SUCCESSFUL IMMUNIZATION AGAINST ACANTHAMOEBA-KERATITIS IN A PIG MODEL

The feasibility of inducing protective immunity to Acanthamoeba kerati tis was tested in a pig model. Experiments were designed to determine if ocular infection with Acanthamoeba trophozoites would elicit protec tion against reinfection. Additional experiments examined whether inje ction of parasite antigens either intramuscularly, subconjunctivally, or by both routes would induce immunity. Therefore, four groups of ani mals were examined: (a) pigs that had resolved a primary corneal infec tion with Acanthamoeba; (b) pigs immunized intramuscularly; (c) pigs i mmunized subconjunctivally; and (d) pigs immunized intramuscularly and subconjunctivally. Animals were subsequently challenged with parasite -laden soft contact lenses and observed clinically for the appearance of Acanthamoeba keratitis. Acanthamoeba-specific serum antibody titers and blastogenic responses of peripheral blood lymphocytes were determ ined weekly. The results indicated that intramuscular injection of Aca nthamoeba antigens failed to protect against ocular infection even tho ugh hosts developed high titers of IgG antibodies and displayed lympho cyte blastogenic responses to parasite antigens. Ocular infection alon e failed to stimulate immunity in any of the animals. By contrast, 50% of the hosts immunized subconjunctivally were protected against corne al disease, and 100% of the animals immunized by a combination of intr amuscular and subconjunctival administration of parasite antigens were completely protected against two separate ocular challenges with infe ctious parasites. Protection did not correlate with either IgG antibod y titers or blastogenic potentials of peripheral blood lymphocytes. In terestingly, ocular infection alone failed to stimulate immunity to su bsequent ocular challenge with infectious parasites. Thus, administrat ion of parasite antigen via the subconjunctival route can protect agai nst Acanthamoeba keratitis.