Equine herpesvirus 1 (EHV-1) glycoprotein M: effect of deletions of transmembrane domains

Construction of a Novel Infectious Clone of Recombinant Herpesvirus of Turkey Fc-126 Expressing VP2 of IBDV

The increased virulence of infectious bursal disease virus (IBDV) is a threat to the chicken industry. The construction of novel herpesvirus of turkey-vectored (HVT) vaccines expressing VP2 of virulent IBDV may be a promising vaccine candidate for controlling this serious disease in chickens. We generated a novel infectious clone of HVT Fc-126 by inserting mini-F sequences in lieu of the glycoprotein C (gC) gene. Based on this bacterial artificial chromosome (BAC), a VP2 expression cassette containing the pMCMV IE promoter and a VP2 sequence from the virulent IBDV NJ09 strain was inserted into the noncoding area between the UL55 and UL56 genes to generate the HVT vector VP2 recombinant, named HVT-VP2-09. The recovered vectored mutant HVT-VP2-09 exhibited higher titers (p = 0.0202 at 36 h) or similar growth kinetics to the parental virus HVT Fc-126 (p = 0.1181 at 48 h and p = 0.1296 at 64 h). The high reactivation ability and strong expression of VP2 by HVT-VP2-09 in chicken embryo fibroblasts (CEFs) were confirmed by indirect immunofluorescence (IFA) and Western blotting. The AGP antibodies against IBDV were detected beginning at 3 weeks post-inoculation (P.I.) of HVT-VP2-09 in 1-day-old SPF chickens. Seven of ten chickens immunized with HVT-VP2-09 were protected post-challenge (P.C.) with the virulent IBDV NJ09 strain. In contrast, all chickens in the challenge control group showed typical IBD lesions in bursals, and eight of ten died P.C. In this study, we demonstrated that (i) a novel HVT BAC with the whole genome of the Fc-126 strain was obtained with the insertion of mini-F sequences in lieu of the gC gene; (ii) HVT-VP2-09 harboring the VP2 expression cassette from virulent IBDV exhibited in vitro growth properties similar to those of the parental HVT virus in CEF cells; and (iii) HVT-VP2-09 can provide efficient protection against the IBDV NJ09 strain.

Identification of non-essential loci within the Meleagrid herpesvirus 1 genome

Background

Meleagrid herpesvirus 1 (MeHV-1) infectious bacterial artificial chromosomes (iBACs) are ideal vectors for the development of recombinant vaccines for the poultry industry. However, the full potential of iBACS as vectors can only be realised after thorough genetic characterisation, including identification of those genetic locations that are non-essential for virus replication. Generally, transposition has proven to be a highly effective strategy for rapid and efficient mutagenesis of iBAC clones. The current study describes the characterisation of 34 MeHV-1 mutants containing transposon insertions within the pMeHV1-C18 iBAC genome.

Methods

Tn5 and MuA transposition methods were used to generate a library of 76 MeHV-1 insertion mutants. The capacity of each mutant to facilitate the recovery of infectious MeHV-1 was determined by the transfection of clone DNA into chicken embryo fibroblasts.

Results

Attempts to recover infectious virus from the modified clones identified 14 genetic locations that were essential for MeHV-1 replication in cell culture. Infectious MeHV-1 was recovered from the remaining 14 intragenic insertion mutants and six intergenic insertion mutants, suggesting that the respective insertion locations are non-essential for MeHV-1 replication in cell culture.

Conclusions

The essential and non-essential designations for those MeHV-1 genes characterised in this study were generally in agreement with previous reports for other herpesviruses homologues. However, the requirement for the mardivirus-specific genes LORF4A and LORF5 are reported for the first time. These findings will help direct future work on the development of recombinant poultry vaccines using MeHV-1 as a vector by identifying potential transgene insertion sites within the viral genome.

Electronic supplementary material

The online version of this article (doi10.1186/s12985-015-0362-9) contains supplementary material, which is available to authorized users.

Construction of a recombinant duck enteritis virus (DEV) expressing hemagglutinin of H5N1 avian influenza virus based on an infectious clone of DEV vaccine strain and evaluation of its efficacy in ducks and chickens

Background

Highly pathogenic avian influenza virus (AIV) subtype H5N1 remains a threat to poultry. Duck enteritis virus (DEV)-vectored vaccines expressing AIV H5N1 hemagglutinin (HA) may be viable AIV and DEV vaccine candidates.

