Parainfluenza 3 virus: plaque-type variants lacking neuraminidase activity

The Sialic Acid Binding Activity of Human Parainfluenza Virus 3 and Mumps Virus Glycoproteins Enhances the Adherence of Group B Streptococci to HEp-2 Cells

In the complex microenvironment of the human respiratory tract, different kinds of microorganisms may synergistically interact with each other resulting in viral-bacterial co-infections that are often associated with more severe diseases than the respective mono-infections. Human respiratory paramyxoviruses, for example parainfluenza virus type 3 (HPIV3), are common causes of respiratory diseases both in infants and a subset of adults. HPIV3 recognizes sialic acid (SA)-containing receptors on host cells. In contrast to human influenza viruses which have a preference for α2,6-linked sialic acid, HPIV3 preferentially recognize α2,3-linked sialic acids. Group B streptococci (GBS) are colonizers in the human respiratory tract. They contain a capsular polysaccharide with terminal sialic acid residues in an α2,3-linkage. In the present study, we report that HPIV3 can recognize the α2,3-linked sialic acids present on GBS. The interaction was evident not only by the binding of virions to GBS in a co-sedimentation assay, but also in the GBS binding to HPIV3-infected cells. While co-infection by GBS and HPIV3 had a delaying effect on the virus replication, it enhanced GBS adherence to virus-infected cells. To show that other human paramyxoviruses are also able to recognize the capsular sialic acid of GBS we demonstrate that GBS attaches in a sialic acid-dependent way to transfected BHK cells expressing the HN protein of mumps virus (MuV) on their surface. Overall, our results reveal a new type of synergism in the co-infection by respiratory pathogens, which is based on the recognition of α2,3-linked sialic acids. This interaction between human paramyxoviruses and GBS enhances the bacterial adherence to airway cells and thus may result in more severe disease.

Complete genome sequence and pathogenicity of two swine parainfluenzavirus 3 isolates from pigs in the United States

Two novel paramyxoviruses, 81-19252 (Texas81) and 92-7783 (ISU92), isolated from the brains of pigs in the United States in the 1980s and 1990s, were characterized. The complete genome of Texas81 virus was 15,456 nucleotides (nt) in length, that of ISU92 was 15,480 nt, and both genomes consisted of six nonoverlapping genes, predicted to encode nine proteins, with conserved and complementary 3' leader and 5' trailer regions and conserved gene starts, gene stops, and trinucleotide intergenic sequences similar to those in paramyxoviruses. The corresponding genes from these two viruses were similar in length, except for the F genes, of which the ISU92 form had an additional 24-nt U-rich 3' untranslated region. The P genes of swine viruses were predicted to produce V and D mRNAs by RNA editing (one to four G insertions in Texas81 and one to nine G insertions in ISU92) or C mRNA by alternative translation initiation. Sequence-specific features related to virus replication and host-specific amino acid signatures indicated that these viruses originated from bovine parainfluenzavirus 3 (bPIV3). Phylogenetic analysis of individual genes suggested that these viruses are novel members of the genus Respirovirus of the Paramyxovirinae subfamily and may be grouped into two subgenotypes of genotype A of bPIV3. Our comprehensive studies revealed that these swine PIV3 are variants of bPIV3 and were possibly transferred from cattle to pigs but failed to establish an active enzootic state. These two viruses were mildly pathogenic to conventionally reared pigs, and results from a limited enzyme-linked immunosorbent assay-based serosurvey of swine farms in Minnesota and Iowa in 2007 and 2008 were negative.

Influence of the human parainfluenza virus 3 attachment protein's neuraminidase activity on its capacity to activate the fusion protein

