BRLF1 suppresses RNA Pol III-mediated RIG-I inflammasome activation in the early EBV lytic lifecycle

Epstein-Barr Virus BRLF1 Induces PD-L1 Expression in Nasopharyngeal Carcinoma Cells

Nasopharyngeal carcinoma (NPC) is a specific human malignancy with unique geographic distribution and genetic backgrounds. Although early treatment with radio-chemotherapy has been proven effective for NPC therapy, its therapeutic efficacy substantially diminishes in the late stages of this malignancy. In the tumor microenvironment of NPC, PD-L1 has been demonstrated as a critical factor in impairing T cell activation. As an etiological role for NPC development, it is found that Epstein-Barr virus (EBV) latent proteins upregulated PD-L1 expression. However, whether EBV lytic protein affects PD-L1 expression remains unclear. In this study, through monitoring the mRNA expression pattern of lytic genes and PD-L1 in EBV-positive NPC cell line NA, EBV immediately-early gene BRLF1(Rta) was found to have the potential for PD-L1 activation. Furthermore, we identified that Rta expression enhanced PD-L1 expression in mRNA and protein levels through quantitative real-time polymerase chain reaction and western blotting analysis. The luciferase reporter assay revealed that Rta expression enhanced PD-L1 promoter activity. We also demonstrated that Rta-induced PD-L1 expressions could impair interleukin 2 secretion of T cells, and this mechanism may be through ERK activation. These results displayed the importance of EBV Rta in PD-L1 expression in NPC and may give an alternative target for NPC therapy.

SARS-CoV-2 Nsp8 suppresses MDA5 antiviral immune responses by impairing TRIM4-mediated K63-linked polyubiquitination

Melanoma differentiation-associated gene-5 (MDA5) acts as a cytoplasmic RNA sensor to detect viral dsRNA and mediates antiviral innate immune responses to infection by RNA viruses. Upon recognition of viral dsRNA, MDA5 is activated with K63-linked polyubiquitination and then triggers the recruitment of MAVS and activation of TBK1 and IKKα/β, subsequently leading to IRF3 and NF-κB phosphorylation. However, the specific E3 ubiquitin ligase for MDA5 K63-polyubiquitination has not been well characterized. Great numbers of symptomatic and severe infections of SARS-CoV-2 are spreading worldwide, and the poor efficacy of treatment with type I interferon and antiviral immune agents indicates that SARS-CoV-2 escapes from antiviral immune responses via several unknown mechanisms. Here, we report that SARS-CoV-2 nonstructural protein 8 (nsp8) acts as a suppressor of antiviral innate immune and inflammatory responses to promote infection of SARS-CoV-2. It downregulates the expression of type I interferon, IFN-stimulated genes and proinflammatory cytokines by binding to MDA5 and TRIM4 and impairing TRIM4-mediated MDA5 K63-linked polyubiquitination. Our findings reveal that nsp8 mediates innate immune evasion during SARS-CoV-2 infection and may serve as a potential target for future therapeutics for SARS-CoV-2 infectious diseases.

Alphaherpesvirus-mediated remodeling of the cellular transcriptome results in depletion of m6A-containing transcripts

The mechanisms by which viruses regulate host mRNAs during infection are still poorly understood. Several host transcripts that encode proteins that contribute to the anti-viral response contain the N6-methyladenosine nucleotide (m6A). In this study, we investigated if and how viruses from different (sub) families specifically affect m6A-containing host transcripts. Systematic analysis of host transcriptomes after infection with diverse types of viruses showed that m6A-methylated transcripts are selectively downregulated during infection with Sendai virus, African swine fever virus and the alphaherpesviruses herpes simplex virus 1 (HSV-1) and pseudorabies virus (PRV). Focusing on PRV and HSV-1, we found that downregulation of m6A-methylated transcripts depends on the YTHDF family of m6A-binding proteins, and correlates with localization of these proteins to enlarged P-bodies. Knockdown of YTHDF proteins in primary cells reduced PRV protein expression and increased expression of antiviral interferon-stimulated genes, suggesting that virus-induced depletion of host m6A-containing transcripts constitutes an immune evasion strategy.

