Definition of a human herpesvirus-6 betaherpesvirus-specific domain in glycoprotein gH that governs interaction with glycoprotein gL: substitution of human cytomegalovirus glycoproteins permits group-specific complex formation

The Contribution of Human Herpes Viruses to γδ T Cell Mobilisation in Co-Infections

γδ T cells are activated in viral, bacterial and parasitic infections. Among viruses that promote γδ T cell mobilisation in humans, herpes viruses (HHVs) occupy a particular place since they infect the majority of the human population and persist indefinitely in the organism in a latent state. Thus, other infections should, in most instances, be considered co-infections, and the reactivation of HHV is a serious confounding factor in attributing γδ T cell alterations to a particular pathogen in human diseases. We review here the literature data on γδ T cell mobilisation in HHV infections and co-infections, and discuss the possible contribution of HHVs to γδ alterations observed in various infectious settings. As multiple infections seemingly mobilise overlapping γδ subsets, we also address the concept of possible cross-protection.

Analyses of Tissue Culture Adaptation of Human Herpesvirus-6A by Whole Genome Deep Sequencing Redefines the Reference Sequence and Identifies Virus Entry Complex Changes

Tissue-culture adaptation of viruses can modulate infection. Laboratory passage and bacterial artificial chromosome (BAC)mid cloning of human cytomegalovirus, HCMV, resulted in genomic deletions and rearrangements altering genes encoding the virus entry complex, which affected cellular tropism, virulence, and vaccine development. Here, we analyse these effects on the reference genome for related betaherpesviruses, Roseolovirus, human herpesvirus 6A (HHV-6A) strain U1102. This virus is also naturally “cloned” by germline subtelomeric chromosomal-integration in approximately 1% of human populations, and accurate references are key to understanding pathological relationships between exogenous and endogenous virus. Using whole genome next-generation deep-sequencing Illumina-based methods, we compared the original isolate to tissue-culture passaged and the BACmid-cloned virus. This re-defined the reference genome showing 32 corrections and 5 polymorphisms. Furthermore, minor variant analyses of passaged and BACmid virus identified emerging populations of a further 32 single nucleotide polymorphisms (SNPs) in 10 loci, half non-synonymous indicating cell-culture selection. Analyses of the BAC-virus genome showed deletion of the BAC cassette via loxP recombination removing green fluorescent protein (GFP)-based selection. As shown for HCMV culture effects, select HHV-6A SNPs mapped to genes encoding mediators of virus cellular entry, including virus envelope glycoprotein genes gB and the gH/gL complex. Comparative models suggest stabilisation of the post-fusion conformation. These SNPs are essential to consider in vaccine-design, antimicrobial-resistance, and pathogenesis.

Accessory proteins 8b and 8ab of severe acute respiratory syndrome coronavirus suppress the interferon signaling pathway by mediating ubiquitin-dependent rapid degradation of interferon regulatory factor 3

Severe acute respiratory syndrome coronavirus (SARS-CoV) is an inefficient inducer of interferon (IFN) response. It expresses various proteins that effectively circumvent IFN production at different levels via distinct mechanisms. Through the construction of recombinant IBV expressing proteins 8a, 8b and 8ab encoded by SARS-CoV ORF8, we demonstrate that expression of 8b and 8ab enables the corresponding recombinant viruses to partially overcome the inhibitory actions of IFN activation to achieve higher replication efficiencies in cells. We also found that proteins 8b and 8ab could physically interact with IRF3. Overexpression of 8b and 8ab resulted in the reduction of poly (I:C)-induced IRF3 dimerization and inhibition of the IFN-β signaling pathway. This counteracting effect was partially mediated by protein 8b/8ab-induced degradation of IRF3 in a ubiquitin-proteasome-dependent manner. Taken together, we propose that SARS-CoV may exploit the unique functions of proteins 8b and 8ab as novel mechanisms to overcome the effect of IFN response during virus infection.

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Recombinant IBV expressing SARS-CoV protein 8b or 8ab replicates better than wild type in cells pre-treated with poly(I:C).

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8b interacts with the IAD domain of IRF3.

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Overexpression of 8b or 8ab reduces poly(I:C)-induced IRF3 dimerization and interferon induction.

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8b and 8ab induce degradation of IRF3 in a ubiquitin-proteasome-dependent manner.

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8b and 8ab suppress interferon response induced by constitutively active IRF3.

