NEW EVIDENCE DETAILS GENETIC IMMUNITY TO HIV.

Byline: Gina Kolata The New York Times

Scientists have long suspected that some people might be immune to the AIDS virus. But now they are accumulating powerful, direct evidence of the extent and strength of such immunity. And many researchers are stunned.

The most recent study, published today in the journal Science, provides the most detailed and convincing evidence yet that one in 100 whites have complete immunity to infection by the AIDS virus and one in five whites have inherited a resistance to the progress of AIDS once infected with the virus.

Almost no African-Americans have the particular protective mutation investigated in the study, but scientists say that other forms of genetic immunity almost certainly exist in both African-Americans and whites.

The findings raise numerous questions and offer remarkable opportunities for research, scientists say. Among the puzzles are why some ethnic groups have such a mutation and others do not, how and when this mutation arose and what other genetic variants exist that protect against AIDS.

One of the study's surprises was that of 1,850 people, all of whom were at risk for infection, about 600 never became infected with HIV, the virus that causes AIDS. Since only 17 of them had the mutation identified as prevalent in whites, presumably the other people had some other form of genetic protection.

``The number that's impressive is the one in five'' who are resistant to the AIDS virus, said Dr. Michael Kaback, a geneticist at the University of California, San Diego. ``Now you're getting up into the frequencies of blood group types or genes for eye colors,'' Kaback said.

A double dose of the mutated gene, called CKR5, confers complete immunity to AIDS, the researchers said. People who have inherited a single dose of the mutated gene can become infected with HIV, but AIDS progresses more slowly and they live on average three years longer than people who do not have the mutated gene.

In addition, the researchers who published in Science, led by Dr. Stephen O'Brien of the National Cancer Institute's research center in Frederick, Md., found that much more genetic resistance remained to be understood. Although African-Americans, for example, virtually never have the mutated gene, they may have other genes that confer resistance to AIDS.

The 1,850 people in the study had been repeatedly exposed to HIV because they were gay men who had sex without condoms, intravenous drug users who shared needles, or hemophiliacs who injected themselves repeatedly with tainted blood products at the beginning of the AIDS epidemic. Since about 600 people in the study never became infected with HIV, the researchers report, the CKR5 mutation, as prevalent as it is, accounted for only 3 percent of the resistance.

The discoveries are making researchers ask why genes that confer resistance to AIDS are so common. Usually a mutation that knocks out a gene, as the CKR5 mutation does, would disappear from a population.