Protective immunity against Acanthahoeba keratitis in a Chinese hamster model

Purpose. To examine the role of serum antibody and cell-mediated immunity in Acanthamoeba keratitis. Methods. Chinese hamsters were immunized either orally or intramuscularly with Acanthamoeba castellanii antigens conjugated to cholera toxin (CT). Hamsters were immunized 4 times at weekly intervals and challenged with A. castellanii-laden contact lenses 7 days after the fourth immunization. Parasite-specific serum IgG antibody was measured by ELISA and delayed-type hypersensitivity (DTH) was assessed by footpad swelling. Results. Oral immunization with antigen conjugated to CT protected against corneal infection (22% infection rate; 6/27), yet did not stimulate serum IgG antibody production or DTH responses to parasite antigens (P>0.05 compared to normal controls). By contrast, intramuscular (IM) immunization with Acanthamoeba antigens conjugated to CT failed to protect against corneal challenge (86% infection; 6/7) even though the hosts produced high titers of parasite-specific IgG antibodies and developed positive DTH to parasite antigens. Conclusions. The route of antigen exposure and the presence of CT adjuvant exert a profound effect on acquired resistance to Acanthamoeba keratitis. The results also suggest that neither serum IgG nor DTH protect against corneal infection with Acanthamoeba.