Fecal microbiota transplantation in allergic diseases

Abstract

Microorganisms such as bacteria, fungi, viruses, parasites living in the human intestine constitute the human intestinal microbiota. Dysbiosis refers to compositional and quantitative changes that negatively affect healthy gut microbiota. In recent years, with the demonstration that many diseases are associated with dysbiosis, treatment strategies targeting the correction of dysbiosis in the treatment of these diseases have begun to be investigated. Faecal microbiota transplantation (FMT) is the process of transferring faeces from a healthy donor to another recipient in order to restore the gut microbiota and provide a therapeutic benefit. FMT studies have gained popularity after probiotic, prebiotic, symbiotic studies in the treatment of dysbiosis and related diseases. FMT has emerged as a potential new therapy in the treatment of allergic diseases as it is associated with the maintenance of intestinal microbiota and immunological balance (T helper 1/T helper 2 cells) and thus suppression of allergic responses. In this article, the definition, application, safety and use of FMT in allergic diseases will be discussed with current data.

Key Words: Microbiota; Dysbiosis; Faecal microbiota transplantation; Allergic diseases

Core Tip: Fecal microbiota transplantation (FMT) studies have gained popularity after probiotic, prebiotic, symbiotic studies in the treatment of dysbiosis and related diseases. FMT is the process of transferring faeces from a healthy donor to another recipient in order to restore the gut microbiota and provide a therapeutic benefit.

INTRODUCTION

The community formed by microorganisms such as bacteria, fungi, viruses and parasites living in the human intestine is called intestinal microbiota. Bacteria dominate the intestinal microbiota, which is a complex ecosystem[1]. Firmicutes, Bacteroidetes, Actinobacteria, Proteobacteria are the 4 main phyla that constitute 90% of the entire bacteriome in the intestine[2]. The intestinal microbiota of an adult human consists of approximately 40 × 10¹² bacteria (approximately 0.2 kg)[3]. The composition of the human gut microbiota varies from region to region (anatomical part of the gut) and is highly dynamic. Differences such as mode of delivery (normal or caesarean section), postnatal food source (breast milk, formula), various diseases, diet, age, medications used, and gender are factors affecting the microbiota[4-7]. A healthy microbiota has immunomodulatory effects (increase in the number of T helper 1 and T regulatory cells) as well as effects such as protection of intestinal integrity by preventing invasion of pathogenic bacteria, synthesis of short-chain fatty acids (SCFAs) and vitamins required for intestinal metabolism, metabolism of drugs, hormones and carcinogens[8]. Compositional and functional disruption of the microbiota affected by many genetic and host-related factors is called dysbiosis[9]. In recent years, an increasing number of diseases (such as inflammatory bowel diseases, liver diseases, diabetes, atherosclerotic diseases, neurological diseases, autoimmune diseases, allergic diseases, psychiatric disorders) have been associated with dysbiosis[10-13]. Therefore, in recent years, many studies have been conducted to investigate the effect of microbiota and correction of dysbiosis in the treatment or prevention of these diseases. Probiotic studies constitute a large part of these studies.

Probiotics reduce intestinal permeability and systemic transmission of allergens by local action. Some of the systematic effects of probiotics are (Table 1): Induction of regulatory T cell (Treg) production, deflection of the response to allergens to T helper 1 (Th1) direction, and anti-inflammatory effect by Toll- like receptor stimulation[14]. In a systematic review conducted in 2014 in which 5 randomized controlled trial (RCT) were evaluated, it was found that probiotic use had no preventive role in allergic rhinitis (AR)[15]. Likewise, in the 2019 meta-analysis including 17 RCTs, it was observed that probiotic use in the perinatal and postnatal period was not effective in the prevention of AR[16]. According to a 2020 meta-analysis including 19 RCTs, taking probiotic supplements during pregnancy or early in life did not reduce the incidence of asthma or wheeze[17]. On the other hand, a meta-analysis by Chen et al[18] showed that the use of probiotics for respiratory allergies in children improved quality of life and reduced symptom severity[18]. In a study conducted by Amalia et al[19] in 2019, it was shown that a probiotic supplement mixture given to the mother while pregnant and continuing breastfeeding, as well as to the high-risk infant, is an effective strategy to reduce the risk of atopic dermatitis (AD) in children. Current studies with probiotics are not sufficient to routinely recommend the use of probiotics in the prevention and treatment of asthma, AR and AD. This may be explained by the heterogeneity of the studies in terms of factors such as probiotic type, duration of administration and dose.

