Regenerative medicine strategies for hair growth and regeneration: A narrative review of literature

Copyright © 2022 The Japanese Society for Regenerative Medicine. Production and hosting by Elsevier B.V.

This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Abstract

Hair loss, or alopecia, is associated with several psychosocial and medical comorbidities, and it remains an economic burden to individuals and the society. Alopecia is attributable to varied mechanisms and features a multifactorial predisposition, and the available conventional medical interventions have several limitations. Thus, several therapeutic strategies for alopecia in regenerative medicine are currently being explored, with increasing evidence suggesting that mesenchymal stem cell (MSC) implantation, MSC-derived secretome treatment, and blood-derived platelet-rich plasma therapies are potential treatment options. In this review, we searched the Cochrane Library, MEDLINE (PubMed), EMBASE, and Scopus using various combinations of terms, such as “stem cell,” “alopecia,” “hair loss,” “Androgenetic alopecia,” “male-pattern hair loss,” “female-pattern hair loss,” “regenerative hair growth,” “cell therapy,” “mesenchymal stem cells,” “MSC-derived extracellular vesicles,” “MSC-derived exosomes,” and “platelet-rich plasma” and summarized the most promising regenerative treatments for alopecia. Moreover, further opportunities of improving efficacy and innovative strategies for promoting clinical application were discussed.

Keywords: Alopecia, Hair loss, Hair follicle regeneration, Hair regrowth, Stem cell therapy, Stem cell extracellular vesicles

Abbreviations: AD-MSCs, adipose tissue derived mesenchymal stromal cells; ADRCs, adipose-derived regenerative cells; ADSVCs, adipose-derived stromal vascular cells; AA, alopecia areata; AE, anagen effluvium; AGA, androgenetic alopecia; AA-PRP, autologous activated PRP; BM-MSCs, bone marrow derived mesenchymal stromal cells; CA, cicatricial alopecia; CD, cluster of differentiation; CM, conditioned medium; DHT, dihydrotestosterone; EGF, epidermal growth factor; EMI, epithelial–mesenchymal interaction; EMA, European Medicines Agency; ECM, extracellular matrix; EVs, extracellular vesicles; FPHL, female-pattern hair loss; FGF, fibroblast growth factor; GMP, good manufacturing practice; GVHD, graft-versus-host disease; HF, hair follicle; HFSCs, hair follicle stem cells; hDPCs, human dermal papilla cells; iPSCs, induced pluripotent stem cells; iPSC-MSCs, induced pluripotent stem cells derived mesenchymal stem cells; IGF, insulin-like growth factor; IL, interleukin; KGF, keratinocyte growth factor; LA, lipedematous alopecia; LS, lipedematous scalp; MCP1, monocyte chemoattractant protein 1; MPHL, male-pattern hair loss; MMP, matrix metalloproteinase; MSCs, mesenchymal stromal cells; MAPK/ERK, mitogen-activated protein kinases or extracellular signal-regulated kinases; non-CA, non-cicatricial alopecia; OS, oxidative stress; PMD-Act, pharmaceuticals and Medical Devices Agency-Act; PI3K/Akt, phosphatidylinositol 3-kinase/protein kinase B; PDGF, platelet-derived growth factor; PRP, platelet-rich plasma; PG, prostaglandin; ROS, reactive oxygen species; Treg, regulatory T cells; SALT, severity of alopecia tool; SCs, stem cells; SVF, stromal vascular fraction; SLE, systemic lupus erythematosus; Th, T helper; TE, telogen effluvium; ISSCR, The International Society for Stem Cell Research; TA, traction alopecia; TGF-β, transforming growth factor β; TTM, trichotillomania; TNF, tumor necrosis factor; hUC-MSCs, umbilical cord blood-derived mesenchymal stromal cells; FDA, United States Food and Drug Administration; VEGF, vascular endothelial growth factor; Wnt, wingless/integrated-1

