Oral tolerance to dietary antigens and Foxp3⁺ regulatory T cells

Summary

Immune tolerance to foods develops in the intestine upon food ingestion and is essential to prevent IgE mediated food allergy and gut inflammation. In homeostasis, the intestine is a tolerogenic environment that favors the formation of food specific Foxp3⁺ regulatory T cells. A tolerogenic intestinal environment depends on colonization by diverse microbiota and exposure to solid foods at a critical period in early life. These early immune responses lead to the induction of antigen-specific Foxp3⁺ regulatory T cells in draining mesenteric lymph nodes. These peripherally induced regulatory cells circulate and seed the lamina propria of the gut, exerting suppressive function systemically and locally in the intestine. Successful establishment of a tolerogenic intestinal environment in early life sets the stage for oral tolerance to new antigens in adult life.

1. INTRODUCTION

The intestinal mucosa is constantly exposed to external antigens, including environmental and dietary components and the microbiota. This ongoing exposure requires the immune system to accurately differentiate between harmful and harmless antigens. The immune system must effectively defend against pathogens while tolerating benign antigens like foods and commensal microbes. Oral tolerance is a key mechanism by which the intestinal mucosa achieves this delicate balance, ensuring tissue homeostasis. If oral tolerance fails, immune reactions to food or microbes can result in food allergies or intestinal inflammation. The modern lifestyle is believed to be responsible for changes in microbial exposure that have contributed to an increase in the incidence of food allergies.¹

2. ORAL TOLERANCE: THE PHYSIOLOGICAL PHENOMENON AND HISTORICAL ASPECTS

Oral tolerance to foods is a fundamental immunological process that begins in the intestinal mucosa upon ingesting dietary antigens and has systemic effects (Figure 1). It is vital for preventing hypersensitivity reactions to food and maintaining a balanced immune system in the gut.^2,3 Oral tolerance can suppress both humoral and cellular immune responses, reducing serum antibodies and inflammation.⁴ Since most encounters with foreign antigens occur through mucosal surfaces, oral tolerance constitutes an essential immunological process occurring in the gastrointestinal tract under homeostasis.

FIGURE 1.

Oral tolerance to foods. After food ingestion (1) food proteins access the lamina propria of the small intestine (2) where they are captured by migratory dendritic cells (DC). DC migrate to mesenteric lymph nodes (LN) (3) where they mediate the differentiation of food specific CD4 T cells into Foxp3⁺ pTregs in a TGFβ and retinoic acid dependent manner (4). Newly formed pTregs acquire homing receptors α4β7 and CCR9/CCR10 and migrate to the small intestinal lamina propria (5), where they proliferate (6). Food specific Foxp3⁺ Tregs disseminate systemically through the efferent lymphatic system and blood circulation, reaching other lymphoid organs and tissues.

Oral tolerance was observed by Wells and Osborne in 1911. Their research demonstrated that guinea pigs fed with a diet containing corn were protected from anaphylactic reactions to zein, the principal protein in corn.⁵ Later, it was reported that a similar suppression of the immune response occurred in experimental animals exposed orally to various proteins, including horse serum, pollen,⁶ bovine albumin,⁷ sheep red blood cells,⁸ chicken ovalbumin (OVA), and other potent immunogens.

Vaz and colleagues and Hanson and colleagues demonstrated that adult mice could develop strong and long-lasting immunological tolerance that prevented specific IgE formation and food allergy after just one exposure to OVA.^9,10 It was also shown that oral antigen administration induced an IgA response.^11–13

Classical experiments on oral tolerance conducted in the ‘80s and ‘90s evaluated the effectiveness of oral tolerance to suppress subsequent inflammation triggered by immunization with the same antigen along with adjuvants in mouse models of autoimmune diseases such as type II collagen-induced arthritis^14,15 and experimental autoimmune encephalomyelitis.^16,17 Studies by van Hoogstraten and collaborators,¹⁸, Haneda and collaborators,¹⁹ and Russo and collaborators,²⁰ showed that oral administration of allergens induced persistent immune tolerance, mitigating subsequent allergic contact hypersensitivity¹⁸ and airway eosinophilic inflammation.^19,20

