Identification and comparative sequence analysis of a gene in equine herpesvirus 1 with homology to the herpes simplex virus glycoprotein D gene

Glycoproteins D of equine herpesvirus type 1 (EHV-1) and EHV-4 determine cellular tropism independently of integrins

Equine herpesvirus type 1 (EHV-1) and EHV-4 are genetically and antigenically very similar, but their pathogenic potentials are strikingly different. The differences in pathogenicity between both viruses seem to be reflected in cellular host range: EHV-1 can readily be propagated in many cell types of multiple species, while EHV-4 entry and replication appear to be restricted mainly to equine cells. The clear difference in cellular tropism may well be associated with differences in the gene products involved in virus entry and/or spread from cell to cell. Here we show that (i) most of the EHV-1 permissive cell lines became resistant to EHV-1 expressing EHV-4 glycoprotein D (gD4) and the opposite was observed for EHV-4 harboring EHV-1 gD (gD1). (ii) The absence of integrins did not inhibit entry into and replication of EHV-1 in CHO-K1 or peripheral blood mononuclear cells (PBMC). Furthermore, integrin-negative K562 cells did not acquire the ability to bind to gD1 when αVβ3 integrin was overexpressed. (iii) PBMC could be infected with similar efficiencies by both EHV-1 and EHV-4 in vitro. (iv) In contrast to results for equine fibroblasts and cells of endothelial or epithelial origin, we were unable to block entry of EHV-1 or EHV-4 into PBMC with antibodies directed against major histocompatibility complex class I (MHC-I), a result that indicates that these viruses utilize a different receptor(s) to infect PBMC. Cumulatively, we provide evidence that efficient EHV-1 and EHV-4 entry is dependent mainly on gD, which can bind to multiple cell surface receptors, and that gD has a defining role with respect to cellular host range of EHV-1 and EHV-4.

Host cell tropism of equine herpesviruses: glycoprotein D of EHV-1 enables EHV-4 to infect a non-permissive cell line

Equine herpesviruses 1 and 4 (EHV-1 and EHV-4) cause equine respiratory disease worldwide. However, only EHV-1 is a cause of abortion and neurological disease, despite the two viruses having all 76 genes in common. In addition EHV-1 has a broader host range in cell culture than EHV-4, as exemplified by the rabbit kidney (RK) cell line that is permissive for EHV-1, but not for EHV-4. Here we describe that when EHV-4 produced in equine cells was inoculated onto RK cells expressing glycoprotein D of EHV-1 (RKgD1), infection developed as clusters of rounded cells, and this infectivity could be passaged in RKgD1 cells. The progeny virus could also infect single RK cells, consistent with EHV-4 acquiring EHV1 gD from the complementing cell line. No such infection was observed for EHV-4 in RK cells expressing EHV-1 glycoprotein C. The results are consistent with gD homologues being major determinants of host cell tropism and raise the possibility that gD may be a factor in the differential pathogenicity of EHV-1 and EHV-4.

Equine herpesvirus 1 glycoprotein D expressed in Pichia pastoris is hyperglycosylated and elicits a protective immune response in the mouse model of EHV-1 disease

Equine herpesvirus 1 glycoprotein D (EHV-1 gD) has been shown in mouse models and in the natural host to have potential as a subunit vaccine, using various expression systems that included Escherichia coli, baculovirus and plasmid DNA. With the aim of producing secreted recombinant protein, we have cloned and expressed EHV-1 gD, lacking its native signal sequence and C-terminal transmembrane region, into the methylotrophic yeast Pichia pastoris. The truncated glycoprotein D (gD) gene was placed under the control of the methanol inducible alcohol oxidase 1 promoter and directed for secretion with the Saccharomyces cerevisiae alpha-factor prepro secretion signal. SDS-PAGE and Western blot analysis of culture supernatant fluid 24 h after induction revealed gD-specific protein products between 40 and 200 kDa. After treatment with PNGase F and Endo H, three predominant bands of 34, 45 and 48 kDa were detected, confirming high mannose N-linked glycosylation of Pichia-expressed gD (Pic-gD). N-terminal sequence analysis of PNGase F-treated affinity-purified protein showed that the native signal cleavage site of gD was being recognised by P. pastoris and the 34 kDa band could be explained by internal proteolytic cleavage effected by a putative Kex2-like protease. Pic-gD, when used in a DNA prime/protein boost inoculation schedule, induced high EHV-1 ELISA and virus neutralizing antibodies and provided protection from challenge infection in BALB/c mice.

