Distinct synovial tissue macrophage subsets regulate inflammation and remission in rheumatoid arthritis

Advances in Regenerative Therapies for Inflammatory Arthritis: Exploring the Potential of Mesenchymal Stem Cells and Extracellular Vesicles

Inflammatory arthritis, including rheumatoid arthritis (RA) and osteoarthritis (OA), is a group of degenerative joint diseases that result in reduced mobility and a prevalent cause of disability. Despite differing etiologies, both conditions involve inflammation, affecting only the joints in OA and systemic in RA due to its autoimmune nature. Regenerative medicine offers promising alternatives, with a focus on the therapy with mesenchymal stem cell (MSC) and their secreted extracellular vesicles (EVs). MSC-derived EVs have shown great potential in modulating inflammatory pathways and promoting tissue repair in the preclinical models of RA and OA. Additionally, EVs from immune cells exhibit strong anti-inflammatory effects, reducing cartilage and bone degeneration. This review highlights the recent progress in MSC-based and EV-based therapies for OA and RA, discussing the bioengineering approaches that enhance the therapeutic efficacy, stability, and targeting of EV. It also addresses the major challenges in translating EV therapy from the laboratory to clinical practice and discusses strategies to overcome these obstacles in the treatment of inflammatory arthritis.

Macrophage heterogeneity in autoimmune diseases

The pathogenesis of autoimmune diseases (AIDs) is complex and their etiology remains unclear, with multiple cell types involved in the disease progression. Macrophages, as a crucial immune cell population in AIDs, play a pivotal role in maintaining immune homeostasis. In traditional research, macrophages are frequently oversimplified into the M1 and M2 polarized subtypes. The advent of single-cell RNA sequencing (scRNA-seq) technology has significantly advanced high-throughput research in the life sciences, enabling in-depth investigations at the cellular and molecular levels. This technology has revealed the significant heterogeneity of macrophages, further enhancing our understanding of their development, phenotypic diversity, and functional plasticity. Additionally, it provides a novel perspective for exploring the molecular mechanisms underlying various diseases. In this review, we comprehensively explore the heterogeneity of macrophages across different AIDs, and summarize potential therapeutic targets for macrophage-directed interventions, aiming to provide valuable theoretical insights and novel research directions to advance precision therapy and related studies in AIDs.

Synovial fibroblast derived small extracellular vesicles miRNA15-29148 promotes articular chondrocyte apoptosis in rheumatoid arthritis

Rheumatoid arthritis (RA) is a systemic autoimmune disease in which synovial fibroblasts (SFs) maintain chronic inflammation by secreting proinflammatory mediators, leading to joint destruction. While the role of proinflammatory mediators in this process is well-established, the contribution of non-inflammatory regulators in SFs to joint pathology remains poorly understood. In this study, we investigated the non-inflammatory role of SFs in RA using a co-culture model, and found that SFs from RA patients promote apoptosis of human chondrocytes. Mechanistic investigations reveal that SFs can secrete small extracellular vesicles (sEVs), which are taken up by chondrocytes and induce chondrocyte apoptosis in both normal chondrocytes and chondrocytes from patients with RA. sEV-derived miRNA 15-29148 are identified as key signaling molecules mediating the apoptosis effects of chondrocytes. Further studies reveal that SF-derived miRNA 15-29148 targeting CIAPIN1 results in increased chondrocyte apoptosis. We further demonstrate that SF-derived miRNA 15-29148 is transferred to chondrocytes, exacerbating cartilage damage in vivo. Moreover, chondrocyte-specific aptamer-modified polyamidoamine nanoparticles not only ameliorated RA but also prevented its onset. This study suggests that, in RA, the secretion of specific sEV-miRNAs from SFs plays a crucial role in promoting chondrocyte apoptosis, potentially through non-inflammatory regulation, and that sEV-miRNA inhibition in SFs may represent an early preventive treatment strategy for cartilage degradation in RA.

The joint accumulation model of arthritis

Most inflammatory arthritides are systemic diseases. In an individual patient, however, disease flares are more common in joints affected previously, a phenomenon termed joint-specific memory. A key driver of localized recurrence is the accumulation of CD8+ T resident memory (TRM) cells in inflamed synovial tissues. These cells remain during remission and initiate recurrent disease when activated by arthritogenic antigens. The joint accumulation model is a paradigm that recognizes the contribution of local as well as systemic immune mechanisms to arthritis chronicity, highlighting new targets for disease intervention, including but not limited to TRM cells. The joint accumulation model underscores the importance of preventing extension of arthritis to new joints, even in established disease, translating into a rolling window of opportunity for optimal long-term arthritis management.

Association of Synovial Innate Immune Exhaustion With Worse Pain in Knee Osteoarthritis

OBJECTIVE

Uncontrolled pain remains a major clinical challenge in the management of knee osteoarthritis (OA), the most common disabling joint disease. Worse pain is associated with synovial innate immune cell infiltration (synovitis), but the role of innate immune-regulatory cells in pain is unknown. Our objective was to identify synovial innate immune cell subsets and pathophysiologic mechanisms associated with worse pain in patients with knee OA.

METHODS

Synovial tissue biopsies from 122 patients with mild-to-severe knee OA pain (Knee Injury and OA Outcome Score [KOOS]) were analyzed to identify associations between synovial histopathology and worse pain. We then used spatial transcriptomics and proteomics of synovial tissue microenvironments (n = 32), followed by single-cell RNA sequencing (n = 8), to identify synovial cell composition and cell-cell communication networks in patients with more severe OA pain.

RESULTS

Histopathological signs of synovial microvascular dysfunction and perivascular edema were associated with worse KOOS pain (-10.76; 95% confidence interval [CI] -18.90 to -2.61). Patients with worse pain had fewer immune-regulatory macrophages, expanded fibroblast subsets, and enrichment in neurovascular remodeling pathways. Synovial macrophages from patients with worse pain expressed markers of immune exhaustion and decreased phagocytic function (-19.42%; 95% CI -35.96 to -2.89) and their conditioned media increased neuronal cell stress in dorsal root ganglia.

CONCLUSION

Although synovitis increases during OA, our findings suggest that exhaustion, dysfunction, and loss of immune-regulatory macrophages is associated with worse pain and may be an important therapeutic target.

Synovium-on-a-Chip Reveals Fibroblast-Macrophage Crosstalk Underpinning Joint Homeostasis and Evaluation of Gout Therapies

The synovium maintains joint homeostasis and regulates immune responses through fibroblast-like synoviocytes (FLS) and macrophage-like synoviocytes (MLS). However, investigation of the intricate FLS-MLS interactions is limited by the lack of physiologically relevant in vitro models. Here, this work presents a synovium-on-a-chip model that faithfully mimics the structural and functional properties of the human synovial lining, established with collagen/alginate hybrid hydrogel (CAHG). The functional phenotype of FLS observed in vivo is replicated on chip when cultured with CAHG. Moreover, co-culture with M2c macrophages derived from CD14+ monocytes enables the reconstruction of key immune functions of the synovial lining, including expression of junction proteins (ZO-1 and CLD5) and immunoregulatory markers (TREM2 and VSIG4). CD44 blockade, disrupting FLS-MLS interactions, significantly suppresses inflammasome-related pathways, underscoring the regulatory role of FLS in synovial immune responses. Our model is further validated by modeling gout, where treatment with monosodium urate crystals trigger NLRP3 inflammasome activation, macrophage polarization, and neutrophil extravasation. Pharmacological interventions with MCC950 and entrectinib effectively inhibit the inflammasome activation, demonstrating the platform's utility for preclinical drug evaluation. This synovium-on-a-chip provides a reliable in vitro model for studying synovial inflammation and serves as a valuable tool for the therapeutic discovery of inflammatory joint diseases.

Amelioration of the rheumatoid arthritis microenvironment using celastrol-loaded silver-modified ceria nanoparticles for enhanced treatment

Rheumatoid arthritis (RA) is characterized by elevated levels of reactive oxygen species (ROS) and a persistent inflammatory microenvironment dominated by M1 macrophages, both of which contribute to disease progression. To address these pathological features, we developed a core-shell nanoplatform consisting of silver-modified ceria nanoparticles loaded with celastrol (Ag-CeNP@Cel). This nanoplatform significantly enhances the water solubility of celastrol and reduces its hepato-renal toxicity by enabling passive accumulation in inflamed joints. The silver-modified ceria nanoparticles synergistically combine with celastrol to scavenge excess ROS and reprogram M1 macrophages into M2 macrophages, thereby mitigating inflammatory responses and improving the rheumatoid arthritis microenvironment (RAM). Ag-CeNP@Cel exhibited robust therapeutic efficacy and safety in preclinical models, presenting an innovative approach to RA treatment by integrating ROS elimination with macrophage modulation to ameliorate inflammatory microenvironment. This study underscores the potential of Ag-CeNP@Cel as a promising therapeutic strategy for RA management.

Epigenomic landscapes define differential Janus kinases inhibitor sensitivity in IFN-γ-primed human macrophages

Janus kinases (JAK) mediate signaling pathways of multiple cytokines, including interferon-γ (IFN-γ), which plays a pivotal role in rheumatoid arthritis (RA) pathogenesis. Although JAK inhibitors (JAKi) have demonstrated efficacy for RA, their molecular effects on macrophages remain incompletely understood. We investigate the impact of JAKi on IFN-γ-induced gene expression in human macrophages, uncovering that JAKi selectively modulates only a subset of IFN-γ-induced genes. Integrated transcriptomic and epigenomic analyses demonstrate that JAKi effectively inhibits IFN-γ signature genes associated with IRF1-STAT1-dependent accessible chromatin regions. However, genes regulated by AP-1 and C/EBP remain insensitive to JAKi and overlap significantly with TNF-induced genes. Single-cell analysis of RA patient samples identifies macrophage subpopulations with variable JAKi sensitivity. Certain JAKi-insensitive genes in IFN-γ-primed macrophages are suppressed by JNK inhibitors. Our findings elucidate JAKi responsiveness mechanisms through IFN-γ-induced epigenomic remodeling, providing insights into inflammatory regulation in RA and suggesting strategies to overcome JAKi resistance.

