Thursday, April 25, 2013

Why haven’t we found a cure for HIV?

     In 1983 the human immunodeficiency virus (HIV) was first isolated and suggested as the root cause of acquired immune deficiency syndrome (AIDS), a universally fatal condition thanks to various opportunistic diseases that take advantage of the sufferers weakened immune system (1). Today HIV is one of the most intensely researched viruses in the world and new drugs are constantly being developed to minimize its effects in HIV+ patients. Yet despite sophisticated cocktails of these drugs that are administered during the most popular treatment for HIV infection, highly active antiretroviral therapy (HAART), we still have not managed to develop a therapeutic strategy to fully eradicate the virus from those infected with it.

     The reason a cure for HIV has been so difficult to obtain has to do with the virus’s latent reservoir. Most cells that become infected with HIV start producing infectious viruses within a few days. And they do this at a high enough rate that the cell eventually dies, either directly due to the viral replication itself or indirectly due to the host’s immune system. These cells die off and are no longer a threat for producing more viruses. A cell involved in the HIV latent reservoir, however, only produces viruses at a low rate or not at all. It can remain dormant, evading the host’s immune system while still containing the HIV genome and, therefore, the ability to produce infective HIV viruses. Recent research has suggested that memory T cells, which are involved in the mechanism that allows the immune system to remember pathogens after infection has cleared, make up the largest proportion of the HIV latent reservoir (P,2). The long-lived nature of this cell type means that it would take an exceedingly long time to wait for each of these proviral cells to die. Recent modeling suggests it could take up to 70 years (3).

     Because so many successful antiretrovirals have been developed, patients adhering to HAART can delay the onset of AIDS indefinitely (P). These drugs are very good at preventing any HIV viruses that persist in the patients bloodstream from infecting new CD4+ T-cells, the viruses target cell type. However, once taken off HAART, these patients start shedding new viruses and progress rapidly to AIDS. Therefore, HAART must be a lifelong treatment, one that is very expensive and very difficult to keep up with. The reason HAART does not fully eradicate HIV is because the cells of the latent reservoir still contain the HIV genome, which enables those cells to manufacture infectious viruses, which are detected in the individuals blood. If the patient is fully adhering to HAART these viruses simply degrade and do not infect new cells. But if the patient is taken off HAART these viruses can infect new cells, which can then go on to shed more viruses.

     If a cure for HIV is to be developed, the latent reservoir must be dealt with (4). One of the most recent ideas is to use histone deacytelase inhibitors (HDACis), commonly used drugs that could theoretically unpack the HIV genome of latently infected cells, allowing for the rapid production of new viruses and subsequent clearance of that cell either directly by HIV replication or indirectly by the host’s immune system (2). If HAART is intensified during this treatment the newly produced viruses would not be able to infect new cells and would eventually die off. At that point the individual would no longer contain any infective HIV viruses or any of the cells that contain its proviral genome.

     Now the question is which HDACi to use. There have been a number of potential candidates but the results of studies involving them have been weak and sometimes contradictory. Bartholomeeusen et al. decided to focus on better understanding the mechanism by which HDACis contribute to the activation of HIV replication in cells of the latent reservoir (P). One of the reasons resting T cells are thought to be involved in the latent reservoir is the combination of their ability to become infected with the HIV genome and their very low levels of the activated form of a protein called Positive Transcription Elongation Factor b (P-TEFb), which is responsible for the initiation of HIV replication.

     In order to determine whether this protein plays a role in the HDACi-mediated reactivation of latently infected cells, Bertholomeeusen et al. infected human T cells with the HIV genome (P). While these cells are not memory T cells they provide a suitable cell line to study HIV latency. After this proviral infection was established the cells were treated with four different HDACis: vorinostat, ST-80, entinostat, and tubastatin A. These cells were then monitored both for the production of new HIV viruses and for their levels of P-TEFb. The HDACi vorinostat successfully reactivated these latently infected cells, leading to the production of new viruses. ST-80 had a similar effect but entinostat showed a weakened effect and tubastatin A had no effect at all.

