Characterization of rabies viruses recovered from persistently infected BHK cells

Generation and Functional Analysis of Defective Viral Genomes during SARS-CoV-2 Infection

Defective viral genomes (DVGs) have been identified in many RNA viruses as a major factor influencing antiviral immune response and viral pathogenesis. However, the generation and function of DVGs in SARS-CoV-2 infection are less known. In this study, we elucidated DVG generation in SARS-CoV-2 and its relationship with host antiviral immune response. We observed DVGs ubiquitously from transcriptome sequencing (RNA-seq) data sets of in vitro infections and autopsy lung tissues of COVID-19 patients. Four genomic hot spots were identified for DVG recombination, and RNA secondary structures were suggested to mediate DVG formation. Functionally, bulk and single-cell RNA-seq analysis indicated the interferon (IFN) stimulation of SARS-CoV-2 DVGs. We further applied our criteria to the next-generation sequencing (NGS) data set from a published cohort study and observed a significantly higher amount and frequency of DVG in symptomatic patients than those in asymptomatic patients. Finally, we observed exceptionally diverse DVG populations in one immunosuppressive patient up to 140 days after the first positive test of COVID-19, suggesting for the first time an association between DVGs and persistent viral infections in SARS-CoV-2. Together, our findings strongly suggest a critical role of DVGs in modulating host IFN responses and symptom development, calling for further inquiry into the mechanisms of DVG generation and into how DVGs modulate host responses and infection outcome during SARS-CoV-2 infection. IMPORTANCE Defective viral genomes (DVGs) are generated ubiquitously in many RNA viruses, including SARS-CoV-2. Their interference activity to full-length viruses and IFN stimulation provide the potential for them to be used in novel antiviral therapies and vaccine development. SARS-CoV-2 DVGs are generated through the recombination of two discontinuous genomic fragments by viral polymerase complex, and this recombination is also one of the major mechanisms for the emergence of new coronaviruses. Focusing on the generation and function of SARS-CoV-2 DVGs, these studies identify new hot spots for nonhomologous recombination and strongly suggest that the secondary structures within viral genomes mediate the recombination. Furthermore, these studies provide the first evidence for IFN stimulation activity of de novo DVGs during natural SARS-CoV-2 infection. These findings set up the foundation for further mechanism studies of SARS-CoV-2 recombination and provide evidence to harness the immunostimulatory potential of DVGs in the development of a vaccine and antivirals for SARS-CoV-2.

Generation and functional analysis of defective viral genomes during SARS-CoV-2 infection.. [PMID: 36172120 PMCID: PMC9516852 DOI: 10.1101/2022.09.22.509123] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Defective viral genomes (DVGs) have been identified in many RNA viruses as a major factor influencing antiviral immune response and viral pathogenesis. However, the generation and function of DVGs in SARS-CoV-2 infection are less known. In this study, we elucidated DVG generation in SARS-CoV-2 and its relationship with host antiviral immune response. We observed DVGs ubiquitously from RNA-seq datasets of in vitro infections and autopsy lung tissues of COVID-19 patients. Four genomic hotspots were identified for DVG recombination and RNA secondary structures were suggested to mediate DVG formation. Functionally, bulk and single cell RNA-seq analysis indicated the IFN stimulation of SARS-CoV-2 DVGs. We further applied our criteria to the NGS dataset from a published cohort study and observed significantly higher DVG amount and frequency in symptomatic patients than that in asymptomatic patients. Finally, we observed unusually high DVG frequency in one immunosuppressive patient up to 140 days after admitted to hospital due to COVID-19, first-time suggesting an association between DVGs and persistent viral infections in SARS-CoV-2. Together, our findings strongly suggest a critical role of DVGs in modulating host IFN responses and symptom development, calling for further inquiry into the mechanisms of DVG generation and how DVGs modulate host responses and infection outcome during SARS-CoV-2 infection.

A Virus Is a Community: Diversity within Negative-Sense RNA Virus Populations

Negative-sense RNA virus populations are composed of diverse viral components that interact to form a community and shape the outcome of virus infections. At the genomic level, RNA virus populations consist not only of a homogeneous population of standard viral genomes but also of an extremely large number of genome variants, termed viral quasispecies, and nonstandard viral genomes, which include copy-back viral genomes, deletion viral genomes, mini viral RNAs, and hypermutated RNAs. At the particle level, RNA virus populations are composed of pleomorphic particles, particles missing or having additional genomes, and single particles or particle aggregates. As we continue discovering more about the components of negative-sense RNA virus populations and their crucial functions during virus infection, it will become more important to study RNA virus populations as a whole rather than their individual parts. In this review, we will discuss what is known about the components of negative-sense RNA virus communities, speculate how the components of the virus community interact, and summarize what vaccines and antiviral therapies are being currently developed to target or harness these components.

Defective viral genomes are key drivers of the virus-host interaction

Viruses survive often harsh host environments, yet we know little about the strategies they utilize to adapt and subsist given their limited genomic resources. We are beginning to appreciate the surprising versatility of viral genomes and how replication-competent and -defective virus variants can provide means for adaptation, immune escape and virus perpetuation. This Review summarizes current knowledge of the types of defective viral genomes generated during the replication of RNA viruses and the functions that they carry out. We highlight the universality and diversity of defective viral genomes during infections and discuss their predicted role in maintaining a fit virus population, their impact on human and animal health, and their potential to be harnessed as antiviral tools.

This Review describes recent findings on the biogenesis and the role of defective viral genomes during replication of RNA viruses and discusses their impact on viral dynamics and evolution.

Defective (interfering) viral genomes re-explored: impact on antiviral immunity and virus persistence

Defective viral genomes (DVGs) are natural products of virus replication that occur in many positive and negative sense RNA viruses, including Ebola, dengue and respiratory syncytial virus. DVGs, which have severe genomic truncations and require a helper virus to replicate, have three well-described functions: interference with standard virus replication, immunostimulation, and establishment of virus persistence. These functions of DVGs were first described almost 50 years ago, yet only recent studies have shown the molecular intersection between their immunostimulatory and pro-persistence activities. Here, we review more than half a century of scientific literature on the immunostimulatory and pro-persistence functions of DVGs. We highlight recent advances in the field and the critical role DVGs have in both the acute and long-term virus-host interactions.

