Restriction of measles virus gene expression in acute and subacute encephalitis of Lewis rats

Suppression of viral RNA polymerase activity is necessary for persistent infection during the transformation of measles virus into SSPE virus

Subacute sclerosing panencephalitis (SSPE) is a fatal neurodegenerative disease caused by measles virus (MV), which typically develops 7 to 10 years after acute measles. During the incubation period, MV establishes a persistent infection in the brain and accumulates mutations that generate neuropathogenic SSPE virus. The neuropathogenicity is closely associated with enhanced propagation mediated by cell-to-cell fusion in the brain, which is principally regulated by hyperfusogenic mutations of the viral F protein. The molecular mechanisms underlying establishment and maintenance of persistent infection are unclear because it is impractical to isolate viruses before the appearance of clinical signs. In this study, we found that the L and P proteins, components of viral RNA-dependent RNA polymerase (RdRp), of an SSPE virus Kobe-1 strain did not promote but rather attenuated viral neuropathogenicity. Viral RdRp activity corresponded to F protein expression; the suppression of RdRp activity in the Kobe-1 strain because of mutations in the L and P proteins led to restriction of the F protein level, thereby reducing cell-to-cell fusion mediated propagation in neuronal cells and decreasing neuropathogenicity. Therefore, the L and P proteins of Kobe-1 did not contribute to progression of SSPE. Three mutations in the L protein strongly suppressed RdRp activity. Recombinant MV harboring the three mutations limited viral spread in neuronal cells while preventing the release of infectious progeny particles; these changes could support persistent infection by enabling host immune escape and preventing host cell lysis. Therefore, the suppression of RdRp activity is necessary for the persistent infection of the parental MV on the way to transform into Kobe-1 SSPE virus. Because mutations in the genome of an SSPE virus reflect the process of SSPE development, mutation analysis will provide insight into the mechanisms underlying persistent infection.

Noncanonical Transmission of a Measles Virus Vaccine Strain from Neurons to Astrocytes

Viruses, including members of the herpes-, entero-, and morbillivirus families, are the most common cause of infectious encephalitis in mammals worldwide. During most instances of acute viral encephalitis, neurons are typically the initial cell type that is infected. However, as replication and spread ensue, other parenchymal cells can become viral targets, especially in chronic infections. Consequently, to ascertain how neurotropic viruses trigger neuropathology, it is crucial to identify which central nervous system (CNS) cell populations are susceptible and permissive throughout the course of infection, and to define how viruses spread between distinct cell types. Using a measles virus (MV) transgenic mouse model that expresses human CD46 (hCD46), the MV vaccine strain receptor, under the control of a neuron-specific enolase promoter (NSE-hCD46+ mice), a novel mode of viral spread between neurons and astrocytes was identified. Although hCD46 is required for initial neuronal infection, it is dispensable for heterotypic spread to astrocytes, which instead depends on glutamate transporters and direct neuron-astrocyte contact. Moreover, in the presence of RNase A, astrocyte infection is reduced, suggesting that nonenveloped ribonucleoproteins (RNP) may cross the neuron-astrocyte synaptic cleft. The characterization of this novel mode of intercellular transport offers insights into the unique interaction of neurons and glia and may reveal therapeutic targets to mitigate the life-threatening consequences of measles encephalitis.IMPORTANCE Viruses are the most important cause of infectious encephalitis in mammals worldwide; several thousand people, primarily the very young and the elderly, are impacted annually, and few therapies are reliably successful once neuroinvasion has occurred. To understand how viruses contribute to neuropathology, and to develop tools to prevent or ameliorate such infections, it is crucial to define if and how viruses disseminate among the different cell populations within the highly complex central nervous system. This study defines a noncanonical mode of viral transmission between neurons and astrocytes within the brain.

