Role of Mesenchymal Stem Cells and Short Chain Fatty Acids in Allergy A Prophylactic Therapy for Future

Allergic diseases are broadly classified as IgE-mediated type-I hypersensitivity immune reactions due to exposure to typically harmless substances known as allergens. These allergenic substances activate antigen presenting cells, which further triggers T-helper 2 cells immune response and class switch B-cells for synthesis of allergen-specific IgE, followed by classical activation of inflammatory mast cells and eosinophils, which releases preformed mediators involved in the cascade of allergic symptoms. However, the role of Mesenchymal stem cells (MSCs) in tissue repair ability and immunomodulation, makes them as an appropriate tool for treatment of various allergic diseases. Several clinical and preclinical studies show that MSCs could be a promising alternative therapy to allergic diseases. Further, short chain fatty acids, produced from gut microbes by breaking down complex fibre-rich


Introduction
Immunoglobulin E (IgE) mediated hypersensitivity reaction develops in atopic individual, who are sensitised to typically harmless food or environmental allergens, which is termed as allergic disorder or Allergy. Allergic diseases include food allergy (FA), allergic asthma (AA), allergic rhinitis (AR) and atopic dermatitis (AD or atopic eczema), which shares some of the common inflammatory cascades caused by dynamic interaction of structural tissue cells and inflammatory cells namely basophils, mast cells, dendritic cells, lymphocytes, neutrophils and eosinophils [1]. The prevalence of allergy affects 20% worldwide amongst the sensitised individuals to various predominant allergens. Nearly one of every four children in high-income countries shows relative symptoms of AR, AA, or AD in the last three to four decades. Importantly, the global incidence of allergic rhinitis and allergic asthma are substantially increased among children and adult [2]. The treatment for allergic disorders with antihistamines, corticosteroids, β2 adrenergic receptor agonists and anti-leukotrienes block inflammatory mediators and immune cells temporarily, which acts as stop gaps [3]. However, the long-term relief from allergic diseases is one of the major concerns worldwide as it burdens quality of life and treatment cost.
Mesenchymal stem cells (MSCs) serve as a new treatment strategy for several diseases including allergic diseases. MSCs are non-hematopoietic, multipotent adult stromal progenitor cells that can self-renew and differentiate into several lineages and cell types.

HLA-DR, CD34
CD45 [20] [21]. A comparative study of the clinical effects of MSCs derived from the amniotic membrane (AMMSC) and umbilical cord (UCMSC) on type 2 diabetes mouse model shows significant improvement of glycolipid metabolism, reduction of hyperglycemia and improvement from insulin resistance conditions [22]. The administration of a therapeutic dose of Adipose-derived mesenchymal stem cells (ADMSC) in a septic rat model, reduced 'cecal ligation and puncture' (CLP)-induced acute lung injury (ALI), and enhanced gut microbiota.
Here, the preclinical application of ADMSCs offers a potential therapeutic approach to treat sepsis [23]. Further, clinical trial in progressive multiple sclerosis (MS) patients using repeated MSC treatments shows safe and clinical benefits lasted for up to four years [24]. Yet another study by Pan X et al. reported that the therapeutic effects of UCMSCs in alleviating inflammatory bowel disease (IBD) symptoms in an animal model by promoting the expression of occludin and claudin-1 (intestinal tight junction proteins), downregulating the expression of the autophagy marker LC3A/B in the colon, and microvascular regeneration of intestinal wall cells [25]. In a study on Leishmania major-infected BALB/c mice model, multiple intravenous (i.v.) injection of AD-MSCs reduced footpad swelling and parasites burden and observed delay on lesion development [26]. Moreover, studies clearly indicate that MSCs could prevent gut dysbiosis and restore normal gut microflora, which summarises involvement dysregulated metabolic pathway. In another study, 2,4,6-trinitrobenzene sulfonic acid (TNBS) induced colitis mouse model was treated with UCMSCs for restoration of the gut microbiota and repairs the intestinal mucosal barrier, which exerts a protective effect [27].
Taken together, MSCs serve as a progressive therapeutic tool for various disease conditions and that more than 950 clinical trials of MSCs in human treatment were registered as therapeutic modality alternative to the main treatment approach [28].

Overview of immune reaction in allergy
The involvement of effector CD4+ T helper type 2 (Th2) cells and IgE-producing classswitched B cells against specific allergens serves as the key indicator of allergic manifestation, followed by elevation of activated mucosal mast cells and eosinophil infiltration [1]. In the first stage of allergic inflammation, which is termed as 'sensitization phase', in which antigen presenting cells (APCs) like macrophages, dendritic cells (DCs), langerhans cell or monocytes process and present the antigen (or allergens) to naive CD4+ T cells, which pave the way for differentiation into Th2 cells. During allergic manifestation, these differentiated Th2 type cells secrete proinflammatory cytokines like interleukin (IL)-4, IL-5, IL-9 and IL-13, which directly play a crucial role in the synthesis of IgE by isotype switching of B cells and induce allergic airway inflammation ( Table 2). Allergen-specific IgE secreted by B-cells binds to the high-affinity receptor FcɛRI present on the surface of mast cells and basophils and aggravates allergic immune response [1], [29], [30]. Further, upon subsequent re-exposure to the same allergens during the second stage of allergic inflammation, also referred to as the effector phase, readily increases the release of proinflammatory mediators by activated mast cells and basophils. In addition, accumulation and activation of effector cells such as eosinophils and neutrophils promote airway hyperreactivity ( Figure 2) [29].

