Identification of a linear heparin binding domain for human respiratory syncytial virus attachment glycoprotein G

Identification of the glycosaminoglycan-binding site on the glycoprotein E(rns) of bovine viral diarrhoea virus by site-directed mutagenesis

Bovine viral diarrhoea virus (BVDV) envelope glycoprotein E(rns) interacts with highly sulphated heparin-like glycosaminoglycans (GAGs) located on the cell surface as an early step in virus infection of cells. Site-directed mutagenesis of recombinant E(rns) was undertaken and analysis of mutants by heparin-affinity chromatography and cell surface binding showed that a cluster of basic amino acids (480KKLENKSK487) near the C terminus of E(rns) was essential for binding. Mutants with amino acid substitutions of lysine residues 481 and 485 in E(rns) reduced the binding of E(rns) to immobilized heparin and cellular GAGs but retained ribonuclease activity. In contrast to normal E(rns), E(rns) that was unable to bind to cells also failed to inhibit BVDV infection of cells when the cells were pre-incubated with E(rns). It is proposed that the cluster of basic residues (480KKLENKSK487) localized at the C-terminal end of E(rns) constitutes a GAG-binding site.

Cleavage at the furin consensus sequence RAR/KR(109) and presence of the intervening peptide of the respiratory syncytial virus fusion protein are dispensable for virus replication in cell culture

Proteolytic processing of the respiratory syncytial virus F (fusion) protein results in the generation of the disulfide-linked subunits F1 and F2 and in the release of pep27, a glycopeptide originally located between the two furin cleavage sites FCS-1 (RKRR(136)) and FCS-2 (RAR/KR(109)). We made use of reverse genetics to study the importance of FCS-2 and of pep27 for BRSV replication in cell culture. Replacement of FCS-2 in the F protein of recombinant viruses by either of the sequences NANR(109), RANN(109) or SANN(109), respectively, abolished proteolytic processing at this position, whereas the cleavage of FCS-1 was not affected. All mutants replicated in calf kidney and Vero cells in the absence of exogenous trypsin, although somewhat higher titers of BRSV containing the NANR(109) or the RANN(109) motif were achieved in the presence of trypsin. The virus mutants showed a reduced cytopathic effect which was lowest in the case of the SANN(109) mutant. These findings demonstrate that cleavage at FCS-2 is dispensable for replication of respiratory syncytial virus in cell culture. A deletion mutant containing FCS-1 but lacking FCS-2 and most of pep27 replicated in cell culture as efficiently as the parental virus, indicating that this domain of the F protein is not essential for virus maturation and infectivity.

Respiratory syncytial virus--viral biology and the host response

Respiratory syncytial virus (RSV) is the most important cause of respiratory tract infection in infants. We have an incomplete understanding of the reasons why some infants are more severely affected by RSV than others. There is no effective antiviral treatment for the infection. Advances in our understanding of the biology of RSV, particularly in relation to the attachment protein G and the fusion protein F, have revealed potential targets for new antiviral therapies and vaccine development. In response to RSV infection an intense inflammatory response is triggered, mediated initially by the infected airway epithelial cells. Cell mediated responses are important in controlling the extent of infection and in viral clearance. Humoral responses are important in protection. There is early evidence that genetic variation of the host response can influence the outcome of RSV-induced bronchiolitis.

A model for the generation of multiple A to G transitions in the human respiratory syncytial virus genome: predicted RNA secondary structures as substrates for adenosine deaminases that act on RNA

Human respiratory syncytial virus (HRSV) escape mutants selected with antibodies specific for the attachment (G) protein contain diverse genetic alterations, including point mutations, premature stop codons, frame shift changes and A to G hypermutations. The latter changes have only been found in mutants selected with antibodies directed against the conserved central region of the G protein. This gene segment fulfils substrate requirements for adenosine deaminases that act on RNA (ADARs): i.e. it is an A+U rich region of 137 residues, and 98 or 106 of them--for A/Mon/3/88 or Long HRSV strains, respectively--are predicted to form intramolecular base pairs leading to a stable RNA secondary structure. In addition, when sequences of the G gene from natural isolates are compared in terms of pairwise substitutions, A to G+G to A changes are preferentially observed in regions where stable intramolecular dsRNA secondary structures are predicted to occur. In this study, a model is proposed in which, in addition to nucleotide misincorporations, reiterative A to G changes in HRSV are generated by ADAR activity operating in short segments (100-200 ribonucleotide residues) of the HRSV genome with high tendency for intramolecular base pairing.

