Consistent with the monocyte activation, circulating levels of proinflammatory cytokines, such as tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β), and interleukin-6 (IL-6), have been shown to be increased in patients with MDD [137] and with PTSD [138], and in animal models of stress [139,140]. Here, IL6 is linked to major depressive disorder.