Human cytomegalovirus strain Towne glycoprotein B is processed by proteolytic cleavage

Identification of Kukoamine A, Zeaxanthin, and Clexane as New Furin Inhibitors

The endogenous protease furin is a key protein in many different diseases, such as cancer and infections. For this reason, a wide range of studies has focused on targeting furin from a therapeutic point of view. Our main objective consisted of identifying new compounds that could enlarge the furin inhibitor arsenal; secondarily, we assayed their adjuvant effect in combination with a known furin inhibitor, CMK, which avoids the SARS-CoV-2 S protein cleavage by means of that inhibition. Virtual screening was carried out to identify potential furin inhibitors. The inhibition of physiological and purified recombinant furin by screening selected compounds, Clexane, and these drugs in combination with CMK was assayed in fluorogenic tests by using a specific furin substrate. The effects of the selected inhibitors from virtual screening on cell viability (293T HEK cell line) were assayed by means of flow cytometry. Through virtual screening, Zeaxanthin and Kukoamine A were selected as the main potential furin inhibitors. In fluorogenic assays, these two compounds and Clexane inhibited both physiological and recombinant furin in a dose-dependent way. In addition, these compounds increased physiological furin inhibition by CMK, showing an adjuvant effect. In conclusion, we identified Kukoamine A, Zeaxanthin, and Clexane as new furin inhibitors. In addition, these drugs were able to increase furin inhibition by CMK, so they could also increase its efficiency when avoiding S protein proteolysis, which is essential for SARS-CoV-2 cell infection.

Dynamin Inhibitors Prevent the Establishment of the Cytomegalovirus Assembly Compartment in the Early Phase of Infection

Cytomegalovirus (CMV) infection initiates massive rearrangement of cytoplasmic organelles to generate assembly compartment (AC). The earliest events, the establishment of the preAC, are initiated in the early phase as an extensive reorganization of early endosomes (EEs), endosomal recycling compartment (ERC), trans-Golgi network (TGN), and the Golgi. Here, we demonstrate that dynamin inhibitors (Dynasore, Dyngo-4a, MiTMAB, and Dynole-34-2) block the establishment of the preAC in murine CMV (MCMV) infected cells. In this study, we extensively analyzed the effect of Dynasore on the Golgi reorganization sequence into the outer preAC. We also monitored the development of the inner preAC using a set of markers that define EEs (Rab5, Vps34, EEA1, and Hrs), the EE-ERC interface (Rab10), the ERC (Rab11, Arf6), three layers of the Golgi (GRASP65, GM130, Golgin97), and late endosomes (Lamp1). Dynasore inhibited the pericentriolar accumulation of all markers that display EE-ERC-TGN interface in the inner preAC and prevented Golgi unlinking and dislocation to the outer preAC. Furthermore, in pulse-chase experiments, we demonstrated that the presence of dynasore only during the early phase of MCMV infection (4-14 hpi) is sufficient to prevent not only AC formation but also the synthesis of late-phase proteins and virion production. Therefore, our results indicate that dynamin-2 acts as a part of the machinery required for AC generation and rearrangement of EE/ERC/Golgi membranes in the early phase of CMV infection.

Common Polymorphisms in the Glycoproteins of Human Cytomegalovirus and Associated Strain-Specific Immunity

Human cytomegalovirus (HCMV), one of the most prevalent viruses across the globe, is a common cause of morbidity and mortality for immunocompromised individuals. Recent clinical observations have demonstrated that mixed strain infections are common and may lead to more severe disease progression. This clinical observation illustrates the complexity of the HCMV genome and emphasizes the importance of taking a population-level view of genotypic evolution. Here we review frequently sampled polymorphisms in the glycoproteins of HCMV, comparing the variable regions, and summarizing their corresponding geographic distributions observed to date. The related strain-specific immunity, including neutralization activity and antigen-specific cellular immunity, is also discussed. Given that these glycoproteins are common targets for vaccine design and anti-viral therapies, this observed genetic variation represents an important resource for future efforts to combat HCMV infections.

Furin-mediated protein processing in infectious diseases and cancer

Proteolytic cleavage regulates numerous processes in health and disease. One key player is the ubiquitously expressed serine protease furin, which cleaves a plethora of proteins at polybasic recognition motifs. Mammalian substrates of furin include cytokines, hormones, growth factors and receptors. Thus, it is not surprising that aberrant furin activity is associated with a variety of disorders including cancer. Furthermore, the enzymatic activity of furin is exploited by numerous viral and bacterial pathogens, thereby enhancing their virulence and spread. In this review, we describe the physiological and pathophysiological substrates of furin and discuss how dysregulation of a simple proteolytic cleavage event may promote infectious diseases and cancer. One major focus is the role of furin in viral glycoprotein maturation and pathogenicity. We also outline cellular mechanisms regulating the expression and activation of furin and summarise current approaches that target this protease for therapeutic intervention.

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.

Multi-antigenic human cytomegalovirus mRNA vaccines that elicit potent humoral and cell-mediated immunity

A cytomegalovirus (CMV) vaccine that is effective at preventing congenital infection and reducing CMV disease in transplant patients remains a high priority as no approved vaccines exist. While the precise correlates of protection are unknown, neutralizing antibodies and antigen-specific T cells have been implicated in controlling infection. We demonstrate that the immunization of mice and nonhuman primates (NHPs) with lipid nanoparticles (LNP) encapsulating modified mRNA encoding CMV glycoproteins gB and pentameric complex (PC) elicit potent and durable neutralizing antibody titers. Since the protective correlates in pregnant women and transplant recipients may differ, we developed an additional mRNA vaccine expressing the immunodominant CMV T cell antigen pp65. Administration of pp65 vaccine with PC and gB elicited robust multi-antigenic T cell responses in mice. Our data demonstrate that mRNA/LNP is a versatile platform that enables the development of vaccination strategies that could prevent CMV infection and consequent disease in different target populations.

Positively Selected Sites at HCMV gB Furin Processing Region and Their Effects in Cleavage Efficiency

Human cytomegalovirus is a ubiquitous infectious agent that affects mainly immunosuppressed, fetuses, and newborns. The virus has several polymorphic regions, in particular in the envelope glycoproteins. The UL55 gene encodes the glycoprotein B that has a variable region, containing a furin cleavage site and according to the variability different genotypes are characterized. Here we investigated variability and existence of selective pressure on the UL55 variable region containing the furin cleavage site in 213 clinical sequences from patients worldwide. We showed the occurrence of positive selective pressure on gB codons 461 and 462, near the furin cleavage site. Cleavage analysis of synthesized peptides demonstrated that most mutations confer better cleavage by furin, suggesting that evolution is acting in order to increase the efficiency cleavage and supporting the hypothesis that gB processing is important in the host. We also demonstrated that peptides containing sequences, that characterize genotypes gB2 and 3, are differentially cleaved by furin. Our data demonstrate for the first time that variability in the cleavage site is related to degree of gB processing by furin.

