Glycoproteins D of Equine Herpesvirus Type 1 (EHV-1) and EHV-4 Determine Cellular Tropism Independently of Integrins (2024)

Viruses.

EHV-1 strain L11Δgp2 (59) was reconstituted after transfection of 2 μg of bacterial artificial chromosome (BAC) DNA into rabbit kidney (RK13) cells, using Lipofectamine 2000 (Invitrogen). Recombinant WA79 was derived from an EHV-4 infectious BAC clone that was generated by the insertion of a loxP-flanked BAC vector into the intergenic region between genes 58 and 59 (7).

Cells.

Fetal horse kidney (FHK), kindly provided by V. Svansson, University of Iceland, human embryonic kidney (cell line 293), RK13, HeLa, Vero, feline kidney (CrFK), and Madin-Darby bovine kidney (MDBK) cells were propagated in Dulbecco's modified Eagle's medium (DMEM) (Biochrom) supplemented with 10% fetal bovine serum (FBS) (Biochrom), 100 U/ml penicillin, and 100 μg/ml streptomycin (1% penicillin-streptomycin). Equine dermal (NBL-6) and CHO-K1 cells were grown in Iscove's modified Dulbecco's medium (IMDM) (Invitrogen) supplemented with 10% FBS. CHO-A, CHO-B, and CHO-C were a kind gift from P. Spear, Northwestern University, Chicago, IL, and express the HveA, HveB, and HveC receptors, respectively. They were grown in IMDM supplemented with 10% FBS and 500 μg/ml G418 (Invitrogen). The human erythroleukemia cell lines K562 and K562_αVβ3, a kind gift from Scott D. Blystone, SUNY Upstate Medical University, NY, were grown in IMDM and IMDM supplemented with 750 μg/ml G418. PBMC were isolated from heparinized blood collected from healthy horses by density gradient centrifugation over Histopaque 1077 (Sigma), following the manufacturer's instructions. After two washing steps, cells were resuspended in RPMI 1640 supplemented with 10% FBS, 0.3 mg/ml glutamine, 100 μg/ml kanamycin, nonessential amino acids (Biochrom), and 1% penicillin-streptomycin. To isolate primary equine vascular endothelial cells (EC) from carotid arteries of healthy horses, collagenase (Sigma) treatment was used as described previously (43). To separate EC from contaminating cells, cultures were labeled with 10 μg/ml of the low-density lipoprotein 1,1′-dioctadecyl-3,3,3′,3′-tetramethylindo-carbocyanide perchlorate (DiI-Ac-LDL) (Biomedical Technologies Inc.) for 4 h at 37°C. After trypsinization, EC were washed, resuspended in medium, and sorted by fluorescence-activated cell sorting (FACS) using a FACS Aria high-speed flow cytometer (Becton Dickinson). The DiI-Ac-LDL fluorescent signal was detected with a 585/45 BP filter, and the brightest 22% of cells were gated and sorted at 65 lb/in² into medium.

Antibodies.

The anti-human CD51/61 monoclonal antibody (MAb), an αVβ3 integrin antagonist, and MAb P1F6, an αVβ5 integrin antagonist, were obtained from Biolegend and Millipore, respectively. An unrelated mouse immunoglobulin G (IgG) isotype control was obtained from Cell Signaling Technologies. EHV-4 polyclonal anti-gD antibodies were kindly provided by Ken Maeda, Yamaguchi University, Japan. Anti-EHV-1 gD 19-mer polyclonal antibodies were kindly provided by Dennis O'Callaghan, Louisiana State University Health Sciences Center, Shreveport, LA. EHV-1 gB MAb 3F6 was described before (5, 48). Anti-equine MHC class I monoclonal antibodies PT85A (isotype IgG2a) and H58A (isotype IgG2a) were obtained from VMRD.

Plasmids.

Transfer plasmids encoding either EHV-1 or EHV-4 gD with a kanamycin resistance (Kan^r) gene were constructed. All of the primers are listed in Table 1. The EHV-1 and EHV-4 gD genes were amplified by PCR using primers P1 and P2 or P3 and P4, respectively (Table 1). The PCR products were digested with the appropriate restriction enzymes and inserted into vector pcDNA3 (Invitrogen), resulting in recombinant plasmids pcDNAgD1 and pcDNAgD4. To construct pcDNAgD1-Kan and pcDNAgD1-4-Kan, the Kan^r gene was amplified from plasmid pEPkan-S by PCR using primers P5, P6, P7, and P8 (Table 1). The PCR products were digested with the appropriate restriction enzymes and inserted into pcDNAgD1 or pcDNAgD4. Correct amplification and insertion were confirmed by nucleotide sequencing (Starseq).

BAC mutagenesis. (i) pL11ΔgD1 and pYOΔgD4.