The central conserved cystine noose of the attachment G protein of human respiratory syncytial virus is not required for efficient viral infection in vitro or in vivo

The G glycoprotein of human respiratory syncytial virus (RSV) was identified previously as the viral attachment protein. Although we and others recently showed that G is not essential for replication in vitro, it does affect the efficiency of replication in a cell type-dependent fashion and is required for efficient replication in vivo. The ectodomain of G is composed of two heavily glycosylated domains with mucin-like characteristics that are separated by a short central region that is relatively devoid of glycosylation sites. This central region contains a 13-amino acid segment that is conserved in the same form among RSV isolates and is overlapped by a second segment containing four cysteine residues whose spacings are conserved in the same form and which create a cystine noose. The conserved nature of the cystine noose and flanking 13-amino acid segment suggested that this region likely was important for attachment activity. To test this hypothesis, we constructed recombinant RSVs from which the region containing the cysteine residues was deleted together with part or all of the conserved 13-amino acid segment. Surprisingly, each deletion had little or no effect on the intracellular synthesis and processing of the G protein, the kinetics or efficiency of virus replication in vitro, or sensitivity to neutralization by soluble heparin in vitro. In addition, neither deletion had any discernible effect on the ability of RSV to infect the upper respiratory tract of mice and both resulted in a 3- to 10-fold reduction in the lower respiratory tract. Thus, although the G protein is necessary for efficient virus replication in vivo, this activity does not require the central conserved cystine noose region.

Respiratory syncytial virus fusion protein mediates inhibition of mitogen-induced T-cell proliferation by contact

Human respiratory syncytial virus (HRSV) and bovine respiratory syncytial virus (BRSV) are major pathogens in infants and calves, respectively. Experimental BRSV infection of calves and lambs is associated with lymphopenia and a reduction in responsiveness of peripheral blood lymphocytes (PBLs) to mitogens ex vivo. In this report, we show that in vitro mitogen-induced proliferation of PBLs is inhibited after contact with RSV-infected and UV-inactivated cells or with cells expressing RSV envelope proteins on the cell surface. The protein responsible was identified as the RSV fusion protein (F), as cells infected with a recombinant RSV expressing F as the single envelope protein or cells transfected with a plasmid encoding F were able to induce this effect. Thus, direct contact with RSV F is necessary and sufficient to inhibit proliferation of PBLs. Interestingly, F derived from HRSV was more efficient in inhibiting human PBL proliferation, while F from BRSV was more efficient in inhibiting bovine PBLs. Since various T-cell activation markers were upregulated after presenter cell contact, T lymphocytes are viable and may still be activated by mitogen. However, a significant fraction of PBLs were delayed or defective in G0/G1 to S-phase transit.

Contribution of the respiratory syncytial virus G glycoprotein and its secreted and membrane-bound forms to virus replication in vitro and in vivo

The surface glycoproteins of viruses can play important roles in viral attachment, entry, and morphogenesis. Here, we investigated the role of the attachment G glycoprotein of human respiratory syncytial virus (RSV) in viral infection. RSV G is produced both as a complete, transmembrane form and as an N-terminally truncated form that is secreted. Using reverse genetics, we created mutant recombinant RSVs (rRSV) that do not express G (DeltaG) or express either the secreted or the membrane-bound form of G only (sG and mG, respectively). In Vero cells, the DeltaG virus formed plaques and grew as efficiently as wild-type rRSV and mG. In contrast, DeltaG replicated less efficiently and did not form distinct plaques in HEp-2 cells. This defect was primarily at the level of the initiation of infection, with only a minor additional effect at the level of packaging. Replication of DeltaG in the respiratory tract of mice was very highly restricted, indicating that G is important in vivo. Although the G protein expressed by the sG virus was confirmed to be secreted, this virus grew at least as efficiently as wild-type in HEp-2 cells and was only moderately attenuated in vivo. Thus, the G protein was important for efficient replication in HEp-2 cells and in vivo, but this function could be supplied in large part by the secreted form and thus does not require the cytoplasmic and transmembrane domains. Amino acids 184-198 have been identified as the major heparin-binding domain of the G protein and were implicated in mediating binding to cells [S. A. Feldman et al., 1999, J. Virol. 73, 6610-6617]. Heparin-like glycosaminoglycans also appeared to be important for infection in vitro by direct clinical isolates of RSV. Deletion of amino acids 187-197 from rRSV did not reduce its sensitivity to neutralization in vitro by incubation with soluble heparin, did not reduce its efficiency of growth in vitro, and resulted in only a modest reduction in vivo. Thus, the putative heparin-binding domain is not the sole determinant of heparin sensitivity and is not a critical functional domain.

