Fig. 1.

Sites of IDO expression and action in cancer. IDO expression has been documented in a variety of cells in tumors and tumor-draining lymph nodes (and other metastatic sites) including malignant cells as well as other stromal, vascular and immune cells indicated. Both tryptophan deprivation and kynurenine production mediated by IDO has been implicated in inflammatory processes and the generation of antigenic immune tolerance (immune escape). The figure summarizes the general effects that have been described on T cell function at each site. APC antigen-presenting cell (e.g., dendritic cell), MDSC myeloid-derived suppressor cell, TAM tumor-associated macrophage, TAN tumor-associated neutrophil, Teff T effector cell, Treg T regulatory cell