Frozen shoulder (FS), also referred to as adhesive capsulitis of the shoulder, is a debilitating condition characterized by shoulder pain and a progressive limitation of both active and passive movement in the glenohumeral joint (1). Epidemiological studies suggest that FS affects approximately 2%–5% of the global population, with its prevalence increasing (1, 2). This condition predominantly manifests in individuals aged 40–60 years, with a peak incidence around age 50, and is approximately 1.23 times more prevalent in women than in men (3, 4). Recent Mendelian randomization evidence further suggests that epigenetic age acceleration is bi-directionally associated with age-related musculoskeletal disorders, supporting a more active role of biological aging in chronic degeneration (5). In line with this age-related biological context, although FS has traditionally been considered a self-limiting disease, clinical studies have demonstrated that a substantial number of patients experience long-term functional impairment even after symptom improvement (1, 6, 7). Furthermore, persistent pain and restricted movement frequently lead to adverse emotional responses, such as anxiety, fear, and depression, which can affect treatment adherence and rehabilitation outcomes to some extent (8). Overall, FS not only severely diminishes the quality of life of patients but also significantly increases the medical and economic burdens on both individuals and society.

Current evidence indicates that FS is a chronic musculoskeletal disorder predominantly characterized by capsular contracture, with dynamic progression in which inflammation initiates and fibrosis consolidates the pathology (9–11). Clinical and histopathological studies have generally classified the disease course into three stages: early inflammatory pain hypersensitivity, intermediate frozen, and late thawing, each exhibiting distinct pathological features and cellular activities (12, 13) (Figure 1A). Although the precise etiology remains unclear, accumulating evidence indicates that an early inflammatory response, potentially triggered by repetitive strain, trauma, rotator cuff injury, or subacromial impingement, activates local immunity and induces the release of inflammatory and growth factors, thereby promoting fibroblast activation, disrupting extracellular matrix (ECM) homeostasis, and accelerating fibrosis (6, 14, 15).

In the context of this pathological progression, macrophages serve as crucial mediators connecting inflammation with fibrotic alterations. Their context-dependent plasticity enables them to undergo phenotypic and functional shifts across various disease stages, thereby influencing downstream outcomes in FS (16–18). Within the classical paradigm, resting macrophages (M0) can polarize into either pro-inflammatory M1-like (classical activation) or pro-restorative M2-like (alternative activation) macrophages. These polarized macrophages actively modulate the local immune–stromal microenvironment by secreting a range of pro-inflammatory and pro-fibrotic mediators, thus linking the initial inflammatory response to fibrotic progression (19–21). Consequently, unresolved inflammation promotes fibroblast activation and excessive ECM deposition (22–24). Single-cell analyses have identified specialized macrophage subsets, including MER tyrosine kinase (MERTK)-positive clusters, which exhibit transcriptional programs associated with the negative regulation of inflammation and may facilitate disease resolution (18). This highlights the pivotal role of macrophage plasticity in determining the inflammatory and fibrotic trajectories of FS. In addition, upstream signals that regulate macrophage homing and programming, such as the C-C motif chemokine ligand 2 (CCL2)/C-C motif chemokine receptor 2 (CCR2) axis and persistent mechanical cues, including matrix stiffness and tissue tension, are likely to affect the rate and extent of the transition from inflammation to fibrosis (18, 25). Despite their importance, these upstream regulatory mechanisms remain insufficiently characterized, warranting an integrative synthesis.

While current research indicates that macrophages may play a crucial regulatory role in FS fibrosis progression, there remains a significant lack of mechanistic studies exploring phenotypic heterogeneity and functional states during inflammation, fibroblast activation, and ECM accumulation. This gap is evident in the understanding of the upstream regulatory pathways and effects of the mechanical microenvironment. This review examines the functional evolution of macrophages during FS, highlighting their roles in the inflammatory and fibrotic stages. This review emphasized the influence of CCL2/CCR2 chemotactic signaling and mechanical stimulation on macrophage polarization and functional expression, and explored the potential applications of related intervention strategies.

Macrophages are deeply involved in the fibrotic process of FS by shaping fibroblast activation and matrix remodeling; however, their net effects are highly context-dependent and are often discussed in relation to polarization-associated programs. In broad terms, M1-like states tend to align with inflammatory amplification, whereas M2-like states are more frequently associated with immune regulation and tissue repair processes. Importantly, macrophage phenotypes are plastic rather than fixed, and cells can transition between these functional programs to match the needs of different disease phases (71, 72). While the M1 and M2 frameworks serve as a useful foundation for categorizing macrophage functional tendencies, in vivo states often extend across a more continuous and multidimensional spectrum that a strict binary classification may not adequately capture. Various disease-associated phenotypes have been identified, including M3, M4, M17, Mreg, Mox, Mhem, and M(hb). In addition to sharing classical M1 or M2 markers, many of these macrophage phenotypes express unique sets of context-specific genes that provide specialized functions tailored to the local tissue needs. These additional programs may enable macrophages to either amplify inflammatory responses for pathogen or tumor clearance or, conversely, to suppress inflammation and aid in the removal of metabolic byproducts and damaged matrix components, thereby contributing to tissue homeostasis (73, 74). This pattern may reflect the plasticity of macrophages, where phenotypes shift in response to local cues and disease stages, rather than remaining fixed. In this context, M1-like and M2-like are better understood as functional orientations, and while marker genes can help stratify states, they may not, on their own, define macrophage function without considering the surrounding microenvironment.