Table 1 Potential roles of probiotics and fecal microbiota transplantation in alleviating or preventing allergic diseases[99].

Probiotics and FMT	Potential roles
Probiotics	Epithelial integrity ↑
	Tight junction protein expression ↑
	Bacterial translocation ↓
	Inflammatory cytokines such as IL-6, IL-8 and TNF-α ↓
	Restoration of Th1/Th2 balance
	Treg cell numbers and function, IL-10 and TGF-β ↑
	SCFAs production (especially butyrate) ↑
FMT	Pro-inflammatory cytokine secretion ↓
	Restoration of Th1/Th2 balance
	Stimulates T cell activity and leukocyte adhesion
	Change in intestinal bacterial diversity (Firmicutes, Bacteroidetes ↑/Proteobacteria, Actinobacteria ↓)
	Intestinal permeability ↓
	Epithelial integrity ↑
	SCFAs production ↑
	Intestinal pH ↓
	Bacterial adhesion to H2O2 ↑

SCFA: Short chain fatty acid; FMT: Fecal microbiota transplantation; TNF-α: Tumor necrosis factor-alpha; IFN-γ: Interferon-gamma; IL: Interleukin.

On the other hand, FMT is thought to be more effective than probiotics in the restoration of altered intestinal microbiota. The reasons for this are that the number and diversity of microorganisms provided by FMT is higher than that provided by probiotics, and while there is a permanent change in the recipient microbiota after FMT, probiotics can colonize the intestinal lumen only temporarily[20-22]. In this article, FMT which is promising in the treatment of allergic diseases and the effect of FMT in allergic diseases will be discussed.

Faecal microbiota transplantation

Faecal microbiota transplantation (FMT) is the process of transferring faeces from a healthy donor into the gastrointestinal tract of the recipient in order to repair the intestinal microbiota and provide a therapeutic benefit[23]. The first FMT application was performed in China and dates back to 300-400 AD. The first applications were recorded as oral administration of human faecal suspension to patients with food poisoning or severe diarrhoea[24]. The first introduction of FMT into the medical literature was in 1958 reported FMT administered via enema as a highly effective treatment in the treatment of severe pseudomembranous enterocolitis[25]. In 2013, in a randomized controlled trial conducted by van Nood et al[26], FMT was found to be 81% successful in the treatment of recurrent C. difficile (rCDI) infection compared to 31% with standard antibiotic treatment and the Food and Drug Administration (FDA) approved the use of FMT in humans in the same year[26,27]. This significant success of FMT in the treatment of rCDI has created great excitement for its use in the treatment of other diseases associated with dysbiosis.

Potential effect mechanisms of FMT in allergic diseases

The main objective is to rebuild the gut microbiota by normalizing immune and inflammatory responses, the amount and activity of neurotransmitters/vasoactive substances and intestinal energy metabolism. This main objective is realized through the following mechanisms (Table 1): (1) The main goal of FMT is to restore the “normal” bacterial population in a dysbiotic colonic environment[25]; (2) A change in the bacterial population in the intestine occurs that mirrors the donor feces[25]; (3) An increase in Firmicutes and Bacteroidetes and a decrease in Proteobacteria and Actinobacteria[25]; (4) It may reduce intestinal permeability and maintain the integrity of the epithelial barrier by increasing the production of SCFAs[23]; (5) It inhibits the secretion of proinflammatory cytokines[23]; (6) It promotes Th1 cell differentiation, T cell activity, leukocyte adhesion and immunostimulating factors[23]; and (7) It lowers intestinal pH and inhibits the transport of pathogenic microorganisms by increasing bacterial adhesion to H₂O₂[23].

FMT procedure, safety and side effects

There is no standard protocol for the preparation of FMT. The recommended and commonly used method of FMT preparation for rCDI is as follows: 50-60 g of faeces and 200-300 mL of diluent (water, saline or milk) are homogenized and suspended. The suspension is left to settle for 5 minutes. Then it is passed through gauze and then through a syringe with filter. The resulting filtrate is ready for FMT[28]. There are routes of administration of FMT such as oral administration in the form of capsules, nasal administration, administration into the upper gastrointestinal tract with the help of nasogastric or nasoduodenal tube, rectal administration via colonoscopy or enema[29].

