Coiled-coil domains in glycoproteins B and H are involved in human cytomegalovirus membrane fusion

Understanding the Neurotrophic Virus Mechanisms and Their Potential Effect on Systemic Lupus Erythematosus Development

Central nervous system (CNS) pathologies are a public health concern, with viral infections one of their principal causes. These viruses are known as neurotropic pathogens, characterized by their ability to infiltrate the CNS and thus interact with various cell populations, inducing several diseases. The immune response elicited by neurotropic viruses in the CNS is commanded mainly by microglia, which, together with other local cells, can secrete inflammatory cytokines to fight the infection. The most relevant neurotropic viruses are adenovirus (AdV), cytomegalovirus (CMV), enterovirus (EV), Epstein-Barr Virus (EBV), herpes simplex virus type 1 (HSV-1), and herpes simplex virus type 2 (HSV-2), lymphocytic choriomeningitis virus (LCMV), and the newly discovered SARS-CoV-2. Several studies have associated a viral infection with systemic lupus erythematosus (SLE) and neuropsychiatric lupus (NPSLE) manifestations. This article will review the knowledge about viral infections, CNS pathologies, and the immune response against them. Also, it allows us to understand the relevance of the different viral proteins in developing neuronal pathologies, SLE and NPSLE.

Insights into the Transcriptome of Human Cytomegalovirus: A Comprehensive Review

Human cytomegalovirus (HCMV) is a widespread pathogen that poses significant risks to immunocompromised individuals. Its genome spans over 230 kbp and potentially encodes over 200 open-reading frames. The HCMV transcriptome consists of various types of RNAs, including messenger RNAs (mRNAs), long non-coding RNAs (lncRNAs), circular RNAs (circRNAs), and microRNAs (miRNAs), with emerging insights into their biological functions. HCMV mRNAs are involved in crucial viral processes, such as viral replication, transcription, and translation regulation, as well as immune modulation and other effects on host cells. Additionally, four lncRNAs (RNA1.2, RNA2.7, RNA4.9, and RNA5.0) have been identified in HCMV, which play important roles in lytic replication like bypassing acute antiviral responses, promoting cell movement and viral spread, and maintaining HCMV latency. CircRNAs have gained attention for their important and diverse biological functions, including association with different diseases, acting as microRNA sponges, regulating parental gene expression, and serving as translation templates. Remarkably, HCMV encodes miRNAs which play critical roles in silencing human genes and other functions. This review gives an overview of human cytomegalovirus and current research on the HCMV transcriptome during lytic and latent infection.

The cytomegalovirus gB/MF59 vaccine candidate induces antibodies against an antigenic domain controlling cell-to-cell spread

Vaccination against human cytomegalovirus (CMV) infection remains high priority. A recombinant form of a protein essential for CMV entry, glycoprotein B (gB), demonstrated partial protection in a clinical trial (NCT00299260) when delivered with the MF59 adjuvant. Although the antibody titre against gB correlated with protection poor neutralising responses against the 5 known antigenic domains (AD) of gB were evident. Here, we show that vaccination of CMV seronegative patients induces an antibody response against a region of gB we term AD-6. Responses to the polypeptide AD-6 are detected in >70% of vaccine recipients yet in <5% of naturally infected people. An AD-6 antibody binds to gB and to infected cells but not the virion directly. Consistent with this, the AD-6 antibody is non-neutralising but, instead, prevents cell-cell spread of CMV in vitro. The discovery of AD-6 responses has the potential to explain part of the protection mediated by gB vaccines against CMV following transplantation.

A Variant Allele in Varicella-Zoster Virus Glycoprotein B Selected during Production of the Varicella Vaccine Contributes to Its Attenuation

Attenuation of the live varicella Oka vaccine (vOka) has been attributed to mutations in the genome acquired during cell culture passage of pOka (parent strain); however, the precise mechanisms of attenuation remain unknown. Comparative sequence analyses of several vaccine batches showed that over 100 single-nucleotide polymorphisms (SNPs) are conserved across all vaccine batches; 6 SNPs are nearly fixed, suggesting that these SNPs are responsible for attenuation. By contrast, prior analysis of chimeric vOka and pOka recombinants indicates that loci other than these six SNPs contribute to attenuation. Here, we report that pOka consists of a heterogenous population of virus sequences with two nearly equally represented bases, guanine (G) or adenine (A), at nucleotide 2096 of the ORF31 coding sequence, which encodes glycoprotein B (gB) resulting in arginine (R) or glutamine (Q), respectively, at amino acid 699 of gB. By contrast, 2096A/699Q is dominant in vOka (>99.98%). gB699Q/gH/gL showed significantly less fusion activity than gB699R/gH/gL in a cell-based fusion assay. Recombinant pOka with gB669Q (rpOka_gB699Q) had a similar growth phenotype as vOka during lytic infection in cell culture including human primary skin cells; however, rpOka_gB699R showed a growth phenotype similar to pOka. rpOka_gB699R entered neurons from axonal terminals more efficiently than rpOka_gB699Q in the presence of cell membrane-derived vesicles containing gB. Strikingly, when a mixture of pOka with both alleles equally represented was used to infect human neurons from axon terminals, pOka with gB699R was dominant for virus entry. These results identify a variant allele in gB that contributes to attenuation of vOka. IMPORTANCE The live-attenuated varicella vaccine has reduced the burden of chickenpox. Despite its development in 1974, the molecular basis for its attenuation is still not well understood. Since the live-attenuated varicella vaccine is the only licensed human herpesvirus vaccine that prevents primary disease, it is important to understand the mechanism for its attenuation. Here we identify that a variant allele in glycoprotein B (gB) selected during generation of the varicella vaccine contributes to its attenuation. This variant is impaired for fusion, virus entry into neurons from nerve terminals, and replication in human skin cells. Identification of a variant allele in gB, one of the essential herpesvirus core genes, that contributes to its attenuation may provide insights that assist in the development of other herpesvirus vaccines.

Neutralizing Monoclonal Antibodies Reduce Human Cytomegalovirus Infection and Spread in Developing Placentas

Congenital human cytomegalovirus (HCMV) infection is a leading cause of birth defects worldwide, yet the most effective strategies for preventing virus transmission during pregnancy are unknown. We measured the efficacy of human monoclonal antibodies (mAbs) to HCMV attachment/entry factors glycoprotein B (gB) and the pentameric complex, gH/gL-pUL128–131, in preventing infection and spread of a clinical strain in primary placental cells and explants of developing anchoring villi. A total of 109 explants from five first-trimester placentas were cultured, and infection was analyzed in over 400 cell columns containing ~120,000 cytotrophoblasts (CTBs). mAbs to gB and gH/gL, 3-25 and 3-16, respectively, neutralized infection in stromal fibroblasts and trophoblast progenitor cells. mAbs to pUL128-131 of the pentameric complex, 1-103 and 2-18, neutralized infection of amniotic epithelial cells better than mAbs 3-25 and 3-16 and hyperimmune globulin. Select mAbs neutralized infection of cell column CTBs, with mAb 2-18 most effective, followed by mAb 3-25. Treatment of anchoring villi with mAbs postinfection reduced spread in CTBs and impaired formation of virion assembly compartments, with mAb 2-18 achieving better suppression at lower concentrations. These results predict that antibodies generated by HCMV vaccines or used for passive immunization have the potential to reduce transplacental transmission and congenital disease.

Identification of adipocyte plasma membrane-associated protein as a novel modulator of human cytomegalovirus infection

Human cytomegalovirus (HCMV) is a ubiquitous pathogen that can cause disability in newborns and serious clinical diseases in immunocompromised patients. HCMV has a large genome with enormous coding potential; its viral particles are equipped with complicated glycoprotein complexes and can infect a wide range of human cells. Although multiple host cellular receptors interacting with viral glycoproteins have been reported, the mechanism of HCMV infection remains a mystery. Here we report identification of adipocyte plasma membrane-associated protein (APMAP) as a novel modulator active in the early stage of HCMV infection. APMAP is necessary for HCMV infection in both epithelial cells and fibroblasts; knockdown of APMAP expression significantly reduced HCMV infection of these cells. Interestingly, ectopic expression of human APMAP in cells refractory to HCMV infection, such as canine MDCK and murine NIH/3T3 cells, promoted HCMV infection. Furthermore, reduction in viral immediate early (IE) gene transcription at 6 h post infection and delayed nucleus translocation of tegument delivered pp65 at 4 h post infection were detected in APMAP-deficient cells but not in the wildtype cells. These results suggest that APMAP plays a role in the early stage of HCMV infection. Results from biochemical studies of APMAP and HCMV proteins suggest that APMAP could participate in HCMV infection through interaction with gH/gL containing glycoprotein complexes at low pH and mediate nucleus translocation of tegument pp65. Taken together, our results suggest that APMAP functions as a modulator promoting HCMV infection in multiple cell types and is an important player in the complex HCMV infection mechanism.

