PROTECTIVE IMMUNITY TO SCHISTOSOMA-MANSONI INDUCED IN THE OLIVE BABOON PAPIO-ANUBIS BY THE IRRADIATED CERCARIA VACCINE

The radiation-attenuated schistosome vaccine induces a high level of p rotective immunity in rodents. In order to assess its potential releva nce to man, we have tested its efficacy in the non-human primate Papio anubis. A vaccination regime consisting of 3 exposures of approximate ly 9000 cercariae irradiated with 30 or 60 krad. of gamma radiation in duced > 50% protection to a challenge with normal larvae. A lower atte nuating dose of 20 krad., optimal for vaccination of mice, was less ef fective. All vaccination regimes elicited a population of PBMC which p roliferated in vitro in response to antigen. These responses peaked af ter the third exposure but were significantly lower after challenge. T hey revealed relatively little cross-reactivity with adult Schistosoma haematobium antigens and provided some evidence for stage-specific an tigens. Circulating IgM reactive with adult S. mansoni antigen was det ected after the second vaccination but levels remained low throughout. In contrast, IgG levels were boosted by successive vaccinations, alth ough they showed a tendency to decline from 14 days after each exposur e. There also appeared to be a lag of about 14 days after challenge be fore levels began to rise. Thus, both proliferation and antibody data suggest a lower responsiveness after challenge which may reflect eithe r the reduced antigenic load or immunogenicity of normal, compared to vaccinating larvae. The data indicate that the attenuated schistosome vaccine is capable of inducing protection in a highly permissive prima te host, with the implication that the mechanisms involved may also be relevant to man.