Hemophilia A is a rare X chromosome-linked recessive disorder that concerns 1/5000 individual. Genetic abnormalities in the FVIII gene result in qualitative or quantitative defects in FVIII production. In its severe form, hemophilia A is a life-threatening, crippling hemorrhagic disease. Treatment of hemophilia patients with therapeutic FVIII results, in up to 30% of the cases, in the emergence of anti-FVIII antibodies (inhibitors) that neutralize the pro-coagulant activity of the therapeutically administered FVIII. The development of FVIII inhibitors represents a major medical hurdle and a critical societal concern. In developed countries, cost of care of patients with inhibitors may reach 200 K€/year. In the last two decades, basic research on the allo-immunisation against FVIII has focused almost exclusively on characterizing the effector arm of the anti-FVIII immune response: the nature and mechanisms of action of FVIII inhibitors, and on deciphering the role of CD4+ T lymphocytes in the process. However, not much is known regarding the nature of the endocytic receptors involved in uptake and presentation of FVIII to CD4+ T cells by professional antigen presenting cells (APCs), which is the very first step in the initiation of deleterious allo-immune responses. We have recently demonstrated that human dendritic cells (DCs), the major professional APCs endocytose FVIII and present FVIII-derived peptide in the context of HLA-DR for the activation of FVIII-specific CD4+T cells. The working hypothesis in the first aim of my project is that a restricted set of endocytic receptors on APCs is responsible for the uptake of FVIII and its subsequent presentation to CD4+ T cells. We will thus identify the receptors implicated in FVIII endocytosis by APCs, using human monocyte-derived DCs and circulating human myeloid DC subsets such as BDCA-1+ and BDCA-3+ myeloid DCs as model APCs. Further, we will also explore the role of plasmacytoid DCs and of type I interferons on the level of expression of endocytic receptors on myeloid DC and in eliciting FVIII immune response. A corollary to our hypothesis is that blocking the interaction of FVIII with endocytic receptors on DCs will reduce FVIII uptake and hence prevent or delay the initiation of allo-immune response to exogenous FVIII in multitransfused hemophilia A patients. Based on FVIII structure, available literature and our preliminary observations, candidate endocytic receptors may belong to receptors for N-linked glycosylations. Using FVIII knock-out and/or endocytic receptor knock-out murine model, we will delineate in vivo the therapeutic relevance of interfering endocytic receptors on DCs to block anti-FVIII immune response. The identification of the endocytic receptors on APCs involved in FVIII endocytosis carries an enormous potential for the delineation of novel therapeutic strategies, based on either cellular/receptor or drug-mediated intervention. The expected results should provide explanations for the yet unexplained elevated immunogenicity of FVIII and provide new tools to modulate the interaction of FVIII with APCs. Furthermore, in the second aim I propose that monitoring the levels of the candidate endocytic receptors, either on the surface of circulating APCs, at the mRNA level or in their soluble form in the plasma, should be of important prognostic value for detecting patients at risk of developing FVIII inhibitors, and thus assisting clinicians in choosing for therapies alternative to FVIII administration. Importantly, because most of the patients with severe hemophilia A develop FVIII inhibitors in infancy, preventing or delaying the onset of the anti-FVIII allo-immune response will have dramatic repercussions in improving the clinical management and quality of life of the patients.