IDO1 and neoplasm: Tregs facilitate the secretion of IL-35 through the PD-1/PD-L1 pathway, and the subsequently secreted IL-35 enhances the secretion of IL-10 by MDSCs.555 Overexpression of indoleamine 2,3-dioxygenase (IDO) or arginase 1 (ARG1) in MDSCs contributes to the expansion mechanisms of Treg cells within the tumor.556 Additionally, there exists a bidirectional crosstalk between MDSCs and TAMs, significantly enhancing the immunosuppressive TME.