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Abstract
In 1996, two leading AIDS researchers shocked and excited participants at an international AIDS meeting by announcing that they planned to stop drug treatment for a number of their patients. The researchers, David Ho and Martin Markowitz of the Aaron Diamond AIDS Research Center in New York City, wanted to know if their new drug combination therapy had achieved the ultimate success - elimination of the virus from the body, which would result in a cure for the deadly disease.
| A novel strategy may identify hidden viral reservoirs. |
The answer to their question was not long in coming. Despite the fact that the HIV levels had been reduced for varying lengths of time to "non-detectable levels" in the patients selected for the experiment, when drug therapy was withdrawn the AIDS virus came roaring back. Obviously, the drugs did not eliminate HIV from the body; somewhere, latent reservoirs remained, reservoirs of the virus just waiting to rebound that were sequestered from treatment by the potent new drugs. Locating these hidden pockets of HIV was recognized as essential if ever the word "cure" were to be uttered in conjunction with AIDS. Now, five years after that first burst of optimism, the regeneration sites for the virus remain a mystery. However, a novel strategy has emerged to identify these reservoirs of HIV infection in the body of patients who have responded well to drug therapy.
In hindsight, the results of this experiment were not surprising. Numerous viral diseases, even those that are controlled by natural immunity such as chicken pox and mononucleosis, are not cured in the sense that all trace of the virus is eliminated from the body. Instead, the virus remains sequestered for life in a reservoir where it is held in check by the action of the immune system. The chicken pox virus, for example, maintains an infection in the nerve cells for the lifetime of the host.
| Monkeys lose every T cell in circulation, yet the virus comes roaring back. |
The AIDS virus acts on the body by infecting and killing T cells, the very cells from the immune system that are necessary to produce a natural immunity to viruses and other microbes. Since HIV carries its genetic material as RNA and early in its cycle of infection produces a DNA copy of this RNA genome, which is inserted into the host genome, the latent reservoir of the virus may be actual viral particles or just the RNA or DNA copies of the viral genome. It does not appear that the infected T cells are the site of the latent reservoir of HIV. Monkeys infected with a form of the AIDS virus lose every T cell in circulation, yet the virus comes roaring back eventually, causing the death of the animal.
T cells are not the only place where HIV is found. There is strong evidence that macrophages are infected by HIV but are not destroyed by the virus, as are the T cells. Moreover, HIV within the macrophages appears to be sequestered from the currently available drugs. The virus may also be found in lymph nodes, testes, intestinal and vaginal epithelium, parts of the brain, spleen, kidney, liver, sperm, and reproductive cells. However, "discovering viruses in these places is a long way from showing that when rebound occurs, those are the sources of the virus," says Stephen J. O'Brien, a researcher at the National Cancer Institute in Frederick, Maryland.
| Local variation can be diagnostic. |
O'Brien is one of a group of scientists using genetic techniques similar to those used for phylogenetic reconstruction in an attempt to identify the location of the viral reservoir that causes HIV rebound. "It's sort of like what evolutionary biologists do when they investigate animals that live in different geographic locales. They simply compare the gene variations to construct a tree to determine if the individuals from one locale are more similar to individuals in another locale, and how they compare to the individual at the next locale," said O.Brien. "So, if you consider the human body as an ecosystem or a geographic unit with lots of little neighbors and that HIV replicates at a really fantastic rate - as many as 10 million copies of virus every day in some individuals, and every one will have one or two mutations - there is accumulated local variation that can be discovered and can be diagnostic" with regard to the origin of the virus.
The strategy for determining the tissue source of the rebound virus requires sequencing of the viral genome from HIV that has been isolated from different tissues within a single individual. Autopsy material from patients who have died of AIDS while not on drug therapy will be used as a source for the virus-infected tissues. O'Brien thinks it likely that once the source of the rebound virus has been identified for one individual it will be the same for others. "All these tissues are different habitats. They have different genes operating that make them more or less favorable for replication of HIV, and I'm sure that every tissue has an edge or a detriment over other tissues [for HIV replication]. That's my speculation," said O'Brien.
| Individual HIV genes accumulate mutations at different rates. |
One of the potential problems with the approach proposed by O'Brien is that the viral genome may accumulate mutations at a rate too slow or too rapid to allow the investigators to determine its migration from one tissue to another. Indeed, this is one critical aspect for using genetic techniques to construct conventional phylogenic trees: a gene must be used as a chronometer that accumulates mutations at a rate compatible with the time-scale of the organism's overall rate of evolution. Fortunately, the individual genes of HIV accumulate mutations at different rates, providing a wide range of possible chronometers. "The envelope gene evolves the fastest; it is driven largely by the immune system in selecting changes that stimulate an immune response. The POL gene evolves slower - it may be the one we want to look at - while the GAG gene has a totally different set of constraints," said O'Brien. "I don't really have enough of a consensus about what restraints exist for these various genes to pick out the one that will be the most useful. One of the things that might come out of this study, even if we do not discover the [viral] reservoir, is at least a better understanding of the [evolutionary] constraints on the various genes."
"The project is in the earliest possible stage," said O'Brien, who went on to explain that the researchers will first use cats as an animal model to work out the details of the approach before working with human tissues. "The cat has a version of the AIDS virus that is very similar to the human virus in terms of what it does: the collapse of the immune cells. Other than humans, the cat is the only animal in the wild that actually suffers from an AIDS-like virus. So, we have an interesting parallel with cats, and they can be used as an animal model to develop techniques to look for the reservoir of the virus."
O'Brien thinks it will be a year before his group has preliminary data to share on the project. "It's going to take a while; we want to do it right."
Robert W. Wallace is a freelance writer based in New York City.
Matt Morrow is a freelance illustrator from Omaha, Nebraska.
HIV Latency in Naive T Cells - describes recent work indicating naïve T cells can be latent reservoirs. From Trends in Molecular Medicine, 2001, 7:7:285. Full text available from BioMedNet.
Therapeutic Targeting of Human Immunodeficiency Virus Type-1 Latency: Current Clinical Realities and Future Scientific Possibilities - a review of recent work and future directions. From Antiviral Research, 2000, 48:3:143 - 176. Full text available from BioMedNet.
Latent Reservoirs of HIV: Obstacles to the Eradication of Virus - reviews the identification, characterization, and persistence of latent reservoirs, as well as strategies for elimination. From the September 28, 1999 issue of the Proceedings of the National Academy of Sciences, 96:20:10958-10961.
A Reservoir for HIV in Patients on Combination Antiretroviral Therapy - Robert F. Siliciano highlights the importance of long-lived reservoirs for HIV. From the John Hopkins AIDS Service.
7th Annual International Discussion Meeting on HIV Dynamics and Evolution - offers abstracts from the conference held in Seattle on April 28-30, 2000.
JAMA HIV/AIDS Resource Center - offers extensive links and in-depth coverage including a special section on HIV's Silent Reservoirs.
National AIDS Treatment Advocacy Program - provides current news, conference reports, and access to the archives including numerous articles on latent reservoirs.
HIV InSite - an exceptionally comprehensive site produced by the University of California, San Francisco AIDS Program at San Francisco General Hospital and the UCSF Center for AIDS Prevention Studies.
National Library of Medicine HIV/AIDS Resources - links to publications and databases, as well as to other federal government resources.
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