On the structure of the influenza virus envelope

Characterization of neurotropic HPAI H5N1 viruses with novel genome constellations and mammalian adaptive mutations in free-living mesocarnivores in Canada

The GsGd lineage (A/goose/Guangdong/1/1996) H5N1 virus was introduced to Canada in 2021/2022 through the Atlantic and East Asia-Australasia/Pacific flyways by migratory birds. This was followed by unprecedented outbreaks affecting domestic and wild birds, with spillover into other animals. Here, we report sporadic cases of H5N1 in 40 free-living mesocarnivore species such as red foxes, striped skunks, and mink in Canada. The clinical presentations of the disease in mesocarnivores were consistent with central nervous system infection. This was supported by the presence of microscopic lesions and the presence of abundant IAV antigen by immunohistochemistry. Some red foxes that survived clinical infection developed anti-H5N1 antibodies. Phylogenetically, the H5N1 viruses from the mesocarnivore species belonged to clade 2.3.4.4b and had four different genome constellation patterns. The first group of viruses had wholly Eurasian (EA) genome segments. The other three groups were reassortant viruses containing genome segments derived from both North American (NAm) and EA influenza A viruses. Almost 17 percent of the H5N1 viruses had mammalian adaptive mutations (E627 K, E627V and D701N) in the polymerase basic protein 2 (PB2) subunit of the RNA polymerase complex. Other mutations that may favour adaptation to mammalian hosts were also present in other internal gene segments. The detection of these critical mutations in a large number of mammals within short duration after virus introduction inevitably highlights the need for continually monitoring and assessing mammalian-origin H5N1 clade 2.3.4.4b viruses for adaptive mutations, which potentially can facilitate virus replication, horizontal transmission and posing pandemic risks for humans.

Influenza A Virus Infection Alters Lipid Packing and Surface Electrostatic Potential of the Host Plasma Membrane

The pathogenesis of influenza A viruses (IAVs) is influenced by several factors, including IAV strain origin and reassortment, tissue tropism and host type. While such factors were mostly investigated in the context of virus entry, fusion and replication, little is known about the viral-induced changes to the host lipid membranes which might be relevant in the context of virion assembly. In this work, we applied several biophysical fluorescence microscope techniques (i.e., Förster energy resonance transfer, generalized polarization imaging and scanning fluorescence correlation spectroscopy) to quantify the effect of infection by two IAV strains of different origin on the plasma membrane (PM) of avian and human cell lines. We found that IAV infection affects the membrane charge of the inner leaflet of the PM. Moreover, we showed that IAV infection impacts lipid-lipid interactions by decreasing membrane fluidity and increasing lipid packing. Because of such alterations, diffusive dynamics of membrane-associated proteins are hindered. Taken together, our results indicate that the infection of avian and human cell lines with IAV strains of different origins had similar effects on the biophysical properties of the PM.

SARS-CoV-2 virus-like-particles via liposomal reconstitution of spike glycoproteins

The SARS-CoV-2 virus, implicated in the COVID-19 pandemic, recognizes and binds host cells using its spike glycoprotein through an angiotensin converting enzyme 2 (ACE-2) receptor-mediated pathway. Recent research suggests that spatial distributions of the spike protein may influence viral interactions with target cells and immune systems. The goal of this study has been to develop a liposome-based virus-like particle (VLP) by reconstituting the SARS-CoV-2 spike glycoprotein within a synthetic nanoparticle membrane, aiming to eventually establish tunability in spike protein presentation on the nanoparticle surface. Here we report on first steps to this goal, wherein liposomal SARS-CoV-2 VLPs were successfully produced via detergent mediated spike protein reconstitution. The resultant VLPs are shown to successfully co-localize in vitro with the ACE-2 receptor on lung epithelial cell surfaces, followed by internalization into these cells. These VLPs are the first step toward the overall goal of this research which is to form an understanding of the relationship between spike protein surface density and cell-level immune response, eventually toward creating better vaccines and anti-viral therapeutics.

Hemagglutinin clusters in the plasma membrane are not enriched with cholesterol and sphingolipids

The clusters of the influenza envelope protein, hemagglutinin, within the plasma membrane are hypothesized to be enriched with cholesterol and sphingolipids. Here, we directly tested this hypothesis by using high-resolution secondary ion mass spectrometry to image the distributions of antibody-labeled hemagglutinin and isotope-labeled cholesterol and sphingolipids in the plasma membranes of fibroblast cells that stably express hemagglutinin. We found that the hemagglutinin clusters were neither enriched with cholesterol nor colocalized with sphingolipid domains. Thus, hemagglutinin clustering and localization in the plasma membrane is not controlled by cohesive interactions between hemagglutinin and liquid-ordered domains enriched with cholesterol and sphingolipids, or from specific binding interactions between hemagglutinin, cholesterol, and/or the majority of sphingolipid species in the plasma membrane.

Lipid composition of viral envelope of three strains of influenza virus - not all viruses are created equal

While differences in the rate of virus fusion and budding from the host cell membrane have been correlated with pathogenicity, no systematic study of the contribution of differences in viral envelope composition has previously been attempted. Using rigorous virus purification, marked differences between virions and host were observed. Over 125 phospholipid species have been quantitated for three strains of influenza (HKx31- H3N2, PR8- H1N1, and VN1203- H5N1) grown in eggs. The glycerophospholipid composition of purified virions differs from that of the host or that of typical mammalian cells. Phosphatidylcholine is the major component in most mammalian cell membranes, while in purified virions phosphatidylethanolamine dominates. Due to its effects on membrane curvature, it is likely that the variations in its content are important to viral processing during infection. This integrated method of virion isolation with systematic analysis of glycerophospholipids provides a tool for the assessment of species specific biomarkers of viral pathogenicity.

