Active involvement of alarmins S100A8 and S100A9 in the regulation of synovial activation and joint destruction during mouse and human osteoarthritis

Bibliometric analysis of synovial in osteoarthritis in the last 10 years

Background

Our aim was to examine trends in the bibliometric analysis of synovial for osteoarthritis over the last 10 years.

Methods

Publications relevant to synovial in osteoarthritis from 2013 to 2022 were retrieved from the Science Citation Index Expanded (SCI-E), Social Sciences Citation Index (SSCI), and Web of Science Core Collection (WoSCC) databases. The countries/regions, institutions, authors, journals, references, and keywords related to this topic were extracted using Citespace and Vosviewer. Citespace and Vosviewer were also used to identify and analyze this field's research hotspots and trends.

Results

Over the past 10 years, 5738 articles addressing the role of synovium in osteoarthritis have been published. Between 2013 and 2022, 2021 had the highest amount of published articles (a total of 756 published articles, or 13.18 % of the total articles) covering synovial in osteoarthritis. China was the country that published the most articles, while Duke University was the institution that published the most articles. Osteoarthritis and Cartilage was the journal with the most publications related to the study of Synovium in osteoarthritis. The National Nature Science Foundation of China provided the most funding. According to the analysis of keyword burst detection, human cartilage, control experiment, and exosomes were the most searched at different points in time.

Conclusion

In the last ten years, both the number of citations and the article discussing synovial in osteoarthritis have increased. The top 10 most searched keywords were "osteoarthritis","synovial fluid", "inflammation", "cartilage", "expression","rheumatoid arthritis","articular cartilage", "knee osteoarthritis", "synovial", "knee". According to the timeline view of co-citation clustering, synovial components and their expressions have emerged as hotspots of research associated with synovial osteoarthritis.

The Role of Alarmins in the Pathogenesis of Rheumatoid Arthritis, Osteoarthritis, and Psoriasis

Alarmins are immune-activating factors released after cellular injury or death. By secreting alarmins, cells can interact with immune cells and induce a variety of inflammatory responses. The broad family of alarmins involves several members, such as high-mobility group box 1, S100 proteins, interleukin-33, and heat shock proteins, among others. Studies have found that the concentrations and expression profiles of alarmins are altered in immune-mediated diseases. Furthermore, they are involved in the pathogenesis of inflammatory conditions. The aim of this narrative review is to present the current evidence on the role of alarmins in rheumatoid arthritis, osteoarthritis, and psoriasis. We discuss their potential involvement in mechanisms underlying the progression of these diseases and whether they could become therapeutic targets. Moreover, we summarize the impact of pharmacological agents used in the treatment of these diseases on the expression of alarmins.

Identification of CD64 as a marker for the destructive potential of synovitis in osteoarthritis

OBJECTIVES

OA is characterized by cartilage degeneration and persistent pain. The majority of OA patients present with synovitis, which is associated with increased cartilage damage. Activated synovial macrophages are key contributors to joint destruction. Therefore, a marker that reflects the activation of these cells could be a valuable tool to characterize the destructive potential of synovitis and benefit monitoring of OA. Here, we aimed to investigate the use of CD64 (FcγRI) as a marker to characterize the damaging potential of synovitis in OA.

METHODS

Synovial biopsies were obtained from end-stage OA patients that underwent joint replacement surgery. CD64 protein expression and localization was evaluated using immunohistochemistry and immunofluorescence and quantified using flow cytometry. qPCR was performed to measure the expression of FCGR1 and OA-related genes in synovial biopsies, and in primary chondrocytes and primary fibroblasts stimulated with OA conditioned medium (OAS-CM).

RESULTS

Our data exposed a wide range of CD64 expression in OA synovium and showed positive correlations between FCGR1 and S100A8, S100A9, IL1B, IL6 and MMP1/2/3/9/13 expression. CD64 protein correlated with MMP1, MMP3, MMP9, MMP13 and S100A9. Furthermore, we observed that synovial CD64 protein levels in source tissue for OAS-CM significantly associated with the OAS-CM-induced expression of MMP1, MMP3 and especially ADAMTS4 in cultured fibroblasts, but not chondrocytes.

CONCLUSION

Together, these results indicate that synovial CD64 expression is associated with the expression of proteolytic enzymes and inflammatory markers related to structural damage in OA. CD64 therefore holds promise as marker to characterize the damaging potential of synovitis.

Inhibition of TLR4 signalling to dampen joint inflammation in osteoarthritis

Local and systemic low-grade inflammation, mainly involving the innate immune system, plays an important role in the development of OA. A receptor playing a key role in initiation of this inflammation is the pattern-recognition receptor Toll-like receptor 4 (TLR4). In the joint, various ligands for TLR4, many of which are damage-associated molecular patterns (DAMPs), are present that can activate TLR4 signalling. This leads to the production of pro-inflammatory and catabolic mediators that cause joint damage. In this narrative review, we will first discuss the involvement of TLR4 ligands and signalling in OA. Furthermore, we will provide an overview of methods for inhibit, TLR4 signalling by RNA interference, neutralizing anti-TLR4 antibodies, small molecules and inhibitors targeting the TLR4 co-receptor MD2. Finally, we will focus on possible applications and challenges of these strategies in the dampening of inflammation in OA.

Proteomic profiling of human menisci from mild joint degeneration and end-stage osteoarthritis versus healthy controls

Objective

To gain new insight into the molecular changes of the meniscus by comparing the proteome profiles of healthy controls with mild degeneration and end-stage osteoarthritis (OA).

Method

We obtained tissue plugs from lateral and medial menisci of 37 individuals (central part of the posterior horn) classified as healthy (n ​= ​12), mild signs of joint damage (n ​= ​13) and end-stage OA (n ​= ​12). The protein profile was analysed by nano-liquid chromatography-mass spectrometry using data-independent acquisition and quantified by Spectronaut. Linear-mixed effects modelling was applied to extract the between-group comparisons.

Results

A similar protein profile was observed for the mild group as compared to healthy controls while the most different group was end-stage OA mainly for the medial compartment. When a pattern of gradual change in protein levels from healthy to end-stage OA was required, a 42-proteins panel was identified, suggesting a potential role in OA development. The levels of QSOX1 were lower and G6PD higher in the mild group following the proposed protein abundance pattern. Qualitative protein changes suggest lower levels of CYTL1 as a potential biomarker of early joint degradation.

Conclusion

For future targeted proteomic approaches, we propose a candidate panel of 42 proteins based on gradually altered meniscal posterior horn protein abundance patterns associated with joint degradation.

Anti-Osteoarthritis Mechanism of the Nrf2 Signaling Pathway

Osteoarthritis (OA) is a chronic degenerative disease and the primary pathogenic consequence of OA is inflammation, which can affect a variety of tissues including the synovial membrane, articular cartilage, and subchondral bone. The development of the intra-articular microenvironment can be significantly influenced by the shift of synovial macrophages between pro-inflammatory and anti-inflammatory phenotypes. By regulating macrophage inflammatory responses, the NF-κB signaling route is essential in the therapy of OA; whereas, the nuclear factor erythroid 2-related factor 2 (Nrf2) signaling pathway appears to manage the relationship between oxidative stress and inflammation. Additionally, it has been demonstrated that under oxidative stress and inflammation, there is a significant interaction between transcriptional pathways involving Nrf2 and NF-κB. Studying how Nrf2 signaling affects inflammation and cellular metabolism may help us understand how to treat OA by reprogramming macrophage behavior because Nrf2 signaling is thought to affect cellular metabolism. The candidates for treating OA by promoting an anti-inflammatory mechanism by activating Nrf2 are also reviewed in this paper.

