Role of saccharides in immunoglobulin--Fc receptor interactions

The rosettes formed by mouse peritoneal macrophages or DCH-5 cells and TNP-erythrocytes coated with anti-TNP antibodies of different isotypes were inhibited to various extent by monosaccharides. The most effective inhibitors were N-acetylglucosamine, glucosamine, mannose and N-acetylneuraminic acid in 1-5 mmol/L concentrations. Even more efficient were glycopeptides isolated from IgG molecules. The Fc receptors (FcRs) released from DCH-5 cells during cultivation and gradually separated by affinity chromatography on immobilized IgG reacted with aggregated IgG and inhibited the rosette formation. The FcRs eluted by monosaccharides influenced mainly the number of rosettes mediated by IgA and IgE while those eluted with a glycine-HCl buffer inhibited preferentially IgG rosettes. As shown by SDS-PAGE the heterogeneity of the fraction eluted with a mixture of monosaccharides revealed one main component with an effective molar mass of 50 kg/mol. The glycine-HCl eluate contained two major components of 55 and 38 kg/mol. The IgG-Sepharose 4B bound all the fractions but only the binding of the 50 kg/mol molecule could be inhibited by monosaccharides.