A dimer is the minimal proton-conducting unit of the influenza a virus M2 channel

When influenza A virus infects host cells, its integral matrix protein M2 forms a proton-selective channel in the viral envelope. Although X-ray crystallography and NMR studies using fragment peptides have suggested that M2 stably forms a tetrameric channel irrespective of pH, the oligomeric states of the full-length protein in the living cells have not yet been assessed directly. In the present study, we utilized recently developed stoichiometric analytical methods based on fluorescence resonance energy transfer using coiled-coil labeling technique and spectral imaging, and we examined the relationship between the oligomeric states of full-length M2 and its channel activities in living cells. In contrast to previous models, M2 formed proton-conducting dimers at neutral pH and these dimers were converted to tetramers at acidic pH. The antiviral drug amantadine hydrochloride inhibited both tetramerization and channel activity. The removal of cholesterol resulted in a significant decrease in the activity of the dimer. These results indicate that the minimum functional unit of the M2 protein is a dimer, which forms a complex with cholesterol for its function.

Membrane-dependent effects of a cytoplasmic helix on the structure and drug binding of the influenza virus M2 protein

The influenza A M2 protein forms a proton channel for virus infection and also mediates virus assembly and budding. The minimum protein length that encodes both functions contains the transmembrane (TM) domain (roughly residues 22-46) for the amantadine-sensitive proton-channel activity and an amphipathic cytoplasmic helix (roughly residues 45-62) for curvature induction and virus budding. However, structural studies involving the TM domain with or without the amphipathic helix differed on the drug-binding site. Here we use solid-state NMR spectroscopy to determine the amantadine binding site in the cytoplasmic-helix-containing M2(21-61). (13)C-(2)H distance measurements of (13)C-labeled protein and (2)H-labeled amantadine showed that in 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) bilayers, the first equivalent of drug bound S31 inside the M2(21-61) pore, similar to the behavior of M2 transmembrane peptide (M2TM) in DMPC bilayers. The nonspecific surface site of D44 observed in M2TM is disfavored in the longer peptide. Thus, the pharmacologically relevant drug-binding site in the fully functional M2(21-61) is S31 in the TM pore. Interestingly, when M2(21-61) was reconstituted into a virus-mimetic membrane containing 30% cholesterol, no chemical shift perturbation was observed for pore-lining residues, whereas M2TM in the same membrane exhibited drug-induced chemical shift changes. Reduction of the cholesterol level and the use of unsaturated phospholipids shifted the conformational equilibrium of M2TM fully to the bound state but did not rescue drug binding to M2(21-61). These results suggest that the amphipathic helix, together with cholesterol, modulates the ability of the TM helix to bind amantadine. Thus, the M2 protein interacts with the lipid membrane and small-molecule inhibitors in a complex fashion, and a careful examination of the environmental dependence of the protein conformation is required to fully understand the structure-function relation of this protein.

Immobilization of the influenza A M2 transmembrane peptide in virus envelope-mimetic lipid membranes: a solid-state NMR investigation

The dynamic and structural properties of membrane proteins are intimately affected by the lipid bilayer. One property of membrane proteins is uniaxial rotational diffusion, which depends on the membrane viscosity and thickness. This rotational diffusion is readily manifested in solid-state NMR spectra as characteristic line shapes and temperature-dependent line narrowing or broadening. We show here that this whole-body uniaxial diffusion is suppressed in lipid bilayers mimicking the composition of eukaryotic cell membranes, which are rich in cholesterol and sphingomyelin. We demonstrate this membrane-induced immobilization on the transmembrane peptide of the influenza A M2 (AM2-TM) proton channel protein. At physiological temperature, AM2-TM undergoes uniaxial diffusion faster than approximately 10(5) s(-1) in DLPC, DMPC, and POPC bilayers, but the motion is slowed by 2 orders of magnitude, to <10(3) s(-1), in a cholesterol-rich virus envelope-mimetic membrane ("viral membrane"). The immobilization is manifested as near rigid-limit (2)H quadrupolar couplings and (13)C-(1)H, (15)N-(1)H, and (13)C-(15)N dipolar couplings for all labeled residues. The immobilization suppresses intermediate time scale broadening of the NMR spectra, thus allowing high-sensitivity and high-resolution spectra to be measured at physiological temperature. The conformation of the protein in the viral membrane is more homogeneous than in model PC membranes, as evidenced by the narrow (15)N lines. The immobilization of the M2 helical bundle by the membrane composition change indicates the importance of studying membrane proteins in environments as native as possible. It also suggests that eukaryote-mimetic lipid membranes may greatly facilitate structure determination of membrane proteins by solid-state NMR.

Membrane Glycoproteins of Enveloped Viruses

This chapter focuses on the recent information of the glycoprotein components of enveloped viruses and points out specific findings on viral envelopes. Although enveloped viruses of different major groups vary in size and shape, as well as in the molecular weight of their structural polypeptides, there are general similarities in the types of polypeptide components present in virions. The types of structural components found in viral membranes are summarized briefly in the chapter. All the enveloped viruses studied to date possess one or more glycoprotein species and lipid as a major structural component. The presence of carbohydrate covalently linked to proteins is demonstrated by the incorporation of a radioactive precursor, such as glucosamine or fucose, into viral polypeptides, which is resolved by sodium dodecyl sulfate (SDS) polyacrylamide gel electrophoresis. Enveloped viruses share many common features in the organization of their structural components, as indicated by several approaches, including electron microscopy, surface-labeling, and proteolytic digestion experiments, and the isolation of subviral components. The chapter summarizes the detailed structure of the glycoproteins of four virus groups: (1) influenza virus glycoproteins, (2) rhabdovirus G protein, (3) togavirus glycoprotein, and (4) paramyxovirus glycoproteins The information obtained includes the size and shape of viral glycoproteins, the number of polypeptide chains in the complete glycoprotein structure, and compositional data on the polypeptide and oligosaccharide portions of the molecules.

