Structure-function analysis of soluble forms of herpes simplex virus glycoprotein D

Structure-based mutagenesis of herpes simplex virus glycoprotein D defines three critical regions at the gD-HveA/HVEM binding interface

Herpes simplex virus (HSV) entry into cells requires the binding of glycoprotein D (gD) to one of several cell surface receptors. The crystal structure of gD bound to one of these receptors, HveA/HVEM, reveals that the core of gD comprises an immunoglobulin fold flanked by a long C-terminal extension and an N-terminal hairpin loop. HveA is a member of the tumor necrosis factor receptor family and contains four cysteine-rich domains (CRDs) characteristic of this family. Fourteen amino acids within the gD N-terminal loop comprise the entire binding site for HveA. To determine the contribution of each gD contact residue to virus entry, we constructed gD molecules mutated in these amino acids. We determined the abilities of the gD mutants to bind receptors, facilitate virus entry, and mediate cell-cell fusion. Seven of the gD mutants exhibited wild-type levels of receptor binding and gD function. Results from the other seven gD mutants revealed three critical regions at the gD-HveA interface. (i) Several gD residues that participate in an intermolecular beta-sheet with HveA were found to be crucial for HveA binding and entry into HveA-expressing cells. (ii) Two gD residues that contact HveA-Y23 contributed to HveA binding but were not required for mediating entry into cells. HveA-Y23 fits into a crevice on the surface of gD and was previously shown to be essential for gD binding. (iii) CRD2 was previously shown to contribute to gD binding, and this study shows that one gD residue that contacts CRD2 contributes to HveA binding. None of the gD mutations prevented interaction with nectin-1, another gD receptor. However, when cotransfected with the other glycoproteins required for fusion, two gD mutants gained the ability to mediate fusion of cells expressing nectin-2, a gD receptor that interacts with several laboratory-derived gD mutants but not with wild-type gD. Thus, results from this panel of gD mutants as well as those of previous studies (A. Carfi, S. H. Willis, J. C. Whitbeck, C. Krummenacher, G. H. Cohen, R. J. Eisenberg, and D. C. Wiley, Mol. Cell 8:169-179, 2001, and S. A. Connolly, D. J. Landsburg, A. Carfi, D. C. Wiley, R. J. Eisenberg, and G. H. Cohen, J. Virol. 76:10894-10904, 2002) provide a detailed picture of the gD-HveA interface and the contacts required for functional interaction. The results demonstrate that of the 35 gD and HveA contact residues that comprise the gD-HveA interface, only a handful are critical for complex formation.

Structure-function analysis of herpes simplex virus type 1 gD and gH-gL: clues from gDgH chimeras

In alphaherpesviruses, glycoprotein B (gB), gD, gH, and gL are essential for virus entry. A replication-competent gL-null pseudorabies virus (PrV) (B. G. Klupp and T. C. Mettenleiter, J. Virol. 73:3014-3022, 1999) was shown to express a gDgH hybrid protein that could replace gD, gH, and gL in cell-cell fusion and null virus complementation assays. To study this phenomenon in herpes simplex virus type 1 (HSV-1), we constructed four gDgH chimeras, joining the first 308 gD amino acids to various gH N-terminal truncations. The chimeras were named for the first amino acid of gH at which each was truncated: 22, 259, 388, and 432. All chimeras were immunoprecipitated with both gD and gH antibodies to conformational epitopes. Normally, transport of gH to the cell surface requires gH-gL complex formation. Chimera 22 contains full-length gH fused to gD308. Unlike PrV gDgH, chimera 22 required gL for transport to the surface of transfected Vero cells. Interestingly, although chimera 259 failed to reach the cell surface, chimeras 388 and 432 exhibited gL-independent transport. To examine gD and gH domain function, each chimera was tested in cell-cell fusion and null virus complementation assays. Unlike PrV gDgH, none of the HSV-1 chimeras substituted for gL for fusion. Only chimera 22 was able to replace gH for fusion and could also replace either gH or gD in the complementation assay. Surprisingly, this chimera performed very poorly as a substitute for gD in the fusion assay despite its ability to complement gD-null virus and bind HSV entry receptors (HveA and nectin-1). Chimeras 388 and 432, which contain the same portion of gD as that in chimera 22, substituted for gD for fusion at 25 to 50% of wild-type levels. However, these chimeras functioned poorly in gD-null virus complementation assays. The results highlight the fact that these two functional assays are measuring two related but distinct processes.

The domains of glycoprotein D required to block apoptosis induced by herpes simplex virus 1 are largely distinct from those involved in cell-cell fusion and binding to nectin1

Glycoprotein D (gD) interacts with two alternative protein receptors, nectin1 and HveA, to mediate herpes simplex virus (HSV) entry into cells. Fusion of the envelope with the plasma membrane requires, in addition to gD, glycoproteins gB, gH, and gL. Coexpression of the four glycoproteins (gD, gB, gH, and gL) promotes cell-cell fusion. gD delivered in trans is also capable of blocking the apoptosis induced by gD deletion viruses grown either in noncomplementing cells (gD(-/-)) or in complementing cells (gD(-/+)). While ectopic expression of cation-independent mannose-6 phosphate receptor blocks apoptosis induced by both stocks, other requirements differ. Thus, apoptosis induced by gD(-/-) virus is blocked by full-length gD (or two gD fragments reconstituting a full-length molecule), whereas ectopic expression of the gD ectodomain is sufficient to block apoptosis induced by gD(-/+) virus. In this report we took advantage of a set of gD insertion-deletion mutants to map the domains of gD required to block apoptosis by gD(-/-) and gD(-/+) viruses and those involved in cell-cell fusion. The mutations that resulted in failure to block apoptosis were the same for gD(-/-) and gD(-/+) viruses and were located in three sites, one within the immunoglobulin-type core region (residues 125, 126, and 151), one in the upstream connector region (residues 34 and 43), and one in the C-terminal portion of the ectodomain (residue 277). A mutant that carried amino acid substitutions at the three glycosylation sites failed to block apoptosis but behaved like wild-type gD in all other assays. The mutations that inhibited polykaryocyte formation were located in the upstream connector region (residues 34 and 43), at the alpha1 helix (residue 77), in the immunoglobulin core and downstream regions (residue 151 and 187), and at the alpha3 helix (residues 243 and 246). Binding of soluble nectin1-Fc to cells expressing the mutant gDs was generally affected by the same mutations that affected fusion, with one notable exception (Delta277-310), which affected fusion without hampering nectin1 binding. This deletion likely identifies a region of gD involved in fusion activity at a post-nectin1-binding step. We conclude that whereas mutations that affected all functions (e.g., upstream connector region and residue 151) may be detrimental to overall gD structure, the mutations that affect specific activities identify domains of gD involved in the interactions with entry receptors and fusogenic glycoproteins and with cellular proteins required to block apoptosis. The evidence that glycosylation of gD is required for blocking apoptosis supports the conclusion that the interacting protein is the mannose-6 phosphate receptor.

