A novel nanobody that detects the gain-of-function phenotype of von Willebrand factor in ADAMTS 13 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 alpha (GpIb alpha) binding conformation. Here, we discuss a llama-derived antibody fragment (AU/VWFa-11) that specifically recognizes the GpIb alpha-binding conformation. AU/VWFa-11 is unable to bind VWF in solution, but efficiently interacts with ristocetin- or 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 (R-2=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.