Treatment of haemophilia A by infusions of the clotting factor VIII (FVIII) results in the development of inhibitors/anti-drug antibodies in up to 25 % of patients. Mechanisms leading to immunogenicity of FVIII products are not yet fully understood. Amongst other factors, danger signals as elicited upon infection or surgery have been proposed to play a role. In the present study, we focused on effects of danger signals on maturation and activation of dendritic cells (DC) in the context of FVIII application. Human monocyte-derived DC were treated with FVIII alone, with a danger signal alone or a combination of both. By testing more than 60 different healthy donors, we show that FVIII and the bacterial danger signal lipopolysaccharide synergise in increasing DC activation, as characterised by increased expression of co-stimulatory molecules and secretion of pro-inflammatory cytokines. The degree and frequency of this synergistic activation correlate with CD86 expression levels on immature DC prior to stimulation. In our assay system, plasma-derived but not recombinant FVIII products activate human DC in a danger signal-dependent manner. Further tested danger signals, such as R848 also induced DC activation in combination with FVIII, albeit not in every tested donor. In our hands, human DC but not human B cells or macrophages could be activated by FVIII in a danger signal-dependent manner. Our results suggest that immunogenicity of FVIII is a result of multiple factors including the presence of danger, predisposition of the patient, and the choice of a FVIII product for treatment.

The most severe side effect in haemophilia A (HA) treatment is the development of anti-factor VIII antibodies, also called inhibitors. Why inhibitors develop in a proportion of treated HA patients and how this can be prevented remains largely unanswered. Among numerous theories, the presence of immunological danger signals, associated with events such as surgery or infection, has been proposed to play a role. In this study, we demonstrate that human dendritic cells (DC) synergistically activated by a combination of factor VIII (FVIII) concentrate plus the bacterial danger signal lipopolysaccharide (LPS) induce a significantly stronger activation of autologous CD4 T cells than DC pretreated with FVIII or LPS alone. The observed T cell activation is dependent on antigen processing, presentation on MHC class II molecules and costimulation via CD86. Of note, FVIII plus LPS pretreated DC predominantly induce the activation of memory T cells and a minor proportion of naive T cells. Collectively, our data support a model in which immunological danger signals plus FVIII concentrates synergistically increase human CD4 T cell responses to FVIII protein.

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The most serious complication in hemophilia A (HA) treatment is the development of factor (F)VIII inhibitors or anti-drug antibodies (ADA) occurring in 25-35% of patients with severe HA. The immunological mechanisms underlying the development of ADA against FVIII products have not been completely understood yet. Immunological danger signals associated with events such as infection or surgery have been suggested to play a critical role. In previous studies, we have demonstrated that plasma-derived (pd)FVIII but not recombinant (r)FVIII can activate human monocyte-derived dendritic cells (DC) in a danger signaldependent manner, which subsequently mediate the proliferation of autologous CD4+ T cells. In this study, we have investigated the ability of plasma components, naturally present in pdFVIII products, to mediate T cell responses. In fact, we show that addition of plasma to rFVIII plus lipopolysaccharide (LPS)-stimulated DC induces proliferation of autologous CD4+ T cells. Interestingly, although DC pulsed with LPS plus plasma induce T cell proliferation upon co-culture, the addition of FVIII significantly increased the number of proliferating as well as FVIII-specific CD4+ T cells. Total proliferating CD4+ T cells and FVIII-specific subsets were identified mainly as central memory T cells. Experiments using blocking antibodies and receptor antagonists revealed that the complement proteins C3a and, to a lesser extent, C5a are critically involved in these LPS-mediated T cell responses. Collectively, our results indicate that complement proteins are potent drivers of T cell responses towards FVIII. Data presented provide a model how event-related substitution of FVIII in HA patients might contribute to inhibitor development.