Glycoprotein IIb/IIIa Cross-Reacting Antigen in Monocyte-Derived Macrophages from the Pigeon

Abstract

The binding of fibrinogen to its receptor on mammalian platelets and avian thrombocytes has been extensively studied; and the receptors, composed of glycoproteins IIb and IIIa, have been characterized in both systems. Recently, monocytes have been implicated in the thrombotic complications of atherosclerosis, and in both the avian and human systems this appears to be through a procoagulant activity which leads to fibrinogen polymerization. Although fibrin polymerization by avian monocytes has been reported, the receptor for fibrinogen on these cells has not been reported previously. The present study describes the presence of glycoprotein IIb- and IIIa-like proteins in avian macrophages and correlates the localization of these glycoproteins with regions to which fibrinogen binds. Through the use of immunofluorescence light microscopy and immunogold electron microscopy in conjunction with monospecific, polyclonal antibodies, GPIIb and GPIIIa cross-reacting antigens were identified on membranes of monocyte/macrophages cultured from White Carneau pigeons. A specific concentration of the antigens was found on membrane ruffles and microvilli, sites to which FITC-labeled fibrinogen also bound. Interaction of the antibodies with pigeon macrophages was confirmed by enzyme-linked immunosorbent assays with cultured cells. Immunoblotting of membranes isolated from pigeon monocyte/macrophages identified a protein of 132,000 Mr that was recognized by anti-GPIIb and a protein of 114,000 Mr that was recognized by anti-GPIIIa. These pigeon monocyte glycoproteins comigrated with glycoproteins IIb and IIIa isolated from human platelets.