Enucleated L929 mouse fibroblasts support invasion and multiplication of Shigella flexneri 5a

Invasive bacteria can induce their own uptake and specify their intracellular localization; hence it is commonly assumed that proximate modulation of host cell transcription is not required for infection. However, bacteria can also modulate, directly or indirectly, the transcription of many host cell genes, whose role in the infection may be difficult to determine by global gene expression. Is the host cell nucleus proximately required for intracellular infection and, if so, for which pathogens and at what stages of infection? Enucleated cells were previously infected with Toxoplasma gondii, Chlamydia psittaci, C. trachomatis, or Rickettsia prowazekii. We enucleated L929 mouse fibroblasts by centrifugation in the presence of cytochalasin B, and compared the infection with Shigella flexneri M90T 5a of nucleated and enucleated cells. Percent infection and bacterial loads were estimated with a gentamicin suppression assay in cultures fixed and stained at different times after infection. Enucleation reduced by about half the percent of infected cells, a finding that may reflect the reduced endocytic ability of L929 cytoplasts. However, average numbers of bacteria and frequency distributions of bacterial numbers per cell at different times were similar in enucleated and nucleated cells. Bacteria with actin-rich tails were detected in both cytoplasts and nucleated cells. Lastly, cytoplasts were similarly infected 2 and 24 h after enucleation, suggesting that short-lived mRNAs were not involved in the infection. Productive S. flexneri infection could thus take place in cells unable to modulate gene transcription, RNA processing, or nucleus-dependent signaling cascades.

Use of shell-vial cell culture assay for isolation of bacteria from clinical specimens: 13 years of experience

The shell-vial culture assay is performed routinely in our laboratory. Recently we revisited our experience of using the shell-vial culture assay for the isolation of microorganisms from various clinical samples. Over a 13-year period, we have isolated 580 bacterial strains (5%) from 11,083 clinical samples tested. Over the same period, 285 isolates of rickettsiae, bartonellae, or Coxiella burnetii were cultured from a total of 7,102 samples tested. These isolates include 55 Rickettsia sp. isolates, 95 Coxiella burnetii isolates, and 135 Bartonella sp. isolates. Based on our experience with the growth of fastidious microorganisms, we have used a centrifugation shell-vial technique called JNSP, for "je ne sais pas" ("I don't know [what I am growing]") for the isolation of other microorganisms. A total of 173 isolates were cultured from the 3,861 clinical samples tested using the JNSP method. Of these, 40 isolates had not been grown before on usual axenic medium. These include 2 Staphylococcus aureus isolates, 7 isolates of Streptococcus sp. and related genera, 6 Mycobacterium sp. isolates, 1 Nocardia asteroides isolate, 1 Actinomyces sp. isolate, 1 Brucella melitensis isolate, 2 Francisella tularensis isolates, 1 Mycoplasma pneumoniae isolate, and 1 Legionella pneumophila isolate. Using this protocol, we have also cultured intracellular bacteria such as Chlamydia trachomatis and we have performed the first culture and establishment of Trophyrema whipplei. Applied in our laboratory, the shell-vial culture generally exhibits a low rate of success. However, in some cases, this technique allowed microbial diagnosis when classical agar procedure and PCR were negative.

CD8 T cell immunome analysis of Listeria monocytogenes

The identification of T cell epitopes is crucial for the understanding of the host response during infections with pathogenic microorganisms. Generally, the identification of relevant T cell responses is based on the analysis of T cell lines propagated in vitro. We used an ex vivo approach for the analysis of the CD8 T cell response against Listeria monocytogenes that is based upon the fractionation of naturally processed antigenic peptides and subsequent analysis with T cells in an enzyme-linked immunospot (ELISPOT) assay. Our data indicate that the direct ex vivo ELISPOT analysis of peptides extracted from infected tissues represents a versatile and potent test system for the analysis of the CD8 T cell immunome of microorganisms that furthermore requires neither the knowledge of the microbial genome nor of the specificity of responding T cells.

Characterization and pathogenic potential of Listeria monocytogenes isolates from the smoked fish industry

This study was designed to evaluate the hypothesis that some of the Listeria monocytogenes subtypes associated with foods, specifically smoked fish, may have an attenuated ability to cause human disease. We tested this hypothesis by using two different approaches: (i) comparison of molecular subtypes found among 117 isolates from smoked fish, raw materials, fish in process, and processing environments with subtypes found among a collection of 275 human clinical isolates and (ii) the evaluation of the cytopathogenicity of industrial isolates. Ribotyping and PCR-restriction fragment length polymorphism typing of the hlyA and actA genes differentiated 23 subtypes among the industrial isolates and allowed classification of the isolates into three genetic lineages. A significantly higher proportion of human isolates (69.1%) than industrial isolates (36.8%) were classified as lineage I, which contains human sporadic isolates and all epidemic isolates. All other industrial isolates (63.2%) were classified as lineage II, which contains only human sporadic isolates. Lineage I ribotypes DUP-1038B and DUP-1042B represented a significantly higher proportion of the human isolates than industrial isolates (5.1%). Lineage II ribotypes DUP-1039C, DUP-1042C, and DUP-1045, shown previously to persist in the smoked fish processing environment, represented nearly 50% of the industrial isolates, compared to 7.6% of the human isolates. Representatives of each subtype were evaluated with a tissue culture plaque assay. Lineage I isolates formed plaques that were significantly larger than those formed by lineage II isolates. Isolates from the smoked fish industry representing three ribotypes formed no plaques or small plaques, indicating that they had an impaired ability to infect mammalian cells. While L. monocytogenes clonal groups linked to human listeriosis cases and outbreaks were isolated, our data also suggest that at least some L. monocytogenes subtypes present in ready-to-eat foods may have limited human-pathogenic potential.

HLA-B27 modulates the survival of Salmonella enteritidis in transfected L cells, possibly by impaired nitric oxide production

Reactive arthritis is triggered by certain microbes that cause primary infections mainly on the gastrointestinal or urogenital mucosa. The disease is strongly associated with HLA-B27. Long persistence of causative microbes or their structures in the body has been thought to have an important role in the pathogenesis of reactive arthritis. This suggests that the elimination of the microbes causing reactive arthritis is ineffective or disturbed in HLA-B27-positive individuals developing this complication. We examined the role of the HLA-B27 antigen in microbe-host interaction in vitro by monitoring the invasion and intracellular survival of Salmonella enteritidis in mouse fibroblasts transfected with HLA-B27, HLA-B7, or beta2-microglobulin only. S. enteritidis invaded into all the three transfectants with the same efficiency. However, at 6 and 10 days after incubation, there were more living intracellular Salmonella organisms in HLA-B27 transfectants than in the other transfected cell lines (P < 0.05), suggesting that the bactericidal effect is impaired in these cells. Impaired NO production in HLA-B27-transfected cells was indicated as a possible mechanism, since the amount of nitrite in the supernatants of the Salmonella-infected HLA-B27-transfected cells was smaller than that in the supernatants of the Salmonella-infected HLA-B7- or beta2-microglobulin-transfected cells (P < 0.001). The inhibition of NO synthesis by N-monomethyl-L-arginine resulted in impaired elimination of Salmonella also in HLA-B7and beta2-microglobulin-transfected cells. The inverse correlation between intracellular survival of Salmonella and the amount of nitrite detected in culture supernatants supports the hypothesis that the L-arginine-dependent NO pathway plays an important role in the murine fibroblast response against Salmonella. We suggest that a major histocompatibility complex class I antigen, HLA-B27, may contribute to the intracellular persistence of Salmonella by a mechanism which involves the NO pathway.

