Therapeutic factor VIII does not trigger TLR1.2 and TLR2.6 signalling in vitro

The administration of therapeutic factor VIII (FVIII) to patients with haemophilia A induces the development of inhibitory anti‐FVIII IgG in a substantial number of patients. For an antigen‐specific immune response to develop, antigen‐presenting cells (APCs) need to mature and procure appropriate co‐stimulatory signals to T cells at the time of presentation of the endocytosed antigen. The nature of the danger signals that induce APC maturation, thus initiating the anti‐FVIII immune response, are yet ill‐characterized. Contradictory reports on a direct effect of therapeutic FVIII on APC maturation have been released. Here, we investigated whether FVIII directly triggers Toll‐like receptor 2 (TLR2) signalling. The capacity of human recombinant FVIII to promote the maturation of a mouse bone marrow macrophage cell line (BMA) was investigated by flow cytometry. In parallel, the triggering of TLR1.2 or TLR2.6‐expressing HEK293 cells by FVIII was analysed following transfection of the cells with a reporter construct for NFκB activity. In contrast, to zymosan, a known TLR2 agonist, human recombinant FVIII did not induce the maturation of mouse BMA macrophages, as analysed by the levels of expression of CD80, CD86, CD40 and I‐Ab at the cell surface. Furthermore, incubation of FVIII with cells expressing TLR2 paired with TLR1 or TLR6, failed to activate NFκB, whereas NKκB activity was triggered in the presence of zymosan. Our results confirm that FVIII alone is insufficient to trigger the maturation of APCs that is required to initiate an immune response.