Engineered Spirulina platensis for treating rheumatoid arthritis and restoring bone homeostasis

Rheumatoid arthritis (RA) is characterized by massive intra-articular infiltration of pro-inflammatory macrophages, leading to articular immune dysfunction, severe synovitis, and ultimately joint erosion. Comprehensive remodeling of articular immune homeostasis and bone homeostasis is essential for alleviating RA. Here we report on Spirulina platensis (SP), a natural microorganism commercially farmed worldwide as a food, as an efficient regulator of both synovial inflammation and osteoclast differentiation in male RA mouse models. SP reduces excessive reactive oxygen species and downregulates pro-inflammatory cytokines in synovial macrophages. Moreover, SP reprograms pro-inflammatory M1-like macrophages to anti-inflammatory M2-like phenotype, suppressing synovitis and remodeling redox balance. Notably, SP inhibits osteoclast activation effectively and blocks the progression of bone erosion. SP is then engineered with macrophage membranes (SP@M) to enable immune evasion and RA-targeting in vivo. SP@M increases LC3-mediated autophagy as well as strengthens ubiquitin-mediated proteasomal degradation toward KEAP1, which promotes the expression and nuclear translocation of NRF2. The NRF2 further activates antioxidant enzymes to terminate macrophages-initiated pathological cascades and reestablish intra-articular immune homeostasis, conferring a bone recovery and chondroprotective effect in collagen-induced arthritis mouse models. This work shows the therapeutic activity of FDA-approved functional food of SP in suppressing synovial inflammation and osteoclast differentiation, offering an off-the-shelf strategy for RA treatment.

Electroacupuncture attenuates bone erosion and promotes macrophage polarization in a mouse model of collagen-induced arthritis

OBJECTIVE

The aim of this study was to investigate the effect of electroacupuncture (EA) on macrophage polarization and bone erosion in a mouse model of collagen-induced arthritis (CIA).

METHODS

C57BL/B6 mice were used to establish a CIA model and were treated with electroacupuncture (EA) at ST36 and SP6. At the end of the experiment, knee joints were harvested for hematoxylin-eosin (H&E) staining to detect knee synovitis. Immunohistochemistry (IHC) was performed to assess the expression of macrophage markers. The degree of bone destruction was evaluated using micro-computed tomography (CT), tartrate-resistant acid phosphatase (TRAP) staining and safranin-O fast green staining. Peripheral blood transcriptome sequencing was performed using Illumina high-throughput sequencing. Synovial membrane proteins were quantitatively analyzed by mass spectrometry. Differentially expressed genes and proteins were identified and the R software package was used to analyze the data.

RESULTS

Compared with the model group, the arthritis index (P < 0.05) and inflammatory infiltration decreased (P < 0.05), cartilage destruction was inhibited (P < 0.01), the number of osteoclasts decreased (P < 0.05), knee bone erosion was alleviated and the M1/M2 macrophage ratio decreased (P < 0.01) in the EA group. The results of bioinformatics analysis showed that the differential genes between the EA and model groups were mainly enriched in rheumatoid arthritis (RA) and the peroxisome proliferator-activated receptor (PPAR) signaling pathway. Differentially expressed proteins were mostly enriched in the toll-like receptor (TLR) signaling and autophagy pathways.

CONCLUSIONS

EA prevents bone erosion, reduces the M1/M2 macrophage ratio in synovial tissue, inhibits the TLR and autophagy pathways and reduces synovial invasion in a mouse model of CIA.

Investigating endogenous immune-mediated monocyte memory in rheumatoid arthritis

OBJECTIVES

Inflammation triggered by endogenous stimuli that signal cellular stress or tissue injury must be tightly controlled to balance robust protection from intrinsic danger while avoiding catastrophic destruction of healthy tissues. Here, we assess the contribution of innate memory to this balance.

METHODS

Memory evoked by the extracellular matrix protein tenascin-C, a damage-associated, toll-like receptor 4 (TLR4) agonist, was compared to that induced by the pathogenic TLR4 agonist lipopolysaccharide (LPS) by transcriptomic and epigenetic profiling of monocytes from healthy individuals or people wirh rheumatoid arthritis (RA), and tissue macrophages from the RA synovium.

RESULTS

Tenascin-C reprograms monocyte response to subsequent threats, inducing concomitantly suppressed and enhanced responses to rechallenge. Comparative analysis of tenascin-C and LPS revealed common and distinct gene expression signatures, effects controlled transcriptionally and associated with stimulus-specific epigenetic mediators. Altered responses following rechallenge after priming with tenascin-C were not limited to subsequent TLR4 activation but were evident in response to various pathogenic and endogenous stimuli detected by different receptors. In healthy monocytes primed with tenascin-C, rechallenge with stimuli found at high levels in the joints of people with RA resulted in trained responses that were not induced by LPS, including genes associated with chronic inflammation, tissue destruction, altered metabolism, and poor treatment response in RA. The expression of a large subset of these genes was elevated in monocytes from people with RA in the absence of any stimulation and in RA synovial macrophage populations associated with disease flare. Moreover, higher levels of permissive complexes within key epigenetic nodes and increased bivalent modification creating poised loci within endogenously trained genes were observed in RA cells.

CONCLUSIONS

These data highlight how innate reprogramming during 'sterile' inflammatory diseases contributes to chronicity, uncovering pathways unique to endogenous immune triggers that could provide disease-specific points of intervention without engendering global immune suppression.

Understanding rheumatic disease through continuous cell state analysis

Autoimmune rheumatic diseases are a heterogeneous group of conditions, including rheumatoid arthritis (RA) and systemic lupus erythematosus. With the increasing availability of large single-cell datasets, novel disease-associated cell types continue to be identified and characterized at multiple omics layers, for example, 'T peripheral helper' (TPH) (CXCR5- PD-1hi) cells in RA and systemic lupus erythematosus and MerTK+ myeloid cells in RA. Despite efforts to define disease-relevant cell atlases, the very definition of a 'cell type' or 'lineage' has proven controversial as higher resolution assays emerge. This Review explores the cell types and states involved in disease pathogenesis, with a focus on the shifting perspectives on immune and stromal cell taxonomy. These understandings of cell identity are closely related to the computational methods adopted for analysis, with implications for the interpretation of single-cell data. Understanding the underlying cellular architecture of disease is also crucial for therapeutic research as ambiguity hinders translation to the clinical setting. We discuss the implications of different frameworks for cell identity for disease treatment and the discovery of predictive biomarkers for stratified medicine - an unmet clinical need for autoimmune rheumatic diseases.

GM-CSF drives IL-6 production by macrophages in polymyalgia rheumatica

OBJECTIVES

Insight into the immunopathology of polymyalgia rheumatica (PMR) is scarce and mainly derived from peripheral blood studies. The limited data available point towards macrophages as potential key players in PMR. This study aimed to identify the factors driving proinflammatory macrophage development and their functions in the immunopathology of PMR.

METHODS

Monocyte phenotypes were investigated by flow cytometry in peripheral blood (PMR, n = 22; healthy controls, n = 20) and paired subacromial-subdeltoid (SASD) bursal fluid (PMR, n = 9). Macrophages in SASD bursa were characterised by immunohistochemistry and immunofluorescence (PMR, n = 12; controls undergoing shoulder replacement surgery, n = 10). The functions of cytokines expressed in PMR-affected tissue were examined using macrophage differentiation cultures (PMR, n = 7; healthy controls, n = 7).

RESULTS

Monocytes (CD14highCD16- and CD14highCD16+) were increased in blood of PMR patients and activated in bursal fluid. Macrophages dominated immune infiltrates in PMR-affected tissue, expressing various proinflammatory cytokines. While interleukin (IL)-6 and interferon-gamma (IFN-γ) expression was abundant in both PMR and control tissue, granulocyte-macrophage colony-stimulating factor (GM-CSF) and macrophage colony-stimulating factor (M-CSF) were significantly increased in PMR tissue. Macrophages in PMR-affected tissue showed an elevated CD206/folate receptor β ratio, reflecting a GM-CSF skewed signature. A combination of GM-CSF/M-CSF/IFN-γ significantly boosted IL-6 production in vitro, while limited IL-6 production was observed without GM-CSF.

CONCLUSIONS

The monocyte compartment is expanded and activated in PMR. Macrophages in PMR-affected tissue produce abundant proinflammatory cytokines such as IL-6. A network of locally expressed cytokines, including GM-CSF, M-CSF, and IFN-γ, may drive the proinflammatory functions of these macrophages. Overall, macrophages may constitute key therapeutic targets for PMR.

