Activity of mesenchymal stem cells in therapies for chronic skin wound healing

Abstract

Chronic or non-healing skin wounds present an ongoing challenge in advanced wound care, particularly as the number of patients increases while technology aimed at stimulating wound healing in these cases remains inefficient. Mesenchymal stem cells (MSCs) have proved to be an attractive cell type for various cell therapies due to their ability to differentiate into various cell lineages, multiple donor tissue types, and relative resilience in ex-vivo expansion, as well as immunomodulatory effects during transplants. More recently, these cells have been targeted for use in strategies to improve chronic wound healing in patients with diabetic ulcers or other stasis wounds. Here, we outline several mechanisms by which MSCs can improve healing outcomes in these cases, including reducing tissue inflammation, inducing angiogenesis in the wound bed, and reducing scarring following the repair process. Approaches to extend MSC life span in implant sites are also examined.

Introduction

Wound healing is a complex multi-stage process that orchestrates the reconstitution of the dermal and epidermal layers of the skin. In many pathological circumstances such as diabetes or severe burns, the normal wound healing process fails to adequately restore function to the skin, leading to potentially severe complications from ulcers or resulting infections. As the incidence of obesity and resulting diabetes continues to increase in the western world,¹ the prevalence of chronic wounds related to these conditions continues to be a major focus of wound care research. In fact, non-healing wounds from these conditions have produced a multi-billion dollar advanced wound care market for technologies aimed at stimulating wound healing in patients that suffer from dysfunctional wound repair, with large projected growth in the near future.² Most current biological technologies for advanced wound care aim to provide antimicrobial support to the open wound and a matrix scaffold (collagen-based in many cases) for invading cells to reestablish the skin, with some focus on growth factor support of the healing process (Table 1).^8,9 However, patient outcomes in this area remain marginal and novel bioengineered approaches to chronic wound repair remain a topic of high interest.

Table 1. Chronic wound healing technologies.

Technology	Company	Product Summary
Apligraf®	Organogenesis	Bilayered human skin directly applied to a wound. Lower layer contains collagen and fibroblasts, upper layer contains expanded keratinocytes.3
Dermagraft®	Shire Regenerative Medicine	Human fibroblasts integrated into a collagen/GAG polymer scaffold (dermal substitute).4
Hydrofiber (Aquacel®)	ConvaTec	Wound hydration system; carboxymethylcellulose fibers that gel upon application to hydrate wound. Silver may be used as an antimicrobial agent.5
Bilayer Matrix Wound Dressing®	Integra	Outer antimicrobial silicone layer, with an inner collagen/GAG matrix layer for cell invasion/remodeling.6
Regranex®	HealthPoint Biotherapeutics	Topical gel (CMC) containing Becaplermin (recombinant PDGF) to stimulate wound healing.7

A sample of current FDA approved bioengineered approaches for advanced wound care, including diabetic wounds and other chronic lesions.

Mesenchymal stem cells (MSCs) are important cells in orchestrating the three main phases of normal wound healing (inflammatory/proliferative/remodeling), directing inflammation and antimicrobial activity and promoting cell migration during epithelial remodeling.¹⁰ However, recently due to advances in understanding of MSC immunosuppression and secretion of pro-angiogenic factors, MSC-based cell therapy in combination with matrix scaffold approaches to improve wound healing outcomes has become a potential strategy in treatment of non-healing wounds.

Traditionally, MSCs have long been identified for their ability to migrate to sites of injury in the body and differentiate into a variety of cell lineages such as bone, fat, and cartilage,^11-14 making them attractive candidates for a variety of cell therapies in recent studies. A variety of easy means of isolating and expanding these cells ex-vivo (bone marrow,^15,16 adipose tissue,^16,17 placenta,¹⁸ peripheral blood,¹⁹ and others) also makes MSCs useful cells for therapeutic approaches to supplementing tissue regeneration (Table 2). Additionally, these cells have been shown to have notable immunomodulatory effects on the surrounding environment following transplantation,^29-32 and can support native cells with the secretion of a variety of pro-survival and pro-migratory cytokines and growth factors.^33,34 As a major problem in chronic wounding is unmitigated inflammation, this characteristic of MSCs has made them good candidates for approaches to cell therapy for chronic wounds in particular.

