Characterization of a temperature-sensitive influenza B virus mutant defective in neuraminidase

Development of Lentiviral Vectors Pseudotyped With Influenza B Hemagglutinins: Application in Vaccine Immunogenicity, mAb Potency, and Sero-Surveillance Studies

Influenza B viruses (IBV) cause respiratory disease epidemics in humans and are therefore components of seasonal influenza vaccines. Serological methods are employed to evaluate vaccine immunogenicity prior to licensure. However, classical methods to assess influenza vaccine immunogenicity such as the hemagglutination inhibition assay (HI) and the serial radial hemolysis assay (SRH), have been proven to have many limitations. As such, there is a need to develop innovative methods that can improve on these traditional assays and provide advantages such as ease of production and access, safety, reproducibility, and specificity. It has been previously demonstrated that the use of replication-defective viruses, such as lentiviral vectors pseudotyped with influenza A hemagglutinins in microneutralization assays (pMN) is a safe and sensitive alternative to study antibody responses elicited by natural influenza infection or vaccination. Consequently, we have produced Influenza B hemagglutinin-pseudotypes (IBV PV) using plasmid-directed transfection. To activate influenza B hemagglutinin, we have explored the use of proteases in increasing PV titers via their co-transfection during pseudotype virus production. When tested for their ability to transduce target cells, the influenza B pseudotypes produced exhibit tropism for different cell lines. The pseudotypes were evaluated as alternatives to live virus in microneutralization assays using reference sera standards, mouse and human sera collected during vaccine immunogenicity studies, surveillance sera from seals, and monoclonal antibodies (mAbs) against IBV. The influenza B pseudotype pMN was found to effectively detect neutralizing and cross-reactive responses in all assays and shows promise as an effective and versatile tool in influenza research.

Oseltamivir expands quasispecies of influenza virus through cell-to-cell transmission

The population of influenza virus consists of a huge variety of variants, called quasispecies, due to error-prone replication. Previously, we reported that progeny virions of influenza virus become infected to adjacent cells via cell-to-cell transmission pathway in the presence of oseltamivir. During cell-to-cell transmission, viruses become infected to adjacent cells at high multiplicity since progeny virions are enriched on plasma membrane between infected cells and their adjacent cells. Co-infection with viral variants may rescue recessive mutations with each other. Thus, it is assumed that the cell-to-cell transmission causes expansion of virus quasispecies. Here, we have demonstrated that temperature-sensitive mutations remain in progeny viruses even at non-permissive temperature by co-infection in the presence of oseltamivir. This is possibly due to a multiplex infection through the cell-to-cell transmission by the addition of oseltamivir. Further, by the addition of oseltamivir, the number of missense mutation introduced by error-prone replication in segment 8 encoding NS1 was increased in a passage-dependent manner. The number of missense mutation in segment 5 encoding NP was not changed significantly, whereas silent mutation was increased. Taken together, we propose that oseltamivir expands influenza virus quasispecies via cell-to-cell transmission, and may facilitate the viral evolution and adaptation.

Molecular epidemiology and evolution of influenza A and B viruses during winter 2013-2014 in Beijing, China

In this study, we investigated the molecular epidemiology and evolution of influenza viruses from patients infected during the 2013-2014 influenza season in Beijing. A phylogenetic analysis of the hemagglutinin (HA) and neuraminidase (NA) sequences of influenza A and B viruses from 18 patients (6 A(H1N1)pdm09, 4 H3N2, and 8 influenza B virus) was performed. Among the influenza A viruses, A(H1N1)pdm09 was the dominant subtype, whereas the B/Yamagata lineage was predominant for influenza B. The influenza B HA and NA strains in Beijing were dominated by reassortants derived from the Yamagata lineage and the Victoria lineage, respectively. All six A(H1N1)pdm09 strains fell into the 6B genetic group with amino acid substitutions D97N, S185T, K163Q, and A256T; the four H3N2 strains fell into genetic group 3C.3 with substitutions T128A, R142G, N145S, and V186G, and the eight influenza B strains were categorized into subgroup 3.1 and harbored an N217S mutation. Two new mutations (K180Q and G187E at the Sa and Ca antigenic sites of the H1 segment, respectively), which were not detected during the preceding influenza season, were identified. Mutations N131K, S165I, N181Y, and D212N in HA of influenza B mapped to the 120-loop, 150-loop, 160-loop, and 190-helix, respectively. Our results reveal the molecular epidemiology and phylogenetic characteristics of influenza viruses within a single geographic location and can have implications for vaccination selection in northern China.

