Molecular characterization of Babesia bovis merozoite surface proteins bearing epitopes immunodominant in protected cattle.

Eight surface-radioiodinated merozoite proteins from a cloned, pathogenic isolate of Babesia bovis can be immunoprecipitated by antibody from cattle that are completely protected against clinical babesiosis. Among these eight surface proteins, the 55- and 42-kDa molecules are biosynthetically labeled with [3H]glucosamine. The 42-kDa glycoprotein can also be labeled with [3H]myristic acid and partitions exclusively into the detergent phase in Triton X-114 extracts, indicating that it is an integral membrane protein and suggesting that it is anchored by a glycosylphosphatidylinositol moiety. Antibody-mediated protection against B. bovis merozoites most probably requires a high level of circulating antibody to ensure antibody-merozoite binding during the parasite's brief extra-erythrocytic phase. Antibodies in diluted sera selectively recognize the 120-, 85-, 55- and 42-kDa surface proteins. Only the 42-kDa integral membrane protein is reactive with serum antibodies diluted greater than or equal to 1:16,000. Thus, we hypothesize that these immunodominant proteins, especially the transmembrane 42-kDa glycoprotein, are important to the induction of the protective immune response and are candidates for an improved vaccine against babesiosis.