Ig class and IgG subclass responses to Treponema pallidum in patients with syphilis

The Ig class and IgG subclasses of anti-Treponema pallidum antibodies in human serum were quantified using solid-phase enzyme-linked immunosorbent assays. Development of these assays with monoclonal antibodies, each specific for a human immunoglobulin class or IgG subclass, provided quantitative data concerning the major antibody specificities. In patients with primary syphilis, anti-T. pallidum activity was limited almost exclusively to IgG1 and IgM. Coordinate, restricted expression of IgG1 and IgG3 responses in T. pallidum-specific assays was observed with sera from patients with active secondary syphilis. IgG1 and IgG3 accounted for roughly 53 and 43% of the total anti-treponemal IgG antibody activity, respectively. While IgM antibody levels were elevated in the patients with secondary syphilis, IgG2 and IgG4 levels, if present at all, represented less than 10 and 2% of the total IgG activity, respectively. Ig in sera from patients who had been treated adequately for secondary syphilis were restricted almost entirely to IgG3 and IgG1. Considering the low level of IgG3 in serum, disproportionately high percentages of antitreponemal antibodies were found in this subclass during and after treatment for secondary syphilis. The restricted, coexpression of the IgG1 and IgG3 isotypes may reflect the close genetic linkage of the gamma 1 and gamma 3 genes and possibly the impact of immunoregulatory mechanisms in response to the induction and expression of autoantibodies which arise during the course of secondary syphilis.