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HIV experts gather to update public on cure research
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amfAR, The Foundation for AIDS Research, recently held a summit in San Francisco on World AIDS Day to update the public on progress made toward a cure for HIV/AIDS.
During the summit, researchers from the amfAR Institute for HIV Cure Research discussed the challenges of eliminating HIV in a small fraction of cells that remain infected with a latent virus, even with the use of ART. Since the immune system is unable to detect and, therefore, fight against this latent reservoir, clinical trials are investigating whether toll-like receptor (TLR) agonists can be used in a “shock and kill” approach to provoke the virus into replicating and enhance the immune system’s ability to kill the cells producing the virus.
Sterilized vs. functional cure
Infectious Disease News Chief Medical Editor Paul A. Volberding, MD, director of the AIDS Research Institute at the University of California, San Francisco (UCSF), said there is debate over the meaning of an HIV cure, which can be defined as “sterilizing” or “functional.” In a sterilizing cure, all HIV–infected cells are eliminated from the body. It is believed that Timothy Brown, known as the Berlin patient, was “cured” of HIV after his viral reservoir was removed during a stem cell transplant. However, researchers suggest this technique is not feasible for all patients with HIV and similar attempts have not reached the same result.
“Brown suggests it is possible; we just need better and safer ways to find a cure. It has to be very effective and safe,” Warner C. Greene, MD, PhD, director and Nick and Susan Hellman distinguished professor of translational medicine at Gladstone Institute of Virology and Immunology, and professor of medicine, microbiology and immunology at UCSF, said during the summit. “It has to be scalable and used in the developing world where the virus hits hardest.”
In a functional cure, patients with HIV are still infected with the latent reservoir; however, they experience long periods of remission in the absence of ART.
“We want to allow an HIV–infected person to live a full life without having to take medications, worry about transmitting the virus, or suffer any complications from HIV,” Volberding, who is also director of the amfAR Institute for HIV Cure Research, said. “If we could do that, we would have done a lot.”
Addressing roadblocks to cure
According to amfAR CEO Kevin Robert Frost, there are four major challenges in cure research. To overcome these roadblocks, researchers must identify the location where the HIV reservoir hides, understand how the virus persists in a dormant state, determine the extent and size of the viral reservoir, and eliminate the virus.
“There are a lot of questions, but not so many answers yet,” Frost said.
Peter Hunt, MD, associate professor of medicine, division of experimental medicine at UCSF, discussed research characterizing the location and phenotype of HIV–infected cells. Researchers have established that most CD4+ T cells — the main reservoir of HIV — live in tissues located in lymph nodes, the spleen and the lining of the gut. More recent examination has shown that T follicular helper cells (Tfh), which are primary targets of HIV in lymph nodes, may represent a barrier to cure. According to Hunt, cytotoxic CD8+ T cells and other natural killer cells that eliminate HIV are prevented from entering the B cell follicle where Tfh cells reside.
“For this reason, infected Tfh cells may be more difficult to clear with immune-based therapies like TLR stimulators,” he said.
Future investigations will quantify HIV in Tfh cells to determine whether HIV burden can predict when a patient will experience viral rebound after stopping therapy. Researchers also are measuring reservoir in resident memory (TRM) T cells, which are suspected of suppressing HIV transcription, and will assess the impact of “shock and kill” treatment approaches on Tfh and TRM cells.
Novel technologies characterize HIV reservoir
In another presentation, Satish Pillai, PhD, associate professor of laboratory medicine at UCSF and associate investigator at the Blood Systems Research Institute, underscored the need for more sensitive assays that can be used to measure the HIV reservoir.
“We are in an era that’s pretty crude,” he said. “We have a lot of expensive technologies. They all do slightly different things and tell different stories, but we don’t really know if any of them are telling us the absolute truth. What we need is a new generation of modern tools that can accurately measure the reservoir in a cost-effective fashion.”
Current methods used to detect the latent reservoir characterize approximately 100,000 cells or less at a time. However, since fewer than 10 out of a million CD4+ T cells may harbor HIV, larger samples are needed to increase the likelihood of encountering latently infected cells. Therefore, Pillai and colleagues have developed a PCR assay with digital droplet technology that can assess 1 million cells or more at a time.
