Crucial role of synovial lining macrophages in the promotion of transforming growth factor beta-mediated osteophyte formation

Osteoimmunology in Osteoarthritis: Unraveling the Interplay of Immunity, Inflammation, and Joint Degeneration

Osteoarthritis (OA) is a degenerative joint disease influenced by multiple factors, with its etiology arising from intricate interactions among mechanical stress, inflammatory processes, and disruptions in bone metabolism. Recent research in bone immunology indicates that immune-mediated mechanisms significantly contribute to the progression of OA, highlighting the interactions among immune cells, cytokine networks, and bone components. Immune cells interact with osteoclasts, osteoblasts, and chondrocytes in a variety of ways. These interactions foster a pro-inflammatory microenvironment, contributing to cartilage breakdown, synovial inflammation, and the sclerosis of subchondral bone. In this article, we present a comprehensive review of bone immunology in OA, focusing on the critical role of immune cells and their cytokine-mediated feedback loops in the pathophysiology of OA. In addition, we are exploring novel therapeutic strategies targeting bone immune pathways, including macrophage polarization, T-cell differentiation, and stem cell therapy to restore the metabolic balance between immunity and bone. By integrating cutting-edge research in bone immunology, this review integrates the latest advancements in bone immunology to construct a comprehensive framework for unraveling the pathogenesis of OA, laying a theoretical foundation for the development of innovative precision therapies.

Development of a macrophage polarization-modulating therapeutic agent for osteoarthritis treatment

Osteoarthritis (OA) is a common chronic degenerative joint disease. Recent studies have emphasized the crucial role of macrophages, particularly tissue-resident macrophages (Tissue-Resident Macrophages, TRMs), in the pathogenesis and progression of OA. Under physiological conditions, TRMs maintain joint homeostasis, but under various stimuli, they can polarize into pro-inflammatory M1 or anti-inflammatory M2 phenotypes. An imbalance in macrophage polarization, favoring the M1 phenotype, leads to sustained inflammation, cartilage degradation, and osteophyte formation, further exacerbating OA symptoms and structural damage. This article reviews the current understanding of macrophage polarization in OA, with a particular emphasis on the mechanisms by which TRMs influence the joint microenvironment. It explores the therapeutic potential of drug molecular platforms aimed at regulating macrophage polarization, shifting the balance from pro-inflammatory M1 to anti-inflammatory M2. The discussion includes various pharmacological agents such as corticosteroids, hyaluronic acid derivatives, monoclonal antibodies, and bioactive molecules like Squid Type II Collagen (SCII) in modulating macrophage function and slowing OA progression. Additionally, the article examines advancements in gene therapy methods targeting macrophages, utilizing nanotechnology-based delivery systems to enhance the specificity and efficiency of macrophage phenotype regulation. Targeting TRMs through sophisticated drug molecular platforms presents a promising strategy for developing novel diagnostic and therapeutic interventions for osteoarthritis.

Innate Immunity and Synovitis: Key Players in Osteoarthritis Progression

Osteoarthritis (OA) is a chronic progressive disease of the joint. Although representing the most frequent cause of disability in the elderly, OA remains partly obscure in its pathogenic mechanisms and is still the orphan of resolutive therapies. The concept of what was once considered a "wear and tear" of articular cartilage is now that of an inflammation-related disease that affects over time the whole joint. The attention is increasingly focused on the synovium. Even from the earliest clinical stages, synovial inflammation (or synovitis) is a crucial factor involved in OA progression and a major player in pain onset. The release of inflammatory molecules in the synovium mediates disease progression and worsening of clinical features. The activation of synovial tissue-resident cells recalls innate immunity cells from the bloodstream, creating a proinflammatory milieu that fuels and maintains a damaging condition of low-grade inflammation in the joint. In such a context, cellular and molecular inflammatory behaviors in the synovium could be the primum movens of the structural and functional alterations of the whole joint. This paper focuses on and discusses the involvement of innate immunity cells in synovitis and their role in the progression of OA.

Characterizing Osteophyte Formation in Knee Osteoarthritis: Application of Machine Learning Quantification of a Computerized Tomography Cohort: Implications for Treatment

BACKGROUND

Osteophytes are commonly used to diagnose and guide knee osteoarthritis (OA) treatment, but their causes are unclear. Although they are not typically the focus of knee arthroplasty surgeons, they can predict case difficulty and length. Furthermore, their extent and location may yield much information about the knee joint status. The aims of this computed tomography-based study in patients awaiting total or partial knee arthroplasty were to: (1) measure osteophyte volume in anatomical subregions and relative change as total volume increases; (2) determine whether medial and/or lateral OA affects osteophyte distribution; and (3) explore relationships between osteophytes and OA severity.

METHODS

Data were obtained from 4,928 computed tomography scans. Machine-learning-based imaging analyses enabled osteophyte segmentation and quantification, divided into anatomical regions. Mean three-dimensional joint space narrowing was assessed in medial and lateral compartments. A Bayesian model assessed the uniformity of osteophyte distribution. We correlated femoral osteophyte volumes with B-scores, a validated OA status measure.

RESULTS

Total tibial (25%) and femoral osteophyte volumes (75%) within each knee correlated strongly (R2 = 0.85). Medial osteophytes (65.3%) were larger than lateral osteophytes (34.6%), with similar proportions in both the femur and tibia. Osteophyte growth was found in all compartments, and as total osteophyte volume increased, the relative distribution of osteophytes between compartments did not markedly change. No evidence of variation was found in the regional distribution of osteophyte volume between knees with medial, lateral, both, or no three-dimensional joint space narrowing in the femur or tibia. There was a direct relationship between osteophyte volume and OA severity.

CONCLUSIONS

Osteophyte volume increased in both medial and lateral compartments proportionally with total osteophyte volume, regardless of OA location. The peripheral position of femoral osteophytes does not appear to contribute to load-bearing. This suggests that osteophytic growth represents a 'whole-knee'/global response. This work may have broad applications for knee OA, both surgically and nonoperatively.

Control of articular degeneration by extracellular vesicles from stem/stromal cells as a potential strategy for the treatment of osteoarthritis

Osteoarthritis (OA) is a degenerative joint condition that contributes to years lived with disability. Current therapeutic approaches are limited as there are no disease-modifying interventions able to delay or inhibit the progression of disease. In recent years there has been an increasing interest in the immunomodulatory and regenerative properties of mesenchymal stem/stromal cells (MSCs) to develop new OA therapies. Extracellular vesicles (EVs) mediate many of the biological effects of these cells and may represent an alternative avoiding the limitations of cell-based therapy. There is also a growing interest in EV modifications to enhance their efficacy and applications. Recent preclinical studies have provided strong evidence supporting the potential of MSC EVs for the development of OA treatments. Thus, MSC EVs may regulate chondrocyte functions to avoid cartilage destruction, inhibit abnormal subchondral bone metabolism and synovial tissue alterations, and control pain behavior. EV actions may be mediated by the transfer of their cargo to target cells, with an important role for proteins and non-coding RNAs modulating signaling pathways relevant for OA progression. Nevertheless, additional investigations are needed concerning EV optimization, and standardization of preparation procedures. More research is also required for a better knowledge of possible effects on different OA phenotypes, pharmacokinetics, mechanism of action, long-term effects and safety profile. Furthermore, MSC EVs have a high potential as vehicles for drug delivery or as adjuvant therapy to potentiate or complement the effects of other approaches.

Surgical restabilization reduces the progression of post-traumatic osteoarthritis initiated by ACL rupture in mice

OBJECTIVE

People who sustain joint injuries such as anterior cruciate ligament (ACL) rupture often develop post-traumatic osteoarthritis (PTOA). In human patients, ACL injuries are often treated with ACL reconstruction. However, it is still unclear how effective joint restabilization is for reducing the progression of PTOA. The goal of this study was to determine how surgical restabilization of a mouse knee joint following non-invasive ACL injury affects PTOA progression.

DESIGN

In this study, 187 mice were subjected to non-invasive ACL injury or no injury. After injury, mice underwent restabilization surgery, sham surgery, or no surgery. Mice were then euthanized on day 14 or day 49 after injury/surgery. Functional analyses were performed at multiple time points to assess voluntary movement, gait, and pain. Knees were analyzed ex vivo with micro-computed tomography, RT-PCR, and whole-joint histology to assess articular cartilage degeneration, synovitis, and osteophyte formation.

RESULTS

Both ACL injury and surgery resulted in loss of epiphyseal trabecular bone (-27-32%) and reduced voluntary movement at early time points. Joint restabilization successfully lowered OA score (-78% relative to injured at day 14, p < 0.0001), and synovitis scores (-37% relative to injured at day 14, p = 0.042), and diminished the formation of chondrophytes/osteophytes (-97% relative to injured at day 14, p < 0.001, -78% at day 49, p < 0.001).

