Optimized bacterial delivery approaches for precision immunotherapy in breast cancer models

Bacteria-mediated cancer therapy is an innovative approach that exploits the tumor-targeting ability of bacteria to deliver anti-cancer drugs directly to tumors. Cytolysin A (ClyA), a bacterial pore-forming toxin, has demonstrated therapeutic efficacy in colorectal cancer but has limited effectiveness in breast cancer. To address this limitation, we engineered an attenuated Salmonella strain to express Clostridium perfringens enterotoxin (CPE), which selectively targets CLDN-4, a tight junction protein overexpressed in breast cancer, thereby minimizing off-target effects. In a 4T1 breast tumor mouse model, CPE-secreting bacteria demonstrated significantly greater therapeutic efficacy than ClyA-secreting bacteria. Mechanistic investigations revealed that ClyA and CPE induced distinct patterns of immune cell infiltration depending on tumor type. In 4T1 tumors, CPE significantly increased the infiltration of CD4⁺ T cells, CD8⁺ T cells, and NK cells, while reducing neutrophil infiltration. In contrast, ClyA promoted immune cell infiltration in CT26 tumors but had negligible effects in 4T1 tumors. Furthermore, CPE treatment markedly reduced granulocyte-colony stimulating factor (G-CSF) expression in 4T1 tumors, a key regulator of neutrophil recruitment, tumor growth, and chemotherapy resistance. Our findings demonstrate that CPE-secreting bacteria exert superior therapeutic efficacy through two synergistic mechanisms: (1) direct tumor cell lysis via pore formation and apoptosis induction, and (2) modulation of the tumor immune microenvironment by enhancing tumor-infiltrating lymphocytes and suppressing neutrophil-associated tumor progression. These results highlight the importance of tailoring bacteria-mediated cancer therapy to tumor specific molecular characteristics to maximize therapeutic efficacy.