Gastroenterology
Volume 137, Issue 4 , Pages 1238-1249, October 2009

Biology and Treatment of Eosinophilic Esophagitis

  • Marc E. Rothenberg

      Affiliations

    • Corresponding Author InformationReprint requests Address requests for reprints to: Marc E. Rothenberg, MD, PhD, Division of Allergy and Immunology, Cincinnati Children's Hospital Medical Center, 3333 Burnet Avenue, Cincinnati, Ohio 45229. fax: (513) 636-3310

Received 26 May 2009; accepted 1 July 2009. published online 13 July 2009.

John P. Lynch and David C. Metz, Section Editors

Article Outline

Eosinophilic esophagitis is a recently recognized but expanding disorder characterized by antigen-driven eosinophil accumulation in the esophagus. Symptoms frequently mimic those of gastroesophageal reflux disease, but the diseases are distinct in their histopathology, gene expression signature, response to therapy, hereditary risk, and association with allergies. The pathogenesis of eosinophilic esophagitis involves environmental and genetic factors, particularly food antigens and expression level of the eosinophil chemoattractant eotaxin-3, respectively. Analyses of gene expression signatures and animal models have indicated the importance of adaptive T-cell immunity that involves interleukin-5 and interleukin-13–induced esophageal epithelial cell responses. Symptoms, dysregulation of esophageal gene expression, and pathology are largely reversible following reduced exposure to specific food antigens as well as anti-inflammatory therapy, but chronic treatment is necessary to prevent relapse. Therefore, eosinophilic esophagitis is a disease with unique features that include chronic esophagitis, atopy, immune sensitization to oral antigens, reversibility, and familial association.

Abbreviations used in this paper: ECP, eosinophil cationic protein, EDN, eosinophil-derived neurotoxin, EE, eosinophilic esophagitis, EPO, eosinophil-derived peroxidase, GERD, gastroesophageal reflux disease, hpf, high-power field, IL, interleukin, MBP, major basic protein, SNP, single nucleotide polymorphism, TGF, transforming growth factor, TLR, Toll-like receptor

 

Unlike all other segments of the gastrointestinal tract, the esophagus is normally devoid of eosinophils,1 so the finding of esophageal eosinophilia denotes pathology. However, the mere presence of esophageal eosinophils is not specific for a particular disorder because eosinophil accumulation in the esophagus occurs in a variety of states, including eosinophilic esophagitis (EE, also referred to as EoE), eosinophilic gastroenteritis, gastroesophageal reflux disease (GERD), chronic (noneosinophilic) esophagitis, parasitic and fungal infections, inflammatory bowel disease, hypereosinophilic syndrome, scleroderma, drug and/or iatrogenic-induced states such as caustic injury,2 multiple convulsive therapy syndrome,3 and immunosuppression, especially following solid organ transplantation (see Supplementary Table 1).4 As part of a spectrum of eosinophilic gastrointestinal disorders, it is notable that EE can evolve into and/or be associated with eosinophilic gastritis, enteritis, and/or colitis.2 The diagnosis of EE requires elimination of other causes of esophagitis, especially GERD; EE is generally distinguished from GERD by persistent esophageal eosinophilia despite adequate acid neutralization therapy before endoscopy.5 EE can be associated with other diseases such as Rubinstein–Taybi syndrome6 and celiac disease.7, 8, 9, 10, 11 The similarities between EE and celiac disease provide insight into possible pathogenic mechanisms of EE. Celiac disease is a prototypic T-cell–mediated immune disease triggered by an oral antigen (gliadin). Although celiac disease has a significant autoimmune component that has not yet been associated with EE, both diseases are associated with immune cell–mediated epithelial cell abnormalities and can be reversed by a food elimination diet, although EE does not typically respond to gliadin avoidance. The EE transcriptome (genes that are up-regulated or down-regulated in esophageal tissue of patients with EE compared with normal esophageal tissue)12 includes genes that regulate the immune response and have been associated with celiac disease. The transcriptome includes overexpression of genes that encode MICA and MICB (ligands that activate NKG2D) and the cytokine interleukin (IL)-15; all have been shown to induce intraepithelial lymphocyte activation.13, 14, 15, 16 Genetic studies of celiac disease have identified the HLA class II genes (encoding DR HLA-DQ2 or -DQ8 molecules), whose products function in the adaptive immune system, as main risk factors along with environmental factors.17 Genome-wide association studies of celiac disease have identified genes that regulate immunity, including IL18RAP, IL12A, and CCR3 (encodes the receptor for the chemokine eotaxin), and genetic variants in the IL2/IL21 locus.18, 19 A recent study linked blood eosinophilia with several genes including celiac disease locus that contains SH2B3 (also known as LNK),20 providing further support for a connection between eosinophils and celiac disease.

