What makes children outgrow food allergy?

Oral tolerance is the fundamental gut immune response towards the ingested soluble antigens 1. It is an active state of systemic non-responsiveness to the ingested antigens. It was first described by Osborne and Wells more than 100 years ago who observed that antigens present in the diet of the mice did not elicit an immune response after parenteral immunization 2. Food allergy is attributed to failure to develop or a breach in previously established oral tolerance to food proteins 3. Food allergy has increased in prevalence worldwide in the past 2 decades; food allergy is estimated to affect up to 6–8% of children and 2–4% of the adults 4. In the westernized societies, peanut allergy in children has become a significant public health problem 5. Nevertheless, food allergy affects the small minority of the population, proving oral tolerance to be an extremely powerful and efficient mechanism. The prognosis of childhood food allergy to cow's milk, hen's egg, soya, and wheat is favourable, with the majority of the affected children becoming tolerant to these foods by school age. Little is known about the natural mechanisms by which childhood food allergies resolve with age and oral tolerance is acquired. The biomarkers associated with food allergy resolution include decreasing skin prick test wheal diameter, lower basophil activation upon food allergen stimulation measured by CD63 surface expression, decreasing food-specific serum IgE antibody levels, and IgE antibodies directed predominantly against the conformational and not sequential epitopes 6-11. In case of childhood milk and egg allergy, lack of reactivity towards baked forms of milk and egg is associated with increased odds of developing tolerance to unheated versions of these foods over time 12, 13. A number of different mechanisms have been implicated in oral tolerance, including active regulation by Tregs, as well as clonal deletion and anergy of T cells, although clonal deletion and anergy of T cells seems to be less relevant in the context of food allergy than regulation by Tregs 3. Different Treg subsets have been identified in oral tolerance, including IL-10-producing Tr1 T cells and Th3 T cells, in addition to the Foxp3+ Tregs that are currently considered to be the most important subset 14. Type 1 Tregs (Tr1 cells) lack Foxp3 expression and are characterized by the high-level secretion of Il-10 and TGF-β and, like Foxp3+ Tregs, by their ability to suppress immune and autoimmune responses. Th3 cells are CD4+ cells characterized by surface expression of latency associated peptide (LAP), a propeptide non-covalently associated with TGF-β. Th3 cells do not express CD25 or Foxp3, and their mechanism of suppression is dependent on TGF-β. Th3 cells are induced by antigen feeding, and their transfer provides protection against experimental autoimmunity. Foxp3+ Tregs can be natural (nTregs) or induced (iTregs). Natural Tregs are selected in the thymus based on their reactivity to self-antigens. Induced Tregs (iTregs) develop from naive conventional CD4+ T cells upon their activation in the presence of TGF-β1 and CD103+ dendritic cells (DCs) in the mesenteric lymph nodes in the gut 15-17. iTreg cells regulate T helper 2 (Th2) cell responses at the mucosal surfaces 18. They are less stable and more susceptible to reprogramming by cytokine milieu than thymic-derived nTregs that have more stable Foxp3 expression and wider demethylation of the Foxp3 gene 19, 20. Transfer of CD4+CD25+ CD4+ T cells, highly enriched in Foxp3+ Tregs, conveys oral tolerance to naïve animals, whereas depletion of CD25+ cells in vivo abrogates oral tolerance 21. In a model of IgE-mediated passive systemic anaphylaxis, induced by intravenous challenge with DNP-HSA in mice passively sensitized with DNP-specific IgE, adoptive transfer of Foxp3+ Tregs from either naïve or DNP-sensitized donors similarly alleviated body temperature loss in Treg-deficient DEREG mice. This suggested that constitutive Foxp3+ Tregs can control IgE-dependent anaphylaxis in mice 22. Mice and humans lacking Foxp3+ T cells develop enteropathy, eczema, elevated IgE, and severe food allergy called immune dysregulation, polyendocrinopathy, enteropathy, X-linked (IPEX) syndrome in humans 23. LPS-responsive beige-like anchor (LRBA) deficiency due to a nonsense mutation has been reported in a child as a novel cause of IPEX-like syndrome and Treg cell deficiency associated with metabolic dysfunction and increased apoptosis of Treg cells 24. Mice with a more selective defect in iTreg cells but normal levels of nTregs develop a Th2-skewed mucosal inflammation and the generation of an antibody response to antigens present in their diet 18. Tregs have been investigated in a limited number of studies focusing on childhood food allergy. Children who became tolerant to milk had an increased frequency of CD4+ CD25+ Tregs in peripheral blood after an oral milk challenge and reduced proliferation of milk-specific T cells; depletion of CD4+ CD25+ Tregs restored the in vitro proliferative response in milk-tolerant children 25. Children tolerant to milk in the baked products had higher levels of peripheral CD4+ CD25+ CD27+Foxp3+ Tregs, compared to the baked milk-allergic children 26. In the context of oral immunotherapy (OIT) to peanut, among treated peanut-allergic children, peanut-specific Foxp3+ T cells increased until 12 months and decreased thereafter 27. In another study, the ratio of Foxp3 (hi): Foxp3 (intermediate) CD4+ CD25+ T cells increased after 12 months in subjects treated with peanut OIT 28. Peanut oral immunotherapy also resulted in increased peanut-induced Tregs function and hypomethylation of Foxp3 29. Collectively, these data suggest that Tregs may be involved in the acquisition of clinical tolerance to food allergens. Qamar and colleagues 30 chose to investigate the role of regulatory T cells in the egg or peanut-allergic children spontaneously acquiring tolerance to hen's egg white or peanut. Their results are reported in the current issue of Clinical and Experimental Allergy. They studied 58 children with either egg or peanut allergy, recent acquisition of natural tolerance to egg or peanut, and healthy children without food allergy. Peripheral blood mononuclear cells (PBMCs) were stimulated with relevant antigen for 48 h, and flow cytometry was performed to characterize both surface (CD3, CD4, CD25, CD14, CD19, CD127) and intracellular markers (IL-10, Foxp3, and IL-5). They found significantly increased CD3+CD4+CD25+CD127loFoxp3+ cells in unstimulated PBMC from naturally tolerant patients compared to persistently allergic or control non-allergic patients. Upon stimulation with relevant antigen, naturally tolerant patients also had significantly increased IL-10-expressing CD25 + CD127lo cells (which they identified as Tr1 cells), Foxp3+ cells, and CD4+ cells. In contrast, no increase was observed in PBMCs from persistently allergic or control patients. This up-regulation was antigen specific and was only seen upon stimulation with a relevant antigen and not with unrelated antigen, suggesting that acquisition of tolerance is an active and antigen-specific process. The authors hypothesized that the increased CD3+CD4+ CD25+CD+127lo cells at baseline and upon stimulation and increased induction of IL-10-producing cells of several types, including Tr1 cells, from naturally tolerant patients suggest an important role for regulatory T cell subsets in the acquisition of natural tolerance. This study contributes to our understanding of the natural tolerance in childhood food allergy and identifies antigen-specific Tregs as an important component of tolerance development. Additional studies refining the role of specific Tregs are necessary to identify the Treg subsets critical for natural tolerance development. The insights into the natural tolerance development may be also useful for targeting specific pathways to enhance tolerance development with food antigen-specific immunotherapy. The authors declare no conflict of interest.