Additive Protection against Congenital Cytomegalovirus Conferred by Combined Glycoprotein B/pp65 Vaccination Using a Lymphocytic Choriomeningitis Virus Vector

Subunit vaccines for prevention of congenital cytomegalovirus (CMV) infection based on glycoprotein B (gB) and pp65 are in clinical trials, but it is unclear whether simultaneous vaccination with both antigens enhances protection. We undertook evaluation of a novel bivalent vaccine based on nonreplicating lymphocytic choriomeningitis virus (rLCMV) vectors expressing a cytoplasmic tail-deleted gB [gB(dCt)] and full-length pp65 from human CMV in mice. Immunization with the gB(dCt) vector alone elicited a comparable gB-binding antibody response and a superior neutralizing response to that elicited by adjuvanted subunit gB. Immunization with the pp65 vector alone elicited robust T cell responses. Comparable immunogenicity of the combined gB(dCt) and pp65 vectors with the individual monovalent formulations was demonstrated. To demonstrate proof of principle for a bivalent rLCMV-based HCMV vaccine, the congenital guinea pig cytomegalovirus (GPCMV) infection model was used to compare rLCMV vectors encoding homologs of pp65 (GP83) and gB(dCt), alone and in combination versus Freund's adjuvanted recombinant gB. Both vectors elicited significant immune responses, and no loss of gB immunogenicity was noted with the bivalent formulation. Combined vaccination with rLCMV-vectored GPCMV gB(dCt) and pp65 (GP83) conferred better protection against maternal viremia than subunit or either monovalent rLCMV vaccine. The bivalent vaccine also was significantly more effective in reducing pup mortality than the monovalent vaccines. In summary, bivalent vaccines with rLCMV vectors expressing gB and pp65 elicited potent humoral and cellular responses and conferred protection in the GPCMV model. Further clinical trials of LCMV-vectored HCMV vaccines are warranted.

Cell Surface THY-1 Contributes to Human Cytomegalovirus Entry via a Macropinocytosis-Like Process

Previously we showed that THY-1 has a critical role in the initial stage of infection of certain cell types with human cytomegalovirus (HCMV) and that THY-1 is important for HCMV-mediated activation of phosphatidylinositol 3-kinase (PI3K)/Akt during virus entry. THY-1 is known to interact with integrins and is a major cargo protein of clathrin-independent endocytic vesicles. Since macropinocytosis involves integrin signaling, is PI3K/Akt dependent, and is a clathrin-independent endocytic process, we determined whether THY-1 has a role in HCMV entry by macropinocytosis. Using electron microscopy in two cell lines that support HCMV infection in a THY-1-dependent manner, we found that HCMV enters these cells by a macropinocytosis-like process. THY-1 associated with HCMV virions on the cell surface and colocalized with virus inside macropinosomes. 5-(N-Ethyl-N-isopropyl)amiloride (EIPA) and soluble THY-1 blocked HCMV infection in the cell lines by ≥80% and 60%, respectively. HCMV entry into the cells triggered increased influx of extracellular fluid, a marker of macropinocytosis, and this increased fluid uptake was inhibited by EIPA and by soluble THY-1. Blocking actin depolymerization, Na⁺/H⁺ exchange, PI3K, and Pak1 kinase, which are critical for macropinocytosis, impaired HCMV infection. Neither internalized HCMV virions nor THY-1 in virus-infected cells colocalized with transferrin as determined by confocal microscopy, indicating that clathrin-mediated endocytosis was not involved in THY-1-associated virus entry. These results suggest that HCMV has adapted to utilize THY-1, a cargo protein of clathrin-independent endocytotic vesicles, to facilitate efficient entry into certain cell types by a macropinocytosis-like process.

IMPORTANCE

Human cytomegalovirus (HCMV) infects over half of the population and is the most common infectious cause of birth defects. The virus is the most important infection occurring in transplant recipients. The mechanism of how HCMV enters cells is controversial. In this study, we show that THY-1, a cell surface protein that is critical for the early stage of entry of HCMV into certain cell types, contributes to virus entry by macropinocytosis. Our findings suggest that HCMV has adapted to utilize THY-1 to facilitate entry of HCMV into macropinosomes in certain cell types. Further knowledge about the mechanism of HCMV entry into cells may facilitate the development of novel inhibitors of virus infection.

Virion Glycoprotein-Mediated Immune Evasion by Human Cytomegalovirus: a Sticky Virus Makes a Slick Getaway

The prototypic herpesvirus human cytomegalovirus (CMV) exhibits the extraordinary ability to establish latency and maintain a chronic infection throughout the life of its human host. This is even more remarkable considering the robust adaptive immune response elicited by infection and reactivation from latency. In addition to the ability of CMV to exist in a quiescent latent state, its persistence is enabled by a large repertoire of viral proteins that subvert immune defense mechanisms, such as NK cell activation and major histocompatibility complex antigen presentation, within the cell. However, dissemination outside the cell presents a unique existential challenge to the CMV virion, which is studded with antigenic glycoprotein complexes targeted by a potent neutralizing antibody response. The CMV virion envelope proteins, which are critical mediators of cell attachment and entry, possess various characteristics that can mitigate the humoral immune response and prevent viral clearance. Here we review the CMV glycoprotein complexes crucial for cell attachment and entry and propose inherent properties of these proteins involved in evading the CMV humoral immune response. These include viral glycoprotein polymorphism, epitope competition, Fc receptor-mediated endocytosis, glycan shielding, and cell-to-cell spread. The consequences of CMV virion glycoprotein-mediated immune evasion have a major impact on persistence of the virus in the population, and a comprehensive understanding of these evasion strategies will assist in designing effective CMV biologics and vaccines to limit CMV-associated disease.

Conservation of the glycoprotein B homologs of the Kaposi׳s sarcoma-associated herpesvirus (KSHV/HHV8) and old world primate rhadinoviruses of chimpanzees and macaques

The envelope-associated glycoprotein B (gB) is highly conserved within the Herpesviridae and plays a critical role in viral entry. We analyzed the evolutionary conservation of sequence and structural motifs within the Kaposi׳s sarcoma-associated herpesvirus (KSHV) gB and homologs of Old World primate rhadinoviruses belonging to the distinct RV1 and RV2 rhadinovirus lineages. In addition to gB homologs of rhadinoviruses infecting the pig-tailed and rhesus macaques, we cloned and sequenced gB homologs of RV1 and RV2 rhadinoviruses infecting chimpanzees. A structural model of the KSHV gB was determined, and functional motifs and sequence variants were mapped to the model structure. Conserved domains and motifs were identified, including an "RGD" motif that plays a critical role in KSHV binding and entry through the cellular integrin αVβ3. The RGD motif was only detected in RV1 rhadinoviruses suggesting an important difference in cell tropism between the two rhadinovirus lineages.

