HEMOSTASIS, THROMBOSIS, AND VASCULAR BIOLOGY A novel nanobody that detects the gain-of-function phenotype of von Willebrand factor in ADAMTS13 deficiency and von Willebrand disease type 2B

Von Willebrand factor (VWF) is unable to interact spontaneously with platelets because this interaction requires a conversion of the VWF A1 domain into a glycoprotein Ib (GpIb ) binding conformation. Here, we discuss a llama-derived antibody fragment (AU/VWFa-11) that specifically recognizes the GpIb -binding conformation. AU/VWFa-11 is unable to bind VWF in solution, but efficiently interacts with ristocetinor botrocetin-activated VWF, VWF comprising type 2B mutation R1306Q, or immobilized VWF. These unique properties allowed us to use AU/ VWFa-11 for the detection of activated VWF in plasma of patients characterized by spontaneous VWF-platelet interactions: von Willebrand disease (VWD) type 2B and thrombotic thrombocytopenic purpura (TTP). For VWD type 2B, levels of activated VWF were increased 12-fold (P < .001) compared to levels in healthy volunteers. An inverse correlation between activated VWF levels and platelet count was observed (R2 0.74; P < .003). With regard to TTP, a 2-fold (P < .001) increase in activated VWF levels was found in plasma of patients with acquired TTP, whereas an 8-fold increase (P < .003) was found in congenital TTP. No overlap in levels of activated VWF could be detected between acquired and congenital TTP, suggesting that AU/VWFa-11 could be used to distinguish between both disorders. Furthermore, it could provide a tool to investigate the role of VWF in the development of thrombocytopenia in various diseases. (Blood. 2005;106: 3035-3042)