Neuraminidase-producing oral mitis group streptococci potentially contribute to influenza viral infection and reduction in antiviral efficacy of zanamivir

Influenza is a serious respiratory disease among immunocompromised individuals, such as the elderly, and its prevention is an urgent social issue. Influenza viruses rely on neuraminidase (NA) activity to release progeny viruses from infected cells and spreading the infection. NA is, therefore, an important target of anti-influenza drugs. A causal relationship between bacteria and influenza virus infection has not yet been established, however, a positive correlation between them has been reported. Thus, in this study, we examined the biological effects of oral mitis group streptococci, which are predominant constituents of human oral florae, on the release of influenza viruses. Among them, Streptococcus oralis ATCC 10557 and Streptococcus mitis ATCC 6249 were found to exhibit NA activity and their culture supernatants promoted the release of influenza virus and cell-to-cell spread of the infection. In addition, culture supernatants of these NA-producing oral bacteria increased viral M1 protein expression levels and cellular ERK activation. These effects were not observed with culture supernatants of Streptococcus sanguinis ATCC 10556 which lacks the ability to produce NA. Although the NA inhibitor zanamivir suppressed the release of progeny viruses from the infected cells, the viral release was restored upon the addition of culture supernatants of NA-producing S. oralis ATCC 10557 or S. mitis ATCC 6249. These findings suggest that an increase in the number of NA-producing oral bacteria could elevate the risk of and exacerbate the influenza infection, hampering the efficacy of viral NA inhibitor drugs.

Separate cellular localizations of human T-lymphotropic virus 1 (HTLV-1) Env and glucose transporter type 1 (GLUT1) are required for HTLV-1 Env-mediated fusion and infection

Interaction of the envelope glycoprotein (Env) of human T-lymphotropic virus 1 (HTLV-1) with the glucose transporter type 1 (GLUT1) expressed in target cells is essential for viral entry. This study found that the expression level of GLUT1 in virus-producing 293T cells was inversely correlated with HTLV-1 Env-mediated fusion activity and infectivity. Chimeric studies between GLUT1 and GLUT3 indicated that the extracellular loop 6 (ECL6) of GLUT1 is important for the inhibition of cell-cell fusion mediated by Env. When GLUT1 was translocated into the plasma membrane from intracellular storage sites by bafilomycin A1 (BFLA1) treatment in 293T cells, HTLV-1 Env-mediated cell fusion and infection also were inhibited without the overexpression of GLUT1, indicating that the localization of GLUT1 in intracellular compartments rather than in the plasma membrane is crucial for the fusion activity of HTLV-1 Env. Immunoprecipitation and laser scanning confocal microscopic analyses indicated that under normal conditions, HTLV-1 Env and GLUT1 do not colocalize or interact. BFLA1 treatment induced this colocalization and interaction, indicating that GLUT1 normally accumulates in intracellular compartments separate from that of Env. Western blot analyses of FLAG-tagged HTLV-1 Env in virus-producing cells and the incorporation of HTLV-1 Env in virus-like particles (VLPs) indicate that the processing of Env is inhibited by either overexpression of GLUT1 or BFLA1 treatment in virus-producing 293T cells. This inhibition probably is due to the interaction of the Env with GLUT1 in intracellular compartments. Taken together, separate intracellular localizations of GLUT1 and HTLV-1 Env are required for the fusion activity and infectivity of HTLV-1 Env.

IMPORTANCE

The deltaretrovirus HTLV-1 is a causative agent of adult T-cell leukemia (ATL) and HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). Although HTLV-1 is a complex retrovirus that has accessory genes, no HTLV-1 gene product has yet been shown to regulate its receptor GLUT1 in virus-producing cells. In this study, we found that a large amount of GLUT1 or translocation of GLUT1 to the plasma membrane from intracellular compartments in virus-producing cells enhances the colocalization and interaction of GLUT1 with HTLV-1 Env, leading to the inhibition of cell fusion activity and infectivity. The results of our study suggest that GLUT1 normally accumulates in separate intracellular compartments from Env, which is indeed required for the proper processing of Env.

