Adipose‐derived stromal cells for nonhealing wounds: Emerging opportunities and challenges

Abstract Wound healing complications affect thousands of people each year, thus constituting a profound economic and medical burden. Chronic wounds are a highly complex problem that usually affects elderly patients as well as patients with comorbidities such as diabetes, cancer (surgery, radiotherapy/chemotherapy) or autoimmune diseases. Currently available methods of their treatment are not fully effective, so new solutions are constantly being sought. Cell‐based therapies seem to have great potential for use in stimulating wound healing. In recent years, much effort has been focused on characterizing of adipose‐derived mesenchymal stromal cells (AD‐MSCs) and evaluating their clinical use in regenerative medicine and other medical fields. These cells are easily obtained in large amounts from adipose tissue and show a high proregenerative potential, mainly through paracrine activities. In this review, the process of healing acute and nonhealing (chronic) wounds is detailed, with a special attention paid to the wounds of patients with diabetes and cancer. In addition, the methods and technical aspects of AD‐MSCs isolation, culture and transplantation in chronic wounds are described, and the characteristics, genetic stability and role of AD‐MSCs in wound healing are also summarized. The biological properties of AD‐MSCs isolated from subcutaneous and visceral adipose tissue are compared. Additionally, methods to increase their therapeutic potential as well as factors that may affect their biological functions are summarized. Finally, their therapeutic potential in the treatment of diabetic and oncological wounds is also discussed.

growth factor beta (TGF-β), platelet-derived growth factor (PDGF), epidermal growth factor (EGF), fibroblast growth factor (FGF), and interleukin (IL)-8. Neutrophils secrete proteases, phagocyte bacteria present in the wound and degrade necrotic tissue. [3][4][5] They are followed by monocytes, which differentiate into macrophages able to phagocytose cell debris and dead neutrophils. 6 In the late inflammatory phase, macrophages secrete growth factors (TGF-β, EGF, PDGF, FGF) and the proinflammatory cytokines IL-1 and IL-6, thus activating keratinocytes, fibroblasts and endothelial cells. 7,8 The second, proliferative phase, which begins 3-4 days after injury and lasts from 2 to 4 weeks, is stimulated by factors secreted in inflammatory phase. In the proliferative phase, angiogenesis and epithelialization occur and ECM and granulation tissue are formed. 9 Angiogenesis, which is essential for the formation of granulation tissue, is induced by growth factors: vascular endothelial growth factor A (VEGF-A), FGF-2, PDGF, and TGF-β. 10 Collagen secreted by fibroblasts gradually replaces the fibrin matrix. Fibroblasts also differentiate into myofibroblasts expressing α-smooth muscle actin, which enables wound contraction. 11 Remodeling of the wound and surrounding tissues by fibroblasts, which is the final stage of wound healing, begins approximately 3 weeks after injury and can last up to 2 years. 12 During remodeling, all processes activated in the earlier phases are terminated, and myofibroblasts, macrophages and endothelial cells undergo apoptosis. 13 Collagen III is converted to collagen I by metalloproteinases, which, together with collagen rearrangement into an organized structure, leads to strengthening of the wound. 14 Typically, wounds reach approximately 20% strength of healthy skin after 3 weeks, and 80% after 12 months. 15 Many methods are used in the treatment of nonhealing wounds, including different kinds of dressings (e.g., films, hydrocolloids, foams, hydrogels, alginates, hydrofibers), tissue-engineered skin substitutes, growth factors, negative pressure therapy, and hyperbaric oxygen. 16 However they are not fully efficient. In recent years, much attention has been paid to the use of cell therapies in the treatment of wounds. For example, epidermal stem cells were first used in wound treatment in 1981 and are now applied to promote healing of burns and chronic wounds. 17,18 Autologous genetically modified cultured epidermal stem cells were also successfully used in a clinical trial for junctional epidermolysis bullosa. 19,20 Recently, therapies with mesenchymal stromal cells (MSCs), especially adipose-derived MSCs (AD-MSCs), have been of great interest around the world. 21 For many years, adipose tissue has been considered medical waste but is in fact a great source of stem cells. AD-MSCs are easy to obtain and have similar properties to bone marrow-derived MSCs (BM-MSCs). 22 Adipose tissue is a more effective source of stem cells, which can be extracted in large amounts (500-fold greater than BM-MSCs when counted per unit volume of fat) without ethical concerns. Additionally, AD-MSCs show higher proliferative capacity, longer life-span and shorter doubling time than BM-MSCs. 23 Stem cells have great potential for chronic wound healing due to increased cell migration, high proliferative potential and release of cytokines and biological factors that regulate angiogenesis, induce repair processes, and inhibit inflammatory and immune responses. 24,25 There is some inconsistency regarding the nomenclature of MSCs and mesenchymal stem cells. According to the International Society for Cellular Therapy (ISCT) the term "mesenchymal stromal cells" refers to the plastic-adherent fraction of cells showing immunomodulatory, secretory and homing properties, while "mesenchymal stem cells" refer to the population expressing progenitor properties such as self-renewal and differentiation potential to multiple cell linages. 26 The abbreviation "MSC" should be used with additional information of the tissue source origin of the cells, for example, AD-MSCs (adipose tissue-derived MSCs) and a functional definition should be provided to clarify whether it refers to stem or stromal cells (i.e., stemness confirmation with in vivo and in vitro tests). 27 Additionally, because MSCs act therapeutically by homing in on the injury site and secreting immunomodulatory and regenerative factors, which makes them therapeutic drugs, Caplan 28 suggested naming them "medicinal signaling cells." In this review, we summarized current knowledge about chronic wound treatment with the use of AD-MSCs with a particular focus on wound healing complications in diabetic and oncological patients. Diabetes and cancer are civilization diseases and the number of patients suffering from them is constantly growing. Wound healing problems are a common complication of diabetes and oncological treatment. Intensive research is underway all over the world on new drug compounds and methods of supporting wound healing in cancer patients and patients with diabetes. The review summarizes not only the biological characteristic of AD-MSCs but also the technical aspects of their isolation, cell culture and transplantation to nonhealing wounds. A comparison of AD-MSCs from different sources (subcutaneous and visceral adipose tissues [SAT and VAT]) was also made. Additionally, factors that can affect AD-MSCs as well as ways to enhance their therapeutic potential were described.

