Genetic variation during persistent reovirus infection: presence of extragenically suppressed temperature-sensitive lesions in wild-type virus isolated from persistently infected L cells

Persistent development of adomavirus and aquareovirus in a novel cell line from marbled eel with petechial skin haemorrhage

In Taiwan, a petechial haemorrhage disease associated with mortality has affected marbled eels (Anguilla marmorata). The eels were revealed to be infected with adomavirus (MEAdoV, previously recognized as a polyoma-like virus). In this study, cell line DMEPF-5 was established from the pectoral fin of a diseased eel. DMEPF-5 was passaged >70 times and thoroughly proliferated in L-15 medium containing 2%-15% foetal bovine serum at 20-30°C. Transcripts of neural cell adhesion molecule 1 and nestin genes, and nucleic acids of MEAdoV and a novel reovirus (MERV) in the cells were demonstrated by reverse transcription-polymerase chain reaction analysis. Phylogenetic analysis revealed that the AdoV LO8 proteins mostly relate to adenovirus adenain, whereas MERV is close to American grass carp reovirus in Aquareovirus G, based on a partial VP2 nucleotide sequence. DMEPF-5 cells are susceptible to additional viral infection. Taken together, the marbled eels with the haemorrhagic disease have coinfection with MEAdoV and MERV, and the pathogenic role of MEAdoV and MERV warrants research. DMEPF-5 has gene expression associated with mesenchymal stem and progenitor cells and is the first cell line persistently infected with adomavirus and aquareovirus. DMEPF-5 can facilitate studies of such viruses and haemorrhagic disease.

Mutant cells selected during persistent reovirus infection do not express mature cathepsin L and do not support reovirus disassembly

Persistent reovirus infections of murine L929 cells select cellular mutations that inhibit viral disassembly within the endocytic pathway. Mutant cells support reovirus growth when infection is initiated with infectious subvirion particles (ISVPs), which are intermediates in reovirus disassembly formed following proteolysis of viral outer-capsid proteins. However, mutant cells do not support growth of virions, indicating that these cells have a defect in virion-to-ISVP processing. To better understand mechanisms by which viruses use the endocytic pathway to enter cells, we defined steps in reovirus replication blocked in mutant cells selected during persistent infection. Subcellular localization of reovirus after adsorption to parental and mutant cells was assessed using confocal microscopy and virions conjugated to a fluorescent probe. Parental and mutant cells did not differ in the capacity to internalize virions or distribute them to perinuclear compartments. Using pH-sensitive probes, the intravesicular pH was determined and found to be equivalent in parental and mutant cells. In both cell types, virions localized to acidified intracellular organelles. The capacity of parental and mutant cells to support proteolysis of reovirus virions was assessed by monitoring the appearance of disassembly intermediates following adsorption of radiolabeled viral particles. Within 2 h after adsorption to parental cells, proteolysis of viral outer-capsid proteins was observed, consistent with formation of ISVPs. However, in mutant cells, no proteolysis of viral proteins was detected up to 8 h postadsorption. Since treatment of cells with E64, an inhibitor of cysteine-containing proteases, blocks reovirus disassembly, we used immunoblot analysis to assess the expression of cathepsin L, a lysosomal cysteine protease. In contrast to parental cells, mutant cells did not express the mature, proteolytically active form of the enzyme. The defect in cathepsin L maturation was not associated with mutations in procathepsin L mRNA, was not complemented by procathepsin L overexpression, and did not affect the maturation of cathepsin B, another lysosomal cysteine protease. These findings indicate that persistent reovirus infections select cellular mutations that affect the maturation of cathepsin L and suggest that alterations in the expression of lysosomal proteases can modulate viral cytopathicity.

The genetic basis of viral virulence

The precise genetic and molecular determ inants of viral virulence are poorly understood. Genetic studies with influenza and reovirus have indicated that virulence is multigenic. The high frequency of m utation of RNA viruses can complicate genetic analyses of virulence, resulting in phenotypes that are difficult to interpret. The ease with which the reoviruses reassort genome segments has made it possible to isolate reassortants from parental viruses causing different patterns of anim al disease. It has thus been feasible to show that each of the three outer capsid proteins plays a m ajor role in the pathogenesis of anim al infection: the viral haem agglutinin determines the specificity of the im mune response and cell and tissue tropism ; the p ic protein plays a central role in determ ining yield at portals of entry as well as in differentiated tissues; the δ3 protein inhibits host m acrom olecular synthesis. Thus virulence is clearly multigenic, with each of the viral components playing distinct roles.