Antiviral activity and structural characteristics of the nonglycosylated central subdomain of human respiratory syncytial virus attachment (G) glycoprotein

Segments of the cystine noose-containing nonglycosylated central subdomain, residues 149-197, of the attachment (G) glycoprotein of human respiratory syncytial virus (HRSV) have been assessed for impact on the cytopathic effect (CPE) of respiratory syncytial virus (RSV). Nalpha-acetyl residues 149-197-amide (G149-197), G149-189, and G149-177 of the A2 strain of HRSV protected 50% of human epithelial HEp-2 cells from the CPE of the A2 strain at concentrations (IC(50)) between 5 and 80 microm. Cystine noose-containing peptides G171-197 and G173-197 did not inhibit the CPE even at concentrations above 150 microm. Systematic C- and N-terminal truncations from G149-189 and G149-177 and alanine substitutions within G154-177 demonstrated that residues 166-170 (EVFNF), within a sequence that is conserved in HRSV strains, were critical for inhibition. Concordantly, G154-177 of bovine RSV and of an antibody escape mutant of HRSV with residues 166-170 of QTLPY and EVSNP, respectively, were not inhibitory. Surprisingly, a variant of G154-177 with an E166A substitution had an IC(50) of 750 nm. NMR analysis demonstrated that G149-177 adopted a well-defined conformation in solution, clustered around F168 and F170. G154-170, particularly EVFNF, may be important in binding of RSV to host cells. These findings constitute a promising platform for the development of antiviral agents for RSV.

Proteolytic activation of respiratory syncytial virus fusion protein. Cleavage at two furin consensus sequences

The F (fusion) protein of the respiratory syncytial viruses is synthesized as an inactive precursor F(0) that is proteolytically processed at the multibasic sequence KKRKRR(136) into the subunits F(1) and F(2) by the cellular protease furin. This maturation process is essential for the F protein to gain fusion competence. We observed that proteolytic cleavage additionally occurs at another basic motif, RARR(109), that also meets the requirements for furin recognition. Cleavage at both sites leads to the removal from the polypeptide chain of a glycosylated peptide of 27 amino acids. When the sequence RARR(109) was changed to NANR(109) or to RANN(109) by site-directed mutagenesis, cleavage by furin was completely prevented. Although the mutants were still processed at position Arg(136), they did not show any syncytia formation. Proteolytic cleavage of the modified motifs was achieved by treatment of transfected cells with trypsin converting the F mutants into their fusogenic forms. Our findings indicate that both furin consensus sequences have to be cleaved in order to activate the fusion protein.

CX3C chemokine mimicry by respiratory syncytial virus G glycoprotein

Chemokines are chemoattractant proteins that are divided into subfamilies based upon cysteine signature motifs termed C, CC, CXC and CX3C. Chemokines have roles in immunity and inflammation that affect cell trafficking and activation of T cells as well as cells of the innate immune system. We report here CX3C chemokine mimicry for the G glycoprotein of respiratory syncytial virus (RSV) and show binding to CX3CR1--the specific receptor for the CX3C chemokine fractalkine--and induction of leukocyte chemotaxis. We also show that CX3CR1 facilitates RSV infection of cells. Thus, G glycoprotein interaction with CX3CR1 probably plays a key role in the biology of RSV infection.

Functional analysis of recombinant respiratory syncytial virus deletion mutants lacking the small hydrophobic and/or attachment glycoprotein gene