| CHRONIC WOUND CHARACTERISTICS AND CLINICAL NEED
Chronic wounds are wounds that do not heal through normal wound healing phases in an orderly and timely manner for at least 1 month. 29 Medical conditions, for example, diabetes, autoimmune diseases, vascular pathologies, obesity, neuropathy or infections, as well as therapeutics, such as cancer chemotherapeutic agents, radiation therapy, nonsteroidal antiinflammatory agents or glucocorticoids, can affect the wound healing process and lead to the formation of nonhealing or chronic wounds. Patient age, nutrition, smoking status and alcohol consumption are also important extrinsic factors. 30 A distinction is made between wounds of various etiologies: venous and ischemic ulcers, diabetic foot syndromes, posttraumatic and postoperative wounds, pressure sores and burn wounds (Figure 2.). [31][32][33][34] Chronic wounds constitute not only a medical issue but also a large economic issue. In developed countries, 1%-2% of the general population has chronic nonhealing wounds. 35 With an aging society and a growing population of patients suffering from diabetes, obesity and vascular diseases, these numbers are expected to rapidly increase. 36 The quality of life of patients with chronic wounds is significantly impaired and they show high morbidity and mortality rates. 37 In addition, treating these wounds is a serious economic burden, estimated to comprise approximately 1%-3% of medical budgets in developed countries. 38 Chronic wounds of different etiologies possess several common features, such as elevated levels of proinflammatory cytokines, proteases, reactive oxygen species (ROS) or senescent cells, dysfunction or deficiency of stem cells, decreased levels of growth factors, abnormalities in ECM functions and weak blood supply. 39,40 Their healing process is mainly arrested in the inflammatory phase, and antimicrobial and phagocytic activity of the immune cells appears to be lower in chronic wounds than in acute wounds, which likely leads to the accumulation of necrotic tissue on the wound edges. 41 The level of growth factors essential for proper wound healing can also be important in the formation of chronic wounds. For example, bFGF, PDGF, TGF-β, and EGF levels are reduced in chronic pressure ulcers compared to acute wounds. 42 In addition, chronic wounds are often gradually colonized by various bacteria, for example, Staphylococcus aureus, Enterococcus faecalis, and Pseudomonas aeruginosa, all of which form a biofilm enabling them to become not only resistant to antibiotics, other antimicrobial agents and the body's defense mechanisms but also more susceptible to other bacterial and fungal infections. [43][44][45] The presence of bacteria and their toxins causes excessive inflammatory reactions and tissue damage and results in intensified local pain. 46 In addition, immune cells and bacteria produce proteases that degrade ECM and growth factors in the wound. 47

| Wound healing in diabetes
Diabetes mellitus is a chronic metabolic disease characterized by hyperglycemia. According to the World Health Organization, 422 million adults suffered from diabetes in 2014, and 1.5 million people died of diabetes-related complications in 2012. 48 It is estimated that the population of diabetic individuals will grow to 592 million by 2035. 49 In 2017, the cost of diabetes in the United States was 237 billion dollars, one-third was allocated to the treatment of diabetic foot ulcers (DFUs). 50 These wounds are one of the most common and serious complications of diabetes and a major cause of morbidity and mortality in individuals with diabetes. Ischemia, neuropathy and infection, often occurring together, constitute the etiological triad, which leads to complications of DFUs. 51 Approximately 15% of people suffering from diabetes have diabetic ulcer during their lifetime, and 85% of amputations in diabetic patients are caused by foot ulceration, which further deteriorates to severe infection. Besides, 50%-70% of all lower limb amputations performed are the result of diabetes. 52,53 DEPTUŁA ET AL.

| 2133
Hyperglycemia can affect wound healing through different mechanisms. Tissue loss may be aggravated by a neuropathic lack of sensation, and wound healing may also be delayed by dysfunctional epithelialization caused by impaired cell proliferation and resistance to growth factors. 54 Impairment in many key processes for proper wound healing including the production of growth factors, the proliferation and migration of keratinocytes and fibroblasts, the angiogenic response, collagen accumulation or the balance between ECM component accumulation and remodeling, plays main roles in the pathophysiology of DFUs (Figure 1). 55 In diabetic wounds, macrophage transition from a proinflammatory to an antiinflammatory state does not occur, and these cells remain mainly in a proinflammatory state. 56 Prolonged inflammation is also caused by IL-1β and tumor necrosis factor α (TNFα), whose levels are increased during the inflammatory phase and remain elevated in wounds for a longer time. The stability and activity of hypoxia-inducible factor 1 affected by hyperglycemia results in the suppression of its target genes, for example, VEGF, and impaired in endothelial progenitor cell recruitment caused by decreased production of nitric oxide (NO) contributes to reduced angiogenesis. 57 In addition, keratinocytes from the margins of diabetic ulcers are activated and highly proliferative (increased expression of Ki67) and do not express the differentiation markers keratins 2 and 10 (K2, K10). They also do not migrate and show reduced expression of the precursor of the α3 chain of laminin 5 (LM-3A32), which is a molecule present in migrating epithelial cells. 60 In vitro cell cultures of fibroblasts isolated from DFUs showed reduced motility, altered secretion of cytokines (lower levels of C-X-C motif ligand 1, IL-6, IL-8, IL-23, monocyte chemoattractant protein 1 (MCP-1), and stromal cell-derived factor 1) and a drop in the maximum mitogenic response to growth factors compared to those of cells from nondiabetic patients. 61,62 Additionally, in diabetic wounds, reduced levels of many growth factors including PDGF, KGF, VEGF, insulin-like growth factors 1 and 2 (IGF-1, IGF-2), nerve growth factor, TGF-β1 and KGF, were observed, which may contribute to the delayed healing of these wounds. 63 The results from a prospective cohort study on diabetic patients showed that serum levels of TNF-α, MCP-1, matrix metalloproteinase 9 (MMP-9) and FGF-2 were higher in patients with nonhealing ulcers than in those whose ulcers had healed. Moreover, increased immune cell infiltration and expression of MMP-9 and protein tyrosine phosphatase 1B (PTP1B) were observed in skin biopsies of diabetic patients.
These factors are associated with, for example, increased inflammation. MMP-9 is involved in the degradation of proteins and growth factors, while PTP1B takes part in negative regulation of insulin, leptin, and growth factors signaling (e.g., PDGF, VEGF, and EGF). Increased expression of extracellular MMP-9 and intracellular PTP1B may not only lead to local inactivation and resistance to the activity of growth factors but also, in a way similar to elevated levels of insulin in insulin resistance, to an increase in circulating growth factors levels. 64 Additionally, impairment in the regulation of ECM was confirmed in diabetes. For example, reduced levels of collagen and elastin were found in biopsies from DFU edges. This changes probably arose as a result of persistent inflammation and fibroblast senescence as well as poor vascularization of the wound. 65 Hyperglycemic conditions may also directly contribute to increased production of MMPs and a reduction of tissue inhibitors of metalloproteinases (TIMPs), which results in disruption of the structures essential for proper wound healing. 66,67

