| The Adeno-associated virus (AAV) is a small ssDNA virus that can package and deliver non-genomic DNA. Its low toxicity and relative lack of pathology, along with broad tropisms, has made it popular as a vector for gene delivery trials. However, despite the critical role of antibodies in the success of gene therapy with AAV vectors, the nature of the antigenic structure of the AAV capsids is not well understood. Here we defined the binding of monoclonal antibodies against AAV serotypes 1, 2, 5, 6, and 8 using cryo-electron microscopy and three-dimensional image reconstructions. Docking of capsid structures with fragment antibody models showed that the footprints on these viruses all fell on or around the three-fold protrusions of the capsid regardless of the serotype examined. Significant overlap between the binding sites led to the conclusion that this is a common region of antigenicity among the AAV capsid surfaces. In preliminary studies, mutagenesis of the proposed sites on the AAV1 capsid showed that certain surface loop changes resulted in viruses that were able to evade previous recognition from parental antibodies. These data support the hypothesis that the three-fold axes are antigenically important and that mutations in this region may avert a pre-existing immune response. Further characterization is necessary to better design escape mutants and fully understand if these mutants are still viable in their original tissue tropisms and transduction efficiencies. |
