Evidence is presented for the association of class I major histocompatibility complex (MHC) antigens with the surface of Treponema pallidum during infection. A monoclonal antibody (IgG2a) directed against a murine H-2Kb epitope of public specificity reacted with the cell surface of T. pallidum, as assayed by the binding of protein A-colloidal gold in immunoelectron microscopy. Monoclonal antibodies directed against class I rabbit MHC antigens also reacted in immunofluorescence assays with material on the surface of rabbit-cultivated T. pallidum. In addition, impression smears of human syphilitic genital ulcers that were darkfield-positive for the presence of spirochetes were tested in immunofluorescence assays with monoclonal antibodies directed against human MHC antigens; antibody directed against HLA-ABC (class I) was reactive whereas antibody directed against HLA-DR (class II) was nonreactive. Results of the study suggest that the association of host-derived class I MHC antigens or molecular mimicry may play a role in T. pallidum evasion of host immune defenses.
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