Metformin enhances the anti-tumor effect of PD-L1 antibodies, and this effect depends on the presence of gut microbiota. Metformin improves anti-tumor immune responses and enhances intestinal mucosal integrity by modulating the gut microbiota. After antibiotic depletion of gut microbiota, the combined therapeutic effect of metformin and PD-L1 antibodies disappears, indicating the crucial role of gut microbiota in their anti-tumor action. This evidence concerns the gene CD274 and neoplasm.