Show me the monkey.
Seeing is believing, and a study in rhesus macaques with a new imaging technique reveals for the first time a real-time map of an AIDS virus replicating in the entire body of a living animal. The results point to some unexpected hideouts of the simian AIDS virus, or SIV. And the experiments also show that when the monkeys are given antiretroviral (ARV) drugs, the amount of virus that persists differs by location in the body. The innovative tool promises to clarify the still-murky details of the initial infection process and may help guide drug, vaccine, and cure research in people.
“It’s fantastic,” says Thomas Hope, an immunologist at Northwestern University Feinberg School of Medicine in Chicago, Illinois, who investigates how HIV, the human counterpart to SIV, infects cells. “The whole monkey shows you things you can’t comprehend by just looking at cells or biopsies of tissues.”
To obtain this unprecedented look at an AIDS virus in action, a research team led by Francois Villinger of Emory University in Atlanta borrowed a technique used with cancer patients. The researchers began by attaching a radioactive molecule to an antibody that targets the surface protein of SIV. They then gave 12 monkeys chronically infected with SIV the harmless, engineered antibody, which sought out and latched onto the SIV in their bodies. A positron emission tomography (PET) scan detected the antibodies and thus revealed exactly where the virus was hanging out inside the monkeys. As expected from previous biopsy studies, the “immunoPET” scan indicated that the gut and lymph nodes harbored high levels of SIV. But there were several startling findings.
The immunoPET method illuminated surprisingly high levels of the SIV antibody in the nasal cavity, the investigators report online today in Nature Methods. “The entire upper respiratory tract is rich with lymphatic tissue, and we just never thought of that,” says Timothy Schacker of the University of Minnesota, Twin Cities, who studies how HIV causes disease. “That’s a really cool finding.”
Villinger was particularly struck by the high levels of SIV in the genital tract of males, given that sexual transmission of the AIDS virus does not occur all that readily. “The epididymis [the tubes that carry sperm] of the monkeys are just lighting up,” Villinger says. “It’s mind-boggling.” The virus also had an unexpected penchant for the lung, an organ that has received relatively little attention from HIV researchers.
Hope says this tool could be used to clarify what takes place in monkeys, and presumably people, during the first weeks of an infection with an AIDS virus. “That sets the tone for what happens years later,” he explains. “We need to know where the virus goes, how it gets there, and why it’s a benefit to the virus. These pieces of the puzzle are really important.”
In another experiment that has practical implications for humans, Villinger and his colleagues gave three of the infected monkeys ARV drugs and then did the same kind of PET scans 5 weeks later. Although none of the monkeys had detectable SIV in their blood by the time of the scans, they all had SIV reproducing in multiple tissues. This meshes with work by Schacker’s group that found evidence of viral replication in biopsied tissue from “fully suppressed” HIV-infected people on treatment who have undetectable levels of the virus in their blood. Schacker’s team has also shown that ARV drugs had difficulty penetrating these tissues. ImmunoPET scans could be an “excellent, more noninvasive way” of evaluating ways to get drugs to those hard-to-reach reservoirs, Schacker says.
If immunoPET can be adapted to tracking HIV in humans, which Villinger and others say is likely, scans might help resolve a related long-standing debate in the field: Just how frequently do fully suppressed people continue producing low levels of the virus? The evidence that antivirals have difficulty reaching some tissues convinces Schacker that it is a common phenomenon. “The drugs are not doing everything we think they’re doing,” he contends.
The other camp of researchers insists that treatment fully suppresses the virus and that it persists only because of “reservoirs” of long-lived cells that have HIV DNA woven into their chromosomes. Neither drugs nor the immune system reach this latent DNA, which sits poised to restart the viral fire at any time.
Figuring out a way to identify which tissues harbor these reservoirs in fully suppressed people tops the agenda of many HIV researchers, as it theoretically could fine-tune attempts to purge them and cure people. Villinger suggests immunoPET might help with this, too. “If you stop antiretroviral treatment, then you can see where the virus rekindles,” he says.
In a similar vein, immunoPET might help clarify the power of different cure strategies. Many investigators are promoting the “shock and kill” idea, which attempts to prod reservoirs into making new copies of HIV, setting up the infected cells for destruction. “We could do shock and image,” Villinger says, theoretically revealing the interventions that have the most impact.
David Margolis, who has pioneered human studies that attempt to shock and kill reservoirs, says immunoPET is a “nice new tool” but cautions that it’s unclear to him how well the signals it detects match viral levels in the body. He also notes that a great deal of latent HIV DNA codes for defective, “dead-end” viruses that cannot cause infection themselves. If the labeled antibodies detect their presence, it could be misleading when it comes to evaluating whether reservoirs are shrinking.
Hope agrees that immunoPET tracking of AIDS viruses needs to be refined and still has technical hurdles to clear, but says the work advances the field. He even expressed surprise that it was in Nature Methods rather than the more prestigious mother journal, Nature. (Villinger says Nature reviewed and rejected the paper after a great deal of back-and-forth.) “It deserves to be there,” Hope says. “This is groundbreaking.”