Differential N-linked glycosylation of human immunodeficiency virus and Ebola virus envelope glycoproteins modulates interactions with DC-SIGN and DC-SIGNR

Baculovirus-derived human immunodeficiency virus type 1 virus-like particles activate dendritic cells and induce ex vivo T-cell responses

We have recently developed a candidate human immunodeficiency virus type 1 (HIV-1) vaccine model based on HIV-1 Pr55(gag) virus-like particles (HIV-VLPs), produced in a baculovirus expression system and presenting a gp120 molecule from a Ugandan HIV-1 isolate of clade A (HIV-VLP(A)s). The HIV-VLP(A)s show the induction in BALB/c mice of systemic and mucosal neutralizing antibodies as well as cytotoxic T lymphocytes, by intraperitoneal as well as intranasal administration. In the present article, the effects of the baculovirus-expressed HIV-VLPs on human immature monocyte-derived dendritic cells (MDDCs) have been evaluated. The HIV-VLPs efficiently induce maturation and activation of MDDCs and are incorporated into MDDCs preferentially via an actin-dependent macropinocytosis and endocytosis. The HIV-VLP-activated MDDCs show enhanced Th1- and Th2-specific cytokine production, and the effects of HIV-VLPs on MDDCs are not mediated through Toll-like receptors 2 and 4 (TLR2 and -4) signaling. Finally, HIV-VLP-loaded MDDCs are able to induce a primary and secondary response in autologous human CD4(+) T cells in an ex vivo immunization assay. Our results on the interaction and processing of baculovirus HIV-VLPs by MDDCs give an insight into the mechanisms underlying the immune response induced by HIV-VLP(A)s in vivo.

The signal peptide of the ebolavirus glycoprotein influences interaction with the cellular lectins DC-SIGN and DC-SIGNR

The C-type lectins DC-SIGN and DC-SIGNR (collectively referred to as DC-SIGN/R) bind to the ebolavirus glycoprotein (EBOV-GP) and augment viral infectivity. DC-SIGN/R strongly enhance infection driven by the GP of EBOV subspecies. Zaire (ZEBOV) but have a much less pronounced effect on infection mediated by the GP of EBOV subspecies. Sudan (SEBOV). For this study, we analyzed the determinants of the differential DC-SIGN/R interactions with ZEBOV- and SEBOV-GP. The efficiency of DC-SIGN engagement by ZEBOV-GP was dependent on the rate of GP incorporation into lentiviral particles, while appreciable virion incorporation of SEBOV-GP did not allow robust DC-SIGN/R usage. Forced incorporation of high-mannose carbohydrates into SEBOV-GP augmented the engagement of DC-SIGN/R to the levels observed with ZEBOV-GP, indicating that appropriate glycosylation of SEBOV-GP is sufficient for efficient DC-SIGN/R usage. However, neither signals for N-linked glycosylation unique to SEBOV- or ZEBOV-GP nor the highly variable and heavily glycosylated mucin-like domain modulated the interaction with DC-SIGN/R. In contrast, analysis of chimeric GPs identified the signal peptide as a determinant of DC-SIGN/R engagement. Thus, ZEBOV- but not SEBOV-GP was shown to harbor high-mannose carbohydrates, and GP modification with these glycans was controlled by the signal peptide. These results suggest that the signal peptide governs EBOV-GP interactions with DC-SIGN/R by modulating the incorporation of high-mannose carbohydrates into EBOV-GP. In summary, we identified the level of GP incorporation into virions and signal peptide-controlled glycosylation of GP as determinants of attachment factor engagement.

Modulation of virion incorporation of Ebolavirus glycoprotein: effects on attachment, cellular entry and neutralization

The filoviruses Ebolavirus (EBOV) and Marburgvirus (MARV) cause severe hemorrhagic fever in humans and are potential agents of biological warfare. The envelope glycoprotein (GP) of filoviruses mediates viral entry into cells and is an attractive target for therapeutic intervention and vaccine design. Here, we asked if the efficiency of virion incorporation of EBOV-GP impacts attachment and entry into target cells and modulates susceptibility to neutralizing antibodies. In order to control the level of EBOV-GP expression, we generated cell lines expressing the GPs of the four known EBOV subspecies in an inducible fashion. Regulated expression of GP on the cell surface allowed production of reporter viruses harboring different amounts of GP. A pronounced reduction of virion incorporation of EBOV-GP had relatively little effect on virion infectivity, suggesting that only a few copies of GP might be sufficient for efficient engagement of cellular receptors. In contrast, optimal interactions with cellular attachment factors like the DC-SIGN protein required incorporation of high amounts of GP. Antibody-mediated neutralization of virions bearing high amounts of GP was slightly more efficient than neutralization of virions harboring low amounts of GP, suggesting that the efficiency of GP incorporation into virions might modulate susceptibility to neutralizing antibodies. Finally, regulated expression of GP in permissive 293 cells did not reduce EBOV-GP-driven infection but diminished vesicular stomatitis virus GP (VSV-G) and amphotropic murine leukemia virus (A-MLV) GP mediated entry in a dose-dependent manner. Therefore, intracellular GP does not seem to downmodulate expression of its receptor(s) but might alter expression and/or function of molecules involved in VSV-G and A-MLV-GP-dependent entry. Our results suggest that the efficiency of virion incorporation of GP could impact EBOV attachment to target cells and might modulate control of viral spread by the humoral immune response.

Measles virus targets DC-SIGN to enhance dendritic cell infection

Dendritic cells (DCs) are involved in the pathogenesis of measles virus (MV) infection by inducing immune suppression and possibly spreading the virus from the respiratory tract to lymphatic tissues. It is becoming evident that DC function can be modulated by the involvement of different receptors in pathogen interaction. Therefore, we have investigated the relative contributions of different MV-specific receptors on DCs to MV uptake into and infection of these cells. DCs express the MV receptors CD46 and CD150, and we demonstrate that the C-type lectin DC-specific intercellular adhesion molecule 3-grabbing nonintegrin (DC-SIGN) is a novel receptor for laboratory-adapted and wild-type MV strains. The ligands for DC-SIGN are both MV glycoproteins F and H. In contrast to CD46 and CD150, DC-SIGN does not support MV entry, since DC-SIGN does not confer susceptibility when stably expressed in CHO cells. However, DC-SIGN is important for the infection of immature DCs with MV, since both attachment and infection of immature DCs with MV are blocked in the presence of DC-SIGN inhibitors. Our data demonstrate that DC-SIGN is crucial as an attachment receptor to enhance CD46/CD150-mediated infection of DCs in cis. Moreover, MV might not only target DC-SIGN to infect DCs but may also use DC-SIGN for viral transmission and immune suppression.

Conserved receptor-binding domains of Lake Victoria marburgvirus and Zaire ebolavirus bind a common receptor

The GP(1,2) envelope glycoproteins (GP) of filoviruses (marburg- and ebolaviruses) mediate cell-surface attachment, membrane fusion, and entry into permissive cells. Here we show that a 151-amino acid fragment of the Lake Victoria marburgvirus GP1 subunit bound filovirus-permissive cell lines more efficiently than full-length GP1. An homologous 148-amino acid fragment of the Zaire ebolavirus GP1 subunit similarly bound the same cell lines more efficiently than a series of longer GP1 truncation variants. Neither the marburgvirus GP1 fragment nor that of ebolavirus bound a nonpermissive lymphocyte cell line. Both fragments specifically inhibited replication of infectious Zaire ebolavirus, as well as entry of retroviruses pseudotyped with either Lake Victoria marburgvirus or Zaire ebolavirus GP(1,2). These studies identify the receptor-binding domains of both viruses, indicate that these viruses utilize a common receptor, and suggest that a single small molecule or vaccine can be developed to inhibit infection of all filoviruses.

