Beyond Eosinophilic Esophagitis: Eosinophils in Gastrointestinal Disease-New Insights, "New" Diseases

Functional dyspepsia (FD) is a highly prevalent disorder. Upper endoscopy is normal, and according to the Rome IV criteria, there is no established pathology. Data accumulated over the last 15 years has challenged the notion FD is free of relevant pathology, and in particular, increased duodenal eosinophils have been observed. Intestinal eosinophils play important roles in microbial defence, immune regulation, tissue regeneration and remodelling, and maintaining tissue homeostasis and metabolism; degranulation of eosinophils releases toxic granule products (e.g., major basic protein, eosinophil-derived neurotoxin) which can damage nerves. Normal cut-offs for eosinophil infiltration into the duodenum histologically are less than five eosinophils per high power field (<25 per five high power fields). In clinical practice there is evidence that pathologically increased intestinal eosinophils may often be overlooked. In a meta-analysis duodenal eosinophils were significantly increased in FD although there was substantial heterogeneity; degranulation of duodenal eosinophils was also significantly higher in FD without significant heterogeneity. In addition, increased duodenal permeability, systemic immune activation, and an altered mucosa-associated duodenal microbiome have been identified that may help explain why symptoms arise, often occur after food with exposure to food antigens, and typically fluctuate. Several potentially reversible risk factors for FD have now been identified. We evaluate the current evidence linking duodenal microinflammation and immune activation with FD and disorders of gut-brain interactions that overlap with FD. We propose a two-hit disease model for eosinophilic functional dyspepsia (EoFD) with management implications.