Synovial tissue research: a state-of-the-art review

Bio-nanoparticles loaded with synovial-derived exosomes ameliorate osteoarthritis progression by modifying the oxidative microenvironment

BACKGROUND AND AIMS

Osteoarthritis (OA) is a prevalent degenerative joint disorder, marked by the progressive degeneration of joint cartilage, synovial inflammation, and subchondral bone hyperplasia. The synovial tissue plays a pivotal role in cartilage regulation. Exosomes (EXOs), small membrane-bound vesicles released by cells into the extracellular space, are crucial in mediating intercellular communication and facilitating the exchange of information between tissues. Our study aimed to devise a hydrogel microsphere infused with SOD3-enriched exosomes (S-EXOs) to protect cartilage and introduce a novel, effective approach for OA treatment.

MATERIALS AND METHODS

We analyzed single-cell sequencing data from 4247 cells obtained from the GEO database. Techniques such as PCR, Western Blot, immunofluorescence (IF), and assays to measure oxidative stress levels were employed to validate the cartilage-protective properties of the identified key protein, SOD3. In vivo, OA mice received intra-articular injections of S-EXOs bearing hydrogel microspheres, and the effectiveness was assessed using safranine O (S.O) staining and IF.

RESULTS

Single-cell sequencing data analysis suggested that the synovium influences cartilage via the exocrine release of SOD3. Our findings revealed that purified S-EXOs enhanced antioxidant capacity of chondrocytes, and maintained extracellular matrix metabolism stability. The S-EXO group showed a significant reduction in mitoROS and ROS levels by 164.2% (P < 0.0001) and 142.7% (P < 0.0001), respectively, compared to the IL-1β group. Furthermore, the S-EXO group exhibited increased COL II and ACAN levels, with increments of 2.1-fold (P < 0.0001) and 3.1-fold (P < 0.0001), respectively, over the IL-1β group. Additionally, the S-EXO group showed a decrease in MMP13 and ADAMTS5 protein expression by 42.3% (P < 0.0001) and 44.4% (P < 0.0001), respectively. It was found that S-EXO-containing hydrogel microspheres could effectively deliver SOD3 to cartilage and significantly mitigate OA progression. The OARSI score in the S-EXO microsphere group markedly decreased (P < 0.0001) compared to the OA group.

CONCLUSION

The study demonstrated that the S-EXOs secreted by synovial fibroblasts exert a protective effect on chondrocytes, and microspheres laden with S-EXOs offer a promising therapeutic alternative for OA treatment.

Laser Capture Microscopy RNA Sequencing for Topological Mapping of Synovial Pathology During Rheumatoid Arthritis

OBJECTIVE

Rheumatoid arthritis (RA) is an autoimmune disease in which the joint lining or synovium becomes highly inflamed and majorly contributes to disease progression. Understanding pathogenic processes in RA synovium is critical for identifying therapeutic targets. We performed laser capture microscopy (LCM) followed by RNA sequencing (LCM-RNAseq) to study regional transcriptomes throughout RA synovium.

METHODS

Synovial lining, sublining, and vessel samples were captured by LCM from seven patients with RA and seven patients with osteoarthritis (OA). RNAseq was performed on RNA extracted from captured tissue. Principal component analysis was performed on the sample set by disease state. Differential expression analysis was performed between disease states based on log2 fold change and q value parameters. Pathway analysis was performed using the Reactome Pathway Database on differentially expressed genes among disease states. Significantly enriched pathways in each synovial region were selected based on the false discovery rate.

RESULTS

RA and OA transcriptomes were distinguishable by principal component analysis. Pairwise comparisons of synovial lining, sublining, and vessel samples between RA and OA revealed substantial differences in transcriptional patterns throughout the synovium. Hierarchical clustering of pathways based on significance revealed a pattern of association between biologic function and synovial topology. Analysis of pathways uniquely enriched in each region revealed distinct phenotypic abnormalities. As examples, RA lining samples were marked by anomalous immune cell signaling, RA sublining samples were marked by aberrant cell cycle, and RA vessel samples were marked by alterations in heme scavenging.

CONCLUSION

LCM-RNAseq confirms reported transcriptional differences between the RA synovium and the OA synovium and provides evidence supporting a relationship between synovial topology and molecular anomalies in RA.

A landscape of gene expression regulation for synovium in arthritis

The synovium is an important component of any synovial joint and is the major target tissue of inflammatory arthritis. However, the multi-omics landscape of synovium required for functional inference is absent from large-scale resources. Here we integrate genomics with transcriptomics and chromatin accessibility features of human synovium in up to 245 arthritic patients, to characterize the landscape of genetic regulation on gene expression and the regulatory mechanisms mediating arthritic diseases predisposition. We identify 4765 independent primary and 616 secondary cis-expression quantitative trait loci (cis-eQTLs) in the synovium and find that the eQTLs with multiple independent signals have stronger effects and heritability than single independent eQTLs. Integration of genome-wide association studies (GWASs) and eQTLs identifies 84 arthritis related genes, revealing 38 novel genes which have not been reported by previous studies using eQTL data from the GTEx project or immune cells. We further develop a method called eQTac to identify variants that could affect gene expression by affecting chromatin accessibility and identify 1517 regions with potential regulatory function of chromatin accessibility. Altogether, our study provides a comprehensive synovium multi-omics resource for arthritic diseases and gains new insights into the regulation of gene expression.