Inflammatory mechanisms in post-traumatic osteoarthritis: a role for CaMKK2

Post-traumatic osteoarthritis (PTOA) is a multifactorial disease of the cartilage, synovium, and subchondral bone resulting from direct joint trauma and altered joint mechanics after traumatic injury. There are no current disease-modifying therapies for PTOA, and early surgical interventions focused on stabilizing the joint do not halt disease progression. Chronic pain and functional disability negatively affect the quality of life and take an economic toll on affected patients. While multiple mechanisms are at play in disease progression, joint inflammation is a key contributor. Impact-induced mitochondrial dysfunction and cell death or altered joint mechanics after trauma culminate in inflammatory cytokine release from synoviocytes and chondrocytes, cartilage catabolism, suppression of cartilage anabolism, synovitis, and subchondral bone disease, highlighting the complexity of the disease. Current understanding of the cellular and molecular mechanisms underlying the disease pathology has allowed for the investigation of a variety of therapeutic strategies that target unique apoptotic and/or inflammatory processes in the joint. This review provides a concise overview of the inflammatory and apoptotic mechanisms underlying PTOA pathogenesis and identifies potential therapeutic targets to mitigate disease progression. We highlight Ca2+/calmodulin-dependent protein kinase kinase 2 (CaMKK2), a serine/threonine protein kinase that was recently identified to play a role in murine and human osteoarthritis pathogenesis by coordinating chondrocyte inflammatory responses and apoptosis. Given its additional effects in regulating macrophage inflammatory signaling and bone remodeling, CaMKK2 emerges as a promising disease-modifying therapeutic target against PTOA.

Fundamentals of osteoarthritis: Inflammatory mediators in osteoarthritis

OBJECTIVES

As more has become known of the pathophysiology of osteoarthritis (OA), evidence that inflammation plays a critical role in its development and progression has accumulated. Here, we aim to review current knowledge of the complex inflammatory network in the OA joint.

DESIGN

This narrative review is presented in three main sections: local inflammation, systemic inflammation, and therapeutic implications. We focused on inflammatory mediators and their link to OA structural changes in the joint.

RESULTS

OA is characterized by chronic and low-grade inflammation mediated mostly by the innate immune system, which results in cartilage degradation, bone remodeling and synovial changes. Synovitis is regarded as an OA characteristic and associated with increased severity of symptoms and joint dysfunction. However, the articular cartilage and the subchondral bone also produce several pro-inflammatory mediators thus establishing a complex interplay between the different tissues of the joint. In addition, systemic low-grade inflammation induced by aging, obesity and metabolic syndrome can contribute to OA development and progression. The main inflammatory mediators associated with OA include cytokines, chemokines, growth factors, adipokines, and neuropeptides.

CONCLUSIONS

Future research is needed to deeper understand the molecular pathways mediating the inflammation in OA to provide new therapeutics that target these pathways, or to repurpose existing drugs.

Neuroimmune Mechanisms Underlying Post-acute Sequelae of SARS-CoV-2 (PASC) Pain, Predictions from a Ligand-Receptor Interactome

PURPOSE OF REVIEW

Individuals with post-acute sequelae of SARS-CoV-2 (PASC) complain of persistent musculoskeletal pain. Determining how COVID-19 infection produces persistent pain would be valuable for the development of therapeutics aimed at alleviating these symptoms.

RECENT FINDINGS

To generate hypotheses regarding neuroimmune interactions in PASC, we used a ligand-receptor interactome to make predictions about how ligands from PBMCs in individuals with COVID-19 communicate with dorsal root ganglia (DRG) neurons to induce persistent pain. In a structured literature review of -omics COVID-19 studies, we identified ligands capable of binding to receptors on DRG neurons, which stimulate signaling pathways including immune cell activation and chemotaxis, the complement system, and type I interferon signaling. The most consistent finding across immune cell types was an upregulation of genes encoding the alarmins S100A8/9 and MHC-I. This ligand-receptor interactome, from our hypothesis-generating literature review, can be used to guide future research surrounding mechanisms of PASC-induced pain.

Identification of S100A8 as a common diagnostic biomarkers and exploring potential pathogenesis for osteoarthritis and metabolic syndrome

Background

Osteoarthritis (OA) is the most frequent musculoskeletal disease and the major contributor to disability worldwide. Metabolic syndrome (MetS) has been recognized as being associated with the pathogenesis of osteoarthritis. However, the exact mechanisms and links between the two are not clear.

Methods

We downloaded clinical information data and gene expression profiles for OA and MetS from the database of Gene Expression Omnibus (GEO), and immune related gene (IRG) from the database of Immunology Database and Analysis Portal (IMMPORT). After screening OA-DEG and MetS-DEG, we identified the common immune hub gene by screening the overlapping genes between OA-DEG, MetS-DEG and IRG. Then we conducted single-gene analysis of S100A8, assessed the correlation of S100A8 with immune cell infiltration, and verified the diagnostic value of S100A8 in OA and MetS database respectively.

Results

323 OA-DEGs,101 MetS-DEGs and an immune-related hub gene, S100A8, were identified. In single gene analysis of S100A8 in OA samples, GSEA suggested that immune-related biological processes were more significantly enriched. The results of immune cell infiltration analysis showed that the enrichment fraction of M2 macrophages was significantly higher in the high S100A8-expressing group, and the level of S100A8 expression was positively correlated with M2 macrophage infiltration. The results of the dataset validation showed that S100A8 expression levels were significantly upregulated in the OA group and performed well in the diagnosis of OA. In single gene analysis of S100A8 in MetS samples, immune cell infiltration analysis showed that monocyte infiltration was higher in the S100A8 high expression samples and that there was a positive correlation between the two. Dataset validation showed that S100A8 is of high value for the diagnosis of MetS. In the validation of the dataset for the four metabolism-related diseases (obesity, diabetes, hypertension and hyperlipidaemia), S100A8 was expressed at higher levels in the disease group and also had a higher diagnostic value for the four metabolism-related diseases.

Conclusion

S100A8 is a common hub gene and diagnostic biomarker for OA and MetS, and the immune regulation involved in S100A8 may play a central role in the pathogenesis of OA and MetS.

Inflammaging and Osteoarthritis

Osteoarthritis is a highly prevalent disease particularly in subjects over 65 years of age worldwide. While in the past it was considered a mere consequence of cartilage degradation leading to anatomical and functional joint impairment, in recent decades, there has been a more dynamic view with the synovium, the cartilage, and the subchondral bone producing inflammatory mediators which ultimately lead to cartilage damage. Inflammaging is defined as a chronic, sterile, low-grade inflammation state driven by endogenous signals in the absence of infections, occurring with aging. This chronic status is linked to the production of reactive oxygen species and molecules involved in the development of age-related disease such as cancer, diabetes, and cardiovascular and neurodegenerative diseases. Inflammaging contributes to osteoarthritis development where both the innate and the adaptive immune response are involved. Elevated systemic and local inflammatory cytokines and senescent molecules promote cartilage degradation, and antigens derived from damaged joints further trigger inflammation through inflammasome activation. B and T lymphocyte populations also change with inflammaging and OA, with reduced regulatory functions, thus implicating self-reactivity as an additional mechanism of joint damage. The discovery of the underlying pathogenic pathways may help to identify potential therapeutic targets for the management or the prevention of osteoarthritis. We will provide a comprehensive evaluation of the current literature on the role of inflammaging in osteoarthritis and discuss the emerging therapeutic strategies.

Soluble and EV-Associated Diagnostic and Prognostic Biomarkers in Knee Osteoarthritis Pathology and Detection

Osteoarthritis (OA) is the most common degenerative disease of the connective tissue of the human musculoskeletal system. Despite its widespread prevalence, there are many limitations in its diagnosis and treatment. OA diagnosis currently relies on the presence of clinical symptoms, sometimes accompanied by changes in joint X-rays or MRIs. Biomarkers help not only to diagnose early disease progression but also to understand the process of OA in many ways. In this article, we briefly summarize information on articular joints and joint tissues, the pathogenesis of OA and review the literature about biomarkers in the field of OA, specifically inflammatory cytokines/chemokines, proteins, miRNA, and metabolic biomarkers found in the blood, synovial fluid and in extracellular vesicles.

Phenotypic and functional characterisation of synovial fluid-derived neutrophils in knee osteoarthritis and knee infection

OBJECTIVE

An increase in the number of neutrophils (NEUs) has long been associated with infections in the knee joints; however, their impact on knee osteoarthritis (KOA) pathophysiology remains largely unexplored.

DESIGN

This study compared the phenotypic and functional characteristics of synovial fluid (SF)-derived NEUs in KOA and knee infection (INF).

