Abstract: The disclosure provides a method for treating a subject afflicted with a cancer comprising administering to the subject a therapeutically effective amount of an anti-PD-1 antagonist, e.g., an anti-PD-1 or anti-PD-L1 antibody, in combination with an indoleamine 2,3-dioxygenase inhibitor, wherein the subject is identified as exhibiting a combined biomarker comprising (a) a high IFN? inflammatory signature score and (b) a low tryptophan 2,3-dioxygenase 2 (TDO2) gene expression score. The high IFN? inflammatory signature score is determined by measuring the expression of a panel of IFN? related inflammatory genes in a cancer sample obtained from the subject, wherein the gene panel comprises, e.g., IFN?, CXCL10, CXCL9, HLA-DRA, IDO1, and STAT1. In some aspects, the gene panel further comprises CCR5, CXCL11, GZMA, and PRF1. In some aspects, the gene panel comprises CXCR6, TIGIT, PD-L1, PD-L2, LAG3, NKG7, PSMB10, CMKLR1, CD8A, IDO1, CCL5, CXCL9, HLA.DQA1, CD276, HLA.DRB1, STAT1, HLA.E, and TDO2.