Methods

To facilitate the generation and further improvement of DEV-vectored HA(H5) vaccines, we first constructed an infectious clone of DEV Chinese vaccine strain C-KCE (DEV^C-KCE). Then, we generated a DEV-vectored HA(H5) vaccine (DEV-H5(UL55)) based on the bacterial artificial chromosome (BAC) by inserting a synthesized HA(H5) expression cassette with a pMCMV IE promoter and a consensus HA sequence into the noncoding area between UL55 and LORF11. The immunogenicity and protective efficacy of the resulting recombinant vaccine against DEV and AIV H5N1 were evaluated in both ducks and chickens.

Results

The successful construction of DEV BAC and DEV-H5(UL55) was verified by restriction fragment length polymorphism analysis. Recovered virus from the BAC or mutants showed similar growth kinetics to their parental viruses. The robust expression of HA in chicken embryo fibroblasts infected with the DEV-vectored vaccine was confirmed by indirect immunofluorescence and western blotting analyses. A single dose of 10⁶ TCID₅₀ DEV-vectored vaccine provided 100 % protection against duck viral enteritis in ducks, and the hemagglutination inhibition (HI) antibody titer of AIV H5N1 with a peak of 8.2 log₂ was detected in 3-week-old layer chickens. In contrast, only very weak HI titers were observed in ducks immunized with 10⁷ TCID₅₀ DEV-vectored vaccine. A mortality rate of 60 % (6/10) was observed in 1-week-old specific pathogen free chickens inoculated with 10⁶ TCID₅₀ DEV-vectored vaccine.

Conclusions

We demonstrate the following in this study. (i) The constructed BAC is a whole genome clone of DEV^C-KCE. (ii) The insertion of an HA expression cassette sequence into the noncoding area between UL55 and LORF11 of DEV^C-KCE affects neither the growth kinetics of the virus nor its protection against DEV. (iii) DEV-H5(UL55) can generate a strong humoral immune response in 3-week-old chickens, despite the virulence of this virus observed in 1-week-old chickens. (iv) DEV-H5(UL55) induces a weak HI titer in ducks. An increase in the HI titers induced by DEV-vectored HA(H5) will be required prior to its wide application.

Ubiquitination and degradation of the ORF34 gene product of equine herpesvirus type 1 (EHV-1) at late times of infection

The equine herpesvirus type 1 (EHV-1) open reading frame 34 (ORF34) is predicted to encode a polypeptide of 161 amino acids. We show that an ORF34 deletion mutant exhibited a significant growth defect in equine peripheral blood mononuclear cells taken directly ex vivo during early but not late times of infection. ORF34 protein (pORF34)-specific antibodies specifically reacted with a 28-kDa early polypeptide present in the cytosol of infected cells. From 10h post infection, multiple smaller pORF34-specific protein moieties were detected indicating that expression of a late viral gene product(s) caused pORF34 degradation. Proteasome inhibitors blocked pORF34 degradation as did treatment of infected cells with a ubiquitin-activating enzyme (E1) inhibitor. Finally, kinetic studies showed that pORF34 is modified by addition of multiple copies of ubiquitin. Taken together, our findings suggest that the ubiquitin proteasome pathway is required for pORF34 degradation that may modulate protein activity in the course of infection.

Glycoprotein M is important for the efficient incorporation of glycoprotein H–L into herpes simplex virus type 1 particles

Herpes simplex virus type 1 glycoprotein M (gM) is a type III membrane protein conserved throughout the family Herpesviridae. However, despite this conservation, gM is classed as a non-essential protein in most alphaherpesviruses. Previous data have suggested that gM is involved in secondary envelopment, although how gM functions in this process is unknown. Using transfection-based assays, we have previously shown that gM is able to mediate the internalization and subcellular targeting of other viral envelope proteins, suggesting a possible role for gM in localizing herpesvirus envelope proteins to sites of secondary envelopment. To investigate the role of gM in infected cells, we have now analysed viral envelope protein localization and virion incorporation in cells infected with a gM-deletion virus or its revertant. In the absence of gM expression, we observed a substantial inhibition of glycoprotein H–L (gH–L) internalization from the surface of infected cells. Although deletion of gM does not affect expression of gH and gL, virions assembled in the absence of gM demonstrated significantly reduced levels of gH–L, correlating with defects of the gM-negative virus in entry and cell-to-cell spread. These data suggest an important role of gM in mediating the specific internalization and efficient targeting of gH–L to sites of secondary envelopment in infected cells.

