A Rhesus Rhadinovirus Viral Interferon (IFN) Regulatory Factor Is Virion Associated and Inhibits the Early IFN Antiviral Response

Of Keeping and Tipping the Balance: Host Regulation and Viral Modulation of IRF3-Dependent IFNB1 Expression

The type I interferon (IFN) response is a principal component of our immune system that allows to counter a viral attack immediately upon viral entry into host cells. Upon engagement of aberrantly localised nucleic acids, germline-encoded pattern recognition receptors convey their find via a signalling cascade to prompt kinase-mediated activation of a specific set of five transcription factors. Within the nucleus, the coordinated interaction of these dimeric transcription factors with coactivators and the basal RNA transcription machinery is required to access the gene encoding the type I IFN IFNβ (IFNB1). Virus-induced release of IFNβ then induces the antiviral state of the system and mediates further mechanisms for defence. Due to its key role during the induction of the initial IFN response, the activity of the transcription factor interferon regulatory factor 3 (IRF3) is tightly regulated by the host and fiercely targeted by viral proteins at all conceivable levels. In this review, we will revisit the steps enabling the trans-activating potential of IRF3 after its activation and the subsequent assembly of the multi-protein complex at the IFNβ enhancer that controls gene expression. Further, we will inspect the regulatory mechanisms of these steps imposed by the host cell and present the manifold strategies viruses have evolved to intervene with IFNβ transcription downstream of IRF3 activation in order to secure establishment of a productive infection.

African Swine Fever Virus Armenia/07 Virulent Strain Controls Interferon Beta Production through the cGAS-STING Pathway

African swine fever, a devastating disease for domestic pigs and wild boar, is currently spreading in Europe, Russia, and China, becoming a global threat with huge economic and ecological consequences. One interesting aspect of ASFV biology is the molecular mechanism leading to high virulence of some strains compared to more attenuated strains, which produce subclinical infections. In this work, we show that the presently circulating virulent Armenia/07 virus blocks the synthesis of IFN-β, a key mediator between the innate and adaptive immune response. Armenia/07 inhibits the cGAS-STING pathway by impairing STING activation during infection. In contrast, the cGAS-STING pathway is efficiently activated during NH/P68 attenuated strain infection, leading to the production of large amounts of IFN-β. Our results show for the first time the relationship between the cGAS-STING pathway and ASFV virulence, contributing to uncover the molecular mechanisms of ASFV virulence and to the rational development of ASFV vaccines.

African swine fever virus (ASFV) is a complex, cytoplasmic double-stranded DNA (dsDNA) virus that is currently expanding throughout the world. Currently, circulating virulent genotype II Armenia/07-like viruses cause fatal disease in pigs and wild boar, whereas attenuated strains induce infections with various levels of chronic illness. Sensing cytosolic dsDNA, mainly by the key DNA sensor cyclic GMP-AMP synthase (cGAS), leads to the synthesis of type I interferon and involves signaling through STING, TBK1, and IRF3. After phosphorylation, STING translocates from the endoplasmic reticulum to the Golgi compartment and to the perinuclear region, acting as an indispensable adaptor connecting the cytosolic detection of DNA to the TBK1-IRF3 signaling pathway. We demonstrate here that attenuated NH/P68, but not virulent Armenia/07, activates the cGAS-STING-IRF3 cascade very early during infection, inducing STING phosphorylation and trafficking through a mechanism involving cGAMP. Both TBK1 and IRF3 are subsequently activated and, in response to this, a high level of beta interferon (IFN-β) was produced during NH/P68 infection; in contrast, Armenia/07 infection generated IFN-β levels below those of uninfected cells. Our results show that virulent Armenia/07 ASFV controls the cGAS-STING pathway, but these mechanisms are not at play when porcine macrophages are infected with attenuated NH/P68 ASFV. These findings show for the first time the involvement of the cGAS-STING-IRF3 route in ASFV infection, where IFN-β production or inhibition was found after infection by attenuated or virulent ASFV strains, respectively, thus reinforcing the idea that ASFV virulence versus attenuation may be a phenomenon grounded in ASFV-mediated innate immune modulation where the cGAS-STING pathway might play an important role.

