Infection of mature dendritic cells with herpes simplex virus type 1 interferes with the interferon signaling pathway

Assessment of Heterozygosity and Genome-Wide Analysis of Heterozygosity Regions in Two Duroc Pig Populations

Heterozygosity can effectively reflect the diverse models of population structure and demographic history. However, the genomic distribution of heterozygotes and the correlation between regions of heterozygosity (runs of heterozygosity, ROHet) and phenotypes are largely understudied in livestock. The objective of this study was to identify ROHet in the Duroc pig genome, and investigate the relationships between ROHet and eight important economic traits. Here, we genotyped 3,770 American Duroc (S21) and 2,096 Canadian Duroc (S22) pigs using 50 K single nucleotide polymorphism array to analyze heterozygosity. A total of 145,010 and 84,396 ROHets were characterized for S21 and S22 populations, respectively. ROHet segments were mostly enriched in 1–2 Mb length classification (75.48% in S21 and 72.25% in S22). The average genome length covered by ROHet was 66.53 ± 12.20 Mb in S21 and 73.32 ± 13.77 Mb in S22 pigs. Additionally, we detected 20 and 13 ROHet islands in S21 and S22 pigs. Genes in these genomic regions were mainly involved in the biological processes of immunity and reproduction. Finally, the genome-wide ROHet-phenotypes association analysis revealed that 130 ROHets of S21 and 84 ROHets of S22 were significantly associated with eight economic traits. Among the candidate genes in the significant ROHet regions, 16 genes related to growth, metabolism, and meat quality were considered as candidate genes for important economic traits of pigs. This work preliminarily explores the effect of heterozygosity-rich regions in the pig genome on production performance and provides new insights for subsequent research on pig genetic improvement.

Herpes simplex virus interference with immunity: Focus on dendritic cells

Herpes simplex virus type 1 (HSV-1) and type 2 (HSV-2) are highly prevalent in the human population. These viruses cause lifelong infections by establishing latency in neurons and undergo sporadic reactivations that promote recurrent disease and new infections. The success of HSVs in persisting in infected individuals is likely due to their multiple molecular determinants involved in escaping the host antiviral and immune responses. Importantly, HSVs infect and negatively modulate the function of dendritic cells (DCs), key immune cells that are involved in establishing effective and balanced immunity against viruses. Here, we review and discuss several molecular and cellular processes modulated by HSVs in DCs, such as autophagy, apoptosis, and the unfolded protein response. Given the central role of DCs in establishing optimal antiviral immunity, particular emphasis should be given to the outcome of the interactions occurring between HSVs and DCs.

Differential roles of interferons in innate responses to mucosal viral infections

Interferons (IFNs) are among the first vertebrate immune pathways activated upon viral infection and are crucial for control of viral replication and dissemination, especially at mucosal surfaces as key locations for host exposure to pathogens. Inhibition of viral establishment and spread at and from these mucosal sites is paramount for preventing severe disease, while concomitantly limiting putative detrimental effects of inflammation. Here, we compare the roles of type I, II, and III IFNs in regulating three archetypal viruses - norovirus, herpes simplex virus, and severe acute respiratory virus coronavirus 2 (SARS-CoV-2) - which infect distinct mammalian mucosal tissues. Emerging paradigms include highly specific roles for IFNs in limiting local versus systemic infection, synergistic activities, and a spectrum of protective versus detrimental effects of IFNs during the infection response.

"Non-Essential" Proteins of HSV-1 with Essential Roles In Vivo: A Comprehensive Review

Viruses encode for structural proteins that participate in virion formation and include capsid and envelope proteins. In addition, viruses encode for an array of non-structural accessory proteins important for replication, spread, and immune evasion in the host and are often linked to virus pathogenesis. Most virus accessory proteins are non-essential for growth in cell culture because of the simplicity of the infection barriers or because they have roles only during a state of the infection that does not exist in cell cultures (i.e., tissue-specific functions), or finally because host factors in cell culture can complement their absence. For these reasons, the study of most nonessential viral factors is more complex and requires development of suitable cell culture systems and in vivo models. Approximately half of the proteins encoded by the herpes simplex virus 1 (HSV-1) genome have been classified as non-essential. These proteins have essential roles in vivo in counteracting antiviral responses, facilitating the spread of the virus from the sites of initial infection to the peripheral nervous system, where it establishes lifelong reservoirs, virus pathogenesis, and other regulatory roles during infection. Understanding the functions of the non-essential proteins of herpesviruses is important to understand mechanisms of viral pathogenesis but also to harness properties of these viruses for therapeutic purposes. Here, we have provided a comprehensive summary of the functions of HSV-1 non-essential proteins.