Structure-based analysis of the herpes simplex virus glycoprotein D binding site present on herpesvirus entry mediator HveA (HVEM)

Binding of herpes simplex virus (HSV) envelope glycoprotein D (gD) to a cell surface receptor is an essential step of virus entry. We recently determined the crystal structure of gD bound to one receptor, HveA. HveA is a member of the tumor necrosis factor receptor family and contains four characteristic cysteine-rich domains (CRDs). The first two CRDs of HveA are necessary and sufficient for gD binding. The structure of the gD-HveA complex reveals that 17 amino acids in HveA CRD1 and 4 amino acids in HveA CRD2 directly contact gD. To determine the contribution of these 21 HveA residues to virus entry, we constructed forms of HveA mutated in each of these contact residues. We determined the ability of the mutant proteins to bind gD, facilitate virus entry, and form HveA oligomers. Our results point to a binding hot spot centered around HveA-Y23, a residue that protrudes into a crevice on the surface of gD. Both the hydroxyl group and phenyl group of HveA-Y23 contribute to HSV entry. Our results also suggest that an intermolecular beta-sheet formed between gD and HveA residues 35 to 37 contributes to binding and that a C37-C19 disulfide bond in CRD1 is a critical component of HveA structure necessary for gD binding. The results argue that CRD2 is required for gD binding mainly to provide structural support for a gD binding site in CRD1. Only one mutant, HveA-R75A, exhibited enhanced gD binding. While some mutations influenced complex formation, the majority did not affect HSV entry, suggesting that most contact residues contribute to HveA receptor function collectively rather than individually. This structure-based dissection of the gD-HveA binding site highlights the contribution of key residues within HveA to gD binding and HSV entry and defines a target region for the design of small-molecule inhibitors.

Heparan sulfate 3-O-sulfotransferase isoform 5 generates both an antithrombin-binding site and an entry receptor for herpes simplex virus, type 1

Heparan sulfate 3-O-sulfotransferase transfers sulfate to the 3-OH position of a glucosamine residue of heparan sulfate (HS) to form 3-O-sulfated HS. The 3-O-sulfated glucosamine residue contributes to two important biological functions of HS: binding to antithrombin and thereby carrying anticoagulant activity, and binding to herpes simplex viral envelope glycoprotein D to serve as an entry receptor for herpes simplex virus 1. A total of five HS 3-O-sulfotransferase isoforms were reported previously. Here we report the isolation and characterization of a novel HS 3-O-sulfotransferase isoform, designated as HS 3-O-sulfotransferase isoform 5 (3-OST-5). 3-OST-5 cDNA was isolated from a human placenta cDNA library and expressed in COS-7 cells. The disaccharide analysis of 3-OST-5-modified HS revealed that 3-OST-5 generated at least three 3-O-sulfated disaccharides as follows: IdoUA2S-AnMan3S, GlcUA-AnMan3S6S, and IdoUA2S-AnMan3S6S. Transfection of the plasmid expressing 3-OST-5 rendered wild type Chinese hamster ovary cells susceptible to the infection by herpes simplex virus 1, suggesting that 3-OST-5-modified HS serves as an entry receptor for herpes simplex virus 1. In addition, 3-OST-5-modified HS bound to herpes simplex viral envelope protein glycoprotein D. Furthermore, we found that 3-OST-5-modified HS also bound to antithrombin, suggesting that 3-OST-5 also produces anticoagulant HS. In summary, our results indicate that a new member of 3-OST family generates both anticoagulant HS and an entry receptor for herpes simplex virus 1. These results provide a new insight regarding the mechanism for the biosynthesis of biologically active HS.

Characterization of a heparan sulfate octasaccharide that binds to herpes simplex virus type 1 glycoprotein D

Herpes simplex virus type 1 utilizes cell surface heparan sulfate as receptors to infect target cells. The unique heparan sulfate saccharide sequence offers the binding site for viral envelope proteins and plays critical roles in assisting viral infections. A specific 3-O-sulfated heparan sulfate is known to facilitate the entry of herpes simplex virus 1 into cells. The 3-O-sulfated heparan sulfate is generated by the heparan sulfate d-glucosaminyl-3-O-sulfotransferase isoform 3 (3-OST-3), and it provides binding sites for viral glycoprotein D (gD). Here, we report the purification and structural characterization of an oligosaccharide that binds to gD. The isolated gD-binding site is an octasaccharide, and has a binding affinity to gD around 18 microm, as determined by affinity coelectrophoresis. The octasaccharide was prepared and purified from a heparan sulfate oligosaccharide library that was modified by purified 3-OST-3 enzyme. The molecular mass of the isolated octasaccharide was determined using both nanoelectrospray ionization mass spectrometry and matrix-assisted laser desorption/ionization mass spectrometry. The results from the sequence analysis suggest that the structure of the octasaccharide is a heptasulfated octasaccharide. The proposed structure of the octasaccharide is DeltaUA-GlcNS-IdoUA2S-GlcNAc-UA2S-GlcNS-IdoUA2S-GlcNH(2)3S6S. Given that the binding of 3-O-sulfated heparan sulfate to gD can mediate viral entry, our results provide structural information about heparan sulfate-assisted viral entry.

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.

Expression and roles of herpesvirus entry mediators A and C in cells of oral origin

The roles of viral glycoprotein D (gD) and cellular herpesvirus entry mediators A (HveA) and C (HveC) in herpes simplex virus entry into oral cells were determined. Studies with purified truncated forms of gD-1, HveA and HveC indicated that these molecules may be involved in herpes simplex virus entry into oral cells. Moreover, HveA was expressed similarly in primary cultures of gingival keratinocytes and fibroblasts, whereas HveC was expressed at higher levels in gingival keratinocytes, as determined by RT-PCR and immunocytochemical staining. Further analysis using immunohistochemistry demonstrated that both HveA and HveC were expressed in epithelial cells, fibroblasts and vascular endothelial cells in gingival tissues. However, only HveC was detected in nerve fibers. Also, HveA was detected throughout the epidermis, whereas HveC was pronounced in the strata basale and spinosum. In conclusion, this study characterized HveA and HveC, molecules that may participate in entry of herpes simplex virus into oral cells.

Substitution in the murine nectin1 receptor of a single conserved amino acid at a position distal from the herpes simplex virus gD binding site confers high-affinity binding to gD

By analogy with its human nectin1 counterpart, murine nectin1 serves as a cellular receptor for the entry of herpes simplex virus (HSV) into murine cells. HSV entry mediated by either receptor is dependent on the viral glycoprotein D (gD). Whereas human nectin1 binds gD at high affinity and in a saturable manner, murine nectin1 binds gD in a barely detectable fashion, depending on the sensitivity of the assay. The immunoglobulin type V domain of murine nectin differs from its human counterpart in 11 amino acids. To identify the key residues responsible for the high-affinity binding of gD to human nectin1, we replaced each of the 11 divergent amino acids with the human counterparts singly or in groups in an incremental manner. Replacement in murine nectin1 of six amino acids that lie within the gD binding region of human nectin1 (previously mapped to residues 64 to 94, likely the CC'C" surface) increased the gD binding activity to a limited extent. In contrast, the single P138L substitution, which lies distal from the gD binding site, markedly increased gD binding. This substitution, when coupled with downstream substitutions, exerted the greatest effect. Three-dimensional modeling of the nectin1 V domain suggested that P138 in murine nectin1 might decrease the stability of the V domain by reducing the size of beta-strand G. The results support the notion that the overall structure of V nectin1 plays a pivotal role in its ability to bind HSV gD.

Chimeric nectin1-poliovirus receptor molecules identify a nectin1 region functional in herpes simplex virus entry

Human nectin1 (hNectin1), an adhesion molecule belonging to the nectin family of the immunoglobulin superfamily, mediates entry of herpes simplex virus (HSV) into cells. The hNectin1 domain that mediates virus entry into cells and also binds glycoprotein D (gD) has been localized to the first N-terminal V-type domain. The poliovirus receptor (PVR) is a structural homolog to nectins, but it cannot function as an HSV entry receptor. hNectin1-PVR chimeras were constructed to functionally locate the site on hNectin1 involved in HSV entry (HSV entry site). The epitope recognized by monoclonal antibody (MAb) R1.302, which is able to block HSV entry, was also located. The chimeric receptors were designed to preserve the overall structure of the V domain. The HSV entry activity mapped entirely to the hNectin1 portion located between residues 64 and 94 (64-94), likely to encode the C, C', and C" beta-strands and intervening loops. In turn, this site consisted of two portions: one with low-level basal activity for HSV entry (77-94), and one immediately upstream (residues 64 to 76) which greatly enhanced the HSV entry activity of the downstream region. The gD-binding site mapped substantially to the same site, whereas the MAb R1.302 epitope also required a further downstream portion (95-102). The involvement of the 64-76 portion is at difference with previous indirect mapping results that were based on competitive binding studies (C. Krummenacher et al., J. Virol. 74:10863-10872, 2000). The A, A', B, D, E, F, and G beta-strands and intervening loops did not appear to play any role in HSV entry. According to the predicted three-dimensional structure of PVR, the C C' C" site is located peripherally in the V domain and very likely represents an accessible portion at the cell surface.

