Identification and partial characterization of Rickettsia tsutsugamushi major protein immunogens

Strains of Rickettsia tsutsugamushi so far examined have either three or four quantitatively predominant proteins, which apparently are surface proteins and which range in size between 50 and 63 kilodaltons. These polypeptides also were the major immunogens detected by polyacrylamide gel electrophoresis of extracted rickettsial proteins which had been precipitated by hyperimmune rabbit sera. The major proteins from different rickettsial strains share some epitopes, as evidenced by the lack of strain specificity of the rabbit sera in the immunoprecipitation tests. However, similar experiments with a limited number of monoclonal antibodies showed that strain-specific determinants also are associated with at least the 58/60-kilodalton polypeptide. A lack of strain-specific epitopes on the rickettsial surface was indicated by our inability to detect binding of heterologous antisera to the rickettsial surface by immunoferritin labeling. Because the three major proteins of the Karp and Gilliam strains are accessible to antibody in unextracted organisms, it is possible that the exteriorly exposed epitopes of these three polypeptides are strain specific and that their common determinants are normally buried in the membrane or otherwise inaccessible. Attempts to absorb out specific antibody with intact rickettsiae gave equivocal results; however, when immune complexes formed before rickettsial extraction were examined by electrophoresis, antibody appeared to have bound strain specifically with at least the 60-kilodalton protein.