Tendon healing in interleukin-4 and interleukin-6 knockout mice

Il-6 signaling exacerbates hallmarks of chronic tendon disease by stimulating reparative fibroblasts

Tendinopathies are debilitating diseases currently increasing in prevalence and associated costs. There is a need to deepen our understanding of the underlying cell signaling pathways to unlock effective treatments. In this work, we screen cell signaling pathways in human tendinopathies and find positively enriched IL-6/JAK/STAT signaling alongside signatures of cell populations typically activated by IL-6 in other tissues. In human tendinopathic tendons, we also confirm the strong presence and co-localization of IL-6, IL-6R, and CD90, an established marker of reparative fibroblasts. To dissect the underlying causalities, we combine IL-6 knock-out mice with an explant-based assembloid model of tendon damage to successfully connect IL-6 signaling to reparative fibroblast activation and recruitment. Vice versa, we show that these reparative fibroblasts promote the development of tendinopathy hallmarks in the damaged explant upon IL-6 activation. We conclude that IL-6 activates tendon fibroblast populations which then initiate and deteriorate tendinopathy hallmarks.

Blood flow restriction-enhanced platelet-rich plasma: A pilot randomised controlled trial protocol

Purpose

To assess platelet-rich plasma (PRP) changes in platelet and leucocyte count, insulin-like growth factor 1 (IGF-1), and interleukin 6 (IL-6) concentration after bilateral low-load knee extensions under blood flow restriction (BFR).

Methods

The present randomised controlled trial protocol will include two groups: the intervention group, which will undergo bilateral knee extensions under BFR, and the control group, which will perform bilateral knee extensions without BFR. Participants will be randomly allocated in a 1:1 ratio. Twenty-two healthy individuals will be enrolled if the predefined inclusion criteria are met: (1) males, (2) ages 18-40, (3) Tegner activity level ≥5 and (4) with no musculoskeletal conditions that would interfere with exercise. Exclusion criteria include (1) individuals with systemic inflammatory diseases, (2) cardiovascular risk factors, (3) any blood dyscrasia, (4) Tegner Activity scale scores <5, (5) under nonsteroidal anti-inflammatory drugs and aspirin treatment within one week before testing or (6) that had previously performed exercises on the testing day. The participant will perform low-load bilateral knee extensions under BFR following a standard protocol of 30-15-15-15 repetitions of consecutive sets with 30-s rest intervals at 80% of limb occlusive pressure and 30% of 1-RM load. PRP platelet and leucocyte count, IGF-1 and IL-6 concentration measurements (via flow cytometry, chemiluminescence testing and immunochromatography, respectively) will be conducted before exercise and 10, 20 and 30 min after the intervention.

Results

The expected outcome is that the standard protocol of low-load bilateral knee extensions under BFR will increase the platelet and leucocyte count, IGF-1 and IL-6 in the PRP preparation.

Conclusion

The current protocol allows the study of an enhanced PRP formulation for its potential implementation in multiple sports injuries.

Decorin Knockdown Improves Aged Tendon Healing by Enhancing Recovery of Viscoelastic Properties, While Biglycan May Not

The objective of the study was to determine the specific roles of decorin and biglycan in the early and late phases of tendon healing in aged mice. Aged (300 day-old) female wildtype (WT), Dcnflox/flox (I-Dcn-/-), Bgnflox/flox (I-Bgn-/-), and compound Dcnflox/flox/Bgnflox/flox (I-Dcn-/-/Bgn-/-) mice with a tamoxifen (TM) inducible Cre underwent a bilateral patellar tendon injury (PT). Cre excision of the conditional alleles was induced at 5 days (samples collected at 3 and 6 weeks) or 21 days post-injury (samples collected at 6 weeks). Scar tissue area, collagen architecture, gene expression and mechanical properties were assessed during re-establishment of tendon architecture after injury. Fibril diameter distribution was impacted by both decorin and biglycan knockdown at 3 and 6 weeks compared to WT. Although early healing appeared to be delayed in the I-Bgn-/- tendons (larger scar tissue area at 3 weeks), no differences in failure properties were detected. By 6 weeks, in the I-Dcn-/- tendons, we observed a better recovery of viscoelastic properties compared to the WT tendons (reduced stress relaxation and increased dynamic modulus) when the knockdown was induced early. This could be explained by the increased expression of other matrix proteins, such as elastin whose gene expression was increased at 3 weeks in the I-Dcn-/- tendons. Despite an impact on collagen fibrillogenesis, decorin and/or biglycan knockdown did not produce a detectable effect on quasi-static properties after patellar tendon injury. However, early decorin knockdown resulted in better recovery of viscoelastic properties. Mechanisms underlying this result remained to be clarified in further studies.

Bletilla striata Polysaccharide-Containing Carboxymethyl Cellulose Bilayer Structure Membrane for Prevention of Postoperative Adhesion and Achilles Tendon Repair

Postoperative tissue adhesion and poor tendon healing are major clinical problems associated with tendon surgery. To avoid postoperative adhesion and promote tendon healing, we developed and synthesized a membrane to wrap the surgical site after tendon suturing. The bilayer-structured porous membrane comprised an outer layer [1,4-butanediol diglycidyl ether cross-linked with carboxymethyl cellulose (CX)] and an inner layer [1,4-butanediol diglycidyl ether cross-linked with Bletilla striata polysaccharides and carboxymethyl cellulose (CXB)]. The morphology, chemical functional groups, and membrane structure were determined. In vitro experiments revealed that the CX/CXB membrane demonstrated good biosafety and biodegradability, promoted tenocyte proliferation and migration, and exhibited low cell attachment and anti-inflammatory effects. Furthermore, in in vivo animal study, the CX/CXB membrane effectively reduced postoperative tendon-peripheral tissue adhesion and improved tendon repair, downregulating inflammatory cytokines in the tendon tissue at the surgical site, which ultimately increased tendon strength by 54% after 4 weeks.

Advanced glycation end products impair the repair of injured tendon: a study in rats

BACKGROUND

The AGEs levels in tissues of diabetics and elderly tend to be higher than in normal individuals. This study aims to determine the effects of AGEs on Achilles tendon repair.

MATERIALS AND METHODS

Thirty-six male eight-week-old Sprague Dawley rats were selected in this study. The rats were randomly divided into two experimental groups and a control group after the transection of the Achilles tendon. During the tendon repair, the experimental groups were injected around the Achilles tendon with 350mmol/L (low dose group) and 1000mmol/L (high dose group) D-ribose 0.2 ml respectively to increase the AGEs level, while in the control group were given the same amount of PBS. The injections were given twice a week for six weeks. Collagen-I, TNF-α, and IL-6 expression in the healed Achilles tendon was assessed. Additionally, macroscopic, pathological, and biomechanical evaluations of Achilles tendon repair were conducted.

RESULTS

The repaired Achilles tendons in the high dose group showed severe swelling and distinctive adhesions. The histological score went up with the increase of the AGEs in the Achilles tendon (p<0.001). TNF- α and IL-6 in the Achilles tendon increased (p<0.001, p<0.001), and the production of collagen-I decreased with the accumulation of AGEs in the repaired Achilles tendon (p<0.001). The tensile strength of Achilles tendon in the high dose group was impaired significantly.

CONCLUSION

In current study, the compromised tendon repair model induced by AGEs was successfully established in rat. The study demonstrated that AGEs significantly impair Achilles tendon repair.

Biology and physiology of tendon healing

Tendon disorders affect people of all ages, from elite and recreational athletes and workers to elderly patients. After an acute injury, 3 successive phases are described to achieve healing: an inflammatory phase followed by a proliferative phase, and finally by a remodeling phase. Despite this process, healed tendon fails to recover its original mechanical properties. In this review, we proposed to describe the key factors involved in the process such as cells, transcription factors, extracellular matrix components, cytokines and growth factors and vascularization among others. A better understanding of this healing process could help provide new therapeutic approaches to improve patients' recovery while tendon disorders management remains a medical challenge.

Treatment of Equine Tarsus Long Medial Collateral Ligament Desmitis with Allogenic Synovial Membrane Mesenchymal Stem/Stromal Cells Enhanced by Umbilical Cord Mesenchymal Stem/Stromal Cell-Derived Conditioned Medium: Proof of Concept

Horses are high-performance athletes prone to sportive injuries such as tendonitis and desmitis. The formation of fibrous tissue in tendon repair remains a challenge to overcome. This impels regenerative medicine to develop innovative therapies that enhance regeneration, retrieving original tissue properties. Multipotent Mesenchymal Stem/Stromal Cells (MSCs) have been successfully used to develop therapeutic products, as they secrete a variety of bioactive molecules that play a pivotal role in tissue regeneration. These factors are released in culture media for producing a conditioned medium (CM). The aforementioned assumptions led to the formulation of equine synovial membrane MSCs (eSM-MSCs)-the cellular pool that naturally regenerates joint tissue-combined with a medium enriched in immunomodulatory factors (among other bioactive factors) produced by umbilical cord stroma-derived MSCs (eUC-MSCs) that naturally contribute to suppressing the immune rejection in the maternal-fetal barrier. A description of an equine sport horse diagnosed with acute tarsocrural desmitis and treated with this formulation is presented. Ultrasonographic ligament recovery occurred in a reduced time frame, reducing stoppage time and allowing for the horse's return to unrestricted competition after the completion of a physical rehabilitation program. This study focused on the description of the therapeutic formulation and potential in an equine desmitis treatment using the cells themselves and their secretomes.