Characterization of a monoclonal antibody resistant variant of MHV

A monoclonal antibody resistant (MAR) variant of MHV was isolated after infection of hybridoma cells secreting the neutralizing and fusion-inhibiting monoclonal antibody, mAb 11F. The isolated variant was able to mediate syncytia formation even in the presence of high concentrations of mAb 11F. The S gene of the variant was cloned and sequenced. There were three nucleotide exchanges in comparison to the wild-type S gene, resulting in two amino acid alterations. However, both amino acid substitutions (at positions 255 and 1116) were located outside the binding site of mAb 11F.

Involvement of lipids in membrane binding of mouse hepatitis virus nucleocapsid protein

Evidence is presented which indicates that membrane binding of the MHV nucleocapsid (N) protein is influenced by membrane lipid composition. Binding of N protein to membranes of mouse fibroblast L-2 cells is very specific and occurs under conditions in which no other viral or cellular proteins show detectable binding. Binding occurs rapidly and does not require the presence of divalent cations such as Ca++ or Mg++. Purified phospholipid liposomes compete against N protein binding to membranes. Phospholipids consisting of cardiolipin are the most effective in inhibiting membrane binding. Because of certain structural similarities between phospholipids and nucleic acids, we speculate that membrane lipid association of the N protein may compete for RNA binding sites on the N protein. Such a mechanism may be important for processes such as nucleocapsid uncoating and nucleocapsid assembly.

Virus-mediated induction of interferon A gene requires cooperation between multiple binding factors in the interferon alpha promoter region

Transcriptional activation of interferon A (IFNA) gene in virus-infected cells is controlled by a 35-nucleotide inducible element that is cell type specific. Within this region, two elements, alpha F1 and IRF-1 binding sites, were shown by mutation analysis to play a crucial role in the expression of inducible element. In this study, we have analyzed the binding of nuclear proteins to the alpha F1 sequence and have shown that the induction is associated with the formation of a novel complex alpha F1/B, which contains at least two DNA binding proteins of 68 and 96 kDa. In contrast, no binding of the purified interferon regulatory factor 1 (IRF-1) either to the alpha F1 or IRF-1 binding sites could be detected in vitro. However, the oligonucleotides corresponding to alpha F1 or IRF-1 binding sites competed efficiently for the induction of IFNA4 promoter region in a transient transfection assay. We suggest that the induction of IFNA promoter region requires cooperation between alpha F1 binding proteins and IRF-1. Interestingly, our data also show that the inability of IFNA6 promoter to be expressed in infected L-cells may be a result of a viral-induced repressor, which could act by binding and inactivating alpha F1 or by competing for the IRF-1 binding site. These results suggest that cell-specific expression of IFNA genes results from core-cruitment of trans-acting factors that bind to alpha F1 and the IRF-1 binding site with the cell-specific virus-induced activator or repressor.

The entry and intracellular multiplication of Francisella tularensis in cultured cells: its correlation with virulence in experimental mice

Five acriflavine agglutination test-positive (acf+) colonies and five negative (acf-) colonies were isolated from each of the four strains (Ebina, CMB2, N9, and Schu) of Francisella tularensis, and the correlation between the virulence in experimental mice and the entry and intracellular multiplication in cultured mouse fibroblast cells (L-929 cells) was examined. All of the acf- colonies derived from the Ebina and CMB2 strains were highly virulent in mice, readily entering and growing well in the cells, while all of the acf- colonies from N9 and Schu strains were of low virulence and neither entered nor grew in the cells effectively. On the other hand, regardless of their parent strains, the acf+ colonies were low virulent and most of those colonies did neither enter nor grow in L-929 cells. In addition, two acf- colonies, one from the N9 and the other from the Schu strain, gained virulence through several passages in mice, and in parallel, their entry and multiplication also improved. However, two acf+ colonies from the Ebina strain and one acf+ colony from the N9 strain showed a moderate degree of the entry and multiplication although they were all low virulent. The overall results indicate that the entry and multiplication in cells are important factors regulating the virulence of F. tularensis. The results also showed, however, that they were not sole factors to elucidate the virulence of the bacterium in mice.

Establishment and characterisation of murine cells constitutively expressing the fusion, nucleoprotein and matrix proteins of measles virus

To advance our understanding of the immunobiology of measles virus (MV) infections, we have investigated the possibility of establishing cell lines constitutively expressing the individual MV antigens. In contrast to previously published studies, we show that it is possible to establish cell lines expressing high levels of fusion (F), nucleoprotein (NP) and matrix (M) MV proteins. Once cloned, the cell lines were stable with high levels of expression for more than six months. The size and cell distribution of the NP and F proteins were similar to those observed in MV- or vaccinia-MV recombinant-infected cells. In contrast, the distribution of the M protein, although being similar to that of MV-infected cells, differed from that of Vaccinia-M recombinant virus-infected cells. Preliminary results suggest that these cell lines will be useful tools for studying the contribution of individual MV antigens to the cell-mediated immune response to this virus.

Evidence for an altered kinetics of DNA excision-repair in cells infected by herpes simplex virus type 1

In cells infected with herpesviruses a series of host cell nuclear changes can be observed in a temporal sequence. Such changes include chromosome aberrations. The precise mechanism by which virus infection produces chromosome damage is not known, but we have previously reported that herpes simplex virus type 1 (HSV-1) induces a significant number of single-stranded breaks in the host cell DNA at early hours of infection and in a time-dependent fashion. Here, it is reported that HSV-1-infected cells subjected to irradiation with ultraviolet light, show an altered kinetics in the normal process of DNA excision-repair at early hours of infection.

