The molecular biology of influenza virus pathogenicity

The critical cut-off temperature of avian influenza viruses

We have measured the pathogenicity for 6-week-old chicks of infection by H7 avian influenza viruses. One virus, strain S3 from A/FPV/Rostock/34(H7N1) showed a temperature sensitive phenotype at 41.5 degrees C and reduced pathogenicity. By analysis of reassortants made between virus S3 and A/FPV/Dobson/27(H7N7), a fully pathogenic virus, two conclusions arise. (1) The critical cut-off temperature for avian influenza virus in 6-week-old chicks is 41.5 degrees. (2) RNA segment 1 of virus S3 is responsible for the lack of pathogenicity in reassortant viruses. Nucleotide sequencing of RNA segment 1 from S3 and its parent, A/FPV/Rostock/34 has revealed a single mutation at nucleotide 1561. This results in a substitution of isoleucine for leucine at amino acid position 512 in the cap-binding protein, PB2.

Inhibition of endoproteolytic cleavage of cytomegalovirus (HCMV) glycoprotein B by palmitoyl-peptidyl-chloromethyl ketone

Endoproteolytic cleavage of glycoprotein B (gB) of human cytomegalovirus (HCMV) is inhibited by palmitoylated peptidyl-chloromethyl ketone (palFAKR-CEK) at concentrations above 30 microM. Inhibitor treatment of HCMV-infected human fibroblasts neither interfered with exposure of gB on the plasma membrane, detected by surface membrane immunostaining, nor reduced production of intracellular infectious viral progeny. Release of infectious virus, on the other hand, was impaired.

Antigenic reactivity and electrophoretic migrational heterogeneity of the three polymerase proteins of type A human and animal influenza viruses

Antigenic reactivity of the three polymerase proteins PB1, PB2, and PA of type A influenza viruses of animal and human origin were analysed by radioimmunoprecipitation using monospecific antisera. Each of the polymerase monospecific antisera made against the polymerase proteins of the human A/WSN/33 (H1N1) influenza virus reacted efficiently with the homologous proteins of all the known thirteen HA subtype viruses of avian influenza virus, three subtypes of human influenza virus, swine and equine influenza viruses. This broad reactivity of each of the antisera indicated that the polymerase proteins are antigenically related among the type A influenza viruses and therefore can be considered as type specific antigens similar to the other viral internal proteins nucleoprotein (NP) and matrix protein (M). No electrophoretic migrational heterogeneity was found among the PB2 proteins of different subtype viruses, whereas PB1 protein exhibited minor variation. However, PA protein from among various viral subtypes showed considerable heterogeneity. Each of the polymerase antisera also immunoprecipitated additional antigenically related polypeptides with distinct electrophoretic mobilities from cells infected with each of the influenza viral subtypes.

Organ tropism of Sendai virus in mice: proteolytic activation of the fusion glycoprotein in mouse organs and budding site at the bronchial epithelium

Wild-type Sendai virus is exclusively pneumotropic in mice, while a host range mutant, F1-R, is pantropic. The latter was attributed to structural changes in the fusion (F) glycoprotein, which was cleaved by ubiquitous proteases present in many organs (M. Tashiro, E. Pritzer, M. A. Khoshnan, M. Yamakawa, K. Kuroda, H.-D. Klenk, R. Rott, and J. T. Seto, Virology 165:577-583, 1988). These studies were extended by investigating, by use of an organ block culture system of mice, whether differences exist in the susceptibility of the lung and the other organs to the viruses and in proteolytic activation of the F protein of the viruses. Block cultures of mouse organs were shown to synthesize the viral polypeptides and to support productive infections by the viruses. These findings ruled out the possibility that pneumotropism of wild-type virus results because only the respiratory organs are susceptible to the virus. Progeny virus of F1-R was produced in the activated form as shown by infectivity assays and proteolytic cleavage of the F protein in the infected organ cultures. On the other hand, much of wild-type virus produced in cultures of organs other than lung remained nonactivated. The findings indicate that the F protein of wild-type virus was poorly activated by ubiquitous proteases which efficiently activated the F protein of F1-R. Thus, the activating protease for wild-type F protein is present only in the respiratory organs. These results, taken together with a comparison of the predicted amino acid substitutions between the viruses, strongly suggest that the different efficiencies among mouse organs in the proteolytic activation of F protein must be the primary determinant for organ tropism of Sendai virus. Additionally, immunoelectron microscopic examination of the mouse bronchus indicated that the budding site of wild-type virus was restricted to the apical domain of the epithelium, whereas budding by F1-R occurred at the apical and basal domains. Bipolar budding was also observed in MDCK monolayers infected with F1-R. The differential budding site at the primary target of infection may be an additional determinant for organ tropism of Sendai virus in mice.

