AAV2 capsid clearance and neuronal trafficking dynamics in the central nervous system.

Adeno-associated virus serotype 2 (AAV2) remains one of the most common vectors for CNS gene delivery. Yet, its long-term intraparenchymal trafficking and the relationship between capsid persistence and transgene expression remain poorly understood. In this study, we tracked the spatial and temporal patterns of AAV2 following direct striatal infusion in rats, analyzing tissue at 3 days, 3 weeks, 12 weeks, 30 weeks, and 67 weeks after injection. Using immunofluorescence for the human aromatic L-amino acid decarboxylase (AADC) transgene and an epitope detecting intact AAV2 capsids (A20), we mapped capsid localization, clearance, and axonal transport over more than a year. Following direct striatal infusion, AAV2 capsids rapidly localized to striatal neurons, with early accumulation in the substantia nigra pars reticulata (SNpr) detectable by 3 days post-infusion, but without evidence of nigral neuron transduction. Transgene expression increased over time and peaked locally at 12 weeks, with a delayed yet robust AADC signal in striatonigral terminals by 30 weeks. By 67 weeks, capsid signal was minimal while AADC expression remained stable. These findings clarify the long-term dynamics of AAV2 distribution, capsid persistence, and axonal transport in the adult brain, informing our understanding of vector behavior and durability following intraparenchymal AAV2 gene delivery.