Complementation of the function of glycoprotein H of human herpesvirus 6 variant A by glycoprotein H of variant B in the virus life cycle

Human herpesvirus 6 (HHV-6) is a T-cell-tropic betaherpesvirus. HHV-6 can be classified into two variants, HHV-6 variant A (HHV-6A) and HHV-6B, based on genetic, antigenic, and cell tropisms, although the homology of their entire genomic sequences is nearly 90%. The HHV-6A glycoprotein complex gH/gL/gQ1/gQ2 is a viral ligand that binds to the cellular receptor human CD46. Because gH has 94.3% amino acid identity between the variants, here we examined whether gH from one variant could complement its loss in the other. Recently, we successfully reconstituted HHV-6A from its cloned genome in a bacterial artificial chromosome (BAC) (rHHV-6ABAC). Using this system, we constructed HHV-6ABAC DNA containing the HHV-6B gH (BgH) gene instead of the HHV-6A gH (AgH) gene in Escherichia coli. Recombinant HHV-6ABAC expressing BgH (rHHV-6ABAC-BgH) was successfully reconstituted. In addition, a monoclonal antibody that blocks HHV-6B but not HHV-6A infection neutralized rHHV-6ABAC-BgH but not rHHV-6ABAC. These results indicate that HHV-6B gH can complement the function of HHV-6A gH in the viral infectious cycle.

Human herpesvirus 6 glycoprotein complex formation is required for folding and trafficking of the gH/gL/gQ1/gQ2 complex and its cellular receptor binding

Human herpesvirus 6 (HHV-6) is a T-cell-tropic betaherpesvirus. A glycoprotein (g) complex that is unique to HHV-6, gH/gL/gQ1/gQ2, is a viral ligand for its cellular receptor, human CD46. However, whether complex formation or one component of the complex is required for CD46 binding and how the complex is transported in cells are open questions. Furthermore, in HHV-6-infected cells the gQ1 protein modified with N-linked glycans is expressed in two forms with different molecular masses: an 80-kDa form (gQ1-80K) and a 74-kDa form (gQ1-74K). Only gQ1-80K, but not gQ1-74K, forms the complex with gQ2, gH, and gL, and this four-component complex is incorporated into mature virions. Here, we characterized the molecular context leading to the maturation of gQ1 by expressing combinations of the individual gH/gL/gQ1/gQ2 components in 293T cells. Surprisingly, only when all four molecules were expressed was a substantial amount of gQ1-80K detected, indicating that all three of the other molecules (gQ2, gH, and gL) were necessary and sufficient for gQ1 maturation. We also found that only the tetrameric complex, and not its subsets, binds to CD46. Finally, a gQ2-null virus constructed in the BAC (bacterial artificial chromosome) system could not be reconstituted, indicating that gQ2 is essential for virus growth. These results show that gH, gL, gQ1, and gQ2 are all essential for the trafficking and proper folding of the gH/gL/gQ1/gQ2 complex and, thus, for HHV-6 infection.

Human herpesvirus-6 entry into host cells

Human herpesvirus-6 (HHV-6) belongs to the herpesvirus family and is categorized into variant A and B (HHV-6A and HHV-6B). Primary HHV-6 infection in children and its related diseases are almost exclusively caused by HHV-6B and no disease caused by HHV-6A has been identified. The cellular receptor of HHV-6 has been shown to be a human CD46, and its viral ligand is an envelope glycoprotein complex, gH/gL/gQ1/gQ2 in HHV-6A. Furthermore, both cellular and viral lipid rafts play an important role in the HHV-6 entry process, suggesting that HHV-6 may enter its target cells through a lipid raft-associated mechanism.

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.

Epitope mapping of herpes simplex virus type 2 gH/gL defines distinct antigenic sites, including some associated with biological function

The gH/gL complex plays an essential role in virus entry and cell-cell spread of herpes simplex virus (HSV). Very few immunologic reagents were previously available to either identify important functional regions or gain information about structural features of this complex. Therefore, we generated and characterized a panel of 31 monoclonal antibodies (MAbs) against HSV type 2 (HSV-2) gH/gL. Fourteen MAbs bound to a conformation-dependent epitope of the gH2/gL2 complex, and all blocked virus spread. The other 17 MAbs recognized linear epitopes of gH (12) or gL (5). Interestingly, two of the gL MAbs and six of the gH MAbs were type common. Overlapping synthetic peptides were used to map MAbs against linear epitopes. These data, along with results of competition analyses and functional assays, assigned the MAbs to groups representing eight distinct antigenic sites on gH (I to VIII) and three sites on gL (A, B, and C). Of most importance, the MAbs with biological activity mapped either to site I of gH2 (amino acids 19 to 38) or to sites B and C of gL2 (residues 191 to 210). Thus, these MAbs constitute a novel set of reagents, including the first such reagents against gH2 and gL2 as well as some that recognize both serotypes of each protein. Several recognize important functional domains of gH2, gL2, or the complex. We suggest a common grouping scheme for all of the known MAbs against gH/gL of both HSV-1 and HSV-2.