Toxin resistance and reduced secretion in a mouse L-cell mutant defective in herpes virus propagation

The mouse L-cell mutant gro29 was selected originally for its inability to propagate herpes simplex virus; it shows severe defects in virus egress and the transport and processing of viral glycoproteins after infection. In this report, we show that uninfected gro29 cells display pleiotropic changes in protein secretion, oligosaccharide processing, and sensitivity to the toxins ricin and modeccin. Specifically, the rate of secretion of a nonglycosylated protein, human growth hormone, was reduced 70% in gro29 cells compared with the parental L cells. A direct measurement of the transport capacity of Golgi membranes in a cell-free assay suggests that gro29 cells contain less functional Golgi than parental cells. Despite this deficiency, N-linked oligosaccharides were processed efficiently in mutant cells, although there were differences in the structure of the mature forms. Lectin intoxication assays revealed that gro29 cells were cross-resistant to killing by the cytotoxic lectins ricin and modeccin, but not to wheat germ agglutinin, Ricinus communis agglutinin RCA120, or leucoagglutinin. Fluorescence labeling using fluorescein-conjugated lectins showed that uninfected gro29 cells expressed relatively few ricin-binding molecules, suggesting a possible mechanism for toxin resistance. These studies provide evidence that the processes of protein secretion, lectin intoxication, and herpes virus maturation and egress may share a common cellular component.

Dissociable antiviral activities directed against cardioviruses are expressed in L cells treated with interferon

Interferon (IFN) restricts a wide variety of viruses. To do so it elicits many antiviral pathways. For example, subclones of the same cell line with a reduced antiviral spectrum are thought to lack one or more antiviral pathways. Our line of L cells exhibits two distinct antiviral activities. The first delays the yield of both wild-type mengovirus (is+) and an IFN-sensitive mutant (is-1). The second specifically inhibits is-1 virus yields 100-fold. From these cells, a subclone was isolated which had lost the second antiviral activity (i.e. in these cells is-1 virus acts like is+ virus). To see whether other cardioviruses are sensitive to these activities, two additional strains [m-mengovirus and encephalomyocarditis-R (EMC-R) virus] were tested in our subclones. Like is+ virus, m-mengovirus yields were delayed by IFN in both subclones; EMC-R virus behaved like is-1 virus in both cell lines. When actinomycin D was added at the time of infection, is-1 virus was phenotypically reversed to is+ virus, but EMC-R virus was still inhibited. The 2-5A synthetase/RNase L pathway is expressed in both clones. Therefore, at least three antiviral activities against cardioviruses can be distinguished in IFN-treated L cells, and two of them appear not to involve the 2-5A synthetase/RNase L pathway.

Eukaryotic cells grown on microcarrier beads offer a cost-efficient way to propagate Chlamydia trachomatis

The use of microcarrier cell culture as a method for the in vitro propagation of the obligate intracellular bacterial parasite, Chlamydia trachomatis, is described. The microcarrier beads proved to be a more cost-effective means to propagate C. trachomatis than traditional tissue culture flasks or roller bottles without sacrificing yields or infectivity. In addition, microcarrier cell culture was found to be a much simpler technique to study the intracellular development of these bacteria.

Effect of 6-thioguanine on Chlamydia trachomatis growth in wild-type and hypoxanthine-guanine phosphoribosyltransferase-deficient cells

Chlamydiae have evolved a biphasic life cycle to facilitate their survival in two discontinuous habitats. The unique growth cycle is represented by two alternating forms of the organism, the elementary body and the reticulate body. Chlamydiae have an absolute nutritional dependency on the host cell to provide ribonucleoside triphosphates and other essential intermediates of metabolism. This report describes the pleiotropic effects of the purine antimetabolite 6-thioguanine on chlamydial replication. In order to display cytotoxicity, 6-thioguanine must first be converted to the nucleotide level by the host cell enzyme hypoxanthine-guanine phosphoribosyltransferase. Our results show that 6-thioguanine is an effective inhibitor of chlamydial growth with either wild-type or hypoxanthine-guanine phosphoribosyltransferase-deficient cell lines as the host. Interestingly, the mechanism of 6-thioguanine-induced inhibition of chlamydial growth is different depending on which cell line is used. With wild-type cells as the host, the cytotoxic effects of 6-thioguanine on chlamydial growth are relatively fast and irreversible. Under these circumstances, cytotoxicity likely results from the combined effect of starving chlamydiae for purine ribonucleotides and incorporation of host-derived 6-thioguanine-containing nucleotides into chlamydial nucleic acids. With hypoxanthine-guanine phosphoribosyltransferase-deficient cells as the host, 6-thioguanine must be present at the start of the chlamydial infection cycle to be effective and the growth inhibition is reversible upon removal of the antimetabolite. These findings suggest that in hypoxanthine-guanine phosphoribosyltransferase-deficient cells, the free base 6-thioguanine may inhibit the differentiation of elementary bodies to reticulate bodies. With hypoxanthine-guanine phosphoribosyltransferase-deficient cells as the host, 6-thioguanine was used as a selective agent in culture to isolate a Chlamydia trachomatis isolate resistant to the effects of the drug. This drug resistant C. trachomatis isolate was completely resistant to 6-thioguanine in hypoxanthine-guanine phosphoribosyltransferase-deficient cells; however, it displayed wildtype sensitivity to 6-thioguanine when cultured in wild-type host cells.

Transduction of the CHO aprt gene into mouse L cells using an adeno-5/APRT recombinant virus

An adenovirus-5 recombinant virus Adapt1 carrying the Chinese hamster ovary (CHO) adenine phosphoribosyltransferase (aprt) gene was constructed by insertion of a 2.5-kb fragment containing the complete CHO aprt structural gene linked to a Moloney murine sarcoma virus (MSV) promoter into the E3 region of adenovirus-5. The CHO aprt gene was in the opposite orientation to the adenovirus E3 promoter. Mouse Lapt- tk- (LAT) cells expressed the CHO aprt gene when infected with the virus, even at low MOI (O.1). APRT activity was detectable from approximately 20 h postinfection. At a low frequency, LAT cells were transformed to aprt+, and four stable transductants were selected in adenine, azaserine (AA) medium. Such cells expressed APRT at approximately 50% wild-type activity and the enzyme was shown to be CHO APRT by starch gel electrophoresis. DNA was isolated from the transductants and probed with CHO aprt-specific DNA and with viral DNA probes. The results indicated that the CHO aprt gene was integrated into the LAT cells at a site other than mouse aprt. Although neighboring viral sequences were integrated and maintained in the transductants, viral sequences further upstream and downstream of the aprt gene were absent.

