Complete elimination of HIV from the body will involve displacing the virus from all its hiding places and preventing the refilling of these reservoirs. A difficult task but perhaps not impossible
By Mario Stevenson
Attempts to develop a vaccine for HIV have indeed failed, but efforts to develop drug treatments, on the other hand, have been impressively successful. More than 25 drugs have been approved so far, and in the right combinations they succeed in suppressing the replication of the virus to such an extent that its level in the blood drops below the detection threshold of the usual tests. Cocktails of these drugs, collectively called "highly active antiretroviral therapy" (HAART), have already extended the lives and improved the health of countless patients. However, unfortunately, these treatments do not have the power to completely cure the disease. If treatment is stopped for any reason, the virus returns and multiplies rapidly.
One of the most important tasks now facing the researchers is to understand how the virus manages to survive in the body in the presence of the drugs. In the last ten years, research has progressed greatly and yielded important insights into this mystery. Hopefully, the answers will eventually clarify whether it is possible to completely eliminate the virus from the body.
To understand the nature of HIV's hiding places in the body and what needs to be done to destroy these reservoirs, it is necessary to understand how the virus behaves in the body. HIV, like all viruses, needs to enter a body cell to replicate itself. The invader uses the cell's mechanisms to make copies of its genome and translate viral genes into proteins. It produces new virus copies, called virions, and these spread to other cells. But unlike most other viruses that harm humans, HIV inserts its genome into the human genome in the cell. Thus, whenever an infected cell multiplies, the viral genes are copied and transferred to the daughter cells, and the existence of the virus in the body is guaranteed as long as the cell and its descendants survive in it.
The immune system succeeds, for the most part, in eliminating viruses by destroying the infected cells. It recognizes such cells easily by the bits of viral protein, the antigens, displayed on their surface to signal the presence of an invader inside. In the case of HIV, the immune system has difficulty destroying the infected cells on its own, partly because the virus attacks components of the immune system itself. Over a period of time, the body manages to attack back and produce new, healthy immune cells capable of recognizing the virus and other invaders. But in people who are infected and do not receive treatment, the virus eventually increases and causes acquired immune deficiency syndrome - AIDS.
The drug combinations given today protect the immune system by suppressing the replication of HIV and limiting the spread of the virus to new cells. Theoretically, these treatments should allow the unaffected parts of the immune system to eliminate any remaining infected cells and cure the disease. If so, why does the immune system fail in this task, despite the drugs?
keep a low profile
It seems that a major reason for this is that cells remain in the body that can genetically produce new virions, but they do not do so and therefore do not inform the immune system of their presence. HIV, as David A. Watkins writes in the article "The search for a vaccine component continues" on page 23, prefers to penetrate immune cells of the type called helper T cells, which are found mainly in the lymph nodes and connective tissues of the digestive system, but also in other lymph nodes and in the bloodstream.
Most of the helper T cells that fight the viral infections die when they are no longer needed. But a small subset of T cells survive as long-lived memory cells, and these will rush to multiply and mobilize the reserve forces if they encounter signs of recurrent infection. But because HIV infected T cells in the first place, it is these memory cells that apparently produce most of the virus in patients. When the cells divide to attack the pathogen they remember, they replicate their own DNA and proteins and at the same time produce new virions. Most of the infected memory cells die from the virus itself or from being attacked by the immune system, but some return to a dormant state. At this point, HIV exists only as viral DNA sitting quietly within the human genome in cells. This viral DNA is not copied and does not produce virus proteins, and therefore no pieces of protein are displayed on the surface of the cell. Therefore, the anti-HIV drugs do not affect the cells, and the immune system does not sense them.
This understanding arose from studies published in 1997. Independent teams, led by Robert P. Siliciano of Johns Hopkins University, Anthony S. Fauci of the US Institutes of Health (HIV), and Douglas D. Richman of the University of California, San Diego, found that inactive T cells isolated from HIV carriers do not produce viruses . But when these cells were awakened, the dormant virus returned and began to multiply. HIV is not the only virus that hides in a dormant state. In fact, some viruses, such as the herpes virus, produce proteins that encourage the virus to go into a dormant state. Estimates based on the lifespan of memory T cells suggest that it will take more than 50 years for the pool of cells infected with dormant HIV to die naturally.