Contribution of cysteine residues to the structure and function of herpes simplex virus gH/gL

In HSV types 1 and 2, gH forms a noncovalent heterodimer with gL. Previous studies demonstrated that the first 323 amino acids of gH1 and the first 161 amino acids of gL1 are sufficient for gH/gL binding. For gL1, substitution of any of its four cysteine (C) residues (all located within the gH/gL binding region) destroyed gH binding and function. Although gH1 contains 8 cysteines in its ectodomain, gH 2 contains 7 (C3 of gH1 is replaced by arginine in gH2). We found that mutation of any of the four C-terminal cysteines led to a reduction or loss of gH/gL function. Mutation of C5 or C6 in gH1 or gH2 rendered the proteins non-functional. However, substitution of C7 and/or C8 in gH1 has a definite negative impact on cell-cell fusion, although these mutations had less effect on complementation. Remarkably, all four gH1 N-terminal cysteines could be mutated simultaneously with little effect on fusion or complementation. As gH2 already lacks C3, we constructed a triple mutant (gH2-C1/2/4) which exhibited a similar phenotype. Since gH1 is known to bind gL2 and vice versa, we wondered whether binding of gH2 to the heterologous gL1 would enhance the fusion defect seen with the gH2-C2 mutant. The combination of mutant gH2-C2 with wild-type gL1 was nonfunctional in a cell-cell fusion assay. Interestingly, the reciprocal was not true, as gH1-C2 could utilize both gL1 and gL2. These findings suggest that there is a structural difference in the gH2 N-terminus as compared to gH1. We also present genetic evidence for at least one disulfide bond within gH2, between cysteines 2 and 4.

Interactions between viruses in transplant recipients

Viral coinfections may modulate disease expression, enhance pathogenicity, and lead to greater cumulative immunosuppression in the host. The pathophysiological basis of these may be direct virus-virus interactions, effect of cohabitating viruses on host cell function, or impaired host immune responses. The interrelationship between viral pathogens has become increasingly more relevant and its scope wider as new or previously unrecognized viruses continue to emerge as pathogens in transplant recipients. The pathways and mediators that modulate biological activity represent potential targets for immunomodulatory interventions as adjunctive therapies for transplant recipients.

Intracellular processing of human herpesvirus 6 glycoproteins Q1 and Q2 into tetrameric complexes expressed on the viral envelope

Human herpesvirus 6 (HHV-6) glycoproteins H and L (gH and gL, respectively) and the 80-kDa form of glycoprotein Q (gQ-80K) form a heterotrimeric complex that is found on the viral envelope and that is a viral ligand for human CD46. Besides gQ-80K, the gQ gene encodes an additional product whose mature molecular mass is 37 kDa (gQ-37K) and which is derived from a different transcript. Therefore, we designated gQ-80K as gQ1 and gQ-37K as gQ2. We show here that gQ2 also interacts with the gH-gL-gQ1 complex in HHV-6-infected cells and in virions. To examine how these components interact in HHV-6-infected cells, we performed pulse-chase studies. The results demonstrated that gQ2-34K, which is endo-beta-N-acetylglucosaminidase H sensitive and which is the precursor form of gQ2-37K, associates with gQ1-74K, which is the precursor form of gQ1-80K, within 30 min of the pulse period. After a 1-h chase, these precursor forms had associated with the gH-gL dimer. Interestingly, an anti-gH monoclonal antibody coimmunoprecipitated mainly gQ1-80K and gQ2-37K, with little gQ1-74K or gQ2-34K. These results indicate that although gQ2-34K and gQ1-74K interact in the endoplasmic reticulum, the gH-gL-gQ1-80K-gQ2-37K heterotetrameric complex arises in the post-endoplasmic reticulum compartment. The mature complex is subsequently incorporated into viral particles.

A functional YNKI motif in the short cytoplasmic tail of varicella-zoster virus glycoprotein gH mediates clathrin-dependent and antibody-independent endocytosis

