Viral determinants of the variable sensitivity of herpes simplex virus strains to gD-mediated interference

Herpes Simplex Virus 1 Glycoprotein B from a Hyperfusogenic Virus Mediates Enhanced Cell-Cell Fusion

Herpes simplex virus 1 (HSV-1) causes significant morbidity and death in humans worldwide. Herpes simplex virus 1 has a complex fusion mechanism that is incompletely understood. The HSV-1 strain ANG has notable fusion and entry activities that distinguish it from wild type. HSV-1 ANG virions fused with the Vero cell surface at 4 °C and also entered cells more efficiently at 15 °C, relative to wild type HSV-1 strain KOS virions, consistent with a hyperfusogenic phenotype. Understanding the molecular basis for the unique entry and fusion activities of HSV-1 strain ANG will help decipher the HSV fusion reaction and entry process. Sequencing of HSV-1 ANG genes revealed multiple changes in gB, gC, gD, gH, and gL proteins relative to wild type HSV-1 strains. The ANG UL45 gene sequence, which codes for a non-essential envelope protein, was identical to wild type KOS. HSV-1 ANG gB, gD, and gH/gL were necessary and sufficient to mediate cell-cell fusion in a virus-free reporter assay. ANG gB, when expressed with wild type KOS gD and gH/gL, increased membrane fusion, suggesting that ANG gB has hyperfusogenic cell-cell fusion activity. Replacing the KOS gD, gH, or gL with the corresponding ANG alleles did not enhance cell-cell fusion. The novel mutations in the ANG fusion and entry glycoproteins provide a platform for dissecting the cascade of interactions that culminate in HSV fusion and entry.

Membrane fusion activity of herpes simplex virus 1 glycoproteins from a hyperfusogenic virus

Herpes simplex virus 1 (HSV-1) causes significant morbidity and death in humans worldwide. Herpes simplex virus 1 has a complex fusion mechanism that is incompletely understood. The HSV-1 strain ANG has notable fusion and entry activities that distinguish it from wild type. HSV-1 ANG virions fused with the Vero cell surface at 4°C and also entered cells more efficiently at 15°C relative to wild type virions, consistent with a hyperfusogenic phenotype. Understanding the molecular basis for the unique entry and fusion activities of HSV-1 strain ANG will help decipher the HSV fusion reaction and entry process. Sequencing of HSV-1 ANG genes revealed multiple changes in gB, gC, gD, gH, and gL proteins relative to wild type HSV-1 strains. The ANG UL45 gene sequence, which codes for a non-essential envelope protein, was identical to wild type. HSV-1 ANG gB, gD, and gH/gL were necessary and sufficient to mediate cell-cell fusion in a virus-free reporter assay. ANG gB, when expressed with wild type gD and gH/gL, increased membrane fusion, suggesting that ANG gB has hyperfusogenic cell-cell fusion activity. Replacing the wild type gD, gH, or gL with the corresponding ANG alleles did not enhance cell-cell fusion. Wild type gC is proposed to facilitate fusion and entry into epithelial cells by optimizing conformational changes in the fusion protein gB. ANG gC substitution or addition also had no effect on cell-cell fusion. The novel mutations in the ANG fusion and entry glycoproteins provide a platform for dissecting the cascade of interactions that culminate in HSV fusion and entry.

Binding of herpesvirus entry mediator (HVEM) and HSV-1 gD affect reactivation but not latency levels

Previously we reported that the HSV-1 latency associated transcript (LAT) specifically upregulates the cellular herpesvirus entry mediator (HVEM) but no other known HSV-1 receptors. HSV-1 glycoprotein D (gD) binds to HVEM but the effect of this interaction on latency-reactivation is not known. We found that the levels of latent viral genomes were not affected by the absence of gD binding to HVEM. However, reactivation of latent virus in trigeminal ganglia explant cultures was blocked in the absence of gD binding to HVEM. Neither differential HSV-1 replication and spread in the eye nor levels of latency influenced reactivation. Despite similar levels of latency, reactivation in the absence of gD binding to HVEM correlated with reduced T cell exhaustion. Our results indicate that HVEM-gD signaling plays a significant role in HSV-1 reactivation but not in ocular virus replication or levels of latency. The results presented here identify gD binding to HVEM as an important target that influences reactivation and survival of ganglion resident T cells but not levels of latency. This concept may also apply to other herpesviruses that engages HVEM.

