Site requirements and kinetics of immune-dependent elimination of intravascularly administered lung stage schistosomula in mice immunized with highly irradiated cercariae of Schistosoma mansoni.

Experiments were performed to compare the migration and survival of 75Se-labeled schistosomes, introduced by percutaneous cercarial exposure or by intravascular administration of 7-day-old lung stage schistosomula, in control and irradiated cercaria-immunized mice. Schistosomula were intravascularly introduced into the lungs, systemic organs and liver by injection via the femoral vein (FV), left ventricle (LV), and superior mesenteric vein (SMV), respectively. The fate of challenge larvae was examined by autoradiography of host tissues and by recovery of adult worms. It was found that both normal and immune elimination were site-dependent. In control mice 45%-60% of cercarial penetrants and lung schistosomula injected into the FV and LV were recoverable as adult worms, while a significantly greater number (70%-85%) were recoverable when lung schistosomula were injected into the SMV. In immunized mice, parasites introduced as either cercariae or FV-injected schistosomula were both highly sensitive to immune elimination. LV-injected schistosomula were also sensitive but to a slightly lesser degree. In contrast, schistosomula placed directly in the liver by SMV injection were totally insensitive to immune elimination. It was concluded that elimination of schistosomula in irradiated cercaria-immunized mice occurs in the lungs and/or in the systemic organs, but not in the liver. Also, it was concluded that immune elimination is not a rapid process, since more than 7 days were required after intravascular challenge for the development of demonstrable differences between control and immunized mice.