Viewed in this light, the macrophage states revealed by single-cell analyses are more appropriately interpreted as context-dependent deployments of shared functional programs rather than fixed or lineage-distinct entities, thereby suggesting that an M1- and M2-like functional orientation may serve as a pragmatic framework for mechanistic discussions in FS. In FS, single-cell profiling of the capsule has identified macrophage states associated with divergent trajectories, including a MERTK^low CD48⁺ population linked to inflammatory activation and a MERTK⁺ LYVE1⁺ MRC1⁺ population enriched with inflammation-restraining and matrix-remodeling features. MERTK-positive macrophages co-express classical repair-associated markers, such as CD163 and CD206, and are also observed in fetal shoulder tissue and remission-phase rheumatoid synovium, suggesting that MERTK expression may mark a conserved pro-resolution macrophage program rather than a disease-specific state. Furthermore, interactions between these macrophages and DKK3⁺ POSTN⁺ fibroblast-like cells have been proposed to facilitate extracellular matrix reorganization and fibrosis regression in FS, potentially contributing to the self-limiting course observed in a subset of patients (18). In the subacromial bursa, macrophages differentiate into regulatory and repair-associated states, which are enriched with CD163, CD206, HSPB1, and F13A1, or more inflammatory or immune-activating states characterized by CD86, PHLDA1, FCER1A, and antigen presentation signatures. Degenerative rotator cuff disease tends to be relatively enriched with the former states, whereas traumatic injury favors the latter, suggesting that distinct macrophage programs may influence divergent inflammatory trajectories in shoulder pathology (75).

Current evidence indicates that macrophage phenotypes exist along a continuum influenced by local microenvironmental cues rather than fixed identities. While single-cell analyses have refined the classification of macrophage phenotypes, many identified subsets retain core functional features that broadly align with either inflammatory or repair-associated programs. Given the limited direct evidence on macrophage polarization in FS, this section refers to conserved mechanisms described in other fibrotic conditions to aid in the interpretation. Accordingly, in this review, M1-like and M2-like are used as functional orientations to characterize macrophage behavior in FS, with selected subsets discussed when they offer disease-relevant mechanistic insights into the coupling of inflammation and fibrosis.

As fully demonstrated, M1-like macrophages are the central force driving the onset and escalation of tissue inflammation, leading to tissue damage by perpetuating the inflammatory process (76, 77). Within the capsule tissue of FS, there is a notable increase in M0 macrophages, which primarily differentiate from monocytes recruited in the early phases of inflammation and tend to polarize into the M1-like phenotype (30). Gene enrichment analysis has shown that the differentially expressed genes linked to these cells are significantly enriched in various biological processes, particularly those involving cytokine receptor interactions, collagen metabolism regulation, and inflammatory cascade reactions (17). M1-like macrophages release inflammatory factors, such as TNF-α, IL-1β, and IL-6, which can induce cytotoxicity, worsen tissue damage, and facilitate fibrosis progression by activating the NF-κB pathway (78, 79). Furthermore, in a fibrotic environment, ROS released by M1-like macrophages not only directly harm parenchymal cells but also promote the activation of latent TGF-β. This activated TGF-β enhances the pro-inflammatory response of macrophages and sets the stage for subsequent pro-fibrotic signals (80) (Figure 2).

While the pro-inflammatory role of M1-like macrophages during the onset of inflammation is well-established, the involvement of M2-like macrophages in fibrosis is more intricate and remains a subject of debate. From a functional perspective, within the broadly defined M2-like orientation, M2 macrophages comprise multiple phenotypically and mechanistically distinct subsets, rather than a single homogeneous population. Based on their inducing signals, M2 macrophages are commonly categorized into M2a, M2b, M2c, and M2d subtypes, each characterized by distinct marker expression and cytokine secretion profiles that translate into nuanced immunoregulatory behaviors (81). Among these subsets, IL-4- or IL-13-induced M2a macrophages exhibit a secretory profile enriched in IL-10, TGF-β, and Th2-associated chemokines and produce matrix-related mediators, such as fibronectin, linking M2a polarization to enhanced tissue repair, ECM synthesis, and fibrosis-associated programs across multiple disease models (82, 83). In contrast, M2c macrophages respond to IL-10 or glucocorticoid signaling and display potent anti-inflammatory and immunosuppressive properties. By suppressing pro-inflammatory cytokines and promoting matrix degradation and clearance, M2c macrophages contribute to the restoration of tissue homeostasis and may exert protective effects against maladaptive fibrosis (82, 84). Although M2b and M2d subsets are also involved in immune regulation, accumulating evidence suggests that the balance between fibrosis-promoting M2a programs and resolution-oriented M2c responses critically influences whether tissue remodeling proceeds toward effective resolution or pathological collagen deposition (85–87). Nonetheless, the presence and functional delineation of these M2 subsets in FS remain to be conclusively demonstrated, and additional evidence is required to define their roles within the capsular microenvironment.

In the context of FS, M2-like macrophages appear to exhibit a comparable functional duality in immune regulation and tissue remodeling, which may be interpreted in light of the functional heterogeneity described above. Under appropriate microenvironmental conditions, M2-like macrophages secrete regulatory and reparative mediators that attenuate inflammation and support tissue homeostasis restoration. However, when these reparative programs become persistently activated or dysregulated, the same pathways may shift toward excessive collagen synthesis, thereby promoting maladaptive capsule remodeling and fibrosis.