THY-1 Cell Surface Antigen (CD90) Has an Important Role in the Initial Stage of Human Cytomegalovirus Infection

Human cytomegalovirus (HCMV) infects about 50% of the US population, is the leading infectious cause of birth defects, and is considered the most important infectious agent in transplant recipients. The virus infects many cell types in vivo and in vitro. While previous studies have identified several cellular proteins that may function at early steps of infection in a cell type dependent manner, the mechanism of virus entry is still poorly understood. Using a computational biology approach, correlating gene expression with virus infectivity in 54 cell lines, we identified THY-1 as a putative host determinant for HCMV infection in these cells. With a series of loss-of-function, gain-of-function and protein-protein interaction analyses, we found that THY-1 mediates HCMV infection at the entry step and is important for infection that occurs at a low m.o.i. THY-1 antibody that bound to the cell surface blocked HCMV during the initial 60 minutes of infection in a dose-dependent manner. Down-regulation of THY-1 with siRNA impaired infectivity occurred during the initial 60 minutes of inoculation. Both THY-1 antibody and siRNA inhibited HCMV-induced activation of the PI3-K/Akt pathway required for entry. Soluble THY-1 protein blocked HCMV infection during, but not after, virus internalization. Expression of exogenous THY-1 enhanced entry in cells expressing low levels of the protein. THY-1 interacted with HCMV gB and gH and may form a complex important for entry. However, since gB and gH have previously been shown to interact, it is uncertain if THY-1 directly binds to both of these proteins. Prior observations that THY-1 (a) interacts with αVβ3 integrin and recruits paxillin (implicated in HCMV entry), (b) regulates leukocyte extravasation (critical for HCMV viremia), and (c) is expressed on many cells targeted for HCMV infection including epithelial and endothelial cells, fibroblast, and CD34+/CD38- stem cells, all support a role for THY-1 as an HCMV entry mediator in a cell type dependent manner. THY-1 may function through a complex setting, that would include viral gB and gH, and other cellular factors, thus links virus entry with signaling in host cells that ultimately leads to virus infection.

Human cytomegalovirus vaccine based on the envelope gH/gL pentamer complex

Human Cytomegalovirus (HCMV) utilizes two different pathways for host cell entry. HCMV entry into fibroblasts requires glycoproteins gB and gH/gL, whereas HCMV entry into epithelial and endothelial cells (EC) requires an additional complex composed of gH, gL, UL128, UL130, and UL131A, referred to as the gH/gL-pentamer complex (gH/gL-PC). While there are no established correlates of protection against HCMV, antibodies are thought to be important in controlling infection. Neutralizing antibodies (NAb) that prevent gH/gL-PC mediated entry into EC are candidates to be assessed for in vivo protective function. However, these potent NAb are predominantly directed against conformational epitopes derived from the assembled gH/gL-PC. To address these concerns, we constructed Modified Vaccinia Ankara (MVA) viruses co-expressing all five gH/gL-PC subunits (MVA-gH/gL-PC), subsets of gH/gL-PC subunits (gH/gL or UL128/UL130/UL131A), or the gB subunit from HCMV strain TB40/E. We provide evidence for cell surface expression and assembly of complexes expressing full-length gH or gB, or their secretion when the corresponding transmembrane domains are deleted. Mice or rhesus macaques (RM) were vaccinated three times with MVA recombinants and serum NAb titers that prevented 50% infection of human EC or fibroblasts by HCMV TB40/E were determined. NAb responses induced by MVA-gH/gL-PC blocked HCMV infection of EC with potencies that were two orders of magnitude greater than those induced by MVA expressing gH/gL, UL128-UL131A, or gB. In addition, MVA-gH/gL-PC induced NAb responses that were durable and efficacious to prevent HCMV infection of Hofbauer macrophages, a fetal-derived cell localized within the placenta. NAb were also detectable in saliva of vaccinated RM and reached serum peak levels comparable to NAb titers found in HCMV hyperimmune globulins. This vaccine based on a translational poxvirus platform co-delivers all five HCMV gH/gL-PC subunits to achieve robust humoral responses that neutralize HCMV infection of EC, placental macrophages and fibroblasts, properties of potential value in a prophylactic vaccine.

Human cytomegalovirus (HCMV) fetal infection during pregnancy and infection of immunocompromised patients are both clinical problems considered extremely important by the Institute of Medicine. Limited efficacy against primary HCMV infection was found using a subunit vaccine based on glycoprotein B, an important neutralizing antibody determinant blocking HCMV entry into fibroblasts. The HCMV field has been transformed by the discovery that a five-member (pentamer) protein complex is a required factor for epithelial and endothelial cell entry and indispensable for transmission as shown in non-human primates. Targeting HCMV with antibodies specific to the pentamer may interrupt horizontal and vertical transmission. We describe an innovative vaccine strategy to induce serum neutralizing antibodies of impressive magnitude against HCMV in two animal models. Using an attenuated poxvirus vector system, we demonstrate that co-expression of all five pentamer components is significantly more potent to induce serum neutralizing antibodies than subunit subsets of the complex or glycoprotein B, reaching peak levels comparable to HCMV hyperimmune globulin. A vaccine that elicits systemic and mucosal antibody responses that prevents infection of multiple cell types crucial to natural history of HCMV infection could play a role in preventing congenital HCMV infection and control of infection in immunocompromised patients.

Cytomegalovirus vaccine strain towne-derived dense bodies induce broad cellular immune responses and neutralizing antibodies that prevent infection of fibroblasts and epithelial cells

Human cytomegalovirus (HCMV), a betaherpesvirus, can cause severe disease in immunosuppressed patients and following congenital infection. A vaccine that induces both humoral and cellular immunity may be required to prevent congenital infection. Dense bodies (DBs) are complex, noninfectious particles produced by HCMV-infected cells and may represent a vaccine option. As knowledge of the antigenicity and immunogenicity of DB is incomplete, we explored characterization methods and defined DB production methods, followed by systematic evaluation of neutralization and cell-mediated immune responses to the DB material in BALB/c mice. DBs purified from Towne-infected cultures treated with the viral terminase inhibitor 2-bromo-5,6-dichloro-1-beta-d-ribofuranosyl benzimidazole riboside (BDCRB) were characterized by nanoparticle tracking analysis (NTA), two-dimensional fluorescence difference gel electrophoresis (2D-DIGE), immunoblotting, quantitative enzyme-linked immunosorbent assay, and other methods. The humoral and cellular immune responses to DBs were compared to the immunogenicity of glycoprotein B (gB) administered with the adjuvant AddaVax (gB/AddaVax). DBs induced neutralizing antibodies that prevented viral infection of cultured fibroblasts and epithelial cells and robust cell-mediated immune responses to multiple viral proteins, including pp65, gB, and UL48. In contrast, gB/AddaVax failed to induce neutralizing antibodies that prevented infection of epithelial cells, highlighting a critical difference in the humoral responses induced by these vaccine candidates. Our data advance the potential for the DB vaccine approach, demonstrate important immunogenicity properties, and strongly support the further evaluation of DBs as a CMV vaccine candidate.

