Characterization of the antigenic determinants and host components in immune complexes from patients with secondary syphilis.

Sera from patients with secondary syphilis were evaluated for abnormal levels of circulating immune complexes (IC), immunoglobulins (Ig), and complement components. Clq-solid-phase assays (Clq-SPA) that made use of monoclonal and polyclonal antibodies directed against IgG subclasses indicated that human IC were composed primarily of IgG3 and IgG1; these findings appeared consistent with subclass profile responses of electrophoretically transferred blots (Western blots) of Treponema pallidum reacted with syphilitic sera. Complexes were isolated from reactive sera by polyethylene glycol precipitation followed by either anti-Clq column chromatography or protein A-Sepharose chromatography. Although qualitative and quantitative differences were noted, all purified materials contained a treponemal polypeptide antigen with a m.w. of approximately 87,000. Subsequent analysis of this polypeptide, which was also present in purified IC from rabbits with experimental syphilis, suggests that it may represent the fibronectin receptor of the organism. The 76,000 and 66,000 materials, earlier identified in purified rabbit IC, appeared to represent C-terminal degradation products of fibronectin presumably of host origin, rather than treponemal antigens. Although fibronectin binds avidly to Clq and could represent a co-precipitable contaminant throughout the isolation procedure, anti-fibronectin antibodies in the sera of patients detectable by radioimmunoassay and the present of antibodies to 76,000 and 66,000 dalton fibronectin fragments in the globulin fractions of disassociated complexes argues against such a conclusion.