IDO2 and major depressive disorder: The relationship between inflammation and depression remains largely unclear, but there are two main mechanisms through which inflammation can contribute to depression: (1) an imbalance in serotonin, norepinephrine, and epinephrine production following hypothalamic–pituitary adrenal (HPA) axis activation; (2) increased activity of the inflammation by-product indoleamine-2,3-dioxygenase (IDO), resulting in serotonin depletion and increases in quinolinic acid (Kopschina Feltes et al., 2017).