Efficiency of viral entry determines the capacity of murine erythroleukemia cells to support persistent infections by mammalian reoviruses

To determine mechanisms by which persistent viral infections are established and maintained, we initiated persistent infections of murine erythroleukemia (MEL) cells by using reovirus strains type 3 Abney and type 3 Dearing. Establishment of persistent reovirus infections of MEL cells was not associated with a significant cytopathic effect despite the presence of high titers of infectious virus in the cultures (>10(5) PFU/ml of culture lysate). Maintenance of persistently infected MEL-cell cultures was associated with coevolution of mutant viruses and cells. Mutant viruses produced greater yields than the parental wild-type (wt) strains in MEL cells cured of persistent infection and in cells treated with ammonium chloride, a weak base that blocks viral disassembly. Mutant cells supported growth of wt infectious subvirion particles, which are disassembly intermediates generated in vitro by treatment of virions with chymotrypsin, substantially better than growth of wt virions. These findings indicate that viral and cellular mutations selected during maintenance of persistently infected MEL-cell cultures affect acid-dependent proteolysis of virions during entry into cells. We also found that wt infectious subvirion particles produce greater yields than wt virions in wt MEL cells, which suggests that inefficient viral disassembly in MEL cells favors establishment of persistent infection. Therefore, steps in reovirus replication leading to viral disassembly appear to be critical determinants of the capacity of MEL cells to support both establishment and maintenance of persistent reovirus infections.

Assembly of the reovirus outer capsid requires mu 1/sigma 3 interactions which are prevented by misfolded sigma 3 protein in temperature-sensitive mutant tsG453

A temperature-sensitive reovirus mutant, tsG453, whose defect was mapped to major outer capsid protein sigma 3, makes core particles but fails to assemble the outer capsid around the core at non-permissive temperature. Previous studies that made use of electron cryo-microscopy and image reconstructions showed that mu 1, the other major outer capsid protein, but not sigma 3, interact extensively with the core capsid. Although wild-type sigma 3 and mu 1 interact with each other, immunocoprecipitation studies showed that mutant sigma 3 protein was incapable of interacting with mu 1 at the non-permissive temperature. In addition, restrictively-grown mutant sigma 3 protein could not be precipitated by some sigma 3-specific monoclonal antibodies. These observations suggest that in a wild-type infection, specific sigma 3 and mu 1 interactions result in changes in mu 1 conformation which are required to allow mu 1/sigma 3 complexes to condense onto the core capsid shell during outer capsid assembly, and that sigma 3 in non-permissive tsG453 infections is misfolded such that it cannot interact with mu 1.

Identification and characterization of a double-stranded RNA- reovirus temperature-sensitive mutant defective in minor core protein mu2

A newly identified temperature-sensitive mutant whose defect was mapped to the reovirus M1 gene (minor core protein mu2) was studied to better understand the functions of this virion protein. Sequence determination of the Ml gene of this mutant (tsH11.2) revealed a predicted methionine-to-threonine alteration at amino acid 399 and a change from proline to histidine at amino acid 414. The mutant made normal amounts of single-stranded RNA, both in in vitro transcriptase assays and in infected cells, and normal amounts of progeny viral protein at early times in a restrictive infection. However, tsH11.2 produced neither detectable progeny protein nor double-stranded RNA at late times in a restrictive infection. These studies indicate that mu2 plays a role in the conversion of reovirus mRNA to progeny double-stranded RNA.

Comparative sequence analysis of the reovirus S4 genes from 13 serotype 1 and serotype 3 field isolates

The reovirus sigma 3 protein is a major outer capsid protein that may function to regulate translation within infected cells. To facilitate the understanding of sigma 3 structure and functions and the evolution of mammalian reoviruses, we sequenced cDNA copies of the S4 genes from 10 serotype 3 and 3 serotype 1 reovirus field isolates and compared these sequences with sequences of prototypic strains of the three reovirus serotypes. We found that the sigma 3 proteins are highly conserved: the two longest conserved regions contain motifs proposed to function in binding zinc and double-stranded RNA. We used the 16 viral isolates to investigate the hypothesis that structural interactions between sigma 3 and the cell attachment protein, sigma 1, constrain their evolution and to identify a determinant within sigma 3 that is in close proximity to the sigma 1 hemagglutination site.

