Morphological and immunocytochemical characterisation of mixed glial cell cultures derived from neonatal canine brain

N-(3-Cyanophenyl)-2-phenylacetamide, an effective inhibitor of morbillivirus-induced membrane fusion with low cytotoxicity

Based on the structural similarity of viral fusion proteins within the family Paramyxoviridae, we tested recently described and newly synthesized acetanilide derivatives for their capacity to inhibit measles virus (MV)-, canine distemper virus (CDV)- and Nipah virus (NiV)-induced membrane fusion. We found that N-(3-cyanophenyl)-2-phenylacetamide (compound 1) has a high capacity to inhibit MV- and CDV-induced (IC(50) μM), but not NiV-induced, membrane fusion. This compound is of outstanding interest because it can be easily synthesized and its cytotoxicity is low [50 % cytotoxic concentration (CC(50)) ≥ 300 μM], leading to a CC(50)/IC(50) ratio of approximately 100. In addition, primary human peripheral blood lymphocytes and primary dog brain cell cultures (DBC) also tolerate high concentrations of compound 1. Infection of human PBMC with recombinant wild-type MV is inhibited by an IC(50) of approximately 20 μM. The cell-to-cell spread of recombinant wild-type CDV in persistently infected DBC can be nearly completely inhibited by compound 1 at 50 μM, indicating that the virus spread between brain cells is dependent on the activity of the viral fusion protein. Our findings demonstrate that this compound is a most applicable inhibitor of morbillivirus-induced membrane fusion in tissue culture experiments including highly sensitive primary cells.

In vitro characterization and preferential infection by canine distemper virus of glial precursors with Schwann cell characteristics from adult canine brain

AIMS

Canine distemper virus (CDV)-induced demyelinating leukoencephalomyelitis is a naturally occurring model for multiple sclerosis. The aim of this study was to establish primary glial cell cultures from adult canine brain for the analysis of CDV spread and cell tropism.

METHODS

Cultures were inoculated with the CDV-R252 and a CDV-Onderstepoort strain expressing the green fluorescent protein (CDV-OndeGFP). CDV antigen expression was studied using cell type-specific antibodies at different days post infection. Glial cells expressing p75(NTR) were purified using antibody-based techniques and characterized with regard to antigen expression and proliferation.

RESULTS

Three weeks after seeding, cultures contained spindle-shaped cells expressing p75(NTR), oligodendrocytic cells, astrocytes, microglia and fibroblasts. Both CDV strains induced a mild to moderate cytopathic effect that consisted of single necrotic and few syncytial giant cells, but displayed in part a differential cell tropism. Whereas CDV-OndeGFP expression in microglia and astrocytes did not exceed 1% and 50%, respectively, CDV-R252 infected 100% and 80% of both cell types, respectively. The cells most early infected by both CDV strains expressed p75(NTR) and may correlate to cells previously identified as aldynoglia. Treatment of p75(NTR+) cells with Schwann cell mitogens and serum deprivation increased proliferation and A2B5 expression, respectively, indicating common properties compared with Schwann cells and oligodendrocyte precursors.

CONCLUSIONS

Infection of adult canine astrocytes and microglia revealed CDV strain-specific cell tropism. Moreover, this is the first identification of a glial cell type with Schwann cell-like properties in adult canine brain and, more importantly, these cells displayed a high susceptibility to CDV infection.

