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Stem cells and chronic wound healing: state of the art

Authors Leavitt T, Hu M, Marshall C, Barnes L, Longaker M, Lorenz P

Received 16 October 2015

Accepted for publication 16 December 2015

Published 2 March 2016 Volume 2016:3 Pages 7—27

DOI https://doi.org/10.2147/CWCMR.S84369

Checked for plagiarism Yes

Review by Single anonymous peer review

Peer reviewer comments 2

Editor who approved publication: Prof. Dr. Marco Romanelli



Tripp Leavitt, Michael S Hu, Clement D Marshall, Leandra A Barnes, Michael T Longaker, H Peter Lorenz

Hagey Laboratory for Pediatric Regenerative Medicine, Division of Plastic Surgery, Department of Surgery, Stanford University School of Medicine, Stanford, CA, USA

Abstract: Currently available treatments for chronic wounds are inadequate. A clearly effective therapy does not exist, and treatment is often supportive. This is largely because the cellular and molecular processes underlying failure of wound repair are still poorly understood. With an increase in comorbidities, such as diabetes and vascular disease, as well as an aging population, the incidence of these intractable wounds is expected to rise. As such, chronic wounds, which are already costly, are rapidly growing as a tremendous burden to the health-care system. Stem cells have garnered much interest as a therapy for chronic wounds due to their inherent ability to differentiate into multiple lineages and promote regeneration. Herein, we discuss the types of stem cells used for chronic wound therapy, as well as the proposed means by which they do so. In particular, we highlight mesenchymal stem cells (including adipose-derived stem cells), endothelial progenitor cells, and induced pluripotent stem cells. We include the results of recent in vitro and in vivo studies in both animal models and human clinical trials. Finally, we discuss the current studies to improve stem cell therapies and the limitations of stem cell-based therapeutics. Stem cells promise improved therapies for healing chronic wounds, but further studies that are well-designed with standardized protocols are necessary for fruition.

Keywords: stem cells, chronic wounds, cell therapy, wound healing

Introduction

Chronic wounds represent a significant burden both financially and in terms of lost quality of life, affecting both individual patients and the health-care system as a whole. In North America alone, management of complex wounds, which include chronic wounds, pressure ulcers, and nonhealing surgical wounds, carries an annual price tag of US$10 billion. Worldwide, the global wound care market is projected to surpass US$22 billion per year by 2020.1 Whereas acute wounds are expected to eventually heal, chronic wounds are defined by a physiologically impaired healing response. Current wound management strategies remain unable to adequately treat chronic wounds, resulting in a vigorous pursuit of novel therapies, including those utilizing stem cells. The inherent regenerative capabilities of stem cells make them ideal targets for addressing the unmet clinical needs associated with chronic wounds, as well as expanding our understanding of the wound healing process. Though numerous clinical trials have measured the effects of stem cells in wound healing, most have been in the context of treating a more systemic illness such as critical limb ischemia (CLI); wound healing is instead often relegated to secondary outcome measures, making analysis of their clinical efficacy more difficult. This review will cover the clinically relevant sources of stem cells for chronic wound therapy, the means by which they modulate wound healing, and the results of recent in vitro and in vivo studies conducted in animals and humans. Both published and ongoing clinical trials will be discussed, in addition to where the field is headed. While this review focuses specifically on stem cells as they relate to chronic wound therapy, the reader is directed to the review by Li and Fu2 for a more in-depth discussion of stem cell mechanisms involved in physiological wound healing.

Stem cell sources for regenerative medicine

Stem cells are defined by their ability to self-renew and differentiate into multiple cell types. They are categorized as either multipotent or pluripotent based on the variety of cell lineages to which they may give rise. Multipotent cells are equivalent to adult stem cells, with the ability to differentiate into several different lineages. Pluripotent cells are embryonic stem cells (ESCs), with an even greater capacity for differentiation, able to form all of the functional cell types of an organism. Various stem cell populations have been the subjects of significant research efforts. The most clinically relevant stem cell populations include mesenchymal stem cells (MSCs), endothelial progenitor cells (EPCs), and induced pluripotent stem cells (iPSCs).

