Absence of interference during high-multiplicity infection by clonally purified vesicular stomatitis virus

Accumulation Dynamics of Defective Genomes during Experimental Evolution of Two Betacoronaviruses

Virus-encoded replicases often generate aberrant RNA genomes, known as defective viral genomes (DVGs). When co-infected with a helper virus providing necessary proteins, DVGs can multiply and spread. While DVGs depend on the helper virus for propagation, they can in some cases disrupt infectious virus replication, impact immune responses, and affect viral persistence or evolution. Understanding the dynamics of DVGs alongside standard viral genomes during infection remains unclear. To address this, we conducted a long-term experimental evolution of two betacoronaviruses, the human coronavirus OC43 (HCoV-OC43) and the murine hepatitis virus (MHV), in cell culture at both high and low multiplicities of infection (MOI). We then performed RNA-seq at regular time intervals, reconstructed DVGs, and analyzed their accumulation dynamics. Our findings indicate that DVGs evolved to exhibit greater diversity and abundance, with deletions and insertions being the most common types. Notably, some high MOI deletions showed very limited temporary existence, while others became prevalent over time. We observed differences in DVG abundance between high and low MOI conditions in HCoV-OC43 samples. The size distribution of HCoV-OC43 genomes with deletions differed between high and low MOI passages. In low MOI lineages, short and long DVGs were the most common, with an additional cluster in high MOI lineages which became more prevalent along evolutionary time. MHV also showed variations in DVG size distribution at different MOI conditions, though they were less pronounced compared to HCoV-OC43, suggesting a more random distribution of DVG sizes. We identified hotspot regions for deletions that evolved at a high MOI, primarily within cistrons encoding structural and accessory proteins. In conclusion, our study illustrates the widespread formation of DVGs during betacoronavirus evolution, influenced by MOI and cell- and virus-specific factors.

Defective Interfering Particles of Negative-Strand RNA Viruses

Viral defective interfering particles (DIPs) were intensely studied several decades ago but research waned leaving open many critical questions. New technologies and other advances led to a resurgence in DIP studies for negative-strand RNA viruses. While DIPs have long been recognized, their exact contribution to the outcome of acute or persistent viral infections has remained elusive. Recent studies have identified defective viral genomes (DVGs) in human infections, including respiratory syncytial virus and influenza, and growing evidence indicates that DVGs influence disease severity and may contribute to viral persistence. Further, several studies have advanced our understanding of key viral and host factors that regulate DIP formation and activity. Here we review these discoveries and highlight key questions moving forward.

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.

Rubella virus RNA replication is cis-preferential and synthesis of negative- and positive-strand RNAs is regulated by the processing of nonstructural protein

Rubella virus (RV) genome encodes nonstructural protein (NSP) in a large open reading frame at its 5' end. It is translated into p200 and further processed into p150 and p90. The NSPs are responsible for viral RNA replication, during which a full-length negative-strand RNA serves as the intermediate for the replication of positive-strand genomic RNA and the transcription of subgenomic RNA. Using complementation experiments, we demonstrated that RV negative-strand RNA is synthesized preferentially in cis while positive-strand RNAs can be synthesized both in cis and in trans but with higher efficiency in cis. During virus infection, negative-strand RNA accumulates until 10 hours postinfection (hpi) and remains nearly constant thereafter. In contrast, positive-strand RNAs (both genomic and subgenomic RNA) do not increase much before 10 hpi and accumulate rapidly thereafter. Previously we demonstrated that p200 synthesizes negative- but not positive-strand RNA, whereas cleavage products p150/p90 are required for efficient production of positive-strand RNAs. In this study, we present evidence demonstrating that a higher concentration of p150/p90 is associated with lower production of negative-strand RNA. Our data support the hypothesis that p200 is the principal replicase for negative-strand RNA, as is p150/p90 for positive-strand RNA. The switch from the synthesis of negative- to positive-strand RNA is thus regulated by NSP processing, which not only activates the efficient production of positive-strand RNA, but also disables negative-strand RNA synthesis. A mechanism for NSP translation, processing, and regulation of RV RNA synthesis is proposed.

