Molecular Mechanism Of Exhausted CD8T Cells During Chronic Viral Infection

BACKGROUND: T cell exhaustion is a status of dysfunction that arised during manychronic infections and cancers. It is defined by poor effector function, sustainedexpression of inhibitory receptors and a transcriptional state distinct from that offunctional effector or memory T cells. The underlying mechanisms of T cell exhaustionare not well understood. Recently, our early work reported that “unhelped” memory CD8T cells were regulated by epigenetic modifications. Low levels of histone acetylation wereexpressed at the ifng and il2loci of the “unhelped” memory CD8T cells, which arefunctionally impaired compared to “helped” memory CD8T cells. In addition, increasedAcH3levels in “unhelped” memory CD8T cells resulted in an improved ability to persistin vivo and provide protection from subsequent infectious challenge, suggesting thathistone acetylation is an important regulator of memory CD8T cell generation. However,it is currently unclear whether histone acetylation regulates CD8T cell exhaustion duringchronic viral infection. Therefore, a more thorough understanding of the impact of histoneacetylation in CD8T cell exhaustion may reveal potential therapeutic targets leading to therestoration of exhausted CD8T cell function and better viral control. METHODS: To study the mechanism of T cell exhaustion, we established the chronicinfection in mice with chronic lymphocytic choriomeningitis virus (LCMV-clone13) tostudy the charicataristics of exhausted CD8T cell; we longitudinally investigated diAcH3(di-acetylated histone H3) expression on virus-specific CD8T cells during acute andchronic LCMV infection by flow cytometery. In addition, we took advantage of cellculture and cell adoptively transfer to examine the effect of in vitro histone deacetylaseinhibitor (HDACi)-Valproic acid sodium (VPA) treatment on diAcH3regulation and itseffect on the restoration of exhausted CD8T cells. Besides in vitro VPA treatment, we alsotook advantage of in vivo approach to evaluate whether VPA can prevent the CD8T cellexhaustion. Moreover, we performed ChIP to detect the level of diAcH3at the cytokinesloci on exhausted CD8T cells. We carried out RTq-PCR to evaluate the cytokineproduction of exhausted CD8T cells after VPA treatment.RESULTS:1). Comapred with the memory CD8T cells generated during acuteinfection, virus-specific CD8T cells were functional exhausted during Cl-13infeciton.2).DiAcH3was highly expressed only in early virus specific CD8T cells after chronic viralinfection and decreased progressively over time. The pattern of decreased diAcH3wasassociated with CD8T cell exhaustion, characterized by gradually impaired secretion ofeffector cytokines such as IFN-γ, TNF-, and IL-2.3). Not only virus-specific CD8T cells,but most of CD8T cells were in a state of dysfunction which associated with decreasedlevel of histone acetylation.4). Increased histone acetylation by VPA in vivo treatment significantly contributed to the functional restoration of exhausted CD8T cells.5). VPA invitro treatment restored exhausted CD8T cells into functional long-lasting memory CD8Tcells which can produce more cytokines compared with the untreated cells after resting invivo. Moreover, restored CD8T cells can provide better recall responses and protectiveimmunity to infectious challenge after resting in vivo.6). In vivo VPA treatment increasedthe level of diAcH3of CD8T cells during Cl-13infection, which delayed the T cellexhaustion.7). Enrichment of histone acetylation at ifng promoter and enhancer loci inexhausted CD8T cells increased after VPA in vitro or in vivo treatment.CONCLUSIONS: We identified a critical role for diAcH3in CD8T cell exhaustiondistinctly different from its role in acute infection. DiAcH3level decreased in exhaustedCD8T cells in response to persistent antigen. Increase of diAcH3by VPA in vitro or invivo treatment restored the function of exhausted CD8T cells. Thus, during chronic viralinfection, diAcH3regulated CD8T cell exhaustion which suggests that targeting histoneacetylation is a potential strategy to improve the immune response to chronic viralinfections.