The discovery, presented at the 12th annual Conference on Retrovirus and Opportunistic Infections in Boston, and simultaneously published in newest issue of the British journal Nature, shows how HIV mutates its form, in turn provoking changes which permit it to enter cells.

The study was done at the Boston Children's Hospital and Harvard Medical School by Stephen Harrison, head of a research team at the Howard Hughes Medical Institute.

The scientists were able to obtain a three-dimensional image of the protein gp120, an element of the HIV membrane, before it metamorphoses itself and attaches to a cell's CD4 receptors.

Once the protein attaches to the receptors, HIV is able to penetrate the cell's interior and reproduce, explained one of the scientists.

The scientists said understanding gp120's form alterations before the attack on cells could lead to the creation of new vaccines to stall HIV infections.

"Knowing how gp120 changes shape is a new route to inhibiting HIV by using compounds that inhibit the shape change," said Harrison.

"The findings also will help us understand why it's so hard to make an HIV vaccine, and will help us start strategizing about new approaches to vaccine development."

In the absence of vaccines to prevent infections, the AIDS pandemic has struck 39 million people around the world. More than three million died in

As with the discovery announced by Harrison, much of the Boston conference focused on the most promising area of AIDS research, vaccine treaments that stimulate the body at the cellular level into defending itself.

The treatments produce antibodies in the blood to neutralize the virus before it penetrates cells, and stimulate an immune reaction against HIV from inside the cell.

In one promising development last year, scientists discovered a protein, TRIM5-alpha, which can block HIV from entering the cell. The protein, whose existence has been long suspected, was discovered during studies of monkeys who could not be infected with the AIDS virus.

Yet another protein, the enzyme APODEC, was shown to interrupt HIV's ability to reproduce genetically once it has entered a cell, effectively preventing its multiplication.

While such cellular vaccines are promising, real prevention against an HIV assault requires stimulating the immune system to produce antibodies capable of hunting and destroying the virus before it enters cells, said Anthony Fauci, director of the US National Institute of Allergies and Infectious Diseases, part of the National Institutes of Health.

"A safe and effective HIV vaccine is critical to the control of HIV globally, and is the most important and diffficult scientific challenge facing AIDS researchers today," said Fauci.

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