CD274 and breast cancer: Currently, immunotherapy for HER2+ breast cancers can be divided into four categories according to the mechanism of action: (1) passive immunotherapy, including monoclonal antibodies; (2) immune checkpoint inhibitors, including programmed death receptor-1 (PD-1)/programmed death ligand 1 (PD-L1) inhibitors; (3) adoptive cell immunotherapy, including chimeric antigen receptor (CAR)-T cell therapy; and (4) active immunotherapy with vaccines.