Hijacking the Fusion Complex of Human Parainfluenza Virus as an Antiviral Strategy.

The receptor binding protein of parainfluenza virus, hemagglutinin-neuraminidase (HN), is responsible for actively triggering the viral fusion protein (F) to undergo a conformational change leading to insertion into the target cell and fusion of the virus with the target cell membrane. For proper viral entry to occur, this process must occur when HN is engaged with host cell receptors at the cell surface. It is possible to interfere with this process through premature activation of the F protein, distant from the target cell receptor. Conformational changes in the F protein and adoption of the postfusion form of the protein prior to receptor engagement of HN at the host cell membrane inactivate the virus. We previously identified small molecules that interact with HN and induce it to activate F in an untimely fashion, validating a new antiviral strategy. To obtain highly active pretriggering candidate molecules we carried out a virtual modeling screen for molecules that interact with sialic acid binding site II on HN, which we propose to be the site responsible for activating F. To directly assess the mechanism of action of one such highly effective new premature activating compound, PAC-3066, we use cryo-electron tomography on authentic intact viral particles for the first time to examine the effects of PAC-3066 treatment on the conformation of the viral F protein. We present the first direct observation of the conformational rearrangement induced in the viral F protein.IMPORTANCE Paramyxoviruses, including human parainfluenza virus type 3, are internalized into host cells by fusion between viral and target cell membranes. The receptor binding protein, hemagglutinin-neuraminidase (HN), upon binding to its cell receptor, triggers conformational changes in the fusion protein (F). This action of HN activates F to reach its fusion-competent state. Using small molecules that interact with HN, we can induce the premature activation of F and inactivate the virus. To obtain highly active pretriggering compounds, we carried out a virtual modeling screen for molecules that interact with a sialic acid binding site on HN that we propose to be the site involved in activating F. We use cryo-electron tomography of authentic intact viral particles for the first time to directly assess the mechanism of action of this treatment on the conformation of the viral F protein and present the first direct observation of the induced conformational rearrangement in the viral F protein.

Authentic Modeling of Human Respiratory Virus Infection in Human Pluripotent Stem Cell-Derived Lung Organoids.

Infectious viruses so precisely fit their hosts that the study of natural viral infection depends on host-specific mechanisms that affect viral infection. For human parainfluenza virus 3, a prevalent cause of lower respiratory tract disease in infants, circulating human viruses are genetically different from viruses grown in standard laboratory conditions; the surface glycoproteins that mediate host cell entry on circulating viruses are suited to the environment of the human lung and differ from those of viruses grown in cultured cells. Polarized human airway epithelium cultures have been used to represent the large, proximal airways of mature adult airways. Here we modeled respiratory virus infections that occur in children or infect the distal lung using lung organoids that represent the entire developing infant lung. These 3D lung organoids derived from human pluripotent stem cells contain mesoderm and pulmonary endoderm and develop into branching airway and alveolar structures. Whole-genome sequencing analysis of parainfluenza viruses replicating in the organoids showed maintenance of nucleotide identity, suggesting that no selective pressure is exerted on the virus in this tissue. Infection with parainfluenza virus led to viral shedding without morphological changes, while respiratory syncytial virus infection induced detachment and shedding of infected cells into the lung organoid lumens, reminiscent of parainfluenza and respiratory syncytial virus in human infant lungs. Measles virus infection, in contrast, induced syncytium formation. These human stem cell-derived lung organoids may serve as an authentic model for respiratory viral pathogenesis in the developing or infant lung, recapitulating respiratory viral infection in the host.IMPORTANCE Respiratory viruses are among the first pathogens encountered by young children, and the significant impact of these viral infections on the developing lung is poorly understood. Circulating viruses are suited to the environment of the human lung and are different from those of viruses grown in cultured cells. We modeled respiratory virus infections that occur in children or infect the distal lung using lung organoids that represent the entire developing infant lung. These 3D lung organoids, derived from human pluripotent stem cells, develop into branching airway and alveolar structures and provide a tissue environment that maintains the authentic viral genome. The lung organoids can be genetically engineered prior to differentiation, thereby generating tissues bearing or lacking specific features that may be relevant to viral infection, a feature that may have utility for the study of host-pathogen interaction for a range of lung pathogens.

