Human monocyte interaction with antibody-coated platelets. I. General characteristics.

The interaction of human monocytes with antibody-coated and normal platelets was studied using an assay system that employed rate zonal centrifugation to separate monocyte-associated (bound) platelets from free platelets. At a monocyte:platelet ratio of 1:10, monocytes bound 21.2 +/- 6.6% of antibody-coated platelets and less than 2.2 +/- 1.3% of control platelets. Monocyte binding of antibody-coated platelets was rapid and inhibited by monomeric IgG and staphylococcal protein A. Specific binding was positively related to the number of monocytes present and the amount of IgG displayed on the platelet surface. Transmission and scanning electron micrographs illustrate the membrane binding of antibody-coated platelets to monocytes and suggest that phagocytosis of platelet targets occurs as well. Thus, these observations indicate that monocytes can participate in Fc-receptor binding of anti-PlA1-sensitized platelets. This novel assay may be useful in the analysis of monocyte-macrophage Fc-receptor interaction with platelets sensitized with IgG from both iso- and autoimmune disorders as well as in the characterization of treatment modalities which may alter monocyte Fc-receptor recognition and binding.