Gamma-herpesvirus kinase actively initiates a DNA damage response by inducing phosphorylation of H2AX to foster viral replication

Interplay between the DNA damage response and the life cycle of DNA tumor viruses

Approximately 20 % of human cancers are associated with virus infection. DNA tumor viruses can induce tumor formation in host cells by disrupting the cell's DNA replication and repair mechanisms. Specifically, these viruses interfere with the host cell's DNA damage response (DDR), which is a complex network of signaling pathways that is essential for maintaining the integrity of the genome. DNA tumor viruses can disrupt these pathways by expressing oncoproteins that mimic or inhibit various DDR components, thereby promoting genomic instability and tumorigenesis. Recent studies have highlighted the molecular mechanisms by which DNA tumor viruses interact with DDR components, as well as the ways in which these interactions contribute to viral replication and tumorigenesis. Understanding the interplay between DNA tumor viruses and the DDR pathway is critical for developing effective strategies to prevent and treat virally associated cancers. In this review, we discuss the current state of knowledge regarding the mechanisms by which human papillomavirus (HPV), merkel cell polyomavirus (MCPyV), Kaposi's sarcoma-associated herpesvirus (KSHV), and Epstein-Barr virus (EBV) interfere with DDR pathways to facilitate their respective life cycles, and the consequences of such interference on genomic stability and cancer development.

Host Genetic Variation Has a Profound Impact on Immune Responses Mediating Control of Viral Load in Chronic Gammaherpesvirus Infection

Chronic infection with the gammaherpesvirus EBV is a risk factor for several autoimmune diseases, and poor control of EBV viral load and enhanced anti-EBV responses elevate this risk further. However, the role of host genetic variation in the regulation of immune responses to chronic gammaherpesvirus infection and control of viral replication remains unclear. To address this question, we infected C57BL/6J (B6) and genetically divergent wild-derived inbred PWD/PhJ (PWD) mice with murine gammaherpesvirus-68 (MHV-68), a gammaherpesvirus similar to EBV, and determined the effect of latent gammaherpesvirus infection on the CD4 T cell transcriptome. Chronic MHV-68 infection of B6 mice resulted in a dramatic upregulation of genes characteristic of a cytotoxic Th cell phenotype, including Gzmb, Cx3cr1, Klrg1, and Nkg7, a response that was highly muted in PWD mice. Flow cytometric analyses revealed an expansion of CX3CR1+KLRG1+ cytotoxic Th cell-like cells in B6 but not PWD mice. Analysis of MHV-68 replication demonstrated that in spite of muted adaptive responses, PWD mice had superior control of viral load in lymphoid tissue, despite an absence of a defect in MHV-68 in vitro replication in PWD macrophages. Depletion of NK cells in PWD mice, but not B6 mice, resulted in elevated viral load, suggesting genotype-dependent NK cell involvement in MHV-68 control. Taken together, our findings demonstrate that host genetic variation can regulate control of gammaherpesvirus replication through disparate immunological mechanisms, resulting in divergent long-term immunological sequelae during chronic infection.

Murine Gammaherpesvirus 68 Efficiently Infects Myeloid Cells Resulting In An Atypical, Restricted Form Of Infection

The gammaherpesviruses (γHVs) establish a lifelong infection in their hosts, with the cellular outcome of infection intimately regulated by target cell type. Murine gammaherpesvirus 68 (MHV68), a small animal model of γHV infection, infects macrophages in vivo, resulting in a range of outcomes, from lytic replication to latent infection. Here, we have further investigated the nature of MHV68 macrophage infection using reductionist and primary in vivo infection studies. While MHV68 readily infected the J774 macrophage cell line, viral gene expression and replication were significantly impaired relative to a fully permissive fibroblast cell line. Lytic replication only occurred in a small subset of MHV68-infected J774 cells, despite the fact that these cells were fully competent to support lytic replication following pre-treatment with interleukin-4, a known potentiator of replication in macrophages. In parallel, we harvested virally-infected macrophages at 16 hours after MHV68 infection in vivo and analyzed gene expression by single cell RNA-sequencing. Among virally infected macrophages, only rare (0.25%) cells had lytic cycle gene expression, characterized by detection of multiple lytic cycle RNAs. In contrast, ~50% of virally-infected macrophages were characterized by expression of ORF75A, ORF75B and/or ORF75C, in the absence of other detectable viral RNAs. Selective transcription of the ORF75 locus also occurred in MHV68-infected J774 cells. In total, these studies indicate that MHV68 efficiently infects macrophages, with the majority of cells characterized by an atypical state of restricted viral transcription, and only rare cells undergoing lytic replication.

DISTINCT ROLES OF VIRAL US3 AND UL13 PROTEIN KINASES IN HERPES VIRUS SIMPLEX TYPE 1 (HSV-1) NUCLEAR EGRESS

Herpesviruses transport nucleocapsids from the nucleus to the cytoplasm by capsid envelopment into the inner nuclear membrane and de-envelopment from the outer nuclear membrane, a process that is coordinated by nuclear egress complex (NEC) proteins, pUL34, and pUL31. Both pUL31 and pUL34 are phosphorylated by the virus-encoded protein kinase, pUS3, and phosphorylation of pUL31 regulates NEC localization at the nuclear rim. pUS3 also controls apoptosis and many other viral and cellular functions in addition to nuclear egress, and the regulation of these various activities in infected cells is not well understood. It has been previously proposed that pUS3 activity is selectively regulated by another viral protein kinase, pUL13 such that its activity in nuclear egress is pUL13-dependent, but apoptosis regulation is not, suggesting that pUL13 might regulate pUS3 activity on specific substrates. We compared HSV-1 UL13 kinase-dead and US3 kinase-dead mutant infections and found that pUL13 kinase activity does not regulate the substrate choice of pUS3 in any defined classes of pUS3 substrates and that pUL13 kinase activity is not important for promoting de-envelopment during nuclear egress. We also find that mutation of all pUL13 phosphorylation motifs in pUS3, individually or in aggregate, does not affect the localization of the NEC, suggesting that pUL13 regulates NEC localization independent of pUS3. Finally, we show that pUL13 co-localizes with pUL31 inside the nucleus in large aggregates, further suggesting a direct effect of pUL13 on the NEC and suggesting a novel mechanism for both UL31 and UL13 in the DNA damage response pathway. IMPORTANCE Herpes simplex virus infections are regulated by two virus-encoded protein kinases, pUS3 and pUL13, which each regulate multiple processes in the infected cell, including capsid transport from the nucleus to the cytoplasm. Regulation of the activity of these kinases on their various substrates is poorly understood, but importantly, kinases are attractive targets for the generation of inhibitors. It has been previously suggested that pUS3 activity on specific substrates is differentially regulated by pUL13 and, specifically, that pUL13 regulates capsid egress from the nucleus by phosphorylation of pUS3. In this study, we determined that pUL13 and pUS3 have different effects on nuclear egress and that pUL13 may interact directly with the nuclear egress apparatus with implications both for virus assembly and egress and, possibly, the host cell DNA- damage response.

Mechanism of herpesvirus protein kinase UL13 in immune escape and viral replication

Upon infection, the herpes viruses create a cellular environment suitable for survival, but innate immunity plays a vital role in cellular resistance to viral infection. The UL13 protein of herpesviruses is conserved among all herpesviruses and is a serine/threonine protein kinase, which plays a vital role in escaping innate immunity and promoting viral replication. On the one hand, it can target various immune signaling pathways in vivo, such as the cGAS-STING pathway and the NF-κB pathway. On the other hand, it phosphorylates regulatory many cellular and viral proteins for promoting the lytic cycle. This paper reviews the research progress of the conserved herpesvirus protein kinase UL13 in immune escape and viral replication to provide a basis for elucidating the pathogenic mechanism of herpesviruses, as well as providing insights into the potential means of immune escape and viral replication of other herpesviruses that have not yet resolved the function of it.

Virus-inspired strategies for cancer therapy

The intentional use of viruses for cancer therapy dates back over a century. As viruses are inherently immunogenic and naturally optimized delivery vehicles, repurposing viruses for drug delivery, tumor antigen presentation, or selective replication in cancer cells represents a simple and elegant approach to cancer treatment. While early virotherapy was fraught with harsh side effects and low response rates, virus-based therapies have recently seen a resurgence due to newfound abilities to engineer and tune oncolytic viruses, virus-like particles, and virus-mimicking nanoparticles for improved safety and efficacy. However, despite their great potential, very few virus-based therapies have made it through clinical trials. In this review, we present an overview of virus-inspired approaches for cancer therapy, discuss engineering strategies to enhance their mechanisms of action, and highlight their application for overcoming the challenges of traditional cancer therapies.

Nuclear antiviral innate responses at the intersection of DNA sensing and DNA repair

The coevolution of vertebrate and mammalian hosts with DNA viruses has driven the ability of host cells to distinguish viral from cellular DNA in the nucleus to induce intrinsic immune responses. Concomitant viral mechanisms have arisen to inhibit DNA sensing. At this virus-host interface, emerging evidence links cytokine responses and cellular homeostasis pathways, particularly the DNA damage response (DDR). Nuclear DNA sensors, such as the interferon (IFN)-γ inducible protein 16 (IFI16), functionally intersect with the DDR regulators ataxia telangiectasia mutated (ATM) and DNA-dependent protein kinase (DNA-PK). Here, we discuss accumulating knowledge for the DDR-innate immunity signaling axis. Through the lens of this infection-driven signaling axis, we present host and viral molecular strategies acquired to regulate autoinflammation and antiviral responses.

