Mesenchymal stem cells and insulin-like growth factor-I gene-enhanced mesenchymal stem cells improve structural aspects of healing in equine flexor digitorum superficialis tendons

Treatment of racehorse superficial digital flexor tendonitis: A comparison of stem cell treatments to controlled exercise rehabilitation in 213 cases

BACKGROUND

Overstrain of the superficial digital flexor tendon (SDFT) is a common Thoroughbred racehorse limb injury requiring treatment.

OBJECTIVES

To determine whether treatment of SDFT lesions in flat Thoroughbred racehorses with autologous bone marrow-derived mesenchymal stem cells (BM-MSCs) or allogenic adipose-derived mesenchymal stem cells (A-MSCs) is associated with improved likelihood of returning to racing, when compared to racehorses managed with a controlled exercise rehabilitation program (CERP) alone.

STUDY DESIGN

Retrospective cohort study combining clinical treatment records with race records.

METHODS

A total of 213 Thoroughbred racehorses were identified. All were prescribed the same 12-month CERP and 66 also received intralesional BM-MSC and 17 A-MSC treatment. Follow-up was a minimum of 2 years after return to full race training. Multivariable logistic regression models were used to investigate associations between the treatments and the likelihood of returning to racing and completing five or more (C5+) races post-injury.

RESULTS

Compared to CERP alone, BM-MSC treatment was associated with increased odds of returning to racing (OR 3.19; 95% CI 1.55-6.81) and C5+ races post-injury (OR 2.64; 95% CI 1.32-5.33). Older age and increasing lesion length were associated with a reduced likelihood of returning to racing. Male sex and increased number of pre-injury starts were associated with increased odds of returning to racing. There was no observed increased likelihood of return to racing or C5+ races associated with treatment with A-MSCs compared to CERP alone.

MAIN LIMITATIONS

Due to the retrospective nature of the study it was not possible to ascertain how strictly the CERP was followed. Due to the novelty of the method, the A-MSC treatment group included a limited number of horses.

CONCLUSIONS

In the study population, intralesional BM-MSC treatment was significantly associated with an increased likelihood of returning to racing and C5+ races post-injury compared to CERP alone. Intralesional A-MSC showed no significant association between treatment and the two investigated outcomes.

Treatment of Naturally Occurring Tendon Disease with Allogeneic Multipotent Mesenchymal Stromal Cells: A Randomized, Controlled, Triple-Blinded Pilot Study in Horses

The treatment of tendinopathies with multipotent mesenchymal stromal cells (MSCs) is a promising option in equine and human medicine. However, conclusive clinical evidence is lacking. The purpose of this study was to gain insight into clinical treatment efficacy and to identify suitable outcome measures for larger clinical studies. Fifteen horses with early naturally occurring tendon disease were assigned to intralesional treatment with allogeneic adipose-derived MSCs suspended in serum or with serum alone through block randomization (dosage adapted to lesion size). Clinicians and horse owners remained blinded to the treatment during 12 months (seven horses per group) and 18 months (seven MSC-group and five control-group horses) of follow-up including clinical examinations and diagnostic imaging. Clinical inflammation, lameness, and ultrasonography scores improved more over time in the MSC group. The lameness score difference significantly improved in the MSC group compared with the control group after 6 months. In the MSC group, five out of the seven horses were free of re-injuries and back to training until 12 and 18 months. In the control group, three out of the seven horses were free of re-injuries until 12 months. These results suggest that MSCs are effective for the treatment of early-phase tendon disease and provide a basis for a larger controlled study.

Preclinical tendon and ligament models: Beyond the 3Rs (replacement, reduction, and refinement) to 5W1H (why, who, what, where, when, how)

Several tendon and ligament animal models were presented at the 2022 Orthopaedic Research Society Tendon Section Conference held at the University of Pennsylvania, May 5 to 7, 2022. A key objective of the breakout sessions at this meeting was to develop guidelines for the field, including for preclinical tendon and ligament animal models. This review summarizes the perspectives of experts for eight surgical small and large animal models of rotator cuff tear, flexor tendon transection, anterior cruciate ligament tear, and Achilles tendon injury using the framework: "Why, Who, What, Where, When, and How" (5W1H). A notable conclusion is that the perfect tendon model does not exist; there is no single gold standard animal model that represents the totality of tendon and ligament disease. Each model has advantages and disadvantages and should be carefully considered in light of the specific research question. There are also circumstances when an animal model is not the best approach. The wide variety of tendon and ligament pathologies necessitates choices between small and large animal models, different anatomic sites, and a range of factors associated with each model during the planning phase. Attendees agreed on some guiding principles including: providing clear justification for the model selected, providing animal model details at publication, encouraging sharing of protocols and expertise, improving training of research personnel, and considering greater collaboration with veterinarians. A clear path for translating from animal models to clinical practice was also considered as a critical next step for accelerating progress in the tendon and ligament field.

Administration and detection of gene therapy in horses: A systematic review

Gene therapy uses genetic modification of cells to produce a therapeutic effect. Defective or missing genes can be repaired or replaced, or gene expression can be modified using a variety of technologies. Repair of defective genes can be achieved using specialized gene editing tools. Gene addition promotes gene expression by introducing synthetic copies of genes of interest (transgenes) into cells where they are transcribed and translated into therapeutic proteins. Protein production can also be modified using therapies that regulate gene expression. Gene therapy is currently prohibited in both human and equine athletes because of the potential to induce production of performance-enhancing proteins in the athlete's body, also referred to as "gene doping." Detection of gene doping is challenging and necessitates development of creative, novel analytical methods for doping control. Methods for detection of gene doping must be specific to and will vary depending on the type of gene therapy. The purpose of this paper is to present the results of a systematic review of gene editing, gene therapy, and detection of gene doping in horses. Based on the published literature, gene therapy has been administered to horses in a large number of experimental studies and a smaller number of clinical cases. Detection of gene therapy is possible using a combination of PCR and sequencing technologies. This summary can provide a basis for discussion of appropriate and inappropriate uses for gene therapy in horses by the veterinary community and guide expansion of methods to detect inappropriate uses by the regulatory community.

Recent advances in tendon tissue engineering strategy

Tendon injuries often result in significant pain and disability and impose severe clinical and financial burdens on our society. Despite considerable achievements in the field of regenerative medicine in the past several decades, effective treatments remain a challenge due to the limited natural healing capacity of tendons caused by poor cell density and vascularization. The development of tissue engineering has provided more promising results in regenerating tendon-like tissues with compositional, structural and functional characteristics comparable to those of native tendon tissues. Tissue engineering is the discipline of regenerative medicine that aims to restore the physiological functions of tissues by using a combination of cells and materials, as well as suitable biochemical and physicochemical factors. In this review, following a discussion of tendon structure, injury and healing, we aim to elucidate the current strategies (biomaterials, scaffold fabrication techniques, cells, biological adjuncts, mechanical loading and bioreactors, and the role of macrophage polarization in tendon regeneration), challenges and future directions in the field of tendon tissue engineering.

Effect of Adipose-Derived Mesenchymal Stem Cells (ADMSCs) Application in Achilles-Tendon Injury in an Animal Model

Background: Achilles-tendon rupture prevails as a common tendon pathology. Adipose-derived mesenchymal stem cells (ADMSCs) are multipotent stem cells derived from adipose tissue with attractive regeneration properties; thus, their application in tendinopathies could be beneficial. Methods: Male rabbit ADMSCs were obtained from the falciform ligament according to previously established methods. After tenotomy and suture of the Achilles tendon, 1 × 106 flow-cytometry-characterized male ADMSCs were injected in four female New Zealand white rabbits in the experimental group (ADMSC group), whereas four rabbits were left untreated (lesion group). Confirmation of ADMSC presence in the injured site after 12 weeks was performed with quantitative sex-determining region Y (SRY)-gene RT-PCR. At Week 12, histochemical analysis was performed to evaluate tissue regeneration along with quantitative RT-PCR of collagen I and collagen III mRNA. Results: Presence of male ADMSCs was confirmed at Week 12. No statistically significant differences were found in the histochemical analysis; however, statistically significant differences between ADMSC and lesion group expression of collagen I and collagen III were evidenced, with 36.6% and 24.1% GAPDH-normalized mean expression, respectively, for collagen I (p < 0.05) and 26.3% and 11.9% GAPDH-normalized mean expression, respectively, for collagen III (p < 0.05). The expression ratio between the ADMSC and lesion group was 1.5 and 2.2 for collagen I and collagen III, respectively. Conclusion: Our results make an important contribution to the understanding and effect of ADMSCs in Achilles-tendon rupture.

