Isolation of protective antigens from Trypanosoma lewisi by using trypanostatic (ablastic) immunoglobulin G from the surface coat.

Antigens were purified from extracts of Trypanosoma lewisi on immunoadsorbent columns of trypanostatic immunoglobulin G eluted from parasite surface coats at 8 days postinfection. Eight absorbed proteins, with molecular weights between 15,000 and 70,000, were identified. These surface coat antigens (SCAgs) were then used to immunize rats. After immunization, sera were assayed in vitro for levels of circulating trypanostatic and trypanocidal antibodies. Approximately half of the immune sera had higher levels of trypanostatic antibody, compared with control sera; no trypanocidal antibodies (agglutinins) were detected in any of the sera. The rats were then challenged intraperitonally, and the parasitemias and division rates of the parasites were monitored. Parasitemias of all immunized rats were significantly (P less than 0.01) lower and of shorter duration than those of the controls. Division rates of trypanosomes were also significantly (P less than 0.01) lower in all immunized rats at all times before total cessation of division compared with control rats. A clear dose-response effect was observed, with greater amounts of SCAg eliciting higher levels of protection. Purified SCAgs were also more effective immunogens than were the crude trypanosome extracts from which they had been purified, and in which other proteins in addition to the SCAgs were present. These data provide conclusive evidence that the immunoglobulin G in the surface coats of T. lewisi, adsorbed during the course of infection, is specific antibody, in that it can be used to isolate parasite antigens that elicit a trypanostatic response in rats immunized with them.