In order to examine functions of the hemagglutinin-neuraminidase (HN) protein that quantitatively influence fusion promotion, human parainfluenza virus 3 (HPIV3) variants with alterations in HN were studied. The variant HNs have mutations that affect either receptor binding avidity, neuraminidase activity, or fusion protein (F) activation. Neuraminidase activity was regulated by manipulation of temperature and pH. F activation was assessed by quantitating the irreversible binding of target erythrocytes (RBC) to HN/F-coexpressing cells in the presence of 4-GU-DANA (zanamivir) to release target cells bound only by HN-receptor interactions; the remaining, irreversibly bound target cells are retained via the fusion protein. In cells coexpressing wild-type (wt) or variant HNs with wt F, the fusion promotion capacity of HN was distinguished from target cell binding by measuring changes with time in the amounts of target RBC that were (i) reversibly bound by HN-receptor interaction (released only upon the addition of 4-GU-DANA), (ii) released by HN's neuraminidase, and (iii) irreversibly bound by F-insertion or fusion (F triggered). For wt HN, lowering the pH (to approach the optimum for HPIV3 neuraminidase) decreased F triggering via release of HN from its receptor. An HN variant with increased receptor binding avidity had F-triggering efficiency like that of wt HN at pH 8.0, but this efficiency was not decreased by lowering the pH to 5.7, which suggested that the variant HN's higher receptor binding activity counterbalanced the receptor dissociation promoted by increased neuraminidase activity. To dissect the specific contribution of neuraminidase to triggering, two variant HNs that are triggering-defective due to a mutation in the HN stalk were evaluated. One of these variants has, in addition, a mutation in the globular head that renders it neuraminidase dead, while the HN with the stalk mutation alone has 30% of wt neuraminidase. While the variant without neuraminidase activity triggered F effectively at 37 degrees C irrespective of pH, the variant possessing effective neuraminidase activity completely failed to activate F at pH 5.7 and was capable of only minimal triggering activity even at pH 8.0. These results demonstrate that neuraminidase activity impacts the extent of HPIV3-mediated fusion by releasing HN from contact with receptor. Any particular HN's competence to promote F-mediated fusion depends on the balance between its inherent F-triggering efficacy and its receptor-attachment regulatory functions (binding and receptor cleavage).

The bovine parainfluenza virus type-3 (BPIV-3) hemagglutinin/neuraminidase glycoprotein expressed in baculovirus protects calves against experimental BPIV-3 challenge

Despite the availability of numerous vaccine schedules, "shipping fever", an acute bronchopneumonia brought on in part by a complex of bovine respiratory viruses, remains a major source of economic loss in the beef and dairy industries. We are exploring new strategies of bovine vaccine design which we hope may provide more effective and more cost-efficient control of these pathogens. In this report, we examined the possible use of subunit vaccines, using as an example the hemagglutinin/neuraminidase (HN) protein of bovine parainfluenza virus type-3 (BPIV-3) expressed in the baculovirus expression system. We showed that the protein was expressed at high levels, and was modified to a similar, but not identical size as the native HN protein expressed from BPIV-3 infected bovine cells. We further demonstrated antigenicity and biological activity of the expressed HN protein. Finally, we vaccinated colostrum deprived sera-negative calves with the baculo HN recombinant protein and challenged with BPIV-3. Vaccination induced excellent serum neutralizing antibody responses, and surprisingly, good mucosal antibody responses, even though the vaccine was administered parenterally. The vaccinated animals were well protected against challenge.

Comparisons of the F and HN gene sequences of different strains of bovine parainfluenza virus type 3: relationship to phenotype and pathogenicity

The genes for the F and HN glycoprotein of a pathogenic field isolate of bovine parainfluenza virus type 3 (BPIV3) were isolated, converted to cDNA, and sequenced using dideoxynucleotides. The resulting nucleotide sequences were converted to protein sequence and were compared to previously sequenced glycoprotein genes with amino acid differences in the glycoproteins of isolates expressing different phenotypes. The HN glycoprotein, involved in the attachment and release of the virus, and the F glycoprotein, involved in penetration and spread of the virus, have been shown to affect pathogenicity of the virus and are the immunodominant proteins of the virus. Both the F and HN proteins have been shown to be required for syncytium formation. Our results suggest that BPIV3 viruses that exhibit greater syncytium-inducing activity in vitro have greater pathogenicity in vivo. By determining which epitopes are involved in syncytium formation and comparing the sequences and enzymatic activities of different strains of virus, it may be possible to design subunit vaccines that protect against disease.

Fusion mutants of Newcastle disease virus selected with monoclonal antibodies to the hemagglutinin-neuraminidase

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.

