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Cells can stop HIV once it gets inside.

One of the more puzzling mysteries in AIDS research is why some people exposed frequently to HIV remain uninfected by the virus. A few answers have begun to emerge. Some of these individuals have mutations in a gene that encodes a cell surface protein commonly used by HIV to infect human immune cells (SN: 8/17/96, p. 103). The mutations apparently alter this receptor protein in a manner that prevents the virus from using it to sneak into cells. In essence, people with these mutations lock the virus out of their immune cells.

What protects other exposed, yet uninfected, people? "The majority remain uninfected for unknown reasons," says Miles W. Cloyd of the University of Texas Medical Branch in Galveston. To address this mystery, Cloyd and his colleagues have been drawing blood from volunteers and testing how easily various HIV strains infect the immune cells in these blood samples. In 1991, the researchers reported findings from a study of a dozen people, some of whose immune cells resist infection by certain strains of the AIDS virus.

While Cloyd acknowledges that the relevance of this observation to real-life HIV resistance remains unclear, his group has since extended the study to 50 randomly chosen people. The investigators found that for some strains of HIV, up to 15 percent of the group has immune cells that are resistant to infection. The protection isn't complete, however. If the cells are exposed to enough virus, they'll succumb. Still, they seem to fend off HIV by a different mechanism than the receptor mutations described last year. The resistance develops after the virus has infected a cell, Cloyd asserts.

HIV carries its genetic material in the form of the nucleic acid RNA. Once inside a cell, the virus copies its RNA into DNA and inserts this DNA into the chromosomes of its host. The virus then takes over the cell and forces it to produce copies of HIV. Yet despite evidence that the virus has copied its RNA into DNA, the resistant cells studied by Cloyd do not churn out new viruses.

Cloyd suggests that the resistant cells somehow prevent HIV from integrating its DNA into host chromosomes. Another possibility, he says, is that the virus does insert its genes but that the cells do not turn those genes on, thus preventing HIV from copying itself.
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Title Annotation:Biology; immunity to HIV in some people due to some unknown intercellular mechanism
Author:Travis, John
Publication:Science News
Article Type:Brief Article
Date:Mar 8, 1997
Previous Article:Toying with a molecular enzyme.
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