Dietary salt swells autoimmune pathogenic T cells in mice
It's not every day multiple sclerosis studies grab news headlines around the world, but T cell research seasoned with salt served up an irresistible new dish of potential autoimmunity mechanism with wide appeal.
Three papers published online Wednesday in Nature poured out compelling evidence that excess salt directly pressures some immature T cells to become the pathogenic type known as T helper 17 (Th17), believed to be one of the most inflammatory pathways in MS and other autoimmune diseases (Kleinewietfeld et al., 2013; Wu et al., 2013; Yosef et al., 2013).
The new findings add dietary salt to the environmental factors, such as smoking and low levels of vitamin D, that may nudge vulnerable immune systems to attack their own tissues instead of just defending against harmful bacteria, viruses, fungi, and parasites. In a twist, the salt connection was made primarily through laboratory studies of mouse and human cells and in experimental autoimmune encephalomyelitis (EAE; see Animal Arsenal), a disorder in mice that mimics some aspects of MS, rather than the epidemiological studies of large groups of people typically used to pin down associations between diseases and environmental exposures.
At Science magazine's news site, MSDF contributor Mitch Leslie wrote:
Researchers converged on the results from different directions. Immunobiologist David Hafler of the Yale School of Medicine and colleagues found that people who admitted to eating a lot of fast food harbored more TH17 cells. One ingredient that fast food contains in prodigious amounts is salt. To determine whether salt accounted for the surfeit of TH17 cells, Hafler and colleagues spiked cultures of unspecialized T cells with sodium chloride. "The results were perhaps among the most dramatic of my career as a research scientist," he says. Modestly raising salt concentrations, mimicking the levels in the tissues of an animal eating a high-salt diet, boosted the number of TH17 cells that matured in the cultures nearly 10 times. And these TH17 cells started making inflammation-provoking molecules, indicating that they'd become the harmful variety.
The scientists next tested whether this ominous effect occurred in animals. They prompted mice to develop experimental autoimmune encephalomyelitis (EAE), a neurological illness similar to multiple sclerosis that is fostered by "bad" TH17 cells. They fed some of the rodents meals that contained about as much salt as a typical Western diet. Compared with animals that lived on low-salt food, mice that munched high-salt chow developed EAE sooner and had more severe symptoms, the team reports in Nature [Kleinewietfeld et al., 2013].
Working independently from Hafler's group, computational biologist Aviv Regev of the Broad Institute in Cambridge, Massachusetts; immunologist Vijay Kuchroo of Harvard Medical School in Boston; and colleagues also hit upon a link between salt and autoimmunity. They tracked gene activity over the 3-day maturation period of a TH17 cell and revealed the molecular circuit that controls the process. One of the most influential genes in this network was SGK1, and it has a salt connection, helping cells manage sodium levels. Using T cell cultures, the team found that salt promotes the specialization of TH17 cells through SGK1.
The Boston Globe takes it from here:
The researchers were intrigued. They administered salt to immune cells and found that the SGK1 gene became more active, turning them into TH17 cells and ramping up production of an inflammatory protein. They found that mice genetically predisposed to develop a form of multiple sclerosis had more severe disease when they were fed a high-salt diet, and that mice lacking the SGK1 gene had less severe disease. …
Three and a half years ago, Dr. Vijay Kuchroo, an immunologist at Brigham and Women’s Hospital, wasn’t thinking about salt. He wanted to better understand TH17 cells, which normally help the body clear infections but can also turn pathogenic, producing proteins that trigger inflammation and are linked to rheumatoid arthritis, psoriasis, and multiple sclerosis. He hoped that understanding those cells in detail could help physicians and researchers sort a needle from a haystack: of the many environmental factors that have changed over the past half century, from diet to lifestyle, which ones might help explain the rise in autoimmune disease?
For example, Type 1 diabetes increased between 2001 and 2009 by 23 percent, according to the American Diabetes Association. The incidence of psoriasis nearly doubled between 1970 and 2000. Pediatric multiple sclerosis, Kuchroo said, was virtually unheard of 20 years ago, and now more and more cases are being reported.
The largest paper of the trio detailed how a new nanowire technology from Hongkun Park, a Harvard University chemist and physicist, helped Regev and Kuchroo identify a network of key pathways and signaling nodes that control Th17 cell differentiation (Shalek et al., 2012).