Modified FGF4 signal peptide inhibits entry of herpes simplex virus type 1

Entry of herpes simplex virus type 1 (HSV-1) into host cells occurs through fusion of the viral envelope with the plasma membrane and involves complex and poorly understood interactions between several viral and cellular proteins. One strategy for dissecting the function of this fusion machine is through the use of specific inhibitors. We identified a peptide with antiviral activity that blocks HSV-1 infection at the entry stage and during cell-to-cell spreading. This peptide (called EB for "entry blocker") consists of the FGF4 signal sequence with an RRKK tetramer at the amino terminus to improve solubility. The activity of EB depends exclusively but not canonically on the signal sequence. Inhibition of virus entry (hrR3) and plaque formation (KOS) strongly depend on virus concentrations and serum addition, with 50% inhibitory concentrations typically ranging from 1 to 10 microM. Blocking preadsorbed virus requires higher EB concentrations. Cytotoxic effects (trypan blue exclusion) are first noted at 50 microM EB in serum-free medium and at > or = 200 microM in the presence of serum. EB does not affect gC-dependent mechanisms of virus attachment and does not block virus attachment at 4 degrees C. Instead, EB directly interacts with virions and inactivates them irreversibly without, however, disrupting their physical integrity as judged by electron microscopy. At subvirucidal concentrations, EB changes the adhesive properties of virions, causing aggregation at high virus concentrations. This peptide may be a useful tool for studying viral entry mechanisms.

Localization of the gD-binding region of the human herpes simplex virus receptor, HveA

During virus entry, herpes simplex virus (HSV) glycoprotein D (gD) binds to one of several human cellular receptors. One of these, herpesvirus entry mediator A (HveA), is a member of the tumor necrosis factor receptor (TNFR) superfamily, and its ectodomain contains four characteristic cysteine-rich pseudorepeat (CRP) elements. We previously showed that gD binds the ectodomain of HveA expressed as a truncated, soluble protein [HveA(200t)]. To localize the gD-binding domain of HveA, we expressed three additional soluble forms of HveA consisting of the first CRP [HveA(76t)], the second CRP [HveA(77-120t)], or the first and second CRPs [HveA(120t)]. Biosensor and enzyme-linked immunosorbent assay studies showed that gD bound to HveA(120t) and HveA(200t) with the same affinity. However, gD did not bind to HveA(76t) or HveA(77-120t). Furthermore, HveA(200t) and HveA(120t), but not HveA(76t) or HveA(77-120t), blocked herpes simplex virus (HSV) entry into CHO cells expressing HveA. We also generated six monoclonal antibodies (MAbs) against HveA(200t). MAbs CW1, -2, and -4 bound linear epitopes within the second CRP, while CW7 and -8 bound linear epitopes within the third or fourth CRPs. None of these MAbs blocked the binding of gD to HveA. In contrast, MAb CW3 recognized a discontinuous epitope within the first CRP of HveA, blocked the binding of gD to HveA, and exhibited a limited ability to block virus entry into cells expressing HveA, suggesting that the first domain of HveA contains at least a portion of the gD binding site. The inability of gD to bind HveA(76t) suggests that additional amino acid residues of the gD binding site may reside within the second CRP.

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.

The novel receptors that mediate the entry of herpes simplex viruses and animal alphaherpesviruses into cells

An extended array of cell surface molecules serve as receptors for HSV entry into cells. In addition to the heparan sulphate glycosaminoglycans, which mediate the attachment of virion to cells, HSV requires an entry receptor. The repertoire of entry receptors into human cells includes molecules from three structurally unrelated molecular families. They are (i) HveA (herpesvirus entry mediator A), (ii) members of the nectin family, (iii) 3-O-sulphated heparan sulphate. The molecules have different attributes and play potentially different roles in HSV infection and spread to human tissues. All the human entry receptors interact physically with the virion envelope glycoprotein D (gD). (i) HveA is a member of the TNF-receptor family. It mediates entry of a restricted range of HSV strains. Its expression is restricted to few lineages (e.g. T-lymphocytes). (ii) The human nectin1alpha (HIgR), nectin1delta (PRR1-HveC), and the nectin2alpha (PRR2alpha-HveB) and nectin2delta (PRR2delta) belong to the immunoglobulin superfamily. They are homologues of the poliovirus receptor (CD155), with which they share the overall structure of the ectodomain. The human nectin1alpha-delta are broadly expressed in cell lines of different lineages, are expressed in human tissue targets of HSV infection, serve as receptors for all HSV-1 and HSV-2 strains tested and mediate entry not only of free virions, but also cell-to-cell spread of virus. (iii) The 3-O-sulphated heparan sulphate is expressed in some selected human cell lines (e.g. endothelial and mast cells) and human tissues, and mediates entry of HSV-1, but not HSV-2. The human nectin2alpha and nectin2delta serve as receptors for a narrow range of viruses. A characteristic of the human nectin1alpha-delta is the promiscuous species non-specific receptor activity towards the animal alphaherpesviruses, pseudorabies virus (PrV) and bovine herpesvirus 1 (BHV-1). By contrast with the human nectin1delta, its murine homologue (mNectin1delta) does not bind gD at detectable level, yet it mediates entry of HSV, as well as of PrV and BHV-1. This provides the first example of a mediator of HSV entry independent of a detectable interaction with gD.

Characterization of cell-cell fusion mediated by herpes simplex virus 2 glycoproteins gB, gD, gH and gL in transfected cells

The mechanisms by which herpes simplex viruses (HSV) mediate fusion between their envelope and the plasma membrane during entry into cells, and between the plasma membranes of adjacent infected and uninfected cells to form multinucleated giant cells, are poorly understood. Four viral glycoproteins (gB, gD, gH and gL) are required for virus-cell fusion, whereas these plus several others are required for cell-cell fusion (syncytium formation). A better understanding would be aided by the availability of a model system, whereby fusion could be induced with a minimal set of proteins, in the absence of infection. A suitable system has now been developed for HSV-2, using transfected COS7, 293 or HEp-2 cells. Insofar as the minimal set of HSV-2 proteins required to cause cell-cell fusion in this system is gB, gD, gH and gL, it would appear to resemble virus-cell fusion rather than syncytium formation. However, the ability of a mutation in gB to enhance the fusion of both transfected cells and infected cells, while having no effect on virus-cell fusion, points to the opposite conclusion. The differential effects of a panel of anti-HSV antibodies, and of the fusion-inhibitor cyclosporin A, confirm that the fusion of transfected cells shares some properties with virus-cell fusion and others with syncytium formation. It may therefore prove useful for determining how these processes differ, and for testing the hypothesis that some viral proteins prevent membrane fusion until the appropriate point in the virus life-cycle, with other proteins then overcoming this block.