Characterization of the guinea pig CMV gH/gL/GP129/GP131/GP133 complex in infection and spread

In human cytomegalovirus (HCMV), the UL128-131A locus plays an essential role in cellular tropism and spread. Here, we report the complete annotation of the GP129-133 locus from guinea pig cytomegalovirus (GPCMV) and the discovery of the UL131A homolog, named GP133. We have found that similar to HCMV the GP129-133 proteins form a pentamer complex with the GPCMV glycoproteins gH and gL. In addition, we find that the GP129-133 proteins play a critical role in entry as the GP129-133 deletion mutant shows a defect in both endothelial and fibroblast cell entry. Although the GP129-133 deletion strain can propagate in vitro, we find that the deletion fails to spread in vivo. Interestingly, the wildtype strain can spontaneously give rise to the GP129-133 deletion strain during in vivo spread, suggesting genetic instability at this locus.

HCMV gB shares structural and functional properties with gB proteins from other herpesviruses

Glycoprotein B (gB) facilitates HCMV entry into cells by binding receptors and mediating membrane fusion. The crystal structures of gB ectodomains from HSV-1 and EBV are available, but little is known about the HCMV gB structure. Using multiangle light scattering and electron microscopy, we show here that HCMV gB ectodomain is a trimer with the overall shape similar to HSV-1 and EBV gB ectodomains. HCMV gB ectodomain forms rosettes similar to rosettes formed by EBV gB and the postfusion forms of other viral fusogens. Substitution of several bulky hydrophobic residues within the putative fusion loops with more hydrophilic residues reduced rosette formation and abolished cell fusion. We propose that like gB proteins from HSV-1 and EBV, HCMV gB has two internal hydrophobic fusion loops that likely interact with target membranes. Our work establishes structural and functional similarities between gB proteins from three subfamilies of herpesviruses.

Neuropathogenesis of congenital cytomegalovirus infection: disease mechanisms and prospects for intervention

Congenital cytomegalovirus (CMV) infection is the leading infectious cause of mental retardation and hearing loss in the developed world. In recent years, there has been an improved understanding of the epidemiology, pathogenesis, and long-term disabilities associated with CMV infection. In this review, current concepts regarding the pathogenesis of neurological injury caused by CMV infections acquired by the developing fetus are summarized. The pathogenesis of CMV-induced disabilities is considered in the context of the epidemiology of CMV infection in pregnant women and newborn infants, and the clinical manifestations of brain injury are reviewed. The prospects for intervention, including antiviral therapies and vaccines, are summarized. Priorities for future research are suggested to improve the understanding of this common and disabling illness of infancy.

Accurate determination of zygosity in transgenic rice by real-time PCR does not require standard curves or efficiency correction

A number of quantitative, real-time PCR methods have been developed for determining transgene copy numbers in plants. Here, we demonstrate that the Roche LightCycler system can be used to determine the zygosity of transgenic lines without the use of standard curves or efficiency correction calculations. We have developed a duplex PCR assay which permits the determination of zygosity, relative to a calibrator sample, in transgenic rice lines containing the gene for a viral glycoprotein. Our data demonstrate that unambiguous 2-fold discrimination of copy number can be attained by calculating relative copy number using the threshold crossing point (Ct) calculated by the LightCycler software combined with delta delta Ct calculations, provided that the appropriate calibrator sample is included in each run. The method presented here is rapid, sensitive, robust and easy to optimise.

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.

Release of the recombinant proteins, human serum albumin, beta-glucuronidase, glycoprotein B from human cytomegalovirus, and green fluorescent protein, in root exudates from transgenic tobacco and their effects on microbes and enzymatic activities in soil

We determined the release in root exudates of human serum albumin (HSA), beta-glucuronidase (GUS), glycoprotein B (gB) from human cytomegalovirus, and green fluorescent protein (GFP) from genetically modified transgenic tobacco expressing the genes for these proteins in hydroponic culture and non-sterile soil. GUS, gB, and GFP were expressed in the plant but were not released in root exudates, whereas HSA was both expressed in the plant and released in root exudates, as shown by a 66.5-kDa band on SDS-PAGE and Western blot and confirmed by ELISA. Root exudates from GUS and gB plants showed no bands that could be attributed to these proteins on SDS-PAGE, and root exudates from GFP plants showed no fluorescence. The concentration of HSA in root exudates was estimated to be 0.021 ng ml(-1), whereas that in the plant biomass was estimated to be 0.087 ng ml(-1). The concentration of HSA in soil was estimated to be 0.049 ng g(-1). No significant differences in the number of microorganisms and the activity of selected enzymes were observed between rhizosphere soil of non-modified and HSA tobacco.

Progress toward an elusive goal: current status of cytomegalovirus vaccines

Although infection with human cytomegalovirus (CMV) is ubiquitous and generally asymptomatic in most individuals, certain patient populations are at high risk for CMV-associated disease. These include HIV-infected individuals with AIDS, transplant patients, and newborn infants with congenital CMV infection. Immunity to CMV infection, both in the transplant setting and among women of childbearing age, plays a vital role in the control of CMV-induced injury and disease. Although immunity induced by CMV infection is not completely protective against reinfection, there is nevertheless a sound basis on which to believe that vaccination could help control CMV disease in high-risk patient populations. Evidence from several animal models of CMV infection indicates that a variety of vaccine strategies are capable of inducing immune responses sufficient to protect against CMV-associated illness following viral challenge. Vaccination has also proven effective in improving pregnancy outcomes following CMV challenge of pregnant guinea pigs, providing a 'proof-of-principle' relevant to human clinical trials of CMV vaccines. Although there are no licensed vaccines currently available for human CMV, progress toward this goal has been made, as evidenced by ongoing clinical trial testing of a number of immunization strategies. CMV vaccines currently in various stages of preclinical and clinical testing include: protein subunit vaccines; DNA vaccines; vectored vaccines using viral vectors, such as attenuated pox- and alphaviruses; peptide vaccines; and live attenuated vaccines. This review summarizes some of the obstacles that must be overcome in development of a CMV vaccine, and provides an overview of the current state of preclinical and clinical trial evaluation of vaccines for this important public health problem.