The Antagonism between the Murine Gammaherpesvirus Protein Kinase and Global Interferon Regulatory Factor 1 Expression Shapes the Establishment of Chronic Infection

Gammaherpesviruses infect most vertebrate species and are associated with B cell lymphomas. Manipulation of B cell differentiation is critical for natural infection and lymphomagenesis driven by gammaherpesviruses. Specifically, human Epstein-Barr virus (EBV) and murine gammaherpesvirus 68 (MHV68) drive differentiation of infected naive B cells into the germinal center to achieve exponential increase in the latent viral reservoir during the establishment of chronic infection. Infected germinal center B cells are also the target of viral lymphomagenesis, as most EBV-positive B cell lymphomas bear the signature of the germinal center response. All gammaherpesviruses encode a protein kinase, which, in the case of Kaposi's sarcoma-associated herpesvirus (KSHV) and MHV68, is sufficient and necessary, respectively, to drive B cell differentiation in vivo. In this study, we used the highly tractable MHV68 model of chronic gammaherpesvirus infection to unveil an antagonistic relationship between MHV68 protein kinase and interferon regulatory factor 1 (IRF-1). IRF-1 deficiency had minimal effect on the attenuated lytic replication of the kinase-null MHV68 in vivo. In contrast, the attenuated latent reservoir of the kinase-null MHV68 was partially to fully rescued in IRF-1-/- mice, along with complete rescue of the MHV68-driven germinal center response. Thus, the novel viral protein kinase-IRF-1 antagonism was largely limited to chronic infection dominated by viral latency and was less relevant for lytic replication during acute infection and in vitro. Given the conserved nature of the viral and host protein, the antagonism between the two, as defined in this study, may regulate gammaherpesvirus infection across species. IMPORTANCE Gammaherpesviruses are prevalent pathogens that manipulate physiological B cell differentiation to establish lifelong infection. This manipulation is also involved in gammaherpesvirus-driven B cell lymphomas, as differentiation of latently infected B cells through the germinal center response targets these for transformation. In this study, we define a novel antagonistic interaction between a conserved gammaherpesvirus protein kinase and a host antiviral and tumor suppressor transcription factor. The virus-host antagonism unveiled in this study was critically important to shape the magnitude of gammaherpesvirus-driven germinal center response. In contrast, the virus-host antagonism was far less relevant for lytic viral replication in vitro and during acute infection in vivo, highlighting the emerging concept that nonoverlapping mechanisms shape the parameters of acute and chronic gammaherpesvirus infection.

Pseudorabies virus kinase UL13 phosphorylates H2AX to foster viral replication

The DNA damage response (DDR) pathway is critical for maintaining genomic integrity and sustaining organismal development. Viruses can either utilize or circumvent the DDR to facilitate their replication. Pseudorabies virus (PRV) infection was shown to induce apoptosis via stimulating DDR. However, the underlying mechanisms have not been fully explored to date. This study showed that PRV infection robustly activates the ATM and DNA-PK signaling pathways shortly after infection. However, inhibition of ATM, but not DNA-PK, could dampen PRV replication in cells. Importantly, we found that PRV-encoded serine/threonine kinase UL13 interacts with and subsequently phosphorylates H2AX. Furthermore, we found that UL13 deletion largely attenuates PRV neuroinvasiveness and virulence in vivo. In addtion, we showed that UL13 contributes to H2AX phosphorylation upon PRV infection both in vitro and in vivo, but does not affect ATM phosphorylation. Finally, we showed that knockdown of H2AX reduces PRV replication, while this reduction can be further enhanced by deletion of UL13. Taken together, we conclude that PRV-encoded kinase UL13 regulates DNA damage marker γH2AX and UL13-mediated H2AX phosphorylation plays a pivotal role in efficient PRV replication and progeny production.

Distinct Oncogenic Transcriptomes in Human Mammary Epithelial Cells Infected With Cytomegalovirus

Human cytomegalovirus is being recognized as a potential oncovirus beside its oncomodulation role. We previously isolated two clinical isolates, HCMV-DB (KT959235) and HCMV-BL (MW980585), which in primary human mammary epithelial cells promoted oncogenic molecular pathways, established anchorage-independent growth in vitro, and produced tumorigenicity in mice models, therefore named high-risk oncogenic strains. In contrast, other clinical HCMV strains such as HCMV-FS, KM, and SC did not trigger such traits, therefore named low-risk oncogenic strains. In this study, we compared high-risk oncogenic HCMV-DB and BL strains (high-risk) with low-risk oncogenic strains HCMV-FS, KM, and SC (low-risk) additionally to the prototypic HCMV-TB40/E, knowing that all strains infect HMECs in vitro. Numerous pro-oncogenic features including enhanced expression of oncogenes, cell survival, proliferation, and epithelial-mesenchymal transition genes were observed with HCMV-BL. In vitro, mammosphere formation was observed only in high-risk strains. HCMV-TB40/E showed an intermediate transcriptome landscape with limited mammosphere formation. Since we observed that Ki67 gene expression allows us to discriminate between high and low-risk HCMV strains in vitro, we further tested its expression in vivo. Among HCMV-positive breast cancer biopsies, we only detected high expression of the Ki67 gene in basal tumors which may correspond to the presence of high-risk HCMV strains within tumors. Altogether, the transcriptome of HMECs infected with HCMV clinical isolates displays an “oncogenic gradient” where high-risk strains specifically induce a prooncogenic environment which might participate in breast cancer development.

Pseudorabies Virus Infection Triggers NF-κB Activation via the DNA Damage Response but Actively Inhibits NF-κB-Dependent Gene Expression

The nuclear factor kappa B (NF-κB) pathway is known to integrate signaling associated with very diverse intra- and extracellular stressors, including virus infections, and triggers a powerful (proinflammatory) response through the expression of NF-κB-regulated genes. Typically, the NF-κB pathway collects and transduces threatening signals at the cell surface or in the cytoplasm leading to nuclear import of activated NF-κB transcription factors. In the current work, we demonstrate that the swine alphaherpesvirus pseudorabies virus (PRV) induces a peculiar mode of NF-κB activation known as "inside-out" NF-κB activation. We show that PRV triggers the DNA damage response (DDR) and that this DDR response drives NF-κB activation since inhibition of the nuclear ataxia telangiectasia-mutated (ATM) kinase, a chief controller of DDR, abolished PRV-induced NF-κB activation. Initiation of the DDR-NF-κB signaling axis requires viral protein synthesis but occurs before active viral genome replication. In addition, the initiation of the DDR-NF-κB signaling axis is followed by a virus-induced complete shutoff of NF-κB-dependent gene expression that depends on viral DNA replication. In summary, the results presented in this study reveal that PRV infection triggers a noncanonical DDR-NF-κB activation signaling axis and that the virus actively inhibits the (potentially antiviral) consequences of this pathway, by inhibiting NF-κB-dependent gene expression. IMPORTANCE The NF-κB signaling pathway plays a critical role in coordination of innate immune responses that are of vital importance in the control of infections. The current report generates new insights into the interaction of the alphaherpesvirus pseudorabies virus (PRV) with the NF-κB pathway, as they reveal that (i) PRV infection leads to NF-κB activation via a peculiar "inside-out" nucleus-to-cytoplasm signal that is triggered via the DNA damage response (DDR), (ii) the DDR-NF-κB signaling axis requires expression of viral proteins but is initiated before active PRV replication, and (iii) late viral factor(s) allow PRV to actively and efficiently inhibit NF-κB-dependent (proinflammatory) gene expression. These data suggest that activation of the DDR-NF-κB during PRV infection is host driven and that its potential antiviral consequences are actively inhibited by the virus.

Conserved Gammaherpesvirus Protein Kinase Counters the Antiviral Effects of Myeloid Cell-Specific STAT1 Expression To Promote the Establishment of Splenic B Cell Latency

Gammaherpesviruses establish lifelong infections and are associated with B cell lymphomas. Murine gammaherpesvirus 68 (MHV68) infects epithelial and myeloid cells during acute infection, with subsequent passage of the virus to B cells, where physiological B cell differentiation is usurped to ensure the establishment of a chronic latent reservoir. Interferons (IFNs) represent a major antiviral defense system that engages the transcriptional factor STAT1 to attenuate diverse acute and chronic viral infections, including those of gammaherpesviruses. Correspondingly, global deficiency of type I or type II IFN signaling profoundly increases the pathogenesis of acute and chronic gammaherpesvirus infection, compromises host survival, and impedes mechanistic understanding of cell type-specific role of IFN signaling. Here, we demonstrate that myeloid-specific STAT1 expression attenuates acute and persistent MHV68 replication in the lungs and suppresses viral reactivation from peritoneal cells, without any effect on the establishment of viral latent reservoir in splenic B cells. All gammaherpesviruses encode a conserved protein kinase that antagonizes type I IFN signaling in vitro. Here, we show that myeloid-specific STAT1 deficiency rescues the attenuated splenic latent reservoir of the kinase-null MHV68 mutant. However, despite having gained access to splenic B cells, the protein kinase-null MHV68 mutant fails to drive B cell differentiation. Thus, while myeloid-intrinsic STAT1 expression must be counteracted by the gammaherpesvirus protein kinase to facilitate viral passage to splenic B cells, expression of the viral protein kinase continues to be required to promote optimal B cell differentiation and viral reactivation, highlighting the multifunctional nature of this conserved viral protein during chronic infection. IMPORTANCE IFN signaling is a major antiviral system of the host that suppresses replication of diverse viruses, including acute and chronic gammaherpesvirus infection. STAT1 is a critical member and the primary antiviral effector of IFN signaling pathways. Given the significantly compromised antiviral status of global type I or type II IFN deficiency, unabated gammaherpesvirus replication and pathogenesis hinders understanding of cell type-specific antiviral effects. In this study, a mouse model of myeloid-specific STAT1 deficiency unveiled site-specific antiviral effects of STAT1 in the lungs and peritoneal cavity, but not the spleen, of chronically infected hosts. Interestingly, expression of a conserved gammaherpesvirus protein kinase was required to counteract the antiviral effects of myeloid-specific STAT1 expression to facilitate latent infection of splenic B cells, revealing a cell type-specific virus-host antagonism during the establishment of chronic gammaherpesvirus infection.