Equid Herpesvirus 1 Targets the Sensitization and Induction Steps To Inhibit the Type I Interferon Response in Equine Endothelial Cells

Equid herpesvirus 1 (EHV-1) is a viral pathogen of horse populations worldwide spread by the respiratory route and is known for causing outbreaks of neurologic syndromes and abortion storms. Previously, we demonstrated that an EHV-1 strain of the neuropathogenic genotype, T953, downregulates the beta interferon (IFN-β) response in vitro in equine endothelial cells (EECs) at 12 h postinfection (hpi). In the present study, we explored the molecular correlates of this inhibition as clues toward an understanding of the mechanism. Data from our study revealed that EHV-1 infection of EECs significantly reduced both Toll-like receptor 3 (TLR3) and TLR4 mRNA expression at 6 hpi and 12 hpi. While EHV-1 was able to significantly reduce IRF9 mRNA at both 6 hpi and 12 hpi, the virus significantly reduced IFN regulatory factor 7 (IRF7) mRNA only at 12 hpi. EHV-1 did not alter the cellular level of Janus-activated kinase 1 (JAK1) at any time point. However, EHV-1 reduced the cellular level of expression of tyrosine kinase 2 (TYK2) at 12 hpi. Downstream of JAK1-TYK2 signaling, EHV-1 blocked the phosphorylation and activation of signal transducer and activator of transcription 2 (STAT2) when coincubated with exogenous IFN, at 12 hpi, although not at 3 or 6 hpi. Immunofluorescence staining revealed that the virus prevented the nuclear translocation of STAT2 molecules, confirming the virus-mediated inhibition of STAT2 activation. The pattern of suppression of phosphorylation of STAT2 by EHV-1 implicated viral late gene expression. These data help illuminate how EHV-1 strategically inhibits the host innate immune defense by limiting steps required for type I IFN sensitization and induction.IMPORTANCE To date, no commercial vaccine label has a claim to be fully protective against the diseases caused by equid herpesvirus 1 (EHV-1), especially the neurologic form. The interferon (IFN) system, of which type I IFN is of great importance, still remains a viable immunotherapeutic option against EHV-1 infection. The type I IFN system has been exploited successfully to treat other viral infections, such as chronic hepatitis B and C in humans. The current state of research on how EHV-1 interferes with the protective effect of type I IFN has indicated transient induction of type I IFN production followed by a rapid shutdown in vitro in equine endothelial cells (EECs). The significance of our study is the identification of certain steps in the type I IFN signaling pathway targeted for inhibition by EHV-1. Understanding this pathogen-host relationship is essential for the long-term goal of developing effective immunotherapy against EHV-1.

What Goes Around, Comes Around - HSV-1 Replication in Monocyte-Derived Dendritic Cells

HSV-1 is a very successful human pathogen, known for its high sero-prevalence and the ability to infect a wide range of different cell types, including dendritic cells (DCs). As very potent antigen-presenting cells DCs play an important role in the induction of antiviral immune responses and therefore represent a strategic target for viral-mediated immune escape mechanisms. It is known that HSV-1 completes its gene expression profile in immature as well as in mature DCs, while lytic infection is only found in immature DCs (iDCs). Notably, HSV-1 infected mature DCs (mDCs) fail to release infectious progeny virions into the supernatant. Apart from HSV-1 dissemination via extracellular routes cell-to-cell spread counteracts a yet unknown mechanism by which the virus is trapped in mDCs and not released into the supernatant. The dissemination in a cell-cell contact-dependent manner enables HSV-1 to infect bystander cells without the exposure toward the extracellular environment. This supports the virus to successfully infect the host and establish latency. In this review the mechanism of HSV-1 replication in iDCs and mDCs and its immunological as well as virological implications, will be discussed.

