Cartilage, which covers the ends of the bones of the joints, gradually breaks down in degenerative joint diseases such as OA (17). As cartilage wears away, bones rub against one another, causing joint pain, stiffness, and reduced mobility (18). As a result of prolonged inactivity or excessive use, OA causes joint pain, swelling, and stiffness. Several risk factors influence the development of OA. As cartilage naturally deteriorates with age, OA may lead to a reduced range of motion, joint instability, and the development of bone spurs over time. Therefore, aging can be another significant risk factor for OA development (5, 19). The development of OA is also associated with obesity since excess weight causes joint stress, with joint injuries and trauma that can accelerate cartilage breakdown (20). Genetic factors, joint overuse, and certain metabolic conditions can cause OA (21). As part of the diagnosis of OA, a physician will assess a patient’s medical history, perform a physical examination, and conduct imaging studies (22). A clinical evaluation aims to determine whether there are characteristic symptoms of joint pain and exclude other possible causes. Symptoms of OA can be managed, and joint function improved through lifestyle modifications, physical therapy, pain medications, and surgical interventions such as joint replacement in severe cases (23). The quality of life of individuals with OA must be improved through early diagnosis and holistic management.

Studies also emphasized the substantial role of trauma in the etiology of OA, particularly focusing on post-traumatic osteoarthritis (PTOA) (24, 25). It highlights that trauma can be responsible for a considerable proportion of OA cases, with U.S. clinical statistics indicating that PTOA represents 12–42% of OA cases (26). Regarding the prevalence and impact of PTOA, noting that knee injuries, mainly those involving the anterior cruciate ligament (ACL) and meniscal damage, considerably increase the risk of developing OA. PTOA has a distinct molecular pathophysiology compared to age-associated OA, which may explain the poor translation of preclinical outcomes to clinical trials. Therefore, there is a requirement for targeted therapeutic interventions and the development of predictive biomarkers to prevent and manage PTOA, emphasizing that understanding trauma-induced OA molecular mechanisms is essential for obtaining promising clinical outcomes (26).

Chondrocytes, the cells that reside in cartilage, play a crucial role in maintaining its structural integrity. Every cell in the joint plays a role in the progression of OA (27). The chondrocytes in OA undergo phenotypic changes and produce enzymes that contribute to cartilage destruction, resulting in an imbalance between cartilage degradation and repair (28). Synoviocytes, found in the synovium surrounding joints, become activated in OA, releasing inflammatory cytokines and enzymes that exacerbate joint inflammation and contribute to disease progression (29). Moreover, synovial inflammation promotes the production of joint fluid, which causes swelling and pain in the joint (30). Osteoblasts and osteoclasts play an imperative role in bone remodeling (31). Osteophytes and bone spurs are often present in OA due to excessive bone formation (32). Multiple studies in both experimental models and human OA have revealed an elevation in subchondral bone turnover, including bone resorption and formation, during the initiation and progression of the disease (33–36). Typically, bone resorption and formation are coupled processes, but in certain conditions, such as postmenopausal osteoporosis, they can become imbalanced. This imbalance is also detected in patients with osteoporosis or progressive OA, emphasizing a potential uncoupling of these processes in OA. Expressly, in human hip OA, increased bone resorption and formation have been noted in the subchondral bone beneath mild, moderate, and severe cartilage lesions (37). However, an increase in bone volume was only observed beneath moderate and severe lesions. These observations indicate an increase in the frequency of osteoclast in areas with subchondral osteosclerosis in advanced knee OA. Moreover, recent experimental OA models have further reported this uncoupled bone remodeling, disclosing that altered mechanical loading triggers subchondral bone resorption, followed by enhanced recruitment of osteoprogenitors and formation of osteoid islets in marrow spaces (35, 38, 39). Given the uncoupled nature of bone remodeling in OA, inhibiting bone resorption may benefit both the early (osteopenia) and advanced (osteosclerosis) stages of OA (40, 41). Several types of immune cells, such as macrophages and T lymphocytes, contribute to joint inflammation (42). While traditionally associated with autoimmune responses, T lymphocytes are increasingly recognized for their involvement in the inflammatory processes of OA. Macrophages release pro-inflammatory mediators and enzymes, further accelerating cartilage degradation (43). It is also known that OA also affects the capsule of the joint as well as ligaments and tendons, which are comprised of fibroblasts (44). Fibroblasts produce extracellular matrix (ECM) proteins, and their dysregulation may impact joint stability and function (45). In addition, it has been shown that a non-immune cell, fibroblast-like synoviocytes (FLSs), reside in the synovium’s inner layer. They are specialized mesenchymal cells that play an essential role in collagen homeostasis of the articular joint, provide ECM materials to cartilage, and contribute to joint destruction through multiple mechanisms (46). The intricate interplay between these types of cells is crucial for understanding how OA progresses. The current research focuses on identifying critical molecular pathways and therapeutic targets within this joint ensemble. In order to provide more effective treatments for individuals affected by OA, researchers intend to develop targeted interventions that alleviate symptoms and modify the disease course by elucidating the role of these cells in OA pathophysiology.

