WASHINGTON: Embryonic stem cells, the controversial and versatile cells that seem able to do just about anything, have now expanded their repertoire into cancer prevention.

A vaccine made from these cells shields mice against developing lung cancer under conditions thought to mimic effects of smoking.

Safety concerns about injecting stem cells into humans mean that regulatory agencies are unlikely to approve human tests of the vaccine, says lead researcher John Eaton at the University of Louisville in Kentucky.

Nevertheless, he thinks the vaccine is worth testing in people at high risk of developing cancer, such as heavy smokers or people with certain genetic mutations.

Other researchers are more cautious. Cancer vaccines, particularly vaccines made from cells, are notoriously more effective in mice than people, says Jeffrey Weber, an immunotherapist at the University of Southern California in Los Angel.

The idea is interesting, but the execution may be impossible," he says. But both Weber and Eaton agree that the finding could lead to new ways to prevent or treat cancer. Eaton's approach was inspired by the similarities between embryos, embryonic stem cells and tumours.

Embryos and tumours both grow as balls, they derive nutrients from the host, and they both express peculiar proteins-some of them in common," he says.

These shared proteins made Eaton think that a vaccine prompting an immune response to embryonic stem cells would also trigger an attack against tumours.

He and his colleagues injected mice with stem cells and gave the mice a booster shot ten days later. The researchers then transplanted lung cancer cells under the animals' skin a standard animal model for the disease.

The stem-cell injection protected 20 out of 25 mice from developing tumours, whereas tumours grew in all unvaccinated mice.