Inhibiting Human Parainfluenza Virus Infection by Preactivating the Cell Entry Mechanism.

Paramyxoviruses, specifically, the childhood pathogen human parainfluenza virus type 3, are internalized into host cells following fusion between the viral and target cell membranes. The receptor binding protein, hemagglutinin (HA)-neuraminidase (HN), and the fusion protein (F) facilitate viral fusion and entry into the cell through a coordinated process involving HN activation by receptor binding, which triggers conformational changes in the F protein to activate it to reach its fusion-competent state. Interfering with this process through premature activation of the F protein has been shown to be an effective antiviral strategy in vitro. Conformational changes in the F protein leading to adoption of the postfusion form of the protein-prior to receptor engagement of HN at the host cell membrane-render the virus noninfectious. We previously identified a small compound (CSC11) that implements this antiviral strategy through an interaction with HN, causing HN to activate F in an untimely process. To assess the functionality of such compounds, it is necessary to verify that the postfusion state of F has been achieved. As demonstrated by Melero and colleagues, soluble forms of the recombinant postfusion pneumovirus F proteins and of their six helix bundle (6HB) motifs can be used to generate postfusion-specific antibodies. We produced novel anti-HPIV3 F conformation-specific antibodies that can be used to assess the functionality of compounds designed to induce F activation. In this study, using systematic chemical modifications of CSC11, we synthesized a more potent derivative of this compound, CM9. Much like CSC11, CM9 causes premature triggering of the F protein through an interaction with HN prior to receptor engagement, thereby preventing fusion and subsequent infection. In addition to validating the potency of CM9 using plaque reduction, fusion inhibition, and binding avidity assays, we confirmed the transition to a postfusion conformation of F in the presence of CM9 using our novel anti-HPIV3 conformation-specific antibodies. We present both CM9 and these newly characterized postfusion antibodies as novel tools to explore and develop antiviral approaches. In turn, these advances in both our molecular toolset and our understanding of HN-F interaction will support development of more-effective antivirals. Combining the findings described here with our recently described physiologically relevant ex vivo system, we have the potential to inform the development of therapeutics to block viral infection.IMPORTANCE Paramyxoviruses, including human parainfluenza virus type 3, are internalized into host cells by fusion between viral and target cell membranes. The receptor binding protein, hemagglutinin-neuraminidase (HN), and the fusion protein (F) facilitate viral fusion and entry into cells through a process involving HN activation by receptor binding, which triggers conformational changes in F to activate it to reach its fusion-competent state. Interfering with this process through premature activation of the F protein may be an effective antiviral strategy in vitro We identified and optimized small compounds that implement this antiviral strategy through an interaction with HN, causing HN to activate F in an untimely fashion. To address that mechanism, we produced novel anti-HPIV3 F conformation-specific antibodies that can be used to assess the functionality of compounds designed to induce F activation. Both the novel antiviral compounds that we present and these newly characterized postfusion antibodies are novel tools for the exploration and development of antiviral approaches.

Effective in Vivo Targeting of Influenza Virus through a Cell-Penetrating/Fusion Inhibitor Tandem Peptide Anchored to the Plasma Membrane.