Table 2. Clinical sources of MSCs.

Cell Source	Isolation source/method	Sample surface markers*	Notes
Bone marrow	Typical bone marrow harvest followed by marrow aspirate density centrifugation/plate adherence with the SVF.20	+ CD13, CD44, CD73, CD90, CD105, CD29 − CD34, CD45, CD14	- Most well-characterized and extensively studied population - More painful isolation procedure, inherent risk factors of marrow harvest - Few cells present in marrow aspirate compared with other methods
Adipose tissue	Processing of residual tissue from liposurgery; tissue digestion/plate adherence to isolate MSC population. Comparatively easy isolation procedure.21	+ CD13, CD44, CD73, CD90, CD105, CD9, CD29, CD106 − CD34, CD45, CD133, CD144, CD14	- Easier isolation method for patient - Abundant and readily available source - Greater number of cells obtained during isolation - Potentially reduced chondrogenic differentiation efficiency22
Placenta	Obtainable from amniotic fluid or placental tissue (various), with standard tissue digestion or fluid fraction segregation. Cells obtained via plate adherence.18	+ CD44, CD29, CD105, CD90, CD144 − CD14, CD34, CD45, CD117	- Readily available and non-invasive cell source - Potentially improved growth potential and life span23 - Substantially low cell yield from isolation, varies with source tissue and individual
Umbilical cord	Can be obtained from cord itself (digested), Wharton’s jelly, or cord blood. Density gradient purification or enzymatic digestion, depending on the sour.16,24	+ CD73, CD105, CD44, CD29 − CD45, CD34, CD14	- Generally highly proliferative MSC populations25 - Heterogeneous MSCs obtained based on UC source tissue - Potentially reduced adipogenic capacity26 - Debatable expression of classic MSC markers CD90 and CD105 - Respond well to hypoxic conditions27
Peripheral blood	Mobilization of MSCs into blood (G-CSF injections), collected by density centrifugation and plating of mononuclear cell fraction; fibrin microbeads have also been used for collection.19	+ CD73, CD90, CD105, CD44, CD166 − CD34, CD45, CD14	- Relatively easy to obtain compared with marrow harvest - Historically controversial whether peripheral blood can contain significant population19,28 - Variable number of cells available for isolation in this method

An overview of sample tissue sources for mesenchymal stem cells, including the basic means of isolation and culture, common surface marker expression patterns, and general considerations for use in clinical applications as well as characteristics of the cell subtype. For most sources, there is occasionally some debate over consensus for surface markers; the most commonly mentioned ones are cited in this case. *Reviewed extensively in references 26, 113, and 117.

In this review, we examine current trends in MSC therapy for chronic wound healing, including several major areas of MSC benefit to the wound repair process. Additionally, potential further MSC applications in wound healing and novel technologies are discussed.

Chronic Skin Wounds

The normal wound healing process is characterized by three main phases that lead to efficient reconstitution of a functional dermis/epidermis and revascularized tissue.^35,36 Briefly, the inflammatory phase immediately follows wounding, serving to stop bleeding in the wound bed via platelet aggregation and fibrin clot formation. This is followed by invasion of neutrophils and mast cells that follow a chemotactic gradient to clear the wound of dead cells, debris, and residual ECM. The proliferative phase then proceeds, including fibroblast migration into the wound bed and deposition of new ECM (collagen). VEGF and B-FGF also stimulate de novo angiogenesis in the skin.^37-39 Finally, the remodeling process resolves the wound by organizing collagen fibers that formed during fibroblast proliferation in parallel with further removal of fibronectin to increase the strength of the new skin.

A chronic or non-healing wound is essentially a wound that does not progress normally through the wound healing process, resulting in an open laceration of varying degrees of severity.^40,41 These conditions can be cause by a number of various pathophysiological conditions (diabetes,⁴² venous stasis ulcer progression,⁴³ and others), though all causes generally lead to a hyper-inflammatory environment, particularly evidenced by the characteristic presence of neutrophils/high MMP activity that leads to high breakdown of new collagen during the wound healing process and inhibition of pro-healing factors (PDGF, TGF-B, and others).^44-47 This excessive inflammation phenotype leads to wounds that cannot resolve under normal circumstances, especially until the inflammation in the wound bed is controlled to a normal level and fibroblasts are able to effectively migrate into the wound space and synthesize new matrix.