EHV-1 strain RacL11 cloned as a BAC (pL11) contains the enhanced green fluorescent protein (EGFP) gene instead of the nonessential gene 71 (59). The EHV-4 BAC clone pYO03 was generated by the insertion of mini F plasmid sequences flanked by loxP sites into the intergenic region between genes 58 and 59 (7). Both pL11 and pYO03 BACs were maintained in Escherichia coli GS1783 cells (a kind gift from Greg Smith, Northwestern University, Chicago, IL). Viruses reconstituted from pL11 and pYO03 were used in this study to make use of EGFP expression for rapid identification of infected cells. Deletion of gD1 and gD4 was done by two-step Red recombination as described before (67). Briefly, PCR primers, P9, P10, P11, and P12 (Table 1) were selected such that the recombination arms of 50 nucleotides (nt) enabled the substitution of nt 1 to 1209 of the EHV-1 or EHV-4 gD gene by the Kan^r gene. PCR products were digested with DpnI in order to remove residual template DNA. The transfer fragments were then electroporated into GS1783 containing the BACs. Kanamycin-resistant colonies were purified and screened by PCR and restriction fragment length polymorphism (RFLP) to detect E. coli harboring mutant clones. Positive clones were subjected to a second round of Red recombination to obtain the final constructs, pL11ΔgD1 and pYOΔgD4, after excision of the Kan^r gene (Fig. 1).

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Fig 1

Schematic diagram of the procedures used to construct mutant genomes. (a and b) Schematic representation of the genomic organization and the BamHI restriction map of the pL11 EHV-1 BAC and the pYO03 EHV-4 BAC. The two unique regions (U_L and U_S), the terminal and internal repeat sequences (TR_S and IR_S), and the inserted mini-F cassettes are shown. The PCR cassette containing the Kan^r gene was first inserted into the gD locus of pL11 or pYO03 using Red recombination. The transfer cassettes (gD4Kan and gD1Kan) were then electroporated into E. coli harboring pL11ΔgD1 or pYOΔgD4, respectively. Recombinant pL11gD4 and pYOgD1 were ultimately transfected into eukaryotic cells to reconstitute infectious viruses (EHV-1gD4 and EHV-4gD1). (c) The Kan^r gene was inserted at nucleotide position 456 of the gD4 gene in pYO03. A point mutation targeting the RSN motif present in gD4 was engineered by converting nucleotide 454 of gD from an adenine to a guanine, changing the asparagine into aspartic acid (gD^152D).

(ii) pL11gD4 and pYOgD1.

The 2,209-bp DNA transfer fragments, gD4Kan and gD1Kan, were amplified by PCR using pcDNAgD4Kan or pcDNAgD1Kan as templates and primers P13, P14, P15, and P16 (Table 1). The resulting PCR products were digested with DpnI and electroporated into GS1783 harboring the BACs. Electroporated cells were selected on LB agar plates containing 25 μg/ml chloramphenicol and kanamycin. Kanamycin-resistant colonies were purified and screened by PCR and RFLP to detect E. coli harboring recombinant pL11gD4Kan and pYOgD1Kan. Positive clones were subjected to a second round of Red recombination to obtain the final constructs, pL11gD4 and pYOgD1, after excision of the Kan^r gene (Fig. 1).

(iii) pL11gD1R and pYOgD4R.

To reintroduce the authentic gD sequences in the respective BACs, we first deleted the gDs from both pL11gD4 and pYOgD1 and inserted back gD1 and gD4 into the corresponding BACs (Fig. 1).

(iv) pYOgD4^152D.

A point mutation targeting the RSN motif present in EHV-4 gD was engineered by converting nucleotide 454 of gD from an adenine to a guanine, changing the asparagine into aspartic acid (gD^152D), by employing two-step Red-mediated recombination. Primers P17, P18, P19, and P20 (Table 1) used for mutant (gD^152D) and revertant (gD^152N) were described earlier.

The respective genotypes of all the mutants and revertants were confirmed by PCR, RFLP, and nucleotide sequencing using primers P21 to P27 (Table 1).

Generation of recombinant viruses.

The EHV-1 mutants and revertants, EHV-1gD4 and EHV-1R, were reconstituted after transfection of 2 μg of purified BAC DNA into RK13 cells using Lipofectamine 2000 (Invitrogen) as described previously (59). For EHV-4 mutants and revertants, EHV-4gD1, EHV-4^152D, and EHV-4R, the viruses were reconstituted by transfection of purified DNA into 293 cells as described earlier (7, 8). Three days later, the supernatant and cells were collected and used to infect confluent NBL-6 cells.

Western blotting.

For Western blot analyses, pellets of infected FHK cells were resuspended in radioimmunoprecipitation assay buffer (50 mM Tris [pH 7.4], 1% Triton X-100, 0.25% Na-deoxycholate, 150 mM sodium chloride, 1 mM EDTA) with a protease inhibitor co*cktail (Roche). Sample buffer (1 M Tris-HCl [pH 6.8], 0.8% sodium dodecyl sulfate [SDS], 0.4% glycerol, 0.15% β-mercaptoethanol, 0.004% bromophenol blue) was added, the mixture was heated at 95°C for 5 min, and proteins were separated by 10% SDS-polyacrylamide gel electrophoresis (PAGE) exactly as described before (70). Expression of gD was detected with EHV-4 polyclonal anti-gD antibodies (1/500 dilution) and anti-EHV-1 gD 19-mer polyclonal antibodies (1/1,000 dilution). Goat anti-mouse or anti-rabbit IgG coupled to peroxidase (Southern Biotech, Birmingham, AL) at 1/10,000 dilutions were used as secondary antibodies. Reactive bands were visualized by enhanced chemiluminescence (ECL Plus; Amersham).

Virus growth assays.