Interleukin-6 upregulates extracellular matrix gene expression and transforming growth factor β1 activity of tendon progenitor cells

BACKGROUND

Prolonged inflammation during tendon healing and poor intrinsic healing capacity of tendon are causal factors associated with tendon structural and functional degeneration. Tendon cells, consisting of mature tenocytes and tendon progenitor cells (TPC) function to maintain tendon structure via extracellular matrix (ECM) synthesis. Tendon cells can succumb to tissue cytokine/chemokine alterations during healing and consequently contribute to tendon degeneration. Interleukin-(IL-)1β, IL-6 and TNFα are key cytokines upregulated in injured tendons; the specific effects of IL-6 on flexor tendon-derived TPC have not been discerned.

METHODS

Passage 3 equine superficial digital flexor tendon (SDFT)-derived TPC were isolated from 6 horses. IL-6 impact on the viability (MMT assay with 0, 1, 5 and 10 ng/mL concentrations), migration (scratch motility assay at 0, 10ng/mL concentration) of TPC in monolayer culture were assessed. IL-6 effect on tendon ECM and chondrogenic gene expression (qRT-PCR), TGFβ1 gene expression and activity (ELISA), and MMP-1, -3 and - 13 gene expression of TPC was evaluated.

RESULTS

IL-6 decreased TPC viability and migration. IL-6 treatment at 10 ng/mL significantly up-regulated TGFβ1 gene expression (6.3-fold; p = 0.01) in TPC, and significantly increased the TGFβ1 concentration in cell culture supernates. IL-6 (at 10 ng/mL) significantly up-regulated both tendon ECM (COL1A1:5.3-fold, COL3A1:5.4-fold, COMP 5.5-fold) and chondrogenic (COL2A1:3.9-fold, ACAN:6.2-fold, SOX9:4.8-fold) mRNA expression in TPC. Addition of SB431542, a TGFβ1 receptor inhibitor, to TPC in the presence of IL-6, attenuated the up-regulated tendon ECM and chondrogenic genes.

CONCLUSION

IL-6 alters TPC phenotype during in vitro monolayer culture. Pro- and anti-inflammatory roles of IL-6 have been implicated on tendon healing. Our findings demonstrate that IL-6 induces TGFβ1 activity in TPC and affects the basal TPC phenotype (as evidenced via increased tendon ECM and chondrogenic gene expressions). Further investigation of this biological link may serve as a foundation for therapeutic strategies that modulate IL-6 to enhance tendon healing.

Platelet-rich plasma in the pathologic processes of tendinopathy: a review of basic science studies

Tendinopathy is a medical condition that includes a spectrum of inflammatory and degenerative tendon changes caused by traumatic or overuse injuries. The pathological mechanism of tendinopathy has not been well defined, and no ideal treatment is currently available. Platelet-rich plasma (PRP) is an autologous whole blood derivative containing a variety of cytokines and other protein components. Various basic studies have found that PRP has the therapeutic potential to promote cell proliferation and differentiation, regulate angiogenesis, increase extracellular matrix synthesis, and modulate inflammation in degenerative tendons. Therefore, PRP has been widely used as a promising therapeutic agent for tendinopathy. However, controversies exist over the optimal treatment regimen and efficacy of PRP for tendinopathy. This review focuses on the specific molecular and cellular mechanisms by which PRP manipulates tendon healing to better understand how PRP affects tendinopathy and explore the reason for the differences in clinical trial outcomes. This article has also pointed out the future direction of basic research and clinical application of PRP in the treatment of tendinopathy, which will play a guiding role in the design of PRP treatment protocols for tendinopathy.

The impact of mild hypercholesterolemia on injury repair in the rat patellar tendon

Hypercholesterolemia is associated with tendon pathology and injury prevalence. Lipids can accumulate in the tendon's extracellular spaces, which may disrupt its hierarchical structure and the tenocytes physicochemical environment. We hypothesized that the tendon's ability to repair after injury would be attenuated with elevated cholesterol levels, leading to inferior mechanical properties. Fifty wild-type (sSD) and 50 apolipoprotein E knock-out rats (ApoE^{-/ -} ) were given a unilateral patellar tendon (PT) injury at 12 weeks old; the uninjured limb served as a control. Animals were euthanized at 3-, 14,- or 42-days postinjury and PT healing was investigated. ApoE^{-/ -} serum cholesterol was double that of SD rats (mean: 2.12 vs. 0.99 mg/mL, p < 0.001) and cholesterol level was related to the expression of several genes after injury; notably rats with higher cholesterol demonstrated a blunted inflammatory response. There was little physical evidence of tendon lipid content or differences in injury repair between groups, therefore we were not surprised that tendon mechanical or material properties did not differ between strains. The young age and the mild phenotype of our ApoE^{-/ -} rats might explain these findings. Hydroxyproline content was positively related to total blood cholesterol, but this result did not translate to observable biomechanical differences, perhaps due to the narrow range of cholesterol levels observed. Tendon inflammatory and healing activity is modulated at the mRNA level even with a mild hypercholesterolemia. These important initial impacts need to be investigated as they may contribute to the known consequences of cholesterol on tendons in humans.

A combination of omega-3 and exercise reduces experimental Achilles tendinopathy induced with a type-1 collagenase in rats

This study aimed to evaluate the effectiveness of omega-3 supplementation with exercise in a collagenase-induced Achilles tendinopathy (AT) rat model. Experimental groups (healthy control (HC), AT, exercise (Ex), omega-3 (W), and Ex+W) were randomly allocated. After a week of adaptation, oral omega-3 was initiated for 8 weeks (5 days/week). The exercise groups performed treadmill running for 30 min/day (5 days/week, 20 m/min, 8 weeks) following one week of adaptation (10 m/min, 15 min/day). Matrix metalloproteinase-13 (MMP-13), interleukin-1 beta (IL-1β), tumor necrosis factor-alpha (TNF-α), and total antioxidant-oxidant status (TAS) levels were determined in serum samples. Tendon samples were obtained for biomechanical, histopathological, and immunohistochemical assessments. Ultimate tensile force, yield force, stiffness values, collagen type-I alpha 1 expression, and serum TAS significantly decreased (P < 0.05) in AT vs. HC. These values and expression significantly increased in the Ex+W group vs. AT. Serum MMP-13, IL-1β, and TNF-α levels decreased in all treatment groups vs. AT. The most significant decrease was found in the Ex+W group (P < 0.01). Histopathologically, the improvement in degeneration was statistically significant in the Ex+W group (P < 0.05). Immunohistochemically, MMP-13, IL-1β, TNF-α, and nitric oxide synthase-2 expression was decreased in all treatment groups vs. AT. In conclusion, omega-3 and exercise might be recommended in AT patients.

Tumour necrosis factor alpha, interleukin 1 beta and interferon gamma have detrimental effects on equine tenocytes that cannot be rescued by IL-1RA or mesenchymal stromal cell-derived factors

Tendon injuries occur commonly in both human and equine athletes, and poor tendon regeneration leads to functionally deficient scar tissue and an increased frequency of re-injury. Despite evidence suggesting inadequate resolution of inflammation leads to fibrotic healing, our understanding of the inflammatory pathways implicated in tendinopathy remains poorly understood, meaning successful targeted treatments are lacking. Here, we demonstrate IL-1β, TNFα and IFN-γ work synergistically to induce greater detrimental consequences for equine tenocytes than when used individually. This includes altering tendon associated and matrix metalloproteinase gene expression and impairing the cells' ability to contract a 3-D collagen gel, a culture technique which more closely resembles the in vivo environment. Moreover, these adverse effects cannot be rescued by direct suppression of IL-1β using IL-1RA or factors produced by BM-MSCs. Furthermore, we provide evidence that NF-κB, but not JNK, P38 MAPK or STAT 1, is translocated to the nucleus and able to bind to DNA in tenocytes following TNFα and IL-1β stimulation, suggesting this signalling cascade may be responsible for the adverse downstream consequences of these inflammatory cytokines. We suggest a superior approach for treatment of tendinopathy may therefore be to target specific signalling pathways such as NF-κB.

