The Elucidation of the Mechanisms of CD8+ T Cell-Mediated Suppression of Human Immunodeficiency Virus Type 1 Replication
Herein we detail the progress made at understanding the overall process of CD8<super>+</super> T-lymphocyte noncytolytic antiviral response (CNAR). This response is comprised of 3 key components, the virus, the effector cell and the target cell, each of which contribute to noncytolytic suppression. During the course of CNAR, the effector cells express antiviral factors that induce intracellular events in the target cell resulting in host-pathogen interactions that inhibit HIV-1 gene expression. The goal of this work was to clarify each step of the process of noncytolytic suppression.
The effector cell was examined to understand the regulation of antiviral factors and to construct a profile of the factors expressed during CNAR. CD8<super>+</super> T-lymphocytes from HIV-1 infected individuals express unidentified factors that suppress viral replication by inhibiting HIV-1 gene expression. Understanding the regulation of these antiviral CD8<super>+</super> T cell-derived factors can provide important insights into how to elicit these factors with therapeutic regimens. For a small subset of human genes, histone deacetylases (HDACs) are epigenetic regulators that condense chromatin to repress transcription. We examined the role of epigenetics in modulating the HIV-1 suppressive factors expressed by primary CD8<super>+</super> T cells from subjects naturally controlling virus replication. HIV-1 suppression by CD8<super>+</super> T-lymphocytes from virus controllers was reversed up to 40% by the addition of an HDAC inhibitor. Therefore, histone deacetylation within CD8<super>+</super> T-lymphocytes was necessary for potent suppression of HIV-1 infection.
Blocking HDACs impairs the ability of CD8<super>+</super> T-lymphocytes to repress HIV-1 transcription, demonstrating the expression of the suppressive factors is regulated by epigenetics. We used this tool to identify the potential antiviral factors that result in decreased noncytolytic suppression. Through real-time PCR analysis of 164 genes we identified 4 genes in primary CD8<super>+</super> T-lymphocytes from a virus controller, and 12 genes in a CD8<super>+</super> T-cell line that were greatly downregulated in response to a HDAC inhibitor. Additionally, we analyzed the chemokine and cytokine profile of these two cell types to characterize what molecules these cells secrete during CNAR. MIP-1 Beta, MIP-1 Alpha, IP-10, and MIG correlated most strongly with the magnitude of CNAR (<italic>p</italic> < 0.0001).
The response of the target cell to the antiviral factors was analyzed to better understand how CD8<super>+</super> T cell antiviral factors exert suppressive activity on the HIV-1 genome in an infected cell. Noncytolytic suppression was not dependent on epigenetic changes within the target cells, as HDAC1 within the target cell was dispensable, and histone acetylation at the HIV-1 LTR remained unchanged in the presence of CD8+ T-lymphocytes.
The genetic elements within HIV-1 and the viral protein Tat were investigated to provide insight into resistance to CNAR. Two virus isolates from the same individual with contrasting sensitivities to CNAR were investigated to identify the genetic elements that confer these phenotypes. Sequence analysis of the two isolates identified mutations in the exon splicing silencers (ESS) 2 and 3 in these viruses. ESS2 and 3 are thought to control splicing of HIV-1 Tat, however levels of spliced Tat RNA levels did not differ between the two isolates. The introduction of the ESS2 mutation into a heterologous HIV-1 isolate moderately boosted resistance to CNAR, suggesting a function for the mutation apart from spliced Tat RNA levels.
In total, a comprehensive analysis of each component of CNAR is discussed here to enhance the overall understanding of the mechanisms of CNAR.
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