The gut probiotic Blautia (Blautia coccoides) significantly enhances the anti-tumor efficacy of PD-1 ICIs in bladder cancer treatment. Mechanistically, Blautia produces the bioactive metabolite trigonelline, which suppresses the Wnt/β-catenin signaling pathway. This promotes CD8+ T cell infiltration and enhances their anti-cancer activity within the TME, leading to potent tumor growth inhibition. These findings position Blautia and its metabolites as promising adjuvant therapeutics to boost the clinical effectiveness of PD-1 inhibitors.​ Here, CD8A is linked to urinary bladder cancer.