Regulating Macrophages through Immunomodulatory Biomaterials Is a Promising Strategy for Promoting Tendon-Bone Healing

The tendon-to-bone interface is a special structure connecting the tendon and bone and is crucial for mechanical load transfer between dissimilar tissues. After an injury, fibrous scar tissues replace the native tendon-to-bone interface, creating a weak spot that needs to endure extra loading, significantly decreasing the mechanical properties of the motor system. Macrophages play a critical role in tendon-bone healing and can be divided into various phenotypes, according to their inducing stimuli and function. During the early stages of tendon-bone healing, M1 macrophages are predominant, while during the later stages, M2 macrophages replace the M1 macrophages. The two macrophage phenotypes play a significant, yet distinct, role in tendon-bone healing. Growing evidence shows that regulating the macrophage phenotypes is able to promote tendon-bone healing. This review aims to summarize the impact of different macrophages on tendon-bone healing and the current immunomodulatory biomaterials for regulating macrophages, which are used to promote tendon-bone healing. Although macrophages are a promising target for tendon-bone healing, the challenges and limitations of macrophages in tendon-bone healing research are discussed, along with directions for further research.

The inhibitory effect of tocilizumab on systemic bone loss and tendon inflammation in a juvenile Collagen-Induced arthritis rat model

PURPOSE OF THE STUDY

Reduced Bone Mineral Density (BMD) is a prevalent comorbidity in Juvenile Idiopathic Arthritis (JIA). Enthesitis and other tendon abnormalities, such as tenosynovitis, tendinitis and tendon ruptures are, also, common extra-articular manifestations of the disease. The aim of the present study was to investigate the effect of tocilizumab, an antibody that binds the Interleukin-6 (IL-6) Receptor, on inflammation-related bone loss and tendon inflammation in an animal model of JIA.

MATERIALS AND METHODS

The Collagen-Induced Arthritis (CIA) model was induced in male rats followed by intraperitoneal administration of tocilizumab for 8 weeks. Methotrexate, the most widely used Disease-Modifying Antirheumatic Drug in the management of JIA, was, also, administered, either as a monotherapy or as an add-on therapy to tocilizumab. BMD was evaluated with Micro-Computed Tomography (Micro-CT) and histopathological examination. Tendon damage was, also, assessed histologically. Finally, two pro-inflammatory cytokines, Tumor Necrosis Factor-alpha (TNF-a) and Interleukin-23 (IL-23) were quantified in tendon tissues by ELISA analysis.

RESULTS

Tocilizumab-treated animals exhibited a significantly improved trabecular microarchitecture on micro-CT analysis and histological examination. Tendon morphology was also improved. Anti-IL-6 treatment led to a significant decrease in TNF-a and IL-23 expression in tendon tissue.

CONCLUSIONS

The results of the present study provide evidence that tocilizumab reduces inflammation-related bone loss and suppresses tendon inflammation in a juvenile CIA rat model. These findings offer perspectives for the management of osteoporosis and enthesitis in JIA.

Tendon 3D Scaffolds Establish a Tailored Microenvironment Instructing Paracrine Mediated Regenerative Amniotic Epithelial Stem Cells Potential

Tendon tissue engineering aims to develop effective implantable scaffolds, with ideally the native tissue’s characteristics, able to drive tissue regeneration. This research focused on fabricating tendon-like PLGA 3D biomimetic scaffolds with highly aligned fibers and verifying their influence on the biological potential of amniotic epithelial stem cells (AECs), in terms of tenodifferentiation and immunomodulation, with respect to fleeces. The produced 3D scaffolds better resemble native tendon tissue, both macroscopically, microscopically, and biomechanically. From a biological point of view, these constructs were able to instruct AECs genotypically and phenotypically. In fact, cells engineered on 3D scaffolds acquired an elongated tenocyte-like morphology; this was different from control AECs, which retained their polygonal morphology. The boosted AECs tenodifferentiation by 3D scaffolds was confirmed by the upregulation of tendon-related genes (SCX, COL1 and TNMD) and TNMD protein expression. The produced constructs also prompted AECs’ immunomodulatory potential, both at the gene and paracrine level. This enhanced immunomodulatory profile was confirmed by a greater stimulatory effect on THP-1-activated macrophages. These biological effects have been related to the mechanotransducer YAP activation evidenced by its nuclear translocation. Overall, these results support the biomimicry of PLGA 3D scaffolds, revealing that not only fiber alignment but also scaffold topology provide an in vitro favorable tenodifferentiative and immunomodulatory microenvironment for AECs that could potentially stimulate tendon regeneration.

Defining the Profile: Characterizing Cytokines in Tendon Injury to Improve Clinical Therapy

Cytokine manipulation has been widely used to bolster innate healing mechanisms in an array of modern therapeutics. While other anatomical locations have a more definitive analysis of cytokine data, the tendon presents unique challenges to detection that make a complete portrayal of cytokine involvement during injury unattainable thus far. Without this knowledge, the advancement of tendon healing modalities is limited. In this review, we discuss what is known of the cytokine profile within the injured tendinous environment and the unique obstacles facing cytokine detection in the tendon while proposing possible solutions to these challenges. IL-1β, TNF-α, and IL-6 in particular have been identified as key cytokines for initiating tendon healing, but their function and temporal expression are still not well understood. Methods used for cytokine evaluation in the tendon including cell culture, tissue biopsy, and microdialysis have their strengths and limitations, but new methods and approaches are needed to further this research. We conclude that future study design for cytokine detection in the injured tendon should meet set criteria to achieve definitive characterization of cytokine expression to guide future therapeutics.

Selective Serotonin Reuptake Inhibitor Promotes Bone-Tendon Interface Healing in a Rotator Cuff Tear Rat Model

BACKGROUND

Selective serotonin reuptake inhibitor (SSRI) is believed to accelerate wound healing, and thus expected to have a positive effect on rotator cuff repair. We hypothesized that SSRI has a positive effect on the healing of the bone-tendon interface (BTI), and improved rotator cuff tear healing would be confirmed by mechanical strength measurements and histological assessment of the restored tendon.

METHODS

The study used 40 adult male Sprague-Dawley wild-type rats. The animals were divided into two groups: group-SSRI, the supraspinatus repair with SSRI injection group, and group-C, conventional supraspinatus repair only without SSRI. Biomechanical and histological analyses were performed 8 weeks after index rotator cuff surgery.

RESULTS

The ultimate load (N) was significantly higher in group-SSRI than in group-C (54.8 ± 56.9 Vs 25.1 ± 11.1, p = .031). In the histological evaluation, the Bonar score confirmed significant differences in collagen fiber density (group-C: 0.6 ± 0.5, group-SSRI: 1.1 ± 0.6, p = .024), vascularity (group-C: 0.1 ± 0.2, group-SSRI: 0.3 ± 0.4, p = .024) and cellularity (group-C: 1.7 ± 0.4, group-SSRI: 2.0 ± 0.0, p = .023) between the groups. Based on the total score, group-SSRI was significantly better compared with group-C (6.3 ± 2.7 Vs 4.3 ± 1.9, p = .019).

CONCLUSION

Our study demonstrated that SSRI could facilitate improved biomechanical and histological outcomes 8 weeks after rotator cuff repair in a rat model. Consequently, SSRI may improve healing after rotator cuff repair.

Tendon Immune Regeneration: Insights on the Synergetic Role of Stem and Immune Cells during Tendon Regeneration

Tendon disorders represent a very common pathology in today's population, and tendinopathies that account 30% of tendon-related injuries, affect yearly millions of people which in turn cause huge socioeconomic and health repercussions worldwide. Inflammation plays a prominent role in the development of tendon pathologies, and advances in understanding the underlying mechanisms during the inflammatory state have provided additional insights into its potential role in tendon disorders. Different cell compartments, in combination with secreted immune modulators, have shown to control and modulate the inflammatory response during tendinopathies. Stromal compartment represented by tenocytes has shown to display an important role in orchestrating the inflammatory response during tendon injuries due to the interplay they exhibit with the immune-sensing and infiltrating compartments, which belong to resident and recruited immune cells. The use of stem cells or their derived secretomes within the regenerative medicine field might represent synergic new therapeutical approaches that can be used to tune the reaction of immune cells within the damaged tissues. To this end, promising opportunities are headed to the stimulation of macrophages polarization towards anti-inflammatory phenotype together with the recruitment of stem cells, that possess immunomodulatory properties, able to infiltrate within the damaged tissues and improve tendinopathies resolution. Indeed, the comprehension of the interactions between tenocytes or stem cells with the immune cells might considerably modulate the immune reaction solving hence the inflammatory response and preventing fibrotic tissue formation. The purpose of this review is to compare the roles of distinct cell compartments during tendon homeostasis and injury. Furthermore, the role of immune cells in this field, as well as their interactions with stem cells and tenocytes during tendon regeneration, will be discussed to gain insights into new ways for dealing with tendinopathies.

