Virus-receptor interactions of human parainfluenza viruses types 1, 2 and 3

Bacterial sialoglycosidases in Virulence and Pathogenesis

Human oral microbiome and dysbiotic infections have been recently evidently identified. One of the major reasons for such dysbiosis is impairment of the immune system. Periodontitis is a chronic inflammatory disease affecting the tissues that surround and support the teeth. In the United States., approximately 65 million people are affected by this condition. Its occurrence is also associated with many important systemic diseases such as cardiovascular disease, rheumatoid arthritis, and Alzheimer’s disease. Among the most important etiologies of periodontitis is Porphyromonas gingivalis, a keystone bacterial pathogen. Keystone pathogens can orchestrate inflammatory disease by remodeling a normally benign microbiota causing imbalance between normal and pathogenic microbiota (dysbiosis). The important characteristics of P. gingivalis causing dysbiosis are its virulence factors which cause effective subversion of host defenses to its advantage allowing other pathogens to grow. Some of the mechanisms involved in these processes are still not well-understood. However, various microbial strategies target host sialoglycoproteins for immune dysregulation. In addition, the enzymes that break down sialoglycoproteins and sialoglycans are the “sialoglycoproteases”, resulting in exposed terminal sialic acid. This process could lead to pathogen-toll like receptor (TLR) interactions mediated through sialic acid receptor ligand mechanisms. Assessing the function of P. gingivalis sialoglycoproteases, could pave the way to designing carbohydrate analogues and sialic acid mimetics to serve as drug targets.

Claudin-1 inhibits human parainfluenza virus type 2 dissemination

Tight junctions enable epithelial cells to form physical barriers that act as an innate immune defense against respiratory infection. However, the involvement of tight junction molecules in paramyxovirus infections, which include various respiratory pathogens, has not been examined in detail. Human parainfluenza virus type 2 (hPIV2) infects airway epithelial cells and causes respiratory illness. In the present study, we found that hPIV2 infection of cultured cells induces expression of claudin-1 (CLDN1), an essential component of tight junctions. This induction seemed to be intrinsically restricted by V, an accessory protein that modulates various host responses, to enable efficient virus propagation. By generating CLDN1 over-expressing and knockout cell lines, we showed that CLDN1 is involved in the restriction of hPIV2 spread via cell-to-cell contact. Taken together, we identified CLDN1 an inhibitory factor for hPIV2 dissemination, and that its V protein acts to counter this.

The Role of Human Parainfluenza Virus Infections in the Immunopathology of the Respiratory Tract

Viral infections are leading causes of both upper and lower airway acute illness in all age groups of healthy persons, and have also been implicated in the acute exacerbations of chronic respiratory disorders like asthma and COPD. Human rhinovirus, respiratory syncytial virus, influenza virus and coronavirus have been considered as the most important respiratory pathogens and relatively little attention has been paid to the role of parainfluenza viruses (hPIVs). Human parainfluenza viruses are single-stranded RNA viruses belonging to the paramyxovirus family that may evoke lower respiratory infections in infants, children and immunocompromised individuals. Among non-immune compromised adults, hPIV infection typically causes mild disease manifested as upper respiratory tract symptoms and is infrequently associated with severe croup or pneumonia. Moreover, hPIV infection may be associated with viral exacerbations of chronic airway diseases, asthma or COPD or chronic rhinosinusitis. In this review, we summarized the basic epidemiology and immunology of hPIVs and addressed the more recent data implicating the role of parainfluenza viruses in the exacerbation of chronic airway disorders.

Parainfluenza virus type 1 induces epithelial IL-8 production via p38-MAPK signalling

Human parainfluenza virus type 1 (HPIV-1) is the most common cause of croup in infants. The aim of this study was to describe molecular mechanisms associated with IL-8 production during HPIV-1 infection and the role of viral replication in MAPK synthesis and activation. An in vitro model of HPIV-1 infection in the HEp-2 and A549 cell lines was used; a kinetic-based ELISA for IL-8 detection was also used, phosphorylation of the mitogen-activated protein kinases (MAPKs) was identified by Western blot analysis, and specific inhibitors for each kinase were used to identify which MAPK was involved. Inactivated viruses were used to assess whether viral replication is required for IL-8 production. Results revealed a gradual increase in IL-8 production at different selected times, when phosphorylation of MAPK was detected. The secretion of IL-8 in the two cell lines infected with the HPIV-1 is related to the phosphorylation of the MAPK as well as viral replication. Inhibition of p38 suppressed the secretion of IL-8 in the HEp-2 cells. No kinase activation was observed when viruses were inactivated.