Equine herpesvirus 1 (EHV-1) glycoprotein D DNA inoculation in horses with pre-existing EHV-1/EHV-4 antibody

We have shown previously that equine herpesvirus 1 (EHV-1) glycoprotein D (gD) DNA elicited protective immune responses against EHV-1 challenge in murine respiratory and abortion models of EHV-1 disease. In this study, 20 horses, all with pre-existing antibody to EHV-4 and two with pre-existing antibody to EHV-1, were inoculated intramuscularly with three doses each of 50, 200 or 500microg EHV-1 gD DNA or with 500microg vector DNA. In 8 of 15 horses, inoculation with EHV-1 gD DNA led to elevated gD-specific antibody and nine horses exhibited increased virus neutralising (VN) antibody titres compared to those present when first inoculated. A lack of increase in gC-specific antibody during the 66 weeks of the experiment showed that the increase in gD-specific antibodies was not due to a natural infection with either EHV-1 or EHV-4. The increase in EHV-1 gD-specific antibodies was predominantly an IgGa and IgGb antibody response, similar to the isotype profile reported following natural EHV-1 infection.

A prime-boost immunization strategy with DNA and recombinant baculovirus-expressed protein enhances protective immunogenicity of glycoprotein D of equine herpesvirus 1 in naïve and infection-primed mice

The immunogenicity and protective efficacy afforded by intramuscular inoculation of plasmid DNA encoding equine herpesvirus 1 (EHV-1) glycoprotein D (gD) followed by EHV-1 gD expressed by a recombinant baculovirus was assessed in a murine model of EHV-1 respiratory infection. Compared with mice inoculated with DNA or protein only, mice inoculated with the combination of gD DNA and protein had enhanced ELISA and neutralizing antibody titres to EHV-1 and had accelerated clearance of virus from lungs following challenge infection. The enhanced protective effects of this consecutive immunization were also evident in mice which had a previous infection with EHV-1 and had pre-existing antibodies. The T-helper 1 (Th1) type of immune response induced by EHV-1 gD DNA was maintained after the protein boost, despite the gD protein alone appearing to direct a Th2 response.

Potential of DNA-mediated vaccination for equine herpesvirus 1

The potential of DNA-mediated immunisation to protect against equine herpesvirus 1 (EHV-1) disease was assessed in a murine model of EHV-1 respiratory infection. Intramuscular injection with DNA encoding the EHV-1 envelope glycoprotein D (gD) in a mammalian expression vector induced a specific antibody response detectable by two weeks and maintained through 23 weeks post injection. Immune responses were proportional to the dose of DNA and a second injection markedly enhanced the antibody response. EHV-1 gD DNA-injected mice developed neutralising antibodies, and a predominance of IgG2a antibodies after the DNA injection was consistent with the generation of a type 1 helper T-cell (Th1) response. Following intranasal challenge with EHV-1, mice immunised with 50 microg of EHV-1 gD DNA were able to clear virus more rapidly from lung tissue and showed reduced lung pathology in comparison with control mice. The data indicate that DNA-mediated immunisation may be a useful strategy for vaccination against EHV-1.

DNA-mediated immunization with glycoprotein D of equine herpesvirus 1 (EHV-1) in a murine model of EHV-1 respiratory infection

DNA-mediated immunization was assessed in a murine model of equine herpesvirus 1 (EHV-1) respiratory infection. A single intramuscular injection with plasmid DNA encoding EHV-1 glycoprotein D (EHV-1 gD), including its predicted C-terminal membrane anchor sequence, induced a specific antibody response detectable by 2 weeks and maintained through 23 weeks post injection. A second injection at 4 weeks markedly enhanced the antibody response and all EHV-1 gD-injected mice developed neutralizing antibodies. A lymphocyte proliferative response to whole EHV-1 was observed and a predominance of IgG2a antibodies after DNA injection was consistent with the generation of a type 1 helper T-cell (Th1) response. Following intranasal challenge with EHV-1, mice immunized with EHV-1 gD DNA were able to clear virus significantly more rapidly from lung tissue and showed reduced lung pathology, in comparison to control mice.

Genomic organization of the canine herpesvirus US region

Canine herpesvirus (CHV) is an alpha-herpesvirus of limited pathogenicity in healthy adult dogs and infectivity of the virus appears to be largely limited to cells of canine origin. CHV's low virulence and species specificity make it an attractive candidate for a recombinant vaccine vector to protect dogs against a variety of pathogens. As part of the analysis of the CHV genome, the authors determined the complete nucleotide sequence of the CHV US region as well as portions of the flanking inverted repeats. Seven full open reading frames (ORFs) encoding proteins larger than 100 amino acids were identified within, or partially within the CHV US: cUS2, cUS3, cUS4, cUS6, cUS7, cUS8 and cUS9; which are homologs of the herpes simplex virus type-1 US2; protein kinase; gG, gD, gI, gE; and US9 genes, respectively. An eighth ORF was identified in the inverted repeat region, cIR6, a homolog of the equine herpesvirus type-1 IR6 gene. The authors identified and mapped most of the major transcripts for the predicted CHV US ORFs by Northern analysis.