Traditional views of OA as a non-inflammatory form of arthritis have been replaced with recognizing that the immune system plays an essential role in its pathogenesis (47). Immune responses contribute to the inflammation and tissue destruction observed in affected joints, even though OA is primarily characterized by cartilage breakdown and bone changes. OA is caused by the activation of immune cells, which release enzymes and cytokines that contribute to cartilage degradation and inflammation of joints (48). Moreover, OA is characterized by low-grade chronic inflammation in the synovium, which contributes to pain and joint damage and perpetuates the progression of the disease (49). Based on available knowledge, in autoimmune diseases, a wide range of cell types, such as macrophages, neutrophils, NK cells, dendritic cells (DCs), and lymphocytes, are involved in innate and adaptive immune responses (50). Activation of the innate immune system is started upon the ligation of pattern recognition receptors (PRRs) to damage-associated molecular patterns (DAMPs) and pathogen-associated molecular patterns (PAMPs). Toll-like receptors (TLR), RIG-1-like receptors (RLR), and NOD-like receptors (NLR) are prominent immune sensors involved in this process, stimulating nuclear factor kappa B (NF-κB) and mitogen-activated protein kinase (MAPK) pathways, upregulating the expression of interleukin-1β (IL-1β) and tumor necrosis factor-alpha (TNF-α), and inducing cartilage catabolism (51–53). It has been widely reported that TLR-2 and TLR-4 are upregulated in OA lesions compared to unaffected OA cartilage and normal cartilage. Additionally, endogenous signals like hyaluronan and fibronectin may activate TLRs 2 and 4 in OA (54). As another essential component of the innate immune system, the complement system, which consists of components C1-C9 and cofactors, is found in an increased abundance of synovial fluid from OA patients. Inflammatory molecules and membrane attack complex (MAC) formation are affected by genetic deficiencies in complement components C5, C6, and CD59a, indicating that complement plays a pivotal role in the progression of OA (55). In OA, cartilage degradation exposes PRRs, leading to persistent innate immune system activation (48).

It has been revealed that the key components of the adaptive immune system are T cells, divided into cytotoxic T cells (CTLs) and T helper (Th) cells. Th cells are categorized into Th1, Th2, Th9, Th17, and Th22 cells (56). Furthermore, B cells are crucial in developing an adaptive immune response through antibody production (humoral immunity). Still, the T cells play an important role in maximizing the protection against intracellular pathogens and tumors (cellular immunity) (57). Depending on the secretion of cytokines, Th2 cells may influence macrophage polarization toward regenerative M2 or inflammatory M1 phenotypes (58). Conventionally, Th cells are involved in microbe elimination; however, these lymphocytes can also participate in inflammatory phenomena and tissue repair processes by inducing macrophage differentiation in response to tissue-derived mediators in an IL-4-dependent manner (59). There is an emerging area of research into the role of T cells in OA progression (48). Evidence reveals that patients with OA have higher T and B cells than healthy controls (60). In OA patients, synovial fluid and tissue have elevated levels of Th1, Th9, Th17 cells, and CTLs, releasing catabolic cytokines like IL-2, IFN-γ, and TNF-α (61). Further research is needed to understand the specific role of T cells in osteoarthritis progression, even though they are present and contribute to cartilage matrix destruction.