Battle between Host Immune Cellular Responses and HCMV Immune Evasion

Human cytomegalovirus (HCMV) is ubiquitously prevalent. HCMV infection is typically asymptomatic and controlled by the immune system in healthy individuals, yet HCMV can be severely pathogenic for the fetus during pregnancy and in immunocompromised persons, such as transplant recipients or HIV infected patients. HCMV has co-evolved with the hosts, developed strategies to hide from immune effector cells and to successfully survive in the human organism. One strategy for evading or delaying the immune response is maintenance of the viral genome to establish the phase of latency. Furthermore, HCMV immune evasion involves the downregulation of human leukocyte antigens (HLA)-Ia molecules to hide infected cells from T-cell recognition. HCMV expresses several proteins that are described for downregulation of the HLA class I pathway via various mechanisms. Here, we review the wide range of immune evasion mechanisms of HCMV. Understanding the mechanisms of HCMV immune evasion will contribute to the development of new customized therapeutic strategies against the virus.

Development of novel vaccines against human cytomegalovirus

Congenital human cytomegalovirus (HCMV) infection and HCMV infection of the immunosuppressed patients cause significant morbidity and mortality, and vaccine development against HCMV is a major public health priority. Efforts to develop HCMV vaccines have been ongoing for 50 y, though no HCMV vaccine has been licensed; encouraging and promising results have obtained from both preclinical and clinical trials. HCMV infection induces a wide range of humoral and T cell-mediated immune responses, and both branches of immunity are correlated with protection. In recent years, there have been novel approaches toward the development of HCMV vaccines and demonstrated that vaccine candidates could potentially provide superior protection over natural immunity acquired following HCMV infection. Further, rationally designed HCMV protein antigens that express native conformational epitopes could elicit optimal immune response. HCMV vaccine candidates, using a multi-antigen approach, to maximize the elicited protective immunity will most likely be successful in development of HCMV vaccine.

Novel trimeric human cytomegalovirus glycoprotein B elicits a high-titer neutralizing antibody response

Human cytomegalovirus (HCMV) is a major cause of disability in congenitally infected infants and in the immunosuppressed. There is currently no licensed prophylactic HCMV vaccine. The HCMV envelope glycoprotein B (gB) is considered a major vaccine target antigen based on its critical role in mediating viral-host cell fusion and thus viral entry. The natural conformation of HCMV gB within the viral envelope is a trimer, but there has been no reported success in producing a recombinant trimeric gB suitable for vaccine use. Phase II clinical trials of a monomeric recombinant gB protein demonstrated 50% efficacy in preventing HCMV infection in seronegative women of reproductive age, and in reducing viremia in solid organ transplantation recipients. We now report the production of a uniformly trimeric recombinant HCMV gB protein in Chinese ovary cells, as demonstrated by Western blot analysis under modified non-reducing conditions and size exclusion chromatography with multi-angle scattering. Immunization of mice with trimeric HCMV gB induced up to 11-fold higher serum titers of total gB-specific IgG relative to monomeric HCMV gB using Alum + CpG as adjuvants. Further, trimeric HCMV gB elicited 50-fold higher complement-independent and 20-fold higher complement-dependent HCMV neutralizing titers compared to monomeric HCMV gB using the fibroblast cell line, MRC-5, and up to 6-fold higher complement-independent and -dependent HCMV neutralizing titers using the epithelial cell line, ARPE-19. The markedly enhanced HCMV neutralizing activity in response to trimeric HCMV gB was also observed using an additional four distinct clinical HCMV isolates. These data support a role for trimeric HCMV gB as an important component for clinical testing of a prophylactic HCMV vaccine.

Identification of a peptide derived from the heptad repeat 2 region of the porcine epidemic diarrhea virus (PEDV) spike glycoprotein that is capable of suppressing PEDV entry and inducing neutralizing antibodies

Heptad repeat (HR) regions are highly conserved motifs located in the glycoproteins of enveloped viruses that form a six-helix bundle structure and is important in the process of virus fusion. Peptides derived from the HR regions of some viruses have also been shown to inhibit viral entry. Porcine epidemic diarrhea virus (PEDV) was predicted to have HR regions (HR1 and HR2) in the spike glycoprotein S2 subunit. Based on this analysis, six peptides derived from HR1 and HR2 were selected, expressed in Escherichia coli, purified, and characterized. Three peptides (HR2M, HR2L and HR2P) were identified as potential competitive inhibitors in PEDV in vitro infection assays, with the HR2P peptide representing the most potent inhibitor. Further study indicated that immunization of HR2P in mice elicited antibodies capable of neutralizing PEDV infection in vitro. These results demonstrate that the HR2P peptide and anti-HR2P antibody can serve as a tool for dissecting the fusion mechanism of PEDV, guiding the search for potent inhibitors with therapeutic value against PEDV infection.

•

Six peptides derived from heptad repeat (HR) 1 and 2 regions of PEDV S glycoprotein were expressed and characterized.

•

Three peptides (HR2M, HR2L and HR2P) exhibited antiviral activity in vitro.

•

Immunization of the HR2P peptide in mice elicited antibodies capable of neutralizing PEDV infection in vitro.

•

HR2P peptide can serve as a potential antiviral drug against PEDV infection.

Infectivity inhibition by overlapping synthetic peptides derived from the gH/gL heterodimer of herpes simplex virus type 1

Herpes simplex virus (HSV) is a human pathogen that infects epithelial cells. The cutaneous lesions, caused by the virus, spread to the nervous system creating several complications. Fusion of host membranes with the viral envelope is mandatory and mediated by a group of glycoproteins conserved in all Herpesviridae subfamilies, such as the glycoproteins B (gB), H (gH), L (gL) and D (gD). We investigated the inhibitory activity mediated by synthetic overlapping peptides spanning the entire ectodomains of gH and gL glycoproteins. We have performed a brute analysis of the complete gH/gL heterodimer in order to explore the inhibitory activity of peptides modelled on these glycoproteins against HSV‐1 infection. Twenty‐four of the gH peptides at a concentration of 150 μM reached the 50% of inhibition cut‐off. Interestingly, they are mainly located in the gH carboxy‐terminal domain. None of the gL peptides had a clear inhibiting effect. No peptide toxicity was observed by lactate dehydrogenase assay at the concentrations used in our experimental conditions. HSV‐1 therapy is based on acyclovir treatment, but some resistant strains are emerging. In this scenario, innovative approaches for HSV‐1 treatment are necessary. Our data support the direct involvement of the described domains in the process of virus penetration; therefore, these results are of relevance to the potential development of novel therapeutic compounds to prevent HSV‐1 infections. Copyright © 2017 European Peptide Society and John Wiley & Sons, Ltd.

Zika Fetal Neuropathogenesis: Etiology of a Viral Syndrome

The ongoing Zika virus epidemic in the Americas and the observed association with both fetal abnormalities (primary microcephaly) and adult autoimmune pathology (Guillain-Barré syndrome) has brought attention to this neglected pathogen. While initial case studies generated significant interest in the Zika virus outbreak, larger prospective epidemiology and basic virology studies examining the mechanisms of Zika viral infection and associated pathophysiology are only now starting to be published. In this review, we analyze Zika fetal neuropathogenesis from a comparative pathology perspective, using the historic metaphor of "TORCH" viral pathogenesis to provide context. By drawing parallels to other viral infections of the fetus, we identify common themes and mechanisms that may illuminate the observed pathology. The existing data on the susceptibility of various cells to both Zika and other flavivirus infections are summarized. Finally, we highlight relevant aspects of the known molecular mechanisms of flavivirus replication.

