OBSERVATIONS ON THE GROWTH OF MOUSE HEPATITIS VIRUS (MHV-3) IN MOUSE MACROPHAGES

Effect of chloroquine on lysosomes and on growth of mouse hepatitis virus (MHV-3)

After a 6-hour treatment with chloroquine, infected mouse peritoneal macrophages produce markedly less mouse hepatitis virus than untreated cells. Macrophages maintained in culture for 72 hours after the treatment produce a higher virus yield. Cytochemical investigations immediately and 3 days after chloroquine treatment show an increased permeability of the lysosomes of the latter. Variations of the enzymes escaping from the lysosomes are thought to be responsible for differences in virus uncoating and consequent virus yield.

How viruses may overcome non-specific defences in the host

The pathogenicity of a virus is mainly related to its capacity to overcome the non-specific defences of a host. Fever limits the multiplication of viruses in vivo , thus allowing recovery; thermoresistant strains are virulent because they are capable of escaping the effect of hyperthermia. Resistance to the antiviral effect of interferon does not seem to exist per se but interferon may play an indirect role in virus virulence: (i) its effect may be minimized by the destruction of interferon-producing cells such as macrophages; (ii) interferon may render virus-infected target cells resistant to natural killer cell lysis. The interaction of viruses with phagocytic cells plays a predominant role; the ability to grow in macrophages or to induce macrophage blockade or paralysis may constitute a marker of virulence. This is particularly important in the liver in which the infection of Kupffer cells often represents the key event of the disease.

The influence of in vivo pretreatment of cyclophosphamide on phagocytic activity of mouse macrophages in vitro

The action of cyclophosphamide on phagocytosis by macrophages from mouse peritoneal exudate cultured in vitro was studied. The mice were pretreated with cyclophosphamide administered for seven days. The experiments were carried out in the presence of serum (with or without complement) from untreated animals or without serum. A significantly increased phagocytic activity was shown in experiments without serum and when decomplemented serum was used. No significant variation was found when macrophages from treated mice were tested with serum with complement from untreated animals. It is tentatively assumed that low dosage administration in vivo of cyclophosphamide does not affect macrophage phagocytic activity while several findings suggest an inhibitory effect of cyclophosphamide on T and B lymphocytes.

Bone-marrow derived macrophages as targets for the replication of mouse hepatitis virus type 3

Bone-marrow (BM) derived macrophages are sensitive target cells for replication of mouse hepatitis virus type 3 (MHV3). These cells can be grown in large numbers and the percentage of defined macrophages increased until day 10 when 100% of the cells represented macrophages. MHV3 replicated within these cells to high titers and caused the formation of multi-nucleated giant cells. This effect was seen with very low virus inocula in BM macrophages of C57BL/6 mice that are highly susceptible to in vivo infection with MHV3 whereas macrophages from resistant A/J mice did not show a cytopathic effect at these virus doses. 1000-fold higher virus doses, however, caused the cytopathic effect in macrophages of both C57BL/6 and A/J mice.

The molecular biology of coronaviruses

Coronaviruses have recently emerged as an important group of animal and human pathogens that share a distinctive replicative cycle. Some of the unique characteristics in the replication of coronaviruses include generation of a 3' coterminal-nested set of five or six subgenomic mRNAs, each of which appears to direct the synthesis of one protein. Two virus-specific RNA polymerase activities have been identified. Many of the distinctive features of coronavirus infection and coronavirus-induced diseases may result from the properties of the two coronavirus glycoproteins. The intracellular budding site, which may be important in the establishment and maintenance of persistent infections, appears to be due to the restricted intracytoplasmic migration of the E1 glycoprotein, which acts as a matrix-like transmembrane glycoprotein. E1 also exhibits distinctive behavior by self-aggregating on heating at 100°C in sodium dodecyl sulfate (SDS) and by its interaction with RNA in the viral nucleocapsid. The E1 of mouse hepatitis virus (MHV) is an O -linked glycoprotein, unlike most other viral glycoproteins. Thus, the coronavirus system may be a useful model for the study of synthesis, glycosylation, and transport of O -linked cellular glycoproteins.

The replication of murine coronaviruses in enucleated cells

Coronaviruses JHMV and A59V have been shown to replicate, produce viral-specific antigens and cytopathic effects (CPE) in enucleated 17CL-1 cells.

Primary monolayer culture of adult mouse hepatocytes -- a model for the study of hepatotropic viruses

Primary monolayer cultures of adult mouse hepatocytes isolated by collagenase perfusion of the liver in situ were exposed to 2 hepatotropic viruses, an avian influenza A virus adapted to grow in mouse liver in vivo and a herpes simplex type I virus. Influenza virus infection led to lysis ofindividual hepatocytes and total monolayer destruction within 18 to 120 hours after infection according to the virus dose used. Virus replication was evidenced by assaying hepatocyte supernates for hemagglutinin and infectivity, by immunofluorescent staining and by electron microscopy. Herpes virus infection resulted in polykaryocyte formation followed by nuclear pycnosis and cell lysis. Virus replication was assayed by titration of supernate infectivity.

Mouse hepatitis virus strain 3 infection of C57, A/Sn and A/J strain mice and their macrophages. Brief report

Mouse hepatitis virus strain 3 replicated in C57, A/Sn and A/J strain mouse macrophages with the production of a clear cytopathic effect, although only C57 and A/Sn strains of mice were killed with similar MHV3 dilutions. We could not confirm a previous report showing that in vitro cultured macrophages from A/J strain mice were resistant to MHV3 infection.