Respiratory syncytial virus (RSV) produces three envelope glycoproteins, the attachment glycoprotein (G), the fusion (F) protein, and the small hydrophobic (SH) protein. It had been assumed, by analogy with other paramyxoviruses, that the G and F proteins would be required for the first two steps of viral entry, attachment and fusion. However, following repeated passage in cell culture, a viable mutant RSV that lacked both the G and SH genes was isolated (R. A. Karron, D. A. Buonagurio, A. F. Georgiu, S. S. Whitehead, J. E. Adamus, M. L. Clements-Mann, D. O. Harris, V. B. Randolph, S. A. Udem, B. R. Murphy, and M. S. Sidhu, Proc. Natl. Acad. Sci. USA 94:13,961--13,966, 1997). To explore the roles of the G, F, and SH proteins in virion assembly, function, and cytopathology, we have modified the full-length RSV cDNA and used it to rescue infectious RSV lacking the G and/or SH genes. The three resulting viruses and the parental virus all contain the green fluorescent protein (GFP) gene that serves to identify infected cells. We have used purified, radiolabeled virions to examine virus production and function, in conjunction with GFP to quantify infected cells. We found that the G protein enhances virion binding to target cells but plays no role in penetration after attachment. The G protein also enhances cell-to-cell fusion, presumably via cell-to-cell binding, and enhances virion assembly or release. The presence or absence of the G protein in virions has no obvious effect on the content of F protein or host cell proteins in the virion. In growth curve experiments, the viruses lacking the G protein produced viral titers that were at least 10-fold lower than titers of viruses containing the G protein. This reduction is due in large part to the less efficient release of virions and the lower infectivity of the released virions. In the absence of the G protein, virus expressing both the F and SH proteins displayed somewhat smaller plaques, lower fusion activity, and slower viral entry than the virus expressing the F protein alone, suggesting that the SH protein has a negative effect on virus fusion in cell culture.

Highly sulfated forms of heparin sulfate are involved in japanese encephalitis virus infection

Japanese encephalitis virus (JEV) infects a broad range of cell types in vitro, though little is known about the initial events of JEV infection. In the present study, we found that highly sulfated glycosaminoglycans (GAGs) are involved in infection of both neurovirulent (RP-9) and attenuated (RP-2ms) JEV strains. Competition experiments using highly sulfated GAGs, heparin and dextran sulfate, demonstrated an inhibition of JEV's attachment and subsequent infection of BHK-21 cells. Treatment of target cells by a potent sulfation inhibitor, sodium chlorate, greatly reduced viral binding ability as well as infection, suggesting a critical role of GAGs' sulfation status on the cellular surface in JEV infection. This phenomenon was confirmed by the manifestation of a distinct binding efficiency of JEV to the wild-type CHO cell line and its mutants with defects in GAG biosynthesis. We also demonstrated the binding of JEV particles and virus envelope glycoprotein to immobilized heparin beads. Furthermore, the addition of heparin suppressed the cytopathic effects induced by JEV infection in cultured cells. Our results establish that the highly sulfated form of GAGs on cell surfaces plays a determining role in the early stage of in vitro JEV infection.

RFI-641 inhibits entry of respiratory syncytial virus via interactions with fusion protein

BACKGROUND

RFI-641, a small dendrimer-like compound, is a potent and selective inhibitor of respiratory syncytial virus (RSV), which is currently a clinical candidate for the treatment of upper and lower respiratory tract infections caused by RSV. RFI-641 inhibits RSV growth with an IC(50) value of 50 nM and prevents syncytia formation in tissue culture. RSV contains of three surface glycoproteins, a small hydrophobic (SH) protein of unknown function, and attachment (G) and fusion (F) proteins that enable binding and fusion of virus, respectively, with target cells. Because of their role in attachment and fusion, the G and F surface proteins are prominent targets for therapeutic intervention. RFI-641 was previously shown to bind purified preparations of RSV fusion protein. Based on this observation, in conjunction with the biological results, it was speculated that the fusion event might be the target of these inhibitors.

RESULTS

A fusion assay based upon the relief of self-quenching of octadecyl rhodamine R18 was used to determine effects of the inhibitors on binding and fusion of RSV. The results show that RFI-641 inhibits both RSV-cell binding and fusion events. The inhibition of RSV is mediated via binding to the fusion protein on the viral surface. A closely related analog, WAY-158830, which is much less active in the virus-infectivity assay does not inhibit binding and fusion of RSV with Vero cells.

CONCLUSIONS

RFI-641, an in vivo active RSV inhibitor, is shown to inhibit both binding and fusion of RSV with cells, events that are early committed steps in RSV entry and pathogenicity. The results described here demonstrate that a non-peptidic, small molecule can inhibit binding and fusion of enveloped virus specifically via interaction with the viral fusion protein.

