DC-SIGN and DC-SIGNR bind ebola glycoproteins and enhance infection of macrophages and endothelial cells

Different potential of C-type lectin-mediated entry between Marburg virus strains

The glycoproteins (GPs) of filoviruses are responsible for virus entry into cells. It is known that GP interacts with cellular C-type lectins for virus attachment to cells. Since primary target cells of filoviruses express C-type lectins, C-type lectin-mediated entry is thought to be a possible determinant of virus tropism and pathogenesis. We compared the efficiency of C-type lectin-mediated entry between Marburg virus strains Angola and Musoke by using a vesicular stomatitis virus (VSV) pseudotype system. VSV pseudotyped with Angola GP (VSV-Angola) infected K562 cells expressing the C-type lectin, human macrophage galactose-type C-type lectin (hMGL), or dendritic cell-specific ICAM-3-grabbing nonintegrin (DC-SIGN) more efficiently than VSV pseudotyped with Musoke GP (VSV-Musoke). Unexpectedly, the binding affinity of the C-type lectins to the carbohydrates on GPs did not correlate with the different efficiency of C-type lectin-mediated entry. Site-directed mutagenesis identified the amino acid at position 547, which switched the efficiency of C-type lectin-mediated entry. In a three-dimensional model of GP, this amino acid was in close proximity to the putative site of cathepsin processing. Interestingly, the cathepsin inhibitors reduced the infectivity of VSV-Angola less efficiently than that of VSV-Musoke in C-type lectin-expressing K562 cells, whereas only a limited difference was found in control cells. The amino acid at position 547 was critical for the different effects of the inhibitors on the virus infectivities. These results suggest that the efficiency of C-type lectin-mediated entry of filoviruses is controlled not only by binding affinity between C-type lectins and GP but also by mechanisms underlying endosomal entry, such as proteolytic processing by the cathepsins.

A forward genetic strategy reveals destabilizing mutations in the Ebolavirus glycoprotein that alter its protease dependence during cell entry

Ebolavirus (EBOV) entry into cells requires proteolytic disassembly of the viral glycoprotein, GP. This proteolytic processing, unusually extensive for an enveloped virus entry protein, is mediated by cysteine cathepsins, a family of endosomal/lysosomal proteases. Previous work has shown that cleavage of GP by cathepsin B (CatB) is specifically required to generate a critical entry intermediate. The functions of this intermediate are not well understood. We used a forward genetic strategy to investigate this CatB-dependent step. Specifically, we generated a replication-competent recombinant vesicular stomatitis virus bearing EBOV GP as its sole entry glycoprotein and used it to select viral mutants resistant to a CatB inhibitor. We obtained mutations at six amino acid positions in GP that independently confer complete resistance. All of the mutations reside at or near the GP1-GP2 intersubunit interface in the membrane-proximal base of the prefusion GP trimer. This region forms a part of the "clamp" that holds the fusion subunit GP2 in its metastable prefusion conformation. Biochemical studies suggest that most of the mutations confer CatB independence not by altering specific cleavage sites in GP but rather by inducing conformational rearrangements in the prefusion GP trimer that dramatically enhance its susceptibility to proteolysis. The remaining mutants did not show the preceding behavior, indicating the existence of multiple mechanisms for acquiring CatB independence during entry. Altogether, our findings suggest that CatB cleavage is required to facilitate the triggering of viral membrane fusion by destabilizing the prefusion conformation of EBOV GP.

Phenylalanines at positions 88 and 159 of Ebolavirus envelope glycoprotein differentially impact envelope function

The envelope glycoprotein (GP) of Ebolavirus (EBOV) mediates viral entry into host cells. Through mutagenesis, we and other groups reported that two phenylalanines at positions 88 and 159 of GP are critical for viral entry. However, it remains elusive which steps of viral entry are impaired by F88 or F159 mutations and how. In this study, we further characterized these two phenylalanines through mutagenesis and examined the impact on GP expression, function, and structure. Our data suggest that F159 plays an indirect role in viral entry by maintaining EBOV GP's overall structure. In contrast, we did not detect any evidence for conformational differences in GP with F88 mutations. The data suggest that F88 influences viral entry during a step after cathepsin processing, presumably impacting viral fusion.

Virus entry paradigms

Viruses, despite being relatively simple in structure and composition, have evolved to exploit complex cellular processes for their replication in the host cell. After binding to their specific receptor on the cell surface, viruses (or viral genomes) have to enter cells to initiate a productive infection. Though the entry processes of many enveloped viruses is well understood, that of most non-enveloped viruses still remains unresolved. Recent studies have shown that compared to direct fusion at the plasma membrane, endocytosis is more often the preferred means of entry into the target cell. Receptor-mediated endocytic pathways such as the dynamin-dependent clathrin and caveolar pathways are well characterized as viral entry portals. However, many viruses are able to utilize multiple uptake pathways. Fluid phase uptake, though relatively non-specific in terms of its cargo, potentially aids viral infection by its ability to intersect with the endocytic pathway. In fact, many viruses despite using specialized pathways for entry are still able to generate productive infection via fluid phase uptake. Macropinocytosis, a major fluid uptake pathway found in epithelial cells and fibroblasts, is stimulated by growth factor receptors. Many viruses can induce these signaling cascades in cells leading to macropinocytosis. Though endocytic trafficking is utilized by both enveloped and non-enveloped viruses, key differences lie in the way membranes are traversed to deposit the viral genome at its site of replication. This review will discuss recent developments in the rapidly evolving field of viral entry.

Segmented helical structure of the neck region of the glycan-binding receptor DC-SIGNR

Carbohydrate-recognition domains (CRDs) in the glycan-binding receptors DC-SIGN (dendritic-cell-specific intercellular adhesion molecule 1-grabbing nonintegrin; CD209) and DC-SIGNR (DC-SIGN-related receptor, also known as L-SIGN and variously designated CD209L and CD299) are projected from the membrane surface by extended neck domains containing multiple repeats of a largely conserved 23-amino-acid sequence motif. Crystals of a fragment of the neck domain of DC-SIGNR containing multiple repeats in which each molecule extends through multiple unit cells, such that the observed crystallographic asymmetric unit represents one repeat averaged over six repeats of the protein, have been obtained. The repeats are largely α-helical. Based on the structure and arrangement of the repeats in the crystal, the neck region can be described as a series of four-helix bundles connected by short, non-helical linkers. Combining the structure of the isolated neck domain with a previously determined overlapping structure of the distal end of the neck region with the CRDs attached provides a model of the almost-complete extracellular portion of the receptor. The results are consistent with previous characterization of the extended structure for the isolated neck region and the extracellular domain. The organization of the neck suggests how CRDs may be disposed differently in DC-SIGN compared with DC-SIGNR and in variant forms of DC-SIGNR assembled from polypeptides with different numbers of repeats in the neck domain.

