Cell killing by viruses. I. Comparison of cell-killing, plaque-forming, and defective-interfering particles of vesicular stomatitis virus

Virus-like Particles: Measures and Biological Functions

Virus-like particles resemble infectious virus particles in size, shape, and molecular composition; however, they fail to productively infect host cells. Historically, the presence of virus-like particles has been inferred from total particle counts by microscopy, and infectious particle counts or plaque-forming-units (PFUs) by plaque assay; the resulting ratio of particles-to-PFUs is often greater than one, easily 10 or 100, indicating that most particles are non-infectious. Despite their inability to hijack cells for their reproduction, virus-like particles and the defective genomes they carry can exhibit a broad range of behaviors: interference with normal virus growth during co-infections, cell killing, and activation or inhibition of innate immune signaling. In addition, some virus-like particles become productive as their multiplicities of infection increase, a sign of cooperation between particles. Here, we review established and emerging methods to count virus-like particles and characterize their biological functions. We take a critical look at evidence for defective interfering virus genomes in natural and clinical isolates, and we review their potential as antiviral therapeutics. In short, we highlight an urgent need to better understand how virus-like genomes and particles interact with intact functional viruses during co-infection of their hosts, and their impacts on the transmission, severity, and persistence of virus-associated diseases.

High-Throughput Single-Cell Kinetics of Virus Infections in the Presence of Defective Interfering Particles

Defective interfering particles (DIPs) are virus mutants that lack essential genes for growth. In coinfections with helper virus, the diversion of viral proteins to the replication and packaging of DIP genomes can interfere with virus production. Mounting cases of DIPs and DIP-like genomes in clinical and natural isolates, as well as growing interest in DIP-based therapies, underscore a need to better elucidate how DIPs work. DIP activity is primarily measured by its inhibition of virus infection yield, an endpoint that masks the dynamic and potentially diverse individual cell behaviors. Using vesicular stomatitis virus (VSV) as a model, we coinfected BHK cells with VSV DIPs and recombinant helper virus carrying a gene encoding a red fluorescent protein (RFP) whose expression correlates with the timing and level of virus release. For single cells within a monolayer, 10 DIPs per cell suppressed the reporter expression in only 1.2% of the cells. In most cells, it slowed and reduced viral gene expression, manifested as a shift in mean latent time from 4 to 6 h and reduced virus yields by 10-fold. For single cells isolated in microwells, DIP effects were more pronounced, reducing virus yields by 100-fold and extending latent times to 12 h, including individual instances above 20 h. Together, these results suggest that direct or indirect cell-cell interactions prevent most coinfected cells from being completely suppressed by DIPs. Finally, a gamma distribution model captures well how the infection kinetics quantitatively depends on the DIP dose. Such models will be useful for advancing a predictive biology of DIP-associated virus growth and infection spread.

IMPORTANCE

During the last century, basic studies in virology have focused on developing a molecular mechanistic understanding of how infectious viruses reproduce in their living host cells. However, over the last 10 years, the advent of deep sequencing and other powerful technologies has revealed in natural and patient infections that viruses do not act alone. Instead, viruses are often accompanied by defective virus-like particles that carry large deletions in their genomes and fail to replicate on their own. Coinfections of viable and defective viruses behave in unpredictable ways, but they often interfere with normal virus growth, potentially enabling infections to evade host immune surveillance. In the current study, controlled levels of defective viruses are coinfected with viable viruses that have been engineered to express a fluorescent reporter protein during infection. Unique profiles of reporter expression acquired from thousands of coinfected cells reveal how interference acts at multiple stages of infection.