Pathogens and infections may play a role in OA development via mechanisms involving chronic low-grade inflammation and immune system activation. Evidence revealed that innate immune system-mediated inflammation in OA can be exacerbated by microbiota imbalances and synovial infections, activating toll-like receptors (TLRs) and the complement system, resulting in the degradation of cartilage and synovitis (62, 63). Moreover, gut microbiota can influence OA progression by inducing systemic inflammation, as shown in pre-clinical studies where microbiota isolated from metabolically compromised mice accelerated OA in these animals (64). Furthermore, specific microbes in the joint synovia have been accompanied by hyperinflammation, contributing to OA pathogenesis (65). As discussed, TLRs are a critical component of the innate immune system and play a remarkable role in the pathogenesis of OA. These receptors recognize PAMPs and DAMPs, triggering inflammatory responses. Specifically, TLRs such as TLR2 and TLR4 have been involved in OA via mediating the expression of chemokines like CCL2 (66). Following infections and activation by PAMPs, TLRs trigger signaling pathways that produce inflammatory mediators, including CCL2. Regarding the role of CCL2 in recruiting monocytes and macrophages to sites of inflammation, its increased levels have been detected in the synovial fluid of OA patients. The CCL2/CCR2 axis is critical in recruiting monocytes/macrophages to the OA joint, resulting in synovitis and cartilage destruction. Accordingly, blocking this pathway considerably reduced inflammation and tissue damage in experimental OA models (67). On the other hand, TLR activation by microbial products or endogenous ligands produces pro-inflammatory mediators and MMPs, degrading the cartilage matrix and provoking inflammation. Studies have reported that TLR2 and TLR4 activation in chondrocytes and synoviocytes upregulates the expression of MMPs, further contributing to joint degradation (68, 69).

Increased levels of lipopolysaccharides (LPS), as an important TLR4 ligand, can occur following impaired hepatic clearance, high dietary fat intake, dysregulation of the endocannabinoid system, decreased intestinal motility, and reduced physical activity (70). Beyond LPS, additional metabolites produced by gut microbiota, capable of promoting OA, might penetrate the circulation via a compromised intestinal barrier. Notably, recent studies have identified microbial DNA, including that from common gut microorganisms, in osteoarthritic cartilage and synovial tissue (71, 72). This evidence suggests that increased intestinal permeability may facilitate the translocation of gut microbes to joint tissues (73).

Collectively, activation of the TLRs and the subsequent expression of CCL2 following infections can play a crucial role in mediating the inflammatory response in OA, leading to synovitis, cartilage destruction, and disease progression. Targeting these pathways holds the potential for developing therapeutic approaches to reduce inflammation and tissue damage in OA.

NSAIDs are commonly used to reduce pain and inflammation associated with OA to alleviate symptoms and slow down its progression (74). A disease-modifying osteoarthritis drug (DMOAD) is a treatment for osteoarthritis aimed at altering the underlying disease process and may have immunomodulatory effects on the immune system (75). Additionally, corticosteroid administration can provide targeted relief in OA (76). The interest in biological therapies that target specific inflammatory pathways involved in OA has increased in recent years (77–79). Specific cytokines, chemokines, or other immune mediators often trigger joint inflammatory responses. Several anti-inflammatory drugs and inhibitors, such as monoclonal antibodies and small molecules, can target and block these ligands or their receptors to reduce inflammation (80–82). OA is a multifactorial disease, and several other factors, such as mechanical stress and genetic predisposition, contribute to its pathogenesis (83). Even though the immune system is known to play a role, several other potential therapeutic targets are involved in the pathogenesis of OA. Various approaches are often used in customized treatment plans to address joint inflammation and structural alterations in OA.

In the symphony of immune responses, CCL2 is the master conductor, also termed monocyte chemoattractant protein-1 (MCP-1) (84). The gene coding for CCL2, a 13 kDa glycoprotein consisting of 76 amino acids, is located on chromosome 17 at position q11.2 (85). Among the G protein-coupled receptors (GPCR) family members, CCL2 is a potent chemoattractant primarily for monocytes, NK cells, and memory T cells (86). Monocytes, astrocytic cells, endothelial cells, fibroblasts, keratinocytes, mesangial cells, smooth muscle cells, microglia, and neurons can release CCL2 (87, 88). Following the ligation of CCL2 to CCR2, immune cells infiltrate inflammation sites due to these molecules’ ability to recruit and migrate monocytes, T lymphocytes, and DCs (89). Initiating and perpetuating inflammatory cascades depend on its indispensable role in coordinating immune cell trafficking. CCL2 levels are elevated in various pathological conditions, including atherosclerosis, rheumatoid arthritis (RA), OA, diabetes, psoriasis, cancer, and infectious diseases, highlighting its importance in disease pathogenesis (90–94).