Highlights

1. Background

Hair is found on various parts of the human body, and it performs various critical functions, such as body protection, heat insulation, camouflage, sebaceous secretion, sensory perception, and social interactions [1]. The hair follicle (HF) grows following a cycle of dynamic and complex processes, which mainly alternate in three phases: rapid growth (anagen), regression (catagen), and quiescence (telogen) [2]. A mature human HF is a complex structure comprising multilayered, concentric epithelial basement cylinders of keratinocytes and distinctive mesenchyme of dermal papilla cells (DPCs) as the main components [3]. Human DPCs (hDPCs) originate from B lymphocyte-induced maturation protein-1 + fibroblasts, also known as dermal stem cells (DSCs), during embryonic development. DSCs, which can stimulate epithelial hair follicle stem cells (HFSCs), are widely studied as key controllers of the HF growth cycle throughout an animal's life cycle [4,5]. Multiple factors within and outside HFs can influence growth [6]. Although HFs are protected and maintained through their association with immune responses against pathogens and different tissue regeneration and healing processes, hair abnormalities or loss (alopecia) commonly occurs in both males and females of all ages, affecting quality of life, attractiveness, and self-esteem. Reportedly, alopecia can lead to psychiatric disorders and increased risks of diseases, such as myocardial infarction and metabolic syndrome [5,7,8].

Alopecia is broadly categorized into two subtypes: scarring or cicatricial alopecia (CA) and non-scarring or non-cicatricial alopecia (non-CA) [9]. CA destroys HFs with or without scar formation [10]. Reportedly, CA comprises approximately 5% of all cases of alopecia and describes multiple subtypes of hair loss caused by unknown inflammatory mechanisms. CA is further subdivided according to the inflammatory response as primary neutrophilic, primary lymphocytic, and mixed subtypes [10,11]. Neutrophilic CA includes folliculitis decalvans and dissecting cellulitis of the scalp; lymphocytic CA comprises central centrifugal cicatricial alopecia, discoid lupus erythematous, lichen planopilaris, frontal fibrosing alopecia, Graham–Little syndrome, pseudopelade of Brocq, follicular mucinosis, and keratosis follicularis spinulosa decalvans; and mixed CA includes acne keloidalis, acne necrotica, and erosive pustular dermatosis of the scalp. Non-CA is often a clinical feature of various diseases, either through direct HF destruction or indirectly via HF dysfunction. Disturbances of HF function lead to excessive terminal hair loss, follicular miniaturization, and progressive hair thinning. Non-CA includes several different types summarized in Table 1 , with the exception of the two most common and widely explored, androgenetic alopecia (AGA) and alopecia areata (AA), which are described in detail as follows:

Table 1

Showing a summary of the different additional types on non-CA.

However, evidently aging scalp shows striking structural and biological changes in HF environment that affect hair growth, and

A phenomenon of gradual thinning of hair with age due to increasing number of HF switching from anagen, to telogen

Common induction factors include stress, nutritional deficiencies, bariatric surgeries, hormonal imbalances in pregnancy and menopause, thyroid dysfunction, diabetes, autoimmune diseases, polymyositis, Sjögren's syndrome, febrile or infectious diseases, neoplastic diseases, chronic poisoning, certain drugs and chronic exposure to low-dose toxic agents

Hair loss in SLE can be either diffuse non-CA, such as TE, AGA, AE, and lupus hair, or patchy non-CA, such as AA, the most common type of alopecia observed in SLE.

Results from both severe catabolic and elevated levels of circulating pro-inflammatory cytokines in the hair growth cycle

Both are considered under repetitive behavior disorders and often lead to significant psychosocial impairment

Reward seeking and loss/harm avoidance play important roles in human behavior, and when there is dysfunction in reward processing, maladaptive behaviors, such as TTM and SPD may occur.

Majorly affects individuals who wear various forms of traumatic hairstyling for a prolonged period of time.

The frequent use of tight buns or ponytails, the attachment of weaves or hair extensions, and tight braids are believed to be the highest risk hairstyles.

Without appropriate intervention TA may progress into an irreversible CA if traumatic hairstyling persists.

TA is characterized by marginal alopecia and non-marginal patchy alopecia features and distinguished by its preservation of the frontal and/or temporal rim, dubbed the “fringe sign and detection of the ongoing traction by the presence of hair casts through a dermoscopy.

These rare alopecias that mostly affect middle-aged women with a thick subcutaneous layer of the scalp and soft and boggy scalps.

