Asthma

Characteristics, classification, and causes

Airway allergic diseases (AAD) are a group of allergic conditions marked by the infiltration of T-helper 2 (Th2) cells, which include allergic rhinitis (AR), allergic asthma, and chronic sinusitis (CRS).^1,2

Asthma is a heterogeneous³ and long-term inflammatory disease defined by inflammation of the airways, increased airway hyperresponsiveness (AHR), and structural changes in the airway remodeling. It is characterized by narrowing of the airways, persistent inflammation of the airways and surrounding tissues, increased growth and response of the airway smooth muscles as well as structural changes in the airways. Alterations in the respiratory tract lead to a wide range of symptoms, including intermittent episodes of breathing difficulty, audible whistling sounds during breathing (wheezing), and persistent coughing. These symptoms are intensified by different ecological influences, such as respiratory viral infections, air pollution, and inhaled allergens.⁵

Asthma is categorized by different approaches, such as the underlying cause (allergic and nonallergic), inflammation type (eosinophilic and non-eosinophilic), severity, and physiological characteristics.⁶ Asthma can also be classified based on the level of Th2 involvement, dividing it into two groups: T2-high and non-T2-high. Since most new biological treatments target T2 cytokines, we aim to explore the underlying mechanisms of T2 asthma, focusing on relevant cytokines and other molecular targets. The imbalance between T1 and T2 cytokines is a key factor in the development of T2-high asthma. T2-low asthma, associated with factors such as obesity, smoking, and age, is linked to the activation of Th1 and/or Th17 cells.⁷ Recent studies highlight the role of Th17/regulatory T cells (Treg) imbalance in steroid-resistant, severe, and neutrophilic asthma.⁷ Quantitative polymerase chain reaction (qPCR) of bronchial biopsies showed higher interleukin-5 (IL-5) and IL-13 mRNA in the Th2-high cluster, compared to Th2-low and controls. The Th2-low group showed no significant difference from the controls.^8,9 The higher levels of Th2 cytokines at sites of allergic inflammation reinforce the notion that Th2 cells play a central role in driving atopic diseases.^10–12 Mice studies show that allergic airway inflammation occurs with Th2 cell transfer, and not with Th1 cells.¹³ The main challenge in asthma is the Th1/Th2 imbalance.¹⁴ Th2 cells mediate humoral immunity by releasing cytokines such as IL-4 and IL-5. IL-4 promotes Th2 differentiation and the production of Th2-associated cytokines (IL-4, IL-5, and IL-13). IL-4 also inhibits Th1 cell proliferation and enhances mast cell degranulation, exacerbating allergic reactions.³ By modulating the levels of IL-10, a protein that helps to regulate the immune system can help to prevent allergic inflammation in the airways.¹⁴ Allergen exposure triggers antigen-presenting cells (APCs) to release IL-10, driving CD4+ T cells (Th0) to differentiate into Th2 cells for an effective immune response.³ A disruption in Th1/Th2 balance can lead to allergic diseases.¹⁵

T-helper 2

Origin, transcription factors, and differentiation

The differentiation of naïve CD4 T cells into Th2 cells is a key process for coordinating immune responses against extracellular parasites and is associated with allergic diseases such as asthma. This transformation is mainly induced by the activation of transcription factors GATA (erythroid transcription factor) binding protein 3 (GATA3) and signal transducer and activator of transcription 6 (STAT6), which are activated in response to IL-4 signaling.¹⁶ GATA3 is a key regulator of Th2 cell differentiation, playing a critical role in promoting the expression of cytokines associated with Th2 immune response,^17,18 modifying the chromatin landscape to facilitate the expression of Th2 cytokine locus in a coordinated manner.¹⁹ Alongside GATA3 and STAT6, additional transcription factors, such as cellular musculoaponeurotic fibrosarcoma (c-Maf), JunB proto-oncogene (JunB), and interferon regulatory factor 4 (IRF-4) also contribute importantly to Th2 differentiation by influencing the expression of essential cytokines and receptors.¹⁶