The hemagglutinating glycoproteins of influenza B and C viruses are acylated with different fatty acids

We present evidence that the hemagglutinin (HA) of influenza B virus and the glycoprotein of influenza C virus (HEF) are acylated. The fatty acid linkage is sensitive to treatment with hydroxylamine and mercaptoethanol, which points to a labile thioester-type linkage. The HA of influenza B virus contains mainly palmitic acid, whereas the HEF glycoprotein of influenza C virus is acylated with stearic acid which has not been observed before as the prevailing fatty acid in viral or cellular acyl proteins.

Generation of seal influenza virus variants pathogenic for chickens, because of hemagglutinin cleavage site changes

Influenza virus A/seal/Mass/1/80 (H7N7) was adapted to grow in MDCK cells and chicken embryo cells (CEC) in the absence of exogenous protease. The biological properties of the virus variants obtained coincided with intracellular activation of the hemagglutinin (HA) by posttranslational proteolytic cleavage and depended on the cell type used for adaptation. MDCK cell-adapted variants contained point mutations in regions of the HA more distant from the cleavage site. It is proposed that these mutations are probably responsible, through an unknown mechanism, for enhanced cleavability of HA in MDCK cells. Such virus variants were apathogenic in chickens. CEC-adapted variants, on the other hand, contained an insertion of basic amino acids at the HA cleavage site, in addition to scattered point mutations. The insertions converted the cleavage sites in the variant virus HAs so that they came to resemble the cleavage site found in highly pathogenic avian influenza viruses. CEC variants with such cleavage site modifications were highly pathogenic for chickens. The lethal outcome of the infection in chickens demonstrated for the first time that an influenza virus derived from a mammalian species can be modified during adaptation to a new cell type to such an extent that the resulting virus variant becomes pathogenic for an avian species.

Comparison of the positions and efficiency of cleavage activation of fusion protein precursors of virulent and avirulent strains of Newcastle disease virus: insights into the specificities of activating proteases

The F1- and F2-polypeptide components of in ovo activated fusion proteins of one virulent (AV or Australia-Victoria) strain, one low-virulence (EG or Eaves-Grimes) strain, and two avirulent (V4 or Queensland and WA2116) strains of Newcastle disease virus (NDV) were isolated and subjected to structural analysis. This included complementary application of amino acid analysis, fast atom bombardment-mass spectrometry, and N-terminal sequence analysis to fragments isolated from AspN protease digests of the F2-polypeptides using HPLC. As a result, the complete sequences of the F2-polypeptides were determined, including documentation of glycosylation of asparagine 54. The sequence of the cleavage-activation site of the WA2116 F0-protein was found to be distinctly different from this site in any other NDV F0-protein. Cleavage activation at the C termini of the F2-polypeptide regions was found to have occurred to approximately equivalent extents at arginines 82 and 85 of the AV and EG strains, but was restricted largely to arginine 85 of the V4 strain and completely to arginine 85 of the WA2116 strain. In each case cleavage activation was apparently succeeded by trimming of the basic residues from the newly formed C termini. Immunochemical analysis with antipeptide antisera showed that the extent of cleavage was influenced by amino acids adjacent to these arginines. These data provide insight into the substrate specificities of the enzymes involved in cleavage activation of the fusion protein precursors.

Structural variation occurring in the hemagglutinin of influenza virus A/turkey/Oregon/71 during adaptation to different cell types

The influenza virus A/turkey/Oregon/71 (H7N3) has been adapted to grow in MDCK or chicken embryo cells (CEC) in the absence of trypsin. Changes occurred in the biological properties of the virus variants selected, depending on the cell type used for adaptation. They coincided with enhanced hemagglutinin (HA) activation by intracellular proteolytic cleavage. In the case of MDCK cell selected variants growth, plaque formation, and HA cleavability were restricted to this cell type, whereas the CEC-derived variants displayed altered activities in a broad range of host cells. Unlike the wild-type virus and its MDCK cell-derived variants, CEC variants had acquired pathogenic properties for chickens. By nucleotide sequence analysis of the HA genes of the MDCK cell variants several point mutations were found, which were localized predominantly at the distal, globular part of the HA molecule. The mechanism by which these point mutations increased HA cleavability has not been defined. In the CEC-derived variants besides point mutations, an insertion of 54 nucleotides adjacent to the cleavage site was observed, which corresponds in its sequence to a region in the 28 S ribosomal RNA. This insertion is probably responsible for the altered cleavability of the CEC variants' HA, leading to increased growth potential and pathogenicity.

