IDO1 and neoplasm: In this context, there are bulk data indicating that kynurenines can influence immune responses in a paracrine fashion, e.g., the overexpression of IDO1 by tumours increases the production of L-Kyn and 3-HAA that can signal to inhibit effector T-cell responses or promote Treg differentiation [52,53]; the latter two outcomes have been recognized as an important mechanism of immune escape by tumours.