Fusion properties of cells infected with human parainfluenza virus type 3: receptor requirements for viral spread and virus-mediated membrane fusion

Cells can be persistently infected with human parainfluenza virus type 3 (HPF3) by using a high multiplicity of infection (MOI) (> or = 5 PFU per cell). The persistently infected cells exhibit no cytopathic effects and do not fuse with each other, yet they readily fuse with uninfected cells. We have previously shown that the failure of the persistently infected cells to fuse with each other is due to the lack of a receptor on these cells for the viral hemagglutinin-neuraminidase glycoprotein, and we have established that both fusion and hemagglutinin-neuraminidase proteins are needed for cell fusion mediated by HPF3. We then postulated that the generation of persistent infection and the failure of cells infected with HPF3 at high MOI to form syncytia are both due to the action of viral neuraminidase in the high-MOI inoculum. In this report, we describe experiments to test this hypothesis and further investigate the receptor requirements for HPF3 infection and cell fusion. A normally cytopathic low-MOI HPF3 infection can be converted into a noncytopathic infection by the addition of exogenous neuraminidase, either in the form of a purified enzyme or as UV-inactivated HPF3 virions. Evidence is presented that the receptor requirements for an HPF3 virus particle to infect a cell are different from those for fusion between cells. By treating infected cells in culture with various doses of neuraminidase, we demonstrate that virus spreads from cell to cell in the complete absence of cell-cell fusion. We compare the outcome of HPF3 infection in the presence of excess neuraminidase with that of another paramyxovirus (simian virus 5) and provide evidence that these two viruses differ in their receptor requirements for mediating fusion.

Fusion properties of cells persistently infected with human parainfluenza virus type 3: participation of hemagglutinin-neuraminidase in membrane fusion

Cells persistently infected with human parainfluenza virus type 3 (HPF3) exhibit a novel phenotype. They are completely resistant to fusion with each other but readily fuse with uninfected cells. We demonstrate that the inability of these cells to fuse with each other is due to a lack of cell surface neuraminic acid. Neuraminic acid is the receptor for the HPF3 hemagglutinin-neuraminidase (HN) glycoprotein, the molecule responsible for binding of the virus to cell surfaces. Uninfected CV-1 cells were treated with neuraminidase and then tested for their ability to fuse with the persistently infected (pi) cells. Neuraminidase treatment totally abolished cell fusion. To extend this result, we used a cell line deficient in sialic acid and demonstrated that these cells, like the neuraminidase-treated CV-1 cells, were unable to fuse with pi cells. We then tested whether mimicking the agglutinating function of the HN molecule with lectins would result in cell fusion. We added a panel of five lectins to the neuraminic acid-deficient cells and showed that binding of these cells to the pi cells did not result in fusion; the lectins could not substitute for interaction of neuraminic acid with the HN molecule in promoting membrane fusion. These results provide compelling evidence that the HN molecule of HPF3 and its interaction with neuraminic acid participate in membrane fusion and that cell fusion is mediated by an interaction more complex than mere juxtaposition of the cell membranes.

Syncytium formation by recombinant vaccinia viruses carrying bovine parainfluenza 3 virus envelope protein genes

The highly syncytium-inducing M strain and the weakly syncytium-inducing SC strain of bovine parainfluenza 3 virus differ by a single amino acid substitution in each of the hemagglutinin-neuraminidase (HN) and membrane (M) proteins, while their fusion (F) proteins are identical (T. Shioda, S. Wakao, S. Suzu, and H. Shibuta, Virology 162:388-396, 1988). We constructed recombinant vaccinia viruses which express separately the M virus HN (Vac-MHN), SC virus HN (Vac-SCHN), M virus M (Vac-MM), SC virus M (Vac-SCM), and common F (Vac-F) proteins. CV-1 cells were infected with the recombinants, singly or in combination, and implanted onto indicator MDBK cells for syncytium formation. Combinations of Vac-MHN plus Vac-F and Vac-SCHN plus Vac-F induced extensive and weak syncytium formation, respectively. Vac-F alone did not induce syncytium formation, and both Vac-MM and Vac-SCM had no effect on syncytium formation. These findings indicated that the syncytium formation by bovine parainfluenza 3 virus requires both the F and HN proteins and that the extensive syncytium formation by the M virus is due to the M virus HN protein. MSC, another weakly syncytium-inducing virus variant, newly isolated from the M virus, was identical to the M virus in the primary structure of the HN and M proteins but differed from the M virus by a single amino acid residue in the F protein. The combination of the recombinant vaccinia virus expressing the MSC virus F protein and Vac-MHN resulted in weak syncytium formation.

