NASA hopes to send humans on a round-trip space flight to Mars by the 2030s that could take up to three years—far longer than any human has spent in space. It’s already well established that long-term exposure to living or travelling through space damages the human body and exposes it great risks. For example, living outside of the pull of enough gravity weakens muscle and bone tissues, possible irreversibly, and plays havoc on a person’ sense of balance. Cosmic particles and radiation from the sun (another group of threats) are known to cause radiation sickness, damage to the central nervous system, and long-term cancers and other degenerative diseases. On Earth, humans are protected by the magnetic field and the radiation belts they create.
Now there is evidence of another risk attributed to cosmic rays: They tend to deactivate or damage NK cells, which help the immune system kill cancer cells and preventing viruses from reactivating, according to a new study led by University of Arizona researchers.
“What NASA and other space agencies are concerned about is whether or not the immune system will be compromised during prolonged spaceflight missions,” says Richard Simpson, senior author and associate professor of nutritional sciences in the College of Agriculture and Life Sciences at UA. “What clinical risks are there to the astronauts during these missions when they’re exposed to things like microgravity, radiation, and isolation stress? Could it be so catastrophic that the astronaut would be unable to complete the mission?”
Simpson and researchers at the UA, the University of Houston, Louisiana State University, and the NASA Johnson Space Center studied the effects of spaceflights of six months or more on natural killer cells, or NK cells, a type of white blood cell.
“Cancer is a major risk to astronauts during long spaceflights because of radiation exposure,” Simpson says. “It‘s true that when you’re in the space station, it’s a nearly sterile environment. You’re unlikely to pick up the flu or a rhinovirus or some community-type infection.
“But the infections that are a problem are the viruses already in your body,” he continues. “These are mostly viruses that cause things such as like shingles, mononucleosis, or cold sores. They stay in your body for the rest of your life and can reactivate when you’re stressed.”
Scientists compared blood samples of eight crewmembers who completed missions to the International Space Station with healthy individuals who remained on Earth. Blood samples were taken before launch, at several points during the mission, and after the astronauts’ return to Earth.
The results showed that NK-cell function is impaired in astronauts as compared with pre-flight levels and ground-based controls. After 90 days in space, NK-cell cytotoxic activity against leukemia cells in vitro was reduced by approximately 50% in International Space Station crew members.
European Space Agency (ESA) astronaut Alexander Gerst and NASA astronaut Serena Auñón-Chancellor, have blood drawn in the Columbus Module’s clinic. (Photo: NASA)
“When we look at the function of astronaut samples during flight compared to their own samples before they flew, it goes down,” Simpson says. “When we compare them to controls who stayed on Earth, it still goes down. I don’t think there’s any doubt that NK-cell function is decreasing in spaceflight environments.”
The effect appears to be more pronounced in first-time astronauts, as opposed to those who have already been in space.
“Serendipitously, we found that half our crew members had flown before, and the other half hadn’t,” Simpson said. “So we were able to see if there was a difference in the effect, and there was. ‘Rookies’ had greater drops in NK-cell function compared to veterans.”
The differences could be chalked up to age or stress, Simpson suggests. Assuming rookie astronauts are generally younger than their veteran counterparts, they might find space travel more stressful than those who had done it before.
Whether the drop in NK-cell function makes astronauts more susceptible to cancer and viral reactivation remains to be seen, Simpson says, adding that he hopes to learn more from future studies.
“The next question would be, how do we mitigate these effects?” he says. “How do we prevent the immune system from declining during space travel? To do that, you have to determine what causes the decline: Is it stress? Is it microgravity? Is it radiation? Is it a combination of things? “When we figure that out,” he concludes, “we can directly target those factors and try to mitigate them.”
Simpson and his fellow researchers at NASA Johnson Space Center, along with European and Russian scientists, are already working on potential countermeasures that could help keep astronauts healthy in space, including nutritional and pharmacological intervention and increased exercise—all of which have been shown to improve the immune system.