Spaceflight can cause a disturbance in the functioning of the genes regulating immune and stress response, which can cause more sickness among astronauts, a new study has revealed.
Astronauts are known to have a higher risk of getting sick compared to their Earth-bound peers.
The stresses that go with weightlessness, confined crew quarters, being away from family and friends and a busy work schedule - all the while not getting enough sleep - are known to wreak havoc on the immune system.
Between spaceflight affecting a crew's susceptibility to infections and previous observations of sickness-causing microbes thriving in a near-zero gravity environment, long journeys to far-away destinations such as Mars pose a big challenge to manned space missions.
"Taken together, our results hint at the possibility that an astronaut's immune system might be compromised in space," said immunobiologist Ty Lebsack at the University of Arizona, the lead author of the study.
The researchers focused their study on the thymus gland, the organ that serves as a "factory" and "training academy" for T-cells that are key players of the immune system.
They compared gene-expression patterns in thymuses from four healthy mice that had spent 13 days aboard NASA's STS-118 Endeavour Space Shuttle to those from an equal number of control mice on the ground.
They found that 970 individual genes in the thymus of space-flown mice were up or down-regulated by a 1.5 fold change or greater.
When these changes were averaged, 12 genes in the thymus tissue of all four space-flown mice were significantly up or down-regulated.
"The altered genes we observed were found to primarily affect signaling molecules that play roles in programmed cell death and regulate how the body responds to stress," said Lebsack.
Programmed cell death plays an important role in a functioning body, for example in the disposal of cells that are no longer needed or damaged beyond repair.
However, cell death must be tightly regulated in the immune system to ensure the process does not get out of hand.
"Many of the genes whose activity was down-regulated in the space-flown mice play important roles in maintaining that balance. Potentially, you could get more cell death aboard a spacecraft because of these differences," said Lebsack.
The results relate with experiments carried out on the ground to study how microgravity affects immune cells.
In these experiments, scientists mimicked weightlessness using clinostats - apparatuses that slowly rotate the study object so the Earth's gravitational pull is never perceived as coming from one consistent direction.
"Previous studies with cell cultures in clinostats showed increased cell death in T-cells when you take away the gravity stimulus, so it was a logical step to test whether we find the same effects in animals exposed to an actual lack of gravity," said Lebsack.
"We observed an overall pattern about the genes whose expression was changed by space flight: All of them are involved, in one way or another, in the development, control and programmed cell death of immune cells," he added.
This study represents the first use of microarray technology to investigate gene expression in thymus tissue of space-flown mice, said the authors.
Complex research undertakings like this require specialists combining their different areas of expertise.