Why Does Acanthamoeba castellanii Not Progress Beyond the Cornea to Produce Intraocular Infections?

Abstract

Abstract: : Purpose: This study examined why A. castellanii remains restricted to the cornea and does not usually produce intraocular infections. The first hypothesis proposes that the amoebae cannot penetrate Descemet’s membrane and the corneal endothelium to enter the anterior chamber (AC). The second hypothesis proposes that the amoebae 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 examine if Acanthamoeba trophozoites can penetrate this membrane in a transwell assay. Additionally, the amoebae were incubated in pig AH in vitro. Their viability was determined at 24 hours, 48 hours and 7 days by direct counts and their ability to form growth trails on a lawn of E. coli. Amoebae (10⁶) were injected into the AC of hamster eyes and eyes were observed for signs of clinical disease for 3 weeks. Eyes were harvested at days 1–29 post infection and the number of amoebae in the AC was determined by histopathology. Results: Amoebae were able to penetrate Descemet’s membrane in 24 hours. Penetration could be prevented by addition of serine protease inhibitors or a monoclonal antibody against the Acanthamoeba serine protease, MIP–133. Although AH induced encystment of the amoebae, cysts remained viable, even after seven days of incubation in 10X AH. However, exposure to AH did not induce encystment if the trophozoites were incubated with iris ciliary body cells as a source of nutrition. Injection of amoebae into the AC induced a robust neutrophil infiltrate. Although large numbers of amoebae were observed in the AC, their numbers dissipated over time and neither cysts nor trophozoites were detectable by day 15 post AC injection. Conclusions: Our findings suggest that A. castellanii is capable of penetrating Descemet’s membrane and entering the AC. Intraocular cells, such as iris ciliary body cells can serve as a source of nutrition for the amoebae. 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 and intraocular infection.

Keywords: Acanthamoeba • keratitis • endophthalmitis