Expression of polymorphic msp1 beta genes during acute Anaplasma marginalerickettsemia

Immunization of cattle with native MSP1 induces protection against Anaplasm a marginale. The native immunogen is composed of a single MSP1a protein and multiple, undefined MSP1b polypeptides. In addition to the originally sequ enced gene, designated msp1 beta(F1), we identified three complete msp1 bet a genes in the Florida strain: msp1 beta(F2), msp1 beta(F3), and msp1 beta( F4). Each of these polymorphic genes encodes a structurally unique MSP1b pr otein, and unique transcripts can be identified during acute A. marginale r ickettsemia. The structural polymorphism is clustered in discrete variable regions, and each MSP1b protein results from a unique mosaic of five variab le regions. Although each of the MSP1b proteins in the Florida strain conta ins epitopes recognized by serum antibody induced by protective immunizatio n with the native MSP1 complex, the variable regions also include epitopes expressed by some but not all of the MSP1b proteins. These data support tes ting recombinant vaccines composed of the multiple antigenically and struct urally unique MSP1b proteins combined with MSP1a in order to mimic the effi cacy of native MSP1 immunization.