Inhibition of primary clinical isolates of human parainfluenza virus by DAS181 in cell culture and in a cotton rat model

DAS181 is a novel drug in development for the treatment of influenza as well as human parainfluenza viruses (hPIVs). Previous studies demonstrated that DAS181 inhibited laboratory strains of hPIV, but no tests were conducted with primary clinical isolates of hPIV. To fill this gap, we studied six primary isolates including hPIV-2 and hPIV-3. First tests showed that the amplification of all viruses in vitro was reproducibly inhibited with DAS181 drug concentrations ranging between 0.1 and 1nM. An hPIV-3 primary clinical isolate was then tested in a cotton rat model for sensitivity to 0.3-1mg/kg drug treatments. Results showed that virus amplification in the lower respiratory tract was significantly and reproducibly inhibited by drug. Together, experiments demonstrated that DAS181 inhibited primary clinical isolates of hPIV in vitro and in vivo at doses similar to those previously described for inhibition of laboratory hPIV and influenza virus isolates.

α2-3- and α2-6- N-linked sialic acids allow efficient interaction of Newcastle Disease Virus with target cells

Receptor recognition and binding is the first step in the viral cycle. It has been established that Newcastle Disease Virus (NDV) interacts with sialylated molecules such as gangliosides and glycoproteins at the cell surface. Nevertheless, the specific receptor(s) that mediate virus entry are not well known. We have analysed the role of the sialic acid linkage in the early steps of the viral infection cycle. Pretreatment of ELL-0 cells with both α2,3 and α2,6 specific sialidases led to the inhibition of NDV binding, fusion and infectivity, which were restored after α2,3(N)- and α2,6(N)-sialyltransferase incubation. Moreover, α2,6(N)-sialyltransferases also restored NDV activities in α2-6-linked sialic acid deficient cells. Competition with α2-6 sialic acid-binding lectins led to a reduction in the three NDV activities (binding, fusion and infectivity) suggesting a role for α2-6- linked sialic acid in NDV entry. We conclude that both α2-3- and α2-6- linked sialic acid containing glycoconjugates may be used for NDV infection. NDV was able to efficiently bind, fuse and infect the ganglioside-deficient cell line GM95 to a similar extent to that of its parental MEB4, suggesting that gangliosides are not essential for NDV binding, fusion and infectivity. Nevertheless, the fact that the interaction of NDV with cells deficient in N-glycoprotein expression such as Lec1 was less efficient prompted us to conclude that NDV requires N-linked glycoproteins for efficient attachment and entry into the host cell.

Fixation of oligosaccharides to a surface may increase the susceptibility to human parainfluenza virus 1, 2, or 3 hemagglutinin-neuraminidase

The hemagglutinin-neuraminidase (HN) protein of human parainfluenza viruses (hPIVs) both binds (H) and cleaves (N) oligosaccharides that contain N-acetylneuraminic acid (Neu5Ac). H is thought to correspond to receptor binding and N to receptor-destroying activity. At present, N's role in infection remains unclear: does it destroy only receptors, or are there other targets? We previously demonstrated that hPIV1 and 3 HNs bind to oligosaccharides containing the motif Neu5Acα2-3Galβ1-4GlcNAc (M. Amonsen, D. F. Smith, R. D. Cummings, and G. M. Air, J. Virol. 81:8341-8345, 2007). In the present study, we tested the binding specificity of hPIV2 on the Consortium for Functional Glycomics' glycan array and found that hPIV2 binds to oligosaccharides containing the same motif. We determined the specificities of N on red blood cells, soluble small-molecule and glycoprotein substrates, and the glycan array and compared them to the specificities of H. hPIV2 and -3, but not hPIV1, cleaved their ligands on red blood cells. hPIV1, -2, and -3 cleaved their NeuAcα2-3 ligands on the glycan array; hPIV2 and -3 also cleaved NeuAcα2-6 ligands bound by influenza A virus. While all three HNs exhibited similar affinities for all cleavable soluble substrates, their activities were 5- to 10-fold higher on small molecules than on glycoproteins. In addition, some soluble glycoproteins were not cleaved, despite containing oligosaccharides that were cleaved on the glycan array. We conclude that the susceptibility of an oligosaccharide substrate to N increases when the substrate is fixed to a surface. These findings suggest that HN may undergo a conformational change that activates N upon receptor binding at a cell surface.

