The role of T cell specific factors and RNA Polymerase II pausing in HIV-1 replication in CD4+ T cells
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In order to eradicate HIV-1 infection the virus needs to be specifically eliminated from latently infected memory CD4+ T cells. There does not seem to be a single mechanism that promotes HIV-1 latency. RNA Polymerase II (RNAP II) pausing, chromatin structure, tissue specific transcriptional repressors and transcriptional interference have been implicated in regulating HIV-1 transcription. The transcription factor B Lymphocyte-Induced Maturation Protein 1 (Blimp-1) is expressed in B and T cells and upregulated in patients chronically infected with HIV-1. I hypothesized that Blimp-1 is a T cell intrinsic factor that binds to HIV-1 LTR, inhibits HIV-1 transcription and contributes to HIV-1 latency. Blimp-1 is expressed in primary peripheral blood CD4+ T cells and is further induced by T cell activation. Importantly, Blimp-1 is highly expressed in memory CD4+ T cells compared to naïve CD4+ T cells. Ectopic expression of Blimp-1 in CD4+ T cells represses HIV-1 transcription, whereas decreasing Blimp-1 in memory CD4+ populations activates HIV-1 transcription. Reduction of Blimp-1 in infected primary T cells increases RNAP II processivity and histone H3 acetylation. Blimp-1 binds downstream of the HIV-1 5'-LTR to the interferon-stimulated response element (ISRE) in resting primary CD4+ T cells and strongly represses Tat-dependent HIV-1 transcription. Upon T cell activation, Blimp-1 is released from the HIV-1 ISRE and this correlates with significant increase in HIV-1 transcription. These results demonstrate that Blimp-1 acts to limit HIV-1 transcription in memory CD4+ T cells and promotes the establishment and maintenance of latency. I also examined whether neighboring host promoters could impact HIV-1 transcription. Using a set of inducible cell lines I observed that neighboring promoters have minimal impact on HIV-1 transcription and that enabling release of paused RNAP II by diminishing negative elongation factor (NELF) is sufficient to reactivate transcriptionally repressed HIV-1 provirus. The implications of my results in the different mechanisms regulating HIV-1 latency are discussed.
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