Restriction of major surface protein 2 (MSP2) variants during tick transmission of the ehrlichia Anaplasma marginale

Anaplasma marginale is an ehrlichial patho gen of cattle that establishes l ifelong persistent infection. Persistence is characterized by rickettsemic cycles in which new A. marginale variant types, defined by the sequence of the expressed msp2 transcripts, emerge. The polymorphic msp2 transcripts en code structurally distinct MSP2 proteins and result in an antigenically div erse and continually changing A. marginale population within the blood. In this manuscript, we used sequence analysis of msp2 transcripts to show that a restricted repertoire of variant types, designated SGV1 and SGV2, is exp ressed within the tick salivary gland. The same SGV1 and SGV2 variant types mere expressed in ticks regardless of the variant types expressed in the b lood of infected cattle at the time of acquisition feeding by the ticks. Im portantly, subsequent tick transmission to susceptible cattle resulted in a cute rickettsemia composed of organisms expressing only the same SGV1 and S GV2 variant types. This indicates that the msp2 expressed by organisms with in the tick salivary gland predicts the variant type responsible for acute rickettsemia and disease. This restriction of transmitted A. marginale vari ant types, in contrast to the marked diversity within persistently infected cattle, supports development of MSP2 vaccines to prevent acute rickettsemi a in tick-transmitted infections.