Monoclonal antibody characterization of Plasmodium falciparum antigens in immune complexes formed when schizonts rupture in the presence of immune serum

When Plasmodium falciparum parasites are cultured with some immune sera, merozoites are agglutinated by antibodies to form immune clusters of merozoites and prevent their invasion into erythrocytes. Within these immune clusters of merozoites, several antigens that are normally found in the soluble fraction after detergent extraction accumulate in relatively insoluble immune complexes. From mice immunized with these immune complexes, we obtained hybridomas secreting monoclonal antibodies (mAb) that react with various immune clusters of merozoites antigens, including mAb 3D5, which recognizes a 101-kDa antigen (p101) and mAb, 5E3, which recognizes a 113-kDa antigen (p113). Both mAb reacted with antigens at the surface of schizonts, in the vacuolar space, and at the surface of merozoites before their release from schizont-infected cells. Both p101 and p113 were synthesized by mature trophozoites and young schizonts. In pulse-chase experiments, p113 was processed to 100-, 70-, 55-, and 50-kDa products. Both p101 and p113 appeared in the culture medium when schizont rupture occurred in normal culture medium but were found in immune complexes when schizont rupture occurred in the presence of immune serum. Antibodies in immune complexes, when dissociated with acid and used to probe immunoblots, reacted with affinity-purified p101 and p113. Antigens such as these, which are accessible at the parasite surface and react with antibodies present in immune serum that inhibits parasite invasion, are logical candidates to study in the search for a vaccine against the erythrocytic stages of malaria.