Some research suggests that M2-like macrophages may contribute to anti-inflammatory responses and help maintain homeostasis during certain phases of FS. Transcriptome data revealed a significant upregulation of M2-related genes (such as Il10, Cd14, and Cd163) in the tissues of patients with FS, while M1-related factors (such as TNF-α, IL-6, and IL-8) and their downstream NF-κB signaling exhibited an inhibitory trend, suggesting a gradual weakening of local inflammatory activity (75, 88). Further research has shown that although fibroblast activation markers in FS tissue are continuously upregulated, the expression of the inflammatory marker CD90 decreases, indicating that fibroblasts may have adopted a repair-oriented activation phenotype (88). This transition is consistent with the polarization state of macrophages, where the M2 subset, especially M2c, contributes to reducing the prolonged stimulation of fibroblasts. These clusters exert this effect by inhibiting the production of pro-inflammatory cytokines, thereby curtailing excessive fibroblast activation and collagen deposition (89, 90). Additionally, as phagocytic cells with clearance functions, M2-like macrophages can directly participate in ECM clearance by engulfing excess collagen, which aids in alleviating matrix barrier abnormalities (80). These findings suggest that at specific stages of FS, M2-like macrophages may exert protective effects through mechanisms such as anti-inflammatory action, repair, and clearance, and their increased proportion may help counteract M1-mediated inflammatory chain reaction.

However, numerous studies have identified M2-like macrophages as the primary agents responsible for exacerbating fibrosis. In FS-related pathology, clinical evidence suggests a tendency for macrophages to skew toward M2-like functional program. Serum from patients who developed secondary FS after rotator cuff repair polarized human macrophages toward an M2-oriented phenotype rather than M1, marked by increased expression of Arginase-1 (Arg-1), TGF-β, and IL-10. This M2-biased polarization is accompanied by enhanced activation of capsule-derived fibroblasts, indicating that M2-like macrophage-driven immunoregulatory programs may contribute to fibrotic formation within the glenohumeral capsule (66). Similarly, single-cell transcriptomic analysis of human osteoarthritic synovium revealed increased infiltration of macrophages enriched for the expression of genes such as CD163, MRC1, C1QC, APOE, and F13A1, along with a higher proportion of M2-polarized macrophages than in non-OA controls. These macrophages exhibited transcriptional programs associated with extracellular matrix organization and synovial remodeling and displayed extensive interactions with fibroblasts, implicating M2-like macrophage states in synovial fibrotic remodeling (91).

Consistent with these observations, evidence from diverse fibrotic disease models supports the crucial role of M2-like macrophage programs in orchestrating matrix remodeling and fibroblast activation. M2-like macrophages are a major source of profibrotic mediators, including TGF-β, platelet-derived growth factor (PDGF), and vascular endothelial growth factor (VEGF), which directly induce fibroblast activation and ECM deposition (21, 92). They also produce MMPs and TIMPs, the MMP/TIMP balance is essential for tissue homeostasis, whereas TIMP overexpression restricts matrix degradation and excessive MMP activity disrupts tissue architecture, both favoring maladaptive repair and collagen accumulation (93, 94). Within this program, TGF-β functions as a central signaling node, guiding the shift from repair to fibrosis. M2-like polarization is frequently accompanied by enhanced TGF-β/Smad3 signaling, which drives fibroblast phenotypic conversion and augments collagen deposition (95). Notably, this regulation appears to be bidirectional: M2-derived growth factors activate fibroblasts, and activated fibroblasts upregulate fibrosis-associated molecules, such as fibroblast activation protein (FAP) and chemokines, which reinforce M2-like polarization, establishing a self-amplifying circuit. Nevertheless, confirmation of this pathway in FS is still required (96) (Figure 2).

The shift from repair signals to pro-fibrotic signals illustrates the dynamic remodeling of macrophage polarization as FS progresses. Macrophage plasticity results in differentiation at various stages. In the early stage of FS marked by pain hypersensitivity, macrophage populations tend to adopt M1-like functional programs, which are associated with elevated inflammatory activity (15, 17, 32) As the disease progresses and capsular stiffness becomes more apparent, M2-like macrophages emerge as the dominant population. Evidence from FS-related studies indicates that patients experiencing secondary FS after rotator cuff repair exhibit more pronounced shoulder stiffness and limited ROM. Additionally, serum-derived factors from these patients can induce M2-like polarization of THP-1 macrophages while simultaneously activating capsule-derived fibroblasts via the TGF-β/Smad3 pathway, promoting capsular thickening and fibrotic remodeling (17, 66).

Consistent with the pathological features of FS, including fibroblast proliferation, synovial thickening, ECM remodeling, and persistent low-grade inflammation (1), these observations support the notion that a transition toward M2-like macrophage dominance may occur during the frozen stage. Although M2-like macrophages are essential for tissue repair, sustained activation of their reparative programs under chronic inflammatory conditions may inadvertently promote excessive collagen deposition, ultimately contributing to capsular fibrosis (Figure 2).

