IDO1 and neoplasm: Second, as within the tumor, inhibitory regulatory molecules such as PD-L1, PD-L2, galectin-9, indoleamine 2,3-dioxygenase 1 (IDO-1), and human B7 homolog 3 (B7-H3) (35–39) can be upregulated on vascular endothelial cells to directly inhibit T cell activity.