To determine virus replication, single-step growth kinetics and plaque areas were determined as described before (9, 70). Briefly, confluent NBL-6 cells were infected at a multiplicity of infection (MOI) of 1 or 0.01. After 1 h of adsorption, cells were washed and overlaid with DMEM containing 10% FBS. Infected cultures were harvested at the indicated times postinfection (p.i.) and stored at −80°C. Viral titers were determined by plating onto NBL-6 cells. Three days p.i., cells were fixed with absolute ethanol and stained with 0.3% crystal violet, and plaques were counted. Growth kinetics were determined in three independent experiments. For plaque size measurements, NBL-6 cells were seeded in six-well plates, infected with viruses at an MOI of 0.001, and overlaid at 1 h p.i. with DMEM containing 0.5% methylcellulose (Sigma). At 3 days p.i., 50 fluorescent plaques were photographed for each virus and average plaque areas were measured by using ImageJ software vl.32j (http://rsb.info.nih.gov/ij/). Values were calculated and compared to those for plaque areas induced by parental viruses, which were set to 100%. Average percentages of plaque areas and standard deviations (SD) were determined from three independent experiments.

Virus infection assay.

A cluster assay, similar to a plaque assay, for measuring the infectivity (efficiency of plating) of recombinant viruses was developed. Cell monolayers were inoculated with different viruses at an MOI of 0.1. After 1 h of adsorption, cells were washed and overlaid with DMEM containing 10% FBS and the infection was allowed to proceed for another 48 h. Infected cells were inspected by using an inverted fluorescence microscope (Zeiss Axiovert 100) and photographed with an Axiocam charge-coupled-device (CCD) camera (Zeiss) for each virus.

Flow cytometry.

To evaluate integrin expression, CHO-K1, Vero, NBL-6, FHK, K562, K562_αVβ3, and PBMC cells were incubated with 2 μg/ml of MAbs targeting αVβ3 (CD51/61) or αVβ5 (P1F6) for 1 h at room temperature (RT). An isotype control mouse IgG (2 μg/ml) was also included. After two washes with phosphate-buffered saline (PBS), cells were incubated with Alexa Fluor 488-labeled goat anti-mouse IgG (1/500 dilution) for 1 h at RT. After a final wash, 10,000 cells were analyzed with a FACSCalibur flow cytometer (BD Biosciences), and the intensity of fluorescence was analyzed using FlowJo software (Treestar).

For infection experiments, a total of 2 × 10⁵ CHO-K1, Vero, NBL-6, and FHK cells as well as K562, K562_αVβ3, and PBMC suspensions were seeded in 24-well plates. Before the addition of antibodies, maintenance media were removed and all cells were washed with PBS containing 2% FBS. Cells were then incubated with integrin and/or anti-equine MHC-I antibody H58A (20 μg/ml) at 4°C for 1 h. After being washed, cells were infected with the parental, mutant, or revertant virus at an MOI of 1 or 5. CHO-K1, Vero, NBL-6, and FHK cells were trypsinized at 24 h p.i. and washed twice, while K562, K562_αVβ3, and PBMC were washed twice at 48 h p.i. After centrifugation, cells were resuspended in PBS supplemented with propidium iodide (PI) (Molecular Probes) at a final concentration of 10 μg/ml, and the intensity of fluorescence of 10,000 cells was analyzed to determine the percentage of infected cells.

Analysis of virus binding to cells by flow cytometry.

K562, K562_αVβ3, and NBL-6 cells were washed with PBS containing 2% FBS. The cells were incubated with either rL11Δgp2 or rWA79 at an MOI of 5 for 2 h at 4°C. In other experiments, CHO-K1 cells were incubated with parental and mutant viruses for 2 h at 4°C. Cells were then washed twice and incubated with either EHV-1 gB MAb 3F6 or EHV-4 polyclonal anti-gD antibodies. The binding levels of both viruses were measured by using Alexa Fluor 488-labeled goat anti-mouse IgG (1/500 dilution) and quantified using flow cytometry. The effect of anti-integrin αVβ3 antibodies on virus binding was also studied by incubating cells with the anti-integrin αVβ3 MAb CD51/61 for 1 h before virus infection. Binding of both viruses was detected and quantified as described above. As a control, cells were also stained with the primary and secondary antibodies without prior virus binding.

Statistical analysis.

Using Microsoft Excel, Student's t test for paired data was used to test for statistically significant differences. Data given are means, and bars show standard deviations.

Generation and genotypic characterization of mutant viruses.

Two-step Red-mediated recombination was used to exchange the gD-encoding sequences between EHV-1 and EHV-4 (Fig. 1). The newly generated viruses included an EHV-1 mutant harboring gD4 in place of authentic gD1 and a corresponding EHV-4 carrying gD1 (Fig. 1a and b). Furthermore, EHV-4 with a point mutation in the RSN motif present in gD was engineered by changing the asparagine into aspartic acid (gD^152D) (Fig. 1c). The appropriate revertant viruses, in which the original sequences were restored, were also generated using the same methodology. PCR analysis showed that all constructs were of the correct size and position (Fig. 2a), and further analysis of the mutant clones was done using RFLP (Fig. 2b) and nucleotide sequencing (data not shown).

