Spherical Body Protein 4 from : A Novel Gene That Contains Conserved B-Cell Epitopes and Induces Cross-Reactive Neutralizing Antibodies in .

Bovine babesiosis is a tick-transmitted disease caused by intraerythrocytic protozoan parasites of the genus . Its main causative agents in the Americas are and while affects cattle in Asia. All species secrete proteins stored in organelles of the apical complex, which are involved in all steps of the invasion process of vertebrate host cells. Unlike other apicomplexans, which have dense granules, babesia parasites instead have large, round intracellular organelles called spherical bodies. Evidence suggests that proteins from these organelles are released during the process of invading red blood cells, where spherical body proteins (SBPs) play an important role in cytoskeleton reorganization. In this study, we characterized the gene that encodes SBP4 in . This gene is transcribed and expressed in the erythrocytic stages of . The gene consists of 834 nucleotides without introns that encode a protein of 277 amino acids. In silico analysis predicted a signal peptide that is cleaved at residue 20, producing a 28.88-kDa protein. The presence of a signal peptide and the absence of transmembrane domains suggest that this protein is secreted. Importantly, when cattle were immunized with recombinant SBP4, antibodies identified and merozoites according to confocal microscopy observations and were able to neutralize parasite multiplication in vitro for both species. Four peptides with predicted B-cell epitopes were identified to be conserved in 17 different isolates from six countries. Compared with the pre-immunization sera, antibodies against these conserved peptides reduced parasite invasion in vitro by 57%, 44%, 42%, and 38% for peptides 1, 2, 3, and 4, respectively ( < 0.05). Moreover, sera from cattle infected with cattle contained antibodies that recognized the individual peptides. All these results support the concept of as a new gene in that should be considered a candidate for a vaccine to control bovine babesiosis.