     The difference in the efficacies of these drugs can be explained by their ability to activate P-TEFb. Treatment with both vorinostat and ST-80 was shown to increase levels of active P-TEFb, an effect not observed with entinostat or tubastatin A. This finding suggests that the unpacking of the HIV genome, which all of the HDACis should be capable of doing, is not sufficient for reactivation of HIV replication in latently infected cells. It is also necessary for there to be activated P-TEFb to initiate HIV replication. This finding was further supported when Bertholomeeusen et al. attempted reactivation of latently infected resting T cells, which were provided by human donors (P). These cells only underwent significant reactivation following treatment with both vorinostat and another drug, bryostatin 1, which increases the levels of active P-TEFb in resting T cells.

     This research provides significant insights into the mechanism of HIV reactivation and can help inform future decisions about which drugs should be investigated during preclinical trials. The major discovery of this study is that the use of an HDACi might not be sufficient without the assistance of an additional drug that can increase the intracellular levels of P-TEFb. It is also important to note that the patient would need to maintain adherence to HAART during treatment. Nonetheless, this study provides new insights into the mechanism of HIV reactivation in the latent reservoir.

     The next step is to identify which HDACi/P-TEFb drug combinations work the best and to get them approved for clinical trials. Unfortunately these trials can take up to 10 years before the treatment reaches consumers. A good HDACi to look into for future studies is valproic acid, which has been shown to target the specific proteins that pack the HIV genome and has already been approved by the FDA for use in humans, which would speed up the approval process for use as a treatment for HIV infection. This study shows that it might be possible that treatment with an HDACi in combination with a drug that upregulates P-TEFb during intensified HAART could facilitate the clearance of the HIV latent reservoir and lead to the complete eradication of the HIV virus from any infected individual. This would allow HIV+ patients to stop receiving complicated and expensive HAART and go on to live normal lives. Even more, on an epidemiological scale, an effective cure for HIV would lead to the decrease and possible elimination of HIV/AIDS from the human population.

Primary Source

Bartholomeeusen, Koen, Koh Fujinaga, Yanhui Xiang, and Matija Peterlin. "HDAC
            Inhibitors That Release Positive Transcription Elongation Factor B (P-TEFb)
            from Its Inhibitory Complex Also Activate HIV Transcription." The Journal of
            Biological Chemistry (in press).

Secondary Sources

1. Barré-Sinoussi, F., J. Chermann, F. Rey, M. Nugeyre, S. Chamaret, J. Gruest, C.
            Dauguet, C. Axler-Blin, F. Vezinet-Brun, C. Rouzioux, W. Rozenbaum, and L.
            Montagnier. "Isolation of a T-lymphotropic Retrovirus from a Patient at Risk
            for Acquired Immune Deficiency Syndrome (AIDS)." Science 220.4599
            (1983): 868-71.
2. Rasmussen, Thomas, Martin Tolstrup, Anni Winckelmann, Lars Ostergaard, and
            Ole Sogaard. "Eliminating the Latent HIV Reservoir by Reactivation
            Strategies." Human Vaccines & Immunotherapeutics 9.4 (2013): 1-10.
3. Conway, Jessica M., and Daniel Coombs. "A Stochastic Model of Latently Infected
            Cell Reactivation and Viral Blip Generation in Treated HIV Patients." PLoS
            Computational Biology 7.4 (2011). PubMed. Public Library of Science, 28 Apr.
4. Lewin, Sharon R., Vanessa A. Evans, Julian H. Elliott, Bruno Spire, and Nicolas
            Chomont. "Finding a Cure for HIV: Will It Ever Be Achievable?" Journal of the
            International AIDS Society 14.4 (2011). PubMed. BioMed Central, 24 Jan.


  1. Interesting how producing more HIV viruses actually helps eliminate them from the body. It seems like there are a lot of drugs involved in the therapeutic strategy proposed, even more so than typical HAART. Do you think that this would have any negative impacts on the liver or any other side effects?

    1. It is kind of counterintuitive but it is the active production of HIV viruses that enables the hosts immune system to identify and eliminate those cells which are infected with HIV. As for the negative side effects I know that many HDACis have already been approved for use in treating other disorders such as epilepsy and a number are currently being evaluated by clinical trials for the treatment of various cancers and other diseases. These studies show that HDACis have relatively few side effects. In addition, the way that the HIV virus replication is initiated only requires a small amount of P-TEFb to be activated. After that occurs the virus can take advantage of non activated P-TEFb as well. So negative side effects from the P-TEFb drugs would likely be minimal as well. However, HAART is already taxing on the body and adding these two other drugs certainly wouldn't help the situation. It might however allow the patient to stop using HAART and without the need for lifelong treatment the body would be allowed to recover. Nonetheless, we are not truly going to understand how this combination of drugs would affect the body until clinical trials are performed to evaluate its toxicity in humans.