Superiority of intramuscular route and full length glycoprotein D for DNA vaccination against herpes simplex 2. Enhancement of protection by the co-delivery of the GM-CSF gene

Immunization with naked DNA has been analyzed in two critical variables: the site of injection and the cellular compartment to which the coded protein is directed. The gene for the full length of the glycoprotein D (gD) of HSV-2 under the control of the citomegalovirus (CMV) promoter was injected via the intradermal (i.d.) or the intramuscular (i.m.) routes in mice. Immunization in the quadricep muscle was superior to the intradermal immunization in the footpads. A stronger activation of IFN-gamma-secreting cells in the spleen and draining lymph nodes (DLN) was induced, resulting in a more efficient protection against an intravaginal challenge. In order to analyze the effect of the cellular localizations of the coded protein, the DNA for the truncated form of the gD (DeltagD) was injected via the i.m. route. Immunization with a vector encoding for DeltagD resulted in higher antibody levels in serum and vaginal washes than immunization with the gene for the full length gD. However, immunization with the DeltagD DNA elicited a much weaker cell-mediated immune response and was inferior to gD DNA in providing protection against a lethal intravaginal challenge with HSV. Co-injection of an expression cassette for the granulocyte-macrophage colony-stimulating factor (GM-CSF) increased both the humoral and cell-mediated immune response with both gD and DeltagD. A strong activation of IL-4-secreting cells was observed in the spleen and DLN together with an increase in the number of IFN-gamma-secreting cells. In addition, a reduction in the vaginal virus titers after an intravaginal challenge was observed in mice co-injected with the GM-CSF gene as compared to those immunized with pCDNAgD only.

The three HveA receptor ligands, gD, LT-alpha and LIGHT bind to distinct sites on HveA

The herpes virus entry mediator A (HveA), a member of the tumor necrosis factor receptor (TNFR) superfamily, interacts with three different protein ligands; lymphotoxin-alpha (LT-alpha) and LIGHT (LIGHT stands for lymphotoxin homolog, which exhibits inducible expression and competes with HSV glycoprotein D for HveA and is expressed on T-lymphocytes) from the host and the herpes simplex virus (HSV) surface glycoprotein gD. It has been reported that the gD binding site on HveA is located within the receptor's two N-terminal CRP domains, and that gD and LIGHT compete for their binding to HveA. However, whether these ligands interact with the same or different sites on the receptor is unclear. We analyzed and compared the sites of interaction between HveA and its TNF ligands, by using two recombinant forms of the receptor, comprising the full-receptor ectodomain (HveA (200t)) and its two first CRP domains (HveA (120t)), as well as several monoclonal antibodies recognizing HveA. Two HveA peptide ligands (BP-1 and BP-2) that differentially inhibit binding of soluble gD and LT-alpha to the receptor were also used to demonstrate that gD, LIGHT and LT-alpha bind to distinct sites on the receptor. Our results suggest that binding of a ligand to HveA may alter the conformation of this receptor, thereby affecting its interaction with its other ligands.

The murine homolog of human Nectin1delta serves as a species nonspecific mediator for entry of human and animal alpha herpesviruses in a pathway independent of a detectable binding to gD

The full-length cDNA of the murine homolog of human nectin1delta (mNectin1delta), also known as human poliovirus receptor related 1 (PRR1) or herpesvirus entry mediator C, was cloned and showed a >90% identity with its human counterpart. mNectin1delta is expressed in some murine cell lines, exemplified by NIH 3T3 and L cells, and in murine tissues. It mediates entry of an extended range of herpes simplex virus (HSV) strains, porcine pseudorabies virus (PrV), and bovine herpesvirus 1. A soluble form of the mediator blocked infectivity in mNectin1delta and human nectin1delta (hNectin1delta)-expressing cells, suggesting a physical interaction of the mediator with virions. The higher concentrations of soluble mNectin1 required to block infectivity relative to soluble hNectin1 suggest that the target of the two molecules is not identical. Entry of HSV, but not PrV, was blocked by soluble mNectin1delta in NIH 3T3 and L cells. Two features were unexpected. First, soluble mNectin1delta failed to physically interact with HSV glycoprotein D (gD) at a detectable level, although it interacted physically with virions. Second, coexpression of mNectin1delta and HSV gD did not restrict HSV or PrV infection, whereas coexpression of hNectin and gD did restrict infection, suggesting that mNectin1delta fails to be sequestered by HSV gD. We conclude that mNectin1delta serves as a species-nonspecific mediator for entry of the human and animal alphaherpesviruses. This activity, at least for HSV, is independent of a detectable binding to gD.

Cell-to-cell spread of wild-type herpes simplex virus type 1, but not of syncytial strains, is mediated by the immunoglobulin-like receptors that mediate virion entry, nectin1 (PRR1/HveC/HIgR) and nectin2 (PRR2/HveB)

The immunoglobulin-like receptors that mediate entry of herpes simplex virus type 1 (HSV-1) into human cells were found to mediate the direct cell-to-cell spread of wild-type virus. The receptors here designated Nectin1alpha and -delta and Nectin2alpha were originally designated HIgR, PRR1/HveC, and PRR2alpha/HveB, respectively. We report the following. (i) Wild-type HSV-1 spreads from cell to cell in J cells expressing nectin1alpha or nectin1delta but not in parental J cells that are devoid of entry receptors. A monoclonal antibody to nectin1, which blocks entry, also blocked cell-to-cell spread in nectin1-expressing J cells. Moreover, wild-type virus did not spread from a receptor-positive to a receptor-negative cell. (ii) The antibody to nectin1 blocked transmission of wild-type virus in a number of human cell lines, with varying efficiencies, suggesting that nectin1 is the principal mediator of wild-type virus spread in a variety of human cell lines. (iii) Nectin1 did not mediate cell fusion induced by the syncytial strains HSV-1(MP) and HFEM-syn. (iv) Nectin2alpha could serve as a receptor for spread of a mutant virus carrying the L25P substitution in glycoprotein D, but not of wild-type virus, in agreement with its ability to mediate entry of the mutant but not of wild-type virus.

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.

The major neutralizing antigenic site on herpes simplex virus glycoprotein D overlaps a receptor-binding domain

Herpes simplex virus (HSV) entry is dependent on the interaction of virion glycoprotein D (gD) with one of several cellular receptors. We previously showed that gD binds specifically to two structurally dissimilar receptors, HveA and HveC. We have continued our studies by using (i) a panel of baculovirus-produced gD molecules with various C-terminal truncations and (ii) a series of gD mutants with nonoverlapping 3-amino-acid deletions between residues 222 and 254. Binding of the potent neutralizing monoclonal antibody (MAb) DL11 (group Ib) was unaffected in forms of gD containing residues 1 to 250 but was greatly diminished in molecules truncated at residue 240 or 234. Both receptor binding and blocking of HSV infection were also affected by these C-terminal truncations. gD-1(234t) bound weakly to both HveA and HveC as determined by enzyme-linked immunosorbent assay (ELISA) and failed to block infection. Interestingly, gD-1(240t) bound well to both receptors but blocked infection poorly, indicating that receptor binding as measured by ELISA is not the only gD function required for blocking. Optical biosensor studies showed that while gD-1(240t) bound HveC with an affinity similar to that of gD-1(306t), the rates of complex formation and dissociation were significantly faster than for gD-1(306t). Complementation analysis showed that any 3-amino-acid deletion between residues 222 and 251 of gD resulted in a nonfunctional protein. Among this set of proteins, three had lost DL11 reactivity (those with deletions between residues 222 and 230). One of these proteins (deletion 222-224) was expressed as a soluble form in the baculovirus system. This protein did not react with DL11, bound to both HveA and HveC poorly as shown by ELISA, and failed to block HSV infection. Since this protein was bound by several other MAbs that recognize discontinuous epitopes, we conclude that residues 222 to 224 are critical for gD function. We propose that the potent virus-neutralizing activity of DL11 (and other group Ib MAbs) likely reflects an overlap between its epitope and a receptor-binding domain of gD.

