Antigenic heterogeneity in high- and low-virulence strains of Rickettsia rickettsii revealed by monoclonal antibodies.

Previously it has been reported that strains of Rickettsia rickettsii that differ greatly in their ability to cause disease in guinea pigs are similar by serological and sodium dodecyl sulfate-polyacrylamide gel electrophoresis analyses. In this study, we used monoclonal antibodies to the virulent R and the relatively avirulent HLP strains to investigate strain differences which might account for the disparate behavior of the strains in guinea pigs. Coomassie blue-stained sodium dodecyl sulfate-polyacrylamide gel electrophoresis profiles of the R and HLP strains were nearly identical for polypeptides with apparent molecular weights greater than 32 kilodaltons (kDa). All of the monoclonal antibodies to a lipopolysaccharide-like antigen reacted equally well with antigen from both strains by immunoblotting. None of the antibodies to the lipopolysaccharide-like antigen protected mice against challenge with viable rickettsiae. Some antibodies reacted with both 120- and 155-kDa polypeptides of both strains in radioimmune precipitation and immunoblotting tests, and other antibodies reacted only with the homologous strain. The monoclonal antibodies cross-reacted with the heterologous strain in the enzyme-linked immunosorbent assay essentially either completely or not at all. The ability of the monoclonal antibodies to the 120- and 155-kDa polypeptides to protect mice against the two strains was correlated with the ability of the antibodies to react with the antigens in the enzyme-linked immunosorbent assay and radioimmune precipitation or immunoblotting tests. These results demonstrate that R and HLP antigens which appear identical in molecular weight differ in their compositions of antigenic determinants.