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Immunoregulation of synovial macrophages for the treatment of osteoarthritis

  • Mingze Xu and Yunhan Ji EMAIL logo
Published/Copyright: January 31, 2023

Abstract

Osteoarthritis (OA) is the most common joint disease affecting approximately 10% of men and 18% of women older than 60. Its pathogenesis is still not fully understood; however, emerging evidence has suggested that chronic low-grade inflammation is associated with OA progression. The pathological features of OA are articular cartilage degeneration in the focal area, including new bone formation at the edge of the joint, subchondral bone changes, and synovitis. Conventional drug therapy aims to prevent further cartilage loss and joint dysfunction. However, the ideal treatment for the pathogenesis of OA remains to be defined. Macrophages are the most common immune cells in inflamed synovial tissues. In OA, synovial macrophages undergo proliferation and activation, thereby releasing pro-inflammatory cytokines, including interleukin-1 and tumor necrosis factor-α, among others. The review article discusses (1) the role of synovial macrophages in the pathogenesis of OA; (2) the progress of immunoregulation of synovial macrophages in the treatment of OA; (3) novel therapeutic targets for preventing the progress of OA or promoting cartilage repair and regeneration.

1 Introduction

Osteoarthritis (OA) is the most prevalent form of arthritis and a major cause of disability worldwide, affecting an estimated 10% of men and 18% of women over 60 [1,2]. However, due to the unclear pathogenesis, few treatments are currently available to prevent the onset or progression of OA. Compared to earlier paradigms, OA is now recognized as a low-grade inflammatory disease affecting the entire joint. It is characterized by articular cartilage destruction, subchondral bone remodeling, osteophyte formation, and synovium inflammation (synovitis) [3].

A recent single-cell RNA-seq study has identified various synovial joint immune cell types and characterized their dynamic alterations during the pathological progression of post-traumatic OA in mouse knee joints following anterior cruciate ligament (ACL) rupture [4]. Multiple immune cell types in joints were detected, including neutrophils, monocytes, macrophages, B cells, T cells, natural killer (NK) cells, and dendritic cells. The monocyte and macrophage populations showed the most dramatic changes after injury. Further characterization of monocytes and macrophages revealed nine major subtypes with distinct transcriptomic signatures, including two macrophage populations with phagocytic genes and enrichment of growth factors [4].

Studies have also found that during the development of OA, many matrix-degrading enzymes, such as matrix metalloproteinases (MMPs), are significantly upregulated. The increased secretion of pro-inflammatory cytokines indicated that the synovium undergoes an inflammatory process, leading to the degradation of the cartilage matrix [5]. Furthermore, increasing evidence suggests that persistent low-grade synovial inflammation exacerbates cartilage damage [6], where synovial macrophages have a critical role [7]. Therefore, immunoregulation of macrophages might limit the pro-inflammatory effects and promote anti-inflammatory effects of synovial macrophages, restoring the normal composition of the extracellular chondrocyte matrix and promoting cartilage repair, which in turn improves joint function and facilitate daily activities of patients with OA [8,9,10,11].

The present review discussed the following: (1) the role of synovial macrophages in the pathogenesis of OA; (2) the progress of immunoregulation of synovial macrophages in the treatment of OA; (3) novel therapeutic targets for preventing the progress of OA or promoting cartilage repair and regeneration.

2 Role of synovial macrophages in OA

Different studies have reported on the role of macrophages in the pathogenesis of OA. In normal synovium, macrophages are the predominant cell type [12]. Synovial macrophages are found on the surface of the synovial membrane in healthy joints, providing regulatory factors for cartilage and bone turnover. Similar to other tissue-resident macrophages, they may also remove cell debris and pathogens to prevent sterile and septic inflammation [13]. Increasing evidence highlights the impact of synovitis and macrophage activation on the occurrence and development of OA [14]. A previous study suggested that monocyte/macrophages are the most abundant immune cells in the synovial fluid of OA patients, accounting for ∼36.5% of the total leukocyte. They are also the CD14 + CD16 + double-positive pro-inflammatory cells [15]. Histological studies have also observed more diffusely distributed macrophages in the synovial lining of OA [12]. Yet, studies have also found that alteration in their functionalities may alter the joints of OA patients. It was reported that macrophages produce various cytokines, including interleukin-1 (IL-1) and tumor necrosis factor-alpha (TNF-α) in the OA synovium [16]. In addition, cytokines such as granulocyte-macrophage colony-stimulating factor (GM-CSF) and interferon-γ (IFN-γ) produced at the site of inflammation can recruit and activate macrophages [17]. This vicious circle of macrophage activation and pro-inflammatory cytokines production causes deterioration of the inflammatory process and cartilage degradation [18] (Figure 1).

Figure 1 
               Mechanisms of macrophages in the pathogenesis of OA. Microenvironment stimuli promote synovial macrophage activation and polarization. M1-polarized macrophages in the synovium contribute to OA by releasing pro-inflammatory cytokines that lead to inflammation and subsequent cartilage degradation and osteophyte formation. Polarized macrophages alter the intercellular signaling pathways in chondrocytes, promoting the degradation of extracellular matrix (ECM) components. ECM, in turn, acts as DAMPs and further stimulates macrophage activation and polarization, resulting in a repeating cycle of inflammation and cartilage degradation. Polarized synovial macrophages and macrophage reprogramming could provide therapeutic targets for OA patients.
Figure 1

Mechanisms of macrophages in the pathogenesis of OA. Microenvironment stimuli promote synovial macrophage activation and polarization. M1-polarized macrophages in the synovium contribute to OA by releasing pro-inflammatory cytokines that lead to inflammation and subsequent cartilage degradation and osteophyte formation. Polarized macrophages alter the intercellular signaling pathways in chondrocytes, promoting the degradation of extracellular matrix (ECM) components. ECM, in turn, acts as DAMPs and further stimulates macrophage activation and polarization, resulting in a repeating cycle of inflammation and cartilage degradation. Polarized synovial macrophages and macrophage reprogramming could provide therapeutic targets for OA patients.

