The Role of B Cells and Antibodies in Eosinophilic Esophagitis

Abstract

Background

Eosinophilic esophagitis (EoE) is a chronic, immune-mediated inflammatory disease affecting the esophagus, characterized by esophageal dysfunction, dysphagia, and tissue remodeling, leading to significant impairment in quality of life. The pathogenesis of EoE involves a complex interplay of genetic, environmental, and immunological factors. Although the disease is strongly associated with type 2 immune responses, emerging evidence suggests that B cells and antibody responses, particularly IgG4, also play a crucial role in disease development and progression.

Summary

This review explores the pathogenesis of EoE with a focus on the role of B cells, plasma cells, and antibodies. Epithelial barrier dysfunction is a pivotal factor in EoE, influenced by genetic predisposition, environmental triggers, and immune responses. The impaired barrier allows for antigen penetration, triggering type 2 immune responses, and chronic inflammation. Elevated levels of IgG4 in esophageal tissues and their potential to drive inflammation and tissue remodeling are highlighted. Animal models and clinical studies provide insights into the involvement of B cells and antibodies in EoE, suggesting that these components may contribute to the chronic inflammatory state and disease severity.

Key Messages

B cells are increasingly recognized for their role in EoE, particularly through their production of antibodies and cytokines that contribute to the inflammatory milieu. EoE is primarily driven by type 2 immune responses involving cytokines such as IL-4, IL-5, and IL-13, which promote eosinophil recruitment and chronic inflammation. Regulatory B cells, which produce anti-inflammatory cytokines such as IL-10 and IL-35, may play a role in modulating immune responses in EoE and contribute to the production of IgG4 antibodies. Elevated levels of IgG4 in esophageal tissues of EoE patients are linked to chronic inflammation and tissue remodeling, suggesting a potential role in disease pathogenesis. Further studies are needed to elucidate the specific mechanisms by which B cells and antibodies contribute to EoE.

Introduction

Eosinophilic esophagitis (EoE) is a chronic, immune-mediated inflammatory disease that primarily affects the esophagus. The condition is characterized by esophageal dysfunction, dysphagia, food impaction, and tissue remodeling, often leading to significant impairment in quality of life [1]. The histopathological hallmark of the disease is a high count of eosinophils in esophageal tissue biopsies. The prevalence of EoE has increased over the past three decades, with estimates reaching up to 40 cases per 100,000 individuals. This condition predominantly affects males and adults [2, 3].

The pathogenesis of EoE involves genetic, environmental, and immunological factors [4–6]. EoE has a strong association with atopic conditions such as asthma, food allergies, and allergic rhinitis. The interaction between a disrupted esophageal mucosal barrier and an aberrant type 2 immune response is believed to drive the progression of EoE. This includes alarmin production by epithelial cells, activation of T helper 2 (Th2) cells, and the subsequent release of cytokines like interleukin (IL)-4, IL-5, and IL-13. However, growing evidence points to the potential involvement of B cells and antibody responses in the disease. Elevated levels of IgG4 antibodies in patients with EoE hint at a modified Th2 response, which may contribute to disease pathogenesis. This raises significant questions regarding the role of B cells and plasma cells in EoE and how these immune components might interact with other pathogenic mechanisms.

This review aimed to provide a comprehensive overview of the pathogenesis of EoE with a specific focus on the role of B cells, plasma cells, and antibodies. By reviewing recent clinical and experimental studies, this work will explore their potential contributions to disease development and progression. Understanding these mechanisms is crucial for identifying new diagnostic markers and therapeutic targets that can improve the management of EoE and patient outcomes.

Immunopathogenesis of EoE

Overview of EoE Pathophysiology

EoE pathogenesis is multifactorial and is influenced by diverse factors, including genetics, sex, race, age, allergic comorbidities, as well as environmental factors, including food and exposure to aeroallergens [4–6]. Its incidence has been increasing over the last 30 years, reaching more than 6 cases per 100,000 inhabitants/year, and its prevalence is estimated to be 40 per 100,000 inhabitants. EoE is also influenced by sex, with a higher prevalence in males than females, and by age, with a greater prevalence in adults compared to children [2, 3].

EoE is characterized by an impaired esophageal mucosal barrier, exhibiting rings, furrows, and/or edema, and a dysregulated immune system [6]. The key indicator of EoE is the number of eosinophils per high power field in esophageal tissue biopsies, and a formal diagnosis requires more than 15 eosinophils per high power field [7]. The abnormal esophageal epithelium leads to the skewing of a type 2-biased immune response that promotes esophageal lesions and mucosal integrity impairment, culminating in chronic inflammation and progressive organ dysfunction [7, 8].

Epithelial Barrier Dysfunction

Epithelial barrier dysfunction is a pivotal factor in the pathophysiology of EoE, characterized by the dilation of intercellular spaces [9]. Moreover, biopsies obtained from individuals with active EoE manifest reduced expression of proteins associated with the apical junction complex, namely, filaggrin (FLG), desmoglein-1 (DSG1), claudin, and zonulin-3 [9–12]. Consequently, the esophagus in EoE patients exhibits heightened barrier permeability relative to healthy counterparts [13].