CONCLUSIONS

This study confirmed that surgical knee restabilization was effective at reducing articular cartilage degeneration and diminishing chondrophyte/osteophyte formation after ACL injury in mice, suggesting that these processes are largely driven by joint instability in this mouse model. However, restabilization was not able to mitigate the early inflammatory response and the loss of epiphyseal trabecular bone, indicating that these processes are independent of joint instability.

Immunoengineering Biomaterials for Musculoskeletal Tissue Repair across Lifespan

Musculoskeletal diseases and injuries are among the leading causes of pain and morbidity worldwide. Broad efforts have focused on developing pro-regenerative biomaterials to treat musculoskeletal conditions; however, these approaches have yet to make a significant clinical impact. Recent studies have demonstrated that the immune system is central in orchestrating tissue repair and that targeting pro-regenerative immune responses can improve biomaterial therapeutic outcomes. However, aging is a critical factor negatively affecting musculoskeletal tissue repair and immune function. Hence, understanding how age affects the response to biomaterials is essential for improving musculoskeletal biomaterial therapies. This review focuses on the intersection of the immune system and aging in response to biomaterials for musculoskeletal tissue repair. The article introduces the general impacts of aging on tissue physiology, the immune system, and the response to biomaterials. Then, it explains how the adaptive immune system guides the response to injury and biomaterial implants in cartilage, muscle, and bone and discusses how aging impacts these processes in each tissue type. The review concludes by highlighting future directions for the development and translation of personalized immunomodulatory biomaterials for musculoskeletal tissue repair.

Development of primary osteoarthritis during aging in genetically diverse UM-HET3 mice

BACKGROUND

Primary osteoarthritis (OA) occurs without identifiable underlying causes such as previous injuries or specific medical conditions. Age is a major contributing factor to OA, and as one ages, various joint tissues undergo gradual change, including degeneration of the articular cartilage, alterations in subchondral bone (SCB) morphology, and inflammation of the synovium.

METHODS

We investigated the prevalence of primary OA in aged, genetically diverse UM-HET3 mice. Articular cartilage (AC) integrity and SCB morphology were assessed in 182 knee joints of 22-25 months old mice using the Osteoarthritis Research Society International (OARSI) scoring system and micro-CT, respectively. Additionally, we explored the effects of methylene blue (MB) and mitoquinone (MitoQ), two agents that affect mitochondrial function, on the prevalence and progression of OA during aging.

RESULTS

Aged UM-HET3 mice showed a high prevalence of primary OA in both sexes. Significant positive correlations were found between cumulative AC (cAC) scores and synovitis in both sexes, and osteophyte formation in female mice. Ectopic chondrogenesis did not show significant correlations with cAC scores. Significant direct correlations were found between AC scores and inflammatory markers in chondrocytes, including matrix metalloproteinase-13, inducible nitric oxide synthase, and the NLR family pyrin domain containing-3 inflammasome in both sexes, indicating a link between OA severity and inflammation. Additionally, markers of cell cycle arrest, such as p16 and β-galactosidase, also correlated with AC scores. In male mice, no significant correlations were found between SCB morphology traits and cAC scores, while in female mice, significant correlations were found between cAC scores and tibial SCB plate bone mineral density. Notably, MB and MitoQ treatments influenced the disease's progression in a sex-specific manner. MB treatment significantly reduced cAC scores at the medial knee joint, while MitoQ treatment reduced cAC scores, but these did not reach significance.

CONCLUSIONS

Our study provides comprehensive insights into the prevalence and progression of primary OA in aged UM-HET3 mice, highlighting the sex-specific effects of MB and MitoQ treatments. The correlations between AC scores and various pathological factors underscore the multifaceted nature of OA and its association with inflammation and subchondral bone changes.

The role of monocyte/macrophage chemokines in pathogenesis of osteoarthritis: A review

Osteoarthritis (OA) is one of the most common degenerative diseases characterised by joint pain, swelling and decreased mobility, with its main pathological features being articular synovitis, cartilage degeneration and osteophyte formation. Inflammatory cytokines and chemokines secreted by activated immunocytes can trigger various inflammatory and immune responses in articular cartilage and synovium, contributing to the genesis and development of OA. A series of monocyte/macrophage chemokines, including monocyte chemotaxis protein (MCP)-1/CCL2, MCP2/CCL8, macrophage inflammatory protein (MIP)-1α/CCL3, MIP-1β/CCL4, MIP-3α/CCL20, regulated upon activation, normal T-cell expressed and secreted /CCL5, CCL17 and macrophage-derived chemokine/CCL22, was proven to transmit cell signals by binding to G protein-coupled receptors on recipient cell surface, mediating and promoting inflammation in OA joints. However, the underlying mechanism of these chemokines in the pathogenesis of OA remains still elusive. Here, published literature was reviewed, and the function and mechanisms of monocyte/macrophage chemokines in OA pathogenesis were summarised. The symptoms and disease progression of OA were found to be effectively alleviated when the expression of these chemokines is inhibited. Elucidating these mechanisms could contribute to further understand how OA develops and provide potential targets for the early diagnosis of arthritis and drug treatment to delay or even halt OA progression.

The Role of Hyaluronan/Receptor for Hyaluronan-Mediated Motility Interactions in the Modulation of Macrophage Polarization and Cartilage Repair

Hyaluronan (HA), a negatively charged linear glycosaminoglycan, is a key macromolecular component of the articular cartilage extracellular matrix. The differential effects of HA are determined by a spatially/temporally regulated display of HA receptors, such as CD44 and receptor for hyaluronan-mediated motility (RHAMM). HA signaling through CD44 with RHAMM has been shown to stimulate inflammation and fibrotic processes. This study shows an increased expression of RHAMM in proinflammatory macrophages. Interfering with HA/RHAMM interactions using a 15-mer RHAMM-mimetic, HA-binding peptide, together with high-molecular-weight (HMW) HA reduced the expression and release of inflammatory markers and increased the expression of anti-inflammatory markers in proinflammatory macrophages. HA/RHAMM interactions were interfered in vivo during the regeneration of a full-thickness cartilage defect after microfracture surgery in rabbits using three intra-articular injections of 15-mer RHAMM-mimetic. HA-binding peptide together with HMWHA reduced the number of proinflammatory macrophages and increased the number of anti-inflammatory macrophages in the injured knee joint and greatly improved the repair of the cartilage defect compared with intra-articular injections of HMWHA alone. These findings suggest that HA/RHAMM interactions play a key role in cartilage repair/regeneration via stimulating inflammatory and fibrotic events, including increasing the ratio of proinflammatory/anti-inflammatory macrophages. Interfering with these interactions reduced inflammation and greatly improved cartilage repair.

Development of primary osteoarthritis during aging in genetically diverse UM-HET3 mice

Background

Primary osteoarthritis (OA) occurs without identifiable underlying causes such as previous injuries or specific medical conditions. Age is a major contributing factor to OA, and as one ages, various joint tissues undergo gradual change, including degeneration of the articular cartilage, alterations in subchondral bone (SCB) morphology, and inflammation of the synovium.

Methods

We investigated the prevalence of primary OA in aged, genetically diverse UM-HET3 mice. Articular cartilage (AC) integrity and SCB morphology were assessed in 182 knee joints of 22-25 months old mice using the Osteoarthritis Research Society International (OARSI) scoring system and micro-CT, respectively. Additionally, we explored the effects of methylene blue (MB) and mitoquinone (MitoQ), two agents that affect mitochondrial function, on the prevalence and progression of OA during aging.

Results

Aged UM-HET3 mice showed a high prevalence of primary OA in both sexes. Significant positive correlations were found between cumulative AC (cAC) scores and synovitis in both sexes, and osteophyte formation in female mice. Ectopic chondrogenesis did not show significant correlations with cAC scores. Significant direct correlations were found between AC scores and inflammatory markers in chondrocytes, including matrix metalloproteinase-13, inducible nitric oxide synthase, and the NLR family pyrin domain containing-3 inflammasome in both sexes, indicating a link between OA severity and inflammation. Additionally, markers of cell cycle arrest, such as p16 and β-galactosidase, also correlated with AC scores. In male mice, no significant correlations were found between SCB morphology traits and cAC scores, while in female mice, significant correlations were found between cAC scores and tibial SCB plate bone mineral density. Notably, MB and MitoQ treatments influenced the disease's progression in a sex-specific manner. MB treatment significantly reduced cAC scores at the medial knee joint, while MitoQ treatment reduced cAC scores, but these did not reach significance.

Conclusions

Our study provides comprehensive insights into the prevalence and progression of primary OA in aged UM-HET3 mice, highlighting the sex-specific effects of MB and MitoQ treatments. The correlations between AC scores and various pathological factors underscore the multifaceted nature of OA and its association with inflammation and subchondral bone changes.

