Abstract
Summary
Treatment of equine naturally occurring over-strain tendinopathy with mesenchymal stem cells suspended in bone marrow supernatant resulted in significant improvements compared to saline treated tendons in the normalisation of biomechanical, morphological, and compositional parameters with no adverse effects.
Introduction
Tendon injuries are a common age-related degenerative condition where natural repair involves scarification, resulting in a functionally inferior tissue1 that frequently re-injures. Naturally-occurring human and equine tendinopathy possess many similarities2 making the horse a good clinically-relevant model. A multitude of treatments are used but few have a strong evidence base. Regenerative approaches using mesenchymal stem cells (MSCs) to improve outcome are supported by clinical data demonstrating reduced re-injury rates in racehorses3. We therefore hypothesised that implantation of autologous MSCs into injured equine tendons would result in a tissue more closely resembling normal tendon matrix than the fibrous scar tissue formed subsequent to natural repair. The aim of this controlled experimental study was to assess the biomechanical, histological and compositional parameters following MSCs implantation into naturally injured tendons.
Methods
This study was carried out following informed consent from the owners of donated horses and under Institutional Ethics and Welfare Committee approval and UK Home Office Licences. Thoroughbred and Thoroughbred cross horses aged between 5 and 15 (mean 7.8±3.0) years, suffering career-ending severe superficial digital flexor tendinopathy (SDFT) within the metacarpal region of the forelimb of less than 2 months duration (average 30 days) were recruited. Autologous bone marrow derived MSCs were expanded and 1×107 MSCs in 2ml of citrated bone marrow supernatant were injected into the central defect of the damaged SDFT as described2 in 6 horses. The control group (6 horses) received an identical volume of isotonic saline. The horses then entered a standardised ascending exercise rehabilitation regimen of walking (3 months) and walking combined with trotting (next 3 months) after which horses were euthanised. Treated and contralateral SDFTs were recovered and analysed for mechanical, histological (blinded), and compositional parameters.
Results
The treated tendons exhibited statistically significant improvements in all parameters compared to saline-injected control tendons towards that of normal tendons and those in the contralateral limbs. Specifically, in comparison to saline-treated tendons, MSC treated tendons had significantly improved elasticity (p<0.05), lower (improved) histological scoring of organisation (p<0.001) and crimp pattern (p<0.05), lower cellularity from histological scoring (p<0.002) and DNA content (p<0.05), reduced vascularity (p<0.03), water content (p<0.05), GAG content (p<0.05). Total collagen content was unaltered between groups and interestingly, while tissue-linked fluorescence (indicator of collagen age) was similar between treatment and control groups, both were significantly lower than the contralateral limb (p<0.014), which suggested similar amounts of new collagen in the injured limbs. A higher remodelling rate, exemplified by increased MMP-13 activity, was also demonstrated for the control group compared to the MSC treated group and contralateral limbs (p<0.02).
Conclusion
This data support the hypothesis that the injection of MSC with BM supernatant results in a tissue more like normal tendon matrix rather than the fibrous scar tissue formed after natural injury and repair. The mechanism of this effect is not clear but the improved tissue elasticity, reduced vascularity, cell content and GAGs is more consistent with an immunomodulatory role for MSCs with a subsequent reduction in fibrosis than a true regenerative effect.