The degree of systemic suppression after oral antigen feeding positively correlated with both the duration of the treatment and the dosage of oral antigen.^12,21 In addition, continuous feeding in the drinking water was more effective than administration of antigen by gavage once a day.^22,23

3. CD4⁺ T CELLS MEDIATE ORAL TOLERANCE

Adoptive cell transfer and cell depletion experiments demonstrated that oral tolerance is a dominant phenomenon mediated by CD4⁺ T cells. Chen and colleagues found that both CD4⁺ and CD8⁺ cells from animals orally tolerized to myelin basic protein (MBP), could adoptively transfer tolerance and protect recipient animals from autoimmune encephalomyelitis; however, CD8⁺ cells were not necessary for the induction of tolerance in the donor mice.²⁴ T cells isolated from the mesenteric lymph nodes of tolerant mice had a regulatory phenotype, secreting molecules like TGFβ, IL-4, and IL-10. Importantly, the suppressor activity of these T cells was neutralized by anti-TGFβ neutralizing antibodies, underscoring the critical role of TGFβ in oral tolerance establishment.²⁴ In a model of allergic eosinophilia, the transfer of splenic CD4⁺ T cells from tolerant mice protected recipient mice from tracheal eosinophilia. Moreover, the neutralization of TGFβ but not IFNγ, abrogated tolerance in the recipient mice, indicating that the adoptively transferred CD4⁺ T cells mediated tolerance through TGFβ rather than through a Th1 mechanism.¹⁹ Other investigators also demonstrated that tolerance could be transferred by CD4⁺ T cells from orally tolerized mice.^25,26

Mechanistically, oral tolerance has been associated with clonal deletion of antigen-specific T cells, anergy, and the induction of regulatory T cells.^3,27

4. ORAL TOLERANCE DEPENDS ON THE INDUCTION OF ANTIGEN-SPECIFIC FOXP3⁺ TREG CELLS

Regulatory T cells, also referred to as suppressor cells, first described almost 30 years ago, are cells that play a crucial role in maintaining immune tolerance to self and down-regulating immune responses to both self and non-self-antigens. CD25 was identified by S. Sakaguchi and colleagues as a main marker of regulatory T cells (Tregs).²⁸ The transcription factor Foxp3 (forkhead box P3) was later shown to be the master regulator of Treg differentiation and function.^29–31 Human deficiency in Foxp3 results in immune dysregulation, polyendocrinopathy, enteropathy, X-linked (IPEX) syndrome, which in associated with autoimmunity and inflammation,^32–34 as well as with severe immuneallergies, including food allergy.³⁵

The cytokine IL-2 and its receptors are essential for maintaining immune homeostasis and preventing inflammation. Although IL-2 is not essential for thymic Foxp3 Treg development,³⁶ it is essential for Treg survival and function.³⁷ Activated conventional CD4 T cells secrete IL-2 that supports the survival and function of CD25⁺ Treg cells.^36–38

While most Foxp3⁺ Tregs were initially thought to be of thymic origin, it was later demonstrated that conventional CD4 T cells could become Foxp3⁺ Treg cells in peripheral tissues. Naïve CD25-CD4⁺ T cells adoptively transferred into lymphopenic adult RAG1 KO mice or wild type neonatal mice generated CD25⁺Foxp3⁺ Tregs by peripheral expansion.³⁶ It is now well accepted that Foxp3⁺ Tregs can be of thymic or extra-thymic origin. Thymic-derived Tregs, referred to as nTregs or tTregs, are induced by self-antigens and have a major role in preventing autoimmunity and overt inflammation.³⁹ Peripherally induced Tregs (referred to as iTregs or pTregs) are induced by the microbiota and by environmental and food antigens. They are essential to establish tolerance and homeostasis at mucosal tissues such as the gastrointestinal and respiratory tracts.^38,40 Peripheral Tregs are also induced in inflamed tissues, where they function to control overt inflammation.^41,42

