Submitted to: Proceedings of the National Academy of Sciences (PNAS)
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/20/1999
Publication Date: N/A
Interpretive Summary: In this study, sequence analysis of Anaplasma marginale transcript msp2 was performed to reveal that a restricted repertoire of major surface protein variants (designated tick salivary gland variants 1 and 2) are expressed regardless of what variant types are present in the blood of infected cattle when the ticks obtain a blood meal from the cattle. When such ticks later were given transmission feeds on uninfected cattle, the resultant disease-associated acute levels of Anaplasma in cattle blood consisted of only tick salivary gland variants 1 and 2. This restriction of transmitted Anaplasma variant types may make feasible the development of vaccines to prevent tick- transmitted acute infection.
Technical Abstract: Anaplasma marginale is an ehrlichial pathogen of cattle that establishes lifelong 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 encode structurally distinct MSP2 proteins and result in an antigenically diverse 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 expressed within the tick salivary gland. The same SGV1 and SGV2 variant types were expressed in ticks regardless of the variant types expressed in the blood of infected cattle at the time of acquisition feeding by the ticks. Importantly, subsequent tick transmission to susceptible cattle resulted in acute rickettsemia composed of organisms expressing only the same SGV1 and SGV2 variant types. This indicates that the msp2 expressed by organisms within the tick salivary gland predicts the variant type responsible for acute rickettsemia and disease. This restriction of transmitted A. marginale variant types, in contrast to the marked diversity within persistently infected cattle, supports development of MSP2 vaccines to prevent acute rickettsemia in tick-transmitted infections.