Contributions of herpes simplex virus 1 envelope proteins to entry by endocytosis

Herpes simplex virus (HSV) proteins specifically required for endocytic entry but not direct penetration have not been identified. HSVs deleted of gE, gG, gI, gJ, gM, UL45, or Us9 entered cells via either pH-dependent or pH-independent endocytosis and were inactivated by mildly acidic pH. Thus, the required HSV glycoproteins, gB, gD, and gH-gL, may be sufficient for entry regardless of entry route taken. This may be distinct from entry mechanisms employed by other human herpesviruses.

TNF Superfamily Networks: bidirectional and interference pathways of the herpesvirus entry mediator (TNFSF14)

The herpesvirus entry mediator (HVEM; TNFRSF14) can activate either proinflammatory or inhibitory signaling pathways. HVEM engages two distinct types of ligands, the canonical TNF-related cytokines, LIGHT and Lymphotoxin-α, and the Ig-related membrane proteins, BTLA (B and T lymphocyte attenuator) and CD160. Recent evidence indicates that the signal generated by HVEM depends on the context of its ligands expressed in trans or in cis. HVEM engagement by all of its ligands in trans initiates bidirectional signaling. In contrast, naïve T cells coexpress BTLA and HVEM forming a cis-complex that interferes with the activation of HVEM by extraneous ligands in the surrounding microenvironment. The HVEM Network is emerging as a key survival system for effector and memory T cells in mucosal tissues.

T cell intrinsic heterodimeric complexes between HVEM and BTLA determine receptivity to the surrounding microenvironment

The inhibitory cosignaling pathway formed between the TNF receptor herpesvirus entry mediator (HVEM, TNFRSF14) and the Ig superfamily members, B and T lymphocyte attenuator (BTLA) and CD160, limits the activation of T cells. However, BTLA and CD160 can also serve as activating ligands for HVEM when presented in trans by adjacent cells, thus forming a bidirectional signaling pathway. BTLA and CD160 can directly activate the HVEM-dependent NF-kappaB RelA transcriptional complex raising the question of how NF-kappaB activation is repressed in naive T cells. In this study, we show BTLA interacts with HVEM in cis, forming a heterodimeric complex in naive T cells that inhibits HVEM-dependent NF-kappaB activation. The cis-interaction between HVEM and BTLA is the predominant form expressed on the surface of naive human and mouse T cells. The BTLA ectodomain acts as a competitive inhibitor blocking BTLA and CD160 from binding in trans to HVEM and initiating NF-kappaB activation. The TNF-related ligand, LIGHT (homologous to lymphotoxins, exhibits inducible expression, and competes with HSV glycoprotein D for HVEM, a receptor expressed by T lymphocytes, or TNFSF14) binds HVEM in the cis-complex, but NF-kappaB activation was attenuated, suggesting BTLA prevents oligomerization of HVEM in the cis-complex. Genetic deletion of BTLA or pharmacologic disruption of the HVEM-BTLA cis-complex in T cells promoted HVEM activation in trans. Interestingly, herpes simplex virus envelope glycoprotein D formed a cis-complex with HVEM, yet surprisingly, promoted the activation NF-kappaB RelA. We suggest that the HVEM-BTLA cis-complex competitively inhibits HVEM activation by ligands expressed in the surrounding microenvironment, thus helping maintain T cells in the naive state.

Structure-function analysis of herpes simplex virus glycoprotein B with fusion-from-without activity

Fusion-from-without (FFWO) is the rapid induction of cell fusion by virions in the absence of viral protein synthesis. The combination of two amino acid mutations in envelope glycoprotein B (gB), one in the ectodomain and one in the cytoplasmic tail, can confer FFWO activity to wild type herpes simplex virus (HSV). In this report, we analyzed the entry and cell fusion phenotypes of HSV that contains FFWO gB, with emphasis on the cellular receptors for HSV, nectin-1, nectin-2 and HVEM. The ability of an HSV strain with FFWO gB to efficiently mediate FFWO via a specific gD-receptor correlated with its ability to mediate viral entry by that receptor. A FFWO form of gB was not sufficient to switch the entry of HSV from a pH-dependent, endocytic pathway to a direct fusion, pH-independent pathway. The conformation of gB with FFWO activity was not globally altered relative to wild type. However, distinct monoclonal antibodies had reduced reactivity with FFWO gB, suggesting an altered antigenic structure relative to wild type. FFWO was blocked by preincubation of virions with neutralizing antibodies to gB or gD. Together with previous studies, the results indicate that the roles of gB in FFWO and in virus-cell fusion during entry are related but not identical. This study also suggests that the FFWO function of gB is not a specific determinant for the selection of HSV entry pathway and that antigenic differences in FFWO gB may reflect its enhanced fusion activity.

