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.
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“If you have a metastatic disease, oncologists always use systemic treatments,” says Fiering. “I think that’s fine, but it’s missing one of the fundamental ideas of immunology, which is that the response you get in one part of the body can distribute throughout the body.”
A familiar example of this is flu shots, which are delivered in your arm but trigger an immune response that protects against a respiratory infection. Fiering started to wonder if a similar approach might be taken with cancer. His idea was that if doctors injected something into a tumor that would cause the body’s immune system to start attacking it, the heightened immune response wouldn’t be limited to just the area around the tumor. The immune system’s T-cells—its frontline soldiers—would also track down any cancer cells that might be lurking elsewhere in the body.
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