Epithelial-Immune Interactions Govern Type 2 Immunity at Barrier Surfaces

Abstract

Allergic diseases are acute and chronic inflammatory conditions resulting from disproportionate responses to environmental stimuli. Affecting approximately 40% of the global population, these diseases significantly contribute to morbidity and increasing healthcare costs. Allergic reactions are triggered by pollen, house dust mites, animal dander, mold, food antigens, venoms, toxins, and drugs. This review explores the pivotal role of the epithelium in the skin, lungs, and gastrointestinal tract in regulating the allergic response, and delves into the mechanisms of tissue-specific epithelial-immune interactions in this context, with recent advances highlighting their roles in the initiation, elicitation, and resolution phases of allergy. Understanding these intricate interactions at epithelial barriers is essential for developing targeted therapies to manage and treat allergic diseases.

Introduction

Allergic diseases such as allergic asthma, food allergy, allergic rhinitis, and atopic dermatitis (AD) are caused by disproportionate responses to environmental stimuli. Allergic diseases affect ~40% of the human population, causing morbidity and increased healthcare costs worldwide [1]. Allergic reactions occur when pollen, house-dust mite (HDM) products, animal dander, mold, pollen, food antigens, venoms, toxins, or drugs are inhaled, ingested, or contact an individual’s skin. Epithelial cells that line body surfaces including the skin, respiratory tract, and gastrointestinal (GI) tract are the first point of contact with allergens. Across tissues, epithelial cells sense allergens and/or are damaged by allergen proteases and produce interleukin (IL)-25, IL-33, and/or thymic stromal lymphopoietin (TSLP). These alarmins activate group 2 innate lymphoid cells (ILC2s) and CD4⁺ T helper Type 2 (Th2) cells that produce the Type 2 cytokines IL-4, −5, −9, and −13, which elicit an array of immune and epithelial effector functions that underlie the signs and symptoms of allergy, including epithelial hyperplasia and increased barrier permeability and mucin production [2–7]. While many of these processes are conserved across tissues, different tissue sites also have unique epithelial features, tied to their specific functions, that impact the regulation of the allergic response [2,5,8]. This review will focus on the central role that skin, lung, and gut epithelial cells play in initiating, perpetuating, and resolving allergic inflammation and the tissue-specific cues and environmental factors that regulate these processes.

Initiation of allergic responses at barrier sites

Epithelial cells as first responders

Epithelial barrier structure and function differ across tissues, which influences the initiation of the Type 2 response. The skin epidermis has a stratified squamous epithelium, composed of multiple layers of keratinized epithelial cells that create a strong barrier. The lung and GI tract both have a single columnar epithelial cell layer to facilitate gas exchange and nutrient absorption, respectively. However, the lung epithelium is composed of ciliated cells that line airways, while GI epithelial cells are organized into crypt-villus units (small intestine only) and have microvilli. The epithelial cell lineages that compose the barrier can also be tissue-specific; for example, the GI and respiratory tracts contain chemosensory tuft cells, while the skin apparently lacks this lineage [9]. However, regardless of tissue site, cytokines such as IL-25, IL-33, and TSLP, and other chemical mediators prompt the first events that underlie allergen detection and initiation of the Type 2 allergic response (Figure 1).

Figure 1. Tissue-specific epithelial structure in Type 2 Immune response.

Diverse epithelial landscapes drive Type 2 immunity across barrier tissues, including the skin (top left), lung (top right), and the gastrointestinal (GI) tract (bottom left). In the skin epithelium, atopic dermatitis (AD) is characterized by barrier disruption, allowing allergen penetration, triggering release of interleukin (IL)-25, IL-33, and thymic stromal lymphopoietin (TSLP) by keratinocytes. This results in activation of T helper type 2 (Th2) and group 2 innate lymphoid (ILC2) cells and secretion of IL-4 and IL-13, contributing to amplification of the Type 2 immune response. In the lung epithelium, allergen exposure can induce IL-33 and IL-25 release from epithelial cells, promoting activation of Th2s and ILC2s. IL-5 signaling results in eosinophilia while IL-13 acts on goblet cells to promote mucus secretion. Additionally, lipoxin A4 can inhibit mucus secretion and dampen inflammation in the airway epithelium. Lastly, in the GI epithelium, IL-33 and TSLP are also secreted by epithelial cells, while IL-25 is secreted specifically by tuft cells, all of which ultimately amplify Th2 and ILC2 responses and Type 2 inflammation in the gut.