Different molecular weights of hyaluronan research in knee osteoarthritis: A state-of-the-art review

Osteoarthritis (OA), the most common form of arthritis, is characterized by progressive cartilage destruction, concomitant adaptive osteogenesis, and loss of joint function. The progression of OA with aging is associated with a decrease in native hyaluronan (HA, hyaluronate or hyaluronic acid) with a high molecular weight (HMW) in synovial fluid and a subsequent increase in lower MW HA and fragments. As HMW HA possesses numerous biochemical and biological properties, we review new molecular insights into the potential of HA to modify OA processes. Different MWs in the formulation of products appear to have varying effects on knee OA (KOA) pain relief, improved function, and postponing surgery. In addition to the safety profile, more evidence indicates that intraarticular (IA) HA administration may be an effective option to treat KOA, with a particular emphasis on the use of HA with fewer injections of higher MW, including potential applications of HA of very HMW. We also analyzed published systemic reviews and meta-analyses of IA HA in treating KOA in order to discuss their conclusions and consensus statements. According to its MW, HA may offer a simple way to refine therapeutic information in selective KOA.

Inside the Joint of Inflammatory Arthritis Patients: Handling and Processing of Synovial Tissue Biopsies for High Throughput Analysis

Inflammatory arthritis is a chronic systemic autoimmune disease of unknown etiology, which affects the joints. If untreated, these diseases can have a detrimental effect on the patient's quality of life, leading to disabilities, and therefore, exhibit a significant socioeconomic impact and burden. While studies of immune cell populations in arthritis patient's peripheral blood have been informative regarding potential immune cell dysfunction and possible patient stratification, there are considerable limitations in identifying the early events that lead to synovial inflammation. The joint, as the site of inflammation and the local microenvironment, exhibit unique characteristics that contribute to disease pathogenesis. Understanding the contribution of immune and stromal cell interactions within the inflamed joint has been met with several technical challenges. Additionally, the limited availability of synovial tissue biopsies is a key incentive for the utilization of high-throughput techniques in order to maximize information gain. This review aims to provide an overview of key methods and novel techniques that are used in the handling, processing and analysis of synovial tissue biopsies and the potential synergy between these techniques. Herein, we describe the utilization of high dimensionality flow cytometric analysis, single cell RNA sequencing, ex vivo functional assays and non-intrusive metabolic characterization of synovial cells on a single cell level based on fluorescent lifetime imaging microscopy. Additionally, we recommend important points of consideration regarding the effect of different storage and handling techniques on downstream analysis of synovial tissue samples. The introduction of new powerful techniques in the study of synovial tissue inflammation, brings new challenges but importantly, significant opportunities. Implementation of novel approaches will accelerate our path toward understanding of the mechanisms involved in the pathogenesis of inflammatory arthritis and lead to the identification of new avenues of therapeutic intervention.

New Druggable Targets for Rheumatoid Arthritis Based on Insights From Synovial Biology

Rheumatoid arthritis (RA) is a multifactorial autoimmune disease characterized by chronic inflammation and destruction of multiple small joints which may lead to systemic complications. Altered immunity via pathogenic autoantibodies pre-date clinical symptom development by several years. Incompletely understood range of mechanisms trigger joint-homing, leading to clinically evident articular disease. Advances in therapeutic approaches and understanding pathogenesis have improved prognosis and likely remission. However, partial/non-response to conventional and biologic therapies witnessed in a subset of patients highlights the need for new therapeutics. It is now evident that joint disease chronicity stems from recalcitrant inflammatory synovial environment, majorly maintained by epigenetically and metabolically reprogrammed synoviocytes. Therefore, interference with effector functions of activated cell types seems a rational strategy to reinstate synovial homeostasis and complement existing anti-inflammatory interventions to mitigate chronic RA. Presenting this newer aspect of fibroblast-like synoviocytes and myeloid cells underlying the altered synovial biology in RA and its potential for identification of new druggable targets is attempted in this review. Major leads from i) molecular insights of pathogenic cell types from hypothesis free OMICS approaches; ii) hierarchy of their dysregulated signaling pathways; and iii) knowledge of druggability of molecular nodes in these pathways are highlighted. Development of such synovial biology-directed therapeutics hold promise for an enriched drug repertoire for RA.

SIX1 attenuates inflammation and rheumatoid arthritis by silencing MyD88-dependent TLR1/2 signaling

OBJECTIVES

Rheumatoid arthritis (RA) is a chronic autoimmune disease that severely affects the patients' quality of life. Sine oculis homeobox 1 (SIX1) has been reported as a key regulator of organogenesis and inflammation. This study aimed to explore the effects of SIX1 on RA.

