Attenuated Listeria monocytogenes as a cancer vaccine vector for the delivery of CD24, a biomarker for hepatic cancer stem cells.

Attenuated Listeria monocytogenes (LM) is a promising candidate vector for the delivery of cancer vaccines. After phagocytosis by antigen-presenting cells, this bacterium stimulates the major histocompatibility complex (MHC)-I and MHC-II pathways and induces the proliferation of antigen-specific T lymphocytes. A new strategy involving genetic modification of the replication-deficient LM strain ΔdalΔdat (Lmdd) to express and secrete human CD24 protein has been developed. CD24 is a hepatic cancer stem cell biomarker that is closely associated with apoptosis, metastasis and recurrence of hepatocellular carcinoma (HCC). After intravenous administration in mice, Lmdd-CD24 was distributed primarily in the spleen and liver and did not cause severe organ injury. Lmdd-CD24 effectively increased the number of interferon (IFN)-γ-producing CD8(+) T cells and IFN-γ secretion. Lmdd-CD24 also enhanced the number of IL-4- and IL-10-producing T helper 2 cells. The efficacy of the Lmdd-CD24 vaccine was further investigated against Hepa1-6-CD24 tumors, which were inguinally inoculated into mice. Lmdd-CD24 significantly reduced the tumor size in mice and increased their survival. Notably, a reduction of T regulatory cell (Treg) numbers and an enhancement of specific CD8(+) T-cell activity were observed in the tumor-infiltrating lymphocytes (TILs). These results suggest a potential application of the Lmdd-CD24 vaccine against HCC.