Neutrophils, as the first responders to tissue injury, play a critical role in the inflammatory response. Their functions include removing bacteria and necrotic tissue through phagocytosis and the release of protease-containing granules. They also generate a hydrogen peroxide gradient via enzyme complexes like NADPH oxidase and ROS, which are essential for initiating the inflammatory cascade (63). However, the formation of neutrophil extracellular traps (NETs) by activated neutrophils requires tight regulation to prevent excessive tissue damage due to their cytotoxic potential (64). Additionally, neutrophils can inhibit healing by releasing soluble mediators, excessive ROS, and pro-inflammatory microRNAs contained in granules, leading to tissue damage (65). Efficient healing relies on the clearance of neutrophils from the inflammatory microenvironment. Neutrophils undergo apoptosis or necrosis and are engulfed by macrophages through phagocytosis or efferocytosis to maintain cellular homeostasis. Some neutrophils may leave the injury site through reverse migration or return to the circulatory system (44). However, failure to clear neutrophils often leads to secondary necrosis, resulting in tissue damage and sustained inflammation through the release of cytotoxic, pro-inflammatory, and immunogenic molecules by the lysing cells (66).

While their involvement in general tissue healing is well-established, their specific role in tendon injury has been a subject of growing interest. Recent studies have highlighted the presence of neutrophils in various tendinopathy models, suggesting their potential contribution to tendon pathology. Published studies have revealed the role of neutrophils in tendon injury. For example, the study by Marsolais et al. demonstrated a sequential accumulation of neutrophils and macrophages in Achilles tendon injury models. Neutrophils were initially elevated, followed by a surge in macrophages, suggesting a coordinated inflammatory response (67). A study by Crowe et al. found that alarm molecules such as S100A8 and S100A9 are upregulated in tendinopathic tissues and that these molecules recruit immune cells such as neutrophils, further exacerbating the inflammatory response (68). Noah et al. observed a robust adaptive immune response in tendons following injury, preceded by an initial innate immune response involving neutrophils and macrophages. This suggests a complex interplay between innate and adaptive immune cells in tendon healing (58). Ribitsch et al. showed differences in neutrophil responses in tendon injury models in fetal and adult animals. Tendon injury in adult animals was accompanied by a stronger inflammatory response and more neutrophil infiltration, whereas the inflammatory response was weaker during the regeneration of fetal tendons (69). These studies provide valuable insights into the nuanced role of neutrophils in tendon injury and provide a basis for exploring whether excessive neutrophil accumulation uniquely contributes to tendon pathology.

In summary, neutrophils play a multifaceted role in tendon injuries. Although neutrophils are essential for removing debris and triggering inflammation, excessive accumulation of neutrophils can lead to tissue damage and delayed healing. Understanding the mechanisms by which neutrophils are involved in these processes is essential for the development of effective therapeutic interventions. Future research should focus on exploring the specific role of neutrophils in scar formation during tendon healing and developing strategies to regulate their activation, migration, and clearance.

Macrophages play a critical role in tendon–bone healing. Macrophages, the predominant subset of immunocytes infiltrating pathologic tendons, play a crucial role in regulating local inflammation. Macrophages, among the various immunocytes, have been observed to infiltrate the damaged tendon, orchestrating local inflammation and regulating the healing process (59). Wong et al.’s model of tendon adhesion showed that there is a large infiltration of inflammatory cells into subcutaneous tissue and tendon wounds within 24 h of tendon injury (70). Injured tendon tissue releases chemokines, with CCL2 recruiting macrophage-dominated immune cells (71). Marsolais et al. demonstrated that in an Achilles tendon injury model, the influx of macrophages leads to further release of chemokines and cytokines, followed by a macrophage surge, suggesting that a synergistic inflammatory response exists and plays a critical role (67).

