Sergio Pistoi Scientific American
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 cells—which mobilize to fight infection—also
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.
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