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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.

Fibroblast-like synoviocytes (FLSs) play a central role in RA pathogenesis and are the main cellular component in the inflamed synovium of patients with rheumatoid arthritis (RA). FLSs are emerging as promising new therapeutic targets in RA. However, fibroblasts perform many essential functions that are required for sustaining tissue homeostasis. Direct targeting of general fibroblast markers on FLSs is challenging because fibroblasts in other tissues might be altered and side effects such as reduced wound healing or fibrosis can occur. To date, no FLS-specific targeted therapies have been applied in the clinical management of RA. With the help of high-throughput technologies such as scRNA-seq in recent years, several specific pathogenic FLS subsets in RA have been identified. Understanding the characteristics of these pathogenic FLS clusters and the mechanisms that drive their differentiation can provide new insights into the development of novel FLS-targeting strategies for RA. Here, we discuss the pathogenic FLS subsets in RA that have been elucidated in recent years and potential strategies for targeting pathogenic FLSs.

Background Endoplasmic reticulum (ER) stress has proinflammatory properties, and transgenic animal studies of rheumatoid arthritis (RA) indicate its relevance in the process of joint destruction. Because currently available studies are focused primarily on myeloid cells, we assessed how ER stress might affect the inflammatory responses of stromal cells in RA. Methods ER stress was induced in RA fibroblast-like synoviocytes (FLS), dermal fibroblasts, and macrophages with thapsigargin or tunicamycin alone or in combination with Toll-like receptor (TLR) ligands, and gene expression and messenger RNA (mRNA) stability was measured by quantitative polymerase chain reaction. Cellular viability was measured using cell death enzyme-linked immunosorbent assays and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assays, and signaling pathway activation was analyzed by immunoblotting. Results No cytotoxicity was observed in FLS exposed to thapsigargin, despite significant induction of ER stress markers. Screening of 84 proinflammatory genes revealed minor changes in their expression (fold change 90th percentile range 2.8–8.3) by thapsigargin alone, but the vast majority were hyperinduced during combined stimulation with thapsigargin and TLR ligands (35% greater than fivefold vs lipopolysaccharide alone). The synergistic response could not be explained by quantitative effects on nuclear factor-κB and mitogen-activated protein kinase pathways alone, but it was dependent on increased mRNA stability. mRNA stabilization was similarly enhanced by ER stress in dermal fibroblasts but not in macrophages, correlating with minimal cooperative effects on gene induction in macrophages. Conclusions RA FLS are resistant to apoptosis induced by ER stress, but ER stress potentiates their activation by multiple TLR ligands. Interfering with downstream signaling pathway components of ER stress may be of therapeutic potential in the treatment of RA.

Background The majority of CD4 + T helper (Th) cells found in the synovial fluid (SF) of patients with rheumatoid arthritis (RA) express CXCR3, a receptor associated with Th1 cells. In blood, subsets of Th2 and Th17 cells also express CXCR3, but it is unknown if these cells are present in RA SF or how cytokines from these subsets affect cytokine/chemokine secretion by fibroblast-like synoviocytes (FLS) from patients with RA. Methods We examined the proportions of Th1, Th2, CXCR3 + Th2, Th17, CXCR3 + Th17, Th1Th17, peripheral T helper (TPh) and T follicular helper (TFh) cells in paired SF and blood, as well as the phenotype of TPh and TFh cells in RA SF ( n  = 8), by the use of flow cytometry. We also examined the cytokine/chemokine profile in paired SF and plasma ( n  = 8) and in culture supernatants of FLS from patients with chronic RA ( n  = 7) stimulated with Th-associated cytokines, by the use of cytometric bead arrays and ELISA. Cytokine receptor expression in FLS ( n  = 3) were assessed by the use of RNA sequencing and qPCR. Results The proportions of Th1 and CXCR3 + Th2 cells were higher in SF than in blood ( P  < 0.05). TPh and PD-1 high TFh in RA SF were primarily of a Th1 and a CXCR3 + Th2 phenotype. Moreover, the levels of CXCL9, CXCL10, CCL20, CCL2, CXCL8, IL-6 and IL-10 were higher in SF than in plasma ( P  < 0.05). Lastly, IL-4, IL-13 and IL-17A induced RA FLS to secrete proinflammatory IL-6, CCL2, CXCL1 and CXCL8, while IFNγ mainly induced CXCL10. Conclusion These findings indicate that not only Th1 but also CXCR3 + Th2 cells may have a pathogenic role in RA synovial inflammation.