Characterization of a Slow-Migrating Component of the Rabies Virus Matrix Protein Strongly Associated with the Viral Glycoprotein

We investigated multiple forms of rabies virus matrix (M) protein. Under non-reducing electrophoretic conditions, we detected, in addition to major bands of monomer forms (23- and 24-kDa) of M protein, an M antigen-positive slow-migrating minor band (about 54 kDa) in both the virion and infected cells. Relative contents of the 54-kDa and monomer components in the virion were about 20-30% and 70-80% of the whole M protein, respectively, while the content of the 54-kDa component was smaller (about 10-20% of the total M protein) in the cell than in the virion. The 54-kDa components could be extracted from the infected cells with sodium deoxycholate, but they were quite resistant to extraction with 1% nonionic detergents by which most monomer components were solubilized. The 54-kDa component was precipitated more efficiently than the monomer by a monoclonal antibody (mAb; #3-9-16), which recognized a linear epitope located at the N-terminal of the M protein. The mAb #3-9-16 coprecipitated the viral glycoprotein (G), which was demonstrated to be due to strong association between the G and 54-kDa component of the M protein. Monomers and the 54-kDa polypeptide migrated to the same isoelectric point (pI) in twodimensional (2-D) gel electrophoresis, implicating that the 54-kDa component was composed of component(s) of the same pI as that of the M protein monomers. From these results, we conclude that the M antigen-positive 54-kDa polypeptide is a homodimer of M protein, taking an N-terminal-exposed conformation, and is strongly associated with the viral glycoprotein. Possible association with a membrane microdomain of the cell will be discussed.

Studies on the Conditions Required for Structural and Functional Maturation of Rabies Virus Glycoprotein (G) in G cDNA-Transfected Cells

When the rabies virus G cDNA was expressed with the help of T7 RNA polymerase provided by a recombinant vaccinia virus (RVV-T7), functional G proteins were produced in terms of their ability to induce low pH-dependent syncytium formation and the formation of conformational epitopes, including the acid-sensitive epitope recognized by mAb #1-30-44. Such an ability and the 1-30-44 epitope formation, however, were not associated with the G gene products when G cDNA was expressed without the help of RVV-T7 using a tetracycline-regulated expression vector (pTet-G), although they were normally transported to the surface of established G protein-producing BHK-21 (G-BHK) cells. But, when the G-BHK cells were treated with 2.5 m M sodium butyrate (NaB) after the removal of tetracycline, we could observe not only a much increased frequency of G protein-producing cells, but also the greatly enhanced maturation of the protein. Another short acylate, sodium propionate (NaP), similarly induced increased G protein synthesis at a concentration of 2.5 m M as NaB; however, such proteins were mostly not endowed with the fusion activity nor the 1-30-44 epitope, while NaP at a higher concentration as 5.0 m M did induce similarly the increased production and enhanced maturation of G protein, including the 1-30-44 epitope formation. From these results, we conclude that functional maturation of G protein to acquire fusogenic activity is correlated with 1-30-44 epitope formation, and 2.5 m M NaB not only stimulates G protein production, but also provides such cellular conditions as are required for the structural and functional maturation of the protein.

Further Studies on the Hyperphosphorylated Form (p40) of the Rabies Virus Nominal Phosphoprotein (P)

We investigated possible mechanisms involved in production of a hyperphosphorylated form (p40) of rabies virus P protein, to which two dimensional (2-D) gel electrophoresis was applied. The P gene products produced in Escherichia coli cells could be detected as a single spot of unphosphorylated 37-kDa form (termed as p37-0) in a 2-D gel. The 37-kDa proteins in the virus-infected cells are composed of some phosphorylated forms, including a major p37-1 and more phosphorylated minor forms (e.g., p37-2, p37-3, etc.), but little p37-0 is detected (Eriguchi et al., 2002). When the E. coli -produced P protein analogues were incubated with BHK-21 cell lysates, heparin-sensitive phosphorylation occurred as described previously (Takamatsu et al., 1998), giving an additional 40-kDa spot. However, such a p40-like derivative displayed a little more basic pI value than that of the authentic p40 produced in the infected cells; hence, the former was termed p40-0 (pI=4.78), while the latter, p40-1 (pI=4.73). In contrast, p40 produced in the P cDNAtransfected animal cell was detected at the p40-1 position. In addition, staurosporine did not affect the p40-1 production in virus-infected nor the P cDNA-transfected animal cells, while the agent reduced production of hyperphosphorylated forms of p37, resulting in accumulation of p37-1, but not of p37-0. These results suggest that, although p37-0 may become a substrate for the heparin-sensitive protein kinase (PK) in vitro, only p37-1 is a substrate for p40 production catalyzed by heparin-sensitive PK in animal cells, and staurosporine-sensitive PK is involved in the production of more phosphorylated forms of p37, but not in p37-1 production from p37-0.

Monoclonal Antibody #3-9-16 Recognizes One of the Two Isoforms of Rabies Virus Matrix Protein That Exposes Its N-Terminus on the Virion Surface

We investigated behaviors of the rabies virus matrix (M) protein using a monoclonal antibody (mAb), #3-9-16, that recognized a linear epitope located at the N-terminus of the protein. Based on the reactivity with this mAb, M proteins could be divided into at least two isoforms; an ordinary major form (Malpha) whose 3-9-16 epitope is hidden, and an N-terminal-exposed epitope-positive form (Mbeta). The Mbeta protein accounted for about 25-30% of the total M proteins in the virion, while its content in the cell ranged from 10 to 15% of total M protein. Fluorescent antibody (FA) staining showed that the Mbeta antigen distributed in the Golgi area where the colocalized viral glycoprotein antigen was also detected. Mbeta antigen was shown to be exposed on the surface of infected cells by both immunoprecipitation and FA staining with the mAb, whereby the cells might have become sensitive to the mAb-dependent complement-mediated cytolysis. Similarly, the Mbeta antigen was shown to be exposed on the virion surface, and the infectivity of the virus was destroyed by the mAb in the presence of a complement. Together with these results, we think that the M protein molecule takes either of two conformations, one (Mbeta) of which exposes the 3-9-16 epitope located in the N-terminal region of the M protein, that are also exposed on the surface of the virion and infected cells, whereby it might play a certain important role(s) in the virus replication process differently from the other form (Malpha), probably through its intimate association with the Golgi area and/or the cell membrane.

A unique transcription mode of rabies virus high egg passage-Flury strain detected in infected baby hamster kidney-21 cells

The transcription mode of rabies virus high egg passage-Flury (HEP) strain was examined and compared with that of Evelyn Rokitniki Abelseth (ERA) strain by northern blot analysis using rabies virus gene-specific probes. The ERA strain was shown to exclusively produce monocistronic mRNAs in transcription. All combinations of multicistronic transcripts, including five monocistronic mRNAs, were detected in the viral RNA transcripts of HEP strain. It was concluded that the unique transcription mode is not due to the nucleotide structure of the genome RNA template, but rather to the viral RNA polymerase of HEP strain. The viral polymerase of HEP strain read through the gene junction at a high frequency. The HEP strain has been passaged many times in chick embryo and cultured cells, and has adapted to propagate well in the baby hamster kidney-21 (BHK-21) cells. Through these passages in various hosts, the HEP strain has acquired a unique transcription mode that might have an advantage in amplification of the virus.

