The extracellular domain of herpes simplex virus gE is indispensable for efficient cell-to-cell spread: evidence for gE/gI receptors

Molecular docking analysis of HSV-1 proteins models with synthetic and plant derived compounds

The atomic resolution model of US9, UL20, and gH protein of HSV is known. Hence, the ligand protein interaction of the US9, UL20, and gH protein models were carried out with synthetic drugs like acyclovir, bexarotene, vinorelbine, foscarnet, famciclovir, cidofovir and two plant derived natural drug acacetin and anthraquinone. Based on structure and docking study, it is predicted that protein US20 and gH binds with particular anti-HSV drug i.e. acyclovir, cidofovir, acacetin and famciclovir, acacetin respectively, while interaction of different protein is different with drugs.

Human immunoglobulins are transported to HCMV viral envelope by viral Fc gamma receptors-dependent and independent mechanisms

Human cytomegaloviruses (HCMVs) employ many different mechanisms to escape and subvert the host immune system, including expression of the viral IgG Fcγ receptors (vFcγRs) RL11 (gp34), RL12 (gp95), RL13 (gpRL13), and UL119 (gp68) gene products. The role of vFcγRs in HCMV pathogenesis has been reported to operate in infected cells by interfering with IgG-mediated effector functions. We found that gp34 and gp68 are envelope proteins that bind and internalize human IgGs on the surface of infected cells. Internalized IgGs are then transported on the envelope of viral particles in a vFcR-dependent mechanism. This mechanism is also responsible for the incorporation on the virions of the anti-gH neutralizing antibody MSL-109. Intriguingly, we show that gp68 is responsible for MSL-109 incorporation, but it is dispensable for other anti-HCMV antibodies that do not need this function to be transported on mature virions. HCMV-infected cells grown in presence of anti-HCMV monoclonal antibodies generate a viral progeny still infective and possible to be neutralized. This is the first example of a virus carrying neutralizing IgGs on its surface and their possible role is discussed.

Pseudorabies Virus Mutants Lacking US9 Are Defective in Cytoplasmic Assembly and Sorting of Virus Particles into Axons and Not in Axonal Transport

Herpes simplex virus (HSV) and varicella zoster virus (VZV) rely on transport of virus particles in neuronal axons to spread from sites of viral latency in sensory ganglia to peripheral tissues then on to other hosts. This process of anterograde axonal transport involves kinesin motors that move virus particles rapidly along microtubules. α-herpesvirus anterograde transport has been extensively studied by characterizing the porcine pseudorabies virus (PRV) and HSV, with most studies focused on two membrane proteins: gE/gI and US9. It was reported that PRV and HSV US9 proteins bind to kinesin motors, promoting tethering of virus particles on the motors, and furthering anterograde transport within axons. Alternatively, other models have argued that HSV and PRV US9 and gE/gI function in the cytoplasm and not in neuronal axons. Specifically, HSV gE/gI and US9 mutants are defective in the assembly of virus particles in the cytoplasm of neurons and the subsequent sorting of virus particles to cell surfaces and into axons. However, PRV US9 and gE/gI mutants have not been characterized for these cytoplasmic defects. We examined neurons infected with PRV mutants, one lacking both gE/gI and US9 and the other lacking just US9, by electron microscopy. Both PRV mutants exhibited similar defects in virus assembly and cytoplasmic sorting of virus particles to cell surfaces. As well, the mutants exhibited reduced quantities of infectious virus in neurons and in cell culture supernatants. We concluded that PRV US9 primarily functions in neurons to promote cytoplasmic steps in anterograde transport.

Suppression of annexin A1 and its receptor reduces herpes simplex virus 1 lethality in mice

Herpes simplex virus 1 (HSV-1)-induced encephalitis is the most common cause of sporadic, fatal encephalitis in humans. HSV-1 has at least 10 different envelope glycoproteins, which can promote virus infection. The ligands for most of the envelope glycoproteins and the significance of these ligands in virus-induced encephalitis remain elusive. Here, we show that glycoprotein E (gE) binds to the cellular protein, annexin A1 (Anx-A1) to enhance infection. Anx-A1 can be detected on the surface of cells permissive for HSV-1 before infection and on virions. Suppression of Anx-A1 or its receptor, formyl peptide receptor 2 (FPR2), on the cell surface and gE or Anx-A1 on HSV-1 envelopes reduced virus binding to cells. Importantly, Anx-A1 knockout, Anx-A1 knockdown, or treatments with the FPR2 antagonist reduced the mortality and tissue viral loads of infected mice. Our results show that Anx-A1 is a novel enhancing factor of HSV-1 infection. Anx-A1-deficient mice displayed no evident physiology and behavior changes. Hence, targeting Anx-A1 and FPR2 could be a promising prophylaxis or adjuvant therapy to decrease HSV-1 lethality.

Herpes simplex virus 1 (HSV-1)-induced encephalitis is the most devastating consequence of HSV-1 infection, even in patients treated with anti-HSV-1 drugs. Moreover, encephalitis induced by drug-resistant HSV-1 has been reported in immunocompromised patients. Identifying the cellular factors in promoting HSV-1 replication, especially those increasing virus attachment and entry, could facilitate the development of alternative or adjuvant therapy. Here, we identified annexin A1 (Anx-A1) and its receptor, formyl peptide receptor 2 (FPR2), facilitating HSV-1 attachment to the cell surface. Suppression of Anx-A1 or blockage of FPR2 impaired HSV-1 attachment to cells, viral yields in cells, and HSV-1 lethality in mice. Moreover, blocking FPR2 decreased the replication of drug-resistant HSV-1 in BABL/c nude mice. Hence, targeting Anx-A1 and FPR2 could be alternative or adjuvant therapy for HSV-1 infection.

Alphaherpesvirus glycoprotein E: A review of its interactions with other proteins of the virus and its application in vaccinology

The viral envelope glycoprotein E (gE) is required for cell-to-cell transmission, anterograde and retrograde neurotransmission, and immune evasion of alphaherpesviruses. gE can also interact with other proteins of the virus and perform various functions in the virus life cycle. In addition, the gE gene is often the target gene for the construction of gene-deleted attenuated marker vaccines. In recent years, new progress has been made in the research and vaccine application of gE with other proteins of the virus. This article reviews the structure of gE, the relationship between gE and other proteins of the virus, and the application of gE in vaccinology, which provides useful information for further research on gE.

Evaluation of the Safety and Immunogenicity of Duck-Plague Virus gE Mutants

Duck plague (DP) is an acute infectious disease in the duck industry. The duck plague virus (DPV) is the pathogen, a subfamily of alphaherpesvirinae. gE is a type I membrane protein that contains three parts: an extracellular domain, a transmembrane domain, and a cytoplasmic domain. gE is the major virulence determinant of α-herpesvirus. However, the functions of the gE extracellular and cytoplasmic domains have not been reported in DPV. In this study, a gE extracellular domain deletion mutant and a gE cytoplasmic domain deletion mutant were constructed from DPV. Virus replication kinetics showed that the growth titers of both the gE ectodomain-deleted mutant virus and the gE cytoplasmic domain-deleted virus in DEFs were lower than that of the parental virus CHv-50. DPV CHv-gEΔET and DPV CHv-gEΔCT were continuously passed to the 20th passage in DEFs and the 10th in ducklings. The mutant virus DNA after passage was extracted for identification. The results showed that the gE ectodomain and gE cytoplasmic domain deletion mutant viruses have good genetic stability. The ducklings in each group (n=10) were inoculated with the same titers of DPV CHv-gEΔET, DPV CHv-gEΔCT, DPV CHv-ΔgE, and parental CHv-50, respectively. Clinical symptoms and serum antibody levels were detected after inoculation. The results showed that the virulence of DPV CHv-gEΔCT to ducklings was reduced compared with parental CHv-50, while the virulence of DPV CHv-gEΔET to ducklings was significantly reduced. 10⁵ TCID₅₀ DPV CHv-gEΔET or DPV CHv-ΔgE can induce ducklings to produce DPV-specific antibodies, protect the ducklings from virulent CHv challenge. Therefore, DPV CHv-gEΔET may serve as a promising vaccine candidate to prevent and control duck plague.

