Multiple heparin binding domains of respiratory syncytial virus G mediate binding to mammalian cells

Advancements in Vaccine Adjuvants: The Journey from Alum to Nano Formulations

Vaccination is a groundbreaking approach in preventing and controlling infectious diseases. However, the effectiveness of vaccines can be greatly enhanced by the inclusion of adjuvants, which are substances that potentiate and modulate the immune response. This review is based on extensive searches in reputable databases such as Web of Science, PubMed, EMBASE, Scopus, and Google Scholar. The goal of this review is to provide a thorough analysis of the advances in the field of adjuvant research, to trace the evolution, and to understand the effects of the various adjuvants. Historically, alum was the pioneer in the field of adjuvants because it was the first to be approved for use in humans. It served as the foundation for subsequent research and innovation in the field. As science progressed, research shifted to identifying and exploiting the potential of newer adjuvants. One important area of interest is nano formulations. These advanced adjuvants have special properties that can be tailored to enhance the immune response to vaccines. The transition from traditional alum-based adjuvants to nano formulations is indicative of the dynamism and potential of vaccine research. Innovations in adjuvant research, particularly the development of nano formulations, are a promising step toward improving vaccine efficacy and safety. These advances have the potential to redefine the boundaries of vaccination and potentially expand the range of diseases that can be addressed with this approach. There is an optimistic view of the future in which improved vaccine formulations will contribute significantly to improving global health outcomes.

Immunopathology of RSV: An Updated Review

RSV is a leading cause of respiratory tract disease in infants and the elderly. RSV has limited therapeutic interventions and no FDA-approved vaccine. Gaps in our understanding of virus-host interactions and immunity contribute to the lack of biological countermeasures. This review updates the current understanding of RSV immunity and immunopathology with a focus on interferon responses, animal modeling, and correlates of protection.

Functional Features of the Respiratory Syncytial Virus G Protein

Respiratory syncytial virus (RSV) is a major cause of serious lower respiratory tract infections in children <5 years of age worldwide and repeated infections throughout life leading to serious disease in the elderly and persons with compromised immune, cardiac, and pulmonary systems. The disease burden has made it a high priority for vaccine and antiviral drug development but without success except for immune prophylaxis for certain young infants. Two RSV proteins are associated with protection, F and G, and F is most often pursued for vaccine and antiviral drug development. Several features of the G protein suggest it could also be an important to vaccine or antiviral drug target design. We review features of G that effect biology of infection, the host immune response, and disease associated with infection. Though it is not clear how to fit these together into an integrated picture, it is clear that G mediates cell surface binding and facilitates cellular infection, modulates host responses that affect both immunity and disease, and its CX3C aa motif contributes to many of these effects. These features of G and the ability to block the effects with antibody, suggest G has substantial potential in vaccine and antiviral drug design.

The larger attachment glycoprotein of respiratory syncytial virus produced in primary human bronchial epithelial cultures reduces infectivity for cell lines

Respiratory syncytial virus (RSV) infects the upper and lower respiratory tracts and can cause lower respiratory tract infections in children and elders. RSV has traditionally been isolated, grown, studied and quantified in immortalized cell lines, most frequently HEp-2 cells. However, in vivo RSV infection is modeled more accurately in primary well differentiated human bronchial epithelial (HBE) cultures where RSV targets the ciliated cells and where the putative RSV receptor differs from the receptor on HEp-2 cells. The RSV attachment (G) glycoprotein in virions produced by HEp-2 cells is a highly glycosylated 95 kDa protein with a 32 kDa peptide core. However, virions produced in HBE cultures, RSV (HBE), contain an even larger, 170 kDa, G protein (LgG). Here we show that LgG is found in virions from both subgroups A and B lab-adapted and clinical isolates. Unexpectedly, RSV (HBE) virions were approximately 100-fold more infectious for HBE cultures than for HEp-2 cells. Surprisingly, the cause of this differential infectivity, was reduced infectivity of RSV (HBE) on HEp-2 cells rather than enhanced infectivity on HBE cultures. The lower infectivity of RSV(HBE) for HEp-2 cells is caused by the reduced ability of LgG to interact with heparan sulfate proteoglycans (HSPG), the RSV receptor on HEp-2 cells. The discovery of different infectivity corresponding with the larger form of the RSV attachment protein when produced by HBE cultures highlights the importance of studying a virus produced by its native host cell and the potential impact on quantifying virus infectivity on cell lines where the virus entry mechanisms differ from their natural target cell.

