Passive transfer of resistance and the site of immune-dependent elimination of the challenge infection in rats vaccinated with highly irradiated cercariae of Schistosoma mansoni.

The immune-dependent elimination of a challenge infection in rats vaccinated with highly-irradiated cercariae of Schistosoma mansoni was analysed by passive transfer of serum, recovery of the challenge from the lungs and liver and by transferring lung-stage schistosomula. Recipients of serum from rats immunized with either unirradiated, 20 or 40 krad.-irradiated cercariae, most of which die in the liver, lungs and skin, respectively, were equally resistant if the serum was injected on the day of infection or 5-7 days after infection. In addition, vaccinated rat serum transferred to mice and vaccinated rabbit serum transferred to rats conferred comparable protection when injected on day 0 or 5 days after infection of the recipients. This apparent susceptibility of the lung schistosomula to immune attack was confirmed by challenging 20 or 40 krad.-irradiated cercariae vaccinated rats with lung-stage schistosomula derived either from mice or rats. All the detectable attrition of a cercarial challenge in vaccinated rats occurred between 7 and 10 days post-challenge, before the parasites reached the liver. Since there was no evidence of damage or attrition in the skin or lungs before day 7 it was concluded that immune-dependent elimination occurred rapidly following a 'window of sensitivity' coinciding with the migration of the parasites from the lungs to the liver.