TGF-β2 enhances expression of equine bone marrow-derived mesenchymal stem cell paracrine factors with known associations to tendon healing

Background

Mesenchymal stem cells (MSCs) secrete paracrine factors and extracellular matrix proteins that contribute to their ability to support tissue healing and regeneration. Both the transcriptome and the secretome of MSCs can be altered by treating the cells with cytokines, but neither have been thoroughly investigated following treatment with the specific cytokine transforming growth factor (TGF)-β2.

Methods

RNA-sequencing and western blotting were used to compare gene and protein expression between untreated and TGF-β2-treated equine bone marrow-derived MSCs (BM-MSCs). A co-culture system was utilized to compare equine tenocyte migration during co-culture with untreated and TGF-β2-treated BM-MSCs.

Results

TGF-β2 treatment significantly upregulated gene expression of collagens, extracellular matrix molecules, and growth factors. Protein expression of collagen type I and tenascin-C was also confirmed to be upregulated in TGF-β2-treated BM-MSCs compared to untreated BM-MSCs. Both untreated and TGF-β2-treated BM-MSCs increased tenocyte migration in vitro.

Conclusions

Treating equine BM-MSCs with TGF-β2 significantly increases production of paracrine factors and extracellular matrix molecules important for tendon healing and promotes the migration of tenocytes in vitro.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-022-03172-9.

Mesenchymal Stem Cell Use in Acute Tendon Injury: In Vitro Tenogenic Potential vs. In Vivo Dose Response

Stem cell therapy for the treatment of tendon injury is an emerging clinical practice in the fields of human and veterinary sports medicine; however, the therapeutic benefit of intralesional transplantation of mesenchymal stem cells in tendonitis cases is not well designed. Questions persist regarding the overall tenogenic potential and efficacy of this treatment alone. In this study, we aimed to isolate a rat mesenchymal stem cell lineage for in vitro and in vivo use, to assess the effects of growth factor exposure in vitro on cell morphology, behavior, and tendon-associated glycoprotein production, and to assess the therapeutic potential of intralesional stem cells, as a function of dose, in vivo. First, rat adipose-derived (rAdMSC) and bone marrow-derived (rBMSC) stem cell lineages were isolated, characterized with flow cytometric analysis, and compared in terms of proliferation (MTS assay) and cellular viability (calcein AM staining). Rat AdMSCs displayed superior proliferation and more homogenous CD 73, CD 44H, and CD 90 expression as compared to rBMSC. Next, the tenogenic differentiation potential of the rAdMSC lineage was tested in vitro through isolated and combined stimulation with reported tenogenic growth factors, transforming growth factor (TGF)-β3 and connective tissue growth factor (CTGF). We found that the most effective tenogenic factor in terms of cellular morphologic change, cell alignment/orientation, sustained cellular viability, and tendon-associated glycoprotein upregulation was TGFβ3, and we confirmed that rAdMSC could be induced toward a tenogenic lineage in vitro. Finally, the therapeutic potential of rAdMSCs as a function of dose was assessed using a rat acute Achilles tendon injury model. Amounts of 5 × 10⁵ (low dose) and 4 × 10⁶ (high dose) were used. Subjectively, on the gross morphology, the rAdMSC-treated tendons exhibited fewer adhesions and less scar tissue than the control tendons; however, regardless of the rAdMSC dose, no significant differences in histological grade or tissue collagen I deposition were noted between the rAdMSC-treated and control tendons. Collectively, rAdMSCs exhibited appropriate stem cell markers and tenogenic potential in vitro, but the clinical efficacy of intralesional implantation of undifferentiated cells in acute tendonitis cases could not be proven. Further investigation into complementary therapeutics or specialized culture conditions prior to implantation are warranted.

Interleukin-1β in tendon injury enhances reparative gene and protein expression in mesenchymal stem cells

Tendon injury in the horse carries a high morbidity and monetary burden. Despite appropriate therapy, reinjury is estimated to occur in 50–65% of cases. Although intralesional mesenchymal stem cell (MSC) therapy has improved tissue architecture and reinjury rates, the mechanisms by which they promote repair are still being investigated. Additionally, reevaluating our application of MSCs in tendon injury is necessary given recent evidence that suggests MSCs exposed to inflammation (deemed MSC licensing) have an enhanced reparative effect. However, applying MSC therapy in this context is limited by the inadequate quantification of the temporal cytokine profile in tendon injury, which hinders our ability to administer MSCs into an environment that could potentiate their effect. Therefore, the objectives of this study were to define the temporal cytokine microenvironment in a surgically induced model of equine tendon injury using ultrafiltration probes and subsequently evaluate changes in MSC gene and protein expression following in vitro inflammatory licensing with cytokines of similar concentration as identified in vivo. In our in vivo surgically induced tendon injury model, IL-1β and IL-6 were the predominant pro-inflammatory cytokines present in tendon ultrafiltrate where a discrete peak in cytokine concentration occurred within 48 h following injury. Thereafter, MSCs were licensed in vitro with IL-1β and IL-6 at a concentration identified from the in vivo study; however, only IL-1β induced upregulation of multiple genes beneficial to tendon healing as identified by RNA-sequencing. Specifically, vascular development, ECM synthesis and remodeling, chemokine and growth factor function alteration, and immunomodulation and tissue reparative genes were significantly upregulated. A significant increase in the protein expression of IL-6, VEGF, and PGE2 was confirmed in IL-1β-licensed MSCs compared to naïve MSCs. This study improves our knowledge of the temporal tendon cytokine microenvironment following injury, which could be beneficial for the development and determining optimal timing of administration of regenerative therapies. Furthermore, these data support the need to further study the benefit of MSCs administered within the inflamed tendon microenvironment or exogenously licensed with IL-1β in vitro prior to treatment as licensed MSCs could enhance their therapeutic benefit in the healing tendon.

Advances in Stem Cell Therapies for Rotator Cuff Injuries

Rotator cuff injury is a common upper extremity musculoskeletal disease that may lead to persistent pain and functional impairment. Despite the clinical outcomes of the surgical procedures being satisfactory, the repair of the rotator cuff remains problematic, such as through failure of healing, adhesion formation, and fatty infiltration. Stem cells have high proliferation, strong paracrine action, and multiple differentiation potential, which promote tendon remodeling and fibrocartilage formation and increase biomechanical strength. Additionally, stem cell-derived extracellular vesicles (EVs) can increase collagen synthesis and inhibit inflammation and adhesion formation by carrying regulatory proteins and microRNAs. Therefore, stem cell-based therapy is a promising therapeutic strategy that has great potential for rotator cuff healing. In this review, we summarize the advances of stem cells and stem cell-derived EVs in rotator cuff repair and highlight the underlying mechanism of stem cells and stem cell-derived EVs and biomaterial delivery systems. Future studies need to explore stem cell therapy in combination with cellular factors, gene therapy, and novel biomaterial delivery systems.

MSC in Tendon and Joint Disease: The Context-Sensitive Link Between Targets and Therapeutic Mechanisms

Mesenchymal stromal cells (MSC) represent a promising treatment option for tendon disorders and joint diseases, primarily osteoarthritis. Since MSC are highly context-sensitive to their microenvironment, their therapeutic efficacy is influenced by their tissue-specific pathologically altered targets. These include not only cellular components, such as resident cells and invading immunocompetent cells, but also components of the tissue-characteristic extracellular matrix. Although numerous in vitro models have already shown potential MSC-related mechanisms of action in tendon and joint diseases, only a limited number reflect the disease-specific microenvironment and allow conclusions about well-directed MSC-based therapies for injured tendon and joint-associated tissues. In both injured tissue types, inflammatory processes play a pivotal pathophysiological role. In this context, MSC-mediated macrophage modulation seems to be an important mode of action across these tissues. Additional target cells of MSC applied in tendon and joint disorders include tenocytes, synoviocytes as well as other invading and resident immune cells. It remains of critical importance whether the context-sensitive interplay between MSC and tissue- and disease-specific targets results in an overall promotion or inhibition of the desired therapeutic effects. This review presents the authors’ viewpoint on disease-related targets of MSC therapeutically applied in tendon and joint diseases, focusing on the equine patient as valid animal model.

