The simian herpesvirus SA8 homologue of the herpes simplex virus gB gene: mapping, sequencing, and comparison to the HSV gB

Primary mouse dermal fibroblast cell cultures as an in vitro model system for the differential pathogenicity of cross-species herpesvirus papio 2 infections

Infection of mice with herpesvirus papio 2 (HVP2) parallels zoonotic monkey B virus infections. A major benefit of the HVP2/mouse model is the existence of two HVP2 subtypes: HVP2nv rapidly invades and destroys the CNS while HVP2ap produces no clinical signs and mild histopathological lesions. However, in the natural baboon host, no difference in pathogenicity of HVP2 subtypes is evident. Primary dermal fibroblast cells were evaluated as a model system for defining virus-host interactions that influence the outcome of a cross-species infection. No differences in plaque formation or virus replication were observed between HVP2 subtypes in primary baboon dermal fibroblast cultures. In contrast, when primary mouse dermal fibroblasts (PMDF) were infected, HVP2nv replicated to higher titers and was more efficient at shutting down host-cell protein synthesis compared to HVP2ap. HVP2ap-infected PMDF cells produced more IFN-beta compared to HVP2nv, and IFN-beta pretreatment of PMDF cultures inhibited HVP2ap replication but did not affect HVP2nv. The differential pathogenicity of HVP2 subtypes in mice and the lack of such differences in the natural baboon host are recapitulated in the primary dermal fibroblast cell culture system. This model may prove useful in examining early, local, host responses that influence the outcome of cross-species infections.

Complete genome sequence of cercopithecine herpesvirus 2 (SA8) and comparison with other simplexviruses

We have obtained the complete sequence of the herpesvirus simian agent 8 (SA8; cercopithecine herpesvirus 2) a baboon simplexvirus closely related to the monkey B virus and herpes simplex virus types 1 and 2. The genome of SA8 is 150,715 bp long, with an overall G/C content of 76%, the highest among the simplexviruses sequenced so far. The sequencing has confirmed that the genomic arrangement of SA8 is similar to that of other simplexviruses: unique long and unique short regions bordered by two sets of inverted repeats. All genes identified in SA8 are homologous and collinear with those of the monkey B virus, including the lack of the RL1 open reading frame, a gene responsible for neurovirulence in human herpes simplex viruses. This latter finding supports the hypothesis that a different pathogenetic mechanism may have developed in human simplexviruses, after their divergence from monkey simplexviruses.

One-step PCR to distinguish B virus from related primate alphaherpesviruses

By adding betaine to the PCR mixture, we previously established a PCR method to amplify a DNA segment of the glycoprotein G gene of B virus (BV) derived from a rhesus macaque. We have found that DNA of other BV strains derived from cynomolgus, pigtail, and lion-tailed macaques can also serve as the template in our PCR assay. Under the same conditions no product was obtained with DNA of simian agent 8 of green monkeys and Herpesvirus papio 2 of baboons, or the human herpes simplex viruses types 1 and 2. Thus, this PCR method is useful to discriminate BV from other closely related primate alphaherpesviruses.

Molecular evidence for distinct genotypes of monkey B virus (herpesvirus simiae) which are related to the macaque host species

Although monkey B virus (herpesvirus simiae; BV) is common in all macaque species, fatal human infections appear to be associated with exposure to rhesus macaques (Macaca mulatta), suggesting that BV isolates from rhesus monkeys may be more lethal to nonmacaques than are BV strains indigenous to other macaque species. To determine if significant differences that would support this supposition exist among BV isolates, we compared multiple BV strains isolated from rhesus, cynomolgus, pigtail, and Japanese macaques. Antigenic analyses indicated that while the isolates were very closely related to one another, there are some antigenic determinants that are specific to BV isolates from different macaque species. Restriction enzyme digest patterns of viral DNA revealed marked similarities between rhesus and Japanese macaque isolates, while pigtail and cynomolgus macaque isolates had distinctive cleavage patterns. To further compare genetic diversity among BV isolates, DNA sequences from two regions of the viral genome containing genes that are conserved (UL27 and US6) and variable (US4 and US5) among primate alphaherpesviruses, as well as from two noncoding intergenic regions, were determined. From these sequence data and a phylogenetic analysis of them it was evident that while all isolates were closely related strains of BV, there were three distinct genotypes. The three BV genotypes were directly related to the macaque species of origin and were composed of (i) isolates from rhesus and Japanese macaques, (ii) cynomolgus monkey isolates, and (iii) isolates from pigtail macaques. This study demonstrates the existence of different BV genotypes which are related to the macaque host species and thus provides a molecular basis for the possible existence of BV isolates which vary in their levels of pathogenicity for nonmacaque species.

