The amino terminus of human cytomegalovirus glycoprotein B contains epitopes that vary among strains

Quantitative temporal viromics: an approach to investigate host-pathogen interaction

A systematic quantitative analysis of temporal changes in host and viral proteins throughout the course of a productive infection could provide dynamic insights into virus-host interaction. We developed a proteomic technique called “quantitative temporal viromics” (QTV), which employs multiplexed tandem-mass-tag-based mass spectrometry. Human cytomegalovirus (HCMV) is not only an important pathogen but a paradigm of viral immune evasion. QTV detailed how HCMV orchestrates the expression of >8,000 cellular proteins, including 1,200 cell-surface proteins to manipulate signaling pathways and counterintrinsic, innate, and adaptive immune defenses. QTV predicted natural killer and T cell ligands, as well as 29 viral proteins present at the cell surface, potential therapeutic targets. Temporal profiles of >80% of HCMV canonical genes and 14 noncanonical HCMV open reading frames were defined. QTV is a powerful method that can yield important insights into viral infection and is applicable to any virus with a robust in vitro model.

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>8,000 proteins quantified over eight time points, including 1,200 cell-surface proteins

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Temporal profiles of 139/171 canonical HCMV proteins and 14 noncanonical HCMV ORFs

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Multiple families of cell-surface receptors selectively modulated by HCMV

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Multiple signaling pathways modulated during HCMV infection

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.

Cytomegalovirus (CMV) genotype in allogeneic hematopoietic stem cell transplantation

Background

Based on sequence variation in the UL55 gene that encodes glycoprotein B (gB), human cytomegalovirus (CMV) can be classified into four gB genotypes. Previous studies have suggested an association between CMV gB genotype and clinical outcome in patients who underwent an allogeneic hematopoietic stem cell transplant (HSCT). The goals of this study were identify patients with active infection caused by CMV in recipients of HSCT; determine the prevalence of CMV genotypes in the study group; correlate genotype with CMV disease, acute GVHD and overall survival.

Methods

The diagnosis of active CMV infection after allogeneic HSCT was detected by antigenemia (AGM) and/or nested-PCR (N-PCR). Positive samples from patients with active CMV infection were submitted to genotyping using N-PCR to amplify a region of UL55, followed by restriction analysis based on HinfI and RsaI digestion. Real-time PCR (qPCR) was used to determine the viral load during active CMV infection and antiviral treatment.

Results

Sixty-three allogeneic HSCT recipients were prospectively evaluated; 49/63 (78%) patients were infected with CMV genotypes – gB1 19/49 (39%), gB2 17/49 (35%), gB3 3/49 (6%), gB4 7/49 (14%) – and 3 (6%) had mixed CMV genotypes (gB1 + gB3, gB1 + gB4 and gB2 + gB4). Characterized by gastrointestinal disease, CMV disease occurred in 3/49 (6.1%) patients, who had CMV gB3 genotype. These gB3 genotype patients presented an increasing AGM number, mean 125 (± 250) (P = 0.70), and qPCR copies/ml, mean 37938 (SD ± 50542) (P = 0.03), during antiviral treatment, when compared with other CMV genotypes. According to CMV genotypes, stratified overall survival was 55% for gB1, 43% for gB2; 0% for gB3 and 57% for gB4 (P = 0.03).

Conclusions

One of the restrictions of the presented study was the low number of CMV gB sub-cohorts). However, we demonstrated that the frequency of active CMV infection in this HSCT population was high, and the most prevalent genotype in these patients with active CMV infection was gB1 and gB2 genotype (74%). In Brazil, HSCT recipients seem to carry mainly gB1 and gB2 CMV genotype.

Antibody evasion by the N terminus of murid herpesvirus-4 glycoprotein B

Herpesviruses characteristically transmit infection from immune hosts. Although their success in escaping neutralization by pre-formed antibody is indisputable, the underlying molecular mechanisms remain largely unknown. Glycoprotein B (gB) is the most conserved component of the herpesvirus entry machinery and its N terminus (gB-NT) is a common neutralization target. We used murid herpesvirus-4 to determine how gB-NT contributes to the virus–antibody interaction. Deleting gB-NT had no obvious impact on virus replication, but paradoxically increased virion neutralization by immune sera. This reflected greater antibody access to neutralization epitopes on gH/gL, with which gB was associated. gB-NT itself was variably protected against antibody by O-linked glycans; on virions from epithelial cells it was protected almost completely. gB-NT therefore provides a protective and largely protected cover for a vulnerable part of gH/gL. The conservation of predicted glycosylation sites in other mammalian herpesvirus gB-NTs suggests that this evasion mechanism is widespread. Interestingly, the gB-NT glycans that blocked antibody binding could be targeted for neutralization instead by a lectin, suggesting a means of therapeutic counterattack.