The trafficking of varicella-zoster virus (VZV) gH was investigated under both infection and transfection conditions. In initial endocytosis assays performed in infected cells, the three glycoproteins gE, gI, and gB served as positive controls for internalization from the plasma membrane. Subsequently, we discovered that gH in VZV-infected cells was also internalized and followed a similar trafficking pattern. This observation was unexpected because all herpesvirus gH homologues have short endodomains not known to contain trafficking motifs. Further investigation demonstrated that VZV gH, when expressed alone with its chaperone gL, was capable of endocytosis in a clathrin-dependent manner, independent of gE, gI, or gB. Upon inspection of the short gH cytoplasmic tail, we discovered a putative tyrosine-based endocytosis motif (YNKI). When the tyrosine was replaced with an alanine, endocytosis of gH was blocked. Utilizing an endocytosis assay dependent on biotin labeling, we further documented that endocytosis of VZV gH was antibody independent. In control experiments, we showed that gE, gI, and gB also internalized in an antibody-independent manner. Alignment analysis of the VZV gH cytoplasmic tail to other herpesvirus gH homologues revealed two important findings: (i) herpes simplex virus type 1 and 2 homologues lacked an endocytosis motif, while all other alphaherpesvirus gH homologues contained a potential motif, and (ii) the VZV gH and simian varicella virus gH cytoplasmic tails were likely longer in length (18 amino acids) than predicted in the original sequence analyses (12 and 16 amino acids, respectively). The longer tails provided the proper context for a functional endocytosis motif.

Human herpesvirus 6: molecular biology and clinical features

Human herpesvirus 6 (HHV-6) exists as distinct variants HHV-6A and HHV-6B. The complete genomes of HHV-6A and HHV-6B have been sequenced. HHV-6B contains 97 unique genes. CD46 is the cell receptor for HHV-6, explaining its broad tissue tropism but its restricted host-species range. HHV-6 utilizes a number of strategies to down-regulate the host immune response, including molecular mimicry by production of a functional chemokine and chemokine receptors. Immunosuppression is enhanced by depletion of CD4 T lymphocytes via direct infection of intra-thymic progenitors and by apoptosis induction. Infection is widespread in infants between 6 months and 2 years of age. A minority of infants develop roseola infantum, but undifferentiated febrile illness is more common. Reactivation from latency occurs in immunocompromised hosts. Organ-specific clinical syndromes occasionally result, but indirect effects including interactions with other viruses such as human immunodeficiency virus type 1 and human cytomegalovirus or graft dysfunction in transplant recipients may be more significant complications in this population. Recent advances in quantitative PCR are providing additional insights into the natural history of infection in paediatric populations and immunocompromised hosts.

Clinical impact of human herpesvirus 6 infection after liver transplantation

BACKGROUND

Reactivation of human herpesvirus 6 (HHV-6) appears to be common after transplant. Viral reactivation may result in febrile illness and may also play an immunomodulatory role that leads to indirect effects such as opportunistic infections and rejection. The objective of this study was to determine the clinical impact of HHV-6 infection after liver transplantation including both direct and indirect effects.

METHODS

This was a prospective single center cohort study of 200 consecutive patients undergoing liver transplantation. Systemic serial HHV-6 viral load measurements and all clinical outcomes including development of opportunistic infections, cytomegalovirus (CMV) disease, and rejection were determined.

RESULTS

HHV-6 infection (defined as viral load > or = 2 log10 copies/microg input DNA) occurred in 56/200 (28%) patients. Symptomatic disease attributable to HHV-6 alone occurred in 2/200 (1%) patients. Univariate analysis revealed HHV-6 infection was associated with the development of opportunistic infection and CMV disease. In a multivariate analysis designed to control for the level of immunosuppression, the risk of opportunistic infection increased by 3.68-fold in patients with HHV-6 infection (95% confidence interval [CI], 1.86-7.27; P=0.001). In a similar multivariate analysis, the risk of CMV disease increased by 3.59-fold in patients with HHV-6 infection (95% CI, 1.53-8.44; P=0.003). HHV-6 infection was not associated with rejection except in the subgroup of patients with rejection after 30 days posttransplant (odds ration 2.27; 95% CI, 1.09-4.77; P=0.029).

CONCLUSIONS

HHV-6 reactivation after transplant is common and is associated with the development of opportunistic infections and CMV disease and possibly with a subgroup of acute rejection episodes. HHV-6 infection likely has a significant impact in transplant recipients through indirect effects of viral replication.

A role for human cytomegalovirus glycoprotein O (gO) in cell fusion and a new hypervariable locus

A cell fusion assay using fusion-from-without (FFWO) recombinant adenoviruses (RAds) and specific antibody showed a role in fusion modulation for glycoprotein gO, the recently identified third component of the gH/gL gCIII complex of human cytomegalovirus (HCMV). As in HCMV, RAd gO expressed multiple glycosylated species with a mature product of 125 kDa. Coexpression with gH/gL RAds showed gCIII reconstitution in the absence of other HCMV products and stabilisation by intermolecular disulfide bonds. Properties of HCMV clinical isolate, Pt, also implicated gO in cell spread. Compared to laboratory strain AD169, Pt was resistant to gH antibody plaque inhibition, but mature gH was identical. However, the gO sequences were highly divergent (20%), with further variation in laboratory strain Towne gO (34%). Thus, gO forms gCIII with gH/gL, performs in cell fusion, and is a newly identified HCMV hypervariable locus which may influence gCIII's function in mediating infection.