Our early work in the 2000s contributed to elucidating the role of antigen-specific peripherally induced Foxp3⁺ Tregs in oral tolerance. We conducted a series of experiments using the TBmc mice, which carry mono-specific non-self-reactive populations of T and B lymphocytes. This unique experimental model involves mice that expressed DO11.10 TCR transgenes specific for OVA, and V(D)J immunoglobulin knock-in genes specific for the hemagglutinin of influenza virus, in a RAG1 knock-out (KO) background.⁴³ TBmc mice lack thymus derived Tregs yet they can form Foxp3⁺ Tregs in peripheral tissues.⁴⁴

Feeding TBmc mice with OVA in the drinking water induced the differentiation of CD25⁺ Foxp3⁺ OVA-specific CD4 T cells and led to the establishment of tolerance. The OVA-fed TBmc mice were protected from inflammation caused by a subsequent intraperitoneal immunization with OVA in alum adjuvant. In contrast, TBmc mice that had not been previously fed with OVA became sensitized after immunization with antigen and adjuvant, developing a Th2 response, IgE antibody production, and allergic inflammation⁴⁴. The generation of the CD25⁺Foxp3⁺ Tregs and the induction of tolerance were largely dependent on the cytokine TGFβ and independent of IL-10. CD25⁺ Tregs induced by oral antigen suppressed the proliferation of CD25^-CD4⁺ T cells in vitro, and prevented the induction of an allergic response in vivo.⁴⁴

Strikingly, TBmc mice carrying the scurfy Foxp3 mutation, which encodes a truncated non-functional protein,⁴⁵ failed to develop functional antigen-specific pTregs afer oral OVA administration, and were susceptible to develop allergic inflammation when immunized with OVA in alum. This findings demonstrated the essential role of Foxp3⁺ in peripheral Treg induction and mucosal tolerance.⁴¹ Interestingly, feeding OVA to TBmc scurfy mice did not by itself induced an inflammatory response, indicating that the antigen-presenting cells involved did not have the ability to induce effector T cells.⁴¹ Consistent with the need of Foxp3⁺ Tregs for oral tolerance, deletion of Foxp3⁺ Tregs by diphtheria toxin treatment in mice expressing the diphtheria toxin receptor under the Foxp3 transcriptional control, suppressed the establishment of oral tolerance in a model of food allergy.⁴⁶

TGFβ was identified as a key differentiation factor for pTregs both in vitro⁴⁷ and in vivo.⁴⁴ The CNS1 enhancer site in the Foxp3 promoter responds to TGFβ signaling and it was found to be necessary for pTreg differentiation but not thymic Treg differentiation.⁴⁸ Mice with Treg specific deletion of CNS1 developed spontaneous type 2 inflammation in respiratory and gastrointestinal mucosal tissues.⁴⁸ Notably, thymic derived nTregs could be distinguished from mucosal induced pTregs by their expression of Helios,⁴⁹ and Neuropilin 1 (Nrp1).^42,50 On the other hand, the expression of Dapl1 and Igfbp4 was higher in pTreg than tTreg.⁴² Nrp1 can be expressed by tissue pTregs under inflammatory conditions, such as in allergic inflammation in the lung, where its upregulation is induced by TGFβ.⁴²

A critical region within the Foxp3 locus, known as the Treg-specific demethylated region (TSDR or CNS2), harbors evolutionary conserved elements crucial for Treg stability. Tregs generated in vitro exhibit higher TSDR methylation, leading to lower stability than their in vivo counterparts.⁵¹ Consistently TSDR demethylation hallmarks the majority of peripheral Tregs.⁵² OVA-specific pTregs induced by oral tolerance, which effectively prevent allergic development, display a demethylated TSDR, indicative of phenotype stability.⁴²

