Characterization of human herpesvirus-6(U1102) and (GS) gp112 and identification of the Z29-specified homolog

Nectin-2 Acts as a Viral Entry Mediated Molecule That Binds to Human Herpesvirus 6B Glycoprotein B

Human herpesvirus 6B (HHV-6B) is a T-lymphotropic virus and the etiological agent of exanthem subitum. HHV-6B is present in a latent or persistent form after primary infection and is produced in the salivary glands or transmitted to this organ. Infected individuals continue to secrete the virus in their saliva, which is thus considered a source for virus transmission. HHV-6B primarily propagates in T cells because its entry receptor, CD134, is mainly expressed by activated T cells. The virus then spreads to the host’s organs, including the salivary glands, nervous system, and liver. However, CD134 expression is not detected in these organs. Therefore, HHV-6B may be entering cells via a currently unidentified cell surface molecule, but the mechanisms for this have not yet been investigated. In this study, we investigated a CD134-independent virus entry mechanism in the parotid-derived cell line HSY. First, we confirmed viral infection in CD134-membrane unanchored HSY cells. We then determined that nectin cell adhesion molecule 2 (nectin-2) mediated virus entry and that HHV-6B-insensitive T-cells transduced with nectin-2 were transformed into virus-permissive cells. We also found that virus entry was significantly reduced in nectin-2 knockout parotid-derived cells. Furthermore, we showed that HHV-6B glycoprotein B (gB) interacted with the nectin-2 V-set domain. The results suggest that nectin-2 acts as an HHV-6B entry-mediated protein.

Comparative Analysis of Roseoloviruses in Humans, Pigs, Mice, and Other Species

Viruses of the genus Roseolovirus belong to the subfamily Betaherpesvirinae, family Herpesviridae. Roseoloviruses have been studied in humans, mice and pigs, but they are likely also present in other species. This is the first comparative analysis of roseoloviruses in humans and animals. The human roseoloviruses human herpesvirus 6A (HHV-6A), 6B (HHV-6B), and 7 (HHV-7) are relatively well characterized. In contrast, little is known about the murine roseolovirus (MRV), also known as murine thymic virus (MTV) or murine thymic lymphotrophic virus (MTLV), and the porcine roseolovirus (PRV), initially incorrectly named porcine cytomegalovirus (PCMV). Human roseoloviruses have gained attention because they can cause severe diseases including encephalitis in immunocompromised transplant and AIDS patients and febrile seizures in infants. They have been linked to a number of neurological diseases in the immunocompetent including multiple sclerosis (MS) and Alzheimer's. However, to prove the causality in the latter disease associations is challenging due to the high prevalence of these viruses in the human population. PCMV/PRV has attracted attention because it may be transmitted and pose a risk in xenotransplantation, e.g., the transplantation of pig organs into humans. Most importantly, all roseoloviruses are immunosuppressive, the humoral and cellular immune responses against these viruses are not well studied and vaccines as well as effective antivirals are not available.

Viral glycoproteomes: technologies for characterization and outlook for vaccine design

It has long been known that surface proteins of most enveloped viruses are covered with glycans. It has furthermore been demonstrated that glycosylation is essential for propagation and immune evasion for many viruses. The recent development of high-resolution mass spectrometry techniques has enabled identification not only of the precise structures but also the positions of such post-translational modifications on viruses, revealing substantial differences in extent of glycosylation and glycan maturation for different classes of viruses. In-depth characterization of glycosylation and other post-translational modifications of viral envelope glycoproteins is essential for rational design of vaccines and antivirals. In this Review, we provide an overview of techniques used to address viral glycosylation and summarize information on glycosylation of enveloped viruses representing ongoing public health challenges. Furthermore, we discuss how knowledge on glycosylation can be translated to means to prevent and combat viral infections.