Advances in nanodelivery systems based on apoptosis strategies for enhanced rheumatoid arthritis therapy

Rheumatoid arthritis (RA) is a chronic systemic autoimmune disorder primarily characterized by persistent synovial inflammation and progressive bone erosion. The pathogenesis of RA involves a complex cascade of cellular and molecular events, including sustained hyperactivation of macrophages, excessive recruitment and activation of neutrophils, pathological proliferation and invasion of fibroblast-like synoviocytes (FLS), and dysregulated differentiation and function of osteoclasts (OCs). The inflammatory factors secreted by these dysregulated cells significantly disrupt the joint microenvironment through multiple pathological mechanisms, primarily by promoting synovial inflammation, cartilage matrix degradation, osteoclast-mediated bone erosion, and pathological angiogenesis. Therapeutic strategies targeting the induction of apoptosis in these malignant cells have demonstrated considerable potential in preclinical studies, offering a promising approach to enhance treatment outcomes by simultaneously reducing inflammatory cytokine production and inhibiting pathogenic cell proliferation. However, conventional therapeutic drugs are limited in clinical applications because of their high toxicity and side effects. Inflammation induces morphological and functional changes in cells within the rheumatoid arthritis microenvironment (RAM), particularly the overexpression of specific receptors on cell membranes. This phenomenon has driven the development of ligand-modified targeted nanodelivery systems (NDSs), which can specifically target and induce apoptosis in specific cell types, thereby enhancing therapeutic efficacy. This paper comprehensively reviews the research progress of targeted NDSs based on apoptosis strategies for RA therapy, with a detailed discussion of their advantages in inducing apoptosis in various disease-associated cells. Furthermore, the potential of combining apoptosis of multiple cell types for RA treatment is explored. This review is expected to improve insights into the apoptosis of malignant cells to enhance RA therapy. STATEMENT OF SIGNIFICANCE: This review highlights recent advances in nanodelivery systems (NDSs) based on apoptotic strategies for enhanced rheumatoid arthritis (RA) therapy. Unlike conventional NDSs, these optimized systems specifically induce apoptosis in malignant cells within the RA microenvironment by integrating multiple therapeutic strategies. By summarizing the latest research, our work demonstrates the potential of these NDSs to suppress inflammatory responses and prevent bone destruction through targeted elimination of malignant cells, offering a novel direction for RA treatment. This review is significant as it provides a comprehensive overview for researchers and clinicians, facilitating the development of more effective therapeutic approaches for RA and other chronic inflammatory diseases.

From rare to more common: The emerging role of omics in improving understanding and treatment of severe inflammatory and hyperinflammatory conditions

Inflammation is a pathogenic driver of many diseases, including atherosclerosis and rheumatoid arthritis. Hyperinflammation can be seen as any inflammatory response that is deleterious to the host, regardless of cause. In medicine, hyperinflammation is defined as severe, deleterious, and fluctuating systemic or local inflammation with presence of a cytokine storm. It has been associated with rare autoinflammatory disorders. However, advances in omics technologies, including genomics, proteomics, and metabolomics, have revealed it to be more common, occurring in sepsis and severe coronavirus disease 2019. With a focus on proteomics, this review highlights the key role of omics in this shift. Through an exploration of research, we present how omics technologies have contributed to improved diagnostics, prognostics, and targeted therapeutics in the field of hyperinflammation. We also discuss the integration of advanced technologies, multiomics approaches, and artificial intelligence in analyzing complex datasets to develop targeted therapies, and we address their potential for revolutionizing the clinical aspects of hyperinflammation. We emphasize personalized medicine approaches for effective treatments and outline challenges, including the need for standardized methodologies, robust bioinformatics tools, and ethical considerations regarding data privacy. This review aims to provide a comprehensive overview of the molecular mechanisms underpinning hyperinflammation and underscores the potential of omics technologies in enabling successful clinical management.

Inflammation-Targeting Multienzyme Activity Carbon Dots Loaded with Methotrexate for Synergistic Immunotherapy in Rheumatoid Arthritis

In rheumatoid arthritis (RA), excessive reactive oxygen species (ROS) and chronic inflammation drive damage to the synovium, cartilage, and bone. Developing precise and synergistic therapy for RA is crucial for improving remission rates. Here, carbon dots (CDs) with multienzyme activity and inflammation-targeting capabilities are designed to deliver methotrexate (MTX) for synergistic RA treatment. Specifically, positively charged CDs with porphyrin iron cores and amino-functionalized surfaces are synthesized to simultaneously scavenge hydrogen peroxide, superoxide anions, and hydroxyl radicals. Conjugation of MTX-loaded CDs with polyethylene glycol (CDs2-P@M) via Schiff base reaction significantly prolongs in vivo circulation time. In collagen-induced arthritis rats, CDs2-P@M accumulates in the diseased joints, reducing ROS and inflammatory cytokines, reprogramming macrophage phenotypes, inhibiting osteoclast activation, and markedly improving arthritis symptoms. This approach targets the RA microenvironment, minimizing MTX toxicity and effectively reshaping immune homeostasis, halting inflammation and tissue destruction, thus offering a new paradigm for RA immunotherapy.

Thermosensitive Light-Driven Smart Platform Induces Apoptosis of Fibroblast-like Synovial Cells and Macrophages for Enhanced Rheumatoid Arthritis Therapy

Macrophage activation induces rapid proliferation and division of fibroblast-like synovial cells (FLSs), resulting in the degradation of cartilage matrix and bone destruction, which are the main pathological characteristics of rheumatoid arthritis (RA). Inducing apoptosis in these inflammatory cells to mitigate the inflammatory response and alleviate bone damage is a potential therapeutic strategy for RA. In this study, we developed a smart platform for synergistic photothermal therapy (PTT) and chemotherapy by utilizing hyaluronic acid (HA)-modified thermally sensitive liposomes loaded with celastrol (CEL) and gold nanorods (GNRs), termed HA/Lipo-CEL-GNRs, for application in a rat RA model. Under laser irradiation, GNRs exhibited excellent photothermal effects due to localized surface plasmon resonance. The resulting increase in temperature not only effectively eliminated hyperproliferative inflammatory cells in the joints but also triggered CEL release from the thermosensitive liposomes, significantly increasing its concentration in the synovium. The synergistic effect of PTT and chemotherapy significantly promoted the apoptosis of FLSs and macrophages and effectively suppressed the inflammatory response in the RA microenvironment. In summary, multifunctional thermosensitive HA/Lipo-CEL-GNRs represent promising nanotherapeutic platforms capable of achieving light-driven enrichment of heat and therapeutic agents, significantly preventing the progression of RA.

Personalised treatment of rheumatoid arthritis based on cytokine profiles and synovial tissue signatures: potentials and challenges

BACKGROUND

Rheumatoid arthritis (RA) is an autoimmune, chronic inflammatory disease that mainly affects the joints and periarticular soft tissues. Although there have been significant advances in RA treatment over the past two decades, approximately 40% of patients do not respond to first-line biological disease-modifying antirheumatic drugs (bDMARDs). Physicians often use an empirical, trial-and-error approach to select bDMARDs to treat patients with RA. This is inefficient and can be costly for healthcare systems which have limited resources. Unlike in oncology, where molecular pathology helps guide targeted therapies, reliable, predictive biomarkers for drug response in RA are yet to be identified. This narrative review aims to summarise current knowledge on novel biomarkers of disease activity and drug response in RA, with a particular focus on serum cytokine profiles and macrophage and fibroblast subsets in synovial tissue. We also highlight key areas of further research that could advance the development of targeted therapies for patients with RA.

METHODS

We searched PubMed to identify studies pertaining to biomarkers of disease activity and drug response in the treatment of RA.

RESULTS

We present a detailed overview of the key studies that have identified serum cytokine profiles and synovial macrophage and fibroblast subsets as novel biomarkers of disease activity and drug response in RA.

CONCLUSION

A novel, evidence-based approach to precision medicine in RA, which involves tailoring treatment based on cytokine profiles and synovial tissue signatures, shows promise for improving patient care. However, more research is needed to identify biomarkers that predict drug response.

Perivascular RELMα-positive synovial macrophages recruit monocytes at the onset of inflammatory arthritis

Macrophages, monocytes and neutrophils are major types of myeloid cells involved in inflammatory diseases, such as rheumatoid arthritis (RA). Recent scRNA-seq studies identified a remarkable diversity of synovial macrophages but, with the exception of lining macrophages, their geographical location and specific roles remain largely unexplored. Here, we localized the RELMα-positive macrophages, predicted to produce high levels of monocyte-recruiting chemokines, to the synovial interstitium and more specifically, to the vicinity of interstitial blood vessels. Using complementary reporter mouse models, CCL2mCherry to label CCL2-producing cells, and CCR2CRE/mKate2 marking CCR2 expressing monocytes, we demonstrated that RELMα-positive perivascular macrophages secrete CCL2 assisting in the recruitment of monocytes predominantly to the synovial interstitium at the onset of antigen-induced arthritis. The inflamed synovial environment guides the differentiation of the recruited monocytes into tissue-resident macrophages, including but not limited to macrophages expressing VSIG4, a characteristic marker of lining macrophages. Thus, RELMα-positive macrophages orchestrate monocyte recruitment to the synovium during articular inflammation, contributing to a local replenishment of synovial lining macrophages.

The role of the immune system in osteoarthritis: mechanisms, challenges and future directions

Osteoarthritis (OA) is a chronic joint disease that has long been considered a simple wear-and-tear condition. Over the past decade, research has revealed that various inflammatory features of OA, such as low-grade peripheral inflammation and synovitis, contribute substantially to the pathophysiology of the disease. Technological advances in the past 5 years have revealed a large diversity of innate and adaptive immune cells in the joints, particularly in the synovium and infrapatellar fat pad. Notably, the presence of synovial lymphoid structures, circulating autoantibodies and alterations in memory T cell and B cell populations have been documented in OA. These data indicate a potential contribution of self-reactivity to the disease pathogenesis, blurring the often narrow and inaccurate line between chronic inflammatory and autoimmune diseases. The diverse immune changes associated with OA pathogenesis can vary across disease phenotypes, and a better characterization of their underlying molecular endotypes will be key to stratifying patients, designing novel therapeutic approaches and ultimately ameliorating treatment allocation. Furthermore, examining both articular and systemic alterations, including changes in the gut-joint axis and microbial dysbiosis, could open up novel avenues for OA management.