Varicella-zoster virus glycoprotein M homolog is glycosylated, is expressed on the viral envelope, and functions in virus cell-to-cell spread

Although envelope glycoprotein M (gM) is highly conserved among herpesviruses, the varicella-zoster virus (VZV) gM homolog has never been investigated. Here we characterized the VZV gM homolog and analyzed its function in VZV-infected cells. The VZV gM homolog was expressed on virions as a glycoprotein modified with a complex N-linked oligosaccharide and localized mainly to the Golgi apparatus and the trans-Golgi network in infected cells. To analyze its function, a gM deletion mutant was generated using the bacterial artificial chromosome system in Escherichia coli, and the virus was reconstituted in MRC-5 cells. VZV is highly cell associated, and infection proceeds mostly by cell-to-cell spread. Compared with wild-type VZV, the gM deletion mutant showed a 90% reduction in plaque size and 50% of the cell-to-cell spread in MRC-5 cells. The analysis of infected cells by electron microscopy revealed numerous aberrant vacuoles containing electron-dense materials in cells infected with the deletion mutant virus but not in those infected with wild-type virus. However, enveloped immature particles termed L particles were found at the same level on the surfaces of cells infected with either type of virus, indicating that envelopment without a capsid might not be impaired. These results showed that VZV gM is important for efficient cell-to-cell virus spread in cell culture, although it is not essential for virus growth.

The cytoplasmic tail of glycoprotein M (gpUL100) expresses trafficking signals required for human cytomegalovirus assembly and replication

The virion envelope of human cytomegalovirus (HCMV) is complex and consists of an incompletely defined number of glycoproteins. The gM/gN protein complex is the most abundant protein component of the envelope. Studies have indicated that deletion of the viral gene encoding either gM or gN is a lethal mutation. Analysis of the amino acid sequence of gM disclosed a C-terminal acidic cluster of amino acids and a tyrosine-containing trafficking motif, both of which are well-described trafficking/sorting signals in the cellular secretory pathway. To investigate the roles of these signals in the trafficking of the gM/gN complex during virus assembly, we made a series of gM (UL100 open reading frame) mutants in the AD169 strain of HCMV. Mutant viruses that lacked the entire C-terminal cytoplasmic tail of gM were not viable, suggesting that the cytoplasmic tail of gM is essential for virus replication. In addition, the gM mutant protein lacking the cytoplasmic domain exhibited decreased protein stability. Mutant viruses with a deletion of the acidic cluster or alanine substitutions in tyrosine-based motifs were viable but exhibited a replication-impaired phenotype suggestive of a defect in virion assembly. Analysis of these mutant gMs using static immunofluorescence and fluorescence recovery after photobleaching demonstrated delayed kinetics of intracellular localization of the gM/gN protein to the virus assembly compartment compared to the wild-type protein. These data suggest an important role of the glycoprotein gM during virus assembly, particularly in the dynamics of gM trafficking during viral-particle assembly.

Gene splicing and mutagenesis by PCR-driven overlap extension

Extension of overlapping gene segments by PCR is a simple, versatile technique for site-directed mutagenesis and gene splicing. Initial PCRs generate overlapping gene segments that are then used as template DNA for another PCR to create a full-length product. Internal primers generate overlapping, complementary 3' ends on the intermediate segments and introduce nucleotide substitutions, insertions or deletions for site-directed mutagenesis, or for gene splicing, encode the nucleotides found at the junction of adjoining gene segments. Overlapping strands of these intermediate products hybridize at this 3' region in a subsequent PCR and are extended to generate the full-length product amplified by flanking primers that can include restriction enzyme sites for inserting the product into an expression vector for cloning purposes. The highly efficient generation of mutant or chimeric genes by this method can easily be accomplished with standard laboratory reagents in approximately 1 week.

In vitro and in vivo characterization of equine herpesvirus type 1 (EHV-1) mutants devoid of the viral chemokine-binding glycoprotein G (gG)

Glycoprotein G (gG) of equine herpesvirus type 1 (EHV-1), a structural component of virions and secreted from virus-infected cells, was shown to bind to a variety of different chemokines and as such might be involved in immune modulation. Little is known, however, about its role in the replication cycle and infection of EHV-1 in vivo. Here we report on the function of gG in context of virus infection in vitro and in vivo. A gG deletion mutant of pathogenic EHV-1 strain RacL11 (vL11DeltagG) was constructed and analyzed. Deletion of gG had virtually no effect on the growth properties of vL11DeltagG in cell culture when compared to parental virus or a rescuant virus vL11DeltagGR, respectively, and virus titers and plaque formation were unaffected in the absence of the glycoprotein. Similarly, in the murine model of EHV-1 infection, no significant differences in virulence between the gG deletion mutant and RacL11 or vL11DeltagGR were found at high doses of infection. However, infection of mice at lower doses revealed that the gG deletion mutant was able to replicate to higher titers in lungs of infected mice. Additionally, these mice lost significantly more weight than those infected with RacL11 and a more pronounced inflammatory response in lungs was observed. Therefore we concluded that deletion of gG in EHV-1 seems to lead to an exacerbation of respiratory disease in the mouse.

