Determining factors of inhibitor formation in hemophilia A

The X-linked bleeding disorder hemophilia A is caused by functional absence of blood coagulation factor VIII (FVIII). The bleeding tendency in patients with hemophilia A can be corrected by regular infusion of FVIII. In approximately 30% of patients with severe hemophilia A, inhibitory antibodies targeting FVIII develop. While it is known that the formation of anti-FVIII antibodies is a classical CD4+ T cell-dependent process, molecular determinants contributing to the risk of inhibitor development have not been fully identified. This thesis describes several studies which focus on the immune recognition and processing of FVIII by antigen presenting cells.
FVIII is efficiently internalized by dendritic cells (DCs) and subsequently presented on major histocompatibility complex class II (MHCII) on the cell surface. We describe that FVIII, in complex with immunoglobulins (FVIII-IC), is more efficiently endocytosed by bone marrow derived DCs when compared to FVIII alone. This enhanced uptake is an Fcγ receptor-dependent process.
We also investigated FVIII-derived peptide presentation on human leukocyte antigen (HLA) DR in the presence or absence of its molecular chaperone von Willebrand factor (VWF). These studies revealed that peptide presentation of FVIII was modulated by the presence of VWF. We used the same mass spectrometry based approach to define the repertoire of FVIII-derived peptides presented on HLA-DQ.
Lastly, we demonstrate that while FVIII is efficiently internalized by DCs, VWF remains bound to the cell surface in as yet undefined membrane microdomains.
Altogether the studies described in this thesis contribute to our current understanding of the molecular events underlying the development of an immune response to therapeutic FVIII in patients with hemophilia A.