Inflammatory stimuli such as IL-1β and TNF-α, as well as microbial products (e.g., Lipopolysaccharide), can increase the expression of CCL2 via various signaling pathways (95, 96). Key pathways include the nuclear factor kappa B (NF-κB) and MAPK pathways, activating transcription factors that bind to the CCL2 promoter, enhancing transcription (97). Extracellular matrix protein cellular communication network factor 1 (CCN1) or Cyr61 has been recognized as a remarkable regulator of CCL2 expression (98). CCN1 can interact with integrins and heparan sulfate proteoglycans to activate NF-κB, increasing CCL2 production (99). This CCN1-related mechanism is critical, as targeting CCN1 could suggest a potential therapeutic tactic for modulating CCL2 levels and subsequent inflammatory responses (100). The induction of CCL2 expression in response to inflammatory stimuli further emphasizes its dynamic role in amplifying immune responses. Upon arrival on the battlefield, this multifaceted chemokine modulates the functions of immune effectors by attracting them. CCL2 expression is regulated by transcriptional, post-transcriptional, and post-translational mechanisms, as shown by studies (101, 102). Understanding CCL2’s role in orchestrating immune responses and its potential as a therapeutic target for various inflammatory conditions requires a deep understanding of its regulatory intricacies.

CCR2 plays a crucial role in cellular migration and activation as the designated receptor for CCL2 (103). The CCR2 amino acid sequence exhibits a high degree of identity between species and is a member of the GPCR superfamily (104). Despite sharing conserved cysteine residues with other members of the CCR family, CCR2 has minimal homology with them. CCR2 comprises seven transmembrane domains containing a putative N‐linked glycosylation at amino acid 14, a sulfotyrosine at amino acid 26, a phosphotyrosine by Janus kinase 2 (JAK2) at amino acid residue 139, and a disulfide bond at amino acid 113 ↔ 190. Human CCR2 contains multiple domains, including α-helixes and β-strands (86). CCR2 expression on the surface of monocytes, macrophages, and specific subsets of T lymphocytes acts as a molecular compass, directing immune cells toward the inflammatory sites with high concentrations of CCL2 (105). Cell migration toward the site of injury or infection is facilitated when CCR2 triggers intracellular signaling cascades upon engagement with CCL2 (84).

Activation of CCR2 has been found to modulate immune cell function beyond its canonical role in chemotaxis. The protein plays a role in regulating macrophage polarization, which affects the balance between pro-inflammatory M1 and anti-inflammatory M2 phenotypes. It has been revealed that the CCL2/CCR2 axis is involved in determining the extent of macrophage polarization because CCL2 enhances the LPS-induced production of IL-10 as an anti-inflammatory and inducer of M2 macrophage polarization. At the same time, CCL2 blockade upregulates the expression of M1 polarization-associated genes and cytokines and downregulates the expression of M2-associated markers in human macrophages (106). In addition, Ccr2^- deficient bone marrow-derived murine macrophages demonstrated an M1-skewed polarization profile at the transcriptomic level and significantly increased proinflammatory cytokine expression when LPS was applied. As a result, the CCL2/CCR2 axis influences macrophage polarization by regulating genes relevant to polarization and proinflammatory cytokines (106). According to the available studies, infiltrating immune cells in an inflammatory milieu is associated with overexpression of CCL2 and CCR2. It has been reported that CCR2-driven activation of NF-κB is involved in regulating DC and Langerhans cell maturation (107). In addition, evidence indicates that CCR2-mediated signaling may modulate T-cell differentiation and activation, influencing adaptive immune responses. Studies conducted in vitro and in vivo have demonstrated that CCR2 directly influences the production of inflammatory cytokines in T cells. In the absence of CCR2 in T cells, the progression of colitis slows down. Additionally, the lack of CCR2 in T cells triggers a process that increases the number of Foxp3⁺ regulatory T cells (Tregs) and reduces the levels of Th17 cells in living organisms. This suggests that CCR2 is crucial in regulating the immune response by balancing the ratio of effector T cells to Tregs (108). Moreover, disorders characterized by dysregulated inflammation may benefit from therapeutic intervention through the intricate regulatory networks governing their expression and activation (109). Therefore, the collaboration between CCL2 and CCR2 in orchestrating immune responses tells an exciting story of molecular interactions influencing inflammation’s course. Understanding the nuances of their roles could lead to the development of targeted therapeutic strategies aimed at fine-tuning immune responses in different pathological conditions.

