Dissociation of HSV gL from gH by αvβ6- or αvβ8-integrin promotes gH activation and virus entry

A Single Amino Acid Substitution in the Transmembrane Domain of Glycoprotein H Functionally Compensates for the Absence of gL in Pseudorabies Virus

Herpesvirus entry requires the coordinated action of at least four viral glycoproteins. Virus-specific binding to a cellular receptor triggers a membrane fusion cascade involving the conserved gH/gL complex and gB. Although gB is the genuine herpesvirus fusogen, it requires gH/gL for fusion, but how activation occurs is still unclear. To study the underlying mechanism, we used a gL-deleted pseudorabies virus (PrV) mutant characterized by its limited capability to directly infect neighboring cells that was exploited for several independent serial passages in cell culture. Unlike previous revertants that acquired mutations in the gL-binding N-terminus of gH, we obtained a variant, PrV-ΔgLPassV99, that unexpectedly contained two amino acid substitutions in the gH transmembrane domain (TMD). One of these mutations, I662S, was sufficient to compensate for gL function in virus entry and in in vitro cell-cell fusion assays in presence of wild type gB, but barely for cell-to-cell spread. Additional expression of receptor-binding PrV gD, which is dispensable for cell-cell fusion mediated by native gB, gH and gL, resulted in hyperfusion in combination with gH V99. Overall, our results uncover a yet-underestimated role of the gH TMD in fusion regulation, further shedding light on the complexity of herpesvirus fusion involving all structural domains of the conserved entry glycoproteins.

A critical review on antiviral peptides derived from viral glycoproteins and host receptors to decoy herpes simplex virus

The health of the human population has been continuously challenged by viral infections. Herpes simplex virus (HSV) is one of the common causes of illness and can lead to death in immunocompromised patients. Existing anti-HSV therapies are not completely successful in eliminating the infection due to anti-viral drug resistance, ineffectiveness against the latent virus and high toxicity over prolonged use. There is a need to update our knowledge of the current challenges faced in anti-HSV therapeutics and realize the necessity of developing alternative treatment approaches. Protein therapeutics are now being explored as a novel approach due to their high specificity and low toxicity. This review highlights the significance of HSV viral glycoproteins and host receptors in the pathogenesis of HSV infection. Proteins or peptides derived from HSV glycoproteins gC, gB, gD, gH and host cell receptors (HSPG, nectin and HVEM) that act as decoys to inhibit HSV attachment, entry, or fusion have been discussed. Few researchers have tried to improve the efficacy and stability of the identified peptides by modifying them using a peptidomimetic approach. With these efforts, we think developing an alternative treatment option for immunocompromised patients and drug-resistant organisms is not far off.

Innovative retargeted oncolytic herpesvirus against nectin4-positive cancers

Nectin4 is a recently discovered tumor associated antigen expressed in cancers that constitute relevant unmet clinical needs, including the undruggable triple negative breast cancer, pancreatic ductal carcinoma, bladder/urothelial cancer, cervical cancer, lung carcinoma and melanoma. So far, only one nectin4-specific drug-Enfortumab Vedotin-has been approved and the clinical trials that test novel therapeutics are only five. Here we engineered R-421, an innovative retargeted onco-immunotherapeutic herpesvirus highly specific for nectin4 and unable to infect through the natural herpes receptors, nectin1 or herpesvirus entry mediator. In vitro, R-421 infected and killed human nectin4-positive malignant cells and spared normal cells, e.g., human fibroblasts. Importantly from a safety viewpoint, R-421 failed to infect malignant cells that do not harbor nectin4 gene amplification/overexpression, whose expression level was moderate-to-low. In essence, there was a net threshold value below which cells were spared from infection, irrespective of whether they were malignant or normal; the only cells that R-421 targeted were the malignant overexpressing ones. In vivo, R-421 decreased or abolished the growth of murine tumors made transgenic for human nectin4 and conferred sensitivity to immune checkpoint inhibitors in combination therapies. Its efficacy was augmented by the cyclophosphamide immunomodulator and decreased by depletion of CD8-positive lymphocytes, arguing that it was in part T cell-mediated. R-421 elicited in-situ vaccination that protected from distant challenge tumors. This study provides proof-of-principle specificity and efficacy data justifying nectin4-retargeted onco-immunotherapeutic herpesvirus as an innovative approach against a number of difficult-to-drug clinical indications.

Herpes simplex virus type I glycoprotein L evades host antiviral innate immunity by abrogating the nuclear translocation of phosphorylated NF-κB sub-unit p65

Nuclear factor (NF)-κB plays an important role in the innate immune response by inducing antiviral genes' expression. However, the herpes simplex virus 1 (HSV-1) virus has developed multiple ways to interfere with NF-κB activity to escape the host antiviral response. Here, we found that HSV-1 envelope glycoprotein L(gL) markedly inhibits interferon (IFN) production and its downstream antiviral genes. Our results showed that ectopic expression of gL inhibited IFN-β promoter activation, and decreased IFN-β production, the expression of IFN-stimulated genes (ISGs), and inhibited immunologic stimulant (poly I:C) induced activation of IFN signaling pathway. Depletion of gL by short interfering RNA (siRNA) significantly upregulated IFN-β and ISG production. Further study showed that the N-terminus of the gL bound to the Rel homology domain (RHD) of the p65 and concealed the nuclear localization signal of p65, thereby impeding the translocation of phosphorylated p65 to the nucleus. In summary, our findings indicated that the N-terminal of HSV-1 gL contributes to immune invasion by inhibiting the nuclear translocation of p65.

LIC12254 Is a Leptospiral Protein That Interacts with Integrins via the RGD Motif

Pathogenic leptospires can bind to receptors on mammalian cells such as cadherins and integrins. Leptospira effectively adheres to cells, overcomes host barriers and spreads into the bloodstream, reaching internal target organs such as the lungs, liver and kidneys. Several microorganisms produce proteins that act as ligands of integrins through the RGD motif. Here, we characterized a leptospiral RGD-containing protein encoded by the gene lic12254. In silico analysis of pathogenic, intermediate and saprophytic species showed that LIC12254 is highly conserved among pathogenic species, and is unique in presenting the RGD motif. The LIC12254-coding sequence is greatly expressed in the virulent Leptospira interrogans L1-130 strain compared with the culture-attenuated L. interrogans M20 strain. We also showed that the recombinant protein rLIC12254 binds to αVβ8 and α8 human integrins most likely via the RGD motif. These interactions are dose-dependent and saturable, a typical property of receptor-ligand interactions. The binding of the recombinant protein lacking this motif-rLIC12254 ΔRAA-to αVβ8 was almost totally abolished, while that with the α8 human integrin was decreased by 65%. Taken together, these results suggest that this putative outer membrane protein interacts with integrins via the RGD domain and may play a key role in leptospirosis pathogenesis.

