Researchers said on Monday that they had found a crucial difference between a gene in humans and one in rhesus monkeys that blocks infection of the virus in the animals.

The finding offers new insights into the origins of Aids and gene therapy.

Had the gene been the same in humans, scientists at the National Institute of Medical Research in London believe, there may not have been the Aids epidemic that now affects 40 million people worldwide.

"If it had recognised HIV, we probably would never have had Aids. I believe it is a key change," said Dr Jonathan Stoye, head of virology at the institute.

Science behind discovery

Scientists had been aware that it was much more difficult to infect monkey cells with HIV than human cells in laboratory experiments, which suggested there was something different in the animal cells that blocked infection.

A gene called Trim 5 alpha was later found to be the reason why. In monkeys, but not in humans, it stops the virus from replicating.

Stoye and his team studied differences in the gene products of the monkey and human Trim 5 alpha genes.

They pinpointed one specific change in a protein that was important in blocking HIV. By substituting a human protein with a monkey-derived protein, they found they could make the human cells resistant to HIV.

"The discovery has significant implications for the development of effective gene therapy to combat Aids," Stoye, who reported the findings in the journal Current Biology, said.

He and his team believe introducing the gene carrying that single change back into human cells would make those cells resistant to HIV infection.

What now?

Gene therapy is an experimental technique which introduces genetic material into cells to fight diseases. The technique has been used for Parkinson's disease and in trials for illnesses such as cystic fibrosis and cancer.

Two French boys with severe immune deficiency were cured with gene therapy but they later developed leukaemia. 

"If you have HIV this might be a form of therapy to prevent progression to Aids. What we are suggesting one might try doing is to purify HIV negative cells out of a patient who has been infected, introduce the gene with this one modification and then put them back into people," said Stoye.

The resistant cells might be sufficient to prevent the progression to Aids, he added.

But Stoye stressed that more laboratory research, followed by animal tests and human trials will be needed.

"If this were the equivalent of adding a drug with a new target to the repertoire of drugs that we have for the treatment of Aids, I think it would be of value."