Inhibition of AIM2 inflammasome activation by SOX/ORF37 promotes lytic replication of Kaposi's sarcoma-associated herpesvirus

Inflammasomes are one kind of important innate immune defense against viral and bacterial infections. Several inflammasome-forming sensors detect molecular patterns of invading pathogens and then trigger inflammasome activation and/or pyroptosis in infected cells, and viruses employ unique strategies to hijack or subvert inflammasome activation. Infection with herpesviruses induces the activation of diverse inflammasomes, including AIM2 and IFI16 inflammasomes; however, how Kaposi's sarcoma-associated herpesvirus (KSHV) counteracts inflammasome activation largely remains unclear. Here, we reveal that the KSHV ORF37-encoded SOX protein suppresses AIM2 inflammasome activation independent of its viral DNA exonuclease activity and host mRNA turnover. SOX interacts with the AIM2 HIN domain through the C-terminal Motif VII region and disrupts AIM2:dsDNA polymerization and ASC recruitment and oligomerization. The Y443A or F444A mutation of SOX abolishes the inhibition of AIM2 inflammasome without disrupting SOX nuclease activity, and a short SOX peptide is capable of inhibiting AIM2 inflammasome activation; consequently, infection with SOX-null, Y443A, or F444A Bac16 recombinant viruses results in robust inflammasome activation, suppressed lytic replication, and increased pyroptosis in human lymphatic endothelial cells in an AIM2-dependent manner. These results reveal that KSHV SOX suppresses AIM2 inflammasome activation to promote KSHV lytic replication and inhibit pyroptosis, representing a unique mechanism for evasion of inflammasome activation during KSHV lytic cycle.

The Role of RNA Sensors in Regulating Innate Immunity to Gammaherpesviral Infections

Kaposi's sarcoma-associated herpesvirus (KSHV) and the Epstein-Barr virus (EBV) are double-stranded DNA oncogenic gammaherpesviruses. These two viruses are associated with multiple human malignancies, including both B and T cell lymphomas, as well as epithelial- and endothelial-derived cancers. KSHV and EBV establish a life-long latent infection in the human host with intermittent periods of lytic replication. Infection with these viruses induce the expression of both viral and host RNA transcripts and activates several RNA sensors including RIG-I-like receptors (RLRs), Toll-like receptors (TLRs), protein kinase R (PKR) and adenosine deaminases acting on RNA (ADAR1). Activation of these RNA sensors induces the innate immune response to antagonize the virus. To counteract this, KSHV and EBV utilize both viral and cellular proteins to block the innate immune pathways and facilitate their own infection. In this review, we summarize how gammaherpesviral infections activate RNA sensors and induce their downstream signaling cascade, as well as how these viruses evade the antiviral signaling pathways to successfully establish latent infection and undergo lytic reactivation.

The RNA polymerase III-RIG-I axis in antiviral immunity and inflammation

Nucleic acid sensors survey subcellular compartments for atypical or mislocalized RNA or DNA, ultimately triggering innate immune responses. Retinoic acid-inducible gene-I (RIG-I) is part of the family of cytoplasmic RNA receptors that can detect viruses. A growing literature demonstrates that mammalian RNA polymerase III (Pol III) transcribes certain viral or cellular DNA sequences into immunostimulatory RIG-I ligands, which elicits antiviral or inflammatory responses. Dysregulation of the Pol III-RIG-I sensing axis can lead to human diseases including severe viral infection outcomes, autoimmunity, and tumor progression. Here, we summarize the newly emerging role of viral and host-derived Pol III transcripts in immunity and also highlight recent advances in understanding how mammalian cells prevent unwanted immune activation by these RNAs to maintain homeostasis.

Immunity and Immune Evasion Mechanisms of Epstein-Barr Virus

Epstein-Barr virus (EBV) is the first human oncogenic virus to be identified, which evades the body's immune surveillance through multiple mechanisms that allow long-term latent infection. Under certain pathological conditions, EBVs undergo a transition from the latent phase to the lytic phase and cause targeted dysregulation of the host immune system, leading to the development of EBV-related diseases. Therefore, an in-depth understanding of the mechanism of developing an immune response to EBV and the evasion of immune recognition by EBV is important for the understanding of the pathogenesis of EBV, which is of great significance for finding strategies to prevent EBV infection, and developing a therapy to treat EBV-associated diseases. In this review, we will discuss the molecular mechanisms of host immunological responses to EBV infection and the mechanisms of EBV-mediated immune evasion during chronic active infection.

