Chasing The Cure

Over the past decade, the pursuit of a cure for HIV infection has gained significant momentum. At one time, the word cure was used rarely and cautiously, for fear of raising false hopes. The immediate focus after the discovery of HIV in 1983 was on treatments capable of suppressing the virus, which arrived in the form of combination antiretroviral therapy (ART) in the mid-1990s, transforming a fatal viral infection into one that now, for many people, is likely to have little or no impact on life expectancy.

But ART is imperfect, with potential toxicities and regular dosing that can negatively impact quality of life. Scientific advances and the availability of new technologies have conspired to create optimism that another step forward is possible. A major research effort, not just in the U.S. but internationally, is now working toward that goal.

To anyone who reads the news, the high profile of cure research has been evident in relatively frequent headlines—not always accurate, unfortunately—about various aspects of the science and newly presented or published studies.

A great deal of attention has focused on the inspiring case of Timothy Ray Brown, who has lacked any sign of active HIV for eight years now and is the first individual considered cured of the infection. Amidst all the media coverage, it can be difficult to ascertain exactly where the research stands and whether or not a broadly effective cure is on the horizon. This article will attempt to offer a brief guide to the state of the field in the spring of 2015, and some of the challenges that lie ahead.

The lone cured man

The circumstances that led to a cure in Timothy Brown have been extensively documented. Brown had been living with HIV for many years when the occurrence of a life-threatening cancer—acute myelogenous leukeumia (AML)—necessitated not one but two stem cell transplants as part of a complex series of treatments. The purpose of the stem cell transplant procedure is to create a new immune system in the recipient, generated by stem cells received from a genetically matched donor.

Brown’s cancer doctor, Gero Hütter, successfully searched for a donor homozygous for the CCR5-Delta32 mutation, which causes immune cells to lack the CCR5 co-receptor that most HIV variants use to infect cells. When Brown ultimately interrupted ART after receiving the transplants, his viral load did not rebound.

With extraordinary altruism, Brown has volunteered samples from just about every possible tissue and while a few times a trace amount of HIV genetic material has been detected, no sign of virus capable of replicating has emerged. He remains off ART today.

Brown’s experience suggests that it is not impossible to cure HIV. But it can’t be applied to most HIV-positive people, because stem cell transplantation is extremely risky, and can lead to death in around one fifth of cases. Part of the risk is attributable to the cell-killing drugs that are typically given to wipe out the existing immune system and make room for the transplanted cells. There is also a potentially fatal complication called graft-versus-host disease (GVHD), in which donated cells are recognized as foreign and attacked by the recipient’s body.

Researchers are investigating whether it might be possible to repeat the HIV cure achieved in Brown in other HIV-positive individuals who require stem cell transplants to treat life-threatening cancers, but to date there have been no reported successes. Gero Hütter recently wrote a letter to the New England Journal of Medicine reviewing the results of all six known cases where HIV-positive individuals received stem cell transplants from donors homozygous for CCR5-Delta32, and sadly the news was not good: all have died either due to the underlying cancer or complications from the procedure. One of the individuals displayed a rebound of viral load due to the presence of an HIV variant capable of using an alternative receptor to CCR5 (named CXCR4), indicating that even if the cancer had been cured, HIV would not have been.

More practical approaches

A number of alternative approaches are being developed and tested, including gene therapies and a therapeutic vaccine or vaccine-like strategies.

Scientists are also attempting to find methods for depleting the body of the HIV that persists despite ART, so that there’ll be less virus for the immune system to deal with when ART is withdrawn. Or, even better, no intact HIV left at all.

To delve into the depletion approaches first, the population of HIV-infected cells that persists on ART is called the “HIV reservoir.” The vast majority are long-lived “resting memory” CD4 T-cells that contain HIV DNA that has integrated into the genome of the cell but not completed the remainder of the viral life cycle. HIV in this form is described as latent, because virus production can begin at a later time—even many, many years later—if the latently infected CD4 T-cell receives signals that cause it to become activated.