The impairment of the epithelial barrier in EoE can be attributed to three principal factors: genetic variation, environmental influences, and immunological factors. Genetic predisposition for EoE is supported by strong familial association [1]. Family and twin studies revealed that genetics, in addition to environmental factors, indeed influence the onset of EoE [14, 15]. Genes encoding eotaxin-3 (CCL26) and calpain 14 (CAPN14) show increased expression in EoE [16–19], while genes encoding FLG, DSG1, and serine peptidase inhibitor Kazal type 7 (SPINK7) are downregulated, leading to an impaired epithelial barrier function [20–22]. The genes encoding the desmosome-associated proteins desmoplakin (DSP) and periplakin (PPL) also appear to have a pathogenic role in EoE in relation to epithelial barrier impairment [23]. Transforming growth factor (TGF)-β may also play a protective role in EoE, as it was shown that mice with a TGF-βR1 loss-of-function variant develop symptoms mirroring EoE, highlighting the role of TGF-β in controlling epithelial cell function and allergic inflammation independently of adaptive immunity [24].

Environmental factors can also affect epithelial barrier function and therefore increase the risk of developing EoE. A study examining dizygotic twins and siblings identified several early life factors implicated in the disease. These included food and penicillin allergies, birth weight, birth season, and breastfeeding versus formula feeding [14]. Furthermore, other environmental factors, like the usage of detergents and surfactants, population growth, industrialization process, and climate change, resulting in increased inhaled substances like PM 2.5, PM 10, nanoparticles, micro- and nanoplastics, aero- and food allergens, all may impact the epithelial barrier function [25–30]. A recent study demonstrated that exposure to the detergent sodium dodecyl sulfate in both esophageal cell cultures and mice led to decreased esophageal barrier integrity, increased inflammation, and upregulation of genes involved in immune response, suggesting that detergents might play a significant role in triggering EoE [30].

Dysregulation of Immune Responses

Epithelial barrier dysfunction may not only trigger type 2 inflammatory responses but also increase the possibility that antigens infiltrate into the esophageal mucosa. In EoE, eosinophils are recruited from the blood and release toxic granules and cytokines, causing tissue damage, fibrosis, and hypersensitivity reactions [31, 32]. The epithelium releases alarmins such as thymic stromal lymphopoietin (TSLP), IL-25, and IL-33 [33, 34]. These cytokines influence the maturation of dendritic cell-mediated T helper cells and stimulate type 2 innate lymphoid cells (ILC2s), culminating in the production of IL-4, IL-5, IL-9, IL-13, and TGF-β, as well as eotaxin. IL-4 and IL-13 drive B cells to class switch to IgE and promote naive T-cell differentiation into Th2 cells, inducing eosinophilic recruitment and chronic inflammation [6, 7, 35]. One remarkable aspect of EoE is its strong association with increased levels of IgG4 antibodies in the inflamed esophagus [36–38]. This is indicative of the occurrence of a process that has been coined “the modified Th2 response,” which promotes the production of IgG4 over IgE in a late-onset sensitization phase (Fig. 1) [39].

Fig. 1.

Potential causative agents and immunological mechanisms in EoE. Food and aeroallergens, environmental factors, and microorganisms can damage the epithelial barrier. This damage leads to increased production of alarmins, including thymic stromal lymphopoietin (TSLP), interleukin (IL)-25, and IL-33, which activate dendritic cells and subsequently promote Th2 differentiation. Chronic antigen exposure may induce regulatory T cells (Tregs). IL-4 and IL-13 drive IgE class switch recombination in B cells, leading to the degranulation of mast cells and basophils. Locally produced IL-10 can skew B cells to switch to IgG4, which can block IgE-mediated mast cell and basophil degranulation. IL-13 triggers the release of eotaxin, which, along with IL-5, drives eosinophil migration into the tissue. The production of transforming growth factor-β1 (TGF-β1) contributes to the tissue fibrosis commonly found in EoE.

Involvement of B Cells in EoE

Many type 2 immune cells are involved in EoE pathogenesis. Eosinophils exhibit migration and infiltration into esophageal tissues, where they release toxic granule proteins, contributing to esophageal damage and inflammation [40–42]. Additionally, mast cells within the EoE esophagus are active participants, generating inflammatory mediators such as histamine, prostaglandins, and leukotrienes [40, 43–45]. CD4+ T cells expressing IL-4, IL-5, and IL-13 play pivotal roles by recruiting and activating eosinophils and basophils, thereby inducing changes in epithelial cells and compromising the esophageal barrier [46–50]. Moreover, EoE is associated with atopic comorbidities, which are IgE-related, including food allergy, allergic rhinitis, asthma, and pollen food allergy syndrome [51, 52]. Elevated levels of IgE and IgG4 have been detected both in serum and esophageal mucosa of EoE patients [53–58].

Direct Effects of Alarmins on B-Cell Functions

While the alarmins IL-25, IL-33, and TSLP are known to be extensively produced in the context of allergic diseases, the B-cell responses to these cytokines have not been studied in EoE. However, there are some indications that alarmins may, under certain circumstances, directly modulate B-cell responses.

IL-25 signals through the IL-17RA and IL-17RB complex. Whereas IL-17RA is ubiquitously expressed by many cell types including B cells, IL-17RB is expressed at high levels by ILC2 cells granulocytes, and myeloid cells as well as a subset of T cells, whereas circulating B cells do not express IL-17RB [59, 60]. However, there are indications that B cells can upregulate IL-17RB expression upon CD40-ligand and IL-4 stimulation [61]. Moreover, human germinal center (GC) B cells were reported to express IL-17RB, and IL-25 stimulation primed them for chemotaxis toward CXCL12 and CXCL13, two chemokines involved in B-cell trafficking in GCs [62]. Thus, there are indications that IL-25 could, under specific conditions, modulate B-cell responses. However, the relevance of this mechanism to EoE remains to be determined.

