Scientists say they have disproved a longstanding theory as to why AIDS victims' immune systems are destroyed so slowly - a finding that could lead to new treatments.

The HIV virus that causes the disease kills T helper cells - a type of white blood cell that can identify and destroy invading bugs.

It often takes at least 10 years for HIV to destroy these immune system cells in patients - believed to have been due to a vicious circle whereby the virus triggered and destroyed new cells at similar rates.

But a study in the journal PLoS Medicine shows this so-called "runaway theory" cannot explain the very slow pace of depletion that occurs in HIV infection.

Using a mathematical model researchers showed if the idea was correct then the T helper cell numbers would fall to very low levels over a number of months - not years.

Mathematical biologist Professor Jaroslav Stark, of Imperial College London, said: "Scientists have never had a full understanding of the processes by which T helper cells are depleted in HIV - and therefore they've been unable to fully explain why HIV destroys the body's supply of these cells at such a slow rate.

"Our new inter-disciplinary research has thrown serious doubt on one popular theory of how HIV affects these cells - and means further studies are required to understand the mechanism behind HIV's distinctive slow process of cellular destruction."

The researchers say one possible explanation could be the virus slowly adapts itself over the course of the infection but stress further analysis is needed to verify this alternative theory.

Prof Stark said: "If the specific process by which HIV depletes this kind of white blood cells can be identified it could pave the way for potential new approaches to treatment."

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