The inflammation-targeted treatment has been confirmed to be effective in alleviating the symptoms of OA [19,20]. Inflammation is a predominant risk factor for OA, which can also affect the function of macrophages. The activation and aging of macrophages affect different processes [21], including Toll-like receptor signal transduction [22,23], phenotypic alterations [24,25], phagocytosis [26,27], and wound repair [24].

2.1 Activation of macrophages

Under healthy conditions, macrophages dynamically and regularly adjust their phenotype and function to stabilize the immune system. However, during pathological conditions, a certain phenotype of macrophages predominates and persists, which is a phenomenon also known as polarization of macrophages [28] (Figure 2).

Figure 2 
                  Schematic representation of macrophage activation and polarization. M1 macrophages (or classical activation pathway) are induced by IFN-γ, LPS, or TNF-α; it promotes the immune response by upregulating pro-inflammatory factors TNF-α and IL-1, and downregulating anti-inflammatory factors such as interleukin 10 (IL-10). M2 macrophages (or alternative activation pathway) have four subpopulations: IL-4- and IL-13-induced M2a macrophages, expressing MRC1 and IL-10; M2b macrophages induced by immune complex signaling, expressing IL-10 and major histocompatibility complex class II; M2c macrophages induced by IL-10 and glucocorticoids, expressing MRC1, IL-10 and TGF-β; M2d macrophages can overexpress vascular endothelial growth factor and inducible nitric oxides synthase (iNOS), or lower expression of TNF-α and arginase 1 (arginase 1, Arg1) and participate in angiogenesis and wound healing. Among them, M2a macrophages are mainly related to anti-inflammatory activity, and M2c macrophages are mainly related to tissue repair.
Figure 2

Schematic representation of macrophage activation and polarization. M1 macrophages (or classical activation pathway) are induced by IFN-γ, LPS, or TNF-α; it promotes the immune response by upregulating pro-inflammatory factors TNF-α and IL-1, and downregulating anti-inflammatory factors such as interleukin 10 (IL-10). M2 macrophages (or alternative activation pathway) have four subpopulations: IL-4- and IL-13-induced M2a macrophages, expressing MRC1 and IL-10; M2b macrophages induced by immune complex signaling, expressing IL-10 and major histocompatibility complex class II; M2c macrophages induced by IL-10 and glucocorticoids, expressing MRC1, IL-10 and TGF-β; M2d macrophages can overexpress vascular endothelial growth factor and inducible nitric oxides synthase (iNOS), or lower expression of TNF-α and arginase 1 (arginase 1, Arg1) and participate in angiogenesis and wound healing. Among them, M2a macrophages are mainly related to anti-inflammatory activity, and M2c macrophages are mainly related to tissue repair.

M1 macrophages can be induced by IFN-γ, lipopolysaccharide (LPS), or Toll-like receptors (TLRs) through the production of reactive oxygen intermediates such as NO. M1 macrophages kill and clear pathogens through lysosomal enzymes and other pathways. They also secrete a variety of chemokines and pro-inflammatory cytokines, such as IL-1β, IL-6, and TNF-α, to participate in the inflammatory response, tissue damage, and cell destruction [29,30,31] (Figure 2). At this time, the immune balance is destroyed, and the corresponding tissues are damaged due to the acute and, later on, chronic inflammatory reaction. In their study, Liu et al. examined the phenotypic status of macrophages in the peripheral blood and synovial fluid of 80 patients with knee OA and observed that the M1/M2 was significantly higher in these patients than in healthy controls. Also, this change was significantly associated with the OA classification, indicating the special significance of controlling the activation and polarization of macrophages for guiding the treatment of OA [32].

M2 macrophages are induced by IL-4 and IL-13. M2 can release anti-inflammatory cytokines such as transforming growth factor-β (TGF-β) and IL-10, inhibit inflammation, and promote tissue repair (Figure 2). M2 macrophages can be divided into four subpopulations [33]: IL-4 and IL-13-induced M2a macrophages, expressing mannose receptor C-type 1 (MRC1) and IL-10; M2b macrophages induced by immune complex signaling, expressing IL-10 and major histocompatibility complex class II; M2c macrophages induced by IL-10 and glucocorticoids, expressing MRC1, IL-10 and TGF-β; M2d macrophages can overexpress vascular endothelial growth factor and inducible nitric oxides synthase (iNOS), or lower expression of TNF-α and arginase 1 (arginase 1, Arg1), which have a role in angiogenesis and wound healing [34]. Among them, M2a macrophages are mainly related to anti-inflammatory activity, while M2c macrophages have an important role in tissue repair [33,35]. In addition, in mouse models of arthritis, IL-10 was identified to inhibit the occurrence and progression of arthritis [36,37].

A research group found significantly increased M1-type macrophages in OA patients and mouse models [38]. They used two OA mouse models (M1 or M2 macrophage conditional knockouts) to identify the role of M1- or M2-type macrophages in the development of OA. The mouse model with accumulated synovial M1-type macrophages presented with increased OA score, thinner articular cartilage, increased surface fibrosis areas, abnormal distribution of chondrocytes, and significantly increased volume and surface area of periarticular osteophytes, which exacerbated the progression of OA [38]. On the contrary, the mouse model with accumulated synovial M2-type macrophages presented with decreased synovial inflammation of the injured ACL and decreased OA score and osteophytes, indicating that synovial M2-type macrophage polarization prevents the development of collagenase-induced OA [38]. Moreover, gene sequencing analysis showed that M1-type macrophages promote the progression of OA by secreting pro-inflammatory factors IL-1, IL-6, and TNF-α, and promote hypertrophic chondrocytes differentiation and maturation, leading to degeneration [38]. Another study reported positive macrophage-specific protein MRP14, indicating the activation of macrophages in an OA animal model [39]. Consecutively, the synovial macrophages were depleted to observe the OA progression. As a result, significantly reduced osteophytes improved the stability of the joints and reduced infiltration of fibroblasts and inflammatory cells (by about 50%) [39]. These findings confirmed the participation of synovial macrophages in the pathological process of OA by promoting synovial fibrosis and osteophyte formation.

