Tumor necrosis factor α (TNFα) is initially sequestered to the cell membrane as a transmembrane protein that is then shed as soluble TNFα via ectodomain cleavage by TNFα-converting enzyme (TACE) (Black et al., 1997; Moss et al., 1997). Soluble and membrane-bound TNFα exerts downstream signaling through binding and activation of either TNF receptor 1 (TNFR1) or TNFR2 (Locksley et al., 2001). TNFR1 is expressed broadly with activation of death-domains (TRADD) upon binding of TNFα leading to inflammation (via NFκB, MAPK), apoptosis (via caspase 9), or necroptosis (via RIPK3) (Hsu et al., 1995; Kalliolias and Ivashkiv, 2016). In contrast, TNFR2 is more selectively expressed in certain populations of leukocytes, endothelial, cardiac, and neural cells (Ware et al., 1991; Irwin et al., 1999; McCoy and Tansey, 2008; Kalliolias and Ivashkiv, 2016). TNFR2 is specifically activated by membrane-bound TNFα and signals through TRADD-independent, TNF-receptor associated factor 2 (TRAF2) (Grell et al., 1995). Upon activation, TNFR2 promotes cell survival through MAPK, NFκB, and AKT pathways (Kalliolias and Ivashkiv, 2016; Yang et al., 2018). Therefore, one model of TNF-mediated inflammation hypothesizes that TNFR1 signaling promotes inflammation while TNFR2 maintains homeostasis, with potential implications for tissue regeneration (Yang et al., 2018).

In human Achilles tendon, TNFR1 and TNFR2 was observed by immunohistochemistry in both control and tendinosis samples (Gaida et al., 2012). While some studies show TNFα in samples of human tendinopathy, others do not (Gaida et al., 2012; Ackermann et al., 2013). In animal models of acute tendon injury, TNFα expression increased within the first 9 days of injury and declined within 2 weeks (Morita et al., 2017). In vitro, stimulation of tenocytes with TNFα results in increased expression of adhesion proteins and inflammatory cytokines (IL6, IL8), that may add to the inflammatory milieu in tendinopathy, independent of immune cells (Stolk et al., 2017). However, TNFα depletion with etanercept following injury in rats failed to improve or worsen tendon healing measured by peak force (Sandberg et al., 2012). In a rat model of rotator cuff repair, TNFα blockade with infliximab improved tendon to bone repair, suggesting TNFα therapies may differ in efficacy. Indeed, infliximab binds both soluble and membrane bound TNFα more stably, while etanercept has less affinity for TNFα. Furthermore, since infliximab is a complement fixing antibody, cells expressing TNFα undergo lysis, which does not occur with etanercept treatment (Mpofu et al., 2004). Therefore, treatment strategies for modifying TNFα signaling will need to consider differences in pharmaceutical mechanisms of action. Lastly, age-dependent differences in TNFα signaling are also likely to modify treatment. A recent study in a repetitive-use tendinopathy model of rat flexor tendons, observed increased TNFα in younger rats (Kietrys et al., 2012).

An acute phase reactant, interleukin-1β (IL1β) is a potent inflammogen that is critical for response to injury, infection, and malignancy (Lopez-Castejon and Brough, 2011; Bent et al., 2018). Along with IL1α, IL18, IL33, IL36α/β/γ, IL1β is a member of the IL1 cytokine family (Garlanda et al., 2013). Unlike the preformed alarmins IL1α and IL33, precursor-IL1β (pre-IL1β) is secreted as a 31 kDa proprotein in response to toll-like receptor (TLR) activation, complement signaling, or secondary cytokine (TNFα, IL1α) stimulation (Lopez-Castejon and Brough, 2011; Garlanda et al., 2013). Following cleavage by caspase-1, pre-IL1β is secreted extracellularly in its active form (Thornberry et al., 1992). Among the IL1 cytokine family, there are 10 distinct receptors (IL1R1 thru IL1R10) (Boraschi and Tagliabue, 2013). Following binding of IL1β to IL1R1, the ligand-receptor dimerizes with IL1R3, bringing together Toll-interleukin receptor (TIR) domains and binding of MyD88 with downstream signaling through activation of MAPK or NFκB (Dinarello, 2018).

