IL6 and major depressive disorder: Experimental animal studies in animal models of immune inflammation and depression have found that lipopolysaccharide (LPS), interleukin-6 (IL-6), tumor necrosis factor-α (TNF-α), interferon-γ (IFN-γ) and other pro-inflammatory factors induce inflammatory responses in the body leading to a lack of pleasure as well as depression-like behavior in mice (8, 9).