Adeno-associated virus (AAV) is a small, non-pathogenic dependovirus that has shown great potential for safe and long-term expression of a genetic payload in the retina. AAV has been used to treat a growing number of animal models of inherited retinal degeneration, though drawbacks-including a limited carrying capacity, slow onset of expression, and a limited ability to transduce some retinal cell types from the vitreous-restrict the utility of AAV for treating some forms of inherited eye disease. Next generation AAV vectors are being created to address these needs, through rational design efforts such as the creation of self-complementary AAV vectors for faster onset of expression and specific mutations of surface-exposed residues to increase transduction of viral particles. Furthermore, directed evolution has been used to create, through an iterative process of selection, novel variants of AAV with newly acquired, advantageous characteristics. These novel AAV variants have been shown to improve the therapeutic potential of AAV vectors, and further improvements may be achieved through rational design, directed evolution, or a combination of these approaches, leading to broader applicability of AAV and improved treatments for inherited retinal degeneration.

Efficient adeno-associated virus-mediated (AAV-mediated) gene delivery remains a significant obstacle to effective retinal gene therapies. Here, we apply directed evolution - guided by deep sequencing and followed by direct in vivo secondary selection of high-performing vectors with a GFP-barcoded library - to create AAV viral capsids with the capability to deliver genes to the outer retina in primates. A replication-incompetent library, produced via providing rep in trans, was created to mitigate risk of AAV propagation. Six rounds of in vivo selection with this library in primates - involving intravitreal library administration, recovery of genomes from outer retina, and extensive next-generation sequencing of each round - resulted in vectors with redirected tropism to the outer retina and increased gene delivery efficiency to retinal cells. These viral vectors expand the toolbox of vectors available for primate retina, and they may enable less invasive delivery of therapeutic genes to patients, potentially offering retina-wide infection at a similar dosage to vectors currently in clinical use.