Project 3 - Project Summary: Liver is arguably the most permissive tissue for recombinant adeno-associated virus (rAAV)-mediated in vivo gene delivery, and liver transduction can be achieved by a simple systemic rAAV administration. However, liver-directed rAAV gene therapy for alpha-1-antitrypsin (AAT) deficiency (AATD) has met multiple challenges, demanding more efficient gene delivery and expression especially in the presence of pre-existing anti-AAV immunity. To address these challenges, one potential solution is to develop new AAV capsids that allow for more efficient gene delivery and/or transgene expression in the liver. We have recently employed single- molecule, real-time (SMRT) sequencing to discover novel full-length cap sequences from natural AAV proviral libraries from the human population to identify novel AAV variants with desirable tissue tropisms and vector properties. In this project, we will employ a high-throughput Illumina barcode sequencing approach to analyze a large collection of new AAV capsids identified from NHP and human tissues. The new capsid variants identified from this project collectively represent a valuable toolbox for liver-directed rAAV gene delivery to multiple species with favorable immunological profiles, and therefore are expected to have broad utilization in gene therapy applications. This project contains four specific aims designed to develop liver- tropic AAV capsids from our primate-derived library to support preclinical AATD gene therapy studies and clinical translation: Aim 1, To screen for mouse liver-targeting AAV capsid variants in wild-type (WT) mice; Aim 2, To screen for ferret liver-targeting AAV capsid variants in WT ferrets; Aim 3, To screen for human liver-targeting, NAB-escaping AAV capsid variants in humanized mice; and Aim 4, Structural-functional studies of new capsid variants. There is considerable interactivity between this project and the other three projects.