Herpesvirus gB: A Finely Tuned Fusion Machine

Production and characterization of biologicals for disease diagnosis and pathological evaluation of elephant endotheliotropic herpesvirus (EEHV)

Elephant endotheliotropic herpesviruses (EEHV) belong to the family Herpesviridae and cause a highly fatal hemorrhagic infection in elephants. EEHV poses a global threat to the already endangered elephant population. Since EEHV is a non-cultivable virus, there is a scarcity of specific diagnostics, therapeutics, and vaccines. In this study, our objective was to develop biologicals for diagnosis and pathological studies against the most prevalent EEHV1A/1B. We expressed two truncated fragments of the DNA polymerase, glycoprotein B (gB), and glycoprotein (gL) of EEHV in the prokaryotic system. Hyperimmune serum against the purified antigens was raised in rabbits and guinea pigs. We validated the reactivity of this hyperimmune serum using western blotting, ELISA, and immune-histochemistry on known positive infected tissues. Samples collected from 270 animals across various states in India were evaluated with these biologicals. The raised antibodies successfully demonstrated virus in immune-cytochemistry. Additionally, all known positive samples consistently exhibited significant inhibition in the OD values when used in the competitive format of ELISA across all four antigens when compared to the serum collected from known negative animals. An apparent sero-prevalence of 10 % was observed in the randomly collected samples. In summary, our study successfully developed and validated biologicals that will be invaluable for EEHV diagnosis and control.

A review of HSV pathogenesis, vaccine development, and advanced applications

Herpes simplex virus (HSV), an epidemic human pathogen threatening global public health, gains notoriety for its complex pathogenesis that encompasses lytic infection of mucosal cells, latent infection within neurons, and periodic reactivation. This intricate interplay, coupled with HSV's sophisticated immune evasion strategies, gives rise to various diseases, including genital lesions, neonatal encephalitis, and cancer. Despite more than 70 years of relentless research, an effective preventive or therapeutic vaccine against HSV has yet to emerge, primarily due to the limited understanding of virus-host interactions, which in turn impedes the identification of effective vaccine targets. However, HSV's unique pathological features, including its substantial genetic load capacity, high replicability, transmissibility, and neurotropism, render it a promising candidate for various applications, spanning oncolytic virotherapy, gene and immune therapies, and even as an imaging tracer in neuroscience. In this review, we comprehensively update recent breakthroughs in HSV pathogenesis and immune evasion, critically summarize the progress made in vaccine candidate development, and discuss the multifaceted applications of HSV as a biological tool. Importantly, we highlight both success and challenges, emphasizing the critical need for intensified research into HSV, with the aim of providing deeper insights that can not only advance HSV treatment strategies but also broaden its application horizons.

Development of a highly effective combination monoclonal antibody therapy against Herpes simplex virus

BACKGROUND

Infections with Herpes simplex virus (HSV)-1 or -2 usually present as mild chronic recurrent disease, however in rare cases can result in life-threatening conditions with a large spectrum of pathology. Monoclonal antibody therapy has great potential especially to treat infections with virus resistant to standard therapies. HDIT101, a humanized IgG targeting HSV-1/2 gB was previously investigated in phase 2 clinical trials. The aim of this study was to develop a next-generation therapy by combining different antiviral monoclonal antibodies.

METHODS

A lymph-node derived phage display library (LYNDAL) was screened against recombinant gB from Herpes simplex virus (HSV) -1 and HDIT102 scFv was selected for its binding characteristics using bio-layer interferometry. HDIT102 was further developed as fully human IgG and tested alone or in combination with HDIT101, a clinically tested humanized anti-HSV IgG, in vitro and in vivo. T-cell stimulating activities by antigen-presenting cells treated with IgG-HSV immune complexes were analyzed using primary human cells. To determine the epitopes, the cryo-EM structures of HDIT101 or HDIT102 Fab bound to HSV-1F as well as HSV-2G gB protein were solved at resolutions < 3.5 Å.

RESULTS

HDIT102 Fab showed strong binding to HSV-1F gB with Kd of 8.95 × 10-11 M and to HSV-2G gB with Kd of 3.29 × 10-11 M. Neutralization of cell-free virus and inhibition of cell-to-cell spread were comparable between HDIT101 and HDIT102. Both antibodies induced internalization of gB from the cell surface into acidic endosomes by binding distinct epitopes in domain I of gB and compete for binding. CryoEM analyses revealed the ability to form heterogenic immune complexes consisting of two HDIT102 and one HDIT101 Fab bound to one gB trimeric molecule. Both antibodies mediated antibody-dependent phagocytosis by antigen presenting cells which stimulated autologous T-cell activation. In vivo, the combination of HDIT101 and HDIT102 demonstrated synergistic effects on survival and clinical outcome in immunocompetent BALB/cOlaHsd mice.

CONCLUSION

This biochemical and immunological study showcases the potential of an effective combination therapy with two monoclonal anti-gB IgGs for the treatment of HSV-1/2 induced disease conditions.

Oncolytic herpes simplex viruses designed for targeted treatment of EGFR-bearing tumors

Oncolytic herpes simplex viruses (oHSVs) have emerged as leading cancer therapeutic agents. Effective oHSV virotherapy may ultimately require both intratumoral and systemic vector administration to target the primary tumor and distant metastases. An attractive approach to enhancing oHSV tumor specificity is engineering the virus envelope glycoproteins for selective recognition of and infection via tumor-specific cell surface proteins. We previously demonstrated that oHSVs could be retargeted to EGFR-expressing cells by the incorporation of a single-chain antibody (scFv) at the N terminus of glycoprotein D (gD). Here, we compared retargeted oHSVs generated by the insertion of scFv, affibody molecule, or VHH antibody ligands at different positions within the N terminus of gD. When compared to the scFv-directed oHSVs, VHH and affibody molecules mediated enhanced EGFR-specific tumor cell entry, spread and cell killing in vitro, and enabled long-term tumor-specific virus replication following intravenous delivery in vivo. Moreover, oHSVs retargeted via a VHH ligand reduced tumor growth upon intravenous injection and achieved complete tumor destruction after intratumoral injection. Systemic oHSV delivery is important for the treatment of metastatic disease, and our enhancements in targeted oHSV design are a critical step in creating an effective tumor-specific oHSVs for safe administration via the bloodstream.

Expression and fusogenic activity of SARS CoV-2 Spike protein displayed in the HSV-1 Virion

Severe acute respiratory syndrome coronavirus (SARS-CoV) is a zoonotic pathogen that can cause severe respiratory disease in humans. The new SARS-CoV-2 is the cause of the current global pandemic termed coronavirus disease 2019 (COVID-19) that has resulted in many millions of deaths world-wide. The virus is a member of the Betacoronavirus family, its genome is a positive strand RNA molecule that encodes for many genes which are required for virus genome replication as well as for structural proteins that are required for virion assembly and maturation. A key determinant of this virus is the Spike (S) protein embedded in the virion membrane and mediates attachment of the virus to the receptor (ACE2). This protein also is required for cell-cell fusion (syncytia) that is an important pathogenic determinant. We have developed a pseudotyped herpes simplex virus type 1 (HSV-1) recombinant virus expressing S protein in the virion envelop. This virus has also been modified to express a Venus fluorescent protein fusion to VP16, a virion protein of HSV-1. The virus expressing Spike can enter cells and generates large multi-nucleated syncytia which are evident by the Venus fluorescence. The HSV-1 recombinant virus is genetically stable and virus amplification can be easily done by infecting cells. This recombinant virus provides a reproducible platform for Spike function analysis and thus adds to the repertoire of pseudotyped viruses expressing Spike.

Cell Intrinsic Determinants of Alpha Herpesvirus Latency and Pathogenesis in the Nervous System

Alpha herpesvirus infections (α-HVs) are widespread, affecting more than 70% of the adult human population. Typically, the infections start in the mucosal epithelia, from which the viral particles invade the axons of the peripheral nervous system. In the nuclei of the peripheral ganglia, α-HVs establish a lifelong latency and eventually undergo multiple reactivation cycles. Upon reactivation, viral progeny can move into the nerves, back out toward the periphery where they entered the organism, or they can move toward the central nervous system (CNS). This latency-reactivation cycle is remarkably well controlled by the intricate actions of the intrinsic and innate immune responses of the host, and finely counteracted by the viral proteins in an effort to co-exist in the population. If this yin-yang- or Nash-equilibrium-like balance state is broken due to immune suppression or genetic mutations in the host response factors particularly in the CNS, or the presence of other pathogenic stimuli, α-HV reactivations might lead to life-threatening pathologies. In this review, we will summarize the molecular virus-host interactions starting from mucosal epithelia infections leading to the establishment of latency in the PNS and to possible CNS invasion by α-HVs, highlighting the pathologies associated with uncontrolled virus replication in the NS.