The impact of influenza virus infection is felt each year on a global scale when approximately 5-10% of adults and 20-30% of children globally are infected. While vaccination is the primary strategy for influenza prevention, there are a number of likely scenarios for which vaccination is inadequate, making the development of effective antiviral agents of utmost importance. Anti-influenza treatments with innovative mechanisms of action are critical in the face of emerging viral resistance to the existing drugs. These new antiviral agents are urgently needed to address future epidemic (or pandemic) influenza and are critical for the immune-compromised cohort who cannot be vaccinated. We have previously shown that lipid tagged peptides derived from the C-terminal region of influenza hemagglutinin (HA) were effective influenza fusion inhibitors. In this study, we modified the influenza fusion inhibitors by adding a cell penetrating peptide sequence to promote intracellular targeting. These fusion-inhibiting peptides self-assemble into ∼15-30 nm nanoparticles (NPs), target relevant infectious tissues in vivo, and reduce viral infectivity upon interaction with the cell membrane. Overall, our data show that the CPP and the lipid moiety are both required for efficient biodistribution, fusion inhibition, and efficacy in vivo.

In Vivo Efficacy of Measles Virus Fusion Protein-Derived Peptides Is Modulated by the Properties of Self-Assembly and Membrane Residence.

Measles virus (MV) infection is undergoing resurgence and remains one of the leading causes of death among young children worldwide despite the availability of an effective measles vaccine. MV infects its target cells by coordinated action of the MV hemagglutinin (H) and fusion (F) envelope glycoproteins; upon receptor engagement by H, the prefusion F undergoes a structural transition, extending and inserting into the target cell membrane and then refolding into a postfusion structure that fuses the viral and cell membranes. By interfering with this structural transition of F, peptides derived from the heptad repeat (HR) regions of F can inhibit MV infection at the entry stage. In previous work, we have generated potent MV fusion inhibitors by dimerizing the F-derived peptides and conjugating them to cholesterol. We have shown that prophylactic intranasal administration of our lead fusion inhibitor efficiently protects from MV infection in vivo We show here that peptides tagged with lipophilic moieties self-assemble into nanoparticles until they reach the target cells, where they are integrated into cell membranes. The self-assembly feature enhances biodistribution and the half-life of the peptides, while integration into the target cell membrane increases fusion inhibitor potency. These factors together modulate in vivo efficacy. The results suggest a new framework for developing effective fusion inhibitory peptides. IMPORTANCE: Measles virus (MV) infection causes an acute illness that may be associated with infection of the central nervous system (CNS) and severe neurological disease. No specific treatment is available. We have shown that fusion-inhibitory peptides delivered intranasally provide effective prophylaxis against MV infection. We show here that specific biophysical properties regulate the in vivo efficacy of MV F-derived peptides.

Prevention of measles virus infection by intranasal delivery of fusion inhibitor peptides.

UNLABELLED: Measles virus (MV) infection is undergoing resurgence and remains one of the leading causes of death among young children worldwide despite the availability of an effective measles vaccine. MV infects its target cells by coordinated action of the MV H and the fusion (F) envelope glycoprotein; upon receptor engagement by H, the prefusion F undergoes a structural transition, extending and inserting into the target cell membrane and then refolding into a postfusion structure that fuses the viral and cell membranes. By interfering with this structural transition of F, peptides derived from the heptad-repeat (HR) regions of F can potently inhibit MV infection at the entry stage. We show here that specific features of H's interaction with its receptors modulate the susceptibility of MV F to peptide fusion inhibitors. A higher concentration of inhibitory peptides is required to inhibit F-mediated fusion when H is engaged to its nectin-4 receptor than when H is engaged to its CD150 receptor. Peptide inhibition of F may be subverted by continued engagement of receptor by H, a finding that highlights the ongoing role of H-receptor interaction after F has been activated and that helps guide the design of more potent inhibitory peptides. Intranasal administration of these peptides results in peptide accumulation in the airway epithelium with minimal systemic levels of peptide and efficiently prevents MV infection in vivo in animal models. The results suggest an antiviral strategy for prophylaxis in vulnerable and/or immunocompromised hosts. IMPORTANCE: Measles virus (MV) infection causes an acute illness that may be associated with infection of the central nervous system (CNS) and severe neurological disease. No specific treatment is available. We have shown that parenterally delivered fusion-inhibitory peptides protect mice from lethal CNS MV disease. Here we show, using established small-animal models of MV infection, that fusion-inhibitory peptides delivered intranasally provide effective prophylaxis against MV infection. Since the fusion inhibitors are stable at room temperature, this intranasal strategy is feasible even outside health care settings, could be used to protect individuals and communities in case of MV outbreaks, and could complement global efforts to control measles.