“We think that this assay that we’ve developed in conjunction with RainDance Technologies represents a meaningful advance over some of the existing tools that are available today,” he said.
To better describe latently HIV–infected cells, researchers have invented a software that allows for multi-dimensional phenotyping of T cells called mass cytometry (CyToF).
“The problem with markers that identify CD4+ T cells is that they are not very sensitive,” Hunt said. “Many infected cells don’t express these markers at all and some cells with these markers are not infected with HIV.”
Researchers assembled a 38-parameter CyToF panel to provide a full characterization of T cells. They are adapting the panel in “shock and kill” studies to detect HIV expression and determine which cells are producing HIV before and after stimulation.
“One of the problems with this approach is that HIV reactivation can alternate the cellular phenotype,” Hunt said. “But the CyToF measurements will help us to identify the original phenotype of the cell.”
Future trials will explore role of TLRs in cure research
Greene, who also is co-director of the UCSF-Gladstone Center for AIDS Research, discussed other research efforts that describe how HIV remains latent and how TLRs can reactivate the reservoir. He highlighted studies with contradictory results, including one that indicated HIV latency accidentally occurs when cells retreat to a resting memory state at the time of infection, and another that suggested latency is a planned event and survival mechanism of the virus.
“It will be important to distinguish between these two models moving forward because it will have implications for how we try and prevent the reservoir from being formed in the first place,” he said.
A series of in vitro studies assessing the efficacy of TLRs demonstrated that TLR4 and TLR7 resulted in moderate yet consistent viral activation in dendritic and latently infected tonsil cells. An in vivo study using latently infected cells from patients on long-term ART identified TLR4, TLR7, TLR8, TLR7/8 and TLR 9 as potential latency-reversing agents.
“Overall, several TLR agonists exhibit moderate ‘shocking’ activity in cells from HIV–infected individuals on ART,” Greene said. “However, these agents appear more active in vivo probably because they also act as adjuvants boosting the adaptive immune response against HIV. We don’t model that in vitro, but in vivo we’re seeing really interesting effects of these agents.”
Steven Deeks, MD, professor of medicine at UCSF, discussed upcoming clinical trials that will further examine the use of TLRs. A study launching in January with support from the amfAR Institute, in collaboration with Gilead, will assess whether the drug Gs9620 — which was safe and effective in animal models — can stimulate TLR7 in patients with medium T-cell responses and boost the cell response to eliminate the virus. Study participants will be randomly assigned to receive Gs9620 or placebo and will be taken off therapy until viral rebound.
“This is controversial in the field, but we’re becoming comfortable with this,” Deeks said. “Almost all clinical studies that are going to look at cure in the modern era will probably have to require very careful, highly monitored interruptions of therapy.”
A second study assessing TLR9 will be conducted in the summer. Further down the line, amfAR will explore combinations of these approaches with a vaccine. The institute also is investigating how anti-inflammatory drugs in cancer and other therapies can enhance the impact of TLRs. Deeks said that a single drug may not lead to a cure. Instead, either gene therapy or combination regimens will be needed to achieve a durable remission.
“People often ask when we’re going to have a cure. I have no idea,” he said. “But to have a combination regimen that is viable and testable, which is the goal of our group now … I’m hoping we’re going to have that in the next few years. That’s an optimistic perspective. I started this job back in 1993. We went from a fatal disease to a disease that was controllable. No one in 1993 had any idea that we would actually be able to control the virus, but yet 3 years later we did. So, I’m optimistic that we will make great success.” - by Stephanie Viguers
Disclosures: Deeks receives research support from ViiV Healthcare, Merck and Gilead Sciences. Greene receives support from JT Pharma and volunteered effort without salary to the Accordia Global Health Foundation, which recently merged with Africare. Hunt received honoraria from Gilead and Merck for consulting. Pillai receives research support from Raindance Technologies. Volberding is a consultant and chairs the Data and Safety Monitoring Board for Merck.