Sixteen years ago, in the relatively early days of the AIDS epidemic, virologist Jay Levy of the University of California (UC), San Francisco, proposed an answer to one puzzle about the AIDS virus and created another. He had tantalizing evidence that immune system cells secrete a chemical that can stop HIV, and he suggested that HIV- infected people who have high levels of this factor could live for decades without damage to their immune systems. This would explain in part why HIV kills at such disparate rates. But just what is the mystery factor? Legions of AIDS researchers have searched for this treasure, unearthing half a dozen different candidates, none of which has been completely convincing. Now a team led by Linqi Zhang and David Ho of the Aaron Diamond AIDS Research Center in New York City claims to have the elusive factor in hand. But other researchers who have been digging for answers and have seen the new data say they're not throwing away their shovels.
In a report published online by Science this week,* Zhang, Ho, and co-workers finger three tiny molecules known as a-defensins. The researchers discovered that white blood cells with CD8 receptors secrete these molecules, but infection by HIV appears to shut down production. However, the CD8 cells of people who remained unharmed by the virus a decade or more after being infected continued to produce them. "It largely solves a mystery that's been out there for 16 years," says Ho. Robert Siliciano, an AIDS virologist at Johns Hopkins University in Baltimore, Maryland, says the work is "certainly a big advance." Robert Lehrer, who first described human defensins in 1985, agrees. "The data are very convincing," says Lehrer, a researcher at UC Los Angeles.
But several leading AIDS researchers aren't convinced. Levy himself applauds "the great effort" to find the defensins in CD8s, but he says that it's not the factor he postulated. Levy says his lab has tested defensins and found that they did not meet his criteria for the factor, which he called CAF (for CD8 antiviral factor). Robert Gallo, head of the Institute of Human Virology in Baltimore, says that although he finds the work "technically sound," the paper relies on too few patients, offers no mechanism for how defensins stop HIV, and dismisses other factors that he believes are likely to be more important. Bruce Walker, whose lab at Harvard Medical School in Boston also recently described a candidate CAF, says the new data show that the defensins have "a very modest effect" against HIV.
In the study, the Aaron Diamond researchers teamed up with scientists from Ciphergen Biosystems Inc. in Fremont, California, to compare secretions of CD8 cells from three HIV-infected "long-term nonprogressors," four HIV-infected "progressors," and 15 uninfected people. Ciphergen makes tiny arrays of proteins that, with the help of mass spectrometry, allowed them to analyze the components in each sample. Company scientists found that only the long-term nonprogressors and uninfected people produced three small, related proteins that a database search revealed as defensins.
As first reported by Lehrer, human defensins are secreted primarily by neutrophils and break down bacterial walls, acting like natural antibiotics. A group in Japan 9 years ago showed that defensins from guinea pigs, rabbits, and rats could inhibit HIV, but the work received little notice. Some AIDS researchers also have incorporated defensins into their vaccines because the molecules can act like an adjuvant, boosting the immune response to the HIV components of the preparation.
When Ho and his colleagues depleted the defensins in the cell secretions from the long-term nonprogressors, they found that the secretions had markedly less anti-HIV activity. And when they depleted both defensins and immune messengers known as b-chemokines--which Gallo's lab in 1995 showed powerfully prevent HIV entry into cells--the secretions had almost no antiviral activity. In what's sure to be the paper's most controversial assertion, the researchers state that the a-defensins "collectively account for the anti-HIV-1 activity of CAF that is not attributable to b-chemokines." As for the mechanism, Ho and Zhang say the shortage of clean defensin material makes it difficult to conduct experiments that might tease out how it combats HIV. But they have now begun those experiments.
Gallo takes exception to the entire concept that a single mysterious, undiscovered CAF exists. "This is ludicrous," he says. He argues that CD8 cells secrete many substances that inhibit HIV, including one his lab has yet to describe that he says appears to be much more powerful than defensins. "We don't use the word 'CAF,' " says Gallo. "Throw it out." Zhang agrees that CD8s might well secrete other, undiscovered molecules that inhibit HIV. "CAF is a black box," he says. "Different molecules could play different roles in different circumstances. We have no idea in vivo." Still, the a-defensins' apparent anti-HIV powers are likely to provide a new focus for research and, if they pan out, open new avenues for treatment.