Human herpesvirus-6 and -7 in transplantation

Infections with the beta-herpesviruses human herpesvirus-6 (HHV-6) and human herpesvirus-7 (HHV-7) are ubiquitous in childhood. The immunosuppression secondary to organ or bone marrow transplantation together with posttransplantation management may favour viral replication and reactivation. HHV-6 and -7 induce immunosuppression by targeting lymphocytes, natural killer cells and monocytes. HHV-6 is commonly detected posttransplantation but variability in definitions of clinical syndromes related to this virus and detection methods have complicated understanding of the clinical relevance of HHV-6 posttransplantation. Clinical symptoms associated with HHV-6 include febrile illness, pneumonitis, hepatitis, encephalitis and bone marrow suppression. However, the majority of HHV-6 infections are asymptomatic. The incidence of HHV-7 infection and its clinical manifestations posttransplantation are even less well characterised. In addition, HHV-6 and HHV-7 are related to CMV disease or acute graft-versus-host disease and, indirectly, to increases in resource utilisation. Based on the potential relevance of these two beta-herpesviruses in transplant recipients, further studies are required to establish their real impact in transplantation. For this, sensitive and specific molecular diagnostic techniques allowing for the rapid detection and quantitation of virus and for the analysis of susceptibility to current antiviral agents are required.

Characterization of interaction of gH and gL glycoproteins of varicella-zoster virus: their processing and trafficking

Varicella-zoster virus (VZV) glycoproteins gH and gL were examined in a recombinant vaccinia virus system. Single expression of glycoprotein gL produced two molecular forms: an 18 kDa form and a 19 kDa form differing in size by one endoglycosidase H-sensitive N-linked oligosaccharide. Coexpression of gL and gH resulted in binding of the 18 kDa gL form with the mature form of gH, while the 19 kDa gL form remained uncomplexed. The glycosylation processing of gL was not dependent on gH; however, gL was required for the conversion of precursor gH (97 kDa) to mature gH (118 kDa). Subsequent analyses indicated that gL (18 kDa) was a more completely processed gL (19 kDa). Screening of the culture media revealed that gH and gL were secreted, but only if coexpressed and complexed together. The secreted form of gL was 18 kDa while that of gH was 114 kDa. The fact that secreted gH was smaller than intracytoplasmic gH suggested a proteolytic processing event prior to secretion. The 19 kDa form of gL was never secreted. These findings support a VZV gL recycling pathway between the endoplasmic reticulum and the cis-Golgi apparatus.

RANTES binding and down-regulation by a novel human herpesvirus-6 beta chemokine receptor

The human herpesvirus 6 (HHV-6) U51 gene defines a new family of betaherpesvirus-specific genes encoding multiple transmembrane glycoproteins with similarity to G protein-coupled receptors, in particular, human chemokine receptors. These are distinct from the HHV-6 U12 and HCMV US28 family. In vitro transcription and translation as well as transient cellular expression of U51 showed properties of a multiple transmembrane protein with a 30-kDa monomer as well as high m.w. aggregates or oligomers. Transient cellularly expressed U51 also appeared to form dimeric intermediates. Despite having only limited sequence similarity to chemokine receptors, U51 stably expressed in cell lines showed specific binding of the CC chemokine RANTES and competitive binding with other beta chemokines, such as eotaxin; monocyte chemoattractant protein 1, 3, and 4; as well as the HHV-8 chemokine vMIPII. In epithelial cells already secreting RANTES, U51 expression resulted in specific transcriptional down-regulation. This correlated with reduced secretion of RANTES protein into the culture supernatants. Regulation of RANTES levels may alter selective recruitment of circulating inflammatory cells that the virus can infect and thus could mediate the systemic spread of the virus from initial sites of infection in epithelia. Alternatively, chemokine regulation could modulate a protective inflammatory response to aid the spread of virus by immune evasion. Such mimicry, by viral proteins, of host receptors leading to down-regulation of chemokine expression is a novel immunomodulatory mechanism.

