PVR and neoplasm: TIGIT and its ligand CD155 are the immune checkpoints expressed on the surface of immune cells; TIGIT is mainly expressed on CD4+ T cells, CD8+ T cells, NK cells, and Treg cells (47); CD155 is mainly expressed on HM1345+ melanoma cells, macrophages, myeloid cells, DCs, and tumor-related infiltration lymphocytes (48).