AGA is the most common cause of hair loss, affecting 30–50% of men (male-pattern hair loss [MPHL]) and approximately 30% of middle-aged women (female-pattern hair loss [FPHL]) [22]. The mechanisms of AGA are multiple, interlinked, and common to both MPHL and FPHL. Among them is the hypothesis of oxidative stress (OS) resulting from increased expression of pro-inflammatory cytokines attributable to chronic perifollicular microinflammation. It is suggested that in the genetically predisposed, OS works in tandem with high androgen levels and various environmental factors to interrupt the corticotropin-releasing hormone pathway and cortisol levels that lead to AGA [23]. Moreover, gene variants in the predisposed are suggested to play a critical role in the etiology of AGA by increasing the activity of 5α-reductase or the sensitivity of androgen receptors [24]. AGA is characterized by HF miniaturization caused by perturbation of the growth cycle via dihydrotestosterone (DHT) accumulation. DHT accumulates because of the inhibition of testosterone metabolism by 5α-reductase [25,26]. AGA is synonymous with the histological observation of lymphocytes and mast cells around the miniaturized HFs and bulge area and reduced numbers of proliferating progenitor cells amidst an intact quantity of HFSCs [27]. Perifollicular inflammatory infiltration and the involvement of inflammatory genes encoding caspase-7 and tumor necrosis factor (TNF) provided proof of the inflammatory hypothesis in AGA [23]. In addition, exposure to high OS levels in the skin leads to the accumulation of reactive oxygen species (ROS), such as superoxide and hydrogen peroxide, in HFs, which overcomes the follicular antioxidant defense capacity and leads to premature and dysfunctional hDPCs [28]. Without intervention and with exacerbation, AGA becomes irreversible.

AA is the second most common form of non-CA, and it affects both sexes equally, with a prevalence of 0.1%–0.2% in the general population and no significant racial preponderance [29]. The etiopathogenesis of AA remains elusive. However, it has been demonstrated that AA is associated with systemic autoimmune activation in isolation as an acquired autoimmune disorder (AD) or as a comorbidity with diseases such as systemic lupus erythematosus (SLE), sometimes referred to as non-CA in SLE [30,31]. SLE has been described as a rare, heterogeneous autoimmune, and autoinflammatory disease with complex etiopathology and clinical manifestations. SLE is characterized by the dysregulation of immune cells, copious amounts of pathogenic autoantibodies, and accumulated immune complexes [31,32]. Immune disorders to SLE include abnormal proliferation, differentiation, activation, and dysfunction of innate and adaptive, immune components such as natural killer (NK) cells, monocytes, macrophages and dendritic cells (DCs), B lymphocytes, and T lymphocytes respectively, which in combination with continuous inflammation culminate in multiple tissue and organ damages. Therefore, patients with AA or non-CA in SLE present significantly dysregulated serum levels of T helper (Th1) cytokines (e.g., interleukin [IL]-1β, IL-2, IL-12, TNF-α, interferon-γ), Th2 cytokines (e.g., IL-4, IL-10, IL-13, IL-25, IL-31), regulatory T cells (Treg), and Th17 cytokines (e.g., IL-17A). Similarly, as observed in AGA, OS caused by interruption of the balance between ROS production and antioxidant activity probably contributes to the pathogenesis of AA [33]. The prognosis of AA is unpredictable, and the disease has variable clinical manifestations, often appearing in patches and sometimes arising in a more extensive distribution pattern [34]. In addition, reports indicate an initial regrowth of white hairs and an association with vitiligo in AA that is proposed to be attributable to immune attack of the hair bulb that mainly comprises melanocytes [30]. Tosti et al. [35,36] described the histopathological distinguishing feature of AA as lesions with peribulbar lymphocytic infiltration comprising cluster of differentiation (CD)8 + T cells in the follicular epithelium and CD4 + T cells around HFs. There are three types of AA based on extent of involvement: patchy AA, alopecia totalis, and alopecia universalis [37]. In addition, based on the pattern of involvement, AA can be of the reticular, ophiasis, or sisaipho type, as well as a new variant described as acute and diffuse total alopecia, which is mainly observed in females. Other unusual patterns are perinevoid alopecia and linear AA [38].

Therapeutic products for alopecia have been extensively studied for many decades because of the continuous high demand by society for effective interventions to reduce the associated rates of mental and physical health disorders and the economic burden [39]. However, only a few pharmacological treatment options have been approved for clinical use by regulatory bodies, such as the United States Food and Drug Administration (FDA) and the European Medicines Agency (EMA). Moreover, these conventional therapies which mainly include corticosteroids, minoxidil, and 5α-reductase inhibitors such as finasteride and dutasteride, face significant challenges [40,41].