Low-Density Lipoprotein Receptor Suppresses the Endogenous Cholesterol Synthesis Pathway To Oppose Gammaherpesvirus Replication in Primary Macrophages

Gammaherpesviruses are ubiquitous pathogens that establish lifelong infections in >95% of adults worldwide and are associated with several cancers. We have shown that endogenous cholesterol synthesis supports gammaherpesvirus replication. However, the role of exogenous cholesterol exchange and signaling during infection remains poorly understood. Extracellular cholesterol is carried in the serum by several lipoproteins, including low-density lipoproteins (LDL). The LDL receptor (LDL-R) mediates the endocytosis of these cholesterol-rich LDL particles into the cell, thereby supplying the cell with cholesterol. We found that LDL-R expression attenuates gammaherpesvirus replication during the early stages of the replication cycle, as evident by increased viral gene expression in LDL-R^-/- primary macrophages. This was not observed in primary fibroblasts, indicating that the antiviral effects of LDL-R are cell type specific. Increased viral gene expression in LDL-R^-/- primary macrophages was due to increased activity of the endogenous cholesterol synthesis pathway. Intriguingly, despite type I interferon-driven increase in LDL-R mRNA levels in infected macrophages, protein levels of LDL-R continually decreased over the single cycle of viral replication. Thus, our study has uncovered an intriguing tug of war between the LDL-R-driven antiviral effect on cholesterol metabolism and the viral targeting of the LDL-R protein. IMPORTANCE LDL-R is a cell surface receptor that mediates the endocytosis of cholesterol-rich low-density lipoproteins, allowing cells to acquire cholesterol exogenously. Several RNA viruses usurp LDL-R function to facilitate replication; however, the role of LDL-R in DNA virus infection remains unknown. Gammaherpesviruses are double-stranded DNA viruses that are associated with several cancers. Here, we show that LDL-R attenuates gammaherpesvirus replication in primary macrophages by decreasing endogenous cholesterol synthesis activity, a pathway known to support gammaherpesvirus replication. In response, LDL-R protein levels are decreased in infected cells to mitigate the antiviral effects, revealing an intriguing tug of war between the virus and the host.

Interferon Regulatory Factor 3 Supports the Establishment of Chronic Gammaherpesvirus Infection in a Route- and Dose-Dependent Manner

Gammaherpesviruses are ubiquitous pathogens that establish lifelong infections and are associated with several malignancies, including B cell lymphomas. Uniquely, these viruses manipulate B cell differentiation to establish long-term latency in memory B cells. This study focuses on the interaction between gammaherpesviruses and interferon regulatory factor 3 (IRF-3), a ubiquitously expressed transcription factor with multiple direct target genes, including beta interferon (IFN-β), a type I IFN. IRF-3 attenuates acute replication of a plethora of viruses, including gammaherpesvirus. Furthermore, IRF-3-driven IFN-β expression is antagonized by the conserved gammaherpesvirus protein kinase during lytic virus replication in vitro In this study, we have uncovered an unexpected proviral role of IRF-3 during chronic gammaherpesvirus infection. In contrast to the antiviral activity of IRF-3 during acute infection, IRF-3 facilitated establishment of latent gammaherpesvirus infection in B cells, particularly, germinal center and activated B cells, the cell types critical for both natural infection and viral lymphomagenesis. This proviral role of IRF-3 was further modified by the route of infection and viral dose. Furthermore, using a combination of viral and host genetics, we show that IRF-3 deficiency does not rescue attenuated chronic infection of a protein kinase null gammaherpesvirus mutant, highlighting the multifunctional nature of the conserved gammaherpesvirus protein kinases in vivo In summary, this study unveils an unexpected proviral nature of the classical innate immune factor, IRF-3, during chronic virus infection.IMPORTANCE Interferon regulatory factor 3 (IRF-3) is a critical component of the innate immune response, in part due to its transactivation of beta interferon (IFN-β) expression. Similar to that observed in all acute virus infections examined to date, IRF-3 suppresses lytic viral replication during acute gammaherpesvirus infection. Because gammaherpesviruses establish lifelong infection, this study aimed to define the antiviral activity of IRF-3 during chronic infection. Surprisingly, we found that, in contrast to acute infection, IRF-3 supported the establishment of gammaherpesvirus latency in splenic B cells, revealing an unexpected proviral nature of this classical innate immune host factor.

Gammaherpesvirus Usurps Host IL-17 Signaling To Support the Establishment of Chronic Infection

Gammaherpesviruses establish lifelong infections in a majority of humans and are associated with B cell lymphomas. IL-17A is a host cytokine that plays a well-established role in the clearance of bacterial and fungal infections; however, the role of IL-17A in viral infections is poorly understood.

Gammaherpesviruses establish lifelong infection and are associated with a variety of cancers, including B cell lymphomas. These viruses manipulate the B cell differentiation process to establish lifelong infection in memory B cells. Specifically, gammaherpesviruses infect naive B cells and promote entry of both infected and uninfected naive B cells into germinal centers, where the virus usurps rapid proliferation of germinal center B cells to exponentially increase its cellular latent reservoir. In addition to facilitating the establishment of latent infection, germinal center B cells are thought to be the target of viral transformation. In this study, we have uncovered a novel proviral role of host interleukin 17A (IL-17A), a well-established antibacterial and antifungal factor. Loss of IL-17A signaling attenuated the establishment of chronic gammaherpesvirus infection and gammaherpesvirus-driven germinal center response in a route of inoculation-dependent manner. Further, IL-17A treatment directly supported gammaherpesvirus reactivation and de novo lytic infection. This study is the first demonstration of a multifaceted proviral role of IL-17 signaling.

Optimized Detection of Acute MHV68 Infection With a Reporter System Identifies Large Peritoneal Macrophages as a Dominant Target of Primary Infection

Investigating the dynamics of virus-host interactions in vivo remains an important challenge, often limited by the ability to directly identify virally infected cells. Here, we utilize a beta-lactamase activated fluorescent substrate to identify primary targets of murine gammaherpesvirus 68 (MHV68) infection in the peritoneal cavity. By optimizing substrate and detection conditions, we were able to achieve multiparameter characterization of infected cells and the ensuing host response. MHV68 infection leads to a pronounced increase in immune cells, with CD8+ T cells increasing by 3 days, and total infiltrate peaking around 8 days post-infection. MHV68 infection results in near elimination of large peritoneal macrophages (LPMs) by 8 days post-infection, and a concordant increase in small peritoneal macrophages (SPMs) and monocytes. Infection is associated with prolonged changes to myeloid cells, with a distinct population of MHC II^high LPMs emerging by 14 days. Targets of MHV68 infection could be readily detected. Between 1 and 3 days post-infection, MHV68 infects ∼5–10% of peritoneal cells, with >75% being LPMs. By 8 days post-infection, the frequency of MHV68 infection is reduced at least 10-fold, with infection primarily in SPMs, with few infected dendritic cells and B cells. Importantly, limiting dilution analysis indicates that at 3 days post-infection, the majority of MHV68-infected cells harbor latent rather than lytic virus at frequencies consistent with those identified based on reporter gene expression. Our findings demonstrate the utility of the beta-lactamase MHV68 reporter system for high throughput single-cell analysis and identify dynamic changes during primary gammaherpesvirus infection.

Interplay Between KSHV and the Host DNA Damage Response

Interactions between viruses and cellular factors are essential for viral replication or host defense. The DNA damage response (DDR) orchestrates a molecular network of cellular mechanisms that integrates cell cycle regulation and DNA repair or apoptosis. Numerous studies have revealed that the DDR is activated by virus infection, aberrant DNA structures generated by viral DNA replication, or the integration of retroviruses. Although the DDR is an essential function for maintaining the genomic integrity of cells, viruses may utilize this mechanism to build a convenient environment for themselves, and the resulting perturbation of the DDR has been shown to increase the risk of tumorigenesis. There have been many studies investigating the roles of the DDR in oncogenic viruses such as Epstein-Barr virus (EBV), human papillomavirus (HPV), hepatitis B virus (HBV), human T-cell leukemia virus type 1 (HTLV-1), and Kaposi’s sarcoma-associated herpesvirus (KSHV). This review summarizes current knowledge on the roles of DDR in the KSHV lifecycle.

Two nuclear localization signals regulate intracellular localization of the duck enteritis virus UL13 protein

Duck enteritis virus (DEV) multifunctional tegument protein UL13 is predicted to be a conserved herpesvirus protein kinase; however, little is known about its subcellular localization signal. In this study, through transfection of 2 predicted nuclear signals of DEV UL13 fused to enhanced green fluorescent protein, 2 bipartite nuclear localization signals (NLS) were identified. We found that ivermectin blocked the NLS-mediated nuclear import of DEV UL13, showing that the nuclear localization signal of DEV UL13 is a classical importin α- and β-dependent process. We constructed a DEV UL13 mutant strain in which the NLS of DEV UL13 was deleted to explore whether deletion of the NLS affects viral replication. Amino acids 4 to 7 and 90 to 96 were predicted to be NLSs, further proving that nuclear import occurs via a classical importin α- and β-dependent process. We also found that the NLS of pUL13 had no effect on DEV replication in cell culture. Our study enhances the understanding of DEV pUL13. Taken together, these results provide significant information regarding the biological function of pUL13 during DEV infection.

Conserved Herpesvirus Protein Kinases Target SAMHD1 to Facilitate Virus Replication

To ensure a successful infection, herpesviruses have developed elegant strategies to counterbalance the host anti-viral responses. Sterile alpha motif and HD domain 1 (SAMHD1) was recently identified as an intrinsic restriction factor for a variety of viruses. Aside from HIV-2 and the related simian immunodeficiency virus (SIV) Vpx proteins, the direct viral countermeasures against SAMHD1 restriction remain unknown. Using Epstein-Barr virus (EBV) as a primary model, we discover that SAMHD1-mediated anti-viral restriction is antagonized by EBV BGLF4, a member of the conserved viral protein kinases encoded by all herpesviruses. Mechanistically, we find that BGLF4 phosphorylates SAMHD1 and thereby inhibits its deoxynucleotide triphosphate triphosphohydrolase (dNTPase) activity. We further demonstrate that the targeting of SAMHD1 for phosphorylation is a common feature shared by beta- and gamma-herpesviruses. Together, our findings uncover an immune evasion mechanism whereby herpesviruses exploit the phosphorylation of SAMHD1 to thwart host defenses.

