Binding specificities of a polyreactive and a monoreactive human monoclonal IgG rheumatoid factor: role of oligosaccharides.

The immunological specificites of two human rheumatoid factor-reactive IgG monoclonal antibodies derived from unstimulated rheumatoid synovial lymphocytes have been analysed. A malaria antigen-reactive IgG monoclonal antibody from an immune donor served as a control. Purified IgG monoclonal antibody from one IgG-RF hybridoma (L1), but not from the other IgG-RF hybridoma (D1) or the anti-malaria monoclonal antibody, exhibited dose-dependent binding to multiple self and non-self antigens such as ds-DNA, cytochrome-c, bovine thyroglobulin, transferrin, cellulose and lipopolysaccharide and therefore was considered polyreactive. The immunological specificity was confirmed by inhibition experiments using the same soluble antigens as inhibitors. The polyreactivity of the IgG-RF MoAb was markedly inhibited by absorption with glycoproteins such as thyroglobulin, a commonly used target for xenoreactive natural antibodies, and cytochrome-c, indicating that the monoclonal antibody is reactive with epitopes expressed on these ligands. Since some naturally occurring antibodies are carbohydrate specific, the authors tested the IgG-RF MoAb for possible carbohydrate specificity. Absorption with certain polysaccharides containing only one or two different sugar moieties did not inhibit the binding reactivities to any of the tested antigens. Polyreactivity of the monoclonal antibody, unlike most xenoreactive natural antibodies, was not caused by reactivity with (gal alpha 1-3gal) as indicated by the remaining binding reactivity after alpha-galactosidase treatment of the antigen. Removal of the N-linked glycosylation sites within the Fc portion of target IgG markedly reduced the antibody binding. The findings suggest that the carbohydrate content of the antigen is necessary for binding of the polyreactive IgG-RF MoAb. Reactivity to carbohydrate antigens may readily explain the so-called multispecificity of certain antibodies.