Development of primary osteoarthritis during aging in genetically diverse UM-HET3 mice

This study investigated the prevalence and progression of primary osteoarthritis (OA) in aged UM-HET3 mice. Using the Osteoarthritis Research Society International (OARSI) scoring system, we assessed articular cartilage (AC) integrity in 182 knee joints of 22-25 months old mice. Aged UM-HET3 mice showed a high prevalence of primary OA in both sexes. Significant positive correlations were found between cumulative AC (cAC) scores and synovitis in both sexes, and osteophyte formation in female mice. Ectopic chondrogenesis did not show significant correlations with cAC scores. Significant direct correlations were found between AC scores and inflammatory markers in chondrocytes, including matrix metalloproteinase-13 (MMP-13), inducible nitric oxide synthase (iNOS), and the NLR family pyrin domain containing-3 (NLRP3) inflammasome in both sexes, indicating a link between OA severity and inflammation. Additionally, markers of cell cycle arrest, such as p16 and β-galactosidase, also correlated with AC scores. Using micro-CT, we examined the correlations between subchondral bone (SCB) morphology traits and AC scores. In male mice, no significant correlations were found between SCB morphology traits and cAC scores, while in female mice, significant correlations were found between cAC scores and tibial SCB plate bone mineral density. Finally, we explored the effects of methylene blue (MB) and mitoquinone (MitoQ), two agents that affect mitochondrial function, on the prevalence and progression of OA during aging. Notably, MB and MitoQ treatments influenced the disease's progression in a sex-specific manner. MB treatment significantly reduced cAC scores at the medial knee joint, while MitoQ treatment reduced cAC scores, but these did not reach significance. In conclusion, our study provides comprehensive insights into the prevalence and progression of primary OA in aged UM-HET3 mice, highlighting the sex-specific effects of MB and MitoQ treatments. The correlations between AC scores and various pathological factors underscore the multifaceted nature of OA and its association with inflammation and subchondral bone changes.

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.

Effects of synovial macrophages in osteoarthritis

Osteoarthritis (OA) is a common degenerative disease in mammals. However, its pathogenesis remains unclear. Studies indicate that OA is not only an aging process that but also an inflammation-related disease. Synovitis is closely related to the progression of OA, and synovial macrophages are crucial participants in synovitis. Instead of being a homogeneous population, macrophages are polarized into M1 or M2 subtypes in OA synovial tissues. Polarization is highly associated with OA severity. However, the M1/M2 ratio cannot be the only factor in OA prognosis because intermediate stages of macrophages also exist. To better understand the mechanism of this heterogeneous disease, OA subtypes of synovial macrophages classified by gene expression were examined. Synovial macrophages do not act alone; they interact with surrounding cells such as synovial fibroblasts, osteoclasts, chondrocytes, lymphocytes and even adipose cells through a paracrine approach to exacerbate OA. Treatments targeting synovial macrophages and their polarization are effective in relieving pain and protecting cartilage during OA development. In this review, we describe how synovial macrophages and their different polarization states influence the progression of OA. We summarize the current knowledge of the interactions between macrophages and other joint cells and examine the current research on new medications targeting synovial macrophages.

Excess Growth Hormone Triggers Inflammation-Associated Arthropathy, Subchondral Bone Loss, and Arthralgia

Growth hormone (GH) is a key mediator of skeletal growth. In humans, excess GH secretion due to pituitary adenoma, seen in patients with acromegaly, results in severe arthropathies. This study investigated the effects of long-term excess GH on the knee joint tissues. One year-old wild-type (WT) and bovine GH (bGH) transgenic mice were used as a model for excess GH. bGH mice showed increased sensitivity to mechanical and thermal stimuli, compared with WT mice. Micro-computed tomography analyses of the distal femur subchondral bone revealed significant reductions in trabecular thickness and significantly reduced bone mineral density of the tibial subchondral bone-plate associated with increased osteoclast activity in both male and female bGH compared with WT mice. bGH mice showed severe loss of matrix from the articular cartilage, osteophytosis, synovitis, and ectopic chondrogenesis. Articular cartilage loss in the bGH mice was associated with elevated markers of inflammation and chondrocyte hypertrophy. Finally, hyperplasia of synovial cells was associated with increased expression of Ki-67 and diminished p53 levels in the synovium of bGH mice. Unlike the low-grade inflammation seen in primary osteoarthritis, arthropathy caused by excess GH affects all joint tissues and triggers severe inflammatory response. Data from this study suggest that treatment of acromegalic arthropathy should involve inhibition of ectopic chondrogenesis and chondrocyte hypertrophy.

Mesenchymal stem/stromal cells-derived extracellular vesicles as a potentially more beneficial therapeutic strategy than MSC-based treatment in a mild metabolic osteoarthritis model

BACKGROUND

Mesenchymal stromal/stem cells (MSCs) and MSC-derived extracellular vesicles (MSC-EVs) hold promise as a disease modifying treatment in osteoarthritis (OA). Obesity, and its associated inflammation, contribute to OA development and metabolic OA represents a specific and significant group of the OA patient population. Given their immunomodulatory properties, MSC and MSC-EVs are especially interesting for this group of patients as a therapeutic option. Here, we were the first to compare the therapeutic efficacy of MSCs and MSC-EVs in a mild OA model taking these metabolic aspects into consideration.

METHODS

Male Wistar-Han rats (Crl:WI(Han) (n = 36) were fed a high fat diet for 24 weeks, with unilateral induction of OA by groove surgery after 12 weeks. Eight days after surgery rats were randomized in three treatment groups receiving MSCs, MSC-EVs or vehicle injection. Pain-associated behavior, joint degeneration, and local and systemic inflammation were measured.

RESULTS

We demonstrated that despite not having a significant therapeutic effect, MSC-EV treatment results in lower cartilage degeneration, less pain behaviour, osteophytosis and joint inflammation, than MSC treatment. Suggesting that MSC-EVs could be a more promising therapeutic strategy than MSCs in this mild metabolic OA model.

CONCLUSION

In summary, we find that MSC treatment has negative effects on the joint in metabolic mild OA. This is an essential finding for the significant group of patients with metabolic OA phenotype, and might help to understand why clinical translation of MSC treatment shows varying therapeutic efficacy thus far. Our results also suggest that MSC-EV-based treatment might be a promising option for these patients, however MSC-EV therapeutic efficacy will need improvement.

In vivo rAAV-mediated human TGF-β overexpression reduces perifocal osteoarthritis and improves osteochondral repair in a large animal model at one year

OBJECTIVE

Osteoarthritis (OA) is a serious consequence of focal osteochondral defects. Gene transfer of human transforming growth factor beta (hTGF-β) with recombinant adeno-associated virus (rAAV) vectors offers a strategy to improve osteochondral repair. However, the long-term in vivo effects of such rAAV-mediated TGF-β overexpression including its potential benefits on OA development remain unknown.

METHOD

Focal osteochondral defects in minipig knees received rAAV-lacZ (control) or rAAV-hTGF-β in vivo. After one year, osteochondral repair and perifocal OA were visualized using validated macroscopic scoring, ultra-high-field MRI at 9.4 T, and micro-CT. A quantitative estimation of the cellular densities and a validated semi-quantitative scoring of histological and immunohistological parameters completed the analysis of microarchitectural parameters.

RESULTS

Direct rAAV-hTGF-β application induced and maintained significantly improved defect filling and safranin O staining intensity and overall cartilage repair at one year in vivo. In addition, rAAV-hTGF-β led to significantly higher chondrocyte densities within the cartilaginous repair tissue without affecting chondrocyte hypertrophy and minimized subarticular trabecular separation. Of note, rAAV-hTGF-β significantly improved the adjacent cartilage structure and chondrocyte density and reduced overall perifocal OA development after one year in vivo.

CONCLUSIONS

rAAV-hTGF-β treatment improves long-term osteochondral repair and delays the progression of perifocal OA in a translational model. These findings have considerable potential for targeted molecular approaches to treat focal osteochondral defects.

Macrophage-Driven Inflammation in Metabolic Osteoarthritis: Implications for Biomarker and Therapy Development

Osteoarthritis (OA) is a common and debilitating joint disorder that leads to progressive joint breakdown and loss of articular cartilage. Accompanied by a state of low-grade inflammation, its etiology extends beyond that of a wear-and-tear disease, and the immune system might have a role in its initiation and progression. Obesity, which is directly associated with an increased incidence of OA, alters adipokine release, increases pro-inflammatory macrophage activity, and affects joint immune regulation. Studying inflammatory macrophage expression and strategies to inhibit inflammatory macrophage phenotype polarization might provide insights into disease pathogenesis and therapeutic applications. In pre-clinical studies, the detection of OA in its initial stages was shown to be possible using imaging techniques such as SPECT-CT, and advances are made to detect OA through blood-based biomarker analysis. In this review, obesity-induced osteoarthritis and its mechanisms in inducing joint degeneration are summarized, along with an analysis of the current developments in patient imaging and biomarker use for diagnostic and therapeutic strategies.

