Inflamed synovial fluid induces a homeostatic response in bone marrow mononuclear cells in vitro: Implications for joint therapy

Abstract

Synovial inflammation is a central feature of osteoarthritis (OA), elicited when local regulatory macrophages (M2‐like) become overwhelmed, activating an inflammatory response (M1‐like). Bone marrow mononuclear cells (BMNC) are a source of naïve macrophages capable of reducing joint inflammation and producing molecules essential for cartilage metabolism. This study investigated the response of BMNC to normal (SF) and inflamed synovial fluid (ISF). Equine BMNC cultured in autologous SF or ISF (n = 8 horses) developed into macrophage‐rich cultures with phenotypes similar to cells native to normal SF and became more confluent in ISF (~100%) than SF (~25%). BMNC cultured in SF or ISF were neither M1‐ nor M2‐like, but exhibited aspects of both phenotypes and a regulatory immune response, characterized by increasing counts of IL‐10+ macrophages, decreasing IL‐1β concentrations and progressively increasing IL‐10 and IGF‐1 concentrations. Changes were more marked in ISF and suggest that homeostatic mechanisms were preserved over time and were potentially favored by progressive cell proliferation. Collectively, our data suggest that intra‐articular BMNC could increase synovial macrophage counts, potentiating the macrophage‐ and IL‐10‐associated mechanisms of joint homeostasis lost during the progression of OA, preserving the production of cytokines involved in tissue repair (PGE2, IL‐10) generally impaired by frequently used corticosteroids.

Comparative analysis of macrophage activation in the synovium of healthy and osteoarthritic equine joints

“…CD86 is constitutively expressed by early myeloid cells and resting macrophages, and increased CD86 expression is part of the cellular checkpoints required for monocytic lineage commitment, activation, and survival (59). Therefore, as observed in the control joints in this study and additional reports (17,25), CD86 is poorly associated with an inflammatory phenotype in vivo.…”

“…markers was mildly increased in OA joints with low-grade inflammation (majority), it was markedly increased in grossly inflamed OA joints, with CD86 most highly expressed. Combined expression of M1-and M2-like macrophage markers (6,23,56) and their increased expression proportionate to inflammation in vivo (9,25) are reported. Although none of the parameters analyzed revealed significant or marginal associations with joint condition, histochemical and histologic findings were consistent with lower concentrations of synovial fluid IL-10 and SDF-1, and higher concentrations of MCP-1 in OA.…”

“…In the chronic inflammation of OA, higher synovial macrophage recruitment and activation, combined with decreased synovial fluid IL-10 and SDF-1 concentrations, as observed in our study, suggests that regulatory functions of macrophages are impaired. As such, macrophagederived pro-resolving cytokines were insufficient, preventing recovery of joint homeostasis (17,18,25,(58)(59)(60)(61)63) The concept of macrophage activation as either inflammatory (M1) or regulatory (M2) originated from monocyte-derived macrophages treated in vitro with defined and overwhelming cytokine stimuli (16,46,64). Clear identification of macrophage phenotypes in vivo is significantly more complex than proposed by in vitro models (6,11,17,27).…”

“…Alternatively activated (M2-like) macrophages are required for efficient chondrogenesis (12). Inflammation in arthritic joints is dampened by M2-like macrophages, improving clinical and histological signs of joint disease (14,24,25). Collectively, these findings suggest that enhancing the M2-like response in diseased joints may provide a mechanism for resolving inflammation and restoring a healthy synovial environment with improved capacity for tissue repair.…”

“…Reports describing comparisons of diseased and healthy joints include extrapolations from other types of arthritides, such as rheumatoid arthritis or are limited to the low numbers of synovial fluid macrophages shedding from the synovium following hyperactivation (6,9,12,23,26,27). Defining patterns of macrophage activation in normal and osteoarthritic synovium is paramount to enhancing the understanding of the roles of macrophages in vivo, and to optimize strategies targeting macrophage-driven joint homeostasis (25). This study was designed to compare the patterns of macrophage activation phenotypes in biopsies of healthy and osteoarthritic equine synovium, and to correlate macrophage phenotypes with gross pathology, histology, and synovial fluid cytokine, chemokine, and growth factor concentrations.…”