Why Astronauts Have Weaker Immune Systems in Space

Gene activity in white blood cells decreased once astronauts got to space—and it didn’t rebound until they returned, a new study finds

An astronaut tethered to the outside of the International Space Station, with a view of Earth behind.
NASA astronaut Andrew Morgan during a 2020 space walk at the International Space Station. Researchers theorize that the weightlessness astronauts experience on the ISS contributes to immune system dysfunction. NASA

Traveling to space is extremely taxing on the body, and for years, scientists have been trying to understand how astronauts’ muscles, bones, organs, blood and mental health are affected when they leave the familiar environment of Earth’s gravity.

Now, scientists say that space travel can reduce gene activity in some white blood cells, leading astronauts’ immune systems to suffer during trips to the International Space Station (ISS).

The research, published last week in the journal Frontiers in Immunology, looks at blood samples taken from 14 astronauts before, during and after stints of about six months at the ISS. Shortly after the astronauts arrived at the station, the expression of genes connected to the immune system decreased—and it didn’t return to normal until after they’d come back to Earth.

The new findings add to the growing body of work documenting the effects of space travel on human health and physiology.

“These results are important considerations of risks to health during spaceflight and space exploration,” Myles Harris, who studies space health at the University College London and did not contribute to the research, tells BBC Science Focus’ Noa Leach.

“Before this paper, we knew of immune dysfunction but not of the mechanisms,” Guy Trudel, a co-author of the study and clinical researcher at the University of Ottawa in Canada, tells Reuters Will Dunham.

While the International Space Station is only a couple hundred miles from the ground—close enough to feel Earth’s gravity—astronauts onboard experience weightlessness because the station is in free fall while orbiting the planet. This microgravity, as well as exposure to space radiation and the psychological impacts of isolation, affects the body in a number of ways.

Living in low gravity can cause people to lose bone density and muscle mass. Without gravity, fluids shift upward in the body, which can put pressure behind the eyes and cause vision problems, per NASA. Fluid-filled cavities in the brain known as ventricles expand during space flight, and a study earlier this month found it could take at least three years for this effect to subside after astronauts return to Earth.

“Short- and long-term spaceflight negatively affects most physiological functions,” write the authors of the new study.

Previous research has also shown that space travelers’ immune systems change, according to NASA. Studies have revealed that astronauts on the ISS sometimes experience cold symptoms and skin rashes and that dormant viruses the astronauts once had, such as herpes or chicken pox, can reactivate while in space, writes National Geographic’s Carrie Arnold.

For the new study, the astronaut participants—11 men and three women—had blood drawn once before takeoff, four times while on the ISS and five times after they returned to Earth.

The researchers looked at the gene expression in leukocytes, which are white blood cells made in the bone marrow that create proteins to ward off pathogens. After just 8 to 12 days in space, genes connected to immune function within these white blood cells decreased in activity. In 247 of the examined genes, expression was down to about one third of normal, according to Reuters.

These changes leveled off after two to six months in space, and they didn’t go back to normal until within a month of astronauts’ return to Earth.

“I was not expecting such a large change in gene expression. Why would the immune system go down in microgravity?” study co-author Odette Laneuville, a molecular biologist at the University of Ottawa, tells National Geographic. “There seems to be something special about space.”

The researchers theorize that these changes were caused by exposure to microgravity as opposed to, say, space radiation, according to Reuters. When people are in microgravity, they experience a shift in how blood plasma is distributed throughout the body, which causes their volume of blood to drop by 10 to 15 percent, the authors write. As a result, there might not be enough room for all the immune cells in the blood, and the decreased gene expression could get rid of some cells, Laneuville tells Ari Daniel on NPR’s “All Things Considered.”

The new findings shed light on how our bodies adapt to, and recover from, extreme environments.

“Within minutes of being in space, your body is changing,” Jamie Foster, an astrobiologist at the University of Florida who did not participate in the research, tells National Geographic. “But I don’t think we have a really good handle on the long-term changes yet.”

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