T Helper 17–Like Regulatory T Cells in Equine Synovial Fluid Are Associated With Disease Severity of Naturally Occurring Posttraumatic Osteoarthritis

Background: Infiltration of cluster of differentiation (CD) 3 + (CD3 + ) T cells into the synovium and synovial fluid occurs in most patients with posttraumatic osteoarthritis. During disease progression, proinflammatory T helper 17 cells and anti-inflammatory regulatory T cells infiltrate the joint in response to inflammation. This study aimed to characterize the dynamics of regulatory T and T helper 17 cell populations in synovial fluid from equine clinical patients with posttraumatic osteoarthritis to determine whether phenotype and function are associated with potential immunotherapeutic targets. Hypothesis: An imbalance of the ratio of regulatory T cells and T helper 17 cells would be associated with disease progression in posttraumatic osteoarthritis, suggesting opportunities for immunomodulatory therapy. Study Design: Descriptive laboratory study. Methods: Synovial fluid was aspirated from the joints of equine clinical patients undergoing arthroscopic surgery for posttraumatic osteoarthritis resulting from intra-articular fragmentation. Joints were classified as having mild or moderate posttraumatic osteoarthritis. Synovial fluid was also obtained from nonoperated horses with normal cartilage. Peripheral blood was obtained from horses with normal cartilage and those with mild and moderate posttraumatic osteoarthritis. Synovial fluid and peripheral blood cells were analyzed by flow cytometry, and native synovial fluid was analyzed by enzyme-linked immunosorbent assay. Results: CD3 + T cells represented 81% of lymphocytes in synovial fluid, which increased in animals with moderate posttraumatic osteoarthritis to 88.3% ( P = .02). CD14 + macrophages were doubled in those with moderate posttraumatic osteoarthritis compared with mild posttraumatic osteoarthritis and controls ( P < .001). Less than 5% of CD3 + T cells found within the joint were forkhead box P3 protein + (Foxp3 + ) regulatory T cells, but a 4- to 8-times higher percentage of nonoperated and mild posttraumatic osteoarthritis joint regulatory T cells secreted interleukin (IL)–10 than peripheral blood Tregs ( P < .005). T regulatory-1 cells that secreted IL-10 but did not express Foxp3 accounted for approximately 5% of CD3 + T cells in all joints. T helper 17 cells and Th17-like regulatory T cells were increased in those with moderate posttraumatic osteoarthritis ( P < .0001) compared with mild and nonoperated patients. IL-10, IL-17A, IL-6, chemokine (C-C motif) ligand (CCL) 2 (CCL2), and CCL5 concentrations detected by enzyme-linked immunosorbent assay in synovial fluid were not different between groups. Conclusions: An imbalance of the ratio of regulatory T cells and T helper 17 cells and an increase in T helper 17 cell–like regulatory T cells in synovial fluid from joints with more severe disease provide novel insights into immunological mechanisms that are associated with posttraumatic osteoarthritis progression and pathogenesis. Clinical Relevance: Early and targeted use of immunotherapeutics in the mitigation of posttraumatic osteoarthritis may improve patient clinical outcomes.