Penaeus monodon chitin-binding protein (PmCBP) is involved in white spot syndrome virus (WSSV) infection

White spot syndrome virus (WSSV) can cause the most serious viral disease of shrimp and has a wide host range among crustaceans. Although researches show a lot about its genome and structure, information concerning the mechanism of how WSSV infects' cells is lacking. In this study, some experiments were applied to confirm the biological meaning of the protein-protein interaction between WSSV envelope protein, VP53A, and Penaeus monodon chitin-binding protein (PmCBP). Immunofluorescent study indicated that PmCBP is located on the cell surface of host cells. PmCBP amounts of about 34kDa can be detected in both P. monodon and Litopenaeus vannamei tissues by Western blotting. In the in vivo neutralization experiment, both rVP53A and rPmCBP that were produced by Esherichia coli can promote resp. a 40% and 20% survival rate of the shrimp which were challenged by WSSV. Furthermore, a yeast-two-hybrid result revealed that PmCBP could interact with at least 11 WSSV envelope proteins. Those findings suggest that PmCBP may be involved in WSSV infection.

Characterization of Chikungunya pseudotyped viruses: Identification of refractory cell lines and demonstration of cellular tropism differences mediated by mutations in E1 glycoprotein

Chikungunya virus (CHIKV) is an alphavirus responsible for a number of large outbreaks. Here we describe the efficient incorporation of CHIKV envelope glycoproteins into lentiviral and rhabdoviral particles. Vectors pseudotyped with CHIKV envelope proteins efficiently transduced many cell types from different species. However, hematopoietic cell types were either partially or completely refractory. A mutation in E1 (A226V) has been linked with expansion of tropism for mosquito species, although differences in in vitro infection of mosquito cell lines have not been noted. However, pseudovirion infectivity assays detected subtle differences in infection of mosquito cells, suggesting an explanation for the changes in mosquito tropism. The presence of C-type lectins increased CHIKV pseudotyped vector infectivity, but not infection of refractory cells, suggesting that they act as attachment factors rather than primary receptors. CHIKV pseudotypes will serve as an important tool for the study of neutralizing antibodies and the analysis of envelope glycoprotein functions.

Capture and transmission of HIV-1 by the C-type lectin L-SIGN (DC-SIGNR) is inhibited by carbohydrate-binding agents and polyanions

It was recently shown that capture of HIV-1 by DC-SIGN-expressing cells and the subsequent transmission of HIV to CD4+ T-lymphocytes can be prevented by carbohydrate-binding agents (CBAs), whereas polyanions were unable to block virus capture by DC-SIGN. In this study, we could show that a short pre-exposure of HIV-1 to both mannose- and N-acetylglucosamine (GlcNAc)-specific CBAs or polyanions dose-dependently prevented virus capture by L-SIGN-expressing 293T-REx/L-SIGN cells and subsequent syncytia formation in co-cultures of the drug-exposed HIV-1-captured 293T-REx/L-SIGN cells and uninfected C8166 CD4+ T-lymphocytes. Additionally, the inhibitory potential of the compounds against L-SIGN-mediated HIV-1 capture and transmission was more pronounced than observed for DC-SIGN expressing293T-REx/DC-SIGN cells. The excess value of CBAs and polyanions to prevent HIV-1 capture and transmission by DC-SIGN and L-SIGN-expressing cells to susceptible T-lymphocytes could be of interest for the development of new drug leads targeting HIV entry/fusion.

Binding-site geometry and flexibility in DC-SIGN demonstrated with surface force measurements

The dendritic cell receptor DC-SIGN mediates pathogen recognition by binding to glycans characteristic of pathogen surfaces, including those found on HIV. Clustering of carbohydrate-binding sites in the receptor tetramer is believed to be critical for targeting of pathogen glycans, but the arrangement of these sites remains poorly understood. Surface force measurements between apposed lipid bilayers displaying the extracellular domain of DC-SIGN and a neoglycolipid bearing an oligosaccharide ligand provide evidence that the receptor is in an extended conformation and that glycan docking is associated with a conformational change that repositions the carbohydrate-recognition domains during ligand binding. The results further show that the lateral mobility of membrane-bound ligands enhances the engagement of multiple carbohydrate-recognition domains in the receptor oligomer with appropriately spaced ligands. These studies highlight differences between pathogen targeting by DC-SIGN and receptors in which binding sites at fixed spacing bind to simple molecular patterns.

Learning from the viral journey: how to enter cells and how to overcome intracellular barriers to reach the nucleus

Viruses deliver their genome into host cells where they subsequently replicate and multiply. A variety of relevant strategies have evolved by which viruses gain intracellular access and utilize cellular machinery for the synthesis of their genome. Therefore, the viral journey provides insight into the cell's trafficking machinery and how it can be best exploited to improve nonviral gene delivery systems. This review summarizes viral internalization pathways and intracellular trafficking of viruses, with an emphasis on the endosomal escape processes of nonenveloped viruses. Intracellular events from viral entry through nuclear delivery of the viral complementary DNA are also discussed.