Viruses Binding to Host Receptors Interacts with Autophagy

Viruses must cross the plasma membrane to infect cells, making them eager to overcome this barrier in order to replicate in hosts. They bind to cell surface receptors as the first step of initiating entry. Viruses can use several surface molecules that allow them to evade defense mechanisms. Various mechanisms are stimulated to defend against viruses upon their entry into cells. Autophagy, one of the defense systems, degrades cellular components to maintain homeostasis. The presence of viruses in the cytosol regulates autophagy; however, the mechanisms by which viral binding to receptors regulates autophagy have not yet been fully established. This review discusses recent findings on autophagy induced by interactions between viruses and receptors. It provides novel perspectives on the mechanism of autophagy as regulated by viruses.

Herpes Simplex Virus 1 Entry Glycoproteins Form Complexes before and during Membrane Fusion

Herpesviruses—ubiquitous pathogens that cause persistent infections—have some of the most complex cell entry mechanisms. Entry of the prototypical herpes simplex virus 1 (HSV-1) requires coordinated efforts of 4 glycoproteins, gB, gD, gH, and gL. The current model posits that the glycoproteins do not interact before receptor engagement and that binding of gD to its receptor causes a “cascade” of sequential pairwise interactions, first activating the gH/gL complex and subsequently activating gB, the viral fusogen. But how these glycoproteins interact remains unresolved. Here, using a quantitative split-luciferase approach, we show that pairwise HSV-1 glycoprotein complexes form before fusion, interact at a steady level throughout fusion, and do not depend on the presence of the cellular receptor. Based on our findings, we propose a revised “conformational cascade” model of HSV-1 entry. We hypothesize that all 4 glycoproteins assemble into a complex before fusion, with gH/gL positioned between gD and gB. Once gD binds to a cognate receptor, the proximity of the glycoproteins within this complex allows for efficient transmission of the activating signal from the receptor-activated gD to gH/gL to gB through sequential conformational changes, ultimately triggering the fusogenic refolding of gB. Our results also highlight previously unappreciated contributions of the transmembrane and cytoplasmic domains to glycoprotein interactions and fusion. Similar principles could be at play in other multicomponent viral entry systems, and the split-luciferase approach used here is a powerful tool for investigating protein-protein interactions in these and a variety of other systems.

Nectin-1 and Non-muscle Myosin Heavy Chain-IIB: Major Mediators of Herpes Simplex Virus-1 Entry Into Corneal Nerves

Herpes Simplex Virus 1 (HSV-1) invades corneal nerves upon its infection of the cornea and then establishes latency in the trigeminal ganglion (TG). The latent virus in TG is often reactivated and travels back to the cornea, causing recurrent herpes simplex keratitis (HSK). The entry of HSV-1 into the corneal nerve is considered the initial step of infection resulting in HSV-1 latency and HSK recurrence. Several gD and gB receptors have been identified, including nectin-1, herpes virus entry medium (HVEM) and 3-O-sulfated heparan sulfate (3-OS-HS) as gD receptors, and non-muscle myosin heavy chain IIA (NMHC-IIA), NMHC-IIB and myelin-associated glycoprotein (MAG) as gB receptors. However, which receptors contribute to the entry of HSV-1 into corneal nerves are yet to be determined. This study observed that receptors nectin-1, HVEM, 3-OS-HS, NMHC-IIA, and NMHC-IIB, not MAG, were expressed in healthy corneal nerves. Further, we cultured TG neurons extracted from mice in vitro to screen for functional gD/gB receptors. Both in vitro siRNA knockdown and in vivo antibody blocking of either nectin-1 or NMHC-IIB reduced the entry and the replication of HSV-1 as shown by qPCR analysis and immunofluorescence measure, respectively. Also, we observed that the re-localization and the upregulation expression of NMHC-IIB after HSV-1 exposure were inhibited when gD receptor nectin-1 was knocked down. These data suggest that nectin-1 was the main gD receptor and NMHC-IIB was the main gB receptor in mediating HSV-1 entry and hold promise as therapeutic targets for resolving HSV-1 latency and HSK recurrence.

The RGD-binding integrins αvβ6 and αvβ8 are receptors for mouse adenovirus-1 and -3 infection

Mammalian adenoviruses (AdVs) comprise more than ~350 types including over 100 human (HAdVs) and just three mouse AdVs (MAdVs). While most HAdVs initiate infection by high affinity/avidity binding of their fiber knob (FK) protein to either coxsackievirus AdV receptor (CAR), CD46 or desmoglein (DSG)-2, MAdV-1 (M1) infection requires arginine-glycine-aspartate (RGD) binding integrins. To identify the receptors mediating MAdV infection we generated five novel reporter viruses for MAdV-1/-2/-3 (M1, M2, M3) transducing permissive murine (m) CMT-93 cells, but not B16 mouse melanoma cells expressing mCAR, human (h) CD46 or hDSG-2. Recombinant M1 or M3 FKs cross-blocked M1 and M3 but not M2 infections. Profiling of murine and human cells expressing RGD-binding integrins suggested that αvβ6 and αvβ8 heterodimers are associated with M1 and M3 infections. Ectopic expression of mβ6 in B16 cells strongly enhanced M1 and M3 binding, infection, and progeny production comparable with mαvβ6-positive CMT-93 cells, whereas mβ8 expressing cells were more permissive to M1 than M3. Anti-integrin antibodies potently blocked M1 and M3 binding and infection of CMT-93 cells and hαvβ8-positive M000216 cells. Soluble integrin αvβ6, and synthetic peptides containing the RGDLXXL sequence derived from FK-M1, FK-M3 and foot and mouth disease virus coat protein strongly interfered with M1/M3 infections, in agreement with high affinity interactions of FK-M1/FK-M3 with αvβ6/αvβ8, determined by surface plasmon resonance measurements. Molecular docking simulations of ternary complexes revealed a bent conformation of RGDLXXL-containing FK-M3 peptides on the subunit interface of αvβ6/β8, where the distal leucine residue dips into a hydrophobic pocket of β6/8, the arginine residue ionically engages αv aspartate215, and the aspartate residue coordinates a divalent cation in αvβ6/β8. Together, the RGDLXXL-bearing FKs are part of an essential mechanism for M1/M3 infection engaging murine and human αvβ6/8 integrins. These integrins are highly conserved in other mammals, and may favour cross-species virus transmission.