Two different conformations of rabies virus glycoprotein taken under neutral pH conditions

We previously reported that the rabies virus glycoprotein (G) takes either of two different conformations (referred to as B and C forms) under neutral pH conditions, that could be differentiated by their reactivity to a monoclonal antibody (mAb), #1-30-44, that recognizes the acid-sensitive conformational epitope, and the formation taken is dependent on two separate regions containing Lys-202 and Asn-336 of the protein (Kankanamge et al., Microbiol. Immunol., 47, 507-519, 2003). Semi-quantitative antibody-binding assays demonstrated that only one-third to one-fourth of mature G proteins on the cell surface were taking the 1-30-44 epitope-positive B form even at pH 7.4. The ratio of B to C varied, depending on the environmental pH, but did not decrease to zero even at pH 5.8-6.2, preserving a certain content (about 15-20%) of B form. Immunoprecipitation studies demonstrated that a portion of G proteins were intimately associated with a dimer form of matrix (M) protein in terms of resistance to treatment with a mixture of 1% deoxycholate and 1% Nonidet P-40, and seemed to preserve the B form even at lower pHs. Similar results were also obtained with the virion-associated G proteins, including the intimate association of a portion of the G proteins with the M protein dimer. From these results, we assume that a certain portion of the rabies virion-associated G proteins are associated with a dimer form of M protein, keeping the 1-30-44 epitope-positive B conformation under various pH conditions, which might possibly assure the virion's recognition of host cell receptor molecules in the body.

Transitions in understanding of RNA viruses: a historical perspective

This chapter documents that RNA viruses have been known for over a century to be genetically variable. In recent decades, genetic and molecular analyses demonstrate that they form RNA quasispecies populations; the most rapidly mutating, highly variable and genetically versatile life forms on earth. Their enormous populations, rapid replication and extreme genetic plasticity can allow rates of evolution that exceed those of their eukaryotic host populations by millions-fold.

Structural difference recognized by a monoclonal antibody #404-11 between the rabies virus nucleocapsid (NC) produced in virus infected cells and the NC-like structures produced in the nucleoprotein (N) cDNA-transfected cells

We investigated structural changes in the rabies virus (HEP-Flury strain) nucleocapsid (NC) during the virus replication, for which we used two anti-nucleoprotein (N) monoclonal antibodies (mAbs), #404-11 (specific for a conformation-dependently exposed linear epitope) and #1-7-11 (specific for a conformational epitope which is exposed after the nucleocapsid formation). Both mAbs recognized the N protein of the viral NC, but not of the RNA-free N-P complex. The 1-7-11 and 404-11 epitopes could be mapped to the N-terminal and the C-terminal regions of N protein, respectively. Immunoprecipitation studies demonstrated that treatment of the NC either with the alkaline phosphatase or sodium deoxycholate (DOC) resulted in dissociation of most P proteins from the NC and in the reduced reactivity to mAb #404-11, but not to mAb #1-7-11. NC-like structures produced in the N cDNA-transfected cells displayed strong reactivity to mAb #1-7-11; however, reactivity to mAb #404-11 was very weak. And, coexpression with viral phosphoprotein (P) resulted in little increase in reactivity to mAb #404-11 of the NC-like structures, while the reactivity was significantly increased by cotransfection with P and the viral minigenome whose 3'- and 5'-end structures were derived from the viral genome. From these results, we assume that, although the 404-11 epitope is a linear one, the epitope-containing region is exposed only when N proteins encapsidate properly the viral RNA in collaboration with the P protein. Further, exposure of the 404-11 epitope region might be function-related, and be regulated by association and dissociation of the P protein.

Association of rabies virus nominal phosphoprotein (P) with viral nucleocapsid (NC) is enhanced by phosphorylation of the viral nucleoprotein (N)

We investigated possible role(s) of N protein phosphorylation in the rabies virus replication process. A large amount of P proteins are associated with the viral nucleocapsid (NC) in the infected cell, the amount which was greatly decreased by phosphatase-treatment of the isolated NC, indicating that the phosphate group of N and/or P proteins is essential for their stable association with the NC. Immunoprecipitation studies were performed on the coexpressed normal N or phosphorylation deficient N(S389A) and P proteins, demonstrating that the P protein associated with phosphorylation-deficient NC-like structures was much less in amount than that associated with the wild type NC. Similar results were also obtained with a mutant P protein, PDeltaN19, which lacked the N-terminal 19 amino acids and was capable of binding to the NC-like structures but incapable of forming the RNA-free N-P complexes. Immunoprecipitation studies with mAb #402-13 further suggested that the NC-specific linear 402-13 epitope was exposed even on the P proteins which were associated with the phosphorylation-deficient NC-like structures, but such association was very weak as demonstrated by greatly decreased amounts of coprecipitated NC-like structures. From these results, we assume that the phosphorylation of N protein enhances the association between the 402-13 epitope-positive P protein and the NC probably by stabilizing such P-NC binding.

Mapping of the low pH-sensitive conformational epitope of rabies virus glycoprotein recognized by a monoclonal antibody #1-30-44

Monoclonal antibody (mAb) #1-30-44 recognized an acid-sensitive conformational epitope of rabies virus glycoprotein (G). The antigenicity of G protein exposed on the cell surface was lost when the infected cells were exposed to pH 5.8. By comparing the deduced amino acid sequence of G protein between the HEP-Flury strain and the epitope-negative CVS strain as well as the mAb-resistant escape mutants, two distant sites that contained Lys-202 and Asn-336 were shown to be involved in the epitope formation. Lys-202 is located in the so-called neurotoxin-like sequence, while Asn-336 is included in antigenic site III and is very near the amino acid at position 333, which is known to affect greatly the neuropathogenicity of rabies virus when changed. Consistent with this finding, antigenicity of a neurovirulent revertant of the HEP-Flury strain, in which Gln-333 of G protein was replaced by Arg, was also affected as shown by its greatly decreased reactivity with mAb #1-30-44 compared to that of the original avirulent HEP virus. Based on these results, we hypothesize that the neurotoxin-like domain and some amino acids in antigenic site III come into contact with each other to form a conformational epitope for mAb #1-30-44, and such a configuration would be lost when exposed to acidic conditions to perform a certain low pH-dependent function of G protein.