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Symptoms and Disease Characteristics 

EE has many different presentations; patients commonly have difficulty eating, failure to thrive, vomiting, epigastric or chest pain, dysphagia, and food impaction.21, 22, 23 These symptoms appear to occur in a progressive order, presenting from infancy into adulthood in the order listed,24, 25 so this could be the natural history of EE. Adult patients typically have recurrent dysphagia and food impactions that are refractory to anti-GERD therapy; in fact, recent studies indicate that 10%–50% of adult male patients with these symptoms have EE.26, 27 Although a fixed stricture could account for the esophageal dysphagia and food impaction observed in some patients with EE,28 evidence is mounting that the esophagus displays impaired smooth muscle function, likely from asynchrony of circular and longitudinal muscle contraction during swallowing.29 A variety of motor disturbances that are reversible with therapy have been reported in patients with EE.30, 31 EE patients are predominantly young males24 who have a high rate of atopic disease and normal esophageal pH compared with patients with GERD. Although EE was originally recognized in pediatric patients, it has similar characteristics (including atopic sensitization) and occurrence rates in adults.32 EE frequently presents during infancy,33 but the disease has also been recognized in patients older than 90 years.34 EE is a chronic disorder that has no significant evidence of spontaneous remission, even over a 14-year period,35 but some patients have seasonal variations in symptoms,36 consistent with an etiology related to airborne allergen exposure.

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Epidemiology 

EE has been reported from all continents except Africa.37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47 Although the exact incidence of EE has not been determined, a mini-epidemic has been noted over the past decade. Liacouras et al found that ∼10% of pediatric patients with GERD-like symptoms are unresponsive to acid blockade and have EE.48, 49 Furuta et al reported that 6% of patients with esophagitis have EE.50 Over a 16-year observation period, Straumann and Simon documented a prevalence of ∼1:4000 adults in Switzerland.51 Croese et al reported EE to be present in 1:70,000 adults in an Australian provincial city,37 and another study revealed a ∼0.4% prevalence in a random adult Swedish population.52 EE was identified in 0.1% of children in the Cincinnati metropolitan area over a 9-year time period24 (M. Rothenberg, unpublished findings). In an outpatient US-based military hospital, EE was identified in 6.5% of adult patients undergoing endoscopy53; this study was notable because the EE population included a large percentage of black patients, suggesting that the predominance of EE in white patients reported in other studies might be due to patient referral patterns. These epidemiology studies indicated that EE has a prevalence that is comparable to that of inflammatory bowel disease but less than that of celiac disease.54, 55 Although EE has only been appreciated as a separate disease entity in the past 10 years, it was probably previously unrecognized among patients diagnosed with reflux esophagitis.56

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Esophageal Numbers of Eosinophils and Histopathology 

