Checkpoint proteins such as CTLA-4, PD-1, LAG-3, TIM-3, VISTA and TIGIT are mostly found on the surface of T-cells, with some also found on dendritic cells (DCs) and monocyte/macrophages (PD-L1, TIM-3, LAG-3, VISTA) or tumour cells (PD-L1, TIM-3) [18,21,22,23]. Here, VSIR is linked to neoplasm.