Discovery and Characterization of an Aberrant Small Form of Glycoprotein I of Herpes Simplex Virus Type I in Cell Culture

The 380-to-393-amino-acid glycoprotein I (gI) encoded by herpes simplex virus 1 (HSV-1) is a critical mediator for viral cell-to-cell spread and syncytium formation. Here we report a previously unrecognized aberrant form of gI in HSV-1-infected cells. Production of this molecule is independent of cell type and viral strains. It had an unexpected gel migration size of approximately 23 kDa, was packaged into viral particles, and could be coimmunoprecipitated by antibodies to both N and C termini of gI. Deep sequencing failed to detect alternative RNA splicing, and the invitro transcribed full-length mRNA gave rise to the 23 kDa protein in transfected cells. Combined mass spectrometry and antibody probing analyses detected peptide information across different regions of gI, suggesting the possibility of a full-length gI but with abnormal migration behavior. In line with this notion, the HA insertion mutagenesis revealed a stable fold in the gI extracellular region aa.38-196 resistant to denaturing conditions, whereas small deletions within this region failed the antibodies to detect the fast, but not the slow-moving species of gI. It is also intriguing that the structure could be perturbed to some extent by a gBsyn mutation, leading to exposure or shielding of the gI epitopes. Thus, the HSV-1 gI apparently adopts a very stable fold in its natural form, rendering it an unusual biophysical property. Our findings provide novel insight into the biological properties of HSV gI and have important implications in understanding the viral spread and pathogenesis.

IMPORTANCE The HSV-1 gI is required for viral cell-to-cell spread within the host, but its behavior during infection has remained poorly defined. Along with the classic 66 kDa product, here we report a previously unrecognized, approximately 23 kDa form of gI. Biochemical and genetics analyses revealed that this molecule represents the full-length form of gI but adopts a stable fold in its extracellular domain that is resistant to denatured conditions, thus contributing to the aberrant migration rate. Our results revealed a novel property of HSV-1 gI and have important implications in understanding viral pathogenesis.

A natural product, (S)-10-Hydroxycamptothecin inhibits pseudorabies virus proliferation through DNA damage dependent antiviral innate immunity

Pseudorabies virus (PRV), a member of the subfamily alphaherpesvirinae, is one of the most important pathogenes that cause acute death in infected pigs and leads to substantial economic losses in the global swine industry. Recently, China's emerging PRV mutant strains resulted in the traditionally commercial vaccines not providing complete protection. Some studies reported that PRV could infect humans and cause endophthalmitis and encephalitis under certain circumstances. It is necessary to develop alternative manners to control the virus infection. Here, by screening a library of natural products, (S)-10-Hydroxycamptothecin (10-HCPT) was revealed to inhibit PRV replication with a selective index of 270.04. And 10-HCPT inhibited PRV replication by blocking the viral genome replication but not inhibiting the viral attachment, internalization, and release. RNA interference assay showed that 10-HCPT inhibited PRV replication by targeting DNA topoisomerase 1 (TOP1). Meanwhile, 10-HCPT treatment induced DNA damage response and stimulated antiviral innate immunity. Animal challenge experiments showed that 10-HCPT effectively alleviated clinical signs and hispathology, and increased INF-β responses in lung and brain tissues of mice induced by PRV infection. The results demonstrate that 10-HCPT is a promising therapeutic agent to control PRV infection.

Evaluation of HSV-2 gE Binding to IgG-Fc and Application for Vaccine Development

Glycoprotein E (gE) and glycoprotein I (gI) are expressed as a heterodimer on the surface of Herpes simplex virus (HSV). Glycoprotein E binds Fc domain of immunoglobulin G (IgG) and inhibits activities mediated by the IgG Fc domain, contributing to immune evasion by HSV. It has been reported that HSV type 1 gE (gE-1) is capable of binding IgG Fc as a monomer and in a heterodimeric complex with gI, with the heterodimer having 50- to100-fold greater affinity for Fc than gE alone. We report the production of both a soluble form of HSV type 2 gE (gE-2) and a soluble HSV-2 gE/gI heterodimer (gE-2/gI-2). Characterization of soluble gE-2 by surface plasmon resonance (SPR) demonstrates that it is incapable of binding human IgG or the IgG Fc domain. Co-expression with HSV-2 gI (gI-2) and purification of the gE-2/gI-2 heterodimer enable gE-2 to bind human IgG through its Fc domain. We hypothesize that functional epitopes of wildtype gE-2 may be masked by plasma IgG Fc and affect the immunogenicity of the gE-2/gI-2 heterodimer as a vaccine antigen. A series of gE-2 mutations within the surface-exposed Fc:gE-2 interface was designed, and gE-2 mutants were co-expressed with gI-2. Evaluation of twelve gE-2 mutant heterodimers by SPR assay identified nine gE-2 mutations which abrogated or reduced Fc binding while maintaining heterodimer formation with gI. Vaccinating rabbits with the four most Fc-binding deficient gE-2/gI-2 heterodimers elicited comparable anti-heterodimer binding antibody titers and statistically significantly higher serum neutralization antibody levels than wildtype heterodimers. Taken together, these data support the concept of rational antigen design for improved vaccine candidates.

Rational Design of Live-Attenuated Vaccines against Herpes Simplex Viruses

Diseases caused by human herpes simplex virus types 1 and 2 (HSV-1 and HSV-2) affect millions of people worldwide and range from fatal encephalitis in neonates and herpes keratitis to orofacial and genital herpes, among other manifestations. The viruses can be shed efficiently by asymptomatic carriers, causing increased rates of infection. Viral transmission occurs through direct contact of mucosal surfaces followed by initial replication of the incoming virus in skin tissues. Subsequently, the viruses infect sensory neurons in the trigeminal and lumbosacral dorsal root ganglia, where they are primarily maintained in a transcriptionally repressed state termed "latency", which persists for the lifetime of the host. HSV DNA has also been detected in other sympathetic ganglia. Periodically, latent viruses can reactivate, causing ulcerative and often painful lesions primarily at the site of primary infection and proximal sites. In the United States, recurrent genital herpes alone accounts for more than a billion dollars in direct medical costs per year, while there are much higher costs associated with the socio-economic aspects of diseased patients, such as loss of productivity due to mental anguish. Currently, there are no effective FDA-approved vaccines for either prophylactic or therapeutic treatment of human herpes simplex infections, while several recent clinical trials have failed to achieve their endpoint goals. Historically, live-attenuated vaccines have successfully combated viral diseases, including polio, influenza, measles, and smallpox. Vaccines aimed to protect against the devastation of smallpox led to the most significant achievement in medical history: the eradication of human disease by vaccination. Recently, novel approaches toward developing safe and effective live-attenuated vaccines have demonstrated high efficacy in various preclinical models of herpetic disease. This next generation of live-attenuated vaccines has been tailored to minimize vaccine-associated side effects and promote effective and long-lasting immune responses. The ultimate goal is to prevent or reduce primary infections (prophylactic vaccines) or reduce the frequency and severity of disease associated with reactivation events (therapeutic vaccines). These vaccines' "rational" design is based on our current understanding of the immunopathogenesis of herpesviral infections that guide the development of vaccines that generate robust and protective immune responses. This review covers recent advances in the development of herpes simplex vaccines and the current state of ongoing clinical trials in pursuit of an effective vaccine against herpes simplex virus infections and associated diseases.

Motor Skills: Recruitment of Kinesins, Myosins and Dynein during Assembly and Egress of Alphaherpesviruses

The alphaherpesviruses are pathogens of the mammalian nervous system. Initial infection is commonly at mucosal epithelia, followed by spread to, and establishment of latency in, the peripheral nervous system. During productive infection, viral gene expression, replication of the dsDNA genome, capsid assembly and genome packaging take place in the infected cell nucleus, after which mature nucleocapsids emerge into the cytoplasm. Capsids must then travel to their site of envelopment at cytoplasmic organelles, and enveloped virions need to reach the cell surface for release and spread. Transport at each of these steps requires movement of alphaherpesvirus particles through a crowded and viscous cytoplasm, and for distances ranging from several microns in epithelial cells, to millimeters or even meters during egress from neurons. To solve this challenging problem alphaherpesviruses, and their assembly intermediates, exploit microtubule- and actin-dependent cellular motors. This review focuses upon the mechanisms used by alphaherpesviruses to recruit kinesin, myosin and dynein motors during assembly and egress.