RESULTS

KOA NEUs were characterised by a lower expression of CD11b, CD54, and CD64 and higher expression of CD62L, TLR2, and TLR4 compared with INF NEUs. Except for CCL2, lower levels of inflammatory mediators and proteases were detected in KOA SF than in INF SF. Functionally, KOA NEUs displayed increased reactive oxygen species production and phagocytic activity compared with INF NEUs. Moreover, KOA and INF NEUs differed in cell sizes, histological characteristics of the surrounding synovial tissues, and their effects on the endothelial cells assessed by human umbilical vein endothelial cells. When KOA patients were subdivided based on the SF NEU abundance, patients with high NEUs (10%-60%) were characterised by i) elevated SF protein levels of TNF-α, IL-1RA, MMP-9, sTREM-1, VILIP-1 and ii) lower CD54, CD64, TLR2 and TLR4 expression compared to patients with low NEUs (<10%). Analysis of paired SF samples suggests that low or high NEU percentages, respectively, persist throughout the course of disease.

CONCLUSIONS

Our findings suggest that NEU may play a significant role in KOA pathophysiology. Further studies should explore the mechanisms that contribute to the increased number of NEUs in SF and the clinical consequences of neutrophilic phenotype in KOA.

From Low-Grade Inflammation in Osteoarthritis to Neuropsychiatric Sequelae: A Narrative Review

Nowadays, osteoarthritis (OA), a common, multifactorial musculoskeletal disease, is considered to have a low-grade inflammatory pathogenetic component. Lately, neuropsychiatric sequelae of the disease have gained recognition. However, a link between the peripheral inflammatory process of OA and the development of neuropsychiatric pathology is not completely understood. In this review, we provide a narrative that explores the development of neuropsychiatric disease in the presence of chronic peripheral low-grade inflammation with a focus on its signaling to the brain. We describe the development of a pro-inflammatory environment in the OA-affected joint. We discuss inflammation-signaling pathways that link the affected joint to the central nervous system, mainly using primary sensory afferents and blood circulation via circumventricular organs and cerebral endothelium. The review describes molecular and cellular changes in the brain, recognized in the presence of chronic peripheral inflammation. In addition, changes in the volume of gray matter and alterations of connectivity important for the assessment of the efficacy of treatment in OA are discussed in the given review. Finally, the narrative considers the importance of the use of neuropsychiatric diagnostic tools for a disease with an inflammatory component in the clinical setting.

Post-traumatic knee osteoarthritis; the role of inflammation and hemarthrosis on disease progression

Knee injuries such as anterior cruciate ligament ruptures and meniscal injury are common and are most frequently sustained by young and active individuals. Knee injuries will lead to post-traumatic osteoarthritis (PTOA) in 25–50% of patients. Mechanical processes where historically believed to cause cartilage breakdown in PTOA patients. But there is increasing evidence suggesting a key role for inflammation in PTOA development. Inflammation in PTOA might be aggravated by hemarthrosis which frequently occurs in injured knees. Whereas mechanical symptoms (joint instability and locking of the knee) can be successfully treated by surgery, there still is an unmet need for anti-inflammatory therapies that prevent PTOA progression. In order to develop anti-inflammatory therapies for PTOA, more knowledge about the exact pathophysiological mechanisms and exact course of post-traumatic inflammation is needed to determine possible targets and timing of future therapies.

Phenotype Diversity of Macrophages in Osteoarthritis: Implications for Development of Macrophage Modulating Therapies

Chronic inflammation is implicated in numerous human pathologies. In particular, low-grade inflammation is currently recognized as an important mechanism of osteoarthritis (OA), at least in some patients. Among the signs of the inflammatory process are elevated macrophage numbers detected in the OA synovium compared to healthy controls. High macrophage counts also correlate with clinical symptoms of the disease. Macrophages are central players in the development of chronic inflammation, pain, cartilage destruction, and bone remodeling. However, macrophages are also involved in tissue repair and remodeling, including cartilage. Therefore, reduction of macrophage content in the joints correlates with deleterious effects in OA models. Macrophage population is heterogeneous and dynamic, with phenotype transitions being induced by a variety of stimuli. In order to effectively use the macrophage inflammatory circuit for treatment of OA, it is important to understand macrophage heterogeneity and interactions with surrounding cells and tissues in the joint. In this review, we discuss functional phenotypes of macrophages and specific targeting approaches relevant for OA treatment development.

Multifunctional Role of S100 Protein Family in the Immune System: An Update

S100 is a broad subfamily of low-molecular weight calcium-binding proteins (9–14 kDa) with structural similarity and functional discrepancy. It is required for inflammation and cellular homeostasis, and can work extracellularly, intracellularly, or both. S100 members participate in a variety of activities in a healthy cell, including calcium storage and transport (calcium homeostasis). S100 isoforms that have previously been shown to play important roles in the immune system as alarmins (DAMPs), antimicrobial peptides, pro-inflammation stimulators, chemo-attractants, and metal scavengers during an innate immune response. Currently, during the pandemic, it was found that several members of the S100 family are implicated in the pathophysiology of COVID-19. Further, S100 family protein members were proposed to be used as a prognostic marker for COVID-19 infection identification using a nasal swab. In the present review, we compiled the vast majority of recent studies that focused on the multifunctionality of S100 proteins in the complex immune system and its associated activities. Furthermore, we shed light on the numerous molecular approaches and signaling cascades regulated by S100 proteins during immune response. In addition, we discussed the involvement of S100 protein members in abnormal defense systems during the pathogenesis of COVID-19.

Alarmins S100A8/A9 promote intervertebral disc degeneration and inflammation-related pain in a rat model through toll-like receptor-4 and activation of the NF-κB signaling pathway

OBJECTIVE

The molecules released from cells undergoing necrosis are recognized as alarmins, and S100A8/9, a typical alarmin, is associated with several inflammation-related diseases. This study was to investigate the molecular role of S100A8/A9 on the process of intervertebral disc degeneration (IVDD) and inflammation-related pain.

METHODS

The expression pattern of S100A8/A9 in different degenerated human nucleus pulposus (NP) tissues were measured by Real-time quantitative reverse transcription PCR (RT-qPCR) and immunohistochemical (IHC). The effects of S100A8/A9 on matrix production were assessed by RT-qPCR, western blotting, and cell immunofluorescence. Involvement of TLR4 and NF-κB signaling pathways were studied by pharmachemical inhibitors and small interfering RNAs (siRNAs). The development of degenerative and pain features in the IVDD model were examed by IHC and pain-behavior testing.

RESULTS

The expression of S100A8/A9 was significantly elevated in severely degenerated human NP tissue with similar expression pattern of TNF-α. In NP cells, S100A8/A9 increased MMP-3/13, TNF-α, IL-6 expression and inhibited aggrecan and collagen II expression. RT-qPCR and western blotting showed that the regulatory effects of S100A8/A9 on IVD were TLR4 dependent. Pharmacological inhibition or siRNA knockdown of the NF-κB signaling attenuated S100A8/A9-induced upregulation of MMP-3/13, TNF-α and IL-6. In vivo, S100A9 inhibitor treatment inhibited disc-puncture induced IVDD and inflammation-related pain.

CONCLUSIONS

This study showed that S100A8/A9 bound to TLR4 and increased the expression of MMPs, TNF-α, and IL-6 through NF-κB signaling pathways in NP cells. Furthermore, S100A8/A9 inhibitor could prevent development of IVDD and inflammation-related pain in the rat model.

A Platelet-Rich Plasma-Derived Biologic Clears Staphylococcus aureus Biofilms While Mitigating Cartilage Degeneration and Joint Inflammation in a Clinically Relevant Large Animal Infectious Arthritis Model

The leading cause of treatment failure in Staphylococcus aureus infections is the development of biofilms. Biofilms are highly tolerant to conventional antibiotics which were developed against planktonic cells. Consequently, there is a lack of antibiofilm agents in the antibiotic development pipeline. To address this problem, we developed a platelet-rich plasma (PRP)-derived biologic, termed BIO-PLY (for the BIOactive fraction of Platelet-rich plasma LYsate) which has potent in vitro bactericidal activity against S. aureus synovial fluid free-floating biofilm aggregates. Additional in vitro studies using equine synoviocytes and chondrocytes showed that BIO-PLY protected these cells of the joint from inflammation. The goal of this study was to test BIO-PLY for in vivo efficacy using an equine model of infectious arthritis. We found that horses experimentally infected with S. aureus and subsequently treated with BIO-PLY combined with the antibiotic amikacin (AMK) had decreased bacterial concentrations within both synovial fluid and synovial tissue and exhibited lower systemic and local inflammatory scores compared to horses treated with AMK alone. Most importantly, AMK+BIO-PLY treatment reduced the loss of infection-associated cartilage proteoglycan content in articular cartilage and decreased synovial tissue fibrosis and inflammation. Our results demonstrate the in vivo efficacy of AMK+BIO-PLY and represents a new approach to restore and potentiate antimicrobial activity against synovial fluid biofilms.

