MDD pathophysiology includes gut dysbiosis/permeability, pro-inflammatory cytokines, IDO induction and the conversion of tryptophan to kynurenine, leading to enhanced AhR activation, thereby dysregulating the tryptophan-melatonin pathway [188], and allowing the MDD pathophysiology to be intimately linked to the pathophysiological processes underpinning ‘autoimmune’ disorders. Here, IDO1 is linked to major depressive disorder.