Determinants of persistence in canine distemper viruses

Viral persistence in the central nervous system is the driving force behind the chronic progressive disease caused by natural canine distemper virus (CDV) infection in dogs. Persistence of CDV is associated with non-cytolytic spread and impaired viral budding. Since budding is to a large extend dependent on the nucleocapsid-(N) and matrixproteins (M) of the virus, we analyzed the nucleotide- and deduced amino acid sequences of the corresponding genes of a spectrum of CDV strains, that differ with respect to virulence and persistence in vivo and in vitro. The wild type CDV (A75/17), which is capable of causing a persistent infection in vivo was compared to two tissue culture adapted CDV strains (passaged A75/17-CDV and Rockborn-CDV), which CDV strains, that differ with respect to virulence and persistence in vivo and in vitro. The wild type CDV (A75/17), which is capable of causing a persistent infection in vivo was compared to two tissue culture adapted CDV strains (passaged A75/17-CDV and Rockborn-CDV), which retain a residual virulence and the capacity to spontaneously persist in vitro. A modified distemper virus (Snyder Hill-CDV), which is neurovirulent but not capable of causing a persistent infection in vivo, and an avianized virus (Onderstepoort-CDV) which is completely apathogenic and spreads by budding in cell cultures were also examined. Differences were found in the C-terminal of the nucleocapsid protein, which--comparing the two extremes of the spectrum (wild A75/17-CDV and OP-CDV)--lead to changes of the predicted protein structure. Such changes could affect the budding process and thus play a role in persistence. Marked changes in the M-gene were found in its non-coding region: the nucleotide sequences of the SH-CDV and OP-CDV differed considerably from the other three strains. Moreover, an additional second open reading frame was detected in the 'non-coding' region of the M gene in the wild A75-CDV, the two tissue culture adapted CDV strains and SH-CDV, but not in OP-CDV. The presence of this additional open reading frame correlated with the ability to cause a spontaneous persistent infection in vitro. Our findings support the notion that both N- and M-genes of CDV harbor determinants of viral persistence.

Restricted infection with canine distemper virus leads to down-regulation of myelin gene transcription in cultured oligodendrocytes

Canine distemper virus (CDV) induces oligodendroglial degeneration and multifocal demyelination in the central nervous system. The mechanism of oligodendrocyte degeneration is not understood but it has been shown that there is a restricted infection of these cells without viral protein production. Using a combination of immunocytochemistry and in situ hybridization we were able to demonstrate the transcription of the entire virus genome throughout the whole observation period (7-35 days after infection) in oligodendrocytes in CDV-infected brain cell cultures. Therefore, the lack of viral protein and particle production can not be explained on the basis of a defective viral transcription. The present study also shows that a restricted infection of oligodendrocytes with CDV down-regulates the transcription of the major myelin genes coding for proteolipid protein, myelin basic protein (MBP) and myelin-associated glycoprotein in a very similar way. Using densitometry for in situ hybridization products of MBP in populations of normal and infected oligodendrocytes, an effect could be observed long before morphological changes were detectable. The present results strongly suggest that demyelination in distemper is induced by a restricted CDV infection of oligodendrocytes which down-regulates the expression of a variety of cellular genes, in particular those coding for myelin proteins. Consequently, the infected cells are no longer able to synthesize all the membrane compounds which are necessary for maintaining their structural integrity.

Canine distemper virus persistence in the nervous system is associated with noncytolytic selective virus spread

Canine distemper virus (CDV), a negative-strand RNA morbillivirus, causes a progressive demyelinating disease in which virus persistence plays an essential role. The antiviral immune response leads to virus clearance in the inflammatory lesions. However, CDV can replicate and persist outside these inflammatory lesions within the brain. How CDV is capable of persisting in the presence of an effective antiviral immune response is poorly understood. In the present investigation, we studied several aspects of virus replication in primary dog brain cell cultures (DBCC), comparing an attenuated CDV strain and a virulent CDV strain. Confluent DBCC were infected with either virulent A75/17-CDV or attenuated Onderstepoort-CDV and monitored for 60 days. Persistence was not associated with defective virus production, because all mRNAs and corresponding proteins were continuously expressed in the noncytolytic infection. Quantitative measurements did not detect a difference between the two types of infection in the rate of virus transcription and protein synthesis at the level of the single cell. However, electron microscopy and virus titration experiments showed that in the persistent CDV infection virus budding is strongly limited compared with that of the attenuated virus. Morphometry and immunocytochemistry showed profound differences in the way the two viruses spread in the culture. The attenuated CDV spread randomly to immediately adjacent cells, whereas persistent CDV spread selectively to more-distant cells by way of cell processes. In conclusion, the present study supports a mechanism of CDV persistence through selective spread by way of cell processes, enabling virulent CDV to invade the central nervous system without the need of releasing much virus into the extracellular space.