METHODS

Wistar rats were immunized with type II collagen to induce an animal model of RA. RA synovial fibroblasts (RASFs) were isolated from the rats. SIX1 expression in RA rats and RASFs was detected by qRT-PCR and western blot. CCK-8, EdU, transwell, flow cytometer, and ELISA were conducted to assay the effects of SIX1 on RASFs. The effects of SIX1 on RA rats were studied by Safranin O staining, H&E staining, and ELISA. Besides, GSEA and KEGG analysis were used to predict the underlying signaling pathways.

RESULTS

SIX1 was low expressed in synovial tissue of RA rats and RASFs. SIX1 overexpression inhibited the proliferation, invasion, and levels of TNF-α, IL-6, and IL-8 in RASFs. However, SIX1 overexpression promoted the apoptosis of RASFs. SIX1 overexpression enhanced body weight, and attenuated the cartilage damage, pathological injury, and pro-inflammatory cytokine release of RA rat model. MyD88-dependent TLR1/2 might be a downstream signaling of SIX1. RelA acted as a transcription factor of TLR1/2, and SIX1 inhibited TLR1/2 signaling possibly via interaction with RelA. Adding with Pam₃CSK₄, a specific agonist of TLR1/2 signaling, attenuated the effects of SIX1 on RASFs.

CONCLUSION

SIX1 attenuated inflammation and RA by silencing MyD88-dependent TLR1/2 signaling. SIX1 may be a promising target for RA treatment.

Herbal Formula Longteng Decoction Promotes the Regression of Synovial Inflammation in Collagen-Induced Arthritis Mice by Regulating Type 2 Innate Lymphocytes

The etiology and pathogenesis of rheumatoid arthritis (RA) have not yet been fully elucidated, with greater adverse drug effects in traditional treatment of RA. It is particularly necessary to develop and study Chinese herbal formula as a supplement and alternative drug for the treatment of RA. The traditional Chinese medicine compound Longteng Decoction (LTD), as an empirical prescription in the treatment of RA in Dongzhimen Hospital of Beijing University of Chinese Medicine, has been widely used in clinic. Type 2 innate lymphocytes (ILC2s) have specific transcription factors and signature cytokines that are very similar to Th cells, which have been proved to be necessary in addressing RA inflammation, and are potential targets for RA prevention and treatment. Our previous studies have confirmed that LTD can intervene in the differentiation of peripheral blood Th17 and Treg cells, reduce joint pain index and swelling degree, shorten the time of morning stiffness, reduce ESR, and inhibit joint inflammation. However, it is unclear whether LTD can promote the regression of RA synovial inflammation by regulating the immune response mechanism of ILC2s.Therefore, our team established a collagen-induced arthritis mouse model and conducted an experimental study with LTD as the intervention object. The results showed that joint swelling, synovial inflammatory infiltration, and articular cartilage destruction were alleviated in CIA mice after intervention with LTD. The proliferation and differentiation of Th17 inflammatory cells and the secretion of proinflammatory cytokines (IL-17 and IFN-γ) were inhibited. In addition, LTD can also activate ILC2s to secrete the anti-inflammatory cytokine IL-4, activate the STAT6 signaling pathway, and act synergistic with Treg cells to inhibit the infiltration of type M1 macrophages in synovial tissue and promote its transformation to M2 phenotype. Taken together, these results confirm that LTD can be used as an adjunct or alternative to RA therapy by modulating the ILC2s immune response network and slowing down the inflammatory process of synovial tissue.

Mesenchymal Stem Cell-Originated Exosomal Circular RNA circFBXW7 Attenuates Cell Proliferation, Migration and Inflammation of Fibroblast-Like Synoviocytes by Targeting miR-216a-3p/HDAC4 in Rheumatoid Arthritis

Background

Rheumatoid arthritis (RA) is a chronic autoimmune disease of articular joint damage and elevated synovial hyperplasia. Abnormal proliferation, invasion inflammatory response of rheumatoid fibroblast-like synoviocytes (RA-FLS) play a critical role in RA progression. Mesenchymal stem cell (MSC)–derived exosomal circular RNAs are promising therapeutic manner for disease treatment. This work aimed to decipher the role of exosomal circFBXW7 in RA.

Methods

The expression of circFBXW7, miR-216a-3p, and HDAC4 were detected in clinical RA samples. The RA rat model was established. Isolation and identification of exosomes from MSCs was conducted. The effects of exosomal circFBXW7 on RA was evaluated by qPCR, CCK-8, transwell assays, flow cytometry, Western blotting, ELISA, and immunohistochemical assay. Interaction between miR-216a-3p and circFBXW7 or HDAC4 was determined by luciferase reporter gene assay and RNA pulldown.