Mesenchymal stem cells

Under suitable conditions, MSCs have the ability to differentiate into bone, fat, cartilage, and muscle. Their ability to adhere to polystyrene tissue culture plastic remains a crude but effective means of isolating what is now understood to be a very heterogeneous population of progenitor cells. In addition to the aforementioned characteristics, the International Society for Cell Therapy has included cell surface expression in its guidelines for defining MSCs: CD73+, CD90+, CD105+, CD11b/14, CD19/CD73b, CD34, CD45, and HLA-DR.3 CD271+ has also been described as the most specific marker for bone marrow-derived mesenchymal stem cells (BM-MSCs) and adipose-derived stem cells (ASCs).4 However, consensus has yet to be reached with regard to their antigen expression profile.5 The inconstant nature of surface marker expression as these cells are removed from their native environment and expanded ex vivo merely worsens the surrounding controversy.6 Lack of standardization can thereby impede our ability to draw comparisons across studies. Despite these shortcomings, MSCs are understood to play a significant role in coordinating the wound healing response as it advances through phases of inflammation, proliferation, and remodeling.7 In addition to their capacity for self-renewal, MSCs have immunomodulatory effects at the local environments into which they are transplanted,8,9 supporting native cells via various paracrine mechanisms that promote cell survival, migration, and proliferation.10,11 This regenerative potential has garnered significant interest for applying MSCs in clinical interventions.

Within the larger group of cells termed MSCs, there are several described subpopulations, differentiated largely by their tissue of origin, but also in terms of sometimes-controversial differences in phenotype. MSCs reside in all mesenchymal tissues, a few of which are more practical for clinical purposes. BM-MSCs are one of the most studied populations. However, harvest of these cells necessitates painful bone marrow aspiration. This, in addition to the scarcity of BM-MSCs (1 in 10,000 mononuclear bone marrow cells)12 has shifted favor toward the more accessible and abundant ASCs. With regards to cell-surface expression, the International Fat Applied Technology Society together with the International Society for Cell Therapy distinguish ASCs from MSCs by their positivity for CD36 and negativity for CD106 in culture.13 ASCs are generally harvested as part of lipoaspirate within the stromal vascular fraction (SVF), which itself has been used for regenerative wound therapy.14 The SVF contains a diverse population of cells, including endothelial, hematopoietic, and pericytic lineages, as well as MSCs. ASCs are believed to account for up to 3% of total cells within isolated SVF, which is orders of magnitude higher than the corresponding proportion of MSCs in bone marrow.15

The relative dearth of MSCs in bone marrow aspirate has also led to the utilization of bone marrow mononuclear cells (BMMNCs) in clinical studies, which demonstrate their potential for accelerating wound healing. BMMNCs are also a heterogeneous group, thought to contain EPCs and MSCs in addition to an array of growth factors and cytokines, together promoting angiogenesis.16,17 Amniotic fluid, placental tissue, and umbilical cord (including Wharton’s jelly and cord blood) are further sources of MSCs. Peripheral blood is also considered by some to be a source of MSCs following mobilization by granulocyte-colony stimulating factor (G-CSF) injection, though these mobilized cells would generally be classified as BMMNCs or peripheral blood mononuclear cells (PBMNCs). Recently, Li et al18 described a coculture system that routinely produces MSCs from peripheral blood without mobilization. Despite sharing the MSC moniker, populations from varying sources differ in terms of cell surface markers and differentiation efficiency, though they maintain their overall multipotent characteristics.

While research has often focused on addressing the issues associated with a heterogeneous and poorly characterized cell population, the improved wound healing associated with MSC administration may in fact depend, at least in part, on multiple cell types working in concert. Rodriguez-Menocal et al19 demonstrated that whole bone marrow is more effective at stimulating angiogenesis relative to cultured bone marrow cells or MSCs, resulting in faster wound healing. A clinical study comparing BMMNCs to MSCs corroborated these findings in patients with chronic wounds.20 Similarly, BMMNCs demonstrate superior osteogenic and angiogenic differentiation potential compared with isolated CD34+ cells.21 Administering heterogeneous groups of stem cells may allow for communication between different cell types, facilitating improved tissue regeneration, as is seen with coculture of MSCs together with EPCs.22

Applied to chronic wound therapies, both MSCs23,24 and ASCs25 have demonstrated an ability to improve wound healing in experimental diabetic models. Findings from in vitro experiments have elucidated a number of ways that BM-MSCs promote tissue regeneration, including production of growth factors, cytokines, collagens, and matrix metalloproteinases.26,27 Their direct interactions with other cells, such as keratinocytes, also accelerate wound healing.28 Despite their ability to differentiate into various cell types, the mechanism of action of MSCs is largely paracrine in nature.29 For example, ASC-conditioned media can stimulate the migration of vascular endothelial cells, fibroblasts, and keratinocytes, suggesting that the secretome can promote wound healing even in the absence of the cells themselves.30