The phosphoprotein genes of measles viruses from subacute sclerosing panencephalitis cases encode functional as well as non-functional proteins and display reduced editing

Products expressed from the second (P/V/C) gene are important in replication and abrogating innate immune responses during acute measles virus (MV) infection. Thirteen clone sets were derived from the P/V/C genes of measles virus (MV) RNA extracted from brains of a unique collection of seven cases of subacute sclerosing panencephalitis (SSPE) caused by persistent MV in the central nervous system (CNS). Whether these functions are fully maintained when MV replicates in the CNS has not been previously determined. Co-transcriptional editing of the P mRNAs by non-template insertion of guanine (G) nucleotides, which generates mRNAs encoding the viral V protein, occurs much less frequently (9%) in the SSPE derived samples than during the acute infection (30-50%). Thus it is likely that less V protein, which is involved in combatting the innate immune response, is produced. The P genes in MV from SSPE cases were not altered by biased hypermutation but exhibited a high degree of variation within each case. Most but not all SSPE derived phospho-(P) proteins were functional in mini genome replication/transcription assays. An eight amino acid truncation of the carboxyl-terminus made the P protein non-functional while the insertion of an additional glycine residue by insertion of G nucleotides at the editing site had no effect on protein function.

Biased hypermutation occurred frequently in a gene inserted into the IC323 recombinant measles virus during its persistence in the brains of nude mice

Measles virus (MV) is the causative agent of measles and its neurological complications, subacute sclerosing panencephalitis (SSPE) and measles inclusion body encephalitis (MIBE). Biased hypermutation in the M gene is a characteristic feature of SSPE and MIBE. To determine whether the M gene is the preferred target of hypermutation, an additional transcriptional unit containing a humanized Renilla reniformis green fluorescent protein (hrGFP) gene was introduced into the IC323 MV genome, and nude mice were inoculated intracerebrally with the virus. Biased hypermutation occurred in the M gene and also in the hrGFP gene when it was inserted between the leader and the N gene, but not between the H and L gene. These results indicate that biased hypermutation is usually found in a gene whose function is not essential for viral proliferation in the brain and that the location of a gene in the MV genome can affect its mutational frequency.

The measles virus replication cycle

This review describes the two interrelated and interdependent processes of transcription and replication for measles virus. First, we concentrate on the ribonucleoprotein (RNP) complex, which contains the negative sense genomic template and in encapsidated in every virion. Second, we examine the viral proteins involved in these processes, placing particular emphasis on their structure, conserved sequence motifs, their interaction partners and the domains which mediate these associations. Transcription is discussed in terms of sequence motifs in the template, editing, co-transcriptional modifications of the mRNAs and the phase of the gene start sites within the genome. Likewise, replication is considered in terms of promoter strength, copy numbers and the remarkable plasticity of the system. The review emphasises what is not known or known only by analogy rather than by direct experimental evidence in the MV replication cycle and hence where additional research, using reverse genetic systems, is needed to complete our understanding of the processes involved.

Measles virus minigenomes encoding two autofluorescent proteins reveal cell-to-cell variation in reporter expression dependent on viral sequences between the transcription units

Transcription from morbillivirus genomes commences at a single promoter in the 3′ non-coding terminus, with the six genes being transcribed sequentially. The 3′ and 5′ untranslated regions (UTRs) of the genes (mRNA sense), together with the intergenic trinucleotide spacer, comprise the non-coding sequences (NCS) of the virus and contain the conserved gene end and gene start signals, respectively. Bicistronic minigenomes containing transcription units (TUs) encoding autofluorescent reporter proteins separated by measles virus (MV) NCS were used to give a direct estimation of gene expression in single, living cells by assessing the relative amounts of each fluorescent protein in each cell. Initially, five minigenomes containing each of the MV NCS were generated. Assays were developed to determine the amount of each fluorescent protein in cells at both cell population and single-cell levels. This revealed significant variations in gene expression between cells expressing the same NCS-containing minigenome. The minigenome containing the M/F NCS produced significantly lower amounts of fluorescent protein from the second TU (TU2), compared with the other minigenomes. A minigenome with a truncated F 5′ UTR had increased expression from TU2. This UTR is 524 nt longer than the other MV 5′ UTRs. Insertions into the 5′ UTR of the enhanced green fluorescent protein gene in the minigenome containing the N/P NCS showed that specific sequences, rather than just the additional length of F 5′ UTR, govern this decreased expression from TU2.

