IDO1 and lung cancer: The use of lung cancer directed vaccines and immune checkpoint inhibitors are driving these activities, however, in the future, it remains to be seen if tumor microenvironment cellular populations such as Tregs, myeloid derived suppressor cells (MDSC), tumor-associated macrophages, or soluble tumor immunosuppressive mediators such as indoleamine 2,3-dioxygenase (IDO), arginase, IL-6, IL-10, and other cytokines/chemokines will also be able to be targeted.