Tenascin-C promotes bone regeneration via inflammatory macrophages

During the early stage of tissue injury, macrophages play important roles in the activation of stem cells for further regeneration. However, the regulation of macrophages during bone regeneration remains unclear. Here, the extracellular matrix (ECM) tenascin-C (TNC) is found to express in the periosteum and recruit inflammatory macrophages. TNC-deficiency in the periosteum delays bone repair. Transplantation of macrophages derived from injured periosteum is able to rescue the decreased skeletal stem cells and impaired bone regeneration caused by TNC deficiency. The cell communication analysis identifies ITGA7 as a TNC receptor contributing to the recruitment of inflammatory macrophages. TNC expression declines in aged mice and the exogenous delivery of TNC significantly promotes bone regeneration after aging through the recruitment of macrophages. Taken together, this study reveals the regulation of macrophage recruitment and its function in the activation of skeletal stem cells after bone injury, providing a strategy to accelerate bone regeneration by TNC delivery.

Staphylococcus Aureus Tames Nociceptive Neurons to Suppress Synovial Macrophage Responses for Sustained Infection in Septic Arthritis

The interaction between the nervous system and immune system during chronic bacterial infection remains unclear. Here, it is reported that Staphylococcus aureus (S. aureus) infection induces calcitonin gene-related peptide (CGRP) secretion from intra-articular transient receptor potential cation channel subfamily V member 1 positive (TRPV1+) nociceptive nerves through its pore-forming toxin (PFT) α-hemolysin. The released CGRP then inhibits the production of chemotactic cytokines by CX3CR1+ tissue-resident synovial lining macrophages via receptor activity modifying protein 1 (RAMP1) receptors at the onset of septic arthritis. During the subsequent chronic course of infection, the continuous release of CGRP triggered by pain has a lasting effect on the antimicrobial capabilities of macrophages, thereby promoting bacterial survival and joint damage. This evidence suggests a critical role for neuroimmune regulation in S. aureus-induced chronic septic arthritis. CGRP receptor antagonism may reduce joint destruction, thus providing a new option for treating bone and joint infections.

Resolution of inflammation during rheumatoid arthritis

Rheumatoid arthritis (RA) is a chronic autoimmune disease that causes synovial joint inflammation as well as bone destruction and erosion, typically characterized by joint pain, swelling, and stiffness, with complications and persistent pain after remission posing a significant health burden for RA patients. The etiology of RA has not yet been fully elucidated, but a large number of studies have shown that the initiation of inflammation in RA is closely related to T-cell activation, the production of a variety of pro-inflammatory cytokines, macrophage M1/M2 imbalance, homeostatic imbalance of the intestinal flora, fibroblast-like synoviocytes (FLSs) and synovial tissue macrophages (STMs) in the synovial lumen of joints that exhibit an aggressive phenotype. While the resolution of RA is less discussed, therefore, we provided a systematic review of the relevant remission mechanisms including blocking T cell activation, regulating macrophage polarization status, modulating the signaling pathway of FLSs, modulating the subpopulation of STMs, and inhibiting the relevant inflammatory factors, as well as the probable causes of persistent arthritis pain after the remission of RA and its pain management methods. Achieving resolution in RA is crucial for improving the quality of life and long-term prognosis of patients. Thus, understanding these mechanisms provide novel potential for further drug development and treatment of RA.

TREM2 Activation Relieves TMJOA by Stabilizing the Synovial Barrier via Siglec1

Triggering receptor expressed on myeloid cells 2 (TREM2) is an immune receptor that plays a vital role in innate immune responses. This study aims to investigate the effect of TREM2 on synovial barrier homeostasis and synovitis during temporomandibular joint osteoarthritis (TMJOA). The expression level of TREM2 is decreased in the synovium of both patients with TMJOA and a mouse model of TMJOA, accompanied by synovial barrier breakdown. TREM2 overexpression inhibits the macrophage inflammatory response ex vivo and relieves synovial inflammation, cartilage degeneration, and synovial barrier destruction in monosodium iodoacetate-induced TMJOA mice. RNA-seq analysis reveals that Siglec1 serves as a downstream signal that is downregulated after TREM2 activation. Further in vivo and in vitro experiments demonstrate that rhSiglec1 treatment promotes the synthesis and release of inflammatory cytokines, such as interleukin-6 and RANTES, in macrophages and reverses the alleviation effect of TREM2 activation on TMJOA synovial barrier disorders, synovial inflammation, cartilage degradation, and bone destruction. Overall, this study verifies that TREM2 activation alleviates TMJOA pathology by maintaining synovial barrier homeostasis and inhibiting synovial inflammation. These findings provide new insight into the mechanism of TREM2 in the pathogenesis of TMJOA.

Activation of proresolving macrophages in dorsal root ganglia attenuates persistent arthritis pain

Pain independent of disease activity is frequently reported by rheumatoid arthritis patients and remains undertreated. Preclinical evidence suggests that imbalance of neuroimmune proresolving interactions within dorsal root ganglia (DRG) rather than at the site of inflammation plays mechanistic roles in persistent arthritis pain. Here, we inhibited production of proresolving lipid mediators by silencing 12/15-lipoxygenase expression in CX3CR1+ monocyte/macrophages conditional knockout (cKO) mice. In an arthritis model, hind paw mechanical hypersensitivity is exacerbated in male and female cKO mice in association with DRG infiltration of neutrophils, which migrate in response to leukotriene B4 released by macrophages through 5-lipoxygenase conversion of arachidonic acid provided by neuron-derived vesicles. Neutrophils apoptosis promotes primary macrophage efferocytosis which is defective in cKO macrophages. In wild-type (WT) and cKO mice, intrathecal injection of MerTK activating antibody, attenuates persistent hypersensitivity and polarizes DRG macrophages toward a proresolving phenotype with production of antinociceptive lipoxin A4. Thus, we delineate a neuron-macrophage-neutrophil bidirectional circuit that can be exploited to reduce persistent arthritis pain.

NaHS@Cy5@MS@SP nanoparticles improve rheumatoid arthritis by inactivating the Hedgehog signaling pathway through sustained and targeted release of H2S into the synovium

BACKGROUND

Aberrant proliferation and inflammation of fibroblast-like synoviocytes (FLSs) significantly contribute to the pathogenesis of rheumatoid arthritis (RA). Deficiency of hydrogen sulfide (H2S) is a driving force for the development of RA, and the short half-life of the H2S-releasing donor sodium hydrosulfide (NaHS) limits its clinical application in RA therapy. Designing a targeted delivery system with slow-release properties for FLSs could offer novel strategies for treating RA.

METHODS

Herein, we designed a strategy to achieve slow release of H2S targeted to the synovium, which was accomplished by synthesizing NaHS-CY5@mesoporous silic@LNP targeted peptide Dil (NaHS@Cy5@MS@SP) nanoparticles.

RESULTS

Our results demonstrated that NaHS@Cy5@MS@SP effectively targets FLSs, upregulates H2S and its-producing enzyme cystathionine-γ-lyase (CSE) in the joints of arthritic mice. Overexpression of CSE inhibited the proliferation, migration, and inflammation of FLSs upon lipopolysaccharide (LPS) exposure, effects that were mimicked by NaHS@Cy5@MS@SP. In vivo studies showed that NaHS@Cy5@MS@SP achieved a threefold higher AUCinf than that of free NaHS, significantly improving the bioavailability of NaHS. Further, NaHS@Cy5@MS@SP inhibited synovial hyperplasia and reduced bone and cartilage erosion in the DBA/1J mouse model of collagen-induced arthritis (CIA), which was superior to NaHS. RNA sequencing and molecular studies validated that NaHS@Cy5@MS@SP inactivated the Hedgehog signaling pathway in FLSs, as evidenced by reductions in the protein expression of SHH, SMO, GLI1 and phosphorylated p38/MAPK.

CONCLUSION

This study highlights NaHS@Cy5@MS@SP as a promising strategy for the controlled and targeted delivery of H2S to synoviocytes, offering potential for RA management.

Adipose-derived stem cells attenuate rheumatoid arthritis by restoring CX3CR1+ synovial lining macrophage barrier

BACKGROUND

Rheumatoid arthritis (RA) is a chronic autoimmune disease and the integrity of CX3CR1+ synovial macrophage barrier significantly impacts its progression. However, the mechanisms driving the dynamic changes of this macrophage barrier remain unclear. Traditional drug therapies for RA have substantial limitations. Mesenchymal stem cells (MSCs)-based cell therapy, especially adipose-derived stem cells (ADSCs), hold therapeutic promise. Nevertheless, the underlying therapeutic mechanism of ADSCs, especially their interactions with CX3CR1+ macrophages, require further investigation.

METHODS

To explore the interaction between ADSCs and CX3CR1+ synovial macrophages during barrier reconstruction, underlying the therapeutic mechanism of ADSCs and the mechanisms on the dynamic changes of the macrophage barrier, scRNA-seq analysis was conducted 4 days after ADSCs injection in serum transfer-induced arthritis model mice. The roles of mitochondria transfer and ADSCs transplantation were also explored. Bulk RNA-seq analysis was performed after the co-culture of ADSCs and CX3CR1+ synovial macrophages. To study the in vivo fate of ADSCs, bulk RNA-seq was performed on ADSCs retrieved at 0, 2, 4, and 7 days post-injection.