| Wound healing in oncological patients
Cancer is one of the greatest challenges of modern medicine and the numbers of newly diagnosed cases and cancerrelated deaths are rapidly growing worldwide. It is estimated that 18.1 million new cases of cancer were diagnosed and that 9.6 million patients died because of cancer in 2018. 68 In the treatment of cancer, chemotherapy and/or radiotherapy can be given to patients preoperatively (neoadjuvant therapy) or after surgical resection of the tumor (adjuvant therapy).
Despite having many advantages, such as increased 5-year survival rates and decreased numbers of local recurrence, neoadjuvant treatment may also cause postoperative complications in wound healing, resulting in reduced blood supply to the wound and regenerative potential as well as a higher incidence of infections. Usually, surgery is postponed to 4-6 weeks after neoadjuvant treatment, but it does not fully prevent the risk of such complications. 69,70 In oncological patients, skin manifestations, wound healing complications and tissue loss may either be caused by the tumor itself or result from the method of treatment used (Figure 3). A common problem is also infections at the surgery site, for example, after breast surgery, which can also delay healing. 71 Chemotherapeutic drugs may interrupt processes responsible for proper wound repair by inhibiting cell division; metabolism and angiogenesis; the synthesis of DNA, RNA, and proteins; cell migration into the wound and the formation of wound matrix. In addition, they impair fibroblast growth, and inhibit collagen production and wound contraction. 72 These drugs are designed to target rapidly dividing cells, so macrophages and fibroblasts are as susceptible to their activity as cancer cells. 73 In addition, chemotherapy weakens patients' immune system which may interfere with the inflammatory phase of wound healing and increase the possibility of wound infections. 74,75 Targeted therapeutics such as epidermal growth factor receptor (EGFR) or VEGF inhibitors should be less toxic to normal cells, but their therapeutic targets involved in cancer growth also participate in wound healing, so the use of these therapeutics is associated with adverse reactions, including skin toxicity. 76 Blocking EGFR signaling in healthy keratinocytes results in inhibition of their growth, proliferation, and migration. 77 The use of DEPTUŁA ET AL.
| 2135 bevacizumab, a monoclonal anti-VEGF antibody, to block angiogenesis, may cause wound dehiscence, bleeding, and infections resulting from the limited delivery of cells, nutrients and oxygen. To avoid wound healing complications, it is recommended to perform surgery 60 days or 6-8 weeks after treatment with this drug and not to take bevacizumab at least 28 days after surgery. [78][79][80] The side effects of radiotherapy can be categorized as acute (hours to weeks after exposure) and late (months to years after exposure). Acute injury includes hyperpigmentation and early ulceration, while late effects present as tissue fibrosis, necrosis, atrophy, vessel damage, and chronic ulcers. 81 Skin damage is one of the most frequent acute side effects F I G U R E 3 Skin complications in oncological patients. (A) 57-year-old patient after a radical mastectomy and postoperative radiotherapy due to breast cancer. The figure shows skin discoloration in the irradiated area, that is, at the breast and armpit. In the postoperative course, features of marginal necrosis of the wound treated in an outpatient setting were observed; (B) 35-year-old patient after a radical surgery of soft tissues sarcoma of a right lower leg above the ankle and a postoperative radiotherapy. Due to ischemia, the wound in the lower limb, in the area from the back and below the ankle, was accompanied by prolonged healing. The figure shows the place of impaired healing visible as a depression with a fragment of an atrophic wound (arrow); (C) 47-year-old patient with the ulceration of the back (dimensions:17 ×10 cm) resulting from skin cancer before the radical procedure. This type of neoplastic ulcer is associated with infection and necrosis. During radical surgery, the most important element is to protect the operated site from infection of the postoperative wound. The arrow marks the site of ulceration as a place with increased risk of postoperative wound infection; (D) Tissue loss of the left tight in a 63-year-old patient after a radical surgery of soft tissues sarcoma and postoperative radiotherapy. A fragment of the thigh bone with a defect in the thigh muscles is visible. It is the most difficult variant to heal, due to the extensive tissue loss and the condition after undergoing oncological treatment. The arrow marks the exposed femur [Color figure can be viewed at wileyonlinelibrary.com] of radiotherapy, with 90% of patients who undergo radiotherapy developing skin reactions. 82 The cytotoxic effect of radiation is a result of direct DNA ionization and interaction of produced ROS with DNA. Base alterations, the formation of dimers and DNA double-strand breaks result in damage to basal keratinocytes and a reduction in the self-renewing properties of the epidermis. Overexpression of proinflammatory factors, including TNF-α, interferon gamma (IFN-γ), IL-1, IL-3, IL-5, IL-6, IL-8, and adhesion molecules, including intercellular adhesion molecule 1, vascular cell adhesion molecule, and E-selectin sustain the inflammatory phase. Wound strength is decreased by the prevention TNF-α and IFN-γmediated collagen deposition, the production of highly disorganized collagen and alterations in the production of ECM proteins resulting from changes in fibroblasts. [83][84][85][86] Moreover, elevated levels of TGF-β in the serum of irradiated patients correlated with a higher risk of radiation-induced fibrosis. 87 Low levels of angiogenic factors (FGF, HGH, and VEGF) in irradiated skin may be responsible for inappropriate vascularization, disrupted angiogenesis and reduced blood supply. 88 Persistent high concentrations of MMP-2 and MMP-9 and an imbalance of the expression of TIMPs and decreased angiogenesis may be the reason why these wounds fail to heal. 89

