Many animals prepare for the onslaught of winter by getting fat, growing luxuriant fur and sleeping in. A new study by two Johns Hopkins researchers suggests that some may also beef up their immune systems to the point where they can fight off cancer.
The scientists say their study is the first to show that the central nervous system, responding to changes in the environment, may trigger changes in the body's immune system that control the growth of tumors.
If so, they said, a better understanding of how this works could lead to new cancer treatments.
Dr. Randy J. Nelson, an associate professor of psychology at Hopkins, and Joan M. C. Blom, a Hopkins graduate student, were scheduled to present their findings in New Orleans today at a meeting of the Society for Neuroscience.
In a telephone interview yesterday, Dr. Nelson cautioned that further study would be needed to confirm a link between the length of the day, the animals' immune system and cancer. And more work would be needed to show that the findings could be applied to human cancer.
"I would call these strong preliminary findings," he said.
"It's a very intriguing observation that the length of exposure to light can affect the growth of a tumor," said Dr. Faye Austin, an immunologist with the National Cancer Institute in Bethesda. "But I think that the conclusions that he draws really need further work to clarify the mechanisms."
But she said the findings were surprising and important, if only because they open up a new avenue for research.
The idea for the study came about a year ago, while Dr. Nelson and Ms. Blom were researching and writing a scholarly article about how animals' bodies change with the seasons.
Studies showed that stress weakened the immune system in animals. Since winter is stressful, they figured, the season probably strains the immune system.
Animals that compensated by revving up their immune systems as winter approached, they reasoned, would have a better chance of surviving and producing offspring.
"In the same way that animals have evolved to select the best time to breed, it struck me that animals ought to be able to predict when conditions would be challenging immunologically," Dr. Nelson said.
So the professor, who holds doctorates in psychology and endocrinology from the University of California at Berkeley, and Ms. Blom, a native of the Netherlands working toward her doctorate in psychology, devised a simple experiment.
They put 25 deer mice in cages and exposed them to eight hours of light a day, mimicking the brief days and long nights of winter. Another 25 mice were placed in cages exposed to 16 hours of light daily, simulating summer.
Animals take their seasonal cues from the length of the day, not from the temperature, Dr. Nelson said.
After eight weeks in their artificial "season," all the deer mice were injected with a standard dose of a known carcinogen.
None of the "winter" deer mice developed cancer, but 85 percent of the "summer" deer mice did.
About 80 percent of the summer mice also developed skin lesions where they were injected. Only 60 percent of the winter deer mice developed lesions, and they healed twice as fast.
Dr. Nelson and Ms. Blom at first doubted the results of their experiment because they appeared so lopsided.
"We thought there can't possibly be this big of an effect," he said. "So we repeated it, and it turned out exactly the same."
Dr. Nelson said there is no proof that humans react in the same way to a reduction in daylight. But he pointed out that there is a seasonal cycle in the detection of some human cancers, and that the seasons affect our body weight, food intake and mood.
"So it's not completely crazy that immune function could also be affected," he said.
The Hopkins researchers are now looking for further evidence for their theory.
It's possible, Dr. Nelson said, that the cancers in the "summer" deer mice were actually caused by estrogen and prolactin. Both hormones are linked to cancer, and both are produced in greater quantities in the summer.
If so, then the researchers would be seeing an increase in summer cancers, not a suppression of winter cancers. That would show that the immune system was not responsible for the differences.
But Dr. Nelson thinks it's more likely the winter deer mice produce more melatonin, a hormone important to the immune system.