Results

Exosomal circFBXW7 treatment suppressed proliferation, migration and inflammatory response of RA-FLSs and damage of RA model. CircFBXW7 could directly sponge miR-216a-3p to upregulate the expression of HDAC4. Inhibition of HDAC4 or upregulation of miR-216a-3p abolished the therapeutic function of exosomal circFBXW7. Our data demonstrated that circFBXW7 and HDAC4 were decreased, and miR-216a-3p was elevated in clinical RA sample compared with healthy samples.

Conclusion

We concluded that MSC-derived exosomal circFBXW7 suppressed proliferation, migration and inflammatory response of RA-FLSs and damage of RA rats via sponging miR-216a-3p and release the activation of HDAC4. These findings may provide a novel therapeutic target for RA.

A convolutional neural network with transfer learning for automatic discrimination between low and high-grade synovitis: a pilot study

Ultrasound-guided synovial tissue biopsy (USSB) may allow personalizing the treatment for patients with inflammatory arthritis. To this end, the quantification of tissue inflammation in synovial specimens can be crucial to adopt proper therapeutic strategies. This study aimed at investigating whether computer vision may be of aid in discriminating the grade of synovitis in patients undergoing USSB. We used a database of 150 photomicrographs of synovium from patients who underwent USSB. For each hematoxylin and eosin (H&E)-stained slide, Krenn's score was calculated. After proper data pre-processing and fine-tuning, transfer learning on a ResNet34 convolutional neural network (CNN) was employed to discriminate between low and high-grade synovitis (Krenn's score < 5 or ≥ 5). We computed test phase metrics, accuracy, precision (true positive/actual results), and recall (true positive/predicted results). The Grad-Cam algorithm was used to highlight the regions in the image used by the model for prediction. We analyzed photomicrographs of specimens from 12 patients with arthritis. The training dataset included n.90 images (n.42 with high-grade synovitis). Validation and test datasets included n.30 (n.14 high-grade synovitis) and n.30 items (n.16 with high-grade synovitis). An accuracy of 100% (precision = 1, recall = 1) was scored in the test phase. Cellularity in the synovial lining and sublining layers was the salient determinant of CNN prediction. This study provides a proof of concept that computer vision with transfer learning is suitable for scoring synovitis. Integrating CNN-based approach into real-life patient management may improve the workflow between rheumatologists and pathologists.

Arthroscopic Guided Synovial Biopsies

Synovial tissue can be safely and reliably collected for research and clinical purposes using arthroscopy. This technique offers the obvious advantage of allowing direct visualization, and targeted biopsy of specific areas of interest within the joint, as well as for the collection of tissue which will include a lining layer. Much has been learnt by studying the synovium retrieved using this technique concerning the pathobiology of inflammatory arthritis. Furthermore, recent evidence suggests that the tissue retrieved may enable the identification of unique pathotypes that will allow for a precise approach to treatment selection in individual patients. Although ultrasound guided techniques for sampling synovial tissue have gained in popularity over the last decade, both methodologies are expected to compliment each other, each having unique benefits and drawbacks. We present here a detailed description of the arthroscopy technique reporting on our collective experience at two centers in Europe.

Blockade of translationally controlled tumor protein attenuated the aggressiveness of fibroblast-like synoviocytes and ameliorated collagen-induced arthritis

Histamine releasing factor/translationally controlled tumor protein (HRF/TCTP) stimulates cancer progression and allergic responses, but the role of HRF/TCTP in rheumatoid arthritis (RA) remains undefined. In this study, we explored the pathogenic significance of HRF/TCTP and evaluated the therapeutic effects of HRF/TCTP blockade in RA. HRF/TCTP transgenic (TG) and knockdown (KD) mice with collagen-induced arthritis (CIA) were used to determine the experimental phenotypes of RA. HRF/TCTP levels in the sera of RA patients were measured and compared to those from patients with osteoarthritis (OA), ankylosing spondylitis, Behçet's disease, and healthy controls. HRF/TCTP expression was also assessed in the synovium and fibroblast-like synoviocytes (FLSs) obtained from RA or OA patients. Finally, we assessed the effects of HRF/TCTP and dimerized HRF/TCTP-binding peptide-2 (dTBP2), an HRF/TCTP inhibitor, in RA-FLSs and CIA mice. Our clinical, radiological, histological, and biochemical analyses indicate that inflammatory responses and joint destruction were increased in HRF/TCTP TG mice and decreased in KD mice compared to wild-type littermates. HRF/TCTP levels in the sera, synovial fluid, synovium, and FLSs were higher in patients with RA than in control groups. Serum levels of HRF/TCTP correlated well with RA disease activity. The tumor-like aggressiveness of RA-FLSs was exacerbated by HRF/TCTP stimulation and ameliorated by dTBP2 treatment. dTBP2 exerted protective and therapeutic effects in CIA mice and had no detrimental effects in a murine tuberculosis model. Our results indicate that HRF/TCTP is a novel biomarker and therapeutic target for the diagnosis and treatment of RA.

