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High‐Affinity Binding of Fibronectin to Cultured Kupffer Cells

Hepatic Kupffer cells are a major component of the reticuloendothelial or macrophage system. They were the first phagocytic cell type whose phagocytosis was shown to be influenced by plasma fibronectin, a dimeric opsonic glycoprotein. In the current study, the binding of soluble radioiodinated fibronectin purified from rat serum to isolated rat hepatic Kupffer cells was investigated using a cultured Kupffer cell monolayer technique. Binding was specific, since unlabeled purified fibronectin competed in a dose‐dependent manner with the 125I‐fibronectin for binding to the Kupffer cells. Addition of gelatin enhanced the binding of 125I‐fibronectin to Kupffer cells. The phagocytosis of gelatinizedcoated red cells by Kupffer cells was increased either by preopsonizing the target particles with purified fibronectin or by the addition of purified fibronectin to the culture medium. In contrast, exposure of the Kupffer cells to medium containing purified fibronectin followed by wash‐removal of the fibronectin did not increase the uptake of gelatin‐coated red blood cells, even though fibronectin was detected on the surface of the Kupffer cells by immunofluorescence. Trypsinized monolayers expressed decreased capacity to bind 125I‐fibronectin as well as fibronectin‐coated sheep erythrocytes. The binding of 125I‐fibronectin‐gelatin complexes was inhibited by excess unlabeled fibronectin. We calculated that specific high‐affinity (Kd = 7.46 × 10‐9 M) binding sites for fibronectin exist on Kupffer cells. There are approximately 2,800–3,500 binding sites or putative “fibronectin receptors” per Kupffer cell. These sites appear to mediate the enhanced phagocytosis of gelatin‐coated particles opsonized by fibronectin.