Stem-cell therapy in dermatology – Challenges and opportunities

Atopic dermatitis (AD)

Current treatment options for AD aim at symptom control and inflammation reduction, but they often lack long-term efficacy and have side effects. SC therapy has emerged as a promising new approach for AD management due to its immunomodulatory and regenerative properties.

SCs, particularly MSCs, exert therapeutic effects in AD through various mechanisms:

• Immunomodulation: MSCs suppress overactive immune responses by inhibiting T-cell activation, decreasing proinflammatory cytokines, and promoting anti-inflammatory molecules.

• Regeneration: MSCs differentiate into keratinocytes, fibroblasts, and endothelial cells, contributing to skin barrier repair and neovascularization.

• Paracrine effects: MSCs secrete a multitude of growth factors and cytokines that promote tissue repair, angiogenesis, and anti-inflammatory responses.

MSCs can dampen both innate and adaptive immune systems, which shows that they have a therapeutic role in immune-related diseases, including AD. Again, MSCs may cause AD by upregulating T helper (Th)1/Th17 and downregulating Th2.

Preclinical studies utilizing MSCs in animal models of AD have demonstrated promising results, including reduced inflammation, improved skin barrier function, and decreased pruritus. Clinical trials investigating MSC therapy (MSCT) for AD are still in their early stages, with limited phase I and II studies showing potential benefits in terms of safety, tolerability, and efficacy. Several delivery routes are being explored for MSC administration in AD, including topical application, intralesional injection, and systemic infusion. Each route has its advantages and disadvantages, requiring further investigation to optimize delivery and improve therapeutic outcomes.

In mice, increased human umbilical blood-derived MSCs (huB-MSCs) reduced AD. The symptoms decrease due to the inhibition of mast cell degranulation and lymphocyte B-cell maturation by increased synthesis of prostaglandin E2 and tumor growth factor-b1 (TGF-b1). In addition, exosomes from adipose tissue-derived stem cells (ADSCs) can decrease proinflammatory cytokine expression, eosinophil count, and serum immunoglobulin E levels in animal models of AD. A recent two-phase clinical experiment using huB-MSCs yielded encouraging outcomes without life-threatening side effects. Since there is not enough data to state whether SC therapy can help improve AD, more studies need to be conducted.^[8,9]

Despite the encouraging preclinical and early clinical data, several limitations and challenges need to be addressed for SC therapy to become a mainstream treatment for AD:

• Heterogeneity of SCs: MSCs isolated from different sources show variable immunomodulatory and regenerative capacity, necessitating standardized isolation and culture protocols.

• Dosage and timing: Determining the optimal dose and timing of SC administration is crucial for maximizing therapeutic benefit and minimizing potential side effects.

• Safety concerns: Long-term safety data on the systemic administration of SCs is still limited, and potential risks such as tumorigenesis and immunogenicity need to be carefully evaluated.

• Cost and scalability: SC therapy currently holds a high cost due to extensive laboratory processing and specialized personnel requirements. Optimizing production and delivery methods is essential for broader accessibility and affordability.

SC therapy holds immense potential for revolutionizing the management of AD. Preclinical and early clinical studies demonstrate promising results in terms of safety, tolerability, and efficacy. However, overcoming limitations related to SC heterogeneity, dosage optimization, safety considerations, and cost-effectiveness is crucial for successful clinical translation. Further, research with larger and well-designed clinical trials is necessary to establish the efficacy and safety of SC therapy as a viable treatment option for AD patients.

Chronic urticarial (CU)

In 2020, Özdemir et al.^[10] conducted the first study of MSCT in humans with CU. This open-label and single-center clinical trial involved ten patients receiving autologous bone marrow-derived MSCs (BM-MSCs) and ten control patients. The MSCs were administered intravenously at a dose of 1 × 10⁶ cells/kg twice, 2 weeks apart, with clinical follow-up for 6 months.

The study included patients with chronic spontaneous urticaria (CSU) for at least 1 year, who had not achieved disease control with omalizumab and cyclosporine for over 6 months, and who had a weekly urticaria activity score (UAS7) >20. Exclusions were those with chronic inducible urticaria, AD, other underlying itchy skin diseases, parasitic infections, or a history of malignancy.

MSCT was well tolerated with no reported adverse effects. Clinical responses were categorized as well-controlled, partially responsive, or unresponsive. On day 14, two patients were well-controlled, and eight partially responded. By the 1^st month, four remained partially responsive, and six became unresponsive. At 6 months, three patients were well-controlled, but two showed no clinical improvement. UAS7 scores of the MSC-treated group significantly decreased compared to the control group at 1, 3, and 6 months.

