Date Published: May 6, 2019
Publisher: Public Library of Science
Author(s): Sandra Koch, Modester Damas, Anika Freise, Elias Hage, Akshay Dhingra, Jessica Rückert, Antonio Gallo, Elisabeth Kremmer, Werner Tegge, Mark Brönstrup, Wolfram Brune, Thomas F. Schulz, Dirk P. Dittmer.
Kaposi’s sarcoma-associated herpesvirus (KSHV; human herpesvirus 8) belongs to the subfamily of Gammaherpesvirinae and is the etiological agent of Kaposi’s sarcoma as well as of two lymphoproliferative diseases: primary effusion lymphoma and multicentric Castleman disease. The KSHV life cycle is divided into a latent and a lytic phase and is highly regulated by viral immunomodulatory proteins which control the host antiviral immune response. Among them is a group of proteins with homology to cellular interferon regulatory factors, the viral interferon regulatory factors 1–4. The KSHV vIRFs are known as inhibitors of cellular interferon signaling and are involved in different oncogenic pathways. Here we characterized the role of the second vIRF protein, vIRF2, during the KSHV life cycle. We found the vIRF2 protein to be expressed in different KSHV positive cells with early lytic kinetics. Importantly, we observed that vIRF2 suppresses the expression of viral early lytic genes in both newly infected and reactivated persistently infected endothelial cells. This vIRF2-dependent regulation of the KSHV life cycle might involve the increased expression of cellular interferon-induced genes such as the IFIT proteins 1, 2 and 3, which antagonize the expression of early KSHV lytic proteins. Our findings suggest a model in which the viral protein vIRF2 allows KSHV to harness an IFN-dependent pathway to regulate KSHV early gene expression.
Kaposi’s sarcoma-associated herpesvirus (KSHV) or human herpesvirus 8 (HHV8) belongs to the genus Rhadinovirus within the subfamily of Gammaherpesvirinae. It was identified in 1994 when herpesvirus-like DNA sequences were discovered in AIDS-associated Kaposi’s sarcoma (KS) tissues . Apart from KS, KSHV is also the cause of two lymphoproliferative diseases, primary effusion lymphoma (PEL) and multicentric Castleman’s disease (MCD) [2, 3]. The KSHV life cycle is divided into two phases: latency and productive (‘lytic’) replication. Early after infection, the viral dsDNA enters the nucleus, is circularized and further chromatinized and maintained as a stable episome within the host . During latency no viral particles are produced and only a few latent proteins are expressed from the so called latency transcript cluster under the control of a constitutively active promoter. These latent proteins function as viral regulators enabling the establishment and maintenance of latency as well as the inhibition of the lytic cycle. Furthermore, they are involved in cell proliferation, survival, differentiation and transformation as well as angiogenesis and the induction of interferon stimulated genes (ISGs) and thereby contribute to KSHV pathogenesis [4–7]. The latent state can be disrupted by lytic reactivation which is characterized by a distinct pattern of gene expression, involving immediate early, early lytic and late lytic transcripts . Lytic cycle activation can be induced by co-factors like hypoxia, oxidative stress and inflammatory cytokines [8–15]. In addition, viral co-infections with human immunodeficiency virus-1 (HIV-1), Herpes simplex virus (HSV) or Human cytomegalovirus (HCMV) are known to induce KSHV reactivation [13, 16–19]. Several chemical compounds, such as histone deacetylase (HDAC) inhibitors like sodium butyrate (SB), or 12-O-tetradecanoylphorbol-13-acetate (TPA) can activate the lytic replication cycle [20, 21]. The immediate early protein, replication and transcription activator (RTA) encoded by ORF50, is a key viral regulator of the KSHV lytic cycle. By trans-activating the expression of other lytic downstream genes as well as its own promoter, RTA is necessary and sufficient to induce the entire viral lytic cycle [22, 23]. Several viral proteins expressed during the early stages of the lytic replication cycle such as vIL6, vGPCR, K1 and K15 contribute to KSHV pathogenesis by promoting proliferation, angiogenesis, invasiveness and may counteract the host antiviral immune response [24, 25].
Among herpesviruses, vIRFs have so far only been found in Old World primate γ2 herpesviruses [30–32]. A number of studies have shown that the KSHV vIRFs can inhibit IFN-signaling and modulate anti-apoptotic as well as cell proliferation pathways . Only a few studies have addressed the role of the KSHV vIRF proteins during KSHV replication. One study showed that KSHV vIRF4 facilitates lytic replication by targeting the expression of cellular IRF4 and c-myc . In addition, vIRF4 interacts with CSF/CBF1, a downstream effector of Notch signaling that is also targeted by KSHV RTA and LANA and that is required for efficient lytic reactivation [42, 91]. Furthermore, vIRF4 has been shown to cooperate with RTA in the activation of several lytic promoters . In contrast, vIRF3 has been shown to suppress, and vIRF1 to promote, lytic replication by recruitment of USP7  or the BH3-only pro-apoptotic Bcl2 family member Bim . However, in apparent contrast, vIRF3 degrades PML NBs in transfected cells, as reported before [44, 45] and shown in Fig 2.