Expression and immunogenicity of recombinant immunoreactive surface protein 2 of Anaplasma phagocytophilum

Human granulocytic anaplasmosis (HGA), caused by Anaplasma phagocytophilum, is an emerging tick-borne zoonotic disease throughout the world. The first HGA cases in China were documented in 2008, and the greatest challenge posed by the disease is rapid and accurate diagnosis during the acute phage of illness. In this study, we successfully cloned and expressed an A. phagocytophilum immunoreactive surface protein (major surface protein 2 [MSP2]) and demonstrated that this recombinant protein has natural immunogenicity by Western blotting and enzyme-linked immunosorbent assay (ELISA) using human HGA-positive sera and reference rabbit HGA-positive sera. The rabbit antisera against the recombinant protein also reacted actively with the natural antigen of A. phagocytophilum by immunofluorescence assay (IFA). No cross-reaction was observed between the recombinant protein and rabbit antisera against 10 common members of the order Rickettsiales by ELISA when the sera were diluted more than 1:100. We concluded that the recombinant MSP2 protein exhibited excellent antigenicity and specificity, results that should lay the foundation for the development of a simple and rapid diagnostic reagent and a vaccination for anaplasmosis.