Other regulatory T cell populations described to play a role in oral tolerance are Th3 and Tr1 cells. Th3 cells, characterized as surface LAP⁺ in an autoimmune model, were induced following antigen ingestion or through oral administration of anti-CD3 antibodies.^53,54 The relationship between LAP⁺ regulatory cells and Foxp3⁺ Tregs is not completely clear. LAP⁺ regulatory T cells can express or not Foxp3.⁵⁵ Moreover, Th3 cells can induce other CD4 T cells to express Foxp3⁺ through TGFβ production.⁵⁶ Tr1 cells have been characterized as Foxp3 negative regulatory CD4⁺ T cells that produce TGFβ and IL-10. Tr1 cells can be induced in vitro by IL-10, IFNα and by immature dendritic cells.⁵⁷

A study by Hong and collaborators used MHCII tetramers to identify gliadin-specific CD4 T cells after oral administration of gliadin peptide. They described several populations of anergic/dysfunctional T cells in addition to Foxp3⁺Tregs.⁵⁸ Notably, some of these dysfunctional CD4 T cells could become Foxp3⁺ Treg cells upon activation.⁵⁸ Investigating intestinal CD4+ T cells induced by dietary antigens, Lockhart and collaborators described clonally expanded conventional and Foxp3⁺ Treg cells in the lamina propria and epithelium, suggesting that both environments contribute to tolerance to foods.⁵⁹

5. FOOD-SPECIFIC TREGS ARE INDUCED IN MESENTERIC LYMPH NODES BY SPECIALIZED ANTIGEN-PRESENTING CELLS

To induce tolerance, the antigens present in the intestinal lumen must penetrate the mucosal barrier and access the lamina propria of the intestine, where they interact with cells of the immune system (Figure 1). It is well known that food proteins undergo degradation in the gastrointestinal tract during the digestive process. However, intact protein absorption is also part of a physiologically normal process.⁶⁰ The transport of food antigens across the small intestine epithelium during homeostasis can occur through microfold cells (M cells), goblet cells, paracellular leak, and antigen-presenting cells uptake through transepithelial dendrites.⁶¹ The transport of cell-bound antigen via afferent lymphatic vessels to draining mesenteric lymph nodes is essential for inducing oral tolerance. Both CCR7 deficiency, which impairs antigen presenting cell migration, and mesenteric lymphadenectomy inhibit the induction of oral tolerance.⁶² After capturing the antigen in the lamina propria of the gut, CD103⁺ dendritic cells migrate to the mesenteric lymph nodes, where they trigger the development of antigen specific Tregs in a TGFβ and retinoic acid dependent manner.^44,63–65 Newly formed Tregs home to the lamina propria of the gut, where they undergo local expansion promoted by IL-10 secreting CX3CR1+ resident macrophages.⁴⁶

Two antigen delivery pathways favor antigen uptake by CD103⁺ dendritic cells in the intestinal lamina propria and are particularly important for establishing oral tolerance. The goblet cell-associated antigen passages (GAPs) preferentially deliver antigens from the intestinal lumen to underlying CD103⁺ dendritic cells in the small intestine lamina propria.⁶⁶ CX3CR1⁺ macrophages were found to take up soluble fed antigens and quickly transfer them to CD103⁺ dendritic cells through gap junctions, and this process was required to induce tolerance to food allergens.⁶⁷

Esterhazy and collaborators used myeloid lineage specific antigen presenting cells (APC) depletion to identify the APC population responsible for inducing the differentiation of Tregs specific for food antigens. They found that CD11b negative, pre-DC-derived classical dendritic cells (cDC), and not CX3CR1⁺ monocyte derived APC, were required to induce pTregs and oral tolerance.⁶⁸ Among cDC, IRF8-dependent cDCs were the main inducers of pTregs.⁶⁸ In a still unpublished study, Canesso, Mucida and collaborators (BioRxiv, 2022) used an intestine adapted version of Labeling Immune Partnerships by SorTagging Intercellular Contacts (LIPSTIC), a method to identify T cell-dendritic cell partners,⁶⁹ to identify dendritic cells presenting antigen in the context of oral tolerance or infection. Combining LIPSTIC with single cell transcriptomics and functional analysis, they demonstrated that, under homeostatic conditions, migratory conventional dendritic cells 1 (cDC1) are the population that induced allergen-specific Tregs. T cell -interacting cDC1 expressed genes associated with pTreg induction, such as Aldh1a2 (RALDH2), Tgfb2 (cytokine TGF-β2), Itgb8 (integrin β8), Ncoa7 and Sod1.

