The role of monogamous bivalency and Fc interactions in the binding of anti-DNA antibodies to DNA antigen

•Fab fragments of lupus IgG preparations bind poorly to DNA by ELISA.•F(abʹ)2 fragments of lupus IgG preparations bind poorly to DNA by ELISA.•Fab and F(abʹ)2 fragments from lupus IgG bind well to tetanus toxoid and EBV antigen.•The poor binding of Fab fragments suggests anti-DNA binding by monogamous bivalency.•The poor binding of F(abʹ)2 fragments suggests a role of Fc for anti-DNA activity.

Antibodies to DNA (anti-DNA) are the serological hallmark of systemic lupus erythematosus. These antibodies can bind DNA avidly by monogamous bivalency, a mechanism which requires the interaction of both Fab combining regions with antigenic determinants on the same polynucleotide. To explore further this mechanism, we tested Fab and F(abʹ)2 fragments prepared from IgG from patient plasmas in an ELISA with native DNA antigen, detecting antibody with a peroxidase conjugated anti-Fab reagent. These studies showed that Fab fragments, which can only bind monovalently, had negligible activity. Although bivalent F(abʹ)2 fragments would be predicted to bind DNA, these fragments also showed poor anti-DNA activity. Control studies showed that the fragments retained antibody activity to tetanus toxoid and an EBV antigen preparation. Together, these findings suggest that anti-DNA avidity depends on monogamous bivalency, with the antibody Fc portion also influencing DNA binding, in a mechanism which can be termed Fc-dependent monogamous bivalency.