M1-like and M2-like macrophages are not mutually exclusive during fibrosis; they frequently coexist within the same lesion, with their relative abundance determined by the disease stage and local microenvironment. Cross-disease studies support this dynamic pattern. M1-like macrophages amplify early inflammation and tissue injury through factors such as TNF-α, IL-1β, IL-6, and CCL2, indirectly promoting fibroblast activation and migration. In contrast, M2-like macrophages drive fibroblast differentiation and directly stimulate ECM deposition by upregulating TGF-β, IL-10, and TIMPs (93, 97, 98). Time-resolved analyses have demonstrated that M1-like macrophages predominate during the early inflammatory phase and gradually give way to M2-like macrophages, a transition closely correlated with increased collagen deposition and fibrotic remodeling (99). Consistent temporal patterns have been observed in myocardial injury models, where M1-like macrophage dominance peaks at approximately day 3 post-injury, followed by an expansion of M2-like macrophages around day 7, coordinating tissue repair and fibrosis (100). Notably, epigenetic regulatory mechanisms may shape the temporal relationship between inflammation and fibrosis. In a spinal cord injury model characterized by significant neuroinflammation and progressive fibrotic or scar-forming changes, circRNA CDR1as was observed to regulate inflammatory and fibrotic responses in a time-dependent manner. This regulation occurs through microglial modulation, activation of the TGF-β/Smads signaling pathway, and progressive accumulation of ECM in the later stages. Together, these observations indicate that immune-to-fibrotic transitions may not be restricted to peripheral tissues (101, 102). Nevertheless, despite these converging lines of evidence, the direct temporal and programmatic characterization of macrophage polarization dynamics in FS remains limited. Consequently, further investigation is needed to understand the precise timing, regulatory cues, and functional transitions of macrophage subsets across different FS stages.

In summary, fibrosis in FS is unlikely to be driven by a single macrophage population but rather reflects the stage-dependent interplay between M1-like and M2-like subsets. Temporal shifts in their polarization states shape the transition from inflammation to fibrosis, underscoring the importance of understanding the coordinated dynamics of disease progression. Current temporal evidence for FS remains limited, highlighting the need for a more systematic characterization of this process.

In the previous section, we highlighted the significance of the CCL2/CCR2 signaling axis in facilitating intercellular communication. Beyond its initial role, this axis is instrumental in recruiting inflammatory monocytes and macrophages, as well as their subsequent polarization. For effective polarization, a sufficient pool of macrophages is necessary; hence, early recruitment driven by CCL2/CCR2 focuses on CCR2⁺ monocytes at the lesion site, thereby expanding the local macrophage population and setting the stage for ensuing differentiation. This section delves into the dual role of the axis in macrophage polarization. Initially, CCL2 aids in the influx of CCR2⁺ cells, creating a conducive environment for inflammatory processes. Subsequently, downstream signals linked to this pathway can influence polarization programs in various directions, affecting local inflammation and fibrosis. Unraveling the mechanisms that guide these pathways will be crucial for pinpointing immune control points in FS progression.

In the initial phase of injury, CCL2 is instrumental in attracting a significant influx of CCR2 positive monocytes and macrophages to the lesions, promoting their polarization into the M1 phenotype and triggering an inflammatory response. These populations subsequently release pro-inflammatory factors, such as TNF-α and IL-1β, which further enhance CCL2 expression, thereby reinforcing the inflammatory cycle. They also facilitate the activation of fibroblasts and the expression of fibrosis markers, including α-SMA and vimentin (103). Once activated, inflammatory fibroblasts can produce CCL2, which further intensifies the local CCL2/CCR2 axis activity and cell communication density (104) (Figure 3). As CCL2/CCR2 signaling persists, some macrophages begin to undergo alternative activation (M2 polarization), marked by increased levels of TGF-β, chitinase-like protein 3, and arginase 1, which are classic indicators of M2 macrophage activation, indicating a shift towards tissue remodeling (105). Notably, the ongoing release of CCL2 by M1 macrophages not only sustains the inflammatory state but also draws a new wave of inflammatory monocytes and immature macrophages to the lesion via CCR2-mediated signaling. In the context of the chronic injury microenvironment, these newly recruited cells are more likely to receive alternative activation signals and adopt the M2 phenotype upon entering the tissue, releasing substantial amounts of pro-repair signals, such as TGF-β1, which continuously stimulate fibroblast activation and exacerbate tissue fibrosis (100) (Figure 3).

Intriguingly, CCL2 appears to have the capacity to simultaneously promote the polarization of both M1 and M2 macrophages, a phenomenon likely due to its dual role as a “resource supplier” and an “effect amplifier.” On the one hand, driven by inflammatory signals, CCL2 preferentially mobilizes Ly6C⁺ CCR2⁺ inflammatory monocytes through CCR2-mediated signaling, thereby determining the size of the polarized macrophage pool. In contrast, even the absence of the CX3C motif chemokine receptor 1 (CX3CR1), which is also highly expressed on macrophage surfaces, does not significantly impact this recruitment process, underscoring the specific role of CCR2 as a core receptor in the ligand-receptor axis (106). From another perspective, the direction of macrophage polarization is primarily influenced by the tissue microenvironment, which is highly complex and dynamic in disease states. Thus, CCL2 is more likely to function as a signal amplifier rather than a sole driving factor, maintaining existing signals while enhancing the polarization trends in specific environments. This regulatory feature may explain why CCL2/CCR2 exhibits markedly different macrophage polarization induction effects across various pathological contexts. For instance, in a chronic obstructive pulmonary disease model, CCL2 overexpression recruits and activates macrophages via the PI3K/AKT pathway, leading to the concurrent upregulation of M1 and M2 markers, ultimately driving chronic inflammation and airway remodeling (107). However, in the context of myocardial infarction, CCL2 overexpression significantly promotes M1 and inhibits M2 polarization by activating the p38 mitogen-activated protein kinase (p38 MAPK) and NF-κB signaling pathways, resulting in a sustained immune response and excessive collagen deposition, thereby exacerbating cardiac remodeling (108).

