von Willebrand factor: factor VIII protector and friend

New technologies in gene therapy for inducing immune tolerance in hemophilia A

INTRODUCTION

Conventional hemophilia treatment is based on repeated infusion of the missing clotting factor. This therapy is lifelong, expensive and can result in the formation of neutralizing antibodies, thus causing failure of the treatment and requiring higher doses of the replacement drug. Areas covered: Gene and cell therapies offer the advantage of providing a definitive and long-lasting correction of the mutated gene, promoting its physiological expression and preventing neutralizing antibody development. This review focuses on the most recent approaches that have been shown to prevent and even eradicate immune response toward the replaced factor. Expert commentary: Despite the encouraging data demonstrated by ongoing clinical trials and pre-clinical studies, more extensive investigations are necessary to establish the long-term safety and efficacy of gene therapy treatments in maintaining immune tolerance.

Alloantibodies to therapeutic factor VIII in hemophilia A: the role of von Willebrand factor in regulating factor VIII immunogenicity

The rising incidence of neutralizing antibodies (inhibitors) against therapeutic factor VIII prompted the conduct of studies to answer the question as to whether this rise is related to the introduction of recombinant factor VIII products. The present article summarizes current opinions and results of non-clinical and clinical studies on the immunogenic potential of recombinant compared to plasma-derived factor VIII concentrates. Numerous studies provided circumstantial evidence that von Willebrand factor, the natural chaperone protein present in plasma-derived factor VIII products, plays an important role in protecting exogenous factor VIII from uptake by antigen presenting cells and from recognition by immune effectors. However, the definite contribution of von Willebrand factor in reducing the inhibitor risk and in the achievement of immune tolerance is still under debate.

Native plasma-derived FVIII/VWF complex has lower sensitivity to FVIII inhibitors than the combination of isolated FVIII and VWF proteins. Impact on Bethesda assay titration of FVIII inhibitors

Sensitivity to FVIII inhibitors of the native plasma-derived (pd) FVIII/VWF complex vs. the complexes formed after exogenous FVIII infusion in the haemophilic patient has not been thoroughly studied. The role of VWF in the interaction of FVIII with inhibitors was studied in vitro using different combinations of VWF and FVIII concentrates. Normal plasma, pdFVIII/VWF and isolated FVIII (recombinant FVIII, B-domain deleted and pdFVIII) were used. Titre (BU) was kinetically determined (up to 2 h) in serial dilutions of inhibitor IgG (purified from a pool of plasmas with inhibitors) mixed with VWF and then incubated with the different FVIII. Inhibitor was also added to previously mixed VWF+FVIII. Residual FVIII:C was determined. TGA assays were performed with FVIII-deficient plasma spiked with the FVIII-VWF mixtures with/without an ESH-8 antibody. Inhibitor titres for plasma and pdFVIII/VWF were comparable at all time points. Titres for all concentrates of isolated FVIII were significantly higher than those for plasma or pdFVIII/VWF (1.4–1.9 fold) even after preincubation with VWF. At t = 0 h, titres for plasma or pdFVIII/VWF were unquantifiable, but were detectable for isolated FVIII (0.6–1.6 BU). In contrast to pdFVIII/VWF, the decrease in thrombin generation parameters by isolated FVIII in the presence of ESH-8 was significant (P < 0.01) even when previously combined with VWF. In conclusion, VWF protection against FVIII inhibitor activity might be higher with native pdFVIII/VWF complex than with the corresponding compound formed from the isolated proteins. Bethesda assay titration using different FVIII concentrates would be advisable to guide the treatment of inhibitor patients.