SEATTLE, WASHINGTON- Antiretroviral drugs (ARV) have transformed HIV infection from a death sentence to a chronic condition. In most tampons, the drugs routinely strain the HIV levels so low that standard tests do not find any virus in blood samples. But inexplicably, about 10% of the infected people HIV remains easily detected in the blood even though they take their daily pills and are not saddled with drug-resistant mutants of the virus.
A study presented last week at the largest annual US HIV / AIDS conference offers a solution to this riddle: "replicons", populations of replicating cells with the HIV genome lying 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 replica clones highlight what can be a serious deficiency in a popular HIV cure strategy. They also pay attention to a radical method of eradicating persistent virus: using the genomic CRISPR to cut HIV genes from infected cells. A monkey study of the approach presented at the same meeting showed signs of success and a biotechnology company now hopes to launch a clinical trial.
HIV-infected people who still have small amounts of virus in the blood even though they are on ARV often leave doctors "exasperated," said virologist John Mellors of the University of Pittsburgh, Pennsylvania. Assuming that the virus has been resistant, they often change their patients' drug regimes and order many extra tests. "Each of them creates anxiety and new side effects," says Mellors. Furthermore, these people may still have sufficiently high viral loads to infect others.
At the conference on retroviruses and opportunistic infections, the virologist Elias Halvas, who works in the middle lab, described carefully to analyze virus isoles and blood cells from eight men and a woman who has had mysterious, persistently low level viremia on average 3 years, despite the fact that ARVs. Halvas and employees noticed something curious. Normally, the virus replicates every time HIV infects a cell's RNA genome into a DNA version that integrates into a new site among the cell's chromosomes. But in each of these patients, all of their infected cells had HIV integrated into the exact same chromosome region – a site that differed from person to person. The sequence of HIV DNA 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, HIV DNA integrated into a chromosome creates new virions that bud from that cell, then infect other cells and acquire mutations each time. ARVs block several steps in that process.
In the other way, HIV essentially gets a free ride because it infects an immune cell that clones itself and makes more cells carrying the viral genome. ARV has no effect on that scenario, and viral DNA ends up in the same chromosomal site in all progeny cells without acquiring any new mutations. These clones can produce new virions themselves, but the ARVs taken by the patients save new infections. Mellor's team showed that this path easily explains the low but persistent viral load in these patients.
Daniel Kuritzkes, an HIV / AIDS clinician at Brigham and Women & # 39; s Hospital in Boston, says that the new data suggests that doctors should not be worried by the low levels of virus in patients who say they stick during the treatment and has no apparent immune system. "It is safe to assume that in the absence of increased viremia one does not need to change [ARVs]," says Kuritzkes, whose own lab reported a similar result in a single patient examination.
But the finding doubts a proposed way of healing an infection: "kicks" cells that end up HIV-DNA in a latent, non-replicating form so that they pump out new copies of the virus and set themselves up for destruction. These replicas are still spying on virions, for whatever reason, they are not self-destructive or eliminated by immune responses, Mellors says. So something extra is needed to kill them. "If we can't beat these guys with our therapies, the kick and death won't work," he says.
But another approach can: directly remove the persistent HIV DNA from a person's chromosomes with the genome editor CRISPR. "It's a science fiction idea that one day might be possible," says Pavlakis. Currently, he argues that the risks are too high that CRISPR's Cas9 enzyme will make cuts in the wrong place and targeting the editor to appropriate cells is not simple. "CRISPR right now is not there," he says.
At the meeting, the neurovirologist Tricia Burdo at Temple University in Philadelphia, Pennsylvania, described a first step: using the editor to check at least part of the analog version of the AIDS virus, SIV, from the chromosomes of two monkeys. Previous work has shown that CRISPR could claim HIV embedded in the cells of mice constructed to have human immune systems. In the new study, researchers infused a harmless adeno-associated virus that transports the genes for CRISPR's targeted molecular scissors in the years of two SIV-infected monkeys. The monkeys were on ARV and had low levels of SIV.
Necropsies of the treated animals showed that CRISPR had cut SIV DNA into blood, spleen, lymph node and lung cells, which apparently inactivated the virus. Blood from the CRISPR monkeys could not infect white blood cells, while blood from a control animal could. The group also found Cas9 in all 14 tissues studied, indicating that the delivery virus had spread through the body as intended. "The data show much stronger effects than previously seen, so it's a step in the right direction," said John Coffin, a retrovirologist at a branch of Tufts University in Boston.
Burdo says that in a future experiment, the team takes the CRISPR-treated monkeys of ARVs to see if the virus is returning. They also plan to transfer millions of blood cells from the CRISPR-treated monkeys to uninfected animals, another sensitive method for determining whether even trace amounts of intact SIV remain hidden.
Burdo's partner, Temple University neurovirologist Kamel Khalili, hopes to develop a human version of this CRISPR gene therapy. Khalili says his company, Excision BioTherapeutics in Philadelphia, is seeking approval to launch HIV CRISPR excision trials in humans at the end of the year.
Some are still skeptical CRISPR can ever beat all HIV-infected cells. Douglas Richman, a virologist at the University of California, San Diego, notes that even a person who has an undetectable virus load can have as many as 100 million cells harboring HIV DNA. "The problem with any cure is that they affect some cells," Richman says. "It still leaves a gigantic amount of virus. And when you eliminate something that is dangerous, you have to get them all."