Scientists are learning that white fat—the fat tissue that acts as a scaffold under our skin, muscles and organs—may serve vital roles beyond cushioning our rears and middle sections. New research from NIAID scientists published last month in Immunity reports that white fat is also an immune organ, storing immune cells called memory T cells that fight against infectious agents.
Immunologists have long been interested in white fat because immune messenger chemicals prevalent in the tissue influence metabolism, especially in people who may have too much fat. However, little was known about the role white fat played in protecting people at a healthy weight from infections. Researchers led by Yasmine Belkaid, Ph.D., chief of the Mucosal Immunology Section in NIAID’s Division of Intramural Research, felt there was a good chance the role could be significant. This was based upon previous work revealing that memory T cells— immune cells that play a critical role in recognizing and responding to infectious agents the body has previously encountered— perform best when exposed to substances produced by fat cells. Dr. Belkaid’s team hypothesized that T cells may be plentiful and highly active in white fat.
To test this hypothesis, researchers analyzed the location of memory T cells in mice that had been infected with either the bacterium Yersinia pseudotuberculosis or the parasite Toxoplama gondii. Once the infection had subsided, researchers found that a large fraction of the pool of memory T cells the mice produced were found in the animals’ white fat tissue. Not only that, but the researchers discovered that when the mice were exposed to the same infection a second time, memory T cells in white fat responded more potently than memory T cells elsewhere in the body.
Researchers were surprised to observe that memory T cells stored in white fat tissue used signaling molecules to activate the fat cells to fight infection as well. These cells, called adipocytes, store lipids to fuel the body, but in the presence of activated T cells, antimicrobial genes within the adipocytes became active, suggesting the cells can play a role in immune protection.
So does more fat mean more immune protection? Not necessarily. Previous studies have shown that white fat tissue in overweight and obese people is prone to unhealthy inflammation that may upset this protective mechanism. So perhaps being a healthy weight is key to utilizing white fat tissue as a lean, mean, infection-fighting machine.
Dr. Belkaid’s group plans to see if this immune activity also takes place in the white fat tissue of humans. If it does, Dr. Belkaid says that these insights may help scientists understand how malnourished people preserve their immunity, or allow scientists to improve upon the success of immunotherapy for cancers and other conditions by using molecules found in the white fat to optimize the protective function of T cells.
S Han et al. White Adipose Tissue Is a Reservoir for Memory T Cells and Promotes Protective Memory Responses to Infection. Immunity DOI: 10.1016/j.immuni.2017.11.009 (2017).