Tendon Tissue Repair in Prospective of Drug Delivery, Regenerative Medicines, and Innovative Bioscaffolds

The natural healing capacity of the tendon tissue is limited due to the hypovascular and cellular nature of this tissue. So far, several conventional approaches have been tested for tendon repair to accelerate the healing process, but all these approaches have their own advantages and limitations. Regenerative medicine and tissue engineering are interdisciplinary fields that aspire to develop novel medical devices, innovative bioscaffold, and nanomedicine, by combining different cell sources, biodegradable materials, immune modulators, and nanoparticles for tendon tissue repair. Different studies supported the idea that bioscaffolds can provide an alternative for tendon augmentation with an enormous therapeutic potentiality. However, available data are lacking to allow definitive conclusion on the use of bioscaffolds for tendon regeneration and repairing. In this review, we provide an overview of the current basic understanding and material science in the field of bioscaffolds, nanomedicine, and tissue engineering for tendon repair.

Use of autologous products for the treatment of joint and soft tissue disease in horses: A systematic review

BACKGROUND

Soft tissue injuries and joint disease are the predominate causes of lameness in the equine athlete and these pathologies carry a guarded prognosis for a return to previous performance. Recently the use of autologous products has become more widespread as a treatment in equine sports medicine. However, the efficacy of these products is yet to be fully established.

OBJECTIVE

To evaluate the current published evidence base regarding the efficacy of autologous products in soft tissue injuries and joint disease.

METHODS

A systematic review of English articles using MEDLINE, EMBASE and Web of Science databases from 1980 to 2017. The search strategy identified 1594 papers for review.

RESULTS

Fifty-eight papers were included in this review, 28 of which were randomised controlled trials. Significant benefit was reported under several parameters, most notably in the use of autologous chondrocytes in artificially induced cartilage defects on histology. One paper documented a significant clinical response under lameness examination.

CONCLUSION

The current literature shows that the treatment of soft tissue injury and cartilage disease with autologous products is safe and that the use of some products can give significant benefit on some outcome measures. True clinical significance is yet to be demonstrated with any product.

Tissue predictability of elastography is low in collagenase induced deep digital flexor tendinopathy

Elastography is an emerging imaging modality for characterizing tendon injury in horses, but its ability to differentiate tissue deformability relative to treatment group and biochemical properties using a prospective, experimental study design remain unknown. Objectives of the current study were to (a) to investigate differences in glycosaminoglycan, DNA, and soluble collagen levels in mesenchymal stem cell (MSC) treated limbs compared to untreated control limbs utilizing a collagenase model of tendinopathy; (b) compare elastographic features between treatment groups; and (c) determine tissue-level predictive capabilities of elastography in relation to biochemical outcomes. Bone marrow was collected for MSC culture and expansion. Tendinopathy of both forelimb deep digital flexor tendons (DDFTs) was induced with collagenase under ultrasonographic guidance. One randomly assigned limb was treated with intra-lesional MSC injection with the opposite limb serving as an untreated control. Horses were placed into a controlled exercise program with elastographic evaluations performed baseline (0) and 14, 60, 90, and 214 days post-treatment. Postmortem biochemical analysis was performed. MSC-treated limbs demonstrated significantly less (42%) glycosaminoglycan (P = .006). Significant differences in elastographic region of interest (ROI) percent hardness, ROI color histogram, and subjective lesion stiffness were appreciated between treatment groups at various study time points. Elastographic outcome parameters were weak predictors of biochemical tissue analysis, with all R² values ≤ 0.50. Within this range of differences in glycosaminoglycan content between treatment groups, elastography outcomes did not predict biochemical differences. Tissue-specific differences between DDFTs treated with MSCs compared to controls were apparent biochemically, but not predicted by elastography.

Evaluation of Autologous Protein Solution Injection for Treatment of Superficial Digital Flexor Tendonitis in an Equine Model

Autologous protein solution (APS) has been used anecdotally for intralesional treatment of tendon and ligament injuries, however, its use in these injuries has never been studied in vivo. Our objective was to evaluate the effect of APS on tendon healing in an equine superficial digital flexor (SDF) tendonitis model. We hypothesized intralesional injection of APS would result in superior structural and biomechanical healing. SDF tendonitis was induced in both forelimbs of eight horses using collagenase injection. One forelimb was randomly assigned to receive an intralesional injection of APS, while the other was injected with saline. Ultrasonographic examinations were performed at weeks −1, 0, 2, 4, 8, and 12 following treatment. At 12 weeks, horses were euthanized and SDF samples harvested. Histologic evaluation, biomechanical testing, gene expression analysis, total glycosaminoglycan (GAG) and total DNA quantification were performed. Collagen type III (COL3A1) expression was significantly higher (p = 0.028) in saline treated tendon than in normal tendon. Otherwise, there were no significant differences in gene expression. There were no significant differences in histologic or ultrasonographic scores between groups. Mean total DNA content was significantly higher (p = 0.024) in saline treated tendons than normal tendons, whereas total DNA content was not significantly different between APS treated tendon and normal tendon. Elastic modulus was higher in APS treated than saline treated tendon, but the difference was not significant. Reduced expression of COL3A1 in APS treated tendon may indicate superior healing. Increased total DNA content in saline treated tendon may indicate ongoing healing processes, vs. APS treated tendons which may be in the later stages of healing. Limitations include a relatively short study period and inconsistency in size and severity of induced lesions. Intralesional injection of APS resulted in some improvements in healing characteristics.

The Lack of a Representative Tendinopathy Model Hampers Fundamental Mesenchymal Stem Cell Research

Overuse tendon injuries are a major cause of musculoskeletal morbidity in both human and equine athletes, due to the cumulative degenerative damage. These injuries present significant challenges as the healing process often results in the formation of inferior scar tissue. The poor success with conventional therapy supports the need to search for novel treatments to restore functionality and regenerate tissue as close to native tendon as possible. Mesenchymal stem cell (MSC)-based strategies represent promising therapeutic tools for tendon repair in both human and veterinary medicine. The translation of tissue engineering strategies from basic research findings, however, into clinical use has been hampered by the limited understanding of the multifaceted MSC mechanisms of action. In vitro models serve as important biological tools to study cell behavior, bypassing the confounding factors associated with in vivo experiments. Controllable and reproducible in vitro conditions should be provided to study the MSC healing mechanisms in tendon injuries. Unfortunately, no physiologically representative tendinopathy models exist to date. A major shortcoming of most currently available in vitro tendon models is the lack of extracellular tendon matrix and vascular supply. These models often make use of synthetic biomaterials, which do not reflect the natural tendon composition. Alternatively, decellularized tendon has been applied, but it is challenging to obtain reproducible results due to its variable composition, less efficient cell seeding approaches and lack of cell encapsulation and vascularization. The current review will overview pros and cons associated with the use of different biomaterials and technologies enabling scaffold production. In addition, the characteristics of the ideal, state-of-the-art tendinopathy model will be discussed. Briefly, a representative in vitro tendinopathy model should be vascularized and mimic the hierarchical structure of the tendon matrix with elongated cells being organized in a parallel fashion and subjected to uniaxial stretching. Incorporation of mechanical stimulation, preferably uniaxial stretching may be a key element in order to obtain appropriate matrix alignment and create a pathophysiological model. Together, a thorough discussion on the current status and future directions for tendon models will enhance fundamental MSC research, accelerating translation of MSC therapies for tendon injuries from bench to bedside.

