Of interest, the activity of the indoleamine 2,3-dioxygenase (IDO)1 enzyme and the IL-10/IL-17 ratio were both decreased in COPD patients [35], a finding further pointing to AhR, known to promote IDO1 activity [36], as a master regulator of inflammation and tolerance in the lung. Here, AHR is linked to chronic obstructive pulmonary disease.