The national blueprint for future basic and translational research to understand factor VIII immunogenicity: NHLBI State of the Science Workshop on factor VIII inhibitors

Introduction Inhibitor formation against coagulation factor VIII (FVIII) is an unresolved serious problem in replacement therapy for the X‐linked bleeding disorder haemophilia A. Although FVIII inhibitors have been extensively studied, much of the basic mechanism of this immune response remains to be uncovered. Aim Within the NHLBI State of the Science Workshop on Factor VIII Inhibitors, Working Group 3 identified three scientific priorities for basic and translational research on FVIII inhibitor formation. Methods A larger list of potential areas of research was initially developed as a basis for subsequent prioritization. Each scientific goal was further evaluated based on required effort, potential impact, approach, methods, technologies and models. Results The three priorities include the following: activation signals and immune regulation that shape the response to FVIII (including innate immunity, microbiome, adaptive immunity and regulatory T cell studies in humans); utility of animal models and non‐animal approaches (in silico, genetic, single‐cell/sorted population “omics,” in vitro) to help predict inhibitor formation and identify novel therapeutics; and impact of the source of FVIII, its structure and von Willebrand factor on immunogenicity and tolerance. Conclusions Early interactions between FVIII and the immune system, biomarker development and studies in different patient groups (previously treated or untreated, with or without inhibitor formation, patients undergoing immune tolerance induction or gene therapy) deserve particular emphasis. Finally, linking basic to clinical studies, development of a repository for biospecimens and opportunities for interdisciplinary research training are important components to solving the urgent problem of inhibitor formation

Characterization and Solution Structure of the Factor VIII C2 Domain in a Ternary Complex with Classical and Non-classical Inhibitor Antibodies

The most significant complication for patients with severe cases of congenital or acquired hemophilia A is the development of inhibitor antibodies against coagulation factor VIII (fVIII). The C2 domain of fVIII is a significant antigenic target of anti-fVIII antibodies. Here, we have utilized small angle x-ray scattering (SAXS) and biochemical techniques to characterize interactions between two different classes of anti-C2 domain inhibitor antibodies and the isolated C2 domain. Multiple assays indicated that antibodies 3E6 and G99 bind independently to the fVIII C2 domain and can form a stable ternary complex. SAXS-derived numerical estimates of dimensional parameters for all studied complexes agree with the proportions of the constituent proteins. Ab initio modeling of the SAXS data results in a long kinked structure of the ternary complex, showing an angle centered at the C2 domain of ∼130°. Guided by biochemical data, rigid body modeling of subunits into the molecular envelope of the ternary complex suggests that antibody 3E6 recognizes a C2 domain epitope consisting of the Arg2209–Ser2216 and Leu2178–Asp2187 loops. In contrast, antibody G99 recognizes the C2 domain primarily through the Pro2221–Trp2229 loop. These two epitopes are on opposing sides of the fVIII C2 domain, are consistent with the solvent accessibility in the context of the entire fVIII molecule, and provide further structural detail regarding the pathogenic immune response to fVIII

B cell–activating factor modulates the factor VIII immune response in hemophilia A

Inhibitors of factor VIII (FVIII) remain the most challenging complication of FVIII protein replacement therapy in hemophilia A (HA). Understanding the mechanisms that guide FVIII-specific B cell development could help identify therapeutic targets. The B cell–activating factor (BAFF) cytokine family is a key regulator of B cell differentiation in normal homeostasis and immune disorders. Thus, we used patient samples and mouse models to investigate the potential role of BAFF in modulating FVIII inhibitors. BAFF levels were elevated in pediatric and adult HA inhibitor patients and decreased to levels similar to those of noninhibitor controls after successful immune tolerance induction (ITI). Moreover, elevations in BAFF levels were seen in patients who failed to achieve FVIII tolerance with anti-CD20 antibody–mediated B cell depletion. In naive HA mice, prophylactic anti-BAFF antibody therapy prior to FVIII immunization prevented inhibitor formation and this tolerance was maintained despite FVIII exposure after immune reconstitution. In preimmunized HA mice, combination therapy with anti-CD20 and anti-BAFF antibodies dramatically reduced FVIII inhibitors via inhibition of FVIII-specific plasma cells. Our data suggest that BAFF may regulate the generation and maintenance of FVIII inhibitors and/or anti-FVIII B cells. Finally, anti-CD20/anti-BAFF combination therapy may be clinically useful for ITI

A microengineered vascularized bleeding model that integrates the principal components of hemostasis