Translational Animal Models Provide Insight Into Mesenchymal Stromal Cell (MSC) Secretome Therapy

The therapeutic potential of the mesenchymal stromal cell (MSC) secretome, consisting of all molecules secreted by MSCs, is intensively studied. MSCs can be readily isolated, expanded, and manipulated in culture, and few people argue with the ethics of their collection. Despite promising pre-clinical studies, most MSC secretome-based therapies have not been implemented in human medicine, in part because the complexity of bioactive factors secreted by MSCs is not completely understood. In addition, the MSC secretome is variable, influenced by individual donor, tissue source of origin, culture conditions, and passage. An increased understanding of the factors that make up the secretome and the ability to manipulate MSCs to consistently secrete factors of biologic importance will improve MSC therapy. To aid in this goal, we can draw from the wealth of information available on secreted factors from MSC isolated from veterinary species. These translational animal models will inspire efforts to move human MSC secretome therapy from bench to bedside.

Cellular Proliferation of Equine Bone Marrow- and Adipose Tissue-Derived Mesenchymal Stem Cells Decline With Increasing Donor Age

Background: Bone marrow (BM)- and adipose tissue (AT)-derived mesenchymal stem cells (MSCs) are used increasingly for autologous cell therapy in equine practice to treat musculoskeletal and other injuries. Current recommendations often call for 10–100 million MSCs per treatment, necessitating the expansion of primary cells in culture prior to therapeutic use. Of concern, human and rodent studies have shown a decline of both MSC recovery from sampled tissue and in vitro proliferative capacity with increasing donor age. This may be problematic for applications of autologous cell-based therapies in the important equine demographic of older patients.

Objectives: To investigate the effect of donor age on the cellular proliferation of equine BM- and AT-MSCs.

Study Design:In vitro study.

Methods: BM- and AT-MSCs and dermal fibroblasts (biological control) were harvested from horses in five different age groups (n = 4, N = 60); newborn (0 days), yearling (15–17 months), adult (5–8 years), middle-aged (12–18 years), and geriatric (≥22 years). Proliferation of the cells was tested using an EdU incorporation assay and steady state mRNA levels measured for targeted proliferation, aging, and senescence biomarkers.

Results: The cellular proliferation of equine BM- and AT-MSCs declined significantly in the geriatric cohort relative to the younger age groups. Proliferation levels in the two MSC types were equally affected by donor age. Analysis of steady state mRNA levels showed an up-regulation in tumor suppressors, apoptotic genes, and multiple growth factors in MSCs from old horses, and a down-regulation of some pro-cycling genes with a few differences between cell types.

Main Limitations: Potential age-dependent differences in cell function parameters relevant to cell-therapy application were not investigated.

Conclusions: The cellular proliferation of equine BM- and AT-MSCs declined at advanced donor ages. High levels of in vitro proliferation were observed in both MSC types from horses in the age groups below 18 years of age.

Mesenchymal Stem Cell-Mediated Immuno-Modulatory and Anti- Inflammatory Mechanisms in Immune and Allergic Disorders

Background: Mesenchymal Stem Cells (MSCs) are present in almost all the tissues of the body and act as the backbone of the internal tissue homeostasis. Among their various characteristic features, immuno-modulatory and/ anti-inflammatory properties play an important role in therapeutics.

Objective: The current topic focuses on the characterization and immuno-modulatory and/ anti-inflammatory properties of MSCs. To present and discuss the current status of MSCs immuno-modulatory properties.

Methods: Available literature on MSCs properties and patents have been detailed, critically interpreted, and discussed based upon available literature. The main focus has been on their characteristic immuno-modulatory and anti-inflammatory properties though some of the basic characterization markers have also been detailed. The databases searched for the literature include PubMed, Med Line, PubMed Central, Science Direct and a few other scientific databases.

Results: MSCs are present in a very limited concentration in the tissues, and as such their culture expansion becomes imperative. MSCs immuno-modulatory and anti-inflammatory roles are achieved through direct cell-cell contact and / by the release of certain factors. Such properties are controlled by micro-environment upon which currently very limited control can be exerted. Besides, further insights in the xeno-protein free culture media as against the fetal bovine serum is required.

Conclusion: MSCs have been well-isolated, cultured and characterized from numerous tissues of the body. The majority of the studies have shown MSCs as immuno-compromised with immunomodulatory and / or anti-inflammatory properties except some of the latest studies that have failed to achieve the desired results and thus, demand further research. Further research is required in the area to translate the results into clinical application.

FGF2: a key regulator augmenting tendon-to-bone healing and cartilage repair

Ligament/tendon and cartilage injuries are clinically common diseases that perplex most clinicians. Because of the lack of blood vessels and nerves, their self-repairing abilities are rather poor. Therefore, surgeries are necessary and also widely used to treat ligament/tendon or cartilage injuries. However, after surgery, there are still many problems that affect healing. In recent years, it has been found that exogenous FGF2 plays an important role in the repair of ligament/tendon and cartilage injuries and exerts a synergistic effect with endogenous FGF2. Therefore, FGF2 can be used as a new type of biomolecule to accelerate tendon-to-bone healing and cartilage repair after injury.

In Vitro Innovation of Tendon Tissue Engineering Strategies

Tendinopathy is the term used to refer to tendon disorders. Spontaneous adult tendon healing results in scar tissue formation and fibrosis with suboptimal biomechanical properties, often resulting in poor and painful mobility. The biomechanical properties of the tissue are negatively affected. Adult tendons have a limited natural healing capacity, and often respond poorly to current treatments that frequently are focused on exercise, drug delivery, and surgical procedures. Therefore, it is of great importance to identify key molecular and cellular processes involved in the progression of tendinopathies to develop effective therapeutic strategies and drive the tissue toward regeneration. To treat tendon diseases and support tendon regeneration, cell-based therapy as well as tissue engineering approaches are considered options, though none can yet be considered conclusive in their reproduction of a safe and successful long-term solution for full microarchitecture and biomechanical tissue recovery. In vitro differentiation techniques are not yet fully validated. This review aims to compare different available tendon in vitro differentiation strategies to clarify the state of art regarding the differentiation process.

Large Animal Models in Regenerative Medicine and Tissue Engineering: To Do or Not to Do

Rapid developments in Regenerative Medicine and Tissue Engineering has witnessed an increasing drive toward clinical translation of breakthrough technologies. However, the progression of promising preclinical data to achieve successful clinical market authorisation remains a bottleneck. One hurdle for progress to the clinic is the transition from small animal research to advanced preclinical studies in large animals to test safety and efficacy of products. Notwithstanding this, to draw meaningful and reliable conclusions from animal experiments it is critical that the species and disease model of choice is relevant to answer the research question as well as the clinical problem. Selecting the most appropriate animal model requires in-depth knowledge of specific species and breeds to ascertain the adequacy of the model and outcome measures that closely mirror the clinical situation. Traditional reductionist approaches in animal experiments, which often do not sufficiently reflect the studied disease, are still the norm and can result in a disconnect in outcomes observed between animal studies and clinical trials. To address these concerns a reconsideration in approach will be required. This should include a stepwise approach using in vitro and ex vivo experiments as well as in silico modeling to minimize the need for in vivo studies for screening and early development studies, followed by large animal models which more closely resemble human disease. Naturally occurring, or spontaneous diseases in large animals remain a largely untapped resource, and given the similarities in pathophysiology to humans they not only allow for studying new treatment strategies but also disease etiology and prevention. Naturally occurring disease models, particularly for longer lived large animal species, allow for studying disorders at an age when the disease is most prevalent. As these diseases are usually also a concern in the chosen veterinary species they would be beneficiaries of newly developed therapies. Improved awareness of the progress in animal models is mutually beneficial for animals, researchers, human and veterinary patients. In this overview we describe advantages and disadvantages of various animal models including domesticated and companion animals used in regenerative medicine and tissue engineering to provide an informed choice of disease-relevant animal models.

