Abstract
BACKGROUND AND OBJECTIVE: Microbiota and their metabolites form a dynamic regulatory network that modulates the tumor microenvironment (TME) and immune cell functionality, emerging as critical targets in cancer immunotherapy. Despite remarkable advances in immune checkpoint inhibitors (ICIs), clinical efficacy remains limited by primary or acquired resistance in a substantial proportion of patients. Accumulating evidence indicates that the gut microbiota-metabolite-immune axis is a critical determinant of ICI responsiveness, but the underlying molecular mechanisms and tumor-specific regulatory patterns remain incompletely elucidated. This narrative review aims to systematically dissect the mechanistic interplay of this axis across diverse cancer types and synthesize current microbiota-targeted therapeutic strategies for improved immunotherapy outcomes. METHODS: We conducted a systematic literature search of PubMed for studies published between January 2019 and December 2025, with a particular focus on basic and translational research elucidating the roles of gut microbiota and their metabolites in cancer immunotherapy across various malignancies. KEY CONTENT AND FINDINGS: This review elucidates the molecular mechanisms by which core metabolites regulate antitumor immunity, synthesizes characteristic microbial signatures across gastrointestinal and non-gastrointestinal cancers. Furthermore, we evaluate the translational potential of intervention strategies, including fecal microbiota transplantation, probiotic supplementation, and engineered microbes, as adjuvants to enhance ICI efficacy and overcome resistance. CONCLUSIONS: Microbiota-based personalized therapeutic strategies are increasingly recognized as promising tools to modulate antitumor immunity and improve immunotherapy outcomes. Systematic profiling of the gut microbiota-metabolite-immune axis across cancer types will facilitate the development of precision interventions to overcome ICI resistance. Future research integrating multi-omics approaches and large-scale clinical trials is pivotal to translate mechanistic insights into clinically viable therapies, ultimately revolutionizing the landscape of cancer immuno-oncology.