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The terminal globular domain of the paramyxovirus hemagglutinin-neuraminidase (HN) glycoprotein spike has a number of conserved residues that are predicted to form its neuraminidase (NA) active site, by analogy to the influenza virus neuraminidase protein. We have performed a site-directed mutational analysis of the role of these residues in the functional activity of the Newcastle disease virus (NDV) HN protein. Substitutions for several of these residues result in a protein lacking both detectable NA and receptor recognition activity. Contribution of NA activity, either exogenously or by coexpression with another HN protein, partially rescues the receptor recognition activity of these proteins, indicating that the receptor recognition deficiencies of the mutated HN proteins result from their lack of detectable NA activity. In addition to providing support for the homology-based predictions for the structure of HN, these findings argue that (i) the HN residues that mediate its NA activity are not critical to its attachment function and (ii) NA activity is required for the protein to mediate binding to receptors.

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The ectodomain of the paramyxovirus haemagglutinin-neuraminidase (HN) glycoprotein spike can be divided into two regions: a membrane-proximal, stalk-like structure and a terminal globular domain. The latter contains all the antibody recognition sites of the protein, as well as its receptor recognition and neuraminidase (NA) active sites. These two activities of the protein can be separated by monoclonal antibody functional inhibition studies and mutations in the globular domain. Herein, we show that mutation of several conserved residues in the stalk of the Newcastle disease virus HN protein markedly decrease its NA activity without a significant effect on receptor recognition. Thus, mutations in the stalk, distant from the NA active site in the globular domain, can also separate attachment and NA. These results add to an increasing body of evidence that the NA activity of this protein is dependent on an intact stalk structure.

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Recent evidence suggests that the attachment (HN) and fusion (F) glycoproteins of Newcastle disease virus interact at the cell surface in a virus-specific manner to promote syncytium formation. Consistent with the existence of such an interaction, we have shown that it is possible to coimmunoprecipitate (co-IP) the two proteins from the surface of transiently expressing cells using a monoclonal antibody to either protein. Further, we show that a point mutation in the globular domain of HN that abolishes its receptor recognition and neuraminidase (NA) and fusion activities also abolishes its ability to interact with F in the co-IP assay. The mechanism by which this mutation might interfere with the interaction between the two proteins is discussed in terms of the postulate that recognition by HN of cellular receptors triggers its interaction with F and the apparently conflicting evidence for an interaction between the two proteins in the endoplasmic reticulum. Also, characterization of a set of chimeric HN proteins, having short overlapping sequences from a heterologous HN protein in the F-specific domain in the protein stalk, reveals that a weakened interaction between HN and F is still sufficient to trigger fusion.

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The production and functional testing of two new bispecific (bs) hybrid antibodies [Abs; bs Ab hemagglutinin-neuraminidase (HN) x CD3 and bs Ab HN x CD28] designed for cancer vaccine modification are described. They allow distinct modifications of the human tumor cell vaccine ATV-NDV, an autologous tumor cell vaccine already modified by infection with Newcastle disease virus. The bs Abs use the viral HN molecule as a common foreign anchoring molecule for attachment to the tumor cells and allow the introduction of anti-CD3 or anti-CD28 T-cell-stimulatory molecules. The bs Abs attached to tumor target cells were able to cross-link CTL effector cells and up-regulate T-cell activation markers on autologous cancer patient-derived CD4 and CD8 T lymphocytes. This strategy of combining a cellular vaccine with a bs Ab is highly specific, quick, and economical and has broad-range applications. Five ng or less of target cell-bound bs Ab HN x CD28 were effective at augmenting T-cell-mediated antitumor cytotoxicity.

