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Although current treatments for human immunodeficiency virus (HIV) can keep the condition at bay, a larger-than-expected quantity of hidden virus may complicate efforts to find a cure, according to the most detailed and comprehensive analysis up to now of the latent reservoir of HIV proviruses.
The three-year study, published Thursday in the journal Cell, deals an unpleasant blow to researchers working hard to locate a cure for HIV/AIDS, an illness that kills nearly 2 million people per year according to the World Health Organization (WHO).
Infectious disease experts at John Hopkins discovered that the quantity of potentially active, dormant forms of HIV hiding in infected immune T cells might 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 some standstill. If antiretroviral therapy is stopped or interrupted, some proviruses can reactivate, allowing HIV to create copies of itself and resume infection of other immune cells, they said.
Senior study investigator Robert Siliciano, M.D., Ph.D., who in 1995 came out that reservoirs of dormant HIV were present in immune cells, asserted as the current study’s results show most proviruses in the latent reservoir are defective, curing the condition is determined by getting a method to target all proviruses with the possibility to restart the infection.
“These results indicate a heightened barrier to cure, as all intact noninduced proviruses have to be eradicated,” Siliciano said. “Although cure of HIV infection might be achievable in special situations, the elimination of the latent reservoir is really a major problem, and it is unclear just how long it will require to find away out to get this done.”
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 % could later still come into action and capable of replicating their genetic material and transmitting infection to other cells. Many of these non-induced proviruses had previously been regarded as defective, with no possible role in resumption from the disease, said Siliciano, a professor in the Johns Hopkins University Med school along with 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 explained. That approach refers to forcing dormant proviruses to “turn back on,” which makes them “visible” and vulnerable to the immune system’s cytolytic “killer” T cells, after which eliminating infected cells in the body while antiretroviral drugs prevent any new cells from becoming infected.
Siliciano said this new discovery could enhance support for alternative methods 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” from the latent reservoir was prompted with a large discrepancy forwards and backwards established approaches for measuring just how much provirus is within defense mechanisms cells.
The team’s original method of calculating only reactivated proviruses yielded numbers that were 300-fold less than a DNA-based technique used to gauge the number of total proviral copies, both dormant and reactivated, can be found.
“If medical researchers are ever going to lure out and reactivate latent HIV, only then do we need to better understand just how much from it is actually there,” Ho said.
In the current study, the researchers sequenced, or explained, 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. Research into 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 demonstrated that the intact, non-induced proviruses could be reconstructed to create active virus, which could replicate in human immune cells.
The researchers also discovered that cloned proviral DNA lacked a latency-inducing chemical methyl group. When researchers looked at where non-induced proviral DNA turned up in infected human immune cells, they found some 92 percent from the non-induced proviral DNA was located in actively transcribed parts of the human cell DNA. This finding shows that non-induced proviral DNA is not permanently hidden in some inaccessible parts of the host chromosomes, but instead is based on 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 way of counting only reactivated viruses.
Additional experiments demonstrated that repeated chemical stimuli could reactivate proviruses that failed to react to initial attempts at reactivation.
Ho says the research results, although disheartening, will galvanize experts to refine and improve methods for detecting proviruses able to reactivation. Siliciano is currently working to organize the “What Will it Decide to try Achieve an AIDS-free World?” conference, that will take place November 3-5 in Bay area.