| CELLS AND TISSUE SOURCES
Adipose tissue may be obtained in two ways: surgical resection of excess fat tissue ( Figure 4) or liposuction of lipoaspirate.
The techniques of fat harvesting are summarized in Table 1. Lipoaspirate from liposuction can be directly clinically used without previous AD-MSCs in vitro culturing. 90 The harvesting procedure uses a tumescent solution of the acidic pH   91 Avoiding in vitro manipulation of clinical material seems to be safer and bypasses some legal regulation. 92 The type of harvesting procedure influences the yield, viability and proliferation of AD-MSCs. There are few main isolation methods: mechanical-assisted liposuction (MAL), power-assisted liposuction (PAL), laser-assisted liposuction (LAL), ultrasound-assisted liposuction (UAL) and surgical resection. PAL has been identified as the best method because cells isolated using this technique show high proliferation potential and low senescence. A comparison between MAL and UAL-derived AD-MSCs did not indicate significant changes in the expression of MSC markers (CD13, CD29, CD73, CD90, CD105; only the level of CD166 was higher in UAL-derived cells). Other comparisons (surgical resection, PAL, and LAL) confirmed that different harvesting methods do not change the expression of basic mesenchymal markers (such as CD90 and CD40). 93-95 AD-MSCs obtained from the abdomen through resection or liposuction yield more cells than either UAL or fat tissue collected from the hip/thigh district. 96

| Isolation of AD-MSCs
Cell isolation is a critical step in obtaining cells for experimental and therapeutic procedures. Enzymatic or nonenzymatic methods of AD-MSCs isolation are described in the literature; the enzymatic method utilizing collagenase ( Figure 5) is most frequently used. However, proteolytic enzymes, for example, trypsin-EDTA, dispase or collagenase may negatively affect the viability and surface antigen expression on AD-MSCs. 97 Moreover, similar to fetal bovine serum (FBS) utilization, there are safety issues regarding the use of xenogeneic collagenase. To overcome this problem, nonenzymatic methods such as sonication, explant culture, and centrifugation were developed. However, none of the proposed nonenzymatic methods isolated the same number of cells as the enzymatic method. 98 Comparison of the explant culture method and enzymatic method showed that the former had higher hematopoiesis potential, a lower percentage of CD34 expression and better quality, but required a large amount of lipoaspirate and resulted in lower overall yield of recovered cell. 99 The enzymatic method elicited a significantly higher number of cells, higher colony-forming efficiency (higher Nanog and Oct4 expression) and better differentiation capability. 100 As we believe that the isolation efficacy is critical, we assume that enzymatic isolation is the best way of obtaining cells for clinical use. Therefore, it is important to develop protocols for AD-MSCs isolation with clinical grade collagenase. Carvalho et al. 101 showed that xeno-free enzymatic products containing collagenases (e.g., Liberase TM) are as effective as research-grade products in the isolation of AD-MSCs. | 2139 compared to other subcutaneous depots. 103,104 AD-MSCs can also be obtained from atypical locations, for example, buccal fat pads. Cells from this area express high levels of osteogenic markers; therefore, they seem to be more appropriate for treating bone defects. 105 Comparison of AD-MSCs lipoaspirates taken from the abdomen and hump showed differences: hump-derived AD-MSCs are smaller in size and narrower in overall appearance than are abdominal AD-MSCs. 106 Additionally, some differences have been observed between subcutaneous AD-MSCs taken from the medial thigh or trochanteric area and those taken from the upper arm, which express higher levels of peroxisome proliferator-activated receptor gamma (PPAR-γ-2). 107

| SVF versus AD-MSCs
The advantage of SVF over AD-MSCs is its heterogeneous cellular composition (preadipocytes, fibroblasts, vascular smooth muscle, endothelial cells, macrophages, and lymphocytes), which is responsible for better therapeutic outcomes, comparable safety and less regulatory criteria. During culture expansion, AD-MSCs can change surface marker expression and cells morphology. However, AD-MSCs can be used in allogeneic and autologous treatments, while SVF can be used only in autologous treatment ( Table 2). [108][109][110] Allogeneic AD-MSCs carry minimal rejection risk, and obtaining AD-MSCs is less deleterious than obtaining other types of MSC. 111 Furthermore, fluids secreted by acute and chronic wounds have an impact on AD-MSCs and may regulate their regenerative potential. 112 For clinical application, it is important to properly store the SVF and AD-MSCs for later therapeutic use. Only an elongated period of cryopreservation at −70°C (>2 years) reduces the number of live cells and their viability. 113 Compared to fresh lipoaspirate-derived SVF, SVF from cryopreserved lipoaspirate has reduced cell viability and a lower colony-forming-unit percentage (16-fold). 114 It was shown that cryopreservation media containing human serum (HS) albumin, HS, or knockout serum replacement has no influence on AD-MSCs conditions (gene expression, immunophenotype, and differentiation ability) for up to 3-4 freeze-thaw cycles. However, their proliferation was significantly reduced, and it was suggested to perform no more than two freeze-thaw cycles on cells for clinical application. 115

| CHARACTERIZATION OF AD-MSCs
The characterization of AD-MSCs is critical before their application in not only laboratory research but also the clinic. Culture conditions can affect the properties of these cells and thus their therapeutic potential.