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Fig 2

Identification of recombinant viruses by PCR, RFLP, and Western blotting. (a) PCR analysis of EHV-1 (left panel) and EHV-4 (middle and right panels) mutants to detect genes 72 encoding gD. PCR products from parental and mutant viruses were electrophoresed in a 1% agarose gel. A molecular weight marker (lane M) was included. (b) Purified DNAs from EHV-4, EHV-4ΔgD, EHV-4gD1, and EHV-4gD^152D (left panel) as well as EHV-1, EHV-1ΔgD, and EHV-1gD4 (right panel) were digested with ScaI, ClaI, and KpnI. Fragments in the mutants that appeared as a consequence of the deletion or insertion of gD sequence are marked by arrows. (c and d) Parental and mutant viruses express gD at similar levels. FHK cells were infected with parental, mutant, or revertant viruses at an MOI of 0.1. For Western blot analysis, cell lysates were prepared and proteins separated by SDS-10% PAGE before transfer to a polyvinylidene difluoride (PVDF) membrane. The blots were incubated with anti-EHV-1 gD 19-mer (1/1,000 dilution) (c) or EHV-4 polyclonal anti-gD antibodies (1/500 dilution) (d) followed by anti-rabbit and anti-mouse IgG peroxidase antibodies (1/7,500 dilution), respectively. Cell lysates from noninfected FHK cells were included as a control.

To determine whether gD1 and gD4 were expressed properly by the mutant viruses, FHK cells were infected with the mutant or revertant viruses and the cell lysates were analyzed by Western blotting. All viruses expressed gD, approximately 55 kDa in size, at levels comparable to those of the parental viruses (Fig. 2c and d). The experiments showed that all mutant viruses expressed either gD1 or gD4 according to the genetic manipulation and as predicted.

Virus growth analyses.

Three independent growth kinetic experiments and plaque size measurements were performed using NBL-6 cells. The results showed that parental, mutant, and revertant viruses grew to similar titers in NBL-6 cells, regardless of whether they harbored gD1 or gD4 (Fig. 3a and b). In addition, there was no significant difference observed between plaque areas of the mutants and their corresponding parental viruses (Fig. 3c and d). We concluded, therefore, that the growth rates of the mutant viruses were not significantly affected by the exchange of gD genes.

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Fig 3

In vitro growth properties of parental and mutant viruses. (a and b) For growth kinetics, NBL-6 cells were infected at an MOI of 1 or 0.01. Infected cells and supernatants were collected and virus titers were determined at the indicated times p.i. The data presented are means ± SD of triplicate measurements. The asterisk indicates a P value of > 0.05 for means when compared to the parental viruses. (c and d) Means ± SD of diameters of 50 plaques measured for each virus are shown. The plaque diameter of parental viruses was set to 100%. The asterisk indicates a P value of > 0.05 for means when compared to the parental viruses.

Type-specific gD determines the host range of EHV-1 and EHV-4 in vitro.

EHV-1 can readily be propagated in many cell lines, including primary equine cells and cell lines derived from other species (75). In contrast, EHV-4 appears to be restricted mainly to cells derived from horses, such as FHK and NBL-6 cells, or replicates only poorly in very few other cell lines, such as Vero cells (44). To test the hypothesis that gD is a major determinant of EHV-1 and EHV-4 tropism, gD mutant viruses were used to explore the viruses' ability to infect different cell lines, using the same infectious doses (MOI, 0.1), which allowed us to define the host range of each virus in vitro. Indeed, most of the cell lines permissive for EHV-1 became resistant to EHV-1 harboring gD4 instead of gD1, and the opposite was observed for EHV-4 expressing gD1 (Fig. 4). We were able to show that RK13, CHO-K1, CHO-A, CHO-B, and CHO-C cells, all of which are highly resistant to infection with EHV-4, can be infected with EHV-4_gD1 at levels comparable to those of EHV-1. In contrast, all of these cell lines became highly resistant to EHV-1_gD4 (Fig. 4). To examine whether EHV-4 gD has a similarly defining role for cellular tropism, we infected Vero and CrFK cells with all mutant viruses (Fig. 5). Vero cells were shown previously to be virtually resistant to EHV-1 but not EHV-4 infection (27, 80); however, no data were available on EHV-1 or EHV-4 infection of CrFK cells. We were able to show that EHV-4 can infect both cell types efficiently, while EHV-1 only poorly infected these cells. Interestingly, EHV-1_gD4 was able to infect Vero and CrFK cells at levels comparable to those of EHV-4; however, the plaque morphology was strikingly different from that of EHV-4. On the other hand, EHV-4_gD1 became virtually unable to infect Vero or CrFK cells (Fig. 5). In addition, we discovered that HeLa cells, MDBK cells, PBMC, EC, and 293 cells can be infected with both EHV-1 and EHV-4 at similar efficiencies (Fig. 6). We concluded from the latter finding that gD1 and gD4 have the same role and facilitate entry of either virus into these cells. However, additional factors such as other glycoproteins, including gB and/or gH/gL or host factors, may contribute to the cellular tropism of the two viruses (Fig. 6). Taken together, the data clearly indicated that gD1 can use host entry receptors, which are not accessible to gD4, and that gD1 confers an extended cellular host range to EHV-4.

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Fig 4

The role of gD in EHV-1 and EHV-4 cell tropism. CHO-K1 (a), CHO-A (b), CHO-B (c), CHO-C (d), or RK13 (e) cells were infected at an MOI of 0.1 with the engineered EHV recombinants, all of which express EGFP. At 48 h p.i., cells were inspected with a fluorescence microscope (Zeiss Axiovert) and images were taken with a CCD camera. The bar represents 100 μm.