Simian varicella virus inhibits the interferon gamma signalling pathway

The alphaherpesvirus simian varicella virus (SVV) causes varicella and zoster in nonhuman primates. Herpesviruses evolved elaborate mechanisms to escape host immunity, but the immune evasion strategies employed by SVV remain ill-defined. We analysed whether SVV impairs the cellular response to key antiviral cytokine interferon-γ (IFNγ). SVV infection inhibited the expression of IFNγ-induced genes like C-X-C motif chemokine 10 and interferon regulatory factor 1. Phosphorylation and nuclear translocation of the signal transducer and activator of transcription 1 (STAT1) was blocked in SVV-infected cells, which did not involve cellular and viral phosphatases. SVV infection did not downregulate IFNγ receptor α and β chain expression on the cell surface. Instead, STAT1, Janus tyrosine kinases 1 (JAK1) and JAK2 protein levels were significantly decreased in SVV-infected cells. Collectively, these results demonstrate that SVV targets three proteins in the IFNγ signal transduction pathway to escape the antiviral effects of IFNγ.

Mapping Murine Corneal Neovascularization and Weight Loss Virulence Determinants in the Herpes Simplex Virus 1 Genome and the Detection of an Epistatic Interaction between the UL and IRS/US Regions

Herpes simplex virus 1 (HSV-1) most commonly causes recrudescent labial ulcers; however, it is also the leading cause of infectious blindness in developed countries. Previous research in animal models has demonstrated that the severity of HSV-1 ocular disease is influenced by three main factors: host innate immunity, host immune response, and viral strain. We have previously shown that mixed infection with two avirulent HSV-1 strains (OD4 and CJ994) results in recombinants with a wide range of ocular disease phenotype severity. Recently, we developed a quantitative trait locus (QTL)-based computational approach (vQTLmap) to identify viral single nucleotide polymorphisms (SNPs) predicted to influence the severity of the ocular disease phenotypes. We have now applied vQTLmap to identify HSV-1 SNPs associated with corneal neovascularization and mean peak percentage weight loss (MPWL) using 65 HSV-1 OD4-CJ994 recombinants. The vQTLmap analysis using Random Forest for neovascularization identified phenotypically meaningful nonsynonymous SNPs in the ICP4, UL41 (VHS), UL42, UL46 (VP11/12), UL47 (VP13/14), UL48 (VP22), US3, US4 (gG), US6 (gD), and US7 (gI) coding regions. The ICP4 gene was previously identified as a corneal neovascularization determinant, validating the vQTLmap method. Further analysis detected an epistatic interaction for neovascularization between a segment of the unique long (UL) region and a segment of the inverted repeat short (IRS)/unique short (US) region. Ridge regression was used to identify MPWL-associated nonsynonymous SNPs in the UL1 (gL), UL2, UL4, UL49 (VP22), UL50, and ICP4 coding regions. The data provide additional insights into virulence gene and epistatic interaction discovery in HSV-1.

IMPORTANCE

Herpes simplex virus 1 (HSV-1) typically causes recurrent cold sores; however, it is also the leading source of infectious blindness in developed countries. Corneal neovascularization is critical for the progression of blinding ocular disease, and weight loss is a measure of infection severity. Previous HSV-1 animal virulence studies have shown that the severity of ocular disease is partially due to the viral strain. In the current study, we used a recently described computational quantitative trait locus (QTL) approach in conjunction with 65 HSV-1 recombinants to identify viral single nucleotide polymorphisms (SNPs) involved in neovascularization and weight loss. Neovascularization SNPs were identified in the ICP4, VHS, UL42, VP11/12, VP13/14, VP22, gG, US3, gD, and gI genes. Further analysis revealed an epistatic interaction between the UL and US regions. MPWL-associated SNPs were detected in the UL1 (gL), UL2, UL4, VP22, UL50, and ICP4 genes. This approach will facilitate future HSV virulence studies.

