A cure for HIV/AIDS seems more distant now that scientists have announced a grim finding: far more potentially active virus lurks in reservoirs of latent HIV than previously thought. Specifically, nearly 12% of latent HIV proviruses isolated from infected immune cells have been shown to harbor intact genomes and retain normal LTR function. In other words, HIV proviruses may lie low, biding their time before eventually stirring, replicating their genetic material and transmitting infection from cell to cell.
Previous investigation of proviruses that persisted in infected T cells considered their response to in vitro activation. The idea was to access the prospects for a “shock and kill” strategy, which would involve inducing proviruses to reactivate, making them vulnerable to the immune system’s cytolytic killer T cells, and then eliminating every last infected cell from the body while antiretroviral drugs prevented any new cells from becoming infected.
Although activation of T cells was known to reverse latency, investigators believed the shock-and-kill strategy showed promise because less than 1% of proviruses could be induced to release infectious virus, even after being subjected to highly potent stimuli. Noninduced proviruses were generally considered defective.
Still, questions remained, and they prompted a team of scientists at Johns Hopkins University to take a closer look at HIV proviruses. Johns Hopkins postdoctoral fellow Ya-Chi Ho, M.D., Ph.D., has said that 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,” said Dr. Ho.
The team decided to sequence the entire genetic code of HIV proviruses that reactivated and those that could not be induced to do so. About 12% of the noninduced isolates, when sequenced, had fully intact genomes when compared to those that did reactivate. Analysis of the remaining 88% showed that all these isolates were defective, possessing genetic deletions and mutations that would forestall viral replication.
These results were published October 24 in Cell, in a paper entitled “Replication-competent non-induced proviruses in the latent reservoir increase barrier to HIV-1 cure.” This paper also describes how the investigators used direct sequencing and genome synthesis to reconstruct full-length, intact proviral clones. This work demonstrated growth kinetics comparable to reconstructed induced proviruses from the same patients. Also, as noted in the paper, “Noninduced proviruses have unmethylated promoters and are integrated into active transcription units. Thus, it cannot be excluded that they may become activated in vivo.”
Most disappointingly, the study points out that the size of the latent reservoir, and hence the barrier to cure, may be up to 60-fold greater than previously estimated.
Summing up, senior study investigator Robert Siliciano, M.D., Ph.D., said, “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 added that the study’s results could boost 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.