It is important to emphasize that there is currently no diagnostic test for EE; instead, diagnosis depends on coordinated clinical and pathologic data. Although a minimum level of 15 eosinophils/high-power field (hpf) has been proposed to be part of the diagnostic criteria of EE,5 any level has to be interpreted in the context of clinical data. Determination of the number and location of eosinophils is helpful in trying to differentiate EE from GERD. As many as 6 eosinophils/hpf (400×) might indicate GERD, whereas more than 20–24 eosinophils/hpf appear to indicate EE,48, 57 especially when these levels are encountered in patients who receive anti-GERD therapy. The number of eosinophils in the esophagus is negatively correlated with response to conventional anti-GERD therapy.48 In particular, numbers of esophageal eosinophils ≥24/hpf have been correlated with lack of responsiveness to anti-GERD therapy. These levels might indicate EE rather than GERD or chronic esophagitis, especially in patients who are already on anti-GERD therapy. Patients with intermediate levels of eosinophils (7–15/hpf) often present a diagnostic dilemma for several reasons. First, the exact cutoff eosinophil concentration value used in the diagnosis of EE has not been determined. Second, EE is often a patchy disease; diagnosis varies based on the number of biopsy specimens obtained, and the maximum level of eosinophils can vary.58 In 2007, an expert panel stated that EE is a clinicopathologic disease that should be considered in patients (1) with symptoms including but not restricted to food impaction and dysphagia (in adults) and feeding intolerance and GERD symptoms (in children); (2) with >15 eosinophils/hpf; and (3) in whom other disorders associated with similar clinical, histologic, or endoscopic features, especially GERD with sustained esophageal eosinophilia (peak numbers of eosinophils >15/400× hpf) after adequate GERD therapy (eg, proton pump inhibitors), have been excluded.5 For research studies, a higher threshold (≥24 eosinophils/hpf) has been recommended.5 Based on histologic analysis of a large cohort of adult patients and the performance of esophageal pH monitoring, 1, 2, 3, and 6 esophageal biopsy specimens from the mid and distal esophagus provide sensitivity levels of 73%, 84%, 97%, and 100%, respectively, using 15 eosinophils/hpf.58 Therefore, at least 3 biopsy specimens are recommended for the diagnosis of EE.5

In addition to high numbers of eosinophils, other features distinguish EE from GERD (see Supplementary Table 2). The presence of eosinophils in both the proximal and the distal esophagus typically denotes EE, whereas the accumulation of eosinophils mainly in the distal esophagus is typical, but not specific, for GERD.59 In addition, esophageal tissue from patients with EE typically contains a thickened mucosa with basal layer hyperplasia (assessed by Ki-67 antigen staining) and papillary lengthening59, 60; this appears to be more pronounced than in GERD.61 Consistent with increased proliferation of the basal cells in response to allergen provocation, rather than acid exposure, biopsy specimens from patients with EE demonstrate down-regulation of cyclooxygenase-2.62 Furthermore, esophageal biopsy specimens from patients with EE can have eosinophil surface layering and eosinophilic microabscesses, processes rarely associated with GERD.63 In addition to eosinophils, esophageal biopsy specimens from patients with EE have increased numbers of dendritic cells and mast cells, generally more than in patients with GERD.64, 65, 66 It was recently proposed that extracellular deposition of the granule protein eosinophil-derived peroxidase (EPO) occurs in the esophagus of patients with EE; this might be more closely correlated with clinical features than eosinophil number.67 Radiographic and endoscopic studies have identified many features of EE, including strictures, mucosal rings, ulcerations, whitish papules, and polyps23, 68; however, nearly 30% of patients with EE have normal endoscopy results,69 indicating the importance of endoscopic biopsies in diagnosis of this disease.

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Disease Pathogenesis 

The pathogenesis of EE is associated with atopy, based on studies of disease co-occurrence, studies in animal models, and the reported success of allergen avoidance (primarily diet control). The majority of patients have evidence of food and aeroallergen hypersensitivity50 and a concurrent history of respiratory allergies (see Figure 1).24 Unlike food anaphylaxis, which occurs in ∼15% of patients with EE,33 polysensitization to a variety of foods (based on skin prick testing) occurs in most patients with EE.70, 71 The key role of food antigen sensitization has been demonstrated by the success of reducing specific food exposures (based on results of skin and patch tests), avoiding the most common 6 food types, or using an amino acid–based formula; all of these approaches induce disease remission.72

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  • Figure 1. 

    Pathogenesis and treatment of EE. Aeroallergens, food allergens, and skin sensitization have been implicated in the pathogenesis of EE. Therapies such as an elemental diet, systemic glucocorticoids, and anti–IL-5 improve the microscopic features of EE, acting at different steps of pathogenesis. Determining the response to proton pump inhibitors (PPIs), which reduce gastric acidity, is important in the diagnosis of EE; inflammation is still present in patients following administration of these drugs. Allergens induce Th2 cells to produce IL-13, which can cause hyperplasic epithelial cells of the esophagus to overexpress eotaxin-3 and fibroblasts to overexpress periostin and down-regulate filaggrin. Eotaxin-3 and periostin overexpression cooperatively chemoattract CCR3+ cells (Th2 cells). Activated Th2 cells also produce IL-5, which regulates eosinophil numbers and their response to eotaxin-3. Inheritance studies indicate that there is a genetic component to EE; an SNP in eotaxin-3 has been associated with the disease. In addition to eosinophils, mast cells and lymphocytes (including B cells) accumulate in the esophagus to contribute to the local inflammatory responses observed in patients with EE.

