Abstract
Covalently cross-linked dimers and oligomers composed of 2-4 subunits of monoclonal human IgG1 were prepared by incubation of purified monomeric IgG1 with glutaraldehyde followed by gelfiltration chromatography. Monomers, dimers, and oligomers then were labeled with (125)I and used to compare the binding properties of IgG Fc receptors on human peripheral blood monocytes and polymorphonuclear leukocytes (PMN). Binding of IgG1 to monocytes at 37 degrees C and of IgG1 polymers to PMN at 4 degrees C could be readily measured and were found to be reversible and saturable. Scatchard plots of binding were linear in each instance. Monocytes bound a mean of 20,200+/-6,800 molecules/cell of IgG1 monomer at saturation and comparable amounts of dimer or oligomer. The mean association constant (Ka) for binding of IgG1 monomer to monocytes was 8.6 x 10(8)M(-1) and the Ka for binding of dimer and oligomer were three-to fivefold greater.In contrast, PMN bound a mean of 460,000+/-130,000 molecules of IgG1 dimer at saturation and comparable amounts of oligomer. The Ka of binding in both cases was 100-1,000-fold lower than the Ka for binding of the same preparations to monocytes. Binding of labeled IgG1 to both cell types was more potently inhibited by unlabeled IgG1 and IgG3 than by IgG4 or IgG2. Binding of labeled polymers of IgG1 to monocytes was 10-100-fold more easily inhibited by monomeric IgG1 than was binding to PMN. Thus, there are significant quantitative and qualitative differences between the binding properties of Fc receptors present on monocytes and PMN.