Where appropriate, the data were analysed using Student's t-test, and a value of P < 0.05 was considered significant. G.F.G. is a leading principal investigator of the Innovative Research Group of the NSFC (81021003). A Conformation-Sensitive Monoclonal Antibody against the A2 Domain of von Willebrand Factor Reduces Its Proteolysis by ADAMTS13 The size of von Willebrand factor (VWF), controlled by ADAMTS13-dependent proteolysis, is associated with its hemostatic activity. Many factors regulate ADAMTS13-dependent VWF proteolysis through their interaction with VWF. These include coagulation factor VIII, platelet glycoprotein 1bα, and heparin sulfate, which accelerate the cleavage of VWF. Conversely, thrombospondin-1 decreases the rate of VWF proteolysis by ADAMTS13 by competing with ADAMTS13 for the A3 domain of VWF. To investigate whether murine monoclonal antibodies (mAbs) against human VWF affect the susceptibility of VWF to proteolysis by ADAMTS13 in vitro, eight mAbs to different domains of human VWF were used to evaluate the effects on VWF cleavage by ADAMTS13 under fluid shear stress and static/denaturing conditions. Additionally, the epitope of anti-VWF mAb (SZ34) was mapped using recombinant proteins in combination with enzyme-linked immunosorbent assay and Western blot analysis. The results indicate that mAb SZ34 inhibited proteolytic cleavage of VWF by ADAMTS13 in a concentration-dependent manner under fluid shear stress, but not under static/denaturing conditions. The binding epitope of SZ34 mAb is located between A1555 and G1595 in the central A2 domain of VWF. These data show that an anti-VWF mAb against the VWF-A2 domain (A1555-G1595) reduces the proteolytic cleavage of VWF by ADAMTS13 under shear stress, suggesting the role of this region in interaction with ADAMTS13. Plasma von Willebrand factor (VWF) is a large multimeric glycoprotein that interacts with platelet surface receptors and is crucial for normal hemostasis. The adhesive activity of VWF is positively correlated with the size of the multimers in plasma [1, 2] . VWF multimer size is regulated by metalloproteinase ADAMTS13, which cleaves the central A2 domain of VWF at the Tyr1605-Met1606 bond [3] [4] . The importance of VWF proteolysis by ADAMTS13 is demonstrated in two syndromes, i.e., thrombotic thrombocytopenic purpura and von Willebrand disease type 2A. The former is associated with the deficiency of plasma ADAMTS13 activity, either due to congenital mutations or acquired autoantibodies [5] [6] [7] . The latter is mostly caused by mutations in the A2 domain of VWF that lead to the increased proteolysis of VWF multimers by ADAMTS13 [8, 9] .