A Novel Trained Immunity Adjuvant Derived From Commensal Bacteria Potentiates Liposome-Hydrogel Cancer Vaccine Against Breast Tumors.

Traditionally, vaccines rely on adaptive immunity to confer protection, yet emerging evidence highlights the importance of trained immunity in enhancing innate immune responses for broad-spectrum defense. The M28 family peptidase (M28), secreted by the commensal bacterium Peribacillus frigoritolerans, acts as a novel inducer of trained immunity with adjuvant potential. In studies using ovalbumin (OVA) as model antigen, both free M28 and M28-loaded liposome (M28-Lipo) significantly enhanced antigen-specific IgG production, stimulated splenic CD4/CD8 T cell proliferation, and induced balanced Th1/Th2 immunity, with M28-Lipo demonstrating superior efficacy. Building on these findings, a biphasic-release breast cancer vaccine (Zfp142-Lipo + M28@ sodium alginate (ALG)) was developed by co-delivering the tumor-specific neoantigen Zfp142 and M28 using a hydrogel-liposome hybrid system. This formulation enabled sustained antigen release and targeted accumulation in lymph nodes and tumor tissues. Prophylactic administration completely inhibited 4T1 tumorigenesis and pulmonary metastasis, whereas therapeutic administration significantly suppressed tumor progression and dissemination. Mechanistic analysis revealed enhanced infiltration of CD4⁺ and CD8⁺ T cells, reduction of regulatory T cells, and M2-to-M1 macrophage repolarization within the tumor microenvironment. These findings establish M28 as a safe and potent adjuvant that reprograms the tumor microenvironment and effectively bridges innate and adaptive immunity, providing a versatile platform for cancer immunotherapy.