The location of asparagine-linked glycans on West Nile virions controls their interactions with CD209 (dendritic cell-specific ICAM-3 grabbing nonintegrin)

Mammalian cell-derived West Nile virus preferentially infects cells expressing the C-type lectin CD209L (dendritic cellspecific ICAM-3 grabbing nonintegrin-related protein; liver- and lymph node-specific ICAM-3 grabbing nonintegrin) but not cells expressing CD209 (dendritic cell-specific ICAM-3 grabbing nonintegrin). In contrast, Dengue virus infection is enhanced in cells expressing either attachment factor. The West Nile virus envelope (E) protein contains a single N-linked glycosylation site at residue 154, whereas Dengue virus E contains sites at residues 153 and 67. We introduced a glycosylation site at position 67 into West Nile virus E. Reporter virus particles pseudotyped with this E protein infected cells using either CD209 or CD209L. We also introduced glycosylation sites at several novel positions. All sites allowed CD209L-mediated infection, but only a subset promoted CD209 use. As seen for other viruses, mannose-rich glycans on West Nile virus were required for its interactions with CD209. Surprisingly, however, mannose-rich glycans were not required for CD209L-mediated infection. Complex glycans, particularly N-acetylglucosamine-terminated structures, were able to mediate reporter virus particle interactions with CD209L. We propose that CD209L recognizes glycosylated flaviviruses with broad specificity, whereas CD209 is selective for flaviviruses bearing mannose-rich glycans. The location of the N-linked glycosylation sites on a virion determines the types of glycans incorporated, thus controlling viral tropism for CD209-expressing cells.     

Critical role of Dengue Virus NS1 protein in viral replication

Dengue virus (DENV) nonstructural protein 1 (NS1) is a highly conserved 46-kDa protein that contains 2 glycosylation sites (Asn-130 and Asn-207) and 12 conserved cysteine (Cys) residues. Here, we performed site-directed mutagenesis to generate systematic mutants of viral strain TSV01. The results of the subsequent analysis showed that an alanine substitution at the second N-linked glycan Asn-207 in NS1 delayed viral RNA synthesis, reduced virus plaque size, and weakened the cytopathic effect. Three mutants at Cys sites (Cys-4, Cys-55, Cys-291) and a C-terminal deletion (ΔC) mutant significantly impaired RNA synthesis, and consequently abolished viral growth, whereas alanine mutations at Asn-130 and Glu-173 resulted in phenotypes that were similar to the wild-type (WT) virus. Further analysis showed that the Asn-207 mutation slightly delayed viral replication. These results suggest that the three conserved disulfide bonds and the second N-linked glycan in NS1 are required for DENV-2 replication.     

N-Linked Glycosylation of GP5 of Porcine Reproductive and Respiratory Syndrome Virus Is Critically Important for Virus Replication In Vivo

It has been proposed that the N-linked glycan addition at certain sites in GP5 of porcine reproductive and respiratory syndrome virus (PRRSV) is important for production of infectious viruses and viral infectivity. However, such specific N-linked glycosylation sites do not exist in some field PRRSV isolates. This implies that the existence of GP5-associated glycan per se is not vital to the virus life cycle. In this study, we found that mutation of individual glycosylation sites at N30, N35, N44, and N51 in GP5 did not affect virus infectivity in cultured cells. However, the mutants carrying multiple mutations at N-linked glycosylation sites in GP5 had significantly reduced virus yields compared with the wild-type (wt) virus. As a result, no viremia and antibody response were detected in piglets that were injected with a mutant without all N-linked glycans in GP5. These results suggest that the N-linked glycosylation of GP5 is critically important for virus replication in vivo. The study also showed that removal of N44-linked glycan from GP5 increased the sensitivity of mutant virus to convalescent-phase serum samples but did not elicit a high-level neutralizing antibody response to wt PRRSV. The results obtained from the present study have made significant contributions to better understanding the importance of glycosylation of GP5 in the biology of PRRSV.     

