High expression reduces an antibody response after neonatal gene therapy with B domain‐deleted human factor VIII in mice

Summary.  Background:  Gene therapy could prevent bleeding in patients with hemophilia A, but might induce antibodies that block factor VIII (FVIII) function. Objectives:  To test the efficacy of gene therapy in the newborn period for preventing a response to human FVIII (hFVIII) because of immaturity of the immune system. Methods:  Varying doses of a retroviral vector (RV) expressing a B domain‐deleted hFVIII cDNA were injected i.v. into newborn hemophilia A C57BL/6 or normal C3H mice. Mice were evaluated for hFVIII expression, hemostasis, and development of anti‐hFVIII antibodies with inhibitory activity. Results and conclusions:  Injection of a high RV dose [10 10  transducing units (TU) kg −1 ] into newborn hemophilia A or C3H mice resulted in 61% and 13% of normal hFVIII antigen in plasma, respectively; most mice did not produce anti‐hFVIII antibodies, and hemophilia A mice did not bleed. Furthermore, most mice with >20 ng mL −1 of hFVIII in plasma (10% normal, 1 × 10 −10   m ) were tolerant to hFVIII, as an antibody response was markedly reduced after challenge with hFVIII with or without adjuvant. However, most RV‐treated animals with lower antigen levels developed antibodies before or after challenge. Thus, initiation of a gene therapy trial with low RV doses might increase inhibitor formation. Furthermore, frequent hFVIII infusions in newborns with hemophilia A might reduce inhibitor formation. Finally, difficulties in achieving tolerance after gene therapy for hemophilia A as compared to hemophilia B may relate to lower expression of FVIII than FIX, as high antigen levels are most effective at inducing tolerance.