This adverse reaction would be interesting to explore further, given that Melssen and colleagues noted that lower levels of indoleamine 2,3-dioxygenase 1 (IDO1), as well as CD8 inhibitory pathways, programmed death-ligand 1 (PD-L1), were committed to a cancer vaccine containing CpG ODN in a liposomal formulation [60]. Here, IDO1 is linked to cancer.