Characterization of human parainfluenza virus type 3 persistent infection in cell culture

How a paramyxovirus fusion/entry complex adapts to escape a neutralizing antibody

Paramyxoviruses including measles, Nipah, and parainfluenza viruses are public health threats with pandemic potential. Human parainfluenza virus type 3 (HPIV3) is a leading cause of illness in pediatric, older, and immunocompromised populations. There are no approved vaccines or therapeutics for HPIV3. Neutralizing monoclonal antibodies (mAbs) that target viral fusion are a potential strategy for mitigating paramyxovirus infection, however their utility may be curtailed by viral evolution that leads to resistance. Paramyxoviruses enter cells by fusing with the cell membrane in a process mediated by a complex consisting of a receptor binding protein (HN) and a fusion protein (F). Existing atomic resolution structures fail to reveal physiologically relevant interactions during viral entry. We present cryo-ET structures of pre-fusion HN-F complexes in situ on surfaces of virions that evolved resistance to an anti-HPIV3 F neutralizing mAb. Single mutations in F abolish mAb binding and neutralization. In these complexes, the HN protein that normally restrains F triggering has shifted to uncap the F apex. These complexes are more readily triggered to fuse. These structures shed light on the adaptability of the pre-fusion HN-F complex and mechanisms of paramyxoviral resistance to mAbs, and help define potential barriers to resistance for the design of mAbs.

Human parainfluenza virus evolution during lung infection of immunocompromised humans promotes viral persistence

The capacity of respiratory viruses to undergo evolution within the respiratory tract raises the possibility of evolution under the selective pressure of the host environment or drug treatment. Long-term infections in immunocompromised hosts are potential drivers of viral evolution and development of infectious variants. We show that intra-host evolution in chronic human parainfluenza virus 3 (HPIV3) infection in immunocompromised individuals elicited mutations that favor viral entry and persistence, suggesting that similar processes may operate across enveloped respiratory viruses. We profiled longitudinal HPIV3 infections from two immunocompromised individuals that persisted for 278 and 98 days. Mutations accrued in the HPIV3 attachment protein hemagglutinin-neuraminidase (HN), including the first in vivo mutation in HN's receptor binding site responsible for activating the viral fusion process. Fixation of this mutation was associated with exposure to a drug that cleaves host cell sialic acid moieties. Longitudinal adaptation of HN was associated with features that promote viral entry and persistence in cells, including greater avidity for sialic acid and more active fusion activity in vitro, but not with antibody escape. Long term infection thus led to mutations promoting viral persistence, suggesting that host-directed therapeutics may support the evolution of viruses that alter their biophysical characteristics to persist in the face of these agents in vivo.

Human parainfluenza virus type 1 regulates cholesterol biosynthesis and establishes quiescent infection in human airway cells

Human parainfluenza virus type 1 (hPIV1) and 3 (hPIV3) cause seasonal epidemics, but little is known about their interaction with human airway cells. In this study, we determined cytopathology, replication, and progeny virion release from human airway cells during long-term infection in vitro. Both viruses readily established persistent infection without causing significant cytopathic effects. However, assembly and release of hPIV1 rapidly declined in sharp contrast to hPIV3 due to impaired viral ribonucleocapsid (vRNP) trafficking and virus assembly. Transcriptomic analysis revealed that both viruses induced similar levels of type I and III IFNs. However, hPIV1 induced specific ISGs stronger than hPIV3, such as MX2, which bound to hPIV1 vRNPs in infected cells. In addition, hPIV1 but not hPIV3 suppressed genes involved in lipid biogenesis and hPIV1 infection resulted in ubiquitination and degradation of 3-hydroxy-3-methylglutaryl-coenzyme A reductase, a rate limiting enzyme in cholesterol biosynthesis. Consequently, formation of cholesterol-rich lipid rafts was impaired in hPIV1 infected cells. These results indicate that hPIV1 is capable of regulating cholesterol biogenesis, which likely together with ISGs contributes to establishment of a quiescent infection.

