HIGHER LEVELS OF PASSIVE COMPARED WITH ACTIVE IMMUNITY IN RATS IMMUNIZED WITH LARVAL ANTIGENS OF SCHISTOSOMA-MANSONI

We have previously shown that it is the lung stage schistosomulum whic h is both the inducer and the target of immunity in rats vaccinated wi th irradiated cercariae but that rats can also be partially protected by immunization with detergent extracts of mechanically-transformed sc histosomula. In the present study we therefore compared the immunogeni city of these and intermediate schistosomular stages. In the first exp eriment low levels of protection (13-24%) were induced by extracts of mechanically-transformed schistosomula cultured for 3 hours or 2 days but not by extracts of lung schistosomula. In a repeat experiment insi gnificant levels of protection were induced by extracts of 3 h schisto somula and lung schistosomula were again non-protective. Nevertheless, high levels of anti-schistosomula antibodies were demonstrated and se ra from actively immunized rats conferred significant passive protecti on in five out of six trials. The levels of protection conferred by pa ssive immunization (26-51%) were in each case higher than the levels o f protection demonstrated in the respective serum donor groups, showin g that some form of immunological blockade suppression is operating to prevent expression of protective immunity in the actively immunized r ats.