Summary/Abstract: Project 2 AATD is among the most common monogenic lung diseases, affecting an estimated 100,000 individuals in the US and causing significant morbidity and mortality. Lung disease in AATD is classically attributed to low levels of AAT in the lung resulting in protease-antiprotease imbalance and typically manifests in the form of emphysema, identified in 100% of NHLBI longitudinal cohort participants with lung tissue available for analysis at the time of death or lung transplant. Studies have found that severely deficient “ZZ” individuals, who inherited two copies of the mutant “Z” allele (as opposed the normal “M” allele), collectively experience lung function decline at an accelerated rate relative to those with “usual” COPD but have likewise demonstrated highly variable rates of lung function decline over time among this population, suggesting that factors in addition to antiprotease levels might contribute to injury. AAT augmentation therapy that normalizes both circulating and epithelial lining fluid AAT levels provides a theoretical solution to “loss-of-function” injury that results from protease-antiprotease imbalance and is the standard of care for “ZZ” individuals with impaired lung function. Despite its ability to restore circulating AAT levels to exceed the therapeutic protective threshold, however, efficacy was surprisingly challenging to establish, with multiple studies unable to conclusively demonstrate significant reduction in FEV1 decline, consistent with the possibility that mechanisms in addition to loss-of-function injury could be contributing. Based on this rationale and supported by provocative preliminary data demonstrating toxicity in human primary AT2s expressing ZAAT, this proposal will test the hypothesis that lung epithelial ZAAT protein production induces gain-of-function toxicity that contributes to lung disease pathogenesis in ZZ AATD. To do so, it will leverage reagents including novel mice engineered to allow conditional expression of ZAAT from specific lineages as well as patient-derived iPSC-type 2 cells to test the effects of ZAAT expression alone or in combination with environmental injury on AT2 function and predisposition to emphysema. It will then examine the effects of modulating specific cellular pathways on the consequences of ZAAT expression in AT2s. Collectively, these experiments have the potential to suggest novel treatment strategies for AATD-associated emphysema.