RESULTS

Intra-articular injection of ADSCs effectively attenuated the pathological progression of mice with serum transfer-induced arthritis. ADSCs gradually adhered to CX3CR1+ macrophages, facilitating the restore of the macrophage barrier, while the absence of this barrier greatly weakened the therapeutic effect of ADSCs. scRNA-seq analysis revealed an Atf3high Ccl3high subset of CX3CR1+ macrophages with impaired oxidative phosphorylation that increased during RA progression. ADSCs-mediated reduction of this subset appeared to be linked to mitochondrial transfer, and transplantation of isolated ADSCs-derived mitochondria also proved effective in treating RA. Both bulk RNA-seq and scRNA-seq analyses revealed multiple interaction mechanisms between ADSCs and CX3CR1+ macrophages, including Cd74/Mif axis and GAS6/MERTK axis, which contribute to barrier restoration and therapeutic effects. Furthermore, bulk RNA-seq analysis showed that ADSCs primarily contribute to tissue repair and immune regulation subsequently.

CONCLUSIONS

Our results suggest that ADSCs ameliorated the energy metabolism signature of CX3CR1+ lining macrophages and may promote barrier restoration through mitochondria transfer. In addition, we elucidated the fate of ADSCs and the therapeutic potential of mitochondria in RA treatment.

Single cell immunoprofile of synovial fluid in rheumatoid arthritis with TNF/JAK inhibitor treatment

Numerous patients with rheumatoid arthritis (RA) manifest severe syndromes, including elevated synovial fluid volumes (SF) with abundant immune cells, which can be controlled by TNF/JAK inhibitors. Here, we apply single-cell RNA sequencing (scRNA-seq) and subsequent validations in SF from RA patients. These analyses of synovial tissue show reduced density of SF-derived pathogenic cells (e.g., SPP1+ macrophages and CXCL13+CD4+ T cells), altered gene expression (e.g., SPP1 and STAT1), molecular pathway changes (e.g., JAK/STAT), and cell-cell communications in drug-specific manners in samples from patients pre-/post-treated with adalimumab/tofacitinib. Particularly, SPP1+ macrophages exhibit pronounced communication with CXCL13+CD4+ T cells, which are abolished after treatment and correlate with treatment efficacy. These pathogenic cell types alone or in combination can augment inflammation of fibroblast-like synoviocytes in vitro, while conditional Spp1 knocking-out reduces RA-related cytokine expression in collagen-induced arthritis mice models. Our study shows the functional role of SF-derived pathogenic cells in progression and drug-specific treatment outcomes in RA.

Residual pain and fatigue are affected by disease perception in rheumatoid arthritis in sustained clinical and ultrasound remission

OBJECTIVE

Regardless of remission status, residual pain (RP) might persist in rheumatoid arthritis (RA). The aim of this study was to characterize RP, its perception, and patient-dependent features and to evaluate its possible association with residual synovitis in patients with RA in remission.

METHODS

Ninety-seven patients with RA, including 68 in sustained clinical and ultrasound remission (Rem/RA) and 29 in high/moderate DAS28-CRP disease activity (H-Mo/RA) were enrolled in the study. Thirty patients with fibromyalgia were enrolled as a control group(FIBRO). At study entry, demographic, clinical, ultrasound characteristics, and pain dimension assessment (VAS-pain, FACIT, CSI, GHQ, and RAID) were collected for each patient. RA patients underwent synovial tissue biopsy to evaluate the degree of synovitis using the Krenn synovitis score (KSS).

RESULTS

Forty-eight percent of Rem/RA still declared unacceptable pain (VAS-Pain > 20) compared to 80% of H-Mo/RA patients (p < 0.0001). Furthermore, Rem/RA patients presented comparable levels of pain dimension assessment regardless of KSS. However, classifying Rem/RA group based on RAID score (< 2 as satisfied SAT-Rem/RA and ≥ 2 as unsatisfied UNSAT-Rem/RA), SAT-Rem/RA group presented a lower grade of VAS-Pain (p < 0.0001), lower percentage of patients with an unacceptable pain (p < 0.0001) and lower grade of fatigue(p < 0.0001) compared to the UNSAT-Rem/RA patients. The percentage of SAT-Rem/RA patients who presented a disease flare did not differ from UNSAT-Rem/RA over the 24 months of follow-up. Finally, female Rem/RA patients presented higher VAS-Pain compared to male Rem/RA (p = 0.0119).

CONCLUSIONS

Moreover,73% satisfied female Rem/Ra patients presented an acceptable pain compared to 23% unsatisfied female Rem/RA patients (p = 0.001). RP in RA patients in remission can represent the way by which the patients communicate their state of non-acceptance of the disease. It can be useful to treat RP with the appropriate treatments. Key Points • Rheumatoid arthritis patients still reported unacceptable residual pain despite sustained clinical and ultrasound remission and despite the low grade/absence of histological synovitis. • Only a small rate of rheumatoid arthritis patients in sustained clinical and ultrasound remission showed residual pain as part of a central sensitivity syndrome or psychiatric disorders. • Rheumatoid arthritis patients in sustained clinical and ultrasound remission complained residual pain and fatigue as part of not acceptance of disease and/or dissatisfaction in the disease management.

LncRNA-MEG3/miR-93-5p/SMAD7 axis mediates proliferative and inflammatory phenotypes of fibroblast-like synoviocytes in rheumatoid arthritis

Synovial hyperplasia, inflammation and immune cell infiltration are the central pathological basis of rheumatoid arthritis (RA). Nonetheless, the cellular, molecular and immunological mechanisms of RA remain poorly understood. An integrated analysis of single-cell RNA (scRNA) and bulk RNA sequencing datasets‌ aimed to unravel the cellular landscape, differentiation trajectory, transcriptome signature, and immunoinfiltration feature of RA synovium. Multilevel experiments were conducted to investigate the role and mechanism of MEG3 in the aggravation and reversal of RA. We screened 97 intergroup differential genes of single-cell transcriptome profiles in the RA versus PsA comparison, which were principally associated with metabolism, inflammation, and proliferation. Clustering and annotation analysis defined 7 key cell subpopulations (monocytes, epithelial cells, CD8+T cells, granulocytes, fibroblasts, HSC_CD34+, and B cells) and their marker genes. Pseudotime analysis demonstrated that fibroblasts could be the end-effector cells, and that downregulation of MEG3 may be responsible for cell differentiation and state transition, followed by the malignant manifestations in the RA synovium. Mechanistically, overexpression of MEG3 could alleviate the proliferative and inflammatory phenotypes of RA synovial fibroblasts by competitively sponging miR-93-5p to promote SMAD7 expression. Taken together, our findings underscore the biological significance of MEG3/miR-93-5p/SMAD7 axis, providing valuable insights into the pathogenesis of RA.

Promise of rheumatoid arthritis therapy: From clinical deep remission to drug-free remission

Achieving clinical deep remission (CliDR) in rheumatoid arthritis (RA) is essential to prevent long-term joint damage, enhance patient quality of life, and possibly reduce and discontinue medication eventually. Recent research advances have raised the possibility of achieving deep remission and even drug-free remission. This comprehensive review examines current strategies of RA therapy, concept of deep remission, challenges, and the emerging prospects of drug-free remission. It also reviews the role of different treatments, including conventional disease-modifying antirheumatic drugs (DMARDs), biologic agents, and targeted synthetic drugs, in the journey from deep remission to drug-free remission. In addition, it emphasizes the importance of patient-centered care, early diagnosis, and individualized treatment approaches in optimizing outcomes for patients with RA.

Single-cell RNA sequencing in autoimmune diseases: New insights and challenges

Autoimmune diseases involve a variety of cell types, yet the intricacies of their individual roles within molecular mechanisms and therapeutic strategies remain poorly understood. Single-cell RNA sequencing (scRNA-seq) offers detailed insights into transcriptional diversity at the single-cell level, significantly advancing research in autoimmune diseases. This article explores how scRNA-seq enhances the understanding of cellular heterogeneity and its potential applications in the etiology, diagnosis, treatment, and prognosis of autoimmune diseases. By revealing a comprehensive cellular landscape, scRNA-seq illuminates the functional regulation of different cell subtypes during disease progression. It aids in identifying diagnostic and prognostic markers, and analyzing cell communication networks to uncover potential therapeutic targets. Despite its valuable contributions, addressing the limitations of scRNA-seq is essential for making further advancements.

Comprehensive analysis of myeloid reporter mice

Macrophages are a pivotal cell type within the synovial lining and sub-lining of the joint, playing a crucial role in maintaining homeostasis of synovium. Although fate-mapping techniques have been employed to differentiate synovial macrophages from other synovial myeloid cells, no comprehensive study has yet been conducted within the mouse synovial macrophage compartment. In this study, we present, for the first time, lineage tracing results from 18 myeloid-specific fate-mapping models in mouse peripheral blood (PB) and synovial tissue. The identification of synovial macrophages and monocyte-lineage cells through flow cytometry was further validated using cellular indexing of transcriptomes and epitopes by sequencing (CITE-seq) datasets. These findings provide a valuable methodological tool for researchers to select appropriate models for studying the function of synovial myeloid cells and serve as a reference for investigations in other tissue types.