Quantitative Trait Locus Based Virulence Determinant Mapping of the HSV-1 Genome in Murine Ocular Infection: Genes Involved in Viral Regulatory and Innate Immune Networks Contribute to Virulence

Herpes simplex virus type 1 causes mucocutaneous lesions, and is the leading cause of infectious blindness in the United States. Animal studies have shown that the severity of HSV-1 ocular disease is influenced by three main factors; innate immunity, host immune response and viral strain. We previously showed that mixed infection with two avirulent HSV-1 strains (OD4 and CJ994) resulted in recombinants that exhibit a range of disease phenotypes from severe to avirulent, suggesting epistatic interactions were involved. The goal of this study was to develop a quantitative trait locus (QTL) analysis of HSV-1 ocular virulence determinants and to identify virulence associated SNPs. Blepharitis and stromal keratitis quantitative scores were characterized for 40 OD4:CJ994 recombinants. Viral titers in the eye were also measured. Virulence quantitative trait locus mapping (vQTLmap) was performed using the Lasso, Random Forest, and Ridge regression methods to identify significant phenotypically meaningful regions for each ocular disease parameter. The most predictive Ridge regression model identified several phenotypically meaningful SNPs for blepharitis and stromal keratitis. Notably, phenotypically meaningful nonsynonymous variations were detected in the UL24, UL29 (ICP8), UL41 (VHS), UL53 (gK), UL54 (ICP27), UL56, ICP4, US1 (ICP22), US3 and gG genes. Network analysis revealed that many of these variations were in HSV-1 regulatory networks and viral genes that affect innate immunity. Several genes previously implicated in virulence were identified, validating this approach, while other genes were novel. Several novel polymorphisms were also identified in these genes. This approach provides a framework that will be useful for identifying virulence genes in other pathogenic viruses, as well as epistatic effects that affect HSV-1 ocular virulence.

In addition to causing recurrent labial lesions, herpes simplex virus type 1 (HSV-1) is also the primary source of infectious blindness in the United States. Animal studies have shown that the severity of infection is influenced by several factors, including viral strain. Conventional studies investigating the genetics of viral virulence have focused on characterizing a naturally occurring strain, and engineering mutations into viruses. The purpose of this study was to develop a quantitative trait locus (QTL) computational analysis of HSV-1 genome to identify ocular virulence determinants and associated viral SNPs. Notably, phenotypically meaningful variations were detected in the UL24, UL29 (ICP8), UL41 (VHS), UL53 (gK), UL54 (ICP27), UL56, ICP4, US1 (ICP22), US3 and gG genes. Several genes previously implicated in virulence were identified, validating this approach, while other genes were novel. This is the first time a QTL based approach has been applied to a herpesvirus and it will also be valuable in future virulence, epistasis, and protein-protein interaction studies.

Herpes simplex virus 1 UL41 protein abrogates the antiviral activity of hZAP by degrading its mRNA

Background

The zinc finger antiviral protein (ZAP) is a host restriction factor that inhibits the replication of various viruses by degradation of certain viral mRNA. However, previous study demonstrated that ectopic expression of rat ZAP did not suppress the replication of herpes simplex virus type 1 (HSV-1), an archetypal member of the alphaherpesvirus subfamily, and the molecular mechanism underneath is still illusive.

Results

Human ZAP (hZAP) does not suppress the replication of herpes simplex virus 1, and HSV-1 UL41 protein was identified as an antagonist of hZAP by degrading its mRNA. Infection of wild-type (WT), but not UL41-null mutant (R2621) virus, diminished the accumulation of hZAP to abrogate its antiviral activity. Moreover, ectopic expression of hZAP inhibited the replication of R2621 but not WT HSV-1.

Conclusion

HSV-1 UL41 was shown for the first time to evade the antiviral function of hZAP via its RNase activity.

Corneal replication is an interferon response-independent bottleneck for virulence of herpes simplex virus 1 in the absence of virion host shutoff

Herpes simplex viruses lacking the virion host shutoff function (Δvhs) are avirulent and hypersensitive to type I and type II interferon (IFN). In this study, we demonstrate that even in the absence of IFN responses in AG129 (IFN-αβγR(-/-)) mice, Δvhs remains highly attenuated via corneal infection but is fully virulent via intracranial infection. The data demonstrate that the interferon-independent inherent replication defect of Δvhs has a significant impact upon peripheral replication and neuroinvasion.

