Characterization of synovial mast cells in knee osteoarthritis: association with clinical parameters

The innate immune response as a mediator of osteoarthritis pain

In this narrative review, we discuss the emerging role of innate immunity in osteoarthritis (OA) joint pain. First, we give a brief description of the pain pathway in the context of OA. Then we consider how neuro-immune signaling pathways may promote OA pain. First, activation of neuronal Pattern Recognition Receptors by mediators released in a damaged joint can result in direct excitation of nociceptors, as well as in production of chemokines and cytokines. Secondly, indirect neuro-immune signaling may occur when innate immune cells produce algogenic factors, including chemokines and cytokines, that act on the pain pathway. Neuro-immune crosstalk occurs at different levels of the pathway, starting in the joint but also in the innervating dorsal root ganglia and in the dorsal horn. Synovitis is characterized by recruitment of immune cells, including macrophages, mast cells, and CD4+ lymphocytes, which may contribute to nociceptor sensitization and OA pain through production of algogenic factors that amplify the activation of sensory neurons. We discuss examples where this scenario has been suggested by findings in human OA and in animal models. Overall, increasing evidence suggests that innate immune pathways play an initiating as well as facilitating role in pain, but information on how these pathways operate in OA remains limited. Since these innate pathways are eminently targetable, future studies in this area may provide fruitful leads towards a better management of symptomatic OA.

GRB10 and E2F3 as Diagnostic Markers of Osteoarthritis and Their Correlation with Immune Infiltration

This study aimed to find potential diagnostic markers for osteoarthritis (OA) and analyze the role of immune cells infiltration in this pathology. We used OA datasets from the Gene Expression Omnibus database. First, R software was used to identify differentially expressed genes (DEGs) and perform functional correlation analysis. Then least absolute shrinkage and selection operator (LASSO) logistic regression and support vector machine-recursive feature elimination algorithms were used to screen and verify the diagnostic markers of OA. Finally, CIBERSORT was used to evaluate the infiltration of immune cells in OA tissues, and the correlation between diagnostic markers and infiltrating immune cells was analyzed. A total of 458 DEGs were screened in this study. GRB10 and E2F3 (AUC = 0.962) were identified as diagnostic markers of OA. Immune cell infiltration analysis found that resting mast cells, T regulatory cells, CD4 memory resting T cells, activated NK cells, and eosinophils may be involved in the OA process. In addition, GRB10 was correlated with NK resting cells, naive CD4 + T cells, and M1 macrophages, while E2F3 was correlated with resting mast cells. In conclusion, GRB10 and E2F3 can be used as diagnostic markers of osteoarthritis, and immune cell infiltration plays an important role in the occurrence and progression of OA.

The Role of Mast Cells in Bone Metabolism and Bone Disorders

Mast cells (MCs) are important sensor and effector cells of the immune system that are involved in many physiological and pathological conditions. Increasing evidence suggests that they also play an important role in bone metabolism and bone disorders. MCs are located in the bone marrow and secrete a wide spectrum of mediators, which can be rapidly released upon activation of mature MCs following their differentiation in mucosal or connective tissues. Many of these mediators can exert osteocatabolic effects by promoting osteoclast formation [e.g., histamine, tumor necrosis factor (TNF), interleukin-6 (IL-6)] and/or by inhibiting osteoblast activity (e.g., IL-1, TNF). By contrast, MCs could potentially act in an osteoprotective manner by stimulating osteoblasts (e.g., transforming growth factor-β) or reducing osteoclastogenesis (e.g., IL-12, interferon-γ). Experimental studies investigating MC functions in physiological bone turnover using MC-deficient mouse lines give contradictory results, reporting delayed or increased bone turnover or no influence depending on the mouse model used. By contrast, the involvement of MCs in various pathological conditions affecting bone is evident. MCs may contribute to the pathogenesis of primary and secondary osteoporosis as well as inflammatory disorders, including rheumatoid arthritis and osteoarthritis, because increased numbers of MCs were found in patients suffering from these diseases. The clinical observations could be largely confirmed in experimental studies using MC-deficient mouse models, which also provide mechanistic insights. MCs also regulate bone healing after fracture by influencing the inflammatory response toward the fracture, vascularization, bone formation, and callus remodeling by osteoclasts. This review summarizes the current view and understanding of the role of MCs on bone in both physiological and pathological conditions.

