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

Long-term correction of hemorrhagic diathesis in hemophilia A mice by an AAV-delivered hybrid FVIII composed of the human heavy chain and the rat light chain

Conventional therapies for hemophilia A (HA) are prophylactic or on-demand intravenous FVIII infusions. However, they are expensive and inconvenient to perform. Thus, better strategies for HA treatment must be developed. In this study, a recombinant FVIII cDNA encoding a human/rat hybrid FVIII with an enhanced procoagulant potential for adeno-associated virus (AAV)-delivered gene therapy was developed. Plasmids containing human FVIII heavy chain (hHC), human light chain (hLC), and rat light chain (rLC) were transfected into cells and hydrodynamically injected into HA mice. Purified AAV viruses were intravenously injected into HA mice at two doses. Results showed that the hHC + rLC protein had a higher activity than the hHC + hLC protein at comparable expression levels. The specific activity of hHC + rLC was about 4- to 8-fold higher than that of their counterparts. Hydrodynamic injection experiments obtained consistent results. Notably, the HA mice undergoing the AAV-delivered hHC + rLC treatment exhibited a visibly higher activity than those treated with hHC + hLC, and the therapeutic effects lasted for up to 40 weeks. In conclusion, the application of the hybrid FVIII (hHC + rLC) via an AAV-delivered gene therapy substantially improved the hemorrhagic diathesis of the HA mice. These data might be of help to the development of optimized FVIII expression cassette for HA gene therapy.

Minimal Essential Human Factor VIII Alterations Enhance Secretion and Gene Therapy Efficiency

One important limitation for achieving therapeutic expression of human factor VIII (FVIII) in hemophilia A gene therapy is inefficient secretion of the FVIII protein. Substitution of five amino acids in the A1 domain of human FVIII with the corresponding porcine FVIII residues generated a secretion-enhanced human FVIII variant termed B-domain-deleted (BDD)-FVIII-X5 that resulted in 8-fold higher FVIII activity levels in the supernatant of an in vitro cell-based assay system than seen with unmodified human BDD-FVIII. Analysis of purified recombinant BDD-FVIII-X5 and BDD-FVIII revealed similar specific activities for both proteins, indicating that the effect of the X5 alteration is confined to increased FVIII secretion. Intravenous delivery in FVIII-deficient mice of liver-targeted adeno-associated virus (AAV) vectors designed to express BDD-FVIII-X5 or BDD-FVIII achieved substantially higher plasma FVIII activity levels for BDD-FVIII-X5, even when highly efficient codon-optimized F8 nucleotide sequences were employed. A comprehensive immunogenicity assessment using in vitro stimulation assays and various in vivo preclinical models of hemophilia A demonstrated that the BDD-FVIII-X5 variant does not exhibit an increased immunogenicity risk compared to BDD-FVIII. In conclusion, BDD-FVIII-X5 is an effective FVIII variant molecule that can be further developed for use in gene- and protein-based therapeutics for patients with hemophilia A.

Modulation of the liver immune microenvironment by the adeno-associated virus serotype 8 gene therapy vector

Adeno-associated viruses (AAVs) are emerging as one of the vehicles of choice for gene therapy. However, the potential immunogenicity of these vectors is a major limitation of their use, leading to the necessity of a better understanding of how viral vectors engage the innate immune system. In this study, we demonstrate the immune response mediated by an AAV vector in a mouse model. Mice were infected intravenously with 4 × 10¹² copies (cp)/kg of AAV8, and the ensuing immune response was analyzed using intravital microscopy during a period of weeks. Administration of AAV8 resulted in the infection of hepatocytes, and this infection led to a moderate, but significant, activation of the immune system in the liver. This host immune response involved platelet aggregation, neutrophil extracellular trap (NET) formation, and the recruitment of monocytes, B cells, and T cells. The resident liver macrophage population, Kupffer cells, was necessary to initiate this immune response, as its depletion abrogated platelet aggregation and NET formation and delayed the recruitment of immune cells. Moreover, the death of liver cells produced by this AAV was moderate and failed to result in a robust, sustained inflammatory response. Altogether, these data suggest that AAV8 is a suitable vector for gene therapy approaches.