Insulin-like growth factor-I binds selectively to human peripheral blood monocytes and B-lymphocytes

Insulin-like growth factor-I (IGF-I) is a potential modulator of responses to a variety of immunological challenges. Previous studies have suggested that specific IGF-I receptors are present on peripheral blood leukocytes, including polymorphonuclear leukocytes, lymphocytes, and monocytes. We sought to determine what type of IGF receptor was present on peripheral blood mononuclear cells and which types of cells possessed these receptors. Binding of [¹²⁵I]IGF-I to mononuclear cells was inhibited by both unlabeled IGF-I and insulin, insulin being 200-fold less potent than IGF-I. Covalent affinity labeling with [¹²⁵I]IGF-I, followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography, revealed a specific binding species with an apparent mol wt of 130 kDa. Two color flow cytometric analysis of mononuclear cells stained with mouse monoclonal antibodies specific for the human IGF-I receptor, the human insulin receptor, and monoclonal antibodies directed against specific monocyte and lymphocyte subset cell surface antigens revealed that both IGF-I receptors and insulin receptors were present on nearly all monocytes and B-lymphocytes, but were present on only 2% of T-lymphocytes. We conclude from these data that among human peripheral blood nonactivated mononuclear cells, IGF-I binds to specific type I IGF receptors found predominately on monocytes and B-lymphocytes.