The equine herpesvirus 1 UL20 product interacts with glycoprotein K and promotes egress of mature particles

The aim of the present study was to identify and functionally characterize the equine herpesvirus 1 (EHV-1) UL20 protein (UL20p). Using a specific antiserum, UL20p was shown to be associated with membranes of infected cells, as well as with envelopes of purified virions. By Western blot analysis, UL20p was detected in two main forms exhibiting M(r)s of 25,000 and 75,000. Both moieties did not enter the separating gel after heating of protein samples to 99 degrees C. The slower-migrating form of UL20p contains N-linked carbohydrates, and its presence is dependent of that of other viral proteins. Infection of cells that either constitutively express UL20p or a gK-green fluorescent protein (GFP) fusion protein with various EHV-1 deletion mutants revealed a relatively stable hetero-oligomer containing gK and UL20p with an apparent M(r) of 75,000. As demonstrated by confocal microscopy, UL20p distribution in Rk13 cells changed from a diffuse granular or netlike appearance to a pattern confined to the Golgi network when gK was coexpressed. Analysis of a UL20 deletion mutant of EHV-1 strain RacL11 indicated an involvement of UL20p in cell-to-cell spread, as well as in very late events in virus egress. Based on these and electron microscopic studies we suggest that the EHV-1 UL20 protein might be necessary to avoid fusion of mature virions with membranes of their transport vesicles.

Bovine herpesvirus 1 UL49.5 protein inhibits the transporter associated with antigen processing despite complex formation with glycoprotein M

Bovine herpesvirus 1 (BHV-1) interferes with peptide translocation by the transporter associated with antigen processing (TAP). Recently, the UL49.5 gene product of BHV-1 was identified as the protein responsible for the observed inhibition of TAP. In BHV-1-infected cells and virions, the UL49.5 protein forms a complex with glycoprotein M (gM). Hence, it was investigated whether UL49.5 can combine the interactions with gM and the TAP complex. In cell lines constitutively expressing both UL49.5 and gM, UL49.5 appears to be required for functional processing of gM. Immunofluorescence-confocal laser scanning microscopy demonstrated that both proteins are interdependent for their redistribution from the endoplasmic reticulum to the trans-Golgi network. Remarkably, expression of cloned gM results in the abrogation of the UL49.5-mediated inhibition of TAP and prevents the degradation of the transporter. However, in BHV-1-infected cells, differences in UL49.5 and gM expression kinetics were seen to create a window of opportunity at the early stages of infection, during which time the UL49.5 protein can act on TAP without gM interference. Moreover, in later periods, non-gM-associated UL49.5 can be detected in addition to the UL49.5/gM complex. Thus, it has been deduced that different functions of UL49.5, editing of gM processing and inhibition of TAP, can be combined during BHV-1 infection.

The alpha-TIF (VP16) homologue (ETIF) of equine herpesvirus 1 is essential for secondary envelopment and virus egress

The equine herpesvirus 1 (EHV-1) alpha-trans-inducing factor homologue (ETIF; VP16-E) is a 60-kDa virion component encoded by gene 12 (ORF12) that transactivates the immediate-early gene promoter. Here we report on the function of EHV-1 ETIF in the context of viral infection. An ETIF-null mutant from EHV-1 strain RacL11 (vL11deltaETIF) was constructed and analyzed. After transfection of vL11deltaETIF DNA into RK13 cells, no infectious virus could be reconstituted, and only single infected cells or small foci containing up to eight infected cells were detected. In contrast, after transfection of vL11deltaETIF DNA into a complementing cell line, infectious virus could be recovered, indicating the requirement of ETIF for productive virus infection. The growth defect of vL11deltaETIF could largely be restored by propagation on the complementing cell line, and growth on the complementing cell line resulted in incorporation of ETIF in mature and secreted virions. Low- and high-multiplicity infections of RK13 cells with phenotypically complemented vL11deltaETIF virus resulted in titers of virus progeny similar to those used for infection, suggesting that input ETIF from infection was recycled. Ultrastructural studies of vL11deltaETIF-infected cells demonstrated a marked defect in secondary envelopment at cytoplasmic membranes, resulting in very few enveloped virions in transport vesicles or extracellular space. Taken together, our results demonstrate that ETIF has an essential function in the replication cycle of EHV-1, and its main role appears to be in secondary envelopment.

