Aryl hydrocarbon receptor and experimental autoimmune arthritis

Aryl hydrocarbon receptor (Ahr) is thought to be a crucial factor that regulates immune responses. Many Ahr-mediated immune regulatory mechanisms have been discovered, which will likely enhance our understanding of the molecular pathogenesis of autoimmune inflammation including rheumatoid arthritis (RA). RA is a systemic inflammatory disease that affects approximately 1 % of the population and is characterized by chronic inflammation of the synovium and subsequent joint destruction. Recent findings showed that Ahr plays critical roles in the development of Th17 cells, which are key effector T cells in a variety of human autoimmune diseases including RA. Consistent with these findings, our previous study demonstrated that Ahr in T cells is important for the development of collagen-induced arthritis, a widely used murine model of human RA, possibly via the induction of Th17 cells. In addition, Ahr is an attractive molecule because tobacco smoke is a well-known environmental risk factor for RA development and Ahr agonists, such as 2,3,7,8-tetrachlorodibenzo-p-dioxin, 3-methyl cholanthrene, and benzo[a]pyrene, are major toxic components in cigarettes. This review summarizes recent findings on Ahr functions in immune cells in the context of RA pathogenesis during stimulation with smoking-derived ligands. We also discuss the potential link between Ahr and novel factors, such as microRNAs, in the development of RA, thereby providing further mechanistic insight into this autoimmune disorder.