A chemiluminescence assay to detect antibodies to brain surface antigens in human sera

A chemiluminescence assay was developed based on the interaction between antibodies binding to the surface of living brain cells in culture and macrophages. Such interaction leads to production of reactive oxygen radicals which can be measured by a chemiluminescence assay. This assay was used to detect anti brain antibodies in serum samples from humans with various neurological diseases. Such antibody activity was found in a high proportion of these patients. Subsequent experiments with purified IgG fractions and corresponding F(ab')2 fragments showed that the observed reactions were highly specific. It was concluded that the chemiluminescence assay is a sensitive and useful technique to detect autoantibodies in neurological diseases.

Identification of canine glial cells by nonradioactive in situ hybridization

Studies on the development of the canine central nervous system and on demyelinating diseases demand unequivocal identification of the glial cells. For that reason, nonradioactive in situ hybridization (ISH) was performed in primary dog brain cell cultures (DBCC) and in brain sections of neonatal dogs. Specific RNA probes were used to detect messenger RNA (mRNA) coding for proteolipid protein (PLP), myelin basic protein (MBP) and glial fibrillary acidic protein (GFAP). PLP and MBP are markers for oligodendrocytes, GFAP for astrocytes. Oligodendrocytes positive for PLP and MBP mRNA were found in both DBCC and brain sections of neonatal dogs. Astrocytes expressing GFAP specific mRNA were detected in DBCC and in brain sections. These cells were evenly distributed in the white matter with additional accumulation in the membrana limitans gliae superficialis, around the ventricles and blood vessels. ISH clearly improves the study of oligodendrocytes in brain sections as, in contrast to the immunohistochemical methods, this technique allows to identify individual cells.

Loss of virulence of canine distemper virus is associated with a structural change recognized by a monoclonal antibody

The monoclonal antibody (mAB) L1, which binds to the nucleocapsid protein of canine distemper virus (CDV), was shown to bind to avirulent CDV obtained after serial passages in Vero cells, but not to two different virulent demyelinating CDV-strains propagated in dog glial cell cultures. However, when both virulent CDV-strains were passaged through Vero cells they expressed, after a number of passages, an epitope recognized by mAB L1. The occurrence of the L1 epitope appeared to coincide with loss of virulence in animal inoculation experiments.

Virulent and attenuated canine distemper virus infects multiple dog brain cell types in vitro

Canine Distemper Virus (CDV) produces an encephalitis in dogs that varies with viral strain. We have studied the cell tropisms of two virulent strains (CDV-SH and CDV A75-17) and an attenuated strain, Rockborn (CDV-RO), in cultured canine brain cells. Infected cell types were identified by double immunofluorescent labeling of specific cell markers and viral antigens. All viral strains studied produced infection in astrocytes, fibroblasts, and macrophages. Neurons were not infected by CDV A75-17 but were rapidly infected by CDV-SH and CDV-RO. Multipolar oligodendrocytes were very rarely infected by any of the virus strains. In contrast, a morphologically distinct subset of bipolar oligodendrocytes were commonly infected by CDV-SH and CDV-RO. The kinetics of infection in the astrocytes, oligodendrocytes, neurons, and macrophages varied between strains. Both CDV-SH and CDV-RO rapidly infected bipolar oligodendrocytes, astrocytes, neurons, and macrophages by 14 days post infection while infection by CDV A75-17 was delayed until after 28-35 days post infection. The differences in the growth kinetics and cell tropisms for some brain cells, exhibited by the three viral strains examined in this in vitro study, may relate to the different CNS symptoms that these strains produce in vivo.

Ultrastructural and biochemical findings in brain cell cultures infected with canine distemper virus

To study the pathomechanism of demyelination in canine distemper (CD), dog brain cell cultures were infected with virulent A75/17-CD virus (CDV) and examined ultrastructurally. Special attention was paid to the oligodendrocytes, which were specifically immunolabelled. In addition, cerebroside sulfotransferase (CST), an enzyme specific for oligodendrocyte activity was assayed during the course of the infection. Infection and maturation as well as CDV-induced changes were found in astrocytes and brain macrophages. Infection of oligodendrocytes was rarely seen, although CST activity of the culture markedly decreased and vacuolar degeneration of these cells occurred, resulting in their complete disappearance. We concluded that the degeneration of oligodendrocytes and demyelination is not due to direct virus-oligodendrocyte interaction, but due to CDV-induced events in other glial cells.