In conclusion, macrophages have an important role in the inflammatory response of OA. In the early stage of inflammation, M1-type macrophages phagocytose pathogens, while in the later stage of inflammation, M2-type macrophages regulate the inflammatory tissue microenvironment by secreting anti-inflammatory cytokines such as IL-10, which is conducive to the regeneration and repair of cartilage tissue. Therefore, timely changes in the polarization state of macrophages are critical for the resolution of inflammation. Therefore, it is of great clinical significance to deeply explore the molecular mechanism of macrophage polarization and achieve targeted induction of anti-inflammatory M2 macrophage polarization.

2.2 Macrophages and synovitis

Although synovial macrophages are the major immune cells in synovial tissues, their role in the pathogenesis of OA remains poorly understood. A few studies have shown synovial macrophages’ abnormal accumulation and phenotypic changes in the OA synovium [40,41]. Compared with healthy synovium, the number of F4/8+ (macrophage marker) cells showed a remarkable increase, with an increased number of iNOS + cells (M1 macrophage marker) and a reduced number of CD206 + cells (M2 macrophage marker) in OA synovium [38]. Up to 90% of patients with end-stage OA have synovitis with the infiltration of CD68-positive macrophages [32]. Other studies suggested greater conspicuous macrophage infiltration in patients with early-stage OA [42]. Also, numerous inflammatory factors and chemokines were found to be elevated in the isolated synoviocytes from minced synovial tissue samples extracted from OA patients [43]. After the depletion of macrophages using anti-CD14-conjugated magnetic beads, TNF-α, IL-1, and other cytokines, including IL-6, IL-8, monocyte chemoattractant protein-1, and MMPs also showed a marked reduction, suggesting that macrophage could secrete pro-inflammatory factors and promote the production of MMPs [16]. Bondeson et al. found that the level of macrophage-secreted pro-inflammatory factor macrophage migration inhibitory factor was positively correlated with the severity of OA-caused pain [16]. Another research group established an in vitro model to study the role of synovial macrophages in OA and found that maintaining the stable phenotype of macrophages is essential for preserving the viability of chondrocytes and maintaining the expression levels of cartilage proteoglycan and collagen [44]. They extracted synovial explants from OA patients for in vitro culture, treated them with different cytokines to stimulate the phenotypic changes of macrophages, and administered dexamethasone, rapamycin, bone morphogenetic protein-7 (BMP-7) or pravastatin to evaluate the inflammatory state of synovitis. Dexamethasone showed an anti-inflammatory effect by inhibiting M1 macrophages, while rapamycin inhibited the M2 phenotype to enhance the inflammatory response [44]. These data suggest the use of macrophage phenotypic modulation to guide the treatment of joint inflammation, which could, in turn, help to develop novel therapies for delaying the progression of OA.

2.3 Macrophages and subchondral bone destruction/repair

The subchondral bone in OA undergoes an uncoupled remodeling process characterized by macrophage infiltration, osteoclast formation, and increased osteoblast activity resulting in local remineralization and bone sclerosis of end-stage OA [45]. Utomo et al. injected clodronate-liposomes to deplete macrophages in the synovium and injected different doses of TGF-β into the knee joint seven days later [46]. In mice without macrophage depletion, osteophytes formed on the inner and outer sides of the patella and femur, while in mice with synovial macrophage depletion, the formed osteophytes were reduced by ∼70% [46]. They also discovered that synovial macrophages could lead to bone morphogenetic protein 2 (BMP-2) and BMP-4 after TGFβ stimulation [46]. These findings suggest that macrophages are a key intermediate factor in TGFβ-induced osteophytes.

Subchondral bone cysts are a common feature in OA [45]. Cysts-derived macrophages promote osteoclast differentiation and contribute to the expansion of OA cysts and osteolysis [47]. Another study reported that the synovial macrophages differentiate into functional osteoclasts, thereby promoting bone resorption and subchondral bone reconstruction [48]. Besides, TNF-α can indirectly induce osteoclast formation by stimulating macrophage differentiation [49]. Furthermore, the M2 polarization of macrophages has been confirmed to be crucial in the regeneration of subchondral bone. In an animal model of the bilateral trochlear cartilage defect, mice were subcutaneously injected with a mixture of chitosan–glycerophosphate and whole blood or serum [50]. This treatment could induce the chemotactic effect of neutrophils and M2 macrophages to concentrate at the injection site and promote trabecular bone repair and bone regeneration by expressing arginase-1 and releasing angiogenic factors [50].

These data suggest macrophages have an important role in the destruction of subchondral bone in OA patients. Therefore, immunoregulation of macrophages, especially polarizing macrophages toward M2 phenotype, might further elucidate the restoring process of subchondral bone.

2.4 Macrophages promote articular cartilage degeneration

The activation of MMPs has been identified as one of the important signs of irreversible damage to articular cartilage. Studies have found that synovial macrophages can mediate the expression of MMPs to induce articular cartilage damage [51]. M1 macrophages induce inflammation and degeneration of OA cartilage explants by up-regulating IL-1, IL-6, and MMP-13, while M2 macrophages have no effect [30]. Utomo et al. established an in vitro three-dimensional co-culture system to evaluate the interaction between activated macrophages and chondrocytes to understand the progression and treatment of OA [30]. It was observed that in the co-culture of activated macrophages and normal chondrocytes, MMPs and pro-inflammatory cytokines were increased while aggrecan and type II collagen were decreased, similar to the microenvironment of early-stage OA in clinical practice; whereas in the co-culture of activated macrophages and OA chondrocytes, the expression levels of MMPs and pro-inflammatory factors were remarkably higher than those in the co-culture system with normal chondrocytes [30]. These results suggest that the activation of pro-inflammatory macrophages is involved in promoting OA development. They also showed that diseased chondrocytes could aggravate the activation of macrophages.

