IDENTIFICATION OF THE ANTIGENIC CONSTITUENTS OF EHRLICHIA-CHAFFEENSIS

Ehrlichia chaffeensis, the novel etiologic agent of human ehrlichiosis in the United States, was first isolated in 1990 and reported in 1991 . To analyze the antigenic components off. chaffeensis, we cultivated these obligate intracellular bacteria in DH82 cells, purified the ehrl ichiae by renografin density gradient centrifugation, and examined the antigens by Western immunoblotting. Rabbit and human antisera to E. c haffeensis revealed more than 20 bands ranging from 20 to 200 kD. The distinct 22-kD protein was heat labile. The rest of the major immunore active components were heat stable. The immunoblots off. chaffeensis w ere highly similar when probed with antisera to E. chaffeensis, E. can is, and E. ewingii, indicating the close antigenic relationships among the three species. The 22-kD protein cross-reacted only with anti-E. canis serum. The antibody against E. sennetsu reacted strongly with th e 66-, 64-, 55-, and 44-kD antigens of E. chaffeensis. The E. risticii antisera reacted strongly with the 55- and 44-kB bands but only faint ly with the 66-kD band. The major immunoreactive antigens of E. chaffe ensis (66, 55, and 44 kD) showed cross-reactions with all the differen t antisera tested. The results indicated that E. chaffeensis is antige nically most closely related to E. canis, is less closely related to E . ewingii, and is only distantly related to E. sennetsu and E. ristici i.