For example, in a murine model of food-induced anaphylaxis following skin sensitization, intestinal epithelial cell-derived IL-25, most likely from tuft cells, the sole source of intestinal epithelial IL-25 [10], was required for ILC2 IL-4/IL-13 production that expanded mast cells to cause anaphylaxis [11]. Similarly, during murine intestinal helminth infection that also elicits Type 2 inflammation, tuft cells released IL-25 that activated ILC2s to produce IL-13, which acted back on the epithelium to promote goblet and tuft cell hyperplasia and increased mucin production that supports helminth expulsion [8,10,12,13]. These data underscore the importance of tuft cell-derived IL-25 in the intestine, though IL-25 also activated pro-allergic ILC2s in the murine lung and induced allergic airway inflammation [14], and keratinocyte-derived IL-25 prompted ILC2 IL-13 expression associated with epithelial hyperplasia in a mouse model of AD [15].

TSLP and IL-33 play key roles in promoting the allergic cascade across tissues. In a murine model of skin sensitization to food allergens, epithelial-derived TSLP and/or IL-33 promoted ILC2 responses to drive anaphylaxis or upper GI inflammation similar to human eosinophilic esophagitis (EoE), an allergic disease of the esophagus [11,16,17]. Following early-life colonization with the skin commensal Staphylococcus lentus, keratinocyte-derived TSLP mobilized ILC2s that drove AD in adult mice [18]. In the murine and human lung, IL-33 has been shown to promote the migration, proliferation, and cytokine producing capacity of pro-allergic lung ILC2s in allergy [19,20]. Together, these studies and others underscore cross-tissue and tissue-specific modules of inflammation that underlie allergic disease mediated by epithelial-derived cytokines (Figure 1).

Epithelial cells also express pattern recognition receptors that allow them to directly respond to allergens [21]. Protease allergens in fungi and HDM degrade epithelial junction proteins to elicit alarmin release and cleave fibrinogen, reviewed in detail in ref [22]. Cleaved fibrinogen and other allergens such as HDM could also signal directly through Toll-like receptor 4 (TLR4) to induce allergic airway inflammation in mice [23,24]. Furthermore, IL-33 was directly cleaved by protease allergens, activating ILC2s and causing eosinophilia in the airway, promoting murine allergic inflammation [25]. However, future studies in humans and murine models will be required to fully understand how direct interactions between epithelial cells and allergens activate the barrier to promote cytokine release in a tissue-specific manner.

Antigen presentation

Antigen uptake, processing, and presentation to allergen-specific CD4⁺ T cells by dendritic cells (DCs) is highly dependent on crosstalk with epithelial cells and underlies the sensitization phase as the adaptive immune response is trained to inappropriately respond to environmental allergens [3]. In murine allergic asthma models, epithelial cell-derived IL-33, TSLP, IL-1a, colony-stimulating factor 1, and granulocyte-monocyte-colony stimulating factor (GM-CSF) acted on classical DC2s [26–28] that have been shown to preferentially drive Th2 polarization [3,29,30]. TSLP induced expression of OX40 ligand on human DCs that promoted Th2 polarization [31] that could drive allergic responses. Similarly, in human skin from AD patients, keratinocytes released TSLP, IL-1, tumor necrosis factor α, GM-CSF, and IL-18 to regulate the maturation of antigen-loaded DCs [32].

While DCs play the preeminent role in Th2 polarization in allergy [29] (Figure 2), some epithelial cells can also express major histocompatibility complex class II (MHC-II) and act as non-professional antigen presenting cells [8,33]. Recent studies have identified functional roles for MHC-II⁺ epithelial cells in interactions with the microbiota, during graft-versus-host disease, preserving lung epithelial barrier integrity, and in cancer [34–37]. In allergy, a human esophageal epithelial cell line could process antigen and activate T cells in vitro under conditions similar to those found in the esophagus of patients with EoE [38], and human nasal epithelial cells from patients with allergic rhinitis were able to activate autologous T cells when stimulated with birch pollen antigen [39]. The study of the role of MHC-II⁺ epithelial cells in allergy remains an active area of investigation, with the potential to reveal novel mechanisms by which the epithelium regulates the initiation or promulgation of adaptive allergic responses.