Human cytomegalovirus virion protein complex required for epithelial and endothelial cell tropism

Human cytomegalovirus replicates in many different cell types, including epithelial cells, endothelial cells, and fibroblasts. However, laboratory strains of the virus, many of which were developed as attenuated vaccine candidates by serial passage in fibroblasts, have lost the ability to infect epithelial and endothelial cells. Their growth is restricted primarily to fibroblasts, due to mutations in the UL131-UL128 locus. We now demonstrate that two products of this locus, pUL130 and pUL128, form a complex with gH and gL, but not gO. The AD169 laboratory strain, which lacks a functional UL131 protein, produces virions containing only the gH-gL-gO complex. An epithelial and endothelial cell tropic AD169 variant in which the UL131 ORF has been repaired, termed BADrUL131, produces virions that carry both gH-gL-gO and gH-gL-pUL128-pUL130 complexes. Antibodies against pUL130 and pUL128 block infection of epithelial and endothelial cells by BADrUL131 and the fusion-inducing factor X clinical human cytomegalovirus isolate but do not affect the efficiency with which fibroblasts are infected.

Predicting coding potential from genome sequence: application to betaherpesviruses infecting rats and mice

Prediction of protein-coding regions and other features of primary DNA sequence have greatly contributed to experimental biology. Significant challenges remain in genome annotation methods, including the identification of small or overlapping genes and the assessment of mRNA splicing or unconventional translation signals in expression. We have employed a combined analysis of compositional biases and conservation together with frame-specific G+C representation to reevaluate and annotate the genome sequences of mouse and rat cytomegaloviruses. Our analysis predicts that there are at least 34 protein-coding regions in these genomes that were not apparent in earlier annotation efforts. These include 17 single-exon genes, three new exons of previously identified genes, a newly identified four-exon gene for a lectin-like protein (in rat cytomegalovirus), and 10 probable frameshift extensions of previously annotated genes. This expanded set of candidate genes provides an additional basis for investigation in cytomegalovirus biology and pathogenesis.

Human cytomegalovirus virion proteins

Human cytomegalovirus (HCMV) is the largest member of the family of human herpesviruses. The number of virus encoded proteins and the complexity of their functions in the life cycle of this virus are reflected in the size of its genome. There continues to be some controversy surrounding the exact protein coding capacity of the virus with estimates ranging from 160 open reading frames to more than 200 open reading frames. Very recent studies using mass spectrometry to determine the viral proteome suggests that the number of viral proteins may be even greater than previous estimates. The proteins of the virion capsid have readily identifiable homologous proteins in the capsid of the more extensively studied herpes simplex virus, likely because of similar capsid structure and assembly pathways. In contrast, the tegument and the envelope of HCMV contain a significant number of proteins that lack structural homology to proteins found in either alpha or gamma-herpesviruses. This brief overview discusses some of the general features and possible functions of the HCMV virion structural proteins in the replicative cycle of this virus.

Characterization of murine gammaherpesvirus 68 glycoprotein B

Murine gammaherpesvirus 68 (MHV-68) glycoprotein B (gB) was identified in purified virions by immunoblotting, immunoprecipitation, and immunoelectron microscopy. It was synthesized as a 120-kDa precursor in infected cells and cleaved into 65-kDa and 55-kDa disulfide-linked subunits close to the time of virion release. The N-linked glycans on the cleaved, virion gB remained partially endoglycosidase H sensitive. The processing of MHV-68 gB therefore appears similar to that of Kaposi's sarcoma-associated herpesvirus gB and human cytomegalovirus gB.

Recombinant modified vaccinia virus Ankara expressing a soluble form of glycoprotein B causes durable immunity and neutralizing antibodies against multiple strains of human cytomegalovirus

Human cytomegalovirus (CMV) is a viral pathogen that infects both genders, who remain asymptomatic unless they receive immunosuppressive drugs or acquire infections that cause reactivation of latent virus. CMV infection also causes serious birth defects following primary maternal infection during gestation. A safe and effective vaccine to limit disease in this population continues to be elusive. A well-studied antigen is glycoprotein B (gB), which is the principal target of neutralizing antibodies (NAb) towards CMV in humans and has been implicated as the viral partner in the receptor-mediated infection by CMV in a variety of cell types. Antibody-mediated virus neutralization has been proposed as a mechanism by which host immunity could modify primary infection. Towards this goal, an attenuated poxvirus, modified vaccinia virus Ankara (MVA), has been constructed to express soluble CMV gB (gB680-MVA) to induce CMV NAb. Very high levels of gB-specific CMV NAb were produced after two doses of the viral vaccine. NAb were durable within a twofold range for up to 6 months. Neutralization titers developed in immunized mice are equivalent to titers found clinically after natural infection. This viral vaccine, expressing gB derived from CMV strain AD169, induced antibodies that neutralized CMV strains of three different genotypes. Remarkably, preexisting MVA and vaccinia virus (poxvirus) immunity did not interfere with subsequent immunizations of gB680-MVA. The safety characteristics of MVA, combined with the robust immune response to CMV gB, suggest that this approach could be rapidly translated into the clinic.

Specific inhibition of human cytomegalovirus glycoprotein B-mediated fusion by a novel thiourea small molecule

A novel small molecule inhibitor of human cytomegalovirus (HCMV) was identified as the result of screening a chemical library by using a whole-virus infected-cell assay. Synthetic chemistry efforts yielded the analog designated CFI02, a compound whose potency had been increased about 100-fold over an initial inhibitor. The inhibitory concentration of CFI02 in various assays is in the low nanomolar range. CFI02 is a selective and potent inhibitor of HCMV; it has no activity against other CMVs, alphaherpesviruses, or unrelated viruses. Mechanism-of-action studies indicate that CFI02 acts very early in the replication cycle, inhibiting virion envelope fusion with the cell plasma membrane. Mutants resistant to CFI02 have mutations in the abundant virion envelope glycoprotein B that are sufficient to confer resistance. Taken together, the data suggest that CFI02 inhibits glycoprotein B-mediated HCMV virion fusion. Furthermore, CFI02 inhibits the cell-cell spread of HCMV. This is the first study of a potent and selective small molecule inhibitor of CMV fusion and cell-cell spread.

Antibody responses to rhesus cytomegalovirus glycoprotein B in naturally infected rhesus macaques

Rhesus cytomegalovirus (RhCMV) exhibits strong parallels with human CMV (HCMV) in terms of nucleic and amino acid identities, natural history, and mechanisms of persistence and pathogenesis in its natural host, rhesus macaques (Macaca mulatta). To determine whether this non-human primate model would be useful to assess vaccine strategies for HCMV, host immune responses to RhCMV glycoprotein B (gB) were evaluated in RhCMV-infected monkeys. Total protein extracts were prepared from cells transiently transfected with an expression plasmid for either the full-length gB or a derivative (gBDelta, 1-680 aa) lacking both the transmembrane domain and cytoplasmic tail. Western blot analysis showed identical reactivity of macaque sera with full-length gB and its derivative gBDelta, indicating that the immunodominant epitopes of gB are contained in the extracellular portion of the protein. Using gBDelta extract as a solid phase, a sensitive and specific ELISA was established to characterize gB antibody responses in monkeys acutely and chronically infected with RhCMV. During primary infection (seroconversion), gB-specific antibodies developed concurrently and in parallel with total RhCMV-specific antibodies. However, during chronic infection gB-specific antibody responses were variable. A strong correlation was observed between neutralizing and gB-specific antibody levels in RhCMV-seropositive monkeys. Taken together, the results of this study indicate that, similar to host humoral responses to HCMV gB, anti-gB antibodies are an integral part of humoral immunity to RhCMV infection and probably play an important protective role in limiting the extent of RhCMV infection. Thus, the rhesus macaque model of HCMV infection is relevant for testing gB-based immune therapies.