Hemagglutinin and neuraminidase genes of influenza B viruses circulating in Riyadh, Saudi Arabia during 2010-2011: evolution and sequence analysis

Influenza viruses are known as continuing threats to human public health every year worldwide. Evolutionary dynamics of influenza B viruses in humans are in a unique progression having two lineages; B/Yam and B/Vic-like viruses, which are circulating simultaneously worldwide. There is a considerable lack of data on influenza B viruses circulating in Saudi Arabia. During the winter-spring season of 2010-2011, 80 nasopharyngeal aspirates were collected from hospitalized patients with flu-like symptoms in Riyadh. Screening of samples by one-step RT-PCR identified three (3.8%) influenza B viruses. Sequencing of hemagglutinin (HA) and neuraminidase (NA) genes was performed to analyze influenza B viruses circulating in Riyadh as compared to the globally circulating strains. Several common and six unique amino acid substitutions were observed for both HA and NA genes of influenza B Saudi strains. Three unique substitutions (T182A, D196N, and K254R) were identified in HA gene of the B/Yam-like Riyadh strains. In NA gene, a unique common substitution (D53G) was found in all Riyadh strains, while two unique substitutions (L38P, G233R) were recognized only in B/Vic-like Riyadh strains. Riyadh strains were also found to contain N-glycosylation site in HA gene of both B/Vic and B/Yam lineages at positions 197-199 (NET) and 196-198 (NNK/DNK), respectively. The significance of these mutations on the antigenicity of both lineages is discussed herein. The unique changes observed in HA and NA genes of influenza B Riyadh strains support strongly the need for continuous surveillance and monitoring of new evolving strains that might pose threat to the Saudi community.

Tamiflu-resistant but HA-mediated cell-to-cell transmission through apical membranes of cell-associated influenza viruses

The infection of viruses to a neighboring cell is considered to be beneficial in terms of evasion from host anti-virus defense systems. There are two pathways for viral infection to “right next door”: one is the virus transmission through cell-cell fusion by forming syncytium without production of progeny virions, and the other is mediated by virions without virus diffusion, generally designated cell-to-cell transmission. Influenza viruses are believed to be transmitted as cell-free virus from infected cells to uninfected cells. Here, we demonstrated that influenza virus can utilize cell-to-cell transmission pathway through apical membranes, by handover of virions on the surface of an infected cell to adjacent host cells. Live cell imaging techniques showed that a recombinant influenza virus, in which the neuraminidase gene was replaced with the green fluorescence protein gene, spreads from an infected cell to adjacent cells forming infected cell clusters. This type of virus spreading requires HA activation by protease treatment. The cell-to-cell transmission was also blocked by amantadine, which inhibits the acidification of endosomes required for uncoating of influenza virus particles in endosomes, indicating that functional hemagglutinin and endosome acidification by M2 ion channel were essential for the cell-to-cell influenza virus transmission. Furthermore, in the cell-to-cell transmission of influenza virus, progeny virions could remain associated with the surface of infected cell even after budding, for the progeny virions to be passed on to adjacent uninfected cells. The evidence that cell-to-cell transmission occurs in influenza virus lead to the caution that local infection proceeds even when treated with neuraminidase inhibitors.

Role of transmembrane domain and cytoplasmic tail amino acid sequences of influenza a virus neuraminidase in raft association and virus budding

