Temperature-sensitive phenomenon of viral maturation observed in BHK cells persistently infected with HVJ

Nairovirus polymerase mutations associated with the establishment of persistent infection in human cells

Crimean-Congo hemorrhagic fever virus (CCHFV), a tick-borne virus of the Orthonairovirus genus, persistently infects tick cells. It has been reported to establish persistent infection in non-human primates, but virological analysis has not yet been performed in human cells. Here, we investigated whether and how nairoviruses persistently infect human cells using Hazara orthonairovirus (HAZV), a surrogate model for CCHFV. We established a human cell line that was persistently infected with HAZV. Surprisingly, virions of persistently infected HAZV (HAZVpi) were not observed in the culture supernatants. There were five mutations (mut1, mut2, mut3, mut4, and mut5) in L protein of HAZVpi. Mutations in L protein of HAZVpi contribute to non-detection of virion in the supernatants. Lmut4 was found to cause low viral growth rate, despite its high polymerase activity. The low growth rate was restored by Lmut2, Lmut3, and Lmut5. The polymerase activity of Lmut1 was extremely low, and recombinant HAZV carrying Lmut1 (rHAZV/Lmut1) was not released into the supernatants. However, genomes of rHAZV/Lmut1 were retained in the infected cells. All mutations (Lmut1-5) found in L protein of HAZVpi were required for experimental reproduction of HAZVpi, and only Lmut1 and Lmut4 were insufficient. We demonstrated that point mutations in viral polymerase contribute to the establishment of persistent HAZV infection. Furthermore, innate immunity was found to be suppressed in HAZVpi-infected cells, which also potentially contributes to viral persistence. This is the first presentation of a possible mechanism behind how nairoviruses establish persistent infection in human cells.

IMPORTANCE

We investigated whether and how nairoviruses persistently infect human cells, using Hazara orthonairovirus (HAZV), a surrogate model for Crimean-Congo hemorrhagic fever virus. We established a human cell line that was persistently infected with HAZV. Five mutations were found in L protein of persistently infected HAZV (HAZVpi): mut1, mut2, mut3, mut4, and mut5. Among them, Lmut1 and Lmut4 restricted viral growth by low polymerase activity and low growth rate, respectively, leading to inhibition of viral overgrowth. The restriction of viral growth caused by Lmut1 and Lmut4 was compensated by other mutations, including Lmut2, Lmut3, and Lmut5. Each of the mutations found in L protein of HAZVpi was concluded to cooperatively modulate viral growth, which facilitates the establishment of persistent infection. Suppression of innate immunity also potentially contributes to virus persistence. This is the first presentation of a possible mechanism behind how nairoviruses establish persistent infection in human cells.

Cationized gelatin-HVJ envelope with sodium borocaptate improved the BNCT efficacy for liver tumors in vivo

BACKGROUND

Boron neutron capture therapy (BNCT) is a cell-selective radiation therapy that uses the alpha particles and lithium nuclei produced by the boron neutron capture reaction. BNCT is a relatively safe tool for treating multiple or diffuse malignant tumors with little injury to normal tissue. The success or failure of BNCT depends upon the 10B compound accumulation within tumor cells and the proximity of the tumor cells to the body surface. To extend the therapeutic use of BNCT from surface tumors to visceral tumors will require 10B compounds that accumulate strongly in tumor cells without significant accumulation in normal cells, and an appropriate delivery method for deeper tissues.Hemagglutinating Virus of Japan Envelope (HVJ-E) is used as a vehicle for gene delivery because of its high ability to fuse with cells. However, its strong hemagglutination activity makes HVJ-E unsuitable for systemic administration.In this study, we developed a novel vector for 10B (sodium borocaptate: BSH) delivery using HVJ-E and cationized gelatin for treating multiple liver tumors with BNCT without severe adverse events.

METHODS

We developed cationized gelatin conjugate HVJ-E combined with BSH (CG-HVJ-E-BSH), and evaluated its characteristics (toxicity, affinity for tumor cells, accumulation and retention in tumor cells, boron-carrying capacity to multiple liver tumors in vivo, and bio-distribution) and effectiveness in BNCT therapy in a murine model of multiple liver tumors.

