J. Microbiol. Biotechnol. (2019), 29(12), 1873–1881 https://doi.org/10.4014/jmb.1910.10004 Research Article Review jmb Beyond Viral Interferon Regulatory Factors: Immune Evasion Strategies Jinjong Myoung1, Shin-Ae Lee2, and Hye-Ra Lee3* 1Korea Zoonosis Research Institute, Genetic Engineering Research Institute and Department of Bioactive Material Science, College of Natural Science, Jeonbuk National University, Jeonju 54531, Republic of Korea 2Department of Molecular Microbiology and Immunology, Keck School of Medicine, University of Southern California, Los Angeles, California, USA 3Department of Biotechnology and Bioinformatics, College of Science and Technology, Korea University, Sejong 30019, Republic of Korea Received: October 8, 2019 Revised: October 8, 2019 The innate immune response serves as a first-line-of-defense mechanism for a host against Accepted: October 24, 2019 viral infection. Viruses must therefore subvert this anti-viral response in order to establish an First published online: efficient life cycle. In line with this fact, Kaposi’s sarcoma-associated herpesvirus (KSHV) October 25, 2019 encodes numerous genes that function as immunomodulatory proteins to antagonize the host *Corresponding author immune system. One such mechanism through which KSHV evades the host immunity is by Phone: +82-44-860-1831 encoding a viral homolog of cellular interferon (IFN) regulatory factors (IRFs), known as Fax: +82-44-860-1598 E-mail: [email protected] vIRFs. Herein, we summarize recent advances in the study of the immunomodulatory strategies of KSHV vIRFs and their effects on KSHV-associated pathogenesis. pISSN 1017-7825, eISSN 1738-8872 Copyright© 2019 by Keywords: KSHV, viral interferon regulatory factor, immune evasion strategy, PRR-mediated The Korean Society for Microbiology signaling pathway, apoptosis pathway and Biotechnology Introduction various transcription factors. In addition to their imperative role in the innate immune response, IRFs are also implicated Kaposi’s sarcoma-associated herpesvirus (KSHV) is a in various physiological processes, such as cellular DNA tumor virus and identified as the causative agent of differentiation, cell growth, and apoptosis. Thus, deregulation Kaposi’s sarcoma (KS), primary effusion lymphoma (PEL), of IRFs occurs in numerous pathological conditions, such and multicentric Castleman’s disease (MCD) [1-3]. In as viral infection, tumorigenesis, and so on. order to efficiently establish life-long persistent infection as To counteract these cellular IRFs, vIRFs have been shown well as KSHV-associated pathogenesis, KSHV encodes to elicit two main biological consequences: inhibition of the numerous immunomodulatory proteins, within a large IFN-mediated innate immunity and suppression of the p53- portion of its genome, that overcome the host immune mediated apoptosis. Furthermore, more recent mounting system to ensure viral persistence [4]. As part of its strategy, data suggested that vIRFs might function as a viral KSHV harbors viral interferon (IFN) regulatory factors transcriptional factor, although they do not comprise the (vIRFs) that have sequence homology with cellular IRFs. five conserved tryptophan residues within their predicted Cellular IRFs are well-known transcriptional factors that DBD region, unlike cellular IRFs. Thus, the aim of this bind and activate IFN-responsible promoters. Activated review is to comprehensively understand the possible IFN response upon virus infection subsequently leads to modes of action of vIRFs in KSHV-associated pathogenesis phosphorylation, dimerization, and nuclear translocation as well as their life cycles. of IRF3/IRF7, which are key factors of type I IFN creating an anti-viral state against viral infection. All cellular IRFs Immune Signaling Pathway have a conserved N-terminal DNA binding domain (DBD) that contains five tryptophan repeats and a characteristic The innate immune response is the first line of host C-terminal IRF-association domain (IAD) responsible for defense mechanism against viral infections. Therefore, it is IRF homo- or heterodimerization, as well as binding with essential for the virus to overcome the innate immune December 2019 ⎪ Vol. 29⎪ No. 12 1874 Myoung et al. response in order to establish a successful viral life cycle STING-mediated IFN production via inhibiting phos- within the host. Upon virus invasion, host pattern phorylation of STING [10]. Interestingly, knockdown of recognition receptors (PRRs) immediately sense pathogen- vIRF1 with siRNA against vIRF1 in KSHV-infected cells associated molecular patterns (PAMPs), which are specific shows suppression of viral lytic replication during molecular features associated with pathogens. Currently, reactivation conditions [10]. Collectively, these