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

Borna disease virus 1 infection in alpacas: Comparison of pathological lesions and viral distribution to other dead-end hosts

Borna disease is a progressive meningoencephalitis caused by spillover of the Borna disease virus 1 (BoDV-1) to horses and sheep and has gained attention due to its zoonotic potential. New World camelids are also highly susceptible to the disease; however, a comprehensive description of the pathological lesions and viral distribution is lacking for these hosts. Here, the authors describe the distribution and severity of inflammatory lesions in alpacas (n = 6) naturally affected by this disease in comparison to horses (n = 8) as known spillover hosts. In addition, the tissue and cellular distribution of the BoDV-1 was determined via immunohistochemistry and immunofluorescence. A predominant lymphocytic meningoencephalitis was diagnosed in all animals with differences regarding the severity of lesions. Alpacas and horses with a shorter disease duration showed more prominent lesions in the cerebrum and at the transition of the nervous to the glandular part of the pituitary gland, as compared to animals with longer disease progression. In both species, viral antigen was almost exclusively restricted to cells of the central and peripheral nervous systems, with the notable exception of virus-infected glandular cells of the Pars intermedia of the pituitary gland. Alpacas likely represent dead-end hosts similar to horses and other spillover hosts of BoDV-1.

Interactions of Equine Viruses with the Host Kinase Machinery and Implications for One Health and Human Disease

Zoonotic pathogens that are vector-transmitted have and continue to contribute to several emerging infections globally. In recent years, spillover events of such zoonotic pathogens have increased in frequency as a result of direct contact with livestock, wildlife, and urbanization, forcing animals from their natural habitats. Equines serve as reservoir hosts for vector-transmitted zoonotic viruses that are also capable of infecting humans and causing disease. From a One Health perspective, equine viruses, therefore, pose major concerns for periodic outbreaks globally. Several equine viruses have spread out of their indigenous regions, such as West Nile virus (WNV) and equine encephalitis viruses (EEVs), making them of paramount concern to public health. Viruses have evolved many mechanisms to support the establishment of productive infection and to avoid host defense mechanisms, including promoting or decreasing inflammatory responses and regulating host machinery for protein synthesis. Viral interactions with the host enzymatic machinery, specifically kinases, can support the viral infectious process and downplay innate immune mechanisms, cumulatively leading to a more severe course of the disease. In this review, we will focus on how select equine viruses interact with host kinases to support viral multiplication.

Transcriptome Analysis of Duck and Chicken Brains Infected with Aquatic Bird Bornavirus-1 (ABBV-1)

Aquatic bird bornavirus 1 (ABBV-1) is a neurotropic virus that infects waterfowls, resulting in persistent infection. Experimental infection showed that both Muscovy ducks and chickens support persistent ABBV-1 infection in the central nervous system (CNS), up to 12 weeks post-infection (wpi), without the development of clinical disease. The aim of the present study was to describe the transcriptomic profiles in the brains of experimentally infected Muscovy ducks and chickens infected with ABBV-1 at 4 and 12 wpi. Transcribed RNA was sequenced by next-generation sequencing and analyzed by principal component analysis (PCA) and differential gene expression. The functional annotation of differentially expressed genes was evaluated by gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis. The PCA showed that the infected ducks sampled at both 4 and 12 wpi clustered separately from the controls, while only the samples from the chickens at 12 wpi, but not at 4 wpi, formed a separate cluster. In the ducks, more genes were differentially expressed at 4 wpi than 12 wpi, and the majority of the highly differentially expressed genes (DEG) were upregulated. On the other hand, the infected chickens had fewer DEGs at 4 wpi than at 12 wpi, and the majority of those with high numbers of DEGs were downregulated at 4 wpi and upregulated at 12 wpi. The functional annotation showed that the most enriched GO terms were immune-associated in both species; however, the terms associated with the innate immune response were predominantly enriched in the ducks, whereas the chickens had enrichment of both the innate and adaptive immune response. Immune-associated pathways were also enriched according to the KEGG pathway analysis in both species. Overall, the transcriptomic analysis of the duck and chicken brains showed that the main biological responses to ABBV-1 infection were immune-associated and corresponded with the levels of inflammation in the CNS.

Immunophenotype of the inflammatory response in the central and enteric nervous systems of cockatiels (Nymphicus hollandicus) experimentally infected with parrot bornavirus 2

Proventricular dilatation disease is a lethal disease of psittacine birds. In this study, we characterized the local cellular immune response in the brain, proventriculus, and small intestine of 27 cockatiels (Nymphicus hollandicus) experimentally infected with parrot bornavirus 2 (PaBV-2). Perivascular cuffs in the brain were composed of CD3+ T-lymphocytes and Iba1+ macrophages/microglia in most cockatiels (n = 26). In the ganglia of the proventriculus, CD3+ T-lymphocytes (n = 17) and Iba1+ macrophages (n = 13) prevailed. The ganglia of the small intestine had a more homogeneous distribution of these leukocytes, including PAX5+ B-lymphocytes (n = 9), CD3+ T-lymphocytes (n = 8), and Iba1+ macrophages (n = 8). Our results indicate that perivascular cuffs in the brain and the inflammatory infiltrate in the proventriculus of PaBV-2-infected cockatiels is predominately composed of T-lymphocytes, while the inflammatory infiltrates in the ganglia of the small intestine are characterized by a mixed infiltrate composed of T-lymphocytes, B-lymphocytes, and macrophages.

Update on immunopathology of bornavirus infections in humans and animals

Knowledge on bornaviruses has expanded tremendously during the last decade through detection of novel bornaviruses and endogenous bornavirus-like elements in many eukaryote genomes, as well as by confirmation of insectivores as reservoir species for classical Borna disease virus 1 (BoDV-1). The most intriguing finding was the demonstration of the zoonotic potential of lethal human bornavirus infections caused by a novel bornavirus of different squirrel species (variegated squirrel 1 bornavirus, VSBV-1) and by BoDV-1 known as the causative agent for the classical Borna disease in horses and sheep. Whereas a T cell-mediated immunopathology has already been confirmed as key disease mechanism for infection with BoDV-1 by experimental studies in rodents, the underlying pathomechanisms remain less clear for human bornavirus infections, infection with other bornaviruses or infection of reservoir species. Thus, an overview of current knowledge on the pathogenesis of bornavirus infections focusing on immunopathology is given.

Central Nervous System Infection with Borna Disease Virus Causes Kynurenine Pathway Dysregulation and Neurotoxic Quinolinic Acid Production