Innate Immunity at the Core of Sex Differences in Osteoarthritic Pain?

Osteoarthritis (OA) is a progressive whole-joint disease; no disease-modifying drugs are currently available to stop or slow its process. Symptoms alleviation is the only treatment option. OA is the major cause of chronic pain in adults, with pain being the main symptom driving patients to seek medical help. OA pathophysiology is closely associated with the innate immune system, which is also closely linked to pain mediators leading to joint pain. Pain research has shown sex differences in the biology of pain, including sexually dimorphic responses from key cell types in the innate immune system. Not only is OA more prevalent in women than in men, but women patients also show worse OA outcomes, partially due to experiencing more pain symptoms despite having similar levels of structural damage. The cause of sex differences in OA and OA pain is poorly understood. This review provides an overview of the involvement of innate immunity in OA pain in joints and in the dorsal root ganglion. We summarize the emerging evidence of sex differences regarding innate immunity in OA pain. Our main goal with this review was to provide a scientific foundation for future research leading to alternative pain relief therapies targeting innate immunity that consider sex differences. This will ultimately lead to a more effective treatment of pain in both women and men.

C/EBPδ-induced epigenetic changes control the dynamic gene transcription of S100a8 and S100a9

The proinflammatory alarmins S100A8 and S100A9 are among the most abundant proteins in neutrophils and monocytes but are completely silenced after differentiation to macrophages. The molecular mechanisms of the extraordinarily dynamic transcriptional regulation of S100a8 and S100a9 genes, however, are only barely understood. Using an unbiased genome-wide CRISPR/Cas9 knockout (KO)-based screening approach in immortalized murine monocytes, we identified the transcription factor C/EBPδ as a central regulator of S100a8 and S100a9 expression. We showed that S100A8/A9 expression and thereby neutrophil recruitment and cytokine release were decreased in C/EBPδ KO mice in a mouse model of acute lung inflammation. S100a8 and S100a9 expression was further controlled by the C/EBPδ antagonists ATF3 and FBXW7. We confirmed the clinical relevance of this regulatory network in subpopulations of human monocytes in a clinical cohort of cardiovascular patients. Moreover, we identified specific C/EBPδ-binding sites within S100a8 and S100a9 promoter regions, and demonstrated that C/EBPδ-dependent JMJD3-mediated demethylation of H3K27me₃ is indispensable for their expression. Overall, our work uncovered C/EBPδ as a novel regulator of S100a8 and S100a9 expression. Therefore, C/EBPδ represents a promising target for modulation of inflammatory conditions that are characterized by S100a8 and S100a9 overexpression.

The Neuroimmune Interplay in Joint Pain: The Role of Macrophages

Chronic pain associated with joint disorders, such as rheumatoid arthritis (RA), osteoarthritis (OA) and implant aseptic loosening (AL), is a highly debilitating symptom that impacts mobility and quality of life in affected patients. The neuroimmune crosstalk has been demonstrated to play a critical role in the onset and establishment of chronic pain conditions. Immune cells release cytokines and immune mediators that can activate and sensitize nociceptors evoking pain, through interaction with receptors in the sensory nerve terminals. On the other hand, sensory and sympathetic nerve fibers release neurotransmitters that bind to their specific receptor expressed on surface of immune cells, initiating an immunomodulatory role. Macrophages have been shown to be key players in the neuroimmune crosstalk. Moreover, macrophages constitute the dominant immune cell population in RA, OA and AL. Importantly, the targeting of macrophages can result in anti-nociceptive effects in chronic pain conditions. Therefore, the aim of this review is to discuss the nature and impact of the interaction between the inflammatory response and nerve fibers in these joint disorders regarding the genesis and maintenance of pain. The role of macrophages is highlighted. The alteration in the joint innervation pattern and the inflammatory response are also described. Additionally, the immunomodulatory role of sensory and sympathetic neurotransmitters is revised.

Human infrapatellar fat pad mesenchymal stem cells show immunomodulatory exosomal signatures

Within the human knee infrapatellar fat pad (IFP) and synovium, resident synoviocytes and macrophages contribute to the onset and progression of inflammatory joint diseases. Our hypothesis is that IFP-derived mesenchymal stem cells (IFP-MSC) robust immunomodulatory therapeutic effects are largely exerted via their exosomal (IFP-MSC EXOs) secretome by attenuating synoviocytes and macrophages pro-inflammatory activation. IFP-MSC EXOs showed distinct miRNA and protein immunomodulatory profiles. Reactome analysis of 24 miRNAs highly present in exosomes showed their involvement in the regulation of six gene groups, including immune system. Exosomes were enriched for immunomodulatory and reparative proteins that are involved in positive regulation of cell proliferation, response to stimulus, signal transduction, signal receptor activity, and protein phosphorylation. Stimulated synoviocytes or macrophages exposed to IFP-MSC EXOs demonstrated significantly reduced proliferation, altered inflammation-related molecular profiles, and reduced secretion of pro-inflammatory molecules compared to stimulated alone. In an acute synovial/IFP inflammation rat model, IFP-MSC EXOs therapeutic treatment resulted in robust macrophage polarization towards an anti-inflammatory therapeutic M2 phenotype within the synovium/IFP tissues. Based on these findings, we propose a viable cell-free alternative to MSC-based therapeutics as an alternative approach to treating synovitis and IFP fibrosis.

Post-traumatic osteoarthritis progression is diminished by early mechanical unloading and anti-inflammatory treatment in mice

OBJECTIVE

Post-traumatic osteoarthritis (PTOA) is a degenerative joint disease initiated by injury. Early phase (0-7 days) treatments often include rest (unloading) and anti-inflammatory medications, but how those early interventions impact PTOA progression is unknown. We hypothesized that early unloading and anti-inflammatory treatment would diminish joint inflammation and slow PTOA progression.

DESIGN

Mice were injured with non-invasive ACL rupture followed by hindlimb unloading (HLU) or normal cage activity (ground control: GC) for 7 days, after which all mice were allowed normal cage activity. HLU and GC mice were treated with daily celecoxib (CXB; 10 mg/kg IP) or vehicle. Protease activity was evaluated using in vivo fluorescence imaging, osteophyte formation and epiphyseal trabecular bone were quantified using micro-computed tomography, and synovitis and articular cartilage were evaluated using whole-joint histology at 7, 14, 21, and 28 days post-injury.

RESULTS

HLU significantly reduced protease activity (-22-30% compared to GC) and synovitis (-24-50% relative to GC) at day 7 post-injury (during unloading), but these differences were not maintained at later timepoints. Similarly, trabecular bone volume was partially preserved in HLU mice at during unloading (-14-15% BV/TV for HLU mice, -21-22% for GC mice relative to uninjured), but these differences were not maintained during reloading. Osteophyte volume was reduced by both HLU and CXB, but there was not an additive effect of these treatments (HLU: -46%, CXB: -30%, HLU + CXB: -35% relative to vehicle GC at day 28).

CONCLUSIONS

These data suggest that early unloading following joint injury can reduce inflammation and potentially slow PTOA progression.

The Emerging Role of Non-Coding RNAs in Osteoarthritis

Osteoarthritis (OS) is the most frequent degenerative condition in the joints, disabling many adults. Several abnormalities in the articular cartilage, subchondral bone, synovial tissue, and meniscus have been detected in the course of OA. Destruction of articular cartilage, the formation of osteophytes, subchondral sclerosis, and hyperplasia of synovial tissue are hallmarks of OA. More recently, several investigations have underscored the regulatory roles of non-coding RNAs (ncRNAs) in OA development. Different classes of non-coding RNAs, including long ncRNAs (lncRNAs), microRNAs (miRNAs), and circular RNAs (circRNAs), have been reported to affect the development of OA. The expression level of these transcripts has also been used as diagnostic tools in OA. In the present article, we aimed at reporting the role of these transcripts in this process. We need to give a specific angle on the pathology to provide meaningful thoughts on it.