| Markers and differentiation potential of AD-MSCs
Since the discovery and isolation of MSCs populations from adipose tissue, much effort has been paid to their characterization, especially in the identification of their surface markers. According to the statement from the ISCT, MSCs are defined by minimal criteria, including adherence to plastic, specific expression pattern of markers  Table 3). Expression of CD36 and a lack of CD106 expression can be used to distinguish AD-MSCs and BM-MSCs.
In addition, the differentiation potential of AD-MSCs into adipocytes, osteocytes and chondrocytes should be qualitatively analyzed by histological staining, and additional quantitative analyses with biochemical methods (Western blot, enzyme-linked immunosorbent assay) or RT-PCR should be considered ( Figure 6). 117 There is some controversy regarding the CD34 marker, whose expression on AD-MSCs is unstable.
MSCs should be negative for these markers, however, the results of various studies indicate their differential expression. 118 In the SVF, a large percentage of cells (up to 85%) were shown to be CD34 + . 119,120 Some authors confirmed that cultured AD-MSCs do not express CD34, [121][122][123] while others reported some fractions of AD-MSCs to be CD34 + . 124,125 Comparison of sorted fractions of early passages of cultured human AD-MSCs showed that the CD34 + cells had greater proliferative potential and colony-forming ability whereas CD34 − cells were characterized by greater differentiation potential into osteocytes and adipocytes. 126 In our studies, in flow cytometric and quantitative polymerase chain reaction (qPCR) analyses, we observed little or no CD34 expression in AD-MSCs cultured in vitro up to the sixth passage. 127 CD34 expression was confirmed to be affected by culture conditions (e.g., plating density and culture medium) and decreased during cell culture. 128,129

| Genetic stability of AD-MSCs and effect of FBS deprivation on AD-MSCs
The genetic stability of AD-MSCs during cell culture should be addressed due to their clinical application. In clinical trials, cells up to the second passage are usually used, but sometimes longer in vitro culture is required. During cell culture, the proliferative and differentiation potential of cells can diminish. Reduced DNA synthesis and DNA repair efficiency may lead to the accumulation of DNA damage, genetic instability, cell senescence, and functional changes and consequently affect the therapeutic efficacy and patient safety. 130 Rubio et al. 131 showed that AD-MSCs can undergo spontaneous transformation in in vitro cell culture. However, the culture was continually passaged for 4-5 months. Our studies showed that in a long-term (up to sixth passage) in vitro cell culture, AD-MSCs from different donors (plastic surgery and oncological patients) maintain their differentiation potential (i.e., ability to differentiate into adipocytes, osteocytes and chondrocytes) and retain their phenotype based on the expression of key surface markers at the transcript (qPCR) and protein level (flow cytometry). These results confirm AD-MSCs stability and safety during long-term culture. 127 AD-MSCs stability was also demonstrated by other research teams. Neri et al. 132 observed an expected slowdown of the proliferation rate during long-term culture but no instances of genetic changes (alterations in chromosome or short repeated sequences), replicative senescence (telomere attrition, expression of significant amounts of active telomerase) or anchorage-independent growth ability, which indicates the therapeutic safety of AD-MSCs. It was also indicated that AD-MSCs do not show signs of senescence up to the seventh passage regardless of culture conditions (oxygen tension and medium supplementation with FBS or human platelet lysate). 133 T A B L E 2 Comparison of AD-MSCs and SVF Another important issue regarding AD-MSCs application is the evaluation of the effect of FBS deprivation on these cells. Traditionally, cells are cultured in medium supplemented with FBS. However, it is recommended to avoid FBS during the culture of cells for preclinical and therapeutic applications. To be considered safe, cells have to be cultured according to GMP standards, so animal-related products need to be eliminated. This is due to the risk of cell product contamination with infectious agents. In addition, FBS contains various growth factors, hormones, nutrients and other components, and its composition varies significantly between batches. FBS use may lead to unspecific activation of cell differentiation and proliferation as well as immune responses. [134][135][136] Accordingly, FBS is removed from cell culture before clinical application and during testing of drug candidates. 137,138 The available results show, that AD-MSCs maintain their stem cell characteristics in serum-deprived medium; they survive, proliferate and are able to differentiate. 139,140 Our research on AD-MSCs after the second passage, which were cultured in the absence of FBS for 48 h, comprised whole transcriptome sequencing followed by gene expression analysis and showed that FBS-deprived AD-MSCs, at the transcription level, show lower metabolic and proliferative activity but retain the expression of key surface markers. Additionally, we did not find evidence of apoptosis and necrosis. These observations suggest that FBS deprivation does not induce changes that could preclude the clinical application AD-MSCs. 127 To overcome the problem of FBS use in cell culture, media containing a combination of recombinant growth factors are used. For example, commercially available chemically modified STK2 medium (DS Pharma Biomedical), which contains FGF2, PDGF, EGF, insulin, lipids, nutrients and minerals, is provided for AD-MSCs in vitro cell expansion. 141 AD-MSCs cultured in STK2 medium showed a higher proliferation rate, increased expression of MSC surface markers and reduced senescence than AD-MSCs grown in DMEM supplemented with FBS. 142 However, serum-free media are mainly applied in laboratory research and not in clinical trials, and the safety of serum-free media in the clinic needs to be evaluated. 143 Other supplements, such as pooled human AB serum, platelet lysates, cord blood serum or thrombin-activated platelet rich plasma (tPRP) were tested as alternatives for FBS in AD-MSCs cell culture. Dessels et al. 136 proved that compared to cells cultured in medium with FBS, AD-MSCs expanded in pooled human platelet lysates (pHPLs) were characterized by higher expression levels of genes involved in the cell cycle, proliferation and division. Additionally, pHPL supplementation did not affect the expression levels of genes responsible for the differentiation of specific developmental processes. Similar findings were also presented in other studies. 144,145 In addition, Kocaoemer et al. 146 reported that AD-MSCs culture in

| SAT versus VAT
Subcutaneous adipose tissue (SAT) and VAT have different characteristics and functional roles in metabolic regulation, for example, VAT has a higher inflammatory response than SAT. 149 Differences between these kinds of fat tissue are probably based on other developmental origins: SAT originates from somatic lateral mesoderm while VAT originates from splanchnic lateral mesoderm. 150 AD-MSCs obtained from visceral (V-AD-MSCs) or subcutaneous fat tissue (S-AD-MSCs) exhibit no difference in reconstructive potential, but collecting VAT is more invasive for patients (Table 4). 103  Both S-AD-MSCs and V-AD-MSCs from obese and Type-2-diabetic patients show higher migration, invasion, and phagocytosis capacity than those from lean subjects. The weight loss in visceral and subcutaneous fat depots is able to at least partially restore their metabolic homeostasis. 158 The above differences should be considered in regenerative therapies.