However, researchers are beginning to understand that it is not only helper T cells that contain dormant virus that are responsible for the return of HIV after stopping treatment. It seems that even during successful treatment, when no viruses are found in the blood, some helper T cells and other cells continue to slowly produce new viruses. This activity occurs "under the radar" of the tests, as the released virus successfully hides in the cells, or it remains trapped in the tissues and does not find its way into the blood. In 2007, for example, the study revealed that helper T cells in the intestines diminish within weeks of being infected with HIV, even before the virus is detected in the blood. It is therefore possible that during the treatment the virus can continue to replicate in tissues such as the intestinal tissue, and that this activity takes place covertly for a long time before the virus reaches the blood.
Another innocent partner to talk about her past
AIDS research has focused, for the most part, on helper T cells because they are found in the blood and are easy to obtain with a simple blood test. Recently, researchers have begun to understand that other cells of the immune system infected with HIV - macrophages and dendritic cells - may contribute to the strengthening of the virus after treatment is stopped or after the virus has developed resistance. The knowledge about macrophages and dendritic cells is not so much, because they are located only in tissues, but new findings indicate that it is possible that the drug treatment does not completely stop the culture of HIV in these cells. The level may be too low for the virus to reach the blood in detectable quantities, but it is enough for it to reach nearby T cells and steadily replenish memory cell stores containing dormant virus. It also seems that some of the infected macrophages are not killed by the virus inside them or by other components of the immune system. Macrophages can therefore wait in the ready state and increase replication when the drug treatment is stopped.
In 2001, for example, Malcolm A. Martin of the US Institutes of Health (NIH) and his colleagues reported that monkeys infected with the simian AIDS virus, SIV, lost most of their helper T cells within a few weeks of infection, but the virus nevertheless continued to form in their bodies in large quantities . It turned out that it was macrophages that produced the virus. When these monkeys were treated with a drug that inhibits the replication of viruses, thereby preventing the infection of new cells, this did not significantly reduce the amount of virus in the blood. This finding meant that the macrophages did not die following the production of the new virus copies.
It also appears that the process of HIV replication in macrophages is somewhat different from the process that occurs in T cells in a way that further benefits the virus. In T cells, the virus components accumulate on the cell membrane, from which they then detach. In macrophages, on the other hand, some virus particles are inserted into compartments in the cell called vacuoles. The bubbles can migrate to the surface of the cell and release the virus particles. The packaging of the virus in closed compartments may help HIV to escape the immune system's detector as it prevents the presentation of antigens on the cell surface, thus not revealing the presence of an invader to the immune system.
And research shows that the drug concentrations needed to suppress viral replication in macrophages are higher than the concentrations needed in T cells. It is not clear why this is so, but it is known that certain cellular proteins, whose normal function is to secrete biological substances from the cell, can interfere with drug treatment by preventing absorption and accumulation of the drugs in the cells. It is possible that these proteins are mainly active in macrophages and prevent the drugs from reaching the desired level. The same may be happening in dendritic cells, but very little is currently known about how these cells respond to HIV.
Anatomical receivers
It is not only the inherent properties of helper T cells and macrophages that enable HIV to survive intensive drug therapy. Some of these cells are located in anatomical compartments that shield them from various drugs, or from the body's defense systems, or both. To completely destroy HIV in the body it is necessary to reach it in these sections.
The central nervous system is one of these sections. Researchers have known for a long time that the central nervous system is vulnerable to HIV. The neurological problems that arise in the later stages of AIDS are due, in large part, to neurotoxins released from macrophages infected with the virus and found in the brain. In order to penetrate the brain, the cells or molecules need to cross the barrier from the blood to the brain, which is essentially a selective membrane that regulates the movement of cells and other substances from the blood to the central nervous system. Macrophages infected with HIV in tissues outside the central nervous system can, it seems, cross the blood-brain barrier and settle in the central nervous system, where the virus can continue to infect specialized macrophages called microglia, whose permanent residence is inside the central nervous system.
The penetration of the virus into the cells of the central nervous system apparently gives it a certain degree of protection from drugs because some of them, especially protease inhibitors which are important for the proper processing of new virus proteins, have difficulty crossing the barrier and reaching the brain. Also, most of the immune cells around the body do not reach the brain. It is not known if infected brain cells can send HIV to other parts of the body, but if the macrophages infected with the virus can cross the barrier between the blood and the brain and reach the central nervous system, they can, obviously, also infiltrate back.
Other areas that are difficult for drugs to penetrate are the walls of the digestive system and the reproductive system. In HIV-positive men, HIV RNA (RNA) is often found in the semen, even when there is no sign of the virus in the blood.