RNA interference with measles virus N, P, and L mRNAs efficiently prevents and with matrix protein mRNA enhances viral transcription

In contrast to studies with genetically modified viruses, RNA interference allows the analysis of virus infections with identical viruses and posttranscriptional ablation of individual gene functions. Using RNase III-generated multiple short interfering RNAs (siRNAs) against the six measles virus genes, we found efficient downregulation of viral gene expression in general with siRNAs against the nucleocapsid (N), phosphoprotein (P), and polymerase (L) mRNAs, the translation products of which form the ribonucleoprotein (RNP) complex. Silencing of the RNP mRNAs was highly efficient in reducing viral messenger and genomic RNAs. siRNAs against the mRNAs for the hemagglutinin (H) and fusion (F) proteins reduced the extent of cell-cell fusion. Interestingly, siRNA-mediated knockdown of the matrix (M) protein not only enhanced cell-cell fusion but also increased the levels of both mRNAs and genomic RNA by a factor of 2 to 2.5 so that the genome-to-mRNA ratio was constant. These findings indicate that M acts as a negative regulator of viral polymerase activity, affecting mRNA transcription and genome replication to the same extent.

Antibody reactivity to individual structural proteins of measles virus in the CSF of SSPE and MS patients

BACKGROUND

Chronic progressive disorders of the central nervous system (CNS) impose diagnostic problems, particularly in younger patients. The demonstration of antibodies against measles virus (MV) in the cerebrospinal fluid (CSF) plays a major role in the laboratory diagnosis of subacute sclerosing panencephalitis (SSPE) as well as multiple sclerosis (MS).

OBJECTIVES

Because intrathecally synthesized antibodies against MV can be found in both diseases, it is necessary to establish easy and reliable methods to improve the differential diagnosis.

STUDY DESIGN

Seventy-one paired serum/CSF samples obtained from patients with the diagnosis of SSPE (n = 23), MS (n = 14), or acute postinfectious measles encephalitis (APME, n = 8) have been examined. The reactivity of intrathecally synthesized immunoglobulin to individual recombinant MV structural proteins was assessed using Western blot analysis, ELISA as well as isoelectric focusing (IEF).

RESULTS

All CSF samples obtained from patients suffering from SSPE showed a strong antibody response to MV-nucleocapsid (N) and phosphoprotein (P). Sera from 15 of the 23 SSPE patients were reactive to MV-fusion protein (F). Faint reactivity was obtained against MV-matrix (M) or hemagglutinin protein (H) in the minority of samples (40 and 20%, respectively). CSF samples of MS patients only revealed a clear response to N, and in two cases to F. The other proteins were not recognized in the CSF samples of MS patients. In contrast to SSPE, the IEF of CSF from MS patients revealed only few MV-specific oligoclonal bands. In the CSF samples from APME patients, intrathecal MV antibodies were not detected.

CONCLUSIONS

This study shows that discrimination between SSPE and MS can be achieved in doubtful cases by IEF using MV-N, P and F proteins.

Expression of the interferon-alpha/beta-inducible MxA protein in brain lesions of subacute sclerosing panencephalitis

The type-I interferon (IFN) inducible human MxA protein exhibits antiviral activity against a variety of RNA viruses including the measles virus (MV). In this study, we investigated the association between the expression of MV antigens and MxA in subacute sclerosing panencephalitis (SSPE) brains. We analyzed the MxA expression in and around lesions in brains of three SSPE patients and compared it with normal brains. Double staining with antibodies against MxA and the MV nucleocapsid revealed that MxA was highly expressed in a belt surrounding MV-antigen-positive lesions in SSPE brains. In normal appearing regions distant from a lesion in SSPE brains and in normal brains, MxA was not detected. Furthermore, MxA was often less or not expressed in the center of lesions expressing high amounts of MV antigens. Such a pattern of MxA expression in SSPE brains clearly indicates that newly infected cells release type I IFN and will become demarcated by a protecting barrier of MxA expressing cells. Double staining with antibodies against MxA and glial fibrillary acidic protein (GFAP) showed that the MxA protein was expressed mainly in the cytoplasm of astrocytes. MxA expression did not correlate with the presence of cellular infiltrates of inflammatory cells, although some lymphoid cells were also positive for MxA. Since MxA inhibits the replication of MV, these findings suggest that the IFN-induced MxA protein plays an important role in slowing down the viral spread in SSPE brains and by doing so may contribute to the persistence of the MV-infection.