The effect of exercise on cytokines: implications for musculoskeletal health: a narrative review

The physiological effects of physical exercise are ubiquitously reported as beneficial to the cardiovascular and musculoskeletal systems. Exercise is widely promoted by medical professionals to aid both physical and emotional wellbeing; however, mechanisms through which this is achieved are less well understood. Despite numerous beneficial attributes, certain types of exercise can inflict significant significant physiological stress. Several studies document a key relationship between exercise and immune activation. Activation of the innate immune system occurs in response to exercise and it is proposed this is largely mediated by cytokine signalling. Cytokines are typically classified according to their inflammatory properties and evidence has shown that cytokines expressed in response to exercise are diverse and may act to propagate, modulate or mitigate inflammation in musculoskeletal health. The review summarizes the existing literature on the relationship between exercise and the immune system with emphasis on how exercise-induced cytokine expression modulates inflammation and the immune response.

Delayed tendon healing after injury in tetranectin-deficient mice

BACKGROUND

Tetranectin, a plasminogen-binding protein, is present in human serum and has a role in tissue remodeling. The wound healing process is established and follows a similar cascade in tendon tissue as in other tissues. In this study, we investigated whether tetranectin has a role in regulating tissue formation of injured tendon.

METHODS

Using the patella tendon injury model in the tetranectin-null mice, healing processes of the injured tendon were evaluated by histological and immunohistochemical analyses, and measurement of the expression of tetranectin, type 1 collagen (Col 1), tenomodulin, scleraxis, TGFβ, IL-1β, IL-6 and TNF-α.

RESULTS

At the inflammatory phase within 7 days after the injury, involvement of inflammatory cells and the expressions of IL-1β, IL-6 and TNF-α were significantly decreased in tetranectin-null mice. Tetranectin expression increased at 1 day and peaked at 3 days, and finally disappeared at 7 days after the injury in wild-type mice. The tendon healing period and maturity were significantly delayed in the tetranectin-null mice. Expression levels of type 1 collagen and tenomodulin in tetranectin-null mice were significantly lower than those in the wild-type mice until 70 days after injury. With regard to the long-term processes, the healing and maturation of the injured tendon in tetranectin-null mice were eventually completed.

CONCLUSION

We believe that tetranectin might have a potential role in enhancing tissue formation of healing tendon at the inflammatory phase after injuries. The characteristics of tetranectin as a purified protein from human serum could be interested in an attractive candidate as a potential agent to enhance tendon healing after injury.

Tendon healing is adversely affected by low-grade inflammation

BACKGROUND

Tendinopathy is common, presents with pain and activity limitation, and is associated with a high risk of recurrence of the injury. Tendinopathy usually occurs as a results of a disrupted healing response to a primary injury where cellular and molecular pathways lead to low grade chronic inflammation.

MAIN FINDINGS

There has been a renewed interest in investigating the role of Inflammation in the pathogenesis of tendinopathy, in particular during the initial phases of the condition where it may not be clinically evident. Understanding the early and late stages of tendon injury pathogenesis would help develop new and effective treatments addressed at targeting the inflammatory pathways.

CONCLUSION

This review outlines the role of low-grade Inflammation in the pathogenesis of tendinopathy, stressing the role of proinflammatory cytokines, proteolytic enzymes and growth factors, and explores how Inflammation exerts a negative influence on the process of tendon healing.

CTRP3 exacerbates tendinopathy by dysregulating tendon stem cell differentiation and altering extracellular matrix composition

Tendinopathy, the most common disorder affecting tendons, is characterized by chronic disorganization of the tendon matrix, which leads to tendon tear and rupture. The goal was to identify a rational molecular target whose blockade can serve as a potential therapeutic intervention for tendinopathy. We identified C1q/TNF-related protein-3 (CTRP3) as a markedly up-regulated cytokine in human and rodent tendinopathy. Overexpression of CTRP3 enhanced the progression of tendinopathy by accumulating cartilaginous proteoglycans and degenerating collagenous fibers in the mouse tendon, whereas CTRP3 knockdown suppressed the tendinopathy pathogenesis. Functional blockade of CTRP3 using a neutralizing antibody ameliorated overuse-induced tendinopathy of the Achilles and rotator cuff tendons. Mechanistically, CTRP3 elicited a transcriptomic pattern that stimulates abnormal differentiation of tendon stem/progenitor cells and ectopic chondrification as an effect linked to activation of Akt signaling. Collectively, we reveal an essential role for CTRP3 in tendinopathy and propose a potential therapeutic strategy for the treatment of tendinopathy.

Tendon Cells Derived From The Long Head Of The Biceps And The Supraspinatus Tendons Of Patients Affected By Rotator Cuff Tears Show Different Expression Of Inflammatory Markers

AIM OF THE STUDY

Tendons are exposed to mechanical stress constantly during movements and thus they are frequently subjected to injuries. Rotator cuff tears are common musculoskeletal disorders, mainly involving the supraspinatus tendon. The characterization of the tenocytes derived from this tendon and the comparison to cells isolated from the long head of the biceps tendon obtained from donors affected by rotator cuff disease may improve the knowledge of the cellular mechanisms involved in the initiation and progression of the pathology. Thus, the aim of the present study was to characterize and compare donor-matched human tendon cells (TCs) isolated from the long head of the biceps (LHB-TCs) and the supraspinatus tendons (SSP-TCs) of patients affected by rotator cuff tears.

METHODS

donor-matched LHB-TCs and SSP-TCs were isolated and cultured up to passage 3. Phenotypic appearance, metabolic activity, DNA content, production of soluble mediators (IL-1Ra, IL-1β, IL-6, and VEGF) and gene expression of tendon markers (SCX, COL1A1, COL3A1), inflammatory (PTGS2), and catabolic enzymes (MMP-1, MMP-3) were evaluated.

RESULTS

LHB-TCs showed an elongated fibroblast-like shape, while SSP-TCs appeared irregular with jagged membrane. SSP-TCs gene expression revealed an augmented production of PTGS2, a marker of inflammation, whereas they produced a reduced amount of IL-6, in respect to LHB-TCs.

CONCLUSION

SSP-TCs showed higher cellular stress and expression of inflammatory markers with respect to donor-matched LHB-TCs, suggesting that addressing the physio-pathological state of supraspinatus tendon cells during treatment of rotator cuff tears could favor tissue healing and possibly prevent relapses.

Reparative and Maladaptive Inflammation in Tendon Healing

Tendon injuries are common and debilitating, with non-regenerative healing often resulting in chronic disease. While there has been considerable progress in identifying the cellular and molecular regulators of tendon healing, the role of inflammation in tendon healing is less well understood. While inflammation underlies chronic tendinopathy, it also aids debris clearance and signals tissue repair. Here, we highlight recent findings in this area, focusing on the cells and cytokines involved in reparative inflammation. We also discuss findings from other model systems when research in tendon is minimal, and explore recent studies in the treatment of human tendinopathy to glean further insights into the immunobiology of tendon healing.

Species variations in tenocytes' response to inflammation require careful selection of animal models for tendon research

For research on tendon injury, many different animal models are utilized; however, the extent to which these species simulate the clinical condition and disease pathophysiology has not yet been critically evaluated. Considering the importance of inflammation in tendon disease, this study compared the cellular and molecular features of inflammation in tenocytes of humans and four common model species (mouse, rat, sheep, and horse). While mouse and rat tenocytes most closely equalled human tenocytes’ low proliferation capacity and the negligible effect of inflammation on proliferation, the wound closure speed of humans was best approximated by rats and horses. The overall gene expression of human tenocytes was most similar to mice under healthy, to horses under transient and to sheep under constant inflammatory conditions. Humans were best matched by mice and horses in their tendon marker and collagen expression, by horses in extracellular matrix remodelling genes, and by rats in inflammatory mediators. As no single animal model perfectly replicates the clinical condition and sufficiently emulates human tenocytes, fit-for-purpose selection of the model species for each specific research question and combination of data from multiple species will be essential to optimize translational predictive validity.

Superior mechanical recovery in male and female MRL/MpJ tendons is associated with a unique genetic profile

Tendon ruptures heal by forming a mechanically inferior scar. We have shown that male Murphy Roths large (MRL/MpJ) mice exhibit improved tendon healing, suggesting that they can inform biological mechanisms that lead to effective tendon healing. As sex impacts healing, we assessed the effect of sex on tendon healing in MRL/MpJ and normal healer C57BL/6 (B6) mice and compared the associated biological environment with identify genes that may be integral to the improved healing outcome. We hypothesized that (a) male MRL/MpJ mice will heal with improved mechanical properties compared to females; and (b) that regenerative tendon healing will be associated with decreased fibrotic pathways, decreased inflammation, and increased activity of matrix metalloproteinases (MMPs). A midsubstance punch was introduced, and tendons were harvested after (a) 1 or 7 days for profiling of 84 genes; (b) 7 or 14 days for the assessment of MMP-2 and MMP-9 activity; and (c) 6 weeks for mechanical assessment. MRL/MpJ tendons healed with the better restoration of mechanical properties than B6 tendons. Sex did not affect the mechanical properties of healing B6 or MRL/MpJ tendons. Comparison of the gene expression profiles in the context of the mechanical outcome revealed several differences between MRL/MpJ and B6 tendon healing, including, lower inflammation, an earlier higher expression of TGF-β-related genes that diminish by 7 days, and genes associated with enhanced cell migration in MRL/MpJ in comparison to B6 tendons. We expect that the timecourse and expression levels of these genes in scarless MRL/MpJ tendon healing represent the balanced environment that leads to improved tendon healing.