indicate several PRRs have been identified, including cytosolic that vIRF1 utilized several different strategies to efficiently sensors, toll-like receptors (TLRs), and nucleotide-binding perturb the anti-IFN activities for establishment of primary oligomerization domain (NOD)-like receptors (NLRs). These infection as well as replication. PRRs comply with the general mechanism of signaling: they recognize PAMPs, and immediately trigger an vIRF2 (K11 and K11.1) intracellular signal transduction pathway that activates the vIRF2 encoded by K11 and K11.1 has different isoforms gene expression and synthesis of inflammatory cytokines, as a result of alternative splicing of exons K11 and K11.1. including IFNs. Subsequently, secreted IFNs bind to the These isoforms of vIRF2 localize both in the nucleus and IFN receptors, further activating the janus kinase-signal the cytoplasm [11]. Interestingly, vIRF2 suppresses the transducer and activator of transcription (JAK-STAT) transcriptional activity of IFN-stimulated promoter and pathway, leading to expression of IFN-stimulated genes IFN-β promoter by interaction with both IRF1 and IRF3, (ISGs). Ultimately, these signaling events amplify the but not IRF7 [12, 13]. Furthermore, vIRF2 interacts with innate immune signaling and coordinate adaptive immune IRF2, ICSBP, RelA (p65), and p300, leading to inhibition of response within the cell to build up an anti-viral state. IFN-α gene expression as well as early pro-inflammatory Therefore, in order for the virus to efficiently complete its cytokine expression [12, 13]. In order to efficiently escape life cycle in the host, KSHV evolutionally employs numerous the type I IFN-driven antiviral response, vIRF2 recruits strategies to evade the host immune signaling pathways. caspase-3 to IRF3 and induces IRF3 degradation [14]. In addition, the short form of vIRF2 inhibits late-state IFN vIRF1 (K9) response via preventing protein kinase-R (PKR) activation, KSHV open reading frame (ORF) K9 encodes vIRF1, which is induced by binding of dsRNA created during viral which effectively downregulates the transcriptional activity replication [15]. Activated PKR leads to phosphorylation of of IFN promoter, ISG promoter, and gamma interferon- the eukaryotic translation initiation factor α (eIF2α) and activated sequence (GAS)-element [5]. These inhibitory subsequently inhibits cellular mRNA synthesis, preventing effects of vIRF1 on IFN-mediated signaling occur via viral protein synthesis. Thereby, KSHV vIRF2 compre- interaction of vIRF1 with cellular proteins. vIRF1 binds hensively suppressed early and late anti-viral states. p300 and selectively prevents p300/CBP-IRF3 complex to suppress the IRF3-mediated transcriptional activity [6, 7]. vIRF3 (K10.5) Interestingly, although vIRF1 binds to IRF3, previous Unlike vIRF1 and vIRF2, vIRF3 facilitates activation of studies have shown that vIRF1-IRF3 interaction did not both IRF3- and IRF7-mediated type I IFN via interaction alter IRF3-mediated homo- or heterodimerization, nuclear with IRF3, IRF7, and p300 [16]. Uniquely, vIRF3 is involved translocation, nor DNA binding activity. However, it has in enhanceosome activity, leading to increased interaction recently been reported by Jacobs et al. that vIRF1 reduced of IRF3 and IRF7 to the IFN-α promoter in infected cells phosphorylation of IRF3 and nuclear localization of IRF3 [16]. Surprisingly, the same group also reported that vIRF3 upon TLR3-mediated upregulation of IFN-β expression [8]. acts as a dominant negative factor of both IRF3 and IRF7 to In addition, the region between amino acids 224 and 349 of suppress virus-induced type I IFN promoter activity [17]. vIRF1 was responsible for interaction with cellular Later, Joo et al. also showed that vIRF3 suppressed IRF7- interferon-stimulated gene 15 (ISG15) E3 ligase, HERC5, mediated IFN-α production by binding specifically with leading to decreased ISGylation of target cellular proteins the DNA-binding domain (DBD) of IRF7 in 293T transfected [5]. Notably, knockdown of ISG15 induced KSHV cells upon Sendai virus treatment [18]. vIRF3 also interacts reactivation to its lytic cycle [5]. As is the case with most with IRF5, a member of the IRF family that was first DNA viruses, cGMP-AMP synthase (cGAS) can recognize identified as a transcriptional factor of type I IFN. vIRF3 viral DNA in the cytoplasm and generate cGMP-AMP interaction with IRF5 inhibits IRF5 from binding to the (cGAMP) that activates STING to cause production of type I ISRE region of the promoter, and leading to suppression of IFN [9]. KSHV, thus, utilizes vIRF1 to ablate the cGAS- IRF5-mediated IFN-response gene activation [19]. J. Microbiol. Biotechnol. Roles of KSHV vIRFs 1875 these pathways subsequently activate initiator caspases and executioner