| BAT versus WAT AD-MSCs
Brown adipose tissue (BAT) which develops early in life is most prominent in human newborns and until recently, it was widely believed that BAT disappears by adulthood. 161 In adults human, it is concentrated in the supraclavicular, neck, axillar, and paravertebral regions and it is inversely correlated with body mass index (BMI). [162][163][164] Brown adipocytes are characterized by the presence of a large number of mitochondria, multilocular lipid droplets, sympathetic innervations, and the expression of uncoupling protein-1 (UCP1) which allows generating heat with little ATP production. 161,165 Brown adipose develops from paraxial mesoderm (dermomyotome) similar to muscles and dorsal dermis. 166 The Ebf2 (early B-cell factor 2) transcription factor is a highly specific marker expressed in both BAT and beige/brite precursors, used for BAT identity and efficient brite/beige cell formation. 164,166 WAT can change to BAT in response to cold exposure or other stimuli (i.e., β-adrenergic) that increase sympathetic tone. 161 Cells from white fat tissue that show Ebf2 expression differentiate into brown-like (or beige) adipocytes, what's more loss of Ebf2 in brown preadipose cells reduce the expression levels of brown preadipose-signature genes. These results indicate that Ebf2 specifically marks and regulates the molecular profile of brown preadipose cells. 166 Human abdominal subcutaneous white-fat preadipocytes have greater brown-adipocyte lineage commitment potential following BMP-7 induction than preadipocytes isolated from visceral white fat. 162 The mesenchymal progenitors that give rise to beige adipocytes express a unique set of cytokines and transcriptional regulators involved in immune cell modulation of adipose tissue browning. An iron accumulation and withstanding with oxidative stress suggest that beige/brite adipocytes are adapted to mitochondrial biogenesis and fatty acid oxidation upon thermogenic stimulation. 167 WAT to BAT conversion is induced in response to β-adrenergic receptor stimulation by β-adrenergic agonist (e.g., norepinephrine, 168 PPAR agonists, 169  Cells exhibit the capacity to undergo osteogenesis, chondrogenesis, and both brown and white adipogenesis. 173

| FACTORS THAT CAN AFFECT AD-MSCs
Obesity, age and related chronic diseases can negatively affect AD-MSCs. The properties of AD-MSCs differ among fat depots and change with age. 175 Cells from younger patients proliferate faster, are more successful in differentiating into mature adipocytes and have more lipolysis activity. 176  higher growth rate and paracrine activity. Additional comparisons showed that in the elderly patients, higher differentiation potential stays substantial only in the arm and subcutaneous thigh. 107,177

| Obesity
AD-MSCs from obese patients show reduced function and differentiation potential compared to those from lean controls. 178 Louwen et al. 179 showed that obesity has an unfavorable impact on AD-MSCs, that is, defective functionalities and properties (differentiation, angiogenesis, motility, multipotent state, metabolism, and immunomodulation).
Additionally, the undifferentiated multipotent state of AD-MSCs is impaired in obese individuals. AD-MSCs from obese individuals have upregulated adipogenic and inflammatory genes, enhanced epithelial-mesenchymal transition, reduced expression of multipotency-associated genes (e.g., OCT4, SAL4, SOX15, and KLF4), and decreased telomerase activity and telomere length (self-renewal capacity). A higher BMI decreases AD-MSCs osteogenesis potential and impairs angiogenic potential. Obesity also alters AD-MSCs secretome profile due to its association with the proinflammatory environment, which can negatively impact on AD-MSCs differentiation potential and regenerative capability. 180,181 Compared to obese AD-MSCs, lean-derived AD-MSCs showed reduced immunosuppressive activities and weaker suppression of lymphocyte proliferation, which protects against obesity-associated inflammation and insulin resistance. 182

AD-MSCs and cancer cells show bidirectional effects. Tumor cells change the AD-MSCs phenotype and function in
a paracrine way. In coculture with lung cancer cells (H358), AD-MSCs differentiate into myofibroblasts. 183 Similar differentiation of AD-MSCs was observed with cells cultured in presence of breast cancer exosomes, breast tumorderived factors and ovarian cancer exosomes. 184 Wang et al. 185 showed that AD-MSCs adipogenesis and adipogenic-specific genes are strongly inhibited by internalization of lung cancer-derived exosomes. AD-MSCs from patients with urological neoplasms show equivalent mesenchymal surface markers, exosome miRNA content, molecular karyotyping and similar growth kinetics to AD-MSCs from healthy subjects, thus confirming the proper use of autologous stem cell transplantation in clinical treatment. 186 In our experiments, we also did not observe differences in the differentiation potential and expression of key MSC markers at the transcriptome and protein levels in in vitro cultured AD-MSCs from plastic surgery and oncological patients. 127 Some in vivo and in vitro preclinical studies indicate AD-MSCs as a factor that increases tumor growth and progression. 187 The interaction between AD-MSCs and the tumor microenvironment has been confirmed in patients with obesity, colorectal tumors, prostate tumors, melanoma and breast cancer, all of which show a higher quantity of circulating AD-MSCs. Tumor stroma and inflammatory cells can release factors (e.g., SD-1 and MCP-1) that stimulate AD-MSCs migration to the cancer microenvironment. 188 AD-MSCs can increase prooncogenic risk but do not differentiate or stimulate angiogenesis in cancer cells. 189 It is suggested that AD-MSCs promote tumor progression and invasiveness, but clinical trials have failed to demonstrate their prooncogenic potential. 163 By contrast, some studies have shown that ASC exosomes exert anticancer immunomodulatory functions and decrease cancer growth, migration and colony formation. 190

| Chemotherapy/radiotherapy
It is important to evaluate the impact of oncological treatment on AD-MSCs in the context of their potential autologous transplantation into the wounds in these patients. In mouse model, it was shown that whole-body irradiation can damage adipose tissue and reduce the cell number and proliferative potential of AD-MSCs. 191 In vitro external radiation reduced the proliferation of AD-MSCs, but the effect was smaller than that for normal human fibroblasts (NHFs). In the coculture of these cells, external radiation did not significantly reduce cell proliferation, which suggests that AD-MSCs may protect NHFs and promote their growth. 192

