IDO1 (indoleamine 2,3-dioxygenase 1) catalyzes the first rate-limiting step of tryptophan catabolism along the kynurenine pathway 12. This metabolic activity generates kynurenine, which serves as substrate for NAD+ biosynthesis and 1-carbon metabolism 34. IDO1 functions as a critical immune regulator through dual mechanisms: tryptophan depletion inhibits T cell proliferation, while kynurenine accumulation induces T cell apoptosis and promotes regulatory T cell (Treg) differentiation via aryl hydrocarbon receptor (AhR) activation 56. Additionally, IDO1 expression in dendritic cells enables metabolic communication with other immune cells to maintain immune tolerance 6. In cancer, IDO1 acts as a suppressor of anti-tumor immunity by creating an immunosuppressive tumor microenvironment enriched with exhausted T cells and Tregs 78. IDO1-derived kynurenine also promotes anti-ferroptotic signaling, protecting cancer cells from ferroptotic death 3. Conversely, IDO1 limits intracellular pathogen growth through tryptophan depletion and protects fetuses from maternal immune rejection 9. In EBV-infected B cells, EBNA2-driven IDO1 expression sustains NAD biosynthesis supporting oncogenic transformation 4. IDO1 inhibition shows therapeutic potential in cancer and EBV-associated lymphoma 47.