Respiratory Syncytial Virus G Protein Sequence Variability among Isolates from St. Petersburg, Russia, during the 2013-2014 Epidemic Season

Human respiratory syncytial virus (RSV) is the most common cause of upper and lower respiratory tract infections in infants and young children. It is actively evolving under environmental and herd immunity influences. This work presents, for the first time, sequence variability analysis of RSV G gene and G protein using St. Petersburg (Russia) isolates. Viruses were isolated in a cell culture from the clinical samples of 61 children hospitalized (January-April 2014) with laboratory-confirmed RSV infection. Real-time RT-PCR data showed that 56 isolates (91.8%) belonged to RSV-A and 5 isolates (8.2%) belonged to RSV-B. The G genes were sequenced for 27 RSV-A isolates and all of them belonged to genotype ON1/GA2. Of these RSV-A, 77.8% belonged to the ON1(1.1) genetic sub-cluster, and 14.8% belonged to the ON1(1.2) sub-cluster. The ON1(1.3) sub-cluster constituted a minor group (3.7%). Many single-amino acid substitutions were identified in the G proteins of St. Petersburg isolates, compared with the Canadian ON1/GA2 reference virus (ON67-1210A). Most of the amino acid replacements were found in immunodominant B- and T-cell antigenic determinants of G protein. These may affect the antigenic characteristics of RSV and influence the host antiviral immune response to currently circulating viruses.

Structures of respiratory syncytial virus G antigen bound to broadly neutralizing antibodies

Respiratory syncytial virus (RSV) is a top cause of severe lower respiratory tract disease and mortality in young children and the elderly. The viral envelope G glycoprotein contributes to pathogenesis through its roles in host cell attachment and modulation of host immunity. Although the G glycoprotein is a target of protective RSV-neutralizing antibodies, its development as a vaccine antigen has been hindered by its heterogeneous glycosylation and sequence variability outside a conserved central domain (CCD). We describe the cocrystal structures of two high-affinity broadly neutralizing human monoclonal antibodies bound to the RSV G CCD. The antibodies bind to neighboring conformational epitopes, which we named antigenic sites γ1 and γ2, that span a highly conserved surface, illuminating an important region of vulnerability. We further show that isolated RSV G CCD activates the chemokine receptor CX3CR1 and that antibodies block this activity. These studies provide a template for rational vaccine design targeting this key contributor to RSV disease.

Preventing Cleavage of the Respiratory Syncytial Virus Attachment Protein in Vero Cells Rescues the Infectivity of Progeny Virus for Primary Human Airway Cultures

All live attenuated respiratory syncytial virus (RSV) vaccines that have advanced to clinical trials have been produced in Vero cells. The attachment (G) glycoprotein in virions produced in these cells is smaller than that produced in other immortalized cells due to cleavage. These virions are 5-fold less infectious for primary well-differentiated human airway epithelial (HAE) cell cultures. Because HAE cells are isolated directly from human airways, Vero cell-grown vaccine virus would very likely be similarly inefficient at initiating infection of the nasal epithelium following vaccination, and therefore, a larger inoculum would be required for effective vaccination. We hypothesized that Vero cell-derived virus containing an intact G protein would be more infectious for HAE cell cultures. Using protease inhibitors with increasing specificity, we identified cathepsin L to be the protease responsible for cleavage. Our evidence suggests that cleavage occurs in the late endosome or lysosome during endocytic recycling. Cathepsin L activity was 100-fold greater in Vero cells than in HeLa cells. In addition, cathepsin L was able to cleave the G protein in Vero cell-grown virions but not in HeLa cell-grown virions, suggesting a difference in G-protein posttranslational modification in the two cell lines. We identified by mutagenesis amino acids important for cleavage, and these amino acids included a likely cathepsin L cleavage site. Virus containing a modified, noncleavable G protein produced in Vero cells was 5-fold more infectious for HAE cells in culture, confirming our hypothesis and indicating the value of including such a mutation in future live attenuated RSV vaccines.