Increase in Tryptase and Its Role in the Synovial Membrane of Overweight and Obese Patients with Osteoarthritis of the Knee

PURPOSE

The mechanisms governing evidence that obesity is a risk factor for osteoarthritis (OA) are not well understood. We previously reported an increase in mast cell (MC) marker expression in the osteoarthritic synovial membrane (SM) of patients with obesity. We hypothesized that an enzyme produced by MC, β-tryptase, may be increased in the SM of obese patients with knee OA and contribute to synovial inflammation. This study investigated the expression of the β-tryptase encoding gene, TPSB2, in the SM of obese patients with knee OA and β-tryptase-mediated regulation of IL-1β in synovial cells.

PATIENTS AND METHODS

A total of 216 patients radiographically diagnosed with knee OA were grouped according to the World Health Organization's body mass index classifications: normal weight (NW; <25 kg/m²), overweight (OW; 25-29.99 kg/m²) and obese (OB; ≥30 kg/m²). Quantitative polymerase chain reaction was conducted to examine TPSB2 expression in the SM among the three groups. We also examined TPSB2 and IL1B expression in MC-rich (CD3-CD14-CD19-CD90-) and MC-poor (CD3+, CD14+, CD19+, or CD90+) fractions freshly isolated from synovial tissue. Further, the effect of β-tryptase on IL1B expression was investigated in cultured CD14-positive (macrophage-rich fraction) and CD14-negative (fibroblast-rich fraction) cells.

RESULTS

There was significantly elevated TPSB2 expression in the OW and OB groups compared to the NW group. The MC-rich fraction had significantly higher levels of TPSB2, CD117 and CD203c than the MC-poor fraction. Recombinant human β-tryptase stimulated IL1B expression in both the synovial fibroblast and macrophage fractions.

CONCLUSION

Obese patients with knee OA showed elevated TPSB2 expression in the SM. Tryptase may play a role in synovial inflammation in obese patients with OA.

Safety and efficacy of a new micronized formulation of the ALIAmide palmitoylglucosamine in preclinical models of inflammation and osteoarthritis pain

Background

Osteoarthritis is increasingly recognized as the result of a complex interplay between inflammation, chrondrodegeneration, and pain. Joint mast cells are considered to play a key role in orchestrating this detrimental triad. ALIAmides down-modulate mast cells and more generally hyperactive cells. Here we investigated the safety and effectiveness of the ALIAmide N-palmitoyl-d-glucosamine (PGA) in inflammation and osteoarthritis pain.

Methods

Acute toxicity of micronized PGA (m-PGA) was assessed in rats following OECD Guideline No.425. PGA and m-PGA (30 mg/kg and 100 mg/kg) were orally administered to carrageenan (CAR)-injected rats. Dexamethasone 0.1 mg/kg was used as reference. Paw edema and thermal hyperalgesia were measured up to 6 h post-injection, when also myeloperoxidase activity and histological inflammation score were assessed. Rats subjected to intra-articular injection of sodium monoiodoacetate (MIA) were treated three times per week for 21 days with PGA or m-PGA (30 mg/kg). Mechanical allodynia and motor function were evaluated at different post-injection time points. Joint histological and radiographic damage was scored, articular mast cells were counted, and macrophages were immunohistochemically investigated. Levels of TNF-α, IL-1β, NGF, and MMP-1, MMP-3, and MMP-9 were measured in serum using commercial colorimetric ELISA kits. One- or two-way ANOVA followed by a Bonferroni post hoc test for multiple comparisons was used.

Results

Acute oral toxicity of m-PGA resulted in LD50 values in excess of 2000 mg/kg. A single oral administration of PGA and m-PGA significantly reduced CAR-induced inflammatory signs (edema, inflammatory infiltrate, and hyperalgesia), and m-PGA also reduced the histological score. Micronized PGA resulted in a superior activity to PGA on MIA-induced mechanical allodynia, locomotor disability, and histologic and radiographic damage. The MIA-induced increase in mast cell count and serum level of the investigated markers was also counteracted by PGA and to a significantly greater extent by m-PGA.

