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AIDS Research Reaches Turning Point

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Oct. 19, 2000 -- A wave of excitement is raising hopes for an AIDS vaccine. Using sophisticated new research tools, scientists have shown that vaccines that don't prevent HIV infection can keep HIV from causing AIDS -- at least in monkeys.

A research team led by Harvard's Norman L. Letvin, MD, gave rhesus monkeys a man-made, immune-boosting protein to boost the effects of an experimental AIDS vaccine. The vaccine is made of DNA from a hybrid monkey/human form of HIV. Normally, this virus is so deadly that animals die of AIDS soon after infection. The immune-boosted vaccine didn't prevent infection -- but the vaccinated animals remained perfectly healthy with no signs of AIDS. The virus stayed in their systems but was kept at very low levels by powerful immune responses.

The results greatly impress Rafi Ahmed, PhD, director of the Emory University Vaccine Center. "I see this as a turning point in our quest to make an AIDS vaccine," says Ahmed, who was not directly involved in the experiments. "Until now, the feeling was [that] it would be difficult to find something protective. The data are spectacular. ... It is a major finding in terms of our perceptions of whether we can control the disease or not. I see this as a landmark paper."

Letvin tells WebMD that the vaccine used in the monkey studies is not intended to be used in humans. "The question is: What is within our grasp with the technologies we have available?" he says. "This is a ... vaccine to see what is possible."

Nobody knows how to make a vaccine that keeps people from becoming infected with HIV. But existing technology should be able to give humans the same kind of immune responses that protected the monkeys. This kind of immune response activates killer blood cells -- called cytotoxic lymphocytes, or CTLs -- that are targeted specifically at HIV. After immunization, some of these cells remain in the blood. When they encounter HIV, the memory cells signal the body to quickly make huge numbers of new, virus-targeted CTLs to attack the virus.

"I don't think the lesson is that this is the only vaccine regimen that will work -- what this shows is that if you have a certain level of memory cells [before infection], there seems to be a good outcome," Ahmed says. "I think several candidate vaccines now in the pipeline may very well be able to demonstrate this kind of CTL response in humans."

In a commentary article accompanying the study, Johns Hopkins researcher Robert F. Siliciano, MD, PhD, suggests that it might be possible to elicit this kind of immune response in HIV-infected people receiving the AIDS drug combinations known as highly active antiretroviral therapy, or HAART.

"I think this is a very exciting demonstration that vaccines that ... fail to protect against HIV infection might still have a huge beneficial effect," Siliciano tells WebMD. "This would be helpful both in terms of preventing people from getting sick and in reducing transmission by holding down the amount of virus in the blood."

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