Dynamics of raft molecules in the cell and artificial membranes: approaches by pulse EPR spin labeling and single molecule optical microscopy

Lipid rafts in the plasma membrane, domains rich in cholesterol and sphingolipids, have been implicated in a number of important membrane functions. Detergent insolubility has been used to define membrane "rafts" biochemically. However, such an approach does not directly contribute to the understanding of the size and the lifetime of rafts, dynamics of the raft-constituent molecules, and the function of rafts in the membrane in situ. To address these issues, we have developed pulse EPR spin labeling and single molecule tracking optical techniques for studies of rafts in both artificial and cell membranes. In this review, we summarize our results and perspectives obtained by using these methods. We emphasize the importance of clearly distinguishing small/unstable rafts (lifetime shorter than a millisecond) in unstimulated cells and stabilized rafts induced by liganded and oligomerized (GPI-anchored) receptor molecules (core receptor rafts, lifetime over a few minutes). We propose that these stabilized rafts further induce temporal, greater rafts (signaling rafts, lifetime on the order of a second) for signaling by coalescing other small/unstable rafts, including those in the inner leaflet of the membrane, each containing perhaps one molecule of the downstream effector molecules. At variance with the general view, we emphasize the importance of cholesterol segregation from the liquid-crystalline unsaturated bulk-phase membrane for formation of the rafts, rather than the affinity of cholesterol and saturated alkyl chains. In the binary mixture of cholesterol and an unsaturated phospholipid, cholesterol is segregated out from the bulk unsaturated liquid-crystalline phase, forming cholesterol-enriched domains or clustered cholesterol domains, probably due to the lateral nonconformability between the rigid planar transfused ring structure of cholesterol and the rigid bend of the unsaturated alkyl chain at C9-C10. However, such cholesterol-rich domains are small, perhaps consisting of only several cholesterol molecules, and are short-lived, on the order of 1-100 ns. We speculate that these cholesterol-enriched domains may be stabilized by the presence of saturated alkyl chains of sphingomyelin or glycosphingolipids, and also by clustered raft proteins. In the influenza viral membrane, one of the simplest forms of a biological membrane, the lifetime of a protein and cholesterol-rich domain was evaluated to be on the order of 100 micro, again showing the short lifetime of rafts in an unstimulated state. Finally, we propose a thermal Lego model for rafts as the basic building blocks for signaling pathways in the plasma membrane.

Suppression of proinflammatory cytokines and induction of IL-10 in human monocytes after coxsackievirus B3 infection

Coxsackievirus B3 (CVB3) causes acute and chronic myocarditis, which is accompanied by an intense mononuclear leukocyte infiltration. Because myocardial tissue damage may either result from viral infections or from a dysregulated immune response, the susceptibility of human monocytes and macrophages to CVB3 was examined in this study with regard to virus replication, virus persistence, and release of cytokines. Monocytes were infected by CVB3 as shown by the intracellular appearance of plus- and minus-strand viral RNA, which was also capable of persisting for more than 10 days. Fresh monocytes were not permissive for full virus replication whereas monocyte-derived macrophages yielded a low amount of new viruses, which led to cell death. Although CVB3 infection induced the mRNA for the proinflammatory cytokines tumor necrosis factor-alpha (TNF-alpha), interleukin (IL)-1, and IL-6, only little cytokine production occurred. When infected monocytes were stimulated in addition by lipopolysaccharides (LPS), cytokine production was partially suppressed. In striking contrast, IL-10 expression was strongly and persistently induced by CVB3 on the mRNA and the protein level. These data show a dysregulated cytokine response in CVB3-exposed human monocytes and macrophages, which is characterized by a suppression of proinflammatory cytokines and a dominance of IL-10. This viral strategy may aid CVB3, causing chronic myocardiopathy.

Pulse EPR detection of lipid exchange between protein-rich raft and bulk domains in the membrane: methodology development and its application to studies of influenza viral membrane

A pulse saturation-recovery electron paramagnetic resonance (EPR) method has been developed that allows estimation of the exchange rates of a spin-labeled lipid between the bulk domain and the protein-rich membrane domain, in which the rate of collision between the spin label and molecular oxygen is reduced (slow-oxygen transport domain, or SLOT domain). It is based on the measurements of saturation-recovery signals of a lipid spin label as a function of concentrations of both molecular oxygen and the spin label. Influenza viral membrane, one of the simplest paradigms for the study of biomembranes, showed the presence of two membrane domains with slow and fast collision rates with oxygen (a 16-fold difference) at 30 degrees C. The outbound rate from and the inbound rate into the SLOT domain (or possibly the rate of the domain disintegration and formation) were estimated to be 7.7 x 10(4) and 4.6 x 10(4) s(-1), (15 micros residency time), respectively, indicating that the SLOT domain is highly dynamic and that the entire SLOT domain represents about one-third of the membrane area. Because the oxygen transport rate in the SLOT domain is a factor of two smaller than that in purple membrane, where bacteriorhodopsin is aggregated, we propose that the SLOT domain in the viral membrane is the cholesterol-rich raft domain stabilized by the trimers of hemagglutinin and/or the tetramers of neuraminidase.