Receptors and ligands for herpes simplex viruses: Novel insights for drug targeting

Human herpes simplex viruses (HSVs) belong to the Herpesviridae family. At present, no vaccine or curative treatment is available for the prevention of HSV infections. Here, we review the cell surface receptors that are recognized by HSV's glycoprotein B, glycoprotein C, glycoprotein D, and the glycoprotein H - glycoprotein L complex to facilitate entry into host cells. These receptors include heparan sulfate (HS), herpesvirus entry mediator (HVEM), and nectin-1/-2, 3-O-sulfated heparan sulfate (3-OS HS).

Two Sides to Every Story: Herpes Simplex Type-1 Viral Glycoproteins gB, gD, gH/gL, gK, and Cellular Receptors Function as Key Players in Membrane Fusion

Herpes simplex virus type-1 (HSV-1) and type-2 (HSV-2) are prototypical alphaherpesviruses that are characterized by their unique properties to infect trigeminal and dorsal root ganglionic neurons, respectively, and establish life-long latent infections. These viruses initially infect mucosal epithelial tissues and subsequently spread to neurons. They are associated with a significant disease spectrum, including orofacial and ocular infections for HSV-1 and genital and neonatal infections for HSV-2. Viral glycoproteins within the virion envelope bind to specific cellular receptors to mediate virus entry into cells. This is achieved by the fusion of the viral envelope with the plasma membrane. Similarly, viral glycoproteins expressed on cell surfaces mediate cell-to-cell fusion and facilitate virus spread. An interactive complex of viral glycoproteins gB, gD/gH/gL, and gK and other proteins mediate these membrane fusion phenomena with glycoprotein B (gB), the principal membrane fusogen. The requirement for the virion to enter neuronal axons suggests that the heterodimeric protein complex of gK and membrane protein UL20, found only in alphaherpesviruses, constitute a critical determinant for neuronal entry. This hypothesis was substantiated by the observation that a small deletion in the amino terminus of gK prevents entry into neuronal axons while allowing entry into other cells via endocytosis. Cellular receptors and receptor-mediated signaling synergize with the viral membrane fusion machinery to facilitate virus entry and intercellular spread. Unraveling the underlying interactions among viral glycoproteins, envelope proteins, and cellular receptors will provide new innovative approaches for antiviral therapy against herpesviruses and other neurotropic viruses.

Halting the Spread of Herpes Simplex Virus-1: The Discovery of an Effective Dual αvβ6/αvβ8 Integrin Ligand

Over recent years, αvβ6 and αvβ8 Arg-Gly-Asp (RGD) integrins have risen to prominence as interchangeable co-receptors for the cellular entry of herpes simplex virus-1 (HSV-1). In fact, the employment of subtype-specific integrin-neutralizing antibodies or gene-silencing siRNAs has emerged as a valuable strategy for impairing HSV infectivity. Here, we shift the focus to a more affordable pharmaceutical approach based on small RGD-containing cyclic pentapeptides. Starting from our recently developed αvβ6-preferential peptide [RGD-Chg-E]-CONH2 (1), a small library of N-methylated derivatives (2-6) was indeed synthesized in the attempt to increase its affinity toward αvβ8. Among the novel compounds, [RGD-Chg-(NMe)E]-CONH2 (6) turned out to be a potent αvβ6/αvβ8 binder and a promising inhibitor of HSV entry through an integrin-dependent mechanism. Furthermore, the renewed selectivity profile of 6 was fully rationalized by a NMR/molecular modeling combined approach, providing novel valuable hints for the design of RGD integrin ligands with the desired specificity profile.

In Vitro Viral Evolution Identifies a Critical Residue in the Alphaherpesvirus Fusion Glycoprotein B Ectodomain That Controls gH/gL-Independent Entry

Herpesvirus entry and spread requires fusion of viral and host cell membranes, which is mediated by the conserved surface glycoprotein B (gB). Upon activation, gB undergoes a major conformational change and transits from a metastable prefusion to a stable postfusion conformation. Although gB is a structural homolog of low-pH-triggered class III fusogens, its fusion activity depends strictly on the presence of the conserved regulatory gH/gL complex and nonconserved receptor binding proteins, which ensure that fusion occurs at the right time and space. How gB maintains its prefusion conformation and how gB fusogenicity is controlled remain poorly understood. Here, we report the isolation and characterization of a naturally selected pseudorabies virus (PrV) gB able to mediate efficient gH/gL-independent virus-cell and cell-cell fusion. We found that the control exerted on gB by the accompanying viral proteins is mediated via its cytosolic domain (CTD). Whereas gB variants lacking the CTD are inactive, a single mutation of a conserved asparagine residue in an alpha-helical motif of the ectodomain recently shown to be at the core of the gB prefusion trimer compensated for CTD absence and uncoupled gB from regulatory viral proteins, resulting in a hyperfusion phenotype. This phenotype was transferred to gB homologs from different alphaherpesvirus genera. Overall, our data propose a model in which the central helix acts as a molecular switch for the gB pre-to-postfusion transition by conveying the structural status of the endo- to the ectodomain, thereby governing their cross talk for fusion activation, providing a new paradigm for herpesvirus fusion regulation.

Herpes Simplex Virus Cell Entry Mechanisms: An Update

Herpes simplex virus (HSV) can infect a broad host range and cause mild to life threating infections in humans. The surface glycoproteins of HSV are evolutionarily conserved and show an extraordinary ability to bind more than one receptor on the host cell surface. Following attachment, the virus fuses its lipid envelope with the host cell membrane and releases its nucleocapsid along with tegument proteins into the cytosol. With the help of tegument proteins and host cell factors, the nucleocapsid is then docked into the nuclear pore. The viral double stranded DNA is then released into the host cell’s nucleus. Released viral DNA either replicates rapidly (more commonly in non-neuronal cells) or stays latent inside the nucleus (in sensory neurons). The fusion of the viral envelope with host cell membrane is a key step. Blocking this step can prevent entry of HSV into the host cell and the subsequent interactions that ultimately lead to production of viral progeny and cell death or latency. In this review, we have discussed viral entry mechanisms including the pH-independent as well as pH-dependent endocytic entry, cell to cell spread of HSV and use of viral glycoproteins as an antiviral target.

Atypical structure and function of integrin αV β8

Integrins are heterodimeric transmembrane proteins that play important roles in various biological processes. Most integrins serve as adhesion molecules and transmit bidirectional signaling across the cell membrane through global conformational changes from the bent closed to the extended open conformation. However, integrin β₈ is distinctive in structure and function. Its cytoplasmic domain lacks the conserved protein-binding sequence, which is important in transmitting inside-out signals, suggesting that integrin β₈ may have a different activation mechanism or lack such signaling. In addition, the ligand-binding or activating metal ion Mn²⁺ does not induce a global conformational change in integrin β₈ . It may have only one conformation, that is, an extended, closed conformation, but with high affinity for ligands under physiological conditions, and is, therefore, considered an atypical integrin member. The extended structure and high ligand-binding affinity of integrin α_v β₈ make it ideal for encountering and binding ligands expressed on an opposing cell or in the extracellular matrix. In this review, we summarize the progress in integrin β₈ research with a focus on its distinctive function and structure among integrin members.