Macrophages exhibit significant functional diversity, classified as proinflammatory M1 and anti-inflammatory M2 phenotypes (72–74). They exhibit a phenotypic shift from M1 pro-inflammatory to M2 anti-inflammatory macrophages during tissue repair (75), releasing different cytokines that contribute to tissue regeneration and wound repair (59, 76). During the early stages of tendon healing, infiltrating macrophages at the site of injury are predominantly of the M1 phenotype (77), and their concentration increases significantly during the first 2 weeks of tendon healing and is localized to newly formed tendon tissues and areas of tissue remodeling (71), where the M1 macrophages migrate to the site of injury and release pro-inflammatory cytokines, such as interleukin-1 (IL-1), tumor necrosis factor-α (TNF-α), and interleukin-6 (IL-6), which may hinder tissue repair (71). Conversely, M2 macrophages are primarily involved in the later stages of healing, demonstrating the ability to eliminate intracellular pathogens and promote tendon regeneration (74, 78). The balance between M1 and M2 macrophages is crucial for successful wound healing ( Figure 2 ). In a previous study, Kawamura et al. examined the interface between the free flexor digitorum longus tendon and bone tunnel and observed that M1 macrophages were present early after surgery and persisted for 4 weeks. In contrast, M2 macrophages were not detected until 11 days after surgery and exhibited high expression levels up to 4 weeks (44). Thus, regulating the transition of macrophages from the M1 to M2 state plays a crucial role in improving tendon–bone healing. Chronic inflammatory models have provided evidence linking the failure of macrophages to transition from M1 to M2 phenotypes with delayed healing and persistent shoulder pain following treatment. For instance, Dakin et al. discovered that the expression of CD206, a cell surface marker associated with M2 macrophages, was higher in pain-free patients after surgical subacromial decompression compared to those experiencing ongoing pain (79). Similarly, Chamberlain et al. demonstrated in a mouse model of Achilles tendon rupture that a decreased M1/M2 ratio promoted the biological response necessary for tendon healing (80). Consequently, modulating the balance between M1 and M2 macrophages presents an appealing target for treating tendinopathy.

Impaired switching from M1 to M2 macrophages can result in poor wound closure and chronic inflammation (73). Dagher et al. performed a study and found a reduction in M1 macrophage aggregation at the tendon–bone interface after 2 and 4 weeks of postoperative fixation. This reduction was associated with improved histological and biomechanical properties at the adhesion (81). In another study by Gulotta et al., after administration of TNF-α antibody in a rat rotator cuff injury model, there was a significant reduction in M1-type macrophages and improved tendon–bone healing (82). The above studies suggest that enhanced macrophage polarization from M1 to M2 accelerates tendon–bone healing and that control of macrophage polarization is essential to prevent tissue damage caused by the prolonged presence of M1 macrophages and to avoid dysregulated or delayed M2 polarization, which leads to chronic inflammation and impaired wound healing.

In contrast to M1 macrophages, M2 macrophages are thought to be involved in the promotion of fibroblast proliferation and new tissue deposition, which are essential for the regeneration and remodeling of tendon tissue (83). The concentration of M2 macrophages increases significantly during the later stages of tendon healing, particularly in the area where the tendon ECM is located (84). It has been shown that 28 days after tendon injury, M2 macrophages increase substantially and become the predominant macrophage phenotype at the site of injury (71). Appropriate mechanical stimulation has been found to promote macrophage polarization towards the M2 phenotype and secretion of high levels of TGF-β1 to promote chondrogenic differentiation of mesenchymal stem cells (MSCs), thereby enhancing tendon–bone healing (85). Although M2 macrophages are critical for anti-inflammatory responses and tissue remodeling in the early stages of tendon repair, their long-term dominance in the later stages may promote exogenous healing, potentially leading to the formation of fibrotic scar tissue (49). Recent studies have shown that M2-type macrophages can also be pro-fibrotic. For example, an increase in M2-type macrophages has been associated with the development of pulmonary fibrosis, and reducing the accumulation of M2-type macrophages has been shown to attenuate fibrosis in a mouse model of idiopathic pulmonary fibrosis (86). These observations have been further validated by studies in patients with hypertrophic scars, whose skin showed significantly more M2 phenotypic macrophages even before the onset of trauma compared to patients with normal scars (87). As in other tissues, M2-type macrophages can be pro-fibrotic in tendons. In tendon injury, TGF-β1 derived from M2 macrophages may also lead to tendon adhesion, recruit MSCs, and promote myofibroblast formation in tendon adhesion (88). During diabetic tendon healing, overproduction of TGF-β1 by M2 macrophages can lead to pathological fibrosis (89). These findings suggest that M2 macrophages play a facilitating role in fibrosis. In conclusion, the role of M2 macrophages in tendon healing is complex, affecting both intrinsic and extrinsic healing processes. Balancing macrophage activity is essential to support intrinsic healing while attenuating excessive exogenous tissue formation. Precise temporal and spatial regulation of macrophage phenotypes is essential for optimal tendon recovery and the development of effective tendon injury treatment strategies.