RESULTS

CG-HVJ-E reduced hemagglutination activity by half and was significantly less toxic in mice than HVJ-E. Higher 10B concentrations in murine osteosarcoma cells (LM8G5) were achieved with CG-HVJ-E-BSH than with BSH. When administered into mice bearing multiple LM8G5 liver tumors, the tumor/normal liver ratios of CG-HVJ-E-BSH were significantly higher than those of BSH for the first 48 hours (p < 0.05). In suppressing the spread of tumor cells in mice, BNCT treatment was as effective with CG-HVJ-E-BSH as with BSH containing a 35-fold higher 10B dose. Furthermore, CG-HVJ-E-BSH significantly increased the survival time of tumor-bearing mice compared to BSH at a comparable dosage of 10B.

CONCLUSION

CG-HVJ-E-BSH is a promising strategy for the BNCT treatment of visceral tumors without severe adverse events to surrounding normal tissues.

A quantitative method for analyzing establishing-efficiency of persistent viral infection

A quantitative method for analyzing establishing-efficiency of persistent infection was devised. The efficiency of hPIV2 CA and SV5 T1 strains was found to be high, that is, 0.1 approximately 0.3 (an efficiency of 1.0 indicates that 100% of the virus-infected cells became persistently infected). The efficiency of the SV5 WR strain was also high, approximately 0.1, though the virus had no ability to immediately establish a steady state of persistent infection in whole cell-culture systems. At about 0.0007, the efficiency of SV41 was almost the same as that of the hPIV2 Toshiba strain. The establishing efficiencies of various rSeV were further analyzed in detail. The efficiencies of the rSeV(PA), rSeV(Ppi) and rSeV(HNpi) were below the limit of detection, while that of rSeV(Lpi) was nearly 1. Although the efficiency was around 0.001, the rSeV(Mpi) and the rSeV(Fpi) were unexpectedly found to be capable of forming persistently-infected cells, indicating that both the Fpi and Mpi proteins contribute to the establishing efficiency of persistent infection of SeVpi.

The properties of recombinant Sendai virus having the P gene of Sendai virus pi strain derived from BHK cells persistently infected with Sendai virus

We prepared the chimeric recombinant Sendai virus [rSeV(Ppi)] by replacing the P gene of the Z strain with that of pi strain for analyzing the function of Ppi, Vpi and Cpi proteins. Intriguingly, HA production by rSeV(Ppi) is significantly lower at 38 degrees C than at 32 degrees C, showing that virus growth of rSeV(Ppi) is slightly suppressed at 38 degrees C. However, the main phenotypes of SeVpi, a marked temperature sensitivity as viral replication and an ability of establishing persistent infection, are not explained by the Ppi, Vpi and Cpi proteins. The V and C proteins form inclusion bodies in L929 cells infected with rSeV(Ppi) and incubated at 38 degrees C. L929 cells infected with rSeV(Ppi) and L929 cells stably expressing the Cpi protein show resistance to interferon-beta at 32 and 38 degrees C, indicating that the Cpi protein per se is not temperature-sensitive to inhibition of IFN signaling. The complete genome sequences of Sendai virus (SeV) pi and parent Nagoya strains were determined. Fifty nucleic acid substitutions are found in the genome sequence of SeV pi strain in comparison with Nagoya strain. There are three nucleic acid substitutions in the leader sequence, while the trailer, intergenic, gene-end and gene-start sequences of both strains are completely identical. Deletions and insertions of nucleotide are not found. There are 32 amino acid substitutions in Sendai virus pi strain. The specific amino acid substitutions unique to the SeVpi are 18. Information about the complete genome sequences of SeVpi strain is important to totally understand the persistent infection and lower pathogenicity of SeV.

Recombinant Sendai viruses with L1618V mutation in their L polymerase protein establish persistent infection, but not temperature sensitivity

The Sendai virus pi strain (SeVpi) isolated from cells persistently infected with SeV shows mainly two phenotypes: (1) temperature sensitivity and (2) an ability of establishing persistent infection (steady state). Three amino acid substitutions are found in the Lpi protein and are located at aa 1088, 1618, and 1664. Recombinant SeV(Lpi) (rSeV(Lpi)) having all these substitutions is temperature sensitive and is capable of establishing persistent infection (steady state). rSeVs carrying the fragment containing L1618V show both phenotypes. rSeV(L1618V), in which leucine at aa 1618 is replaced with valine, has the ability of establishing persistent infection, but is not a temperature-sensitive mutant, indicating that the ability of a virus to establish persistent infection can be separated from temperature sensitivity. The amino acid change at 1618(L-->V) coexisting with aa 1169 threonine is required for acquirement of a temperature-sensitive phenotype. Three amino acid substitutions are also found in the Ppi protein, but rSeV(Ppi) does not show these phenotypes.