Synovial-derived cells, found in the synovial membrane of human joints, were obtained by digestion of the synovial membrane and were subsequently expanded in vitro. The identity of synovial-derived cells has long been a topic of debate. The terms “type B synoviocytes,” “fibroblast-like synoviocytes (FLS),” “synovium-derived mesenchymal stem cells (MSCs),” and “synovial fibroblasts (SF)” appeared in different articles related to human synovial-derived cells in various disease models, yet they seemed to be describing the same cell type. However, to date, there is no clear standard to distinguish these terms; thus, the hypothesis that they represent the same cell type is currently inconclusive. Therefore, this review aims to clarify the similarities and differences between these terms and to diffuse the chaotic nomenclature of synovial-derived cells.

Background Anti-citrullinated protein/peptide antibodies (ACPAs) are present in patients at onset and have important pathogenic roles during the course of rheumatoid arthritis (RA). The characteristics of several molecules recognized by ACPA have been studied in RA, but the positivity rate of autoantibodies against each antigen is not high, and the pathogenic mechanism of each antibody is not fully understood. We investigated the role of anti-citrullinated osteopontin (anti-cit-OPN) antibodies in RA pathogenesis. Methods Enzyme-linked immunosorbent assays on RA patients’ sera were used to detect autoantibodies against OPN. Fibroblast-like synoviocytes (FLS) isolated from RA patients were used to test the binding activity and inflammatory response of OPN mediated by anti-cit-OPN antibodies, and their effect was tested using an inflammatory arthritis mouse model immunized with cit-OPN. Anti-cit-OPN antibody positivity and clinical characteristics were investigated in the patients as well. Results Using sera from 224 RA patients, anti-cit-OPN antibodies were positive in approximately 44% of RA patients, while approximately 78% of patients were positive for the cyclic citrullinated peptide (CCP2) assay. IgG from patients with anti-cit-OPN antibody increased the binding activity of OPN to FLSs, which further increased matrix metalloproteinase and interleukin-6 production in TNF-stimulated FLSs. Mice immunized with cit-OPN antibodies experienced severe arthritis. Anti-cit-OPN antibodies in RA patients decreased the drug survival rate of tumor necrosis factor (TNF) inhibitors, while it did not decrease that of CTLA4-Ig. Conclusions Anti-cit-OPN antibodies were detected in patients with RA. IgG from patients with anti-cit-OPN antibodies aggravated RA, and anti-cit-OPN antibody was a marker of reduced the survival rate of TNF inhibitors in RA patients.

Background: Rheumatoid arthritis (RA) is a chronic inflammatory autoimmune disease. The fruits of Illicium verum, which is a medicinal and edible resource, have been shown to have anti-inflammatory properties. Methods: In this study, we investigated the effects of I. verum extracts (IVEs) on human RA fibroblasts-like synoviocytes (RA-FLS) by using a sensitive and selective ultra-high-performance liquid chromatography with high-definition mass spectrometry (UPLC-HDMS) method. We subsequently analyzed the metabolites produced after the incubation of cultured RA-FLS with IVEs. Results: IVEs inhibited the proliferation and suppressed the migration of RA-FLS, and reduced the levels of inflammatory factors including TNF-α and IL-6. Twenty differential metabolites responsible for the effects of IVEs were screened and annotated based on the UPLC-HDMS data by using a cell metabolomics approach. Discussion: Our findings suggest that treating RA-FLS with IVEs can regulate lipid and amino acid metabolism, indicating that this extract has the potential to modify the metabolic pathways that cause inflammation in RA. Conclusions: This might lead to novel therapeutic strategies for managing patients with RA.

Background/Objectives: Rheumatoid arthritis (RA) is an autoimmune disorder manifesting as joint destruction and synovial inflammation, with the aberrant activation of fibroblast-like synoviocytes (FLSs) functioning as a critical pathological mechanism. Liquiritigenin (LIQ), a natural flavonoid extracted from licorice root, possesses anti-inflammatory and antioxidant activities; however, its efficacy and mechanism in RA pathological models remain unclear. This study aimed to investigate the anti-RA effects of LIQ mediated through FLSs and its underlying mechanisms. Methods: Complete Freund’s adjuvant (CFA)-induced rat model and TNF-α-stimulated MH7A cell model were employed to assess the anti-RA effects and underlying mechanisms. In vivo experiments examined the effects of LIQ on RA manifestations, joint damage, and inflammatory responses in CFA-induced rats, while in vitro experiments explored its effects on aberrant activation, oxidative stress, and inflammation in TNF-α-stimulated MH7A cells. The regulatory effects of LIQ on the Nrf2/NF-κB/NLRP3 signaling pathway were validated by immunofluorescence and Western blotting in vivo and in vitro. Results: LIQ alleviated joint swelling and bone damage, reducing synovial cellular infiltration and hyperplastic changes in CFA-induced rats. Furthermore, LIQ inhibited proliferation, migration, and invasion while reducing reactive oxygen species levels in TNF-α-stimulated MH7A cells, and decreased IL-1β and IL-18 levels in rat serum and MH7A cell supernatants. Moreover, LIQ activated Nrf2 and inhibited NF-κB and NLRP3, thereby attenuating inflammatory responses and alleviating oxidative stress. Administration of the Nrf2 inhibitor ML385 partially reversed its suppressive effects on inflammatory responses and oxidative stress in vivo and in vitro. Conclusions: LIQ exerted anti-RA effects in FLSs by suppressing inflammation and aberrant activation. Its mechanism may involve modulation of the Nrf2/NF-κB/NLRP3 signaling pathway.