Exploring biomolecular recognition using optical biosensors

Understanding the basic forces that determine molecular recognition helps to elucidate mechanisms of biological processes and facilitates discovery of innovative biotechnological methods and materials for therapeutics, diagnostics, and separation science. The ability to measure interaction properties of biological macromolecules quantitatively across a wide range of affinity, size, and purity is a growing need of studies aimed at characterizing biomolecular interactions and the structural elements that drive them. Optical biosensors have provided an increasingly impactful technology for such biomolecular interaction analyses. These biosensors record the binding and dissociation of macromolecules in real time by transducing the accumulation of mass of an analyte molecule at the sensor surface coated with ligand molecule into an optical signal. Interactions of analytes and ligands can be analyzed at a microscale and without the need to label either interactant. Sensors enable the detection of bimolecular interaction as well as multimolecular assembly. Most notably, the method is quantitative and kinetic, enabling determination of both steady-state and dynamic parameters of interaction. This article describes the basic methodology of optical biosensors and presents several examples of its use to investigate such biomolecular systems as cytokine growth factor-receptor recognition, coagulation factor assembly, and virus-cell docking.

A novel role for 3-O-sulfated heparan sulfate in herpes simplex virus 1 entry

Herpes simplex virus type 1 (HSV-1) binds to cells through interactions of viral glycoproteins gB and gC with heparan sulfate chains on cell surface proteoglycans. This binding is not sufficient for viral entry, which requires fusion between the viral envelope and cell membrane. Here, we show that heparan sulfate modified by a subset of the multiple D-glucosaminyl 3-O-sulfotransferase isoforms provides sites for the binding of a third viral glycoprotein, gD, and for initiation of HSV-1 entry. We conclude that susceptibility of cells to HSV-1 entry depends on (1) presence of heparan sulfate chains to which virus can bind and (2) 3-O-sulfation of specific glucosamine residues in heparan sulfate to generate gD-binding sites or the expression of other previously identified gD-binding receptors.

The first immunoglobulin-like domain of HveC is sufficient to bind herpes simplex virus gD with full affinity, while the third domain is involved in oligomerization of HveC

The human herpesvirus entry mediator C (HveC/PRR1) is a member of the immunoglobulin family used as a cellular receptor by the alphaherpesviruses herpes simplex virus (HSV), pseudorabies virus, and bovine herpesvirus type 1. We previously demonstrated direct binding of the purified HveC ectodomain to purified HSV type 1 (HSV-1) and HSV-2 glycoprotein D (gD). Here, using a baculovirus expression system, we constructed and purified truncated forms of the receptor containing one [HveC(143t)], two [HveC(245t)], or all three immunoglobulin-like domains [HveC(346t)] of the extracellular region. All three constructs were equally able to compete with HveC(346t) for gD binding. The variable domain bound to virions and blocked HSV infection as well as HveC(346t). Thus, all of the binding to the receptor occurs within the first immunoglobulin-like domain, or V-domain, of HveC. These data confirm and extend those of Cocchi et al. (F. Cocchi, M. Lopez, L. Menotti, M. Aoubala, P. Dubreuil, and G. Campadelli-Fiume, Proc. Natl. Acad. Sci. USA 95:15700, 1998). Using biosensor analysis, we measured the affinity of binding of gD from HSV strains KOS and rid1 to two forms of HveC. Soluble gDs from the KOS strain of HSV-1 had the same affinity for HveC(346t) and HveC(143t). The mutant gD(rid1t) had an increased affinity for HveC(346t) and HveC(143t) due to a faster rate of complex formation. Interestingly, we found that HveC(346t) was a tetramer in solution, whereas HveC(143t) and HveC(245t) formed dimers, suggesting a role for the third immunoglobulin-like domain of HveC in oligomerization. In addition, the stoichiometry between gD and HveC appeared to be influenced by the level of HveC oligomerization.

Three-dimensional structure of herpes simplex virus type 1 glycoprotein D at 2.4-nanometer resolution

Herpes simplex virus type 1 glycoprotein D (gD) is essential for virus infectivity and is responsible for binding to cellular membrane proteins and subsequently promoting fusion between the virus envelope and the cell. No structural data are available for gD or for any other herpesvirus envelope protein. Here we present a three-dimensional model for the baculovirus-expressed truncated protein gD1(306t) based on electron microscopic data. We demonstrate that gD1(306t) appears as a homotetramer containing a pronounced pocket in the center of the molecule. Monoclonal antibody binding demonstrates that the molecule is oriented such that the pocket protrudes away from the virus envelope.

Inhibition of herpes simplex virus gD and lymphotoxin-alpha binding to HveA by peptide antagonists

The herpesvirus entry mediator A (HveA) is a recently characterized member of the tumor necrosis factor receptor family that mediates the entry of most herpes simplex virus type 1 (HSV-1) strains into mammalian cells. Studies on the interaction of HSV-1 with HveA have shown that of all the viral proteins involved in uptake, only gD has been shown to bind directly to HveA, and this binding mediates viral entry into cells. In addition to gD binding to HveA, the latter has been shown to interact with proteins of tumor necrosis factor receptor-associated factor family, lymphotoxin-alpha (LT-alpha), and a membrane-associated protein referred to as LIGHT. To study the relationship between HveA, its natural ligands, and the viral proteins involved in HSV entry into cells, we have screened two phage-displayed combinatorial peptide libraries for peptide ligands of a recombinant form of HveA. Affinity selection experiments yielded two peptide ligands, BP-1 and BP-2, which could block the interaction between gD and HveA. Of the two peptides, only BP-2 inhibited HSV entry into CHO cells transfected with an HveA-expressing plasmid. When we analyzed these peptides for the ability to interfere with HveA binding to its natural ligand LT-alpha, we found that BP-1 inhibited the interaction of cellular LT-alpha with HveA. Thus, we have dissected the sites of interaction between the cell receptor, its natural ligand LT-alpha and gD, the virus-specific protein involved in HSV entry into cells.

Glycoprotein gL-independent infectivity of pseudorabies virus is mediated by a gD-gH fusion protein

Envelope glycoproteins gH and gL, which form a complex, are conserved throughout the family Herpesviridae. The gH-gL complex is essential for the fusion between the virion envelope and the cellular cytoplasmic membrane during penetration and is also required for direct viral cell-to-cell spread from infected to adjacent noninfected cells. It has been proposed for several herpesviruses that gL is required for proper folding, intracellular transport, and virion localization of gH. In pseudorabies virus (PrV), glycoprotein gL is necessary for infectivity but is dispensable for virion localization of gH. A virus mutant lacking gL, PrV-DeltagLbeta, is defective in entry into target cells, and direct cell-to-cell spread is drastically reduced, resulting in only single or small foci of infected cells (B. G. Klupp, W. Fuchs, E. Weiland, and T. C. Mettenleiter, J. Virol. 71:7687-7695, 1997). We used this limited cell-to-cell spreading ability of PrV-DeltagLbeta for serial passaging of cells infected with transcomplemented virus by coseeding with noninfected cells. After repeated passaging, plaque formation was restored and infectivity in the supernatant was observed. One single-plaque isolate, designated PrV-DeltagLPass, was further characterized. To identify the mutation leading to this gL-independent infectious phenotype, Southern and Western blot analyses, radioimmunoprecipitations, and DNA sequencing were performed. The results showed that rearrangement of a genomic region comprising part of the gH gene into a duplicated copy of part of the unique short region resulted in a fusion fragment predicted to encode a protein consisting of the N-terminal 271 amino acids of gD fused to the C-terminal 590 residues of gH. Western blotting and radioimmunoprecipitation with gD- and gH-specific antibodies verified the presence of a gDH fusion protein. To prove that this fusion protein mediates infectivity of PrV-DeltagLPass, cotransfection of PrV-DeltagLbeta DNA with the cloned fusion fragment was performed, and a cell line, Nde-67, carrying the fusion gene was established. After cotransfection, infectious gL-negative PrV was recovered, and propagation of PrV-DeltagLbeta on Nde-67 cells produced infectious virions. Thus, a gDH fusion polypeptide can compensate for function of the essential gL in entry and cell-to-cell spread of PrV.