Mouse models of EE have been developed through exposure of animals to allergens and overexpression of cytokines produced by Th2 cells.73, 74 Repeated intranasal exposure to the aeroallergen Aspergillus fumigatus induces simultaneous eosinophilic airway and esophageal inflammation (without inducing lower gastrointestinal eosinophilia).73 Intratracheal delivery of human or mouse IL-13 induces experimental EE in a dose-dependent manner75; this process can be blocked with a therapeutic antibody against human IL-13.76 Epicutaneous allergen sensitization potently primes for respiratory allergen–induced experimental EE77; this finding could be particularly important for understanding the pathogenesis of EE, because a large fraction of patients with EE had preceding allergic skin disease (atopic dermatitis).22 Collectively, these experimental systems have shown a connection between development of eosinophilic inflammation in the respiratory tract and esophagus, not only in response to external allergic triggers but also to intrinsic Th2 cytokines, and highlight the potential for sensitization to occur via cutaneous antigen exposure. It is notable that patients with allergic rhinitis have seasonal increases in esophageal eosinophils and patients with EE have seasonal variations in symptoms,78 providing clinical evidence to support a role for aeroallergen-driven eosinophil-associated responses in the esophagus.

Studies in mouse models have shown that Th2 signaling is required for induction of experimental EE. In particular, mice with a targeted deletion of STAT6 are protected (in part) from allergen- and IL-13–induced experimental EE.75, 77 Further, IL-13–deficient mice have reduced levels of allergen-induced experimental EE.77 IL-13 is overexpressed in the esophagus of patients with EE and selectively induces the eosinophil-activating chemoattractant eotaxin-3 by a transcriptional mechanism in esophageal epithelial cells.79

There is marked overexpression of approximately 1% of the genome in the esophagus of patients with EE compared with healthy individuals and patients with chronic esophagitis.12 This EE transcriptome is highly conserved across patient phenotypes, regardless of sex, age, or familial variants; eotaxin-3 is the most highly induced gene.12, 80, 81 In paraffin-embedded esophageal biopsy samples, levels of eotaxin-3 messenger RNA can be used to distinguish patients with EE from those with GERD.82 A comparison of transcriptomes of patients with allergic and nonallergic EE revealed that the gene expression signature is conserved between these 2 major phenotypes.12 This indicates that the effector phase of the disease is the same between phenotypes, regardless of what factors activated the inflammation. Interestingly, a specific group of genes in the EE transcriptome is directly induced by exposure of primary esophageal epithelial cells to IL-13 (including eotaxin-3, the gene that is most highly induced by IL-13).79 Other genes in the EE transcriptome that are regulated by IL-13 include periostin (markedly induced by IL-13 and overexpressed in EE tissues)83 and filaggrin (markedly down-regulated by IL-13 and decreased in EE tissues).12 Periostin is a fasciclin domain–containing extracellular matrix molecule that regulates eosinophil adhesion and promotes eotaxin-induced eosinophil recruitment.83 Filaggrin is a skin structural barrier protein; its loss of function is associated with increased skin permeability and susceptibility to atopic dermatitis in humans84 and atopic sensitization in mice.85 Notably, in contrast to atopic dermatitis that is associated with loss of function genetic variants of filaggrin, EE is associated with a functional impairment in filaggrin expression. Notably, IL-13 down-regulates filaggrin expression in skin keratinocytes,86 providing a mechanism by which food antigen–elicited Th2 cell adaptive immunity might impair esophageal barrier function, perhaps propagating local inflammatory processes (including sensitivity to acid) and increasing antigen uptake by cells in the esophagus. These processes might be particularly important because of the increased levels of activated mast cells and B cells and evidence for in situ production of immunoglobulins in the esophagus of patients with EE, demonstrated by histology and transcriptome analyses.12, 64, 66, 87