These differences underscore how the CCL2/CCR2 signaling axis shapes tissue repair paths through context-dependent regulation of the local microenvironment and immune dynamics. The fibrotic outcome ultimately depends on the functional reprogramming of macrophages at different stages and the persistence of their recruitment. In this setting, a single biomarker is insufficient to predict disease progression; reliable assessment requires the integration of phenotypic signatures with functional programs (109). Notably, the sustained dominance of any macrophage phenotype can disrupt tissue homeostasis, with persistent CCL2/CCR2 signaling potentially amplifying this imbalance. Rather than dictating a fixed polarization fate, this axis primarily modulates the inflammatory–repair equilibrium by expanding the recruitment of inflammatory cells and reinforcing existing microenvironmental cues.

The mechanisms involved have significant implications for the pathogenesis of FS. The presence of both inflammatory signals and fibrotic responses in FS lesion tissues indicates that CCL2 may not only facilitate monocyte recruitment via CCR2 and increase the local pool of inflammatory cells, but also enhance pro-inflammatory or pro-repair/fibrotic tendencies, depending on the specific signaling context. In a highly inflammatory environment, CCL2/CCR2 may sustain M1 dominance, resulting in chronic inflammation and tissue injury. Conversely, when repair signals prevail, this axis may shape the functional orientation of M2 macrophages to adapt to the shifting microenvironment, with fibrosis potentially arising as a maladaptive consequence (Figure 3). Understanding this context dependency is crucial for elucidating the intertwined mechanisms of inflammation and fibrosis in FS, as well as for developing therapeutic strategies targeting the CCL2/CCR2 axis.

Mechanical stress is a fundamental physical cue through which cells sense and respond to their microenvironment, and mechanotransduction critically regulates inflammation, hyperplasia, and fibrotic remodeling following tissue injury (110). The glenohumeral capsule is an extracellular matrix–dominated dense fibrous connective tissue primarily composed of type I and type III collagen, with collagen fibers arranged in a multi-axial orientation to accommodate multidirectional joint loading (9, 111, 112). Based on these intrinsic mechanical characteristics of the capsule, accumulating evidence indicates that pathological alterations in ECM organization and stiffness are closely linked to the progression of FS. A study constructing a three-dimensional model of the glenohumeral joint based on CT arthrography demonstrated that the capsular tissue in FS exhibits increased stiffness and a persistently high-tension state, reflecting a mechanically imbalanced intracapsular environment (113). This mechanical dysregulation is largely attributable to the excessive accumulation of ECM together with enhanced collagen cross-linking, which progressively disrupts the physiological mechanical balance of the capsule, leading to restricted glenohumeral mobility and driving disease progression (Figure 4) (13, 112).

Macrophages represent a significant subset of infiltrating immune cells in FS and are increasingly acknowledged as potential sensors of stiffness-related mechanical cues within the capsule microenvironment. Emerging evidence suggests that macrophages can detect matrix stiffness and mechanical forces, resulting in functional reprogramming toward inflammatory or pro-fibrotic states. In vitro, Meizlish et al. (114) demonstrated that IL-4–polarized bone marrow–derived macrophages perceive substrate stiffness and selectively modify the transcriptional programs associated with tissue repair in response to the mechanical state of the ECM. Similarly, Chen et al. (115) reported that substrate stiffness dynamically influences macrophage polarization, with softer matrices favoring an M1-like pro-inflammatory phenotype and intermediate to higher stiffness promoting an M2-like repair-oriented phenotype. Progressive matrix stiffening further enhances M2-associated markers, indicating that a stiff ECM may bias macrophages toward a profibrotic polarization state, thereby reinforcing matrix stiffening (116). Within the musculoskeletal system, changes in physiological tendon stiffness have been shown to affect the proliferation and morphology of human tendon-derived cells and alter the cytokine secretion profile and functional behavior of THP-1–derived macrophages in ways that may favor tendinopathy development (117). Thus, although these models do not fully recapitulate the complexity of the in vivo microenvironment, they are largely based on rodent bone marrow–derived macrophages or THP-1–derived human macrophages, which retain conserved mechanosensitive signaling properties, and therefore provide a reasonable stiffness-dependent mechanistic reference for capsular fibrosis (Figure 4). To integrate these observations across disease contexts and experimental systems, the major mechanotransduction-associated immune and fibrotic responses are summarized in Table 1.

Macrophages possess the ability to actively interpret variations in matrix stiffness, raising a critical mechanistic inquiry regarding the manner in which these cells translate external mechanical stimuli into intracellular signaling pathways. Emerging evidence suggests that this conversion process relies on specialized mechanotransduction mechanisms, with mechanosensitive ion channels serving as the primary structural and functional components that facilitate the detection and transduction of mechanical forces by cells (118). Mechanosensitive ion channels, notably Piezo1 and Piezo2, are known to directly respond to mechanical stimuli, such as tension and shear stress, facilitating Ca²⁺ influx and initiating signal transduction pathways (118). Among these channels, Piezo1 has been identified as the primary and functionally specific mechanotransducer in load-bearing connective tissues. Evidence from human tendon cells and corresponding mouse models indicates that mechanical stress–induced Ca²⁺ signaling is predominantly dependent on Piezo1, whereas other mechanosensitive channels, such as transient receptor potential vanilloid 4 (TRPV4) and polycystin-2 (PKD2), appear to play limited roles in this response (119).

Beyond its role in structural mechanosensing within matrix-resident cells, Piezo1 activation plays a crucial role in regulating key immune functions, such as migration, inflammatory activation, and phenotype reprogramming in macrophages, dendritic cells, and T cells (120, 121). Through these mechanisms, Piezo1 links mechanical cues to downstream immune responses that are vital for tissue repair and regeneration (122, 123). Furthermore, a recent study revealed that Piezo1 coordinates interactions between mesenchymal stromal cells and Th17 cells to regulate bone metabolism in a murine temporomandibular joint arthritis model (124), underscoring its broader role as a mechanosensitive integrator between the stromal and immune compartments.