Conclusions

The results of the present study showed that PGA is endorsed with anti-inflammatory, pain-relieving, and joint-protective effects. Moreover, it proved that particle size reduction greatly enhances the activity of PGA, particularly on joint pain and disability. Given these results, m-PGA could be considered a valuable option in the management of osteoarthritis.

Subchondral bone remodelling in osteoarthritis

Subchondral bone remodelling is an integral part of osteoarthritis and involves the development of subchondral sclerosis seen on plain imaging, along with osteophyte formation.

The development of these changes is due to persistent abnormal mechanical stresses which create a cellular and biomolecular response to microfractures in the subchondral bone and osteochondral junction.

An early sign is bone marrow lesions seen on MRI scanning. Healing can occur at this stage by correcting the abnormal loads. Persistence leads to what is thought to be a delayed union or nonunion response by the bone.

Microfractures of the osteochondral junction, coupled with articular cartilage fissuring and loss, allows synovial fluid to penetrate the subchondral bone along with cytokines and other molecules reacting with the bone cells to increase the pathological effects.

This review gives an overview of the current thoughts on subchondral bone remodelling in osteoarthritis that is aimed at orthopaedic surgeons to help in the understanding of the pathogenesis of osteoarthritis and the role of surgical management.

Cite this article: EFORT Open Rev 2019;4 DOI: 10.1302/2058-5241.4.180102

IgE-mediated mast cell activation promotes inflammation and cartilage destruction in osteoarthritis

Osteoarthritis is characterized by articular cartilage breakdown, and emerging evidence suggests that dysregulated innate immunity is likely involved. Here, we performed proteomic, transcriptomic, and electron microscopic analyses to demonstrate that mast cells are aberrantly activated in human and murine osteoarthritic joint tissues. Using genetic models of mast cell deficiency, we demonstrate that lack of mast cells attenuates osteoarthritis in mice. Using genetic and pharmacologic approaches, we show that the IgE/FcεRI/Syk signaling axis is critical for the development of osteoarthritis. We find that mast cell-derived tryptase induces inflammation, chondrocyte apoptosis, and cartilage breakdown. Our findings demonstrate a central role for IgE-dependent mast cell activation in the pathogenesis of osteoarthritis, suggesting that targeting mast cells could provide therapeutic benefit in human osteoarthritis.

Editorial note: This article has been through an editorial process in which the authors decide how to respond to the issues raised during peer review. The Reviewing Editor's assessment is that all the issues have been addressed (see decision letter).

Mast Cells in Early Rheumatoid Arthritis

Rheumatoid arthritis (RA) is a chronic systemic autoimmune disease characterized by inflammation of the synovial membrane, with thickening of the synovial layer, cellular hyperplasia, and infiltration of immune cells. Mast cells (MCs) are cells of the innate immunity present in healthy synovia and part of the cellular hyperplasia characterizing RA synovitis. Although their presence in synovia has been well described, the exact functions and the correlation of MCs with disease development and progression have been debated, particularly because of contradictory data obtained in animal models and from patients with longstanding disease. Here, we present a revision of the literature on MCs in RA, including the most recent observations obtained from patients with early RA, indicating MCs as relevant markers of disease severity in early RA.

Human mast cells and basophils-How are they similar how are they different?

Mast cells and basophils are key contributors to allergies and other inflammatory diseases since they are the most prominent source of histamine as well as numerous additional inflammatory mediators which drive inflammatory responses. However, a closer understanding of their precise roles in allergies and other pathological conditions has been marred by the considerable heterogeneity that these cells display, not only between mast cells and basophils themselves but also across different tissue locations and species. While both cell types share the ability to rapidly degranulate and release histamine following high-affinity IgE receptor cross-linking, they differ markedly in their ability to either react to other stimuli, generate inflammatory eicosanoids or release immunomodulating cytokines and chemokines. Furthermore, these cells display considerable pharmacological heterogeneity which has stifled attempts to develop more effective anti-allergic therapies. Mast cell- and basophil-specific transcriptional profiling, at rest and after activation by innate and adaptive stimuli, may help to unravel the degree to which these cells differ and facilitate a clearer understanding of their biological functions and how these could be targeted by new therapies.