Two engineered eglin c mutants potently and selectively inhibiting kexin or furin

Eglin c with mutants L45R and D42R at the P(1) and P(4) positions has been reported to become a stable inhibitor toward the proprotein convertases (PC), furin and kexin, with a K(i) of 2.3x10(-8) and 1.3x10(-10) M, respectively. The mutant was further engineered at the P(2)'-P(4)' positions to create a more potent and selective inhibitor for each enzyme. The residue Asp at P(1)' which is crucial for stabilizing the conformation of eglin c remained unchanged. The eglin c mutants cloned into the vector pGEX-2T and expressed in Escherichia coli (DH5alpha) were purified to homogeneity, and their inhibitory activities toward the purified recombinant furin and kexin were examined. The results showed that (1) Leu47 at P(2)' replaced with either a positively or negatively charged residue resulted in a decrease in inhibitory activities to both enzymes; (2) the replacement of Arg with Asp at P(3)' was favorable for inhibiting furin with a K(i) of 7.8 x 10(-9) M, but not for inhibiting kexin; (3) the replacement of Tyr with Glu at P(4)' increased the inhibitory activity to kexin with a K(i) of 3 x 10(-11) M, but was almost without any influence on furin inhibition. It was indicated that the inhibitory specificity of eglin c could be changed from inhibiting elastase to inhibiting PCs by site-directed mutation at the P positions, while the inhibitory selectivity to furin or kexin could be optimized by mutation at the P' positions.

Identification of the authentic varicella-zoster virus gB (gene 31) initiating methionine overlapping the 3' end of gene 30

The varicella-zoster virus (VZV) gB sequence was re-examined in light of recent knowledge about unusually long gB signal peptides in other herpesviral gB homologs. Through mutational analysis, the discovery was made that the authentic initiating methionine for VZV gB is a codon beginning at genome nucleotide 56,819. The total length for the VZV gB primary translation product was 931 amino acids (aa) with a 71-aa signal sequence. Considering the likely signal sequence cleavage site to be located between Ser 71 and Val 72, the length of the mature VZV gB polypeptide would then be 860 amino acids prior to further internal endoproteolytic cleavage between amino acids Arg 494 and Ser 495. In this report, we also produced a full-length gB and demonstrated its association with VZV gE, suggesting a possible gE-gB interaction during gB trafficking before its cleavage in the Golgi.

Binding characteristics determine the neutralizing potential of antibody fragments specific for antigenic domain 2 on glycoprotein B of human cytomegalovirus

Site I of antigenic domain 2 (AD-2) on human cytomegalovirus glycoprotein B (gB) is poorly immunogenic in both man and mouse and knowledge about antibody repertoires reactive with this epitope is thus limited. Here we have characterized a phage display-derived repertoire of antibody fragments specific for this epitope in terms of antigen recognition, fine-specificity, and virus-neutralizing capacity. Our results show that the functional properties within a closely related repertoire may differ widely and that the effectiveness of the members of the repertoire to neutralize the virus is determined by the fine-specificity and kinetics of the interaction with the antigen. The half-life of the interaction between monomeric antibody fragments and gB seems to be particularly critical for the neutralizing capacity. We also demonstrate that sequence variation within gB allows virus variants to escape at least a part of the AD-2-specific neutralizing antibody repertoire, apparently without preventing antibody binding to the epitope.

Excretion of herpes simplex virus type 2 glycoprotein D into the culture medium

Glycoprotein D (gD) of herpes simplex virus type 2 (HSV-2) was excreted from infected cells into the medium. Peptide mapping analysis and lectin binding assays suggested that the gD in the medium is secreted after full glycosylation and cleavage at its C terminus. Release of HSV-2 gD was inhibited by addition of either tunicamycin or brefeldin A, suggesting that the gD in the medium was secreted through the endoplasmic reticulum and Golgi apparatus.

The genotype of mice influences the autoimmune response to spliceosome proteins induced by cytomegalovirus gB immunization

In previous studies we have established a link between cytomegalovirus (CMV) infection and an autoimmune response to the U1-70 k protein of the spliceosome in man. This autoimmune response, generally referred to as the anti-RNP (ribonucleoprotein) antibodies, is observed in about 30% of patients with systemic lupus erythematosus (SLE). We have also found that the CMV glycoprotein B (CMV gB) when expressed in a adenovirus vector (Ad) could induce a significant anti-U1-70 k antibody response in several strains of mice, such as C3H, MRL and BALB/c. In the present study we examined the autoimmune response induced by immunization with Ad-gB in A/J and C57BL/6 (B6) mice and determined whether there was any autoimmune phenotype similar to that observed in patients with SLE. Thus groups of A/J and B6 mice were immunized with Ad/gB or with Ad alone and then observed for possible skin or kidney disease. In addition the autoantibody response to the spliceosome was measured, and the target antigens identified by immunoblot techniques. All of the A/J mice mounted a very high IgG response primarily to the U1-70 k protein of the spliceosome, with evidence of a rapid spreading of the autoantibody response to other components of the complex. In contrast, B6 mice mounted only a very low titre autoantibody response and failed to show signs or symptoms of autoimmunity. The A/J but not the B6 mice were found to have deposits of IgG in their kidneys, which were consistent with abnormal levels of blood urea nitrogen in the A/J but not B6 mice. This study demonstrates the importance of the genetic background in the susceptibility to autoimmunity.

