Impact of gut microbiome on radiotherapy and immunotherapy efficacy in microsatellite-stable colorectal cancer: role of propionic acid and B. fragilis

Abstract Background Colorectal cancer (CRC) is the third most common cancer and the second leading cause of cancer-related deaths worldwide. While immunotherapy is effective in microsatellite instability-high (MSI-H) CRC, its benefits in microsatellite-stable (MSS) CRC are limited. Radiotherapy may modify the immune microenvironment in MSS-CRC, enhancing immunotherapy efficacy, but individual responses vary. Methods We employed MSS-CRC mouse models to examine the effects of combined radiotherapy and immunotherapy, with and without antibiotics (ABX). Various analyses, including metagenomic, nontargeted metabolomic, and gas chromatography-mass spectrometry (GC-MS), were performed to identify factors influencing treatment outcomes. Flow cytometry, immunohistochemistry and in vivo antibody blockade experiments assessed the role of metabolites and bacteria on CD8 + T cell infiltration and treatment responses, complemented by transcriptomic sequencing and molecular biology experiments. Results Our analyses identified propionic acid and Bacteroides fragilis ( B. fragilis ) as crucial factors enhancing the efficacy of combined therapies in MSS-CRC. Both propionic acid and B. fragilis improved CD8 + T cell infiltration and treatment outcomes, with molecular assays indicating that propionic acid facilitates H3K14 acetylation, activating the Meox1-Cxcr6/Ccl5 axis. Conclusions This study highlights the pivotal role of the gut microbiome, specifically propionic acid and B. fragilis , in modulating the efficacy of combined radiotherapy and immunotherapy in MSS-CRC.