Trio of research teams discover potential link between salt, genetic factors in autoimmune disease

A five-year effort has yielded new insights on how the immune cell Th17 develops and influences immune response and has also helped to unveil a surprising connection between autoimmunity and salt consumption

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CAMBRIDGE, Mass.—A five-year effort from three academicinstitutions on the East Coast has yielded new insights on how the immune cellTh17 develops and influences immune response. In doing so, the research trioalso made a surprising connection between autoimmunity and salt consumption,unearthing new information on how genetics and environmental factors makepeople susceptible to autoimmune diseases.
The team, composed of researchers from the BroadInstitute's Klarman Cell Observatory, Brigham and Women's Hospital (BWH) andYale University, published these results in three companion papers in Nature in early March. At the center ofthis small consortium is Vijay Kuchroo, co-director of the Center for Infectionand Immunity at BWH's Biomedical Research Institute and a Broad associatemember. A specialist in veterinary pathology in his native India, Kuchroo wenton to develop many animal models of disease, including multiple sclerosis, andhas since worked for BWH, the U.S. National Institutes of Health (NIH) andHarvard Medical School.
Working closely with Aviv Regev, a Broad Institute coremember and an associate professor of biology at the Massachusetts Institute ofTechnology, David Hafler's group at Yale and the laboratory of Harvardprofessor and Broad associate member Hongkun Park, Kuchroo set out toinvestigate the role that T cells play in clearing foreign pathogens and therelevance of this function in the development of autoimmune diseases. Althoughrecent studies have reconstructed regulatory networks in mammalian cells, thisperturbation approach cannot be readily applied to primary T cells. As aresult, the molecular circuits that control the differentiation of naive Tcells remain largely unknown.
Th17 cells can promote inflammation that is important forprotection against pathogens, but they have also been implicated in diseaseslike multiple sclerosis, psoriasis, rheumatoid arthritis and ankylosingspondylitis. Treatment options for some of these diseases, such as psoriasis,include manipulating T cell function. Some genes have been previously tied toTh17 development, but the research team wanted a more comprehensive view.
"The question we wanted to pursue was: how does the highlypathogenic, pro-inflammatory T cell develop?" says Kuchroo. "Once we have amore nuanced understanding of the development of the pathogenic Th17 cells, wemay be able to pursue ways to regulate them or their function." 
Over the course of three days, the researchers took 18snapshots to see what was happening within T cells as they grew from a naïvestate to more specialized Th17 cells. They then applied computationalalgorithms to record the molecular changes that happened as the cells matured.
Armed with that information, the researchers systematicallytested their model by silencing genes one by one, a process designed to revealthe most important points in the network and decipher their biological meaning.This was done using a bed of silicon nanowires, developed by Park's lab, topierce cells.
Using this new technology, the team deleted each of the keygenes required in the development of Th17 cells. The team observed that onenetwork positively regulates the cells, coaxing them to multiply whilesuppressing the development of other cells, while another network negativelyregulates them, having the opposite effect.
One gene, SGK1, which plays an important role in celldevelopment, stood out. When turned off in mice, Th17 cells were not produced.
"Ultimately, we found that SGK1 is uniquely expressed inTh17 cells when they are activated," says Kuchroo. "This seems to be important,as once you get rid of the kinase, which is one of the major hubs involved indevelopment, T cells won't become Th17 cells once you get rid of it."
Interestingly, SGK1 had previously been found to play a rolein the absorption of salt in cells in the gut and kidneys. This then led to aquest to test the connection between salt and autoimmunity, with Kuchroo usingmouse models, and Hafler's team using human cells. The researchers found thatthey could induce more severe forms of autoimmune diseases in mice that werefed a high-salt diet, compared to mice that were fed a normal diet.
Kuchroo notes that the high-salt diet alone did not causeautoimmune disease, as the researchers had to inject a self-antigen to inducedisease, but adds, "salt could be one more thing on the list of predisposingenvironmental factors that may promote the development of autoimmunity."
"Clearly, we have not tested the impact of a high-salt dietin humans. Careful, precise clinical trials must be done," Kuchroo says.
Other environmental factors suspected to play a role inautoimmune disease development include infection, smoking and lack of sunlightand vitamin D.
Next, the researchers plan to revisit the cell circuitrydata to follow up on potential drug targets.
"One of the things we hope to discover is how to build maps,bridges and networks to identify these major hubs that could potentially becomedrug targets, and to explore the downstream effects of blocking this pathway,"he says.
The team will also work to identify whether salt-inducedgenes are identified in the genetic analysis of diseases like multiplesclerosis or inflammatory bowel disease.
Kuchroo stresses that none of the published work—and plannedfuture studies—would be possible without the interdisciplinary effort deployedhere. 
"Modern science can't be done in a lab, sitting in a cornersomewhere. Modern science needs experts in different areas working together. Wemay put ourselves into defined bins, but nature doesn't define those bins," hesays.
Support for the three studies was provided by the NationalHuman Genome Research Institute, the NIH, the National Multiple Sclerosis Society,the Klarman Cell Observatory, the Guthy Jackson Foundation and the AustrianScience Fund.

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