A putative WAVE regulatory complex (WRC) interacting receptor sequence (WIRS) in the cytoplasmic tail of HSV-1 gE does not function in WRC recruitment or neuronal transport

HSV-1 envelope glycoprotein E (gE) is important for viral egress and cell-to-cell spread but the host protein(s) involved in these functions have yet to be determined. We aimed to investigate a role for the Arp2/3 complex and actin regulation in viral egress based on the identification of a WAVE Regulatory Complex (WRC) Interacting Receptor Sequence (WIRS) in the cytoplasmic tail (CT) of gE. A WIRS-dependent interaction between the gE(CT) and subunits of the WRC was demonstrated by GST-pulldown assay and a role for the Arp2/3 complex in cell-to-cell spread was also observed by plaque assay. Subsequent study of a recombinant HSV-1 gE WIRS-mutant found no significant changes to viral production and release based on growth kinetics studies, or changes to plaque and comet size in various cell types, suggesting no function for the motif in cell-to-cell spread. GFP-Trap pulldown and proximity ligation assays were unable to confirm a WIRS-dependent interaction between gE and the WRC in human cell lines though the WIRS-independent interaction observed in situ warrants further study. Confocal microscopy of infected cells of neuronal origin identified no impairment of gE WIRS-mutant HSV-1 anterograde transport along axons. We propose that the identified gE WIRS motif does not function directly in recruitment of the WRC in human cells, in cell-to-cell spread of virus or in anterograde transport along axons. Further studies are needed to understand how HSV-1 manipulates and traverses the actin cytoskeleton and how gE may contribute to these processes in a WIRS-independent manner.

Anterograde transport of α-herpesviruses in neuronal axons

α-herpesviruses have been very successful, principally because they establish lifelong latency in sensory ganglia. An essential piece of the lifecycle of α-herpesviruses involves the capacity to travel from sensory neurons to epithelial tissues following virus reactivation from latency, a process known as anterograde transport. Virus particles formed in neuron cell bodies hitchhike on kinesin motors that run along microtubules, the length of axons. Herpes simplex virus (HSV) and pseudorabies virus (PRV) have been intensely studied to elucidate anterograde axonal transport. Both viruses use similar strategies for anterograde transport, although there are significant differences in the form of virus particles transported in axons, the identity of the kinesins that transport viruses, and how certain viral membrane proteins, gE/gI and US9, participate in this process. This review compares the older models for HSV and PRV anterograde transport with recent results, which are casting a new light on several aspects of this process.

Evaluation of Recombinant Herpes Zoster Vaccine for Primary Immunization of Varicella-seronegative Transplant Recipients

BACKGROUND

Immunization of VZV-seronegative solid organ transplant (SOT) patients using the live-attenuated varicella vaccine is generally contraindicated, leaving no widely applicable immunization option. The recombinant subunit herpes zoster vaccine (RZV) is indicated for VZV seropositive persons to prevent shingles but could potentially also protect VZV-seronegative persons against varicella. We performed a safety and immunogenicity evaluation of RZV in VZV-seronegative SOT recipients as an option for protection.

METHODS

VZV-seronegative adult SOT patients with no history of varicella/shingles vaccine or disease were given 2 doses of RZV vaccine 2-6 months apart. Blood was drawn prevaccination (V1), prior to the second dose (V2) and 4 weeks after second dose (V3). Humoral (anti-gE) and cell-mediated immunity was evaluated, with polyfunctional cells defined as cells producing ≥2 cytokines.

RESULTS

Among 31 eligible VZV-seronegative SOT patients screened, 23 were enrolled. Median age was 38 years and median time since transplant procedure was 38 years. The most frequent transplant types were liver (35%) and lung (30%). Median anti-gE levels significantly increased from V1 to V3 (p=0001) and V2 to V3 (p<0001), even though only 55% had a positive seroresponse. Median polyfunctional CD4 T-cells counts increased from V1 to V2 (54/10 vs 104/10 cells; p=0041), and from V2 to V3 (380/10; p=0002). Most adverse events were mild with no rejection episodes.

CONCLUSION

RZV was safe and elicited significant humoral and cellular responses in VZV-seronegative SOT patients, and has the potential to be considered as a preventive strategy against primary varicella.

HSV-1 Cytoplasmic Envelopment and Egress

Herpes simplex virus type 1 (HSV-1) is a structurally complex enveloped dsDNA virus that has evolved to replicate in human neurons and epithelia. Viral gene expression, DNA replication, capsid assembly, and genome packaging take place in the infected cell nucleus, which mature nucleocapsids exit by envelopment at the inner nuclear membrane then de-envelopment into the cytoplasm. Once in the cytoplasm, capsids travel along microtubules to reach, dock, and envelope at cytoplasmic organelles. This generates mature infectious HSV-1 particles that must then be sorted to the termini of sensory neurons, or to epithelial cell junctions, for spread to uninfected cells. The focus of this review is upon our current understanding of the viral and cellular molecular machinery that enables HSV-1 to travel within infected cells during egress and to manipulate cellular organelles to construct its envelope.

Induction of Rod-Shaped Structures by Herpes Simplex Virus Glycoprotein I

The envelope glycoprotein I (gI) of herpes simplex virus 1 (HSV-1) is a critical mediator of virus-induced cell-to-cell spread and cell-cell fusion. Here, we report a previously unrecognized property of this molecule. In transfected cells, the HSV-1 gI was discovered to induce rod-shaped structures that were uniform in width but variable in length. Moreover, the gI within these structures was conformationally different from the typical form of gI, as a previously used monoclonal antibody mAb3104 and a newly made peptide antibody to the gI extracellular domain (ECD) (amino acids [aa] 110 to 202) both failed to stain the long rod-shaped structures, suggesting the formation of a higher-order form. Consistent with this observation, we found that gI could self-interact and that the rod-shaped structures failed to recognize glycoprotein E, the well-known binding partner of gI. Further analyses by deletion mutagenesis and construction of chimeric mutants between gI and gD revealed that the gI ECD is the critical determinant, whereas the transmembrane domain served merely as an anchor. The critical amino acids were subsequently mapped to proline residues 184 and 188 within a conserved PXXXP motif. Reverse genetics analyses showed that the ability to induce a rod-shaped structure was not required for viral replication and spread in cell culture but rather correlated positively with the capability of the virus to induce cell fusion in the UL24syn background. Together, this work discovered a novel feature of HSV-1 gI that may have important implications in understanding gI function in viral spread and pathogenesis.IMPORTANCE The HSV-1 gI is required for viral cell-to-cell spread within the host, but the molecular mechanisms of how gI exactly works have remained poorly understood. Here, we report a novel property of this molecule, namely, induction of rod-shaped structures, which appeared to represent a higher-order form of gI. We further mapped the critical residues and showed that the ability of gI to induce rod-shaped structures correlated well with the capability of HSV-1 to induce cell fusion in the UL24syn background, suggesting that the two events may have an intrinsic link. Our results shed light on the biological properties of HSV-1 gI and may have important implications in understanding viral pathogenesis.

Environmental Restrictions: A New Concept Governing HIV-1 Spread Emerging from Integrated Experimental-Computational Analysis of Tissue-Like 3D Cultures

HIV-1 can use cell-free and cell-associated transmission modes to infect new target cells, but how the virus spreads in the infected host remains to be determined. We recently established 3D collagen cultures to study HIV-1 spread in tissue-like environments and applied iterative cycles of experimentation and computation to develop a first in silico model to describe the dynamics of HIV-1 spread in complex tissue. These analyses (i) revealed that 3D collagen environments restrict cell-free HIV-1 infection but promote cell-associated virus transmission and (ii) defined that cell densities in tissue dictate the efficacy of these transmission modes for virus spread. In this review, we discuss, in the context of the current literature, the implications of this study for our understanding of HIV-1 spread in vivo, which aspects of in vivo physiology this integrated experimental-computational analysis takes into account, and how it can be further improved experimentally and in silico.