In addition to the conventional DC described above, various populations of RORγt⁺ APC cells have been recently described to play a prominent role in mucosal tolerance to the intestinal microbiota.^70,71 The Sonnenberg group described MHCII⁺ LTi-like ILC3s with antigen-presenting capability.⁷² These ILC3s were present in barrier tissues and lymph nodes in early life and adulthood. They mediated the differentiation of RORγt⁺ pTregs and restrained antigen specific Th1, Th17 and Th2 responses.⁷³ Intestinal lamina propria RORγt⁺ APCs migrated to the colonic mesenteric lymph nodes, where they promoted the differentiation of microbe-specific RORγt⁺ Tregs.⁷⁴ RORγt⁺ APC with characteristics of medular epithelial dendritic cells (mTECs) and dendritic cells, referred to as Thetis cells, were found in early-life in lymph nodes and declined by weaning time. They mediated the differentiation of microbe-specific Tregs in early life.⁷⁵

In spite of the importance of the microbiota for the development of tolerance to foods, there are differences in phenotype, LN induction, and lamina propria localization of diet induced Tregs and microbiota-induced Tregs (Figure 2). Esterhazy and collaborators found that pTregs specific for orally fed OVA developed preferentially in the duodenum draining lymph nodes, following an ascending proximal to distal gradient.⁷⁶ Consistent with this, diet induced pTregs were preferentially found in the small intestine lamina propria and were by largely RORγt negative.⁷⁷ In contrast, microbiota-induced Tregs were RORγt⁺ and were found predominantly in the colonic lamina propria, and at a much lower frequency in the small intestinal lamina propria.^77–79

FIGURE 2.

Differences between food specific and microbe specific pTregs. Dietary antigens presented by CD103⁺ classical dendritic cells 1 (cDC1) to antigen specific CD4⁺ T cells, mediate the formation of Foxp3⁺ RORgt^- pTregs in duodenal draining lymph nodes. The food specific pTregs migrate to the small intestine lamina propria where they undergo expansion. Microbe induced Foxp3⁺RORγt⁺ pTregs are formed in the colon draining lymph nodes through antigen-presentation by RORγt⁺ antigen presenting cells (APC). These MHCII⁺RORγt⁺ APC include ILC3s, AIRE⁺ APCs, and DC-like APC referred. Microbe induced pTregs preferentially home to the colonic lamina propria.

In sum, intestinal tolerance requires the induction of microbe specific and food specific Foxp3⁺ Tregs with distinct intestinal inductive and homing sites. Classical CD11c⁺ dendritic cells and RORγt⁺ antigen-presenting cells mediate the induction of Foxp3⁺ Tregs in mesenteric lymph nodes. The functional relationships between these antigen-presenting cell populations, their specificity for induction of microbe versus food specific Tregs, and the spatiotemporal regulation of these populations still needs to be elucidated.

6. THE INTESTINAL MICROBIOTA, THE WEANING REACTION, AND TOLERANCE TO FOODS

Colonization of the gut by microbes is essential for immune homeostasis and prevention of intestinal inflammation.⁸⁰ This is evident in the increased susceptibility to allergic diseases in germ-free mice and mice with low-diversity microbiota.^81,82 Gut microbes induce the differentiation of RORγt⁺ Tregs that localize predominantly to the colonic lamina propria.^77–79 These RORγt⁺ Tregs are absent in the colon of germ-free mice, and inactivation of RORγt⁺ in Foxp3⁺ cells leads to the disappearance of the Helios negative Treg population in the colonic lamina propria of mice in special pathogen free (SPF) facility.^78,79

