Herpes simplex virus type 1 (HSV-1)-induced apoptosis in human dendritic cells as a result of downregulation of cellular FLICE-inhibitory protein and reduced expression of HSV-1 antiapoptotic latency-associated transcript sequences

Anti-HSV nucleoside and non-nucleoside analogues: spectroscopic characterisation of naphthyl and coumarinyl amides and their mode and mechanism of antiviral action

Nucleoside analogues acyclovir, valaciclovir, and famciclovir are the preferred drugs against human Herpes Simplex Viruses (HSVs). However, the viruses rapidly develop resistance against these analogues which demand safer, more efficient, and nontoxic antiviral agents. We have synthesized two non-nucleoside amide analogues, 2-Oxo-2H-chromene-3-carboxylic acid [2-(pyridin-2-yl methoxy)-phenyl]-amide (HL1) and 2-hydroxy-1-naphthaldehyde-(4-pyridine carboxylic) hydrazone (HL2). The compounds were characterized by different physiochemical methods including elementary analysis, FT-IR, Mass spectra, 1H-NMR; and evaluated for their antiviral efficacy against HSV-1F by Plaque reduction assay. The 50% cytotoxicity (CC50), determined by MTT test, revealed that HL1 (270.4 μg/ml) and HL2 (362.6 μg/ml) are safer, while their antiviral activity (EC50) against HSV-1F was 37.20 μg/ml and 63.4 μg/ml against HL1 and HL2 respectively, compared to the standard antiviral drug Acyclovir (CC50 128.8 ± 3.4; EC50 2.8 ± 0.1). The Selectivity Index (SI) of these two compounds are also promising (4.3 for HL1 and 9.7 for HL2), compared to Acyclovir (49.3). Further study showed that these amide derivatives block the early stage of the HSV-1F life cycle. Additionally, both these amides make the virus inactive, and reduce the number of plaques, when infected Vero cells were exposed to HL1 and HL2 for a short period of time.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-023-03658-0.

Low Prevalence of HSV-1 and Helicobacter pylori in HNSCC and Chronic Tonsillitis Patients Compared to Healthy Individuals

Recent studies identified viral and bacterial factors, including HSV-1 and H. pylori, as possible factors associated with diseases such as chronic tonsillitis and cancers, including head and neck squamous cell carcinoma (HNSCC). We assessed the prevalence of HSV-1/2 and H. pylori in patients with HNSCC, chronic tonsillitis, and healthy individuals using PCR after DNA isolation. Associations were sought between the presence of HSV-1, H. pylori, and clinicopathological and demographic characteristics and stimulant use. HSV-1 and H. pylori were most frequently identified in controls (HSV-1: 12.5% and H. pylori: 6.3%). There were 7 (7.8%) and 8 (8.6%) patients with positive HSV-1 in HNSCC and chronic tonsillitis patients, respectively, while the prevalence of H. pylori was 0/90 (0%) and 3/93 (3.2%), respectively. More cases of HSV-1 were observed in older individuals in the control group. All positive HSV-1 cases in the HNSCC group were associated with advanced tumor stage (T3/T4). The prevalence of HSV-1 and H. pylori was highest in the controls compared to HNSCC and chronic tonsillitis patients, which indicates that the pathogens were not risk factors. However, since all positive HSV-1 cases in the HNSCC group were observed only in patients with advanced tumor stage, we suggested a possible link between HSV-1 and tumor progression. Further follow-up of the study groups is planned.

Apoptosis and Phagocytosis as Antiviral Mechanisms

Viruses are infectious entities that make use of the replication machinery of their hosts to produce more progenies, causing disease and sometimes death. To counter viral infection, metazoan hosts are equipped with various defense mechanisms, from the rapid-evoking innate immune responses to the most advanced adaptive immune responses. Previous research demonstrated that cells in fruit flies and mice infected with Drosophila C virus and influenza, respectively, undergo apoptosis, which triggers the engulfment of apoptotic virus-infected cells by phagocytes. This process involves the recognition of eat-me signals on the surface of virus-infected cells by receptors of specialized phagocytes, such as macrophages and neutrophils in mice and hemocytes in fruit flies, to facilitate the phagocytic elimination of virus-infected cells. Inhibition of phagocytosis led to severe pathologies and death in both species, indicating that apoptosis-dependent phagocytosis of virus-infected cells is a conserved antiviral mechanism in multicellular organisms. Indeed, our understanding of the mechanisms underlying apoptosis-dependent phagocytosis of virus-infected cells has shed a new perspective on how hosts defend themselves against viral infection. This chapter explores the mechanisms of this process and its potential for developing new treatments for viral diseases.

