IDO1 and pancreatic neoplasm: MDSCs recruited by tumor cells inhibit the function of T cells by regulating PD-L1 [46]. Pancreatic cancer cells inhibit NK cell activity by direct toxicity to NK cells and by secretion of indoleamine 2,3-dioxygenase (IDO)、 IL-10 and TGF-β [47]. At distant metastatic sites, pancreatic cancer cells recruit bone-derived suppressor cells via EVs and inhibit regulatory T cell function via TGF-β, IL-10, and IFN-γ [48, 49]. Neutrophils are also recruited to neutralize other immune cells through IL-12 and TNF-α [45].