Intervertebral disc degeneration and osteoarthritis: a common molecular disease spectrum

Intervertebral disc degeneration (IDD) and osteoarthritis (OA) affecting the facet joint of the spine are biomechanically interdependent, typically occur in tandem, and have considerable epidemiological and pathophysiological overlap. Historically, the distinctions between these degenerative diseases have been emphasized. Therefore, research in the two fields often occurs independently without adequate consideration of the co-dependence of the two sites, which reside within the same functional spinal unit. Emerging evidence from animal models of spine degeneration highlight the interdependence of IDD and facet joint OA, warranting a review of the parallels between these two degenerative phenomena for the benefit of both clinicians and research scientists. This Review discusses the pathophysiological aspects of IDD and OA, with an emphasis on tissue, cellular and molecular pathways of degeneration. Although the intervertebral disc and synovial facet joint are biologically distinct structures that are amenable to reductive scientific consideration, substantial overlap exists between the molecular pathways and processes of degeneration (including cartilage destruction, extracellular matrix degeneration and osteophyte formation) that occur at these sites. Thus, researchers, clinicians, advocates and policy-makers should consider viewing the burden and management of spinal degeneration holistically as part of the OA disease continuum.

MRI underestimates presence and size of knee osteophytes using CT as a reference standard

OBJECTIVE

To explore the diagnostic performance of routine magnetic resonance imaging (MRI) for the cross-sectional assessment of osteophytes (OPs) in all three knee compartments using computed tomography (CT) as a reference standard.

METHODS

The Strontium Ranelate Efficacy in Knee Osteoarthritis (SEKOIA) trial explored the effect of 3 years of treatment with strontium ranelate in patients with primary knee OA. OPs were scored for the baseline visit only using a modified MRI Osteoarthritis Knee Score (MOAKS) scoring system in the patellofemoral (PFJ), the medial tibiofemoral (TFJ) and the lateral TFJ. Size was assessed from 0 to 3 in 18 locations. Descriptive statistics were used to describe differences in ordinal grading between CT and MRI. In addition, weighted-kappa statistics were employed to assess agreement between scoring using the two methods. Sensitivity, specificity, positive predictive value and negative predictive value as well as area under the curve (AUC) measures of diagnostic performance were employed using CT as the reference standard.

RESULTS

Included were 74 patients with available MRI and CT data. Mean age was 62.9 ± 7.5 years. Altogether 1,332 locations were evaluated. For the PFJ, MRI detected 141 (72%) of 197 CT-defined OPs with a w-kappa of 0.58 (95% CI [0.52-0.65]). In the medial TFJ, MRI detected 178 (81%) of 219 CT-OPs with a w-kappa of 0.58 (95% CI [0.51-0.64]). For the lateral compartment these numbers were 84 (70%) of 120 CT-OPs with a w-kappa of 0.58 (95% CI [0.50-0.66]).

CONCLUSION

MRI underestimates presence of osteophytes in all three knee compartments. CT may be helpful particularly regarding assessment of small osteophytes particularly in early disease.

Controlling Microenvironments with Organs-on-Chips for Osteoarthritis Modelling

Osteoarthritis (OA) remains a prevalent disease affecting more than 20% of the global population, resulting in morbidity and lower quality of life for patients. The study of OA pathophysiology remains predominantly in animal models due to the complexities of mimicking the physiological environment surrounding the joint tissue. Recent development in microfluidic organ-on-chip (OoC) systems have demonstrated various techniques to mimic and modulate tissue physiological environments. Adaptations of these techniques have demonstrated success in capturing a joint tissue's tissue physiology for studying the mechanism of OA. Adapting these techniques and strategies can help create human-specific in vitro models that recapitulate the cellular processes involved in OA. This review aims to comprehensively summarise various demonstrations of microfluidic platforms in mimicking joint microenvironments for future platform design iterations.

Targeting Inflammation and Regeneration: Scaffolds, Extracellular Vesicles, and Nanotechnologies as Cell-Free Dual-Target Therapeutic Strategies

Osteoarthritis (OA) affects over 250 million people worldwide and despite various existing treatment strategies still has no cure. It is a multifactorial disease characterized by cartilage loss and low-grade synovial inflammation. Focusing on these two targets together could be the key to developing currently missing disease-modifying OA drugs (DMOADs). This review aims to discuss the latest cell-free techniques applied in cartilage tissue regeneration, since they can provide a more controllable approach to inflammation management than the cell-based ones. Scaffolds, extracellular vesicles, and nanocarriers can be used to suppress inflammation, but they can also act as immunomodulatory agents. This is consistent with the latest tissue engineering paradigm, postulating a moderate, controllable inflammatory reaction to be beneficial for tissue remodeling and successful regeneration.

Interpretation of Cartilage Damage at Routine Clinical MRI: How to Match Arthroscopic Findings

This review is intended to aid in the interpretation of damage to the articular cartilage at routine clinical MRI to improve clinical management. Relevant facets of the histologic and biochemical characteristics and clinical management of cartilage are discussed, as is MRI physics. Characterization of damage to the articular cartilage with MRI demands a detailed understanding of the normal and damaged appearance of the osteochondral unit in the context of different sequence parameters. Understanding the location of the subchondral bone plate is key to determining the depth of the cartilage lesion. Defining the bone plate at MRI is challenging because of the anisotropic fibrous organization of articular cartilage, which is susceptible to the "magic angle" phenomenon and chemical shift artifacts at the interface with the fat-containing medullary cavity. These artifacts may cause overestimation of the thickness of the subchondral bone plate and, therefore, overestimation of the depth of a cartilage lesion. In areas of normal cartilage morphology, isolated hyperintense and hypointense lesions often represent degeneration of cartilage at arthroscopy. Changes in the subchondral bone marrow at MRI also increase the likelihood that cartilage damage will be visualized at arthroscopy, even when a morphologic lesion cannot be resolved, and larger subchondral lesions are associated with higher grades at arthroscopy. The clinical significance of other secondary features of cartilage damage are also reviewed, including osteophytes, intra-articular bodies, and synovitis. Online supplemental material is available for this article. Work of the U.S. Government published under an exclusive license with the RSNA.

The effects of metformin in the treatment of osteoarthritis: Current perspectives

Osteoarthritis is a chronic and irreversible disease of the locomotor system which is closely associated with advancing age. Pain and limited mobility frequently affect the quality of life in middle-aged and older adults. With a global population of more than 350 million, osteoarthritis is becoming a health threat alongside cancer and cardiovascular disease. It is challenging to find effective treatments to promote cartilage repair and slow down disease progression. Metformin is the first-line drug for patients with type 2 diabetes, and current perspectives suggest that it cannot only lower glucose but also has anti-inflammatory and anti-aging properties. Experimental studies applying metformin for the treatment of osteoarthritis have received much attention in recent years. In our review, we first presented the history of metformin and the current status of osteoarthritis, followed by a brief review of the mechanism that metformin acts, involving AMPK-dependent and non-dependent pathways. Moreover, we concluded that metformin may be beneficial in the treatment of osteoarthritis by inhibiting inflammation, modulating autophagy, antagonizing oxidative stress, and reducing pain levels. Finally, we analyzed the relevant evidence from animal and human studies. The potential of metformin for the treatment of osteoarthritis deserves to be further explored.

Activation of circulating monocytes by low-density lipoprotein-a risk factor for osteoarthritis?

Synovial macrophages are key mediators of OA pathology, and skewing of macrophage phenotype in favour of an M1-like phenotype is thought to underlie the chronicity of synovial inflammation in OA. Components of the metabolic syndrome (MetS), such as dyslipidaemia, can affect macrophage phenotype and function, which could explain the link between MetS and OA development. Recently published studies have provided novel insights into the different origins and heterogeneity of synovial macrophages. Considering these findings, we propose an important role for monocyte-derived macrophages in particular, as opposed to yolk-sac derived residential macrophages, in causing a pro-inflammatory phenotype shift. We will further explain how this can start even prior to synovial infiltration; in the circulation, monocytes can be trained by metabolic factors such as low-density lipoprotein to become extra responsive to chemokines and damage-associated molecular patterns. The concept of innate immune training has been widely studied and implicated in atherosclerosis pathology, but its involvement in OA remains uncharted territory. Finally, we evaluate the implications of these insights for targeted therapy directed to macrophages and metabolic factors.

Source and hub of inflammation: The infrapatellar fat pad and its interactions with articular tissues during knee osteoarthritis

Knee osteoarthritis, the most prevalent degenerative joint disorder worldwide, is driven by chronic low-grade inflammation and subsequent cartilage degradation. Clinical data on the role of the Hoffa or infrapatellar fat pad in knee osteoarthritis are, however, scarce. The infrapatellar fat pad is a richly innervated intracapsular, extrasynovial adipose tissue, and an abundant source of adipokines and proinflammatory and catabolic cytokines, which may contribute to chronic synovial inflammation, cartilage destruction, and subchondral bone remodeling during knee osteoarthritis. How the infrapatellar fat pad interacts with neighboring tissues is poorly understood. Here, we review available literature with regard to the infrapatellar fat pad's interactions with cartilage, synovium, bone, menisci, ligaments, and nervous tissue during the development and progression of knee osteoarthritis. Signaling cascades are described with a focus on immune cell populations, pro- and anti-inflammatory cytokines, adipokines, mesenchymal stromal cells, and molecules derived from conditioned media from the infrapatellar fat pad. Understanding the complex interplay between the infrapatellar fat pad and its neighboring articular tissues may help to better understand and treat the multifactorial pathogenesis of osteoarthritis.

