Enhanced uptake of blood coagulation factor VIII containing immune complexes by antigen presenting cells

Background: A major complication in the treatment of hemophilia A is the development of inhibitory antibodies targeting coagulation factor VIII (FVIII). Eradication of these inhibitors can be established by immune tolerance induction (ITI), which consists of daily administration of high dosages of FVIII. FVIII immune complexes (FVIII-IC) could be formed following FVIII infusion in patients with pre-existing anti-FVIII antibodies. Objectives: Here we studied endocytosis of FVIII-IC by bone marrow-derived dendritic cells (BMDCs). Methods: BMDCs were pulsed with FVIII/FVIII-IC and uptake was assessed by flow cytometry and confocal imaging. Results: BMDCs were able to efficiently internalize FVIII-IC in a dose-dependent manner, 3-4-fold more efficiently when compared with equimolar concentrations of non-complexed FVIII. Uptake of FVIII-IC, but not FVIII alone, could be inhibited with anti-Fcc receptor (FccR) antibody 2.4G2, indicating functional involvement of FccR. No internalization of FVIIIIC was observed in BMDCs lacking FccRI, FccRIIb, FccRIII and FccRIV. Genetic ablation of FccRIIb, FccRIII or FccRIV individually did not affect the ability of anti-FVIII IgG to promote the uptake of FVIII. BMDCs lacking FccRI showed lower FVIII-IC uptake levels when compared with other single FccR null BMDCs. Expression of the inhibitory FccRIIb alone was sufficient to internalize FVIII-IC more efficiently than FVIII. Conclusions: FccR are critical in the internalization of FVIII-IC by BMDCs and multiple FccR can contribute independently to this process. Our findings provide a basis for future studies to address whether the outcome of ITI is dependent on the interplay between FVIII-IC and inhibitory and activating FccR