Characterization of feline herpesvirus-1 deletion mutants in tissue explant cultures

Feline herpesvirus-1 (FHV-1) is the primary cause of viral respiratory and ocular disease in cats. While commercial vaccines can provide clinical protection, they do not protect from infection or prevent latency. Moreover, they are not safe for intranasal administration. Our overall objective is to develop a new mucosal vaccine against FHV-1 disease to address these shortcomings. Feline herpesvirus-1 deletion mutants of glycoprotein C (gC-), gE (gE-), US3-encoded serine/threonine protein kinase (PK-), and both gE and thymidine kinase (gE-TK-) were generated by bacterial artificial chromosome (BAC) mutagenesis. Tracheal tissue explants from eight cats were used to compare the pattern of viral infection and associated tissue damage, as well as virus spread through the basement membrane following inoculation with wild-type virus (WT), and gE-, gE-TK-, PK-, and gC- mutants. Tissues were collected at 24, 48, or 72  hours post-inoculation (hpi) followed by immunohistochemistry (IHC) for FHV-1. Histological changes were graded based on the distribution of virus infected cells and the severity of tissue damage. Inoculations with the WT virus resulted in maximal scores at 72 hpi both at a multiplicity of infection (MOI) of 1 and 0.1. Inoculation with the gE- mutant produced scores similar to scores of explants inoculated with the WT virus at 24 and 48 hpi, but scores were significantly decreased at 72 hpi. Explants inoculated with the gE-TK- mutant showed significantly decreased scores at all time points. Further, the majority of explants inoculated with the PK- mutant resulted in scores of zero at all time points, regardless of MOI. Finally, inoculation with WT resulted in significant stromal invasion below the infected epithelium, while stromal invasion was observed in less than 50 % of the samples following inoculation with gE-, gE-TK-, PK-, or gC- mutants and confined closely to the area surrounding the infected epithelium. In conclusion, the gE-TK- and PK- mutants exhibited significantly reduced virulence, tissue damage and spread to the underlying stroma, suggesting that they may be good vaccine candidates for in vivo testing.

Duck plague virus gE serves essential functions during the virion final envelopment through influence capsids budding into the cytoplasmic vesicles

Duck plague virus (DPV), a member of the alphaherpesviruses subfamily, causes massive ducks death and results in a devastating hit to duck industries in China. It is of great significance for us to analyze the functions of DPV genes for controlling the outbreak of duck plague. Thus, glycoproteins E (gE) of DPV, which requires viral cell-to-cell spreading and the final envelopment in herpes simplex virus 1 (HSV-1) and pseudorabies virus (PRV), was chosen herein. The gE mutant virus BAC-CHv-ΔgE was constructed by using a markerless two-step Red recombination system implemented on the DPV genome cloned into a bacterial artificial chromosome (BAC). Viral plaques on duck embryo fibroblast (DEF) cells of BAC-CHv-ΔgE were on average approximately 60% smaller than those produced by BAC-CHv virus. Viral replication kinetics showed that BAC-CHv-ΔgE grew to lower titers than BAC-CHv virus did in DEF cells. Electron microscopy confirmed that deleting of DPV gE resulted in a large number of capsids accumulating around vesicles and very few of them could bud into vesicles. The drastic inhibition of virion formation in the absence of the DPV gE indicated that it played an important role in virion morphogenesis before the final envelopment of intracytoplasmic nucleocapsids.

Herpes Simplex Virus 1 Spread in Oligodendrocytic Cells Is Highly Dependent on MAL Proteolipid

Herpes simplex virus 1 (HSV-1) is a neurotropic pathogen that can infect many types of cells and establish latent infections in neurons. HSV-1 may spread from infected to uninfected cells by two main routes: by cell-free virus or by cell-to-cell spread. In the first case, virions exit into the extracellular space and then infect another cell from the outside. In the second case, viral transmission occurs through cell-to-cell contacts via a mechanism that is still poorly understood. A third mode of spread, using extracellular vesicles, also exists. In this study, we demonstrate the important role for a myelin protein, myelin and lymphocyte protein (MAL), in the process of cell-to-cell viral spread in oligodendrocytes. We show that MAL is involved in trafficking of virions along cell processes and that MAL depletion produces a significant alteration in the viral cycle, which reduces cell-to cell spread of HSV-1.

Myelin and lymphocyte protein (MAL) is a tetraspan integral membrane protein that resides in detergent-insoluble membrane fractions enriched in condensed membranes. MAL is expressed in oligodendrocytes, in Schwann cells, where it is essential for the stability of myelin, and at the apical membrane of epithelial cells, where it has a critical role in transport. In T lymphocytes, MAL is found at the immunological synapse and plays a crucial role in exosome secretion. However, no involvement of MAL in viral infections has been reported so far. Here, we show that herpes simplex virus 1 (HSV-1) virions travel in association with MAL-positive structures to reach the end of cellular processes, which contact uninfected oligodendrocytes. Importantly, the depletion of MAL led to a significant decrease in infection, with a drastic reduction in the number of lytic plaques in MAL-silenced cells. These results suggest a significant role for MAL in viral spread at cell contacts. The participation of MAL in the cell-to-cell spread of HSV-1 may shed light on the involvement of proteolipids in this process.

IMPORTANCE Herpes simplex virus 1 (HSV-1) is a neurotropic pathogen that can infect many types of cells and establish latent infections in neurons. HSV-1 may spread from infected to uninfected cells by two main routes: by cell-free virus or by cell-to-cell spread. In the first case, virions exit into the extracellular space and then infect another cell from the outside. In the second case, viral transmission occurs through cell-to-cell contacts via a mechanism that is still poorly understood. A third mode of spread, using extracellular vesicles, also exists. In this study, we demonstrate the important role for a myelin protein, myelin and lymphocyte protein (MAL), in the process of cell-to-cell viral spread in oligodendrocytes. We show that MAL is involved in trafficking of virions along cell processes and that MAL depletion produces a significant alteration in the viral cycle, which reduces cell-to cell spread of HSV-1.

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.

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.

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.

Molecular Basis for the Recognition of Herpes Simplex Virus Type 1 Infection by Human Natural Killer Cells

Primary infection with Herpes simplex virus type 1 (HSV1) is subclinical or only mildly symptomatic in normal individuals, yet the reason for the body's effective immune defense against this pathogen in the absence of antigen-specific immunity has not been well understood. It is clear that human natural killer (NK) cells recognize and kill HSV1-infected cells, and those individuals who either lack or have functionally impaired NK cells can suffer severe, recurrent, and sometimes fatal HSV1 infection. In this article, we review what is known about the recognition of HSV1 by NK cells, and describe a novel mechanism of innate immune surveillance against certain viral pathogens by NK cells called Fc-bridged cell-mediated cytotoxicity.

Radiotherapy-induced reactivation of neurotrophic human herpes viruses: Overview and management

PURPOSE

Infection by Human Herpes Viruses (HHV) types 1-3, are prevalent throughout the world. It is known that radiotherapy can reactivate HHVs, but it is unclear how and to what extent reactivations can interact with or affect radiotherapeutic efficacy, patient outcomes and mortality risk. Herein, we aim to summarize what is known about Herpes Simplex Virus (HSV)-1,2 and Varicella Zoster Virus (VZV) pathophysiology as it relates to tumor biology, radiotherapy, chemo-radiotherapy, diagnosis and management so as to optimize cancer treatment in the setting of active HHV infection. Our secondary aim is to emphasize the need for further research to elucidate the potential adverse effects of active HHV infection in irradiated tumor tissue and to design optimal management strategies to incorporate into cancer management guidelines.

MATERIALS AND METHODS

The literature regarding herpetic infection, herpetic reactivation, and recurrence occurring during radiotherapy and that regarding treatment guidelines for herpetic infections are reviewed. We aim to provide the oncologist with a reference for the infectious dangers of herpetic reactivation in patients under their care and well established methods for prevention, diagnosis, and treatment of such infections. Pain management is also considered.

CONCLUSIONS

In the radiotherapeutic setting, serologic assays for HSV-1 and HSV-2 are feasible and can alert the clinician to patients at risk for viral reactivation. RT-PCR is specific in identifying the exact viral culprit and is the preferred diagnostic method to measure interventional efficacy. It can also differentiate between herpetic infection and radionecrosis. The MicroTrak^® HSV1/HSV2/VZV staining kit has high sensitivity and specificity in acute lesions, is also the most rapid means to confirm diagnosis. Herpetic reactivation and recurrences during radiotherapy can cause interruptions, cessations, or prolongations of the radiotherapeutic course, thus decreasing the biologically effective dose, to sub-therapeutic levels. Active HHV infection within the treatment volume results in increased tumor radio-resistance and potentially sub-therapeutic care if left untreated. Visceral reactivations may result in fatality and therefore, a high index of suspicion is important to identify these active infections. The fact that such infections may be mistaken for acute and/or late radiation effects, leading to less than optimal treatment decisions, makes knowledge of this problem even more relevant. To minimize the risk of these sequelae, prompt anti-viral therapy is recommended, lasting the course of radiotherapy.