Intra-articular injection of triamcinolone acetonide sustains macrophage levels and aggravates osteophytosis during degenerative joint disease in mice

BACKGROUND AND PURPOSE

Corticosteroids such as triamcinolone acetonide (TAA) are potent drugs administered intra-articularly as an anti-inflammatory therapy to relieve pain associated with osteoarthritis (OA). However, the ability of early TAA intervention to mitigate OA progression and modulate immune cell subsets remains unclear. Here, we sought to understand the effect of early intra-articular injection of TAA on OA progression, local macrophages, and peripheral blood monocytes.

EXPERIMENTAL APPROACH

Degenerative joint disease was induced by intra-articular injection of collagenase into the knee joint of male C57BL/6 mice. After 1 week, TAA or saline was injected intra-articularly. Blood was taken throughout the study to analyse monocyte subsets. Mice were killed at days 14 and 56 post-induction of collagenase-induced OA (CiOA) to examine synovial macrophages and structural OA features.

KEY RESULTS

The percentage of macrophages relative to total live cells present within knee joints was increased in collagenase- compared with saline-injected knees at day 14 and was not altered by TAA treatment. However, at day 56, post-induction of CiOA, TAA-treated knees had increased levels of macrophages compared with the knees of untreated CiOA-mice. The distribution of monocyte subsets present in peripheral blood was not altered by TAA treatment during the development of CiOA. Osteophyte maturation was increased in TAA-injected knees at day 56.

CONCLUSION AND IMPLICATIONS

Intra-articular injection of TAA increases long-term synovial macrophage numbers and osteophytosis. Our findings suggest that TAA accentuates the progression of osteoarthritis-associated features when applied to an acutely inflamed knee.

Synovial inflammation in osteoarthritis progression

Osteoarthritis (OA) is a progressive degenerative disease resulting in joint deterioration. Synovial inflammation is present in the OA joint and has been associated with radiographic and pain progression. Several OA risk factors, including ageing, obesity, trauma and mechanical loading, play a role in OA pathogenesis, likely by modifying synovial biology. In addition, other factors, such as mitochondrial dysfunction, damage-associated molecular patterns, cytokines, metabolites and crystals in the synovium, activate synovial cells and mediate synovial inflammation. An understanding of the activated pathways that are involved in OA-related synovial inflammation could form the basis for the stratification of patients and the development of novel therapeutics. This Review focuses on the biology of the OA synovium, how the cells residing in or recruited to the synovium interact with each other, how they become activated, how they contribute to OA progression and their interplay with other joint structures.

Sprague Dawley Rats Show More Severe Bone Loss, Osteophytosis and Inflammation Compared toWistar Han Rats in a High-Fat, High-Sucrose Diet Model of Joint Damage

In animal models, joint degeneration observed in response to obesogenic diet varies in nature and severity. In this study, we compare joint damage in Sprague Dawley and Wistar-Han rats in response to a high-fat, high-sucrose (HFS) diet groove model of osteoarthritis (OA). Wistar Han (n = 5) and Sprague Dawley (n = 5) rats were fed an HFS diet for 24 weeks. OA was induced 12 weeks after the diet onset by groove surgery in the right knee joint. The left knee served as a control. Outcomes were OARSI histopathology scoring, bone changes by µCT imaging, local (synovial and fat pad) and systemic (blood cytokine) inflammation markers. In both rat strains, the HFS diet resulted in a similar change in metabolic parameters, but only Sprague Dawley rats showed a large, osteoporosis-like decrease in trabecular bone volume. Osteophyte count and local joint inflammation were higher in Sprague Dawley rats. In contrast, cartilage degeneration and systemic inflammatory marker levels were similar between the rat strains. The difference in bone volume loss, osteophytosis and local inflammation suggest that both rat strains show a different joint damage phenotype and could, therefore, potentially represent different OA phenotypes observed in humans.

Gadolinium-free assessment of synovitis using diffusion tensor imaging

The dynamic contrast-enhanced (DCE)-MRI parameter Ktrans can quantify the intensity of synovial inflammation (synovitis) in knees with osteoarthritis (OA), but requires the use of gadolinium-based contrast agent (GBCA). Diffusion tensor imaging (DTI) measures the diffusion of water molecules with parameters mean diffusivity (MD) and fractional anisotropy (FA), and has been proposed as a method to detect synovial inflammation without the use of GBCA. The purpose of this study is to (1) determine the ability of DTI to quantify the intensity of synovitis in OA by comparing MD and FA with our imaging gold standard Ktrans within the synovium and (2) compare DTI and DCE-MRI measures with the semi-quantitative grading of OA severity with the Kellgren-Lawrence (KL) and MRI Osteoarthritis Knee Score (MOAKS) systems, in order to assess the relationship between synovitis intensity and OA severity. Within the synovium, MD showed a significant positive correlation with Ktrans (r = 0.79, p < 0.001), while FA showed a significant negative correlation with Ktrans (r = -0.72, p = 0.0026). These results show that DTI is able to quantify the intensity of synovitis within the whole synovium without the use of exogenous contrast agent. Additionally, MD, FA, and Ktrans values did not vary significantly when knees were separated by KL grade (p = 0.15, p = 0.32, p = 0.41, respectively), while MD (r = 0.60, p = 0.018) and Ktrans (r = 0.62, p = 0.013) had a significant positive correlation and FA (r = -0.53, p = 0.043) had a negative correlation with MOAKS. These comparisons indicate that quantitative measures of the intensity of synovitis may provide information in addition to morphological assessment to evaluate OA severity. Using DTI to quantify the intensity of synovitis without GBCA may be helpful to facilitate a broader clinical assessment of the severity of OA.

Intra-Articular Injections of Curcumin Monoglucuronide TBP1901 Suppresses Articular Cartilage Damage and Regulates Subchondral Bone Alteration in an Osteoarthritis Rat Model

OBJECTIVE

Curcumin monoglucuronide (TBP1901) is highly water soluble and can convert to free form curcumin, which has pharmacological effects, on intravenous administration. This study aimed to investigate the effectiveness of TBP1901 intra-articular injections in an osteoarthritis (OA) rat model.

METHODS

Sixty-four male Wistar rats (12 weeks old) who underwent destabilized medial meniscus (DMM) surgery were randomly separated into the TBP1901 injection or saline solution (control) injection group. They were sacrificed at 1, 2, 6, or 10 weeks postoperatively (weeks 1, 2, 6, and 10; n = 8 for each group). TBP1901 (30 mg/mL) or saline solution of 50 μL was injected into the knee joints twice a week during weeks 1 and 2 to investigate the effects in the acute phase of posttraumatic (PT) OA or once a week during weeks 6 and 10 to investigate it in the chronic phase of PTOA. Histology, immunohistochemistry, and micro-computed tomography were performed to evaluate the changes in OA.

RESULTS

TBP1901 injections significantly reduced synovial inflammation at weeks 1 and 2, and tumor necrosis factor-α expression in the articular cartilage at week 6. The TBP1901 injections also significantly suppressed articular cartilage damage, subchondral bone (SB) plate thickening, SB plate perforation, and osteophyte formation at week 10.

CONCLUSIONS

TBP1901 intra-articular injections suppressed synovial inflammation in the acute phase of PTOA in DMM rats. In the chronic phase, TBP1901 suppresses articular cartilage damage and regulates SB plate changes.

Proximal tibial osteophyte volumes are correlated spatially and with knee alignment: a quantitative analysis suggesting the influence of biochemical and mechanical factors in the development of osteophytes

OBJECTIVE

To characterize the differences and correlations in osteophyte volumes between and within proximal tibial compartments, and to assess the correlations between osteophyte volumes and the femorotibial angle.

DESIGN

CT scans of 73 knees with predominantly medial femorotibial osteoarthritis (21 K/L2, 28 K/L3, 24 K/L4) were retrospectively analyzed using a new, reproducible method measuring total and subregional osteophyte volumes in the medial and lateral compartments. Non-parametric statistics was used for comparison and correlation analyses.

