Alteration in TRP catabolites was monitored in atherosclerosis and an elevated expression of indoleamine-2,3-dioxygenase (IDO)-1/2, the first and rate-limiting enzyme catabolizing both D and L-tryptophan to KYN was observed in the macrophage-rich cores of human advanced atherosclerotic plaques [49]. Here, IDO1 is linked to atherosclerosis.