In clinical samples of tendinopathy, the presence of IL1β is ambiguous. Microdialysis of human Achilles tendon 2 weeks following repair of acute rupture showed no significant difference compared to uninjured tendons (Ackermann et al., 2013); however, early stage subscapularis tendinopathy showed increased expression of IL1β (Abraham et al., 2019). In preclinical studies, IL1β is generally observed in the early stages of tendon injury with stimulation leading to increased expression of IL6 and IL8 (Morita et al., 2017; Stolk et al., 2017). In rats, in vivo fatigue testing of patellar tendons resulted in increased expression of MMP13 and IL1β, with observed microstructural damage (Sun et al., 2008). Interestingly in bioartificial tendon constructs, IL1β decreased ultimate tensile strength and elasticity (Qi et al., 2006a). In tenocytes, silencing of IL1β signaling decreased load-dependent MMP13 expression, suggesting that IL1β may drive MMP13-mediated ECM degradation (Corps et al., 2004; Qi et al., 2006a, b; Sun et al., 2008). In mouse tenocytes, constitutively active NFκB signaling sensitized cells to IL1β, with increased expression of TNFα, prostaglandin-endoperoxide synthase 2 (PTGS2), granulocyte colony-stimulating factor (G-CSF) and CXCL2 following IL1β stimulation (Abraham et al., 2019). Knockdown of NFκB signaling desensitized tenocytes to IL1β insult (Abraham et al., 2019). More directly, deletion of NFκB in mice resulted in increased apoptosis of myofibroblasts indicating NFκB signaling promotes myofibroblast survival (Best et al., 2020). In humans, tenocytes isolated from tendon biopsies 2 to 4 years post-treatment demonstrated increased expression of IL6, IL8, and activated fibroblast marker podoplanin (PDPN) when stimulated with IL1β, in comparison to IL1β-treated healthy controls (Dakin et al., 2017). Together, these data indicate that tendon healing may be impeded in the background of persistent NFκB signaling via increased IL1β sensitivity, and that inflammation is a feature of chronic tendinopathy.

Interferon gamma (IFNγ) is a cytokine involved in orchestrating the innate and adaptive immune response to bacterial, fungal, and viral pathogens (Bhat et al., 2018). IL12 and IL2 costimulation of Th0 (naive) T cells drives differentiation into Th1 cells, marked by secretion of IFNγ (Del Prete et al., 1994; Spellberg and Edwards, 2001). In addition, IFNγ can be produced by ILC1s and CD8⁺ cytotoxic T cells (CTLs) (Klose et al., 2014; Bhat et al., 2017). IFNγ binds to the IFNγ Receptor Complex (IFNGR), which leads to receptor dimerization and downstream expression of interferon stimulated genes (ISGs) and interferon response factors (IRF) via JAK-STAT signaling (Bhat et al., 2018). IRFs and ISGs act broadly to promote innate and adaptive type I inflammatory responses, including increasing inflammatory cytokine production, nitric oxide synthase (important to generate free radicals to kill pathogens), antigen presentation, NK cell activity, and leukocyte migration (Volk et al., 1986; Martin et al., 1994; Bhat et al., 2018). Moreover, recent data indicates that IFNγ signaling leads to broad transcriptional and epigenetic signatures that favors differentiation of immune cells into a type I phenotype (e.g., M1-like macrophages and Th1 T cells) (Ivashkiv, 2018). Importantly, secretion of IFNγ is also important for negative regulation of type II inflammation (Gajewski and Fitch, 1988; Gajewski et al., 1988). For example, binding of IFNγ leads to phosphorylation of STAT1 that inhibits IL4 mediated STAT6 signaling via SOCS1 (Zhao et al., 2019). Conversely, phosphorylation of STAT6 via IL4, inhibits downstream STAT1 signaling via inhibition of IRF1, thereby blocking upstream IFNγ signaling (D’andrea et al., 1993; Ohmori and Hamilton, 1997; Ito et al., 1999; Zhao et al., 2019).