Identification of sequence elements containing signals for replication and encapsidation of the reovirus M1 genome segment

In reovirus the genetic signals that control genome replication and encapsidation are unknown. Serial passage of reovirus results in the accumulation of deletion mutants that contain fragments of genome segments. The smallest fragments found in deletion mutants will consist of the minimum essential sequences for genome replication and assembly. T1 x T3 reassortants containing the L2 segment from T3 and the M3 segment derived from T1 generate deletions in segment M1 on serial passage. Fragments of M1 segments were produced by serial passage, characterized by PAGE and Northern blotting before amplification by PCR, cloning, and sequencing. Thirteen of the smallest deletion fragments were sequenced. All of the smallest fragments contained sequences from both termini of segment M1. The smallest fragment was 344 nucleotides long. The consensus sequences consisted of 132-135 nucleotides from the 5' end of the plus strand and 183-185 nucleotides from the 3' end of the plus strand. It is concluded that these regions contain all the signals necessary for the replication and assembly of the M1 genome segment.

Differential effects of defective interfering Semliki Forest virus on cellular and virus polypeptide synthesis

Defective interfering Semliki Forest virus (DI SFV) inhibited virus RNA and virus polypeptide synthesis in cells coinfected with standard virus but did not delay or alter kinetics of RNA synthesis. Inhibition of polypeptide synthesis was 20-fold greater than that of RNA synthesis which presumably reflected the amplification resulting from cumulative translation of mRNAs. At high concentration, DI virus p12e inhibited the shutoff of host protein synthesis and allowed no synthesis of structural or nonstructural polypeptides. Dilution of DI virus restored the inhibition of host protein synthesis but further dilution was necessary before virus-specified polypeptide synthesis could be demonstrated. Another DI virus (p20a) with the same interference titre as p12e also inhibited shutoff of host protein synthesis but synthesis of virus-induced polypeptides was inhibited differentially: significant amounts of polypeptides comigrating with the structural precursor polypeptide p62 and the nonstructural polypeptide nsp63 were present and the synthesis of nsp90 was little affected at any concentration of DI virus p20a tested. None of the DI viruses tested induced the synthesis of any viral or novel polypeptide. It was concluded that DI SFV preparations have qualitatively different interfering activities in relation to their effects on virus and host cell polypeptide synthesis.

Extragenic suppression of temperature-sensitive phenotype in reovirus: mapping suppressor mutations

Independently isolated, spontaneous pseudorevertants of temperature-sensitive (ts) mutants of reovirus type 3 have previously been genetically characterized (R. F. Ramig and B. N. Fields, 1979, Virology 92, 155-167). Eighteen of these pseudorevertants were backcrossed to wild-type reovirus type 1 and reassortant progeny expressing the parental ts phenotype were selected. Analysis of segregation of genome segments in the reassortant, parental ts, progeny clones allowed the determination of the genome segment bearing the suppressor mutation of four pseudorevertants. The suppressor of tsA(201) phenotype mapped to segment S4 in the pseudorevertants RtsA(201)101 and RtsA(201)121 and to segment L3 in pseudorevertant RtsA(201)122. The suppressor of tsB(352) phenotype mapped to segment S1 in the pseudorevertant RtsB(352)b. In two other pseudorevertants the suppressor could not be mapped to a single genome segment due to the small number of progeny clones examined. These genetic results indirectly support the "compensating protein interactions" hypothesis for the mechanism of suppression.