Caspase-8 is required for HSV-1-induced apoptosis and promotes effective viral particle release via autophagy inhibition

Regulated cell death (RCD) plays an important role in the progression of viral replication and particle release in cells infected by herpes simplex virus-1 (HSV-1). However, the kind of RCD (apoptosis, necroptosis, others) and the resulting cytopathic effect of HSV-1 depends on the cell type and the species. In this study, we further investigated the molecular mechanisms of apoptosis induced by HSV-1. Although a role of caspase-8 has previously been suggested, we now clearly show that caspase-8 is required for HSV-1-induced apoptosis in a FADD-/death receptor-independent manner in both mouse embryo fibroblasts (MEF) and human monocytes (U937). While wild-type (wt) MEFs and U937 cells exhibited increased caspase-8 and caspase-3 activation and apoptosis after HSV-1 infection, respective caspase-8-deficient (caspase-8−/−) cells were largely impeded in any of these effects. Unexpectedly, caspase-8−/− MEF and U937 cells also showed less virus particle release associated with increased autophagy as evidenced by higher Beclin-1 and lower p62/SQSTM1 levels and increased LC3-I to LC3-II conversion. Confocal and electron microscopy revealed that HSV-1 stimulated a strong perinuclear multivesicular body response, resembling increased autophagy in caspase-8−/− cells, entrapping virions in cellular endosomes. Pharmacological inhibition of autophagy by wortmannin restored the ability of caspase-8−/− cells to release viral particles in similar amounts as in wt cells. Altogether our results support a non-canonical role of caspase-8 in both HSV-1-induced apoptosis and viral particle release through autophagic regulation.

Direct cleavage of caspase-8 by herpes simplex virus 1 tegument protein US11

The HSV-1 tegument protein Us11 counteracts the antiviral defense mechanisms by precluding the host protein shutoff. Previous works demonstrated that Us11 prevents heat-and staurosporine-induced apoptosis and inhibits autophagy. Therefore, in the present study, we investigated the hypothesis that HSV-1, through Us11, could recruit caspase-8, a key enzyme regulating programmed cell death. We first show that HSV-1 promotes the accumulation of caspase-8-p18 active fragments in both semi permissive THP-1 cells and fully permissive HEp-2 cells to HSV-1 replication. Using a recombinant virus R3630 (ΔUs11/ΔUs12) and a plasmid encoding Us11-recombinant protein we have proven that Us11 promotes p18 accumulation, which does not trigger the apoptotic signaling. Additional, in an in vitro model, we demonstrated that Us11-recombinant protein induces caspase-8-p18 cleavage by physically interacting with the caspase-8 recombinant protein. Finally, we found that, during HSV-1 replication, activated-caspase-8 cleaves Atg3 protein to potentially block autophagy and support its replication.

Interaction between the Hepatitis B Virus and Cellular FLIP Variants in Viral Replication and the Innate Immune System

During viral evolution and adaptation, many viruses have utilized host cellular factors and machinery as their partners. HBx, as a multifunctional viral protein encoded by the hepatitis B virus (HBV), promotes HBV replication and greatly contributes to the development of HBV-associated hepatocellular carcinoma (HCC). HBx interacts with several host factors in order to regulate HBV replication and evolve carcinogenesis. The cellular FADD-like IL-1β-converting enzyme (FLICE)-like inhibitory protein (c-FLIP) is a major factor that functions in a variety of cellular pathways and specifically in apoptosis. It has been shown that the interaction between HBx and c-FLIP determines HBV fate. In this review, we provide a comprehensive and detailed overview of the interplay between c-FLIP and HBV in various environmental circumstances. We describe strategies adapted by HBV to establish its chronic infection. We also summarize the conventional roles of c-FLIP and highlight the functional outcome of the interaction between c-FLIP and HBV or other viruses in viral replication and the innate immune system.

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.

How dendritic cells sense and respond to viral infections

The ability of dendritic cells (DCs) to sense viral pathogens and orchestrate a proper immune response makes them one of the key players in antiviral immunity. Different DC subsets have complementing functions during viral infections, some specialize in antigen presentation and cross-presentation and others in the production of cytokines with antiviral activity, such as type I interferons. In this review, we summarize the latest updates concerning the role of DCs in viral infections, with particular focus on the complex interplay between DC subsets and severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2). Despite being initiated by a vast array of immune receptors, DC-mediated antiviral responses often converge towards the same endpoint, that is the production of proinflammatory cytokines and the activation of an adaptive immune response. Nonetheless, the inherent migratory properties of DCs make them a double-edged sword and often viral recognition by DCs results in further viral dissemination. Here we illustrate these various aspects of the antiviral functions of DCs and also provide a brief overview of novel antiviral vaccination strategies based on DCs targeting.

Herpes Simplex Virus type 1 infects Langerhans cells and the novel epidermal dendritic cell, Epi-cDC2s, via different entry pathways

Skin mononuclear phagocytes (MNPs) provide the first interactions of invading viruses with the immune system. In addition to Langerhans cells (LCs), we recently described a second epidermal MNP population, Epi-cDC2s, in human anogenital epidermis that is closely related to dermal conventional dendritic cells type 2 (cDC2) and can be preferentially infected by HIV. Here we show that in epidermal explants topically infected with herpes simplex virus (HSV-1), both LCs and Epi-cDC2s interact with HSV-1 particles and infected keratinocytes. Isolated Epi-cDC2s support higher levels of infection than LCs in vitro, inhibited by acyclovir, but both MNP subtypes express similar levels of the HSV entry receptors nectin-1 and HVEM, and show similar levels of initial uptake. Using inhibitors of endosomal acidification, actin and cholesterol, we found that HSV-1 utilises different entry pathways in each cell type. HSV-1 predominantly infects LCs, and monocyte-derived MNPs, via a pH-dependent pathway. In contrast, Epi-cDC2s are mainly infected via a pH-independent pathway which may contribute to the enhanced infection of Epi-cDC2s. Both cells underwent apoptosis suggesting that Epi-cDC2s may follow the same dermal migration and uptake by dermal MNPs that we have previously shown for LCs. Thus, we hypothesize that the uptake of HSV and infection of Epi-cDC2s will stimulate immune responses via a different pathway to LCs, which in future may help guide HSV vaccine development and adjuvant targeting.

