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

The DC-SIGN–related lectin LSECtin mediates antigen capture and pathogen binding by human myeloid cells

Liver and lymph node sinusoidal endothelial cell C-type lectin (LSECtin [CLEC4G]) is a C-type lectin encoded within the liver/lymph node–specific intercellular adhesion molecule-3–grabbing nonintegrin (L-SIGN)/dendritic cell–specific intercellular adhesion molecule-3–grabbing nonintegrin (DC-SIGN)/CD23 gene cluster. LSECtin expression has been previously described as restricted to sinusoidal endothelial cells of the liver and lymph node. We now report LSECtin expression in human peripheral blood and thymic dendritic cells isolated ex vivo. LSECtin is also detected in monocyte-derived macrophages and dendritic cells at the RNA and protein level. In vitro, interleukin-4 (IL-4) induces the expression of 3 LSECtin alternatively spliced isoforms, including a potentially soluble form (Δ2 isoform) and a shorter version of the prototypic molecule (Δ3/4 isoform). LSECtin functions as a pathogen receptor, because its expression confers Ebola virus–binding capacity to leukemic cells. Sugar-binding studies indicate that LSECtin specifically recognizes N-acetyl-glucosamine, whereas no LSECtin binding to Mannan- or N-acetyl-galactosamine–containing matrices are observed. Antibody or ligand-mediated engagement triggers a rapid internalization of LSECtin,which is dependent on tyrosine and diglutamic-containing motifs within the cytoplasmic tail. Therefore, LSECtin is a pathogen-associated molecular pattern receptor in human myeloid cells. In addition, our results suggest that LSECtin participates in antigen uptake and internalization, and might be a suitable target molecule in vaccination strategies.

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.

N-linked glycosylation attenuates H3N2 influenza viruses

Over the last four decades, H3N2 subtype influenza A viruses have gradually acquired additional potential sites for glycosylation within the globular head of the hemagglutinin (HA) protein. Here, we have examined the biological effect of additional glycosylation on the virulence of H3N2 influenza viruses. We created otherwise isogenic reassortant viruses by site-directed mutagenesis that contain additional potential sites for glycosylation and examined the effect on virulence in naïve BALB/c, C57BL/6, and surfactant protein D (SP-D)-deficient mice. The introduction of additional sites was consistent with the sequence of acquisition in the globular head over the past 40 years, beginning with two sites in 1968 to the seven sites found in contemporary influenza viruses circulating in 2000. Decreased morbidity and mortality, as well as lower viral lung titers, were seen in mice as the level of potential glycosylation of the viruses increased. This correlated with decreased evidence of virus-mediated lung damage and increased in vitro inhibition of hemagglutination by SP-D. SP-D-deficient animals displayed an inverse pattern of disease, such that more highly glycosylated viruses elicited disease equivalent to or exceeding that of the wild type. We conclude from these data that increased glycosylation of influenza viruses results in decreased virulence, which is at least partly mediated by SP-D-induced clearance from the lung. The continued exploration of interactions between highly glycosylated viruses and surfactant proteins may lead to an improved understanding of the biology within the lung and strategies for viral control.

Virus glycosylation: role in virulence and immune interactions

The study of N-linked glycosylation as it relates to virus biology has become an area of intense interest in recent years due to its ability to impart various advantages to virus survival and virulence. HIV and influenza, two clear threats to human health, have been shown to rely on expression of specific oligosaccharides to evade detection by the host immune system. Additionally, other viruses such as Hendra, SARS-CoV, influenza, hepatitis and West Nile rely on N-linked glycosylation for crucial functions such as entry into host cells, proteolytic processing and protein trafficking. This review focuses on recent findings on the importance of glycosylation to viral virulence and immune evasion for several prominent human pathogens.