Functional comparison of mouse CIRE/mouse DC-SIGN and human DC-SIGN

CIRE/mDC-SIGN is a C-type lectin we originally identified as a molecule differentially expressed by mouse dendritic cell (DC) populations. Immunostaining with a CIRE/mDC-SIGN-specific mAb revealed that CIRE/mDC-SIGN is indeed on the surface of some CD4⁺, CD4⁻8⁻ DCs and plasmacytoid pre-DCs, but not on CD8⁺ DCs. It has been proposed that CIRE/mDC-SIGN is the functional orthologue of human DC-SIGN (hDC-SIGN), a molecule that both enhances T cell responses and facilitates antigen uptake. We assessed if CIRE/mDC-SIGN and hDC-SIGN exhibit functional similarities. CIRE/mDC-SIGN is down-regulated upon activation, but unlike hDC-SIGN, incubation with IL-4 and IL-13 did not enhance CIRE/mDC-SIGN expression, indicating differences in gene regulation. Like hDC-SIGN, CIRE/mDC-SIGN bound mannosylated residues. However, we could detect no role for CIRE/mDC-SIGN in T cell–DC interactions and the protein did not bind to pathogens known to interact with hDC-SIGN, including Leishmania mexicana, cytomegalovirus, HIV and lentiviral particles bearing the Ebolavirus glycoprotein. The binding of CIRE/mDC-SIGN to hDC-SIGN ligands was not rescued when CIRE/mDC-SIGN was engineered to express the stalk region of hDC-SIGN. We conclude that there are significant differences in the fine specificity of the C-type lectin domains of hDC-SIGN and CIRE/mDC-SIGN and that these two molecules may not be functional orthologues.

West Nile virus discriminates between DC-SIGN and DC-SIGNR for cellular attachment and infection

The C-type lectins DC-SIGN and DC-SIGNR bind mannose-rich glycans with high affinity. In vitro, cells expressing these attachment factors efficiently capture, and are infected by, a diverse array of appropriately glycosylated pathogens, including dengue virus. In this study, we investigated whether these lectins could enhance cellular infection by West Nile virus (WNV), a mosquito-borne flavivirus related to dengue virus. We discovered that DC-SIGNR promoted WNV infection much more efficiently than did DC-SIGN, particularly when the virus was grown in human cell types. The presence of a single N-linked glycosylation site on either the prM or E glycoprotein of WNV was sufficient to allow DC-SIGNR-mediated infection, demonstrating that uncleaved prM protein present on a flavivirus virion can influence viral tropism under certain circumstances. Preferential utilization of DC-SIGNR was a specific property conferred by the WNV envelope glycoproteins. Chimeras between DC-SIGN and DC-SIGNR demonstrated that the ability of DC-SIGNR to promote WNV infection maps to its carbohydrate recognition domain. WNV virions and subviral particles bound to DC-SIGNR with much greater affinity than DC-SIGN. We believe this is the first report of a pathogen interacting more efficiently with DC-SIGNR than with DC-SIGN. Our results should lead to the discovery of new mechanisms by which these well-studied lectins discriminate among ligands.

Interactions between virus proteins and host cell membranes during the viral life cycle

The structure and function of cells are critically dependent on membranes, which not only separate the interior of the cell from its environment but also define the internal compartments. It is therefore not surprising that the major steps of the life cycle of viruses of animals and plants also depend on cellular membranes. Indeed, interactions of viral proteins with host cell membranes are important for viruses to enter into host cells, replicate their genome, and produce progeny particles. To replicate its genome, a virus first needs to cross the plasma membrane. Some viruses can also modify intracellular membranes of host cells to create a compartment in which genome replication will take place. Finally, some viruses acquire an envelope, which is derived either from the plasma membrane or an internal membrane of the host cell. This paper reviews recent findings on the interactions of viral proteins with host cell membranes during the viral life cycle.

Homozygous L-SIGN (CLEC4M) plays a protective role in SARS coronavirus infection

Severe acute respiratory syndrome (SARS) is caused by infection of a previously undescribed coronavirus (CoV). L-SIGN, encoded by CLEC4M (also known as CD209L), is a SARS-CoV binding receptor that has polymorphism in its extracellular neck region encoded by the tandem repeat domain in exon 4. Our genetic risk association study shows that individuals homozygous for CLEC4M tandem repeats are less susceptible to SARS infection. L-SIGN is expressed in both non-SARS and SARS-CoV-infected lung. Compared with cells heterozygous for L-SIGN, cells homozygous for L-SIGN show higher binding capacity for SARS-CoV, higher proteasome-dependent viral degradation and a lower capacity for trans infection. Thus, homozygosity for L-SIGN plays a protective role during SARS infection.

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.

LSECtin interacts with filovirus glycoproteins and the spike protein of SARS coronavirus

Cellular attachment factors like the C-type lectins DC-SIGN and DC-SIGNR (collectively referred to as DC-SIGN/R) can augment viral infection and might promote viral dissemination in and between hosts. The lectin LSECtin is encoded in the same chromosomal locus as DC-SIGN/R and is coexpressed with DC-SIGNR on sinusoidal endothelial cells in liver and lymphnodes. Here, we show that LSECtin enhances infection driven by filovirus glycoproteins (GP) and the S protein of SARS coronavirus, but does not interact with human immunodeficiency virus type-1 and hepatitis C virus envelope proteins. Ligand binding to LSECtin was inhibited by EGTA but not by mannan, suggesting that LSECtin unlike DC-SIGN/R does not recognize high-mannose glycans on viral GPs. Finally, we demonstrate that LSECtin is N-linked glycosylated and that glycosylation is required for cell surface expression. In summary, we identified LSECtin as an attachment factor that in conjunction with DC-SIGNR might concentrate viral pathogens in liver and lymph nodes.

Characterization of a highly glycosylated form of the human cytomegalovirus HLA class I homologue gpUL18

Human cytomegalovirus (HCMV) gpUL18 is a HLA class I (HLA-I) homologue with high affinity for the inhibitory receptor LIR-1/ILT2. The previously described 67 kDa form of gpUL18 is shown here to be sensitive to endoglycosidase-H (EndoH). A novel form of gpUL18 with a molecular mass of approximately 160 kDa and resistance to EndoH was identified in cells infected with HCMV strain AD169 or the low passage HCMV isolates Merlin and Toledo. The 67 kDa EndoH-sensitive gpUL18 glycoform was detected earlier in a productive infection (from 24 h post-infection) than the slower-migrating EndoH-resistant glycoform (from 72 h post-infection). Deletion of the US2-US11 region from the HCMV genome was associated with a substantial up-regulation of endogenous HLA-I in infected cells, but had no obvious effect on the gpUL18 expression pattern. Vaccinia virus and adenovirus vectors were used to further analyse gpUL18 expression. Depending on the delivery vector system, differences in the electrophoretic motility of the EndoH-resistant >105 kDa form of gpUL18, but not the EndoH-sensitive 67 kDa form, were observed; post-translational modification of the higher molecular mass glycoform appears to be influenced by active virus infection and vector delivery. The EndoH-sensitive 67 kDa gpUL18 had a rapid turnover, while the maturation to the EndoH-resistant >105 kDa form was relatively slow and inefficient. However, synthesis of the EndoH-resistant >105 kDa form was enhanced with elevated levels of beta2-microglobulin. When expressed by using an adenovirus vector, both the EndoH-sensitive 67 kDa and the EndoH-resistant >105 kDa gpUL18 forms could be detected on the cell surface.

