Aryl hydrocarbon receptor inhibition and non-canonical signaling activation impact Treg function in autoimmune hepatitis

Abstract

Abstract Treg are dysfunctional in autoimmune hepatitis (AIH), a severe hepatopathy characterized by hypergammaglobulinemia, presence of serum autoantibodies and interface hepatitis on histology. AIH-derived Treg display impaired expression of CD39, an ectonucleotidase that scavenges extracellular ATP to generate immunosuppressive adenosine. CD39 can be regulated upon activation of aryl-hydrocarbon-receptor (AHR), a mediator of toxin responses that also modulates T-cell immunity. Upon binding to endogenous or exogenous ligands, AHR dimerizes with aryl-hydrocarbon-receptor-nuclear-translocator (ARNT) or other non-canonical factors like estrogen-receptor-alpha (Era) or Kruppel-like-factor-6 (KLF6) to modulate gene expression. AHR is in turn regulated by the aryl-hydrocarbon-receptor-repressor (AHRR) that inhibits the transcriptional activity of the AHR/ARNT complex. We note that, compared to healthy subjects (HS), Treg obtained from AIH patients display increased AHRR levels and heightened expression of non-canonical binding factors Era and KLF6, but comparable AHR and ARNT levels. Differently from HS, AIH Treg do not upregulate CD39 expression and activity upon exposure to the AHR ligands unconjugated bilirubin (UCB), L-Kynurenine and quercetin. Silencing of AHRR and Era via siRNA boosts CD39 in AIH Treg in the presence of UCB and quercetin. We further note that, in AIH Treg, AHR binds Era with higher affinity than ARNT. In conclusion, multiple AHR signaling alterations might impact CD39 and Treg function in AIH. Targeted blockade of AHRR and Era represents a potential therapeutic approach to restore immune homeostasis in AIH and other chronic inflammatory conditions by boosting CD39.