Macrophage-mediated regulation of joint homeostasis
Carvalho Menarim, Bruno
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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.
General Audience Abstract
Osteoarthritis (OA) is a common cause of joint deterioration in people and horses. Current treatments provide limited recovery of joint function, creating an urgent need for more efficient therapies; however, development of new treatments requires better understanding of the mechanism causing OA. A shared characteristic among many arthritic conditions is long-standing inflammation. Cells called macrophages are the main drivers of joint inflammation and can exert pro- and anti-inflammatory effects. Macrophages promote joint health by clearing aggressor agents and secreting molecules required for optimal joint function. However, when these housekeeping functions are overwhelmed by damage, macrophages drive inflammation recruiting more cells to cope with increased demands for repair. If this process is efficiently accomplished, macrophages then resolve inflammation, recovering joint health. Macrophages in the bone marrow (BMNC - bone marrow mononuclear cells) are used to treat inflammation in several tissues and are known to produce molecules essential for joint health. Although little information exists regarding their use in joints, studies treating different organs suggest it can provide high rewards. The studies presented in this dissertation focused on understanding the dual function of macrophages in driving and controlling joint inflammation, and harnessed their therapeutic potential. In the first study, macrophages were investigated in normal and OA-affected joints, and curiously exhibited a hybrid pro- and anti-inflammatory identity in both groups. The indicators of this mixed identity were more markedly expressed in arthritic joints showing gross inflammation. Low levels of a macrophage-derived anti-inflammatory protein called IL-10 were detected in OA joints. The results of this study suggest that anti-inflammatory mechanisms from macrophages may be overwhelmed in OA-affected joints, preventing inflammation to be resolved, and that recovering this anti-inflammatory function may aid in the treatment of OA. In the second study we investigated how the incubation of BMNC in fluid from normal and inflamed joints affects the response of macrophages. Similar to what we observed in the first study, BMNC incubated in both normal and inflamed joint fluid induced macrophages to develop a hybrid identity that was ultimately similar to native cells from normal joints. Macrophages proliferated more when incubated in fluid from inflamed joints. Macrophages in both groups produced anti-inflammatory effects with high levels of IL-10 that were highest in ISF cultures. These observations suggest that higher proliferation of macrophages in inflamed joint fluid helped preserve anti-inflammatory mechanisms. Therefore, our study suggests that joint injection with BMNC could maximize macrophage- and IL-10-associated mechanisms required to resolve joint inflammation. The third and final study investigated the response of normal and inflamed joints to BMNC injection using a model of joint inflammation in horses. Inflamed joints treated with BMNC showed visual and laboratorial markers of improvement, with increasing numbers of macrophages and concentrations of IL-10 in the joint fluid, which remained lower in joints treated with placebo. BMNC provide means to recover macrophage-associated effects required to control joint inflammation and can benefit thousands of patients with OA. Together, the results of these studies show that macrophages are biased promoters of joint health, leading to inflammation when their anti-inflammatory mechanisms are overwhelmed. Replenishing inflamed joints with healthy macrophages maximizes their anti-inflammatory effects, favoring the recovery of a healthy articular environment.
- Doctoral Dissertations