CCN2 mediates fibroblast-macrophage interaction in knee arthrofibrosis based on single-cell RNA-seq analysis

Knee arthrofibrosis, characterized by excessive matrix protein production and deposition, substantially impairs basic daily functions, causing considerable distress and financial burden. However, the underlying pathomechanisms remain unclear. Here, we characterized the heterogeneous cell populations and cellular pathways by combination of flow cytometry and single-cell RNA-seq analysis of synovial tissues from six patients with or without knee arthrofibrosis. Increased macrophages and fibroblasts were observed with decreased numbers of fibroblast-like synoviocytes, endothelial cells, vascular smooth muscle cells, and T cells in the arthrofibrosis group compared with negative controls. Notably, fibroblasts were discovered to interact with macrophages, and lead to fibrosis through TGF-β pathway induced CCN2 expression in fibroblasts. CCN2 was demonstrated to be required for fibroblast pro-fibrotic functions (activation, proliferation, and migration) through TGFBR/SMAD pathway. The expression of CCN2 was positively correlated with the collagen volume and TGF-β expression and negatively associated with patient-reported outcome measures in another cohort of patients with knee arthrofibrosis. Our study reveals the role of CCN2 in the fibroblast-macrophage interaction through TGF-β pathway which might help to shed light on CCN2 as a potential biomarker.

Spatial transcriptomics in autoimmune rheumatic disease: potential clinical applications and perspectives

Spatial transcriptomics is a cutting-edge technology that analyzes gene expression at the cellular level within tissues while integrating spatial location information. This concept, which combines high-plex RNA sequencing with spatial data, emerged in the early 2010s. Spatial transcriptomics has rapidly expanded with the development of technologies such as in situ hybridization, in situ sequencing, in situ spatial barcoding, and microdissection-based methods. Each technique offers advanced mapping resolution and precise spatial assessments at the single-cell level. Over the past decade, the use of spatial transcriptomics on clinical samples has enabled researchers to identify gene expressions in specific diseased foci, significantly enhancing our understanding of cellular interactions and disease processes. In the field of rheumatology, the complex and elusive pathophysiology of diseases such as rheumatoid arthritis, systemic lupus erythematosus, and Sjögren's syndrome remains a challenge for personalized treatment. Spatial transcriptomics provides insights into how different cell populations interact within disease foci, such as the synovial tissue, kidneys, and salivary glands. This review summarizes the development of spatial transcriptomics and current insights into the pathophysiology of autoimmune rheumatic diseases, focusing on immune cell distribution and cellular interactions within tissues. We also explore the potential of spatial transcriptomics from a clinical perspective and discuss the possibilities for translating this technology to the bedside.

Synovial macrophage diversity and activation of M-CSF signaling in post-traumatic osteoarthritis

Synovium is home to immune and stromal cell types that orchestrate inflammation following a joint injury; in particular, macrophages are central protagonists in this process. We sought to define the cellular and temporal dynamics of the synovial immune niche in a mouse model of post-traumatic osteoarthritis (PTOA), and to identify stromal-immune crosstalk mechanisms that coordinate macrophage function and phenotype. We induced PTOA in mice using a non-invasive tibial compression model of anterior cruciate ligament rupture (ACLR). Single-cell RNA-sequencing and flow cytometry were used to assess immune cell populations in healthy (Sham) and injured (7 and 28 days post-ACLR) synovium. Characterization of synovial macrophage polarization states was performed, alongside computational modeling of macrophage differentiation, as well as implicated transcriptional regulators and stromal-immune communication axes. Immune cell types are broadly represented in healthy synovium, but experience drastic expansion and speciation in PTOA, most notably in the macrophage portion. We identified several polarization states of macrophages in synovium following joint injury, underpinned by distinct transcriptomic signatures, and regulated in part by stromal-derived macrophage colony-stimulating factor signaling. The transcription factors Pu.1, Cebpα, Cebpβ, and Jun were predicted to control differentiation of systemically derived monocytes into pro-inflammatory synovial macrophages. In summary, we defined different synovial macrophage subpopulations present in healthy and injured mouse synovium. Nuanced characterization of the distinct functions, origins, and disease kinetics of macrophage subtypes in PTOA will be critical for targeting these highly versatile cells for therapeutic purposes.

Energy metabolism in health and diseases

Energy metabolism is indispensable for sustaining physiological functions in living organisms and assumes a pivotal role across physiological and pathological conditions. This review provides an extensive overview of advancements in energy metabolism research, elucidating critical pathways such as glycolysis, oxidative phosphorylation, fatty acid metabolism, and amino acid metabolism, along with their intricate regulatory mechanisms. The homeostatic balance of these processes is crucial; however, in pathological states such as neurodegenerative diseases, autoimmune disorders, and cancer, extensive metabolic reprogramming occurs, resulting in impaired glucose metabolism and mitochondrial dysfunction, which accelerate disease progression. Recent investigations into key regulatory pathways, including mechanistic target of rapamycin, sirtuins, and adenosine monophosphate-activated protein kinase, have considerably deepened our understanding of metabolic dysregulation and opened new avenues for therapeutic innovation. Emerging technologies, such as fluorescent probes, nano-biomaterials, and metabolomic analyses, promise substantial improvements in diagnostic precision. This review critically examines recent advancements and ongoing challenges in metabolism research, emphasizing its potential for precision diagnostics and personalized therapeutic interventions. Future studies should prioritize unraveling the regulatory mechanisms of energy metabolism and the dynamics of intercellular energy interactions. Integrating cutting-edge gene-editing technologies and multi-omics approaches, the development of multi-target pharmaceuticals in synergy with existing therapies such as immunotherapy and dietary interventions could enhance therapeutic efficacy. Personalized metabolic analysis is indispensable for crafting tailored treatment protocols, ultimately providing more accurate medical solutions for patients. This review aims to deepen the understanding and improve the application of energy metabolism to drive innovative diagnostic and therapeutic strategies.

Exploring the Role of Macrophages and Their Associated Structures in Rheumatoid Arthritis

BACKGROUND

Rheumatoid arthritis (RA) is a chronic, invasive autoimmune disease characterized by symmetrical polyarthritis involving synovial inflammation. Epidemiological studies indicate that the incidence of RA continues to rise, yet the pathogenesis of this disease remains not fully understood. A significant infiltration of macrophages is observed in the synovium of RA patients. It can be inferred that macrophages likely play a crucial role in the onset and progression of RA.

SUMMARY

This review aims to summarize the research progress on the mechanisms by which macrophages and their associated structures contribute to RA, as well as potential therapeutic approaches, aiming to provide new insights into the study of RA pathogenesis and its clinical treatment.

KEY MESSAGES

During the course of RA, besides the inherent roles of macrophages, these cells respond to microenvironmental changes such as pathogen invasion or tissue damage by undergoing polarization, pyroptosis, or forming macrophage extracellular traps (METs), all of which influence inflammatory responses and immune homeostasis, thereby mediating the occurrence and development of RA. Additionally, macrophages secrete exosomes, which participate in intercellular communication and signal transduction processes, thus contributing to the progression of RA. Therefore, it is critical to elucidate how macrophages and their related structures function in RA.

BACKGROUND

Rheumatoid arthritis (RA) is a chronic, invasive autoimmune disease characterized by symmetrical polyarthritis involving synovial inflammation. Epidemiological studies indicate that the incidence of RA continues to rise, yet the pathogenesis of this disease remains not fully understood. A significant infiltration of macrophages is observed in the synovium of RA patients. It can be inferred that macrophages likely play a crucial role in the onset and progression of RA.

SUMMARY

This review aims to summarize the research progress on the mechanisms by which macrophages and their associated structures contribute to RA, as well as potential therapeutic approaches, aiming to provide new insights into the study of RA pathogenesis and its clinical treatment.

KEY MESSAGES

During the course of RA, besides the inherent roles of macrophages, these cells respond to microenvironmental changes such as pathogen invasion or tissue damage by undergoing polarization, pyroptosis, or forming macrophage extracellular traps (METs), all of which influence inflammatory responses and immune homeostasis, thereby mediating the occurrence and development of RA. Additionally, macrophages secrete exosomes, which participate in intercellular communication and signal transduction processes, thus contributing to the progression of RA. Therefore, it is critical to elucidate how macrophages and their related structures function in RA.

Spatially restricted and ontogenically distinct hepatic macrophages are required for tissue repair

Our understanding of the functional heterogeneity of resident versus recruited macrophages in the diseased liver is limited. A population of recruited lipid-associated macrophages (LAMs) has been reported to populate the diseased liver alongside resident Kupffer cells (KCs). However, the precise roles of these distinct macrophage subsets remain elusive. Here, using proteogenomics, we have identified LAMs in multiple models of liver injury. Moreover, we found that this phenotype is not specific to recruited macrophages, as a subset of resident KCs can also adopt a LAM-like phenotype in the mouse and human liver. By combining genetic mouse models targeting the distinct populations, we determined that both recruited LAMs and resident LAM-like KCs play crucial roles in tissue repair. Specifically, triggering receptor expressed on myeloid cells 2 (TREM2) expression on either resident or recruited macrophages is required for the efficient clearance of dying cells, enhancing repair and preventing exacerbated fibrosis.

ITGA5+ synovial fibroblasts orchestrate proinflammatory niche formation by remodelling the local immune microenvironment in rheumatoid arthritis

OBJECTIVES

To investigate the phenotypic heterogeneity of tissue-resident synovial fibroblasts and their role in inflammatory response in rheumatoid arthritis (RA).

METHODS

We used single-cell and spatial transcriptomics to profile synovial cells and spatial gene expressions of synovial tissues to identify phenotypic changes in patients with osteoarthritis, RA in sustained remission and active state. Immunohistology, multiplex immunofluorescence and flow cytometry were used to identify synovial fibroblasts subsets. Deconvolution methods further validated our findings in two cohorts (PEAC and R4RA) with treatment response. Cell coculture was used to access the potential cell-cell interactions. Adoptive transfer of synovial cells in collagen-induced arthritis (CIA) mice and bulk RNA sequencing of synovial joints further validate the cellular functions.

