Neuro-immune regulation of macromolecular permeability in the normal human colon and in ulcerative colitis
Abstract: Background and aim: Persistent stress and life events affect the course of ulcerative colitis (UC) by largely unknown mechanisms. Regulation of epithelial permeability to antigens is crucial for the balance between inflammation and immuno-surveillance, and increased intestinal permeability has been shown in patients with ulcerative colitis. Corticotropin releasing hormone (CRH) has been implicated as an important mediator of stress-induced abnormalities in intestinal mucosal function in animal models. Further cholinergic signalling during stresshas been reported to increase bowel ion secretion in humans and uptake of HRP in rodents via activation of mast cells.The overall aim of this thesis was to examine the role of CRH-mediated and cholinergic signalling, and their interaction with mast cells and eosinophils, in the regulation of the mucosal barrier function in the normal human colon and in UC. In vivo studies or the use of surgical specimens for such studies have major shortcomings. Therefore a method with endoscopic biopsies in Ussing chambers was established for studies of protein antigen uptake and electrophysiology in human colonic biopsies, and used in subsequent investigations.Materials and methods: In the four studies a total of 91 healthy volunteers, 3 patients with rectal cancer, and 15 UC patients were included. Biopsies from the sigmoid colon were assessed for macromolecular permeability (Horseradish peroxidase (HRP), and 51Cr-EDTA), and electrophysiology during challenge with sodium caprate (C10), CRH or carbachol. Experiments were repeated with CRH receptor antagonists, carbachol receptor antagonists, mast cell stabilizers and nerve conductance blockers in Ussing chambers. The biopsies were examined by electron and light microscopy for endocytosis of HRP, morphological changes and receptor expression. Moreover, the human mast cell line, HMC-1; was used in studying expression of CRH receptors on mast cells.Results: Endoscopic biopsies of human colon were viable in Ussing chambers, and the technique was shown to be a reliable tool for studies of mucosal permeability to HRP. CRH stimulates transcellular uptake of HRP in human colon via CRH receptor subtypes R1 and R2 on subepithelial mast cells. Further, carbachol acts on muscarinic receptors, located on subepithelial eosinophils. Activated muscarinic M2 and M3 receptors on increased numbers of CRHproducing eosinophils in UC, lead to activation of mast cells and increased macromolecular uptake across the colonic mucosa. This signalling cascade is previously unrecognized, and may be involved in the inflammatory process in UC.Conclusions: In conclusion, we have demonstrated a chain of events leading to increased permeability to the protein antigen HRP in biopsies from healthy volunteers and patients with UC. The important steps begin with a cholinergic signal to muscarinic receptors on the CRH containing eosinophils. The next step includes activation of CRH receptors on mast cells leading to degranulation and increased macromolecular uptake across the epithelium. This explanatory model will have implications for understanding of the pathogenesis of UC and future treatment of the disease.
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