Clinically, these wounds present a large problem for wound care specialists globally, with approximately 1–2% prevalence and a greater than 50% recurrence rate for diabetic patients.^48,49 This need has generated a large interest in new treatments for improving patient outcomes in chronic wound therapies. Mesenchymal stem cells, given their immunomodulatory and angiogenic properties, have therefore been studied extensively with regards to cell therapy to supplement wound dressings. With over 350 listed clinical trials for MSC therapies (clinicaltrials.gov), many include studies utilizing MSCs for healing ischemic/diabetic foot ulcers and similar wounds (Table 3).

Table 3. Clinical trials for MSCs and chronic wounds.

Study	Trial ID	Research Location	Status
Induced Wound Healing by Application of Expanded Bone Marrow Stem Cells in Diabetic Patients With Critical Limb Ischemia	NCT01065337	Ruhr University of Bochum	Completed50
Human Adipose Derived Mesenchymal Stem Cells for Critical Limb Ischemia in Diabetic Patients	NCT01257776	University Hospital Virgen Macarena	Unknown
Umbilical Cord Mesenchymal Stem Cells Injection for Diabetic Foot	NCT01216865	Qingdao University	Unknown
Autologous Bone Marrow Stem Cell Transplantation for Critical, Limb-threatening Ischemia (BONMOT)	NCT00434616	Franziskus Hospital Berlin Vascular Center	Unknown
Autologous Bone Marrow Stem Cell Transfer in Patients With Chronical Critical Limb Ischemia and Diabetic Foot	NCT01232673	University Hospital Ostrava	Completed51
Study of the Effectiveness of Autologous Bone Marrow-Derived Mesenchymal Stem Cells in Fibrin to Treat Chronic Wounds	NCT01751282	Roger Williams Medical Center	Completed; Recruiting for next phase52
Comparison of Autologous Mesenchymal Stem Cells and Mononuclear Cells on Diabetic Critical Limb Ischemia and Foot Ulcer	NCT00955669	Third Military Medical University	Completed53
The Role of Lipoaspirate Injection in the Treatment of Diabetic Lower Extremity Wounds and Venous Stasis Ulcers	NCT00815217	Washington DC. Veterans Affairs Medical Center	Unknown
Safety Study of Stem Cells Treatment in Diabetic Foot Ulcers	NCT01686139	Sheba Medical Center	Pre-recruitment
Intramuscular Mononuclear Cells and Mesenchymal Stem Cells Transplantation to Treat Chronic Critical Limb Ischemia	NCT01456819	UKM Medical Centre	Recruiting

A sample of currently listed international clinical trials involving mesenchymal stem cells and applications in advancing chronic wound healing. Data collected from clinicaltrials.gov; reference ID’s refer to listings from that database.

Ultimately, this interest in MSCs for cell therapies in wound healing revolves around several key aspects, including immunosuppression, angiogenesis stimulation, and scar reduction. As MSCs play a normal role in the wound healing process, they are an obvious candidate for study in this context as opposed to embryonic stem cells or other regenerative sources. Recent studies have outlined some successful approaches to promoting wound healing with MSCs, including autologous bone marrow-derived MSCs in fibrin matrix⁵² or, more recently, intramuscular injection of autologous MSCs to improve diabetic wound closure.⁵⁴ While some trials have been aimed primarily at safety of MSC use for wound healing,⁵⁰ several clinical trials have shown the potential benefit of MSCs for inclusion in wound healing devices, including improved average rate of wound healing and general limb perfusion after treatment⁵³ and also improved acute wound healing correlating to the number of injected cells.⁵² Despite any effects on healing, there was some doubt as to any reduction in limb amputation rate or relative pain levels among groups, a major consideration for effective therapy in chronic wounds.⁵³ In general, the consensus from completed trials has been an overall improvement in chronic wound closure with application of mesenchymal stem cells, particularly as a part of a matrix delivery system (wound gel, etc.).