Proteolytic activation of the 1918 influenza virus hemagglutinin

Proteolytic activation of the hemagglutinin (HA) protein is indispensable for influenza virus infectivity, and the tissue expression of the responsible cellular proteases impacts viral tropism and pathogenicity. The HA protein critically contributes to the exceptionally high pathogenicity of the 1918 influenza virus, but the mechanisms underlying cleavage activation of the 1918 HA have not been characterized. The neuraminidase (NA) protein of the 1918 influenza virus allows trypsin-independent growth in canine kidney cells (MDCK). However, it is at present unknown if the 1918 NA, like the NA of the closely related strain A/WSN/33, facilitates HA cleavage activation by recruiting the proprotease plasminogen. Moreover, it is not known which pulmonary proteases activate the 1918 HA. We provide evidence that NA-dependent, trypsin-independent cleavage activation of the 1918 HA is cell line dependent and most likely plasminogen independent since the 1918 NA failed to recruit plasminogen and neither exogenous plasminogen nor the presence of the A/WSN/33 NA promoted efficient cleavage of the 1918 HA. The transmembrane serine protease TMPRSS4 was found to be expressed in lung tissue and was shown to cleave the 1918 HA. Accordingly, coexpression of the 1918 HA with TMPRSS4 or the previously identified HA-processing protease TMPRSS2 allowed trypsin-independent infection by pseudotypes bearing the 1918 HA, indicating that these proteases might support 1918 influenza virus spread in the lung. In summary, we show that the previously reported 1918 NA-dependent spread of the 1918 influenza virus is a cell line-dependent phenomenon and is not due to plasminogen recruitment by the 1918 NA. Moreover, we provide evidence that TMPRSS2 and TMPRSS4 activate the 1918 HA by cleavage and therefore may promote viral spread in lung tissue.

The primed ebolavirus glycoprotein (19-kilodalton GP1,2): sequence and residues critical for host cell binding

Entry of ebolavirus (EBOV) into cells is mediated by its glycoprotein (GP(1,2)), a class I fusion protein whose structure was recently determined (J. E. Lee et al., Nature 454:177-182, 2008). Here we confirmed two major predictions of the structural analysis, namely, the residues in GP(1) and GP(2) that remain after GP(1,2) is proteolytically primed by endosomal cathepsins for fusion and residues in GP(1) that are critical for binding to host cells. Mass spectroscopic analysis indicated that primed GP(1,2) contains residues 33 to 190 of GP(1) and all residues of GP(2). The location of the receptor binding site was determined by a two-pronged approach. We identified a small receptor binding region (RBR), residues 90 to 149 of GP(1), by comparing the cell binding abilities of four RBR proteins produced in high yield. We characterized the binding properties of the optimal RBR (containing GP(1) residues 57 to 149) and then conducted a mutational analysis to identify critical binding residues. Substitutions at four lysines (K95, K114, K115, and K140) decreased binding and the ability of RBR proteins to inhibit GP(1,2)-mediated infection. K114, K115, and K140 lie in a small region modeled to be located on the top surface of the chalice following proteolytic priming; K95 lies deeper in the chalice bowl. Combined with those of Lee et al., our findings provide structural insight into how GP(1,2) is primed for fusion and define the core of the EBOV RBR (residues 90 to 149 of GP(1)) as a highly conserved region containing a two-stranded beta-sheet, the two intra-GP(1) disulfide bonds, and four critical Lys residues.

Ebolavirus glycoprotein structure and mechanism of entry

Ebolavirus (EBOV) is a highly virulent pathogen capable of causing a severe hemorrhagic fever with 50-90% lethality. The EBOV glycoprotein (GP) is the only virally expressed protein on the virion surface and is critical for attachment to host cells and catalysis of membrane fusion. Hence, the EBOV GP is a critical component of vaccines as well as a target of neutralizing antibodies and inhibitors of attachment and fusion. The crystal structure of the Zaire ebolavirus GP in its trimeric, prefusion conformation (3 GP(1) plus 3 GP(2)) in complex with a neutralizing antibody fragment, derived from a human survivor of the 1995 Kikwit outbreak, was recently determined. This is the first near-complete structure of any filovirus glycoprotein. The overall molecular architecture of the Zaire ebolavirus GP and its role in viral entry and membrane fusion are discussed in this article.

Selective receptor expression restricts Nipah virus infection of endothelial cells

Nipah virus (NiV) is a highly pathogenic paramyxovirus that causes severe diseases in animals and humans. Endothelial cell (EC) infection is an established hallmark of NiV infection in vivo. Despite systemic virus spread via the vascular system, EC in brain and lung are preferentially infected whereas EC in other organs are less affected. As in vivo, we found differences in the infection of EC in cell culture. Only brain-derived primary or immortalized EC were found to be permissive to NiV infection. Using a replication-independent fusion assay, we could show that the lack of infection in non-brain EC was due to a lack of receptor expression. The NiV entry receptors ephrinB2 (EB2) or ephrinB3 were only expressed in brain endothelia. The finding that EB2 expression in previously non-permissive aortic EC rendered the cells permissive to infection then demonstrated that EB2 is not only necessary but also sufficient to allow the establishment of a productive NiV infection. This strongly suggests that limitations in receptor expression restrict virus entry in certain EC subsets in vivo, and are thus responsible for the differences in EC tropism observed in human and animal NiV infections.

DC-SIGN and L-SIGN: the SIGNs for infection

Two closely related trans-membrane C-type lectins dendritic cell-specific intracellular adhesion molecules (ICAM)-3 grabbing non-integrin (DC-SIGN or CD209) and liver/lymph node-specific ICAM-3 grabbing non-integrin (L-SIGN also known as DC-SIGNR, CD209L or CLEC4M) directly recognize a wide range of micro-organisms of major impact on public health. Both genes have long been considered to share similar overall structure and ligand-binding characteristics. This review presents more recent biochemical and structural studies, which show that they have distinct ligand-binding properties and different physiological functions. Of importance in both these genes is the presence of an extra-cellular domain consisting of an extended neck region encoded by tandem repeats that support the carbohydrate-recognition domain, which plays a crucial role in influencing the pathogen-binding properties of these receptors. The notable difference between these two genes is in this extra-cellular domain. Whilst the tandem-neck-repeat region remains relatively constant size for DC-SIGN, there is considerable polymorphism for L-SIGN. Homo-oligomerization of the neck region of L-SIGN has been shown to be important for high-affinity ligand binding, and heterozygous expression of the polymorphic variants of L-SIGN in which neck lengths differ could thus affect ligand-binding affinity. Functional studies on the effect of this tandem-neck-repeat region on pathogen-binding, as well as genetic association studies for various infectious diseases and among different populations, are discussed. Worldwide demographic data of the tandem-neck-repeat region showing distinct differences in the neck-region allele and genotype distribution among different ethnic groups are presented. These findings support the neck region as an excellent candidate acting as a functional target for selective pressures exerted by pathogens.