Characterization of retinoic acid-inducible gene-I (RIG-I) expression corresponding to viral infection and UVB in human keratinocytes

BACKGROUND

Retinoic acid-inducible gene-I (RIG-I) is a cytoplasmic protein that recognizes viral double-stranded RNA to induce the type I interferon (IFN) response. In human keratinocytes, RIG-I is induced by IFN-γ and tumor necrosis factor-α stimulation, and is abundantly expressed in psoriatic keratinocytes of the spinous and basal layers.

OBJECTIVE

This study investigated the effects of extraneous stimuli including viral infection and UVB exposure on RIG-I expression in human keratinocytes.

METHODS

Human skin keratinocytes (HaCaT cells) were stimulated by polyinosinic-polycytidylic acid (poly(I:C)), which mimics viral infection, and UVB exposure. We assessed the expression of RIG-I and IFN-regulatory factor (IRF)-1 in HaCaT cells by RT-PCR and Western blot analysis. Moreover, we investigated the effect of IRF-1 binding site of RIG-I gene promoter on the regulation of RIG-I expression by luciferase promoter assay and electrophoretic mobility shift assay.

RESULTS

Poly(I:C) induced RIG-I expression, while UVB inhibited basal RIG-I expression and the poly(I:C)-induced RIG-I overexpression in HaCaT cells. IRF-1, which binds to a regulatory element located on the RIG-I gene promoter, was required for both inductions of RIG-I expression. IRF-1 expression was enhanced three hours after the poly(I:C) stimulation, consistent with the RIG-I response to poly(I:C), and thereafter was suppressed. Moreover, UVB exposure promptly decreased IRF-1 expression, resulting in decreased IRF-1 protein binding to the RIG-I promoter, and consequently, decreased RIG-I expression.

CONCLUSION

Thus, suppression of RIG-I and IRF-1 expression caused by UVB exposure may partly explain the inhibition of skin-based immune responses, leading to viral infection and recrudescence.

The virion host shutoff protein of herpes simplex virus 1 blocks the replication-independent activation of NF-κB in dendritic cells in the absence of type I interferon signaling

Immune evasion is a defining feature of the virus-host relationship. During infection, herpes simplex virus type 1 (HSV-1) utilizes multiple proteins to manipulate the host immune response. In the present study, we investigated the mechanism by which the virion host shutoff (vhs) protein blocks the activation of dendritic cells (DCs). Previously, we found that coinfection of wild-type HSV-1 with a panel of RNA viruses resulted in a block to DC activation that was attributable to vhs. These observations led us to hypothesize that the vhs-mediated inhibition was dependent on signaling through the RIG-I-like receptor (RLR) signaling pathway. By examining DCs generated from MAVS (IPS-1) knockout (KO) mice, we determined that RLR/MAVS signaling is not essential for the DC response to HSV-1. We also evaluated the requirement for the type I interferon (IFN) signaling pathway in DC activation following infection with HSV-1 and found that stimulation of DCs with wild-type HSV-1 required intact type I IFN signaling for the production of cytokines, whereas the vhs deletion (vhs(-)) mutant virus activated DCs without the need for exogenous IFN signaling. Comparisons of transcription factor activation in DCs infected with wild-type HSV and the vhs(-) mutant virus revealed that NF-κB activation was inhibited by vhs in the early phase of the infection. In contrast, IRF3 activation was not influenced by vhs. In these studies, measurement of proinflammatory cytokines and type I IFN release from the infected DCs reflected the activation status of these transcription factors. Taken together, the work presented here (i) describes a novel role for the vhs protein as an inhibitor of the early activation of NF-κB during HSV-1 infection of DCs and (ii) offers a mechanistic explanation of how this protein interferes with DC activation.

