IDO1 and neoplasm: Lymphodepletion, as an important cornerstone of CAR T-cell therapy, has direct anti-tumor cytotoxicity, activates innate immunity, enhances the production of homeostatic cytokines (MCP1, IL-7, and IL-15) [44], and reduces the immune regulation number of cells, such as myeloid-derived suppressor cells and regulatory T cells [91], and decreases indoleamine 2,3-dioxygenase (IDO) expression [92].