A V region-connected autoreactive subfraction of normal human serum immunoglobulin G.

Mouse and human natural IgM autoantibodies have been shown to be polyreactive and "connected" through V region-dependent interactions. In the present study, we have identified a connected subfraction of normal human serum IgG by using affinity chromatography of F(ab')2 fragments of pooled IgG (IVIg) or of IgG from a single donor on Sepharose-bound F(ab')2 fragments of the same source of IgG. The connected fraction of IgG exhibited a high content of autoantibodies directed against a wide panel of evolutionarily conserved self antigens and of self antigens that may be targets of autoantibodies in autoimmune diseases. Connected IgG also contained higher amounts of antibodies directed against commonly encountered microbial antigens than unfractionated IgG. The connected fraction did not, however, differ from unchromatographed IgG nor from non-connected IgG in its content of antibodies to vaccinal antigens and to distant foreign antigens. Thus, in humans as in mice, connectivity is a prominent feature of autoantibodies. Our observations are suggestive of a tight control by IgG of the expressed autoreactive repertoire in healthy individuals and strengthen the concept that the therapeutic infusion of pooled normal IgG (IVIg) may be effective in autoimmune diseases by bringing to patients normal regulatory components of the immunoglobulin network.