ILCs in food allergy and oral tolerance development
IgE-mediated food allergy is the most common type of food allergy, and affecting up to 10% of infants in western populations.85, 86 It is characterized by specific IgE (sIgE) production and IgE interaction with different immune cells (Th2 cells, mast cells and basophils).86, 87 The intestinal mucosa is capable of secreting alarmins which can activate ILC2s.88 In an experimental food allergy model, it was found that deficiency of IL33 receptor (ST2) prevented the development of food allergy.10 In another study, TSLP and basophils were both found to be important in allergic cutaneous sensitization while IL-33 was found to be necessary for the induction of IgE-dependent anaphylaxis.89 The release of IL-33 following skin injury has been shown to induce IgE-mediated mast cell degranulation and anaphylaxis; blocking ST2 prior to oral challenge reduced the severity of reaction without affecting the Th2 response to the allergen.6 Another important alarmin, IL-25, has also been shown to increase following food challenges.7 The authors also demonstrated that mice deficient in IL-25 were more resistant to the symptoms of food allergy. Following allergic sensitization, IL-25 and IL-2 derived from CD4+ Th2 cells stimulate ILC2s resulting in the production of large amounts of IL-13 and amplifying the IgE-mediated response in this experimental model. In addition, ILC2s with defective IL-13 production had a reduced ability to develop allergic inflammation, resulting in resistance to food allergy.7 In a murine model of food allergy, IgE‐stimulated mast cells induced IL‐13 production from ILC2s in an IL‐4Rα−dependent manner; in addition, ILC2s amplified systemic anaphylaxis by increasing target tissue sensitivity to mast cell mediators.90 These findings support a key role of ILC2s in food allergy in an IL-13-dependent manner.
Tregs regulate the functions of ILC2s and suppress their type 2 cytokine production. Reciprocally, ILC2s secrete IL-4 which downregulates Treg functions and increases mast cell activation.10 In the steady state, these networks function to facilitate tolerance; however, genetic and environmental factors, such as microbiota dysregulation, may stimulate alarmin production from the intestinal epithelial cells resulting in ILC2 activation.88 Of note, medium-chain triglycerides in peanut have been shown to increase alarmin production, thus promoting allergic sensitization and anaphylaxis.91 Inducible T regulatory cells (iTregs) are inhibited by the effect of IL-4,10 and after re-exposure to food allergens, activated mast cells can stimulate IL-33 production and ILC2 activation. This forms a positive feedback loop on mast cell activation and a negative feedback loop on iTregs, promoting the persistence of food allergy.92, 93
Commensal bacteria also have indirect effects on host immune responses towards food antigens. Some Clostridia strains induce the accumulation of Tregs in the colon.94, 95Clostridia also trıgger ILC3s to produce IL-22, strengthening the epithelial barrier. In mice, Clostridia containing microbiota suppressed food allergic responses.96 In response to the microbial signals, macrophages secrete IL-1β which mediates GM-CSF release from ILC3s. GM-CSF induces IL-10 and retinoic acid production by DCs and macrophages and subsequently promotes the induction of Tregs. Any interference with this crosstalk can result in loss of oral tolerance to food allergens.97
Eosinophilic esophagitis (EoE) is a chronic disease characterized by gastroesophageal reflux disease, and dysphagia leading to food impactions and strictures. Pathologically, esophageal remodeling consists of epithelial hyperplasia with barrier dysfunction, subepithelial fibrosis, and smooth muscle dysmotility.98 Although the role of IgE in the pathophysiology of EoE is unclear, affected individuals usually have elevated levels total IgE and IgE sensitization to foods and aeroallergens.99 In children with EoE, sensitization to both food and environmental allergens may be observed in 75% of patients.100 Elimination of food allergens, or elemental diets can reduce disease symptoms in 70–99% of patients, however relapse is very common after re-introduction of food allergens.101 This suggests that chronic food antigen hypersensitivity is an essential feature of EoE. In addition, many patients with EoE have a history of IgE-mediated food allergy,100, 102 and there may be progression from IgE-mediated food allergy to the later development of EoE towards the same food.103EoE has been reported as an adverse event in up to 10% of patients undergoing oral immunotherapy (OIT) for IgE-mediated food allergy , although fortunately in this context symptoms tend to resolve after discontinuation of the triggering allergen (presumably on the basis that the development of symptoms in the context of OIT is less chronic in natüre).104, 105Furthermore, OIT is known to stimulate the production of IgG4, which may indicate its in the pathogenesis of EoE.106, 107 Historically, EoE was proposed to ocur through an IgE-mediated mechanism, however targeted dietary elimination and the use of IgE-blocking strategies have failed to support this notion. Measures of IgE sensitisation are not helpful in identifying the responsible food allergen in clinical practice.108-111This, while EoE is associated with IgE sensitization, it is not considered to be an IgE-mediated disease.
