Antibodies define multiple proteins with epitopes exposed on the surface of live Babesia bigemina merozoites

Babesia bigemina is one of several tick-borne hemoparasitic diseases of cattle that are inadequately controlled and cause substantial livestock production losses in tropical and subtropical climates. Recovery from acute babesiosis is associated with development of protective immunity against subsequent challenge with both homologous and heterologous parasites. Viable and infectious merozoites, the intraerythrocytic stage of B. bigemina responsible for clinical disease, were separated from contaminating host cells by density gradient centrifugation. Monoclonal antibodies developed against gradient-separated merozoites were screened for surface reactivity against live merozoites in an immunofluorescent binding assay. Surface-reactive antibodies immunoprecipitated five major biosynthetically radiolabeled merozoite proteins with relative m.w. of 72,000, 58,000, 55,000, 45,000, and 36,000 in SDS-PAGE. Two additional proteins immunoprecipitated with the 45,000 m.w. protein were unreactive with monoclonal antibody in western blots and are apparently part of a membrane complex co-precipitated by this antibody. In contrast, additional proteins of m.w. of 36,000, 35,000, and 33,000, immunoprecipitated with the 58,000 protein, all contain the surface-exposed epitope bound by monoclonal antibody. Immune serum from an animal that had recovered from infection with a Mexico isolate of B. bigemina immunoprecipitated five radiolabeled proteins from the Mexico isolate that co-migrated in SDS-PAGE with major proteins precipitated by surface-reactive monoclonal antibodies. In addition, antibodies against a Kenya isolate of B. bigemina immunoprecipitated the same co-migrating proteins from radio-labeled Mexico isolate, demonstrating epitope conservation between surface proteins of geographically different isolates. The identification of proteins with epitopes exposed on the surface of live merozoites and accessible to antibody provides candidates to be tested as protective immunogens in cattle.