Replication of Borna disease virus in cell cultures

Borna disease virus and schizophrenia

The development of a new serological assay method to detect antibodies in human sera recognizing Borna disease virus (BDV) proteins and a clinical pilot study are presented. Psychiatric patients from a schizophrenia research clinic in Baltimore, Maryland, were examined for antibodies to BDV antigen with traditional indirect immunofluorescence assays (IFA) that used both single and double labeling techniques and also with a Western blot assay capable of detecting antibodies to the three BDV proteins from a human neuroblastoma cell line. Thirteen of 90 (14.4%) patients and 0/20 control subjects had antibodies that recognized more than one BDV protein on the Western blot. Three patients had antibodies that recognized all three BDV proteins. Magnetic resonance imaging assessments of the volume of the putamen (with controls for total cranial volume) differentiated BDV+ from BDV- patients, and there were trend differences for bilateral amygdalae and the left amygdala-hippocampal process. We conclude that: (1) the Western blot assay is superior to IFA assays in BDV serology studies, (2) detection of antibodies to more than one BDV protein is a useful working criterion for seropositivity, (3) the 14.5 kDa BDV protein is 10 times more predictive of seropositivity than either the 38/40 kDa or the 24 kDa protein, (4) there is tentative evidence for a schizophrenia-control difference in the prevalence of anti-BDV antibodies, and (5) it is likely that there are neuroanatomical/behavioral features that differentiate seropositive from seronegative schizophrenic patients.

Induction of protection against Borna disease by inoculation with high-dose-attenuated Borna disease virus

Borna disease is a chronic neurological disease caused by an enveloped negative-strand RNA virus (BDV). Experimental disease can be reproduced in rats with brain homogenates derived from infected animals or with virus derived from infected cells in culture. The virus replicates in cultured cells without evidence of cytopathic effect or production of significant levels of cell-free virus. Borna disease is caused by an immunopathological response to viral infection of neural cells. To further investigate the pathogenesis of Borna disease, rats were inoculated with different doses of BDV attenuated by culture in MDCK cells. Low doses of attenuated BDV (10(2)-10(4) TCID50) resulted in typical clinical disease and severe encephalitis; however, the lag period between inoculation and disease was considerably longer than that with virulent BDV. In contrast, animals inoculated with a high dose of attenuated BDV (10(5)-10(6) TCID50) did not develop clinical disease, although a mild encephalitic response was present that did not progress beyond the mild encephalitis. Animals inoculated with a high dose of BDV developed high titers of anti-BDV antibody and were protected against virulent challenge. Protection was correlated with the rapid induction of an immune response in the animals and the lack of any biologically detectable virus in the CNS.

Sequence analyses of the p24 gene of Borna disease virus in naturally infected horse, donkey and sheep

By reverse transcriptase/PCR amplification and subsequent sequence determination of the p24 gene, the relatedness of Borna disease virus (BDV) in various naturally infected animal species was determined. These results are indicative of a common ancestral virus pool and a remarkably low species barrier of BDV. Comparison of 11 sequences to that of tissue culture adapted virus revealed that the homology among all isolates was at least 96.2% at the nucleotide level, and 97% at the amino acid level. Viral sequences from sheep, donkey and horse were found to be not more distantly related to each other than sequences from different infected horses. Tissue-specific virus variants were detected in one horse: the sequences established from infected cerebrum and kidney showed 10 mutations, whereas sequences obtained from parotid gland contained 20 mutations in comparison to the nucleotide sequence of MDCK cell adapted BDV.

Characterization of a Borna disease virus glycoprotein, gp18

Borna disease virus is a nonsegmented negative-strand RNA virus that causes neurologic disease in a wide variety of animal hosts. Here we describe identification and characterization of the first glycoprotein in this viral system. The 18-kDa glycoprotein, gp18, has been purified from infected rat brain. Isolation and microsequencing of this protein allowed identification of a 16.2-kDa open reading frame in the viral antigenome. Lectin binding and endoglycosidase sensitivity assays indicate that gp18 is an unusual N-linked glycoprotein.