αvβ8 integrin adhesion and signaling pathways in development, physiology and disease

Cells must interpret a complex milieu of extracellular cues to modulate intracellular signaling events linked to proliferation, differentiation, migration and other cellular processes. Integrins are heterodimeric transmembrane proteins that link the extracellular matrix (ECM) to the cytoskeleton and control intracellular signaling events. A great deal is known about the structural and functional properties for most integrins; however, the adhesion and signaling pathways controlled by αvβ8 integrin, which was discovered nearly 30 years ago, have only recently been characterized. αvβ8 integrin is a receptor for ECM-bound forms of latent transforming growth factor β (TGFβ) proteins and promotes the activation of TGFβ signaling pathways. Studies of the brain, lung and immune system reveal that the αvβ8 integrin-TGFβ axis mediates cell-cell contact and communication within complex multicellular structures. Perturbing components of this axis results in aberrant cell-cell adhesion and signaling leading to the initiation of various pathologies, including neurodegeneration, fibrosis and cancer. As discussed in this Review, understanding the functions for αvβ8 integrin, its ECM ligands and intracellular effector proteins is not only an important topic in cell biology, but may lead to new therapeutic strategies to treat human pathologies related to integrin dysfunction.

Role of Herpes Simplex Envelope Glycoprotein B and Toll-Like Receptor 2 in Ocular Inflammation: An ex vivo Organotypic Rabbit Corneal Model

It has been recently reported, using in vitro studies, that the herpes simplex virus 1 (HSV-1) encoded envelope glycoprotein B (gB1) interacts with cell surface toll-like receptor 2 (TLR2) and induces the secretion of interleukin-8 (IL8), a representative marker of inflammatory cytokine activation. The purpose of this study is to investigate the role of gB1 in activating host inflammatory responses by using a secreted form of gB1 (gB1s) and an ex vivo organotypic rabbit corneal model. Abraded corneas exposed to gB1s alone or to the recombinant protein mixed with anti gB polyclonal antibody were cultured in an air-liquid interface. The corneas exposed to gB1s show the appearance of mydriasis and high levels of TLR2 and IL-8 mRNAs transcripts were detected in the superficial layer of corneal epithelial cells. Histological stain and immunohistochemical analyses revealed morphological changes in the epithelium of the treated corneas and variations in expression and localization of TLR2. Collectively these findings provide new insight into the pathogenesis of HSV-1 ocular infection by demonstrating the leading role of gB in activating an inflammatory response and in the appearance of mydriasis, a sign of HSV-1 anterior uveitis.

Common characteristics and unique features: A comparison of the fusion machinery of the alphaherpesviruses Pseudorabies virus and Herpes simplex virus

Membrane fusion is a fundamental biological process that allows different cellular compartments delimited by a lipid membrane to release or exchange their respective contents. Similarly, enveloped viruses such as alphaherpesviruses exploit membrane fusion to enter and infect their host cells. For infectious entry the prototypic human Herpes simplex viruses 1 and 2 (HSV-1 and -2, collectively termed HSVs) and the porcine Pseudorabies virus (PrV) utilize four different essential envelope glycoproteins (g): the bona fide fusion protein gB and the regulatory heterodimeric gH/gL complex that constitute the "core fusion machinery" conserved in all members of the Herpesviridae; and the subfamily specific receptor binding protein gD. These four components mediate attachment and fusion of the virion envelope with the host cell plasma membrane through a tightly regulated sequential activation process. Although PrV and the HSVs are closely related and employ the same set of glycoproteins for entry, they show remarkable differences in the requirements for fusion. Whereas the HSVs strictly require all four components for membrane fusion, PrV can mediate cell-cell fusion without gD. Moreover, in contrast to the HSVs, PrV provides a unique opportunity for reversion analyses of gL-negative mutants by serial cell culture passaging, due to a limited cell-cell spread capacity of gL-negative PrV not observed in the HSVs. This allows a more direct analysis of the function of gH/gL during membrane fusion. Unraveling the molecular mechanism of herpesvirus fusion has been a goal of fundamental research for years, and yet important mechanistic details remain to be uncovered. Nevertheless, the elucidation of the crystal structures of all key players involved in PrV and HSV membrane fusion, coupled with a wealth of functional data, has shed some light on this complex puzzle. In this review, we summarize and discuss the contemporary knowledge on the molecular mechanism of entry and membrane fusion utilized by the alphaherpesvirus PrV, and highlight similarities but also remarkable differences in the requirements for fusion between PrV and the HSVs.

Characteristics of herpes simplex virus infection and pathogenesis suggest a strategy for vaccine development

Herpes simplex virus (HSV) can cause oral or genital ulcerative lesions and even encephalitis in various age groups with high infection rates. More seriously, HSV may lead to a wide range of recurrent diseases throughout a lifetime. No vaccines against HSV are currently available. The accumulated clinical research data for HSV vaccines reveal that the effects of HSV interacting with the host, especially the host immune system, may be important for the development of HSV vaccines. HSV vaccine development remains a major challenge. Thus, we focus on the research data regarding the interactions of HSV and host immune cells, including dendritic cells (DCs), innate lymphoid cells (ILCs), macrophages, and natural killer (NK) cells, and the related signal transduction pathways involved in immune evasion and cytokine production. The aim is to explore possible strategies to develop new effective HSV vaccines.

Oncolytic herpes simplex virus tumor targeting and neutralization escape by engineering viral envelope glycoproteins

Oncolytic herpes simplex viruses (oHSVs) have been approved for clinical usage and become more and more popular for tumor virotherapy. However, there are still many issues for the oHSVs used in clinics and clinical trials. The main issues are the limited anti-tumor effects, intratumor injection, and some side effects. To overcome such challenges, here we review the genetic engineering of the envelope glycoproteins for oHSVs to target tumors specifically, and at the same time we summarize the many neutralization antibodies against the envelope glycoproteins and align the neutralization epitopes with functional domains of the respective glycoproteins for future identification of new functions of the glycoproteins and future engineering of the epitopes to escape from host neutralization.

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.