Ross River virus envelope glycans contribute to type I interferon production in myeloid dendritic cells

Alphaviruses are mosquito-transmitted viruses that cause significant human disease, and understanding how these pathogens successfully transition from the mosquito vector to the vertebrate host is an important area of research. Previous studies demonstrated that mosquito and mammalian-cell-derived alphaviruses differentially induce type I interferons (alpha/beta interferon [IFN-alpha/beta]) in myeloid dendritic cells (mDCs), where the mosquito cell-derived virus is a poor inducer of IFN-alpha/beta compared to the mammalian-cell-derived virus. Furthermore, the reduced IFN-alpha/beta induction by the mosquito cell-derived virus is attributed to differential N-linked glycosylation. To further evaluate the role of viral envelope glycans in regulating the IFN-alpha/beta response, studies were performed to assess whether the mosquito cell-derived virus actively inhibits IFN-alpha/beta induction or is simply a poor inducer of IFN-alpha/beta. Coinfection studies using mammalian- and mosquito cell-derived Ross River virus (mam-RRV and mos-RRV, respectively) indicated that mos-RRV was unable to suppress IFN-alpha/beta induction by mam-RRV in mDC cultures. Additionally, a panel of mutant viruses lacking either individual or multiple N-linked glycosylation sites was used to demonstrate that N-linked glycans were essential for high-level IFN-alpha/beta induction by the mammalian-cell-derived virus. These results suggest that the failure of the mosquito cell-derived virus to induce IFN-alpha/beta is due to a lack of complex carbohydrates on the virion rather than the active suppression of the DC antiviral response.     

Identification of two N-linked glycosylation sites within the core of the simian immunodeficiency virus glycoprotein whose removal enhances sensitivity to soluble CD4

Using PCR mutagenesis to disrupt the NXT/S N-linked glycosylation motif of the Env protein, we created 27 mutants lacking 1 to 5 of 14 N-linked glycosylation sites within regions of gp120 lying outside of variable loops 1 to 4 within simian immunodeficiency virus strain 239 (SIV239). Of 18 mutants missing N-linked glycosylation sites predicted to lie within 10 A of CD4 contact sites, the infectivity of 12 was sufficient to measure sensitivity to neutralization by soluble CD4 (sCD4), pooled immune sera from SIV239-infected rhesus macaques, and monoclonal antibodies known to neutralize certain derivatives of SIV239. Three of these 12 mutants (g3, lacking the 3rd glycan at position 79; g11, lacking the 11th glycan at position 212; and g3,11, lacking both the 3rd and 11th glycans) were approximately five times more sensitive to neutralization by sCD4 than wild-type (WT) SIV239. However, these same mutants were no more sensitive to neutralization than WT by pooled immune sera. The other 9 of 12 replication-competent mutants in this group were no more sensitive to neutralization than the WT by any of the neutralizing reagents. Six of the nine mutants that did not replicate appreciably had three or more glycosylation sites eliminated; the other three replication-deficient strains involved mutation of site 15. Our results suggest that elimination of glycan attachment sites 3 and 11 enhanced the exposure of contact residues for CD4. Thus, glycans at positions 3 and 11 of SIV239 gp120 may be particularly important for shielding the CD4-binding site from antibody recognition.     

N-linked glycosylation of west nile virus envelope proteins influences particle assembly and infectivity

West Nile virus (WNV) encodes two envelope proteins, premembrane (prM) and envelope (E). While the prM protein of all WNV strains contains a single N-linked glycosylation site, not all strains contain an N-linked site in the E protein. The presence of N-linked glycosylation on flavivirus E proteins has been linked to virus production, pH sensitivity, and neuroinvasiveness. Therefore, we examined the impact of prM and E glycosylation on WNV assembly and infectivity. Similar to other flaviviruses, expression of WNV prM and E resulted in the release of subviral particles (SVPs). Removing the prM glycosylation site in a lineage I or II strain decreased SVP release, as did removal of the glycosylation site in a lineage I E protein. Addition of the E protein glycosylation site in a lineage II strain that lacked this site increased SVP production. Similar results were obtained in the context of either reporter virus particles (RVPs) or infectious lineage II WNV. RVPs or virions bearing combinations of glycosylated and nonglycosylated forms of prM and E could infect mammalian, avian, and mosquito cells (BHK-21, QT6, and C6/36, respectively). Those particles lacking glycosylation on the E protein were modestly more infectious per genome copy on BHK-21 and QT6 cells, while this absence greatly enhanced the infection of C6/36 cells. Thus, glycosylation of WNV prM and E proteins can affect the efficiency of virus release and infection in a manner that is cell type and perhaps species dependent. This suggests a multifaceted role for envelope N-linked glycosylation in WNV biology and tropism.     