In order to establish a causal relationship between macrophages and tendon–bone healing, Hays et al. conducted a study using liposomal clodronate to eliminate macrophages in rats. Their findings demonstrated improved tendon–bone healing at the graft and bone tunnel site (56, 90). Subsequently, several studies focused on developing various modalities targeting macrophages to enhance tendon–bone healing. One such modality was the use of an electrospun nano-scaffold developed by Song et al., which exhibited the ability to inhibit macrophage accumulation. In a rat model of rotator cuff injury, this scaffold was shown to promote tendon–bone healing (90). Another approach was taken by Lu et al., who utilized fresh bone marrow to facilitate the M2 polarization of macrophages. This intervention resulted in the formation of better-organized connective tissue between the tendon and bone (91). These studies highlight the significance of macrophages in tendon–bone healing and demonstrate the potential of macrophage-targeting strategies to promote this process.

Successful tendon–bone healing after tendon injury repair requires both mechanical and biological enhancement. Macrophages play a crucial role in the inflammatory response of the tendon-bone interface induced by surgery, which can inhibit tendon-bone healing. (92). M1 macrophages are initially present and exhibit proinflammatory functions, while M2 macrophages gradually replace them with anti-inflammatory functions. Accelerating the transition from M1 to M2 polarization of macrophages can expedite tissue repair. Failure to convert macrophages to the M2 phenotype in a timely manner can lead to prolonged local inflammation and delayed healing (93). Therefore, modulating the polarization state of macrophages may be a valuable strategy to promote early healing in tendon injury repair. Macrophages play a vital role in the inflammatory response and subsequent wound-healing processes. Their timely polarization from proinflammatory M1 to anti-inflammatory M2 phenotypes is critical for effective tissue repair and regeneration. Failure to regulate macrophage polarization can lead to chronic inflammation, impaired healing, and excessive scar formation. Understanding the role of macrophages in tendon–bone healing after tendon injury provides insights into potential therapeutic strategies for promoting efficient and accelerated healing in these conditions.

The accumulation of T cells in tendon injuries and diseases suggests their potential role in the healing process. In spondyloarthritis, a specific type of tendon-specific T cell characterized by RORγt expression has been identified, lacking CD4 and CD8 markers (94). These T cells respond to IL-23 and secrete IL-22, stimulating osteoblast-mediated bone remodeling and promoting bone formation. Additionally, muscle-specific regulatory T cells (Tregs) influence tissue repair by reducing inflammation and secreting factors that directly impact satellite cell growth (95).

Following tendon injury, CD4+ T cells migrate to the tissue, releasing pro-inflammatory cytokines including IL-2 to enhance the innate immune response for tendon repair. The phenotypic heterogeneity of immune cell populations at the tendon–bone interface underscores the importance of understanding their specific functions (96). Th2 cells are associated with promoting collagen synthesis and fibronectin deposition, while Th1 cells alleviate fibrosis by inhibiting fibroblast proliferation and collagen gene expression (97).

Interestingly, recent studies using in vitro culture systems have revealed a positive inflammatory feedback loop between tendon cells and T cells, although the specific subtypes of T cells remain undetermined (60). While these studies suggest a potential pathological role of T cells in tendon healing, recent findings indicate that neonatal tendon regeneration relies on Tregs, as depletion of Tregs leads to impaired structural and functional healing. Unlike adult Tregs, neonatal Tregs promote regeneration by shifting macrophages from pro-inflammatory to anti-inflammatory phenotypes (98). Transfer of neonatal Tregs to adult hosts improves macrophage polarization and consequently facilitates functional recovery. Moreover, various injury models suggest a protective role of IL-33 in promoting Treg expansion across multiple tissues, although it may also exhibit pathological effects (99).

In conclusion, these research findings highlight the impact of T cells on tendon–bone healing and underscore the importance of comprehensively understanding their contributions to strengthening therapeutic strategies.