Coupling between genome translation and replication in an RNA virus

The replication of poliovirus RNA genomes containing amber mutations was studied to test whether viral proteins provided in trans could rescue the replication of an RNA genome that could not be completely translated itself. Mutants containing amber codons at different positions in the genome displayed vastly different abilities to be rescued by wild-type proteins provided by a helper genome. Amber-suppressing cell lines were used to ensure that the defects in the amber mutants arose from their failure to be translated, not from defects in RNA sequence or structure. An internal region of the poliovirus genome was identified whose translation is required in cis; failure to translate this region was shown to inhibit RNA replication. This coupling between translation and RNA replication could provide a late proofreading mechanism that enables poliovirus, and possibly many other RNA viruses, to prevent the replication of defective genomes.

Wide occurrence of measles virus subgenomic RNAs in attenuated live-virus vaccines

Nine measles vaccine preparations, including four different viral strains, provided by eight different manufacturers were analysed by Northern blot for the nature of their nucleocapsid RNAs. Out of nine preparations, six were shown to contain subgenomic RNAs, along with the full length genomic RNA. Presence or absence of the subgenomic RNAs correlated strictly with the viral strains used. The role of the defective interfering particles in measles virus vaccine attenuation, and in its seroconversion efficacy upon vaccination, as well as the potential hazard of the presence of defective interfering particles in live-virus vaccine preparations, is discussed.

Platelet-derived growth factor and double-stranded ribonucleic acids stimulate expression of the same genes in 3T3 cells

Platelet-derived growth factor (PDGF) stimulates expression of a "competence" gene family in Balb/c-3T3 cells. The competence family contains the c-myc and c-fos genes together with several functionally uncharacterized genes (JE, KC, and r-fos) that have been isolated as cDNA clones. We show that double-stranded ribonucleic acid is a potent inducer of the competence gene family. Infection with vesicular stomatitis virus also induces expression of this gene family. Conversely, PDGF stimulates expression of genes hitherto characterized as responsive to double-stranded ribonucleic acids, including the beta-fibroblast interferon and (2'-5')-oligoadenylate synthetase genes. These PDGF-inducible genes could conceivably function in a feedback loop to control 3T3 cell growth. Some of the genes, such as c-fos and c-myc, are induced quickly by PDGF and may initiate a round of cell division. Others, such as beta-fibroblast interferon and (2'-5')-oligoadenylate synthetase, are induced more slowly and may function as feedback inhibitors of the growth response to PDGF.

Characterization of New Jersey vesicular stomatitis virus isolates from horses and black flies during the 1982 outbreak in Colorado

Vesicular stomatitis viruses isolated from horses, afflicted during the recent outbreak in the western United States, and from black flies (Simuliidae) were characterized with respect to the homology of their genomic RNAs and the mobility of their proteins in polyacrylamide gels. All the isolates were very similar, if not identical, with respect to these two parameters. When the black fly isolate was compared to other VSV isolates, this virus appeared to belong in the Hazelhurst subgroup of the New Jersey serotype of VSV. Since the other viruses in this division were obtained from infected swine, the natural host range of this subgroup has been extended to horses.

