Immune escape is a fundamental trait of
cancer in which the Th1-type
cytokine interferon- γ (IFN-γ) seems to play a key role. Among other tumoricidal biochemical pathways, IFN-γ induces the
tryptophan-degrading
enzyme indoleamine 2,3-dioxygenase (IDO) in a variety of cells including macrophages, dendritic cells (DCs) and
tumor cells. IDO activity has been shown to reflect the extent and the course in a plethora of
malignancies including prostate, colorectal, pancreatic, cervical, endometrial, gastric, lung, bladder, ovarian, esophageal and
renal cell carcinomas,
glioblastomas,
mesotheliomas, and
melanomas. Furthermore IDO activity during malignant
tumor diseases seems to be part of the tumoricidal immune defense strategy, which in the long run is detrimental to the host, when
tryptophan deprivation and production of pro-apoptotic
tryptophan catabolites counteract T-cell responsiveness.