Differences in bovine parainfluenza 3 virus variants studied by sequencing of the genes of viral envelope proteins

By determining gene nucleotide sequences we compared the primary structures of the membrane (M), fusion (F), and hemagglutinin-neuraminidase (HN) proteins of bovine parainfluenza 3 virus strains, M, SC, and MR which are substrains derived from a wild strain YN. The M and SC viruses are indistinguishable in having very weak hemagglutination (HA) and neuraminidase (NA) activities, but M virus' syncytium-inducing (SI) activity is considerably higher than that of the SC virus. However, the results showed that the amino acid sequence of the F protein was identical in M and SC viruses, demonstrating that M virus' high SI activity was not due to alteration of its F protein. Two differences in M and SC viruses' other proteins then seemed to be important, although their significance in the SI activity is not clear at present; the first being the 70th amino acid residue of the M protein, which was Asp in the M virus and Gly in the SC virus, and the other being the 539th residue of the HN protein, which was Tyr in the M virus and His in the SC virus. The nucleocapsid proteins of both M and SC viruses were identical. The MR virus, which is a variant derived from the M virus and has high HA and NA activities but very weak SI activity, was different from the M virus at only one site throughout the M, F, and HN proteins; the 193rd amino acid residue of the HN protein was Leu in the MR virus and Phe in the M virus. This result strongly suggested that the substitution of Leu with Phe at this particular site was closely linked to the drastic reduction in both HA and NA activities.

Nucleotide sequence of the bovine parainfluenza 3 virus genome: its 3' end and the genes of NP, P, C and M proteins

We present the nucleotide sequence of bovine parainfluenza 3 virus (BPIV3) genome from its 3' end to the opening region of the F gene, through the NP, P plus C, and M genes. Comparison of the sequence with those reported for other paramyxoviruses indicated that BPIV3 was most similar to human parainfluenza 3 virus (HPIV3), and also very similar to Sendai virus in the structural make-up of its genome and the amino acid sequences of its gene products, suggesting that these three viruses constitute a paramyxovirus subgroup from which Newcastle disease and measles viruses are separable. In BPIV3 and Sendai virus, the NP and M proteins, the main structural elements, were more highly conserved than the functionally important P and C proteins. This tendency was also observed even in BPIV3 and HPIV3. Virus-specific amino acid sequences of the NP and M proteins were found at the carboxyl and amino terminal regions, respectively. BPIV3 M mRNA was found to have aberrations in its poly A attachment site.

Nucleotide sequence of the bovine parainfluenza 3 virus genome: the genes of the F and HN glycoproteins

By analysing complementary DNA clones constructed from genomic RNA of bovine parainfluenza 3 virus (BPIV3), we determined the nucleotide sequence of the region containing the entire F and HN genes. Their deduced amino acid sequences showed about 80% homologies with those of human parainfluenza 3 virus (HPIV3), about 45% with those of Sendai virus, and about 20% with those of SV5 and Newcastle disease virus (NDV), indicating, together with the results described in the preceding paper on the NP, P, C and M proteins of BPIV3, that BPIV3, HPIV3 and Sendai virus constitute a paramyxovirus subgroup, and that BPIV3 and HPIV3 are very closely related. The F and HN proteins of all these viruses, including SV5 and NDV, however, were shown to have protein-specific structures as well as short but well-conserved amino acid sequences, suggesting that these structures and sequences are related to the activities of these glycoproteins.

