Effect of Monoclonal Antibodies Against von Willebrand Factor and Platelet

The platelet retention test provides a measure of the number of platelets retained in a column of glass beads and is one of the few in vitro platelet function tests that is abnormal in von Willebrand’s disease (vWd). In a two-stage test, 1 mL of blood (designated A) was passed through the column, followed by 5 mL of isotonic saline and then 5 mL of blood (B) in which platelet retention was measured. With normal blood as A and B, retention is very high in all 5 mL of blood B. In the first stage, platelets adhere to the glass beads; this requires fibrinogen but not von Willebrand factor (vWf). The platelet-platelet adhesion in the second stage requires vWf, is dependent on release of ADP, and fails to occur if thrombasthenic platelets are tested. Retention was normal when blood from a patient with afibrinogenemia was used as blood B. We have now used monoclonal antibodies to elucidate further the mechanism of platelet retention. Five antibodies to different epitopes on vWf essentially abolished retention in the one-stage test and in the second stage of the two-stage test, but had no effect on the first stage. Thus, the entire vWf molecule must be free of antibody to function in the platelet-platelet adhesion of the second stage of this test. Binding of the T HE PLATELET retention test is an in vitro measure of platelet function that is abnormal in blood from patients with von Willebrand’s disease (vWd).’ The test provides a measure of the number of platelets retained when native or heparinized blood is pumped through a column of glass beads. Under appropriate conditions, retention is low in the first milliliter to emerge from the column, and increases progressively so that >80% of the platelets from the fourth and fifth milliliter are retained.2 If flow is rapid, retention of platelets in vWd blood is low.2’3 Retention is also low when normal blood is disturbed.4 ADP is released during passage of the blood through the column and is required for normal retention; prior release of ADP is responsible for the inhibitory effects of disturbance. 9 Monoclonal antibodies that react with platelet glycoprotein (GP) lb’#{176} or GP lIb/Illa” inhibit retention, suggesting that binding of vWf to GP lb and binding of fibrinogen to GP lib/Illa may be important. We developed a two-stage test in which 1 mL of blood (blood A) was passed through the column, washed out with isotonic saline or plasma, and was followed by 5 mL of blood (B), in each milliliter of which retention was measured.’2