Involvement of HVEM receptor in activation of nuclear factor kappaB by herpes simplex virus 1 glycoprotein D

The UV-inactivated herpes simplex virus 1 (HSV-1) and glycoprotein D (gD) of HSV-1 have been shown to activate nuclear factor kappaB (NF-kappaB) in U937 cells, but mechanisms involved in this activation have not been elucidated. Here we report that: (i) UV-inactivated HSV-1 induced an increased NF-kappaB activation in cells expressing human HVEM (for herpesvirus entry mediator) at surface level, naturally or following stable transfection, but not in cells in which this receptor was not detected by flow cytometry analysis, (ii) treatment with soluble gD induced a dose-dependent NF-kappaB activation in THP-1 cells naturally expressing HVEM, and a monoclonal antibody that prevents binding of gD to HVEM significantly reduced NF-kappaB activation by soluble gD in the same cells, (iii) coculture with transfectants expressing wild-type gD on their surface induced an approximately twofold increase in NF-kappaB activation in cells naturally expressing HVEM, while coculture with transfectants expressing a mutated form of gD, lacking its capability to bind HVEM, did not induce a similar effect and (iv) treatment with soluble gD induced a dose-dependent NF-kappaB activation in CHO transfectants expressing HVEM, but not in control CHO transfectants lacking any functional gD receptor. Overall, these results establish that HVEM is involved in NF-kappaB activation by HSV-1 gD.

Involvement of gD/HVEM interaction in NF-kB-dependent inhibition of apoptosis by HSV-1 gD

In the present paper, we aimed to verify whether the interaction of the glycoprotein D (gD) of herpes simplex 1 (HSV-1) with the HSV-1 receptor HVEM is involved in NF-kappaB-dependent protection against apoptosis by gD. To this purpose, first we utilized MAbs that interfere with gD/HVEM interaction and U937 cells that naturally express human HVEM on their surface. Pre-incubation with these MAbs, but not with a control antibody, partially reverted the protection of infectious HSV-1 towards anti-Fas induced apoptosis in U937 cells. Similarly, pre-incubation of UV-inactivated HSV-1 (UV-HSV-1) or recombinant gD with the same MAbs, significantly reduced the inhibition of Fas-mediated apoptosis by UV-HSV-1 or gD, respectively, in U937 cells. Moreover, coculture with stable transfectants expressing at surface level wild type gD protected U937 cells against Fas-induced apoptosis, while coculture with transfectants expressing a mutated form of gD, incapable to bind HVEM, did not protect. Finally, UV-HSV-1 protected against staurosporine-induced apoptosis in U937 cells as well as in the CHO transfectants expressing human HVEM on their surface, but not in the control CHO transfectants, which did not express HVEM. These results suggest that signaling triggered by binding of gD to HVEM could represent an additional mechanism of evasion from premature apoptotic death exerted by HSV-1-gD in HVEM-expressing cells, disclosing new opportunities of cell death manipulation by using gD preparations.

Nectin-2-mediated entry of a syncytial strain of herpes simplex virus via pH-independent fusion with the plasma membrane of Chinese hamster ovary cells

BACKGROUND

Herpes simplex virus (HSV) can utilize multiple pathways to enter host cells. The factors that determine which route is taken are not clear. Chinese hamster ovary (CHO) cells that express glycoprotein D (gD)-binding receptors are model cells that support a pH-dependent, endocytic entry pathway for all HSV strains tested to date. Fusion-from-without (FFWO) is the induction of target cell fusion by addition of intact virions to cell monolayers in the absence of viral protein expression. The receptor requirements for HSV-induced FFWO are not known. We used the syncytial HSV-1 strain ANG path as a tool to evaluate the complex interplay between receptor usage, membrane fusion, and selection of entry pathway.