Central nervous system infection of neonatal and adult rats with Borna disease virus (BDV) results in neuronal destruction and behavioral abnormalities with differential immune-mediated involvement. Neuroactive metabolites generated from the kynurenine pathway of tryptophan degradation have been implicated in several human neurodegenerative disorders. Here, we report that brain expression of key enzymes in the kynurenine pathway are significantly, but differentially, altered in neonatal and adult rats with BDV infection. Gene expression analysis of rat brains following neonatal infection showed increased expression of kynurenine amino transferase II (KATII) and kynurenine-3-monooxygenase (KMO) enzymes. Additionally, indoleamine 2,3-dioxygenase (IDO) expression was only modestly increased in a brain region- and time-dependent manner in neonatally infected rats; however, its expression was highly increased in adult infected rats. The most dramatic impact on gene expression was seen for KMO, whose activity promotes the production of neurotoxic quinolinic acid. KMO expression was persistently elevated in brain regions of both newborn and adult BDV-infected rats, with increases reaching up to 86-fold. KMO protein levels were increased in neonatally infected rats and colocalized with neurons, the primary target cells of BDV infection. Furthermore, quinolinic acid was elevated in neonatally infected rat brains. We further demonstrate increased expression of KATII and KMO, but not IDO, in vitro in BDV-infected C6 astroglioma cells. Our results suggest that BDV directly impacts the kynurenine pathway, an effect that may be exacerbated by inflammatory responses in immunocompetent hosts. Thus, experimental models of BDV infection may provide new tools for discriminating virus-mediated from immune-mediated impacts on the kynurenine pathway and their relative contribution to neurodegeneration.IMPORTANCE BDV causes persistent, noncytopathic infection in vitro yet still elicits widespread neurodegeneration of infected neurons in both immunoincompetent and immunocompetent hosts. Here, we show that BDV infection induces expression of key enzymes of the kynurenine pathway in brains of newborn and adult infected rats and cultured astroglioma cells, shunting tryptophan degradation toward the production of neurotoxic quinolinic acid. Thus, our findings newly implicate this metabolic pathway in BDV-induced neurodegeneration. Given the importance of the kynurenine pathway in a wide range of human infections and neurodegenerative and neuropsychiatric disorders, animal models of BDV infection may serve as important tools for contrasting direct viral and indirect antiviral immune-mediated impacts on kynurenine pathway dysregulation and the ensuing neurodevelopmental and neuropathological consequences.

Neurons are MHC class I-dependent targets for CD8 T cells upon neurotropic viral infection

Following infection of the central nervous system (CNS), the immune system is faced with the challenge of eliminating the pathogen without causing significant damage to neurons, which have limited capacities of renewal. In particular, it was thought that neurons were protected from direct attack by cytotoxic T lymphocytes (CTL) because they do not express major histocompatibility class I (MHC I) molecules, at least at steady state. To date, most of our current knowledge on the specifics of neuron-CTL interaction is based on studies artificially inducing MHC I expression on neurons, loading them with exogenous peptide and applying CTL clones or lines often differentiated in culture. Thus, much remains to be uncovered regarding the modalities of the interaction between infected neurons and antiviral CD8 T cells in the course of a natural disease. Here, we used the model of neuroinflammation caused by neurotropic Borna disease virus (BDV), in which virus-specific CTL have been demonstrated as the main immune effectors triggering disease. We tested the pathogenic properties of brain-isolated CD8 T cells against pure neuronal cultures infected with BDV. We observed that BDV infection of cortical neurons triggered a significant up regulation of MHC I molecules, rendering them susceptible to recognition by antiviral CTL, freshly isolated from the brains of acutely infected rats. Using real-time imaging, we analyzed the spatio-temporal relationships between neurons and CTL. Brain-isolated CTL exhibited a reduced mobility and established stable contacts with BDV-infected neurons, in an antigen- and MHC-dependent manner. This interaction induced rapid morphological changes of the neurons, without immediate killing or impairment of electrical activity. Early signs of neuronal apoptosis were detected only hours after this initial contact. Thus, our results show that infected neurons can be recognized efficiently by brain-isolated antiviral CD8 T cells and uncover the unusual modalities of CTL-induced neuronal damage.

When a virus infects the brain, it is important to quickly block viral replication without causing excessive damage to neurons, which are not easily renewed. Cytotoxic T lymphocytes (CTL) are one of the main actors for virus elimination. However, the question of whether CTL are indeed capable of destroying infected neurons remains controversial. For this work, we analyzed the characteristics of interactions between infected neurons and CTL using neurotropic Borna disease virus (BDV). This virus infects neurons and triggers severe inflammation in the brain. We isolated CTL directly from the brains of rats infected with BDV and analyzed their interaction with primary cultures of neurons. Using live-cell fluorescence microscopy, we observed that CTL were arrested upon encounter with infected neurons and that they established stable contacts with them. Thereafter, infected neurons exhibited rapid changes in permeability but remained alive and electrically active for several hours, before ultimately being destroyed. Our study shows that neurons can indeed be recognized by CTL, an important observation for a better understanding of the physiopathology of virus-induced brain inflammation. In addition, it reveals that neurons are relatively resistant to CTL-induced killing, which may open a window of opportunity for new treatments.

A synthetic cannabinoid agonist promotes oligodendrogliogenesis during viral encephalitis in rats

Chronic CNS infection by several families of viruses can produce deficits in prefrontal cortex (PFC) and striatal function. Cannabinoid drugs have been long known for their anti-inflammatory properties and their ability to modulate adult neuro and gliogenesis. Therefore, we explored the effects of systemic administration of the cannabinoid agonist WIN55,212-2(WIN) on prefrontal cortex (PFC) and striatal cytogenesis in a viral model of CNS injury and inflammation based on Borna Disease (BD) virus encephalitis. Active BrdU(+) progenitor populations were significantly decreased 1 week after BrdU labeling in BD rats [p<0.001 compared to uninfected (NL) controls] while less than 5% of BrdU(+) cells colabeled for BDV protein. Systemic WIN (1mg/kg i.p. twice daily×7 days) increased the survival of BrdU(+) cells in striatum (p<0.001) and PFC of BD rats, with differential regulation of labeled oligodendroglia precursors vs microglia/macrophages. WIN increased the percentage of BrdU(+) oligodendrocyte precursor cells and decreased BrdU(+) ED-1-labeled phagocytic cells, without producing pro- or antiviral effects. BDV infection decreased the levels of the endocannabinoid anandamide (AEA) in striatum (p<0.05 compared to NL rats), whereas 2-AG levels were unchanged. Our findings indicate that: 1) viral infection is accompanied by alterations of AEA transmission in the striatum, but new cell protection by WIN appears independent of its effect on endocannabinoid levels; and 2) chronic WIN treatment alters the gliogenic cascades associated with CNS injury, promoting oligodendrocyte survival. Limiting reactive gliogenesis and macrophage activity in favor of oliogodendroglia development has significance for demyelinating diseases. Moreover, the ability of cannabinoids to promote the development of biologically supportive or symbiotic oligodendroglia may generalize to other microglia-driven neurodegenerative syndromes including NeuroAIDS and diseases of aging.

Tracking antigen-specific CD8+ T cells in the rat using MHC class I multimers

Studies of the quantitative and qualitative aspects of anti-microbial, anti-tumoral or autoreactive immune responses have been greatly facilitated by the possibility to stain antigen-specific CD8(+) T cells using fluorescently labeled multimeric major histocompatibility complex (MHC) class I/peptide complexes. So far, this technology has been developed for human and mouse, but not yet in the rat. Here, we describe the generation of the first rat MHC multimer. We produced a rat RT1(l) Pro5 MHC Pentamer combined with the immunodominant peptide for Borna disease virus (BDV), in order to study the characteristics of the antiviral CD8(+) T cell response. BDV is an RNA virus that can cause persistent infections of the central nervous system (CNS), often associated with prominent brain inflammation. In adult Lewis rats, of the RT1(l) MHC haplotype, BDV infection leads to severe immune-mediated neurological symptoms. The pathogenic role of the immune response is due primarily to antiviral CD8(+) T cells, many of them being specific for an immunodominant epitope located in the BDV nucleoprotein (N(230-238)). Ex vivo flow cytometry analyses revealed that 3 to 12% of CD8(+) T cells found in the brains of BDV-infected rats stained positively with the BDV-Pentamer. Interestingly, the frequency of Pentamer-positive cells increased up to 3.3 fold after a short resting period in culture. Virus-specific CD8(+) T cells were mainly detected in the brain and were virtually undetectable in peripheral lymphoid organs. This novel rat Pro5 MHC Pentamer represents an attractive tool for the detection, isolation and characterization of antigen-specific CD8(+) T cell responses in the rat.