Early stage of establishment of persistent Sendai virus infection: unstable dynamic phase and then selection of viruses which are tightly cell associated, temperature sensitive, and capable of establishing persistent infection

We obtained 157 cloned cell lines persistently infected with Sendai virus; these cell lines were generated independently of each other. Infectious viruses could be isolated from 123 of these cloned cell lines by inoculation of culture fluids or infected cells into embryonated eggs. The majority of the viruses carried by cells persistently infected with viruses showed high cytotoxicity and did not have the ability to establish persistent infection. The association of carried virus with cells became stronger and virus isolation correspondingly became more difficult as cells persistently infected with virus were subcultured. Viruses derived from virus-infected cells eventually acquired the ability to establish persistent infection, although the ways in which the viruses acquired this ability varied. The viruses also acquired temperature sensitivity as persistently infected cells were subcultured. First, the hemagglutinin-neuraminidase and M proteins acquired temperature sensitivity, and then the polymerase(s) did so. The M proteins were localized in the nuclei of cells infected with cloned viruses that had the ability to establish persistent infection. Cells infected with viruses capable of establishing persistent infection showed no or slight staining by terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling. Specific amino acid substitutions accumulated in the M protein and the L protein as virus-infected cells were subcultured. This study shows that there is an unstable dynamic phase at an early stage of the establishment of persistent Sendai virus infection (steady state), and then viruses capable of establishing persistent infection are selected.

Characterization of Sendai virus persistently infected L929 cells and Sendai virus pi strain: recombinant Sendai viruses having Mpi protein shows lower cytotoxicity and are incapable of establishing persistent infection

It is commonly accepted that the temperature-sensitive phenotype of Sendai virus (SeV) persistently infected cells is caused by the M and/or HN proteins. Expression level of the L, M, HN, and V proteins is extremely low in L929 cells persistently infected with SeVpi (L929/SeVpi cells) incubated at 38 degrees C. The HN protein quickly disappears in L929/SeVpi cells following a temperature shift up to 38 degrees C, and pulse-chase experiments show that the Lpi, HNpi, and Mpi proteins are unstable at 38 degrees C. Following a temperature shift either upward or downward, M protein is translocated into the nucleus and then localizes to the perinuclear region. None of virus-specific polypeptides are detected in the cells primarily infected with SeVpi and incubated at 38 degrees C and virus proteins are not pulse-labeled at 38 degrees C, indicating that temperature-sensitive step is at an early stage of infection. The Mpi protein is transiently located in the nucleus of the SeVpi primarily infected cells. Recombinant SeVs possessing the HNpi or/and Mpi proteins are not temperature-sensitive. The HN protein is expressed at very low levels and the F protein localizes to the perinuclear region in rSeV(Mpi)-infected cells incubated at 38 degrees C for 18 h. rSeVs having the Mpi protein exhibit lower cytotoxicity and are incapable of establishing persistent infection. Amino acid 116 of the Mpi protein is related to the nuclear translocation and lower cytopathogenesis, whereas aa183 is involved in the interaction between M protein and viral glycoproteins.

Induction of interferon by virus glycoprotein(s) in lymphoid cells through interaction with the cellular receptors via lectin-like action: an alternative interferon induction mechanism

When animals and cells are infected with a virus, interferon is produced. Viral-nucleic acid is considered to be one of actual components for interferon induction. In addition, viral glycoproteins trigger interferon induction in lymphoid cells by membrane-membrane interaction via a lectin-like activity. A biological significance of lectin-like activity of viral glycoproteins is discussed.