Preclinical evaluations of peptide-conjugate vaccines targeting the antigenic domain-2 of glycoprotein B of human cytomegalovirus

The Antigenic Domain 2 (AD-2) is a short region near the N-terminus of glycoprotein B of human cytomegalovirus (HCMV). AD-2 has been shown to contain linear epitopes that are targets for neutralizing monoclonal antibodies from human subjects with natural HCMV infection. However, AD-2 appears to be masked by the adjacent immunodominant AD-1 region. We assessed a serum panel from HCMV-seropositive individuals and found a wide range of antibody titers to AD-2; these did not correlate to serum neutralization. To expose potential epitopes in AD-2, we constructed a series of AD-2 peptide-conjugate vaccines. Mice were immunized 3 times and produced high and sustained antibody titers to AD-2 peptides, but neutralization was weak even after a single boost with whole HCMV virions. Rabbits were likewise immunized with AD-2 peptide vaccines, and produced a robust antibody response, but neutralization was inferior to a recombinant gB vaccine with an oil-in-water adjuvant. These results highlight the challenges of developing a peptide-based vaccine specific to the HCMV gB AD-2 region.

Crystal Structure of the Human Cytomegalovirus Glycoprotein B

Human cytomegalovirus (HCMV), a dsDNA, enveloped virus, is a ubiquitous pathogen that establishes lifelong latent infections and caused disease in persons with compromised immune systems, e.g., organ transplant recipients or AIDS patients. HCMV is also a leading cause of congenital viral infections in newborns. Entry of HCMV into cells requires the conserved glycoprotein B (gB), thought to function as a fusogen and reported to bind signaling receptors. gB also elicits a strong immune response in humans and induces the production of neutralizing antibodies although most anti-gB Abs are non-neutralizing. Here, we report the crystal structure of the HCMV gB ectodomain determined to 3.6-Å resolution, which is the first atomic-level structure of any betaherpesvirus glycoprotein. The structure of HCMV gB resembles the postfusion structures of HSV-1 and EBV homologs, establishing it as a new member of the class III viral fusogens. Despite structural similarities, each gB has a unique domain arrangement, demonstrating structural plasticity of gB that may accommodate virus-specific functional requirements. The structure illustrates how extensive glycosylation of the gB ectodomain influences antibody recognition. Antigenic sites that elicit neutralizing antibodies are more heavily glycosylated than those that elicit non-neutralizing antibodies, which suggest that HCMV gB uses glycans to shield neutralizing epitopes while exposing non-neutralizing epitopes. This glycosylation pattern may have evolved to direct the immune response towards generation of non-neutralizing antibodies thus helping HCMV to avoid clearance. HCMV gB structure provides a starting point for elucidation of its antigenic and immunogenic properties and aid in the design of recombinant vaccines and monoclonal antibody therapies.

Human cytomegalovirus (HCMV) establishes lifelong infection in a majority of the world’s population and causes disease in neonates and the immunocompromised patients such as organ transplant recipients or persons with AIDS. There is no vaccine against HCMV, and current HCMV antivirals are toxic and an increasing prevalence of resistance. Glycoprotein B (gB), displayed on the viral surface is a major viral immunogen and is necessary for viral penetration into cells. The crystal structure of gB reported here provides a detailed 3D map of gB. A thick glycan layer covers a large surface area, which may explain why anti-gB neutralizing antibodies are relatively rare. The structure is expected to aid in the development of a HCMV vaccine and monoclonal antibody therapies.

GBNV encoded movement protein (NSm) remodels ER network via C-terminal coiled coil domain

Plant viruses exploit the host machinery for targeting the viral genome-movement protein complex to plasmodesmata (PD). The mechanism by which the non-structural protein m (NSm) of Groundnut bud necrosis virus (GBNV) is targeted to PD was investigated using Agrobacterium mediated transient expression of NSm and its fusion proteins in Nicotiana benthamiana. GFP:NSm formed punctuate structures that colocalized with mCherry:plasmodesmata localized protein 1a (PDLP 1a) confirming that GBNV NSm localizes to PD. Unlike in other movement proteins, the C-terminal coiled coil domain of GBNV NSm was shown to be involved in the localization of NSm to PD, as deletion of this domain resulted in the cytoplasmic localization of NSm. Treatment with Brefeldin A demonstrated the role of ER in targeting GFP NSm to PD. Furthermore, mCherry:NSm co-localized with ER-GFP (endoplasmic reticulum targeting peptide (HDEL peptide fused with GFP). Co-expression of NSm with ER-GFP showed that the ER-network was transformed into vesicles indicating that NSm interacts with ER and remodels it. Mutations in the conserved hydrophobic region of NSm (residues 130-138) did not abolish the formation of vesicles. Additionally, the conserved prolines at positions 140 and 142 were found to be essential for targeting the vesicles to the cell membrane. Further, systematic deletion of amino acid residues from N- and C-terminus demonstrated that N-terminal 203 amino acids are dispensable for the vesicle formation. On the other hand, the C-terminal coiled coil domain when expressed alone could also form vesicles. These results suggest that GBNV NSm remodels the ER network by forming vesicles via its interaction through the C-terminal coiled coil domain. Interestingly, NSm interacts with NP in vitro and coexpression of these two proteins in planta resulted in the relocalization of NP to PD and this relocalization was abolished when the N-terminal unfolded region of NSm was deleted. Thus, the NSm interacts with NP via its N-terminal unfolded region and the NSm-NP complex could in turn interact with the ER membrane via the C-terminal coiled coil domain of NSm to form vesicles that are targeted to PD and there by assist the cell to cell movement of the viral genome complex.

Identification of a neutralizing epitope within antigenic domain 5 of glycoprotein B of human cytomegalovirus

Human cytomegalovirus (HCMV) is an important, ubiquitous pathogen that causes severe clinical disease in immunocompromised individuals, such as organ transplant recipients and infants infected in utero. The envelope glycoprotein B (gB) of HCMV is a major antigen for the induction of virus-neutralizing antibodies. We have begun to define target structures within gB that are recognized by virus-neutralizing antibodies. Antigenic domain 5 (AD-5) of gB has been identified as an important target for neutralizing antibodies in studies using human monoclonal antibodies (MAbs). Anti-AD-5 MAbs share a target site on gB, despite originating from different, healthy, HCMV-infected donors. Mutational analysis of AD-5 identified tyrosine 280 in combination with other surface-exposed residues (the YNND epitope) as critical for antibody binding. The YNND epitope is strictly conserved among different HCMV strains. Recombinant viruses carrying YNND mutations in AD-5 were resistant to virus-neutralizing MAbs. Competition enzyme-linked immunosorbent assays (ELISAs) with human HCMV-convalescent-phase sera from unselected donors confirmed the conserved antibody response for the YNND epitope in HCMV-infected individuals and, because a significant fraction of the gB AD-5 response was directed against the YNND epitope, further argued that this epitope is a major target of anti-AD-5 antibody responses. In addition, affinity-purified polyclonal anti-AD-5 antibodies prepared from individual sera showed reactivity to AD-5 and neutralization activity toward gB mutant viruses that were similar to those of AD-5-specific MAbs. Taken together, our data indicate that the YNND epitope represents an important target for anti-gB antibody responses as well as for anti-AD-5 virus-neutralizing antibodies.

IMPORTANCE

HCMV is a major global health concern, and a vaccine to prevent HCMV disease is a widely recognized medical need. Glycoprotein B of HCMV is an important target for neutralizing antibodies and hence an interesting molecule for intervention strategies, e.g., vaccination. Mapping the target structures of neutralizing antibodies induced by naturally occurring HCMV infection can aid in defining the properties required for a protective capacity of vaccine antigens. The data presented here extend our knowledge of neutralizing epitopes within gB to include AD-5. Collectively, our data will contribute to optimal vaccine design and development of antibody-based therapies.