Advancing therapeutic strategies for Epstein-Barr virus-associated malignancies through lytic reactivation

Epstein-Barr virus (EBV) is a widespread human herpes virus associated with lymphomas and epithelial cell cancers. It establishes two separate infection phases, latent and lytic, in the host. Upon infection of a new host cell, the virus activates several pathways, to induce the expression of lytic EBV antigens and the production of infectious virus particles. Although the carcinogenic role of latent EBV infection has been established, recent research suggests that lytic reactivation also plays a significant role in carcinogenesis. In this review, we summarize the mechanism of EBV reactivation and recent findings about the role of viral lytic antigens in tumor formation. In addition, we discuss the treatment of EBV-associated tumors with lytic activators and the targets that may be therapeutically effective in the future.

Exploiting RIG-I-like receptor pathway for cancer immunotherapy

RIG-I-like receptors (RLRs) are intracellular pattern recognition receptors that detect viral or bacterial infection and induce host innate immune responses. The RLRs family comprises retinoic acid-inducible gene 1 (RIG-I), melanoma differentiation-associated gene 5 (MDA5) and laboratory of genetics and physiology 2 (LGP2) that have distinctive features. These receptors not only recognize RNA intermediates from viruses and bacteria, but also interact with endogenous RNA such as the mislocalized mitochondrial RNA, the aberrantly reactivated repetitive or transposable elements in the human genome. Evasion of RLRs-mediated immune response may lead to sustained infection, defective host immunity and carcinogenesis. Therapeutic targeting RLRs may not only provoke anti-infection effects, but also induce anticancer immunity or sensitize "immune-cold" tumors to immune checkpoint blockade. In this review, we summarize the current knowledge of RLRs signaling and discuss the rationale for therapeutic targeting RLRs in cancer. We describe how RLRs can be activated by synthetic RNA, oncolytic viruses, viral mimicry and radio-chemotherapy, and how the RNA agonists of RLRs can be systemically delivered in vivo. The integration of RLRs agonism with RNA interference or CAR-T cells provides new dimensions that complement cancer immunotherapy. Moreover, we update the progress of recent clinical trials for cancer therapy involving RLRs activation and immune modulation. Further studies of the mechanisms underlying RLRs signaling will shed new light on the development of cancer therapeutics. Manipulation of RLRs signaling represents an opportunity for clinically relevant cancer therapy. Addressing the challenges in this field will help develop future generations of cancer immunotherapy.

The lytic phase of Epstein-Barr virus plays an important role in tumorigenesis

Epstein-Barr virus (EBV) is a recognized oncogenic virus that is related to the occurrence of lymphoma, nasopharyngeal carcinoma (NPC), and approximately 10% of gastric cancer (GC). EBV is a herpesvirus, and like other herpesviruses, EBV has a biphasic infection mode made up of latent and lytic infections. It has been established that latent infection promotes tumorigenesis in previous research, but in recent years, there has been new evidence that suggests that the lytic infection mode could also promote tumorigenesis. In this review, we mainly discuss the contribution of the EBV lytic phase to tumorigenesis, and graphically illustrate their relationship in detail. In addition, we described the relationship between the lytic cycle of EBV and autophagy. Finally, we also preliminarily explored the influence of the tumorigenesis effect of the EBV lytic phase on the future treatment of EBV-associated tumors.

Suppression of KSHV lytic replication and primary effusion lymphoma by selective RNF5 inhibition

Primary effusion lymphoma (PEL), a rare aggressive B-cell lymphoma in immunosuppressed patients, is etiologically associated with oncogenic γ-herpesvirus infection. Chemotherapy is commonly used to treat PEL but usually results in poor prognosis and survival; thus, novel therapies and drug development are urgently needed for PEL treatment. Here, we demonstrated that inhibition of Ring finger protein 5 (RNF5), an ER-localized E3 ligase, suppresses multiple cellular pathways and lytic replication of Kaposi sarcoma-associated herpesvirus (KSHV) in PEL cells. RNF5 interacts with and induces Ephrin receptors A3 (EphA3) and EphA4 ubiquitination and degradation. RNF5 inhibition increases the levels of EphA3 and EphA4, thereby reducing ERK and Akt activation and KSHV lytic replication. RNF5 inhibition decreased PEL xenograft tumor growth and downregulated viral gene expression, cell cycle gene expression, and hedgehog signaling in xenograft tumors. Our study suggests that RNF5 plays the critical roles in KSHV lytic infection and tumorigenesis of primary effusion lymphoma.

