Tendon disorders represent the most common musculoskeletal complaint for which patients seek medical attention; inflammation drives tendon degeneration before tearing and impairs healing after repair. Clinical evidence has implicated the nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) pathway as a correlate of pain-free return to function after surgical repair. However, it is currently unknown whether this response is a reaction to or a driver of pathology. Therefore, we aimed to understand the clinically relevant involvement of the NF-κB pathway in tendinopathy, to determine its potential causative roles in tendon degeneration, and to test its potential as a therapeutic candidate. Transcriptional profiling of early rotator cuff tendinopathy identified increases in NF-κB signaling, including increased expression of the regulatory serine kinase subunit IKKβ, which plays an essential role in inflammation. Using cre-mediated overexpression of IKKβ in tendon fibroblasts, we observed degeneration of mouse rotator cuff tendons and the adjacent humeral head. These changes were associated with increases in proinflammatory cytokines and innate immune cells within the joint. Conversely, genetic deletion of IKKβ in tendon fibroblasts partially protected mice from chronic overuse–induced tendinopathy. Furthermore, conditional knockout of IKKβ improved outcomes after surgical repair, whereas overexpression impaired tendon healing. Accordingly, targeting of the IKKβ/NF-κB pathway in tendon stromal cells may offer previously unidentified therapeutic approaches in the management of human tendon disorders.

IntroductionMacrophage-driven inflammation is a key feature of the early period following tendon repair, but excessive inflammation has been associated with poor clinical outcomes. Modulation of the inflammatory environment using molecular or cellular treatments may provide a means to enhance tendon healing.MethodsTo examine the effect of pro-inflammatory cytokines secreted by macrophages on tendon fibroblasts (TF), we established in vitro models of cytokine and macrophage-induced inflammation. Gene expression, protein expression, and cell viability assays were used to examine TF responses. In an effort to reduce the negative effects of inflammatory cytokines on TFs, adipose-derived mesenchymal stromal cells (ASCs) were incorporated into the model and their ability to modulate inflammation was investigated.ResultsThe inflammatory cytokine interleukin 1 beta (IL-1β) and macrophages of varying phenotypes induced up-regulation of pro-inflammatory factors and matrix degradation factors and down-regulation of factors related to extracellular matrix formation by TFs in culture. ASCs did not suppress these presumably negative effects induced by IL-1β. However, ASC co-culture with M1 (pro-inflammatory) macrophages successfully suppressed the effects of M1 macrophages on TFs by inducing a phenotypic switch from a pro-inflammatory macrophage phenotype to an anti-inflammatory macrophage phenotype, thus resulting in exposure of TFs to lower levels of pro-inflammatory cytokines (e.g., IL-1β, tumor necrosis factor alpha (TNFα)).ConclusionsThese findings suggest that IL-1β and M1 macrophages are detrimental to tendon healing and that ASC-mediated modulation of the post-operative inflammatory response may be beneficial for tendon healing.Electronic supplementary materialThe online version of this article (doi:10.1186/s13287-015-0059-4) contains supplementary material, which is available to authorized users.

Kormpakis, Ioannis; Havlioglu, Necat; Ominsky, Michael S.; Galatz, Leesa M.; and Thomopoulos, Stavros, ,"Sclerostin antibody treatment enhances rotator cuff tendon-to-bone healing in an animal model

The objective of this study was to understand the effect of pre-repair rotator cuff chronicity on post-repair healing outcomes using a chronic and acute multi-tendon rat rotator cuff injury model. Full-thickness dual tendon injuries (supra- and infraspinatus) were created unilaterally in adult male Sprague Dawley rats, and left chronically detached for 8 or 16 weeks. After chronic detachment, tears were repaired and acute dual tendon injuries were created and immediately repaired on contralateral shoulders. Tissue level outcomes for bone, tendon, and muscle were assessed 4 or 8 weeks after repair using histology, microcomputed tomography, biomechanical testing, and biochemical assays. Substantial gap formation was seen in 35% of acute repairs and 44% of chronic repairs. Gap formation negatively correlated with mechanical and structural outcomes for both healing time points regardless of injury duration. Bone and histomorphometry, as well as biomechanics, were similar between acute and chronic injury and repair regardless of chronicity and duration of healing. This study was the first to implement a multi-tendon rotator cuff injury with surgical repair following both chronic and acute injuries. Massive tear in a rodent model resulted in gap formation regardless of injury duration which had detrimental effects on repair outcomes.

Rotator cuff degeneration is a common affliction that results in pain and disability. Tendinopathy was historically classified with or without the involvement of the immune system. However, technological advancements in screening have shown that the immune system is both present and active in all forms of tendinopathy. During injury and healing, the coordinated effort of numerous immune cell populations work with the resident stromal cells to break down damaged tissues and stimulate remodeling. These cells deploy a wide array of tools, including phagocytosis, enzyme secretion, and chemotactic gradients to direct these processes. Yet, there remains a knowledge gap in our understanding of the sequence of critical events and regulatory factors that mediate this is process in injury and healing. Furthermore, current treatments do not specifically target inflammation at the molecular level. Typical regimens include non-steroidal anti-inflammatory drugs or corticosteroids; however, researchers have found irrevocable functional deficits following treatment, and have disputed their long-term efficacy. Therefore, developing therapeutics that specifically consider the nuances of the immune system are necessary to improve patient outcomes.