Schistosoma mansoni: Parasitology and immunology of baboons vaccinated with irradiated cryopreserved schistosomula

Abstract

Young, captivity-born male baboons ( Papio cynocephalus) were vaccinated with γ-irradiated (500 Gy) cryopreserved Puerto Rican strain schistosomula of S. mansoni. Protection against heterologous, normal Kenyan Strain S. mansoni challenge infection was erratic and partial; and two putative correlates of immunity, reduced worm fecundity and change in worm location (anterior shift) were not observed. However, immunization of baboons with this vaccine resulted in a stimulated immune system. Both cellular and humoral anamnesis were demonstrable in vaccinated-challenged baboons. Schistosome infection-associated IgM hypergammaglobulinema was also greatly reduced in vaccinated-challenged baboons. On the other hand, IgG antibodies to adult, egg, and cercarial antigens were increased after challenge infection in preimmunized baboons. Vaccination appears to have resulted in a redirection of the immune system into anti-parasite channels, but this more specific immune response was insufficient to confer good protection against challenge infection in this experiment. The dampening effect of the vaccine on the hypergammaglobulinemia of schistosomiasis is another candidate for a possible “anti-pathogenesis” effect of irradiated schistosome larval vaccines, to be added to the reduced granuloma size already reported (Damian, Roberts, Powell, Clark, Lewis & Stirewalt, 1984) to occur in these vaccinated baboons. Together, they could increase the potential benefit of live, irradiated vaccines for schistosomiasis. Human trials with vaccines only partially protective against challenge infections may therefore ultimately be warranted.