PROJECT SUMMARY Tissue engineering provides a strategy for developing better biomaterials for use in congenital heart surgery. Results of our clinical trials evaluating the use of tissue engineered vascular grafts (TEVGs) in congenital heart surgery have demonstrated the growth capacity of the TEVG making it the first man made graft with growth potential. However, results of these trials have also revealed that stenosis is the most common graft-related complication and the principle hurdle preventing its widespread clinical use. Recently, we have identified a novel immune-regulatory protein encoded by the LYST gene. Mutations of the LYST gene dramatically reduce the incidence of TEVG stenosis in murine models. In this proposal, we will investigate the cellular and molecular mechanisms underlying the formation of LYST-mediated TEVG stenosis. We will use a on-demand inducible LYST-mutant mouse to determine the critical temporal factors underling this process. Next, we will use a conditional LYST-mutant model to elucidate the roles of macrophages and determine the critical cell type(s) responsible for driving LYST-mediated TEVG stenosis. Finally, we will evaluate the role of extracellular vesicle- dependent intercellular signaling on the formation of LYST-mediated TEVG stenosis. Successful completion of these studies would open the door to rationally designing strategies to inhibit the formation of TEVG stenosis based on modulating LYST function. The development of an improved TEVG with growth capacity has the potential to improve outcomes for children born with congenital heart disease.