IMPORTANCE African swine fever, a devastating disease for domestic pigs and wild boar, is currently spreading in Europe, Russia, and China, becoming a global threat with huge economic and ecological consequences. One interesting aspect of ASFV biology is the molecular mechanism leading to high virulence of some strains compared to more attenuated strains, which produce subclinical infections. In this work, we show that the presently circulating virulent Armenia/07 virus blocks the synthesis of IFN-β, a key mediator between the innate and adaptive immune response. Armenia/07 inhibits the cGAS-STING pathway by impairing STING activation during infection. In contrast, the cGAS-STING pathway is efficiently activated during NH/P68 attenuated strain infection, leading to the production of large amounts of IFN-β. Our results show for the first time the relationship between the cGAS-STING pathway and ASFV virulence, contributing to uncover the molecular mechanisms of ASFV virulence and to the rational development of ASFV vaccines.

Rhesus Macaque Rhadinovirus Encodes a Viral Interferon Regulatory Factor To Disrupt Promyelocytic Leukemia Nuclear Bodies and Antagonize Type I Interferon Signaling

Interferon (IFN) production and the subsequent induction of IFN-stimulated genes (ISGs) are highly effective innate strategies utilized by cells to protect against invading pathogens, including viruses. Critical components involved in this innate process are promyelocytic leukemia nuclear bodies (PML-NBs), which are subnuclear structures required for the development of a robust IFN response. As such, PML-NBs serve as an important hurdle for viruses to overcome to successfully establish an infection. Both Kaposi's sarcoma-associated herpesvirus (KSHV) and the closely related rhesus macaque rhadinovirus (RRV) are unique for encoding viral homologs of IFN regulatory factors (termed vIRFs) that can manipulate the host immune response by multiple mechanisms. All four KSHV vIRFs inhibit the induction of IFN, while vIRF1 and vIRF2 can inhibit ISG induction downstream of the IFN receptor. Less is known about the RRV vIRFs. RRV vIRF R6 can inhibit the induction of IFN by IRF3; however, it is not known whether any RRV vIRFs inhibit ISG induction following IFN receptor signaling. In our present study, we demonstrate that the RRV vIRF R12 aids viral replication in the presence of the type I IFN response. This is achieved in part through the disruption of PML-NBs and the inhibition of robust ISG transcription.IMPORTANCE KSHV and RRV encode a unique set of homologs of cellular IFN regulatory factors, termed vIRFs, which are hypothesized to help these viruses evade the innate immune response and establish infections in their respective hosts. Our work elucidates the role of one RRV vIRF, R12, and demonstrates that RRV can dampen the type I IFN response downstream of IFN signaling, which would be important for establishing a successful infection in vivo.

The Molecular Basis of Viral Inhibition of IRF- and STAT-Dependent Immune Responses

The antiviral innate immunity is the first line of host defense against virus infections. In mammalian cells, viral infections initiate the expression of interferons (IFNs) in the host that in turn activate an antiviral defense program to restrict viral replications by induction of IFN stimulated genes (ISGs), which are largely regulated by the IFN-regulatory factor (IRF) family and signal transducer and activator of transcription (STAT) family transcription factors. The mechanisms of action of IRFs and STATs involve several post-translational modifications, complex formation, and nuclear translocation of these transcription factors. However, many viruses, including human immunodeficiency virus (HIV), Zika virus (ZIKV), and herpes simplex virus (HSV), have evolved strategies to evade host defense, including alteration in IRF and STAT post-translational modifications, disturbing the formation and nuclear translocation of the transcription complexes as well as proteolysis/degradation of IRFs and STATs. In this review, we discuss and summarize the molecular mechanisms by which how viral components may target IRFs and STATs to antagonize the establishment of antiviral host defense. The underlying host-viral interactions determine the outcome of viral infection. Gaining mechanistic insight into these processes will be crucial in understanding how viral replication can be more effectively controlled and in developing approaches to improve virus infection outcomes.

