SEATTLE, WASHINGTON- Antiretroviral drugs (ARV) have transformed the HIV infection from a death sentence to a chronic condition. In most people the drugs usually buffer HIV levels so low that standard tests do not find viruses in blood samples. But inexplicably, in about 10% of people infected with HIV it remains easily detectable in the blood even if they take their daily pills and are not affected by mutants resistant to the drugs of the virus.
A study presented last week at the largest annual US conference The HIV / AIDS conference offers a solution to this riddle: "repliclons", the populations of cells replicating with the HIV yes genome they nest within them. "It's the most interesting presentation I've seen here," says George Pavlakis, a retrovirologist at the National Cancer Institute in Frederick, Maryland.
These replicas highlight what could be a serious gap in a popular HIV care strategy. They are also calling new attention to a radical approach to eradicating the persistent virus: using the CRISPR genome editor to cut off HIV genes from infected cells. A study on monkeys on the approach presented at the same meeting showed signs of success, and a biotechnology company now hopes to start a clinical trial.
People living with HIV who still have a small amount of virus in their blood despite being on ARV often leave doctors "exasperated," says virologist John Mellors of the University of Pittsburgh, Pennsylvania. Assuming that the virus has acquired resistance, they often change the pharmacological regimes of their patients and order many additional tests. "Each of these creates anxiety and new side effects," says Mellors. Furthermore, these people may still have viral loads high enough to infect others.
At the conference on retroviruses and opportunistic infections here, the virologist Elias Halvas, who works in Mellors' laboratory, carefully described the analysis of viral isolates and blood cells from eight men and a woman who had mysterious and persistent low-level viremia for an average of 3 years, despite having taken ARV. Halvas and colleagues have noticed something curious. Normally, every time the HIV infects a cell, the virus copies its genome of the RNA into a version of DNA that integrates into a new point between the chromosomes of the cell. But in each of these patients, all their infected cells had HIV integrated into the exact same chromosomal region, a place that differed from person to person. The DNA sequence of the HIV taken from different cells in the same person was also identical.
Researchers have long known that HIV can make new copies of itself in two ways. In the basic replication cycle, the DNA of HIV integrated into a chromosome creates new virions that sprout from that cell, then infect other cells, acquiring mutations every time. ARVs block multiple steps in that process.
In the second path, HIV essentially gets a free ride because it infects an immune cell that clones itself, producing more cells that carry the viral genome. ARVs have no impact on this scenario and viral DNA ends up in the same chromosomal position in all progenitor cells, without acquiring new mutations. These clones can independently produce new virions, but the ARVs taken by the patients derail new infections. Mellors' team has shown that this path alone explains the low but persistent viral loads in these patients.
Daniel Kuritzkes, an HIV / AIDS clinician at Brigham and Women's Hospital in Boston, says the new data suggests that doctors should not be alarmed by low virus levels in patients who claim to be adhering to their treatment and do not show obvious immune damage. "It is safe to assume that in the absence of increasing viremia it is not necessary to change [ARVs]," says Kuritzkes, whose laboratory reported a similar finding in a single patient examination.
But the discovery casts doubt on a proposed approach to cure an infection: "kicking" the cells that host HIV's DNA in a latent, not duplicating form, so as to pump out new copies of the virus, preparing for destruction. These replicates are spitting up the virions and, for whatever reason, do not self-destruct rapidly or are eliminated by the immune responses, says Mellors. So something extra is needed to kill them. "If we can't defeat those guys with our therapies, then kicking and killing won't work," he says.
But another approach could: directly remove the DNA of the persistent HIV from the chromosomes of a person with the CRISPR publisher genome. "It's a science fiction idea that could one day be possible," says Pavlakis. Currently, he argues, the risks are too high for the CRISPR Cas9 enzyme to make cuts in the wrong place and not direct the editor to the appropriate cells. "CRISPR is not there now," he says.
During the meeting, neurovirologist Tricia Burdo of Temple University in Philadelphia, Pennsylvania described a first step: using the editor to remove at least part of the simian version of the AIDS virus, SIV, from the chromosomes of two monkeys. Previous work has shown that CRISPR could remove HIV immersed in the cells of mice designed to have immune systems similar to humans. In the new study, researchers infused a harmless adeno-associated virus that carries genes for targeted CRISPR molecular scissors into the veins of two SIV-infected monkeys. The monkeys were on ARV and had low levels of SIV.
Necropsies of treated animals showed that CRISPR had cut SIV DNA into the blood, spleen, lymph node and lung cells, apparently disabling the virus. CRISPR monkey blood could not infect white blood cells, while a control animal's blood could do so. The group also found Cas9 in all 14 tissues studied, suggesting that the administration virus had spread through the body as expected. "The data shows effects much more intense than those seen before, so this is a step in the right direction," says John Coffin, a retrovirologist at a branch of Tufts University in Boston.
Burdo says that in a future experiment, you will take the monkeys treated with CRISPR from the ARVs to see if the virus is bounced. They also intend to transfer millions of blood cells from CRISPR-treated monkeys to uninfected animals, another sensitive way to determine if even traces of intact SIV remain hidden.
Burdo's collaborator, Temple University neurovirologist Kamel Khalili, hopes to develop a human version of this CRISPR gene therapy. Khalili says his company, Excision Bio Therapeutics of Philadelphia, is seeking approval to launch HIV excision tests with CRISPR in humans by the end of this year.
Some remain skeptical. CRISPR could never affect all HIV-infected cells. Douglas Richman, a virologist at the University of California, San Diego, notes that even a person who has an undetectable viral load can have up to 100 million cells that host HIV DNA. "The problem with all treatment interventions is that they have an impact on some cells," says Richman. "That still leaves a huge amount of viruses. And when you are eliminating something dangerous, you have to take them all."