Mannose-binding lectin binds to Ebola and Marburg envelope glycoproteins, resulting in blocking of virus interaction with DC-SIGN and complement-mediated virus neutralization

Mannose-binding lectin (MBL), a serum lectin that mediates innate immune functions including activation of the lectin complement pathway, binds to carbohydrates expressed on some viral glycoproteins. In this study, the ability of MBL to bind to virus particles pseudotyped with Ebola and Marburg envelope glycoproteins was evaluated. Virus particles bearing either Ebola (Zaire strain) or Marburg (Musoke strain) envelope glycoproteins bound at significantly higher levels to immobilized MBL compared with virus particles pseudotyped with vesicular stomatitis virus glycoprotein or with no virus glycoprotein. As observed in previous studies, Ebola-pseudotyped virus bound to cells expressing the lectin DC-SIGN (dendritic cell-specific intercellular adhesion molecule 3-grabbing non-integrin). However, pre-incubation of virus with MBL blocked DC-SIGN-mediated binding to cells, suggesting that the two lectins bind at the same or overlapping sites on the Ebola glycoprotein. Neutralization experiments showed that virus pseudotyped with Ebola or Marburg (Musoke) glycoprotein was neutralized by complement, while the Marburg (Ravn strain) glycoprotein-pseudotyped virus was less sensitive to neutralization. Neutralization was partially mediated through the lectin complement pathway, since a complement source deficient in MBL was significantly less effective at neutralizing viruses pseudotyped with filovirus glycoproteins and addition of purified MBL to the MBL-deficient complement increased neutralization. These experiments demonstrated that MBL binds to filovirus envelope glycoproteins resulting in important biological effects and suggest that MBL can interact with filoviruses during infection in humans.

Antigenic Properties of Peptide Mimotopes of HIV-1-associated Carbohydrate Antigens

The glycan shield of the human immunodeficiency virus (HIV) envelope protein presents many potential epitopes for vaccine development. To augment immune responses to HIV, type 1 (HIV-1), envelope-associated carbohydrate antigens, we are defining peptide mimics of HIV-associated carbohydrate antigens that function as antigen mimotopes that upon immunization will induce antibodies cross-reactive with carbohydrate antigens. We have previously defined peptides with a putative sequence tract RYRY that mimic concanavalin A-binding glycans. To imitate the multivalent binding of carbohydrates, we compared the avidity of a linear (911) and cyclic peptide (D002) reactive with concanavalin A presented in a multiple antigen peptide (MAP) format. The affinity of the MAP-D002 peptide was higher than that of the peptide MAP-911, whereas the avidity of D002 peptide was lower than that of 911. Serum from mice immunized with MAP-911 had lower titer for oligomannose-9 than those elicited by MAP-D002 under the same conditions, but both immunogens elicited antibodies that can block the binding of GP120 to dendritic cells. Antibodies that bind to the studied MAPs were found in a preparation of normal human immunoglobulin for intravenous use. Those that were purified on 911 bound back to 911 and D002, whereas anti-D002 antibodies were specific only for D002. Human antibodies reactive with both mimotopes and with a mannosyl preparation were observed to bind to envelope protein. These results suggested the potential to fine-tune the antibody response to carbohydrate antigens by modifying structural features of peptide mimotope-based immunogens.

Analysis and function of prototype foamy virus envelope N glycosylation

The prototype foamy virus (PFV) glycoprotein, which is essential for PFV particle release, displays a highly unusual biosynthesis, resulting in posttranslational cleavage of the precursor protein into three particle-associated subunits, i.e., leader peptide (LP), surface (SU), and transmembrane (TM). Glycosidase digestion of metabolically labeled PFV particles revealed the presence of N-linked carbohydrates on all subunits. The differential sensitivity to specific glycosidases indicated that all oligosaccharides on LP and TM are of the high-mannose or hybrid type, whereas most of those attached to SU, which contribute to about 50% of its molecular weight, are of the complex type. Individual inactivation of all 15 potential N-glycosylation sites in PFV Env demonstrated that 14 are used, i.e., 1 out of 2 in LP, 10 in SU, and 3 in TM. Analysis of the individual altered glycoproteins revealed defects in intracellular processing, support of particle release, and infectivity for three mutants, having the evolutionarily conserved glycosylation sites N8 in SU or N13 and N15 in the cysteine-rich central "sheets-and-loops" region of TM inactivated. Examination of alternative mutants with mutations affecting glycosylation or surrounding sequences at these sites indicated that inhibition of glycosylation at N8 and N13 most likely is responsible for the observed replication defects, whereas for N15 surrounding sequences seem to contribute to a temperature-sensitive phenotype. Taken together these data demonstrate that PFV Env and in particular the SU subunit are heavily N glycosylated and suggest that although most carbohydrates are dispensable individually, some evolutionarily conserved sites are important for normal Env function of FV isolates from different species.

Synthesis, assembly, and processing of the Env ERVWE1/syncytin human endogenous retroviral envelope

Syncytin is a fusogenic protein involved in the formation of the placental syncytiotrophoblast layer. This protein is encoded by the envelope gene of the ERVWE1 proviral locus belonging to the human endogenous retrovirus W (HERV-W) family. The HERV-W infectious ancestor entered the primate lineage 25 to 40 million years ago. Although the syncytin fusion property has been clearly demonstrated, little is known about this cellular protein maturation process with respect to classical infectious retrovirus envelope proteins. Here we show that the cellular syncytin protein is synthesized as a glycosylated gPr73 precursor cleaved into two mature proteins, a gp50 surface subunit (SU) and a gp24 transmembrane subunit (TM). These SU and TM subunits are found associated as homotrimers. The intracytoplasmic tail is critical to the fusogenic phenotype, although its cleavage requirements seem to have diverged from those of classical retroviral maturation.

Dendritic Cell-specific Intercellular Adhesion Molecule 3-grabbing Non-integrin (DC-SIGN)-mediated Enhancement of Dengue Virus Infection Is Independent of DC-SIGN Internalization Signals

Dengue virus (DV) is a mosquito-borne flavivirus that causes hemorrhagic fever in humans. In the natural infection, DV is introduced into human skin by an infected mosquito vector where it is believed to target immature dendritic cells (DCs) and Langerhans cells (LCs). We found that DV productively infects DCs but not LCs. We show here that the interactions between DV E protein, the sole mannosylated glycoprotein present on DV particles, and the C-type lectin dendritic cell-specific intercellular adhesion molecule 3-grabbing non-integrin (DC-SIGN) are essential for DV infection of DCs. Binding of mannosylated N-glycans on DV E protein to DC-SIGN triggers a rapid and efficient internalization of the viral glycoprotein. However, we observed that endocytosis-defective DC-SIGN molecules allow efficient DV replication, indicating that DC-SIGN endocytosis is dispensable for the internalization step in DV entry. Together, these results argue in favor of a mechanism by which DC-SIGN enhances DV entry and infection in cis. We propose that DC-SIGN concentrates mosquito-derived DV particles at the cell surface to allow efficient interaction with an as yet unidentified entry factor that is ultimately responsible for DV internalization and pH-dependent fusion into DCs.