RESULTS

We identified a novel tissue-remodelling CD45-CD31-PDPN+ITGA5+ synovial fibroblast population with unique transcriptome of POSTN, COL3A1, CCL5 and TGFB1, and enriched in immunoregulatory pathways. This subset was upregulated in active and lympho-myeloid type of RA, associated with an increased risk of multidrug resistance. Transforming growth factor (TGF)-β1 might participate in the differentiation of this subset. Moreover, ITGA5+ synovial fibroblasts might occur in early stage of inflammation and induce the differentiation of CXCL13hiPD-1hi peripheral helper T cells (TPHs) from naïve CD4+ T cells, by secreting TGF-β1. Intra-articular injection of ITGA5+ synovial fibroblasts exacerbates RA development and upregulates TPHs in CIA mice.

CONCLUSIONS

We demonstrate that ITGA5+ synovial fibroblasts might regulate the RA progression by inducing the differentiation of CXCL13hiPD-1hi TPHs and remodelling the proinflammatory microenvironments. Therapeutic modulation of this subpopulation could therefore be a potential treatment strategy for RA.

A multiparametric risk table for loss of clinical remission status in patients with rheumatoid arthritis: a STARTER study post-hoc analysis

OBJECTIVE

This post-hoc analysis was carried out on data acquired in the longitudinal Sonographic Tenosynovitis Assessment in RheumaToid arthritis patiEnts in Remission (STARTER) study. Our primary aim was to determine the predictive clinical and musculoskeletal ultrasonographic (MSUS) features associated with disease flare in RA patients in clinical remission, while our secondary aim was to evaluate the probability of disease flare based on clinical and MSUS features.

METHODS

We analysed data for a total of 389 RA patients in DAS28-defined remission. All patients underwent a MSUS examination according to the OMERACT guidelines. Logistic regression and results, presented as odds ratio and 95% CI, were used for the evaluation of the association between selected variables and disease flare. Significant clinical and MSUS features were incorporated into a risk table for predicting disease flare within at least 12 months of follow-up in patients with RA remission.

RESULTS

Within 12 months, 137 (35%) RA patients experienced a disease flare. RA patients who experienced a flare disease differed from those with persistent remission in terms of ACPA positivity (75.9% vs 62.3%, respectively; P = 0.007), percentage of sustained clinical remission at baseline (44.1% vs 68.5%, respectively; P = 0.001) and synovium power Doppler signal presence (58.4% vs 33.3%, respectively; P < 0.001). Based on these results, these three features were considered in a predictive model of disease flare with an adjusted odds ratio of 3.064 (95% CI 1.728-5.432). Finally, a risk table was constructed including the three significant predictive factors of disease flare occurring within 12 months from the enrolment.

CONCLUSION

An adaptive flare-prediction model tool, based on data available in outpatient settings, was developed as a multiparametric risk table. If confirmed by external validation, this tool might support the defining of therapeutic strategies in RA patients in DAS28-defined remission status.

Resident synovial macrophages in synovial fluid: Implications for immunoregulation

Resident synovial macrophages (RSMs) are anti-inflammatory, self-renewing macrophages that provide physical immune sequestration of the joint space from the peripheral immune system. Increased permeability of this structure is associated with peripheral immune cells in the synovial fluid (SF). Direct measures of synovial barrier integrity are possible with tissue histology, but after barrier breakdown, if these cells perpetuate or initiate chronic inflammation in SF remains unknown. We sought to identify RSM in human SF as an indirect measure of synovial barrier integrity. To validate findings, we created a novel ex vivo explant model using human synovium. scRNA-seq revealed these SF RSMs upregulated pro-fibrotic and pro-osteoclastic pathways in inflammatory arthritis, but not septic arthritis. Increased frequencies of RSMs in SF was associated with increased sRANKL regardless of underlying pathology. These findings suggest the frequency of RSMs in SF may correlate with synovial barrier damage and in turn, potential damage to joint structures.

Metabolic checkpoints in rheumatoid arthritis

BACKGROUND

Rheumatoid Arthritis is a systemic autoimmune disease affecting 0.5-1 % of the population. Despite a growing therapeutic armamentarium, RA remains incurable, and many patients suffer significant morbidity over time. The strongest genetic risk derives from HLA class II polymorphisms, implicating T cells as pathogenic drivers. Innate immune cells, e.g. monocytes and macrophages (Mⱷ) contribute to chronic tissue inflammation through an array of pro-inflammatory functions but also present antigen to autoreactive T cells. Differentiation, survival, and effector functions of both T cells and Mⱷ are ultimately controlled by their bioenergetic and biosynthetic programs, identifying cellular metabolism as a critical disease mechanism in RA.

OBJECTIVES

Summarize current knowledge about metabolic conditions in the RA joint and disease-relevant metabolic circuits shaping the effector repertoire of RA T cells and Mⱷ.

RESULTS

The rheumatoid joint is a glucose deplete tissue environment, selecting for invading immune cells that can survive on non-glucose fuel sources. Inflamed synovium instead offers the amino acid glutamine and RA CD4+ T cells and RA Mⱷ rely on glutamine and glutamate to support their pathogenic functions. The metabolic hallmark of RA T cells is their low mitochondrial performance, resulting in low ATP production, low generation of reactive oxygen species (ROS) and low availability of tricarboxylic acid (TCA) cycle intermediates, all shifting RA T cells towards autoreactivity. The underlying defect stems from insufficient repair of mitochondrial DNA (mtDNA). Functional consequences include reversal of the TCA cycle, accumulation of citrate and lack of malate production. Excessive citrate promotes cytoskeletal hyperacetylation, creating hypermigratory and tissue-invasive T cells. Surplus acetyl-CoA supports lipid droplet formation and lipotoxicity. Lack of malate production disrupts the malate-aspartate shuttle, restricts recovery of cytosolic NAD and drives the endoplasmic reticulum (ER) into expansion. The bioenergetically stressed ER accumulates TNF mRNA and turns RA T cells into TNF superproducers. ATP low production renders RA T cells susceptible to cell death, depositing highly inflammatory mtDNA in the tissue. Mitochondrial deficiency leads to a slowdown in glycolysis and pyruvate processing, such that RA CD4+ T cells shunt glucose towards the pentose phosphate pathway to support nucleotide synthesis and clonal proliferation. Metabolically deprived CD4+ T cells partner with Mⱷ that have highly functional mitochondria. A hallmark of RA Mⱷ is the high expression of the DNA binding protein RFX5, which co-ordinates adaptations to metabolic needs with function. RFX5 upregulates HLA-DR expression and induces the glutaminolytic enzyme glutamate dehydrogenase 1 (GLUD1), providing bioenergetic resources for antigen presentation and survival in the tissue. In essence, RA CD4+ T cells and Mⱷ function in a metabolically challenging environment and rewire their cellular metabolism to survive. Metabolic adaptations promote immunostimulation and tissue inflammation, triggering and sustaining rheumatoid synovitis.

Mitochondrial Oxidative Stress Related Diagnostic Model Accurately Assesses Rheumatoid Arthritis Risk Stratification and Immune Infiltration Characterization

Rheumatoid arthritis (RA) is a chronic autoimmune disease that affects synovial joints, leading to joint destruction, impaired physical function, and reduced quality of life. However, no accurate method for assessing RA risk currently exists. Given the critical role of early detection and intervention in RA management, further comprehensive risk assessments are essential. Mitochondrial oxidative stress (MOS) is a key factor in the initiation and progression of RA. The bidirectional interaction between RA and MOS supports the feasibility of MOS-based risk stratification for RA. Using public databases, we first applied the weighted gene co-expression network analysis (WGCNA) model to identify key genes involved in RA among MOS-related genes. Differential expression patterns of MOS-related genes were then analyzed using various machine learning algorithms to identify potential biomarkers. A nomogram model was established using CDKN1A, GADD45B, and MAFF genes to predict RA risk, followed by an evaluation of its reliability and stability. Additionally, we analyzed MOS-associated molecular subtypes and immune infiltration characteristics. Our findings highlight the significant role of MOS in RA development and underscore the clinical value of personalized treatment strategies.

IĸBζ as a Central Modulator of Inflammatory Arthritis Pathogenesis

OBJECTIVE

Current therapies targeting individual factors in inflammatory arthritis show variable efficacy, often requiring treatment with combinations of drugs, and are associated with undesirable side effects. NF-ĸB is critical for the production and function of most inflammatory cytokines. However, given its essential role in physiologic processes, targeting NF-ĸB is precarious. Hence, identifying pathways downstream of NF-ĸB that selectively govern the expression of inflammatory cytokines in inflammatory arthritis would be advantageous. We have previously identified IĸBζ as a unique inflammatory signature of NF-ĸB that controls the transcription of inflammatory cytokines only under pathologic conditions while sparing physiologic NF-ĸB signals.

METHODS

We generated mice harboring myeloid, lymphoid, and global deletion of Nfkbiz (the gene encoding IĸBζ). These models were subjected to serum transfer-induced arthritis. Additionally, pharmacologic inhibitors of IĸBζ were injected intraperitonially. Joint swelling, microcomputed tomography, immunohistochemistry, flow cytometry, and cytokine measurements were conducted using synovial tissue samples.

