PDCD1 and graft versus host disease: To circumvent graft-versus-host diseases, allogeneic T cells are produced through genetic disruption of HLA-A, HLA-B, class II major histocompatibility complex transactivator (CIITA), T-cell receptor alpha constant (TRAC), and Programmed Cell Death Protein 1 (PD-1), leading to their inability in recognizing allogeneic antigens (57, 59, 60).