The responses of extrinsic fibroblasts infiltrating the devitalised patellar tendon to IL-1beta are different from those of normal tendon fibroblasts

Histological and biochemical changes in a rat rotator cuff tear model with or without the subacromial bursa

The subacromial bursa (SAB) plays an important role in the tendon healing process. Based on previous reports, co-culture of the rotator cuff (RC) and SAB have been shown to increase the tendon-related gene expressions, inflammatory cytokines, and tensile strength. However, the nature of the specific biochemical alterations during the inflammatory and repair phases of tendon healing with or without the SAB remain unknown. Using a full-thickness RC tear rat model, we determined how the presence or absence of the SAB alters the histological characteristics and gene expressions. After 3 and 6 weeks, tissues were collected for histological and real-time quantitative polymerase chain reaction (RT-qPCR) evaluations. Results showed greater cell density at 3 weeks, neovascularization and tendon thickening at 6 weeks with SAB preservation. Immunostaining revealed significant increases in type 3 collagen (COL3) expression at 6 weeks with SAB preservation. The RT-qPCR results showed that SAB preservation induced significant increases in the expression of scleraxis, matrix metalloproteinase-13 (MMP-13), interleukin-1β (IL-1β), and inducible nitric oxide synthase (iNOS) at 3 weeks and significant increases in COL3, IL-10, and arginase-1 (Arg-1) at 6 weeks. An RC tear undergoes more appropriate inflammatory and repair phases during the tendon healing process when the SAB is retained.

Inflammation-related signaling pathways in tendinopathy

Tendon is a connective tissue that produces movement by transmitting the force produced by muscle contraction to the bones. Most tendinopathy is caused by prolonged overloading of the tendon, leading to degenerative disease of the tendon. When overloaded, the oxygen demand of tenocytes increases, and the tendon structure is special and lacks blood supply, which makes it easier to form an oxygen-deficient environment in tenocytes. The production of reactive oxygen species due to hypoxia causes elevation of inflammatory markers in the tendon, including PGE2, IL-1β, and TNF-α. In the process of tendon healing, inflammation is also a necessary stage. The inflammatory environment formed by cytokines and various immune cells play an important role in the clearance of necrotic material, the proliferation of tenocytes, and the production of collagen fibers. However, excessive inflammation can lead to tendon adhesions and hinder tendon healing. Some important and diverse biological functions of the body originate from intercellular signal transduction, among which cytokine mediation is an important way of signal transduction. In particular, NF-κB, NLRP3, p38/MAPK, and signal transducer and activator of transcription 3, four common signaling pathways in tendinopathy inflammatory response, play a crucial role in the regulation and transcription of inflammatory factors. Therefore, summarizing the specific mechanisms of inflammatory signaling pathways in tendinopathy is of great significance for an in-depth understanding of the inflammatory response process and exploring how to inhibit the harmful part of the inflammatory response and promote the beneficial part to improve the healing effect of the tendon.

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.

ERK1/2 drives IL-1β-induced expression of TGF-β1 and BMP-2 in torn tendons

Diseased and injured tendons develop fibrosis, driven by factors including TGF-β, BMPs and CTGF. IL-1β and its signal transducer Erk1/2 are known to regulate TGF-β expression in animal tendons. We utilised tissues and cells isolated from patients with shoulder tendon tears and tendons of healthy volunteers to advance understanding of how inflammation induces fibrosis in diseased human tendons. ERK1/2 expression was reduced in torn (diseased) compared to healthy patient tendon tissues. We next investigated the fibrotic responses of tendon-derived cells isolated from healthy and diseased human tendon tissues in an inflammatory milieu. IL-1β treatment induced profound ERK1/2 signalling, TGFB1 and BMP2 mRNA expression in diseased compared to healthy tendon-derived cells. In the diseased cells, the ERK1/2 inhibitor (PD98059) completely blocked the IL-1β-induced TGFB1 and partially reduced BMP2 mRNA expression. Conversely, the same treatment of healthy cells did not modulate IL-1β-induced TGFB1 or BMP2 mRNA expression. ERK1/2 inhibition did not attenuate IL-1β-induced CTGF mRNA expression in healthy or diseased tendon cells. These findings highlight differences between ERK1/2 signalling pathway activation and expression of TGF-β1 and BMP-2 between healthy and diseased tendon tissues and cells, advancing understanding of inflammation induced fibrosis during the development of human tendon disease and subsequent repair.

