Reversible and irreversible loss of Fc receptor function of human monocytes as a consequence of interaction with immunoglobulin G.

The effects of IgG in different configurations on the Fc receptor function of human monocytes were studied. Receptor function was assessed by quantitating immune adherence and/or ingestion of human erythrocytes coated with IgG anti-D antibody. Monomeric IgGl in solution inhibited the Fc receptor function of monocytes, but this function was restored completely after washing. In contrast, monomeric IgG that was adsorbed nonspecifically to a plastic surface inhibited the Fc receptor function of monocytes even after washing away unbound IgGl. This loss of function could be blocked by sodium azide and was reversed when the IgG adsorbed to plastic was degraded by trypsin, suggesting that loss of function was the reversible consequence of localized binding of most of the monocyte's receptors at the point of contact with immobilized IgGl. Fluid-phase aggregates of IgGl also reduced the Fc receptor function of monocytes as a consequence of direct binding to the monocyte surface. High concentrations of purified aggregates rapidly reduced Fc receptor function but function was reversed by trypsin even after incubation for 18 h. Lower concentrations of aggregates reduced Fc receptor function more slowly, but after 18 h of incubation, lost function was not restored by trypsin treatment. Because the transition from reversible to irreversible loss was blocked by sodium azide, an energy-dependent process of ingestion, shedding or denaturation of receptors is responsible for this irreversible loss of Fc receptor function. Rabbit IgG anti-human IgG bound to IgG adsorbed to the surface of monocytes also mediated a loss of Fc receptor function as a result of the binding of Fc receptors to the Fc portion of the rabbit IgG molecule, a process analogous to the binding of aggregated IgG. After irreversible depletion of Fc receptor function by anti-IgG, partial recovery of function was detectable within 12-24 h of incubation in vitro, and this recovery was blocked by cycloheximide, suggesting that new receptor synthesis was required for restoration of function.