Role of IRF4 in the Regulation of Cellular Interferon Stimulated Genes and KSHV Lytic Gene Expression in Primary Effusion Lymphoma

The interferon regulatory factors (IRF) are a family of transcription factors that control intrinsic cellular responses to viral infections. The regulation of target gene expression by IRF proteins is mediated by binding to interferon-stimulated response elements (ISRE) DNA motifs located in the 5' regulatory region of Interferon-stimulated genes (ISGs). Interferon regulatory factor 4 (IRF4), a hematopoietic-specific transcription factor, is a potential dual regulator of ISRE-mediated gene expression although its transcriptional signature is still poorly understood. Primary effusion lymphoma (PEL), a rare B-cell malignancy is characterized by the expression of high level of cellular IRF4, a disrupted B-cell transcriptional program, and latent infection with Kaposi's sarcoma-associated herpesvirus (KSHV). The goal of this study is to elucidate the ability of IRF4 to alter the transcriptional expression of ISRE and ISRE-like sequence regulated genes in the absence of B-cell specific binding partners. Small-scale gene expression assays showed that IRF4 is capable of differentially regulating the expression of ISRE-responsive genes. The positive regulation of IRF4 target genes (ISG60 and Cig5) was shown to be in response to direct binding of IRF4 to chromatin regions corresponding to ISRE-motifs located at the 5' promoter regulator regions of ISG60 and Cig5. To understand the role of IRF4 beyond cellular gene expression, we tested KSHV-encoded latency associated genes that modulated IRF4-mediated gene expression. We identified the viral FLICE inhibitory protein (vFLIP) as an enhancer of IRF4-mediated ISG induction and showed that this function is dependent on the activation of NF-ƒUB. Finally, we examined the role of IRF4 in the regulation of lytic gene expression. The replicator and transcription activator (RTA) protein, is a sequence specific transcription factor that regulates the expression of a subset of early genes through binding ISRE-like motifs on their promoters. Studies aimed at understanding the consequences of IRF4 expression in PEL cells showed that IRF4 acts as a negative regulator of lytic gene expression by inhibiting RTA expression and RTA-mediated gene transactivation. These data support a model in which IRF4 mediates an antiviral cellular response, inhibiting lytic replication of KSHV while contributing to the transformative effects of KSHV by promoting viral latency.