At the macrophage level, mechanistic investigations have elucidated that Piezo1 functions as the principal sensor of stiffness, linking matrix rigidity to macrophage polarization. It demonstrates significantly elevated expression and Ca²⁺ signal transduction capacity compared to Piezo2. Enhanced substrate stiffness activates Piezo1-dependent Ca²⁺ signaling, which subsequently facilitates the activation of macrophage inflammatory programs. Conversely, deletion of Piezo1 specifically in myeloid cells suppresses pro-inflammatory macrophage polarization, augments the production of anti-inflammatory mediators, and significantly diminishes immune infiltration and fibrotic capsule formation around rigid subcutaneous implants (125). Current evidence indicates that mechanical overload plays a crucial role in driving inflammation and structural remodeling of synovial joints, such as the knee and shoulder, with Piezo1-mediated mechanotransduction serving as the central regulatory mechanism in this process (126). In rat models of osteoarthritis, increased Piezo1 expression has been found to be closely associated with synovial inflammation and fibrotic remodeling (127, 128). Additionally, Shen et al. (129) demonstrated in a murine model of temporomandibular joint arthritis that an imbalance between M1-like and M2-like macrophage populations plays a critical role in synovial inflammation and fibrosis, with this polarization disequilibrium being dynamically regulated by Piezo1 signaling. Collectively, these findings suggest that Piezo1-mediated mechanotransduction links abnormal mechanical cues to immune activation fibrotic diseases. Currently, there is no direct evidence demonstrating Piezo1 activation in macrophages within FS, either in patient-derived tissues or in preclinical models, which limits our understanding of how mechanical signals shape the progression and clinical outcomes of this disease. However, considering the established role of Piezo1 in various fibrotic disorders and the unique mechanical properties of the glenohumeral capsule, it is reasonable to hypothesize that FS involves similar mechanosensitive processes (Figure 4). Future research examining the role of Piezo1-driven mechanotransduction within the specific mechanical environment of the glenohumeral capsule could further substantiate the applicability of this hypothesis in FS.

Piezo1-mediated Ca²⁺ influx represents the initial phase of macrophage mechanical perception, with the subsequent activation of transcriptional regulatory modules playing a significant role. The yes-associated protein (YAP) and transcriptional coactivator with PDZ-binding motif (TAZ) are known for tension sensing and gene expression reprogramming across cell types, acting as mechanotransduction transcription factor axes (130). YAP and TAZ are homologous transcription coactivators that operate in the nucleus upon activation. In response to mechanical stimuli, such as increased ECM stiffness or stress overload, YAP/TAZ translocates into the nucleus and forms transcriptional complexes with TEA domain transcription factors (TEAD), thereby governing the expression of genes involved in cell proliferation, differentiation, and extracellular matrix remodeling (131, 132). Through these downstream transcriptional programs, YAP/TAZ modulates synovial cell functions and drives arthritic pathological progression characterized by synovial hyperplasia, pannus formation, immune cell infiltration, and ultimately synovial fibrosis (130, 133). In line with this concept, in vitro substrates with adjustable stiffness have demonstrated that variations in matrix rigidity modulate Piezo1 expression and YAP/TAZ activation in macrophages. This modulation subsequently influences whether macrophages predominantly assume M1- or M2-like polarization states (134). In vivo models of visceral fibrosis have demonstrated that targeting YAP/TAZ signaling in macrophages can shift them between pro-inflammatory and pro-fibrotic states, thereby affecting the severity of fibrosis. This indicates that YAP/TAZ plays a crucial role in determining macrophage phenotype in fibrotic tissues (135, 136). In the musculoskeletal system, Li et al. (137) demonstrated that activating the YAP axis in skeletal stem cells using microgel-based carriers enhances their immunomodulatory reprogramming. This, in turn, modulates macrophage polarization and alleviates osteoarthritic pathology. While direct evidence of a similar YAP/TAZ-mediated mechanism in FS is still lacking, a recent study on fibroblast-like cells derived from the subacromial bursa of patients with chronic shoulder disease revealed that these cells exhibit YAP-dependent mechano-responsiveness. Increased cyclic loading promotes YAP nuclear localization, disrupts the MMP/TIMP balance, and drives ECM production and tissue remodeling (138). Collectively, these findings suggest that YAP/TAZ-driven mechanotransduction is relatively conserved across fibrotic contexts. Its ability to reprogram macrophages and stromal cell phenotypes may offer a valuable framework for understanding the mechanically driven inflammation-fibrosis coupling in FS, warranting further investigation.

Piezo1 activation not only directly influences macrophage function but is also intricately involved in modulating chemotactic signaling pathways. The process of mechanical loading is linked to the upregulation of CCL2 and CCR2 expression, which appears to be contingent on Piezo1 activation. In rodent models of osteoarthritis, transcriptomic profiling of joint tissues has indicated a persistent upregulation of several genes implicated in inflammatory responses and mechanosensing, such as Ccl2 and Piezo1. This is accompanied by a significant enrichment of gene programs associated with synovial hyperplasia and fibrosis within the synovium (127, 139). These findings suggest that Piezo1-mediated mechanosignaling may be crucial for regulating the CCL2/CCR2 axis in response to mechanical stress.

