Asthma and stem cell therapy

Abstract

The global incidence of asthma, a leading respiratory disorder affecting more than 235 million people, has dramatically increased in recent years. Characterized by chronic airway inflammation and an imbalanced response to airborne irritants, this chronic condition is associated with elevated levels of inflammatory factors and symptoms such as dyspnea, cough, wheezing, and chest tightness. Conventional asthma therapies, such as corticosteroids, long-acting β-agonists, and anti-inflammatory agents, often evoke diverse adverse reactions and fail to reduce symptoms and hospitalization rates over the long term effectively. These limitations have prompted researchers to explore innovative therapeutic strategies, including stem cell-related interventions, offering hope to those afflicted with this incurable disease. In this review, we describe the characteristics of stem cells and critically assess the potential and challenges of stem cell-based therapies to improve disease management and treatment outcomes for asthma and other diseases.

Key Words: Asthma; Stem cell; Therapy; Embryonic stem cells; Induced pluripotent stem cells; Mesenchymal stem cells; Adult stem cells

Core Tip: In this review, we provide an overview of the characteristics of stem cells, including embryonic stem cells, induced pluripotent stem cells, mesenchymal stem cells and adult stem cells, along with a summary of stem cell therapies for asthma and associated challenges. This review aims to guide future research endeavors on developing innovative stem cell therapies for asthma and other disorders.

INTRODUCTION

Asthma, a chronic inflammatory airway disease affecting both children and adults, has seen a notable increase in incidence in recent years. The World Health Organization 2020 report indicates that approximately 235 million individuals suffer from asthma. Despite accounting for less than 1% of the total mortality rate from all causes, asthma significantly impacts human health and imposes a considerable economic burden. The cost of treating severe asthma cases can be 10 times higher than that of conventional therapy, consuming over 50% of global medical resources allotted for asthma[1].

Patients with asthma present with variable clinical and pathological manifestations including restricted airflow, lung tissue remodeling, and typical symptoms such as coughing, wheezing, and chest tightness. Infants frequently present with wheezing, and a significant proportion progress to chronic asthma by 6 years of age. However, three-quarters of school-aged children with asthma outgrow the disease by adulthood[2], while adults with asthma often experience incomplete remission[3,4]. Asthma is driven by an exaggerated T helper type 2 (Th2) immune response characterized by excess numbers of CD4+ T cells that produce interleukin 4 (IL-4) and IL-5. This response leads to the production of allergen-specific immunoglobulin E (IgE) and eosinophil accumulation that trigger chronic airway inflammation, culminating in airway remodeling marked by basement membrane thickening, goblet cell hyperplasia, smooth muscle cell proliferation, inflammatory cell infiltration, and mucus plug formation[5,6].

Traditional asthma therapies, such as corticosteroids, long-acting β-agonists, and anti-inflammatory agents, are often associated with a broad spectrum of adverse effects such as immunosuppression induced by long-term glucocorticoids use that can increase susceptibility to infection, as well as adrenocortical insufficiency, bone damage, electrolyte disorders, high blood pressure, and hyperglycemia[7-9]. Another drawback of conventional treatments lies in their inability to effectively reverse the asthma pathogenic process, thereby contributing to the high prevalence of severe and refractory cases that underscores the urgent need for innovative prevention and treatment strategies.

Stem cells (SCs), which have self-renewal and differentiation capabilities, were first identified in the hematopoietic system in the mid-20^th century. A landmark 1963 study conducted by Becker et al[10] showed that simple infusion of bone marrow-derived cells into the blood of lethally irradiated animals can reconstitute all blood cell populations, rescuing the animals from death, while recent studies have highlighted their potential as therapies for lung diseases such as asthma[11]. Notably, SCs exhibit a spectrum of potencies, ranging from totipotent to pluripotent, multipotent, and unipotent, each with a progressively narrower range of differentiation potential. Totipotent SCs are capable of differentiating into all cell types found within the embryonic and extraembryonic tissues (e.g., placenta) of a developing organism; pluripotent cells can only differentiate into cell types found within the embryo proper, including germ cells and cells within any germ layer that can differentiate into any cell type found within a mature organism; multipotent cells are capable of differentiating into any cell type found within a specific germ layer; and unipotent cells can only differentiate into a limited number of cell types. Importantly, SCs can be derived using diverse methods from human embryos, adult somatic cells, or by enhancing the SC potential of differentiated somatic cells (Figure 1).