Molecular determinants of antiviral potency of paramyxovirus entry inhibitors.

Hendra virus (HeV) and Nipah virus (NiV) constitute the Henipavirus genus of paramyxoviruses, both fatal in humans and with the potential for subversion as agents of bioterrorism. Binding of the HeV/NiV attachment protein (G) to its receptor triggers a series of conformational changes in the fusion protein (F), ultimately leading to formation of a postfusion six-helix bundle (6HB) structure and fusion of the viral and cellular membranes. The ectodomain of paramyxovirus F proteins contains two conserved heptad repeat regions, the first (the N-terminal heptad repeat [HRN]) adjacent to the fusion peptide and the second (the C-terminal heptad repeat [HRC]) immediately preceding the transmembrane domain. Peptides derived from the HRN and HRC regions of F are proposed to inhibit fusion by preventing activated F molecules from forming the 6HB structure that is required for fusion. We previously reported that a human parainfluenza virus 3 (HPIV3) F peptide effectively inhibits infection mediated by the HeV glycoproteins in pseudotyped-HeV entry assays more effectively than the comparable HeV-derived peptide, and we now show that this peptide inhibits live-HeV and -NiV infection. HPIV3 F peptides were also effective in inhibiting HeV pseudotype virus entry in a new assay that mimics multicycle replication. This anti-HeV/NiV efficacy can be correlated with the greater potential of the HPIV3 C peptide to interact with the HeV F N peptide coiled-coil trimer, as evaluated by thermal unfolding experiments. Furthermore, replacement of a buried glutamic acid (glutamic acid 459) in the C peptide with valine enhances antiviral potency and stabilizes the 6HB conformation. Our results strongly suggest that conserved interhelical packing interactions in the F protein fusion core are important determinants of C peptide inhibitory activity and offer a strategy for the development of more-potent analogs of F peptide inhibitors.

Inhibition of hendra virus fusion.

Hendra virus (HeV) is a recently identified paramyxovirus that is fatal in humans and could be used as an agent of bioterrorism. The HeV receptor-binding protein (G) is required in order for the fusion protein (F) to mediate fusion, and analysis of the triggering/activation of HeV F by G should lead to strategies for interfering with this key step in viral entry. HeV F, once triggered by the receptor-bound G, by analogy with other paramyxovirus F proteins, undergoes multistep conformational changes leading to a six-helix bundle (6HB) structure that accomplishes fusion of the viral and cellular membranes. The ectodomain of paramyxovirus F proteins contains two conserved heptad repeat regions (HRN and HRC) near the fusion peptide and the transmembrane domains, respectively. Peptides derived from the HRN and HRC regions of F are proposed to inhibit fusion by preventing F, after the initial triggering step, from forming the 6HB structure that is required for fusion. HeV peptides have previously been found to be effective at inhibiting HeV fusion. However, we found that a human parainfluenza virus 3 F-peptide is more effective at inhibiting HeV fusion than the comparable HeV-derived peptide.

Contribution of the human parainfluenza virus type 3 HN-receptor interaction to pathogenesis in vivo.