Substrate fiber alignment mediates tendon cell response to inflammatory signaling

Healthy tendon tissue features a highly aligned extracellular matrix that becomes disorganized with disease. Recent evidence suggests that inflammation coexists with early degenerative changes in tendon, and that crosstalk between immune-cells and tendon fibroblasts (TFs) can contribute to poor tissue healing. We hypothesized that a disorganized tissue architecture may predispose tendon cells to degenerative extracellular matrix remodeling pathways, particularly within a pro-inflammatory niche. This hypothesis was tested by analyzing human TFs cultured on electrospun polycaprolactone (PCL) mats with either highly aligned or randomly oriented fiber structures. We confirmed that fibroblast morphology, phenotype, and markers of matrix turnover could be significantly affected by matrix topography. More strikingly, the TF response to paracrine signals from polarized macrophages or by stimulation with pro-inflammatory cytokines featured significant downregulation of signaling related to extracellular synthesis, with significant concomitant upregulation of gene and protein expression of matrix degrading enzymes. Critically, this tendency towards degenerative re-regulation was exacerbated on randomly oriented PCL substrates. These novel findings indicate that highly aligned tendon cell scaffolds not only promote tendon matrix synthesis, but also play a previously unappreciated role in mitigating adverse resident fibroblast response within an inflammatory milieu.

STATEMENT OF SIGNIFICANCE

Use of biomaterial scaffolds for tendon repair often results in tissue formation characteristic of scar tissue, rather than the highly aligned type-1 collagen matrix of healthy tendons. We hypothesized that non-optimal biomaterial surfaces may play a role in these outcomes, specifically randomly oriented biomaterial surfaces that unintentionally mimic structure of pathological tendon. We observed that disorganized scaffold surfaces do adversely affect early cell attachment and gene expression. We further identified that disorganized fiber surfaces can prime tendon cells toward pro-inflammatory signaling. These findings represent provocative evidence unstructured fiber surfaces may underlie inflammatory responses that drive aberrant collagen matrix turnover. This work could be highly relevant for the design of cell instructive biomaterial therapies that yield positive clinical outcomes.

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.

Cytokines in tendon disease: A Systematic Review

OBJECTIVES

Emerging evidence indicates that tendon disease is an active process with inflammation that is critical to disease onset and progression. However, the key cytokines responsible for driving and sustaining inflammation have not been identified.

METHODS

We performed a systematic review of the literature using MEDLINE (U.S. National Library of Medicine, Bethesda, Maryland) in March 2017. Studies reporting the expression of interleukins (ILs), tumour necrosis factor alpha (TNF-α) and interferon gamma in diseased human tendon tissues, and animal models of tendon injury or exercise in comparison with healthy control tissues were included.

RESULTS

IL-1β, IL-6, IL-10, and TNF-α are the cytokines that have been most frequently investigated. In clinical samples of tendinopathy and tendon tears, the expression of TNF-α tended not to change but IL-6 increased in tears. Healthy human tendons showed increased IL-6 expression after exercise; however, IL-10 remained unchanged. Animal tendon injury models showed that IL-1β, IL-6, and TNF-α tend to increase from the early phase of tendon healing. In animal exercise studies, IL-1β expression showed a tendency to increase at the early stage after exercise, but IL-10 expression remained unchanged with exercise.

CONCLUSIONS

This review highlights the roles of IL-1β, IL-6, IL-10, and TNF-α in the development of tendon disease, during tendon healing, and in response to exercise. However, there is evidence accumulating that suggests that other cytokines are also contributing to tendon inflammatory processes. Further work with hypothesis-free methods is warranted in order to identify the key cytokines, with subsequent mechanistic and interaction studies to elucidate their roles in tendon disease development.Cite this article: W. Morita, S. G. Dakin, S. J. B. Snelling, A. J. Carr. Cytokines in tendon disease: A Systematic Review. Bone Joint Res 2017;6:656-664. DOI: 10.1302/2046-3758.612.BJR-2017-0112.R1.

