Researchers at NYU College of Dentistry and NYU Grossman School of Medicine reported the findings of a study in mice that outlines new discoveries about the roles of calcium signaling, regulatory T cells, and interferon, in Sjögren’s disease (SjD). The autoimmune disorder is characterized by dry eyes and mouth (sicca syndrome) resulting from lacrimal gland (LG) and salivary gland (SG) dysfunction, inflammation, and destruction.
The newly reported study showed that impaired regulatory T cells are a critical contributing factor to Sjögren’s disease in both mice and humans, and identified an existing rheumatology drug, baricitinib, as a promising therapy for the disease.
“Not only did we dissect the underlying cause for Sjögren’s disease in our mouse model, but we correlated these findings to the disease’s classification criteria and genetic signatures in humans,” said Stefan Feske, MD, the Jeffrey Bergstein Professor of Medicine in the department of pathology at NYU Grossman School of Medicine. “Moreover, I think using baricitinib has great promise in the treatment of Sjögren’s disease going forward.”
Feske and colleagues reported on their findings in Science Translational Medicine, in a paper titled, “IFN-γ–producing TH1 cells and dysfunctional regulatory T cells contribute to the pathogenesis of Sjögren’s disease.”
“Sjögren’s disease (SjD) is a common autoimmune disorder with a prevalence of 0.5 to 1% in the general population,” the authors wrote. In Sjögren’s disease, the immune system attacks the glands that produce saliva and tears, resulting in dry mouth and eyes. It can also affect other parts of the body, with some patients experiencing fatigue, joint and muscle pain, rashes, and lung inflammation.
“Not being able to produce tears or saliva can have a great impact on one’s life,” said co-senior author Rodrigo Lacruz, PhD, professor of molecular pathobiology at NYU College of Dentistry. “If you can’t produce saliva, it may hamper your speech and your ability to process food, increases your risk for developing cavities, and overall worsens one’s health.” And as the authors further pointed out, “In some patients, the systemic disease becomes life threatening, with leading causes of death related to interstitial lung disease (ILD), renal failure, cryoglobulinemic vasculitis, and B cell lymphoma.”
Autoantibodies in the blood and lymphocytes in the salivary glands are hallmarks of Sjögren’s disease that help with diagnosis. There is no cure for Sjögren’s disease, and some treatments alleviate symptoms but may not provide patients with full relief. “The pathophysiology of SjD remains incompletely understood, and clinical trials of immunomodulatory drugs that were successful in other autoimmune diseases have shown mixed results in SjD,” the scientists further pointed out. “Sjögren’s disease is an inflammation-driven disease,” added Feske. “There are therapies for Sjögren’s that deplete B cells with antibodies, but they showed mixed results in clinical trials.”
Lacruz and Feske study the role of calcium signaling in human diseases, including disorders of the immune system and those that affect the mouth. Calcium signaling is important for saliva production, but it is unclear to what extent it may be involved in the development of Sjögren’s disease.
Feske and Lacruz undertook two studies focusing on different tissues—salivary gland cells and immune cells—to better understand what’s happening at the cellular level to contribute to Sjögren’s disease. They specifically looked at cells lacking the Stim1 and Stim2 genes, which interrupt calcium signaling.
In a recently published study the researchers, led by Lacruz, showed that in mice without the Stim1 and Stim2 genes in salivary gland cells, there was a lack of calcium uptake into these cells, and the mice had reduced saliva production as a result of reduced calcium levels and signaling. However, the animals did not demonstrate salivary gland inflammation or increased levels of autoantibodies that are characteristic of Sjögren’s disease in humans, suggesting that the loss of calcium signaling in salivary gland cells might actually suppress inflammatory responses rather than rendering these mice susceptible to inflammation and autoantibodies.
“We found that a specialized calcium channel activated by STIM1 and STIM2 proteins, the ORAI1 channel, is essential for driving saliva secretion, which is an important discovery,” Lacruz stated. “Lack of calcium signals not only impairs function, it may also decrease the effect of inflammatory molecules that have been associated with Sjögren’s disease.”
Previous studies have also demonstrated that mice genetically engineered to lack calcium signaling in their immune system’s T cells exhibited dysfunction in regulatory T cells, and these animals exhibited inflammation and autoimmune disease. “We had previously shown that the Ca2+ function and development of Treg cells depend on store-operated entry (SOCE), which is mediated by ORAI1 Ca2+ channels and stromal interaction protein 1 (STIM1) and STIM2,” the authors noted.
Regulatory T cells help control the immune system’s response, but when they don’t work as they should, they fail to prevent autoimmune disease. Because prior research on regulatory T cells and Sjögren’s disease has yielded mixed results, these cells also became another key focus of Feske’s and Lacruz’s work.
In the newly reported study in Science Translational Medicine, the researchers, led by Feske, again studied mice that were lacking Stim1 and Stim2 genes and thus calcium signaling, but this time, focused on regulatory T cells instead of salivary gland cells. The team found that the dysfunction in regulatory T cells resulted in severe inflammation in the mice that aligned with the classification criteria for Sjögren’s disease: dry eyes, dry mouth, autoantibodies, and lymphocytes in the salivary glands. Some mice also developed lung inflammation, which can be a symptom of Sjögren’s disease. “Knocking out these two genes drove a cascade of immune dysfunction,” said Feske. The authors further wrote in their paper, “… we show that mice with a Foxp3+ Treg cell–specific deletion of Stim1 and Stim2 develop a phenotype that fulfills all classification criteria of human SjD.”
A question was whether it was the impaired calcium signaling that prompted the autoimmune response akin to Sjögren’s disease in mice. The team carried out further analyses in mice and in human blood cells, concluding that the key issue was the dysfunction of regulatory T cells, which can occur through different pathways, not just calcium signaling. A likely culprit causing Sjögren’s disease symptoms in mice was interferon-gamma.
“It came down to a defect in regulatory T cells and an overactivation of the cells that produce an inflammatory cytokine called interferon-gamma,” said Feske. “Interferon-gamma was absolutely critical for causing dysfunction of salivary glands in our mouse model.”
Regulatory T cells typically inhibit other immune cells, including those that produce interferon-gamma. Knocking out calcium signaling in regulatory T cells unleashed the cells that produce interferon-gamma, allowing them to produce more of the cytokine. However, genetically depleting interferon-gamma from the mice’s T cells led to improved salivary gland function.
To test whether a drug could do the same, the researchers turned to baricitinib, which is currently used to treat rheumatoid arthritis, alopecia, and more recently, hospitalized COVID-19 patients. Baricitinib is a Janus kinase (JAK) inhibitor that reduces inflammation by suppressing signals downstream of the interferon receptor.
Experiments in mice showed that baricitinib treatment suppressed salivary gland dysfunction and inflammation. “Inhibition of IFN signaling with the JAK1/2 inhibitor baricitinib alleviated CD4+ T cell–induced SjD in mice,” they wrote. Given the success of the drug in mice with Sjögren’s symptoms, both in this study and others, the researchers think that baricitinib could be a candidate for treating Sjögren’s disease.
To determine if their findings in mice translated to humans, the researchers also examined blood samples from patients with Sjögren’s disease. Using single-cell RNA sequencing to study white blood cells, they found a strong correlation between the gene expression in cells of mice and humans with Sjögren’s disease. “Impaired Treg function in mice correlated with altered expression of genes associated with memory Treg function in patients with SjD, suggesting that Treg function is critical to prevent SjD in mice and human patients,” they wrote. “Together, our findings in mice and human patients provide evidence for a critical role of Treg cells and IFN-γ–producing TH1 cells in the pathogenesis of SjD.”