Seasonal epidemics caused by parainfluenza viruses result in a significant burden of disease in children. These viruses infect airway epithelial cells and cause acute respiratory infection. Humans are the only known hosts for these viruses, but how these viruses are maintained within the population is not known. In this study, we analyzed human airway cells infected with type 1 and 3 parainfluenza viruses. Both viruses readily established persistent infection without causing major cytopathic effects. However, assembly and release of hPIV1 rapidly declined over time in sharp contrast to hPIV3. HPIV1 infected cells formed large aggregates of viral nucleocapsid at late time points, suggesting impaired nucleocapsid trafficking and virus assembly. Transcriptomic analysis of infected cells showed no major difference in IFN induction between the viruses, while hPIV1 induced elevated levels of interferon stimulated genes (ISGs) compared to hPIV3. Interestingly, hPIV1 infection specifically downregulated genes involved in cholesterol biogenesis. We also found that hPIV1 infection induced ubiquitination and degradation of 3-hydroxy-3-methylglutaryl-coenzyme A reductase, a rate limiting enzyme in cholesterol biosynthesis. These results suggest that induction of IFN-independent ISGs and suppression of cholesterol by hPIV1 likely play a role in establishing quiescent infection in human respiratory epithelial cells.

Features of Circulating Parainfluenza Virus Required for Growth in Human Airway

Respiratory paramyxoviruses, including the highly prevalent human parainfluenza viruses, cause the majority of childhood croup, bronchiolitis, and pneumonia, yet there are currently no vaccines or effective treatments. Paramyxovirus research has relied on the study of laboratory-adapted strains of virus in immortalized cultured cell lines. We show that findings made in such systems about the receptor interaction and viral fusion requirements for entry and fitness—mediated by the receptor binding protein and the fusion protein—can be drastically different from the requirements for infection in vivo. Here we carried out whole-genome sequencing and genomic analysis of circulating human parainfluenza virus field strains to define functional and structural properties of proteins of circulating strains and to identify the genetic basis for properties that confer fitness in the field. The analysis of clinical strains suggests that the receptor binding-fusion molecule pairs of circulating viruses maintain a balance of properties that result in an inverse correlation between fusion in cultured cells and growth in vivo. Future analysis of entry mechanisms and inhibitory strategies for paramyxoviruses will benefit from considering the properties of viruses that are fit to infect humans, since a focus on viruses that have adapted to laboratory work provides a distinctly different picture of the requirements for the entry step of infection.

Mechanistic information about viral infection—information that impacts antiviral and vaccine development—is generally derived from viral strains grown under laboratory conditions in immortalized cells. This study uses whole-genome sequencing of clinical strains of human parainfluenza virus 3—a globally important respiratory paramyxovirus—in cell systems that mimic the natural human host and in animal models. By examining the differences between clinical isolates and laboratory-adapted strains, the sequence differences are correlated to mechanistic differences in viral entry. For this ubiquitous and pathogenic respiratory virus to infect the human lung, modulation of the processes of receptor engagement and fusion activation occur in a manner quite different from that carried out by the entry glycoprotein-expressing pair of laboratory strains. These marked contrasts in the viral properties necessary for infection in cultured immortalized cells and in natural host tissues and animals will influence future basic and clinical studies.

Expression of novel genes encoded by the paramyxovirus J virus

Characterization of the J virus or, in keeping with recent nomenclature recommendations, J paramyxovirus (JPV) genome revealed a unique genome structure, consisting of eight genes in the order 3'-N-P/V/C-M-F-SH-TM-G-L-5'. The small hydrophobic (SH) protein and the transmembrane (TM) protein genes are predicted to encode proteins 69 and 258 aa in size, respectively. The 4401 nt attachment (G) protein gene, much larger than most other paramyxovirus attachment protein genes sequenced to date, encodes a putative 709 aa attachment protein and contains distally a second open reading frame (ORF-X) 2115 nt long. Experiments undertaken in this study were intended to confirm the sequence-based gene allocation of JPV and to determine if proteins encoded by the SH gene, the novel TM gene and ORF-X are expressed. Northern blot analyses carried out on mRNA purified from JPV-infected cells indicated that the putative transcription initiation and termination sequences flanking the SH and TM genes are functional, consistent with their allocation as discrete genes, although a high level of read-through was observed across almost all transcriptional boundaries. Probes specific to the G protein coding region and ORF-X both identified an mRNA species corresponding to the predicted length of the G gene, confirming sequence-based predictions. While the SH and TM proteins were both detected in infected cells, no evidence was found for the expression of ORF-X. Preliminary studies indicate that the novel TM protein is a type II glycosylated integral membrane protein, orientated with its C terminus exposed at the cell surface.