Severe Acute Respiratory Syndrome Coronavirus Entry as a Target of Antiviral Therapies

The identification in 2003 of a coronavirus as the aetiological agent of severe acute respiratory syndrome (SARS) intensified efforts to understand the biology of corona-viruses in general and SARS coronavirus (SARS-CoV) in particular. Rapid progress was made in describing the SARS-CoV genome, evolution and lifecycle. Identification of angiotensin-converting enzyme 2 (ACE2) as an obligate cellular receptor for SARS-CoV contributed to understanding of the SARS-CoV entry process, and helped to characterize two targets of antiviral therapeutics: the SARS-CoV spike protein and ACE2. Here we describe the role of these proteins in SARS-CoV replication and potential therapeutic strategies aimed at preventing entry of SARS-CoV into target cells.

Severe Acute Respiratory Syndrome Coronavirus (SARS-CoV)

Severe acute respiratory syndrome (SARS) is caused by a coronavirus (CoV), SARSCoV. SARS-CoV belongs to the family Coronaviridae, which are enveloped RNA viruses in the order Nidovirales. Global research efforts are continuing to increase the understanding of the virus, the pathogenesis of the disease it causes (SARS), and the “heterogeneity of individual infectiousness” as well as shedding light on how to prepare for other emerging viral diseases. Promising drugs and vaccines have been identified. The milestones achieved have resulted from a truly international effort. Molecular studies dissected the adaptation of this virus as it jumped from an intermediary animal, the civet, to humans, thus providing valuable insights into processes of molecular emergence.

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.

Highly conserved regions within the spike proteins of human coronaviruses 229E and NL63 determine recognition of their respective cellular receptors

We have recently demonstrated that the severe acute respiratory syndrome coronavirus (SARS-CoV) receptor angiotensin converting enzyme 2 (ACE2) also mediates cellular entry of the newly discovered human coronavirus (hCoV) NL63. Here, we show that expression of DC-SIGN augments NL63 spike (S)-protein-driven infection of susceptible cells, while only expression of ACE2 but not DC-SIGN is sufficient for entry into nonpermissive cells, indicating that ACE2 fulfills the criteria of a bona fide hCoV-NL63 receptor. As for SARS-CoV, murine ACE2 is used less efficiently by NL63-S for entry than human ACE2. In contrast, several amino acid exchanges in human ACE2 which diminish SARS-S-driven entry do not interfere with NL63-S-mediated infection, suggesting that SARS-S and NL63-S might engage human ACE2 differentially. Moreover, we observed that NL63-S-driven entry was less dependent on a low-pH environment and activity of endosomal proteases compared to infection mediated by SARS-S, further suggesting differences in hCoV-NL63 and SARS-CoV cellular entry. NL63-S does not exhibit significant homology to SARS-S but is highly related to the S-protein of hCoV-229E, which enters target cells by engaging CD13. Employing mutagenic analyses, we found that the N-terminal unique domain in NL63-S, which is absent in 229E-S, does not confer binding to ACE2. In contrast, the highly homologous C-terminal parts of the NL63-S1 and 229E-S1 subunits in conjunction with distinct amino acids in the central regions of these proteins confer recognition of ACE2 and CD13, respectively. Therefore, despite the high homology of these sequences, they likely form sufficiently distinct surfaces, thus determining receptor specificity.

Human hepatic sinusoidal endothelial cells can be distinguished by expression of phenotypic markers related to their specialised functions in vivo

The hepatic sinusoids are lined by a unique population of hepatic sinusoidal endothelial cells (HSEC), which is one of the first hepatic cell populations to come into contact with blood components. However, HSEC are not simply barrier cells that restrict the access of blood-borne compounds to the parenchyma. They are functionally specialised endothelial cells that have complex roles, including not only receptor-mediated clearance of endotoxin, bacteria and other compounds, but also the regulation of inflammation, leukocyte recruitment and host immune responses to pathogens. Thus understanding the differentiation and function of HSEC is critical for the elucidation of liver biology and pathophysiology. This article reviews methods for isolating and studying human hepatic endothelial cell populations using in vitro models. We also discuss the expression and functions of phenotypic markers, such as the presence of fenestrations and expression of VAP-1, Stabilin-1, L-SIGN, which can be used to identify sinusoidal endothelium and to permit discrimination from vascular and lymphatic endothelial cells.