Adaptation of tick-borne encephalitis virus to BHK-21 cells results in the formation of multiple heparan sulfate binding sites in the envelope protein and attenuation in vivo

Propagation of the flavivirus tick-borne encephalitis virus in BHK-21 cells selected for mutations within the large surface glycoprotein E that increased the net positive charge of the protein. In the course of 16 independent experiments, 12 different protein E mutation patterns were identified. These were located in all three of the structural domains and distributed over almost the entire upper and lateral surface of protein E. The mutations resulted in the formation of local patches of predominantly positive surface charge. Recombinant viruses carrying some of these mutations in a defined genetic backbone showed heparan sulfate (HS)-dependent phenotypes, resulting in an increased specific infectivity and binding affinity for BHK-21 cells, small plaque formation in porcine kidney cells, and significant attenuation of neuroinvasiveness in adult mice. Our results corroborate the notion that the selection of attenuated HS binding mutants is a common and frequent phenomenon during the propagation of viruses in cell culture and suggest a major role for HS dependence in flavivirus attenuation. Recognition of this principle may be of practical value for designing attenuated flavivirus strains in the future.

RhoA is activated during respiratory syncytial virus infection

Respiratory syncytial virus (RSV) is an important human pathogen that can cause severe and life-threatening respiratory infections in infants and immunocompromised adults. We have recently shown the RSV F glycoprotein, which mediates viral fusion and entry, interacts with the cellular protein RhoA in two-hybrid and in vitro binding assays. Whether this interaction occurs in living cells remains an open question. However, because RhoA signaling is associated with many cellular functions relevant to RSV pathogenesis such as actin cytoskeleton organization, expression of proinflammatory cytokines, and smooth muscle contraction, we asked whether RhoA activation occurred during RSV infection of HEp-2 cells. We found that the amount of isoprenylated and membrane-bound RhoA in RSV-infected cultures was increased. Further evidence of RhoA activation was demonstrated by downstream signaling activity mediated by RhoA. There was an increase in p130(cas) phosphorylation during RSV infection, which was prevented by Y-27632, a specific inhibitor of Rho kinase, or lovastatin, an HMG-CoA reductase inhibitor that reduces the synthesis of groups needed for isoprenylation. In addition, RSV infection of HEp-2 cells resulted in an increase in the formation of actin stress fibers. Pretreatment of HEp-2 cells with Clostridium botulinum C3 exotoxin, an enzyme that specifically ADP-ribosylates and inactivates RhoA, prevented RSV-induced stress fiber formation. These observations indicate that RhoA and subsequent downstream signaling events are activated during RSV infection, which has implications for RSV pathogenesis.

N-glycans of F protein differentially affect fusion activity of human respiratory syncytial virus

The human respiratory syncytial virus (Long strain) fusion protein contains six potential N-glycosylation sites: N27, N70, N116, N120, N126, and N500. Site-directed mutagenesis of these positions revealed that the mature fusion protein contains three N-linked oligosaccharides, attached to N27, N70, and N500. By introducing these mutations into the F gene in different combinations, four more mutants were generated. All mutants, including a triple mutant devoid of any N-linked oligosaccharide, were efficiently transported to the plasma membrane, as determined by flow cytometry and cell surface biotinylation. None of the glycosylation mutations interfered with proteolytic activation of the fusion protein. Despite similar levels of cell surface expression, the glycosylation mutants affected fusion activity in different ways. While the N27Q mutation did not have an effect on syncytium formation, loss of the N70-glycan caused a fusion activity increase of 40%. Elimination of both N-glycans (N27/70Q mutant) reduced the fusion activity by about 50%. A more pronounced reduction of the fusion activity of about 90% was observed with the mutants N500Q, N27/500Q, and N70/500Q. Almost no fusion activity was detected with the triple mutant N27/70/500Q. These data indicate that N-glycosylation of the F2 subunit at N27 and N70 is of minor importance for the fusion activity of the F protein. The single N-glycan of the F1 subunit attached to N500, however, is required for efficient syncytium formation.

Echoviruses bind heparan sulfate at the cell surface

Some echoviruses (EV) that bind decay-accelerating factor (DAF) also bind cells of human and murine origins in a DAF-independent manner. Pretreatment of cells with heparinase 1 or heparin blocks the binding of radiolabeled virus to the cell surface, and heparin prevents infection of rhabdomyosarcoma cells by certain EV, including several low-passage clinical isolates of EV 6 and some EV that do not bind DAF. These studies suggest that heparan sulfate may be of in vivo relevance as an attachment molecule for EV.

