Induction of a protective immune response in rats by injection of live or killed S. mansoni worms

Resistance to S. mansoni cercarial challenge, induced by prior infection of rats with cercariae, is analysed by substitution of alternatives to the primary cercarial exposure. Intravenous injections of either live or killed 4-week stage worms have been examined, utilizing two different routes of injection, one or two injected doses, and two different methods to kill worms. In addition to assessing peripheral blood eosinophil levels, anti-schistosome antibody titres, and challenge worm burdens in the injected and control recipients, sera from these rat groups collected 3 weeks after challenge were used for passive immunization studies in rats. Recipients of intravenous live worm injections were resistant to a cercarial challenge and yielded sera capable of passively immunizing rats. Recipients of intravenous killed worm injections were not significantly resistant to a challenge infection, yet paradoxically, yielded sera capable of passively immunizing rats. Serum from challenge control rats was not significantly protective in the passive immunization assay. There were no correlations between peripheral blood eosinophil levels or anti-schistosome antibody titres and resistance (either active or passive). We conclude that killed worms induce an immune response which, although insufficient to provide protection by itself, can be recalled or augmented by a challenge infection to result in significant levels of serum protective activity.