Identification and functional characterization of a bovine orthologue to DC-SIGN

Dendritic cell-specific ICAM-3-grabbing nonintegrin (DC-SIGN) C-type lectin is almost exclusively expressed at the cell surface of DC. In addition to its normal function facilitating contact of DC with T cells, DC-SIGN has been shown to bind a variety of pathogens, including Mycobacterium bovis, and HIV-1 envelope protein gp120. In this study, we identified the bovine ortholog of the human DC-SIGN gene within the bovine genome, which exists as a single copy. PCR amplified a product, showing a 100% match with the predicted sequences as well as a sequence predicted to be similar to that of SIGNR7. Furthermore, a protein with the same molecular weight as human DC-SIGN was detected by Western blot in cell lysate derived from bovine DC. To characterize this molecule functionally, the uptake of FITC-labeled OVA and FITC-labeled gp120 (FITC-gp120) by bovine and human DC was assessed. FITC-gp120 was shown to bind to bovine DC in a time- and temperature-dependent manner. Binding was blocked by a polyclonal anti-DC-SIGN antibody but not by a control antibody. Furthermore, blocking of this molecule also reduced the binding of M. bovis bacillus Calmette-Guerin expressing GFP. Confocal microscopy showed that DC-SIGN was expressed on the surface of bovine DC. Subsequent pulse-chase studies revealed that FITC-gp120 was internalized by bovine monocyte-derived DC as early as 10 min. Thus, there is evidence of a DC-SIGN-like molecule expressed specifically by bovine DC. This molecule may play an important role in the infection of bovine (DC) cells with M. bovis.

Interactions of LSECtin and DC-SIGN/DC-SIGNR with viral ligands: Differential pH dependence, internalization and virion binding

The calcium-dependent lectins DC-SIGN and DC-SIGNR (collectively termed DC-SIGN/R) bind to high-mannose carbohydrates on a variety of viruses. In contrast, the related lectin LSECtin does not recognize mannose-rich glycans and interacts with a more restricted spectrum of viruses. Here, we analyzed whether these lectins differ in their mode of ligand engagement. LSECtin and DC-SIGNR, which we found to be co-expressed by liver, lymph node and bone marrow sinusoidal endothelial cells, bound to soluble Ebola virus glycoprotein (EBOV-GP) with comparable affinities. Similarly, LSECtin, DC-SIGN and the Langerhans cell-specific lectin Langerin readily bound to soluble human immunodeficiency virus type-1 (HIV-1) GP. However, only DC-SIGN captured HIV-1 particles, indicating that binding to soluble GP is not necessarily predictive of binding to virion-associated GP. Capture of EBOV-GP by LSECtin triggered ligand internalization, suggesting that LSECtin like DC-SIGN might function as an antigen uptake receptor. However, the intracellular fate of lectin-ligand complexes might differ. Thus, exposure to low-pH medium, which mimics the acidic luminal environment in endosomes/lysosomes, released ligand bound to DC-SIGN/R but had no effect on LSECtin interactions with ligand. Our results reveal important differences between pathogen capture by DC-SIGN/R and LSECtin and hint towards different biological functions of these lectins.

The mechanism of Axl-mediated Ebola virus infection

We previously reported that expression of the receptor-type tyrosine kinase Axl, which regulates cell survival and activation, enhances both pseudotype and live Ebola virus (EBOV) infection. To clarify the mechanistic basis of this enhancement, we created a series of Axl mutants and identified amino acids/domains necessary for this function, by using a pseudotype virus carrying the EBOV glycoprotein (GP). Analyses of the Axl mutants showed the importance of extracellular and intracellular regions for Axl functions, including ligand binding and signal transduction, in EBOV GP-mediated infection. These data suggest that EBOV uses the physiological functions of Axl to enter cells.

Analysis of the interaction of Ebola virus glycoprotein with DC-SIGN (dendritic cell-specific intercellular adhesion molecule 3-grabbing nonintegrin) and its homologue DC-SIGNR

BACKGROUND

The lectin DC-SIGN (dendritic cell-specific intercellular adhesion molecule 3-grabbing nonintegrin) augments Ebola virus (EBOV) infection. However, it its unclear whether DC-SIGN promotes only EBOV attachment (attachment factor function, nonessential) or actively facilitates EBOV entry (receptor function, essential).

METHODS

We investigated whether DC-SIGN on B cell lines and dendritic cells acts as an EBOV attachment factor or receptor.

RESULTS

Engineered DC-SIGN expression rendered some B cell lines susceptible to EBOV glycoprotein (EBOV GP)-driven infection, whereas others remained refractory, suggesting that cellular factors other than DC-SIGN are also required for susceptibility to EBOV infection. Augmentation of entry was independent of efficient DC-SIGN internalization and might not involve lectin-mediated endocytic uptake of virions. Therefore, DC-SIGN is unlikely to function as an EBOV receptor on B cell lines; instead, it might concentrate virions onto cells, thereby allowing entry into cell lines expressing low levels of endogenous receptor(s). Indeed, artificial concentration of virions onto cells mirrored DC-SIGN expression, confirming that optimization of viral attachment is sufficient for EBOV GP-driven entry into some B cell lines. Finally, EBOV infection of dendritic cells was only partially dependent on mannose-specific lectins, such as DC-SIGN, suggesting an important contribution of other factors.

CONCLUSIONS

Our results indicate that DC-SIGN is not an EBOV receptor but, rather, is an attachment-promoting factor that boosts entry into B cell lines susceptible to low levels of EBOV GP-mediated infection.

A novel mechanism for LSECtin binding to Ebola virus surface glycoprotein through truncated glycans

LSECtin is a member of the C-type lectin family of glycan-binding receptors that is expressed on sinusoidal endothelial cells of the liver and lymph nodes. To compare the sugar and pathogen binding properties of LSECtin with those of related but more extensively characterized receptors, such as DC-SIGN, a soluble fragment of LSECtin consisting of the C-terminal carbohydrate-recognition domain has been expressed in bacteria. A biotin-tagged version of the protein was also generated and complexed with streptavidin to create tetramers. These forms of the carbohydrate-recognition domain were used to probe a glycan array and to characterize binding to oligosaccharide and glycoprotein ligands. LSECtin binds with high selectivity to glycoproteins terminating in GlcNAcbeta1-2Man. The inhibition constant for this disaccharide is 3.5 microm, making it one of the best low molecular weight ligands known for any C-type lectin. As a result of the selective binding of this disaccharide unit, the receptor recognizes glycoproteins with a truncated complex and hybrid N-linked glycans on glycoproteins. Glycan analysis of the surface glycoprotein of Ebola virus reveals the presence of such truncated glycans, explaining the ability of LSECtin to facilitate infection by Ebola virus. High mannose glycans are also present on the viral glycoprotein, which explains why DC-SIGN also binds to this virus. Thus, multiple receptors interact with surface glycoproteins of enveloped viruses that bear different types of relatively poorly processed glycans.