The envelope of human parainfluenza virus type 3 (HPF3) contains two viral glycoproteins, the hemagglutinin-neuraminidase (HN) protein and the fusion (F) protein. In a previous study, highly fusogenic variant HPF3 viruses were isolated, including two, C-0 and C-22, that exhibit increased avidity for sialic acid receptors due to single amino acid changes in the HN protein and one, C-28, that has decreased neuraminidase activity relative to that of the wild type (wt) and is delayed in the release of virus particles into the supernatant fluid. These variants form very large plaques and destroy a cell monolayer more rapidly than does wt HPF3 in cell culture. These variant viruses allowed us to formulate hypotheses about the roles of HN in pathogenesis. We investigated the behavior of wt HPF3 and the three variant viruses in the cotton rat model. In the cotton rat, there was no delayed clearance of any of the variant viruses compared to that of the wt. The variant plaque morphology was preserved in vivo, and there was no reversion to the wt phenotype in the infected animals. In spite of a slight advantage of wt virus in viral titer, there were no differences in the severities of peribronchiolitis between wt viruses and the variants. However, there were marked differences in severities in alveolitis and interstitial pneumonitis when each of the three variants was compared to the wt, with the variants causing enhanced disease. Thus, despite similar or lower viral titers and similar clearance rates, the variants caused more extensive disease in the lung. The results show that mutations in HN conferring altered fusion properties in cell culture also confer striking differences in the ability of HPF3 to cause extensive disease in the cotton rat lung and that this effect is dissociated from any effect on viral replication.

The role of quantitative Epstein-Barr virus polymerase chain reaction and preemptive immunosuppression reduction in pediatric liver transplantation: a preliminary experience.

BACKGROUND: Risk factors for the development of posttransplant lymphoproliferative disease (PTLD), a major cause of morbidity and mortality after pediatric liver transplantation, are primary Epstein-Barr virus (EBV) infection and intensity of immunosuppression. The authors assessed monitoring of EBV replication and preemptive immunosuppression reduction in pediatric liver transplant recipients. METHODS: The authors prospectively followed monthly EBV-quantitative competitive polymerase chain reaction to measure EBV replication in 23 patients who underwent liver transplant between July 1997 and November 1998. Preemptive immunosuppression reduction was instituted for significant EBV replication. Patients were followed up for at least 1 year and divided in two groups for analysis (group 1, pretransplant seronegative for EBV [13 patients]; group 2, seropositive for EBV [10 patients]). RESULTS: In group 1, 9 of 13 patients had positive polymerase chain reaction results at a mean time of 22.4 weeks after transplantation. All but one of these patients were asymptomatic. In seven of nine patients, preemptive immunosuppression reduction was undertaken without development of PTLD or rejection. In two of nine patients, immunosuppression could not be continuously reduced, and both patients experienced low-grade and medically responsive PTLD. In no patient in group 2 did an EBV-positive viral load or PTLD develop. CONCLUSIONS: Prospective longitudinal measurement of EBV by quantitative competitive polymerase chain reaction permits early detection of asymptomatic viral replication. Subsequent preemptive reduction of immunosuppression may prevent the progression to PTLD.

Human parainfluenza virus type 3 HN-receptor interaction: effect of 4-guanidino-Neu5Ac2en on a neuraminidase-deficient variant.

The envelope of human parainfluenza virus type 3 (HPF3) contains two viral glycoproteins, the hemagglutinin-neuraminidase (HN) and the fusion protein (F). HN, which is responsible for receptor attachment and for promoting F-mediated fusion, also possesses neuraminidase (receptor-destroying) activity. We reported previously that 4-guanidino-neu5Ac2en (4-GU-DANA) and related sialic acid-based inhibitors of HPF3 neuraminidase activity also inhibit HN-mediated receptor binding and fusion processes not involving neuraminidase activity. We have now examined this mechanism, as well as neuraminidase's role in the viral life cycle, using a neuraminidase-deficient HPF3 variant (C28a) and stable cell lines expressing C28a or wild-type (wt) HN. C28a, which has a wt F sequence and two point mutations in the HN gene corresponding to two amino acid changes in the HN protein, is the first HPF3 variant with insignificant neuraminidase activity. Cells expressing C28a HN did not bind erythrocytes at 4 degrees C unless pretreated with neuraminidase, but no such pretreatment was required for hemadsorption activity (HAD) at 22 or 37 degrees C. HAD was blocked by 4-GU-DANA, attesting to the ability of this compound to inhibit HN's receptor-binding activity. C28a or wt plaque enlargement, a process that involves cell-cell fusion and does not depend on virion release, is diminished by the presence of 4-GU-DANA, confirming the inhibitory effect of 4-GU-DANA on the fusogenic function of C28a HN. In C28a-infected cell monolayers, virion release and thus multicycle replication are severely restricted. This defect was corrected by supplementation of exogenous neuraminidase and also by the addition of 4-GU-DANA; neuraminidase destroys the receptors whereby newly formed C28a virions would remain attached to the cell surface, whereas 4-GU-DANA prevents the attachment itself, obviating the need for receptor cleavage. In accord with the ability of 4-GU-DANA to prevent attachment, the neuraminidase inhibitory effect of 4-GU-DANA on wt HPF3 did not diminish virion release into the medium. Thus, it is by inhibition of viral entry and syncytium formation that sialic acid analogs like 4-GU-DANA may counteract wt HPF3 infection.

