Tendons, dense connective tissues responsible for transferring force from muscles to bones, play a crucial role in movement by storing elastic energy and withstanding immense tensile forces. (Docheva et al., 2015). Various injury mechanisms, such as acute overload, tearing, overuse, or age-related degeneration, can lead to tendon injuries. (Thomopoulos et al., 2015). Despite significant advancements in surgical and rehabilitation techniques, tendon repair may encounter postoperative complications. (Voleti et al., 2012). Tendon adhesion, a major complication following tendon injury, affects approximately 40% of patients after surgery, restricting tendon gliding ability and potentially leading to lifelong disability. (de Putter et al., 2012; Titan et al., 2019).

To understand tendon adhesion and healing, it is essential to firstly explore tendon biology, focusing on its collagen composition. Natural tendons are characterized by a sophisticated hierarchy of collagen intermingled with tenocytes and non-collagenous elements. (Benjamin et al., 2008). The tendon as a whole is wrapped in a thin layer called the epitenon. Beneath this layer are the fascicles, aligned with the tendon’s length and visible post-dissection, each surrounded by the endotenon, which also provides blood and nerve supply to the tendon. (Voleti et al., 2012).

Based on this structural foundation, we examine the detailed composition of tendon fascicles, crucial components of tendon architecture. Tendon fascicles consist of collagen fibers interspersed with tenocytes and are identifiable under optical microscopy at about 10 μm in diameter. (Screen et al., 2004). Electron microscopy further uncovers collagen fibrils, marked by periodic crimping in relaxed states. (Sharma and Maffulli, 2005). At a more minute level, microfibrils, formed from cross-linked tropocollagen molecules, lay the foundation of this structure, with tropocollagen being a water-soluble triple helix of polypeptide chains. (Screen et al., 2004). These complex structures, down to tropocollagen, are essential for understanding tendon response to stress and repair initiation.

The critical issue of tendon adhesion following injury or surgery was a key focus of current study. (Legrand et al., 2017). Further research has been dedicated to understanding the genesis and prevention of these adhesion, particularly in healing intrasynovial flexor tendons. (Voleti et al., 2012). Animal studies have shed light on critical elements influencing tendon repair and adhesion development, including initial injury severity, quality of surgical repair, and the significance of mechanical loading. (Thomopoulos et al., 2015). While mechanical loading facilitates collagen type III synthesis and boosts growth factor levels as well as cellular and matrix activities at the injury site, its excess can compromise healing. (Wong et al., 2009). Additionally, prolonged immobility is also implicated in adhesion development, as evidenced in various animal models. (Wong et al., 2014).

In the context of these findings, adhesion prevention has become a central goal in tendon repair research, especially considering its role in functional recovery, as adhesions can complicate the healing process and impair functional recovery. (Hu et al., 2023). In light of this, researchers have come to realize that the immune system plays an important role in tendon healing and adhesion formation, many studies have focused on how to modulate the immune response at the injured site. (Chisari et al., 2020). Bao et al. (Bao et al., 2024) expand the horizon of the anti-inflammatory effects primarily driven by sympathetic nerve through β2 adrenergic signals on macrophages, and made a history of using sympathetic stimulation to significantly prevent macrophage-mediated peritendious inflammation. In this context, the role of macrophages, particularly their involvement in the inflammatory response and modulation of the healing process, has garnered increasing attention. This focus stems from the understanding that inflammatory processes, driven largely by macrophages, are critical in the formation of adhesions during tendon healing. (Sunwoo et al., 2020). Additionally, researchers have recognized the significant impact of macrophages on tendon healing and adhesion formation, leading to a shift in focus towards understanding how macrophages specifically regulate tendon adhesion. (Xu et al., 2020) (Table 1).

The natural healing process of injured tendons involves three consecutive and overlapping stages: the inflammatory phase, proliferative phase, and remodeling phase. (Nichols et al., 2019). During the inflammatory phase, cytokines derived from platelets signal an elevation in vascular permeability, attracting circulating inflammatory cells, including phagocytic neutrophils, monocytes, and macrophages, to the injury site. (Marsolais et al., 2001; Chisari et al., 2020). The subsequent proliferative phase is characterized by the release of growth factors, such as vascular endothelial growth factor and members of the transforming growth factor beta (TGF-β) family, stimulating angiogenesis, granulation tissue formation, and fibroblast proliferation. (Wong et al., 2009). In the final remodeling phase, newly synthesized collagen fibers realign along the longitudinal axis of the tendon until they can withstand load. This process may take up to 2 years to complete. (Lomas et al., 2015).