Pathobiology and treatment of viral keratitis

Keratitis is one of the most prevalent ocular diseases manifested by partial or total loss of vision. Amongst infectious (viz., microbes including bacteria, fungi, amebae, and viruses) and non-infectious (viz., eye trauma, chemical exposure, and ultraviolet exposure, contact lens) risk factors, viral keratitis has been demonstrated as one of the leading causes of corneal opacity. While many viruses have been shown to cause keratitis (such as rhabdoviruses, coxsackieviruses, etc.), herpesviruses are the predominant etiologic agent of viral keratitis. This chapter will summarize current knowledge on the prevalence, diagnosis, and pathobiology of viral keratitis. Virus-mediated immunomodulation of host innate and adaptive immune components is critical for viral persistence, and dysfunctional immune responses may cause destruction of ocular tissues leading to keratitis. Immunosuppressed or immunocompromised individuals may display recurring disease with pronounced severity. Early diagnosis of viral keratitis is beneficial for disease management and response to treatment. Finally, we have discussed current and emerging therapies to treat viral keratitis.

The latency-associated transcript locus of herpes simplex virus 1 is a virulence determinant in human skin

Herpes simplex virus 1 (HSV-1) infects skin and mucosal epithelial cells and then travels along axons to establish latency in the neurones of sensory ganglia. Although viral gene expression is restricted during latency, the latency-associated transcript (LAT) locus encodes many RNAs, including a 2 kb intron known as the hallmark of HSV-1 latency. Here, we studied HSV-1 infection and the role of the LAT locus in human skin xenografts in vivo and in cultured explants. We sequenced the genomes of our stock of HSV-1 strain 17syn⁺ and seven derived viruses and found nonsynonymous mutations in many viral proteins that had no impact on skin infection. In contrast, deletions in the LAT locus severely impaired HSV-1 replication and lesion formation in skin. However, skin replication was not affected by impaired intron splicing. Moreover, although the LAT locus has been implicated in regulating gene expression in neurones, we observed only small changes in transcript levels that were unrelated to the growth defect in skin, suggesting that its functions in skin may be different from those in neurones. Thus, although the LAT locus was previously thought to be dispensable for lytic infection, we show that it is a determinant of HSV-1 virulence during lytic infection of human skin.

Herpes simplex virus type 1 (HSV-1) infects and destroys the outer layer of skin cells, producing lesions known as cold sores. Although these lesions heal, the virus persists in the host for the lifetime and can reactivate to cause new lesions. This is possible because the virus enters the axons of neurones in the skin and moves to their cell bodies located in spinal or cranial nerve bundles called ganglia, where the virus becomes dormant (latent). The most abundant viral RNAs expressed during this state are the latency associated transcripts (LATs), which have been considered a hallmark of HSV-1 latency. Here, we studied HSV-1 infection and spread in human skin. Unexpectedly, we found that the LAT locus is necessary for lesion formation in skin. HSV-1 viruses that were genetically mutated to delete the start of the locus could not spread in skin, whereas viruses with many other genetic mutations had this capacity. Our results suggest that an antiviral drug that inhibits transcripts from this region of the viral genome could block viral spread in skin, or a vaccine could possibly be produced by genetically modifying the virus at the LAT locus and by doing so, limit the virus’ ability become latent in neurones.

Viral FLIP blocks Caspase-8 driven apoptosis in the gut in vivo

A strict cell death control in the intestinal epithelium is indispensable to maintain barrier integrity and homeostasis. In order to achieve a balance between cell proliferation and cell death, a tight regulation of Caspase-8, which is a key player in controlling apoptosis, is required. Caspase-8 activity is regulated by cellular FLIP proteins. These proteins are expressed in different isoforms (cFLIP_long and cFLIP_short) which determine cell death and survival. Interestingly, several viruses encode FLIP proteins, homologous to cFLIP_short, which are described to regulate Caspase-8 and the host cell death machinery. In the current study a mouse model was generated to show the impact of viral FLIP (vFLIP) from Kaposi’s Sarcoma-associated Herpesvirus (KSHV)/ Human Herpesvirus-8 (HHV-8) on cell death regulation in the gut. Our results demonstrate that expression of vFlip in intestinal epithelial cells suppressed cFlip expression, but protected mice from lethality, tissue damage and excessive apoptotic cell death induced by genetic cFlip deletion. Finally, our model shows that vFlip expression decreases cFlip mediated Caspase-8 activation in intestinal epithelial cells. In conclusion, our data suggests that viral FLIP neutralizes and compensates for cellular FLIP, efficiently counteracting host cell death induction and facilitating further propagation in the host organism.

