Expression of Polymorphic msp1β Genes during Acute Anaplasma marginale Rickettsemia

Immunization of cattle with native MSP1 induces protection against Anaplasma marginale. The native immunogen is composed of a single MSP1a protein and multiple, undefined MSP1b polypeptides. In addition to the originally sequenced gene, designated msp1β(F1), we identified three complete msp1β genes in the Florida strain: msp1β(F2), msp1β(F3), and msp1β(F4). Each of these polymorphic genes encodes a structurally unique MSP1b protein, and unique transcripts can be identified during acute A. marginale rickettsemia. The structural polymorphism is clustered in discrete variable regions, and each MSP1b protein results from a unique mosaic of five variable regions. Although each of the MSP1b proteins in the Florida strain contains epitopes recognized by serum antibody induced by protective immunization with the native MSP1 complex, the variable regions also include epitopes expressed by some but not all of the MSP1b proteins. These data support testing recombinant vaccines composed of the multiple antigenically and structurally unique MSP1b proteins combined with MSP1a in order to mimic the efficacy of native MSP1 immunization.