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

PURPOSE: Instillation of sterile 1.0 microM 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.

The immunobiology of Acanthamoeba keratitis.

Acanthamoeba spp. are free-living amoebae that cause Acanthamoeba keratitis, a blinding corneal infection. The innate immune apparatus is crucial for the resolution of the disease. With the exception of mucosal antibody, elements of the adaptive immune system fail to prevent infection or contribute to its resolution in experimental animals.

The pathophysiology of Acanthamoeba keratitis.

Acanthamoeba keratitis is a sight-threatening infection of the ocular surface that is produced by several free-living amebae of the genus Acanthamoeba. Infection is usually initiated by Acanthamoeba-contaminated contact lenses and produces exquisite pain and ulceration of the ocular surface. The pathophysiology of this infection involves an intricate series of sequential events that includes the production of several pathogenic proteases that degrade basement membranes and induce cytolysis and apoptosis of the cellular elements of the cornea, culminating in dissolution of the collagenous corneal stroma. Targeting such proteases could lead to the development of vaccines that target the disease process rather than the pathogen itself.

Failure of Acanthamoeba castellanii to produce intraocular infections.

PURPOSE: This study examined possible mechanisms to explain why Acanthamoeba castellanii remains restricted to the cornea and rarely produces intraocular infections. The first hypothesis proposed that trophozoites cannot penetrate Descemet's membrane and the corneal endothelium to enter the anterior chamber (AC). The second hypothesis proposed that the trophozoites can enter the AC; however, the aqueous humor (AH) contains factors that either induce encystment or kill the amoebae. METHODS: Descemet's membrane was isolated from pig corneas and was used to determine whether Acanthamoeba trophozoites could penetrate this membrane in vitro. In addition, the capacity of trophozoites to survive in AH was determined in vitro. Trophozoites (10(6)) were injected into the AC of hamster eyes, and the number of amoebae in the AC was determined by histopathology 1 to 15 days later. RESULTS: The amoebae penetrated Descemet's membrane within 24 hours of in vitro culture. Penetration was prevented by addition of serine protease inhibitors or a chicken monoclonal antibody against the Acanthamoeba serine protease MIP-133. Although AH induced encystment of the amoebae, cysts remained viable. Injection of amoebae into the AC induced a robust neutrophil infiltrate, which was associated with complete clearance by day 15 after AC injection. CONCLUSIONS: The findings suggest that A. castellanii is capable of penetrating Descemet's membrane and entering the AC. However, a robust neutrophil response is associated with the disappearance of intraocular trophozoites and suggests that cells of the innate immune apparatus are important in preventing Acanthamoeba keratitis from progressing to become an intraocular infection.