Important note: Information in this article was accurate in 1998. The state of the art may have changed since the publication date.

Reuters NewMedia, Inc.; Monday June 29 3:06 PM EDT
Patricia Reaney

The virus invades the body by entering host cells, using the cell's machinery to replicate and produce new viral particles.

It latches on to a receptor site on the surface of the cell -- known as CD4 -- but needs a second, co-receptor site to enter it.

Scientists from the U.S. Centers for Disease Control and Prevention (CDC) have discovered how certain variations of HIV are able to enter cells through multiple co-receptors which may lead to rapid development of AIDS in some patients.

"Basically, instead of having only one door to enter the cell, the virus can adapt and learn how to enter through multiple doors," biologist Renu Lal said during a presentation at the 12th World AIDS Conference.

In early infection, HIV normally invades through the CCR5 co-receptor, which is critical for most sexual transmission of HIV.

In a study that began in 1982, Lal and her team examined blood samples from 80 HIV-positive gay men in various stages of the illness to see how the viral variants affected the progression of the HIV virus.

She used levels of CD4 cells -- the white blood cells which help boost the immune system and are used to determine the severity of the illness -- to divide the men into three categories.

In the first group, men whose disease progressed most rapidly had CD4 cell counts below 200 per microliter within five years of starting the study. The second group included men whose disease progressed more slowly and who had counts that declined gradually. In the final category were men with CD4 cell counts above 500 cells per microliter, who showed no signs of illness for more than 12-14 years.

Lal and her team found viral variants, which enabled entry through multiple co-receptors, in all of the men in the first group but in only half of the second. In the men with no symptoms, the virus used only the CCR5 co-receptor. "There is a correlation between co-receptor adaptation and disease progression," Lal said, adding that a single viral variation is capable of using multiple co-receptors.

But she emphasized there were probably other factors, including genetic variations, that regulate the progression of the illness.

The HIV virus is incredibly complicated and its ability to adapt and enter through multiple doors has important implications. Most interesting for Lal was that in men without any symptoms of illness, who were HIV-positive for more than a decade, the virus still only used the CCR5 co-receptor.

"An understanding of why the virus in these patients is not evolving may provide insight about how to delay disease progression in people infected with HIV," she said.

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Always watch for outdated information. This article first appeared in 1998. This material is designed to support, not replace, the relationship that exists between you and your doctor.

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