Although FMT is generally considered safe, the adverse event rate was reported as 28.5% in a systematic review. However, most of these side effects were self-limiting and included abdominal pain, gas, increased stool frequency, vomiting and fever. Serious adverse events were more rare and occurred in 5%. Although more serious side effects such as aspiration and intestinal perforation have been reported after FMT, these risks are mostly related to the route of administration[30]. Administration of FMT via capsules is a less invasive approach and can be used to overcome complications related with the route of administration[31,32]. FMT also has reported side effects including sepsis, peritonitis and toxic megacolon[30]. To prevent disease transmission from donor to recipient, it should be ensured that the donor is completely healthy. The donor is examined for the presence of human immunodeficiency virus, syphilis, hepatitis A, B, C, autoimmune and atopic diseases. The donor's faecal material is also examined for the presence of parasites, helminth eggs and pathogenic bacterial toxins. The donor should be free of tumors, inflammation, diabetes, infectious diseases and metabolic syndrome and the donor should not be obese. In addition, the donor should not be using immunosuppressants, steroids, probiotics, aspirin, proton pump inhibitors and antibiotics[33]. FDA has recently recommended additional donor screening and testing protocols to reduce the risk of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and monkeypox transmission with FMT[34].

FMT in disease treatments

As mentioned above, FMT is a method applied to restore the gut microbiota and provide a therapeutic benefit and dates back to 300-400 AD and China[23]. The first clinical applications were oral administration of human fecal suspension to patients with food poisoning or severe diarrhea[24]. Studies conducted in the last decade have demonstrated the potential efficacy of FMT in conditions such as ulcerative colitis (UC), Crohn's disease, epilepsy, autism and recurrent urinary tract infections (UTIs)[20-24].

FMT has also been proven to be highly effective in the treatment of rCDI (approximately 90% cure rate) and FMT has become an undisputed treatment modality for rCDI[35,36]. A recent systematic review revealed that FMT can treat 85 specific diseases. When evaluated according to the number of cases treated using FMT, the diseases such as intestinal infections (rDCI), UC, irritable bowel syndrome, constipation, hypertension, fatty liver disease, autism spectrum disorders, radiation enteritis are the leading diseases[35]. In addition to these diseases, FMT has also been found to be effective in diseases such as hepatic encephalopathy, epilepsy, depression, metabolic syndrome, obesity, primary sclerosing cholangitis, anorexia and recurrent UTI[37-45]. Various animal studies and human case reports for Parkinson's disease, Alzheimer's disease, multiple sclerosis and stroke show the positive effect of FMT in these diseases[46-50].

FMT in allergy and allergic diseases

The increase in the prevalence of allergic diseases in modern societies over the last 50-60 years is remarkable. This allergy pandemic is explained by 2 hypotheses: Hygiene hypothesis[51] and microbiota hypothesis[52]. T helper 2 (Th 2) response is dominant in humans during pregnancy and the first years of life[53]. Microbial agents encountered in the early period of life cause interleukin (IL) 12 production from macrophages, the innate cells of the immune system. IL-12 plays a key role in the progression of the immune system towards Th 1. According to the hygiene hypothesis, if IL-12 production does not occur in the early stages of childhood, Th 2 cells will predominate and as a result, atopy will develop in genetically predisposed children[54]. A healthy gut microbiota helps to maintain intestinal integrity by preventing the invasion of pathogenic bacteria and to prevent the development of allergies through immunomodulatory effects (increase in the number of Th 1 and Treg cells)[8]. Today, factors such as excessive use of antibiotics, consumption of more germ-free foods, better personal hygiene, and smaller family size are held responsible for changes in intestinal microbiota and allergy risk[55-57]. Many studies have shown that traditional agriculture has a preventive effect on the prevalence and incidence of asthma in childhood; exposure to farm animals and their feed and consumption of unpasteurized cow's milk have a protective effect on asthma. Higher microbial exposure is hypothesized to have a protective effect on allergy development[58,59]. Decreased microbiotal diversity in the gastrointestinal tract has been found to be associated with diseases such as atopy, asthma and eczema[60-63]. For all these reasons, the effect of FMT in allergic diseases has started to be investigated more recently (Table 2). In the following sections of this article, the use of FMT in allergic diseases will be emphasized.

Table 2 Impact of fecal microbiota transplantation on allergic diseases.