Figure 2. Effector mechanisms of allergic inflammation in the gastrointestinal tract.

Epithelial cells are initiators and drivers of allergic inflammation upon exposure to allergens. Epithelial-derived cytokines such as interleukin (IL)-25, IL-33, and thymic stromal lymphopoietin (TSLP) play critical roles in recruiting and activating various immune cells that propagate the Type 2 response. IL-4, IL-5, IL-9, and IL-13 further promote inflammation, and IL-13 in particular acts back on the epithelium to drive a whole-tissue remodeling response. In the gastrointestinal (GI) tract, this remodeling is often associated with increased mucus production, goblet and tuft cell hyperplasia, and increased barrier permeability. Nerve growth factor (NGF); Neuromedin U (NMU).

Allergic effector mechanisms

Epithelial support of immune responses

Following sensitization, epithelial cells again take center stage as critical effectors in allergic inflammation, coordinating an interplay with immune and neural cells. In response to allergen challenge, epithelial cells again release IL-25, IL-33, and TSLP that activate ILC2s, Ag-specific CD4⁺ T cells, and innate granulocytes such as eosinophils, mast cells, and basophils [8,10,16,40,41]. Epithelial-derived factors can also provide support for immunoglobulin E (IgE) and memory Th2 responses. For example, TSLP could program human DCs to drive emergence of T follicular helper cell responses in vitro that promoted IgE production by B cells [42]. Such support of IgE production is critical, as IgE binds to the high-affinity IgE receptor, FcεRIα, on mast cells and basophils, with subsequent allergen exposure and crosslinking prompting degranulation [43] that releases histamine, neurotrophic factors, bioactive lipids, Type 2 cytokines, and proteases that further propagate the immune response [3,8]. However, some allergic responses are largely IgE-independent, including EoE [16,44]. In IgE-dependent and -independent responses, epithelial-derived cytokines act on Th2 memory cells to drive IL-4 and IL-5 production, which contributes to the recruitment and activation of eosinophils [16,45]. TSLP has pleotropic effects on Th2 memory cells (reviewed in ref [8]) and can promote basophil activities in AD and EoE [8,16]. Thus, epithelial-derived cytokines are critical in promoting immune responses in the allergic cascade when allergen challenge occurs, in the GI tract and other tissues (Figure 2).

In addition to their role as cytokine-producers, epithelial cells, particularly tuft cells, are an emerging source of bioactive lipids called eicosanoids that can tune inflammation. Eicosanoids include the prostanoids, leukotrienes, lipoxins, and other species that derive from arachidonic acid and are synthesized via species-specific enzymatic synthesis cascades [46–48]. Cysteinyl leukotrienes (cysLTs) in particular have a pathogenic role in allergic airway inflammation [49]. Loss of the transcription factor FoxA2 in the airway epithelium drove increased cysLT production and allergic airway disease [50]. A recent study showed that in mice, tracheal tuft cells, termed brush cells, produced cysLTs, dependent upon the ATP sensor P2Y2, that drove ILC2 activation and allergic responses to Alternaria allergen [14,51]. Similarly, prostaglandin D₂ (PGD₂) could also promote allergic airway inflammation, synergizing with airway epithelium-derived IL-25 and IL-33 to activate ILC2s [52] and recruit them to the inflamed lung [19]. However, the role of tuft cell-derived eicosanoids in allergic immune responses remains unclear in many contexts; tuft cell-derived cysLTs contributed to intestinal Type 2 inflammation in helminth infection [53], and small intestinal tuft cells have been shown to produce PGD₂ [54], but whether tuft cells across tissues are a key eicosanoid source that activate or recruit immune cells in allergy requires further study.

