Anti-cancer vaccines are something of a holy grail of medicine, but what if vaccines that have already been administered could be repurposed? Scientists have demonstrated in mice a method that tricks the immune system into attacking the tumor by mistaking it for a pathogen to which the immune system is already responding.
Generally speaking, vaccines work by stimulating the immune system to respond to specific antigens associated with viruses, bacteria, or other pathogens, including cancer. Of course, vaccines are typically targeted before being injected into patients, but in this new study, researchers at the University of Massachusetts Amherst investigated how existing immune responses can be reactivated to target cancer.
"The idea is that everyone gets a bunch of vaccines, and if we could give that vaccination to cancer, we could use it to destroy the cancer," said Neil Forbes, senior author of the study. "But cancer obviously doesn't display viral molecules on its surface. So the question is, can we use Salmonella to extract a molecule inside a cancer cell and then have the immune system attack that cancer cell as if it were an invading virus?"
The team genetically engineered a strain of salmonella to seek out cancer cells and, once found, deliver a special protein - in this case, the ovalbumin found in eggs. This protein disperses into the fluid within the cancer cells.
The bacterial therapy worked in mice with pancreatic cancer that, importantly, had been previously vaccinated with an ovalbumin vaccine. As the protein diffuses through the cell fluid, it attracts the attention of the previously primed immune system to respond to the tumor.
Three out of seven test mice (43%) were completely cured of their cancer, and all lived significantly longer. Next, the researchers reintroduced pancreatic cancer cells into the mice and found that this response was enough to prevent the disease from recurring.
"None of the tumors grew, which means the mice had developed immunity, not just to the ovalbumin protein, but to the cancer itself. The immune system already knew the tumor was an immunogen," Forbes said. "I'm doing further work to understand exactly how this happens."
Researchers hope that cancer patients will eventually be able to receive a vaccine in which the active ingredient is a protein from a pathogen they have already been vaccinated against - perhaps a routine rubella vaccination as a child, or more recently, a COVID-19 vaccine. While this research sounds promising so far, it's important to note that these are very early results, were only tested in mice, and in small numbers. Further animal testing is needed to ensure the safety of this technology before human trials can begin.
The research was published in the journal Frontiers in Immunology.