Increase in mast cell marker expression in the synovium of obese patients with osteoarthritis of the knee

Purpose: While research suggests that obesity is a risk factor for knee osteoarthritis (KOA), the mechanisms are not fully understood. Mast cell (MC) numbers are increased in the osteoarthritic synovium and in the adipose tissue of obese individuals. We hypothesized that MC numbers are increased in the synovium of obese KOA patients. This study investigated MC marker and MC-generated cytokine/growth factor expression in the synovium of obese KOA patients. Patients and methods: Patients radiographically diagnosed with KOA (male: 38, female: 132) were allocated to three groups based on their body mass index (BMI): normal (<25 kg/m²), overweight (25-29.99 kg/m²) and obese (≥30 kg/m²), according to the World Health Organization BMI classification. We used real-time polymerase chain reaction to compare the expression of MC markers (CD117, CD203c) and growth factors/cytokines (FGF2, VEGFA, TNFA, and IL8) in patients' synovium among the groups. Results: CD117 expression was significantly higher in the obese group than in the normal and overweight groups. CD203c and FGF2 expression were higher in the obese group than in the normal group. FGF2 expression levels were significantly correlated with those of CD117 (ρ=0.487) and CD203c (ρ=0.751). Conclusion: MC markers CD117 and CD203c, and FGF2 were highly expressed in the synovium of obese KOA patients. Further investigations are needed to reveal the role of MCs in the relationship between obesity and osteoarthritis pathology.

Effects of anticoagulants and storage conditions on clinical oxylipid levels in human plasma

Metabolomics and lipidomics are of fundamental importance to personalized healthcare. Particularly the analysis of bioactive lipids is of relevance to a better understanding of various diseases. Within clinical routines, blood derived samples are widely used for diagnostic and research purposes. Hence, standardized and validated procedures for blood collection and storage are mandatory, in order to guarantee sample integrity and relevant study outcomes. We here investigated different plasma storage conditions and their effect on plasma fatty acid and oxylipid levels. Our data clearly indicate the importance of storage conditions for plasma lipidomic analysis. Storage at very low temperature (-80 °C) and the addition of methanol directly after sampling are the most important measures to avoid ex vivo synthesis of oxylipids. Furthermore, we identified critical analytes being affected under certain storage conditions. Finally, we carried out chiral analysis and found possible residual enzymatic activity to be one of the contributors to the ex vivo formation of oxylipids even at -20 °C.

Gender-related differences observed among immune cells in synovial fluid in knee osteoarthritis

OBJECTIVE

There is no existing comprehensive report on the cellular composition of synovial fluids (SFs) from knee osteoarthritis (OA). We therefore aimed to characterise the immune cell composition in SFs from knee OA (KOA) and in subgroups according to gender.

DESIGN

The immunophenotyping of monocyte/macrophage lineage cells, T and B cells, NK cells, neutrophils, dendritic and mast cells (MC) present in SFs from 53 patients (24 males/29 females) with KOA was performed using 6-colour flow cytometry.

RESULTS

SFs from patients with OA contained 90% hematopoietic cells. Lymphocytes were the predominant cell population (44.8%) in the SFs of OA patients, with CD4⁺ T lymphocytes being more prevalent than CD8⁺ T cells (CD4⁺/CD8⁺ ratio = 1.3). Within the monocyte/macrophage lineage gating, monocytes accounted for 33.9%, macrophages 14.8%, myeloid dendritic cells 16.4%. The rest of the hematopoietic cells were comprised of neutrophils (8%), NK cells (3.8%), T regulatory cells (1.2%), plasmacytoid dendritic cells (1.1%), mast cells (0.3%). In OA females, a higher percentage of CD4⁺ T cells (P = 0.023), macrophages (P = 0.012), and a lower percentage of monocytes (P = 0.008) and CD8⁺ T cells (P = 0.002) were detected in comparison to OA males.

CONCLUSIONS

Based on the immune cell composition of SFs, data mining analysis revealed distinct phenotypes (monocyte- and lymphocyte-predominant) within each gender group. This first study on the cellular complexity of SFs in KOA showed marked differences between male and female patients. The findings give a rational starting point for patient stratification according to their phenotypes, as is required for phenotype-specific treatment strategies.