Differential RNA Sequence Requirement for Dengue Virus Replication in Mosquito and Mammalian Cells

Dengue virus cycles between mosquitoes and humans. Each host provides a different environment for viral replication, imposing different selective pressures. We identified a sequence in the dengue virus genome that is essential for viral replication in mosquito cells but not in mammalian cells. This sequence is located at the viral 3′ untranslated region and folds into a small hairpin structure. A systematic mutational analysis using dengue virus infectious clones and reporter viruses allowed the determination of two putative functions in this cis-acting RNA motif, one linked to the structure and the other linked to the nucleotide sequence. We found that single substitutions that did not alter the hairpin structure did not affect dengue virus replication in mammalian cells but abolished replication in mosquito cells. This is the first sequence identified in a flavivirus genome that is exclusively required for viral replication in insect cells.     

Mutational analysis of N-linked glycosylation sites of Friend murine leukemia virus envelope protein.

The roles played by the N-linked glycans of the Friend murine leukemia virus envelope proteins were investigated by site-specific mutagenesis. The surface protein gp70 has eight potential attachment sites for N-linked glycan; each signal asparagine was converted to aspartate, and mutant viruses were tested for the ability to grow in NIH 3T3 fibroblasts. Seven of the mutations did not affect virus infectivity, whereas mutation of the fourth glycosylation signal from the amino terminus (gs4) resulted in a noninfectious phenotype. Characterization of mutant gene products by radioimmunoprecipitation confirmed that glycosylation occurs at all eight consensus signals in gp70 and that gs2 carries an endoglycosidase H-sensitive glycan. Elimination of gs2 did not cause retention of an endoglycosidase H-sensitive glycan at a different site, demonstrating that this structure does not play an essential role in envelope protein function. The gs3- mutation affected a second posttranslational modification of unknown type, which was manifested as production of gp70 that remained smaller than wild-type gp70 after removal of all N-linked glycans by peptide N-glycosidase F. The gs4- mutation decreased processing of gPr80 to gPr90, completely inhibited proteolytic processing of gPr90 to gp70 and Pr15(E), and prevented incorporation of envelope products into virus particles. Brefeldin A-induced mixing of the endoplasmic reticulum and parts of the Golgi apparatus allowed proteolytic processing of wild-type gPr90 to occur in the absence of protein transport, but it did not overcome the cleavage defect of the gs4- precursor, indicating that gs4- gPr90 is resistant to the processing protease. The work reported here demonstrates that the gs4 region is important for env precursor processing and suggests that gs4 may be a critical target in the disruption of murine leukemia virus env product processing by inhibitors of N-linked glycosylation.     

Site occupancy and glycan compositional analysis of two soluble recombinant forms of the attachment glycoprotein of Hendra virus

Hendra virus (HeV) continues to cause morbidity and mortality in both humans and horses with a number of sporadic outbreaks. HeV has two structural membrane glycoproteins that mediate the infection of host cells: the attachment (G) and the fusion (F) glycoproteins that are essential for receptor binding and virion-host cell membrane fusion, respectively. N-linked glycosylation of viral envelope proteins are critical post-translation modifications that have been implicated in roles of structural integrity, virus replication and evasion of the host immune response. Deciphering the glycan composition and structure on these glycoproteins may assist in the development of glycan-targeted therapeutic intervention strategies. We examined the site occupancy and glycan composition of recombinant soluble G (sG) glycoproteins expressed in two different mammalian cell systems, transient human embryonic kidney 293 (HEK293) cells and vaccinia virus (VV)-HeLa cells, using a suite of biochemical and biophysical tools: electrophoresis, lectin binding and tandem mass spectrometry. The N-linked glycans of both VV and HEK293-derived sG glycoproteins carried predominantly mono- and disialylated complex-type N-glycans and a smaller population of high mannose-type glycans. All seven consensus sequences for N-linked glycosylation were definitively found to be occupied in the VV-derived protein, whereas only four sites were found and characterized in the HEK293-derived protein. We also report, for the first time, the existence of O-linked glycosylation sites in both proteins. The striking characteristic of both proteins was glycan heterogeneity in both N- and O-linked sites. The structural features of G protein glycosylation were also determined by X-ray crystallography and interactions with the ephrin-B2 receptor are discussed.     