Selective degeneration of oligodendrocytes mediated by reactive oxygen species

The mechanism underlying demyelination in inflammatory canine distemper encephalitis is uncertain. Macrophages and their secretory products are thought to play an important effector role in this lesion. Recently, we have shown that anti-canine distemper virus antibodies, known to occur in chronic inflammatory lesions, stimulate macrophages leading to the secretion of reactive oxygen species (ROS). To investigate whether ROS could be involved in demyelination, dog glial cell cultures were exposed to xanthine/xanthine oxidase (X/XO), a system capable of generating O2-. This treatment resulted in a specific time-dependent degeneration and loss of oligodendrocytes, the myelin producing cells of the central nervous system. Initial degeneration was not associated with a decrease in viability of oligodendrocytes as judged by trypan blue and propidium iodide exclusion. Astrocytes and brain macrophages were not affected morphologically by this treatment. Further, an evaluation of the effect of several ROS scavengers, transition metal chelators and inhibitors of poly (ADP-ribose) polymerase suggests that a metal dependent formation of .OH or a similar highly oxidizing species could be responsible for the observed selective damage to oligodendrocytes.

Antibody-induced generation of reactive oxygen radicals by brain macrophages in canine distemper encephalitis: a mechanism for bystander demyelination

The mechanism of inflammatory demyelination in canine distemper encephalitis (CDE) is uncertain but macrophages are thought to play an important effector role in this lesion. Serum and cerebrospinal fluid (CSF), containing anti-canine distemper virus and anti-myelin antibodies from dogs with CDE were tested for their ability to generate reactive oxygen species (ROS) in macrophages in primary dog brain cell cultures using a chemiluminescence (CL) assay. The majority of serum samples and several CSF samples from animals with inflammatory demyelination elicited a CL signal in infected dog brain cell cultures. In contrast, none of these samples induced a positive response in uninfected cultures which contained large numbers of myelin antigen-presenting cells, although defined anti-myelin antibodies lead to a marked secretion of ROS in this system. It was concluded that antiviral antibody-induced secretion of ROS, known to be highly toxic for brain tissue, may play an important role in white matter damage in inflammatory lesions supporting a previous hypothesis of bystander demyelination in CDE. No evidence was found for a similar antibody-dependent cellular cytotoxicity-like mechanism mediated by anti-myelin antibodies in CDE, which does not support the concept of autoimmunity in this disease.

Antiviral antibodies stimulate production of reactive oxygen species in cultured canine brain cells infected with canine distemper virus

Canine distemper is characterized mainly by respiratory, enteric, and nervous symptoms. Infection of the central nervous system results in demyelination, to which inflammation has been shown to contribute significantly. It has been proposed that macrophages play a major role as effector cells in this process. We report that cultured dog brain cells contain a population of macrophages capable of producing reactive oxygen species as measured by luminol-dependent chemiluminescence. In cultures infected with canine distemper virus, a burst of reactive oxygen is triggered by antiviral antibody. This response depends on the presence of viral antigens on the surfaces of infected cells and is mediated by the interaction of antigen-bound antibody with Fc receptors on the macrophages. Since there is no evidence in vitro or in vivo that oligodendrocytes, the cells forming myelin, are infected, our observation supports the hypothesis that "innocent bystander killing" is important in demyelination caused by canine distemper virus. Reactive oxygen species released from macrophages may contribute to destruction of myelin.

