IDO1 and atherosclerosis: In kynurenine pathway, the indoleamine 2,3-dioxygenase (IDO) is firstly induced by inflammatory cytokines, such as interferon-γ (IFN-γ) and tumor necrosis factor-α (TNF-α), and then leads to the catabolic of kynurenine, causing an increase of Kynurenine/Tryptophan ratio, which is associated with the greater carotid plaque in atherosclerosis patients and in coronary heart disease patients22, 23.