Natural Killer T-Cells Guard Against Obesity

Discovery expands understanding of immuno-metabolic interactions

Invariant natural killer T-cells (iNKT), a unique subset of immune cells, are known to influence inflammatory responses. Now, Harvard Medical School researchers at Beth Israel Deaconess Medical Center have found that iNKT cells also play a protective role, guarding against obesity and metabolic syndrome, a group of medical conditions that, when they occur together, can increase the risk heart disease and diabetes.

The researchers also found that although iNKT cells are lost when humans become obese, they can be restored through weight loss. Their research further suggests that therapies that activate iNKT cells could help manage obesity, diabetes and metabolic disease.

The findings were published on-line Sept. 13 in the journal Immunity.

NKT cells had been thought to be rare in humans until work by Lydia Lynch found they were plentiful in human adipose tissue (fat). Lynch is an HMS research fellow in the Department of Hematology/Oncology at Beth Israel Deaconess and the study’s first author.

Lydia Lynch and Mark Exley. Courtesy Beth Israel Deaconess.

“Our previous work had revealed a large population of iNKT cells in fat tissue in both mice and humans,” said Lynch. “Now we have identified them in mice and identified a role for them in the regulation of body weight and the metabolic state, likely by regulating inflammation in adipose tissue.”

Together with senior author Mark Exley, HMS assistant professor of medicine at Beth Israel Deaconess and a leader in the field of NKT investigations, the team also discovered that a lipid called alpha-galactosylceramide (aGC) can lead to a dramatic improvement in metabolism, weight loss and fatty liver disease and can reverse diabetes by bolstering cells that have been depleted.

Lynch first began this line of investigation in 2007 in her native Ireland, where her work in the Obesity Clinic at St. Vincent’s University Hospital in Dublin focused on the immune systems of obese patients.

“We knew that not only did obese patients have more heart attacks and a greater incidence of Type 2 diabetes than lean individuals, but they also developed more infections than non-obese individuals,” she said.

Blood samples taken from the patients revealed that both NKT cells and iNKT cells were decreased. Subsequent studies of fat tissue from a group of obese patients who had lost weight following bariatric surgery showed that iNKT cells had increased to normal levels.

Researchers conducted a series of animal experiments to test their hypothesis that iNKT cells play a physiological role in fat tissue regulation, and thereby protect against the development of inflammation and metabolic syndrome.

Knowing that mice have iNKT cells in their liver tissue, the scientists first ascertained that, like humans, the animals also harbored these cells in fat tissue.

“We found loads of them,” said Lynch, explaining that the research team next proceeded to put the mice on a high-fat diet (60 percent calories from fat) and studied the outcomes.

 “Similar to the human subjects we had previously studied, the animals lost their iNKT cells when they became obese,” Lynch said. “Once we took them off this diet and put them back on a normal standard-fat diet, they lost the weight—and their iNKT cells increased.”

In the next experiment, the authors set out to better understand the exact role of the iNKT cells by examining two strains of knockout mice, the CD1d-/- and the Ja18-/-, both of which are deficient in iNKT cells.

“We put these knockout mice and a group of control animals on high-fat diets,” said Lynch. “While all of the animals grew obese, the response in the knockout mice was much more severe in that they grew 30 percent fatter than the control animals and developed the mouse equivalent of Type 2 diabetes very quickly, over just six weeks.”

The mice also had greatly increased triglyceride levels, larger adipocytes and fatty liver disease.

Next, the authors removed iNKT cells from a normal mouse and injected them into obese NKT knockout mice.

“By doing this, we actually reversed the diabetes, and even though the mice continued to eat a high-fat diet, they lost one to two grams of weight [normal mouse weight being 20 to 25 grams] and exhibited a host of features that suggested reduced inflammation, including improved insulin sensitivity, lower triglycerides and leptin and shrunken adipocytes.”

Finally, in order to determine if the remaining diminished pool of iNKT cells in obesity could be activated to improve metabolism, the scientists tested alpha-galactosylceramide (aGC), a lipid known to activate iNKT cells.

They found that administering a single dose of aGC caused a dramatic improvement in metabolism and fatty liver disease, loss of much of the weight gained, and reversal of diabetes in the obese animals.

Exley noted that aGC has been tested in clinical trials for the treatment of certain cancers and shown to be safe and to produce few side effects in humans.

“The effect of NKT stimulation, whether by aGC or other means, on weight loss, obesity and metabolic disorder has not been investigated until now and may provide a new avenue for the treatment of obesity and metabolic syndrome, which have now reached epidemic proportions worldwide,” he said. 

In addition to Lynch and Exley, study coauthors included HMS and Beth Israel Deaconess investigators Steven Balk, Michael Nowak, Bindu Varghese, Justice Clark and Vasillis Toxavidis; Andrew Hogan and Donal O’Shea of St. Vincent’s University Hospital, Dublin, Ireland; and Cliona O’Farrelly of Trinity College, Dublin, Ireland.

This study was supported by grants from the National Institutes of Health (NIH R21 CA143748, R01 DK066917, U19 AI066313), the U.S. Department of Defense (US DOD W81XWH-09-1-0156); the UNESCO L’Oreal Fellowship; the European Commission Marie Curie Fellowship; the Science Foundation Ireland; and the Health Research Board, Ireland.

Mark Exley and Steven Balk have consulting relationships with NKT Therapeutics, Inc.

Adapted from a Beth Israel Deaconess News Release.