Figure 1.

Summary of immune checkpoints in different immune cells and tumor cells. Each ICI was targeted to different cell types. Although two of the ICIs’ target receptors and corresponding ligands are in the same checkpoint signaling pathway, there are also differences in therapeutic efficacy and side effects between them. In addition, there may be overlapping inhibitory effects between different checkpoint signaling pathways in the same immune cell type. This is one of the reasons for ICI resistance.