AD-MSCs cocultured with NHF and microvascular endothelial cells (HDMECs)
showed increased expression of cytokines and adhesion molecules (in NHFs and HDMECs) after radiation, which suggests that AD-MSCs may have a stabilizing effect on irradiated wounds. 193 The injection of AD-MSCs is a promising therapeutic strategy in wound healing, especially after laryngectomy combined with radiotherapy. 194

| THE ROLE OF AD-MSCs IN WOUND HEALING
AD-MSCs play an essential role in wound healing, however, the mechanism of their action is still under investigation. Endogenous AD-MSCs may be activated after wounding. During the proliferative phase mature adipocytes and their precursors, together with fibroblasts, populate the wound site. 198 AD-MSCs can also differentiate into fibroblasts, keratinocytes and endothelial cells and secrete various factors (e.g., cytokines and growth factors) that stimulate the proliferation and migration of these cells. In addition, through the secretion of growth factors (e.g., VEGF, bFGF, EGF, PDGF, hepatocyte growth factor, TGF-α), cytokines (e.g., IL-6, and IL-8) and chemokines in a paracrine manner, AD-MSCs can promote angiogenesis, the immune response, epithelial regeneration, and wound remodeling. 199 They were also reported to exert antioxidant effects in wound healing. 200

| Fat grafting by the Coleman technique
Fat grafting was first described by Coleman as a cosmetic facial filler in the 1980s. In 1994, Coleman first introduced his technique of processing fat for structural fat grafting. This technique is called the Coleman technique ( Figure 6) or structural fat grafting or "lipostructure" and uses syringes, cannulas, centrifuges and centrifugation protocols. Fat can be harvested under local or general anesthesia, depending on patient preference, pain tolerance and the volume of fat needed. [201][202][203] The first step is to prepare fat tissue for harvesting and transplantation. For this purpose, fat tissue needs to be infiltrated through the miniature holes in skin, with a specialized cannula ( Figure 6A) with a solution, known as Klein solution, which consists 0.5% lidocaine, 1:1000 epinephrine, sodium bicarbonate and Ringer's solution. 204,205 The next step is to uptake fat tissue by gentle manual suction of fat with 10 ml Luer-Lock syringes and the specialized Coleman cannula ( Figure 6B). The plunger of the syringe is gently pulled back to create light negative DEPTUŁA ET AL.
| 2147 pressure to harvest the fat. This method produces a very low and constant vacuum that minimizes the destruction of adipocytes ( Figure 6E). 206 The lipoaspirate is processed for removal of the lipid and aqueous portions to isolate the adipose stroma for grafting. There are a few techniques for this isolation process, such as centrifugation, decantation, sedimentation, filtration, and mesh/gauze rolling; the Coleman protocol recommends centrifugation. Freshly harvested fat is centrifuged using appropriate gravitational force (3000g for 3 min) to separate the fat from unnecessary pollutants and nonviable components. Processed fat is transferred to 1 ml syringes and is ready for placement using the specialized cannulas ( Figure 6D). There are several types of cannulas with different diameters, lengths and ends depending on the tissues into which the lipoaspirate is grafted. [207][208][209] Coleman fat grafts have a greater number of viable adipocytes and sustain better cellular function than fat grafts harvested with other methods, and the Colman technique is currently the most common method of autologous fat transfer. 210,211 This procedure can also be used in wound healing ( Figure 6G).

| Clinical application of AD-MSCs in wound healing
The should possess several features, such as nontoxicity, nonimmunogenicity, good biodegradability and biocompatibility, and be easy to handle. Additionally, they should also exhibit good chemical and mechanical surface properties (i.e., high porosity) to support cell resistance; promote the adhesion, proliferation, and differentiation of stem cells and allow retention of metabolic futures. [213][214][215] Clinical trials of fat grafting and SVF application in wound healing are described in Table 5, and clinical trials of AD-MSCs application in wound healing are summarized in Table 6 Attempts to transplant AD-MSCs into radiotherapy-or cancer-damaged tissues were also made. For example, fat transfer followed by split-thickness skin grafting was performed in a 67-year-old woman with a chronic, nonhealing ulcer on her leg resulting from squamous cell carcinoma excision and radiotherapy, this resulted in complete healing of the ulcer. 195 Fat transfer was also used in patients after radiotherapy for breast or head and neck cancers (Table 7).

