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Macrophages are considered to contribute to chronic inflammatory diseases such as rheumatoid arthritis 1 . However, both the exact origin and the role of macrophages in inflammatory joint disease remain unclear. Here we use fate-mapping approaches in conjunction with three-dimensional light-sheet fluorescence microscopy and single-cell RNA sequencing to perform a comprehensive spatiotemporal analysis of the composition, origin and differentiation of subsets of macrophages within healthy and inflamed joints, and study the roles of these macrophages during arthritis. We find that dynamic membrane-like structures, consisting of a distinct population of CX 3 CR1 + tissue-resident macrophages, form an internal immunological barrier at the synovial lining and physically seclude the joint. These barrier-forming macrophages display features that are otherwise typical of epithelial cells, and maintain their numbers through a pool of locally proliferating CX 3 CR1 − mononuclear cells that are embedded into the synovial tissue. Unlike recruited monocyte-derived macrophages, which actively contribute to joint inflammation, these epithelial-like CX 3 CR1 + lining macrophages restrict the inflammatory reaction by providing a tight-junction-mediated shield for intra-articular structures. Our data reveal an unexpected functional diversification among synovial macrophages and have important implications for the general role of macrophages in health and disease. Analysis of macrophage subsets within joints reveals a population of CX 3 CR1 + tissue-resident macrophages that form a tight-junction-mediated barrier at the synovial lining, protecting the joint from the invasion of inflammatory cells.

Rheumatoid arthritis (RA) is driven by hyperplastic fibroblast‐like synoviocytes (FLS) that adopt a persistent inflammatory and invasive phenotype, often resistant to conventional therapies. Here, we investigated whether cold gas plasma exposure modulates inflammatory FLS (iFLS) in vitro. Primary murine FLS were primed with TNF‐α to induce an inflammatory phenotype and exposed to an argon plasma jet for 30–150 s; argon gas alone served as a control. Gas plasma treatment caused a rapid, dose‐dependent rise in intracellular reactive oxygen species and free thiols, followed by partial resolution by 24 h. This oxidative burst coincided with mitochondrial dysfunction and progressive loss of cell viability. Surviving iFLS exhibited markedly reduced inflammatory features, such as surface levels of ICAM‐1 (CD54), VCAM‐1 (CD106), and Thy‐1 (CD90.2), along with diminished production of IL‐6 and CCL2 at higher doses. Reflecting the impaired expression of inflammation‐associated adhesion molecules. Furthermore, plasma treatment significantly delayed iFLS migration. Together, these findings suggest that cold gas plasma has cytotoxic effects on a subset of iFLS induced by acute oxidative stress, as well as reprogramming surviving iFLS towards a less aggressive phenotype. These in vitro data establish a foundation for further research in synovial explants and arthritis models, as well as for systematic safety testing in non‐inflammatory joint‐resident cells.

AbstractFibroblast-like synoviocytes (FLSs) are important non-immune cells located mostly in the inner layer of the synovium. Indeed, these cells are specialized mesenchymal cells, implicated in collagen homeostasis of the articular joint and provide extracellular matrix (ECM) materials for cartilage and contribute to joint destruction via multiple mechanisms. RA FLS interactions with immune and non-immune cells lead to the development and organization of tertiary structures such as ectopic lymphoid-like structures (ELSs), tertiary lymphoid organs (TLOs), and secretion of proinflammatory cytokines. The interaction of RA FLS cells with immune and non-immune cells leads to stimulation and activation of effector immune cells. Pathological role of RA FLS cells has been reported for many years, while molecular and cellular mechanisms are not completely understood yet. In this review, we tried to summarize the latest findings about the role of FLS cells in ELS formation, joint destruction, interactions with immune and non-immune cells, as well as potential therapeutic options in rheumatoid arthritis (RA) treatment. Our study revealed data about interactions between RA FLS and immune/non-immune cells as well as the role of RA FLS cells in joint damage, ELS formation, and neoangiogenesis, which provide useful information for developing new approaches for RA treatment.

The severity of osteoarthritis (OA) and cartilage degeneration is highly associated with synovial inflammation. Although recent investigations have revealed a dysregulated crosstalk between fibroblast‐like synoviocytes (FLSs) and macrophages in the pathogenesis of synovitis, limited knowledge is available regarding the involvement of exosomes. Here, increased exosome secretion is observed in FLSs from OA patients. Notably, internalization of inflammatory FLS‐derived exosomes (inf‐exo) can enhance the M1 polarization of macrophages, which further induces an OA‐like phenotype in co‐cultured chondrocytes. Intra‐articular injection of inf‐exo induces synovitis and exacerbates OA progression in murine models. In addition, it is demonstrated that inf‐exo stimulation triggers the activation of glycolysis. Inhibition of glycolysis using 2‐DG successfully attenuates excessive M1 polarization triggered by inf‐exo. Mechanistically, HIF1A is identified as the determinant transcription factor, inhibition of which, both pharmacologically or genetically, relieves macrophage inflammation triggered by inf‐exo‐induced hyperglycolysis. Furthermore, in vivo administration of an HIF1A inhibitor alleviates experimental OA. The results provide novel insights into the involvement of FLS‐derived exosomes in OA pathogenesis, suggesting that inf‐exo‐induced macrophage dysfunction represents an attractive target for OA therapy. Exosome secretion from fibroblast‐like synoviocytes (FLSs) is elevated in synovium during osteoarthritis (OA). The exosomes from inflammatory FLSs lead to inflammation activation and M1 polarization in macrophages through HIF1A‐mediated hyperglycolysis, which further exert detrimental effects on chondrocytes and exacerbate OA. Inhibition of HIF1A can alleviate such macrophage dysfunction triggered by exosomes and attenuate experimental OA.

