After his best friend from high school had a daughter with Down syndrome, immunologist Dusan Bogunovic of the Icahn School of Medicine at Mount Sinai in New York pored over decades of research to better understand the condition. As he read through the literature, he began to notice overlaps between Down syndrome and more severe genetic disorders that stem from over-producing interferons—molecules that open the innate immune system’s floodgates by activating hundreds of genes that guard against viral infections and trigger inflammation to curb disease after an infection occurs.
Now, about six years later, Bogunovic and colleagues report evidence that a hypersensitivity to interferons, specifically those classified as type I, may paradoxically blunt responses to those signaling molecules after infection, causing an immunosuppressed state that can let inflammation run rampant. The findings, published today (October 14) in Immunity, may help explain why viruses tend to infect people with Down syndrome less often than people without the condition, but when they do, illness is more severe.
“I think [this research] is a very elegant way to test how interferon receptors might trigger different feedback,” says Xiao-Fei Kong, an immunology researcher and clinician at UT Southwestern Medical Center in Texas who has worked previously with Bogunovic but was not involved in this research. “This is also an important question to answer: why those patients with Down syndrome suffer from severe viral infection.”
Down syndrome, also called trisomy 21, is the most common chromosomal disease in the US; around 1 out of every 700 babies are born with the condition. The study authors write that while recent research indicates people with Down syndrome are less likely than the general population to catch certain viruses (such as influenza), they are significantly more likely to be hospitalized or die from illness caused by respiratory viruses. For example, post-infection mortality due to COVID-19 is about 10 times higher in people with Down syndrome compared to those without the condition, says Bogunovic.
Because Down syndrome arises from having a third copy of chromosome 21, people with the condition have an extra copy of the more than 200 genes on that chromosome. That includes genes like IFNAR1 and IFNAR2, which code for receptors that bind to interferons.
Transcriptomic analyses confirmed the researchers’ expectation that these interferon receptors are expressed at levels roughly 1.5 times higher in fibroblast cell lines derived from people with Down syndrome (DS cells) than lines derived from controls without trisomy 21, says study coauthor Lousie Malle, an immunology researcher at Icahn. However, knowing that a receptor gene is expressed more highly “doesn’t necessarily tell you how the target genes of the pathway are going to be expressed,” says Malle. “It’s usually not linear.”
To measure the downstream impact of having more receptors, the researchers exposed cells to interferons to activate the receptors, and found that the expression of infection-fighting interferon-simulated genes (ISGs) was up to six-fold higher in the DS cells than in control cells. The results suggest that “individuals with Down syndrome are hypersensitive to small amounts of interferon,” Bogunovic says. “So something that me and you would barely sense—because they have more of these receptors, they’re hyper-responding.”
After a virus infects us, Bogunovic says that our bodies harness the power of our innate immune systems, fighting the pathogen through inflammation such as with a fever. “But we need to stop [the] fever, otherwise we’re going to kill ourselves,” he says, so a negative feedback mechanism kicks in to subdue inflammation. Some of the ISGs responsible for fighting infection also lead to the expression of inhibitory molecules like USP18 that block interferon receptors. “These are the brakes that come on,” he says, because they prevent interferon from continuing to push the immune system’s gas pedal.
However, because DS cells are more stimulated by interferons initially, these brakes slam on like a car screeching to a halt, overly suppressing inflammation, Bogunovic says. This happens to such an extent that thereafter, the DS cells hardly responded to interferons, resulting in “what we call the inflammation-caused immunosuppressed state, which is a paradox,” he says, because the inflammation normally relied upon to fight infection apparently dampens the innate immune response.
Thanks to elevated levels of interferon receptors, people with Down syndrome (solid line) exhibit a higher initial response to a viral pathogen than those without trisomy 21 (dashed line). However, their greater initial defense is accompanied by overzealous tamping down of the immune response, leading to susceptibility later on, Dusan Bogunovic and colleagues posit.
He and his colleagues hypothesized that individuals with Down syndrome could be more vulnerable to viral disease while this suppression occurs. To test this, the researchers first exposed control and DS cells to a dose of interferon, then exposed them to influenza A virus. This initial dose prevented some infection in both types of cells: about 48 percent of the healthy control cells were infected compared to about 43 percent of the DS cells. Then the researchers repeated the experiment, this time giving the cells an additional dose of interferons before exposing them to the virus. Only the control cells showed a reduction in infection rate (with only 33 percent of cells infected) from the second interferon jolt; the percent of infected DS cells was once again about 43 percent. To the team, this suggested that during the hyposensitive stage, viruses may be more able to spread throughout the body.
Kong isn’t as certain about DS cells being more susceptible to the flu virus, saying that the in vitro data only show “very mild effects.” Bugonavic agrees but says a mild response was expected considering Down syndrome patients do not often die of the flu. Testing SARS-CoV-2, because of its higher mortality rate and worse health outcomes, could show a more pronounced effect, he says.
The researchers also experimented with blood cells from humans with and without Down syndrome to get a better indication of how their findings might translate to human health. In line with previous research, blood cells from people with Down syndrome showed higher steady-state interferon levels compared to controls. In addition, immunoblotting revealed evidence of USP18—one of the molecular brakes—in the blood cells taken from people with Down syndrome, but not in the controls. Finally, exposure to interferons induced less of an innate immune response in these cells than the controls.
Taken together, the authors argue these results indicate the innate immune systems of people with Down syndrome may generally exist in a partially desensitized state. Malle says that their interferon levels may be “high enough to provide some baseline defense,” but may not be enough for “the really well-orchestrated response that you need to fight off a severe infection.”
“I think this [provides] new insights into understanding interferon, but there are still a lot of questions,” says Kong. His interpretation is that the response of DS blood cells to subsequent interferon stimulation was “still pretty strong,” and describes the USP18 expression as so low that it might not significantly inhibit interferon’s ability to activate receptors. “I’m not really convinced that monocytes or myeloid cells are desensitized.”
Kelly Sullivan, a molecular biologist at the Linda Crnic Institute for Down Syndrome at the University of Colorado, Anschutz Medical Campus who was not involved in the work, agrees that the data showing USP18 in the blood cells aren’t the most compelling, but notes that low levels of a protein don’t preclude it from having a large effect. He says that the data comparing DS and healthy blood cells do validate the authors’ claim that the DS cells are partially desensitized. “I think there’s still quite a lot of work to do to really unravel what [the] clinical implications could be,” he says, but taken together, these data suggest that “by restoring that basal level of interferon signaling, we might be able to rescue that appropriate secondary response to the viral infection.”
And that’s the overall goal, says Bogunovic. In additional in vitro experiments, the team demonstrated that well-timed pulses of JAK inhibitors—immunosuppressants that he likens to artificial USP18—can help smooth out how the innate immune system’s brakes are applied in DS cells and normalize the immune response after infection. “We hope that [by] understanding the dynamics of the immune system—which goes up and then goes down—that we can start manipulating [it] in a way that can benefit the health [of] individuals with Down syndrome,” Bogunovic says, adding that he and his colleagues are planning future clinical trials to evaluate this possibility.
The authors write that it’s likely people with Down syndrome are more susceptible to disease from viral infections because of a combination of factors: the initial hypersensitivity and subsequent hyposensitivity of their innate immune systems as indicated in these findings, previously reported deficiencies in their adaptive immune systems, and anatomical anomalies in their respiratory tracts.
“Individuals with Down syndrome have a complex immune dysregulation. And there are many aspects of disease that need untangling,” says Bogunovic. “What we have discovered here is a piece of that puzzle, which we think is very important.”