The tumor microbiome and cancer immunotherapy: A systematic review of a new frontier beyond the gut

This systematic review explores the role of tumor-local microbiota in modulating anti-tumor immune responses. We screened five databases for original studies evaluating intratumoral bacteria and immune outcomes in solid tumors. Seventeen studies met inclusion criteria. Tumor-resident microbes were found to influence immune cell infiltration, cytokine expression, checkpoint regulation, and responses to immune therapies. Three key patterns emerged: immune activation, immune suppression, and modulation of checkpoint pathways. These spatially localized microbial effects offer new insights into the tumor–immune microenvironment. Integrating microbiome profiling into cancer immunotherapy may inform biomarker discovery and guide precision interventions.