Functional domains of the poliovirus receptor

A number of mutant cDNAs of the human poliovirus receptor were constructed to identify essential regions of the molecule as the receptor. All mutant cDNAs carrying the sequence coding for the entire N-terminal immunoglobulin-like domain (domain I) confer permissiveness for poliovirus to mouse L cells, but a mutant cDNA lacking the sequence for domain I does not. The transformants permissive for poliovirus were able to bind the virus and were also recognized by monoclonal antibody D171, which competes with poliovirus for the cellular receptor. These results strongly suggest that the poliovirus binding site resides in domain I of the receptor. Mutant cDNAs for the sequence encoding the intracellular peptide were also constructed and expressed in mouse L cells. Susceptibility of these cells to poliovirus revealed that the entire putative cytoplasmic domain is not essential for virus infection. Thus, the cytoplasmic domain of the molecule appears not to play a role in the penetration of poliovirus.

Identification of the viral genes responsible for growth of strains of reovirus in cultured mouse heart cells

Viral growth in specific tissue is usually required in order to lead to pathology. Two reovirus isolates (type 1 Lang and type 3 Dearing) differ in their capacity to grow in cultured mouse heart cells. The mammalian reoviruses contain a genome of 10 double-stranded RNA gene segments. By the use of 37 reassortant viruses (consisting of viruses with different combinations of genes derived from the two parents), difference in capacity of different strains to grow in heart cells was mapped to three different genes, all of which encode viral core proteins: the M1 gene (P less than 0.000044); the L1 gene (P = 0.00094); and the L3 gene (P = 0.019). Using the same set of reassortant viruses, the L1 (P = 0.00015) and L3 (P = 0.0065) genes were involved in differences of the ability of viral strains to grow in mouse L cells (fibroblasts), but the M1 gene (P = 0.12) was not. These findings suggest that the M1 gene plays an important and specific role in determining the relative capacity of certain viral strains to grow in the heart. Thus, we have identified viral genes responsible for differing growth capacity in heart muscle cells in culture. These findings provide a novel system for studies of viral myocarditis at a molecular genetic level.

Infectious cDNA clones of the DA strain of Theiler's murine encephalomyelitis virus

The DA strain and other members of the TO subgroup of Theiler's murine encephalomyelitis viruses cause a persistent demyelinating infection, whereas the GDVII strain and other GDVII subgroup strains cause an acute lethal polioencephalomyelitis. We generated an infectious DA cDNA clone inserted into a transcription vector. Virus derived from transfection of transcripts produced a demyelinating disease indistinguishable from that of wild-type virus. The infectious clone provides a critical reagent for the production of interstrain recombinant viruses to help identify genetic loci responsible for the biological activities of the strains.

Derivation and characterization of an efficiently myocarditic reovirus variant

A reovirus variant, 8B, was isolated from a neonatal mouse which had been inoculated with a mixture of two reovirus strains: type 1 Lang (T1L) and type 3 Dearing (T3D) (E. A. Wenske, S.J. Chanock, L. Krata, and B. N. Fields, J. Virol. 56:613-616, 1985). 8B is a reassortant containing eight gene segments derived from the T1L parent and two gene segments derived from the T3D parent. Upon infection of neonatal mice, 8B produced a generalized infection characteristic of many reoviruses, but it also efficiently induced numerous macroscopic external cardiac lesions, unlike either of its parents. Microscopic examination of hearts from infected mice revealed myocarditis with necrotic myocytes and both polymorphonuclear and mononuclear cellular infiltration. Electron microscopy revealed viral arrays in necrotic myocytes and dystrophic calcification accompanying late lesions. Determination of viral titers in hearts from T1L-, T3D-, or 8B-infected mice indicated that growth was not the primary determinant of myocardial necrosis. Results from inoculations of athymic mice demonstrated that T cells were not a requirement for the 8B-induced myocarditis. Finally, 8B was more cytopathic than either of the parent viruses in cultured mouse L cells. Together, the data suggest that 8B-induced myocardial necrosis is due to a direct effect of reovirus on myocytes. Reovirus thus provides a useful model for the study of viral myocarditis.

The herpes simplex virus virion host shutoff function

The virion host shutoff (vhs) function of herpes simplex virus (HSV) limits the expression of genes in the infected cells by destabilizing both host and viral mRNAs. vhs function mutants have been isolated which are defective in their ability to degrade host mRNA. Furthermore, the half-life of viral mRNAs is significantly longer in cells infected with the vhs-1 mutant virus than in cells infected with the wild-type (wt) virus. Recent data have shown that the vhs-1 mutation resides within the open reading frame UL41. We have analyzed the shutoff of host protein synthesis in cells infected with a mixture of the wt HSV-1 (KOS) and the vhs-1 mutant virus. The results of these experiments revealed that (i) the wt virus shutoff activity requires a threshold level of input virions per cell and (ii) the mutant vhs-1 virus protein can irreversibly block the wt virus shutoff activity. These results are consistent with a stoichiometric model in which the wt vhs protein interacts with a cellular factor which controls the half-life of cell mRNA. This wt virus interaction results in the destabilization of both host and viral mRNAs. In contrast, the mutant vhs function interacts with the cellular factor irreversibly, resulting in the increased half-life of both host and viral mRNAs.

The reovirus M1 gene, encoding a viral core protein, is associated with the myocarditic phenotype of a reovirus variant

Reoviruses contain a genome composed of 10 double-stranded RNA gene segments. A reovirus reassortant, 8B, derived from type 1 Lang (T1L) and type 3 Dearing (T3D), displayed a phenotype unlike that of either of its parents in that it efficiently induced numerous macroscopic external cardiac lesions in neonatal mice (B. Sherry, F. J. Schoen, E. Wenske, and B. N. Fields, J. Virol. 63:4840-4849, 1989). A panel of T1L/T3D reassortants and a panel of reassortants derived from 8B were used to determine whether novel T1L/T3D gene associations in 8B were responsible for its myocarditic phenotype. The results eliminated the possibility that any T1L/T3D gene combination found in 8B, from 2 genes to all 10 genes, was the explanation for its induction of cardiac lesions. This suggested that a mutation(s) in an 8B gene(s) might be responsible for induction of the myocarditis. Statistical analysis of experiments with 31 reassortants derived from 8B revealed a highly significant association (P = 0.002) of the 8B M1 gene with induction of cardiac lesions. The reovirus M1 gene encodes a viral core protein of unknown function, although evidence suggests a potential role in core structure and/or viral RNA synthesis. This represents the first report of the association of a viral gene with induction of myocarditis.

Introduction and expression of the bacterial PaeR7 restriction endonuclease gene in mouse cells containing the PaeR7 methylase

To study the factors essential for a functional restriction system, the PaeR7 restriction-modification system has been introduced and expressed in murine cells. Transfer of this system was accomplished in two steps. First, cells containing sufficient PaeR7 methylase to completely methylate the mouse genome were constructed. In the second step, the mouse metallothionein promoter-regulated, endonuclease expression vector linked to the hygromycin B resistance selection marker was used to transfect the high methylase-expressing cells. Sixty percent of the clones isolated contained PaeR7 endonuclease enzymatic activity. Transfected cells expressing both methylase and endonuclease were incapable of blocking infection by DNA viruses, and possible explanations are discussed.