Upregulation of ATF4-LAMP3 Axis by ORF45 Facilitates Lytic Replication of Kaposi's Sarcoma-Associated Herpesvirus

Kaposi's sarcoma-associated herpesvirus (KSHV) is a γ-oncogenic herpesvirus, and both lytic and latent infections play important roles in its pathogenesis and tumorigenic properties. Multiple cellular pathways and diverse mediators are hijacked by viral proteins and are used to support KSHV lytic replication. In previous studies, we revealed that KSHV ORF45 promoted KSHV transcription and translation by inducing sustained p90 ribosomal S6 kinase (RSK) activation and the phosphorylation of its substrates c-Fos and eIF4B. However, the cellular mediators required for lytic replication remain largely unknown. Here, we reveal that ORF45 activates eIF2α phosphorylation and ATF4 translation and then upregulates the expression of lysosome-associated membrane protein 3 (LAMP3) in an ATF4-dependent manner during KSHV lytic replication. Consequently, LAMP3 promotes Akt and ERK activation and then facilitates lytic gene expression and virion production. Furthermore, ATF4 enhances lytic replication through LAMP3, and LAMP3 acts in an ATF4-independent manner. Our findings suggest that the ATF4-LAMP3 axis is upregulated by ORF45 through ER stress activation during the KSHV lytic life cycle and, in turn, facilitates optimal lytic replication. IMPORTANCE The lytic replication of Kaposi's sarcoma-associated herpesvirus (KSHV) reprograms cellular transcription and translation to generate viral proteins and virion particles. Here, we show that the mediator of ER stress ATF4 and the expression of the downstream gene LAMP3 are upregulated by ORF45 during lytic replication. Consequently, increased LAMP3 expression activates Akt and ERK and promotes lytic replication. Although several UPR transcription factors are able to promote KSHV lytic replication, the proviral effect of ATF4 on lytic replication is attenuated by LAMP3 silencing, whereas the effect of LAMP3 does not directly require ATF4 expression, indicating that LAMP3 primarily exerts effects on KSHV lytic replication downstream of ATF4 and ER stress. Taken together, our findings suggest that the ORF45-upregulated ATF4-LAMP3 axis plays an essential role in KSHV lytic replication.

Clinical and genetic characterization of Epstein-Barr virus-associated T/NK-cell lymphoproliferative diseases

BACKGROUND

Epstein-Barr virus (EBV)-associated T-/natural killer (T/NK)-cell lymphoproliferative diseases clinically take on various forms, ranging from an indolent course to an aggressive condition.

OBJECTIVE

Clinically, failure to establish precise diagnosis and provide proper treatment makes it difficult to help patients. We sought to better understand the underlying pathogenesis and to identify genetic prognostic factors to achieve better treatment efficacy.

METHODS

In this study, 119 cases of EBV-associated lymphoproliferative diseases, including EBV-associated hemophagocytic lymphohistiocytosis (n = 46) and chronic active EBV disease of T/NK cell type (n = 73), were retrospectively examined.

RESULTS

Adults aged >20 years at onset accounted for 71.4% of our cohort. About 54.6% patients with unfavorable overall survival developed hemophagocytic lymphohistiocytosis and had higher plasma EBV load. Allogenic hematopoietic stem-cell transplantation was the sole independent favorable factor. We systematically screened germline and somatic aberrations by whole-exome and targeted sequencing. Among 372 antiviral immunity genes, germline variants of 8 genes were significantly enriched. From a panel of 24 driver genes, somatic mutations were frequently identified in dominant EBV-infected T/NK cells. Patients carrying any germline/somatic aberrations in epigenetic modifiers and RIG-I-like receptor (RLR) pathway had worse overall survival than those without 2 type aberrations. Importantly, patients with IFIH1 and/or DDX3X aberrations in the RLR pathway had higher plasma and NK-cell EBV load. Knockdown of DDX3X in NKYS cells downregulated RLR signaling activities and elevated the expression of EBV-encoded oncogenes such as LMP1 and EBNA1.