Reversible susceptibility to natural cell-mediated cytotoxicity observed in altered BHK cells resistant to HVJ (Sendai virus) infection

Altered baby hamster kidney (BHK-R) cells which were subcultured in the continuous presence of HVJ (hemagglutinating virus of Japan--the Sendai strain of parainfluenza 1 virus) showed a high susceptibility to natural cell-mediated cytotoxicity, although BHK-R cells are not transiently or persistently infected with HVJ but contain the restricted amount of sialic acid. By repeated subcultivation of BHK-R cells in growth medium free of HVJ, the sensitivity to natural killer cytotoxicity decreased to the level of normal BHK cells with a counter increase of cellular sialic acid, and the subsequent treatment of the cells with neuraminidase caused a loss of proper sialic acid residues, once again resulting in a significant enhancement of lysis by natural killer cells. In the BHK-R cell system which exhibits a reversible resistance to the interferon action, the enhancing effect induced by interferon on target cell susceptibility to natural killer activity became more pronounced in accord with the recovery of sensitivity to the antiviral action of interferon.

Persistence of mumps virus in mouse L929 cells

The characteristics of a persistent infection of L929 cells with mumps virus (MuV) is presented. The persistent infection (L-MuV cells) was regulated by interferon (IFN) produced endogenously and almost all the properties showed that the carrier culture was maintained by horizontal transmission of the virus. Small-plaque mutants, but not temperature-sensitive variants, were selected during the persistent infection. MuV released from L-MuV cells (MuV-pi) replicated efficiently in L929 cells, while infection of L929 cells with the original MuV-o resulted in an abortive infection. The efficient replication of MuV-pi in L929 cells can be explained by the findings that MuV-pi induced IFN more slowly and had lower susceptibility to IFN in L929 cells than MuV-o did. M protein was synthesized to a considerable degree in MuV-pi-infected cells, while it could not be detected in MuV-o-infected cells. By contrast, MuV-pi formed small plaques in Vero cell monolayers and the yield of MuV-pi in Vero cells was lower than that of MuV-o. M protein induced by MuV-pi decayed easily in Vero cells. M protein was considered to be a limiting factor for MuV replication in both cell lines.

Reversible resistance to the antiviral action of interferon observed in altered BHK cells resistant to HVJ (Sendai virus) infection. Brief report

Altered baby hamster kidney (BHK-R) cells which were subcultured in the continuous presence of HVJ (hemagglutinating virus of Japan--the Sendai strain of parainfluenza 1 virus) showed a resistance to the antiviral action of both type I and II interferons. No evidence for a direct inactivation of interferon molecules during incubation of BHK-R cells was obtained. After serial subculture of BHK-R cells in growth medium free of HVJ, surface membranes with the proper sialic acid residues were restored and the cells became susceptible to the interferon action. It is suggested that binding sites for interferons might be ranked above HVJ receptors in the "receptor gradient".

Search for Sendai 6/94 viral RNA in the antigen-free cell line Cl-C-2 isolated from human multiple sclerosis brain tissue

The viral antigen-free cell line Cl-C-2, obtained from multiple sclerosis brain tissue by cell fusion with CV-1 cells, was examined for the presence of intracellular virus-specific RNA sequences of the persistent Sendai 6/94 virus by nucleic acid hybridization. As a specific probe for this assay, an in vitro synthesized cDNA was used. Oligodeoxyguanidylic acid served as a primer for the initiation of cDNA synthesis. The 6/94 RNA was detectable as expected in the viral antigen-expressing cell lines Cl-E-8 and Cl-F-2, which were used as a reference of the same source. In the viral antigen-free cell line Cl-C-2, however, no viral RNA sequences have been found by hybridization experiments. Corresponding superinfection studies confirmed the conclusion that in cell line Cl-C-2 no viral components are present. The lack of expression of viral proteins and of protection against superinfection seems to be correlated with the lack of viral RNA in Cl-C-2 cells, which may eliminate the persistent virus by a cellular defense mechanism.