A critical time for bacterial colonization and intestinal Treg formation occurs around weaning,^78,79 when mice are exposed to solid food and there is an expansion and diversification of the gut microbiota.⁸³ The ‘weaning reaction’ (Figure 3) occurs in a narrow interval of time and is associated with an acute production of cytokines in the gut and the induction of RORγt⁺ Tregs by the microbiota.⁸³ At this time, the transition to solid food also leads to the development of food-specific Tregs in the small intestine.⁷⁷ The weaning reaction is regulated by epidermal growth factor (EGF) in breast milk of neonates. High levels of EGF in the first days of life inhibit GAP formation, preventing an early response to the microbiota.⁸⁴ The weaning reaction can be inhibited by prolonging a milk-based diet or by antibiotic treatment. This leads to increased susceptibility to colitis, allergic inflammation, and cancer later in life.⁸³

FIGURE 3.

Weaning reaction and the intestinal immune system maturation. Neonates have low competence to establish mucosal tolerance due to their immature gut immune system, low levels of vitamin A and maternal breast milk factors such as EGF. Declining levels of EGF in the second and third week of life, increased microbial colonization and exposure to solid foods trigger an intense albeit transient immune activation known as the weaning reaction. The weaning reaction is associated with the formation of food and microbiota induced pTregs. This process is crucial for establishing intestinal homeostasis and creating a tolerogenic environment that prevents food allergies and intestinal inflammation.

Consistent with the idea of a critical period for tolerance around weaning time, feeding neonate mice during the first week of age with OVA did not induced tolerance to OVA, while feeding OVA during the 2^nd and 3^rd week of life did.⁸⁵ Similar findings were reported by Turfkruyer and collaborators. They administered OVA through the breast milk several times between birth and weaning. They found that efficient tolerance was induced at three weeks of age, which is when mice are routinely weaned. Tolerant mice were protected from adjuvant induced allergic inflammation in adult life.⁸⁶ The inefficiency of inducing tolerance in neonates was found ascribed to the immature immune system and diminished levels of vitamin A in the neonates, which persisted until the third week of life.⁸⁶ On the other hand, immunocomplexes of IgG+allergen in the breast milk of previously immunized mothers protected breast-fed offspring against allergic sensitization in adulthood.⁸⁷ Uptake of IgG immunocomplexes was dependent on the expression of the neonatal Fc receptor FcRn.^87,88 FcRn dependent antigen presentation by CD11c+ dendritic cells induced allergen-specific Foxp3⁺ Treg cells in the offspring.⁸⁸

Recently, several groups have shown that RORγt⁺ Tregs are necessary to prevent allergic conditions, including food allergy. Mice with RORγt deficient Tregs developed increased frequencies of Gata3⁺ CD4⁺ T cells and Gata3⁺ Tregs producing IL-4, IL-5 and IL-13, were more susceptible to type 2 colitis and were more resistant to nematode infection.⁷⁸ In food allergic mice with enhanced IL-4R signaling, (Il4ra^F709 mice) and in human food allergic individuals, Noval Rivas and collaborators described an increase in pathogenic Th2 cell-like Tregs marked by expression of Gata3 and type 2 cytokines. These Th2 cell-like Tregs lose their regulatory function and exacerbate allergic inflammation.⁸⁹ The mechanism involved STAT6-dependent IL-4R signaling in Il4ra^F709 mice suppressing TGFβ1 transcription in Treg cells.⁹⁰ TGFβ regulated the balance between Gata3⁺ and RORγt expressing Tregs, as selective deletion of TGFβ or its receptor on Tregs disrupted the differentiation of RORγt⁺ Treg cells in the intestine while promoting the generation of Gata3⁺ Tregs.⁹⁰

In contrast to the pathogenic role of Gata3⁺ Tregs discussed above, Gata3⁺ Tregs were also found to have a regulatory function in the gut during homeostasis and during inflammation.^91,92 ST2⁺ Gata3⁺ RORγt negative colonic Tregs found in homeostatic conditions included Helios positive and Helios negative populations, suggesting that they are from thymic and peripheral origin.⁹¹

