Intraepithelial Eosinophils: Key Insights for GI Immunity

Eosinophils are a type of white blood cell traditionally associated with allergic responses and parasitic infections, but they also play a crucial role in gastrointestinal (GI) immunity. Within the GI tract, intraepithelial eosinophils interact with epithelial cells and immune components, influencing both homeostasis and disease processes.

Understanding their function is essential for deciphering immune regulation in the gut. Research continues to uncover how these cells contribute to barrier integrity, inflammation, and tissue remodeling.

Distribution in Gastrointestinal Layers

Intraepithelial eosinophils are present throughout the GI tract, though their density varies by region and physiological conditions. The small intestine, particularly the duodenum and jejunum, harbors the highest concentrations under normal conditions, as shown in histological studies using eosinophil-specific markers like major basic protein (MBP) and eosinophil peroxidase (EPO). In contrast, the stomach and esophagus contain fewer eosinophils, though their numbers can rise significantly in response to pathological stimuli. This regional variation suggests local environmental factors, including microbiota composition and epithelial-derived cytokines, influence eosinophil residency.

Within the intestinal epithelium, eosinophils are interspersed between enterocytes, maintaining close contact with tight junctions and mucin-secreting goblet cells. This positioning allows them to interact with epithelial-derived signals that regulate their retention and function. Transmission electron microscopy studies show eosinophils have an elongated morphology, enabling them to navigate intercellular spaces without disrupting epithelial integrity. Their proximity to Paneth cells in the crypts of Lieberkühn suggests they may influence antimicrobial peptide secretion, though the exact mechanisms remain under investigation.

The colonic epithelium, while generally hosting fewer eosinophils than the small intestine, shows a more dynamic distribution. In healthy individuals, eosinophils are primarily localized to the basal regions of the crypts, but environmental or dietary changes can trigger migration toward the luminal surface. Studies using confocal microscopy and immunohistochemical staining for eosinophil granule proteins have documented this redistribution. Colonic eosinophils also exhibit a distinct transcriptional profile compared to their counterparts in the small intestine, indicating regional specialization beyond numerical differences.

Tissue Recruitment Mechanisms

The recruitment of intraepithelial eosinophils to the GI tract is a highly regulated process involving chemokines, adhesion molecules, and signaling pathways that direct their migration from the bloodstream into the epithelial layer. This process begins with eosinophil mobilization from the bone marrow, where they develop under the influence of interleukin-5 (IL-5), a cytokine essential for eosinophil differentiation and survival. Once in circulation, eosinophils traverse the endothelial barrier of gut-associated blood vessels, aided by adhesion molecules such as vascular cell adhesion molecule-1 (VCAM-1) and intercellular adhesion molecule-1 (ICAM-1), which facilitate attachment to the endothelium. Intravital microscopy studies show eosinophils exhibit rolling and arrest behaviors along postcapillary venules in the intestinal lamina propria, highlighting the role of selectins and integrins in their initial recruitment.

Once inside intestinal tissue, eosinophils follow chemoattractant signals guiding them toward the epithelium. Eotaxins, particularly eotaxin-1 (CCL11) and eotaxin-3 (CCL26), bind to the CCR3 receptor on eosinophils, promoting directed migration. Elevated eotaxin levels in the intestinal mucosa under both homeostatic and disease conditions highlight their role in maintaining eosinophil presence. Beyond eotaxins, epithelial-derived alarmins such as thymic stromal lymphopoietin (TSLP) and IL-33 contribute to eosinophil positioning by modulating chemokine gradients. Murine models show genetic deletion of CCR3 or eotaxin-1 significantly reduces intraepithelial eosinophil numbers, reinforcing the necessity of these signals for recruitment.

Eosinophils must then traverse the lamina propria and basement membrane to reach the epithelium, a process requiring extracellular matrix remodeling. Matrix metalloproteinases (MMPs), particularly MMP-9, degrade basement membrane components, creating pathways for cellular movement. Interactions between eosinophils and epithelial cell-derived integrins, such as α4β7, promote adhesion to the basolateral surface of enterocytes, enabling final positioning within the epithelium. Live-cell imaging studies show eosinophils actively extending pseudopodia between epithelial cells, indicating a dynamic migration mechanism that integrates them into the intraepithelial niche without disrupting epithelial integrity.