Subcellular location of the major protein antigens of paramyxoviruses revealed by immunoperoxidase cytochemistry

The major structural proteins of Newcastle disease virus and Sendai virus were localized in infected BHK-21 and MDBK cells by ultrastructural immunoperoxidase cytochemistry using antibodies against the individual viral protein antigens. The intracellular glycoproteins were strictly membrane bound, being localized in the rough endoplasmic reticulum (RER), perinuclear spaces, smooth membrane vesicles, and presumed Golgi apparatus. The nucleocapsid proteins were detected exclusively in membrane free cytosol and accumulated there, forming inclusions. The membrane (M) protein was found both in cytosol and on RER. The viral proteins on RER exhibited a distinct site specificity; the glycoproteins were facing the lumen of RER whereas M protein was present at the outer cytoplasmic surface. All the viral proteins were detectable at the plasma membrane where virus assembly takes place. However, their modes of distribution differed remarkably. The glycoproteins were spread widely over the entire cell surface including the areas of virus budding and those of normal morphology, whereas M protein was localized in restricted areas of the membrane, frequently forming a patch of virus specific membrane. The presence of nucleocapsids was confined to the virus particles budding from the plasma membrane. These results complement and extend the earlier morphological and biochemical data on the assembly or morphogenesis of paramyxoviruses.

Persistent baculovirus infections: Spodoptera frugiperda NPV and Autographa californica NPV in Spodoptera frugiperda cells

Establishment of a persistent infection of Spodoptera frugiperda nuclear polyhedrosis virus (NPV) in Spodoptera frugiperda (S.f.) cells occurred in three phases: the first phase was characterised by high levels of cell infection and death, the second phase by decreasing cell infection levels leading to the final phase where less than one per cent of the cells were infected during any subculture. The virus persisted at this level of infection provided the cells were maintained by regular subculturing and incubated at the optimum growth temperature of 27 degrees C. Because of the low proportion of cells infected, cultures of virus-free cells could be selected ('cured') by dilution of the persistent infection without the use of viral antiserum. Unlike the parent S.f. cells, cultures of cured cells were partially resistant to infection with S. frugiperda NPV or infection with an unrelated baculovirus Autographa californica NPV. A. californica NPV, which is cytolytic for the parent S.f. cell line, established a persistent infection in the cured cells. The establishment pattern was similar to that previously found for S. frugiperda NPV and only one to five per cent of the cells were infected at equilibrium. Cured cells from the A. californica NPV persistent infection were highly resistant to infection with both S. frugiperda NPV and A. californica NPV. All attempts to find a viral interference phenomenon to explain the resistance of the cured cells were unsuccessful. All cell types adsorbed virus equally well. Slower growth of S.f. cells cured from the persistent A. californica NPV infection is the only difference so far observed between any of the S.f. cell types.

Persistent viral infections as models for research in virus chemotherapy

The acute systemic virus infection is commonly used as an experimental model in chemotherapy research despite the fact that the chance for an effective chemotherapy of acute virus infections is small. In most acute infections, virus multiplication is well advanced before the disease is expressed and treatment will, in many cases, come too late. However, control by chemotherapy might be promising for persistent virus infections, where, owing to the slow progression of the disease, sufficient time for treatment is available. Although there are various ways in which viruses can persist in their hosts, comparative studies in vitro and in vivo reveal common features that shall be briefly reviewed. Animal models with persistent virus infections are usually difficult to experiment with because of the varying length of the incubation period brought about by the complex relationship among virus replication, immune reactions, and disease.

Growth of poliovirus in HeLa cells persistently infected with HVJ (Sendai virus)

The growth of poliovirus in a HeLa cell culture persistently infected with the hemagglutinating virus of Japan (HVJ, the Sendai strain of parainfluenza 1 virus) (HeLaHVJ) was studied. Plaques produced by poliovirus on HeLaHVJ cell monolayers were hazier, smaller and fewer than those on HeLa cells. HeLaHVJ cells were indistinguishable from normal HeLa cells with respect to adsorption rate and penetration efficiency of poliovirus. Extracellular yields of poliovirus in HeLaHVJ cells were lower, and the cytopathic changes were less than those in normal HeLa cells, while cell-associated virus growth in HeLaHVJ cells was nearly equal to that in HeLa cells. HeLaHVJ cells responded more effectively to the action of magnesium chloride, which facilitates virus release from infected cells, resulting in an cytopathic effects. No reduction in poliovirus yield could be detected in HeLa cells acutely infected with HVJ. The relationship between the inhibition of the release of poliovirus from HeLaHVJ cells and the persistent infection of the cells with HVJ is discussed.