RESULTS

Inhibitors of endocytosis and endosome acidification blocked ANG path entry into CHO cells expressing nectin-1 receptors, but not CHO-nectin-2 cells. Thus, under these conditions, nectin-2 mediates pH-independent entry at the plasma membrane. In addition, CHO-nectin-2 cells supported pH-dependent, endocytic entry of different strains of HSV-1, including rid1 and HFEM. The kinetics of ANG path entry was rapid (t1/2 of 5-10 min) regardless of entry route. However, HSV-1 ANG path entry by fusion with the CHO-nectin-2 cell plasma membrane was more efficient and resulted in larger syncytia. ANG path virions added to the surface of CHO-nectin-2 cells, but not receptor-negative CHO cells or CHO-nectin-1 cells, induced rapid FFWO.

CONCLUSION

HSV-1 ANG path can enter CHO cells by either endocytic or non-endocytic pathways depending on whether nectin-1 or nectin-2 is present. In addition to these cellular receptors, one or more viral determinants is important for the selection of entry pathway. HSV-induced FFWO depends on the presence of an appropriate gD-receptor in the target membrane. Nectin-1 and nectin-2 target ANG path to divergent cellular pathways, and these receptors may have different roles in triggering viral membrane fusion.

Prediction of Activity, Synthesis and Biological Testing of anti-HSV Active Peptides

Herpes simplex virus infections can be treated with a number of drugs, but as for all pathogens, there is a constant need for new therapies. In the search for lead compounds some peptides have proven to possess an antiviral effect, but it is still unclear what mechanisms are responsible for this effect. We wish to report on the use of principal properties of amino acids for developing quantitative structure-activity relationships (QSAR:s) as a tool for modelling peptide activity and predicting the activity of new peptides. In order to test the reliability of the method, new peptides have been designed by using multivariate methodology, synthesized and tested for a number of responses. Two of the new peptides synthesized were active at lower concentrations than experienced before regarding entry and herpes simplex virus activity, but they were not able to completely inhibit viral infection. This may reflect differences in mode of action of peptides depending on the amino acid content.

Cellular and viral requirements for rapid endocytic entry of herpes simplex virus

It was recently demonstrated that herpes simplex virus (HSV) successfully infects Chinese hamster ovary (CHO) cells expressing glycoprotein D (gD) receptors and HeLa cells by an endocytic mechanism (A. V. Nicola, A. M. McEvoy, and S. E. Straus, J. Virol. 77:5324-5332, 2003). Here we define cellular and viral requirements of this pathway. Uptake of intact, enveloped HSV from the cell surface into endocytic vesicles was rapid (t(1/2) of 8 to 9 min) and independent of the known cell surface gD receptors. Following uptake from the surface, recovery of intracellular, infectious virions increased steadily up to 20 min postinfection (p.i.), which corresponds to accumulation of enveloped virus in intracellular compartments. There was a sharp decline in recovery by 30 min p.i., suggesting loss of the virus envelope as a result of capsid penetration from endocytic organelles into the cytosol. In the absence of gD receptors, endocytosed virions did not successfully penetrate into the cytosol but were instead transported to lysosomes for degradation. Inhibitors of phosphatidylinositol (PI) 3-kinase, such as wortmannin, blocked transport of incoming HSV to the nuclear periphery and virus-induced gene expression but had no effect on virus binding or uptake. This suggests a role for PI 3-kinase activity in trafficking of HSV through the cytosol. Viruses that lack viral glycoproteins gB, gD, or gH-gL were defective in transport to the nucleus and had reduced infectivity. Thus, similar to entry via direct penetration at the cell surface, HSV entry into cells by wortmannin-sensitive endocytosis is efficient, involves rapid cellular uptake of viral particles, and requires gB, gD, and gH-gL.