Influenza virus neuraminidase (NA), a type II transmembrane glycoprotein, possesses receptor-destroying activity and thereby facilitates virus release from the cell surface. Among the influenza A viruses, both the cytoplasmic tail (CT) and transmembrane domain (TMD) amino acid sequences of NA are highly conserved, yet their function(s) in virus biology remains unknown. To investigate the role of amino acid sequences of the CT and TMD on the virus life cycle, we systematically mutagenized the entire CT and TMD of NA by converting two to five contiguous amino acids to alanine. In addition, we also made two chimeric NA by replacing the CT proximal one-third amino acids of the NA TMD [NA(1T2N)NA] and the entire NA TMD (NATRNA) with that of human transferrin receptor (TR) (a type II transmembrane glycoprotein). We rescued transfectant mutant viruses by reverse genetics and examined their phenotypes. Our results show that all mutated and chimeric NAs could be rescued into transfectant viruses. Different mutants showed pleiotropic effects on virus growth and replication. Some mutants (NA2A5, NA3A7, and NA4A10) had little effect on virus growth while others (NA3A2, NA5A27, and NA5A31) produced about 50- to 100-fold-less infectious virus and still some others (NA5A14, NA4A19, and NA4A23) exhibited an intermediate phenotype. In general, mutations towards the ectodomain-proximal sequences of TMD progressively caused reduction in NA enzyme activity, affected lipid raft association, and attenuated virus growth. Electron microscopic analysis showed that these mutant viruses remained aggregated and bound to infected cell surfaces and could be released from the infected cells by bacterial NA treatment. Moreover, viruses containing mutations in the extreme N terminus of the CT (NA3A2) as well as chimeric NA containing the TMD replaced partially [NA(1T2N)NA] or fully (NATRNA) with TR TMD caused reduction in virus growth and exhibited the morphological phenotype of elongated particles. These results show that although the sequences of NA CT and TMD per se are not absolutely essential for the virus life cycle, specific amino acid sequences play a critical role in providing structural stability, enzyme activity, and lipid raft association of NA. In addition, aberrant morphogenesis including elongated particle formation of some mutant viruses indicates the involvement of NA in virus morphogenesis and budding.

Analysis of the desialidation process of the haemagglutinin protein of influenza B virus: the host-dependent desialidation step

It was reported previously that haemadsorption by the haemagglutinin (HA) protein of influenza B virus required that the protein must undergo desialidation. When MDCK and COS cells were infected with influenza B/Kanagawa/73 virus in the presence of a neuraminidase (NA) inhibitor, Zanamivir, haemadsorption on MDCK cells was inhibited but that on COS cells was not. The activity of the NA protein of the two types of infected cells was similar and both were inhibited by Zanamivir in a dose-dependent manner. A comparison of the desialidation of the HA protein was made on MDCK and COS cells in the presence of bacterial NA and both cells were found to have similar sensitivity. On the accumulation of the HA and NA proteins in the trans-Golgi network of MDCK cells by means of low-temperature treatment, desialidation of the HA protein in the presence of Zanamivir was detected by two-dimensional gel electrophoresis. Because this agent was reported to be unable to penetrate cells, these data suggest that, in MDCK cells, desialidation of the HA protein occurs on the cell surface but, in COS cells, the HA and NA proteins might accumulate in the trans-Golgi network, thus allowing NA desialidation before their migration to the cell surface.

Adaptation of influenza A viruses to cells expressing low levels of sialic acid leads to loss of neuraminidase activity

Influenza A viruses possess two virion surface proteins, hemagglutinin (HA) and neuraminidase (NA). The HA binds to sialyloligosaccharide viral receptors, while the NA removes sialic acids from the host cell and viral sialyloligosaccarides. Alterations of the HA occur during adaptation of influenza viruses to new host species, as in the 1957 and 1968 influenza pandemics. To gain a better understanding of the contributions of the HA and possibly the NA to this process, we generated cell lines expressing reduced levels of the influenza virus receptor determinant, sialic acid, by selecting Madin-Darby canine kidney cells resistant to a lectin specific for sialic acid linked to galactose by alpha(2-3) or alpha(2-6) linkages. One of these cell lines had less than 1/10 as much N-acetylneuraminic acid as its parent cell line. When serially passaged in this cell line, human H3N2 viruses lost sialidase activity due to a large internal deletion in the NA gene, without alteration of the HA gene. These findings indicate that NA mutations can contribute to the adaptation of influenza A virus to new host environments and hence may play a role in the transmission of virus across species.