IL-33 signals through a receptor complex comprised of IL1RAP and ST2. B-2 cells, which comprise follicular and marginal zone B cells, do not express ST2 under steady-state conditions, and the direct effects of IL-33 on B cells were mainly observed on B-1 cells in mouse models of mucosal inflammation. Purified murine B-1 cells, in particular B-1b cells, showed proliferation, IgM production, as well as IL-5 and IL-13 production in response to IL-33 stimulation and could confer increased contact hypersensitivity responses in ST2^−/− mice that were challenged with IL-33 [63]. IL-33 induced IL-10 production in a subset of B-1 cells termed Breg^IL-33 and adoptive transfer of these cells protected against spontaneous colitis in IL-10^−/− mice [64]. It is unknown whether similar mechanisms are involved in the pathophysiology of EoE, and so far, no studies on the role of B-1 cells in EoE have been published.

TSLP plays a role in B-cell development as it supports human B-cell differentiation from hematopoietic progenitor cells [65]. However, a recent study demonstrated elevated TSLP expression in patients with IgG4-related disease. Moreover, TSLP induced B-cell proliferation and activation of JAK-STAT signaling. TSLP-stimulated B cells could drive the differentiation of naïve CD4 T cells to Tfh cells, suggesting a role for a TSLP-B-cell-Tfh axis in the pathogenesis of IgG4-related disease, a disease that exhibits similarities with EoE [66, 67]. A recent study also demonstrated that TSLP signaling in B cells is crucial for GC function and the generation of antigen-specific memory B cells [68].

Distribution and Function of B Cells in EoE

Although the esophagus of EoE patients shows low numbers of intraepithelial B cells, research indicates an increase in B cells in the esophageal mucosa following allergen exposure [48, 69]. B cells can also generate IgE and IgG4 locally, demonstrated by the expression of epsilon and gamma 4 germline transcripts, activation-induced deaminase, IgE heavy chain (Epsilon), and mature IgE transcripts in esophageal biopsies. Moreover, IgE-bearing mast cells were present in the esophageal mucosa of EoE patients [53]. However, in a mouse model for allergen-induced EoE, B cells were not required for the development of the condition, whereas both CD4+ and CD4− T cells were [48]. While these findings do not rule out the involvement of B cells or antibodies in the pathophysiology of EoE, they suggest that T cells are the primary drivers of the initial inflammatory response.

B Cell-Derived Cytokines in EoE

To date, there are few studies on the function of B cell and B cell-derived cytokines in EoE, although B-cell antigen presentation properties and cytokine secretion should still be considered in the pathogenesis of EoE. Granulocyte-macrophage colony-stimulating factor can be produced by B cells, and its blockade reduces esophageal eosinophilia, basal cell hyperplasia, and epithelial remodeling [70]. Moreover, angiogenesis and tissue remodeling play a role in the pathogenesis of EoE [71–73]. In this context, we reported that IgG4⁺CD73⁺CD49b⁺ B-cell clones generated various proangiogenic factors and supported the tube formation of endothelial cells [73]. In EoE patients, increased CD73⁺CD49b⁺ B-cell frequencies indicate a possible connection between IgG4 and angiogenesis [73]. It remains to be determined where this population originates and how its differentiation is regulated.

B cell-derived cytokines can also affect other immune cells. T-cell polarization is affected by B-cell products: IL-12 leads to Th1 differentiation, IL-6 to Th2 phenotype, and TGF-β to Th17 [74–76]. The maturation and function of dendritic cells can also be regulated by B cells in some diseases, like systemic lupus erythematosus [77]. Notably, a large set of data implicates B-cell cytokines in the pathophysiology of several chronic inflammatory responses and EoE-related allergic comorbidities, such as asthma [78–80]. In EoE, there are elevated blood levels of IL-1α, IL-6, and IL-8 but lower levels of IL-12, IL-17, and CD40L compared to healthy controls. An upregulation of IL-1, IL-9, and IL-17 receptor gene expression is also evident in EoE lesions. Analysis of the transcriptome in EoE tissue revealed a distinctive Th2 pattern, demonstrating significantly increased mRNA levels of eotaxin-3, IL-5, IL-5 receptor α-chain, and IL-13 [81].

In a murine model of allergen-induced EoE, B cells were not required for the development of EoE symptoms, and CD4+ T cells were identified as key drivers of the initiation of experimental esophagitis in that model [48]. Interestingly, there are indications that B cells play an important role in the recruitment of eosinophils to intestinal draining lymph nodes during gastrointestinal helminth infections, a strongly type 2-associated condition. This recruitment depends on IL-4Rα expression on B cells and involves B-cell-stromal cell crosstalk, which enhances eosinophil homing and supports the development of the adaptive immune response [82]. Overall, while murine experiments indicated that B-cell deficiency does not significantly alter the development of allergen-induced EoE, further investigation is needed into the roles of cytokine-producing B-cell subsets and the complex interaction networks between B cells and other immune cells.