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Fig 5

gD4 confers an extended cellular host range to EHV-1. CrFK (a) and Vero (b) cells were infected with parental and mutant viruses at an MOI of 0.1. At 48 h p.i., cells were inspected with a fluorescence microscope (Zeiss Axiovert) and images were taken with a CCD camera. The bar represents 100 μm.

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Fig 6

Infection of various cells by parental and mutant viruses. 293 cells (a), HeLa cells (b), MDBK cells (c), PBMC (d), or EC (e) cells were infected with various viruses. At 48 h p.i., cells were inspected with a fluorescence microscope (Zeiss Axiovert) and images were taken with a CCD camera. The bar represents 100 μm.

EHV-1 gD binds to CHO-K1 cells independently of integrin αVβ5.

Recently, it was described that EHV-1 enters CHO-K1 cells predominantly via the endocytic pathway and that this entry is triggered by the interaction between cellular integrins and the RSD motif present in EHV-1 but not EHV-4 gD (28, 68). We constructed several mutants to shed more light on the process. First, CHO-K1 cells were infected with EGFP-expressing parental EHV-1 and EHV-4 strains as well as the EHV-1_gD4, EHV-4_gD1, and EHV-4_gD^152D mutants. As shown above, CHO-K1 cells can only be infected with parental EHV-1 and EHV-4_gD1, showing that EHV-1 gD is essential for infection of these cells.

We further explored if integrins are essential for EHV-1 entry. EHV-1 gD was shown to contain an RSD amino acid sequence, a motif that mimics the canonical integrin-binding motif RGD, at position 152 (33, 68). Furthermore, αVβ5, one of the integrins that can recognize RGD motifs (60), is known to be expressed on the surface of CHO-K1 cells (1, 68). First, the proper expression of αVβ5 on CHO-K1 cells using the function-blocking MAb P1F6 was confirmed (Fig. 7a); however, these antibodies failed to block the entry of EHV-1 or EHV-4_gD1 (Fig. 7b). We also confirmed the previous results of Sasaki et al. (61), that anti-MHC class I antibody H58A failed to block entry of these viruses into CHO-K1 cells (data not shown). Furthermore, our results indicated that, like parental EHV-4, EHV-4_gD^152D was unable to infect CHO-K1 cells (Fig. 7c). Next, we quantitated by FACS analysis the binding of parental or mutant viruses to these cells. Binding was assessed in the presence or absence of αVβ5 MAb P1F6. We discovered that EHV-1 and EHV-4_gD1 can bind to CHO-K1 cells, while EHV-4, EHV-1_gD4, and EHV-4_gD^152D bound only poorly, regardless of whether the integrin MAb was present or not (Fig. 7d). As the latter viruses were unable to enter into CHO-K1 cells, the data suggested that EHV-1 gD is essential for stable virus attachment and entry into CHO-K1 cells but that these processes are independent of gD interaction with αVβ5 integrins or equine MHC-I. The receptor(s) for EHV-1 gD utilized on CHO-K1 for stable binding and initiation of entry still needs to be identified.

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Fig 7

Inhibition of integrins has no effect on EHV-1 infectivity in CHO-K1 cells. (a) CHO-K1 cells in suspension were incubated with anti-αVβ5, MAb P1F6, for 1 h at RT, followed by incubation with Alexa Fluor 488-labeled goat anti-mouse IgG for 1 h at RT. As controls, cells were incubated with irrelevant MAbs of the same IgG isotype. Integrin expression was determined by flow cytometry. (b) Cells were preincubated with medium or αVβ5 function-blocking MAb P1F6, for 1 h at 4°C, followed by infection with L11Δgp2 or EHV-4gD1 at an MOI of 5 for 2 h at 37°C. At 16 h p.i., cells were detached and the percentage of infected cells was determined by flow cytometry. (c) CHO-K1 cells were infected with EHV-4 or EHV-4gD^152D at an MOI of 2. At 48 h p.i., cells were inspected with a fluorescence microscope (Zeiss Axiovert) and images were taken with a CCD camera. The bar represents 100 μm. (d) Binding of various viruses to CHO-K1 cells. CHO-K1 cells were incubated with parental and mutant viruses for 2 h at 4°C in the presence of MAb P1F6. Cell surface binding was detected by flow cytometry. All data represent the mean ± SD of three independent experiments.

See Also

Awakening the Dormant Dragon: Neurological Form of Equine Herpesvirus-1

To further elucidate the role of integrin in virus entry, we determined the expression of different integrins on the surface of different cells. Our results showed that the RGD-binding αVβ3 integrin (60) was highly expressed on the surface of Vero and FHK cells (data not shown). However, another integrin, α4β1, which can bind to a different motif (32, 38), was found to be only moderately expressed on the surface of NBL-6 cells. Blocking of integrin receptors, using specific antibodies, on these cells did not affect the entry of parental or mutant viruses (data not shown).

Integrins are not essential for virus entry into PBMC.