Activation and cytokine production

The first stage of naïve cell differentiation is influenced by the cytokine environment, the concentration of antigens, the type of APCs, and the presence of costimulatory molecules.²⁰ When activated, Th2 cells release several cytokines, including IL-4, IL-5, and IL-13, which impact different immune cells and tissues.²¹ IL-4 and IL-13 are essential for inducing immunoglobulin E (IgE) class switching in B cells, which enhances the production of allergen-specific IgE antibodies. This mechanism is vital for the development of allergic reactions, as IgE attaches to allergens and helps mast cells and basophils recognize them, leading to degranulation and release of inflammatory mediators.²² IL-5 is widely believed to be a key factor in the maturation and differentiation of eosinophils,²³ the accumulation of which tends to be at the site of allergic inflammation, and plays a role in the development of bronchial asthma.²⁴ The presence of eosinophils can lead to airway inflammation and damage, which exacerbates asthma symptoms. Eosinophils in the lungs accumulate and are linked to mucus hypersecretion, AHR, and tissue remodeling in Th2-driven asthma.²⁵

Pathophysiological effects in asthma

Various pathological changes in the airways are driven by the cytokines secreted by Th2 cells. For instance, increased mucus production is a result of IL-13 enhancement of goblet cell hyperplasia, which can lead to airway obstruction.²⁶ Additionally, bronchial hyperresponsiveness can be promoted by IL-4 and IL-13,²⁷ which is a characteristic feature of asthma that leads to breathing difficulty and wheezing.²⁸ Th2 cells are responsible for more than just recruiting eosinophils. Other immune cells, such as mast cells and dendritic cells (DCs), are also influenced by them, which amplify inflammatory response.²⁹ This network of interactions creates a feedback loop that prolongs airway inflammation and leads to chronic symptoms in asthma patients.²⁹

Different immunotherapeutic paths

Among various drug treatments for asthma, inhaled corticosteroids (ICS) are the most effective controllers of asthma. ICS is effective in the expression of infectious cytokine genes, such as IL-4, IL-5, and IL-13, but these must be consumed regularly, otherwise the initial symptoms reappear; moreover, it also has adverse effects, such as growth suppression and cataract.³⁰

Another treatment method is allergen immunotherapy (AIT), which treats allergic diseases by imparting increasing doses of relevant allergens to patients. This method modulates asthma by targeting Th2 and promoting T-naïves differentiation into Th1.³¹ Although it can trigger a systemic reaction, some meta-analyses presented conflicting results on AIT’s effectiveness in preventing new sensitizations, particularly in pediatric populations, highlighting a need for further research to clarify these outcomes. It should be noted that these therapies might lead to systemic reactions. Additionally, the 3- to 5-year treatment duration poses significant challenges, limiting their practical applications in some patients.³²

Moreover, there are some novel immunotherapeutic modes of treating this disease, for instance the use of monoclonal antibody-based treatments. These treatments are designed to reduce infection by targeting cytokines.³³ For example, mepolizumab works by blocking IL-5,³⁴ while dupilumab targets the receptor for two other cytokines, IL4 and IL13, by focusing on interleukin-4 receptor alpha (IL-4Rα).³³ As a new growing cure, stem cells are mentioned,³⁵ which is the focus of this article to examine their mechanism of action in Th2 functioning in controlling of asthma. As a new growing cure, stem cells are mentioned.

Mesenchymal stem cells

Pioneers in regenerative medicine and immune modulation

Mesenchymal stem cells are remarkable cells with the ability to renew themselves and transform into various cell types, such as fat, bone, cartilage, epithelial, and endothelial cells.^36–39 MSCs are an invaluable source of regenerative medicine and are sourced from a variety of tissues. These include bone marrow, peripheral blood, inner organs, and adipose tissue. Additionally, they are harvested from neonatal tissues such as the umbilical cord, amniotic membrane, amniotic fluid, and placenta, making them widely accessible for therapeutic applications.^40,41 The use of MSC treatments has demonstrated significant potential in managing various conditions, including hematological diseases, graft-versus-host disease, diabetes, multiple sclerosis, Crohn’s disease, ulcerative colitis, lupus, and in patients undergoing kidney transplantation.⁴² MSCs are crucial for tissue regeneration and immune system regulation.^37,38 They possess an incredible ability to regulate the immune system by inhibiting both innate and acquired immune cells. This powerful function is enabled through direct cell-to-cell communication as well as the release of a diverse array of secretory factors, collectively known as secretome, which includes growth factors, cytokines, and chemokines.⁴³ Given their potential to differentiate and their immune-regulatory capabilities, MSCs are emerging as a new therapeutic tool for a wide range of immune and degenerative diseases,^44,45 although overcoming certain limitations is important to fully realize the clinical potential of MSCs.⁴⁶ MSCs have been shown to suppress inflammatory responses and pathological remodeling in asthma models by modulating immune cells, particularly T cells, and reducing the levels of inflammatory cytokines, such as IL-4, IL-5, and IL-13.^47–50

