Summary
Thrombocytopenia is frequently reported in type 2B von Willebrand disease (vWD), and
thought to be related to the abnormally high affinity of 2B von Willebrand factor
(vWF) for platelet GPIb-IX. To gain an insight into the nature of this thrombocytopenia,
we measured plasma glycocalicin (GC) levels (as a marker of platelet turnover), and
platelet surface expression of the alpha granule protein P-selectin (as a marker of
platelet activation) in 9 patients with type 2B vWD before, and in 4 patients also
following the infusion of 1-desamino-8-d-arginine vasopressin (DDAVP). Three patients
presented a persistent decrease of platelet counts in the resting condition. GC levels
were within the normal range, regardless of the platelet counts, in all but one patient
who presented, on the other hand, a normal platelet count. Moreover, platelets expressed
normal amounts of P-selectin on their surface, regardless of platelet counts. These
findings suggest that the thrombocytopenia observed in type 2B vWD is not due to platelet
activation and subsequent consumption in circulation.
Despite a significant, albeit transient, decrease in platelet count, DDAVP did not
induce an increase in plasma GC levels, nor enhance P-selectin expression. These observations
indicate that the acute post-DDAVP thrombocytopenia in type 2B vWD is not related
to platelet activation and consumption. We advance that the post-DDAVP 2B vWF is hemostatically
more active, and able to induce agglutination but not aggregation of circulating platelets.
This would explain both the prompt recovery of basal platelet counts after the post-DDAVP
decrease, and the lack of reported thrombotic complications in this disorder.
Therefore, even though 2B vWF is characterized by an enhanced affinity for the platelet
surface, its binding to platelet GPIb-IX in the soluble phase is not able to induce
true platelet aggregation; vWF thus appears to be mainly an adhesive protein, rather
than an aggregating agent.