3 Immunomodulatory macrophages in the treatment of OA

3.1 Depletion of macrophages

As macrophages are important in the immune pathogenesis of OA, several studies have tried to deplete macrophages to examine their effect on cartilage health and joint integrity. Previous studies found that depleting the synovial macrophages by intra-articular injection of clodronate-loaded liposomes can significantly decrease the expression of MMP-3 and MMP-9 in the synovium and reduce TGF-β-mediated osteophyte formation in the collagenase-induced OA mouse model [51,52]. In another study, anti-CD14 binding magnetic beads were used to achieve the specific depletion of synovial macrophages in OA synoviocytes in vitro, resulting in downregulation of the expression MMPs and fibroblasts-produced cytokines, including IL-6, IL-8, and MCP-1 [16]. However, other studies have reported increased synovial inflammation after the depletion of macrophages, which could not prevent the progression of OA [53,54]. Chamberlain et al. showed that compared with the medial collateral ligament (MCL) of untreated rats, the mechanical strength of MCL was decreased in rats with macrophage depletion [55]. These findings demonstrated that depleting macrophages may affect the inflammatory response around the injured joints while inhibiting the function of macrophages could profoundly impact joint inflammation and bone homeostasis after joint injury.

Currently, a variety of in vitro or animal models are available to study macrophage depletion. Yet, these technologies cannot precisely target the specific phenotype of macrophages without affecting other bone marrow lineages, such as dendritic cells and neutrophils. Simply depleting macrophages without considering the polarization of macrophages may not permanently address the OA progression.

3.2 Immunomodulatory macrophages

The continued existence of the pro-inflammatory M1 macrophages is generally thought to be detrimental to tissue repair, while the anti-inflammatory M2 macrophages can benefit tissue regeneration. Several cell or animal studies have attempted to improve or treat OA using immunomodulatory macrophages, including regulating and targeting specific signaling pathways [38,56], and other interventions such as extracts of traditional Chinese medicine [57,58], anti-inflammatory drugs [8], and mesenchymal stem cell therapy [59]. Studies highlighting the potential targets of macrophage immunomodulation are listed in Table 1.

Table 1

Genes and targets of interest on the immunoregulation of macrophages in OA

References Relevant gene/treatment Disease model Genotype Upregulated cytokines Downregulated cytokines Effect on macrophages Remark
[77] NFAT5 DMM-induced OA in mice NFAT5 haplo-insufficient (NFAT5 +/−) mice CCL2, IL-1β, MMP-13, ADMATS-5 NA Macrophage infiltration NA
[78] Alpha defensin-1 Meniscal/ligamentous injury, rat Wistar rats COL2A1, ACN,MMP3, MMP13 and ADAMTS5 NA Promoting M1 to M2 macrophage polarization via insulin and Toll-like receptor signaling pathway Candidate treatment
[79] Artificial M2 macrophages Injecting papain, mice Kunming mice NA IL-Iβ, IL-6, IL-17 A promising strategy
[80] Basic calcium phosphate crystals Macrophage isolated from Human blood monocyte NA CXCL9, CXCL10, HIF1a, GLUT1and hexokinase 2 CCL13, MRC1 Promoting M1 macrophage polarization Potential therapeutic target
[81] Lumican Synovial fluid of OA patients NA TLR4 NA Up-regulating primary macrophage activation and polarization towards the M1-like phenotype NA
[82] GM-CSF The collagenase-induced osteoarthritis (CiOA) in mice C57BL/6 mice NA NA NA Potential benefits of anti-GM-CSF (and anti-CCL17) mAb therapy in OA
[83] The E3 ubiquitin ligase, Itch Post-traumatic OA joints C57BL/6J mice; Itch global knockout (Itch −/−) mice, macrophage-specific Itch knockout (MΔItch) mice NA NF-kB, JNK, and MARK12 Inhibiting macrophage pro-inflammatory polarization NA
[84] PTP-001 DMM-induced OA in rat Rat NA MMP-13, TNFa, IL-1b Inhibiting macrophage polarization A promising biologic treatment
[85] IL-4 DMM-induced OA in mice C57BL/6J, BALB/cJ mice CD206, CCL24, CCL18 TNFa Promoting macrophages polarize towards an M2 phenotype Could provide therapeutic benefit

Glucocorticoids can decrease the CD68 + macrophages in the synovial fluid of patients with symptomatic knee OA and increase the expression of CD163 in synovial macrophages [60]. A decreased number of macrophages were reported in advanced knee OA after intra-articular injection of hyaluronic acid (HA) or methylprednisolone [61]. Mechanistically, HA mainly stimulates the repair process, while corticosteroids mainly reduce inflammation. Another study on dexamethasone found its anti-inflammatory effect on the synovial explants of OA patients. Dexamethasone inhibited the pro-inflammatory M1 macrophages and promoted the anti-inflammatory M2 macrophages in the culture of polarized primary human monocytes [46]. This study also carried out similar experiments using rapamycin, BMP-7, and pravastatin, finding that rapamycin and BMP-7 could enhance the inflammatory response of synovial explants and inhibit M2 macrophages. Moreover, pravastatin did not affect the inflammatory state of synovial explants, though it could inhibit M2 macrophages [46]. In the papain-induced OA rat model, triamcinolone acetonide (TA) intra-articular injection limited the osteophyte formation but could not affect cartilage degeneration or subchondral sclerosis [62]. The results indicated that TA could induce the differentiation of monocytes into M2 macrophages.