Global aspects of viral glycosylation

Enveloped viruses encompass some of the most common human pathogens causing infections of different severity, ranging from no or very few symptoms to lethal disease as seen with the viral hemorrhagic fevers. All enveloped viruses possess an envelope membrane derived from the host cell, modified with often heavily glycosylated virally encoded glycoproteins important for infectivity, viral particle formation and immune evasion. While N-linked glycosylation of viral envelope proteins is well characterized with respect to location, structure and site occupancy, information on mucin-type O-glycosylation of these proteins is less comprehensive. Studies on viral glycosylation are often limited to analysis of recombinant proteins that in most cases are produced in cell lines with a glycosylation capacity different from the capacity of the host cells. The glycosylation pattern of the produced recombinant glycoproteins might therefore be different from the pattern on native viral proteins. In this review, we provide a historical perspective on analysis of viral glycosylation, and summarize known roles of glycans in the biology of enveloped human viruses. In addition, we describe how to overcome the analytical limitations by using a global approach based on mass spectrometry to identify viral O-glycosylation in virus-infected cell lysates using the complex enveloped virus herpes simplex virus type 1 as a model. We underscore that glycans often pay important contributions to overall protein structure, function and immune recognition, and that glycans represent a crucial determinant for vaccine design. High throughput analysis of glycosylation on relevant glycoprotein formulations, as well as data compilation and sharing is therefore important to identify consensus glycosylation patterns for translational applications.

Evaluation of a method to measure HHV-6B infection in vitro based on cell size

BACKGROUND

Human herpesvirus 6 (HHV-6A and HHV-6B) infection of cell cultures can be measured by different methods, including immunofluorescence microscopy, flow cytometry, or quantification of virus DNA by qPCR. These methods are reliable and sensitive but require long processing times and can be costly. Another method used in the field relies on the identification of enlarged cells in the culture; this method requires little sample processing and is relatively fast. However, visual inspection of cell cultures can be subjective and it can be difficult to establish clear criteria to decide if a cell is enlarged. To overcome these issues, we explored a method to monitor HHV-6B infections based on the systematic and objective measurement of the size of cells using an imaging-based automated cell counter.

RESULTS

The size of cells in non-infected and HHV-6B-infected cultures was measured at different times post-infection. The relatively narrow size distribution observed for non-infected cultures contrasted with the broader distributions observed in infected cultures. The average size of cultures shifted towards higher values after infection, and the differences were significant for cultures infected with relatively high doses of virus and/or screened at longer times post-infection. Correlation analysis showed that the trend observed for average size was similar to the trend observed for two other methods to measure infection: amount of virus DNA in supernatant and the percentage of cells expressing a viral antigen. In order to determine the performance of the size-based method in differentiating non-infected and infected cells, receiver operating characteristic (ROC) curves were used to analyze the data. Analysis using size of individual cells showed a moderate performance in detecting infected cells (area under the curve (AUC) ~ 0.80-0.87), while analysis using the average size of cells showed a very good performance in detecting infected cultures (AUC ~ 0.99).

CONCLUSIONS

The size-based method proved to be useful in monitoring HHV-6B infections for cultures where a substantial fraction of cells were infected and when monitored at longer times post-infection, with the advantage of being relatively fast and easy. It is a convenient method for monitoring virus production in-vitro and bulk infection of cells.

Glycoproteins of HHV-6A and HHV-6B

Recently, human herpesvirus 6A and 6B (HHV-6A and HHV-6B) were classified into distinct species. Although these two viruses share many similarities, cell tropism is one of their striking differences, which is partially because of the difference in their entry machinery. Many glycoproteins of HHV-6A/B have been identified and analyzed in detail, especially in their functions during entry process into host cells. Some of these glycoproteins were unique to HHV-6A/B. The cellular factors associated with these viral glycoproteins (or glycoprotein complex) were also identified in recent years. Detailed interaction analyses were also conducted, which could partially prove the difference of entry machinery in these two viruses. Although there are still issues that should be addressed, all the knowledges that have been earned in recent years could not only help us to understand these viruses' entry mechanism well but also would contribute to the development of the therapy and/or prophylaxis methods for HHV-6A/B-associated diseases.

