The role of carbohydrates in the IgG binding and suppressive activities of shed human Fc receptors.

IgG Fc receptor (Fc gamma R) shed from human peripheral mononuclear cells and purified by IgG affinity chromatography bound to ConA, Pisum sativum, Ulex europeus and peanut lectins. The soluble Fc gamma R was not absorbed by wheat germ, soyabean, lentil, Sophora japonica or Helix pomotia lectins. Out of 14 different sugars, none interfered with the interaction between IgG Fc and membrane Fc gamma R. These results show that shed Fc gamma R had glucosyl, mannosyl, fucosyl and galactosyl groups, but that these components were not part of the IgG-binding site(s). The shed FcR had IgG-binding factor (BF) activity; it suppressed secondary in vitro antibody production. In accordance with the binding studies, IgG-BF activity could be adsorbed by ConA and PNA, but not by lentil lectins. The presence of 10 mM galactose or alpha-methylmannoside had no influence on biological activity, suggesting that, although present on human shed Fc gamma R, these sugars do not play a major role in its suppressive activity.