A STANDARD APPROACH TO MAKING A VACCINE is to select a key substance from the outside of a virus or bacterium, then use it to prod an uninfected body into making antibodies that can neutralize any invading strain of the bug. But when scientists tried this approach on HIV, early in the AIDS epidemic, it failed. They synthesized parts of HIV's envelope glycoproteins, called gp120, which did indeed stimulate antibodies. But these were no match for the AIDS virus. A key reason, researchers learned, is that gp120 changes shape as the virus fuses with the cells it infects--and the antibodies are unable to block it.

But what if antibodies could be generated against gp120's new shape? A team of scientists at the University of Montana and New York University conducted an experiment, reported in the Jan. 15 issue of Science, in which they managed to capture the glycoprotein in the act of fusing with cell receptors and hold it in the new shape before injecting it into mice. "Perhaps to the surprise of all involved, this ambitious experiment worked," write AIDS experts David C. Montefiori and John P. Moore in an accompanying commentary. Confronted with a new protein shape that offers a more vulnerable attack site, the mice's immune systems made antibodies capable of neutralizing a wide variety of HIV strains.

Still, scientists caution that a truly effective vaccine may also have to stimulate the production of killer T-cells capable of killing HIV-infected cells, because the antibodies alone may not be enough.

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