Toxoplasma gondii (T gondii) is a single-celled parasite found in a cat's intestines, but it can live in any warm blooded animal.
T gondii affects about one-third of the world's population. Most people have no symptoms, but some experience a flu-like illness. Those with suppressed immune systems, however, can develop a serious infection if they are unable to fend off T gondii.
A healthy immune system responds vigorously to T gondii in a manner that parallels how the immune system attacks a tumour.
"We know biologically this parasite has figured out how to stimulate the exact immune responses you want to fight cancer," said David J Bzik, professor of Microbiology and Immunology, Geisel School of Medicine at Dartmouth.
In response to T gondii, the body produces natural killer cells and cytotoxic T cells. These cell types wage war against cancer cells. Cancer can shut down the body's defensive mechanisms, but introducing T gondii into a tumour environment can jump start the immune system.
"The biology of this organism is inherently different from other microbe-based immunotherapeutic strategies that typically just tickle immune cells from the outside," said Barbara Fox, senior research associate of Microbiology and Immunology.
"By gaining preferential access to the inside of powerful innate immune cell types, our mutated strain of T gondii reprogrammes the natural power of the immune system to clear tumour cells and cancer," Fox said.
Since it isn't safe to inject a cancer patient with live replicating strains of T gondii, Bzik and Fox created "cps," an immunotherapeutic vaccine.
Based on the parasite's biochemical pathways, researchers delete a Toxoplasma gene needed to make a building block of its genome and create a mutant parasite that can be grown in the laboratory but is unable to reproduce in animals or people.
Even when the host is immune deficient, cps still retains that unique biology that stimulates the ideal vaccine responses.
"Aggressive cancers too often seem like fast moving train wrecks. Cps is the microscopic, but super strong, hero that catches the wayward trains, halts their progression, and shrinks them until they disappear," said Bzik.
Published laboratory studies from the Geisel School of Medicine labs have tested the cps vaccine in extremely aggressive lethal mouse models of melanoma or ovarian cancer and found unprecedented high rates of cancer survival.