The review builds on the concept of the microbiota-gut-brain axis, a bidirectional communication network linking the gastrointestinal tract with the central nervous system through neural, endocrine, immune, and metabolic pathways. Gut-derived signals can reach the brain via vagal afferents, circulating immune mediators, microbial metabolites, and gut hormones, while autonomic and hypothalamic-pituitary-adrenal signaling can in turn modify intestinal motility, barrier function, and microbial composition. Microbial products such as short-chain fatty acids, tryptophan derivatives, and lipopolysaccharide are known to influence blood-brain barrier homeostasis, microglial activity, immune-cell differentiation, and epigenetic regulation. Much of this mechanistic evidence, however, comes from germ-free animals and non-cancerous neurological models, and its direct relevance to human glioblastoma remains incompletely established.

The tumor microenvironment of glioblastoma provides a plausible target for such systemic signals. The tumor is dominated by myeloid cells, particularly tumor-associated macrophages that arise from both resident microglia and infiltrating monocytes. Single-cell studies show these populations occupy distinct spatial niches and transcriptional states, rendering the conventional M1/M2 classification overly simplistic. Glioma cells recruit and shape myeloid cells through mediators including colony-stimulating factor 1, CCL2, transforming growth factor-beta, and interleukin-10, while regulatory T cells and exhausted T cells expressing PD-1, TIM-3, LAG-3, and CTLA-4 further entrench immune suppression. Immune dysfunction also extends beyond the tumor itself: treatment-naive patients show sequestration of naive T cells in the bone marrow, impairing T-cell egress and helping explain why checkpoint blockade alone has failed in this disease.

Against this backdrop, experimental evidence from mouse models has begun to suggest that gut microbes can indeed modulate glioma-associated immunity. In one of the earliest glioma-specific studies, chronic antibiotic exposure in mice bearing intracranial GL261 tumors altered intestinal bacterial composition, reduced cytotoxic natural-killer-cell subsets, modified microglial inflammatory proteins, and was accompanied by accelerated tumor growth. A subsequent orthotopic model showed that antibiotic-induced dysbiosis reduced short-chain fatty acid concentrations in blood and tumor tissue and sped tumor progression, while oral supplementation with these metabolites increased glycolytic activity in tumor-associated macrophages and improved outcomes. More strikingly, fecal microbiota transfer experiments demonstrated that mice receiving microbiota from patients with high-grade glioma developed faster-growing tumors than mice receiving microbiota from healthy donors, with bacterial genera such as Eisenbergiella and Merdimonas enriched in the recipients.

Metabolite signaling sits at the heart of these mechanisms, but the review stresses a critical caveat: a metabolite relevant to glioblastoma is not necessarily microbiota-derived. Short-chain fatty acids such as butyrate act through G-protein-coupled receptors and epigenetic mechanisms, inhibiting histone deacetylases and influencing regulatory T-cell differentiation. Sodium butyrate has reduced proliferation and promoted apoptosis in human glioma cell lines, and in one mouse study it increased PD-L1 expression and enhanced the efficacy of anti-PD-1 therapy. Yet the millimolar concentrations used in vitro do not demonstrate that microbiota-derived butyrate reaches human glioblastoma tissue at comparable levels. Similarly, the kynurenine pathway engages the aryl hydrocarbon receptor, but in glioblastoma the strongest evidence points to kynurenine produced by tumor cells themselves rather than by gut microbes. A recent study also showed that tumor-cell-derived spermidine suppressed CD8-positive T-cell activity and that blocking its synthesis prolonged survival in mice, again without any established microbial contribution.

Perhaps the most provocative findings concern microbial material detected within human brain tumors themselves. A prospective multi-institutional study of 243 samples from 221 patients combined RNA-based fluorescence in situ hybridization, immunohistochemistry, spatial imaging, sequencing, and culturomics, detecting bacterial 16S rRNA and lipopolysaccharide signals in subsets of glioma and metastatic samples, including within tumor, immune, and stromal cells. Signals assigned to genera including Fusobacterium, Prevotella, and Veillonella partially overlapped with matched oral or gut communities. Separately, an immunopeptidomic analysis of tumors from nineteen patients identified 344 distinct bacteria-associated peptides bound to HLA class II molecules, and some of these peptides were recognized by tumor-infiltrating T cells, including a CD4-positive T-cell clone that cross-recognized both a mutated glioblastoma antigen and several microbial peptides.