In animal models of OA, different traditional Chinese medicine extracts, such as ginsenoside [63] and squid type II collagen [58], have been verified to alter the polarization state of synovial macrophages and alleviate cartilage degradation in OA.

Other treatments, such as TissueGene-C (TG-C), a novel cell-mediated gene therapy, can also immunomodulate macrophages through local transduction of TGF-β1. In a rat model of monosodium iodoacetate, IL-10 and other M2 macrophage markers were increased in the knee joints of the TG-C group compared with the control group, indicating that TG-C could induce an anti-inflammatory microenvironment in the knee joint [64]. Furthermore, stem cell therapy could alleviate OA by regulating macrophage activation [65]. The stem cells are effective in cartilage repair, as they can differentiate in chondrocytes and replace degraded or dead chondrocytes [66]. The potential of mesenchymal stem cells to repair OA has been shown to rely on their ability to immunomodulate macrophages [67]. In osteochondral defect models, human embryonic stem cell-derived exosomes increased intra-articular CD163 + macrophages (M2), decreased CD86 + macrophages (M1), and reduced intra-articular pro-inflammatory cytokines [67].

The CCR2 signaling pathway has long been of interest to the rheumatology research community due to its pronounced pro-inflammatory and chemoattractive effects. As a major chemotactic pathway for monocytes, the CCL2/CCR2 axis is critical for recruiting CCR2-expressing circulating monocytes to sites of inflammation. However, studies in Ccr2-null mice reported controversial data in terms of mitigating OA. Miller et al. found severe allodynia and structural knee joint damage in ccr2-null mice equal to wild-type mice; yet, ccr2-null mice did not develop movement-provoked pain behaviors within 8 weeks in a surgical model of OA induced by medial meniscus (DMM) instability [68]. Another study found that the absence of CCR2 strongly suppressed selective inflammatory response genes in the joint with a lower average chondropathy score and delays pain-related behavior DMM [69]. On the contrary, Raghu et al. reported that mice lacking CCR2 were protected against OA by attenuating macrophage accumulation in the synovial joints [70], thus indicating that the CCL2/CCR2 signaling axis preferentially mediates monocyte trafficking and promotes inflammation and tissue damage in OA. These conflicting results might be due to differences in experimental design, including older mice model (20-week-old vs 10-week-old) and duration of OA development (20 weeks after DMM vs 8–12 weeks). Therefore, the function of CCR2 remains unclear in the development of OA, and CCL2/CCR2 inhibition in the treatment of OA should be regarded with caution.

However, there are still some limitations. For instance, diversity and plasticity are hallmarks of macrophages, and the M1/M2 paradigm is a limited attempt to define its complexity. In vivo, macrophages respond to environmental cues by acquiring distinct functional phenotypes. In mice, during the progression of the inflammatory response, the M1-to-M2 switch enables macrophages to perform different activities at different stages [71]. Previous studies have also shown that macrophages can undergo dynamic transitions between different functional states with a mixture of M1 and M2 phenotypes [72,73]. In addition, differences in macrophage biology between mice and humans in terms of phenotype, homology, transcription factors, and functions may confound the interpretation of results. For instance, murine and human macrophages express different cell markers [74]. Macrophages from mice or humans also exhibit differential metabolic responses to LPS [75]. Therefore, study results on mice should be interpreted in relation to the latent differences when implementing potential therapeutic approaches in humans. In addition, inflammatory processes may substantially vary between patients. The role of macrophages in OA pathogenesis differs by disease stage and endotype [76]. A clear understanding of the immunopathological patterns of OA is critical for further research.

4 Summary and future directions

OA is the main cause of lower-limb disability in the elderly [86]. Age is the leading risk factor for OA. Due to the aging population worldwide, an increasing number of patients are at risk of developing OA, which imposes a tremendous economic burden, including productivity and health care. Macrophages have been identified as the main pathological features of OA. They regulate the immune-inflammatory response of synovial tissues, secrete various inflammatory factors such as TNF-α and IL-1β, promote the infiltration of other inflammatory cells, and directly produce cytokines such as MMPs, which in turn accelerate articular cartilage damage and mediate osteophyte formation upon TGFβ stimulation. The damaged articular cartilage fragments subsequently trigger more macrophage activation, forming a vicious circle. Several studies have highlighted the impact of the phenotypic changes of macrophages in the development of OA [76]. The role of macrophages in synovitis and OA has gradually become the focus of therapeutic interventions. Overall, inhibiting the M1 polarization of macrophages and blocking the expression of TNF-α and MMPs may provide novel insights to guide the clinical treatment of OA.


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  1. Funding information: Authors state no funding involved.

  2. Author contributions: M.X.: drafted the manuscript; Y.J.: reviewed and made modifications​ to the manuscript. All authors read and approved the final manuscript.​

  3. Conflict of interest: Authors state no conflict of interest.

  4. Data availability statement: The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.

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Received: 2022-07-11
Revised: 2023-01-07
Accepted: 2023-01-09
Published Online: 2023-01-31

© 2023 the author(s), published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 International License.