While the review’s main focus is the CCL2/CCR2 axis, it is advantageous to briefly mention other pivotal pathways responsible for OA pathogenesis, influencing chondrocytes and other cell types. Depending on the context, the Wnt/β-catenin signaling pathway has been revealed to play a dual role, both provoking and suppressing chondrocyte activity (110). NF-κB signaling is another essential pathway that mediates inflammation and catabolic responses, including upregulating MMPs and inducible iNOS (54). The TGF-β pathway, as a double-edged sword, can either stimulate or inhibit cartilage degradation based on the specific receptors and involved downstream signaling (111). Furthermore, AMP-activated protein kinase (AMPK) signaling has been recognized to regulate energy homeostasis and inflammatory responses in chondrocytes (112). The role of fibroblast growth factor 2 (FGF2) signaling in the pathogenesis of OA via modulation of Wnt/β-catenin and other signaling pathways emphasizes its potential therapeutic relevance (113). Hypoxia-inducible factors (HIFs) contribute to chondrocyte survival under low oxygen states typical in OA joints (111). These signaling pathways collectively play expressive roles in OA pathogenesis and are imperative considerations alongside the CCL2/CCR2 axis.

The expression of CCL2 in chondrocytes, osteoblasts, and synovial cells is associated with bone metabolism and OA. In the synovium of patients with OA, CCR2⁺ cells are prevalent (114). The CCL2/CCR2 axis has been demonstrated to regulate inflammation in OA and other pathological conditions (115). Some previous studies showed that this axis orchestrates an intricate cellular choreography during OA pathogenesis (66, 116, 117). In OA, the dynamic interplay between CCL2 and CCR2 is mediated by synovial tissues, chondrocytes, and infiltrating immune system cells. In the synovium, pro-inflammatory cytokines such as IL-1β and TNF-α are responsible for activating synovial fibroblast, releasing CCL2, and recruiting CCR2⁺ monocytes and macrophages (118). Infiltrating macrophages contributes to developing a pro-inflammatory milieu and sustaining inflammation within the joint (119). In addition to their role in innate immunity and tissue remodeling, macrophages produce cytokines and activate matrix metalloproteinases (MMPs), damaging cartilage. Depleting macrophages in mouse models of OA reduced cartilage degeneration and osteophytosis, demonstrating their role in pathologic bone formation (120). A study reported that S100A8 and S100A9 proteins are mainly produced by GM-CSF–differentiated macrophages in the synovium, with lesser production by M-CSF–differentiated macrophages and not by fibroblasts (121). S100A9 stimulation of OA synovial tissue led to increased production of IL-1β, IL-6, IL-8, and TNF-α, as well as upregulation of MMPs. Further experiments revealed that these effects were mainly due to the stimulation of GM-CSF–differentiated macrophages and, to a lesser extent, M-CSF-differentiated macrophages, while fibroblasts remained unaffected by S100A9. Additionally, S100A9 activated canonical Wnt signaling in GM-CSF-differentiated macrophages but not M-CSF-differentiated ones. Using the S100A9 inhibitor paquinimod on OA synovium reduced the activation of this signaling pathway (121, 122). CCL2/CCR2 dance is also actively performed by chondrocytes, the primary cells of articular cartilage. The expression of CCL2 is increased in chondrocytes in response to mechanical stress and inflammatory signals, resulting in a positive feedback loop that promotes inflammation and cartilage degeneration (123). This process is further exacerbated by the infiltration of CCR2⁺ immune cells into the cartilage, resulting in a progressive loss of cartilage integrity (66).

However, an investigation explored the role of CCR2 in the pathogenesis of injury-induced OA. Utilizing a murine model of injury-induced OA through DMM, CCR2 was systemically blocked using the antagonist RS504393 at various stages of disease progression. Joint degeneration was assessed by evaluating cartilage (including metrics such as cartilage loss, chondrocyte hypertrophy, and MMP-13 expression) and bone lesions (such as bone sclerosis and osteophyte formation) with and without CCR2 inhibition. Pain-related behaviors were also studied by measuring weight distribution between normal and arthritic hind paws using the IITS incapacitance meter. This study showed that the timing of CCR2 antagonist administration was crucial for mitigating joint damage. Early intervention with the CCR2 antagonist within the first four weeks post-injury significantly reduced OA-related cartilage and bone damage. In contrast, later interventions proved less effective. Furthermore, early to moderate CCR2 blockade (1–4 weeks and 4–8 weeks post-surgery, respectively) resulted in decreased pain measures, with sustained pain improvement even after cessation of treatment. These results indicate the potential of early CCR2 antagonism to not only decelerate the progression of post-injury OA but also to alleviate associated pain symptoms, suggesting a therapeutic window for optimizing the efficacy of CCR2-targeted treatments in OA management (124).