Nox2 Deficiency Reduces Cartilage Damage and Ectopic Bone Formation in an Experimental Model for Osteoarthritis

Osteoarthritis (OA) is a destructive disease of the joint with age and obesity being its most important risk factors. Around 50% of OA patients suffer from inflammation of the synovial joint capsule, which is characterized by increased abundance and activation of synovial macrophages that produce reactive oxygen species (ROS) via NADPH-oxidase 2 (NOX2). Both ROS and high blood levels of low-density lipoprotein (LDL) are implicated in OA pathophysiology, which may interact to form oxidized LDL (oxLDL) and thereby promote disease. Therefore, targeting NOX2 could be a viable treatment strategy for OA. Collagenase-induced OA (CiOA) was used to compare pathology between wild-type (WT) and Nox2 knockout (Nox2^−/−) C57Bl/6 mice. Mice were either fed a standard diet or Western diet (WD) to study a possible interaction between NOX2-derived ROS and LDL. Synovial inflammation, cartilage damage and ectopic bone size were assessed on histology. Extracellular ROS production by macrophages was measured in vitro using the Amplex Red assay. Nox2^−/− macrophages produced basal levels of ROS but were unable to increase ROS production in response to the alarmin S100A8 or the phorbol ester PMA. Interestingly, Nox2 deficiency reduced cartilage damage, synovial lining thickness and ectopic bone size, whereas these disease parameters were not affected by WD-feeding. These results suggest that NOX2-derived ROS are involved in CiOA development.

Pro-regenerative Dialogue Between Macrophages and Mesenchymal Stem/Stromal Cells in Osteoarthritis

Osteoarthritis (OA), the most common degenerative and inflammatory joint disorder, is multifaceted. Indeed, OA characteristics include cartilage degradation, osteophytes formation, subchondral bone changes, and synovium inflammation. The difficulty in discovering new efficient treatments for OA patients up to now comes from the adoption of monotherapy approaches targeting either joint tissue repair/catabolism or inflammation to address the diverse components of OA. When satisfactory, these approaches only provide short-term beneficial effects, since they only result in the repair and not the full structural and functional reconstitution of the damaged tissues. In the present review, we will briefly discuss the current therapeutic approaches used to repair the damaged OA cartilage. We will highlight the results obtained with cell-based products in clinical trials and demonstrate how the current strategies result in articular cartilage repair showing restricted early-stage clinical improvements. In order to identify novel therapeutic targets and provide to OA patients long-term clinical benefits, herein, we will review the basis of the regenerative process. We will focus on macrophages and their ambivalent roles in OA development and tissue regeneration, and review the therapeutic strategies to target the macrophage response and favor regeneration in OA.

A single dose of anti-IL-1β antibodies prevents Western diet-induced immune activation during early stage collagenase-induced osteoarthritis, but does not ameliorate end-stage pathology

OBJECTIVE

Metabolic dysfunction can cause IL-1β mediated activation of the innate immune system, which could have important implications for the therapeutic efficacy of IL-1β neutralizing drugs as treatment for OA in the context of metabolic syndrome (MetS). In the present study, we investigated whether early treatment with a single dose of IL-1β blocking antibodies could prevent Western diet (WD) induced changes to systemic monocyte populations and their cytokine secretion profile and herewith modulate collagenase induced osteoarthritis (CiOA) pathology.

METHODS

CiOA was induced in female C57Bl/6 mice fed either a standard diet (SD) or WD and treated with a single dose of either polyclonal anti-IL-1β antibodies or control. Monocyte subsets and granulocytes in bone marrow and blood were analyzed with flow cytometry, and cytokine expression by bone marrow cells was analyzed using qPCR. Synovial cellularity, cartilage damage and osteophyte formation were assessed on histology.

RESULTS

WD feeding of C57Bl/6 mice led to increased serum levels of low-density lipoprotein (LDL) and innate immune activation in the form of an increased number of Ly6C^high cells in bone marrow and blood and increased cytokine expression of IL-6 and TNF-α by bone marrow cells. The increase in monocyte number and activity was ameliorated by anti-IL-1β treatment. However, anti-IL-1β treatment did not significantly affect synovial lining thickness, cartilage damage and ectopic bone formation during WD feeding.

CONCLUSIONS

Single-dose systemic anti-IL-1β treatment prevented WD-induced innate immune activation during early stage CiOA in C57Bl/6 mice, but did not ameliorate joint pathology.

Macrophage phenotypes and monocyte subsets after destabilization of the medial meniscus in mice

Macrophages play an important role in the development and progression of osteoarthritis (OA). The aim of this study was to identify macrophage phenotypes in synovium and monocyte subsets in peripheral blood in C57BL/6 mice by destabilizing the medial meniscus (DMM), and the association of macrophage subsets with OA features. DMM, sham, and non-operated knees were histologically assessed between 1 and 56 days for macrophage polarization states by immunohistochemistry (IHC), cartilage damage, synovial thickening, and osteophytes (n = 9 per timepoint). Naive knees (n = 6) were used as controls. Monocyte and polarized synovial macrophage subsets were evaluated by flow cytometry. CD64 and CD206 levels on IHC were higher at early timepoints in DMM and sham knees compared to naive knees. iNOS labeling intensity was higher in DMM and sham knees than in naive knees from d3 onwards. CD163 expression was unaltered at all timepoints. Even though macrophage polarization profiles were similar in DMM and sham knees, only in DMM knees the presence of iNOS and CD206 associated with synovial thickness, and CD163 staining inversely correlated with osteophyte presence. At day 14, monocyte subset distribution was different in peripheral blood of DMM mice compared with sham mice. In conclusion, monocyte subsets in blood and synovial macrophage phenotypes vary after joint surgery. High levels of iNOS⁺ , CD163⁺ , and CD206⁺ cells are found in both destabilized and sham-operated knees, and coexistence with joint instability may be a requirement to initiate and exacerbate OA progression.

Efficacy and Mechanism of Electroacupuncture Treatment of Rabbits With Different Degrees of Knee Osteoarthritis: A Study Based on Synovial Innate Immune Response

Knee osteoarthritis (KOA) is a chronic degenerative bone and joint disease, which is often clinically manifested as pain, joint swelling, and deformity. Its pathological manifestations are mainly synovial inflammation and cartilage degeneration. This study aims to investigate the efficacy of electro-acupuncture (EA) on model rabbits with varying degrees of KOA and to study the mechanism of EA on KOA based on the innate immune response. Mild and moderate rabbit KOA models were established using a modified Hluth method, and EA was given to both the mild and moderate model groups. The Lequesne-MG index was used to evaluate the behavioral changes in the rabbits before and after EA treatment. Morphological changes in the synovial membrane and cartilage of each group were observed by H&E staining. The Mankin scoring standard and the Krenn scoring standard were used to score the pathology of the cartilage tissue and synovial tissue, respectively. The inflammatory factors and metalloproteinases were detected in the serum of each group by ELISA. The protein and messenger RNA (mRNA) expressions of important elements related to Toll-like receptors (TLRs)-mediated innate immune response in the synovial tissue were detected by Western blot and quantitative PCR (qPCR). The Lequesne-MG index score of the rabbits gradually increased with the modeling prolonged but decreased significantly after EA treatment, indicating that EA has a better effect on alleviating the pain and improving the dysfunction. The morphological analysis showed that the inflammation of and the damage to the synovial membrane and the cartilage tissue gradually deteriorated with the modeling prolonged. However, the synovial membrane inflammation was significantly relieved after EA treatment, and the cartilage injury showed signs of repair. The ELISA analysis showed that, with the modeling prolonged, the serum-related inflammatory factors and mechanism of metalloproteinases gradually increased but decreased after EA treatment. The tumor necrosis factor alpha (TNF-α), interleukin 6 (IL-6), and matrix metalloproteinase3 (MMP3) of EA1 group were significantly lower than those of EA2 group. Both Western blot and qPCR results showed that the protein and mRNA expressions of the elements related to the innate immune response in the synovial membrane increased gradually with the modeling prolonged, but decreased significantly after EA treatment. Additionally, the expression of some components in EA1 group was significantly lower than that in EA2 group. These results confirm that synovial inflammation gradually aggravated with time from the early to mid-stage of KOA. EA alleviated the inflammation and histological changes in KOA rabbits by inhibiting the TLRs-mediated innate synovial immune response. This suggests that using EA in the early stage of KOA may achieve a desirable efficacy.