  2. So interesting and hopeful! How do you think the development of this new treatment would affect cost and distribution?

    1. It would definitely be more costly to distribute this treatment involving HDACis and a P-TEFb stimulating drug. The costs of HAART alone is the reason why so many people in sub saharan Africa, by far the densest area of HIV infected patients, are not receiving it. However, if this method truly allows the virus to be eliminated from the body, then the long term costs would be significantly decreased. Patients would not need to pay for HAART for the rest of their life but rather just until the virus is eliminated. It is also likely that in this situation the United States Government and the governments of other countries would subsidize the distribution of this treatment in an effort to lower the prevalence of HIV/AIDS on an epidemiological scale, as was done with the polio virus.

  3. This is an extremely interesting and important post. Very eye-opening! I am very interested in the beginnings of their research into HIV/AIDS. Do you have any background on the first steps taken to figuring out how to deal with these two diseases? How did they come up with HAART and their earlier drug treatments?

    1. Back when AIDS was known as gay-related immune disease (GRID) it was universally fatal within a few years. The viral origin of the disease was not understood very well at all and treatments were mainly focused on symptomatic relief. The first real treatment came with the advent of antiretrovirals, specifically AZT, which is a nucleoside analogue that interrupt the viruses reverse transcriptase protein, preventing its replication in cells. However, the HIV virus is possibly the most rapidly evolving entity on the planet and it quickly develops resistance to individual drugs. HAART was introduced as a way to prevent the virus from developing resistance to AZT and other antiretrovirals. To develop these kinds of drugs researchers usually use a kind of forward genetic approach, adding various agents to cells and then infecting them with the HIV virus, those that develop resistance to the HIV virus would survive and then the researchers can go back and figure out which drug or which mutation conferred this resistance to them.

  4. This is a very interesting and well thought-out article. I'm very passionate about HIV/AIDS awareness and prevention so it was refreshing to hear helpful suggestions for steps to take to create more effective treatments. I was also surprised that the solution would be to produce more HIV viruses. I'm curious as to what the HAART treatments actually do to defer HIV/AIDS effects if they don't eliminate the latent reservoir cells.

    1. HAART is a combination of antiretrovirals that are taken to prevent the virus from developing resistance to any one of them. Usually it is a mix of three antiretrovirals, which target the HIV replication cycle at different points. For example, the HIV protein reverse transcriptase is responsible for copying the HIV RNA genome into DNA so that it can be inserted into the hosts genome. One of the common antiretrovirals targets this protein so that a virus that has newly infected a cell cannot reverse transcribe its genome, preventing it from fully infecting the cell and leading to the production of new viruses. However, these antiretrovirals are rarely administered immediately after a person has come into contact with the virus. Any viruses in the bloodstream can infect CD4+ T cells, inserting their viral genome into the cells genome. If the cell is of the latent reservoir it would produce HIV viruses for an extended period of time without being cleared by the immune system. The antiretrovirals used in HAART prevent viruses in the bloodstream from infecting new cells, but do not kill cells that already have HIV DNA inserted into their genome, allowing them to continue producing more viruses.

  5. Unfortunately, resting T cells are not the only potential reservoir...macrophages, microglia, and some other cell types (such as hematopoietic stem cells) may also serve as a reservoir. Did the authors comment on whether this treatment would be effective against these reservoirs? Also, the use of HDACi's also poses some risks, as you might also activate expression of endogenous retroviruses encoded in the genome (this has been a problem in other studies); while promising, I think there needs to be a lot more research on this before it will be of practical clinical use.

    1. The authors did not mention other cells involved in the latent reservoir. I believe that as long as these drugs could reach those cells they would be able to have a similar impact. Unfortunately, a lot of these other cells are not nearly as accessible as resting T cells. For example, microglia of the central nervous system are significantly more difficult to reach and the HDACis would have to overcome the blood brain barrier in order to reach them. More research is definitely necessary to determine the clinical possibilities of using this form of combined therapy, perhaps utilizing BLT mice models, which have been treated surgically in order to develop a human-like immune system.