| Abstract: | The valence for ligand of the 72 kD high-affinity IgG FcR present on human mononuclear phagocytes was evaluated. Lysates of U937 cells whose high-affinity FcR had been saturated with equivalent quantities of 125I-IgG1 kappa and unlabeled IgG1 lambda or with 125I-IgG1 lambda and unlabeled IgG1 kappa were incubated with Sepharose-anti-kappa. Eighty-nine percent of the applied 125I-IgG1 kappa was bound, whereas 0.35% of the applied 125I-IgG1 lambda bound (mean of two experiments), indicating that if the receptors are occupied with ligand, the receptors bind only one ligand molecule at a time. Two experiments were performed to show that the receptors were ligand-occupied. First, a monoclonal antibody directed against the 72 kD FcR (FcRmab32) was added to lysates of U937 cells saturated with equal quantities of 125I-IgG1 lambda and IgG1 kappa. This anti-FcR antibody caused a dose-dependent sevenfold increase in the amount of 125I-IgG1 lambda bound to the anti-kappa immunoadsorbent (presumably by cross-linking receptors bearing 125I-IgG1 lambda with receptors bearing IgG1 kappa), whereas monoclonal antibodies (MMA and IV3) directed against two other determinants on U937 caused no such increase. In the second experiment, Sepharose-FcRmab32 adsorbed 60% of the 125I-IgG1 kappa and 46% of the 125I-IgG1 lambda applied in a U937 lysate (bearing high-affinity FcR), whereas only 3% of 125I-IgG1 kappa and 6% of 125I-IgG1 lambda applied in a K562 lysate (bearing no high-affinity FcR) were adsorbed. We interpret these data to indicate that in detergent solution the valency of the high-affinity FcR on U937 cells is one. |