Demyelinating, non-demyelinating and attenuated canine distemper virus strains induce oligodendroglial cytolysis in vitro

A virulent canine distemper virus (CDV) strain that causes demyelination in vivo has been shown to induce oligodendroglial degeneration in vitro. In order to investigate if this effect on oligodendrocytes is specific for demyelinating strains only, primary brain cell cultures were infected with either virulent demyelinating strains (A75/17 and CH84-CDV), a virulent non-demyelinating strain (SH-CDV) or a non-virulent strain (OP-CDV). All virulent viruses caused a persistent type infection with moderate cytolysis whereas the non-virulent strain was highly cytolytic. All strains induced a similar pattern of oligodendroglial degeneration. It was concluded that the ability to induce oligodendroglial degeneration, which is thought to be the in vitro correlate of demyelination in vivo, is inherent to CDV irrespective of the strain. The discrepancy between biological behaviour of CDV strains in brain cell cultures and in vivo can be explained by the more complex virus-cell interactions in vivo than in vitro.

Oligodendroglial pathology in canine distemper virus infection in vitro

Dog brain cell cultures were infected with different canine distemper virus (CDV) strains to study the oligodendrocytes, which were characterized with eight different antibodies to cover the whole oligodendroglial population in the culture. A few weeks after infection all oligodendroglial cell types started to degenerate and disappeared from the culture. However, since no CDV protein could be demonstrated in the degenerating oligodendrocytes with extensive double-labelling studies, this lesion can not be explained as being a result of cytolytic infection. This conclusion was further supported in experiments with plaque-forming CDV, in which viral replication is restricted to the cytolytic areas only; oligodendrocytes also degenerated in virus-free areas between the plaques. The hypothesis of toxic factors released by other infected cell types in the culture leading to secondary damage of the oligodendrocyte could not be confirmed by transferring supernatants from infected to normal cultures. Whereas the presence of toxic factors can not be completely excluded, the possibility of an abortive infection of the oligodendrocytes with no or very limited viral protein synthesis is discussed.

Cell and tissue culture of the central nervous system: recent developments and current applications

A survey of methods for cell and tissue culture of the central nervous system (CNS) is given. This includes a brief historical outline and description of methods in current use. Recent methodological improvements are emphasized, and it is shown how these are applied in modern neurobiological research. Both monolayer cell cultures and three-dimensional organ culture systems are widely used, each having advantages and limitations. In recent years, there has been considerable improvement of culture for prolonged periods in chemically defined media. Brain tissue from a wide spectrum of species have been used, including different types of human brain cells which can be propagated for several months. At present, these culture systems are employed for dynamic studies of the developing, the adult and ageing brain. It is possible to select neurons and the different classes of glial cells for culture purposes. Cell culture of the CNS has given new insights into the biology of brain tumours. Culture systems for experimental tumour therapy in vitro are also available. Recently, it has been shown that organ cultures of brain tissue can be used as targets for invasive glioma cells, enabling a direct study of the interactions between tumour cells and normal tissue to take place.

A monoclonal antibody that binds to both astrocytes and myelin sheaths

A monoclonal antibody designated III 5H8 was shown to bind both to astrocytes and to myelin sheaths as studied with immunocytochemical techniques on brain sections and cell cultures. Binding to astrocytes was confirmed by double immunofluorescent labelling of frozen sections and cell cultures with anti-GFAP, and appeared to be sensitive to formalin treatment. Binding to myelin sheaths was confirmed by comparing sections labelled with III 5H8 with sections labelled with antibodies against axons and myelin basic protein as well as by staining of sections of hypomyelinated spinal cord with III 5H8. On immunoblots of separated white matter III 5H8 revealed two bands, while on immunoblots of purified myelin only one band was seen. The findings are discussed with respect to the function of astrocytes in white matter and shared antigenic determinants between astrocytes and oligodendrocytes.

Canine distemper virus does not infect oligodendrocytes in vitro

Dissociated canine brain cell cultures were infected with virulent canine distemper virus (CDV). Double immunofluorescent labelling was done to simultaneously demonstrate viral antigen and specific glial cell markers. Virus containing oligodendrocytes were not found at any stage of the infection. A certain proportion of the infected cells were shown to be astrocytes. It was concluded that CDV has no obvious tropism for oligodendrocytes which could explain the mechanism of demyelination in distemper in vivo.