B cells can also exert immunosuppressive functions through the secretion of anti-inflammatory cytokines such as IL-10 and IL-35. Several types of so-called regulatory B (Breg) cells have been described during the past 20 years [83]. Concerning EoE, there are several potential ways in which Breg cells could play a role. A subset of inducible Breg cells, termed BR1 cells, was characterized by high expression of surface CD25 and CD71 and low CD73. These cells produced high levels of IL-10 and effectively suppressed antigen-specific T-cell proliferation. In addition, BR1 cells were skewed toward the production of IgG4 antibodies [84]. Given the high levels of IgG4 antibodies observed in the esophagus of EoE patients, as well as increased levels of IL-10 [37], it could be speculated that BR1 cells contribute to IgG4 production. As mentioned earlier, another type of B cells with regulatory capacity was reported in the context of intestinal inflammation. IL-33 induced IL-10 production in a subset of B-1 cells termed Breg^IL-33, which protected mice against intestinal inflammation in a mouse model of inflammatory bowel disease [64]. Therefore, Breg cells may play a role in the regulation of inflammatory responses in EoE and potentially confer some degree of protection against excessive inflammation. Further studies are required to address these questions.

Antibodies in EoE

EoE is considered to be a mixed or a non-IgE-mediated allergic response to food and environmental allergens where IgE is not the main driver of the impaired immune reaction [85]. However, IgE levels have been actively investigated in EoE, especially in relation to food triggers [86–88]. Total IgE levels were elevated in esophageal biopsies and serum of EoE patients compared to healthy controls in some studies [47, 53], while others showed no difference [37, 89]. Specific IgE against several trigger foods such as wheat, soy, peanuts, milk, eggs, rice, corn, and seafood has also been reported. Of the known potential trigger foods for EoE, IgE against cow’s milk was found most consistently. Sensitization to aeroallergens was found as commonly as that to food allergens, and a significant portion of the patients had a cluster of sensitization to multiple allergens [54, 90]. However, the role of IgE in the development of EoE remains uncertain. Moreover, the ability of serum allergen-specific IgE to reliably forecast genuine food triggers for EoE has proven inconsistent [91]. The total and food allergen-specific IgE levels were not different between individuals who did and did not respond to food elimination treatment [88, 89]. These findings indicate that IgE is not likely to play a predominant role in mediating EoE.

In a landmark study by Clayton et al. [36], EoE patients were treated with the anti-IgE drug omalizumab. This treatment was not effective in reducing symptoms and histological findings, suggesting that IgE is not a major driver of EoE. Interestingly, total IgG4 in esophageal tissue biopsies was strongly elevated, ranging from a 4-fold to a 45-fold increase of IgG4 compared to the total IgG [36–38]. IgG4 staining in esophageal biopsies from both adult and pediatric patients could identify 48% of patients with EoE and a strong association between IgG4 staining in the distal esophagus and basal zone hyperplasia further connected heightened IgG4 levels to EoE [92]. Staining of IgG4-positive cells also showed differentiation between patients with active and inactive forms of EoE [92, 93]. Furthermore, relatively large numbers of IgG4-positive cells as well as intrasquamous IgG4 were found in esophageal biopsies [56, 58]. Unlike esophageal IgG4, total IgG4 in serum was not always as elevated, with results ranging from no increase at all to a 12-fold increase compared to controls [36, 58].

Production of IgG4 is associated with chronic antigen exposure. In the context of allergic disease, IgG4 is considered an anti-inflammatory immunoglobulin isotype that can interfere with IgE-mediated allergic responses. Increased allergen-specific IgG4 is frequently observed during allergen-specific immunotherapy [94]. Allergen-specific IgG4 antibodies that can compete with IgE for allergen epitopes are considered to play a key role in the protection against type I hypersensitivity reactions [95, 96]. Its inability of IgG4 to fix complement and its capacity to undergo Fab arm exchange are key factors conferring anti-inflammatory properties to IgG4. Fab arm exchange involves the swapping of half molecules, or Fab arms, between different IgG4 antibodies. As a result, IgG4 antibodies acquire a bispecific nature, meaning they contain two different antigen-binding sites rather than two identical ones. Bispecific IgG4 antibodies are less likely to cross-link antigens to form immune complexes. This reduces their capacity to trigger immune responses compared to other immunoglobulin subclasses [97].

Both the causes as well as the consequences of high levels of IgG4 in EoE patients remain largely unexplored. Impaired epithelial barrier integrity may be a cause for chronic allergen exposure locally in the esophagus of EoE patients and subsequent IgG4 production. Whether IgG4 plays a role in the pathogenesis of EoE or it should be considered a bystander effect is still open for exploration. This raises the question of whether IgG4 plays a role in the pathogenesis of EoE. One might speculate that high local concentrations of IgG4 antibodies for a limited range of antigens may reduce the likelihood of Fab arm exchange with IgG4 of different specificities, resulting in fewer bispecific antibodies. This would allow IgG4 antibodies to cross-link specific allergens and form immune complexes that can drive inflammation. This possibility is supported by the finding that there is co-localization of IgG4 and antigens that are commonly thought to trigger EoE, such as cow’s milk proteins. These allergens and IgG4 bind to each other to form immune complexes. In the vicinity of these complexes, eosinophil-derived proteins can also be found [98].