The role of IFNγ in tendinopathy and healing remains poorly characterized. While some studies show little to no observable levels of IFNγ in acute Achilles tendon rupture 2 weeks postoperatively, others show increased expression of IFNγ in early stage tendinopathy samples of human subscapularis (Ackermann et al., 2013; Abraham et al., 2019). These inconsistencies may be due to differences in mechanism of injury, tendon origin (Achilles vs. rotator cuff), or expression level (protein vs. gene expression). Moreover, a study comparing gene expression of cytokines across the proximal, middle, and distal torn edge of the supraspinatus tendon found significant differences in IL1β, IFNγ, IL4, and IL13 with corresponding changes in collagen I expression (Fabiś et al., 2014). Interestingly, IFNγ (type I cytokine) expression was greatest at the distal edge, and lowest at the proximal edge, while IL4 (type II cytokine) had an inverse expression profile. This suggests that local immune profiles may play mutually exclusive and distinct roles in different zones of the injured tendon that might be lost in aggregate profiling. The role of IFNγ in tendon healing remains unclear. In vitro, tenocytes isolated from tendinopathic and ruptured Achilles tendon have increased expression of interferon response cytokines when stimulated with IFNγ, compared to tenocytes derived from healthy control tendons (Dakin et al., 2018). However, given that the expression of cytokines and inflammatory genes were more dramatic when stimulated with other inflammatory cytokines (IL6, IL1β), it is possible that IFNγ may play a more dominant role in recruited immune cells vs. directly in tenocytes (Stolk et al., 2017; Dakin et al., 2018).

IL6 is a cytokine that typically signals infection or tissue injury, and stimulates production of acute phase reactants (e.g., fibrinogen, C-reactive protein, serum amyloid A, hepcidin) to promote host defenses (Castell et al., 1989). Paradoxically, the beneficial role of IL6 in tissue protection and healing has also been observed. In rats, IL6 administration prior to CCL4 challenge with partial hepatectomy promoted liver regeneration and survival (Tiberio et al., 2007, 2008). Similarly in the gut, IL6 mediated STAT3 signaling is required for epithelial healing in a mouse model of colitis (Grivennikov et al., 2009). Binding of IL6 to its receptor (IL6R), facilitates recruitment and dimerization of glycoprotein-130 (gp130) which leads to downstream JAK-SHP2-MAPK, JAK-AKT, and JAK-STAT3 signaling (Tanaka et al., 2014). IL6 has no appreciable affinity for gp130, therefore signaling requires expression of IL6R which is not ubiquitously expressed in all cells (Jostock et al., 2001; Wolf et al., 2014). Interestingly, recent studies have demonstrated a non-canonical mechanism of IL6 signaling known as trans-signaling. In trans-signaling, IL6R is shedded in a soluble form (sIL6R) following cleavage by ADAM17. This soluble form can then bind IL6 to stimulate gp130 signaling in cells that do not natively express IL6R (Rose-John, 2012). It is hypothesized that the paradoxical differences in inflammatory vs. reparative mechanisms of IL6 signaling may be explained by trans- vs. canonical IL6 signaling (Rose-John, 2012; Galun and Rose-John, 2013). In a series of elegant experiments, the contribution of trans- and canonical IL6 signaling was delineated by expressing the extracellular soluble domain of gp130 (sgp130), which was used to selectively block IL6 trans-signaling by binding to and sequestering sIL6R (Jostock et al., 2001; Rose-John, 2012). Using a cecal ligation and puncture (CLP) model, it was shown that blockade of IL6 trans-signaling but not global IL6 led to improved survival following CLP, indicating that trans-signaling may largely contribute to the inflammatory arm of IL6 signaling while canonical-membrane bound IL6 signaling may be largely responsible for improved healing (Barkhausen et al., 2011; Rose-John, 2012).