Genetic polymorphisms among human cytomegalovirus (HCMV) wild-type strains

Human cytomegalovirus (HCMV) clinical isolates display genetic polymorphisms in multiple genes. Some authors have suggested that those polymorphisms may be implicated in HCMV-induced immunopathogenesis, as well as in strain-specific behaviours, such as tissue-tropism and ability to establish persistent or latent infections. This review summarises the features of the main clustered HCMV polymorphic open reading frames and also briefly cites other variable loci within the viral genome. The implications of gene polymorphisms are discussed in terms of potentially advantageous higher fitness obtained by the strain, but also taking into account that the published data are often speculative. The last section of this review summarises and critically analyses the main literature reports about the linkage of strain specific genotypes with clinical manifestations of HCMV disease in different patient populations affected by severe cytomegalovirus infections, namely immunocompromised subjects and congenitally infected newborns.

Proteolytic cleavage of the amino terminus of the U(L)15 gene product of herpes simplex virus type 1 is coupled with maturation of viral DNA into unit-length genomes

The U(L)15 gene of herpes simplex virus type 1 (HSV-1), like U(L)6, U(L)17, U(L)28, U(L)32, and U(L)33, is required for cleavage of concatameric DNA into genomic lengths and for packaging of cleaved genomes into preformed capsids. A previous study indicated that the U(L)15 gene encodes minor capsid proteins. In the present study, we have shown that the amino-terminal 509 amino acids of the U(L)15-encoded protein are sufficient to confer capsid association inasmuch as a carboxyl-terminally truncated form of the U(L)15-encoded protein with an M(r) of approximately 55,000 readily associated with capsids. This and previous studies have shown that, whereas three U(L)15-encoded proteins with apparent M(r)s of 83,000, 80,000, and 79,000 associated with wild-type B capsids, only the full-length 83,000-M(r) protein associated with B capsids purified from cells infected with viruses lacking functional U(L)6, U(L)17, U(L)28, U(L)32, and U(L)33 genes (B. Salmon and J. D. Baines, J. Virol. 72:3045-3050, 1998). Thus, all viral mutants that fail to cleave viral DNA into genomic-length molecules also fail to produce capsid-associated U(L)15 80,000- and 79,000-M(r) proteins. In contrast, the 80,000- and 79,000-M(r) proteins were readily detected in capsids purified from cells infected with a U(L)25 null virus that cleaves, but does not package, DNA. The conclusion that the amino terminus of the 83,000-M(r) protein is truncated to produce the 80,000- and/or 79,000-M(r) protein was supported by the following observations. (i) Whereas the C termini of the 83,000-, 80, 000-, and 79,000-M(r) proteins are identical, immunoreactivity dependent on the first 35 amino acids of the U(L)15 83,000-M(r) protein was absent from the 80,000- and 79,000-M(r) proteins. (ii) The 79,000- and 80,000-M(r) proteins were detected in capsids from cells infected with HSV-1(U(L)15M36V), an engineered virus encoding valine rather than methionine at codon 36. Thus, initiation at codon 36 is unlikely to account for production of the 80,000- and/or 79, 000-M(r) protein. Taken together, these data strongly suggest that capsid-associated U(L)15-encoded protein is proteolytically cleaved near the N terminus and indicate that this modification is tightly linked to maturation of genomic DNA.

Human cytomegalovirus glycoprotein B contains autonomous determinants for vectorial targeting to apical membranes of polarized epithelial cells