Virus demyelination

A number of viruses can initiate central nervous system (CNS) diseases that include demyelination as a major feature of neuropathology. In humans, the most prominent demyelinating diseases are progressive multifocal leukoencephalopathy, caused by JC papovirus destruction of oligodendrocytes, and subacute sclerosing panencephalitis, an invariably fatal childhood disease caused by persistent measles virus. The most common neurological disease of young adults in the developed world, multiple sclerosis, is also characterized by lesions of inflammatory demyelination; however, the etiology of this disease remains an enigma. A viral etiology is possible, because most demyelinating diseases of known etiology in both man and animals are viral. Understanding of the pathogenesis of virus-induced demyelination derives for the most part from the study of animal models. Studies with neurotropic strains of mouse hepatitis virus, Theiler's virus, and Semliki Forest virus have been at the forefront of this research. These models demonstrate how viruses enter the brain, spread, persist, and interact with immune responses. Common features are an ability to infect and persist in glial cells, generation of predominantly CD8(+) responses, which control and clear the early phase of virus replication but which fail to eradicate the infection, and lesions of inflammatory demyelination. In most cases demyelination is to a limited extent the result of direct virus destruction of oligodendrocytes, but for the most part is the consequence of immune and inflammatory responses. These models illustrate the roles of age and genetic susceptibility and establish the concept that persistent CNS infection can lead to the generation of CNS autoimmune responses.

Infection of human oligodendroglioma cells by a recombinant measles virus expressing enhanced green fluorescent protein

One of the hallmarks of the human CNS disease subacute sclerosing panencephalitis (SSPE) is a high level of measles virus (MV) infection of oligodendrocytes. It is therefore surprising that there is only one previous report of MV infection of rat oligodendrocytes in culture and no reports of human oligodendrocyte infection in culture. In an attempt to develop a model system to study MV infection of oligodendrocytes, time-lapse confocal microscopy, immunocytochemistry, and electron microscopy (EM) were used to study infection of the human oligodendroglioma cell line, MO3.13. A rat oligodendrocyte cell line, OLN-93, was also studied as a control. MO3.13 cells were shown to be highly susceptible to MV infection and virus budding was observed from the surface of infected MO3.13 cells by EM. Analysis of the infection in real time and by immunocytochemistry revealed that virus spread occurred by cell-to-cell fusion and was also facilitated by virus transport in cell processes. MO3.13 cells were shown to express CD46, a MV receptor, but were negative for the recently discovered MV receptor, signaling leucocyte activation molecule (SLAM). Immunohistochemical studies on SSPE tissue sections demonstrated that CD46 was also expressed on populations of human oligodendrocytes. SLAM expression was not detected on oligodendrocytes. These studies, which are the first to show MV infection of human oligodendrocytes in culture, show that the cells are highly susceptible to MV infection and this model cell line has been used to further our understanding of MV spread in the CNS.

Recombinant measles viruses expressing altered hemagglutinin (H) genes: functional separation of mutations determining H antibody escape from neurovirulence

Measles virus (MV) strain CAM/RB, which was adapted to growth in the brain of newborn rodents, is highly neurovirulent. It has been reported earlier that experimentally selected virus variants escaping from the monoclonal antibodies (MAbs) Nc32 and L77 to hemagglutinin (H) preserved their neurovirulence, whereas mutants escaping MAbs K71 and K29 were found to be strongly attenuated (U. G. Liebert et al., J. Virol. 68:1486-1493, 1994). To investigate the molecular basis of these findings, we have generated a panel of recombinant MVs expressing the H protein from CAM/RB and introduced the amino acid substitutions thought to be responsible for antibody escape and/or neurovirulence. Using these recombinant viruses, we identified the amino acid changes conferring escape from the MAbs L77 (377R-->Q and 378M-->K), Nc32 (388G-->S), K71 (492E-->K and 550S-->P), and K29 (535E-->G). When the corresponding recombinant viruses were tested in brains of newborn rodents, we found that the mutations mediating antibody escape did not confer differential neurovirulence. In contrast, however, replacement of two different amino acids, at positions 195G-->R and 200S-->N, which had been described for the escape mutant set, caused the change in neurovirulence. Thus, antibody escape and neurovirulence appear not to be associated with the same structural alterations of the MV H protein.

