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The largest study to date to examine immune differences between sexes at single-cell resolution has identified over 1,000 genetic switches that operate in distinct ways when comparing immune cells from men and women. Published today in The American Journal of Human Genetics, these findings could explain why women are much more likely to be affected by autoimmune conditions than men.
“Our findings show that the immune system needs to be studied with sex in mind,” says Seyhan Yazar, PhD, group leader of the precision immunology program at the Garvan Institute of Medical Research in Australia. “Even though we know men’s and women’s immune systems differ, many studies still overlook these differences, which can limit how well we understand disease, and in turn bias treatment options.”
Yazar’s team analyzed single-cell RNA sequencing data from over 1.25 million circulating immune cells from nearly 1,000 healthy individuals who participated in the OneK1K cohort. This Australian research program maps how individual immune cells respond to disease and pathogens to determine why some individuals respond to treatment but others don’t.
Results revealed distinct genetic and cellular profiles between both sexes. While men were found to have a higher proportion of monocytes, women showed higher levels of B cells and regulatory T cells. In men, genetic activity seemed to focus on basic cellular maintenance processes, but in women genetic activity heavily skewed towards the activation of inflammatory pathways.
“While this highly reactive immune profile gives females an advantage in fighting viral infections, it comes with a biological trade-off: a greater predisposition to autoimmune diseases,” says Sara Ballouz, PhD, senior lecturer at the University of New South Wales (UNSW). “On the other hand, male immune cells are less primed for inflammation, making men generally more susceptible to infections and non-reproductive cancers.”
Interestingly, most of the genetic switches found to be active in individuals of one sex but not the other were not found to be located in sex chromosomes. More than 1,000 sex-specific genetic switches were identified on autosomes, with many of them being directly linked to autoimmune conditions.
“This is the first time we have shown that these differences occur at the genetic control level, providing a new layer of insight into human immunity,” Ballouz says. “Having shown that female-biased genes are heavily enriched in inflammatory pathways, we now have another biological rationale for why the immune system can more easily mistakenly attack the body’s own tissues in women.”
The analysis found female-specific genetic variants that affected the expression of two genes linked to systemic lupus erythematosus (SLE), an autoimmune condition that is nine times more likely to affect women than men. Although conditions like SLE are multifactorial, uncovering the contribution of genetic variants to their development is an important step forward towards better understanding disease susceptibility between sexes.
“Our findings add strong evidence that female and male autoimmune diseases may not be the same, and the way we should treat them may not necessarily be the same,” says Yazar. “Currently, clinicians rely on a one-size-fits-all management approach for most autoimmune diseases—a more inclusive approach is needed.”
Currently, autoimmune conditions are often treated with broad immunosuppressants that reduce the activity of the entire immune system. Research is striving to move towards treatments that more precisely target each person’s unique needs, which is only possible through the identification of distinct genetic pathways driving autoimmune disease.
“If we want to realize the potential of precision medicine, we have to understand these fundamental biological variables,” says Joseph E. Powell, PhD, director of the translational genomics program at the Garvan Institute. “Treatments need to be tailored not just to the disease, but to how a patient’s immune system operates at a baseline genetic level.”