Disease	Species	Main result	Ref.
Food allergy	Mice	Food allergy protection was achieved in food allergy-prone recipient mice receiving FMT from a healthy donor	[69]
	Mice	It has been shown that recipient germ-free mice after FMT from healthy and cow's milk allergy-free donors are protected against anaphylactic reactions to cow's milk allergen	[70]
	Human	Phase 1 and phase 2 study investigating the efficacy of oral capsule FMT in people with peanut allergy is ongoing	[71,72]
Eosinophilic gastritis	Human	Combination of FMT and steroid therapy in a patient with steroid-resistant eosinophilic gastritis improved the patient's symptoms	[76]
Allergic proctocolitis (FPIAP)	Human	After 2 days after administration of FMT via rectal tube to 19 infants with FPIAP, 17 infants showed symptomatic regression	[80]
Atopic dermatitis	Mice	In recipient mice after FMT administered from non-atopic dermatitis patients to atopic dermatitis patients: SCFA ↑, Th1 cytokines (IL-12, IFN-γ, TNF-α) ↑, Th2 cytokines (IL-4, IL-5, IL-13) ↓, Total IgE ↓, SCORAD ↓	[86,87]
	Human	After FMT in patients with moderate to severe AD: SCORAD ↓, need for topical corticosteroids ↓	[88]
Allergic rhinitis	Mice	In recipient mice after FMT: nasal symptoms ↓, Total IgE ↓, histopathological recovery (decrease in eosinophilia)	[91]

SCFA: Short chain fatty acid; FMT: Fecal microbiota transplantation; FPIAP: Food protein-induced allergic proctocolitis; SCORAD: Scoring atopic dermatitis; TNF-α: Tumor necrosis factor-alpha; IFN-γ: Interferon-gamma; IL: Interleukin.

Food allergy and FMT

Low microbial diversity in the intestines and high Enterobacteriaceae/Bacteroidaceae ratio have been associated with the development of food sensitivity in children[64]. Similarly, the risk of allergic diseases was found to be higher in children born after caesarean section who did not receive vaginal microbiota from their mothers[65]. In the absence of healthy microbiota, the decrease in SCFA, which are also involved in the recovery of epithelial barrier integrity, will lead to increased sensitivity to food allergens[66,67]. The first studies with germ-free mice showed the importance of gut microbiota in modulating food allergy and revealed that germ-free mice could not develop tolerance against food allergens[68]. Recent studies with germ-free mice demonstrated the importance of gut microbiota in modulating food allergy and revealed that germ-free mice without healthy gut microbiota could not develop tolerance to food allergens[69,70]. These findings obtained with FMT in mouse models have formed the basis of new studies to be conducted in humans. A Phase 1 study with 10 adult subjects to monitor the safety and tolerability of oral capsule FMT administered for 2 days in the treatment of peanut allergy has been completed but the data have not yet been published[71]. A phase II randomized double-blind placebo-controlled phase II randomized double-blind placebo-controlled study to evaluate the safety and tolerability of oral encapsulated FMT in 24 patients with peanut allergy is still ongoing[72].

Eosinophilic gastroenteritis, allergic colitis and FMT

Eosinophilic gastrointestinal disorders are a group of rare disorders characterized by pathological eosinophilic infiltration of the gastrointestinal tract with symptoms such as dysphagia, abdominal pain, nausea, vomiting, early satiety, diarrhoea and weight loss. These Th 2 -mediated disorders include eosinophilic oesophagitis, eosinophilic gastritis (EG), eosinophilic gastroenteritis (EGE), eosinophilic enteritis (EE) and eosinophilic colitis. Symptoms of the disease vary according to the site of involvement and depth of involvement (mucosa, submucosa, serosa)[73-75]. Although there is no consensus on the treatment of EGE so far, steroid treatment forms the basis of treatment. In the literature, there is a case report in which the combination of FMT and steroid treatment improved the symptoms of a 35-year-old patient with EG who did not respond to steroid treatment alone[76].