In T cells, the role of IL6 is similarly complex. IL6 promotes differentiation of naive Th0 cells into Th17 cells and inhibits Treg differentiation. IL6 also inhibits IFNγ-mediated differentiation of Th1 cells by inducing expression of SOCS1. In contrast, IL6 induces IL4 expression in an autocrine fashion that stimulates Th2 differentiation. Taken together, the role of IL6 in wound healing can be both reparative and inflammatory and mitigating factors will likely continue to be elucidated.

Given the wide role of IL6, there is much interest in its role in tendinopathy and tendon healing. In human specimens of acute Achilles tendon or rotator cuff rupture, IL6 expression by protein microdialysis and RNA gene expression was significantly increased when compared to uninjured tendons (Nakama et al., 2006; Ackermann et al., 2013). Importantly, inflammatory cytokines TNFα and IL1β are known inducers of IL6 expression in human tenocytes (Tsuzaki et al., 2003; John et al., 2010). However, in specimens of chronic tendinopathy, there is little evidence of elevated IL6, suggesting a more prominent role of IL6 in healing as a response to inflammation vs. as a driver (Morita et al., 2017). In IL6^–/– mice, tendons developed normally, however tendon injury results in impaired mechanical restoration and collagen organization (Lin T. W. et al., 2005; Lin et al., 2006). More broadly, it was proposed that IL6 may play a role in tendon homeostasis in addition to its role in response to acute tendon rupture. Following exercise, peritendinous IL6 concentration was increased 100-fold relative to serum concentration in human Achilles tendon (Langberg et al., 2002; Andersen et al., 2011; Gump et al., 2013). Consistent with this, cyclic loading of bovine tendon fascicles explants or human tenocytes in vitro led to an increase in IL6 expression (Skutek et al., 2001; Legerlotz et al., 2013). Interestingly, Achilles tendon production of procollagen propeptide increased in response to peritendinous injection of IL6 in humans (Andersen et al., 2011). Expression of IL6 was also increased in mouse Achilles tendons treated with collagenase (Ueda et al., 2019). In sum, IL6 plays an important role in tendon healing; however, the mechanisms by which IL6 signals tendon homeostasis and repair remain unclear.

Upon stimulation, type 2 innate and adaptive cells (e.g., eosinophils, Th2 T cells, ILC2s) release a milieu of type 2 cytokines, most notably IL4, IL5, and IL13. Among these, IL4 is a potent inducer of type 2 polarization and can suppress IFNγ-producing CD4⁺ T cells that promote type 1 polarization (Tanaka et al., 1993). Classically, induction of type 2 inflammation following parasitic infiltration results in IL4 production which promotes Th2 activation, IgE class switching, and eosinophil activation that cooperatively function in parasite elimination (Spellberg and Edwards, 2001; Henry et al., 2017). In parallel, IL4 functions broadly to activate resident macrophages and fibroblasts to repair damaged tissue resulting from parasitic insult. Since parasitic infiltration most commonly occurs via mucosal entry, much of what is known regarding IL4 mediated wound healing comes from studies of the mucosal lungs and gut. In the lungs, IL4 signaling following helminth infection results in phenotypic polarization of macrophages to an alternatively activated (M2) fate (Chen et al., 2012). M2 macrophages facilitate tissue repair with production of growth factors (TGFβ1, IGF1, VEGF, PDGF, and RELM), and suppress type 1 inflammatory cytokines (e.g., IL17A) via IL10 (Chen et al., 2012; Wynn and Vannella, 2016). More recently, it was revealed in the lung that surfactant protein A enhanced IL4 sensing in macrophages suggesting tissue specific regulatory loops that promote IL4 mediated tissue repair (Minutti et al., 2017). In the gut, conditioned media from IL4-stimulated M2 macrophages improved epithelial barrier integrity in a mouse model of inflammatory colitis (Jayme et al., 2020). Consistent with these organ systems, the major contributor of IL4 mediated tissue repair is dependent on macrophage polarization to an M2 fate, with subsequent scavenging of apoptotic bodies and growth factor release (Bosurgi et al., 2017). In non-lymphoid muscle, however, production of IL4 from infiltrating eosinophils directly controls fate switching of resident fibroadipogenic progenitor cells to promote myogenesis and inhibit adipogenesis (Heredia et al., 2013).