Selection of a mutant S1 gene during reovirus persistent infection of L cells: role in maintenance of the persistent state

LR-7 cells, variant L cells derived from a type 3 reovirus persistently infected (p.i.) carrier culture (R. Ahmed, W. M. Canning, R. S. Kauffman, A. H. Sharpe, J. V. Hallum, and B. N. Fields, Cell 25, 325-332, 1983) were used to define the viral genes critical for maintenance of the persistent state. A cloned viral isolate (L/C virus) derived from the p.i. culture replicated normally in LR-7 cells, while wild-type (wt) viruses of the three reovirus serotypes replicated less efficiently. To identify the viral gene(s) permitting enhanced replication of L/C virus in LR-7 cells, viral reassortants were prepared by mixed infection of L cells with L/C virus and type 1 wt. Study of the one-step growth curves and final yields of large numbers of reassortants in both L cells and LR-7 cells revealed that the presence of the S1 gene from L/C virus was critical for normal viral replication in LR-7 cells. However, this phenotype was suppressed by the simultaneous presence in reassortants of both the M2 and S4 genes from the type 1 wt parent. The critical change in the S1 gene occurred by passage 13 (63 days) after initiation of the carrier culture. Although multiple mutations are present in the viral population from p.i. cultures, certain specific mutations can be identified as critical for maintenance of the persistent state.

Genetic variation during persistent reovirus infection: isolation of cold-sensitive and temperature-sensitive mutants from persistently infected L cells

We have examined the evolution of reovirus in two independently established persistently infected (p.i.) cell lines. We found that reovirus undergoes extensive mutation during persistent infection in L cells. However, there was no consistent pattern of virus evolution; in one p.i. cell line temperature-sensitive (ts) mutants were selected, whereas cold-sensitive (cs) mutants were isolated from the second p.i. culture. Neither the cs nor the ts mutants isolated from the carrier cultures expressed their defect at 37 degrees, the temperature at which the p.i. cells were maintained, indicating that the cs and ts phenotypes were nonselected markers. These results emphasize the point that emergence of the ts or cs mutants during persistent infection only signifies that the virus has changed; it does not necessarily imply that the particular mutant is essential for the maintenance of the persistent infection. Given the high mutation rate of viruses, and the wide spectrum of viral mutants present in carrier cultures, it is essential to distinguish the relevant changes from those which may simply represent an epiphenomenon. In the accompanying paper (R. S. Kauffman, R. Ahmed, and B. N. Fields Virology, 130, 79-87, 1983), we show that by using a genetic approach, it is possible to identify the viral gene(s) which are critical for the maintenance of persistent reovirus infection.

A genetic map of reovirus: assignment of the newly defined mutant groups H, I, and J to genome segments

Mutants representing three previously undefined reovirus type 3 mutant groups have been isolated following backcross of suppressed pseudorevertants to wild type (R.F. Ramig and B.N. Fields, 1979, Virology 92, 155-167; R. Ahmed, P.R. Chakraborty, A.F. Graham, R.F. Ramig, and B.N. Fields, 1980, J. Virol. 34, 383-389). The prototype mutant of each of the three new mutant groups was mapped by analysis of genome segment segregation in intertypic recombinants derived from crosses between the type 3 ts mutants and ts mutants of type 1 or type 2. Segregation analysis revealed the location of the group H prototype mutant tsH(26/8) to be genome segment M1, that of the group I prototype mutant tsI(138) to be segment L3, and that of the group J prototype mutant tsJ(128) to be segment S1. Mapping of the group I and J lesions required the identification of suppressed ts lesions in some of the intertypic rcombinant clones.

Novel phenotype of RNA synthesis expressed by vesicular stomatitis virus isolated from persistent infection

Vesicular stomatitis virus (VSV) stocks isolated from two persistently infected mouse L-cell lines (designated VSV-PI stocks) express an altered phenotype of RNA synthesis. This phenotype is different from the RNA synthesis phenotype expressed by the viruses used to initiate the persistently infected lines, wild-type VSV and VSV ts-0-23 (a group III, ts-, RNA+ mutant). At 34 and 37 degrees C in L cells productively infected with VSV-PI stocks derived from the two cell lines, transcription of virus mRNA was significantly reduced, whereas replication of the 40S genomic RNA species was enhanced compared with wild-type VSV or ts-0-23. At 34 and 37 degrees C, both VSV-PI stocks replicated with equal or greater efficiency than wild-type VSV; 37 degrees C was the temperature at which the persistently infected cultures were maintained. At 40 degrees C, both VSV-PI stocks were temperature sensitive, and clonal VSV-PI isolates from both cell lines belong to complementation group I (RNA-). Standard ts- mutants (derived by mutagenesis of wild-type VSV) belonging to RNA- complementation groups I, II, and IV do not express the VSV-PI RNA synthesis phenotype at the permissive temperature, making this phenotype distinctive to persistent infection. Since the two VSV-PI populations from persistently infected cell lines initiated with different viruses both evolved this unique phenotype of RNA synthesis, the expression of this phenotype may play an important role in the maintenance of persistence.