Herpesviruses have evolved elegant strategies to dampen the host anti-viral responses. Zhang et al. discover a mechanism by which herpesviruses evade SAMHD1-mediated host defenses through phosphorylation, expanding the functional repertoire of viral protein kinases in herpesvirus biology.

Cross-regulation of viral kinases with cyclin A secures shutoff of host DNA synthesis

Herpesviruses encode conserved protein kinases (CHPKs) to stimulate phosphorylation-sensitive processes during infection. How CHPKs bind to cellular factors and how this impacts their regulatory functions is poorly understood. Here, we use quantitative proteomics to determine cellular interaction partners of human herpesvirus (HHV) CHPKs. We find that CHPKs can target key regulators of transcription and replication. The interaction with Cyclin A and associated factors is identified as a signature of β-herpesvirus kinases. Cyclin A is recruited via RXL motifs that overlap with nuclear localization signals (NLS) in the non-catalytic N termini. This architecture is conserved in HHV6, HHV7 and rodent cytomegaloviruses. Cyclin A binding competes with NLS function, enabling dynamic changes in CHPK localization and substrate phosphorylation. The cytomegalovirus kinase M97 sequesters Cyclin A in the cytosol, which is essential for viral inhibition of cellular replication. Our data highlight a fine-tuned and physiologically important interplay between a cellular cyclin and viral kinases.

Conserved Herpesvirus Kinase ORF36 Activates B2 Retrotransposons during Murine Gammaherpesvirus Infection

Short interspersed nuclear elements (SINEs) are RNA polymerase III (RNAPIII)-transcribed, retrotransposable noncoding RNA (ncRNA) elements ubiquitously spread throughout mammalian genomes. While normally silenced in healthy somatic tissue, SINEs can be induced during infection with DNA viruses, including the model murine gammaherpesvirus 68 (MHV68). Here, we explored the mechanisms underlying MHV68 activation of SINE ncRNAs. We demonstrate that lytic MHV68 infection of B cells, macrophages, and fibroblasts leads to robust activation of the B2 family of SINEs in a cell-autonomous manner. B2 ncRNA induction requires neither host innate immune signaling factors nor involvement of the RNAPIII master regulator Maf1. However, we identified MHV68 ORF36, the conserved herpesviral kinase, as playing a key role in B2 induction during lytic infection. SINE activation is linked to ORF36 kinase activity and can also be induced by inhibition of histone deacetylases 1 and 2 (HCAC 1/2), which is one of the known ORF36 functions. Collectively, our data suggest that ORF36-mediated changes in chromatin modification contribute to B2 activation during MHV68 infection and that this activity is conserved in other herpesviral protein kinase homologs.IMPORTANCE Viral infection dramatically changes the levels of many types of RNA in a cell. In particular, certain oncogenic viruses activate expression of repetitive genes called retrotransposons, which are normally silenced due to their ability to copy and spread throughout the genome. Here, we established that infection with the gammaherpesvirus MHV68 leads to a dramatic induction of a class of noncoding retrotransposons called B2 SINEs in multiple cell types. We then explored how MHV68 activates B2 SINEs, revealing a role for the conserved herpesviral protein kinase ORF36. Both ORF36 kinase-dependent and kinase-independent functions contribute to B2 induction, perhaps through ORF36 targeting of proteins involved in controlling the accessibility of chromatin surrounding SINE loci. Understanding the features underlying induction of these elements following MHV68 infection should provide insight into core elements of SINE regulation, as well as disregulation of SINE elements associated with disease.

[Dynamic changes of cellular environment during Epstein-Barr virus productive replication]

Productive (lytic) replication of DNA viruses elicits host cell DNA damage responses, which cause both beneficial and detrimental effects on viral replication. Viruses utilize them and selectively cancel the 'noisy' downstream signaling pathways, leading to maintain high S-phase CDK activities required for viral replication. To achieve this fine tuning of cellular environment, herpesviruses encode many (>70) genes in their genome, which are expressed in a strictly regulated temporal cascade (immediate-early, early, and late). Here, I introduce and discuss how Epstein-Barr virus, an oncogenic herpesvirus, hijacks the cellular environment and adapt it for the progeny production.

Zika Virus Infection Induces DNA Damage Response in Human Neural Progenitors That Enhances Viral Replication

Zika virus (ZIKV) infection attenuates the growth of human neural progenitor cells (hNPCs). As these hNPCs generate the cortical neurons during early brain development, the ZIKV-mediated growth retardation potentially contributes to the neurodevelopmental defects of the congenital Zika syndrome. Here, we investigate the mechanism by which ZIKV manipulates the cell cycle in hNPCs and the functional consequence of cell cycle perturbation on the replication of ZIKV and related flaviviruses. We demonstrate that ZIKV, but not dengue virus (DENV), induces DNA double-strand breaks (DSBs), triggering the DNA damage response through the ATM/Chk2 signaling pathway while suppressing the ATR/Chk1 signaling pathway. Furthermore, ZIKV infection impedes the progression of cells through S phase, thereby preventing the completion of host DNA replication. Recapitulation of the S-phase arrest state with inhibitors led to an increase in ZIKV replication, but not of West Nile virus or DENV. Our data identify ZIKV's ability to induce DSBs and suppress host DNA replication, which results in a cellular environment favorable for its replication.IMPORTANCE Clinically, Zika virus (ZIKV) infection can lead to developmental defects in the cortex of the fetal brain. How ZIKV triggers this event in developing neural cells is not well understood at a molecular level and likely requires many contributing factors. ZIKV efficiently infects human neural progenitor cells (hNPCs) and leads to growth arrest of these cells, which are critical for brain development. Here, we demonstrate that infection with ZIKV, but not dengue virus, disrupts the cell cycle of hNPCs by halting DNA replication during S phase and inducing DNA damage. We further show that ZIKV infection activates the ATM/Chk2 checkpoint but prevents the activation of another checkpoint, the ATR/Chk1 pathway. These results unravel an intriguing mechanism by which an RNA virus interrupts host DNA replication. Finally, by mimicking virus-induced S-phase arrest, we show that ZIKV manipulates the cell cycle to benefit viral replication.

Illuminating the Role of Vpr in HIV Infection of Myeloid Cells

Vpr is a 14 kDa accessory protein conserved amongst extant primate lentiviruses that is required for virus replication in vivo. Although many functions have been attributed to Vpr, its primary role, and the function under selective pressure in vivo, remains elusive. The minimal importance of Vpr in infection of activated CD4+ T cells in vitro suggests that its major importance lies in overcoming restriction to virus replication in non-cycling myeloid cell populations, such as macrophages and dendritic cells. HIV-1 replication is attenuated in the absence of Vpr in myeloid cells such as monocyte-derived dendritic cells (MDDCs) and macrophages, and is correlated with the ability of Vpr to overcome a post-integration transcriptional defect in these cells. Intriguingly, recent identification of the human hub silencing (HUSH) complex as a target for DCAF^CRL4-mediated degradation by numerous ancestral SIV Vpr alleles, and the Vpr paralog Vpx, signifies the potential function of HIV-1 Vpr to alter yet-to-be identified chromatin remodeling complexes and prevent host-mediated transcriptional repression of both invading viral genomes and pro-inflammatory responses. Myeloid cells constitute an important bridge between innate and adaptive immune responses to invading pathogens. Here, we seek to illustrate the numerous means by which Vpr manipulates the myeloid cellular environment and facilitates virus replication, myeloid cell-dependent HIV transmission, and systemic virus dissemination.

Utility of macrophages in an antitumor strategy based on the vectorization of iron oxide nanoparticles

Many solid tumors and their metastases are still resistant to current cancer treatments such as chemo- and radiotherapy. The presence of a small population of Cancer Stem Cells in tumors is held responsible for relapses. Moreover, the various physical barriers of the organism (e.g. blood-brain barrier) prevent many drugs from reaching the target cells. In order to alleviate this constraint, we suggest a Trojan horse strategy consisting of intravascular injection of macrophages loaded with therapeutic nanoparticles (an iron nanoparticle-based solution marketed under the name of FERINJECT®) to bring a high quantity of the latter to the tumor. The aim of this article is to assess the response of primary macrophages to FERINJECT® via functional assays in order to ensure that the macrophages loaded with these nanoparticles are still relevant for our strategy. Following this first step, we demonstrate that the loaded macrophages injected into the bloodstream are able to migrate to the tumor site using small-animal imaging. Finally, using synchrotron radiation, we validate an improvement of the radiotherapeutic effect when FERINJECT®-laden macrophages are deposited at the vicinity of cancer cells and irradiated.

In Vivo Models of Oncoproteins Encoded by Kaposi's Sarcoma-Associated Herpesvirus

Kaposi's sarcoma-associated herpesvirus (KSHV) is a human oncogenic virus. KSHV utilizes its proteins to modify the cellular environment to promote viral replication and persistence. Some of these proteins are oncogenic, modulating cell proliferation, apoptosis, angiogenesis, genome stability, and immune responses, among other cancer hallmarks. These changes can lead to the development of KSHV-associated malignancies. In this Gem, we focus on animal models of oncogenic KSHV proteins that were developed to enable better understanding of KSHV tumorigenesis.

Epstein–Barr virus: overcoming the DNA damage response

Infections by DNA viruses including, Epstein–Barr virus (EBV), typically induce cellular DNA damage responses (DDR), in particular double-stranded break signaling. To avoid apoptosis associated with constitutive DDR signaling, downstream steps of this pathway must be inactivated. EBV has developed multiple ways of disabling the DDR using several different viral proteins expressed at various stages of EBV infection. Here the interplay between EBV and host DDRs is discussed at each stage of EBV infection, along with the EBV proteins and miRNAs that are known to interfere with DDR signaling. The newly discovered APOBEC editing of EBV DNA and protection from this mutation is also discussed.

