Role of stem cell therapies in treating chronic wounds: A systematic review

Table 1 Inclusion and exclusion criteria

Inclusion criteria
Articles published in English
Stem cell therapies with demonstrated utility in the healing of chronic wounds either alone or as a complementary modality
Systematic reviews, literature reviews, case reports, case series, retrospective and prospective studies, and clinical trials between the years 2000-2019
Exclusion criteria
Articles not published in English
No full text availability
No report of stem cell therapy application
Animal or non-human, ex vivo, or in vitro studies
Letters, comments, and editorials

Table 2 Stem cell therapies

Stem cell therapy	Cell markers	Indications	Mechanisms of action
Adipose-derived stem cells (ADSCs)	CD90+, CD105+, CD73+, CD44+, CD166+; CD34-, CD45-	Severe radiation injury, chronic ulcers, venous leg ulcers, chronic fistulae	Kim et al[61]: Promote angiogenesis, secrete growth factors and cytokines, and allow for human dermal fibroblast proliferation through direct cell contact and paracrine activation in the re-epithelialization phase; Trottier et al[62]: When combined with skin substitute with human extracellular matrix (ECM), ADSCs produce subcutaneous, dermal, and epidermal regenerated tissues
Bone marrow-derived stem cells (BMMSCs)	CD105+, CD73+, CD90+; CD13-, CD34-, CD45-	Severe radiation-associated wounds, chronic diabetic ulcers, advanced pressure ulcers in patients with spinal cord injury, and other intractable wounds	Han et al[64]: Synthesize high amounts of collagen, fibroblast growth factor (FGF), and vascular endothelial growth factor (VEGF); Ren et al[76]: Induce proliferation and potent differentiation of cells under low oxygen tension with morphologic and cell cycle changes towards bone and fat; Stoff et al[77]: Increase tensile strength of postoperative incisional wounds; Maxson et al[46]: Secrete antimicrobial factors and promote host immune response
Bone marrow-derived mononuclear cells (BMMNCs)	CD133+, CD117+, CD34+	Chronic ulcers	Amato et al[15]: Secrete angiogenic growth factors to decrease local inflammation and promote vascularization
Epidermally-derived mesenchymal stem cells (EMSCs)	CD90+, CD73+, CD105+/-; CD34-, CD271-	Chronic ulcers, burns, generalized junctional epidermolysis bullosa (JEMB)	Yang et al[89]: Promote re-epithelialization in wound healing and regenerate functional epidermal layer
Fibroblast stem cells (FSCs)	CD34+, CD11b+, CD13+, MHC II+, CD86+, CD45+, collagen-1+, procollagen-1+; CD44-	Chronic ulcers	Amato et al[15]: Increase cell proliferation, ECM deposition, wound contraction, and vascularization with additional secretion of growth factors and cytokines
Keratinocyte stem cells (KSCs)	K5, K14, K15, integrins; CD34-	Chronic ulcers	Lampert et al[94]: Enable the formation of the stratified keratinizing epidermis; Domaszewska-Szostek et al[26]: Proliferate, migrate, and differentiate during re-epithelialization with mechanical, antibacterial, and nutritious roles. Keratinocytes also interact with fibroblasts during the wound healing process and tissue regeneration. They perform autocrine secretion of IL-6 and nitric oxide, release growth factors, and help restore the barrier function of skin
Placental mesenchymal stem cells (PMSCs)	CD105+, CD73+, CD90+, CD44+; CD34-, CD45-	Chronic venous ulcers	Farivar et al[30]: Stimulate tissue regeneration and repair for improved wound healing
Umbilical cord mesenchymal stem cells (UMSCs)	CD105+, CD73+, CD90+; CD34-, CD45-	Chronic diabetic ulcers	Hashemi et al[37]: Secrete growth factors for wound healing and can differentiate into fibroblast, epithelial, and endothelial cells for wound healing
Embryonic stem cells (ESCs)	Oct-4, Stage specific embryonic antigens (SSEAs)	Intractable wounds	Guenou et al[102]: Capable of differentiating into all three germ cell layers and can form functional human basal keratinocytes