Flow cytometry analysis revealed that on the 14^th day, frequencies of CD4+ interferon gamma+ and CD4+ Gata3+ cells were significantly higher in the treated group than in the control group. In addition, there were significant differences in TGF-b1 and IDO values between the treated and control groups, suggesting that these immunological effects might result from transient cytokine changes.^[11]

Clinical perspectives on MSCT in CU and AD

The immunologic functions of MSCs imply that MSCT could offer symptomatic, causative, and even preventive treatment benefits. In a clinical trial for AD, allogeneic MSCs were well tolerated without rejection.

Animal studies revealed that systemically administered MSCs through intravenous injection tend to localize in the lungs and show positive effects in CSU, indicating systemic action. Similarly, in AD, subcutaneous administration of MSCs resulted in local and systemic improvements.

MSCT has shown efficacy in both in vitro and in vivo animal studies, leading to successful predictions and outcomes in human trials. Although human diseases and animal models differ, the promising results from animal studies suggest that future human clinical trials are likely to be successful.

MSCs can be administered subcutaneously or intravenously. Both methods have been effective, but subcutaneous injection is considered safer due to the risk of pulmonary embolism associated with intravenous injection. Despite intravenous administration showing higher efficacy in animal models of AD, subcutaneous administration was preferred for subsequent human trials for safety reasons.

In the limited clinical trials conducted for AD and CSU, the effective MSC dose was approximately 5 × 10⁷ cells. In CSU, two injections 2 weeks apart proved successful. Both studies demonstrated that MSCT was effective and safe, with no severe adverse effects.

Notably, in a human study on MSCT for CSU, clinical scores improved and remained stable for 6 months in patients treated with MSCs, in contrast to recurrence in those treated with omalizumab, a new and effective biologic for CSU. These findings indicate that MSCs hold significant potential as a novel therapeutic approach. ^[10,12]

Autoimmune bullous disorders

Skin rejuvenation

SCs are used for the remodeling of skin after promising preclinical findings. Subcutaneously, injecting ASCs into mice with UVB-induced wrinkles improved wrinkles and increased skin thickness and collagen content. Epidermal penetration has been facilitated by the combination of SCs with microneedling. In two randomized, controlled, and blinded split-facial studies, one-half of the face received microneedling, and the other half received SC-conditioned medium (CM). Compared to baseline and placebo, therapy reduced wrinkles, erythema pigmentation, and pore size and improved overall patient satisfaction.^[40] Laser resurfacing has been combined with SC treatment. Following fractional carbon dioxide laser resurfacing, ASC-CM was administered to half of the face, and the other half received a placebo in the form of Dulbecco’s Modified Eagle’s Medium (DMEM) free of fetal bovine serum (FBS). ASC-CM treatment of the experimental half of the face improved patient satisfaction and all objective clinical assessment scores, such as pigmentation, hydration, and skin roughness, compared to baseline and placebo after three sessions. At 21 days post-procedure, histology showed enhanced skin collagen and elastin content on the treatment side compared to placebo. ^[41] Autologous fat transfer procedures, which are performed after liposuction, are convenient and popular due to their advantage of volume loss reversal. Due to SCs in fat grafts, they have positive effects on skin regeneration, such as a reduction in wrinkles, pores, and pigmentation. ^[42]

Fat transfer can be either macrofat (conventional utilizing large multiport cannulas), microfat, or nanofat (emulsified and filtered into liquid suspension). Given the final solution contents and viscosity, macrofat is employed for lipofilling major defects. Because nanofat does not contain live adipocytes, it is mostly employed for skin regeneration, whereas microfat is used for more delicate areas. ^[43]

Studies on SC therapy for skin rejuvenation are shown in Table 6. ^[40,44-46]

Role of SCs in cosmeceuticals

The utility of SCs in cosmetology is unclear. Hydrophilic compounds beyond 500 daltons cannot permeate intact stratum corneum. Topical formulations containing SCs with a larger molecular weight and their growth factors can enter both the epidermis and dermis, notwithstanding this limitation. The bulk of cosmeceutical SCs are also produced from plant sources, which cannot mimic human SCs and are not long-lasting. According to in vitro research, the application of the SCs increased the cellular proliferation of human SCs in a concentration-dependent manner. In a second in vitro experiment, they discovered that when the plant SCs were stressed with ultraviolet radiation, their vitality increased.^[47] Numerous other plants have been investigated, such as edelweiss, ginger, and tomato vines (Lycopersicon esculentum), among others. However, in-depth clinical trials are still lacking. These have demonstrated encouraging outcomes in terms of preventing oxidative stress on the human SCs that are now in existence, reducing inflammation, and scavenging free radicals. A vehicle-controlled, multicenter, and double-blind study was conducted using preparations of a- and b-defensin. A total of 44 female patients were randomized to receive either a placebo or the entire recipe, which included a cream, mask, and serum containing a- and b-defensins. These items were administered twice daily. Epidermal thickness increased significantly at 12 weeks on histology. Appearance of pores and superficial wrinkles decreased with the overall improvement in skin tone and evenness.^[48]