Endothelial progenitor cells

Recruited from the bone marrow, EPCs circulate in the blood expressing hematopoietic and endothelial surface markers, localizing to sites of tissue injury and ischemia.31 At the wound bed they contribute to vasculogenesis, the process by which new vessels are formed from circulating progenitor cells. Like MSCs and ASCs, EPCs represent another heterogeneous population, with variations in classification that impede standardization of their clinical applications. The definition of EPCs overlaps with PBMNCs and BMMNCs (Figure 1), which may lead to confusion. In terms of clinical applications, EPCs are harvested from the peripheral blood (except Wettstein et al32 who harvested from bone marrow), often following G-CSF mobilization from the bone marrow, and are commonly enriched for CD34 positivity. The angiogenic potential of these cells has made them promising targets for treating chronic wounds with underlying ischemic pathologies.

Figure 1 Stem cell populations administered in clinical trials.
Note: Dashed lines represent more ambiguously characterized populations.
Abbreviations: ALDHbr, aldehyde dehydrogenase bright cell; EPC, endothelial progenitor cell; ASC, adipose-derived stem cell; BMMNC, bone-marrow mononuclear cell; BM-MSC, bone-marrow mesenchymal stem cell; SVF, stromal vascular fraction; PBMNC, peripheral blood mononuclear cell; G-CSF, granulocyte-colony stimulating factor; UC-MSC, umbilical cord mesenchymal stem cell.

Induced pluripotent stem cells

In the search for regenerative cell therapies, ESCs provide an excellent early possibility given their pluripotent nature. However, ethical concerns regarding embryological tissue usage have limited their applications. Furthermore, when used in adults, these cells would be allogeneic, potentially resulting in immune-mediated rejection. In 2007, the first human iPSCs were produced in vitro.33,34 iPSCs circumvent the aforementioned barriers to ESC use in that they are derived from adult somatic cells, allowing for autologous tissue generation without need of posttransplant immunosuppressive therapy. While the first human iPSCs were derived from adult fibroblasts, more recent advances have allowed for faster and more efficient production from ASCs,35 demonstrating further potential of MSCs in regenerative medicine.

iPSCs may overcome current limitations in wound healing therapies, but their use is not without risk. A common concern associated with cell-based therapies is that in utilizing cells with a nearly unlimited ability to self-renew and regenerate, there is the potential for malignant transformation. Reverting cells from a fully differentiated to a pluripotent state requires the use of reprogramming factors. Despite demonstrating reduced tumorigenicity over previous viral integration methods,36 alternative adenoviral, plasmid-based, and recombinant protein-based strategies37,38 continue to rely on protooncogenic reprogramming factors.39 More studies are thus needed to establish a safety profile for iPSC interventions. As such, there are currently no clinical trials utilizing iPSCs underway in the US. However, worldwide, their first use in a clinical trial began in 2014. While this Japanese study is aimed at treating age-related macular degeneration, the findings from this study may hopefully assuage initial concerns allowing for further clinical study, including iPSC use in chronic wound therapy.

Interactions between stem cells and chronic wounds

Chronic wound characteristics

Physiological wound healing occurs within a microenvironment conducive to tissue repair; high levels of growth factors and mitigated degradative enzymes promote the functionality of fibroblasts, keratinocytes, and vascular endothelial cells, which are key instigators of wound healing.40 Conversely, the chronic wound bed is an environment of unabated inflammation, low mitogenic activity, excessive matrix metalloproteinases, extracellular matrix degradation, reduced angiogenesis, and premature fibroblast senescence, resulting in an overall delayed time to healing.4144 While chronic wounds have a variety of causes such as pressure, diabetes, and peripheral arterial disease, the majority of chronic wounds share at least some of these pathological mechanisms.45 Additionally, failed reepithelialization may perpetuate these processes.46 Chronic wounds demonstrate a pathological level of underhealing. The use of stem cells in addressing these lesions is geared toward restoring the wound’s ability to heal, either by supplanting ineffective healing mechanisms or by augmenting muted physiological processes.

