Plasma membranes of macrophages and lymphoid cells, both normal and of tumor origin, bear receptors for IgG (Fc receptors). These receptors are vital to phagocytosis of antibody-coated particles and to antibody-mediated cell cytotoxicity (ADCC). They are implicated as well in a number of biological processes, including regulation of antibody production, modification of cytotoxic and mitogenic responses, stimulation of suppressor cell function and mediation of enhancing effects of immune complexes on tumors and allografts. A more precise knowledge of the structure of these receptors is needed. The aims of the research plan are twofold: to isolate and characterize IgG Fc receptors on human monocytes and macrophages and to determine the minimum necessary signal for Fc receptor-mediated endocytosis of small immune complexes. The U937 human macrophage cell line will be utilized, although for selected experiments, human peripheral blood monocytes and cultured macrophages will be obtained. Receptors will be isolated by three distinct methods: ligand-immobilized affinity chromatography, affinity adsorption with hybridoma anti-Fc receptor antibody and cross-linking of receptor-ligand complexes with cleavable bifunctional cross-linking reagents followed by purification of the complexes with anti-immunoglobulin reagents. After determining the valence of Fc receptors for IgG, the minimum necessary signal for Fc receptor-mediated endocytosis will be evaluated by determining the fate (dissociation or endocytosis) of small IgG oligomers of specific size made with bivalent cross-linking reagents and by cross-linking Fc receptors with anti-Fc receptor hybridoma antibody. The long-term objectives of this study are to develop a model for understanding the molecular events involved in Fc receptor-mediated endocytosis of small immune complexes. An understanding of the cell biology and membrane chemistry of these events will aid in analysis of related immunological problems in which Fc receptors participate, such as antibody-dependent, cell-mediated killing of tumor cells, secretion of effector molecules from phagocytic cells and immunoregulation by lymphocytes.