Epithelial cells as effectors in remodeled allergic tissues

Allergic disease involves drastic tissue remodeling mediated by epithelial changes driven by Type 2 cytokines [3,8] (Figure 1). In the GI tract, IL-13 acts on intestinal epithelial stem cells to skew differentiation towards tuft and goblet cells [8,33,54]. Single cell RNA-sequencing has shown that that IL-13 drove stem cells away from self-renewal and towards tuft and goblet cell gene signatures in mouse intestinal epithelial cells [55] or small intestinal organoid cultures [56]. Accumulation of tuft and goblet cells is a critical driver of intestinal Type 2 inflammation. Tuft cells produce IL-25, leukotrienes, and acetylcholine (Ach) that can alter endothelial, epithelial, and immune function and increase intestinal permeability to facilitate allergen translocation across the barrier [9,10,12–14,53,57]. Goblet cells produce mucus that elicits allergic pathology, and in naive wild-type mice, goblet cells formed goblet cell-associated antigen passages (GAPs) that allowed luminal antigens to access lamina propria immune cells [58], potentially contributing to sensitization and tolerance (Figures 1 and 2).

In the lung, IL-13-mediated epithelial remodeling in allergic asthma is particularly well-characterized. Allergic asthma features airway hyperresponsiveness, mucus hypersecretion, and remodeling characterized by changes in the composition of epithelial cells in the airways and epithelial cell apoptosis (smooth muscle cell proliferation and fibroblast activation also occur), contributing to airflow limitation and respiratory symptoms, reviewed in ref [21]. In this context, granulocytes and other immune cells release factors, such as IL-13, that trigger goblet cell mucus hyperproduction, and human eosinophils produced Charcot-Leyden crystals composed of galectin 10 that embedded in mucus and acted as a Type 2 adjuvant [59]. In addition, using human nasal polyp epithelial cells and murine models, tuft cells have been shown to be a source of PGE₂ that prompted increased mucociliary transport function [60]. IL-13 and other cytokines, like IL-31, also drive epithelial remodeling and functional changes in the skin during allergy, supporting the development of pruritic acute or chronic lesions with distinct epithelial features and a breach in the skin barrier [61] that can predispose to other allergic diseases (a process termed “the allergic march,” reviewed in detail in ref [62]). The pruritis that leads to scratching and related pathology in AD speaks to the importance of the neuro-immune-epithelial link in the skin (discussed below). Thus, epithelial cells serve as end-point effectors that execute a suite of pro-inflammatory allergic responses in the GI tract (Figure 2), lung, and skin (Figure 1).

Importantly, tissue remodeling that occurs during allergy is not an acute event. Epithelial cells in allergy and other contexts “remember” past inflammatory responses, elevating baseline inflammation even in periods of reduced symptoms and increasing sensitivity to subsequent exposures. Epigenetic modifications in epithelial stem cells may play a critical role in these processes, altering gene expression in response to past exposures [63–66]. In the respiratory tract, airway epithelial basal stem cells from chronic rhinosinusitis patients generated less diverse progeny compared to controls, associated with distinct alterations in the chromatin landscape, and retained a Type 2-skewed signature and elevated Wnt pathway activity after exposure to Type 2 cytokines [66]. Whether and how epithelial inflammatory memory functions in the allergic GI tract and skin remains unknown, but a better understanding of the processes that underlie lasting changes in epithelial function and structure during allergy may offer novel avenues for intervention in chronic disease.

Neural-immune link

The rapid response to allergens is believed to be an evolutionary mechanism to expel toxins and establish avoidance behavior, suggesting a direct link between immune-epithelial function and physiology controlled by the nervous system – itch, sneezing, gut motility, and pain. Immune and epithelial cells express receptors for neural factors and secrete neurotropic factor/neurotransmitters. In turn, neurons express cytokine receptors, suggesting bi-directional communication [8,67,68]. In addition, neurons can directly sense a diverse array of enzymatically active allergens. Recent work has highlighted the critical role for this sensory mechanism in initiating allergic inflammation in the skin and DC migration via Substance P [69].