Inflammation

One of the proposed advantages of stem cell therapies is that their immunomodulatory effects can shift the wound equilibrium away from degradation toward tissue synthesis. In diabetic and venous ulcers for example, the cellular infiltrate and extracellular matrix demonstrate a lower CD4+ (T-helper)/CD8+ (cytotoxic) T-cell ratio relative to acute wounds, as well as increased B-cells and plasma cells.47 This proinflammatory state can be reversed through BM-MSC suppression of T-lymphocytes48 and B-cells.49 However, the effects of chronic wounds on stem cells may impair their ability to modulate the wound microenvironment. For example, relative to acute wound fluid, chronic wound fluid is less chemotactic to ASCs and inhibits rather than stimulates their proliferation.50

Infection

The chronic inflammatory state is in part related to bacterial colonization. Interestingly, not only do MSCs decrease inflammation at the wound bed, they also enhance bacterial clearance and improve survival in sepsis via secretion of the antimicrobial peptide LL-37.51,52 However, there may be limitations to this antimicrobial effect. Long-term bacterial colonization of chronic wounds is often facilitated by polysaccharide biofilms, which may negatively impact MSCs. Exposure to biofilm-conditioned media as well as isolated soluble biofilm factors alone are both sufficient to impair MSC migration and differentiation while promoting apoptosis.53 These findings suggest that chronic wounds provide a suboptimal environment for transplanted stem cells, which may thus impede the utility of stem cell therapy in chronic wounds, at least in those with ongoing bacterial colonization with biofilm forming organisms.

Hypoxia

MSCs have a high tolerance for oxidative stress in vitro, which suggests that they are ideally suited to treating ischemic pathology, promoting tissue regeneration, and reducing reactive oxygen species burden.54 ASC survival and ensuing tissue regeneration in nonvascularized fat grafts via adipogenesis and angiogenesis suggests this phenomenon also applies in vivo.55 Additionally, MSCs may promote wound healing in response to hypoxia, increasing paracrine secretion of transforming growth factor-β1, which in turn can restore fibroblast wound healing functionality inhibited under hypoxic conditions.56 Despite tolerance, and even activation of stem cells in hypoxic environments, byproducts of tissue hypoxia may be detrimental to their regenerative capabilities. As an example, elevated lactate levels, as are found in chronic wounds, are associated with gene expression in MSCs associated with inflammation and apoptosis.57

Fibroblasts

Fibroblasts are fundamental to the wound-healing cascade, but in the diabetic wound environment they display decreased proliferation and migration.47,58 However, restoration of fibroblast function is possible via paracrine signals from MSCs, such as those elucidated by Shabbir et al.59 Exosomes secreted by MSCs are taken up by fibroblasts from both normal and diabetic wounds leading to increased cellular migration, with a greater increase observed in the chronic wound fibroblasts.59

Cytotoxicity

Patients undergoing chemotherapy experience suboptimal wound healing. Paclitaxel was found to be more cytotoxic to ASCs compared to fibroblasts,60 suggesting patients undergoing chemotherapy may demonstrate a diminished response to stem cell-based therapies. Conversely, in an off-label trial, plerixafor was found to be better than G-CSF at mobilizing hematopoietic stem cells in diabetic patients, despite being indicated for lymphoma and multiple myeloma.61 Of note, however, is that the patients in this trial were not undergoing chemotherapy for cancer treatment, and thus they do not represent the traditional patient sample receiving this drug. Radiation is another cancer-related treatment with numerous side effects, including chronic ulcer formation. Using a rat model, Huang et al62 demonstrated that ASCs have the ability to accelerate healing of these ulcers. Impairments in wound healing vary greatly across patients undergoing cancer therapy. This variability may lead to a wide variety of wound healing responses to stem cells, necessitating further study.