Quantitative characterization of defective virus emergence by deep sequencing

Populations of RNA viruses can spontaneously produce variants that differ in genome size, sequence, and biological activity. Defective variants that lack essential genes can nevertheless reproduce by coinfecting cells with viable virus, a process that interferes with virus growth. How such defective interfering particles (DIPs) change in abundance and biological activity within a virus population is not known. Here, a prototype RNA virus, vesicular stomatitis virus (VSV), was cultured for three passages on BHK host cells, and passages were subjected to Illumina sequencing. Reads from the initial population, when aligned to the full-length viral sequence (11,161 nucleotides [nt]), distributed uniformly across the genome. However, during passages two plateaus in read counts appeared toward the 5' end of the negative-sense viral genome. Analysis by normalization and a simple sliding-window approach revealed plateau boundaries that suggested the emergence and enrichment of at least two truncated species having medium (∼5,900 nt) and short (∼4,000 nt) genomes. Relative measures of full-length and truncated species based on read counts were validated by quantitative reverse transcription-PCR (qRT-PCR). Limit-of-detection analysis suggests that deep sequencing can be more sensitive than complementary measures for detecting and quantifying defective particles in a population. Further, particle counts from transmission electron microscopy, coupled with infectivity assays, linked the rise in smaller genomes with an increase in truncated particles and interference activity. In summary, variation in deep sequencing coverage simultaneously shows the size, location, and relative level of truncated-genome variants, revealing a level of population heterogeneity that is masked by other measures of viral genomes and particles.

IMPORTANCE

We show how deep sequencing can be used to characterize the emergence, diversity, and relative abundance of truncated virus variants in virus populations. Adaptation of this approach to natural isolates may elucidate factors that influence the stability and persistence of virus populations in nature.

Dynamics of biologically active subpopulations of influenza virus: plaque-forming, noninfectious cell-killing, and defective interfering particles

The dynamic changes in the temporal appearance and quantity of a new class of influenza virus, noninfectious cell-killing particles (niCKP), were compared to defective interfering particles (DIP). After a single high-multiplicity passage in MDCK cells of an egg-derived stock that lacked detectable niCKP or DIP, both classes of particles appeared in large numbers (>5 x 10(8)/ml), and the plaque-forming particle (PFP) titer dropped approximately 60-fold. After two additional serial high-multiplicity passages the DIP remained relatively constant, the DIP/niCKP ratio reached 10:1, and the PFP had declined by about 10,000-fold. Together, the niCKP and DIP subpopulations constituted ca. 20% of the total hemagglutinating particle population in which these noninfectious biologically active particles (niBAP) were subsumed. DIP neither killed cells nor interfered with the cell-killing (apoptosis-inducing) activity of niCKP or PFP (infectious CKP), even though they blocked the replication of PFP. Relative to the UV-target of approximately 13,600 nucleotides (nt) for inactivation of PFP, the UV target for niCKP was approximately 2,400 nt, consistent with one of the polymerase subunit genes, and that for DIP was approximately 350 nt, consistent with the small DI-RNA responsible for DIP-mediated interference. Thus, niCKP and DIP are viewed as distinct particles with a propensity to form during infection at high multiplicities. These conditions are postulated to cause aberrations in the temporally regulated replication of virus and its packaging, leading to the production of niBAP. DIP have been implicated in the virulence of influenza virus, but the role of niCKP is yet unknown.

Clonogenic assay of type a influenza viruses reveals noninfectious cell-killing (apoptosis-inducing) particles

Clonogenic (single-cell plating) assays were used to define and quantify subpopulations of two genetically closely related variants of influenza virus A/TK/OR/71 that differed primarily in the size of the NS1 gene product; they expressed a full-size (amino acids [aa] 1 to 230) or truncated (aa 1 to 124) NS1 protein. Monolayers of Vero cells were infected with different amounts of virus, monodispersed, and plated. Cell survival curves were generated from the fraction of cells that produced visible colonies as a function of virus multiplicity. The exponential loss of colony-forming capacity at low multiplicities demonstrated that a single virus particle sufficed to kill a cell. The ratios of cell-killing particles (CKP) to plaque-forming particles (PFP) were 1:1 and 7:1 in populations of variants NS1(1-124) and NS1(1-230), respectively. This study revealed a new class of particles in influenza virus populations-noninfectious CKP. Both infectious and noninfectious CKP were 6.3 times more resistant to UV radiation than PFP activity. Based on UV target theory, a functional polymerase subunit was implicated in a rate-limiting step in cell killing. Since influenza viruses kill cells by apoptosis (programmed cell death), CKP are functionally apoptosis-inducing particles. Noninfectious CKP are present in excess of PFP in virus populations with full-size NS1 and induce apoptosis that is temporally delayed and morphologically different than that initiated by infectious CKP present in the virus population expressing truncated NS1. The identification and quantification of both infectious and noninfectious CKP defines new phenotypes in influenza virus populations and presents a challenge to determine their role in regulating infectivity, pathogenesis, and vaccine efficacy.