Impact of Isolation Procedures on the Development of a Preclinical Synovial Fibroblasts/Macrophages in an In Vitro Model of Osteoarthritis

Simple Summary

In vitro models able to represent osteoarthritis (OA) synovial tissue (ST) inflammation are lacking. Therefore, we aimed to characterize OA ST and to compare mechanical and enzymatic digestion procedures to find the one that better preserve the heterogeneity of the main OA synovial cell populations: fibroblasts and macrophages. We demonstrated that typical macrophage phenotypical markers, like CD68, CD80 and CD163, were higher expressed on cells isolated with mechanical than enzymatic procedure. Moreover, we found that typical cytokines of inflammatory macrophages (i.e., TNFα) and anti-inflammatory macrophages (i.e., IL10) were also higher on mechanically isolated cells. Synovial fibroblasts were well conserved in both procedures. The definition of an OA ST model in vitro that better preserve the heterogeneity of specific cell populations represents a prerequisite for defining the real effects of new cell therapies or drugs for OA treatment, and could contribute to the reduction or avoidance of animal models.

Abstract

There is a lack ofin vitromodels able to properly represent osteoarthritis (OA) synovial tissue (ST). We aimed to characterize OA ST and to investigate whether a mechanical or enzymatic digestion procedures influence synovial cell functional heterogeneity in vitro. Procedures using mechanical nondigested fragments (NDF), synovial digested fragments (SDF), and filtrated synovial digested cells (SDC) were compared. An immunophenotypic profile was performed to distinguish synovial fibroblasts (CD55, CD73, CD90, CD106), macrophages (CD14, CD68), M1-like (CD80, CD86), and M2-like (CD163, CD206) synovial macrophages. Pro-inflammatory (interleukin 6 IL6), tumor necrosis factor alpha (TNFα), chemokine C-C motif ligand 3 (CCL3/MIP1α), C-X- motif chemokine ligand 10 (CXCL10/IP10) and anti-inflammatory (interleukin 10 (IL10)), transforming growth factor beta 1 (TGFβ1), C-C motif chemokine ligand 18 (CCL18) cytokines were evaluated. CD68 and CD163 markers were higher in NDF and SDF compared to the SDC procedure, while CD80, CD86, and CD206 were higher only in NDF compared to the SDC procedure. Synovial fibroblast markers showed similar percentages. TNFα, CCL3/MIP1α, CXCL10/IP10, and CCL18 were higher in NDF compared to SDC, but not compared to SDF. IL10 and TGFβ1 were higher in NDF than SDC at the molecular level, while IL6 did not show differences among procedures. We demonstrated that NDF isolation procedures better preserved the heterogeneity of specific OA synovial populations (fibroblasts, macrophages), fostering their use for testing new cell therapies or drugs for OA, reducing or avoiding the use of animal models.

Infrapatellar Fat Pad/Synovium Complex in Early-Stage Knee Osteoarthritis: Potential New Target and Source of Therapeutic Mesenchymal Stem/Stromal Cells

The infrapatellar fat pad (IFP) has until recently been viewed as a densely vascular and innervated intracapsular/extrasynovial tissue with biomechanical roles in the anterior compartment of the knee. Over the last decade, secondary to the proposition that the IFP and synovium function as a single unit, its recognized tight molecular crosstalk with emerging roles in the pathophysiology of joint disease, and the characterization of immune-related resident cells with varying phenotypes (e.g., pro and anti-inflammatory macrophages), this structural complex has gained increasing attention as a potential therapeutic target in patients with various knee pathologies including osteoarthritis (KOA). Furthermore, the description of the presence of mesenchymal stem/stromal cells (MSC) as perivascular cells within the IFP (IFP-MSC), exhibiting immunomodulatory, anti-fibrotic and neutralizing activities over key local mediators, has promoted the IFP as an alternative source of MSC for cell-based therapy protocols. These complementary concepts have supported the growing notion of immune and inflammatory events participating in the pathogenesis of KOA, with the IFP/synovium complex engaging not only in amplifying local pathological responses, but also as a reservoir of potential therapeutic cell-based products. Consequently, the aim of this review is to outline the latest discoveries related with the IFP/synovium complex as both an active participant during KOA initiation and progression thus emerging as a potential target, and a source of therapeutic IFP-MSCs. Finally, we discuss how these notions may help the design of novel treatments for KOA through modulation of local cellular and molecular cascades that ultimately lead to joint destruction.

Molecular and Cellular Pathways Contributing to Joint Damage in Rheumatoid Arthritis

Rheumatoid arthritis is a chronic autoimmune syndrome associated with several genetic, epigenetic, and environmental factors affecting the articular joints contributing to cartilage and bone damage. Although etiology of this disease is not clear, several immune pathways, involving immune (T cells, B cells, dendritic cells, macrophages, and neutrophils) and nonimmune (fibroblasts and chondrocytes) cells, participate in the secretion of many proinflammatory cytokines, chemokines, proteases (MMPs, ADAMTS), and other matrix lysing enzymes that could disturb the immune balance leading to cartilage and bone damage. The presence of autoantibodies preceding the clinical onset of arthritis and the induction of bone erosion early in the disease course clearly suggest that initiation events damaging the cartilage and bone start very early during the autoimmune phase of the arthritis development. During this process, several signaling molecules (RANKL-RANK, NF-κB, MAPK, NFATc1, and Src kinase) are activated in the osteoclasts, cells responsible for bone resorption. Hence, comprehensive knowledge on pathogenesis is a prerequisite for prevention and development of targeted clinical treatment for RA patients that can restore the immune balance improving clinical therapy.