MSC Immunomodulation In Wound Beds

Chronic hyperinflammation in the wound bed is the most substantial barrier to treatment in non-healing wounds, as outlined previously. Mesenchymal stem cells have recently been shown to hold a variety of immunomodulatory effects on host immune cells in both wound healing and transplant biology contexts. These characteristics are potentially what make MSCs the most attractive cell type for cell therapy in chronic wounds, as they exert pleiotropic effects on the inflammatory mechanisms to move the wound past static inflammation and fibrosis.

It has been known for some time that donor MSCs are able to suppress host T cell proliferation, a key activity in reducing wound bed inflammation.⁵⁵ More recently, this was demonstrated to be dependent on MSC induction of IL-10⁵⁶ in native T cells and macrophages, as well as TGF-β activity.⁵⁷ Additionally MSCs have been shown to be capable of modulating host TNF-α production to mediate excessive inflammatory effects, and reduce NK cell function in the inflammatory phase, lowering IFN-γ activity in the process.³² Conversely, in the later stages of inflammation, active TGF-α is able to stimulate implanted MSCs to produce a variety of pro-healing growth factors and cytokines, including VEGF to stimulate angiogenesis in the wound bed.⁵⁸ In 2008 Ren et al. showed a dependence on pro-inflammatory factors for these processes to be effective, suggesting a potential time window for application of allogeneic MSCs to be efficient in reducing inflammation.⁵⁹

Importantly, recent research into the immune response to allogeneic MSCs has shown that in most systems, the donor MSCs are ‘immunoprivileged’ and do not induce a significant response in the host, suggesting that allogeneic cell sources may be possible for chronic wound therapies, where diabetic patients may have already-defective endogenous MSC populations making autologous therapy less than optimal. This characteristic of allogeneic MSCs is crucial in this particular wound environment, where excessive inflammation already drives the chronic phenotype and additional immune response from cell implantation must be as low as possible. However, several studies have shown that this immunoprivileged characteristic is lost as the MSCs differentiate, leading to a gradual host response to the implanted cells.^60,61 Thus, approaches to keeping MSCs undifferentiated may be key in future chronic wound therapies.

MSCs have also recently been identified as having antimicrobial effects, a significant advantage in reducing excess inflammation from any contaminants in the wound during injury and treatment.⁶² This was identified in 2008 by Krasnodembskaya and colleagues⁶³ as a mechanism based on secretion of LL-37, a peptide with a wide array of antimicrobial properties including broad spectrum microbial defense via disruption of bacterial cell membranes⁶⁴ and directly limiting bacterial macrophage activity via upregulation of chemokine receptors, all while ignoring pro-inflammatory cytokine activation.⁶⁵ In terms of cell therapeutics, the concern for reducing infection is great and many products attempt to seal wounds with silicone barriers to dressings. Silver nanoparticles have also been examined for antimicrobial properties, which can be conveniently included in wound healing gels and allowed to leach into the wound locally.^66,67 Combined with MSC antimicrobial activity, this would help to reduce any additional inflammation seen during the healing process.

As a whole, all of the effects produced by MSCs here help to solve the problem of chronic hyperinflammation in the wound bed and advance wounds such as diabetic ulcers into the next stages of wound healing. Allogeneic application of MSCs in gel-based products for wound healing holds promise for combatting these issues, as has been done in various applications for MSC therapy using fibrin-based gel systems^68,69 and related mimetics.⁷⁰

Stimulation of Angiogenesis

Revascularization of the wound bed is a crucial stage of the normal wound healing process, where new vessels form as granulation tissue develops to supply blood to the wound area, which is in need of oxygen and nutrients. Endothelial cells therefore need to be able to break through the dermis of the wound and form tubes in the newly developing tissue, a process that is balanced by the growth factor production cascade during wound healing. Mesenchymal stem cells play a normal role in this process as they are recruited to the wound bed following mobilization from endogenous sources.^71,72 The ability of mesenchymal stem cells to promote angiogenesis in vivo is not necessarily unique, as several other cell types have been shown to been integral in stimulating angiogenesis via cell therapy, such as hematopoietic stem cells⁷³ or resident cardiac progenitor cells.⁷⁴ However, the unique role of MSCs during normal wound repair and additional effects of MSCs discussed in this review make the application of MSCs to stimulate vessel growth in chronic wounds particularly interesting for future studies in clinical MSC application for wound therapy.

