PVR and cancer: The inhibitory receptor TIGIT competes together with the inhibitory receptor PVRIG (poliovirus receptor-related immunoglobulin) against the activating receptor CD226 (DNAX accessory molecule-1, DNAM-1) for their shared ligands including CD112 (poliovirus receptor-related 2, PVRL2), and CD155 (poliovirus receptor, PVR) which are expressed on cancer cells and antigen-presenting cells [19–22].