A comprehensive review of the medical and cosmetic applications of exosomes in dermatology

Exosomes are a subset of extracellular vesicles that are released by all cell types and are theorized to play a crucial role in intercellular communication. Ranging from 40 to 160 nm in diameter, exosomes contain a variety of genetic materials including DNA, RNA, mRNA, metabolites, proteins, and lipids depending on their cellular origin.


| INTRODUC TI ON
Exosomes are one of the primary subtypes of extracellular vesicles important for intercellular communication.Upon release, through a process known as exocytosis, exosomes travel through the extracellular fluid until they reach target cells.Importantly, given that exosomes are themselves comprised of a plasma membrane-derived phospholipid bilayer, their membrane prevents the degradation of their contents. 1This makes it possible to safely transport vesicular contents from one cell to another over a long distance.Upon reaching their target cells, exosomes are taken into a cell via endocytosis, ultimately leading to the discharge of the exosome's contents into the cell's cytosol.As such, exosomes have the ability to effectively alter their target's cellular physiology.[3][4] Figure 1 illustrates these pathways for exosomes to transport and utilize their properties for intracellular communications.
For instance, exosomal excretion is increased in response to stress and/or pathological conditions such as cardiovascular disease 5 and cancer. 4While pathologic exosomes transfer contents that may be harmful to recipient cells, and could ultimately lead to cell death, exosomes can also ameliorate or prevent disease progression, which is why this field of study has become so popular.It has been reported that exosomes secreted from stem cells can confer protection in a variety of diseases.A list of disease states where exosomes are being evaluated is potential areas for the use of exosomes are shown in Figure 2. Mesenchymal stem cell-derived exosomes are cardioprotecive in models of cardiovascular disease. 6More specifically, it was found that intravenous administration of stem cell-derived exosomes reduced infarct size by 50%-60%, preserved cardiac function, and decreased oxidative stress in murine and porcine models of myocardial ischemia/reperfusion injury.It was further theorized that the cardioprotection conferred by these exosomes was due to the paracrine factors (i.e., growth factors, chemokines, cytokines, etc.) they secrete, which ultimately play a pivotal role in cardiac tissue regeneration following injury. 6Additionally, exosomal excretion has been shown to aid or strengthen the immunological response mounted against microbial pathogens.Studies have found that animals injected with macrophage-derived exosomes helped prime or boost a protective immune response in naive and BCG-vaccinated mice exposed to aerosolized M. tuberculosis, respectively. 7It was determined that exosomes play a role in the immune response by presenting antigens to target cells, thus conferring protection against infection. 7Specifically, it was found that exosomes may contain major histocompatibility complex (MHC) class II molecules bound with mycobacterial proteins, capable of activating antigen-specific T cells. 7Given the intrinsic properties and the ability of exosomes to alter the biological response of target cells by regulating critical intracellular pathways, their potential use as novel vehicles of drug delivery for the treatment of an amalgamation of clinical diseases and conditions seems promising. 8

| MED I C AL APPLI C ATI ON OF E XOSO ME S
The mechanisms behind exosome excretion, overall role in intercellular communication in disease and healthy states, and biomedical application remain an active topic of investigation.In addition F I G U R E 1 Mode of actions for exosomes.From mode of actions to therapies.Exosomes are promising players where their polymorph uses (orange) can influence their purpose (green) in clinical settings. 28o the use of exosomes in cardiovascular disease, as was previously mentioned, exosome-based therapies have also been studied in the context of other medical conditions.0][11] Due to their aforementioned role in regulating the tumor microenvironment, exosomes may serve as noninvasive biomarkers for the early detection of cancer. 9Still, though their role in vivo primarily involves promoting tumor proliferation and progression, exosomes can also be utilized in the treatment of cancer, for they can be used for the therapeutic delivery of molecules, proteins, etc. to cancer cells. 4other debilitating disorder for which exosome therapy has proven efficacious is traumatic brain injury. 12Initial studies have shown that transplantation of mesenchymal stem cells into injured brain areas have improved functional outcomes, likely as a result of a reduction in both neuroinflammation and secondary neurodegeneration, and an induction of neurogenesis and angiogenesis. 12,13Furthermore, although exosomes have not been utilized as a treatment for traumatic brain injury in a clinical setting, clinical trials support the role of exosomes as a potential therapy.Namely, transplantation of bone mesenchymal stem cells in traumatic brain injury patients has been shown to decrease inflammation, prevent neurodegeneration, and promote angiogenesis. 12Importantly, these studies suggest that the functional improvement seen after stem cell therapy is mediated by the immunomodulation orchestrated by the molecular factors secreted by mesenchymal stem cells via exosomes. 13,14Given the increased risk of tumor development that is associated with stem cell therapy, these studies highlight the advantage of mesenchymal stem cell-derived exosomes in models of traumatic brain injury.Moreover, though clinical studies investigating the efficacy of mesenchymal stem cell-derived exosomes in attenuating the severity of neurologic injury are lacking (or other medical diseases for that matter), these studies support the therapeutic potential of exosomes in clinically realistic large animal models.
In addition to cancer and traumatic brain injury, the therapeutic potential of exosomes has also been studied in the context of F I G U R E 2 Potential use areas for exosomes.Engineering strategies to refine a specific function and/or tropism to exosomes.Depending on their parent cell line, exosomes can express intrinsic ligands or can be engineered to express specific targeting ligands, stimuli response peptide, fusion protein, immune-evasive components, or viral glycoproteins.Their cargos can vary from small-sized genetic material such as noncoding RNAs to components as large as AAV.Exosome engineering can enable specific targeting of the centrak nervous system, systemic organs, or tumours.AAV, adeno-associated vector; HEK, human embryonic kidney; MSC, mesenchymal stem cells; PEG, polyethylene glycol; TRAIL, TNF-related apoptosis-inducing ligand. 28nflammatory skin conditions like atopic dermatitis, psoriasis, scleroderma, etc. 15,16 In the case of atopic dermatitis, for example, both intravenous and subcutaneous injection of exosomes derived from human adipose tissue-derived mesenchymal stem cells were shown to decrease the expression profiles of inflammatory cytokines, clinical score, the number of eosinophils present in blood, IgE serum levels, and infiltration of mast cells, CD86+, and CD206+ cells into wound sites.16 Though this study was carried out in a murine model of atopic dermatitis, its findings illustrate the therapeutic potential of exosomes in the treatment of dermatological conditions like atopic dermatitis.16