Mutations in the N termini of herpes simplex virus type 1 and 2 gDs alter functional interactions with the entry/fusion receptors HVEM, nectin-2, and 3-O-sulfated heparan sulfate but not with nectin-1

Multiple cell surface molecules (herpesvirus entry mediator [HVEM], nectin-1, nectin-2, and 3-O-sulfated heparan sulfate) can serve as entry receptors for herpes simplex virus type 1 (HSV-1) or HSV-2 and also as receptors for virus-induced cell fusion. Viral glycoprotein D (gD) is the ligand for these receptors. A previous study showed that HVEM makes contact with HSV-1 gD at regions within amino acids 7 to 15 and 24 to 32 at the N terminus of gD. In the present study, amino acid substitutions and deletions were introduced into the N termini of HSV-1 and HSV-2 gDs to determine the effects on interactions with all of the known human and mouse entry/fusion receptors, including mouse HVEM, for which data on HSV entry or cell fusion were not previously reported. A cell fusion assay was used to assess functional activity of the gD mutants with each entry/fusion receptor. Soluble gD:Fc hybrids carrying each mutation were tested for the ability to bind to cells expressing the entry/fusion receptors. We found that deletions overlapping either or both of the HVEM contact regions, in either HSV-1 or HSV-2 gD, severely reduced cell fusion and binding activity with all of the human and mouse receptors except nectin-1. Amino acid substitutions described previously for HSV-1 (L25P, Q27P, and Q27R) were individually introduced into HSV-2 gD and, for both serotypes, were found to be without effect on cell fusion and the binding activity for nectin-1. Each of these three substitutions in HSV-1 gD enhanced fusion with cells expressing human nectin-2 (ordinarily low for wild-type HSV-1 gD), but the same substitutions in HSV-2 gD were without effect on the already high level of cell fusion observed with the wild-type protein. The Q27P or Q27R substitution in either HSV-1 and HSV-2 gD, but not the L25P substitution, significantly reduced cell fusion and binding activity for both human and mouse HVEM. Each of the three substitutions in HSV-1 gD, as well as the deletions mentioned above, reduced fusion with cells bearing 3-O-sulfated heparan sulfate. Thus, the N terminus of HSV-1 or HSV-2 gD is not necessary for functional interactions with nectin-1 but is necessary for all of the other receptors tested here. The sequence of the N terminus determines whether nectin-2 or 3-O-sulfated heparan sulfate, as well as HVEM, can serve as entry/fusion receptors.

High level expression and secretion of truncated forms of herpes simplex virus type 1 and type 2 glycoprotein D by the methylotrophic yeast Pichia pastoris

Herpes simplex virus type 1 and 2 (HSV-1 and -2) glycoproteins D (gD-1 and gD-2) play a role in the entry of the virus into the host cell. Availability of substantial amounts of these proteins, or large fragments thereof, will be needed to allow studies at the molecular level. We studied the potency of the Pichia pastoris yeast expression system to produce soluble forms of gD. The DNA sequences encoding the extracellular domains of gD [amino acids 1-314 (gD-1(1-314)) and amino acids 1-254 (gD-1(1-254)) of gD-1 and amino acids 1-314 of gD-2 (gD-2(1-314))] were cloned into the P. pastoris yeast expression vector pPIC9. Two truncated forms of gD-1 were fitted with a His tail (designated as gD-1(1-314His) and gD-1(1-254His)) to facilitate their purification. Large amounts of gD-1(1-314) and gD-1(1-314His) (280-300mg/L induction medium) were produced. The yields of recombinant gD-1(1-254) and gD-1(1-254His) were lower: 20-36mg/L, and the yield of the gD-2(1-314) fragment was much lower: 6mg/L. SDS-PAGE analysis revealed multiple glycosylated species of the larger gD fragments, ranging in apparent molecular weight from 31 to 78kDa. The smaller gD-1(1-254) fragment appeared as two bands with molecular weights of 33 and 31kDa. All recombinant proteins produced by P. pastoris were recognized, as expected, by a panel of MAbs (A16, DL6, A18, DL11, HD1, ABDI, and AP7). In addition, we showed that gD-1(1-314), gD-2(1-314), and gD-1(1-254His) were able to interfere with binding of HSV to susceptible cells. These results indicate that the conformations of the recombinant proteins closely resemble those of native gD.

Mutations in herpes simplex virus glycoprotein D distinguish entry of free virus from cell-cell spread

Herpes simplex virus type 1 (HSV-1) glycoprotein D (gD) is an essential component of the entry apparatus that is responsible for viral penetration and subsequent cell-cell spread. To test the hypothesis that gD may serve distinguishable functions in entry of free virus and cell-cell spread, mutants were selected for growth on U(S)11cl19.3 cells, which are resistant to both processes due to the lack of a functional gD receptor, and then tested for their ability to enter as free virus and to spread from cell to cell. Unlike their wild-type parent, HSV-1(F), the variants that emerged from this selection, which were named SP mutants, are all capable of forming macroscopic plaques on the resistant cells. This ability is caused by a marked increase in cell-cell spread without a concomitant increase in efficiency of entry of free virus. gD substitutions that arose within these mutants are sufficient to mediate cell-cell spread in U(S)11cl19.3 cells but are insufficient to overcome the restriction to entry of free virions. These results suggest that mutations in gD (i) are sufficient but not necessary to overcome the block to cell-cell spread exhibited by U(S)11cl19.3 cells and (ii) are insufficient to mediate entry of free virus in the same cells.