Upregulation of Toll-like receptor 2 expression in colorectal cancer infected by human cytomegalovirus

The aim of the present study was to investigate the association between Toll-like receptor 2 (TLR2) expression and human cytomegalovirus (HCMV) in colorectal carcinoma by detecting the expression of IE1-72, TLR2, TLR4 and tumor necrosis factor (TNF)-α in colorectal carcinoma and colon adenoma samples, as well as by analyzing the mRNA levels of the proteins in colon cancer cell lines, following HCMV infection. For this study, 56 colorectal cancer and 36 colon adenoma samples were collected, and normal mucosal tissue adjacent to the tumor was used as the control. The expression of the IE1-72, TLR2, TLR4, nuclear factor (NF)-κB and TNF-α protein was detected by immunohistochemistry. Cells from the SW480 human colon carcinoma cell line were infected with HCMV. The expression of IE1-72, TLR2, TLR4, NF-κB and TNF-α mRNA was quantified at different time points prior to and following infection. The positive expression rate of IE1-72 was 44.6% (25/56) in colorectal cancer and 41.7% (15/36) in colon adenoma. These rates were significantly higher when compared with the 12.5% (7/56) observed in the normal tissues adjacent to the cancer tissues (P<0.05). The expression levels of TLR2, TLR4, NF-κB and TNF-α in colorectal cancer and adenoma were also higher than those in the control tissues. Furthermore, the expression of IE1-72 in colorectal cancer tissues was found to correlate with TLR2 and TLR4, and the correlation coefficients were 0.515 and 0.462, respectively. Following the infection of SW480 cells, the mRNA levels of TLR2 and TNF-α increased gradually from 6 h, peaked at 48 h, and then decreased gradually. No significant differences in TLR4 and NF-κB expression were identified. The results of the present study indicated that there is a specific association between HCMV and the occurrence and development of colorectal cancer, which may be facilitated by the TLR2 signaling pathway.

Processing bodies accumulate in human cytomegalovirus-infected cells and do not affect viral replication at high multiplicity of infection

Translationally silenced mRNAs are recruited to two major classes of RNA granules in the cytoplasm, processing bodies (PBs) and stress granules (SGs). We show that PBs accumulated after human cytomegalovirus (HCMV) infection. PB assembly after HCMV infection was also detected in the presence of the protein synthesis inhibitor, cycloheximide, but required active RNA synthesis. UV-inactivated HCMV virions were sufficient to induce PB accumulation in HFF cells treated with cycloheximide. Viral IE1 RNA did not colocalize with PBs, and we could not detect an effect of PB accumulation on viral growth. These results may indicate that HCMV inhibits the colocalization of IE1 mRNA with PBs, preventing IE1 mRNA decay and translational inhibition.

Impact of sequence variation in the UL128 locus on production of human cytomegalovirus in fibroblast and epithelial cells

The human cytomegalovirus (HCMV) virion envelope contains a complex consisting of glycoproteins gH and gL plus proteins encoded by the UL128 locus (UL128L): pUL128, pUL130, and pUL131A. UL128L is necessary for efficient infection of myeloid, epithelial, and endothelial cells but limits replication in fibroblasts. Consequently, disrupting mutations in UL128L are rapidly selected when clinical isolates are cultured in fibroblasts. In contrast, bacterial artificial chromosome (BAC)-cloned strains TB40-BAC4, FIX, and TR do not contain overt disruptions in UL128L, yet no virus reconstituted from them has been reported to acquire mutations in UL128L in vitro. We performed BAC mutagenesis and reconstitution experiments to test the hypothesis that these strains contain subtle mutations in UL128L that were acquired during passage prior to BAC cloning. Compared to strain Merlin containing wild-type UL128L, all three strains produced higher yields of cell-free virus. Moreover, TB40-BAC4 and FIX spread cell to cell more rapidly than wild-type Merlin in fibroblasts but more slowly in epithelial cells. The differential growth properties of TB40-BAC4 and FIX (but not TR) were mapped to single-nucleotide substitutions in UL128L. The substitution in TB40-BAC4 reduced the splicing efficiency of UL128, and that in FIX resulted in an amino acid substitution in UL130. Introduction of these substitutions into Merlin dramatically increased yields of cell-free virus and increased cell-to-cell spread in fibroblasts but reduced the abundance of pUL128 in the virion and the efficiency of epithelial cell infection. These substitutions appear to represent mutations in UL128L that permit virus to be propagated in fibroblasts while retaining epithelial cell tropism.

Characterization of a discontinuous neutralizing epitope on glycoprotein B of human cytomegalovirus

Human cytomegalovirus (HCMV) is a ubiquitously distributed pathogen that causes severe disease in immunosuppressed patients and newborn infants infected in utero. The viral envelope glycoprotein B (gB) is an attractive molecule for active vaccination and passive immunoprophylaxis and therapy. Using human monoclonal antibodies (MAbs), we have recently identified antigenic region 4 (AD-4) on gB as an important target for neutralizing antibodies. AD-4 is formed by a discontinuous sequence comprising amino acids 121 to 132 and 344 to 438 of gB of HCMV strain AD169. To map epitopes for human antibodies on this protein domain, we used a three-dimensional (3D) model of HCMV gB to identify surface-exposed amino acids on AD-4 and selected juxtaposed residues for alanine scans. A tyrosine (Y) at position 364 and a lysine (K) at position 379 (the YK epitope), which are immediate neighbors on the AD-4 surface, were found to be essential for binding of the human MAbs. Recognition of AD-4 by sera from HCMV-infected individuals also was largely dependent on these two residues, indicating a general importance for the antibody response against AD-4. A panel of AD-4 recombinant viruses harboring mutations at the crucial antibody binding sites was generated. The viruses showed significantly reduced susceptibility to neutralization by AD-4-specific MAbs or polyclonal AD-4-specific antibodies, indicating that the YK epitope is dominant for the AD-4-specific neutralizing antibody response during infection. To our knowledge, this is the first molecular identification of a functional discontinuous epitope on HCMV gB. Induction of antibodies specific for this epitope may be a desirable goal following vaccination with gB.

Diverse immune evasion strategies by human cytomegalovirus

Members of the Herpesviridae family have the capacity to undergo both lytic and latent infection to establish a lifelong relationship with their host. Following primary infection, human cytomegalovirus (HCMV) can persist as a subclinical, recurrent infection for the lifetime of an individual. This quiescent portion of its life cycle is termed latency and is associated with periodic bouts of reactivation during times of immunosuppression, inflammation, or stress. In order to exist indefinitely and establish infection, HCMV encodes a multitude of immune modulatory mechanisms devoted to escaping the host antiviral response. HCMV has become a paradigm for studies of viral immune evasion of antigen presentation by both major histocompatibility complex (MHC) class I and II molecules. By restricting the presentation of viral antigens during both productive and latent infection, HCMV limits elimination by the human immune system. This review will focus on understanding how the virus manipulates the pathways of antigen presentation in order to modulate the host response to infection.

Peptide inhibitors against herpes simplex virus infections

Herpes simplex virus (HSV) is a significant human pathogen causing mucocutaneous lesions primarily in the oral or genital mucosa. Although acyclovir (ACV) and related nucleoside analogs provide successful treatment, HSV remains highly prevalent worldwide and is a major cofactor for the spread of human immunodeficiency virus. Encephalitis, meningitis, and blinding keratitis are among the most severe diseases caused by HSV. ACV resistance poses an important problem for immunocompromised patients and highlights the need for new safe and effective agents; therefore, the development of novel strategies to eradicate HSV is a global public health priority. Despite the continued global epidemic of HSV and extensive research, there have been few major breakthroughs in the treatment or prevention of the virus since the introduction of ACV in the 1980s. A therapeutic strategy at the moment not fully addressed is the use of small peptide molecules. These can be either modeled on viral proteins or derived from antimicrobial peptides. Any peptide that interrupts protein-protein or viral protein-host cell membrane interactions is potentially a novel antiviral drug and may be a useful tool for elucidating the mechanisms of viral entry. This review summarizes current knowledge and strategies in the development of synthetic and natural peptides to inhibit HSV infectivity.

Interaction domain of glycoproteins gB and gH of Marek's disease virus and identification of an antiviral peptide with dual functions

Our previous study reported that both glycoproteins gB and gH of the herpesvirus Marek's disease virus (MDV) contain eleven potential heptad repeat domains. These domains overlap with α-helix-enriched hydrophobic regions, including the gH-derived HR1 (gHH1) and HR3 (gHH3) and gB-derived HR1 (gBH1) regions, which demonstrate effective antiviral activity, with 50% inhibitory concentrations (IC(50)) of less than 12 µM. Plaque formation and chicken embryo infection assays confirmed these results. In this study, biochemical and biophysical analyses detected potential interactions between these peptides. gHH1, gHH3, and gBH1 were found to interact with each other in pairs. The complex formed by gHH3 and gBH1 showed the most stable interaction at a molar ratio of 1:3, the binding between gHH1 and gBH1 was relatively weak, and no interaction was observed between the three HR peptides. These results indicate that gHH3 and gBH1 are likely the key contributors to the interaction between gB and gH. Furthermore, each HR peptide from herpesvirus glycoproteins did not effectively inhibit virus infection compared with peptides from a class I enveloped virus. In this report, the HR mimic peptide modified with a double glutamic acid (EE) or a double lysine (KK) at the non-interactive sites (i.e., solvent-accessible sites) did not noticeably affect the antiviral activity compared with the wild-type HR peptide, whereas tandem peptides from gH-derived gHH1 and gB-derived gBH1 (i.e., gBH1-Linker-gHH1) produced efficient antiviral effects, unlike the individual peptides. The proposed interpretation of inhibition of entry has been addressed. Our results support the hypothesis that the interaction domain between glycoproteins gH and gB is a critical target in the design of inhibitors of herpesvirus infection.

