HTLV-III virus: themes and variations.

HTLV-III, the virus that causes AIDS, consists of a whole spectrum of closely related but genetically distinct viruses, reports a team of researchers from the National Cancer Institute in Bethesda, Md., Litton Bionetics Inc. in Kensington, Md., and the Walter Reed Army Institute of Research in Washington, D.C., in the Aug. 23 SCIENCE.

The researchers isolated the AIDS-related virus from the blood of one healthy homosexual and nine patients who had either AIDS or the AIDS-related complex (ARC) -- which has some but not all AIDS characteristics -- and from lymph and brain tissue from eight deceased AIDS or ARC patients. Although all of the virus isolates had the same basic structure, no two were identical, and some varied considerably from the others. "The way we see the virus now is that there aren't strains -- A, B and C -- but rather a continuum of virus isolates," National Cancer Institute researcher Robert Gallo told SCIENCE NEWS.

None of the different types of virus could be associated with whether the patient had AIDS or ARC or was healthy. However, Gallo suggests that genetic differences in the virus may explain why different AIDS patients have such different sets of symptoms.

Two of the 18 patients were infected with more than one form of the virus, leading researchers to wonder whether the virus had infected the two patients more than once, or whether the virus changes while in the body.

Because none of the other patients in the group had such multiple infections, in spite of presumed ample exposure to other forms of the virus, the researchers suggest that one form of the virus tends to become dominant and somehow interferes with infections by other forms. However, the rarity of multiple infections might be only an artifact of in vitro culturing, the researchers say.

In culture, the HTLV-III virus doesn't change much, so many of the genetic changes in the virus probably occur when the viral DNA is transcribed into DNA in the body, says Gallo.

Whether the virus's genetic diversity will affect the difficulty of developing an AIDS vaccine in unknown.

Meanwhile, the virus has been discovered in the tears of an AIDS patient. "I don't think that tears are a major mode of transmission," says Gallo. "But this tells us that the virus is in places where we didn't know it could exist." The virus, which has been found in blood, lymph nodes, semen, saliva and now tears, is generally thought to replicate almost exclusively in the T4 white blood cells. Now Gallo says he thinks the virus is replicating somewhere else -- exactly where, he says, will be revealed in a research report to be published in LANCET.