Age-related changes in susceptibility of rat brain slice cultures including hippocampus to encephalomyocarditis virus

Replication of the D variant of encephalomyocarditis virus (EMC-D) and its cytopathic effects were studied in the brain slice cultures including hippocampus (hippocampal slice) obtained from postnatal 1-, 4-, 7-, 14-, 28-and 56-day-old Fischer 344 rats. At 0, 12, 24, 36 and 48 h after infection, virus titres of the slices and culture media were assayed. Viral replication was observed in cultures from 1-to 28-day-old rats, and the highest titre was recorded in the slice and culture medium from the youngest rat. The peak of virus titre decreased with age and no distinct viral replication was observed in the cultures from 56-day-old rats. Light microscopy revealed that degenerative and necrotic changes appeared in the infected hippocampal slices from 1- to 28-day-old rats, and the changes became less prominent with age. In situ hybridization and indirect immunofluorescence staining showed that positive signals of viral RNA and antigen were prominent in younger rats and decreased with age. These results suggest that an age-related decrease in the susceptibility of rat brain to EMC-D is less related to the maturation of the immune system but possibly to that of the neurone.

Observation of measles virus cell-to-cell spread in astrocytoma cells by using a green fluorescent protein-expressing recombinant virus

A recombinant measles virus (MV) which expresses enhanced green fluorescent protein (EGFP) has been rescued. This virus, MVeGFP, expresses the reporter gene from an additional transcription unit which is located prior to the gene encoding the measles virus nucleocapsid protein. The recombinant virus was used to infect human astrocytoma cells (GCCM). Immunocytochemistry (ICC) together with EGFP autofluorescence showed that EGFP is both an early and very sensitive indicator of cell infection. Cells that were EGFP-positive and ICC-negative were frequently observed. Confocal microscopy was used to indirectly visualize MV infection of GCCM cells and to subsequently follow cell-to-cell spread in real time. These astrocytoma cells have extended processes, which in many cases are intimately associated. The processes appear to have an important role in cell-to-cell spread, and MVeGFP was observed to utilize them in the infection of surrounding cells. Heterogeneity was seen in cell-to-cell spread in what was expected to be a homogeneous monolayer. In tissue culture, physical constraints govern the integrity of the syncytia which are formed upon extensive cell fusion. When around 50 cells were fused, the syncytia rapidly disintegrated and many of the infected cells detached. Residual adherent EGFP-positive cells were seen to either continue to be involved in the infection of surrounding cells or to remain EGFP positive but no longer participate in the transmission of MV infection to neighboring cells.

Pathogenic aspects of measles virus infections

Measles virus (MV) infections normally cause an acute self limiting disease which is resumed by a virus-specific immune response and leads to the establishment of a lifelong immunity. Complications associated with acute measles can, on rare occasions, involve the central nervous system (CNS). These are postinfectious measles encephalitis which develops soon after infection, and, months to years after the acute disease, measles inclusion body encephalitis (MIBE) and subacute sclerosing panencephalitis (SSPE) which are based on a persistent MV infection of brain cells. Before the advent of HIV, SSPE was the best studied slow viral infection of the CNS, and particular restrictions of MV gene expression as well as MV interactions with neural cells have revealed important insights into the pathogenesis of persistent viral CNS infections. MV CNS complication do, however, not large contribute to the high rate of mortality seen in association with acute measles worldwide. The latter is due to a virus-induced suppression of immune functions which favors the establishment of opportunistic infections. Mechanisms underlying MV-mediated immunosuppression are not well understood. Recent studies have indicated that MV-induced disruption of immune functions may be multifactorial including the interference with cytokine synthesis, the induction of soluble inhibitory factors or apoptosis and negative signalling to T cells by the viral glycoproteins expressed on the surface of infected cells, particularly dendritic cells.