New attack plans
Complete elimination of HIV from the body of an infected person would require, at a minimum, the elimination of all T cells containing latent virus. One of the ways currently being researched to solve the problem of hidden reservoirs is treatment with substances that stimulate dormant T cells that have been infected to wake up and divide, in the hope that the cells will produce a virus and be exposed to the antiviral treatment. Two limited human trials have tested this approach with commercially available drugs approved for use in other conditions. The results were inconclusive.
The perfect substances will stimulate the T cells to the extent required to resume production of the virus proteins displayed on the cell surface, but not enough to stimulate them to produce new copies of the virus. To this end, researchers are currently looking for drugs that will stimulate the synthesis of HIV proteins by changing the way in which the chromatin is organized (conjugations of DNA and protein that make up the chromosomes) in dormant T cells that have been infected with the virus. But even these agents will be of little use if they only work on T cells when the virus also resides in macrophages.
A second arrowhead in the attack to rid the body of HIV will have to block any replication of the virus, so that it disappears not only from the blood but also from the tissues and all types of cells in which it hides. The drugs currently in use disrupt the action of one of two enzymes: reverse-transcriptase, which changes the genetic material of the virus from RNA to DNA in order to insert it into the cell genome, or protease, which helps to mature new virus particles. Within weeks of starting the usual treatment, the level of the virus in the patient's blood drops to undetectable levels. The decline slope was found to be fairly constant from patient to patient, and the researchers interpreted this as evidence that the treatment completely prevented the virus from replicating. However, recent studies have shown that if you increase the standard treatment with the new drug raltegravir, which damages a viral enzyme that has not been treated so far (the viral enzyme integrase, which inserts the DNA of the virus into the DNA of the cell), the decline of the virus is accelerated. This success shows that it is possible to damage infected cells more quickly and more efficiently than is currently done. If this hypothesis is correct, it implies that even more vigorous treatment of HIV may limit the size of the original latent reservoir, prevent its refilling, and perhaps even reduce replication so much that the immune system can overcome all the virus-producing reservoirs left after all cells are destroyed The latent memory infected with the virus.
In 2007, several new drugs entered clinical trials, disrupting steps in the replication of the virus that were not previously attacked. Besides the integrase inhibitor, there is another drug that prevents infection by disrupting the ability of the virus to attach to the CCR5 molecular receptor located on the cell surface. Various studies show that certain cellular proteins may also serve as good targets for drug therapy. Although HIV recruits some of these proteins to help with its replication (CCR5, for example), it now appears that other cellular proteins, or cellular restrictions, as they are called, actually work against the replication of the virus.
Six years ago, Michael H. Mallim of King's College London and his research group identified the first cellular marker, A3G. This protein is abundant in macrophages and lymphocytes. Unfortunately, the virus has developed a countermeasure to A3G: it produces a protein, Vif, which causes the degradation of A3G. The good news is that both A3G and the viral protein Vif are promising targets for drug therapy. Drugs that inhibit Vif or otherwise protect A3G from degradation would, in theory, make human cells resistant to HIV infection.
In 2008, two independent research teams, Paul D. Beniasch's team at the Aaron Diamond Center for AIDS Research in New York City and John S. Guately's team at the University of California, San Diego, identified a second sag. The caveat, called tetherin, prevents the release of new virus copies from infected cells. It turns out that HIV has also developed a defense against tetherin through the viral protein Vpu. Drugs that would block the action of Vpu could prevent the spread of HIV to new cells.
In basic research, new targets for drug treatment will be added and discovered, most likely, and these will lead to the development of new substances that will harm HIV in a variety of ways. If we are able to plan medicines that will complement and enhance the effects of the existing treatments, we will eventually succeed in depleting the reservoirs of the dormant virus and eliminating it completely from the body. To this end, large studies are currently being conducted examining the effect of more vigorous drug treatments on the virus. The results are expected within two years, and from them we will learn if complete elimination of the virus in the body is an achievable goal. We wait with great hope.
key concepts
The medical treatments given today, the "cocktails", can suppress the AIDS virus, HIV, in the bodies of carriers, but they do not have the power to completely eliminate the virus.
To remove HIV from the body, researchers need to understand where it hides and how to damage it in those places.