Virus-like particles in MDCK cells persistently infected with Borna disease virus

A line of Madin Darby canine kidney (MDCK) cells persistently infected with Borna disease virus was examined by electron microscopy. Thin sections revealed the presence of intracytoplasmic virus-like particles ranging from 50-100 nm in diameter. Nuclei of the infected cells exhibited accumulation of electron-dense granular structures 15-18 nm in diameter. The intracytoplasmic particles were roughly spherical with a limiting membrane, suggesting the presence of a lipid-containing envelope. The internal structure consisted of strand-like material which in some cases was condensed underneath the envelope. The possible relationship of these particles to Borna disease virions is discussed.

Borna disease virus-specific T cells protect against or cause immunopathological Borna disease

In this report we show that passive immunization of Lewis rats with viable CD4+, Borna disease virus (BDV)-specific T cells before infection with BDV resulted in protection against BD, whereas inoculation of these T cells after BDV infection induced clinical disease with more rapid onset than seen in BDV control animals. The protective as well as encephalitogenic effector functions of BDV-specific CD4+ T cells were mediated only by viable BDV-specific T cells. The protective situation was obtained by passive transfer of BDV-specific T cells into animals inoculated later with virus, whereas the immunopathological situation was observed when virus-specific T cells developed normally or after adoptive transfer, and appeared on the scene after considerable virus replication in the brain.

Borna disease virus (BDV), a nonsegmented RNA virus, replicates in the nuclei of infected cells where infectious BDV ribonucleoproteins are present

Borna disease virus (BDV) causes neurological disease in a wide range of animal species, providing an important model for studies of persistent viral infection of the central nervous system. In addition, the detection of antibodies that react with BDV antigen in serum from psychiatric patients suggests a role for BDV, or related viruses, in human mental disorders, providing further reason for study of this poorly characterized neurotropic virus. We present evidence that BDV has a nonsegmented negative single-strand RNA genome with the property that viral replication and transcription take place in the nuclei of infected cells where infectious BDV ribonucleoproteins are present. Our results support the view that BDV has unique biological features among animal viruses. Furthermore, the finding that BDV ribonucleoproteins are able to infect susceptible cells raises interesting questions regarding the mechanisms by which some neurotropic viruses may spread through the central nervous system of the infected host without requiring the production of mature infectious virus.

Sequence and genome organization of Borna disease virus

We have previously demonstrated that Borna disease virus (BDV) has a negative nonsegmented single-stranded (NNS) RNA genome that replicates in the nucleus of infected cells. Here we report for the first time the cloning and complete sequence of the BDV genome. Our results revealed that BDV has a genomic organization similar to that of other members of the Mononegavirales order. We have identified five main open reading frames (ORFs). The largest ORF, V, is located closest to the 5' end in the BDV genome and, on the basis of strong homology with other NNS-RNA virus polymerases, is a member of the L-protein family. The intercistronic regions vary in length and nucleotide composition and contain putative transcriptional start and stop signals. BDV untranslated 3' and 5' RNA sequences resemble those of other NNS-RNA viruses. Using a set of overlapping probes across the BDV genome, we identified nine in vivo synthesized species of polyadenylated subgenomic RNAs complementary to the negative-strand RNA genome, including monocistronic transcripts corresponding to ORFs I, II, and IV, as well as six polycistronic polyadenylated BDV RNAs. Interestingly, although ORFs III and V were detected within polycistronic transcripts, their corresponding monocistronic transcripts were not detected. Our data indicate that BDV is a member of the Mononegavirales, specially related to the family Rhabdoviridae. However, in contrast to the rest of the NNS-RNA animal viruses, BDV replication and transcription occur in the nucleus of infected cells. These findings suggest a possible relationship between BDV and the plant rhabdoviruses, which also replicate and transcribe in the nucleus.

Sequence conservation in field and experimental isolates of Borna disease virus

Coding and noncoding sequences were analyzed from field and experimental isolates of Borna disease virus. For a 24-kDa protein, maximum divergence was 1.5% at the predicted amino acid level and 3.1% at the nucleotide level. For a 40-kDa protein, maximum divergence was 1.1% at the predicted amino acid level and 4.1% at the nucleotide level. The highest variability in sequence (10%) was found in a 40-nucleotide stretch of genomic RNA between coding sequences for the 40- and 24-kDa proteins. The degree of sequence conservation in these isolates, passaged in various host species in vivo and in vitro over a period of 64 years, is unusual for negative-strand RNA viruses.