It has been demonstrated that CCL2 treatment activated ERK and p38 via CCR2 in healthy chondrocytes. CCL2 treatment of healthy chondrocytes for short (6h) or long (7–72h) periods resulted in upregulating CCR2, MMP-1, MMP-3, MMP-13, and tissue inhibitor of matrix metalloproteinase 1 (TIMP1). CCL2-mediated upregulation of CCR2 and cartilage catabolic markers was mediated by ERK and p38 signaling in healthy and OA chondrocytes (125). Further, the CCL2/CCR2 axis influences how different types of cells interact in the joint microenvironment. CCR2⁺ osteoclast precursors are recruited by CCL2, resulting in bone resorption and the characteristic subchondral changes observed in patients with OA (126).

A study explored whether the specific inactivation of the Ccr2 gene in osteoblasts at various stages of PTOA can improve joint structure, bone parameters, and pain outcomes (126). This study used a tamoxifen-inducible system to inactivate Ccr2 in collagen1α-expressing cells, resulting in osteoblasts deficient in Ccr2 (CCR2-Col1αKO). The PTOA model was induced in both CCR2-Col1αKO and control (CCR2^+/+ ) mice using the medial meniscus (DMM) method destabilization. Recombination was triggered before or after the DMM procedure to compare early versus late inactivation effects. Joint damage was assessed at two, four, eight, and twelve weeks post-DMM using various metrics, including articular cartilage structure (ACS), Safranin-O staining, histomorphometry, osteophyte size and maturity, subchondral bone thickness, and synovial hyperplasia. Pain levels, both spontaneous and evoked, were monitored for up to 20 weeks. Results demonstrated that early inactivation of Ccr2 in osteoblasts delayed the progression of articular cartilage damage and matrix degradation compared to CCR2^+/+ controls, and it also decreased DMM-induced bone thickening. The formation and maturation of osteophytes were only slightly influenced. Conversely, late inactivation of collagen1α-Ccr2 resulted in less pronounced improvements. Furthermore, Ccr2 deletion in osteoblasts improved static pain measures, though evoked pain remained unchanged. This study demonstrates that Ccr2 expression in osteoblasts plays a critical role in the progression of PTOA and associated pain by impacting both cartilage and bone tissues (126).

Taken together, due to the intricate choreography orchestrated by the CCL2/CCR2 axis, OA progression is a multifaceted phenomenon.

Dysregulation of the CCL2/CCR2 dance can lead to pathology, crucial to maintaining a delicate balance. It has been suggested that disruptions in the equilibrium between CCL2 and CCR2 are responsible for the severity and progression of OA (127). A CCL2/CCR2 axis-mediated mechanism is thought to mediate the pain of OA, suggesting that increased CCL2 levels during OA may indicate altered bone and cartilage metabolism, causing pain and destruction of joints (114). There is evidence that elevated CCL2 levels in synovial fluid and serum are related to OA severity and pain (128, 129). As a result of dysregulated CCL2 production in response to inflammatory stimuli, chronic inflammation is perpetuated, leading to relentless cartilage degradation. A positive correlation has been reported between synovial fluid CCL2 concentrations, the Western Ontario and McMaster Universities Arthritis Index (WOMAC) pain ratings, WOMAC function scores, and total WOMAC scores (128). This association remained considerable even after adjusting for potential confounding factors through multivariate linear regression analysis. Additionally, the correlation between synovial fluid and serum CCL2 concentrations with the K-L grading system has been evaluated, and no significant difference was found, suggesting that CCL2 concentrations associated with radiographic changes in knee OA patients were not linked to symptomatic severity (128). These findings suggest that CCL2 synovial fluid levels could be a novel biochemical marker reflecting pain severity and functioning as a pain mediator in knee OA patients. These outcomes may pave the way for new, targeted therapeutic approaches to alleviate pain and its associated symptoms in patients with knee OA.

