 |
Mechanisms of HIV Persistence
Summary: Robert Siliciano's research deals with the dynamics of HIV replication in vivo and mechanisms of HIV persistence in patients on highly active antiretroviral therapy.
For the staggering number of people infected with HIV (40 million), the best current hope for avoiding the fatal consequences of the infection lies in treatment with highly active antiretroviral therapy (HAART), which consists of combinations of three or more drugs that inhibit HIV reverse transcriptase or protease. The benefits of HAART in reducing mortality are clear, but major questions remain about how best to use HAART and how to make it available to all who need it. Our work has shown that current HAART regimens cannot cure the infection in most patients because the virus persists in a very stable latent reservoir in resting memory CD4+ T cells. Because HAART is not curative, treatment of HIV infection is a lifelong challenge. Most infected individuals will ultimately have to depend upon HAART to avoid fatal immunodeficiency. Problems of drug resistance and drug toxicity make this an alarming prospect. We are interested in understanding viral persistence and in applying basic studies of viral dynamics in HIV infection to optimizing antiretroviral therapy.
Our work on viral persistence began with the idea that the capacity of HIV to establish a state of silent or latent infection at the level of individual cells might provide a mechanism for viral persistence in the face of immune responses and antiretroviral therapy. We hypothesized that HIV might capitalize on an extremely fundamental aspect of the immune system, immunologic memory, to ensure its persistence in the host. At any given time, most of the lymphocytes in the body are in a resting state. When a lymphocyte encounters a bacterial or viral protein that it is programmed to recognize, it becomes activated and begins to proliferate, generating effector cells that eliminate the invading microorganism. Most of these effector cells die, but some survive and return to a resting state as memory cells. These cells persist indefinitely, allowing effective responses to future challenges with the relevant microorganism. HIV preferentially infects activated CD4+ T lymphocytes, inserting its genetic information into the genome of the host cells and directing the production of new virus particles in a process that usually leads to the death of the infected cells. However, a small subset of the activated CD4+ T cells that are infected with HIV survive long enough to revert back to a resting memory state. Because the expression of HIV genes depends on host transcription factors induced in activated T cells, viral gene expression is automatically extinguished when these cells return to a quiescent state. The result is a stably integrated but transcriptionally silent form of the HIV genome in a memory T cell, a cell whose function it is to survive for years in a quiescent state. Upon subsequent reexposure to the relevant microorganism, the latently infected cell is reactivated and becomes competent for HIV gene expression and virus production.
Over the past several years, we have demonstrated the presence and persistence of latently infected resting memory CD4+ T cells with integrated HIV DNA in infected individuals. The cells are present only at low frequencies, reflecting the fact that most productively infected CD4+ T cells die before they can revert back to a resting memory state. Particularly important is whether this small reservoir of latent virus persists in patients on HAART. In the years following the advent of HAART, there was considerable optimism that virus eradication might be possible with prolonged treatment, based on analysis of the rapid decay of plasma virus to undetectable levels following the initiation of HAART. We have shown, however, that the frequency of latently infected cells does not decrease even in patients on HAART who have had suppression of viremia to undetectable levels for as long as 7 years. As a result of this discovery, the overall approach to the treatment of HIV infection has significantly changed. This work raised the possibility that the virus could persist indefinitely in all patients on HAART, leading many investigators to question the wisdom of beginning aggressive therapy with the goal of eradicating the infection, particularly in light of the substantial long-term toxicities of HAART regimens.
Several additional findings add to the seriousness of the problem presented by the latent reservoir. We have shown that this reservoir is a permanent archive for drug-resistant viruses that are generated by inadequate treatment. Once drug-resistant viruses have entered the reservoir, they persist there indefinitely, permanently restricting the patient's therapeutic options. The problem of stored drug-resistance mutations is particularly severe in the case of perinatally infected children, who face a lifetime of treatment. Recently, we demonstrated the presence and persistence of this latent reservoir in these children. In addition, we have demonstrated that latency operates at the transcriptional level. Latently infected cells carry integrated HIV DNA but contain little translatable HIV RNA. Unfortunately, the last hope for detecting and targeting latently infected cells was that the cells might be expressing low levels of particular viral proteins, allowing recognition by immune effector mechanisms. It now appears that we may be dealing with a completely silent form of latent infection that will be difficult to target with antiretroviral drugs or HIV-specific immune responses.
Most recently, we have become interested in understanding the nature of the low-level virus production that continues in patients on HAART whose plasma virus levels are below the limit of detection of standard assays. We have developed methods for cloning and characterizing the extremely low levels of plasma virus that are present in such patients. We have shown that this virus is generally archival in nature, is devoid of new drug-resistance mutations, and may be derived from the activation of latently infected cells. Most importantly, we do not see evidence for the continued evolution of drug resistance in most patients on suppressive HAART regimens. This provides a counterpoint to our disheartening findings on the stability of the latent reservoir. Although current HAART regimens cannot produce eradication because of the extraordinary stability of the latent reservoir, they can largely halt virus evolution, affording patients the possibility of lifelong suppression of viremia if the problem of drug toxicity can be overcome.
In the future, we will address several critical questions related to the molecular mechanism of HIV latency and the clinical implications of this form of viral persistence. We are interested in whether it will ever be possible to eliminate this reservoir. We hope to translate our findings on mechanisms of viral persistence into new approaches for optimizing antiretroviral therapy. The correct choice of a HAART regimen is literally a matter of life and death for many patients, and we feel basic studies of viral persistence can be applied to improving decisions about how and when antiretroviral therapy should be given.
This research has received support from the National Institutes of Health and the Doris Duke Charitable Foundation.
Last updated: June 16, 2008
|
|
|
|
