Immune responses to AAV in canine muscle monitored by cellular assays and noninvasive imaging.

We previously demonstrated that direct intramuscular injection of rAAV2 or rAAV6 in wild-type dogs resulted in robust T-cell responses to viral capsid proteins, and others have shown that cellular immunity to adeno-associated virus (AAV) capsid proteins coincided with liver toxicity and elimination of transgene expression in a human trial of hemophilia B. Here, we show that the heparin-binding ability of a given AAV serotype does not determine the induction of T-cell responses following intramuscular injection in dogs, and identify multiple epitopes in the AAV capsid protein that are recognized by T cells elicited by AAV injection. We also demonstrate that noninvasive magnetic resonance imaging (MRI) can accurately detect local inflammatory responses following intramuscular rAAV injection in dogs. These studies suggest that pseudotyping rAAV vectors to remove heparin-binding activity will not be sufficient to abrogate immunogenicity, and validate the utility of enzyme-linked immunosorbent spot (ELISpot) assay and MRI for monitoring immune and inflammatory responses following intramuscular injection of rAAV vectors in preclinical studies in dogs. These assays should be incorporated into future human clinical trials of AAV gene therapy to monitor immune responses.