A numerical framework for mechano-regulated tendon healing-Simulation of early regeneration of the Achilles tendon

Mechano-regulation during tendon healing, i.e. the relationship between mechanical stimuli and cellular response, has received more attention recently. However, the basic mechanobiological mechanisms governing tendon healing after a rupture are still not well-understood. Literature has reported spatial and temporal variations in the healing of ruptured tendon tissue. In this study, we explored a computational modeling approach to describe tendon healing. In particular, a novel 3D mechano-regulatory framework was developed to investigate spatio-temporal evolution of collagen content and orientation, and temporal evolution of tendon stiffness during early tendon healing. Based on an extensive literature search, two possible relationships were proposed to connect levels of mechanical stimuli to collagen production. Since literature remains unclear on strain-dependent collagen production at high levels of strain, the two investigated production laws explored the presence or absence of collagen production upon non-physiologically high levels of strain (>15%). Implementation in a finite element framework, pointed to large spatial variations in strain magnitudes within the callus tissue, which resulted in predictions of distinct spatial distributions of collagen over time. The simulations showed that the magnitude of strain was highest in the tendon core along the central axis, and decreased towards the outer periphery. Consequently, decreased levels of collagen production for high levels of tensile strain were shown to accurately predict the experimentally observed delayed collagen production in the tendon core. In addition, our healing framework predicted evolution of collagen orientation towards alignment with the tendon axis and the overall predicted tendon stiffness agreed well with experimental data. In this study, we explored the capability of a numerical model to describe spatial and temporal variations in tendon healing and we identified that understanding mechano-regulated collagen production can play a key role in explaining heterogeneities observed during tendon healing.

Bone marrow mesenchymal stem cell-derived exosomes promote rotator cuff tendon-bone healing by promoting angiogenesis and regulating M1 macrophages in rats

BACKGROUND

Rotator cuff tears (RCTs) often require reconstructive surgery. Tendon-bone healing is critical for the outcome of rotator cuff reconstruction, but the process of tendon-bone healing is complex and difficult. Mesenchymal stem cells (MSCs) are considered to be an effective method to promote tendon-bone healing. MSCs have strong paracrine, anti-inflammatory, immunoregulatory, and angiogenic potential. Recent studies have shown that MSCs achieve many regulatory functions through exosomes. The purpose of this study was to explore the role of bone marrow mesenchymal stem cell-derived exosomes (BMSC-Exos) in tendon-bone healing.

METHODS

Our study found that BMSC-Exos promote the proliferation, migration, and angiogenic tube formation of human umbilical vein endothelial cells (HUVECs). The mechanism by which BMSC-Exos achieve this may be through the regulation of the angiogenic signaling pathway. In addition, BMSC-Exos can inhibit the polarization of M1 macrophages and inhibit the secretion of proinflammatory factors by M1 macrophages. After rotator cuff reconstruction in rats, BMSC-Exos were injected into the tail vein to analyze their effect on the rotator cuff tendon-bone interface healing.

RESULTS

It was confirmed that BMSC-Exos increased the breaking load and stiffness of the rotator cuff after reconstruction in rats, induced angiogenesis around the rotator cuff endpoint, and promoted growth of the tendon-bone interface.

CONCLUSION

BMSC-Exos promote tendon-bone healing after rotator cuff reconstruction in rats by promoting angiogenesis and inhibiting inflammation.

Hip abductor tendon tears: where are we now?

Hip abductor tendon tear is a difficult problem to manage. The hip abductor mechanism is made up of the gluteus medius and minimus muscles, both of which contribute to stabilising the pelvis through the gait cycle. Tears of these tendons are likely due to iatrogenic injury during arthroplasty and chronic degenerative tendinopathy. Ultrasound and magnetic resonance imaging have provided limited clues regarding the pattern of disease and further work is required to clarify both the macro and microscopic pattern of disease. While surgery has been attempted over the last 2 decades, the outcomes are variable and the lack of high-quality studies have limited the uptake of surgical repair. Hip abductor tendon tears share many features with rotator cuff tears, hence, innovations in surgical techniques, materials and biologics may apply to both pathologies.

The role of the macrophage in tendinopathy and tendon healing

The role of the macrophage is an area of emerging interest in tendinopathy and tendon healing. The macrophage has been found to play a key role in regulating the healing process of the healing tendon. The specific function of the macrophage depends on its functional phenotype. While the M1 macrophage phenotype exhibits a phagocytic and proinflammatory function, the M2 macrophage phenotype is associated with the resolution of inflammation and tissue deposition. Several studies have been conducted on animal models looking at enhancing or suppressing macrophage function, targeting specific phenotypes. These studies include the use of exogenous biological and pharmacological substances and more recently the use of transgenic and genetically modified animals. The outcomes of these studies have been promising. In particular, enhancement of M2 macrophage activity in the healing tendon of animal models have shown decreased scar formation, accelerated healing, decreased inflammation and even enhanced biomechanical strength. Currently our understanding of the role of the macrophage in tendinopathy and tendon healing is limited. Furthermore, the roles of therapies targeting macrophages to enhance tendon healing is unclear. Clinical Significance: An increased understanding of the significance of the macrophage and its functional phenotypes in the healing tendon may be the key to enhancing tendon healing. This review will present the current literature on the function of macrophages in tendinopathy and tendon healing and the potential of therapies targeting macrophages to enhance tendon healing.

Autologous microfragmented adipose tissue reduces inflammatory and catabolic markers in supraspinatus tendon cells derived from patients affected by rotator cuff tears

PURPOSE

Rotator cuff tears are common musculoskeletal disorders, and surgical repair is characterized by a high rate of re-tear. Regenerative medicine strategies, in particular mesenchymal stem cell-based therapies, have been proposed to enhance tendon healing and reduce the re-tear rate. Autologous microfragmented adipose tissue (μFAT) allows for the clinical application of cell therapies and showed the ability to improve tenocyte proliferation and viability in previous in vitro assessments. The hypothesis of this study is that μFAT paracrine action would reduce the catabolic and inflammatory marker expression in tendon cells (TCs) derived from injured supraspinatus tendon (SST).

METHODS

TCs derived from injured SST were co-cultured with autologous μFAT in transwell for 48 h. Metabolic activity, DNA content, the content of soluble mediators in the media, and the gene expression of tendon-specific, inflammatory, and catabolic markers were analyzed.

RESULTS

μFAT-treated TCs showed a reduced expression of PTGS2 and MMP-3 with respect to untreated controls. Increased IL-1Ra, VEGF, and IL-6 content were observed in the media of μFAT-treated samples, in comparison with untreated TCs.

CONCLUSION

μFAT exerted an anti-inflammatory action on supraspinatus tendon cells in vitro through paracrine action, resulting in the reduction of catabolic and inflammatory marker expression. These observations potentially support the use of μFAT as adjuvant therapy in the treatment of rotator cuff disease.

The Effects of Photobiomodulation on Vocal Fold Wound Healing: In Vivo and In Vitro Studies

Background: Photobiomodulation (PBM) is increasingly used in dermatology and dentistry due to its benefit of promoting wound healing and relieving pain; however, there is no corresponding research report on the application of PBM to vocal fold wound healing. Objective: To assess the potential wound-healing effects of PBM on the vocal folds via in vivo and in vitro experiments. Materials and methods: In in vitro study, vocal fold fibroblasts (VFFs) were irradiated under a diode laser with wavelength of 635 nm at energy density of 8 J/cm². The Cell Counting Kit-8 (CCK-8) assay was used to study the viability of VFFs, and the gene expressions of COL1A2, COL3A1, IL-6, HAS2, and COX-2 were investigated by real-time polymerase chain reaction (RT-PCR). In in vivo study, 15 rabbits were used. Lamina propria of the left vocal folds of 12 rabbits was unilaterally stripped, and 6 of them were treated with PBM. The remaining three rabbits served as normal controls. After 3 months, all animals were sacrificed to obtain histological results. We used laryngoscope to record images of the healing phase. Results: Irradiation with energy density of 8 J/cm² resulted in a 2.8% increase in cell proliferation (p < 0.05). However, the difference between the experimental and the control group became larger after 48 and 72 h of subsequent irradiation. RT-PCR results showed that the expression of COL1A2, COL3A1, and HAS2 was higher, and the expression of IL-6 and COX-2 was lower. Histological examination showed that, compared with the injury group, hyaluronic acid (HA) increased significantly, collagen deposition decreased, and the configuration of collagen was more organized after PBM treatment. Conclusions: PBM can inhibit inflammatory reaction and promote the secretion of HA to decrease the deposition of collagen and regenerate vocal fold tissue without scar.

