974 Platelet Membrane Glycoproteins

Glycoproteins I Ib / I I Ia form a Ca2+-dependent complex that functions as the fibrinogen receptor on human platelets (1). Several reports have indicated that this fibrinogen receptor is also expressed on the surface of human monocytes (2-4). Of three monoclonal antibodies directed against platelet I Ib/ I I Ia , two were reported to give a positive reaction with monocytes as well (4). Other investigators reported that, in patients with Glanzmann's thrombasthenia, a bleeding disorder characterized by the absence of or marked reduction in I Ib / IIIa on platelets, both glycoproteins were also missing in monocytes (3). It has also been shown that monocytes bind fibrin (5, 6), which would be consistent with the presence of a fibrinogen receptor on their surface. The presence of I Ib / I I Ia on monocytes would offer a good opportunity to study the biosynthesis and molecular biology of these glycoproteins and hence to determine the genetic defect in bleeding disorders like Glanzmann's thrombasthenia. Monocytes are accessible in large numbers and are easily kept in culture, in contrast to megakaryocytes, the platelet precursor cells. Despite much evidence, however, the presence of I Ib / I I Ia on mononuclear phagocytes is still not fully established. Monocytes that are isolated from blood are often contaminated with adherent platelets unless scrupulous precautions are taken (7-9). Thorough washing may remove most platelets but fragments of platelet membrane can remain at tached to the monocyte surface by an interaction mechanism that is still unclear. We have recently worked out a method for the large scale preparation of highly purified monocytes from donor blood using centrifugal elutriation. Since our preparations were virtually free of platelets, we decided to reinvestigate I Ib / I I Ia on mononuclear phagocytes. Using two different techniques based on monoand polyclonal antibodies, as well as immunoelectron microscopy, we found no evidence for the presence of these glycoproteins on monocytes and monocyte-derived macrophages. This work was supported in part by grants 3.302.082 and 3.353.082 from the Swiss National Research Foundation, grant 817016 from INSERM, and grant 31610 from the National Institutes of Health.