EVALUATION OF ANAPLASMA-MARGINALE MAJOR-SURFACE-PROTEIN-3 (MSP3) AS ADIAGNOSTIC-TEST ANTIGEN

An immunodominant surface protein, major surface protein 3 (MSP3), has been proposed as an antigen suitable for use in the diagnosis of bovi ne anaplasmosis. We further characterized MSP3 to examine its potentia l as a test antigen for the serological diagnosis of carrier cattle. T he specificity of this antigen in detecting infected cattle as well as the conservation of MSP3 between strains of Anaplasma marginale was e valuated by using immunoblots of A. marginale proteins separated by on e- and two dimensional polyacrylamide gel electrophoreses. Immune sera from animals infected with Anaplasma ovis, Ehrlichia risticii, and Eh rlichia ewingii reacted with the MSP3 antigen of A. marginale. One-dim ensional gel electrophoresis of A, marginale proteins demonstrated siz e polymorphism of MSP3 between different geographic isolates. Two-dime nsional gel electrophoresis revealed at least three different antigens migrating at the 86-kDa molecular size, and sera from animals Infecte d with different strains of A. marginale reacted with different 86-kDa antigens, These results indicate that MSP3 may share cross-reactive e pitopes with antigens found in A. ovis and some Ehrlichia spp. In addi tion, MSP3 is not conserved between different isolates of A. marginale , and at least in the isolate from Florida, what was previously identi fied 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 mult iple 86-kDa antigens recognized by various sera suggest that MSP3 is n ot a suitable candidate for use as a recombinant test antigen.