We previously reported that human cytomegalovirus (CMV) glycoprotein B (gB) is vectorially transported to apical membranes of CMV-infected polarized human retinal pigment epithelial cells propagated on permeable filter supports and that virions egress predominantly from the apical membrane domain. In the present study, we investigated whether gB itself contains autonomous information for apical transport by expressing the molecule in stably transfected Madine-Darby canine kidney (MDCK) cells grown on permeable filter supports. Laser scanning confocal immunofluorescence microscopy and domain-selective biotinylation of surface membrane domains showed that CMV gB was transported to apical membranes independently of other envelope glycoproteins and that it colocalized with proteins in transport vesicles of the biosynthetic and endocytic pathways. Determinants for trafficking to apical membranes were located by evaluating the targeting of gB derivatives with deletions in the lumen, transmembrane (TM) anchor, and carboxyl terminus. Derivative gB(Delta717-747), with an internal deletion in the luminal juxtamembrane sequence that preserved the N- and O-glycosylation sites, retained vectorial transport to apical membranes. In contrast, derivatives that lacked the TM anchor and cytosolic domain (gBDelta646-906) or the TM anchor alone (gBDelta751-771) underwent considerable basolateral targeting. Likewise, derivatives lacking the entire cytosolic domain (gBDelta772-906) or the last 73 amino acids (gBDelta834-906) showed disrupted apical transport. Site-specific mutations that deleted or altered the cluster of acidic residues with a casein kinase II phosphorylation site at the extreme carboxyl terminus, which can serve as an internalization signal, caused partial missorting of gB to basolateral membranes. Our studies indicate that CMV gB contains autonomous information for apical targeting in luminal, TM anchor, and cytosolic domain sequences, forming distinct structural elements that cooperate in vectorial transport in polarized epithelial cells.

Human monoclonal antibodies to cytomegalovirus. Characterization and recombinant expression of a glycoprotein-B-specific antibody

Human monoclonal antibodies (mAb) to human cytomegalovirus (HCMV) were established from spleen cells of a HCMV-positive donor. The antibodies (gamma 3, lambda) secreted from a stable heterohybridoma cell line were further characterized by immunoprecipitation and immune-fluorescence microscopy using HCMV infected cells and recombinant cell lines expressing HCMV glycoprotein B. The antibody reacted with the entire glycoprotein B or the extracellular domain expressed as glycoprotein-B--beta-galactosidase fusion protein in the native state, but the antibody was not neutralizing HCMV. Denatured and reduced forms of glycoprotein B were not recognized by this antibody, however, native glycoprotein B on the surface of infected cells was detected efficiently. The genes encoding the Fab part of the antibody were cloned and expressed in Escherichia coli. Recombinant Fab fragments specifically binding the extracellular domain of glycoprotein B could easily be isolated from the periplasmic space. Recombinant antibodies provide the opportunity to modify effector functions and to add tags to diagnostic antibodies for more efficient detection of CMV-infected cells.

The herpes simplex virus 1 UL15 gene encodes two proteins and is required for cleavage of genomic viral DNA

Previous studies have shown that a ts mutant [herpes simplex virus 1 (mP)ts66.4] in the UL15 gene fails to package viral DNA into capsids (A. P. W. Poon and B. Roizman, J. Virol. 67:4497-4503, 1993) and that although the intron separating the first and second exons of the UL15 gene contains UL16 and UL17 open reading frames, replacement of the first exon with a cDNA copy of the entire gene does not affect viral replication (J.D. Baines, and B. Roizman, J. Virol. 66:5621-5626, 1992). We report that (i) a polyclonal rabbit antiserum generated against a chimeric protein consisting of the bacterial maltose-binding protein fused in frame to the majority of sequences contained in the second exon of the UL15 gene reacted with two proteins with M(r) of 35,000 and 75,000, respectively, in cells infected with a virus containing the authentic gene yielding a spliced mRNA or with a virus in which the authentic UL15 gene was replaced with a cDNA copy. (ii) Insertion of 20 additional codons into the C terminus of UL15 exon II caused a reduction in the electrophoretic mobility of both the apparently 35,000- and 75,000-M(r) proteins, unambiguously demonstrating that both share the carboxyl terminus of the UL15 exon II. (iii) Accumulation of the 35,000-M(r) protein was reduced in cells infected and maintained in the presence of phosphonoacetate, an inhibitor of viral DNA synthesis. (iv) The UL15 proteins were localized in the perinuclear space at 6 h after infection and largely in the nucleus at 12 h after infection. (v) Viral DNA accumulating in cells infected with herpes simplex virus 1(mP)ts66.4 and maintained at the nonpermissive temperature was in an endless (concatemeric) form, and therefore UL15 is required for the cleavage of mature, unit-length molecules for packaging into capsids.