A single amino acid alteration in the human parainfluenza virus type 3 hemagglutinin-neuraminidase glycoprotein confers resistance to the inhibitory effects of zanamivir on receptor binding and neuraminidase activity.

Entry and fusion of human parainfluenza virus type 3 (HPF3) requires interaction of the viral hemagglutinin-neuraminidase (HN) glycoprotein with its sialic acid receptor. 4-Guanidino-2,4-dideoxy-2,3-dehydro-N-acetylneuraminic acid (4-GU-DANA; zanamivir), a sialic acid transition-state analog designed to fit the influenza virus neuraminidase catalytic site, possesses antiviral activity at nanomolar concentrations in vitro. We have shown previously that 4-GU-DANA also inhibits both HN-mediated binding of HPF3 to host cell receptors and HN's neuraminidase activity. In the present study, a 4-GU-DANA-resistant HPF3 virus variant (ZM1) was generated by serial passage in the presence of 4-GU-DANA. ZM1 exhibited a markedly fusogenic plaque morphology and harbored two HN gene mutations resulting in two amino acid alterations, T193I and I567V. Another HPF3 variant studied in parallel, C-0, shared an alteration at T193 and exhibited similar plaque morphology but was not resistant to 4-GU-DANA. Neuraminidase assays revealed a 15-fold reduction in 4-GU-DANA sensitivity for ZM1 relative to the wild type (WT) and C-0. The ability of ZM1 to bind sialic acid receptors was inhibited 10-fold less than for both WT and C-0 in the presence of 1 mM 4-GU-DANA. ZM1 also retained infectivity at 15-fold-higher concentrations of 4-GU-DANA than WT and C-0. A single amino acid alteration at HN residue 567 confers these 4-GU-DANA-resistant properties. An understanding of ZM1 and other escape variants provides insight into the effects of this small molecule on HN function as well as the role of the HN glycoprotein in HPF3 pathogenesis.

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.

The anti-influenza virus agent 4-GU-DANA (zanamivir) inhibits cell fusion mediated by human parainfluenza virus and influenza virus HA.