Viral-induced inflammation is accompanied by beta-amyloid plaque reduction in brains of amyloid precursor protein transgenic Tg2576 mice

Amyloid plaques, one of the neuropathological hallmarks of Alzheimer's disease, and their main constituent, the amyloid beta-peptide (Abeta), are triggers of the activation of innate inflammatory mechanisms involving the activation of microglia. To dissect the effects of a non-Abeta-specific microglial activation on the Abeta metabolism, we employed a viral infection-based model. Transgenic mice expressing a mutated form of the human amyloid precursor protein (Tg2576) were used. In preceding experiments, 2-week-old transgenic mice and non-transgenic littermates were infected intracerebrally with the neurotropic Borna disease virus and investigated at 2, 4 and 14 weeks post-infection. The Borna disease virus-inoculated mice showed a persisting, subclinical infection of cortical and limbic brain areas characterized by slight T-cell infiltrates, expression of cytokines and a massive microglial activation in the hippocampus and neocortex. Viral-induced effects reached their peak at 4 weeks post-infection. In 14-month-old Tg2576 mice, characterized by the deposition of diffuse and dense-core amyloid plaques in cortical brain regions, Borna disease virus-induced microglial activation in the vicinity of Abeta deposits was used to investigate the influence of a local inflammatory response on these deposits. At 4 weeks post-infection, histometric analyses employing Abeta immunohistochemistry revealed a decrease of the cortical and hippocampal Abeta-immunopositive area. This overall decrease was accompanied by a decrease of parenchymal thioflavin-S-positive amyloid deposits and an increase of such deposits in the walls of cerebral vessels, which indicates that the elicitation of a non-Abeta-specific microglial activation may contribute to a reduction of Abeta in the brain parenchyma.

Spatiotemporal analysis of purkinje cell degeneration relative to parasagittal expression domains in a model of neonatal viral infection

Infection of newborn Lewis rats with Borna disease virus (neonatal Borna disease [NBD]) results in cerebellar damage without the cellular inflammation associated with infections in later life. Purkinje cell (PC) damage has been reported for several models of early-life viral infection, including NBD; however, the time course and distribution of PC pathology have not been investigated rigorously. This study examined the spatiotemporal relationship between PC death and zonal organization in NBD cerebella. Real-time PCR at postnatal day 28 (PND28) revealed decreased cerebellar levels of mRNAs encoding the glycolytic enzymes aldolase C (AldoC, also known as zebrin II) and phosphofructokinase C and the excitatory amino acid transporter 4 (EAAT4). Zebrin II and EAAT4 immunofluorescence analysis in PND21, PND28, PND42, and PND84 NBD rat cerebella revealed a complex pattern of PC degeneration. Early cell loss (PND28) was characterized by preferential apoptotic loss of zebrin II/EAAT4-negative PC subsets in the anterior vermis. Consistent with early preferential loss of zebrin II/EAAT4-negative PCs in the vermis, the densities of microglia and the Bergmann glial expression of metallothionein I/II and the hyaluronan receptor CD44 were higher in zebrin II/EAAT4-negative zones. In contrast, early loss in lateral cerebellar lobules did not reflect a similar discrimination between PC phenotypes. Patterns of vermal PC loss became more heterogeneous at PND42, with the loss of both zebrin II/EAAT4-negative and zebrin II/EAAT4-positive neurons. At PND84, zebrin II/EAAT4 patterning was abolished in the anterior cerebellum, with preferential PC survival in lobule X. Our investigation reveals regional discrimination between patterns of PC subset loss, defined by zebrin II/EAAT4 expression domains, following neonatal viral infection. These findings suggest a differential vulnerability of PC subsets during the early stages of virus-induced neurodegeneration.

Disturbance of the cortical cholinergic innervation in Borna disease prior to encephalitis

Rats experimentally infected with the highly neurotropic Borna disease virus (BDV) display a wide variety of dysfunction such as learning deficiencies and behavioral abnormalities. Prior to the onset of encephalitis alterations of one of the major cortical neurotransmitters, acetylcholine, were monitored immunohistochemically by light and electron microscopy of its synthesizing enzyme choline acetyltransferase (ChAT). We found a progressing decrease in the number of ChAT-positive fibers, starting with discrete changes at day 6 post infection (p.i.) and ending with a nearly complete loss of cholinergic fibers, especially in the hippocampus and neocortex, suggesting a massive disturbance of the cholinergic innervation by day 15 p.i.. The fiber pathways (e.g., fimbria-fornix) connecting the basal forebrain with these target areas in the cortex displayed axon spheroids which are often linked to axonal transport dysfunction. No evidence for significant cellular destruction was seen in the brain, including the cells of origin of these axons in the basal forebrain. We conclude that the motor, mood, learning and memory disabilities in BDV-infected rats are likely to result, in part, from cortical cholinergic denervation. The present study gives new insights into the pathogenesis of neurological disease caused by a noncytopathogenic virus.

Expression of allograft inflammatory factor-1 and haeme oxygenase-1 in brains of rats infected with the neurotropic Borna disease virus

Experimental infection of Lewis rats with Borna disease virus (BDV) causes an immune-mediated nonpurulent meningoencephalitis. Viral persistence in the central nervous system is accompanied by mononuclear infiltrates, activated monocytic/microglial cells and reactive astrocytes. The immune-mediated process was further characterized by expression analysis of allograft inflammatory factor-1 (AIF-1), a novel marker of monocyte/microglial activation and of glial fibrillary acid protein (GFAP) between day 3 and day 50 post infection (p.i.). Potential neuroprotective effects of these cells were studied by the induction of haeme oxygenase-1 (HO-1), a defensive molecule against oxidative stress in various brain insults. In BDV-infected rat brains, mononuclear infiltrates and AIF-1 expression increased up to day 28 p.i. During early time points p.i., AIF-1 expression was mainly found in inflammatory lesions and adjacent brain parenchyma. Already 24 days p.i., a widespread upregulation of AIF-1 was observed which declined only moderately beyond day 28 p.i. HO-1 induction was maximal between days 18 and 28 p.i. Increased amounts of GFAP-positive astrocytes were present beyond 24 days p.i. Viral antigen expression increased simultaneously to the inflammatory reaction and persisted up to 50 days p.i. Widespread upregulation of AIF-1 indicates an early, long-lasting microglial activation, which might be involved in the immunesurveillance of the immune-mediated inflammatory events. The early peak of HO-1 most likely represents a neuroprotective, anti-inflammatory response by invading monocytes, microglial cells and astrocytes during the formation of encephalitic lesions and acute viral replication.