Review: Emerging concepts in the pathogenesis of tendinopathy

Tendinopathy is a common clinical problem and has a significant disease burden attached, not only in terms of health care costs, but also for patients directly in terms of time off work and impact upon quality of life. Controversy surrounds the pathogenesis of tendinopathy, however the recent systematic analysis of the evidence has demonstrated that many of the claims of an absence of inflammation in tendinopathy were more based around belief than robust scientific data. This review is a summary of the emerging research in this topical area, with a particular focus on the role of neuronal regulation and inflammation in tendinopathy.

IL-17A mediates inflammatory and tissue remodelling events in early human tendinopathy

Increasingly, inflammatory mediators are considered crucial to the onset and perpetuation of tendinopathy. We sought evidence of interleukin 17A (IL-17A) expression in early human tendinopathy and thereafter, explored mechanisms whereby IL-17A mediated inflammation and tissue remodeling in human tenocytes. Torn supraspinatus tendon (established pathology) and matched intact subscapularis tendon (representing 'early pathology') along with control biopsies were collected from patients undergoing shoulder surgery. Markers of inflammation and IL-17A were quantified by RT-PCR and immunohistochemistry. Human tendon cells were derived from hamstring tendon obtained during ACL reconstruction. In vitro effects of IL-17A upon tenocytes were measured using RT-PCR, multiplex cytokine assays, apoptotic proteomic profiling, immunohistochemistry and annexin V FACS staining. Increased expression of IL-17A was detected in 'early tendinopathy' compared to both matched samples and non-matched control samples (p < 0.01) by RT-PCR and immunostaining. Double immunofluoresence staining revealed IL-17A expression in leukocyte subsets including mast cells, macrophages and T cells. IL-17A treated tenocytes exhibited increased production of proinflammatory cytokines (p < 0.001), altered matrix regulation (p < 0.01) with increased Collagen type III and increased expression of several apoptosis related factors. We propose IL-17A as an inflammatory mediator within the early tendinopathy processes thus providing novel therapeutic approaches in the management of tendon disorders.

MicroRNA29a regulates IL-33-mediated tissue remodelling in tendon disease

MicroRNA (miRNA) has the potential for cross-regulation and functional integration of discrete biological processes during complex physiological events. Utilizing the common human condition tendinopathy as a model system to explore the cross-regulation of immediate inflammation and matrix synthesis by miRNA we observed that elevated IL-33 expression is a characteristic of early tendinopathy. Using in vitro tenocyte cultures and in vivo models of tendon damage, we demonstrate that such IL-33 expression plays a pivotal role in the transition from type 1 to type 3 collagen (Col3) synthesis and thus early tendon remodelling. Both IL-33 effector function, via its decoy receptor sST2, and Col3 synthesis are regulated by miRNA29a. Downregulation of miRNA29a in human tenocytes is sufficient to induce an increase in Col3 expression. These data provide a molecular mechanism of miRNA-mediated integration of the early pathophysiologic events that facilitate tissue remodelling in human tendon after injury.

IL-21 receptor expression in human tendinopathy

The pathogenetic mechanisms underlying tendinopathy remain unclear, with much debate as to whether inflammation or degradation has the prominent role. Increasing evidence points toward an early inflammatory infiltrate and associated inflammatory cytokine production in human and animal models of tendon disease. The IL-21/IL-21R axis is a proinflammatory cytokine complex that has been associated with chronic inflammatory diseases including rheumatoid arthritis and inflammatory bowel disease. This project aimed to investigate the role and expression of the cytokine/receptor pair IL-21/IL-21R in human tendinopathy. We found significantly elevated expression of IL-21 receptor message and protein in human tendon samples but found no convincing evidence of the presence of IL-21 at message or protein level. The level of expression of IL-21R message/protein in human tenocytes was significantly upregulated by proinflammatory cytokines (TNFα/IL-1β) in vitro. These findings demonstrate that IL-21R is present in early human tendinopathy mainly expressed by tenocytes and macrophages. Despite a lack of IL-21 expression, these data again suggest that early tendinopathy has an inflammatory/cytokine phenotype, which may provide novel translational targets in the treatment of tendinopathy.