A glycoprotein I- and glycoprotein E-deficient mutant of infectious laryngotracheitis virus exhibits impaired cell-to-cell spread in cultured cells

In alphaherpesviruses, glycoprotein I (gI) and glycoprotein E (gE) form a heterodimer that functions in cell-to-cell spread of the virus. Generally, alphaherpesvirus mutants that lack these glycoproteins are replication competent in cell culture but show a reduced capacity for cell-to-cell spread and hence smaller plaque sizes. Infectious laryngotracheitis virus (ILTV), or Gallid herpesvirus 1, is an alphaherpesvirus that causes respiratory disease in chickens. The roles of gI and gE in ILTV have not been investigated previously. In this study, a glycoprotein I and glycoprotein E deletion mutant of ILTV (gI/gE-ve ILTV) was generated by replacing the region of the ILTV genome coding for the adjacent gI and gE genes with the gene for enhanced green fluorescent protein (eGFP). This gI/E-ve ILTV was readily propagated in cell culture in the presence of wildtype ILTV (wt ILTV). However, in the absence of wt ILTV the propagation of gI/gE-ve ILTV was severely impaired. Infection of permissive cell cultures with gI/gE-ve ILTV failed to produce plaques but single infected cells could be identified by fluorescence microscopy. This suggests that gI/gE has a more significant role in the cell-to-cell spread of ILTV in vitro than in many other alphaherpesviruses.

Varicelloviruses avoid T cell recognition by UL49.5-mediated inactivation of the transporter associated with antigen processing

Detection and elimination of virus-infected cells by cytotoxic T lymphocytes depends on recognition of virus-derived peptides presented by MHC class I molecules. A critical step in this process is the translocation of peptides from the cytoplasm into the endoplasmic reticulum by the transporter associated with antigen processing (TAP). Here, we identified the bovine herpesvirus 1-encoded UL49.5 protein as a potent inhibitor of TAP. The expression of UL49.5 results in down-regulation of MHC class I molecules at the cell surface and inhibits detection and lysis of the cells by cytotoxic T lymphocytes. UL49.5 homologs encoded by two other varicelloviruses, pseudorabies-virus and equine herpesvirus 1, also block TAP. Homologs of UL49.5 are widely present in herpesviruses, acting as interaction partners for glycoprotein M, but in several varicelloviruses UL49.5 has uniquely evolved additional functions that mediate its participation in TAP inhibition. Inactivation of TAP by UL49.5 involves two events: inhibition of peptide transport through a conformational arrest of the transporter and degradation of TAP by proteasomes. UL49.5 is degraded along with TAP via a reaction that requires the cytoplasmic tail of UL49.5. Thus, UL49.5 represents a unique immune evasion protein that inactivates TAP through a unique two-tiered process.

Alphaherpesvirus glycoprotein M causes the relocalization of plasma membrane proteins

Herpesvirus glycoprotein M (gM) is a multiple-spanning integral membrane protein found within the envelope of mature herpesviruses and is conserved throughout the Herpesviridae. gM is defined as a non-essential glycoprotein in alphaherpesviruses and has been proposed as playing a role in controlling final envelopment in a late secretory-pathway compartment such as the trans-Golgi network (TGN). Additionally, gM proteins have been shown to inhibit cell-cell fusion in transfection-based assays by an as yet unclear mechanism. Here, the effect of pseudorabies virus (PRV) gM and the herpes simplex virus type 1 (HSV-1) gM/UL49A complex on the fusion events caused by the HSV-1 glycoproteins gB, gD, gH and gL was investigated. Fusion of cells expressing HSV-1 gB, gD, gH and gL was efficiently inhibited by both PRV gM and HSV-1 gM/UL49A. Furthermore, expression of PRV gM or HSV-1 gM/UL49A, which are themselves localized to the TGN, caused both gD and gH/L to be relocalized from the plasma membrane to a juxtanuclear compartment, suggesting that fusion inhibition is caused by the removal of 'fusion' proteins from the cell surface. The ability of gM to cause the relocalization of plasma membrane proteins was not restricted to HSV-1 glycoproteins, as other viral and non-viral proteins were also affected. These data suggest that herpesvirus gM (gM/N) can alter the membrane trafficking itineraries of a broad range of proteins and this may have multiple functions.