Perhaps the most well characterized neural-immune connection during allergy is between neurons and mast cells [8,68,70]. Broadly, murine and human mast cell-derived factors such as histamine, PGE₂, and leukotriene C₄ signal to neurons, and mast cells produce neurotrophic factors such as serotonin and nerve growth factor (NGF) that induce itch in the skin and increase gut motility. Mast cell degranulation is regulated by neurotransmitters including Substance P, calcitonin gene-related peptide (CGRP), Ach, and vasoactive intestine peptide [68]. In vivo, recent studies have shown that mast cells loaded with dietary-specific IgE antibodies led to meal-induced abdominal pain and subsequent avoidance behavior in mice [71,72]. In addition to this important role for mast cells, in the murine intestine, eosinophils responded to neuron-derived neuromedin U (NMU) to initiate goblet cell differentiation [73] (Figure 2), and NMU could also activate pro-allergic ILC2s from humans and mice to drive lung allergy [74–77]. Finally, a very recent study has shown that γδ T cells produced IL-3 that acted on sensory neurons to increase their sensitivity to the protease allergen papain and promote itch and allergic skin inflammation in mice [78].

There is also mounting evidence to support epithelial-neural connections during Type 2 responses [8,68]. In the GI tract, enteric neurons and epithelial tuft cells released Ach to stimulate mucus and epithelial fluid secretion in helminth-induced Type 2 inflammation [57], though whether this axis functions in intestinal allergy remains unclear. During allergic lung inflammation, epithelial-derived IL-25 synergized with NMU that activated ILC2s [77]. In addition, innervated pulmonary neuroendocrine cells, a rare epithelial cell type in the lung, produced CGRP and γ-aminobutyric acid that drove pulmonary ILC2 and goblet cell responses, respectively [79]. In AD, sensory neurons expressed receptors for Th2-derived IL-31 that promotes itch and epithelial cell-derived IL-33, TSLP, and NGF [80–82]. Also in the skin, neural-derived CGRP and Substance P drove Langerhans cells towards a Type 2 phenotype and induced keratinocyte release of tumor necrosis factor α (TNFα), IL-1b, and NGF [83,84]. Thus, evidence is emerging that neuro-epithelial functions are highly intertwined in allergic tissues. However, further studies are required to dissect the complex interplay of these interactions, particularly in humans, and whether they could be targeted therapeutically to treat allergy.

The environment shapes epithelial cell-dependent initiation and promotion of allergic disease

Environmental factors such as climate, pollution, and nutrition affect epithelial responses and are key contributors to allergic disease, driving pro-allergic pathways and, through climate change, prolonging exposure to seasonal allergens [1,85–88]. For example, exposing human airway epithelial cells to pollutant fine particulate matter resulted in inflammatory cytokine and mucin gene expression [89] and enhanced MUC5AC production, downstream of ER stress [90]. Diesel exhaust particles activated the aryl hydrocarbon receptor (AhR) on primary asthmatic human airway epithelial cells, associated with enhanced AhR binding to the IL25, IL33, and TSLP promoters [91]. In human skin, particulate matter exposure resulted in decreased filaggrin expression, dependent upon TNFa and the AhR, associated with epithelial barrier defects that could predispose to allergic sensitization [92]. While additional research is required, these studies suggest a key role for pollutants in shaping pro-allergic epithelial initiation and effector responses (Figure 3).

Figure 3. Environmental and nutritional factors mediating allergic disease.

The link between environmental factors and allergic disease has become apparent in recent years. Airborne pollutants such as particulate matter and greenhouse gases contribute to allergic disease exacerbation by altering the climate and promoting inflammatory epithelial responses. Additionally, the “Western lifestyle” can influence nutrition, limit beneficial breastfeeding behaviors, and alter commensal microbial communities, promoting the occurrence of allergy and exacerbating existing allergic responses. Levels of iron, zinc, vitamin D and other nutrients have been linked to inflammatory diseases like atopic dermatitis and asthma. The mechanisms by which environmental factors drive pro-allergic epithelial activities requires further study. House dust mite (HDM).