Glycoengineered keratinocyte library reveals essential functions of specific glycans for all stages of HSV-1 infection

Viral and host glycans represent an understudied aspect of host-pathogen interactions, despite potential implications for treatment of viral infections. This is due to lack of easily accessible tools for analyzing glycan function in a meaningful context. Here we generate a glycoengineered keratinocyte library delineating human glycosylation pathways to uncover roles of specific glycans at different stages of herpes simplex virus type 1 (HSV-1) infectious cycle. We show the importance of cellular glycosaminoglycans and glycosphingolipids for HSV-1 attachment, N-glycans for entry and spread, and O-glycans for propagation. While altered virion surface structures have minimal effects on the early interactions with wild type cells, mutation of specific O-glycosylation sites affects glycoprotein surface expression and function. In conclusion, the data demonstrates the importance of specific glycans in a clinically relevant human model of HSV-1 infection and highlights the utility of genetic engineering to elucidate the roles of specific viral and cellular carbohydrate structures.

Novel monoclonal antibody-based therapies: implications for the treatment and prevention of HCMV disease

Human cytomegalovirus (HCMV) is an important pathogen worldwide. Although HCMV infection is often asymptomatic in immunocompetent individuals, it can cause severe or even life-threatening symptoms in immunocompromised patients. Due to limitations of antiviral treatments, it is necessary to search for new therapeutic alternatives. Recent studies have highlighted the contribution of antibodies in protecting against HCMV disease, including neutralizing and non-neutralizing antibodies. Given the immunocompromised target population, monoclonal antibodies (mAbs) may represent an alternative to the clinical management of HCMV infection. In this context, we provide a synthesis of recent data revising the literature supporting and arguing about the role of the humoral immunity in controlling HCMV infection. Additionally, we review the state of the art in the development of therapies based on mAbs.

Glycoprotein B Antibodies Completely Neutralize EBV Infection of B Cells

The Epstein-Barr virus (EBV) is the first reported oncogenic herpesvirus that establishes persistent infection in B lymphocytes in 95% of adults worldwide. Glycoprotein B (gB) plays a predominant role in the fusion of the viral envelope with the host cell membrane. Hence, it is of great significance to isolate gB-specific fusion-inhibiting neutralizing antibodies (NAbs). AMMO5 is the only gB NAb but fails to antagonize B-cell infection. It is essential to isolate potent NAbs that can completely block EBV infection of B cells. Using hybridoma technology and neutralization assay, we isolate two gB NAbs 8A9 and 8C12 that are capable of completely neutralizing B-cell infection in vitro. In addition, 8A9 shows cross-reactivity with rhesus lymphocryptovirus (rhLCV) gB. Competitive binding experiments demonstrate that 8A9 and 8C12 recognize novel epitopes that are different from the AMMO5 epitope. The epitopes of 8A9 and 8C12 are mapped to gB D-II, and the AMMO5 epitope is located precisely at gB aa 410-419. We find that 8A9 and 8C12 significantly inhibit gB-derived membrane fusion using a virus-free fusion assay. In summary, this study identifies two gB-specific NAbs that potently block EBV infection of B cells. Our work highlights the importance of gB D-II as a predominant neutralizing epitope, and aids in the rational design of therapeutics or vaccines based on gB.

Neutralizing Antibodies Limit Cell-Associated Spread of Human Cytomegalovirus in Epithelial Cells and Fibroblasts

Human cytomegalovirus (HCMV) can cause severe clinical disease in immunocompromised individuals, such as allograft recipients and infants infected in utero. Neutralizing activity of antibodies, measured as the ability to prevent the entry of cell-free virus, has been correlated with the reduction in HCMV transmission and the severity of HCMV-associated disease. However, in vivo HCMV amplification may occur mainly via cell-to-cell spread. Thus, quantifying the inhibition of cell-to-cell transmission could be important in the evaluation of therapeutic antibodies and/or humoral responses to infection or immunization. Here, we established a quantitative plaque reduction assay, which allowed for the measurement of the capacity of antibodies to limit HCMV spread in vitro. Using an automated fluorescence spot reader, infection progression was assayed by the expansion of viral plaques during the course of infection with various GFP-expressing viruses. We found that in contrast to non-neutralizing monoclonal antibodies (mAbs), neutralizing mAbs against both glycoprotein B and H (gB and gH) could significantly inhibit viral plaque expansion of different HCMV strains and was equally efficient in fibroblasts as in epithelial cells. In contrast, an anti-pentamer mAb was active only in epithelial cells. Taken together, our data demonstrate that specific anti-HCMV mAbs can significantly limit cell-associated virus spread in vitro.

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).

Cell Fusion and Syncytium Formation in Betaherpesvirus Infection

Cell–cell fusion is a fundamental and complex process that occurs during reproduction, organ and tissue growth, cancer metastasis, immune response, and infection. All enveloped viruses express one or more proteins that drive the fusion of the viral envelope with cellular membranes. The same proteins can mediate the fusion of the plasma membranes of adjacent cells, leading to the formation of multinucleated syncytia. While cell–cell fusion triggered by alpha- and gammaherpesviruses is well-studied, much less is known about the fusogenic potential of betaherpesviruses such as human cytomegalovirus (HCMV) and human herpesviruses 6 and 7 (HHV-6 and HHV-7). These are slow-growing viruses that are highly prevalent in the human population and associated with several diseases, particularly in individuals with an immature or impaired immune system such as fetuses and transplant recipients. While HHV-6 and HHV-7 are strictly lymphotropic, HCMV infects a very broad range of cell types including epithelial, endothelial, mesenchymal, and myeloid cells. Syncytia have been observed occasionally for all three betaherpesviruses, both during in vitro and in vivo infection. Since cell–cell fusion may allow efficient spread to neighboring cells without exposure to neutralizing antibodies and other host immune factors, viral-induced syncytia may be important for viral dissemination, long-term persistence, and pathogenicity. In this review, we provide an overview of the viral and cellular factors and mechanisms identified so far in the process of cell–cell fusion induced by betaherpesviruses and discuss the possible consequences for cellular dysfunction and pathogenesis.

A Novel Strain-Specific Neutralizing Epitope on Glycoprotein H of Human Cytomegalovirus

Human cytomegalovirus (HCMV) is a ubiquitous pathogen that causes severe clinical disease in immunosuppressed patients and congenitally infected newborn infants. Viral envelope glycoproteins represent attractive targets for vaccination or passive immunotherapy. To extend the knowledge of mechanisms of virus neutralization, monoclonal antibodies (MAbs) were generated following immunization of mice with HCMV virions. Hybridoma supernatants were screened for in vitro neutralization activity, yielding three potent MAbs, 6E3, 3C11, and 2B10. MAbs 6E3 and 3C11 blocked infection of all viral strains that were tested, while MAb 2B10 neutralized only 50% of the HCMV strains analyzed. Characterization of the MAbs using indirect immunofluorescence analyses demonstrated their reactivity with recombinantly derived gH. While MAbs 6E3 and 3C11 reacted with gH when expressed alone, 2B10 detected gH only when it was coexpressed with gB and gL. Recognition of gH by 3C11 was dependent on the expression of the entire ectodomain of gH, whereas 6E3 required residues 1 to 629 of gH. The strain-specific determinant for neutralization by Mab 2B10 was identified as a single Met→Ile amino acid polymorphism within gH, located within the central part of the protein. The polymorphism is evenly distributed among described HCMV strains. The 2B10 epitope thus represents a novel strain-specific antibody target site on gH of HCMV. The dependence of the reactivity of 2B10 on the simultaneous presence of gB/gH/gL will be of value in the structural definition of this tripartite complex. The 2B10 epitope may also represent a valuable tool for diagnostics to monitor infections/reinfections with different HCMV strains during pregnancy or after transplantation. IMPORTANCE HCMV infections are life threatening to people with compromised or immature immune systems. Understanding the antiviral antibody repertoire induced during HCMV infection is a necessary prerequisite to define protective antibody responses. Here, we report three novel anti-gH MAbs that potently neutralized HCMV infectivity. One of these MAbs (2B10) targets a novel strain-specific conformational epitope on gH that only becomes accessible upon coexpression of the minimal fusion machinery gB/gH/gL. Strain specificity is dependent on a single amino acid polymorphism within gH. Our data highlight the importance of strain-specific neutralizing antibody responses against HCMV. The 2B10 epitope may also represent a valuable tool for diagnostics to monitor infections/reinfections with different HCMV strains during pregnancy or after transplantation. In addition, the dependence of the reactivity of 2B10 on the simultaneous presence of gB/gH/gL will be of value in the structural definition of this tripartite complex.

