By Marc Kaufman
Washington Post Staff Writer
Monday, December 10, 2007
With NASA now actively planning for the day when astronauts will live for months on the moon or make the years-long flight to Mars and back, a potentially troublesome question is being raised with increasing urgency: Is the human body -- even a well-protected human body -- capable of living in space for long periods without suffering serious damage?
That question has focused for some time on concerns about exposures to cosmic and solar radiation, as well as the loss of bone strength and muscle tone in weightlessness. But researchers are also coming up against another more surprising physical risk for future long-haul space travelers -- their immune systems appear to become less capable in space, leaving them more susceptible to stowaway bacteria and viruses.
At the same time, researchers studying the microbes that could infect astronauts recently found that at least one, salmonella, becomes significantly more virulent in weightlessness.
"The question of immunity is a potentially big problem for astronauts on long trips and those who may be living on the moon in the future," said Millie Hughes-Fulford, a former astronaut who is researching the effects of "microgravity" on immunity. Her NASA-supported research has led her to conclude that weightlessness itself is a major cause of the problem.
"Human beings evolved in gravity, and it makes perfect sense that some systems -- especially the immune and skeletal systems -- might not do well without it," she said.
Duane Pierson, senior microbiologist at NASA's Johnson Space Center, said he also fears immunity problems in the future, although they have been contained so far.
"Even though astronauts are not now getting sick on their missions, we see very clearly statistically significant and reproducible change in immune functioning after two weeks in space," he said. "We don't need a crash program or anything like that, but many of us feel this issue is very much on the table because of long-duration flight."
NASA learned that the immune systems of astronauts in space become temporarily less robust more than three decades ago during the Apollo program -- when all three members of one crew got colds at the same time, and an Apollo 13 crew member came down with severe chills from a bug that only affects those with suppressed immunity. The solution of the time was to isolate astronauts before launch to keep germs at bay.
Research into the phenomenon of decreased immunity was not a particularly high priority because, for the most part, astronauts and cosmonauts were not getting seriously sick. The combination of their good health, the clean -- though not sterile -- environments in which they lived, and their relatively short stays in space apparently were enough to keep harmful infections from exploiting their immune deficiencies.
But with longer stays in space becoming more common at the international space station, and scheduled to lengthen substantially in a few years, researchers have been stepping up studies into how and why astronauts' immunity falters and how to find ways to better protect space travelers.
Pierson now focuses on the well-documented reactivation during space missions of dormant viruses, including herpes and chicken pox -- a clear result of lowered immunity. Others are studying immune reactions in both human and non-human subjects to answer what remains the central question: Why does spaceflight reduce immunity, and by what mechanisms?
Hughes-Fulford, now at the University of California at San Francisco, has been studying human blood cells that have been sent into space, and she has found that some genes that turn immune system cells on and off do not work in weightlessness as they do on Earth. The result is that the body cannot deploy some important infection-fighters.
Deborah Kimbrell of University of California at Davis is lead investigator for a NASA-supported experiment involving fruit flies, whose immune systems operate in ways that are surprisingly similar to those of humans. Her team sent 10 vials of fruit flies on a recent shuttle mission, along with fungus and bacteria to later infect them, to test their immunity.
The research is not complete, but Kimbrell said that as with humans, the flies lost some of their ability to fight off infection after returning to Earth. She, too, believes that weightlessness itself plays a major role in disrupting the immune response.
The human immune system, which Pierson said is as complex as the central nervous system, includes an arsenal of responses to invading bacteria, viruses and other pathogens -- ranging from first responders such as neutrophils and microphages to more sophisticated T-cells and B-cells that act on a molecular level. Although research into the workings of these infection-fighters during space missions remains limited -- because of technical problems with collecting samples in space, very limited crew time and, more recently, deep funding cuts -- Pierson said many key infection-fighting molecules appear to be less available to people in space.
A NASA study published in 2005, for instance, found that neutrophil function in blood samples of 25 astronauts decreased significantly, in some cases by as much as 60 percent below preflight levels. Other research found that virus-specific antibodies increased significantly during flight, indicating that viruses were active and spreading.
Pierson and others say this appears to be caused by a number of factors -- including stress (known to decrease immunity), the fact that weightlessness changes the basic dynamics of cells, and the still-undetermined effects of increased exposure to radiation. Researchers said there have been anecdotal reports of increased fever blisters (a form of herpes), other minor outbreaks and colds.
But astronauts appear to have had sufficient immune reserves to avoid serious illness so far, and no missions have been compromised or cut short as a result, Pierson said.
Adding to the challenge is a recent finding that salmonella bacteria, a common food-borne pathogen, become more virulent in space. Researchers are now studying whether this phenomenon -- which also involves changes in how genes are expressed -- is specific to salmonella or occurs with other bacteria.
"Microbes do better and the host does worse," said Kimbrell. "A perfect, but unfortunate fit."