Shifting commensal microbial communities, in part attributable to a “Western” lifestyle that lacks microbial exposure and often features early life antibiotic use, also contribute to epithelial allergic inflammation. While this topic has been reviewed extensively elsewhere [87], a few examples related to the epithelium are notable. TLR4 on lung structural cells, most likely epithelial cells, was required for the HDM response in mice and promoted production of IL-25, IL-33, TSLP, and GM-CSF [24], though chronic exposure to farm dust or low-dose endotoxin protected mice against HDM-elicited allergic airway inflammation, dependent upon lung epithelial-intrinsic A20, a ubiquitin-modifying enzyme that regulates cytokine signaling [93]. Certain microbial species are associated with protection against allergy. Clostridia sp. colonization of germ-free mice elicited epithelial barrier-protective IL-22 that limited food allergy [94], and Anaerostipes caccae from the feces of healthy infants was protective against anaphylaxis in response to cow’s milk in colonized mice, associated with unique metabolic gene expression profiles in ileal epithelial cells [95]. Some species associated with allergy can promulgate allergic inflammation; monolayers of A549 airway epithelial cells exposed to Streptococcus salivarius from allergic rhinitis patients increased expression of IL33 and TSLP, and S. salivarius treatment exacerbated inflammation in a mouse model of rhinitis [96].

Gut microbes also release products such as short-chain fatty acids that tune allergic inflammation. The metabolites butyrate and propionate restored IL-4-, IL-13-, and HDM-elicited loss of barrier function in human airway epithelial cells [97]. In the skin, mice given butyrate had improved skin barrier integrity, decreased Il33 expression, and enhanced keratinocyte differentiation driven by increased mitochondrial fatty acid oxidation in a model of AD-like disease [98], and topical application of propionate in AD and AD-like disease in humans and mice led to improved symptoms and decreased IL-33 levels, with propionate inhibiting IL-33 production, dependent on increased AhR expression and/or function [99]. Together, these studies show that microbial signals can control epithelial initiation and effector responses that underlie allergy, providing exciting opportunities to modulate allergic disease using microbes or their products (Figure 3).

Finally, deficiencies in essential micronutrients and vitamins can be associated with allergy [88]. For example, zinc deficiency resulted in increased mucus-secreting goblet cells and caspase-3-driven epithelial apoptosis in murine allergic airway disease [100]. Vitamin D receptor levels were decreased in nasal epithelial cells from rhinosinusitis patients, leading to an increase in the epithelial-derived, inflammatory cytokine IL-36γ [101]. Similarly, individuals with EoE were more likely to have low vitamin D levels, correlated with an increased risk of sensitization through dysregulated epithelial barriers; in vitro, vitamin D-deprived esophageal epithelial cells in air-liquid interface cultures treated with IL-13 displayed exaggerated epithelial thickness and decreased expression of epithelial structural markers [102]. However, overall, while there are known associations between allergy and other nutritional deficiencies, including iron deficiency [103], we are just beginning to explore the role of nutrition in allergic disease. Mechanistic studies in murine models coupled with translational human studies will be required to fully dissect how nutrient status alters epithelial function in allergy (Figure 3).

Resolution of allergic responses

Immune cells and cytokines

Coordinated resolution of Type 2 inflammation is critical to restore tissue homeostasis, repair damage, and allow for responses to subsequent stimuli [104,105]. T regulatory cells (Tregs) play a key role in initiating tolerance to potential allergens (reviewed in refs [106,107]). During Type 2 responses, Tregs, as well as M2-like macrophages, produce anti-inflammatory cytokines such as transforming growth factor-β (TGF-β) and IL-10 that are critical in restraining ongoing responses [4,104–108]. Treg depletion in mice resulted in a loss of barrier integrity in the lung, with increased airway permeability and a loss of tight junction protein expression; these changes were partially reversed by anti-IL-4 treatment, suggesting a direct connection between the presence of Tregs and epithelial Type 2 inflammation [109]. More recently, the transcription factor RORα in Tregs was shown to drive expression of the death receptor DR3 that sequestered TNF ligand–related molecule 1, limiting ILC2 activation and downstream epithelial inflammation in allergic skin disease [110]. However, while Tregs clearly restrain the immune responses that promote allergy, the precise role that Tregs play in directly suppressing epithelial-intrinsic inflammation modules in the intestine (Figure 4), lung, and skin is still being studied [106,107].