These discoveries come with heavy methodological caveats. Brain tumors contain extremely low microbial biomass, making sequencing studies vulnerable to reagent contamination, environmental DNA, and computational misclassification. The 2024 retraction of a prominent TCGA-based cancer microbial-signature study underscored the danger. Standard culture failed to recover a readily cultivable bacterial community from the prospective brain-tumor samples, the signals were sparse and heterogeneous, and no association with clinical outcome was established. The review therefore recommends reserving the terms microbiota and microbiome for settings where a diverse, biologically active community is supported by adequate evidence, and instead using phrases like microbial signals or bacterial elements until viability, replication, and function are demonstrated. Rigorous negative controls, orthogonal spatial validation, and viability assessment are deemed essential before any claim of stable intratumoral colonization can stand.

On the translational front, the review finds everything still at the earliest stage. No gut- or tumor-derived microbial biomarker has been independently validated for glioblastoma diagnosis, prognosis, or treatment selection, and current prospective studies such as THERABIOME-GBM remain observational. Fecal microbiota transplantation has shown clinical proof of principle in melanoma, where it can promote responses to checkpoint inhibitors, but no therapeutic trial exists in glioblastoma, and safety concerns are serious: FMT has transmitted multidrug-resistant organisms to immunocompromised recipients, including a fatal infection. Probiotic combinations of Bifidobacterium lactis and Lactobacillus plantarum reduced tumor volume in mice, and even dietary manipulations such as a short-term high-glucose drink or aspartame exposure altered tumor biology in animal models, but the authors explicitly caution that none of these findings justifies recommending any microbiome-directed intervention to patients.

Looking ahead, the review identifies several promising but speculative directions. One is the intersection between microbiota-associated inflammation and the circadian-glymphatic-melatonergic network, where experimental sleep-restriction data suggest melatonin can preserve glymphatic function, and a proposed but unproven pathway links aryl hydrocarbon receptor activity to melatonin metabolism and N-acetylserotonin signaling through the TrkB receptor. Another is bacteriophage-based modulation, which in colorectal cancer models has successfully targeted Fusobacterium nucleatum to augment chemotherapy, though no viable bacterial driver has been validated in glioblastoma and delivery across the blood-brain barrier remains unsolved. The authors conclude that progress will depend on source attribution, orthogonal validation, perturbation-and-rescue experiments, longitudinal human sampling, and replication across centers. The key distinction, they argue, is between a microbiota-associated signal and a clinically established mechanism, and only evidence that a defined microbial feature is reproducible, mechanistically linked to tumor or immune phenotypes, and safely modifiable will allow microbiota research to contribute meaningfully to precision neuro-oncology.

Subject of Research: The role of the gut microbiome and microbial metabolites in glioblastoma tumor-immune interactions and translational oncology

Article Title: The Microbiome in Glioblastoma: Mechanisms, Tumor–Immune Interactions, and Translational Perspectives

Article References: AlRamadneh, T. N., Jyothi‐S, R., Priyadarshini‐Nayak, P., Nanda, A., Al‐Hasnaawei, S., Bhatt, A., Singh‐Chauhan, A., Singla, S., & Mishra, M. K. (2026). The Microbiome in Glioblastoma: Mechanisms, Tumor–Immune Interactions, and Translational Perspectives. MicrobiologyOpen, 15(5), Article e70431. https://doi.org/10.1002/mbo3.70431

Image Credits: AI Generated

DOI: 10.1002/mbo3.70431

Keywords: glioblastoma, gut microbiome, microbiota-gut-brain axis, tumor microenvironment, short-chain fatty acids, kynurenine, aryl hydrocarbon receptor, tumor-associated macrophages, immune checkpoint inhibitors, fecal microbiota transplantation, intratumoral bacteria, HLA peptides

The biological rationale behind the trial was compelling. Many tumors defend themselves not just by hiding from immune cells but by actively poisoning the local environment in which those cells operate. One of the most notorious mechanisms involves indoleamine 2,3-dioxygenase-1, or IDO1, an enzyme that catabolizes the essential amino acid tryptophan into kynurenine. When IDO1 is highly active, tryptophan levels in the tumor microenvironment fall while kynurenine levels rise. This metabolic shift is doubly damaging: T cells deprived of tryptophan become dysfunctional and can even undergo stress responses, while kynurenine itself promotes immunosuppressive signaling. The ratio of kynurenine to tryptophan, often abbreviated Kyn/Trp, therefore serves as a convenient biochemical readout of IDO1 activity and of the metabolic hostility of a tumor’s surroundings.

