Evaluation of Anaplasma marginale major surface protein 3 (MSP3) as a diagnostic test antigen.

An immunodominant surface protein, major surface protein 3 (MSP3), has been proposed as an antigen suitable for use in the diagnosis of bovine anaplasmosis. We further characterized MSP3 to examine its potential as a test antigen for the serological diagnosis of carrier cattle. The specificity of this antigen in detecting infected cattle as well as the conservation of MSP3 between strains of Anaplasma marginale was evaluated by using immunoblots of A. marginale proteins separated by one- and two-dimensional polyacrylamide gel electrophoreses. Immune sera from animals infected with Anaplasma ovis, Ehrlichia risticii, and Ehrlichia ewingii reacted with the MSP3 antigen of A. marginale. One-dimensional gel electrophoresis of A. marginale proteins demonstrated size polymorphism of MSP3 between different geographic isolates. Two-dimensional gel electrophoresis revealed at least three different antigens migrating at the 86-kDa molecular size, and sera from animals infected with different strains of A. marginale reacted with different 86-kDa antigens. These results indicate that MSP3 may share cross-reactive epitopes with antigens found in A. ovis and some Ehrlichia spp. In addition, MSP3 is not conserved between different isolates of A. marginale, and at least in the isolate from Florida, what was previously identified as MSP3 is actually a group of three or more 86-kDa antigens with different isoelectric points. The cross-reactivity of MSP3 with some Ehrlichia spp., the variability of MSP3 between isolates, and the multiple 86-kDa antigens recognized by various sera suggest that MSP3 is not a suitable candidate for use as a recombinant test antigen.