Temperature-sensitive virus derived from BHK cells persistently infected with HVJ (Sendai virus)

Effective photodynamic therapy in drug-resistant prostate cancer cells utilizing a non-viral antitumor vector (a secondary publication)

BACKGROUND AND AIMS

There is an urgent need to develop an efficient strategy for the treatment of drug-resistant prostate cancer. Photodynamic therapy (PDT), in which low incident levels of laser energy are used to activate a photosensitizer taken up by tumor cells, is expected as a novel therapy for the treatment of prostate cancer because of the minimal invasive nature of PDT. The present study was designed to assess the efficacy of a novel vector approach combined with a conventional porphyrin-based photosensitizer.

MATERIALS AND METHODS

Our group focused on a non-viral vector (hemagglutinating virus of Japan envelope; HVJ-E) combined with protoporphyrin IX (PpIX) lipid, termed the porphyrus envelope (PE). It has been previously confirmed that HVJ-E has drug-delivering properties and can induce cancer-specific cell death. The PE (HVJ-E contained in PpIX lipid) was developed as a novel photosensitizer. In this study, the antitumor and PDT efficacy of the PE against hormone-antagonistic human prostate cancer cells (PC-3) were evaluated.

RESULTS AND CONCLUSIONS

Our results demonstrated that, under specific circumstances, PDT using the PE was very effective against PC-3 cells. A novel therapy for drug-resistant prostate cancer based on this vector approach is eagerly anticipated.

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.

Peroral vaccination with a temperature-sensitive mutant of parainfluenza virus type 1 protects mice against respiratory challenge infection

Peroral vaccination for preventing respiratory infectious diseases was investigated in a murine model using a temperature-sensitive (ts) mutant of parainfluenza virus type 1. The ts mutant virus administered perorally in drinking water neither multiplied nor caused lesions in the respiratory tract or the central nervous system. However, ts virus antigen-positive cells appeared in oropharyngeal lymphoid tissues. This type of antigenic stimulation was capable of inducing both humoral and cellular immune responses, together with an augmentation of interferon production and natural killer cell activity, making it possible to protect the mice against challenge infection with a virulent wild-type virus. These results suggest that the oral cavity, a constituent member of the common mucosal immune system, is a candidate organ applicable as a vaccine route against virus respiratory diseases.

Sendai virus blocks alpha interferon signaling to signal transducers and activators of transcription

We demonstrate here that Sendai virus (SeV) blocks alpha interferon (IFN-alpha) signaling to signal transducers and activators of transcription (STATs) in HeLa cells. IFN-alpha-stimulated tyrosine phosphorylation of STATs and subsequent formation of the IFN-stimulated gene factor 3 transcription complex were inhibited in SeV-infected cells, resulting in inefficient induction of IFN-stimulated gene products. None of the components of the signaling pathway-type I IFN receptor subunits Jak1, Tyk2, Stat1, Stat2, and p48-was degraded. Moreover, tyrosine phosphorylation of Jak1 in response to IFN-alpha was unaffected at the early phase of infection, suggesting that oligomerization of the receptor subunits proceeded normally. In contrast to Jak1, IFN-alpha-stimulated tyrosine phosphorylation of Tyk2 was partially inhibited. Therefore, this partial inhibition of activation of Tyk2 probably contributes to the subsequent failure in the activation of STATs.

Sendai virus gene expression in lytically and persistently infected cells

Sendai virus RNA species were quantitated in lytically and persistently infected cultured cells by Northern blot hybridization to region- and strand-specific cloned cDNA probes. Levels of NP, P and M mRNA in lytically infected cells were equally high, but F and HN mRNA were present in about 3-fold, and L mRNA in 30-fold, lower amounts, reflecting transcriptional attenuation especially at the M-F and HN-L gene junction. Two persistently infected cell lines, which release only 1% of the virus particles of lytically infected cells, were shown to contain only 4- to 8-fold-less amounts of each viral mRNA and 2- to 3-fold-less genomic RNA than lytically infected cells. Additionally, transcription was neither defective nor more attenuated as compared to the lytical infection. Taken together the results suggest the existence of an additional regulatory mechanism for the virus release. A cell-associated state of infection therefore seems to be achievable by a relatively weak general reduction of the copy numbers of viral mRNA and genomic RNA.