Therapeutic Efficacy of Intratendinous Delivery of Dexamethasone Using Porous Microspheres for Amelioration of Inflammation and Tendon Degeneration on Achilles Tendinitis in Rats

Achilles tendinitis caused by overuse, aging, or gradual wear induces pain, swelling, and stiffness of Achilles tendon and leads to tendon rupture. This study was performed to investigate the suppression of inflammation responses in interleukin-1β- (IL-1β-) stimulated tenocytes in vitro and the suppression of the progression of Achilles tendinitis-induced rat models in vivo using dexamethasone-containing porous microspheres (DEX/PMSs) for a sustained intratendinous DEX delivery. DEX from DEX/PMSs showed the sustained release of DEX. Treatment of IL-1β-stimulated tenocytes with DEX/PMSs suppressed the mRNA levels for COX-2, IL-1β, IL-6, and TNF-α. The intratendinous injection of DEX/PMSs into Achilles tendinitis rats both decreased the mRNA levels for these cytokines and increased mRNA levels for anti-inflammatory cytokines IL-4 and IL-10 in tendon tissues. Furthermore, DEX/PMSs effectively prevented tendon degeneration by enhancing the collagen content and biomechanical properties. Our findings suggest that DEX/PMSs show great potential as a sustained intratendinous delivery system for ameliorating inflammation responses as well as tendon degeneration in Achilles tendinitis.

Tendon healing in presence of chronic low-level inflammation: a systematic review

BACKGROUND

Tendinopathy is a common musculoskeletal condition affecting subjects regardless of their activity level. Multiple inflammatory molecules found in ex vivo samples of human tendons are related to the initiation or progression of tendinopathy. Their role in tendon healing is the subject of this review.

SOURCES OF DATA

An extensive review of current literature was conducted using PubMed, Embase and Cochrane Library using the term 'tendon', as well as some common terms of tendon conditions such as 'tendon injury OR (tendon damage) OR tendonitis OR tendinopathy OR (chronic tendonitis) OR tendinosis OR (chronic tendinopathy) OR enthesitis' AND 'healing' AND '(inflammation OR immune response)' as either key words or MeSH terms.

AREAS OF AGREEMENT

An environment characterized by a low level of chronic inflammation, together with increased expression of inflammatory cytokines and growth factors, may influence the physiological tendon healing response after treatment.

AREAS OF CONTROVERSY

Most studies on this topic exhibited limited scientific translational value because of their heterogeneity. The evidence associated with preclinical studies is limited.

GROWING POINTS

The role of inflammation in tendon healing is still unclear, though it seems to affect the overall outcome. A thorough understanding of the biochemical mediators of healing and their pathway of pain could be used to target tendinopathy and possibly guide its management.

AREAS TIMELY FOR DEVELOPING RESEARCH

We require further studies with improved designs to effectively evaluate the pathogenesis and progression of tendinopathy to identify cellular and molecular targets to improve outcomes.

Autologous Microfragmented Adipose Tissue Reduces the Catabolic and Fibrosis Response in an In Vitro Model of Tendon Cell Inflammation

Background

Mesenchymal stem cells (MSCs) emerged as a promising therapy for tendon pathologies. Microfragmented adipose tissue (μFAT) represents a convenient autologous product for the application of MSC-based therapies in the clinical setting. In the present study, the ability of μFAT to counteract inflammatory processes induced by IL-1β on human tendon cells (TCs) was evaluated.

Methods

Cell viability and proliferation were evaluated after 48 hours of transwell coculture of TCs and autologous μFAT in the presence or absence of IL-1β. Gene expression of scleraxis, collagen type I and type III, metalloproteinases-1 and -3, and cyclooxygenase-2 was evaluated by real-time RT-PCR. The content of VEGF, IL-1Ra, TNFα, and IL-6 was evaluated by ELISA.

Results

IL-1β-treated TCs showed augmented collagen type III, metalloproteases, and cyclooxygenase-2 expression. μFAT was able to reduce the expression of collagen type III and metalloproteases-1 in a significant manner, and at the same time, it enhanced the production of VEGF, IL-1Ra, and IL-6.

Conclusions

In this in vitro model of tendon cell inflammation, the paracrine action of μFAT, exerted by anti-inflammatory molecules and growth factors, was able to inhibit the expression of fibrosis and catabolic markers. Then, these results suggest that the application of μFAT may represent an effective conservative or adjuvant therapy for the treatment of tendon disorders.

Murine patellar tendon transplantation requires transosseous cerclage augmentation - development of a transplantation model for investigation of systemic and local drivers to healing

Background

Tendon injuries are common musculoskeletal injuries that heal with scar tissue formation, often achieving reduced biomechanical and functional properties. The murine patellar tendon is a research tool that holds potential for investigating tendon healing and can be useful for exploring therapeutic strategies. Since healing is a complex process that results from the collaboration between the systemic and local tissue environment, a murine tendon transplantation model that can be applied to transgenic mice and genetic mutants would allow isolation of systemic versus local tendon factors in driving effective tendon healing. Preliminary studies have shown that transplantation with simple tendon sutures results in a proximalization of the patellar bone due to the involuntary quadriceps muscle force leading to tearing of the graft and failure of the knee extensor mechanism. To avoid this elongation of the graft, two cerclage techniques for murine patellar tendon transplantation were introduced and validated.

Methods

Three developed surgical techniques (no-cerclage-augmentation (NCA)), transfascial suture cerclage with encirclement of the patellar tendon (TFSC), and dual-cerclage-augmentation with a transosseous bone-to-bone cerclage through the patella bone and an additional musculotendinous cerclage (DCA)) were compared at 4 and 8 weeks macroscopically in regards to graft continuity, cerclage integrity, gap formation, and radiologically by measuring the patello-tibial distance and using a patella bone position grading system.

Results

The NCA group showed complete failure at 5–7 days after surgery. The TFSC has led to 69% functional failure of the cerclage. In contrast, the DCA with a has led to 78% success with improvement in patellar bone position and a similar patello-tibial distance to the naïve contralateral murine knees over the time period of 8 weeks.

Conclusions

This study shows that a bone-to-bone cerclage is necessary to maintain a desired graft length in murine patellar tendon models. This surgery technique can serve for future graft trans- and implantations in the murine patellar tendon.

A Tppp3+Pdgfra+ tendon stem cell population contributes to regeneration and reveals a shared role for PDGF signalling in regeneration and fibrosis

Tendon injuries cause prolonged disability and never recover completely. Current mechanistic understanding of tendon regeneration is limited. Here we use single cell transcriptomics to identify a tubulin polymerization-promoting protein family member 3-expressing (Tppp3⁺) cell population as potential tendon stem cells. Through inducible lineage tracing, we demonstrated that these cells can generate new tenocytes and self-renew upon injury. A fraction of Tppp3⁺ cells expresses platelet-derived growth factor receptor alpha (Pdfgra). Ectopic platelet-derived growth factor-AA (PDGF-AA) protein induces new tenocyte production while inactivation of Pdgfra in Tppp3⁺ cells blocks tendon regeneration. These results support Tppp3⁺Pdgfra⁺ cells as tendon stem cells. Unexpectedly, Tppp3⁻Pdgfra⁺ fibro-adipogenic progenitors coexist in tendon stem cell niche and give rise to fibrotic cells, revealing a clandestine origin of fibrotic scars in healing tendons. Our results explain why fibrosis occurs in injured tendons and present clinical challenges to enhance tendon regeneration without a concurrent increase in fibrosis by PDGF application.

A New Understanding of the Role of IL-1 in Age-Related Intervertebral Disc Degeneration in a Murine Model

Increased cytokine expression, in particular interleukin-1β (IL-1β), is considered a hallmark of intervertebral disc degeneration. However, the causative relationship between IL-1 and age-dependent degeneration has not been established. To investigate the role of IL-1 in driving age-related disc degeneration, we studied the spine phenotype of global IL-1α/β double knockout (IL-1KO) mice at 12 and 20 months. Multiplex ELISA analysis of blood revealed significant reductions in the concentrations of IFN-γ, IL-5, IL-15, TNF-α, IP-10, and a trend of reduced concentrations of IL-10, macrophage inflammatory protein 1α (MIP-1α), keratinocyte chemoattractant/human growth-regulated oncogene (KC/GRO), and IL-6. However, the circulating level of MIP-2, a neutrophil chemoattractant, was increased in the IL-1KO. The alterations in systemic cytokine levels coincided with altered bone morphology-IL-1KO mice exhibited significantly thicker caudal cortical bone at 12 and 20 months. Despite these systemic inflammatory and bony changes, IL-1 deletion only minimally affected disc health. Both wild-type (WT) and IL-1KO mice showed age-dependent disc degeneration. Unexpectedly, rather than protecting the animals from degeneration, the aging phenotype was more pronounced in IL-1KO animals: knockout mice evidenced significantly more degenerative changes in the annulus fibrosis (AF) together with alterations in collagen type and maturity. At 20 months, there were no changes in nucleus pulposus (NP) extracellular matrix composition or cellular marker expression; however, the IL-1KO NP cells occupied a smaller proportion of the NP compartment that those of WT controls. Taken together, these results show that IL-1 deletion altered the systemic inflammatory environment and vertebral bone morphology. However, instead of protecting discs from age-related disc degeneration, global IL-1 deletion amplified the degenerative phenotype. © 2019 American Society for Bone and Mineral Research.