Herpes Simplex Virus Type 1 and Host Antiviral Immune Responses: An Update

Herpes simplex virus type 1 (HSV-1) infection activates a rapid stimulation of host innate immune responses and a delicate interplay between virus and host immune elements regulates the whole events. Although host immune elements play well in limiting the HSV-1 infection by interfering viral replication, they are still unable to remove the virus completely, because HSV-1 proteins are efficient enough to bypass the host antiviral immune responses and virus succeed to reactivate again from latency at opportune time. Type 1 interferon signaling pathway is the central point of innate immunity along with some of the activated neutrophils, monocytes, macrophages, and dendritic cells, and some natural killer cells play role, while the CD8⁺ T cells are crucial in adaptive immunity. In this review, the current knowledge of host and HSV-1 interaction has been described that how the host antiviral immune responses occur and what are the mechanisms of viral evasion adapted by virus to counteract with both arms of immunity.

The PD-1/PD-L1 Axis and Virus Infections: A Delicate Balance

Programmed cell death protein (PD-1) and its ligands play a fundamental role in the evasion of tumor cells from antitumor immunity. Less well appreciated is the fact that the PD-1/PD-L1 axis also regulates antiviral immune responses and is therefore modulated by a number of viruses. Upregulation of PD-1 and its ligands PD-L1 and PD-L2 is observed during acute virus infection and after infection with persistent viruses including important human pathogens such as human immunodeficiency virus (HIV), hepatitis C virus (HCV), and hepatitis B virus (HBV). Experimental evidence suggests that insufficient signaling through the PD-1 pathway promotes immunopathology during acute infection by exaggerating primary T cell responses. If chronic infection is established, however, high levels of PD-1 expression can have unfavorable immunological consequences. Exhaustion and suppression of antiviral immune responses can result in viral immune evasion. The role of the PD-1/PD-L1 axis during viral infections is further complicated by evidence that PD-L1 also mediates inflammatory effects in the acute phase of an immune response. In this review, we discuss the intricate interplay between viruses and the PD-1/PD-L1 axis.

Herpes Simplex Virus Evasion of Early Host Antiviral Responses

Herpes simplex viruses type 1 (HSV-1) and type 2 (HSV-2) have co-evolved with humans for thousands of years and are present at a high prevalence in the population worldwide. HSV infections are responsible for several illnesses including skin and mucosal lesions, blindness and even life-threatening encephalitis in both, immunocompetent and immunocompromised individuals of all ages. Therefore, diseases caused by HSVs represent significant public health burdens. Similar to other herpesviruses, HSV-1 and HSV-2 produce lifelong infections in the host by establishing latency in neurons and sporadically reactivating from these cells, eliciting recurrences that are accompanied by viral shedding in both, symptomatic and asymptomatic individuals. The ability of HSVs to persist and recur in otherwise healthy individuals is likely given by the numerous virulence factors that these viruses have evolved to evade host antiviral responses. Here, we review and discuss molecular mechanisms used by HSVs to evade early innate antiviral responses, which are the first lines of defense against these viruses. A comprehensive understanding of how HSVs evade host early antiviral responses could contribute to the development of novel therapies and vaccines to counteract these viruses.

US6 Gene Deletion in Herpes Simplex Virus Type 2 Enhances Dendritic Cell Function and T Cell Activation

Herpes simplex virus (HSV) type 1 (HSV-1) and type 2 (HSV-2) produce lifelong infections that are associated with frequent asymptomatic or clinically apparent reactivation. Importantly, HSV express multiple virulence factors that negatively modulate innate and adaptive immune components. Notably, HSV interfere with dendritic cell (DC) viability and function, likely hindering the capacity of the host to mount effective immunity against these viruses. Recently, an HSV-2 virus that was deleted in glycoprotein D was engineered (designated ΔgD-2). The virus is propagated on a complementing cell line that expresses HSV-1 gD, which permits a single round of viral replication. ΔgD-2 is safe, immunogenic, and provided complete protection against vaginal or skin challenges with HSV-1 and HSV-2 in murine models. Here, we sought to assess the interaction of ΔgD-2 with DCs and found that, in contrast to wild-type (WT) virus which induces DC apoptosis, ΔgD-2 promoted their migration and capacity to activate naïve CD8⁺ and CD4⁺ T cells in vitro and in vivo. Furthermore, DCs exposed to the WT and ΔgD-2 virus experienced different unfolded protein responses. Mice primed with DCs infected with ΔgD-2 in vitro displayed significantly reduced infection and pathology after genital challenge with virulent HSV-2 compared to non-primed mice, suggesting that DCs play a role in the immune response to the vaccine strain.

The suppression of apoptosis by α-herpesvirus

Apoptosis, an important innate immune mechanism that eliminates pathogen-infected cells, is primarily triggered by two signalling pathways: the death receptor pathway and the mitochondria-mediated pathway. However, many viruses have evolved various strategies to suppress apoptosis by encoding anti-apoptotic factors or regulating apoptotic signalling pathways, which promote viral propagation and evasion of the host defence. During its life cycle, α-herpesvirus utilizes an elegant multifarious anti-apoptotic strategy to suppress programmed cell death. This progress article primarily focuses on the current understanding of the apoptosis-inhibition mechanisms of α-herpesvirus anti-apoptotic genes and their expression products and discusses future directions, including how the anti-apoptotic function of herpesvirus could be targeted therapeutically.