Demonstration of Borna disease virus-specific RNA in secretions of naturally infected horses by the polymerase chain reaction

The presence of Borna disease virus (BDV)-specific RNA was traced by the reverse transcriptase-polymerase chain reaction in conjunctival fluid, nasal secretions and saliva of horses which were seropositive but did not have any history of clinical Borna disease. Positive reactions encompassed sequences encoding the p24 BDV-specific protein. Virus specificity of the amplified product was confirmed by hybridization with the respective oligomer probe. Viral infectivity or virus-specific antigen was not found in any of these secretions by conventional assays in cell culture and immunoblotting.

T cell memory specific for self and non-self antigens in rats persistently infected with Borna disease virus

We have studied CD4+ Th1 T cell responses in Borna disease (BD), a virus-mediated immune disease of the central nervous system (CNS), and demonstrate the priming of virus-specific as well as autoreactive T cells specific for myelin antigens in the course of viral infection. The fate of these in vivo generated T cells was subsequently assessed by in vitro proliferation assays with lymphocytes from different lymphoid organs of diseased animals over a long period of time. Virus-specific T cell responses continuously decreased during the establishment of persistent infection and could no longer be detected after 5-6 months post infectionem, when inflammatory reactions in the brain had ceased. By contrast, autoantigen-specific T cells kept their ability to mount characteristic secondary responses--although at an overall rather low level--over long periods of time; these autoreactive T cells homed to a specific lymphoid organ, the perithymic lymph node. Our study thus describes for the first time a complete decline of virus-specific T cell memory in a persistent viral infection, and raises the question how long-lasting T cell autoreactivity is controlled.

Lysis of major histocompatibility complex class I-bearing cells in Borna disease virus-induced degenerative encephalopathy

CD8+ as well as CD4+ T cells and macrophages are of crucial importance for the pathogenesis of Borna disease in rats. This virus-induced immunopathological disease of the brain is characterized by neurological symptoms in the acute phase and chronic debility associated with severe loss of brain tissue in the late stage. We demonstrate here the cytotoxic activity of T lymphocytes in the brain of intracerebrally infected rats. T cells isolated from the brain of infected rats lyse major histocompatibility complex (MHC) class I-bearing target cells in the absence of MHC class II. Borna disease virus (BDV)-infected syngeneic skin cells and astrocytes, the latter one of the relevant target cells in vivo, were significantly lysed whereas infected allogeneic target cells were not. Most relevant to the in vivo situation, primary brain cell cultures propagated from the hippocampus of BDV-infected rats containing considerable numbers of neurons were lysed in vitro. Blocking experiments using antibodies directed against MHC class I antigen provided further evidence for the presence and activity of classical cytotoxic T lymphocytes. Antibodies against MHC class II antigen did not influence lysis of skin target cells but had an effect on lysis of astrocytes at late time points. Lymphocytes isolated from spleen, peripheral blood, or lymph nodes did not show cytotoxic activity. These results verify, on the cellular level, earlier findings that strongly suggest the involvement of CD8+ T cells in brain cell lesions, resulting in brain atrophy long after infection of rats with BDV. This is further evidenced by the presence of CD8+ T cells in direct proximity to neuronal cell lesions. Interestingly, the cytolytic capacity, demonstrated in vitro and strongly correlated to organ destruction, does not result in elimination of the virus but the virus persists in the central nervous system.

Borna disease virus in peripheral blood mononuclear and bone marrow cells of neonatally and chronically infected rats

Borna disease virus (BDV) establishes a persistent infection in cells of the nervous system in rats. The response, or lack thereof, of the immune system to BDV infection of neurons is responsible for the presence or absence, respectively, of Borna disease. We recently demonstrated transmission of BDV by bone marrow cells from neonatally infected rats. Our findings suggested the possibility of a heretofore unsuspected interaction between BDV and the immune system, that of direct effects of BDV infection on the cells of the immune system. This report enlarges upon the previous findings and confirms the presence of BDV RNA in bone marrow cells of neonatally infected rats, using a reverse transcription-polymerization chain reaction-enzyme immunosorbent assay (RT-PCR-EIA). In addition, we detected BDV RNA in peripheral blood mononuclear cells of neonatally infected rats, and in rats inoculated as adults in the chronic, but not the acute, stage of infection. In addition, the RT-PCR-EIA technique identified BDV RNA in cerebrospinal fluid, nasal secretions, saliva, urine and stool. BDV-sequences were not detected in the plasma of infected animals nor in the body fluids and tissues of normal rats.

