PB0339 - The impact of von Willebrand factor on fibrin formation and structure unveiled with type 3 von Willebrand disease plasma.

Background: Normally, VWF remains inactive unless its A1A2 domains undergo a shear stress-triggered conformational change. We demonstrated the capacity of a recombinant A2 domain of VWF to bind and to play an active role in fibrin formation, altering the fibrin clot structure. The data indicated that VWF contains an additional binding site for fibrin in the A2 domain that plays a role in the incorporation of VWF to the polymerizing fibrin.

Aims: The aim of this study is to use plasma from individuals with type 3 VWD to examine the hypothesis that VWF directly influence fibrin polymerization and the structure of fibrin clots.

Methods: Plasma from healthy donors and type 3 VWD, ristocetin, recombinant gain-of-function A1A2A3 mutant (exposes A1A2 domain), fibrin polymerization assays, confocal microscopy and scanning electron microscopy were utilized to test the hypothesis.

Results: Our findings indicate that the exposed A2 domain in active VWF harbors additional binding sites for fibrin(ogen), and significantly potentiates fibrin formation. This positive effect was markedly observed with ristocetin in healthy plasma or with A1A2A3 mutant in type 3 VWD plasma without ristocetin. Antibody against the A2 domain of VWF effectively reduced fibrin formation in healthy plasma. Clot analyses revealed a notable and significant difference in porosity between normal and type 3 VWD plasma, further supported by scanning electron microscopy, which demonstrated thicker fibrin fibers in the presence of plasma VWF. Confocal immunofluorescence microscopy showed punctate VWF staining along fibrin fibrils, providing visual evidence of the integration of plasma VWF into the fibrin matrix.

Conclusion(s): This study with type 3 VWD plasma supports the hypothesis that VWF directly influences fibrin polymerization and clot structure. We conclude that a conformational change in the A1A2 domains exposes a hidden fibrin(ogen) binding site, indicating that plasma VWF determines the fibrin clot structure.