Explaining the female bias in autoimmunity
Females are at a disadvantage when it comes to susceptibility to lupus and many other autoimmune conditions. Eighty-five percent of lupus patients are women.
Research led by Montserrat C. Anguera, an assistant professor in the Department of Biomedical Sciences in the School of Veterinary Medicine, provides the first explanation of why this is the case, on the molecular level. The reason is tied up in the quintessential element that distinguishes women from men: their second X chromosome.
X inactivation is a process by which genes on one of a female’s two X chromosomes are silenced. The end result is that gene dosage from the X is balanced between males and females. Anguera and colleagues wondered if some female-biased autoimmune diseases could arise from incomplete X inactivation.
“What caught my attention about autoimmunity and specifically lupus,” Anguera says, “was that there were genes on the X chromosome that were immunity-related and that had been shown to have higher expression levels in lupus patients.”
The research team focused attention on lymphocytes, specifically the T cells and B cells responsible for much of the pathology in lupus. They looked at patterns of Xist, a non-coding RNA molecule that mediates the process of X inactivation, examining “naïve,” or unstimulated lymphocytes donated by healthy human females as well as from female mice. In contrast to other cell types where Xist is associated tightly with the inactive X chromosome, the female lymphocytes lacked this Xist “cloud,” suggesting that Xist wasn’t properly localizing to the inactive X to keep it silent. The Xist clouds reappeared when the immune cells were stimulated, similar to when cells are presented with a pathogen.
While Anguera and colleagues suspected that the lupus patients might have had unusual patterns of X inactivation, they were surprised to see such seeming abnormalities in samples from healthy females.
“Our hypothesis was that the lupus samples were going to be dysregulated and the healthy females would be fine,” Montserrat says. “So it was really shocking to us that lymphocytes in normal females lacked these markers of X inactivation, as well.”
Further experiments showed that a small percentage of female lymphocytes—but not male lymphocytes—expressed two copies of some immunity-related genes. And when they looked specifically at the lymphocytes of pediatric lupus patients, they found more Xist mis-localization and more instances of two copies of immunity-related genes than in health patients.
The research not only provides a mechanistic explanation for differences in autoimmune risk, but also for the advantage that females have in fighting off infections. Further work could lead to biomarkers for disease risk for lupus as well as other autoimmune diseases that may share a common dysregulated pathway of Xist mis-localization.