Tyrosine phosphorylation of measles virus P-phosphoprotein in persistently infected neuroblastoma cells

Replication and encapsidation of measles virus (MV) requires the interaction between the nuclear protein (N) and the phosphoprotein (P). It is known that both proteins are phosphorylated on serine and threonine residues. Recently we have shown that N is phosphorylated on tyrosine in persistently-infected mouse neuroblastoma cells (NS20Y/MS). Here, we show that P in NS20Y/MS is also phosphorylated on tyrosine. To investigate whether cellular tyrosine kinases can bind and phosphorylate P, a solid phase kinase assay was employed. We show that bacterially-expressed MV P fragments, were phosphorylated on tyrosine by purified mouse c-Src protein-tyrosine kinase and when mixed with uninfected neuroblastoma cell (NS20Y) extracts, these P fragments were phosphorylated on tyrosine in addition to serine and threonine. These results imply that MV P is a substrate for tyrosine phosphorylation by cellular tyrosine kinase(s).

Tyrosine phosphorylation of measles virus nucleocapsid protein in persistently infected neuroblastoma cells

Subacute sclerosing panencephalitis is a slowly progressing fatal human disease of the central nervous system which is a delayed sequel of measles virus (MV) infection. A typical pathological feature of this disease is the presence of viral ribonucleocapsid structures in the form of inclusion bodies and the absence of infectious virus or budding viral particles. The mechanisms governing the establishment and maintenance of a persistent MV infection in brain cells are still largely unknown. To understand the mechanisms underlying MV persistence in neuronal cells, a tissue culture model was studied. Clone NS20Y/MS of the murine neuroblastoma C1300 persistently infected with the wild-type Edmonston strain of MV secretes relatively high levels of alpha/beta interferon (IFN). As shown previously, treatment of the persistently infected cultures with anti-IFN serum converted the persistent state into a productive infection indicated by the appearance of multinucleated giant cells. In this study, we have investigated whether alpha/beta IFN produced by NS20Y/MS cells activates cellular protein tyrosine kinases which will induce tyrosine phosphorylating activity specific to virus-infected cells. We present data to show augmented protein tyrosine kinase activity in the persistently infected cells. We demonstrate that the MV N protein is phosphorylated on tyrosine in addition to serine and threonine in the persistent state but not in NS20Y cells acutely infected with MV.

Analysis of the molecular basis of neuropathogenesis of RNA viruses in experimental animals: relevance for human disease?

RNA viruses with segmented genomes were the first model used for molecular analysis of viral neuropathogenesis, since they could be analysed genetically by reassortment. Four viruses with non-segmented genomes have been used as models of neurovirulence and demyelinating disease: JHM coronavirus, Theiler's virus, Sindbis virus and Semliki Forest virus (SFV). Virus gene expression in the central nervous system of infected animals has been measured by in situ hybridization and immunocytochemistry. Cell tropism has been analysed by neural cell culture. Infectious clones have been constructed for Theiler's virus, Sindbis virus and SFV, and these allow analysis of the sequences involved in the determination of neuropathogenesis, through the construction of chimeric viruses and site-specific mutagenesis. Measles and rubella viruses have been studied in animal systems because of their importance for human disease. The importance of two recently discovered mechanisms of neuropathogenesis, antibody-induced modulation of virus multiplication, and persistence of virus in the absence of multiplication, remains to be assessed.

MxA-dependent inhibition of measles virus glycoprotein synthesis in a stably transfected human monocytic cell line

The alpha/beta (type I) interferon-inducible human MxA protein confers resistance to vesicular stomatitis virus (VSV) and influenza A virus in MxA-transfected mouse 3T3 cells (3T3/MxA). We investigated the inhibitory effects of the MxA protein on measles virus (MV) and VSV in the human monocytic cell line U937. In transfected U937 clones which constitutively express MxA (U937/MxA), the release of infectious MV and VSV was reduced approximately 100-fold in comparison with control titers. Transcription of VSV was inhibited similar to that observed for 3T3/MxA cells, whereas no difference was detected for MV in the rates of transcription or the levels of MV-specific mRNAs. In contrast, analysis of MV protein expression by immunofluorescence and immunoprecipitation revealed a significant reduction in the synthesis of MV glycoproteins F and H in U937/MxA cells. These data demonstrate a virus-specific effect of MxA which may, in the case of MV, contribute to the establishment of a persistent infection in human monocytic cells.