Equine herpesvirus type 1 (EHV-1) glycoprotein K is required for efficient cell-to-cell spread and virus egress

The function of the equine herpesvirus type 1 (EHV-1) glycoprotein K (gK) homologue was investigated. Deletion of 88% of the UL53-homologous open reading frame in EHV-1 strain RacH resulted in a severe growth defect of the gK-negative virus (HDeltagK) as reflected by a significant decrease in the production of infectious virus progeny on RK13 cells. The HDeltagK virus induced only minute plaques, was unable to form syncytia, and its penetration efficiency into RK13 cells was reduced by approximately 40%. To further analyze gK function and intracellular trafficking, gK of strain RacH was replaced by a C-terminally truncated gK-green fluorescent protein fusion protein (gK-GFP). The generated recombinant virus was shown to replicate well on non-complementing cells, and virus penetration and syncytium formation were comparable to parental RacH. A reduction in plaque size and slightly decreased intra- and extracellular virus titers, however, were observed. The gK-GFP fusion protein was expressed with early-late kinetics, and multiple forms of the protein exhibiting M(r)s between 50,000 and 85,000 were detected by Western blot analysis. The various gK-GFP forms were shown to be N-glycosylated, associated with membranes of the Golgi apparatus, and were incorporated into extracellular virions. Complete processing of gK-GFP was only observed within the context of viral infection. From the results, we concluded that EHV-1 gK is required for efficient virus growth in vitro and that the carboxy-terminal amino acids are not required for its function, because the gK-GFP fusion protein was able to complement for EHV-1 growth in the absence of authentic gK.

The truncated form of glycoprotein gp2 of equine herpesvirus 1 (EHV-1) vaccine strain KyA is not functionally equivalent to full-length gp2 encoded by EHV-1 wild-type strain RacL11

Most equine herpesvirus 1 (EHV-1) strains, including the naturally occurring virulent RacL11 isolate, encode a large glycoprotein, gp2 (250 kDa), which is expressed from gene 71. Besides other alterations in the viral genome, the avirulent strain KyA harbors an in-frame deletion of 1,242 nucleotides in gene 71. To examine the contributions of gp2 variation to virus growth and virulence, mutant RacL11 and KyA viruses expressing full-length or truncated gp2 were generated. Western blot analyses demonstrated expression of a 250-kDa gp2 in cells infected with RacL11 virus or a mutant KyA virus harboring full-length gene 71, whereas a 75- to 80-kDa gp2 was detected in cells infected with KyA or mutant RacL11 virus expressing KyA gp2. The RacL11 gp2 precursor of 250 kDa in size and its truncated KyA counterpart of 80 kDa, as well as the 42-kDa carboxy-terminal gp2 subunit, were incorporated into virus particles. Absence of gp2 in RacL11 resulted in a 6-fold reduction of extracellular virus titers and a 13% reduction of plaque diameters, whereas gp2-negative KyA exhibited a 55% reduction in plaque diameter and a 51-fold decrease in extracellular virus titers. The massive growth defects of gp2-negative KyA could be restored by reinsertion of the truncated but not the full-length gp2 gene. The virulence of the generated gp2 mutant viruses was compared to the virulence of KyA and RacL11 in a murine infection model. RacL11 lacking gp2 was apathogenic for BALB/c mice, and insertion of the truncated KyA gp2 gene into RacL11 was unable to restore virulence. Similarly, replacement in the KyA genome of the truncated with the full-length RacL11 gene 71 did not result in the generation of virulent virus. From the results we conclude that full-length and truncated EHV-1 gp2 are not functionally equivalent and cannot compensate for the action of their homologues in allogeneic virus backgrounds.

The equine herpesvirus 1 UL11 gene product localizes to the trans-golgi network and is involved in cell-to-cell spread

