Polypeptides constituting the antigenic basis for identification of Rickettsia sibirica species by the standard serotyping method for spotted fever group Rickettsiae

The antigens of Rickettsia sibirica, Rickettsia rickettsii, and Rickettsia conorii were examined by Western immunoblots and the standard immunofluorescent serotyping assay with mouse anti-Rickettsia sibirica serum that is used for determining the species identification of spotted fever group rickettsiae. Serum was employed prior to absorption and after absorption with purified native or heated Rickettsia sibirica. The unabsorbed antiserum, the antiserum absorbed with heated Rickettsia sibirica and the antibodies eluted from native Rickettsia sibirica recognized two antigenic polypeptides of Rickettsia sibirica (130 and 118 kDa), Rickettsia rickettsii (151 and 133 kDa), and Rickettsia conorii (136 and 113 kDa), respectively when examined in native state, not denatured by heat. The antiserum absorbed with native Rickettsia sibirica reacted most strongly with one polypeptide of Rickettsia sibirica (118 kDa), Rickettsia rickettsii (133 kDa), and Rickettsia conorii (136 kDa); at the endpoint of the immunofluorescence assay there was minimal reactivity with the 130 kDa polypeptide by immunoblotting. Both Western immunoblots and immunofluorescence assay showed that the antiserum absorbed with native Rickettsia sibirica reacted with homologous and heterologous antigens at a much lower titer than did the antiserum that was not absorbed. However, the titers of reaction with Rickettsia sibirica, Rickettsia rickettsii, or Rickettsia conorii were not diminished by absorption of the native antiserum with heated Rickettsia sibirica. The antibody eluted from native Rickettsia sibirica reacted by immunofluorescence with all the rickettsiae at a higher dilution than the antibody eluted from heated Rickettsia sibirica. Above the endpoint dilution in the IFA test, the serum eluted from native Rickettsia sibirica reacted mainly with the 118 kDa polypeptide. The antigenic polypeptides of Rickettsia sibirica that are detected by early mouse antiserum and are responsible for the species-specific immunofluorescence reaction are heat-labile polypeptides of 118 and 130 kDa and contain both species-specific and group reactive antigens. Although the 118 kDa polypeptide quantitatively contains more of the species-specific, heat labile antigen, there is also an important contribution of the 130 kDa antigen.