IL6 and major depressive disorder: In MDD, the most recent meta-analysis (29 studies, 822 MDD, 726 healthy controls) of serum proinflammatory cytokines confirmed that soluble IL-2 receptor, IL-6 and TNF-α levels are increased in MDD (trait markers) [91], while, IL-1β, IL-2, IL-4, IL-8 and IL-10, are not statistically different from controls [91].