TNF and major depressive disorder: TNF-α has long been thought to be involved in the etiology of depression (Himmerich et al., 2008; Liu et al., 2015) and is suggested to be a major and common risk factor for inflammation-associated depressive disorders, as its mRNA expression levels were elevated in models of depression induced by both chronic stress and repeated central LPS infusion (Guan et al., 2015).