Uncovering the colorectal cancer immunotherapeutic potential: Evening primrose (Oenothera biennis) root extract and its active compound oenothein B targeting the PD-1/PD-L1 blockade

•Antitumor potential of EPRE and its component, oenothein B, were evaluated.•EPRE and oenothein B blocked human PD-1/PD-L1 interaction.•Oenothein B inhibited cancer growth by activating tumor-specific CD8+ T cells.•The blocking of PD-1/PD-L1 interaction by EPRE and oenothein B is a novel finding.•Oenothein B is a promising natural small molecule with potential anticancer effects.