Functional analysis of N-linked glycosylation mutants of the measles virus fusion protein synthesized by recombinant vaccinia virus vectors.

The role of N-linked glycosylation in the biological activity of the measles virus (MV) fusion (F) protein was analyzed by expressing glycosylation mutants with recombinant vaccinia virus vectors. There are three potential N-linked glycosylation sites located on the F2 subunit polypeptide of MV F, at asparagine residues 29, 61, and 67. Each of the three potential glycosylation sites was mutated separately as well as in combination with the other sites. Expression of mutant proteins in mammalian cells showed that all three sites are used for the addition of N-linked oligosaccharides. Cell surface expression of mutant proteins was reduced by 50% relative to the wild-type level when glycosylation at either Asn-29 or Asn-61 was abolished. Despite the similar levels of cell surface expression, the Asn-29 and Asn-61 mutant proteins had different biological activities. While the Asn-61 mutant was capable of inducing syncytium formation, the Asn-29 mutant protein did not exhibit any significant cell fusion activity. Inactivation of the Asn-67 glycosylation site also reduced cell surface transport of mutant protein but had little effect on its ability to cause cell fusion. However, when the Asn-67 mutation was combined with mutations at either of the other two sites, cleavage-dependent activation, cell surface expression, and cell fusion activity were completely abolished. Our data show that the loss of N-linked oligosaccharides markedly impaired the proteolytic cleavage, stability, and biological activity of the MV F protein. The oligosaccharide side chains in MV F are thus essential for optimum conformation of the extracellular F2 subunit that is presumed to bind cellular membranes.     

Identification of two surface proteins from C6/36 cells that bind dengue type 4 virus.

Dengue viruses infect cells by attaching to a surface receptor, probably through the envelope (E) glycoprotein, located on the surface of the viral membrane. However, the identity of the dengue virus receptor in the mosquito and in mammalian host cells remains unknown. To identify and characterize the molecules responsible for binding dengue virus, overlay protein blot and binding assays were performed with labeled virus. Two glycoproteins of 40 and 45 kDa located on the surface of C6/36 cells bound dengue type 4 virus. Virus binding by total and membrane proteins obtained from trypsin-treated cells was inhibited, while neuraminidase treatment did not inhibit binding. Periodate treatment of cell proteins did not reduce virus binding, but it modified the molecular weight of the polypeptide detected by overlay assays. Preincubation of C6/36 cells with electroeluted 40- and 45-kDa proteins or with specific antibodies raised against these proteins inhibited virus binding. These results strongly suggest that the 40- and 45-kDa surface proteins are putative receptors or part of a receptor complex for dengue virus.     

The highly conserved glycan at asparagine 260 of HIV-1 gp120 is indispensable for viral entry

Carbohydrate-binding agents bind to the N-glycans of HIV-1 envelope gp120 and prevent viral entry. Carbohydrate-binding agents can select for mutant viruses with deleted envelope glycans. Not all glycosylation motifs are mutated to the same extent. Site-directed mutagenesis revealed that deletions destroying the highly conserved (260)NGS(262) glycosylation motif resulted in non-infectious virus particles. We observed a significant lower CD4 binding in the case of the N260Q mutant gp120 virus strains, caused by a strikingly lower expression of gp120 and gp41 in the virus particle. In addition, the mutant N260Q HIV-1 envelope expressed in 293T cells was unable to form syncytia in co-cultures with U87.CD4.CXCR4.CCR5 cells, due to the lower expression of envelope protein on the surface of the transfected 293T cells. The detrimental consequence of this N-glycan deletion on virus infectivity could not be compensated for by the creation of novel glycosylation sites near this amino acid, leaving this uncovered envelope epitope susceptible to neutralizing antibody binding. Thus, the Asn-260 glycan in the gp120 envelope of HIV-1 represents a hot spot for targeting suicidal drugs or antibodies in a therapeutic effort to efficiently neutralize a broad array of virus strains.     