Immune response to HHV-6 and implications for immunotherapy

Most adults remain chronically infected with HHV-6 after resolution of a primary infection in childhood, with the latent virus held in check by the immune system. Iatrogenic immunosuppression following solid organ transplantation (SOT) or hematopoetic stem cell transplantation (HSCT) can allow latent viruses to reactivate. HHV-6 reactivation has been associated with increased morbidity, graft rejection, and neurological complications post-transplantation. Recent work has identified HHV-6 antigens that are targeted by the CD4+ and CD8+ T cell response in chronically infected adults. T cell populations recognizing these targets can be expanded in vitro and are being developed for use in autologous immunotherapy to control post-transplantation HHV-6 reaction.

Herpesvirus 6 glycoproteins B (gB), gH, gL, and gQ are necessary and sufficient for cell-to-cell fusion

The human herpesvirus 6 (HHV-6) envelope glycoprotein gH/gL/gQ1/gQ2 complex associates with host cell CD46 as its cellular receptor. Although gB has been suggested to be involved in HHV-6 infection, its function in membrane fusion has remained unclear. Here, we have developed an HHV-6A (strain GS)and HHV-6B (strain Z29) virus-free cell-to-cell fusion assay and demonstrate that gB and the gH/gL/gQ1/gQ2 complex are the minimum components required for membrane fusion by HHV-6.

Generation and characterization of a monoclonal antibody specific for human herpesvirus 6 variant A immediate-early 2 protein

Two variants (A and B) of human herpesvirus 6 (HHV-6) can be isolated from humans, with each variant having unique biological properties. HHV-6 variant typing is mainly done following amplification of viral genomic DNA followed by restriction endonuclease digestion. Our objective was to generate a monoclonal antibody (mAb) that would allow us to discriminate between variants A and B of HHV-6. BALB/c mice were immunized with a recombinant glutathione-S-transferase protein fused to the immediate-early (IE) 2 protein from HHV-6 variant A. Following splenocytes fusion, one IgG1 kappa light chain mAb (P6H8) was isolated and found to react specifically with variant A IE2 protein in immunofluorescence and western blot assays. The P6H8 antibody represents a useful tool for both fundamental research and clinical applications allowing for the discrimination of infections caused by HHV-6 variants A or B.

Trafficking to the plasma membrane of the seven-transmembrane protein encoded by human herpesvirus 6 U51 gene involves a cell-specific function present in T lymphocytes

The sequence of human herpesvirus 6 (HHV-6) U51 open reading frame predicts a protein of 301 amino acid residues with seven transmembrane domains. To identify and characterize U51, we derived antipeptide polyclonal antibodies and developed a transient expression assay. We ascertained that U51 was synthesized in cord blood mononuclear cells infected with either variant A- or variant B-HHV-6 and was transported to the surface of productively infected cells. When synthesized in transient expression systems, U51 intracellular trafficking was regulated in a cell-type-dependent fashion. In human monolayer HEK-293 and 143tk- cells, U51 accumulated predominantly in the endoplasmic reticulum and failed to be transported to the cell surface. In contrast, in T-lymphocytic cell lines J-Jhan, Molt-3, and Jurkat, U51 was successfully transported to the plasma membrane. We infer that transport of U51 to the cell surface requires a cell-specific function present in activated T lymphocytes and T-cell lines.

Human herpesvirus 6: An emerging pathogen

Infections with human herpesvirus 6 (HHV-6), a beta-herpesvirus of which two variant groups (A and B) are recognized, is very common, approaching 100% in seroprevalence. Primary infection with HHV-6B causes roseola infantum or exanthem subitum, a common childhood disease that resolves spontaneously. After primary infection, the virus replicates in the salivary glands and is shed in saliva, the recognized route of transmission for variant B strains; it remains latent in lymphocytes and monocytes and persists at low levels in cells and tissues. Not usually associated with disease in the immunocompetent, HHV-6 infection is a major cause of opportunistic viral infections in the immunosuppressed, typically AIDS patients and transplant recipients, in whom HHV-6 infection/reactivation may culminate in rejection of transplanted organs and death. Other opportunistic viruses, human cytomegalovirus and HHV-7, also infect or reactivate in persons at risk. Another disease whose pathogenesis may be correlated with HHV-6 is multiple sclerosis. Data in favor of and against the correlation are discussed.

