Summary
The Bernard-Soulier syndrome (BSS) is characterized by thrombocytopenia with giant
platelets, a prolonged bleeding time with defective platelet adhesion to the subendothelium
related to a defect in platelet membrane glycoprotein lb (GPIb) and a decreased prothrombin
consumption. The mechanism of the latter abnormality remains unknown. In this study,
we showed that this defect was corrected by the addition of purified human factor
VIII (FVIII) to blood from four patients with BSS. The correction of prothrombin consumption
was almost complete at concentrations between 1.5 and 3 IU/ml of FVIII procoagulant
activity (VIII.'C) and partially abolished by a monoclonal antibody which neutralizes
VIII:C. This correction was specific for FVIII and was not observed after addition
of purified human FIX. It was obtained, in the same magnitude range, with FVIII complexed
to von Willebrand factor (vWF) but not with free vWF. These data provide a new insight
into the knowledge of the physiological interaction between the platelet membrane
and the vWF-FVIII complex facilitating plasma coagulation activation and may lead
to helpful therapeutic advances.