The intestinal epithelium (IEC) with its luminal microflora serves as an ideal experimental and medical model that merges innate immune recognition in the context of a tissue regeneration environment. Genetic predispositions that cause imbalanced microbe-host interaction may contribute to the pathogenesis of inflammatory bowel disease (IBD). Various susceptibility loci for Crohn's disease and ulcerative colitis have been identified; however, their contributions to the disease mechanism remain poorly defined. A Crohn's disease susceptibility locus at chromosome 15q22 is immediately linked to RAB11A. This gene encodes a small guanosine triphosphatase (GTPase) that regulates the recycling endosome function. The applicant's laboratory has genetically targeted the mouse Rab11a, and analyzed IEC-specific Rab11a knockout mice. Preliminary data suggested that Rab11a controls epithelial-cell-intrinsic inflammatory cytokine response to enteric microbiota. Enterocytes deficient in Rab11a in Drosophila and mouse intestines overproduced proinflammatory cytokines, and caused early-onset enteritis, higher susceptibility to inflammatory neoplasia, and premature mortality. The long- term goal is to understand how the endosomal sorting of microbial receptors in IECs influences innate mucosal immunity and microbe-host homeostasis. The objective of this proposal is to establish the molecular mechanisms, by which Rab11a endosome-mediated sorting of Toll-like receptors (TLRs), in particular TLR9, support intestinal mucosal tolerance to microbiota. The central hypothesis is that Rab11a sequesters TLR9 to an inactive endosomeal compartment and dampens unwanted immune response to enteric microbiota at steady state conditions. Information learned about IEC-mediated modulation of inflammatory cytokine production and immune response may help reduce the adverse inflammatory response in IBD. This hypothesis will be tested with 2 specific aims: (1) to establish Rab11a endosomal compartment as an epithelial-cell-intrinsic modulator of cytokine response to microbiota; and (2) to determine the mechanism of Rab11a-controlled TLR9 transport and activation in response to microbial agonists. The experiments will use the novel Rab11a conditional mouse model that facilitates inducible Rab11a deletion, IEC-specific Drosophila Rab11 RNA interference (RNAi) lines, RAB11A-depleted human colon epithelial cell lines, and in vivo perfusion of microbial agonists. The combination of these genetic models with in vivo physiologic analyses in the study of endocytic control of microbe-host homeostasis is innovative and a major step toward understanding IBD.
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