On the structure of the epidemic spread of AIDS: the influence of an infectious coagent

The reported AIDS cases in the USA and the Federal Republic of Germany are growing almost exponentially. Considering the epidemiological curves for different risk groups (homosexual men, i.v. drug abusers, heterosexual partners etc.) it is extremely surprising, that nearly exactly the same development occurs in all risk groups. One only has to consider a certain time shift for each group. The conformity of the development for all groups is evident by the fact, that the curves representing the reported AIDS cases for different risk groups are straight lines (in a logarithmic scale) running nearly exactly in parallel. This remarkable parallelism can be understood only if the spread of AIDS is independent of the sexual or drug risk in a certain sense. On the other hand, the drastic overrepresentation of the sexually highly active groups and drug abusers in the number of AIDS cases obviously requires that the transmission of AIDS unequivocally depends on the sexual and drug risk. We present a mathematical model that is suitable to reconcile this apparent contradiction in the interpretation of the epidemiological data: the observed parallel time series for the spread of AIDS in groups with different risk of infection can be realized by computer simulation, if one assumes that the outbreak of full-blown AIDS only occurs if HIV and a certain infectious coagent (cofactor) CO are present. Such a situation is not uncommon, see, e.g. the influenza virus--Staphylococcus aureus system. According to the mathematical model this cofactor would spread independently of the sexual and drug risk--in contrast to HIV. However, due to its analytical properties the simulated cofactor cannot be identified so far with any known infectious agent.

Rapid detection of influenza virus H1 by the polymerase chain reaction

We applied a combination of reverse transcription (RT) with the polymerase chain reaction (PCR) for a rapid detection of influenza virus H1 subtype. We amplified a 441 bp segment of relatively high genetic stability of the hemagglutinin gene. Experimental conditions were established using plasmid DNA and infected cell cultures. The test was applied to 28 nasopharyngeal lavages from patients, two of which were positive for influenza virus H1. When the amplified DNA of a positive sample was sequenced we found 97% homology with the recent strain A/USSR/70.

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.

Inhibition of proteolytic activation of influenza virus hemagglutinin by specific peptidyl chloroalkyl ketones

Lysates of cultured cells have been analyzed for arginine-specific endoproteases using peptidyl-p-nitroanilides as chromogenic substrates. The enzymes present in MDBK, MDCK, VERO, BHK, and chick embryo cells required lysine-arginine or arginine-arginine pairs as cleavage sites, whereas chorioallantoic membrane cells contained, in addition, an activity that could cleave at a single arginine. The effect of peptidyl chloroalkyl ketones on the activation of the fowl plague virus hemagglutinin by the proteases specific for paired basic residues has been investigated. When virions containing uncleaved hemagglutinin were incubated with lysates of uninfected cells, cleavage was completely inhibited by peptidyl chloroalkyl ketones containing paired basic residues at a concentration of 1 mM. In contrast a compound containing a single arginine had no inhibitory activity. When dibasic peptidyl chloroalkyl ketones were added to infected cell cultures, cleavage of hemagglutinin and multiple cycles of virus replication were inhibited at 10 mM. However, a 100- to 200-fold increase of the inhibitory activity in intact cells could be achieved by N-terminal acylation. These studies suggest a potential role of peptidyl chloroalkyl ketones as antiviral agents.

