PDCD1 and graft versus host disease: GVHD pathogenesis is regulated by multiple factors, such as T regulatory CD4+ cells (Treg), donor APCs, B regulatory cells (Breg), myeloid-derived suppressor cells (MDSC), and immune checkpoints including interactions of programmed death ligand 1 (PD-L1) with programmed death-1 (PD-1) and CD80 (20–26).