In vivo modulation of vaccine-induced immune responses toward a Th1 phenotype increases potency and vaccine effectiveness in a herpes simplex virus type 2 mouse model

Several vaccines have been investigated experimentally in the herpes simplex virus type 2 (HSV-2) model system. While it is believed that CD4(+)-T-cell responses are important for protection in general, the correlates of protection from HSV-2 infection are still under investigation. Recently, the use of molecular adjuvants to drive vaccine responses induced by DNA vaccines has been reported in a number of experimental systems. We sought to take advantage of this immunization model to gain insight into the correlates of immune protection in the HSV-2 mouse model system and to further explore DNA vaccine technology. To investigate whether the Th1- or Th2-type immune responses are more important for protection from HSV-2 infection, we codelivered the DNA expression construct encoding the HSV-2 gD protein with the gene plasmids encoding the Th1-type (interleukin-2 [IL-2], IL-12, IL-15, and IL-18) and Th2-type (IL-4 and IL-10) cytokines in an effort to drive immunity induced by vaccination. We then analyzed the modulatory effects of the vaccine on the resulting immune phenotype and on the mortality and the morbidity of the immunized animals following a lethal challenge with HSV-2. We observed that Th1 cytokine gene coadministration not only enhanced the survival rate but also reduced the frequency and severity of herpetic lesions following intravaginal HSV challenge. On the other hand, coinjection with Th2 cytokine genes increased the rate of mortality and morbidity of the challenged mice. Moreover, of the Th1-type cytokine genes tested, IL-12 was a particularly potent adjuvant for the gD DNA vaccination.

The V domain of herpesvirus Ig-like receptor (HIgR) contains a major functional region in herpes simplex virus-1 entry into cells and interacts physically with the viral glycoprotein D

The herpesvirus entry mediator C (HveC), previously known as poliovirus receptor-related protein 1 (PRR1), and the herpesvirus Ig-like receptor (HIgR) are the bona fide receptors employed by herpes simplex virus-1 and -2 (HSV-1 and -2) for entry into the human cell lines most frequently used in HSV studies. They share an identical ectodomain made of one V and two C2 domains and differ in transmembrane and cytoplasmic regions. Expression of their mRNA in the human nervous system suggests possible usage of these receptors in humans in the path of neuron infection by HSV. Glycoprotein D (gD) is the virion component that mediates HSV-1 entry into cells by interaction with cellular receptors. We report on the identification of the V domain of HIgR/PRR1 as a major functional region in HSV-1 entry by several approaches. First, the epitope recognized by mAb R1. 302 to HIgR/PRR1, capable of inhibiting infection, was mapped to the V domain. Second, a soluble form of HIgR/PRR1 consisting of the single V domain competed with cell-bound full-length receptor and blocked virion infectivity. Third, the V domain was sufficient to mediate HSV entry, as an engineered form of PRR1 in which the two C2 domains were deleted and the V domain was retained and fused to its transmembrane and cytoplasmic regions was still able to confer susceptibility, although at reduced efficiency relative to full-length receptor. Consistently, transfer of the V domain of HIgR/PRR1 to a functionally inactive structural homologue generated a chimeric receptor with virus-entry activity. Finally, the single V domain was sufficient for in vitro physical interaction with gD. The in vitro binding was specific as it was competed both by antibodies to the receptor and by a mAb to gD with potent neutralizing activity for HSV-1 infectivity.

The ectodomain of a novel member of the immunoglobulin subfamily related to the poliovirus receptor has the attributes of a bona fide receptor for herpes simplex virus types 1 and 2 in human cells

We report on the functional cloning of a hitherto unknown member of the immunoglobulin (Ig) superfamily selected for its ability to confer susceptibility to herpes simplex virus (HSV) infection on a highly resistant cell line (J1.1-2 cells), derived by exposure of BHKtk- cells to a recombinant HSV-1 expressing tumor necrosis factor alpha (TNF-alpha). The sequence of herpesvirus Ig-like receptor (HIgR) predicts a transmembrane protein with an ectodomain consisting of three cysteine-bracketed domains, one V-like and two C-like. HIgR shares its ectodomain with and appears to be an alternative splice variant of the previously described protein PRR-1 (poliovirus receptor-related protein). Both HIgR and PRR-1 conferred on J1.1-2 cells susceptibility to HSV-1, HSV-2, and bovine herpesvirus 1. The viral ligand of HIgR and PRR-1 is glycoprotein D, a constituent of the virion envelope long known to mediate viral entry into cells through interaction with cellular receptor molecules. Recently, PRR-1, renamed HveC (herpesvirus entry mediator C), and the related PRR-2, renamed HveB, were reported to mediate the entry of HSV-1, HSV-2, and bovine herpesvirus 1, and the homologous poliovirus receptor was reported to mediate the entry of pseudorabies virus (R. J. Geraghty, C. Krummenacher, G. H. Cohen, R. J. Eisenberg, and P. G. Spear, Science 280:1618-1620, 1998; M. S. Warner, R. J. Geraghty, W. M. Martinez, R. I. Montgomery, J. C. Whitbeck, R. Xu, R. J. Eisenberg, G. H. Cohen, and P. G. Spear, Virology 246:179-189, 1998). Here we further show that HIgR or PRR-1 proteins detected by using a monoclonal antibody to PRR-1 are widely distributed among human cell lines susceptible to HSV infection and commonly used for HSV studies. The monoclonal antibody neutralized virion infectivity in cells transfected with HIgR or PRR-1 cDNA, as well as in the human cell lines, indicating a direct interaction of virions with the receptor molecule, and preliminarily mapping this function to the ectodomain of HIgR and PRR-1. Northern blot analysis showed that HIgR or PRR-1 mRNAs were expressed in human tissues, with the highest expression being detected in nervous system samples. HIgR adds a novel member to the cluster of Ig superfamily members able to mediate the entry of alphaherpesviruses into cells. The wide distribution of HIgR or PRR-1 proteins among human cell lines susceptible to HSV infection, coupled with the neutralizing activity of the antibody in the same cells, provides direct demonstration of the actual use of this cluster of molecules as HSV-1 and HSV-2 entry receptors in human cell lines. The high level of expression in samples from nervous system makes the use of these proteins in human tissues very likely. This cluster of molecules may therefore be considered to constitute bona fide receptors for HSV-1 and HSV-2.

Functional region IV of glycoprotein D from herpes simplex virus modulates glycoprotein binding to the herpesvirus entry mediator

Glycoprotein D (gD) of herpes simplex virus (HSV) is essential for virus entry and has four functional regions (I to IV) important for this process. We previously showed that a truncated form of a functional region IV variant, gD1(Delta290-299t), had an enhanced ability to block virus entry and to bind to the herpesvirus entry mediator (HveAt; formerly HVEMt), a cellular receptor for HSV. To explore this phenotype further, we examined other forms of gD, especially ones with mutations in region IV. Variant proteins with deletions of amino acids between 277 and 300 (region IV), as well as truncated forms lacking C-terminal residues up to amino acid 275 of gD, were able to block HSV entry into Vero cells 1 to 2 logs better than wild-type gD1(306t). In contrast, gD truncated at residue 234 did not block virus entry into Vero cells. Using optical biosensor technology, we recently showed that gD1(Delta290-299t) had a 100-fold-higher affinity for HveAt than gD1(306t) (3.3 x 10(-8) M versus 3.2 x 10(-6) M). Here we found that the affinities of other region IV variants for HveAt were similar to that of gD1(Delta290-299t). Thus, the affinity data follow the same hierarchy as the blocking data. In each case, the higher affinity was due primarily to a faster kon rather than to a slower koff. Therefore, once the gDt-HveAt complex formed, its stability was unaffected by mutations in or near region IV. gD truncated at residue 234 bound to HveAt with a lower affinity (2.0 x 10(-5) M) than did gD1(306t) due to a more rapid koff. These data suggest that residues between 234 and 275 are important for maintaining stability of the gDt-HveAt complex and that functional region IV is important for modulating the binding of gD to HveA. The binding properties of any gD1(234t)-receptor complex could account for the inability of this form of gDt to block HSV infection.