Analyses of lymphocyte-deficient mice have indicated the roles of T cells in the pathogenesis of EE. T-cell–deficient, but not B-cell–deficient, mice fail to develop antigen-induced EE.88 CD8+ and CD4+ T cells do not seem to be required for induction of Aspergillus fumigatus–induced experimental EE,88 indicating the involvement of a unique component of the adaptive immune system. There is evidence for the contribution of Th2 cell–derived IL-5 in the pathogenesis of EE; overexpression of IL-5 (by pharmacologic administration or in transgenic mice) induces experimental EE, whereas neutralization of IL-5 (with antibodies or gene targeting) blocks allergen- or IL-13–induced experimental EE in mice.73, 74, 75 Local eosinophils that have been activated by IL-5 have been shown to contribute to esophageal remodeling in mice with experimental EE.89 IL-5 is overproduced by circulating CD4+ T cells in patients with EE90 and in response to food antigen stimulation in vitro.91 Additionally, some (but not all) preliminary clinical studies have shown that administration of a humanized antibody against IL-5 reduces symptoms of EE.92, 93, 94, 95 A coordinated mechanism of disease pathogenesis is presented in Figure 1.

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Genetics of EE 

There is evidence that EE has a strong familial association.24, 96 Nearly 10% of parents of patients with EE have a history of esophageal strictures and approximately 8% have biopsy-proven EE.24 In a study of 798 pediatric patients, 27 were found to have at least one sibling or parent with EE; we have recently reported 26 multiplex families with EE and demonstrated conserved clinical, pathologic, and genetic features compared with patients with simplex EE.81 Familial EE is typically identified among siblings or between children and parents; however, 3 generations of affected distal relatives have been reported. Patel and Falchuk reported the occurrence of EE among 3 adult brothers with dysphagia.97

One widely used measure of familial aggregation is the sibling recurrence risk ratio (λS), which compares disease rates among siblings with the prevalence in the general population.98 A λS >1 indicates an increased risk of disease among siblings in the proband compared with the general population. Based on a population prevalence for EE of approximately 5 per 10,000 people, the estimated λS for EE is approximately 80.80, 99 Compared with common allergic disorders, such as atopy or asthma (λS is estimated to be approximately 2),99 the considerably higher λS for EE indicate that this disorder is likely to have a relatively large genetic component. One study has associated a single nucleotide polymorphism (SNP) in eotaxin-3 (+2496T>G, rs2302009) with EE using population-based case-control and family-based transmission disequilibrium analyses, but the disease-associated allele is only present in 14% of patients.12 Clearly, other genes are involved in EE risk, phenotype, and patient outcome.

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Potential Role of Eosinophils in EE 

Eosinophil granule constituents are readily detected in extracellular locations in the esophagus of patients with EE; there is strong evidence for in situ eosinophil activation and degranulation.100 In vitro studies have shown that eosinophil granule constituents are toxic to a variety of tissues, including the intestinal epithelium.101 Eosinophil granules contain a crystalloid core composed of major basic protein (MBP)-1 and -2 and a matrix composed of eosinophil cationic protein (ECP), eosinophil-derived neurotoxin (EDN), and EPO.102 These cationic proteins share certain proinflammatory properties but differ in other ways. For example, MBP, EPO, and ECP have cytotoxic effects on the epithelium in concentrations similar to those found in biological fluids from patients with eosinophilia. Additionally, ECP and EDN belong to the ribonuclease A superfamily and possess antiviral and ribonuclease activity.103, 104 EDN is an endogenous ligand for the Toll-like receptor (TLR)-2 that has the capacity to activate myeloid dendritic cells by triggering the TLR2/myeloid differentiation factor 88 signaling pathway.105 Importantly, EDN promotes the ability of dendritic cells to induce antigen-specific Th2 responses; in this manner, EDN participates in the adaptive immune system. ECP can insert voltage-insensitive, ion-nonselective toxic pores into the membranes of target cells; these pores can facilitate the entry of other toxic molecules.106