Role of hemagglutinin cleavage for the pathogenicity of influenza virus

Although human epidemics of influenza occur on nearly an annual basis and result in a significant number of "excess deaths," the viruses responsible are not generally considered highly pathogenic. On occasion, however, an outbreak occurs that demonstrates the potential lethality of influenza viruses. The human pandemic of 1918 spread worldwide and killed millions, and the limited human outbreak of highly pathogenic avian viruses in Hong Kong at the end of 1997 is a warning that this could happen again. In avian species such as chickens and turkeys, several outbreaks of highly pathogenic influenza viruses have been documented. Although the reason for the lethality of the human 1918 viruses remains unclear, the pathogenicity of the avian viruses, including those that caused the human 1997 outbreak, relates primarily to properties of the hemagglutinin glycoprotein (HA). Cleavage of the HA precursor molecule HA0 is required to activate virus infectivity, and the distribution of activating proteases in the host is one of the determinants of tropism and, as such, pathogenicity. The HAs of mammalian and nonpathogenic avian viruses are cleaved extracellularly, which limits their spread in hosts to tissues where the appropriate proteases are encountered. On the other hand, the HAs of pathogenic viruses are cleaved intracellularly by ubiquitously occurring proteases and therefore have the capacity to infect various cell types and cause systemic infections. The x-ray crystal structure of HA0 has been solved recently and shows that the cleavage site forms a loop that extends from the surface of the molecule, and it is the composition and structure of the cleavage loop region that dictate the range of proteases that can potentially activate infectivity. Here influenza virus pathogenicity is discussed, with an emphasis on the role of HA0 cleavage as a determining factor.

The methyltransferase inhibitor Neplanocin A interferes with influenza virus replication by a mechanism different from that of 3-deazaadenosine

Neplanocin A (NeplA) and 3-deazaadenosine (3DA-Ado) are both inhibitors of methyltransferases, and both interfere with influenza virus replication. Their modes of action, however, are different. In chicken embryo cells NeplA inhibits only in media depleted of or low in methionine, while 3DA-Ado acts independently of the concentration of methionine. While homocysteine partially reverses the effect of NeplA, it strongly potentiates the effect of 3DA-Ado. While NeplA inhibits the synthesis of all viral proteins to nearly the same extent, 3DA-Ado interferes only with the production of late proteins (Fischer et al. (1990) Virology 177, 523-531). In NeplA-pretreated cells there is an extreme accumulation of S-adenosylhomocysteine, independent of the concentration of methionine in the medium, although NeplA inhibits influenza virus replication only in methionine-depleted medium. Therefore an accumulation of this intermediate by NeplA cannot account for the inhibitory effect, as has been implicated in the inhibition of the replication of other viruses. Our results indicate that at least two different methyltransferases are involved in influenza virus replication.

Identification of annexin 33 kDa in cultured cells as a binding protein of influenza viruses

The binding of three influenza A and one influenza B virus strains to proteins of three continuously cultured cell lines was studied using protein overlay and immunostaining methods. The results obtained indicated the presence of both sialic acid-dependent and -independent binding of the virus strains; virus binding to proteins in the molecular mass range from about 40 to 103 kDa was dependent on sialic acid, whereas binding to the 33 kDa protein was independent of sialic acid. The 33 kDa binding protein was identified as annexin, a widely distributed non-glycosylated calcium-dependent phospholipid-binding protein.

The potentiating effect of LPS on tumor necrosis factor-alpha production by influenza A virus-infected macrophages

Infection of murine PU5-1.8 macrophages and human monocytes by influenza A virus was associated with virus replication, release of tumor necrosis factor-alpha (TNF-alpha) and subsequent cell death. In the presence of small and by itself rather inefficient concentrations of lipopolysaccharide (LPS) or free lipid A (1 to 10 ng/ml), TNF-alpha production of virus-infected macrophages was strongly potentiated. LPS-triggered and enhanced TNF-alpha release from virus-infected macrophages was neither due to increased cell survival nor altered virus replication, potentiated TNF-alpha gene transcription, release of intracellularly stored TNF-alpha or shifts in the kinetics of TNF-alpha secretion. Influenza A virus infection alone induced a massive TNF-alpha mRNA accumulation which, however, was only weakly translated into bioactive TNF-alpha protein. When these virus-primed macrophages were exposed to LPS either simultaneously or up to 4 h after infection, an efficient and high translation into TNF-alpha protein occurred. Although the LPS-induced biochemical pathways leading to an augmented TNF-alpha production by virus-infected macrophages still remains unsolved, the findings suggest that the frequently observed serious clinical complications in the course of combined influenza A virus and bacterial infections may be due, at least in part, to an excessive release of cytokines such as TNF-alpha.

A H1 hemagglutinin of a human influenza A virus with a carbohydrate-modulated receptor binding site and an unusual cleavage site

Two receptor binding variants of the influenza virus A/Tübingen/12/85 (H1N1) were separated by their different plaque formation in MDCK cells. Hemagglutination of variant I was restricted to red blood cells of guinea pigs, whereas variant II also hemagglutinated chicken cells. The variants differed also in their ability to bind to alpha 2,6-linked sialic acid. Evidence is presented that this difference is determined by a complex carbohydrate side chain at asparagine131 near the receptor binding site which is absent in variant II. With both variants, the arginine found at the cleavage site of all other human isolates analyzed so far was replaced by lysine.

Genetic relatedness of the nucleoprotein (NP) of recent swine, turkey, and human influenza A virus (H1N1) isolates

The sequences of nucleoprotein (NP) genes of recent human and turkey isolates of influenza A viruses, which serologically could be correlated to contemporary swine viruses, were determined. These sequences were closely related to the NPs of these swine viruses and they formed a separate branch on the phylogenetic tree. While the early swine virus from 1931 resembled the avian strains in consensus amino acids of the NP and in its ability to rescue NP ts mutants of fowl plague virus in chicken embryo cells, the later strains on that branch were different: at 15 positions they have their own amino acids and they rescued the NP ts mutants only poorly. Of the NPs of the human New Jersey/76 isolates analysed, one clustered with the recent H1N1 swine viruses of the U.S.A., the other one with contemporary human strains. Since the NP is one of the main determinants of species specificity it is concluded that, although the H1N1 swine isolates from the U.S.A. form their own branch in the phylogenetic tree, they can be transmitted to humans and turkeys, but they do not spread further in these populations and so far have not contributed to human pandemics. It is not very likely that they will do so in future, since its branch in the phylogenetic tree develops further away from the human and avian branch.

