Berlin patient,” she says. “I’d like to take him out to dinner.”
In trying to cripple CCR5, Cannon is building on the efforts of many others. Sangamo Biosciences, a biotech company in Richmond, CA, designed an enzyme called a zinc finger nuclease that can specifically target the CCR5 gene and disrupt its function. Working with Sangamo, Carl June, a gene therapy investigator at the University of Pennsylvania, now has a human study under way in which CD4 cells are pulled from HIV-infected people, infected with an adenovirus that carries the zinc finger nuclease, and then reinfused into the patients. But Cannon’s work would take things one step further. By targeting the CCR5 gene in the stem cells that give rise to the CD4s, Cannon, who is also working with Sangamo, thinks she ultimately has a better chance of achieving an effective and durable cure.
To test the idea, Cannon’s lab transplants human stem cells into one group of mice that serve as controls. A second group of mice receive human stem cells that have been modified with the zinc finger nuclease. The researchers then infect the mice with HIV. Experiments on numerous groups of mice show that the virus initially does equally well in all the animals, but after a few weeks, viral levels nosedive in the treated mice.
The zinc finger nucleases successfully mangle the CCR5 gene in only about 5 percent of the mouse immune cells. But HIV selectively kills the cells whose CCR5 receptors are intact. Thus, Cannon contends, the proportion of cells with a broken CCR5 receptor will increase over a few weeks, until the virus can no longer spread: even if a latently infected cell starts churning out HIV, it has nowhere to go. So the treated mice remain infected, but at such low levels that they suffer no ill effects. “ ’Cure’ doesn’t mean you have to eliminate the virus,” says Cannon. “You just have to eliminate the consequences of viruses. It’s a Herculean task to remove every cell in the body that has HIV in it.”
Some AIDS researchers still find that Herculean task worth pursuing. To them, the functional cures that Cannon and others are pushing for have merit but do not ultimately solve the problem. After all, an HIV strain that does not need CCR5 may be hiding in the body. Or maybe a latent virus will pop out and somehow mutate in such a way that it does not need CCR5 either. History, of course, is not on the side of those who want to wipe out the virus completely. “It’s incredibly heartening to see more people looking at eradication more carefully,” says David Margolis, a clinician at the University of North Carolina in Chapel Hill, who has done some of the first drug studies in humans that seek to purge the reservoir of latent HIV. “But it’s going to take a lot of hard work by a lot of people for a long time to really make progress. Who knows where the next real advance will come from?”
If current antiretrovirals do indeed completely stop HIV from copying itself, the remaining steps toward eradication will be to identify the location of the latent reservoirs and to flush the virus out of them and into the bloodstream, so the drugs can do their work. Researchers know that one place the latent virus hides is in resting CD4 cells, but Siliciano has published molecular evidence that this cannot be the only reservoir. One recent report from scientists at the University of Michigan suggests that inactive HIV can lurk in bone-marrow stem cells, and the virus could also be in the brain, gut, and lymph nodes. Checking for HIV in any of these tissues is much more difficult than analyzing a blood sample, so it won’t be easy to determine how effective a therapy has been at wiping it out.
Regardless of where latent HIV is, the virus must be awakened before drugs can target it. In the late 1990s, David Ho and a few other research groups made a crude attempt to do this. They explored the idea of prodding resting CD4 cells to “activate” and start making copies of themselves; in the process, those latent cells that harbored HIV would transcribe their viral DNA and then die. Ho’s group treated one patient with a monoclonal antibody that triggers activation. “He got pretty sick, and we just stopped it,” Ho remembers. “It was too scary.” A similar attempt almost killed another patient. “For the past decade, it’s just been thought of as way too high risk,” says Daria Hazuda, who does HIV drug discovery at Merck