Constitutive activation of the transcription factor NF-kappaB results in impaired borna disease virus replication

The inducible transcription factor NF-kappaB is commonly activated upon RNA virus infection and is a key player in the induction and regulation of the innate immune response. Borna disease virus (BDV) is a neurotropic negative-strand RNA virus, which replicates in the nucleus of the infected cell and causes a persistent infection that can lead to severe neurological disorders. To investigate the activation and function of NF-kappaB in BDV-infected cells, we stably transfected the highly susceptible neuronal guinea pig cell line CRL with a constitutively active (IKK EE) or dominant-negative (IKK KD) regulator of the IKK/NF-kappaB signaling pathway. While BDV titers were not affected in cells with impaired NF-kappaB signaling, the expression of an activated mutant of IkappaB kinase (IKK) resulted in a strong reduction in the intracellular viral titer in CRL cells. Electrophoretic mobility shift assays and luciferase reporter gene assays revealed that neither NF-kappaB nor interferon regulatory factors (IRFs) were activated upon acute BDV infection of wild-type or vector-transfected CRL cells. However, when IKK EE-transfected cells were used as target cells for BDV infection, DNA binding to an IRF3/7-responsive DNA element was detectable. Since IRF3/7 is a key player in the antiviral interferon response, our data indicate that enhanced NF-kappaB activity in the presence of BDV leads to the induction of antiviral pathways resulting in reduced virus titers. Consistent with this observation, the anti-BDV activity of NF-kappaB preferentially spread to areas of the brains of infected rats where activated NF-kappaB was not detectable.

Prevention of virus persistence and protection against immunopathology after Borna disease virus infection of the brain by a novel Orf virus recombinant

The Parapoxvirus Orf virus represents a promising candidate for novel vector vaccines due to its immune modulating properties even in nonpermissive hosts such as mouse or rat. The highly attenuated Orf virus strain D1701 was used to generate a recombinant virus (D1701-VrVp40) expressing nucleoprotein p40 of Borna disease virus, which represents a major antigen for the induction of a Borna disease virus-specific humoral and cellular immune response. Infection with Borna disease virus leads to distinct neurological symptoms mediated by the invasion of activated specific CD8+ T cells into the infected brain. Usually, Borna disease virus is not cleared from the brain but rather persists in neural cells. In the present study we show for the first time that intramuscular application of the D1701-VrVp40 recombinant protected rats against Borna disease, and importantly, virus clearance from the infected brain was demonstrated in immunized animals. Even 4 and 8 months after the last immunization, all immunized animals were still protected against the disease. Initial characterization of the immune cells attracted to the infected brain areas suggested that D1701-VrVp40 mediated induction of B cells and antibody-producing plasma cells as well as T cells. These findings suggest the induction of various defense mechanisms against Borna disease virus. First studies on the role of antiviral cytokines indicated that D1701-VrVp40 immunization did not lead to an enhanced early response of gamma or alpha interferon or tumor necrosis factor alpha. Collectively, this study describes the potential of the Orf virus vector system in mediating long-lasting, protective antiviral immunity and eliminating this persistent virus infection without provoking massive neuronal damage.

Neurological Disorder after Borna Disease Virus Infection in the Absence of Either Interferon-γ, Fas, Inducible NO Synthase, or Chemokine Receptor CXCR3

Borna disease virus (BDV) can induce severe neurological disorder in Lewis rats and MRL mice. Antiviral CD8 T cells have been shown to be the mediators of disease in these animals. To define molecules involved in the disease process, we performed infection studies in MRL mice lacking either interferon-gamma, a functional Fas/FasL system, chemokine receptor CXCR3, or inducible NO synthase. We further used transgenic MRL mice expressing interferon-gamma-inducible, T cell-attracting chemokine CXCL10 in brain astrocytes. After intracerebral infection with BDV, wild-type and mutant mice developed CD8 T cell responses and neurological disease at similar frequency and with similar kinetics, suggesting that these factors are not required for initiation and maintenance of the immunopathological process. Similarly, the course of disease could not be altered by treating infected MRL mice or Lewis rats with the drug L-N(6)-(1-iminoethyl)-lysine (L-NIL) that specifically blocks the activity of the inducible NO synthase. We therefore have excluded a number of important factors that have been demonstrated to be crucial in the pathogenesis of a broad number of pathologic conditions. Thus, BDV-induced disease may not result from the action of a single dominant T cell-dependent effector molecule. Disease rather reflects a combined influence of several as yet undefined factors from CD8 T cells.

Precursors of Borna disease virus-specific T cells in secondary lymphatic tissue of experimentally infected rats

Borna disease in rats represents an experimental model to study the immunopathological role of T cells in central nervous system disease. Adoptive transfer experiments were performed to investigate homing properties of T cells that infiltrate the brains of infected animals. Lymphocytes isolated from the brains of diseased rats were labelled with 5,6-carboxyfluorescein diacetate succinimidyl ester (CFSE) and transferred into immunosuppressed infected recipients. In recipient rats displaying neurological disease, labeled lymphocytes were demonstrated in the vicinity of brain cell lesions, suggesting that the neuronal destruction was dependent on the presence of transferred lymphocytes. Furthermore, the presence of virus-specific cytotoxic T cells was scrutinized in secondary lymphatic tissue and the functional activity of lymphocytes isolated from spleens, cervical lymph nodes, and mesenteric lymph nodes of infected animals was tested immediately after isolation and after in vitro restimulation. The data presented here indicate that precursors of Borna disease virus (BDV)-specific CD8(+) T cells are present and cytotoxic activity was demonstrated after in vitro cocultivation with infected cells in cervical lymph nodes and spleens but not in mesenteric lymphoid tissue. Adoptive transfer of in vitro restimulated T cells induced alterations in BDV-infected, immunosuppressed rats that resemble the well-defined clinical symptoms and neuropathology of Borna disease. This report provides for the first time formal evidence that virus-specific cytotoxic T cells are primed in the periphery after BDV infection, a disease that exclusively manifests itself in the central nervous system.

Accumulation of the proinflammatory cytokine endothelial-monocyte-activating polypeptide II in ramified microglial cells in brains of Borna virus infected Lewis rats

Borna disease virus (BDV) infection of adult Lewis rats induces a severe and often fatal neurologic disease characterized by a massive mononuclear meningo-encephalitis, and activation of microglial cells. Therefore, we analyzed expression of endothelial monocyte activating polypeptide II (EMAP II) by immunohistology as a marker for activation of microglial cells in BDV infected rat brains. EMAP II is a chemotactic peptide, inducing activation of macrophages and endothelial cells, and is considered a proinflammatory mediator of the innate immune system. An up to 30-fold increase in numbers of EMAP II+ microglial cells and a massive expression by infiltrating macrophages at perivascular inflammatory foci was observed in infected brains, with a maximum on day 25 after infection. These results provide evidence that EMAP II contributes to immune responses in inflammatory processes caused by viral infections.

Upregulation of chemokine receptor gene expression in brains of Borna disease virus (BDV)-infected rats in the absence and presence of inflammation

Infection of adult rats with Borna disease virus (BDV) causes CD8 T cell-mediated meningoencephalitis. Previously, we described a complex pattern of chemokine gene expression in the central nervous system (CNS) of such rats. We now found that expression of chemokine receptor genes CXCR3, CCR5, CX(3)CR1, and CXCR4 was also upregulated, which is in agreement with the predominance in brains of adult infected rats of T cells and monocytes/macrophages that express these receptors. In contrast to these rats, neonatally infected rats (designated PTI-NB) develop a persistent CNS infection associated with neurodegenerative processes in the absence of inflammation. In brains of PTI-NB rats, sustained expression of chemokines also takes place. We therefore analyzed mRNA expression of selected chemokine receptor genes, as well as of the chemokine fractalkine in brains of PTI-NB rats. We observed a marked increase of CCR5 and CX(3)CR1 transcripts in brains of these rats. CX(3)CR1 expressing cells were predominantly microglia, and upregulation of CX(3)CR1 was mainly due to an increase in the number of CX(3)CR1 expressing microglia. Fractalkine gene expression was found to be reduced to similar extents in brains of adult and newborn infected rats. These findings might be of relevance with respect to the selective neuronal cell loss observed in brains of PTI-NB rats.

