Immunoregulatory roles of chemokines and epithelium in inflammatory bowel disease - CCL20 in mucosa, human colonoids and peripheral blood mononuclear cells
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Inflammatory bowel diseases (IBD) are characterized by chronic inflammation in the intestinal tract, which leads to substantial cost for the individual and for society. The pathogenesis is described as an inflammation occurring in an individual with a genetic risk profile, under the influence of environmental factors, where the mucosal immune cells aberrantly react to the microbiota, resulting in a chronic disease. The epithelium forms a barrier between the immune cells and the microbiota and produces mediators which influence both, including immunoregulatory cytokines. Chemokines are examples of immunoregulatory mediators which recruit immune cells to the epithelium. CCL20 and CCR6 are a chemokine its receptor, which have been investigated in IBD due to their effect on both suppressive and proinflammatory immune cells. In paper I, we performed extensive investigation of CCL20 and CCR6 expression in colonic biopsies from IBD patents. Both were upregulated on gene expression level in a large microarray analysis (n=92), and on IHC staining, epithelial CCL20 protein increased substantially in inflammation. On the other hand, the number of CCR6 protein expressing cells in lamina propria decreased in inflammation. We continued with exploring mechanisms for CCL20 expression in the epithelium in the HT29 cell line, where TLR3 stimulation induced a potent CCL20 response and TLR3 siRNAs significantly reduced the release. We found increased number of CCL20 expressing cells in lamina propria in inflamed mucosa, and in paper II we further explored immune cell expression of CCL20 and CCR6 by using peripheral blood mononuclear cells (PBMCs) from 40 IBD patients and healthy controls. Here we found that TLR1/2 and NOD2 activation as well as IL1β stimulation increased CCL20 release. Our observations indicated that the CCL20/CCR6 axis was more active in UC than in CD, including higher CCL20 release from UC PBMCs and reduced CCR6 expression on PBMCs from UC patients with active inflammation. The healthy intestinal epithelium is in a state of physiologic hypoxia (physioxia) which is pushed towards hypoxia in active IBD. In paper III, we confirmed a hypoxia response in an RNA-Seq dataset of mucosal biopsies from UC patients with active disease (n=24). To investigate the impact of reduced oxygen and inflammatory triggers on epithelial cell functions, we assessed the transcriptome of patient-derived colonic epithelial organoids (colonoids). Here we found that reduced oxygen concentration increased barrier-related genes, while inflammatory gene networks induced by TNF or TNF/IL17 were dampened. This included chemokines, like CCL20. Lastly, we observed a dysregulated hypoxia response in colonoids from UC patients compared to healthy control colonoids. In sum, we provide conclusive evidence of an epithelial CCL20 expression in the IBD mucosa and implicate PRR activation in CCL20 release from both epithelium and immune cells. Based on findings in PBMCs, we suggest a stronger involvement of the CCL20/CCR6 axis in UC than CD. We present evidence of dysregulated chemokine responses and hypoxia responses in the mucosa in active IBD and show in patient-derived colonoids that the anti-inflammatory effect of a hypoxia response may include a reduction in chemokine production.