HIV’s Secret Hideout Frustrates Efforts To Develop A Cure
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redOrbit Staff & Wire Reports – Your Universe Online
Although current treatments for human immunodeficiency virus (HIV) can keep the disease at bay, a larger-than-expected amount of hidden virus may complicate efforts to find a cure, according to the most detailed and comprehensive analysis to date of the latent reservoir of HIV proviruses.
The three-year study, published Thursday in the journal Cell, deals a painful blow to researchers working hard to find a cure for HIV/AIDS, a disease that kills nearly two million people per year according to the World Health Organization (WHO).
Infectious disease experts at John Hopkins found that the amount of potentially active, dormant forms of HIV hiding in infected immune T cells may actually be 60 times greater than previously thought.
This hidden HIV is part of the so-called latent reservoir of functional proviruses that remains long after antiretroviral drug therapy has successfully brought viral replication to a standstill. If antiretroviral therapy is stopped or interrupted, some proviruses can reactivate, allowing HIV to make copies of itself and resume infection of other immune cells, the researchers said.
Senior study investigator Robert Siliciano, M.D., Ph.D., who in 1995 first showed that reservoirs of dormant HIV were present in immune cells, said that while the current study’s results show most proviruses in the latent reservoir are defective, curing the disease will depend on finding a way to target all proviruses with the potential to restart the infection.
“These results indicate an increased barrier to cure, as all intact noninduced proviruses need to be eradicated,” Siliciano said. “Although cure of HIV infection may be achievable in special situations, the elimination of the latent reservoir is a major problem, and it is unclear how long it will take to find a way to do this.”
The study’s results showed that among 213 HIV proviruses that were isolated from the reservoirs of eight patients and that were initially unresponsive to highly potent biological stimuli, some 12 percent could later still become active and capable of replicating their genetic material and transmitting infection to other cells. All of these non-induced proviruses had previously been thought to be defective, with no possible role in resumption of the disease, said Siliciano, a professor at the Johns Hopkins University School of Medicine and a Howard Hughes Medical Institute investigator.
These disappointing findings pose a serious problem to prevailing hopes for the so-called “shock and kill” approach to curing HIV, he said. That approach refers to forcing dormant proviruses to “turn back on,” making them “visible” and vulnerable to the immune system’s cytolytic “killer” T cells, and then eliminating infected cells from the body while antiretroviral drugs prevent any new cells from becoming infected.
Siliciano said this new discovery could enhance support for alternative approaches to a cure, including renewed efforts to develop a therapeutic vaccine to stimulate immune system cells that attack and kill all HIV.
Lead study investigator and Johns Hopkins postdoctoral fellow Ya-Chi Ho, M.D., Ph.D., said the team’s investigation of “the true size” of the latent reservoir was prompted by a large discrepancy between the two established techniques for measuring how much provirus is in immune system cells.
The team’s original method of calculating only reactivated proviruses yielded numbers that were 300-fold lower than a DNA-based technique used to gauge how many total proviral copies, both dormant and reactivated, are present.
“If medical researchers are ever going to lure out and reactivate latent HIV, then we need to better understand exactly how much of it is really there,” Ho said.
In the current study, the researchers sequenced, or spelled out, the entire genetic code of HIV proviruses that reactivated and those that could not be induced to do so. Twenty-five of the 213 non-induced isolates, when sequenced, had fully intact genomes when compared to those that did reactivate. Analysis of the remaining 88 percent of non-induced proviruses showed that all were defective, possessing genetic deletions and mutations that would forestall viral replication.
Additional experiments on the cloned proviruses showed that the intact, non-induced proviruses could be reconstructed to produce active virus, which in turn could replicate in human immune cells.
The researchers also found that cloned proviral DNA lacked a latency-inducing chemical methyl group. When researchers looked at where non-induced proviral DNA showed up in infected human immune cells, they found some 92 percent of the non-induced proviral DNA was located in actively transcribed regions of the human cell DNA. This finding suggests that non-induced proviral DNA is not permanently hidden in some inaccessible regions of the host chromosomes, but instead lies in regions where it could become reactivated, the researchers said.
Statistical modeling later revealed that these figures equated to a 60-fold increase in the potential size of the latent reservoir when compared to the team’s original method for counting only reactivated viruses.
Additional experiments showed that repeated chemical stimuli could reactivate proviruses that failed to respond to initial attempts at reactivation.
Ho says the study results, although disheartening, will galvanize experts to refine and improve methods for detecting proviruses capable of reactivation. Siliciano is currently working to organize the “What Will it Take to Achieve an AIDS-free World?” conference, which will take place November 3-5 in San Francisco.