Transient resistance of influenza virus to interferon action attributed to random multiple packaging and activity of NS genes

Interferon (IFN) action survival curves for an avian influenza virus (AIV) in chicken or quail cells showed that 40-60% of the virions in a stock of virus were highly sensitive to the inhibitory effects of chicken IFN-alpha (ChIFN-alpha), whereas the rest were up to 100 times less sensitive. This greater resistance to IFN was transient, that is, was not a stable characteristic, in that virus stocks grown from plaques that formed in the presence of 50-800 U/ml IFN gave rise to virus populations that contained both sensitive and resistant virions. If AIV was serially passaged several times in the presence of IFN, the proportion of transiently IFN-resistant virus was greater. We propose a model to account for this transient resistance of AIV to IFN action based on the reported inactivation of the dsRNA-dependent protein kinase (PKR) and its activator dsRNA by the NS1 protein of influenza virus and also on the increase in the survival of AIV in IFN-treated cells exposed to 2-aminopurine, a known inhibitor of PKR. We suggest that IFN-resistant AIV is generated from a random packaging event that results in virions that contain two or more copies of RNA segment 8, the gene segment that encodes the NS1 protein of AIV, and that these virions will produce correspondingly elevated levels of NS1. The experimental data fit well to theoretical curves based on this model and constructed from the fraction of virus in the population expected by chance to contain one, two, or three copies of the NS gene when packaging an average of 12 influenza gene segments that include the 8 segments essential for infectivity.

Mycoplasmas produce double-stranded ribonuclease

Mycoplasmas (Mollicutes) constitute a constant threat as insidious contaminants of animal cell cultures. They are responsible for myriad biochemical reactions associated with the cells they infect, and undoubtedly have been the source of metabolic and physiological activities attributed to their hosts. In an attempt to demonstrate a dsRNA-inducible double-stranded ribonuclease (dsRNase) in mammalian cells, comparable to that reported in avian cells, we discovered high levels of dsRNase "induced" by a particular stock of vesicular stomatitis virus. We now report that the double-stranded ribonuclease resulted from the activity of a contaminant in that stock--a "noncultivable" Mycoplasma hyorhinis. This report demonstrates the ubiquitous distribution of dsRNase among mycoplasmas, presents some characteristics of the enzyme and its production, and implicates once again mycoplasmas as contaminants of cell culture and potential perturbers of cellular physiology.

Interferon induction by viruses. XIX. Vesicular stomatitis virus--New Jersey: high multiplicity passages generate interferon-inducing, defective-interfering particles

The infectious particles of plaque-derived, low multiplicity passaged wild-type VSV of New Jersey origin consistently induce about 1800 units of interferon (IFN)/10(7) aged chick embryo cells. This inducing capacity is sensitive to both uv radiation and heat (50 degrees). Virus obtained after two successive high multiplicity passages in GMK-Vero cells consistently induced over 25,000 units of IFN/10(7) cells. The IFN induction dose-response curve showed that one IFN-inducing particle (IFP) per cell sufficed to produce a quantum yield of IFN, but infection with two or more IFPs led initially to a marked suppression in the yield of IFN. IFN induction was attributed to two distinct defective particles that differed in size, both containing snap-back RNA, i.e., covalently linked, self-complementary [+/-]RNA. The IFN-inducing capacity of these defective-interfering particles was not inactivated by uv or heat. However heat did eliminate the IFN suppressing activity observed at higher multiplicities, implicating a heat-sensitive component in the virion as a regulator of IFN yield, and involving possibly the virion transcriptase and 3'-leader RNA product.

