Multiple glycosylated forms of the respiratory syncytial virus fusion protein are expressed in virus-infected cells

Evidence that an interaction between the respiratory syncytial virus F and G proteins at the distal ends of virus filaments mediates efficient multiple cycle infection

Evidence for a stable interaction between the respiratory syncytial virus (RSV) F and G proteins on the surface of virus filaments was provided using antibody immunoprecipitation studies on purified RSV particles, and by the in situ analysis on the surface of RSV-infected cells using the proximity ligation assay. Imaging of the F and G protein distribution on virus filaments suggested that this protein complex was localised at the distal ends of the virus filaments, and suggested that this protein complex played a direct role in mediating efficient localised cell-to-cell virus transmission. G protein expression was required for efficient localised cell-to-cell transmission of RSV in cell monolayers which provided evidence that this protein complex mediates efficient multiple cycle infection. Collectively, these data provide evidence that F and G proteins form a complex on the surface of RSV particles, and that a role for this protein complex in promoting virus transmission is suggested.

Characterization of a Panel of Monoclonal Antibodies Targeting the F-Protein of the Respiratory Syncytial Virus (RSV) for the Typing of Contemporary Circulating Strains

Respiratory syncytial virus (RSV) is the most common cause of upper and lower respiratory tract infections in infants and young children. Virus-specific monoclonal antibodies (mAbs) can be used for diagnosis, prophylaxis, and research of RSV pathogenesis. A panel of 16 anti-RSV mAbs was obtained from mice immunized by RSV strain Long. Half of them had virus-neutralizing activity. According to Western blot all of these mAbs effectively bound native oligomeric (homodimeric and homotrimeric) forms of the RSV fusion (F) protein. Only five of the mAbs interacted with the monomeric form, and only one of these possessed neutralizing activity. None of these mAbs, nor the commercial humanized neutralizing mAb palivizumab, reacted with the denaturated F protein. Thus, interaction of all these mAbs with F protein had clear conformational dependence. Competitive ELISA and neutralization assays allowed the identification of nine antigenic target sites for the interaction of mAb with the F protein. Five partially overlapping sites may represent a complex spatial structure of one antigenic determinant, including one neutralizing and four non-neutralizing epitopes. Four sites (three neutralizing and one non-neutralizing) were found to be distinct. As a result of virus cultivation RSV-A, strain Long, in the presence of a large amount of one of the neutralizing mAbs, an escape mutant with a substitution, N240S, in the F protein, was obtained. Thus, it was shown for the first time that position 240 is critical for the protective effect of an anti-RSV antibody. To assess the ability of these mAbs to interact with modern RSV strains circulating in St. Petersburg (Russia) between 2014 and 2022, 73 RSV-A and 22 RSV-B isolates were analyzed. Six mAbs were directed to conserved epitopes of the F protein as they interacted most efficiently with both RSV subtypes in a fixed cell-ELISA and could be used for diagnostic assays detecting RSV.

Defining the Assembleome of the Respiratory Syncytial Virus

During respiratory syncytial virus (RSV) particle assembly, the mature RSV particles form as filamentous projections on the surface of RSV-infected cells. The RSV assembly process occurs at the / on the cell surface that is modified by a virus infection, involving a combination of several different host cell factors and cellular processes. This induces changes in the lipid composition and properties of these lipid microdomains, and the virus-induced activation of associated Rho GTPase signaling networks drives the remodeling of the underlying filamentous actin (F-actin) cytoskeleton network. The modified sites that form on the surface of the infected cells form the nexus point for RSV assembly, and in this review chapter, they are referred to as the RSV assembleome. This is to distinguish these unique membrane microdomains that are formed during virus infection from the corresponding membrane microdomains that are present at the cell surface prior to infection. In this article, an overview of the current understanding of the processes that drive the formation of the assembleome during RSV particle assembly is given.