Further characterization of the rabies virus glycoproteins produced by virus-infected and G cDNA-transfected cells using a monoclonal antibody, #1-30-44, which recognizes an acid-sensitive epitope

Expression of rabies virus glycoprotein (G) by G cDNA-transfected mammalian cells resulted in the production of only a fusion-negative form. Low pH-dependent fusion activity, however, was seen when the expression was done under control of the T7 promoter with the help of recombinant vaccinia virus (RVV-T7) that provided T7 RNA polymerase. Fusion-inactive G proteins were transported to the cell surface as being detected by a conformational epitope-specific monoclonal antibody (mAb; #1-46-12). The fusion-inactive G proteins were recognized by most of our 13 conformation-specific mAbs, except for one mAb, #1-30-44, that recognized the low pH-sensitive conformational epitope. When the G gene expression was done with the help of RVV-T7, although most G proteins remained in the epitope-negative form, a small fraction of G gene products were 1-30-44 epitope-positive, and cell fusion activity could be seen when cells were exposed to low pH conditions. From these results, we conclude that acquisition of low pH-dependent fusion activity is closely related to structural maturation of the G protein to form the low pH-sensitive 1-30-44 epitope. Such maturation seems to be dependent on certain rabies virus-induced cellular conditions or functions, which might also be provided in part by the vaccinia virus infection. We further assume that expression of G cDNA alone mostly results in the production of mis-folded and/or differently folded forms of G protein, and only a small fraction is correctly folded even under RVV-T7-mediated expression conditions.

Studies on the rabies virus RNA polymerase: 3. Two-dimensional electrophoretic analysis of the multiplicity of non-catalytic subunit (P protein)

We described previously (Takamatsu et al., 1998. Microbiol. Immunol. 42: 761-771) the rabies virus P protein as being composed of several components of different sizes, among which the full-sized major components were termed as p40 and p37 according to their electrophoretic mobilities, and radiolabeling studies with [32P]phosphate implied that p40 was a hyperphosphorylated form. We further examined here these proteins by two-dimensional (2-D) gel electrophoresis and immunoblotting, showing that a major component, p37, was composed of multiply modified subcomponents of different pIs (termed p37-1, p37-2, p37-3, etc., based on their acidity) in the virion and infected cells, but the unmodified precursor (termed p37-0) was little in amount. The viral nucleocapsid (NC)-bound P proteins were composed of multiple forms of p37 (the major one was p37-1) and also a minor component, p40-1. P proteins which were bound to newly synthesized free N proteins were mostly composed of p37-1, indicating that hyperphosphorylation of P proteins occurred after their being used for the encapsidation. Treatment of the infected cells with okadaic acid induced accumulation of the more acidic forms of P proteins, suggesting that heterogeneity in the full-sized P proteins is a reflection of their dynamic aspects of multiple cycles of phosphorylations and dephosphorylations in the cell. Two-D gel analyses demonstrated also that p40 was not so acidic as we expected, and implied that our previous data of apparent hyperphosphorylation of p40 was due to very frequently recycled utilization of the protein, and preformed non-labeled P proteins were also 32P-phosphorylated in a radiolabeling period and were converted to the p40.

Structural relationship between nucleocapsid-binding activity of the rabies virus phosphoprotein (P) and exposure of epitope 402-13 located at the C terminus

The structural changes of the nominal phosphoprotein (P) of rabies virus using a monoclonal antibody, mAb #402-13, was investigated. This mAb recognized a linear epitope that was mapped roughly to a C-terminal region of the P protein, ranging from aa 256 to 297. The P gene products were detected by the mAb in immunoblot assays, the products of which were produced either in BHK-21 cells or in Escherichia coli cells. The mAb, however, detected very low levels of P gene products in immunoprecipitation assays. The mAb recognized the nucleocapsid (NC)-associated P proteins but recognized free P protein and free N-P complex produced in the infected cells much less efficiently. When the P proteins were released from the NC, however, they were no longer recognized by the mAb. Similar results were obtained from BHK-21 cells co-transfected with P and N cDNAs. Furthermore, studies with C-terminally truncated P protein mutants revealed that the NC-binding ability of the P protein was dependent on the presence of the C-terminal epitope region. From these results, it is thought that the 402-13 epitope region is concealed when the P protein is present in a free form or free N-P complex but is exposed when it is associated with the NC. The C-terminal epitope region seemed to be essential for the P protein to be associated with the NC but not for the formation of free N-P complexes with newly synthesized N protein.

The rabies virion-associated 100-kDa polypeptide (VAP100) is a host-derived minor component of the viral envelope

We investigated a minor polypeptide component of 100-kDa detected in the rabies virion (referred to as VAP100) by using a monoclonal antibody (mAb), #16743, which was shown to recognize the SDS-denatured VAP100 antigen by immunoblot analyses. Although the VAP100 antigen was hardly detectable in the cell by usual immunoblot methods with this mAb, we could detect the antigen by a luminescent immunoblot method as well as by immunoprecipitation from the metabolically radiolabeled cell lysates and virions. Fluorescent antibody (FA) staining with mAb #16743 detected the uniformly distributed antigen on the formalin-fixed normal BHK-21 cells, while slight accumulation of the antigen was also seen in the Golgi area when the cells were permeabilized by treatment with Triton X-100 after fixation. Rabies virus infection induced alteration of the behavior of VAP100 to show a spotted distribution pattern in virus-infected cells. Double FA staining with mAb #16743 and rabbit antibody against the rabies virus envelope antigen demonstrated colocalized distribution of the viral envelope antigens and VAP100 in the cell. From these results, we think that VAP100 is a membrane-associated component of the cell, and its colocalized distribution with the viral envelope antigens in the cell implicates an intimate association of the VAP100 with viral envelope protein(s) and a reflection of possible involvement in the efficient incorporation of VAP100 into the virion.