Table 3 Stem cell therapy clinical applications

Stem cell therapy	Clinical outcomes	Considerations for therapy optimization	Challenges associated with use
Adipose-derived stem cells (ADSCs)	Studies that demonstrated chronic wound healing: Akita et al[14]: No recurrences or wound abnormalities at 8 ± 2.2 wk (n = 5 patients); Lee et al: Wound healing rate of 66.7%, with improvement in pain and claudication walking distance (n = 15 patients); García-Olmo et al[72]: Epithelial covering of chronic Crohn's fistulas and healing by 8 wk (n = 4 patients); Rigotti et al[63]: Improved tissue ultrastructure, hydration, and neo-vessel formation in chronically radiated wounds over 31 mo (n = 20 patients); Studies that demonstrated a decrease in peripheral arterial disease ulcer size, number, and pain; Marino et al[71]: 6 patients had complete healing over 3 mo (n = 10 patients); Bura et al[106]: Improved transcutaneous saturation over 6 mo (n = 7 patients). Studies that showed higher chronic wound closure: Raposio et al[66]: Higher chronic wound closure than with control treatment (n = 16 patients); Carstens et al[67]: Complete chronic wound closure in 9 mo (n = 4 patients) with reduced pain; Han et al[64]: 100% chronic diabetic wound closure at 8 wk (n = 28 patients) vs 62% in the control group (n = 26 patients). Studies that demonstrated ulcer closure: Chopinaud et al[68]: Hypertensive leg ulcer closure of 93.1% at 6 mo with reduced fibrin, necrosis, and pain (n = 10 patients); Konstantinow et al[69]: 100% wound venous and arterial-venous ulcer closure over 6 mo (n = 13 patients) with reduced pain within days; Darinskas et al[70]: Complete ulcer healing (n = 15 patients) with less pain and walking improvement.	Stem cell delivery: Akita et al[14]: A 2-layered (atelocollagen + collagen) artificial dermis scaffold with injected ADSCs protects cells from infection and ambient dryness; Garg et al[73]: Capillary force ADSC seeding of hydrogels increases cell genetic expression and survival; Larsen et al[43]: ADSCs can be administered on an OASIS wound matrix for improved ulcer healing. Stem cell harvest: Akita et al[14]: ADSCs need to be cultured in very lean patients. When harvesting from subcutaneous adipose tissue, take care not to penetrate deeper viscera and vasculature. When injecting in chronic radiation injury sites, avoid surface rupture or laceration.	Gauglitz et al[31]: ADSCs are not immortal and display signs of "old age" when subject to culturing. Adipose tissue varies in metabolic activity and capacity for proliferation and differentiation, depending on the location of tissue harvest and other patient variables (age, gender); Marfia et al[45]: Autologous ADSCs have an altered genotype in diabetic patients, resulting in decreased potency, and decreased expression of vascular endothelial growth factor A and chemokine receptor CXCR4; Rezaie et al[50]: Short survival, poor transplantation, inferior homing, possibility of tumor formation, and loss of chemokine markers during ex vivo expansion.
Bone marrow-derived stem cells (BMMSCs)	Badiavas et al[78]: Demonstrated complete closure, dermal rebuilding, reduced scarring, and successful engraftment of cells over 1 year (n = 3 patients); Vojtassák et al[82]: Increase in dermal vascularity and dermal thickness of the wound bed after 29 d of treatment (n = 1 patient). Studies that demonstrated improved ulcer healing: Lu et al[79]: Improved healing in chronic diabetic ulcers at 6 wk with 100% healing 4 wk earlier than treatment with BMMNCs (n = 18 patients); Dash et al[80]: Improved Buerger disease (n = 9 patients) and diabetic (n = 3 patients) ulcer healing at 12 wk; Sarasúa et al[33]: Full healing of longstanding stage IV pressure ulcers in patients with spinal cord injury (n = 19 patients). Studies that showed decrease in chronic wound size: Gupta et al[35]: 70% reduction in chronic wound size over 3 wk followed by complete closure at 1 mo with application of cultured BMMSCs, rather than application of bone marrow aspirate alone (n = 19 patients); Humpert et al[75]: Reduction in chronic venous and neuroischemic wound size, increased vascularization, and infiltration of mononuclear cells after 7 d of treatment (n = 1 patient); Rogers et al[81]: Reduction in size in chronic wounds of different etiologies (n = 3 patients); Wettstein et al[54]: Reduction in chronic wound size of 50% over 3 wk of therapy (n = 3 patients).	Stem cell delivery: Yoshikawa et al[83]: Improved skin generation when BMMSCs are cultured in an artificial collagen dermis (n = 18 patients); Falanga et al[29]: 40% reduction in chronic wound size when BMMSCs are cultured in fibrin spray (n = 5 patients); Ravari et al[84]: Demonstrated significant reduction in chronic diabetic wounds after 4 wk of treatment when BMMSCs are co-administered with platelets, fibrin glue, and bone marrow-impregnated collagen matrix (n = 8 patients).	Rezaie et al[50]: Short survival, poor transplantation, inferior homing, possibility of tumor formation, and loss of chemokine markers during ex vivo expansion.