Conserved Gammaherpesvirus Protein Kinase Selectively Promotes Irrelevant B Cell Responses

Gammaherpesviruses are ubiquitous pathogens that are associated with B cell lymphomas. In the early stages of chronic infection, these viruses infect naive B cells and subsequently usurp the B cell differentiation process through the germinal center response to ensure latent infection of long-lived memory B cells. A unique feature of early gammaherpesvirus chronic infection is a robust differentiation of irrelevant, virus-nonspecific B cells with reactivities against self-antigens and antigens of other species. In contrast, protective, virus-specific humoral responses do not reach peak levels until a much later time. While several host factors are known to either promote or selectively restrict gammaherpesvirus-driven germinal center response, viral mechanisms that contribute to the irrelevant B cell response have not been defined. In this report we show that the expression and the enzymatic activity of the gammaherpesvirus-encoded conserved protein kinase selectively facilitates the irrelevant, but not virus-specific, B cell responses. Further, we show that lack of interleukin-1 (IL-1) receptor attenuates gammaherpesvirus-driven B cell differentiation and viral reactivation. Because germinal center B cells are thought to be the target of malignant transformation during gammaherpesvirus-driven lymphomagenesis, identification of host and viral factors that promote germinal center responses during gammaherpesvirus infection may offer an insight into the mechanism of gammaherpesvirus pathogenesis.IMPORTANCE Gammaherpesviruses are ubiquitous cancer-associated pathogens that usurp the B cell differentiation process to establish life-long latent infection in memory B cells. A unique feature of early gammaherpesvirus infection is the robust increase in differentiation of B cells that are not specific for viral antigens and instead encode antibodies that react with self-antigens and antigens of other species. Viral mechanisms that are involved in driving such irrelevant B cell differentiation are not known. Here, we show that gammaherpesvirus-encoded conserved protein kinase and host IL-1 signaling promote irrelevant B cell responses and gammaherpesvirus-driven germinal center responses, with the latter thought to be the target of viral transformation.

LXR Alpha Restricts Gammaherpesvirus Reactivation from Latently Infected Peritoneal Cells

Gammaherpesviruses are ubiquitous viruses that establish lifelong infections. Importantly, these viruses are associated with numerous cancers and lymphoproliferative diseases. While risk factors for developing gammaherpesvirus-driven cancers are poorly understood, it is clear that elevated viral reactivation from latency often precedes oncogenesis. Here, we demonstrate that the liver X receptor alpha isoform (LXRα) restricts gammaherpesvirus reactivation in an anatomic-site-specific manner. We have previously demonstrated that deficiency of both LXR isoforms (α and β) leads to an increase in fatty acid and cholesterol synthesis in primary macrophage cultures, with a corresponding increase in gammaherpesvirus replication. Interestingly, expression of fatty acid synthesis genes was not derepressed in LXRα-deficient hosts, indicating that the antiviral effects of LXRα are independent of lipogenesis. Additionally, the critical host defenses against gammaherpesvirus reactivation, virus-specific CD8⁺ T cells and interferon (IFN) signaling, remained intact in the absence of LXRα. Remarkably, using a murine gammaherpesvirus 68 (MHV68) reporter virus, we discovered that LXRα expression dictates the cellular tropism of MHV68 in the peritoneal cavity. Specifically, LXRα^-/- mice exhibit reduced latency within the peritoneal B cell compartment and elevated latency within F4/80⁺ cells. Thus, LXRα restricts gammaherpesvirus reactivation through a novel mechanism that is independent of the known CD8⁺ T cell-based antiviral responses or changes in lipid synthesis and likely involves changes in the tropism of MHV68 in the peritoneal cavity.IMPORTANCE Liver X receptors (LXRs) are nuclear receptors that mediate cholesterol and fatty acid homeostasis. Importantly, as ligand-activated transcription factors, LXRs represent potential targets for the treatment of hypercholesterolemia and atherosclerosis. Here, we demonstrate that LXRα, one of the two LXR isoforms, restricts reactivation of latent gammaherpesvirus from peritoneal cells. As gammaherpesviruses are ubiquitous oncogenic agents, LXRs may represent a targetable host factor for the treatment of poorly controlled gammaherpesvirus infection and associated lymphomagenesis.

Tumor suppressor Interferon Regulatory Factor 1 selectively blocks expression of endogenous retrovirus

Endogenous retroviruses (ERVs) comprise 10% of the genome, with many of these transcriptionally silenced post early embryogenesis. Several stimuli, including exogenous virus infection and cellular transformation can reactivate ERV expression via a poorly understood mechanism. We identified Interferon Regulatory Factor 1 (IRF-1), a tumor suppressor and an antiviral host factor, as a suppressor of ERV expression. IRF-1 decreased expression of a specific mouse ERV in vitro and in vivo. IRF-3, but not IRF-7, also decreased expression of distinct ERV families, suggesting that suppression of ERVs is a relevant biological function of the IRF family. Given the emerging appreciation of the physiological relevance of ERV expression in cancer, IRF-1-mediated suppression of specific ERVs may contribute to the overall tumor suppressor activity of this host factor.

Liver X Receptors Suppress Activity of Cholesterol and Fatty Acid Synthesis Pathways To Oppose Gammaherpesvirus Replication

Gammaherpesviruses are oncogenic pathogens that persist in ~95% of the adult population. Cellular metabolic pathways have emerged as important regulators of many viral infections, including infections by gammaherpesviruses that require several lipid synthetic pathways for optimal replication. Liver X receptors (LXRs) are transcription factors that are critical regulators of cellular fatty acid and cholesterol synthesis pathways. Not surprisingly, LXRs are attractive therapeutic targets in cardiovascular disease. Here we describe an antiviral role for LXRs in the context of gammaherpesvirus infection of primary macrophages. We show that type I interferon increased LXR expression following infection. Surprisingly, there was not a corresponding induction of LXR target genes. Rather, LXRs suppressed the expression of target genes, leading to decreased fatty acid and cholesterol synthesis, two metabolic pathways that support gammaherpesvirus replication. This report defines LXR-mediated restriction of cholesterol and lipid synthesis as an intrinsic metabolic mechanism to restrict viral replication in innate immune cells.IMPORTANCE Fatty acid and cholesterol synthesis pathways of the host play important roles in diverse biological systems. Importantly, these two metabolic pathways are also usurped by a number of viruses to facilitate viral replication. In this report, we show that suppression of these pathways by liver X receptors in primary macrophages creates an intrinsic antiviral state that attenuates gammaherpesvirus replication by limiting viral access to the two metabolic pathways.

The Major Tegument Protein of Bovine Herpesvirus 1, VP8, Interacts with DNA Damage Response Proteins and Induces Apoptosis

VP8, the U_L47 gene product in bovine herpesvirus-1 (BoHV-1), is a major tegument protein that is essential for virus replication in vivo The major DNA damage response protein, ataxia telangiectasia mutated (ATM), phosphorylates Nijmegen breakage syndrome (NBS1) and structural maintenance of chromosome-1 (SMC1) proteins during the DNA damage response. VP8 was found to interact with ATM and NBS1 during transfection and BoHV-1 infection. However, VP8 did not interfere with phosphorylation of ATM in transfected or BoHV-1-infected cells. In contrast, VP8 inhibited phosphorylation of both NBS1 and SMC1 in transfected cells, as well as in BoHV-1-infected cells, but not in cells infected with a VP8 deletion mutant (BoHV-1ΔU_L47). Inhibition of NBS1 and SMC1 phosphorylation was observed at 4 h postinfection by nuclear VP8. Furthermore, UV light-induced cyclobutane pyrimidine dimer (CPD) repair was reduced in the presence of VP8, and VP8 in fact enhanced etoposide or UV-induced apoptosis. This suggests that VP8 blocks the ATM/NBS1/SMC1 pathway and inhibits DNA repair. VP8 induced apoptosis in VP8-transfected cells through caspase-3 activation. The fact that BoHV-1 is known to induce apoptosis through caspase-3 activation is in agreement with this observation. The role of VP8 was confirmed by the observation that BoHV-1 induced significantly more apoptosis than BoHV-1ΔU_L47. These data reveal a potential role of VP8 in the modulation of the DNA damage response pathway and induction of apoptosis during BoHV-1 infection.IMPORTANCE To our knowledge, the effect of BoHV-1 infection on the DNA damage response has not been characterized. Since BoHV-1ΔU_L47 was previously shown to be avirulent in vivo, VP8 is critical for the progression of viral infection. We demonstrated that VP8 interacts with DNA damage response proteins and disrupts the ATM-NBS1-SMC1 pathway by inhibiting phosphorylation of DNA repair proteins NBS1 and SMC1. Furthermore, interference of VP8 with DNA repair was correlated with decreased cell viability and increased DNA damage-induced apoptosis. These data show that BoHV-1 VP8 developed a novel strategy to interrupt the ATM signaling pathway and to promote apoptosis. These results further enhance our understanding of the functions of VP8 during BoHV-1 infection and provide an additional explanation for the reduced virulence of BoHV-1ΔU_L47.

Human herpesvirus-encoded kinase induces B cell lymphomas in vivo

Kaposi's sarcoma-associated herpesvirus (KSHV) is a gammaherpesvirus that is the etiological agent of the endothelial cell cancer Kaposi's sarcoma (KS) and 2 B cell lymphoproliferative disorders, primary effusion lymphoma (PEL) and multicentric Castleman's disease (MCD). KSHV ORF36, also known as viral protein kinase (vPK), is a viral serine/threonine kinase. We previously reported that KSHV vPK enhances cell proliferation and mimics cellular S6 kinase to phosphorylate ribosomal protein S6, a protein involved in protein synthesis. We created a mouse model to analyze the function of vPK in vivo. We believe this is the first mouse tumor model of a viral kinase encoded by a pathogenic human virus. We observed increased B cell activation in the vPK transgenic mice compared with normal mice. We also found that, over time, vPK transgenic mice developed a B cell hyperproliferative disorder and/or a high-grade B cell non-Hodgkin lymphoma at a greatly increased incidence compared with littermate controls. This mouse model shows that a viral protein kinase is capable of promoting B cell activation and proliferation as well as augmenting lymphomagenesis in vivo and may therefore contribute to the development of viral cancers.