Interplay between human cytomegalovirus and intrinsic/innate host responses: a complex bidirectional relationship

The interaction between human cytomegalovirus (HCMV) and its host is a complex process that begins with viral attachment and entry into host cells, culminating in the development of a specific adaptive response that clears the acute infection but fails to eradicate HCMV. We review the viral and cellular partners that mediate early host responses to HCMV with regard to the interaction between structural components of virions (viral glycoproteins) and cellular receptors (attachment/entry receptors, toll-like receptors, and other nucleic acid sensors) or intrinsic factors (PML, hDaxx, Sp100, viperin, interferon inducible protein 16), the reactions of innate immune cells (antigen presenting cells and natural killer cells), the numerous mechanisms of viral immunoevasion, and the potential exploitation of events that are associated with early phases of virus-host interplay as a therapeutic strategy.

Herpes virus fusion and entry: a story with many characters

Herpesviridae comprise a large family of enveloped DNA viruses all of whom employ orthologs of the same three glycoproteins, gB, gH and gL. Additionally, herpesviruses often employ accessory proteins to bind receptors and/or bind the heterodimer gH/gL or even to determine cell tropism. Sorting out how these proteins function has been resolved to a large extent by structural biology coupled with supporting biochemical and biologic evidence. Together with the G protein of vesicular stomatitis virus, gB is a charter member of the Class III fusion proteins. Unlike VSV G, gB only functions when partnered with gH/gL. However, gH/gL does not resemble any known viral fusion protein and there is evidence that its function is to upregulate the fusogenic activity of gB. In the case of herpes simplex virus, gH/gL itself is upregulated into an active state by the conformational change that occurs when gD, the receptor binding protein, binds one of its receptors. In this review we focus primarily on prototypes of the three subfamilies of herpesviruses. We will present our model for how herpes simplex virus (HSV) regulates fusion in series of highly regulated steps. Our model highlights what is known and also provides a framework to address mechanistic questions about fusion by HSV and herpesviruses in general.

Glycoprotein B of herpes simplex virus 2 has more than one intracellular conformation and is altered by low pH

The crystal structure of herpes simplex virus (HSV) gB identifies it as a class III fusion protein, and comparison with other such proteins suggests this is the postfusion rather than prefusion conformation, although this is not proven. Other class III proteins undergo a pH-dependent switch between pre- and postfusion conformations, and a low pH requirement for HSV entry into some cell types suggests that this may also be true for gB. Both gB and gH undergo structural changes at low pH, but there is debate about the extent and significance of the changes in gB, possibly due to the use of different soluble forms of the protein and different assays for antigenic changes. In this study, a complementary approach was taken, examining the conformations of full-length intracellular gB by quantitative confocal microscopy with a panel of 26 antibodies. Three conformations were distinguished, and low pH was found to be a major influence. Comparison with previous studies indicates that the intracellular conformation in low-pH environments may be the same as that of the soluble form known as s-gB at low pH. Interestingly, the antibodies whose binding was most affected by low pH both have neutralizing activity and consequently must block either the function of a neutral pH conformation or its switch from an inactive form to an activated form. If one of the intracellular conformations is the fusion-active form, another factor required for fusion is presumably absent from wherever that conformation is present in infected cells so that inappropriate fusion is avoided.

B cell repertoire analysis identifies new antigenic domains on glycoprotein B of human cytomegalovirus which are target of neutralizing antibodies

Human cytomegalovirus (HCMV), a herpesvirus, is a ubiquitously distributed pathogen that causes severe disease in immunosuppressed patients and infected newborns. Efforts are underway to prepare effective subunit vaccines and therapies including antiviral antibodies. However, current vaccine efforts are hampered by the lack of information on protective immune responses against HCMV. Characterizing the B-cell response in healthy infected individuals could aid in the design of optimal vaccines and therapeutic antibodies. To address this problem, we determined, for the first time, the B-cell repertoire against glycoprotein B (gB) of HCMV in different healthy HCMV seropositive individuals in an unbiased fashion. HCMV gB represents a dominant viral antigenic determinant for induction of neutralizing antibodies during infection and is also a component in several experimental HCMV vaccines currently being tested in humans. Our findings have revealed that the vast majority (>90%) of gB-specific antibodies secreted from B-cell clones do not have virus neutralizing activity. Most neutralizing antibodies were found to bind to epitopes not located within the previously characterized antigenic domains (AD) of gB. To map the target structures of these neutralizing antibodies, we generated a 3D model of HCMV gB and used it to identify surface exposed protein domains. Two protein domains were found to be targeted by the majority of neutralizing antibodies. Domain I, located between amino acids (aa) 133–343 of gB and domain II, a discontinuous domain, built from residues 121–132 and 344–438. Analysis of a larger panel of human sera from HCMV seropositive individuals revealed positivity rates of >50% against domain I and >90% against domain II, respectively. In accordance with previous nomenclature the domains were designated AD-4 (Dom II) and AD-5 (Dom I), respectively. Collectively, these data will contribute to optimal vaccine design and development of antibodies effective in passive immunization.

The development of antibodies is a major defense mechanism against viruses. Understanding the repertoire of antiviral antibodies induced during infection is a necessary prerequisite to defining the protective activities of an antiviral antibody response. The isolation of antigen specific memory B cells and subsequent stimulation to antibody producing cells provides a powerful tool to study the antibody repertoire in infected individuals. We have used this approach to analyze the antibody repertoire against glycoprotein B (gB) of human cytomegalovirus (HCMV), a major antigen for the induction of antiviral antibodies during infection and a constituent of experimental vaccines in humans. We find in different infected individuals that the vast majority of gB-specific B cells produce antibodies that cannot neutralize free virus. Antibodies with antiviral capacity target two domains of gB that have not been previously identified. The identification of these new antigenic domains was possible with the aid of a 3D molecular model of HCMV gB. Our results will be useful for vaccine development since comparison of the immune response after natural infection with that induced by vaccination can be readily accomplished. Moreover, neutralizing human monoclonal antibodies could constitute powerful therapeutics to combat the infection in populations at risk for HCMV disease.

Early inhibitors of human cytomegalovirus: state-of-art and therapeutic perspectives

Human cytomegalovirus (HCMV) infection is associated with severe morbidity and mortality in immunocompromised individuals, mainly transplant recipients and AIDS patients, and is the most frequent cause of congenital malformations in newborn children. To date, few drugs are licensed for the treatment of HCMV infections, most of which target the viral DNA polymerase and suffer from many drawbacks, including long-term toxicity, low potency, and poor bioavailability. In addition, the emergence of drug-resistant viral strains is becoming an increasing problem for disease management. Finally, none of the current anti-HCMV drugs have been approved for the treatment of congenital infections. For all these reasons, there is still a strong need for new anti-HCMV drugs with novel mechanisms of action. The first events of the virus replication cycle, including attachment, entry, immediate-early gene expression, and immediate-early functions—in particular that of Immediate-Early 2 protein—represent attractive targets for the development of novel antiviral compounds. Such inhibitors would block not only the expression of viral immediate-early proteins, which play a key role in the pathogenesis of HCMV infection, but also the host immunomodulation and the changes to cell physiology induced by the first events of virus infection. This review describes the current knowledge on the initial phases of HCMV replication, their validation as potential novel antiviral targets, and the development of compounds that block such processes.

Structural characteristics and antiviral activity of multiple peptides derived from MDV glycoproteins B and H

Background

Marek's disease virus (MDV), which is widely considered to be a natural model of virus-induced lymphoma, has the potential to cause tremendous losses in the poultry industry. To investigate the structural basis of MDV membrane fusion and to identify new viral targets for inhibition, we examined the domains of the MDV glycoproteins gH and gB.