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Newcastle disease virus (NDV) is primarily a respiratory tract pathogen of birds, particularly chickens, but it occasionally produces infection in man. Human parainfluenza virus type 3 (hPIV3) is a common respiratory pathogen, particularly in young children. These two viruses gain entry to host cells via direct fusion between the viral envelope and the cell membrane, mediated by the two surface glycoproteins: the hemagglutinin-neuraminidase (HN) and fusion (F) proteins. Promotion of fusion by HN and F requires that they are derived from homologous viruses. We have constructed chimeric proteins composed of domains from heterologous HN proteins. Their ability to bind cellular receptors and to complement the F protein of each virus in the promotion of fusion were evaluated in a transient expression system. The fusion specificity was found to segregate with a segment extending from the middle of the transmembrane anchor to the top of the putative stalk region of the ectodomain. All of the chimeras, in which the globular domain is derived from the NDV HN and various lengths of the stalk region are derived from the hPIV3 HN maintain receptor binding activity, but some have markedly reduced neuraminidase (NA) activity. Decrease in the NA activity of the chimeras correlates with alteration in the antigenic structure of the globular domain. This suggests that the stalk region of the HN spike is important for maintenance of the structure and function of the globular domain of the HN protein spike.

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Paramyxoviruses possess neuraminidase (NA) activity, i.e., the ability to cleave sialic acid from membrane-bound and soluble glycoconjugates. The activity is associated with a homotetrameric, surface glycoprotein spike, called the hemagglutinin-neuraminidase. This structure also mediates viral attachment to sialic acid-containing receptors and constitutes the major neutralizing antigen for the virus. Cooperativity has been demonstrated for the NA activity of an isolate of one of the paramyxoviruses, Newcastle disease virus. Although all known viral NA proteins are homooligomeric, this is the first demonstration of cooperativity in this family of proteins. A variant virus, selected by escape from neutralization by a monoclonal antibody thought to bind close to the NA active site, has lost cooperativity. Conversion to the noncooperative state correlates with increases in both avidity for cellular receptors and fusogenic activity.

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The promotion of membrane fusion by the paramyxovirus hemagglutinin-neuraminidase (HN) and fusion (F) proteins requires that they be derived from homologous viruses, suggesting the possibility that the promotion of fusion requires a virus-specific communication between the two glycoprotein spikes. We have evaluated the ability of chimeric HN proteins, composed of domains from the HN proteins of two heterologous members of the group, human parainfluenza virus 3 (hPIV3) and Newcastle disease virus (NDV), to complement the F protein of each virus in the promotion of fusion. Specificity for the F protein of hPIV3 segregates with a segment composed of the transmembrane anchor and the first 82 residues of the ectodomain of its HN protein. Specificity of NDV HN for its homologous F protein is determined by a similar domain. These findings suggest that determinants specific to this segment of the attachment protein spike may be involved in the triggering of the fusion process.

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The binding of monoclonal antibodies to antigenic site 3 on the hemagglutinin-neuraminidase (HN) glycoprotein of Newcastle disease virus neutralizes viral infectivity and prevents syncytium formation by a mechanism other than the prevention of viral attachment. The virus can escape neutralization by these antibodies by the addition of an N-glycan at a site introduced by a D287N mutation in HN. The variant has significantly reduced ability to induce fusion from within, the mode of fusion promoted by the viral glycoproteins deposited on the cell surface late in infection. Conversely, and unlike the parent virus, the variant has acquired the ability to promote fusion from without, the mode of fusion directly mediated by input virions at high multiplicity. This finding is consistent with different roles for the HN protein in virion-cell and cell-cell fusion. D287N-mutated HN with its additional N-glycan shows a markedly reduced ability, compared to wild-type HN, to complement the viral fusion protein in the promotion of fusion in a BHK cell transient expression system. This confirms that the addition of an N-glycan in HN antigenic site 3 and the deficiency in syncytium formation are causally related. Moreover, no alteration in cell surface expression, hemadsorption, or neuraminidase activity was detected in the mutated protein. This monoclonal antibody-selected mutation suggests that a fusion-related function, secondary to receptor recognition, may be defined by the globular head of the HN spike. However, D287C or D287S-mutated HN is as effective as the wild-type protein in the promotion of fusion in the coexpression system. This suggests that the diminished fusogenicity of the D287N-mutated protein is probably due to a more global effect of glycosylation in site 3 rather than an alteration at the amino acid level.