Finding the molecular switches that cause the body to overproduce TH17 cells has been difficult, in part because conventional methods of activating native immune cells in the laboratory often harm the cells or alters the course of their development.
So when researchers heard a talk by Hongkun Park, a physicist at Harvard University in Cambridge, Massachusetts, about the use of silicone nanowires to disarm single genes in cells, they approached him immediately, recalls Aviv Regev, a biologist at the Massachusetts Institute of Technology (also in Cambridge) and a co-author on two of the studies.
Park showed last year that these nanowires can be used to manipulate genes in immune cells without affecting the cells’ functions. For [their] Nature studies, Regev and her colleagues used Park's technology to piece together a functional model of how TH17 cells are controlled, she says.
Our favorite description of the nanowire technique comes from the Boston Globe, where staff writer Carolyn Y. Johnson writes,
They laid their delicate TH17 cells on a bed of tiny wires coated with the genetic material, a technique Kuchroo compares to a yogi lying on a bed of nails. It worked, allowing the scientists to turn key genes off so they could tweak their cell circuit diagram.
Regev saw that all genes could be grouped into two big networks. One network fired up activity of the immune cells, the other tamped it down. It was, she said, a yin and yang—unless that tension was yanked out of balance.
Scientifically, the salt connection to MS in people remains to be proven. "Our data raises a hypothesis that needs to be tested" in people, Kuchroo said in an interview. "We're not asking people to stop eating salt. We're not even saying the increase in autoimmunity is because of the high-salt Western diet. We need to do a careful and precise clinical trial to see if a high-salt diet is an environmental trigger."
On the other hand, people eating a Western diet typically eat about twice as much salt as they need, which public health experts believe puts them at higher risk for high blood pressure and heart disease.
It [will] likely be years before this link is confirmed, but Hafler says for patients already at risk of autoimmune disease, reducing dietary salt may be a good idea.
"If I had MS, I would think very much about not eating processed foods and really cutting down my salt intake," he said.
A low-salt diet is so difficult to achieve that their follow-up clinical research will necessitate preparing special meals for people in the study, says Yale immunologist Markus Kleinewietfeld, first author of one of the studies. Hafler, Kleinewietfeld, their German co-authors, and collaborators at the Massachusetts Institute of Technology in Cambridge are also exploring the influence of the other fast-food villain—fat. As for salt, the preliminary findings have piqued the interest of autoimmune researchers.
From The Scientist:
“I thought the papers were very exciting and provocative,” said John O’Shea, a doctor at the National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS), who wrote a Nature commentary accompanying the new findings and was not involved in the study [O'Shea and Jones, 2013]. …
But O’Shea cautioned that salt has not been explicitly shown to have an effect on human autoimmune diseases. “This is an artificial model of autoimmunity,” he said. The scientists induced autoimmunity in the mice, and the salt only exacerbated their condition. It remains to be seen, he said, whether salt can induce or worsen disease in humans.
From New Scientist:
Ari Waisman, who studies autoimmune diseases at the Johannes-Gutenberg University of Mainz in Germany, says that if the same mechanisms are at work in humans, psoriasis may be particularly affected because the condition recedes dramatically in people receiving antibodies that inhibit IL-17A. "Levels of IL-17A increase in MS patients, but the disease with a clear pathogenic role for IL-17A is psoriasis," he says.
Larry Steinman, a neurologist at Stanford University in California, concurs with both the caveats and the keen interest in the implications. "They're elegant pieces of work," he tells MSDF. "It's important to get this translated to humans." For Steinman, the salt connection reinforces an earlier finding in mice that suggests that a common medicine for high blood pressure might be effective against the inflammatory process in the brains of people with MS. For his previous study, Steinman published side by side with the same German research lab that collaborated with Hafler on the latest papers (Platten et al., 2009; Stegbauer et al., 2009). The phase II human study to test the drug as an adjunctive therapy for MS has gotten the green light from the U.S. Food and Drug Administration to begin.
"I had a smile on my face the first time I read them," Steinman says. Or maybe that grin came from musings about the broader role of salt in normal immune responses (albeit still at levels well below those of the normal Western diet) and the memory of the slightly salty chicken soup his mother served him when he was sick.
Key open questions
- Is a high-salt diet an environmental trigger of MS in people? And if so, what combination of genes (and biological pathways) confers protection or increases risk?