Coexpression of respiratory syncytial virus (RSV) fusion (F) protein and attachment glycoprotein (G) in a vesicular stomatitis virus (VSV) vector system provides synergistic effects against RSV infection in a cotton rat model

Respiratory syncytial virus (RSV) is one of the most important causes of respiratory disease in infants, immunocompromised individuals, and the elderly. Natural infection does not result in long-term immunity, and there is no licensed vaccine. Vesicular stomatitis virus (VSV) is a commonly used vaccine vector platform against infectious diseases, and has been used as a vector for a licensed Ebola vaccine. In this study, we expressed the RSV fusion (F) protein, the RSV F protein stabilized in either a pre-fusion or a post-fusion configuration, the attachment glycoprotein (G), or the G and F proteins of RSV in combination in a VSV vector. Cotton rats were immunized with these recombinants intranasally or subcutaneously to test immunogenicity. RSV F stabilized in either a pre-fusion or a post-fusion configuration proved to be poorly immunogenic and protective when compared to unmodified F. RSV G provided partial protection and moderate levels of neutralizing antibody production, both of which improved with intranasal administration compared to subcutaneous inoculation. The most successful vaccine vector was VSV expressing both the G and F proteins after intranasal inoculation. Immunization with this recombinant induced neutralizing antibodies and provided protection from RSV challenge in the upper and lower respiratory tract for at least 80 days. Our results demonstrate that co-expression of F and G proteins in a VSV vector provides synergistic effects in inducing RSV-specific neutralizing antibodies and protection against RSV infection.

Respiratory Syncytial Virus: Infection, Detection, and New Options for Prevention and Treatment

Respiratory syncytial virus (RSV) infection is a significant cause of hospitalization of children in North America and one of the leading causes of death of infants less than 1 year of age worldwide, second only to malaria. Despite its global impact on human health, there are relatively few therapeutic options available to prevent or treat RSV infection. Paradoxically, there is a very large volume of information that is constantly being refined on RSV replication, the mechanisms of RSV-induced pathology, and community transmission. Compounding the burden of acute RSV infections is the exacerbation of preexisting chronic airway diseases and the chronic sequelae of RSV infection. A mechanistic link is even starting to emerge between asthma and those who suffer severe RSV infection early in childhood. In this article, we discuss developments in the understanding of RSV replication, pathogenesis, diagnostics, and therapeutics. We attempt to reconcile the large body of information on RSV and why after many clinical trials there is still no efficacious RSV vaccine and few therapeutics.

Mammalian Cell-Derived Respiratory Syncytial Virus-Like Particles Protect the Lower as well as the Upper Respiratory Tract

Globally, Respiratory Syncytial Virus (RSV) is a leading cause of bronchiolitis and pneumonia in children less than one year of age and in USA alone, between 85,000 and 144,000 infants are hospitalized every year. To date, there is no licensed vaccine. We have evaluated vaccine potential of mammalian cell-derived native RSV virus-like particles (RSV VLPs) composed of the two surface glycoproteins G and F, and the matrix protein M. Results of in vitro testing showed that the VLPs were functionally assembled and immunoreactive, and that the recombinantly expressed F protein was cleaved intracellularly similarly to the virus-synthesized F protein to produce the F1 and F2 subunits; the presence of the F1 fragment is critical for vaccine development since all the neutralizing epitopes present in the F protein are embedded in this fragment. Additional in vitro testing in human macrophage cell line THP-1 showed that both virus and the VLPs were sensed by TLR-4 and induced a Th1-biased cytokine response. Cotton rats vaccinated with RSV VLPs adjuvanted with alum and monophosphoryl lipid A induced potent neutralizing antibody response, and conferred protection in the lower as well as the upper respiratory tract based on substantial virus clearance from these sites. To the best of our knowledge, this is the first VLP/virosome vaccine study reporting protection of the lower as well as the upper respiratory tract: Prevention from replication in the nose is an important consideration if the target population is infants < 6 months of age. This is because continued virus replication in the nose results in nasal congestion and babies at this age are obligate nose breathers. In conclusion, these results taken together suggest that our VLPs show promise to be a safe and effective vaccine for RSV.

