| Babesia bovis merozoites and tick-derived sporozoites invade the same target cell, the bovine erythrocyte. We hypothesized that molecules with a proposed role in invasion will be expressed by both merozoites and sporozoites. First, we examined B. bovis sporozoites for the expression of the merozoite surface antigen 1 (MSA-1) and rhoptry-associated protein 1 (RAP-1). Both msa-1 and rap-1 were transcribed and expressed on the surface of sporozoites. Importantly, monospecific MSA-1 and RAP-1 antisera each inhibited sporozoite invasion of erythrocytes in vitro. Second, we analyzed the B. bovis merozoite surface antigen (MSA) 2 locus, which encodes four glycoproteins, MSA-2a1, 2a2, 2b, and 2c, proposed to be involved in erythrocyte invasion. Using specific antibodies, each MSA-2 glycoprotein was shown to be expressed on the surface of live extracellular merozoites and co-expression on single merozoites was confirmed. Individual antisera against MSA-2a, MSA-2b, and MSA-2c inhibited merozoite invasion of erythrocytes. Expression of each MSA-2 on the sporozoite surface was demonstrated and statistically significant inhibition of sporozoite binding to the erythrocytes was shown using antisera specific for MSA-2a, MSA-2b, and MSA-2c. These results indicate an important role for RAP-1, MSA-1 and MSA-2 proteins in the initial binding and invasion of host erythrocytes and support the hypothesis that sporozoites and merozoites use common surface molecules in erythrocyte invasion. |
