Roles of the gut microbiota in cancer immunotherapy: Mechanistic foundations and therapeutic opportunities

Cancer immunotherapy has revolutionized oncological treatment through diverse modalities including immune checkpoint blockade, adoptive cell therapy, therapeutic vaccines, and cytokine-based approaches. Despite these advances, clinical responses remain heterogeneous, with sustained benefit limited to a minority of patients. Emerging evidence now implicates gut microbiota as a critical systemic regulator of immunotherapy efficacy across multiple treatment platforms, mechanistically linking intestinal dysbiosis to antitumor immunity through the gut-immune-tumor axis. Specific commensal taxa and their metabolites, including short-chain fatty acids and tryptophan derivatives, regulate anti-tumor immunity through effector T cell enhancement, dendritic cell activation, and regulatory T cell suppression. This review systematically examines the microbial-metabolite-immune axis, elucidating mechanisms whereby intestinal microbes and metabolites mediate immunotherapy responses. We comprehensively evaluate microbiota-targeting strategies including dietary interventions, probiotics, prebiotics, and fecal microbiota transplantation, providing mechanistic insights and translational frameworks. We further discuss current challenges in transitioning from associative microbiome studies to mechanistic causality, standardizing intervention protocols, and integrating multi-modal microecological data, proposing future directions for engineered probiotics and precision microbial therapeutics to optimize outcomes under current immunotherapy.