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 which is both the inducer and the target of immunity in rats vaccinated with irradiated cercariae but that rats can also be partially protected by immunization with detergent extracts of mechanically-transformed schistosomula. In the present study we therefore compared the immunogenicity of these and intermediate schistosomular stages. In the first experiment 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 insignificant levels of protection were induced by extracts of 3 h schistosomula and lung schistosomula were again non-protective. Nevertheless, high levels of anti-schistosomula antibodies were demonstrated and sera from actively immunized rats conferred significant passive protection in five out of six trials. The levels of protection conferred by passive immunization (26-51%) were in each case higher than the levels of protection demonstrated in the respective serum donor groups, showing that some form of immunological blockade suppression is operating to prevent expression of protective immunity in the actively immunized rats.