Interferon type I downregulates human parainfluenza virus type 3-induced major histocompatibility complex class II expression

Human parainfluenza virus type 3 (HPIV3) induces major histocompatibility complex (MHC) class II expression in a signal transducer and activator of transcription-1 (STAT1)- and class II transactivator (CIITA)-independent manner. Interferon (IFN)-gamma, the potent inducer of MHC class II, on the other hand, requires both STAT1 and CIITA in the induction process. IFN-alpha/beta has been shown to inhibit the IFN-y-induced expression of MHC class II by targeting a step(s) downstream of CIITA. Here we report that IFN-alpha/beta also inhibits the CIITA-independent expression of HPIV3-induced MHC class II. The inhibitory role of IFN-alpha/beta on HPIV3-induced MHC class II was confirmed by using anti-IFN-alpha/beta antibody and mutant cell lines defective in the IFN signaling components STAT1 and STAT2. IFN-alpha/beta inhibits virus-induced MHC class II expression just as it does IFN-gamma-induced MHC class II. The inhibition by IFN-alpha/beta of MHC class II expression may play a regulatory role in virus induced autoimmune disease mediated by MHC class II aberrant expression.

Human parainfluenza virus type 3 inhibits gamma interferon-induced major histocompatibility complex class II expression directly and by inducing alpha/beta interferon

Human parainfluenza virus type 3 (HPIV3) is one of the major causes of bronchiolitis, pneumonia, and croup in newborns and infants. Cellular immunity involving major histocompatibility complex (MHC) class I and class II molecules plays an important role in controlling virus infection. Several viruses have been shown to down-regulate gamma interferon (IFN-gamma)-mediated MHC class II expression. In this communication, we show that HPIV3 strongly inhibits the IFN-gamma-induced MHC class II expression in HT1080 human fibrosarcoma cells. The culture supernatant of HPIV3-infected cells also inhibited IFN-gamma-induced MHC class II expression, a phenomenon that was found to be due, in large part, to alpha/beta interferon (IFN-alpha/beta). Expression of MHC class I and intercellular adhesion molecule 1 occurred efficiently in cells simultaneously infected with HPIV3 and treated with IFN-gamma, indicating that the inhibitory effect of HPIV3 was specific to MHC class II. STAT1 activation was not affected by HPIV3 at early postinfection times but was partially inhibited at later times. These data suggested that the potent inhibition of MHC class II expression was, in major part, due to a defect downstream of STAT1 activation in the IFN-gamma-induced MHC class II expression pathway. Class II transactivator (CIITA) is the unique mediator of IFN-gamma-induced transcription from the MHC class II promoter. By RNase protection analysis, CIITA expression was found to be strongly inhibited in HPIV3-infected cells. The culture supernatant containing IFN-alpha/beta, on the other hand, inhibited MHC class II expression without affecting STAT1 and CIITA expression. These data indicate that HPIV3 inhibits IFN-gamma-induced MHC class II expression primarily by the viral gene products targeting CIITA and additionally by inducing IFN-alpha/beta to target one or more steps further downstream.