Application of immunoassay of encephalomyocarditis virus in cell culture with enzyme-labeled virus-specific monoclonal antibodies for rapid detection of virus, neutralizing antibodies, and interferon

Encephalomyocarditis virus (EMCV)-specific monoclonal antibody UM 21.1 labeled with horseradish peroxidase was used to detect EMCV in L-cell monolayers. This direct enzyme immunoassay of EMCV, performed in wells of 96-well plates, could be applied for various purposes, such as early detection of virus multiplication, determination of 50% tissue culture infective doses, and rapid titration of interferon and EMCV-neutralizing antibodies. Multiplication of EMCV is indicated by a rapid increase of the absorbance values measured against EMCV-infected L cells starting as early as 4.5 h after virus inoculation. The early rise of absorbance (i.e., virus multiplication) is inhibited by interferon, allowing its rapid titration. Preincubation of the virus inoculum with neutralizing antibodies also yielded decreased absorbance values. With the latter enzyme immunoassay for neutralizing antibodies, performed after an infection period of 8 h, antibody titers measured were comparable to those obtained with a conventional plaque reduction test. We assume that similar assays could be developed for other picornaviruses (e.g., polioviruses).

Sequences of the cell-attachment sites of reovirus type 3 and its anti-idiotypic/antireceptor antibody: modeling of their three-dimensional structures

Previous studies have identified an area of amino acid sequence similarity shared by the reovirus type 3 cell-attachment protein sigma 1 and an anti-idiotypic/antireceptor monoclonal antibody (mAb) 87.92.6 that mimics reovirus type 3 by attaching to the same cell-surface receptor. We found that synthetic peptides corresponding to this area of primary sequence similarity bind a neutralizing mAb 9BG5 against which the mAb 87.92.6 is directed. The synthetic peptides compete with mAb 87.92.6 and reovirus type 3 for binding by mAb 9BG5 and displace mAb 87.92.6 and reovirus type 3 from binding to the cell-surface reovirus type 3 receptor. Such observations show that the shared primary structure between reovirus type 3 sigma 1 polypeptide and antireceptor mAb 87.92.6 defines the oligopeptide neutralizing/cell-attachment epitope of reovirus type 3. Computer modeling of this epitope, by use of sequence similarities of known immunoglobulin hypervariable loop conformations, permits an examination of the rudimentary three-dimensional structure of this epitope.

Biological and molecular analysis of LCV, an endogenous retrovirus with defective env gene

Retrovirus infectivity is the result of a cooperative interaction of three structural genes, gag, pol, and env. Since the L-cell endogenous retrovirus (LCV) lacks the env gene translation product, our aim was to study the biological and molecular basis of its non-infectiousness. Fusion experiments between LCV and homologous or heterologous cells demonstrated that virus production could be obtained only after LCV artificial penetration in murine cells and that the new progeny was still noninfectious. Northern blot analysis and heteroduplex mapping of the genomic RNA revealed a 0.99 kb deletion including the 3' region of the pol reading frame, the whole xenotropic and part of the ecotropic domain of the env gene. The results suggest that the observed deletion is responsible for the absence of the gp 70 and the gp 15 E molecules in the virion and seems therefore to be the molecular basis for the non-infectiousness of this retrovirus.

Antibody-nucleic acid interactions. Antibodies to psoralen-modified RNA as probes of RNA structure

Antisera elicited by immunization of rabbits with 4'-aminomethyl-trioxsalen (AMT)-modified poly(A,U) complexed with methylated bovine serum albumin was characterized in competition radioimmunoassays (RIA) and enzyme-linked immunosorbent assays (ELISA). AMT-poly(A,U) was over 10,000-fold more reactive than unmodified poly(A,U) or AMT alone. The antiserum cross-reacted to varying extents with AMT-modified-RNA's and -DNA's. The presence of AMT-uridine usually assured strong reactivity. The amino group of AMT contributed to antibody binding to a small degree. Binding was not significantly affected by high ionic strength, suggesting that binding does not involve ion pair formation. Murine encephalomyocarditis virus replicative intermediates, as well as cellular RNA and DNA were modified by psoralen in intact cells, suggesting that EMCV RNA and cellular RNA's in intact cells possess detectable stretches of base pairs. The antibodies described here will be useful in studying the secondary and tertiary structure of RNA's in vitro and in intact cells.

Increased turnover of vaccinia virus-specific immediate early RNAs in interferon-treated chick embryo fibroblasts

The synthesis and steady state level of immediate early vaccinia virus-specific RNAs in interferon-treated chick embryo fibroblasts were determined by blot hybridization analysis using the cloned restriction endonuclease fragment pEJ 18 containing the gene of vaccinia virus WR-specific DNA polymerase as a probe. Even though early vaccinia virus WR RNA was still synthesized, accumulation of immediate early viral RNAs was strongly inhibited. Accumulation of beta-actin RNA was not affected. This indicated an enhanced degradation of vaccinia virus WR-specific early RNAs in interferon-treated chick embryo fibroblasts. This notion was supported by Northern blot analysis which revealed degradation of residual RNA of vaccinia virus WR-specific DNA polymerase. In contrast to interferon-treated mouse L 929 cells, ribosomal RNA is not degraded in interferon-treated vaccinia WR-infected chick embryo fibroblasts.

Vesicular stomatitis virus P function depends on cellular growth cycle

The P function of vesicular stomatitis virus (VSV) is defined as the viral function which results in a reduced rate of total protein synthesis (viral plus cellular) arising from a nonspecific reduction in the efficiency of the translational machinery in infected cells. The existence of P function has been challenged by Lodish and Porter who were unable to detect it in L-strain mouse cells infected with wild-type VSV (HR) or, as expected, with the P- mutant, T1026-R1. Although other groups have subsequently confirmed the existence of P function and the difference between HR and T1026-R1, we have sought an explanation for the difference between Lodish and Porter's results and those of other laboratories. We show that the VSV P function depends on the phase of the growth cycle of infected L-cell cultures. In very early exponential phase, as used by Lodish and Porter, HR has very little demonstrable P function; as the growth cycle proceeds toward stationary phase, P function becomes more and more manifest. Under the same conditions, T1026-R1 shows no P function throughout the growth cycle. Furthermore we show that the VSV M protein mutant tsG31 has a P++ phenotype reducing total protein synthesis below that seen with wild-type HR. P function can be observed in cells infected with tsG31, even early in the exponential phase of the cellular growth cycle.