There is evidence that MSCs can differentiate into a variety of skin cell types, contributing to repopulation of the wound bed with normal dermal structure, as well as endothelial cells to yield new vessels.^72,75 Recently several groups have focused on differentiation of MSCs into endothelial cells, an approach that has potential to be useful in direct transplant into anti-angiogenic environments such as ischemic wound beds. Results have shown endothelial-like cell populations derived from human MSCs in vitro with varying degrees of donor variation,^76,77 while Bago et al. showed similar results for amnion-derived MSCs in glioma tumors.⁷⁸ Furthermore, pericytes that stabilize vessel walls and promote vessel maturation during angiogenesis have been shown to be derived from bone marrow populations following injury.⁷⁹ Recent evidence suggests that these pericytes in fact represent a sub-population of mesenchymal stem cells that contribute to the healing process.⁸⁰ These cells all have the potential to support new vessel growth in a chronic wound bed, a critical aspect of overcoming barriers to current therapies.

Perhaps more important than differentiation, secreted factors also play a substantial role in MSC regulation of angiogenesis in the wound bed. Chronic wounds are often subject to anti-angiogenic conditions, including reduced growth factor production as a result of increased MMP production in the wound bed, as outlined by Krisp et al. recently in a global secretome analysis of wound exudates.⁸¹ MSCs naturally produce a variety of pro-angiogenic factors following recruitment to the wound bed that stimulate endothelial cell proliferation and tube formation in the wound bed, most notably VEGF, a potent stimulator for angiogenesis that is regulated by IL-6 and TGF-α in the wound bed.⁸² Though it has been shown that exogenous VEGF application to wounds can stimulate angiogenesis,⁸³ MSCs used in cell therapeutics also have been shown to stimulate EC recruitment and wound healing via VEGF secretion^33,75 or via pre-differentiation into angiogenic precursors.⁸⁴ Ultimately, MSCs are able to stimulate de novo angiogenesis in wound beds upon transplantation, a crucial factor in stimulating healing in chronic wounds that lack this normally due to the hyperinflammatory environment.

Reduction in Scar Formation

Another consideration in repair of wounds under all circumstances is the formation of scars, caused by deposition of excess ECM by fibroblasts in the wound bed. These structures carry a variety of undesirable consequences, including unsightly appearance on the skin and, more critically, scars lack many of the normal makeup of the skin such as follicles and nerve endings and also do not retain the normal tensile strength of undamaged skin.³⁵ While scar reduction research has been a field in of itself for quite some time, it is a notable consideration for patients with large non-healing ulcers.

As discussed previously, anti-inflammatory mechanisms of MSCs have several effects on fibrotic phenotypes in the wound, and thus play a major role in reducing scar formation following wound healing. Most notably, MSC production of PGE2 drives a variety of changes in the scarring phenotype. PGE2 from MSCs has been shown to increase secretion of IL-10 by T cells and macrophages,⁸⁵ an important anti-inflammatory cytokine in the wound environment. PGE2 secreted by MSCs in response to the inflammatory wound bed plays a crucial role in the healing process, reducing T cell migration and NK cell proliferation during the inflammatory phase.^86,87 The upregulation of IL-10 in the wound by MSCs also has a multitude of effects on general scar formation, including downregulation of IL-6 and IL-8 to reduce collagen production in the wound⁸⁸ and inhibition of neutrophil invasion and macrophage activity to suppress ROS generation,⁸⁹ all leading to support of regenerative healing in recent experimental scar formation models.⁹⁰ ROS generation is also affected by nitric oxide secreted by MSCs, acting as a scavenger to prevent the fibrotic activity of the oxygen radicals.^91,92 Though these anti-inflammatory mechanisms are part of normal MSC function following homing to acute wound sites, the hyperinflammatory environment of a chronic wound makes the MSC ability to modulate excessive inflammation and reduce excessive scarring critical. Ultimately, reduced scar formation is not an outcome desired specifically for chronic wounds, but nevertheless is a significant potential benefit of utilizing MSCs to promote closure of such non-healing wounds. Experimentally, recent experimental evidence has shown that MSCs can indeed reduce a fibrotic phenotype in a mouse model,⁹³ showing promise for reduced scar formation in future MSC therapeutics.