Kaposi sarcoma herpesvirus pathogenesis

Kaposi sarcoma herpesvirus (KSHV), taxonomical name human gammaherpesvirus 8, is a phylogenetically old human virus that co-evolved with human populations, but is now only common (seroprevalence greater than 10%) in sub-Saharan Africa, around the Mediterranean Sea, parts of South America and in a few ethnic communities. KSHV causes three human malignancies, Kaposi sarcoma, primary effusion lymphoma, and many cases of the plasmablastic form of multicentric Castleman's disease (MCD) as well as occasional cases of plasmablastic lymphoma arising from MCD; it has also been linked to rare cases of bone marrow failure and hepatitis. As it has colonized humans physiologically for many thousand years, cofactors are needed to allow it to unfold its pathogenic potential. In most cases, these include immune defects of genetic, iatrogenic or infectious origin, and inflammation appears to play an important role in disease development. Our much improved understanding of its life cycle and its role in pathogenesis should now allow us to develop new therapeutic strategies directed against key viral proteins or intracellular pathways that are crucial for virus replication or persistence. Likewise, its limited (for a herpesvirus) distribution and transmission should offer an opportunity for the development and use of a vaccine to prevent transmission.

This article is part of the themed issue ‘Human oncogenic viruses’.

Localization of Double-Strand Break Repair Proteins to Viral Replication Compartments following Lytic Reactivation of Kaposi's Sarcoma-Associated Herpesvirus

Double-strand breaks (DSBs) in DNA are recognized by the Ku70/80 heterodimer and the MRE11-RAD50-NBS1 (MRN) complex and result in activation of the DNA-PK and ATM kinases, which play key roles in regulating the cellular DNA damage response (DDR). DNA tumor viruses such as Kaposi's sarcoma-associated herpesvirus (KSHV) are known to interact extensively with the DDR during the course of their replicative cycles. Here we show that during lytic amplification of KSHV DNA, the Ku70/80 heterodimer and the MRN complex consistently colocalize with viral genomes in replication compartments (RCs), whereas other DSB repair proteins form foci outside RCs. Depletion of MRE11 and abrogation of its exonuclease activity negatively impact viral replication, while in contrast, knockdown of Ku80 and inhibition of the DNA-PK enzyme, which are involved in nonhomologous end joining (NHEJ) repair, enhance amplification of viral DNA. Although the recruitment of DSB-sensing proteins to KSHV RCs is a consistent occurrence across multiple cell types, activation of the ATM-CHK2 pathway during viral replication is a cell line-specific event, indicating that recognition of viral DNA by the DDR does not necessarily result in activation of downstream signaling pathways. We have also observed that newly replicated viral DNA is not associated with cellular histones. Since the presence and modification of these DNA-packaging proteins provide a scaffold for docking of multiple DNA repair factors, the absence of histone deposition may allow the virus to evade localization of DSB repair proteins that would otherwise have a detrimental effect on viral replication.IMPORTANCE Tumor viruses are known to interact with machinery responsible for detection and repair of double-strand breaks (DSBs) in DNA, although detail concerning how Kaposi's sarcoma-associated herpesvirus (KSHV) modulates these cellular pathways during its lytic replication phase was previously lacking. By undertaking a comprehensive assessment of the localization of DSB repair proteins during KSHV replication, we have determined that a DNA damage response (DDR) is directed to viral genomes but is distinct from the response to cellular DNA damage. We also demonstrate that although recruitment of the MRE11-RAD50-NBS1 (MRN) DSB-sensing complex to viral genomes and activation of the ATM kinase can promote KSHV replication, proteins involved in nonhomologous end joining (NHEJ) repair restrict amplification of viral DNA. Overall, this study extends our understanding of the virus-host interactions that occur during lytic replication of KSHV and provides a deeper insight into how the DDR is manipulated during viral infection.

The regulatory role of protein phosphorylation in human gammaherpesvirus associated cancers

Activation of specific sets of protein kinases by intracellular signal molecules has become more and more apparent in the past decade. Phosphorylation, one of key posttranslational modification events, is activated by kinase or regulatory protein and is vital for controlling many physiological functions of eukaryotic cells such as cell proliferation, differentiation, malignant transformation, and signal transduction mediated by external stimuli. Moreovers, the reversible modification of phosphorylation and dephosphorylation can result in different features of the target substrate molecules including DNA binding, protein-protein interaction, subcellular location and enzymatic activity, and is often hijacked by viral infection. Epstein-Barr virus (EBV) and Kaposi’s sarcomaassociated herpesvirus (KSHV), two human oncogenic gamma-herpesviruses, are shown to tightly associate with many malignancies. In this review, we summarize the recent progresses on understanding of molecular properties and regulatory modes of cellular and viral proteins phosphorylation influenced by these two tumor viruses, and highlight the potential therapeutic targets and strategies against their related cancers.

B Cell-Specific Expression of Ataxia-Telangiectasia Mutated Protein Kinase Promotes Chronic Gammaherpesvirus Infection

Manipulation of host cellular pathways is a strategy employed by gammaherpesviruses, including mouse gammaherpesvirus 68 (MHV68), in order to negotiate a chronic infection. Ataxia-telangiectasia mutated (ATM) plays a unique yet incompletely understood role in gammaherpesvirus infection, as it has both proviral and antiviral effects. Chronic gammaherpesvirus infection is poorly controlled in a host with global ATM insufficiency, whether the host is a mouse or a human. In contrast, ATM facilitates replication, reactivation, and latency establishment of several gammaherpesviruses in vitro, suggesting that ATM is proviral in the context of infected cell cultures. The proviral role of ATM is also evident in vivo, as myeloid-specific ATM expression facilitates MHV68 reactivation during the establishment of viral latency. In order to better understand the complex relationship between host ATM and gammaherpesvirus infection, we depleted ATM specifically in B cells, a cell type critical for chronic gammaherpesvirus infection. B cell-specific ATM deficiency attenuated the establishment of viral latency due to compromised differentiation of ATM-deficient B cells. Further, we found that during long-term infection, peritoneal B-1b, but not related B-1a, B cells display the highest frequency of gammaherpesvirus infection. While ATM expression did not affect gammaherpesvirus tropism for B-1 B cells, B cell-specific ATM expression was necessary to support viral reactivation from peritoneal cells during long-term infection. Thus, our study reveals a role of ATM as a host factor that promotes chronic gammaherpesvirus infection of B cells.IMPORTANCE Gammaherpesviruses infect a majority of the human population and are associated with cancer, including B cell lymphomas. ATM is a unique host kinase that has both proviral and antiviral roles in the context of gammaherpesvirus infection. Further, there is insufficient understanding of the interplay of these roles in vivo during chronic infection. In this study, we show that ATM expression by splenic B cells is required for efficient establishment of gammaherpesvirus latency. We also show that ATM expression by peritoneal B cells is required to facilitate viral reactivation during long-term infection. Thus, our study defines a proviral role of B cell-specific ATM expression during chronic gammaherpesvirus infection.

ATM supports gammaherpesvirus replication by attenuating type I interferon pathway

Ataxia-Telangiectasia mutated (ATM) kinase participates in multiple networks, including DNA damage response, oxidative stress, and mitophagy. ATM also supports replication of diverse DNA and RNA viruses. Gammaherpesviruses are prevalent cancer-associated viruses that benefit from ATM expression during replication. This proviral role of ATM had been ascribed to its signaling within the DNA damage response network; other functions of ATM have not been considered. In this study increased type I interferon (IFN) responses were observed in ATM deficient gammaherpesvirus-infected macrophages. Using a mouse model that combines ATM and type I IFN receptor deficiencies we show that increased type I IFN response in the absence of ATM fully accounts for the proviral role of ATM during gammaherpesvirus replication. Further, increased type I IFN response rendered ATM deficient macrophages more susceptible to antiviral effects of type II IFN. This study identifies attenuation of type I IFN responses as the primary mechanism underlying proviral function of ATM during gammaherpesvirus infection.

Interferon Regulatory Factor 1 and Type I Interferon Cooperate To Control Acute Gammaherpesvirus Infection

Gammaherpesviruses are ubiquitous pathogens that establish lifelong infection in >95% of adults worldwide and are associated with a variety of malignancies. Coevolution of gammaherpesviruses with their hosts has resulted in an intricate relationship between the virus and the host immune system, and perturbation of the virus-host balance results in pathology. Interferon regulatory factor 1 (IRF-1) is a tumor suppressor that is also involved in the regulation of innate and adaptive immune responses. Here, we show that type I interferon (IFN) and IRF-1 cooperate to control acute gammaherpesvirus infection. Specifically, we demonstrate that a combination of IRF-1 and type I IFN signaling ensures host survival during acute gammaherpesvirus infection and supports IFN gamma-mediated suppression of viral replication. Thus, our studies reveal an intriguing cross talk between IRF-1 and type I and II IFNs in the induction of the antiviral state during acute gammaherpesvirus infection.

IMPORTANCE

Gammaherpesviruses establish chronic infection in a majority of adults, and this long-term infection is associated with virus-driven development of a range of malignancies. In contrast, a brief period of active gammaherpesvirus replication during acute infection of a naive host is subclinical in most individuals. Here, we discovered that a combination of type I interferon (IFN) signaling and interferon regulatory factor 1 (IRF-1) expression is required to ensure survival of a gammaherpesvirus-infected host past the first 8 days of infection. Specifically, both type I IFN receptor and IRF-1 expression potentiated antiviral effects of type II IFN to restrict gammaherpesvirus replication in vivo, in the lungs, and in vitro, in primary macrophage cultures.

