The feasibility of inducing protective immunity to
Acanthamoeba keratitis was tested in a pig model. Experiments were designed to determine if
ocular infection with Acanthamoeba trophozoites would elicit protection against
reinfection. Additional experiments examined whether injection of parasite
antigens either intramuscularly, subconjunctivally, or by both routes would induce immunity. Therefore, four groups of animals were examined: (a) pigs that had resolved a primary corneal
infection with Acanthamoeba; (b) pigs immunized intramuscularly; (c) pigs immunized 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 determined weekly. The results indicated that
intramuscular injection of Acanthamoeba
antigens failed to protect against
ocular infection even though hosts developed high titers of
IgG antibodies and displayed lymphocyte blastogenic responses to parasite
antigens.
Ocular infection alone failed to stimulate immunity in any of the animals. By contrast, 50% of the hosts immunized subconjunctivally were protected against
corneal disease, and 100% of the animals immunized by a combination of intramuscular and subconjunctival administration of parasite
antigens were completely protected against two separate ocular challenges with infectious parasites. Protection did not correlate with either
IgG antibody titers or blastogenic potentials of peripheral blood lymphocytes. Interestingly,
ocular infection alone failed to stimulate immunity to subsequent ocular challenge with infectious parasites. Thus, administration of parasite
antigen via the subconjunctival route can protect against
Acanthamoeba keratitis.