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  91. Granulomatous polyangiitis involving the fourth ventricle: Report of a rare case and a literature review
  92. Studying infant mortality: A demographic analysis based on data mining models
  93. Metaplastic breast carcinoma with osseous differentiation: A report of a rare case and literature review
  94. Protein Z modulates the metastasis of lung adenocarcinoma cells
  95. Inhibition of pyroptosis and apoptosis by capsaicin protects against LPS-induced acute kidney injury through TRPV1/UCP2 axis in vitro
  96. TAK-242, a toll-like receptor 4 antagonist, against brain injury by alleviates autophagy and inflammation in rats
  97. Primary mediastinum Ewing’s sarcoma with pleural effusion: A case report and literature review
  98. Association of ADRB2 gene polymorphisms and intestinal microbiota in Chinese Han adolescents
  99. Tanshinone IIA alleviates chondrocyte apoptosis and extracellular matrix degeneration by inhibiting ferroptosis
  100. Study on the cytokines related to SARS-Cov-2 in testicular cells and the interaction network between cells based on scRNA-seq data
  101. Effect of periostin on bone metabolic and autophagy factors during tooth eruption in mice
  102. HP1 induces ferroptosis of renal tubular epithelial cells through NRF2 pathway in diabetic nephropathy
  103. Intravaginal estrogen management in postmenopausal patients with vaginal squamous intraepithelial lesions along with CO2 laser ablation: A retrospective study
  104. Hepatocellular carcinoma cell differentiation trajectory predicts immunotherapy, potential therapeutic drugs, and prognosis of patients
  105. Effects of physical exercise on biomarkers of oxidative stress in healthy subjects: A meta-analysis of randomized controlled trials
  106. Identification of lysosome-related genes in connection with prognosis and immune cell infiltration for drug candidates in head and neck cancer
  107. Development of an instrument-free and low-cost ELISA dot-blot test to detect antibodies against SARS-CoV-2
  108. Research progress on gas signal molecular therapy for Parkinson’s disease
  109. Adiponectin inhibits TGF-β1-induced skin fibroblast proliferation and phenotype transformation via the p38 MAPK signaling pathway
  110. The G protein-coupled receptor-related gene signatures for predicting prognosis and immunotherapy response in bladder urothelial carcinoma
  111. α-Fetoprotein contributes to the malignant biological properties of AFP-producing gastric cancer
  112. CXCL12/CXCR4/CXCR7 axis in placenta tissues of patients with placenta previa
  113. Association between thyroid stimulating hormone levels and papillary thyroid cancer risk: A meta-analysis
  114. Significance of sTREM-1 and sST2 combined diagnosis for sepsis detection and prognosis prediction
  115. Diagnostic value of serum neuroactive substances in the acute exacerbation of chronic obstructive pulmonary disease complicated with depression
  116. Research progress of AMP-activated protein kinase and cardiac aging
  117. TRIM29 knockdown prevented the colon cancer progression through decreasing the ubiquitination levels of KRT5
  118. Cross-talk between gut microbiota and liver steatosis: Complications and therapeutic target
  119. Metastasis from small cell lung cancer to ovary: A case report
  120. The early diagnosis and pathogenic mechanisms of sepsis-related acute kidney injury
  121. The effect of NK cell therapy on sepsis secondary to lung cancer: A case report
  122. Erianin alleviates collagen-induced arthritis in mice by inhibiting Th17 cell differentiation
  123. Loss of ACOX1 in clear cell renal cell carcinoma and its correlation with clinical features
  124. Signalling pathways in the osteogenic differentiation of periodontal ligament stem cells
  125. Crosstalk between lactic acid and immune regulation and its value in the diagnosis and treatment of liver failure
  126. Clinicopathological features and differential diagnosis of gastric pleomorphic giant cell carcinoma
  127. Traumatic brain injury and rTMS-ERPs: Case report and literature review
  128. Extracellular fibrin promotes non-small cell lung cancer progression through integrin β1/PTEN/AKT signaling
  129. Knockdown of DLK4 inhibits non-small cell lung cancer tumor growth by downregulating CKS2
  130. The co-expression pattern of VEGFR-2 with indicators related to proliferation, apoptosis, and differentiation of anagen hair follicles
  131. Inflammation-related signaling pathways in tendinopathy
  132. CD4+ T cell count in HIV/TB co-infection and co-occurrence with HL: Case report and literature review
  133. Clinical analysis of severe Chlamydia psittaci pneumonia: Case series study
  134. Bioinformatics analysis to identify potential biomarkers for the pulmonary artery hypertension associated with the basement membrane
  135. Influence of MTHFR polymorphism, alone or in combination with smoking and alcohol consumption, on cancer susceptibility
  136. Catharanthus roseus (L.) G. Don counteracts the ampicillin resistance in multiple antibiotic-resistant Staphylococcus aureus by downregulation of PBP2a synthesis
  137. Combination of a bronchogenic cyst in the thoracic spinal canal with chronic myelocytic leukemia
  138. Bacterial lipoprotein plays an important role in the macrophage autophagy and apoptosis induced by Salmonella typhimurium and Staphylococcus aureus
  139. TCL1A+ B cells predict prognosis in triple-negative breast cancer through integrative analysis of single-cell and bulk transcriptomic data
  140. Ezrin promotes esophageal squamous cell carcinoma progression via the Hippo signaling pathway
  141. Ferroptosis: A potential target of macrophages in plaque vulnerability
  142. Predicting pediatric Crohn's disease based on six mRNA-constructed risk signature using comprehensive bioinformatic approaches
  143. Applications of genetic code expansion and photosensitive UAAs in studying membrane proteins
  144. HK2 contributes to the proliferation, migration, and invasion of diffuse large B-cell lymphoma cells by enhancing the ERK1/2 signaling pathway
  145. IL-17 in osteoarthritis: A narrative review
  146. Circadian cycle and neuroinflammation
  147. Probiotic management and inflammatory factors as a novel treatment in cirrhosis: A systematic review and meta-analysis
  148. Hemorrhagic meningioma with pulmonary metastasis: Case report and literature review
  149. SPOP regulates the expression profiles and alternative splicing events in human hepatocytes
  150. Knockdown of SETD5 inhibited glycolysis and tumor growth in gastric cancer cells by down-regulating Akt signaling pathway
