IDO1 and neoplasm: In patients, CAR T cells targeting EGFRvIII in a phase I trial led to marked expansion of tumor-infiltrating T cells but also increased expression of inhibitory regulatory T cells and upregulation of immunosuppressive markers indoleamine 2,3-dioxygenase 1 (IDO1) and PDL1 (O’Rourke et al., 2017).