IDO1 and cancer: Thus, as documented, features of the immune milieu in PDAC tumors include infiltration of cancer-supportive cells (e.g., cancer-associated fibroblasts (CAFs), Treg cells, suppressive neutrophils, indoleamine 2,3-dioxygenase (IDO)-producing DCs, M2-like TAMs and MDSCs), upregulation of suppressive cytokines (e.g., nitric oxide, hyaluronic acid, IL-6, IL-10, VEGF, TGF-β, CSF-1, GM-CSF, CXCL1, CXCL8, CXCL12 and CXCL13), angiogenesis and ‘T cell exclusion’ [7,52,53] (Figure 1).