IDO1 and melanoma: The treatment with 1MT partially restored iNKT cell proliferation in a concentration-dependent manner (Figure 7A,B), and, as expected, the concomitant reduction in L-kynurenine levels (Supplementary Figure S1) confirmed that IDO1 was involved in the melanoma-mediated reduction in iNKT cell proliferation through the production of L-kynurenine.