High-dose Borna disease virus infection induces a nucleoprotein-specific cytotoxic T-lymphocyte response and prevention of immunopathology

Experimental Borna disease virus (BDV) infection of rats and natural infection of horses and sheep leads to severe central nervous system disease based on immunopathological pathways. The virus replicates slowly, and the cellular immune response results in immunopathology. CD8(+) T cells exert effector cell functions, and their activity results in the destruction of virus-infected cells. Previously, Oldach and colleagues (D. Oldach, M. C. Zink, J. M. Pyper, S. Herzog, R. Rott, O. Narayan, and J. E. Clements, Virology 206:426-434, 1995) have reported protection against Borna disease after inoculation of high-dose cell-adapted BDV. Here we show that the outcome of the infection, i.e., immunopathology versus protection, is simply dependent on the amount of virus used for infection. High-dose BDV (10(6) FFU) triggers an early virus-specific reaction of the immune system, as demonstrated by strong cellular and humoral responses. In particular, the early presence and function of nucleoprotein-specific CD8(+) T cells could be demonstrated in the brain. We present evidence that in a noncytolytic and usually persistent virus infection, high-dose input virus mediates early control of the pathogen due to an efficient induction of an antiviral immune mechanism. From these data, we conclude that immune reactivity, in particular the cytotoxic T-cell response, determines whether the virus is controlled with prevention of the ensuing immunopathological disease or whether a persistent infection is established.

A naturally processed rat major histocompatibility complex class I-associated viral peptide as target structure of borna disease virus-specific CD8+ T cells

The first naturally processed peptide synthesized by a virus and recognized by classical CD8(+) T cells in association with the RT1.A(l) major histocompatibility complex class I molecule of the Lewis rat is reported. Borna disease virus-specific CD8(+) T cells recognize syngeneic target cells pulsed with peptides extracted from Borna disease virus-infected cells. The predicted peptide sequence ASYAQMTTY from the viral p40 protein coeluted with the cytotoxic T-lymphocyte-reactive fraction was identified among natural ligands by tandem mass spectrometry. Numerous naturally processed peptides derived from intracellular bacteria, viruses, or tumors and recognized by CD8(+) T cells of man and mice are known, leading to a better understanding of cellular immune mechanisms against pathogens in these two species. In contrast, for the rat little information exists with regard to the function and role of CD8(+) T cells as part of their cellular immune defense system. This first naturally processed viral epitope in the rat contributes to the understanding of the rat cellular immune response and might trigger the identification of more cytotoxic T-lymphocyte epitopes in this animal.

Borna disease virus

Borna disease virus (BDV) is unique amongst animal RNA viruses in its molecular biology and capacity to cause persistent, noncytolytic CNS-infection in a wide variety of host species. Unlike other non-segmented negative-strand RNA animal viruses, BDV replicates in the nucleus of the host cell where splicing is employed for expression of a very compact genome. Epidemiological studies indicate a broad host range and geographical distribution, and some investigators have proposed that human infection may result in neuropsychiatric disorders. Experimental Borna disease in neonatal and adult rats provides an intriguing model for immune-mediated disturbances of brain development and function.

Borna disease virus: a mystery as an emerging zoonotic pathogen

For Central European veterinarians, Borna disease (BD) has been known for a long time as a sporadically occurring, progressive viral polioencephalomyelitis predominantly affecting horses and sheep and-as discovered in the last decade-an increasing number of domestic and zoo animals. The aetiological agent, the Borna disease virus (BDV), a negative-sense, single-stranded RNA virus classified in the new virus family Bornaviridae within the order Mononegavirales, can induce severe clinical signs typically of a viral encephalitis with striking behavioural disturbances. After an incubation period lasting a few weeks to several months, BDV-infection causes locomotor and sensory dysfunctions followed by paralysis and death. Natural infections seem to be subclinical in most cases. BD received world-wide attention when it was reported that sera and/or cerebrospinal fluids from neuro-psychiatric patients can contain BDV-specific antibodies. Since infected animals produce BDV-specific antibodies only after virus replication, it was assumed that the broad spectrum of BDV-susceptible species also includes man. However, reports describing the presence of other BDV-markers, i.e. BDV-RNA or BDV-antigen, in peripheral blood leukocytes or brain tissue of neuro-psychiatric patients are highly controversial and, therefore, the role of BDV in human neuro-psychiatric disorders is questionable. (c) 2001 Harcourt Publishers Ltd.

Neutralizing antibodies in persistent borna disease virus infection: prophylactic effect of gp94-specific monoclonal antibodies in preventing encephalitis

Borna disease virus (BDV) infection triggers an immune-mediated encephalomyelitis and results in a persistent infection. The immune response in the acute phase of the disease is characterized by a cellular response in which CD8(+) T cells are responsible for the destruction of virus-infected brain cells. CD4(+) T cells function as helper cells and support the production of antiviral antibodies. Antibodies generated in the acute phase of the disease against the nucleoprotein and the phosphoprotein are nonneutralizing. In the chronic phase of the disease, neutralizing antibodies directed against the matrix protein and glycoprotein are synthesized. In the present work, the biological role of the neutralizing-antibody response to BDV was further investigated. By analyzing the blood of rats infected intracerebrally with BDV, a highly neurotropic virus, nucleic acid could be detected between 30 and 50 days after infection. Neutralizing antibodies were found between 60 and 100 days after infection. Furthermore, we produced hybridomas secreting BDV-specific neutralizing monoclonal antibodies. These antibodies, directed against the major glycoprotein (gp94) of BDV, were able to prevent Borna disease if given prophylactically. These data suggest that the late appearance of BDV-specific neutralizing antibodies is due to the presence of BDV in the blood of chronically infected rats. Furthermore, these antibodies have the potential to neutralize the infectious virus when given early, which is an important finding with respect to the development of a vaccine.

Borna disease in horses

Borna disease is a sporadically occurring, progressive viral polioencephalomyelitis that primarily affects horses and sheep. The etiological agent, Borna disease virus (BDV), is an enveloped, single-stranded RNA virus that has been classified in the new virus family Bornaviridae within the order Mononegavirales. Serological evidence of BDV infection has been found in an increasing number of countries throughout the world. After an incubation period lasting a few weeks to several months, BDV infection can cause locomotor and sensory dysfunction followed by paralysis and death. Borna disease is the result of a virus-induced immunopathological reaction. BDV-specific antibodies and viral RNA have been found in humans with various psychiatric disorders.

Chemokine gene expression in astrocytes of Borna disease virus-infected rats and mice in the absence of inflammation

Borna disease virus (BDV) causes CD8(+) T-cell-mediated meningoencephalitis in immunocompetent mice and rats, thus providing a valuable animal model for studying the mechanisms of virus-induced central nervous system (CNS) immunopathology. Chemokine-mediated leukocyte recruitment to the CNS is a crucial step in the development of neurological disease. We found increased mRNA levels of IP-10 and other chemokines in brains of adult rats following infection with BDV. The marked increase in chemokine gene expression at about day 8 postinfection seemed to immediately precede the inflammatory process. In brains of rats infected as newborns, in which inflammation was only mild and transient, sustained expression of IP-10 and RANTES genes was observed. In situ hybridization studies revealed that astrocytes were the major source of IP-10 mRNAs in brains of rats infected as newborns and as adults. In brains of infected mice lacking CD8(+) T cells (beta2m(0/0)), transcripts encoding IP-10 and RANTES were also observed. IP-10 transcripts were also present in a small number of scattered astrocytes of infected knockout mice lacking mature B and T cells as well as functional alpha/beta and gamma interferon receptors, indicating that BDV can induce chemokine synthesis in the absence of interferons and other B- or T-cell-derived cytokines. These data provide strong evidence that CNS-resident cells are involved in the early localized host immune response to infection with BDV and support the concept that chemokines are pivotal for the initiation of virus-induced CNS inflammation.