Osteoarthritis-Related Inflammation Blocks TGF-β's Protective Effect on Chondrocyte Hypertrophy via (de)Phosphorylation of the SMAD2/3 Linker Region

Osteoarthritis (OA) is a degenerative joint disease characterized by irreversible cartilage damage, inflammation and altered chondrocyte phenotype. Transforming growth factor-β (TGF-β) signaling via SMAD2/3 is crucial for blocking hypertrophy. The post-translational modifications of these SMAD proteins in the linker domain regulate their function and these can be triggered by inflammation through the activation of kinases or phosphatases. Therefore, we investigated if OA-related inflammation affects TGF-β signaling via SMAD2/3 linker-modifications in chondrocytes. We found that both Interleukin (IL)-1β and OA-synovium conditioned medium negated SMAD2/3 transcriptional activity in chondrocytes. This inhibition of TGF-β signaling was enhanced if SMAD3 could not be phosphorylated on Ser213 in the linker region and the inhibition by IL-1β was less if the SMAD3 linker could not be phosphorylated at Ser204. Our study shows evidence that inflammation inhibits SMAD2/3 signaling in chondrocytes via SMAD linker (de)-phosphorylation. The involvement of linker region modifications may represent a new therapeutic target for OA.

Serum Inflammatory Markers in Patients With Knee Osteoarthritis: A Proteomic Approach

OBJECTIVES

Osteoarthritis (OA) is known to be a slowly progressive disease that alters all tissue compartments of the joint involved with a characteristic degradation of the cartilage, bone remodeling, and inflammation. One of the prominent symptoms in OA patients is pain, but a few radiologic, inflammatory, or structurally related biomarkers have shown few if any associations with pain. This study aimed to assess serum levels of 92 markers involved in inflammatory pathways in patients with knee osteoarthritis (KOA) and evaluate their possible associations with the clinical pain intensity.

MATERIALS AND METHODS

Serum samples were collected from 127 KOA patients and 39 healthy participants with no knee pain. Each serum sample was analyzed for 92 inflammatory markers using the Proximity Extension Array (PEA) technology. Clinical pain intensity was assessed using a Visual Analog Scale, and patients completed the Knee Injury and Osteoarthritis Outcome Score (KOOS) questionnaire.

RESULTS

Fifteen markers were significantly different when comparing KOA patients and healthy participants. Two markers, fibroblast growth factor-21 and Eukaryotic translation initiation factor 4E-binding protein 1 (4E-BP1), correlated positively with pain intensity (R=0.235, P=0.008; R=0.233, P=0.008). Moreover, a linear regression model showed interleukin-6, macrophage colony-stimulating factor 1, fibroblast growth factor-21, and tumor necrosis factor superfamily member 12 (TWEAK) as significant independent parameters for pain intensity.

DISCUSSION

The associations between specific cytokines and KOA pain intensities provide new insights into the understanding of the underlying factors driving the pain in OA.

The role of inflammation in mesenchymal stromal cell therapy in osteoarthritis, perspectives for post-traumatic osteoarthritis: a review

OA is a complex and highly prevalent degenerative disease affecting the whole joint, in which factors like genetic predisposition, gender, age, obesity and traumas contribute to joint destruction. ∼50-80% of OA patients develop synovitis. OA-associated risk factors contribute to joint instability and the release of cartilage matrix fragments, activating the synovium to release pro-inflammatory factors and catabolic enzymes in turn damaging the cartilage and creating a vicious circle. Currently, no cure is available for OA. Mesenchymal stromal cells (MSCs) have been tested in OA for their chondrogenic and anti-inflammatory properties. Interestingly, MSCs are most effective when administered during synovitis. This review focusses on the interplay between joint inflammation and the immunomodulation by MSCs in OA. We discuss the potential of MSCs to break the vicious circle of inflammation and describe current perspectives and challenges for clinical application of MSCs in treatment and prevention of OA, focussing on preventing post-traumatic OA.

S100A9 induces nucleus pulposus cell degeneration through activation of the NF-κB signaling pathway

Oxidative stress in the lumbar disc leads to the degeneration of nucleus pulposus (NP). However, the molecular mechanisms underlying this process remain unclear. In this study, we delineated a key calcium-binding protein, S100A9, which was induced by oxidative stress and was highly expressed in the degenerative NP. Immunofluorescence staining and Western blotting revealed that S100A9 induced NP cell apoptosis in vitro by up-regulating the expression of pro-apoptotic markers, including cleaved caspase-3, cytochrome c and Bax. Moreover, RT-PCR analyses revealed that the expression of S100A9 caused NP matrix degradation by up-regulating the expression of matrix degradation enzymes and increased the inflammatory response by up-regulating cytokine expression. Therefore, S100A9 induced NP cell degeneration by exerting pro-apoptotic, pro-degradation and pro-inflammatory effects. The detailed mechanism underlying S100A9-induced NP degeneration was explored by administering SC75741, a specific NF-κB inhibitor in vitro. We concluded that S100A9 induced NP cell apoptosis, caused matrix degradation and amplified the inflammatory response through the activation of the NF-κB signalling pathway. Inhibition of these pro-apoptotic, pro-degradation and pro-inflammatory effects induced by S100A9 in NP may be a favourable therapeutic strategy to slow lumbar disc degeneration.

Neuroimmune interactions and osteoarthritis pain: focus on macrophages

Bidirectional interactions between the immune system and the nervous system are increasingly appreciated as playing a pathogenic role in chronic pain. Unraveling the mechanisms by which inflammatory pain is mediated through communication between nerves and immune cells may lead to exciting new strategies for therapeutic intervention. In this narrative review, we focus on the role of macrophages in the pathogenesis of osteoarthritis (OA) pain. From regulating homeostasis to conducting phagocytosis, and from inducing inflammation to resolving it, macrophages are plastic cells that are highly adaptable to their environment. They rely on communicating with the environment through cytokines, growth factors, neuropeptides, and other signals to respond to inflammation or injury. The contribution of macrophages to OA joint damage has garnered much attention in recent years. Here, we discuss how macrophages may participate in the initiation and maintenance of pain in OA. We aim to summarize what is currently known about macrophages in OA pain and identify important gaps in the field to fuel future investigations.

The P2X7 Receptor in Osteoarthritis

Osteoarthritis (OA) is the most common joint disease. With the increasing aging population, the associated socio-economic costs are also increasing. Analgesia and surgery are the primary treatment options in late-stage OA, with drug treatment only possible in early prevention to improve patients' quality of life. The most important structural component of the joint is cartilage, consisting solely of chondrocytes. Instability in chondrocyte balance results in phenotypic changes and cell death. Therefore, cartilage degradation is a direct consequence of chondrocyte imbalance, resulting in the degradation of the extracellular matrix and the release of pro-inflammatory factors. These factors affect the occurrence and development of OA. The P2X7 receptor (P2X7R) belongs to the purinergic receptor family and is a non-selective cation channel gated by adenosine triphosphate. It mediates Na⁺, Ca²⁺ influx, and K⁺ efflux, participates in several inflammatory reactions, and plays an important role in the different mechanisms of cell death. However, the relationship between P2X7R-mediated cell death and the progression of OA requires investigation. In this review, we correlate potential links between P2X7R, cartilage degradation, and inflammatory factor release in OA. We specifically focus on inflammation, apoptosis, pyroptosis, and autophagy. Lastly, we discuss the therapeutic potential of P2X7R as a potential drug target for OA.

Blood and synovial fluid calprotectin as biomarkers to diagnose chronic hip and knee periprosthetic joint infections

AIMS

Calprotectin (CLP) is produced in neutrophils and monocytes and released into body fluids as a result of inflammation or infection. The aim of this study was to evaluate the utility of blood and synovial CLP in the diagnosis of chronic periprosthetic joint infection (PJI).

METHODS

Blood and synovial fluid samples were collected prospectively from 195 patients undergoing primary or revision hip and knee arthroplasty. Patients were divided into five groups: 1) primary total hip and knee arthroplasty performed due to idiopathic osteoarthritis (OA; n = 60); 2) revision hip and knee arthroplasty performed due to aseptic failure of the implant (AR-TJR; n = 40); 3) patients with a confirmed diagnosis of chronic PJI awaiting surgery (n = 45); 4) patients who have finished the first stage of the PJI treatment with the use of cemented spacer and were qualified for replantation procedure (SR-TJR; n = 25), and 5) patients with rheumatoid arthritis undergoing primary total hip and knee arthroplasty (RA; n = 25). CLP concentrations were measured quantitatively in the blood and synovial fluid using an immunoturbidimetric assay. Additionally, blood and synovial CRP, blood interleukin-6 (IL-6), and ESR were measured, and a leucocyte esterase (LE) strip test was performed.