PBMC are highly relevant to EHV-1 pathogenesis as outlined above, whereas EHV-4 infection of leukocyte is reported to be a rare event. Moreover, it was postulated that the RSD integrin motif, present in EHV-1 gD but not EHV-4 gD, is an important determinant for efficient infection of PBMC in vitro (68). In order to address the hypothesis that gD1 but not gD4 can mediate viral entry into PBMC, we first infected PBMC with EHV-1 and EHV-4 strains and analyzed the percentages of infected cells by flow cytometry. Surprisingly, the percentages of infected PBMC, after inoculation with EHV-1 or EHV-4, were nearly identical, ranging from approximately 3% (MOI, 1) to 8% (MOI, 5; Fig. 8a and b). In a second round of experiments, we included all of our mutants in the PBMC infection experiments. Similar to our previous results, the rate of infection was almost the same for all viruses used (Fig. 8c). Furthermore, we used anti-integrin MAb CD51/61 to investigate the role of integrins during entry into PBMC. Flow cytometric analysis showed that, in contrast to CHO-K1 cells, PBMC express αVβ3 instead of αVβ5 integrin. However, the rate of αVβ3 expression was very low (7%) (Fig. 9a) compared to the rate of αVβ5 expression on the surface of CHO-K1 cells (25%) (Fig. 7a). Preincubation of PBMC with the αVβ3 function-blocking MAb CD51/61 did not affect the rate of infection in PBMC for any of the parental or mutant viruses (Fig. 9b). When similar experiments were performed using anti-MHC class I antibody, H58A, and PT85A, either alone (Fig. 9c) or together with the integrin antibodies CD51/61 and P1F6 (Fig. 9d), no difference in infectivity could be observed.

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Fig 8

Infection of equine PBMC with recombinant viruses. PBMC were incubated with EHV-1 or EHV-4 at an MOI of 1 (a) or 5 (b) for 1 h at 37°C. After 48 h, the percentage of infected cells was determined by flow cytometry. In another experiment, PBMC were infected with all mutant viruses (c). The data represent the mean ± SD of at least three independent experiments.

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Fig 9

Integrins have no role in EHV-1 entry into equine PBMC. (a) PBMC were incubated with anti-αVβ3 MAb CD51/61 or αVβ5 MAb P1F6 for 1 h at RT, followed by incubation with Alexa Fluor 488-labeled goat anti-mouse IgG for 1 h at RT. As controls, cells were incubated with irrelevant MAbs of the same IgG isotype. Integrin expression was determined by flow cytometry. Cells were preincubated with αVβ3 function-blocking MAb CD51/61 (b), anti-equine MHC-I MAb H58A (c), or both MHC-I and integrin antibodies (d) for 1 h at 4°C, followed by infection with recombinant viruses at an MOI of 2 for 2 h at 37°C. At 48 h p.i., cells were washed and the percentage of infected cells was determined by flow cytometry. The rate of infection of parental viruses was set to 100%. All data represent the mean ± SD of three independent experiments.

We concluded from our data that PBMC can be infected with similar efficiencies by both EHV-1 and EHV-4 in vitro. Furthermore, integrins apparently do not serve as the (only) receptor for EHV-1 or EHV-4 entry into PBMC. In addition, in contrast to other equine cells, we were unable to block entry of EHV-1 or EHV-4 into PBMC with MHC-I antibodies, which indicates that these viruses likely utilize a different receptor(s) to infect PBMC.

Virus binding to K562 and K562_αVβ3 cells.

To further confirm our finding that EHV-1 and EHV-4 enter cells independently of integrins, we conducted experiments using the K562 and K562_αVβ3 cell lines. The erythroleukemia cell line K562 was chosen because it expresses a very limited number of integrins and no detectable αV integrins (12). K562_αVβ3 are stably expressing αVβ3 integrins (12), which can bind to the RGD motif. We first confirmed the expression of αVβ3 integrins on the surface of K562_αVβ3 cells using anti-αVβ3 MAb CD51/61 (Fig. 10a). Then, we infected K562 and K562_αVβ3 cells with the parental EHV-1 and EHV-4 strains in the presence or absence of the antibody. Our results showed that both cell types could not be infected with either of the viruses (data not shown). Next, we analyzed the binding of either EHV-1 or EHV-4 to these cells using FACS. Cells were incubated with either EHV-1 or EHV-4 for 2 or 24 h at 4°C and reacted with either EHV-1 gB MAb or EHV-4 polyclonal anti-gD antibody, respectively. Both viruses failed to bind K562 cells, irrespective of whether they expressed αVβ3 integrins (Fig. 10b). NBL-6 cells were included as a positive control and allowed binding of both EHV-1 and EHV-4 (Fig. 10b). We concluded from this experiment that neither the RSD motif in EHV-1 gD nor other related motifs in the EHV-1 or EHV-4 envelope are capable of mediating binding to αVβ3 integrins and that, therefore, integrins do not play a major role in EHV-1 or EHV-4 tropism.

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Fig 10

Binding of EHV-1 to K562 cells. (a) K562 or K562_αVβ3 cells were incubated with anti-αVβ3 MAbs for 1 h at RT, followed by incubation with Alexa Fluor 488-labeled goat anti-mouse IgG for 1 h at RT. As controls, cells were incubated with irrelevant MAbs of the same IgG isotype. Integrin expression was determined by flow cytometry. (b) K562, K562_αVβ3, or NBL-6 cells were incubated with EHV-1 or EHV-4 virus for 2 h at 4°C in the presence of αVβ5 function-blocking antibodies. Cell surface binding was detected by flow cytometry. All data represent the mean ± SD of three independent experiments.

gD plays essential roles in virus entry and cell tropism.

