IL6 and major depressive disorder: Remarkably, elevated levels of circulating markers of inflammation predict a positive response to tricyclic antidepressants, ketamine and electroconvulsive therapy and a poor response to selective SSRI (O’Brien et al., 2007; Yoshimura et al., 2009; Carvalho et al., 2013); in particular, patients with SSRI resistant depression had significantly higher production of the pro-inflammatory cytokines IL-6, TNFα, and IL-6R compared with healthy controls.