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Restriction of major surface protein 2 (MSP2) variants during tick transmission of the ehrlichia Anaplasma marginale
F R Rurangirwa1, D Stiller, D M French
1Program in Vector-Borne Diseases, Department of Veterinary Microbiology and Pathology, Washington State University, Pullman, WA 99164-7040, USA.
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.
Insights
Anaplasma marginale variant types expressed in tick salivary glands predict disease. This finding supports developing MSP2 vaccines against cattle ehrlichial pathogen infections.
Area of Science:
- Veterinary Microbiology
- Tick-borne Diseases
- Molecular Epidemiology
Background:
- Anaplasma marginale causes persistent ehrlichial infections in cattle.
- Infection is marked by rickettsemic cycles with emergence of new A. marginale variant types.
- Antigenic variation of MSP2 proteins contributes to pathogen diversity.
Purpose of the Study:
- To investigate the repertoire of A. marginale variant types expressed in tick salivary glands.
- To determine if tick-expressed variants predict those causing acute rickettsemia upon transmission.
- To evaluate the potential for MSP2-based vaccines.
Main Methods:
- Sequence analysis of msp2 transcripts from tick salivary glands.
- Comparison of tick-expressed variants with those in infected cattle blood.
- Monitoring rickettsemia and variant types following experimental tick transmission.
Main Results:
- A restricted repertoire of A. marginale variant types (SGV1 and SGV2) was identified in tick salivary glands.
- These SGV1 and SGV2 variants were consistently expressed in ticks, irrespective of cattle blood variants at acquisition.
- Tick transmission resulted in acute rickettsemia solely composed of SGV1 and SGV2 variants.
Conclusions:
- A. marginale variant type expression in tick salivary glands predicts the variants responsible for acute rickettsemia.
- This restriction contrasts with the diversity observed in persistently infected cattle.
- Targeting MSP2 offers a promising strategy for vaccines against tick-transmitted anaplasmosis.