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  • Title: Kaposi's Sarcoma-Associated Herpesvirus Latency-Associated Nuclear Antigen Inhibits Major Histocompatibility Complex Class II Expression by Disrupting Enhanceosome Assembly through Binding with the Regulatory Factor X Complex.
    Author: Thakker S, Purushothaman P, Gupta N, Challa S, Cai Q, Verma SC.
    Journal: J Virol; 2015 May; 89(10):5536-56. PubMed ID: 25740990.
    Abstract:
    UNLABELLED: Major histocompatibility complex class II (MHC-II) molecules play a central role in adaptive antiviral immunity by presenting viral peptides to CD4(+) T cells. Due to their key role in adaptive immunity, many viruses, including Kaposi's sarcoma-associated herpesvirus (KSHV), have evolved multiple strategies to inhibit the MHC-II antigen presentation pathway. The expression of MHC-II, which is controlled mainly at the level of transcription, is strictly dependent upon the binding of the class II transactivator (CIITA) to the highly conserved promoters of all MHC-II genes. The recruitment of CIITA to MHC-II promoters requires its direct interactions with a preassembled MHC-II enhanceosome consisting of cyclic AMP response element-binding protein (CREB) and nuclear factor Y (NF-Y) complex and regulatory factor X (RFX) complex proteins. Here, we show that KSHV-encoded latency-associated nuclear antigen (LANA) disrupts the association of CIITA with the MHC-II enhanceosome by binding to the components of the RFX complex. Our data show that LANA is capable of binding to all three components of the RFX complex, RFX-associated protein (RFXAP), RFX5, and RFX-associated ankyrin-containing protein (RFXANK), in vivo but binds more strongly with the RFXAP component in in vitro binding assays. Levels of MHC-II proteins were significantly reduced in KSHV-infected as well as LANA-expressing B cells. Additionally, the expression of LANA in a luciferase promoter reporter assay showed reduced HLA-DRA promoter activity in a dose-dependent manner. Chromatin immunoprecipitation assays showed that LANA binds to the MHC-II promoter along with RFX proteins and that the overexpression of LANA disrupts the association of CIITA with the MHC-II promoter. These assays led to the conclusion that the interaction of LANA with RFX proteins interferes with the recruitment of CIITA to MHC-II promoters, resulting in an inhibition of MHC-II gene expression. Thus, the data presented here identify a novel mechanism used by KSHV to downregulate the expressions of MHC-II genes. IMPORTANCE: Kaposi's sarcoma-associated herpesvirus is the causative agent of multiple human malignancies. It establishes a lifelong latent infection and persists in infected cells without being detected by the host's immune surveillance system. Only a limited number of viral proteins are expressed during latency, and these proteins play a significant role in suppressing both the innate and adaptive immunities of the host. Latency-associated nuclear antigen (LANA) is one of the major proteins expressed during latent infection. Here, we show that LANA blocks MHC-II gene expression to subvert the host immune system by disrupting the MHC-II enhanceosome through binding with RFX transcription factors. Therefore, this study identifies a novel mechanism utilized by KSHV LANA to deregulate MHC-II gene expression, which is critical for CD4(+) T cell responses in order to escape host immune surveillance.
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