Viral glycoproteins accumulate in newly formed annulate lamellae following infection of lymphoid cells by human herpesvirus 6

Ultrastructural analysis of HSB-2 T-lymphoid cells and human cord blood mononuclear cells infected with human herpesvirus 6 revealed the presence, in the cell cytoplasm, of annulate lamellae (AL), which were absent in uninfected cells. Time course analysis of the appearance of AL following viral infection showed that no AL were visible within the first 72 h postinfection and that their formation correlated with the expression of the late viral glycoprotein gp116. The requirement of active viral replication for AL neoformation was further confirmed by experiments using inactivated virus or performed in presence of the viral DNA polymerase inhibitor phosphonoacetic acid. Both conventional electron microscopic examination and immunogold fracture labeling with anti-endoplasmic reticulum antibodies indicated a close relationship of AL with the endoplasmic reticulum and nuclear membranes. However, when the freeze-fractured cells were immunogold labeled with an anti-gp116 monoclonal antibody, AL membranes were densely labeled, whereas nuclear membranes and endoplasmic reticulum cisternae appeared virtually unlabeled, showing that viral envelope glycoproteins selectively accumulate in AL. In addition, gold labeling with Helix pomatia lectin and wheat germ agglutinin indicated that AL cisternae, similar to cis-Golgi membranes, contain intermediate, but not terminal, forms of glycoconjugates. Taken together, these results suggest that in this cell-virus system, AL function as a viral glycoprotein storage compartment and as a putative site of O-glycosylation.

Selection and characterization of two specific monoclonal antibodies directed against the two variants of human herpesvirus-6

Monoclonal antibodies (mAbs) specific for human herpesvirus-6 (HHV6) proteins were derived from the splenocytes of mice immunized with HHV6 TAN isolate-infected peripheral blood mononuclear cells. The two mAbs 8C8 and 7C7 reacted by means of immunofluorescence and immunoperoxidase assays with both variant A and variant B isolates giving two different staining patterns. In infected cells, cytoplasmic diffuse staining was observed with mAb 8C8, whereas intense nuclear staining was obtained with mAb 7C7. These different locations of viral target proteins were confirmed by confocal microscopy. The mAb 8C8 reacted with a family of six glycoproteins designated as the gp72 complex in the case of variant A strains and gp63 complex in the case of variant B strains. The endoglycosidases H and F reduced those glycoproteins to a putative precursor molecule of 58 kDa. The mAb 7C7 reacted with 116 and 109 kDa proteins with the two HHV6 variants. These two mAbs did not neutralize virion infectivity in the absence of complement. No cross-reactivity was observed when these mAbs were used in immunoperoxidase assay and immunoblotting against the proteins of human cytomegalovirus or other human herpesviruses. Thus, the two mAbs 8C8 and 7C7 may be valuable tools for the diagnosis and biological investigation of HHV6 infections.

Identification and analyses of glycoprotein B of human herpesvirus 7

The gene for the human herpes virus 7 (HHV-7) glycoprotein B (gB) has been identified by sequencing a molecularly cloned HHV-7 DNA fragment. A 2.5-kb open reading frame (ORF) encoded a protein of 822 amino acids with characteristics of a transmembrane glycoprotein, and showed the strongest similarity (56.5%) with the human herpesvirus 6 (HHV-6) gB. The genes for the transport/capsid assembly protein (tp/cap) and the DNA polymerase (pol) existed upstream and downstream of the gB gene, respectively. This arrangement was the same as that of HHV-6. Antisera were generated by immunizing mice with a glutathione S-transferase-carboxy terminal gB fusion protein. Immunofluorescent tests demonstrated that the antisera reacted specifically with HHV-7 antigens in cytoplasm of infected cells. The antisera immunoprecipitated proteins with apparent molecular masses of 51, 63 and 112 kDa from HHV-7 infected cells by pulse-chase analysis. In the presence of tunicamycin, the protein with a molecular mass of 112 kDa was replaced by a protein with a molecular mass of 88 kDa, and this size was consistent with the predicted size of the primary translation product of the HHV-7 gB gene. These results suggested that the protein with a molecular mass of 112 kDa was a glycoprotein synthesized by addition of N-linked oligosaccharides to a non-glycosylated precursor of the protein with a molecular mass of 88 kDa and then cleaved into the proteins with molecular masses of 51 and 63 kDa in HHV-7 infected cells.