MBP directly increases smooth muscle reactivity by disrupting function of vagal muscarinic M2 receptors.107 MBP also induces degranulation of mast cells and basophils. MBP directly binds the extracellular calcium–sensing receptor on esophageal epithelial cells, resulting in release of fibroblast growth factor 9 and autocrine stimulation of epithelial cell proliferation.108 Engagement of cytokine receptors, immunoglobulins, and complement causes eosinophils to produce of a wide range of inflammatory cytokines, including IL-1, IL-3, IL-4, IL-5, IL-13, granulocyte monocyte colony-stimulating factor, transforming growth factor (TGF)-α, TGF-β, tumor necrosis factor α, RANTES, macrophage inflammatory protein 1α, and eotaxin-1; eosinophils thereby have the potential to modulate multiple aspects of the immune response.109 In fact, eosinophil-derived TGF-β has been associated with epithelial growth, fibrosis, and tissue remodeling,110, 111 processes that occur even in pediatric patients with EE. One study showed that eosinophils are the chief source of TGF-β1 in pediatric patients with EE and their numbers of eosinophils correlate with esophageal fibrosis and phosphorylation of the transcription factor SMAD2/3.112 Eosinophils rapidly release mitochondrial DNA in response to exposure to bacteria, C5a, or CCR3 ligands.113 In contrast to neutrophils, eosinophils do not undergo cell death upon release of their DNA; in addition, DNA release requires free radical production via nicotinamide adenine dinucleotide phosphate oxidase. Eosinophil DNA traps contain ECP and MBP and display antimicrobial activity,113 so these cells might have an essential role in innate immunity against bacteria via this unique mechanism. Perhaps mitochondrial DNA release by esophageal eosinophils contributes to epithelial function and/or innate immunity during the pathogenesis of EE.114 Eosinophils can directly present antigen to and activate T cells and regulate T-cell recruitment to allergic tissue by controlling the expression of T-cell–directed chemokines.115, 116 Further eosinophil-mediated damage is caused by toxic hydrogen peroxide and halide acids generated by EPO and by superoxide generated by the respiratory burst oxidase enzyme pathways in eosinophils. Eosinophils also generate large amounts of the cysteinyl leukotriene C4, which is metabolized to leukotriene D4 and leukotriene E4. These 3 lipid mediators increase vascular permeability and mucus secretion and are potent stimulators of smooth muscle contraction, which might contribute to dysmotility associated with EE and/or peristalsis abnormalities in eosinophilic gastrointestinal disorders. Electron microscopy studies have revealed ultrastructural changes, including inversion of core-to-matrix densities and lucency of secondary granules (indicating eosinophil degranulation and mediator release), in esophageal samples from patients with EE.117 In addition, in the intestine, eosinophils are juxtaposed to nerves and have been shown to participate in axonal necrosis.118 As such, eosinophils are indeed pleiotropic cells that initiate adaptive immune responses and sustain and propagate inflammatory reactions (see Figure 2).

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  • Figure 2. 

    Effects of eosinophils. Eosinophils are bilobed granulocytes with secondary granules that are stained by eosin; these granules contain 4 primary cationic proteins: EPO, MBP, ECP, and EDN. EDN is a ligand for TLR-2 that induces the Th2 polarizing activity of dendritic cells. EPO, MBP, ECP, and EDN are cytotoxic molecules; ECP and EDN are ribonucleases. Eosinophils respond to diverse stimuli, including nonspecific tissue injury, infections, allergens, and tumors. In addition to releasing the preformed cationic proteins, eosinophils also produce a variety of cytokines, chemokines, lipid mediators, and neuromodulators. Eosinophils communicate directly with T cells and mast cells in a bidirectional manner. Eosinophils can release mitochondrial DNA, which functions as an extracellular trap for bacteria. Eosinophil-derived TGF-β induces fibrosis.

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Therapy for Patients With EE 