Dendritic cell subsets in health and disease

The dendritic cell (DC) system of antigen-presenting cells controls immunity and tolerance. DCs initiate and regulate immune responses in a manner that depends on signals they receive from microbes and their cellular environment. They allow the immune system to make qualitatively distinct responses against different microbial infections. DCs are composed of subsets that express different microbial receptors and express different surface molecules and cytokines. Our studies lead us to propose that interstitial (dermal) DCs preferentially activate humoral immunity, whereas Langerhans cells preferentially induce cellular immunity. Alterations of the DC system result in diseases such as autoimmunity, allergy, and cancer. Conversely, DCs can be exploited for vaccination, and novel vaccines that directly target DCs in vivo are being designed.

Role of homozygous DC-SIGNR 5/5 tandem repeat polymorphism in HIV-1 exposed seronegative North Indian individuals

Despite multiple sexual exposures to HIV-1 virus, some individuals remain HIV-1 seronegative. Although several genetic factors have been related to HIV-1 resistance, the homozygosity for a mutation in CCR5 gene (the 32-bp deletion, i.e., CCR5-Delta32 allele) is presently considered the most relevant one. The C-type lectins, DC-SIGN (present on dendritic cells and macrophages) and DC-SIGNR (present on endothelial cells in liver and lymph nodes) efficiently bind and transmit HIV-1 to susceptible cell in trans, thereby augmenting the infection. A potential association of the DC-SIGN and DC-SIGNR neck domain repeat polymorphism and risk of HIV-1 infection is currently under debate. To determine the influence of host genetic factors on HIV-1 resistance, we conducted genetic risk association study in HIV-1-exposed seronegative (n = 47) individuals, HIV-1 seronegative (n = 262) healthy control, and HIV-1-infected seropositive patients (n = 168) for polymorphism in neck domain of DC-SIGN and DC-SIGNR genes. The DC-SIGN and DC-SIGNR genotypes were identified by polymerase chain reaction method in DNA extracted from peripheral blood and confirmed by sequencing. Fisher exact or χ² test was used for static analysis. DC-SIGN genotype and allele distribution was fairly similar in HIV-1-exposed seronegative, HIV-1 seropositive, and HIV-1 seronegative control. There was no statistical significance in the differences in the distribution of DC-SIGN genotypes. A total of 13 genotypes were found in DC-SIGNR neck repeat region polymorphism. Among all the genotypes, only 5/5 homozygous showed significant reduced risk of HIV-1 infection in HIV-1-exposed seronegative individuals (p = 0.009). A unique genotype 8/5 heterozygous was also found in HIV-1 seropositive individual, which is not reported elsewhere.

How Ebola and Marburg viruses battle the immune system

The filoviruses Ebola and Marburg have emerged in the past decade from relative obscurity to serve now as archetypes for some of the more intriguing and daunting challenges posed by such agents. Public imagination is captured by deadly outbreaks of these viruses and reinforced by the specter of bioterrorism. As research on these agents has accelerated, it has been found increasingly that filoviruses use a combination of familiar and apparently new ways to baffle and battle the immune system. Filoviruses have provided thereby a new lens through which to examine the immune system itself.

Ebola virus-like particle-induced activation of NF-kappaB and Erk signaling in human dendritic cells requires the glycoprotein mucin domain

Dendritic cells (DCs), important early targets of Ebola virus (EBOV) infection in vivo, are activated by Ebola virus-like particles (VLPs). To better understand this phenomenon, we have systematically assessed the response of DCs to VLPs of different compositions. VLPs containing the viral matrix protein (VP40) and the viral glycoprotein (GP), were found to induce a proinflammatory response highly similar to a prototypical DC activator, LPS. This response included the production of several proinflammatory cytokines, activation of numerous transcription factors including NF-kappaB, the functional importance of which was demonstrated by employing inhibitors of NF-kappaB activation, and activation of ERK1/2 MAP kinase. In contrast, VLPs constituted with a mutant GP lacking the heavily glycosylated mucin domain showed impaired NF-kappaB and Erk activation and induced less DC cytokine production. We conclude that the GP mucin domain is required for VLPs to stimulate human dendritic cells through NF-kappaB and MAPK signaling pathways.

Modulation of HIV and SIV neutralization sensitivity by DC-SIGN and mannose-binding lectin

The C-type lectin DC-SIGN binds to oligosaccharides on the human and simian immunodeficiency virus (HIV, SIV) envelope glycoproteins and promotes infection of susceptible cells. Here, we show that DC-SIGN recognizes glycans involved in SIV sensitivity to neutralizing antibodies and that binding to DC-SIGN confers neutralization resistance to an otherwise sensitive SIV variant. Moreover, we provide evidence that mannose-binding lectin (MBL) can interfere with HIV-1 neutralization by the carbohydrate-specific antibody 2G12.

The Immunobiology of SARS*

Severe acute respiratory syndrome (SARS) presented as an atypical pneumonia that progressed to acute respiratory distress syndrome in approximately 20% of cases and was associated with a mortality of about 10%. The etiological agent was a novel coronavirus (CoV). Angiotensin-converting enzyme 2 is the functional receptor for SARS-CoV; DC-SIGN and CD209L (L-SIGN) can enhance viral entry. Although the virus infects the lungs, gastrointestinal tract, liver, and kidneys, the disease is limited to the lungs, where diffuse alveolar damage is accompanied by a disproportionately sparse inflammatory infiltrate. Pro-inflammatory cytokines and chemokines, particularly IP-10, IL-8, and MCP-1, are elevated in the lungs and peripheral blood, but there is an unusual lack of an antiviral interferon (IFN) response. The virus is susceptible to exogenous type I IFN but suppresses the induction of IFN. Innate immunity is important for viral clearance in the mouse model. Virus-specific neutralizing antibodies that develop during convalescence prevent reinfection in animal models.