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

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

The Fc Domain of Immunoglobulin Is Sufficient to Bridge NK Cells with Virally Infected Cells

Clearance of pathogens or tumor cells by antibodies traditionally requires both Fab and Fc domains of IgG. Here, we show the Fc domain of IgG alone mediates recognition and clearance of herpes simplex virus (HSV1)-infected cells. The human natural killer (NK) cell surface is naturally coated with IgG bound by its Fc domain to the Fcγ receptor CD16a. NK cells utilize the Fc domain of bound IgG to recognize gE, an HSV1-encoded glycoprotein that also binds the Fc domain of IgG but at a site distinct from CD16a. The bridge formed by the Fc domain between the HSV1-infected cell and the NK cell results in NK cell activation and lysis of the HSV1-infected cell in the absence of HSV1-specific antibody in vitro and prevents fatal HSV1 infection in vivo. This mechanism also explains how bacterial IgG-binding proteins regulate NK cell function and may be broadly applicable to Fcγ-receptor-bearing cells.

Molecular association of herpes simplex virus type 1 glycoprotein E with membrane protein Us9

Herpes simplex virus type 1 (HSV-1) glycoprotein E (gE), glycoprotein I (gI), and Us9 promote efficient anterograde axonal transport of virus from the neuron cytoplasm to the axon terminus. HSV-1 and PRV gE and gI form a heterodimer that is required for anterograde transport, but an association that includes Us9 has not been demonstrated. NS-gE380 is an HSV-1 mutant that has five amino acids inserted after gE residue 380, rendering it defective in anterograde axonal transport. We demonstrated that gE, gI and Us9 form a trimolecular complex in Vero cells infected with NS-gE380 virus in which gE binds to both Us9 and gI. We detected the complex using immunoprecipitation with anti-gE or anti-gI monoclonal antibodies in the presence of ionic detergents. Under these conditions, Us9 did not associate with gE in cells infected with wild-type HSV-1; however, using a nonionic detergent, TritonX-100, an association between Us9 and gE was detected in immunoprecipitates of both wild-type and NS-gE380-infected cells. The results suggest that the interaction between Us9 and gE is weak and disrupted by ionic detergents in wild-type infected cells. We postulate that the tight interaction between Us9 and gE leads to the anterograde spread defect in the NS-gE380 virus.

Connections matter--how viruses use cell–cell adhesion components

The epithelium is a highly organized type of animal tissue. Except for blood and lymph vessels, epithelial cells cover the body, line its cavities in single or stratified layers and support exchange between compartments. In addition, epithelia offer to the body a barrier to pathogen invasion. To transit through or to replicate in epithelia, viruses have to face several obstacles, starting from cilia and glycocalyx where they can be neutralized by secreted immunoglobulins. Tight junctions and adherens junctions also prevent viruses to cross the epithelial barrier. However, viruses have developed multiple strategies to blaze their path through the epithelium by utilizing components of cell–cell adhesion structures as receptors. In this Commentary, we discuss how viruses take advantage of the apical junction complex to spread. Whereas some viruses quickly disrupt epithelium integrity, others carefully preserve it and use cell adhesion proteins and their cytoskeletal connections to rapidly spread laterally. This is exemplified by the hidden transmission of enveloped viruses that use nectins as receptors. Finally, several viruses that replicate preferentially in cancer cells are currently used as experimental cancer therapeutics. Remarkably, these viruses use cell adhesion molecules as receptors, probably because--to reach tumors and metastases--ncolytic viruses must efficiently traverse or break epithelia.

The herpes simplex virus 1 UL51 protein interacts with the UL7 protein and plays a role in its recruitment into the virion

The alphaherpesvirus UL51 protein is a tegument component that interacts with the viral glycoprotein E and functions at multiple steps in virus assembly and spread in epithelial cells. We show here that pUL51 forms a complex in infected cells with another conserved tegument protein, pUL7. This complex can form in the absence of other viral proteins and is largely responsible for recruitment of pUL7 to cytoplasmic membranes and into the virion tegument. Incomplete colocalization of pUL51 and pUL7 in infected cells, however, suggests that a significant fraction of the population of each protein is not complexed with the other and that they may accomplish independent functions.

IMPORTANCE

The ability of herpesviruses to spread from cell to cell in the face of an immune response is critical for disease and shedding following reactivation from latency. Cell-to-cell spread is a conserved ability of herpesviruses, and the identification of conserved viral genes that mediate this process will aid in the design of attenuated vaccines and of novel therapeutics. The conserved UL51 gene of herpes simplex virus 1 plays important roles in cell-to-cell spread and in virus assembly in the cytoplasm, both of which likely depend on specific interactions with other viral and cellular proteins. Here we identify one of those interactions with the product of another conserved herpesvirus gene, UL7, and show that formation of this complex mediates recruitment of UL7 to membranes and to the virion.

Herpes simplex virus gE/gI extracellular domains promote axonal transport and spread from neurons to epithelial cells

Following reactivation from latency, there are two distinct steps in the spread of herpes simplex virus (HSV) from infected neurons to epithelial cells: (i) anterograde axonal transport of virus particles from neuron bodies to axon tips and (ii) exocytosis and spread of extracellular virions across cell junctions into adjacent epithelial cells. The HSV heterodimeric glycoprotein gE/gI is important for anterograde axonal transport, and gE/gI cytoplasmic domains play important roles in sorting of virus particles into axons. However, the roles of the large (∼400-residue) gE/gI extracellular (ET) domains in both axonal transport and neuron-to-epithelial cell spread have not been characterized. Two gE mutants, gE-277 and gE-348, contain small insertions in the gE ET domain, fold normally, form gE/gI heterodimers, and are incorporated into virions. Both gE-277 and gE-348 did not function in anterograde axonal transport; there were markedly reduced numbers of viral capsids and glycoproteins compared with wild-type HSV. The defects in axonal transport were manifest in neuronal cell bodies, involving missorting of HSV capsids before entry into proximal axons. Although there were diminished numbers of mutant gE-348 capsids and glycoproteins in distal axons, there was efficient spread to adjacent epithelial cells, similar to wild-type HSV. In contrast, virus particles produced by HSV gE-277 spread poorly to epithelial cells, despite numbers of virus particles similar to those for HSV gE-348. These results genetically separate the two steps in HSV spread from neurons to epithelial cells and demonstrate that the gE/gI ET domains function in both processes.

IMPORTANCE

An essential phase of the life cycle of herpes simplex virus (HSV) and other alphaherpesviruses is the capacity to reactivate from latency and then spread from infected neurons to epithelial tissues. This spread involves at least two steps: (i) anterograde transport to axon tips followed by (ii) exocytosis and extracellular spread from axons to epithelial cells. HSV gE/gI is a glycoprotein that facilitates this virus spread, although by poorly understood mechanisms. Here, we show that the extracellular (ET) domains of gE/gI promote the sorting of viral structural proteins into proximal axons to begin axonal transport. However, the gE/gI ET domains also participate in the extracellular spread from axon tips across cell junctions to epithelial cells. Understanding the molecular mechanisms involved in gE/gI-mediated sorting of virus particles into axons and extracellular spread to adjacent cells is fundamentally important for identifying novel targets to reduce alphaherpesvirus disease.

The herpes virus Fc receptor gE-gI mediates antibody bipolar bridging to clear viral antigens from the cell surface

The Herpes Simplex Virus 1 (HSV-1) glycoprotein gE-gI is a transmembrane Fc receptor found on the surface of infected cells and virions that binds human immunoglobulin G (hIgG). gE-gI can also participate in antibody bipolar bridging (ABB), a process by which the antigen-binding fragments (Fabs) of the IgG bind a viral antigen while the Fc binds to gE-gI. IgG Fc binds gE-gI at basic, but not acidic, pH, suggesting that IgG bound at extracellular pH by cell surface gE-gI would dissociate and be degraded in acidic endosomes/lysosomes if endocytosed. The fate of viral antigens associated with gE-gI-bound IgG had been unknown: they could remain at the cell surface or be endocytosed with IgG. Here, we developed an in vitro model system for ABB and investigated the trafficking of ABB complexes using 4-D confocal fluorescence imaging of ABB complexes with transferrin or epidermal growth factor, well-characterized intracellular trafficking markers. Our data showed that cells expressing gE-gI and the viral antigen HSV-1 gD endocytosed anti-gD IgG and gD in a gE-gI-dependent process, resulting in lysosomal localization. These results suggest that gE-gI can mediate clearance of infected cell surfaces of anti-viral host IgG and viral antigens to evade IgG-mediated responses, representing a general mechanism for viral Fc receptors in immune evasion and viral pathogenesis.