Uses of flow cytometry in virology

This article reviews some of the published applications of flow cytometry for in vitro and in vivo detection and enumeration of virus-infected cells. Sample preparation, fixation, and permeabilization techniques for a number of virus-cell systems are evaluated. The use of flow cytometry for multiparameter analysis of virus-cell interactions for simian virus 40, herpes simplex viruses, human cytomegalovirus, and human immunodeficiency virus and its use for determining the effect of antiviral compounds on these virus-infected cells are reviewed. This is followed by a brief description of the use of flow cytometry for the analysis of several virus-infected cell systems, including blue tongue virus, hepatitis C virus, avian reticuloendotheliosis virus, African swine fever virus, woodchuck hepatitis virus, bovine viral diarrhea virus, feline leukemia virus, Epstein-Barr virus, Autographa californica nuclear polyhedrosis virus, and Friend murine leukemia virus. Finally, the use of flow cytometry for the rapid diagnosis of human cytomegalovirus and human immunodeficiency virus in peripheral blood cells of acutely infected patients and the use of this technology to monitor patients on antiviral therapy are reviewed. Future prospects for the rapid diagnosis of in vivo viral and bacterial infections by flow cytometry are discussed.

Strategies for mapping and imitating viral B-cell epitopes

Identification of viral B-cell epitopes is of importance in the selection of peptides for inclusion into subunit vaccines, the development of virus-specific serological tests and understanding the interaction of antibodies with viruses at a molecular level. B-cell epitopes can often be determined unequivocally by X-ray analyses of antibody-antigen complexes. This technique is, however, time-consuming and alternative strategies have now been developed for identifying epitopes. This article provides an overview of approaches which are currently available for mapping and imitating B-cell epitopes.

Different antibody response to a neutralizing epitope of human cytomegalovirus glycoprotein B among seropositive individuals

The amino-terminal portion of human cytomegalovirus glycoprotein B (HCMV-gB) was expressed as a fusion protein to analyze the neutralizing epitope recognized by human monoclonal antibody C23 and the humoral immune response to this epitope. The linear neutralizing epitope was further localized to the peptide within 17 amino acids (position 68-84) which were conserved between two HCMV laboratory strains. Ten out of 17 HCMV-seropositive human sera contained the antibody against this epitope. Although seven sera were negative for reacting with the fusion protein, the viruses isolated from the same patients retained the epitope. The immunogenicity of the epitope and the possible application of C23 human monoclonal antibody for passive immunization against HCMV infections are discussed.

The UL21 gene products of herpes simplex virus 1 are dispensable for growth in cultured cells

A viral deletion mutant (delta UL21) that lacked the sequences encoding 484 of the predicted first 535 amino acids of the UL21 open reading frame was genetically engineered and studied with respect to its phenotype in cells in culture. We report the following. (i) The replication of delta UL21 was identical to that of the parent herpes simplex virus 1 (HSV-1) strain F in Vero cells, but the yields were three- to fivefold lower than those of the parent virus in human embryonic lung cells. (ii) To characterize the UL21 protein, we immunized rabbits against a purified bacterial fusion protein consisting of glutathione S-transferase fused to the majority of the coding domain of the UL21 gene. Rabbit antiserum directed against the fusion protein recognized a broad band with an apparent M(r) of 62,000 to 64,000 in lysates of cells infected with HSV-1 strain F and in virions purified from the infected cell cytoplasm. This band was absent from lysates of mock-infected cells or cells infected with the delta UL21 virus. The band was significantly reduced in intensity in lysates of cells infected in the presence of phosphonoacetic acid, indicating that it is expressed as a late (gamma 1) gene. (iii) Immunofluorescence studies localized the UL21 antigen primarily in brightly staining granules in the cytoplasms of infected cells. Taken together, the data indicate that the UL21 protein is a virion component dispensable for all aspects of replication of HSV-1 in the cells tested. The electrophoretic mobility of the UL21 protein suggests that it is extensively modified posttranslationally.

Ontogeny of glycoprotein gB-specific antibody and neutralizing activity during natural cytomegalovirus infection

The envelope glycoprotein gB (gpUL55) is a candidate for inclusion in subunit cytomegalovirus (CMV) vaccines, although data on gB antibody responses after natural infection are limited. [35S]-labeled gB was partially purified from cells infected with an adenovirus recombinant expressing gB and used in radioimmunoprecipitation assays to characterize responses in solid organ transplant recipients with primary (n = 11) or secondary (n = 8) CMV infection. Seropositive transplant patients without evidence of infection (n = 5) and healthy seroconverters (n = 7) were also studied. gB antibody developed concurrently with CMV-specific IgG, IgM, and neutralizing activity in transplant patients with primary infection. Sustained boosts in gB antibody were seen in patients with secondary infection, and healthy seroconverters developed early gB responses. These data imply that gB antibody is an integral part of the humoral response to CMV infection, and, in view of experimental data regarding immunogenicity, support a role for gB in subunit vaccines.

