The synovial membrane is a complex, highly cellular, and metabolically active tissue. A diverse array of fibroblast-type and immune cells contribute to synovial fluid production, nutrient exchange, and local immune responses. The role of the synovial membrane, and particularly the immune cell population, has been recognized in the development of osteoarthritis (OA) in recent years. Several types of immune cells, including macrophages and T-cells, appear to play key roles in the development and progression of OA. The knowledge regarding immune cell populations within equine joints with OA is lacking, with very few studies delving into the cell types present in health and disease. Characterizing the immune cell populations present within equine joints in healthy and OA-affected joints is a key step towards understanding the role of these cells in the development of disease and where therapeutics are acting. The overarching goal of this study is to define the immune cell populations present in naturally occurring equine OA and the response to intra-articular therapeutics.