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Background: The development of factor VIII (FVIII) inhibitors remains the major hurdle in the clinical management of patients with hemophilia A. FVIII uptake by professional antigen-presenting cells (APC) is the first step involved in initiation of immune responses to FVIII. Studies on FVIII catabolism have highlighted the role played by CD91/LRP as a potential target for increasing FVIII half-life in patients and prolonging treatment efficiency. We investigated the involvement of CD91 in FVIII endocytosis by human dendritic cells (DC), a model of professional APC., Design and Methods: Immature DC were generated from circulating monocytes from healthy donors. Surface expression of CD91 was assessed by flow cytometry. Uptake of fluorescein isothiocyanate-conjugated ligands by immature DC was studied in the presence of various blocking agents., Results: CD91 was expressed on approximately 20% of DC and mediated the internalization of its model ligand, alpha2-macroglobulin. DC internalized FVIII and activated a human FVIII-specific T-cell clone in a dose-dependent manner. FVIII uptake by DC and subsequent T-cell activation were not inhibited by receptor-associated protein., Conclusions: Our results indicate that CD91 and other members of the LDL receptor family are not strongly implicated in FVIII internalization by monocyte-derived DC, and suggest the involvement of alternative divalent ion-dependent endocytic receptors.

Von Willebrand factor (VWF) is a chaperone molecule for procoagulant factor VIII (FVIII). Its role in the reduction of the immunogenicity of therapeutic FVIII in patients with hemophilia A has been evoked but lacks clear cellular and molecular rationale. Here, we demonstrate that VWF protects FVIII from being endocytosed by human dendritic cells (DCs) and subsequently presented to FVIII-specific T cells. The immunoprotective effect of VWF requires a physical interaction with FVIII because the endocytosis of FVIII was significantly restored on hindering the formation of the VWF-FVIII complex. Interestingly, VWF had no direct inhibitory effect either on the ability of DCs to present antigenic peptides or on the activation potency of CD4+ T cells. We thus propose that VWF may reduce the immunogenicity of FVIII by preventing, upstream from the activation of immune effectors, the entry of FVIII in professional antigen-presenting cells.