Antiviral strategies for pandemic and seasonal influenza

While vaccines are the primary public health response to seasonal and pandemic flu, short of a universal vaccine there are inherent limitations to this approach. Antiviral drugs provide valuable alternative options for treatment and prophylaxis of influenza. Here, we will review drugs and drug candidates against influenza with an emphasis on the recent progress of a host-targeting entry-blocker drug candidate, DAS181, a sialidase fusion protein.

A recombinant sialidase fusion protein effectively inhibits human parainfluenza viral infection in vitro and in vivo

BACKGROUND

The first step in infection by human parainfluenza viruses (HPIVs) is binding to the surface of respiratory epithelial cells via interaction between viral receptor-binding molecules and sialic acid-containing receptors. DAS181, a recombinant sialidase protein containing the catalytic domain of Actinomyces viscosus sialidase, removes cell surface sialic acid, and we proposed that it would inhibit HPIV infection.

METHODS

Depletion of sialic acid receptors by DAS181 was evaluated by lectin-binding assays. Anti-HPIV activity in cultured cell lines and in human airway epithelium was assessed by the reduction in viral genomes and/or plaque forming units on treatment. In vivo efficacy of intranasally administered DAS181 was assessed using a cotton rat model.

RESULTS

DAS181-mediated desialylation led to anti-HPIV activity in cell lines and human airway epithelium. Intranasal DAS181 in cotton rats, a model for human disease, significantly curtailed infection.

CONCLUSIONS

Enzymatic removal of the sialic acid moiety of HPIV receptors inhibits infection with all tested HPIV strains, both in vitro and in cotton rats. Enzyme-mediated removal of sialic acid receptors represents a novel antiviral strategy for HPIV. The results of this study raise the possibility of a broad spectrum antiviral agent for influenza virus and HPIVs.

Sialidase-based anti-influenza virus therapy protects against secondary pneumococcal infection

BACKGROUND

DAS181 (Fludase) is a sialidase fusion protein in clinical development as a broad-spectrum anti-influenza virus (IFV) therapeutic agent. Previous reports by other investigators have raised the concern that desialylation of airway epithelium might increase susceptibility to Streptococcus pneumoniae infection.

METHODS

To address whether DAS181 would lead to an increased risk of pneumococcal infection, we tested S. pneumoniae colonization after DAS181 treatment of human A549 cells, healthy mice, and mice challenged with a lethal dose of IFV A/PR/8/34 (H1N1) or A/Victoria/3/75 (H3N2), followed by 10(4) cfu of S. pneumoniae (D39) on day 3 or day 7. DAS181 treatment was given 24-48 h after IFV challenge.

RESULTS

DAS181 treatment did not increase S. pneumoniae colonization in vitro or in vivo in healthy animals. In IFV-infected mice, DAS181 prevented pneumonia and significantly prolonged survival and inhibited the IFV titer by > or = 3 logs. None of the treated animals showed enhanced S. pneumoniae colonization of the lung. In addition, opportunistic infections with Citrobacter species or Klebsiella species occurred only in mice receiving vehicle, not in animals treated with DAS181.

CONCLUSIONS

These data indicate that DAS181 treatment does not exacerbate secondary bacterial infection in mice. DAS181 may reduce the risk of secondary bacterial infection by inhibiting IFV.

Human parainfluenza virus infection of the airway epithelium: viral hemagglutinin-neuraminidase regulates fusion protein activation and modulates infectivity

Three discrete activities of the paramyxovirus hemagglutinin-neuraminidase (HN) protein, receptor binding, receptor cleaving (neuraminidase), and triggering of the fusion protein, each affect the promotion of viral fusion and entry. For human parainfluenza virus type 3 (HPIV3), the effects of specific mutations that alter these functions of the receptor-binding protein have been well characterized using cultured monolayer cells, which have identified steps that are potentially relevant to pathogenesis. In the present study, proposed mechanisms that are relevant to pathogenesis were tested in natural host cell cultures, a model of the human airway epithelium (HAE) in which primary HAE cells are cultured at an air-liquid interface and retain functional properties. Infection of HAE cells with wild-type HPIV3 and variant viruses closely reflects that seen in an animal model, the cotton rat, suggesting that HAE cells provide an ideal system for assessing the interplay of host cell and viral factors in pathogenesis and for screening for inhibitory molecules that would be effective in vivo. Both HN's receptor avidity and the function and timing of F activation by HN require a critical balance for the establishment of ongoing infection in the HAE, and these HN functions independently modulate the production of active virions. Alterations in HN's F-triggering function lead to the release of noninfectious viral particles and a failure of the virus to spread. The finding that the dysregulation of F triggering prohibits successful infection in HAE cells suggests that antiviral strategies targeted to HN's F-triggering activity may have promise in vivo.

