TNF and depressive disorder: Some of the factors that link the development and progression of depression in PD through the promotion of neuroinflammation are considered to be microgliosis in the SNpc and elevated levels of pro-inflammatory cytokines in the brain and blood of PD patients with elevated levels of the soluble receptor interleukin-2 (sIL-2R), tumor necrosis factor alpha (TNFα) and C-reactive protein (CRP) in addition to the dysregulation of genes involved in signaling pathways related to the immune system and lipid and glucose metabolism.