L929 cells infected with temperature sensitive mutants of vesicular stomatitis virus: virus replication is necessary for induction of changes in membrane permeability

Infection of L929 murine cells with vesicular stomatitis virus (VSV) results in inhibition of host protein synthesis and appearance of membrane alterations at a time when cells are still actively engaged in viral protein synthesis. VSV temperature-sensitive (ts) mutants have been used to explore the role(s) played by the virus-coded proteins in the genesis of these effects. Cells were infected with each of five ts mutants representing the known complementation groups of VSV Indiana serotype, and incubated at permissive (32 degrees C) and non-permissive temperatures (39 degrees C). Protein synthesis in the presence and absence of Hygromycin B (Hyg. B) was analyzed during virus infection via incorporation of 35S-methionine in acid-precipitable material and SDS-polyacrylamide gel electrophoresis. Data indicate that mutants belonging to groups I (L protein), II (NS protein) and IV (N protein) do not inhibit host protein synthesis and do not induce any membrane changes when grown at the non-permissive temperature. Mutants of group III (M protein) and V (G protein), instead, do inhibit cell protein synthesis and induce membrane changes also when grown at the non-permissive temperature; this suggests that these effects do not correlate with the biological activity of these proteins and their interaction with the cellular membrane. On the other hand, mutants exhibiting defective steps of nucleocapsid replication are apparently unable to induce these effects once more suggesting that virus replication per se is essential, as also indirectly shown by experiments employing cycloheximide to mimic shut-off.

Translational regulation in mouse hepatitis virus infection is not mediated by altered intracellular ion concentrations

Infection of mouse L-2 fibroblasts with mouse hepatitis virus (MHV) results in strong inhibition of host cell protein synthesis. Since it has been suggested in other virus systems that translational control is modulated by changes in the intracellular ionic environment, we investigated the possible occurrence of similar changes during MHV infection. Membrane permeability to extracellular sodium ions was measured by culturing MHV-infected cells in the presence of 22Na+. Sodium influx into MHV-infected cells rose dramatically from 4 to 6 h post-infection. This influx correlated chronologically with the expression of MHV-mediated cell fusion. Cell fusion was blocked by the addition of a monoclonal antibody against the MHV E2 glycoprotein. This addition also resulted in a reduction in the normal influx of 22Na+, suggesting that E2 expression was responsible, directly or indirectly, for the increased permeability to sodium ions in infected cells. Cultures of MHV-infected cells were labelled with [35S]methionine in the presence of medium supplemented with sodium chloride at final concentrations ranging from 150 mM to 350 mM. Incorporation of radiolabel into proteins decreased with increasing NaCl concentration; however, the ratio of viral to cellular protein synthesis remained relatively constant. Similarly, alteration of intracellular Na+ and K+ levels by treatment of infected cells with ouabain had little effect on the pattern of viral/cellular protein synthesis. Using monoclonal anti-E2 antibody to inhibit Na+ influx, we demonstrated normal inhibition of host cell protein synthesis. We therefore conclude that MHV-induced shut-off of host translation is not mediated by changes in intracellular Na+ concentrations.

Vaccinia virus and the EGF receptor: a portal for infectivity?

We previously demonstrated that occupancy of the epidermal growth factor (EGF) receptor reduced the ability of vaccinia virus to infect L cells [Eppstein et al: Nature 318:663, 1985]. This result suggested that vaccinia virus was utilizing the EGF receptor as one pathway to infect cells. We have studied this system further, and now find that antibodies to the EGF receptor also reduce the ability of vaccinia virus to infect cells productively. Inclusion of both EGF and antibodies to the EGF receptor did not cause inhibition over that obtained by EGF alone, providing another line of evidence that the antiviral effects on vaccinia virus were at the level of the EGF receptor. The antiviral effects of EGF or synthetic peptides corresponding to the third disulfide loop of TGF-alpha or the vaccinia virus growth factor were specific to vaccinia virus and did not inhibit replication of herpes simplex virus type 2 or vesicular stomatitis virus. The inhibitory effects on replication of vaccinia virus were obtained when EGF (but not insulin or growth hormone) was present prior to, but not after, productive viral adsorption. These results provided further evidence that the antivaccinia viral effects of EGF were at the level of initial receptor occupancy. As interferon (IFN) treatment has been shown to interfere with the action of some growth factors, including EGF, we examined the effects of IFN treatment of cells on the antivaccinia viral activity of EGF. Our results show that the antivaccinia effect of IFN-beta either interfered with or partially coalesced with the inhibitory effects of EGF.(ABSTRACT TRUNCATED AT 250 WORDS)

Association of reovirus proteins with the structural matrix of infected cells

The interaction of reovirus with the cytoskeleton was investigated. The soluble components of infected cells were extracted with the nonionic detergent NP-40 in a physiological buffer, and a cytoskeletal extract was prepared from the detergent-insoluble fraction. We observed a selective association of viral-specified products with the cytoskeleton that was temporally controlled. Viral dsRNA appeared first on the framework but after several hours was found also in the soluble phase, encapsidated in mature virions. The initial viral translation products were associated exclusively with the soluble fraction, but concomitant with the appearance of dsRNA, viral proteins microNS and sigma 3 were detected on the cytoskeleton. Several hours later, all viral proteins were detected on the framework. Viral polypeptide microNS exhibited unique spatial distribution patterns that correlated with viral assembly: Before dsRNA replication, it appeared as diffusely distributed protein; a few hours later, it was detected in punctate foci interconnected by tiny filaments; several hours later, it appeared as an extensive fiber network that traversed the foci. The other viral proteins were detected only within viral foci. MicroNS remained bound to the matrix fraction after treatment with DNase, Mg2+, and high salt, treatments that released other viral proteins. This distribution pattern was virus-directed because passage of virus at high multiplicity of infection induced mutations that prevented assembly of the microNS-coated filament organization. A small fraction of the viral-specified products that included polypeptide microNS, but not viral dsRNA, was coprecipitated from cytoskeletal extracts with proteins of mol wt approximately 55K by monoclonal antibodies that recognized tubulin and vimentin. Disruption of this interaction by long exposure to colchicine did not prevent association of viral proteins or RNA with the matrix, indicating that viral products were not transported through these interactions. The results indicate that reovirus morphogenesis includes temporal and spatial controls not described previously.

Intracellular human gamma-interferon triggers an antiviral state in transformed murine L cells

Interaction of human gamma-interferon (IFN-gamma) with a cell-surface receptor is known to be essential for the cell to become resistant to viral infection. Here we demonstrate that IFN-gamma, when present inside the cell, is also capable of inducing a permanent antiviral state. Mouse cells transformed with a truncated human cDNA encoding a mature IFN-gamma protein lacking the signal peptide accumulate high levels of intracellular human IFN-gamma. Not only do these cells acquire a permanent resistance to viral infection, they also exhibit all the biochemical characteristics normally observed after exposure to exogenous IFN. The observed loss of species specificity normally associated with IFN-gamma suggests that this restriction is strictly dependent on the interaction of the molecule with the cell-surface receptor.