Effect of cytolytic infection on maintenance of resistance to HVJ (Sendai virus) in an altered BHK cell culture

Altered baby hamster kidney (BHK-R) cells were serially cultured in the continuous presence of hemagglutinating virus of Japan (HVJ). These cells showed a distinct resistance to superinfection with the homologous HVJ. This resistance of BHK-R cells gradually disappeared after serial passages in the presence of ultraviolet-irradiated HVJ particles which lost infectivity but still preserved hemagglutinating and neuraminidase activities. When BHK-R cells were serially cultured in the presence of a temperature-sensitive mutant of HVJ at non-permissive temperature, the cells also lost the resistance. The resistance of BHK-R cells remained unchanged, even after prolonged incubation in virus-free maintenance medium under the conditions of no cell division. It was suggested that killing of virus-sensitive cells, which were generated during cell proliferation, was required for maintenance of the resistance.

Relation of HVJ (Sendai virus) production to cell growth phase in persistently infected mouse 3T3 cells

Using 3T3 mouse fibroblasts persistently infected with temperature-sensitive HVJ (3T3-HVJ pi), the relationship between production of HVJ and cell growth was investigated. In these cells the highest virus release into the culture media occurred from 24 to 48 hours after seeding. The release gradually decreased as the cultures approached confluency, at which time it was reduced below 1-2 per cent of that on day 2 in parallel with the decrease of both cellular DNA and RNA synthesis rates. Comparative examination in growing and resting HVJ carrier 3T3 cells showed that amounts of nucleocapsids and rate of viral structural protein synthesis in the latter phase was reduced to 4-5 per cent of those in the former phase. In addition, viral replication and transcription rates in the resting phase were found to be suppressed to 8-9 per cent of levels detected in the growing phase. These results suggest that the reduced virus production in the resting HVJ carrier cells may be mainly due to the suppression of viral RNA synthesis.

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.

Sendai-virus-induced cell-mediated cytotoxicity in vitro. The role of viral glycoproteins in cell-mediated cytotoxicity

Treatment of peripheral blood lymphocytes from normal donors with small amounts of purified Sendai virions results in enhanced cellular cytotoxicity in vitro to uninfected tissue culture target cells (virus-dependent cellular cytotoxicity (VDCC)), without any obvious correlation to the natural cytotoxicity (NK) displayed by the lymphocytes in the absence of virus. Removal from the virions of the two surface components present in the viral envelope, the HN glycoprotein (gp 71), carrying haemagglutinating and neuraminidase activity, and the F glycoprotein (gp 49), carrying fusion activity, by treatment with pronase abrogated their capacity to induce VDCC. Similar results were obtained when virions lacking the HN glycoprotein after treatment with chymotrypsin were added to the lymphocytes. In contrast, treatment of the virus particles with trypsin, which removed the F glycoprotein, did not affect their capacity to induce VDCC. When the solubilized and separated peplomers were used for lymphocyte treatment, either alone or in combination, the purified HN glycoprotein had full capacity to induce VDCC, whereas the F glycoprotein was inactive. These results suggest that the HM peplomer is solely or primarily responsible for the cytolytic activity arising in non-sensitized lymphocytes when confronted with certain viruses.

Biological properties of plaque-size variants of Sendai virus

Large (RL)-and small (RS)-plaque variants of Sendai virus were isolated in culture of LLCMK2 cells in the presence of trypsin and their biological properties were determined. The RL variant was more virulent to mice than the RS variant. The RL variant had a higher growth rate than the RS variant in multiple-step growth in the presence of trypsin, but the two variants had an almost equal growth rate in its absence. Restoration of hemolytic activity in cleavage of the F protein of the RL variant were achieved by milder trypsin treatment than was needed for the RS variant.

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.

Isolation and characterization of temperature-sensitive mutants of Sendai virus

Sixteen temperature-sensitive mutants of Sendai virus were isolated from mutagenized stocks (10 mutants, designated numerically) and persistently infected cultures (6 mutants, designated alphabetically). Based on complementation tests, virion-associated activities, thermal inactivation, and viral RNA and hemadsorbing antigen synthesis as well as virion production in chick lung embryo cells at nonpermissive temperature, these mutants were divided into seven groups as follows. i) HANA group mutants (ts-5, -9, -10, -201), defective in hemagglutinin-neuraminidase protein, complementation group I. ii) F group mutants (ts-18, -108), defective in hemolytic and cell-fusing activity, complementation group II. iii) Ts-43, defective in RNA polymerase activity, complementation group III. iv) Ts-23, defective in RNA polymerase activity, interfered with the other mutants in complementation tests. v) Ts-25, defective in the incorporation of hemagglutinin-neuraminidase protein into the virion at the stage of virus assembly. vi) Ts-110, belongs to F group mutants on one hand, but is considered to carry another undetermined defect. vii) C group (carrier culture-borne group) mutants (ts-a, -b, -c, -d, -e, -f), defective lesion not yet determined and belong to neither complementation group I nor II. Assignment of mutants in groups iv), v), vi), and vii) to complementation groups could not be achieved.