Allergic colitis, also known as food protein-induced allergic proctocolitis (FPIAP), is a clinical entity characterized by inflammatory changes developing in the distal colon in response to one or more food proteins. Although the underlying mechanism in allergic colitis is not known, IgE is thought not to play a role[77]. Symptoms in infants with FPIAP usually start in the first months of life. Patients present with red blood and mucus mixed with stools with or without diarrhoea[78]. The most common trigger for FPIAP is cow's milk and elimination diet, which involves the removal of the responsible food from the diet, is the mainstay of treatment of allergic enteritis[79]. In a 2017 study, 19 infants with proctocolitis who had severe diarrhoea/haematochezia and who did not recover completely with routine treatment were treated with FMT via rectal tube and clinical results were followed up. In 17 infants, allergic colitis symptoms were alleviated within 2 days after FMT and no recurrence was observed during the following 15 months. According to 16S rDNA analysis performed in 10 of these babies, an increase in microbiota diversity was observed in most of these babies after FMT[80].

AD and FMT

AD (eczema) is a condition that causes dry, itchy and inflamed skin. Atopic dermatitis is one of the chronic inflammatory skin diseases affecting 15%-30% of children and 10% of adults in which impaired barrier function, immune response and microbial factors play a role in its pathogenesis[81,82]. In recent years, as in many diseases, the treatment of AD has focused on gut microbiota in relation to immune modulation. Different results have been reported in various studies investigating the effects of probiotic treatment on AD and the efficacy of probiotics in AD treatment has not been proven[83-85]. In recent years, studies on the effects of FMT in the treatment of AD have started to be carried out because of the reconstitution of the intestinal microbiota and the long-term change of the recipient microbiome. In a study conducted with mice with and without AD, an increase in SCFA levels was found in recipient mice after FMT. As a result of the measurement of cytokine levels before and after FMT in mice with AD, Th-2 cytokines (IL-4, IL-5, IL-13) decreased and Th1 cytokines (IL-12, IFN-γ and TNF-α) increased, total IgE level decreased and dermatitis scores decreased after FMT[86]. Similar results were found in another mouse study conducted in 2023[87]. In a human study conducted with 9 patients with moderate to severe AD, a significant improvement in AD SCORAD (SCORing Atopic Dermatitis) scores and a decrease in the frequency of weekly topical corticosteroid use were observed in 7 patients after FMT. No side effects were reported during the study[88]. These studies show that FMT may be effective through immune modulation by gut microbiota and are promising for the future of AD treatment, but more clinical studies are needed.

AR and FMT

AR is an IgE-mediated inflammatory disease of the upper airway induced by inhaled allergens affecting 10-30% of the world population[89,90]. In an experimental mouse model investigating the effect of FMT on AR, it was found that nasal symptoms were significantly alleviated in recipient mice after transplantation of normal mouse faecal microbiota into AR mice. In contrast, it was observed that AR symptoms developed in healthy mice that received FMT from allergic mice. In the study, it was found that total IgE levels decreased in AR mice treated with FMT. In addition, histopathological evaluation of the nasal mucosa was performed after FMT and a decrease in eosinophil levels was observed in the recipient group after FMT[91]. Although there is no human study on the effect of FMT in the treatment of AR, improvement in AR symptoms was found incidentally in a patient with AR who received FMT for UC and reported on a case basis[92]. These studies show promise that FMT may be an effective treatment in the treatment of AR.

Asthma and FMT

Asthma is one of the most common chronic diseases thought to affect more than 300 million people worldwide[93]. Asthma is related with the predominance of the immune response in the Th2 direction and its etiology has not yet been fully elucidated. Genetic and environmental factors are also thought to contribute to the pathogenesis of asthma[94,95]. Intestinal microbial triggers are an important environmental factor associated with asthma. Recent studies have shown that gut microbiota plays a role in the etiology of asthma through its effects on innate and adaptive immunity[96,97]. In line with this information, FMT represents a possible treatment modality to improve asthma. However, scientific studies in this field have only recently begun[98].

CONCLUSION

After the high success rate of FMT, especially in the treatment of rCDI, and the increase in data proving the relationship between disorders in the intestinal microbiota and many diseases in recent years, the number of studies investigating the effects of FMT in these diseases has increased. Until this time, studies targeting microbiota in disease treatment have mostly been probiotic studies. The most important limitation of probiotic treatment in the treatment of diseases is that the variety of microorganisms given in probiotic treatment is limited and the duration of action is short. FMT is thought to be more successful in the treatment of diseases because it contains more and various microorganisms and can provide a more permanent microbial environment. Studies on the efficacy of FMT in the treatment of allergic diseases have only recently started and the first results are promising for the use of FMT in the treatment of allergic diseases in the future. However, more clinical studies are needed to recommend the routine use of FMT in the treatment of diseases.