| Allogeneic versus autologous therapy and the immunological properties of AD-MSCs
The use of allogeneic cells gives rise to the possibility of rapid application of cells to the patient and enables full cell characterization before therapy. Additionally, a small number of donors would provide treatment for many patients; hence, immunogenicity tests and analyses of transplantation safety in allogeneic systems are very important. Allogeneic transplants allow the pooling of cells from many donors and standardization of the cell product. 222 The application of allogeneic, stored cells is already in use. However, there are still some issue to be considered, for example, a lack of standardized parameters for handling frozen cells. Another issue is the lack of uniform guidelines for all AD-MSCs banking processes, such as donor recruitment, manipulation, banking procedures, exemption and specific postthaw qualification tests. It would ensure multiple treatment of the patients, avoiding recurrent liposuction. 223,224 AD-MSCs are suitable candidates for allogeneic cell therapies due to their low immunogenic profile, which was demonstrated by low expression of major histocompatibility complex (MHC) Class II molecules, and T and B cell costimulatory molecules CD80, CD86, and CD40 in vitro. 225 However, in vivo studies have shown that AD-MSCs can induce a humoral and cellular immune response; hence, they are not fully immunologically privileged cells.
AD-MSCs may also be under the influence of inflammation-based signaling and induce the expression of MHC II molecules and Toll-like receptors. In addition, AD-MSCs may also increase first-and second-class MHC expression as a result of differentiation. [226][227][228] In vitro cell culture can also influence immunogenicity; for example, cell culture with bovine serum slightly reduces cell immunogenicity compared to that in culture without serum. 229 Following modulation of the local inflammatory environment, it is believed that the final determination of immunomodulatory responses is likely elicited by the differentiation of cells, a combined action of direct cell-cell contacts and the secretion of soluble factors. 229,230 The immunological properties of AD-MSCs are also dependent on the donor's concomitant diseases. Cells derived from obese Type 2 diabetes (T2D) patients show increased expression of inflammatory markers, activation of the NLRP3 inflammasome, and a greater capacity for migration and phagocytosis than those derived from lean donors. Interestingly, AD-MSCs derived from obese and T2D individuals also show a reduction in typical immunosuppressive activities attributed to MSCs. 231 Moreover, it has recently been proven that Crohn's disease disturbs the immune activities of AD-MSCs, which is related to inflammasome activation. 232

| MSCs INTERACTIONS WITH IMMUNE CELLS
It should be noted that MSCs also take part in regenerative processes by affecting the immune system. MSCs cells produce number of modulating and immunosuppressive cytokines as well as they can affect immune cells through direct interactions. 233,234 MSCs may inhibit the activity of natural killer (NK) cells, lymphocytes Tc and dendritic cells (DCs). 235 MSCs can also induce the transition of M1 macrophages into M2 macrophages, thus promoting tissue regeneration. M2 produce various cytokines and growth factors such as IL-10, TGF-β1, PDGF, IGF-1, and VEGF, which may stimulate cell proliferation, granulation tissue formation, angiogenesis and wound healing. 7,8 One of the immunosuppressive mechanisms of MSCs activity is also the production of indoleamine 2,3-dioxygenase, which is involved in the L-tryptophan catabolism. This process leads to a reduction in the level of tryptophan in the microenvironment at the same time increasing the level of kynurenine which inhibits the activity of T cells, NK cells, and DCs. 236 MSCs also use other immunosuppressive and antiinflammatory agents such as nitric oxide synthase (iNOS), TNF-α-stimulated gene-6 (TSG6) and prostaglandin E2 (PGE2). MSCs have been shown to increase the expression of iNOS in response to proinflammatory cytokines (TNF-α, IL-1) and IFN-γ. 237,238 Another factor produced by MSCs is a glycoprotein TSG-6 (35 kDa). It is believed that TSG-6 counteract inflammatory effects of TNF and IL-1 thus inhibiting inflammation. Interestingly, proper expression of the TSG-6 is crucial to achieve the therapeutic effect of MSC cells. 239 PGE2 is also involved in modulating the activity of the immune system by inhibiting the inflammatory response and stimulating Treg lymphocytes. 233 It was shown that AD-MSCs cultured in endothelial growth medium enhance their proangiogenic properties, which may increase their therapeutic potential in ischemic diseases, for example, ischemic wounds. 242 In another study, AD-MSCs administrated with bFGF via sustained release from a gelatin hydrogel showed increased secretion of angiogenic growth factors and vessel maturation in a murine ischemic hind limb model. 243   VEGF and bFGF expression on messenger RNA and protein level. Additionally, the conditioned medium from AD-MSCs cultured in hypoxic conditions compared to normoxia-cultured AD-MSCs conditioned medium, stimulated collagen synthesis and migration of human dermal fibroblasts as well as accelerated wound closure in a mice full-thickness wound model. 249 Culturing MSCs under hypoxic conditions also helps to improve their self-renewal capacity and retain undifferentiated phenotype. 250 Genetic modifications were also shown to increase AD-MSCs activity in wound healing. Kim et al. 251 proved that overexpression of CXCR4 in AD-MSCs leads to an increase in their homing and engraftment into ischemic areas after transplantation. These cells also promoted long-term engraftment and muscle tissue regeneration in diabetic mice with hindlimb ischemia. Similarly, AD-MSCs with gene transfer of manganese superoxide dismutase (SOD2) that were transplanted into syngeneic mice elicited cytoprotective effects and improved survival and engraftment rates. 252 Additionally, overexpression of Oct4 and Sox2 in AD-MSCs increases their proliferation and differentiation and can be helpful in expanding and enhancing their stemness. 253 Direct application of AD-MSCs-derived exosomes may also have therapeutic potential in cutaneous wound healing. Conducted studies revealed that their application accelerates wound healing in a murine model by optimizing fibroblast functions. AD-MSCs exosomes stimulate the production of collagen I and III in the early stages of wound healing and reduce scar formation by decreasing collagen expression in the late stages. 254 In a continued study, it was found that AD-MSCs-derived exosomes promote scarless wound healing through regulation of ECM remodeling (increases in TGFβ3:TGFβ1 and MMP3:TIMP1 ratios and prevention of myofibroblast differentiation), and the mechanism involves activation of the ERK/MAPK pathway. 255 It was also demonstrated that stimulation with inflammatory cytokines (TNF-α/IFN-γ) can enhance the immunosuppressive and antiinflammatory potential of AD-MSCs-derived exosomes. 256 9 | CONCLUSIONS  Table 1; Aneta Skoniecka: provided photos of cell isolation, described cell isolation and cells and tissue sources, compared SAT and VAT (with Table 4), compared AD-MSCs and SVF (with Table 2) and described factors affecting AD-MSCs and BAT and WAT AD-MSCs; Jacek Zieliński: provided photos of fat harvesting through surgical resection and wound healing complications in oncological patients and critically revised the manuscript; Michał Pikuła: participated in manuscript conceptualization, described immunological properties of AD-MSCs, MSCs interactions with immune cells, supervised work, critically revised the manuscript and provided financial support.
All authors approved final version of the manuscript. The authors declare that there are no conflict of interests.