Although Piezo1-mediated mechanosignaling is closely associated with the expression of CCL2, it does not appear to directly regulate CCL2 or CCR2 transcription, implying the involvement of additional intermediary regulators. In pulmonary fibrosis models, YAP/TAZ activation results in increased CCL2/CCR2 expression, facilitating the recruitment of monocytes and macrophages, thereby intensifying inflammatory infiltration and promoting fibrosis. Remarkably, neutralizing CCL2 has been shown to reverse these effects, indicating that the development of pulmonary fibrosis driven by YAP/TAZ is at least partially dependent on the CCL2/CCR2 axis (135). This phenomenon was consistently observed in a cardiac fibrosis model. Subsequent mechanistic analyses demonstrated that YAP/TAZ can bind to the Ccl2 promoter, thereby directly enhancing its transcription (140). This finding contributes to a deeper understanding of the mechanism by which CCL2 responds to mechanical cues (Figure 4). Interestingly, a study reported that under lumbar instability, abnormal mechanical stress interferes with YAP/TEAD signaling, thereby reducing YAP activity in the cartilage endplate cells. This interference results in increased CCL3 expression, which recruits and activates osteoclasts originating from bone marrow monocytes, leading to pathological remodeling of the endplate. Mechanistically, YAP/TEAD complexes consistently function by occupying the Ccl3 promoter region and regulating its transcriptional activity (141). Although these studies focused on different cellular processes and produced tissue-specific effects, they both highlighted that YAP-dependent regulation of CC chemokine transcription is a conserved mechanotransductive mechanism. This mechanism allows mechanical stress to direct immune cell recruitment and target tissue remodeling in a context-dependent manner.

In summary, the Piezo1–YAP/TAZ–CCL2/CCR2 signaling pathway discussed here should be viewed as a hypothesis-generating framework rather than a confirmed pathway in FS. Currently, there is no direct evidence from FS tissues or disease models demonstrating that Piezo1-driven YAP/TAZ activation enhances CCL2/CCR2-dependent macrophage recruitment and functional reprogramming. This concept is inferred from mechanistic studies on various fibrotic disorders. Nevertheless, growing clinical and biomechanical data underscore the significant role of mechanical stress in the development of capsular fibrosis in FS. Future research using FS-specific samples and experimental models is required to determine whether this mechanotransduction axis functions in the glenohumeral capsule and to clarify its translational relevance in clinical settings.

FS is a chronic and progressive fibrotic condition marked by ongoing pain and restricted joint movement, which significantly diminishes the quality of life and hampers functional autonomy. Although some patients experience gradual improvement, many endure prolonged disability and economic burdens. Therefore, symptom relief and shortening of the disease course remain key clinical priorities. Current treatments, including physical therapy, manual manipulation, nonsteroidal anti-inflammatory drugs, intra-articular steroid injections, capsular distension, and arthroscopic synovectomy, are frequently used to reduce pain and moderately improve the glenohumeral function. However, multimodal combinations are often necessary to achieve satisfactory outcomes and involve procedural risks, while lacking strategies that address the underlying fibrotic mechanisms (3, 142, 143). Accumulating evidence implicates macrophages as central regulators of FS pathogenesis and highlights their potential as mechanistic nodes influenced by diverse interventions. Exploring the impact of existing or emerging therapies on macrophage activity may provide a foundation for developing more precise mechanism-oriented treatment strategies in the future. Given the anatomically confined nature of the glenohumeral capsule and spatially localized inflammatory–fibrotic lesions in FS, therapeutic strategies that consider both immunoregulatory mechanisms and delivery context may be particularly relevant.

FS is a musculoskeletal disorder marked by the progressive development of chronic inflammation and fibrosis, ultimately resulting in contraction and adhesion of the glenohumeral capsule. In the initial stages, aseptic inflammation within the capsule prompts the sustained release of cytokines and chemokines, which subsequently induces fibroblast activation and abnormal ECM accumulation. As the condition progresses, inflammatory signals and fibrosis remodeling mutually reinforce each other and gradually establish a pathological circuit that is challenging to self-limit. Throughout this process, macrophages play a pivotal role as central hubs linking the amplification of inflammation and progression of fibrosis, exhibiting clear situational dependence and stage-specific responses. Their recruitment, polarization, and functional status are synergistically regulated by various microenvironmental factors, including inflammatory mediators, chemotactic signals, and mechanical stimuli, and they display distinct temporal patterns. Recognizing this dynamism can facilitate the identification of intervention windows at specific disease stages.

While previous research has underscored the critical pathological role of macrophages in FS, the mechanisms driving the phenotypic switching of these communities and their functional patterns across different pathological environments require further exploration. A key unresolved question is not only which macrophage programs are involved, but also where these programs are located within the capsular lesion and how their spatial organization correlates with the transition from inflammation to fibrosis. Spatially resolved multiomics marks a significant methodological advancement that enables the coordinated assessment of multiple molecular layers, such as transcripts, proteins, and tissue architecture, within intact tissue sections. By maintaining spatial context, this approach allows for the delineation of lesion microdomains and direct inference of immune–stromal neighborhood relationships in situ. When paired with artificial intelligence (AI)-assisted analytical frameworks, these high-dimensional spatial datasets can be systematically integrated through automated spatial clustering, domain identification, and cell–cell interaction inference, thereby enhancing their scalability, reproducibility, and interpretability across samples (182). In the context of FS, spatial multiomics could potentially elucidate whether distinct macrophage subsets preferentially localize to specific capsular regions, such as the synovial lining, subsynovial layer, or areas of significant ECM accumulation and increased stiffness, and whether these cells are spatially adjacent to fibroblasts or other immune populations. Furthermore, comparisons across disease stages may provide insights into whether inflammatory and fibrotic programs tend to occupy separate capsular regions or emerge sequentially within the same microenvironment, and whether the dominant pathological focus shifts spatially as FS progresses. Through AI-enabled integration, such spatially anchored information may support more refined, stage-relevant mechanistic models and help guide the prioritization of locally targeted and stratified intervention strategies for FS.