Adult and Cord Blood-Derived High-Affinity gB-CAR-T Cells Effectively React Against Human Cytomegalovirus Infections

Human cytomegalovirus (HCMV) reactivations are associated with lower overall survival after transplantations. Adoptive transfer of HCMV-reactive expanded or selected T cells can be applied as a compassionate use, but requires that the human leukocyte antigen-matched donor provides memory cells against HCMV. To overcome this, we developed engineered T cells expressing chimeric antigen receptors (CARs) targeted against the HCMV glycoprotein B (gB) expressed upon viral reactivation. Single-chain variable fragments (scFvs) derived from a human high-affinity gB-specific neutralizing monoclonal antibody (SM5-1) were fused to CARs with 4-1BB (BBL) or CD28 (28S) costimulatory domains and subcloned into retroviral vectors. CD4⁺ and CD8⁺ T cells obtained from HCMV-seronegative adult blood or cord blood (CB) transduced with the vectors efficiently expressed the gB-CARs. The specificity and potency of gB-CAR-T cells were demonstrated and compared in vitro using the following: 293T cells expressing gB, and with mesenchymal stem cells infected with a HCMV TB40 strain expressing Gaussia luciferase (HCMV/GLuc). BBL-gB-CAR-T cells generated with adult or CB demonstrated significantly higher in vitro activation and cytotoxicity performance than 28-gB-CAR-T cells. Nod.Rag.Gamma (NRG) mice transplanted with human CB CD34⁺ cells with long-term human immune reconstitution were used to model HCMV/GLuc infection in vivo by optical imaging analyses. One week after administration, response to BBL-gB-CAR-T cell therapy was observed for 5/8 mice, defined by significant reduction of the bioluminescent signal in relation to untreated controls. Response to therapy was sporadically associated with CAR detection in spleen. Thus, exploring scFv derived from the high-affinity gB-antibody SM5-1 and the 4-1BB signaling domain for CAR design enabled an in vitro high on-target effect and cytotoxicity and encouraging results in vivo. Therefore, gB-CAR-T cells can be a future clinical option for treatment of HCMV reactivations, particularly when memory T cells from the donors are not available.

Immune Prophylaxis and Therapy for Human Cytomegalovirus Infection

Human Cytomegalovirus (HCMV) infection is widespread and can result in severe sequelae in susceptible populations. Primary HCMV infection of naïve individuals results in life-long latency characterized by frequent and sporadic reactivations. HCMV infection elicits a robust antibody response, including neutralizing antibodies that can block the infection of susceptible cells in vitro and in vivo. Thus, antibody products and vaccines hold great promise for the prevention and treatment of HCMV, but to date, most attempts to demonstrate their safety and efficacy in clinical trials have been unsuccessful. In this review we summarize publicly available data on these products and highlight new developments and approaches that could assist in successful translation of HCMV immunotherapies.

Antibody binding to native cytomegalovirus glycoprotein B predicts efficacy of the gB/MF59 vaccine in humans

Human cytomegalovirus (CMV) is the most common infectious cause of infant brain damage and posttransplant complications worldwide. Despite the high global burden of disease, vaccine development to prevent infection remains hampered by challenges in generating protective immunity. The most efficacious CMV vaccine candidate tested to date is a soluble glycoprotein B (gB) subunit vaccine with MF59 adjuvant (gB/MF59), which achieved 50% protection in multiple historical phase 2 clinical trials. The vaccine-elicited immune responses that conferred this protection have remained unclear. We investigated the humoral immune correlates of protection from CMV acquisition in populations of CMV-seronegative adolescent and postpartum women who received the gB/MF59 vaccine. We found that gB/MF59 immunization elicited distinct CMV-specific immunoglobulin G (IgG)-binding profiles and IgG-mediated functional responses in adolescent and postpartum vaccinees, with heterologous CMV strain neutralization observed primarily in adolescent vaccinees. Using penalized multiple logistic regression analysis, we determined that protection against primary CMV infection in both cohorts was associated with serum IgG binding to gB present on a cell surface but not binding to the soluble vaccine antigen, suggesting that IgG binding to cell-associated gB is an immune correlate of vaccine efficacy. Supporting this, we identified gB-specific monoclonal antibodies that differentially recognized soluble or cell-associated gB, revealing that there are structural differences in cell-associated and soluble gB are relevant to the generation of protective immunity. Our results highlight the importance of the native, cell-associated gB conformation in future CMV vaccine design.

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.

Role of Envelope Glycoprotein Complexes in Cell-Associated Spread of Human Cytomegalovirus

The role of viral envelope glycoproteins, particularly the accessory proteins of trimeric and pentameric gH/gL-complexes, in cell-associated spread of human cytomegalovirus (HCMV) is unclear. We aimed to investigate their contribution in the context of HCMV variants that grow in a strictly cell-associated manner. In the genome of Merlin pAL1502, the glycoproteins gB, gH, gL, gM, and gN were deleted by introducing stop codons, and the mutants were analyzed for viral growth. Merlin and recent HCMV isolates were compared by quantitative immunoblotting for expression of accessory proteins of the trimeric and pentameric gH/gL-complexes, gO and pUL128. Isolates were treated with siRNAs against gO and pUL128 and analyzed regarding focal growth and release of infectious virus. All five tested glycoproteins were essential for growth of Merlin pAL1502. Compared with this model virus, higher gO levels were measured in recent isolates of HCMV, and its knockdown decreased viral growth. Knockdown of pUL128 abrogated the strict cell-association and led to release of infectivity, which allowed cell-free transfer to epithelial cells where the virus grew again strictly cell-associated. We conclude that both trimer and pentamer contribute to cell-associated spread of recent clinical HCMV isolates and downregulation of pentamer can release infectious virus into the supernatant.

Development of a Colloidal Gold Immunochromatographic Assay for Duck Enteritis Virus Detection Using Monoclonal Antibodies

Duck viral enteritis is a highly contagious and fatal disease of commercial waterfowl flocks. The disease occurs sporadically or epizootically in mainland China due to insufficient vaccinations. Early and rapid diagnosis is important for preventive intervention and the control of epizootic events in clinical settings. In this study, we generated two monoclonal antibodies (MAbs) that specifically recognized the duck enteritis virus (DEV) envelope glycoprotein B and tegument protein UL47, respectively. Using these MAbs, a colloidal gold-based immunochromatographic assay (ICA) was developed for the efficient detection of DEV antigens within 15 min. Our results showed that the detection limit of the developed ICA strip was 2.52 × 10³ TCID₅₀/mL for the virus infected cell culture suspension with no cross-reactivity with other pathogenic viruses commonly encountered in commercially raised waterfowl. Using samples from experimentally infected ducks, we demonstrated that the ICA detected the virus in cloacal swab samples on day three post-infection, demonstrating an 80% concordance with the PCR. For tissue homogenates from ducks succumbing to infection, the detection sensitivity was 100%. The efficient and specific detection by this ICA test provides a valuable, convenient, easy to use and rapid diagnostic tool for DVE under both laboratory and field conditions.

Receptors in host pathogen interactions between human cytomegalovirus and the placenta during congenital infection

Cellular receptors in human cytomegalovirus (HCMV) mother to child transmission play an important role in congenital infection. Placental trophoblast cells are a significant cell type in placental development, placental functional processes, and in HCMV transmission. Different cells within the placental floating and chorionic villi present alternate receptors for HCMV cell entry. Syncytiotrophoblasts present neonatal Fc receptors that bind and transport circulating maternal immunoglobulin G across the placental interface which can also be bound to HCMV virions, facilitating viral entry into the placenta and foetal circulation. Cytotrophoblast express HCMV receptors including integrin-α1β1, integrin-αVβ3, epidermal growth factor receptor and platelet-derived growth factor receptor alpha. The latter interacts with HCMV glycoprotein-H, glycoprotein-L and glycoprotein-O (gH/gL/gO) trimers (predominantly in placental fibroblasts) and the gH/gL/pUL128, UL130-UL131A pentameric complex in other placental cell types. The pentameric complex allows viral tropism of placental trophoblasts, endothelial cells, epithelial cells, leukocytes and monocytes. This review outlines HCMV ligands and target receptor proteins in congenital HCMV infection.

