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Origin and Pathogenesis of Human Immunodeficiency Viruses
Summary: George Shaw is interested in the origins and pathogenesis of the human AIDS virus, HIV-1, and in novel strategies for vaccine development.
Retroviruses, including the human immunodeficiency virus type 1 (HIV-1) and HIV-2 and the human T cell leukemia viruses, are responsible for a number of important human illnesses, the most notable being the acquired immunodeficiency syndrome, or AIDS.
AIDS results from chronic infection by HIV-1 (or HIV-2) and a relentless destruction of the immune system that occurs as a consequence of persistent viral replication. Sixty-five million adults and children worldwide have been infected by HIV-1, and most will die of disease unless effective therapies are developed. These numbers will increase substantially in the next decade if effective preventive measures, including vaccines, are not developed. The magnitude of the HIV-1 pandemic, the insidious nature of virus transmission and persistence, and the enormity of human suffering that occurs as a consequence of HIV infection have made the development of a fundamental understanding of HIV biology and the discovery of effective therapies and vaccines top scientific priorities.
Since the first recognition of AIDS as a new human malady, and the discovery of HIV-1 as its cause, elucidation of the origin of the HIV-1 virus has been a high priority. A principal reason for this interest is the hope that understanding the natural history and evolution of simian immunodeficiency viruses will yield clues about the pathogenesis and prevention of HIV disease. Similar insights might also derive from understanding the unique features of the HIV-1 life cycle that make the goals of virus eradication (curative therapy) and prevention (effective vaccination) especially challenging.
HIV-1 infects integral components of the immune system—lymphocytes, macrophages, and dendritic cells—by entry mechanisms that involve the cell surface molecule CD4 and a family of chemokine receptors. After entry, the HIV genome becomes stably integrated within the host cell genome. From this point, the virus persists in the form of a chromosomally integrated provirus for the life of the cell. Depending on the activation state of the infected cell, virus expression may be explosive or restricted, even latent, with infected cells exhibiting life spans as short as a few days or as long as many years. HIV persistence in vivo is thus a consequence of a complicated viral life cycle involving stable incorporation of the viral genome into cellular chromosomal DNA; a complex viral genomic organization that allows for variable gene expression; the propensity of the virus to target and disable CD4+ T lymphocytes and other cells whose normal function is essential to the integrity of the host immune system; and the ability of HIV to evolve rapidly by genetic mutation in response to selective pressures, including the host immune response as well as antiretroviral drug therapy.
Our laboratory's goal is to develop a molecular and biological understanding of the origin of HIV-1 and of viral and host factors responsible for HIV-1 persistence and pathogenesis. In parallel, we are examining the evolutionary origins of HIV-1 with respect to other human and simian immunodeficiency viruses and studying at a molecular level the basis for HIV-1 replication and persistence in infected humans. Using a combination of genetic sequencing and phylogenetic analyses, we have shown that HIV-1 viruses that infect humans worldwide are closely related to—and indeed derive from—a simian virus naturally infecting the Pan troglodytes troglodytes subspecies of common chimpanzee. Furthermore, we have found that this chimpanzee virus, termed SIVcpz (P.t.t.), has been transmitted to humans on no fewer than three occasions, sometime in the first half of the 20th century.
Using analogous genetic and phylogenetic approaches for analyzing virus populations within infected individuals, we have probed the molecular mechanisms of HIV-1 persistence and pathogenesis. The replicative intermediate forms of HIV-1 in blood and lymphoreticular tissues of infected humans include virion-associated RNA, pre-integrative viral DNA intermediate forms, integrated proviral DNA, and expressed viral RNA and proteins. By quantifying each of these species as a function of disease stage, we can develop a dynamic model of viral replication and persistence. Already this work has led to an appreciation of the dynamic nature of HIV-1 persistence, with cell-free virus exhibiting a life span measured in hours, virus-producing cells a life span of a few days, and latently infected cells (which can reactivate infection even after months or years of potent antiretroviral therapy) a life span of years.
