No one welcomes a serious infection, but in some cases it can help to protect you against tumors. New York surgeon William B. Coley was the first to observe in 1893 that some tumors actually regress in people with severe infections. Not surprisingly, he had little success in treating cancer by infecting patients with harmful bacteria. Now, however, researchers from University of Pennsylvania may have figured out where Coley went wrong by finding a new twist to this long-standing phenomenon.

Traditionally scientists explained infection-tumor interference by assuming that immune cellswhich mobilize to fight infectionalso become more active against tumors. Penn researchers Andrei Thomas Tikhonenko, Christopher Hunter and collaborators, whose results will be published in the May 15th issue of the Journal of Immunology, showed instead that infection blocks the growth of new blood vessels that nourish tumors. Solid tumors must develop an extensive network of veins and arteries (a process called angiogenesis) to get enough oxygen and nutrients to survive. Molecules that block angiogenesis, such as the well-known endostatin, can therefore choke tumors and are being investigating as anticancer drugs in humans.

The researchers first injected mice with melanoma cells, which form large tumors within a few days. Then they observed that tumor growth was dramatically reduced when the same mice were infected with Toxoplasma gondii, the parasite that causes toxoplasmosis. The infection also blocked cancer growth when the animals' immune systems were severely impaired by genetic defect, proving that immunity did not play a major role in inhibiting the tumors. Moreover, mice with toxoplasmosis failed to develop new blood vessels, even after they were injected with strong stimulators of angiogenesis. Scientists believe that mice produce powerful antiangiogenic factors in response to the infection. Infected animals could be a source of new, extremely potent inhibitors of angiogenesis, the "Holy Grails of modern anti-cancer research," Tikhonenko says.

As exciting as these results are, the scientists do not know whether a new molecule or well-known compound is stopping the blood vessel growth in the mice, and they stress that it may take years to answer that question.