Molecular Docking Based Screening of Plant Flavonoids as Dengue NS1 Inhibitors

Dengue infection has turned into a serious health concern globally due to its high morbidity rate and a high possibility of increase in its mortality rate on the account of unavailability of any proper treatment for severe dengue infection. The situation demands an urgent development of efficient and practicable treatment to deal with Dengue virus (DENV). Flavonoids, a class of phytochemicals present in medicinal plants, possess anti-viral activity and can be strong drug candidates against viruses. NS1 glycoprotein of Dengue virus is involved in its RNA replication and can be a strong target for screening of drugs against this virus. Current study focuses on the identification of flavonoids which can block Asn-130 glycosylation site of Dengue virus NS1 to inhibit viral replication as glycosylation of NS1 is required for its biological functioning. Molecular docking approach was used in this study and the results revealed that flavonoids have strong potential interactions with active site of NS1. Six flavonoids (Deoxycalyxin A; 3,5,7,3'',4''-pentahydroxyflavonol-3-O-beta-D-galactopyranoside; (3R)-3'',8-Dihydroxyvestitol; Sanggenon O; Epigallocatechin gallate; Chamaejasmin) blocked the Asn-130 glycosylation site of NS1 and could be able to inhibit the viral replication. It can be concluded from this study that these flavonoids could serve as antiviral drugs for dengue infections. Further in-vitro analyses are required to confirm their efficacy and to evaluate their drug potency.     

Role of N-linked glycans of HCV glycoprotein E1 in folding of structural proteins and formation of viral particles

Envelope proteins E1 and E2 of the hepatitis C virus (HCV) play a major role in the life cycle of a virus. These proteins are the main components of the virion and are involved in virus assembly. Envelope proteins are modified by N-linked glycosylation, which is supposed to play a role in their stability, in the assembly of the functional glycoprotein heterodimer, in protein folding, and in viral entry. The effects of N-linked glycosylation of HCV protein E1 on the assembly of structural proteins were studied using site-directed mutagenesis in a model system of Sf9 insect cells producing three viral structural proteins with the formation of virus-like particles due to the baculovirus expression system. The removal of individual N-glycosylation sites in HCV protein E1 did not affect the efficiency of its expression in insect Sf9 cells. The electrophoretic mobility of E1 increased with a decreasing number of N-glycosylation sites. The destruction of E1 glycosylation sites N1 or N5 influenced the assembly of the noncovalent E1E2 glycoprotein heterodimer, which is the prototype of the natural complex within the HCV virion. It was also shown that the lack of glycans at E1 sites N1 and N5 significantly reduced the efficiency of E1 expression in mammalian HEK293 T cells.     

N-linked glycans with similar location in the fusion protein head modulate paramyxovirus fusion

N-linked glycans not only orchestrate the folding and intracellular transport of viral glycoproteins but also modulate their function. We have characterized the three glycans attached to fusion (F) proteins of the morbilliviruses canine distemper virus and measles virus. The individual Morbillivirus glycans have similar functional properties: the glycan at position 68 is essential for protein transport, and those at positions 36 and 75 modulate fusion (numbering according to the Newcastle disease virus [NDV] F protein sequence). Based on the crystal structure of the NDV F protein, we then predicted the locations of the Morbillivirus glycans: the glycan at position 36 is located in the F protein head, and those at positions 68 and 75 are located near the neck-stalk interface. NDV position 36 is not occupied by a glycan; the only glycan in that F protein head also has a fusion control function and grows from residue 366, located only 6 A from residue 36. We then exchanged the glycan at position 36 with the glycan at position 366 and showed functional complementation. Thus, structural information about the F proteins of Paramyxoviridae coupled with functional analysis disclosed a location in the protein head into which fusion-modulating glycans independently evolved.     

Release of Dengue Virus Genome Induced by a Peptide Inhibitor

Dengue virus infects approximately 100 million people annually, but there is no available therapeutic treatment. The mimetic peptide, DN59, consists of residues corresponding to the membrane interacting, amphipathic stem region of the dengue virus envelope (E) glycoprotein. This peptide is inhibitory to all four serotypes of dengue virus, as well as other flaviviruses. Cryo-electron microscopy image reconstruction of dengue virus particles incubated with DN59 showed that the virus particles were largely empty, concurrent with the formation of holes at the five-fold vertices. The release of RNA from the viral particle following incubation with DN59 was confirmed by increased sensitivity of the RNA genome to exogenous RNase and separation of the genome from the E protein in a tartrate density gradient. DN59 interacted strongly with synthetic lipid vesicles and caused membrane disruptions, but was found to be non-toxic to mammalian and insect cells. Thus DN59 inhibits flavivirus infectivity by interacting directly with virus particles resulting in release of the genomic RNA.     

