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Tendon injuries are a common clinical condition with limited treatment options. The cellular components of the innate immune system, such as neutrophils and macrophages, have been studied in tendon injuries. However, the adaptive immune system, comprising specialized lymphocytes, plays an important role in orchestrating the healing of numerous tissues, but less is known about these cells in tendon healing. To gain a greater understanding of the biological processes that regulate tendon healing, we determined how the cellular components of the adaptive and innate immune system respond to a tendon injury using two-month-old male mice. We observed that lymphatic vasculature is present in the epitenon and superficial regions of Achilles tendons, and that the lymphatics drain into the popliteal lymph node. We then created an acute Achilles tenotomy followed by repair, and collected tendons and popliteal lymph nodes 1, 2, and 4 wk after injury. Tendon injury resulted in a robust adaptive immune cell response that followed an initial innate immune cell response in tendons and lymph nodes. Monocytes, neutrophils, and macrophages initially accumulated at 1 wk after injury in tendons, while dendritic cells and CD4 + T cells peaked at 2 wk after injury. B cells and CD8 + T cells progressively increased over time. In parallel, immune cells of the popliteal lymph node demonstrated a similarly coordinated response to the injury. These results suggest that there is an adaptive immune response to tendon injury, and adaptive immune cells may play a role in regulating tendon healing. NEW & NOTEWORTHY While the innate immune system, consisting of macrophages and related hematopoietic cells, has been studied in tendon injury, less is known about the adaptive immune system. Using a mouse model of Achilles tendon tenotomy and repair, we observed an adaptive immune cell response, consisting of CD4 + and CD8 + T cells, and B cells, which occur through 4 wk after tendon injury. This response appeared to be coordinated by the draining popliteal lymph node.

Background: Fat accumulation commonly occurs in chronically torn rotator cuff muscles, and increased fat within the rotator cuff is correlated with poor clinical outcomes. The extent of lipid deposition is particularly pronounced in injured rotator cuff muscles compared with other commonly injured muscles such as the gastrocnemius. Satellite cells, which are a tissue-resident muscle stem-cell population, can differentiate into fat cells. We hypothesized that satellite cells from the rotator cuff have greater intrinsic adipogenic differentiation potential than do gastrocnemius satellite cells, and this difference is due to variations in epigenetic imprinting between the cells., Methods: Satellite cells from gastrocnemius and rotator cuff muscles of mice were cultured in adipogenic media, and the capacity to differentiate into mature muscle cells and adipogenic cells was assessed (n ≥ 9 plates per muscle group). We also performed DNA methylation analysis of gastrocnemius and rotator cuff satellite cells to determine whether epigenetic differences were present between the 2 groups (n = 5 mice per group)., Results: Compared with the gastrocnemius, satellite cells from the rotator cuff had a 23% reduction in myogenic differentiation and an 87% decrease in the expression of the differentiated muscle cell marker MRF4 (myogenic regulatory factor 4). With respect to adipogenesis, rotator cuff satellite cells had a 4.3-fold increase in adipogenesis, a 12-fold increase in the adipogenic transcription factor PPARγ (peroxisome proliferator-activated receptor gamma), and a 65-fold increase in the adipogenic marker FABP4 (fatty-acid binding protein 4). Epigenetic analysis identified 355 differentially methylated regions of DNA between rotator cuff and gastrocnemius satellite cells, and pathway enrichment analysis suggested that these regions were involved with lipid metabolism and adipogenesis., Conclusions: Satellite cells from rotator cuff muscles have reduced myogenic and increased adipogenic differentiation potential compared with gastrocnemius muscles. There appears to be a cellular and genetic basis behind the generally poor rates of rotator cuff muscle healing., Clinical Relevance: The reduced myogenic and increased adipogenic capacity of rotator cuff satellite cells is consistent with the increased fat content and poor muscle healing rates often observed for chronically torn rotator cuff muscles. For patients undergoing rotator cuff repair, transplantation of autologous satellite cells from other muscles less prone to fatty infiltration may improve clinical outcomes.

Background: The repair of rotator cuff tears is often complicated by fatty degeneration, which is the combination of lipid accumulation, fibrosis, inflammation, and muscle weakness. A signaling molecule that plays a central role in these processes is p38 mitogen-activated protein kinase (MAPK). The purpose of this study was to evaluate the ability of a small molecule inhibitor of p38 MAPK, SB203580, to reduce fatty degeneration in a preclinical model of rotator cuff injury and repair., Materials and Methods: Adult rats underwent a bilateral supraspinatus tenotomy that was repaired 30 days later. Rats were treated with SB203580 or vehicle every 2 days, with injections beginning 3 days before surgery and continuing until 7 days after surgery. Two weeks after surgical repair, muscles were analyzed using histology, lipid profiling, gene expression, and permeabilized muscle fiber contractility., Results: Inhibition of p38 MAPK resulted in a nearly 49% reduction in fat accumulation and a 29% reduction in collagen content, along with changes in corresponding genes regulating adipogenesis and matrix accumulation. There was also a marked 40% to 80% decrease in the expression of several proinflammatory genes, including IL1B, IL6, and COX2, and a 360% increase in the anti-inflammatory gene IL10. No differences were observed for muscle fiber force production., Conclusion: Inhibition of p38 MAPK was found to result in a significant decrease in intramuscular lipid accumulation and fibrosis that is usually seen in the degenerative cascade of rotator cuff tears, without having negative effects on the contractile properties of the rotator cuff muscle tissue., (Copyright © 2016 Journal of Shoulder and Elbow Surgery Board of Trustees. Published by Elsevier Inc. All rights reserved.)

Mechanical loading can increase tendon cross-sectional area (CSA), but the mechanisms by which this occurs are largely unknown. To gain a greater understanding of the cellular mechanisms of adult tendon growth in response to mechanical loading, we used a synergist ablation model whereby a tenectomy of the Achilles tendon was performed to induce growth of the synergist plantaris tendon. We hypothesized that after synergist ablation progenitor cells in the epitenon would proliferate and increase the size of the existing tendon matrix. Adult male mice were subjected to a bilateral Achilles tenectomy, and plantaris tendons were isolated from mice at 0, 2, 7, 14, and 28 days after surgery. Tendons were sectioned stained with either fast green and hematoxylin, prepared for fluorescent microscopy, or prepared for gene expression of scleraxis and type I collagen. After overload, there was a dramatic increase in total CSA of tendons, whereas the size of the original tendon matrix was not changed. Growth primarily occurred through the formation of a neotendon matrix between the original tendon and the epitenon, and contained cells that were proliferative and scleraxis positive. Additionally, an initial expansion of fibroblast cells occurred before the synthesis of new extracellular matrix. Fibroblasts in the original tendon did not re-enter the cell cycle. The results from this study provide new insight into the mechanisms of tendon growth, indicate tendon consists mostly of postmitotic cells, and that growth of tendon primarily occurs from the most superficial layers outward., (Copyright © 2014 the American Physiological Society.)