Effects of hemagglutinin-neuraminidase protein mutations on cell-cell fusion mediated by human parainfluenza type 2 virus

The monoclonal antibody M1-1A, specific for the hemagglutinin-neuraminidase (HN) protein of human parainfluenza type 2 virus (HPIV2), blocks virus-induced cell-cell fusion without affecting the hemagglutinating and neuraminidase activities. F13 is a neutralization escape variant selected with M1-1A and contains amino acid mutations N83Y and M186I in the HN protein, with no mutation in the fusion protein. Intriguingly, F13 exhibits reduced ability to induce cell-cell fusion despite its multistep replication. To investigate the potential role of HPIV2 HN protein in the regulation of cell-cell fusion, we introduced these mutations individually or in combination to the HN protein in the context of recombinant HPIV2. Following infection at a low multiplicity, Vero cells infected with the mutant virus H-83/186, which carried both the N83Y and M186I mutations, remained as nonfused single cells at least for 24 h, whereas most of the cells infected with wild-type virus mediated prominent cell-cell fusion within 24 h. On the other hand, the cells infected with the mutant virus, carrying either the H-83 or H-186 mutation, mediated cell-cell fusion but less efficiently than those infected with wild-type virus. Irrespective of the ability to cause cell-cell fusion, however, every virus could infect all the cells in the culture within 48 h after the initial infection. These results indicated that both the N83Y and M186I mutations in the HN protein are involved in the regulation of cell-cell fusion. Notably, the limited cell-cell fusion by H-83/186 virus was greatly promoted by lysophosphatidic acid, a stimulator of the Ras and Rho family GTPases.

Human parainfluenza viruses hPIV1 and hPIV3 bind oligosaccharides with alpha2-3-linked sialic acids that are distinct from those bound by H5 avian influenza virus hemagglutinin

We investigated the binding of human parainfluenza virus types 1 and 3 (hPIV1 and hPIV3, respectively) to the glycan array of the Consortium for Functional Glycomics and binding and their release from erythrocytes under conditions where neuraminidase is inactive or active. hPIV1 and hPIV3 bind modifications of Neu5Acalpha2-3Galbeta1-4GlcNAc, including the sialyl-Lewis(x) motif and structures containing 6-sulfogalactose. hPIV1 and hPIV3 thus bind typical N-linked glycans, in contrast to avian influenza virus H5 hemagglutinin (J. Stevens, O. Blixt, T. M. Tumpey, J. K. Taubenberger, J. C. Paulson, and I. A. Wilson, Science 312:404-410, 2006), which binds less-common motifs. While the receptor is not the sole determinant of tropism, hPIV or H5 influenza virus infection of specific cells that express receptors may contribute to their different pathologies.

Human parainfluenza virus 3 neuraminidase activity contributes to dendritic cell maturation

Mechanisms of dendritic cells (DCs) immunomodulation by parainfluenza viruses have not been characterized. We analyzed whether the human parainfluenza 3 (HPF3) virus hemagglutinin-neuraminidase glycoprotein (HN) might influence DC maturation. HN possesses a receptor binding function and a neuraminidase or desialidating activity. To assess whether the neuraminidase activity of HN affects DC maturation, human myeloid DCs were exposed to either live or UV-inactivated HPF3 viruses containing wild type or a mutated form of HN with decreased neuraminidase activity. Exposure of human DCs to either UV-inactivated or live virus induced up-regulation of CD83 and CD86 surface markers, morphological changes, and a cytokine expression pattern consistent with maturation. However, the level of maturation was found to be lower in DCs infected with the neuraminidase deficient variant as compared to the wild type. These results suggest that during the course of viral infection, HN's neuraminidase activity may play an important role contributing to maturation and activation of DCs.

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).

