Alternative strategies for gene therapy of hemophilia

Donor antigen-primed regulatory T cells permit liver regeneration and phenotype correction in hemophilia A mouse by allogeneic bone marrow stem cells

INTRODUCTION

Cell replacement therapy may be considered as an alternate approach to provide therapeutic dose of plasma factor VIII (FVIII) in patients with hemophilia A (HA). However, immune rejection limits the use of allogeneic cells in this mode of therapy. Here, we have examined the role of donor major histocompatibility complex (MHC)-stimulated host CD4(+)CD25(+) regulatory T (Treg) cells in suppressing immune responses against allogeneic uncommitted (Lin(-)) bone marrow cells (BMCs) for correction of bleeding disorder in HA mice.

METHODS

Allogeneic donor Lin(-) BMCs were co-transplanted with allo-antigen sensitized Treg cells in HA mice having acetaminophen-induced acute liver injury. Plasma FVIII activity was determined by in vitro functional assay, and correction of bleeding phenotype was assessed on the basis of capillary blood clotting time and tail-clip challenge. The immunosuppression potential of the sensitized Treg cells on CD4(+) T cells was studied both in vitro and in vivo. Suppression of inflammatory reactions in the liver against the homed donor cells by sensitized Treg cells was analysed by histopathological scoring. Allo-specificity of sensitized Treg cells and long-term retention of immunosuppression were examined against a third-party donor and by secondary challenge of allogeneic donor cells, respectively. The engraftment and phenotype change of donor BMCs in the liver and their role in synthesis of FVIII and liver regeneration were also determined.

RESULTS

Co-transplantation of allogeneic Lin(-) BMCs with sensitized Treg cells led to systemic immune modulation and suppression of inflammatory reactions in the liver, allowing better engraftment of allogeneic cells in the liver. Allo-antigen priming led to allo-specific immune suppression even after 1 year of transplantation. Donor-derived endothelial cells expressed FVIII in HA mice, leading to the correction of bleeding phenotype. Donor-derived hepatocyte-like cells, which constitute the major fraction of engrafted cells, supported regeneration of the liver after acute injury.

CONCLUSIONS

A highly proficient FVIII secreting core system can be created in regenerating liver by transplanting allogeneic Lin(-) BMCs in HA mice where transplantation tolerance against donor antigens can be induced by in vitro allo-antigen primed Treg cells. This strategy can be beneficial in treatment of genetic liver disorders for achieving prophylactic levels of the missing proteins.

Adenoviral Vectors for Hemophilia Gene Therapy

Hemophilia is an inherited blood clotting disorder resulting from deficiency of blood coagulation factors. Current standard of care for hemophilia patients is frequent intravenous infusions of the missing coagulation factor. Gene therapy for hemophilia involves the introduction of a normal copy of the deficient coagulation factor gene thereby potentially offering a definitive cure for the bleeding disorder. A variety of approaches have been pursued for hemophilia gene therapy and this review article focuses on those that use adenoviral vectors.

The gene therapy journey for hemophilia: are we there yet?

Since the isolation and characterization of the genes for FVIII and FIX some 30 years ago, a longstanding goal of the field has been development of successful gene therapy for the hemophilias. In a landmark study published in 2011, Nathwani et al demonstrated successful conversion of severe hemophilia B to mild or moderate disease in 6 adult males who underwent intravenous infusion of an adeno-associated viral (AAV) vector expressing factor IX. These 6 subjects have now exhibited expression of FIX at levels ranging from 1% to 6% of normal for periods of > 2 years. This review discusses obstacles that were overcome to reach this goal and the next steps in clinical investigation. Safety issues that will need to be addressed before more widespread use of this approach are discussed. Efforts to extend AAV-mediated gene therapy to hemophilia A, and alternate approaches that may be useful for persons with severe liver disease, who may not be candidates for gene transfer to liver, are also discussed.

The gene therapy journey for hemophilia: are we there yet?

Since the isolation and characterization of the genes for FVIII and FIX some 30 years ago, a longstanding goal of the field has been development of successful gene therapy for the hemophilias. In a landmark study published in 2011, Nathwani et al demonstrated successful conversion of severe hemophilia B to mild or moderate disease in 6 adult males who underwent intravenous infusion of an adeno-associated viral (AAV) vector expressing factor IX. These 6 subjects have now exhibited expression of FIX at levels ranging from 1% to 6% of normal for periods of > 2 years. This review discusses obstacles that were overcome to reach this goal and the next steps in clinical investigation. Safety issues that will need to be addressed before more widespread use of this approach are discussed. Efforts to extend AAV-mediated gene therapy to hemophilia A, and alternate approaches that may be useful for persons with severe liver disease, who may not be candidates for gene transfer to liver, are also discussed.

Modern haemophilia care

Haemophilia care has undergone substantial improvements during the past 40-50 years. Early clotting factor concentrates were not sufficiently refined to enable self-administered treatment at home until the 1970s. Unfortunately, these advances led to transmission of viral diseases including HIV and hepatitis, resulting in an increased burden of morbidity and mortality, especially during the 1980s. Throughout the past two decades, product development, including the advent of recombinant concentrates, has greatly improved the safety and availability of therapy and the focus of care is shifting towards prevention and management of disease sequelae. Long-term substitution therapy (prophylaxis) of the missing clotting factor is the recommended treatment in severe haemophilia, but several research issues remain to be elucidated such as when to start and how to optimise these regimens, and when or whether to stop this expensive treatment. The major side-effect of treatment, development of inhibitors to the infused concentrate, is the main threat to the health of patients and consequently the goal of intense research. Development of new products with improved pharmacokinetics is the next step to improved therapy.