Use of Human Placenta-Derived Cells in a Preclinical Model of Tendon Injury

BACKGROUND

Emerging data suggest that human cells derived from extraembryonic tissues may have favorable musculoskeletal repair properties. The purpose of this study was to determine whether the injection of human placenta-derived mesenchymal-like stromal cells, termed placental expanded cells (PLX-PAD), would improve tendon healing in a preclinical model of tendinopathy.

METHODS

Sixty male Sprague-Dawley rats underwent bilateral patellar tendon injection with either saline solution (control) or PLX-PAD cells (2 × 10 cells/100 µL) 6 days after collagenase injection to induce tendon degeneration. Animals were killed at specific time points for biomechanical, histological, and gene expression analyses of the healing patellar tendons.

RESULTS

Biomechanical testing 2 weeks after the collagenase injury demonstrated better biomechanical properties in the tendons treated with PLX-PAD cells. The load to failure of the PLX-PAD-treated tendons was higher than that of the saline-solution-treated controls at 2 weeks (77.01 ± 10.51 versus 58.87 ± 11.97 N, p = 0.01). There was no significant difference between the 2 groups at 4 weeks. There were no differences in stiffness at either time point. Semiquantitative histological analysis demonstrated no significant differences in collagen organization or cellularity between the PLX-PAD and saline-solution-treated tendons. Gene expression analysis demonstrated higher levels of interleukin-1β (IL-1β) and IL-6 early in the healing process in the PLX-PAD-treated tendons.

CONCLUSIONS

Human placenta-derived cell therapy induced an early inflammatory response and a transient beneficial effect on tendon failure load in a model of collagenase-induced tendon degeneration.

CLINICAL RELEVANCE

Human extraembryonic tissues, such as the placenta, are an emerging source of cells for musculoskeletal repair and may hold promise as a point-of-care cell therapy for tendon injuries.

Tendon healing affects the multiscale mechanical, structural and compositional response of tendon to quasi-static tensile loading

Tendon experiences a variety of multiscale changes to its extracellular matrix during mechanical loading at the fascicle, fibre and fibril levels. For example, tensile loading of tendon increases its stiffness, with organization of collagen fibres, and increases cell strain in the direction of loading. Although applied macroscale strains correlate to cell and nuclear strains in uninjured tendon, the multiscale response during tendon healing remains unknown and may affect cell mechanosensing and response. Therefore, this study evaluated multiscale structure-function mechanisms in response to quasi-static tensile loading in uninjured and healing tendons. We found that tendon healing affected the macroscale mechanical and structural response to mechanical loading, evidenced by decreases in strain stiffening and collagen fibre realignment. At the micro- and nanoscales, healing resulted in increased collagen fibre disorganization, nuclear disorganization, decreased change in nuclear aspect ratio with loading, and decreased indentation modulus compared to uninjured tendons. Taken together, this work supports a new concept of nuclear strain transfer attenuation during tendon healing and identifies several multiscale properties that may contribute. Our work also provides benchmarks for the biomechanical microenvironments that tendon cells may experience following cell delivery therapies.

Bayesian inference of constitutive model parameters from uncertain uniaxial experiments on murine tendons

Constitutive models for biological tissue are typically formulated as a mixture of constituents and the overall response is then assembled by superposition or compatibility. This ensures the stress response of the biological tissue to be in the range of a given constitutive relationship, guaranteeing that at least one parameter combination exists so that an experimental response can be sufficiently well captured. Another, perhaps more challenging, problem is to use constitutive models as a proxy to infer the structure/function of a biological tissue from experiments. In other words, we determine the optimal set of parameters by solving an inverse problem and use these parameters to infer the integrity of the tissue constituents. In previous studies, we focused on the mechanical stress-stretch response of the murine patellar tendon at various age and healing timepoints and solved the inverse problem using three constitutive models, i.e., the Freed-Rajagopal, Gasser-Ogden-Holzapfel and Shearer in order of increasing microstructural detail. Herein, we extend this work by adopting a Bayesian perspective on parameter estimation and implement the constitutive relations in the tulip library for uncertainty analysis, critically analyzing parameter marginals, correlations, identifiability and sensitivity. Our results show the importance of investigating the variability of parameter estimates and that results from optimization may be misleading, particularly for models with many parameters inferred from limited experimental evidence. In our study, we show that different age and healing conditions do not correspond to statistically significant separation among the Gasser-Ogden-Holzapfel and Shearer model parameters, while the phenomenological Freed-Rajagopal model is instead characterized by better indentifiability and parameter learning. Use of the complete experimental observations rather than averaged stress-stretch responses appears to positively constrain inference and results appear to be invariant with respect to the scaling of the experimental uncertainty.

Embryonic and postnatal tendon cells respond differently to interleukin-1β

Adult tendons heal as scar tissue, whereas embryonic tendons heal scarlessly via unknown mechanisms. Scarred tendon healing results from inflammation-driven imbalances in anabolic and catabolic functions. To test scarless versus scarring age tendon cell responses to inflammatory conditions, we treated embryonic and postnatal tendon cells with interleukin (IL)-1β and characterized expression of collagens, matrix metalloproteinases (MMPs), inflammatory mediators, and phosphorylation of signaling molecules. At baseline, postnatal cells expressed significantly higher levels of inflammatory mediators. When treated with IL-1β, both postnatal and embryonic cells upregulated inflammatory mediators and MMPs. Notably, postnatal cells secreted inflammatory factors up to 12.5 times the concentration in embryonic cultures. IL-1β activated NF-κB p65 and p38 mitogen-activated protein kinase (MAPK) pathways in both cell types, but phosphorylated p38 MAPK levels were two times higher in postnatal than embryonic cells. Our results suggest that scarred healing tendon cells respond to proinflammatory cytokines by promoting an imbalance in anabolic and catabolic functions, and that the heightened response involves p38 MAPK signaling activity. In contrast, embryonic cell responses are smaller in magnitude. These intriguing findings support a potential role for tendon cells in determining scarless versus scarred healing outcomes by regulating the balance between anabolic and catabolic functions during tendon healing.

S100A8 & S100A9: Alarmin mediated inflammation in tendinopathy

Alarmins S100A8 and S100A9 are endogenous molecules released in response to environmental triggers and cellular damage. They are constitutively expressed in immune cells such as monocytes and neutrophils and their expression is upregulated under inflammatory conditions. The molecular mechanisms that regulate inflammatory pathways in tendinopathy are largely unknown therefore identifying early immune effectors is essential to understanding the pathology. Based on our previous investigations highlighting tendinopathy as an alarmin mediated pathology we sought evidence of S100A8 & A9 expression in a human model of tendinopathy and thereafter, to explore mechanisms whereby S100 proteins may regulate release of inflammatory mediators and matrix synthesis in human tenocytes. Immunohistochemistry and quantitative RT-PCR showed S100A8 & A9 expression was significantly upregulated in tendinopathic tissue compared with control. Furthermore, treating primary human tenocytes with exogenous S100A8 & A9 significantly increased protein release of IL-6, IL-8, CCL2, CCL20 and CXCL10; however, no alterations in genes associated with matrix remodelling were observed at a transcript level. We propose S100A8 & A9 participate in early pathology by modulating the stromal microenvironment and influencing the inflammatory profile observed in tendinopathy. S100A8 and S100A9 may participate in a positive feedback mechanism involving enhanced leukocyte recruitment and release of pro-inflammatory cytokines from tenocytes that perpetuates the inflammatory response within the tendon in the early stages of disease.

Cyclic Stretching Exacerbates Tendinitis by Enhancing NLRP3 Inflammasome Activity via F-Actin Depolymerization

Modern molecular techniques have highlighted the presence of inflammation throughout the spectrum of tendinopathy. Previous studies have suggested that excessive inflammation in the tendon is a major factor leading to poor clinical treatment. Furthermore, the NLRP3 inflammasome, as a new term, is closely associated with the pathogenesis of many diseases. In the present study, we examined whether the NLRP3 inflammasome contributes to the development of tendinitis and whether cyclic stretching plays a prominent role in inflammation in the tendon. In the present study, we showed that hydrogen peroxide (H₂O₂) remarkably enhances the expression and release of IL-1β, TNF-α, and IL-6. The maturation of IL-1β, induced by H₂O₂, depends on the activation of the NLRP3 inflammasome. Cyclic stretching enhances the maturation of IL-1β via promoting H₂O₂-induced NLRP3 inflammasome activation in tenocytes. Furthermore, we also found that the depolymerization of filamentous actin (F-actin) was required for cyclic stretching-enhanced NLRP3 inflammasome activation. The present study suggests that NLRP3 inflammasome plays an important regulatory role in the pathogenesis of tendinitis. Disruption of the cytoskeleton by cyclic stretching exerts a proinflammatory effect via further activating the NLRP3/IL-1β pathway and hence contributes to tendinitis. These results may provide theoretical support for a new treatment strategy for preventing excessive inflammation in the tendon.