Influenza A viruses lacking sialidase activity can undergo multiple cycles of replication in cell culture, eggs, or mice

Influenza A viruses possess both hemagglutinin (HA), which is responsible for binding to the terminal sialic acid of sialyloligosaccharides on the cell surface, and neuraminidase (NA), which contains sialidase activity that removes sialic acid from sialyloligosaccharides. Interplay between HA receptor-binding and NA receptor-destroying sialidase activity appears to be important for replication of the virus. Previous studies by others have shown that influenza A viruses lacking sialidase activity can undergo multiple cycles of replication if sialidase activity is provided exogenously. To investigate the sialidase requirement of influenza viruses further, we generated a series of sialidase-deficient mutants. Although their growth was less efficient than that of the parental NA-dependent virus, these viruses underwent multiple cycles of replication in cell culture, eggs, and mice. To understand the molecular basis of this viral growth adaptation in the absence of sialidase activity, we investigated changes in the HA receptor-binding affinity of the sialidase-deficient mutants. The results show that mutations around the HA receptor-binding pocket reduce the virus's affinity for cellular receptors, compensating for the loss of sialidase. Thus, sialidase activity is not absolutely required in the influenza A virus life cycle but appears to be necessary for efficient virus replication.

An analysis of the role of neuraminidase in the receptor-binding activity of influenza B virus: the inhibitory effect of Zanamivir on haemadsorption

We analysed the role of neuraminidase (NA) on haemadsorption by the haemagglutinin (HA) protein of influenza B virus. The influenza B virus mutant ts-7 has a temperature-sensitive mutation in the NA protein. At high temperature, cells infected with this virus did not exhibit haemadsorption activity, but the addition of bacterial neuraminidase (bNA) restored haemadsorption activity. COS cells transfected with HA cDNAs of B/Kanagawa/73 or B/Lee/40 virus showed no evidence of haemadsorption. However, with the addition of bNA or co- transfection with NA cDNA of the B/Lee/40 virus, haemadsorption was observed. Experiments with point-mutated HA cDNAs of B/Lee/40 virus showed that two N-acetyl glycosylation sites at amino acid residues 160 and 217 were responsible for the inability of the HA protein to adsorb to erythrocytes. These results indicated that haemadsorption by the HA protein of influenza B virus required the involvement of NA. Because the NA inhibitor Zanamivir was reported not to penetrate cells, we investigated the action of this inhibitor and found that Zanamivir inhibited haemadsorption on MDCK cells infected with B/Kanagawa/73 or B/Lee/40 virus. After removing Zanamivir by washing, the addition of bNA restored the haemadsorption activity on the infected cells. Scanning electron microscopy indicated that at 0.4 microM Zanamivir, HA protein did not adsorb to erythrocytes but retained the ability to aggregate virions. However, at 4 microM Zanamivir, distinct virion formation could not be observed.

Isolation and characterization of a transfectant influenza B virus altered in RNA segment 6

This report describes the successful generation of an influenza B transfectant virus altered in RNA segment 6, which encodes the neuraminidase (NA) protein. The procedure for selection of the transfectant virus relies on the use of strain-specific anti-NA monoclonal antibodies to inhibit growth of the helper virus within the system. A transfectant virus has been engineered which has a coding change in the NA protein. This change resulted in attenuated growth in vitro that could be rescued by addition of exogenous bacterial NA. The mutant virus-associated NA activity was unstable as a result of the engineered changes. The ability to genetically manipulate influenza B virus segment 6 will allow us to assess the function of both NA and the small protein NB, also coded from this RNA, within the context of the virus infectious cycle.

Approaches and strategies for the treatment of influenza virus infections

Influenza A and B viruses belong to the Orthomyxoviridae family of viruses. These viruses are responsible for severe morbidity and significant excess mortality each year. Infection with influenza viruses usually leads to respiratory involvement and can result in pneumonia and secondary bacterial infections. Vaccine approaches to the prophylaxis of influenza virus infections have been problematic owing to the ability of these viruses to undergo antigenic shift by exchanging genomic segments or by undergoing antigenic drift, consisting of point mutations in the haemagglutinin (HA) and neuraminidase (NA) genes as a result of an error-prone viral polymerase. Historically, antiviral approaches for the therapy of both influenza A and B viruses have been largely unsuccessful until the elucidation of the X-ray crystallographic structure of the viral NA, which has permitted structure-based drug design of inhibitors of this enzyme. In addition, recent advances in the elucidation of the structure and complex function of influenza HA have resulted in the discovery of a number of diverse compounds that target this viral protein. This review article will focus largely on newer antiviral agents including those that inhibit the influenza virus NA and HA. Other novel approaches that have entered clinical trials or been considered for their clinical utility will be mentioned.