4-GU-DANA (zanamivir) (as well as DANA and 4-AM-DANA) was found to inhibit the neuraminidase activity of human parainfluenza virus type 3 (HPF3). The viral neuraminidase activity is attributable to hemagglutinin-neuraminidase (HN), an envelope protein essential for viral attachment and for fusion mediated by the other envelope protein, F. While there is no evidence that HN's neuraminidase activity is essential for receptor binding and syncytium formation, we found that 4-GU-DANA prevented hemadsorption and fusion of persistently infected cells with uninfected cells. In plaque assays, 4-GU-DANA reduced the number (but not the area) of plaques if present only during the adsorption period and reduced plaque area (but not number) if added only after the 90-min adsorption period. 4-GU-DANA also reduced the area of plaques formed by a neuraminidase-deficient variant, confirming that its interference with cell-cell fusion is unrelated to inhibition of neuraminidase activity. The order-of-magnitude lower 50% inhibitory concentrations of 4-GU-DANA (and also DANA and 4-AM-DANA) for plaque area reduction and for inhibition in the fusion assay than for reducing plaque number or blocking hemadsorption indicate the particular efficacy of these sialic acid analogs in interfering with cell-cell fusion. In cell lines expressing influenza virus hemagglutinin (HA) as the only viral protein, we found that 4-GU-DANA had no effect on hemadsorption but did inhibit HA2b-red blood cell fusion, as judged by both lipid mixing and content mixing. Thus, 4-GU-DANA can interfere with both influenza virus- and HPF3-mediated fusion. The results indicate that (i) in HPF3, 4-GU-DANA and its analogs have an affinity not only for the neuraminidase active site of HN but also for sites important for receptor binding and cell fusion and (ii) sialic acid-based inhibitors of influenza virus neuraminidase can also exert a direct, negative effect on the fusogenic function of the other envelope protein, HA.

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.

Immunization of pediatric solid organ transplant candidates and recipients.

Organ transplantation has evolved from an experimental procedure to an accepted treatment for otherwise irreversible or congenital disorders. The immunosuppression necessary to prevent rejection enhances the severity of many infectious diseases and may potentially attenuate the response to vaccines designed to prevent disease. In spite of the frequency and severity of infectious diseases in organ transplant recipients, many children are not fully vaccinated before transplantation. The safety and efficacy of many of the currently available vaccines for solid organ transplant recipients have not been evaluated. We review the currently available data on immunization safety and efficacy, discuss experimental vaccines, and outline strategies to avoid vaccine-preventable diseases in pediatric organ transplant recipients.

The use of a quantitative fusion assay to evaluate HN-receptor interaction for human parainfluenza virus type 3.

Sialic acid is the receptor determinant for the human parainfluenza virus type 3 (HPF3) hemagglutinin-neuraminidase (HN) glycoprotein, the molecule responsible for binding of the virus to cell surfaces. In order for the fusion protein (F) of HPF3 to promote membrane fusion, HN must interact with its receptor. In addition to its role in receptor binding and fusion promotion, the HPF3 HN molecule contains receptor-destroying (sialidase) activity. The putative active sites are in the extracellular domain of this type II integral membrane protein. However, HN is not available in crystalline form; the exact locations of these sites, and the structural requirements for binding to the cellular receptor, which has not yet been isolated, are unknown. Nor have small molecular synthetic inhibitors of attachment or fusion that would provide insight into these processes been identified. The strategy in the present study was to develop an assay system that would provide a measure of a specific step in the viral cycle-functional interaction between viral glycoproteins and the cell during attachment and fusion-and serve to screen a variety of substances for inhibitory potential. The assay is based on our previous finding that CV-1 cells persistently infected (p.i.) with HPF3 do not fuse with one another but that the addition of uninfected CV-1 cells, supplying the critical sialic acid containing receptor molecules that bind HN, results in rapid fusion. In the present assay two HeLa cell types were used: we persistently infected HeLa-LTR-betagal cells, assessed their fusion with uninfected HeLa-tat cells, and then quantitated the beta-galactosidase (betagal) produced as a result of this fusion. The analog alpha-2-S-methyl-5-N-thioacetylneuraminic acid (alpha-Neu5thioAc2SMe) interfered with fusion, decreasing betagal production by 84% at 50 mM and by 24% at 25 mM. In beginning to extend our studies to different types of molecules, we tested an unsaturated derivative of sialic acid, 2,3-dehydro-2-deoxy-n-acetyl neuraminic acid (DANA), which is known to inhibit influenza neuraminidase by virtue of being a transition-state analog. We found that 10 mM DANA inhibited neuraminidase activity in HPF3 viral preparations. More significantly, this compound was active in our assay of HN-receptor interaction; 10 mM DANA completely blocked fusion and betagal production, and hemadsorption inhibition by DANA suggested that DANA blocks attachment. In plaque reduction assays performed with the compounds, the active analog alpha-Neu5thioAc2SMe reduced plaque formation by 50% at a 50 mM concentration; DANA caused a 90% inhibition in the plaque reduction assay at a concentration of 25 mM. Our results indicate that specific sialic acid analogs that mimic the cellular receptor determinant of HPF3 can block virus cell interaction and that an unsaturated n-acetyl-neuraminic acid derivative with affinity to the HN site responsible for neuraminidase activity also interferes with HN-receptor binding. Strategies suggested by these findings are now being pursued to obtain information regarding the relative locations of the active sites of HN and to further elucidate the relationship between the receptor-binding and receptor-destroying activities of HN during the viral life cycle. The quantitative assay that we describe is of immediate applicability to large-scale screening for potential inhibitors of HPF3 infection in vivo.