Characterization of a glial cell line persistently infected with borna disease virus (BDV): influence of neurotrophic factors on BDV protein and RNA expression

Borna disease virus (BDV) infects cells of the nervous system in a wide range of species. Previous work suggests that there are differences in BDV replication in neuronal cells and glial cells. Many neurons are lysed by the immunopathologic response to BDV; lysis of dentate gyrus neurons in the absence of encephalitis is seen in rats inoculated with BDV as neonates. In contrast, persistently BDV-infected astrocytes increase over the course of BDV infection. Therefore, we compared BDV replication in neuronal (SK-N-SH and SK-N-SHEP) and astrocytic (C6) cell lines. While SK-N-SH cells produced more infectious virions per cell, the C6 cells contained more BDV proteins and RNA. BDV sequences in the supernatants of both cell types were identified, despite low titers of infectious virus, suggesting the release of incomplete virions into the medium. C6 cells secreted a factor or factors into the medium that enhanced the production of BDV proteins and RNA in other cell lines. In addition, nerve growth factor treatment produced the same enhancement. Thus, BDV replication in certain neural cells in vitro may be linked to the production of cell-specific factors which affect viral replication.

Brain cell lesions in Borna disease are mediated by T cells

Experimental Borna Disease (BD) in rats is characterized by severe lymphocytic encephalitis and by massive brain cell lesions finally leading to brain atrophy. Treatment of BDV-infected rats with monoclonal antibodies directed against CD4+ and CD8+ T cells could almost completely inhibit the immunopathological reactions and revealed less BDV-infected neurons and astrocytes that expressed MHC class I antigen. Brain cell lesions were minimal, and no obvious brain atrophy could be observed even late after infection. Since BDV itself is not known to exert cytopathic effects and since brain cell damage was independent of antibody titers, brain cell destruction correlates well with the intracerebral presence of CD8+ T cells and the expression of MHC class I antigens. Moreover, BDV-infected brain cells in vitro could be demonstrated to be lysed in a MHC class I-restricted manner. These findings provide evidence that virus-infected neurons can be destructed by T cell mediated cytotoxicity which results in organ atrophy and dementia.

Analysis of virus-specific RNA species and proteins in Freon-113 preparations of the Borna disease virus

Treatment of homogenates from Borna disease virus (BDV)-infected brain tissue or cell cultures with Freon-113 yielded infectious particles with a buoyant density of 1.16-1.22 g/ml. Positive- and negative-stranded BDV-specific RNA species as well as three virus-specific proteins, known to be present in BDV-infected cell extracts, were demonstrated in these Freon-treated fractions. When the Freon-purified virus preparations were treated with RNase A prior to RNA extraction, only negative-stranded, genomic RNA was detected in Northern blot hybridizations using sense and antisense RNA probes. These data substantiate that BDV is a negative-stranded RNA virus.

Pathogenesis of Borna disease

Borna disease represents a unique model of a virus-induced immunological disease of the brain. Naturally occurring in horses and sheep, the mechanisms of pathogenesis have been studied in experimental animals, namely in the rat. Many investigations have revealed that the infection of the natural hosts principally follows the same pathogenic pathways as observed in rats, leading to a severe encephalomyelitis. This affliction of the central nervous system results in severe neurological disorders that again, are fully comparable in laboratory animals to those in the natural and the different experimental hosts. In addition, alterations have been reported which are also based on the infection of the brain and do not result in the classical encephalitic clinical picture but rather in alterations of behavior. However, to all of our knowledge, the various clinical pictures of Borna disease are not caused by the infecting virus itself but rather by the hosts immune response towards it, i.e. by a virus-induced cell-mediated immunopathological reaction. The importance of virus-specific CD4+ T cells as exemplified by a cultured T cell line and of CD8+ T cells as shown by immunomodulatory substances and specific antibody treatment in vivo for the pathogenesis of acute Borna disease will be elucidated here. In addition, evidence will be provided that virus-specific CD8+ T cells are also responsible for the dramatic brain atrophy in the chronic phase of the disease in rats. Therefore, Borna disease not only lends itself exquisitely well to the study of the pathogenesis of an immunopathological disease of the brain but also represents one of the few models for immune-mediated tissue destruction that eventually leads to brain atrophy and clinically to dementia.