Experimental Dissection of the Lytic Replication Cycles of Herpes Simplex Viruses in vitro

Herpes simplex viruses type 1 and type 2 (HSV-1 and HSV-2) produce lifelong infections and are highly prevalent in the human population. Both viruses elicit numerous clinical manifestations and produce mild-to-severe diseases that affect the skin, eyes, and brain, among others. Despite the existence of numerous antivirals against HSV, such as acyclovir and acyclovir-related analogs, virus variants that are resistant to these compounds can be isolated from immunosuppressed individuals. For such isolates, second-line drugs can be used, yet they frequently produce adverse side effects. Furthermore, topical antivirals for treating cutaneous HSV infections usually display poor to moderate efficacy. Hence, better or novel anti-HSV antivirals are needed and details on their mechanisms of action would be insightful for improving their efficacy and identifying specific molecular targets. Here, we review and dissect the lytic replication cycles of herpes simplex viruses, discussing key steps involved in cell infection and the processes that yield new virions. Additionally, we review and discuss rapid, easy-to-perform and simple experimental approaches for studying key steps involved in HSV replication to facilitate the identification of the mechanisms of action of anti-HSV compounds.

Co-localization of and interaction between duck enteritis virus glycoprotein H and L

BACKGROUND

Duck Enteritis Virus (DEV), belonging to the α-herpesvirus subfamily, is a linear double-stranded DNA virus. Glycoprotein H and L (gH and gL), encoded by UL22 and UL1, are conserved in the family of herpesviruses. They play important roles as gH/gL dimers during viral entry into host cells through cell-cell fusion. The interaction between gH and gL has been confirmed in several human herpesviruses, such as Herpes Simplex Virus (HSV), Epstein-Barr virus (EBV) and Human Cytomegalovirus (HCMV). In this paper, we studied the interaction between DEV gH and gL.

RESULTS

Recombinant plasmids pEGFP-N-gH and pDsRED-N-gL were constructed successfully. Expressions of both DEV gH and gL were observed after incubation of COS-7 cells transfected with pEGFP-N-gH and pDsRED-N-gL plasmids after 12 h, respectively. Also, the co-localization of a proportion of the gH and gL was detected in the cytoplasm of COS-7 cells after co-transfection for 24 h. Then, pCMV-Flag-gL and pCMV-Myc-gH recombinant plasmids were constructed and co-transfected into COS-7 cells. It was showed that both gH and gL were tested with positive results through co-immunoprecipitation and Western-blotting.

CONCLUSIONS

Our results demonstrated not only the co-localization of DEV gH and gL in COS-7 cells, but also the interaction between them. It will provide an insight for the further studies in terms of protein-protein interaction in DEV.

Virus-Receptor Interactions: The Key to Cellular Invasion

Virus–receptor interactions play a key regulatory role in viral host range, tissue tropism, and viral pathogenesis. Viruses utilize elegant strategies to attach to one or multiple receptors, overcome the plasma membrane barrier, enter, and access the necessary host cell machinery. The viral attachment protein can be viewed as the “key” that unlocks host cells by interacting with the “lock”—the receptor—on the cell surface, and these lock-and-key interactions are critical for viruses to successfully invade host cells. Many common themes have emerged in virus–receptor utilization within and across virus families demonstrating that viruses often target particular classes of molecules in order to mediate these events. Common viral receptors include sialylated glycans, cell adhesion molecules such as immunoglobulin superfamily members and integrins, and phosphatidylserine receptors. The redundancy in receptor usage suggests that viruses target particular receptors or “common locks” to take advantage of their cellular function and also suggests evolutionary conservation. Due to the importance of initial virus interactions with host cells in viral pathogenesis and the redundancy in viral receptor usage, exploitation of these strategies would be an attractive target for new antiviral therapeutics.

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Viral receptors are key regulators of host range, tissue tropism, and viral pathogenesis.

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Many viruses utilize common viral receptors including sialic acid, cell adhesion molecules such as immunoglobulin superfamily members and integrins, and phosphatidylserine receptors.

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Detailed molecular interactions between viruses and receptors have been defined through elegant biochemical analyses including glycan array screens, structural–functional analyses, and cell-based approaches providing tremendous insights into these initial events in viral infection.

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Commonalities in virus–receptor interactions present promising targets for the development of broad-spectrum antiviral therapies.

Initial Contact: The First Steps in Herpesvirus Entry

The entry process of herpesviruses into host cells is complex and highly variable. It involves a sequence of well-orchestrated events that begin with virus attachment to glycan-containing proteinaceous structures on the cell surface. This initial contact tethers virus particles to the cell surface and results in a cascade of molecular interactions, including the tight interaction of viral envelope glycoproteins to specific cell receptors. These interactions trigger intracellular signaling and finally virus penetration after fusion of the viral envelope with cellular membranes. Based on the engaged cellular receptors and co-receptors, and the subsequent signaling cascades, the entry pathway will be decided on the spot. A number of viral glycoproteins and many cellular receptors and molecules have been identified as players in one or several of these events during virus entry. This chapter will review viral glycoproteins, cellular receptors and signaling cascades associated with the very first interactions of herpesviruses with their target cells.

Simultaneous Insertion of Two Ligands in gD for Cultivation of Oncolytic Herpes Simplex Viruses in Noncancer Cells and Retargeting to Cancer Receptors

Insertion of a single-chain variable-fragment antibody (scFv) to HER2 (human epidermal growth factor receptor 2) in gD, gH, or gB gives rise to herpes simplex viruses (HSVs) specifically retargeted to HER2-positive cancer cells, hence to highly specific nonattenuated oncolytic agents. Clinical-grade virus production cannot rely on cancer cells. Recently, we developed a double-retargeting strategy whereby gH carries the GCN4 peptide for retargeting to the noncancer producer Vero-GCN4R cell line and gD carries the scFv to HER2 for cancer retargeting. Here, we engineered double-retargeted recombinants, which carry both the GCN4 peptide and the scFv to HER2 in gD. Novel, more-advantageous detargeting strategies were devised so as to optimize the cultivation of the double-retargeted recombinants. Nectin1 detargeting was achieved by deletion of amino acids (aa) 35 to 39, 214 to 223, or 219 to 223 and replacement of the deleted sequences with one of the two ligands. The last two deletions were not attempted before. All recombinants exhibited the double retargeting to HER2 and to the Vero-GCN4R cells, as well as detargeting from the natural receptors HVEM and nectin1. Of note, some recombinants grew to higher yields than others. The best-performing recombinants carried a gD deletion as small as 5 amino acids and grew to titers similar to those exhibited by the singly retargeted R-LM113 and by the nonretargeted R-LM5. This study shows that double retargeting through insertion of two ligands in gD is feasible and, when combined with appropriate detargeting modifications, can result in recombinants highly effective in vitro and in vivo.

