Immune response to Staphylococcus aureus endophthalmitis in a rabbit model

Staphylococcus aureus is an important cause of severe bacterial endophthalmitis. Both immunoglobulin (Ig) G and A antibody titers to ribitol teichoic acid (RTA), the major antigenic determinant of the S. aureus cell wall, were measured by an enzyme-linked immunosorbent assay in serum, tears, aqueous, and vitreous on days 3, 7, 10, 14, 21, and 30 after intravitreal injection of viable S. aureus in rabbits. Clinical examination showed vitreous opacification in all rabbits from days 7-30. Histopathologic examination showed acute inflammation on day 3 and chronic inflammation on days 7-30 in the conjunctiva, cornea, iris, ciliary body, and trabecular meshwork. The vitreous cavity contained neutrophils and necrotic cells on all days. Retinal necrosis was present on days 14-30. Lymphoid follicles with plasma cells were identified in the conjunctiva, ciliary body, and choroid. The vitreous of experimental eyes showed increasing numbers of bacteria from days 3-14, followed by a decrease in numbers on day 21 and absence of viable bacteria on day 30. Increases in IgG antibody levels to RTA were first detected in serum where they were higher than in tears, aqueous, and vitreous until day 14. Vitreous IgG antibody levels to RTA in experimental eyes exceeded all other samples on day 14 and progressively increased thereafter; the other samples declined. The IgA antibody levels were increased in tears on day 14 and in the vitreous of experimental eyes on days 14, 21, and 30. Vitreous IgG antibody levels to RTA were substantially higher than vitreous IgA antibody levels. An inverse correlation was found between vitreous IgG antibody levels and positive vitreous cultures, suggesting that the humoral immune response may be important in the spontaneous sterilization of the vitreous in this model.