The herpes simplex virus 1 UL51 gene product has cell type-specific functions in cell-to-cell spread

The herpes simplex virus 1 (HSV-1) UL51 gene encodes a 244-amino-acid (aa) palmitoylated protein that is conserved in all herpesviruses. The alphaherpesvirus UL51 (pUL51) protein has been reported to function in nuclear egress and cytoplasmic envelopment. No complete deletion has been generated because of the overlap of the UL51 coding sequence 5' end with the UL52 promoter sequences, but partial deletions generated in HSV and pseudorabies virus (PrV) suggest an additional function in epithelial cell-to-cell spread. Here we show partial uncoupling of the replication, release, and cell-to-cell spread functions of HSV-1 pUL51 in two ways. Viruses in which aa 73 to 244 were deleted from pUL51 or in which a conserved YXXΦ motif near the N terminus was altered showed cell-specific defects in spread that cannot be accounted for by defects in replication and virus release. Also, a cell line that expresses C-terminally enhanced green fluorescent protein (EGFP)-tagged pUL51 supported normal virus replication and release into the medium but the formation of only small plaques. This cell line also failed to support normal localization of gE to cell junctions. gE and pUL51 partially colocalized in infected cells, and these two proteins could be coimmunoprecipitated from infected cells, suggesting that they can form a complex during infection. The cell-to-cell spread defect associated with the pUL51 mutation was more severe than that associated with gE-null virus, suggesting that pUL51 has gE-independent functions in epithelial cell spread.

IMPORTANCE

Herpesviruses establish and reactivate from lifelong latency in their hosts. When they reactivate, they are able to spread within their hosts despite the presence of a potent immune response that includes neutralizing antibody. This ability is derived in part from a specialized mechanism for virus spread between cells. Cell-to-cell spread is a conserved property of herpesviruses that likely relies on conserved viral genes. An understanding of their function may aid in the design of vaccines and therapeutics. Here we show that one of the conserved viral genes, UL51, has an important role in cell-to-cell spread in addition to its previously demonstrated role in virus assembly. We find that its function depends on the type of cell that is infected, and we show that it interacts with and modulates the function of another viral spread factor, gE.

Cell-cell contact-mediated hepatitis C virus (HCV) transfer, productive infection, and replication and their requirement for HCV receptors

Hepatitis C virus (HCV) infection is believed to begin with interactions between cell-free HCV and cell receptors that include CD81, scavenger receptor B1 (SR-B1), claudin-1 (CLDN1), and occludin (OCLN). In this study, we have demonstrated that HCV spreading from infected hepatocytes to uninfected hepatocytes leads to the transfer of HCV and the formation of infection foci and is cell density dependent. This cell-cell contact-mediated (CCCM) HCV transfer occurs readily and requires all these known HCV receptors and an intact actin cytoskeleton. With a fluorescently labeled replication-competent HCV system, the CCCM transfer process was further dissected by live-cell imaging into four steps: donor cell-target cell contact, formation of viral puncta-target cell conjugation, transfer of viral puncta, and posttransfer. Importantly, the CCCM HCV transfer leads to productive infection of target cells. Taken together, these results show that CCCM HCV transfer constitutes an important and effective route for HCV infection and dissemination. These findings will aid in the development of new and novel strategies for preventing and treating HCV infection.

Rapid host immune response and viral dynamics in herpes simplex virus-2 infection

Herpes simplex virus-2 (HSV-2) is periodically shed throughout the human genital tract. Although a high viral load correlates with the development of genital ulcers, shedding also commonly occurs even when ulcers are absent, allowing for silent transmission during coitus and contributing to high seroprevalence of HSV-2 worldwide. Frequent viral reactivation occurs within ganglia despite diverse and complementary host and viral mechanisms that predispose toward latency, suggesting that viral replication may be constantly occurring in a small minority of neurons at these sites. Within genital mucosa, the in vivo expansion and clearance rates of HSV-2 are extremely rapid. Resident dendritic cells and memory HSV-2 specific T cells persist at prior sites of genital tract reactivation and, in conjunction with prompt innate recognition of infected cells, lead to rapid containment of infected cells. The fact that immune responses usually control viral replication in genital skin before lesions develop provides hope that enhancing such responses could lead to effective vaccines and immunotherapies.

Griffithsin protects mice from genital herpes by preventing cell-to-cell spread

Griffithsin, which binds N-linked glycans on gp120 to prevent HIV entry, has the most potent HIV-1 inhibitory activity described for any antiviral lectin and is being developed for topical preexposure prophylaxis. The current studies were designed to further assess its potential by exploring its activity against herpes simplex virus 2 (HSV-2), a cofactor for HIV acquisition, in vitro and in a murine model. Safety was evaluated by examining its impact on epithelial barrier integrity in polarized cultures and testing whether repeated intravaginal dosing potentiates the susceptibility of mice to genital herpes. Griffithsin displayed modest inhibitory activity against HSV-2 if present during viral entry but completely blocked plaque formation if present postentry, reduced plaque size, and prevented cell-to-cell spread. These in vitro findings translated to significant protection against genital herpes in mice treated with 0.1% griffithsin gel. Griffithsin, but not placebo gel, prevented viral spread (visualized with a luciferase-expressing virus), significantly reduced disease scores, and resulted in greater survival (P < 0.05, log rank test). Protection persisted when HSV-2 was introduced in seminal plasma. Although griffithsin triggered a small decline in transepithelial electrical resistance in polarized cultures, this did not translate to any significant increase in the ability of HIV to migrate from the apical to the basolateral chamber nor to an increase in susceptibility to HSV-2 in mice treated with griffithsin gel for 7 days. These findings demonstrate that griffithsin inhibits HSV-2 by a unique mechanism of blocking cell-to-cell spread and support its further development for HIV and HSV-2 prevention.

Entry of herpesviruses into cells: the enigma variations

The entry of herpesviruses into their target cells is complex at many levels. Virus entry proceeds by a succession of interactions between viral envelope glycoproteins and molecules on the cell membrane. The process is divided into distinct steps: attachment to the cell surface, interaction with a specific entry receptor, internalization of the particle (optional and cell specific), and membrane fusion. Several viral envelope glycoproteins are involved in one or several of these steps. The most conserved entry glycoproteins in the herpesvirus family (gB, gH/gL) are involved in membrane fusion. Around this functional core, herpesviruses have a variety of receptor binding glycoproteins, which interact with cell surface proteins often from different families. This interaction activates and controls the actual fusion machinery. Interactions with cellular receptors and between viral glycoproteins have to be tightly coordinated and regulated to guarantee successful entry. Although additional entry receptors for herpesviruses continue to be identified, the molecular interactions between viral glycoproteins remain mostly enigmatic. This chapter will review our current understanding of the molecular interactions that occur during herpesvirus entry from attachment to fusion. Particular emphasis will be placed on structure-based representation of receptor binding as a trigger of fusion during herpes simplex virus entry.

Function of glycoprotein E of herpes simplex virus requires coordinated assembly of three tegument proteins on its cytoplasmic tail

Glycoprotein E (gE) of HSV plays a key role in cell-to-cell spread and virus-induced cell fusion. Here, we report that this function of gE requires the cooperation of tegument proteins UL11, UL16, and UL21. We found that the four proteins come together with very high efficiency to form a complex in transfected cells and in a manner that is regulated and coordinated. In particular, the inefficient interaction of UL16 with each membrane protein (UL11 and gE) observed in pairwise transfections became efficient when other binding partners were present. The significance of these interactions was revealed in studies of viral mutants, which showed that each of these tegument proteins is critical for processing, transport, and biological activity of gE. These findings provide insights into the mechanisms of how gE executes its function and also have implications in understanding HSV assembly and budding.