In the context of macrophage-mediated regulation, the dominance of macrophages becomes significant beyond the initial 24-h period following injury. (Wong et al., 2009). Understanding how macrophages modulate the inflammatory and proliferative phases, and their potential impact on the subsequent remodeling phase, is essential for unraveling the intricacies of tendon healing. As we delve into the macrophage-specific aspects of tendon healing, the focus shifts towards strategies that enhance intrinsic healing while minimizing the impact of extrinsic healing, with the ultimate goal of improving the functional recovery of tendons (Stauber et al., 2020).

Macrophages are key components of the human innate immune system, widely distributed across connective tissues and various solid organs (Murray, 2017). Their high heterogeneity and plasticity enable them to play diverse roles in human diseases, driven by their ability to differentiate into distinct phenotypes under varying stimuli in the local microenvironment (Mosser and Edwards, 2008; Mould et al., 2019).

Classically activated macrophages, or M1 macrophages, emerge under the induction of lipopolysaccharide (LPS), interferon-gamma (IFN-γ), or tumor necrosis factor-alpha (TNF-α) (Di Benedetto et al., 2019). These M1 macrophages are known for their involvement in phagocytosis and display pro-inflammatory characteristics, essential in the body’s response to pathogens and injury.

Conversely, alternative activated macrophages, or M2 macrophages, develop in response to interleukin-4 (IL-4) or interleukin-13 (IL-13) (Viola et al., 2019). These M2 macrophages exhibit anti-inflammatory and pro-healing functions, playing a vital role in tissue repair and regeneration. Notably, M2 macrophages are further subclassified into M2a, M2b, M2c, and M2d subtypes, each characterized by unique activation stimuli, molecular expressions, and functional attributes (Paoli et al., 2014).

Furthermore, Lehner et al (Lehner et al., 2019) have identified a specific population of tissue-resident macrophages in murine and human tendons. These macrophages are key to phagocytosis, inflammatory cytokine secretion, and extracellular matrix-related proteins, playing a crucial role in tendon health and response to injury. Fujii et al. (2022) extend this understanding in the context of anterior cruciate ligament reconstruction (ACLR) in mice, identifying two distinct macrophage populations that infiltrate the tendon/bone interface post-surgery: the CD9⁺ IL1+ and CX3CR1+ CCR2+ macrophages. The CD9⁺ IL1+ macrophages peak 1 day after surgery with a highly inflammatory profile, transitioning later to a homeostatic state, while the CX3CR1+ CCR2+ macrophages accumulate more gradually and express interferon signature genes that might suppress bone formation. In addition, Li et al. (2023) provides the first evidence that macrophages are a primary source of TGF-β1, which is crucial in recruiting stem cell and make it differentiate into myofibroblasts to the surrounding site of injured tendon. This breakthrough sheds light on a crucial antiadhesion landmark of drug therapies, as it identifies an accurate cellular target to reduce peritendinous adhesion by manipulating the TGF-β1 pathway.

In normal tendon tissue, both in the outer membrane and inner fibrous layers, macrophages are sparsely distributed. (de la Durantaye et al., 2014). However, this changes during the acute inflammatory phase following tendon injury. (Wong et al., 2009). In such instances, injured tendon tissue releases chemokines, including C-C chemokine ligand 2 (CCL2), which recruit immune cells, predominantly macrophages. (Sugg et al., 2014). This influx of macrophages leads to the further release of chemokines and cytokines, amplifying the inflammatory response and playing a crucial role in the initial phase of tendon healing. (Marsolais et al., 2001) (Figure 1).

The C-C chemokine receptor type 2 (CCR2) is particularly significant in this context, as it plays a vital role in the activation of macrophages, especially those exhibiting pro-inflammatory characteristics. (Liu et al., 2017). Utilizing CCR2 knockout models, researchers have demonstrated reduced recruitment of monocytes and a subsequent decrease in the inflammatory environment at wound healing sites. (Willenborg et al., 2012). This discovery has prompted exploration into CCR2 inhibition as a potential therapeutic approach, with studies in various conditions, including traumatic brain injury and nonalcoholic fatty liver disease, showing promising results. (Morganti et al., 2015; Flores-Toro et al., 2020). In tendon healing, particularly in the context of rotator cuff repair in CCR2 knockout mice models, there has been an observed decline in macrophage infiltration and suppression of interferon pathways. (Eliasberg et al., 2023).

Recent insights into macrophage-mediated adhesion pathology reveal the significant role of macrophage-secreted Secreted Phosphoprotein 1 (SPP1) in aggravating tendon adhesions. (Liu et al., 2022). Moreover, the interaction of SPP1 with fibroblasts, via cytokine secretion and cellular communication, leads to enhanced fibroblast activation and migration, further exacerbating adhesion formation. (Kapur et al., 2019). Wang et al. (Wang et al., 2024) further reveal that elevated SPP1 expression in macrophages enhanced fibroblasts activation to myofibroblasts through CD44 positive feedback pathway, which indicates a crucial cascade amplification between macrophages and myofibroblasts in the field of in inflammatory hyperplasia.