Established rheumatoid arthritis. The pathogenic aspects

The development of rheumatoid arthritis (RA), at least in its autoantibody-positive subset, evolves through a series of events starting well before the appearance of synovitis. The distinction between 'early' and 'established' RA is, therefore, an evolving concept. In routine practice, however, the management of RA still starts with the occurrence of clinically detectable synovitis. As such, the synovial membrane remains a major target for the exploitation of possible stage-specific drivers of the disease. The recognition of a 'window of opportunity', in which treatment is more likely to succeed, raises the hypothesis that there might be a period in which the biological processes of RA are less mature and potentially reversible. The present review aims to provide a general picture of the modifications occurring in RA synovium, analysing the contribution of both infiltrating immune cells and stromal cells. When available, differences between early and established RA will be discussed.

A Clinical Approach for the Use of VIP Axis in Inflammatory and Autoimmune Diseases

The neuroendocrine and immune systems are coordinated to maintain the homeostasis of the organism, generating bidirectional communication through shared mediators and receptors. Vasoactive intestinal peptide (VIP) is the paradigm of an endogenous neuropeptide produced by neurons and endocrine and immune cells, involved in the control of both innate and adaptive immune responses. Exogenous administration of VIP exerts therapeutic effects in models of autoimmune/inflammatory diseases mediated by G-protein-coupled receptors (VPAC1 and VPAC2). Currently, there are no curative therapies for inflammatory and autoimmune diseases, and patients present complex diagnostic, therapeutic, and prognostic problems in daily clinical practice due to their heterogeneous nature. This review focuses on the biology of VIP and VIP receptor signaling, as well as its protective effects as an immunomodulatory factor. Recent progress in improving the stability, selectivity, and effectiveness of VIP/receptors analogues and new routes of administration are highlighted, as well as important advances in their use as biomarkers, contributing to their potential application in precision medicine. On the 50th anniversary of VIP's discovery, this review presents a spectrum of potential clinical benefits applied to inflammatory and autoimmune diseases.

Origin and function of synovial macrophage subsets during inflammatory joint disease

Mononuclear phagocytes, including monocytes and macrophages, are a central component of the host's innate immune system designated to protect against invading pathogens. However, these cells do not only interact with various parts of the innate and adaptive immune system, but also fulfill indispensable duties during the control of tissue homeostasis and organ function. Moreover, macrophages are crucially involved in tissue remodeling and repair in response to damage. Simultaneously, mononuclear phagocytes might also contribute to the pathogenesis of various inflammatory and autoimmune diseases. In particular, their potential role in inflammatory joint diseases such as rheumatoid arthritis (RA) has drawn increasing attention and substantially shaped our general understanding of the role of monocytes and macrophages during health and disease. This review summarizes our current knowledge about the origin and function of mononuclear phagocytes within the joint and addresses their involvement in joint inflammation.

Synovial Tissue: Cellular and Molecular Phenotyping

PURPOSE OF REVIEW

This review provides a summary of recent molecular findings that have refined our understanding of the cell types that constitute human synovial tissue, particularly in patients with rheumatoid arthritis (RA).

RECENT FINDINGS

Recent advances in high-dimensional and single-cell assays have elucidated upwards of 20 cell subsets in the RA synovium. This includes novel fibroblast populations and lymphocyte phenotypes, which in many cases exhibit features that have not been found in other tissues thus far. Molecular profiling studies over the past several years have rapidly generated a comprehensive and detailed outline of the cellular phenotypes in synovial tissue affected by RA. Molecular features of these newly identified cell subsets immediately represent reasonable therapeutic targets and provide the opportunity to design the most clinically relevant mechanistic experiments. Broadly speaking, the ~ 20 cell types thus far identified in RA synovium seem to be fairly well conserved across patients, despite extensive heterogeneity in patient clinical features, stage of disease, and treatment responses. Thus, a next phase in molecular profiling may benefit from quantifying patient samples in terms of the ratios of cell types, with the rationale that certain cellular interactions will predominate in an individual and medications targeting these interactions may be more efficacious for that individual. Such cellular profiling in tissues combined with studies examining how the compendium of these cells interact in their three-dimensional tissue ultrastructures will be important in understanding how collectively these cells drive the disease process and ultimately how best to treat patients.