Infection of human T lymphoid cells by human herpesvirus 6 is blocked by two unrelated protein tyrosine kinase inhibitors, biochanin A and herbimycin

Human herpesvirus 6 is a T lymphotropic herpesvirus that causes exanthem subitum in infants and is considered a potential cofactor in AIDS etiopathogenesis and progression owing to its in vivo and in vitro interactions with human immunodeficiency virus. We report that no differences in phosphorylation on tyrosine residues of cellular proteins were detectable at early times following HHV-6 infection in comparison to uninfected cells. On the contrary, several cellular proteins appeared phosphorylated on tyrosine at 24-48 hr postinfection. In addition, when tyrosine phosphorylation induced by HHV-6 infection was inhibited by the tyrosine kinase inhibitor biochanin A, the infection of HSB-2 cells was also coordinately reduced, as judged by inhibition of cytopathic effect and by inhibition of early and late viral antigen expression. Similar results were obtained with a second unrelated tyrosine kinase inhibitor, herbimycin. The inhibitors seem to act at a late stage of the viral infectious cycle, since neither viral binding nor internalization were affected. Thus, our results indicate that HHV-6 infection leads to the phosphorylation of protein tyrosine kinases, which may play a role in the course of viral infection, probably by participating in the cytopathic effect induced by the virus.

Selection of a monoclonal antibody specific for variant B human herpesvirus 6-infected mononuclear cells

A monoclonal antibody, designated as MAb 6E2, specific for human herpesvirus 6 variant B (HHV-6B) was derived from the spleen of a mouse immunized with lysates of HHV-6B(Z29) cord blood mononuclear cells. MAb 6E2 reacts by immunofluorescence with all the HIV-6B strains tested (Z29, CV, Hashimoto and SF) and fails to react with variant A prototypes, GS and U1102. The immunofluorescence staining was punctate and localized to the cytoplasm. The protein reacting with MAb 6E2 was identified as protein 48,000 in apparent M(r) value by immunoaffinity chromatography of lysates of HHV-6B-infected mononuclear cells.

Human herpesvirus 6 envelope glycoproteins B and H-L complex are undetectable on the plasma membrane of infected lymphocytes

Membrane immunofluorescence analysis of cells infected with either variant (A or B) of human herpesvirus 6 revealed a typical punctate staining, after labeling with several HHV-6-positive human sera or with two monoclonal antibodies directed to gB and gH. Immunoprecipitation studies showed a sharp difference in glycoprotein content in whole-cell extracts versus on the cell surface, suggesting the occurrence of gB in the extracellular virions juxtaposed to plasma membranes. By immunoelectron microscopy, the extracellular virions still attached to the cell surface appeared consistently and specifically labeled, whereas the plasma membrane was always unlabeled, independent of viral variant, antibody, or target cell used. These findings may reflect an atypical maturation pathway of HHV-6, and could have important implications in the control of cellular immune response to HHV-6-infected lymphocytes.

HHV-6 infection in Italy: characterization of an endemic isolate and seroepidemiologic analysis

A biologic, immunologic and molecular characterization of an HHV-6 isolate (BA92) rescued by the peripheral blood mononuclear cells of a child affected by Exanthem subitum is reported. The comparison with the known HHV-6 prototype strains showed that BA92 is indistinguishable from the Z29 isolate, and can be included in the variant B group of HHV-6. A seroepidemiologic analysis of the antibody response to BA92 of normal individuals as well as patients affected by diseases potentially associated to HHV-6 infection has shown an overall seroprevalence of 81%, and that no variations in seroprevalence or in antibody geometric mean titer are observed assaying the sera also against G.S., U1102, or Z29 infected cells, respectively. These findings indicate: (1) HHV-6 infection is widely diffuse in Italy; (2) it is not possible to discriminate between the viral variants by the currently available IF assays, and (3) no conclusions can be drawn on the potential association of HHV-6 with any of the diseases examined.