SLAM (CD150)-independent measles virus entry as revealed by recombinant virus expressing green fluorescent protein

Wild-type measles virus (MV) strains use human signaling lymphocyte activation molecule (SLAM) as a cellular receptor, while vaccine strains such as the Edmonston strain can use both SLAM and CD46 as receptors. Although the expression of SLAM is restricted to cells of the immune system (lymphocytes, dendritic cells, and monocytes), histopathological studies with humans and experimentally infected monkeys have shown that MV also infects SLAM-negative cells, including epithelial, endothelial, and neuronal cells. In an attempt to explain these findings, we produced the enhanced green fluorescent protein (EGFP)-expressing recombinant MV (IC323-EGFP) based on the wild-type IC-B strain. IC323-EGFP showed almost the same growth kinetics as the parental recombinant MV and produced large syncytia exhibiting green autofluorescence in SLAM-positive cells. Interestingly, all SLAM-negative cell lines examined also showed green autofluorescence after infection with IC323-EGFP, although the virus hardly spread from the originally infected individual cells and thus did not induce syncytia. When the number of EGFP-expressing cells after infection was taken as an indicator, the infectivities of IC323-EGFP for SLAM-negative cells were 2 to 3 logs lower than those for SLAM-positive cells. Anti-MV hemagglutinin antibody or fusion block peptide, but not anti-CD46 antibody, blocked IC323-EGFP infection of SLAM-negative cells. This infection occurred under conditions in which entry via endocytosis was inhibited. These results indicate that MV can infect a variety of cells, albeit with a low efficiency, by using an as yet unidentified receptor(s) other than SLAM or CD46, in part explaining the observed MV infection of SLAM-negative cells in vivo.

Replication of rhesus cytomegalovirus in life-expanded rhesus fibroblasts expressing human telomerase

The kinetics of rhesus cytomegalovirus (RhCMV) infection were compared in primary and telomerase-immortalized (Telo-) rhesus fibroblasts (RF). Equivalent viral titers were achieved with both cell types. However, the production of infectious virions was slightly faster and plaque size was larger in Telo-RF, compared with primary cells. Comparable RhCMV growth curves and viral susceptibility were observed using Telo-RF passaged for different time periods in culture, whereas the ability of primary cells to support robust RhCMV replication declined as the cells approached senescence. Analysis of cell growth kinetics suggested that the rate of RhCMV replication was directly related to the rate of cell proliferation. RT-PCR analysis of representative RhCMV genes demonstrated that the presence of telomerase did not alter the temporal profile of RhCMV gene expression. In addition, Telo-RF cells were observed to have a significantly increased efficiency of transfection with cationic lipids, compared with primary RF. These results demonstrated that Telo-RF represents a stable, permissive cell line for RhCMV infection, facilitating standardization of in vitro assays for this important non-human primate CMV. The ease of transfection will enable molecular analyses and the generation of complementing cell lines for the propagation of defective RhCMV variants.

Open reading frame UL26 of human cytomegalovirus encodes a novel tegument protein that contains a strong transcriptional activation domain

A selection strategy, the activator trap, was used in order to identify genes of human cytomegalovirus (HCMV) that encode strong transcriptional activation domains in mammalian cells. This approach is based on the isolation of activation domains from a GAL4 fusion library by means of selective plasmid replication, which is mediated in transfected cells by a GAL4-inducible T antigen gene. With this screening strategy, we were able to isolate two types of plasmids encoding transactivating fusion proteins from a library of random HCMV DNA inserts. One plasmid contained the exon 3 of the HCMV IE-1/2 gene region, which has previously been identified as a strong transcriptional activation domain. In the second type of plasmid, the open reading frame (ORF) UL26 of HCMV was fused to the GAL4 DNA-binding domain. By quantitative RNA mapping using S1 nuclease analysis, we were able to classify UL26 as a strong enhancer-type activation domain with no apparent homology to characterized transcriptional activators. Western blot analysis with a specific polyclonal antibody raised against a prokaryotic UL26 fusion protein revealed that two protein isoforms of 21 and 27 kDa are derived from the UL26 ORF in both infected and transfected cells. Both protein isoforms, which arise via alternative usage of two in-frame translational start codons, showed a nuclear localization and could be detected as early as 6 h after infection of primary human fibroblasts. By performing Western blot analysis with purified virions combined with fractionation experiments, we provide evidence that pUL26 is a novel tegument protein of HCMV that is imported during viral infection. Furthermore, we observed transactivation of the HCMV major immediate-early enhancer-promoter by pUL26, whereas several early and late promoters were not affected. Our data suggest that pUL26 is a novel tegument protein of HCMV with a strong transcriptional activation domain that could play an important role during initiation of the viral replicative cycle.

Genetic variations of human herpesvirus 7 by analysis of glycoproteins B and H, and R2-repeat regions

Clinical isolates of human herpesvirus 7 (HHV-7) from the saliva of healthy individual were investigated for genetic variations in the regions of two immediate-early (IE) genes, the glycoprotein B (gB) and glycoprotein H (gH) genes, and in R2-repeat. The genomic DNA of 24 isolates from citizens of Thailand, Japan, and the United States was amplified to detect size variations in the IE-1 and IE-2 loci, but none was observed, suggesting that there was no deletion or insertion in these genes, in contrast with an IE gene of human herpesvirus 6 (HHV-6). The sequences of the gB gene from isolates acquired from 5 Japanese and 8 Thai subject were then compared with those of American strains JI and RK with respect to codons that are known to differentiate gB alleles. All the isolates were found to have gB allele C except for the JI strain, which has allele F. Variability was also observed in five specific gH codons, resulting in 6 different groups. The HHV-7 isolates might be classified into two major genetic variants by combining their gB and gH allelic groupings. In the present study, only JI belonged to variant 1, while the rest of the isolates appeared to belong to variant 2. In the R2-repeat region, size heterogeneities were observed among the 24 isolates, due to different repeat numbers (17, 15, 14, 13, or 12 repeats). Therefore, we used the R2-repeat to identify the origins of isolates in a study of HHV-7 transmission, and found HHV-7 to be transmitted within a family from both mothers and fathers to their children.

Efficient downregulation of major histocompatibility complex class I molecules in human epithelial cells infected with cytomegalovirus

Liver and intestinal epithelial cells are a major target of infection by cytomegaloviruses (CMV), causing severe disease in affected organs of immunocompromised patients. CMV downregulates major histocompatibility complex class I (MHC-I) molecule expression in fibroblasts in order to avoid lysis by CD8(+) cytotoxic T lymphocytes. However, MHC-I expression in human cytomegalovirus (HCMV)-infected hepatic tissue was reported to be increased. As it is unclear at present whether HCMV affects MHC-I expression in epithelial cells, new cell culture models for HCMV infection of differentiated hepatobiliary cell lines were established. HCMV immediate early gene expression was achieved in 60 to 95% of cells. Progression of the HCMV replication cycle differed from prototypic infection of fibroblasts, since structural early and late proteins were produced at low levels and HCMV progeny yielded much lower titres in hepatobiliary cells. In contrast, HCMV glycoproteins, gpUS2, gpUS3, gpUS6 and gpUS11, that downregulate MHC-I expression were synthesized with temporal kinetics and in a similar quantity to that seen in fibroblasts. As a result, HCMV infection led to a drastic and selective downregulation of MHC-I expression in epithelial cells and was uniformly observed irrespective of the hepatic or biliary origin of the cells. The new models document for the first time a stealth function of HCMV in epithelial cells and indicate that the downregulation of MHC-I expression by HCMV can occur in the virtual absence of virus replication.

