London: Scientists have identified stem cells which rapidly rebuild alveoli, the tiny air sacs in lungs; a major breakthrough which they claim may soon pave the way for new treatments for people with damaged lungs.

An international team, led by the Genome Institute of Singapore, says that the discovery of the stem cells would offer new hope for heavy smokers, people with asthma and those with chronic lung scarring.

The hitherto unknown stem cells were identified after the scientists infected rodents with a variant of H1N1 almost identical to the one that caused the Spanish flu pandemic of 1918. Another variant of H1N1 caused 2009 swine flu pandemic.

Other lung stem cells have been studied already, but they were grown from embryonic stem cells. The new cells occur naturally within the lungs, an agency reported.

Samples taken from the mice showed that straight after infection the virus destroyed over half the original tissue in alveoli -- the sacs in lungs vital for absorbing oxygen from inhaled air.

Just three months later, however, all the tissue had naturally repaired itself, thanks mainly to the stem cells.

"We saw essentially pristine lungs at three months after a loss of 50 per cent of lung tissue," said Frank McKeon, who led the team. The stem cells multiplied rapidly, creating hundreds of times their original number within a week. Then they migrated to sites of damage where they formed pod-like structures as a prelude to becoming new air sacs, say the scientists.

They isolated similar cells from human lung tissue and found that they form the same pod-like structures.

The scientists say that they have probably not been discovered before because the damage caused by the flu virus is more amenable to repair than that caused by bleomycin, a drug used to deliberately damage mouse lungs in experiments.

They claim the best hope for treating damaged may be through identification of the key signalling molecules that order the cells to multiply and migrate to sites of damage.

The team is now screening potential growth factors in fluid from repair sites in the mouse lungs to work out which do the job. "There are probably 20 to 30 of these factors that we're looking at," McKeon said.

Injecting the right growth factor could aid healing in people with both acute and chronic lung damage. "It's too early to say common lung diseases will be treatable, but it's a start, and there's a lot of potential," he added.