Experiments were conducted to identify and characterize the equine herpesvirus type 1 (EHV-1) UL11 homologous protein. At early-late times after EHV-1 infection of Rk13 cells several proteins at an M(r) of 8000 to 12,000 were detected using a UL11 protein-specific antiserum. Particularly, an M(r) of 11,000 protein was found abundantly in purified virions and could be assigned to the tegument fraction. As demonstrated by confocal laser scanning microscopy, UL11 reactivity localized predominantly to the trans-Golgi network of infected cells, but was also noted at the plasma membrane, specifically of transfected cells. Deletion of UL11 sequences in EHV-1 vaccine strain RacH (Hdelta11) and in the virulent isolate RacL22 (Ldelta11) resulted in viruses that were able to replicate on noncomplementing cells. It was shown in one-step growth kinetics on Rk13 cells that the reduction of intracellular and of extracellular virus titers caused by the absence of UL11 expression in either virus was somewhat variable, but approximately 10- to 20-fold. In contrast, a marked influence on the plaque phenotype was noted, as mean maximal diameters of plaques were reduced to 23.2% (RacL22) or 34.7% (RacH) of parental virus plaques and as an effect on the ability of RacH to cause syncytia upon infection was noted. It was therefore concluded that the EHV-1 UL11 product is not essential for virus replication in Rk13 cells but is involved in cell-to-cell spread.

The products of the UL10 (gM) and the UL49.5 genes of Marek's disease virus serotype 1 are essential for virus growth in cultured cells

The role of the products of the UL10 and the UL49.5 homologous genes of Marek's disease virus serotype 1 (MDV-1) in virus replication was investigated. Deletion of either open reading frame in an infectious bacterial artificial chromosome clone (BAC20) of MDV-1 resulted in progeny viruses that were unable to spread from cell to cell. After transfection of UL10- or UL49.5-negative BAC20 DNA into chicken or quail cells, only single infected cells were observed by indirect immunofluorescence analysis. In contrast, plaque formation was restored when mutant BAC DNAs were co-transfected with the corresponding expression plasmid encoding either the UL10-encoded gM or the UL49.5 gene product. These data demonstrate that gM and its putative complex partner, the UL49.5 homologous protein, are essential for MDV-1 growth in cultured cells. Thus, MDV-1 represents the first example of a member of the family Herpesviridae for which the highly conserved membrane proteins are indispensable for cell-to-cell spread.

The gene 10 (UL49.5) product of equine herpesvirus 1 is necessary and sufficient for functional processing of glycoprotein M

The functional cooperation of equine herpesvirus 1 (EHV-1) glycoprotein M (gM) and the gene 10 (UL49.5) product was analyzed. Transient-transfection experiments using gM and UL49.5 expression plasmids as well as RK13 cell lines constitutively expressing UL49.5 (RK49.5) or gM (RKgM) demonstrated that the endo-beta-N-acetylglucosaminidase H (endo H)-resistant mature form of gM was detectable only after coexpression of the two proteins. Deletion of the EHV-1 UL49.5-homologous gene 10 in strain KyA resulted in a small-plaque phenotype and up to 190-fold-reduced virus titers. The growth defects of the mutant KyA Delta 49.5 virus, which were very similar to those of a gM-negative KyA virus, could be completely compensated for by growth of the mutant virus on RK49.5 cells or by repairing the deletion of gene 10 in the revertant virus KyA Delta 49.5R. Analysis of cells infected with the UL49.5-negative EHV-1 demonstrated that gM was not transported to the trans-Golgi network in the absence of the UL49.5 product. In contrast, gM was efficiently transported and processed to the endo H-resistant mature form in KyA Delta 49.5-infected RK49.5 cells. Furthermore, radioimmunoprecipitation experiments demonstrated that gM maturation was observed only if a 10,000-M(r) protein was coprecipitated with gM in KyA- or KyA Delta 49.5R-infected cells or virions. This protein was absent in cells infected with Ky Delta 49.5 or KyA Delta gM, suggesting that it was the EHV-1 UL49.5 product. Taken together, our results demonstrate that the expression of the EHV-1 UL49.5 product is necessary and sufficient for gM processing and that it is required for efficient virus replication.

Equine herpesvirus type 1 devoid of gM and gp2 is severely impaired in virus egress but not direct cell-to-cell spread

