Fc receptors of a human promyelocytic leukemic cell line: evidence for two types of receptors defined by binding of the staphylococcal protein A-IgG1 complex

Abstract

The specificity and kinetics of binding of purified monomeric human and murine myeloma immunoglobulins to Fc receptors were studied in a human promyelocytic cell line (HL-60). HL-60 cells contain approximately 20,000 Fc receptors per cell and bind human IgG1, IgG3 and mouse IgG2a with high affinity (dissociation constant of 5 to 10 nM). Kinetic studies of the binding of IgG1 to HL-60 cells demonstrate rapid exchange with ambient immunoglobulin with approximately one-half of the surface-bound IgG1 exchanging every 25 to 30 min at 37 degrees C. Estimation of the equilibrium binding constant from the rates of association and dissociation of IgG1 agrees well with the values obtained from Scatchard analysis of equilibrium binding of radioiodinated IgG1. Approximately one-half of the Fc receptors of HL-60 cells are capable of binding IgG1 complexed to Protein A. This result was independent of the concentration of Protein A (0.5 to 200 microM) or the time of incubation of IgG1 with Protein A. Studies in which Protein A was incubated with HL-60 cells at 37 degrees C then rapidly washed at 0 degrees C indicated that Protein A did not degrade Fc receptors or interact with the Fc receptor sites on HL-60 cells. The complexes formed between Protein A and IgG1 sedimented at 7 to 9S by ultracentrifugation. These results suggest that there are two types of Fc receptors on HL-60 cells, which can be distinguished by their ability to bind the IgG1-Protein A complex.