Bcl11b, a T-cell commitment factor, and its role in human immunodeficiency virus-1 transcription
Woerner, Andrew James
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Advancements of antiretroviral therapies (ART) have made significant strides in reducing human immunodeficiency virus (HIV) viral loads in patients to undetectable levels. Upon interruption of ART, viral load rebounds and AIDS symptoms return. Latent reservoirs of virus are responsible for this phenomenon because they contain integrated provirus, which is transcriptionally silent, thus unaffected by ART and hidden from host immune surveillance. A commonly proposed mechanism for HIV latency is the presence of host cell transcription factors that lead to transcriptional silencing. CD4+ T cells and other immune cells, whether due to their subset phenotype, activation state, or stage in development, will vary in their battery of transcription factors. Of particular interest is Bcl11b, a critical transcription factor involved in the commitment to a T-cell fate during thymocyte development that has recently been shown to play a role in silencing HIV-1 transcription. Bcl11b is required for inhibiting the development of natural killer cell-like traits during the early development of T cells. The repressive role of this zinc-finger transcription factor has recently been shown to inhibit HIV-1 transcription in the context of microglial cells via recruitment of chromatin remodeling factors. Also, Bcl11b has been shown to interact with other HIV-1 transcriptional silencing factors such as NuRD and NCoR. Preliminary mass spectrophotometry results have pointed to a physical interaction of Bcl11b with NELF, another proven repressive factor of HIV transcription. We hypothesize that Bcl11b represses HIV transcription and is recruited to the HIV-1 long terminal repeat (LTR) through a paused RNA polymerase II complex, contributing to the establishment and maintenance of latency. Our studies confirm Bcl11b's repressive role in T cells, and investigate the mechanism with NELF. Transfection of HEK293T cells with HIV-LUC shows nearly 50% reduction in HIV transcription in the presence of Bcl11b, and analysis of viral protein output by p24 ELISA confirms this result. Furthermore, when co-transfected with NELF-B, the two transcription factors lead to nearly 90% reduction in HIV transcription. Results suggest that these factors cooperate to repress HIV transcriptional elongation. Protein and chromatin immunoprecipitations (ChIP) were also performed to see a direct interaction between the two transcription factors and the HIV LTR. Physical interaction of the two factors was not witnessed, while ChIP analysis shows enrichment of RNA polymerase II at the transcriptional start site suggesting Bcl11b increasing RNA polymerase II pausing. We conclude that Bcl11b plays a repressive role in HIV transcription through promoter-proximal pausing with a synergistic effect with NELF, but a yet to be identified factor is responsible for the coordination of the two factors. As an important T-cell commitment factor, Bcl11b may play an important role in establishing and maintaining cellular latency through transcriptional repression via a complex with NELF. Confirming Bcl11b's role as a repressive transcription factor and providing further support for a synergistic role with NELF, could highlight a new target for therapeutic strategies against the elusive latent reservoir.