RESULTS

Patients with PJI had higher CLP concentrations than those undergoing aseptic revision in blood (median PJI 2.14 mg/l (interquartile range (IQR) 1.37 to 3.56) vs AR-TJR 0.66 mg/l (IQR 0.3 to 0.83); p < 0.001) and synovial fluid samples (median PJI 20.46 mg/l (IQR 14.3 to 22.36) vs AR-TJR 0.7 mg/l (IQR 0.41 to 0.95); p < 0.001). With a cut-off value of 1.0 mg/l, blood CLP showed a sensitivity, specificity, positive predictive value, and negative predictive value of 93.3%, 87.5%, 89.4%, and 92.1%, respectively. For synovial fluid with a cut-off value of 1.5 mg/l, these were 95.6%, 95%, 95.5%, and 95%, respectively.

CONCLUSION

This small study suggests that synovial and blood CLP are useful markers in chronic PJI diagnosis with similar or higher sensitivity and specificity than routinely used markers such as CRP, ESR, IL-6, and LE. CLP was not useful to differentiate patients with PJI from those with rheumatoid arthritis. Cite this article: Bone Joint J 2021;103-B(1):46-55.

The Damage-Associated Molecular Patterns (DAMPs) as Potential Targets to Treat Osteoarthritis: Perspectives From a Review of the Literature

During the osteoarthritis (OA) process, activation of immune systems, whether innate or adaptive, is strongly associated with low-grade systemic inflammation. This process is initiated and driven in the synovial membrane, especially by synovium cells, themselves previously activated by damage-associated molecular patterns (DAMPs) released during cartilage degradation. These fragments exert their biological activities through pattern recognition receptors (PRRs) that, as a consequence, induce the activation of signaling pathways and beyond the release of inflammatory mediators, the latter contributing to the vicious cycle between cartilage and synovial membrane. The primary endpoint of this review is to provide the reader with an overview of these many molecules categorized as DAMPs and the contribution of the latter to the pathophysiology of OA. We will also discuss the different strategies to control their effects. We are convinced that a better understanding of DAMPs, their receptors, and associated pathological mechanisms represents a decisive issue for degenerative joint diseases such as OA.

Interleukin-22 and connective tissue diseases: emerging role in pathogenesis and therapy

Interleukin-22 (IL-22), a member of the IL-10 family of cytokines, is produced by a number of immune cells involved in the immune microenvironment of the body. IL-22 plays its pivotal roles by binding to the IL-22 receptor complex (IL-22R) and subsequently activating the IL-22R downstream signalling pathway. It has recently been reported that IL-22 also contributes to the pathogenesis of many connective tissue diseases (CTDs). In this review, we will discuss the role of IL-22 in several CTDs, such as system lupus erythematosus, rheumatoid arthritis, Sjögren’s syndrome, systemic sclerosis and dermatomyositis, suggesting that IL-22 may be a potential therapeutic target in CTDs.

Cartilage tissue engineering for obesity-induced osteoarthritis: Physiology, challenges, and future prospects

Osteoarthritis (OA) is a multifactorial joint disease with pathological changes that affect whole joint tissue. Obesity is acknowledged as the most influential risk factor for both the initiation and progression of OA in weight-bearing and non-weight-bearing joints. Obesity-induced OA is a newly defined phenotypic group in which chronic low-grade inflammation has a central role. Aside from persistent chronic inflammation, abnormal mechanical loading due to increased body weight on weight-bearing joints is accountable for the initiation and progression of obesity-induced OA. The current therapeutic approaches for OA are still evolving. Tissue-engineering-based strategy for cartilage regeneration is one of the most promising treatment breakthroughs in recent years. However, patients with obesity-induced OA are often excluded from cartilage repair attempts due to the abnormal mechanical demands, altered biomechanical and biochemical activities of cells, persistent chronic inflammation, and other obesity-associated factors. With the alarming increase in the number of obese populations globally, the need for an innovative therapeutic approach that could effectively repair and restore the damaged synovial joints is of significant importance for this sub-population of patients. In this review, we discuss the involvement of the systemic and localized inflammatory response in obesity-induced OA and the impact of altered mechanical loading on pathological changes in the synovial joint. Moreover, we examine the current strategies in cartilage tissue engineering and address the critical challenges of cell-based therapies for OA. Besides, we provide examples of innovative ways and potential strategies to overcome the obstacles in the treatment of obesity-induced OA.

THE TRANSLATIONAL POTENTIAL OF THIS ARTICLE

Altogether, this review delivers insight into obesity-induced OA and offers future research direction on the creation of tissue engineering-based therapies for obesity-induced OA.

Toll-Like Receptors, Associated Biochemical Signaling Networks, and S100 Ligands

ABSTRACT

Host cells recognize molecules that signal danger using pattern recognition receptors (PRRs). Toll-like receptors (TLRs) are the most studied class of PRRs and detect pathogen-associated molecular patterns and danger-associated molecular patterns. Cellular TLR activation and signal transduction can therefore contain, combat, and clear danger by enabling appropriate gene transcription. Here, we review the expression, regulation, and function of different TLRs, with an emphasis on TLR-4, and how TLR adaptor protein binding directs intracellular signaling resulting in activation or termination of an innate immune response. Finally, we highlight the recent progress of research on the involvement of S100 proteins as ligands for TLR-4 in inflammatory disease.

Common risk factors and therapeutic targets in obstructive sleep apnea and osteoarthritis: An unexpectable link?

Osteoarthritis (OA) and Obstructive Sleep Apnea (OSA) are two highly prevalent chronic diseases for which effective therapies are urgently needed. Recent epidemiologic studies, although scarce, suggest that the concomitant occurrence of OA and OSA is associated with more severe manifestations of both diseases. Moreover, OA and OSA share risk factors, such as aging and metabolic disturbances, and co-morbidities, including cardiovascular and metabolic diseases, sleep deprivation and depression. Whether this coincidental occurrence is fortuitous or involves cause-effect relationships is unknown. This review aims at collating and integrating present knowledge on both diseases by providing a brief overview of their epidemiology and pathophysiology, analyzing current evidences relating OA and OSA and discussing potential common mechanisms by which they can aggravate each other. Such mechanisms constitute potential therapeutic targets whose pharmacological modulation may provide more efficient ways of reducing the consequences of OA and OSA and, thus, lessen the huge individual and social burden that they impose.

Of mice and men: converging on a common molecular understanding of osteoarthritis

Despite an increasing burden of osteoarthritis in developed societies, target discovery has been slow and there are currently no approved disease-modifying osteoarthritis drugs. This lack of progress is due in part to a series of misconceptions over the years: that osteoarthritis is an inevitable consequence of ageing, that damaged articular cartilage cannot heal itself, and that osteoarthritis is driven by synovial inflammation similar to that seen in rheumatoid arthritis. Molecular interrogation of disease through ex-vivo tissue analysis, in-vitro studies, and preclinical models have radically reshaped the knowledge landscape. Inflammation in osteoarthritis appears to be distinct from that seen in rheumatoid arthritis. Recent randomised controlled trials, using treatments repurposed from rheumatoid arthritis, have largely been unsuccessful. Genome-wide studies point to defects in repair pathways, which accords well with recent promise using growth factor therapies or Wnt pathway antagonism. Nerve growth factor has emerged as a robust target in osteoarthritis pain in phase 2-3 trials. These studies, both positive and negative, align well with those in preclinical surgical models of osteoarthritis, indicating that pathogenic mechanisms identified in mice can lead researchers to valid human targets. Several novel candidate pathways are emerging from preclinical studies that offer hope of future translational impact. Enhancing trust between industry, basic, and clinical scientists will optimise our collective chance of success.

Impact of smoking on femorotibial and hip osteoarthritis progression: 3-year follow-up data from the KHOALA cohort

OBJECTIVES

To evaluate the clinical and structural impact of smoking on knee and hip osteoarthritis at baseline and after 3years.

METHODS

Observational data on the progressive effects of smoking at baseline and after 3years were collected from The Knee and Hip Osteoarthritis Long-Term Assessment cohort comprising a French population of patients aged 40-75years with symptomatic lower limb osteoarthritis. Clinical (the Western Ontario and McMaster Universities Arthritis Index and Harris scores) and structural (radiography for osteophyte detection and joint-space narrowing assessment) were conducted. The tobacco usage categories were 'never smoker', 'former smoker', and 'current smoker'.