RESULTS

Global deletion of Nfkbiz or depletion of neutrophils (vastly IĸBζ+ cells) reduced inflammatory synovial cells and increased anti-inflammatory and regenerative synovial cells, plummeted expression of inflammatory factors and ameliorated experimental mouse inflammatory arthritis. Further, expression of immune responsive gene-1, the enzyme responsible for itaconate production, was increased in synovial cells. Accordingly, the itaconate derivative dimethyl itaconate (DI) inhibited IĸBζ-mediated inflammatory factors. Further, in silico screen identified 8-hydroxyquinoline (HQ) as a putative inhibitor of IĸBζ not affecting physiologic NF-ĸB activity. Congruently, systemic administration of either DI or HQ inhibited joint swelling and damage.

CONCLUSION

Our study positions IĸBζ as an inflammation-specific target for therapeutic consideration in rheumatoid arthritis because its inhibition spares the beneficial functions of NF-ĸB.

Prediction of long-term drug-free outcomes in ACPA-positive and ACPA-negative rheumatoid arthritis by combined clinical and ultrasound assessment of residual disease: a 5-year prospective study

OBJECTIVE

To delineate, within the framework of current clinical practice and criteria, the sustainability of first-line immuno-suppressive treatment discontinuation in rheumatoid arthritis (RA) and the impact of residual disease in remission on long-term drug-free (DF) outcomes.

METHODS

RA patients, referring to the Pavia early arthritis clinic (EAC) between 2009 and 2021 and achieving remission after Disease Activity Score-driven methotrexate (MTX) monotherapy, were recruited. Eligible patients underwent DF follow-up at 3-month intervals over 5 years after MTX discontinuation. Pre-selected clinical, serological and ultrasound (US) exposure variables at MTX withdrawal were analysed using multivariable Cox regression to predict time-to-flare.

RESULTS

Of 761 EAC patients with RA, 132 started DF follow-up (person-months: 3678). 62 experienced a flare after a median (range) of 9 (3-60) months, resulting in a progressive decline in flare-free survival throughout the observation period. Whole-cohort multivariate Cox regression identified anti-citrullinated protein antibody (ACPA) positivity (HR: 4.20, 95% CI 2.37 to 7.44) and hands' joints with grey scale (US-GS) alterations (GS>1; HR: 2.18, 95% CI 1.20 to 3.93) as independent predictors. ACPA-positive patients in Simplified Disease Activity Index (SDAI) remission displayed a flare-free survival estimate at 5 years of 6.4% (95% CI 1.2 to 35.7) versus 78.2% (95% CI 67.4 to 90.8) for ACPA-negative patients in SDAI remission without residual US-GS alteration in hands' joints (n=59); the latter group showing no evidence of radiographic progression and functional deterioration.

CONCLUSIONS

Long-term DF remission is attainable in a niche subset of ACPA-negative RA. Examining clinical and subclinical residual synovial abnormalities during remission allows for effective preemptive identification of this subset in real life.

Regulation of macrophage polarization and pyroptosis by 4-methylcatechol alleviates collagen-induced arthritis via Nrf2/HO-1 and NF-κB/NLRP3 signaling pathways

Rheumatoid arthritis (RA) is a chronic inflammatory disease that leads to joint deformity and ultimately disability. The metabolite of quercetin, 4-Methylcatechol (4-MC), has been acknowledged for its anti-inflammatory and antioxidant properties; however, the protective effects of 4-MC on RA and its underlying mechanisms remain incompletely elucidated. In a collagen-induced arthritis (CIA) model, we observed that 4-MC effectively mitigated joint inflammation and bone destruction in CIA mice. Additionally, it significantly suppressed the upregulated expression of inflammatory cytokines in synovial tissues. Mechanistically, upon lipopolysaccharide (LPS) stimulation, 4-MC inhibited M1 polarization of macrophages and induced a phenotypic switch from M1 to M2 phenotype, thereby reducing the release of pro-inflammatory cytokines by M1 macrophages while increasing the release of anti-inflammatory cytokines by M2 macrophages. Furthermore, it attenuated LPS/adenosine triphosphate (ATP)-induced pyroptosis in macrophages by downregulating NLRP3 expression levels along with cleaved caspase-1, cleaved IL-1β, and GSDMD-NT expression levels. Notably, our findings revealed that 4-MC exerted inhibitory effects on the NF-κB signaling pathway through specific modulation of the NF-κB complex as well as phosphorylation of the upstream IKK kinase complex. Collectively, these results highlight significant therapeutic potential for utilizing 4-MC in RA treatment.

CD34hi subset of synovial fibroblasts contributes to fibrotic phenotype of human knee osteoarthritis

Osteoarthritis (OA) shows various clinical manifestations depending on the status of its joint components. We aimed to identify the synovial cell subsets responsible for OA pathophysiology by comprehensive analyses of human synovium samples in single-cell resolution. Two distinct OA synovial tissue groups were classified by gene expression profiles in RNA-Seq: inflammatory and fibrotic. The inflammatory group exhibited high expression of inflammatory cytokines, histologically inflammatory infiltrate, and a more severe pain score. The fibrotic group showed higher expression of fibroblast growth factor (FGFs) and bone morphogenetic proteins (BMPs), showed histologically perivascular fibrosis, and showed a lower pain score. In single-cell RNA-Seq (scRNA-Seq) of synovial cells, MERTKloCD206lo macrophages and CD34hi fibroblasts were associated with the inflammatory and fibrotic groups, respectively. Among the 3 fibroblast subsets, CD34loTHY1lo and CD34loTHY1hi fibroblasts were influenced by synovial immune cells, whereas CD34hi fibroblasts were influenced by mural and endothelial cells. Particularly, in CD34hi fibroblast subsets, CD34hiCD70hi fibroblasts promoted proliferation of Tregs, potentially suppressing synovitis and protecting articular cartilage. Elucidation of the mechanisms underlying the regulation of these synovial cell subsets may lead to novel strategies for OA therapeutics.

Suberosin attenuates rheumatoid arthritis by repolarizing macrophages and inhibiting synovitis via the JAK/STAT signaling pathway

BACKGROUND

Rheumatoid arthritis (RA) is a systemic disease that primarily manifests as chronic synovitis of the symmetric small joints. Despite the availability of various targeted drugs for RA, these treatments are limited by adverse reactions, warranting new treatment approaches. Suberosin (SBR), isolated from Plumbago zeylanica-a medicinal plant traditionally used to treat RA in Asia-possesses notable biological activities. This study aimed to investigate the effects and potential underlying pathways of SBR on RA.

METHODS

Tumor necrosis factor-alpha (TNF-α) induced inflammation in RA-derived fibroblast-like synoviocytes (RA-FLS), and the expression of proinflammatory mediators was assessed using q-RT PCR and ELISA after treatment with various SBR concentrations. Bone marrow-derived macrophages (BMDMs) were induced to differentiate into M1 and M2 macrophages, followed by treatment with various SBR concentrations and macrophage polarization assessment. Low-dose (0.5 mg/kg/d) and high-dose (2 mg/kg/d) SBR regimens were administered to a collagen-induced arthritis (CIA) mouse model for 21 days, and the anti-arthritic effects of SBR were evaluated. Network pharmacology and molecular docking analyses were used to predict the anti-arthritic targets of SBR. The effect of SBR on the Janus kinase/signal transducer and activator of transcription (JAK/STAT) pathway was evaluated.

RESULTS

SBR suppressed macrophage polarization toward the M1 phenotype while enhancing their polarization toward the M2 phenotype. SBR reduced the levels of proinflammatory mediators in TNF-α-induced RA-FLS. Mechanistically, SBR inhibited the phosphorylation of the JAK1/STAT3 signaling pathway in RA-FLS and M1 macrophages and promoted the phosphorylation of the JAK1/STAT6 pathway in M2 macrophages, enhancing M2 polarization. In vivo, prophylactic treatment of low-dose SBR reduced M1 macrophage infiltration into synovial tissue, increased the proportion of M2 macrophages, and decreased the expression of inflammatory mediators in the serum and synovial tissue, alleviating synovial inflammation. SBR significantly alleviated arthritis in CIA mice through macrophage repolarization and inhibition of inflammation.

CONCLUSION

SBR significantly reduced clinical symptoms, joint pathological damage, and expression inflammatory cytokine expression in CIA mice. SBR exhibited anti-arthritic effects via the JAK1/STAT3 and JAK1/STAT6 signaling pathways, inhibiting synovial tissue inflammation and M1 macrophage polarization while promoting M2 macrophage polarization. Therefore, SBR may be an effective candidate for RA treatment.

Exploring perinatal mesenchymal stromal cells as a potential therapeutic strategy for rheumatoid arthritis

Rheumatoid arthritis (RA) is a chronic inflammatory autoimmune disease characterized by inflammation in the synovial tissue, driven by aberrant activation of both the innate and adaptive immune systems, which can lead to irreversible disability. Despite the increasing therapeutic approaches for RA, only a low percentage of patients achieve sustained disease remission, and the persistence of immune dysregulation is likely responsible for disease recurrence once remission is attained. Cell therapy is an attractive, wide-spectrum strategy to modulate inflammation, and mesenchymal stromal cells (MSC) derived from perinatal tissues provide valuable tools for their use in regenerative medicine, mainly due to their immunomodulatory properties. Several in vitro studies have shown that perinatal MSC modulate the proliferation, maturation, and cytokine secretion profile of both innate and adaptive immune cells. Moreover, different beneficial effects have also been described when perinatal MSC were used to treat animal models of diseases associated with inflammatory conditions and degenerative processes. Specifically, in experimental models of RA, treatment with perinatal MSC resulted in a strong reduction of articular damage, which was associated with the modulation of both inflammation and activation of stromal resident cells in the synovial tissue. Here, we present in vitro and in vivo evidence supporting the use of perinatal MSC in RA. We also highlight the promising results from the few published clinical trials, which demonstrate the safety of perinatal MSC.