Structural and functional characterization of the RNA-positive complementation groups, C and D, of the New Jersey serotype of vesicular stomatitis virus: assignment of the M gene to the C complementation group

The structural and functional lesions in the RNA-positive complementation groups, C and D, of the New Jersey serotype (Hazelhurst subtype) of vesicular stomatitis virus have been characterized. The M protein of the temperature-sensitive mutant C1, the prototype of the C complementation group, was degraded at the restrictive temperature in vivo, and was resolved from the wild-type M protein by SDS-polyacrylamide gel electrophoresis and nonequilibrium pH gradient electrophoresis. Coreversion of these properties and the temperature-sensitive phenotype was observed in a spontaneous revertant. On the basis of these results, the M gene was assigned to the C complementation group. Intracellular nucleocapsids could not be isolated from New Jersey serotype infections by procedures developed for Indiana serotype infections. Therefore, in order to assess the ability of New Jersey ts mutants to accumulate nucleocapsids at the restrictive temperature, a procedure for their isolation was developed. Hypertranscription was observed in C1-infected cells incubated at the restrictive temperature, but was not accompanied by proportionate increases in intracellular viral nucleocapsids or protein synthesis. The G and N proteins of the temperature-sensitive mutant D1, the sole representative of the D complementation group, were electrophoretic variants. The relative yield of intracellular D1 N protein was lower at the restrictive than at the permissive temperature, and the D1 L protein was thermolabile. No intracellular viral nucleocapsids were detected in D1 infected cells incubated at the restrictive temperature; however, more 40 S and less message-sized RNA were synthesized at the restrictive than at the permissive temperature. These results suggested functional defects in both the N protein and polymerase of D1.

Amphotericin B selection of mutant Chinese hamster cells with defects in the receptor-mediated endocytosis of low density lipoprotein and cholesterol biosynthesis

This paper describes a rapid and efficient two-step procedure for the isolation of mutant cells with defects in receptor-mediated endocytosis. The procedure takes advantage of two fungal metabolites, compactin (ML236B), a potent inhibitor of cholesterol biosynthesis, and amphotericin B, a polyene antibiotic that forms toxic complexes with sterols in membranes. Mutagen-treated Chinese hamster ovary cells were preincubated overnight in a medium containing mevalonate, low density lipoprotein (LDL), and compactin (Mev/LDL/Com). At the end of the preincubation period, wild-type cells were cholesterol replete while mutant cells that could not utilize the cholesterol in LDL were cholesterol deficient. Subsequent incubation with amphotericin B for 6 hr killed most of the wild-type cells. After a second round of Mev/LDL/Com-amphotericin B selection, endocytosis-defective clones appeared at a frequency of approximately equal to 2.6 X 10(-5). Some of these clones expressed LDL receptor-defective phenotypes and fell into one of two previously defined classes of mutation. Sensitivity of the mutants to infection by vesicular stomatitis virus suggested that the mutations do not disrupt the coated pit-coated vesicle pathway of endocytosis. Minor modifications in the Mev/LDL/Com-amphotericin B selection permit the isolation of cholesterol auxotrophs and might allow the isolation of conditional-lethal mutations. Because LDL can be coupled to ligands that bind to receptors other than the LDL receptor, Mev/LDL/Com-amphotericin B selection may permit the isolation of mutant cells with defects that specifically disrupt other endocytic pathways.

Effect of intracellular vesicular stomatitis virus mRNA concentration on the inhibition of host cell protein synthesis

Inhibition of host cellular protein synthesis by vesicular stomatitis virus (VSV) has been suggested to be primarily the result of competition for ribosomes between cellular and viral mRNAs (H. F. Lodish and M. Porter, J. Virol., 36:719-733, 1980; Lodish and Porter, J. Virol. 38:504-517, 1981). This hypothesis was investigated by regulating the extent of VSV mRNA synthesis through the use of defective interfering particles. Although intracellular VSV mRNA concentrations decreased by as much as a factor of 14 at high multiplicities of infection of defective interfering particles, the inhibition of host cell protein synthesis by VSV decreased by a maximum of only 10%. The data also indicated that under these conditions the protein-synthesizing capacity of the cells was not exhausted. We concluded that competition for cellular ribosomes could not have been the major factor in the inhibition of host cell protein synthesis by VSV. This conclusion was further supported by inhibition data obtained with VSV mutants. The ts G22 mutant, defective in replication but not in primary transcription, inhibited host protein synthesis at the nonpermissive temperature (39 degrees C) to the same extent as did wild-type virus, even though it generated only 30 to 50% of the amount of viral mRNA as did wild-type virus. Conversely, in infections with the R1 mutant, which did not inhibit host cell protein synthesis, the amount of total and polysome-bound viral mRNA was indistinguishable from that obtained in infections by wild-type virus.