The job of memory CD4 T-cells is to coordinate the immune response to a particular pathogen that your body has encountered sometime in the past (for example, if you were vaccinated against measles as a child, you will still have some memory CD4 T-cells specific for the measles virus antigens contained in the vaccine), so they are designed to be able to survive in a resting state and only become active if they see the antigen that triggered their development.

Importantly, most latently infected CD4 T-cells do not show any outward signs of containing HIV because no viral proteins are being made, and this prevents the immune system from recognizing and targeting them for elimination. Estimates suggest that the average number of latently infected memory CD4 T-cells in an individual on ART is around a million, although recently it has been reported that this could be an underestimate and that the true number could be as much as 60-fold higher.

Not so fast

A large amount of evidence points to the potential benefits of limiting or reducing the size of the HIV reservoir. The most widely publicized involves three individuals who, for a brief and tantalizing period, were thought to have possibly joined Timothy Brown as examples of HIV cures. One is the “Mississippi baby,” who acquired HIV infection from her mother and was started on aggressive ART extremely early after being born. An interruption in ART occurred at around 18 months of age and very unusually, viral load did not rebound; furthermore no HIV reservoir was detectable.

The infant remained off ART for 27 months and there were hopes that she was cured, but then in mid-2014 HIV viral load became detectable again and ART was restarted.

The two other cases are known as the “Boston patients”: these are two HIV-positive men who underwent stem cell transplants to treat cancers. Unlike Timothy Brown, they received stem cells from normal donors lacking the CCR5-Delta32 mutation. But they remained on ART throughout the procedure and it was thought this might prevent HIV from infecting the newly transplanted immune system cells.

After the procedures, an HIV reservoir was not detectable and both ultimately underwent an ART interruption. The virus remained undetectable for three and eight months, respectively, but then viral load returned and ART was restarted.

Although the lack of a cure in these individuals was disappointing, their experience is in line with mathematical modeling indicating that significant reductions in the HIV reservoir can equate to extended periods of “remission” from viral replication. These models predict that diminishing the reservoir even further could lead to a lifelong absence of viral load rebound in a majority of HIV-positive people.

But the magnitude of the task is daunting: it’s estimated the amount of latent HIV declined around 3 logs (1,000-fold) in the Boston patients, but that reductions of 5–6 logs (100,000–1 million-fold) would be needed for lifelong remission.

Nevertheless, the correlation between smaller reservoir size and longer time off ART provides a starting point for one route toward a cure.

Early ART

The Mississippi baby case highlights the potential for starting ART soon after infection to greatly limit the size of the reservoir, and a trial testing whether similar—or longer—periods of remission can be obtained in other perinatally infected newborns began recently. The closest adult equivalent is individuals identified very soon after HIV acquisition, and rapid initiation of ART in this setting is also associated with very small or even undetectable HIV reservoirs. A number of studies are investigating whether early ART, with or without additional interventions, can lead to remission in adults. Some encouragement comes from an unusual group of 20 early-treated individuals in France—known as the VISCONTI cohort—in whom a degree of remission may have occurred; they have maintained extremely low or undetectable viral loads for an average of over nine years after an ART interruption, in the presence of very modest but detectable HIV reservoirs.

Latency reversal

Efforts to deplete the HIV reservoir currently center on compounds referred to as latency-reversing agents (LRAs). As the name implies, they aim to awaken latent HIV, thus either flagging the infected CD4 T-cell for elimination by the immune system or provoking its destruction by HIV’s cell-killing effects. A class of anticancer agents called HDAC inhibitors have emerged as lead LRA candidates, and three—vorinostat, panobinostat, and romidepsin—have demonstrated the capacity to cause HIV production by latently infected CD4 T-cells in clinical trials.

At least six different LRAs are now in human testing, from multiple different classes (HDAC inhibitors, PKC agonists, and toll-like receptor agonists); this represents significant progress given that it was only a few years ago that the first trial of an LRA got underway.