Hemostasis encompasses an ensemble of interactions among platelets, coagulation factors, blood cells, endothelium, and hemodynamic forces, but current assays assess only isolated aspects of this complex process. Accordingly, here we develop a comprehensive in vitro mechanical injury bleeding model comprising an “endothelialized” microfluidic system coupled with a microengineered pneumatic valve that induces a vascular “injury”. With perfusion of whole blood, hemostatic plug formation is visualized and “in vitro bleeding time” is measured. We investigate the interaction of different components of hemostasis, gaining insight into several unresolved hematologic issues. Specifically, we visualize and quantitatively demonstrate: the effect of anti-platelet agent on clot contraction and hemostatic plug formation, that von Willebrand factor is essential for hemostasis at high shear, that hemophilia A blood confers unstable hemostatic plug formation and altered fibrin architecture, and the importance of endothelial phosphatidylserine in hemostasis. These results establish the versatility and clinical utility of our microfluidic bleeding model

Emerging benefits of Fc fusion technology in the context of recombinant factor VIII replacement therapy

International audienceAlthough the primary reason for recombinant factor VIII Fc fusion protein (rFVIIIFc) development was to reduce treatment burden associated with routine prophylaxis, new evidence suggests additional benefits of Fc fusion technology in the treatment of people with haemophilia A. Preclinical research has been utilized to characterize the potential immunomodulatory properties of rFVIIIFc, including an ability to reduce inflammation and induce tolerance to factor VIII. This has since been expanded into clinical research in immune tolerance induction (ITI) with rFVIIIFc, results of which suggest the potential for rapid tolerization in first-time ITI patients and therapeutic benefit in patients undergoing rescue ITI. The potential for improved joint health through the anti-inflammatory properties of rFVIIIFc has also been suggested. In addition, a new avenue of research into the role of rFVIIIFc in promoting bone health in patients with haemophilia A, potentially through reduced osteoclast formation, has yielded encouraging results that support further study. This review summarizes the existing preclinical and clinical studies of immunomodulation and tolerization with rFVIIIFc, as well as studies in joint and bone health, to elucidate the potential benefits of rFVIIIFc in haemophilia A beyond the extension of factor VIII half-life

Potentiation of thrombin generation in hemophilia A plasma by coagulation factor VIII and characterization of antibody-specific inhibition.

Development of inhibitory antibodies to coagulation factor VIII (fVIII) is the primary obstacle to the treatment of hemophilia A in the developed world. This adverse reaction occurs in 20-30% of persons with severe hemophilia A treated with fVIII-replacement products and is characterized by the development of a humoral and neutralizing immune response to fVIII. Patients with inhibitory anti-fVIII antibodies are treated with bypassing agents including recombinant factor VIIa (rfVIIa). However, some patients display poor hemostatic response to bypass therapy and improved treatment options are needed. Recently, we demonstrated that fVIII inhibitors display widely variable kinetics of inhibition that correlate with their respective target epitopes. Thus, it was hypothesized that for antibodies that display slow rates of inhibition, supplementation of rfVIIa with fVIII would result in improved thrombin generation and be predictive of clinical responses to this novel treatment regimen. In order to test this hypothesis, 10 murine monoclonal antibodies (MAbs) with non-overlapping epitopes spanning fVIII, differential inhibition titers, and inhibition kinetics were studied using a thrombin generation assay. Of the 3 MAbs with high inhibitory titers, only the one with fast and complete (classically defined as "type I") kinetics displayed significant inhibition of thrombin generation with no improvement upon supplementation of rfVIIa with fVIII. The other two MAbs that displayed incomplete (classically defined as "type II") inhibition did not suppress the potentiation of thrombin generation by fVIII. All antibodies that did not completely inhibit fVIII activity demonstrated potentiation of thrombin generation by the addition of fVIII as compared to rfVIIa alone. In conclusion, fVIII alone or in combination with rfVIIa corrects the thrombin generation defect produced by the majority of anti-fVIII MAbs better than single agent rfVIIa. Therefore, combined fVIII/rfVIIa therapy may provide better hemostatic control than current therapy in some patients with anti-fVIII inhibitors

Characteristics of anti-fVIII MAbs.

<p>Characteristics of anti-fVIII MAbs.</p

TGA parameters for anti-fVIII MAbs.

<p>ETP, peak thrombin and index velocity are presented as ratios compared to fVIII deficient plasma supplemented with 1 U/ml fVIII in the absence of any anti-fVIII MAb.</p><p>ETP – endogenous thrombin potential.</p

Influence of inhibitory titer on residual fVIII activity and peak thrombin generation.

<p>The inhibitor titer for each MAb was compared with the peak thrombin generation of fVIII alone or fVIII + rfVIIa at the immediate (solid circles, r² = 0.36) or 1 hr (open circles, r² = 0.09) time points (A.) and with residual fVIII activity at the immediate (solid circles, r² = 0.004) or 1 hr (r² = 0.07) time points (B.).</p