Isolation and characterization of defective simian virus 40 genomes which complement for infectivity

A new variant of simian virus 40 (EL SV40), containing the complete viral DNA separated into two molecules, was isolated. One DNA species contains nearly all of the early (E) SV40 sequences, and the other DNA contains nearly all of the late (L) viral sequences. Each genome was encircled by reiterated viral origins and termini and migrated in agarose gels as covalently closed supercoiled circles. EL SV40 or its progenitor appears to have been generated in human A172 glioblastoma cells, as defective interfering genomes during acute lytic infections, but was selected during the establishment of persistently infected (PI) green monkey cells (TC-7). PI TC-7/SV40 cells contained EL SV40 as the predominant SV40 species. EL SV40 propagated efficiently and rapidly in BSC-1, another line of green monkey cells, where it also formed plaques. EL SV40 stocks generated in BSC-1 cells were shown to be free of wild-type SV40 by a number of criteria. E and L SV40 genomes were also cloned in the bacterial plasmid pBR322. When transfected into BSC-1 cell monolayers, only the combination of E and L genomes produced a lytic infection, followed by the synthesis of EL SV40. However, transfection with E SV40 DNA alone did produce T-antigen, although at reduced frequency.

The involvement of Lyt-2 antigens in the lysis of virus-infected target cells by antigen specific anti-vesicular stomatitis virus cytotoxic T-lymphocytes

The involvement of cell surface components associated with anti-viral cytotoxic T-lymphocytes (CTLs) in the specific lysis of virus-infected target cells was analyzed by the ability of different monoclonal antibodies to specifically inhibit CTL lytic activity. Utilizing monoclonal anti-Lyt antibodies, we showed that Lyt-2 antigens are closely associated with the CTL lytic event. The nature and limitations of this association are discussed.

Elicitation of anti-vesicular stomatitis virus cytotoxic T lymphocytes by using purified viral and cellular antigens incorporated into phospholipid vesicles

We evaluated the minimal molecular and cellular requirements for elicitation of anti-vesicular stomatitis virus (VSV) cytotoxic T lymphocytes (CTL). The results indicated that lipid vesicles containing the purified major surface glyco-protein of VSV (G protein) and purified H-2K(k) glycoproteins elicited specific H-2K(k)-restricted anti-VSV CTL. These antiviral CTL were shown to be Ly 1(-),2(+). However, both Ly 1(+),2(-) and Ly1(-),2(+) T-cell subpopulations were shown to be required for elicitation of these CTL.

Elicitation of natural killer cells in beige mice by infection with vesicular stomatitis virus

Vesicular stomatitis virus (VSV) infection of beige (bg/bg) mice induced levels of natural killer cytolytic activity comparable to that of uninfected normal bg/+ controls, but considerably less than natural killer activity in VSV-infected bg/+ mice. In contrast, VSV-infected bg/bg and bg/+ mice had essentially equivalent amounts of anti-VSV cytotoxic T-lymphocyte and antibody activity. VSV infection induced comparable levels of interferon in both bg/bg and bg/+ mice. Therefore, the decreased natural killer activity in bg/bg mice could not be attributed to an inability to produce interferon.