Failure to obtain drug-resistant variants of influenza virus after treatment with inhibiting doses of 3-deazaadenosine and H7

3-Deazaadenosine and H7 specifically inhibit influenza virus replication under conditions at which they have no effect on other tested RNA viruses. This effect can be significantly potentiated by concomitant application of both compounds. Even under the most stringent conditions we failed to obtain any drug resistant variants. A possible explanation for this failure is that these compounds presumably do not act on a viral component like amantadine which was used as a control, but they interfere with cellular enzymes (factors) absolutely essential for influenza virus replication but more or less dispensable for the survival of the cell.

Retarded processing of influenza virus hemagglutinin in insect cells

When expressed in Spodoptera frugiperda cells by a baculovirus vector, the hemagglutinin of fowl plague virus has been found to contain palmitic acid in covalent hydroxylamine-sensitive linkage, indicating that these cells have the capacity to acylate foreign proteins at cysteine residues. Centrifugation on sucrose density gradients and immune precipitation with conformation-specific antibodies were used to compare trimerization of the hemagglutinin in insect cells and in fowl plague virus-infected MDCK cells. Trimerization of the hemagglutinin was incomplete in insect cells, and the kinetics of this reaction were about three times slower than in vertebrate cells. Similarly, post-translational proteolytic cleavage occurred in insect cells with a half-time of 90 min, and a substantial fraction of the hemagglutinin persisted in uncleaved form. In contrast, hemagglutinin was almost completely cleaved in MDCK cells, and the half-time of cleavage was only 30 min. The data indicate that in insect cells trimerization and, as a result, the subsequent processing steps of the hemagglutinin, are retarded and less efficient. The possible roles of aberrant glycosylation, acidic milieu, and lack of other influenza virus proteins in hemagglutinin trimerization are discussed.

Hemagglutinating encephalomyelitis virus attaches to N-acetyl-9-O-acetylneuraminic acid-containing receptors on erythrocytes: comparison with bovine coronavirus and influenza C virus

The receptors for the hemagglutinating encephalomyelitis virus (HEV, a porcine coronavirus) on chicken erythrocytes were analyzed and compared to the receptors for bovine coronavirus (BCV) and influenza C virus. Evidence was obtained that HEV requires the presence of N-acetyl-9-O-acetylneuraminic acid (Neu5,9Ac2) on the cell surface for agglutination of erythrocytes as has been previously shown for BCV and influenza C virus: (i) Incubation of red blood cells with sialate 9-O-acetylesterase, the receptor-destroying enzyme of influenza C virus, rendered the erythrocytes resistant against agglutination by each of the three viruses; (ii) Human erythrocytes which are resistant to agglutination by HEV acquire receptors for HEV after resialylation with Neu5,9Ac2. Sialylation of red blood cells with limiting amounts of sialic acid indicated that strain JHB/1/66 of influenza C virus requires less Neu5,9Ac2 for agglutination of erythrocytes than the two coronaviruses, both of which were found to be similar in their reactivity with Neu5,9Ac2-containing receptors.

Inhibition of influenza virus A/WSN replication by a trypsin inhibitor, 6-amidino-2-naphthyl p-guanidinobenzoate

A trypsin inhibitor, 6-amidino-2-naphthyl p-guanidinobenzoate (FUTHAN) reduced both the number and size of plaques of influenza virus A/WSN/33 (H1N1) that can grow without trypsin treatment in MDCK cells. The resulting virus particles with uncleaved hemagglutinin (HA) in the presence of FUTHAN was activated to produce infectious virions by trypsin treatment. Uncleaved HA of WSN virus grown in the presence of FUTHAN was found to be accumulated by protein analysis of WSN virus labeled biosynthetically with [35S]-methionine. It was strongly suggested that FUTHAN inhibited viral replication by preventing proteolytic cleavage of HA.

The oligosaccharides of influenza virus hemagglutinin expressed in insect cells by a baculovirus vector

The hemagglutinin of fowl plague virus has been expressed in Spodoptera frugiperda (SF) cell cultures using a baculovirus vector. To elucidate the structure of the carbohydrate side chains, radioactively labeled oligosaccharides were liberated by treatment with endoglucosaminidase H and glycopeptidase F. Sequential degradation with exoglycosidases and chromatographic analyses revealed the presence of oligomannosidic side chains, predominantly of the structures Man5-9GlcNAc2, and the truncated oligosaccharide cores Man3GlcNAc2 and Man3[Fuc]GlcNAc2. Polyacrylamide gel electrophoresis of endoglycosidase-treated hemagglutinin showed that most side chains of the HA1 subunit are truncated, whereas the HA2 subunit has one oligomannosidic and one truncated oligosaccharide. Comparison of these results with the glycosylation pattern of hemagglutinin obtained from vertebrate cells allowed a tentative allocation of the oligosaccharides to individual glycosylation sites. The results indicate that SF cells have the capacity to trim N-glycans to trimannosyl cores and to further process these by the addition of fucose. Thus, the complex oligosaccharides found on hemagglutinin from vertebrate hosts are replaced on hemagglutinin derived from insect cells by small truncated side chains.

