Osteoarthritis is the most common cause of performance limiting lameness in horses and affects all professions, from pleasure horses to racing. Racehorses are particularly susceptible to osteoarthritis both during and following their racing careers due to the cyclic stresses applied to their joints. A complex interaction exists between tissues of the diarthrodial joint and the immune system in the development and progression of osteoarthritis. Several types of immune cells, including M1 polarized macrophages, T lymphocytes, and NK cells, increase during the development and progression of osteoarthritis in humans and rodents. Recently similar increases in M1 polarized macrophages in equine osteoarthritis. Lubricin is a key component to normal synovial fluid, and alterations in the glycosylation has been documented in human and equine osteoarthritis. Lubricin has a primary role in boundary lubrication, however anti-inflammatory and chondroprotective properties have also been documented, giving lubricin excellent potential as a therapeutic molecule.

The overarching goal of this proposal is to gain additional knowledge of immune function in the development and progression of osteoarthritis, identify trends in immune populations and lubricin glycosylation, and modulation of the immune response using lubricin or well documented intra-articular treatments. To answer these questions, we will start by utilizing tissues from the metacarpophalangeal joints of horses with and without osteoarthritis. Flow cytometry will be used to identify different immune cell types in articular cartilage, synovial membrane, and synovial fluid. Changes in immune cell type between osteoarthritic and healthy joints, as well as in relation to disease severity will be evaluated. Glycosylation patterns of synovial fluid lubricin will be evaluated using lectin microarray and mass spectrometry (MALDI/TOF-MS), followed by correlation with immune cell distributions. For evaluating the effects of lubricin and common intra-articular therapeutics explants of articular cartilage and synovial membrane will be treated in co-culture with lubricin, triamcinolone, or platelet rich plasma; cultures will then be activated using IL-1β. Treatment effects on macrophage polarization, cytokine production, and articular cartilage degeneration will be evaluated using flow cytometry, NF-κB assays, ELISA analysis of culture supernatant, and cartilage histology.

The long-term objectives are to identify immune cell targets that will provide improved management of osteoarthritis in the horse, particularly in identifying methods that will provide disease-modifying treatment options instead of only symptom-modifying. Lubricin shows promise in early studies as a disease-modifying therapeutic, and evaluation of the anti-inflammatory and chondroprotective effects in an equine model will provide the invaluable evidence needed to determine the suitability of lubricin as an osteoarthritis treatment. Lubricin’s glycosylation profile in healthy and osteoarthritic joints, as well as early versus late osteoarthritis, may provide a synovial fluid biomarker that can identify horses with early joint disease, before significant clinical or radiographic signs may be present, and allow clinicians to institute prompt treatment to prevent irreparable joint damage.