Protection of mice against virulent virus infection by a temperature-sensitive mutant derived from an HVJ (Sendai virus) carrier culture

Experimental infection with HVJ (haemagglutinating virus of Japan-the Sendai strain of parainfluenza 1 virus) in mice was studied. Aerosol infection of newborn mice with the wild-type virus (HVJ-W) retarded the development of body weight and killed the animals within a few weeks. Large amounts of virus were isolated from both the lungs and the nasal turbinates of infected mice. In contrast, newborn mice exposed by inhalation to a temperature-sensitive (ts) mutant (HVJ-pB) derived from an HVJ carrier culture showed no clinical signs and grew equally well as mock-infected animals. No infectious virus could be recovered from the lungs although the ts mutant grew to moderate titre in the nasal turbinates. The prior inoculation of newborn mice with the ts mutant virus induced a state of significant resistance to subsequent challenge with the virulent wild-type virus. No replication of challenge virus in both lungs and nasal turbinates could be detected and the animals were protected a lethal infection. It is suggested that an avirulent temperature-sensitive mutant which has lost the capacity to replicate in the lower respiratory tract but is still capable of multiplying in the nasal turbinates may be a promising candidate for use in live vaccines especially against the infectious disease of the lower respiratory tract.

Characterization of the polypeptides synthesized in cells infected with a temperature-sensitive mutant derived from an HVJ (Sendai virus) carrier culture

The intracellular synthesis of virus-specific polypeptides in cells infected with the wild-type virus of HVJ (HVJ-W) (haemagglutinating virus of Japan--the Sendai strain of parainfluenza 1 virus) and with a temperature-sensitive (ts) mutant (HVJ-pB) derived from an HVJ carrier culture has been analysed by polyacrylamide gel electrophoresis. At the permissive temperature (32 degrees C), all of the known virus structural polypeptides were identified in cells infected with each strain of virus and in addition to the non-structural polypeptides B and C, another polypeptide at the region with a molecular weight of 26,000 to 27,000 (26 to 27K) could be detected in infected cells. At the non-permissive temperature (38 degrees C), the synthesis of the polypeptide M was markedly restrained in cells infected with HVJ-pB, while other major virus polypeptides were present in approximately comparable amounts to cells infected with the wild-type virus. A non-structural polypeptide with a molecular weight of 105K was dominant in ts mutant infected cells at higher temperatures and disappeared after temperature-shift from 38 degrees to 32 degrees C. The production of the non-structural polypeptides B and 27K was also temperature-sensitive. The molecular weights of the polypeptides B, M and 27K in HVJ-pB infected cells were larger than those of the corresponding polypeptides in HVJ-W infected cells. The synthesis of the M protein in HVJ-prinfected cells started just after lowering the incubation temperature and the newly made M protein was successfully incorporated into virus particles.

Recovery of a Sendai virus variant with temperature-sensitive hemolytic activity from persistently infected cells from mouse brain

A persistently infected cell line designated MB/Senas was established by cultivation of mouse brain cells from four-day-old C3H mice infected intracerebrally at birth with 10(6) PFU of Sendai virus, strain 52. After 5 passages, 0.16 per cent of Sendai52 antiserum (containing two 50 per cent plaque reducing doses/ml of serum) was introduced into the culture medium. The addition of antiserum was accompanied by a rise in cell-associated viral antigen from a level of 5 per cent antigen positive cells to 100 per cent demonstrable by both intracellular and membrane immunofluorescence. A variant of Sendai52 virus, designated Sendaias, was recovered from MB/Senas by inoculation of supernatant medium into chick embryos. Infection of chick embryos at 37 degrees C was abortive. Fifty per cent or less of chick embryos infected at dilutions 10(-1) to 10(-9) yielded detectable virus. Hemagglutination (HA) was weak but could be improved by trypsinization of allantoic fluids. Neuraminidase (NA) activity was barely detectable. Hemolysis (HE) was absent. Propagation of Sendaias virus at 33 degrees C showed no change from weak HA and NA activities but HE activity was now apparent which was temperature sensitive. Mortality of infected chick embryos increased to 100 per cent. HE activity and lethality for chick embryos was thermolabile at 45 degrees C.