To neutralize this mechanism, the investigators used epacadostat, known in earlier development as INCB024360, an oral small-molecule inhibitor of IDO1 supplied by Incyte Corporation. Patients received 300 milligrams twice daily starting on day zero. Three weeks later, beginning on day 21, they also received MELITAC 12.1, a multipeptide vaccine developed to stimulate T cells against multiple melanoma antigens simultaneously. The idea was a classic one-two punch: the vaccine would expand armies of T cells primed to recognize melanoma, while the IDO1 inhibitor would strip away the metabolic defenses that tumors use to disable those armies once they arrive. The trial enrolled eleven eligible patients with measurable regional or distant melanoma metastases.

A distinctive strength of the study was its insistence on looking inside the tumor, not just at the bloodstream. Patients underwent tumor biopsies at three time points: before treatment, on day 21, and on day 42. Some patients also underwent resection of all clinically evident disease by day 42, providing additional tissue for analysis. The prespecified primary immunologic endpoint was an increase in CD8-positive tumor-infiltrating lymphocytes, the cytotoxic T cells that can directly kill tumor cells, measured by immunohistochemistry. Circulating T cell responses were tracked using interferon-gamma ELIspot assays, while inhibition of IDO1 was assessed by measuring kynurenine and tryptophan levels in both serum and tumor tissue.

On the safety front, the combination performed respectably. Two of the eleven subjects, about 18 percent, experienced dose-limiting toxicities that were transient, and the regimen was otherwise well tolerated. This matters because combination immunotherapies often stack toxicities on top of one another, and a tolerable pairing preserves options for future development. The biochemical data were likewise encouraging. Epacadostat normalized serum kynurenine-to-tryptophan ratios in 91 percent of subjects, demonstrating that the drug was doing exactly what it was designed to do systemically. Perhaps more importantly, the drug also reduced Kyn/Trp ratios inside the tumors themselves, although pretreatment intratumoral ratios varied widely from patient to patient, hinting that IDO1-driven immunosuppression may be relevant in some tumors but not others.

Here, however, the trial’s central hypothesis ran aground. The prespecified primary endpoint, an increase in total CD8-positive T cell infiltration within tumors, was not met. Blocking IDO1, whether alone or in combination with the vaccine, did not meaningfully increase the number of cytotoxic T cells crowding into the tumor microenvironment. This finding lands as part of a broader reckoning with the IDO1 field. Enthusiasm for IDO1 inhibitors was once enormous, but large randomized trials of epacadostat in combination with checkpoint inhibitors in melanoma failed to improve clinical outcomes, and the present study adds mechanistic texture to that disappointment: the drug reliably lowers kynurenine, yet lowering kynurenine alone does not summon more killer T cells into tumors.

The vaccine component also produced puzzling results. In one subject, a day-42 tumor contained T cells reactive to peptides included in the vaccine, a tantalizing sign that vaccine-primed cells could reach tumor tissue. Unfortunately, no baseline tumor sample was available from that patient, making it impossible to determine whether those cells had been recruited or expanded as a result of treatment. More broadly, circulating T cell responses measured by ELIspot were lower than those seen in prior studies of the vaccine given alone, raising the possibility that the IDO1 inhibitor, or the sequencing of the two treatments, somehow blunted rather than enhanced vaccine-induced immunity. That counterintuitive observation alone justifies further mechanistic study.

Clinically, the trial was too small to draw firm conclusions about efficacy, but the signals were modest at best. Among five patients evaluable for clinical response, the best response was a partial remission in one subject, stable disease in three, and progressive disease in one. The partial response was transient. The authors conclude that these findings do not support routine combination of the MELITAC 12.1 vaccine with epacadostat, an honest negative result that spares future patients from an unpromising regimen and redirects resources toward more rational strategies.

Yet the study is far from a dead end. The observation that pretreatment intratumoral Kyn/Trp ratios varied widely suggests a biomarker-driven path forward: rather than giving IDO1 inhibitors to everyone, clinicians might identify the subset of patients whose tumors are genuinely dependent on IDO1-mediated metabolic suppression and treat only those individuals. The authors explicitly note that there may be value in targeted study of IDO1 inhibitors for tumors with the highest kynurenine-to-tryptophan levels. This kind of sequential tumor biopsy design, sampling the microenvironment before and during therapy, is exactly the sort of translational rigor that turns failed trials into roadmaps.

