Expression of a gene encoding the major antigenic protein 2 homolog of Ehrlichia chaffeensis and potential application for serodiagnosis

The major antigenic protein 2 (MAP2) homolog of Ehrlichia chaffeensis was c loned and expressed. The recombinant protein was characterized and tested i n an enzyme-linked immunosorbent assay (ELISA) format for potential applica tion in the serodiagnosis of human monocytic ehrlichiosis. The recombinant protein, which contained a C-terminal polyhistidine tag, had a molecular ma ss of approximately 26 kDa, The antigen was clearly identified by Western i mmunoblotting using antihistidine antibody. However, immune sera failed to react with the recombinant on immunoblots when the antigen was denatured by heat or reduced using beta-mercaptoethanol. The recombinant MAP2, (rMAP2) was used in an ELISA format with 60 blinded serum samples. Twenty of the se rum samples were previously demonstrated to contain antibodies reactive wit h E. chaffeensis by indirect immunofluorescence assays (IFAs), The remainin g 40 samples were seronegative. All samples negative by IFA were also found to be negative for antibodies against the rMAP2 of E. chaffeensis by using the ELISA, Only 1 of 20 IFA-positive samples tested negative in the rMAP2 ELISA. There was 100% agreement using HA-negative samples and 95% agreement using IFA-positive samples, resulting in a 97.5% overall agreement between the two assays. These data suggest that the rMAP2 homolog of E. chaffeensi s may have potential as a test antigen for the serodiagnosis of human monoc ytic ehrlichiosis. To our knowledge, this recombinant is unique because it is thus far the only E. chaffeensis recombinant antigen that has been shown to work in an ELISA format.