Analysis of VSV mutant with attenuated cytopathogenicity: mutation in viral function, P, for inhibition of protein synthesis

T1026, a ts mutant of VSV which is much less cytopathogenic than its parent, HR, and which can establish persistent infection under certain conditions, is a double mutant. In addition to its ts mutation in the virion RNA polymerase, T1026 has a second non-ts mutation in a viral function termed "P". This function is responsible for the inhibition of total protein synthesis in infected cells and acts chiefly at the level of translational initiation. In some cell systems, the inhibition of protein synthesis produced by P appears to be selective for cellular protein synthesis, whereas in other cell systems, both cellular and viral protein synthesis are inhibited. T1026 and its ts revertants are phenotypically P- -that is, cells infected with them show total protein synthesis rates equal to or greater than uninfected cells, while synthesizing viral proteins at the same or even greater rates than HR-infected cells. The P- mutation is correlated with failure to increase plaque size after 2-3 days of incubation. Since viral mutants obtained from persistently infected cultures in a variety of systems appear to be double mutants with a ts mutation in the virion RNA polymerase and a small plaque marker, we suggest that T1026 could represent a model for such mutants.

Surface antigens on HeLa cells persistently infected with HVJ (Sendai virus)

Surface antigens of HeLaHVJ cells, a cell line persistently infected with HVJ, were studied by fluorescent antibody staining. After absorption with concentrated HVJ virions and HeLa cells, anti-HeLaHVJ antiserum was able to demonstrate specific surface fluorescence on HeLaHVJ cells, while this serum no longer reacted with original HeLa cells nor with HVJ virions. During cytolytic infection of HeLa cells with HVJ, this specific surface antigen appeared at an early stage of infection prior to the appearance of newly synthesized HVJ viral antigens and moreover appeared in spite of the inhibition of viral protein synthesis. This antigen was detected neither on HeLa cells infected with other myxoviruses except HVJ nor on various other kinds of cells infected with HVJ. The specific surface antigen was still found on the HeLaHVJ cell surface after incubation at 38 degrees C for two days, while HVJ structural antigens on the cell surface no longer could be detected. Mild short-term treatment of HeLa cells with trypsin, neuraminidase from vibrio cholerae, phospholipase-C and hyaluronidase failed to expose specific antigen. The antigen was distinguishable from the Forssman and human blood type antigens. The mechanism of appearance of a new antigen on the surface of HeLaHVJ cells remains unclear.

Persistent infection of tissue culture cells by RNA viruses

In this paper, the characteristics of cultured cells persistently infected with RNA viruses, other than leuko viruses are described. The roles that the host cell, interferon, virus mutants and defective interfering particles may play in the establishment and maintenance of persistent infection are discussed. It is proposed that the interaction of viruses with certain types of host cells can lead to persistent infection. The differences in virus-host interactions may be attributable to differences in membrane properties of various cells. Defective interfering particles may play a role in the establishment of persistent infections in cells which normally undergo lytic virus development. Mutant types of virus appear to be prominent in the virus released from persistently infected cells, but the role that various mutants play in the maintenance of persistent infections remains unclear.

Accidental persistent infection of cell lines by Newcastle disease virus, showing three unusual features--defective neuraminidase, temperature sensitivity and intranuclear inclusions

A persistent, defective infection by an unknown strain of Newcastle disease virus (NDV) appeared accidentally in established lines of pig, ox and sheep kidney cells. Virus particles released from the persistently infected cells were not infectious and were deficient in neuraminidase activity. Synthesis of some of the virus-specified proteins in the persistently infected cells was temperature-sensitive. Co-cultivation of mixed populations of carrier cells and healthy chick embryo cells induced cell fusion with the formation of multinucleate heterokaryons and intra-nuclear inclusions. The development of inclusions in the chicken nuclei was not accompanied by 'rescue' of infectious NDV.

Analysis of a viral agent isolated from multiple sclerosis brain tissue: characterization as a parainfluenzavirus type 1

A virus originally isolated from cell cultures obtained by lysolecithin-induced fusion of human multiple sclerosis brain cells with CV-1 cells has been analyzed for its antigenic, RNA, and polypeptide compositions, and for selective biological properties. Our findings establish that this isolate, designated 6/94 virus, contains a 50S RNA genome and is, as yet, indistinguishable from Sendai virus in its antigenic and total polypeptide compositions. Despite these similarities, the 6/94 and Sendai viruses differ in certain phenotypic properties. 6/94 virus is markedly less cytocidal for chick fibroblasts, especially at 37 C and, after beta-propiolactone inactivation, it possesses a greater capacity for cell fusion and a lower toxicity than does comparably treated Sendai virus. In addition, 6/94 virus shows greater hemolytic activity.