Increased viral pathogenicity after insertion of a 28S ribosomal RNA sequence into the haemagglutinin gene of an influenza virus

The haemagglutinin glycoprotein HA of influenza viruses is responsible for the attachment of the virus to neuraminic acid-containing receptors at the cell surface and subsequent penetration by triggering fusion of the viral envelope with cellular membranes. To express full activity of the newly synthesized precursor, HA has to be modified by post-translational proteolytic cleavage into the polypeptides HA1 and HA2 by cellular enzymes. If proteases suitable for cleavage are not present in the host cell, the resulting virus particles are non-infectious. During adaptation of the apathogenic influenza virus A/turkey/Oregon/71 to chicken embryo cells, which are not permissive for HA cleavage, we obtained an infectious virus variant with increased pathogenicity. Sequence analysis revealed that during adaptation 54 nucleotides were inserted into the HA gene; their sequence corresponds to a region of the 28S ribosomal RNA. This insertion is probably responsible for increased cleavability of HA, as well as for infectivity and pathogenicity of the adapted virus.

Mutations at the cleavage site of the hemagglutinin after the pathogenicity of influenza virus A/chick/Penn/83 (H5N2)

Six variants that form plaques in chick embryo cells in the absence of trypsin have been isolated from the apathogenic avian influenza virus A/chick/Pennsylvania/1/83 (H5N2). Unlike the wild-type, the plaque variants contain a hemagglutinin that is cleaved in chick embryo cells and MDCK cells. The variants differ also from the wild-type in their pathogenicity for chickens. Nucleotide sequence and oligosaccharide analysis of the hemagglutinin have revealed that, unlike natural isolates with increased pathogenicity (Y. Kawaoka et al., 1984, Virology 139, 303-316; Y. Kawaoka and R. G. Webster, 1985, Virology 146, 130-137), the variants obtained in vitro have retained an oligosaccharide at asparagine 11 that is believed to interfere with the cleavage site of the wild-type. However, all variants showed mutations in the hemagglutinin resulting in an increased number of basic groups at the cleavage site. These observations demonstrate that masking of the cleavage site by an oligosaccharide is overcome by an enhancement of the basic charge at the cleavage site.

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.

Characterization of a pantropic variant of Sendai virus derived from a host range mutant

A variant (F1-R) was isolated from a temperature-sensitive host range mutant (ts-f1) of Sendai virus. F1-R was no longer temperature-sensitive but it retained the host range phenotype. Unlike wild-type virus, F1-R and ts-f1 undergo multiple cycles of replication in several cell lines in the absence of trypsin. This was attributed to proteolytic activation of the fusion (F) glycoprotein of the host range mutants, in cell nonpermissive to wild-type virus. In mice infected intranasally the variant F1-R caused a generalized infection. This was shown by immunohistology and with infectious virus being recovered from several organs whereas infection with wild-type virus was restricted to the lung. These observations indicate that the pantropic property of F1-R is the result of proteolytic activation of the virus by ubiquitous proteases. Nucleotide sequence analyses revealed that ts-f1 and F1-R differed from the wild-type virus by mutations at the region of the cleavage site of F and at the glycosylation site of the F2 subunit. The findings indicated that these mutations are responsible for the increased cleavability of the F protein of ts-f1 and F1-R and therefore are important determinants for the pantropism of F1-R.

Structural features unique to each of the three antigenic sites on the hemagglutinin-neuraminidase protein of Newcastle disease virus

Antigenic variants of D26 strain of Newcastle disease virus (NDV) were selected with monoclonal antibodies directed to the three nonoverlapping antigenic sites on the hemagglutinin-neuraminidase (HN) protein, and their HN genes were sequenced to identify the amino acids important for the integrity of each site. Seven variants for site I, which is immunodominant and conserved among NDV strains, had a change of glutamic acid at position 347, mostly to lysine, and in a single case, to glycine. In the second group of two variants for site IV, a change of asparagine to aspartic acid was found at position 481. This resulted in elimination of the oligosaccharide attached to this asparagine residue of the parental virus. Together with the finding that the site IV was destroyed by treatment with endoglycosidase F, it was suggested that the oligosaccharide is important for maintaining the structure of site IV. The oligosaccharide appeared to contribute to exposing a nearby determinant by conferring hydrophilicity on it. A variant for site II had also a nonconservative mutation resulting in the change of glutamic acid to valine at position 495. The site I recognized by antibodies which inhibit neuraminidase activity with a small substrate neuraminlactose was located closer to the predicted sialic acid-binding site than to the other sites recognized by antibodies lacking the enzyme-inhibiting capacity. The sequence of the parental virus HN gene revealed that the HNo precursor for the HN protein is an extra-long protein whose C terminus is elongated by 45 amino acids, compared with the usual HN protein sequenced in parallel.