New research suggests even brief forays into space can weaken the immune systems of astronauts.
During the final flight of the US space shuttle Atlantis in the summer of 2011, one of the most important pieces of scientific cargo on board was a medical experiment designed to test how the human immune system responds to stress and disease in the microgravity conditions of Earth orbit.
The experiment contained samples of living human cells housed in a sterile, temperature-controlled module. Astronauts pushed a button to infect the cells with a common bacterial toxin that causes sepsis, a severe and potentially lethal skin infection.
Cells in space didn't fight infection
The module spent two weeks in space and researchers at the U.S. Army Medical Command have spent the past two years analyzing the data.
They compared that data with the results of a parallel experiment conducted on Earth at the same time, under normal gravity conditions.
The same cell line was used to minimize variation between the two experiments.
"We did a micro-array analysis, where we screen 30,000 to 40,000 genes at a time," said Rasha Hammamieh, deputy director of the Integrative Systems Biology Program at the Medical Command and lead scientist on the project.
The cell samples that went into space showed a diminished ability to activate a normal immune response, according to Hammamieh.
"This means that the cells are not able to respond to a pathogen anymore," she said. "For an astronaut, that means that it will be easier to get sick because their immune system is weakening."
Parallels to stress in war
The cells in the space shuttle experiment were so busy dealing with microgravity that they barely put up a fight against infections, said Marti Jett, director of the Integrative Systems Biology Program at the Medical Command.
Researchers from US Army Medical Research and Material Command and the Department of Defense Space Test Program collaborate on the space health experiments. (USAMRMC)Researchers from US Army Medical Research and Material Command and the Department of Defense Space Test Program collaborate on the space health experiments. (USAMRMC)
These results were very similar to those of a study she and her colleagues conducted a couple of years ago with Army Ranger trainees under battlefield conditions.
"We saw a rather similar thing there," Jett said, "that these young men were so stressed from reduced sleep, their heavy exercise, their activities, that their immune cells simply did not respond very well, when ex-vivo [outside the body] exposed the cells to a variety of pathogens."
This weakened immune response echoed the results with the space shuttle samples. But the Atlantis experiment revealed other effects.
Researchers found the reduced gravity also activated certain genes in the human tissue involved with rheumatoid arthritis and tumor growth, raising additional health concerns for human space flight.
Could lead to new therapies
Identifying which gene molecules were responsible for the weakened immune response offers hope for therapeutic strategies, both in space and on Earth.
"If one molecule is not as active as it should be in space, how can we make this molecule more active?" Hammamieh said. "This way we can induce the immune system of astronauts or humans in space."
She hopes to follow up with a study that will send mice with induced wounds up to the International Space Station to explore how microgravity affects healing. The results could be especially important for astronauts heading into space on longer missions, like those planned to an asteroid and Mars.