To understand the role of antibodies in EoE, analysis of allergen-specific antibodies rather than total immunoglobulin concentrations is preferred. Since the finding that total esophageal IgG4 was increased significantly in EoE patients, several groups have investigated the specificity of IgG4 in esophageal biopsies and sera. Food allergen-specific IgG4 was found against wheat, milk, egg, and nuts in EoE tissue and serum. The highest titers of sIgG4 were found against cow’s milk proteins and gluten, confirming the findings of sIgE studies that these are common trigger foods [36, 37, 99]. Interestingly, gliadin and casein were found in the esophageal mucosa up to 96 h after elimination from the diet along with IgG4 staining, giving further evidence for these foods as potential triggers for IgG4 production [100]. Serum sIgG4 levels to both food and aeroallergen proteins were higher in EoE patients than in non-EoE allergic controls [101].

Other immunoglobulin isotypes have also been investigated in relation to EoE. Tissue total IgM measurements showed no or little significant difference between EoE patients and controls [36, 37]. Total IgD was elevated in 60% of pediatric patients [102]. Total IgA levels have been reported with similar conclusions: one study reported no measurable difference in mucosal total IgA to controls [36], others have found a 2-fold increase in mucosal IgA with only weak correlations with esophageal eosinophil numbers [37, 38] or an increase with greatly varying concentrations [103]. Notably, the total and specific IgA levels vary widely between studies, possibly due to the difference in IgA concentration between mucosal brushings and the underlying tissue. Food allergen-specific IgA was also measured in EoE patients. However, food-specific IgA was not or barely detectable in esophageal tissue biopsies and was not significantly raised compared to total IgA in patients or controls [103]. In the mucosa, IgA specific for casein, gluten, soy, and egg was found in significantly greater quantities than in controls.

In summary, the current evidence suggests that, although IgE is often present in EoE patients, it does not appear to play a major role in the pathogenesis of the disease. On the other hand, the elevated levels of IgG4 in biopsies and sera suggest a potential role in the development of the condition, although further studies are required to explore this further (Fig. 2).

Fig. 2.

Systemic and local humoral immune responses in EoE. The ingestion of allergens or microorganisms induces a humoral immune response both in the circulation and in the esophagus. Systemically, total and food allergen-specific IgE and IgG4 are increased together with total IgD and aeroallergen-specific IgG4, while IgM levels do not change. Locally, food allergen-specific IgA and total IgG4 show increased levels.

Animal Models of EoE

Animal models have been essential in revealing fundamental pathophysiological mechanisms of EoE and confirming potential therapeutic interventions. Murine models were mainly used, and they have significantly enhanced the comprehension of the immune mechanisms involved in EoE. The initial instance of such models was delineated by Mishra et al. [104] in 2001, involving mice exposed intranasally to Aspergillus fumigatus. This administration led to the accumulation of eosinophils in both the bronchial and gastrointestinal regions, along with esophageal epithelial hyperplasia. Similar to the majority of EoE models, this particular model is most effectively employed in a BALB/C mouse, known for its elevated susceptibility to type 2 inflammatory response, leading to eosinophil reactions beyond the esophagus. The aspergillus model was also beneficial for enlightening the crucial roles of α4β7-integrin and periostin in eosinophil recruitment [16, 105, 106]. Maskey et al. [107] showed that increased levels of Th2 cytokines IL-4 and IL-13, rather than IL-5, along with elevated Th2 chemokine CCL11 and compromised IgA and IgG, could play a role in the severe pathogenesis of EoE and widespread organ inflammation in C57BL/KaLawRij-Sharpin^cpdm mice. Camilleri et al. [108] developed a gene therapy-oriented mouse model for peanut-induced EoE, replicating the human disease through sensitization and challenge with peanut extract. Thus, this murine model demonstrated that a single treatment with AAVrh.10mAnti-Eos has the potential to provide a persistent therapeutic benefit to EoE cases. Animal models have also proven that experimental EoE can be generated in the absence of IgE, providing further evidence that IgE is not a major driver for the disease [33, 109].

The continuation of these studies involved diverse animal models, such as guinea pigs, resulting in valuable findings [110]. However, the translation of findings from murine models to human disease is subject to certain limitations. For instance, mice lack esophageal submucosal glands (related to esophageal repair) that exist in humans. The frequency of IgE response in mice is considerably higher (reaching up to 80%) under sensitization with IgE antibodies in contrast to the range of 0.1–15% observed in humans [111]. Another limitation of the use of murine models is the fact that mice do not have IgG4. Given the strong association between elevated IgG4 and EoE, this may hamper accurate replication of the human situation in these models. Further, EoE in humans is a chronic disease; thus, the EoE model should demonstrate long-term disease characteristics. The short life span of mice does not permit the observation of symptoms occurring over an extended period [112]. Plundrich et al. [113] induced food allergy using hen egg white protein in young pigs. In this experimental model, pigs that received food allergen displayed OVA-specific CD4⁺ T cells and exhibited clinical signs of esophagitis. The study provided promising results leading to the conclusion that young pigs may be a useful large animal model for EoE to mimic the human counterpart. More recently, in 2022, Cortes et al. [112] established a novel swine model as a relevant large animal source for translational biomedical research in EoE with a potential for therapeutic development. Through intraperitoneal sensitization and oral challenge with hen egg white protein, the swine developed esophageal eosinophilia and exhibited immunological, pathological, and endoscopic markers associated with human EoE. The significance of animal models in elucidating the complex mechanisms of EoE, identifying therapeutic targets, and advancing our understanding displays the need of further research in this area.