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

Persistent reovirus infection of L cells was established with a serially passaged stock of temperature-sensitive (ts) mutant C(447) containing greater than 90% defective interfering particles. Within a month after establishment of the carrier culture, the ts mutant was replaced by virus that expressed the wild-type (ts(+)) temperature phenotype (R. Ahmed and A. F. Graham, J. Virol. 23:250-262, 1977). To determine whether the ts(+) phenotype of the virus was due to intragenic reversion or to the presence of an extragenic mutation suppressing the original ts defect, several clones were backcrossed to wild-type reovirus, and the progeny of each cross were screened for temperature sensitivity. The results indicated that the original tsC lesion had reverted. However, in two of the seven clones examined, new ts lesions were found. These new ts lesions appeared phenotypically as ts(+) due to the presence of extragenic suppressor mutations. Temperature-sensitive mutants representing three different groups were rescued from one suppressed clone, indicating that this ts(+) clone contained multiple ts lesions. Among the ts mutants rescued were the initial isolates of a new recombination group which we have designated H. Some of the ts mutants rescued from the suppressed clones are capable of interfering with the growth of wild-type reovirus and may play a role in maintaining the carrier state. The results of this study show that persistently infected L cells contain a genetically heterogeneous population of reovirus even though all virus clones express the ts(+) phenotype. It is thus critical to distinguish between genotype and phenotype when analyzing viruses that emerge during persistent infection.

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.

Tryptic peptide analysis of the structural proteins of a temperature-sensitive mutant derived from an HVJ (Sendai virus) carrier culture

The individual structural polypeptides of HVJ (haemagglutinating virus of Japan--the Sendai strain of parainfluenza 1 virus) were examined by tryptic peptide analysis. [3H]-methionine-labelled structural proteins of the wild-type virus of HVJ (HVJ-W) and [35S]-methionine-labelled corresponding constituent proteins of a temperature-sensitive (ts) mutant (HVJ-pB) derived from an HVJ carrier culture were compared by ion-exchange chromatography on columns of P-type chromobeads. The tryptic peptides of the individual structural proteins showed characteristic elution profiles. In all the structural proteins tested, the chromatographic elution profiles of both strains generally showed a close resemblance. However, certain minor peaks which were present in one strain but absent in the other strain were detected in the preparations of the P, HN, and F polypeptides. Further, analysis of the NP polypeptide showed that a major peak of one strain appeared at a position in the pH gradient different from a seemingly corresponding major peak of the other strain. In the M protein some possibly homologous minor peaks were found to differ between the two strains.

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.

Assembly of viral structural proteins in cells infected with a temperature-sensitive mutant derived from an HVJ (Sendai virus) carrier culture. Brief report

The processing of virus polypeptides synthesized in cells infected with HVJ (haemagglutinating virus of Japan--the Sendai strain of parainfluenza 1 virus) was studied. Maturation of a temperature-sensitive (ts) mutant (HVJ-pB) derived from an HVJ carrier culture was inhibited at 38 degrees C incubation. A considerable amount of viral components were made at the restrictive temperature. They were, with the exception of the polypeptide HN, well preserved without a great loss of their function and successfully incorporated into virus particles released after lowering the incubation temperature. The membrane (M) protein seems to be essential for virus morphogenesis.

In vivo and in vitro models of demyelinating diseases. II. Persistence and host-regulated thermosensitivity in cells of neural derivation infected with mouse hepatitis and measles viruses

Following inoculation of continuous cell lines of neural and other derivations, persistent infections are established with facility by mouse hepatitis and measles viruses. This occurs equally with the prototype MHV3 and its neurotropic variant JHM as well as with the Edmonston vaccine and SSPE Hallé measles variants. In almost every instance that the infection becomes persistent at 32.5°, virus replication is found to be thermosensitive at 39.5°; however, progeny virus derived from such infections at 32.5° is itself thermostable when replicating in the indicator, fully permissive cell lines. The new data, therefore, reveal the existence of a host-conferred interrelationship between persistence and virus restriction at elevated temperature. They indicate that the two agents with neurotropic potential, when they become established as pathogens in the nervous system, could be under close host cell regulation involving as yet unknown mechanisms.

