PDCD1 and graft versus host disease: which increased PD-L1/PD-1 interplay between donor CD8+ T cells and recipient tissues contributes to preventing GVHD by promoting the apoptosis and exhaustion of T-cell in GVHD target tissues, and enhanced PD-L1/CD80 interplay between CD8+ T cells contributes to retaining GVL responses by improving T-cell expansion and survival.