Contributions of the Four Essential Entry Glycoproteins to HSV-1 Tropism and the Selection of Entry Routes

Herpes simplex viruses (HSV-1 and HSV-2) encode up to 16 envelope proteins, four of which are essential for entry. However, whether these four proteins alone are sufficient to dictate the broad cellular tropism of HSV-1 and the selection of different cell type-dependent entry routes is unknown. To begin addressing this, we previously pseudotyped vesicular stomatitis virus (VSV), lacking its native glycoprotein G, with only the four essential entry glycoproteins of HSV-1: gB, gH, gL, and gD. This novel VSVΔG-BHLD pseudotype recapitulated several important features of HSV-1 entry: the requirement for gB, gH, gL, gD, and a cellular receptor and sensitivity to anti-gB and anti-gH/gL neutralizing antibodies. However, due to the use of a single cell type in that study, the tropism of the VSVΔG-BHLD pseudotype was not investigated. Here, we show that the cellular tropism of the pseudotype is severely limited compared to that of wild-type HSV-1 and that its entry pathways differ from the native HSV-1 entry pathways. To test the hypothesis that other HSV-1 envelope proteins may contribute to HSV-1 tropism, we generated a derivative pseudotype containing the HSV-1 glycoprotein C (VSVΔG-BHLD-gC) and observed a gC-dependent increase in entry efficiency in two cell types. We propose that the pseudotyping platform developed here has the potential to uncover functional contributions of HSV-1 envelope proteins to entry in a gain-of-function manner.IMPORTANCE Herpes simplex viruses (HSV-1 and HSV-2) contain up to 16 different proteins in their envelopes. Four of these, glycoproteins gB, gD, gH, and gL, are termed essential with regard to entry, whereas the rest are typically referred to as nonessential based on the entry phenotypes of the respective single genetic deletions. However, the single-gene deletion approach, which relies on robust loss-of-function phenotypes, may be confounded by functional redundancies among the many HSV-1 envelope proteins. We have developed a pseudotyping platform in which the essential four entry glycoproteins are isolated from the rest, which can be added back individually for systematic gain-of-function entry experiments. Here, we show the utility of this platform for dissecting the contributions of HSV envelope proteins, both the essential four and the remaining dozen (using gC as an example), to HSV entry.

Elephant Endotheliotropic Herpesvirus Is Omnipresent in Elephants in European Zoos and an Asian Elephant Range Country

Elephant endotheliotropic herpesviruses (EEHVs) may cause acute, often lethal, hemorrhagic disease (EEHV-HD) in young elephants. Prevalence of EEHV in different elephant populations is still largely unknown. In order to improve diagnostic tools for the detection of EEHV infections and to obtain insight into its spread among elephants, we developed novel ELISAs based on EEHV1A gB and gH/gL. Performance of the ELISAs was assessed using sera from 41 European zoo elephants and 69 semi-captive elephants from Laos, one of the Asian elephant range countries. Sera from all (sub)adult animals tested (≥5 years of age) showed high reactivity with both gB and gH/gL, indicating that EEHV prevalence has been highly underestimated so far. Reactivity towards the antigens was generally lower for sera of juvenile animals (1 > 5 years). Only one (juvenile) animal, which was sampled directly after succumbing to EEHV-HD, was found to be seronegative for EEHV. The two other EEHV-HD cases tested showed low antibody levels, suggesting that all three cases died upon a primary EEHV infection. In conclusion, our study suggests that essentially all (semi-)captive (sub)adult elephants in European zoos and in Laos carry EEHV, and that young elephants with low antibody levels are at risk of dying from EEHV-HD.

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.

Immunogenicity and protective efficacy induced by an mRNA vaccine encoding gD antigen against pseudorabies virus infection

Messenger RNA-based vaccines represent new tools with prophylactic and therapeutic potential characterized by high flexibility of application for infectious diseases. Pseudorabies virus (PRV) is one of the major viruses affecting the pig industry. PRV has serious effects in piglets, sows, and growing-fattening pigs and can lead to huge economic losses. In this study, an envelope glycoprotein D (gD) gene-based specific mRNA vaccine was generated, and a mouse model was used to investigate the protective efficacy of the vaccine. The gD mRNA vaccine and the recombinant plasmid pVAX-gD were transfected into BHK21 cells, and the antigenicity of the expressed proteins was detected by Western blot analysis. Groups of mice were vaccinated with the gD mRNA vaccine, pVAX-gD, and PBS. T cell immune responses were measured by flow cytometry or ELISA and serum neutralization tests every two weeks. The challenge with the PRV-XJ strain was performed eight weeks after the primary immunization, and the response was monitored for 15 days. The levels of specific and neutralizing antibodies in the gD mRNA vaccine group were significantly increased in 8 weeks compared to those in the control group, and cytokine levels, including that of IFN-γ/IL-2, were considerably higher than those in the control animal. Additionally, the proportion of CD4+/CD8+ cells in peripheral lymphocytes was remarkably increased. Our data demonstrate that mRNA is a promising and effective tool for the development of vaccines. The PRV-gD-based mRNA vaccine can elicit an efficient neutralizing antibody response and induce effective protection in mice in defense against PRV infection.

The structural basis of herpesvirus entry

Herpesviruses are ubiquitous, double-stranded DNA, enveloped viruses that establish lifelong infections and cause a range of diseases. Entry into host cells requires binding of the virus to specific receptors, followed by the coordinated action of multiple viral entry glycoproteins to trigger membrane fusion. Although the core fusion machinery is conserved for all herpesviruses, each species uses distinct receptors and receptor-binding glycoproteins. Structural studies of the prototypical herpesviruses herpes simplex virus 1 (HSV-1), HSV-2, human cytomegalovirus (HCMV) and Epstein-Barr virus (EBV) entry glycoproteins have defined the interaction sites for glycoprotein complexes and receptors, and have revealed conformational changes that occur on receptor binding. Recent crystallography and electron microscopy studies have refined our model of herpesvirus entry into cells, clarifying both the conserved features and the unique features. In this Review, we discuss recent insights into herpesvirus entry by analysing the structures of entry glycoproteins, including the diverse receptor-binding glycoproteins (HSV-1 glycoprotein D (gD), EBV glycoprotein 42 (gp42) and HCMV gH-gL-gO trimer and gH-gL-UL128-UL130-UL131A pentamer), as well gH-gL and the fusion protein gB, which are conserved in all herpesviruses.

Understanding the Activated Form of a Class-I Fusion Protein: Modeling the Interaction of the Ebola Virus Glycoprotein 2 with a Lipid Bilayer

The fusion of the viral and target cell membranes is a key step in the life cycle of all enveloped viruses. Here, a range of structural data is used to generate an evidence-based model of the active conformation of an archetypical type-I fusion protein, the Ebola glycoprotein 2 (GP2). The stability of the trimeric complex is demonstrated using molecular dynamics and validated by simulating the interaction of the complex with a lipid bilayer. In this model, the fusion peptides project away from the central helix bundle parallel to the target membrane. This maximizes contact with the host membrane, enhances lateral stability, and would explain why, when activated, viral fusion proteins are trimeric.

Involvement of autophagy in MHC class I antigen presentation

MHC class I molecules on the cellular surface display peptides that either derive from endogenous proteins (self or viral), or from endocytosis of molecules, dying cells or pathogens. The conventional antigen‐processing pathway for MHC class I presentation depends on proteasome‐mediated degradation of the protein followed by transporter associated with antigen‐processing (TAP)‐mediated transport of the generated peptides into the endoplasmic reticulum (ER). Here, peptides are loaded onto MHC I molecules before transportation to the cell surface. However, several alternative mechanisms have emerged. These include TAP‐independent mechanisms, the vacuolar pathway and involvement of autophagy. Autophagy is a cell intrinsic recycling system. It also functions as a defence mechanism that removes pathogens and damaged endocytic compartments from the cytosol. Therefore, it appears likely that autophagy would intersect with the MHC class I presentation pathway to alarm CD8⁺ T cells of an ongoing intracellular infection. However, the importance of autophagy as a source of antigen for presentation on MHC I molecules remains to be defined. Here, original research papers which suggest involvement of autophagy in MHC I antigen presentation are reviewed. The antigens are from herpesvirus, cytomegalovirus and chlamydia. The studies point towards autophagy as important in MHC class I presentation of endogenous proteins during conditions of immune evasion. Because autophagy is a regulated process which is induced upon activation of, for example, pattern recognition receptors (PRRs), it will be crucial to use relevant stimulatory conditions together with primary cells when aiming to confirm the importance of autophagy in MHC class I antigen presentation in future studies.