The Adipokine Network in Rheumatic Joint Diseases

Rheumatic diseases encompass a diverse group of chronic disorders that commonly affect musculoskeletal structures. Osteoarthritis (OA) and rheumatoid arthritis (RA) are the two most common, leading to considerable functional limitations and irreversible disability when patients are unsuccessfully treated. Although the specific causes of many rheumatic conditions remain unknown, it is generally accepted that immune mechanisms and/or uncontrolled inflammatory responses are involved in their etiology and symptomatology. In this regard, the bidirectional communication between neuroendocrine and immune system has been demonstrated to provide a homeostatic network that is involved in several pathological conditions. Adipokines represent a wide variety of bioactive, immune and inflammatory mediators mainly released by adipocytes that act as signal molecules in the neuroendocrine-immune interactions. Adipokines can also be synthesized by synoviocytes, osteoclasts, osteoblasts, chondrocytes and inflammatory cells in the joint microenvironment, showing potent modulatory properties on different effector cells in OA and RA pathogenesis. Effects of adiponectin, leptin, resistin and visfatin on local and systemic inflammation are broadly described. However, more recently, other adipokines, such as progranulin, chemerin, lipocalin-2, vaspin, omentin-1 and nesfatin, have been recognized to display immunomodulatory actions in rheumatic diseases. This review highlights the latest relevant findings on the role of the adipokine network in the pathophysiology of OA and RA.

A novel function of artesunate on inhibiting migration and invasion of fibroblast-like synoviocytes from rheumatoid arthritis patients

Introduction

Anti-malarial drug artesunate can suppress inflammation and prevent cartilage and bone destruction in collagen-induced arthritis model in rats—suggesting it may be a potent drug for rheumatoid arthritis (RA) therapy. We aimed to investigate its effect on the invasive property of fibroblast-like synoviocytes (FLS) from patients with RA.

Methods

Synovial tissues were obtained by closed needle biopsy from active RA patients, and FLS were isolated and cultured in vitro. RA-FLS were treated with artesunate at various concentrations, while methotrexate or hydroxychloroquine was employed as comparator drugs. Cell viability, proliferation, cell cycle, apoptosis, migration, invasion, and pseudopodium formation of RA-FLS were assessed by CCK-8 assays, EdU staining, Annexin V-FITC/PI staining, transwell assays, or F-actin staining, respectively. Further, relative changes of expressed proteases were analyzed by Proteome profiler human protease array and verified by quantitative real-time PCR (qPCR), Western blot, and ELISA. The expression of signaling molecules of MAPK, NF-κB, AP-1, and PI3K/Akt pathways were measured by qPCR and Western blot. PDK-1 knockdown by specific inhibitor AR-12 or siRNA transfection was used to verify the pharmacological mechanism of artesunate on RA-FLS.

Results

Artesunate significantly inhibited the migration and invasion of RA-FLS in a dose-dependent manner with or without TNF-α stimulation. The effect was mediated through artesunate inhibition of MMP-2 and MMP-9 production, and pre-treatment with exogenous MMP-9 reversed the inhibitory effect of artesunate on RA-FLS invasion. Artesunate had a stronger inhibitory effect on migration and invasion of RA-FLS as well as greater anti-inflammatory effect than those of hydroxychloroquine. Similar inhibitory effect was detected between artesunate and methotrexate, and synergy was observed when combined. Mechanistically, artesunate significantly inhibited PDK-1 expression as well as Akt and RSK2 phosphorylation—in a similar manner to PDK-1-specific inhibitor AR-12 or PDK-1 knockdown by siRNA transfection. This inhibition results in suppression of RA-FLS migration and invasion as well as decreased MMP-2 and MMP-9 expression.

Conclusions

Our study demonstrates artesunate is capable of inhibiting migration and invasion of RA-FLS through suppression of PDK1-induced activation of Akt and RSK2 phosphorylation—suggesting that artesunate may be a potential disease-modifying anti-rheumatic drug for RA.

Electronic supplementary material

The online version of this article (10.1186/s13075-019-1935-6) contains supplementary material, which is available to authorized users.

Clinical Applications of Synovial Biopsy

The synovial tissue is a primary target of multiple diseases characterized by different pathogenic mechanisms, including infective, deposition, neoplastic, and chronic immune-inflammatory pathologies. Synovial biopsy can have a relevant role in differential diagnosis of specific conditions in clinical practice, although its exploitation remains relatively limited. In particular, no validated synovial-tissue-derived biomarkers are currently available in the clinic to aid in the diagnosis and management in most frequent forms of chronic inflammatory arthropathies, namely rheumatoid arthritis (RA) and the spondyloarthritides (SpA). In this brief review, we will discuss the current spectrum of clinical applications of synovial biopsy in routine rheumatologic care and will provide an analysis of the perspectives for its potential exploitation in patients with chronic inflammatory arthritides.

