The study investigates the role of commensal Bifidobacterium in enhancing antitumor immunity and improving the efficacy of anti-PD-L1 therapy. Researchers compared melanoma growth in mice with different gut microbiota and found that mice with a specific microbiota (JAX) had better spontaneous antitumor immunity compared to those with a different microbiota (TAC). cohousing or fecal transfer of JAX microbiota eliminated the differences in tumor growth and immune responses. Sequencing identified Bifidobacterium as associated with antitumor effects. Oral administration of Bifidobacterium alone improved tumor control to the same extent as PD-L1-specific antibody therapy, and combination treatment nearly abolished tumor growth. The enhanced immune response was mediated by augmented dendritic cell function, leading to better CD8+ T cell priming and accumulation in the tumor microenvironment. These findings suggest that manipulating the gut microbiota could modulate cancer immunotherapy.The study investigates the role of commensal Bifidobacterium in enhancing antitumor immunity and improving the efficacy of anti-PD-L1 therapy. Researchers compared melanoma growth in mice with different gut microbiota and found that mice with a specific microbiota (JAX) had better spontaneous antitumor immunity compared to those with a different microbiota (TAC). cohousing or fecal transfer of JAX microbiota eliminated the differences in tumor growth and immune responses. Sequencing identified Bifidobacterium as associated with antitumor effects. Oral administration of Bifidobacterium alone improved tumor control to the same extent as PD-L1-specific antibody therapy, and combination treatment nearly abolished tumor growth. The enhanced immune response was mediated by augmented dendritic cell function, leading to better CD8+ T cell priming and accumulation in the tumor microenvironment. These findings suggest that manipulating the gut microbiota could modulate cancer immunotherapy.