Precise manipulation of site and stoichiometry of capsid modification enables optimization of functional adeno-associated virus conjugates

The ability to engineer adeno-associated virus (AAV) vectors for targeted infection of specific cell types is critically important to fully harness its potential of human gene therapy. A promising approach to achieve this objective involves chemically attaching retargeting ligands onto the virus capsid. Site-specific incorporation of a bioorthogonal noncanonical amino acid (ncAA) into the AAV capsid proteins provides a particularly attractive strategy to introduce such modifications with exquisite precision. In this study, we show that using ncAA mutagenesis, it is possible to systematically alter the attachment site of a retargeting ligand (cyclic-RGD) on the AAV capsid to create diverse conjugate architectures, and that the site of attachment heavily impacts the retargeting efficiency. We further demonstrate that the performance of these AAV conjugates is highly sensitive to the stoichiometry of capsid labeling (labels per capsid), with an intermediate labeling density (∼12 per capsid) providing optimal activity. Finally, we developed technology to precisely control the number of attachment sites per AAV capsid, by selectively incorporating a ncAA into the minor capsid proteins with high fidelity and efficiency, such that AAV-conjugates with varying stoichiometry can be synthesized in a homogeneous manner. Together, this platform provides unparalleled control over site and stoichiometry of capsid modification, which will enable the development of next-generation AAV vectors tailored with desirable attributes.