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

Human parainfluenza viruses types 1, 2 and 3 (HPF 1, 2 and 3) are important pathogens in children. While these viruses share common structures and replication strategies, they target different parts of the respiratory tract; the most common outcomes of infection with HPF3 are bronchiolitis and pneumonia, while HPF 1 and 2 are associated with croup. While the HPF3 fusion protein (F) is critical for membrane fusion, our previous work revealed that the receptor binding hemagglutinin-neuraminidase (HN) is also essential to the fusion process; interaction between HN and its sialic acid-containing receptor on cell surfaces is required for HPF3 mediated cell fusion. Using our understanding of HPF3 HN's functions in the cell-binding and viral entry process, we are investigating the ways in which these processes differ in HPF 1 and 2, in part by manipulating receptor availability. Three experimental treatments were used to compare the HN-receptor interaction of HPF 1, 2 and 3: infection at high multiplicity of infection (m.o.i.); bacterial neuraminidase treatment of cells infected at low m.o.i.; and viral neuraminidase treatment of cells infected at low m.o.i. (using Newcastle disease virus [NDV] neuraminidase or UV irradiated HPF3 as sources of neuraminidase). In cells infected with HPF3, we have shown that infection with high m.o.i. blocks fusion, by removing sialic acid receptors for the viral HN. However, in cells infected with HPF 1 and 2, infection with high m.o.i. did not block fusion; the fusion increases with increasing m.o.i. In cells infected with HPF 1 and 2, neither bacterial nor NDV neuraminidase blocked cell fusion, using amounts of neuraminidase that completely block fusion of HPF3 infected cells. However, when inactivated HPF3 was used as a source of viral neuraminidase, the treatment inhibited fusion of cells infected with HPF 1 and 2 as well as 3. The differences found between these viruses in terms of their interaction with the cell, ability to modulate cell-cell fusion and response to exogenous neuraminidases of various specificities, may reflect salient differences in biological properties of the three viruses.

Prospective longitudinal analysis of quantitative Epstein-Barr virus polymerase chain reaction in pediatric liver transplant recipients.

BACKGROUND: Posttransplant lymphoproliferative disease (PTLD) remains a significant cause of morbidity and mortality in pediatric liver transplant recipients. Epstein-Barr Virus (EBV) mismatch associated with more prevalent use of split-liver, reduced size, and living-related transplants has increased the risk of primary EBV infection and subsequent PTLD. Early identification of EBV viremia may reduce the risk of PTLD, because it allows for early adjustment of immunosuppression and antiviral therapy. METHODS: EBV viral load was measured monthly by quantitative competitive polymerase chain reactions in three pediatric liver transplant recipients. RESULTS: Onset of EBV viremia was documented in one recipient. Established EBV viremia was followed in the other two recipients (one with chronic rejection and one with PTLD) who were initially tested once monitoring was initiated in our program. CONCLUSIONS: EBV quantitative competitive polymerase chain reactions may represent a promising way to follow EBV viral load and potentially prevent the development of PTLD.