Since IL4 is critical for wound healing in various tissues, recent studies have sought to elucidate potential roles for IL4 in tendon repair. In human samples of torn supraspinatus tendons, IL4 and IFNγ are counter-expressed with high expression of IL4 at the proximal segment of the torn tendon that decreases distally. Interestingly, cell proliferation and collagen I expression follows a pattern of expression similar to IL4 suggesting shared responses to injury (Courneya et al., 2010). In IL4^–/– mice, only minor differences in stress relaxation and collagen organization were observed after tendon injury, which may suggest potential compensation by other cytokines such as IL13 and IL10 (Lin et al., 2006). Alternatively, since the pro-regenerative effect of IL4 is likely mediated via macrophages, impaired IL4:M2 macrophage polarization associated with aging may explain minimal differences between WT and IL4^–/– mice (Mahbub et al., 2012). In bone fracture healing, aging was associated with increased activation of M1 macrophages associated with increased expression of TNFα following stimulation, compared to macrophages from younger mice. Moreover in adult mice, blockade of macrophage recruitment improved bone fracture healing (Gibon et al., 2016; Clark et al., 2020). These studies, conducted in bone, suggest that IL4:macrophage polarization may be restricted neonatally, with aging resulting in an M1 macrophage bias that is associated with poor functional healing. While M2 macrophages are known to improve tendon healing, future studies to compare differences in neonatal and adult healing may shed further light on immune mechanisms that promote regeneration (Mauro et al., 2016).

IL5 is a type 2 cytokine that plays a critical role in the activation and survival of eosinophils (Adachi and Alam, 1998). Binding of IL5 ligand to the cognate receptor drives eosinophil activation/degranulation and survival. Activation of Lyn, Syk, and JAK2 by IL5 promotes survival, while Raf1 kinase is primarily involved in degranulation and expression of adhesion proteins (van der Bruggen et al., 1995; Yousefi et al., 1996; Pazdrak et al., 1998). Clinically, IL5:eosinophil activation is classically associated with atopic diseases, most notably asthma (Adachi and Alam, 1998). However more recently, several studies have uncovered novel roles for eosinophils in wound healing. Shortly after injury, recruited eosinophils assist in thrombosis and platelet plug formation to assist in hemostasis (Coden and Berdnikovs, 2020). Additionally, eosinophils exposed to fibrogen undergo degranulation, releasing polarizing type 2 cytokines that promote pro-regenerative M2 macrophages (Heredia et al., 2013). In muscle following injury, eosinophil deficient mice exhibit increased fatty degeneration with poor debris clearance, which was shown to be mediated via FAP:IL4 signaling (Yang et al., 1997; Heredia et al., 2013). While eosinophil recruitment is critical for restorative tissue repair, excessive eosinophilia can also be deleterious to healing. In mice, overexpression of IL5 with characteristic eosinophilia showed delayed epithelial wound healing with impaired collagen deposition and gap closure (Yang et al., 1997). Therefore, it is likely that pro-regenerative healing requires a well orchestrated cascade of signals that must be properly balanced to promote healing. Currently, there are no studies that have investigated the role of IL5 in tendon repair. However, given the role of IL5 in wound healing in other systems, it is likely that IL5 is also present in tendon injury. Future experiments that investigate the role of IL5 will provide valuable insights into the immunobiology of tendon healing.