Clinical Studies Exploring B Cell and Antibody Involvement

Cross-Sectional Studies

Recently, increasing data supports the involvement of IgG4 in EoE [36, 55]. Moreover, the association between esophageal IgG4 levels and histopathologic features in EoE patients has been established, providing insights into the potential significance and regulation of IgG4. Rosenberg et al. [37] found that tissue IgG4 levels correlated with eosinophil counts, histologic grade/stage scores, IL-4, IL-10, and IL-13. Esophageal IgG subclasses, in addition to IgA and IgM, were also elevated in EoE patients [37]. Children diagnosed with EoE exhibit notably elevated levels of IgG4 to milk allergen components [99, 114]. Indeed, the serum IgG4/IgE ratio was >10,000 among the pediatric cohort with EoE [99]. Similar to total IgG4 values, food-specific IgG4 was elevated in EoE patients. Masuda et al. [115] demonstrated that food-specific IgG4 levels for milk and wheat are increased in plasma and throughout the upper gastrointestinal tract in individuals with EoE. These levels correlate with endoscopic results and esophageal eosinophilia [115]. In addition to its potential involvement in immune tolerance to allergens, IgG4 may be associated with structural tissue remodeling [73]. Heightened angiogenesis is evident in the esophageal mucosa among pediatric patients diagnosed with EoE [71]. These findings point out that further studies are needed to provide a better understanding of the roles of IgG4 in EoE.

Longitudinal Studies and Treatment Outcomes

Various molecular aspects have been adopted in the context of EoE. Recently, there has been a reported association between EoE and IgG4, as opposed to IgE. Weidlich et al. [58] evaluated the IgG4 and IgE expression in EoE patients before and after 8 weeks of budesonide treatment. In their prospective investigation, a high number of esophageal IgG4-positive plasma cells were evident in EoE patients, and the frequency of these cells was significantly reduced by therapy. Notably, there were no differences in IgE levels. IL-10 plays a crucial role in guiding the switch to IgG4 in activated B cells [116]. Correspondingly, cells expressing IL-10, potentially associated with the Treg-cell lineage, have been identified at increased frequencies in the mucosa of individuals with EoE [117, 118]. In their investigation of the impact of the two-food elimination diet (excluding milk and wheat) on individuals with EoE, Appana et al. [119] demonstrated that the reduction in IL-10 cell frequencies was substantiated by histological variations, emphasizing the crucial role of IL-10 in the pathogenesis of EoE. Agents that stimulate the production of IL-10, such as butyrate derived from microbiota, might downregulate the expression of eosinophil-specific chemokines and the migration of eosinophils. Additionally, they could potentially restore the disrupted barrier function caused by IL-13 in the esophageal mucosa [120].

IL-9, a T-cell-associated cytokine, could also potentially play a role in the production of IgG4 in EoE, as it enhances the production of immunoglobulins induced by IL-4 [121]. Specifically, IL-7 and IL-35 favor IgG4 expression by inducing IL-9 production in T cells [121, 122].

Therapeutic Implications

Given the relatively recent emergence of EoE, there is a limited array of treatment options. Most of these options are also nonspecific, as they are commonly used or have been developed for other disorders, and their effectiveness varies among patients.

Proton pump inhibitors (PPIs) have been the first treatment used in the management of EoE. Initially, PPIs were used to distinguish EoE from gastroesophageal reflux disease with eosinophilia, as the consensus was that EoE was a consequence of gastroesophageal reflux disease. EoE would not respond to the therapy, therefore allowing for a full diagnosis [123]. However, a subset of EoE patients does respond to high doses of therapy, in a condition denoted as PPI-responsive esophageal eosinophilia, making PPIs effectively the first line of treatment used in EoE [124–126].

The second line of treatment is swallowed topical corticosteroids. They are more efficient and have fewer adverse effects compared to systemic corticosteroids, although they still cannot ensure long-lasting remission in all patients [127, 128]. Budesonide and fluticasone, originally developed for the management of asthma, are recognized as highly effective medications and none is preferred over the other [129, 130]. Budesonide orodispersible tablet showed the ability to maintain EoE patients in remission in several studies and is now widely used in the management of EoE, although it is often used as an off-label drug and is only approved for the specific use in EoE in Europe [131–135].

Immunosuppressive agents are a third class of drugs currently in use for EoE treatment. Th2 cytokines are the main target of potential therapies, as it has been shown that they are present in EoE biopsy specimens and are thought to have a role in the pathophysiology [136–138].

Dupilumab, approved as the first targeted medicine for the treatment of EoE both in Europe and in the USA, is a monoclonal antibody that targets IL-4 and IL-13 by binding to the IL-4 receptor alpha subunit to suppress Th2 response and reduce the inflammatory state [139–141]. Other biologics are being studied, including the anti-IL-5 mepolizumab and reslizumab and the IL-5 receptor blocker benralizumab. Considering the direct relation between eosinophils and IL-5, this target was promising and proved to effectively reduce the number of eosinophils in the esophagus. However, some of these studies showed that targeting IL-5 did not improve symptoms nor conferred complete clinical remission, while others proved the opposite, revealing heterogeneity in the response to this treatment [142–145].

Current treatments are still under investigation and need further characterization and optimization to be specifically used in EoE. Notably, no drug has been exclusively formulated for EoE; instead, existing treatments have been adapted from those used in the management of conditions such as asthma or other Th2-related diseases. This limitation underlines the necessity of finding an ideal formulation that can effectively address the diverse EoE manifestations.