H2AX phosphorylation and DNA damage kinase activity are dispensable for herpes simplex virus replication

BACKGROUND

Herpes simplex virus type 1 (HSV-1) can establish both lytic and latent infections in humans. The phosphorylation of histone H2AX, a common marker of DNA damage, during lytic infection by HSV-1 is well established. However, the role(s) of H2AX phosphorylation in lytic infection remain unclear.

METHODS

Following infection of human foreskin fibroblasts by HSV-1 or HSV-2, we assayed the phosphorylation of H2AX in the presence of inhibitors of transcription, translation, or viral DNA replication, or in the presence of inhibitors of ATM and ATR kinases (KU-55933 and VE-821, respectively). We also assayed viral replication in fibroblasts in the presence of the kinase inhibitors or siRNAs specific for ATM and ATR, as well as in cell lines deficient for either ATR or ATM.

RESULTS

The expression of viral immediate-early and early proteins (including the viral DNA polymerase), but not viral DNA replication or late protein expression, were required for H2AX phosphorylation following HSV-1 infection. Inhibition of ATM kinase activity prevented HSV-stimulated H2AX phosphorylation but had only a minor effect on DNA replication and virus yield in HFF cells. These results differ from previous reports of a dramatic reduction in viral yield following chemical inhibition of ATM in oral keratinocytes or following infection of ATM(-/-) cells. Inhibition of the closely related kinase ATR (whether by chemical inhibitor or siRNA disruption) had no effect on H2AX phosphorylation and reduced viral DNA replication only moderately. During infection by HSV-2, H2AX phosphorylation was similarly dispensable but was dependent on both ATM activity and viral DNA replication.

CONCLUSION

H2AX phosphorylation represents a cell type-specific and virus type-specific host response to HSV infection with little impact on viral infection.

Type I Interferon Counteracts Antiviral Effects of Statins in the Context of Gammaherpesvirus Infection

The cholesterol synthesis pathway is a ubiquitous cellular biosynthetic pathway that is attenuated therapeutically by statins. Importantly, type I interferon (IFN), a major antiviral mediator, also depresses the cholesterol synthesis pathway. Here we demonstrate that attenuation of cholesterol synthesis decreases gammaherpesvirus replication in primary macrophages in vitro and reactivation from peritoneal exudate cells in vivo. Specifically, the reduced availability of the intermediates required for protein prenylation was responsible for decreased gammaherpesvirus replication in statin-treated primary macrophages. We also demonstrate that statin treatment of a chronically infected host attenuates gammaherpesvirus latency in a route-of-infection-specific manner. Unexpectedly, we found that the antiviral effects of statins are counteracted by type I IFN. Our studies suggest that type I IFN signaling counteracts the antiviral nature of the subdued cholesterol synthesis pathway and offer a novel insight into the utility of statins as antiviral agents.

IMPORTANCE

Statins are cholesterol synthesis inhibitors that are therapeutically administered to 12.5% of the U.S.

POPULATION

Statins attenuate the replication of diverse viruses in culture; however, this attenuation is not always obvious in an intact animal model. Further, it is not clear whether statins alter parameters of highly prevalent chronic herpesvirus infections. We show that statin treatment attenuated gammaherpesvirus replication in primary immune cells and during chronic infection of an intact host. Further, we demonstrate that type I interferon signaling counteracts the antiviral effects of statins. Considering the fact that type I interferon decreases the activity of the cholesterol synthesis pathway, it is intriguing to speculate that gammaherpesviruses have evolved to usurp the type I interferon pathway to compensate for the decreased cholesterol synthesis activity.

Infection of a Single Cell Line with Distinct Strains of Human Cytomegalovirus Can Result in Large Variations in Virion Production and Facilitate Efficient Screening of Virus Protein Function

Previously, we reported that the absence of the ataxia telangiectasia mutated (ATM) kinase, a critical DNA damage response (DDR) signaling component for double-strand breaks, caused no change in HCMV Towne virion production. Later, others reported decreased AD169 viral titers in the absence of ATM. To address this discrepancy, human foreskin fibroblasts (HFF) and three ATM(-) lines (GM02530, GM05823, and GM03395) were infected with both Towne and AD169. Two additional ATM(-) lines (GM02052 and GM03487) were infected with Towne. Remarkably, both previous studies' results were confirmed. However, the increased number of cell lines and infections with both lab-adapted strains confirmed that ATM was not necessary to produce wild-type-level titers in fibroblasts. Instead, interactions between individual virus strains and the cellular microenvironment of the individual ATM(-) line determined efficiency of virion production. Surprisingly, these two commonly used lab-adapted strains produced drastically different titers in one ATM(-) cell line, GM05823. The differences in titer suggested a rapid method for identifying genes involved in differential virion production. In silico comparison of the Towne and AD169 genomes determined a list of 28 probable candidates responsible for the difference. Using serial iterations of an experiment involving virion entry and input genome nuclear trafficking with a panel of related strains, we reduced this list to four (UL129, UL145, UL147, and UL148). As a proof of principle, reintroduction of UL148 largely rescued genome trafficking. Therefore, use of a battery of related strains offers an efficient method to narrow lists of candidate genes affecting various virus life cycle checkpoints.

IMPORTANCE

Human cytomegalovirus (HCMV) infection of multiple cell lines lacking ataxia telangiectasia mutated (ATM) protein produced wild-type levels of infectious virus. Interactions between virus strains and the microenvironment of individual ATM(-) lines determined the efficiency of virion production. Infection of one ATM(-) cell line, GM05823, produced large titer differentials dependent on the strain used, Towne or AD169. This discrepancy resolved a disagreement in the literature of a requirement for ATM expression and HCMV reproduction. The titer differentials in GM08523 cells were due, in part, to a decreased capacity of AD169 virions to enter the cell and traffic genomes to the nucleus. In silico comparison of the Towne, AD169, and related variant strains' genomes was coupled with serial iterations of a virus entry experiment, narrowing 28 candidate proteins responsible for the phenotype down to 4. Reintroduction of UL148 significantly rescued genome trafficking. Differential behavior of virus strains can be exploited to elucidate gene function.

Murine Gammaherpesvirus 68 LANA and SOX Homologs Counteract ATM-Driven p53 Activity during Lytic Viral Replication

Tumor suppressor p53 is activated in response to numerous cellular stresses, including viral infection. However, whether murine gammaherpesvirus 68 (MHV68) provokes p53 during the lytic replication cycle has not been extensively evaluated. Here, we demonstrate that MHV68 lytic infection induces p53 phosphorylation and stabilization in a manner that is dependent on the DNA damage response (DDR) kinase ataxia telangiectasia mutated (ATM). The induction of p53 during MHV68 infection occurred in multiple cell types, including splenocytes of infected mice. ATM and p53 activation required early viral gene expression but occurred independently of viral DNA replication. At early time points during infection, p53-responsive cellular genes were induced, coinciding with p53 stabilization and phosphorylation. However, p53-related gene expression subsided as infection progressed, even though p53 remained stable and phosphorylated. Infected cells also failed to initiate p53-dependent gene expression and undergo apoptosis in response to treatment with exogenous p53 agonists. The inhibition of p53 responses during infection required the expression of the MHV68 homologs of the shutoff and exonuclease protein (muSOX) and latency-associated nuclear antigen (mLANA). Interestingly, mLANA, but not muSOX, was necessary to prevent p53-mediated death in MHV68-infected cells under the conditions tested. This suggests that muSOX and mLANA are differentially required for inhibiting p53 in specific settings. These data reveal that DDR responses triggered by MHV68 infection promote p53 activation. However, MHV68 encodes at least two proteins capable of limiting the potential consequences of p53 function.

IMPORTANCE

Gammaherpesviruses are oncogenic herpesviruses that establish lifelong chronic infections. Defining how gammaherpesviruses overcome host responses to infection is important for understanding how these viruses infect and cause disease. Here, we establish that murine gammaherpesvirus 68 induces the activation of tumor suppressor p53. p53 activation was dependent on the DNA damage response kinase ataxia telangiectasia mutated. Although active early after infection, p53 became dominantly inhibited as the infection cycle progressed. Viral inhibition of p53 was mediated by the murine gammaherpesvirus 68 homologs of muSOX and mLANA. The inhibition of the p53 pathway enabled infected cells to evade p53-mediated cell death responses. These data demonstrate that a gammaherpesvirus encodes multiple proteins to limit p53-mediated responses to productive viral infection, which likely benefits acute viral replication and the establishment of chronic infection.

ATM facilitates mouse gammaherpesvirus reactivation from myeloid cells during chronic infection

Gammaherpesviruses are cancer-associated pathogens that establish life-long infection in most adults. Insufficiency of Ataxia-Telangiectasia mutated (ATM) kinase leads to a poor control of chronic gammaherpesvirus infection via an unknown mechanism that likely involves a suboptimal antiviral response. In contrast to the phenotype in the intact host, ATM facilitates gammaherpesvirus reactivation and replication in vitro. We hypothesized that ATM mediates both pro- and antiviral activities to regulate chronic gammaherpesvirus infection in an immunocompetent host. To test the proposed proviral activity of ATM in vivo, we generated mice with ATM deficiency limited to myeloid cells. Myeloid-specific ATM deficiency attenuated gammaherpesvirus infection during the establishment of viral latency. The results of our study uncover a proviral role of ATM in the context of gammaherpesvirus infection in vivo and support a model where ATM combines pro- and antiviral functions to facilitate both gammaherpesvirus-specific T cell immune response and viral reactivation in vivo.

