IDO1 and neoplasm: Immune checkpoint receptors are often upregulated in tumor tissue and promote tumor evasion from host immunosurveillance; (4) Cancer vaccination is performed using effector T cells to activate tumor-specific immune responses that can specifically decrease tumor load and control tumor relapse; (5) Indoleamine 2,3-dioxygenase (IDO), which degrades L-tryptophan along the kynurenine pathway, suppresses CD4+ T-cell function.