Higher levels of QA are related to impaired cognition, while the flux of tryptophan through IDO depletes serotonin and may contribute to HIV-associated neuropsychiatric manifestations, such as depression and sleep disturbances.13,16,20–24 Apart from damaging neurons, reactive oxygen species and QA further perpetuate the inflammatory response leading to a vicious cycle of neuroinflammation and neural injury. Here, IDO1 is linked to major depressive disorder.