Presence of lysine at aa 335 of the hemagglutinin-neuraminidase protein of mumps virus vaccine strain Urabe AM9 is not a requirement for neurovirulence.

The recent global resurgence of mumps has drawn attention to the continued need for robust mumps immunization programs. Unfortunately, some vaccines derived from inadequately attenuated vaccine strains of mumps virus have caused meningitis in vaccinees, leading to withdrawal of certain vaccine strains from the market, public resistance to vaccination, or in some cases, cessation of national mumps vaccination programs. The most widely implicated mumps vaccine in cases of postvaccination meningitis is derived from the Urabe AM9 strain, which remains in use in some countries. The Urabe AM9 vaccine virus has been shown to exhibit a considerable degree of nucleotide and amino acid heterogeneity. Some studies have specifically implicated variants containing a lysine residue at amino acid position 335 in the hemagglutinin-neuraminidase (HN) protein with neurotoxicity, whereas a glutamic acid residue at this position was associated with attenuation. To test this hypothesis we generated two modified Urabe AM9 cDNA clones coding either for a lysine or a glutamic acid at position 335 in the HN gene. The two viruses were rescued by reverse genetics and characterized in vitro and in vivo. Both viruses exhibited similar growth kinetics in neuronal and non-neuronal cell lines and were of similar neurotoxicity when tested in rats, suggesting that amino acid 335 is not a crucial determinant of Urabe AM9 growth or neurovirulence.

Antibody induced by immunization with the Jeryl Lynn mumps vaccine strain effectively neutralizes a heterologous wild-type mumps virus associated with a large outbreak.

Recent mumps outbreaks in older vaccinated populations were caused primarily by genotype G viruses, which are phylogenetically distinct from the genotype A vaccine strains used in the countries affected by the outbreaks. This finding suggests that genotype A vaccine strains could have reduced efficacy against heterologous mumps viruses. The remote history of vaccination also suggests that waning immunity could have contributed to susceptibility. To examine these issues, we obtained consecutive serum samples from children at different intervals after vaccination and assayed the ability of these samples to neutralize the genotype A Jeryl Lynn mumps virus vaccine strain and a genotype G wild-type virus obtained during the mumps outbreak that occurred in the United States in 2006. Although the geometric mean neutralizing antibody titers against the genotype G virus were approximately one-half the titers measured against the vaccine strain, and although titers to both viruses decreased with time after vaccination, antibody induced by immunization with the Jeryl Lynn mumps vaccine strain effectively neutralized the outbreak-associated virus at all time points tested.

Genetic contributions to influenza virus attenuation in the rat brain.

Influenza is generally regarded as an infection of the respiratory tract; however, neurological involvement is a well-recognized, although uncommon, complication of influenza A virus infection. The authors previously described the development of a rat model for studying influenza virus infection of the central nervous system (CNS). This model was used here to study the role of virus genes in virus replication and spread in brain. In the present work, an infectious cDNA clone of the neurotoxic WSN strain of influenza virus (rWSN) was altered by site-directed mutagenesis at five loci that corresponded to changes previously shown to confer temperature sensitivity and attenuation of the A/Ann Arbor/6/60 strain (PB1Delta 391, PB1Delta 581, and PB1Delta 661; PB2Delta 265, and NPDelta 34). Whereas rWSN and its mutated derivative (mu-rWSN) replicated equally well in MDCK cells at 37 degrees C (the body temperature of rats), rWSN grew to higher titers and infection was more widespread compared to mu-rWSN in rat brain. These results demonstrate that the five mutations that confer attenuation of the A/Ann Arbor/6/60 influenza virus strain for the respiratory system also confer attenuation for the central nervous system. Further in vivo and in vitro examination of these five mutations, both individually and in combination, will likely provide important information on the role of specific virus genes in virulence and pathogenesis.

A single nucleotide change in the mumps virus F gene affects virus fusogenicity in vitro and virulence in vivo.

Mumps virus is highly neurotropic, with evidence of infection of the central nervous system in more than half of clinical cases. In the prevaccine era, mumps was a major cause of viral meningitis in most developed countries. Despite efforts to attenuate the virus, some mumps vaccines have retained virulence properties and have caused aseptic meningitis in vaccinees, resulting in public resistance to vaccination in some countries. Ensuring the safety of mumps vaccines is an important public health objective, as the need for robust immunization programs has been made clear by the recent resurgence of mumps outbreaks worldwide, including the United States, which in 2006 experienced its largest mumps outbreak in 20 years. To better understand the molecular basis of mumps virus attenuation, the authors developed two infectious full-length cDNA clones for a highly neurovirulent strain of mumps virus. The clones differed at only one site, possessing either an A or G at nucleotide position 271 in the F gene, to represent the heterogeneity identified in the original virulent clinical isolate. In comparison to the clinical isolate, virus rescued from the A-variant cDNA clone grew to higher cumulative titers in vitro but exhibited similar cytopathic effects in vitro and virulence in vivo. In contrast, virus rescued from the G-variant cDNA clone, in comparison to the clinical isolate and the A-variant, was more fusogenic in vitro but replicated to lower cumulative titers and was less neurovirulent in vivo. These data suggest that nucleotide position 271 in the F gene plays a significant role in virus pathogenesis. This infectious clone system will serve as a key tool for further examination of the molecular basis for mumps virus neurovirulence and neuroattenuation.

