PROTECTION AGAINST SCHISTOSOMA-MANSONI AND ASSOCIATED IMMUNE-RESPONSES INDUCED IN THE VERVET MONKEY CERCOPITHECUS-AETHIOPS BY THE IRRADIATED CERCARIA VACCINE

The efficacy of the radiation-attenuated schistosome vaccine in elicit ing protective immunity was investigated in the vervet Cercopithecus a ethiops. Three groups of test animals received one, three, or five exp osures to 5,000-9,000 cercariae attenuated with 30 krad of gamma radia tion from a Cs-137 source, while a fourth group served as challenge co ntrols. After a single vaccination, no statistically significant prote ction was found; after three vaccinations it reached its highest peak (48%) and five exposures resulted in reduced protection (39%). Examina tion of immunologic parameters supported these findings, with prolifer ative responses of peripheral blood lymphocytes being greatest in the group vaccinated three times. Specific immunoglobulin G (IgG) levels a lso peaked after three vaccinations, with clear evidence of a reductio n after the fourth and fifth exposures. There was a clear correlation (r = 0.66) between the antibody level al the time of challenge and the protection observed in individual vervets. These data demonstrate tha t protective immunity to Schistosoma mansoni can be achieved with the attenuated vaccine in a highly susceptible nonhuman primate (i.e., 82% maturation of penetrant cercariae was recorded in challenge control a nimals in this study). However, they also indicate that there is a cei ling to this protection. Furthermore, increasing the number of vaccina tions beyond a certain point, instead of boosting protection may cause it to diminish, possibly due to the induction of immunoregulatory mec hanisms.