Results

Four peptides derived from the MDV glycoprotein gH (gHH1, gHH2, gHH3, and gHH5) and one peptide derived from gB (gBH1) could efficiently inhibit plaque formation in primary chicken embryo fibroblast cells (CEFs) with 50% inhibitory concentrations (IC₅₀) of below 12 μM. These peptides were also significantly able to reduce lesion formation on chorioallantoic membranes (CAMs) of infected chicken embryos at a concentration of 0.5 mM in 60 μl of solution. The HR2 peptide from Newcastle disease virus (NDVHR2) exerted effects on MDV specifically at the stage of virus entry (i.e., in a cell pre-treatment assay and an embryo co-treatment assay), suggesting cross-inhibitory effects of NDV HR2 on MDV infection. None of the peptides exhibited cytotoxic effects at the concentrations tested. Structural characteristics of the five peptides were examined further.

Conclusions

The five MDV-derived peptides demonstrated potent antiviral activity, not only in plaque formation assays in vitro, but also in lesion formation assays in vivo. The present study examining the antiviral activity of these MDV peptides, which are useful as small-molecule antiviral inhibitors, provides information about the MDV entry mechanism.

Cell entry of enveloped viruses

Enveloped viruses penetrate their cell targets following the merging of their membrane with that of the cell. This fusion process is catalyzed by one or several viral glycoproteins incorporated on the membrane of the virus. These envelope glycoproteins (EnvGP) evolved in order to combine two features. First, they acquired a domain to bind to a specific cellular protein, named "receptor." Second, they developed, with the help of cellular proteins, a function of finely controlled fusion to optimize the replication and preserve the integrity of the cell, specific to the genus of the virus. Following the activation of the EnvGP either by binding to their receptors and/or sometimes the acid pH of the endosomes, many changes of conformation permit ultimately the action of a specific hydrophobic domain, the fusion peptide, which destabilizes the cell membrane and leads to the opening of the lipidic membrane. The comprehension of these mechanisms is essential to develop medicines of the therapeutic class of entry inhibitor like enfuvirtide (Fuzeon) against human immunodeficiency virus (HIV). In this chapter, we will summarize the different envelope glycoprotein structures that viruses develop to achieve membrane fusion and the entry of the virus. We will describe the different entry pathways and cellular proteins that viruses have subverted to allow infection of the cell and the receptors that are used. Finally, we will illustrate more precisely the recent discoveries that have been made within the field of the entry process, with a focus on the use of pseudoparticles. These pseudoparticles are suitable for high-throughput screenings that help in the development of natural or artificial inhibitors as new therapeutics of the class of entry inhibitors.

The glycoprotein B disintegrin-like domain binds beta 1 integrin to mediate cytomegalovirus entry

Cellular integrins were identified as human cytomegalovirus (HCMV) entry receptors and signaling mediators in both fibroblasts and endothelial cells. The goal of these studies was to determine the mechanism by which HCMV binds to cellular integrins to mediate virus entry. HCMV envelope glycoprotein B (gB) has sequence similarity to the integrin-binding disintegrin-like domain found in the ADAM (a disintegrin and metalloprotease) family of proteins. To test the ability of this region to bind to cellular integrins, we generated a recombinant soluble version of the gB disintegrin-like domain (gB-DLD). The gB-DLD protein bound to human fibroblasts in a specific, dose-dependent and saturable manner that required the expression of an intact beta1 integrin ectodomain. Furthermore, a physical association between gB-DLD and beta1 integrin was demonstrated through in vitro pull-down assays. The function of this interaction was shown by the ability of cell-bound gB-DLD to efficiently block HCMV entry and the infectivity of multiple in vivo target cells. Additionally, rabbit polyclonal antibodies raised against gB-DLD neutralized HCMV infection. Mimicry of the ADAM family disintegrin-like domain by HCMV gB represents a novel mechanism for integrin engagement by a virus and reveals a unique therapeutic target for HCMV neutralization. The strong conservation of the DLD across beta- and gammaherpesviruses suggests that integrin recognition and utilization may be a more broadly conserved feature throughout the Herpesviridae.

Crystal structure of the conserved herpesvirus fusion regulator complex gH-gL

Herpesviruses, which cause many incurable diseases, infect cells by fusing viral and cellular membranes. Whereas most other enveloped viruses use a single viral catalyst called a fusogen, herpesviruses, inexplicably, require two conserved fusion-machinery components, gB and the heterodimer gH-gL, plus other nonconserved components. gB is a class III viral fusogen, but unlike other members of its class, it does not function alone. We determined the crystal structure of the gH ectodomain bound to gL from herpes simplex virus 2. gH-gL is an unusually tight complex with a unique architecture that, unexpectedly, does not resemble any known viral fusogen. Instead, we propose that gH-gL activates gB for fusion, possibly through direct binding. Formation of a gB-gH-gL complex is critical for fusion and is inhibited by a neutralizing antibody, making the gB-gH-gL interface a promising antiviral target.

The presence of a single N-terminal histidine residue enhances the fusogenic properties of a Membranotropic peptide derived from herpes simplex virus type 1 glycoprotein H

Herpes simplex virus type 1 (HSV-1)-induced membrane fusion remains one of the most elusive mechanisms to be deciphered in viral entry. The structure resolution of glycoprotein gB has revealed the presence of fusogenic domains in this protein and pointed out the key role of gB in the entry mechanism of HSV-1. A second putative fusogenic glycoprotein is represented by the heterodimer comprising the membrane-anchored glycoprotein H (gH) and the small secreted glycoprotein L, which remains on the viral envelope in virtue of its non-covalent interaction with gH. Different domains scattered on the ectodomain of HSV-1 gH have been demonstrated to display membranotropic characteristics. The segment from amino acid 626 to 644 represents the most fusogenic region identified by studies with synthetic peptides and model membranes. Herein we have identified the minimal fusogenic sequence present on gH. An enlongation at the N terminus of a single histidine (His) has proved to profoundly increase the fusogenic activity of the original sequence. Nuclear magnetic resonance (NMR) studies have shown that the addition of the N-terminal His contributes to the formation and stabilization of an alpha-helical domain with high fusion propensity.

Helical membrane peptides to modulate cell function

In recent years there has been an abundance of research into the potential of helical peptides to influence cell function. These peptides have been used to achieve a variety of different outcomes from cell repair to cell death, depending upon the peptide sequence and the nature of its interactions with cell membranes and membrane proteins. In this critical review, we summarise several mechanisms by which helical peptides, acting as either transporters, inhibitors, agonists or antibiotics, can have significant effects on cell membranes and can radically affect the internal mechanisms of the cell. The various approaches to peptide design are discussed, including the role of naturally-occurring proteins in the design of these helical peptides and current breakthroughs in the use of non-natural (and therefore more stable) peptide scaffolds. Most importantly, the current successful applications of these peptides, and their potential uses in the field of medicine, are reviewed (131 references).

Structure of a trimeric variant of the Epstein-Barr virus glycoprotein B

Epstein-Barr virus (EBV) is a herpesvirus that is associated with development of malignancies of lymphoid tissue. EBV infections are life-long and occur in >90% of the population. Herpesviruses enter host cells in a process that involves fusion of viral and cellular membranes. The fusion apparatus is comprised of envelope glycoprotein B (gB) and a heterodimeric complex made of glycoproteins H and L. Glycoprotein B is the most conserved envelope glycoprotein in human herpesviruses, and the structure of gB from Herpes simplex virus 1 (HSV-1) is available. Here, we report the crystal structure of the secreted EBV gB ectodomain, which forms 16-nm long spike-like trimers, structurally homologous to the postfusion trimers of the fusion protein G of vesicular stomatitis virus (VSV). Comparative structural analyses of EBV gB and VSV G, which has been solved in its pre and postfusion states, shed light on gB residues that may be involved in conformational changes and membrane fusion. Also, the EBV gB structure reveals that, despite the high sequence conservation of gB in herpesviruses, the relative orientations of individual domains, the surface charge distributions, and the structural details of EBV gB differ from the HSV-1 protein, indicating regions and residues that may have important roles in virus-specific entry.