The not so good news is that no decline in the HIV reservoir has yet been observed with any of these agents, meaning additional interventions are likely needed to prompt the killing of infected CD4 T-cells after viral latency is reversed. But the first tentative steps in this direction are occurring: a trial combining romidepsin with a therapeutic HIV vaccine in people on ART is ongoing.

Several other therapeutic HIV vaccine candidates are also being evaluated with a view to being paired with LRAs; the hope is that vaccination will boost or create immune responses capable of delivering the coup de grace to latently-infected cells after the LRA has done its work.

Infusions of neutralizing antibodies are being eyed as another means to achieving this end, as the antibodies can potentially bind to infected cells and mark them for destruction via a mechanism called antibody-mediated cellular cytotoxity (ADCC). An antibody/LRA combination is being tested in macaques infected with SIV (simian immunodeficiency virus, a virus similar to HIV that is  found in certain monkeys).

Coping strategies

Uncertainty about the degree to which the HIV reservoir can be drained is not so much of an issue for approaches that aim to prepare the body to be able to deal with whatever virus is present when ART is interrupted. Gene therapies that modify vulnerable cells are at the forefront of this aspect of cure research, and several are in clinical trials.

It is early days however, and only hints of progress have been seen.

The most extensively studied is Sangamo’s SB728-T, which involves extracting CD4 T-cells from HIV-positive individuals, genetically modifying them so they no longer express the CCR5 receptor, and then reinfusing them in large numbers. In clinical trials, a few isolated examples of prolonged control of viral load after an ART interruption have been reported, suggesting modified CD4 T-cells may be able to coordinate a more effective immune response against HIV.

The challenge is to attain more robust viral load control in greater numbers of people, and ways of modifying greater numbers of CD4 T-cells are being tested. A new trial that will use the technique on stem cells—which could potentially give rise to HIV-resistant CD4 T-cells in the body—has recently been given the green light by the FDA.

A company called Calimmune is also testing a gene therapy that seeks to generate HIV-resistant CD4 T-cells by altering and infusing stem cells (results from this trial are pending).

Over the past two decades, an array of different therapeutic HIV vaccines have been tested with the goal of promoting control of viral load when ART is stopped. But perhaps because CD4 T-cells—HIV’s primary target—play such a crucial role in sustaining vaccine-induced immune responses, significant success has proven elusive.

Hence the recent shift in focus to combining therapeutic vaccines with LRAs to reduce the viral reservoir in people on ART, which may be a more modest task than controlling viral replication after ART withdrawal. There is a seemingly uniquely potent therapeutic vaccine candidate derived from a cytomegalovirus (CMV) vector that has shown success in macaques, but it has yet to be tested in humans.

An innovative alternative to vaccination made headlines recently and is worthy of mention: it comprises a highly potent inhibitor of HIV named eCD4-Ig delivered using an adeno-associated virus (AAV) vector. The AAV takes up residence in muscle tissue and acts as a factory for churning out eCD4-Ig, which could theoretically equip a person with a means of lifelong HIV suppression. The idea has shown promise as a preventive in macaques exposed to SIV, and therapeutic studies in this animal model are now planned.

Collaboration

There is so much activity in the cure research realm that this brief review has only skimmed the surface, focusing on ideas that are in human trials or may soon get there. Progress is certainly being made, but most scientists suspect it will likely be decades before a widely applicable curative intervention might come to light.

Solving the many challenges on the horizon will require collaboration, and encouragingly many collaborative endeavors are underway or being formed. These include the International AIDS Society’s Towards an HIV Cure initiative which is currently updating its global strategy recommendations; amfAR’s Research Consortium for HIV Eradication (ARCHE); the National Institutes of Health’s Martin Delaney Collaboratories (named after the activist and founder of Project Inform) which are soon to be expanded; the Forum for Collaborative HIV Research’s cure project focused on the pathways toward regulatory approval; and a growing number of consortia globally such as CHERUB (Collaborative HIV Eradication of viral Reservoirs: UK BRC) in the United Kingdom.

Advocacy continues to be key in supporting and guiding this work, so that one day HIV can be conquered once and for all.