Functional consequences of attenuating mutations in the haemagglutinin neuraminidase, fusion and polymerase proteins of a wild-type mumps virus strain.

Wild-type mumps viruses (MuVs) are highly neurotropic and, prior to widespread vaccination programmes, were a major cause of viral meningitis and encephalitis in most developed countries. At present, there are no markers for virus attenuation, apart from the failure of a passaged isolate to produce clinical symptoms in vaccinees. Indeed, some MuV vaccines have retained residual neurovirulence properties and have caused aseptic meningitis in vaccinees. Three amino acid changes associated with the neuroattenuation of a wild-type MuV strain were identified previously. This study evaluated the impact of these changes on the function of the respective proteins. The data demonstrated that the Ser-->Asp amino acid substitution at position 466 in the haemagglutinin-neuraminidase protein resulted in decreased receptor binding and neuraminidase activity, the Ala/Thr-->Thr selection in the fusion protein resulted in decreased fusion activity, and the Ile-->Val substitution in the polymerase resulted in increased replicative/transcriptional activity. These data suggest a polygenic component (i.e. specific and inter-related roles of these amino acid changes) to MuV neuroattenuation.

Activation of microglia by borna disease virus infection: in vitro study.

Neonatal Borna disease virus (BDV) infection of the rat brain is associated with microglial activation and damage to the certain neuronal populations. Since persistent BDV infection of neurons in vitro is noncytolytic and noncytopathic, activated microglia have been suggested to be responsible for neuronal cell death in vivo. However, the mechanisms of activation of microglia in neonatally BDV-infected rat brain have not been investigated. To address these issues, activation of primary rat microglial cells was studied following exposure to purified BDV or to persistently BDV-infected primary cortical neurons or after BDV infection of primary mixed neuron-glial cultures. Neither purified virus nor BDV-infected neurons alone activated primary microglia as assessed by the changes in cell shape or production of the proinflammatory cytokines. In contrast, in the BDV-infected primary mixed cultures, we observed proliferation of microglia cells that acquired the round morphology and expressed major histocompatibility complex molecules of classes I and II. These manifestations of microglia activation were observed in the absence of direct BDV infection of microglia or overt neuronal toxicity. In addition, compared to uninfected mixed cultures, activation of microglia in BDV-infected mixed cultures was associated with a significantly greater lipopolysaccharide-induced release of tumor necrosis factor alpha, interleukin 1beta, and interleukin 10. Taken together, the present data are the first in vitro evidence that persistent BDV infection of neurons and astrocytes rather than direct exposure to the virus or dying neurons is critical for activating microglia.

Changes in mumps virus neurovirulence phenotype associated with quasispecies heterogeneity.

Mumps virus is a highly neurotropic virus with evidence of central nervous system invasion (CNS) in approximately half of all cases of infection. In countries where live attenuated mumps virus vaccines were introduced, the number of mumps cases declined dramatically; however, recently, the safety of some vaccine strains has been questioned. For example, one of the most widely used vaccines, the Urabe AM9 strain, was causally associated with meningitis, leading to the withdrawal of this product from the market in several countries. This highlights the need for a better understanding of the attenuation process and the identification of markers of attenuation. To this end, we further attenuated the Urabe AM9 strain by serial passage in cell culture and compared the complete nucleotide sequences of the parental and passaged viruses. Interestingly, despite a dramatic decrease in virus virulence (as assayed in rats), the only genomic changes were in the form of changes in the level of genetic heterogeneity at specific genome sites, i.e., either selection of one nucleotide variant at positions where the starting material exhibited nucleotide heterogeneity or the evolution of an additional nucleotide to create a heterogenic site. This finding suggests that changes in the level of genetic heterogeneity at specific genome sites can have profound neurovirulence phenotypic consequences and, therefore, caution should be exercised when evaluating genetic markers of virulence or attenuation based only on a consensus sequence.

The rat-based neurovirulence safety test for the assessment of mumps virus neurovirulence in humans: an international collaborative study.

