IL6 and major depressive disorder: Consistent with a role for cytokines in depression, meta-analyses have established that circulating concentrations of the pro-inflammatory cytokines interleukin (IL)-6 and tumor necrosis factor (TNF)-α were elevated in non-medicated depressed patients, whereas variations of IL-1β were less consistently demonstrated (Dowlati et al., 2010; Liu et al., 2012a), possibly because of the difficulty in detecting the very low levels of this interleukin in human circulation.