Important note: Information in this article was accurate in 1997. The state of the art may have changed since the publication date.

Reuters NewMedia, Inc. - Thursday November 20, 8:43 pm Eastern Time
Maggie Fox, Health and Science Correspondent

They said the method might be used to ferret out some of the last reserves of the virus that lurk in immune system cells even after years of therapy with powerful drugs.

The new approach uses HIV's own tactics against it.

It all boils down to the receptors, or chemical doorways, the virus uses to infect cells, Michael Endres and colleagues at the University of Pennsylvania in Philadelphia said.

When it invades immune system cells, HIV very sneakily uses the receptors that are supposed to be used by immune signaling chemicals known as chemokines.

Once it hooks onto a cell using one of these receptors, the HIV virus injects its own genetic material, forcing the cell to crank out more virus as it replicates itself.

Endres and colleagues used this strategy in a gene-therapy approach.

They disabled an HIV virus so it could not reproduce on its own, and equipped it with genes coding for the right receptors as well as a "marker" gene that let them see if the virus entered the cell.

The virus homed in on infected cells and pumped the marker gene into them, they reported in the journal Science.

"It's basically a gene delivery system that targets infected cells specifically (and) enables to us to deliver genes only to HIV-infected cells," Endres said in a telephone interview.

HIV needs to use at least two of several different receptors to get into cells. Endres's team equipped their virus with the gene for the CD4 receptor and either CCR5 or CXCR4 -- both chemokine receptors used by HIV.

The cells they targeted started expressing, or producing, these receptors. What especially pleased Endres's group was that their system worked on macrophages -- the immune system cells suspected of harboring the last vestiges of HIV infection that remain after years of drug cocktail therapy.

In September, another team of researchers did something similar using a rabies virus. "What we have done that is a little bit different is that rather than using rabies we have an HIV vector itself," Endres said.

The next step would be to incorporate genes that would help kill off HIV. "This would be used with existing anti-viral genes," Endres said. "The hope would be you could only selectively kill cells that are infected with the HIV virus."

There are several possible approaches and Endres said his team would start testing one soon, although he declined to give details.

He said the system depended on viral proteins appearing on the surface of infected cells. Sometimes an infected cell may not express viral proteins on its surface -- as with memory T-cells. The method would not work in these if this was the case, Endres said.

A second study published in Science pointed to another possible way to improve AIDS drugs.

Norma Gerard and colleagues at Harvard Medical School in Boston found that very minor changes in the cell surface receptor through which HIV enters cells can have a major impact on whether infection succeeds.

They said their findings could help in the design of highly specific AIDS drugs.

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