RESULTS

Total osteophyte volumes were larger in the medial than in the lateral compartment for all severity groups (p < 0.05). Additionally, statistically significant differences were observed among subregions of the lateral compartment in K/L3 and K/L4 knees. Statistically significant positive correlations were found between the medial and lateral total osteophyte volumes in K/L3 and K/L4 knees (ρ ≥ 0.44, p = 0.03), and among most subregional osteophyte volumes within each compartment in K/L3 knees. Markedly fewer statistically significant correlations were present in K/L2 and K/L4 knees. In K/L3 knees, the femorotibial angle was statistically significantly positively correlated with the total osteophyte volume in the medial compartment (ρ = 0.50, p = 0.01), with osteophyte volumes in most medial subregions, and with the osteophyte volume in the lateral posterior subregion (ρ = 0.40, p = 0.05).

CONCLUSIONS

Quantitative assessment of osteophytes may bring insight on factors influencing their development. Positive correlations of osteophyte volumes found between and within compartments suggest the influence of biochemical mediators acting on the entire joint, while positive correlations between the femorotibial angle and osteophyte volumes suggest a role of mechanical factors. These hypotheses are to be further confirmed.

The nature of coxofemoral joint pathology across family Canidae

We evaluated coxofemoral joints from museum specimens of: Vulpes lagopus; Vulpes vulpes; Vulpes velox; Nyctereutes procyonoides; Urocyon cinereoargenteus; Aenocyon [Canis] dirus; Canis latrans; Canis lupus lupus; Canis lupus familiaris; C. l. familiaris × latrans; and Canis dingo. Acetabular components included: fossa; articular surface; medial and lateral articular margins; and periarticular surfaces. Acetabular components variably revealed: osteophyte-like features; varying appearance of articular margin rims (especially contour changes); rough bone surfaces (especially fossa and articular surface); and surface wear. Proximal femoral components included: articular surface; articular margin; periarticular surfaces; and joint capsule attachment. Femoral components variably revealed: rough bone surface; bone loss; articular margin osteophyte-like features; caudal post-developmental mineralized prominence; and enthesophytes along the joint capsule attachment. Non-metric multidimensional scaling was used to analyze right-left asymmetric relationships between observed traits, across taxa. Significantly different acetabular trait asymmetry involved only C. latrans-C. l. familiaris; V. vulpes-N. procyonoides, and U. cinereoargenteus-N. procyonoides. There were no significant lateralized differences in proximal femoral traits involving modern canids, ancient and modern C. l. familiaris, or modern vulpines. Thus, the observations were strongly bilateral. We hypothesized high similarity of traits across taxa. The data confirm the hypothesis and strongly suggest broad and deep morphological and mechanistic conservation that almost certainly pre-existed (at least) all modern canids. Further zoological studies are needed to evaluate phylogenic implications in greater detail.

Macrophages in heterotopic ossification: from mechanisms to therapy

Heterotopic ossification (HO) is the formation of extraskeletal bone in non-osseous tissues. It is caused by an injury that stimulates abnormal tissue healing and regeneration, and inflammation is involved in this process. It is worth noting that macrophages are crucial mediators of inflammation. In this regard, abundant macrophages are recruited to the HO site and contribute to HO progression. Macrophages can acquire different functional phenotypes and promote mesenchymal stem cell (MSC) osteogenic differentiation, chondrogenic differentiation, and angiogenesis by expressing cytokines and other factors such as the transforming growth factor-β1 (TGF-β1), bone morphogenetic protein (BMP), activin A (Act A), oncostatin M (OSM), substance P (SP), neurotrophin-3 (NT-3), and vascular endothelial growth factor (VEGF). In addition, macrophages significantly contribute to the hypoxic microenvironment, which primarily drives HO progression. Thus, these have led to an interest in the role of macrophages in HO by exploring whether HO is a "butterfly effect" event. Heterogeneous macrophages are regarded as the "butterflies" that drive a sequence of events and ultimately promote HO. In this review, we discuss how the recruitment of macrophages contributes to HO progression. In particular, we review the molecular mechanisms through which macrophages participate in MSC osteogenic differentiation, angiogenesis, and the hypoxic microenvironment. Understanding the diverse role of macrophages may unveil potential targets for the prevention and treatment of HO.

Cellular senescence in musculoskeletal homeostasis, diseases, and regeneration

Emerging insights into cellular senescence highlight the relevance of senescence in musculoskeletal disorders, which represent the leading global cause of disability. Cellular senescence was initially described by Hayflick et al. in 1961 as an irreversible nondividing state in in vitro cell culture studies. We now know that cellular senescence can occur in vivo in response to various stressors as a heterogeneous and tissue-specific cell state with a secretome phenotype acquired after the initial growth arrest. In the past two decades, compelling evidence from preclinical models and human data show an accumulation of senescent cells in many components of the musculoskeletal system. Cellular senescence is therefore a defining feature of age-related musculoskeletal disorders, and targeted elimination of these cells has emerged recently as a promising therapeutic approach to ameliorate tissue damage and promote repair and regeneration of the skeleton and skeletal muscles. In this review, we summarize evidence of the role of senescent cells in the maintenance of bone homeostasis during childhood and their contribution to the pathogenesis of chronic musculoskeletal disorders, including osteoporosis, osteoarthritis, and sarcopenia. We highlight the diversity of the senescent cells in the microenvironment of bone, joint, and skeletal muscle tissue, as well as the mechanisms by which these senescent cells are involved in musculoskeletal diseases. In addition, we discuss how identifying and targeting senescent cells might positively affect pathologic progression and musculoskeletal system regeneration.

[18F]NaF PET-MRI provides direct in-vivo evidence of the association between bone metabolic activity and adjacent synovitis in knee osteoarthritis: a cross-sectional study

OBJECTIVE

Synovitis is hypothesized to play a role in the development and growth of osteophytes. Our objectives were to use hybrid positron emission tomography-magnetic resonance imaging (PET-MRI) to (1) determine whether synovitis adjacent to peripheral bone subregions with increased metabolic activity is greater than adjacent to regions without increased metabolic activity and (2) assess the association between subregional bone metabolic activity and adjacent synovitis.

DESIGN

We recruited 11 participants (22 knees) with a diagnosis of OA in at least one knee. Simultaneous bilateral knee PET-MRI was performed. We quantified bone metabolic activity using the radiotracer [18F]sodium fluoride ([18F]NaF) with calculation of maximum standardized uptake values (SUVmax). Synovitis was quantified using dynamic contrast-enhanced MRI with calculation of Ktrans. Bone subregions were coded as osteophyte (OP), focal increased [18F]NaF uptake without osteophyte (FIU), or normal (no osteophyte or FIU). We used robust linear mixed effects models to assess differences in adjacent Ktrans between different subregion types and to assess association between Ktrans and adjacent SUVmax.

RESULTS

94 OPs were detected (59 MOAKS grade 1, 30 grade 2, 5 grade 3), along with 28 FIU and 18 normal subregions. Ktrans was higher adjacent to FIU (adjusted mean [95% CI] = 0.06 [0.03,0.09]) and OPs (0.08 [0.05,0.11]) when compared to normal bone subregions (0.03 [0.00,0.09]). PET SUVmax was positively associated with adjacent Ktrans (β[95% CI] = 0.018 [0.008,0.027]).

CONCLUSIONS

Synovitis is more intense adjacent to peripheral bone regions with increased metabolic activity than those without, although there is some overlap. Subregional bone metabolic activity is positively associated with intensity of adjacent synovitis.

Association between Oncostatin M Expression and Inflammatory Phenotype in Experimental Arthritis Models and Osteoarthritis Patients

Pro-inflammatory cytokines are considered to play a major role in osteoarthritis (OA), yet so far, the specific cytokines involved in the pathology of OA have not been identified. Oncostatin M (OSM) is a cytokine from the interleukin 6 (IL-6) family that has been shown to be elevated in synovial fluid of most rheumatoid arthritis (RA) patients, but only in a limited subset of OA patients. Little is known about OSM in the different joint tissues during OA and how its expression correlates with hallmarks of disease. Here, we mapped OSM expression in the joint tissues of two rat models of arthritis: an acute inflammatory model and an instability-induced osteoarthritic model. OSM expression was correlated with hallmarks of OA, namely cartilage damage, synovitis, and osteophyte formation. Reanalysis of an existing dataset on cytokine profiling of OA synovial fluid was performed to assess pattern differences between patients positive and negative for OSM. In the inflammatory model, OSM expression correlated with synovitis and osteophyte formation but not with cartilage damage. On the contrary, in the instability model of OA, an increase in synovitis, cartilage damage, and osteophyte formation was observed without changes in OSM expression. In line with these findings, synovial fluid of OA patients with detectable OSM contained higher levels of other inflammatory cytokines, namely interferon gamma (IFN-γ), IL-1α and tumor necrosis factor alpha (TNF-α), likely indicating a more inflammatory state. Taken together these data indicate OSM might play a prominent role in inflammatory phenotypes of OA.

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.