Differential maturation and subcellular localization of severe acute respiratory syndrome coronavirus surface proteins S, M and E

Post-translational modifications and correct subcellular localization of viral structural proteins are prerequisites for assembly and budding of enveloped viruses. Coronaviruses, like the severe acute respiratory syndrome-associated virus (SARS-CoV), bud from the endoplasmic reticulum-Golgi intermediate compartment. In this study, the subcellular distribution and maturation of SARS-CoV surface proteins S, M and E were analysed by using C-terminally tagged proteins. As early as 30 min post-entry into the endoplasmic reticulum, high-mannosylated S assembles into trimers prior to acquisition of complex N-glycans in the Golgi. Like S, M acquires high-mannose N-glycans that are subsequently modified into complex N-glycans in the Golgi. The N-glycosylation profile and the absence of O-glycosylation on M protein relate SARS-CoV to the previously described group 1 and 3 coronaviruses. Immunofluorescence analysis shows that S is detected in several compartments along the secretory pathway from the endoplasmic reticulum to the plasma membrane while M predominantly localizes in the Golgi, where it accumulates, and in trafficking vesicles. The E protein is not glycosylated. Pulse-chase labelling and confocal microscopy in the presence of protein translation inhibitor cycloheximide revealed that the E protein has a short half-life of 30 min. E protein is found in bright perinuclear patches colocalizing with endoplasmic reticulum markers. In conclusion, SARS-CoV surface proteins S, M and E show differential subcellular localizations when expressed alone suggesting that additional cellular or viral factors might be required for coordinated trafficking to the virus assembly site in the endoplasmic reticulum-Golgi intermediate compartment.

Characterization of DC-SIGN/R interaction with human immunodeficiency virus type 1 gp120 and ICAM molecules favors the receptor's role as an antigen-capturing rather than an adhesion receptor

The dendritic cell (DC)-specific intercellular adhesion molecule 3 (ICAM-3)-grabbing nonintegrin binding receptor (DC-SIGN) was shown to bind human immunodeficiency virus type 1 (HIV-1) viral envelope protein gp120 and proposed to function as a Trojan horse to enhance trans-virus infection to host T cells. To better understand the mechanism by which DC-SIGN and DC-SIGNR selectively bind HIV-1 gp120, we constructed a series of deletion mutations in the repeat regions of both receptors. Different truncated receptors exist in different oligomeric forms. The carbohydrate binding domain without any repeats was monomeric, whereas the full extracellular receptors existed as tetramers. All reconstituted receptors retained their ability to bind gp120. The dissociation constant, however, differed drastically from micromolar values for the monomeric receptors to nanomolar values for the tetrameric receptors, suggesting that the repeat region of these receptors contributes to the avidity of gp120 binding. Such oligomerization may provide a mechanism for the receptor to selectively recognize pathogens containing multiple high-mannose-concentration carbohydrates. In contrast, the receptors bound to ICAMs with submicromolar affinities that are similar to those of two nonspecific cell surface glycoproteins, FcgammaRIIb and FcgammaRIII, and the oligomerization of DC-SIGNR resulted in no increase in binding affinity to ICAM-3. These findings suggest that DC-SIGN may not discriminate other cell surface glycoproteins from ICAM-3 binding. The pH dependence in DC-SIGN binding to gp120 showed that the receptor retained high-affinity gp120 binding at neutral pH but lost gp120 binding at pH 5, suggesting a release mechanism of HIV in the acidic endosomal compartment by DC-SIGN. Our work contradicts the function of DC-SIGN as a Trojan horse to facilitate HIV-1 infection; rather, it supports the function of DC-SIGN/R (a designation referring to both DC-SIGN and DC-SIGNR) as an antigen-capturing receptor.

Comprehensive analysis of ebola virus GP1 in viral entry

Ebola virus infection is initiated by interactions between the viral glycoprotein GP1 and its cognate receptor(s), but little is known about the structure and function of GP1 in viral entry, partly due to the concern about safety when working with the live Ebola virus and the difficulty of manipulating the RNA genome of Ebola virus. In this study, we have used a human immunodeficiency virus-based pseudotyped virus as a surrogate system to dissect the role of Ebola virus GP1 in viral entry. Analysis of more than 100 deletion and amino acid substitution mutants of GP1 with respect to protein expression, processing, viral incorporation, and viral entry has allowed us to map the region of GP1 responsible for viral entry to the N-terminal 150 residues. Furthermore, six amino acids in this region have been identified as critical residues for early events in Ebola virus entry, and among these, three are clustered and are implicated as part of a potential receptor-binding pocket. In addition, substitutions of some 30 residues in GP1 are shown to adversely affect GP1 expression, processing, and viral incorporation, suggesting that these residues are involved in the proper folding and/or overall conformation of GP. Sequence comparison of the GP1 proteins suggests that the majority of the critical residues for GP folding and viral entry identified in Ebola virus GP1 are conserved in Marburg virus. These results provide information for elucidating the structural and functional roles of the filoviral glycoproteins and for developing potential therapeutics to block viral entry.

Identification of the mycobacterial carbohydrate structure that binds the C-type lectins DC-SIGN, L-SIGN and SIGNR1

Mycobacterium tuberculosis represents a worldwide health risk and although macrophages are primarily infected, dendritic cells (DC) are important in inducing cellular immune responses against M. tuberculosis. Recent studies have demonstrated that M. tuberculosis targets the DC-specific C-type lectin DC-SIGN to inhibit the immuno-stimulatory function of DC through the interaction of the mycobacterial mannosylated lipoarabinomannan (ManLAM) to DC-SIGN, which prevents DC maturation and induces the immuno-suppressive cytokine IL-10. This may contribute to survival and persistence of M. tuberculosis. Here, we have identified the specific pathogen-derived carbohydrate structure on ManLAM that is recognized by DC-SIGN. We have synthesized the mannose-cap oligosaccharides man-ara, (man)2-ara and (man)3-ara, and demonstrate that these neoglycoconjugates are specifically bound by DC-SIGN. Moreover, we demonstrate that the human and murine DC-SIGN homologue L-SIGN and SIGNR1, respectively, also interact with mycobacteria through ManLAM. Both homologues have the highest affinity for the (man)3-ara structure, similar to DC-SIGN. This study provides information about the specific carbohydrate structures on pathogens that are recognized by DC-SIGN, and may provide strategies to develop vaccines against these pathogens. Moreover, the identification of SIGNR1 as a receptor for ManLAM will enable in vivo studies to investigate the role of DC-SIGN in M. tuberculosis pathogenesis.