Replication of parainfluenza type 3 virus in alveolar macrophages: evidence of in vivo infection and of in vitro temperature sensitivity in virus maturation

Approximately 2% of cultured alveolar macrophages (AM), originally lavaged from the lungs of parainfluenza type 3 virus (PI-3V)-infected calves, were observed to contain viral antigen (by fluorescent antibody method) or viral nucleocapsids (by electron microscopy). Plaque assays, however, indicated that virus titers were generally low when cultures were incubated at 37 degrees C for 10 days. AM, obtained from "in vivo infected" and "noninfected" calves, were found to be equally susceptible to further in vitro PI-3V infection when cultures were incubated at 37 degrees C. AM that were obtained from the lungs of normal calves, cultured at 37 degrees C, and inoculated with PI-3V were observed to produce relatively high virus titers when the incubation temperature was shifted down to 32 degrees C. Results from hemagglutinin assays showed that considerable amounts of hemagglutinin were detected when AM cultures were incubated at 32 degrees C, but only limited amounts were detected at 37 degrees C. Results from electron microscopic examinations at both temperatures substantiated the results of plaque and hemagglutinin assays. The PI-3V, isolated from AM cultures incubated at 32 degrees C, grew well in Madin-Darby bovine kidney cells at 32 degrees C, but little virus was produced at 37 degrees C. In contrast, parent PI-3V grew equally well at both temperatures. The results are discussed in terms of host susceptibility, temperature-sensitivity and virus maturation, and surface viral antigens and persistent viral infection.

In vivo and in vitro models of demyelinating diseases: tropism of the JHM strain of murine hepatitis virus for cells of glial origin

Infection of mice with the neurotropic JHM strain of murine hepatitis virus causes demyelinating lesions resulting from an infection of the oligodendroglia. This was most evident in mice inoculated intraperitoneally with JHM. Such CNS lesions were not observed in mice inoculated intraperitoneally with the MHV₃ strain. An in vitro system is described in which the rat glial RN2 cell line functions as a discriminating host for the JHM virus. Shortly after inoculation, this virus establishes a persistent infection in which there is a cyclical rise and fall in titer with an accompanying cytopathology. Furthermore, this host cell confers a thermal lability which the virus does not demonstrate in the fully permissive host cell, L-2. By comparison, infection of RN2 cells with the prototype MHV₃ is aborted immediately. In the persistent infection of RN2 cells with measles virus, Hallé strain, the cell again confers a temperature sensitivity which the virus does not possess when replicating in Vero cells.

This appears to be the first instance in which a cloned cell line of glial origin determines the outcome of the infectious process, discriminating in favor of a neurotropic variant which possesses a tropism for the glia in vivo. Systems such as the one described here may now offer a specific screening procedure for selecting, identifying and characterizing the nature of neurotropic viruses.

Integration of genomes on infectious viruses

The article contains a review relating to the integration of genomes of infectious viruses in Eukaryotic cells. Data are presented that were obtained in the study of tissue cultures chronically infected with viruses, in the study of pathologic and ecologic processes. Based on the evidence accumulated, some theoretical and applied aspects of the problem are discussed.

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.

Growth and genetic stability of the ts-1 mutant of respiratory syncytial virus at restrictive temperatures

An in vitro study was performed to define in greater detail those factors which favored the growth of the ts-1 mutant of respiratory syncytial virus under restrictive conditions and the emergence of genetically altered virus with decreased temperature sensitivity. Replication of ts-1 occurred at each of the restrictive temperatures of 37, 38, and 39 C, even through plaque formation was not observed. The level of virus growth under restrictive conditions was inversely related to the incubation temperature and directly related to the multiplicity of infection. These relationships appeared to reflect the effect of restrictive temperature in reducing the quantity of virus produced and released from an infected cell. Under restrictive conditions the production of genetically altered virus which exhibited reduced temperature sensitivity was directly related to the multiplicity of infection and inversely related to temperature. Production of genetically altered virus was not observed under permissive conditions.