(Philadelphia, PA) - A team from the University of Pennsylvania School of Medicine has shown that by using a cancer vaccine based on the bacterium Listeria monocytogenes, they can cure mice with established breast tumors. Cancer vaccines, which are more properly described as immunotherapy, work by boosting an immune response against tumor-associated antigens. Using Listeria, the researchers, led by Yvonne Paterson, PhD, Professor of Microbiology, delivered the tumor-associated antigen HER-2/Neu to immune cells. HER-2/Neu is overexpressed in 20 to 40 percent of all breast cancers and also present in many cancers of the ovaries, lung, pancreas, and gastrointestinal tract. These cells eventually enlist killer T cells to seek out and destroy the tumor cells that display the HER-2/Neu molecule.

"We found that we can stop the tumor from growing out to 100 days, at which time we stopped measuring since this is a long time for experiments of this type," says Paterson. "The tumors stopped growing or went completely away." The researchers published their findings in the September 15 issue of The Journal of Immunology.

"The problem that we encounter is that often by the time a patient presents with cancer, they've developed immune tolerance to the tumor antigen, particularly when the antigen is expressed at low levels on normal tissue as with Her2/Neu," explains Paterson. "So how is the body to mount a strong enough immune reaction?"

In general, bacteria are good at inducing both innate and adaptive immune responses, activating such immune cells as macrophages, dendritic cells, and T cells. This helps jump-start the immune response to break tolerance.

But, why Listeria over other bacteria as a vehicle to deliver a tumor-associated antigen? Because of Listeria's unusual life style. Normally, when bacteria get taken up into an antigen-presenting cell, they are engulfed by a phagocytic vacuole where they get killed-whereupon their proteins get broken down into smaller pieces (peptides) and attached to MHC Class 2 molecules. These egress to the cell surface, where they expand and activate helper T cells, which are enlisted into the immune response.

But Listeria has evolved to escape from this vacuole and survive inside the cytosol of antigen-presenting cells, where it can replicate and grow, unlike other bacteria. So, although some of the bacteria are destroyed in the vacuole that feeds the MHC class II pathway of antigen presentation with the induction of helper T cells, others survive by escaping into the cytosol of the cell. This is important because the antigen-processing pathway that feeds antigenic peptides to the surface of the cell for recognition by killer T cells is generated in this cellular compartment. "We reasoned that if we could get Listeria to secrete a foreign protein into the interior of the cell, it would target that pathway and would elicit a strong killer T cell response, and we have shown that," says Paterson. "Listeria is almost unique in the bacterial kingdom in doing this."

In this model, pieces of the very large HER-2/Neu molecule are broken up into little fragments and bound to the MHC Class 1 molecule within the antigen-presenting cell. This is what the killer T cell "sees" at the cell surface. These killer T cells, which are being produced in the spleen, where Listeria usually colonizes, seek out and destroy the tumor. This system ensures an increase in the production of killer T cells that can recognize the HER-2/Neu pieces on the surface of the tumor cell. In addition, the Penn team helped the immune system along by fusing the tumor antigen to a bacterial protein that seems to activate antigen-presenting cells. They have found that by doing this the immune system now recognizes regions of the HER-2/neu molecule that are not immunogenic when presented by other vaccine approaches.

Paterson first hit on the idea of using Listeria as a cancer vaccine vector over ten years ago. "It took a while to dissect what elements of an immune response were best able to cause the rejection of established tumors," she says. "But in the last couple of years it has paid off and we are very excited to see the technology finally being tested in cancer patients. The dream of the cancer immunotherapist is to provide an alternative and more humane way of controlling metastatic disease than current chemotherapies."

The Listeria vector is currently being prepared for a clinical trial targeting a tumor antigen associated with cervical cancer by Advaxis Inc., a cancer vaccine biotech company that has licensed Penn patents on the use of Listeria monocytogenes as a vaccine vector. Paterson is the scientific founder of Advaxis and Chair of the Scientific Advisory Board. The successful demonstration that the Listeria vector technology can also be used with the HER-2/neu molecule paves the way for applying this promising cancer vaccine approach to breast cancer.

This research was funded by the Department of Defense and the National Cancer Institute. Co-authors are Reshma Singh and Mary E. Dominiecki, both from Penn, as well as Elizabeth M. Jaffee from the Johns Hopkins University School of Medicine.

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