The UL13 and US3 Protein Kinases of Herpes Simplex Virus 1 Cooperate to Promote the Assembly and Release of Mature, Infectious Virions

Herpes simplex virus type 1 (HSV-1) encodes two bona fide serine/threonine protein kinases, the US3 and UL13 gene products. HSV-1 ΔUS3 mutants replicate with wild-type efficiency in cultured cells, and HSV-1 ΔUL13 mutants exhibit <10-fold reduction in infectious viral titers. Given these modest phenotypes, it remains unclear how the US3 and UL13 protein kinases contribute to HSV-1 replication. In the current study, we designed a panel of HSV-1 mutants, in which portions of UL13 and US3 genes were replaced by expression cassettes encoding mCherry protein or green fluorescent protein (GFP), respectively, and analyzed DNA replication, protein expression, and spread of these mutants in several cell types. Loss of US3 function alone had largely negligible effect on viral DNA accumulation, gene expression, virion release, and spread. Loss of UL13 function alone also had no appreciable effects on viral DNA levels. However, loss of UL13 function did result in a measurable decrease in the steady-state levels of two viral glycoproteins (gC and gD), release of total and infectious virions, and viral spread. Disruption of both genes did not affect the accumulation of viral DNA, but resulted in further reduction in gC and gD steady-state levels, and attenuation of viral spread and infectious virion release. These data show that the UL13 kinase plays an important role in the late phase of HSV-1 infection, likely by affecting virion assembly and/or release. Moreover, the data suggest that the combined activities of the US3 and UL13 protein kinases are critical to the efficient assembly and release of infectious virions from HSV-1-infected cells.

Regulation of DNA Damage Signaling and Cell Death Responses by Epstein-Barr Virus Latent Membrane Protein 1 (LMP1) and LMP2A in Nasopharyngeal Carcinoma Cells

Nasopharyngeal carcinoma (NPC) is closely associated with latent Epstein-Barr virus (EBV) infection. Although EBV infection of preneoplastic epithelial cells is not immortalizing, EBV can modulate oncogenic and cell death mechanisms. The viral latent membrane proteins 1 (LMP1) and LMP2A are consistently expressed in NPC and can cooperate in bitransgenic mice expressed from the keratin-14 promoter to enhance carcinoma development in an epithelial chemical carcinogenesis model. In this study, LMP1 and LMP2A were coexpressed in the EBV-negative NPC cell line HK1 and examined for combined effects in response to genotoxic treatments. In response to DNA damage activation, LMP1 and LMP2A coexpression reduced γH2AX (S139) phosphorylation and caspase cleavage induced by a lower dose (5 μM) of the topoisomerase II inhibitor etoposide. Regulation of γH2AX occurred before the onset of caspase activation without modulation of other DNA damage signaling mediators, including ATM, Chk1, or Chk2, and additionally was suppressed by inducers of DNA single-strand breaks (SSBs) and replication stress. Despite reduced DNA damage repair signaling, LMP1-2A coexpressing cells recovered from cytotoxic doses of etoposide; however, LMP1 expression was sufficient for this effect. LMP1 and LMP2A coexpression did not enhance cell growth, with a moderate increase of cell motility to fibronectin. This study supports that LMP1 and LMP2A jointly regulate DNA repair signaling and cell death activation with no further enhancement in the growth properties of neoplastic cells.

IMPORTANCE

NPC is characterized by clonal EBV infection and accounts for >78,000 annual cancer cases with increased incidence in regions where EBV is endemic, such as southeast Asia. The latent proteins LMP1 and LMP2A coexpressed in NPC can individually enhance growth or survival properties in epithelial cells, but their combined effects and potential regulation of DNA repair and checkpoint mechanisms are relatively undetermined. In this study, LMP1-2A coexpression suppressed activation of the DNA damage response (DDR) protein γH2AX induced by selective genotoxins that promote DNA replication stress or SSBs. Expression of LMP1 was sufficient to recover cells, resulting in outgrowth of LMP1 and LMP1-2A-coexpressing cells and indicating distinct LMP1-dependent effects in the restoration of replicative potential. These findings demonstrate novel properties for LMP1 and LMP2A in the cooperative modulation of DDR and apoptotic signaling pathways, further implicating both proteins in the progression of NPC and epithelial malignancies.

DNA Damage Signaling Is Induced in the Absence of Epstein-Barr Virus (EBV) Lytic DNA Replication and in Response to Expression of ZEBRA

Epstein Barr virus (EBV), like other oncogenic viruses, modulates the activity of cellular DNA damage responses (DDR) during its life cycle. Our aim was to characterize the role of early lytic proteins and viral lytic DNA replication in activation of DNA damage signaling during the EBV lytic cycle. Our data challenge the prevalent hypothesis that activation of DDR pathways during the EBV lytic cycle occurs solely in response to large amounts of exogenous double stranded DNA products generated during lytic viral DNA replication. In immunofluorescence or immunoblot assays, DDR activation markers, specifically phosphorylated ATM (pATM), H2AX (γH2AX), or 53BP1 (p53BP1), were induced in the presence or absence of viral DNA amplification or replication compartments during the EBV lytic cycle. In assays with an ATM inhibitor and DNA damaging reagents in Burkitt lymphoma cell lines, γH2AX induction was necessary for optimal expression of early EBV genes, but not sufficient for lytic reactivation. Studies in lytically reactivated EBV-positive cells in which early EBV proteins, BGLF4, BGLF5, or BALF2, were not expressed showed that these proteins were not necessary for DDR activation during the EBV lytic cycle. Expression of ZEBRA, a viral protein that is necessary for EBV entry into the lytic phase, induced pATM foci and γH2AX independent of other EBV gene products. ZEBRA mutants deficient in DNA binding, Z(R183E) and Z(S186E), did not induce foci of pATM. ZEBRA co-localized with HP1β, a heterochromatin associated protein involved in DNA damage signaling. We propose a model of DDR activation during the EBV lytic cycle in which ZEBRA induces ATM kinase phosphorylation, in a DNA binding dependent manner, to modulate gene expression. ATM and H2AX phosphorylation induced prior to EBV replication may be critical for creating a microenvironment of viral and cellular gene expression that enables lytic cycle progression.

Regulation of Epstein-Barr virus reactivation from latency

The Epstein-Barr virus (EBV) is a human gamma-herpesvirus that is implicated in various types of proliferative diseases. Upon infection, it predominantly establishes latency in B cells and cannot ever be eradicated; it persists for the host's lifetime. Reactivation of the virus from latency depends on expression of the viral immediate-early gene, BamHI Z fragment leftward open reading frame 1 (BZLF1). The BZLF1 promoter normally exhibits only low basal activity but is activated in response to chemical or biological inducers, such as 12-O-tetradecanoylphorbol-13-acetate, calcium ionophore, histone deacetylase inhibitor, or anti-Ig. Transcription from the BZLF1 promoter is activated by myocyte enhancer factor 2, specificity protein 1, b-Zip type transcription factors and mediating epigenetic modifications of the promoter, such as histone acetylation and H3K4me3. In contrast, repression of the promoter is mediated by transcriptional suppressors, such as ZEB, ZIIR-BP, and jun dimerization protein 2, causing suppressive histone modifications like histone H3K27me3, H3K9me2/3 and H4K20me3. Interestingly, there is little CpG DNA methylation of the promoter, indicating that DNA methylation is not crucial for suppression of BZLF1. This review will focus on the molecular mechanisms by which the EBV lytic switch is controlled and discuss the physiological significance of this switching for its survival and oncogenesis.

Chromatin Structure of Epstein-Barr Virus Latent Episomes

EBV latent infection is characterized by a highly restricted pattern of viral gene expression. EBV can establish latent infections in multiple different tissue types with remarkable variation and plasticity in viral transcription and replication. During latency, the viral genome persists as a multi-copy episome, a non-integrated-closed circular DNA with nucleosome structure similar to cellular chromosomes. Chromatin assembly and histone modifications contribute to the regulation of viral gene expression, DNA replication, and episome persistence during latency. This review focuses on how EBV latency is regulated by chromatin and its associated processes.

Role of ATM in the formation of the replication compartment during lytic replication of Epstein-Barr virus in nasopharyngeal epithelial cells

Epstein-Barr virus (EBV), a type of oncogenic herpesvirus, is associated with human malignancies. Previous studies have shown that lytic reactivation of EBV in latently infected cells induces an ATM-dependent DNA damage response (DDR). The involvement of ATM activation has been implicated in inducing viral lytic gene transcription to promote lytic reactivation. Its contribution to the formation of a replication compartment during lytic reactivation of EBV remains poorly defined. In this study, the role of ATM in viral DNA replication was investigated in EBV-infected nasopharyngeal epithelial cells. We observed that induction of lytic infection of EBV triggers ATM activation and localization of DDR proteins at the viral replication compartments. Suppression of ATM activity using a small interfering RNA (siRNA) approach or a specific chemical inhibitor profoundly suppressed replication of EBV DNA and production of infectious virions in EBV-infected cells induced to undergo lytic reactivation. We further showed that phosphorylation of Sp1 at the serine-101 residue is essential in promoting the accretion of EBV replication proteins at the replication compartment, which is crucial for replication of viral DNA. Knockdown of Sp1 expression by siRNA effectively suppressed the replication of viral DNA and localization of EBV replication proteins to the replication compartments. Our study supports an important role of ATM activation in lytic reactivation of EBV in epithelial cells, and phosphorylation of Sp1 is an essential process downstream of ATM activation involved in the formation of viral replication compartments. Our study revealed an essential role of the ATM-dependent DDR pathway in lytic reactivation of EBV, suggesting a potential antiviral replication strategy using specific DDR inhibitors.

IMPORTANCE

Epstein-Barr virus (EBV) is closely associated with human malignancies, including undifferentiated nasopharyngeal carcinoma (NPC), which has a high prevalence in southern China. EBV can establish either latent or lytic infection depending on the cellular context of infected host cells. Recent studies have highlighted the importance of the DNA damage response (DDR), a surveillance mechanism that evolves to maintain genome integrity, in regulating lytic EBV replication. However, the underlying molecular events are largely undefined. ATM is consistently activated in EBV-infected epithelial cells when they are induced to undergo lytic reactivation. Suppression of ATM inhibits replication of viral DNA. Furthermore, we observed that phosphorylation of Sp1 at the serine-101 residue, a downstream event of ATM activation, plays an essential role in the formation of viral replication compartments for replication of virus DNA. Our study provides new insights into the mechanism through which EBV utilizes the host cell machinery to promote replication of viral DNA upon lytic reactivation.