A Common Plant Virus Is an Unlikely Ally in the War on Cancer

Jack Hoopes spends a lot of time with dying dogs. A veterinary radiation specialist at Dartmouth College, Hoopes has spent his decades-long career treating canine cancers with the latest experimental therapies as a pathway for developing human treatments. Recently, many of Hoopes’ furry patients have come to him with a relatively common oral cancer that will almost certainly kill them within a few months if left untreated. Even if the cancer goes into remission after radiation treatment, there’s a very high chance it will soon reemerge.

For Hoopes, it’s a grim prognosis that’s all too familiar. But these pups are in luck. They’re patients in an experimental study exploring the efficacy of a new cancer treatment derived from a common plant virus. After receiving the viral therapy, several of the dogs had their tumors disappear entirely and lived into old age without recurring cancer. Given that around 85 percent of dogs with oral cancer will develop a new tumor within a year of radiation therapy, the results were striking. The treatment, Hoopes felt, had the potential to be a breakthrough that could save lives, both human and canine. “If a treatment works in dog cancer, it has a very good chance of working, at some level, in human patients,” says Hoopes.

The new cancer therapy is based on the cowpea mosaic virus, or CPMV, a pathogen that takes its name from the mottled pattern it creates on the leaves of infected cowpea plants, which are perhaps best known as the source of black-eyed peas. The virus doesn’t replicate in mammals like it does in plants, but as the researchers behind the therapy discovered, it still triggers an immune response that could be the key to more effective treatments for a wide variety of cancers.

The idea is to use the virus to overcome one of the gnarliest problems in oncology: A doctor’s best ally, their patient’s own immune system, doesn’t always recognize a cancerous cell when it sees one. It’s not the body’s fault; cancer cells have properties that trick the immune system into thinking nothing is wrong. Oncologists have puzzled over this for nearly a century, and it's only in the past decade that researchers have really started to get a grip on cancer’s immunosuppressive properties. Immunotherapy, which has emerged as one of the most promising types of cancer treatment, is all about developing techniques to help the body’s immune system recognize cancerous cells so it can fight back. It’s the medical equivalent of putting a big flashing neon sign on the tumor that reads “ATTACK HERE.” And that’s where the cowpea mosaic virus could help.

To treat his canine patients, Hoopes typically injects 200 micrograms of virus-like particles—about three times the dose of a typical flu vaccine—directly into their tumors. These particles are not live cowpea mosaic viruses; rather, they’re viruses that have had their genetic material removed or have been inactivated so they can’t replicate. Each pup receives four doses of the viral particles over two weeks while also taking standard radiation therapy. The dog’s immune system recognizes the pathogens as foreign bodies and goes into attack mode. When the body goes after the particles, it takes the cancerous cells down with them.

While other viruses could theoretically be used as immune system bait, CPMV has proven far more effective at triggering a response than any other pathogens the researchers have tried so far. They’re still not sure what makes this particular virus so uniquely effective, but the important thing is that it works. “It’s worked better than radiation by itself, which is a huge positive for us,” says Hoopes. “The immune system is more powerful than we thought.”