CXCL8 and major depressive disorder: (1) Excessive smoking triggers COPD patients’ pulmonary injury, which stimulates the release of Tumor Necrosis Factor-α(TNF-α), Nitric Oxide(NO), Interleukin-8(IL-8) and other inflammatory mediators [55], which can directly regulate the metabolism of 5-hydroxytryptamine, noradrenaline, dopamine, and other neurotransmitters in the central nervous system, consequently affecting emotional and conscious motor dominated areas of the brain and inducing anxiety and depression [56].