Role of mast cells in autoimmunity

Autoimmune diseases result from an immune response against structures of the organism itself. Their etiology is unknown, although they have been associated with genetic and environmental factors and the participation of mast cells has recently been proposed. The objective of this review was to provide an update on the role of mast cells in these diseases. According to the literature, mast cells not only intervene in immune processes as a defense mechanism but are also involved in the development of disease. Thus, mast cells may participate in important ways in autoimmune diseases through the secretion of cytokines or release of enzymes from their cytoplasmic granules or through the activity of proinflammatory mediators released by these cells. Further research is warranted to elucidate the role of mast cells in autoimmune diseases.

Subchondral bone fragility with meniscal tear accelerates and parathyroid hormone decelerates articular cartilage degeneration in rat osteoarthritis model

The aims of this study were to investigate the influence of subchondral bone fragility (SBF) on the progression of the knee osteoarthritis by using a novel rat model, and to examine the preventive effect of parathyroid hormone (PTH) on cartilage degeneration. First, 40 rats were assigned to the following four groups: Sham, SBF, Medial meniscal tear (MMT), and MMT + SBF groups. In SBF and MMT + SBF groups, we induced SBF by microdrilling the subchondral bone. Second, 10 additional rats were randomly assigned to the following two groups: MMT + SBF + saline and MMT + SBF + PTH groups. Osteoarthritic changes in the articular cartilage and subchondral bone were evaluated using safranin-O/fast green staining, matrix metalloproteinase-13 (MMP-13), and type X collagen immunohistochemistry, toluidine blue staining, and micro-CT scanning. The combination of SBF and meniscal tear increased the number of mast cells in the subchondral bone, and led to the abnormal subchondral bone microarchitecture, such as abnormally decreased trabecular number and increased trabecular thickness, compared with meniscal tear alone. Moreover, SBF with meniscal tear enhanced articular cartilage degeneration and increased the expression of MMP-13 and type X collagen, compared with meniscal tear alone. The administration of PTH decreased the number of mast cells in the subchondral bone and improved the microstructural parameters of the subchondral bone, and delayed the progression of articular cartilage degeneration. These results suggest that SBF is one of the factors underlying the osteoarthritis development, especially in knees with traumatic osteoarthritis, and that the administration of PTH is a potential therapeutic treatment for preventing OA progression. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:1959-1968, 2018.

H1-antihistamines are associated with lower prevalence of radiographic knee osteoarthritis: a cross-sectional analysis of the Osteoarthritis Initiative data

Background

There is growing evidence that mast cells (MCs) play a role in knee osteoarthritis (OA). H₁-antihistamines block H₁-receptors of histamine, which is an important mediator of MCs. There is a lack of data on whether H₁-antihistamines can influence OA. We hypothesized that the use of H₁-antihistamines may be linked to the reduced prevalence of knee OA.

Methods

Baseline data from the Osteoarthritis Initiative cohort were analysed cross-sectionally. Unadjusted and adjusted logistic regression models were performed to compare the prevalence of knee OA in H₁-antihistamine users and non-users. Generalized estimating equations were used to adjust for the correlation between knees. Knee OA was defined as (1) Kellgren-Lawrence (KL) grade ≥ 2 or total joint replacement or (2) KL grade ≥ 2 and joint space narrowing or total joint replacement.

Results

The analysed sample consisted of 8545 knees (664 knees of H₁-antihistamine users and 7881 knees of H₁-antihistamine non-users). The use of H₁-antihistamines was associated with reduced prevalence of knee OA in unadjusted and adjusted models using both the first (adjusted OR, 0.77; 95% CI, 0.62, 0.96; P < 0.02) and second (adjusted OR, 0.75; 95% CI, 0.62, 0.93; P < 0.008) definitions of knee OA.

Conclusions

H₁-antihistamines are associated with a reduced prevalence of knee OA. The findings indicate that this class of drugs should be further evaluated for possible structure-modifying properties in knee OA.

Peripheral Mechanisms Contributing to Osteoarthritis Pain

PURPOSE OF REVIEW

Osteoarthritis (OA) is the most common form of arthritis and a major source of pain and disability worldwide. OA-associated pain is usually refractory to classically used analgesics, and disease-modifying therapies are still lacking. Therefore, a better understanding of mechanisms and mediators contributing to the generation and maintenance of OA pain is critical for the development of efficient and safe pain-relieving therapies.