Spontaneous and differentiation-dependent regulation of measles virus gene expression in human glial cells

The expression of measles virus (MV) in six different permanent human glioma cell lines (D-54, U-251, U-138, U-105, U-373, and D-32) was analyzed. Although all cell lines were permissive for productive replication of all MV strains tested, U-251, D-54, and D-32 cells spontaneously revealed restrictions of MV transcription similar to those observed for primary rat astroglial cells and brain tissue. In vitro differentiation of D-54 and U-251 cells by substances affecting the intracellular cyclic AMP level caused a significant reduction of the expression of the viral proteins after 18, 72, and 144 h of infection. This pronounced restriction was not paralleled to a comparable level by an inhibition of the synthesis and biological activity in vitro of virus-specific mRNAs as shown by quantitative Northern (RNA) blot analyses and in vitro translation. The block in viral protein synthesis could not be attributed to the induction of type I interferon by any of the substances tested. Our findings indicate that down-regulation of MV gene expression in human brain cells can occur by a cell type-dependent regulation of the viral mRNA transcription and a differentiation-dependent regulation of translation, both of which may be crucial for the establishment of persistent MV infections in the central nervous system.

Restricted expression of viral surface proteins in canine distemper encephalitis

Sixteen dogs with naturally occurring acute and chronic canine distemper virus (CDV) encephalitis were examined immunohistochemically for the presence of the five major CDV-specific proteins in the central nervous system. Monoclonal antibodies (mAbs) directed against two, three, four and five epitopes of the nucleo- (N), phospho- (P), fusion (F), and hemagglutinin (H) protein, respectively, and a polyclonal monospecific antibody recognizing the matrix (M) protein were used. Both core proteins and their epitopes, three F protein epitopes and the M protein were demonstrated in all animals examined. A fourth F protein epitope was found only in 13 animals. The H-2 and H-3 epitope of the H protein were detected in 15, the H-1 and H-5 epitope in 14, and the H-4 epitope in 3 animals. All viral proteins were observed in the same types of brain cells including neurons and astrocytes. The N and P protein were demonstrated in nucleus, cytoplasm and cell processes, whereas M, H and F protein were observed in the cytoplasm only and rarely in cell processes. In addition, the M protein was detected occasionally in the nucleus of neurons and reactive astrocytes. Intralesional distribution of CDV-specific proteins varied between core and surface proteins. In acute and subacute lesions without associated inflammation, expression of the M, H and F protein was only slightly diminished compared to the N and P protein. However, plaques with severe inflammation were either devoid of viral antigen or exhibited N- and P protein-specific immunoreactivity exclusively at the periphery, whereas expression of surface proteins was severely reduced or absent. These results are suggestive of restricted synthesis of CDV envelope proteins in acute, and more prominent in chronic, distemper encephalitis.

Cell fusion by the envelope glycoproteins of persistent measles viruses which caused lethal human brain disease

Measles virus (MV) rarely induces lethal diseases of the human central nervous system characterized by reduced expression of the viral envelope proteins and by lack of viral budding. The MV envelope contains two integral membrane proteins, termed fusion (F) protein and hemagglutinin (H) protein, and a membrane-associated matrix (M) protein. Previously, analysis of MV genes from autopsy material indicated that the M protein and the F protein intracellular domain are often drastically altered by mutations. Here, we present evidence that truncation of the F protein intracellular domain does not impair fusion function, and we suggest that this alteration interferes with viral budding. Unexpectedly, certain combinations of functional F and H proteins were unable to induce syncytium formation, an observation suggesting that specific F-H protein interactions are required for cell fusion. We also found that three of four H proteins of persistent MVs are defective in intracellular transport, oligosaccharide modification, dimerization, and fusion helper function. Thus, MVs replicating in the brain at the terminal stage of infection are typically defective in M protein and in the two integral membrane proteins. Whereas the M protein appears dispensable altogether, partial preservation of F-protein function and H-protein function seems to be required, presumably to allow local cell fusion. Certain subtle alterations of the F and H proteins may be instrumental for disease development.