Herpes simplex virus membrane proteins gE/gI and US9 act cooperatively to promote transport of capsids and glycoproteins from neuron cell bodies into initial axon segments

Herpes simplex virus (HSV) and other alphaherpesviruses must move from sites of latency in ganglia to peripheral epithelial cells. How HSV navigates in neuronal axons is not well understood. Two HSV membrane proteins, gE/gI and US9, are key to understanding the processes by which viral glycoproteins, unenveloped capsids, and enveloped virions are transported toward axon tips. Whether gE/gI and US9 function to promote the loading of viral proteins onto microtubule motors in neuron cell bodies or to tether viral proteins onto microtubule motors within axons is not clear. One impediment to understanding how HSV gE/gI and US9 function in axonal transport relates to observations that gE(-), gI(-), or US9(-) mutants are not absolutely blocked in axonal transport. Mutants are significantly reduced in numbers of capsids and glycoproteins in distal axons, but there are less extensive effects in proximal axons. We constructed HSV recombinants lacking both gE and US9 that transported no detectable capsids and glycoproteins to distal axons and failed to spread from axon tips to adjacent cells. Live-cell imaging of a gE(-)/US9(-) double mutant that expressed fluorescent capsids and gB demonstrated >90% diminished capsids and gB in medial axons and no evidence for decreased rates of transport, stalling, or increased retrograde transport. Instead, capsids, gB, and enveloped virions failed to enter proximal axons. We concluded that gE/gI and US9 function in neuron cell bodies, in a cooperative fashion, to promote the loading of HSV capsids and vesicles containing glycoproteins and enveloped virions onto microtubule motors or their transport into proximal axons.

Investigation of varicella-zoster virus neurotropism and neurovirulence using SCID mouse-human DRG xenografts

Varicella-zoster virus (VZV) is a medically important human alphaherpesvirus. Investigating pathogenic mechanisms that contribute to VZV neurovirulence are made difficult by a marked host restriction. Our approach to investigating VZV neurotropism and neurovirulence has been to develop a mouse-human xenograft model in which human dorsal root ganglia (DRG) are maintained in severe compromised immunodeficient (SCID) mice. In this review, we will describe our key findings using this model in which we have demonstrated that VZV infection of SCID DRG xenograft results in rapid and efficient spread, enabled by satellite cell infection and polykaryon formation, which facilitates robust viral replication and release of infectious virus. In neurons that persist following this acute replicative phase, VZV genomes are present at low frequency with limited gene transcription and no protein synthesis, a state that resembles VZV latency in the natural human host. VZV glycoprotein I and interaction between glycoprotein I and glycoprotein E are critical for neurovirulence. Our work demonstrates that the DRG model can reveal characteristics about VZV replication and long-term persistence of latent VZV genomes in human neuronal tissues, in vivo, in an experimental system that may contribute to our knowledge of VZV neuropathogenesis.

Human cytomegalovirus glycoprotein gO complexes with gH/gL, promoting interference with viral entry into human fibroblasts but not entry into epithelial cells

A complex of five human cytomegalovirus virus (HCMV) proteins, gH, gL, UL128, UL130, and UL131 (gH/gL/UL128-131), is essential for virus entry into epithelial cells. We previously showed that gH/gL/UL128-131 expressed in epithelial cells interferes with subsequent HCMV entry into cells. There was no interference with only gH/gL or gB. We concluded that the expression of gH/gL/UL128-131 causes a mislocalization or downregulation of epithelial cell proteins that HCMV requires for entry. In contrast, gH/gL/UL128-131 expression in fibroblasts did not produce interference, suggesting a different mechanism for entry. Here, we show that the coexpression of another HCMV glycoprotein, gO, with gH/gL in human fibroblasts interferes with HCMV entry into fibroblasts but not epithelial cells. However, the coexpression of gO with gH/gL did not increase the cell surface expression level of gH/gL and did not enhance cell-cell fusion, a process that depends upon cell surface gH/gL. Instead, gO promoted the export of gH/gL from the endoplasmic reticulum (ER) and the accumulation of gH/gL in the trans-Golgi network. Thus, interference with gH/gL or gH/gL/gO, i.e., the mislocalization or blocking of entry mediators, occurs in cytoplasmic membranes and not in cell surface membranes of fibroblasts. Together, the results provide additional support for our hypotheses that epithelial cells express putative gH/gL/UL128-1331 receptors important for HCMV entry and that fibroblasts express distinct gH/gL receptors.

Interaction and interdependent packaging of tegument protein UL11 and glycoprotein e of herpes simplex virus

The UL11 tegument protein of herpes simplex virus plays a critical role in the secondary envelopment; however, the mechanistic details remain elusive. Here, we report a new function of UL11 in the budding process in which it directs efficient acquisition of glycoprotein E (gE) via a direct interaction. In vitro binding assays showed that the interaction required only the first 28, membrane-proximal residues of the cytoplasmic tail of gE, and the C-terminal 26 residues of UL11. A second, weaker binding site was also found in the N-terminal half of UL11. The significance of the gE-UL11 interaction was subsequently investigated with viral deletion mutants. In the absence of the gE tail, virion packaging of UL11, but not other tegument proteins such as VP22 and VP16, was reduced by at least 80%. Reciprocally, wild-type gE packaging was also drastically reduced by about 87% in the absence of UL11, and this defect could be rescued in trans by expressing U(L)11 at the U(L)35 locus. Surprisingly, a mutant that lacks the C-terminal gE-binding site of UL11 packaged nearly normal amounts of gE despite its strong interaction with the gE tail in vitro, indicating that the interaction with the UL11 N terminus may be important. Mutagenesis studies of the UL11 N terminus revealed that the association of UL11 with membrane was not required for this function. In contrast, the UL11 acidic cluster motif was found to be critical for gE packaging and was not replaceable with foreign acidic clusters. Together, these results highlight an important role of UL11 in the acquisition of glycoprotein-enriched lipid bilayers, and the findings may also have important implications for the role of UL11 in gE-mediated cell-to-cell spread.

Direct and specific binding of the UL16 tegument protein of herpes simplex virus to the cytoplasmic tail of glycoprotein E

The UL16 tegument protein of herpes simplex virus (HSV) is conserved throughout all of the herpesvirus families. Previous studies have shown that the binding of HSV to heparan sulfate molecules on the host cell triggers the release of UL16 from the capsid, but the mechanism by which the signal is sent from the virion surface into the tegument is unknown. Here, we report that a glutathione S-transferase chimera bearing the cytoplasmic tail of viral glycoprotein E (gE) is capable of binding to UL16 in lysates of eukaryotic cells or purified from bacteria. Moreover, mass spectrometry studies of native-UL16 complexes purified from infected cells also revealed the presence of gE. Proof that UL16-gE can interact within cells required the fortuitous discovery of a mutant possessing only the first 155 residues of UL16. Confocal microscopy of cotransfected cells revealed that this mutant colocalized with gE in the cytoplasm, whereas it was found throughout the cytoplasm and nucleus when expressed alone. In contrast, the full-length UL16 molecule was very poorly capable of finding gE. Moreover, membrane flotation assays showed that UL16(1-155) was able to float to the top of sucrose step gradients when coexpressed with gE, whereas full-length UL16 was not. Thus, the discovery of the UL16(1-155) mutant confirmed the specific in vitro interaction with gE and provides evidence that a binding domain at the N terminus of UL16 may be controlled by a regulatory domain within the C terminus. These findings suggest the possibility that the UL16-gE interaction may play roles in the tegument signaling mechanism, virus budding, and the gE-mediated mechanism of cell-to-cell spread.