HSV1 latent transcription and non-coding RNA: A critical retrospective

Virologists have invested great effort into understanding how the herpes simplex viruses and their relatives are maintained dormant over the lifespan of their host while maintaining the poise to remobilize on sporadic occasions. Piece by piece, our field has defined the tissues in play (the sensory ganglia), the transcriptional units (the latency-associated transcripts), and the responsive genomic region (the long repeats of the viral genomes). With time, the observed complexity of these features has compounded, and the totality of viral factors regulating latency are less obvious. In this review, we compose a comprehensive picture of the viral genetic elements suspected to be relevant to herpes simplex virus 1 (HSV1) latent transcription by conducting a critical analysis of about three decades of research. We describe these studies, which largely involved mutational analysis of the notable latency-associated transcripts (LATs), and more recently a series of viral miRNAs. We also intend to draw attention to the many other less characterized non-coding RNAs, and perhaps coding RNAs, that may be important for consideration when trying to disentangle the multitude of phenotypes of the many genetic modifications introduced into recombinant HSV1 strains.

HSV-1-induced activation of NF-κB protects U937 monocytic cells against both virus replication and apoptosis

The transcription factor nuclear factor-kappa B (NF-κB) is a crucial player of the antiviral innate response. Intriguingly, however, NF-κB activation is assumed to favour herpes simplex virus (HSV) infection rather than restrict it. Apoptosis, a form of innate response to viruses, is completely inhibited by HSV in fully permissive cells, but not in cells incapable to fully sustain HSV replication, such as immunocompetent cells. To resolve the intricate interplay among NF-κB signalling, apoptosis and permissiveness to HSV-1 in monocytic cells, we utilized U937 monocytic cells in which NF-κB activation was inhibited by expressing a dominant-negative IκBα. Surprisingly, viral production was increased in monocytic cells in which NF-κB was inhibited. Moreover, inhibition of NF-κB led to increased apoptosis following HSV-1 infection, associated with lysosomal membrane permeabilization. High expression of late viral proteins and induction of apoptosis occurred in distinct cells. Transcriptional analysis of known innate response genes by real-time quantitative reverse transcription-PCR excluded a contribution of the assayed genes to the observed phenomena. Thus, in monocytic cells NF-κB activation simultaneously serves as an innate process to restrict viral replication as well as a mechanism to limit the damage of an excessive apoptotic response to HSV-1 infection. This finding may clarify mechanisms controlling HSV-1 infection in monocytic cells.

Cleaved c-FLIP mediates the antiviral effect of TNF-α against hepatitis B virus by dysregulating hepatocyte nuclear factors

BACKGROUND & AIMS

Cytokines are key molecules implicated in the defense against virus infection. Tumor necrosis factor-alpha (TNF-α) is well known to block the replication of hepatitis B virus (HBV). However, the molecular mechanism and the downstream effector molecules remain largely unknown.

METHODS

In this study, we investigated the antiviral effect and mechanism of p22-FLIP (FLICE-inhibitory protein) by ectopic expression in vitro and in vivo. In addition, to provide the biological relevance of our study, we examined that the p22-FLIP is involved in TNF-α-mediated suppression of HBV in primary human hepatocytes.

RESULTS

We found that p22-FLIP, a newly discovered c-FLIP cleavage product, inhibited HBV replication at the transcriptional level in both hepatoma cells and primary human hepatocytes, and that c-FLIP conversion to p22-FLIP was stimulated by the TNF-α/NF-κB pathway. p22-FLIP inhibited HBV replication through the upregulation of HNF3β but downregulation of HNF4α, thus inhibiting both HBV enhancer elements. Finally, p22-FLIP potently inhibited HBV DNA replication in a mouse model of HBV replication.

CONCLUSIONS

Taken together, these findings suggest that the anti-apoptotic p22-FLIP serves a novel function of inhibiting HBV transcription, and mediates the antiviral effect of TNF-α against HBV replication.

Phylogenetically Distant Viruses Use the Same BH3-Only Protein Puma to Trigger Bax/Bak-Dependent Apoptosis of Infected Mouse and Human Cells

Viruses can trigger apoptosis of infected host cells if not counteracted by cellular or viral anti-apoptotic proteins. These protective proteins either inhibit the activation of caspases or they act as Bcl-2 homologs to prevent Bax/Bak-mediated outer mitochondrial membrane permeabilization (MOMP). The exact mechanism by which viruses trigger MOMP has however remained enigmatic. Here we use two distinct types of viruses, a double stranded DNA virus, herpes simplex virus-1 (HSV-1) and a positive sense, single stranded RNA virus, Semliki Forest virus (SFV) to show that the BH3-only protein Puma is the major mediator of virus-induced Bax/Bak activation and MOMP induction. Indeed, when Puma was genetically deleted or downregulated by shRNA, mouse embryonic fibroblasts and IL-3-dependent monocytes as well as human colon carcinoma cells were as resistant to virus-induced apoptosis as their Bax/Bak double deficient counterparts (Bax/Bak-/-). Puma protein expression started to augment after 2 h postinfection with both viruses. Puma mRNA levels increased as well, but this occurred after apoptosis initiation (MOMP) because it was blocked in cells lacking Bax/Bak or overexpressing Bcl-xL. Moreover, none of the classical Puma transcription factors such as p53, p73 or p65 NFκB were involved in HSV-1-induced apoptosis. Our data suggest that viruses use a Puma protein-dependent mechanism to trigger MOMP and apoptosis in host cells.