Homologous and heterologous interference requires bovine herpesvirus-1 glycoprotein D at the cell surface during virus entry

Expression of glycoprotein D (gD) of alphaherpesviruses protects cells from superinfection by homologous and heterologous viruses by a mechanism termed interference. We recently showed that MDBK cells expressing bovine herpesvirus (BHV)-1gD (MDBK(gD)) resist BHV-1, pseudorabies virus (PRV) and herpes simplex virus-1 (HSV-1) but not the more closely related BHV-5 infection as determined by the number of plaques produced. However, the plaque size is reduced in all four viral infections suggesting a block in cell-to-cell transmission. Here, we show that MDBK cells expressing truncated BHV-1 gD, designated MDBK(t-gD), secreted soluble gD and were fully susceptible to infection by all the four viruses when the cells were washed prior to infection. When MDBK cells or MDBK(t-gD) cells were treated with medium containing truncated gD prior to infection, they partially resisted BHV-1, PRV and HSV-1 but not BHV-5. Interestingly, both BHV-1 and BHV-5 formed normal-sized plaques in MDBK(t-gD) cells suggesting that the viruses were able to spread efficiently. Thus BHV-1 gD is required at the cell surface at the time of infection in order to block BHV-1, HSV-1 and PRV infections, consistent with a common coreceptor for the three gDs.

Cellular expression of alphaherpesvirus gD interferes with entry of homologous and heterologous alphaherpesviruses by blocking access to a shared gD receptor

Several human and animal alphaherpesviruses can enter cells via human herpesvirus entry mediator C (HveC), a receptor for viral glycoprotein D (gD). In previous studies with cells expressing unknown entry mediators, cellular expression of alphaherpesvirus gD was shown to inhibit entry of the homologous virus and sometimes also of heterologous alphaherpesviruses. To investigate the mechanism of gD-mediated interference and the basis for cross-interference among alphaherpesviruses, HveC was expressed in cells as the sole entry mediator, in the presence or absence of one of the gDs encoded by herpes simplex virus type 1, pseudorabies virus, or bovine herpesvirus type 1. Cells expressing HveC alone were highly susceptible to entry of all three viruses, whereas cells coexpressing HveC and any one of the gDs were at least partially resistant to infection by each virus. Coexpression of gD with HveC did not cause reduced levels of cell-surface HveC but the HveC had reduced ability to bind to exogenous gD. Coimmunoprecipitation experiments revealed that HveC was complexed with gD in lysates of cells expressing both. Thus, cellular expression of gD can interfere with alphaherpesvirus entry by blocking ligand-binding sites of the gD receptor(s) used for entry and cross-interference can occur because different forms of alphaherpesvirus gD can compete for shared entry receptors.

Absence of superinfection exclusion during asynchronous reovirus infections of mouse, monkey, and human cell lines

Reovirus is a gastroenteric virus with a genome that consists of ten segments of double-stranded RNA. The segmented nature of the genome allows for genetic mixing when cells are simultaneously infected with two different viral serotypes. The ability of viral reassortment to take place in asynchronous infections has not previously been investigated with mammalian reoviruses. In this study, five different cell lines, representing mouse, monkey, and human, were infected synchronously or asynchronously with various sets of two different temperature-sensitive (ts) reovirus mutants in order to study the genetic interactions which occur. Recombinant viruses were detected at high frequency when infection by the two different ts mutants was separated by as much as 24 h, suggesting that superinfection exclusion does not play a role in reovirus mixed infections. The apparent lack of superinfection exclusion in reovirus infections may have important implications in its evolution.

