Hybrid bacteriophage hydrogels deplete intratumoral bacteria to potentiate oncolytic immunotherapy

Oncolytic viruses act as in situ cancer vaccines by inducing antigen release to boost antitumor immunity. Specific intratumoral bacteria exacerbate immunosuppression; however, their influence on oncolytic immunotherapy remains unknown. Here, we report a hybrid bacteriophage hydrogel composed of engineered oncolytic M13 phage (eM13) and Escherichia coli (E. coli) phage Tequatrovirus YZ2, which suppresses breast cancer progression and metastasis by boosting antitumor immunity through selective elimination of intratumoral E. coli. Particularly, YZ2 is conjugated with photosensitizer CyI to produce YZ2-CyI (YC), and the hydrogel is prepared by cross-linking YC and eM13 using pH-sensitive coordinating molecules through the formation of dynamic Schiff base bonds. Within the acidic tumor microenvironment, eM13 and YC are gradually released for targeted oncolysis and E. coli eradication, respectively. Intratumoral bacterial clearance enhances dendritic cell and CD8⁺ T cell activation, while suppressing immunosuppressive M2 macrophages, regulatory T cells, and myeloid-derived suppressor cells. In poorly immunogenic triple-negative breast cancer models, depleting intratumoral E. coli potentiates oncolytic therapy against tumor progression, recurrence, and metastasis. Notably, the bacteriophage formulation shows favorable safety without disrupting gut microbiota homeostasis. These results support its potential clinical application in microbiome-infiltrated tumors and ability to improve oncolytic immunotherapy outcomes.