Acute Lung Injury Results from Innate Sensing of Viruses by an ER Stress Pathway

Incursions of new pathogenic viruses into humans from animal reservoirs are occurring with alarming frequency. The molecular underpinnings of immune recognition, host responses, and pathogenesis in this setting are poorly understood. We studied pandemic influenza viruses to determine the mechanism by which increasing glycosylation during evolution of surface proteins facilitates diminished pathogenicity in adapted viruses. ER stress during infection with poorly glycosylated pandemic strains activated the unfolded protein response, leading to inflammation, acute lung injury, and mortality. Seasonal strains or viruses engineered to mimic adapted viruses displaying excess glycans on the hemagglutinin did not cause ER stress, allowing preservation of the lungs and survival. We propose that ER stress resulting from recognition of non-adapted viruses is utilized to discriminate “non-self” at the level of protein processing and to activate immune responses, with unintended consequences on pathogenesis. Understanding this mechanism should improve strategies for treating acute lung injury from zoonotic viral infections.

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ER stress pathways can mediate immune recognition of zoonotic viruses

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Glycosylation status of viral proteins regulates activation of ER stress

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Acute lung injury from pandemic influenza viruses is dependent on this activation

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Adaptation through glycan addition mediates immune escape of seasonal IAV

Genetic variability among complete human respiratory syncytial virus subgroup A genomes: bridging molecular evolutionary dynamics and epidemiology

Human respiratory syncytial virus (RSV) is an important cause of severe lower respiratory tract infections in infants and the elderly. In the vast majority of cases, however, RSV infections run mild and symptoms resemble those of a common cold. The immunological, clinical, and epidemiological profile of severe RSV infections suggests a disease caused by a virus with typical seasonal transmission behavior, lacking clear-cut virulence factors, but instead causing disease by modifying the host’s immune response in a way that stimulates pathogenesis. Yet, the interplay between RSV-evoked immune responses and epidemic behavior, and how this affects the genomic evolutionary dynamics of the virus, remains poorly understood. Here, we present a comprehensive collection of 33 novel RSV subgroup A genomes from strains sampled over the last decade, and provide the first measurement of RSV-A genomic diversity through time in a phylodynamic framework. In addition, we map amino acid substitutions per protein to determine mutational hotspots in specific domains. Using Bayesian genealogical inference, we estimated the genomic evolutionary rate to be 6.47×10⁻⁴ (credible interval: 5.56×10⁻⁴, 7.38×10⁻⁴) substitutions/site/year, considerably slower than previous estimates based on G gene sequences only. The G gene is however marked by elevated substitution rates compared to other RSV genes, which can be attributed to relaxed selective constraints. In line with this, site-specific selection analyses identify the G gene as the major target of diversifying selection. Importantly, statistical analysis demonstrates that the immune driven positive selection does not leave a measurable imprint on the genome phylogeny, implying that RSV lineage replacement mainly follows nonselective epidemiological processes. The roughly 50 years of RSV-A genomic evolution are characterized by a constant population size through time and general co-circulation of lineages over many epidemic seasons – a conclusion that might be taken into account when developing future therapeutic and preventive strategies.

Immunogenic properties of RSV-B1 fusion (F) protein gene-encoding recombinant adenoviruses

Two recombinant adenoviruses designated rAd-F0DeltaTM and rAd-F0 carrying the transmembrane truncated and full length of the F gene of the RSV-B1 strain, respectively, were engineered. Comparative immunogenicity studies in BALB/c mice showed that each vector was capable of inducing RSV-B1-specific antibodies that cross-reacted with the RSV-long and RSV-A2 viruses. The anti-RSV-B1 antibodies were neutralizing, and exhibited strong cross-neutralizing activity against the RSV-long and RSV-A2 isolates as well. Analysis of the cellular responses revealed that animals immunized with rAd-F0DeltaTM and rAd-F0 elicited CD4(+) T-cell responses of the Th1 and Th2 phenotypes, as well as F protein-specific CTLs. Production of Th2 cytokines (IL-4, IL-5 and IL-13) by splenocytes of the rAd-F0DeltaTM and rAd-F0 immunized mice was markedly lower than those released by animals administered with heat-inactivated RSV-B1 (HIRSV-B1). Comparison of the overall humoral and cellular responses suggests that rAd-F0DeltaTM is significantly more immunogenic than rAd-F0. The anti-viral immunity generated by both recombinant adenovirus vectors has conferred protection against live RSV-B1 challenge as judged by the lower viral load recovered in the lungs, a faster rate of recovery of body weight loss, and a lower count of eosinophils as compared to eosinophilia in mice immunized with HIRSV-B1. Results from these studies suggest that rAd-F0DeltaTM or rAd-F0 possess immunogenic properties that meet the requirements expected from potential RSV vaccine candidates.