Human cytomegalovirus glycoprotein B is required for virus entry and cell-to-cell spread but not for virion attachment, assembly, or egress

Glycoprotein B (gB) homologs are conserved throughout the family Herpesviridae and appear to serve essential, universal functions, as well as specific functions unique to a particular herpesvirus. Genetic analysis is a powerful tool to analyze protein function, and while it has been possible to generate virus mutants, complementation of essential virus knockouts has been problematic. Human cytomegalovirus (HCMV) gB (UL55) plays an essential role in the replication cycle of the virus. To define the function(s) of gB in HCMV infection, the BAC system was used to generate a recombinant virus in which the UL55 gene was replaced with galK (pAD/CreDeltaUL55). UL55 deletions in the viral genome have been made before, demonstrating that UL55 is an essential gene. However, without being able to successfully complement the genetic defect, a phenotypic analysis of the mutant virus was impossible. We generated fibroblasts expressing HCMV gB that complement pAD/CreDeltaUL55 and produce infectious virions lacking the UL55 gene but containing wild-type gB on the virion surface (DeltaUL55-gB HCMV). This is the first successful complementation of an HCMV mutant with a glycoprotein deleted. To characterize DeltaUL55 infection in the absence of gB, noncomplementing cells were infected with DeltaUL55-gB virus. All stages of gene expression were detected, and significant amounts of DNase-resistant viral DNA genomes, representing whole intact virions, were released into the infected cell supernatant. Gradient purification of these virions revealed they lacked gB but contained other viral structural proteins. The gB-null virions were able to attach to the cell surface similarly to wild-type gB-containing virions but were defective in virus entry and cell-to-cell spread. Glycoprotein B-null virions do, however, contain infectious DNA, as IE gene expression can be detected in fibroblasts following treatment of attached gB-null virions with a membrane fusion agent, polyethylene glycol. Taken together, our results indicate that gB is required for virus entry and cell-to-cell spread of the virus. However, HCMV gB is not absolutely required for virus attachment or assembly and egress from infected cells.

Multiple peptides homologous to herpes simplex virus type 1 glycoprotein B inhibit viral infection

The 773-residue ectodomain of the herpes simplex virus type 1 (HSV-1) glycoprotein B (gB) has been resistant to the use of mutagenic strategies because the majority of the induced mutations result in defective proteins. As an alternative strategy for the identification of functionally important regions and novel inhibitors of infection, we prepared a library of overlapping peptides homologous to the ectodomain of gB and screened for the ability of the peptides to block infection. Seven of 138 15-mer peptides inhibited infection by more than 50% at a concentration of 100 microM. Three peptides (gB94, gB122, and gB131) with 50% effective concentrations (EC(50)s) below 20 microM were selected for further studies. The gB131 peptide (residues 681 to 695 in HSV-1 gB [gB-1]) was a specific entry inhibitor (EC(50), approximately 12 microM). The gB122 peptide (residues 636 to 650 in gB-1) blocked viral entry (EC(50), approximately 18 microM), protected cells from infection (EC(50), approximately 72 microM), and inactivated virions in solution (EC(50), approximately 138 microM). We were unable to discern the step or steps inhibited by the gB94 peptide, which is homologous to residues 496 to 510 in gB-1. Substitution of a tyrosine in the gB122 peptide (Y640 in full-length gB-1) reduced the antiviral activity eightfold, suggesting that this residue is critical for inhibition. This peptide-based strategy could lead to the identification of functionally important regions of gB or other membrane proteins and identify novel inhibitors of HSV-1 entry.

Analysis of Epstein-Barr virus glycoprotein B functional domains via linker insertion mutagenesis

Epstein-Barr Virus (EBV) glycoprotein B (gB) is essential for viral fusion events with epithelial and B cells. This glycoprotein has been studied extensively in other herpesvirus family members, but functional domains outside of the cytoplasmic tail have not been characterized in EBV gB. In this study, a total of 28 linker insertion mutations were generated throughout the length of gB. In general, the linker insertions did not disrupt intracellular expression and variably altered cell surface expression. Oligomerization was disrupted by insertions located between residues 561 and 620, indicating the location of a potential site of oligomer contacts between EBV gB monomers. In addition, a novel N-glycosylated form of wild-type gB was identified under nonreducing Western blot conditions that likely represents a mature form of the protein. Fusion activity was abolished in all but three variants containing mutations in the N-terminal region (gB30), within the ectodomain (gB421), and in the intracellular C-terminal domain (gB832) of the protein. Fusion activity with variants gB421 and gB832 was comparable to that of the wild type with epithelial and B cells, and only these two mutants, but not gB30, were able to complement gB-null virus and subsequently function in virus entry. The mutant gB30 exhibited a low level of fusion activity with B cells and was unable to complement gB-null virus. The mutations generated here indicate important structural domains, as well as regions important for function in fusion, within EBV gB.

Kaposi's sarcoma-associated herpesvirus gH/gL: glycoprotein export and interaction with cellular receptors

The attachment, entry, and fusion of Kaposi's sarcoma-associated herpesvirus (KSHV) with target cells are mediated by complex machinery containing, among others, viral glycoprotein H (gH) and its alleged chaperone, gL. We observed that KSHV gH, in contrast to its homologues in several other herpesviruses, is transported to the cytoplasm membrane independently from gL, but not vice versa. Mutational analysis revealed that the N terminus of gH is sufficient for gL interaction. However, the entire extracellular part of gH is required for efficient gL secretion. The soluble ectodomain of gH was sufficient to interact with the surfaces of potential target cells in a heparin-dependent manner, and binding was further enhanced by coexpression of gL. Surface plasmon resonance revealed a remarkably high affinity of gH for glycosaminoglycans. Heparan sulfate (HS) proteoglycans of the syndecan family act as cellular receptors for the gH/gL complex. They promoted KSHV infection, and expression of gH/gL on target cells inhibited subsequent KSHV infection. Whereas gH alone was able to bind to HS, we observed that only the gH/gL complex adhered to heparan sulfate-negative cells at lamellipodium-like structures.

Peptides containing membrane-interacting motifs inhibit herpes simplex virus type 1 infectivity

Herpes simplex virus (HSV) membrane fusion represents an attractive target for anti-HSV therapy. To investigate the structural basis of HSV membrane fusion and identify new targets for inhibition, we have investigated the different membranotropic domains of HSV-1 gH envelope glycoprotein. We observed that fusion peptides when added exogenously are able to inhibit viral fusion likely by intercalating with viral fusion peptides upon adopting functional structure in membranes. Interestingly, peptides analogous to the predicted HSV-1 gH loop region inhibited viral plaque formation more significantly. Their inhibitory effect appears to be a consequence of their ability to partition into membranes and aggregate within them. Circular dichroism spectra showed that peptides self-associate in aqueous and lipidic solutions, therefore the inhibition of viral entry may occur via peptides association with their counterpart on wild-type gH. The antiviral activity of HSV-1 peptides tested provides an attractive basis for the development of new fusion peptide inhibitors corresponding to regions outside the fusion protein heptad repeat regions.

Virus entry and innate immune activation

Human cytomegalovirus (HCMV) exhibits an exceptionally broad cellular tropism as it is capable of infecting most major organ systems and cell types. Definitive proof of an essential role for a cellular molecule that serves as an entry receptor has proven very challenging. It is widely hypothesized that receptor utilization, envelope glycoprotein requirements and entry pathways may all vary according to cell type, which is partially supported by the data. What has clearly emerged in recent years is that virus entry is not going undetected by the host. Robust and rapid induction of innate immune response is intimately associated with entry-related events. Here we review the state of knowledge on HCMV cellular entry mediators confronting the scientific challenges by accruing a definitive data set. We also review the roles of pattern recognition receptors such as Toll-like receptors in activation of specific innate immune response and discuss how entry events are tightly coordinated with innate immune initiation steps.

Analysis of a membrane interacting region of herpes simplex virus type 1 glycoprotein H

Glycoprotein H (gH) of herpes simplex virus type I (HSV-1) is involved in the complex mechanism of membrane fusion of the viral envelope with the host cell. Membrane interacting regions and potential fusion peptides have been identified in HSV-1 gH as well as glycoprotein B (gB). Because of the complex fusion mechanism of HSV-1, which requires four viral glycoproteins, and because there are only structural data for gB and glycoprotein D, many questions regarding the mechanism by which HSV-1 fuses its envelope with the host cell membrane remain unresolved. Previous studies have shown that peptides derived from certain regions of gH have the potential to interact with membranes, and based on these findings we have generated a set of peptides containing mutations in one of these domains, gH-(626-644), to investigate further the functional role of this region. Using a combination of biochemical, spectroscopic, and nuclear magnetic resonance techniques, we showed that the alpha-helical nature of this stretch of amino acids in gH is important for membrane interaction and that the aromatic residues, tryptophan and tyrosine, are critical for induction of fusion.

