Weightlessness may sound relaxing, but a new study shows its effects may be stressful for organs that create armies of disease-fighting cells.

Scientists conducted an experiment with mice that simulated zero-gravity on the ground and showed that a protein called osteopontin (OPN), a stress hormone connected with bone loss in space, may also be connected with the dangerous wasting of the spleen and thymus organs.

These immune system organs create white bloods cells that battle infections--without them, the body would be open season for disease.

"We didn't have any reason to think osteopontin would have any effect on immune organ damage," said David Denhardt, a cell biologist at Rutgers University in New Jersey. "But when we did [the experiement], we were surprised to see that it is involved in this stress response."

Denhardt and his colleagues' work will be detailed in the Sept. 11 issue of the journal Proceedings of the National Academy of Sciences.

Wasting away

Denhardt explained that osteopontin is an instigator, signaling cells to push for survival and stay on the offensive during injury or stress.

Although Denhardt isn't uncertain how the process works, his team found that lifting up mice's hind legs--a stressful simulation of weightlessness--for three days caused about a 70 percent reduction in spleen and thymus tissue, compared to normal mice. The breaking down of organ tissue, called atrophy, also occurred in mice that were stressed out due to isolation.

"The atrophy was dramatic. It appears as if the cells simple destroy themselves, which contradicts OPN's known role of keeping cells alive," Denhardt told SPACE.com. In spite of the contradiction, he explained there is a surprising connection. When his team performed the same experiment on mice bred with an inability to produce osteopontin, they showed far less dramatic thymus and spleen tissue loss.

"We think osteopontin is controlling a class of hormones which suppress the immune system," he said. When osteopontin isn't around to control the hormones, immune tissue carries on as normal.

Show me the money

While mice aren't substitutes for astronauts in space, Denhardt explained that the research may eventually cut down the increased risk of getting sick in space--especially during long-term excursions to the moon and eventually Mars.

"Osteopontin is somehow important in permitting increased susceptibility to immune problems and bone loss," Denhardt said. "It's a long shot, but if we find an antibody able to lock up osteopontin, then we could reduce its impact on many microgravity related health problems."

Denhardt imagines astronauts receiving an injection of such antibodies before rocketing into space. He and his team have already isolated a group of potential osteopontin-silencing antibodies, but he said money available for bringing the research into the realm of medicine is shockingly low.

"Funding is a big problem," Denhardt said. "But the more we can get, the faster this will move forward."

In the meantime, Denhardt and his team are trying to piece together the mystery of how osteopontin causes immune organ atrophy.