Lentivirus‐mediated platelet gene therapy of murine hemophilia A with pre‐existing anti‐factor VIII immunity

Summary.  Background:  The development of inhibitory antibodies, referred to as inhibitors, against exogenous factor VIII in a significant subset of patients with hemophilia A remains a persistent challenge to the efficacy of protein replacement therapy. Our previous studies using the transgenic approach provided proof‐of‐principle that platelet‐specific expression could be successful in treating hemophilia A in the presence of inhibitory antibodies. Objective:  To investigate a clinically translatable approach for platelet gene therapy of hemophilia A with pre‐existing inhibitors. Methods:  Platelet FVIII expression in preimmunized FVIII null mice was introduced by transplantation of lentivirus‐transduced bone marrow or enriched hematopoietic stem cells. FVIII expression was determined with a chromogenic assay. The transgene copy number per cell was quantitated with real‐time PCR. Inhibitor titer was measured with the Bethesda assay. Phenotypic correction was assessed by the tail clipping assay and an electrolytically induced venous injury model. Integration sites were analyzed with linear amplification‐mediated PCR. Results:  Therapeutic levels of platelet FVIII expression were sustained in the long term without evoking an anti‐FVIII memory response in the transduced preimmunized recipients. The tail clip survival test and the electrolytic injury model confirmed that hemostasis was improved in the treated animals. Sequential bone marrow transplants showed sustained platelet FVIII expression resulting in phenotypic correction in preimmunized secondary and tertiary recipients. Conclusions:  Lentivirus‐mediated platelet‐specific gene transfer improves hemostasis in mice with hemophilia A with pre‐existing inhibitors, indicating that this approach may be a promising strategy for gene therapy of hemophilia A even in the high‐risk setting of pre‐existing inhibitory antibodies.