Sodium dodecyl sulfate-polyacrylamide gel electrophoresis immunoblotting as a serological tool in the diagnosis of syphilitic infections.

The utility of sodium dodecyl sulfate-polyacrylamide gel electrophoresis immunoblotting as a serological tool in the diagnosis of human syphilitic infections was examined. In model experiments, rabbits were immunized with Treponema pallidum or T phagedenis, and the antisera were tested for cross-reactivities with both sets of antigens. A major T. pallidum antigen with a molecular weight of ca. 17,000 appeared to be the most reliable specific antigenic marker as assessed by the immunoblotting technique with peroxidase-labeled second antibodies. Antibodies to this antigen were never detected in hyperimmune rabbit anti-T. phagedenis sera or in the sera of nonsyphilitic humans. In contrast, reactive antibodies were found in all syphilitic human sera and also in liquor samples that were positive in the passive hemagglutination test. Differentiation between immunoglobulin M and immunoglobulin G antibodies was directly possible by applying the respective specific second antibodies. Immunoblotting tests were performed with sera exhibiting low passive hemagglutination test titers and equivocal fluorescent treponemal antibody and rapid plasma reagin card reactions. In more than 60% of these cases, immunoblot positivity with respect to the 17,000-molecular-weight antigen was found. The same results were obtained with partially purified 17,000-molecular-weight antigen. The immunoblot technique should be useful as an additional diagnostic tool for differentiating between true and false-positive serological reactions.