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The sequence NRKSCS constitutes the longest linear stretch in the amino acid sequence of the hemagglutinin-neuraminidase (HN) glycoprotein of the paramyxoviruses that is completely conserved among all viruses in the group. We have used site-directed mutagenesis and expression of the mutated HN protein of one member of the group, Newcastle disease virus, to explore the role of this highly conserved sequence in the structure and function of the protein. Any substitution introduced for each of four residues in the sequence, N-234, R-235, K-236, or S-237, results in a drastic decrease in neuraminidase activity relative to that of the wild-type protein. Only substitutions for the terminal serine residue in the sequence had comparatively little effect on this activity. These findings are consistent with prior computer-based predictions of protein secondary structure which had suggested that this domain corresponds to one in the beta-sheet propeller structure of the neuraminidase protein of influenza virus closest to the center of the sialic acid binding site and forms part of the enzyme active site. Four of the substitutions, N-234-->Y and K-236-->E, -->Q, and -->S, apparently cause a local alteration in the antigenic structure of the protein. This is evidenced by (i) the diminished recognition of the protein only by monoclonal antibodies thought to bind at the neuraminidase active site, among an extensive panel of conformation-specific antibodies, and (ii) the slower rate of migration in sodium dodecyl sulfate-polyacrylamide gel electrophoresis for all except the K-236-->Q mutation. One of the mutations, K-236-->S, completely abolishes the ability of the protein to promote cellular fusion when coexpressed with the fusion protein. The latter cannot be explained by a decrease in the relative hemadsorption activity of the protein and suggests that the globular head of the protein may contribute to this process beyond providing receptor recognition.

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The hemagglutinin-neuraminidase (HN) glycoprotein of paramyxoviruses is anchored in the virion membrane near its amino terminus, protruding from the virion surface to mediate attachment to cellular receptors. Solubilization of HN spikes can be achieved by treatment of virions with detergent and high salt concentrations. When the solubilized HN protein from the Australia-Victoria (AV) isolate of the virus is incubated at 37 degrees C, a chymotrypsin-sensitive site between residues 112 and 113 is exposed. A chymotrypsin-cleaved soluble form of the protein, named CT-HN, has been prepared using this approach. It is membrane anchor-less, due to removal of a 14-kDa fragment from the NH2 terminus of HN. It retains all potential glycosylation sites and cysteines present in the ectodomain of the native protein. It migrates in nonreducing gels and sediments in sucrose gradients at the rate expected for homodimeric HN. The latter is also consistent with our demonstration by site-directed mutagenesis that cysteine residues at positions 6 and 123, respectively, mediate disulfide-linked homotetramer and homodimer formation. CT-HN retains almost total antigenicity, suggesting that it is conformationally very similar to the intact molecule, as well as receptor recognition function and, at low pH, neuraminidase activity. It should prove to be a useful tool for further studies of the structure and function of this important viral glycoprotein.

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The Australia-Victoria (AV) isolate of Newcastle disease virus (NDV) induces fusion from within but not fusion from without. L1, a neuraminidase (NA)-deficient virus derived from AV, has the opposite fusion phenotype from the wild-type virus. It fails to induce the former mode of fusion, but has gained a limited ability to promote the latter. Monoclonal antibodies to antigenic site 23 on the hemagglutinin-neuraminidase (HN) glycoprotein have previously been shown to select variants of the AV isolate that have altered NA activity or receptor-binding affinity. By using an antibody to this site, variants of L1 have been selected. Three of the variants have gained an increased affinity for sialic acid-containing receptors, as evidenced by the resistance of their hemagglutinating activity to the presence of reduced amounts of sialic acid on the surface of chicken erythrocytes. All four variants still have very low levels of NA activity, comparable to that of the parent virus, L1. The alteration in receptor-binding affinity results in a decreased potential for elution from cellular receptors and correlates with an increased ability to promote both modes of fusion. A single amino acid substitution in the HN protein of each variant, responsible for its escape from neutralization, has been identified. These studies identify two HN residues, 193 and 203, at which monoclonal antibody-selected substitution influences the receptor recognition properties of NDV and may influence its ability to promote syncytium formation.