The pathogenesis of influenza in humans

The rapid evolution of influenza A and B viruses contributes to annual influenza epidemics in humans. In addition, pandemics of influenza are also caused by influenza A viruses, whereas influenza B does not have the potential to cause pandemics because there is no animal reservoir of the virus. Study of the genetic differences between influenza A and influenza B viruses, which are restricted to humans, may be informative in understanding the factors that govern mammalian adaptation of influenza A viruses. Aquatic birds provide the natural reservoir for influenza A viruses, but in general, avian influenza is asymptomatic in feral birds. Occasionally, however, highly pathogenic strains of influenza cause serious systemic infections in domestic poultry. The pathogenicity of these strains is related to the presence of a polybasic cleavage sequence in the precursor of the surface glycoprotein haemagglutinin, which makes the glycoprotein susceptible to activation by ubiquitous proteases such as furin and PC6. However, the mechanism of pathogenicity may differ in highly pathogenic strains of human influenza, such as the H1N1 pandemic strain of 1918 and the H5N1 strain involved in the outbreak in Hong Kong in 1997. Binding of host proteases by the viral neuraminidase to assist activation of the haemagglutinin, shortening of the neuraminidase and substitutions in the polymerase gene, PB2, have all been suggested as alternative molecular correlates of pathogenicity of human influenza viruses. Additionally, systemic spread in humans of pathogenic subtypes has not been demonstrated and host factors such as interferons may be crucial in preventing the spread of the virus outside the respiratory tract.

Antibodies to the major linear neutralizing domains of cytomegalovirus glycoprotein B among natural seropositives and CMV subunit vaccine recipients

The gB protein (gpUL55) of human cytomegalovirus (CMV) contains C-terminal (AD-1) and N-terminal (AD-2) linear immunodominant neutralizing domains. To measure antibodies to these epitopes, a modified protein (delta-gB) lacking heavily glycosylated intervening regions, the transmembrane domain, and the cytoplasmic domain, was expressed in recombinant baculovirus-infected cells. Eighty-six percent of 600 naturally CMV-seropositive individuals and 93% of 121 gB vaccine recipients had antibodies to delta-gB as detected by enzyme-linked immunosorbent assay (ELISA). The antibody level in vaccinees (median optical density [OD] = 1.73) exceeded that in natural seropositives (median OD = 0.94; p < .0001). Eleven percent of 95 natural seropositives and 7% of 120 gB vaccinees lacked A-gB antibodies but had neutralizing activity. Among subjects with delta-gB antibody, there were weak correlations between antibody level and neutralizing titer. These data suggest that antibodies to linear neutralizing gB domains are highly prevalent in naturally-infected individuals and regularly develop in gB vaccinees. However, for some individuals, discontinuous and/or linear epitopes not represented on delta-gB may be more important in the generation of neutralizing responses.

Glycoprotein B of human herpesvirus 8 is a component of the virion in a cleaved form composed of amino- and carboxyl-terminal fragments

Human herpesvirus 8 (HHV-8) or Kaposi's sarcoma-associated herpesvirus (KSHV) is the only known human member of the Rhadinovirus genus of the gammaherpesvirus subfamily. Antibodies against peptides representing portions of the amino- and carboxyl-termini of HHV-8 gB were produced and used to detect gB expression in Vero cells transfected with the gB gene, in the HHV-8-harboring cell line, BCBL-1, and in purified virions. Expression of gB was detected in approximately 3% of uninduced BCBL-1 cells, while up to 30% of the cells expressed gB after 12-O-tetradecanoylphorbol-13-acetate (TPA) induction of virus replication. Indirect immunofluorescence assays and confocal microscopy showed that gB was distributed throughout the cytoplasm of BCBL-1 cells and transfected Vero cells. Immunoblot analyses of virion preparations revealed the presence of full-length as well as two smaller than full-length gB-derived species corresponding to the amino- and carboxy-terminal portions of gB, respectively. Biochemical analysis of the gB carbohydrate moieties using glycosylation inhibitors revealed that gB contained N-linked oligosaccharides of the high-mannose type, characteristic of precursor carbohydrate chains added in the endoplasmic reticulum.

Identification of a novel signal sequence that targets transmembrane proteins to the nuclear envelope inner membrane

Herpesvirus maturation requires translocation of glycoprotein B homologue from the endoplasmic reticulum to the inner nuclear membrane. Glycoprotein B of human cytomegalovirus was used in this context as a model protein. To identify a specific signal sequence within human cytomegalovirus glycoprotein B acting in a modular fashion, coding sequences were recombined with reporter proteins. Immunofluorescence and cell fractionation demonstrated that a short sequence element within the cytoplasmic tail of human cytomegalovirus glycoprotein B was sufficient to translocate the membrane protein CD8 to the inner nuclear membrane. This carboxyl-terminal sequence had no detectable nuclear localization signal activity for soluble beta-Galactosidase and could not be substituted by the nuclear localization signal of SV40 T antigen. For glycoprotein B of herpes simplex virus, a carboxyl-terminal element with comparable properties was found. Further experiments showed that the amino acid sequence DRLRHR of human cytomegalovirus glycoprotein B (amino acids 885-890) was sufficient for nuclear envelope translocation. Single residue mutations revealed that the arginine residues in positions 4 and 6 of the DRLRHR sequence were essential for its function. These results support the view that transmembrane protein transport to the inner nuclear membrane is controlled by a mechanism different from that of soluble proteins.

Characterization of a panel of insertion mutants in human cytomegalovirus glycoprotein B

Glycoprotein B (gB; gpUL55) of human cytomegalovirus (HCMV) plays a critical role in virus entry and cell-to-cell spread of infection. To define the structure-function relationships in gB, a panel of linker-insertion mutations was generated throughout the coding region. This strategy yielded a panel of 22 mutants with four amino acid insertions and 3 large truncation mutants. Assessment of the mutant proteins' biosynthetic properties and folding patterns analyzed in context with predicted secondary features revealed novel insights into gB's structure and trafficking properties. All of the insertion mutants were able to assemble into oligomers, suggesting that oligomerization is tolerant of small insertions and/or that multiple regions of the protein may be involved. Computer algorithm predictions of gB's secondary structure indicate that the furin-recognized cleavage site falls within an exposed loop. This loop may be particularly sensitive to structural alterations, since insertions upstream and downstream of the cleavage site rendered the mutant proteins cleavage defective. In addition, a strong correlation existed between terminal folding and cleavage of gB. Interestingly, terminal folding was not correlated with delivery to the cell surface but may influence the rate of transport to the cell surface. Nine mutants, containing insertions in both the extracellular and intracellular portions of gB, retained wild-type structural properties. This panel of characterized gB mutants, the first of this type for an HCMV protein, will be a useful tool in dissecting the role of gB during HCMV infection.