Combating Actions of Green 2D-Materials on Gram Positive and Negative Bacteria and Enveloped Viruses

Interactions of novel bi-dimensional nanomaterials and live matter such as bacteria and viruses represent an extremely hot topic due to the unique properties of the innovative nanomaterials, capable in some cases to exhibit bactericide and antiviral actions. The interactions between bacteria and viruses and two dimensional nanosheets are here investigated. We extensively studied the interaction between a gram-negative bacterium, Escherichia coli, and a gram-positive bacterium, Staphylococcus aureus, with two different types of 2D nanoflakes such as MoS₂, belonging to the Transition Metal Dichalcogenides family, and Graphene Oxide. The same two types of nanomaterials were employed to study their antiviral action toward the Herpes simplex virus type-1, (HSV-1). The experimental results showed different bactericide impacts as well as different antiviral power between the two nanomaterials. The experimental findings were interpreted in bacteria on the base of the Derjaguin–Landau–Verwey–Overbeek theory. A simple kinetic model of bacterial growth in the presence of the interacting nanosheets is also elaborated, to explain the observed results. The experimental results in viruses are really novel and somewhat surprising, evidencing a stronger antiviral action of Graphene Oxide as compared to MoS₂. Results in viruses are complicated to quantitatively interpret due to the complexity of the system under study, constituted by virus/host cell and nanoflake, and due to the lack of a well assessed theoretical context to refer to. Thus, these results are interpreted in terms of qualitative arguments based on the chemical properties of the interactors in the given solvent medium.

Cell Fusion Induced by a Fusion-Active Form of Human Cytomegalovirus Glycoprotein B (gB) Is Inhibited by Antibodies Directed at Antigenic Domain 5 in the Ectodomain of gB

Human cytomegalovirus (HCMV) is a ubiquitous pathogen that can cause severe clinical disease in allograft recipients and infants infected in utero Virus-neutralizing antibodies defined in vitro have been proposed to confer protection against HCMV infection, and the virion envelope glycoprotein B (gB) serves as a major target of neutralizing antibodies. The viral fusion protein gB is nonfusogenic on its own and requires glycoproteins H (gH) and L (gL) for membrane fusion, which is in contrast to requirements of related class III fusion proteins, including vesicular stomatitis virus glycoprotein G (VSV-G) or baculovirus gp64. To explore requirements for gB's fusion activity, we generated a set of chimeras composed of gB and VSV-G or gp64, respectively. These gB chimeras were intrinsically fusion active and led to the formation of multinucleated cell syncytia when expressed in the absence of other viral proteins. Utilizing a panel of virus-neutralizing gB-specific monoclonal antibodies (MAbs), we could demonstrate that syncytium formation of the fusogenic gB/VSV-G chimera can be significantly inhibited by only a subset of neutralizing MAbs which target antigenic domain 5 (AD-5) of gB. This observation argues for differential modes of action of neutralizing anti-gB MAbs and suggests that blocking the membrane fusion function of gB could be one mechanism of antibody-mediated virus neutralization. In addition, our data have important implications for the further understanding of the conformation of gB that promotes membrane fusion as well as the identification of structures in AD-5 that could be targeted by antibodies to block this early step in HCMV infection.IMPORTANCE HCMV is a major global health concern, and antiviral chemotherapy remains problematic due to toxicity of available compounds and the emergence of drug-resistant viruses. Thus, an HCMV vaccine represents a priority for both governmental and pharmaceutical research programs. A major obstacle for the development of a vaccine is a lack of knowledge of the nature and specificities of protective immune responses that should be induced by such a vaccine. Glycoprotein B of HCMV is an important target for neutralizing antibodies and, hence, is often included as a component of intervention strategies. By generation of fusion-active gB chimeras, we were able to identify target structures of neutralizing antibodies that potently block gB-induced membrane fusion. This experimental system provides an approach to screen for antibodies that interfere with gB's fusogenic activity. In summary, our data will likely contribute to both rational vaccine design and the development of antibody-based therapies against HCMV.

Tetrameric glycoprotein complex gH/gL/gQ1/gQ2 is a promising vaccine candidate for human herpesvirus 6B

Primary infection of human herpesvirus 6B (HHV-6B) occurs in infants after the decline of maternal immunity and causes exanthema subitum accompanied by a high fever, and it occasionally develops into encephalitis resulting in neurological sequelae. There is no effective prophylaxis for HHV-6B, and its development is urgently needed. The glycoprotein complex gH/gL/gQ1/gQ2 (called 'tetramer of HHV-6B') on the virion surface is a viral ligand for its cellular receptor human CD134, and their interaction is thus essential for virus entry into the cells. Herein we examined the potency of the tetramer as a vaccine candidate against HHV-6B. We designed a soluble form of the tetramer by replacing the transmembrane domain of gH with a cleavable tag, and the tetramer was expressed by a mammalian cell expression system. The expressed recombinant tetramer is capable of binding to hCD134. The tetramer was purified to homogeneity and then administered to mice with aluminum hydrogel adjuvant and/or CpG oligodeoxynucleotide adjuvant. After several immunizations, humoral and cellular immunity for HHV-6B was induced in the mice. These results suggest that the tetramer together with an adjuvant could be a promising candidate HHV-6B vaccine.

Human herpesvirus 6B (HHV-6B) is known as the cause of the common childhood febrile illness exanthem subitum in its primary infection, and it develops into a lifelong latent infection in almost all individuals. Severe complications such as meningitis and encephalitis can occur in both the primary infection and reactivation. There is no established treatment or vaccine. The tetrameric glycoprotein complex gH/gL/gQ1/gQ2 (tetramer) on the viral envelope is the ligand for the entry of HHV-6B, which is the critical part for its infection. Here, we established a soluble form of the tetramer and purified it to homogeneity. After several immunizations of tetramer along with different combinations of adjuvants in mice, we observed that it greatly induced defensive immunity against HHV-6B, indicating that the tetramer has the potential to become a vaccine candidate. Moreover, our results also revealed that combinations of distinct adjuvants with the tetramer would be useful as an HHV-6B vaccine strategy for different purposes.

High Preventive Effect of G2-S16 Anionic Carbosilane Dendrimer against Sexually Transmitted HSV-2 Infection

Anionic carbosilane dendrimers such as G2-S16 are very effective in preventing HSV-2 infection both in vitro and in vivo. We present the main achievements obtained for the G2-S16 dendrimer in vivo, especially related to its efficacy against HSV-2 infection. Moreover, we discuss the mechanisms by which the G2-S16 dendrimer applied vaginally as a topical microbicide has been demonstrated to be safe and harmless for the vaginal microbiome balance, as both conditions present an essential step that has to be overcome during microbicide development. This review points to the marked protective effect of the G2-S16 dendrimer against sexually transmitted HSV-2 infection, supporting its role as a possible microbicide against HSV-2 infection.

Polymorphisms in Human Cytomegalovirus Glycoprotein O (gO) Exert Epistatic Influences on Cell-Free and Cell-to-Cell Spread and Antibody Neutralization on gH Epitopes

Human cytomegalovirus (HCMV) glycoproteins H and L (gH/gL) can be bound by either gO or the UL128 to UL131 proteins (referred to here as UL128-131) to form complexes that facilitate entry and spread, and the complexes formed are important targets of neutralizing antibodies. Strains of HCMV vary considerably in the levels of gH/gL/gO and gH/gL/UL128-131, and this can impact infectivity and cell tropism. In this study, we investigated how natural interstrain variation in the amino acid sequence of gO influences the biology of HCMV. Heterologous gO recombinants were constructed in which 6 of the 8 alleles or genotypes (GT) of gO were analyzed in the backgrounds of strains TR and Merlin (ME). The levels of gH/gL complexes were not affected, but there were impacts on entry, spread, and neutralization by anti-gH antibodies. AD169 (AD) gO (GT1a) [referred to here as ADgO(GT1a)] drastically reduced cell-free infectivity of both strains on fibroblasts and epithelial cells. PHgO(GT2a) increased cell-free infectivity of TR in both cell types, but spread in fibroblasts was impaired. In contrast, spread of ME in both cell types was enhanced by Towne (TN) gO (GT4), despite similar cell-free infectivity. TR expressing TNgO(GT4) was resistant to neutralization by anti-gH antibodies AP86 and 14-4b, whereas ADgO(GT1a) conferred resistance to 14-4b but enhanced neutralization by AP86. Conversely, ME expressing ADgO(GT1a) was more resistant to 14-4b. These results suggest that (i) there are mechanistically distinct roles for gH/gL/gO in cell-free and cell-to-cell spread, (ii) gO isoforms can differentially shield the virus from neutralizing antibodies, and (iii) effects of gO polymorphisms are epistatically dependent on other variable loci.IMPORTANCE Advances in HCMV population genetics have greatly outpaced understanding of the links between genetic diversity and phenotypic variation. Moreover, recombination between genotypes may shuffle variable loci into various combinations with unknown outcomes. UL74(gO) is an important determinant of HCMV infectivity and one of the most diverse loci in the viral genome. By analyzing interstrain heterologous UL74(gO) recombinants, we showed that gO diversity can have dramatic impacts on cell-free and cell-to-cell spread as well as on antibody neutralization and that the manifestation of these impacts can be subject to epistatic influences of the global genetic background. These results highlight the potential limitations of laboratory studies of HCMV biology that use single, isolated genotypes or strains.