RECENT FINDINGS

Both peripheral and central mechanisms contribute to OA pain. Clinical evidence suggests that a strong peripheral nociceptive drive from the affected joint maintains pain and central sensitization associated with OA. Mediators present in the OA joint, including nerve growth factor, chemokines, cytokines, and inflammatory cells can contribute to sensitization. Furthermore, structural alterations in joint innervation and nerve damage occur in the course of OA. Several interrelated pathological processes, including joint damage, structural reorganization of joint afferents, low-grade inflammation, neuroplasticity, and nerve damage all contribute to the pain observed in OA. It can be anticipated that elucidating exactly how these mechanisms are operational in the course of progressive OA may lead to the identification of novel targets for intervention.

Role of TrkA signalling and mast cells in the initiation of osteoarthritis pain in the monoiodoacetate model

OBJECTIVE

Aiming to delineate novel neuro-immune mechanisms for NGF/TrkA signalling in osteoarthritis (OA) pain, we evaluated inflammatory changes in the knee joints following injection of monoiodoacetate (MIA) in mice carrying a TrkA receptor mutation (P782S; TrkA KI mice).

METHOD

In behavioural studies we monitored mechanical hypersensitivity following intra-articular MIA and oral prostaglandin D₂ (PGD₂) synthase inhibitor treatments. In immunohistochemical studies we quantified joint mast cell numbers, calcitonin gene-related peptide expression in synovia and dorsal root ganglia, spinal cord neuron activation and microgliosis. We quantified joint leukocyte infiltration by flow cytometry analysis, and PGD₂ generation and cyclooxygenase-2 (COX-2) expression in mast cell lines by ELISA and Western blot.

RESULTS

In TrkA KI mice we observed rapid development of mechanical hypersensitivity and amplification of dorsal horn neurons and microglia activation 7 days after MIA. In TrkA KI knee joints we detected significant leukocyte infiltration and mast cells located in the vicinity of synovial nociceptive fibres. We demonstrated that mast cells exposure to NGF results in up-regulation of COX-2 and increase of PGD₂ production. Finally, we observed that a PGD₂ synthase inhibitor prevented MIA-mechanical hypersensitivity in TrkA KI, at doses which were ineffective in wild type (WT) mice.

CONCLUSION

Using the TrkA KI mouse model, we delineated a novel neuro-immune pathway and suggest that NGF-induced production of PGD₂ in joint mast cells is critical for referred mechanical hypersensitivity in OA, probably through the activation of PGD₂ receptor 1 in nociceptors: TrkA blockade in mast cells constitutes a potential target for OA pain.

Do immune cells lead the way in subchondral bone disturbance in osteoarthritis?

Osteoarthritis (OA) is a whole-joint disorder, and non-cartilage articular pathologies, e.g. subchondral bone disturbance, contribute substantially to the onset and progression of the disease. In the early stage of OA, abnormal mechanical loading leads to micro-cracks or micro-fractures that trigger a reparative process with angiogenesis and inflammatory response. With the progression of disease, cystic lesion, sclerosis and osteophytosis occur at tissue level, and osteoblast dysfunction at cellular level. Osteoblasts derived from OA sclerotic bone produce increased amount of type I collagen with aberrant Col1A1/A2 ratio and poor mineralization capability. The coupling mechanism of bone resorption with formation is also impaired with elevated osteoclastic activities. All these suggest a view that OA subchondral bone presents a defective fracture repair process in a chronic course. It has been found that T and B cells, the major effectors in the adaptive immunity, take part in the hard callus formation at fracture site in addition to the initial phase of haematoma and inflammation. Infiltration of lymphocytes could interplay with osteoclasts and osteoblasts via a direct physical cell-to-cell contact. Several lines of evidence have consistently shown the involvement of T and B cells in osteoclastogenesis and bone erosion in arthritic joints. Yet the biological link between immune cells and osteoblastic function remains ambiguous. This review will discuss the current knowledge regarding the role of immune cells in bone remodelling, and address its implications in emerging basic and clinical investigations into the pathogenesis and management of subchondral bone pathologies in OA.

