Essmat et al. mentioned that the activation of the hypothalamic-pituitary-adrenal axis (HPAA), production of indoleamine 2, 3-dioxygenase (IDO), and loss of BDNF have all been linked to an increase in neuroinflammatory cytokines and chemokines in the etiology of depression. Here, IDO1 is linked to depressive symptom measurement.