Wound-healing effects of 635-nm low-level laser therapy on primary human vocal fold epithelial cells: an in vitro study

Low-level laser therapy (LLLT) has been promoted for its beneficial effects on tissue healing and pain relief for skin and oral applications. However, there is no corresponding literature reporting on vocal fold wound healing. Our purpose was to assess the potential wound-healing effects of LLLT on primary human vocal fold epithelial cells (VFECs). In this study, normal vocal fold tissue was obtained from a 58-year-old male patient who was diagnosed with postcricoid carcinoma without involvement of the vocal folds and underwent total laryngectomy. Primary VFECs were then cultured. Cells were irradiated at a wavelength of 635 nm with fluences of 1, 4, 8, 12, 16, and 20 J/cm² (50 mW/cm²), which correspond to irradiation times of 20, 80, 160, 240, 320, and 400 s, respectively. Cell viability of VFECs in response to varying doses of LLLT was investigated by the Cell Counting Kit-8 (CCK-8) method. The most effective irradiation dose was selected to evaluate the cell migration capacity by using the scratch wound-healing assay. Real-time polymerase chain reaction (RT-PCR) was used to detect the gene expression of TGF-β1, TGF-β3, EGF, IL-6, and IL-10. Irradiation with doses of 8 J/cm² resulted in 4% increases in cell proliferation differing significantly from the control group (p < 0.05). With subsequent doses at 48 and 72 h after irradiation, the differences between the experimental and the control groups became greater, up to 9.8% (p < 0.001) and 19.5% (p < 0.001), respectively. It also increased cell migration and the expression of some genes, such as EGF, TGF-β1, TGF-β3, and IL-10, involved in the tissue healing process. This study concludes that LLLT at the preset parameters was capable of stimulating the proliferation and migration of human vocal fold epithelial cells in culture as well as increase the expression of some genes involved in tissue healing process. Additionally, successive laser treatments at 24 h intervals have an additive beneficial effect on the healing of injured tissues.

Dynamic Loading and Tendon Healing Affect Multiscale Tendon Properties and ECM Stress Transmission

The extracellular matrix (ECM) is the primary biomechanical environment that interacts with tendon cells (tenocytes). Stresses applied via muscle contraction during skeletal movement transfer across structural hierarchies to the tenocyte nucleus in native uninjured tendons. Alterations to ECM structural and mechanical properties due to mechanical loading and tissue healing may affect this multiscale strain transfer and stress transmission through the ECM. This study explores the interface between dynamic loading and tendon healing across multiple length scales using living tendon explants. Results show that macroscale mechanical and structural properties are inferior following high magnitude dynamic loading (fatigue) in uninjured living tendon and that these effects propagate to the microscale. Although similar macroscale mechanical effects of dynamic loading are present in healing tendon compared to uninjured tendon, the microscale properties differed greatly during early healing. Regression analysis identified several variables (collagen and nuclear disorganization, cellularity, and F-actin) that directly predict nuclear deformation under loading. Finite element modeling predicted deficits in ECM stress transmission following fatigue loading and during healing. Together, this work identifies the multiscale response of tendon to dynamic loading and healing, and provides new insight into microenvironmental features that tenocytes may experience following injury and after cell delivery therapies.

Pooled Platelet-Rich Plasma Lysate Therapy Increases Synoviocyte Proliferation and Hyaluronic Acid Production While Protecting Chondrocytes From Synoviocyte-Derived Inflammatory Mediators

Platelet-rich plasma (PRP) preparations are being used with moderate success to treat osteoarthritis (OA) in humans and in veterinary species. Such preparations are hindered, however, by being autologous in nature and subject to tremendous patient and processing variability. For this reason, there has been increasing interest in the use of platelet lysate preparations instead of traditional PRP. Platelet lysate preparations are acellular, thereby reducing concerns over immunogenicity, and contain high concentrations of growth factors and cytokines. In addition, platelet lysate preparations can be stored frozen for readily available use. The purpose of this study was to evaluate the effects of a pooled allogeneic platelet-rich plasma lysate (PRP-L) preparation on equine synoviocytes and chondrocytes challenged with inflammatory mediators in-vitro to mimic the OA joint environment. Our hypothesis was that PRP-L treatment of inflamed synoviocytes would protect chondrocytes challenged with synoviocyte conditioned media by reducing synoviocyte pro-inflammatory cytokine production while increasing synoviocyte anti-inflammatory cytokine production. Synoviocytes were stimulated with either interleukin-1β (IL-1β) or lipopolysaccharide (LPS) for 24 h followed by no treatment or treatment with platelet-poor plasma lysate (PPP-L) or PRP-L for 48 h. Synoviocyte growth was evaluated at the end of the treatment period and synoviocyte conditioned media was assessed for concentrations of hyaluronic acid (HA), IL-1β, tumor necrosis factor alpha (TNF-α), and interleukin-6 (IL-6). Chondrocytes were then challenged for 48 h with synoviocyte conditioned media from each stimulation and treatment group and examined for gene expression of collagen types I (COL1A1), II (COL2A1), and III (COL3A1), aggrecan (ACAN), lubricin (PRG4), and matrix metallopeptidase 3 (MMP-3) and 13 (MMP-13). Treatment of inflamed synoviocytes with PRP-L resulted in increased synoviocyte growth and increased synoviocyte HA and IL-6 production. Challenge of chondrocytes with conditioned media from PRP-L treated synoviocytes resulted in increased collagen type II and aggrecan gene expression as well as decreased MMP-13 gene expression. The results of this study support continued investigation into the use of pooled PRP-L for the treatment of osteoarthritis and warrant further in-vitro studies to discern the mechanisms of action of PRP-L.

The roles of inflammatory mediators and immunocytes in tendinopathy

Tendinopathy is a common disease of the musculoskeletal system, particularly in athletes and sports amateurs. In this review, we will present evidence for the critical role of inflammatory mediators and immunocytes in the pathogenesis of tendinopathy and the efficacy of current antiinflammatory therapy and regenerative medicine in the clinic. We hereby propose a hypothesis that in addition to pulling force there may be compressive forces being exerted on the tendon during physical activities, which may initiate the onset of tendinopathy. We performed literature searches on MEDLINE from the inception of this review to February 2018. No language restrictions were imposed. The search terms were as follows: ("Tendinopathy"[Mesh] OR "Tendon Injuries"[Mesh] OR "Tendinitis"[Mesh] OR "Tendon"[Mesh]) AND (Inflammation OR "Inflammatory mediator*" OR Immunocyte*) OR ("anti inflammatory*" OR "regenerative medicine"). Inclusion criteria included articles that were original and reliable, with the main contents being highly relevant to our review. Exclusion criteria included articles that were not available online or have not been published. We scanned the abstract of these articles first. This was then followed by a careful screening of the articles which might be suitable for our review. Finally, 84 articles were selected as references. This review article is written in the narrative form. The translational potential of this article: Understanding the mechanisms of inflammation and existing antiinflammatory and regenerative therapies is key to the development of therapeutic strategies in tendinopathy.

Interleukin-6 Promotes Proliferation but Inhibits Tenogenic Differentiation via the Janus Kinase/Signal Transducers and Activators of Transcription 3 (JAK/STAT3) Pathway in Tendon-Derived Stem Cells

Background

Previous studies demonstrated that tendon-derived stem cells (TDSCs) were vital healing cells and that mRNA expression of anti-inflammatory cytokine IL-6 was significantly upregulated in injured tendons. The aim of the present study was to investigate the effects of IL-6 on the TDSCs in vitro.

Material/Methods

TDSCs isolated from the Achilles tendons in SD rats were co-cultured with various concentrations of IL-6. Cell proliferation, cell cycle analysis, quantitative real-time PCR, western blotting analysis, and statistical analysis were used in the study.

Results

The result showed that IL-6 strongly increased proliferation capability, and induced cell cycle activation and transition into G2/M phase from G1 phase in TDSCs. However, IL-6 treatment strongly inhibited gene expression of Scleraxis, Collagen 1, Tenomodulin, Collagen 3, Early Growth Response Protein 1, Decorin, Lumican, Biglycan and Fibromodulin in TDSCs. It also strongly inhibited protein expression of tendon cell markers like scleraxis, collagen 1, collagen 3, and tenomodulin. IL-6 treatment strongly activated the JAK/Stat3 signaling pathway in TDSCs. Furthermore, WP1066, a JAK/Stat3 signaling pathway inhibitor, abrogated the effects of IL-6 on TDSCs.

Conclusions

These findings indicated that IL-6 might exert dual effects on TDSCs in vitro: strongly enhancing their proliferation but inhibiting their tenogenic differentiation via the JAK/Stat3 pathway.