The trial also stands as a testament to collaborative infrastructure. It was funded by an NIH/NCI grant supporting the Cancer Immunotherapy Trials Network, with additional support from cancer center grants at the Fred Hutch/University of Washington/Seattle Children’s Cancer Consortium and the University of Virginia Cancer Center, along with philanthropic support for vaccine peptide production. Specialized analyses of gene expression, IDO1 content, and tumor metabolites were performed with partners including NanoString Technologies and ImaBiotech. In an era when immunotherapy headlines often celebrate dramatic successes, this study is a reminder that negative results, when paired with deep biological measurement, are among the most instructive outputs of clinical science. Eleven patients, three biopsies each, and a meticulously measured metabolic axis have clarified where one immunotherapeutic combination does not work, and pointed toward where it still might.

Subject of Research: A phase II trial combining an IDO1 inhibitor with a multipeptide vaccine in advanced melanoma

Article Title: A phase II pilot trial of an indoleamine 2,3-dioxygenase-1 (IDO1) inhibitor (INCB024360) plus a multipeptide melanoma vaccine (MELITAC 12.1) in patients with advanced melanoma, with sequential evaluation of the tumor microenvironment

Article References: Slingluff, C. L., Ernstoff, M. S., Lawson, D., Delman, K. A., Hanks, B. A., Gastman, B., Mauldin, I. S., Olson, W. C., Ninmer, E. K., Gradecki, S. E., Smith, K. T., Gru, A., Ramchurren, N., Morishima, C., Fling, S. P., Kaiser, J. C., & Cheever, M. A. (2026). A phase II pilot trial of an indoleamine 2,3-dioxygenase-1 (IDO1) inhibitor (INCB024360) plus a multipeptide melanoma vaccine (MELITAC 12.1) in patients with advanced melanoma, with sequential evaluation of the tumor microenvironment. Cancer Immunology, Immunotherapy. https://doi.org/10.1007/s00262-026-04572-z

Image Credits: AI Generated

DOI: 10.1007/s00262-026-04572-z

Keywords: melanoma, IDO1, epacadostat, cancer vaccine, immunotherapy, tumor microenvironment, kynurenine, tryptophan, CD8 T cells, clinical trial, Cancer Immunotherapy Trials Network, tumor-infiltrating lymphocytes

At the heart of the review is the concept of metabolic reprogramming, the process by which cancer cells fundamentally alter how they generate energy and build the molecular components needed for relentless growth. The most famous example is the Warburg effect, a phenomenon first described nearly a century ago in which cancer cells preferentially burn glucose through glycolysis even in the presence of abundant oxygen, a strategy that would seem inefficient but turns out to support rapid proliferation. By enhancing glycolytic flux, colorectal cancer cells rapidly consume glucose from their surroundings while flooding the tumor microenvironment with lactate, a metabolic byproduct that acidifies the tissue and undermines the function of immune cells that would otherwise attack the tumor.

The consequences of this metabolic competition extend far beyond glucose. The review details how alterations in lipid metabolism allow tumor cells to synthesize the fatty acids required for building new membranes as they divide, while amino acid metabolism provides the nitrogen and carbon skeletons for producing proteins and nucleotides. One particularly important amino acid is glutamine, which fuels multiple biosynthetic pathways, and another is tryptophan, whose breakdown has emerged as a critical immune escape mechanism. Together, these metabolic shifts do not merely feed the tumor; they actively sculpt the surrounding environment into one that suppresses immune activity and fosters therapeutic resistance.

The tumor microenvironment in colorectal cancer, as the review describes it, becomes a battlefield defined by metabolites. Immune cells that infiltrate the tumor find themselves starved of the glucose and amino acids they need to sustain their anti-tumor activity, while simultaneously being exposed to an accumulating pool of immunosuppressive metabolites. Among the most notorious of these is kynurenine, a product of tryptophan catabolism that directly inhibits T cell activity. The metabolic conditions within the tumor also promote the expansion of regulatory T cells, or Tregs, a specialized immune cell type that normally restrains excessive immune responses but which tumors co-opt to shield themselves from attack. Myeloid-derived suppressor cells, or MDSCs, another population of immunosuppressive cells, are likewise favored by the altered metabolic landscape, compounding the barrier to effective anti-tumor immunity.

This is where the concept of metabolic modulators as gatekeepers becomes central to the review’s argument. Immune checkpoint inhibitors, the class of immunotherapy drugs that have revolutionized treatment of several cancers, depend on the presence of functional, energized T cells to work. In colorectal cancer, however, the metabolically hostile tumor microenvironment often renders T cells exhausted and dysfunctional, meaning that releasing the brakes on the immune system achieves little when the engine itself has run out of fuel. The review emphasizes that targeting the metabolic pathways responsible for this dysfunction, including glycolysis, oxidative phosphorylation, and amino acid metabolism, can simultaneously disrupt tumor proliferation and restore the anti-tumor immune responses that immunotherapy requires.