Fatal Elephant Endotheliotropic Herpesvirus Infection of Two Young Asian Elephants

Elephant endotheliotropic herpesvirus (EEHV) can cause a devastating haemorrhagic disease in young Asian elephants worldwide. Here, we report the death of two young Asian elephants after suffering from acute haemorrhagic disease due to EEHV-1A infection. We detected widespread distribution of EEHV-1A in various organs and tissues of the infected elephants. Enveloped viral particles accumulated within and around cytoplasmic electron-dense bodies in hepatic endothelial cells were detected. Attempts to isolate the virus on different cell cultures showed limited virus replication; however, late viral protein expression was detected in infected cells. We further showed that glycoprotein B (gB) of EEHV-1A possesses a conserved cleavage site Arg-X-Lys/Arg-Arg that is targeted by the cellular protease furin, similar to other members of the Herpesviridae. We have determined the complete 180 kb genome sequence of EEHV-1A isolated from the liver by next-generation sequencing and de novo assembly. As virus isolation in vitro has been unsuccessful and limited information is available regarding the function of viral proteins, we have attempted to take the initial steps in the development of suitable cell culture system and virus characterization. In addition, the complete genome sequence of an EEHV-1A in Europe will facilitate future studies on the epidemiology and diagnosis of EEHV infection in elephants.

Highly Effective and Safe Polymeric Inhibitors of Herpes Simplex Virus in Vitro and in Vivo

A series of poly(ethylene glycol)-block-poly(3-(methacryloylamino)propyl trimethylammonium chloride) (PEG-b-PMAPTAC) water-soluble block copolymers consisting of PEG and PMPTAC were obtained by reversible addition-fragmentation chain-transfer (RAFT) polymerization and demonstrated to function as highly effective herpes simplex virus type 1 (HSV-1) inhibitors as shown by in vitro tests (Vero E6 cells) and in vivo experiments (mouse model). Half-maximal inhibitory concentration (IC₅₀) values were determined by quantitative polymerase chain reaction to be 0.36 ± 0.08 μg/mL for the most effective polymer PEG₄₅-b-PMAPTAC₅₂ and 0.84 ± 1.24 μg/mL for the less effective one, PEG₄₅-b-PMAPTAC₇₄. The study performed on the mouse model showed that the polymers protect mice from lethal infection. The polymers are not toxic to the primary human skin fibroblast cells up to the concentration of 100 μg/mL and to the Vero E6 cells up to 500 μg/mL. No systemic or topical toxicity was observed in vivo, even with mice treated with concentrated formulation (100 mg/mL). The mechanistic studies indicated that polymers interacted with the cell and blocked the formation of the entry/fusion complex. Physicochemical and biological properties of PEG_x-b-PMAPTAC_y make them promising drug candidates.

Differential Requirements for gE, gI, and UL16 among Herpes Simplex Virus 1 Syncytial Variants Suggest Unique Modes of Dysregulating the Mechanism of Cell-to-Cell Spread

Like all the herpesviruses, herpes simplex virus encodes machinery that enables it to move through cell junctions to avoid neutralizing antibodies. This cell-to-cell spread mechanism requires the viral fusion machinery (gD, gH/gL, and gB) and numerous accessory proteins. Of all of these, minor alterations to only four proteins (gB, gK, UL20, or UL24) will dysregulate the fusion machinery, allowing the formation of syncytia. In contrast, removal of individual accessory proteins will block cell-to-cell spread, forcing the virus to transmit in a cell-free manner. In the context of a Syn variant, removal of a required accessory protein will block cell fusion, again forcing cell-free spread. This has been investigated most thoroughly for gBsyn variants, which lose their syncytial phenotype in the absence of several accessory proteins, including gE, gI, UL16, and UL21, which are known to physically interact. Recently it was found that UL21 is not needed for gKsyn-, UL20syn-, or UL24syn-induced cell fusion, and hence it was of interest to ascertain whether gE, gI, and UL16 are required for Syn variants other than gBsyn. Null mutants of these were each combined with seven syncytial variants distributed among gK, UL20, and UL24. Surprisingly, very different patterns of accessory protein requirements were revealed. Indeed, for the three gKsyn variants tested, two different patterns were found. Also, three mutants were able to replicate without causing cytopathic effects. These findings show that mutations that produce Syn variants dysregulate the cell-to-cell-spread machinery in unique ways and provide clues for elucidating how this virus moves between cells.IMPORTANCE Approximately 2/3 of adults worldwide are latently infected with herpes simplex virus 1. Upon reactivation, the virus has the ability to evade neutralizing antibodies by moving through cell junctions, but the mechanism of direct cell-to-cell spread is poorly understood. The machinery that assembles between cells includes the viral fusion proteins and various accessory proteins that prevent cells from fusing. Alterations in four proteins will dysregulate the machinery, allowing neighboring cells to fuse to make syncytia, but this can be prevented by removing various individual accessory proteins to further disable the machinery. Previously, the accessory protein UL21 was found to be important for the activity of some syncytial variants but not others. In this study, we discovered that UL16, gE, and gI all act differently in how they control the fusion machinery. A better understanding of the mechanism of cell-to-cell spread may enable the development of drugs that block it.

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.

Entry of betaherpesviruses

In this chapter, we present an overview on betaherpesvirus entry, with a focus on human cytomegalovirus, human herpesvirus 6A and human herpesvirus 6B. Human cytomegalovirus (HCMV) is a complex human pathogen with a genome of 235kb encoding more than 200 genes. It infects a broad range of cell types by switching its viral ligand on the virion, using the trimer gH/gL/gO for infection of fibroblasts and the pentamer gH/gL/UL128/UL130/UL131 for infection of other cells such as epithelial and endothelial cells, leading to membrane fusion mediated by the fusion protein gB. Adding to this scenario, however, accumulating data reveal the actual complexity in the viral entry process of HCMV with an intricate interplay among viral and host factors. Key novel findings include the identification of entry receptors platelet-derived growth factor-α receptor (PDGFRα) and Netropilin-2 (Nrp2) for trimer and pentamer, respectively, the determination of atomic structures of the fusion protein gB and the pentamer, and the in situ visualization of the state and arrangement of functional glycoproteins on virion. This is covered in the first part of this review. The second part focusses on HHV-6 which is a T lymphotropic virus categorized as two distinct virus species, HHV-6A and HHV-6B based on differences in epidemiological, biological, and immunological aspects, although homology of their entire genome sequences is nearly 90%. HHV-6B is a causative agent of exanthema subitum (ES), but the role of HHV-6A is unknown. HHV-6B reactivation occasionally causes encephalitis in patients with hematopoietic stem cell transplant. The HHV-6 specific envelope glycoprotein complex, gH/gL/gQ1/gQ2 is a viral ligand for the entry receptor. Recently, each virus has been found to recognize a different cellular receptor, CD46 for HHV 6A amd CD134 for HHV 6B. These findings show that distinct receptor recognition differing between both viruses could explain their different pathogenesis.

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.

Analysis of herpes simplex type 1 gB, gD, and gH/gL on production of infectious HIV-1: HSV-1 gD restricts HIV-1 by exclusion of HIV-1 Env from maturing viral particles

BACKGROUND

We previously showed that the gM of HSV-1 could restrict the release of infectious HIV-1 from cells. In this study, we analyzed if the four HSV-1 glycoproteins (gD, gB, and gH/gL), which are the minimum glycoproteins required for HSV-1 entry, restricted the release of infectious HIV-1.

RESULTS

Of these four glycoproteins, gD and gH/gL restricted the production of infectious HIV-1 from cells transfected with an infectious molecular clone of HIV-1 (strain NL4-3) while gB had no significant effect. Pulse-chase analyses indicated that gD did not affect the biosynthesis and processing of gp160 into gp120/gp41, the transport of the gp120/gp41 to the cell surface, or the release of HIV-1 particles from the cell surface. Our analyses revealed that gD was incorporated into HIV-1 virus particles while gp120/gp41 was excluded from released virus particles. Truncated mutants of gD revealed that the cytoplasmic domain was dispensable but that a membrane bound gD was required for the restriction of release of infectious HIV-1. Finally, cell lines expressing gD also potently restricted the release of infectious virus.