Figure 1 Development and progression and stem cell potency. The figure was drawn by Figdraw. During the earliest embryonic developmental stages, totipotent stem cells, capable of giving rise to any cell type, are present in the zygote during the initial cell divisions following fertilization. With the formation of the blastocyst, stem cells evolve into a pluripotent state, enabling them to differentiate into nearly any cell type, marking a critical step in embryonic development. As development progresses and the primitive streak forms, cells predominantly exhibit multipotent capabilities, restricting their differentiation potential to a narrow array of cell lineages excluding embryonic germ cells. In subsequent developmental phases, stem cells are classified as either ‘embryonic’ or ‘adult,’ reflecting their specific differentiation potentials and roles within an organism’s cellular hierarchy.

An overview of the characteristics of SCs is presented in this review, including embryonic SCs (ESCs), induced pluripotent SCs (iPSCs), mesenchymal SCs (MSCs), and adult SCs, along with a summary of SC therapies for asthma and associated challenges. This review aims to guide future research endeavors towards developing innovative SC therapies for asthma and other disorders.

ESCs

ESCs, typically harvested from preimplantation blastocysts around 7 to 10 days post-fertilization, offer unparalleled regenerative capabilities. Their capacity for indefinite propagation and differentiation into essentially any specialized cell type within the body sets them apart as a powerful tool for disease treatment and tissue repair[12].

Lin et al[13] described a murine asthma model employed to uncover the association between the therapeutic benefits of human ESC (hESC)-derived MSCs (hESC-MSCs) and the expression of crucial mRNAs in lung tissue. These mRNAs are transcribed from genes encoding chemokine C-C motif ligand 11 (CCL11), CCL24, IL-13, IL-33, and the eosinophil-associated, ribonuclease A family, member 11, as assessed using a polymerase chain reaction array[13]. Ultimately, MSCs were successfully obtained from hESCs through a simple two-step protocol that eliminated the need for fluorescence-activated cell sorting, a process that could potentially damage the cells. Following intravenous injection into the tail veins of allergic mice, the transplanted hESC-MSCs suppressed allergic inflammation in the lung tissues of animals by reducing the expression of CCL11, CCL24, IL13, IL33 (expressed in Th2 cells) and the eosinophil-associated, ribonuclease A family, member 11 gene (expressed in eosinophils). Additionally, the transplanted cells restored previously diminished regulatory T cells (Tregs) levels in lung tissues to normal levels. These results suggest that allergic reactions induce elevated expression of these mRNAs, which MSC-mediated immunomodulation inhibits. It is noteworthy that hESC-MSCs share biological characteristics with bone marrow-derived MSCs (BMSCs), including expression of surface markers related to multilineage differentiation and immunomodulatory states, robust proliferation and regenerative capacity and low levels of heterogeneity.

Nevertheless, challenges persist, particularly in ensuring hESC-derived cells’ purity, safety, and efficacy before they are widely applied as asthma treatments in clinical settings. For example, MSC therapy has sometimes resulted in poor clinical outcomes due to inherent differences in characteristics among MSCs obtained from different donors[14]. It is, therefore, crucial to evaluate the efficiency of hESC-MSC differentiation by testing hESC-MSC cultures generated using the abovementioned protocol for potential contamination with other cell types, which could trigger unexpected side effects when transplanted. However, it is worth noting that in another study the vast majority of hESC-MSCs strongly expressed classical MSC markers cluster of differentiation 73 (CD73) (> 96.3%), CD90 (> 75.4%), and CD105 (> 99.7%) and were positive for other known MSC markers CD166 (> 45.0%), CD44 (> 98.2%), and CD146 (> 89.0%), while testing negative for the hematopoietic SC marker CD45 (< 8.0%)[13].

Although significant strides have been made in the field of ESC-related regenerative medicine, even highly pure populations of transplantable hESC-derived tissue-specific cells may be unsuitable for tissue regeneration due to low-level expression of human leukocyte antigen I that can trigger immunorejection despite their otherwise low immunogenicity. Accordingly, our laboratory developed a reliable culture and genetic selection procedure yielding the first pure population of transplantable hESCs derived from lung alveolar type II epithelial cells (hESC-ATIICs). After these cells were transplanted into the lungs of severe-combined immunodeficiency mice with bleomycin-induced acute lung injury[15], in vivo differentiation of hESC-ATIICs into ATICs was observed associated with repair of damaged lung tissue and long-term restoration of pulmonary function without teratoma formation. At study completion (10 days post-injury), the engrafted cells expressed ATIC phenotypic markers, strongly indicating ongoing or complete differentiation of transplanted ATIICs into ATICs, a noteworthy step towards overcoming the challenge of immunorejection within the context of ESC-derived cell transplantation-induced tissue repair. These results highlight an approach for overcoming immune rejection, paving the way for expanded clinical applications of hESCs and positioning them as a potential “universal donor” SC line with enhanced therapeutic potential.

