Studies with recombinant proteins of Ehrlichia risticii: identification of strain-specific antigen as a protective antigen.

Ehrlichia risticii is the causative agent of Potomac horse fever, an acute infectious disease of equines. To study the role of major antigens of E. risticii in protective immune response, we have expressed the genes of the 55 kDa, 51 kDa and 85/50 kDa-strain-specific antigens of the 90-12 (85 kDa antigen) and 25-D (50 kDa antigen) strains in Escherichia coli using pRSET A, B, C system (Invitrogen, San Diego, CA). Mice immunized with these purified recombinant proteins of E. risticii developed strong and specific humoral immune response. The recombinant 85 kDa antigen of the 90-12 strain protected mice against challenge infection with both E. risticii strains, whereas its homologue from the 25-D strain, the recombinant 50 kDa antigen, protected mice against only the homologous strain challenge, but not against the heterologous 90-12 strain. Sera from mice immunized with the 85- or 50-kDa antigens did not inhibit the replication of cell-free Ehrlichiae in in vitro neutralization assays. Sera from normal mice and mice immunized with other antigens caused non-specific neutralization of E. risticii. Immunoglobulin G from mice immunized with the 51 kDa protein of the 90-12 strain caused partial in vitro neutralization of both strains of E. risticii. These studies demonstrate that the 85/50-kDa-strain-specific antigen of E. risticii is involved in immunoprotection against PHF.