Mast cells play a crucial role in tendon healing and are usually located locally in tendon tissue or adjacent connective tissue, such as tendon, muscle–tendon junction, or bone–tendon junction. They can migrate to the site of injury in response to inflammation and nerve growth (100). It is uncertain whether the increase in mast cells at injured tendon sites is entirely due to migration or whether mast cells also proliferate or originate from circulating progenitor cells (101). The complex and fascinating role of mast cells in tendinopathy is a subject of ongoing research.

Activated mast cells release a variety of pre-formed and synthesized bioactive substances, including vascular endothelial growth factor (VEGF) and nerve growth factor (NGF), which contribute to new blood vessel formation and nerve growth during tendon healing (102). Mast cells also regulate collagen renewal, a key process in tendon tissue repair. By producing growth factors such as TGFβ and FGF2, mast cells stimulate collagen synthesis in fibroblasts. Mast cell-derived proteases (e.g., trypsin-like and rennet) can further enhance collagen synthesis in fibroblasts and may have similar effects on tendon cells (103). In addition, mast cells activate matrix metalloproteinases that promote collagen degradation. The proximity of mast cells to peripheral nerve endings suggests that they can be activated by neurotransmitters released by nerves after tendon injury (104).

Mast cells express receptors for various neurotransmitters such as substance P, glutamate, CGRP, and neurokinin A, suggesting their responsiveness to these signaling molecules (105). Activation of mast cells by neurotransmitters leads to the release of cytokines and other pro-inflammatory mediators that may contribute to the pathology of tendon injury (106). In addition, mast cells can activate peripheral nerve cells by secreting neurotransmitters such as histamine, serotonin, and dopamine. Upregulation of histamine receptor 1 in injured nerve fibers may be associated with neuropathic pain (53).

Clinical studies have shown an increased number of mast cells in tendinopathic tissues compared to healthy tissues, especially within injured tendons (61). Mast cells are closely associated with neovascularization, suggesting their influence on vascular growth in inflamed tendons (107). The density of mast cells correlated with the area of blood vessels and higher mast cell numbers were observed in patients with longer duration of symptoms (107). Similar findings were found in animal models of tendon injury and tendinopathy (106, 108, 109).

In conclusion, mast cells play a complex role in the inflammatory process of tendon healing. They migrate to the site of injury, release bioactive compounds, regulate collagen renewal, respond to neurotransmitters, and interact with nerve cells. Understanding the involvement of mast cells in tendon pathology may pave the way for potential therapeutic interventions to treat tendon injuries. Further studies are needed to delve into the specific role of mast cells in the healing process after tendon injury.

Natural killer (NK) cells also play an important role in tissue injury repair (110). However, other studies have shown that NK cells may cause delayed wound healing due to the production of pro-inflammatory cytokines (62). Studies have shown that more NK cells are found in chronic tendinopathic Achilles tendons than in healthy tendons (111). Although the exact mechanism of action of NK cells in tendon injury healing is not fully understood, it has been shown that by regulating the activity of NK cells, it is possible to influence the intensity and duration of the inflammatory response, and thus the outcome of tissue repair. For example, MSCs can inhibit the activity of NK cells, thereby exerting an anti-inflammatory effect (112). Therefore, therapeutic strategies targeting NK cells may provide new ideas for the treatment of chronic tendinopathy.

In addition to neutrophils and macrophages, other immune cell lineages such as B cells, T cells, and NK cells have been observed in the tendon during the healing process. While macrophages have received considerable attention in tendon–bone healing research, the functions of these other immune cells remain poorly understood (44, 113). For example, T helper cells have been detected in the tendon 3 days after injury and are believed to promote fibronectin production by migrating fibroblasts (114). The phenotypic heterogeneity and tissue-specific functions of immune cells underscore the complexity of the immune response in tendon–bone healing. Gaining a comprehensive understanding of the involvement of these immune cell lineages is crucial for unraveling the intricate mechanisms underlying tendon–bone healing and for optimizing therapeutic approaches in this context. Further research is necessary to elucidate the contributions of these immune cell lineages and develop targeted therapeutic interventions.

Fibroblasts play a key role in tendon healing. In addition to their cell proliferation and collagen synthesis functions, fibroblasts are deeply involved in the inflammatory response of tendon tissue (117).