The pharmacological treatments are mostly tailored to specific types of alopecia. For instance, in CA, treatment majorly aims to stop inflammation using topical or intralesional corticosteroids, antimalarials, immunosuppressants, and anti-microbials. Several other potential remedies for intractable alopecia are under investigation, albeit with limited efficacy [42], and non-CA treatment mostly utilizes corticosteroids, minoxidil, and the 5α-reductase inhibitors such as finasteride and dutasteride, and depends on the type of alopecia as well. AGA being progressive with a risk of irreversibility, treatment is challenged by multiple factors including early initiation before the occurrence of overt HF miniaturization and management of the disease that must aim at identifying and eliminating exacerbating factors. The treatment of AA is commonly based on professional assessment with the Severity of Alopecia Tool (SALT), and to a great extent remains contestable with the multiple associated recurrences negating long-term outcomes of the conventional approaches [43]. AA treatment aiming to stop the immune episodes is sometimes effective in reversing the disease. However, it is suggested that hair regeneration in AA requires more than immunosuppression to restore normal HF function following the stoppage of the immune response [43].

Corticosteroids are associated with adverse events including hyperglycemia, blood pressure alterations and edema, hypothalamic–pituitary–adrenal axis suppression, osteoporosis, immunosuppression, muscle wasting, and physical appearance changes such as moon facies, buffalo hump, and central trunk obesity [44]. Minoxidil is associated with adverse reactions in approximately 20% of patients, which include facial hypertrichosis observed in approximately 80% of patients during treatment, hypotension, acute pulmonary edema, and pulmonary hypertension. Contact dermatitis occurs probably because of propylene glycol, a common ingredient in topical minoxidil preparations, and other side effects include dizziness, sodium, and fluid retention, reflex tachycardia, headaches and, less commonly, electrocardiogram changes, pericardial effusion, and congestive heart failure [45,46]. Moreover, termination of minoxidil treatment results in progressive hair loss within a short period (12–24 weeks) [47]. Similarly, the 5α-reductase inhibitors are associated with undesirable events including a collection of sexual dysfunction effects such as decreased libido, erectile dysfunction, and reduced ejaculate volume, which are observed in approximately 2.1%–3.8% of patients, as well as mood disturbance and gynecomastia [48]. In addition, the drugs are potentially teratogenic, which is critically an irreversible event. Meanwhile, the drugs are mostly effective in younger adults, such as women younger than 50 years and men younger than 40 years.

Taken together, the conventional treatment approaches for alopecia are contestable, and present significant challenges, which are prohibitive to regular clinical use. Thus, deciphering the pathogenesis and exploring the biology of hair toward identifying novel therapies and/or improving the effectiveness of existing interventions remains paramount. Recently, evidence-based regenerative medicine reports found that the application of various related therapeutic approaches is safe and effective in restoring the normal function of diseased tissues or organs including HFs [27,49]. The approaches include stem cell (SC) therapy and blood-derived cellular therapies such as platelet-rich plasma (PRP) therapy for hair growth and regeneration. This review highlights current information on different regenerative strategies and discusses innovative clinical application plans for improving safety and effectiveness in the future.

2. Methodology

2.1. Literature search

The literature search mainly focused on original English-language articles on regenerative medicine and skin aging treatment approaches. Non-English articles were evaluated for pertinence. Databases including Medline, Embase, and Web of Science were searched using various combinations of the following search terms: “stem cell,” “alopecia,” “hair loss,” “AGA,” “MPHL,” “FPHL,” “regenerative hair growth,” “therapy,” “mesenchymal stem cells,” and “platelet-rich plasma.” A web search was conducted between March 2022 and June 2022 to investigate the clinical trial implementation status of cell therapy for hair growth and regeneration. The following websites were surveyed: ClinicalTrials.gov (https://clinicaltrials.gov/), a list of submitted regenerative medicine provision plans (Ministry of Health, Labour and Welfare, https://saiseiiryo.mhlw.go.jp/published_plan/index/1/2 and https://saiseiiryo.mhlw.go.jp/published_plan/index/1/3), and jRCT (https://jrct.niph.go.jp/search).