Rheumatoid arthritis (RA) is a rheumatic disease charactered by severe bone destruction. Evidence suggests that fatty acid metabolism (FAM)-related proteins can regulate inflammation of synoviocytes in RA. However, the fundamental roles of FAM regulators in RA remain to be elucidated. We selected the GSE93272 dataset sourced from the Gene Expression Omnibus (GEO) for the classification of FAM-associated molecular subtypes and immune microenvironments in RA. Subsequently, bone marrow-derived macrophages (BMMs) with or without receptor activator of nuclear factor kappa-B ligand (RANKL) intervention were harvested for RNA sequencing (RNA-seq) to verify FAM hub gene expressions. Difference analysis between RA samples and controls screened 53 significant FAM regulators. Random forest algorithm for RA risk prediction was utilized to identify ten diagnostic FAM regulators (hub genes). A nomogram incorporating hub genes was developed, and decision curve analysis suggested its potential utility in clinical practice. Additionally, consensus clustering analysis of these hub genes categorized RA patients to different FAM clusters (cluster A and cluster B). To quantify FAM clusters, principal component analysis (PCA) was adopted to count FAM score of every sample. ClusterB may be more linked with osteoclastogenesis in RA characterized by RXRA, IL17RA, and TBXA2R. Additionally, cases in cluster A were associated with the immunity of activated CD4 T cell, activated CD8 T cell, eosinophil, Gamma delta T cell, immature dendritic cell, MDSC, macrophage, regulatory T cell, and Type 2 T helper cell, while cluster B was linked to CD56dim natural killer cell, Natural killer T cell, T follicular helper cell, Type 1 T helper cell immunity, which has a higher FAM score. Remarkably, RNA-seq analysis confirmed the expression trend of SREBF1, FASN, CD36, SCD1 and SCD2, consistent with bioinformatics predictions. This scoring system of FAM subtypes provided promising markers and immunotherapeutic strategies for future RA treatment.

Rheumatoid arthritis (RA) is an inflammatory condition that causes severe cartilage degradation and synovial damage in the joints with multiple systemic implications. Previous studies have revealed that fibroblast-like synoviocytes (FLSs) play a pivotal role in the pathogenesis of RA. The appropriate regulation of FLS function is an efficient approach for the treatment of this disease. In the present study, we explored the effects of methyl canthin-6-one-2-carboxylate (Cant), a novel canthin-6-one alkaloid, on the function of FLSs. Our data showed that exposure to Cant significantly suppressed RA-FLS migration and invasion properties in a dose-dependent manner. Meanwhile, pre-treatment with Cant also had an inhibitory effect on the release of several pro-inflammatory cytokines, including IL-6 and IL-1β, as well as the production of MMP1 and MMP3, which are important mediators of FLS invasion. In further mechanistic studies, we found that Cant had an inhibitory effect on the Hippo/YAP signaling pathway. Treatment with Cant suppressed YAP expression and phosphorylation on serine 127 and serine 397 while enhancing LATS1 and MST1 levels, both being important upstream regulators of YAP. Moreover, YAP-specific siRNA or YAP inhibition significantly inhibited wound healing as well as the migration and invasion rate of FLS cells, an impact similar to Cant treatment. Meanwhile, the over-expression of YAP significantly reversed the Cant-induced decline in RA-FLS cell migration and invasion, indicating that YAP was required in the inhibitory effect of Cant on the migration and invasion of RA-FLS cells. Additionally, supplementation of MMP1, but not MMP3, in culture supernatants significantly reversed the inhibitory effect of Cant on RA-FLS cell invasion. Our data collectively demonstrated that Cant may suppress RA-FLS migration and invasion by inhibiting the production of MMP1 via inhibiting the YAP signaling pathway, suggesting a potential of Cant for the further development of anti-RA drugs.