  151. PTX3 promotes IVIG resistance-induced endothelial injury in Kawasaki disease by regulating the NF-κB pathway
  152. Pancreatic ectopic thyroid tissue: A case report and analysis of literature
  153. The prognostic impact of body mass index on female breast cancer patients in underdeveloped regions of northern China differs by menopause status and tumor molecular subtype
  154. Report on a case of liver-originating malignant melanoma of unknown primary
  155. Case report: Herbal treatment of neutropenic enterocolitis after chemotherapy for breast cancer
  156. The fibroblast growth factor–Klotho axis at molecular level
  157. Characterization of amiodarone action on currents in hERG-T618 gain-of-function mutations
  158. A case report of diagnosis and dynamic monitoring of Listeria monocytogenes meningitis with NGS
  159. Effect of autologous platelet-rich plasma on new bone formation and viability of a Marburg bone graft
  160. Small breast epithelial mucin as a useful prognostic marker for breast cancer patients
  161. Continuous non-adherent culture promotes transdifferentiation of human adipose-derived stem cells into retinal lineage
  162. Nrf3 alleviates oxidative stress and promotes the survival of colon cancer cells by activating AKT/BCL-2 signal pathway
  163. Favorable response to surufatinib in a patient with necrolytic migratory erythema: A case report
  164. Case report of atypical undernutrition of hypoproteinemia type
  165. Down-regulation of COL1A1 inhibits tumor-associated fibroblast activation and mediates matrix remodeling in the tumor microenvironment of breast cancer
  166. Sarcoma protein kinase inhibition alleviates liver fibrosis by promoting hepatic stellate cells ferroptosis
  167. Research progress of serum eosinophil in chronic obstructive pulmonary disease and asthma
  168. Clinicopathological characteristics of co-existing or mixed colorectal cancer and neuroendocrine tumor: Report of five cases
  169. Role of menopausal hormone therapy in the prevention of postmenopausal osteoporosis
  170. Precisional detection of lymph node metastasis using tFCM in colorectal cancer
  171. Advances in diagnosis and treatment of perimenopausal syndrome
  172. A study of forensic genetics: ITO index distribution and kinship judgment between two individuals
  173. Acute lupus pneumonitis resembling miliary tuberculosis: A case-based review
  174. Plasma levels of CD36 and glutathione as biomarkers for ruptured intracranial aneurysm
  175. Fractalkine modulates pulmonary angiogenesis and tube formation by modulating CX3CR1 and growth factors in PVECs
  176. Novel risk prediction models for deep vein thrombosis after thoracotomy and thoracoscopic lung cancer resections, involving coagulation and immune function
  177. Exploring the diagnostic markers of essential tremor: A study based on machine learning algorithms
  178. Evaluation of effects of small-incision approach treatment on proximal tibia fracture by deep learning algorithm-based magnetic resonance imaging
  179. An online diagnosis method for cancer lesions based on intelligent imaging analysis
  180. Medical imaging in rheumatoid arthritis: A review on deep learning approach
  181. Predictive analytics in smart healthcare for child mortality prediction using a machine learning approach
  182. Utility of neutrophil–lymphocyte ratio and platelet–lymphocyte ratio in predicting acute-on-chronic liver failure survival
  183. A biomedical decision support system for meta-analysis of bilateral upper-limb training in stroke patients with hemiplegia
  184. TNF-α and IL-8 levels are positively correlated with hypobaric hypoxic pulmonary hypertension and pulmonary vascular remodeling in rats
  185. Stochastic gradient descent optimisation for convolutional neural network for medical image segmentation
  186. Comparison of the prognostic value of four different critical illness scores in patients with sepsis-induced coagulopathy
  187. Application and teaching of computer molecular simulation embedded technology and artificial intelligence in drug research and development
  188. Hepatobiliary surgery based on intelligent image segmentation technology
  189. Value of brain injury-related indicators based on neural network in the diagnosis of neonatal hypoxic-ischemic encephalopathy
  190. Analysis of early diagnosis methods for asymmetric dementia in brain MR images based on genetic medical technology
  191. Early diagnosis for the onset of peri-implantitis based on artificial neural network
  192. Clinical significance of the detection of serum IgG4 and IgG4/IgG ratio in patients with thyroid-associated ophthalmopathy
  193. Forecast of pain degree of lumbar disc herniation based on back propagation neural network
  194. SPA-UNet: A liver tumor segmentation network based on fused multi-scale features
  195. Systematic evaluation of clinical efficacy of CYP1B1 gene polymorphism in EGFR mutant non-small cell lung cancer observed by medical image
  196. Rehabilitation effect of intelligent rehabilitation training system on hemiplegic limb spasms after stroke
  197. A novel approach for minimising anti-aliasing effects in EEG data acquisition
  198. ErbB4 promotes M2 activation of macrophages in idiopathic pulmonary fibrosis
  199. Clinical role of CYP1B1 gene polymorphism in prediction of postoperative chemotherapy efficacy in NSCLC based on individualized health model
  200. Lung nodule segmentation via semi-residual multi-resolution neural networks
  201. Evaluation of brain nerve function in ICU patients with Delirium by deep learning algorithm-based resting state MRI
  202. A data mining technique for detecting malignant mesothelioma cancer using multiple regression analysis
  203. Markov model combined with MR diffusion tensor imaging for predicting the onset of Alzheimer’s disease
  204. Effectiveness of the treatment of depression associated with cancer and neuroimaging changes in depression-related brain regions in patients treated with the mediator-deuterium acupuncture method
  205. Molecular mechanism of colorectal cancer and screening of molecular markers based on bioinformatics analysis
  206. Monitoring and evaluation of anesthesia depth status data based on neuroscience
  207. Exploring the conformational dynamics and thermodynamics of EGFR S768I and G719X + S768I mutations in non-small cell lung cancer: An in silico approaches
  208. Optimised feature selection-driven convolutional neural network using gray level co-occurrence matrix for detection of cervical cancer
  209. Incidence of different pressure patterns of spinal cerebellar ataxia and analysis of imaging and genetic diagnosis