Cell-surface expression of CD4 reduces HIV-1 infectivity by blocking Env incorporation in a Nef- and Vpu-inhibitable manner

BACKGROUND

Human immunodeficiency virus-1 (HIV-1) infection decreases the cell-surface expression of its cellular receptor, CD4, through the combined actions of Nef, Env and Vpu. Such functional convergence strongly suggests that CD4 downregulation is critical for optimal viral replication, yet the significance of this phenomenon has so far remained a puzzle.

RESULTS

We show that high levels of CD4 on the surface of HIV-infected cells induce a dramatic reduction in the infectivity of released virions by the sequestering of the viral envelope by CD4. CD4 is able to accumulate in viral particles while at the same time blocking incorporation of Env into the virion. Nef and Vpu, through their ability to downregulate CD4, counteract this effect.

CONCLUSIONS

The CD4-mediated 'envelope interference' described here probably explains the plurality of mechanisms developed by HIV to downregulate the cell-surface expression of its receptor.

The transmembrane domain in viral fusion: essential role for a conserved glycine residue in vesicular stomatitis virus G protein

The transmembrane (TM) domains of viral fusion proteins are required for fusion, but their precise role is unknown. G protein, the fusion protein of vesicular stomatitis virus, was previously shown to lose syncytia-forming ability if six residues (GLIIGL) were deleted from its TM domain. The 20-residue TM domain of wild-type (TM20) G protein was thus changed into a TM domain of 14 residues (TM14). To assess possible sequence specificity for this loss of function, the two Gly residues in TM20 were replaced with either Ala or Leu. Both mutations resulted in complete loss of fusion activity, as measured by fusion-dependent reporter gene transfer. Single substitutions decreased activity by about half. TM14 was weakly active (15%) but reintroduction of a Gly residue into TM14 by a single Ile --> Gly substitution increased activity to 80%. All mutants retained normal hemifusion activity, i.e., lipid mixing between the outer leaflets of the reacting membranes. Thus, at least one TM Gly residue is required for a late step in fusion mediated by G protein. Gly residues were significantly (2.6-fold; P = 0.004) more abundant in the TM domains of viral fusion proteins than in those of nonfusion proteins and were distributed differently within the TM domain. Thus, Gly residues in the TM domain of other viral fusion proteins may also prove to be important for fusion activity.

Expression of influenza B virus hemagglutinin containing multibasic residue cleavage sites

The hemagglutinin (HA) protein of influenza B virus contains a single arginine residue at its cleavage site and the HA0 precursor is not cleaved to the HA1 and HA2 subunits by tissue culture cell-associated proteases. To investigate if an HA protein could be obtained that could be cleaved by an endogenous cellular protease, the cDNA for HA of influenza B/MD/59 virus was subjected to site-specific mutagenesis. Three HA mutant proteins were constructed, through substitution or insertion of arginine residues, that have 4, 5, or 6 basic residues at their cleavage sites. Chemical cross-linking studies indicated that all three HA cleavage site mutants could oligomerize to a trimeric species, like WT HA. The three HA cleavage site mutant proteins were efficiently transported to the cell surface and bound erythrocytes in hemadsorption assays. The mutants were cleaved at a low level to HA1 and HA2 by an endogenous host cell protease and cleavage could be increased somewhat by addition of exogenous trypsin. The fusogenic activities of the HA cleavage site mutants were assessed in comparison to the WT HA protein by determining their syncytium formation ability and by using an R18 lipid-mixing assay and a NBD-taurine aqueous-content mixing assay. While the fusion activity of the WT HA protein was dependent on exogenous trypsin to activate HA, the three HA cleavage site mutant proteins were able to induce fusion in the absence of trypsin when assayed with the R18 lipid-mixing and NBD-taurine aqueous-content mixing assays, but were unable to induce syncytium formation in either the presence or absence of exogenous trypsin. Our results suggest that while the presence of a subtilisin-like protease cleavage sequence at the influenza B virus HA1/HA2 boundary does enable some HA0 molecules to be cleaved intracellularly, it alone is not sufficient for efficient cleavage.

