IDO1 and esophageal squamous cell carcinoma: For example, as esophageal squamous cell carcinoma (ESCC) progresses, changes in the TME are accompanied by an increase in immunosuppressive cells such as regulatory T (Treg) cells, myeloid-derived suppressor cells (MDSCs), and immuno-suppressive DCs, as well as soluble inhibitory molecules such as indole-2,3 dioxygenase (IDO) (45) and fibroblast growth factor 2 (FGF2) (46), resulting in reduced infiltration and functional inhibition of CD8+ T cells (47).