Evidence is growing about the many ways that traveling in the microgravity environment of space tampers with the human body, with new research showing how it dials down the activity of genes in white blood cells crucial to the immune system.
A study involving 14 astronauts who spent 4½ to 6½ months aboard the International Space Station found that gene expression in these cells, also called leukocytes, quickly decreased when they reached space and then returned to normal not long after returning to Earth, researchers said Thursday.
The findings offer insight into why astronauts are more susceptible to infections during flights, showing how the body's system for fighting off pathogens is weakened in space.
"A weaker immunity increases the risk of infectious diseases limiting astronauts' ability to perform their very demanding work in space. If an infection or an immune-related condition was to evolve to a severe state requiring medical care, astronauts while in space would have limited access to care and medication," said molecular biologist Odette Laneuville of the University of Ottawa in Canada, lead author of the research published in the journal Frontiers in Immunology.
Leukocytes are produced in the bone marrow and travel through the bloodstream and tissues. Once they detect bodily invaders like a virus or bacterium, they produce antibody proteins to attack the pathogen. Specific genes govern the release of such proteins.
The researchers examined leukocytes isolated in blood drawn from astronauts -- 11 men and three women -- from the Canadian Space Agency and U.S. space agency NASA, once before the flight, four times aboard the space station and five times after returning to Earth.
Gene expression in 247 genes in leukocytes was at about one third the normal levels while in space, the study found. This occurred within the first few days in space, but then remained at a stable level. The genes typically returned to normal behavior within about a month of an astronaut's return to Earth.
"White blood cells are very sensitive to the environment of space. They trade their specialized immune functions to take care of cell maintenance or housekeeping roles. Before this paper, we knew of immune dysfunction but not of the mechanisms," said study co-author Guy Trudel, an Ottawa Hospital rehabilitation medicine specialist.
Discovering altered gene behavior in leukocytes is "a significant step toward understanding human immune dysregulation in space," Trudel added.
This altered behavior, the researchers said, may result from a phenomenon called "fluid shift" in which blood in the absence of Earth's gravitational pull is redistributed from the lower to the upper part of the body. It is unlikely that greater solar radiation exposure in space was the culprit, they added.
"New and specific countermeasures will be needed," Trudel said.
Scientists previously documented astronauts experiencing immune dysfunction in space. This has included reactivation of latent viruses such as: Epstein-Barr, responsible for infectious mononucleosis; varicella-zoster, responsible for shingles; and herpes simplex 1, responsible for cold sores.
It also has been shown that astronauts in space shed more viral particles in their biological fluids -- saliva and urine -- increasing the risk of spreading pathogens to other astronauts whose own immune systems may be weakened.
The study, funded by the Canadian Space Agency, follows NASA-funded research published June 8 that detailed brain changes in astronauts -- expansion of spaces in the brain containing fluid that cushions it to protect against sudden impact and remove waste products.
Other documented effects of space travel include bone and muscle atrophy, cardiovascular changes, issues with the balance system in the inner ear and a syndrome involving the eyes.
Cancer risk from greater radiation exposure is another concern.