Increased elastic modulus of the synovial membrane in a rat ACLT model of osteoarthritis revealed by atomic force microscopy

This study aimed to explore changes in nanoscale elastic modulus of the synovium using atomic force microscopy (AFM) in addition to investigate changes in synovial histomorphology and secretory function in osteoarthritis (OA) in a rat anterior cruciate ligament transection (ACLT) model. Sprague-Dawley rats were randomly assigned to sham control and ACLT OA groups. All right knee joints were harvested at 4, 8, or 12 weeks (W) after surgery for histological assessment of cartilage damage and synovitis in both the anterior and posterior capsules. AFM imaging and nanoscale biomechanical testing were conducted to measure the elastic modulus of the synovial collagen fibrils. Immunohistochemistry was used to visualize the expression of interleukin-1β (IL-1β), tumor necrosis factor-α (TNF-α), and matrix metalloproteinase-3 (MMP-3) in the synovium. The OA groups exhibited progressive development of disease in the cartilage and synovium. Histopathological scores of the synovium in the OA groups increased gradually. Significant differences were observed between all OA groups except for the posterior 4W group. The synovial fibril arrangement in all OA groups was significantly disordered. The synovial fibrils in all ACLT OA groups at each time point were stiffer than those in the sham controls. OA rats displayed a significantly higher expression of IL-1β and MMP3 in the anterior capsule. In summary, synovial stiffening was closely associated with joint degeneration and might be a factor contributing to synovitis and increased production of proinflammatory mediators. Our data provided insights into the role of synovitis, particularly stiffening of the synovium, in OA pathogenesis.

The role of macrophages in osteoarthritis and cartilage repair

Osteoarthritis (OA) is a family of degenerative diseases affecting multiple joint tissues. Despite the diverse etiology and pathogenesis of OA, increasing evidence suggests that macrophages can play a significant role in modulating joint inflammation, and thus OA severity, via various secreted mediators. Recent advances in next-generation sequencing technologies coupled with proteomic and epigenetic tools have greatly facilitated research to elucidate the embryonic origin of macrophages in various tissues including joint synovium. Furthermore, scientists have now begun to appreciate that macrophage polarization can span beyond the conventionally recognized binary states (i.e., pro-inflammatory M1-like vs anti-inflammatory M2-like) and may encompass a broad spectrum of phenotypes. Although the presence of these cells has been shown in multiple joint tissues, additional mechanistic studies are required to provide a comprehensive understanding of the precise role of these diverse macrophage populations in OA onset and progression. New approaches that can modulate macrophages into desired functional phenotypes may provide novel therapeutic strategies for preventing OA or enhancing cartilage repair and regeneration.

Cartilage repair mediated by thermosensitive photocrosslinkable TGFβ1-loaded GM-HPCH via immunomodulating macrophages, recruiting MSCs and promoting chondrogenesis

Repairing cartilage defects using thermosensitive hydrogels is an attractive treatment strategy, but the poor mechanical properties and limited understanding of the interactions between hydrogels and cells limit their application. Methods: In this study, a thermosensitive hydroxypropyl chitin hydrogel (HPCH) was functionalized with methacrylate groups to synthesize photocrosslinkable glycidyl methacrylate-modified HPCH (GM-HPCH). GM-HPCH could form a gel in situ through a thermosensitive sol-gel transition and its mechanical properties can be improved by UV irradiation. Cell viability, cell adhesion and anti-apoptosis activity of GM-HPCH were evaluated. Transforming growth factor-β1 (TGFβ1) was introduced into the GM-HPCH hydrogel to fabricate the composite hydrogel. The macrophage immunomodulation, MSC recruitment and chondrogenesis of the composite hydrogel were evaluated. Results: With high biocompatibility, GM-HPCH could protect chondrocytes from apoptosis. Both the in vitro and in vivo experiments showed that GM-HPCH + TGFβ1 shifted the recruited macrophages from M1 to M2 and promoted chondrogenic gene expression. Additionally, the composite hydrogel could promote the migration of marrow stromal cells (MSCs) in the Transwell test and increase migrated gene expression. The fluorescent tracking of MSCs confirmed MSC homing in the rat chondral defect with the help of GM-HPCH. The macroscopic evaluation and histological results at 6 weeks and 12 weeks postsurgery showed that GM-HPCH + TGFβ1 can achieve superior cartilage healing. Conclusions: The GM-HPCH + TGFβ1 hydrogel effectively promoted cartilage repair via immunomodulating macrophages, recruiting MSCs and promoting chondrogenesis; thus it is a promising injectable hydrogel for cartilage regeneration.

Longitudinal MRI structural findings observed in accelerated knee osteoarthritis: data from the Osteoarthritis Initiative

OBJECTIVE

To analyze structural, longitudinal MRI findings during the development of accelerated knee osteoarthritis (AKOA) over 4 years.

MATERIALS AND METHODS

From the Osteoarthritis Initiative (OAI), knees with no radiographic osteoarthritis (KL 0/1) developing advanced-stage osteoarthritis (KL 3/4; AKOA) within a 4-year (y) timeframe were selected. MRIs were graded using the modified Whole-Organ Magnetic Resonance Imaging Score (WORMS) at the beginning of the 4-year timeframe (index visit), at 2-year, and 4-year follow-up. Morphological and clinical findings associated with KL 3/4 onset within 2 years compared to 4 years were assessed using generalized estimating equations.

RESULTS

AKOA was found in 162 knees of 149 subjects (age 63.25 ± 8.3; 103 females; BMI 29.4 ± 3.9). Moderate to severe meniscal lesions WORMS ≥ 3 were present in 25% (41/162) at the index visit, 64% (104/162) at 2-year and 93% (151/162) at 4-year follow-up. Meniscal extrusion was the most prevalent finding (ranging from 18% at the index visit, 45% at 2-year and 94% at 4-year follow-up) and root tears were the most common types of tears (9% at the index visit; 22% at 2 years and 38% at 4 years). Risk factors associated with KL 3/4 onset within 2 years included root tears at the index visit (adjusted OR, 2.82; 95% CI: 1.33, 6.00; p = 0.007) and incident knee injury (42%, 49/116 vs. 24%, 11/46, p = 0.032).

CONCLUSIONS

Meniscal abnormalities, in particular extrusion and root tears, were the most prevalent morphological features found in subjects with AKOA. These results suggest that meniscal abnormalities have a significant role in accelerated progression of OA.

Musculoskeletal pain: determination of clinical phenotypes and the rational treatment approach

Personalized treatment is one of the basic principles of modern medicine. When administering a treatment, one should consider individual patient characteristics, comorbidities and, what is most important, the prevailing symptoms, as well as the clinical phenotype of a disease. This is directly related to chronic musculoskeletal pain (MSP), which occurs with underlying most prevalent joint and vertebral disorders. At present, MSP is considered to be an independent clinical syndrome.Predominant mechanisms of MSP pathophysiology allow for determination of its special phenotypes: “inflammatory”, “mechanical”, related to enthesopathy and central sensitization. Treatment strategies for MSP phenotypes should obviously be differentiated and based on a tailored and pathophysiologically sound of medical agents and non-medical measures with different mechanisms of pharmacological effects. Effective treatment of the “inflammatory” phenotype requires the use of non-steroidal anti-inflammatory drugs, topical glucocorticoids, disease modifying anti-inflammatory agents. The “mechanical” phenotype necessitates the correction of biomechanical abnormalities, the use of hyaluronic acid containing agents, whereas the “enthesopathic” phenotype is treated with local therapy. Treatment of the phenotype with central sensitization is performed with agents effective for neuropathic pain (anticonvulsants, anti-depressants).

Autologous bone marrow mononuclear cells modulate joint homeostasis in an equine in vivo model of synovitis

Osteoarthritis (OA) is characterized by macrophage-driven synovitis. Macrophages promote synovial health but become inflammatory when their regulatory functions are overwhelmed. Bone marrow mononuclear cells (BMNCs) are a rich source of macrophage progenitors used for treating chronic inflammation and produce essential molecules for cartilage metabolism. This study investigated the response to autologous BMNC injection in normal and inflamed joints. Synovitis was induced in both radiocarpal joints of 6 horses. After 8 h, 1 inflamed radiocarpal and 1 normal tarsocrural joint received BMNC injection. Contralateral joints were injected with saline. Synovial fluid was collected at 24, 96, and 144 h for cytology, cytokine quantification, and flow cytometry. At 144 h, horses were euthanatized, joints were evaluated, and synovium was harvested for histology and immunohistochemistry. Four days after BMNC treatment, inflamed joints had 24% higher macrophage counts with 10% more IL-10⁺ cells than saline-treated controls. BMNC-treated joints showed gross and analytical improvements in synovial fluid and synovial membrane, with increasing regulatory macrophages and synovial fluid IL-10 concentrations compared with saline-treated controls. BMNC-treated joints were comparable to healthy joints histologically, which remained abnormal in saline-treated controls. Autologous BMNCs are readily available, regulate synovitis through macrophage-associated effects, and can benefit thousands of patients with OA.-Menarim, B. C., Gillis, K. H., Oliver, A., Mason, C., Ngo, Y., Werre, S. R., Barrett, S. H., Luo, X., Byron, C. R., Dahlgren, L. A. Autologous bone marrow mononuclear cells modulate joint homeostasis in an equine in vivo model of synovitis.