Role of Tregs in immune suppression
Tregs (regulatory T cells) are one among the subsets of CD4 + T cells that express the IL-2 receptor α chain (CD25) and help to sustain homeostasis and self-tolerance by controlling immune responses. Foxp3 (forkhead box P3 transcription factor), a member of FOX protein family, plays a prominent role in the development and immunosuppressive activity of Tregs.
Naturally occurring Tregs (nTregs) derived from the thymus and secondary lymphoid-derived inducible Tregs (iTregs) from peripheral naive T cells are the two types of Foxp3+ Tregs.
Both play a predominant role in the immune homeostasis maintenance. It has shown in humans that a lower level of Tregs is linked to airway inflammation [30], [31]. There are three 2. Secretion of immune-regulatory cytokines such as IL-10 and transforming growth factor-β (TGF-β) by Treg cells is another mechanism to induce immune suppression [35], [36].
3. Treg cells' suppressive action can also be mediated by destruction of target cell metabolism. The IL-2 receptor α-chain express on the surface of Treg cells facilitates IL-2 binding. Since IL-2 is needed for Teff cell proliferation and activation, IL-2 deficiency can cause Treg cells to suppress Teff cell growth. Furthermore, hydrolysis of ATP or ADP to cAMP followed by suppression of Teff cell function or DC maturation occurs due to the expression of CD39 and CD73 on Treg cell surface [31], [37]- [39].

Role of MSC in allergy treatment
MSCs has emerged as appealing therapeutic resources in transplantation, tissue regeneration, and autoimmune disorders due to their high potential expansion capability ex vivo, multilineage differentiation potential, and immune suppression functions [40]. In autoimmune and inflammation-related diseases such as inflammatory bowel disease, collagen-induced arthritis, multiple sclerosis, graft-versus-host disease, sepsis, and type I diabetes, MSCs therapy has shown greater potential in regulation of inflammatory responses. So far, the clinical data from several studies demonstrate the role of MSCs in ameliorating allergic conditions such as AA, AR, AD, and FA as well [41].

4.1.Clinical trials with MSCs on Allergic diseases
Only

4.2.MSCs therapy in Allergic Airway Diseases
MSCs has the potential to alleviate allergic airway inflammation and improve lung function.
Administration of AT-MSCs in asthmatic mouse model significantly reduces levels of serum IgE and IgG, as well as Th2 cytokines (IL-4, IL-5, and IL-13), whereas the levels of IFN-γ, regulatory cytokines such as IL-10 and TGF-β and Treg cell ratio were significantly increased compared to untreated asthmatic mice [44]. Tonsil-derived MSCs (T-MSCs) administered on mouse model of AR significantly reduced allergic symptoms and inflammatory parameters such as eosinophil infiltration, serum IgE levels, Th2 cytokines, innate cytokines such as IL-25 and IL-33 and chemokines such as CCL11 and CCL24 [45]. These collective evidence from preclinical studies suggests the role of MSC based therapy in the allergic airway disease-animal models may offer an alternative, highly promising approach with more reliability and productivity to benefit patients with allergic airway disease who are unable to be cured with conventional therapies. However, there is still a long way from the current allergic airway disease-animal model trials to the final therapeutic implementation in human allergic airway disease therapy for a safer, efficient, and routine manner. In conclusion, whether MSC therapy would be a standalone treatment for human allergic airway diseases needs further investigation [23].

4.3.MSCs therapy in Atopic dermatitis (AD)
Kim production, which shows to reduce MC degranulation by repressing high-affinity FCɛR1 receptor expression (39). In the same mouse model, the therapeutic effect of administering human adipose tissue-derived MSCs (hAT-MSCs) shows cyclooxygenase-2 (COX-2) signalling mediated inhibition of B cell proliferation and maturation by significant downregulation of the serum IgE level [49]. The effective therapy of hUCB-MSCs preexposed to MC granules by suppressing the allergic immune response and stimulating the mechanism of the tissue regeneration more effectively than naive cells have been observed in experimental AD, which suggests a potential improvement strategy for stem cell therapy [50].
To determine the safety and efficacy of hUCB-MSCs in AD treatment, a two phase clinical trial has been conducted by Kim et al., (2017) in moderate to severe AD patients. A marked improvement of AD symptoms such as downregulation of serum IgE levels, and blood eosinophils, 50% reduction in Eczema Area and Severity Index (EASI) and Severity Scoring for Atopic Dermatitis (SCORAD) scores, and 33% reduction in Investigator's Global assessment (IGA) score were resulted from the study without any serious adverse events.
These findings indicate that infusing UCB-MSCs into patients with moderate-to-severe AD may be a potential treatment option [51].