Characterization of vesicular stomatitis virus mutants by partial proteolysis

Structural proteins of temperature-sensitive (ts) mutants of vesicular stomatitis virus, Indiana serotype, were compared with those of wild-type and revertant virions by electrophoresis on polyacrylamide gels of partial digests with Staphylococcus aureus V8 protease. Mutants of complementation groups III (tsG31 and tsG33), II (tsG22), and IV (tsG41) differed from the wild-type virion in peptide profiles of their M, NS, and N proteins, respectively. The differences were only detectable over a narrow range of enzyme-substrate ratios and were due to peptides transiently generated during incomplete digestion. Proteins of revertants to tsG31, tsG22, and tsG41 exhibited the wild-type virion peptide pattern, indicating that reversion had restored their original conformation. However, in the case of tsG22, the NS peptide profile reverted to the wild-type phenotype only partially, suggesting that a silent mutation might have taken place during either the original chemical mutagenesis or the following repeated laboratory passages. The apparent alteration in protein conformation and its restoration upon reversion of the mutants indicated that the lesions of groups III and IV were located in the M and N proteins, respectively. Moreover, for the first time, the site of mutation of group II could be positively identified as the NS protein cistron.

UV inactivation of the biological activity of defective interfering particles generated by vesicular stomatitis virus

UV inactivation of vesicular stomatitis virus and its defective interfering (DI) particles was measured in order to obtain the target size for interference. In the case of DI particles whose genomes mapped at the 5' end of the virion RNA, this target size corresponded to the entire DI particle RNA molecule regardless of whether it amounted to 10, 30, or 50% of the viral genome. These data were interpreted as demonstrating that both termini of the DI particle RNAs were required for their replication and for interference with virion RNA replication. The unique heat-resistant DI particle, with an RNA molecule corresponding to the 3' half of the viral genome, exhibited an inactivation target size of approximately 42% of its RNA molecule with respect to both homotypic and heterotypic interference. Unlike other DI particles, this particle interfered with virion primary transcription. The unusual inactivation target size of the heat-resistant DI particle was interpreted as being a compromise between the requirements for replication of its genome and those for interference with virion primary transcription.

Persistent vesicular stomatitis virus infection mediates base substitutions in viral RNA termini

We have sequenced (via a product RNA) the 3' RNA terminus of a defective interfering particle that was generated from the standard virus isolated from a culture of BHK-21 cells persistently infected with vesicular stomatitis virus for over 5 years. By hybridization and RNA sequencing, seven mutations were identified in the 46 nucleotides at the terminus of this defective-interfering-particle RNA. It is likely that these mutations are a reflection of altered protein-nucleic acid interactions that the virus has evolved to maintain its persistently infected carrier state in vitro.

Synthesis of a small RNA in cells coinfected by standard and defective interfering particles of vesicular stomatitis virus

A small RNA, containing approximately 50 nucleotides, is synthesized by cells coinfected with standard vesicular stomatitis virus and its defective interfering (DI) particles. Infection of cells by standard virus or DI particles alone does not lead to synthesis of significant amounts of small RNA. The RNA is initiated at its 5' end with (p)ppXp and is not polyadenylylated at the 3' end despite a content of 51% adenosine. It has sequences complementary to the genome of a DI particle. The synthesis of the small RNA correlates with the replication of the genome of DI particles with molar ratio small RNA/genome RNA of DI particles greater than 50. When replication of DI genomes is prevented by the addition of cycloheximide or prior UV irradiation of DI particles, small RNA is not synthesized in coinfected cells. These results indicate that the small RNA is not the result of transcriptional initiation and that it may relate to interference mediated by DI particles.

RNA synthesis of vesicular stomatitis virus. VIII. Oligonucleotides of the structural genes and mRNA

The single-stranded RNA genome of vesicular stomatitis virus (VSV, Indiana serotype, San Juan strain) yields approx. 75 RNase T1-resistant oligonucleotides ranging in size from 10 to 50 bases. Each of the five structural genes, isolated as duplex RNA molecules hybridized to complementary mRNA, contains two or more of these large oligonucleotides. One of the oligonucleotides is identified as part of the non-coding region near the 3' end of the genome. Comparison of these results with others indicate that the RNA sequence of VSV is apparently stable in the laboratory but not in the wild. RNase T1-resistant oligonucleotides are also shown for all five VSV mRN species. Whether the mRNA for these digestions are are isolated from duplex RNA molecules or as single-stranded RNA species, the oligonucleotide patterns for each mRNA are virtually identical, indicating that each mRNA is transcribed from contiguous sequences on the genome. Comparison with published oligonucleotide patterns obtained from other isolates of VSV or from VSV deletion mutants indicate that identity and changes in their genome structure can be correlated with specific structural genes.