Decrease of Lewis frequency in HIV-infected patients: possible competition of fucosylated antigens with HIV for binding to DC-SIGN

We explored the impact of human ABO glycosyltransferase and Lewis and secretor fucosyltransferase polymorphisms in HIV infection. We found that, compared with healthy blood donors, HIV-infected patients display a significant decrease in Le(a-b+) phenotype frequencies. We showed that HIV binding on DC-SIGN-transduced Jurkat cells was inhibited by fucosyl bovine serum albumin. Our results suggest a slight protective effect of Lewis b antigen on HIV infection, possibly by the competition of Lewis antigens with HIV for binding to DC-SIGN.

DC-SIGN and DC-SIGNR interact with the glycoprotein of Marburg virus and the S protein of severe acute respiratory syndrome coronavirus

The lectins DC-SIGN and DC-SIGNR can augment viral infection; however, the range of pathogens interacting with these attachment factors is incompletely defined. Here we show that DC-SIGN and DC-SIGNR enhance infection mediated by the glycoprotein (GP) of Marburg virus (MARV) and the S protein of severe acute respiratory syndrome coronavirus and might promote viral dissemination. SIGNR1, a murine DC-SIGN homologue, also enhanced infection driven by MARV and Ebola virus GP and could be targeted to assess the role of attachment factors in filovirus infection in vivo.

Infection of specific dendritic cells by CCR5-tropic human immunodeficiency virus type 1 promotes cell-mediated transmission of virus resistant to broadly neutralizing antibodies

The tropism of human immunodeficiency virus type 1 for chemokine receptors plays an important role in the transmission of AIDS. Although CXCR4-tropic virus is more cytopathic for T cells, CCR5-tropic strains are transmitted more frequently in humans for reasons that are not understood. Phenotypically immature myeloid dendritic cells (mDCs) are preferentially infected by CCR5-tropic virus, in contrast to mature mDCs, which are not susceptible to infection but instead internalize virus into a protected intracellular compartment and enhance the infection of T cells. Here, we define a mechanism to explain preferential transmission of CCR5-tropic viruses based on their interaction with mDCs and sensitivity to neutralizing antibodies. Infected immature mDCs differentiated normally and were found to enhance CCR5-tropic but not CXCR4-tropic virus infection of T cells even in the continuous presence of neutralizing antibodies. Infectious synapses also formed normally in the presence of such antibodies. Infection of immature mDCs by CCR5-tropic virus can therefore establish a pool of infected cells that can efficiently transfer virus at the same time that they protect virus from antibody neutralization. This property of DCs may enhance infection, contribute to immune evasion, and could provide a selective advantage for CCR5-tropic virus transmission.

Carbohydrate arrays as tools for the glycomics revolution

Determining the roles of carbohydrates in cell biology is hindered by the tremendous efforts that must be taken either to synthesize carbohydrates chemically or to isolate them from natural sources. Several platforms have been developed to study glycobiology. These platforms include the covalent and non-covalent immobilization of carbohydrates on microtiter plates, nitrocellulose membranes, coated slides, glass slides, gold surfaces and microspheres. Each platform has both positive and negative attributes for studying carbohydrate biology, such as flexibility and the amount of material required for study. Here we discuss these techniques with the goal of developing a ‘gold standard’ in the field of glycomics.

Domains of macaque DC-SIGN essential for capture and transfer of simian immunodeficiency virus

The C-type lectin DC-SIGN mediates the capture and transfer of simian immunodeficiency virus (SIV) from macaque dendritic cells (DCs) to permissive T-cells. To further identify the determinants in macaque DC-SIGN required for capture and transfer of virus, we created mutants containing deletions or point mutations in the extracellular domains, and tested their ability to capture and transmit SIV. We found that SIV bound to the carbohydrate recognition domain (CRD) of macaque DC-SIGN via the envelope protein. In addition, deleting the C-terminal half of the CRD, or mutating amino acids within this region that contact Ca(2+) or mannose, disrupted virion capture activity. However, an N-terminal CRD deletion mutant was capable of binding SIV, indicating that this region was not necessary for binding. Finally, deletion of the neck domain also reduced the capacity for macaque DC-SIGN to capture SIV. Interestingly, ICAM-3, the cellular ligand for DC-SIGN, did not bind to any of the DC-SIGN mutants, including mutants with amino acid changes in the N-terminal region of the CRD. These data suggest that the binding sites for SIV and ICAM-3 may be distinct but overlapping. Together, the data demonstrate the importance of both the neck and the CRD of macaque DC-SIGN for efficient capture of SIV and binding to ICAM-3.

Ebola virus glycoproteins: guidance devices for targeting gene therapy vectors

Replacing the native viral envelope protein on the surface of a retrovirus or lentivirus with the glycoprotein of a foreign enveloped virus, a process called pseudotyping, can expand the set of potential target cells for a viral vector or can restrict entry to specific cells. The Ebola virus glycoprotein, because of its evolutionary origins and the route of viral entry promoted by it, possesses distinct advantages in forming the outer shell of such pseudotyped retroviruses for gene therapy applications. Studies of the transduction of human airway epithelia by lentivirus pseudotyped with a modified Ebola virus glycoprotein from which the region of O- glycosylation has been removed have demonstrated that such recombinant viruses possess particular promise for the treatment of cystic fibrosis. This result highlights the synergism between basic studies of virus entry and gene therapy advances.

Trans-dominant cellular inhibition of DC-SIGN-mediated HIV-1 transmission

Background

Dendritic cell (DC) transmission of human immunodeficiency virus (HIV) to CD4+ T cells occurs across a point of cell-cell contact referred to as the infectious synapse. The relationship between the infectious synapse and the classically defined immunological synapse is not currently understood. We have recently demonstrated that human B cells expressing exogenous DC-SIGN, DC-specific intercellular adhesion molecule-3 (ICAM-3)-grabbing nonintegrin, efficiently transmit captured HIV type 1 (HIV-1) to CD4+ T cells. K562, another human cell line of hematopoietic origin that has been extensively used in functional analyses of DC-SIGN and related molecules, lacks the principal molecules involved in the formation of immunological synaptic junctions, namely major histocompatibility complex (MHC) class II molecules and leukocyte function-associated antigen-1 (LFA-1). We thus examined whether K562 erythroleukemic cells could recapitulate efficient DC-SIGN-mediated HIV-1 transmission (DMHT).

Results

Here we demonstrate that DMHT requires cell-cell contact. Despite similar expression of functional DC-SIGN, K562/DC-SIGN cells were inefficient in the transmission of HIV-1 to CD4+ T cells when compared with Raji/DC-SIGN cells. Expression of MHC class II molecules or LFA-1 on K562/DC-SIGN cells was insufficient to rescue HIV-1 transmission efficiency. Strikingly, we observed that co-culture of K562 cells with Raji/DC-SIGN cells impaired DMHT to CD4+ T cells. The K562 cell inhibition of transmission was not directly exerted on the CD4+ T cell targets and required contact between K562 and Raji/DC-SIGN cells.

