High Efficacy of a Listeria-Based Vaccine against Metastatic Breast Cancer Reveals a Dual Mode of Action

Abstract

Most cancer vaccines induce CTL responses to tumorassociated antigens (TAA). Killing of tumor cells occurs through TAA-specific CTL-mediated cytolysis. Here, we show that one preventive followed by two therapeutic immunizations with an attenuated Listeria monocytogenes (LM)-based vaccine eradicates all metastases and almost the entire primary tumor in the syngeneic, aggressive mouse breast tumor model 4T1. We provide strong evidence that this is due to the combined result of direct kill by Listeria infecting the tumor cells and by CTL responses against Listeria antigens. We showed by electron microscopy that LM expressing truncated listeriolysin O (LLO) and amino acid fragments 311 to 660 of TAA Mage-b (LM-LLO-Mage-b 311-660 ) and the control strain LM-LLO infect tumor cells in vitro and in vivo. In vitro data indicate that tumor cell death occurs through activation of NADP + oxidase and increased intracellular Ca 2+ levels, both resulting in the production of high ROS levels. Because both LM-LLO and LM-LLO-Mage-b 311-660 showed equally strong efficacies in vivo, we concluded that LM-LLO was crucial and Mage-b was of less importance. We found strong CTL responses to LM-LLO in the spleen, and depletion of CD8 T cells in vivo resulted in significant tumor regrowth (52%) in LM-LLO-vaccinated mice, indicating that LM-LLOspecific CTL indeed partially contributed to tumor cell kill in vivo. This dual mode of action of a Listeria-based vaccine has not been described before and may provide new directions in the development of more effective vaccines against metastatic breast cancer.

Development of a live and highly attenuated Listeria monocytogenes-based vaccine for the treatment of Her2/neu-overexpressing cancers in human

“…Another mechanism for the anti-tumoral effects of Listeria was recently suggested by the discovery that the Listeria can infect and reside in the tumors and mediate the direct lysis of tumor cells. 22 Still other effects include Listeria's ability to stimulate myelopoesis and the maturation of immature myeloid cells such as dendritic cell and their ability to facilitate the extravasation of activated immune cells into tumors. 23 In addition to the role of innate immunity stimulated by Listeria-based vaccines, the delivery of a TAA by this vector has a crucial role in its strong and specific antitumor responses.…”

Cancer Vaccination at Older Age

“…In another more recent vaccine study with Mage-b delivered through a highly attenuated L. monocytogenes, we found a dramatic effect on the metastases in the 4T1 model at young age [40]. However, we discovered that this was not solely due to Mage-b, but rather to the direct infection and kill of tumor cells by Listeria [40]. Since Listeria-infected tumor cells highly express Listeria proteins, the tumor cells become a highly sensitive target for NK cells and Listeria-specific CTL [40].…”

“…Interestingly, we found that Mage-b vaccination activated macrophages and NK cells (producing IFNγ) in old mice [3]. In another more recent vaccine study with Mage-b delivered through a highly attenuated L. monocytogenes, we found a dramatic effect on the metastases in the 4T1 model at young age [40]. However, we discovered that this was not solely due to Mage-b, but rather to the direct infection and kill of tumor cells by Listeria [40].…”

“…Finally, we have shown that an attenuated L. monocytogenes can be used to deliver genes directly and selectively in tumor cells in vivo [40]. Also other non-pathogenic bacteria are currently under investigation for the delivery of genes selectively into tumor cells such as Lactococcus lactis and Escherichia coli [47].…”

“…However, we discovered that this was not solely due to Mage-b, but rather to the direct infection and kill of tumor cells by Listeria [40]. Since Listeria-infected tumor cells highly express Listeria proteins, the tumor cells become a highly sensitive target for NK cells and Listeria-specific CTL [40]. We found that the Listeria-based vaccine was equally effective against metastatic breast cancer at young and old age [unpubl.…”