Regulating Th2 to ameliorate asthma

Mesenchymal stem cells exert their therapeutic effects through various mechanisms, including mitochondrial donation,⁵¹ protection against apoptosis and oxidative stress,⁵² and the secretion of extracellular vesicles (EVs) along with soluble factor, such as transforming growth factor-β (TGF-β), prostaglandin E2 (PGE2), and indoleamine 2,3-dioxygenase (IDO), effectively transforming the asthma patient’s environment from a proinflammatory state to an anti-inflammatory state.⁵³ MSCs play a crucial role in alleviating AHR and preventing goblet cell metaplasia, both of which are key contributors to the development of asthma. Their therapeutic potential could significantly enhance asthma management strategies.^54–56 MSCs can effectively modulate the Th2 immune pathway by balancing Th1 and Th2 responses. They suppress key Th2 cytokines such as IL-4, IL-5, and IL-13, reducing allergic inflammation and AHR.^57–62 MSCs can promote the production of anti-inflammatory cytokines such as IL-10 and interferon gamma (IFN-γ), which help to reduce Th2-dominated allergic responses.⁶² They significantly enhance IL-10 expression while suppressing the expression of surface receptors.⁶³ Furthermore, MSCs can modulate DC maturation⁶⁴ and macrophage differentiation to suppress Th2 responses.⁶⁵

Exosomes

Exosomes, cystic vesicles released by MSCs, are packed with messenger RNA (mRNA), microRNA (miRNA), and proteins.^66,67 They transfer these materials into target cells, impacting their growth, death, and movement.⁶⁸ Studies have shown that miRNAs can control gene expression by binding to specific regions of mRNA.⁶⁹ For example, upregulating MSC-derived exosomal miR-222-3p leads to reduced expression of interferon regulatory factor 2 (IRF2).⁷⁰ IRF2 is found to promote Th2 cell immune response and inhibit Th1 cell immune response.^71–73 Therefore, overexpression of miR-222-3p through engineering enhances the Th1/Th2 cell ratio, which could be useful in controlling asthma.⁷⁰

Transforming growth factor-β

Mesenchymal stem cells possess an extraordinary capacity to direct the transformation of CD4+ T cells into CD25+Foxp3+regulatory T cells (induced Tregs) by expressing TGF-β, which produce anti-inflammatory cytokines, such as IL-10 and TGF-β that suppress airway inflammation and cause the cycle to continue.^60,74,75 This illustrates the remarkable impact of MSCs in regulating immune response.⁷⁵ This potential is associated with the stimulation of TGF-β^66,67 and IFN-γ⁶⁸ as well as the recruitment of Tregs,^69–71 thereby contributing to the balance between Th1 and Th2 cells,⁶⁸ so that MSCs secreting TGF-β can be helpful in modulating asthma.

Prostaglandin E2

Prostaglandins, such as lipid-derived autacoids, are crucial in modulating the immune system.⁷⁶ The secretion of PGE2 by MSCs activates the EP2 and EP4 receptors on macrophages, stimulating them to produce and release IL-10, a cytokine with anti-inflammatory properties.⁷⁷ Studies have shown that stimulation of EP2 and EP4 receptor subtypes promotes monocyte-derived DC maturation.⁷⁸ However, the exact reason for the necessity of both EP2 and EP4 receptors for this effect remains unknown. It is possible that the two receptors may synergistically increase cyclic adenosine monophosphate (AMP) levels, or they could have non-overlapping actions that are crucial for induction.⁷⁶ Additionally, the formation of functional heterodimers by these receptors in macrophages is a potential area of interest for further research.⁷⁷

Indoleamine 2,3-dioxygenase

The expression of IDO is important for MSCs to suppress T-cell proliferation effectively. The varied levels of IDO expression among MSC donors are strongly associated with their capacity to suppress T-cell responses.^79,80 IDO works by hindering T-cell proliferation through tryptophan depletion,⁸¹ and the production of kynurenine, a tryptophan metabolite that inhibits T-cell proliferation by activating the stress response kinase GCN2.^82,83 Furthermore, IDO expression triggers the transformation of naïve CD4+ T-cells into regulatory CD4+CD25+FOXP3+regulatory T-cells.⁸⁴

