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Mechanical loading stimulates tendon healing both when applied in the inflammatory phase and in the early remodeling phase of the process, although not necessarily via the same mechanisms. We investigated the gene response to mechanical loading in these two phases of tendon healing. The right Achilles tendon in rats was transected, and the hindlimbs were unloaded by tail suspension. The rats were exposed to 5 min of treadmill running 3 or 14 days after tendon transection. Thereafter, they were resuspended for 15 min or 3 h until euthanasia. The controls were suspended continuously. Gene analysis was first performed by microarray analysis followed by quantitative RT-PCR on selected genes, focusing on inflammation. Fifteen minutes after loading, the most important genes seemed to be the transcription factors EGR1 and C-FOS, regardless of healing phase. These transcription factors might promote tendon cell proliferation and differentiation, stimulate collagen production, and regulate inflammation. Three hours after loading on day 3 , inflammation was strongly affected. Seven inflammation-related genes were upregulated according to PCR: CCL20, CCL7, IL-6, NFIL3, PTX3, SOCS1, and TLR2. These genes can be connected to macrophages, T cells, and recruitment of leukocytes. According to Ingenuity Pathway Analysis, the recruitment of leukocytes was increased by loading on day 3 , which also was confirmed by histology. This inflammation-related gene response was not seen on day 14 Our results suggest that the immediate gene response after mechanical loading is similar in the early and late phases of healing but the late gene response is different. NEW & NOTEWORTHY This study investigates the direct effect of mechanical loading on gene expression during different healing phases in tendon healing. One isolated episode of mechanical loading was studied in otherwise unloaded healing tendons. This enabled us to study a time sequence, i.e., which genes were the first ones to be regulated after the loading episode., (Copyright © 2017 the American Physiological Society.)

Background and purpose — In 3 papers in Acta Orthopaedica 10 years ago, we described that platelet-rich plasma (PRP) improves tendon healing in a rat Achilles transection model. Later, we found that microtrauma has similar effects, probably acting via inflammation. This raised the suspicion that the effect ascribed to growth factors within PRP could instead be due to unspecific influences on inflammation. While testing this hypothesis, we noted that the effect seemed to be related to the microbiota. Material and methods — We tried to reproduce our old findings with local injection of PRP 6 h after tendon transection, followed by mechanical testing after 11 days. This failed. After fruitless variations in PRP production protocols, leukocyte concentration, and physical activity, we finally tried rats carrying potentially pathogenic bacteria. In all, 242 rats were used. Results — In 4 consecutive experiments on pathogen-free rats, no effect of PRP on healing was found. In contrast, apparently healthy rats carrying Staphylococcus aureus showed increased strength of the healing tendon after PRP treatment. These rats had higher levels of cytotoxic T-cells in their spleens. Interpretation — The failure to reproduce older experiments in clean rats was striking, and the difference in response between these and Staphylococcus-carrying rats suggests that the PRP effect is dependent on the immune status. PRP functions may be more complex than just the release of growth factors. Extrapolation from our previous findings with PRP to the situation in humans therefore becomes even more uncertain. [ABSTRACT FROM PUBLISHER]

Background: During early tendon healing, the cells within the regenerating tissue are, to a large part, inflammatory leukocytes (CD45+). In a rat Achilles tendon healing model, the inflammation resolves between 5 and 10 days. In the same model, Cox inhibitors (NSAIDs) impair healing when given during the first 5 days, but have a positive effect if given later. We tested the hypothesis that a Cox inhibitor would exert these effects by influencing inflammation, and thereby the composition of the inflammatory cell subpopulations. Methods: Achilles tendon transection was performed in 44 animals. Animals were randomized to either parecoxib or saline injections. Healing was evaluated by mechanical testing day 7 after surgery and by flow cytometry day 3 and 10. Results: Cross-sectional area, peak force and stiffness were reduced by parecoxib 31, 33, and 25% respectively (p=0.005, p=0.002, and p=0.005). By flow cytometry, there was a strong effect of time (p<0.001) on virtually all inflammatory cell subpopulations (CD45, CD11b, CD68, CCR7, CD163, CD206, CD3, CD4), but no significant effect of parecoxib at any time point. Conclusion: The results suggest that the negative effects of Cox inhibitors on tendon healing might be exerted mainly via mechanisms not directly related to inflammatory cells. [ABSTRACT FROM AUTHOR]