Expanding the paradigm of eosinophilic esophagitis: mast cells and IL-9

Eosinophilic esophagitis (EoE) is a disease characterized by the aberrant infiltration of eosinophils into the esophageal mucosa. Symptoms of EoE vary by patient age, are typically refractory to acid suppressive therapy, and include difficulties with swallowing, vomiting, failure to thrive, abdominal pain, and dysphagia1. The severity of EoE correlates positively with the esophageal eosinophil level and the degree of epithelial cell hyperplasia, substantiating that eosinophils are likely the primary effector cells involved in the pathogenesis of EoE. While the etiology of EoE remains unclear, it has been hypothesized that an uncontrolled immune response to food allergens and aeroallergens triggers the development of EoE2. Several clinical and animal studies have addressed the potential immunological mechanisms that underlie the recruitment and accumulation of esophageal eosinophils in EoE. Using a whole-genome expression analysis approach, the transcription of CCL26 (eotaxin-3), a potent eosinophil chemoattractant in humans, was revealed to be highly dysregulated in the esophageal tissue of patients with EoE. Notably, the loss of the CCL26 receptor (CCR3) attenuated eosinophil recruitment into the esophagus in an experimental model of EoE3. In addition to CCL26, experimental studies and clinical reports support a role of interleukin (IL)-5, a CD4+ T cell-derived TH2 cytokine, in contributing to the accumulation of esophageal eosinophils in the pathogenesis of EoE. Mice overexpressing IL-5 were shown to be prone to develop experimental EoE4, 5, whereas the neutralization of IL-5 blocked allergen- and IL-13-induced EoE pathogenesis in mouse models6, 7. In humans, the expression level of IL5 transcript in the esophageal biopsies of patients with EoE was positively associated with esophageal eosinophil numbers and the disease activity8. The role of IL-5 in regulating EoE pathogenesis was further evaluated in patient studies using the humanized monoclonal antibody against IL-5, which was originally designed to specifically target eosinophils for the treatment of asthma. Clinical trials conducted in patients with EoE treated with either mepolizumab (Bosatria®, GlaxoSmithKline) or reslizumab (Cinquil®, Cephalon) demonstrated that humanized anti-IL-5 antibody could effectively attenuate eosinophils in the esophagitis or peripheral blood9–12. However, while the mepolizumab-treated patients experienced improvements in clinical symptoms and epithelial hyperplasia, there were no significant differences between the mepolizumab- and placebo-treated groups. The ambiguous results may be due to the usage of non-standardized patient-reported outcome metrics and to a subset of patients with EoE failing to respond to anti-IL-5 antibody treatment9, 11–13. Thus, alternative measurements of EoE-specific clinical outcomes need to be identified, and additional research to elucidate other inflammatory mechanisms involved in EoE pathogenesis in patients who fail to respond to anti-IL-5 treatments is required. In this report14, Otani et al. performed a detailed analysis of the biopsy specimens from a previous trial using a quantitative immunochemistry approach. Among the studied 43 patient biopsies, 40% of the subjects that showed reduced esophageal eosinophil numbers (<15 eosinophils per high power field (hpf)) after anti-IL-5 antibody treatment were defined as “responders”. Notably, the effect of anti-IL-5 antibody treatment in reducing esophageal eosinophil number was most pronounced in a subgroup of responders that also displayed a marked reduction of tryptase-positive mast cell numbers (pretreatment 62 vs. post treatment 19 per hpf). Interestingly, these esophageal mast cells were found adjacent to eosinophils, and the frequency of these mast cell/eosinophil couplets in the esophagus of the responders was reduced significantly 12 weeks after anti-IL-5 antibody treatment. Several lines of evidence from previous studies suggest the involvement of mast cells in EoE pathogenesis15, 16. First, the presence of activated mast cells in the intraepithelial layer of the esophagus was found to be a useful diagnosis marker that differentiates the pathology of EoE from that of gastroesophageal reflux disease (GERD)15. In a later transcriptome expression profile analysis, the