Distribution of Borna disease virus in the brain of rats infected with an obesity-inducing virus strain

Experimental infection of Lewis rats with Borna disease virus (BDV), a nonsegmented, single-stranded RNA virus, usually causes an immune-mediated biphasic neurobehavioral disorder. Such animals develop a persistent infection of the CNS with viral antigen expression in all brain regions and a disseminated nonpurulent meningoencephalitis. Interestingly, intracerebral infection of Lewis rats with a BDV-variant (BDV-ob) causes a rapid increase of body weight with the development of an obesity syndrome without obvious neurological signs. The obese phenotype is correlated with a characteristic distribution of inflammatory lesions and BDV-antigen in the rat brain. Infiltration with mononuclear immune cells and viral antigen expression are restricted to the septum, hippocampus, amygdala and ventromedian tuberal hypothalamus. Therefore, infection with the obesity-inducing BDV-ob results most likely in neuroendocrine dysregulations leading to the development of an obesity syndrome. This might be due to the restriction of viral antigen expression and inflammatory lesions to brain areas which are involved in the regulation of body weight and food intake. The BDV-induced obesity syndrome represents a model for the study of immune-mediated neuroendocrine disorders caused by viral infections of the CNS.

High prevalence of Borna disease virus in domestic cats with neurological disorders in Japan

A total of 15 (T-1-T-15) domestic cats with neurological disorders in Tokyo area were examined for association with Borna disease virus (BDV). None had detectable antibodies to feline immunodeficiency virus (FIV), feline leukemia virus, feline infectious peritonitis virus and Toxoplasma gondii, and only cat T-8 had detectable antibody to FIV. Serological and molecular epidemiological studies revealed a significantly high prevalence of BDV infection in these cats: antibodies against BDV p24 and/or p40 proteins in 10/15 (66.7%) and p24 and/or p40 RNA in peripheral blood mononuclear cells in 8/15 (53.3%). Further, in situ hybridization and immunohistochemistry analyses of the autopsied brain samples derived from one of the cats (T-15) revealed BDV RNA predominantly in neuronal cells in restricted regions, such as olfactory bulb and medulla of cerebrum. Thus, BDV is present in Japanese domestic cats with neurological disorders at a high prevalence.

Cytokine expression in the rat central nervous system following perinatal Borna disease virus infection

Borna disease virus (BDV) causes central nervous system (CNS) disease in several vertebrate species, which is frequently accompanied by behavioral abnormalities. In the adult rat, intracerebral (i.c.) BDV infection leads to immunomediated meningoencephalitis. In contrast, i.c. infection of neonates causes a persistent infection in the absence of overt signs of brain inflammation. These rats (designated PTI-NB) display distinct behavioral and neurodevelopmental abnormalities. However, the molecular mechanisms for these virally induced CNS disturbances are unknown. Cytokines play an important role in CNS function, both under normal physiological and pathological conditions. Astrocytes and microglia are the primary resident cells of the central nervous system with the capacity to produce cytokines. Strong reactive astrocytosis is observed in the PTI-NB rat brain. We have used a ribonuclease protection assay to investigate the mRNA expression levels of proinflammatory cytokines in different brain regions of PTI-NB and control rats. We show here evidence of a chronic upregulation of proinflammatory cytokines interleukin-6, tumor necrosis factor alpha, interleukins-1alpha, and -1beta in the hippocampus and cerebellum of the PTI-NB rat brain. These brain regions exhibited only a very mild and transient immune infiltration. In contrast, in addition to reactive astrocytes, a strong and sustained microgliosis was observed in the PTI-NB rat brains. Our data suggest that CNS resident cells, namely astrocytes and microglia, are the major source of cytokine expression in the PTI-NB rat brain. The possible implications of these findings are discussed.

High susceptibility of Mongolian gerbil (Meriones unguiculatus) to Borna disease virus

Borna disease virus (BDV) is a neurotropic enveloped virus with a nonsegmented, single-, negative-stranded RNA genome. This virus induced encephalitis in experimentally infected adult rats, but in newborn rats BDV established a persistent, tolerant infection with no apparent clinical signs. Here, we report evidence that newborn Mongolian gerbils (Meriones unguiculatus) are more susceptible to experimental intracranial inoculation of horse-derived BDV in persistently infected MDCK cells, compared with similar inoculation in newborn rats. All inoculated newborn gerbils, but not rats, died 30 days after infection. Reverse transcriptase-polymerase chain reaction amplified BDV-specific sequences in several regions including the brain. Histopathological analysis revealed apparent inflammatory reactions in the brains of inoculated gerbils but not rats, although similar levels of BDV RNA were detected in both gerbil and rat brains. BDV-specific antigen and RNA were identified predominantly in neurons in the brains by immunohistochemistry with antibodies to BDV and in situ hybridization with BDV-specific riboprobes, respectively. BDV in the gerbil brain was easily rescued by co-cultivation of the brain homogenate with human oligodendroglioma cells. Thus, gerbils seem to be a useful animal model for studying BDV-induced pathogenesis in the brain.

Borna disease virus nucleoprotein (p40) is a major target for CD8(+)-T-cell-mediated immune response

Experimental infection of rats with Borna disease virus (BDV) and natural BDV infection of horses and sheep leads to a virus-induced T-cell-mediated immunopathology in the central nervous system. Earlier work revealed the importance of the BDV-specific T-cell response and of CD8(+) effector cells in particular in the destruction of virus-infected cells. Evidence was also presented that this major histocompatibility complex class I-restricted lysis detected in vitro might play a functional role in the immunopathogenesis of Borna disease. The present study employed different vaccinia virus recombinants expressing single BDV-specific proteins to investigate the specificity of the cytolytic CD8(+)-T-cell response, revealing a major epitope on the BDV nucleoprotein p40. In contrast, no direct evidence in favor of the presence of in vivo relevant cytotoxic T-lymphocyte epitopes on other BDV-specific proteins was found.

Humoral immunity in the central nervous system of Lewis rats infected with Borna disease virus

The aim of this study was to investigate the humoral immune response to Borna disease (BD) virus in the brain of experimentally infected Lewis rats. Abundant IgG was detected in BD-rat brain with isotype variation throughout infection. IgG was locally produced as indicated by an intact blood-brain barrier, Ig kappa light chain mRNA-containing cells in brain and accumulation of virus-specific antibodies in cerebrospinal fluid. Treatment with BD-rat serum altered viral gene expression in persistently infected cultured rat glioblastoma cells. These data suggest that antibodies, produced in the brain, may influence viral gene expression.

Evolution of the immune response in the central nervous system following infection with Borna disease virus

Borna disease virus infection of Lewis rats results in an immune-mediated disease associated with transient meningoencephalitis and persistent viral infection. In the acute phase of disease, perivascular immune cell infiltrates consisted of CD4 + and CD8 + T cells, macrophages and NK cells with peak expression of mRNAs encoding the cytokines IL1alpha, IL2, IL6, TNFalpha, and IFNgamma. In the chronic phase of disease, numbers of NK cells, B cells and activated microglia increased in the brain parenchyma with peak expression of IL4 mRNA. These data were consistent with a switch from a Th1-like, cellular immune response to a Th2-like, humoral immune response.