Mesenchymal stem cells also produce a variety of anti-fibrotic factors throughout the wound healing process. Aside from IL-10, HGF is a major contributor to reduced fibrosis, which has been shown to be effective in advancing clean wound healing in a variety of tissues such as liver⁹⁴ and various skin contexts.^95,96 HGF has also been attributed to chronic wounds, with differential regulation of HGF production and presence of c-Met in chronic wound dermis.⁹⁷ Specifically in relation to fibrosis, HGF has been demonstrated to reduce TGF-β and collagen production in fibroblasts,⁹⁸ and also have a multitude of effects on cell recruitment to the wound bed, including endothelial cells and promotion of keratinocyte migration.⁹⁹ Ultimately, HGF production by transplanted MSCs would yield a more normal state of cell migration and matrix production than what is normally seen in chronic wound beds.

Together in concert with the other immunomodulatory mechanisms of MSC function in wound repair, addition of MSCs to chronic wounds may prove to be an effective means of promoting cleaner healing on a smaller time scale than traditional treatments.

Promotion of MSC Survival

In typical cutaneous wound healing, MSCs are mobilized from host sources and home to the site of injury, persisting to support immunomodulation and improved angiogenesis in the wound bed as the skin repairs itself. These host MSCs are able to perform these normal functions despite somewhat challenging conditions in the wound site, such as hypoxia or lack of nutrients. However, in the case of chronic wounds, the normally-ischemic wound environment becomes even harsher, with excessive inflammation¹⁰⁰ and an environment not conducive to angiogenesis compared with normal wounds.⁸¹ Therefore, a significant barrier to successful use of MSCs in any potential cell therapy has been post-implant cell survival in a variety of ischemic injury models. Past studies have shown marginal MSC preservation in various models including cardiac infarct¹⁰¹ or cerebral injury,¹⁰² but still MSC use in any of these models is limited by MSC death due to the harsh wound environment. As all of the benefits of MSC therapy for any type of wound healing are dependent on cell survival in the wound, strategies to improve survival following implantation are of interest in future research efforts. Recent studies have examined the effectiveness of preconditioning human MSCs with varying oxygen concentrations or pan-caspase inhibitors to improve the MSC survival response immediately following implantation. Saini et al. showed hyperoxic and pan-caspase pre-treatment of the cells substantially decreased MSC apoptosis in a cardiac infarct model, a scenario that produces an ischemic environment for implanted MSCs.¹⁰³ Conversely, Chang et al. recently demonstrated the advantages of preconditioning MSCs in hypoxia, which was shown to improve the secretory capabilities of the cells (VEGF, HGF, and others), a main benefit of MSC therapy.¹⁰⁴ Gene therapy in MSCs has also received some attention, as Wang et al. recently showed that adenoviral upregulation of protein kinase G1α improved MSC survival following implantation into a similar cardiac infarct model.¹⁰¹

Additional recent studies have examined the possibility of exploiting endogenous signaling pathways in promoting MSC survival in a variety of wound healing contexts. The possibility of activating pro-survival pathways via matrikine moieties is a relatively novel concept that has been demonstrated to affect MSC signaling during wound healing normally.¹⁰⁵ This research led to the idea that EGFR could be activated by EGF molecules tethered to growth scaffolds, which was shown to improve MSC survival in vitro during cell death assays.^106,107 This system of activating EGFR artificially to promote survival signaling has been applied to several tissue engineered surfaces, and provides the MSCs with a variety of survival advantages that could be used to combat the ischemic wound environment. More recently, Rodrigues et al. showed that the matrix protein Tenascin C could produce similar effects in vitro.¹⁰⁸ Tenascin is easily incorporated into collagen-based scaffolds, and could potentially be combined with current therapeutic gels to modulate MSC survival. This could be beneficial in scaffold design for MSC delivery to chronic wound beds, as biomaterials used in scaffold design can at times produce a more robust artificial inflammatory response.