Nucleocapsid formation and/or subsequent conformational change of rabies virus nucleoprotein (N) is a prerequisite step for acquiring the phosphatase-sensitive epitope of monoclonal antibody 5-2-26

We investigated the antigenic maturation of rabies virus N protein, for which we used some conformational epitope-specific monoclonal antibodies (MAbs) and an MAb (5-2-26) against a phosphorylation-dependent linear epitope. Infected cells were lysed with a deoxycholate-free lysis buffer and separated by ultracentrifugation into the soluble top and the nucleocapsid fractions. None of the study MAbs recognized N proteins in the top fraction, whereas nucleocapsid-associated N proteins were recognized by all of the MAbs. Immunoprecipitation with polyclonal anti-N antibodies coprecipitated the P proteins from the top fraction, indicating that soluble N proteins are mostly associated with the P protein. The N proteins dissociated from both the N-P complex and nucleocapsids were recognized by none of the study MAbs, whereas the MAb 5-2-6 recognized the SDS-denatured N proteins of the nucleocapsid but not of the top fraction. In addition, the phosphorylation-deficient mutant N proteins were shown to be similarly accumulated as the wild-type N proteins into the viral inclusion bodies, defined as the virus-specific structures composed of viral nucleocapsids, that are produced in the cytoplasm of the infected cells. Based on these results, we believe that newly synthesized N proteins are not immediately phosphorylated at serine-389 (a common phosphorylation site) but are first associated with the P protein. After being used for encapsidation of the viral RNA, the N proteins undergo conformational changes, whereby epitopes for the conformation-specific MAbs are formed and become phosphorylated at serine-389.

Association of a cellular 21-kDa transmembrane protein (VAP21) with enveloped viruses

We reported previously that the rabies virions contained a 21-kDa cellular transmembrane protein (referred to as VAP21) as a minor component (Sagara, J. et al, Microbiol. Immunol. 41(12): 947-955, 1997). In this study, we further examined the possible interactions of VAP21 with other enveloped viruses, including the vesicular stomatitis virus (VSV; negative-stranded RNA virus), Sindbis virus (positive-stranded RNA virus) and herpes simplex virus type 1 (HSV-1; double-stranded DNA virus). An immunoblot analysis demonstrated that all of these enveloped viruses contained VAP21 in the virion as a minor component. Immunoprecipitation studies suggested that VAP21 was associated with certain viral proteins in the cell, such as the matrix (M) protein of VSV, a capsid protein of Sindbis virus, and at least a capsid protein (VP5) of HSV-1. The association was disrupted by treatment with 0.5% sodium dodecyl sulfate, but resistant to the treatment with 1% NP-40 plus 1% deoxycholate. These results suggest that: 1) VAP21 is not primarily associated with the viral transmembrane glycoprotein but rather with the internal viral protein, and, 2) this association would cause the efficient incorporation of VAP21 into the virion.

Intracellular behavior of rabies virus matrix protein (M) is determined by the viral glycoprotein (G)

To investigate the nature and intracellular behavior of the matrix (M) protein of an avirulent strain (HEP-Flury) of rabies virus, we cloned and sequenced the cDNA of the protein. Using expression vectors pZIP-NeoSV(X)1 and pCDM8, the cDNA was transfected to animal cells (BHK-21 and COS-7) with or without coexpression of viral glycoprotein (G). When M protein alone was expressed in the cells, it displayed homogeneous distribution in the whole cell including the nucleus. In contrast, coexpression with G protein resulted in the abolishment of nuclear distribution of M antigen, and both of the antigens displayed a colocalized distribution in the cell, especially at the cellular membrane as seen in the virus-infected cells, while the distribution of G antigen was not affected by coexpressed M antigen. Immunoprecipitation studies revealed that M protein was coprecipitated with G protein by anti-G antibody, and vice versa, although cross-linking with dithiobis(succinimidyl propionate) was necessary for coprecipitation because of their easier dissociation in the presence of sodium deoxycholate. These results suggest that M protein intimately associates with G protein, which may affect or regulate the behavior (e.g., intracellular localization) of M protein. Studies with deletion mutants of M protein indicate that an internal region around the amino acids from 115 to 151 is essential for the M protein to preserve its binding ability to G protein.

A new quantitative method for rabies virus by detection of nucleoprotein in virion using ELISA

We have developed a new quantitative method for rabies virus (RV) detection using enzyme-linked immunosorbent assay (ELISA). The method named N-ELISA was based on the quantitation of nucleoprotein (N) in RV virions captured by RV-specific polyclonal antibodies on an ELISA plate. Both infective and defective interfering (DI) particles of RV could be detected by this method. When viruses were propagated in a medium of pH 7.4 adjusted with 7% NaHCO3, N-ELISA could detect them with titers of more than 10(6) pfu/ml, though the result did not correlate highly with that of the infectivity assay. The reason for this was considered to be that RVs included spikeless and damaged particles which were produced under conditions of low or high pH. However, in the time course of virus yield, titers of N-ELISA correlated well with those of the infectivity assay.

Studies on the rabies virus RNA polymerase: 2. Possible relationships between the two forms of the non-catalytic subunit (P protein)

We investigated the relationship between the two forms of rabies virus P protein, a non-catalytic subunit of rabies virus RNA polymerase. The two displayed different electrophoretic mobilities as 37- and 40-kDa polypeptides, hence termed as p37 and p40, respectively. Double labeling experiments with [3H]leucine and [32P]orthophosphate demonstrated that p40 was much more phosphorylated than p37. Treatment of the virion proteins with alkaline phosphatase eliminated only p40, and not 37-kDa polypeptide. The p37 was a major product of the P gene, and was accumulated in the infected cell and incorporated into the virion. On the other hand, p40 was apparently detected only in the virion, and little detected in the cells. Treatment of infected cells with okadaic acid, however, resulted in significant accumulation of p40 in the cell, suggesting that p40 was continuously produced in the cell but dephosphorylated quickly. We detected both 37- and 40-kDa products in P cDNA-transfected animal cells, while only a 37-kDa product was produced in Escherichia coli. Incubation of 37-kDa products from E. coli with the lysates of animal cells in vitro resulted in the production of a 40-kDa product, which was also shown to be suppressed by the heparin. From these results, it is suggested that p40 is produced by the hyperphosphorylation of a 37-kDa polypeptide, which depends on certain heparin-sensitive cellular enzyme(s) and occurs even in the absence of the other viral gene products, and that p40 is reverted quickly to p37 in the infected cells, probably being dependent on some virus-induced factor(s).

A defective interference-like phenomenon of human hepatitis B virus in chronic carriers

Defective interfering (DI) particles have been found in many RNA and DNA viruses of bacteria, plants, and animals since their first discovery in influenza virus. However, this fundamental phenomenon has not been demonstrated in human natural infections. Using a new approach, here we provide the first experimental evidence for the existence of DI-like viruses in human chronic carriers of hepatitis B virus (HBV). Functional characterization of naturally occurring core internal deletion (CID) variants of HBV revealed all of the features of DI particles. When equal amounts of wild-type and CID variant DNAs were cotransfected into a human hepatoma cell line, Huh7, a three- to fivefold enrichment of CID variants was most often observed. The fluctuations of the virus populations between CID variants and helper HBV in three chronic carriers are reminiscent of the cycling phenomenon in other DI viral systems. This finding has important implications for chronic viral hepatitis and other chronic progressive viral diseases.