Firstly, fibroblasts are active participants in the critical microenvironment. They express Toll-like receptors (TLRs) that recognize risk- and danger-related molecular patterns, thereby triggering an immune response (118). In addition, fibroblasts activate NF-κB factors in response to epidemic stimuli, further promoting the recruitment of inflammatory cells (119). It has been shown that fibroblasts exhibit sustained activation in injured or inflamed tendon tissue, which is strongly associated with the development of chronic inflammation and tendinopathy (120).

Secondly, fibroblasts play an important role in the regulation of the immune response. They are not only involved in immune cell uptake but also influence the function of immune cells through various cytokines and growth factors. For example, VEGF-A of fibroblasts promotes angiogenesis, providing nutrients and oxygen for tissue repair (121). In addition, fibroblasts are antimicrobial. Their antimicrobial peptides and defense molecules help clear the risk of infection (122).

In summary, fibroblasts not only are involved in tissue reconstruction during tendon healing but also actively regulate the response. They influence the activation and function of immune cells through a variety of mechanisms, thus playing an important role in all stages of tendon healing.

Vascular endothelial cells play a crucial role in regulating inflammation during tendon–bone healing. Key aspects of their involvement include the following:

Inflammatory signaling is a crucial aspect of the role played by vascular endothelial cells in tendon–bone healing. Upon stimulation by trauma signals, these cells produce inflammatory cytokines and chemokines, such as interleukins and tumor necrosis factor, initiating an inflammatory response (123). Furthermore, the expression of cell adhesion molecules, including selectins and integrins, by vascular endothelial cells facilitates interactions with circulating inflammatory cells. This expression promotes the capture, rolling, adhesion, and migration of inflammatory cells on the surface of endothelial cells (124, 125). Moreover, vascular endothelial cells exert influence over the infiltration of inflammatory cells by regulating endothelial permeability. Through the opening of intercellular gaps during inflammation, these cells enable inflammatory cells to enter the injured tissue via the interstitium (126). Additionally, vascular endothelial cells release inflammatory mediators, such as chemokines and cytokines, which further modulate the inflammatory response. These mediators attract and activate inflammatory cells, thus promoting tissue repair and regeneration processes (127). In addition to their roles in inflammation, vascular endothelial cells participate in immune modulation by interacting with immune cells. They regulate the intensity and duration of the inflammatory response, playing a significant role in maintaining immune balance and controlling excessive inflammatory reactions (128).

Therefore, by orchestrating inflammatory signaling, expressing adhesion molecules, influencing inflammatory cell infiltration, releasing inflammatory mediators, and participating in immune modulation, vascular endothelial cells play a crucial regulatory role in tendon–bone healing. A comprehensive understanding of their functions and mechanisms in this process can contribute to optimizing treatment strategies, promoting tendon–bone healing, and reducing the occurrence of related complications. Understanding the function and mechanisms of vascular endothelial cells in tendon–bone healing is essential for optimizing treatment strategies to promote healing and reduce associated complications.

In summary, non-inflammatory cells such as fibroblasts and vascular endothelial cells play active roles in the inflammatory phase of tendon–bone healing ( Table 2 ). Their secretion of various molecules, expression of TLRs, and involvement in immune cell recruitment contribute to the complex processes of inflammation and tissue repair. Further research is necessary to fully understand the mechanisms underlying their immunomodulatory effects and explore their potential therapeutic applications in the healing of tendon injuries.

NF-κB, a pivotal transcription factor governing inflammation, manifests as a heterodimer (p50/p60) post-tendon injury, instigating the release of inflammatory mediators like IL-1β, IL-6, TNF-α, CCL2, and CXCL10 (147). It orchestrates multiple facets of tendon healing—from inflammation to cell proliferation, angiogenesis, and tissue adhesion formation (148). In the inflammatory phase, NF-κB acts as a pro-inflammatory signaling pathway, chiefly modulating the expression of pro-inflammatory genes and cytokines (149). Immune cells such as neutrophils and macrophages release inflammatory factors like IL-1β and TNF-α post-tendon injury (139). The canonical NF-κB activation route involves ligand-receptor binding for TNF or IL-1, alongside pattern recognition and antigen receptors, with resident tendon fibroblasts housing these receptors. In tendinopathy, there is an upsurge in corresponding cytokine expression (144, 150, 151). Recently, in early rotator cuff tendinopathy, interleukin-33 (IL-33) was found to spur canonical NF-κB signaling via ST2, suppressing micro-RNA-29a in tendon cells and prompting type III collagen production for scar formation. NF-κB activation unfolds through the canonical pathway involving IκB phosphorylation and degradation, alongside the non-classical pathway spurred by CD40 and hypoxia (152–154).

