Three major surface antigens of Schistosoma mansoni are linked to the membrane by glycosylphosphatidylinositol

Schistosomula of Schistosoma mansoni were examined for the presence of glycosylphosphatidylinositol (GPI) anchored surface membrane Ag. Parasites were surface iodinated and cultured in the presence or absence of a crude phospholipase C (PLC) preparation or phosphatidylinositol-specific PLC (PIPLC). Culture supernatants were then analyzed: 1) by centrifugation to ascertain which molecules released from the surface were soluble or contained in membrane vesicles; 2) by immunoprecipitation with antibodies specific for the 'cross-reacting determinant,' an epitope revealed on some GPI-anchored proteins only after cleavage of the diacylglycerol from the protein by PIPLC, and 3) by immunoprecipitation with immune mouse sera to establish co-identity with previously described, immunologically relevant surface Ag. By using these techniques, schistosomula were shown to possess three GPI-anchored surface Ag of m.w. 38,000, 32,000 and 18,000 which are spontaneously released from the surface of schistosomula in association with membrane, but remain insoluble until cleaved by PIPLC. All three molecules were recognized by antibodies from mice vaccinated with irradiated cercariae and/or chronically infected mice. Moreover, the m.w. 38,000 component was recognized by a previously described protective mAb (E.1). A major developmental modification appears to occur in the expression of these molecules because, by the same techniques, no GPI-anchored surface Ag were detectable on 7-day-old lung stage parasites. The finding that these important parasite immunogens are GPI-anchored and released from the surface of the parasite in membrane vesicles may, in part, explain why they elicit strong immune responses capable of damaging the schistosomulum tegument.