Stem cells harvested from patients with chronic disease

Further potential hurdles to autologous stem cell use in chronic wound therapy relate to the quality and quantity of cells harvested from patients with systemic disease. Systemic disease may be linked to depletion of angiogenic precursor cells both in the bone marrow and peripheral circulation.63 In vitro experiments by Cianfarani et al64 found ASCs isolated from diabetic mice to have lower proliferative and migration potential, muted stem cell surface marker expression, and less paracrine secretion of cytokines involved in wound healing, including vascular endothelial growth factor-A, hepatocyte growth factor, and insulin-like growth factor-1. Compared to ASCs harvested from nondiabetic mice, this also translates into a blunted improvement in diabetic wound healing.64 Human ASCs harvested from ischemic limbs of diabetic patients also display muted functionality.65 Similarly, bone marrow cells harvested from chronic wound patients and EPCs from diabetic patients demonstrate reduced growth in culture and diminished stem cell potency relative to healthy controls.66,67 In stratifying patients receiving BMMNCs for CLI based on clinical outcome, Altaner et al68 found that patients who responded to therapy have higher CD44 and CD90 MSC expression and greater interleukin-4 and interleukin-6 secretion. However, the diabetic status of the patients in this trial did not correlate with response,68 which is surprising given recent studies showing diabetes impairs the ability of ASCs to promote neovascularization (via subpopulation depletion).69 Not all studies have shown that chronic systemic disease impairs stem cell functionality. Smadja et al70 noted that BM-MSCs harvested from CLI patients and healthy controls demonstrate similar proangiogenic effects when transplanted into CLI-induced mice. Increasing age and BMI are risk factors for chronic disease. Therefore, it is understandable that they also correlate negatively with stem cell yield and proliferative capacity.7173 Overall, the effects of long-term exposure to chronic pathophysiology on endogenous stem cell properties is likely quite variable, with corresponding differences in response to autologous cell therapy between patients. It is unfortunate that the patients most in need of stem cell-based wound therapies are also the least likely to have an optimal progenitor population to draw from.

Review of the clinical evidence

To the authors’ knowledge, no systematic review has been conducted that includes all types of stem cells used to manage chronic wounds. Several meta-analyses of stem cell therapies of CLI exist, but other forms of chronic wounds are omitted. Inconsistencies across clinical trials are often cited as making statistical comparisons difficult, if not impossible. Our own findings echo this sentiment, with variations between studies that include (but are not limited to) type of stem cell; method of cell harvest, purification, expansion, and administration; number of cells administered; graft immunophenotype; targeted pathophysiology; nebulous outcome measures (eg, complete wound closure vs improved wound healing vs time for given reduction in wound area); and follow-up time. We have identified 45 published trials measuring the effects of stem cell therapies on chronic wound healing, the majority of which are early Phase I trials assessing safety and efficacy, without sample randomization or placebo control. Many patients with wounds refractory to treatment over months to years may be considered to provide fairly adequate internal historical controls, though the inherent decrease in validity precludes their incorporation into meta-analysis. The difficulty of conducting randomized controlled trials has also spurred some investigators, such as Lu et al,20 to use contralateral ischemic limbs within the same patient as an internal control. Because of the overall small number of true randomized trials, these have sometimes been included in meta-analyses. This practice is likely to introduce confounding given the importance of paracrine-mediated MSC functionality.10 Given the incredible variability in trial designs that include stem cell-based wound therapy, our literature search was systematic (Figure 2, Tables S1–S3), but statistical meta-analysis was not performed. The full list of relevant studies can be found in Table 1.

Figure 2 Literature search flowchart
Abbreviations: NCT, National Clinical Trial; CLI, critical limb ischemia.

Table 1 Published studies
Notes: aWhen not stated in original publication, clinical outcomes were analyzed with Fisher’s exact test to calculate P-value. (Y), Internal control used eg, different area of wound or contralateral limb; (*), statistically significant but used internal control; “*”, statistical significance not directly related to improved wound healing in cell therapy group vs control.
Abbreviations: Y, yes; N, no; ALDHbr, aldehyde dehydrogenase bright cell; APC, angiogenic progenitor cell; ASC, adipose-derived stem cell; BMMNC, bone-marrow mononuclear cell; BM-MSC, bone-marrow mesenchymal stem cell; DFU, diabetic foot ulcer; e-SVF, enhanced stromal vascular fraction; IA, intra-arterial; IM, intramuscular; IV, intravenous; PAD, peripheral arterial disease; PBMNC, peripheral blood mononuclear cell; PBNM, peripheral blood non-mobilized cells; PNS, Panax notoginseng saponins; PTA, percutaneous transluminal angioplasty; TAO, thromboangiitis obliterans (Buerger’s disease); UC-MSC, umbilical cord mesenchymal stem cell.

