PDCD1 and pancreatic intraductal papillary-mucinous neoplasm: PD-1 was expressed at a significantly higher level on peripheral CD8+ T lymphocytes in PDAC than that in IPMN (51.08 ± 13.22 vs. 31.96 ± 12.72; P < 0.001) and in healthy donors (51.08 ± 13.22 vs. 28.04 ± 10.27; P < 0.001) (Fig. 2A), whereas there was no significant difference in the expression of PD-1 on CD4+ T lymphocytes among the three groups (Fig. 2B).