New findings reveal some of the HIV shelters in the body, and raise new treatment options.
on the tip of the fork
In 2007 there were 33 million HIV carriers worldwide
Every day about 6,000 people die from AIDS and about 6,800 become infected with the virus
Treatment against HIV is currently accessible to less than a third of those who need it
Highly active antiretroviral therapy (HAART), "cocktail", increases the patient's chances of survival by an average of 13.3 years.
hiding place
Most of the HIV in the blood comes, it seems, from lymphocytes of the immune system, called memory T cells, that have been infected with the virus. These cells, on the surface of which pieces of HIV are displayed, die for the most part from the infection itself or from the attack of the immune system directed at the displayed pieces. But some survive and enter a dormant state for many years (far right). In this state, they shelter the HIV genome within their DNA and can produce new copies of the virus if they are reactivated.
The many reservoirs of HIV
HIV survives in the body not only by patiently waiting in dormant T-type memory cells, but also by multiplying at a slow rate in other cells of the immune system, especially in macrophages and dendritic cells, which seem to have an inherent ability to repel immune defenses and anti-HIV drugs to a certain extent. Moreover, in some areas of the body, cells infected with HIV may gain some degree of protection from the immune system and certain drugs. HIV created in cellular and anatomical reservoirs has difficulty reaching the blood in patients receiving aggressive treatment, but may break out with great force when treatment is stopped.
promising treatment approaches
To completely eliminate HIV from the body, it would be necessary, at the very least, to cause infected dormant T cells to produce new viruses or viral proteins, actions that would attract an attack by drugs or the immune system. Such treatments will be given together with the routine drugs that block the spread of the virus from cell to cell. There is new evidence that increasing the control of viral replication, by hitting new viral or cellular targets, may also be beneficial. Potential targets for drug therapy to achieve these goals (orange) are described on the page to the right. Medicines that are already on the market (in green) target the protein coat of the virus and the CCR5 receptor of T cells (to block the entry of the virus into the cells) and try to inhibit the viral enzymes reverse-transcriptase, integrase and protease (to stop the copying of the HIV genome, its insertion into the DNA "A of the cells and the maturation of the HIV proteins, respectively).
Potential targets for drug therapy
Vif (viral infectivity factor):
The cellular protein called A3G causes extensive mutations in the HIV genes and thus impairs its ability to continue to exist. HIV's Vif protein interferes. Inhibiting Vif or otherwise protecting A3G will allow A3G to do its job against the virus.
LEDGF (lens epithelium-derived growth factor):
In HIV-infected cells, the cellular protein LEDGF helps integrase to join the DNA of the virus into the cell's genome. Studies show that inhibiting LEDGF reduces HIV replication.
Chromatin (linking DNA and protein that make up chromosomes):
Drugs that change chromatin organization will act on infected dormant T cells so that the cell will resume synthesizing HIV proteins. As a result, the cells will be revealed to the immune system, which will attack them and destroy them.
Vpu (viral protein U):
In HIV-infected cells, the newly produced viruses are bound to the front of the cell, but the HIV Vpu protein releases them. A Vpu inhibitor will prevent the virus from spreading to other cells.
About the author
Mario Stevenson is a professor of AIDS research in the molecular medicine program at the University of Massachusetts Medical School. He received his doctorate from the University of Strathclyde in Glasgow, Scotland, where he studied the introduction of drugs into macrophages using liposomes. He was invited to lecture at the prestigious Shipley Symposium of Harvard Medical School, and serves as chairman of the research committee of the American Foundation for AIDS Research and director of the Center for AIDS Research at the University of Massachusetts Medical School. He was also a consultant to the pharmaceutical company Merck. In his spare time he enjoys playing the piano and speed skating in the hall.
More on the subject
Macrophages Are the Principal Reservoir and Sustain High Virus Loads in Rhesus Macaques after the Depletion of CD4+ T Cells by a Highly Pathogenic Simian Immunodeficiency Virus/HIV Type 1 Chimera (SHIV): Implications for HIV-1 Infections of Humans. T. Igarashi et al. in Proceedings of the National Academy of Sciences USA, Vol. 98, no. 2, pages 658-663; January 16, 2001.
Isolation of a Human Gene That Inhibits HIV-1 Infection and Is Suppressed by the Viral Vif Protein. AM Sheehy et al. in Nature, Vol. 418, pages 646-650; August 8, 2002.
Antiretroviral Therapy with the Integrase Inhibitor Raltegravir Alters Decay Kinetics of HIV, Significantly Reducing the Second Phase. JM Murray et al. in AIDS, Vol. 21, no. 17, pages 2315-2321; November 12, 2007.
Tetherin Inhibits Retrovirus Release and Is Antagonized by HIV-1 Vpu. SJD Neil et al. in Nature, Vol. 451, pages 425-430; January 24, 2008.