Advances in the development of gene therapy, noncoding RNA, and exosome-based treatments for tendinopathy

Tendinopathy is a common musculoskeletal disorder characterized by chronic low-grade inflammation and tissue degeneration. Tendons have poor innate healing ability and there is currently no cure for tendinopathy. Studies elucidating mechanisms underlying the pathogenesis of tendinopathy and mechanisms mediating the genesis of tendons during development have provided novel targets and strategies to enhance tendon healing and repair. This review summarizes the current understanding and treatments for tendinopathy. The review also highlights recent advances in gene therapy, the potential of noncoding RNAs, such as microRNAs, and exosomes, which are nanometer-sized extracellular vesicles secreted from cells, for the treatment of tendinopathy.

Quantitative Assessment of Tendon Hierarchical Structure by Combined Second Harmonic Generation and Immunofluorescence Microscopy

Histological evaluation of healing tendons is primarily focused on monitoring restoration of longitudinal collagen alignment, although the elastic property of energy-storing flexor tendons is largely attributed to interfascicular sliding facilitated by the interfascicular matrix (IFM). The objectives of this study were to explore the utility of second harmonic generation (SHG) imaging to objectively assess cross-sectional tendon fascicle architecture, to combine SHG microscopy with elastin immunofluorescence to assess the ultrastructure of collagen and elastin in longitudinal and transverse sections, and lastly, to quantify changes in IFM elastin and fascicle collagen alignment of normal and collagenase-injured flexor tendons. Paraffin-embedded transverse and longitudinal histological sections (10-μm thickness) derived from normal and collagenase-injured (6- and 16-week time-points) equine superficial digital flexor tendons were de-paraffinized, treated with Tris EDTA at 80°C for epitope retrieval, and incubated with mouse monoclonal anti-elastin antibody (1:100 dilution) overnight. Anti-mouse IgG Alexa Flour 546 secondary antibody was applied, and sections were mounted with ProLong Gold reagent with 4',6-diamidino-2-phenylindole (DAPI). Nuclei (DAPI) and elastin (Alexa Fluor 546) signals were captured by using standard confocal imaging with 405 and 543 nm excitation wavelengths, respectively. The SHG signal was captured by using a tunable Ti:Sapphire laser tuned to 950 nm to visualize type I collagen. Quantitative measurements of fascicle cross-sectional area (CSA), IFM thickness in transverse SHG-DAPI merged z-stacks, fascicle/IFM elastin area fraction (%), and elastin-collagen alignment in longitudinal SHG-elastin merged z-stacks were conducted by using ImageJ software. Using this methodology, fascicle CSA, IFM thickness, and IFM elastin area fraction (%) at 6 weeks (∼2.25-fold; ∼2.8-fold; 60% decrease; p < 0.001) and 16 weeks (∼2-fold; ∼1.5-fold; 70% decrease; p < 0.001) after collagenase injection, respectively, were found to be significantly different from normal tendon. IFM elastin and fascicle collagen alignment characterized via fast Fourier transform (FFT) frequency plots at 16 weeks demonstrated that collagen re-alignment was more advanced than that of elastin. The integration of SHG-derived quantitative measurements in transverse and longitudinal tendon sections supports comprehensive assessment of tendon structure. Our findings demonstrate the importance of including IFM and non-collagenous proteins in tendon histological evaluations, tasks that can be effectively carried out by using SHG and immunofluorescence microscopy. Impact statement This work demonstrated that second harmonic generation microscopy in conjunction with elastin immunofluorescence provided a comprehensive assessment of multiscale structural re-organization in healing tendon than when restricted to longitudinal collagen fiber alignment alone. Utilizing this approach for tendon histomorphometry is ideal not only to improve our understanding of hierarchical structural changes that occur after tendon injury and during remodeling but also to monitor the efficacy of therapeutic approaches.

Clinical safety of intratesticular transplantation of allogeneic bone marrow multipotent stromal cells in stallions

Although stem cell therapy is a promising alternative for treatment of degenerative diseases, there are just few reports on the use of stem cells therapy in horse's reproductive system. This study aims to evaluate the effect of intratesticular injection of bone marrow mesenchymal stromal/stem cells (MSCs) in healthy stallions, and its outcome on seminal parameters and fertility. In Experiment 1, 24 stallions were divided into treatment group (TG) and control group (CG). In the TG, an intratesticular application of MSC was performed, and in the CG, only PBS was used. Measurements of testicular volume, surface temperature and Doppler ultrasonography were performed 24 and 48 hr after treatments. Fifteen days after application, the testicles were removed and submitted to histological analysis. In Experiment 2, 3 fertile stallions received similarly treatment with MSCs. Physical examination and sperm analysis were performed weekly during 60 days after treatment, and at the end, semen from one of them was used for artificial inseminations of 6 healthy mares. In Experiment 1, clinical examinations showed no signals of acute inflammation on both groups according to the analysed variables (p > .05). Also, no signal of chronic inflammation was observed on histological evaluation. In Experiment 2, stallions presented no physical alterations or changes in sperm parameters, and a satisfactory fertility rate (83%; 5/6) was observed after AI. The results support the hypothesis that intratesticular application of bone marrow MSCs is a safe procedure, and this could be a promising alternative to treat testicular degenerative conditions.

Histological and immunohistochemical evaluation of two cell therapy protocols in equine suspensory ligament repair

ABSTRACT This study aimed to histologically evaluate the quality of tissue repair in equine suspensory ligament treated with two cell therapy protocols. All four limbs of six animals were operated simultaneously to remove a fragment in each ligament using a skin biopsy punch. Two days later, intralesional injections were performed using bone marrow mononuclear fraction (BM group), cultivated cells derived from adipose tissue (AT group), saline (positive control group), or no treatment (negative control group), in such way that each horse received all treatments. After sixty days biopsies were performed for histological analysis (H & E, Masson’s trichrome and picrosirius red) and immunohistochemistry analysis (collagen type III). Histological findings (H & E and Masson’s trichrome), birefringence intensity (through picrosirius) and collagen type III expression (through immunohistochemistry) were analyzed. Samples from treated groups had better birefringence intensity (P=0.007) and fiber alignment scores were superior compared to controls, though not statistically significant (P=0.08). Presence of inflammatory cells and intense staining for collagen type III occurred in all groups demonstrating an active healing process. In conclusion, both protocols resulted in improvement of tissue repair indicating their potential to be used as an adjuvant treatment of equine suspensory ligament disorders.

Current Trends in Viral Gene Therapy for Human Orthopaedic Regenerative Medicine

Background

Viral vector-based therapeutic gene therapy is a potent strategy to enhance the intrinsic reparative abilities of human orthopaedic tissues. However, clinical application of viral gene transfer remains hindered by detrimental responses in the host against such vectors (immunogenic responses, vector dissemination to nontarget locations). Combining viral gene therapy techniques with tissue engineering procedures may offer strong tools to improve the current systems for applications in vivo.

Methods

The goal of this work is to provide an overview of the most recent systems exploiting biomaterial technologies and therapeutic viral gene transfer in human orthopaedic regenerative medicine.

Results

Integration of tissue engineering platforms with viral gene vectors is an active area of research in orthopaedics as a means to overcome the obstacles precluding effective viral gene therapy.

Conclusions

In light of promising preclinical data that may rapidly expand in a close future, biomaterial-guided viral gene therapy has a strong potential for translation in the field of human orthopaedic regenerative medicine.

Strategies of tenogenic differentiation of equine stem cells for tendon repair: current status and challenges

Tendon injuries, as one of the most common orthopedic disorders, are the major cause of early retirement or wastage among sport horses which mainly affect the superficial digital flexor tendon (SDFT). Tendon repair is a slow process, and tendon tissue is often replaced by scar tissue. The current treatment options are often followed by an incomplete recovery that increases the susceptibility to re-injury. Recently, cell therapy has been used in veterinary medicine to treat tendon injuries, although the risk of ectopic bone formation after cell injection is possible in some cases. In vitro tenogenic induction may overcome the mentioned risk in clinical application. Moreover, a better understanding of treatment strategies for musculoskeletal injuries in horse may have future applications for human and vice versa. This comprehensive review outlines the current strategies of stem cell therapy in equine tendon injury and in vitro tenogenic induction of equine stem cell.