Ebola virus glycoprotein 1: identification of residues important for binding and postbinding events

The filoviruses Ebola virus (EBOV) and Marburg virus (MARV) are responsible for devastating hemorrhagic fever outbreaks. No therapies are available against these viruses. An understanding of filoviral glycoprotein 1 (GP1) residues involved in entry events would facilitate the development of antivirals. Towards this end, we performed alanine scanning mutagenesis on selected residues in the amino terminus of GP1. Mutant GPs were evaluated for their incorporation onto feline immunodeficiency virus (FIV) particles, transduction efficiency, receptor binding, and ability to be cleaved by cathepsins L and B. FIV virions bearing 39 out of 63 mutant glycoproteins transduced cells efficiently, whereas virions bearing the other 24 had reduced levels of transduction. Virions pseudotyped with 23 of the poorly transducing GPs were characterized for their block in entry. Ten mutant GPs were very poorly incorporated onto viral particles. Nine additional mutant GPs (G87A/F88A, K114A/K115A, K140A, G143A, P146A/C147A, F153A/H154A, F159A, F160A, and Y162A) competed poorly with wild-type GP for binding to permissive cells. Four of these nine mutants (P146A/C147A, F153A/H154A, F159A, and F160A) were also inefficiently cleaved by cathepsins. An additional four mutant GPs (K84A, R134A, D150A, and E305/E306A) that were partially defective in transduction were found to compete effectively for receptor binding and were readily cleaved by cathepsins. This finding suggested that this latter group of mutants might be defective at a postbinding, cathepsin cleavage-independent step. In total, our study confirms the role of some GP1 residues in EBOV entry that had previously been recognized and identifies for the first time other residues that are important for productive entry.

Development of vaccines for Marburg hemorrhagic fever

Marburg (MARV) and Ebola viruses (EBOV) emerged from the rainforests of Central Africa more than 30 years ago causing outbreaks of severe and, usually, fatal hemorrhagic fever. EBOV has garnered the lion's share of the attention, fueled by the higher frequency of EBOV outbreaks, high mortality rates and importation into the USA, documented in such popular works as the best-selling novel 'The Hot Zone'. However, recent large outbreaks of hundreds of cases of MARV infection in the Democratic Republic of the Congo and Angola with case fatalities approaching 90% dramatically highlight its lethal potential. Although no vaccines or antiviral drugs for MARV are currently available, remarkable progress has been made over the last few years in developing potential countermeasures against MARV in nonhuman primate models. In particular, a vaccine based on attenuated recombinant vesicular stomatitis virus was recently shown to have both preventive and postexposure efficacy.

Multiple modes of binding enhance the affinity of DC-SIGN for high mannose N-linked glycans found on viral glycoproteins

The dendritic cell surface receptor DC-SIGN and the closely related endothelial cell receptor DC-SIGNR specifically recognize high mannose N-linked carbohydrates on viral pathogens. Previous studies have shown that these receptors bind the outer trimannose branch Manalpha1-3[Manalpha1-6]Manalpha present in high mannose structures. Although the trimannoside binds to DC-SIGN or DC-SIGNR more strongly than mannose, additional affinity enhancements are observed in the presence of one or more Manalpha1-2Manalpha moieties on the nonreducing termini of oligomannose structures. The molecular basis of this enhancement has been investigated by determining crystal structures of DC-SIGN bound to a synthetic six-mannose fragment of a high mannose N-linked oligosaccharide, Manalpha1-2Manalpha1-3[Manalpha1-2Manalpha1-6]Manalpha1-6Man and to the disaccharide Manalpha1-2Man. The structures reveal mixtures of two binding modes in each case. Each mode features typical C-type lectin binding at the principal Ca2+-binding site by one mannose residue. In addition, other sugar residues form contacts unique to each binding mode. These results suggest that the affinity enhancement displayed toward oligosaccharides decorated with the Manalpha1-2Manalpha structure is due in part to multiple binding modes at the primary Ca2+ site, which provide both additional contacts and a statistical (entropic) enhancement of binding.

Endothelial cell infection and hemostasis

As an important component of the vasculature, endothelial cell lining covers the inner surface of blood vessels and provides an active barrier interface between the vascular and perivascular compartments. In addition to maintaining vasomotor equilibrium and organ homeostasis and communicating with circulating blood cells, the vascular endothelium also serves as the preferred target for a number of infectious agents. This review article focuses on the roles of interactions between vascular endothelial cells and invading pathogens and resultant endothelial activation in the pathogenesis of important human diseases with viral and bacterial etiologies. In this perspective, the signal transduction events that regulate vascular inflammation and basis for endothelial cell tropism exhibited by certain specific viruses and pathogenic bacteria are also discussed.

Attachment of human immunodeficiency virus to cells and its inhibition

The entry of enveloped viruses involves virus adsorption followed by close apposition of the viral and plasma membranes. This multistep process is initiated by specific binding interactions between glycoproteins in the viral envelope and appropriate receptors on the cell surface. In the case of HIV-1, attachment of virions to the cell surface is attributed to a high affinity interaction between envelope spike glycoproteins (Env, composed of the surface protein gp120 and the transmembrane protein gp41) and a complex made of the primary CD4 receptor and a seven-transmembrane co-receptor (e.g., CXCR4 or CCR5) (reviewed in [1]). Then a chain of dynamic events take place that enable the viral nucleocapsid to penetrate within the target cell following the destabilization of membrane microenvironment and the formation of a fusion pore.

The C type lectins DC-SIGN and L-SIGN: receptors for viral glycoproteins

DC-SIGN and L-SIGN are C-type lectins that recognize carbohydrate structures present on viral glycoproteins and function as attachment factors for several enveloped viruses. DC-SIGN and L-SIGN enhance viral entry and facilitate infection of cells that express the cognate entry receptor (cis-infection). They are also able to capture viruses and transfer viral infections to other target cells (trans-infection). In this chapter, we will give an overview of protocols used to produce soluble viral glycoproteins at high levels and to study the molecular basis of viruses/DC-SIGN and L-SIGN binding and internalization. We will also describe techniques to investigate the molecular mechanisms by which DC-SIGN or L-SIGN spread viral infections.