Virus-specific CD4+ T cells eliminate borna disease virus from the brain via induction of cytotoxic CD8+ T cells

Persistent Borna disease virus infection of the brain can be prevented by treatment of naive rats with a virus-specific CD4+ T-cell line prior to infection. In rats receiving this treatment, only a transient low-level encephalitis was seen compared to an increasingly inflammatory reaction in untreated infected control rats. Virus replication was found in the brain for several days after infection before the virus was cleared from the central nervous system. The loss of infectivity from the brain was confirmed by negative results by reverse transcription-PCR with primers for mRNA, by in situ hybridization for both genomic and mRNA, and by immunohistology. Most importantly, in vitro assays revealed that the T-cell line used for transfusion had no cytotoxic capacity. The kinetics of virus clearance were paralleled by the appearance of CD8+ T cells and the expression of perforin in the brain. Testing of lymphocytes isolated from the brains of CD4+ T-cell-treated rats after challenge revealed high cytotoxic activity due to the presence of CD8+ cytotoxic T cells at time points when brain lymphocytes from infected control rats induced low-level cytolysis of target cells. Neutralizing antiviral antibodies and gamma interferon were shown not to be involved in the elimination of virus from the brain.

Borna disease virus-induced neurological disorder in mice: infection of neonates results in immunopathology

Borna disease virus (BDV) is a neurotropic nonsegmented negative-stranded RNA virus that persistently infects warm-blooded animals. In horses and other natural animal hosts, infections with BDV cause meningoencephalitis and behavioral disturbances. Experimental infection of adult mice takes a nonsymptomatic course, an observation previously believed to indicate that this animal species is not suitable for pathogenesis studies. We now demonstrate that BDV frequently induces severe neurological disease in infected newborn mice. Signs of neurological disease were first observed 4 to 6 weeks after intracerebral infection. They included a characteristic nonphysiological position of the hind limbs at an early stage of the disease and paraparesis at a later stage. Histological examination revealed large numbers of perivascular and meningeal inflammatory cells in brains of diseased mice and, unexpectedly, no increase in immunoreactivity to glial fibrillar acidic protein. The incidence and severity of BDV-induced disease varied dramatically among mouse strains. While only 13% of the infected C57BL/6 mice showed disease symptoms, which were mostly transient, more than 80% of the infected MRL mice developed severe neurological disorder. In spite of these differences in susceptibility to disease, BDV replicated to comparable levels in the brains of mice of the various strains used. Intracerebral infections of newborn beta2-microglobulin-deficient C57BL/6 and MRL mice, which both lack CD8+ T cells, did not result in meningoencephalitis or neurological disease, indicating that the BDV-induced neurological disorder in mice is a cytotoxic T-cell-mediated immunopathological process. With this new animal model it should now be possible to characterize the disease-inducing immune response to BDV in more detail.

Spontaneous Borna disease in sheep and horses: immunophenotyping of inflammatory cells and detection of MHC-I and MHC-II antigen expression in Borna encephalitis lesions

Borna disease (BD) has been recognized as a virally induced T-cell dependent immunopathological disorder of the central nervous system (CNS), as shown by experimental infection of rats with Borna disease virus (BDV). In contrast to the rat model, little is known about the pathogenesis of spontaneous BD in sheep and horses. The present study describes the brain lesions of 12 ovine and 11 equine cases of naturally occurring BD. A set of monoclonal and polyclonal antibodies was used in order to determine the cells operative in encephalitic lesions and to detect expression of MHC-I and MHC-II products in the brains of affected animals. In all cases investigated, a reaction pattern similar to that reported for the acute phase of BD in experimentally infected rats was noted. In brief, the majority of inflammatory cells in perivascular infiltrates (PVI) as well as parenchymal and meningeal infiltrates were CD3 +. CD4 + cells outnumbered CD8 + cells in PVI as well as in the parenchyma. Macrophages (defined by lysozyme immunoreactivity) were seen less often and B-cells or plasma cells (cells positive for lambda or kappa light chains) were demonstrated at lower numbers. TCR-1 + cells were found on very rare occasions in PVI of some sheep. MHC-I and MHC-II products were constantly expressed on inflammatory cells but inconsistently on astrocytes and neurons. Neuronal degeneration was not a major feature.

Evidence that deficient IFN-gamma production is a biological basis of herpes simplex virus type-2 neurovirulence

Although immune response control of herpes simplex virus (HSV) has been well demonstrated, numerous HSV-2 strains are neurovirulent in immunocompetent mice. Using an RNase protection assay and an ELISA, we found that HSV-2-infected mice exhibited a deficient IFN-gamma response, an inability to clear virus, and eventual death. An HSV-based amplicon vector expressing mouse IFN-gamma was constructed and packaged into HSV-1-helper virus (HSV(pIFN-gamma)). In mice treated with HSV(pIFN-gamma), (i) the LD50 of HSV-2(G) increased 5000-fold, (ii) intracerebral IFN-gamma expression increased 10-fold, and (iii) HSV titer rapidly decreased. We suggest that the deficient IFN-gamma response is a basis for HSV-2 neurovirulence in mice.

Enriched immune-environment of blood-brain barrier deficient areas of normal adult rats

The circumventricular organs (CVOs) in the brain are without a blood-brain barrier (BBB) and as such directly exposed to blood plasma constituents and blood-borne pathogens. In light of previous studies showing discrepancies regarding the immunocompetence of these organs, we initiated the present study to provide a comprehensive immunohistochemical analysis of the cellular expression of immune-associated antigens within the pineal gland, area postrema and the subfornical organ. In all CVOs, subpopulations of cells morphologically similar to complement receptor type 3 immunoreactive microglial/macrophage cells expressed major histocompatibility complex (MHC) class II antigen, leucocyte common antigen (LCA/CD45), as well as CD4 and ED1 antigen. Based on morphological criteria the MHC class II antigen expressing cells could be grouped into a major population of classical parenchymal and perivascular ramified microglial cells and a minor population presenting itself as scattered or small groups of rounded macrophage-like cells. CD4 and ED1 antigen were expressed by both cell types. CD45 was preferentially expressed by macrophage-like cells. MHC class I antigen was expressed by the vascular endothelium in both BBB-protected and BBB-deficient areas and was additionally present as a lattice-like network throughout the BBB-deficient parenchyma in all CVOs. The results suggest that the BBB-free areas of the brain besides being constantly surveyed by blood-borne macrophages, possess an intrinsic immune surveillance system based on resting and activated microglial cells, which may function as a non-endothelial, cellular barrier against blood-borne pathogens.