Synthesis of biologically active influenza virus hemagglutinin in insect larvae

The hemagglutinin of influenza (fowl plague) virus was expressed in larvae of Heliothis virescens by using recombinant Autographa californica nuclear polyhedrosis virus (AcNPV) as a vector. Animals were infected with the recombinant virus either by parenteral injection or by feeding. For oral uptake, recombinant virus occluded in polyhedra obtained from cultured Spodoptera frugiperda cells after coinfection with authentic AcNPV was used. Immunohistological analyses of infected animals revealed that the hemagglutinin was expressed only in those tissues that are also permissive for the replication of authentic AcNPV. These tissues included hypodermis, fat body, and tracheal matrix. After oral infection, hemagglutinin was also detected in individual gut cells. The amount of hemagglutinin synthesized in larvae after parenteral infection was 0.3% of the total protein, compared with 5% obtained in cultured insect cells. The hemagglutinin was transported to the cell surface and expressed in polarized cells only at the apical plasma membrane. It was processed by posttranslational proteolysis into the cleavage products HA1 and HA2. Oligosaccharides were attached by N-glycosidic linkages and were smaller than those found on hemagglutinin obtained from vertebrate cells. Hemagglutinin from larvae expressed receptor binding and cell fusion activities, but quantitation of the hemolytic capacity revealed that it was only about half as active as hemagglutinin from vertebrate or insect cell cultures. Chickens immunized with larval tissues containing hemagglutinin were protected from infection with fowl plague virus. These observations demonstrate that live insects are able to produce a recombinant membrane protein of vertebrate origin in biologically active form.

The structure of serotype H10 hemagglutinin of influenza A virus: comparison of an apathogenic avian and a mammalian strain pathogenic for mink

The primary structure of the hemagglutinin of the apathogenic avian influenza virus A/chick/Germany/N/49 (H10N7) and of the serologically related strain A/mink/Sweden/84 (H10N4) pathogenic for mink has been elucidated by nucleotide sequence analysis, and the carbohydrates attached to the polypeptide have been determined. The H10 hemagglutinin has 65, 52, 46, 45, and 44% amino acid sequence homology with serotypes H7, H3, H1, H2, and H5, respectively. H10 and H7 hemagglutinins are also most closely related in their glycosylation patterns. There is a high sequence homology between both H10 strains supporting the concept that the mink virus has obtained its hemagglutinin from an avian strain. The sequence homology includes the cleavage site which consists of a single arginine as is the case with most other hemagglutinins exhibiting low susceptibility to proteolytic activation. The similarity in hemagglutinin structure between both H10 strains is discussed in light of the distinct differences in the pathogenicity of both viruses.

Effect of dimethylsulfoxide (DMSO) on virus replication and maturation

At intermediate concentrations of DMSO the yields of infectious virus and biologically active hemagglutinin and neuraminidase of an influenza A virus (fowl plague virus) and of reassortants therefrom are enhanced severalfold, even though viral protein synthesis is not significantly affected. A corresponding enhancing effect was also found with New castle disease and Semliki Forest viruses. At elevated concentrations of DMSO virus yield decreases, and under these conditions the synthesis of the late influenza virus proteins is specifically inhibited. The results indicate that DMSO can facilitate the assembly of virus particles, and viral components, which are normally produced in surplus amounts, now contribute to the maturation of infectious particles.

Mutants of an influenza A reassortant which are cold-sensitive (cs) as well as temperature-sensitive (ts): on the role of the neuraminidase activity for influenza virus infection

Temperature-sensitive (ts) mutants were obtained by undiluted passage of the cold-sensitive (cs) influenza A reassortant 113/Ho. This reassortant produces normal yields of infectious virus with negligible neuraminidase (NA) at 40 degrees. The mutants obtained from it had a narrow temperature optimum for plaque formation in chick embryo cells, since they were cs as well as ts. Such cs/ts mutants have not been described before. In contrast to mutants derived from FPV, most of the mutants derived from 113/Ho carried a ts defect in the NA gene. NA activity was not detectable after infection with these mutants at 40 degrees. The results are interpreted to mean that, although NA activity is not completely dispensible for influenza A virus replication in tissue cultures, the viruses possess a surplus of NA activity. The normally high activity of NA of influenza viruses seems to be necessary only for the natural infection of the respiratory tract.

Influence of insulin and 12-O-tetradecanoylphorbol-13-acetate (TPA) on influenza virus multiplication

Insulin and 12-O-tetradecanoylphorbol-13-acetate (TPA) interfere with the multiplication of fowl plague virus, an influenza A virus, in primary chick embryo cells. Specifically the production of the viral glycoproteins hemagglutinin and neuraminidase are affected by the drugs. A decrease or omission of glucose from the culture medium enhances this effect, which is in agreement with the idea that these drugs act on virus replication via a shortage of glucose in the host cell. Virus replication in cells of different organs is affected to different extents by insulin and TPA.

A reassortant between influenza A viruses (H7N2) synthesizing an enzymatically inactive neuraminidase at 40 degrees which is not incorporated into infectious particles

Cells infected with a reassortant (113/Ho, H7N2) between A/fowl plague/Rostock/34 (FPV, H7N1) and A/Hong Kong/1/68 (H3N2) carrying RNA segments 1 and 6 of the Hong Kong virus and the residual genes of FPV, synthesized at 40 degrees a neuraminidase (NA) which is enzymatically not active and which is not incorporated into infectious particles. At 40 degrees NA accumulates in the rough endoplasmic reticulum. It contains mainly carbohydrate side chains of the mannose type, and fucose is only scarcely incorporated. At 33 degrees NA of the reassortant is overproduced, and at least some of it is active and is incorporated into viral particles. Under nonreducing conditions during PAGE its NA migrates to the same position as after heating with mercaptoethanol, in contrast to the Hong Kong parent virus. It is speculated that at 40 degrees the tetramerization of the NA in the rough endoplasmic reticulum does not function, and in this way its migration to the cytoplasmic membrane and its incorporation into infectious particles does not occur. Since 113/Ho is as pathogenic for the chicken (body temperature of 41 degrees) as is FPV, the question arises which role the NA plays in virus replication and spread in the infected organism.

