(The Associated Press) -- Scientists say they've discovered how some monkeys resist infection with the AIDS virus, a finding that might lead to a treatment that blocks HIV in people.

Researchers found that once HIV enters monkey cells, it encounters a protein that stifles its attempts to replicate. That stops the virus from spreading in the animal.

"This is really important because it will help build a basis for hammering the virus before it gets started," said Paul Luciw, a University of California at Davis microbiologist who specializes in AIDS research.

The protein, called TRIM5-alpha, was identified in rhesus macaques by a team of Harvard researchers at the Dana-Farber Cancer Institute in Boston. It's not clear exactly how the protein acts again HIV, said Dr. Joseph Sodroski, who led the Harvard study published Thursday in the journal Nature.

Humans have their own version of TRIM5-alpha, but it's not as effective as the monkey version in countering HIV. However, researchers may be able to design a drug that makes it work better, Sodroski said.

"We expect that now that we've identified this protein factor, it is likely we'll find ways to manipulate it and increase its potency," Sodroski said, "and we hope to stimulate our own natural resistance to HIV by doing so."

The mechanism may even work against other viruses, he said.

"What we're really uncovering is the first example of a natural system of defense that may be operating against other viruses besides HIV," Sodroski said. "We're looking at 'example one' here, and I highly doubt it will be the only example in nature."

Normally, HIV enters a cell and hijacks its "factory" for making proteins. It orders the machinery to churn out proteins to make new copies of HIV, which then leave to infect other cells.

The monkey protein blocks this process, apparently by interfering with HIV's attempts to remove the coating that surrounds its genetic material, researchers said. This coating must be shed before HIV can insert its genetic material into the cell's own DNA, a key step in replication.

The Nature study could help researchers test the effectiveness of a potential HIV vaccine on animals now that scientists have a better understanding of why monkeys resist the disease, said Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases.

Little is known about the family of proteins that includes TRIM5-alpha, said Stephen Goff, a Columbia University biochemist.

But the Harvard study suggests there are probably many virus-resistant proteins "that act at different stages in the viral life cycle, and these are only in the last few years coming to light," Goff said.

"Gradually now, they're being identified and hopefully they will be understood in terms of how they work," Goff said.

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