Monoclonal antibody #5-2-26 recognizes the phosphatase-sensitive epitope of rabies virus nucleoprotein

We prepared monoclonal antibodies (MAbs) against the rabies virus N protein, among which one antibody (MAb 5-2-26) was shown to lack reactivity with the phosphatase-treated N protein. The MAb was able to recognize the sodium dodecyl sulfate (SDS)-denatured N protein. The MAb did not recognize the N-protein analogues produced in Escherichia coli (E. coli), indicating that the N-gene products were not normally processed in E. coli after translation. On the other hand, the MAb reacted normally with N-gene products produced in COS-7 cells, but not with those produced in the presence of K-252a (a protein kinase inhibitor of a broad spectrum). The MAb displayed weak cross-reactivity with the Triton-insoluble network structures composed of several components, while another phosphoprotein (M1) of the virus was not recognized at all. These results suggest that MAb 5-2-26 preferentially recognizes a phosphatase-sensitive linear epitope of N protein, which may enable further investigations to be conducted on the mechanism of N-protein phosphorylation and its role(s) in virus replication.

Identification of a phosphatase-sensitive epitope of rabies virus nucleoprotein which is recognized by a monoclonal antibody 5-2-26

We have investigated a phosphatase-sensitive sequential epitope of the nucleoprotein (N), one of the phosphoproteins of rabies virus, which is recognized by the monoclonal antibody (MAb) #5-2-26. The epitope was shared in common by all of the rabies virus strains we tested, including the HEP, ERA, CVS and Japanese strains (Nishigahara and Komatsukawa). Thin layer chromatography of the acid hydrolyzates of 32P-labeled N protein showed that the protein contained phosphoserine and phosphothreonine at a molar ratio of about 4 to 1, while no phosphotyrosine was detected. Immunoprecipitation studies with several deletion mutants of the N protein showed that the epitope is located in a region spanning from amino acid 344 to 415. If the phosphatase-sensitive epitope is located at or near the phosphoamino acid, the location of the latter could be narrowed further to a region from amino acid 354 to 389 by comparing the amino-acid sequences among the viral strains. To examine this assumption, point mutation was introduced by amino-acid substitution with alanine at either of five potential phosphorylation sites (i.e., positions 354, 375, 377, 386 and 389) in the 354-389 region. Among those, only one substitution, at position 389, greatly affected the antigenicity. Substitution of serine-389 by threonine also reduced the antigenicity. These results strongly suggest that serine-389 is a phosphorylation site and essential for constructing or stabilizing the antigenic structure for MAb 5-2-26.

Mapping and characterization of a sequential epitope on the rabies virus glycoprotein which is recognized by a neutralizing monoclonal antibody, RG719

We have established a murine hybridoma cell line RG719 which produces a rabies virus-neutralizing IgM-type monoclonal antibody (referred to as MAb RG719). Immunoblot analysis indicated that the antibody recognized a sequential epitope of G protein. Among four rabies virus strains tested, the antigenicity to MAb RG719 was absent from the Nishigahara strain, while the other three strains (HEP, ERA and CVS) reacted to the MAb. Studies with deletion mutants of the G protein indicated that the epitope was located in a middle region of the primary structure of G protein, ranging from position 242 to 300. By comparing the estimated amino acid sequence of the four strains, we found in this region two amino acids (at positions 263 and 291) which are common to three of those strains but are not shared by the Nishigahara strain. The site-directed point mutagenesis revealed that replacement of phenylalanine-263 by leucine destroyed the epitope of the HEP G protein, while the epitope was generated on the Nishigahara G protein whose leucine-263 was replaced by phenylalanine. These observations suggest that phenylalanine-263 is essential for constructing the epitope for MAb RG719. The synthetic 20-mer peptide produced by mimicking the amino acid sequence (ranging from amino acid positions 249 to 268) of the presumed epitope region was shown to bind specifically to MAb RG719 and also to raise the virus-neutralizing antibodies in rabbits. Vaccination with the HEP vaccine produced in Japan induced in humans and rabbits production of significant amounts of the antibodies which reacted with the 20-mer peptide.

Detection of truncated virus particles in a persistent RNA virus infection in vivo

Infectious hematopoietic necrosis virus (IHNV) is a rhabdovirus which causes devastating epizootics of trout and salmon fry in hatcheries around the world. In laboratory and field studies, epizootic survivors are negative for infectious virus by plaque assay at about 50 days postexposure. Survivors are considered virus free with no sequelae and, thus, are subsequently released into the wild. When adults return to spawn, infectious virus can again be isolated. Two hypotheses have been proposed to account for the source of virus in these adults. One hypothesis contends that virus in the epizootic survivors is cleared and that the adults are reinfected with IHNV from a secondary source during their migration upstream. The second hypothesis contends that IHNV persists in a subclinical or latent form and the virus is reactivated during the stress of spawning. Numerous studies have been carried out to test these hypotheses and, after 20 years, questions still remain regarding the maintenance of IHNV in salmonid fish populations. In the study reported here, IHNV-specific lesions in the hematopoietic tissues of rainbow trout survivors, reared in specific-pathogen-free water, were detected 1 year after the epizootic. The fish did not produce infectious virus. The presence of viral protein detected by immunohistochemistry, in viral RNA by PCR amplification, and in IHNV-truncated particles by immunogold electron microscopy confirmed the presence of IHNV in the survivors and provided the first evidence for subclinical persistence of virus in the tissues of IHNV survivors.

Rabies virus replication in primary murine bone marrow macrophages and in human and murine macrophage-like cell lines: implications for viral persistence