Delayed formation of defective interfering particles in vesicular stomatitis virus-infected cells: kinetic studies of viral protein and RNA synthesis during autointerference

The time course of defective interfering (DI) particle and B particle release from vesicular stomatitis virus-infected BHK-21 cells was studied at different multiplicities of defective and infective particles. Particle release was progressively delayed in cells infected with an increasing DI-to-B particle ratio. The delayed particle release during interference was found to be connected with a reduced but prolonged synthesis of viral proteins, a slower accumulation of viral proteins, and a delayed shutoff of cellular protein synthesis. The relative synthesis of M and G proteins was reduced during interference, whereas the relative synthesis of N and NS proteins was increased. On the level of genomic RNA replication, we found that DI RNA was replicated more slowly during interference than the standard genomic RNA was during acute infection. The ratio of DI particles to B particles which were released increased throughout the infectious cycle. At a given time in the infectious cycle, this ratio was independent of the multiplicity of infecting DI and B particles. On the basis of the kinetic studies, we argue that cells infected with higher amounts of DI particles compared with B particles synthesize a higher DI-to-B particle ratio and release these progeny particles later than cells infected with a low DI-to-B particle ratio.

Interferon induction by viruses. XV. Biological characteristics of interferon induction-suppressing particles of vesicular stomatitis virus

A single interferon (IFN) induction-suppressing particle (ISP) of vesicular stomatitis virus (VSV) blocked completely the yield of IFN in a cell otherwise programmed to produce IFN. With mouse L cells as hosts, one lethal hit of UV radiation (D37 = 52.5 ergs/mm2) to the VSV genome sufficed to inactivate ISP activity; however, with "aged" primary chick embryo cells as hosts, it took 198 lethal hits (D37 = 10,395 ergs/mm2). ISP expression in chick cells did not require virus replication or amplified RNA synthesis, but did involve functional virion-associated L protein. ISP in chick cells also were capable of inhibiting, in a multiplicity-dependent manner, the plaquing efficiency of two viruses that require cellular polymerase II (pol II) for replication, e.g., pseudorabies and influenza. The refractory state to IFN inducibility that resulted from infection of chick cells with ISP (VSV tsO5 [UV = 100 hits]) was still extant after 6 days. In contrast, the plaquing efficiency of pseudorabies virus returned to control levels by 5 h after ISP infection. Chick cells infected with UV ISP remained viable, served as hosts for the replication of other viruses, and could be subcultured. Models are presented to account for these contrasting effects. The involvement of viral plus-strand leader RNA as an inhibitor of cellular pol II-dependent RNA synthesis, and the multifunctional activities of the virion-associated L protein, are discussed as possible molecules involved in the action of ISP in chick cells.

Primary structure of leader RNA and nucleoprotein genes of the rabies genome: segmented homology with VSV

We have determined the nucleotide sequence of the 3'region of the rabies genome (PV strain). This work is a first step in a project aimed at establishing the complete primary structure. From the 3'nucleotide sequence of the RNA genome, an octadecanucleotide complementary to the 3'extremity was constructed and used to prime cDNA synthesis. Two overlapping recombinant cDNA clones hybridizing with the nucleoprotein mRNA (NmRNA) were isolated and sequenced. The 1500 first nucleotides of the rabies genome cover two transcriptional units: the leader RNA and the NmRNA which was shown to be initiated around residue 59 by S1 nuclease protection experiments. Comparison between rabies PV and CVS strains up to residue 180 suggests a rapid evolution in the leader region. Studies of the sequence relationships between the 3'regions of two Rhabdoviruses, rabies virus and Vesicular Stomatitis Virus (VSV), demonstrate that there is a segmented homology. Stretches of highly conserved amino acids possibly involved in the interaction with the RNA genome were observed in the N protein, despite a wide divergence in the remaining sequence. In addition, the high homology between the transcription start and stop signals reflects the conservation of a similar transcriptional mechanism in these two non segmented negative strand RNA viruses.

Vesicular stomatitis virus-infected cells fuse when the intracellular pool of functional M protein is reduced in the presence of G protein

Five highly cytolytic strains of both Indiana and New Jersey serotypes of vesicular stomatitis virus were shown to induce cell fusion in BHK-21 and R(B77) cells. Inhibition of protein synthesis after the eclipse period of viral replication is a prerequisite for vesicular stomatitis virus-induced cell fusion. Pulse-chase experiments showed that inhibition of protein synthesis would lead to a drastic reduction in the intracellular pool of M protein as compared with other proteins. A temperature-sensitive mutant defective in M protein function (G31) was the only mutant of the five complementation groups to spontaneously induce polykaryocytes at the nonpermissive temperature. Previously, G protein has been shown to play a role in vesicular stomatitis virus-induced cell fusion. These results suggest that the combination of the presence of G protein on the virus-infected cell surface and the absence of functional M protein or a reduced level of intracellular M protein promotes cell fusion. On the basis of this study, we propose that vesicular stomatitis virus infection can induce cell fusion when the functional M protein pool declines to a critical level while G protein remains on the cell surface.

