IDO1 and neoplasm: Paradoxically, the upregulation of PD-L1, indoleamine-2,3-dioxygenase (IDO), and Tregs in tumor microenvironment has been shown to be driven by IFN-γ-producing CD8+ T cells themselves [67,68] and these three factors can all contribute to disable T cell responses and impair vaccine efficacy [69].