Mechanisms of viral persistence can also be studied by examining genetic mutations in HIV-1 and the dynamics of viral evolution in vivo. HIV-1 exists in humans as a "quasispecies," since with each cycle of infection in which a virion productively infects a new cell, one or more mutations are introduced into the proviral genome. Some of these are lethal, but those that are not provide the basis for evolution of the viral quasispecies in response to selective pressure, be it immunologic, biologic, or induced by an antiretroviral drug.
A novel aspect of our work has been to take advantage of insights into virus population dynamics in the design of experiments to look for virus selection and escape. Since the life span of plasma virus and the cells producing the majority (99 percent) of this virus is only a day or less, we reasoned that a population-based analysis of the genetic content of plasma virus would provide a sensitive and dynamic assessment of biologically important selection pressures operating in vivo. By linking such an analysis to molecularly identifiable biological determinants, we showed for the first time that under drug or cytotoxic T lymphocyte (CTL) pressure, virtually the entire replicating viral population in vivo can be replaced by escape mutants, often within a matter of a few weeks. We have extended this work to include neutralizing antibodies and have shown that such antibodies exert selection pressures in humans, comparable to those of antiviral drugs and CTLs. Moreover, we have discovered a novel mechanism of virus escape from neutralizing antibodies that enables HIV-1 to persist despite an evolving antibody repertoire. We describe this model of HIV-1 immune escape as the evolving glycan shield.
Most recently, we have extended our work on neutralizing antibodies to HIV-1 vaccine development. Immunogenic, broadly reactive epitopes of the HIV-1 envelope glycoprotein could serve as important components of an effective AIDS vaccine. However, variability in exposed epitopes and a combination of highly effective envelope-cloaking strategies have made the identification of such epitopes problematic. We have now shown that the chemokine coreceptor-binding site of HIV-1 from clades A, B, C, and D elicits high titers of antibody during natural human infection and that these antibodies bind and neutralize viruses as divergent as HIV-2. We have found that CD4-induced (CD4i) monoclonal antibodies elicited by HIV-1 infection (including 17b, 21c, 19e, E51, ED49, ED47, and X5) bind and neutralize HIV-2 pretreated with soluble CD4 and that polyclonal neutralizing antibodies from HIV-1–infected humans compete specifically with such monoclonal antibodies for HIV-2 binding. We have also shown that site-directed mutations in the HIV-2 envelope—corresponding to amino acids 419, 422, and 434 in the bridging sheet of HIV-1—contribute to coreceptor binding and alter the neutralization potency of CD4i monoclonal and polyclonal antibodies by as much as 150-fold. These findings indicate that the coreceptor-binding surface of HIV-1 is highly immunogenic, antigenically cross-reactive, and elicits neutralizing antibodies of extraordinary breadth and titer. We are thus exploring the role of coreceptor-specific antibodies in virus containment in natural infection and the potential for utilizing the HIV-1 coreceptor as an immunogen in HIV-1 vaccines.
We also postulated that the HIV-2 envelope might serve as a molecular scaffold for presenting HIV-1–neutralizing epitopes in a functional context. By “transplanting the membrane-proximal external region (MPER) of the HIV-1 gp41 into HIV-2, we could test for the first time for the presence of MPER epitope-specific neutralizing antibodies in HIV-1–infected humans. We made the surprising finding that while most humans lack neutralizing antibodies that share epitopes with the prototype MPER human monoclonals 2F5 and 4E10, a substantial proportion of patients have neutralizing antibodies directed toward other MPER epitopes. These findings have important implications for HIV-1 vaccine design.
In summary, my laboratory studies the dynamics and mechanisms of HIV-1 replication and persistence in vivo. Our goal is to develop a comprehensive mathematical model and biological understanding of HIV-1 persistence and a quantitative appreciation of viral and host factors contributing to the all-important and clinically predictive plasma virus load set point. Such insights will be instrumental in the development of effective treatment and vaccine strategies for HIV-1, with the ultimate goal of curative (or long-term suppressive) therapy and an effective vaccine for HIV/AIDS.
Last updated July 29, 2005
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