Intestinal epithelial paracellular permeability and immune tolerance

Project Summary The long-term goal of our laboratory is to elucidate the molecular basis for intestinal homeostasis and its dysregulation in intestinal inflammation, and to develop novel approaches for prevention and therapy of inflammatory bowel diseases (IBD). The apically located inter-cellular tight junctions (TJ) within the intestinal epithelium act as a selective barrier between luminal and host environment. Although increased intestinal permeability with loss of intestinal TJ barrier function is associated with intestinal disorders and IBD, the TJ barrier is complex and context dependent, and the physiological need of epithelial paracellular permeation for the appropriate gut immune response is being recognized. The aryl hydrocarbon receptor (AHR), a susceptibility locus for IBD and an environmental sensor, acts as a class I, basic helix-loop-helix transcriptional regulator. Deficiency of AhR increases severity of experimental colitis by perturbing intestinal stem cell homeostasis and differentiation and dysregulating the gut immune responses. On the other hand, activation of the AhR pathway improves colitis outcomes in animal IBD models. Thus, there is a lot of scientific and clinical interest in the regulation of AhR activity. In our present studies, and consistent with published reports, we found that the severity of colitis is increased in intestinal epithelial specific AhR deleted (Ahr∆IEC) mice in several IBD models. Interestingly, we found that the baseline colonic TJ barrier permeability is reduced in Ahr∆IEC mice. This reduction in colonic paracellular permeability was accompanied by increased inflammatory tone (increased pro-inflammatory cytokines and inflammatory phenotype of dendritic cells and macrophages) and impairment of immune tolerance (reduced IL-10, TGF-β, Foxp3, Programmed death-ligand 1 (PD-L1), and cytotoxic T-lymphocyte- associated protein 4 (CTLA4)) in Ahr∆IEC mouse colonic mucosa. Based on these findings, we hypothesize that reduction in the homeostatic paracellular permeability, in the absence of AhR, affects immune tolerance. We will address this hypothesis with the following specific aims: 1) To determine the mechanisms of reduced intestinal epithelial permeability in the absence of AhR and 2) To delineate the role of AhR-regulated paracellular permeability in gut immune tolerance. The proposed studies in Ahr∆IEC mouse model, presents a unique opportunity to study the role of paracellular permeation in maintaining the gut homeostasis. Once completed, this study will provide unique knowledge about the role of intestinal barrier in shaping up the immune homeostasis which can provide a foundation for further investigations to explore pathogenesis-based therapeutic tools for IBD and other immune-dysregulated diseases.