The requirement of protein synthesis for VSV inhibition of host cell RNA synthesis

Published ultraviolet (uv) inactivation data and in vitro transcription studies have suggested that vesicular stomatitis virus (VSV) leader RNA was solely responsible for the inhibition of host cell RNA synthesis by this virus. Since no protein product is encoded in leader RNA, this conclusion implied that no protein synthesis should be required for this effect. Therefore, the inhibitory activity of VSV was examined in the presence of the protein synthesis inhibitors, cycloheximide, pactamycin, and emetine. Protein synthesis inhibitors are known not to interfere with VSV primary transcription, but in their presence viral replication and amplification of transcription do not take place. Although at 39 degrees the VSV mutant tsG22 could undergo only primary transcription, maximum inhibition of host cell RNA synthesis took place. However, in the presence of the protein synthesis inhibitors the VSV mutant was no longer able to interfere with host cell RNA synthesis. These results could not be explained by a change in the concentration of intracellular leader RNA which remained unaltered by the drugs. Similar results were also obtained with wild-type VSV in the presence of cycloheximide. Upon removal of the drug, inhibition of host cell RNA synthesis was reestablished in parallel with the restoration of protein synthesis. It is concluded that protein synthesis is required for the inhibitory activity of VSV, presumably because the active inhibitory complex is a nucleoprotein containing leader RNA and either a cellular protein or the viral N protein. The cellular protein would have to be in limiting supply since de novo protein synthesis was required for the inhibition to take place.

Intracellular events following the infection of different cell types with vesicular stomatitis subviral particles

Vesicular stomatitis virus (VSV) subviral particles (nucleocapsids and G-depleted particles) were used to infect various cells (chicken embryo, HeLa, and BHK21 cells). These particles bind to and penetrate into host cells; the association of G-depleted particles to cells was even better than that of normal virions. The parental genomes of subviral particles and virions were degraded at the same rate in the infected cells. Nevertheless, these subviral particles had a very low infectivity, synthesized very little viral macromolecules, and had very little, if any, effect on the various host cells used. Furthermore , subviral particles could be rescued in chicken embryo cells by uv-irradiated VSV virions, demonstrating that subviral particles actually penetrated into cells, and that their arrested cycle could be unblocked up to a certain point. On the other hand, subviral particles were not rescued in HeLa cells, suggesting a dependence on the host cell system.

Interferon induction by viruses. XI. Early events in the induction process

The mechanism by which NDV, Sindbis virus, and VSV enter "aged" primary chick embryo cells to initiate IFN induction was studied by using NH4Cl, a lysosomotropic weak base that compromises low pH-dependent membrane fusion. NH4Cl was used to perturb the early steps in virus entry into the cytosol thought to result ultimately in presentation of dsRNA to the cell's first-stage recognition system for IFN induction. Three parameters were monitored: (i) the nature of the dose (multiplicity)-response (IFN yield) curve, (ii) the activity of IFN-inducing particles (IFP), and (iii) the quantum yield of IFN. In all tests, the qualitative nature of the dose-response curve was not altered by NH4Cl treatment. NH4Cl had no effect on infectivity of IFN induction by NDV in keeping with a mode of entry involving acidic-independent fusion through the plasma membrane. Sindbis virus infectivity and IFN-inducing particle activity were inhibited similarly in an NH4Cl concentration-dependent manner. While the infectivity of VSV was very sensitive to the action of NH4Cl, virtually all IFN-inducing particles were functional; however, the quantum yield of IFN they induced was reduced in an NH4Cl concentration-dependent manner. Only 1 of 6 VSV [+/-]DI-011 particles registered in NH4Cl-treated cells; however, they induced more than one-half the normal quantum yield of IFN. The mechanism of entry of Sindbis virus (acidic-fusion) and VSV (acidic-endocytosis) was distinguished by the action of cytochalasin B. Infectivity and IFN induction by Sindbis virus and VSV share a common, limiting step in NH4Cl treated cells: transfer of viral RNA from basic vacuoles into the cell cytosol. The similar sensitivity of Sindbis IFP and PFP to NH4Cl suggests that both activities respond the same to increased intravacuolar pH; neither can be expressed. VSV-IFP, requiring only partial genomic expression (or none in the case of DI-011) can register in NH4Cl-treated cells to varying degrees under conditions that prevent expression of the full genome.

