Germ-killing blood cells that ordinarily protect the body from disease can also cause cancer in lab animals, and that may explain why years of chronic inflammation heighten people's risk of some kinds of tumors, researchers say.
Their work, conducted at Massachusetts General Hospital, provides clues to the origins of a variety of common human malignancies, including cancers of the colon, lung and breast.
The results of the research, directed by Dr. Sigmund A. Weitzman, were published in today's issue of the journal Science.
The white cells, called phagocytes, are the blood's foot soldiers and garbage cleaners. They surround bacteria and other germs and kill them, and they also gobble up worn-out normal cells. In the course of both of these duties, they release toxic chemicals known as free radicals.
The new work suggests that these same chemicals can damage ordinary tissue and even make it turn cancerous if the exposure persists for a long time.
So far, researchers have observed this effect in the test tube and in mice, providing indirect evidence, they say, that the same thing may be happening in people.
"There has long been known to be an association between chronic inflammation and cancer," said Dr. Thomas P. Stossel, one of the researchers.
One example is ulcerative colitis, a chronic bowel inflammation that constantly bathes the bowel with phagocytes. During the first 20 years of this disease, the cancer risk is relatively low. But then the rate of colon cancer rises quickly.
When persons smoke cigarettes, bits of soot become lodged in the lungs, and phagocytes roam about, eating them up.
"It's possible that the association between lung cancer and smoking is in part due to these radicals," Stossel said in an interview Thursday.
Throughout the body, cells are constantly wearing out and being replaced. This turnover is especially rapid in the ducts of the female breast, where new tissue is built every month. Essential to this process are the phagocytes, which cart away the dead cells. The researchers believe that, over time, this constant exposure to free radicals could contribute to the development of breast cancer.
The work may someday provide new strategies for warding off cancer.
"One can design chemicals that sop up these free radicals," Stossel said. "If the evidence continued to accumulate that this was something to take seriously, one could begin to design preventative measures to keep this from happening."