TLR4 and major depressive disorder: Clinical studies have discovered the presence of various inflammatory mediators such as IL-1β, IL-6, tumor necrosis factor α (TNF-α), Toll-like receptor 3 (TLR3), and Toll-like receptor 4 (TLR4) in brains of patients with MDD patients who committed suicide, and the levels of these inflammatory factors were significantly higher than those of normal individuals (25).