To determine whether rabies viruses replicate in macrophage or macrophage-like cells, several human and murine macrophage-like cell lines, as well as primary cultures of murine bone marrow macrophages, were incubated with the Evelyn-Rokitnicki-Abelseth (ERA) virus and several different street rabies viruses (SRV). ERA rabies virus replicated well in human monocytic U937 and THP-1 cells and murine macrophage IC-21 cells, as well as primary cultures of murine macrophages. Minimal replication was detected in murine monocytic WEHI-3BD- and PU5-1R cells, and ERA virus did not replicate in murine monocytic P388D1 or J774A.1 cells. A tissue culture-adapted SRV of bat origin also replicated in IC-21 and U937 cells. Non-tissue culture-adapted SRV isolated from different animal species, particularly bats, replicated minimally in U937, THP-1, IC-21 cells and primary murine bone marrow macrophages. To determine whether rabies virus replication is dependent upon the state of differentiation of the macrophage-like cell, human promyelocytic HL-60 cells were differentiated with 12-O-tetradecanoylphorbol-13-acetate (TPA). ERA rabies virus replicated in the differentiated HL-60 cells but not in undifferentiated HL-60 cells. Persistent infections were established in macrophage-like U937 cells with ERA rabies virus and SRV, and infectious SRV was isolated from adherent bone marrow cells of mice that had been infected 96 days previously. Virus harvested from persistently infected U937 cells and the adherent bone marrow cells had specifically adapted to each cell. This specificity was shown by the inability of the viruses to infect macrophages other than U937 cells and primary bone marrow macrophages, respectively. Virus titers of the persistently infected U937 cells fluctuated with extended cell passage. After 30 passages, virus released from the cells had lost virulence as shown by its inability to kill intracranially inoculated mice. However, the avirulent virus released from the persistently infected cells was more efficient in infecting and replicating in naive U937 cells than the virus which was used to establish the persistent infection. These results suggest that macrophages may serve as reservoirs of infection in vivo, sequestering virus which may subsequently be activated from its persistent state, resulting in clinical infection and death.

Identification of heat shock protein 70 in the rabies virion

We investigated a 73-kDa polypeptide (p73), a minor component of the rabies virion (HEP-Flury and ERA strains), accounting for as much as 1% of total virion proteins. Two-dimensional gel electrophoresis and immunoblotting with the antiserum against the heat shock protein 70 (hsp70) demonstrated that p73 was identical to a constitutive type of cellular hsp70. The antiserum also detected p73/hsp70 in the purified virions of other negative-stranded RNA viruses, such as vesicular stomatitis virus (New Jersey serotype), Newcastle disease virus (Miyadera strain), and influenza A virus (PR8 strain), among which, however, the contents were variable.

Syncytium formation is induced in the murine neuroblastoma cell cultures which produce pathogenic type G proteins of the rabies virus

We investigated comparatively the interactions of host cells with two types of rabies virus G protein, an avirulent type G (Gln) and a virulent type G (Arg) protein, having glutamine and arginine at position 333, respectively. For this purpose, we established four types of cell lines (referred to as G(Gln)-NA, G(Arg)-NA, G(Gln)-BHK, and G(Arg)-BHK cells, respectively) by transfecting either the G(Gln)-cDNA or G(Arg)-cDNA into two kinds of cells, murine neuroblastoma C1300 (clone NA) and nonneuronal BHK-21. Both G(Gln)-NA and G(Arg)-NA cells produced G proteins when they were treated with 5 mM sodium butyrate, but only G(Arg)-NA cells formed syncytia at the neutral pH, which was suppressed by anti-G antiserum. The sodium butyrate-treated G(Arg)-NA cells fused also with sodium butyrate-treated NA cells under coculture conditions, but neither with untreated NA cells nor with BHK-21 cells. On the other hand, both G(Gln)-BHK and G(Arg)-BHK cells constitutively produced G proteins, but no syncytium was produced at the neutral pH. G(Arg)-BHK cells, however, formed syncytia with the sodium butyrate-treated NA cells when they were cocultured. These results suggest that only G(Arg) has a potential ability to produce syncytia of NA cells regardless of cell types by which G(Arg) protein was produced and also suggest that a certain cellular factor(s) is required for the syncytium formation, the factor(s) which is lacking in BHK-21 and untreated NA cells but is produced by the sodium butyrate-treated NA cells.

Temperature-sensitivity of the replication of rabies virus (HEP-flury strain) in BHK-21 cells. I. Alteration of viral RNA synthesis at the elevated temperature

We investigated the nature of temperature sensitivity of the HEP strain of rabies virus. After initial incubation for appropriate period (more than 12 hr) at the permissive temperature (36-37 degrees), incubation temperature of the rabies virus infected cultures was shifted to a nonpermissive temperature (39.5-40.5 degrees). Upon the upshift, virion production was ceased, but the rate of viral RNA synthesis was greatly increased and reached almost 10 times that of 36 degrees-infection within 8-10 hr, and then the activity quickly decreased together with the onset of accelerated CPE. Little or no 42S genome-sized RNA was produced at the elevated temperature, and almost all RNAs produced in large amounts seemed to be viral mRNAs and were shown to be functional in t he cell-free translation system. Consistent with these observations, the viral ribonucleoprotein complex isolated from the temperature-upshifted culture was associated with relatively large amounts of small sized RNAs, which might reflect their increased transcriptive activity. These observations suggest that the viral RNA polymerase itself is not temperature-sensitive and the temperature-induced defect may reside in the regulatory factor which plays a role in turning on the synthesis of viral genome-sized RNA.

Effects of defective interfering viruses on virus replication and pathogenesis in vitro and in vivo

DI viruses and defective viruses generally are widespread in nature. Laboratory studies show that they can sometimes exert powerful disease-modulating effects (either attenuation or intensification of symptoms). Their role in nature remains largely unexplored, despite recent suggestive evidence for their importance in a number of systems.

Viral RNA and protein synthesis in two LLC-MK2 cell lines persistently infected with human parainfluenza virus 3

Two lines of LLC-MK2 cells persistently infected with human parainfluenza virus 3 (HPIV-3) have been maintained in culture for approximately 3 years. Subgenomic RNAs (putative defective interfering particle genomes) were detected in virions released from both persistently infected cultures. In one of the persistently infected cell lines cyclic variation in the production of virions containing standard virus genomic-size (50S) RNA and subgenomic RNA was observed. The molar ratio of subgenomic RNA to 50S RNA ranged from less than 0.1/1 to 8.7/1. Northern blot analyses revealed that the patterns of viral mRNA synthesis in persistently infected cells from both cultures were similar to those of standard virus infected cells. Furthermore, the intracellular viral-specific proteins had electrophoretic mobilities similar to the corresponding proteins in standard virus-infected cells. Nucleotide sequence analysis of cloned M gene from virus after 29 months of persistence (147 passages) revealed only one variable conservative amino acid change in two clones analyzed from each cell line, indicating that the M protein is not likely to be involved in the maintenance of the persistent infections. The possible mechanisms by which the persistent state is maintained are discussed.

