Systemically delivered Bacteroides thetaiotaomicron -derived bacterial extracellular vesicles inhibit primary and metastatic melanoma growth

ABSTRACT The gut microbiome can contribute to anti-tumor immunity and cancer therapy responses, but translating live microbe-based interventions remains challenging due to safety, controllability, and delivery constraints. Bacterial extracellular vesicles (BEVs) are an attractive cell-free alternative, as they package bacterial cargo into a nanoscale format capable of host-cell engagement, immunological activation, and systemic distribution. Here, we investigated the anti-tumor potential of BEVs derived from the human gut commensal Bacteroides thetaiotaomicron (Bt). We show that the delivery route is a major determinant of efficacy. Intravenous, but not intraperitoneal, administration produced robust anti-tumor activity in a B16F10 melanoma mouse model. Intravenously delivered Bt BEVs suppressed primary tumor growth in a dose-dependent manner and reduced metastatic outgrowth in the lung. Bt BEVs did not reduce B16F10 metabolic viability during a 24-h in vitro exposure, but they activated NF-κB and Toll-like receptor signaling in human reporter systems, and NanoLuc-associated signal from labeled BEV preparations was detected in excised tumor tissue following systemic administration. Although the host pathways responsible for tumor control remain to be defined, these findings identify naturally produced commensal-derived Bt BEVs as a candidate cell-free microbial therapeutic modality for cancer therapy. IMPORTANCE Gut bacteria can influence cancer immunity, but using live microorganisms as treatments creates practical and safety challenges. Bacterial extracellular vesicles offer a cell-free way to deliver microbial molecules to the host, yet the anti-cancer potential of vesicles naturally produced by common gut bacteria remains poorly defined. Here, we show that vesicles from the human gut bacterium Bacteroides thetaiotaomicron suppress primary melanoma growth and reduce lung tumor burden in mice when delivered intravenously, whereas delivery into the abdominal cavity is much less effective. The vesicle preparations did not reduce melanoma-cell viability directly in culture, suggesting that the in vivo effect is not explained by simple toxicity to tumor cells. These findings establish that naturally produced vesicles from a common gut bacterium can exert anti-tumor activity after systemic delivery and identify the administration route as an important determinant of their activity. These findings support their further evaluation as cell-free therapeutic platforms for cancer.