Experiments were conducted to analyze the effects of a simultaneous deletion of glycoprotein M (gM) and glycoprotein 2 (gp2) of equine herpesvirus type 1 (EHV-1). EHV-1 strain RacH was cloned as a bacterial artificial chromosome (pRacH) by homologous recombination of a mini F plasmid into the unique short region of the genome, thereby deleting gene 71 encoding gp2. Upon transfection of the pRacH DNA into rabbit kidney RK13 cells, virus plaques were visible from day 1 after transfection. The mutant RacH virus (H Delta gp2) reconstituted from pRacH lacked gene 71 and did not express gp2 as assayed by indirect immunofluorescence analysis using gp2-specific monoclonal antibodies. The H Delta gp2 virus exhibited 10-fold reduced extracellular titers and an approximately 10% reduction in mean plaque diameters when compared to parental or gp2-revertant virus. The gM open reading frame was deleted from pRacH by recE/T mediated mutagenesis in Escherichia coli. The gM-gp2 double negative virus mutant (H Delta gp2gM) did not express either of the deleted glycoproteins as demonstrated by indirect immunofluorescence analysis. The H Delta gp2gM virus exhibited a 200-fold reduction of end-point extracellular titers when compared to parental RacH virus, which could not be compensated for by growth of the mutant virus on gM-expressing cells. After restoration of the gM open reading frame, however, growth of the mutant virus was comparable to the H Delta gp2 virus. Plaque diameters of the gM-gp2 double-negative mutant were reduced by only 16% when compared to that of parental RacH virus. From the results it was concluded that the simultaneous absence of gM and gp2 had an additive effect on egress but not secondary envelopment or cell-to-cell spread of EHV-1.

Cloning of the genomes of equine herpesvirus type 1 (EHV-1) strains KyA and racL11 as bacterial artificial chromosomes (BAC)

The genome of equine herpesvirus type 1 (EHV-1) strain RacL11, a highly virulent isolate obtained from an aborted foal, and that of the modified live vaccine strain KyA, were cloned as bacterial artificial chromosomes (BAC) in Eseherichia coli. Mini F plasmid sequences were inserted into the viral genomes by homologous recombination instead of the gene 71 (EUS4) open reading frame after co-transfection of viral DNA and recombinant plasmid pdelta71-pHA2 into RK13 cells. After isolation of recombinant viruses by three rounds of plaque purification, viral DNA was isolated from RK13 cells infected with RacL11 or KyA virus mutants expressing the green fluorescent protein (GFP), and electroporated into Escherichia coli DH10B cells. Several bacterial colonies were shown to contain high-molecular weight BAC DNA with a restriction enzyme fragment pattern indicative of the presence of full-length RacL11 or KyA genomes. Two selected BAC clones were analysed by restriction enzyme analysis and Southern blotting, and were eventually termed pRacLI I and pKyA. respectively. Upon transfection of pRacL11 or pKyA DNA into RK13 cells, GFP-expressing fluorescing virus plaques could be identified from day 1 after transfection. Infectivity after transfection of pRacL11 or pKyA could be readily propagated on RK13 or equine cells, indicating that infectious full-length DNA clones of strains RacL11 and KyA were successfully cloned in Escherichia coli as BACs. The glycoprotein 2-negative progeny reconstituted from pRacL11 and pKyA (L11deltagp2 and KyAdeltagp2) exhibited different growth properties. Whereas both L11deltagp2 and KyAdeltagp2 extracellular titres were reduced by 15- to 32-fold, plaque diameters were only markedly (50%) reduced in the case of KyAdeltagp2.

Glycoproteins E and I of Marek's disease virus serotype 1 are essential for virus growth in cultured cells

The role of glycoprotein E (gE) and gI of Marek's disease virus serotype 1 (MDV-1) for growth in cultured cells was investigated. MDV-1 mutants lacking either gE (20DeltagE), gI (20DeltagI), or both gE and gI (20DeltagEI) were constructed by recE/T-mediated mutagenesis of a recently established infectious bacterial artificial chromosome (BAC) clone of MDV-1 (D. Schumacher, B. K. Tischer, W. Fuchs, and N. Osterrieder, J. Virol. 74:11088-11098, 2000). Deletion of either gE or gI, which form a complex in MDV-1-infected cells, resulted in the production of virus progeny that were unable to spread from cell to cell in either chicken embryo fibroblasts or quail muscle cells. This was reflected by the absence of virus plaques and the detection of only single infected cells after transfection, even after coseeding of transfected cells with uninfected cells. In contrast, growth of rescuant viruses, in which the deleted glycoprotein genes were reinserted by homologous recombination, was indistinguishable from that of parental BAC20 virus. In addition, the 20DeltagE mutant virus was able to spread from cell to cell when cotransfected into chicken embryo fibroblasts with an expression plasmid encoding MDV-1 gE, and the 20DeltagI mutant virus exhibited cell-to-cell spread capability after cotransfection with a gI expression plasmid. The 20DeltagEI mutant virus, however, was not able to spread in the presence of either a gE or gI expression plasmid, and only single infected cells were detected by indirect immunofluorescence. The results reported here demonstrate for the first time that both gE and gI are absolutely essential for cell-to-cell spread of a member of the Alphaherpesvirinae.