Therapy for EE is based on avoidance diets, anti-inflammatory approaches, and physical dilatation when strictures are present. Dilatation is associated with a relatively high rate of perforation, warranting cautious use.119 Patients with EE are initially given anti-GERD therapies because acid can trigger esophageal eosinophilia, albeit generally of a lower magnitude than that associated with EE.5 Even if pathologic reflux is not present, acid exposure has the potential to irritate the inflamed esophagus. If anti-GERD therapy is unsuccessful (based on histologic assessment), elimination of specific food allergens (via a restricted diet) or an exclusive elemental (amino acid–based) formula is recommended. In a retrospective study of 381 patients over a 10-year period, Liacouras et al found that the removal of dietary antigens (primarily in the form of an elemental diet) significantly improved clinical symptoms and esophageal histology in 98% of patients.23 Although dietary elimination is an efficient strategy, it can be difficult because patients are typically sensitized to multiple food groups that include common and uncommon food types. In addition, skin prick test results do not uniformly identify the best foods to remove from the diet.33 Skin patch testing has been proposed to identify foods that should be eliminated from the diet and might induce disease remission,120, 121 but there have not been consistent findings in support of this approach.122 Although a diet consisting of an exclusive elemental (amino acid–based) formula is effective in reducing disease, it is often not well tolerated (especially in older individuals) because it frequently requires a surgically placed feeding tube, which can be costly (thousands of dollars per month) and is unpalatable. Glucocorticoids (systemic or topical) have been used with satisfactory results in some patients. Systemic corticosteroids are often used in patients with acute exacerbations, whereas topical corticosteroids are used to provide long-term control.123 In a randomized placebo-controlled trial, topical therapy with swallowed fluticasone and oral prednisone have comparable efficacy but are associated with a high rate of relapse on discontinuation.124 Levels of eotaxin-3 and IL-13 messenger RNA (overexpressed in the EE transcriptome) are reduced following successful topical glucocorticoid therapy.79 However, a significant fraction of patients do not respond to swallowed fluticasone, likely due to corticosteroid resistance.60, 65 Smaller body weight and shorter stature increase responsiveness to corticosteroids, suggesting dose dependence.65 In addition, atopic individuals have reduced responsiveness to glucocorticoid therapy,65 likely due to the ongoing exposure to the triggering antigens. Antibodies against IL-5 prevent the development of experimental EE in mice73, 74 and appeared to reduce eosinophil infiltration of the human esophagus in early-stage clinical trials93; large-scale controlled trials of the effects of anti–IL-5 in patients with EE are under way. In a preclinical analysis (experimental EE induced by human IL-13 in mice), anti-human IL-13 reduced esophageal eosinophilia; it will be of interest to examine the impact of IL-13 blockade in patients with EE.79 Other anti-inflammatory agents such as leukotriene receptor antagonists and tumor necrosis factor inhibitors have been advocated but have not been shown to reverse esophageal pathology.125, 126 Preliminary studies with azathioprine and 6-mercaptopurine have variable benefits,127 warranting further study. Because immunoglobulin E effector cells such as mast cells and basophils are a source of proinflammatory chemokines, cytokines, and proteases, anti–immunoglobulin E therapy might have anti-inflammatory effects in EE.128 It is important to note that EE is a chronic disorder that requires ongoing therapy; the disease almost uniformly returns when therapy is discontinued (eg, glucocorticoid therapy is stopped or the diet is liberated).

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Future Directions 

EE is a recently recognized and growing clinical disorder previously misdiagnosed as GERD, but the 2 diseases are distinct in terms of their histopathology, gene expression pattern, response to therapy, hereditary risk, and association with allergies. Atopic disorders such as asthma and eczema are complex diseases; susceptibility depends on multiple genes that interact with environmental factors. There is a relatively strong genetic component of EE compared with other atopic diseases, indicating that fewer genes might be involved in pathogenesis compared with the 100 or more genes that have been implicated in respiratory allergy.129 It is important to examine the association between EE and candidate genes that are biologically relevant to its pathogenesis (see Figure 3). Apart from the candidate gene approach, with the advance of genotyping technology, genome-wide linkage analysis can be conducted to identify genetic variants associated with EE susceptibility. It will be of interest to investigate whether there is overlap among genes associated with atopy, inflammatory bowel disease, celiac disease, and EE.

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  • Figure 3. 

    Regulation of the Th2 inflammatory response in EE. Food antigens induce Th2 cells to release IL-5 and IL-13, which activate eosinophils and esophageal epithelial cells, respectively. IL-13 induces epithelial cells to produce eotaxin-3 (an eosinophil chemoattractant and activating factor) and down-regulate filaggrin expression. IL-5 and eotaxin-3 activate eosinophils to release MBP and EDN, which activate mast cells and dendritic cells, respectively; mast cell activation contributes to fibrosis. Eosinophils also produce TGF-β, which activates epithelial cells and contributes to hyperplasia, fibrosis, and dysmotility. Reduced production of filaggrin might inhibit esophageal barrier function and perpetuates these processes by promoting local food antigen uptake. Genetic variations that affect expression of these regulatory molecules could contribute to the risk of EE.