IPSCs

In 2006, Takahashi and Yamanaka[16] introduced four transcription factors (TFs), organic cation transporter 3/4, sex determining region (SRY) box 2, c-Myc, and Kruppel-like factor 4, into mature cells lacking their expression. Intriguingly, a subset of these modified mature cells reverted to a significantly less-developed ESC-like state, highlighting the successful artificial expansion of cell pluripotency as a transformative achievement ushering in a new era in SC biology. Subsequently, iPSCs collected from an individual could differentiate into any other cell type found within that individual’s body, underscoring their potential value as a tool for evaluating the efficacy and safety of ‘personalized’ drug therapies. However, after extended culture, these cells exhibit noticeable changes in mRNA copy number during reprogramming and epigenetic memory following differentiation and increased in vivo tumorigenicity.

Interestingly, treatment of asthmatic mice with iPSC-MSCs or BMSCs prior to the antigenic challenge has been shown to reduce levels of Th2-induced immunoglobulins (e.g., IgE) and cytokines (e.g., IL-4, IL-5, IL-13) in bronchoalveolar and/or nasal lavage fluid[17,18]. Royce et al[19] demonstrated the superior protective effect of intranasally administered iPSCs and mesenchymoangioblast-derived MSCs against ovalbumin (OVA)-induced chronic allergic airway disease and asthma compared to corticosteroids.

In a study by Gao et al[20], two distinct sets of human iPSCs were employed to generate MSCs with robust proliferative capacity; heightened expression of recognized adult BMSC markers; and enhanced abilities to engage in adipogenesis, osteogenesis, and chondrogenesis. Specifically, urine cell-derived iPSCs were generated from cells isolated from human urine through reprogramming induced by electroporation of the plasmid pEP4EO2SET2K into the cells. Meanwhile, amniocyte-derived iPSCs were produced through retrovirus-mediated transduction of genes encoding organic cation transporter 4, SRY box 2, Kruppel-like factor 4, and c-Myc TFs into cells isolated from amniotic fluid. Notably, both types of iPSC-MSCs exhibited superior proliferative ability, longer life spans (over 50 passages), and lower rates of cell senescence than MSCs, highlighting their promise as a source of easily generated and well-tolerated MSCs for use in clinical applications. Additionally, these iPSC-MSCs inhibited dendritic cell differentiation, an effect attributed to both cell-cell interactions and iPSC-MSC secretion of IL-10. Unfortunately, safety assessments, including testing of iPSC-MSCs in immunodeficient mice, were lacking in these studies[20]. However, regarding the safety issue, previous studies demonstrated that two other iPSC-MSC clones suppressing allergic airway inflammation[17] were devoid of carcinogenic drifts during four months following their subcutaneous transplantation into severe-combined immunodeficiency mice[21]. In practice, incomplete and random reprogramming of iPSCs by TFs has been observed, associated with abnormal gene expression profiles and necessitating the rigorous screening of iPSCs using various techniques, such as whole genome sequencing, comparative genomic hybridization, single nucleotide polymorphism analysis, before deeming these iPSCs suitable for clinical applications. Within this context, our laboratory has successfully established a novel site-specific insertion-driven targeting strategy for efficiently generating mutation-free, reprogramming factor-free human iPSCs[22]. A refinement of the iPSC methodology is currently in progress, and it is poised to substantially increase the future utilization of iPSCs in therapeutic applications.

MSCs

Over the past half-century, understanding the basic and clinical aspects of asthma MSC-mediated mechanisms has advanced significantly, resulting in the emergence of MSCs as the most extensively studied cell type in experimental cell therapy (Figure 2). Beyond their potential cell replacement applications, certain MSC cell types may be capable of altering the course of a disease without undergoing engraftment. This realization prompted researchers to explore the potential of MSCs to modulate cellular responses to injury or aberrant immune cell activity. In turn, these efforts led to the early identification of a population of BMSCs capable of generating various MSC-derived populations ex vivo, giving rise to the concept of MSCs as a customizable tool for regenerating specific types of tissues[23,24]. Currently, MSC-derived colonies arising in culture are recognized for their ability to generate cells that could be induced to differentiate into osteoblasts, adipocytes, or chondrocytes in vitro. MSCs may be administered in vivo via intravenous, intranasal, or intratracheal routes. However, an ongoing debate persists regarding the criteria for identifying these cells and their specific functions after in vivo administration.