Uncoating of parental bluetongue virus to core and subcore particles in infected L cells

A study was made of the fate of parental bluetongue virus (BTV) in infected cells. Within the first hour after infection, the BTV particles are converted to core particles with the loss of major capsid polypeptides P2 and P5. The particles are able to synthesize mRNA in vitro in a transcription reaction characterized by a temperature-dependent inhibition at high core concentrations. From about 6 hr after infection a second uncoating event is observed in which the 470 S core particles are converted to 390 S subcore particles. These particles are morphologically strikingly different from core particles and have a skeletonlike structure with a hexagonal profile and a side to side diameter of 40 nm. These subcore particles contain only one major structural protein, P3, and three minor proteins, P1, P4, and P6. They do, however, contain all 10 double-stranded RNA segments. The results suggest that the characteristic capsomeres on the surface of core particles are composed mainly of P7, the soluble group-specific antigen of BTV. The subcore particles are stable only at very low salt concentrations. Under these conditions no transcriptase activity can be demonstrated.

Ultrastructural study of mode of entry of Chlamydia psittaci into L-929 cells

The entry of Chlamydia psittaci into L-929 cells was studied morphologically by transmission electron microscopy and quantitatively by a method that discriminates between attachment and uptake. Upon adsorption of 3H-labeled elementary bodies (EBs) to host cells at 4 degrees C, the EBs bound efficiently to the L-cell surface. Binding reached an equilibrium level of 55% in 3 h. Ultrastructural analysis revealed that EBs were bound preferentially to the tips and sides of microvilli at this temperature. The EBs were also observed in coated pits located at the bases of microvilli and along smooth surfaces of the host cell. No internalization was observed at 4 degrees C. When cells with prebound 3H-labeled EBs were warmed to 37 degrees C, the EBs rapidly became resistant to proteinase K removal (half time = 5 min), indicating ingested chlamydiae. At 37 degrees C, the EBs were internalized within tightly bound vesicles surrounded by an electron-dense coat of fibrillar material. EBs were also present in smooth-surfaced pits and vesicles of the host cell. Using alpha 2-macroglobulin coupled to colloidal gold (a known marker for receptor-mediated endocytosis), we observed that the entry of EBs into cells via coated pits was identical in appearance to the internalization of alpha 2-macroglobulin. Also, when the two ligands were mixed together, they could be seen within the same coated pits and were cointernalized within endocytic vesicles of the host cell. These results suggest that C. psittaci can enter nonprofessional phagocytic cells by a pathway which is similar to that of receptor-mediated endocytosis of many physiologically important macromolecules, bacterial toxins, and viruses.

Site specificity of DNA methylation and expression of herpes simplex thymidine kinase gene

Analysis of the methylation pattern of a single-copy herpes simplex virus thymidine kinase gene integrated into the genome of mouse L cells revealed that hypomethylation of five specific AvaI sites correlates with expression of the TK gene in all of the cell lines tested. Of these specific sites, one lies 5' to the coding region, one 3' to the coding region, and three lie within the coding region of the thymidine kinase gene. Analysis of methylation at a variety of other sites using other methylation-sensitive endonucleases revealed considerable variation in the methylation patterns, apparently unrelated to gene expression and subject to variation with time in culture.

[Ultrastructural aspects of the process of Salmonella typhi bacterial interaction with L929-line cells]

The study of the use of scanning electron microscopy and the analysis of the initial stages of interaction between S. typhi and eukaryotic cells by the method of three-dimensional reconstruction has revealed that the infective agent penetrates into the cytoplasm on the principle of internalization. The internalization of S. typhi occurs with the active participation of the eukaryotic cells which, at the beginning, envelopes the bacteria with its processes, and the infective agents firmly adhere to the glycocalyx of the host cell by means of special fimbria-like formations differing from fimbriae by their lesser rigidity and thickness; then the microbes fixed to the membrane penetrate inside the cell without destroying its cytoplasmic membrane. Differences in the processes of the interaction of eukaryotic cells with S. typhi initial strain 238 and its variant free from the plasmid with a molecular weight of 6 Md, characterized by its lower capacity for association with cells of continuous cell culture L929, have been revealed. The factors stimulating the ingestion of S. typhi by eukaryotic cells are under study at present.

Translational control in murine hepatitis virus infection

High multiplicity infection of mouse fibroblast L-2 cells with mouse hepatitis virus (MHV) resulted, within 6 h, in a decline in total protein synthesis to about 7% of that observed in uninfected cells. The amount of intracellular total translatable RNA, however, increased approximately threefold, as a result of the accumulation of virus-encoded mRNAs. MHV-infected cells could be superinfected with vesicular stomatitis virus, demonstrating that MHV infection did not irreversibly alter the cellular translational machinery to the exclusion of non-MHV mRNAs. Comparative polysome analysis from MHV-infected and uninfected L-2 cells showed that MHV infection resulted in an increase in single 80S ribosomes and in a shift from longer to shorter polysomes. These observations suggest first, that MHV infection inhibits total protein synthesis at a very early stage, as evidenced by the increase in 80S ribosomes, and, second, that the increased number of viral mRNAs produced after infection compete with cellular mRNAs for cellular ribosomes. In vitro translation of RNA extracted from MHV-infected and mock-infected cells suggested that levels of cellular mRNAs were decreased after infection. This suggestion was confirmed by demonstrating the loss of cellular actin mRNA, using a radiolabelled cDNA probe, as a consequence of MHV infection.

Interferon inhibits the growth of Legionella micdadei in mouse L cells

The intracellular growth of Legionella micdadei was inhibited in mouse L cells treated with interferon (IFN). This IFN-mediated restriction was dose-dependent and required preincubation of the L cells with high doses of IFN (1,000 U/ml) for maximal inhibition. Incubation of L. micdadei with IFN alone had no detectable effect on growth of the bacteria. The IFN-mediated growth restriction was not dependent upon tryptophan concentration in the culture medium.

Studies on interactions of dTK-HSV mutants with neurons in vitro

Thymidine kinase negative (dTK-) mutants of herpes simplex virus type 1 (HSV-1) multiplied well in rat brain glioma cells. A proportion (less than 1%) of glioma cells survived the infection with HSV and were designated "survivor" glioma cells. Survivor cells of dTK- mutant virus infection ceased to produce infectious virus after two passages and were highly resistant to both HSV-1 and HSV-2 but not to vesicular stomatitis virus (VSV). Flow cytometric studies indicated morphological differences between parental and survivor glioma cells, and HSV-1 specific antigens as well as DNA were detected in the survivor glioma cells, but only in early passages. Sensitivity to superinfection with HSV appears to correlate to loss of HSV-specific viral DNA in the survivor glioma cells. Survivor glioma cells after several subcultures lost their ability to resist superinfecting HSV, reverted morphologically to the appearance of parental glioma cells and also lost significant amount of HSV-1 specific DNA. These transient survivor glioma cells became persistently infected-virus producer cells upon HSV infection.

