times the number of granulocytes had migrated into the vitreous of eyes infected with 5000 CFU, in comparison to eyes infected with 500 CFU. Conclusion: Infection of the mouse eye with 5000 CFU S.aureus leads to massive intraocular inflammation, rapid loss of retinal function, and eventually results in destruction of the eye. Infection with 500 CFU also results in a robust inflammatory response, but can be cleared without overt sequelae. This novel murine model will permit the sequence of events in the inflammatory cascade to be characterized and the role of immune privilege in limiting the eye's capacity to recruit sufficient quantities of granulocytes and potentially impairing their function to be determined quantitatively through flow cytometric analysis and qualitatively through the use of knockout mice.

Keywords: 316 animal model • 328 bacterial disease • 437 inflammation