The Characteristics of Binding of Human Recombinant Interferon-gamma to Its Receptor on Human Monocytes and Human Monocyte-like Cell Lines

Interferon-gamma (IFN-gamma) induces many effector functions in macrophages, presumably through binding to cell surface receptors. Such receptors have been documented in murine macrophages. This report demonstrates that IFN-gamma interacts with a specific receptor on human monocytes and monocyte-like cell lines, U937 and HL60. Recombinant IFN-gamma (rIFN-gamma) was radioiodinated by using Bolton-Hunter reagent to high specific activity. 125I-rIFN-gamma bound in a specific and saturable manner. Saturation of binding sites occurred at 10(-9) M (300 U/ml); there were 4000 specific binding sites per cell for both monocytes and the cell lines. Purified lymphocyte-derived IFN-gamma as well as rIFN-gamma competed for binding sites with 125I-rIFN-gamma. There was no inhibition of binding of the 125I-rIFN-gamma by rIFN-alpha or rIFN-beta. The forward rate constant, k1, at 4 degrees C was about 8 X 10(5) M-1 sec-1 for each cell type studied. In the presence of excess ligand, the dissociation rate constant, k-1, was about 2 X 10(-4) sec-1. The Ka calculated from these constants (4 X 10(9) M-1) agreed closely with that calculated from experiments performed at equilibrium (8 X 10(9) M-1). Because the dissociation of rIFN-gamma from cells was enhanced in the presence of ligand, interactions between binding sites of a negative cooperative type could be operative. These studies demonstrate that rIFN-gamma binds in a specific and saturable manner and with high affinity to a receptor on human monocytes as well as monocyte-like cell lines.