Demonstration and Partial Characterization of the Interferon-Gamma Receptor on

Radioiodinated recombinant human interferon-gamma (IFN-y) bound to human monocytes, U937, and HL60 cells in a specific, saturable, and reversible manner. At 40C, the different cell types bound 3,000-7,000 molecules of IFNy, and binding was of com-parable affinity (K. = 4-12 X 10 M-l). No change in the receptor was observed after monocytes differentiated to macrophages or when the cell lines were pharmacologically induced to differ-entiate. The functional relevance of the receptor was validated by the demonstration that receptor occupancy correlated with induction of Fc receptors on U937. Binding studies using U937 permeabilized with digitonin showed that only 46 % of the total receptor pool was expressed at the cell surface. The receptor appears to be a protein, since treatment of U937 with trypsin or pronase reduced '251-IFNy binding by 87 and 95%, respectively. At 370C, ligand was internalized, since 32 % of the cell-associated IFNy became resistant to trypsin stripping. Monocytes degraded '251-IFNy into trichloroacetic acid-soluble counts at 370C but not at 40C, at an approximate rate of 5,000 molecules/cell per h. The receptor was partially characterized by SDS-polyacryl-amide gel electrophoresis analysis of purified U937 membranes that had been incubated with _251-IFNy. After cross-linking, the receptor-ligand complex migrated as a broad band that displayed an M, of 104,000±18,000 at the top and 84,000±6,000 at the bottom. These results thereby define and partially characterize the IFNy receptor of human mononuclear phagocytes