A Jack of All Trades: Impact of Glucocorticoids on Cellular Cross-Talk in Osteoimmunology

Glucocorticoids (GCs) are known to have a strong impact on the immune system, metabolism, and bone homeostasis. While these functions have been long investigated separately in immunology, metabolism, or bone biology, the understanding of how GCs regulate the cellular cross-talk between innate immune cells, mesenchymal cells, and other stromal cells has been garnering attention rather recently. Here we review the recent findings of GC action in osteoporosis, inflammatory bone diseases (rheumatoid and osteoarthritis), and bone regeneration during fracture healing. We focus on studies of pre-clinical animal models that enable dissecting the role of GC actions in innate immune cells, stromal cells, and bone cells using conditional and function-selective mutant mice of the GC receptor (GR), or mice with impaired GC signaling. Importantly, GCs do not only directly affect cellular functions, but also influence the cross-talk between mesenchymal and immune cells, contributing to both beneficial and adverse effects of GCs. Given the importance of endogenous GCs as stress hormones and the wide prescription of pharmaceutical GCs, an improved understanding of GC action is decisive for tackling inflammatory bone diseases, osteoporosis, and aging.

Neuronal FcγRI mediates acute and chronic joint pain

Although joint pain in rheumatoid arthritis (RA) is conventionally thought to result from inflammation, arthritis pain and joint inflammation are at least partially uncoupled. This suggests that additional pain mechanisms in RA remain to be explored. Here we show that FcγRI, an immune receptor for IgG immune complex (IgG-IC), is expressed in a subpopulation of joint sensory neurons and that, under naïve conditions, FcγRI crosslinking by IgG-IC directly activates the somata and peripheral terminals of these neurons to evoke acute joint hypernociception without obvious concurrent joint inflammation. These effects were diminished in both global and sensory neuron-specific Fcgr1 knockout mice. In murine models of inflammatory arthritis, FcγRI signaling was upregulated in joint sensory neurons. Acute blockade or global genetic deletion of Fcgr1 significantly attenuated arthritis pain and hyperactivity of joint sensory neurons without measurably altering joint inflammation. Conditional deletion of Fcgr1 in sensory neurons produced similar analgesic effects in these models. We therefore suggest that FcγRI expressed in sensory neurons contributes to arthritis pain independently of its functions in inflammatory cells. These findings expand our understanding of the immunosensory capabilities of sensory neurons and imply that neuronal FcγRI merits consideration as a target for treating RA pain.

Proinflammatory polarization of stifle synovial macrophages in dogs with cruciate ligament rupture

OBJECTIVE

To determine polarization of synovial macrophages during development of cruciate ligament rupture (CR) and determine whether differences in synovial macrophage polarization in CR, osteoarthritis (OA), and healthy joints exist.

STUDY DESIGN

Prospective case-controlled study.

ANIMALS

Client-owned dogs with unstable stifles with CR (n = 22), paired stable contralateral stifles with partial CR (pCR; n = 7), joints with OA not related to CR (n = 6), and clinically normal (Normal; n = 7) joints.

METHODS

Synovial fluid samples were collected. Smears were made for differential cytology counts and estimated total nucleated cell counts. Cytospin preparations were made, and immunocytochemical staining was performed with the pan-macrophage marker CD68, M1 macrophage markers inducible nitric oxide synthase (iNOS) and chemokine (C-C motif) receptor 7 (CCR7), and M2 macrophage markers arginase 1 and CD163. Positively stained cells were counted.

RESULTS

Numbers of lymphocytes were increased in the CR group compared with the OA and Normal groups (P < .05). Numbers of CD68⁺ , CCR7⁺ , and iNOS⁺ cells in the CR and OA groups were increased compared with the Normal group (P < .05). Globally, the ratio of positively stained M1 polarized CD68⁺ cells to M2 polarized CD68⁺ cells was highest for the OA group (2.49), followed by the pCR (2.1), CR (1.63), and Normal (0.7) groups.

CONCLUSION

Polarization of synovial macrophages toward an M1 proinflammatory phenotype is an early event in the development of canine CR.

CLINICAL SIGNIFICANCE

M1 polarization in pCR stifles provides evidence of a possible role for macrophages in progressive development of cruciate ligament fiber damage. Lymphocytes may play a role in the synovitis found in CR joints. Our findings provide evidence that these cells are therapeutic targets.

Enhancement of the chondrogenic differentiation of mesenchymal stem cells and cartilage repair by ghrelin

Transforming growth factor beta (TGF-β) is commonly utilized in chondrogenic differentiation protocols, but this often results in incomplete maturation of the derived chondrocytes. Gene expression analysis, quantitation of sulfated glycosaminoglycan and collagen, and histological staining were performed to assess the effects of ghrelin. The signaling pathways involved were investigated with inhibitors or targeted by shRNAs. Joint cavity delivery of TGF-β with or without ghrelin, within a rat cartilage defect model was performed to evaluate the in vivo effects of ghrelin. Ghrelin dramatically enhanced gene expression levels of SOX9, ACAN, and COL II and resulted in increased synthesis of sulfated glycosaminoglycan (sGAG) and collagen in vitro. Combined treatment with TGF-β and ghrelin synergistically enhanced the phosphorylation of ERK1/2 and DMNT3A, which accounted for increased expression of chondrogenic genes. Delivery of ghrelin in combination with TGF-β after MSC implantation within a rat osteochondral defect model significantly enhanced de novo cartilage regeneration, as compared to delivery with TGF-β alone. In conclusion, ghrelin could significantly enhance MSC chondrogenic differentiation in vitro and can also enhance cartilage regeneration in vivo when used in combination with TGF-β. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:1387-1397, 2019.

Macrophage proliferation distinguishes 2 subgroups of knee osteoarthritis patients

Osteoarthritis (OA) is a leading cause of disability, globally. Despite an emerging role for synovial inflammation in OA pathogenesis, attempts to target inflammation therapeutically have had limited success. A better understanding of the cellular and molecular processes occurring in the OA synovium is needed to develop novel therapeutics. We investigated macrophage phenotype and gene expression in synovial tissue of OA and inflammatory-arthritis (IA) patients. Compared with IA, OA synovial tissue contained higher but variable proportions of macrophages (P < 0.001). These macrophages exhibited an activated phenotype, expressing folate receptor-2 and CD86, and displayed high phagocytic capacity. RNA sequencing of synovial macrophages revealed 2 OA subgroups. Inflammatory-like OA (iOA) macrophages are closely aligned to IA macrophages and are characterized by a cell proliferation signature. In contrast, classical OA (cOA) macrophages display cartilage remodeling features. Supporting these findings, when compared with cOA, iOA synovial tissue contained higher proportions of macrophages (P < 0.01), expressing higher levels of the proliferation marker Ki67 (P < 0.01). These data provide new insight into the heterogeneity of OA synovial tissue and suggest distinct roles of macrophages in pathogenesis. Our findings could lead to the stratification of OA patients for suitable disease-modifying treatments and the identification of novel therapeutic targets.

Effects of carrageenan induced synovitis on joint damage and pain in a rat model of knee osteoarthritis

OBJECTIVE

Knee osteoarthritis (OA) is associated with ongoing pain and joint damage that can be punctuated by acute flares of pain and inflammation. Synovitis in normal knees might resolve without long-term detriment to joint function. We hypothesised that osteoarthritis is associated with impaired resilience to inflammatory flares.

DESIGN

We induced synovitis by injecting carrageenan into rat knees with or without meniscal transection (MNX)-induced OA, and measured synovitis, weightbearing asymmetry (pain behaviour), and joint damage up to 35 days after OA induction (23 days after carrageenan-injection).

RESULTS

Carrageenan injection induced weightbearing asymmetry for 1 week, transient increase in knee diameter for 2 days, and a sustained increase in synovial macrophages, endothelial cell proliferation and vascular density compared with naive vehicle-injected controls. MNX surgery induced weightbearing asymmetry and histological evidence of OA. Carrageenan-injection in MNX-operated knees was followed for 2 days by increased weightbearing asymmetry compared either to MNX+vehicle or to sham+carrageenan groups. OA structural damage and synovitis at day 35 were greater in MNX+carrageenan compared to MNX+vehicle and sham+carrageenan groups. Carrageenan injection did not induce OA in Sham-operated knees.

CONCLUSION

Intra-articular injection of the pro-inflammatory compound carrageenan in OA and sham-operated control knees induced a short term increase in joint pain. Even though pain flares resolved in both groups and damage was not induced in sham-operated knees, carrageen injection exacerbated long-term joint damage in OA knees. OA knees display less resilience to inflammatory episodes. Preventing inflammatory flares may be particularly important in preventing symptoms and long term joint damage in OA.