Herpes simplex virus glycoprotein D can bind to poliovirus receptor-related protein 1 or herpesvirus entry mediator, two structurally unrelated mediators of virus entry

Several cell membrane proteins have been identified as herpes simplex virus (HSV) entry mediators (Hve). HveA (formerly HVEM) is a member of the tumor necrosis factor receptor family, whereas the poliovirus receptor-related proteins 1 and 2 (PRR1 and PRR2, renamed HveC and HveB) belong to the immunoglobulin superfamily. Here we show that a truncated form of HveC directly binds to HSV glycoprotein D (gD) in solution and at the surface of virions. This interaction is dependent on the native conformation of gD but independent of its N-linked glycosylation. Complex formation between soluble gD and HveC appears to involve one or two gD molecules for one HveC protein. Since HveA also mediates HSV entry by interacting with gD, we compared both structurally unrelated receptors for their binding to gD. Analyses of several gD variants indicated that structure and accessibility of the N-terminal domain of gD, essential for HveA binding, was not necessary for HveC interaction. Mutations in functional regions II, III, and IV of gD had similar effects on binding to either HveC or HveA. Competition assays with neutralizing anti-gD monoclonal antibodies (MAbs) showed that MAbs from group Ib prevented HveC and HveA binding to virions. However, group Ia MAbs blocked HveC but not HveA binding, and conversely, group VII MAbs blocked HveA but not HveC binding. Thus, we propose that HSV entry can be mediated by two structurally unrelated gD receptors through related but not identical binding with gD.

Examination of the kinetics of herpes simplex virus glycoprotein D binding to the herpesvirus entry mediator, using surface plasmon resonance

Previously, we showed that truncated soluble forms of herpes simplex virus (HSV) glycoprotein D (gDt) bound directly to a truncated soluble form of the herpesvirus entry mediator (HveAt, formerly HVEMt), a cellular receptor for HSV. The purpose of the present study was to determine the affinity of gDt for HveAt by surface plasmon resonance and to compare and contrast the kinetics of an expanded panel of gDt variants in binding to HveAt in an effort to better understand the mechanism of receptor binding and virus entry. Both HveAt and gDt are dimers in solution and interact with a 2:1 stoichiometry. With HveAt, gD1(306t) (from the KOS strain of HSV-1) had a dissociation constant (KD) of 3.2 x 10(-6) M and gD2(306t) had a KD of 1.5 x 10(-6) M. The interaction between gDt and HveAt fits a 1:1 Langmuir binding model, i.e., two dimers of HveAt may act as one binding unit to interact with one dimer of gDt as the second binding unit. A gD variant lacking all signals for N-linked oligosaccharides had an affinity for HveAt similar to that of gD1(306t). A variant lacking the bond from cysteine 1 to cysteine 5 had an affinity for HveAt that did not differ from that of the wild type. However, variants with double cysteine mutations that eliminated either of the other two disulfide bonds showed decreased affinity for HveAt. This result suggests that two of the three disulfide bonds of gD are important for receptor binding. Four nonfunctional gDt variants, each representing one functional domain of gD, were also studied. Mutations in functional regions I and II drastically decreased the affinity of gDt for HveAt. Surprisingly, a variant with an insertion in functional region III had a wild-type level of affinity for HveAt, suggesting that this domain may function in virus entry at a step other than receptor binding. A variant with a deletion in functional region IV [gD1(Delta290-299t)] exhibited a 100-fold enhancement in affinity for HveAt (KD = 3.3 x 10(-8) M) due mainly to a 40-fold increase in its kinetic on rate. This agrees with the results of other studies showing the enhanced ability of gD1(Delta290-299t) to block infection. Interestingly, all the variants with decreased affinities for HveAt exhibited decreased kinetic on rates but only minor changes in their kinetic off rates. The results suggest that once the complex between gDt and HveAt forms, its stability is unaffected by a variety of changes in gD.

Monoclonal antibodies to distinct sites on herpes simplex virus (HSV) glycoprotein D block HSV binding to HVEM

HVEM (for herpesvirus entry mediator) is a member of the tumor necrosis factor receptor superfamily and mediates entry of many strains of herpes simplex virus (HSV) into normally nonpermissive Chinese hamster ovary (CHO) cells. We used sucrose density centrifugation to demonstrate that purified HSV-1 KOS virions bind directly to a soluble, truncated form of HVEM (HVEMt) in the absence of any other cell-associated components. Therefore, HVEM mediates HSV entry by serving as a receptor for the virus. We previously showed that soluble, truncated forms of HSV glycoprotein D (gDt) bind to HVEMt in vitro. Here we show that antibodies specific for gD, but not the other entry glycoproteins gB, gC, or the gH/gL complex, completely block HSV binding to HVEM. Thus, virion gD is the principal mediator of HSV binding to HVEM. To map sites on virion gD which are necessary for its interaction with HVEM, we preincubated virions with gD-specific monoclonal antibodies (MAbs). MAbs that recognize antigenic sites Ib and VII of gD were the only MAbs which blocked the HSV-HVEM interaction. MAbs from these two groups failed to coprecipitate HVEMt in the presence of soluble gDt, whereas the other anti-gD MAbs coprecipitated HVEMt and gDt. Previous mapping data indicated that site VII includes amino acids 11 to 19 and site Ib includes 222 to 252. The current experiments indicate that these sites contain residues important for HSV binding to HVEM. Group Ib and VII MAbs also blocked HSV entry into HVEM-expressing CHO cells. These results suggest that the mechanism of neutralization by these MAbs is via interference with the interaction between gD in the virus and HVEM on the cell. Group Ia and II MAbs failed to block HSV binding to HVEM yet still neutralized HVEM-mediated entry, suggesting that these MAbs block entry at a step other than HVEM binding.

The glycoprotein D (US6) homolog is not essential for oncogenicity or horizontal transmission of Marek's disease virus

RB1BUS6lacgpt, a Marek's disease virus (MDV) mutant having a disrupted glycoprotein D (gD) homolog gene, established infection and induced tumors in chickens exposed to it by inoculation or by contact. Lymphoblastoid cell lines derived from RB1BUS6lacgpt-induced tumors harbored only the mutant virus. These results provide strong evidence that an intact gD homolog gene is not essential for oncogenicity or horizontal transmission of MDV.

