IDO1 and neoplasm: Ferroptosis occurring in tumor-infiltrating neutrophils—an extrinsic microenvironmental factor—suppresses CD8+ T cell proliferation and cytotoxicity through the secretion of prostaglandin E2 (PGE2), indoleamine 2,3-dioxygenase (IDO), and oxidized lipids, and is accompanied by the accumulation of IL1β+ CXCL3+ CD4+ T cells.