IL1B and major depressive disorder: In the pathophysiology of depression, IL-1β synergistically contributes to central 5-HT system hypofunction: it activates IDO1, diverting Trp metabolism away from 5-HT synthesis, and upregulates the SERT to accelerate the clearance of 5-HT from the synaptic cleft (Zhu et al., 2006).