Selective inhibition of NF-κB activity, especially via IKKβ inhibition, can dampen the inflammatory response (155). A canine study on tendon repair showcased that oral ACHP (an IKKβ inhibitor) administration diminished phosphorylated p65 (p-p65) levels by day 14, curbing inflammation-related gene expression and boosting cell proliferation and neovascularization, expediting healing (156). Likewise, administering ACHP in a rat rotator cuff injury model demonstrated reduced inflammation-related gene expression in the NF-κB pathway and enhanced ECM production at the injury site. In vitro experiments with stimulated rat tendon fibroblasts yielded promising results (157). NF-κB-induced cytokine-stimulated fibroblasts proliferate, synthesize collagen fibers, and transition into myofibroblasts, fostering scar tissue formation (131, 147). Elevated p65 expression in adhesion tissues implies NF-κB signaling’s ties to fibrotic adhesion formation in tendon repair (158).

NF-κB signaling cascades influence IL-6 production, a cytokine with dual roles. While traditionally viewed as proinflammatory for T-cell activation, macrophage activation, and osteoclast formation, IL-6 now emerges as an anti-inflammatory myokine, responding to physical exercise by aiding inhibitory factor secretion like IL-1ra and IL-10 (159). IL-6’s pro-inflammatory tendencies are modulated by TNF-α (160). In patients with RCTs, IL-6 gene expression and cytokine production correlate with degeneration extent and genes linked to tissue remodeling (161–164). Studies in rotator cuff repair patients have linked increased joint stiffness to IL-6 gene polymorphisms (130). Exploration into inhibiting adhesion formation via IL-10 or hyaluronic acid injections is ongoing, necessitating further research to elucidate molecular mechanisms and refine clinical applications (165).

The inflammasome, a signaling complex comprising multiple proteins recognizing pathogen-associated molecular patterns (PAMPs) and damage-associated molecular patterns (DAMPs), plays a crucial role in various diseases (166). The NLRP3 inflammasome, a common complex in tendinopathy inflammation, consists of the NLRP3 sensor, ASC adapter, and pro-caspase1 (167). Activation of NLRP3 by diverse signals, both exogenous and endogenous, leads to cytokine release, intensifying the inflammatory response (133). Following tendon injury, HMGB1 release from necrotic cells is a primary pathway for NLRP3 activation, stimulating the assembly and activation of the NLRP3 inflammasome (137). Activated NLRP3 triggers cytokine release, disrupts the ECM, and amplifies inflammation, impeding tendon healing. Various cellular signals, such as K+ efflux, Ca2+ signaling, and mitochondrial dysfunction, can activate the NLRP3 inflammasome, necessitating further investigation into their specific roles (168).

HMGB1 binds to cell membrane receptors (RAGE, TLR2, TLR4, and TREM-1), activating NLRP3, which assembles apoptosis-associated speckle-like protein (ASC) and recruits caspase-1 for processing (169). Active caspase-1 converts pro-IL-1β and pro-IL-18 into their mature, biologically active forms. Elevated Ca2+ levels disrupt F-actin function, releasing NLRP3 inhibition and enhancing IL-1β maturation and release (170).

Persistent adipose tissue infiltration in tenocytes prolongs inflammation and hampers tendon healing (171). Studies in hepatocytes suggest a synergistic effect between TLR2, TLR4, TREM-1, and RAGE in upregulating HMGB1 and promoting adipose tissue infiltration (172). Inhibition of NLRP3 activity prevents adipose tissue accumulation in hepatocytes, hinting at a potential role of NLRP3 in adipose tissue infiltration across tissues (173). Ion fluxes, including K+, Ca2+, and Cl−, activate the NLRP3 inflammasome. F-actin interacts with calcium-dependent proteins, inhibiting NLRP3 activation. Cyclic stretching of tenocytes increases Ca2+ levels, depolymerizing F-actin and relieving NLRP3 inhibition, leading to heightened IL-1β release (174).

