Researchers at Ohio State University and their colleagues found that the hamsters had higher levels of certain immune cells in their bloodstream during the short days of winter. In addition, during acute stress, hamsters kept in winter-like conditions launched a more vigorous immune response in preparation for potential injury or infection.
Hamsters take a cue from the decreasing length of days as winter approaches as a signal to boost their immune function, according to the study, which will be published in the March 19 edition of the Proceedings of the National Academy of Sciences.
"Our results suggest that Siberian hamsters are prepared to mount a greater and more rapid immune response during short days compared to long days because that may be a more efficient strategy for survival," said Firdaus Dhabhar, co-author of the study and assistant professor of oral biology and molecular virology, immunology and medical genetics at Ohio State University.
The results also suggest that short periods of stress may actually enhance immune responses, Dhabhar said, even though research has shown that long-term, chronic stress can suppress the immune system. This short-term enhancement is stronger during winter than during summer. Staci Bilbo, a doctoral student in psychology at The Johns Hopkins University, was the other lead author on the study. Bilbo is currently working in the lab of Randy Nelson, another co-author and professor of psychology and neuroscience at Ohio State.
The study involved 50 Siberian hamsters. Using artificial light, the researchers exposed half of them to long days (15 hours of light per day) to simulate summer conditions. The other half lived in short days (9 hours of light per day) to simulate winter conditions.
After 10 weeks of living under these conditions, some hamsters were tested for levels of key immune cells in their bloodstream, including leukocytes, lymphocytes, T cells and NK cells.
The results showed that hamsters living in winter-like conditions have higher baseline levels of these immune cells than do hamsters living in summer-like daylight conditions.
"It is more difficult for hamsters to survive in the winter because of stressors like cold and reduced availability of food," Bilbo said. "We found that the animals seem to use the length of day to warn of the onset of these stressors and then to enhance their immune system when winter approaches."
The researchers also wanted to find out what happens to hamsters during acute stress -- such as what might happen when a wild hamster is attacked by another animal -- and see if the reactions are different during summer and winter conditions.
Some hamsters were put in restraints that increased stress in the animals but did not harm them. An antigen -- a substance that provokes an immune response -- was then applied to their skin to determine the vigor of the inflammatory immune reaction.
Results showed that immune cells rushed out of the bloodstream, presumably to the site where the antigen was placed on the skin. However, among hamsters living in shorter-day conditions, this immune reaction was quicker and stronger than for those experiencing the longer days.
For example, Bilbo said that after just 10 minutes of restraint stress, the short-day hamsters showed a 30 percent decrease in the levels of some immune cells in the bloodstream. Previous research by Dhabhar has indicated that immune cells do not die due to the stress, but leave the bloodstream to rush to areas such as the skin where the antigen is placed.
For the long-day hamsters, there was no significant decrease in blood levels of immune cells after 10 minutes, although differences were found after two hours.
"The skin is one of the first places where a hamster could be injured or infected," Bilbo said. "Thus, it makes sense that immune cells should leave the bloodstream and travel to those areas of the body where they are needed the most. The immune system reacts more strongly to injuries or infections during winter."
Dhabhar said the results show that short periods of stress can actually help strengthen the immune system, even though research has shown long periods of chronic stress may be harmful.
"It doesn't make sense that stress would always suppress immunity, Dhabhar said. "Stress can help prepare the immune system for a flight or fight response just as it prepares some of the body's other systems. The ideal stress response is one where the individual very rapidly mounts a physiological response during stress and then it very rapidly shuts it down when the stress is over."
Bilbo said the results also help explain earlier results that she and Nelson found in their lab. In the earlier study, they found Siberian hamsters had shorter reactions to immune challenges during winter-like daylight conditions than they did in the summer-like conditions. For example, the study found that fevers didn't last as long in winter as they did during the summer. These new results suggest the immune response may be shorter in the winter because it is also stronger.
"Our results suggest hamsters may be able to clear their infections more quickly in the winter and thus return to normal in less time," she said.
Other co-authors of the study were Kavitha Viswanathan and Alison Saul, both in the Department of Oral Biology at Ohio State; and Steven Yellon, from the department of physiology and pharmacology at Loma Linda University.
The study was supported by grants from the National Science Foundation, the National Institutes of Health, and The Dana Foundation.
Contact: Firdaus Dhabhar, (614) 688-8562; Dhabhar.1@osu.edu Staci Bilbo, (614) 538-9540; Bilbo.1@osu.edu Randy Nelson, (614) 538-9526; Rnelson@osu.edu Written by Jeff Grabmeier, (614) 292-8457; Grabmeier.1@osu.edu
Journal
Proceedings of the National Academy of Sciences