Protective immunity toSchistosoma mansoniinduced in the olive baboonPapio anubisby the irradiated cercaria vaccine

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Abstract
SUMMARY: The radiation-attenuated schistosome vaccine induces a high level of protective immunity in rodents. In order to assess its potential relevance to man, we have tested its efficacy in the non-human primatePapio anubis. A vaccination regime consisting of 3 exposures of approximately 9000 cercariae irradiated with 30 or 60krad. of gamma radiation induced > 50% protection to a challenge with normal larvae. A lower attenuating dose of 20 krad., optimal for vaccination of mice, was less effective. All vaccination regimes elicited a population of PBMC which proliferatedin vitroin response to antigen. These responses peaked after the third exposure but were significantly lower after challenge. They revealed relatively little cross-reactivity with adultSchistosoma haematobiumantigens and provided some evidence for stage-specific antigens. Circulating IgM reactive with adultS. mansoniantigen was detected after the second vaccination but levels remained low throughout. In contrast, IgG levels were boosted by successive vaccinations, although they showed a tendency to decline from 14 days after each exposure. There also appeared to be a lag of about 14 days after challenge before levels began to rise. Thus, both proliferation and antibody data suggest a lower responsiveness after challenge which may reflect either 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 primate host, with the implication that the mechanisms involved may also be relevant to man.

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