3. Regenerative medicine-based treatment strategies

Regenerative medicine aims to restore or establish normal body function by replacing or regenerating cells, tissues, and organs. The replacement or regeneration process may utilize SCs, soluble or trophic molecules such as SC- or hematopoietic tissue-derived cytokines and growth factors, and gene-based therapies, as well as tissue engineering and reprogramming [[49], [50], [51]]. The reported current regenerative medicine-based therapies with promise in the treatment of alopecia include SC and PRP therapies [52,53]. The molecular mechanisms underlying the action of regenerative therapies remain elusive. SCs possess characteristic properties such as self-renewal, migration, anti-inflammatory, and immunomodulation functions, essential for the repair and restoration of injured tissues or organs [49]. The therapeutic effects of SCs were initially believed to be based on their abilities to migrate into damaged tissue (homing) and subsequently differentiate to replace the damaged tissues or organs. However, Gnecchi et al. [54] indicated that the therapeutic impact of SCs on diseased tissue occurs, at least in part, through the release of trophic (paracrine) factors. PRP exerts hematopoietic effects through the release of cytokines and various growth factors, such as platelet-derived growth factor (PDGF), transforming growth factor β (TGF-β), vascular endothelial growth factor (VEGF), epidermal growth factor (EGF), insulin-like growth factor (IGF), and fibroblast growth factor (FGF), and it plays important roles in regenerating or restoration of tissues such as hair [53]. The use of PRP in the treatment of alopecia will be reviewed in detail in this article.

Although still in infancy, several preclinical studies indicated that SC-based therapies for different pathologies are generally safe and potentially promising in the treatment of alopecia [52,53]. Nevertheless, given their recent inception compared to widely studied conventional pharmacological treatments for alopecia, bottlenecks to the clinical application of SC-based therapies likely exist. The geneses of these bottlenecks are multifactorial, and they may include the origin tissue of SCs, donor factors such as age, and challenges to large-scale cell production or cell manipulation. Such factors could lead to low SC transplant cell viability, poor homing and engraftment into injured tissues, SC heterogeneity, challenges related to optimum SC dose determination, and adverse events, such as graft-versus-host disease (GVHD) and increased risks of tumorigenicity [55]. However, there is increasing motivation in research and clinical practice to explore the potential of SCs for enhancing HF growth and regeneration through several robust clinical investigations and advanced innovation and manufacturing technology in industry and academia to continually discover novel and/or improve the existing treatment applications [53]. Moreover, to ensure consistency regarding safety, efficiency, and effectiveness, regenerative therapies are strictly regulated in accordance with set guidelines and standards or principles. The International Society for Stem Cell Research (ISSCR) set global standards for stem cell research and clinical translation, stipulating guidelines for preclinical research, clinical translation, and practice, to emphasize the importance of high standards of cell processing, manufacturing, and good manufacturing practice (GMP) [56]. The ISSCR and other international standards such as the Declaration of Helsinki, which is the cornerstone document on human research ethics, provide public reassurance that the production or handling of SCs adheres to quality standards in addition to the approval processes set by the different national regulatory authorities, such as the Pharmaceuticals and Medical Devices Agency (PMDA) and the Ministry of Health, Labor and Welfare in Japan [57], the US FDA [58], the EMA and its Committee for Advanced Therapies [59], and the Australian Therapeutic Goods Administration [60].

3.1. SC-based therapies for alopecia

Currently, SC therapies majorly utilize adult tissue-specific derived SCs, such as mesenchymal stromal cells (MSCs), SC-derived chemokines or growth factors (secretome), and biomaterials with regenerative properties, such as Lipogems therapy, in addition to the existing SC niche (microenvironment), to provoke an impactful restoration process [51]. It is against this background that this review discusses SC-based therapies for alopecia, which can be autologous or allogeneic, through a classification based on the use of purely isolated SCs with manipulation (culturing) or with minimal manipulation (cell-based therapies) and the utilization of an SC-derived secretome or extracellular matrix (ECM) (conditioned medium [CM]) or extracellular vesicles (EVs) such as exosomes (cell-free therapies).