CONCLUSION

Genetic defects were prevalent in adult EBV-associated hemophagocytic lymphohistiocytosis patients and patients with chronic active EBV disease of T/NK cell type; these defects were associated with unfavorable prognosis. These findings can help clinicians work out more precise staging of the condition and provide new insights into these EBV-associated diseases.

The oncogenic gamma herpesviruses Epstein-Barr virus (EBV) and Kaposi's sarcoma-associated herpesvirus (KSHV) hijack retinoic acid-inducible gene I (RIG-I) facilitating both viral and tumour immune evasion

Herpesviruses evade host immunity to establish persistent lifelong infection with dormant latent and replicative lytic phases. Epstein-Barr virus (EBV) and Kaposi's Sarcoma-associated virus (KSHV) are double-stranded DNA herpesviruses that encode components to activate RNA sensors, (Retinoic acid-inducible gene I (RIG-I) and melanoma differentiation-associated protein 5 (MDA5). Yet both viruses can effectively evade the antiviral immune response. The ability of these viruses to disarm RIG-I to evade immunity allowing viral persistency can contribute to the creation of a protected niche that facilitates tumour growth and immune evasion. Alternatively, viral nucleic acids present in the cytosol during the replicative phase of the viral lifecycle can activate pro-inflammatory signaling downstream of RIG-I augmenting tumour promoting inflammation. Understanding how these viral proteins disrupt innate immune pathways could help identify mechanisms to boost immunity, clearing viral infection and enhancing the efficacy of immunotherapy for virally induced cancers. Here we review literature on the strategies EBV and KSHV use to either enhance or inhibit RLR signaling.

Strategies of Epstein-Barr virus to evade innate antiviral immunity of its human host

Epstein-Barr virus (EBV) is a double-stranded DNA virus of the Herpesviridae family. This virus preferentially infects human primary B cells and persists in the human B cell compartment for a lifetime. Latent EBV infection can lead to the development of different types of lymphomas as well as carcinomas such as nasopharyngeal and gastric carcinoma in immunocompetent and immunocompromised patients. The early phase of viral infection is crucial for EBV to establish latency, but different viral components are sensed by cellular sensors called pattern recognition receptors (PRRs) as the first line of host defense. The efficacy of innate immunity, in particular the interferon-mediated response, is critical to control viral infection initially and to trigger a broad spectrum of specific adaptive immune responses against EBV later. Despite these restrictions, the virus has developed various strategies to evade the immune reaction of its host and to establish its lifelong latency. In its different phases of infection, EBV expresses up to 44 different viral miRNAs. Some act as viral immunoevasins because they have been shown to counteract innate as well as adaptive immune responses. Similarly, certain virally encoded proteins also control antiviral immunity. In this review, we discuss how the virus governs innate immune responses of its host and exploits them to its advantage.

BRLF1-dependent viral and cellular transcriptomes and transcriptional regulation during EBV primary infection in B lymphoma cells

The immediate-early protein BRLF1 plays important roles in lytic infection of Epstein-Barr virus (EBV), in which it activates lytic viral transcription and replication. However, knowledge of the influence of BRLF1 on cellular gene expression and transcriptional reprogramming during the early lytic cycle remains limited. In the present study, deep RNA-sequencing analysis identified all differentially expressed genes (DEGs) and alternative splicing in B lymphoma cells subjected to wild-type and BRLF1-deficient EBV primary infection. The BRLF1-dependent cellular DEGs were annotated, and major differentially enriched pathways were related to DNA replication and transcription, immune and inflammatory responses, cytokine-receptor interactions and chemokine signaling and metabolic processes. Furthermore, analysis of BRLF1-binding proteins by mass spectrometry shows that BRLF1 binds to and cooperates with several transcription factors and components of the spliceosome and then influences both RNA polymerase II-dependent transcription and pre-mRNA splicing. The RTA-binding RRE motifs or specific motifs of unique cooperative transcription factors in viral and cellular DEG promoter regions indicate that BRLF1 employs different strategies for regulating viral and cellular transcription. Thus, our study characterized BRLF1-dependent cellular and viral transcriptional profile during primary infection and then revealed the comprehensive virus-cell interaction and alterations of transcription during EBV primary infection and lytic replication.