Figure 2 Experimental and clinical investigations into mesenchymal stem cell-mediated mechanisms of asthma causation. The figure was drawn by Figdraw. Mesenchymal stem cells (MSCs) reduce the production of various inflammatory factors, promote the production and release of anti-inflammatory factors, suppress the inflammatory response, restore the T helper type 1/2 balance, and repair damaged epithelial cells, while modulating the immune response through direct cell-to-cell contact. The paracrine action of MSCs is attributed to the release of exosomes containing biologically active substances that effectively mimic MSC immunomodulatory effects. After the MSC-mediated asthma mechanism was elucidated, the first clinical report describing the beneficial effects of bone marrow-derived mononuclear cell therapy in a patient with severe asthma was published in 2020. Currently, four clinical trials grounded in animal experiments are in progress, and they are evaluating the therapeutic effects of systemically administered MSCs registered on the clinicaltrials.gov platform.

MSCs effectively modulate immune responses by suppressing activated T and B cells, inhibiting M1 macrophage differentiation and reducing antigen-presenting cell costimulation[25]. They regulate the Th1/Th2 balance, suppress pathological T cell proliferation, and offer anti-inflammatory[26], antifibrotic[27], anti-apoptotic[28], antimicrobial[29], antioxidative[30], and pro-angiogenic benefits[31]. Additionally, MSCs enhance alveolar fluid clearance[32] and repair pulmonary endothelial and epithelial cell damage[33].

MSCs, specialized cells with critical roles in regulating immune system functions and managing immune responses triggering inflammatory diseases, do not home efficiently to target tissues when infused intravenously[34], resulting in limited MSC colonization and differentiation within target tissues[35]. Despite this limitation, MSCs exert immunoregulatory effects through cell-cell contact involving two key intercellular interaction molecules: Programmed death ligand 1, a costimulatory molecule, and tumor necrosis factor (TNF) ligand superfamily member 6 (TNFSF6)[36]. In one study, during T cell recruitment, BMSCs were found to regulate monocyte chemoattractant protein-1 secretion via a Fas-dependent mechanism, leading to T cell apoptosis through a TNFSF6-based mechanism. Subsequently, macrophages were observed to ingest apoptotic T cell debris and release elevated quantities of transforming growth factor beta (TGF-β), triggering enhanced Treg activity and immunotolerance.

MSCs have been evaluated as potential treatments for asthma. In a study conducted by Shin et al[37], the therapeutic effects of human umbilical cord-MSCs were evaluated in two murine models of severe asthma, alternaria (alternata-induced) and house dust mite/diesel exhaust particle-induced asthma. Their results revealed significant post-treatment reductions in airway hyperresponsiveness, lung eosinophil levels, and direct inhibition of Th2 cell and type 2 innate lymphoid cell activities. However, Volarevic et al[38] revealed that MSC populations exhibited notable diversity in secreted immunoregulatory factor profiles, encompassing TGF-β, hepatocyte growth factor, nitric oxide, indoleamine 2,3-dioxygenase, IL-10, IL-6, leukemia inhibitory factor, IL-1 receptor antagonist, galectins, TNF-stimulated gene 6 protein, human leukocyte antigen-G5, heme oxygenase-1, and prostaglandin E2[38].

Previously, researchers had speculated that the multidirectional differentiation potential of MSCs might lead to their differentiation into fibroblasts and myofibroblasts during asthma progression, triggering the pathological process of airway remodeling[39]. However, recent animal studies demonstrated that MSCs can improve airway remodeling in asthmatic mice[40,41], although the specific mechanism underlying this effect remains unclear. Meanwhile, the paracrine action of MSCs has been linked to the exosome release of biologically active substances that mirrors the observed MSC-induced immunomodulatory effect. MSC exosomes (MSC-Exo) maintained in a conditioned serum-free medium and isolated via serial centrifugations were non-immunogenic, well-tolerated by the human body, and equipped with membrane penetration and intrinsic homing capabilities.

