TNF and major depressive disorder: In a systematic review and meta-analysis of 82 studies measuring cytokine levels in healthy controls and participants with major depressive disorder (MDD), the latter had elevated peripheral levels of chemokine ligand 2, interleukin (IL)-1 receptor antagonist, 2, 6, 10, 12, 18, and tumor necrosis factor-alpha (TNF-α) and lower interferon-gamma (IFN-γ) levels (12).