Effects of L-glutamine deprivation on growth of HVJ (Sendai virus) in BHK cells

l-Glutamine requirement for viral maturation was found in BHK-HVJ cells, a cell line of baby hamster kidney cells persistently infected with HVJ (Sendai virus). Synthesis of envelope protein in BHK-HVJ cells was markedly suppressed by deprivation of l-glutamine, whereas development of nucleocapsid (S) antigen was less affected. More detailed examination of this phenomenon was carried out by using a cytolytic system. Growth of HVJ in BHK cells cultured in media deprived of various amino acids was investigated, and omission of l-glutamine from culture medium resulted in a marked inhibitory effect on the release of infectious virus and synthesis of envelope protein, although synthesis of virus-specific RNA and nucleocapsid antigen in the cells was readily detected. When l-glutamine was restored to the culture medium, infectious virus and envelope protein could be detected. l-Glutamic acid, l-aspartic acid, or l-alanine could be substituted for l-glutamine. Effects of l-glutamine deprivation on HVJ growth in several other cells were also investigated. The growth of HVJ in the cells other than BHK and FL cells was not suppressed by lack of l-glutamine. Growth of Sindbis virus in BHK cells was also markedly retarded in the absence of l-glutamine.

Temperature-sensitive virus from Aedes albopictus cells chronically infected with Sindbis virus

Cultures of Aedes albopictus cells persistently infected with wild-type Sindbis virus (SV-W) give rise to small plaque-forming mutants which are also temperature sensitive. These mutants, designated SV-C, are neutralized by antiserum produced against SV-W. Mutant ts clones were isolated from SV-C by plaque purification. After serial undiluted passage in BHK or mosquito cells, each of the clones gave rise to ts(+) revertants which, however, remained mutant with respect to plaque morphology. Nineteen of 20 clones derived from SV-C were RNA(+), and one was RNA(-) (SV-C-2). The RNA synthesizing activity, once induced in infected cells by SV-C-2, was stable at the nonpermissive temperature (39.5 C). All clones derived from SV-C were inactivated at 60 C much more quickly than was SV-W. It was not possible to demonstrate complementation between any of the SV-C clones.

Selection of temperature-sensitive mutants during persistent infection: role in maintenance of persistent Newcastle disease virus infections of L cells

Virus mutants (NDV(pi)) recovered from L cells persistently infected with Newcastle disease virus (NDV, Herts strain) are temperature-sensitive (ts) at 43 C, although the wild-type virus (NDV(o)) which initiated the persistent infection replicates normally at that temperature. To study the relationship between the ts marker of NDV(pi) and the other properties which distinguish this virus from NDV(o), NDV(pi) ts(+) revertants were selected at the nonpermissive temperature and NDV(o) ts mutants were generated by treating NDV(o) with nitrous acid. Spontaneously-occurring ts mutants in the Herts NDV population were also isolated. The different virus populations were characterized with regard to plaque size, virulence for eggs, and thermal stability of infectivity, hemagglutinin, and neuraminidase. The NDV(pi) ts(+) revertants, although no longer temperature-sensitive, retained NDV(pi) properties, whereas both spontaneously-occurring and mutagen-induced ts mutants remained wild-type in their other properties. These findings showed that the properties which characterized NDV(pi) were independent of the ts marker. However, the ts marker and the other markers of NDV(pi) were coselected during the persistent infection, and the combination of those markers appeared to be important in the outcome of NDV infection of L cells. NDV(pi) replicated productively in L cells, whereas NDV(o), the NDV(pi) ts(+) revertants, and the spontaneously-occurring ts mutants all yielded covert infections in L cells. The role of the selection of ts mutants in persistent infection was confirmed as follows: L cells were persistently infected with NDV(pi) ts(+) revertants and NDV(o) ts mutants. Virus recovered from the persistently infected cultures after eight cell passages was always temperature-sensitive and of smaller plaque size than the parental virus in chicken embryo cell cultures. Similar results were obtained with virus recovered from L-cell cultures persistently infected with two other velogenic strains of NDV, the Texas-GB and Kansas-Man strains. These results strongly suggest that selection of ts mutants during the persistent infection was not random and played a role in establishment or maintenance of the persistent infection, or both.