Similar to IL4, the role of IL13 in tissue healing has been heavily studied. In parasitized organs, release of IL13 by Th2 cells promotes activation of tissue resident fibroblasts to polarize macrophages toward a M2 phenotype (Murray and Wynn, 2011; O’Reilly et al., 2016). In contrast, following wounding in the lung, alveolar epithelial cells secrete IL13 suggesting differences in upstream mediators that distinguish septic vs. aseptic inflammation (Allahverdian et al., 2008). IL13 signals via an IL4Rα and IL13Rα1 heterodimer to activate the transcription factors STAT3 and STAT6, which act broadly to activate healing responses in the gut, neonatal heart, and lung (Hershey, 2003; Allahverdian et al., 2008; Yang et al., 2019; Zhu et al., 2019). In the gut, IL13 is produced by ILC2s in response to parasitic infection or tissue damage. Interestingly, stimulation of Lgr5⁺ intestinal crypt stem cells by IL13 results in expression of Foxp1 which stabilizes nuclear translocation of β-catenin, thereby driving intestinal stem cell proliferation and self-renewal (Bouchery et al., 2019; Zhu et al., 2019). Similarly, in neonatal heart regeneration, activation of ERK/AKT signaling by IL13 drives cardiomyocyte proliferation that is blocked in IL13^–/– mice (Wodsedalek et al., 2019). In the lung, production of IL13 by Tregs results in macrophage polarization toward a Ly6c^lo (reparative) phenotype and secretion of amphiregulin, which drives proliferation of type II alveolar epithelial cells (Liu et al., 2019).

Given the broad role of IL13 in tissue repair, it is unsurprising that several studies also implicate IL13 in tendon repair. Histological assessment and gene expression of IL13 in human tendinopathy samples demonstrated robust presence of IL13 indicating clinical relevance (Fabiś et al., 2014; Akbar, 2018). In vitro stimulation of human tenocytes with IL13 demonstrated a twofold increase in proliferation, in a dose-responsive fashion (Courneya et al., 2010; Akbar, 2018). Future studies may further determine whether IL13 mediates tendon healing via macrophage polarization, as has been shown in other tissues.

Part of the IL1 cytokine family, IL33 is a type 2 cytokine that plays an important role in innate and adaptive immunity. Despite the relatively recent discovery of IL33, numerous studies have demonstrated broad and robust roles of IL33 in homeostasis and disease (Liew et al., 2016; Jin et al., 2020). An alarmin, IL33 is produced by tissue resident stromal cells and sequestered in the nucleus. Following damage induced necroptosis, release of IL33 triggers downstream responses in ILC2s, macrophages, Tregs, and other innate immune cells (Liew et al., 2016; Jin et al., 2020). IL33 binds to the suppression of tumorigenicity 2 (ST2) receptor which leads to downstream activation of NFκB and MAPK signaling via activation of MYD88, IRAK1/4, and TRAF6 (Schmitz et al., 2005; Liew et al., 2016). Non-canonically, nuclear IL33 can act in an intracrine fashion by binding to chromatin or inactivating transcription factors to modulate gene expression (Carriere et al., 2007; Ali et al., 2011). Interestingly, this second layer of IL33 signaling has been shown to suppress IL1β mediated expression of target genes IκBα, TNFα, and CREL by sequestration of the p65 subunit involved in NFκB signaling (Ali et al., 2011).