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Sequence determinations of the hemagglutinin-neuraminidase (HN) glycoproteins of a temperature-sensitive mutant of Newcastle disease virus and two sequentially selected revertants had previously shown that substitution at a pair of residues, 129 and 175, resulted in a deficiency in neuraminidase (NA) activity, which was partially restored by a third substitution at residue 193. To evaluate the role of the substitution at residue 175 in diminished NA activity, the mutation was introduced into HN and the protein expressed in COS cells. The mutated HN not only had minimal NA activity but also was unable to absorb chicken erythrocytes, even though it was transported to the cell surface in normal amounts, in an apparently antigenic form. Attachment function was restored to the protein by the introduction of the additional substitution(s) at 129 and/or 193. These results indicate that residue 175 influences not only NA activity but also receptor recognition.

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The majority of neutralizing monoclonal antibodies (MAbs) to the haemagglutinin-neuraminidase (HN) glycoprotein of Newcastle disease virus prevent attachment of the virus to cellular receptors and inhibits virion-induced fusion from without (FFWO) and fusion from within (FFWI) mediated by the virus glycoprotein-laden infected cell surface. For these antibodies, the inhibition of fusion is presumed to be the result of the prevention of HN-mediated bridging of potential fusion partners. MAbs against antigenic sites 3 and 4 neutralize virus infectivity, but by a mechanism other than the prevention of attachment, the exact nature of which remains to be established. Antibodies to both of these sites effectively inhibit virion-induced FFWO, even when the inducing virus is not infectious. This is consistent with the mechanism of neutralization of these MAbs involving the inhibition of an early, post-attachment step in infection. MAbs to site 3 also inhibit FFWI, but those to site 4 do not, even when added at high concentrations. This suggests that the requirement for HN may be different in the two modes of fusion. The epitopes recognized by MAbs to sites 3 and 4 have been delineated by the identification of individual nucleotide substitutions in the HN genes of neutralization escape variants. Some of the deduced amino acid substitutions result in additional N-linked glycosylation sites in HN, which are utilized and presumably account for the escape from neutralization.

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Monoclonal antibodies (MAbs) to the hemagglutinin-neuraminidase (HN) glycoprotein of Newcastle disease virus delineate seven overlapping antigenic sites which form a continuum on the surface of the molecule. Antibodies to five of these sites neutralize viral infectivity principally by preventing attachment of the virion to cellular receptors. Through the identification of single amino acid substitutions in variants which escape neutralization by MAbs to these five antigenic sites, a neutralization map of HN was constructed, identifying several residues that contribute to the epitopes recognized by MAbs which block the attachment function of the molecule. These epitopes are defined, at least in part, by three domains on HN: residues 193 to 201; 345 to 353 (which include the only linear epitope we have identified in HN); and a C-terminal domain composed of residues 494, 513 to 521, and 569. To identify HN residues directly involved in receptor recognition, each of the variants was tested for its ability to agglutinate periodate-modified chicken erythrocytes. One variant with a single amino acid substitution at residue 193 was 2.5- to 3-fold more resistant to periodate treatment of erythrocytes than the wild-type virus, suggesting that this residue influences the binding of virus to a sialic acid-containing receptor(s) on the cell surface.

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Monoclonal antibodies (MAbs) to three overlapping antigenic sites (designated 12, 2, and 23) on the hemagglutinin-neuraminidase glycoprotein (HN) of Newcastle disease virus (NDV) were previously shown to inhibit neuraminidase activity (NA) on neuraminlactose (R. M. Iorio and M. A. Bratt, 1984a, J. Immunol. 133, 2215-2219; R. M. Iorio et al., 1989, Virus Res. 13, 245-262). However, a competitive inhibitor of NA blocks the binding of only MAbs to site 23, suggesting that the domain they recognize may be closely related to the NA site. Antigenic variants selected with site 23 MAbs have single amino acid substitutions at HN residues 192, 193, or 200. Virions of variants, which have a substitution at residue 193 or 200, have alterations in NA which are not attributable to a commensurate change in HN content. A revertant of a temperature-sensitive mutant, which has markedly diminished NA relative to the wild type, has an amino acid substitution at residue 175. A second step revertant having partially restored NA has an additional substitution at residue 192 identical to that in one of the site 23 variants, which, in turn, also makes the revertant resistant to neutralization by site 23 MAbs. Thus, an amino acid substitution at residue 175, 193, or 200 of the HN of NDV can have marked effects on the NA of the protein. The amino acids in the region around residue 175 are highly conserved between the HNs of NDV and other paramyxoviruses, suggesting that this domain is important to the integrity of the NA site in this group of viruses.