Placental Stem Cells from Domestic Animals: Translational Potential and Clinical Relevance

The field of regenerative medicine is moving toward clinical practice in veterinary science. In this context, placenta-derived stem cells isolated from domestic animals have covered a dual role, acting both as therapies for patients and as a valuable cell source for translational models. The biological properties of placenta-derived cells, comparable among mammals, make them attractive candidates for therapeutic approaches. In particular, stemness features, low immunogenicity, immunomodulatory activity, multilineage plasticity, and their successful capacity for long-term engraftment in different host tissues after autotransplantation, allo-transplantation, or xenotransplantation have been demonstrated. Their beneficial regenerative effects in domestic animals have been proven using preclinical studies as well as clinical trials starting to define the mechanisms involved. This is, in particular, for amniotic-derived cells that have been thoroughly studied to date. The regenerative role arises from a mutual tissue-specific cell differentiation and from the paracrine secretion of bioactive molecules that ultimately drive crucial repair processes in host tissues (e.g., anti-inflammatory, antifibrotic, angiogenic, and neurogenic factors). The knowledge acquired so far on the mechanisms of placenta-derived stem cells in animal models represent the proof of concept of their successful use in some therapeutic treatments such as for musculoskeletal disorders. In the next future, legislation in veterinary regenerative medicine will be a key element in order to certify those placenta-derived cell-based protocols that have already demonstrated their safety and efficacy using rigorous approaches and to improve the degree of standardization of cell-based treatments among veterinary clinicians.

Effect of intra-ovarian injection of mesenchymal stem cells in aged mares

PURPOSE

This study aims to determine if intra-ovarian injection of bone marrow-derived mesenchymal stem cells (MSCs) improves or restores ovarian function in aged females.

METHODS

Prospective randomized study of eight aged mares and six young mares receiving intra-ovarian injection of MSCs or vehicle. Main outcome measures were antral follicle count and serum anti-Müllerian hormone (AMH) (aged and young mares), and for aged mares, oocyte meiotic and developmental competence; gross and histological ovarian assessment; evaluation of presence of chimerism in recovered granulosa cells and in ovarian tissue samples; and gene expression in ovarian tissue as assessed by RNA sequencing.

RESULTS

Injection of MSCs was not associated with significant changes in follicle number, oocyte recovery rate on follicle aspiration, oocyte maturation rate, or blastocyst rate after ICSI in aged mares, or in changes in follicle number in young mares. There were no significant changes in peripheral AMH concentrations, indicating a lack of effect on growing follicles. MSC donor DNA was not recovered in granulosa cells or in ovarian tissue, indicating lack of persistence of injected MSC. RNA sequencing revealed significant differences in gene expression between MSC- and vehicle-injected ovaries.

CONCLUSIONS

Intra-ovarian injection of bone marrow-derived MSCs altered gene expression but did not improve ovarian function in aged mares.

Equine Mesenchymal Stem Cells: Properties, Sources, Characterization, and Potential Therapeutic Applications

Properties like sustained multiplication and self-renewal, and homing and multilineage differentiation to undertake repair of the damaged tissues make stem cells the lifeline for any living system. Therefore, stem cell therapy is regarded to carry immense therapeutic potential. Though the dearth of understanding about the basic biological properties and pathways involved in therapeutic benefits currently limit the application of stem cells in humans as well as animals, there are innumerable reports that suggest clinical benefits of stem cell therapy in equine. Among various stem cell sources, currently adult mesenchymal stem cells (MSCs) are preferred for therapeutic application in horse owing to their easy availability, capacity to modulate inflammation, and promote healing. Also the cells carry very limited teratogenic risk compared to the pluripotent stem cells. Mesenchymal stem cells were earlier considered mainly for musculoskeletal tissues, but now may also be utilized in other diverse clinical problems in horse, and the results may be extrapolated even for human medicine. The current review highlights biological properties, sources, mechanisms, and potential therapeutic applications of stem cells in equine practice.

Current trends in tendinopathy: consensus of the ESSKA basic science committee. Part II: treatment options

The treatment of painful chronic tendinopathy is challenging. Multiple non-invasive and tendon-invasive methods are used. When traditional non-invasive treatments fail, the injections of platelet-rich plasma autologous blood or cortisone have become increasingly favored. However, there is little scientific evidence from human studies supporting injection treatment. As the last resort, intra- or peritendinous open or endoscopic surgery are employed even though these also show varying results. This ESSKA basic science committee current concepts review follows the first part on the biology, biomechanics and anatomy of tendinopathies, to provide a comprehensive overview of the latest treatment options for tendinopathy as reported in the literature.

Effects of mesenchymal stromal cells versus serum on tendon healing in a controlled experimental trial in an equine model

BACKGROUND

Mesenchymal stromal cells (MSC) have shown promising results in the treatment of tendinopathy in equine medicine, making this therapeutic approach seem favorable for translation to human medicine. Having demonstrated that MSC engraft within the tendon lesions after local injection in an equine model, we hypothesized that they would improve tendon healing superior to serum injection alone.

METHODS

Quadrilateral tendon lesions were induced in six horses by mechanical tissue disruption combined with collagenase application 3 weeks before treatment. Adipose-derived MSC suspended in serum or serum alone were then injected intralesionally. Clinical examinations, ultrasound and magnetic resonance imaging were performed over 24 weeks. Tendon biopsies for histological assessment were taken from the hindlimbs 3 weeks after treatment. Horses were sacrificed after 24 weeks and forelimb tendons were subjected to macroscopic and histological examination as well as analysis of musculoskeletal marker expression.

RESULTS

Tendons injected with MSC showed a transient increase in inflammation and lesion size, as indicated by clinical and imaging parameters between week 3 and 6 (p < 0.05). Thereafter, symptoms decreased in both groups and, except that in MSC-treated tendons, mean lesion signal intensity as seen in T2w magnetic resonance imaging and cellularity as seen in the histology (p < 0.05) were lower, no major differences could be found at week 24.

CONCLUSIONS

These data suggest that MSC have influenced the inflammatory reaction in a way not described in tendinopathy studies before. However, at the endpoint of the current study, 24 weeks after treatment, no distinct improvement was observed in MSC-treated tendons compared to the serum-injected controls. Future studies are necessary to elucidate whether and under which conditions MSC are beneficial for tendon healing before translation into human medicine.

Practical considerations for clinical use of mesenchymal stem cells: From the laboratory to the horse

Since the clinical use of mesenchymal stem cells (MSCs) for treating musculoskeletal injuries is gaining popularity, practitioners should be aware of the factors that may affect MSCs from tissue harvesting for MSC isolation to cell delivery into the injury site. This review provides equine practitioners with up-to-date, practical knowledge for the treatment of equine patients using MSCs. A brief overview of laboratory procedures affecting MSCs is provided, but the main focus is on shipping conditions, routes of administration, injection methods, and which commonly used products can be combined with MSCs and which products should be avoided as they have deleterious effects on cells. There are still several knowledge gaps regarding MSC-based therapies in horses. Therefore, it is important to properly manage the factors which are currently known to affect MSCs, to further strengthen the evidence basis of this treatment.

A Protocol to Acquire the Degenerative Tenocyte from Humans

Tendinopathy, a painful condition that develops in response to tendon degeneration, is on the rise in the developed world due to increasing physical activity and longer life expectancy. Despite its increasing prevalence, the underlying pathogenesis still remains unclear, and treatment is generally symptomatic. Recently, numerous therapeutic options, including growth factors, stem cells, and gene therapy, were investigated in hopes of enhancing the healing potency of the degenerative tendon. However, the majority of these research studies were conducted only on animal models or healthy human tenocytes. Despite some studies using pathological tenocytes, to the best of our knowledge there is currently no protocol describing how to obtain human degenerative tenocytes. The aim of this study is to describe a standard protocol for acquiring human degenerative tenocytes. Initially, the tendon tissue was harvested from a patient with lateral epicondylitis during surgery. Then biopsy samples were taken from the extensor carpi radialis brevis tendon corresponding to structural changes observed at the time of surgery. All of the harvested tendons appeared to be dull, gray, friable, and edematous, which made them visually distinct from the healthy ones. Tenocytes were cultured and used for experiments. Meanwhile, half of the harvested tissues were analyzed histologically, and it was shown that they shared the same key features of tendinopathy (angiofibroblastic dysplasia or hyperplasia). A secondary analysis by immunocytochemistry confirmed that the cultured cells were tenocytes with the majority of the cells having positive stains for mohawk and tenomodulin proteins. The qualities of the degenerative nature of tenocytes were then determined by comparing the cells with the healthy control using a proliferation assay or qRT-PCR. The degenerative tenocyte displayed a higher proliferation rate and similar gene expression patterns of tendinopathy that matched previous reports. Overall, this new protocol might provide a useful tool for future studies of tendinopathy.