Excessive activation of inflammatory pathways due to dysregulated CCR2 signaling can lead to joint inflammation and tissue damage. In contrast, inadequate activation of CCR2 may result in a lack of immune cells recruited to repair tissues and resolve inflammation (130). Furthermore, the interplay between the CCL2/CCR2 axis and other signaling pathways adds another layer of complexity to the dance. Cross-talk with Transforming Growth Factor-beta (TGF-β), NF-κB, and MAPK pathways modulate the inflammatory response in OA (131). Consequently, Disease progression can be positively or negatively affected depending on how these interactions are balanced.

A potential role for TGFα-EGFR signaling in OA progression has been identified, particularly in its untested role in vivo (132). Except for endothelin A, the downstream effectors of TGFα signaling in cartilage are poorly understood. It has been shown that EGFR signaling induces the expression of CCL2 in osteoblasts, and CCL2 is elevated in human OA cartilage, which promotes proteoglycan loss (127). When CCL2 is elevated, its expression is further driven by a positive feedback loop that prevents mesenchymal progenitor cells from differentiating along chondrogenic lines, inhibiting natural joint repair processes (133). It was revealed that exposure to CCL2 prompts synovial multipotent progenitor cells (sMPCs) pro-inflammatory activation (increasing the expression of IL-6 in sMPCs) while impeding their ability to undergo chondrogenesis in vitro, the process by which these cells mature into cartilage. Furthermore, exposure to CCL2 at levels typically found in OA knee joint synovial fluid induces significant alterations in the gene expression profile of sMPCs. Additionally, prolonged exposure to CCL2 results in the heightened expression of CCL2 itself within sMPCs, setting up a positive feedback loop from which these cells struggle to disengage. Consequently, elevated CCL2 can attract sMPCs to the site of injury but also disrupt the normal transcriptional regulation of these cells, hampering their progression toward chondrogenesis (133). These findings present promising avenues for further exploring the mechanisms underlying the observed lack of intrinsic repair following articular cartilage injury and/or arthritis.

The interaction between TGF-α and CCL2 plays a critical role in the pathogenesis of OA (134, 135). A study elucidated that TGF-α stimulates the expression of CCL2 through various intracellular signaling pathways, including MEK/ERK, p38 MAPK, and phosphoinositide 3-kinases (PI3Ks) in chondrocytes (136)​​. TGF-α-induced CCL2 expression subsequently promotes the production of MMPs, specifically MMP-3, which contributes to the degradation of type II collagen and aggrecan, essential components of cartilage extracellular matrix​​. Pharmacologic inhibition of either TGF-α or CCL2 signaling has been revealed to reduce cartilage degradation and disease progression in a rat model of posttraumatic OA, suggesting that the TGF-α/CCL2 axis is a significant pathway in OA pathophysiology (136). These findings indicate that targeting the TGF-α/CCL2 signaling pathway could be a promising therapeutic approach to mitigate cartilage destruction in OA​​.

Polymorphisms, while not directly involved in the pathogenesis of OA, can considerably increase the risk of developing the disease. These genetic variations can impact the biological pathways associated with OA, thereby acting as imperative prognostic biomarkers (137). By identifying individuals with specific polymorphisms, it is possible to predict the probability of OA onset and progression, enabling more personalized approaches to prevention and management. In this context, a study found that specific variations in the CCL2 gene, rs1024611 and rs4586, significantly increased the risk of developing OA, emphasizing the central role played by the CCL2/CCR2 axis in disease susceptibility (138). After further analysis, it was determined that the rs1024611 polymorphism was associated with increased risk for males and non-drinkers. In contrast, the rs4586 polymorphism was associated with heightened risk for smokers and drinkers. As a result of the rs4586 polymorphism, individuals with the CC genotype showed significantly higher levels of CCL2 compared to those with TT genotypes. As a result, variation in CCL2, particularly rs1024611 and rs4586, is associated with OA susceptibility and could serve as early diagnostic markers (138). Another study revealed that certain genetic variations within the CCL2 gene might be linked to an increased risk of developing knee OA (139). Specifically, the rs2857657 variant (G) allele appears more common in female knee OA cases than in individuals without OA. Additionally, identifying a haplotype (H5) exclusively in the control group implies a potential protective effect against knee OA (139). These findings entail that CCL2-associated genetic factors could contribute to the susceptibility to knee OA.

Collectively, CCL2 appears to be a crucial downstream mediator in the cartilage degradation process, including the MEK/ERK pathway, according to the study.