PDCD1 and osteosarcoma: Abbreviations – ACT: adoptive cellular transfer; ATCT: adoptive T-cell transfer; CTLA-4: cytotoxic T lymphocyte-associated protein 4; ICB: immune checkpoint blockade; MSCs: mesenchymal stem cells; OS: osteosarcoma; PD-1: programmed death cell receptor 1; TME: tumor microenvironment; YOP: year of publication.