In the early stages of tendon healing, the infiltrating macrophages at the site of injury are predominantly of the M1 phenotype (Sunwoo et al., 2020). Their concentration significantly increases within the first 2 weeks of tendon healing, and they localize to the newly formed tendon tissue and areas of tissue remodeling (Marsolais et al., 2001; Sugg et al., 2014). M1 macrophages contribute to the propagation of inflammatory responses by releasing a range of pro-inflammatory cytokines and mediators, such as interleukin-1 (IL-1), IL-6, IL-12, tumor necrosis factor-alpha (TNF-α), and reactive nitrogen and oxygen species (Barrientos et al., 2008; Koh and DiPietro, 2011). While they exhibit stronger microbicidal properties, M1 macrophages also have an increased potential for causing collateral damage to surrounding healthy tissues (Chamberlain et al., 2011; Sica and Mantovani, 2012). Additionally, M1 macrophages contribute to the degradation of the extracellular matrix, engaging in processes such as phagocytosis of cellular debris and apoptosis (Mosser and Edwards, 2008).

Given the pivotal role of M1 macrophages in the early stages of tendon healing, it is important to understand the underlying mechanisms that regulate their activity. One such critical aspect is epigenetic regulation, particularly through DNA methylation (Chen et al., 2023). Crucial DNA methyltransferases like DNA methyltransferase 3b (DNMT3b) and DNMT1 are involved in the polarization of these macrophages. DNMT3b inhibits peroxisome proliferator activated receptor (PPAR)γ1, a regulator of the anti-inflammatory M2 phenotype, thereby promoting the M1 phenotype crucial for the early inflammatory response in tendon healing (Yang et al., 2014). DNMT1 contributes by mediating the hypermethylation of genes that are essential for the pro-inflammatory activities of M1 macrophages (Denis et al., 2011). This intricate regulation of gene expression through epigenetic mechanisms underscores the complexity of M1 macrophage behavior in tendon healing and their critical role in initiating the inflammatory response that is essential for early stages of tissue repair.

Furthermore, the emerging role of non-coding RNAs (ncRNAs) in macrophage polarization is gaining attention (Ning and Liu, 2013). MicroRNAs (miRNAs) and other ncRNAs, such as long non-coding RNAs (lncRNAs) and circular RNAs (circRNAs), are significant regulators of macrophage behavior. For example, under the stimulation of IL-13 or TGF-β, miR-155 targets IL-13Rα1 and SMAD2, leading to a bias towards M1-like gene expression (Louafi et al., 2010). Beyond miRNAs, there is increasing interest in the role of long non-coding RNAs (lncRNAs) (Wang et al., 2020). LncRNA cyclooxygenase-2 (cox-2), for instance, is more prevalently expressed in LPS-induced M1 macrophages than in IL-4-induced M2 macrophages, and its suppression results in a decrease in M1 macrophage markers (Ye et al., 2018). Additionally, circular RNAs (circRNAs), which have a unique covalently closed loop structure, are also being studied for their relationship with macrophage polarization. Notably, circRNA Cdyl has been found to promote M1 polarization by inhibiting the nuclear translocation of interferon regulatory factor 4 (IRF4) (Song et al., 2022). In a similar vein, circRNA PPM1F is known to enhance the NF-κB signaling pathway following LPS stimulation, promoting M1 polarization (Zhang et al., 2020). This highlights the growing importance of understanding ncRNA-mediated regulation in macrophages, particularly in the context of their role in tissue healing and adhesion mechanisms.

Besides, an increasing number of studies indicate that M1 macrophages may influence the tissue microenvironment through the secretion of exosomes, which function to transport molecules containing biological information (Momen-Heravi et al., 2014). Lou et al. (2023) discovered that miRNA-155-5p, which is highly expressed in exosomes derived from M1-polarized macrophages, exerts antiangiogenic effects by targeting the GDF6-Akt axis, ultimately impacting the healing process in diabetic conditions.