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We have previously identified five antigenic sites on the hemagglutinin-neuraminidase (HN) glycoprotein of the Australia-Victoria isolate of Newcastle disease virus (Iorio and Bratt, J. Virol. 48, 440-450; Iorio et al., J. Gen. Virol. 67, 1393-1403). Two additional sites (designated 12 and 23) are now described, bringing to a total of seven the number of antigenic sites defined by our panel of neutralizing anti-HN antibodies. Competition antibody binding and additive neutralization assays reveal that each of these newly-identified sites overlaps two previously-defined ones. The seven HN antigenic sites thus form a continuum in the three-dimensional conformation of the molecule. Studies on the inhibition of hemagglutination (HA), neuraminidase (NA) and the attachment of virus to chick cell monolayers have been used to construct a functional profile of each antigenic site. Monoclonal antibodies (mAbs) to three overlapping sites (12, 2 and 23) inhibit HA and NA and prevent viral attachment to chick cell monolayers. These findings are consistent with the domains recognized by these mAbs being close to the NA and receptor-binding sites. MAbs to two other overlapping sites, 14 and 1 (which in turn, overlap site 12), inhibit HA quite effectively, and attachment to a lesser extent. Sites 14 and 1 probably identify a second domain involved in receptor recognition. MAbs to the two remaining sites (3 and 4), though neutralizing, are negative in all three assays, thus recognizing domains not involved in HA or NA or attachment to chick cells.

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Newcastle disease virus exhibits a wide range of pathogenicity and virulence which, as with all paramyxoviruses, is directly related to the cleavability of a precursor (F0) of the fusion glycoprotein by cellular proteases. Sequence analyses of the cleavage site of several virulent and avirulent isolates of the Newcastle disease virus serotype reveal a correlation between virulence or pathogenicity and a high content of basic amino acid residues at the cleavage site. A similar correlation has been seen for other paramyxoviruses.

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Viruses within the Newcastle disease virus (NDV) serotype induce a wide array of disease manifestations ranging from an almost apathogenic pattern to the high mortality caused by avirulent or virulent isolates, respectively. A disulfide-linked dimer form of the NDV hemagglutinin-neuraminidase (HN) glycoprotein can be demonstrated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis under nonreducing conditions for only some of these isolates. For others, indeed the majority of those we have studied, no such reducing agent-sensitive dimeric form of HN is demonstrable. Apparently, there is no causal relationship between disulfide-linked dimeric HN and virulence. Using the deduced amino acid sequence of the dimeric HN of isolate AV as a basis for selection of oligonucleotide primers, we sequenced three additional reducing agent-sensitive dimeric HN glycoproteins and eight for which a disulfide-linked dimer has not been identified, using primer extension and dideoxy sequencing. The deduced amino acid sequences reveal a strict correlation between the presence of cysteine at residue 123 and reducing agent-sensitive dimerization of HN.

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Previously, a panel of monoclonal antibodies recognizing epitopes in four antigenic sites on the haemagglutinin-neuraminidase (HN) glycoprotein of the Australia-Victoria strain of Newcastle disease virus were used in strain comparisons. Epitopes in three sites were found to be conserved while the epitope recognized by the single antibody to site 3 was not. A new panel of antibodies is described, two of which bind to epitopes in site 3 and six of which bind to a site (site 1,4) that overlaps with sites 1 and 4 as determined by analyses of variants, temperature-sensitive mutants, and strains by assays of neutralization of infectivity and binding in a radioimmunoassay. Neutralization of heterologous strains with the panel of antibodies revealed that both new site 3 epitopes are also highly divergent, while three additional epitopes outside site 3 (those in site 1,4) are highly conserved. The new site 3 antibodies can bind to virions of several heterologous strains without neutralizing infectivity. Thus, of the 10 epitopes we have now examined, all of three in site 3 are specific with respect to neutralization of infectivity for the homologous strain, while all of seven in other sites are conserved in heterologous strains. This suggests that the strain specificity originally described for a single site 3 epitope is, instead, a property of a much more extensive, poorly conserved domain on the HN molecule.

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