Despite these advances, the current understanding of macrophage mechanisms in the progression from inflammation to fibrosis remains incomplete, resulting in interventions that primarily focus on blocking inflammation or inhibiting recruitment, without precise regulatory strategies for their functional status and dynamic evolution. Existing evidence indicates that relying solely on pharmacological approaches to block inflammation or inhibit macrophage recruitment is insufficient for reversing existing tissue fibrosis. Future research should focus on the precise reprogramming and local regulation of pathogenic macrophage subpopulations at various stages of disease. This approach aims to minimize systemic adverse reactions while enhancing therapeutic specificity. Achieving this will likely require engineered platforms capable of spatially confined and temporally controlled modulation within the capsular microenvironment. Recent advancements in bioengineering indicate that controllable, locally retained delivery systems may provide a practical means of achieving this modulation. Microsphere-based carriers offer a promising approach for translating localized and sustained drug delivery concepts into FS treatment. Biodegradable polymer microspheres can achieve high drug-loading efficiency and programmable release kinetics, allowing therapeutic agents to remain within the target tissue for extended periods while minimizing systemic exposure (183). These features are particularly relevant for FS, where pathological inflammation and fibrosis are confined to the glenohumeral capsule, and short-lived intra-articular exposure may not be sufficient for durable immunomodulatory effects. Notably, microelectromechanical system–based bioprinting has been proposed as a scalable fabrication method to enhance microsphere uniformity and batch-to-batch consistency, potentially leading to more predictable release profiles and improving the translational feasibility of localized sustained-release strategies in capsular diseases (183). Simultaneously, advances in three-dimensional (3D) printing for tissue engineering, particularly in bone regeneration, illustrate a broader principle in which material composition and structural design can be deliberately programmed to jointly modulate mechanical cues and biologically active signals. Studies using polylactic acid/nano-hydroxyapatite composite scaffolds further modified with bioactive ions, such as magnesium or lithium, have shown that precisely engineered material properties can be coupled with pathway-level activation of osteogenic and angiogenic programs, supporting the concept of 3D bioprinting as a platform for microenvironmental regulation beyond structural fabrication (184–186). Although FS is characterized by fibrotic remodeling rather than tissue loss, the glenohumeral capsule remains a mechanically sensitive soft-tissue compartment that requires dynamic compliance under complex loading. Therefore, these engineering concepts may inform FS-oriented designs of minimally invasive, soft-tissue-compatible biomaterials that combine controlled drug loading and sustained local release with modulation of lesion mechanics, potentially creating conditions that are more permissive for stage-appropriate macrophage remodeling. In future FS-oriented investigations, incorporating macrophage-centered readouts alongside conventional clinical endpoints may provide a more informative framework for assessing the biological impact of such platforms.

It is crucial to emphasize that local biomechanical steady-state disruption and increased capsular stiffness significantly contribute to the progression of FS. However, effective targeted strategies that directly address these biomechanical abnormalities are lacking. Current FS management primarily relies on invasive procedures, which are associated with procedural risks, limited durability of benefits, and substantial economic burden. Therefore, reducing systemic exposure and procedural risks while developing more adaptable and stage-sensitive treatment strategies remains a key goal for improving FS care. Non-invasive interventions, particularly device-based mechanical stretching, offer advantages in terms of repeatability, safety, and patient adherence, making them attractive candidates for broader clinical adoption in FS. Beyond their mechanical effects, such approaches may indirectly influence the local immune microenvironment by improving tissue compliance and restoring the capsular mechanical balance. Recent advances in digital health research have further highlighted how the integration of flexible sensing technologies with AI can enhance rehabilitation by enabling continuous physiological monitoring, data-driven adjustments, and scalable remote management. Flexible sensors can capture high-sensitivity biomechanical and electrophysiological signals over time, whereas AI-based analytical frameworks are well-suited for interpreting the resulting complex and high-dimensional datasets, thereby supporting adaptive control and individualized rehabilitation strategies (187). Against this backdrop, the application of AI-assisted flexible sensing may hold particular promise for refining non-invasive mechanical interventions in FS. When incorporated into device-based stretching systems, such platforms could potentially enable real-time detection of insufficient or excessive mechanical loading, support individualized adjustment of stretching intensity and frequency, and characterize patient-specific response patterns across the treatment and recovery phases. Coupled with remote data integration and cloud-based management, these technologies may also facilitate home-based rehabilitation and longitudinal monitoring, with potential benefits for the accessibility, adherence, and personalization of FS care. Nevertheless, although non-invasive mechanical stretching is already applied in FS management, the biological mechanisms through which mechanical interventions influence macrophage behavior and fibrotic remodeling within the capsular microenvironment remain insufficiently understood. Whether emerging sensor-assisted and AI-supported rehabilitation systems can meaningfully enhance these mechanobiological effects remains an open question, highlighting the need for future studies that integrate mechanical, immunological, and functional readouts in FS.

Overall, staged research integrating immunology and mechanobiology, along with emerging bioengineering and digital health approaches, has the potential to yield more actionable and precise intervention strategies for FS, providing a conceptual framework that links fundamental mechanisms with clinically translatable solutions. .