HIV Infection Dynamics: From Underlying the Establishment and Maintenance to the Molecular Mechanisms of Latency

Abstract

Human immunodeficiency virus (HIV) is a lentivirus characterized by its ability to establish chronic infection by evading the immune system. Combination antiretroviral therapy (cART) inhibits HIV spread and reduces transmission as well as the onset of immunodeficiency. Despite effective treatment, HIV persists in optimally treated people as a transcriptionally silent provirus. HIV requires its receptor CD4, plus a co-receptor, either CCR5 or CXCR4, to enter cells and primarily replicates in CD4+ T cells, which express high levels of CD4. A barrier to curing HIV is that a proportion of replication-competent HIV proviruses exist in a latent state that cannot be eradicated by current therapies or detected by the anti-HIV immune response. Besides CD4+ T cells, CD4+ hematopoietic stem and progenitor cells (HSPCs) are a potential viral reservoir. There is evidence that they can be infected in vivo and that they amplify integrated viral genomes by cellular proliferation and differentiation into a variety of cell types increasing the size of the latent reservoir. Thus, latently infected cells evade the immune system and the harmful effects of the virus, thereby creating a long-lasting reservoir of HIV. The work described in Chapter 2 provides deeper insight into the molecular mechanisms of HIV latency establishment. Here, we describe a series of HIV-1 fluorescent reporter viruses that distinguish active versus latent infection. We unexpectedly observed that the proportion of active-to-latent infection depended on a limiting viral factor, which created a bottle neck that could be overcome by superinfection of the cell. T cell activation or overexpression of HIV-1 trans activator of transcription (Tat) were sufficient to overcome this bottleneck and activate HIV gene expression. In addition, we found that tat and rev expression levels vary amongst HIV molecular clones and that tat levels were an important variable in latency establishment. Lower rev levels limited viral protein expression whereas lower Tat levels or mutation of the Tat binding element promoted latent infection that was resistant to reactivation even in fully activated primary T cells. Nevertheless, we found that combinations of latency reversal agents targeting both cellular activation and histone acetylation pathways overcame deficiencies in the Tat-TAR axis of transcription regulation. These results provide additional insight into the mechanisms of latency establishment and inform Tat-centered approaches to cure HIV. The tools and results described in Chapter 2 will enable investigators to answer important questions in future experiments, some of which are detailed in Chapter 3. Overall, this research provides important new results and strategies to significantly inform future studies on HIV latency that will aid in the development of clinically practical and effective latency reversal agents. These advancements could improve HIV treatment by reducing the impact of persistent reservoirs of latent virus, potentially leading toward a cure.