Osteoarthritis is the most common cause of performance limiting lameness in horses and affects individuals across all disciplines. A complex interaction exists between synovial joint tissues and the immune system in osteoarthritis, with many immune cells contributing to the pathogenesis of disease. Human and rodent studies dominate the literature, with little research on immune cells in equine osteoarthritis. Macrophage polarization, specifically shifts from M2 anti-inflammatory to M1 pro-inflammatory phenotypes, have been consistently documented in experimental and naturally occurring osteoarthritis. Equine intra-articular therapeutics are purported to function via a wide range of mechanisms of action, but often with minimal knowledge of their effects on joint immunology. In addition to several commercially available intraarticular therapeutics (corticosteroid, polyacrylamide hydrogel, and hyaluronic acid), we will evaluate the effects of a novel therapeutic—recombinant equine lubricin, on macrophage immunomodulation. Recombinant lubricin has shown boundary lubricating and anti-inflammatory effects in rodent models, with promise as a new potential therapeutic target for equine OA.

Our research goal is 2-fold: (1) to identify the immune cell populations present in healthy and osteoarthritic equine joints, and (2) to evaluate the effects of intra-articular therapeutics on macrophage polarization. Synovial fluid and synovial membrane tissues will be obtained from horses undergoing arthroscopy for osteochondral fragmentation of carpal and fetlock joints, followed by flow cytometry to identify the immune cell distributions. The effects of triamcinolone acetonide, polyacrylamide hydrogel, hyaluronic acid, and recombinant lubricin on cultured macrophages will be used to assess macrophage polarization, protein secretion, and foreign body responses in an in vitro inflammatory model system. Secreted cytokines/chemokines, cell surface markers, and nitric oxide metabolism will provide insight into macrophage phenotype shifts. A more complete understanding of local immune responses in joint disease, and how it can be manipulated through therapeutics, is key in improving the management of osteoarthritis in equine athletes.

Specific Aim 1: To compare immune cell distributions and immunophenotypes in synovial fluid and synovial membrane from control (healthy) joints and joints with early OA (eOA) and late OA (lOA). Specific Aim 1 Hypothesis: Carpal and fetlock joints with OA will display a skew from M2 to M1 polarized macrophages, in addition to altered distributions of T cells and NK cells.

Specific Aim 2: To determine the effects of intra-articular joint medications (triamcinolone, polyacrylamide hydrogel, hyaluronic acid, and recombinant equine lubricin) on macrophage polarization and microscopic morphology in response to IL-1β. Specific Aim 2 Hypothesis: PBMC derived macrophages will preferentially M1 polarize in response to IL-1β, which will be inhibited by equine lubricin and promoted by polyacrylamide hydrogel.