IMPORTANCE There is increasing interest in oncolytic viruses following the FDA and European Medicines Agency (EMA) approval of the oncolytic HSV Oncovex^GM-CSF and, mainly, because they greatly boost the immune response to the tumor and can be combined with immunotherapeutic agents, particularly immune checkpoint inhibitors. A strategy to gain high cancer specificity and avoid virus attenuation is to retarget the virus tropism to cancer-specific receptors of choice. However, cultivation of retargeted oncolytics in cells expressing the cancer receptor may not be approvable by regulatory agencies. We devised a strategy for their cultivation in noncancer cells. Here, we describe a double-retargeting strategy, based on the simultaneous insertion of two ligands in gD, one for retargeting to a producer, universal Vero cell derivative and one for retargeting to the HER2 cancer receptor. These insertions were combined with novel, minimally disadvantageous detargeting modifications. The current and accompanying studies indicate how to best achieve the clinical-grade cultivation of retargeted oncolytics.

First Impressions-the Potential of Altering Initial Host-Virus Interactions for Rational Design of Herpesvirus Vaccine Vectors

Purpose

The earliest host-virus interactions occur during virus attachment and entry into cells. These initial steps in the virus lifecycle influence the outcome of infection beyond delivery of the viral genome into the cell. Herpesviruses alter host signaling pathways and processes during attachment and entry to facilitate virus infection and modulate innate immune responses. We suggest in this review that understanding these early signaling events may inform the rational design of therapeutic and prevention strategies for herpesvirus infection, as well as the engineering of viral vectors for immunotherapy purposes.

Recent Findings

Recent reports demonstrate that modulation of Herpes Simplex Virus Type-1 (HSV-1) entry results in unexpected enhancement of antiviral immune responses.

Summary

A variety of evidence suggests that herpesviruses promote specific cellular signaling responses that facilitate viral replication after binding to cell surfaces, as well as during virus entry. Of particular interest is the ability of the virus to alter innate immune responses through these cellular signaling events. Uncovering the underlying immune evasion strategies may lead to the design of live-attenuated vaccines that can generate robust and protective anti-viral immune responses against herpesviruses. These adjuvant properties may be extended to a variety of heterologous antigens expressed by herpesviral vectors.

Herpes simplex viruses activate phospholipid scramblase to redistribute phosphatidylserines and Akt to the outer leaflet of the plasma membrane and promote viral entry

Herpes simplex virus (HSV) entry is associated with Akt translocation to the outer leaflet of the plasma membrane to promote a complex signaling cascade. We hypothesized that phospholipid scramblase-1 (PLSCR1), a calcium responsive enzyme that flips phosphatidylserines between membrane leaflets, might redistribute Akt to the outside during entry. Confocal imaging, biotinylation of membrane proteins and flow cytometric analysis demonstrated that HSV activates PLSCR1 and flips phosphatidylserines and Akt to the outside shortly following HSV-1 or HSV-2 exposure. Translocation was blocked by addition of a cell permeable calcium chelator, pharmacological scramblase antagonist, or transfection with small interfering RNA targeting PLSCR1. Co-immunoprecipitation and proximity ligation studies demonstrated that PLSCR1 associated with glycoprotein L at the outer leaflet and studies with gL deletion viruses indicate that this interaction facilitates subsequent restoration of the plasma membrane architecture. Ionomycin, a calcium ionophore, also induced PLSCR1 activation resulting in Akt externalization, suggesting a previously unrecognized biological phenomenon.

Defining the mechanisms by which herpes simplex viruses (HSV) enter cells will facilitate the development of new strategies to prevent or treat infections and provide insights into cell biology. We report the novel observation that HSV activates the enzyme, scramblase, which redistributes phosphatidylserines, the major component of the inner leaflet of the plasma membrane, and the associated protein, Akt, between the inner and outer leaflet of the plasma membrane, to promote viral entry.

Analyses of Tissue Culture Adaptation of Human Herpesvirus-6A by Whole Genome Deep Sequencing Redefines the Reference Sequence and Identifies Virus Entry Complex Changes

Tissue-culture adaptation of viruses can modulate infection. Laboratory passage and bacterial artificial chromosome (BAC)mid cloning of human cytomegalovirus, HCMV, resulted in genomic deletions and rearrangements altering genes encoding the virus entry complex, which affected cellular tropism, virulence, and vaccine development. Here, we analyse these effects on the reference genome for related betaherpesviruses, Roseolovirus, human herpesvirus 6A (HHV-6A) strain U1102. This virus is also naturally “cloned” by germline subtelomeric chromosomal-integration in approximately 1% of human populations, and accurate references are key to understanding pathological relationships between exogenous and endogenous virus. Using whole genome next-generation deep-sequencing Illumina-based methods, we compared the original isolate to tissue-culture passaged and the BACmid-cloned virus. This re-defined the reference genome showing 32 corrections and 5 polymorphisms. Furthermore, minor variant analyses of passaged and BACmid virus identified emerging populations of a further 32 single nucleotide polymorphisms (SNPs) in 10 loci, half non-synonymous indicating cell-culture selection. Analyses of the BAC-virus genome showed deletion of the BAC cassette via loxP recombination removing green fluorescent protein (GFP)-based selection. As shown for HCMV culture effects, select HHV-6A SNPs mapped to genes encoding mediators of virus cellular entry, including virus envelope glycoprotein genes gB and the gH/gL complex. Comparative models suggest stabilisation of the post-fusion conformation. These SNPs are essential to consider in vaccine-design, antimicrobial-resistance, and pathogenesis.

Making Oncolytic Virotherapy a Clinical Reality: The European Contribution

Oncolytic viruses (OVs) are quickly moving toward the forefront of modern medicines. The reward for the decades of research invested into developing viral platforms that selectively replicate in and lyse tumor cells while sparking anticancer adaptive immunity is presenting in the form of durable therapeutic responses. While this has certainly been a concerted global effort, in this review for the 25th anniversary of the European Society of Gene and Cell Therapy, we focus on the contributions made by European researchers. Research centers across Europe have held central roles in advancing OVs, from the earliest reports of coincidental viral infections leading to antitumor efficacy, to advanced mechanistic studies, and now through Phase I-III trials to imminent regulatory approvals. While challenges still remain, with limitations in preclinical animal models, antiviral immune clearance, and manufacture restrictions enforced by poor viral yields in certain cases, the field has come a very long way in recent years. Thoughtful mechanistic integration of OVs with standard of care strategies and other newly approved therapies should provide potent novel approaches. Combination with immunotherapeutic regimes holds significant promise, and the ability to arm the viral platform with therapeutic proteins for localized expression at the tumor site provides an opportunity for creating highly effective synergistic treatments and brings a new age of targeted cancer therapeutics.

