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HIV-1 envelope protein determinants of viral tropism and antiviral drug resistance
thesisposted on 14.02.2017, 00:10 by Roche, Michael John
HIV-1 replication and thus, the progression of infected individuals to AIDS can now be largely controlled by combined antiretroviral therapy (cART). However the ability of HIV-1 to; 1) infect and become latent in long-lived cells of the immune system, and 2) persist in tissue anatomical sites where antiretroviral drug penetration is poor, means that cART can not eradicate HIV-1 infection. The application of strategies aimed at exposing HIV-1 reservoirs of infection will require greater understanding of the mechanisms that allow HIV-1 persistence in long-lived cells and tissue sites. Longterm use of cART leads to toxicity and resistance issues in patients with HIV-1 and to address these problems, new classes of HIV-1 antiretrovirals are required. Recently a new drug called Maraviroc (MVC) was approved for use in HIV-1 therapy. MVC comes from a novel class of HIV-1 antiretrovirals called CCR5 antagonists that inhibit HIV-1 entry by altering CCR5 such that it is no longer recognised by the HIV-1 Env. As is the case for all current antiretrovirals, HIV-1 can develop resistance to CCR5 antagonists, although the mechanisms that underlie resistance are poorly understood. The HIV-1 envelope glycoproteins (Env) have been demonstrated to play an important role in influencing HIV-1 entry and tropism. The hypothesis of this study was that changes in the way that the HIV-1 Env interacts with the primary coreceptors, CCR5 and CXCR4, will enhance and/or alter the HIV-1 entry process which will lead to expansion of viral tropism for cells and tissue sites as well as promote resistance to CCR5 antagonists. The central nervous system (CNS) is an immune privileged site that has poor antiretroviral drug penetration. To examine the role of the HIV-1 Env in viral tropism for the CNS, the Env-coreceptor interactions of a unique panel of dual-tropic (R5X4) Envs isolated from the CNS and lymphoid tissue were analysed. It was found that although functionally dual-tropic, R5X4 Envs isolated from the CNS displayed a preference for CCR5 in viral entry assays whilst R5X4 Envs isolated from lymphoid tissue displayed a preference for CXCR4. These results identify mechanisms underlying R5X4 HIV-1 persistence in different tissue reservoirs and demonstrate tissue-specific adaptation that enhances the tropism of R5X4 strains for CCR5-expressing macrophage-lineage cells in the CNS, and CXCR4-expressing T-cells in lymphoid tissues. Macrophages are long-lived target cells for HIV-1 infection, and enhanced viral replication in macrophages appears to be an important factor in HIV-1 disease progression. To better understand the Env-coreceptor interactions important for HIV-1 macrophage (M)-tropism, the phenotypes of a panel of sixteen CCR5-using (R5) Envs and six R5X4 or CXCR4-using (X4) Envs were studied. Enhanced M-tropism of R5 HIV-1 Envs was associated with an enhanced and altered interaction with CCR5, such that M-tropic R5 Envs had reduced CCR5 dependence and reduced reliance on the CCR5 N-terminus. M-tropism by R5X4 or X4 HIV-1 Envs was associated with increased dependence on the CXCR4 N-terminus, thus displaying a differential coreceptor requirement compared to R5 Envs. These novel results suggest that the mechanism by which HIV-1 Envs engage with coreceptor is an important factor in viral tropism. R5 HIV-1 can become resistant to the CCR5 antagonist MVC by recognising and binding to the MVC-modified form of CCR5. However, the molecular mechanisms behind this altered engagement with CCR5 are unknown. The phenotype of a HIV-1Env from a resistant virus generated in vitro was compared to that of an Env from the parental sensitive virus. Resistance to MVC was associated with a critical reliance on the CCR5 N-terminus, which led to a less efficient interaction with CCR5. The presence of MVC abolished the M-tropic properties of the MVC resistant Env, and these results suggest that continuing MVC even after resistance develops could be clinically beneficial by sparing or reducing the macrophage reservoir of HIV-1. Finally, it was discovered that the CC1/85 HIV-1 clinical isolate, which appears to be uniquely predisposed to acquiring resistance to CCR5 antagonists in vitro, displayed a low-level baseline ability to utilise the MVC-modified form of CCR5 for viral entry. This recognition of the MVC-CCR5 complex was only observed in cells expressing high levels of CCR5 and not in cells that are typically used in the tropism pre-screening assays that are performed prior to a patient starting MVC therapy. These results indicate that a low-level ability to recognise the MVC-modified form of CCR5 may drive full resistance to MVC and detection of these variants could help identify patients at higher risk of virologic failure on MVC. Furthermore, these results suggest that cell lines expressing relatively higher levels of CCR5 may be more appropriate for the tropism pre-screening assays.
Awards: Winner of the Mollie Holman Doctoral Medal for Excellence, Faculty of Medicine, Nursing and Health Sciences, 2012.