BACKGROUND: Patients with breast cancer (BC) who benefit from the PD-1/PD-L1 inhibitor (PDi) is limited, necessitating novel strategies to improve immunotherapy efficacy of BC. Here we aimed to investigate the inhibitory effects of flaxseed lignans (FL) on the biological behaviors of BC and evaluate the roles of FL in enhancing the anticancer effects of PDi. METHODS: HPLC was used to detect the content of enterolactone (ENL), the bacterial transformation product of FL. Transcript sequencing was performed and identified CD38 as a downstream target gene of ENL. CD38-overexpressing cells were constructed and cell proliferation, colony formation, wound healing and transwell assays were used to assess the function of ENL/CD38 axis on BC cells in vitro. Multiplexed immunohistochemistry (mIHC) and CyTOF were used to detect the changes of the tumor immune microenvironment (TIM). 16S rDNA sequencing was used to explore the changes of gut microbiota in mice. A series of in vivo experiments were conducted to investigate the anticancer effects and mechanisms of FL and PDi. RESULTS: FL was converted to ENL by gut microbiota and FL administration inhibited the progression of BC. ENL inhibited the malignant behaviors of BC by downregulating CD38, a key gene associated with immunosuppression and PD-1/PD-L1 blockade resistance. The mIHC assay revealed that FL administration enhanced CD3 CONCLUSIONS: The FL/ENL/CD38 axis inhibited BC progression. FL enhanced the anticancer effects of PDi by modulating gut microbiota and host immunity.