Differences in bovine parainfluenza 3 virus variants studied by monoclonal antibodies against viral glycoproteins

We previously showed that of three bovine parainfluenza 3 virus strains the M strain, which is neurovirulent for young mice, has an extensive syncytium-inducing activity, whereas avirulent SC and 910N strains are weak in this activity. It was also demonstrated that both M and SC strains have very low hemagglutination and neuraminidase activities, while the 910N strain shows these activities to high levels. In the present study, monoclonal antibodies (Mabs) were raised against the glycoproteins of the 910N strain, and utilized to further characterize these three viral strains. Five Mabs against the hemagglutinin-neuraminidase protein, which were classified into four different epitope-recognizing groups, neutralized the M strain much more effectively than the 910N and SC strains, while the Mabs showed lower hemagglutination inhibition (HI) titers against the M and SC strains than the 910N strain. Three Mabs against the fusion protein neutralized the M strain but not the 910N and SC strains, while they showed no HI activity against any of these strains. These findings suggested that the M strain is considerably different from other strains in the structure of the viral envelope proteins.

Experimental parainfluenza virus infection in mice: growth and spread of a highly pathogenic variant of parainfluenza 3 virus in the mouse brain

We had previously showed that following intracerebral inoculation of newborn mice, the 910 N and M strains of bovine parainfluenza 3 virus induce a non-lethal hydrocephalus and a lethal disease with marked thymic and splenic atrophy, respectively. Moreover, only the M virus was lethal for 2-week-old mice. In the present study, we demonstrate that the M virus multiplies and spreads in the mouse brain invading the thalamus, hypothalamus and brain stem beyond the ependyma whereas the 910 N virus causes only slight ependymitis. This growth and spread of M virus was blocked by passive immunization 3 days after infection. Mouse embryo brain cell cultures were infected with M and 910 N viruses, about 50 per cent became antigen-positive for M whereas only a small proportion of cells were positive for the 910 N virus. However, the latter did produce higher yeilds of infectious virus than M.

Structural characterization of virion proteins and genomic RNA of human parainfluenza virus 3

The virion proteins and genomic RNA of human parainfluenza virus 3 have been characterized. The virion contains seven major and two minor proteins. Three proteins of 195 X 10(3) molecular weight (195K), 87K, and 67K are associated with the nucleocapsid of the virion and have been designated L, P, and NP, respectively. Three proteins can be labeled with [14C]glucosamine and have molecular weights of 69K, 60K, and 46K. We have designated these proteins as HN, F0, and F1, respectively. HN protein has interchain disulfide bonds, but does not participate in disulfide bonding to form homomultimeric forms. F1 appears to be derived from a complex, F1,2, that has an electrophoretic mobility similar to that of F0 under nonreducing conditions. A protein of 35K is associated with the envelope components of the virion and aggregates under low-salt conditions; this protein has been designated M. The genome of human parainfluenza virus 3 is a linear RNA molecule with a molecular weight of approximately 4.6 X 10(6).

Neuraminidase activity and syncytial formation in variants of parainfluenza 3 virus

By a sensitive fluorometric assay method, we could definitely demonstrate neuraminidase activity for two variants of parainfluenza 3 virus, M and SC, which were previously shown to have no detectable neuraminidase activity. The enzyme activities of these viruses were very similar to each other, showing a much lower catalytic rate, a much higher Km value, and a more acidic pH optimum than those of the virus variants of high neuraminidase activity, 910N, LT, and MR. M and SC viruses eluted from guinea pig erythrocytes very poorly, whereas 910N and LT viruses eluted readily. M virus required the aid of a bacterial neuraminidase for effective growth and plaque formation in MDBK cells, but the virus grew well and formed plaques in R66 and Vero cells without the enzyme. SC virus required no exogenous neuraminidase for growth in all of these cell types. Depending on cell type, SC virus induced slight to extensive syncytial formation which was greatly inhibited by exogenous neuraminidase. In contrast, M virus induced extensive syncytial formation in all these cells regardless of the presence or absence of exogenous neuraminidase, although development and disintegration of the syncytia were more or less retarded by the enzyme, especially in MDBK cells. These results indicate that M virus possesses highly potent inducibility of syncytial formation which is further fortified by being low in viral neuraminidase activity.

Experimental parainfluenzavirus infection in mice: fatal illness with atrophy of thymus and spleen in mice caused by a variant of parainfluenza 3 virus

Intracerebral inoculation of the YN strain of parainfluenza 3 virus was found to induce an acute fatal illness characterized by marked thymic and splenic atrophy in newborn mice. Previously we showed that the YN strain contains three distinct plaque-type variants, LT, SC, and M. Of these, the M-type variant induced this fatal illness, whereas the other two variants induced hydrocephalus.