Exploring the Role of RGD-Recognizing Integrins in Cancer

Integrins are key regulators of communication between cells and with their microenvironment. Eight members of the integrin superfamily recognize the tripeptide motif Arg-Gly-Asp (RGD) within extracelluar matrix (ECM) proteins. These integrins constitute an important subfamily and play a major role in cancer progression and metastasis via their tumor biological functions. Such transmembrane adhesion and signaling receptors are thus recognized as promising and well accessible targets for novel diagnostic and therapeutic applications for directly attacking cancer cells and their fatal microenvironment. Recently, specific small peptidic and peptidomimetic ligands as well as antibodies binding to distinct integrin subtypes have been developed and synthesized as new drug candidates for cancer treatment. Understanding the distinct functions and interplay of integrin subtypes is a prerequisite for selective intervention in integrin-mediated diseases. Integrin subtype-specific ligands labelled with radioisotopes or fluorescent molecules allows the characterization of the integrin patterns in vivo and later the medical intervention via subtype specific drugs. The coating of nanoparticles, larger proteins, or encapsulating agents by integrin ligands are being explored to guide cytotoxic reagents directly to the cancer cell surface. These ligands are currently under investigation in clinical studies for their efficacy in interference with tumor cell adhesion, migration/invasion, proliferation, signaling, and survival, opening new treatment approaches in personalized medicine.

Insertion of a ligand to HER2 in gB retargets HSV tropism and obviates the need for activation of the other entry glycoproteins

Herpes simplex virus (HSV) entry into the cells requires glycoproteins gD, gH/gL and gB, activated in a cascade fashion by conformational modifications induced by cognate receptors and intermolecular signaling. The receptors are nectin1 and HVEM (Herpes virus entry mediator) for gD, and αvβ6 or αvβ8 integrin for gH. In earlier work, insertion of a single chain antibody (scFv) to the cancer receptor HER2 (human epidermal growth factor receptor 2) in gD, or in gH, resulted in HSVs specifically retargeted to the HER2-positive cancer cells, hence in highly specific non-attenuated oncolytic agents. Here, the scFv to HER2 was inserted in gB (gBHER2). The insertion re-targeted the virus tropism to the HER2-positive cancer cells. This was unexpected since gB is known to be a fusogenic glycoprotein, not a tropism determinant. The gB-retargeted recombinant offered the possibility to investigate how HER2 mediated entry. In contrast to wt-gB, the activation of the chimeric gBHER2 did not require the activation of the gD and of gH/gL by their respective receptors. Furthermore, a soluble form of HER2 could replace the membrane-bound HER2 in mediating virus entry, hinting that HER2 acted by inducing conformational changes to the chimeric gB. This study shows that (i) gB can be modified and become the major determinant of HSV tropism; (ii) the chimeric gBHER2 bypasses the requirement for receptor-mediated activation of other essential entry glycoproteins.

Herpes simplex virus Membrane Fusion

Herpes simplex virus mediates multiple distinct fusion events during infection. HSV entry is initiated by fusion of the viral envelope with either the limiting membrane of a host cell endocytic compartment or the plasma membrane. In the infected cell during viral assembly, immature, enveloped HSV particles in the perinuclear space fuse with the outer nuclear membrane in a process termed de-envelopment. A cell infected with some strains of HSV with defined mutations spread to neighboring cells by a fusion event called syncytium formation. Two experimental methods, the transient cell-cell fusion approach and fusion from without, are useful surrogate assays of HSV fusion. These five fusion processes are considered in terms of their requirements, mechanism, and regulation. The execution and modulation of these events require distinct yet often overlapping sets of viral proteins and host cell factors. The core machinery of HSV gB, gD, and the heterodimer gH/gL is required for most if not all of the HSV fusion mechanisms.

DNA from Dust: Comparative Genomics of Large DNA Viruses in Field Surveillance Samples

The intensification of the poultry industry over the last 60 years facilitated the evolution of increased virulence and vaccine breaks in Marek's disease virus (MDV-1). Full-genome sequences are essential for understanding why and how this evolution occurred, but what is known about genome-wide variation in MDV comes from laboratory culture. To rectify this, we developed methods for obtaining high-quality genome sequences directly from field samples without the need for sequence-based enrichment strategies prior to sequencing. We applied this to the first characterization of MDV-1 genomes from the field, without prior culture. These viruses were collected from vaccinated hosts that acquired naturally circulating field strains of MDV-1, in the absence of a disease outbreak. This reflects the current issue afflicting the poultry industry, where virulent field strains continue to circulate despite vaccination and can remain undetected due to the lack of overt disease symptoms. We found that viral genomes from adjacent field sites had high levels of overall DNA identity, and despite strong evidence of purifying selection, had coding variations in proteins associated with virulence and manipulation of host immunity. Our methods empower ecological field surveillance, make it possible to determine the basis of viral virulence and vaccine breaks, and can be used to obtain full genomes from clinical samples of other large DNA viruses, known and unknown. IMPORTANCE Despite both clinical and laboratory data that show increased virulence in field isolates of MDV-1 over the last half century, we do not yet understand the genetic basis of its pathogenicity. Our knowledge of genome-wide variation between strains of this virus comes exclusively from isolates that have been cultured in the laboratory. MDV-1 isolates tend to lose virulence during repeated cycles of replication in the laboratory, raising concerns about the ability of cultured isolates to accurately reflect virus in the field. The ability to directly sequence and compare field isolates of this virus is critical to understanding the genetic basis of rising virulence in the wild. Our approaches remove the prior requirement for cell culture and allow direct measurement of viral genomic variation within and between hosts, over time, and during adaptation to changing conditions.

Integrins as Herpesvirus Receptors and Mediators of the Host Signalosome

The repertoire of herpesvirus receptors consists of nonintegrin and integrin molecules. Integrins interact with the conserved glycoproteins gH/gL or gB. This interaction is a conserved biology across the Herpesviridae family, likely directed to promote virus entry and endocytosis. Herpesviruses exploit this interaction to execute a range of critical functions that include (a) relocation of nonintegrin receptors (e.g., herpes simplex virus nectin1 and Kaposi's sarcoma-associated herpesvirus EphA2), or association with nonintegrin receptors (i.e., human cytomegalovirus EGFR), to dictate species-specific entry pathways; (b) activation of multiple signaling pathways (e.g., Ca²⁺ release, c-Src, FAK, MAPK, and PI3K); and (c) association with Rho GTPases, tyrosine kinase receptors, Toll-like receptors, which result in cytoskeletal remodeling, differential cell type targeting, and innate responses. In turn, integrins can be modulated by viral proteins (e.g., Epstein-Barr virus LMPs) to favor spread of transformed cells. We propose that herpesviruses evolved a multipartite entry system to allow interaction with multiple receptors, including integrins, required for their sophisticated life cycle.