Evidence for supercoiled hepatitis B virus DNA in chimpanzee liver and serum Dane particles: possible implications in persistent HBV infection

In chimpanzee hepatitis B virus (HBV) carriers, the mechanism of viral persistence has been examined by analyzing viral DNA molecules in liver and serum. Chimpanzee liver DNA contained two extrachromosomal HBV DNA molecules migrating on hybridization blots at 4.0 kb and 2.3 kb. There was no evidence for integration of HBV DNA into the host genome. The extrachromosomal molecules were distinct from Dane particle DNA and were converted to linear 3.25 kb full-length double-stranded HBV DNA on digestion with Eco RI. Nucleases S1 and Bal 31 converted "2.3 kb" HBV DNA to 3.25 kb via an intermediate of "4.0 kb" apparent length. The HBV DNA molecule that migrated at 2.3 kb represents a supercoiled form I of the HBV genome, and the molecule that migrated at 4.0 kb represents a full-length "nicked," relaxed circular form II. Evidence for supercoiled HBV DNA in serum Dane particles was obtained by production of form II molecules upon digestion with nuclease S1 or Bal 31. It is proposed that most Dane particles represent interfering noninfectious virus containing partially double-stranded DNA circles and that particles containing supercoiled HBV DNA may represent infectious hepatitis B virus.

Characterization of viral genomes in the liver and serum of chimpanzee long-term hepatitis B virus carriers: a possible role for supercoiled HBV-DNA in persistent HBV infection

In chimpanzee hepatitis B virus (HBV) carriers, the molecular mechanism for viral persistence has been examined by analyzing the properties of viral DNA molecules in liver and serum. Two extrachromosomal HBV-DNA molecules migrating on Southern blots at 4.0 kb and 2.3 kb were observed in chimpanzee liver DNA. There was no evidence for integration of HBV sequences into the host genome. The HBV-DNA molecule which migrated at 4.0 kb position represents a full-length "nicked," relaxed circular form, and the DNA molecules migrating at 2.3 kb position represents a supercoiled form of the HBV genome. Evidence for supercoiled HBV-DNA in serum was obtained by production of the relaxed circular intermediate upon digestion of Dane particle DNA with specific nucleases S1 and Bal 31. A possible role of these two extrachromosomal HBV-DNA molecules in the biology of hepatitis B virus infection and the mechanism for viral persistence are discussed.

Role of the host cell in persistent viral infection: coevolution of L cells and reovoirus during persistent infection

Mutant L cells, designated LR cells, were isolated after "curing" a persistently infected cell line (L/C) with antireovirus serum. The LR cells were shown to be virus-free; no reovirus was detectable by infectious center assays, plaque assays, presence of viral proteins, presence of viral dsRNA and immunofluorescence studies. Persistent infections were readily established n LR cells following infection with either cloned, low passage wild-type reovirus or cloned, low passage reovirus isolated from carrier cultures. Reovirus isolated from carrier cultures, however, grew much better than wild-type reovirus in LR cells and showed complete dominance over wild-type reovirus in coinfection experiments. Infection of LR cells with wild-type reovirus resulted in a low-level persistent infection with inefficient viral replication; these mutant L cells were partially resistant to infection with wild-type reovirus. In contrast, infection of the mutant L cells with virus isolated from the persistently infected cells resulted in a persistent infection accompanied with efficient viral replication. Infection of the original L cells with either wild-type reovirus or reovirus isolated from the persistently infected cells resulted in a lytic infection with no surviving cells. Thus the host cell plays a crucial role in the maintenance of persistent reovirus infection. Our results show that there is a coevolution of both mutant L cells and mutant reovirus during persistent infection.