Preventive effects of early anti-CD4 or anti-CD8 treatment on Borna disease in rats

Borna disease is a virus-induced, immunopathological encephalomyelitis in which CD4+ cells and macrophages dominate the pathological picture. However, significant numbers of CD8+ cells have been morphologically identified in perivascular infiltrates as well. To determine the contribution of different T-cell subsets to the pathogenesis of Borna disease, virus-infected rats were treated with monoclonal antibodies specific for CD4+ and CD8+ cells. Both types of monoclonal antibodies were able to significantly decrease or even prevent the local inflammatory reaction in the brain if given early during the infection. However, CD8-specific monoclonal antibodies appeared to be more effective than antibodies directed against CD4+ cells. Treatment initiated 4 days postinfection did not result in inhibition of encephalitis and disease. Virus titers in the brain of infected rats treated with T-cell-specific antibodies did not differ from titers in untreated infected control animals. The results indicate an important functional role of CD8+ cells, in addition to CD4+ cells, in the pathogenesis of Borna disease.

Borna disease virus-infected astrocytes function in vitro as antigen-presenting and target cells for virus-specific CD4-bearing lymphocytes

Astrocytes isolated from the brain of newborn Lewis rats and an astrocytic cell line were susceptible to infection with the neurotropic Borna disease virus in vitro. Since astrocytes also have been found to be infected in vivo it seemed appropriate to test this cell type for interaction with a Borna disease virus-specific CD4⁺ T cell line. Borna disease virus-infected astrocytes were found to be capable of presenting virus-specific antigen to virus-specific T cells in vitro. However, the response was significantly enhanced if the purified 38/39 kDa Borna disease virus-specific protein was added exogenously to the cultures. Beside the function as antigen-presenting cells for various antigens including virus-specific protein and myelin basic protein, persistently infected astrocytes were also found to act as target cells for a CD4⁺ T cell line as shown in conventional⁵¹Cr release assays after induction of MHC class II expression by gamma interferon. Infection of astrocytes alone did not cause expression of this self antigen. It could be shown that the ability of CD4⁺ BDV-specific T cells to mediate lysis was in part dependent on the stage of activation. Lymphocytes “activated” before testing exerted high lysis after only 4h of coincubation with target cells, whereas “resting” T cells did not cause significant lysis until 12h of coincubation. The dependence of the interaction between effector and target cells on MHC class II antigen was demonstrated by the finding that antibodies to Ia antigens reduced lysis of target cells.

Borna disease, a possible hazard for man?

Evidence is presented that Borna disease (BD) virus, which is known to cause encephalopathy in horses, sheep, and a broad range of experimental animals, or a related agent, can infect man and may induce mental disorders. BD virus-specific antibodies could be demonstrated in 4-7% of sera (depending on origin) from more than 5000 psychiatric or neurological patients from Germany, U.S.A. and Japan. Antibodies from seropositive patients reacted with a BD virus-specific protein translated by RNAs which were transcribed from a cDNA clone obtained from BD virus-infected tissues. When the cerebrospinal fluid from three seropositive patients was inoculated into rabbits or rabbit embryonic brain cell cultures, evidence was obtained that suggests the presence of BD virus or a related agent.

Atypical dissemination of the highly neurotropic Borna disease virus during persistent infection in cyclosporine A-treated, immunosuppressed rats

In adult rats infected with Borna disease virus, the virus was found exclusively in the brain, whereas in cyclosporine A-treated rats, infectious virus was also detected in peripheral nerve fibers and, unexpectedly, in adjacent organ-specific cells. In contrast to untreated virus-infected rats, no major histocompatibility complex class II expression was found in the brain of cyclosporine A-treated animals.

