PDCD1 and neoplasm: PD-1/PD-L1 interaction has been implicated in promoting the induction of Treg cells [28]; the blocking of PD-1 by antibodies suppresses TGF-β and retinoic acid-induced Treg conversion from naïve T cells, and treatment with a PD-1-blocking antibody in combination with a tumor vaccine was shown to reduce Treg infiltration into tumors in vivo [29].