Under normal circumstances, small intestinal pTreg cells are triggered by dietary antigens upon transitioning to solid food.⁷⁷ Following weaning, this rise in Treg cells is vital for preventing an excessive immune response to food antigens.⁷⁷ In germ-free mice, which are devoid of the microbiota, diet-induced, Nrp1 negative Tregs were found in the small intestine, indicating that food-specific Tregs can develop independently of microbiota-induced Tregs.^42,77 Furthermore, while antibiotic treatment of SPF mice led to a significant reduction of RORγt⁺ Tregs in the lamina propria of the small intestine and the colon, weaning SPF mice into an antigen-free diet significantly reduced the frequency of RORγt negative Foxp3⁺ Tregs in the lamina propria of the small intestine.⁷⁷

The microbiome is altered in humans and mice with food allergies, causing dysbiosis.^1,93 Transfer of the fecal microbiome from food allergic patients to germ-free mice promoted the development of food allergy, while fecal transfer from non-allergic individuals protected from food allergy development.^94,95 Metabolites produced by the healthy microbiome, such as short chain fatty acids, tryptophan metabolites, and bile acid metabolites, contribute to intestinal tolerance and gut barrier integrity.^1,93,96

In sum, mice develop tolerance to both microbes and the diet around the time of weaning. The proper development of intestinal tolerance at this critical period appears essential for establishing competence for tolerant responses to new antigens in post weaning life and adulthood.

7. HUMAN TOLERANCE TO FOODS AND THE EARLY LIFE PERIOD

The incidence of food allergy in the pediatric population has increased significantly in recent decades.^97,98 In humans as in mice, there is evidence of a ‘critical period’ for tolerance to foods in early life, and of factors that may favor or be detrimental to establishing tolerance to foods. Breast feeding, microbial colonization, and the time of introduction of solid foods are recognized factors impacting the development of food allergies. Breast feeding affects the intestinal microbiota composition in early life. An important component of breast milk are human milk oligosaccharides (HMO), which are metabolized by bacteria into indole-3 lactic acid and promote the growth of beneficial bacteria such as Bifidobacterium longum subsp infantis.⁹⁹ On the other hand, antibiotic treatment during the first year of life significantly increases the risk of developing food allergy.¹⁰⁰ The occurrence of eczema is a known risk factor for food allergy. Allergic sensitization to foods may occur in infants even before the allergenic food is consumed, by sensitization through disrupted inflamed skin.¹⁰¹

The concern of when to introduce solid foods to lactating children has been a matter of debate.¹⁰² There is now a consensus that delaying solid food introduction may be a factor contributing to the food allergy epidemic. An important breakthrough in early food introduction and food allergy was the Learning About Peanut Allergy (LEAP) study. LEAP tested the effect of early peanut introduction on the development of peanut allergy in an at-risk pediatric cohort from 4–11 months of age. The proportion of peanut-allergic participants in groups that consumed or avoided peanut consumption was evaluated at 60 months of age. In the peanut avoidance group, 13,7% developed peanut allergy, while only 1.9% developed peanut allergy in the peanut consumption group.¹⁰³ A follow-up into adolescence of the LEAP trial demonstrated a long-lasting tolerance to peanuts in participants of the consumption group.¹⁰⁴ The Efficacy of the Enquiring About Tolerance (EAT) study has also provided evidence for the beneficial effect of early introduction of allergenic foods on the prevention of food allergy.¹⁰⁵

8. CONCLUSIONS

The oral tolerance to foods induced by their ingestion prevents the development of food allergy. The establishment of tolerance to a food depends on the formation of food specific CD4⁺Foxp3⁺ Tregs, which exert dominant tolerance systemically. Food tolerance can successfully develop during both infancy and adulthood. However, establishing a long term tolerogenic intestinal environment is determined by an early immune reaction to microbial intestinal colonization and ingestion of solid foods. In mice, this occurs around weaning time, while in human infants it happens during the first year of life. Developing strategies to promote tolerance in early life holds promise to reduce the food allergy epidemic.

DATA AVAILABILITY STATEMENT

No datasets were generated or analyzed during this current study.