The effect of recombinant hybrid interferon (IFN)-αA/D either alone or in combination with retinoic acid (RA) on induction of differentiation of the human promyelocytic leukemia cell line HL-60 was studied. The combination of 10 nM RA and IFN-αA/D at concentrations of 120 units/ml and higher induces differentiation into cells having many monocytic characteristics such as monocyte-like morphology, ability of Superoxide anion production, antibody-dependent cellular cytotoxicity, and nonspecific esterase activity. However, this combination does not induce an increase in IgG Fc receptors (FcγR) and immunoerythrophagocytosis. IFN-αA/D alone shows essentially no effect on these parameters. Pretreatment of HL-60 with IFN-αA/D blocks the FcγR-increasing activity of IFN-αA, IFN-αD, and IFN-γ. This block is complete after a 2-h pretreatment for IFN-αA and IFN-αD and after a 6-h pretreatment for IFN-γ. Furthermore, an increase in FcγR-mediated phagocytosis is also suppressed by the pretreatment. However, IFN-αA/D does not suppress Superoxide anion production. These results indicate that the block of an FcγR-increase by IFN-αA/D has not resulted in the competitive inhibition at IFN receptors, but may occur in intracellular events such as postreceptor transduction for the expression of FcγR.
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