Blood cells that normally protect the body from bacterial infection sometimes can produce enough germ-killing toxic substances over a long enough time to cause normal tissue to become malignant, researchers at Massachusetts General Hospital and Harvard Medical School in Boston report in the March 8 SCIENCE. They believe some cancers can be caused not only by toxic substances in the environment but also by toxic substances released by cells called phagocytes to fend off environmental carcinogens.
The researchers showed that human neutrophils -- phagocytic white blood cells that ingest bacteria and foreign substances--release toxic free radicals (oxygen metabolites) that can cause normal mouse connective tissue to become malignant. They injected 43 mice with cells treated with human neutrophils activated to produce toxic oxygen metabolites and injected a control group of 53 mice with untreated cells. Five of the mice given treated cells developed malignant tumors and six developed benign tumors. None of the control mice developed tumors.
Humans need phagocytes to protect against bacteria in the environment. The body has several elaborate chemical mechanisms to detoxify phagocytes' oxygen products, "but it's a relative resistance," says Sigmund Weitzman, who directed the research. "If there are too many of these metabolites, they can damage normal tissue."
The most common human model in which phagocytes accumulate and ultimately cause cancer is ulcerative colitis, a chronic bowel inflammation. …