The type of cell used in chronic wound therapy varies widely across clinical trials, with different degrees of overlap between them, further complicated by often-controversial classification standards. We have attempted to categorize the cell types used in the 45 trials identified as being relevant to the use of stem cells in chronic wound therapy (Figure 1, Table 1). BMMNCs are a frequently used source of progenitor cells for clinical use comprising a heterogeneous population that includes MSCs and EPCs. Numerous study protocols, including Huang et al,74 harvest cells originally localized to the bone marrow, but which have been mobilized by G-CSF into the peripheral blood (characterizing them as PBMNCs), circumventing the need for more invasive bone marrow aspiration. Within a given cell population, certain studies also include populations enriched for particular markers, such as aldehyde dehydrogenase bright cells (ALDHbr), CD90+, or CD34+ cells.7577 MSCs are another population frequently used in clinical trials, sourced from various tissues including bone marrow, adipose tissue, and the umbilical cord. SVF represents an additional collection of progenitor cells, including MSCs and EPCs.78

Of the 45 selected trials, only 17 include independent placebo controls (3 additional studies relied on internal controls) (Table 1). To date, the JUVENTAS trial is the largest published randomized controlled trials utilizing stem cells in wound healing. Results from this study indicate that there is no statistical significance between the placebo and treatment groups.79 The fact that this study categorized major amputations as nonhealing ulcers is likely to have skewed outcomes defined as complete wound healing relative to other studies, which traditionally exclude these patients from analysis. However, the measurements of wound area reduction, which did exclude patients who underwent amputation, also demonstrated a clinically insignificant advantage of BMMNC administration.

The two largest ongoing trials, NCT01245335 and NCT02099500, predict enrollments of 200 or more each, and utilize cells from bone marrow and adipose tissue, respectively. NCT01245335 is a double-blinded, randomized, placebo-controlled trial. Though it does not specifically include wound healing as an outcome, patients must have Rutherford Category 5 peripheral artery disease to qualify for enrollment, which is defined as minor tissue loss (nonhealing ulcer, focal gangrene with diffuse pedal ischemia).80 Given that the primary outcome is improvement in Rutherford classification, this study will likely provide valid data as to the effect of bone marrow stem cells on wound healing in a much larger population than previously studied. While NCT02099500 addresses the relative scarcity of published clinical trials using ASCs for chronic wound therapy and includes a large sample size, this trial lacks a placebo control arm. Overall, with regard to large studies involving placebo controls, 4 of 11 ongoing trials with an enrollment ≥50 do not have a placebo or sham control arms (NCT02099500, NCT01903044, NCT02089828, and NCT01456819).

Comparisons of published studies with ongoing or unpublished studies (Table 2) demonstrate a shift in focus (Figure 3). The relative increase in studies utilizing ASCs coincides with a more recent appreciation for the higher yield of readily accessible stem cells existing within adipose tissue. Harvesting these cells from what is generally classified as biohazardous waste following liposuction may allow patients to forgo painful bone marrow aspiration. Additionally, BMMNCs appear to be favored in ongoing or unpublished trials, potentially due to several studies documenting the superiority of more heterogeneous cell transplants.1921 The number of studies and their respective sample sizes receiving allogeneic cells is also greater among ongoing trials, reflecting an understanding that autologous stem cell potency may be blunted in patients with chronic systemic illness. Conversely, allogeneic cells have been shown to significantly enhance diabetic wound healing.81

Table 2 Unpublished studies
Notes: Results of clinicaltrials.gov search showing ongoing trials or unpublished studies pertaining to stem cell therapies in patients with chronic wounds, verified within the last 4 years.
Abbreviations: ALDHbr, aldehyde dehydrogenase bright cell; APC, angiogenic progenitor cell; ASC, adipose-derived stem cell; BMMNC, bone-marrow mononuclear cell; BM-MSC, bone-marrow mesenchymal stem cell; SVF, enhanced stromal vascular fraction; PBMNC, peripheral blood mononuclear cell; UC-MSC, umbilical cord mesenchymal stem cell; ePTFE, expanded polytetrafluoroethylene.

Figure 3 Relative proportions of stem cell population use in analyzed published and unpublished clinical studies.
Note: Chart area is proportional to sample size.
Abbreviations: ASC, adipose-derived stem cell; EPC, endothelial progenitor cell; MSC, mesenchymal stem cell; BMMNC, bone marrow mononuclear cell.

Taken together, the published studies demonstrate that stem cell therapies can indeed lead to healing of chronic wounds resistant to traditional therapy. Meta-analyses corroborate this impression; most recently Liew et al82 calculated an odds ratio of 2.90 (95% confidence interval [CI], 1.44–5.82) when comparing stem cell therapies with control treatment for complete ulcer healing. However, the 2015 meta-analysis by Liu et al83 suggests that this benefit may wane with longer follow-up times. While there is sufficient evidence to support the belief that stem cells improve chronic wound healing in clinical trials, the limited number of placebo control groups and inconsistent means of reporting wound healing (eg, using median wound area84 rather than complete wound closure) prevents us from establishing the true extent of this benefit. In our literature search, 9 of the 17 placebo-controlled trials showed a statistically significant improvement in wound healing with cell therapy. Interestingly, these studies include an array of methods to administer cells (intramuscular, intra-arterial, and direct application) and cell populations (PBMNCs, SVF, BM-MSCs, and BMMNCs). This suggests that many tissues and extraction methods offer means of reliably harvesting cells that can augment the healing process.