Modulate DC functions to suppress Th2 responses

Dendritic cells are crucial in driving the development of allergic diseases by steering naïve T cells toward becoming T helper 2 cells.⁸⁵ However, MSCs have been demonstrated to impair the maturation and migration of lung DCs to lymph nodes, thereby alleviating Th2-driven inflammation.⁴ Additionally, MSCs can suppress the activation of both naïve and effector Th2 cells mediated by DCs and reduce the production of chemokines responsible for recruiting Th2 cells to the airways.⁴ MSC therapy significantly reduces the population of DCs and macrophages that possess antigen-presenting capabilities.⁶³ MSC treatment leads to a reduction in MHCII expression in DCs, which is responsible for presenting antigens to Th2 cells.⁶³

Regulate macrophage differentiation to inhibit Th2 responses

Along with Th2 cells, M2 macrophages are a major source of type 2 cytokines that worsen asthmatic inflammation.⁸⁶ Macrophages are the primary immune cells found in the airway, able to differentiate into M1 or M2 subtypes depending on particular signals.⁸⁷ M1 macrophages are primarily induced by pro-inflammatory signals and are characterized by the secretion of IFN-γ, which is derived from Th1 cells, thereby promoting inflammation. In contrast, M2 macrophages are activated by anti-inflammatory signals, such as IL-4 and IL-13, and play a key role in tissue repair and the resolution of inflammation.⁸⁸ Many studies have discovered a bidirectional effect of direct contact between MSCs and macrophages that affects both cell types and is important for beneficial effects. Direct contact between MSCs and pro-inflammatory macrophages enhances TSG-6 production by MSCs and leads macrophages to M2-like phenotype while suppressing T cell proliferation.⁸⁹ IL-6, a cytokine secreted by MSCs and present in inflammatory conditions, plays a pivotal role in driving macrophages toward an anti-inflammatory M2-like phenotype, which is characterized by the production of IL-10.⁹⁰ A summary of this section is provided in Figure 1.

Figure 1 How MSCs exhibit immunosuppressive modulation to induce immune tolerance against asthma. Upregulating miR-222-3p from MSC-derived exosomes reduces the expression of IRF2, which promotes Th2 cell immune responses. Therefore, miR-222-3p expression improves the Th1/Th2 cell ratio in MSCs. The release of soluble factors such as TGF-β, PGE2, and IDO shifts the asthma patient’s environment from pro-inflammatory to anti-inflammatory. MSCs aid in transforming CD4+ T cells into induced Tregs by expressing TGF-β, which produces anti-inflammatory cytokines such as IL-10 and TGF-β, helping balance Th1 and Th2 cells. PGE2 from MSCs activates macrophage EP2 and EP4 receptors, promoting IL-10 production, while IDO suppresses T-cell proliferation and encourages the conversion of naive CD4+ T cells into induced Tregs. MSCs can inhibit DC functions such as directing naive T cells to become Th2 cells, facilitating the maturation and movement of lung DCs to lymph nodes, producing chemokines that attract Th2 cells to the airway, and enhancing MHCII expression for antigen presentation to Th2 cells. Cell contact between MSCs and M1 macrophages enhances TSG-6 production in MSCs, promoting M2-like polarization of macrophages and suppressing T-cell proliferation. Additionally, IL-6 produced by MSCs helps to regulate macrophage polarization toward an IL-10-producing M2-like phenotype.