3.1.1. Cell-based therapies

The adult tissue sources of MSCs with multipotent regenerative potential include adipose tissue (AD-MSCs) [61], bone marrow (BM-MSCs) [62], HFs of non-affected areas such as HFSCs and hDPCs [63], and perinatal sources, such as the placenta and its fetal adnexa (this review mainly provides details of umbilical cord blood-derived MSCs [hUC-MSCs]) [64]. Regarding induced pluripotent stem cells (iPSCs), because of the associated increased risk of tumorigenicity, therapies envisage the use of iPSC-derived SCs with lower risk known as iPSC-derived mesenchymal stem cells (iPSC-MSCs) [65]. MSCs can regenerate HFs and other organs in the skin, such as the sebaceous glands, through several mechanisms including the reversal of pathological mechanisms, regeneration of HFs, and creation of new HFs with organoid technology [66]. Conversely, in AA or non-CA in SLE, MSCs show promise in alleviating the often severe and refractory SLE under appropriate conditions, and thus indirectly treating the alopecia [32]. Management of ADs such as SLE is quite challenging and is beyond the scope of this article. However, of note, SLE flares can essentially be abated by gaining immune homeostasis and improve hair growth [67]. MSCs have strong anti-inflammatory and modulatory effects on innate and adaptive immunity [31,32,68,69]. Indeed, although Traggiai et al. [70] raised a special concern regarding MSCs in the treatment of SLE by demonstrating improved B cell proliferation and differentiation following the culture of MSCs obtained from patients with SLE and healthy donors with B cells in vitro, increasing evidence suggests that MSC therapy could effectively treat SLE. In studies reported by Kamen et al. [71] and Liang et al. [72] MSC treatment was well tolerated and effective against the disease conditions. Wang et al. [73] also reported that MSC transplantation effectively treated SLE by increasing regulatory T cell (Treg) counts and decreasing Th17 counts in TGF-β–and PGE2-dependent manners. Allogeneic MSC transplantation was also well tolerated and resulted in long-term clinical remission in patients with SLE [74]. Furthermore, Liang et al. [72] observed that patients with refractory SLE who underwent MSC transplantation experienced clinical improvement without severe side effects. In another study by Barbado et al. [75], MSC transplantation exhibited low immunogenicity, improved proteinuria levels, reduced medication doses, and improved renal function without clinical signs of acute immune rejection.

Notwithstanding, the molecular mechanisms underlying the MSC therapeutic effects in SLE are not yet completely deciphered [69]. However, potency of the anti-inflammatory and immunomodulatory effects of the MSCs exerted through intercellular contact and paracrine pathways have been described and discussed [31,32,68,69]. Triggers of anti-inflammatory effects of MSCs are multifactorial including the inflammatory environment and its associated proinflammatory factors such as IL-6, TNF-stimulated gene 6, IGF1, human leukocyte antigen G (HLA-G), and PGE2. MSC immunosuppression is through the production of nitric oxide, Indoleamine-pyrrole 2,3-dioxygenase (IDO), PGE2, TGF-β, IL-6, and HLA-G protein [32]. MSC modulatory effects are dose and cell type dependent; i.e., high numbers of allogeneic MSCs are suppressive, whereas lower numbers could up-regulate the immune cell responses, and MSCs obtained from SLE patients (autologous MSCs) are reportedly dysfunctional [69,76]. Tang et al. [32] highlighted different published findings on the interactions between MSCs and immune cells. (i) MSCs secreted factors inhibit NK cell cytotoxicity and secretion of inflammatory cytokines, such as IFN-γ or TNF-α. IDO and PGE2 reportedly mediate the downregulation of activating NK receptors: natural cytotoxicity triggering receptor 3 (CD337 or NKp30), natural cytotoxicity triggering receptor 2 (CD 336 or NKp44), and NKG2D by MSCs in inhibiting NK cell proliferation and cytotoxicity. Additionally, TGF-β and IL-6 can reduce NK cell effector function and encourage NK cell differentiation. (ii) MSCs induce differentiation of pro-inflammatory M1 macrophages to anti-inflammatory M2 macrophages, which express macrophage mannose receptor, CD206, and haptoglobin–hemoglobin scavenger receptor, CD163, and secrete high levels of IL-10 and chemokine (C–C motif) ligand 18 that induce strong immunosuppressive effects. (iii) MSC nourish the endurance of monocytes by inducing morphological and physiological transformation following phagocytosis of infused MSCs to mediate a subsequent immunomodulatory effect. (iv) MSCs modulate DC activation and tolerogenic characteristics and program neutrophils to develop an immunosuppressive phenotype. A study by Yuan et al. [69] demonstrated the possibility of allogeneic MSCs in suppressing inflammation in SLE by up-regulating tolerogenic CD1c + DCs and the levels of serum cytokine Fms-related tyrosine kinase 3-ligand (FLT3L), a key regulator of DC commitment in hematopoiesis. FLT3L expression in MSCs is induced by IFN-γ through JAK/STAT signaling pathway. (v) B cells exhibit hyperactivity in SLE and increased expression of especially toll-like receptors 7/9, which contribute to the inflammatory state [77]. MSCs inhibit the differentiation of B cells into plasma cells, reduce the production of immunoglobulin, and downregulate the expression of chemokine receptors, such as CXCR4, CXCR5, and CCR7 on the surface of B cells, which determines the response to the chemotactic ligands, stromal cell-derived factor 1, B lymphocyte chemoattractant and chemokine (C–C motif) ligand 19/21 respectively. In addition, although reduced monocyte chemoattractant protein 1 (MCP1) expression in SLE impairs B cell inhibition, the presence of T cells and cell–cell contacts between MSCs and T cells enhances B cell inhibition. It is known as well that MSCs stimulate IL-10-producing B regulatory cells under latent immunological conditions. (vi) MSCs inhibition of T cell proliferation and production is dependent on MCP1 and other MSC secreted factors, such as TGF-β1/2/3, and IL-10 via inhibiting cleavage of caspases [78]. MSCs decrease the ratio of Th1 to Th2 cells and increase the number of Treg cells to regulate the immune environment in SLE. The regulation of Th17 cells is complicated, however, MSCs down-regulate Th17 cells through the regulation of TGF-β and PGE2 in SLE patients [79].