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

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

The herpes simplex virus 1 IgG fc receptor blocks antibody-mediated complement activation and antibody-dependent cellular cytotoxicity in vivo

Herpes simplex virus 1 (HSV-1) glycoprotein E (gE) mediates cell-to-cell spread and functions as an IgG Fc receptor (FcγR) that blocks the Fc domain of antibody targeting the virus or infected cell. Efforts to assess the functions of the HSV-1 FcγR in vivo have been hampered by difficulties in preparing an FcγR-negative strain that is relatively intact for spread. Here we report the FcγR and spread phenotypes of NS-gE264, which is a mutant strain that has four amino acids inserted after gE residue 264. The virus is defective in IgG Fc binding yet causes zosteriform disease in the mouse flank model that is only minimally reduced compared with wild-type and the rescue strains. The presence of zosteriform disease suggests that NS-gE264 spread functions are well maintained. The HSV-1 FcγR binds the Fc domain of human, but not murine IgG; therefore, to assess FcγR functions in vivo, mice were passively immunized with human IgG antibody to HSV. When antibody was inoculated intraperitoneally 20 h prior to infection or shortly after virus reached the dorsal root ganglia, disease severity was significantly reduced in mice infected with NS-gE264, but not in mice infected with wild-type or rescue virus. Studies of C3 knockout mice and natural killer cell-depleted mice demonstrated that the HSV-1 FcγR blocked both IgG Fc-mediated complement activation and antibody-dependent cellular cytotoxicity. Therefore, the HSV-1 FcγR promotes immune evasion from IgG Fc-mediated activities and likely contributes to virulence at times when antibody is present, such as during recurrent infections.

Varicella-zoster virus glycoprotein E is a critical determinant of virulence in the SCID mouse-human model of neuropathogenesis

Varicella-zoster virus (VZV) is a neurotropic alphaherpesvirus. VZV infection of human dorsal root ganglion (DRG) xenografts in immunodeficient mice models the infection of sensory ganglia. We examined DRG infection with recombinant VZV (recombinant Oka [rOka]) and the following gE mutants: gEΔ27-90, gEΔCys, gE-AYRV, and gE-SSTT. gEΔ27-90, which lacks the gE domain that interacts with a putative receptor insulin-degrading enzyme (IDE), replicated as extensively as rOka, producing infectious virions and significant cytopathic effects within 14 days of inoculation. Since neural cells express IDE, the gE/IDE interaction was dispensable for VZV neurotropism. In contrast, gEΔCys, which lacks gE/gI heterodimer formation, was significantly impaired at early times postinfection; viral genome copy numbers increased slowly, and infectious virus production was not detected until day 28. Delayed replication was associated with impaired cell-cell spread in ganglia, similar to the phenotype of a gI deletion mutant (rOkaΔgI). However, at later time points, infection of satellite cells and other supportive nonneuronal cells resulted in extensive DRG tissue damage and cell loss such that cytopathic changes observed at day 70 were more severe than those for rOka-infected DRG. The replication of gE-AYRV, which is impaired for trans-Golgi network (TGN) localization, and the replication of gE-SSTT, which contains mutations in an acidic cluster, were equivalent to that of rOka, causing significant cytopathic effects and infectious virus production by day 14; genome copy numbers were equivalent to those of rOka. These experiments suggest that the gE interaction with cellular IDE, gE targeting to TGN sites of virion envelopment, and phosphorylation at SSTT are dispensable for VZV DRG infection, whereas the gE/gI interaction is critical for VZV neurovirulence.

Herpes simplex virus type 1/adeno-associated virus hybrid vectors

Herpes simplex virus type 1 (HSV-1) amplicons can accommodate foreign DNA of any size up to 150 kbp and, therefore, allow extensive combinations of genetic elements. Genomic sequences as well as cDNA, large transcriptional regulatory sequences for cell type-specific expression, multiple transgenes, and genetic elements from other viruses to create hybrid vectors may be inserted in a modular fashion. Hybrid amplicons use genetic elements from HSV-1 that allow replication and packaging of the vector DNA into HSV-1 virions, and genetic elements from other viruses that either direct integration of transgene sequences into the host genome or allow episomal maintenance of the vector. Thus, the advantages of the HSV-1 amplicon system, including large transgene capacity, broad host range, strong nuclear localization, and availability of helper virus-free packaging systems are retained and combined with those of heterologous viral elements that confer genetic stability to the vector DNA. Adeno-associated virus (AAV) has the unique capability of integrating its genome into a specific site, designated AAVS1, on human chromosome 19. The AAV rep gene and the inverted terminal repeats (ITRs) that flank the AAV genome are sufficient for this process. HSV-1 amplicons have thus been designed that contain the rep gene and a transgene cassette flanked by AAV ITRs. These HSV/AAV hybrid vectors direct site-specific integration of transgene sequences into AAVS1 and support long-term transgene expression.

Functions of the unique N-terminal region of glycoprotein E in the pathogenesis of varicella-zoster virus infection

Varicella-zoster virus (VZV) is an alphaherpesvirus that infects skin, lymphocytes, and sensory ganglia. VZV glycoprotein E (gE) has a unique N-terminal region (aa1-188), which is required for replication and includes domains involved in secondary envelopment, efficient cell-cell spread, and skin infection in vivo. The nonconserved N-terminal region also mediates binding to the insulin-degrading enzyme (IDE), which is proposed to be a VZV receptor. Using viral mutagenesis to make the recombinant rOka-DeltaP27-G90, we showed that amino acids in this region are required for gE/IDE binding in infected cells; this deletion reduced cell-cell spread in vitro and skin infection in vivo. However, a gE point mutation, linker insertions, and partial deletions in the aa27-90 region, and deletion of a large portion of the unique N-terminal region, aa52-187, had similar or more severe effects on VZV replication in vitro and in vivo without disrupting the gE/IDE interaction. VZV replication in T cells in vivo was not impaired by deletion of gE aa27-90, suggesting that these gE residues are not essential for VZV T cell tropism. However, the rOka-DeltaY51-P187 mutant failed to replicate in T cell xenografts as well as skin in vivo. VZV tropism for T cells and skin, which is necessary for its life cycle in the human host, requires this nonconserved region of the N-terminal region of VZV gE.

EBV-positive human sera contain antibodies against the EBV BMRF-2 protein

We previously showed that the EBV glycoprotein BMRF-2 contains a functional integrin-binding Arg-Gly-Asp (RGD) domain that plays an important role in viral infection and cell-to-cell spread of progeny virions in oral epithelial cells. In this study, we found that EBV-seropositive human sera contain antibodies against BMRF-2. The inhibitory effect of EBV-positive sera on EBV infection of oral epithelial cells was substantially reduced by pre-incubation of serum samples with the BMRF-2 RGD peptide, suggesting that anti-BMRF-2 human antibodies possess neutralizing activity. EBV-specific sera reacted strongly with the BMRF-2 extracellular domain (170-213 aa) containing the RGD motif, whereas they reacted only weakly or not at all with a mutated form of the BMRF-2 extracellular domain containing AAA instead of RGD. These data indicate that RGD motif of BMRF-2 is part of an immunodominant antigenic determinant within the extracellular domain of BMRF-2 that may contribute to EBV neutralization during EBV reactivation.

Investigation of the mechanism by which herpes simplex virus type 1 LAT sequences modulate preferential establishment of latent infection in mouse trigeminal ganglia

We previously demonstrated that herpes simplex virus type 1 (HSV-1) preferentially establishes latent infection in monoclonal antibody (MAb) A5-positive ganglionic neurons and that a 2.8-kb portion of the HSV-1 genome, corresponding to the 5' end of the LAT (latency-associated transcript) coding region, is responsible for this phenotype (38, 65). In the current study we carried out further genetic mapping of this latency phenotype and investigated some of the mechanisms that might be responsible. Studies with the chimeric virus HSV-1 17syn+/LAT2, an HSV-1 virus engineered to express HSV-2 LAT, demonstrated that this virus exhibited an HSV-2 latency phenotype, preferentially establishing latency in MAb KH10-positive neurons. This result is complementary to that previously described for the chimeric virus HSV-2 333/LAT1 and indicate that the HSV-1 latency phenotype can be changed to that of HSV-2 by substitution of a 2.8-kb piece of complementary viral DNA. Sequential studies in which we evaluated the pattern of HSV-1 latent infection of the mouse trigeminal ganglion following ocular inoculation with viruses with deletions of functional thymidine kinase, glycoprotein E, ICP0, and US9 protein demonstrate that preferential establishment of HSV-1 latent infection in A5-positive neurons is not a consequence of (i) differential access of HSV-1 to A5-positive neurons,(ii) differential cell-to-cell spread of HSV-1 to A5-positive neurons, (iii) differential "round-trip" spread of HSV-1 to A5-positive neurons, or (iv) expression of ICP0. Additional mapping studies with the HSV-1 LAT deletion viruses dLAT371, 17DeltaSty, and 17Delta348 indicate that most of the LAT 5' exon is not required for HSV-1 to preferentially establish latent infection in A5-positive neurons.