MSC-Exo modulate Treg activity by upregulating levels of immunosuppressive cytokines IL-10 and TGF-β1 produced by peripheral blood mononuclear cells of asthmatic patients, a process potentially influenced by antigen-presenting cells[42]. Furthermore, MSC-Exo reversed airway hyperresponsiveness, histopathological changes, and inflammation when administered intratracheally to a severe, steroid-resistant asthma mouse model by reshaping the macrophage polarization profile[43]. MSC-Exo may be an effective therapeutic strategy for Th17-dominant neutrophilic airway inflammation by inhibiting Th17 polarization through the Janus kinase 2/signal transducer and activator of transcription 3 pathway[44]. Kun et al[45] found that inhibiting the Notch1/Jagged1 pathway could facilitate the migration of allogeneic BMSCs to injured lung tissues, which promotes immune regulation, corrects the Th1/Th2 imbalance, and enhances the treatment of asthmatic airway inflammation. These effects suggest that MSCs can sense their environment and restore the T cell balance in individuals with disorders primarily associated with aberrant Th1 or Th2 responses[46]. Here, we highlight strategies of MSC-derived extracellular vesicle treatment in asthma models (Figure 3) and summarize findings from the past 3 years[44,47-57], showing that MSC-Exo hold promise as a cell-free therapeutic approach (Table 1). Exosomes are administered via intravenous, inhalation, intratracheal, and intranasal. Combined with chemotherapy, small inhibitors, nucleic acids, or immunotherapy could enhance the outcomes in treatment.

Figure 3 Mesenchymal stem cell-exosomes treatment of asthma models purified exogenous and autologous exosomes derived from mesenchymal stem cells express various functions in treating asthma in animal models. The figure was drawn by Figdraw. Intravenous, inhalation, intratracheal, and intranasal administration for delivering exosomes, as well as the cargo carried by the exosomes, could have important effects on treatment.

Table 1 Details regarding mesenchymal stem cell-derived extracellular vesicles treatment of asthma models in the past 3 years (2022-2024).

Ref.	MSC-dosage and frequency	MSC-EV source	Asthma animal model	Asthma replication model	Animal numbers	Delivery	Cargo	EV markers
Firouzabadi et al[47], 2024	15 μg, 1 time	HBMSC	Male BALB/c mice	Sensitized and challenged with OVA	Total = 43, C = 11, A = 11, T = 21	IV + IT	N/A	CD105, CD63
Shan et al[48], 2022	20 μg, 9 times	HBMSC	BALB/c mice	Sensitized and challenged with OVA	Total = 30, C = 10, A = 10, T = 10	IV	MiR-188, miR-124, miR-410, miR-223, miR-130a	CD81, TSG101
Liu et al[49], 2022	100 μg, 3 times	MBMSC	BALB/c mice	Sensitized and challenged with OVA	Total = 24, C = 8, A = 8, T = 8	IT	N/A	CD9, CD63, CD81, TSG101
Feng et al[50], 2022	NP	NP	Male BALB/c mice	Sensitized and challenged with OVA	Total = 15, C = 5, A = 5, T = 5	NP	Some transfected with miR-301a-3	CD63, CD9
Li et al[51], 2023	NP	MBMSC	Male SD rats	Sensitized and challenged with OVA	Total = 32, C = 8, A = 8, T = 16	IV	MiR-223-3p	CD9, CD63, CD81
Xu et al[52], 2023	40 μg, 4 times	Hypoxic, HUCMSC	Female BALB/c mice	Sensitized and challenged with OVA	Total = 21, C = 4, A = 5, IV T = 6, INH T = 6	INH and IV	Some vesicles were transfected with miR-146a-5p	TSG101, HSP70
Bandeira et al[53], 2023	2 × 109 particles, 1 time	HBMSC	C57BL/6 male mice	Sensitized and challenged with OVA	Total = 15, C = 5, A = 5, T = 5	IN	N/A	Flotillin-1, CD81, and β-actin
Dehnavi et al[54], 2023	NP	MAMSC	Female BALB/c mice	Sensitized and challenged with OVA	Total = 20, C = 5, A = 5, OVA-EV T = 5, normal EV T = 5	SL	OVA	CD9, CD63
Asadirad et al[55], 2023	NP, 6 times	MAMSC	Female BALB/c mice	Sensitized and challenged with OVA	Total = 20, C = 5, A = 5, T = 10	SL	OVA	CD9, CD63
Luo et al[56], 2024	40 μg, 4 times	Hypoxic, HUCMSC	Female BALB/c mice	Sensitized and challenged with OVA	Total = 16, C = 4, A = 6 T = 6	INH	N/A	TSG101, HSP70
Liu et al[57], 2024	NP, 3 times	HUCMSC	Female C57BL/6 mice	Sensitized and challenged with DFE	Total = 24, C = 6, A = 6, DFE + EVs T = 6, DFE + 146a-EVs T = 6	INH	MiR-146a-5p	CD63, HSP70, TSG101
He et al[44], 2024	2 × 1010 particles, 3 times	Human iPSC-MSCs	Female C57BL/6 mice	Sensitized with OVA and LPS, challenged with OVA	Total = 15, C = 5, A = 5, T = 5	IV	N/A	CD9, CD63, Alix, TSG101, calnexin