CONCLUSIONS

Due to its ability to exclude HIV-1 gp120/gp41 from maturing virus, gD may provide a useful tool in deciphering mechanisms of Env incorporation into maturing virus particles.

Glycoprotein D of HSV-1 is dependent on tegument protein UL16 for packaging and contains a motif that is differentially required for syncytia formation

Glycoprotein D (gD) of herpes simplex virus type 1 (HSV-1) plays a key role in multiple events during infection including virus entry, cell-to-cell spread, and virus-induced syncytia formation. Here, we provide evidence that an arginine/lysine cluster located at the transmembrane-cytoplasm interface of gD critically contributes to viral spread and cell-cell fusion. Our studies began with the discovery that packaging of gD into virions is almost completely blocked in the absence of tegument protein UL16. We subsequently identified a novel, direct, and regulated interaction between UL16 and gD, but this was not important for syncytia formation. However, a mutational analysis of the membrane-proximal basic residues of gD revealed that they are needed for the gBsyn phenotype, salubrinal-induced fusion of HSV-infected cells, and cell-to-cell spread. Finally, we found that these same gD tail basic residues are not required for cell fusion induced by a gKsyn variant.

Natural Selection of Glycoprotein B Mutations That Rescue the Small-Plaque Phenotype of a Fusion-Impaired Herpes Simplex Virus Mutant

Glycoprotein B (gB) is a conserved viral fusion protein that is required for herpesvirus entry. To mediate fusion with the cellular membrane, gB refolds from a prefusion to a postfusion conformation. We hypothesize that an interaction between the C-terminal arm and the central coiled coil of the herpes simplex virus 1 (HSV-1) gB ectodomain is critical for fusion. We previously reported that three mutations in the C-terminal arm (I671A/H681A/F683A, called gB3A) greatly reduced cell-cell fusion and that virus carrying these mutations had a small-plaque phenotype and delayed entry into cells. By serially passaging gB3A virus, we selected three revertant viruses with larger plaques. These revertant viruses acquired mutations in gB that restore the fusion function of gB3A, including gB-A683V, gB-S383F/G645R/V705I/A855V, and gB-T509M/N709H. V705I and N709H are novel mutations that map to the portion of domain V that enters domain I in the postfusion structure. S383F, G645R, and T509M are novel mutations that map to an intersection of three domains in a prefusion model of gB. We introduced these second-site mutations individually and in combination into wild-type gB and gB3A to examine the impact of the mutations on fusion and expression. V705I and A855V (a known hyperfusogenic mutation) restored the fusion function of gB3A, whereas S383F and G645R dampened fusion and T509M and N709H worked in concert to restore gB3A fusion. The results identify two regions in the gB ectodomain that modulate the fusion activity of gB, potentially by impacting intramolecular interactions and stability of the prefusion and/or postfusion gB trimer.IMPORTANCE Glycoprotein B (gB) is an essential viral protein that is conserved in all herpesviruses and is required for virus entry. gB is thought to undergo a conformational change that provides the energy to fuse the viral and cellular membranes; however, the details of this conformational change and the structure of the prefusion and intermediate conformations of gB are not known. Previously, we demonstrated that mutations in the gB "arm" region inhibit fusion and impart a small-plaque phenotype. Using serial passage of a virus carrying these mutations, we identified revertants with restored plaque size. The revertant viruses acquired novel mutations in gB that restored fusion function and mapped to two sites in the gB ectodomain. This work supports our hypothesis that an interaction between the gB arm and the core of gB is critical for gB refolding and provides details about the function of gB in herpesvirus-mediated fusion and subsequent virus entry.

A new era in cytomegalovirus vaccinology: considerations for rational design of next-generation vaccines to prevent congenital cytomegalovirus infection

Human cytomegalovirus (HCMV), a member of the beta-herpesvirus family, is the most common cause of congenital infection worldwide as well as an important cause of morbidity in transplant recipients and immunosuppressed individuals. An estimated 1 in 150 infants are infected with HCMV at birth, which can result in lifelong, debilitating neurologic sequelae including microcephaly, sensorineural hearing loss, and cognitive impairment. Natural maternal immunity to HCMV decreases the frequency of reinfection and reduces risk of congenital transmission but does not completely protect against neonatal disease. Thus, a vaccine to reduce the incidence and severity of infant infection is a public health priority. A variety of candidate HCMV vaccine approaches have been tried previously, including live-attenuated viruses, glycoprotein subunit formulations, viral vectors, and single/bivalent DNA plasmids, but all have failed to reach target endpoints in clinical trials. Nevertheless, there is a great deal to be learned from the successes and failures of the HCMV vaccine field (both congenital and transplant-associated), as well as from vaccine development efforts for other herpesvirus pathogens including herpes simplex virus 1 and 2, varicella zoster virus, and Epstein-Barr virus. Here, we review those successes and failures, evaluating recent cutting-edge discoveries that have shaped the HCMV vaccine field and identifying topics of critical importance for future investigation. These considerations will inform rational design and evaluation of next-generation vaccines to prevent HCMV-associated congenital infection and disease.

Membranotropic peptides mediating viral entry

The means used by enveloped viruses to bypass cellular membranes are well characterized; however, the mechanisms used by non-enveloped viruses to deliver their genome inside the cell remain unresolved and poorly defined. The discovery of short, membrane interacting, amphipathic or hydrophobic sequences (known as membranotropic peptides) in both enveloped and non-enveloped viruses suggests that these small peptides are strongly involved in breaching the host membrane and in the delivery of the viral genome into the host cell. Thus, in spite of noticeable differences in entry, this short stretches of membranotropic peptides are probably associated with similar entry-related events. This review will uncover the intrinsic features of viral membranotropic peptides involved in viral entry of both naked viruses and the ones encircled with a biological membrane with the objective to better elucidate their different functional properties and possible applications in the biomedical field.

Functional Relevance of the Transmembrane Domain and Cytoplasmic Tail of the Pseudorabies Virus Glycoprotein H for Membrane Fusion

Herpesvirus membrane fusion depends on the core fusion machinery, comprised of glycoproteins B (gB) and gH/gL. Although gB structurally resembles autonomous class III fusion proteins, it strictly depends on gH/gL to drive membrane fusion. Whether the gH/gL complex needs to be membrane anchored to fulfill its function and which role the gH cytoplasmic (CD) and transmembrane domains (TMD) play in fusion is unclear. While the gH CD and TMD play an important role during infection, soluble gH/gL of herpes simplex virus 1 (HSV-1) seems to be sufficient to mediate cell-cell fusion in transient assays, arguing against an essential contribution of the CD and TMD. To shed more light on this apparent discrepancy, we investigated the role of the CD and TMD of the related alphaherpesvirus pseudorabies virus (PrV) gH. For this purpose, we expressed C-terminally truncated and soluble gH and replaced the TMD with a glycosylphosphatidylinositol (gpi) anchor. We also generated chimeras containing the TMD and/or CD of PrV gD or HSV-1 gH. Proteins were characterized in cell-based fusion assays and during virus infection. Although truncation of the CD resulted in decreased membrane fusion activity, the mutant proteins still supported replication of gH-negative PrV, indicating that the PrV gH CD is dispensable for viral replication. In contrast, PrV gH lacking the TMD, membrane-anchored via a lipid linker, or comprising the PrV gD TMD were nonfunctional, highlighting the essential role of the gH TMD for function. Interestingly, despite low sequence identity, the HSV-1 gH TMD could substitute for the PrV gH TMD, pointing to functional conservation.IMPORTANCE Enveloped viruses depend on membrane fusion for virus entry. While this process can be mediated by only one or two proteins, herpesviruses depend on the concerted action of at least three different glycoproteins. Although gB has features of bona fide fusion proteins, it depends on gH and its complex partner, gL, for fusion. Whether gH/gL prevents premature fusion or actively triggers gB-mediated fusion is unclear, and there are contradictory results on whether gH/gL function requires stable membrane anchorage or whether the ectodomains alone are sufficient. Our results show that in pseudorabies virus gH, the transmembrane anchor plays an essential role for gB-mediated fusion while the cytoplasmic tail is not strictly required.