Tyro3 family-mediated cell entry of Ebola and Marburg viruses

Filoviruses, represented by the genera Ebolavirus and Marburgvirus, cause a lethal hemorrhagic fever in humans and in nonhuman primates. Although filovirus can replicate in various tissues or cell types in these animals, the molecular mechanisms of its broad tropism remain poorly understood. Here we show the involvement of members of the Tyro3 receptor tyrosine kinase family-Axl, Dtk, and Mer-in cell entry of filoviruses. Ectopic expression of these family members in lymphoid cells, which otherwise are highly resistant to filovirus infection, enhanced infection by pseudotype viruses carrying filovirus glycoproteins on their envelopes. This enhancement was reduced by antibodies to Tyro3 family members, Gas6 ligand, or soluble ectodomains of the members. Live Ebola viruses infected both Axl- and Dtk-expressing cells more efficiently than control cells. Antibody to Axl inhibited infection of pseudotype viruses in a number of Axl-positive cell lines. These results implicate each Tyro3 family member as a cell entry factor in filovirus infection.

DC-SIGN and CLEC-2 mediate human immunodeficiency virus type 1 capture by platelets

Platelets can engulf human immunodeficiency virus type 1 (HIV-1), and a significant amount of HIV-1 in the blood of infected individuals is associated with these cells. However, it is unclear how platelets capture HIV-1 and whether platelet-associated virus remains infectious. DC-SIGN and other lectins contribute to capture of HIV-1 by dendritic cells (DCs) and facilitate HIV-1 spread in DC/T-cell cocultures. Here, we show that platelets express both the C-type lectin-like receptor 2 (CLEC-2) and low levels of DC-SIGN. CLEC-2 bound to HIV-1, irrespective of the presence of the viral envelope protein, and facilitated HIV-1 capture by platelets. However, a substantial fraction of the HIV-1 binding activity of platelets was dependent on DC-SIGN. A combination of DC-SIGN and CLEC-2 inhibitors strongly reduced HIV-1 association with platelets, indicating that these lectins are required for efficient HIV-1 binding to platelets. Captured HIV-1 was maintained in an infectious state over several days, suggesting that HIV-1 can escape degradation by platelets and might use these cells to promote its spread. Our results identify CLEC-2 as a novel HIV-1 attachment factor and provide evidence that platelets capture and transfer infectious HIV-1 via DC-SIGN and CLEC-2, thereby possibly facilitating HIV-1 dissemination in infected patients.

Protective efficacy of neutralizing antibodies against Ebola virus infection

Ebola virus causes lethal hemorrhagic fever in humans and nonhuman primates, but no effective antiviral compounds are available for the treatment of this infection. The surface glycoprotein (GP) of Ebola virus is an important target of neutralizing antibodies. Although passive transfer of GP-specific antibodies has been evaluated in mouse and guinea pig models, protection was achieved only by treatment shortly before or after virus challenge. Using these animal models, we evaluated the protective efficacy of two monoclonal antibodies whose epitopes are distinct from those of the antibodies tested by others. Treatment of mice with these antibodies 2 days after challenge completely protected most of the animals; even treatment 3 or 4 days after challenge was partially effective. Although antibody treatment in the guinea pig model was not as effective as in the mouse model, single-dose treatment of guinea pigs 1 day before, or 1 or 2 days after challenge did protect some animals. Interestingly, the protective effects seen in these animal models did not correlate with the in vitro neutralizing activity of the antibodies, suggesting different mechanisms of the neutralization by these antibodies. These results underscore the potential therapeutic utility of monoclonal antibodies for postexposure treatment of Ebola virus infections.

Viral hemorrhagic fevers

A taxonomically diverse set of single-stranded ribonucleic acid(ssRNA) viruses from four diverse viral families Arenaviridae,Bunyaviridae, Filoviridae, and Flaviviridae cause an acute systemic febrile syndrome called viral hemorrhagic fever (VHF). The syndrome produces combinations of prostration, malaise, increased vascular permeability, and coagulation maladies. In severe illness,VHF may include generalized bleeding but the bleeding does not typically constitute a life-threatening loss of blood volume. To a certain extent, it is a sign of damage to the vascular endothelium and is an indicator of disease severity in specific target organs. Although the viruses that cause hemorrhagic fever (HF) can productively replicate in endothelial cells, much of the disease pathology including impairment to the vascular system is thought to result primarily from the release of a variety of mediators from virus-infected cells, such as monocytes and macrophages that subsequently alter vascular function and trigger the coagulation disorders that epitomize these infections. While significant progress has been made over the last several years in dissecting out the molecular biology and pathogenesis of the HF viruses, there are currently no vaccines or drugs licensed available for most of the VHFs.

Characterization of Marburg virus glycoprotein in viral entry

One major determinant of host tropism for filoviruses is viral glycoprotein (GP), which is involved in receptor binding and viral entry. Compared to Ebola GP (EGP), Marburg GP (MGP) is less well characterized in viral entry. In this study, using a human immunodeficiency virus-based pseudotyped virus as a surrogate system, we have characterized the role of MGP in viral entry. We have shown that like EGP, the mucin-like region of MGP (289-501) is not essential for virus entry. We have developed a viral entry interference assay for filoviruses, and using this assay, we have demonstrated that transfection of EGP or MGP in target cells can interfere with EGP/HIV and MGP/HIV pseudotyped virus entry in a dose-dependent manner. These results are consistent with the notion that Ebola and Marburg viruses use the same or a related host molecule(s) for viral entry. Substitutions of the non-conserved residues in MGP1 did not impair MGP-mediated viral entry. Unlike that of EGP1, individual substitutions of many conserved residues of MGP1 exerted severe defects in MGP expression, incorporation to HIV virions, and thus its ability to mediate viral entry. These results indicate that MGP is more sensitive to substitutions of the conserved residues, suggesting that MGP may fold differently from EGP.

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.

