Aberrant lectin-binding activity of immunoglobulin G in serum from rheumatoid arthritis patients

Structural studies of oligosaccharide chains of immunoglobulin G (IgG) in serum have revealed a specific galactosylation deficiency associated with rheumatoid arthritis (RA). Using a two-site lectin-immunofluorometric assay, I studied the interaction of IgG with immobilized lectins. Compared with control IgG, IgG purified from RA patients' sera bound up to 40-fold more strongly to immobilized Bandeiraea simplicifolia agglutinin II, a lectin that specifically binds agalacto forms of other glycoproteins. However, inhibition studies and treatment of IgG with glycosidase suggested that only a minor part of this binding was mediated by agalacto oligosaccharides of IgG. Furthermore, these IgG samples bound even more intensively to some other immobilized lectins, including Ricinus communis agglutinin (RCA). The binding to RCA was not inhibited by lactose, a hapten sugar of RCA, whereas other lectin species in solution effectively inhibited it. Compared with intact RA IgG, isolated F(ab')2 fragments displayed only low RCA-binding activity. These results indicate the existence of a carbohydrate-nondependent interaction between RA IgG and different plant lectins. With use of immobilized RCA, the lectin-immunofluorometric assay was rapid and reproducible for measuring the aberrant lectin-binding activity of IgG directly in diluted serum samples.