Restoration of normoglycemia in diabetic models via insulin gene therapy

Steady glycaemic control is crucial for T1D patients, however it remains difficult to achieve with exogenous insulin therapy. An interesting and unexplored therapeutic option is the use of genetically modified adeno-associated virus (AAV) vectors that have proved successful in the cure of monogenic disorders such as haemophilia B and have shown negligible toxicity and immunogenicity. In the present study, we have investigated ways of restoring a base level of insulin production in various diabetic mouse models via administration of liver tropic AAV2/8 vector containing a codon-optimised human insulin gene. We achieved restoration of euglycemia in chemically induced diabetic C57BL/6 mice and spontaneously diabetic Non Obese Diabetic (NOD) mice with the former showing no immunological barriers to efficacy of insulin gene therapy and enjoying long lasting correction of hyperglycemia up to 250 days. Euglycemia was also restored in spontaneously diabetic NOD mice, although these mice required a 7-10 fold higher dose of vector to achieve similar efficacy as the C57BL/6 mice and the immunodeficient NOD^scid mice. We detected CD8⁺ T cell reactivity to insulin and mild inflammatory infiltration in the livers of gene therapy recipient NOD mice, neither of which were observed in the treated C57BL/6 mice. Efficacy of the gene therapy in NOD mice was partially improved by targeting the immune system with anti-CD4 antibody treatment, while transfer of NOD mouse AAV2/8-reactive serum to NOD^scid recipients prevented successful restoration of euglycemia in AAV8/hInsulin treated NOD^scid mice. Our data indicate that both immune cells and antibodies form a barrier to successful restoration of euglycemia in autoimmune diabetic recipient mice with insulin gene therapy, but that this barrier can be overcome by increasing the dose of vector and by suppressing immune responses.