[Cultivation of Neisseria gonorrhoeae in an L-929 cell culture]

N. gonorrhoeae strain b has been found to be capable of retaining its viability in medium 199 with 10% of inactivated cattle serum added and in monolayer cell culture L-929 in the above medium. The characteristics obtained in the present investigation permit simulating the mixed association of gonococci and chlamydiae in the culture system used in this work.

[Characteristics of rickettsial morphology during intracellular development]

The normal anatomy of rickettsiae has been characterized with the use of R. prowazekii, R. conorii and R. akari in continuous cell cultures L-929, Al, FL and in primary chick embryo fibroblast culture. Rickettsiae are short rod-shaped cells with the dense cytoplasm and the regular structure of the cell wall--cytoplasmic membrane complex. The study has shown the absence of polymorphism in rickettsiae growing under permissive conditions, but at the same time these organisms easily develop into pathological forms. Pathological forms can be detected alongside normal rickettsiae in the same cells. The classification of the pathological forms of rickettsiae is presented. In this classification the compensating (reversible) and destructive (irreversible) forms of alterations, as well as hypertrophic and dystrophic processes on the level of the whole rickettsial cell or its organelles, are pointed out.

Difference in sensitivity to interferon among mouse hepatitis viruses with high and low virulence for mice

Mouse hepatitis viruses (MHV) of different virulence for mice were studied with respect to interferon (IFN) sensitivity. The growth of low-virulent MHV-S and intermediately virulent MHV-JHM was significantly suppressed in IFN-treated L cells compared with untreated cells. However, a comparable suppression of the growth of highly virulent MHV-2 was not observed in IFN-treated cells. This differential effect of IFN treatment could also be demonstrated at the level of viral mRNA and viral proteins. In cells infected with MHV-S or MHV-JHM the amount of viral mRNAs was remarkably reduced by IFN treatment. Also the levels of the major intracellular viral proteins, in particular the E1 protein, were affected by IFN treatment. Similar effects could not be demonstrated in MHV-2-infected cells. These results suggest that during MHV-S or MHV-JHM infection IFN treatment suppresses virus replication at several stages. The significance of these results is discussed in terms of the pathogenecity of these viruses.

In vivo and in vitro models of demyelinating diseases. XII. Persistence and expression of corona JHM virus functions in RN2-2 Schwannoma cells during latency

The coronavirus JHMV persistently infects rat Schwannoma cells RN2-2 at 32.5 degrees C and enters a host-imposed reversible, latent state at 39.5 degrees C. JHMV can remain up to 20 days in the latent state and about 14 days before the cultures lose the capacity to resume virus production upon return to 32.5 degrees C. Although persistently and latently infected RN2-2 cells display resistance to superinfection by a heterologous agent VSV, these cells do not release detectable soluble mediators (e.g., interferon) of the antiviral state. Nevertheless, RN2-2 cells are competent to synthesize and release interferon when treated with the appropriate inducers. These observations suggest that interferon does not play any role or may not be the major factor in the control of latency in the Schwannoma cell. Hybridization with virus-specific cDNAs shows that all viral mRNAs are present during latency and that viral mRNAs are present in the polysomes of infected cells at 39.5 degrees C. Western immunoblotting with hybridoma antibodies demonstrates that viral specific proteins are produced at the restrictive temperature. These results suggest that despite the absence of production of infectious virus at 39.5 degrees C, there is active transcription and translation into virus-specified products.

Flavivirus infection enhancement in macrophages: an electron microscopic study of viral cellular entry

The mode of entry of West Nile virus (WNV) into the macrophage-like cell line P388D1 was investigated at the electron microscopical level using synchronized infections. The presence of the antiviral monoclonal antibody F6/16A at a concentration that enhanced viral attachment to P388D1 cells ninefold made no difference to the entry pathway of WNV. In both the absence and presence of F6/16A the initial uptake of single viral particles was mediated by coated pits, and started within 30 s of warming the cells to 37 degrees C. Viral particles later appeared in fully or partially coated vesicles and later in uncoated prelysosomal endocytic vacuoles before degradation in lysosomes. However, aggregates of viral particles (five or more virus particles in cross-section), appeared to be phagocytosed whole by cells in a process which involved aggregates being engulfed by extensions of the plasma membrane. This process exhibited a slower time course than the uptake of single viral particles, becoming prominent 15 to 30 min after warming the cells to 37 degrees C. The involvement of a prelysosomal vacuolar compartment in the entry process was shown by a failure to stain for acid phosphatase. This compartment could be specifically loaded with viral particles when viral internalization occurred at 20 degrees C in the presence of 50 mM-ammonium chloride.

[Morphological characteristics of the brain lesions in mice infected with a homogenate of L cells latently infected with the scrapie agent]

Degeneration of neurons of the Ammon horn lower branch both in the early and terminal stages of the disease of mice infected with the homogenate of L cells latently infected with the scrapie agent (the L-S system) was frequently detected alongside with brain lesions typical of slow infections (vacuolation). Examinations of chromosomes in metaphase plates of L-S cells carried out by several methods including the TAC system for texture analysis of the image (Leutz, BRD) revealed three marker chromosomes new for continuous L cells, the appearance of true chromatid translocations as well as significant changes in chromosome numbers. Besides, ultrastructural features of L-S cells at later stages of cultivation were revealed. It is assumed that the active effect of the scrapie agent on L cells infected with it resulted in the emergency of a new antigen capable to induce selective affection of the neurons of the Ammon horn lower branch in susceptible mice.

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.

In vitro studies with the scrapie agent

Persistent infection with the scrapie agent has been established in L cells. The agent was propagated in homogenates of the cell line designated L-S. The L-S cells showed slower growth rate and morphological changes like pycnosis of nuclei and vacuolization of their cytoplasm. Cytogenetic analysis revealed rearrangement of chromosomes in L-S cells. In the course of passaging the number of cells with the characteristic marker chromosome decreased. Along with this cells were found with deletion of one arm of the marker chromosome. In addition, 3 new marker chromosomes were detected in infected cells, suggesting the influence of the scrapie agent on cytogenetic processes in scrapie-carrier cultures. The infectious activity of nucleic acids isolated from L-S cells was determined in BALB/c mice inoculated with untreated, DNase-treated and pronase-treated nucleic acid preparations. A slightly decreased infectious activity has been noted after DNase and pronase treatments.