The important role of von Willebrand factor in platelet-derived FVIII gene therapy for murine hemophilia A in the presence of inhibitory antibodies

BackgroundOur previous studies have demonstrated that targeting FVIII expression to platelets results in FVIII storage together with von Willebrand factor (VWF) in platelet -granules and that platelet-derived FVIII (2bF8) corrects the murine hemophilia A phenotype even in the presence of high-titer anti-FVIII inhibitory antibodies (inhibitors). ObjectiveTo explore how VWF has an impact on platelet gene therapy for hemophilia A with inhibitors. Methods2bF8 transgenic mice in the FVIII-/- background (2bF8(tg+/-)F8(-/-)) with varying VWF phenotypes were used in this study. Animals were analyzed by VWF ELISA, FVIII activity assay, Bethesda assay and tail clip survival test. ResultsOnly 18% of 2bF8(tg+/-)F8(-/-)VWF(-/-) animals, in which VWF was deficient, survived the tail clip challenge with inhibitor titers of 3-8000BUmL(-1). In contrast, 82% of 2bF8(tg+/-)F8(-/-)VWF(+/+) mice, which had normal VWF levels, survived tail clipping with inhibitor titers of 10-50000BUmL(-1). All 2bF8(tg+/-)F8(-/-)VWF(-/-) mice without inhibitors survived tail clipping and no VWF(-/-)F8(-/-) mice survived this challenge. Because VWF is synthesized by endothelial cells and megakaryocytes and is distributed in both plasma and platelets in peripheral blood, we further investigated the effect of each compartment of VWF on platelet-FVIII gene therapy for hemophilia A with inhibitors. In the presence of inhibitors, 42% of animals survived tail clipping in the group with plasma-VWF and 50% survived in the platelet-VWF group. ConclusionVWF is essential for platelet gene therapy for hemophilia A with inhibitors. Both platelet-VWF and plasma-VWF are required for optimal platelet-derived FVIII gene therapy for hemophilia A in the presence of inhibitors.