Bone marrow-derived mononuclear cells (BMMNCs)	Studies that demonstrated a higher wound healing rate: Yamaguchi et al[87]: Epidermal grafting significantly accelerated chronic diabetic foot ulcer healing (n = 10 patients); Jain et al[86]: Decreased wound area by 17.4% (n = 25 patients) compared to 4.84% with control (n = 23 patients) at 2 wk with average decrease of 36.4% vs 27.24% in wound area at 12 wk; Deng et al[24]: Demonstrated wound healing rate of 34.55% ± 11.18%, compared to control wound healing of 10.16% ± 2.67% (n = 10 patients); Deng et al[25]: Demonstrated wound healing of 26.5% ± 9.51% when administered as high density nanofat combined with negative pressure wound therapy (NPWT), compared to control healing of 12.02+4.2% with NPWT alone (n = 8 patients).	Not reported in reviewed articles.	Not reported in reviewed articles.
Epidermally-derived mesenchymal stem cells (EMSCs)	Bauer et al[90]: Regeneration of functional epidermis with gene-corrected EMSCs in previously infected, non-healing chronic ulcers due to junctional epidermolysis bullosa (n = 1 patient).	Teng et al[52]: Cultured epidermal autografts enriched with EMSCs on an ECM-compatible substrate can overcome EMSC deficiency in chronic wounds and provide ECM materials to stabilize the wound site.	Not reported in reviewed articles.
Fibroblast stem cells (FSCs)	Yamada et al[107]: Chronic wound size reduction of 33.3% (n = 5 patients); You et al[105]: Complete diabetic foot ulcer healing in 84% of patients (n = 26 patients) over 36.4+17.6 d.	Stem cell delivery: Brower et al[20]: Enhanced cell adherence, proliferation, and migration when delivered in a 2-chamber fibrin sealant; Yonezawa et al[92]: Improved wound healing of 92% (n = 13 patients) when fibroblasts were cultured on a hyaluronic acid and atelo-collagen matrix; Marcelo et al[108]: More complete wound healing achieved when administered with autologous fibrin glue over the ulcer site.	Not reported in reviewed articles.
Keratinocyte stem cells (KSCs)	Studies that demonstrated improved wound healing: Vanscheidt et al[95]: Complete and faster wound healing in 38.3% of patients receiving autologous keratinocytes, compared to 22.4% in the control group (n = 44 patients); Moustafa et al[96]: improved wound healing over 6 wk when administered as cultured autologous keratinocytes on cell-free discs (n = 12 patients); Bayram et al[99]: Reduction in diabetic foot wound size of 92% (n = 20 patients), compared to control wound size reduction of 30%. Studies that demonstrated complete ulcer healing: Hartmann et al[97]: Complete ulcer healing at a mean of 14.5 wk when administered as keratinocytes transplanted in a fibrin carrier (n = 4 patients); De Luca et al[98]: Complete ulcer healing in 20 wk (n = 20 patients) with a 30%-84.4% reduction in size after 3 wk (n = 4 patients); Beele et al[100]: Demonstrated complete venous leg ulcer closure within 1 wk (n = 11 patients) over a period of 4.1-24.9 wk with decreased pain.	Stem cell delivery: Hartmann et al[97]: Better keratinocyte graft fixation and epithelial monolayer formation with low-density fibrin; Bayram et al[99]: More effective delivery when cultured keratinocytes are attached to microcarriers made of polyethylene and silica; Teepe et al[109]: Radial progression toward wound closure when administered as cryopreserved cultured allografts (n = 43 patients); Shukla et al[101]: Complete chronic wound healing over 12-48 wk when administered as keratinocytes along with epidermal cell suspension (n = 12 patients).	Rezaie et al[50]: Difficult to isolate, short lifespan during serial cultivation, and possible tumor formation.
Placental mesenchymal stem cells (PMSCs)	Farivar et al[30]: Complete chronic venous ulcer healing in 53% (n = 21 patients with 30 total ulcers) with 79% mean reduction in wound surface area over an average of 10.9 wk.	Farivar et al[30]: Improved cell survival when MSCs are administered in cryopreserved, aseptic placental tissue (hVWM).	Duscher et al[27]: Appropriate donor selection is necessary to avoid immune-mediated rejection or transmission of genetic diseases. More efficient cell isolation, culture, and expansion techniques are needed, along with close surveillance for malignant transformation.
Umbilical cord mesenchymal stem cells (UMSCs)	Hashemi et al[37]: Significant decrease in chronic diabetic ulcer size over 9 d with decreased wound healing time (n = 5 patients).	Hashemi et al[37]: Improved tissue regeneration and wound healing when cells are seeded on an acellular amniotic membrane scaffold.	Duscher et al[27]: Appropriate donor selection is necessary to avoid immune-mediated rejection or transmission of genetic diseases. More efficient cell isolation, culture, and expansion techniques are needed, along with close surveillance for malignant transformation.
Embryonic stem cells (ESCs)	Not reported in reviewed articles.	Not reported in reviewed articles.	Okano et al[104]: Patients will need to be monitored for tumorigenesis and teratoma formation.