Monkey Trial May Show Possible Way to AIDS Vaccine

A new study of monkeys vaccinated against simian immunodeficiency virus (SIV) -- considered the closest model of human infection -- may provide important clues about how HIV destroys the immune system and how to track the health of infected people, researchers said Thursday.

"A vaccine of this type does not appear to prevent infection," said Dr. Norman Letvin of Harvard Medical School. But what the vaccine may do is help infected people live longer without becoming sick.

Most vaccines stimulate the body to produce antibodies that in turn create an immune response against a particular virus or bacteria. However, this approach does not work for HIV. Scientists believe a second type of immune response, a cell-mediated immune response, is required to fight HIV. In the current study, Letvin and colleagues tested a vaccine that triggers a strong immune response by those T cells.

Monkeys who received the SIV vaccine lived much longer when they were later infected, up to 900 days, compared to unvaccinated monkeys who died on average within 300 days, Letvin and colleagues reported.

"The magnitude of the immune response that is generated by vaccination predicts how long the animals will live after infection," said Letvin. "The more potent the immune response after injection, the longer the monkey lived."

Letvin said counter to many assumptions among AIDS experts, the amount of virus in the blood, or viral load, was not especially important. "What is useful [to measure] is the subpopulation of helper CD4 T cells -- the central memory CD4 T lymphocyte population. This tells us something profoundly important about why AIDS progresses clinically -- the preservation of this central memory population of CD4 helper lymphocytes appears to be absolutely crucial for maintaining immunological competence," he said.

Currently, more than 30 different vaccines are in various stages of human testing. "There are two human vaccines that are similar to this that are now going forward into advanced efficacy trials," said Letvin.

The study, "Preserved CD4+ Central Memory T Cells and Survival in Vaccinated SIV-Challenged Monkeys," was published in Science (2006;312)5779):1530-1533).

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