Osteoarthritis (OA), the most ubiquitous degenerative disease affecting the entire joint, is characterized by cartilage degradation and synovial inflammation. Although the pathogenesis of OA remains poorly understood, synovial inflammation is known to play an important role in OA development. However, studies on OA pathophysiology have focused more on cartilage degeneration and osteophytes, rather than on the inflamed and thickened synovium. Fibroblast‐like synoviocytes (FLS) produce a series of pro‐inflammatory regulators, such as inflammatory cytokines, nitric oxide (NO) and prostaglandin E2 (PGE2). These regulators are positively associated with the clinical symptoms of OA, such as inflammatory pain, joint swelling and disease development. A better understanding of the inflammatory immune response in OA‐FLS could provide a novel approach to comprehensive treatment strategies for OA. Here, we have summarized recently published literatures referring to epigenetic modifications, activated signalling pathways and inflammation‐associated factors that are involved in OA‐FLS‐mediated inflammation. In addition, the current related clinical trials and future perspectives were also summarized.

Recent evidence has suggested that synovial inflammation and macrophage polarization were involved in the pathogenesis of osteoarthritis (OA). Additionally, high-molecular-weight hyaluronic acid (HMW-HA) was often used clinically to treat OA. GRP78, an endoplasmic reticulum (ER) stress chaperone, was suggested to contribute to the hyperplasia of synovial cells in OA. However, it was still unclear whether HMW-HA affected macrophage polarization through GRP78. Therefore, we aimed to identify the effect of HMW-HA in primary synovial cells and macrophage polarization and to investigate the role of GRP78 signaling. We used IL-1β to treat primary synoviocytes to mimic OA, and then treated them with HMW-HA. We also collected conditioned medium (CM) to culture THP-1 macrophages and examine the changes in the phenotype. IL-1β increased the expression of GRP78, NF-κB (p65 phosphorylation), IL-6, and PGE2 in primary synoviocytes, accompanied by an increased macrophage M1/M2 polarization. GRP78 knockdown significantly reversed the expression of IL-1β-induced GRP78-related downstream molecules and macrophage polarization. HMW-HA with GRP78 knockdown had additive effects in an IL-1β culture. Finally, the synovial fluid from OA patients revealed significantly decreased IL-6 and PGE2 levels after the HMW-HA treatment. Our study elucidated a new form of signal transduction for HMW-HA-mediated protection against synovial inflammation and macrophage polarization and highlighted the involvement of the GRP78-NF-κB signaling pathway.

Introduction Anti-malarial drug artesunate can suppress inflammation and prevent cartilage and bone destruction in collagen-induced arthritis model in rats—suggesting it may be a potent drug for rheumatoid arthritis (RA) therapy. We aimed to investigate its effect on the invasive property of fibroblast-like synoviocytes (FLS) from patients with RA. Methods Synovial tissues were obtained by closed needle biopsy from active RA patients, and FLS were isolated and cultured in vitro. RA-FLS were treated with artesunate at various concentrations, while methotrexate or hydroxychloroquine was employed as comparator drugs. Cell viability, proliferation, cell cycle, apoptosis, migration, invasion, and pseudopodium formation of RA-FLS were assessed by CCK-8 assays, EdU staining, Annexin V-FITC/PI staining, transwell assays, or F-actin staining, respectively. Further, relative changes of expressed proteases were analyzed by Proteome profiler human protease array and verified by quantitative real-time PCR (qPCR), Western blot, and ELISA. The expression of signaling molecules of MAPK, NF-κB, AP-1, and PI3K/Akt pathways were measured by qPCR and Western blot. PDK-1 knockdown by specific inhibitor AR-12 or siRNA transfection was used to verify the pharmacological mechanism of artesunate on RA-FLS. Results Artesunate significantly inhibited the migration and invasion of RA-FLS in a dose-dependent manner with or without TNF-α stimulation. The effect was mediated through artesunate inhibition of MMP-2 and MMP-9 production, and pre-treatment with exogenous MMP-9 reversed the inhibitory effect of artesunate on RA-FLS invasion. Artesunate had a stronger inhibitory effect on migration and invasion of RA-FLS as well as greater anti-inflammatory effect than those of hydroxychloroquine. Similar inhibitory effect was detected between artesunate and methotrexate, and synergy was observed when combined. Mechanistically, artesunate significantly inhibited PDK-1 expression as well as Akt and RSK2 phosphorylation—in a similar manner to PDK-1-specific inhibitor AR-12 or PDK-1 knockdown by siRNA transfection. This inhibition results in suppression of RA-FLS migration and invasion as well as decreased MMP-2 and MMP-9 expression. Conclusions Our study demonstrates artesunate is capable of inhibiting migration and invasion of RA-FLS through suppression of PDK1-induced activation of Akt and RSK2 phosphorylation—suggesting that artesunate may be a potential disease-modifying anti-rheumatic drug for RA.