Expression and characterization of equine herpesvirus 1 glycoprotein D in mammalian cell lines

Equine herpesvirus 1 glycoprotein D (EHV-1 gD) expressed constitutively in mammalian cell lines had similar electrophoretic mobility to gD produced in EHV-1 infected cells but lacked a possibly complexed higher molecular weight form seen in the latter. Recombinant gD was N-terminally cleaved at the same site as gD in EHV-1 infected cells and expression was associated with enhanced levels of cell-cell fusion, indicating a role for EHV-1 gD in cell-to-cell transmission of virus.

Evidence for involvement of equine herpesvirus 1 glycoprotein B in cell-cell fusion

Monoclonal antibodies specific for equine herpesvirus 1 (EHV-1) glycoproteins (gB, gD, gp2 and a cleaved translation product of gene 71) were tested for ability to inhibit cell-cell fusion as measured by syncytium formation in EHV-1 infected cell cultures. Syncytium formation was inhibited by a complement-dependent neutralising antibody (7B10) which recognised the large subunit of EHV-1 gB. This indicated that EHV-1 gB, in common with gB homologues of herpes simplex virus and other herpesviruses, plays a role in the cell-cell fusion process.

Expression of membrane-bound and secreted forms of equine herpesvirus 1 glycoprotein D by recombinant baculovirus

Analyses of the synthesis and processing of recombinant full-length glycoprotein D of equine herpesvirus type 1 (EHV-1; gD392) or recombinant truncated gD (gD352) expressed in baculovirus-infected Sf9 cells revealed the following: (1) gD polypeptides encoded by both recombinant baculoviruses react with gD-specific antibodies including peptide-specific antiserum that neutralizes EHV-1 in a plaque reduction assay, (2) both the full-length recombinant gD392 and the truncated gD352 are expressed predominantly as gD species that contain high mannose-type oligosaccharides (55 kDa and 52 kDa, respectively), (3) both the full-length recombinant gD392 and the truncated gD352 are also expressed in lesser amounts as gD species that contain complex-type oligosaccharides (58 kDa and 55 kDa, respectively) as well as the unglycosylated forms of gD (43 kDa and 37 kDa, respectively), (4) flow cytometric analyses of cells expressing gD392 revealed that gD first appears on the cell surface at 24 h post infection; by 60 h, 95% of the cells express high levels of cell surface gD, (5) cells expressing gD352, in contrast to cells expressing gD392, secrete gD into the extracellular medium. This initial demonstration that immunoreactive EHV-1 glycoprotein D can be produced as a secreted polypeptide in the baculovirus system should provide reagents to assess the potential use of gD as a subunit vaccine in an animal model.

Expression of equine herpesvirus 1 glycoprotein D by using a recombinant baculovirus

Glycoprotein D (gD) of equine herpesvirus 1 (EHV-1) was expressed at the surface of insect cells infected by a recombinant baculovirus. EHV-1 gD was detected as multiple forms (56, 52, and 48 kDa) from 18 to 96 h postinfection. Laboratory animals inoculated with the recombinant EHV-1 gD developed neutralizing antibody responses against both EHV-1 and EHV-4.

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.

The DNA sequence of equine herpesvirus-1

The complete DNA sequence was determined of a pathogenic British isolate of equine herpesvirus-1, a respiratory virus which can cause abortion and neurological disease. The genome is 150,223 bp in size, has a base composition of 56.7% G + C, and contains 80 open reading frames likely to encode protein. Since four open reading frames are duplicated in the major inverted repeat, two are probably expressed as a spliced mRNA, and one may contain an internal transcriptional promoter, the genome is considered to contain 76 distinct genes. The genes are arranged collinearly with those in the genomes of the two previously sequenced alphaherpesviruses, varicella-zoster virus, and herpes simplex virus type-1, and comparisons of predicted amino acid sequences allowed the functions of many equine herpesvirus 1 proteins to be assigned.

Characterization of the glycoprotein D gene products of equine herpesvirus 1 using a prokaryotic cell expression vector

The gene encoding equine herpesvirus 1 (equine abortion virus; EHV-1) glycoprotein D was engineered into the prokaryotic vector pEX, and expressed as a beta-galactosidase fusion product, which was recognized by pooled equine sera and anti-EHV-1 rabbit sera. Antibodies raised against the EHV-1 gD fusion product identified strong bands in infected cells at 66 and 68 K and at 138 K in purified virus, thus characterizing the several forms of this major envelope glycoprotein which is an important candidate for inclusion in subunit vaccines.