IL6 and major depressive disorder: Moreover, increased circulating levels of inflammatory factors, such as IL-6, tumor necrosis factor-alpha (TNF-α), and the acute phase protein C-reactive protein (CRP) have been reported in MDD (Goldsmith et al., 2016; Haapakoski et al., 2015; Howren et al., 2009; Miller and Raison, 2016).