Entry of hepatitis C virus and human immunodeficiency virus is selectively inhibited by carbohydrate-binding agents but not by polyanions

We studied the antiviral activity of carbohydrate-binding agents (CBAs), including several plant lectins and the non-peptidic small-molecular-weight antibiotic pradimicin A (PRM-A). These agents efficiently prevented hepatitis C virus (HCV) and human immunodeficiency virus type 1 (HIV-1) infection of target cells by inhibiting the viral entry. CBAs were also shown to prevent HIV and HCV capture by DC-SIGN-expressing cells. Surprisingly, infection by other enveloped viruses such as herpes simplex viruses, respiratory syncytial virus and parainfluenza-3 virus was not inhibited by these agents pointing to a high degree of specificity. Mannan reversed the antiviral activity of CBAs, confirming their association with viral envelope-associated glycans. In contrast, polyanions such as dextran sulfate-5000 and sulfated polyvinylalcohol inhibited HIV entry but were devoid of any activity against HCV infection, indicating that they act through a different mechanism. CBAs could be considered as prime drug leads for the treatment of chronic viral infections such as HCV by preventing viral entry into target cells. They may represent an attractive new option for therapy of HCV/HIV coinfections. CBAs may also have the potential to prevent HCV/HIV transmission.

Functional analysis of the N-linked glycans within the fusion protein of respiratory syncytial virus

The respiratory syncytial virus fusion (F) protein is initially expressed as a single polypeptide chain (F0). The F0 subsequently undergoes posttranslational cleavage-by-cell protease activity to produce the F1 and F2 subunits. Each of the two subunits within the mature F protein is modified by the addition of N-linked glycans. The individual N-linked glycans on the F protein were selectively removed by using site-directed mutagenesis to mutate the individual glycan-acceptor sites. In this way the role of these individual glycans in targeting of the F protein to the cell surface, and on the ability of the F protein to induce membrane fusion, was examined.

Respiratory syncytial virus (RSV) evades the human adaptive immune system by skewing the Th1/Th2 cytokine balance toward increased levels of Th2 cytokines and IgE, markers of allergy--a review