Conclusions

DMHT is cell type dependent and requires cell-cell contact. We also find that the cellular milieu can negatively regulate DC-SIGN transmission of HIV-1 in trans.

pH-dependent entry of severe acute respiratory syndrome coronavirus is mediated by the spike glycoprotein and enhanced by dendritic cell transfer through DC-SIGN

The severe acute respiratory syndrome coronavirus (SARS-CoV) synthesizes several putative viral envelope proteins, including the spike (S), membrane (M), and small envelope (E) glycoproteins. Although these proteins likely are essential for viral replication, their specific roles in SARS-CoV entry have not been defined. In this report, we show that the SARS-CoV S glycoprotein mediates viral entry through pH-dependent endocytosis. Further, we define its cellular tropism and demonstrate that virus transmission occurs through cell-mediated transfer by dendritic cells. The S glycoprotein was used successfully to pseudotype replication-defective retroviral and lentiviral vectors that readily infected Vero cells as well as primary pulmonary and renal epithelial cells from human, nonhuman primate, and, to a lesser extent, feline species. The tropism of this reporter virus was similar to that of wild-type, replication-competent SARS-CoV, and binding of purified S to susceptible target cells was demonstrated by flow cytometry. Although myeloid dendritic cells were able to interact with S and to bind virus, these cells could not be infected by SARS-CoV. However, these cells were able to transfer the virus to susceptible target cells through a synapse-like structure. Both cell-mediated infection and direct infection were inhibited by anti-S antisera, indicating that strategies directed toward this gene product are likely to confer a therapeutic benefit for antiviral drugs or the development of a SARS vaccine.

Inhibiting sexual transmission of HIV-1 infection

The worldwide infection rate for HIV-1 is estimated to be 14,000 per day, but only now, more than 20 years into the epidemic, are the immediate events between exposure to infectious virus and the establishment of infection becoming clear. Defining the mechanisms of HIV-1 transmission, the target cells involved and how the virus attaches to and fuses with these cells, could reveal ways to block the sexual spread of the virus. In this review, we will discuss how our increasing knowledge of the ways in which HIV-1 is transmitted is shaping the development of new, more sophisticated intervention strategies based on the application of vaginal or rectal microbicides.

Actinohivin, a novel anti-human immunodeficiency virus protein from an actinomycete, inhibits viral entry to cells by binding high-mannose type sugar chains of gp120

We searched human immunodeficiency virus (HIV) entry inhibitors and found a novel anti-HIV protein, actinohivin (AH), in a culture filtrate of the newly discovered genus actinomycete Longispora albida gen. nov., sp. nov. This paper deals with the mechanism of action of the anti-HIV activity of AH. AH exhibited potent anti-HIV activities against various strains of HIV-1 and HIV-2. AH bound to the glycoprotein gp120 of various strains of HIV-1 and gp130 of simian immunodeficiency virus (SIV), but did not bind to non-glycosylated gp120 nor to cells having CD4 and coreceptors, suggesting that AH inhibits viral entry to cells by binding to the envelope glycoprotein. The investigation of the effects of various sugars on AH-gp120 binding by ELISA revealed that yeast mannan alone strongly inhibited the binding (IC50 = 3.0 microg/ml). Experiments investigating the binding of AH to other glycoproteins revealed that AH binds to ribonuclease B and thyroglobulin that have a high-mannose type saccharide chain, but not to other glycoproteins having a N-glycoside type saccharide chain. The above results indicate that high-mannose type saccharide chains of gp120 are molecular targets of AH in its anti-HIV activity.

Human macrophage C-type lectin specific for galactose and N-acetylgalactosamine promotes filovirus entry

Filoviruses cause lethal hemorrhagic disease in humans and nonhuman primates. An initial target of filovirus infection is the mononuclear phagocytic cell. Calcium-dependent (C-type) lectins such as dendritic cell- or liver/lymph node-specific ICAM-3 grabbing nonintegrin (DC-SIGN or L-SIGN, respectively), as well as the hepatic asialoglycoprotein receptor, bind to Ebola or Marburg virus glycoprotein (GP) and enhance the infectivity of these viruses in vitro. Here, we demonstrate that a recently identified human macrophage galactose- and N-acetylgalactosamine-specific C-type lectin (hMGL), whose ligand specificity differs from DC-SIGN and L-SIGN, also enhances the infectivity of filoviruses. This enhancement was substantially weaker for the Reston and Marburg viruses than for the highly pathogenic Zaire virus. We also show that the heavily glycosylated, mucin-like domain on the filovirus GP is required for efficient interaction with this lectin. Furthermore, hMGL, like DC-SIGN and L-SIGN, is present on cells known to be major targets of filoviruses (i.e., macrophages and dendritic cells), suggesting a role for these C-type lectins in viral replication in vivo. We propose that filoviruses use different C-type lectins to gain cellular entry, depending on the cell type, and promote efficient viral replication.

Raji B cells, misidentified as THP-1 cells, stimulate DC-SIGN-mediated HIV transmission

A number of studies examining interactions of dendritic cell (DC)-specific ICAM-3 grabbing nonintegrin (DC-SIGN) with viral pathogens have relied on monocytic transfectants as models for primary DCs. Here we show that the presumed "THP-1" monocytic cells used in these studies are instead Raji B cells. Moreover, we demonstrate that true THP-1 cells do not support DC-SIGN-mediated HIV-1 transmission, whereas human B cell lines efficiently enhance this process. These data indicate that there are features common to B cells and DCs that facilitate transmission of HIV-1 and provide new insights toward the mechanism of DC-SIGN-mediated HIV-1 transmission.

DC-SIGN: Binding receptor for HCV?

DC-SIGN, a dendritic Cell-specific adhesion receptor and a type II transmembrane mannose-binding C-type lectin, is very important in the function of DC, both in mediating naive T cell interactions through ICAM-3 and as a rolling receptor that mediates the DC-specific ICAM-2-dependent migration processes. It can be used by viral and bacterial pathogens including Human Immunodeficiency Virus (HIV), HCV, Ebola Virus, CMV and Mycobacterium tuberculosis to facilitate infection. Both DC-SIGN and DC-SIGNR can act either in cis, by concentrating virus on target cells, or in trans, by transmission of bound virus to a target cell expressing appropropriate entry receptors. Recent work showed that DC-SIGN are high-affinity binding receptors for HCV. Besides playing a role in entry into DC, HCV E2 interaction with DC-SIGN might also be detrimental for the interaction of DC with T cells during antigen presentation. The clinical strategies that target DC-SIGN may be successful in restricting HCV dissemination and pathogenesis as well as directing the migration of DCs to manipulate appropriate immune responses in autoimmunity and tumorigenic situations.

Self- and nonself-recognition by C-type lectins on dendritic cells

Dendritic cells (DCs) are highly efficient antigen-presenting cells (APCs) that collect antigen in body tissues and transport them to draining lymph nodes. Antigenic peptides are loaded onto major histocompatibility complex (MHC) molecules for presentation to naive T cells, resulting in the induction of cellular and humoral immune responses. DCs take up antigen through phagocytosis, pinocytosis, and endocytosis via different groups of receptor families, such as Fc receptors for antigen-antibody complexes, C-type lectin receptors (CLRs) for glycoproteins, and pattern recognition receptors, such as Toll-like receptors (TLRs), for microbial antigens. Uptake of antigen by CLRs leads to presentation of antigens on MHC class I and II molecules. DCs are well equipped to distinguish between self- and nonself-antigens by the variable expression of cell-surface receptors such as CLRs and TLRs. In the steady state, DCs are not immunologically quiescent but use their antigen-handling capacities to maintain peripheral tolerance. DCs are continuously sampling and presenting self- and harmless environmental proteins to silence immune activation. Uptake of self-components in the intestine and airways are good examples of sites where continuous presentation of self- and foreign antigens occurs without immune activation. In contrast, efficient antigen-specific immune activation occurs upon encounter of DCs with nonself-pathogens. Recognition of pathogens by DCs triggers specific receptors such as TLRs that result in DC maturation and subsequently immune activation. Here we discuss the concept that cross talk between TLRs and CLRs, differentially expressed by subsets of DCs, accounts for the different pathways to peripheral tolerance, such as deletion and suppression, and immune activation.