The matrix (M) protein of vesicular stomatitis virus regulates transcription

Temperature-sensitive mutants of vesicular stomatitis virus (VSV) belonging to complementation group III contain a lesion in the matrix (M) protein. This results in a 2--5 fold increase in transcription at the nonpermissive temperature. Co-infection of cells with one of these mutants and wild-type virus reverses this mutant phenotype. Separation of the transcriptional and translational products from mutant-infected cells reveals an overall increase in each of the viral mRNA species concomitant with degradation of the M protein at the nonpermissive temperature. The increase in mRNA, however, does not lead to increased synthesis of viral proteins. Quantitation of individual mRNA species indicates that M protein acts as a direct inhibitor of transcription as well as an attenuator of sequential transcription.

Effects of phosphorylation and pH on the association of NS protein with vesicular stomatitis virus cores

The proteins of vesicular stomatitis virus (VSV) were analyzed on the basis of charge as well as size in polyacrylamide gels containing urea and acetic acid. The phosphorprotein NS was resolved into two major species. The less phosphorylated NS1 species contained about 10% fewer phosphate residues than the second species, NS2. These two phosphorylated forms were compartmentalized both in the virus and in the infected cell cytoplasm. Cores from virions and the core-containing fraction of the infected cell cytoplasm contained only the NS1 form. All of the more highly phosphorylated NS2 form and some of the NS1 form were found to be free of cores, whether they were derived from virions or from the infected cell. Therefore, the degree of phosphorylation appeared to determine whether or not the NS protein became bound to VSV cores. Moreover, the amount of bound NS1 protein relative to nucleocapsids increased as the pH of the culture medium was raised from 6.6 to 7.4. Because an increased in pH increases VSV replication (Fiszman et al., J. Virol. 13:801-808, 1974; Palma and Huang, in W.S. Robinson and C.F. Fox, ed., Mechanisms of Virus Disease, ICN-UCLA Symposia, p. 87-100, 1974), the NS1 protein may either regulate overall VSV RNA synthesis or regulate the switch between transcription and replication.

Vesicular stomatitis virus glycoprotein is necessary for H-2-restricted lysis of infected cells by cytotoxic T lymphocytes

Vesicular stomatitis virus (VSV) elicited cytotoxic thymus-derived lymphocytes (CTLs) in mice of the BALB/c and three congenic strains (BALB.b, BALB.k, BALB.HTG). CTL lysis of VSV-infected fibroblasts from the four strains was restricted by the target cells' major histocompatibility complex (H-2). Target cells were also infected with two temperature-sensitive mutants of VSV, tsM and tsG in which, respectively, the viral matrix protein and glycoprotein are not expressed at 39 degrees (restrictive temperature) on the infected cell's surface membrane. At the restrictive temperature, cells infected with wild-type VSV or tsM were lysed by CTLs, but cells infected with tsG were not. The requirement for the glycoprotein on the target cell was also evident from the ability of antisera to the glycoprotein to block completely CTL lysis of VSV-infected cells.