A: Asthmatic; C: Control; CD: Cluster of differentiation; DFE: Dermatophagoides farinae extract; EVs: Extracellular vesicles; HBMSC: Human bone marrow mesenchymal stem cell; HSP70: 70-kDa heat shock protein; HUMSC: Human umbilical cord mesenchymal stem cell; IN: Intranasal INH: Inhalation; iPSC-MSC: Induced pluripotent stem cell derived mesenchymal stem cell; IV: Intravenous; IT: Intratracheal; M-AMSC: Murine adipose tissue mesenchymal stem cell; MBMSC: Murine bone marrow mesenchymal stem cell; MSC: Mesenchymal stem cell; N/A: Not applicable; NP: Not provided; OVA: Ovalbumin; SL: Sublingual; T: Treated; TSG101: Tumor susceptibility gene 101.

Moreover, the differential expression of numerous microRNAs (miRNAs), including miR-146a-5p[57], miR-223-3p[51], miR-138-5p[58], miRNA-let-7, miRNA-155, and miRNA-126, is associated with allergic airway inflammation[59], making these miRNAs potential therapeutic targets in human MSC-based therapy (part of these references are discussed in Table 1). At the same time, the miR-21/activin A receptor type 2A axis has emerged as a crucial mechanism implicated in asthmatic inflammation in both a mouse model of asthma and in human subjects with asthma[60]. Furthermore, human MSCs genetically modified to afford inhibition of miR-138-5p, showed enhanced ability to reduce inflammation and allergic reactions by activating sirtuin 1 and inhibiting the high-mobility group box 1/Toll-like receptor 4 pathway in an asthma mouse model[58]. Shan et al[48] found that human bone marrow-MSC-derived exosomes reduced BMSC proliferation and lung injury in asthmatic mice via the miR-188/Jumonji, AT-rich interactive domain 2/Wnt/β-catenin pathway. In addition, externally originating MSCs have been observed to migrate towards lung tissue, accumulating at damage sites and then differentiating into type I and type II alveolar epithelial cells that participate in tissue restoration and repair in vivo[61,62]. Although their roles in this process remain unclear, the perceived weak differentiation ability of MSCs has prompted researchers to focus increasingly on studying their immunomodulatory effects on the repair of damaged and diseased tissues instead of their transplantation and differentiation behaviors.

Placenta-derived MSCs exerted an anti-IL-5 effect in vitro and reduced IL-5 level in culture with peripheral blood mononuclear cells that had been isolated from different subgroups of children with asthma[63]. Concurrently, another study conducted around the same time demonstrated that therapeutic administration of human MSCs to a mouse model of allergic asthma positively impacted oxidative stress by decreasing nitrotyrosine levels in lung tissues[64]. Similarly, Hu et al[65] reported a reduction in inflammation and oxidative stress to improve therapeutic outcomes in lung injury of acute respiratory distress syndrome by a new technique of integrating biomaterials and therapeutic agents through the fusion of mitochondria with liposomes. Interactions with probiotics also exerted antioxidant effects through the action of antioxidant enzymes[66]. Surprisingly, even nonviable MSCs, such as apoptotic MSCs, have been shown to exert immunosuppressive effects in vivo[67].

Shortly after the abovementioned studies were completed, a 12-month study was conducted to evaluate the significance of repeated intravenous MSC infusions. The study’s findings revealed that 12-month administration of MSCs initiated after the onset of chronic allergic feline asthma did not lead to reduced airway inflammation and hyperresponsiveness, as assessed using computed tomographic measurements of airway remodeling. However, reduced airway remodeling was observed earlier, after 8 months of treatment[68]. More recently, researchers discovered that a single injection of human MSCs was significantly more effective than double injections in reducing OVA-induced airway inflammation in a mouse model. Although systemic delivery of cell therapy through infusion is thought to achieve optimal therapeutic efficiency and targeting of lung tissues after optimization of MSC size and treatment frequency[69], both local and systemic administration of a single dose of MSCs were shown to reduce inflammation and lung tissue remodeling in OVA- and aspergillus hyphal extract-induced allergic asthma models[70,71]. Conversely, a single dose of MSCs did not improve lung function or remodeling in house dust mite extract-induced allergic asthma. In contrast, multiple MSC doses reduce lung inflammation, stimulate tissue remodeling, improve lung mechanics, and promote T cell-mediated immunosuppression[70]. Consequently, these outcomes should be carefully considered in future clinical trials when evaluating potential MSC-based asthma therapies.