  210. Pathogenic bacteria and treatment resistance in older cardiovascular disease patients with lung infection and risk prediction model
  211. Adoption value of support vector machine algorithm-based computed tomography imaging in the diagnosis of secondary pulmonary fungal infections in patients with malignant hematological disorders
  212. From slides to insights: Harnessing deep learning for prognostic survival prediction in human colorectal cancer histology
  213. Ecology and Environmental Science
  214. Monitoring of hourly carbon dioxide concentration under different land use types in arid ecosystem
  215. Comparing the differences of prokaryotic microbial community between pit walls and bottom from Chinese liquor revealed by 16S rRNA gene sequencing
  216. Effects of cadmium stress on fruits germination and growth of two herbage species
  217. Bamboo charcoal affects soil properties and bacterial community in tea plantations
  218. Optimization of biogas potential using kinetic models, response surface methodology, and instrumental evidence for biodegradation of tannery fleshings during anaerobic digestion
  219. Understory vegetation diversity patterns of Platycladus orientalis and Pinus elliottii communities in Central and Southern China
  220. Studies on macrofungi diversity and discovery of new species of Abortiporus from Baotianman World Biosphere Reserve
  221. Food Science
  222. Effect of berrycactus fruit (Myrtillocactus geometrizans) on glutamate, glutamine, and GABA levels in the frontal cortex of rats fed with a high-fat diet
  223. Guesstimate of thymoquinone diversity in Nigella sativa L. genotypes and elite varieties collected from Indian states using HPTLC technique
  224. Analysis of bacterial community structure of Fuzhuan tea with different processing techniques
  225. Untargeted metabolomics reveals sour jujube kernel benefiting the nutritional value and flavor of Morchella esculenta
  226. Mycobiota in Slovak wine grapes: A case study from the small Carpathians wine region
  227. Elemental analysis of Fadogia ancylantha leaves used as a nutraceutical in Mashonaland West Province, Zimbabwe
  228. Microbiological transglutaminase: Biotechnological application in the food industry
  229. Influence of solvent-free extraction of fish oil from catfish (Clarias magur) heads using a Taguchi orthogonal array design: A qualitative and quantitative approach
  230. Chromatographic analysis of the chemical composition and anticancer activities of Curcuma longa extract cultivated in Palestine
  231. The potential for the use of leghemoglobin and plant ferritin as sources of iron
  232. Investigating the association between dietary patterns and glycemic control among children and adolescents with T1DM
  233. Bioengineering and Biotechnology
  234. Biocompatibility and osteointegration capability of β-TCP manufactured by stereolithography 3D printing: In vitro study
  235. Clinical characteristics and the prognosis of diabetic foot in Tibet: A single center, retrospective study
  236. Agriculture
  237. Biofertilizer and NPSB fertilizer application effects on nodulation and productivity of common bean (Phaseolus vulgaris L.) at Sodo Zuria, Southern Ethiopia
  238. On correlation between canopy vegetation and growth indexes of maize varieties with different nitrogen efficiencies
  239. Exopolysaccharides from Pseudomonas tolaasii inhibit the growth of Pleurotus ostreatus mycelia
  240. A transcriptomic evaluation of the mechanism of programmed cell death of the replaceable bud in Chinese chestnut
  241. Melatonin enhances salt tolerance in sorghum by modulating photosynthetic performance, osmoregulation, antioxidant defense, and ion homeostasis
  242. Effects of plant density on alfalfa (Medicago sativa L.) seed yield in western Heilongjiang areas
  243. Identification of rice leaf diseases and deficiency disorders using a novel DeepBatch technique
  244. Artificial intelligence and internet of things oriented sustainable precision farming: Towards modern agriculture
  245. Animal Sciences
  246. Effect of ketogenic diet on exercise tolerance and transcriptome of gastrocnemius in mice
  247. Combined analysis of mRNA–miRNA from testis tissue in Tibetan sheep with different FecB genotypes
  248. Isolation, identification, and drug resistance of a partially isolated bacterium from the gill of Siniperca chuatsi
  249. Tracking behavioral changes of confined sows from the first mating to the third parity
  250. The sequencing of the key genes and end products in the TLR4 signaling pathway from the kidney of Rana dybowskii exposed to Aeromonas hydrophila
  251. Development of a new candidate vaccine against piglet diarrhea caused by Escherichia coli
  252. Plant Sciences
  253. Crown and diameter structure of pure Pinus massoniana Lamb. forest in Hunan province, China
  254. Genetic evaluation and germplasm identification analysis on ITS2, trnL-F, and psbA-trnH of alfalfa varieties germplasm resources
  255. Tissue culture and rapid propagation technology for Gentiana rhodantha
  256. Effects of cadmium on the synthesis of active ingredients in Salvia miltiorrhiza
  257. Cloning and expression analysis of VrNAC13 gene in mung bean
  258. Chlorate-induced molecular floral transition revealed by transcriptomes
  259. Effects of warming and drought on growth and development of soybean in Hailun region
  260. Effects of different light conditions on transient expression and biomass in Nicotiana benthamiana leaves
  261. Comparative analysis of the rhizosphere microbiome and medicinally active ingredients of Atractylodes lancea from different geographical origins
  262. Distinguish Dianthus species or varieties based on chloroplast genomes
  263. Comparative transcriptomes reveal molecular mechanisms of apple blossoms of different tolerance genotypes to chilling injury
  264. Study on fresh processing key technology and quality influence of Cut Ophiopogonis Radix based on multi-index evaluation
  265. An advanced approach for fig leaf disease detection and classification: Leveraging image processing and enhanced support vector machine methodology
  266. Erratum
  267. Erratum to “Protein Z modulates the metastasis of lung adenocarcinoma cells”
  268. Erratum to “BRCA1 subcellular localization regulated by PI3K signaling pathway in triple-negative breast cancer MDA-MB-231 cells and hormone-sensitive T47D cells”
  269. Retraction
  270. Retraction to “Protocatechuic acid attenuates cerebral aneurysm formation and progression by inhibiting TNF-alpha/Nrf-2/NF-kB-mediated inflammatory mechanisms in experimental rats”
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