Traces of a monkey virus that contaminated early batches of polio vaccine have been spotted for the first time in non-Hodgkin's lymphoma. The finding, reported independently by two research teams, suggests the virus might trigger this common cancer, which ranks among the top five causes of cancer deaths. The results also suggest that the virus is spreading in humans, and it could point to new ways of diagnosing, treating, and preventing the disease.
Simian virus 40 (SV40) was discovered in 1960 and was soon found to be contaminating most U.S. stocks of the injectable polio vaccine. It got there because polio virus for the vaccine was grown in cultured kidney cells that had been isolated from infected monkeys. SV40 was then shown to cause four types of cancer in hamsters. By 1963 the contaminated vaccine had been taken out of use. But by then, tens of millions of people could have received the contaminated vaccine. Researchers have been arguing ever since whether SV40 from the vaccine has caused any human cancers.
Earlier studies turned up bits of SV40 DNA in tumor cells from people with three of the cancers SV40 causes in hamsters. To determine whether virus DNA is present in other cancers, Janet Butel and her colleagues at Baylor College of Medicine in Houston fished for tiny snippets of SV40 DNA in tumor tissue from patients with non-Hodgkin's lymphoma. They found that 42% of samples taken from 154 patients had SV40 DNA. Moreover, DNA from the virus strain found in three patients matched that isolated from a 1955 batch of the vaccine. DNA from this strain had never been seen anywhere else previously. No virus was found in samples from 186 other kinds of cancers or from 54 samples from healthy lymph tissue, the team reports in the 9 March issue of The Lancet.
A second study in the same issue, led by Adi Gazdar of the University of Texas Southwestern Medical Center in Dallas, yielded similar results. Because some of the patients in Butel's study were born after the polio vaccine was free of SV40, the results mean that the virus could now be spreading among people, says Butel.
The studies are "very well done," says pediatric oncologist Bob Garcea of the University of Colorado School of Medicine in Denver, but they don't yet prove conclusively that the virus causes the cancer. But if the connection holds up, researchers could use an SV40 vaccine to prompt a cancer-killing immune response, says molecular virologist Michele Carbone of Loyola University in Chicago. "At the moment you have a virus, you have a target."