Date of Award

Spring 2021

Document Type

Open Access Dissertation

Department

Biomedical Science

First Advisor

Mitzi Nagarkatti

Abstract

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor and an environmental sensor that is highly conserved evolutionarily. It was first discovered in the early 1970s for its ability to bind with high affinity to 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD), an environmental pollutant and induce a xenobiotic-metabolizing enzyme known as aryl hydrocarbon hydroxylase (AHH). AhR was shown to regulate TCDD-mediated toxicity against a variety of tissues including the immune system. More recently, AhR has been characterized as key regulator of the immune system, specifically the differentiation of T cells. Also, several AhR ligands have been characterized in addition to TCDD such as the endogenous AhR ligands, dietary AhR ligands, and gut microbiota-derived tryptophan metabolites. This has spiked significant interest in understanding how such ligands regulate the immune functions. In the current study, we investigated the effects of two AhR ligands: 1) TCDD an exogenous environmental pollutant with high affinity for AhR, and 2) 6-Formylindolo[3,2-b]carbazole (FICZ), one of the most potent naturally occurring endogenous ligand.

Delayed-type hypersensitivity (DTH) is a Type-IV hypersensitivity response involved in a large number of clinical disorders including contact dermatitis, drug-induced allergy, allogenic transplant rejection, and pathogenesis of certain autoimmune disease. It also plays a significant role in protection against intracellular bacterial infections while causing major tissue injury when not regulated. In the current study, we investigated the effect of AhR ligands, TCDD and FICZ, on the DTH response against methylated Bovine Serum Albumin (mBSA) in C57BL/6 mice. Treatment of C57BL/6 mice with TCDD attenuated mBSA-mediated DTH response, induced Tregs, decreased Th-17 cells, and caused upregulation of miRNA-132. TCDD caused an increase in several Treg subsets including inducible peripheral, natural thymic, and Th3 cells. Also, TCDD increased TGF-β and Foxp3 expression. In contrast, treating mice with FICZ exacerbated the DTH response, induced inflammatory Th17 cells, induced IL-17, and RORγ. Transfection studies revealed that miRNA-132 targeted High Mobility Group Box 1 (HMGB1) and downregulation of HMGB1 caused an increase in FoxP3+ Treg differentiation and suppression of Th-17 cells. Additionally, mice with DTH also showed significant changes in gut microbiota including an increased abundance of Bacteroidetes and decreased Firmicutes at the phylum level. Also, there was also a decrease in Clostridium cluster XIV and IV, which promote anti-inflammatory responses, and an increase in Prevotella copri that facilitates pro-inflammatory responses. Interestingly, TCDD reversed the gut microbiota composition towards normalcy. In contrast, FICZ failed to cause a major shift in gut microbiota. Furthermore, TCDD but not FICZ caused an increase in the levels of short-chain fatty acids (SCFA), n-butyric acid, and acetic acid. Administration of sodium butyrate into mice with DTH suppressed the response, increased Tregs, and reduced Th17 cells IL17. Lastly, TCDD, as well as butyrate but not FICZ, were able to inhibit proinflammatory Histone deacetylases (HDSCs) class I and II. Together, our data suggest that AhR ligands such as TCDD that suppress DTH response may mediate this effect by altering the miRNA and reversing the gut dysbiosis induced during this inflammatory response, while FICZ may fail to suppress the DTH response because of its inability to overturn the miRNA alterations and dysbiosis. Our study forms the basis of using AhR ligands to treat diseases in which DTH triggers autoimmune disease or to boost the DTH response against certain infections.

Rights

© 2021, Osama Azeldeen Abdulla

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