Recombinant bovine respiratory syncytial virus with deletions of the G or SH genes: G and F proteins bind heparin

Bovine respiratory syncytial virus (BRSV) encodes three transmembrane envelope glycoproteins, namely the small hydrophobic (SH) protein, the attachment glycoprotein (G) and the fusion glycoprotein (F). The BRSV reverse genetics system has been used to generate viable recombinant BRSV lacking either the G gene or the SH gene or both genes. The deletion mutants were fully competent for multicycle growth in cell culture, proving that, of the BRSV glycoprotein genes, the SH and G genes are non-essential. Virus morphogenesis was not impaired by either of the deletions. The deletion mutants were used to study the role of the F glycoprotein and the contributions of SH and G with respect to virus attachment. Attachment mediated by the F protein alone could be blocked by soluble heparin, but not by chondroitin sulphate. Heparin affinity chromatography revealed that both the BRSV G and F glycoproteins have heparin-binding activity, with the affinity of the F glycoprotein being significantly lower than that of G. Therefore, the roles of the BRSV glycoproteins in virus attachment and receptor binding have to be reconsidered.

Glycosaminoglycan sulfation requirements for respiratory syncytial virus infection

Glycosaminoglycans (GAGs) on the surface of cultured cells are important in the first step of efficient respiratory syncytial virus (RSV) infection. We evaluated the importance of sulfation, the major biosynthetic modification of GAGs, using an improved recombinant green fluorescent protein-expressing RSV (rgRSV) to assay infection. Pretreatment of HEp-2 cells with 50 mM sodium chlorate, a selective inhibitor of sulfation, for 48 h prior to inoculation reduced the efficiency of rgRSV infection to 40%. Infection of a CHO mutant cell line deficient in N-sulfation was three times less efficient than infection of the parental CHO cell line, indicating that N-sulfation is important. In contrast, infection of a cell line deficient in 2-O-sulfation was as efficient as infection of the parental cell line, indicating that 2-O-sulfation is not required for RSV infection. Incubating RSV with the purified soluble heparin, the prototype GAG, before inoculation had previously been shown to neutralize its infectivity. Here we tested chemically modified heparin chains that lack their N-, C6-O-, or C2-O-sulfate groups. Only heparin chains lacking the N-sulfate group lost the ability to neutralize infection, confirming that N-sulfation, but not C6-O- or C2-O-sulfation, is important for RSV infection. Analysis of heparin fragments identified the 10-saccharide chain as the minimum size that can neutralize RSV infectivity. Taken together, these results show that, while sulfate modification is important for the ability of GAGs to mediate RSV infection, only certain sulfate groups are required. This specificity indicates that the role of cell surface GAGs in RSV infection is not based on a simple charge interaction between the virus and sulfate groups but instead involves a specific GAG structural configuration that includes N-sulfate and a minimum of 10 saccharide subunits. These elements, in addition to iduronic acid demonstrated previously (L. K. Hallak, P. L. Collins, W. Knudson, and M. E. Peeples, Virology 271:264-275, 2000), partially define cell surface molecules important for RSV infection of cultured cells.

Binding of human respiratory syncytial virus to cells: implication of sulfated cell surface proteoglycans

Binding of human respiratory syncytial virus (HRSV) to cultured cells was measured by flow cytometry. Using this assay and influenza virus as a control virus with a well-characterized receptor, a systematic search of cell surface molecules that might be implicated in HRSV binding was carried out. Treatment of cells with different enzymes or with other reagents suggested that heparin-like glycosaminoglycans (GAGs) were involved in attachment of HRSV, but not influenza virus, to host cells. This was further confirmed by a lack of binding of HRSV to CHO-K1 mutant cell lines deficient in glycosylation or GAGs biosynthesis and by an inhibition of binding after preincubation of virus with heparin and other GAGs. The degree of sulfation, more than the polysaccharide backbone of GAGs, seems to be critical for virus binding.

