Binding of human C-reactive protein to monocytes: analysis by flow cytometry

An opsonic role has been proposed as a major function of C-reactive protein (CRP) in humans. In support of this hypothesis, recent radiolabelled ligand binding studies have provided evidence for the presence of specific receptors for soluble human CRP on human phagocytic cells, including neutrophils and monocytes. In order to confirm specific binding of CRP to monocytes and to quantify the percentage of such cells capable of expressing binding sites, we employed a sensitive biotin-avidin fluorescence assay to study the CRP-monocyte interaction. It was observed that 67% of monocytes bound biotinylated CRP in a dose-dependent manner, that the binding was calcium dependent, and that it could be inhibited by 60% in the presence of a greater than 20-fold excess of competing native CRP. In other experiments, neither IgG nor heat-aggregated IgG inhibited the binding of CRP to monocytes; and no significant binding to lymphocyte population could be detected. These studies confirm the ability of human CRP to bind to a majority of human monocytes in a calcium-dependent and specific manner, and provide further support for a biologically important interaction of this acute-phase protein with phagocytic cells.