It has been proposed that a peak eosinophil count of 15 cells/hpf be used as the cutoff value for diagnosis of EE.5 Although an empiric threshold level of eosinophils might prove useful for disease classification, it is recommended that the EE gene expression signature, which includes eotaxin-3 overexpression, be considered for inclusion in definition of the disease. Molecular diagnostics could help differentiate EE from GERD as well as predict patients' response to therapy and long-term outcome (see Figure 4). This type of approach is already being used to classify cancer types and is likely to be cost-efficient, given the improving and readily available technologies.130 Transcriptome analysis might be particularly useful in disease classification in patients who are on therapy; although the EE transcriptome normalizes once patients receive therapy (to a level of 95% correction), a specific expression signature persists and is distinct from that of normal individuals (see Figure 4).79

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  • Figure 4. 

    Diagnosis of EE. Endoscopy is used to obtain biopsy samples from patients with proton pump inhibitor (PPI)-refractory upper gastrointestinal symptoms. These tissue samples are analyzed by microscopy; a minimum of 15 eosinophils/hpf indicates that a patient has EE. Analysis of transcription profiles has indicated dysregulated expression of 1% of the human genome, including overexpression of eotaxin-3, in samples from patients with EE. This gene expression profile can be used to distinguish between biopsy specimens from patients with EE, patients with reflux esophagitis (RE), and normal individuals (NL). In patients successfully treated (Rx) with dietary modification and/or glucocorticoids, eosinophil numbers in biopsy samples are reduced to <1/hpf and the transcriptome more closely resembles that of NL, although there are residual gene expression (*) differences between patients treated for EE and RE and NL.

Unlike classic food anaphylaxis, which occurs in some patients with EE and is typically limited to a select group of foods,131, 132 EE is associated with hypersensitivity to a broad panel of food antigens, indicating a general breakdown in oral antigen tolerance. Studies in mice showed that a limited repertoire of regulatory T cells might mediate a predisposition to mucosal Th2 responses, including esophagitis.133 The impaired local barrier function in the esophagus of patients with EE, combined with the increased level of immunoreactive cells (including mast cells, eosinophils, B cells, T cells, and dendritic cells), likely perpetuates food antigen–driven inflammation and perhaps local antigen sensitization. Whereas glucocorticoids are effective therapeutics for patients with EE, strategies to curtail upstream antigen-induced inflammation are essential. Empiric elimination diets and amino acid–based formulas are the primary form of allergen control; however, future trials are likely to be conducted to induce oral antigen tolerance, an approach now under way to treat food anaphylaxis.134

Adaptive lymphocyte immunity is an important mediator of eosinophilic esophageal inflammation; cytokines produced by Th2 cells are involved in pathogenesis and their function can be manipulated for therapeutic benefit. Notably, IL-13 expression is both sufficient and required for the induction of experimental EE. Furthermore, IL-13 is overexpressed in the esophagus of patients with EE and is capable of inducing a gene expression profile in esophageal epithelial cells that includes up-regulation of eotaxin-3 and overlaps with the esophageal transcriptome observed in vivo. Th2 cell–derived IL-5 regulates the pool of circulating eosinophils and their responsiveness to local activating signals, especially eotaxin-3. Therapeutic interventions that target Th2 cell development, cytokine production, and signaling, along with eosinophils and epithelial/eosinophil interactions (eg, anti–eotaxin-3, CCR3 antagonists, anti–IL-5, and anti–IL-13), might be developed for patients with EE.

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Acknowledgments 

The author thanks numerous colleagues, especially those associated with the Cincinnati Center for Eosinophilic Disorders, for their vast contributions to the content of this review.

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Supplementary data 

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Supplementary Tables 1 and 2.

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 Conflicts of interest The author discloses the following: Dr Rothenberg is a paid consultant for Centocor, Ception Therapeutics, Merck & Co, Novartis, and Nycomed.

 Funding Supported by National Institutes of Health grants R01 AI45898, R01 DK067255, U19 AI070235, R01 DK076893, R01 AI057803, and P30 DK078392; the Campaign Urging Research for Eosinophilic Disorders; the Food Allergy and Anaphylaxis Network; the Food Allergy Project; and the Buckeye Foundation.

PII: S0016-5085(09)01156-1

doi:10.1053/j.gastro.2009.07.007

Gastroenterology
Volume 137, Issue 4 , Pages 1238-1249, October 2009