Herpes simplex virus type 2 virion host shutoff protein suppresses innate dsRNA antiviral pathways in human vaginal epithelial cells

Viruses that establish persistent infections have evolved numerous strategies to evade host innate antiviral responses. We functionally assessed the role of herpes simplex virus type 2 (HSV-2) virion host shutoff (vhs) protein on innate immune sensing pathways in human vaginal epithelial cells (VK2 ECs). Infection of cells with wild-type (WT) HSV-2 significantly decreased expression of innate immune sensors of viral infection, Toll-like receptor (TLR)2, TLR3, retinoic acid inducible gene I (RIG-I) and melanoma differentiation-associated gene 5 (Mda-5), relative to cells infected with a mutant that lacks vhs (vhsB) or mock-infected cells. Transfection with HSV-2 vhs similarly decreased expression of TLR2, TLR3, RIG-I and Mda-5, which was also confirmed in human embryonic kidney (HEK) 293 cells. vhsB infection of VK2 cells caused robust increases in the active form of interferon regulatory factor (IRF)3 and its translocation to the nucleus compared with the WT. Additionally, IRF3 activation by Sendai virus and polyinosinic : polycytidylic acid-induced stimulation of beta interferon (IFN-β) was significantly inhibited in vhs-transfected cells. Overall, our findings provide the first evidence that HSV-2 vhs plays roles in selectively inhibiting TLR3 and RIG-I/Mda-5, as well as TLR2-mediated antiviral pathways for sensing dsRNA and effectively suppresses IFN-β antiviral responses in human vaginal ECs.

Classical swine fever virus suppresses maturation and modulates functions of monocyte-derived dendritic cells without activating nuclear factor kappa B

Classical swine fever virus (CSFV) compromises the host immune system, causing the severe disease of pigs. Dendritic cells (DCs) are the most potent inducers of immune responses. In the present study, we investigated the functional properties of porcine monocyte-derived DCs (Mo-DCs) affected by CSFV. Results showed that the expression of surface markers of DCs such as major histocompatibility complex class II (MHC-II), CD80, CD83 and CD86 were unimpaired, but an obviously increased expression of CD172a in DCs was noticed 48 h after CSFV infection. The expression profiles of cytokines were detected in cultured Mo-DCs after various treatments for 48 h by Q-RT-PCR. The findings suggested that CSFV infection significantly increased the mRNA expression of IL-10 and TNF-α, and inhibited IL-12 expression, with little effect on IFN-α and IFN-γ expression. We further demonstrated that CSFV was incapable of activating the nuclear factor kappa B (NF-κB) in infected DCs, which was characterized by an unvaried DNA binding activity of NF-κB, the lack of translocation of p65/RelA from the cytoplasm to the nucleus and the stabilization of p65/RelA expression. Furthermore, Western blot analysis indicated that the inactivation of NF-κB was due to the failure of IκBα degradation. The data demonstrated that CSFV could be replicated in DCs and CSFV infection could modulate the secretion of crucial co-stimulatory molecules and cytokines which down-regulated maturation of DCs, without activating NF-κB in DCs. Thus, the results suggested a possible mechanism for CSFV evasion of innate host defenses, providing the basis for understanding molecular pathways in CSFV pathogenesis.

Herpes simplex virus type I (HSV-1) replicates in mature dendritic cells but can only be transferred in a cell-cell contact-dependent manner

HSV-1 is a very successful representative of the α-herpesvirus family, and ∼ 90% of the population is seropositive for this particular virus. Although the pathogen usually causes the well-known mild lesions on the lips, also, severe infections of the eye or the brain can be observed in rare cases. It is well known, that this virus can efficiently infect the most potent APCs, i.e., the DCs, in their immature and mature state. Although the infection of the iDC has been shown to be productive, infection of mMDDCs is believed to be abortive in the early phase of the viral replication cycle. In line with these findings, no virus particles can be detected in the supernatant of HSV-1-infected mMDDC. In this study, however, we show for the first time that this pathogen completes its replication cycle in mMDDCs. We detected the presence of viral gene transcripts of all three phases of the replication cycle, as well as of late viral proteins, and even the generation of small amounts of progeny virus. Although we could confirm the findings that these particles are not released into the supernatant, surprisingly, the newly generated viral particles can be passed on to Vero cells, as well as to primary keratinocytes in a cell-cell contact-dependent manner. Finally, we provide evidence that the viral gE is involved in the transfer of infectious virus from mMDDCs to other permissive cells.

