High density lipoprotein (HDL) is a complex and heterogeneous macromolecular assembly of multiple proteins and lipids. Defined simply by its buoyant density, its compositional heterogeneity is mirrored by its functional heterogeneity, with cholesterol efflux, anti-inflammatory, anti-apoptotic, anti-thrombotic, microRNA delivery, and innate immune functions just a few of the activities ascribed to the lipoprotein. Apolipoprotein A1 (apoA1) serves as the primary protein scaffolding upon which the lipid cargo-carrying particle is built. Epidemiology studies consistently show an inverse association between circulating HDL cholesterol (HDLc) and apoA1 levels with both coronary artery disease (CAD) and its adverse events, and animal studies using both genetic and direct infusion models similarly show global anti-atherosclerotic functions of the lipoprotein. Recent failures of HDLc elevating clinical intervention studies and Mendelian genetic studies of HDLc, however, underscore both the complexity and incomplete understanding of HDL. We first showed apoA1 is a selective target for oxidative modification in the human artery wall. More recently, using new antibody tools, we showed that the function and HDL particle distribution of apoA1 within the human artery wall are distinct from in the circulation. We have identified multiple high abundance dysfunctional apoA1 forms in the artery wall, and developed tools for both monitoring them in the circulation and studying their functions. Our overall goals are to structurally define site-specific modifications o apoA1 and HDL within human atherosclerotic plaque that adversely impact function, and explore the clinical significance of dysfunctional apoA1/HDL forms in vivo. We will achieve these goals by: 1) testing the hypothesis that specific post translational modifications of apoA1/HDL represent dysfunctional forms of the lipoprotein that influence atherosclerosis; and 2) testing the hypothesis that knowledge of site-specific modifications of apoA1/HDL in vivo may be leveraged for development of clinically relevant diagnostics and therapeutics for CAD. Successful completion of the proposed studies will functionally and clinically define the significance of structurally specific oxidized forms of apoA1/HDL abundant in human atheroma. They also will help develop both novel diagnostic tests and potential therapeutic agents for the treatment of CAD.