Infection of infants in their first year of life, children and elderly people with the respiratory syncytial virus (RSV) endangers the life of the patient. An attempt to develop a formalin-inactivated RSV (FI-RSV) vaccine during the 1960s resulted in an aggravated infection in immunized children, leading to hospitalization, while infection of non-immunized children produced much milder symptoms. The reason for this remained an enigma, one which was gradually solved over the last decade by many researchers who studied the molecular biology of RSV infection of respiratory ciliary cells. Clinical studies of RSV-infected patients indicated increased levels of Th2 cytokines and IgE in the patients' sera, suggesting that an allergy-like condition developed during infection. The biomarkers of allergy caused by endogenous or environmental allergens include a marked increase of the Th2 cytokine IL-4 and IgE non-neutralizing antibodies to the allergen. The way allergens trigger allergy was deciphered recently, and will be discussed later. Studies of RSV infection led to the suggestion that RSV patients suffer from allergy prior to RSV infection, a concept that was later abandoned. Studies on HIV-1 [Y. Becker, Virus Genes 28, 319-331 (2005)] research led me to the hypothesis that since HIV-1 infection induces a marked increase of IL-4 and IgE in serum, an allergy-like condition, the AIDS stage is the result of an allergen motif that is embedded in the shed viral gp120 molecules. It is hypothesized that the viral-soluble G glycoprotein (sG) contains a T cell superantigen (Tsag) that is capable of binding to the V(H)3 domain of IgE/FcepsilonRI(+) hematopoietic cells, basophils, mast cells and monocytes, similar to the case of allergens, and that this aggregation causes these innate system cells to degranulate and release large amounts of Th2 cytokines (IL-4, IL-5, IL-10, IL-13) into the blood. The way these Th2 cytokines skew the Th1/Th2 balance toward Th2 > Th1 will be discussed. The aim of the present review is to base RSV pathogenicity on the numerous very good analyses of the virus genes and to suggest a therapeutic approach to treatment that is directed at preventing the inhibitory effects of Th2 cytokines on the adaptive immune system of the patients, instead of inhibiting RSV replication by antivirals. The review of the molecular research on the role of the viral fusion (F) and attachment (G) glycoproteins of RSV provided information on their role in the virus infection: early in infection the F glycoprotein induces Th1 cells to release the Th1 cytokines IL-2, IL-12 and IFN-gamma to activate precursors CTLs (pCTLs) to become anti-RSV CTLs. The G and sG glycoproteins attach to FKNR1(+) ciliary respiratory epithelial cells as well as directly to eosinophils to the lungs. The sG T cell antigen can also induce the release of large amounts of Th2 cytokines from CD4(+) T cells and from FCepsilonRI(+) mast cells, basophils and monocytes. By comparison to HIV-1 gp120 it is possible to show that in the G and sG proteins the T cell antigen resembles the CD4(+) T cell superantigen (=allergen) domain of HIV-1 gp120 which aggregates with IgE/FCepsilonRI(+) hematopoietic cells. The increased IL-4 level in the serum inhibits the adaptive immune response: IL-4Ralpha(+) Th1 cells stop Th1 cytokine synthesis and IL-4Ralpha(+) B cells stop the synthesis of antiviral IgG and IgA and switch to IgE synthesis. In addition, the hematopoietic cells release histamine and prostaglandin which induce wheezing. The gradual increase of sG molecules creates a gradient of fractalkine (FKN) which directs IL-5-activated eosinophils to the lungs of the patient.

Contribution of cysteine residues in the extracellular domain of the F protein of human respiratory syncytial virus to its function

The mature F protein of all known isolates of human respiratory syncytial virus (HRSV) contains fifteen absolutely conserved cysteine (C) residues that are highly conserved among the F proteins of other pneumoviruses as well as the paramyxoviruses. To explore the contribution of the cysteines in the extracellular domain to the fusion activity of HRSV F protein, each cysteine was changed to serine. Mutation of cysteines 37, 313, 322, 333, 343, 358, 367, 393, 416, and 439 abolished or greatly reduced cell surface expression suggesting these residues are critical for proper protein folding and transport to the cell surface. As expected, the fusion activity of these mutations was greatly reduced or abolished. Mutation of cysteine residues 212, 382, and 422 had little to no effect upon cell surface expression or fusion activity at 32 degrees C, 37 degrees C, or 39.5 degrees C. Mutation of C37 and C69 in the F2 subunit either abolished or reduced cell surface expression by 75% respectively. None of the mutations displayed a temperature sensitive phenotype.

The cytoplasmic domain of the F protein of Human respiratory syncytial virus is not required for cell fusion

The cytoplasmic domains of the fusion proteins encoded by several viruses play a role in cell fusion and contain sites for palmitoylation associated with viral protein trafficking and virus assembly. The fusion (F) protein ofHuman respiratory syncytial virus(HRSV) has a predicted cytoplasmic domain of 26 residues containing a single palmitoylated cysteine residue that is conserved in bovine RSV F protein, but not in the F proteins of other pneumoviruses such as pneumonia virus of mice, human metapneumovirus and avian pneumovirus. The cytoplasmic domains in other paramyxovirus fusion proteins such as Newcastle disease virus F protein play a role in fusion. In this study, it was shown that deletion of the entire cytoplasmic domain or mutation of the single cysteine residue (C550S) of the HRSV F protein had no effect on protein processing, cell-surface expression or fusion.