IMPORTANCE

Worldwide, RSV is the second leading infectious cause of infant death, but no vaccine is available. Experimental live attenuated RSV vaccines are grown in Vero cells, but during production the virion attachment (G) glycoprotein is cleaved. Virions containing a cleaved G protein are less infectious for primary airway epithelial cells, the natural RSV target. In the study described here we identified the protease responsible, located the cleavage site, and demonstrated that cleavage likely occurs during endocytic recycling. Moreover, we showed that the infectivity of Vero cell-derived virus for primary airway epithelial cells is increased 5-fold if the virus contains a mutation in the G protein that prevents cleavage. The blocking of cleavage should improve RSV vaccine yield, consequently reducing production costs. Posttranslational cleavage of the fusion glycoprotein of many viruses plays an essential role in activation; however, cleavage of the RSV G protein is a novel example of a detrimental effect of cleavage on virus infectivity.

Cholesterol-rich microdomains as docking platforms for respiratory syncytial virus in normal human bronchial epithelial cells

Respiratory syncytial virus (RSV) is one of the major causes of respiratory infections in children, and it is the main pathogen causing bronchiolitis in infants. The binding and entry mechanism by which RSV infects respiratory epithelial cells has not yet been determined. In this study, the earliest stages of RSV infection in normal human bronchial epithelial cells were probed by tracking virions with fluorescent lipophilic dyes in their membranes. Virions colocalized with cholesterol-containing plasma membrane microdomains, identified by their ability to bind cholera toxin subunit B. Consistent with an important role for cholesterol in RSV infection, cholesterol depletion profoundly inhibited RSV infection, while cholesterol repletion reversed this inhibition. Merger of the outer leaflets of the viral envelope and the cell membrane appeared to be triggered at these sites. Using small-molecule inhibitors, RSV infection was found to be sensitive to Pak1 inhibition, suggesting the requirement of a subsequent step of cytoskeletal reorganization that could involve plasma membrane rearrangements or endocytosis. It appears that RSV entry depends on its ability to dock to cholesterol-rich microdomains (lipid rafts) in the plasma membrane where hemifusion events begin, assisted by a Pak1-dependent process.

Role of cellular glycosaminoglycans and charged regions of viral G protein in human metapneumovirus infection

Human metapneumovirus (hMPV) is an important cause of lower respiratory tract disease, particularly in infants and young children. hMPV has two major glycoproteins, G and F, which are responsible for virus attachment and membrane fusion, respectively. We investigated the role of cellular glycosaminoglycans (GAGs) and G protein in hMPV infection. The pretreatment of hMPV with soluble heparin markedly inhibited the infection of HEp-2 cells. Recombinant G protein, comprising the extracellular domain of G, bound to heparin-agarose columns and also to HEp-2 cells. hMPV infection and G protein binding to HEp-2 cells was inhibited by other soluble GAGs, including chondroitin sulfates, by the enzymatic removal of cell surface GAGs with GAG lyases or by the pretreatment of cells with basic fibroblast growth factor. The role of cellular GAGs was confirmed by the binding of G protein to wild-type CHO cells but not to GAG-deficient CHO-pgsA745 cells. An analysis of the G protein sequence revealed two adjacent clusters of positively charged amino acids ((149)EKKKTRA(155) and (159)QRRGKGKE(166)). Truncated G fragments were expressed, and only the fragment containing these putative heparin binding domains retained heparin binding. The alanine mutagenesis of charged residues in either of these regions resulted in the loss of binding to heparin and to HEp-2 cells, suggesting that both sites are likely to be required for hMPV attachment. These results, taken together with the inhibition of hMPV infection by soluble G protein, indicate an important role for G protein and cellular GAGs in hMPV infection.