HBsAg uptake by macrophages in vitro: an immunofluorescence study

The uptake of HBsAg by in vitro cultured macrophages was studied by immunofluorescence method. Intracytoplasmic fluorescent particles appeared 3 h after the contact with HBsAg-positive serum, while after 24-48 h only a few cells contained these particles, which are probably destroyed within the cytoplasm.

Effect of pretreatment with prednisolone on the phagocytic activity of mouse peritoneal macrophages in vitro

The phagocytic activity on in vitro cultured mouse peritoneal macrophages derived from animals treated with 6-alpha-methyl-prednisolone was examined. The statistical evaluation of results showed an increase of phagocytic activity of macrophages derived from treated animals in comparison with controls.

Physico-chemical properties of mouse hepatitis virus (MHV-2) grown on DBT cell culture

Some properties of a strain of mouse hepatitis virus, MHV-2, grown on DBT cells were determined using a plaque assay on the cells. Viral growth was not inhibited by the presence of actinomycin D or 5-iodo-2-deoxyuridine. MHV-2 was completely inactivated by ether, chloroform, sodium deoxycholate or beta-propiolactone, but showed a moderate resistance to trypsin. Heating at 56 C for 30 min did not completely abolish the virus infectivity. The virus was stable after heating at 50 C for 15 min in 1M-MgCl2 or 1M-MgSO4 as well as at 37 C for 60 min at pH 3.0 to 9.0. Infectivity was decreased to 1/100 and 1/400 after storing at 4 C for 30 days and 37 C for 24 hr, respectively. The virus passed through a 200-nm but not a 50-nm Sartorius membrane filter. The buoyant density of MHV-2 was 1.183 g/cm3 in sucrose gradient, and the fraction contained coronavirus-like particles measuring 70 to 130 nm in diameter. Survival rate was 10% after exposure to ultraviolet at 150 ergs/mm2. Freezing and thawing or sonication at 20 kc for 3 min did not affect the virus titer. No hemagglutinin was demonstrable with red blood cells of the chicken, Japanese quail, mouse, rat, hamster, guinea pig, sheep, bovine or human.

In vitro growth characteristics and heterogeneity of mouse hepatitis virus type 3

The in vitro virus yield of MHV3 reached 10(7) PFU/ml in mouse DBT cells infected with a virus suspension in HEPES-buffered medium containing DEAE-dextran. The virus titer was 10(6) PFU/ml in the presence of 10 micrograms actinomycin D/ml MHV3 growth in DBT cells gave three peaks of density (1.10--1.14 g/cm3, 1.18--1.20 g/cm3, and 1.25--1.31 g/cm3) in sucrose gradients. All these peaks retained infectivity.

Regeneration of oligodendroglia during recovery from demyelinating disease

Infection of mice with the JHM strain of mouse hepatitis virus causes demyelination as a result of a cytolytic infection of oligodendroglia. In recovery, animals show remyelination, which could result either from surviving oligodendrocytes extending their territory or by generation of new oligodendroglia. Electron microscopic autoradiographic studies with 3H-labeled thymidine demonstrate that the cells associated with remyelination are newly generated oligodendroglia.

Correlation of persistent mouse hepatitis virus (MHV-3) infection with its effect on mouse macrophage cultures

MHV 3 has three distinct effects in different strains of mice: strain A mice are completely resistant, most strains (including C57BL, DBA/2, BALB/c and NZB strains) die of acute hepatitis whereas in certain strains (eg. C3H and A2G) the virus produces a persistent infection with neurological symptoms. In cultures of peritoneal macrophages from susceptible strains, MHV-3 replicated freely, with giant cell formation. No replication was observed in macrophages from strain A mice. In contrast to this full susceptibility or resistance, macrophage cultures from strains of mice in which persistent infections occur showed an intermediate susceptibility, as judged by the intensity of the cytopathic effect, the presence of viral antigens in the cytoplasm and levels of viral replication. Possible ways in which the intermediate susceptibility of macrophages and persistent infections might be related are discussed.

An improved method for titration of mouse hepatitis virus type 3 in a mouse cell culture

Plaque assay in DBT cells with DEAE-dextran and trypsin presents a titration system for MHV3 as sensitive as the LD₅₀ assay in mice.

Murine hepatitis virus: effect on liver RNA

After infection of mice with hepatitis virus MHV3, the RNA in the liver undergoes changes. The fraction extracted with phenol at 0 degrees C does not alter. However, the fraction extracted with hot phenol at elevated pH (60 degrees C, pH 8.3) shows a 16S peak on sucrose-density-gradient centrifugation. This fraction shows actinomycin D-resistant incorporation of C(14)-orotic acid in infected but not in control livers-possible evidence of the RNA nature of MHV3.

Morphogenesis of avian infectious bronchitis virus and a related human virus (strain 229E)

Avian infectious bronchitis virus (IBV) and strain 229E, a virus recently recovered from patients with colds, have been shown to possess a similar distinctive morphology in negatively stained preparations. An electron microscopic study of the morphogenesis of IBV in the chorioallantoic membrane and of strain 229E in WI-38 cells was performed. In infected cells, round electron-dense particles 82 mmu in diameter were observed to form by a process of budding from membranes of the endoplasmic reticulum and cytoplasmic vesicles. The particles in IBV-infected cells were similar in size and shape to those in strain 229E-infected cells but showed certain differences in internal structure. The evidence that the particles represent virions and the implications of these findings in the classification of this virus group are discussed.