Constitutive expression of human cytomegalovirus (HCMV) glycoprotein gpUL75 (gH) in astrocytoma cells: a study of the specific humoral immune response

The humoral immune response to gpUL75 (gH) was determined in different groups of human cytomegalovirus (HCMV) infected subjects using a full-length glycoprotein constitutively expressed in an astrocytoma cell line. The recombinant molecule consisted of two distinct isoforms resembling the authentic protein of infected cells. Separated from the interactions of other viral gene products gH failed to form an oligomeric complex, thus exhibiting exclusively epitopes present on the monomer. Ninety five percent of serum samples from latently-infected healthy adults revealed the presence of gH-specific IgG. Moreover, examination of sequential sera from immunocompromised and immunocompetent individuals undergoing active HCMV infection demonstrated that antibodies to gH occurred in most cases simultaneously with those to the abundant surface antigen gpUL55 (gB) and at similar titres. Appearance of this response was correlated with a considerable increase of the virus-neutralizing activity and most likely associated with restriction of viral dissemination during subsequent viremic episodes. Together, these results suggest that glycoprotein H of HCMV is like gB, a highly immunogenic component of the infectious particle.

Domains of glycoprotein H of herpes simplex virus type 1 involved in complex formation with glycoprotein L

The complex formation between glycoproteins H (gH) and L (gL) of herpes simplex virus type 1 (HSV-1) was studied by using five recombinant baculoviruses expressing open reading frames that contain deletions in the coding region of the extracellular domain of gH. In addition, the gH-deletion mutants contained a C-terminal tag. Complex formation of gL and the gH-deletion mutants was studied by immunoprecipitations with anti-tag monoclonal antibody (MAb) A16 and with the gH-specific MAbs 37S, 46S, and 52S. All gH-deletion mutants were complexed to gL when analyzed by MAb A16. MAb 37S precipitated complexes between gL and the two gH-deletion mutants that contain the epitope of this MAb. When the gH conformation-dependent MAbs 46S and 52S were used, gL was coprecipitated together with the gH-deletion mutant lacking amino acids 31-299, but gL was not coprecipitated with the gH-deletion mutant lacking amino acids 31-473. The data from the precipitation studies do allow at least two interpretations. There is either one site for gL binding on gH (residue 300-473) or gL contacts multiple regions of gH. We were unable to demonstrate gL-dependent cell surface expression of either of the gH-deletion mutants. This suggests that the coassociation of gH with gL is necessary but not sufficient for transport of gH to the cell surface.

Human herpesvirus 6 seronegativity before transplantation predicts the occurrence of fungal infection in liver transplant recipients

BACKGROUND

Invasive fungal infection has a major impact on the morbidity and mortality of liver transplant recipients. Human herpesvirus (HHV)-6 infection after transplantation is associated with an immunosuppressive state and the development of cytomegalovirus disease. Because cytomegalovirus infection is a risk factor for invasive fungal infection after transplantation, we have examined whether HHV-6 and fungal infection are associated after transplantation.

METHODS

Pretransplantation sera from 247 consecutive liver transplant recipients were analyzed for IgG to HHV-6. Thirty-three (13%) HHV-6-seronegative recipients were identified. Six of 33 (18%) seronegative recipients experienced fungal infection as compared with 15 of 214 (7%) seropositive recipients (P=0.034).

RESULTS

In a univariate analysis of risk factors for fungal infection, pretransplantation seronegativity to HHV-6 (P=0.034), intraoperative cryoprecipitate requirements greater than the 75th percentile (P=0.035), reoperation (P=0.005), biliary stricturing postoperatively (P=0.046), and gastrointestinal or vascular complications postoperatively (P=0.030) were identified as significant risk factors. Moreover, in pairwise multivariate analysis, pretransplantation HHV-6 seronegativity remained a significant variable even in the presence of each of the other variables.

CONCLUSIONS

These results suggest that HHV-6 seronegativity before transplantation is a valuable clinical marker that identifies patients at risk for developing fungal infection after transplantation.

Human herpesvirus 6

Human herpesvirus (HHV) 6 is a beta-herpes, DNA virus. This virus shows closest homology with cytomegalovirus and HHV-7. Infection usually occurs in infants 6 to 24 months of age, and primary infection may result in roseola. HHV-6 infection in infants is the commonest cause of fever-induced seizures. Infection in adults is seen primarily in immunocompromised hosts with solid organ transplants or in those with human immunodeficiency virus infection. The virus is capable of pronounced interaction in vitro with cytomegalovirus and human immunodeficiency virus and induces immunosuppression and apoptosis. The importance of these interactions in vivo necessitates further investigation. HHV-6 infection may contribute to the pathogenesis of multiple sclerosis. HHV-6 may be diagnosed by viral culture, serology, or polymerase chain reaction.