Asparagine-linked glycosylation of human chymotrypsin C is required for folding and secretion but not for enzyme activity

Human chymotrypsin C (CTRC) plays a protective role in the pancreas by mitigating premature trypsinogen activation through degradation. Mutations that abolish activity or secretion of CTRC increase the risk for chronic pancreatitis. The aim of the present study was to determine whether human CTRC undergoes asparagine-linked (N-linked) glycosylation and to examine the role of this modification in CTRC folding and function. We abolished potential sites of N-linked glycosylation (Asn-Xaa-Ser/Thr) in human CTRC by mutating the Asn residues to Ser individually or in combination, expressed the CTRC mutants in HEK 293T cells and determined their glycosylation state using PNGase F and endo H digestion. We found that human CTRC contains a single N-linked glycan on Asn52. Elimination of N-glycosylation by mutation of Asn52 (N52S) reduced CTRC secretion about 10-fold from HEK 293T cells but had no effect on CTRC activity or inhibitor binding. Overexpression of the N52S CTRC mutant elicited endoplasmic reticulum stress in AR42J acinar cells, indicating that N-glycosylation is required for folding of human CTRC. Despite its important role, Asn52 is poorly conserved in other mammalian CTRC orthologs, including the rat which is monoglycosylated on Asn90. Introduction of the Asn90 site in a non-glycosylated human CTRC mutant restored full glycosylation but only partially rescued the secretion defect. We conclude that N-linked glycosylation of human CTRC is required for efficient folding and secretion; however, the N-linked glycan is unimportant for enzyme activity or inhibitor binding. The position of the N-linked glycan is critical for optimal folding, and it may vary among the otherwise highly homologous mammalian CTRC sequences.     

Role of N-linked oligosaccharides in processing and intracellular transport of E2 glycoprotein of rubella virus.

The role of N-linked glycosylation in processing and intracellular transport of rubella virus glycoprotein E2 has been studied by expressing glycosylation mutants of E2 in COS cells. A panel of E2 glycosylation mutants were generated by oligonucleotide-directed mutagenesis. Each of the three potential N-linked glycosylation sites was eliminated separately as well as in combination with the other two sites. Expression of the E2 mutant proteins in COS cells indicated that in rubella virus M33 strain, all three sites are used for the addition of N-linked oligosaccharides. Removal of any of the glycosylation sites resulted in slower glycan processing, lower stability, and aberrant disulfide bonding of the mutant proteins, with the severity of defect depending on the number of deleted carbohydrate sites. The mutant proteins were transported to the endoplasmic reticulum and Golgi complex but were not detected on the cell surface. However, the secretion of the anchor-free form of E2 into the medium was not completely blocked by the removal of any one of its glycosylation sites. This effect was dependent on the position of the deleted glycosylation site.     

Mosquito and mammalian cells grown on microcarriers for four-serotype dengue virus production: variations in virus titer, plaque morphology, and replication rate

Dengue (DEN) viruses consisting of four distinct serotypes cause diseases such as dengue fever, dengue hemorrhagic fever, and dengue shock syndrome in humans. Most of the dengue viruses can be effectively propagated in some mosquito and mammalian cell lines. In this study, we applied microcarrier cell culture technology to study two relevant aspects involving dengue virus, one on biotechnology of cell growth and virus production, and the other on virus biology concerning genetic variation of a virus population. We investigated the growth of C6/36 mosquito cells and Vero cells grown on Cytodex 1 microcarriers. High-titer DEN virus production can be achieved in C6/36 and Vero cells infected at low cell inoculation density, in the lag-phase cell stage, and at low multiplicity of infection (MOI). The maximum titers produced for DEN-1, DEN-3, and DEN-4 viruses were approximately 10- to 10,000-fold lower than for DEN-2 virus produced in C6/36 and Vero cells grown on microcarriers. The DEN-2 virus produced in C6/36 cells displayed far more extensive plaque heterogeneity than in Vero cells. Microcarrier C6/36 mosquito cell culture appeared to be the most effective system for four-serotype DEN virus production. Interestingly, some selected variants of DEN virus may outgrow in Vero cells when using a T-flask culture. These results may provide useful information for DEN vaccine development.