Rheumatoid arthritis (RA) is a systemic autoimmune disease characterized by chronic inflammation of the synovial membrane, leading to cartilage destruction and bone erosion. Due to the complex pathogenesis of RA and the limitations of current therapies, increasing research attention has been directed towards novel strategies targeting fibroblast-like synoviocytes (FLS), which are key cellular components of the hyperplastic pannus. Recent studies have highlighted the pivotal role of FLS in the initiation and progression of RA, driven by their tumour-like transformation and the secretion of pro-inflammatory mediators, including cytokines, chemokines and matrix metalloproteinases. The aggressive phenotype of RA-FLS is marked by excessive proliferation, resistance to apoptosis, and enhanced migratory and invasive capacities. Consequently, FLS-targeted therapies represent a promising avenue for the development of next-generation RA treatments. The efficacy of such strategies – particularly those aimed at modulating FLS signalling pathways – has been demonstrated in both preclinical and clinical settings, underscoring their therapeutic potential. This review provides an updated overview of the pathogenic mechanisms and functional roles of FLS in RA, with a focus on critical signalling pathways under investigation, including Janus kinase/signal transducer and activator of transcription (JAK/STAT), mitogen-activated protein kinase (MAPK), nuclear factor kappa B (NF-κB), Notch and interleukin-1 receptor-associated kinase 4 (IRAK4). In addition, we discuss the emerging understanding of FLS-subset-specific contributions to immunometabolism and explore how computational biology is shaping novel targeted therapeutic strategies. A deeper understanding of the molecular and functional heterogeneity of FLS may pave the way for more effective and precise therapeutic interventions in RA.

Mitochondrial fission and fusion are important for mitochondrial function, and dynamin 1‐like protein (DNM1L) is a key regulator of mitochondrial fission. We investigated the effect of mitochondrial fission on mitochondrial function and inflammation in fibroblast‐like synoviocytes (FLSs) during rheumatoid arthritis (RA). DNM1L expression was determined in synovial tissues (STs) from RA and non‐RA patients. FLSs were isolated from STs and treated with a DNM1L inhibitor (mdivi‐1, mitochondrial division inhibitor 1) or transfected with DNM1L‐specific siRNA. Mitochondrial morphology, DNM1L expression, cell viability, mitochondrial membrane potential, reactive oxygen species (ROS), apoptosis, inflammatory cytokine expression and autophagy were examined. The impact of mdivi‐1 treatment on development and severity of collagen‐induced arthritis (CIA) was determined in mice. Up‐regulated DNM1L expression was associated with reduced mitochondrial length in STs from patients with RA and increased RA severity. Inhibition of DNM1L in FLSs triggered mitochondrial depolarization, mitochondrial elongation, decreased cell viability, production of ROS, IL‐8 and COX‐2, and increased apoptosis. DNM1L deficiency inhibited IL‐1β–mediated AKT/IKK activation, NF‐κBp65 nuclear translocation and LC3B‐related autophagy, but enhanced NFKBIA expression. Treatment of CIA mice with mdivi‐1 decreased disease severity by modulating inflammatory cytokine and ROS production. Our major results are that up‐regulated DNM1L and mitochondrial fission promoted survival, LC3B‐related autophagy and ROS production in FLSs, factors that lead to inflammation by regulating AKT/IKK/NFKBIA/NF‐κB signalling. Thus, inhibition of DNM1L may be a new strategy for treatment of RA.

We elucidated the molecular cross-talk between cartilage and synovium in osteoarthritis, the most widespread arthritis in the world, using the powerful tool of single-cell RNA-sequencing. Multiple cell types were identified based on profiling of 10,640 synoviocytes and 26,192 chondrocytes: 12 distinct synovial cell types and 7 distinct articular chondrocyte phenotypes from matched tissues. Intact cartilage was enriched for homeostatic and hypertrophic chondrocytes, while damaged cartilage was enriched for prefibro- and fibro-, regulatory, reparative and prehypertrophic chondrocytes. A total of 61 cytokines and growth factors were predicted to regulate the 7 chondrocyte cell phenotypes. Based on production by > 1% of cells, 55% of the cytokines were produced by synovial cells (39% exclusive to synoviocytes and not expressed by chondrocytes) and their presence in osteoarthritic synovial fluid confirmed. The synoviocytes producing IL-1beta (a classic pathogenic cytokine in osteoarthritis), mainly inflammatory macrophages and dendritic cells, were characterized by co-expression of surface proteins corresponding to HLA-DQA1 , HLA-DQA2 , OLR1 or TLR2 . Strategies to deplete these pathogenic intra-articular cell subpopulations could be a therapeutic option for human osteoarthritis.