Gene transfer to the CNS is efficacious in immune-primed mice harboring physiologically relevant titers of anti-AAV antibodies.

Central nervous system (CNS)-directed gene therapy with recombinant adeno-associated virus (AAV) vectors has been used effectively to slow disease course in mouse models of several neurodegenerative diseases. However, these vectors were typically tested in mice without prior exposure to the virus, an immunological scenario unlikely to be duplicated in human patients. Here, we examined the impact of pre-existing immunity on AAV-mediated gene delivery to the CNS of normal and diseased mice. Antibody levels in brain tissue were determined to be 0.6% of the levels found in systemic circulation. As expected, transgene expression in brains of mice with relatively high serum antibody titers was reduced by 59-95%. However, transduction activity was unaffected in mice that harbored more clinically relevant antibody levels. Moreover, we also showed that markers of neuroinflammation (GFAP, Iba1, and CD3) and histopathology (hematoxylin and eosin (H&E)) were not enhanced in immune-primed mice (regardless of pre-existing antibody levels). Importantly, we also demonstrated in a mouse model of Niemann Pick Type A (NPA) disease that pre-existing immunity did not preclude either gene transfer to the CNS or alleviation of disease-associated neuropathology. These findings support the continued development of AAV-based therapies for the treatment of neurological disorders.

Preexisting immunity and low expression in primates highlight translational challenges for liver-directed AAV8-mediated gene therapy.

Liver-directed gene therapy with adeno-associated virus (AAV) vectors effectively treats mouse models of lysosomal storage diseases (LSDs). We asked whether these results were likely to translate to patients. To understand to what extent preexisting anti-AAV8 antibodies could impede AAV8-mediated liver transduction in primates, commonly preexposed to AAV, we quantified the effects of preexisting antibodies on liver transduction and subsequent transgene expression in mouse and nonhuman primate (NHP) models. Using the highest viral dose previously reported in a clinical trial, passive transfer of NHP sera containing relatively low anti-AAV8 titers into mice blocked liver transduction, which could be partially overcome by increasing vector dose tenfold. Based on this and a survey of anti-AAV8 titers in 112 humans, we predict that high-dose systemic gene therapy would successfully transduce liver in >50% of human patients. However, although high-dose AAV8 administration to mice and monkeys with equivalent anti-AAV8 titers led to comparable liver vector copy numbers, the resulting transgene expression in primates was ~1.5-logs lower than mice. This suggests vector fate differs in these species and that strategies focused solely on overcoming preexisting vector-specific antibodies may be insufficient to achieve clinically meaningful expression levels of LSD genes using a liver-directed gene therapy approach in patients.