Immunosuppression Modulates Immune Responses to AAV Capsid in Human Subjects Undergoing Intramuscular Gene Transfer for Lipoprotein Lipase Deficiency

Abstract Administration of adeno-associated viral vectors (AAV) has resulted in long-term therapeutic gene transfer in multiple large animal models of disease, but attempts to translate systemic administration of AAV to humans have been limited in some cases by an immune response to the vector capsid (Nature Med12:342–7, 2006; Nature Med13:419–422, 2007). To overcome this obstacle, we have proposed that a short course of immunosuppression (IS) be administered with vector injection. Here we report the safety and efficacy results of this maneuver in a trial of AAV-1 administered to skeletal muscle. Lipoprotein lipase (LPL) deficiency is a familial disorder in which insufficient levels of LPL enzyme result in the accumulation of triglycerides in plasma. In a clinical study to correct this disorder, an AAV-1 vector encoding the therapeutic transgene LPL was administered to the skeletal muscle of affected individuals. Eight subjects were assigned to two dose cohorts, receiving 1×1011 genome copies (gc)/kg or 3×1011gc/kg. In this study, one subject receiving the high vector dose experienced a transient, asymptomatic increase in the muscle enzyme creatinine phosphokinase beginning 4 weeks after gene transfer, persisting for several weeks. This was associated with capsid-specific CD4+ and CD8+ T cell activation detectable by IFN-γ ELISPOT and intracellular cytokine staining on PBMC. In total, a T cell response to the AAV capsid, but not to the LPL transgene, was detectable in 4/8 subjects. In some of these subjects, T cell responses were detectable in peripheral blood up to 2 years after gene transfer. To prevent potentially harmful immune responses directed to the AAV capsid, a follow up study in LPL deficient subjects was initiated in which a 12-week regimen of mycophenolate mofetil and cyclosporine A was administered orally starting at the time of AAV-1 intramuscular gene transfer. Two additional subjects were administered AAV-1-LPL in the absence of immunosuppression, to compare the safety and efficacy of two different vector production methods. Overall, IS was well tolerated and no adverse events were reported. At a dose of 3×1011 gc/kg, IS effectively blocked T cell responses to capsid, which were undetectable by IFN-γ ELISPOT in 4/4 subjects, even after IS was discontinued. However, at a dose of 1×1012gc/kg, a delayed IFN-γ response to capsid antigen was observed in 3/5 subjects. In two subjects the T cell response was still detectable after IS was discontinued. T cell responses did not correlate with pre-existing antibody titers in any of the subjects, as positivity for antibodies against the AAV capsid was not predictive of ELISPOT results. Antibody analysis revealed that IS did not have any effect on the development of antibodies against AAV-1 capsid, as all subjects developed humoral immunity against capsid, with predominance of IgG1 antibody subclass. None of the subjects receiving IS developed humoral or cellular immunity to the LPL transgene product. In conclusion, the use of IS in the context of AAV-1 gene transfer for LPL deficiency is safe and at least partially effective in blocking T cell responses directed to the capsid antigen. Ongoing long-term evaluation of transgene expression in these subjects will allow further assessment of the effects of IS on efficacy of gene transfer.

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