Figure 4. Epithelial resolution pathways in allergic inflammation in the gastrointestinal tract.

Coordinated resolution of effector responses in Type 2 inflammation is vital for restoring tissue homeostasis and preventing chronic pathology. Recent work has highlighted a role for cytokines such as interleukin(IL)-10 and transforming growth factor-β (TGF-β) that can promote regeneration, derived from T regulatory cells (Tregs) and other immune cells; prostaglandins; and specialized pro-resolving mediators such as lipoxins, resolvins, protectins, and maresins in acting on and regulating epithelial responses. Prostaglandin D₂ (PGD₂) and PGE₂ act directly on the epithelium, with PGD₂ and its receptor chemoattractant receptor homologous molecule expressed by Th2 cells (CRTH2) counteracting the IL-13-driven inflammatory program in the epithelium. Damage-associated molecular pattern molecules (DAMPs), extracellular matrix (ECM).

Likewise, while the effects of anti-inflammatory cytokines such as IL-10 and TGF-β on the epithelium have been investigated in other contexts [4,108], less is known regarding their effects on the epithelium during allergy. Recent work has highlighted that culture of nasal epithelial cells with IL-10 or IL-10-producing ILC2s reversed grass pollen-induced loss of expression of tight junction protein zonula occludens-1 (and increased expression of IL-6 and IL-8, supporting barrier integrity [111]. In the lung, TGF-β plays roles in the epithelial-to-mesenchymal transition, mucin production, and cell survival, which can shape epithelial barrier integrity and contribute to allergy, reviewed in ref [112] (Figure 4). However, specific effects of TGF-β on the epithelium during the resolution of allergic inflammation across tissues remain to be fully described. Future studies that selectively delete IL-10 or TGF-β responsiveness in the epithelium in murine and human in vitro models of allergic inflammation will be required to determine how immunomodulatory cytokines act on the epithelium to resolve or suppress allergic inflammation.

Eicosanoids and lipid mediators

Eicosanoids can have potent pro-inflammatory effects, as described above, but they are pleiotropic factors and also have anti-inflammatory properties associated with Type 2 inflammation. However, their role in resolving allergic epithelial responses is only just emerging [46–48]. During intestinal helminth-elicited Type 2 inflammation, we have demonstrated that the eicosanoid PGD₂, potentially derived from tuft cells, and its receptor chemoattractant receptor-homologous molecule expressed on Th2 cells (CRTH2) counteract IL-13-driven epithelial reprogramming, establishing an epithelial-intrinsic negative regulatory mechanism in intestinal Type 2 inflammation [54] (Figure 4). However, it remains unclear whether this circuit functions in allergy. In addition, PGD₂ elicited a pro-remodeling response in CRTH2-expressing airway epithelial cells from human asthmatic donors and increased differentiation of goblet cells in air-liquid interface cultures from human samples, though decreased CRTH2 expression was observed in allergic compared to healthy epithelium in this context [113]. Thus, given the known pro-inflammatory functions of this axis on immune cells [19,52], how PGD₂ and CRTH2 regulate epithelial responses in various allergic tissues must be studied further.

In addition to PGD₂, other prostanoids can act on epithelial cells to resolve inflammation. In contact hypersensitivity, PGE₂ suppressed skin inflammation via inhibition of keratinocyte activation, and the PGE₂ receptor EP3 on conjunctival epithelium promoted resolution of murine experimental allergic conjunctivitis through inhibition of CXCL1 [114]. However, in human bronchial epithelial air-liquid interface cultures, PGE₂ induced mucus secretion, potentially exacerbating asthma [115], demonstrating that the role of this prostanoid, like PGD₂, can be context-dependent. Other lipid mediators such as lipoxins impact epithelial cell biology; for example, lipoxin B₄ promoted resolution of allergic lung inflammation through inhibition of goblet cell hyperplasia [116], and in human airway epithelial cells, lipoxin A₄ suppressed mucus secretion via inhibition of p38 and ERK phosphorylation [117]. Overall, the eicosanoid family is complex, with questions remaining regarding its function in vivo in humans, but as many eicosanoid pathways are highly druggable [46], an increased understanding of how eicosanoids promote resolution of inflammation may inform the development of new drugs to treat allergy (Figure 4).