CD4 down-modulation during infection of human T cells with human immunodeficiency virus type 1 involves independent activities of vpu, env, and nef

The human immunodeficiency virus type 1 (HIV-1) genes vpu, env, and nef have all been implicated in modulating the levels of cell surface CD4 on infected cells. To quantitatively assess the relative contribution of each gene product to the regulation of CD4 during HIV infection of Jurkat T cells and peripheral blood mononuclear cells, we have developed an infectious HIV reporter system which expresses different combinations of these genes. To distinguish infected cells in the early or late stages of infection from uninfected cells, these viruses were designed to express human placental alkaline phosphatase with the kinetics of either early or late viral genes. Flow cytometry to detect placental alkaline phosphatase and CD4 in infected cells showed that vpu, env, and nef are independently capable of down-modulation of CD4. As predicted by their respective expression patterns, nef down-modulated CD4 rapidly during the early phase of virus infection whereas vpu and env functioned late in the infection. In both Jurkat cells and peripheral blood mononuclear cells, a combination of the three genes was more efficient than any one or two genes, demonstrating that all three genes are required to achieve maximal CD4 down-modulation. In primary cells, down-modulation of CD4 was less efficient than in Jurkat cells and there was a stronger dependence on nef function for reducing cell surface CD4. HIV therefore has three genes that are able to independently down-modulate CD4; together, they can eliminate the bulk of cell surface CD4.

Temperature-sensitive phenotype of the human parainfluenza virus type 3 candidate vaccine strain (cp45) correlates with a defect in the L gene

We have previously demonstrated that the temperature sensitivity of a human parainfluenza virus type 3 (HPIV-3) candidate vaccine strain (cp45), which is currently under evaluation in humans, is associated with poor transcriptional activity of the virus at the nonpermissive temperature (R. Ray, K. Meyer, F. Newman, and R. B. Belshe, J. Virol. 69:1959-1963, 1995). In this study, the temperature sensitivity of cp45 virus was further investigated by the complementation of a specific gene function. CV-1 cells were transfected with cloned genes from wild-type HPIV-3 encoding the large protein (L), phosphoprotein (P), and nucleocapsid protein (NP), alone or together, for the expression of biologically active proteins. Only cells expressing the L gene were able to rescue cp45 replication when incubated at the nonpermissive temperature (39.5 degrees C), whereas cells transiently expressing NP or P were incapable of rescuing the virus. The virus titers obtained following complementation of the L protein were 190 to 2,300 PFU/ml of culture medium, compared with the undetectable growth of the cp45 temperature-sensitive mutant at the nonpermissive temperature. Rescued progeny virus still maintained the temperature-sensitive phenotype. Results from this study suggest that the temperature sensitivity of the cp45 candidate vaccine strain is associated primarily with L-protein function and that the defect can be complemented by transient expression of the wild-type protein. This study underscores the importance of the L protein in RNA polymerase activity and its critical role in virus replication.

Characterization of a live, attenuated human parainfluenza type 3 virus candidate vaccine strain

Characterization of a temperature-sensitive and live, attenuated human parainfluenza type 3 virus strain (cp45) grown at a permissive temperature (32 degrees C) suggested that the virus efficiently multiplies in cell lines and retains antigenic and functional properties of the envelope glycoproteins. When grown at a nonpermissive temperature (39.5 degrees C), the cp45 virus exhibited poor replication; however, shifting to a permissive temperature allowed virus growth. Although at a nonpermissive temperature virus polypeptide synthesis was significantly reduced, the hemagglutinin-neuraminidase and fusion glycoproteins were transported to cell surfaces and retained their characteristic biologic activities. Studies on mRNA synthesis from the P protein gene suggested a poor transcriptional activity of the cp45 virus at a nonpermissive temperature. Results from this study indicate that the temperature sensitivity of cp45 virus is related to altered transcriptional activity and a marked reduction in virus polypeptide synthesis.