Food elimination diets have been proven to be effective in the management of EoE since the early reports about the disease [86, 146–149]. Identifying food triggers holds promise for designing specific elimination diets. IgE testing is not able to predict the success of a diet, but other antibody isotypes might be used instead [88, 91, 150, 151]. One study found that esophageal total and food allergen-specific IgG4 can be decreased with food elimination [55]. From a different perspective, it has been found that IgA- and IgG4-specific for gluten, casein, soy, and egg are elevated in EoE patients that had these triggers [139]. Additionally, patients with a known dairy trigger showed higher dairy-specific IgG1, IgG2, IgG4, IgM, IgA, and IgE [38, 152]. On the other hand, one study showed that food-specific IgA collected from EoE biopsies is not associated with known food triggers [38, 103].

All this taken into consideration, profiling allergen-specific immunoglobulins should be explored to determine their value to formulate a targeted diet. However, it must be noted that the screening methodologies, i.e., whether antibodies are taken from brushings, saliva, or biopsies, have a role in determining the success of a diet.

Future Directions

Unanswered Questions in B Cell and Antibody Research in EoE

So far, B cells have received limited attention concerning their role in the pathogenesis of EoE, rendering this field of research open to discoveries and new insights. Despite several studies demonstrating the potential involvement of B cells in EoE, their precise role in the pathogenesis of EoE remains to be elucidated.

Recent advances in EoE research suggested that EoE can also manifest without significant eosinophil infiltration and that different variants of EoE may exist [153, 154]. In the lymphocytic variant, esophageal infiltration by eosinophils is absent, while lymphocytic infiltration better characterizes the patients’ esophagus, leading to EoE not being diagnosed. This hints that symptoms, immunohistology, and general inflammatory status are better EoE markers than eosinophil numbers. The suggested lymphocytic variant should be further investigated, particularly focusing on B- and T-cell subsets and their relative role [154].

The role and the presence of B cells must be carefully assessed. Different studies have shown that B lymphocytes and plasma cells are present in the esophagus of EoE patients and experimental animal models [48, 49, 53]. Local and circulation antibody production has also been confirmed, although the effective role of these antibodies is not yet clear. Rituximab, a B cell-depleting antibody, successfully reduces the levels of autoantibodies and the antigen-presenting functions of B cells in autoimmune diseases [155]. Although not specifically studied in EoE, this treatment seems to benefit IgG4-related disease patients [156].

Indeed, the inflammatory milieu provides an opportunity for all immune cells to interact and have a role in the pathogenesis of EoE. Research questions are mainly directed toward clarifying whether these cells are actively participating in esophageal inflammation, disruption, and dysfunction, rather than being bystanders. While the humoral response, likely triggered by food allergy, leads to skewed production of food allergen-specific antibodies toward IgG4, it is not excluded that this process contributes to the underlying mechanism of EoE. However, the specific mechanisms driving this phenomenon have yet to be fully elucidated. Food-specific antibody levels can, sometimes, predict the success of an elimination diet [38, 55, 88, 91, 150–152]. Several studies evaluated systemic immunoglobulin levels in individuals with food allergies and successfully predicted symptom improvement. In particular, high IgG4 against specific allergens at baseline determined which food allergens should be excluded from the diet in allergic and EoE patients, and the efficacy of the diet was accompanied by lower IgG4 levels [157, 158]. Additional studies could facilitate clinicians’ work and patients’ lives, by formulating a targeted elimination diet that can effectively reduce the symptoms. On the other hand, studying antibody levels in EoE could also lead to potential therapeutic targets, if it is discovered that reducing a particular isotype results in improvement of the typical EoE features. To achieve this goal, a focus on locally and systemically produced antibodies should be intensified. Specifically, it is not well understood whether IgG4 directly influences the course of the disease, despite being elevated [36–38]. Moreover, other isotypes should be further investigated to fill the existing gap in our understanding of the disease mechanisms, as they also might be differentially expressed in EoE patients.

Identification of Specific Triggers

Antibody research in EoE is mainly focused on food antigens. The starting point of most of these studies is food triggers, which are usually patient-dependent, and not EoE-dependent. The food-induced immediate response of the esophagus (FIRE) syndrome has been recently identified in EoE patients [159]. The onset of this condition occurs within minutes of ingesting a trigger food, manifesting symptoms distinct from dysphagia, such as esophageal discomfort [160]. Due to its recentness, further studies are needed to understand the mechanisms underlying food-induced immediate response of the esophagus. This highlights the need to identify specific trigger antigens, which could also mean broadening the investigation to environmental allergens. Recent explorations of the topic reveal that aeroallergens may trigger EoE symptoms [161–163]. This is also supported by the fact that EoE is usually correlated with asthma and allergic rhinitis, both conditions directly induced by aeroallergens, and with the occurrence of EoE after immunotherapy [164, 165]. Our comprehension in this domain is still incomplete and requires further investigation, particularly directed toward identifying antigens that are specific to either patients or EoE.

Precision Medicine and Multi-Omics Approaches

Although EoE diagnosis is confirmed by a precise count of eosinophils, its treatment is not straightforward due to the variability in patient response. Personalized medicine is thus the preferred method for addressing nonresponsive patients. One approach involves identifying potential targets for elimination diets via antibody profiling. More recently, new technologies have been applied in research and clinical practice to identify potential therapies, to enhance treatment options and outcomes.