In contrast to M1 macrophages, M2 macrophages play a significant role in promoting fibroblast proliferation and stimulation of new tissue deposition. (Mantovani et al., 2002; Sun et al., 2023). The increase in the concentration of M2 macrophages primarily occurs in the later stages of the tendon healing process, especially in the region where the tendon extracellular matrix is located (Nichols et al., 2019). Sugg et al. (2014) found that in healing mouse tendons, the concentration of M2 macrophages within the first 28 days post-injury was similar to that of normal, uninjured tendon tissue. However, after 28 days of tendon injury, there was a significant increase in M2 macrophages, becoming the predominant macrophage phenotype at the site of injury. Wang L. et al. (2023) found that mechanical stimulation promotes the polarization of macrophages into the M2 phenotype and secretion of elevated levels of TGF-β1, ultimately, facilitates the chondrogenic differentiation of MSCs and enhances the process of tendon to bone healing in an acute rotator cuff repair model. Interestingly, while the study found that TGF-β1 secreted by M2 macrophages promotes tendon repair, another study revealed a potential drawback of TGF-β1, as it may contribute to tendon adhesion. The research findings by Li et al. (2023) suggest that TGF-β1 derived from M2 macrophages recruits mesenchymal stem cells and promotes the formation of myofibroblasts in tendon adhesion. Besides, due to its association with the inhibition of pro-inflammatory cytokines including IL-1β, IL-8, GM-CSF, and TNF-α, TGF-β1 is involved in terminating the inflammatory response during tendon healing, with M2 macrophages playing a role in this process (Fadok et al., 1998). Another research has been demonstrated that M2 macrophages facilitate tendon healing by secreting phospholipase A1 member A (Pla1a) (Jing et al., 2023). The secretion of Pla1a not only promotes tendon cell proliferation and reduces apoptosis but also leads to decreased Etv1 expression. This dual action of Pla1a, regulated by M2 macrophages, plays a critical role in reducing tendon adhesion and enhancing cell viability.

Contrasting with the earlier discussed role of M1 macrophage-derived exosomes, Liu et al (Liu et al., 2023) found that exosomes derived from M2 macrophages uniquely influence the differentiation of fibro-adipogenic progenitors, highlighting their distinct role in tendon healing mechanisms. The study provides detailed insights into the dynamic interactions between these exosomes and fibro-adipogenic progenitors, emphasizing that M2 macrophages significantly promote brown/beige fat differentiation, a crucial factor for effective muscle regeneration and reducing fatty infiltration.

M2 macrophage could also be associated with an elevated propensity for scar tissue formation. Wojciak et al (Wojciak and Crossan, 2008) found that the existence of inflammatory cells within the synovial sheath and epitenon during the healing process of tendons prompts synovial fibroblasts and epitenon cells to augment their fibronectin synthesis, thereby establishing a framework that facilitates the subsequent formation of adhesions. Moreover, an elevated M2 macrophage activity was detected in the fibrotic healing process observed in murine flexor digitorum longus tendons with Type II Diabetes (Ackerman et al., 2017). These fibrotic tendons displayed diminished biomechanical strength when compared to the repaired tendons of the nondiabetic control group. This excessive fibrosis could potentially be attributed to the excessive production of TGF-β1 by the M2 macrophages, which has been associated with the development of pathological fibrotic conditions in various tissues (Colwell et al., 2005).

In summary, the complex role of M2 macrophage is greatly due to the distinction between intrinsic and extrinsic healing processes in tendon repair. Intrinsic healing, originating from within the tendon, involves tenocytes and internal collagen synthesis, aiming for the restoration of normal tendon structure and function (Stauber et al., 2020). This contrasts with extrinsic healing, where repair is facilitated by external cells including fibroblasts and macrophages, often leading to the formation of adhesion (Voleti et al., 2012). Therefore, while M2 macrophages are integral to anti-inflammatory responses and promote tissue remodeling, which is beneficial in early stages of tendon repair, their prolonged predominance in later stages can inevitably promote extrinsic healing. Excessive M2 activity may lead to an imbalance in the healing process, deviating from the desired intrinsic repair pathway. Thus, in developing therapeutic strategies for tendon injuries, a critical goal is to modulate macrophage activity to encourage a balance that supports intrinsic healing, while mitigating the risk of excessive extrinsic tissue formation. This balance is essential for optimal tendon recovery, emphasizing the need for precise temporal and spatial control of macrophage phenotypes during the healing process.

An increasing amount of evidence suggests that macrophages exhibit a vast spectrum of phenotypes and functions, shaped by specific differentiation signals, surrounding cell types, and the molecular context of different tissues. (Williams et al., 2018; Lehner et al., 2019). This diversity is more intricately understood through advanced technologies such as single-cell RNA-seq and single-cell mass cytometry by time of flight, which allow for the analysis of macrophage phenotypes with unprecedented resolution. (Murray, 2017; Arlauckas et al., 2021). These studies reveal that macrophages exist in a continuum of numerous subtypes, emphasizing the complexity of their roles in various physiological contexts. While the classification of macrophages into M1 and M2 phenotypes provides a useful framework, it serves primarily as a starting point for exploring the regulation of the optimal balance between inflammation and regeneration during tendon healing.

In conclusion, while this review has highlighted significant advancements in understanding macrophage polarization in tendon healing and adhesion mechanisms, the path forward calls for focused exploration. Future research should aim to unravel the complex molecular pathways influencing macrophage behavior, particularly the role of non-coding RNAs. Additionally, the development and clinical application of innovative biomaterials that can modulate macrophage activity presents a promising avenue for enhancing tendon repair and reducing adhesion formation. These focused areas of research hold the potential to significantly advance our understanding and treatment of tendon injuries.