Constitutive and Inducible Innate Responses in Cells Infected by HSV-1-Derived Amplicon Vectors

Amplicons are helper-dependent herpes simplex virus type 1 (HSV-1)-based vectors that can deliver very large foreign DNA sequences and, as such, are good candidates both for gene delivery and vaccine development. However, many studies have shown that innate constitutive or induced cellular responses, elicited or activated by the entry of HSV-1 particles, can play a significant role in the control of transgenic expression and in the induction of inflammatory responses. Moreover, transgene expression from helper-free amplicon stocks is often weak and transient, depending on the particular type of infected cells, suggesting that cellular responses could be also responsible for the silencing of amplicon-mediated transgene expression. This review summarizes the current experimental evidence underlying these latter concepts, focusing on the impact on transgene expression of very-early interactions between amplicon particles and the infected cells, and speculates on possible ways to counteract the cellular protective mechanisms, thus allowing stable transgene expression without enhancement of vector toxicity.

Mechanisms employed by herpes simplex virus 1 to inhibit the interferon response

The interferon (IFN) family of cytokines constitutes potent inducers of innate antiviral responses that also influence adaptive immune processes. Despite eliciting such formidable cellular defense responses, viruses have evolved ways to interfere with the IFN response. Herpes simplex virus 1 (HSV-1) is an enveloped, dsDNA virus and a member of the herpesvirus family. Like other herpesvirus family members, HSV-1 has become highly specialized for its host and establishes a lifelong infection by undergoing latency within neurons. A leading reason for the success of HSV-1 as a pathogen results from its ability to evade the IFN response. Specifically, HSV-1 encodes several proteins that function to inhibit both IFN production and subsequent signal transduction. This review will identify and summarize the current understanding of viral proteins encoded by HSV-1 involved in the evasion of the IFN response.

The role of dendritic cells in alphaherpesvirus infections: archetypes and paradigms

Dendritic cells (DCs) play a critical role in orchestrating both innate and adaptive components of the immune system and are therefore of pivotal importance in the initiation of immune responses to control and eliminate viral infections. A major focus of this review is to give an overview on the recent findings that point out the importance of DCs in controlling alphaherpesvirus infections, but also indicate that these viruses have evolved several strategies to inhibit and/or exploit DC functions to delay or escape elimination by the immune system. In addition, we point out the common features and interspecies differences between DCs from man and animal, and discuss the potential use of animal alphaherpesvirus homologues to gain further insights into the interaction between alphaherpesviruses and DCs in their natural virus-host environment. Finally, recent knowledge on the potential of alphaherpesviruses as vectors for DC stimulation and their use for immunotherapy is presented.

Herpes simplex virus type I infection of mature dendritic cells leads to reduced LMP7-mRNA-expression levels

Mature dendritic cells (mDCs) are the most potent antigen presenting cells within the human immune system known today. However, several viruses, including herpes simplex virus type 1 (HSV-1) have developed numerous immune escape mechanisms, such as the avoidance of peptide presentation through the major histocompatibility complex (MHC) class I to CD8(+) cytotoxic T-cells. Within the MHC class I pathway, the majority of antigenic peptides are generated by the proteasome, a multicatalytic protease complex. Upon exposure to IFN-gamma, the constitutive proteasome is partially replaced by the immunoproteasome, which contains the IFN-gamma-inducible subunits LMP2, MECL1 and LMP7. In this study, we report the downregulation of LMP7 on mRNA level in HSV-1 infected mDCs. Interestingly, this reduction was not vhs-mediated since using a virus strain lacking the vhs gene we obtained similar results. However, on protein level, LMP7-expression was not affected, which is probably due the high stability of the LMP7 protein. Also the incorporation of LMP7 into the immunoproteasome was not affected by HSV-1. However, for the in vivo situation, in which DC reside for a prolonged time period in peripheral tissues, the reduced LMP7-mRNA level could be of biological importance, since the virus could escape/hide from immune system of the host and establish latency processes.