Several classes of metabolic drugs have already shown promise in preclinical and clinical contexts, according to the authors. Fatty acid synthase inhibitors, known as FASN inhibitors, aim to cut off the tumor’s capacity to manufacture its own lipids, a vulnerability that many rapidly dividing cancers cannot easily bypass. Metformin, the widely used and inexpensive diabetes drug, has attracted intense oncology interest because of its effects on cellular energy sensing pathways, and the review highlights its potential to improve immunotherapy responses in colorectal cancer. Inhibitors of indoleamine 2,3-dioxygenase 1, or IDO-1, target the enzyme responsible for degrading tryptophan into kynurenine, thereby addressing one of the most direct mechanisms by which tumors starve and suppress T cells. By combining these metabolic interventions with checkpoint blockade, the authors suggest, clinicians may be able to overcome the resistance mechanisms that currently limit immunotherapy in many colorectal cancer patients.

The technical logic behind these combinations is grounded in the interdependence of glucose, lipid, and amino acid metabolism. Blocking a single pathway rarely defeats a tumor outright, because cancer cells are metabolically flexible and can compensate through alternative routes. However, metabolic stress imposed on tumor cells can tipping the balance of the microenvironment: less lactate means less acidity, more available glucose means better-fueled cytotoxic T cells, and preserved tryptophan levels mean less kynurenine-mediated suppression. When these favorable metabolic shifts are layered onto immunotherapy, the checkpoint inhibitors encounter a population of T cells that is genuinely capable of proliferating and killing tumor cells, rather than a depleted and exhausted contingent. The review frames this metabolic recalibration as a strategy that could extend the benefits of immunotherapy to patients whose tumors are currently classified as unresponsive.

Beyond treatment, the review underscores the growing role of metabolomic and multi-omics profiling in personalized colorectal cancer care. These techniques allow researchers to measure hundreds of metabolites and metabolic gene signatures in a patient’s tumor simultaneously, generating a metabolic fingerprint that could predict how an individual will respond to immunotherapy before treatment begins. Such biomarkers would represent a significant advance over current approaches, in which immunotherapy is often administered empirically and its effectiveness assessed only after weeks of treatment. By identifying which metabolic pathways dominate in a given patient’s tumor microenvironment, clinicians could in principle select the most appropriate metabolic modulator to pair with immunotherapy, moving the field closer to truly personalized therapeutic regimens.

The authors, whose work was supported by Rafsanjan University of Medical Sciences, are careful to position their analysis as a synthesis of the current evidence landscape rather than a report of a single new experiment. As a review, the article draws together findings from preclinical models and clinical studies to build a coherent framework connecting tumor metabolism, immune evasion, and immunotherapy response. The framework carries important caveats that the field itself continues to grapple with: systemically delivered metabolic drugs affect both tumor cells and immune cells, sometimes with opposing consequences, and patients’ overall metabolic state, influenced by diet, obesity, diabetes, and the gut microbiome, adds further layers of complexity that laboratory models struggle to fully replicate.

Nevertheless, the review’s core thesis offers a compelling reframing of an old problem. Colorectal cancer has long been considered relatively resistant to immunotherapy compared with melanoma and lung cancer, and much of the research effort has focused on genetic and immune-cell-based explanations. By spotlighting metabolism as the decisive variable, Najafi, Falak, Khorramdelazad and their colleagues point toward a therapeutic frontier in which the tumor’s own fuel supply becomes its greatest vulnerability. If ongoing and future clinical studies validate the promise of metabolic modulators, the gatekeeper metaphor may prove apt in the most literal sense: controlling what crosses the metabolic gates of the tumor microenvironment could determine who benefits from the immunotherapy revolution and who is left behind.

Subject of Research: Metabolic reprogramming and metabolic modulators as determinants of immunotherapy response in colorectal cancer

Article Title: Metabolic modulators as gatekeepers of immunotherapy response in colorectal cancer

Article References: Najafi, A., Rahimi, A., Baghernejadian, Z., Hazrati, A., Malekpour, K., Samimi, L. N., Falak, R., & Khorramdelazad, H. (2026). Metabolic modulators as gatekeepers of immunotherapy response in colorectal cancer. Cancer Cell International. https://doi.org/10.1186/s12935-026-04471-4

Image Credits: AI Generated

DOI: 10.1186/s12935-026-04471-4

Keywords: colorectal cancer, immunotherapy, tumor metabolism, Warburg effect, kynurenine, IDO-1 inhibitors, metformin, FASN inhibitors, tumor microenvironment, Tregs, MDSCs, metabolomics