Sustained Phenotypic Correction of Murine Hemophilia A with Pre-Existing Anti-FVIII Immunity Using Lentivirus-Mediated Platelet-Specific FVIII Gene Transfer.

Abstract 29

The development of inhibitory antibodies to exogenous factor VIII (FVIII) is considered a severe and important complication of FVIII infusion in hemophilia A patients. Gene therapy of hemophilia A with inhibitors is especially challenging because functional FVIII activity may be inactivated by circulating inhibitory antibodies if transgene protein is constitutively secreted into the blood circulation. Our previous studies have demonstrated that syngeneic transplantation of hematopoietic stem cells from 2bF8 transgenic mice that express platelet-specific FVIII can efficiently restore hemostasis to hemophilic mice with pre-existing inhibitory antibodies. In the current study, we assessed whether lentivirus-mediated 2bF8 gene transfer could efficiently introduce 2bF8 transgene expression and ameliorate the hemorrhagic phenotype in hemophilic mice with pre-existing immunity. To mimic the clinical situation of an autologous transplant in an inhibitor patient, both donor and recipient FVIII^null mice were immunized with recombinant human B-domain deleted FVIII to induce inhibitory antibody development. Platelet-derived FVIII expression in FVIII^null mice was introduced by 2bF8 lentiviral-mediated bone marrow (BM) transduction and syngeneic transplantation. Following BM reconstitution, mice were analyzed by PCR, quantitative real-time PCR, platelet lysate FVIII activity assay, and inhibitor assay. Phenotypic correction was assessed by tail clip survival test and electrolytic-induced thrombus formation. Expression of the 2bF8 product was detected in all recipients that received 2bF8 lentivirus transduced BM cells, indicating viable engraftment of BM genetically modified with the 2bF8 lentivirus transfer vector. Functional platelet-FVIII activity levels in the transduced mice with pre-existing immunity ranged from 0.36 to 6.18 mU/10⁸ platelets (mean 1.56 ± 1.76 mU/10⁸ platelets, n = 10), which was not significantly different from the levels obtained from a parallel non-inhibitor model (1.46 ± 0.87 mU/10⁸ platelets, n = 4). Real-time PCR demonstrated that there was an average of 0.17 ± 0.05 LV DNA copies per cell in peripheral white blood cells from transduced mice. FVIII inhibiter titer gradually declined with the time, indicating that transduced platelet FVIII is well protected from exposure to the immune system, avoiding activation of a memory response. The tail clip survival test showed that 90% of mice survived tail clip challenge. The electrolytic injury model demonstrated that hemostasis was improved in recipients that received 2bF8 lentivirus-transduced BM cells. Furthermore, BM transferred from the primary transplant recipients into immunized FVIII^null secondary recipients demonstrated sustained platelet-FVIII expression, resulting in the correction of the hemophilia A phenotype with pre-existing immunity. This shows that gene transfer has occurred within long-term repopulating hematopoietic stem cells even in the presence of inhibitory antibodies. These results demonstrate that lentivirus-mediated bone marrow transduction/transplantation can provide sustained improvement of hemostasis in hemophilic mice with pre-existing immunity, indicating that this approach may be a promising strategy for gene therapy of hemophilia A with inhibitory antibodies in humans.