4.4.MSCs therapy in Food allergy
Food allergy incidence has risen gradually over the last few decades, and it makes immunotherapeutic treatment options more popular. Variable treatment efficacies, adverse allergic reactions, and clinical safety deficiencies make the clinical trials of immunotherapeutic treatment methods more difficult [52].

Interplay of microbial Metabolites and MSCs in allergy: future outcomes
Gut microbiome has a beneficial impact on the biological processes of host by conversion of complex fibre-rich food to highly relevant metabolites. However, modern diet and lifestyle has led to dysbiosis of the gut microbiome environment, which play a prominent role in the emergence of immune-mediated disorders including allergic diseases [53]. The primary metabolites generated by the gut microbiota are short-chain fatty acids (SCFAs), specifically acetate (C2), propionate (C3), and butyrate (C4) (respectively in a 60:20:20 ratio). SCFAs, produced by gut microbes by anaerobic fermentation of fibre-rich foods, have a multifaceted role in human metabolism, for instance regulation of hunger, lipid metabolism, and glucose homeostasis [54]- [56]. Further, G protein-coupled receptors (GPCRs) such as GPCR41 (also known as free fatty acid receptor-3 or FFAR3), GPCR43 (free fatty acid receptor-2 or FFAR2), GPCR109A and OR51E2 plays pivotal role in the SCFA-mediated signalling pathways of various cells and involved in the epigenetic modifications such as inhibition of histone deacetylase (HDAC) activity [54]. The binding of SCFAs on GPCRs are selective based on their length. GPCR43 prefers to bind with the shorter SCFAs such as acetate and propionate, whereas GPCR41 binds propionate, butyrate, and valerate preferentially with a lesser affinity for acetate. GPCRs signals through heterotrimeric G-proteins such as G s , G i / o , G q / 11 , or G 12 / 13 , however, the signalling mechanism occur upon the binding of SCFAs to GPCRs are poorly understood and needed more research attention in this specific area [57].

5.1.Short Chain Fatty Acids in Immunomodulation
Preclinical studies have reported that SCFAs activates DCs, which supresses Th2 activation and ameliorate allergic inflammation [58], [59]. A recent study reported that, acetate produced from a high-fibre diet by gut microbiome enhanced Treg cell mediated protection from asthma development in pregnant mice. Further, acetate induces FoxP3 promoter acetylation by inhibition of HDAC and enhance the number of Tregs and its function [60]. shows that the mechanism occurs independent of GPR41, GPR43 and peroxisome proliferator-activated receptors (PPAR) [61], [62]. Another study shows SCFA dietary intake leads to reduction in severe asthma symptoms, which was associated with the T cells and DCs in a vancomycin-treated mice. Vancomycin treatment depletes the level of SCFA level in mice [63]. Further, a study conducted by Theiler et al (2019), shows that SCFAs deplete human eosinophils, and that interestingly butyrate ameliorates allergen-induced lung and airway eosinophilia, and Th2 cytokines [64]. Figure 4 represents the mechanism of SCFAs on alleviation of allergic airway diseases. one year old infants, and in a followed-up study of these children at the age of 3 and 6 years respectively showed no signs in the development of atopic sensitization and asthma symptoms. Further, food allergies and allergic rhinitis diagnosis were less common in children with the highest butyrate levels. In addition, mice administered with SCFAs orally experienced a marked reduction in the intensity of allergic airway inflammation [66]. A cohort study of infants (6-24 months) shows lower levels of butyrate and valerate in transient AD subjects compared to healthy and persistent AD subjects [67]. Further, the direct role of butyrate in the regulation of mitochondrial metabolism of epidermal keratinocytes and the changes in the synthesis of essential structural elements has improved the function of skin barrier. Moreover, dietary fibres and SCFAs strengthen the epidermal barrier, which eventually prevents early allergic sensitivity and disease progression [68]. Different SCFAs and their role in the amelioration of allergic conditions are listed in Table 4.

Conclusion
In the recent past, allergic disorders such as allergic-asthma, -rhinitis, -skin diseases and food allergies have gained global public health interest. MSCs acts in tissue repair mechanism due to their ability to self-renew and differentiate. Further, multilineage differentiation potential and immune suppression functions of MSCs shows therapeutic potential in transplantation, tissue regeneration, and autoimmune disorders. Based on numerous studies, the emergence of MSC as a viable therapeutic option over the current treatment approaches might lead to substantial improvement in the various allergic disorders. In addition, human gut microbial metabolites like SCFAs play significant role in the maintenance of human metabolism and homeostasis, which might influence the proliferation and differentiation potential of MSCs.
Therefore, the combinatorial treatment of SCFA and MSCs might be a promising area of research for the development of better therapeutic option to allergic disordered. However, studies on therapeutic effectiveness of SCFAs and MSCs are limited. To conclude, evidencebased study for understanding the role of SCFAs and MSCs requires further investigation, followed by pharmacodynamic studies and subsequent clinical trials.