Programmed hepatocytes cell death associated with FLIP downregulation in response to extracellular preS1/2

Chronic hepatitis B virus (HBV) infection involves liver damage resulting in continuous cell injury and death. During HBV infection, hepatocytes exhibit changes in death receptor expression and in their susceptibility to death. These changes are observed not only in infected cells but also in bystander cells. Because excess viral surface protein (HBsAg) is secreted in large amounts as soluble particles containing preS proteins, the role of soluble preS1/2 in hepatocyte (HepG2) death modulation is an important issue to be explored. An increase of cell death induced by preS1/2 was observed. Also, cell death was associated with the down-regulation of FLIP and activation of caspase 8, caspase 9, and BID. Additionally, hepatocytes exhibited a sensitization to death mediated by the Fas receptor. These results, may contribute to understanding the role of envelope proteins (preS1/2) in the pathogenesis of HBV infection.

Immunological control of herpes simplex virus infections

Herpes simplex virus type 1 (HSV-1) is capable of causing a latent infection in sensory neurons that lasts for the lifetime of the host. The primary infection is resolved following the induction of the innate immune response that controls replication of the virus until the adaptive immune response can clear the active infection. HSV-1-specific CD8⁺ T cells survey the ganglionic regions containing latently infected neurons and participate in preventing reactivation of HSV from latency. The long-term residence and migration dynamics of the T cells in the trigeminal ganglia appear to distinguish them from the traditional memory T cell subsets. Recently described tissue resident memory (T_RM) T cells establish residence and survive for long periods in peripheral tissue compartments following antigen exposure. This review focuses on the immune system response to HSV-1 infection. Particular emphasis is placed on the evidence pointing to the HSV-1-specific CD8⁺ T cells in the trigeminal belonging to the T_RM class of memory T cells and the role of T_RM cells in virus infection, pathogenesis, latency, and disease.

Bovine Herpes Virus 1 (BHV-1) and Herpes Simplex Virus Type 1 (HSV-1) Promote Survival of Latently Infected Sensory Neurons, in Part by Inhibiting Apoptosis

α-Herpesvirinae subfamily members, including herpes simplex virus type 1 (HSV-1) and bovine herpes virus 1 (BHV-1), initiate infection in mucosal surfaces. BHV-1 and HSV-1 enter sensory neurons by cell-cell spread where a burst of viral gene expression occurs. When compared to non-neuronal cells, viral gene expression is quickly extinguished in sensory neurons resulting in neuronal survival and latency. The HSV-1 latency associated transcript (LAT), which is abundantly expressed in latently infected neurons, inhibits apoptosis, viral transcription, and productive infection, and directly or indirectly enhances reactivation from latency in small animal models. Three anti-apoptosis genes can be substituted for LAT, which will restore wild type levels of reactivation from latency to a LAT null mutant virus. Two small non-coding RNAs encoded by LAT possess anti-apoptosis functions in transfected cells. The BHV-1 latency related RNA (LR-RNA), like LAT, is abundantly expressed during latency. The LR-RNA encodes a protein (ORF2) and two microRNAs that are expressed in certain latently infected neurons. Wild-type expression of LR gene products is required for stress-induced reactivation from latency in cattle. ORF2 has anti-apoptosis functions and interacts with certain cellular transcription factors that stimulate viral transcription and productive infection. ORF2 is predicted to promote survival of infected neurons by inhibiting apoptosis and sequestering cellular transcription factors which stimulate productive infection. In addition, the LR encoded microRNAs inhibit viral transcription and apoptosis. In summary, the ability of BHV-1 and HSV-1 to interfere with apoptosis and productive infection in sensory neurons is crucial for the life-long latency-reactivation cycle in their respective hosts.

Herpes simplex virus 2 (HSV-2) prevents dendritic cell maturation, induces apoptosis, and triggers release of proinflammatory cytokines: potential links to HSV-HIV synergy

Herpes simplex virus 2 (HSV-2) may cause frequent recurrences, highlighting its ability to evade host defense. This study tested the hypothesis that HSV-2 interferes with dendritic cell (DC) function as an escape mechanism, which may contribute to enhanced HIV replication in coinfected populations. Immature monocyte-derived human DCs were exposed to live or UV-inactivated HSV-2 or lipopolysaccharide. Little or no increase in the maturation marker CD83 was observed in response to HSV-2 and HSV-2 exposed DCs were impaired in their ability to present antigen (influenza) to T cells. Exposure to UV-inactivated virus stimulated a modest, but significant increase in CD83, suggesting that viral gene expression contributes to the block in DC maturation. The functional impairment of HSV-2-exposed DCs could be partially attributed to the induction of apoptosis. Live and inactivated HSV-2 triggered an increase in the number of early and late apoptotic cells in both the infected and bystander cell populations; apoptosis was associated with a decrease in cellular FLICE-inhibitory protein (c-FLIP). Paradoxically, HSV-2 induced Akt phosphorylation, which typically promotes DC maturation and survival. Despite these aberrant responses, live and inactivated HSV-2 induced the release of cytokines into culture supernatants, which were sufficient to activate HIV-1 replication in latently infected U1 cells. Together, these findings suggest that in the presence of overt or subclinical HSV-2, the function of mucosal DCs would be impaired. These responses may allow HSV to escape immune surveillance but may also promote HIV infection and contribute to the epidemiological link between HIV and HSV.