Herpes simplex virus type 2 glycoprotein G is targeted by the sulfated oligo- and polysaccharide inhibitors of virus attachment to cells

Variants of herpes simplex virus type 2 (HSV-2) generated by virus passage in GMK-AH1 cells in the presence of the sulfated oligosaccharide PI-88 were analyzed. Many of these variants were substantially resistant to PI-88 in their initial infection of cells and/or their cell-to-cell spread. The major alteration detected in all variants resistant to PI-88 in the initial infection of cells was a frameshift mutation(s) in the glycoprotein G (gG) gene that resulted in the lack of protein expression. Molecular transfer of the altered gG gene into the wild-type background confirmed that the gG-deficient recombinants were resistant to PI-88. In addition to PI-88, all gG-deficient variants of HSV-2 were resistant to the sulfated polysaccharide heparin. The gG-deficient virions were capable of attaching to cells, and this activity was relatively resistant to PI-88. In addition to having a drug-resistant phenotype, the gG-deficient variants were inefficiently released from infected cells. Purified gG bound to heparin and showed the cell-binding activity which was inhibited by PI-88. Many PI-88 variants produced syncytia in cultured cells and contained alterations in gB, including the syncytium-inducing L792P amino acid substitution. Although this phenotype can enhance the lateral spread of HSV in cells, it conferred no virus resistance to PI-88. Some PI-88 variants also contained occasional alterations in gC, gD, gE, gK, and UL24. In conclusion, we found that glycoprotein gG, a mucin-like component of the HSV-2 envelope, was targeted by sulfated oligo- and polysaccharides. This is a novel finding that suggests the involvement of HSV-2 gG in interactions with sulfated polysaccharides, including cell surface glycosaminoglycans.

Type 1 and 2 immunity following vaccination is influenced by nanoparticle size: formulation of a model vaccine for respiratory syncytial virus

Previous studies compared uptake by dendritic cells (DC) of 20, 40, 100, 200, 500, 1000, and 2000 nm beads in vivo. When beads were used as antigen carriers, bead size influenced antibody responses and induction of IFN-gamma-producing CD4 and CD8 T cells. Beads of 40-50 nm were taken up preferentially by DC and induced particularly strong immunity. Herein, we examine immunity induced by minute differences in nanobead size, specifically within a narrow viral-sized range (20, 40, 49, 67, 93, 101, and 123 nm), to see if bead carrier size influenced the induction of type 1 or type 2 cells as demonstrated by the production of IFN-gamma or IL-4. In vivo uptake by DC was assessed for selected sizes in this range. Responses to whole ovalbumin (OVA) or the OVA-derived CD8 T cell peptide epitope (SIINFEKL) were tested. After one immunization with beads-OVA, IFN-gamma responses to both OVA and SIINFEKL were significantly better with 40 and 49 nm beads than other sizes, while, in contrast, IL-4 responses to OVA were higher after immunization with OVA conjugated to larger beads (93, 101, and 123 nm). Thus IFN-gamma induction from CD8 T cells was limited to 40-49 nm beads, while CD4 T cell activation and IL-4 were induced by 93-123 nm beads-OVA. After two immunizations, there were comparable high levels of IFN-gamma produced with 40 and 49 beads and IL-4 reactivity was still higher for larger beads (93, 101, 123 nm). Production of IgG1 was seen across the full range of bead sizes, increasing after two immunizations. Since protection against respiratory syncytial virus (RSV) depends on strong IFN responses, while IL-4 responses are reported to cause asthma-like symptoms, immunization with RSV antigens on the 49 nm carrier beads could provide the basis for a suitable vaccine. When the 49 nm beads were conjugated to RSV proteins G88 (surface) or M2.1 (internal capsid), one immunization with G88 induced high levels of IFN-gamma and low levels of IL-4. IL-4 increased with two immunizations. Beads-M2.1 induced only moderate levels of IFN-gamma and low titer antibody after two immunizations. Mice vaccinated once with G88-conjugated 49 nm beads and challenged intranasally with RSV strain A2 subtype showed reduced viral titers and recovered from weight loss more rapidly than mice immunized with M2.1-conjugated 49 nm beads or naive control mice. These results show that precise selection of nanobead size for vaccination can influence the type 1/type 2 cytokine balance after one immunization, and this will be useful in the development of effective vaccines against common human pathogens such as RSV.