Because of the highly neurotropic and neurovirulent properties of wild-type mumps viruses, most national regulatory organizations require neurovirulence testing of virus seeds used in the production of mumps vaccines. Such testing has historically been performed in monkeys; however, some data suggest that testing in monkeys does not necessarily discriminate among the relative neurovirulent risks of mumps virus strains. To address this problem, a collaborative study was initiated by the National Institute for Biological Standards and Control in the United Kingdom and the Food and Drug Administration in the United States, to test a novel rat-based mumps virus neurovirulence safety test. Results indicate that the assay correctly assesses the neurovirulence potential of mumps viruses in humans and is robust and reproducible.

Mouse neurotoxicity test for vaccinia-based smallpox vaccines.

The only US FDA licensed smallpox vaccine, Dryvax, was associated with rare but serious neurological adverse events. After smallpox was eradicated in the United States, mass vaccination ceased in 1971. As counter-bioterrorism/biowarfare measures, new smallpox vaccines are now being investigated. However, there are no established pre-clinical neurotoxicity assays with which to evaluate these new vaccines prior to licensure. Here we report the development and initial characterization of a small animal neurotoxicity assay for vaccinia-based smallpox vaccines using Dryvax virus as a reference vaccine strain and the neuroadapted Western Reserve (WR) strain as a neurotoxic positive control. In neonatally inoculated mice, the WR strain produced significantly greater and more rapid onset of mortality than the Dryvax vaccine reference. Expression of virus antigen in neural cells and infectious virus replication in the brain was also significantly different between the two strains. In addition, the appearance of high titer virus antibody correlated with the clearance of virus from brain. With further validation, this assay incorporating a licensed vaccine reference standard and positive control strain may provide important pre-clinical neurotoxicity data on new vaccinia-based smallpox vaccine strains.

Changes in mumps virus gene sequence associated with variability in neurovirulent phenotype.

Mumps virus is highly neurotropic and, prior to widespread vaccination programs, was the major cause of viral meningitis in the United States. Nonetheless, the genetic basis of mumps virus neurotropism and neurovirulence was until recently not understood, largely due to the lack of an animal model. Here, nonneurovirulent (Jeryl Lynn vaccine) and highly neurovirulent (88-1961 wild type) mumps virus strains were passaged in human neural cells or in chicken fibroblast cells with the goal of neuroadapting or neuroattenuating the viruses, respectively. When tested in our rat neurovirulence assay against the respective parental strains, a Jeryl Lynn virus variant with an enhanced propensity for replication (neurotropism) and damage (neurovirulence) in the brain and an 88-1961 wild-type virus variant with decreased neurotropic and neurovirulent properties were recovered. To determine the molecular basis for the observed differences in neurovirulence and neuroattenuation, the complete genomes of the parental strains and their variants were fully sequenced. A comparison at the nucleotide level associated three amino acid changes with enhanced neurovirulence of the neuroadapted vaccine strain: one each in the nucleoprotein, matrix protein, and polymerase and three amino acid changes with reduced neurovirulence of the neuroattenuated wild-type strain: one each in the fusion protein, hemagglutinin-neuraminidase protein, and polymerase. The potential role of these amino acid changes in neurotropism, neurovirulence, and neuroattenuation is discussed.

Exploring the cerebellum with a new tool: neonatal Borna disease virus (BDV) infection of the rat's brain.

Cerebellar pathology has been associated with a number of developmental behavioral disorders, including autism spectrum disorders. Despite the fact that perinatal virus infections have been implicated in neurodevelopmental damage, few animal models have been developed to study the pathogenesis involved. One of the most interesting in vivo models of virus-induced cerebellar damage is the neonatal Borna disease virus (BDV) infection of the rat brain. The present review describes molecular, cellular, neuroanatomical, neurochemical and behavioral features of the BDV model and also provides a basis for a new understanding of the pathogenic mechanisms of cerebellar malformation and associated behavioral deficits.

Peripheral expression of self-MHC-II influences the reactivity and self-tolerance of mature CD4(+) T cells: evidence from a lymphopenic T cell model.

While intrathymic MHC expression influences the specificity of developing thymocytes, we considered that peripheral MHC expression might influence the reactivity of postthymic T cells. We now report for CD4(+) T cells that peripheral MHC-II expression does influence their reactivity and self-tolerance. Upon transfer into MHC-II-deficient lymphopenic hosts, mature CD4(+) T cells were found to acquire an activated memory phenotype and to become: (1) autoreactive against syngeneic MHC-II(+) skin grafts, (2) hyperreactive against third-party MHC-II(+) skin grafts, and (3) functionally dysregulated, resulting in a lymphoproliferative disorder characterized by intraepithelial infiltrations. Peripheral MHC-II expression appeared to influence CD4(+) T cell reactivity by two complementary mechanisms: maintenance of CD4(+)CD25(+) regulatory T cells ("suppression") and direct dampening of CD4(+) T cell reactivity ("tuning").

Enhanced neurovirulence of borna disease virus variants associated with nucleotide changes in the glycoprotein and L polymerase genes.