Regulatory dendritic cells: there is more than just immune activation

The immune system exists in a delicate equilibrium between inflammatory responses and tolerance. This unique feature allows the immune system to recognize and respond to potential threats in a controlled but normally limited fashion thereby preventing a destructive overreaction against healthy tissues. While the adaptive immune system was the major research focus concerning activation vs. tolerance in the immune system more recent findings suggest that cells of the innate immune system are important players in the decision between effective immunity and induction of tolerance or immune inhibition. Among immune cells of the innate immune system dendritic cells (DCs) have a special function linking innate immune functions with the induction of adaptive immunity. DCs are the primary professional antigen presenting cells (APCs) initiating adaptive immune responses. They belong to the hematopoietic system and arise from CD34(+) stem cells in the bone marrow. Particularly in the murine system two major subgroups of DCs, namely myeloid DCs (mDCs) and plasmacytoid DCs (pDCs) can be distinguished. DCs are important mediators of innate and adaptive immunity mostly due to their remarkable capacity to present processed antigens via major histocompatibility complexes (MHC) to T cells and B cells in secondary lymphoid organs. A large body of literature has been accumulated during the last two decades describing which role DCs play during activation of T cell responses but also during the establishment and maintenance of central tolerance (Steinman et al., 2003). While the concept of peripheral tolerance has been clearly established during the last years, the role of different sets of DCs and their particular molecular mechanisms of immune deviation has not yet fully been appreciated. In this review we summarize accumulating evidence about the role of regulatory DCs in situations where the balance between tolerance and immunogenicity has been altered leading to pathologic conditions such as chronic inflammation or malignancies.

CD8+ T cells patrol HSV-1-infected trigeminal ganglia and prevent viral reactivation

A hallmark of herpes viruses is their capacity to cause recurrent disease. Recurrences of herpes simplex virus (HSV)-1 disease do not result from reinfection from external sources, but rather from reactivation of virus that is maintained in a latent state in sensory neurons and periodically reactivates from latency to cause recurrent disease. Recent findings implicate HSV-specific CD8(+) T cells in immune surveillance of HSV-1 latently infected sensory neurons in trigeminal ganglia (TG) and inhibition of HSV-1 reactivation from latency. This review summarizes recent findings regarding the characteristics of the TG-resident CD8(+) T cell population and certain unique obstacles that might complicate the development of therapeutic vaccines.

The ribonucleotide reductase R1 subunits of herpes simplex virus types 1 and 2 protect cells against TNFα- and FasL-induced apoptosis by interacting with caspase-8

We previously reported that HSV-2 R1, the R1 subunit (ICP10; UL39) of herpes simplex virus type-2 ribonucleotide reductase, protects cells against apoptosis induced by the death receptor (DR) ligands tumor necrosis factor-alpha- (TNFα) and Fas ligand (FasL) by interrupting DR-mediated signaling at, or upstream of, caspase-8 activation. Further investigation of the molecular mechanism underlying HSV-2 R1 protection showed that extracellular-regulated kinase 1/2 (ERK1/2), phosphatidylinositol 3-kinase (PI3-K)/Akt, NF-κB and JNK survival pathways do not play a major role in this antiapoptotic function. Interaction studies revealed that HSV-2 R1 interacted constitutively with caspase-8. The HSV-2 R1 deletion mutant R1(1-834)-GFP and Epstein-Barr virus (EBV) R1, which did not protect against apoptosis induced by DR ligands, did not interact with caspase-8, indicating that interaction is required for protection. HSV-2 R1 impaired caspase-8 activation induced by caspase-8 over-expression, suggesting that interaction between the two proteins prevents caspase-8 dimerization/activation. HSV-2 R1 bound to caspase-8 directly through its prodomain but did not interact with either its caspase domain or Fas-associated death domain protein (FADD). Interaction between HSV-2 R1 and caspase-8 disrupted FADD-caspase-8 binding. We further demonstrated that individually expressed HSV-1 R1 (ICP6) shares, with HSV-2 R1, the ability to bind caspase-8 and to protect cells against DR-induced apoptosis. Finally, as the long-lived Fas protein remained stable during the early period of infection, experiments with the HSV-1 UL39 deletion mutant ICP6∆ showed that HSV-1 R1 could be essential for the protection of HSV-1-infected cells against FasL.

The herpes simplex virus 1 latency-associated transcript promotes functional exhaustion of virus-specific CD8+ T cells in latently infected trigeminal ganglia: a novel immune evasion mechanism

Following ocular herpes simplex virus 1 (HSV-1) infection of C57BL/6 mice, HSV-specific (HSV-gB(498-505) tetramer(+)) CD8(+) T cells are induced, selectively retained in latently infected trigeminal ganglia (TG), and appear to decrease HSV-1 reactivation. The HSV-1 latency-associated transcript (LAT) gene, the only viral gene that is abundantly transcribed during latency, increases reactivation. Previously we found that during latency with HSV-1 strain McKrae-derived viruses, more of the total TG resident CD8 T cells expressed markers of exhaustion with LAT(+) virus compared to LAT(-) virus. Here we extend these findings to HSV-1 strain 17syn+-derived LAT(+) and LAT(-) viruses and to a virus expressing just the first 20% of LAT. Thus, the previous findings were not an artifact of HSV-1 strain McKrae, and the LAT function involved mapped to the first 1.5 kb of LAT. Importantly, to our knowledge, we show here for the first time that during LAT(+) virus latency, most of the HSV-1-specific TG resident CD8 T cells were functionally exhausted, as judged by low cytotoxic function and decreased gamma interferon (IFN-γ) and tumor necrosis factor alpha (TNF-α) production. This resulted in LAT(-) TG having more functional HSV-gB(498-505) tetramer(+) CD8(+) T cells compared to LAT(+) TG. In addition, LAT expression, in the absence of other HSV-1 gene products, appeared to be able to directly or indirectly upregulate both PD-L1 and major histocompatibility complex class I (MHC-I) on mouse neuroblastoma cells (Neuro2A). These findings may constitute a novel immune evasion mechanism whereby the HSV-1 LAT directly or indirectly promotes functional exhaustion (i.e., dysfunction) of HSV-specific CD8(+) T cells in latently infected TG, resulting in increased virus reactivation.