Partial resistance to gD-mediated interference conferred by mutations affecting herpes simplex virus type 1 gC and gK

Cells expressing herpes simplex virus (HSV) gD can be resistant to HSV entry as a result of gD-mediated interference. HSV strains differ in sensitivity to this interference, which blocks viral penetration but not binding. Previous studies have shown that mutations or variations in virion-associated gD can confer resistance to gD-mediated interference. Here we show that HSV-1 mutants selected for enhanced ability to bind and penetrate in the presence of inhibitory concentrations of heparin were partially resistant to gD-mediated interference. The resistance was largely due to the presence of two mutations: one in gC (the major heparin-binding glycoprotein) resulting in the absence of gC expression and the other in gK resulting in a syncytial phenotype. The results imply that heparin selected for mutants with altered postbinding requirements for entry. Resistance to gD-mediated interference conferred by mutations affecting gC and gK has not been previously described.

Antigenic structure of soluble herpes simplex virus (HSV) glycoprotein D correlates with inhibition of HSV infection

Soluble forms of herpes simplex virus (HSV) glycoprotein D (gD) block viral penetration. Likewise, most HSV strains are sensitive to gD-mediated interference by cells expressing gD. The mechanism of both forms of gD-mediated inhibition is thought to be at the receptor level. We analyzed the ability of different forms of soluble, truncated gD (gDt) to inhibit infection by different strains of HSV-1 and HSV-2. Strains that were resistant to gD-mediated interference were also resistant to inhibition by gDt, thereby suggesting a link between these two phenomena. Virion gD was the major viral determinant for resistance to inhibition by gDt. An insertion-deletion mutant, gD-1(delta 290-299t), had an enhanced inhibitory activity against most strains tested. The structure and function of gDt proteins derived from the inhibition-resistant viruses rid1 and ANG were analyzed. gD-1(ridlt) and gD-1(ANGt) had a potent inhibitory effect on plaque formation by wild-type strains of HSV but, surprisingly, little or no effect on their parental strains. As measured by quantitative enzyme-linked immunosorbent assay with a diverse panel of monoclonal antibodies, the antigenic structures of gD-1(rid1t) and gD-1(ANGt) were divergent from that of the wild type yet were similar to each other and to that of gD-1 (delta 290-299t). Thus, three different forms of gD have common antigenic changes that correlate with enhanced inhibitory activity against HSV. We conclude that inhibition of HSV infectivity by soluble gD is influenced by the antigenic conformation of the blocking gDt as well as the form of gD in the target virus.

Adaptability in herpesviruses: glycoprotein D-independent infectivity of pseudorabies virus

Initial contact between herpesviruses and host cells is mediated by virion envelope glycoproteins which bind to cellular receptors. In several alphaherpesviruses, the nonessential glycoprotein gC has been found to interact with cell surface proteoglycans, whereas the essential glycoprotein gD is involved in stable secondary attachment. In addition, gD is necessary for penetration, which involves fusion between virion envelope and cellular cytoplasmic membrane. As opposed to other alphaherpesvirus gD homologs, pseudorabies virus (PrV) gD is not required for direct viral cell-to-cell spread. Therefore, gD- PrV can be passaged in noncomplementing cells by cocultivating infected and noninfected cells. Whereas infectivity was found to be strictly cell associated in early passages, repeated passaging resulted in the appearance of infectivity in the supernatant, finally reaching titers as high as 10(7) PFU/ml (PrV gD- Pass). Filtration experiments indicated that this infectivity was not due to the presence of infected cells, and the absence of gD was verified by Southern and Western blotting and by virus neutralization. Infection of bovine kidney cells constitutively expressing PrV gD interfered with the infectivity of wild-type PrV but did not inhibit that of PrV gD- Pass. Similar results were obtained after passaging of a second PrV mutant, PrV-376, which in addition to gD also lacks gG, gI, and gE. Penetration assays demonstrated that PrV gD- Pass entered cells much more slowly than wild-type PrV. In summary, our data demonstrate the existence of a gD-independent mode of initiation of infection in PrV and indicate that the essential function(s) that gD performs in wild-type PrV infection can be compensated for after passaging. Therefore, regarding the requirement for gD, PrV seems to be intermediate between herpes simplex virus type 1, in which gD is necessary for penetration and cell-to-cell spread, and varicella-zoster virus (VZV), which lacks a gD gene. Our data show that the relevance of an essential protein can change under selective pressure and thus demonstrate a way in which VZV could have evolved from a PrV-like ancestor.