Molecular Basis for Variable Expression of Merozoite Surface Antigen gp45 among American Isolates ofBabesia bigemina

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Abstract
Immunization with the merozoite surface glycoprotein gp45 induces protection against challenge using the homologousBabesia bigeminastrain. However, gp45 B-cell epitopes are highly polymorphic amongB. bigeminastrains isolated from different geographical locations within North and South America. The molecular basis for this polymorphism was investigated using the JG-29 biological clone of a Mexico strain ofB. bigeminaand comparison with the Puerto Rico, St. Croix, and Texcoco strains. The molecular size and antibody reactivity of gp45 expressed by the JG-29 clone were identical to those of the parental Mexico strain.gp45cDNA and the genomic locus encompassinggp45were cloned and sequenced from JG-29. The locus sequence and Southern blot data were consistent with a singlegp45copy in the JG-29 genome. The JG-29 cDNA expressed the full-length protein recognized by the gp45-specific monoclonal antibody 14/1.3.2. The genomes of the Puerto Rico and St. Croix strains ofB. bigeminawere shown to lack a closely relatedgp45-like gene by PCR using multiple primer sets and by Southern blots using both full-length and region-specificgp45probes. This genomic difference was confirmed using unpassaged isolates from a 1999 disease outbreak in Puerto Rico. In contrast, the Texcoco strain retains agp45gene, encoding an open reading frame identical to that of JG-29. However, the Texcocogp45gene is not transcribed. These two mechanisms, lack of a closely relatedgp45-like gene and failure to transcribegp45, result in generation of antigenic polymorphism amongB. bigeminastrains, and the latter mechanism is unique compared to prior mechanisms of antigenic polymorphism identified in babesial parasites.

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