CHICAGO, June 25, 2007 (AFP) - A popular theory on how HIV attacks the body's immune system is wrong, a new study has found.

Scientists have long believed that HIV causes the slow depletion of healthy white blood cells -- the T cells which recognize infections so the body can fight them off -- by causing infected T cells to produce virus particles before dying.

This ongoing cycle of infection, HIV production, reinfection and cell destruction has been called the "runaway" hypothesis.

But if this were so, the T cells would be killed off far too quickly, the researchers found.

Using a simple mathematical model, researchers in the United States and Britain showed the "runaway" model would deplete the body's healthy T cells in a matter of months, instead of the years it actually takes.

The results show that a "slow process must be active" in the depletion of the T cells, the authors wrote in the current issue of the journal PLoS Medicine.

Identifying this process "will provide a key insight into the nature of HIV disease and indicate potential new approaches to therapy," they concluded.

A potential explanation for the slow process could be that the virus slowly adapts over the course of the infection, the authors said.

"The virus is constantly mutating and there may be selection - in a Darwinian sense -- over time for 'fitter' mutants of the virus in an infected person," said lead author Andrew Yates of Emory University in Atlanta, Georgia.

"There are several ways the virus could become fitter - for example by being able to infect new cells more readily, or by infected cells producing more new virus, or producing it more rapidly," he said in an e-mail interview.

"The virus adaptation hypothesis requires a lot more experimental investigation, however, and is only a tentative conclusion."

Reference:

Andrew Yates, Jaroslav Stark, Nigel Klein, et al., “Understanding the Slow Depletion of Memory CD4þ T Cells in HIV Infection”, PLoS Med 4(5): e177.doi:10.1371/journal.pmed.0040177.

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