Molecular characterization of the Borna disease agent

Borna disease (BD) is a neurologic syndrome characterized by profound disturbances in behavior and the accumulation of specific antigens in limbic system neurons. The potency of brain homogenates from animals with BD to cause disease in normal animals is reduced by exposure to detergents. We have recently described isolation and characterization of clones derived from the BD agent. Here we present evidence that suggests that the BD agent is a negative-sense, single-strand RNA virus. The 8.5-kb genome of this virus appears to be associated with nuclei and encodes two major RNA transcripts of 2.1 and 0.8 kb.

Determination of immune cells and expression of major histocompatibility complex class II antigen in encephalitic lesions of experimental Borna disease

After intracerebral infection with Borna disease virus adult Lewis rats develop a virus-induced immunopathological reaction resulting in severe neurological symptoms and a non-purulent meningoencephalitis. The composition of inflammatory cells and major histocompatibility complex (MHC) class II antigen expression during the course of the infection was investigated using immunocytochemistry with a panel of monoclonal antibodies (mAb). Macrophages and lymphocytes of the T helper phenotype (CD4+) were dominant at all stages of infection, whereas T suppressor/cytotoxic lymphocytes (CD8+) were less frequent. B lymphocytes and plasma cells occurred mainly during later stages of the disease and marked parenchymal deposition of immunoglobulin developed. Beginning 10 days after infection massive expression of MHC class II antigen was noted up to the termination of experiments 70 days after infection. Besides lymphatic cells and macrophages, cells morphologically resembling microglia expressed this antigen. Furthermore, ependymal cells were found positive for MHC class II expression during infection whereas astrocytes remained negative. These findings are consistent with previous results which provide evidence for a delayed-type hypersensitivity reaction being operative in the pathogenesis of Borna disease.

Borna disease, a progressive meningoencephalomyelitis as a model for CD4+ T cell-mediated immunopathology in the brain

A homogeneous T cell line NM1 with Borna disease (BD) virus reactivity could be established. The NM1 cells have been characterized as CD4+ T cells. Adoptive transfer revealed that this MHC class II-restricted immune cell is responsible for the immunopathological effect leading to BD, a progressive meningoencephalomyelitis.

Immune-mediated pathogenesis of Borna disease

Borna disease is an endemic progressive encephalomyelitis of horses and sheep prevalent in central Europe. A wide variety of animal species, ranging from chickens to primates can be infected experimentally with the causative virus, which is only poorly characterized. Furthermore, BD virus-specific antibodies have been detected in sera and cerebrospinal fluids of psychiatric patients. Our studies on the pathogenesis of BD have shown that-at least in rats-the disease is not caused by the infecting virus itself, but by a virus-induced immunopathological reaction. Thus, after intracerebral infection immunoincompetent rats do not get the disease despite persistent virus replication in cells of the central nervous system. However, after adoptive transfer of immune cells from diseased rats, immunoincompetent rats exhibit full-blown BD. Recently, we have been successful in establishing a virus-specific T cell line of the helper/inducer phenotype (CD4+). This T cell was shown to play an important role in the pathogenesis of BD, suggesting that the disease is caused by a delayed type hypersensitivity reaction.

Axonal transport of Borna disease virus along olfactory pathways in spontaneously and experimentally infected rats

In this study it has been shown that infection of mother rats by Borna disease virus (BDV) from infected newborns led to a fatal disease. This differed both in clinical symptoms and in histological alterations from the form of the disease which occurred after intracerebral (i.c.) infection. Both parameters were, however, similar to those seen after experimental intranasal (i.n.) infection of adult rats. Detailed immunohistological studies showed clearly that after experimental i.n. infection, the infecting virus migrates intraaxonally from the neuroreceptors in the olfactory epithelium into the brain. It is therefore suggested that i.n. transmission is an important route of natural BDV infection.

