Microbiota-dependent presence of murine enteric glial cells requires myeloid differentiation primary response protein 88 signaling.

Enteric glial cells (EGCs) were shown to maintain the barrier integrity and immune homeostasis of the bowel. Postnatally, EGCs develop from progenitor cells located in the myenteric plexus and are continuously replenished through adulthood. Both, murine EGC development and replenishment were shown to depend on the microbiome. The underlying mechanisms are still unknown, and we hypothesized that the myeloid differentiation primary response protein 88 (Myd88) or toll-like receptor signaling pathways may be involved. Adult and neonatal C57BL/6 wild-type (wt) and Myd88-/- mice were housed under specific pathogen-free (SPF) or germ-free (GF) conditions. GF mice were further conventionalized by gavaging stools from, and cohousing with, SPF mice having intact microbiomes. The small bowels were harvested at various time points, and immunohistochemistry and qPCR analysis of EGC markers in the muscularis externa and mucosa were performed. In wt mice, after conventionalization, the glial cell-specific markers, glial fibrillary acidic protein (GFAP) and S100 calcium-binding protein ß (S100ß), were upregulated in the mucosa and muscularis externa. In Myd88-/- mice, this upregulation did not occur. Importantly, GFAP (only in the mucosa) and S100ß (in both the mucosa and muscularis externa) were significantly reduced in conventionalized Myd88-/- mice compared with the conventionalized wt mice. In neonatal mice, the gene expressions of GFAP and S100ß increased between the day of birth (P0) and postnatal day 15 (P15) in the mucosa and muscularis externa of both wt and Myd88-/- mice. Notably, in the mucosa but not the muscularis externa, at P15, the gene expressions of GFAP and S100ß were significantly reduced in Myd88-/-. Our data demonstrated that postnatal development and replenishment of EGCs require intestinal microbiota and depend on Myd88. The specific upstream mechanisms may involve toll-like-receptor recognition of the microbiota and will be the subject of further research.

The Role of the Environment in Eosinophilic Esophagitis.

Eosinophilic esophagitis (EoE) is a chronic, immune-mediated inflammatory disease with incompletely understood pathogenesis. Though disease manifestations were initially ascribed to a delayed reaction to food allergens, emerging evidence suggests that modifiable host factors and environmental allergen exposure may also play critical roles in the pathogenesis and ongoing manifestations of EoE. As with other atopic diseases, lack of early-life exposure to microbial pathogens leads to an immune tolerance defect and reprograms the commensal gut microflora toward a type 2 T helper (Th2) phenotype; the esophageal microbiota, a rich environment consisting of diverse bacterial species, is greatly altered by inflammation. Although multiple early life microbiome-altering factors are associated with EoE development, no causative, direct relationships have been identified. Interestingly, large, cross-sectional analyses of several populations identify an inverse relationship between Helicobacter pylori presence and EoE, likely via virulence factors that downregulate Th2 inflammation, though causality has not been proven. In regard to environmental allergens, some studies support seasonal variation in EoE diagnosis and flares, and EoE can be generated after a large, identifiable aeroallergen exposure. Examples include mouse models of intranasal Aspergillus dosing and following initiation of oral immunotherapy to foods or environmental allergens. Conversely, treatment of allergic rhinoconjunctivitis may improve EoE symptoms, though data is limited to case reports and small series. Unfortunately, biologic therapies for atopic conditions have failed to improve EoE symptoms despite improvement in esophageal eosinophil count, though dupilumab shows promise in ongoing studies. Overall, this chapter shows that EoE pathogenesis is likely multifactorial, and the environment is a key component in our understanding of EoE.