How Gut Microbiome Metabolites Control Cancer Immunity and Immunotherapy Response
A comprehensive review reveals how SCFAs, bile acids, and tryptophan metabolites reshape the tumor immune microenvironment in GI cancers.
Summary
This review from Lanzhou University synthesizes evidence on how gut bacteria-derived metabolites — including short-chain fatty acids, bile acids, tryptophan derivatives, inosine, TMAO, and urolithin A — regulate immune cells and influence responses to cancer immunotherapy. These compounds act on receptors like GPR41/43, FXR, TGR5, and AhR to modulate T-cell activity, macrophage polarization, and tumor microenvironment composition. Critically, their effects are context-dependent: some enhance anti-tumor immunity while others promote immune suppression. The review also highlights organoid co-culture systems as tools for testing metabolite-based adjuvants alongside PD-1/PD-L1 checkpoint inhibitors, pointing toward precision strategies for gastrointestinal cancer treatment.
Detailed Summary
Gastrointestinal cancers — including colorectal, gastric, hepatocellular, and biliary tract malignancies — remain among the deadliest cancers worldwide. While immune checkpoint inhibitors (ICIs) targeting PD-1/PD-L1 and CTLA-4 have transformed oncology, response rates remain heterogeneous and resistance is common. This 2025 review in Frontiers in Immunology, authored by Luo et al. from Lanzhou University, comprehensively maps how gut microbiota-derived metabolites mediate the critical interface between microbial composition, immune regulation, and tumor progression in GI cancers.
The review organizes metabolites into four primary classes and several emerging categories. Short-chain fatty acids (SCFAs) — acetate, propionate, and butyrate — are produced by Clostridium, Faecalibacterium, and Bacteroides through fermentation of dietary fibers. They signal through GPR41/43 and GPR109A receptors and inhibit histone deacetylases (HDACs), enhancing epithelial barrier integrity, promoting the IL-10–STAT3 anti-inflammatory axis, regulating M1/M2 macrophage balance, and improving CD8+ T-cell effector function. Notably, butyrate can also promote immune tolerance via Treg induction, illustrating the dual-edged nature of these metabolites.
Bile acids present similarly complex biology. Primary bile acids (CDCA) are converted by gut bacteria to secondary forms including deoxycholic acid (DCA) and lithocholic acid (LCA), which promote tumor progression, while ursodeoxycholic acid (UDCA) and tauroursodeoxycholic acid (TUDCA) exert protective effects. These molecules signal through FXR, TGR5, and PXR receptors to regulate macrophage and T-cell polarization, suppress NF-κB activation, modulate reactive oxygen species, influence m6A RNA methylation, and maintain Treg stability. The balance between tumor-promoting and tumor-suppressing bile acid species is thus a critical determinant of GI cancer outcomes.
Tryptophan metabolism bifurcates into immunosuppressive and immune-protective branches. The kynurenine pathway, driven by IDO/TDO enzymes, produces kynurenine which suppresses T-cell function and promotes Treg differentiation via AhR signaling. In contrast, indole derivatives produced by Lactobacillus and Bifidobacterium — including indole-3-acetic acid (IAA), indole-3-propionic acid (IPA), and indole-3-aldehyde (IAld) — reinforce barrier integrity, induce IL-22, and modulate CD8+ T-cell infiltration. Serotonin (5-HT) represents a third tryptophan-derived pathway with additional neuroimmune implications.
Emerging metabolites receive dedicated attention. Inosine, produced by Bifidobacterium and Faecalibaculum via purine degradation, activates A2AR/cAMP-PKA signaling to enhance CD8+ T-cell activation and synergize with PD-1/CTLA-4 blockade — though excessive adenosine promotes immunosuppression. Urolithin A (UroA), derived from microbial conversion of dietary ellagitannins by Gordonibacter and Ellagibacter, improves CD8+ mitochondrial metabolism via ERK1/2 and ULK1 signaling, enhances NK and CAR-T cytotoxicity, and potentiates PD-1 blockade. TMAO activates the NLRP3 inflammasome and promotes IFN-γ+TNF-α+ T-cell responses but has dual effects on angiogenesis. Formate links exercise-induced microbiota changes to antitumor immunity via Nrf2 activation and IFN-γ release.
A key translational contribution of this review is its emphasis on immune-competent organoid co-culture systems as platforms for quantifying exposure-response thresholds, dissecting context-dependent metabolite effects, and pre-evaluating the safety and feasibility of metabolite-based immunologic adjuvants combined with PD-1/PD-L1 blockade. The authors also highlight dietary interventions, probiotics, engineered microbes, and plant-derived nanoparticles as strategies to reshape the microbiota-metabolite-immune axis. Three figures and three summary tables organize the mechanistic landscape, microbial sources, and clinical implications across 216 references.
Key Findings
- SCFAs (butyrate, propionate, acetate) enhance CD8+ T-cell effector function and synergize with PD-1 therapy by inhibiting HDACs and signaling through GPR41/43 and GPR109A receptors
- Bile acids display opposing tumor effects: DCA and LCA promote GI tumor progression while UDCA and TUDCA counteract these effects via FXR/TGR5/PXR receptor signaling
- Kynurenine pathway tryptophan metabolism suppresses T-cell immunity via AhR/IDO/TDO, while indole derivatives (IAA, IPA, IAld) from Lactobacillus/Bifidobacterium enhance barrier integrity and CD8+ infiltration
- Inosine from Bifidobacterium activates A2AR/cAMP-PKA to enhance CD8+ T-cell activation and synergize with PD-1/CTLA-4 blockade, though excess adenosine drives immunosuppression
- Urolithin A improves CD8+ mitochondrial metabolism via ERK1/2 and ULK1 signaling, enhances NK and CAR-T cytotoxicity, and potentiates PD-1 blockade in GI cancer models
- TMAO activates the NLRP3 inflammasome and promotes IFN-γ+TNF-α+ T-cell responses, with dual effects on tumor angiogenesis and immune activation
- Formate, produced by Bacteroides and Clostridium via anaerobic fermentation, induces Nrf2 activation and IFN-γ release, linking exercise-induced microbiota changes to antitumor T-cell function
Methodology
This is a narrative review article (not a primary study) synthesizing 216 references from the published literature on gut microbiota metabolites and GI cancer immunology. The authors organized evidence across four major metabolite classes and several emerging compounds, using three summary tables and three figures to map microbial sources, biosynthetic pathways, receptor targets, and immunoregulatory functions. No original experimental data, patient cohorts, or statistical analyses were generated; the review draws on preclinical models, clinical association studies, and mechanistic investigations from the existing literature.
Study Limitations
As a narrative review rather than a systematic review or meta-analysis, the paper is subject to selection bias in the literature cited and does not provide quantitative effect size estimates or quality assessments of included studies. The authors acknowledge that most mechanistic evidence derives from preclinical models, and the context-dependent, often opposing effects of the same metabolites (e.g., butyrate promoting both CD8+ activity and Treg induction) complicate direct clinical translation. The study was funded by regional Chinese government grants (Gansu Province Traditional Chinese Medicine Research Project and Lanzhou City Science and Technology Program), and no conflicts of interest were declared.
Enjoyed this summary?
Get the latest longevity research delivered to your inbox every week.