IDO1 and cancer: Cancer cells are closely related to extracellular matrix, stromal cells, and immune cells, which together constitute TME (5) and these components in TME impact the efficacy of ICIs, for example, the number of regulatory T cells (Tregs), myeloid suppressor cells (MDSCs), dendritic cells (DCs) and the activity of indoleamine 2,3-dioxygenase (IDO).