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David Bricker
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Last modified: Thursday, March 27, 2003

Sudden removal of fat impairs immune function in rodents, biologists find

A liposuction-like procedure called lipectomy results in a loss of humoral immune protection in two commonly studied rodent models, the prairie vole and the Siberian hamster, scientists have found.

The report by a team of researchers at Indiana University, Ohio State University and Johns Hopkins University was made available online this week by The Royal Society. Their study is the first to show that even a moderate loss of fat leads to decreased amounts of infection-fighting IgG antibodies.

"We were able to show that even a subtle decrease in fat can decrease humoral immunity, which has the potential to increase disease susceptibility," said Indiana University biologist Gregory Demas, who led the study. "We knew that immune function is energetically costly, but it is now clear that animals use energy stored as fat to bolster immunity and likely to combat infection."

The researchers also found that immune system function improved after the regrowth of fat tissue that had been removed.

The research team divided 54 adult male prairie voles and 36 adult male Siberian hamsters into three experimental groups. The individuals from one of these groups had epididymal white adipose tissue removed, the second group had inguinal white adipose tissue removed, and the third group had a surgical procedure but had no adipose tissue removed.

Half of the rodents in each of the three groups were then exposed to an immune system-roiling antigen called keyhole limpet hemocyanin (KLH) four weeks after surgery. The other half of each group received KLH 12 weeks after surgery. KLH is known to induce a strong immune response in voles and hamsters without making the animals sick.

After a few days, the researchers measured each rodent's bloodstream concentrations of IgG antibodies raised in response to the presence of KLH. Lower concentrations of the antibodies signify impaired immune function. The researchers found that immune function in KLH-inoculated voles appeared normal four weeks after surgery but was impaired 12 weeks after surgery. KLH-inoculated hamsters, however, showed impaired immune function four weeks after surgery but seemed to have recovered normal immune function 12 weeks after surgery, when they had regained normal levels of body fat. Demas attributed the different responses to the two species' unique physiologies.

Demas said the study strengthens biologists' perception that immune systems require a lot of energy to perform "optimally" in any mammal.

"It's not yet known whether there is also a strong connection in humans between energy stored as fat and immune function, but it's clearly important to see whether it holds true in humans as well," he said.

All animals used in the study were treated in accordance with the standards of the Johns Hopkins University Institutional Animal Care and Use Committee. Deborah Drazen of Johns Hopkins University and Randy Nelson of Ohio State University also contributed to the report, which will appear in the May 7 issue of Proceedings B of the Royal Society. Sue Carter of the University of Illinois at Chicago provided the prairie vole line, and Katherine Wynne-Edwards of Queens University (Canada) provided the Siberian hamster line. The study was funded by a National Research Service Award grant from the National Institutes of Health.

The full text of the published report is available at http://giorgio.ingentaselect.com/vl=4681112/cl=38/ini=rsl/nw=1/rpsv/catchword/rsl/09628452/previews/w02pb0787

To speak with Demas, contact David Bricker at 812-856-9035 or brickerd@indiana.edu.