Influence on Barrier Function

The GI epithelial barrier regulates nutrient absorption while preventing harmful substance entry. Intraepithelial eosinophils contribute by interacting with structural components like tight junctions, mucins, and antimicrobial peptides. Their proximity to epithelial cells allows them to influence barrier integrity through secreted granule proteins such as eosinophil-derived neurotoxin (EDN) and eosinophil cationic protein (ECP), which affect epithelial cell turnover. Studies using intestinal organoid models show eosinophil-derived factors enhance epithelial regeneration following injury, supporting barrier continuity under physiological stress.

Eosinophils also influence the mucus layer, a critical barrier component against mechanical and microbial insults. Goblet cells, which secrete mucins such as MUC2, respond to eosinophil-derived mediators. Mucin-deficient mouse models indicate eosinophils help stabilize mucins, preventing excessive degradation by luminal proteases. This regulation is particularly relevant in diseases like eosinophilic esophagitis, where compromised mucus integrity correlates with increased epithelial permeability.

The extracellular matrix (ECM) underlying the epithelium provides structural support and regulates cellular adhesion. Eosinophils secrete matrix-modulating enzymes such as MMPs, which help maintain ECM flexibility. However, excessive MMP activity can remodel the basement membrane, altering epithelial anchoring. Human intestinal biopsies show increased eosinophil-associated MMP expression in areas of epithelial disruption, highlighting their dual role in supporting or compromising barrier function depending on context.

Relevance in Inflammatory Processes

Eosinophils are implicated in GI inflammatory disorders, where their accumulation within the epithelium correlates with tissue remodeling and disease severity. Their presence in conditions such as eosinophilic gastroenteritis and inflammatory bowel diseases (IBD) suggests they shape the inflammatory landscape. Unlike immune cells mediating acute responses, eosinophils contribute to chronic inflammation through sustained degranulation and cytokine release, perpetuating epithelial dysfunction and fibrosis. Biopsy analyses from eosinophilic esophagitis patients reveal dense eosinophilic infiltrates within the epithelium, often accompanied by microabscess formation and basal cell hyperplasia, indicating prolonged tissue irritation.

Eosinophilic granule proteins, including major basic protein (MBP) and eosinophil-derived neurotoxin (EDN), exacerbate epithelial damage by disrupting cellular membranes and inducing oxidative stress. Experimental colitis models show excessive eosinophil activity intensifies mucosal injury, with eosinophil depletion reducing epithelial apoptosis and improving barrier integrity. This cytotoxicity is particularly relevant in chronic inflammation, where unresolved eosinophilic activity drives fibrotic changes, as seen in eosinophilic esophagitis, where prolonged inflammation leads to esophageal strictures and impaired motility.

Methods for Histological Identification

Identifying intraepithelial eosinophils in GI tissues requires specialized staining techniques and imaging methods to distinguish them from other immune and epithelial cells. Hematoxylin and eosin (H&E) staining provides a general overview, as eosinophils exhibit a bright pink cytoplasm due to their granule content. However, this method lacks specificity, necessitating additional histochemical and immunohistochemical approaches for precise quantification and localization.

Major basic protein (MBP) is a widely used marker for eosinophil identification, detected through immunohistochemical staining. Antibodies targeting MBP allow high-resolution visualization of eosinophils within epithelial layers. Eosinophil peroxidase (EPO) and eosinophil cationic protein (ECP) are also common markers, particularly for studying eosinophil degranulation. In situ hybridization, which detects eosinophil-specific mRNA transcripts, adds specificity by differentiating intact eosinophils from extracellular granules.

Advanced imaging modalities further enhance eosinophil detection. Confocal and multiphoton microscopy enable three-dimensional visualization of eosinophil positioning relative to epithelial structures, clarifying their interactions with tight junctions and mucin-secreting goblet cells. Transmission electron microscopy (TEM) reveals ultrastructural details like elongated cell bodies facilitating intercalation between enterocytes. Flow cytometry and mass cytometry characterize eosinophil surface markers and activation states, providing quantitative insights into their role in health and disease.