MOLECULAR CHARACTERIZATION OF A 28 KDA SURFACE-ANTIGEN GENE FAMILY OFTHE TRIBE EHRLICHIAE

Antisera against different Ehrlichiae recognize an immunodominant, cro ss-reacting similar to 28 kDa surface antigen defined as the MAP1 in C owdria ruminantium. These antigens are considered valuable in developi ng serodiagnostic tests and recombinant vaccines for Ehrlichiae infect ions. To evaluate the relationship in three closely related Ehrlichiae , Ehrlichia chaffeensis, Ehrlichia canis, and C. ruminantium, the stru cture of the 28 kDa antigen genes was analyzed. We describe the clonin g and characterization of DNA encoding genes homologous to MAP1 from E . chaffeensis and E. canis. The cloned segment of E. chaffeensis conta ins one expressed and four transcriptionally silent tandemly arranged, nonidentical genes; the E. canis locus consists of two nonidentical g enes. Comparative analysis of these genes revealed the presence of fou r conserved regions separated by three highly variable regions. B-cell epitope analysis identified three major cross-reacting epitopes that map to the variable regions. Location of the epitopes at the variable regions and the presence of multigene family with only one expressed c opy suggest a mechanism of immune evasion in these Ehrlichiae. (C) 199 8 Academic Press.