A recent study in a mouse model of middle cerebral artery occlusion demonstrated that the macrophages/microglia could enhance indoleamine 2,3-dioxygenase 1 (IDO1)-dependent neurotoxic kynurenine metabolism during ischemic pathogenesis, which was closely related to the post-stroke depression (Koo et al., 2018). Here, IDO1 is linked to major depressive disorder.