Project Summary: Symptomatic knee osteoarthritis (OA) affects 14 million adults in the U.S. and is a leading cause of disability. There is growing recognition of distinct phenotypes within OA and the need to tailor therapies to specific phenotypes. Intra-articular inflammation manifesting as synovitis has emerged as an important phenotypic feature associated with pain and structural progression. This proposal centers on understanding the role of synovitis and its topographic and temporal effects on pain as well as its potential as a treatment target. The first aim investigates whether discordance in effusion-synovitis between an individual’s knees portends differential pain reporting between the knees. Longitudinally we will investigate whether changes in synovitis over 24 months are associated with changes in pain. These analyses will include adjustment for both knee and individual level contributors to pain to better ascertain the independent effect of synovitis on pain. The second aim uses ultrasound, a readily available and inexpensive imaging technique, to examine whether the presence of effusion-synovitis in the knee identifies a subgroup that will have a greater reduction in pain after intra-articular steroid injection. Lastly, aim 3 will utilize single-cell RNA sequencing of the synovium in patients with mild, moderate and severe OA to better define cell types and transcriptional profiles active at various OA stages. If, as we suspect, inflammation is more predominant early in the disease process, anti- inflammatory therapies may be most effective in early OA. This funding opportunity and project are specifically geared toward my transitioning to an independent investigator and R01 funding. I will have protected time for this project and a uniquely supportive and rich academic environment with access to world-leading mentors, coursework, and career development series through the Harvard Medical School, Harvard T.H. Chan School of Public Health and Brigham and Women’s Hospital. Specifically, the aims in this proposal will provide experience with advanced hierarchical, clustered and longitudinal statistical modeling, use of ultrasound and magnetic resonance imaging variables in analysis, and RNA sequencing of the synovium. These aims and educational opportunities will give me an in-depth understanding of methods to investigate key facets of inflammation in OA, positioning me to lead future work on the timing and targeting of novel OA therapies.