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.

Hosting the severe acute respiratory syndrome coronavirus: specific cell factors required for infection

As with all viruses, the severe acute respiratory syndrome coronavirus (SARS‐CoV) utilizes specific host cell factors during its infection cycle. Some of these factors have been identified and are now increasingly scrutinized as targets to intervene with infection. In this brief review, we describe the current understanding of how the SARS‐CoV is able to use the cellular machinery for its replication.

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.

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.

How the SARS coronavirus causes disease: host or organism?

The previous epidemic of severe acute respiratory syndrome (SARS) has ended. However, many questions concerning how the aetiological agent, the novel SARS coronavirus (CoV), causes illness in humans remain unanswered. The pathology of fatal cases of SARS is dominated by diffuse alveolar damage. Specific histological changes are not detected in other organs. These contrast remarkably with the clinical picture, in which there are apparent manifestations in multiple organs. Both pathogen and host factors are important in the pathogenesis of SARS. The choice of specific receptors and the unique genome of the SARS‐CoV are important elements in understanding the biology of the pathogen. For the host cells, the outcome of SARS‐CoV infection, whether there are cytopathic effects or not, depends on the cell types that are infected. At the whole‐body level, immune‐mediated damage, due to activation of cytokines and/or chemokines and, perhaps, autoimmunity, may play key roles in the clinical and pathological features of SARS. Continued research is still required to determine the pathogenetic mechanisms involved and to combat this new emerging human infectious disease. Copyright © 2006 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Significant redox insensitivity of the functions of the SARS-CoV spike glycoprotein: comparison with HIV envelope

The capacity of the surface glycoproteins of enveloped viruses to mediate virus/cell binding and membrane fusion requires a proper thiol/disulfide balance. Chemical manipulation of their redox state using reducing agents or free sulfhydryl reagents affects virus/cell interaction. Conversely, natural thiol/disulfide rearrangements often occur during the cell interaction to trigger fusogenicity, hence the virus entry. We examined the relationship between the redox state of the 20 cysteine residues of the SARS-CoV (severe acute respiratory syndrome coronavirus) Spike glycoprotein S1 subdomain and its functional properties. Mature S1 exhibited ∼4 unpaired cysteines, and chemically reduced S1 displaying up to ∼6 additional unpaired cysteines still bound ACE2 and enabled fusion. In addition, virus/cell membrane fusion occurred in the presence of sulfhydryl-blocking reagents and oxidoreductase inhibitors. Thus, in contrast to various viruses including HIV (human immunodeficiency virus) examined in parallel, the functions of the SARS-CoV Spike glycoprotein exhibit a significant and surprising independence of redox state, which may contribute to the wide host range of the virus. These data suggest clues for molecularly engineering vaccine immunogens.

The severe acute respiratory syndrome (SARS)

The world was shocked in early 2003 when a pandemic of severe acute respiratory syndrome (SARS) was imminent. The outbreak of this novel disease, caused by a novel coronavirus (the SARS-coronavirus), hit hardest in the Asian Pacific region, though eventually it spread to five continents. The speed of the spread of the SARS epidemic was unprecedented due to the highly efficient intercontinental transportation. An international collaborative effort through the World Health Organization (WHO) has helped to identify the aetiological agent about 1 month after the onset of the epidemic. The power of molecular biology and bioinformatics has enabled the complete decoding of the viral genome within weeks. Over 1000 publications on the phylogeny, epidemiology, genomics, laboratory diagnostics, antiviral, immunization, pathogenesis, clinical disease, and management accumulated within just 1 year. Although the exact animal reservoir of virus and how it evolved into a human pathogen are still obscure, accurate diagnosis and epidemiological control of the disease are now possible. This article reviews what is currently known about the virus and the disease.