Comparative inhibition of cellular transcription by vesicular stomatitis virus serotypes New Jersey and Indiana: role of each viral leader RNA

We compared the ability of the leader RNAs of the New Jersey and Indiana serotypes of vesicular stomatitis virus to inhibit transcription in infected host cells. The level of cellular RNA synthesis in cells infected with either serotype was drastically reduced by 5 h after infection. Studies with UV-inactivated virus demonstrated that shutoff of cellular RNA synthesis directly correlated with the ability of the infecting virus to transcribe its plus-stranded leader RNA. Although both serotypes inhibited cellular RNA synthesis, the Indiana serotype reduced synthesis to lower levels. In addition, an examination of the kinetics of leader RNA synthesis in vivo indicated that up to four times more leader RNA was produced in cells infected with the Indiana serotype than in those infected with the New Jersey serotype. However, in vivo studies also suggested that the leader RNA of the New Jersey serotype was a more efficient RNA inhibitor than was the Indiana serotype leader RNA. Although up to 2,900 copies of the leader RNA per cell could be detected in infected cells, only 550 copies of the Indiana and 100 copies of the New Jersey leader RNAs per cell were present in infected cells that were demonstrating 50% of the maximal inhibition of RNA synthesis. In an in vitro system, leader RNAs of both serotypes inhibited DNA-dependent transcription of the adenovirus late promoter and adenovirus-associated RNA genes, but the New Jersey serotype leader was also a better inhibitor in this reconstituted system. Data from the dose response of inhibition by each leader suggest that polymerase III transcription was more sensitive to inhibition by viral leaders than was polymerase II transcription. Polyadenylated viral mRNAs and the NS and N gene starts transcribed by both serotypes did not significantly inhibit transcription at levels at which the corresponding leader RNAs were inhibitory. Overall, our results strongly suggest a role for the plus-stranded leader RNAs of the New Jersey and Indiana serotypes of vesicular stomatitis virus in inhibiting cellular transcription in vivo. We discuss differences in the nucleotide sequences of the two leader RNAs in relation to their differences in biological activity and to potential regulatory sequences.

Inhibition of cellular DNA synthesis by vesicular stomatitis virus

DNA synthesis in mouse myeloma (MPC-11) cells and L cells was rapidly and progressively inhibited by infection with vesicular stomatitis virus (VSV). No significant difference in cellular DNA synthesis inhibition was noted between synchronized and unsynchronized cells, nor did synchronized cells vary in their susceptibility to VSV infection after release from successive thymidine and hydroxyurea blocks. Cellular RNA synthesis was inhibited to about the same extent as DNA synthesis, but cellular protein synthesis was less affected by VSV at the same multiplicity of infection. The effect of VSV on cellular DNA synthesis could not be attributed to degradation of existing DNA or to decreased uptake of deoxynucleoside triphosphates, nor were DNA polymerase and thymidine kinase activities significantly different in VSV-infected and uninfected cell extracts. Analysis by alkaline sucrose gradients of DNA in pulse-labeled uninfected and VSV-infected cells indicated that VSV infection did not appear to influence DNA chain elongation. Cellular DNA synthesis was not significantly inhibited by infection with the VSV polymerase mutant tsG114(I) at the restrictive temperature or by infection with defective-interfering VSV DI-011 (5' end of the genome), but DI-HR-LT (3' end of genome) exhibited initially rapid but not prolonged inhibition of MPC-11 cell DNA synthesis. DNA synthesis inhibitory activity of wild-type VSV was only slowly and partially inactivated by very large doses of UV irradiation. These data suggest that, as in the effect of VSV on cellular RNA synthesis (Weck et al., J. Virol. 30:746-753, 1979), inhibition of cellular DNA synthesis by VSV requires transcription of a small segment of the viral genome.

Is cytoskeleton involved in vesicular stomatitis virus reproduction?

