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 (10⁶) 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.