Current therapeutics that target epithelial activities in allergy

As our understanding of the mechanisms that control Type 2 epithelial-driven inflammation has increased, novel monoclonal antibodies (mAbs) that block or neutralize epithelial-derived cytokines have been developed to treat allergic diseases [118]. Tezepelumab, an anti-TSLP mAb, was the first allergy treatment to directly target an epithelial-derived cytokine. Tezepelumab reduced asthma severity and is approved for use in moderate-severe asthma [119]. Therapies that target IL-25 and IL-33 are not available yet, though a number of anti-IL-33/IL-33R compounds are in clinical trials. Itepekimab (anti-IL-33) reduced asthma-related events in a clinical trial [120], and tozorakimab (anti-IL-33) blocked IL-33R-dependent effects in human primary cells [121]. Etokimab (anti-IL-33) was effective in improving symptoms of AD [122] and reduced reactivity to peanut challenge in allergic subjects [123]. Astegolimab (anti-IL-33R/ST2) was effective in reducing asthma exacerbation rates [124] but was not effective in AD [125], suggesting that targeting IL-33 to treat allergic disease must be context-specific. These studies highlight that understanding epithelial function can underpin successful efforts to treat allergic disease in the clinic.

Future work that unravels how other epithelial products contribute to allergy will hopefully lead to the development of new therapies that target a diverse array of epithelial functions. The eicosanoids represent a particularly attractive target [46–48]; the leukotriene inhibitor montelukast is already commonly used in the clinic to treat allergic asthma [126]. However, major unknowns related to epithelial cell-derived eicosanoids remain. For example, inhibition of the PGD₂ receptor CRTH2 to treat allergic airway inflammation did not offer substantial benefit over placebo [127]. While this approach aimed to dampen CRTH2-dependent, pro-allergic immune responses, data showing that the PGD₂-CRTH2 pathway suppressed Type 2 epithelial inflammation [54] suggests that a more complete understanding of the role of epithelial-derived eicosanoids is needed before we can successfully target these pathways therapeutically. However, developing therapies that enhance epithelial-intrinsic, pro-resolution pathways could be an attractive alternate route to treat allergic disease.

Conclusion

Epithelial barriers are central to the pathogenesis of allergic diseases, serving as critical interfaces for the initiation, propagation, and resolution of Type 2 inflammatory responses. Recent advances have highlighted how environmental factors, including pollutants, microbiota, and nutrient deficiencies, profoundly influence epithelial integrity and function in allergy, in a tissue-specific manner. Areas of particular interest in the field center around how these environmental factors and genetics intersect to mediate susceptibility to the development of allergic disease, how the neuro-immune-epithelial interplay influences allergic effector mechanisms, and how we can leverage our growing knowledge of the tissue-dependent mechanisms that control allergy to develop next-generation therapeutics to prevent and treat allergy. A deeper understanding of the epithelial-intrinsic mechanisms that promote allergy will be pivotal in developing targeted strategies to mitigate allergic disease burden and improve patient outcomes.

Abbreviations

Ach

acetylcholine

AD

atopic dermatitis

AhR

aryl hydrocarbon receptor

CGRP

calcitonin gene-related peptide

CRTH2

chemoattractant receptor-homologous molecule expressed on Th2 cells

cysLT

cysteinyl leukotriene

DC

dendritic cell

EoE

eosinophilic esophagitis

GAP

goblet cell-associated antigen passages

GM-CSF

granulocyte-monocyte colony-stimulating factor

GI

gastrointestinal

HDM

house dust mite

IgE

immunoglobulin E

IL

interleukin

ILC2

group 2 innate lymphoid cells

mAb

monoclonal antibody

MHC-II

major Histocompatibility Complex II

NGF

nerve growth factor

NMU

neuromedin U

PG

prostaglandin

SCFA

short chain fatty acid

TGF-β

transforming growth factor-β

Th2 - CD4⁺

T helper Type 2

TLR4

Toll-like receptor 4

TNFα

tumor necrosis factor α

TSLP

thymic stromal lymphopoietin