Role of nucleolin in human parainfluenza virus type 3 infection of human lung epithelial cells

Human parainfluenza virus type 3 (HPIV-3) is an airborne pathogen that infects human lung epithelial cells from the apical (luminal) plasma membrane domain. In the present study, we have identified cell surface-expressed nucleolin as a cellular cofactor required for the efficient cellular entry of HPIV-3 into human lung epithelial A549 cells. Nucleolin was enriched on the apical cell surface domain of A549 cells, and HPIV-3 interacted with nucleolin during entry. The importance of nucleolin during HPIV-3 replication was borne out by the observation that HPIV-3 replication was significantly inhibited following (i). pretreatment of cells with antinucleolin antibodies and (ii). preincubation of HPIV-3 with purified nucleolin prior to its addition to the cells. Moreover, HPIV-3 cellular internalization and attachment assays performed in the presence of antinucleolin antibodies and purified nucleolin revealed the requirement of nucleolin during HPIV-3 internalization but not during attachment. Thus, these results suggest that nucleolin expressed on the surfaces of human lung epithelial A549 cells plays an important role during HPIV-3 cellular entry.

Parainfluenza viruses

Human parainfluenza viruses (HPIV) were first discovered in the late 1950s. Over the last decade, considerable knowledge about their molecular structure and function has been accumulated. This has led to significant changes in both the nomenclature and taxonomic relationships of these viruses. HPIV is genetically and antigenically divided into types 1 to 4. Further major subtypes of HPIV-4 (A and B) and subgroups/genotypes of HPIV-1 and HPIV-3 have been described. HPIV-1 to HPIV-3 are major causes of lower respiratory infections in infants, young children, the immunocompromised, the chronically ill, and the elderly. Each subtype can cause somewhat unique clinical diseases in different hosts. HPIV are enveloped and of medium size (150 to 250 nm), and their RNA genome is in the negative sense. These viruses belong to the Paramyxoviridae family, one of the largest and most rapidly growing groups of viruses causing significant human and veterinary disease. HPIV are closely related to recently discovered megamyxoviruses (Hendra and Nipah viruses) and metapneumovirus.

Role of heparan sulfate in human parainfluenza virus type 3 infection

Our current studies have demonstrated that human parainfluenza virus type 3 (HPIV-3) utilizes heparan sulfate (HS) for its efficient cellular entry. HPIV-3 interacted with HS-agarose in vitro and the cellular entry and infection of HPIV-3 were reduced following (a) infection of human epithelial lung A549 cells with HPIV-3 pre-incubated with soluble HS; (b) treatment of A549 cells with heparinase to remove cell surface HS and sodium chlorate (NaClO(3)), a potent inhibitor of proteoglycan sulfation; and (c) infection of HS-deficient mutant CHO cell lines. However, in each instance, complete inhibition of HPIV-3 entry did not occur, suggesting the presence of additional nonproteoglycan cell surface molecule(s) that is required for HPIV-3 entry. Thus the cell surface HS appears to play an important role in efficient cellular entry of HPIV-3.

Expression and characterization of soluble human parainfluenza virus type 1 hemagglutinin-neuraminidase glycoprotein

Human parainfluenza virus types 1 (hPIV-1), 2, and 3 represent significant respiratory pathogens for which no antiviral treatment is currently available. To characterize the biochemical functions of the hPIV-1 hemagglutinin-neuraminidase (HN) glycoprotein, a potential target for antiviral therapy, we cloned and expressed a soluble portion of hPIV-1 HN (amino acid residues 137-575), lacking the N-terminal hydrophobic membrane anchorage region, in insect cells using the baculovirus secretion expression system. The expressed HN protein was purified through cation-exchange chromatography followed by metal affinity chromatography, using the 6xHis epitope introduced at the carboxyl terminus of the recombinant protein. N-terminal amino acid sequence analysis of purified HN indicated that the honeybee melittin secretion signal peptide was correctly removed during post-translational processing. Further characterization revealed that the purified HN protein was N-glycosylated and exhibited neuraminidase activity whose characteristics resembled those of the native HN protein of hPIV-1 virions. The establishment of this expression and purification system has allowed us to further explore the biochemical characteristics of paramyxovirus HN and to obtain material that could be suitable for X-ray crystallography studies.