Heparan sulfate proteoglycans initiate dengue virus infection of hepatocytes

Dengue viruses (DEN) cause a broad spectrum of clinical manifestations including potentially life-threatening conditions such as hemorrhagic shock syndrome and less frequently acute hepatitis with liver failure and encephalopathy. In addition, dengue viruses provide a potential model to understand the initiation of hepatocyte infection by the structurally closely related hepatitis C virus (HCV), because this virus at present cannot be grown in cell culture. Although the initial steps of viral infection are a critical determinant of tissue tropism and therefore pathogenesis, little is known about the molecular basis of binding and endocytic trafficking of DEN or of any other flavivirus. Our studies revealed that binding of radiolabeled DEN to the human hepatoma cell line HuH-7 was strictly pH dependent and substantially inhibitable by the glycosaminoglycan heparin. Ligand-blot analysis, performed as a viral overlay assay, showed two heparan sulfate (HS) containing cell-surface binding proteins resolving at 33 and 37 kd. Based on the sensitivity of unprotected virus and the viral binding site on the cell surface to trypsin, viral internalization was quantified as an increase in trypsin protected virus over time. Virus trafficking to the site of degradation was inhibited by pH dissociation of the clathrin coat and dependent on IP(3)-mediated homotypic endosomal fusion. These findings confirm the hypothesis that binding and internalization of DEN by hepatocytes are mediated primarily by HS containing proteoglycans and suggest that flaviviruses traffic the major clathrin-dependent endocytic pathway during infection.

Respiratory syncytial virus infection in adults

Respiratory syncytial virus (RSV) is now recognized as a significant problem in certain adult populations. These include the elderly, persons with cardiopulmonary diseases, and immunocompromised hosts. Epidemiological evidence indicates that the impact of RSV in older adults may be similar to that of nonpandemic influenza. In addition, RSV has been found to cause 2 to 5% of adult community-acquired pneumonias. Attack rates in nursing homes are approximately 5 to 10% per year, with significant rates of pneumonia (10 to 20%) and death (2 to 5%). Clinical features may be difficult to distinguish from those of influenza but include nasal congestion, cough, wheezing, and low-grade fever. Bone marrow transplant patients prior to marrow engraftment are at highest risk for pneumonia and death. Diagnosis of RSV infection in adults is difficult because viral culture and antigen detection are insensitive, presumably due to low viral titers in nasal secretions, but early bronchoscopy is valuable in immunosuppressed patients. Treatment of RSV in the elderly is largely supportive, whereas early therapy with ribavirin and intravenous gamma globulin is associated with improved survival in immunocompromised persons. An effective RSV vaccine has not yet been developed, and thus prevention of RSV infection is limited to standard infection control practices such as hand washing and the use of gowns and gloves.

Immune-mediated disease pathogenesis in respiratory syncytial virus infection

Respiratory syncytial virus (RSV) is an important cause of severe respiratory disease in persons at both extremes of age. Wheezing is a cardinal sign of infection and the illness is associated with an increased incidence of childhood asthma. Data from both humans and animal models have linked severe disease in infants and the syndrome of vaccine-enhanced illness with an aberrant composition of CD4+ T cells, suggestive of an exaggerated Th2 response. Studies in murine models have shown that prior vaccination, coexisting allergic inflammation, or direct modulation of the cytokine milieu can profoundly influence the immune response to RSV and thereby affect the expression of disease. In addition, there are intrinsic antigenic properties of the RSV G glycoprotein that promote Th2 responses and eosinophilia. This paper proposes an integrated working model of how host and virus factors interact to determine the characteristics of RSV-induced illness. This model suggests strategies for the development of new vaccine and immunotherapeutic interventions, and creates a framework for asking additional questions about the immunopathogenesis of RSV.

Respiratory syncytial virus infection in adults

Respiratory syncytial virus (RSV) is now recognized as a significant problem in certain adult populations. These include the elderly, persons with cardiopulmonary diseases, and immunocompromised hosts. Epidemiological evidence indicates that the impact of RSV in older adults may be similar to that of nonpandemic influenza. In addition, RSV has been found to cause 2 to 5% of adult community-acquired pneumonias. Attack rates in nursing homes are approximately 5 to 10% per year, with significant rates of pneumonia (10 to 20%) and death (2 to 5%). Clinical features may be difficult to distinguish from those of influenza but include nasal congestion, cough, wheezing, and low-grade fever. Bone marrow transplant patients prior to marrow engraftment are at highest risk for pneumonia and death. Diagnosis of RSV infection in adults is difficult because viral culture and antigen detection are insensitive, presumably due to low viral titers in nasal secretions, but early bronchoscopy is valuable in immunosuppressed patients. Treatment of RSV in the elderly is largely supportive, whereas early therapy with ribavirin and intravenous gamma globulin is associated with improved survival in immunocompromised persons. An effective RSV vaccine has not yet been developed, and thus prevention of RSV infection is limited to standard infection control practices such as hand washing and the use of gowns and gloves.