Detection and differentiation of primate alpha-herpesviruses by PCR

A rapid method for detection and differentiation of 5 primate alpha-herpesviruses (human herpes simplex virus types 1 and 2 [HSV1, HSV2], green monkey simian agent 8, baboon herpesvirus 2 [HVP2], and macaque B virus [BV]) was developed utilizing the polymerase chain reaction (PCR). PCR primers were located in conserved regions of the gene encoding the glycoprotein B, which flanks an intervening region that is highly divergent among the 5 viruses. Amplified PCR products from the 5 viruses were readily differentiated by their unique restriction enzyme digestion patterns. No variation in digestion patterns was noted among strains of HSV1, HSV2, or HVP2. One clinical isolate of BV exhibited variation in a single restriction site, but its overall restriction pattern remained typical of BV. This method (PCR/RFLP) allowed the presence of herpesvirus DNA in clinical swabs from primates to be readily detected and the virus unambiguously identified.

Disulfide bonds of herpes simplex virus type 2 glycoprotein gB

Glycoprotein B (gB) is the most highly conserved envelope glycoprotein of herpesviruses. The gB protein is required for virus infectivity and cell penetration. Recombinant forms of gB being used for the development of subunit vaccines are able to induce virus-neutralizing antibodies and protective efficacy in animal models. To gain structural information about the protein, we have determined the location of the disulfide bonds of a 696-amino-acid residue truncated, recombinant form of herpes simplex virus type 2 glycoprotein gB (HSV gB2t) produced by expression in Chinese hamster ovary cells. The purified protein, which contains virtually the entire extracellular domain of herpes simplex virus type 2 gB, was digested with trypsin under nonreducing conditions, and peptides were isolated by reversed-phase high-performance liquid chromatography (HPLC). The peptides were characterized by using mass spectrometry and amino acid sequence analysis. The conditions of cleavage (4 M urea, pH 7) induced partial carbamylation of the N termini of the peptides, and each disulfide peptide was found with two or three different HPLC retention times (peptides with and without carbamylation of either one or both N termini). The 10 cysteines of the molecule were found to be involved in disulfide bridges. These bonds were located between Cys-89 (C1) and Cys-548 (C8), Cys-106 (C2) and Cys-504 (C7), Cys-180 (C3) and Cys-244 (C4), Cys-337 (C5) and Cys-385 (C6), and Cys-571 (C9) and Cys-608 (C10). These disulfide bonds are anticipated to be similar in the corresponding gBs from other herpesviruses because the 10 cysteines listed above are always conserved in the corresponding protein sequences.

Intracellular compartmentalization of the glycoprotein B of herpesvirus Simian agent 8 expressed with a baculovirus vector in insect cells

The intracellular localization of the glycoprotein B of herpesvirus simian agent 8 expressed with a baculovirus system in insect cells was studied. Cell fractionation and immunoprecipitation revealed that gB is present in microsomal as well as in nuclear membranes. Both fractions contain oligomers, probably dimers, of gB with endoglycosidase-H sensitive, mannose-rich carbohydrates. Nuclear transport of gB was further analysed by immuno electron microscopy of recombinant baculovirus-infected cells. The glycoprotein is present both in the outer and the inner nuclear membrane as well as in cytoplasmic structures and at the cell surface. This study precludes the possibility that glycosylation and/or oligomerisation of SA8 gB are responsible for nuclear targeting.

Glycoprotein B of herpes simplex virus type 1 oligomerizes through the intermolecular interaction of a 28-amino-acid domain