From our experiments, we concluded that gD (i) is interchangeable between EHV-1 and EHV-4 without altering the replication kinetics of the respective viruses in conventional equine cell cultures and (ii) determines the host range of both EHV-1 and EHV-4. Interestingly, gD1 was able to considerably expand the host range of EHV-4 and provide the ability to infect cells that were not accessible through gD4. On the other hand, although gD4 was associated with a narrow host range, it similarly endowed EHV-1 with the ability to infect cells that were originally resistant to infection. Two of the most indicative cell lines used in this study were RK13 and Vero cells. It has long been known that RK13 cells are permissive for EHV-1, yet these cells are highly resistant to EHV-4 infection (22, 75). On the other hand, EHV-1 is known to be severely impaired with respect to entry into Vero cells while EHV-4 can infect these cells efficiently (27, 80). By infecting these cells with parental EHV-1 and EHV-4 as well as the EHV-1_gD4 and EHV-4_gD1 viruses, we observed that the cytopathic effect (CPE) induced by the mutant viruses was not identical to that induced by parental viruses, which had a cluster-like morphology and only minimal signs of syncytia. Our results are consistent with the previous finding of Whalley and coworkers, who showed that morphological changes upon EHV-4 infection in RK13 cells constitutively expressing gD1 were different from those induced by EHV-1 (75). As gB, gH/L, and gK also control the process of entry, syncytium formation, and egress, one may expect this difference in CPE morphology (25, 50, 74).

As with other alphaherpesviruses, we confirmed that efficient EHV-1 and EHV-4 entry was mainly dependent on gD, which can bind to several cell surface receptors. As stated before (27), we also proved that EHV-1 can efficiently enter and replicate in CHO-K1, CHO-A, CHO-B, and CHO-C cells. On the other hand, our results show that EHV-4 gD cannot bind to any of the well-established herpesvirus entry receptors HveA, HveB, and HveC or even the unique receptor on the surface of CHO-K1 cells. However, EHV-4 gD allows virus entry into Vero cells, which is one of the cell lines highly permissive for HSV-1, but the cellular receptors, which interact with gD1 or gD4 in the case of CHO-K1 or Vero cells, respectively, still need to be elucidated. Since equine cell lines such as FHK and NBL-6 cells are permissive for both EHV-1 and EHV-4, both viruses may use the same receptor in the case of these cells. Recently, two independent studies have identified equine MHC class I as a cellular entry receptor for EHV-1 (40, 61). Equine MHC-I acts as a gD receptor for EHV-1 entry into equine cells, including NBL-6 and EC (CNS). Our own data also show that EHV-4 also utilizes equine MHC-I to infect different equine cells, including FHK, NBL6, and EC cells (unpublished data).

The crystal structure of gD1 or gD4 has not been determined yet. However, on the basis of overall similarity, one may derive some information from their HSV-1 counterpart. X-ray structural analysis revealed that gD residues 7 to 32 are required for binding of gD to different cell receptors (15, 20, 45, 79). The EHV-1 and EHV-4 gD hom*ologues have approximately 77% amino acid identity with each other and 20 and 25%, respectively, with HSV gD (66, 76). The alignment supports the notion of conservation of tertiary structures across the gD family, with positional identity of six cysteine residues and other sequence motifs. Therefore, it is likely that regions similar to those in HSV-1 gD are involved in receptor binding of EHV-1 and EHV-4 gD (24, 73). The N-terminal region is among the most variable between the gDs of EHV-1 and EHV-4 and may therefore be implicated in the differences in host cell tropism.

In order for gD to initiate entry, the glycoprotein must interact with other viral glycoproteins. Previous studies have shown that the C-terminal region of HSV-1 gD (encompassing residues 260 to 310) is required to trigger fusion (18, 42). In addition, the C terminus partially masks receptor-binding sites and prevents access to key residues located in the N-terminal part. Upon receptor binding, the C terminus is pushed aside, and this movement is necessary to activate the fusion machinery represented by gB and gH/gL (18). As it seems likely that the mechanism of gD triggering fusion is shared among alphaherpesviruses (30), EHV-1 or EHV-4 gD may bind to the cell surface receptor and still be able to activate the fusion machinery of the other virus. This theoretical possibility is supported by the observation that gD1 and gD4 differ in only 5 amino acids in the respective region encompassing residues 260 to 310.

gD interacts with cell surface proteins independently of integrins.

One of the main objectives of this work was to study the role of gD in determining the host range of EHV-1. Therefore, we included many cell lines and primary cells, including PBMC, in our study. Surprisingly, we found that both EHV-1 and EHV-4 can infect PBMC with the same efficiency when using the same MOI. This directed our attention toward the role of integrins in EHV-1, but not EHV-4, entry into PBMC as stated before (68). It has been shown that EHV-1 can enter certain cell types, mainly CHO-K1 and PBMC, via endocytosis upon binding of the RSD motif of gD1 to integrins αVβ3 and αVβ5 (68), although, in the case of HSV-1, there was no effect on plaque formation in epithelial cells by using an RGD peptide or monoclonal antibodies to the human β1 or β4 integrin subunit (37). More recently, several lines of evidence have shown that HSV-1 gH/gL bind to cells independently of αVβ3 integrin (31). Using different mutants and revertants, we asked whether gD is a ligand to αVβ3 and αVβ5 integrins. We observed that function-blocking antibodies against αVβ5 and αVβ3 had no effect on productive EHV-1 or EHV-4 infection in CHO-K1 cells and PBMC. While αVβ5 integrins are moderately expressed on the surface of CHO-K1 cells, αVβ3 integrins are poorly expressed on the surface of PBMC. This may indicate that αVβ3 integrins may not be implicated in virus entry. Furthermore, we observed that EHV-1 and EHV-4 failed to infect or bind to K562 cells, irrespective of whether they were negative or positive for αVβ3 integrins.