Structural and antigenic analysis of a truncated form of the herpes simplex virus glycoprotein gH-gL complex

The herpes simplex virus (HSV) gH-gL complex is essential for virus infectivity and is a major antigen for the host immune system. The association of gH with gL is required for correct folding, cell surface trafficking, and membrane presentation of the complex. Previously, a mammalian cell line was constructed which produces a secreted form of gHt-gL complex lacking the transmembrane and cytoplasmic tail regions of gH. gHt-gL retains a conformation similar to that of its full-length counterpart in HSV-infected cells. Here, we examined the structural and antigenic properties of gHt-gL. We first determined its stoichiometry and carbohydrate composition. We found that the complex consists of one molecule each of gH and gL. The N-linked carbohydrate (N-CHO) site on gL and most of the N-CHO sites on gH are utilized, and both proteins also contain O-linked carbohydrate and sialic acid. These results suggest that the complex is processed to the mature form via the Golgi network prior to secretion. To determine the antigenically active sites of gH and gL, we mapped the epitopes of a panel of gH and gL monoclonal antibodies (MAbs), using a series of gH and gL C-terminal truncation variant proteins produced in transiently transfected mammalian cells. Sixteen gH MAbs (including H6 and 37S) reacted with the N-terminal portion of gH between amino acids 19 and 276. One of the gH MAbs, H12, reacted with the middle portion of gH (residues 476 to 678). Nine gL MAbs (including 8H4 and VIII 62) reacted with continuous epitopes within the C-terminal portion of gL, and this region was further mapped within amino acids 168 to 178 with overlapping synthetic peptides. Finally, plasmids expressing the gH and gL truncations were employed in cotransfection assays to define the minimal regions of both gH and gL required for complex formation and secretion. The first 323 amino acids of gH and the first 161 amino acids of gL can form a stable secreted hetero-oligomer with gL and gH792, respectively, while gH323-gL168 is the smallest secreted hetero-oligomer. The first 648 amino acids of gH are required for reactivity with MAbs LP11 and 53S, indicating that a complex of gH648-gL oligomerizes into the correct conformation. The data suggest that both antigenic activity and oligomeric structure require the amino-terminal portions of gH and gL.

Identification and characterization of the guinea-pig cytomegalovirus glycoprotein H gene

Subunit vaccines which target viral envelope glycoproteins offer promise for the prevention of congenital cytomegalovirus (CMV) infection. The guinea pig model of CMV infection is uniquely well suited to testing vaccines for prevention of congenital infection, since, in contrast to other animal cytomegaloviruses, the guinea pig CMV (GPCMV) crosses the placenta, producing intrauterine infection. Antibody to the CMV glycoproteins B (gB) and H (gH) appears to be important in conferring protective immunity. Unfortunately, little is known about specific GPCMV envelope glycoproteins. Sequencing of GPCMV genome fragments was therefore undertaken to test whether GPCMV encodes a gH homologue. Partial sequencing of the Hind III A fragment of the GPCMV genome revealed an open reading frame of 2,169 nucleotides capable of encoding a protein of 723 amino acids. Computer matrix analyses demonstrated identity between this ORF and the gH coding sequences of other herpesviruses. The GPCMV gH ORF encodes 12 highly conserved cysteine residues, contains 9 potential N-linked glycosylation sites, and has a predicted M(r) of 81.6 kDa. Northern blot hybridizations with gH-specific probes identified an abundant 5.1 kb mRNA with expression kinetics of an "early" gene. A polyclonal antiserum raised against a synthetic peptide derived from the deduced amino acid sequence of the gH ORF identified a virion-associated protein with an approximate M(r) of 85-kDa, the putative GPCMV gH, in immunoblot assays.

Study of the protective immunity of co-expressed glycoprotein H and L of equine herpesvirus-1 in a murine intranasal infection model

Equine herpesvirus-1 (EHV-1) glycoproteins H, and L (gH and gL) expressed individually or co-expressed by recombinant baculoviruses were used to immunise BALB/c mice prior to intranasal challenge in a murine model of respiratory infection. Only the co-expressed material (EHV-1 gH/gL) induced neutralising antibody (low levels). The same immunogen also produced the strongest cellular responses. Immunisation with gH/gL and, to a lesser extent, with gH alone was associated with a reduction of virus load in nasal turbinates and olfactory bulbs after challenge infection. Viraemia, detected by polymerase chain reaction, was also reduced. No such protective effects were observed for gL alone. Adoptive transfer of lymphocytes from gH/gL-immunised mice to näive mice subsequently challenged with EHV-1 indicated that both CD4+ and CD8+ cells had a role in protective immunity. Although clearance of EHV-1 from respiratory tissue was not as effective as previously found for glycoproteins D or C, these experiments provide evidence that the co-expression of EHV-1 gL with gH generates a conformational neutralising epitope which is not present in either molecule alone, and suggests that gH/gL antigen may have a better potential as a component of an EHV-1 vaccine than gH alone.