The identification and characterization of fusogenic domains in herpes virus glycoprotein B molecules

The molecular mechanism of entry of herpes viruses requires a multicomponent fusion system. Virus entry and cell-cell fusion of Herpes simplex virus (HSV) requires four glycoproteins: gD, gB and gH/gL. The role of gB remained elusive until recently, when the crystal structure of HSV-1 gB became available. Glycoprotein B homologues represent the most highly conserved group of herpes virus glycoproteins; however, despite the high degree of sequence and structural conservation, differences in post-translational processing are observed for different members of this virus family. Whereas gB of HSV is not proteolytically processed after oligomerization, most other gB homologues are cleaved by a cellular protease into subunits that remain linked through disulfide bonds. Proteolytic cleavage is common for activation of many other viral fusion proteins, so it remains difficult to envisage a common role for different herpes virus gB structures in the fusion mechanism. We selected bovine herpes virus type 1 (BoHV-1) and herpes simplex virus type 1 (HSV-1) as representative viruses expressing cleaved and uncleaved gBs, and have screened their amino acid sequences for regions of highly interfacial hydrophobicity. Synthetic peptides corresponding to such regions were tested for their ability to induce the fusion of large unilamellar vesicles and to inhibit herpes virus infection. These results underline that several regions of the gB protein are involved in the mechanism of membrane interaction.

Cytomegalovirus UL131-128 products promote gB conformational transition and gB-gH interaction during entry into endothelial cells

Herpesviruses use gB and gH-gL glycoproteins to execute fusion. Other virus-specific glycoproteins are required for receptor binding and fusion activation. The human cytomegalovirus (HCMV) UL131-128 proteins are essential for the infection of leukocytes, endothelial cells (ECs), and many epithelial cell lines. Here we show that UL131-128 play a role in a chain of events involving gB and gH during HCMV entry into ECs. An HCMV strain bearing the wild-type (wt) UL131-128 locus exhibited a gB transition from a protease-resistant to protease-sensitive form, a conformational change that was suppressed by a thiourea inhibitor of fusion (WY1768); in contrast, gH was susceptible to proteolysis throughout entry. Moreover, gB and gH transiently interacted, and a lipid mixing assay showed that the wt strain had carried out fusion by 60 min postinfection. However, these events were greatly altered when UL131-128-defective strains were used for infection or when there was an excess of soluble pUL128 during wt infection: the gB conformational change became WY1768 resistant, the gB-gH complex was no longer observed, and fusion was prevented. Both gB and gH in this case showed late protease resistance, related to their endocytic uptake. Our data point to the involvement of UL131-128 proteins in driving gB through a WY1768-sensitive fold transition, thus promoting a short-lived gB-gH complex and fusion; they also suggest that HCMV fuses with the EC plasma membrane and that endocytosis ensues only when the virus cannot trigger UL131-128-dependent steps.

Evidence for a role of the membrane-proximal region of herpes simplex virus type 1 glycoprotein H in membrane fusion and virus inhibition

We have identified a putative membrane-interacting domain preceding the transmembrane domain of the Herpes simplex virus type 1 (HSV-1) glycoprotein H (gH). Peptides derived from this region interact strongly with membranes and show a high tendency to partition at the interface. This region is predicted to bind at the membrane interface by adopting an alpha helical structure. Peptides representing either the HSV-1 gH pretransmembrane region or a scrambled control with a different hydrophobic profile at the point of interface have been studied. The peptides derived from this domain of gH induce the fusion of liposomal membranes, adopt helical conformations in membrane mimetic environments and are able to inhibit HSV-1 infectivity. The pretransmembrane region appears to be a common feature in viral fusion proteins of several virus families, and such a feature might be related to their fusogenic function. The identification of membrane-interacting regions capable of modifying the biophysical properties of phospholipid membranes lends weight to the view that such domains might function directly in the fusion process and could facilitate the future development of HSV-1 entry inhibitors.

The cytoplasmic tail of glycoprotein M (gpUL100) expresses trafficking signals required for human cytomegalovirus assembly and replication

The virion envelope of human cytomegalovirus (HCMV) is complex and consists of an incompletely defined number of glycoproteins. The gM/gN protein complex is the most abundant protein component of the envelope. Studies have indicated that deletion of the viral gene encoding either gM or gN is a lethal mutation. Analysis of the amino acid sequence of gM disclosed a C-terminal acidic cluster of amino acids and a tyrosine-containing trafficking motif, both of which are well-described trafficking/sorting signals in the cellular secretory pathway. To investigate the roles of these signals in the trafficking of the gM/gN complex during virus assembly, we made a series of gM (UL100 open reading frame) mutants in the AD169 strain of HCMV. Mutant viruses that lacked the entire C-terminal cytoplasmic tail of gM were not viable, suggesting that the cytoplasmic tail of gM is essential for virus replication. In addition, the gM mutant protein lacking the cytoplasmic domain exhibited decreased protein stability. Mutant viruses with a deletion of the acidic cluster or alanine substitutions in tyrosine-based motifs were viable but exhibited a replication-impaired phenotype suggestive of a defect in virion assembly. Analysis of these mutant gMs using static immunofluorescence and fluorescence recovery after photobleaching demonstrated delayed kinetics of intracellular localization of the gM/gN protein to the virus assembly compartment compared to the wild-type protein. These data suggest an important role of the glycoprotein gM during virus assembly, particularly in the dynamics of gM trafficking during viral-particle assembly.

N-terminal mutants of herpes simplex virus type 2 gH are transported without gL but require gL for function

Glycoprotein H (gH) is conserved among all herpesviruses and is essential for virus entry and cell fusion along with gL, gB, and, in most alphaherpesviruses, gD. Within the gH/gL heterodimer, it is thought that gH accounts for the fusion function and gL acts as a chaperone for the folding and transport of gH. Here, we found that the N terminus of gH2 contains important elements involved in both its folding and its transport. Our conclusions are based on the phenotypes of a series of gH deletion mutants in which the signal sequence (residues 1 to 18) was retained and N-terminal residues were removed up to the number indicated. The first mutant, gH2Delta29 (deletion of residues 19 to 28), like wild-type (WT) gH, required gL for both transport and function. To our surprise, two other mutants (gH2Delta64 and gH2Delta72) were transported to the cell surface independent of gL but were nonfunctional, even when complexed with gL. Importantly, a fourth mutant (gH2Delta48) was transported independent of gL but was functional only when complexed with gL. Using a panel of monoclonal antibodies against gH2, we found that when gH2Delta48 was expressed alone, its antigenic structure differed from that of gH2Delta48/gL or gH2-WT/gL. Mutation of gH2 residue R39, Y41, W42, or D44 allowed gL-independent transport of gH. Our results also show that gL is not merely required for gH transport but is also necessary for the folding and function of the complex. Since gH2Delta64/gL and gH2Delta72/gL were nonfunctional, we hypothesized that residues critical for gH/gL function lie within this deleted region. Additional mutagenesis identified L66 and L72 as important for function. Together, our results highlight several key gH residues: R39, Y41, W42, and D44 for gH transport and L66 and L72 for gH/gL structure and function.

Point mutations in EBV gH that abrogate or differentially affect B cell and epithelial cell fusion

Cell fusion mediated by Epstein-Barr virus requires three conserved glycoproteins, gB and gHgL, but activation is cell type specific. B cell fusion requires interaction between MHC class II and a fourth virus glycoprotein, gp42, which complexes non-covalently with gHgL. Epithelial cell fusion requires interaction between gHgL and a novel epithelial cell coreceptor and is blocked by excess gp42. We show here that gp42 interacts directly with gH and that point mutations in the region of gH recognized by an antibody that differentially inhibits epithelial and B cell fusion significantly impact both the core fusion machinery and cell-specific events. Substitution of alanine for glycine at residue 594 completely abrogates fusion with either B cells or epithelial cells. Substitution of alanine for glutamic acid at residue 595 reduces fusion with epithelial cells, greatly enhances fusion with B cells and allows low levels of B cell fusion even in the absence of gL.