Identification of two amino acid residues on Ebola virus glycoprotein 1 critical for cell entry

Using site-directed mutagenesis and retroviral vector pseudotyping of the wild type or mutated glycoprotein of Zaire ebolavirus (ZEBOV), we analyzed 15 conserved residues in the N-terminus of the filovirus glycoprotein 1 (GP1) in order to identify residues critical for cell entry. Results from infectivity assays and Western blot analyses identified two phenylalanine residues at positions 88 and 159 that appear to be critical for ZEBOV entry in vitro. We extended this observation by introduction of alanines at either position 88 or 159 of Ivory Coast Ebolavirus (CIEBOV) and observed the same phenotype. Further, we showed that introduction of each of the two mutations in a recombinant full-length clone of ZEBOV (Mayinga strain) that also carried the coding sequence for GFP could not be rescued, suggesting the mutants rendered the virus non-infectious. The two phenylalanines that are critical for both ZEBOV and CIEBOV entry are found in two linear domains of GP1 that are highly conserved among filoviruses, and thus could provide a target for rational development of broadly cross-protective vaccines or antiviral therapies.

Immunologic aspects of select agents

We are on the cusp of exciting new developments in vaccine biology. The use of DNA constructs allows virtually unlimited access to previously inaccessible organisms. Next-generation adjuvants will boost innate and acquired immunity, and will provide protection from infection with any potential biowarfare organism. We are limited only by our imaginations in the construction of a protective armamentarium.

Expression of DC-SIGN and DC-SIGNR on human sinusoidal endothelium: a role for capturing hepatitis C virus particles

Hepatic sinusoidal endothelial cells are unique among endothelial cells in their ability to internalize and process a diverse range of antigens. DC-SIGNR, a type 2 C-type lectin expressed on liver sinusoids, has been shown to bind with high affinity to hepatitis C virus (HCV) E2 glycoprotein. DC-SIGN is a closely related homologue reported to be expressed only on dendritic cells and a subset of macrophages and has similar binding affinity to HCV E2 glycoprotein. These receptors function as adhesion and antigen presentation molecules. We report distinct patterns of DC-SIGNR and DC-SIGN expression in human liver tissue and show for the first time that both C-type lectins are expressed on sinusoidal endothelial cells. We confirmed that these receptors are functional by demonstrating their ability to bind HCV E2 glycoproteins. Although these lectins on primary sinusoidal cells support HCV E2 binding, they are unable to support HCV entry. These data support a model where DC-SIGN and DC-SIGNR on sinusoidal endothelium provide a mechanism for high affinity binding of circulating HCV within the liver sinusoids allowing subsequent transfer of the virus to underlying hepatocytes, in a manner analogous to DC-SIGN presentation of human immunodeficiency virus on dendritic cells.

DC-SIGN facilitates fusion of dendritic cells with human T-cell leukemia virus type 1-infected cells

Interactions between the oncogenic retrovirus human T-cell leukemia virus type 1 (HTLV-1) and dendritic cells (DCs) are poorly characterized. We show here that monocyte-derived DCs form syncytia and are infected upon coculture with HTLV-1-infected lymphocytes. We examined the role of DC-specific ICAM-3-grabbing nonintegrin (DC-SIGN), a C-type lectin expressed in DCs, in HTLV-1-induced syncytium formation. DC-SIGN is known to bind with high affinity to various viral envelope glycoproteins, including human immunodeficiency virus (HIV) and hepatitis C virus, as well as to the cellular receptors ICAM-2 and ICAM-3. After cocultivating DCs and HTLV-1-infected cells, we found that anti-DC-SIGN monoclonal antibodies (MAbs) were able to decrease the number and size of HTLV-1-induced syncytia. Moreover, expression of the lectin in epithelial-cell lines dramatically enhanced the ability to fuse with HTLV-1-positive cells. Interestingly, in contrast to the envelope (Env) glycoproteins of HIV and other viruses, that of HTLV-1 does not bind directly to DC-SIGN. The facilitating role of the lectin in HTLV-1 syncytium formation is mediated by its interaction with ICAM-2 and ICAM-3, as demonstrated by use of MAbs directed against these adhesion molecules. Altogether, our results indicate that DC-SIGN facilitates HTLV-1 infection and fusion of DCs through an ICAM-dependent mechanism.

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.

All but the Shortest Polymorphic Forms of the Viral Receptor DC-SIGNR Assemble into Stable Homo- and Heterotetramers

Polymorphisms that affect the length of the extracellular neck region of the endothelial receptor DC-SIGNR (dendritic cell-specific intercellular adhesion molecule-3-grabbing nonintegrin-related protein) have been linked to differences in susceptibility to infection by enveloped viruses. We have characterized the effects of these polymorphisms on the ability of DC-SIGNR to form tetramers containing the clusters of sugar-binding sites needed for binding to viral envelope glycoproteins. Chemical cross-linking and analytical ultracentrifugation experiments have been used to show that only the smallest form of DC-SIGNR is defective in homotetramer assembly. A novel affinity-tagging approach has been employed to demonstrate that, contrary to previous speculation, heterotetramers can be assembled efficiently from DC-SIGNR polypeptides of different lengths. The heterotetramers are stable and can be detected in fibroblasts transfected with multiple forms of DC-SIGNR. These results provide a molecular basis for interpreting the way polymorphisms affect interactions with viruses.

Impact of polymorphisms in the DC-SIGNR neck domain on the interaction with pathogens

The lectins DC-SIGN and DC-SIGNR augment infection by human immunodeficiency virus (HIV), Ebolavirus (EBOV) and other pathogens. The neck domain of these proteins drives multimerization, which is believed to be required for efficient recognition of multivalent ligands. The neck domain of DC-SIGN consists of seven sequence repeats with rare variations. In contrast, the DC-SIGNR neck domain is polymorphic and, in addition to the wild type (wt) allele with seven repeat units, allelic forms with five and six sequence repeats are frequently found. A potential association of the DC-SIGNR genotype and risk of HIV-1 infection is currently under debate. Therefore, we investigated if DC-SIGNR alleles with five and six repeat units exhibit defects in pathogen capture. Here, we show that wt DC-SIGNR and patient derived alleles with five and six repeats bind viral glycoproteins, augment viral infection and tetramerize with comparable efficiency. Moreover, coexpression of wt DC-SIGNR and alleles with five repeats did not decrease the interaction with pathogens compared to expression of each allele alone, suggesting that potential formation of hetero-oligomers does not appreciably reduce pathogen binding, at least under conditions of high expression. Thus, our results do not provide evidence for diminished pathogen capture by DC-SIGNR alleles with five and six repeat units. Albeit, we cannot exclude that subtle, but in vivo relevant differences remained undetected, our analysis suggests that indirect mechanisms could account for the association of polymorphisms in the DC-SIGNR neck region with reduced risk of HIV-1 infection.