Curing HIV/AIDS Gets Tougher: Study Shows Far More 'Hidden' and Potentially Active Virus Than Once Thought
Discovery of a larger than expected latent reservoir of HIV confounds "shock and kill" cure strategy
10/24/2013 - Just when some scientists were becoming more hopeful about finding a strategy to
outwit HIV's ability to resist, evade and otherwise survive efforts to rid it from the body, another hurdle has emerged to foil
their plans, new research from Johns Hopkins shows.
In a cover-story report on the research to be published in the journal Cell online Oct. 24, Johns Hopkins infectious
disease experts say the amount of potentially active, dormant forms of HIV hiding in infected immune T cells may actually be
60-fold greater than previously thought.
The hidden HIV, researchers say, 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.
The disappointing finding comes after a three-year series of lab experiments, which they say represents the
most detailed and comprehensive analysis to date of the latent reservoir of HIV proviruses. 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.
Senior study investigator Robert Siliciano, M.D., Ph.D., who in 1995 first showed that reservoirs of dormant HIV were present
in immune cells, says that while the latest study 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.
Study results showed that among 213 HIV proviruses isolated from the reservoirs of eight patients and initially unresponsive
to highly potent biological stimuli, some 12 percent could later still become active, and were capable of replicating their genetic
material and transmitting infection to other cells. Siliciano says that all of these non-induced proviruses had previously been
thought to be defective, with no possible role in resumption of the disease.
Siliciano, a professor at the Johns Hopkins University School of Medicine and a Howard Hughes Medical Institute investigator,
says his team's latest study findings pose a serious problem to prevailing hopes for the so-called "shock and kill" approach to curing HIV.
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 every last infected cell from the body while antiretroviral drugs prevent
any new cells from becoming infected.
Siliciano says this new discovery could boost support for alternative approaches to a cure, including renewed efforts to
a therapeutic vaccine to stimulate immune system cells that attack and kill all HIV. "Our study results certainly show that finding a
cure for HIV disease is going to be much harder than we had thought and hoped for," he says.
Lead study investigator and Johns Hopkins postdoctoral fellow Ya-Chi Ho, M.D., Ph.D., says the team's investigation
"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. She says 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," says Ho.
In the latest study, 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.
Further lab 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. 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, they say,
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.
Statistical modeling later showed 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 discouraging, will energize HIV experts to refine and improve methods for detecting
proviruses capable of reactivation.
Siliciano is next helping to organize in November a San Francisco conference, jointly sponsored by the journals Cell and The
Lancet, entitled "What Will it Take to Achieve an AIDS-free World?"
Funding support for this research was provided by the amfAR Research Collaboration on HIV Eradication, and the Martin
Delaney CARE and DARE Collaboraties of the National Institutes of Health and the National Institute of Allergy and Infectious
Diseases, the Johns Hopkins Center for AIDS Research, and the Howard Hughes Medical Institute. Corresponding federal grant
numbers are AI096113, 1U19AI096109 and AI043222.
Besides Siliciano and Ho, other researchers involved in this study, performed entirely at Johns Hopkins, were Liang
Shan, Ph.D.; Nina Hosmane, B.A.; Jeffrey Wang, B.A.; Sarah Laskey, B.A.; Daniel Rosenbloom, Ph.D. Jun Lai, B.A.; Joel
Blankson, M.D., Ph.D.; and Janet Siliciano, Ph.D.
Currently, there are more than 34 million people in the world living with HIV, including an estimated 1,178,000 in the
United States and 23,000 in the state of Maryland.
