Slight differences in five amino acids in a protein called HLA-B may explain why certain people resist the human immunodeficiency virus, U.S. researchers said on Thursday in a study that lends new clues about how to make a vaccine to prevent AIDS.

“For a long time, we’ve known that some people progress extremely rapidly when they get infected, and others can stay well for three decades and never need treatment and still look entirely well,” said Dr. Bruce Walker of Massachusetts General Hospital and Harvard University, whose study appears in the journal Science.

“We thought we could apply new techniques from the human genome project to understand what the genetic basis was for that,” he said.

About one in 300 people infected with HIV can suppress the virus with the immune system, keeping the virus at extremely low levels.

The team searched the genetic makeup of nearly 1,000 people with that ability and compared it with the genetic code of 2,600 others who were infected with HIV.

That helped them identify some 300 different sites in the genetic code that were linked with immune control of HIV, all located on chromosome 6.

They narrowed that down to four single-letter changes in the DNA, known as single nucleotide polymorphisms or SNPs - pronounced “snips” - all related to the immune system.

“We did a second study where we looked amino acid by amino acid in that region,” Walker said.

They found five amino acids in the HLA-B protein linked with differences in a person’s ability to control HIV.

That protein is important for helping the immune system tag and destroy cells infected by a virus, and Walker said those genetic variants could make a big difference in a person’s ability to control HIV.

Knowing how some people mount an effective immune response to HIV could be an important step in understanding how to make a vaccine to fight the virus.

It was not a vaccine yet, Walker cautions, but it is promising.

“We’ve got a clearer indication of why people can survive in the face of HIV, and we’ve gotten more focused in terms of the research we need to do to get where we’ve got to go,” he said.

No vaccine exists against the human immunodeficiency virus that causes AIDS. Since the AIDS pandemic started in the early 1980s, almost 60 million people have been infected with HIV, many of them in Africa, and it has killed 25 million.

In September 2009, scientists reported their biggest success yet with an experimental vaccine that showed a modest effect and appeared to slow the rate of infection by about 30 percent. In July, U.S. researchers found antibodies that can protect against a wide range of AIDS viruses and said they may be able to use them to design a vaccine.

(Editing by Peter Cooney)
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By Julie Steenhuysen

CHICAGO