There is increasing evidence for epithelial barrier dysfunction in EoE followed by eosinophilic inflammation like AD. The expression of FLG, zonula occludens (ZO)-3 and claudin-1 is decreased in EoE patients, correlating with spongiosis,112 and mutations in FLG are higher in patients with EoE.113 EoE pathogenesis is determined by TSLP produced by epithelial cells and IL-5 and IL-13 produced by inflammatory cells, including ILC2s. IL-5 production leads to esophageal eosinophilia and IL-13 induces EoE-specific epithelial gene expression and remodeling like angiogenesis.114Eosinophil granulocyte-derived extracellular traps stimulate alarmin (IL-33, TSLP) production in airway epithelial cells that enhances ILC2 activation and cytokine production.115 It remains unknown as to whether eosinophil cells contribute to ILC2 activation in patients with EoE. IL-33 gene expression has been implicated in pediatric EoE development.12 Inhibition of the esophageal Treg function by IL-33 may induce loss of antigen specific tolerance and provide a mechanistic rationale for EoE development.12 IL-33 and TSLP induce human ILC2s to produce large amounts of Th2 cytokines. Recently it was shown that ILC2s were significantly increased in tissues from patients with active EoE versus inactive EoE and were also higher than ILC2s in controls. Importantly, levels of ILC2s in biopsy specimens correlated with numbers of eosinophils in esophagus tissue. This supports a role for ILC2s in EoE pathogenesis.13 Mast cells and IL-9 have also been implicated. ILC2s produce IL-9 and are located close to mast cells in human lung, suggesting a crosstalk between these two cell types.116, 117 Both IL-33 and TSLP activate ILC2s from esophageal lymphoid tissue which provides a proof of concept for ILC2s induction in EoE (Figure 4).13 Indeed, TSLP is highly expressed in EoE tissue.98, 118
Atopic dermatitis (AD) is a relapsing chronic inflammatory skin disease characterized by epithelial barrier dysfunction and type 2 inflammation.119 During the development of AD, environmental allergens and genetic factors affect the skin barrier function and trigger type 2 inflammation. This may lead to the failure of the mechanisms of oral tolerance, resulting in the development of food allergy. Infants with AD at 4 months of age are at increased risk for allergy to hen’s egg.79 An important risk factor for AD is the FLG gene mutation. FLG mutations cause skin barrier disruption and decrease the level of E-cadherin which is an important inhibitor of ILC2s through its receptor (Killer cell lectin-like receptor subfamily G member 1 (KLRG1).51 In an experimental model of AD with FLG mutations, skin inflammation was associated with the expansion of IL-5-producing ILC2s. Additionally, mice with FLG mutations showed increased frequency of ILC2s in the skin in comparison to control subjects.27
Interaction of ILC2s with other cell types may also be important. Although basophils in healthy human skin are rare, they are enriched in the dermis of AD lesions and located in close proximity to ILC2. ILC2s express IL-4 receptor α (IL-4Rα) and proliferate in an IL-4-dependent manner. IL-4 is supplied by basophils during AD inflammation, which suggests a relationship between these two cell types in AD pathogenesis.120 Dermal ILC2s locate perivascularly and are in close proximity to the skin resident mast cells. ILC2s produce IL-9 and IL-13 and these cytokines may activate mast cells. On the other hand, mast cells produce IL-25, Prostaglandin D2 (PGD2), and Leukotriene C4 (LTC4) which can activate the ILC2s.121Both mast cells and ILC2s can be activated by TSLP which may potentiate the function of both of these cell types.