expression of carboxypeptidase A3 (CPA3) and tryptase, but not chymase transcript, were found to be elevated in esophagus of patients with EoE16. Notably, the expression levels of these mast cell-associated transcripts correlated positively with that of KIT ligand (KITLG), a surrogate marker for the degranulation and activation of mast cells16, suggesting that esophageal mastocytosis and degranulation can occur in patients with EoE. Consistent with this possibility, a recent experimental model of IL-13-driven EoE established that esophageal pathology could occur independent of eosinophils17. Thus, in addition to eosinophils, esophageal mast cells may be critical for disease manifestations, and the complex interplay between these different infiltrating cell types, through their cytokine production, may be involved in the immunopathogenesis of EoE. IL-9, a pleiotropic cytokine, can promote the activation and maturation of mast cells18. Notably, the expression of the IL9 gene transcript is elevated in the esophagus of EoE patients with EoE8. The finding that the reduction of esophageal mast cell numbers could occur primarily in a subgroup of patients EoE who respond to the anti-IL-5 antibody treatment raises the question of the cellular source of IL-9. In a double-immunofluorescence analysis, the authors show that activated MBP+ eosinophils and other unidentified cells that are adjacent to the tryptase+ mast cells in the esophagus produced IL-9. This intriguing observation suggests that the esophageal eosinophils represent only one of the IL-9-producing immune cell types involved in the pathogenesis of EoE. One study identified an IL-9-producing CD4+ cell subset, termed TH9 cells19. Distinct from classical CD4+TH2 cells that can produce IL-9 and other TH2 cytokines (IL-4, IL-5, and IL-13), TH9 cells that expressed the transcription factors GATA3, PU.1, and IRF4 primarily produced IL-9 but few of the other TH2 cytokines18. Moreover, a recently discovered type-2 cytokine-producing innate lymphoid cell (ILC2), also called natural helper cells or nuocytes, also transiently produced IL-9 that facilitated IL-5 and IL-13 production in an autocrine manner20. Whether CD4+TH2 or TH9 cells or ILC2 represent the unidentified esophageal IL-9 producers that are involved in the pathogenesis of EoE remains to be determined (Figure 1). Since TGF-β and IL-4 together can induce IL-9 production19, it will be interesting to determine whether these two cytokines, produced by esophageal mast cells16 and CD4+TH2 cells, respectively, may induce the adjacent eosinophils to produce IL-9, which in turn, activates the accompanying mast cells to release TGF-β and other inflammatory mediators, thus amplifying the pathogenesis of EoE (Figure 1). Figure 1 A potential immunological mechanism involved in the pathogenesis of EoE. An uncontrolled TH2 immune response initiated by an allergic insult results in the transition of the esophagus from a normal (NL) to EoE phenotype through enhanced IL-13 production ... One of the significant findings of this report is that the interplay between esophageal eosinophils and mast cells can be associated with the severity of EoE symptoms. On the basis of a severity scale from 1–6 for reported symptoms employed in the clinical trial, the authors reported that the esophageal mast cell numbers correlated with the severity of EoE symptoms only in a subgroup of patients who exhibited a >70% decrease of esophageal mast cells after anti-IL-5 antibody treatment. In contrast, the reduction of eosinophil numbers did not correlate with symptom severity. Since many studies have demonstrated a role of mast cells in regulating smooth muscle contractile and vagal nerve activity21–23, it is reasonable to speculate that the activated mast cells in the esophagus are the primary drivers of the pathophysiology of EoE in a subgroup of patients. Thus, among patients with EoE who respond to anti-IL-5 antibody treatment, only the subset of responders who exhibit significant reduction of esophageal mastocytosis may experience diminished clinical symptoms of EoE. In summary, the findings in this study revealed an additional pathological function of esophageal eosinophils, providing IL-9 that promotes esophageal mastocytosis, and that the interactions between mast cells and eosinophils may regulate the severity of EoE symptoms.