Most MSC-based trials are currently in their early stages (primarily Phase 1 or 2), with only a limited number progressing to Phase 3[72]. As of May 2020, 68 clinical trials related to MSCs and respiratory diseases were underway, as documented in the clinical registration research database[73]. Among these, coronavirus disease 2019 emerged as the most frequent target condition (31 ongoing trials), followed closely by acute respiratory distress syndrome and chronic obstructive pulmonary disease (10 trials each), idiopathic pulmonary fibrosis (six trials) and asthma (two trials). The remaining nine trials focused on a broad range of diseases, including cystic fibrosis, lung transplantation, pneumoconiosis, radiation-induced injury, and unspecified lung injury. In terms of trial phases, thirty trials were classified as Phase 1, 17 as combined Phase 1/2, 14 as Phase 2, two as combined Phase 2/3, and one as Phase 3.

In 2020, a report revealed treatment outcomes for three individuals who have severe asthma refractory to conventional therapies, including steroids, bronchodilators, and anti-IgE medications are not effective for severe asthma, but SC therapy is effective. These patients received a single intravenous treatment with autologous BMSCs (2 × 10⁷ cells/patient) and then were monitored for 1 year for therapeutic and adverse effects. The study’s results demonstrated that administering autologous BMSCs via intravenous infusion was safe and effective in improving self-perceived quality of life, as assessed during the early post-procedure phase. Lung function and the 6-minute walk test measurements remained stable throughout[74]. These findings paved the way for future clinical investigations of treatments based on BMSCs or alternative cell types for patients diagnosed with severe asthma.

In 2023, Sharan et al[75] reported preliminary findings related to the first participant with asthma enrolled in a Phase 1 clinical trial (Safety of cultured allogeneic adult umbilical cord derived mesenchymal SC intravenous infusion for the treatment of pulmonary diseases, NCT05147688). This trial involved intravenous infusion of cultured MSCs derived from umbilical cord tissue, administered at a dosage of 100 million cells over a 40-minute period. Encouragingly, no adverse events or complications were noted at 2 months and 6 months post-treatment, while improvement in the participant’s condition persisted throughout the 6-month follow-up period. These results underscore the potential safety and efficacy of MSC-based therapies for pulmonary diseases, warranting further validation in more extensive clinical trials.

To date, four early-stage clinical studies employing SC therapies for asthma are registered in the clinical trials database, of which three are Phase 1 trials (http://clinicaltrials.gov) (Table 2). Notably, umbilical cord MSCs are the prevailing MSC type utilized in two of these trials, while allogeneic MSCs under evaluation in three of the four trials have exhibited robust immunomodulatory properties. As anticipated, MSCs were predominantly administered via the intravenous route (two trials), while intranasal delivery was employed in one trial. It is important to note that these studies are not designed to rule out potential adverse effects of therapy, including uncontrolled MSC proliferation, vascular blockage occurring after intravascular administration, and abnormal differentiation of injected MSCs. Nonetheless, before MSCs can be used for asthma treatment and other clinical applications, MSC dose/dosage, formulation, route of administration, frequency, and indications[76] must be optimized in animal models.

Table 2 Brief description of four clinical asthma trials related to stem cells in the clinical trials database.

NCT No	Title	Disease	Source MSCs	Auto/Allo?	Delivery	Phases	Enrollment	Ages eligible for study	Locations
02192736	Safety and feasibility study of intranasal MTF for treatment of asthma	Asthma	UCMSC-CM	Allogeneic	Intra-nasal	1/2	20	18-65 years old	Panama
03137199	Allogeneic human MSCs via intravenous delivery in patients with mild asthma	Asthma	BMSCs	Allogeneic	IV	1	6	18-65 years old	United States
04883320	Stem cell strategies for the treatment of chronic asthma	Asthma	MSC	Unspecified	Not provided	Not applicable	15	18-70 years old	United Kingdom
05147688	Safety of cultured allogeneic adult umbilical cord-derived mesenchymal stem cells for pulmonary diseases	Pulmonary diseases, asthma	UCMSC	Allogeneic	IV	1	20	Child, adult, older adult	Antigua and Barbuda

BMSCs: Bone marrow mesenchymal stem cells; MSC: Mesenchymal stem cell; MTF: Mesenchymal trophic factor; UCMSC-CM: Conditioned medium prepared from umbilical cord mesenchymal stem cell; UCMSC: Umbilical cord mesenchymal stem cell.

