London, Aug 27 : While humankind aims for manned missions to the Moon and Mars, a new study showed that T-cells of the immune system can get affected by weightlessness in space and render them ineffective to fight infections.  Space is an extremely hostile environment that poses threats to human health. One such threat is changes to the immune system that occur in astronauts while in space and that persist after their return to Earth, said researchers in the study, published in the journal Science Advances. This immune deficiency can leave them more vulnerable to infection and lead to the reactivation of latent viruses in the body. "If astronauts are to be able to undergo safe space missions, we need to understand how their immune systems are affected and try to find ways to counter harmful changes to it," said Lisa Westerberg, principal researcher at the Department of Microbiology, Tumour and Cell Biology, Karolinska Institutet in Sweden. "We've now been able to investigate what happens to T-cells, which are a key component of the immune system, when exposed to weightless conditions," Westerberg added. In the study, the researchers have tried to simulate weightlessness in space using a method called dry immersion. This involves a custom-made waterbed that tricks the body into thinking it is in a weightless state. The researchers examined T-cells in the blood of eight healthy individuals for three weeks of exposure to simulated weightlessness. Blood analyses were performed before the experiment started, at seven, 14 and 21 days after the start, and at seven days after the experiment ended. They found that the T-cells significantly changed their gene expression -- that is to say, which genes were active and which were not -- after seven and 14 days of weightlessness and that the cells became more immature in their genetic program. The greatest effect was seen after 14 days. "The T cells began to resemble more so-called naive T-cells, which have not yet encountered any intruders. This could mean that they take longer to be activated and thus become less effective at fighting tumour cells and infections. Our results can pave the way for new treatments that reverse these changes to the immune cells' genetic programme," said Carlos Gallardo Dodd, doctoral student at the Institutet's Department of Microbiology, Tumour and Cell Biology. After 21 days, the T-cells had "adapted" their gene expression to weightlessness so that it had almost returned to normal, but analyses carried out seven days after the experiment ended showed that the cells had regained some of the changes. /IANS