Performing a Better Bone Marrow Aspiration

Bone marrow aspiration (BMA) is increasingly being used to harvest stem cells for use in regenerative medicine. The focus of BMA in interventional orthopedics is to maximize the yield of mesenchymal stem cells. The authors present an improved method for BMA that involves fluoroscope or ultrasound guidance combined with anesthesia; in the authors' experience, it produces the highest possible stem cell yield and is well tolerated by patients. The authors provide a step-by-step guide to the process, along with a discussion of technical and other considerations and quick reference guides for ultrasound- and fluoroscope-guided BMA.

Rescue plan for Achilles: Therapeutics steering the fate and functions of stem cells in tendon wound healing

Due to the increasing age of our society and a rise in engagement of young people in extreme and/or competitive sports, both tendinopathies and tendon ruptures present a clinical and financial challenge. Tendon has limited natural healing capacity and often responds poorly to treatments, hence it requires prolonged rehabilitation in most cases. Till today, none of the therapeutic options has provided successful long-term solutions, meaning that repaired tendons do not recover their complete strength and functionality. Our understanding of tendon biology and healing increases only slowly and the development of new treatment options is insufficient. In this review, following discussion on tendon structure, healing and the clinical relevance of tendon injury, we aim to elucidate the role of stem cells in tendon healing and discuss new possibilities to enhance stem cell treatment of injured tendon. To date, studies mainly apply stem cells, often in combination with scaffolds or growth factors, to surgically created tendon defects. Deeper understanding of how stem cells and vasculature in the healing tendon react to growth factors, common drugs used to treat injured tendons and promising cellular boosters could help to develop new and more efficient ways to manage tendon injuries.

Engineering the Second Generation of Therapeutic Cells with Enhanced Targeting of Injured Tissues

Experimental approaches to improving tissue repair utilize cells and growth factors needed to restore the architecture and function of damaged tissues and organs. Key limitations of these approaches include poor delivery of therapeutic cells and growth factors into injury sites, as well as their short-term retention in target areas. In our earlier studies, we demonstrated that artificial collagen-specific anchor (ACSA) expressed on the surface of therapeutic cells directs them into collagen-rich sites of injury. Moreover, we demonstrated that the ACSA improves the retention of these cells in target sites, thereby promoting tissue repair. To advance the ACSA-based technology, we engineered the second generation of the ACSA-expressing cells able to deliver growth factors to target sites. In this study, we specifically focused on insulin growth factor 1 (IGF1), which enhances the repair of a number of collagen-rich connective tissues, including ligament and tendon. Utilizing gene engineering, we produced IGF1 in the ACSA-expressing cells. Using relevant experimental models, we demonstrated that recombinant IGF1 secreted by these cells maintains its specificity and biological activity. Moreover, our studies show that IGF1 produced by the ACSA-expressing cells cultured in three-dimensional environment promotes the formation of the collagen-rich fibrillar matrix. Furthermore, the engineered cells integrated well with the native collagen-rich tendon tissue. Our study provides strong evidence for the great potential of cells with rationally engineered target-specific receptors to restore damaged connective tissues. Future studies in relevant animal models will determine the utility of these cells in vivo.

Cardiomyocyte differentiation of mesenchymal stem cells from bone marrow: new regulators and its implications

In the past years, cardiac mortality has decreased, but cardiac diseases are still responsible for millions of deaths every year worldwide. Bone-marrow mesenchymal stem cells (BMSCs) transplantation may be a promising therapeutic strategy because of its capacity to differentiate into cardiac cells. Current research indicates that chemical substances, microRNAs, and cytokines have biological functions that regulate the cardiomyocytes differentiation of BMSCs. In this review, we chiefly summarize the regulatory factors that induce BMSCs to differentiate into cardiomyocytes.

Molecular Characteristics of the Equine Periodontal Ligament

The equine periodontal ligament (PDL) is a fibrous connective tissue that covers the intra-alveolar parts of the tooth and anchors it to the alveolar bone-it, therefore, provides a similar function to a tendinous structure. While several studies have considered the formation and structure of tendons, there is insufficient information particularly on the molecular composition of the PDL. Especially for the equine PDL, there is limited knowledge concerning the expression of genes commonly regarded as typical for tendon tissue. In this study, the gene expression of, e.g., collagen type 1 alpha 1 (COL1), collagen type 3 alpha 1 (COL3), scleraxis (SCX), and fibrocartilage markers was examined in the functional mature equine PDL compared with immature and mature equine tendon tissue. PDL samples were obtained from incisor, premolar, and molar teeth from seven adult horses. Additionally, tendon samples were collected from four adult horses and five foals at different sampling locations. Analyses of gene expression were performed using real-time quantitative polymerase chain reaction (qRT-PCR). Significantly higher expression levels of COL1 and 3 were found in the mature equine PDL in comparison with mature tendon, indicating higher rates of collagen production and turnover in the mature equine PDL. The expression levels of SCX, a specific marker for tenogenic-differentiated cells, were on a similar level in functional mature PDL and in mature tendon tissue. Evidence of chondrogenic metaplasia, often found in tendon entheses or in pressurized regions of tendons, was not found in the mature equine PDL. The obtained results justify further experiments focused on the possible use of equine PDL cells for cell-based regenerative therapies.

Accuracy of open magnetic resonance imaging for guiding injection of the equine deep digital flexor tendon within the hoof

Lesions of the distal deep digital flexor tendon (DDFT) are frequently diagnosed using MRI in horses with foot pain. Intralesional injection of biologic therapeutics shows promise in tendon healing; however, accurate injection of distal deep digital flexor tendon lesions within the hoof is difficult. The aim of this experimental study was to evaluate accuracy of a technique for injection of the deep digital flexor tendon within the hoof using MRI-guidance, which could be performed in standing patients. We hypothesized that injection of the distal deep digital flexor tendon within the hoof could be accurately guided using open low-field MRI to target either the lateral or medial lobe at a specific location. Ten cadaver limbs were positioned in an open, low-field MRI unit. Each distal deep digital flexor tendon lobe was assigned to have a proximal (adjacent to the proximal aspect of the navicular bursa) or distal (adjacent to the navicular bone) injection. A titanium needle was inserted into each tendon lobe, guided by T1-weighted transverse images acquired simultaneously during injection. Colored dye was injected as a marker and postinjection MRI and gross sections were assessed. The success of injection as evaluated on gross section was 85% (70% proximal, 100% distal). The success of injection as evaluated by MRI was 65% (60% proximal, 70% distal). There was no significant difference between the success of injecting the medial versus lateral lobe. The major limitation of this study was the use of cadaver limbs with normal tendons. The authors conclude that injection of the distal deep digital flexor tendon within the hoof is possible using MRI guidance.