Mesenchymal stem cells-based therapies

Preclinical trials and limitation

Multiple preclinical studies have shown the effectiveness of MSCs in various animal models of asthma, including ovalbumin (OVA)-induced allergic asthma and house dust mite (HDM)-induced asthma.^91–93 Recent animal studies have shown notable outcomes with MSC treatments.^57,94–96 Research has demonstrated that MSCs can diminish inflammatory cell infiltration,⁹³ reduce mucus production,⁹³ lessen airway remodeling,⁹⁷ and enhance lung functioning. The use of chemical agents that simulate in vivo microenvironment, combined with genetic modifications, is studied as a strategy to improve the survival, proliferation, and homing efficiency of MSCs, thereby enhancing their therapeutic effectiveness.^98–101 In damaged tissues, the potential for MSC engraftment is limited due to cellular death.^102,103 Despite the proven safety of MSCs as a form of cellular treatment, their therapeutic efficacy in human patients is limited in general.¹⁰⁴ Decrease in proliferative capacity, pluripotent gene expression, and differentiation potential in MSCs are partially attributed to senescence, which is associated with low telomerase activity.^105,106 Engineering of MSC is a strategic approach aimed at enhancing their therapeutic efficacy and overcoming several challenges related to their use. Research has shown that modifying MSCs can significantly improve their beneficial effects. Licensing or pre-stimulation of MSCs, including their transfection with specific genes before application, can significantly enhance their immunomodulatory and tissue-repair capabilities. For instance, cytokine pre-activation with IL-1β, tumor necrosis factor α (TNF-α), or IFN-γ has proved to strengthen MSC-driven repair and the resolution of ventilator-induced lung injury.¹⁰⁷ In addition, improving the microenvironment after acid-primed lung injury or using MSCs containing the human IL-10 gene or hepatocyte growth factor (HGF) has shown to lessen the negative effects of untreated MSCs.¹⁰⁸ In conclusion, modifying MSCs provides a comprehensive strategy to boost their therapeutic effectiveness for asthma by enhancing their survival, targeting ability, and anti-inflammatory effects.

Anti-inflammatory and immunomodulatory properties

The remarkable therapeutic potential of MSCs can be significantly enhanced through strategic pre-treatment methods, such as exposure to hypoxia, cytokines, and cultivation in a three-dimensional (3D) environment. The priming of MSCs induces a shift in their properties, promoting anti-inflammatory, pro-trophic, and regenerative characteristics, thereby enhancing their overall therapeutic potential.^109–115 Furthermore, the dynamic production of both anti- and pro-inflammatory factors by MSCs allows for a fine-tuned regulation of their immunoregulatory properties.¹¹⁶ The conditions of cell culture are crucial in influencing and enhancing the beneficial characteristics of MSCs, highlighting the importance of proper conditioning for improved therapeutic outcomes.^110,117,118

Survival enhancement

In the context of cell therapies, it is critical for MSCs to navigate effectively the tissue microenvironments characterized by ischemia, inflammation, and oxidative stress to deliver their therapeutic benefits. However, harsh conditions within these microenvironments often result in low MSC survival rates, with as little as 7% of transplanted MSCs surviving for a few days in infarcted animal myocardium.¹¹⁹ To tackle this challenge, researchers are exploring strategies to boost the survival of transplanted MSCs. Using nonviral gene delivery of pro-survival or anti-apoptotic factors into MSCs is an effective strategy.^120,121 Moreover, miRNA delivery provides a different approach by modulating gene expression pathways that support cell survival, eliminating the need for the external expression of growth factors or cytoprotective proteins.¹²²

Improved homing efficiency

Attributed to MSCs’ distinct components, such as cytoskeletal protein filaments (mainly microtubules, actin filaments, and intermediate filaments), along with organized myofilaments and pseudopods around the cell periphery, MSC homing demonstrates strong migratory potential.¹²³ It is a complex process that entails a dynamic interaction of cytokines, chemokines, adhesion molecules, and integrins on the surface of administered MSCs, directing their migration from the bloodstream to damaged tissues.¹²⁴ To achieve successful transplantation outcomes, MSCs must reach target tissues effectively. Researchers have applied nonviral gene delivery approaches to guide MSCs toward specific tissues or improve their intrinsic tissue-homing ability.^125,126 This method holds promise for enhancing the therapeutic effectiveness of MSCs while reducing the required dosage. The homing efficiency of MSCs can be improved by strategically adjusting cell culture conditions, increasing the expression of homing molecules, and employing genetic engineering to modify cell surface receptors. This understanding paves the way for developing more targeted and effective MSC-based therapies for conditions such as asthma, where MSC homing plays a pivotal role in tissue regeneration and immunomodulation.^127–130

Optimized gene delivery

Mesenchymal stem cells offer incredible potential in regenerative medicine, and advancements in gene delivery techniques can further enhance their therapeutic benefits. By introducing specific genes, MSCs are tailored to improve survival,^121,131 target specific cell membrane receptors,^132–134 differentiate into desired cell types,^135,136 and produce beneficial growth factors,^137,138 cytokines,^139,140 and miRNA in exosomes.^141,142 Gene delivery is the process of introducing exogenous DNA or RNA into cells to modulate gene expression. Viral gene delivery systems capitalize on the natural efficiency of viruses to encapsulate and transport genetic material into target cells.¹⁴³ While viral gene delivery systems are highly efficient, they present safety concerns such as the risk of mutations and tumorigenicity.^144,145 As such, it’s imperative to explore and develop nonviral gene delivery strategies that not only ensure safety but also maximize efficiency and minimize toxicity.¹⁴⁶ Effective nonviral techniques, such as membrane disruption methods for nucleic acid delivery into cells (e.g., microinjection, electroporation, etc.) or the use of nanocarriers for nucleic acid packaging, hold significant potential and require further investigation to fully realize the capabilities of MSCs in regenerative therapies.¹⁴⁷