Interaction between respiratory syncytial virus and glycosaminoglycans, including heparan sulfate

Glycosaminoglycans (GAGs), including heparan sulfate (HS), are expressed on the surface of nearly all cells, linked to transmembrane proteins. These GAGs are sulfated to varying extents, lending a negative charge, and are used by a large number of viruses to initiate infection of immortalized cell lines. Here we describe the rationale and methods for analyzing GAG usage by one such virus, respiratory syncytial virus (RSV). The protocols presented allow the determination of which GAG(s) is employed by the virus, which GAG modification(s) is important, and whether the important GAG is on the cell or on the virus. We also discuss the finding that many viruses are selected for GAG usage during passage in culture and present a method for rapidly determining whether GAG usage is characteristic of a wild virus or is limited to laboratory-adapted virus.

Interaction between the respiratory syncytial virus G glycoprotein cytoplasmic domain and the matrix protein

Paramyxovirus assembly at the cell membrane requires the movement of viral components to budding sites and envelopment of nucleocapsids by cellular membranes containing viral glycoproteins, facilitated by interactions with the matrix protein. The specific protein interactions during assembly of respiratory syncytial virus (RSV) are unknown. Here, the postulated interaction between the RSV matrix protein (M) and G glycoprotein (G) was investigated. Partial co-localization of M with G was demonstrated, but not with a truncated variant lacking the cytoplasmic domain and one-third of the transmembrane domain, in cells infected with recombinant RSV or transfected to express G and M. A series of G mutants was constructed with progressively truncated or modified cytoplasmic domains. Data from co-expression in cells and a cell-free binding assay showed that the N-terminal aa 2-6 of G play a key role in G-M interaction, with serine at position 2 and aspartate at position 6 playing key roles.

The soluble form of human respiratory syncytial virus attachment protein differs from the membrane-bound form in its oligomeric state but is still capable of binding to cell surface proteoglycans

The soluble (Gs) and membrane-bound (Gm) forms of human respiratory syncytial virus (HRSV) attachment protein were purified by immunoaffinity chromatography from cultures of HEp-2 cells infected with vaccinia virus recombinants expressing either protein. Sucrose gradient centrifugation indicated that Gs, which is secreted into the culture medium, remains monomeric, whereas Gm is an oligomer, probably a homotetramer. Nevertheless, Gs was capable of binding to the surface of cells in vitro, as assessed by a flow cytometry-based binding assay. The attachment of Gs to cells was inhibited by previous heparinase treatment of living cells, and Gs did not bind to CHO cell mutants defective in proteoglycan biosynthesis. Thus, Gs, as previously reported for the G protein of intact virions, binds to glycosaminoglycans presented at the cell surface as proteoglycans. Deletion of a previously reported heparin binding domain from Gs protein substantially inhibited its ability to bind to cells, but the remaining level of binding was still sensitive to heparinase treatment, suggesting that other regions of the Gs molecule may contribute to attachment to proteoglycans. The significance of these results for HRSV infection is discussed.