MOLECULAR-BASIS FOR ANTIGENIC VARIATION OF A PROTECTIVE STRAIN-SPECIFIC ANTIGEN OF EHRLICHIA-RISTICII

Ehrlichia risticii, the causative agent of Potomac horse fever, has re cently been isolated from many vaccinated horses with typical clinical signs of the disease. The heterogeneity of the E, risticii isolates o btained from the vaccinated horses necessitates the identification of the molecular basis of strain variations to elucidate the vaccine fail ure and to aid in the development of an efficient vaccine against this disease. As an attempt, two major cross-reacting surface antigen gene s of 50- and 85-kDa antigens, present separately in strains 25-D (isol ated in 1984) and 90-12 (isolated in 1990 from a vaccinated horse), re spectively, were cloned and sequenced. A comparative sequence analysis revealed differences and similarities between these two antigens with strain-specific sizes (SSA), The 2.5- and 1.6-kb genes coding for the 85- and 50-kDa proteins, respectively, contained many different tande m repeats. The identical repeat motifs were more frequent in the middl e of both genes, but the numbers and positions of the repeats were alt ogether different in the genes. Many of these direct repeats of both g enes had exact sequence homology and coded for the same amino acids, T he homology of the 5'- and 3'-flanking regions of the two genes was gr eater than that of the regions in the central part of the genes. A com parative analysis of the deduced amino acid sequences of these two ant igen genes indicated eight common domains, which were designated ident ical domains. Although the sequence homologies of these identical doma ins were the same, the positions of the domains in their respective st rains were completely different. This finding might be one of the base s of antigenic variation between the strains. In addition, there were a few unique regions in both antigen genes where no sequence homology existed. These specific regions were designated unique domains. The 50 -kDa protein had two such unique domains, and the 85-kDa protein had s ix such unique domains. The presence of such unique domains contribute d to the large size variation of these SSA. The cross-reactivity of re combinant proteins confirmed the presence of conserved epitopes betwee n these two antigens, The SSA have been determined to be apparent prot ective antigens of E, risticii.