The Enteric Nervous System as Modulator of Mucosal Immune Cells

Hirschsprung's disease (HD) is diagnosed shortly after birth and is characterized by the absence of enteric nerves in parts of colon [Amiel et al.]. Following surgical correction many patients develop HD-associated enterocolitis (HAEC), a condition distinguished by intestinal inflammation resulting in abdominal distension, severe diarrhea, fever and sepsis [Demehri et al.]. The underlying factors leading to HAEC remain poorly understood and likely involve a defect in epithelial barrier, including decreased mucin production and insufficient immunoglobulin translocation. The establishment of the epithelial barrier is dependent on epithelial recognition of microbial products by innate immune receptors, like toll-like receptors (TLRs) [Peterson et al.]. TLR-dependent epithelial recognition of microflora also coordinates the immune response away from harmless commensal bacteria and towards pathogenic invaders. Both innate and adaptive effector cell functions are influenced by epithelial-derived signals. Under homeostatic conditions commensal bacteria induce anti-inflammatory cytokines in epithelial cells which trigger a tolerogenic phenotype in mucosal antigen presenting cells (APC) resulting in generation of commensal-specific regulatory T cells (Tregs) [Curotto de Lafaille et al.]. During infection, recognition of pathogenic organisms by epithelial cells leads to secretion of inflammatory cytokines thereby inducing an inflammatory APC phenotype which promotes T effector cell (Th1, Th17) generation. The enteric nervous system is directly located underneath the epithelium and controls epithelial cell function. Ablation of enteric glia cells, one of the two cell types of the ENS, in mice is associated with inflammation and enterocolitis [Cornet et al.]. In a study from 2011 Flamant and co-workers demonstrate that enteric glia cells protect from a shigella flexneri invasion by preventing lesions in the epithelial barrier mediated by the glia cell derived neurothrophic factor S-nitrosoglutathione (GSNO) [Flamant et al.]. We hypothesize that the lack of an enteric nervous system in HD patients modulates the microbial recognition of epithelial cells and thereby the phenotype of underlying mucosal APCs and effector T cells; this might be associated with the manifestation of HAEC.