Human herpesvirus 6

Human herpesvirus 6 variant A (HHV-6A) and human herpesvirus 6 variant B (HHV-6B) are two closely related yet distinct viruses. These visuses belong to the Roseolovirus genus of the betaherpesvirus subfamily; they are most closely related to human herpesvirus 7 and then to human cytomegalovirus. Over 95% of people older than 2 years of age are seropositive for either or both HHV-6 variants, and current serologic methods are incapable of discriminating infection with one variant from infection with the other. HHV-6A has not been etiologically linked to any human disease, but such an association will probably be found soon. HHV-6B is the etiologic agent of the common childhood illness exanthem subitum (roseola infantum or sixth disease) and related febrile illnesses. These viruses are frequently active and associated with illness in immunocompromised patients and may play a role in the etiology of Hodgkin's disease and other malignancies. HHV-6 is a commensal inhabitant of brains; various neurologic manifestations, including convulsions and encephalitis, can occur during primary HHV-6 infection or in immunocompromised patients. HHV-6 and distribution in the central nervous system are altered in patients with multiple sclerosis; the significance of this is under investigation.

Glycoprotein H-related complexes of human cytomegalovirus: identification of a third protein in the gCIII complex

Previous studies have described three disulfide-bonded glycoprotein complexes within the envelope of human cytomegalovirus (HCMV). These have been designated gCI, gCII, and gCIII. Although gCI has been identified as homodimeric glycoprotein B (gB, gpUL55), the compositions of gCII and gCIII remain incompletely defined. Earlier studies suggested that gCIII was composed of glycoprotein H (gH, gpUL75) complexed with a second glycoprotein, the gL homolog of HCMV. We characterized the gCIII complex of HCMV using recombinant vaccinia virus-expressed gH and gL. Our results indicated that authentic gCIII was not reconstituted by coexpression of gH and gL. The presence of a third, structurally and antigenically unique glycoprotein with an estimated molecular mass of 125,000 Da in virion-derived gCIII complexes suggested that at least three proteins were necessary for formation of this envelope glycoprotein complex. This third glycoprotein, gp125, contained both simple and complex N-linked carbohydrates and had an estimated deglycosylated mass of 64,000 Da. Furthermore, we demonstrated that mature gH existed as both a covalently complexed and noncovalently associated component of the gCIII complex within the envelope of infectious extracellular virions. These findings provide further evidence for the structural complexity of the envelope of HCMV and emphasize the uncertainties associated with the previous assignment of specific functions to envelope proteins of HCMV.

Site-directed and linker insertion mutagenesis of herpes simplex virus type 1 glycoprotein H

The gH-gL complex of herpes simplex virus type 1 (HSV-1) is essential for virion infectivity and virus-induced cell fusion, but functional domains of the gH molecule remain to be defined. We have addressed this question by mutagenesis. A set of linker insertion mutants in HSV-1 gH was generated and tested in transient assays for their ability to complement a gH-negative virus. Insertions at three sites in the C-terminal third of the external domain affected the ability of gH to function in cell-cell fusion and virus entry, while insertions at six sites in the N-terminal half of the external domain induced conformational changes in gH such that it was not recognized by monoclonal antibody LP11, although expression at the cell surface was unchanged. A recombinant virus in which a potential integrin-binding motif, RGD, in gH was changed to the triplet RGE entered cells as efficiently as the wild type, indicating that HSV-1 entry is not mediated by means of the gH-RGD motif binding to cell surface integrins. Furthermore, mutagenesis of the glycosylation site which is positionally conserved in all herpesvirus gH sequences in close proximity to the transmembrane domain generated a recombinant virus that grew in vitro with wild-type single-step kinetics.

Chaperone functions common to nonhomologous Epstein-Barr virus gL and Varicella-Zoster virus gL proteins

Herpesviruses encode the complex-forming, essential glycoproteins gH and gL. Maturation and transport of gH are dependent on coexpression of its chaperone, gL. The gL proteins of alpha herpesviruses and gamma herpesviruses do not have a significant percentage of amino acid sequence homology. Yet, as we report herein, the diverse gL glycoproteins of Epstein-Barr virus (EBV) and varicella-zoster virus (VZV) were functionally interchangeable, although membrane expression and maturation of gH were separate functions for these viruses. In VZV both functions were performed by a single protein. EBV required two separate glycoproteins, one of which can be replaced by its homologous protein from VZV, a distant relative of EBV. Collectively, these results suggested that VZV gL is a simpler form of the gL chaperone protein than EBV gL.