IL6 and major depressive disorder: Compared to healthy controls, several studies reported increased IL-6 levels in MDD patients (Dowlati et al., 2010; Goldsmith et al., 2016; Howren et al., 2009; Köhler et al., 2017), but Zou et al. (2018) found no significant differences in IL-6 concentrations between MDD patients and healthy controls.