3.1.1.1. Treatment of alopecia using AD-MSCs

The use of MSCs obtained from adipose tissue in treating alopecia is a two-component process: freshly derived primary multipotent MSCs that are part of the stromal vascular fraction (SVF), referred to as adipose-derived stromal vascular cells (ADSVCs) or adipose-derived regenerative cells (ADRCs), and the isolated and cultured pure MSCs, termed AD-MSCs [50]. SVF is obtained by removing fat cells from subcutaneous adipose tissue and is composed of peripheral blood-derived cell groups such as macrophages and neutrophils and cell groups such as vascular endothelial cells and ADSVCs or ADRCs. SVF plays an important role in the repair and regeneration of chronically damaged tissue, and it has the potential to promote hair growth by enhancing the capacity of DP to grow or regenerate hair as indicated in the NCT02729415 and NCT02865421 clinical trials. Recent reports indicated that ADRCs were safe and effective for treating AA [50]. In the report, 19 of 20 patients exhibited an increased hair diameter and significantly increased hair density within 3–6 months of treatment. In addition, the impact of ADRCs on hair growth was investigated by combining it with adipose tissue in the transplantation procedure (similar to the cell-assisted lipotransfer reviewed in Ref. [50]) in a study of six patients with MPHL or FPHL [80]. The study results revealed a statistically significant 23% increase in the mean hair count, versus a 7.5% increase in patients treated only with non-assisted adipose tissue.

A review by Owczarczyk-Saczonek et al. [66] re-emphasized several previously reported important details on the use of AD-MSCs in alopecia treatment. First, the interfollicular dermal macroenvironment composed of HFs surrounded by subcutaneous adipocytes and skin is important for maintaining normal cell growth in the bulge and HFs. Second, AD-MSCs secrete growth factors that play critical roles in the activation of epidermal stem cells and hDPCs, summarized in Table 2 . Third, through direct interactions between the cells and secretion of PGE2, leukemia inhibitory factor, and kynurenine, AD-MSCs exert immunomodulatory and immunosuppressive effects on epidermal SCs and hDPCs. Fourth, it is suggested that adipose tissue is essential to the extension of anagen because of the ability of progenitor cells, which were adipocytes in the transition from telogen to anagen, to proliferate and (ii) the significant increase in the thickness of the subcutaneous adipocyte layer during the anagen phase versus that in the telogen phase. Finally, AD-MSCs stimulate HF cells via peroxisome proliferator-activated receptors. Conversely, mature adipocytes negatively affect the proliferation of HFs and the surrounding fibroblasts in cocultures. Several studies evaluated the effect of AD-MSCs on alopecia. Zanzottera et al. [61] used the Rigenera® device to prepare autologous AD-MSCs and applied them to the scalps of three patients with AGA. The patients were followed up monthly, and they exhibited more accelerated healing of the AD-MSC transplant-induced wounds and improvements of hair growth with a shorter telogen phase after 2 months of treatment. In Japan, the use of AD-MSCs on the scalps of patients with alopecia is mostly performed by beauty or cosmetic practitioners as a class II risk approved regenerative product in accordance with the PMD Act [57].

Table 2

Summarizes the activities of different AD-MSCs and the MSC-derived paracrine factors as reviewed in [51,65].