MAPKs, critical protein kinases involved in phosphorylation, transmit extracellular signals within organisms (175). In tendinopathy, MAPKs regulate inflammation, ECM dynamics, and apoptosis (176). The p38/MAPK pathway, activated under stress, governs inflammation, cell growth, and apoptosis (177). Activation involves multiple kinase phosphorylations for signaling transmission (145). GTPases activate MAPKKK, which then activates MAPKK (MKK3 and MKK6 in p38 pathway), phosphorylating MAPK and inducing inflammatory factor production like TNF-α, IL-6, and IL-8 to regulate inflammation (175).

Enhanced NF-κB and p38/MAPK activity prompts inflammation and heterotopic ossification (HO) (178). In vitro studies using MAPK inhibitors (e.g., SB203580, PD98059, and SP600125) showed reduced phosphorylation of ERK, JNK, p38, NF-κB, and IKKβ, along with decreased HO, suggesting MAPK inhibition as a therapeutic strategy for attenuating inflammation, warranting further human in vivo research (179).

The STAT pathway is a vital signaling pathway comprising STAT proteins and their receptors. Within the STAT protein family, STAT3 stands out as a significant member, contributing to immune response, cell proliferation and differentiation, angiogenesis, apoptosis, and tumorigenesis (180). Various chemical signals such as interferons, interleukins, and growth factors bind to cell surface receptors, initiating a cascade of events involving associated kinases. These kinases phosphorylate themselves, the receptor, and downstream target proteins, including the transcription factor STAT3. Phosphorylated STAT3 forms dimers that translocate to the nucleus, binding to target genes and regulating downstream gene transcription (181).

In the context of the tendon’s inflammatory response, STAT3 displays dual regulatory roles, although the exact mechanisms remain unclear. On one hand, inhibiting JNK/STAT3 signaling has been shown to significantly increase the expression of the proinflammatory factor IL-6, which promotes wound healing through fibrosis and scarring (33). However, persistent inflammation often leads to excessive myofibroblast proliferation and the excessive deposition of ECM components, resulting in scar tissue formation (182). On the other hand, the STAT signaling pathway is essential for IL-10 to exert its anti-inflammatory effects and stimulate IL-10 production (33). For instance, IL-10 regulates the TLR4/NF-κB signaling axis in dermal fibroblasts via the IL-10R/STAT3 pathway, effectively reducing the inflammatory response and preventing scar formation (165).

IL-10 forms honor heterodimers by binding to the IL-10R receptor, activating the receptor-linked tyrosine-protein kinase 1 (JAK1) (183). Stimulation of tendon stem cells (TSCs) with connective tissue growth factor induces the production of the anti-inflammatory cytokine IL-10 through the JNK/STAT3 signaling pathway, effectively reducing inflammation, ECM secretion, and scar tissue (184). Additionally, the JNK/STAT3 signaling pathway plays a crucial role in aspirin-induced expression of IL-10 and TIMP-3 in TSCs. Aspirin upregulates the expression of the anti-inflammatory factor IL-10 via the JNK/STAT3 signaling axis, inhibiting the migration and proliferation of TSCs induced by IL-1β. Consequently, the reduction in TSCs may decrease ECM deposition in the injured tendon, ultimately reducing scar formation. As such, aspirin indirectly improves tendon healing by regulating inflammation through the JNK/STAT3 signaling pathway (33). Besides pharmacological interventions, recent research has explored the use of mechanical stimulation to modulate the inflammatory response. For example, mechanical stimulation of tendons through treadmill running increases the expression of IL-4 at the tendon–bone junction. The binding of IL-4 to its receptor activates JAK1 and JAK3, leading to the activation of STAT and its translocation to the nucleus. Ultimately, the JAK/STAT signaling pathway shifts macrophages from a pro-inflammatory to an anti-inflammatory phenotype, resulting in the production of various anti-inflammatory cytokines that regulate the local inflammatory microenvironment and promote tissue healing (185).

However, further in-depth studies, including sequencing analysis, are necessary in the future to elucidate the specific mechanisms and targets involved in the overall inflammatory regulatory process. Therefore, STAT3 plays a crucial role in maintaining the balance between inflammation and ECM during the healing of injured tendons.