IDO1 and depressive symptom measurement: Three mechanisms on how cytokines may lead to depression or depressive symptoms were hypothesized [47]: (1) the stimulation of the indoleamine-2-3-dioxygenase (IDO) [73], (2) the modulation of the serotoninergic neurotransmission (not IDO-mediated), and (3) the activation of the HPA axis.