The ribonucleotide reductase R1 subunits of herpes simplex virus 1 and 2 protect cells against poly(I · C)-induced apoptosis

We recently provided evidence that the ribonucleotide reductase R1 subunits of herpes simplex virus types 1 and 2 (HSV-1 and -2) protect cells against tumor necrosis factor alpha- and Fas ligand-induced apoptosis by interacting with caspase 8. Double-stranded RNA (dsRNA) is a viral intermediate known to initiate innate antiviral responses. Poly(I · C), a synthetic analogue of viral dsRNA, rapidly triggers caspase 8 activation and apoptosis in HeLa cells. Here, we report that HeLa cells after HSV-1 and HSV-2 infection were quickly protected from apoptosis caused by either extracellular poly(I · C) combined with cycloheximide or transfected poly(I · C). Cells infected with the HSV-1 R1 deletion mutant ICP6Δ were killed by poly(I · C), indicating that HSV-1 R1 plays a key role in antiapoptotic responses to poly(I · C). Individually expressed HSV R1s counteracted caspase 8 activation by poly(I · C). In addition to their binding to caspase 8, HSV R1s also interacted constitutively with receptor-interacting protein 1 (RIP1) when expressed either individually or with other viral proteins during HSV infection. R1(1-834)-green fluorescent protein (GFP), an HSV-2 R1 deletion mutant protein devoid of antiapoptotic activity, did not interact with caspase 8 and RIP1, suggesting that these interactions are required for protection against poly(I · C). HSV-2 R1 inhibited the interaction between the Toll/interleukin-1 receptor domain-containing adaptor-inducing beta interferon (IFN-β) (TRIF) and RIP1, an interaction that is essential for apoptosis triggered by extracellular poly(I · C) plus cycloheximide or TRIF overexpression. TRIF silencing reduced poly(I · C)-triggered caspase 8 activation in mock- and ICP6Δ-infected cells, confirming that TRIF is involved in poly(I · C)-induced apoptosis. Thus, by interacting with caspase 8 and RIP1, HSV R1s impair the apoptotic host defense mechanism prompted by dsRNA.

Herpes simplex virus type 1-induced FasL expression in human monocytic cells and its implications for cell death, viral replication, and immune evasion

Herpes simplex virus type 1 (HSV-1) is a ubiquitously occurring pathogen that infects humans early in childhood. The virus persists as a latent infection in dorsal root ganglia, especially of the trigeminal nerve, and frequently becomes reactivated in humans under conditions of stress. Monocytic cells constitute an important component of the innate and adaptive immune responses. We show here for the first time that HSV-1 stimulates human FasL promoter and induces de novo expression of FasL on the surface of human monocytic cells, including monocytes and macrophages. This virus-induced FasL expression causes death of monocytic cells growing in suspension, but not in monolayers (e.g., macrophages). The addition of a broad-spectrum caspase inhibitor, as well as anti-FasL antibodies, reduced cell death but increased viral replication in the virus-infected cell cultures. We also show here for the first time that the virus-induced de novo expression of FasL on the cell surface acts as an immune evasion mechanism by causing the death of interacting human CD4+ T cells, CD8+ T cells, and natural killer (NK) cells. Our study provides novel insights on FasL expression and cell death in HSV-infected human monocytic cells and their impact on interacting immune cells.

Heliothis zea nudivirus 1 gene hhi1 induces apoptosis which is blocked by the Hz-iap2 gene and a noncoding gene, pag1

Heliothis zea nudivirus 1 (HzNV-1 or Hz-1 virus), previously regarded as a nonoccluded baculovirus, recently has been placed in the Nudivirus genus. This virus generates HzNV-1 HindIII-I 1 (hhi1) and many other transcripts during productive viral infection; during latent viral infection, however, persistency-associated gene 1 (pag1) is the only gene expressed. In this report, we used transient expression assays to show that hhi1 can trigger strong apoptosis in transfected cells, which can be blocked, at least partially, by the inhibitor of apoptosis genes Autographa californica iap2 (Ac-iap2) and H. zea iap2 (Hz-iap2). In addition to these two genes, unexpectedly, pag1, which encodes a noncoding RNA with no detectable protein product, was found to efficiently suppress hhi1-induced apoptosis. The assay of pro-Sf-caspase-1 processing by hhi1 transfection did not detect the small P12 subunit at any of the time intervals tested, suggesting that hhi1 of HzNV-1 induces apoptosis through alternative caspase pathways.