This electron microscope image shows white blood cells with virus particles (shown in red circles). These cells can reactivate the virus as it travels to the tumor site, allowing it to retain its cancer-killing ability. Photo courtesy of Leiden University Medical Center

Sept. 13 (UPI) -- Certain cell-killing viruses, with help from white blood cells, are showing promise for cancer treatment.

Scientists from the University of Leeds in England and the Institute of Cancer Research in London found that white blood cells can reactivate the virus when traveling to the tumor site and retain its ability to destroy the cancer cells. Their findings were published Wednesday in the journal Cancer Immunology Research.


"We are only just beginning to understand how viruses can help us tackle cancer, but it is exciting to learn that our bodies are actually capable of helping them to destroy cancer tumors," Dr. Elizabeth Ilett, a researcher at Leeds, said in a press release.

Many viruses generally attack the body and cause harm, but others destroy cancer tissue while causing minimal side effects to the patient -- known as oncolytic virus therapy.

Scientists originally believed the human body's normal immune response neutralizes viruses injected into the bloodstream in an effort to destroy cancer tumors.

"We previously believed that follow-up doses were useless once the body had produced antibodies against the virus," llett said.

Britain's National Health Service has approved a virus, called T-vec, for advanced melanoma skin cancers given to patients by injection directly into their tumors -- but the tumors must be located somewhere that can be reached with a needle.

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Patients with this therapy in the bloodstream in clinical trials quickly developed antibodies that could deactivate the virus. Researchers believe this is why they stopped functioning.

But new research shows that white blood cells carrying the virus to the tumor can also reactivate these deactivated viruses, allowing them to infect and kill the cancer cells.

The researchers, using antibodies from patients undergoing virus therapy, neutralized viruses and added them to melanoma cells in the laboratory with no effect. Adding white blood cells, called monocytes, allowed the virus to become reactivated and kill the cancer cells.

They found that neutralized forms of two of three viruses currently being used in clinical trials could be reactivated by white blood cells.

"Our study shows that, crucially, viruses retain their cancer-killing ability even in the bloodstream," said Alan Melcher, a researcher at the Institute of Cancer Research. "This research has profound implications for how we might use viruses to treat cancer in future, opening up virus therapy to many more patients with hard-to-reach tumors of different cancer types."

They believe other viruses might also be effective even after being neutralized by antibodies.

"Viruses are a hugely exciting new type of treatment for cancer," Melcher said. "Not only do virus therapies kill cancer cells directly, they do so with only mild side effects compared to traditional cancer treatments and they attract the immune system to the site of the tumor -- which means they can work well when combined with other types of immunotherapy."

This means the tumor doesn't have to be reachable with a needle, as previously required.

"This discovery suggests that cancer treatments using virus therapy could be significantly expanded in future, as we previously believed that follow-up doses were useless once the body had produced antibodies against the virus," Llett said.