HSV-1 upregulates the ARE-binding protein tristetraprolin in a STAT1- and p38-dependent manner in mature dendritic cells

Dendritic cells are the sentinels of the immune system and as such represent the first-line of defense against incoming pathogens. Upon encounter with harmful antigens, these antigen-presenting cells start to mature and migrate towards the draining lymph nodes to display the antigen to T-lymphocytes, thereby eliciting the immune response of the host. Viruses, including human herpesvirus type I (HSV-1), seek to avoid such immune reactions. Therefore, they developed an arsenal of immune evasion strategies, some of which have been described earlier by our group and others. The secretion of tumor necrosis factor (TNF) represents a typical defense line of the host and it has been shown that this cytokine contributes to the inhibition of viral replication and augments the proliferation of cytotoxic T-lymphocytes. Here we report, that upon infection of mature dendritic cells, HSV-1 very strongly induces the expression of the AU-rich elements (ARE)-binding protein tristetraprolin (TTP), an mRNA-destabilizing protein. One of the best described targets of TTP is the TNF mRNA. This induction is dependent on the phosphorylation of both signal transducer and activator of transcription (STAT1) and p38 in a collaborative manner. By repressing this phosphorylation with specific inhibitors, we were able to reduce TTP mRNA levels. At the same time TNF mRNA levels were increased, suggesting that TNF mRNA is indeed a target of TTP in this setting. In summary, these data underline that HSV-1 induces TTP transcription in order to reduce TNF levels generated by infected mature dendritic cell.

Myxoma virus selectively disrupts type I interferon signaling in primary human fibroblasts by blocking the activation of the Janus kinase Tyk2

Poxviruses currently are known to disrupt Jak-STAT signal transduction induced by interferon (IFN) through two distinct mechanisms: (1) secreted poxviral IFN decoy receptors that prevent the initiation of IFN signaling from type I or II receptors at the cell surface; and (2) poxviral phosphatase that dephosphorylates STAT1 intracellularly. Here, we report a novel mechanism by which poxviruses can inhibit Jak-STAT signaling in response to type I IFN. Myxoma virus (MV) is a highly species-restricted member of the poxvirus family that infects only rabbits under the natural setting. Interestingly, primary human fibroblasts support a permissive MV infection that is only partially sensitive to the antiviral state induced by type I IFN. In this study we show that when type I IFN is added to primary human fibroblasts following MV infection, the tyrosine phosphorylation of the Janus kinase Tyk2 is specifically blocked, thereby preventing the subsequent activation of downstream STAT1 and STAT2. In stark contrast, type II IFN-induced activation of Jak1, Jak2 and STAT1 remains largely unaffected in MV-infected human fibroblasts. Unlike the de-activation of STAT1 by the poxvirus phosphatase, which is delivered into the cell by the input virions, the Tyk2 inhibition by MV infection requires new viral gene expression. Thus, our study documents a previously unrecognized immune evasion mechanism exploited by a poxvirus to selectively disrupt the type I IFN-Jak-STAT signaling cascade.

Expression of gamma interferon-dependent genes is blocked independently by virion host shutoff RNase and by US3 protein kinase

Gamma interferon receptor alpha (IFN-gammaR alpha) is stable but posttranslationally modified in herpes simplex virus 1(F) [HSV-1(F)]-infected cells. Studies with antibody directed to the phosphorylation site indicate that IFN-gammaR alpha is phosphorylated by the U(S)3 kinase. The modification is abolished in cells infected with DeltaU(S)3, DeltaU(L)13, or Delta(U(S)3/U(L)13) mutant virus. Transcripts of the IFN-gamma-dependent genes do not accumulate in cells transduced with the U(S)3 protein kinase and treated with IFN-gamma. In contrast, the accumulation of IFN-gamma-dependent gene transcripts is suppressed in cells infected with the wild-type virus, in cells infected with the DeltaU(S)3 mutant virus, and to a lesser extent in the DeltaU(L)41 virus-infected cells. The accumulation of IFN-gamma-dependent gene transcripts in DeltaU(L)41-infected cells could be due at least in part to a significant delay and reduction in the accumulation of the U(S)3 protein. The results suggest that the expression of IFN-gamma-dependent genes is blocked independently by the degradation of IFN-gamma-dependent gene transcripts--a function of the virion host shutoff RNase--and by posttranslational modification of the IFN-gammaR alpha protein.