Pathogenesis of Borna disease in rats: evidence that intra-axonal spread is the major route for virus dissemination and the determinant for disease incubation

Borna disease virus is an uncharacterized agent that causes sporadic but fatal neurological disease in horses and sheep in Europe. Studies of the infection in rats have shown that the agent has a strict tropism for neural tissues, in which it persists indefinitely. Inoculated rats developed encephalitis after an incubation period of 17 to 90 days. This report shows that the incubation period is the time required for transport of the agent in dendritic-axonal processes from the site of inoculation to the hippocampus. The immune responses to the agent had no effect on replication or transport of the virus. The neural conduit to the brain was proven by intranasal inoculation of virus that resulted in rapid transport of the agent via olfactory nerves to the hippocampus and in development of disease in 20 days. Virus inoculation into the feet resulted in spread along nerve fibers from neuron to neuron. There was sequential replication in neurons of the dorsal root ganglia adjacent to the lumbar spinal cord, the gracilis nucleus in the medulla, and pyramidal cells in the cerebral cortex, followed by infection of the hippocampal neurons and onset of disease. This progression required 50 to 60 days. The exclusiveness of the neural conduit was proven by failure to cause infection after injection of the virus intravenously or into the feet of neurectomized rats.

Detection of serum antibodies to Borna disease virus in patients with psychiatric disorders

Borna disease virus causes a rare meningoencephalitis in horses and sheep and has been shown to produce behavioral effects in some species. The possibility that the Borna virus is associated with mental disorders in humans was evaluated by examining serum samples from 979 psychiatric patients and 200 normal volunteers for the presence of Borna virus-specific antibodies. Antibodies were detected by the indirect immunofluorescence focus assay. Antibodies to the virus were demonstrated in 16 of the patients but none of the normal volunteers. The patients with the positive serum samples were characterized by having histories of affective disorders, particularly of a cyclic nature. Further studies are needed to define the possible involvement of Borna virus in human psychiatric disturbances.

Increase of virus yields and releases of Borna disease virus from persistently infected cells

Borna disease virus grows to low titres in persistently infected cells with an infectious particle to cell ratio of 0.01 to 0.05. Inclusion of n-butyrate in the growth medium enhances infectivity yields up to 1 log. This effect is time and concentration dependent. In hypertonic medium with an excess of NaCl, KCl or Na2SO4 up to 50% of the total infectious virus yield is released from the cells. Released supernatant virus (buoyant density in sucrose rho = 1.22 g/cm3) is more heat stabile than cell-bound virus (rho = 1.18 g/cm3). The access to cell-free (released) virus opens new possibilities for the characterization of this neurotropic agent.

Replication of Borna disease virus in rats: age-dependent differences in tissue distribution

There are age-dependent differences in the tissue distribution of Borna disease (BD) virus in rats infected intracerebrally. While in adult rats BD virus replication is restricted to neural cells, in neonatally infected rats infectious virus or viral antigens were found in the cells of most organs. The possibility that differences in the immune status between newborn and adult animals are responsible for different tissue susceptibility could be excluded.

Borna disease of horses. An immunohistological and virological study of naturally infected animals

The brains of eight horses that had suffered from natural Borna disease were examined with virologic, immunohistological, and electron-microscopic methods. All brains harbored infectious virus as shown by inoculation of experimental animals. Regional assessment of the infectivity exhibited the highest titers in the hippocampus and piriform cortex and the lowest in the cerebellum. Conventional histology yielded pathologic alterations very similar to those of the classical description of the disease. Immunohistology demonstrated the highest amounts of Borna disease virus-specific antigen in the nuclei of neurons. In the perikarya, dendrites, and axons smaller amounts of antigen were found. A comparison of the antigen distribution with the inflammatory reaction established a high concordance of these two parameters. The presence of virus-specific antigen seems to trigger the exsudation of inflammatory cells, which reflect the extension of the infectious process. Heavy inflammatory exsudates in the white matter underlying diseased cortical areas can be explained by the axonal presence of virus-specific antigen. Virus particles could not be demonstrated with the electron microscope. The most significant findings at the ultrastructural level were stacks of fine filaments, adhering closely to cytoplasmic cisterns. These structures might be related to virus components or might be involved in virus morphogenesis.

Behavioral disease in rats caused by immunopathological responses to persistent borna virus in the brain

Borna virus replicated persistently in the brains of rats, causing frenzied and apathetic behavioral states in sequence but no mortality. The transient frenzied behavior was caused by an immune-mediated, cytolytic, encephalitic response that was unexpectedly self-limiting. Cessation of active pathological processes coincided with the onset of the passive phase of the disease. This study thus demonstrates suppression of virus-specific inflammation despite continuous viral replication and describes a new mechanism by which chronic encephalitis may become established.