Improving stem cell yield, efficacy, and lifespan – current and future techniques

Unfortunately, though MSCs have demonstrated an ability to improve wound healing, their lifespans are short in vitro, reducing the efficacy of ex vivo expansion. Furthermore, they demonstrate suboptimal engraftment, survivability, and retention at the wound when transplanted,85,86 with several of the underlying mechanisms discussed previously. Many studies rely on intradermal or intramuscular injection to administer stem cells in suspension. Though technically simple, there is a relative loss of therapeutic efficacy, potentially caused by subsequent anoikis in the absence of cell–matrix attachment or shear forces during the injection.87 When compared, stem cells delivered intramuscularly and intra-arterially demonstrate no significant difference in terms of improved wound healing.88,89 Conversely, intravenous administration of stem cells is uncommon in chronic wound therapy because of cell entrapment in the pulmonary vasculature (the pulmonary “first-pass” effect).90 Stem cell localization to the wound bed is notably impaired in chronic (but not acute) wounds partly due to downregulated stromal cell-derived factor 1 (a chemokine attracting MSC to wound bed) secondary to uncontrolled inflammation.91 The resulting decrease of ASC migration to the wound bed further elaborates on the difficulties of systemic stem cell therapy for chronic wound healing, necessitating consideration when developing mechanisms of administration for translational medicine. Advances in stem cell surface modification offer a potential solution to this problem by targeting cells to specific tissues.92

Poor cell engraftment and survivability are problematic considering the dose–effect relationship observed by Falanga et al,93 though the number of administered cells has not universally been shown to correlate with response.77 Defining dose in heterogeneous cell populations can also be difficult, but perhaps subpopulation composition may be less relevant (given BMMNC:MSC ratios resulted in similar clinical improvements).94 Efficacy is most likely related to a minimum required dose,93 while the lack of consensus may be related to inconsistent methodologies (eg, wound type, cell type, harvest, expansion, and administration). Increasing stem cell potency with adjuvants such as platelet-rich plasma or Panax notoginseng saponins are possible alternatives for decreasing the minimum required dose.95,96

Means of prolonging transplanted cell lifespan are now heavily sought, as MSCs must survive to influence healing. Numerous possibilities have arisen, such as hyperoxic and pan-caspase pretreatment, which reduces MSC apoptosis in ischemic microenvironments.97 Hypoxic preconditioning has also been shown to increase paracrine secretion by MSCs.98 Mohanty et al99 showed that small molecule-induced prion protein upregulation also results in increased lifespan and yield of MSCs in culture, as well as improved engraftment. Gene therapy provides further opportunity, such as protein kinase G1α overexpression via adenovirus vector to promote MSC survival.100 Low-level light irradiation is another recent tool for increasing stem cell wound healing potency.101

Endogenous wound healing pathways provide further means by which to optimize MSC survival. In the presence of proapoptotic cytokines (FasL, ubiquitous in chronic wound microenvironments), endothelial growth factor (EGF) molecules tethered to growth scaffolds demonstrated an ability to improve MSC survival via activation of the EGF-receptor.102 Surface-tethered EGF generated a superior MSC response relative to saturating concentrations of soluble EGF,103 supporting the use of other endogenous mediators such as the matrix protein Tenascin-C combined with biosynthetic scaffolds to enhance survival of transplanted MSCs.104 Such combinations may reduce the inflammatory response to the scaffolds themselves.105

Scaffolds are valuable additions to stem cell-based wound therapy as they provide an external niche for transplanted cells outside of the hostile wound environment, while still allowing them to facilitate wound healing. In an excisional wound model, Rustad et al87 showed both a faster time to complete wound closure and a return of skin appendages in wounds treated with a biomimetic pullulan–collagen hydrogel scaffold seeded with MSCs. Moreover, this hydrogel led to longer MSC viability, increased engraftment efficiency, and enhanced angiogenesis. Clinically, Yoshikawa et al106 used a composite graft of BM-MSCs incorporated into a collagen sponge to successfully treat decubitus ulcers refractory to artificial skin grafting. ASCs embedded in silk fibroin scaffolds and fibrin gels have demonstrated a similar ability to accelerate wound healing in vivo.107,108 Autologous MSCs applied with a fibrin spray system also resulted in some improvement in patients with chronic ulcers.93 Scaffolds offer a viable means for enhancing stem cell engraftment and survivability. It is therefore likely that their incorporation into cell-based therapies will increase markedly in the near future.