The V protein of human parainfluenza virus 2 antagonizes type I interferon responses by destabilizing signal transducer and activator of transcription 2

Type I interferon (IFN) induces antiviral responses through the activation of the ISGF3 transcription factor complex that contains the subunit proteins STAT1, STAT2, and p48/ISGF3 gamma/IRF9. The ability of some human paramyxoviruses to overcome IFN actions by specific proteolysis of STAT proteins has been examined. Infection of cells with type 2, but not type 1 or type 3 human parainfluenza virus (HPIV) leads to a loss of cellular STAT2 protein. Expression of a single HPIV2 protein derived from the V open reading frame blocks IFN-dependent transcriptional responses in the absence of other viral proteins. The loss of IFN response is due to V-protein-induced proteolytic degradation of STAT2. Expression of HPIV2 V causes the normally stable STAT2 protein to be rapidly degraded, and this proteolytic activity can be partially alleviated by proteasome inhibition. No V-protein-specific effects on STAT2 mRNA levels were observed. The results indicate that the V protein of HPIV2 is sufficient to recognize and target a specific cellular transcription factor for destruction by cellular machinery.

Receptor specificities of human respiroviruses

Through their hemagglutinin-neuraminidase glycoprotein, parainfluenza viruses bind to sialic acid-containing glycoconjugates to initiate infection. Although the virus-receptor interaction is a key factor of infection, the exact nature of the receptors that human parainfluenza viruses recognize has not been determined. We evaluated the abilities of human parainfluenza virus types 1 (hPIV-1) and 3 (hPIV-3) to bind to different types of gangliosides. Both hPIV-1 and hPIV-3 preferentially bound to neolacto-series gangliosides containing a terminal N-acetylneuraminic acid (NeuAc) linked to N-acetyllactosamine (Galbeta1-4GlcNAc) by the alpha2-3 linkage (NeuAcalpha2-3Galbeta1-4GlcNAc). Unlike hPIV-1, hPIV-3 bound to gangliosides with a terminal NeuAc linked to Galbeta1-4GlcNAc through an alpha2-6 linkage (NeuAcalpha2-6Galbeta1-4GlcNAc) or to gangliosides with a different sialic acid, N-glycolylneuraminic acid (NeuGc), linked to Galbeta1-4GlcNAc (NeuGcalpha2-3Galbeta1-4GlcNAc). These results indicate that the molecular species of glycoconjugate that hPIV-1 recognizes are more limited than those recognized by hPIV-3. Further analysis using purified gangliosides revealed that the oligosaccharide core structure is also an important element for binding. Gangliosides that contain branched N-acetyllactosaminoglycans in their core structure showed higher avidity than those without them. Agglutination of human, cow, and guinea pig erythrocytes but not equine erythrocytes by hPIV-1 and hPIV-3 correlated well with the presence or the absence of sialic acid-linked branched N-acetyllactosaminoglycans on the cell surface. Finally, NeuAcalpha2-3I, which bound to both viruses, inhibited virus infection of Lewis lung carcinoma-monkey kidney cells in a dose-dependent manner. We conclude that hPIV-1 and hPIV-3 preferentially recognize oligosaccharides containing branched N-acetyllactosaminoglycans with terminal NeuAcalpha2-3Gal as receptors and that hPIV-3 also recognizes NeuAcalpha2-6Gal- or NeuGcalpha2-3Gal-containing receptors. These findings provide important information that can be used to develop inhibitors that prevent human parainfluenza virus infection.

Mechanism of interference mediated by human parainfluenza virus type 3 infection

Viral interference is characterized by the resistance of infected cells to infection by a challenge virus. Mechanisms of viral interference have not been characterized for human parainfluenza virus type 3 (HPF3), and the possible role of the neuraminidase (receptor-destroying) enzyme of the hemagglutinin-neuraminidase (HN) glycoprotein has not been assessed. To determine whether continual HN expression results in depletion of the viral receptors and thus prevents entry and cell fusion, we tested whether cells expressing wild-type HPF3 HN are resistant to viral infection. Stable expression of wild-type HN-green fluorescent protein (GFP) on cell membranes in different amounts allowed us to establish a correlation between the level of HN expression, the level of neuraminidase activity, and the level of protection from HPF3 infection. Cells with the highest levels of HN expression and neuraminidase activity on the cell surface were most resistant to infection by HPF3. To determine whether this resistance is attributable to the viral neuraminidase, we used a cloned variant HPF3 HN that has two amino acid alterations in HN leading to the loss of detectable neuraminidase activity. Cells expressing the neuraminidase-deficient variant HN-GFP were not protected from infection, despite expressing HN on their surface at levels even higher than the wild-type cell clones. Our results demonstrate that the HPF3 HN-mediated interference effect can be attributed to the presence of an active neuraminidase enzyme activity and provide the first definitive evidence that the mechanism for attachment interference by a paramyxovirus is attributable to the viral neuraminidase.