Borna disease virus (BDV) infection produces a variety of clinical diseases, from behavioral illnesses to classical fatal encephalitis (i.e., Borna disease [BD]). Since the genomes of most BDV isolates differ by less than 5%, host factors are believed responsible for much of the reported variability in disease expression. The contribution of BDV genomic differences to variation in BD expression is largely unexplored. Here we compared the clinical outcomes of rats infected with one of two related BDV variants, CRP3 or CRNP5. Compared to rats inoculated with CRP3, adult and newborn Lewis rats inoculated with CRNP5 had more severe and rapidly fatal neurological disease, with increased damage to the hippocampal pyramidal neurons and rapid infection of brain stem neurons. To identify possible virus-specific contributions to the observed variability in disease outcome, the genomes of CRP3 and CRNP5 were sequenced. Compared to CRP3, there were four nucleotide changes in the CRNP5 variant, two each in the G protein and in the L polymerase, resulting in four amino acid changes. These results suggest that small numbers of genomic differences between BDV variants in the G protein and/or L polymerase can contribute to the variability in BD outcomes.

Effects of genetic background on neonatal Borna disease virus infection-induced neurodevelopmental damage. I. Brain pathology and behavioral deficits.

The pathogenic mechanisms of gene-environment interactions determining variability of human neurodevelopmental disorders remain unclear. In the two consecutive papers, we used the neonatal Borna disease virus (BDV) infection rat model of neurodevelopmental damage to evaluate brain pathology, monoamine alterations, behavioral deficits, and responses to pharmacological treatments in two inbred rat strains, Lewis and Fisher344. The first paper reports that despite comparable virus replication and distribution in the brain of both rat strains, neonatal BDV infection produced significantly greater thinning of the neocortex in BDV-infected Fisher344 rats compared to BDV-infected Lewis rats, while no strain-related differences were found in BDV-induced granule cell loss in the dentate gyrus of the hippocampus and cerebellar hypoplasia. Unlike BDV-infected Lewis rats, more severe BDV-induced brain pathology in Fisher344 rats was associated with (1) greater locomotor activity to novelty and (2) impairment of habituation and prepulse inhibition of the acoustic startle response. The present data demonstrate that the same environmental insult can produce differential neuroanatomical and behavioral abnormalities in genetically different inbred rat strains.

Effects of genetic background on neonatal Borna disease virus infection-induced neurodevelopmental damage. II. Neurochemical alterations and responses to pharmacological treatments.

The gene-environment interplay is thought to determine variability in clinical conditions and responses to therapy in human neurodevelopmental disorders. Studying abnormal brain and behavior development in inbred strains of rodents can help in the identification of the complex pathogenic mechanisms of the host-environment interaction. This paper is the second one in a series of the two reports of the use of the Borna disease virus (BDV) infection model of neurodevelopmental damage to characterize effects of genetic background on virus-induced neurodevelopmental damage in inbred rat strains, Lewis and Fisher344. The present data demonstrate that neonatal BDV infection produced regional and strain-related alterations in levels of serotonin, norepinephrine and in levels of serotonin turnover at postnatal day 120. Neonatal BDV infection also induced upregulation of hippocampal 5-HT(1a) and cortical 5-HT(2a) receptors in Lewis rats and downregulation of cortical 5-HT(2a) receptors in Fisher344 rats. BDV-associated regional downregulation of D(2) receptors and dopamine transporter sites were noted in Fisher344 rats. In addition to the neurochemical disturbances, neonatal BDV infection induced differential responses to serotonin compounds. While 8-OH-DPAT suppressed virus-enhanced ambulation in BDV-infected Fisher344, fluoxetine inhibited virus-induced hyperactivity in BDV-infected Lewis rats only. The present data provide new insights into the pathogenic events that lead to differential responses to pharmacological treatments in genetically different animals following exposure to the same environmental challenge.

Borna disease virus infection of the neonatal rat: developmental brain injury model of autism spectrum disorders.

Autism spectrum disorders (ASD) have been the focus of a great deal of research and clinical speculation. This intense interest relates to both the perplexing pathogenesis and devastating consequences of these disorders. One of the obstacles to understanding the pathogenesis of autism and its efficient treatment has been the paucity of animal models that could be used for hypotheses-driven mechanistic studies of abnormal brain and behavior development and for the pre-clinical testing novel pharmacological treatments. The present review provides a detailed analysis of a new animal model of ASD. This model utilizes neonatal Borna disease virus (BDV) infection of the rat brain as a unique experimental teratogen to study the pathogenesis of neurodevelopmental damage. For more than a decade, studies of the BDV animal model have yielded much insight into the pathogenic processes of abnormal brain development and resulting autistic-like behavioral abnormalities in rats. The most recent experiments demonstrate the utility of the BDV model for studying the pathophysiological mechanisms of the gene-environment interaction that determines differential disease outcomes and variability in responses to treatments.