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Dec. 16 —

A part of the AIDS virus that was considered vulnerable to attack can camouflage itself by changing shapes, says a study that helps show why HIV is so hard to target and kill.

HIV cripples the immune system by infecting and killing T-cells. It uses a protein structure on its surface called gp120 to gain entry to the cells.

In 1998, scientists announced that they had figured out much about the structure of gp120 and hoped that finding a vulnerability in it could lead to vaccines against HIV.

But finding gp120's weakness has remained elusive, in part because the protein varies from strain to strain. Some scientists believed that the best hope was in targeting an area of gp120 common to all strains a vulnerable region where it must expose its core in order to bind to a T-cell.

But the new work shows that this region is more elusive than previously thought, because it is composed of very flexible parts that let it take on different shapes.

That camouflages gp120 against the blood proteins called antibodies launched by the immune system as a defense, researchers report in the Dec. 12 issue of the journal Nature.

Antibodies that do manage to latch on to the protein are less potent at killing the virus, possibly because of the trouble it takes to fix gp120 rigidly, said Joseph Sodroski of the Dana-Farber Cancer Institute in Boston, one of the researchers.

"There's a lot of mobility within the protein. It's a blurry, moving structure that is very difficult for the immune system to deal with," he said.

The scientists made the discovery by studying 20 different antibodies, measuring their interactions with gp120.

Another study author, Peter Kwong of the National Institutes of Health, said the finding doesn't rule out the possibility of finding an AIDS vaccine. He said there may be ways to overcome the mobile nature of gp120. Exactly how remains uncertain.

Theodore Jardetzky, a Northwestern University molecular biologist not connected with the study, called the findings surprising.

"How big a deal this is, we're going to have to wait and see. They've pointed us in a new direction," he said.

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