Validation in patient-derived immune cell models of breast cancer demonstrated that both circFAM53B and its encoded peptide induce dendritic cell maturation, promote effector T cell differentiation, and significantly enhance expression of cytokines including interferon gamma and granzyme B. In mouse melanoma and breast cancer models, vaccination with dendritic cells pulsed with circFAM53B or its encoded peptide increased tumor-infiltrating cytotoxic T lymphocytes and suppressed tumor growth. Here, GZMB is linked to breast cancer.