Human parainfluenza virus type 3 upregulates ICAM-1 (CD54) expression in a cytokine-independent manner

Human parainfluenza virus type 3 (HPIV3) causes bronchiolitis, pneumonia, and croup in newborns and infants. Several studies have implicated intercellular adhesion molecule-1 (ICAM-1) in inflammation during infection by viruses. In this study, we investigated the potential for HPIV3 to induce ICAM-1 in HT1080 cells. FACS analysis showed that HPIV3 strongly induced ICAM-1 expression in these cells. The ICAM-1 induction was significantly reduced when the virions were UV inactivated prior to infection, indicating that ICAM-1 induction was mostly viral replication dependent. Culture supernatant of HPIV3-infected cells induced ICAM-1 at an extremely low level, indicating that virus-induced cytokines played only a minor role in the induction process. Consistent with this, potent inducers of ICAM-1 such as IFN-gamma, TGF-beta, and TNF-alpha were absent in the culture supernatant, but a significant amount of IFN type 1 was present. By using U2A cells, which are defective in IFN type I signaling, we confirmed that ICAM-1 induction by HPIV3 occurred in a JAK/STAT signaling-independent manner. These data strongly indicate that HPIV3 induces ICAM-1 directly by viral antigens in a cytokine-independent manner; this induction may play a role in the inflammation during HPIV3 infection.

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.

Human parainfluenza virus type 3 up-regulates major histocompatibility complex class I and II expression on respiratory epithelial cells: involvement of a STAT1- and CIITA-independent pathway

Human parainfluenza virus type 3 (HPIV3) infection causes severe damage to the lung epithelium, leading to bronchiolitis, pneumonia, and croup in newborns and infants. Cellular immunity that plays a vital role in normal antiviral action appears to be involved, possibly because of inappropriate activation, in the infection-related damage to the lung epithelium. In this study, we investigated the expression of major histocompatibility complex (MHC) class I and II molecules on human lung epithelial (A549) and epithelium-like (HT1080) cells following HPIV3 infection. MHC class I was induced by HPIV3 in these cells at levels similar to those observed with natural inducers such as beta and gamma interferon (IFN-beta and -gamma). MHC class II was also efficiently induced by HPIV3 in these cells. UV-irradiated culture supernatants from infected cells were able to induce MHC class I but not MHC class II, suggesting involvement of released factors for the induction of MHC class I. Quantitation of IFN types I and II in the culture supernatant showed the presence of IFN-beta as the major cytokine, while IFN-gamma was undetectable. Anti-IFN-beta, however, blocked the HPIV3-mediated induction of MHC class I only partially, indicating that viral antigens, besides IFN-beta, are directly involved in the induction process. The induction of MHC class I and class II directed by the viral antigens was confirmed by using cells lacking STAT1, an essential intermediate of the IFN signaling pathways. HPIV3 induced both MHC class I and class II molecules in STAT1-null cells. Furthermore, MHC class II was also induced by HPIV3 in cells defective in class II transactivator, an important intermediate of the IFN-gamma-mediated MHC class II induction pathway. Together, these data indicate that the HPIV3 gene product(s) is directly involved in the induction of MHC class I and II molecules. The induction of MHC class I and II expression by HPIV3 suggests that it plays a role in the infection-related immunity and pathogenesis.

Sequence and genome organization of Borna disease virus

We have previously demonstrated that Borna disease virus (BDV) has a negative nonsegmented single-stranded (NNS) RNA genome that replicates in the nucleus of infected cells. Here we report for the first time the cloning and complete sequence of the BDV genome. Our results revealed that BDV has a genomic organization similar to that of other members of the Mononegavirales order. We have identified five main open reading frames (ORFs). The largest ORF, V, is located closest to the 5' end in the BDV genome and, on the basis of strong homology with other NNS-RNA virus polymerases, is a member of the L-protein family. The intercistronic regions vary in length and nucleotide composition and contain putative transcriptional start and stop signals. BDV untranslated 3' and 5' RNA sequences resemble those of other NNS-RNA viruses. Using a set of overlapping probes across the BDV genome, we identified nine in vivo synthesized species of polyadenylated subgenomic RNAs complementary to the negative-strand RNA genome, including monocistronic transcripts corresponding to ORFs I, II, and IV, as well as six polycistronic polyadenylated BDV RNAs. Interestingly, although ORFs III and V were detected within polycistronic transcripts, their corresponding monocistronic transcripts were not detected. Our data indicate that BDV is a member of the Mononegavirales, specially related to the family Rhabdoviridae. However, in contrast to the rest of the NNS-RNA animal viruses, BDV replication and transcription occur in the nucleus of infected cells. These findings suggest a possible relationship between BDV and the plant rhabdoviruses, which also replicate and transcribe in the nucleus.