Use of a novel cell-based fusion reporter assay to explore the host range of human respiratory syncytial virus F protein

Human respiratory syncytial virus (HRSV) is an important respiratory pathogen primarily affecting infants, young children, transplant recipients and the elderly. The F protein is the only virion envelope protein necessary and sufficient for virus replication and fusion of the viral envelope membrane with the target host cell. During natural infection, HRSV replication is limited to respiratory epithelial cells with disseminated infection rarely, if ever, occurring even in immunocompromised patients. However, in vitro infection of multiple human and non-human cell types other than those of pulmonary tract origin has been reported. To better define host cell surface molecules that mediate viral entry and dissect the factors controlling permissivity for HRSV, we explored the host range of HRSV F protein mediated fusion. Using a novel recombinant reporter gene based fusion assay, HRSV F protein was shown to mediate fusion with cells derived from a wide range of vertebrate species including human, feline, equine, canine, bat, rodent, avian, porcine and even amphibian (Xenopus). That finding was extended using a recombinant HRSV engineered to express green fluorescent protein (GFP), to confirm that viral mRNA expression is limited in several cell types. These findings suggest that HRSV F protein interacts with either highly conserved host cell surface molecules or can use multiple mechanisms to enter cells, and that the primary determinants of HRSV host range are at steps post-entry.

Amino acid variation within the fusion protein of respiratory syncytial virus subtype A and B strains during annual epidemics in South Africa

Recent evidence of positive selection within the cytotoxic T-cell (CTL) epitopes of the highly conserved nucleoprotein of influenza virus raised the question of whether the CTL epitopes of Respiratory syncytial virus (RSV) are also affected by immune driven change over annual epidemics. The fusion protein (F-protein) of RSV is highly conserved within the two subtypes (A and B) and the most important target for the protective response. The position of various neutralizing epitopes has been mapped and characterized between RSV subtypes. CTL epitopes have also recently been mapped for the F-protein of subtype A, however variation within these epitopes between and within the subtypes has not been determined. To address this question, the F-proteins of 18 strains representative of all subgroup A and B genotypes identified in South Africa over a period of 5 years were sequenced. F-protein sequences were highly conserved within and between South African genotypes, with most variability occurring at the nucleotide level. Most of the amino acid differences identified within neutralizing and CTL epitopes were conserved within the subtypes, and therefore does not indicate immune selection. However, out of three CTL epitopes previously identified in subtype A, two (restricted to HLA B*57 and HLA A *01) were conserved only within subtype A, while the third (restricted to Cw*12) contained both subtype- and genotype-specific changes. These results suggest that most of the identified CTL epitopes are subtype A-specific and may not be recognized in subtype B viruses, while the HLA Cw*12 restricted epitope may also not be recognized efficiently in GA5 strains.

The small hydrophobic (SH) protein accumulates within lipid-raft structures of the Golgi complex during respiratory syncytial virus infection

The cellular distribution of the small hydrophobic (SH) protein in respiratory syncytial virus (RSV)-infected cells was examined. Although the SH protein was distributed throughout the cytoplasm, it appeared to accumulate in the Golgi complex within membrane structures that were enriched in the raft lipid, GM1. The ability of the SH protein to interact with lipid-raft membranes was further confirmed by examining its detergent-solubility properties in Triton X-100 at 4 degrees C. This analysis showed that a large proportion of the SH protein exhibited detergent-solubility characteristics that were consistent with an association with lipid-raft membranes. Analysis of virus-infected cells by immuno-transmission electron microscopy revealed SH protein clusters on the cell surface, but only very low levels of the protein appeared to be associated with mature virus filaments and inclusion bodies. These data suggest that during virus infection, the compartments in the secretory pathway, such as the endoplasmic reticulum (ER) and Golgi complex, are major sites of accumulation of the SH protein. Furthermore, although a significant amount of this protein interacts with lipid-raft membranes within the Golgi complex, its presence within mature virus filaments is minimal.