Borna disease virus (BDV), a (zoonotic?) worldwide pathogen. A review of the history of the disease and the virus infection with comprehensive bibliography

A comprehensive history of Borna disease virus (BDV) and this infection, including the complete bibliography, is presented. Over the last 200 years, descriptions of this 'head disease' of horses ('Kopfkrankheit der Pferde') have been given. Considerable losses in the horse population (< 0.8%) led to intensive clinical and (neuro-)pathological investigations of this meningitis cerebrospinalis which occurs with faint behavioural changes, occasionally followed by severe neurological symptomatology and death. The broad experimental host range reflects infections in nature which include horses, sheep, cattle, cats, dogs, rodents, ostriches, and some zoo animals. BDV infections are associated with phylogentically old brain areas, and the retina. Occasionally, expression in the autonomic nervous system occurs, besides its neurotropism BDV can spread to peripheral organs, especially to epithelial tissues and peripheral blood mononuclear cells. Infections of humans that can be monitored by antibodies, antigens or nucleic acids in blood samples are prominent features of future interest. BDV, the prototype of the family Bornaviridae is an enveloped spherical virus carrying an 8.9 kb single-stranded, non-segmented RNA with negative polarity which replicates in the nucleus. These features together with its considerable genetic stability make this non-cytopathogenic virus an evolutionary 'old pathogen' in nature.

Induction of degenerative brain lesions after adoptive transfer of brain lymphocytes from Borna disease virus-infected rats: presence of CD8+ T cells and perforin mRNA

Lymphocytes were isolated from the brains of Borna disease virus-infected donor Lewis rats at various time points after infection. Cell populations were characterized by cytofluorometry, with special emphasis on CD4+ and CD8+ cells. Testing of isolated lymphocytes revealed major histocompatibility complex class I-restricted cytotoxic activity. Reverse transcription-PCR analyses of brain homogenates of infected donors revealed the presence of CD8 mRNA after day 11 of infection and of perforin mRNA between days 13 and 25 after infection. Adoptive transfers of lymphocytes isolated from the brain at days 13 and 21 resulted in severe neurological symptoms, resembling experimental Borna disease. The onset of disease was dependent on the cell numbers transferred and was clearly related to the appearance of T cells in the brain. CD8+ T cells were found in the parenchyma, whereas CD4+ T cells were found predominantly in perivascular locations. A disseminated lymphocytic infiltration in the parenchyma was accompanied by severe morphological alterations, including significant necrosis of neurons. Furthermore, a prominent spongiform-like degeneration was observed; this increased over time and finally resulted in severe cortical brain atrophy. Lymphocytes obtained during the beginning chronic phase of experimental Borna disease in rats had no significant cytolytic capacity in vitro and were also not able to induce neurological symptoms typical of Borna disease after adoptive transfer. The data presented here show for the first time that lymphocytes isolated from the site of the inflammatory lesions, namely, the brains of diseased rats, induce the immunopathological reaction and cause Borna disease. After transfer, the pathological alterations induced in the recipients exactly reflect those observed during experimentally induced Borna disease in rats, including necrosis of neurons and glial cells and gross degeneration resulting in cortical brain atrophy. Evidence that the immunopathology of Borna disease is closely related to the presence of CD8+ T cells in the brain parenchyma is provided.

Borna disease virus and the brain

Viruses with the ability to establish persistent infection in the central nervous system (CNS) can induce progressive neurologic disorders associated with diverse pathological manifestations. Clinical, epidemiological, and virological evidence supports the hypothesis that viruses contribute to human mental diseases whose etiology remains elusive. Therefore, the investigation of the mechanisms whereby viruses persist in the CNS and disturb normal brain function represents an area of research relevant to clinical and basic neurosciences. Borna disease virus (BDV) causes CNS disease in several vertebrate species characterized by behavioral abnormalities. Based on its unique features, BDV represents the prototype of a new virus family. BDV provides an important model for the investigation of the mechanisms and consequences of viral persistence in the CNS. The BDV paradigm is amenable to study virus-cell interactions in the CNS that can lead to neurodevelopmental abnormalities, immune-mediated damage, as well as alterations in cell differentiated functions that affect brain homeostasis. Moreover, seroepidemiological data and recent molecular studies indicate that BDV is associated with certain neuropsychiatric diseases. The potential role of BDV and of other yet to be uncovered BDV-related viruses in human mental health provides additional impetus for the investigation of this novel neurotropic infectious agent.

Immunopathogenic role of T-cell subsets in Borna disease virus-induced progressive encephalitis

Borna disease is an immunopathological virus-induced encephalopathy comprising severe inflammation and degenerative brain cell lesions which results in organ atrophy and chronic debility in rats. CD4+ and CD8+ T cells have been reported to be involved in the development of this disease of the central nervous system. A virus-specific homogeneous T-cell line, established in vitro after immunization of rats with the recombinant 24-kDa virus-specific protein, showed antigen-specific proliferation in the presence of the 24-kDa but not the 38-kDa Borna disease virus-specific protein, another major virus-specific antigen. This T-cell line, P205, was found to exhibit characteristics of a T-helper cell: CD4+ CD8- IL-2- IL-4- IFN-gamma+ IL-6+ IL-10+. Furthermore, this T-cell line expressed the alpha/beta T-cell receptor and the alpha 4 integrin (VLA-4). Adoptive transfer of this helper cell resulted in an increase of antibody titers and two different types of disease in virus-infected rats after cyclophosphamide-induced immunosuppression. (i) Rats receiving T cells between 10 and 18 days after treatment with cyclophosphamide showed an acute lymphoproliferative disease in the gut and lungs within 9 days after adoptive transfer and died. (ii) Passive transfer within the first 5 days after immunosuppressive treatment resulted in typical Borna disease associated with neurological symptoms such as ataxia and paresis starting 14 to 16 days after transfer. Immunohistological analysis of the brains of rats with Borna disease uniformly revealed the presence of CD8+ T cells in encephalitic lesions in addition to CD4+ cells that were found in the brains of recipients of the virus-specific CD4+ T-cell line, irrespective of whether neurological symptoms developed or not. However, recipient rats treated with antibodies against CD8+ T cells developed neither encephalitis nor disease. Therefore, CD4+ T cells appear to accumulate in the brain and cause perivascular inflammatory lesions which alone obviously do not cause disease. In contrast, the presence of CD8+ cells apparently directly correlates with the development of neurological symptoms.

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.

Immunoreactivity of the central nervous system in cats with a Borna disease-like meningoencephalomyelitis (staggering disease)

The inflammatory cell composition and the expression of major histocompatibility complex (MHC) antigens in the central nervous system (CNS) of 13 cats with a spontaneous, Borna disease-like meningoencephalomyelitis (staggering disease) was investigated by immunohistochemistry with a panel of monoclonal and polyclonal antibodies. T lymphocytes were the predominating inflammatory cells within the adventitial space. CD4+ T cells were more abundant than CD8+ T cells. Scattered IgG-, IgA- and IgM-containing cells were found in the adventitial space and surrounding neuropil, often adjacent to neurons. There was a markedly increased MHC class II expression in cells morphologically resembling microglia. In several cats, Borna disease virus specific antigen was detected, but only in a few cells, mainly of macrophage character. Our findings indicate a long-standing inflammatory reaction in the CNS of cats with staggering disease, possibly triggered and sustained by a persistent viral infection.

Alteration of intracerebral cytokine production in mice infected with herpes simplex virus types 1 and 2

Previously we reported that a lethal strain of herpes simplex virus type 2 (HSV-2) infects the brain following ocular inoculation of mice. We now demonstrate that HSV-2 mediates an unusual intracellular sequestering of class II major histocompatibility complex (MHC) antigens. With use of an RNase protection assay, we observed a selective inhibition of IFN-gamma and IL-6 gene transcription in brains of mice infected with HSV-2. It is likely that the inhibition of cytokine gene expression was mediated through a failure to activate CD4+ lymphocytes. These data suggest that the infecting herpesvirus can influence the profile of intracerebrally produced cytokines, which in turn may determine the outcome of the infection.