We could demonstrate by mutational analysis that EHV-4_gD^152D, a mutant specifying an RSD motif in gD, failed to infect and/or bind to CHO-K1 and that the function-blocking αVβ5 had no effect on productive EHV-1 or EHV-4_gD1 infection in CHO-K1 cells. Furthermore, blocking of αVβ3 integrin receptors on the surface of Vero cells using specific antibodies did not affect the entry of EHV-4_gD^152D. In addition, PBMC were infectible at virtually identical levels by parental EHV-1, EHV-4, and mutant viruses (EHV-1_gD4, EHV-4_gD1, and EHV-4_gD^152D) in the presence or absence of integrin antibodies. This is in contrast to the earlier results (68) where anti-integrin antibodies were shown to have a significant effect on productive EHV-1 infection in CHO-K1 cells and PBMC. However, the low expression levels of αVβ3 integrin on the surface of PBMC or αVβ5 on CHO-K1 cells reported here, compared to the extremely high expression of MHC-I, may indicate that integrins may not be “convenient” cellular receptors for EHV-1 or EHV-4 entry. Therefore, it is possible that at least the PBMC used here would have different activation levels and likely higher MHC-I levels. Our results are, however, in agreement with those obtained before with HSV-1, where a recombinant virus in which the potential integrin-binding motif RGD present in gH was mutated entered cells as efficiently as the wild type (29). Also in another study, Gianni and coworkers state that the presence of αVβ3 integrins is not a requirement for gH/gL binding to different cells (31). Based on the data obtained in this study, we conclude that the presence of integrins is not necessary for EHV-1 or EHV-4 cellular entry. Consistent with this interpretation, expression of αVβ3 integrins in K562 cells that are negative for both subunits did not confer the ability to bind gD. While anti-MHC-I antibodies could block the entry of EHV-1 and EHV-4 into equine cells (data not shown), a function-blocking αVβ3 integrin MAb failed to reduce the infection of integrin-positive cells. To further assess the role of integrins in the entry of EHV-1 into PBMC, two other mutant viruses generated in our laboratory, EHV-1_gH4 and EHV-4_gH1, were used to infect PBMC in the presence or absence of α₄β₁ or α₄β₇ antibodies. EHV-1 gH is known to harbor an integrin-binding motif (LDI) (32, 38), which may mediate entry into PBMC through the respective integrins. Again, and very similar to the results reported here, PBMC were infected at similar rates by all parental and mutant viruses (L. Zajic, W. Azab, and N. Osterrieder, unpublished observations).

Interestingly, our results showed that PBMC were infected at similar rates and with comparable kinetics by all viruses, including the mutant viruses. Previous studies on EHV-4 showed that viral genomic DNA can be initially detected in PBMC of infected foals. However, viral DNA loads were low and lasted only for a short period of time (56). In contrast, in the case of EHV-1, high levels of cell-associated viremia were detected for prolonged periods (3, 51). In another study, it was reported that EHV-4 is incapable of efficiently infecting PBMC in vitro (53). However, one has to carefully interpret these results, as the EHV-4 strain used in that study was a mutant in which the viral gM gene was absent. The resulting virus mutant was shown to exhibit a significant defect in viral replication (80). From our experiments and the existing literature, one may hypothesize that the difference in pathogenic potential between EHV-1 and EHV-4 may be caused by the decreased ability of EHV-4 to manipulate mononuclear cells in vivo but not by reduced infectivity.

Taken together, we provide evidence here of the following. (i) gD of EHV-1 confers an extended cellular host range to EHV-4, and we consider it likely that it represents an important virulence factor. Therefore, it will be critical to relate our findings in vitro to cells and tissues in vivo. However, while EHV-1 can infect mice and cause respiratory disease, in vivo studies of EHV-4 pathogenesis have been restricted owing to the lack of suitable small-animal models for this virus (6, 10, 54). (ii) Integrins apparently do not play a decisive role in the entry of EHV-1 or EHV-4 into PBMC or any other cells tested in this study. However, we cannot rule out the possibility that, in specific cells, integrins may play a role in EHV-1 or EHV-4 entry or postentry steps, particularly in signaling cascades. (iii) PBMC can be infected with similar efficiency by both EHV-1 and EHV-4 in vitro. Our findings therefore raise the question of why EHV-4 is obviously unable to establish a systemic infection in the natural host. Finally, in contrast to other equine cells, we were unable to block entry of EHV-1 or EHV-4 into PBMC with MHC-I antibodies, which indicates that these viruses utilize a different receptor(s) to infect PBMC and that the alternative receptor is the one preferably used in these cells.

Published ahead of print 14 December 2011

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