With respect to wound healing, in the past 5 years several studies showed that IL33 is critical for tissue repair. Broadly, release of IL33 following injury results in polarization or recruitment of type 2 immune cells that release growth factors to stimulate resident stem and progenitor cell proliferation. Interestingly, Tregs seem to have an essential role in mediating IL33-driven tissue repair in the gut, brain, muscle, and lung (Burzyn et al., 2013; Monticelli et al., 2015; Ito et al., 2019; Liu et al., 2019). In all four systems, activation via the ST2 receptor results in release of amphiregulin from Tregs that promotes tissue repair, suggesting shared healing programs across tissues with diverse origins. In the gut, IL33 also stimulates the release of amphiregulin by ILC2s which promotes intestinal stem cell proliferation (Monticelli et al., 2015). In the lung and brain, IL33 seems to be dependent on a Treg:macrophage axis. Treg activation via IL33 was critical for functional recovery and promoted polarization of pro-regenerative Ly6c^lo macrophages and neuroprotective microglia in the lung and brain, respectively (Ito et al., 2019; Liu et al., 2019). Recently, dysregulated IL33 signaling associated with aging was revealed as a contributor to poor muscle healing in mice. In aged (>20 month) mice, decreased IL33 signaling was associated with impaired Treg recruitment and increased fatty infiltration, resulting in poor muscle repair (Kuswanto et al., 2016). These data highlight differences in immunity with age that may contribute to regenerative vs. fibrotic healing. In tendon, the role of IL33 in repair has not been fully characterized. However, IL33 is present in human samples of tendinopathy (Millar et al., 2015). Strikingly, in contrast to other tissues, knockout of IL33 or ST2 resulted in improved tendon healing in adult mice. Mechanistically, IL33 in adult mice promoted a switch from type I to type III collagen, resulting in mechanically inferior tendon, which was rescued with deletion of IL33 signaling (Millar et al., 2015). In chronic parasitic infections that invoke a sustained type 2 response, chronically elevated IL33 contributes to liver fibrosis (Peng et al., 2016). Therefore, it is possible that while IL33 may be required for tissue repair, if it is not resolved, sustained IL33 signaling may paradoxically contribute to fibrosis. Future studies that further elucidate the functions of IL33 signaling in tendon healing will undoubtedly add to the understanding of tissue regeneration and disease.

In mammals, transforming growth factor beta (TGFβ) exists in three isoforms (TGFβ1, TGFβ2, TGFβ3) which together have diverse roles in proliferation, differentiation, apoptosis, and migration that are covered with great detail in other excellent reviews (Massagué, 1998; Derynck and Budi, 2019). TGFβ is produced by cells in an inactive state where it is bound to latency associated peptide (LAP) forming a latency complex (Worthington et al., 2011). Large latency complexes (LLC) bind to ECM, where they can be readily cleaved and available to receptive cells. In this manner, abundant sources of TGFβ are readily available with minimal transcriptional or translational latency. Following injury, ECM disruption liberates TGFβ ligands which can then stimulate downstream signaling. Importantly, activated TGFβ has important roles in immune modulation that may have implications in wound healing.

The role of TGFβ in inflammation is generally considered to promote immune tolerance or decrease inflammation (Ulloa et al., 1999; Park et al., 2007). In conjunction with naive CD4⁺ T cell stimulation, TGFβ drives expression of Foxp3 that results in induction of Treg fate (Lu et al., 2010). Tregs provide anti-inflammatory signals (e.g., IL10) that prevent overactive inflammation (Palomares et al., 2014). Moreover, there is extensive counter-regulation among TGFβ and IFNγ that regulate the balance between type 2 (e.g., Th2, M2 macrophage) vs. type 1 polarity (e.g., Th1, M1 macrophage) of leukocytes. Repression of Tbet by TGFβ via MEK/ERK signaling suppresses IFNγ, while upregulation of SMAD7 inhibits SMAD3/6 activation by TGFβ (Ulloa et al., 1999; Lin J. T. et al., 2005; Park et al., 2007). In tendon, latent TGFβ is present in the tendon matrix and is critical for tendon maintenance and regeneration in neonatal mice (Maeda et al., 2011; Kaji et al., 2020; Tan et al., 2020). However, while it is clear that TGFβ signaling in Scx⁺ tenocytes is critical for tendon healing as well as scar and adhesion formation (Katzel et al., 2011; Goodier et al., 2016; Kallenbach et al., 2021), it is unclear whether TGFβ released following ECM disruption during injury plays an additional role in immune modulation.