Rhadinoviral interferon regulatory factor homologues

Kaposi's sarcoma-associated herpesvirus (KSHV), or human herpesvirus 8 (HHV8) is a gammaherpesvirus and the etiological agent of Kaposi's sarcoma, primary effusion lymphoma and multicentric Castleman disease. The KSHV genome contains genes for a unique group of proteins with homology to cellular interferon regulatory factors, termed viral interferon regulatory factors (vIRFs). This review will give an overview over the oncogenic, antiapoptotic and immunomodulatory characteristics of KSHV and related vIRFs.

KSHV-encoded viral interferon regulatory factor 4 (vIRF4) interacts with IRF7 and inhibits interferon alpha production

Before an infection can be completely established, the host immediately turns on the innate immune system through activating the interferon (IFN)-mediated antiviral pathway. Kaposi's sarcoma-associated herpesvirus (KSHV) utilizes a unique antagonistic mechanism of type I IFN-mediated host antiviral immunity by incorporating four viral interferon regulatory factors (vIRF1-4). Herein, we characterized novel immune evasion strategies of vIRF4 to inhibit the IRF7-mediated IFN-α production. KSHV vIRF4 specifically interacts with IRF7, resulting in inhibition of IRF7 dimerization and ultimately suppresses IRF7-mediated activation of type I IFN. These results suggest that each of the KSHV vIRFs, including vIRF4, subvert IFN-mediated anti-viral response via different mechanisms. Therefore, it is indicated that KSHV vIRFs are indeed a crucial immunomodulatory component of their life cycles.

Viral Evasion Strategies in Type I IFN Signaling - A Summary of Recent Developments

The immune system protects the organism against infections and the damage associated with them. The first line of defense against pathogens is the innate immune response. In the case of a viral infection, it induces the interferon (IFN) signaling cascade and eventually the expression of type I IFN, which then causes an antiviral state in the cells. However, many viruses have developed strategies to counteract this mechanism and prevent the production of IFN. In order to modulate or inhibit the IFN signaling cascade in their favor, viruses have found ways to interfere at every single step of the cascade, for example, by inducing protein degradation or cleavage, or by mediate protein polyubiquitination. In this article, we will review examples of viruses that modulate the IFN response and describe the mechanisms they use.

Animal models of tumorigenic herpesviruses--an update

Any one model system, be it culture or animal, only recapitulates one aspect of the viral life cycle in the human host. By providing recent examples of animal models for Epstein-Barr virus and Kaposi sarcoma-associated herpesvirus, we would argue that multiple animal models are needed to gain a comprehensive understanding of the pathogenesis associated with human oncogenic herpesviruses. Transgenic mice, homologous animal herpesviruses, and tumorgraft and humanized mouse models all complement each other in the study of viral pathogenesis. The use of animal model systems facilitates the exploration of novel anti-viral and anti-cancer treatment modalities for diseases associated with oncogenic herpesviruses.

The Modulation of Apoptotic Pathways by Gammaherpesviruses

Apoptosis or programmed cell death is a tightly regulated process fundamental for cellular development and elimination of damaged or infected cells during the maintenance of cellular homeostasis. It is also an important cellular defense mechanism against viral invasion. In many instances, abnormal regulation of apoptosis has been associated with a number of diseases, including cancer development. Following infection of host cells, persistent and oncogenic viruses such as the members of the Gammaherpesvirus family employ a number of different mechanisms to avoid the host cell’s “burglar” alarm and to alter the extrinsic and intrinsic apoptotic pathways by either deregulating the expressions of cellular signaling genes or by encoding the viral homologs of cellular genes. In this review, we summarize the recent findings on how gammaherpesviruses inhibit cellular apoptosis via virus-encoded proteins by mediating modification of numerous signal transduction pathways. We also list the key viral anti-apoptotic proteins that could be exploited as effective targets for novel antiviral therapies in order to stimulate apoptosis in different types of cancer cells.