RESULTS

Of the 873 subjects included, 215 (25%) were former smokers and 119 (14%) were current smokers. Multivariate analyses revealed that former and current smokers had fewer knee osteophytes in the medial compartment at baseline (odds ratio [OR]=0.64 [0.41-0.99] and 0.63 [0.36-1.11], respectively), lower osteophyte development in the lateral condyle after 3years (OR=011 [0.03-0.45] and 0.15 [0.03-0.97]), and lower osteophyte development in the lateral tibial plateau after 3years (OR=0.22 [0.06-0.75] and 0.68 [0.14-3.35]). Higher tobacco consumption and longer duration of consumption were significantly associated with fewer knee osteophytes at baseline and lower osteophyte development at 3years.

CONCLUSION

Although cigarette smoking did not influence knee function, pain, or the need for replacement surgery, current and former smokers developed fewer osteophytes. This relationship may be linked to the quantity and duration of consumption. Our results provide further insight into the smoking-related pathophysiology of osteoarthritis.

Gene Expression Profiling Studies Using Microarray in Osteoarthritis: Genes in Common and Different Conditions

Osteoarthritis (OA), which is characterized mainly by cartilage degradation, is the most prevalent joint disorder worldwide. Although OA is identified as a major cause of joint pain, disability, and socioeconomic burden, the etiology of OA is still not clearly known. Recently, gene microarray analysis has become an efficient method for the research of complex diseases and has been employed to determine what genes and pathways are involved in the pathological process of OA. In this review, OA study results over the last decade are summarized for gene expression profiling of various tissues, such as cartilage, subchondral bone, and synovium in human OA and mouse OA models. Many differentially expressed genes, which mainly involve matrix metabolism, bone turnover, and inflammation pathways, were identified in diseased compared with "normal" tissues. Nevertheless, rare common genes were reported from studies using different tissue sources, microarray chips, and research designs. Thus, future novel and carefully designed microarray studies are required to elucidate underlying genetic mechanisms in the pathogenesis of OA as well as new directions for potential OA-targeted pharmaceutical therapies.

Proteins involved in the endoplasmic reticulum stress are modulated in synovitis of osteoarthritis, chronic pyrophosphate arthropathy and rheumatoid arthritis, and correlate with the histological inflammatory score

It is now well recognized that osteoarthritis (OA) synovial membrane presents inflammatory components. The aim of this work is to provide evidence that similar inflammatory mechanisms exist in synovial membrane (n = 24) obtained from three pathologies presenting altogether an inflammatory gradient: OA, chronic pyrophosphate arthropathy (CPPA) and rheumatoid arthritis (RA). Synovial biopsies were first characterized by a histological score based on synovial hyperplasia and infiltration of lymphocytes, plasma cells, polymorphonuclear and macrophages. All biopsies were also analyzed by 2D-nano-UPLC-ESI-Q-Orbitrap for protein identification and quantification. Protein levels were correlated with the histological score. Histological score was in the range of 3 to 8 for OA, 5 to 13 for CPPA and 12 to 17 for RA. Of the 4,336 proteins identified by mass spectrometry, 51 proteins were selected for their strong correlation (p < 0.001) with the histological score of which 11 proteins (DNAJB11, CALR, ERP29, GANAB, HSP90B1, HSPA1A, HSPA5, HYOU1, LMAN1, PDIA4, and TXNDC5) were involved in the endoplasmic reticulum (ER) stress. Protein levels of S100A8 and S100A9 were significantly higher in RA compared to OA (for both) or to CPPA (for S100A8 only) and also significantly correlated with the histological score. Eighteen complement component proteins were identified, but only C1QB and C1QBP were weakly correlated with the histological score. This study highlights the inflammatory gradient existing between OA, CPPA and RA synovitis either at the protein level or at the histological level. Inflamed synovitis was characterized by the overexpression of ER stress proteins.

The alarmins S100A8 and S100A9 mediate acute pain in experimental synovitis

Background

Synovitis-associated pain is mediated by inflammatory factors that may include S100A8/9, which is able to stimulate nociceptive neurons via Toll-like receptor 4. In this study, we investigated the role of S100A9 in pain response during acute synovitis.

Methods

Acute synovitis was induced by streptococcal cell wall (SCW) injection in the knee joint of C57Bl/6 (WT) and S100A9^−/− mice. The expression of S100A8/A9 was determined in serum and synovium by ELISA and immunohistochemistry. Inflammation was investigated by ^99mTc accumulation, synovial cytokine release, and histology at days 1, 2, and 7. To assess pain, weight distribution, gait analysis, and mechanical allodynia were monitored. Activation markers in afferent neurons were determined by qPCR and immunohistochemistry in the dorsal root ganglia (DRG). Differences between groups were tested using a one-way or two-way analysis of variance (ANOVA). Differences in histology were tested with a non-parametric Mann–Whitney U test. p values lower than 0.05 were considered significant.

Results

Intra-articular SCW injection resulted in increased synovial expression and serum levels of S100A8/A9 at day 1. These increased levels, however, did not contribute to the development of inflammation, since this was equal in S100A9^−/− mice. WT mice showed a significantly decreased percentage of weight bearing on the SCW hind paw on day 1, while S100A9^−/− mice showed no reduction. Gait analysis showed increased “limping” behavior in WT, but not S100A9^−/− mice. Mechanical allodynia was observed but not different between WT and S100A9^−/− when measuring paw withdrawal threshold. The gene expression of neuron activation markers NAV1.7, ATF3, and GAP43 in DRG was significantly increased in arthritic WT mice at day 1 but not in S100A9^−/− mice.

Conclusions

S100A8/9, released from the synovium upon inflammation, is an important mediator of pain response in the knee during the acute phase of inflammation.

Cathelicidin and Calprotectin Are Disparately Altered in Murine Models of Inflammatory Arthritis and Airway Inflammation

Cationic host defense peptides (CHDP) are immunomodulatory molecules that control infections and contribute to immune homeostasis. CHDP such as cathelicidin and calprotectin expression is altered in the arthritic synovium, and in the lungs of asthma and COPD patients. Recent studies suggest a link between airway inflammation and the immunopathology of arthritis. Therefore, in this study we compared the abundance of mouse cathelicidin (CRAMP), defensins, and calprotectin subunits (S100A8 and S100A9) in murine models of collagen-induced arthritis (CIA) and allergen house dust mite (HDM)-challenged airway inflammation. CRAMP, S100A8, and S100A9 abundance were significantly elevated in the joint tissues of CIA mice, whereas these were decreased in the lung tissues of HDM-challenged mice, compared to naïve. We further compared the effects of administration of two different synthetic immunomodulatory peptides, IG-19 and IDR-1002, on cathelicidin and calprotectin abundance in the two models. Administration of IG-19, which controls disease progression and inflammation in CIA mice, significantly decreased CRAMP, S100A8, and S100A9 levels to baseline in the joints of the CIA mice, which correlated with the decrease in cellular influx in the joints. However, administration of IDR-1002, which suppresses HDM-induced airway inflammation, did not prevent the decrease in the levels of cathelicidin and calprotectin in the lungs of HDM-challenged mice. Cathelicidin and calprotectin levels did not correlate with leukocyte accumulation in the lungs of the HDM-challenged mice. Results of this study suggest that endogenous cathelicidin and calprotectin abundance are disparately altered, and may be differentially regulated, within local tissues in airway inflammation compared to arthritis.

Degenerative osteoarthritis a reversible chronic disease

Osteoarthritis (OA) is the most common chronic musculoskeletal disorder. It can affect any joint and is the most frequent single cause of disability in older adults.

OA is a progressive degenerative disease involving the entire joint structure in a vicious circle that includes the capsule-bursa tissue inflammation, synovial fluid modifications, cartilage breakdown and erosions, osteochondral inflammatory damage leading to bone erosion and distortion.

Research has identified the initial inflammatory-immunologic process that starts this vicious cycle leading to so-called early OA. Research has also identified the role played in the disease advancement by synoviocytes type A and B, chondrocytes, extracellular matrix, local immune-inflammatory mediators and proteases.

This article investigates the joint-resident MSCs that play an essential local homeostatic role and regulate cell turn over and tissue repair. Resident MSCs establish and maintain a local regenerative microenvironment.

The understanding of OA physiopathology clarifies the core mechanisms by which minimally invasive interventions might be able to halt and reverse the course of early stage OA. Interventions employing PRP, MSCs and exosomes are considered in this article.