The gH-gL complex of herpes simplex virus (HSV) stimulates neutralizing antibody and protects mice against HSV type 1 challenge

The herpes simplex virus type 1 (HSV-1) gH-gL complex which is found in the virion envelope is essential for virus infectivity and is a major antigen for the host immune system. However, little is known about the precise role of gH-gL in virus entry, and attempts to demonstrate the immunologic or vaccine efficacy of gH and gL separately or as the gH-gL complex have not succeeded. We constructed a recombinant mammalian cell line (HL-7) which secretes a soluble gH-gL complex, consisting of gH truncated at amino acid 792 (gHt) and full-length gL. Purified gHt-gL reacted with gH- and gL-specific monoclonal antibodies, including LP11, which indicates that it retains its proper antigenic structure. Soluble forms of gD (gDt) block HSV infection by interacting with specific cellular receptors. Unlike soluble gD, gHt-gL did not block HSV-1 entry into cells, nor did it enhance the blocking capacity of gD. However, polyclonal antibodies to the complex did block entry even when added after virus attachment. In addition, these antibodies exhibited high titers of complement-independent neutralizing activity against HSV-1. These sera also cross-neutralized HSV-2, albeit at low titers, and cross-reacted with gH-2 present in extracts of HSV-2-infected cells. To test the potential for gHt-gL to function as a vaccine, BALB/c mice were immunized with the complex. As controls, other mice were immunized with gD purified from HSV-infected cells or were sham immunized. Sera from the gD- or gHt-gL-immunized mice exhibited high titers of virus neutralizing activity. Using a zosteriform model of infection, we challenged mice with HSV-1. All animals showed some evidence of infection at the site of virus challenge. Mice immunized with either gD or gHt-gL showed reduced primary lesions and exhibited no secondary zosteriform lesions. The sham-immunized control animals exhibited extensive secondary lesions. Furthermore, mice immunized with either gD or gHt-gL survived virus challenge, while many control animals died. These results suggest that gHt-gL is biologically active and may be a candidate for use as a subunit vaccine.

LIGHT, a new member of the TNF superfamily, and lymphotoxin alpha are ligands for herpesvirus entry mediator

Herpes simplex virus (HSV) 1 and 2 infect activated T lymphocytes by attachment of the HSV envelope glycoprotein D (gD) to the cellular herpesvirus entry mediator (HVEM), an orphan member of the tumor necrosis factor receptor superfamily. Here, we demonstrate that HVEM binds two cellular ligands, secreted lymphotoxin alpha (LTalpha) and LIGHT, a new member of the TNF superfamily. LIGHT is a 29 kDa type II transmembrane protein produced by activated T cells that also engages the receptor for the LTalphabeta heterotrimer but does not form complexes with either LTalpha or LTbeta. HSV1 gD inhibits the interaction of HVEM with LIGHT, and LIGHT and gD interfere with HVEM-dependent cell entry by HSV1. This characterizes herpesvirus gD as a membrane-bound viokine and establishes LIGHT-HVEM as integral components of the lymphotoxin cytokine-receptor system.

Glycoprotein D of herpes simplex virus (HSV) binds directly to HVEM, a member of the tumor necrosis factor receptor superfamily and a mediator of HSV entry

Glycoprotein D (gD) is a structural component of the herpes simplex virus (HSV) envelope which is essential for virus entry into host cells. Chinese hamster ovary (CHO-K1) cells are one of the few cell types which are nonpermissive for the entry of many HSV strains. However, when these cells are transformed with the gene for the herpesvirus entry mediator (HVEM), the resulting cells, CHO-HVEM12, are permissive for many HSV strains, such as HSV-1(KOS). By virtue of its four cysteine-rich pseudorepeats, HVEM is a member of the tumor necrosis factor receptor superfamily of proteins. Recombinant forms of gD and HVEM, gD-1(306t) and HVEM(200t), respectively, were used to demonstrate a specific physical interaction between these two proteins. This interaction was dependent on native gD conformation but independent of its N-linked oligosaccharides, as expected from previous structure-function studies. Recombinant forms of gD derived from HSV-1(KOS)rid1 and HSV-1(ANG) did not bind to HVEM(200t), explaining the inability of these viruses to infect CHO-HVEM12 cells. A variant gD protein, gD-1(delta290-299t), showed enhanced binding to HVEM(200t) relative to the binding of gD-1(306t). Competition studies showed that gD-1(delta290-299t) and gD-1(306t) bound to the same region of HVEM(200t), suggesting that the differences in binding to HVEM are due to differences in affinity. These differences were also reflected in the ability of gD-1(delta290-299t) but not gD-1(306t) to block HSV type 1 infection of CHO-HVEM12 cells. By gel filtration chromatography, the complex between gD-1(delta290-299t) and HVEM(200t) had a molecular mass of 113 kDa and a molar ratio of 1:2. We conclude that HVEM interacts directly with gD, suggesting that HVEM is a receptor for virion gD and that the interaction between these proteins is a step in HSV entry into HVEM-expressing cells.

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.

CD4-independent association between HIV-1 gp120 and CXCR4: functional chemokine receptors are expressed in human neurons

BACKGROUND

Chemokines are a family of proteins that chemoattract and activate immune cells by interacting with specific receptors on the surface of their targets. We have shown previously that chemokine receptors including the interleukin-8 receptor B (CXCR2) and the Duffy blood group antigen are expressed on subsets of neurons in various regions of the adult nervous system.

RESULTS

Using a combination of immunohistochemical staining and receptor binding studies, we show that hNT cells, which are differentiated human neurons derived from the cell line NTera2, express functional chemokine receptors of the C-X-X and C-C types. These chemokine receptors include CXCR2, CXCR4, CCR1 and CCR5. We demonstrate high-affinity binding of both types of chemokines to hNT neurons and dose-dependent chemotactic responses to these chemokines in differentiated, but no t undifferentiated, NTera 2 cells. In addition, we show that the envelop glycoprotein from the T-cell-tropic human immunodeficiency virus 1 (HIV-1) strain IIIB is a CD4-independent, dose-dependent inhibitor of the binding of stromal cell-derived factor 1 to its receptor, CXCR4.

CONCLUSIONS

These data support recent findings that members of the chemokine family, including CCR5 and LESTR/Fusin (CXCR4), function as coreceptors in combination with CD4 for HIV-1 invasion. This is the first report of functional expression of chemokine receptors on human neurons. Furthermore, our studies provide for direct CD4-independent association of the viral envelope protein of the HIV-1 strain III with the chemokine receptor CXCR4.

From essential to beneficial: glycoprotein D loses importance for replication of bovine herpesvirus 1 in cell culture

Glycoprotein D (gD) of bovine herpesvirus 1 (BHV-1) has been shown to be an essential component of virions involved in virus entry. gD expression in infected cells is also required for direct cell-to-cell spread. Therefore, BHV-1 gD functions are identical in these aspects to those of herpes simplex virus 1 (HSV-1) gD. In contrast, the gD homolog of pseudorabies virus (PrV), although essential for penetration, is not necessary for direct cell-to-cell spread. Cocultivation of cells infected with phenotypically gD-complemented gD- mutant BHV-1/80-221 with noncomplementing cells resulted in the isolation of the cell-to-cell-spreading gD-negative mutant ctcs+BHV-1/80-221, which was present in the gD-null BIV-1 stocks. ctcs+BHV-1/80-221 could be propagated only by mixing infected with uninfected cells, and virions released into the culture medium were noninfectious. Marker rescue experiments revealed that a single point mutation in the first position of codon 450 of the glycoprotein H open reading frame, resulting in a glycine-to-tryptophan exchange, enabled complementation of the gD function for cell-to-cell spread. After about 40 continuous passages of ctcs+BHV-1/80-221-infected cells with noninfected cells, the plaque morphology in the cultures started to change from roundish to comet shaped. Cells from such plaques produced infectious gD- virus, named gD-infBHV-1, which entered cells much more slowly than wild-type BHV-1. In contrast, integration of the gD gene into the genomes of gD-infBHV-1 and ctcs+BHV-1/80-221 resulted in recombinants with accelerated penetration in comparison to wild-type virions. In summary, our results demonstrate that under selective conditions, the function of BHV-1 gD for direct cell-to-cell spread and entry into cells can be compensated for by mutations in other viral (glyco)proteins, leading to the hypothesis that gD is involved in formation of penetration-mediating complexes in the viral envelope of which gH is a component. Together with results for PrV, varicella-zoster virus, which lacks a gD homolog, and Marek's disease virus, whose gD homolog is not essential for infectivity, our data may open new insights into the evolution of alphaherpesviruses.