Since space organizations such as NASA are becoming more ambitious with research plans, further research is needed to see what the long-term health effects of space flight on humans are.
A research team led by scientists from the University of Arizona now finds that long-term space flights affect certain cells found in the immune system called natural killer cells (NK cells) – white blood cells that kill cancer cells in the body and prevent older viruses from reactivating.
"What NASA and other space agencies are concerned about is whether the immune system will be compromised during very long space flights," said Richard Simpson, senior author and assistant professor of nutritional science at Arizona. – What clinical risks do astronauts have during these missions when exposed to things like microgravity, radiation, and isolation? Could it be catastrophic to the extent that the astronaut could not complete the mission?
The research team, which included scientists at the University of Houston, Louisiana State University and NASA-Johnson Space Center, has tested blood samples of eight crew members who have completed missions for six months or more to the International Space Station (ISS).
After taking blood samples before and after the space flight, as well as at several different points during the mission, the scientists compared samples of blood samples to healthy individuals.
The researchers found that after about three months in space, the cytotoxic activity of NK cells against leukemia cells in vitro was reduced by about 50%.
"When we look at the function of astronauts in flight, compared to their own samples, before they fly, it drops down," Simpson says. "When we compare them with the controls that are left on Earth, it still falls. I do not think there is any doubt that the function of NK cells decreases in the midst of space flight when analyzed in a cell culture system.
The researchers also found that the effect on NK cell function was more pronounced in the recruited astronauts, unlike veterans already in the cosmos. According to Simpson, this may be due to a number of factors, including the age and stress of astronauts for the first time.
Simpson explained that the team is now planning to investigate whether the reduction in NK cell function will make space researchers more susceptible to cancer and viral reactivation, what exactly reduces this function, and whether the impact can be mitigated.
"Cancer is a big risk for astronauts during very long space flights because of exposure to radiation," said Simpson. "[NK-cells] are also very important for the destruction of viral infected cells. When in the space station, it's a very sterile environment – you are not likely to take flu or rhinovirus or any kind of common infection, but the infections that are a problem are the viruses that are already in your body. These are primarily viruses that cause things like shingles, mononucleosis or cold sores; they stay in your body for the rest of your life and they are activated again when you are under stress.
Researchers are working on a number of countermeasures, including nutritional and pharmacological intervention, and increased physical workloads that have proven their benefit to the immune system.
Other studies indicate several health risks associated with space flight, including muscle and bone loss.
The study was published in Journal of Applied Physiology,