Interaction of mannose-binding lectin with HIV type 1 is sufficient for virus opsonization but not neutralization

Mannose-binding lectin (MBL), a microbe-recognition protein in serum, binds to high mannose glycans on HIV-1 gp120 and has been reported to neutralize the cell line-adapted strain HIV(IIIB). Because HIV primary isolates (PI) are generally more resistant to neutralization by antibodies and considering that PI are produced in primary cells that could alter the number of high mannose glycans on HIV relative to cell lines, we assessed the ability to MBL to neutralize HIV PI. MBL at concentrations up to 50 microg/ml mediated relatively little neutralization (<20%) of HIV PI infection of peripheral blood mononuclear cells (PBMCs). MBL-neutralizing activity was slightly higher for cell line-adapted HIV infection of the H9 T cell line (up to 64% at 50 microg/ml). However, this effect was specific for H9 cells since MBL did not neutralize cell line-adapted virus infection of PBMCs, HIV PI infection of the GHOST cell line, or VSV pseudotyped with HIV gp160 from cell line-derived virus or PI. In contrast to its low activity in neutralization assays, MBL efficiently bound infectious HIV PI and opsonized HIV PI for uptake by monocytic cells. These results show that both PI and cell line-adapted HIV, despite binding of MBL, are relatively resistant to neutralization by levels of MBL normally present in serum. However, binding and opsonization of HIV by MBL may alter virus trafficking and viral-antigen presentation during HIV infection.

DC-SIGN binds to HIV-1 glycoprotein 120 in a distinct but overlapping fashion compared with ICAM-2 and ICAM-3

DC-SIGN is a C-type lectin that binds to endogenous adhesion molecules ICAM-2 and ICAM-3 as well as the viral envelope glycoprotein human immunodeficiency virus, type 1, glycoprotein (gp) 120. We wished to determine whether DC-SIGN binds differently to its endogenous ligands ICAM-2 and ICAM-3 versus HIV-1 gp120. We found that recombinant soluble DC-SIGN bound to gp120-Fc more than 100- and 50-fold better than ICAM-2-Fc and ICAM-3-Fc, respectively. This relative difference was maintained using DC-SIGN expressed on three different CD4-negative cell lines. Although the cell surface affinity for gp120 varied by up to 4-fold on the cell lines examined, the affinity for gp120 was not a correlate of the ability of the cell line to transfer virus. Monosaccharides with equatorial 4-OH groups competed as well as D-mannose for gp120 binding to DC-SIGN, regardless of how the other hydroxyl groups were positioned. Disaccharide competitors and glycan chip analysis showed that DC-SIGN has a preference for oligosaccharides linked in an alpha-anomeric configuration. Alanine-scanning mutagenesis of DC-SIGN revealed that highly conserved residues that coordinate calcium (Asp-366) and/or are involved in both calcium and specific carbohydrate interactions (Glu-347, Asn-349, Glu-354, and Asp-355) significantly compromised binding to all three ligands. Mutating non-conserved residues (Asn-311, Arg-345, Val-351, Gly-352, Glu-353, Ser-360, Gly-361, and Asn-362) minimally affected binding except for the Asp-367 mutant, which enhanced gp120 binding but diminished ICAM-2 and ICAM-3 binding. Conversely, mutating the moderately conserved residue (Gly-346) abrogated gp120 binding but enhanced ICAM-2 and ICAM-3 binding. Thus, DC-SIGN appears to bind in a distinct but overlapping manner to gp120 when compared with ICAM-2 and ICAM-3.

Binding of human immunodeficiency virus type 1 to immature dendritic cells can occur independently of DC-SIGN and mannose binding C-type lectin receptors via a cholesterol-dependent pathway

Interactions of human immunodeficiency virus type 1 (HIV-1) with immature dendritic cells (DC) are believed to be multifactorial and involve binding to the CD4 antigen, DC-specific ICAM-3-grabbing nonintegrin (DC-SIGN), mannose binding C-type lectin receptors (MCLR), and heparan sulfate proteoglycans (HSPG). In this study we assessed the relative contributions of these previously defined virus attachment factors to HIV binding and accumulation in DC and the subsequent transfer of the bound virus particle to CD4(+) T cells. Using competitive inhibitors of HIV-1 attachment to DC, we have identified the existence of DC-SIGN-, MCLR-, and HSPG-independent mechanism(s) of HIV attachment and internalization. Furthermore, virus particles bound by DC independently of CD4, DC-SIGN, MCLR, and HSPG are efficiently transmitted to T cells. Treatment of virus particles with the protease subtilisin or treatment of immature DC with trypsin significantly reduced virus binding, thus demonstrating the role of HIV envelope glycoprotein interactions with unidentified DC-surface factor(s). Finally, this DC-mediated virus binding and internalization are dependent on lipid rafts. We propose that pathways to HIV-1 attachment and uptake in DC exhibit functional redundancy; that is, they are made up of multiple independent activities that can, at least in part, compensate for one another.

DC-SIGN and L-SIGN can act as attachment receptors for alphaviruses and distinguish between mosquito cell- and mammalian cell-derived viruses

C-type lectins such as DC-SIGN and L-SIGN, which bind mannose-enriched carbohydrate modifications of host and pathogen proteins, have been shown to bind glycoproteins of several viruses and facilitate either cis or trans infection. DC-SIGN and L-SIGN are expressed in several early targets of arbovirus infection, including dendritic cells (DCs) and cells of the reticuloendothelial system. In the present study, we show that DC-SIGN and L-SIGN can function as attachment receptors for Sindbis (SB) virus, an arbovirus of the Alphavirus genus. Human monocytic THP-1 cells stably transfected with DC-SIGN or L-SIGN were permissive for SB virus replication, while untransfected controls were essentially nonpermissive. The majority of control THP-1 cells were permissive when attachment and entry steps were eliminated through electroporation of virus transcripts. Infectivity for the DC-SIGN/L-SIGN-expressing cells was largely blocked by yeast mannan, EDTA, or a DC-SIGN/L-SIGN-specific monoclonal antibody. Infection of primary human DCs by SB virus was also dependent upon SIGN expression by similar criteria. Furthermore, production of virus particles in either C6/36 mosquito cells or CHO mammalian cells under conditions that limited complex carbohydrate content greatly increased SB virus binding to and infection of THP-1 cells expressing these lectins. C6/36-derived virus also was much more infectious for primary human DCs than CHO-derived virus. These results suggest that (i) lectin molecules such as DC-SIGN and L-SIGN may represent common attachment receptor molecules for arthropod-borne viruses, (ii) arbovirus particles produced in and delivered by arthropod vectors may preferentially target vertebrate host cells bearing these or similar lectin molecules, and (iii) a cell line has been identified that can productively replicate alphaviruses but is deficient in attachment receptors.