Sequence variation of the amino-terminal antigenic domains of glycoprotein B of human cytomegalovirus strains isolated from Chinese patients

To study the genetic variation of human cytomegalovirus (HCMV) in Asian populations, the amino-terminal antigenic domains of glycoprotein B of HCMVs isolated from ethnic Chinese transplant patients were cloned and sequenced. The nucleotide and encoded amino acid sequences were compared with published sequences of AD169 and Towne laboratory strains. Within the region sequenced, 9 out of 15 clinical isolates (60%) possessed a peptide configuration similar to that of strain AD169 while 6 isolates (40%) displayed a peptide configuration similar to that of strain Towne. The nucleotide and amino acid identities of AD169-like clinical isolates exhibited variations of 95.4%-99.6% and 95.4%-100% respectively, whereas the identities of Towne-like clinical isolates were within the range of 97.3%-100% and 96.6%-100% at the nucleotide and amino acid levels. The previously defined neutralizing epitope was conserved among the clinical isolates sequenced while unique non-conservative amino acid substitutions were detected in the non-neutralizing epitope within the amino-terminal antigenic domain of glycoprotein B of all AD169-like isolates (Y- > S) and one of the Towne-like isolates (R- > Q).

The UL10 gene of herpes simplex virus 1 encodes a novel viral glycoprotein, gM, which is present in the virion and in the plasma membrane of infected cells

The herpes simplex virus 1 UL10 gene encodes a hydrophobic membrane protein dispensable for viral replication in cell culture (J.D. Baines and B. Roizman, J. Virol. 65:938-944, 1991). We report the following. (i) A fusion protein consisting of glutathione S-transferase fused to the C-terminal 93 amino acids of the UL10 protein was used to produce a rabbit polyclonal antiserum. The antiserum reacted with infected-cell proteins which formed in denaturing polyacrylamide gels a sharp band (apparent M(r) of 50,000) and a very broad band (M(r) of 53,000 to 63,000). These bands were not formed by lysates of UL10- virus or by lysates of infected cells boiled in the presence of sodium dodecyl sulfate before electrophoresis. (ii) The proteins forming both bands were labeled by [3H]glucosamine, indicating that they were glycosylated. (iii) The UL10 protein in cells treated with tunicamycin formed a single band (apparent M(r) of 47,000) reactive with the anti-UL10 antibody, indicating that the 47,000-M(r) protein was a precursor of N-glycosylated, more slowly migrating forms of UL10. Treatment of the immunoprecipitate with endoglycosidase H increased the electrophoretic mobility of the 50,000-M(r) species to that of the 47,000-M(r) species, indicating that the 50,000-M(r) species contained high-mannose polysaccharide chains, whereas the proteins forming the 53,000- to 63,000-M(r) bands contained mature chains inasmuch as they were resistant to digestion by the enzyme. (iv) The UL10 protein of R7221 carrying a 20-amino-acid epitope formed only one band with an M(r) of 53,000. This band was sensitive to endoglycosidase H, suggesting that the epitope inserted in the R7221 UL10 protein may have interfered with glycosylation. (v) The UL10 protein does not contain a cleavable signal sequence inasmuch as the first UL10 methionine codon was reflected in the 50,000-M(r) protein. (vi) The UL10 protein is present in virions and plasma membranes of unfixed cells that were reacted with the polyclonal rabbit antibody. In accordance with the current nomenclature, the UL10 protein is designated glycoprotein M.

Homology of the envelope glycoprotein B of human herpesvirus-6 and cytomegalovirus

The envelope glycoprotein B (gB) coding sequences of two strains of human herpesvirus-6 (HHV6 GS and Z29) were determined by sequencing a 2.5-kb open reading frame adjacent to the DNA polymerase sequence. The deduced primary translation product is 830 amino acids in length and is 96% conserved between the two divergent strains with no localized hypervariability noted. It contains the expected signal and transmembrane sequence motifs as well as a putative site of protease cleavage. There was 39% amino acid identity with the gB of human cytomegalovirus (CMV, strain AD169). HHV6-CMV gB peptide homology was evident through the entire sequence, but was especially strong in the amino-terminal portion of CMV gp55, which contains linear and conformational epitopes recognized by CMV-neutralizing antibodies. All 10 cysteine residues of HHV6 gB match corresponding residues of CMV gB. Sequence data suggest strong structural similarity and possible immunologic cross-reactivity of gB from the two viruses.