Importantly, MSCs derived from adult or newborn tissues of different donors exhibit limited proliferative capacities, significant variability in quality, rapid loss of differentiation potential, and lower therapeutic efficacy compared to corresponding features of iPSCs and ESCs. Nevertheless, MSCs are still considered an ideal therapeutic option due to their lower immunogenicity and greater ease of preparation. At the same time, iPSCs and ESCs, due to their high proliferation rates, could potentially serve as progenitor cells for generating artificially induced MCSs with therapeutic value. Meanwhile, explorations of the effects of MSC-Exo on the expression of genes related to asthma progression through gene editing could enhance MSC regenerative capacities and effectiveness. Notably, MSC-Exo hold great promise as an alternative to MSCs that may alleviate asthma by regulating the expression of novel miRNAs and other disease-related targets awaiting identification through more comprehensive approaches, such as transcriptomics and proteomics. Such research will enhance understanding of asthma-related pathways and pave the way for developing more targeted and effective therapeutic strategies.

Adult SCs

Adult SCs, also known as tissue-specific SCs, are believed to reside in most tissues and persist throughout an individual’s lifetime. These cells are considered crucial for tissue maintenance and repair, particularly in tissues with high cell turnover, such as blood, skin, and intestines, where adult SCs have been clearly identified and studied experimentally[77-79]. Meanwhile, potential adult SC populations have also been reported in tissues with low cell turnover, such as muscle, brain, and kidney[80-83], although one report of adult SCs in lung tissues was retracted[84,85].

In animal models, substantial numbers of bone marrow-derived adult SCs that produce collagen type I and α-smooth muscle actin have been detected in specific tissues, such as lung tissues of mice with OVA-induced chronic asthma[86]. However, adult SC isolation from different tissues can be challenging and an obstacle hindering exploring its potential in regenerative medicine. Within this context, the application of iPSC technology has emerged as a significantly important tool that can stimulate local tissue adult SCs to engage in tissue repair through paracrine signaling and other mechanisms, offering a promising avenue for advancing regenerative medicine.

CHALLENGES ASSOCIATED WITH THE CLINICAL USE OF SCS

Although MSCs demonstrate promising therapeutic potential for asthma, MSC-based treatment strategies remain challenging. For example, restricted in vitro growth, stemness decline, and harsh microenvironments hinder transplanted MSCs’ therapeutic potential and clinical application prospects[87]. Allogeneic MSC infusion triggers immune memory and boosts innate responses. If their immunosuppressive function is not activated, MSCs can act like antigen-presenting cells and promote inflammation[88]. Adverse effects post-transplantation include fever, chills, headache, back pain, and numbness[89]. Several issues must be addressed before SCs can be harnessed for patient treatment in clinical settings.

The first concern revolves around tissue integration, whereby transplanted cells must seamlessly integrate into surrounding tissues to ensure physiologically beneficial outcomes. Intriguingly, certain types of SCs, such as hESC-derived endothelial cells, exhibit an inherent ability to assemble into tubular structures that can integrate within the vasculature of tissues when inoculated into animals as dispersed cells[90].

A second challenge is the high likelihood that transplanted cells will develop into tumours, with a particularly high risk noted for transplanted pluripotent cells, given their ability to generate teratomas in animal models[91]. Therefore, it is crucial to precisely determine the differentiation states of transplanted cells to avoid the delivery of residual pluripotent cells that might undergo abnormal differentiation in vivo. Additionally, culture procedures must be designed to minimize the proliferation of genetically aberrant and potentially hazardous cell types[92]. To address these concerns, assessing pluripotent or other cell types for genetic integrity before transplantation in vivo is essential.

A third challenge relates to the ability to control the differentiation of cells into specific cell types, as it can sometimes be challenging to obtain a desired cell type from pluripotent cells. Furthermore, achieving uniformity and consistency of differentiated cells may be difficult, especially when derived from certain progenitor cell types. While these challenges are daunting, they are not insurmountable, although overcoming each obstacle will require substantial effort and focus. Nevertheless, we remain confident that these hurdles will be overcome to pave the way for the continued incorporation of SCs in asthma treatment strategies[12].

CONCLUSION

This review covers fundamental SC traits, including ESCs, iPSCs, MSCs, and adult SCs. It sheds light on SC therapies for alleviating asthma. Although the mechanisms by which SCs alleviate asthma are unclear, results of animal experiments suggest that SC-based treatments may relieve asthma symptoms by effectively reducing both airway inflammation and tissue remodeling, improving oxidative stress responses and paracrine functions. Based on these findings, human clinical trials are underway to evaluate SC safety and effectiveness as treatments for asthma and other diseases.

ACKNOWLEDGEMENTS

Professor Da-Chun Wang, the important author of this review, died March 29, 2024. We would like to thank Professor Wang for the support, encouragement, and care. We will always miss him.