Structure and composition of influenza virus. A small-angle neutron scattering study

A detailed analysis is presented of the small-angle neutron scattering curves of homogeneous solutions of influenza B virus, both intact and after treatment with bromelain, which removes the external glycoprotein spikes. The two sets of data are consistent with the following low-resolution structure: the virus particles are spherical, about 1200 A in diameter and of Mr about 180 X 10(6). The lipid bilayer is centred at a radius of 425 A, is 40 A to 50 A thick and constitutes 25% to 28% of the virus mass. The surface glycoproteins, predominantly haemagglutinin, contribute 40% to 46% of the total mass. Surprisingly little protein is found in the interior of the virus. It is suggested that the reason for this is that many particles do not contain the full complement of ribonucleoprotein complexes. These results are in good agreement with recent scanning transmission electron microscopic measurements of molecular mass and cryo-electron microscopic observations of the same preparations. Appendix 1 describes a new method of deriving spherical shell models from contrast variation neutron scattering data on viruses, in which scattering curves from all measured contrasts are used simultaneously. There is also a discussion of the assumptions and limitations implicit in the structural interpretation of such models, with emphasis on viruses containing lipid bilayers. Appendix 2 examines the effect on the scattering curves of various arrangements of the surface glycoproteins.

The nucleoprotein as a possible major factor in determining host specificity of influenza H3N2 viruses

In an attempt to assess the importance of the nucleoprotein (NP) in the determination of host specificity, a series of experiments was performed on influenza A viruses of the H3N2 subtype. We have examined rescue of mutants of A/FPV/Rostock/34 with temperature-sensitive (ts) lesions in the nucleoprotein (NP) gene by double infection of chick embryo cells with H3N2 strains isolated from different species. The ts mutants could be rescued by all avian H3N2 strains but not by any of the human H3N2 isolates. Only two of the swine H3N2 strains tested were able to rescue our mutants. The NP gene of these two swine isolates resembled the NP gene of the avian strains genetically in the hybridization test. However, their NPs reacted differently with a set of monoclonal antibodies when compared with NPs of avian H3N2 strains. Concerning multiplication in ducks they behaved like the other swine and human strains. The phosphopeptide fingerprints of all swine isolates tested were alike and were different from those of human or avian origin. Our observations are compatible with the idea that human H3N2 strains might not be able to cross the species barrier to birds directly, and possibly also not the other way around, without prior reassortment in pigs, which seem to have a broader host range concerning the compatibility of the NP gene in reassortants.

Sensitivity of ortho- and paramyxovirus replication to human interferon alpha

Replication of the influenza virus strains Influenza Ao/WSN (H0N1), fowl plague (Hav1N1) and B-Lee/40 (ATCC) and the paramyxovirus, New Castle disease virus (Victoria) are highly sensitive to human interferon type alpha in Madin Darby bovine kidney cells. Pretreatment of cells with human interferon type alpha resulted in protection of the cells against viral cytopathic effect. The inhibition of the orthomyxovirus strains used in this study and New Castle disease virus replication is mediated by an inhibition of viral protein synthesis. Residual WSN virus particles released from interferon treated cells showed the same structural protein pattern as virus particles isolated from control cells. Glycosylation of the viral structural components appeared to be unaffected by interferon.

Host antigens associated with haemagglutinin

A delayed-type hypersensitivity (DTH) test has been used to detect the presence of host-specific antigen(s) is highly purified egg-grown influenza virus, virus components and isolated haemagglutinin. Cyclophosphamide-treated mice, primed with an extract from allantoic fluid, showed DTH reactions when challenged with preparations containing envelope lipid or polymer-type subunits, whereas lipid-free derivatives such as internal virus protein complexes and monomer haemagglutinin preparations lacked the activity. This approach has been extended to demonstrate the presence of associated host antigens in SV40-vector expressed haemagglutinin produced in simian cells.

Stability of infectious influenza A viruses to treatment at low pH and heating

We have measured the infectivity of influenza A virus strains grown either in embryonated eggs or in chick embryo cells in culture after treatment at low pH. At pH values at which hemolysis occurs there was an irreversible loss of infectivity. The threshold pH, at which the infectivity was lost, depended on the hemagglutinin subtype of the virus strain. All H5 and H7 strains tested were extremely labile at low pH. In contrast, all H3 strains were relatively stable, independent of the species from which the viruses were isolated. With several H1 viruses the hemagglutination (HA) activity was irreversibly lost at intermediate pH values causing inactivation of infectivity. Strains with noncleaved hemagglutinins were much more stable. These observations might explain why duck influenza viruses can easily survive in lake water and wet faeces, and multiply in the intestinal tract, where trypsin is present. There are also significant differences in heat stability exhibited by influenza A strains. In contrast to pH stability this is not a specific trait of the hemagglutinin, since it can be influenced by reassortment. There is no correlation between the stability of infectivity at low pH and heat.

A mutant of fowl plague virus (influenza A) with an altered glycosylation pattern in its hemagglutinin

A temperature-sensitive mutant (ts 1/1) with a defect in the hemagglutinin (HA) gene, which was obtained by undiluted passage of fowl plague virus (FPV) at 33 degrees, is described. At 33 degrees proteolytic cleavage of the abnormal HA yielded an altered HA2 (XHA2) which migrated ahead of the NS1 protein and lacked the complex oligosaccharide side chain. At the nonpermissive temperature of 40 degrees, the migration of the HA of ts 1/1 from the rough endoplasmic reticulum (RER) via the Golgi apparatus to the cell surface was rate limiting for virus maturation. The HA was only slowly cleaved and migrated during polyacrylamide gel electrophoresis ahead of the HA of wild type FPV. Some revertants of ts 1/1 exhibited the same protein pattern as the mutant, others resembled wild type FPV, while one revertant gave rise to a mixture of HA2 and XHA2 at 40 degrees. These results suggest that (1) the loss of the complex oligosaccharide side chain is not responsible for the ts phenotype, (2) the mutation is presumably not at the site where the oligosaccharide side chain is linked to the protein backbone, and (3) ts 1/1 presumably carries a mutation located in RNA segment 4, which by pseudoreversion (suppressor mutation) in the same gene leads to different ts+ phenotypes.