HIV-1 transmission and cytokine-induced expression of DC-SIGN in human monocyte-derived macrophages

Dendritic cell-specific intercellular adhesion molecule-3-grabbing nonintegrin (DC-SIGN) has been described as an attachment molecule for human immunodeficiency virus type 1 (HIV-1) with the potential to mediate its transmission. We examined DC-SIGN expression in monocyte-derived macrophages (MDM) and its role in viral transmission when MDM were exposed to interleukin (IL)-13, IL-4, or interferon-gamma (IFN-gamma). We show that IL-13 and IL-4 increase transcripts, total protein, and cell-surface expression of DC-SIGN in all MDM tested, IFN-gamma results ranged from no change to up-regulation of surface expression, and message and total protein were, respectively, induced in all and 86% of donors tested. Transmission experiments of HIV-1 X4 between cytokine-treated MDM to Sup-T1 cells showed no association between total transmission and DC-SIGN up-regulation. IL-4 but not IL-13 resulted in a less than twofold increase in MDM viral transmission to CD4+ T cells in spite of a fourfold up-regulation in DC-SIGN expression by either cytokine. In contrast, IFN-gamma treatment induced a decrease in total transmission by at least two-thirds, despite its induction of DC-SIGN. Soluble mannan resulted in a greater inhibition of viral transmission to CD4+ T cells than neutralizing anti-DC-SIGN monoclonal antibody (67-75% vs. 39-48%), supporting the role of mannose-binding receptors in viral transmission. Taken together, results show that DC-SIGN regulation in MDM does not singly predict the transmission potential of this cell type.

Automated Synthesis of a Protected N-Linked Glycoprotein Core Pentasaccharide

[structure: see text] Described is the first automated solid-phase synthesis of the core N-linked pentasaccharide, common to all N-linked glycoproteins via stepwise assembly from mono- and disaccharide building blocks. The challenging beta-mannosidic linkage was incorporated by the inclusion of a disaccharide trichloroacetimidate. This automated synthesis provides rapid access to an oligosaccharide common to an entire class of glycoconjugates.

The role of dendritic cell C-type lectin receptors in HIV pathogenesis

Dendritic cells play a major role in HIV pathogenesis. Epithelial dendritic cells appear to be one of the first cells infected after sexual transmission and transfer of the virus to CD4 lymphocytes, simultaneously activating these cells to produce high levels of HIV replication. Such transfer may occur locally in inflamed mucosa or after dendritic cells have matured and migrated to local lymph nodes. Therefore, the mechanism of binding, internalization, infection and transfer of HIV to CD4 lymphocytes is of great interest. Recently, the role of the C-type lectin DC-SIGN as a dendritic cell receptor for HIV has been intensively studied with in vitro monocyte-derived dendritic cells. However, it is clear that other C-type lectin receptors such as Langerin on Langerhan cells and mannose receptor on dermal dendritic cells are at least equally important for gp120 binding on epithelial dendritic cells. C-type lectin receptors play a role in virus transfer to T cells, either via de novo infection ("cis transfer") or without infection ("in trans" or transinfection). Both these processes are important in vitro, and both may have a role in vivo, although the low-level infection of immature dendritic cells may be more important as it leads to R5 HIV strain selection and persistence of virus within dendritic cells for at least 24 h, sufficient for these cells to transit to lymph nodes. The exact details of these processes are currently the subject of intense study.

Dendritic-cell-specific ICAM3-grabbing non-integrin is essential for the productive infection of human dendritic cells by mosquito-cell-derived dengue viruses

Dengue virus (DV) is a mosquito-borne flavivirus that causes haemorrhagic fever in humans. DV primarily targets immature dendritic cells (DCs) after a bite by an infected mosquito vector. Here, we analysed the interactions between DV and human-monocyte-derived DCs at the level of virus entry. We show that the DC-specific ICAM3-grabbing non-integrin (DC-SIGN) molecule, a cell-surface, mannose-specific, C-type lectin, binds mosquito-cell-derived DVs and allows viral replication. Conclusive evidence for the involvement of DC-SIGN in DV infection was obtained by the inhibition of viral infection by anti-DC-SIGN antibodies and by the soluble tetrameric ectodomain of DC-SIGN. Our data show that DC-SIGN functions as a DV-binding lectin by interacting with the DV envelope glycoprotein. Mosquito-cell-derived DVs may have differential infectivity for DC-SIGN-expressing cells. We suggest that the differential use of DC-SIGN by viral envelope glycoproteins may account for the immunopathogenesis of DVs.

DC-SIGN and L-SIGN are high affinity binding receptors for hepatitis C virus glycoprotein E2

The hepatitis C virus (HCV) genome codes for highly mannosylated envelope proteins, which are naturally retained in the endoplasmic reticulum. We found that the HCV envelope glycoprotein E2 binds the dendritic cell-specific intercellular adhesion molecule 3-grabbing nonintegrin (DC-SIGN) and the related liver endothelial cell lectin L-SIGN through high-mannose N-glycans. Competing ligands such as mannan and an antibody directed against the carbohydrate recognition domains (CRD) abrogated binding. While no E2 interaction with distant monomeric CRDs on biosensor chips could be detected, binding is observed if CRDs are closely seeded (Kd = 48 nm) and if the CRD is part of the oligomeric-soluble extracellular domain of DC-SIGN (Kd = 30 nm). The highest affinity is seen for plasma membrane-expressed DC-SIGN and L-SIGN (Kd = 3 and 6 nm, respectively). These results indicate that several high-mannose N-glycans in a structurally defined cluster on E2 bind to several subunits of the oligomeric lectin CRD. High affinity interaction of viral glycoproteins with oligomeric lectins might represent a strategy by which HCV targets to and concentrates in the liver and infects dendritic cells.

Vaccine research efforts for filoviruses

Ebola and Marburg viruses belong to the family Filoviridae, and cause acute, frequently fatal, haemorrhagic fever in humans and non-human primates. No vaccines are available for human use. This review describes the status of research efforts to develop vaccines for these viruses and to identify the immune mechanisms of protection. The vaccine approaches discussed include DNA-based vaccines, and subunit vaccines vectored by adenovirus, alphavirus replicons, and vaccinia virus.

Hepatitis C virus glycoproteins interact with DC-SIGN and DC-SIGNR

DC-SIGN and DC-SIGNR are two closely related membrane-associated C-type lectins that bind human immunodeficiency virus (HIV) envelope glycoprotein with high affinity. Binding of HIV to cells expressing DC-SIGN or DC-SIGNR can enhance the efficiency of infection of cells coexpressing the specific HIV receptors. DC-SIGN is expressed on some dendritic cells, while DC-SIGNR is localized to certain endothelial cell populations, including hepatic sinusoidal endothelial cells. We found that soluble versions of the hepatitis C virus (HCV) E2 glycoprotein and retrovirus pseudotypes expressing chimeric forms of both HCV E1 and E2 glycoproteins bound efficiently to DC-SIGN and DC-SIGNR expressed on cell lines and primary human endothelial cells but not to other C-type lectins tested. Soluble E2 bound to immature and mature human monocyte-derived dendritic cells (MDDCs). Binding of E2 to immature MDDCs was dependent on DC-SIGN interactions, while binding to mature MDDCs was partly independent of DC-SIGN, suggesting that other cell surface molecules may mediate HCV glycoprotein interactions. HCV interactions with DC-SIGN and DC-SIGNR may contribute to the establishment or persistence of infection both by the capture and delivery of virus to the liver and by modulating dendritic cell function.