Herpes simplex virus type 1 glycoprotein B (gB) is an envelope component that plays an essential role in virus infection. The biologically active form of gB is an oligomer that contributes to the process of viral envelope fusion with the cell surface membrane, resulting in viral penetration and initiation of the replication cycle. In previous studies, two discontinuous sites for oligomer formation were identified: a nonessential upstream site located between residues 93 and 282 and an essential downstream site located between residues 596 and 711. In this study, in vitro-transcribed and -translated gB test molecules were used to characterize the more active essential membrane-proximal domain. A series of gB test polypeptides mutated in this downstream oligomerization domain were assayed for their abilities to form oligomers with a mutant gB capture polypeptide containing the analogous wild-type domain. Detection of oligomers was achieved by coimmunoprecipitation of two gB mutant molecules by using a monoclonal antibody specific for a hemagglutinin epitope tag introduced into the coding sequence of the capture polypeptide. Analysis of the immune-precipitated products by sodium dodecyl sulfate-polyacrylamide gel electrophoresis demonstrated that the downstream oligomerization domain resided within residues 626 to 676. This region was further resolved into two segments, residues 626 to 653 and 653 to 675, each of which was independently sufficient to form oligomers. However, residues 626 to 653 provided for a stronger interaction between gB monomers. Moreover, this stretch of 28 amino acids was shown to form oligomers when introduced into the carboxy-terminal region of gB monomers lacking this domain at the normal site, thus indicating that this domain was functionally independent of its natural location within the gB molecule. Further analysis of the sequence within residues 596 to 653 by using mutant test polypeptides altered in individual amino acids revealed that cysteines 9 and 10 located at positions 596 and 633, respectively, were not required for oligomer formation but contributed to dimer formation and/or stabilization. The results of this study suggest that oligomerization of gB monomers is induced by interactions between contiguous residues localized within the ectodomain near the site of molecule insertion into the viral envelope membrane.

Herpesvirus papio 2, an SA8-like alpha-herpesvirus of baboons

Several SA8 isolates obtained from baboons were compared to the prototype SA8 herpesvirus of African green monkeys. SDS-PAGE and restriction enzyme analyses revealed definite differences between green monkey and baboon isolates. DNA and amino acid sequences of the gB, gD and gJ glycoprotein genes exhibited substantial differences in variable regions. For the gB and gD, the amount of amino acid substitutions between SA8 and the baboon viruses was comparable to levels observed between analogous genes of SA8 & B virus or HSV1 & HSV2. Although a high degree of antigenic cross-reactivity was apparent, virus-specific antigenic determinants were also readily detected. Phylogenetic analyses supported separation of the baboon isolates and SA8 as distinct viruses. Taken together these results suggest that although closely related to SA8, the baboon viruses represent a distinct simian alpha-herpesvirus which we propose be designated Herpesvirus papio 2.

Glycoprotein B of bovine herpesvirus type 4: its phylogenetic relationship to gB equivalents of the herpesviruses

In order to estimate the phylogenetic relationship of BHV-4 among the herpesviruses, we have cloned and sequenced its glycoprotein B (gB). The 2.6 kb open reading frame codes for a 874 amino acid long protein. The comparison of its deduced amino acid sequence with those of its counterparts in 19 distinct herpesviruses groups BHV-4 into the gamma-herpesvirinae. The calculation of an evolutionary tree emphasized that BHV-4 is more closely related to herpesvirus saimiri (HVS) than to Epstein-Barr virus (EBV). However, in contrast to EBV and HVS, the gB of BHV-4 contains a putative protease cleavage site and 20 potential N-glycosylation sites. The alignment of the amino acid sequences revealed that 10 cysteine and 7 proline residues, as well as the motifs SPF and GQLG, were completely conserved among the 20 investigated gBs.

Simian agent 8 (SA8): morphogenesis and ultrastructure

Electron microscopic studies on the morphogenesis of SA8 in primary rabbit brain cell cultures revealed that in early stages of infection, envelopment of nucleocapsids commonly occurred at the inner nuclear membrane. From the perinuclear space, enveloped virus particles moved into the cisternae of the endoplasmic reticulum (ER) in which they were transported, through the cytoplasm, to the plasma membrane. Alternatively, de-envelopment at the outer nuclear membrane and egress of naked capsids into the cytoplasm were frequently observed. Non-enveloped cytoplasmic capsids were also a consistent feature of cells in late stages of infection, when nuclear membranes became ruptured. In these cases, the envelopment of naked capsids took place by budding either into the cisternae of ER or into cytoplasmic vesicles and vacuoles, in which transport to and exocytosis at the cell membrane occurred. Budding at the cell membrane was rarely found. Capsids of enveloped particles were asymmetrically surrounded by an electron-dense layer which may be identical to the tegument. Because only enveloped cytoplasmic and free virions were tegumented we suggested that the tegumentation must occur during the envelopment (budding) into cytoplasmic vesicles and at the plasma membrane.