Clinical implementation of cell-based therapies has opened a new frontier in the development of biomedical devices aimed at optimizing current therapies.109 The fundamental risk of contamination associated with cell products has spurred the development of closed systems for harvesting and/or culturing cells.110 Widespread clinical use also necessitates scalable technologies, such as tissue bioreactors.111 Increased automation of stem cell harvest, isolation, and expansion will allow for more standardized therapies, and subsequently more generalizable results. Novel approaches to cell population characterization such as kinome analysis may also improve clinical efficacy, or at least provide a better measure of prognosis.112

Promoting stem cell yield, survival, and efficacy at the wound bed are worthy goals. However, it is also possible that truly successful chronic wound therapy requires a deeper understanding of how the various types of stem cell therapies can modulate systemic pathophysiology. For example, in type 2 diabetes, inhibiting the local proinflammatory phenotype at the wound bed may be insufficient to completely restore wound healing; restoration of an anti-inflammatory M1/M2 macrophage equilibrium is required to allow for physiological wound healing.113

Limitations

Some of the limitations of stem cells in wound therapies have already been discussed, such as phenotypic drift in culture, heterogeneity of cell populations, and the variable quality of cells depending on their source. Beyond barriers to therapeutic efficacy, there are also potential risks, as is the case with any medical intervention. The possibility of malignant transformation exists whenever stem cells are transplanted. While this may be a greater risk with pluripotent iPSCs, study of the more commonly used multipotent MSCs has generated less of a concern. While malignant transformation has been observed in long-term culture,114 the larger body of evidence suggests that the risks of malignant transformation are low, especially prior to MSCs undergoing senescence.115 Follow-up to one of the original studies thought to demonstrate spontaneous MSC transformation has since shown a small number of malignant cells to be the culprit.116 Furthermore, the beneficial immunomodulatory properties of stem cells are also not without theoretical risks. MSC immunomodulation and homing to different target organs can increase risks of opportunistic or disseminated infections, as well as susceptibility to malignancy.117,118 Finally, transplant of biological material also carries risks of directly transmitting infectious agents.117 Overall, clinical trials demonstrate that stem cell applications to wound healing are safe, but physicians must continue to reevaluate the risks and benefits of their use as the results of more long-term follow-up studies are published.

Conclusion

Stem cells can provide the next step in advancing wound care, particularly for chronic wounds resistant to current therapies. Meta-analyses consistently show that stem cells provide a safe and effective means for promoting chronic wound healing. However, the statistically similar improvement observed in both trial arms of the large JUVENTAS study offers a solid reminder of the importance of placebo controls for measuring the true efficacy of stem cell therapy. As development of commercial devices for stem cell therapies increases, standardization of protocols will allow for greater study validity. Therapies can then be fine-tuned and catered to specific pathologies, whereas currently the different stem cell populations and routes of administration provide only roughly comparative results across several head-to-head studies. As we move forward, stem cells are likely to become a common tool available to clinicians for wound management, but clinical practice must follow evidence of safety and efficacy, so frequent reevaluation of the literature will be critical as new therapies are described.

Disclosure

The authors report no conflicts of interest in this work.


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Supplementary materials

Table S1 CENTRAL search strategy
Abbreviations: BM-MNC, bone marrow mononuclear cell; PB-MNC, peripheral blood mononuclear cell; AT-MSC, adipose tissue-derived mesenchymal stem cells.

Table S2 PubMed search strategy
Abbreviations: iPS cells, induced pluripotent stem cells; BM-MNC, bone marrow mononuclear cell; PB-MNC, peripheral blood mononuclear cell; AT-MSC, adipose tissue-derived mesenchymal stem cells; ASC, adipose-derived stem cell.

Table S3 clinicaltrials.gov search strategy
Abbreviations: iPS cells, induced pluripotent stem cells; MSC, mesenchymal stem cells; ASC, adipose-derived stem cell.

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