Inhibition of IGF-1 receptor kinase blocks the differentiation into cardiomyocyte-like cells of BMSCs induced by IGF-1

Bone marrow mesenchymal stem cells (BMSCs) have the potential to transdifferentiate into cardiomyocyte-like cells (CLCs) if an appropriate cardiac environment is provided. Insulin-like growth factor-1 (IGF-1) plays an important role in the cell migration, survival and differentiation of BMSCs. However, the effect of IGF-1 on the cellular differentiation remains unclear. In the present study, BMSCs were isolated from rat femurs and tibias and the cells were purified at passage 6 (P6). IGF-1 and IGF-1 receptor (IGF-1R) kinase inhibitor I-OMe AG538 were added to detect if IGF-1 could induce BMSCs to transdifferentiate into CLCs and if I-OMe AG538 could inhibit IGF-1-mediated receptor activation and downstream signaling. Immunostaining demonstrated that all P6 BMSCs express CD29 and CD44 but not CD45. BMSCs induced by 15 ng/ml IGF-1 revealed positivity for cardiac troponin-T and cardiac troponin-I. The optimal induction time was 14 days but the expression of these proteins were incompletely inhibited by 300 nmol/l I-OMe AG538 and completely inhibited by 10 µmol/l I-OMe AG538. Western blotting showed that the level of IGF-1R autophosphorylation and the expression of cTnT and cTnI were higher when BMSCs were induced for 14 days. I-OMe AG538 selectively inhibited IGF-1-mediated growth and signal transduction and the inhibitory effect of I-OMe AG538 were not reverted in the presence of exogenous IGF-1. In addition, when a time course analysis of the effects of I-OMe AG538 on mitogen-activated protein kinase kinase and phosphatidylinositol 3-kinase signaling were done, we observed a transient inhibitory effect on Erk1/2 and Akt phosphorylation, in keeping with the inhibitory effects on cell growth. Taken together, these data indicate that I-OMe AG538 could inhibit IGF-1-induced CLCs in BMSCs and this effect is time- and concentration-dependent.

Effect of bone marrow-derived mesenchymal stem cells and stem cell supernatant on equine corneal wound healing in vitro

Background

We aimed to determine and compare the in vitro effects of autologous bone marrow-derived mesenchymal stem cells (BM-MSCs) and mesenchymal stem cell supernatant (MSC-Sp) on the wound healing capacity of equine corneal fibroblasts using a scratch assay.

Methods

Bone marrow aspirates and eyes were collected from normal, euthanized horses with subsequent isolation and culture of BM-MSCs and corneal stromal cells. Corneal stromal cells were culture-expanded in the culture well of transwell plates and then treated with an autologous BM-MSC suspension (dose: 2.5 × 10⁵/100 μL media with the BM-MSCs contained within the insert well), MSC-Sp solution, or naive culture media (control) for 72 h. A linear defect in confluent cell cultures was created (i.e., corneal scratch assay) to assess the cellular closure (“healing”) over time. Three representative areas of the scratch in each culture were photographed at each time point and the scratch area was quantitated using image analysis software (ImageJ). Media from the scratches were analyzed for various growth factors using human enzyme-linked immunosorbent assay (ELISA) kits that crossreact with the horse.

Results

There was a significant percentage decrease in the scratch area remaining in the BM-MSC and MSC-Sp groups compared to the control group. There was also a significant percentage decrease in the scratch area remaining in the BM-MSC group compared to the MSC-Sp group at 36 h post-scratch and all time points thereafter. The concentration of transforming growth factor (TGF)-β1 in the media was significantly higher in the BM-MSC group compared to the control group.

Conclusions

The significant decrease in scratch area in equine corneal fibroblast cultures treated with autologous BM-MSCs compared to MSC-Sp or control treatments suggests that BM-MSCs may substantially improve corneal wound healing in horses. MSC-Sp may also improve corneal wound healing given the significant decrease in scratch area compared to control treatments, and would be an immediately available and cost-effective treatment option.

Electronic supplementary material

The online version of this article (doi:10.1186/s13287-017-0577-3) contains supplementary material, which is available to authorized users.

Effect of needle diameter on the viability of equine bone marrow derived mesenchymal stem cells

OBJECTIVES

Mesenchymal stem cells (MSCs) are frequently delivered via needle injection for treatment of musculoskeletal injuries. The purpose of this study was to evaluate the effect of needle diameter on the viability of MSCs.

METHODS

Equine bone marrow-derived MSCs from 5 horses were suspended in PBS, and held at room temperature for 7 hours to mimic shipping conditions. Two replicate samples for each needle size (20, 22, 23, or 25-gauge [ga]) were aspirated into a 3 mL syringe and re-injected into the holding vial 3 times, to reproduce the resuspension of cells prior to injection in clinical cases. Cells were stained with fluorescein diacetate and propidium iodide to measure viability. Flow cytometry (FC) was performed to compare cell debris and intact cells between groups.

RESULTS

MSC viability was higher when cells were passed through a 20-ga rather than a 25-ga needle. Cell suspensions passed through a 20-ga needle contained a larger percentage of intact cells, compared to 25-ga samples. The percentage of debris present in cell suspensions tended to increase with decreasing needle diameter. Neither horse nor passage had a significant effect on viability.

CONCLUSIONS

Cell damage is more likely when MSCs are passed through 25-ga rather than 20-ga needles.

CLINICAL RELEVANCE

Use of needles larger than 25-ga is recommended to maintain the viability of MSCs injected in horses.

Allogeneic major histocompatibility complex-mismatched equine bone marrow-derived mesenchymal stem cells are targeted for death by cytotoxic anti-major histocompatibility complex antibodies

Background

Allogeneic mesenchymal stem cells (MSCs) are a promising cell source for treating musculoskeletal injuries in horses. Controversy exists, however, over whether major histocompatibility complex (MHC)‐mismatched MSCs are recognised by the recipient immune system and targeted for death by a cytotoxic antibody response.

Objectives

To determine if cytotoxic anti‐MHC antibodies generated in vivo following MHC‐mismatched MSC injections are capable of initiating complement‐dependent cytotoxicity of MSCs.

Study design

Experimental controlled study.

Methods

Antisera previously collected at Days 0, 7, 14 and 21 post‐injection from 4 horses injected with donor MHC‐mismatched equine leucocyte antigen (ELA)‐A2 haplotype MSCs and one control horse injected with donor MHC‐matched ELA‐A2 MSCs were utilised in this study. Antisera were incubated with ELA‐A2 MSCs before adding complement in microcytotoxicity assays and cell death was analysed via eosin dye exclusion. ELA‐A2 peripheral blood leucocytes (PBLs) were used in the assays as a positive control.

Results

Antisera from all 4 horses injected with MHC‐mismatched MSCs contained antibodies that caused the death of ELA‐A2 haplotype MSCs in the microcytotoxicity assays. In 2 of the 4 horses, antibodies were present as early as Day 7 post‐injection. MSC death was consistently equivalent to that of ELA‐A2 haplotype PBL death at all time points and antisera dilutions. Antisera from the control horse that was injected with MHC‐matched MSCs did not contain cytotoxic ELA‐A2 antibodies at any of the time points examined.

Main limitations

This study examined MSC death in vitro only and utilized antisera from a small number of horses.

Conclusions

The cytotoxic antibody response induced in recipient horses following injection with donor MHC‐mismatched MSCs is capable of killing donor MSCs in vitro. These results suggest that the use of allogeneic MHC‐mismatched MSCs must be cautioned against, not only for potential adverse events, but also for reduced therapeutic efficacy due to targeted MSC death.

Long-Term Cell Tracking following Local Injection of Mesenchymal Stromal Cells in the Equine Model of Induced Tendon Disease

Tendon disease has been treated with multipotent mesenchymal stromal cells (MSCs) in the equine large-animal model with promising success. The aim of this study was to gain more insight into the fate and biodistribution of MSCs after local application into tendon lesions by long-term cell tracking in this large-animal model. Superficial digital flexor tendon lesions were induced in all limbs in six horses and injected with 10 × 10 6 Molday ION Rhodamine B™-labeled MSCs suspended in serum or serum alone. Follow-up was performed using low-field magnetic resonance imaging (MRI), flow cytometry, and histology. Cell tracking based on the hypointense artifacts induced by the superparamagnetic iron oxide (SPIO) labeling agent in MRI as well as based on Rhodamine B fluorescence was feasible. However, Prussian blue staining for assessment of histology was not entirely specific for SPIO. Labeled cells could be traced at their injection site by MRI as well as histology for the whole follow-up period of 24 weeks. Although the numbers of labeled cells within the injected tendon lesions decreased over time, part of the applied cells appeared to remain viable and integrated within the injured tissue. Furthermore, small numbers of labeled cells were identified in peripheral blood within the first 24 h after cell injection and could also be found until week 24 within the contralateral control tendon lesions that had been injected with serum. The present findings unveil details on MSC biodistribution and persistence after their local application, which are of clinical relevance with regard to MSC safety and mechanisms of action.