IDO1 and lung carcinoma: Data in metastatic tissue and blood have suggested that the expression of IDO-1 and other well-known immune checkpoint such as CTLA-4 and PD-L1 may be significantly interconnected (31–33); consequently, new possible combination therapies in lung cancer with IDO-1 and PD-1/PD-L1/CTLA-4 inhibitors may be explored.