Induction of protective immunity against Schistosoma mansoni by a non-living vaccine. V. Effects of varying the immunization and infection schedule and site.

Intradermal (i.d.) injection with non-living schistosome antigens plus bacterial adjuvant protects mice against subsequent infection with Schistosoma mansoni. This protection became apparent within 2 weeks after a single inoculation and persisted for at least 8 weeks. Administration of one or more booster immunizations enhanced the level of protection but never produced complete resistance to cercarial challenge under the conditions tested. All immunized mice recognized the Sm 97 antigen in adult worms, as measured by antibody reactivity in ELISA, although the level of reactivity did not correlate with the level of resistance. Significant protection was observed when mice were immunized in the skin of the chest, the footpad or at the base of the tail if challenge infection was administered percutaneously either on the abdomen or back. However, when mice were infected by cercarial exposure of the tail skin, the level of resistance was consistently lower regardless of the immunization site. Vaccinated mice were not resistant to infection with lung stage parasites. These results demonstrate that the i.d. vaccine induces significant and persistent resistance in mice, the level of which is strengthened by booster immunization. The resistance is unrelated to inflammation at the site of immunization. However, immune response at the challenge site may play a critical role in the effector mechanism of resistance in this model.