The fusion glycoprotein of human respiratory syncytial virus facilitates virus attachment and infectivity via an interaction with cellular heparan sulfate

Human respiratory syncytial virus (RSV) F glycoprotein (RSV-F) can independently interact with immobilized heparin and facilitate both attachment to and infection of cells via an interaction with cellular heparan sulfate. RSV-glycosaminoglycan (GAG) interactions were evaluated using heparin-agarose affinity chromatography. RSV-F from A2- and B1/cp-52 (cp-52)-infected cell lysates, RSV-F derived from a recombinant vaccinia virus, and affinity-purified F protein all bound to and were specifically eluted from heparin columns. In infectivity inhibition studies, soluble GAGs decreased the infectivity of RSV A2 and cp-52, with bovine lung heparin exhibiting the highest specific activity against both A2 (50% effective dose [ED(50)] = 0.28 +/- 0.11 microg/ml) and cp-52 (ED(50) = 0.55 +/- 0. 14 microg/ml). Furthermore, enzymatic digestion of cell surface GAGs by heparin lyase I and heparin lyase III but not chondroitinase ABC resulted in a significant reduction in cp-52 infectivity. Moreover, bovine lung heparin inhibited radiolabeled A2 and cp-52 virus binding up to 90%. Taken together, these data suggest that RSV-F independently interacts with heparin/heparan sulfate and this type of interaction facilitates virus attachment and infectivity.

Iduronic acid-containing glycosaminoglycans on target cells are required for efficient respiratory syncytial virus infection

Respiratory syncytial virus (RSV) is an important human respiratory pathogen, particularly in infants. Glycosaminoglycans (GAGs) have been implicated in the initiation of RSV infection of cultured cells, but it is not clear what type of GAGs and GAG components are involved, whether the important GAGs are on the virus or the cell, or what the magnitude is of their contribution to infection. We constructed and rescued a recombinant green fluorescent protein (GFP)-expressing RSV (rgRSV) and used this virus to develop a sensitive system to assess and quantify infection by flow cytometry. Evaluation of a panel of mutant Chinese hamster ovary cell lines that are genetically deficient in various aspects of GAG synthesis showed that infection was reduced up to 80% depending on the type of GAG deficiency. Enzymatic removal of heparan sulfate and/or chondroitin sulfate from the surface of HEp-2 cells also reduced infection, and the removal of both reduced infection even further. Blocking experiments in which RSV was preincubated with various soluble GAGs revealed the relative blocking order of: heparin > heparan sulfate > chondroitin sulfate B. Iduronic acid is a component common to these GAGs. GAGs that do not contain iduronic acid, namely, chondroitin sulfate A and C and hyaluronic acid, did not inhibit infection. A role for iduronic acid-containing GAGs in RSV infection was confirmed by the ability of basic fibroblast growth factor to block infection, because basic fibroblast growth factor binds to GAGs containing iduronic acid. Pretreatment of cells with protamine sulfate, which binds and blocks GAGs, also reduced infection. In these examples, infection was reduced by pretreatment of the virus with soluble GAGs, pretreatment of the cells with GAG-binding molecules, pretreatment of the cells with GAG-destroying enzymes or in cells genetically deficient in GAGs. These results establish that the GAGs involved in RSV infection are present on the cell rather than on the virus particle. Thus, the presence of cell surface GAGs containing iduronic acid, like heparan sulfate and chondroitin sulfate B, is required for efficient RSV infection in cell culture.

A RhoA-derived peptide inhibits syncytium formation induced by respiratory syncytial virus and parainfluenza virus type 3

The fusion glycoproteins of human respiratory syncytial virus (RSV) and human parainfluenza virus type-3 (PIV-3) mediate virus entry and syncytium formation. Interaction between the fusion protein of RSV and RhoA, a small GTPase, facilitates virus-induced syncytium formation. We show here a RhoA-derived peptide inhibits RSV and syncytium formation induced by RSV and PIV-3, both in vitro by inhibition of cell-to-cell fusion and in vivo by reduction of peak titer by 2 log10 in RSV-infected mice. These findings indicate that the interaction between these two paramyxovirus fusion proteins and RhoA is an important target for new antiviral strategies.