Structure and transcription of the glycoprotein gene of attenuated HEP-Flury strain of rabies virus

The mRNA-encoding G protein of the attenuated HEP-Flury strain of rabies virus was sequenced by the cDNA cloning technique. The G-mRNA was composed of 2059 nucleotides, with the coding region located from the 28th to 1602nd nucleotide, and was capable of encoding a polypeptide of 524 amino acids. Although the coding region was highly homologous (90% or more) to that of ERA and PV strains, the 3' noncoding region of the HEP virus G-mRNA was longer than that reported for other strains by some 400 nucleotides. The extra sequence was homologous to the long G-L intergenic sequence of the PV viral genome. The HEP virus genome lacked the postulated polyadenylating signal (TG-AAAAAAAA) that should have been found just before the "long G-L intergenic region," which indicates that the long G-L intergenic region of the HEP virus is integrated into the preceding G gene, and is transcribed only as a portion of the G-mRNA molecule. In the ERA virus-infected cells, however, two species of G-mRNA (1.9 and 2.3 kb long) were produced. The longer G-mRNA also contained the sequence complementary to the long G-L intergenic region and the shorter one did not. These findings suggest that two different poly(A)-tailing signals (one is present just before and another at the end of the long G-L intergenic region) work toward terminating the transcription of the ERA virus G gene and that the longer G-mRNA is produced as a readthrough product.

Budding efficiency of Sendai virus nucleocapsids: influence of size and ends of the RNA

The budding efficiency of Sendai virus antigenomes, as well as of defective interfering (DI) nucleocapsids of the deletion and copy-back types, was compared to that of the viral genome during infections of baby hamster kidney (BHK) cells. The antigenomes were shown to bud into virus particles as efficiently as the genomes, arguing for the irrelevance of the nucleocapsid-RNA ends in regulating the efficiency of budding. The DI nucleocapsids, however, were restricted in their budding by factors inversely proportional to their size, arguing for an effect of nucleocapsid size in this process. This restriction in budding, however, appeared to be only expressed under conditions of very efficient DI-RNA replication.

Generation of measles virus defective interfering particles and their presence in a preparation of attenuated live-virus vaccine

By starting from a thrice-purified wild-type measles virus plaque, the generation of detectable subgenomic RNAs was achieved within a series of five serial infections of Vero cells. The evolution of these subgenomic RNAs was followed for seven serial passages and ended with the preparation of a highly interfering viral stock. On the other hand, the detection of discrete subgenomic RNAs was achieved during the first infection of Vero cells with at least one of three measles virus vaccine preparations tested. These subgenomic RNAs, which interfered very efficiently with the replication of the endogenous standard genomes upon vaccine infection but showed a moderate interfering activity with a standard virus stock derived by plaque purification from the vaccine preparation, resulted from the presence of defective interfering particles in the vaccine preparation. The relevance of this finding for the attenuation, stability, and potential capacity for persistent infection of such a vaccine is discussed.

The intracellular half-lives of nonreplicating nucleocapsids of DI particles of wild type and mutant strains of vesicular stomatitis virus

We have used defective interfering (DI) particles purified free of all infectious virus to determine the intracellular biological stability of nonreplicating nucleocapsids of vesicular stomatitis virus (VSV). Following infection of BHK-21 cells or tc-7 cells with a low multiplicity of pure DI particles, we superinfected them at varying times afterward with a high multiplicity of infectious helper virus to allow replication of those DI particle nucleocapsids retaining biological activity. Careful quantitation of DI particles in the yields from each time point showed that the biological half-life of intracellular VSV Indiana wild type DI particle nucleocapsids was 6 hr in BHK-21 cells and 5.3 hr in tc-7 cells (not significantly different). However, a DI particle from a temperature-sensitive mutant (tsG31) of VSV exhibited a biological nucleocapsid half-life of 12.5 hr and a DI particle isolated following 5 years of persistent infection had a half-life of 18 hr. These findings have significance for the stability of DI particle activity in vivo during acute infections where virus and DI particles are not always present together in the same cells due to cycling interactions. The increased half-life of DI nucleocapsids after years of persistent infection contrasts with the decreased stability and debilitation generally observed in infectious virus from persistent infection. Finally, transcapsidation studies showed that the intracellular half-life differences between DI particles are due mainly to their RNA genomes rather than to the N protein which encapsidates them.

Persistent infection of a temperature-sensitive G31 vesicular stomatitis virus mutant in neural and nonneural cells: biological and virological characteristics

Mouse L-929 cells (L cells), human oligodendroglioma cells, and rat glioma cells were persistently infected with vesicular stomatitis virus (VSV) mutant tsG31 and maintained for at least 4 years at 37 degrees C. The striking observation in this study was that there is a marked difference in neurovirulence among the persistent infections (PIs) derived from the three cell lines. tsG31 VSV derived from persistently infected L cells and oligodendroglioma cells remained highly virulent as assayed by intracerebral (i.c.) inoculation into 3-week-old Swiss mice. In contrast, tsG31 VSV isolated from glioma cells lost neurovirulence by passage 20. Persistently infected glioma cells were carried through more than 180 passages without reemergence of neurovirulent virus. Importantly, glioma PI virus neurovirulence was restored quickly by i.c. passage in mice and more slowly by passage through normal L cells. In contrast, the neurovirulence of L-cell PI virus was enhanced by i.c. passage in mice and slowly reduced by passage through normal glioma cells. Furthermore, no alteration in neurovirulence was observed in the case of oligodendroglioma PI virus. Although the mechanism(s) underlying the loss of virulence in glioma cells is unclear, our studies suggest that either strict temperature sensitivity or the presence of a heat-labile transcriptase or both play a major role in this phenomenon.

The biological relationship of mouse hepatitis virus (MHV) strains and interferon: in vitro induction and sensitivities

Five prototype strains of mouse hepatitis virus (MHV) -1, -3, -S, -A59 and -JHM were analyzed for their ability to induce interferon (IFN) in seven cell lines of rodent origin. Induction of IFN by all of the prototype MHV strains was infrequent and unpredictable, while IFN was produced consistently by five cell lines treated with known inducers. Priming and/or aging of cells did not enhance IFN induction by the MHV strains except in the case of MHV-A59 which consistently induced moderate levels of IFN on L-cells which were both primed and aged. Kinetic studies of MHV-A59-induced IFN on primed and aged L-cells demonstrated that detectable levels of IFN were not produced until 24 hours post-inoculation (p.i.). Peak levels were attained at 30 hours p.i. with no additional IFN produced through 48 hours p.i. MHV-induced IFN was similar in composition and properties to Newcastle disease virus-induced IFN. The sensitivities of the five MHV strains to eight concentrations of preformed L-cell IFN were also assessed. All strains except MHV-S fit a linear model with MHV-3, MHV-A59 and MHV-JHM having similar slopes. At most concentrations MHV-3 was less sensitive than MHV-1, -A59 or -JHM to IFN. The response curve for MHV-S was non-linear. This strain was more sensitive to the antiviral effects of the pre-formed IFN except at the highest concentrations of IFN used.