Macrophage-mediated regulation of joint homeostasis

Osteoarthritis (OA) is the leading cause of musculoskeletal disability in people and horses, and is characterized by progressive joint degeneration. There is a critical need for a better understanding of disease processes leading to OA in order to develop more efficient therapies. A shared feature among different arthritic conditions is chronic synovitis. Macrophages are the main drivers of synovitis and can display pro-inflammatory (M1) or pro-resolving responses (M2). Macrophages promote joint health through phagocytic and secretory activities; however, when these functions are overwhelmed, macrophages upregulate inflammation, recruiting more cells to counteract damage. Once cell recruitment is efficiently accomplished, macrophages coordinate tissue repair and further resolution of inflammation. Bone marrow mononuclear cells (BMNC) are a source of macrophages used to treat inflammation and produce essential molecules for cartilage metabolism; however, little information exists regarding their use in joints. The studies presented in this dissertation focus on understanding the dual role of macrophages in driving and resolving synovitis and how to harness their therapeutic potential.

In the first study, patterns of macrophage phenotypes (M1:M2) in healthy and osteoarthritic equine synovium were compared and correlated with gross pathology, histology, and synovial fluid cytokines. M1 and M2 markers were co-expressed in normal and osteoarthritic joints, varying in intensity of expression according to degree of inflammation. Concentrations of synovial fluid IL-10, a macrophage-produced cytokine that is vital for chondrocyte recovery from injury, was lower in OA joints. The combined findings of this study suggest homeostatic mechanisms from synovial macrophages in OA may be overwhelmed, preventing inflammation resolution.

In the second study we investigated the response of BMNC to normal (SF) and inflamed synovial fluid (ISF). BMNC cultured in autologous SF or ISF developed into macrophage cultures that were more confluent in ISF (~100%) than SF (~25%), and exhibited phenotypes that were ultimately similar to cells native to normal joints. BMNC cultured in SF or ISF were neither M1 nor M2, but exhibited aspects of both phenotypes and a regulatory response, characterized by increasing counts of IL-10+ macrophages, decreasing concentrations of IL-1β, and progressively increasing concentrations of IL-10 and IGF-1, all more marked in ISF. These findings suggest that homeostatic mechanisms were preserved over time, and potentially favored by macrophage proliferation. Our data suggest that BMNC therapy could potentiate the macrophage- and IL-10-associated mechanisms of joint homeostasis lost in OA.

Finally, using an equine model of synovitis, the last study investigated the response of normal and inflamed joints to autologous BMNC injection. Inflamed joints treated with BMNC showed gross and analytical improvements in synovial fluid and synovial membrane, with increasing numbers of regulatory macrophages and synovial fluid concentrations of IL-10, not observed in saline-treated controls. Autologous BMNC are readily available, downregulate synovitis through macrophage-associated effects, and can benefit thousands of patients with OA.

Combined, the results of these studies support the role of macrophage-driven synovial homeostasis and identified a therapeutic way to recover homeostatic mechanisms of synovial macrophages lost during chronic inflammation. Our findings also uncover new research directions and methods for future studies targeting modulation of joint inflammation.