The role of pro-inflammatory and immunoregulatory cytokines in tendon healing and rupture: new insights

Owing to limited self-healing capacity, tendon ruptures and healing remain major orthopedic challenges. Increasing evidence suggests that post-traumatic inflammatory responses, and hence, cytokines are involved in both cases, and also in tendon exercise and homeostasis. This review summarizes interrelations known between the cytokines interleukin (IL)-1β, tumor necrosis factor (TNF)α, IL-6 and vascular endothelial growth factor (VEGF) in tendon to assess their role in tendon damage and healing. Exogenic cytokine sources are blood-derived leukocytes that immigrate in damaged tendon. Endogenous expression of IL-1β, TNFα, IL-6, IL-10 and VEGF was demonstrated in tendon-derived cells. As tendon is a highly mechanosensitive tissue, cytokine homeostasis and cell survival underlie an intimate balance between adequate biomechanical stimuli and disturbance through load deprivation and overload. Multiple interrelations between cytokines and tendon extracellular matrix (ECM) synthesis, catabolic mediators e.g. matrix-degrading enzymes, inflammatory and angiogenic factors (COX-2, PGE2, VEGF, NO) and cytoskeleton assembly are evident. Pro-inflammatory cytokines affect ECM homeostasis, accelerate remodeling, amplify biomechanical adaptiveness and promote tenocyte apoptosis. This multifaceted interplay might both contribute to and interfere with healing. Much work must be undertaken to understand the particular interrelation of these inflammatory and regulatory mediators in ruptured tendon and healing, which has relevance for the development of novel immunoregulatory therapeutic strategies.

Effects of stress shielding and subsequent restressing on mechanical properties of regenerated and residual tissues in rabbit patellar tendon after resection of its central one-third

Central third of patellar tendon (PT) is used as an autograft for anterior cruciate ligament (ACL) reconstruction. Previous studies investigated temporal changes in material properties of healing tissues in PT after resection of the central third. However, no study has been performed on effects of stress shielding (SS) and restressing (RS) on the properties of healing tissues. The present study hypothesised that SS adversely affects the mechanical integrity of healing tissues, which is recovered by subsequent RS. An entire rectangular defect was created in the central third of rabbit PT. Operated PTs were subjected to either SS or no stress shielding (NSS). A subgroup of stress-shielded PTs was followed by the resumption of normal loading, namely RS. Tensile properties of tissues regenerated in the defect and residual tendons were evaluated. Regenerated tissues of SS for 3 weeks resulted in significantly lower strength than NSS, which was recovered to NSS level by 3 weeks of RS. Strength of residual tissues in RS reversed SS effects, leading to the strength at NSS level after 12 weeks. However, tangent modulus of residual tissues in RS was still significantly lower than that of NSS at 12 weeks. Therefore, SS induces detrimental effects on the mechanical integrity of healing PTs, and the response to RS was different between regenerate and residual tissues, the latter of which took longer period to reach NSS level.

The influence of the mechanical environment on remodelling of the patellar tendon

An understanding of the remodelling of tendon is crucial for the development of scientific methods of treatment and rehabilitation. This study tested the hypothesis that tendon adapts structurally in response to changes in functional loading. A novel model allowed manipulation of the mechanical environment of the patellar tendon in the presence of normal joint movement via the application of an adjustable external fixator mechanism between the patella and the tibia in sheep, while avoiding exposure of the patellar tendon itself. Stress shielding caused a significant reduction in the structural and material properties of stiffness (79%), ultimate load (69%), energy absorbed (61%), elastic modulus (76%) and ultimate stress (72%) of the tendon compared with controls. Compared with the material properties the structural properties exhibited better recovery after re-stressing with stiffness 97%, ultimate load 92%, energy absorbed 96%, elastic modulus 79% and ultimate stress 80%. The cross-sectional area of the re-stressed tendons was significantly greater than that of stress-shielded tendons. The remodelling phenomena exhibited in this study are consistent with a putative feedback mechanism under strain control. This study provides a basis from which to explore the interactions of tendon remodelling and mechanical environment.