CER cells infected with vesicular stomatitis virus showed a morphology similar to that observed after cytochalasin B treatment. Temperature-sensitive mutants affected in envelope protein maturation did not induce those morphological changes at a nonpermissive temperature. In addition, the cytoskeleton was not implicated in vesicular stomatitis virus reproduction.

Quantitative analysis of defective interfering particles in infectious pancreatic necrosis virus preparations

Infectious pancreatic necrosis virus exhibited an interference phenomenon that resulted in the survival of the infected cell with one hit kinetics. The responsible factor was found to co-purify with standard virus through a purification regime that employed two CsCl gradients and a sucrose gradient. This result suggested that a defective interfering (DI) viral particle was involved. It was possible to estimate the number of DI particles by a statistical method using the Poisson distribution that related cell survival to input DI/cell, which indicated that virus samples from dilute passage contained as many DI particles as samples from undiluted passage; this means that multiple undiluted virus passage did not increase the yield of DI particles. In isopycnic CsCl gradient centrifugation, the DI particles were found in a broad band superimposed over the standard virus peak and extending above it, such that the ratio DI/PFU varied from 0.3--20 in different fractions. These centrifugation methods did not completely separate DI particles from standard virus.

Defective interfering particles with covalently linked [+/-]RNA induce interferon

Defective interfering (DI) particles of vesicular stomatitis virus which contain covalently linked complementary [+]message and [-]anti-message RNA as a single-stranded ribonucleoprotein complex within the particle, are extremely efficient inducers of interferon. A single particle can induce a quantum yield of interferon. A single molecule of double-stranded RNA presumed to form, at least in part, on entry into the cell is thought to induce interferon synthesis. Conventional [-]RNA DI particles with the same polypeptide composition as [+/-]RNA DI particles fail to induce interferon.

Inhibition of cellular DNA synthesis in hamster kidney cells infected with western equine encephalitis virus

Infection of BHK cells with western equine encephalitis (WEE) virus resulted in rapid inhibition of cellular DNA synthesis. The rate of inhibition of DNA synthesis depended on the multiplicity of infection, and was closely related to virus replication. Cellular DNA synthesis was not inhibited in infected BHK cells that had been irradiated with ultraviolet radiation. These results indicated that a functional viral genome was required for the inhibition of DNA synthesis by WEE virus. The sharp decrease in thymidine incorporation into the acid-insoluble fraction was not due to a change in the intracellular pool of the acid-soluble fraction. Sedimentation analysis in alkaline sucrose gradients was used to show that cellular DNA was not degraded during WEE viirus infection. DNA polymerase activity in infected cells was not significantly reduced.

Cell-killing by Epstein-Barr virus: analysis by colony inhibition procedure

The P3HR-1 and B95-8 strains of Epstein-Barr virus (EBV) were cytocidal for EBV-carrier human cell lines, as revealed by a colony inhibition procedure. The cytocidal activity was proportional to virus dose added. The cell killing was neutralized by anti-EBV antibody-positive but not -negative human sera. When the relative sensitivity to ultraviolet light of EBV activities was examined, the cytocidal actitivy was much more resistant than the viral infectivity as assayed by early antigen-forming activity (P3HR-1 virus) or leukocyte-transforming activity (B95-8 virus), but it closely paralleled the ability to adsorb to cells.

Interfering passages of Sindbis virus: concomitant appearance of interference, morphological variants, and trucated viral RNA

Serial passage of Sindbis at high multiplicities of infection resulted in cyclical variations in virus titer. Decreases in virus titer were correlated with the appearance of smaller-sized virions, interference and truncated viral RNA. The smaller particles were 37 nm in diameter, exclusive of the hemagglutinin spikes as compared with a diameter of 50 nm for standard virions. Passages which contained 37-nm partilces also interfered with infectious center formation by standard, plaque-purified virus. Polyacrylamide gel analysis of RNA isolated from virions present in interfering passages demonstrated the sequential appearance of three RNA species smaller than standard RNA with approximate molecular weights of 3.3 X 106, 2.7 X 106, and 2.2 X 106. The 3.3 X 106 RNA was evident in passage 5, by passage 8 both the 3.3 X 106 and 2.7 X 106 RNAs were present, and by passage 13 all three were present with the 2.2 X 106 RNA predominating.