Alterations of aryl hydrocarbon receptor signaling in autoimmune hepatitis

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Abstract Autoimmune hepatitis (AIH) is a severe liver disease of unknown etiology that typically follows a relapsing- remitting course and may often become refractory to conventional immunosuppression. This important illness may progress over months or years to end-stage liver disease with jaundice, requiring transplantation in about 20% of cases. Dysfunctional Tregs co-exist, in AIH, along with heightened Th17 immune responses, which are refractory to immunoregulation. CD39 is an ectonucleotidase that hydrolyzes extracellular ATP to ultimately generate immunosuppressive adenosine. This ectoenzyme is expressed by Tregs and a subset of effector Th17 cells, where it marks the acquisition of regulatory properties. Induction of CD39 results, at least in part, from activation of aryl hydrocarbon receptor (AhR), which mediates toxin responses to modulate Treg and Th17 cell immunity. AhR binds exogenous and endogenous ligands, including xenobiotics and heme derivatives e.g. unconjugated bilirubin. AhR functions are mediated through interactions with the aryl hydrocarbon receptor nuclear translocator or other ‘non-canonical’ binding factors like the estrogen receptor-a (Era) or the Kruppel- like factor 6. AhR is also regulated by hypoxia inducible factor 1 alpha (HIF-1a), the AhR repressor (AhRR) and TGF-b and is induced by NF-kB through the Rel-A subunit. We have noted that AIH-derived Tregs and Th17 cells express decreased levels of CD39. This alteration is associated with defective Treg function and persistence of pro-inflammatory Th17 cells. We also observe that both Tregs and Th17 cells fail to upregulate CD39 in response to AhR activation. Tregs and Th17 cells in AIH also express heightened Era, KLF6, AhRR and HIF-1a levels. These data suggest that AhR binding to non-canonical partners and/or aberrant AhR regulation might interfere with T cell responses, ultimately resulting in CD39 downregulation in AIH. We therefore hypothesize that Treg and Th17 cell purinergic dysfunction in AIH can be linked to aberrant AhR signaling and/or regulation. These alterations perpetuate Treg/Th17 cell imbalances that favor tissue damage and disease progression in AIH. In Aim 1, we will determine whether aberrant AhR interactions with non-canonical partners impact the purinergic response, metabolic profiles and function of Tregs and Th17 cells; either derived from the peripheral blood or liver biopsies of AIH patients. In Aim 2, we will define the cellular mechanisms regulating AhR signaling and how these impact CD39 in AIH-derived Tregs and Th17 cells. In both Aims 1 and 2, the effects of altered AhR signaling and regulation will be also tested in an in vivo model of acute hepatitis induced by Concanavalin-A in immunodeficient NOD/scid/gamma mice. Lastly, in Aim 3, we will test innovative strategies a) to enhance AhR signaling, either by pharmacological blockade of HIF-1a or AhRR; and/or b) directly boost CD39 activity through administration of exogenous ADPase. Our investigations will provide mechanistic insights into autoimmune tissue damage in AIH and, notably, will aid identifying novel therapeutic targets to control inflammation and halt disease progression in AIH and other chronic liver illnesses.