Differential Influences of the Aryl Hydrocarbon Receptor on Th17 Mediated Responses in vitro and in vivo

The aryl hydrocarbon receptor (AhR) has been attributed with anti-inflammatory effects in the development of pathological
immune responses leading to experimental autoimmune encephalomyelitis (EAE) via the induction of regulatory T cells. In
agreement with previously published findings, we find that TCDD administration confers protection from EAE, however, this
immuno-modulatory effect was not the consequence of de novo Treg generation, but the inhibition of Th17 cell
differentiation. Systemic application of FICZ at the time of immunization also reduced EAE pathology albeit to a lesser
degree than TCDD. In vitro Th17 differentiation in the presence of AhR agonists, including TCDD, promoted IL-17 and IL-22
expression, but did not induce Treg differentiation. AhR affinity influenced the amounts of IL-17 and IL-22 protein that was
secreted by Th17 cells, but did not seem to affect susceptibility to EAE in vivo. Making use of conditional AhR-deficient mice,
we show that the anti-inflammatory effect of TCDD depends on AhR activation in both T cells and dendritic cells, further
emphasising the ability of TCDD to interfere with T effector cell differentiation in vivo. The dichotomy between the in vivo
and in vitro effects of AhR reveals the complexity of the AhR pathway, which has the capacity of affecting different AhRexpressing
cell types involved in mounting immune responses, thus participating in defining their outcome.