Adsorptive Endocytosis of Fibrin Monomer by Macrophages: Evidence of a Receptor for the Amino Terminus of the Fibrin α Chain

Human fibrinogen and fibrin monomer were labeled with heme-octapeptide for cytochemical examination of their interaction with rabbit peritoneal macrophages in vitro. Upon short exposure to labeled fibrin monomer in solution with unlabeled fibrinogen, the cells became covered with surface-adsorbed monomer in nonaggregated form and having a characteristic trinodular shape. Within 30 min, the adsorbed monomer became fully internalized by vesicular uptake, with much of it being incorporated into lysosomal bodies. A concomitant loss of adsorptive capacity of the cell surface for uptake of more monomer accompanied the internalization. By contrast, labeled fibrinogen was not adsorbed by the macrophage surface. Some internalization of fibrinogen by passive fluid-phase pinocytosis was evident, but it was not accompanied by loss of adsorptive capacity for fibrin monomer. The active uptake of monomer may have depended on binding to the amino terminus that is blocked by fibrinopeptide A in fibrinogen, because addition of synthetic peptide corresponding to the terminal Gly-Pro-Arg segment inhibited both the adsorption and the internalization of monomer.