Linking impaired skin barrier function to esophageal allergic inflammation via IL-33

Eosinophilic esophagitis (EoE) is an emerging chronic allergic inflammatory disorder that selectively affects the esophagus and is characterized clinically by symptoms of esophageal dysfunction, including vomiting and dysphagia.¹ Molecularly, EoE is characterized by a T_H2 immune response based on increased expression of the cytokines IL-4, IL-5, and IL-13.² IL-13 promotes immune cell infiltration by inducing production of chemokines, such as CCL26 (eotaxin-3),³ and by impairing epithelial barrier function through reduced expression of the epithelial adhesion molecule desmoglein 1⁴ and overexpression of the protease calpain 14,⁵ which is encoded by the CAPN14 gene at chromosome 2p23 and is a major susceptibility genetic locus for EoE.^6,7 Patients with EoE often present with comorbid conditions, including asthma and atopic dermatitis,¹ which also involve dysregulated T_H2 responses and impaired barrier function. The similarity of the pathogeneses of these disorders is evidenced by the significant degree of shared genetic associations that exists among patients with these allergic disorders, including the overabundance of genetic variants in the thymic stromal lymphopoietin (TSLP)^8,9 and filaggrin (FLG) loci.¹⁰

There is a critical need to elucidate the factors that initiate and propagate the T_H2 responses present in patients with EoE. The disease is considered to be driven by food antigens because strict elimination diets cause complete remission of the disease¹¹ and experimental EoE can be induced in mice through allergen exposure.¹² Currently, the route of sensitization to these food antigens is not clear. It is notable that many patients with EoE are sensitized to aeroallergens¹³ and that there are seasonal variations in the clinical symptoms of patients with EoE.¹⁴ Additionally, experimental murine studies indicate that EoE-like disease can be induced by allergen challenge after epicutaneous¹⁵ or respiratory¹² sensitization. However, after sensitization occurs, it remains unclear how the T_H2 immune response is initiated and propagated upon re-exposure to food antigens, although this likely includes activation of innate immune cells, including antigen-presenting cells, by epithelium-derived, T_H2-promoting innate cytokines. Notably, esophageal TSLP is expressed at increased levels in patients with EoE^16,17 and activates dendritic cells¹⁸ and basophils¹⁹ to induce T_H2 polarization of CD4¹ T cells. Additionally, Noti et al¹⁶ showed a critical role for TSLP in the pathogenesis of EoE using an experimental mouse model that induces eosinophilic infiltration and T_H2 cytokine production in the esophagus after epicutaneous sensitization and repeated intragastric challenge with the food antigen ovalbumin. Genetic deficiency in TSLP or its receptor prevented induction of disease in this model, and administering TSLP-neutralizing antibodies after establishing esophageal eosinophilia reversed the features of the disease. Preliminary evidence was presented that basophils were the critical target of TSLP because depleting basophils phenocopied loss of TSLP, although the definitive experiment of adoptive transfer of wild-type and TSLP receptor–deficient basophils was not performed.

There has been much recent interest in investigating whether another epithelium-derived, T_H2-promoting innate cytokine, IL-33, could serve as an additional signal to initiate and propagate the T_H2 response in patients with EoE. IL-33 is an alarmin that is present during homeostasis in the nuclei of epithelial cells^20,21 and is released during necrosis or cellular damage.²² Upon extracellular release, it can activate a wide variety of immune cells that express its receptor, ST2, including type 2 innate lymphoid cells,²³ CD4¹ T cells,²⁴ eosinophils,²⁵ mast cells,²⁶ and basophils.²⁶ IL-33 and ST2 are required for induction of allergic asthma in murine models.^27,28 Two recent studies have shown that esophageal biopsy specimens from patients with EoE express higher levels of IL-33 protein than those from control subjects.^17,29 Additionally, there is association between genetic variants in the IL33 locus and disease risk.⁶ Finally, Judd et al²⁹ showed that intraperitoneal injection of recombinant IL-33 induces esophageal eosinophilia, epithelial hyperplasia, and production of T_H2-associated cytokines.

In this issue of the Journal, Venturelli et al³⁰ provide evidence of a requirement for IL-33 in the pathogenesis of experimental murine EoE-like disease. Eosinophil infiltration and T_H2-associated cytokine production were induced in the esophagus by means of repeated intranasal challenge with ovalbumin after epicutaneous sensitization by disrupting the skin epithelial barrier through tape-stripping or genetic deficiency of FLG. Disrupting the skin epithelial barrier was required because allergic sensitization did not occur in wild-type mice without tape-stripping of the skin. IL-33 was a critical mediator in this model because esophageal symptoms were not present in mice either genetically deficient in ST2 or given ST2-neutralizing antibodies. Furthermore, genetic deficiency in both FLG and ST2 prevented esophageal symptoms. Basophil depletion prevented disease induction, and esophageal symptoms were restored on adoptive transfer of wild-type, but not ST2-deficient, basophils. This observation supports that basophils were the target of IL-33, at least in part. Collectively, the data show that IL-33–ST2 interactions in basophils and impaired skin epithelial barrier integrity mediate induction of EoE-like disease induced by epicutaneous allergen sensitization (summarized in Fig 1).

FIG 1.

Proposed model for the development of allergic inflammation in patients with EoE. Impaired skin barrier integrity, such as that caused by injury or FLG mutations, allows for increased allergen penetration through the epithelium, causing allergic sensitization and release of IL-33 and TSLP from the epithelium. Secondary to activation by these cytokines, there is increased trafficking of basophils through the bloodstream to the esophagus. Later, direct exposure of allergens to the esophageal epithelium leads to the release of TSLP and IL-33, which induce IL-4 production from basophils and subsequent release of IL-5 and IL-13 from T_H2 cells. IL-13 causes esophageal epithelial barrier impairment and increased allergen exposure by decreasing desmoglein 1 (DSG1) and increasing calpain 14 (CAPN14) expression. Additionally, IL-13 induces CCL26 production from the epithelium, which, in combination with IL-5, causes eosinophil infiltration into the esophagus.

The work presented in this article builds on the aforementioned work by Noti et al¹⁶ showing the requirement of basophils and TSLP for induction of EoE-like disease in mice, which used a different model of allergen challenge (intragastric vs intranasal). Although this work provides strong evidence for the importance of basophils in 2 different experimental mouse models, concerns remain with regard to translating these findings to human disease. Although Noti et al¹⁶ show accumulation of basophils in human biopsy specimens, this finding has not been independently confirmed by others, including Venturelli et al.³⁰ In contrast, esophageal biopsy specimens from patients with active EoE are notable for infiltration of mast cells and eosinophils,³¹ which are both potently activated by IL-33. Therefore, it is likely that IL-33 activating these cells also contributes to the human disease. Additionally, the experimental antigen exposure is of supraphysiologic doses of a single antigen with one route of sensitization, whereas the human disease likely involves multiple allergens at more physiologic exposure doses and with several routes of sensitization. Finally, the mechanism through which skin sensitization leads to esophageal priming, whether involving basophils or not, remains unclear.

In summary, this study identifies that epicutaneous sensitization promotes induction of EoE-like disease in mice through a mechanism that involves IL-33/ST2–mediated activation of basophils. This study further supports the view that deficiency of FLG predisposes patients to EoE and suggests that blocking the IL-33–ST2 axis and protecting or enhancing the skin barrier would be of therapeutic value for treating EoE. It will also be of interest to apply these results directly to the esophagus because esophageal epithelial barrier function in patients with EoE is impaired, likely through IL-13–mediated dysregulation of calpain 14^5,6 and desmoglein 1.⁴