Genetic engineering of MSCs

Mesenchymal stem cells can be engineered into highly effective drug-delivery systems through their modification to enhance their survival and tissue-specific targeting, along with incorporating nonviral gene delivery techniques.¹⁴⁸ Furthermore, studies have shown that immunomodulatory factors can be introduced into MSCs for sustained expression, highlighting the versatility of MSCs in reducing inflammation and supporting cell-based therapies.¹⁴⁹ Modifying the genetic makeup of MSCs empowers them to produce specific factors that can shield them from cell death, enhance their survival in low-oxygen environments, and improve their natural abilities, such as movement and specialization into particular cell types. Genetic engineering of MSCs is typically carried out using viral vectors, although there is a growing use of nonviral vectors for this purpose.¹⁵⁰ Genetic engineering of MSCs can be achieved through the overexpression or suppression of specific genes by using advanced gene-editing approaches, including knock-in and knock-out strategies such as the clustered regularly interspaced short palindromic repeats-associated protein 9 (CRISPR/Cas9) system and RNA silencing techniques. These methods enable precise control of native MSC gene activity or the introduction of exogenous genes for specific therapeutic purposes.¹⁵¹

Interleukin-10

IL-10 is a powerful anti-inflammatory cytokine produced naturally.¹⁵² Its remarkable abilities include the capacity to protect and mitigate diseases by staunching inflammation and upholding self-tolerance. This cytokine has demonstrated its therapeutic effects in immune-related disorders, including psoriasis, rheumatoid arthritis, and allergic asthma.^153–156 It is clear that MSCs, with their immunomodulatory potential, can be further empowered through IL-10 gene expression to manage effectively the pathophysiology of allergic asthma.¹⁴ Recent studies have indicated that IL-10 can suppress Th2 cells, significantly decreasing their production of IL-4 and IL-5.¹⁵⁷ The manipulation of MSCs as immunomodulatory cells using immunosuppressive biofactors such as IL-10 could very well emerge as the primary method for not only controlling but also curing asthma.¹⁴

Interleukin-35

IL-35 is shown to have an impressive ability to inhibit the growth of CD4+ T cells and elevate the presence of Treg cells, thereby potentially reducing the risk of autoimmune and inflammatory diseases.¹⁵⁸ Furthermore, IL-35 can effectively suppress the activity of Th1, Th2, and Th17.¹⁵⁸ It is important to highlight that Treg cells, whose proliferation and functioning are enhanced by IL-35, play a key role in suppressing the functioning of other T cells to regulate immune response.^159,160 In a study, MSCs were successfully transduced using the pUNO1-mIL35 elastic vector, leading to the production of IL-35 in modified cells.⁴ Their findings revealed that MSCs transfected with the IL-35 gene exhibited a remarkable ability to control effectively allergic asthma symptoms, including AHR, inflammation of the airways, difficulty breathing, wheezing, and persistent coughing.^4,5,41 This discovery holds immense promise for advancing the treatment of allergic asthma and underscores the potential of IL-35 gene expression in enhancing therapeutic outcomes.⁴¹

MicroRNA-138-5p (miR-138-5p)

MiR-138-5p has a significant role in the pathophysiology of asthma.¹⁶¹ It is shown that miR-138-5p is involved in various mechanisms that contribute to airway inflammation and hyperresponsiveness, the defining characteristics of asthma.¹⁶² The presence of pro-inflammatory cytokines, such as TNF-α and IL-6, reduces inflammatory responses in human MSCs (hMSCs) when miR-138-5p is inhibited.¹⁶³ This inhibition leads to a reduction in allergic symptoms in mice models of asthma, suggesting that targeting miR-138-5p could be a promising means to manage asthma and improve treatment outcomes.^161,163 MiR-138-5p has emerged as a key regulator of deacetylase sirtuin 1 (SIRT1), with its elevated levels being strongly linked to a diverse range of human malignancies.^164–166 Notably, the relationship between miR-138-5p and SIRT1 has been associated with pancreatic cancer, highlighting its crucial role in the disease.¹⁶⁴ Consider engineering of MSCs with the miR-138-5p gene, a method that offers a well-established set of targets. This approach is anticipated to accelerate the integration of innovative hMSC-based therapy. A recent study has discovered that hMSCs significantly decreased the levels of IL-4, IL-6, IL-5, IL-13, TNF-α, and GATA-3. Notably, hMSCs modified with an miR-138-5p inhibitor exhibited an even greater decrease in these factors. This highlights the potential of this approach for enhanced therapeutic outcomes.¹⁶³