Viral pathogenesis and molecular biology

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Mechanism of formation of pseudotypes between vesicular stomatitis virus and murine leukemia virus

Pseudotypes of vesicular stomatitis virus (VSV) and Moloney murine leukemia virus (MuLV), defined by their resistance to neutralization by anti-VSV antiserum, are released preferentially at early times after infection of MuLV-producing cells with VSV. At later times, after synthesis of MuLV proteins has been inhibited by the VSV infection, neither MuLV virions nor the VSV (MuLV) pseudotypes are made. Infection of MuLV-producing cells with mutants of VSV having temperature-sensitive lesions in either G or M protein does not generate pseudotypes at nonpermissive temperature, indicating that both proteins are needed for pseudotypes to form. Although the pseudotypes resist neutralization by anti-VSV serum, they are inactivated by anti-VSV serum plus complement, and they can be precipitated by rabbit anti-VSV serum plus goat anti-rabbit IgG. These results, coupled with experiments using a temperature-sensitive mutant of VSV G protein grown at partly restrictive temperature, suggest that small numbers of VSV G protein are obligately incorporated into VSV(MuLV) pseudotypes. There appears to be a stringent requirement for recognition of the viral core by homologous envelope components as the nucleating step in the budding process. Only after such a nucleation can the envelope components of the second virus substitute into the membrane of the budding particle.

Human nasal epithelial cell culture system: evaluation of response to human interferons

This report describes the response of human nasal epithelial cells to exogenously applied human interferon (IF). To measure the response, a system developed to study human nasal epithelial cells in vitro was used. Nasal epithelial cells responded equally to both leukocyte and fibroblast IF, with development of resistance to virus replication that was time and dose dependent. Cells required contact with 1,000 U of IF (400 69/19 reference units) for 4 h to produce consistently a significant reduction in virus yield. This in vitro cell cultured system may be useful in predicting the in vivo response of human IF in respiratory viral infections.

Localization of two cellular forms of the vesicular stomatitis viral glycoprotein

Two cell-associated forms of the glycoprotein (G) of vesicular stomatitis virus, termed G1 and G2, have been resolved by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. G1 has the higher electrophoretic mobility, but both forms migrate more slowly than G protein synthesized in a wheat germ cell-free system (G0), which presumably is the unglycosylated form. G1 is a kinetic precursor of the G2 form, and the apparent cause of the electrophoretic difference between the two species is the presence of N-acetylneuraminic acid on the G2 form. Conversion of G1 to G2 occurs 10 to 20 min prior to the appearance of the G2 form of the protein on the cell surface. This suggests that the G protein may be completely glycosylated several minutes prior to its migration to the cell surface and that glycosylation is not the limiting step in its maturation. No glycoprotein comigrating with G0 can be detected in the infected cells, even after 5-min labeling periods; this suggests that partial clycosylation of G occurs concomitantly with or immediately after its synthesis.

Mechanism of restriction of ecotropic and xenotropic murine leukemia viruses and formation of pseudotypes between the two viruses

Ecotropic and xenotropic murine leukemia viruses (MuLV's) constitute separate interference groups; within each group there is cross-interference, but between the groups there is no detectable interference. Interference is manifest against pseudotypes in which the vesicular stomatitis virus genome is contained within the coat of one of the murine leukemia viruses. The pseudotypes display the cell specificity of the leukemia viruses: pseudotypes with an ecotropic MuLV coat infect mouse cells but not rabbit or mink cells; pseudotypes with a xenotropic MuLV coat infect rabbit or mink cells well but mouse cells very poorly. Efficient pseudotype formation also occurs between the two MuLV classes, and both the interference patterns and the cell specificity of these pseudotypes are entirely determined by their envelope. Using these pseudotypes, ecotropic MuLV infection could be established in xenogeneic cells, and the resulting progeny could be scored by using a conventional XC cell assay. Also, xenotropic MuLV infection could be established in a mouse cell, showing that no absolute intracellular barrier against xenotropic virus growth exists in murine cells. The major barriers against both xenotropic and ecotropic MuLV therefore are cell surface barriers. Xenogeneic cells probably lack receptors for ecotropic MuLV, but murine cells may either lack receptors for xenotropic MuLV or have receptors that are blocked by endogenous expression of the glycoprotein of endogenous xenotropic MuLV.