Host subspecific viral strains in European house mice: Murine cytomegalovirus in the Eastern (Mus musculus musculus) and Western house mouse (Mus musculus domesticus)

Murine cytomegalovirus (MCMV) has been reported from house mice (Mus musculus) worldwide, but only recently from Eastern house mice (M. m. musculus), of particular interest because they form a semi-permeable species barrier in Europe with Western house mice, M. m. domesticus. Here we report genome sequences of EastMCMV (from Eastern mice), and set these in the context of MCMV genomes from genus Mus hosts. We show EastMCMV and WestMCMV are genetically distinct. Phylogeny splitting analyses show a genome wide (94%) pattern consistent with no West-East introgression, the major exception (3.8%) being a genome-terminal region of duplicated genes involved in host immune system evasion. As expected from its function, this is a region of maintenance of ancestral polymorphism: The lack of clear splitting signal cannot be interpreted as evidence of introgression. The EastMCMV genome sequences reported here can therefore serve as a well-described resource for exploration of murid MCMV diversity.

From recognition to execution-the HCMV Pentamer from receptor binding to fusion triggering

The β-herpesvirus human cytomegalovirus (HCMV) is the leading viral cause of neonatal developmental disabilities. In HCMV, the conserved herpesvirus glycoprotein B (gB) mediates membrane fusion between the viral and host cell membranes, whereas the trimeric gH/gL/gO or the pentameric gH/gL/UL128/UL130/UL31A complexes (Pentamer) bind to cell-specific receptors and provide the triggering signal to gB. Recent structural and functional studies have provided new insights into Pentamer structure, conformational flexibility, location of epitopes for neutralizing antibodies and potential binding sites for cell surface receptors. Together, these data suggest a model where receptor binding triggers a conformational change in Pentamer, allowing it to interact with gB and initiate the membrane fusion process.

The HSV-1 mechanisms of cell-to-cell spread and fusion are critically dependent on host PTP1B

All herpesviruses have mechanisms for passing through cell junctions, which exclude neutralizing antibodies and offer a clear path to neighboring, uninfected cells. In the case of herpes simplex virus type 1 (HSV-1), direct cell-to-cell transmission takes place between epithelial cells and sensory neurons, where latency is established. The spreading mechanism is poorly understood, but mutations in four different HSV-1 genes can dysregulate it, causing neighboring cells to fuse to produce syncytia. Because the host proteins involved are largely unknown (other than the virus entry receptor), we were intrigued by an earlier discovery that cells infected with wild-type HSV-1 will form syncytia when treated with salubrinal. A biotinylated derivative of this drug was used to pull down cellular complexes, which were analyzed by mass spectrometry. One candidate was a protein tyrosine phosphatase (PTP1B), and although it ultimately proved not to be the target of salubrinal, it was found to be critical for the mechanism of cell-to-cell spread. In particular, a highly specific inhibitor of PTP1B (CAS 765317-72-4) blocked salubrinal-induced fusion, and by itself resulted in a dramatic reduction in the ability of HSV-1 to spread in the presence of neutralizing antibodies. The importance of this phosphatase was confirmed in the absence of drugs by using PTP1B^-/- cells. Importantly, replication assays showed that virus titers were unaffected when PTP1B was inhibited or absent. Only cell-to-cell spread was altered. We also examined the effects of salubrinal and the PTP1B inhibitor on the four Syn mutants of HSV-1, and strikingly different responses were found. That is, both drugs individually enhanced fusion for some mutants and reduced fusion for others. PTP1B is the first host factor identified to be specifically required for cell-to-cell spread, and it may be a therapeutic target for preventing HSV-1 reactivation disease.

It is estimated that 67% of the global population is infected with herpes simplex virus type 1 (HSV-1). This virus resides in sensory neurons in a quiescent state but periodically reactivates, producing virus particles that travel down the axon to infect epithelial cells of the skin, where it can be transmitted to additional people. To avoid neutralizing antibodies, herpesviruses have evolved mechanisms for moving directly from one cell to another through their sites of intimate contact; however, the mechanism of cell-to-cell spread is poorly understood. Studies of HSV-1 mutants have implicated numerous viral proteins, but the necessary cellular factors are unknown except for the one that the virus uses to enter cells. Our experiments have identified a cellular enzyme (PTP1B, a tyrosine phosphatase) that is dispensable for the production of infectious virions but is critically important for the cell-to-cell spreading mechanism. Promising drugs targeting PTP1B have already been tested in early clinical trials for possible treatment of obesity and type-2 diabetes, and thus, our study may have immediate utility for attenuating HSV-1 reactivation disease in immunocompromised patients.

Functional Role of N-Linked Glycosylation in Pseudorabies Virus Glycoprotein gH

Many viral envelope proteins are modified by asparagine (N)-linked glycosylation, which can influence their structure, physicochemical properties, intracellular transport, and function. Here, we systematically analyzed the functional relevance of N-linked glycans in the alphaherpesvirus pseudorabies virus (PrV) glycoprotein H (gH), which is an essential component of the conserved core herpesvirus fusion machinery. Upon gD-mediated receptor binding, the heterodimeric complex of gH and gL activates gB to mediate fusion of the viral envelope with the host cell membrane for viral entry. gH contains five potential N-linked glycosylation sites at positions 77, 162, 542, 604, and 627, which were inactivated by conservative mutations (asparagine to glutamine) singly or in combination. The mutated proteins were tested for correct expression and fusion activity. Additionally, the mutated gH genes were inserted into the PrV genome for analysis of function during virus infection. Our results demonstrate that all five sites are glycosylated. Inactivation of the PrV-specific N77 or the conserved N627 resulted in significantly reduced in vitro fusion activity, delayed penetration kinetics, and smaller virus plaques. Moreover, substitution of N627 greatly affected transport of gH in transfected cells, resulting in endoplasmic reticulum (ER) retention and reduced surface expression. In contrast, mutation of N604, which is conserved in the Varicellovirus genus, resulted in enhanced in vitro fusion activity and viral cell-to-cell spread. These results demonstrate a role of the N-glycans in proper localization and function of PrV gH. However, even simultaneous inactivation of all five N-glycosylation sites of gH did not severely inhibit formation of infectious virus particles.IMPORTANCE Herpesvirus infection requires fusion of the viral envelope with cellular membranes, which involves the conserved fusion machinery consisting of gB and the heterodimeric gH/gL complex. The bona fide fusion protein gB depends on the presence of the gH/gL complex for activation. Viral envelope glycoproteins, such as gH, usually contain N-glycans, which can have a strong impact on their folding, transport, and functions. Here, we systematically analyzed the functional relevance of all five predicted N-linked glycosylation sites in the alphaherpesvirus pseudorabies virus (PrV) gH. Despite the fact that mutation of specific sites affected gH transport, in vitro fusion activity, and cell-to-cell spread and resulted in delayed penetration kinetics, even simultaneous inactivation of all five N-glycosylation sites of gH did not severely inhibit formation of infectious virus particles. Thus, our results demonstrate a modulatory but nonessential role of N-glycans for gH function.

The Neutralizing Linear Epitope of Human Herpesvirus 6A Glycoprotein B Does Not Affect Virus Infectivity

Human herpesvirus 6A (HHV-6A) glycoprotein B (gB) is a glycoprotein consisting of 830 amino acids and is essential for the growth of the virus. Previously, we reported that a neutralizing monoclonal antibody (MAb) called 87-y-13 specifically reacts with HHV-6A gB, and we identified its epitope residue at asparagine (Asn) 347 on gB. In this study, we examined whether the epitope recognized by the neutralizing MAb is essential for HHV-6A infection. We constructed HHV-6A bacterial artificial chromosome (BAC) genomes harboring substitutions at Asn347, namely, HHV-6A BACgB(N347K) and HHV-6A BACgB(N347A). These mutant viruses could be reconstituted and propagated in the same manner as the wild type and their revertants, and MAb 87-y-13 could not inhibit infection by either mutant. In a cell-cell fusion assay, Asn at position 347 on gB was found to be nonessential for cell-cell fusion. In addition, in building an HHV-6A gB homology model, we found that the epitope of the neutralizing MAb is located on domain II of gB and is accessible to solvents. These results indicate that Asn at position 347, the linear epitope of the neutralizing MAb, does not affect HHV-6A infectivity.IMPORTANCE Glycoprotein B (gB) is one of the most conserved glycoproteins among all herpesviruses and is a key factor for virus entry. Therefore, antibodies targeted to gB may neutralize virus entry. Human herpesvirus 6A (HHV-6A) encodes gB, which is translated to a protein of about 830 amino acids (aa). Using a monoclonal antibody (MAb) for HHV-6A gB, which has a neutralizing linear epitope, we analyzed the role of its epitope residue, N347, in HHV-6A infectivity. Interestingly, this gB linear epitope residue, N347, was not essential for HHV-6A growth. By constructing a homology model of HHV-6A gB, we found that N347 was located in the region corresponding to domain II. Therefore, with regard to its neutralizing activity against HHV-6A infection, the epitope on gB might be exposed to solvents, suggesting that it might be a target of the immune system.