An Overview of VPAC Receptors in Rheumatoid Arthritis: Biological Role and Clinical Significance

The axis comprised by the Vasoactive Intestinal Peptide (VIP) and its G protein-coupled receptors (GPCRs), VPAC1, and VPAC2, belong to the B1 family and signal through Gs or Gq proteins. VPAC receptors seem to preferentially interact with Gs in inflammatory cells, rather than Gq, thereby stimulating adenylate cyclase activity. cAMP is able to trigger various downstream pathways, mainly the canonical PKA pathway and the non-canonical cAMP-activated guanine nucleotide exchange factor (EPAC) pathway. Classically, the presence of VPACs has been confined to the plasma membrane; however, VPAC1 location has been described in the nuclear membrane in several cell types such as activated Th cells, where they are also functional. VPAC receptor signaling modulates a number of biological processes by tipping the balance of inflammatory mediators in macrophages and other innate immune cells, modifying the expression of TLRs, and inhibiting MMPs and the expression of adhesion molecules. Receptor signaling also downregulates coagulation factors and acute-phase proteins, promotes Th2 over Th1, stimulates Treg abundance, and finally inhibits a pathogenic Th17 profile. Thus, the VIP axis signaling regulates both the innate and adaptive immune responses in several inflammatory/autoimmune diseases. Rheumatoid arthritis (RA) is a complex autoimmune disease that develops on a substrate of genetically susceptible individuals and under the influence of environmental factors, as well as epigenetic mechanisms. It is a heterogeneous disease with different pathogenic mechanisms and variable clinical forms between patients with the same diagnosis. The knowledge of VIP signaling generated in both animal models and human ex vivo studies can potentially be translated to clinical reality. Most recently, the beneficial effects of nanoparticles of VIP self-associated with sterically stabilized micelles have been reported in a murine model of RA. Another novel research area is beginning to define the receptors as biomarkers in RA, with their expression levels shown to be associated with the activity of the disease and patients-reported impairment. Therefore, VPAC expression together VIP genetic variants could allow patients to be stratified at the beginning of the disease with the purpose of guiding personalized treatment decisions.

Transcriptional Profiling of Synovial Macrophages Using Minimally Invasive Ultrasound-Guided Synovial Biopsies in Rheumatoid Arthritis

OBJECTIVE

Currently, there are no reliable biomarkers for predicting therapeutic response in patients with rheumatoid arthritis (RA). The synovium may unlock critical information for determining efficacy, since a reduction in the numbers of sublining synovial macrophages remains the most reproducible biomarker. Thus, a clinically actionable method for the collection of synovial tissue, which can be analyzed using high-throughput strategies, must become a reality. This study was undertaken to assess the feasibility of utilizing synovial biopsies as a precision medicine-based approach for patients with RA.

METHODS

Rheumatologists at 6 US academic sites were trained in minimally invasive ultrasound-guided synovial tissue biopsy. Biopsy specimens obtained from patients with RA and synovial tissue from patients with osteoarthritis (OA) were subjected to histologic analysis, fluorescence-activated cell sorting, and RNA sequencing (RNA-seq). An optimized protocol for digesting synovial tissue was developed to generate high-quality RNA-seq libraries from isolated macrophage populations. Associations were determined between macrophage transcriptional profiles and clinical parameters in RA patients.

RESULTS

Patients with RA reported minimal adverse effects in response to synovial biopsy. Comparable RNA quality was observed from synovial tissue and isolated macrophages between patients with RA and patients with OA. Whole tissue samples from patients with RA demonstrated a high degree of transcriptional heterogeneity. In contrast, the transcriptional profile of isolated RA synovial macrophages highlighted different subpopulations of patients and identified 6 novel transcriptional modules that were associated with disease activity and therapy.

CONCLUSION

Performance of synovial tissue biopsies by rheumatologists in the US is feasible and generates high-quality samples for research. Through the use of cutting-edge technologies to analyze synovial biopsy specimens in conjunction with corresponding clinical information, a precision medicine-based approach for patients with RA is attainable.

Normative ultrasound references for the paediatric wrist; dorsal soft tissues

Objective

To report novel ultrasound (US) references for the healthy paediatric wrist.

Methods

Healthy children and adolescents had an US examination including Doppler, of the right wrist, using a mid-sagittal image through the radiocarpal (RC)/midcarpal (MC) joints. These features were noted: appearances and depth of the recess, with an additional measurement in the flexed position and number of Doppler signals within the recess in close proximity to the joint. In a second, blinded session, all images were reassessed by the same observer.

Results

In total, 116 subjects (59 girls) aged 6–16 years (mean 10 years and 11 months) were included. The RC recess was visible in 71, of which 60 were non-bulging and 11 were bulging. In flexion, the thickness decreased in 60 (84.5%), suggestive of joint fluid. Of the 54 visible MC recesses, 45 (83.3%) thinned in flexion. The mean depths of the RC and MC recesses were 0.4 mm (SD 0.5, range 0–2.9) and 0.5 mm (SD 0.6, range 0–2.4), respectively, with no differences according to sex, but with increasing depths by age.

For the RC joints, at least one Doppler signal was seen in the recess in 9/116 joints (7.8%), whereas this was true for 3.5% of the MC recesses. There were no statistically significant association between the depth of the RC or MC recesses and number of hand active sport activities (P=0.733 and P=0.091, respectively).

Conclusion

The appearances of the dorsal RC and MC recesses in healthy children may mimic pathology. Hence, findings should be interpreted carefully.