| COS ME TI C APPLI C ATI ON OF E XOSO ME S
Wound healing is a highly variable process whose mechanism is incompletely understood.This process varies depending on the location of the wound, as well as the age, ethnicity, and gender of the patient.Moreover, one of the major challenges during the wound repair process is scar formation. 3,17,18Preclinical studies have shown that exosomes promote wound healing in vitro, as they reduce scar formation, stimulate dermal fibroblast proliferation, migration, and increase gene expression of collagen type 1, KGF, CD34, elastin, and VEGF. 3,17,18As noted earlier, the majority of our knowledge on exosome-mediated improvements in wound healing arise from preclinical studies in cells and animal models.While they certainly illustrate the therapeutic potential of exosomes in wound healing and skin rejuvenation, preclinical studies may not faithfully translate to human patients due to species-dependent differences in the wound healing process or expression profiles of exosomes. 19Still, this has not stopped the advertisement of exosome therapy post radiofrequency microneedling (to reach deeper layers of the skin) as a procedure to promote more rapid skin repair.Similarly, in a split-face study, the efficacy and safety of a combinatorial treatment consisting of the topical application of adipose tissue stem cell-derived exosomes following fractional CO 2 laser therapy for acne scars was assessed in 25 patients. 20Following a 12-week period, it was found that the combination treatment was associated with a significant improvement in scar appearance and a decrease in post-treatment adverse effects such as erythema, dryness, pain, etc., compared to laser therapy alone. 202][23] Just as in the context of wound healing and skin rejuvenation, the majority of literature reports arise from in vitro and in vivo animal models, as well as tissue/cell culture work. 212][23][24] Of the few studies conducted in patients, exosomes from mesenchymal stem cells were found to increase hair density, thickness, and hair follicle proliferation in 20 human patients, thus recapitulating the findings reported by preclinical studies. 22Similarly, other studies found that topical application of adipose-derived stem cell exosomes increased both hair density and thickness in patients with adrogenetic alopecia. 25nally, while the injection of exosomes into the skin is prohibited in the United States, as was mentioned previously, others around the world have found that intradermal biologic injection of a mixture containing exosomes increased hair growth in a majority of patients (i.e., 20/31 patients), as well hair density in 11 patients. 26Of note, one important limitation with the aforementioned clinical studies is that the lack of appropriate controls cannot discount the possibility that the observed increases in hair growth, density, thickness, or hair follicle proliferation were actually due to other confounding variables, and not to exosome vesicles and their contents. 22,25,26Still, these studies do highlight the need for additional studies employing the use (i.e., topical or biologic injection) of exosomes in healthy humans and those who suffer from hair loss, for they are imperative to our continued understanding of exosome therapy and the extent of its application.

| PREC AUTI ON S IN THE UTILIZ ATI ON OF E XOSOME S A S THER APEUTI C AG ENTS
Given the recency of exosome therapy, the majority of studies supporting its efficacy have been conducted in cells and animal models of disease, 6 highlighting a need for more clinical trials to assess its efficacy and safety in humans.Presently, exosome therapy has not yet been approved by the Food and Drug Administration (FDA) due to a lack of standardization in isolation and analysis procedures, despite its current topical use in wound healing, hair loss, etc. 27 This is concerning, for the use of non-FDA approved exosome products not only hinders the regulation of these therapies, but also poses a potential health risk to patients, as the adverse effects associated with exosome therapy have not been fully elucidated.Patients should proceed with caution when seeking exosome therapy for any given medical or cosmetic procedure.If a patient wishes to enroll in a clinical trial involving the use of exosome products, he/she should be made aware of any potential health risks associated with the experimental procedure.Considering that non-FDA approved exosome products have led to serious adverse events, patients should also ask for the FDA-issued Investigational New Drug Application number to verify and review the information provided by the clinical investigator.

AUTH O R CO NTR I B UTI O N
We verified that all authors had access to the data and a role in writing the manuscript.

DATA AVA I L A B I L I T Y S TAT E M E N T
Data sharing not applicable to this article as no datasets were generated or analysed during the current study.