Genital Herpes: Insights into Sexually Transmitted Infectious Disease

Etiology, transmission and protection: Herpes simplex virus-2 (HSV-2) is a leading cause of sexually transmitted infections with recurring manifestations throughout the lifetime of infected hosts. Currently no effective vaccines or prophylactics exist that provide complete protection or immunity from the virus, which is endemic throughout the world. Pathology/Symptomatology: Primary and recurrent infections result in lesions and inflammation around the genital area and the latter accounts for majority of genital herpes instances. Immunocompromised patients including neonates are susceptible to additional systemic infections including debilitating consequences of nervous system inflammation. Epidemiology, incidence and prevalence: More than 500 million people are infected worldwide and most reported cases involve the age groups between 16-40 years, which coincides with an increase in sexual activity among this age group. While these numbers are an estimate, the actual numbers may be underestimated as many people are asymptomatic or do not report the symptoms. Treatment and curability: Currently prescribed medications, mostly nucleoside analogs, only reduce the symptoms caused by an active infection, but do not eliminate the virus or reduce latency. Therefore, no cure exists against genital herpes and infected patients suffer from periodic recurrences of disease symptoms for their entire lives. Molecular mechanisms of infection: The last few decades have generated many new advances in our understanding of the mechanisms that drive HSV infection. The viral entry receptors such as nectin-1 and HVEM have been identified, cytoskeletal signaling and membrane structures such as filopodia have been directly implicated in viral entry, host motor proteins and their viral ligands have been shown to facilitate capsid transport and many host and HSV proteins have been identified that help with viral replication and pathogenesis. New understanding has emerged on the role of autophagy and other innate immune mechanisms that are subverted to enhance HSV pathogenesis. This review summarizes our current understanding of HSV-2 and associated diseases and available or upcoming new treatments.

αvβ3 Integrin Boosts the Innate Immune Response Elicited in Epithelial Cells through Plasma Membrane and Endosomal Toll-Like Receptors

We report that αvβ3 integrin strongly affects the innate immune response in epithelial cells. αvβ3 integrin greatly increased the response elicited via plasma membrane Toll-like receptors (TLRs) by herpes simplex virus or bacterial ligands. The endosomal TLR3, not the cytosolic sensor interferon gamma-inducible protein 16 (IFI16), was also boosted by αvβ3 integrin. The boosting was exerted specifically by αvβ3 integrin but not by αvβ6 or αvβ8 integrin. Current and previous work indicates that integrin-TLR cooperation occurs in epithelial and monocytic cells. The TLR response should be considered an integrin-TLR response.

Novel therapeutic investigational strategies to treat severe and disseminated HSV infections suggested by a deeper understanding of in vitro virus entry processes

The global burden of herpes simplex virus (HSV) legitimates the critical need to develop new prevention strategies, such as drugs and vaccines that are able to fight either primary HSV infections or reactivations. Moreover, the ever-growing number of patients receiving transplants increases the number of severe HSV infections that are unresponsive to current therapies. Finally, the high global incidence of genital HSV-2 infection increases the risk of perinatal transmission to newborns, in which disseminated infection or central nervous system (CNS) involvement is frequent, with associated high morbidity and mortality rates. There are several key features shared by novel anti-HSV drugs, from currently available optimized drugs to small molecules able to interfere with various virus replication steps. However, several virological aspects of the disease and associated clinical needs highlight why an ideal anti-HSV drug has yet to be developed.

Retargeting Strategies for Oncolytic Herpes Simplex Viruses

Most of the oncolytic herpes simplex viruses (HSVs) exhibit a high safety profile achieved through attenuation. They carry defects in virulence proteins that antagonize host cell response to the virus, including innate response, apoptosis, authophagy, and depend on tumor cell proliferation. They grow robustly in cancer cells, provided that these are deficient in host cell responses, which is often the case. To overcome the attenuation limits, a strategy is to render the virus highly cancer-specific, e.g., by retargeting their tropism to cancer-specific receptors, and detargeting from natural receptors. The target we selected is HER-2, overexpressed in breast, ovarian and other cancers. Entry of wt-HSV requires the essential glycoproteins gD, gH/gL and gB. Here, we reviewed that oncolytic HSV retargeting was achieved through modifications in gD: the addition of a single-chain antibody (scFv) to HER-2 coupled with appropriate deletions to remove part of the natural receptors' binding sites. Recently, we showed that also gH/gL can be a retargeting tool. The insertion of an scFv to HER-2 at the gH N-terminus, coupled with deletions in gD, led to a recombinant capable to use HER-2 as the sole receptor. The retargeted oncolytic HSVs can be administered systemically by means of carrier cells-forcedly-infected mesenchymal stem cells. Altogether, the retargeted oncolytic HSVs are highly cancer-specific and their replication is not dependent on intrinsic defects of the tumor cells. They might be further modified to express immunomodulatory molecules.

Role of Proteolipid Protein in HSV-1 Entry in Oligodendrocytic Cells

Herpes simplex virus type 1 (HSV-1) has the ability to enter many different hosts and cell types by several strategies. This highly prevalent alphaherpesvirus can enter target cells using different receptors and different pathways: fusion at a neutral pH, low-pH-dependent and low-pH-independent endocytosis. Several cell receptors for viral entry have been described, but several observations suggest that more receptors for HSV-1 might exist. In this work, we propose a novel role for the proteolipid protein (PLP) in HSV-1 entry into the human oligodendrocytic cell line HOG. Cells transfected with PLP-EGFP showed an increase in susceptibility to HSV-1. Furthermore, the infection of HOG and HOG-PLP transfected cells with the R120vGF virus–unable to replicate in ICP4-defficient cells- showed an increase in viral signal in HOG-PLP, suggesting a PLP involvement in viral entry. In addition, a mouse monoclonal antibody against PLP drastically inhibited HSV-1 entry into HOG cells. PLP and virions colocalized in confocal immunofluorescence images, and in electron microscopy images, which suggest that PLP acts at the site of entry into HOG cells. Taken together these results suggest that PLP may be involved in HSV-1 entry in human oligodendrocytic cells.

Herpesvirus gB: A Finely Tuned Fusion Machine

Enveloped viruses employ a class of proteins known as fusogens to orchestrate the merger of their surrounding envelope and a target cell membrane. Most fusogens accomplish this task alone, by binding cellular receptors and subsequently catalyzing the membrane fusion process. Surprisingly, in herpesviruses, these functions are distributed among multiple proteins: the conserved fusogen gB, the conserved gH/gL heterodimer of poorly defined function, and various non-conserved receptor-binding proteins. We summarize what is currently known about gB from two closely related herpesviruses, HSV-1 and HSV-2, with emphasis on the structure of the largely uncharted membrane interacting regions of this fusogen. We propose that the unusual mechanism of herpesvirus fusion could be linked to the unique architecture of gB.