AIDS viral burden far exceeds estimates.

AIDS viral burden far exceeds estimates

People with AIDS carry at least 100 times more of the AIDS virus, HIV, in their white blood cells than previously estimated, researchers reported this week. The new findings may help settle the contentious issue of how such a serious syndrome could be triggered by what seemed so few infectious particles.

Some critics of mainstream AIDS research -- most notably Peter H. Duesberg of the University of California, Berkeley--have argued that some factor in addition to HIV must underlie AIDS. Duesberg and others question, for example, how a virus detected in only one of 10,000 targeted white blood cells could so cripple a person's immune system.

The new research indicates that AIDS patients harbor HIV in more than one of every 100 CD4-positive T4 cells -- the white blood cells that serve as the major target and reservoir of HIV. In many of these cells, HIV has incorporated itself into the DNA of its host cell, thus resisting detection by standard laboratory techniques. Researchers knew HIV could sequester itself in white blood cells but until now had not shown that the practice was so prevalent.

The new ratio of one infected CD4-positive cell per 100 represents "a very, very high number of infected cells," says Miltiades C. Psallidopoulos of the Georgetown University School of Medicine in Washington, D.C., who performed the work with colleagues from George-town, the National Institutes of Health in Bethesda, Md., and Program Resources, Inc., in Frederick, Md. Their data also indicate that AIDS patients carry actively replicating HIV in a concentration 10 times that previously seen.

The researchers accomplished the unprecedented tally by combining two extremely sensitive technologies: a laser-driven flow cytometer capable of separating different types of cells with about 99 percent accuracy, and the polymerase chain reaction, which can detect extremely small quantities of specific DNA sequences such as those from HIV. Their high-tech cell sorter is one of the very few in the United States sealed in a special facility to keep aerosolized, AIDS-infected droplets from escaping. The novel arrangement allowed the researchers to test for HIV DNA in individual cells of known subtypes, providing the best data yet regarding the types and numbers of cells harboring HIV in patients at various stages of AIDS.

In describing the unexpectedly large percentage of infected cells, the researchers also express surprise about two other findings. First, very few of the patients' circulating monocytes -- another type of white blood cell -- contained HIV, even though scientists have found several monocyte-infecting strains of HIV in laboratory studies of cultured cells. Second, patients' so-called CD4-negative cells contained essentially no HIV, although such cells often contain HIV when grown in culture. These observations suggest that some laboratory studies of AIDS infection may not reflect clinical realities, say research leaders Steven M. Schnittman and Anthony S. Fauci.

"The in vitro studies are still very valid," Fauci told SCIENCE NEWS. "But this gives us a caveat that what is going on in vitro, although very helpful, may not reflect the very subtle things that are going on in vivo." The report appears in the July 21 SCIENCE.

A related study by Psallidopoulos and others, due to appear in an upcoming JOURNAL OF VIROLOGY, shows that as asymptomatic, HIV-infected individuals progress to full-blown AIDS, the percentage of CD4-positive T4 cells infected with HIV increases, as does the ratio of active to latent HIV in those cells. By examining subtle genetic and biochemical differences between individual cells harboring active versus latent HIV, the researchers hope to learn how to maintain viral latency indefinitely in infected individuals.