Androgenetic alopecia (AGA)

A commercialized adipose-derived SC-CM (ADSC) containing lyophilized proteins is cultivated from ASCs of healthy adult women. In a study, ASC-CM was injected intradermally into AGA patients’ scalps every 3–5 weeks for 4–6 months for a total of six sessions. Initial investigations showed subjective improvement in terms of major visual analog scales and patient satisfaction. Trichograms taken 1–3 months after the last treatment revealed significant hair growth over the baseline. In a different trial, saline was injected into half of the scalp and ASC-CM into the other. Compared to the untreated half, the trichogram’s hair counts were much greater. In a different experiment, commercial ASC-CM was used for 12 weekly AGA therapy sessions along with a microneedle roller. Hair density and thickness increased significantly at the end of 12 weeks in terms of trichogram imaging results.^[49]

A study evaluated the role of SVF in AGA. A pure fat graft and SVF were prepared from tumescent liposuction lipoaspirate and injected subcutaneously into the scalp. Six patients reached the 24-week objective improvement scores when hair counts increased significantly compared to baseline. However, anagen, telogen hair count, and thickness changes were not statistically significant. Treatment of hair loss has been explored from other SC sources.^[50] Another study used scalp punch biopsies to mechanically disintegrate and centrifuge autologous SCs from hair follicles. Microscopy showed dermal papilla CD44+ SCs and bulge with CD200+ SCs. After that, 11 AGA patients had two intradermal injections of this suspension of SCs, separated by 60 days, which after 23 weeks enhanced both the number and density of hair compared to the baseline and placebo. These results were validated by another clinical research.^[51]

Exosome-based SC therapy for wound healing

Exosome derived from SC (EdSC) is an innovative method of enhancing the wound-healing process. The smallest SC extracellular vesicle, EdSC, has a lipid bilayer structure with a diameter of 30–150 nm. EdSC has been shown to regulate inflammatory cells, activate keratinocytes, fibroblasts, and endothelial cells, and alter collagen I and III ratios during wound healing. Injecting EdSC intravenously is the method of administration used most frequently. For instance, Hu et al. found that injecting EdSC directly into a mouse wound site can enhance the fibroblasts’ activity and hasten wound healing. EdSC can also be administered subcutaneously to promote wound healing. Furthermore, subcutaneously administered melatonin stimulated EdSCs improved diabetic wound healing by controlling the polarization of macrophages.^[54]

Although injection as a process is straightforward and effective, its rapid clearance rate limits EdSC therapeutic activity. In recent years, numerous studies have paired EdSC with hydrogel to extend its efficacy and durability to expedite wound healing. Hydrogels can absorb a lot of EdSC-containing solution and increase wound hydration when applied to the wound bed. It helps loaded cells repair faster by maintaining a favorable wound bed microenvironment.^[55]

Melanoma

Even with oncogene-directed therapy and immunotherapy, melanoma is an aggressive, chemo- and radio-resistant malignancy. Recently, it has been proposed that tumor SCs, which are a tiny population of tumor-initiating cells, promote carcinogenesis. Melanoma SCs have been considered for this notion, originally developed for human myeloid leukemia. Self-renewing tumor SCs can proliferate indefinitely, reject numerous therapies, and cause tumor relapse. Thus, future medicines can target biomarkers, microenvironments, and melanoma progenitor cells.^[56] Melanoma SCs are positive for CD133, CD271, and aldehyde dehydrogenase. In clinical trials, rituximab has regressed chemotherapy-refractory melanoma because its SCs express CD20. A monoclonal antibody targeting ABCB5+ melanoma SCs inhibited tumors in mice. Since mutant B-RAF activates the mitogen-activated protein kinase (MAPK) signaling pathway to drive melanoma tumor proliferation and B-RAF inhibitors can cause MAPK pathway reactivation through a pathway known as an “escape route,” MAPK signaling molecules may be promising targets to overcome melanoma resistance. Inhibiting critical SC factors or administering differentiation factors reduces tumor SC tumorigenicity.^[57,58]

Merkel cell carcinoma

Metastatic treatment for this rare cutaneous tumor is unknown. In recent years, the Merkel cell polyomavirus as the causal agent has been identified and understood. It demonstrates that transitory or resident polyomaviruses capable of neoplastic transformation are present in healthy human skin. Positive for polyomavirus Merkel cell carcinomas are distinct from polyomavirus-negative malignancies in every way, including morphology, gene expression, signaling pathways, microRNA profiles, dysregulated immune surveillance, and post-translational modifications. This affects the prognosis. Recent studies imply that polyomavirus-positive and negative Merkel cell carcinomas originate from dermal and epidermal SCs, respectively. Research on Merkel cell carcinoma SCs may help create targeted therapies. Studies combining polychemotherapy with autologous peripheral blood SC transplantation produced improvement in terms of a 6-month remission.^[59,60]