Influenza type A virus neuraminidase does not play a role in viral entry, replication, assembly, or budding

We have used a neuraminidase-deficient influenza virus, NWS-Mvi, which was selected by supplying bacterial neuraminidase in the medium (C. Liu and G. M. Air, Virology 194:403-407, 1993), to define the role of neuraminidase in influenza virus replication. Electron microscopy showed that virions of the NWS-Mvi mutant assembled normally and formed large aggregates associated with cell surfaces. The NWS-Mvi virus grown in the absence of neuraminidase was able to carry out a second round of replication in MDCK cells without added neuraminidase, indicating that the virus particles contained in these aggregates were infectious. Aggregates of virus were also found in cytoplasmic vacuoles. When virus-infected cells were incubated in the presence of ferritin, such aggregates were found to be labeled with ferritin, indicating that they are derived from uptake at the cell surface. When the neuraminidase-deficient virus was administered intranasally to C57BL/6 mice, low titers of virus were recovered from the lungs and major histocompatibility complex class I-restricted cytotoxic T cells were generated: evidence that cells were infected in vivo. In C57BL/6 nu/nu mice, the low level of virus persisted for at least 28 days but never increased. These results suggest that neuraminidase is not required for influenza virus entry, replication, or assembly in cell culture or in mice.

Approach to the involvement of influenza B neuraminidase in the cleavage of HA by host cell protease using low pH-induced cell fusion reaction

ts7, a temperature-sensitive mutant defective in neuraminidase (NA) of influenza B/Kanagawa/73, lacks NA enzymatic activity at the nonpermissive temperature (37.5 C). When MDCK cells were infected with the mutant at the permissive temperature (32 C) and exposed to pH 5.2 medium, extensive cell fusion occurred. In contrast, at the nonpermissive temperature cells did not show cell fusion at all unless they were pretreated with trypsin, suggesting that at 37.5 C the hemagglutinin (HA) of ts7 is expressed at the cell surface in an uncleaved form. It was also found that the replacement of RNA segment 6 of ts7 with that of wild-type B/Lee resulted in the emergence of low pH-induced fusion activity as well as NA enzymatic activity at the incubation temperature of 37.5 C and that the addition of bacterial NA to the cultures infected with ts7 at 37.5 C early in infection brought about low pH-induced cell fusion. We suggest that the removal of neuraminic acid from the carbohydrate moiety of HA by NA is essential for the cleavage of HA by cellular protease.

Isolation and classification of temperature-sensitive mutants of influenza B virus

We isolated 25 temperature-sensitive mutants of B/Kanagawa/73 strain generated by mutagenesis with 5-fluorouracil and classified them into seven recombination groups by pair-wise crosses. All mutants showed a ratio of plaquing efficiency at the nonpermissive temperature (37.5 C) to the permissive temperature (32 C) of 10(-4) or less. At 37.5 C most of group I, II, and III mutants did not produce appreciable amounts of protein, but all other group mutants were protein synthesis-positive. A group VII mutant produced active hemagglutinin (HA) and neuraminidase (NA) at the nonpermissive temperature, but Group V mutants produced only active NA and were defective in the HA molecule. The other group mutants, including group IV mutants with mutation only in the NA gene (8, 10), lacked both activities at the nonpermissive temperature. One of nine influenza B virus isolates in 1989 had EOP 37.5/32 of 1/3 x 10(-2) and belonged to recombination group VII.

Role of neuraminidase in the morphogenesis of influenza B virus

When ts7, a temperature-sensitive (ts) mutant of influenza B/Kanagawa/73 virus, infected MDCK cells at the nonpermissive temperature (37.5 degrees C), infectious virus was produced at very low levels compared with the yield at the permissive temperature (32 degrees C) and hemagglutinating activity and enzymatic activity of neuraminidase (NA) were negligible. However, viral protein synthesis and transport of hemadsorption-active hemagglutinin to the cell surface were not affected. When the cell lysate was treated with bacterial NA, hemagglutinating activity was recovered but infectivity was not, even after further treatment with trypsin. It was found that ts7 was defective in transport of NA to the cell surface and formation of virus particles. Analysis of the genomes of non-ts recombinants obtained by crossing ts7 and UV-inactivated B/Lee showed that ts7 had the ts mutation only in RNA segment 6 coding for NA and the glycoprotein NB. Nucleotide sequence analysis of the RNA segment revealed that ts7 had four amino acid changes in the NA molecule but not in NB. We suggest that assembly or budding of influenza B virus requires the presence of NA at the plasma membrane, unlike influenza A virus.