Abstract High-grade gliomas (HGGs) are invariably lethal brain tumors defined by a heterogeneous profile owing to their dynamic cellular and molecular landscape. As such, treatment efforts have focused on targeting genetic alterations but have failed to yield significant advancements in patient outcomes over recent decades. This underscores the need to explore other avenues that drive HGG pathogenesis and affect therapeutic potency, such as microenvironmental interactions. The gut-brain axis, defined as the bidirectional communication between the gastrointestinal tract and the central nervous system, has been implicated in neurological disorders such as Alzheimer’s disease and autism spectrum disorder and is now an emerging area of interest in gliomagenesis. Although largely unexplored in brain tumors, growing evidence suggests that the gut microbiota can exert systemic effects on brain integrity and neuroinflammation through circulating metabolites that can cross the blood-brain barrier and modulate homeostatic brain function. Particularly, short-chain fatty acids (SCFAs) can regulate microglia maturation and function, which is key in establishing the inflammatory signature associated with HGGs. In this sense, our study aims to uncover the role of gut-derived metabolites, such as SCFAs, in promoting an inflammatory microenvironment and thereby driving gliomagenesis. To address the following, we established a chemical-genetic mouse model of HGG (Nestin-Cre;Trp53fl/fl) in which Trp53 deletion in neural precursor cells is combined with N-ethyl-N-nitrosourea administration to pregnant females at embryonic day 13. 5. Through genetic and histological characterization of tumor samples at premalignant, early, and late stages, we observed close recapitulation of human HGG development in this model, exhibiting stemness features and a strong immune signature. We then re-derived these mice in distinct housing conditions to model various microbiological statuses: specific pathogen-free (SPF), SPF with antibiotics (ampicillin, metronidazole, and neomycin), germ-free (GF), and GF with SCFA supplementation (acetate, propionate, and butyrate). Survival analysis reveals an increase in survival for both antibiotic-treated and GF mice, with reduced survival upon SCFA treatment. To investigate changes in the immune landscape, we generated 3D reconstructions of microglia from FFPE brain sections of tumour-bearing mice. Microglia in the tumour core consistently displayed an amoeboid-like morphology, whereas peripheral microglia appeared ramified, corresponding to activated and homeostatic states, respectively. Notably, peripheral microglia displayed diminished activation in GF conditions, with increased activation upon SCFA reintroduction; this re-establishment of an inflammatory environment coincides with reduced survival. Together, we implicate gut-derived metabolites as modulators of the microglial activation and inflammatory signalling that underlie HGG pathogenesis, highlighting the gut microbiome as a tractable target for therapeutic interventions. Citation Format: Margaret Mary P. Javier, Morgane Morabito, Heather Whetstone, Michelle Kushida, Peter B. Dirks. Defining the role of the gut microbiota on high-grade glioma initiation and progression abstract. In: Proceedings of the AACR Special Conference in Cancer Research: Brain Cancer; 2026 Mar 23-25; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2026;86 (6Suppl): Abstract nr A004.
Javier et al. (Mon,) studied this question.