Relative affinity of the human parainfluenza virus type 3 hemagglutinin-neuraminidase for sialic acid correlates with virus-induced fusion activity

The ability of enveloped viruses to cause disease depends on their ability to enter the host cell via membrane fusion events. An understanding of these early events in infection, crucial for the design of methods of blocking infection, is needed for viruses that mediate membrane fusion at neutral pH, such as paramyxoviruses and human immunodeficiency virus. Sialic acid is the receptor 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, the HN must interact with its receptor. In the present report, two variants of HPF3 with increased fusion-promoting phenotypes were selected and used to study the function of the HN glycoprotein in membrane fusion. Increased fusogenicity correlated with single amino acid changes in the HN protein that resulted in increased binding of the variant viruses to the sialic acid receptor. These results suggest that the avidity of binding of the HN protein to its receptor regulates the level of F protein-mediated fusion and begin to define one role of the receptor-binding protein of a paramyxovirus in the membrane fusion process.

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.

Subacute sclerosing panencephalitis virus dominantly interferes with replication of wild-type measles virus in a mixed infection: implication for viral persistence

Interaction between the Edmonston or Nagahata strain of acute measles virus (MV) and the defective Biken strain of MV isolated from a patient with subacute sclerosing panencephalitis (SSPE) was examined by a cell fusion protocol. Biken-CV-1 cells nonproductively infected with Biken strain SSPE virus were fused with neomycin-resistant CV-1 cells. All the fused cells selected with the neomycin analog G418 expressed Biken viral proteins, as determined by an immunofluorescence assay. This procedure enabled the transfer of Biken viral genomes into cells previously infected with MV. In the fused cells coinfected by Biken strain SSPE virus and Edmonston or Nagahata strain MV, early MV gene expression was suppressed, as determined by immunoprecipitation with strain-specific antibodies. Maturation of Edmonston strain MV was also suppressed. When the coinfected fused cells were selected with G418, Biken viral proteins remained at a constant level for up to 7 weeks. Wild-type MV proteins gradually decreased to a barely detectable level after 4 weeks and became undetectable after 7 weeks. Immunofluorescence studies showed a steady decline in cells expressing wild-type MV proteins in the coinfected cultures. These results suggest that Biken strain SSPE virus dominantly interferes with the replication of wild-type MV. The possible mechanisms of dominant interference and the implication for evolution of a persistent MV infection are discussed.

Properties of human parainfluenza virus type 3 RNA polymerase/replicase activity in vitro: consensus with other negative-stranded RNA viruses

A cell-free system supporting transcription, replication, and nucleocapsid assembly of the genome RNA of human parainfluenza virus type 3 (HPF3) is described. Cytoplasmic extracts from infected CV-1 or BHK cells catalyzed the transcription of the entire HPF3 genome, the replication of genome RNA, and the assembly of this RNA into nucleocapsidlike structures. Newly replicated RNA was resistant to micrococcal nuclease digestion and was stable in CsCl gradients, exhibiting the density of authentic HPF3 nucleocapsids. After fractionation of the extracts, the nucleocapsid-containing pellet fraction synthesized viral mRNAs. Reconstitution with the soluble protein fraction was necessary for genome RNA replication and nucleocapsid assembly.

Defective interfering particles of human parainfluenza virus type 3 are associated with persistent infection in cell culture

CV-1 cell lines persistently infected with human parainfluenza virus type 3 (HPF3) contain one or more distinct subgenomic RNAs in addition to standard viral genomes. These RNAs are shown to be the genomes of defective-interfering (DI) particles of the virus; they are present in particles in the culture fluid, and they interfere with the growth of wild-type virus. Removal of the particles from the culture fluid by ultracentrifugation yields a supernatant fluid free from inhibitory activity, demonstrating that the anti-viral effect is not mediated by soluble factors. A role for the DI particles in persistence of HPF3 is considered.

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.