215 - Gut-Derived Short-Chain Fatty Acids Orchestrate Tumor Metabolic and T-cell Immune Reprogramming to Potentiate Radiotherapy Efficacy in Head and Neck Cancer

Purpose/Objective(s):

Head and neck cancer (HNC) remains a major cause of cancer-related morbidity and mortality worldwide. Radiotherapy (RT), with or without chemotherapy, is central to curative treatment, yet radioresistance limits durable tumor control in many patients. Emerging evidence implicates the gut microbiota in modulating anticancer therapy responses. Our previous work showed that dietary fiber enhances tumor radiosensitivity in vivo, but the mechanisms linking gut microbial activity to RT response remain unclear.

Materials/Methods:

Patients with HNC undergoing definitive RT or chemoradiotherapy were prospectively recruited, with pre-treatment fecal samples collected for gut microbiota analysis. Species-level taxonomic and functional pathway analyses focused on carbohydrate metabolism and short-chain fatty acid (SCFA)-related pathways and were correlated with six-month treatment response. In vivo, syngeneic MOC2 tumors were established in immunocompetent mice treated with oral SCFAs, tumor irradiation, or both. Gut microbiota composition (16S rRNA sequencing), metabolomics (LC–MS of feces, plasma and tumor), immune profiling (flow cytometry), and tumor transcriptomics (bulk RNA-seq) were performed. In vitro, MOC2 cells were treated with physiologically relevant SCFA concentrations and irradiation to assess viability, oxidative stress, clonogenic survival and DNA damage signaling.

Results:

Responding patients showed enrichment of SCFA-producing genera, including Blautia, Anaerobutyricum and Bacteroides, with increased inferred fermentative capacity, suggesting functional priming for SCFA biosynthesis. In mice, SCFA supplementation enhanced tumor control only when combined with RT. This combination induced distinct gut microbial shifts enriched for SCFA- and formate-associated taxa, alongside coordinated metabolomic changes across the gut–circulation–tumor axis. Intratumoural acetate and formate accumulated markedly, with formate increasing nearly seven-fold. Combined SCFA and RT treatment increased intratumoural CD8? T-cell activation, reduced regulatory T-cell infiltration, and tended to increase the CD8?/Treg ratio without significantly altering colonic Tregs, indicating tumor-selective immune reprogramming. In vitro, SCFAs increased oxidative stress, suppressed ATM and PARP signaling, and promoted apoptotic and autophagy-related stress responses. Dietary fiber and next-generation probiotics independently improved tumor control in irradiated mice.

Conclusion:

Gut-derived SCFAs act as RT-dependent metabolic and immunological modulators rather than nonspecific immune enhancers. SCFAs enhance RT response in HNC through tumor-intrinsic radiosensitisation and selective immune reprogramming. These effects are achievable through practical interventions, supporting microbiota-based radiosensitization and future biomarker-driven clinical translation.