Limitations of MSC Use in Chronic Wound Repair

As discussed in this article, one potential limitation to use of MSCs for treating chronic wounds is varying degrees of cell survival following implantation that might curtail any therapeutic effect in the long-term. However, there are other more fundamental hurdles to use of MSCs. One clear limitation to using MSCs as a standard therapy in any context is a general functional heterogeneity that makes a “standardized” MSC for manufacturing and quality control purposes a serious challenge. Changes in cell proliferation rate and differentiative capacity between donor sources have been reported for many years,¹⁰⁹ as well as functional differences among subpopulations of MSCs from a single source based on variation in RNA production.¹¹⁰ Additionally, use of MSCs as therapeutic agents requires ex vivo expansion that, while somewhat more accessible than other stem cell types, can remain problematic due to the aforementioned heterogeneity as well as a very limited natural in vitro lifespan.¹¹¹ This all ultimately leads to issues in commercialization, where there remains no real set of guidelines for MSC expansion and general use in therapeutics for companies using the cells, despite their relatively common use in clinical trials and some marketed products.¹¹²

Finally, the source of MSCs used can create a great deal of variation among therapeutic cells and also have an effect on harvesting methods. Many sources of MSCs have been discovered over time, including those mentioned in this article. There are complex and sometimes vast differences between MSCs obtained from bone marrow, adipose tissue, and other sources, ranging from cell surface marker expression to differentiation limitations and, importantly, immunomodulatory capacity (recently reviewed by Hass et al.¹¹³). In terms of therapeutic strategy, this makes selection of a cell source an important consideration and/or limitation for any given therapy. For cutaneous wound healing, bone marrow-derived MSCs are the most often-used source, as evidenced by their inclusion in most of the clinical trials cited in this article (Table 3). However, adipose-derived cells and umbilical cord-derived cells have also been used for treating diabetic ulcers in clinical trials, suggesting continued disparity among researchers as to the optimal source.

Conclusions and Future Directions

Cell therapies for improving wound healing have become a topic of great interest, particularly for non-healing wounds such as diabetic or venous stasis ulcers. Patients with these wounds continue to be subject to inefficient wound care technologies that do nothing to stimulate the wound environment itself, instead providing secondary support via antimicrobial action or space filling matrix. Despite some potential limitations, we have here outlined several reasons why mesenchymal stem cells provide unique and effective support for stimulating the wound healing process in a chronic wound bed. Ultimately, these cells have the ability to suppress excessive inflammation and reduce scarring while stimulating de novo angiogenesis in the wound bed, all leading to promising outcomes in chronic wound repair.

Future directions for research in this field might focus on optimization of MSC function in chronic wound contexts, both in delivery systems and scaffold designs as well as improving cell survival via independent technologies or in combination with these delivery systems. The most promising current technologies, as outlined in several clinical trials cited in this review, include basic fibrin mesh scaffolds in gel form for seeding of MSCs; however, technologies for improving MSC efficiency in wound healing continue to emerge, such as recent studies for microsphere delivery in wound gel.¹¹⁴ MSC therapy also holds promise in improving wound healing outcomes in other wound care settings, such as surgical wounds or burns.^115,116 Ongoing studies into MSC immunomodulation and wound support in other wound models (myocardial infarction, brain, long bone defects, and others) may continue support advanced wound care research with insights into novel biomaterials and beneficial properties of MSCs in cell therapy.

Key Points

Chronic wound therapies present an ongoing problem in the advanced wound care sector with increasing prevalence of diabetes and related complications.

Mesenchymal stem cells provide a promising approach to healing these wounds.

MSC immunomodulation and related attenuation of scar formation are critical aspects of treating the hyperinflammatory chronic wound environment.

Cytokines produced from normal MSC function support angiogenesis in the healing wound.

Strategies to extend MSC survival and optimize cell delivery will improve these cell therapies in the future.

10.4161/org.27405