Immune Contributions to Osteoarthritis

PURPOSE OF THE REVIEW

Mounting evidence supports a role of low-grade inflammation in the pathophysiology of osteoarthritis (OA). We review and discuss the role of synovitis, complement activation, cytokines, and immune cell population in OA.

RECENT FINDINGS

Using newer imaging modalities, synovitis is found in the majority of knees with OA. Complement activation and pro-inflammatory cytokines play a significant role in the development of cartilage destruction and synovitis. Immune cell infiltration of OA synovial tissue by sub-populations of T cells and activated macrophages correlates with OA disease progression and pain. The innate and acquired immune system plays a key role in the low-grade inflammation found associated with OA. Targets of these pathways my hold promise for future disease-modifying osteoarthritis drugs (DMOADs).

What drives osteoarthritis?-synovial versus subchondral bone pathology

Subchondral bone and the synovium play an important role in the initiation and progression of OA. MRI often permits an early detection of synovial hypertrophy and bone marrow lesions, both of which can precede cartilage damage. Newer imaging modalities including CT osteoabsorptiometry and hybrid SPECT-CT have underlined the importance of bone in OA pathogenesis. The subchondral bone in OA undergoes an uncoupled remodelling process, which is notably characterized by macrophage infiltration and osteoclast formation. Concomitant increased osteoblast activity leads to spatial remineralization and osteosclerosis in end-stage disease. A plethora of metabolic and mechanical factors can lead to synovitis in OA. Synovial tissue is highly vascularized and thus exposed to systemic influences such as hypercholesterolaemia or low grade inflammation. This review aims to describe the current understanding of synovitis and subchondral bone pathology and their connection in OA.

Inflammation and its resolution and the musculoskeletal system

Inflammation, an essential tissue response to extrinsic/intrinsic damage, is a very dynamic process in terms of complexity and extension of cellular and metabolic involvement. The aim of the inflammatory response is to eliminate the pathogenic initiator with limited collateral damage of the inflamed tissue, followed by a complex tissue repair to the preinflammation phenotype. Persistent inflammation is a major contributor to the pathogenesis of many musculoskeletal diseases including ageing-related pathologies such as osteoporosis, osteoarthritis, and sarcopaenia. Understanding the mechanisms of inflammation and its resolution is therefore critical for the development of effective regenerative, and therapeutic strategies in orthopaedics.

PPAR-α Agonist WY-14643 Inhibits LPS-Induced Inflammation in Synovial Fibroblasts via NF-kB Pathway

Osteoarthritis (OA), the most prevalent form of arthritis that results from breakdown of joint cartilage and underlying bone, has been viewed as a chronic condition manifested by persistence of inflammatory responses and infiltration of lymphocytes. Regulation of the inflammatory responses in synovial fibroblasts might be useful to prevent the development and deterioration of osteoarthritis. WY-14643, a potent peroxisome proliferator activator receptor-α (PPAR-α) agonist, has been described to beneficially regulate inflammation in many mammalian cells. Here, we investigate the potential anti-inflammatory role of WY-14643 in lipopolysaccharide (LPS)-induced synovial fibroblasts. WY-14643 greatly inhibited the production of NO and PGE2 induced by LPS. In addition, the mRNA expression of intracellular adhesion molecule-1 (ICAM-1), vascular cell adhesion molecule-1 (VCAM-1), endothelin-1 (ET-1), and tissue factor (TF) was significantly suppressed by WY-14643, as well as the secretion of pro-inflammatory cytokines including interleukin-6 (IL-6), IL-1β, tumor necrosis factor-α (TNF-α), and monocyte chemotactic protein-1 (MCP-1). Furthermore, the transcription activity and nuclear translocation of NF-kB were found to be markedly decreased by WY-14643, while the phosphorylation of IkB was enhanced, indicating that the anti-inflammatory role of WY-14643 was meditated by NF-kB-dependent pathway. The application of WY-14643 failed to carry out its anti-inflammatory function in PPAR-α silenced cells, suggesting the role of PPAR-α. These findings may facilitate further studies investigating the translation of pharmacological PPAR-α activation into clinical therapy of OA.