Factors involved in the generation and replication of rhabdovirus defective T particles

Previous indications that cloned B virions might be genetically predisposed to generate a particular defective T particle are shown to be inaccurate. T particle generation was found to be a much more random process than was previously believed. We show that the previously observed generation of particular sizes of T particles by B virion pools is due to the random generation of T particles during preparation of first-passage pools of cloned B virions, and these breed true during the additional passages needed to produce visible quantities of T particles. It is also shown that different host cell lines selectively amplify different T particles, suggesting a strong role of host cell factors in T particle replication. Surprisingly, our line of HeLa cells did not generate or replicate detectable T particles of vesicular stomatitis virus (VSV) Indiana after either serial undiluted passage or direct addition of T particles, even though the added T particles strongly interfered with B virion replication. In contrast to VSV, rabies virus generates large amounts of T particles during the first passage of cloned B virions, and every rabies-infected baby hamster kidney-21 cell culture evolves into a persistent carrier state. We find that T particle RNA is biologically inactive although T particle nucleocapsid ribonucleoprotein replicates and interferes in cells coinfected with B virions. Efforts to study the mechanism of T particle generation by in vitro attempts to generate T particles or modify their size (using sheared ribonucleoprotein or chemical or UV mutagenesis) were unsuccessful. The kinetics of UV and nitrous acid inactivation of T particles indicate a smaller target size relative to B virions, even after correcting for lengths of RNA molecules. The intercalating dye proflavine does not photosensitize VSV B virions or T particles when present during replication, indicating that there is little or no RNA base pairing in the helical nucleocapsids of either.

Physical properties of New Jersey serotype of vesicular stomatitis virus and its defective particles

The wild-type New Jersey serotype of vesicular stomatitis virus generated two types of defective interfering T-particles. The physical properties of these particles and the wild-type virion were determined by laser light scattering spectroscopy, sedimentation measurements, and electron microscopy.

Screening procedure for complementation-dependent mutants of vesicular stomatitis virus

To isolate new types of vesicular stomatitis virus (VSV) mutants, a four-stage screen was developed which identifies and characterizes mutants capable of complementing the defect in the VSV temperature-sensitive mutant tsG11. Two types of mutants of VSV, Indiana serotype, have been found by using the screen; they are new temperature-sensitive mutants which are, of necessity, not in complementation group I and mutants which do not produce plaques under conditions of single infection at 31 C (the normal permissive temperature) and are, therefore, called complementation-dependent mutants. The newly isolated, temperature-sensitive mutants fall into three complementation groups, two of which are congruent with known complementation groups; the newly identified group extends to six the number of complementation groups of VSV Indiana. The nature of the complementation-dependent mutants has not been established, but one was shown to not contain a significant deletion in its nucleic acid.

Transcribing complexes in cells infected by vesicular stomatitis virus and rabies virus

We analyzed cell extracts from BHK(21) cells infected with vesicular stomatitis virus (VSV) and rabies virus for in vitro RNA polymerase activity. Cells infected with VSV B virions exhibited several complexes with in vitro RNA polymerase activity in sucrose gradients. These complexes synthesize VSV transcriptase product (4 to 18S) polyadenylated in RNA complementary to virion RNA. Cells infected with a high multiplicity of B virions and T particles show only one RNA polymerase complex active in vitro. This complex sediments at 110S and makes only small (2S) RNA. Carrier BHK(21) cells persistently noncytopathically infected with VSV contain several complexes active in RNA polymerase, but both exhibit very low activity. Cytoplasms of cells noncytopathically infected with rabies virus also show very low levels of a complex containing RNA polymerase activity. No transcriptase nor any other in vitro polymerase activity could be found associated with purified rabies virions, although they do carry out primary transcription in cells treated with actinomycin D and cycloheximide before infection.