Intracorneal instillation of latex beads induces macrophage-dependent protection against Acanthamoeba keratitis

PURPOSE. Instillation of sterile 1.0 μM latex beads into the central corneal epithelium renders Chinese hamsters resistant to corneal infection with Acanthamoeba castellanii. By contrast, activation of the adaptive immune response by subcutaneous immunization with A. castellanii antigens fails to protect against Acanthamoeba keratitis. This study was undertaken to examine the mechanisms that mediate latex bead-induced resistance to Acanthamoeba keratitis. METHODS. In vitro experiments examined the effect of latex bead treatment on the capacity of A. castellanii trophozoites to adhere to and kill corneal epithelial cells. In vivo administration of antineutrophil antiserum was used to evaluate the role of neutrophils in latex-bead-induced protection against Acanthamoeba keratitis. Liposomes containing the macrophagicidal drug clodronate were used to deplete conjunctival macrophages and determine the role of macrophages in the latex-bead-induced resistance. RESULTS. Latex bead treatment did not affect adherence of trophozoites to the corneal epithelium or protect corneal epithelial or stromal cells from trophozoite-mediated cytolysis in vitro. Neutrophil depletion did not abrogate the latex beads' protective effect. Latex bead treatment induced a significant infiltration of macrophages into the corneas that peaked at day 4 of infection. Moreover, depletion of conjunctival macrophages with the macrophagicidal drug clodronate eliminated the latex beads' protective effect. CONCLUSIONS. The results indicate that intracorneal injection of latex beads induces a remarkable resistance to Acanthamoeba keratitis that is largely, if not entirely, mediated by macrophages. These results underscore the importance of the innate immune apparatus in the resistance to Acanthamoeba keratitis.