Gene mapping and sequence analysis of the unique short region of the simian herpesvirus SA 8 genome

A 10.5 kbp BamHI restriction fragment representing most of the unique short (Us) region of the genome of the simian alpha-herpesvirus SA8 was identified and cloned. Partial sequencing of this DNA fragment identified regions of sequence homology with eight open reading frames (ORFs) of HSV1 and/or HSV2. Sequence and size analysis of subcloned fragments of the SA8 Us region and comparison with homologous HSV Us sequences determined that the number, order, size, and orientation of SA8 Us ORFs are comparable to those of HSV. Based on the location of transcriptional control elements, transcription of SA8 Us genes appears to be organized into 3' co-terminal mRNA sets as in HSV, although the grouping of the gene sets is different. The SA8 US4 (gG) ORF is more similar to that of HSV2 than HSV1, both in size and predicted amino acid sequence. Complete sequences were determined for five SA8 genes which represent homologs of the HSV gD, gE, gI, US5, and US9 genes. The predicted polypeptides encoded by SA8 are similar to the corresponding HSV polypeptides. All SA8 Us genes were more closely related to those of HSV than to related gene homologs of other mammalian alpha-herpesviruses.

Structure, function, and intracellular localization of glycoprotein B of herpesvirus simian agent 8 expressed in insect and mammalian cells

The cloned gene of glycoprotein B (gB) of herpesvirus simian agent 8 (SA 8) was expressed with a baculovirus system in insect cells. Expression of gB was easily detectable over the cellular background by Coomassie staining of electrophoretically separated proteins. Endoglycosidase digestion of immunoprecipitated gB revealed that the gene product is N-glycosylated, but only with unprocessed, endoglycosidase-H sensitive carbohydrates. The lack of terminal glycosylation of gB is consistent with the observation that gB expressed in insect cells has a molecular weight slightly lower than gB synthesized during an SA 8 infection in mammalian cells. The truncated carbohydrates of gB from insect cells have no measurable effect on the tertiary structure of gB. Immunofluorescence studies on mammalian cells expressing gB from a simian virus 40 based vector revealed that the glycoprotein is localized to cytoplasmic membranes, to the plasma membrane and to the nuclear envelope. Cells expressing gB were fused to polykaryons, which shows that gB has cell fusing activity in the absence of any other SA 8 gene product.

Sequence analysis of herpes simplex virus gB gene homologs of two platyrrhine monkey alpha-herpesviruses

Homologs of the herpes simplex virus gB gene were identified in two alpha-herpesviruses of platyrrhine monkeys, Herpesvirus saimiri 1 (HVS 1) and H. ateles 1 (HVA 1). These genes were cloned and sequenced in their entirety. Analysis of the predicted amino acid sequences indicated that the gB glycoproteins of these two viruses are of similar size, have 10 Cys residues and 5 potential N-linked glycosylation sites which align exactly with those in other primate alpha-herpesvirus gB polypeptides, and have a similar distribution of predicted secondary structural features, all of which indicate a conserved structure of the gB polypeptide. Alignment of these two gB sequences with those of four other primate alpha-herpesviruses (SA 8, B virus, HSV 1 and HSV 2) revealed localized regions of extensive sequence divergence as well as highly conserved regions. On comparison of the six primate virus gB sequences, the gBs of the two platyrrhine monkey viruses form a subgroup separate from that of the four catarrhine virus gBs. The degree of relatedness of the HVA 1 and HVS 1 gB sequences to each other was equivalent to the degree of relatedness between the human and the cercopithecine monkey virus gB sequences.

Identification and nucleotide sequence of a gene in feline herpesvirus type 1 homologous to the herpes simplex virus gene encoding the glycoprotein B

The nucleotide sequence of the glycoprotein B (gB) homologous gene of feline herpesvirus type 1 (FHV-1) was determined. The gene was found to be located within a 9.6 kbp SalI fragment by Southern-blot hybridization with a probe derived from the herpes simplex virus type 1 (HSV-1) gB DNA sequence. Furthermore, the predominant portion of the coding sequences was mapped to a 1.9 kbp Hin cII-EcoRI and its flanking 2.7 kbp Eco RI-Eco RI subfragments in the 9.6 kbp SalI fragment. The entire nucleotide sequence revealed that the FHV-1 gB homologous gene is capable of encoding a polypeptide of 948 amino acids. The predicted precursor polypeptide derived from this open reading frame could have a calculated M(r) of 106 kDa in unglycosylated form and contains ten potential N-linked glycosylation sites and a probable internal proteolytic cleavage site. By Northern-blot analysis using portions of the open reading frame as a probe, 3.9 and 3.3 kb RNA transcripts were identified in FHV-1 infected cells. The alignment of the amino acid sequence of the FHV-1 gB homologue with those of 14 other herpesviruses revealed that 10 cysteine residues were completely conserved. Meanwhile, when evolutionary trees were generated among these herpesvirus gB counterparts, the FHV-1 gB homologous nucleotide sequence seems to be closely related to equine herpesvirus type 4 and its amino acid sequence to pseudorabies virus.