Correlation between loss of the temperature-sensitive phenotype and pathogenicity of fowl plague virus mutants in the chicken

The reversion of temperature-sensitive (ts) mutants of fowl plague virus to the ts+ phenotype was correlated with pathogenicity for chicken. Two types of ts mutants were investigated: those obtained by mutagenesis with 5-fluorouracil and those obtained by undiluted passages at 33 degrees C. The reversion frequency of the former mutants depended on the RNA segment in which the ts defect was located, mutations in RNA segments 1 and 2 having the highest reversion frequency, those in the RNA segments coding for the glycoproteins the lowest. ts mutants obtained by undiluted passages behaved differently in this respect. There was an approximate correlation between frequency of reversion and pathogenicity for chicken. Double mutants induced by 5-fluorouracil, having one tight and one leaky mutation, reverted easily without loss of the leaky mutation. These double mutants were still to a limited extent pathogenic for the chicken. Only one double mutant with two tight mutations (ts 293) was completely nonpathogenic after intramuscular inoculation. Two ts mutants with multiple tight defects (ts 1/1 and ts 3/18) obtained by undiluted passage did not revert to wild-type after injection into embryonated eggs and incubation at 33 degrees C, but they were still slightly pathogenic for the chicken. There was no obvious correlation between the shut-off temperature and pathogenicity of mutants carrying a single ts defect. However, for mutants with multiple tight mutations a high shut-off temperature seemed to be essential for reversion during serial passages as well as for pathogenicity in the chicken, when different routes of inoculation were examined. ts mutants seem to be safe as live vaccines only, (1) if they carry at least two tight ts defects, (2) if they have a relatively low shut-off temperature, and (3) if they could be administered other than via the respiratory tract.

Characterization of a cold-sensitive (cs) recombinant between two influenza A strains

Recombinants between fowl plague virus (FPV, H7N1) and the Hong Kong (H3N2) or Singapore (H2N2) influenza virus strains carrying the hemagglutinin of FPV and the neuraminidase of the human strains form only very tiny plaques at 33 degrees, but normal plaques at 37 degrees. One recombinant (113/Ho) has been studied in more detail. It multiplies only very slowly at 33 degrees, the nonpermissive temperature. Adsorption and penetration are normal at 33 degrees, but synthesis of protein is impeded. Temperature-shift experiments suggest that the synthesis of viral mRNA is slowed at 33 degrees. 113/Ho does not agglutinate chicken erythrocytes at 40 degrees, as the parent viruses do. 113/Ho can be adapted to grow normally at 33 degrees. The frequency of adaptation is comparable to reversion of a single point mutation (ca. 10(-5) ). Recombinants which grow well at 37 degrees but not at 33 degrees are called cold-sensitive (cs) recombinants.

The estimation of host antigen in experimental and commercial influenza subunit preparations by delayed-type hypersensitivity reaction

A delayed-type hypersensitivity (DTH) test has been used to detect the presence of host-specific antigen(s) in highly-purified egg-grown influenza virus and virus components. Cyclophosphamide-treated mice, primed with an extract from allantoic fluid, showed DTH reactions when challenged with whole virus, spikeless virus particles or polymer-type subunits. All commercial egg-grown influenza vaccines showed the presence of host antigen, whereas lipid-free derivatives such as internal virus protein complexes and monomer HA preparations lacked the activity. These findings are discussed in relation to vaccine application.

Disulfide bonding in influenza virus proteins as revealed by polyacrylamide gel electrophoresis

Disulfide bonding in the major proteins of influenza virus A, WSN strain, was studied by electrophoresis in sodium dodecyl sulfate-polyacrylamide gels under reducing and nonreducing conditions. The electrophoretic behavior of the proteins correlated with their localization in the virions and their chemical composition. The internal proteins of the viral particles, i.e. matrix and nucleoproteins, were shown to contain a relatively small number of cysteine residues. Electrophoresis under nonreducing conditions yielded multiple forms of the proteins which could be discriminated by small but readily observable, reproducible differences in their migration rates in the gel. the multiplicity of the protein forms was caused by the formation of intramolecular disulfide bonds in matrix and nucleoproteins that arose during or after solubilization in sodium dodecyl sulfate. On the other hand, we failed to detect native inter- and intramolecular linkages in matrix and nucleoproteins. External glycoproteins of the virions (HA and NA) had, in contrast to the internal ones, a higher number of cysteine residues and native disulfide bonds. At least three disulfide linkages were revealed in HA and NA in our experiments. In uncleaved HA all of the linkages were intramolecular. In NA at least one disulfide bond linked two identical polypeptides into a dimer. It was established that the reduction of the different disulfide linkages in HA and NA required different concentrations of the reducing agent.

Acylation of viral spike glycoproteins: a feature of enveloped RNA viruses

The covalent attachment of fatty acids to the glycoproteins of orthomyxo-, paramyxo, alpha-, and coronavirus was studied. All enveloped viruses analyzed afford covalently bound fatty acid in at least one species of their spike glycoproteins. No internal components of the viruses studied including the hydrophobic M proteins of myxo- and rhabdoviruses contained fatty acid. Analysis of myxovirus particles devoid of the exposed portions of their spikes revealed that fatty acids are linked to the hydrophobic tail fragment of the glycoprotein which is associated with the viral lipid bilayer. With influenza virus hemagglutinin the fatty acid attachment site could be located at the cyanogen bromide peptide of the small subunit (HA₂) which contains the membrane-embedded region of the polypeptide. The binding of fatty acids to viral glycoproteins occurs in a wide range of host cells including mammalian, avian, and insect cells.