Omics technologies are gaining attention because they can stratify patients based on specific markers and reveal immunological mechanisms. Bulk RNA sequencing and single-cell RNA sequencing are currently used to study tissue samples, yielding promising results. Differences in expressed transcripts have been found between EoE and control samples and between active and inactive EoE samples [21, 166–168]. RNA-seq can help effectively monitor disease pathogenesis before, during, and after treatment, helping uncover underlying mechanisms [169]. Notably, transcriptomic analysis revealed that EoE shares a set of disease-specific transcripts with atopic dermatitis, suggesting shared therapeutic strategies [170].

B cells and plasma cells were not analyzed in detail in most single-cell transcriptomic studies of EoE esophageal biopsies [45, 50, 171]. This may be due to the low number of these cells identified, possibly because of biopsy depth and the limited subepithelial tissue captured [172]. Furthermore, these studies included limited numbers of cells and focused on more abundant cell types such as T cells, mast cells, and epithelial cells [45, 50, 171]. A recent study in which >400,000 cells from esophageal biopsies from 7 healthy controls and 15 EoE patients (8 active and 7 in remission) were profiled using single-cell transcriptomics provides a valuable overview of the cellular composition of the EoE esophagus. Besides identifying various populations of eosinophils, mast cells, macrophages, dendritic cells, inflammatory fibroblasts, ILC2s, T cells, and natural killer cells, five B-cell subsets were also observed. The study highlighted a significant relationship between IgG4-expressing plasma cells and IL13RA2+ inflammatory fibroblasts, which were nearly absent in healthy individuals but expanded in some active EoE patients. IgG4 levels correlated with esophageal eosinophil counts and disease severity, while IL13RA2+ fibroblasts were linked to increased proportions of IgG+ plasma B cells and SEPP1+ macrophages. This connection underscores the role of IgG4 and plasma cells, alongside inflammatory fibroblasts, in driving inflammation and tissue damage in EoE. It was found that active EoE is characterized by a noticeable increase in plasma B cells, particularly those expressing IgG and IgM, along with cycling plasma cells. In non-active EoE cases, these cells were rarely observed or absent. IgG+ plasma B cells exhibited distinct subsets of IgG genes, and cycling plasma B cells significantly increased. Additionally, the study identified rare IgE+ memory B cells and IgE-expressing plasma B cells, which were absent in healthy individuals, indicating their unique link to the active phase of the disease [173].

Bulk and single-cell data can be merged via multi-omics integration. Recent investigations using this approach revealed that a subset of patients were characterized by microbial dysbiosis, suggesting that in this group, antigen-mediated response might be different than in others [174]. Such approaches have the potential to increase our understanding of hidden disease mechanisms and determine a specific EoE signature, which would allow a precise formulation of therapeutic agents [168].

Conclusion

B cells may contribute to the pathogenesis of EoE through cellular effector mechanisms as well as through the production of antibodies (Fig. 3). Emerging evidence highlights that B cells, particularly through the production of IgG4, may contribute significantly to the disease’s chronic inflammatory state and tissue remodeling. Despite the complexity and multifaceted nature of EoE, understanding these immunological mechanisms opens avenues for novel diagnostic and therapeutic strategies. Future research should aim to further clarify the interactions between B cells, antibodies, and other immune components to develop targeted treatments that can more effectively manage and potentially ameliorate EoE symptoms. Addressing these gaps will not only improve patient outcomes but also enhance our overall comprehension of immune-mediated diseases.

Fig. 3.

Putative mechanisms of B cell and plasma cell involvement in the immunopathogenesis of EoE. Regulatory B cells, capable of secreting immunosuppressive cytokines, may reduce inflammation by suppressing effector T-cell responses. IL-10 can act autocrinally on B cells, shifting antibody production from IgE toward IgG4. In EoE patients, increased frequencies of proangiogenic B cells producing various proangiogenic and pro-fibrotic cytokines have been identified. These cells may contribute to angiogenesis and tissue remodeling in EoE. The type 2 cytokine environment in EoE can drive IgE production, especially in response to food antigens, leading to mast cell degranulation. Elevated levels of food-specific IgG4 and potentially other IgG subclasses may inhibit this process. In parallel, food-specific IgG could contribute to inflammation in EoE through immune complex formation, triggering eosinophil degranulation and activating dendritic cells, which subsequently activate allergen-specific T cells.

Conflict of Interest Statement

W.V. has received research grants from Promedica Stiftung, Switzerland, and EoE Stiftung, Switzerland, and consulting fees from Mabylon AG, Switzerland. Hang Du received a training grant from the China Government (China Scholarship Council Programme, project ID: 202306350024). Other authors have no conflicts of interest to share.

Funding Sources

This study was supported by the Promedica Stiftung (Grant #1515/M) (W.V.) and by a training grant from the China Government (China Scholarship Council Programme, project ID: 202306350024) (H.D). The funders had no role in the study design, data collection or analysis, the decision to publish, or the preparation of the manuscript.

Author Contributions

Willem van de Veen designed the study and revised the manuscript. Manal Bel Imam, Hang Du, Özge Ardicli, and Jenne Meinema conducted literature research and contributed parts of the manuscript text. Özge Ardicli, Manal Bel Imam, and Hang Du created the figures. Manal Bel Imam wrote the first draft of the manuscript. All authors approved the final version of the article.

Funding Statement

This study was supported by the Promedica Stiftung (Grant #1515/M) (W.V.) and by a training grant from the China Government (China Scholarship Council Programme, project ID: 202306350024) (H.D). The funders had no role in the study design, data collection or analysis, the decision to publish, or the preparation of the manuscript.