Environmental signals that control T lymphocytes in the gastrointestinal tract

  • George Ramsay

    Student thesis: Doctoral ThesisDoctor of Philosophy


    The Aryl Hydrocarbon Receptor Nuclear Translocator (ARNT) binds the Aryl Hydrocarbon Receptor (AHR) and also the Hypoxia Inducible Factor-1α (HIF-1α). Both the AHR and HIF-1α are key sensors of the microenvironment. Dietary tryptophan metabolites and microbiota catabolites are AHR ligands and HIF-1α is a sensor of low oxygen tension. Deletion of the AHR results in a loss of key lymphocyte subsets in the barrier organs of the skin and the intestine, but whether these effects are solely mediated by AHR–ARNT complexes is unclear. Moreover, the influence of HIF-1α-ARNT complexes in mucosal lymphocytes has yet to be addressed. Here, we investigated the roles of AHR-ARNT and HIF-1α-ARNT signalling in lymphocyte maintenance at barrier surfaces. Firstly, we demonstrated that AHR-ARNT signalling can be detected in vivo at the intestinal barrier sites. We also show that ARNT deletion results in a reduction in barrier lymphocyte populations, pheno-copying that of AHR deletion. However, HIF-1α is not required for barrier surface lymphocyte maintenance.

    We then analysed AHR, ARNT and HIF-1α signalling in effector CD4+ cells. Under Th17 differentiating conditions, AHR null and ARNT null activated CD4+ cells produce lower amounts of IL-17 and IL-22 than wild type cells. HIF-1α null cells, however, showed only a small reduction in these cytokines when compared to wild type cells, arguing that AHR-ARNT is the key pathway for the differentiation of these cells. We then describe a novel system to study acute AHR-ARNT downstream signalling. We measured changes in transcript expression in activated CD4+ cells grown in Th17 polarising conditions to which AHR ligand had been added for 2 or 24 hours. This microarray-based analysis has enabled us to discover previously unrecognised AHR-ARNT signalling gene targets, and provided insights into the crosstalk between AHR and HIF-1α in lymphocytes.
    Date of Award2015
    Original languageEnglish
    SupervisorDoreen Cantrell (Supervisor)

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