Von Willebrand Factor Protects Factor VIII from Endocytosis by Human Monocytes−Derived Dendritic Cells and Subsequent Presentation to Immune Effectors.

The role of Von Willebrand Factor (VWF) as a chaperone molecule for procoagulant factor VIII (FVIII) has been extensively documented. Under physiological conditions, VWF binds to FVIII after its release in the circulation. VWF protects FVIII from proteolysis by lipid−bound proteases, stabilizes FVIII and further regulates its elimination by lipoprotein receptors. We investigated whether VWF might modulate FVIII endocytosis by professionnal antigen presenting cells (APCs). Immature dendritic cells (DCs), utilised as model of APCs, were generated from circulating monocytes of healthy blood donors and incubated with FITC−labeled FVIII (FVIII−FITC). T cell assays were performed by co−culturing the human FVIII−specific CD4+ T cell clone, D9E9, with immature DCs from MHC−matched donors in presence of unconjugated FVIII and increasing concentrations of VWF (1:1 to 1:130). Preincubation of FVIII−FITC with VWF at 1 to 130−fold molar excess resulted in a dose−dependent inhibition of FVIII endocytosis (12 ± 2% to 94 ± 18%, respectively). Human serum albumin used in equivalent molar ratios did not prevent FVIII internalization. In contrast, the protective effect of VWF on FVIII uptake was significantly restored by co−incubation of FVIII and VWF in the presence of F(ab’)2 fragments of BO2C11, an anti−FVIII IgG, and Ac418, an anti−VWF IgG, that both disrupt the interaction between FVIII and VWF. In addition, blocking DC entry of FVIII by VWF resulted in a dose−dependent reduction of the activation of D9E9 (up to 75%). Interestingly, D9E9 activation by DCs loaded with a synthetic FVIII−derived peptide, I²¹⁴⁴–T²¹⁶¹, was not altered in the presence of equivalent concentrations of VWF, indicating that VWF does not have a direct inhibitory effect on T cell activation. The administration of exogenous FVIII to patients with hemophilia A, results in up to 30% of the cases, in the developement of FVIII−specific antibodies that inhibit FVIII procoagulant activity. In vivo experimental evidences and clinical observations have documented that the presence of VWF in FVIII therapeutic concentrates may be associated with a lower incidence of FVIII inhibitors. Here, we demonstrate that VWF may reduce the immunogenicity of FVIII by preventing its internalization by professionnal APCs, and reducing the subsequent presentation of FVIII−derived peptides to T lymphocytes.