Bacterial membrane vesicles as effective immunotherapeutic agents in melanoma, colon, and breast cancer

Aim: Bacterial outer membrane vesicles are highly immunogenic due to their native bacterial antigens, however，their toxic components trigger systemic inflammation. We previously developed non-toxic synthetic bacterial vesicles (SyBV), and confirmed their anti-tumor effects when co-administered with tumor antigens. Building on this, the present study aimed to extensively assess the immunotherapeutic potential of local SyBV administration, leveraging endogenous tumor antigens across multiple cancer models. Methods: SyBV were generated from Escherichia coli membranes via lysozyme-induced cell lysis and ionic stress. Multiple intratumoral administrations of SyBV at different doses were evaluated in melanoma-bearing mice. Therapeutic efficacy was further assessed according to initial tumor size and across multiple tumor models, including melanoma, colon, breast, kidney, and lung cancers. Systemic anti-tumor effects were also examined by evaluating tumor growth at distant, non-injected tumor sites following local SyBV immunization. Results: High-dose SyBV treatment effectively suppressed tumor growth in melanoma-bearing mice. Anti-tumor efficacy varied depending on initial tumor size, with the strongest effects observed when treatment was initiated at smaller tumor volumes. Local SyBV immunization also induced systemic anti-tumor responses, reducing tumor growth at distant tumor sites. Furthermore, SyBV demonstrated significant immunotherapeutic efficacy in colon and breast cancer models, but not in kidney or lung cancer models, highlighting their therapeutic potential in selected cancer types. Conclusion: Our findings show that engineered bacterial vesicles serve as effective standalone cancer immunotherapeutics, allowing simpler drug formulations and potentially improving their clinical translation.