Erythropoietin (EPO)

The latest research has confirmed that EPO has remarkable antioxidant and anti-inflammatory properties that serve an essential function in enhancing the proliferation and migration of MSCs.¹⁶⁷ Furthermore, EPO has been shown to effectively mitigate prolonged histopathological transformations in the lungs of asthma patients.¹⁶⁸ It is essential to note that EPO also suppresses the synthesis of IL-4 and IL-5, both of which are pivotal for developing asthma.¹⁶⁹ A study conducted in 2018 employed MSCs engineered to express EPO in a mouse model of chronic asthma triggered by OVA.¹⁶⁷ The results revealed that both MSCs and EPO–MSCs significantly reduced IL-5, IL-4, and IL-13 levels, with EPO–MSCs showing a stronger inhibitory effect. EPO–MSCs also considerably inhibited the TGF-β1-TAK1-p38MAPK pathway, effectively reducing airway inflammation, mucus secretion, AHR, and airway remodeling.

Indoleamine 2,3-dioxygenase

Indoleamine 2,3-dioxygenase degrades tryptophan, an essential amino acid vital for T-cell metabolism, thereby directly inhibiting T-cell activation and proliferation while promoting T-cell inactivation, apoptosis, and differentiation into Treg cells.^170–172 IDO-transfected MSCs enhance the antigen-specific immunosuppressive functioning of Treg cells.¹⁷³ They also stimulate Treg cells to secrete IL-10 and TGF-β1.¹⁷⁴ These findings show that IDO–MSCs not only suppress CD4+ T cell proliferation but also promote Treg cell induction, which helps to lessen asthma by balancing Th1 and Th2 cell activity.^7,68,173

Soluble suppression of tumorigenicity 2 (sST2)

sST2 functions as an IL-33 receptor antagonist, which triggers Th2 cytokine production. In a model of ammonium persulfate-induced occupational asthma, human-AT-MSCs overexpressing soluble ST2 effectively inhibit IL-33 production, reduce IgE levels, and significantly alleviate neutrophilic inflammation and airway remodeling. This significant advancement shows great potential for revolutionizing the management of occupational asthma.¹⁷⁵ A comprehensive summary of engineering of MSC is presented in Table 1.

Advancements in genetic engineering of MSCs offer a compelling path toward revolutionizing the treatment of allergic asthma. Through strategic genetic modifications, such as overexpressing or silencing specific genes, including the introduction of the genes such as IL-10, IL-35, miR-138-5p, EPO, IDO, and sST2, researchers have unlocked the potential to enhance the immunomodulatory and anti-inflammatory properties of MSCs. The resulting engineered MSCs demonstrate unparalleled abilities to effectively control allergic asthma symptoms, reduce inflammation, and regulate immune response. These findings not only underscore the promise of genetic modification of MSCs but also present a compelling case for the development of more potent treatments for allergic asthma and other immune-mediated diseases.

Conclusion

Mesenchymal stem cells are a new therapeutic way of controlling asthma and other allergic diseases by releasing their exosomes and soluble factors. They suppress Th2 responses by modulating the mechanism of macrophages and DCs. However, we are facing a series of limitations in the use of MSCs; in order to overcome this constraint, engineering of MSCs is growing as a new therapeutic technique for controlling asthma, which can be engineered by special genes to overexpress or inhibit the production of interleukins, RNAs, and different molecules. Major new developments in modifying MSCs provide a considerable mode to change how we treat allergic asthma and other immune-related diseases. The strategic genetic modifications of MSCs not only maximize the therapeutic effects of the delivered genes but also have the potential to improve treatment outcomes. Although all these are at a pre-clinical level, with continued research and the expansion of genetic engineering applications, these approaches could develop into more effective and sustainable treatments for asthma. In addition, the combination of transfecting several genes with one another as utilized in different diseases may become an effective therapy for asthma.