Pathogenesis of virus-induced demyelination

Demyelination is a component of several viral diseases of humans. The best known of these are subacute sclerosing panencephalitis (SSPE) and progressive multifocal leukoencephalopathy (PML). There are a number of naturally occurring virus infections of animals that involve demyelination and many of these serve as instructive models for human demyelinating diseases. In addition to the naturally occurring diseases, many viruses have been shown to be capable of producing demyelination in experimental situations. In discussing virus-associated demyelinating disease, the chapter reviews the architecture and functional organization of the CNS and considers what is known of the interaction of viruses with CNS cells. It also discusses the immunology of the CNS that differs in several important aspects from that of the rest of the body. Experimental models of viral-induced demyelination have also been considered. Viruses capable of producing demyelinating disease have no common taxonomic features; they include both DNA and RNA viruses, enveloped and nonenveloped viruses. The chapter attempts to summarize the important factors influencing viral demyelination, their common features, and possible mechanisms.

Antibody-dependent transcriptional regulation of measles virus in persistently infected neural cells

Application of neutralizing anti-hemagglutinin antibodies to mouse neuroblastoma cells (NS20Y/MS) persistently infected with measles virus (MV) leads to a significant reduction of viral structural proteins within 6 days. While the transcriptional gradient for MV-specific mRNAs remained unaffected upon antibody treatment, the total amount of MV-specific transcripts dropped by 80% after 24 h. The expression of genomic RNA was affected similarly, with slightly slower time kinetics. Both transcription and expression of the viral structural proteins could be completely reactivated when viral antibodies were removed from the tissue culture. The same findings could be obtained in rat glioma cells persistently infected with subacute sclerosing panencephalitis virus (C6/SSPE) but not in cells of nonneural origin. The data indicate that antibody-induced antigenic modulation affects the early stages of viral transcription within a few hours after the addition of antibodies and leads to an almost complete repression of viral gene expression in cells of neural origin.

Expression and properties of the V protein in acute measles virus and subacute sclerosing panencephalitis virus strains

Measles virus (MV) inserts one guanosine (G) residue at a specific site in a subpopulation of the mRNA transcribed from the phosphoprotein (P) gene to produce V mRNA. Using an antiserum against the unique carboxyl-terminal region of the predicted V protein, we found that a phosphorylated V protein was expressed in two acute MV strains (Edmonston and Nagahata) and three SSPE virus strains (Biken, Yamagata, and Niigata). The V protein of Biken strain SSPE virus was electrophoretically and antigenically indistinguishable from the V protein of Nagahata strain acute MV, the likely progenitor of the Biken strain. The V protein of these two viruses was not present in the intracellular viral nucleocapsids, but was found only in the cytosolic free protein pool. Pulse-chase experiments failed to show transport of the V protein to the plasma membrane. The V protein was also absent in the extracellular virions. The P protein synthesized from the cloned gene associated with the MV nucleocapsids in vitro, but the V protein had no affinity to the MV nucleocapsids. These results suggest that expression and properties of the V protein are conserved in chronic MV infection.

Constant and variable regions of measles virus proteins encoded by the nucleocapsid and phosphoprotein genes derived from lytic and persistent viruses

The nucleotide sequences of the N and P genes of two wild type measles virus strains JM and CM in two distinct lineages of the virus have been analyzed and compared with those of other MV strains in order to assess which parts of the internal proteins are variable. Most variations in the P protein appear to occur in the N-terminus, while the middle part of the protein (residues 201-350) and the C-terminus are conserved. The C protein varies primarily in its N-terminal amino acids. The C-terminal amino acid residues of the V protein, which are unique to this protein, do not vary significantly between measles virus strains. The data show that evolutionary trees determined on the basis of the N, P, or M genes are the same and that probably no recombination has taken place between these genes in the strains investigated so far. The M protein appears to be less variable than the other genes and thus changes observed in this gene in some SSPE and MIBE viruses may be of greater significance than were assumed earlier.

Subacute sclerosing panencephalitis is typically characterized by alterations in the fusion protein cytoplasmic domain of the persisting measles virus

Our recent extensive analysis of three cases of subacute sclerosing panencephalitis (SSPE) revealed intriguing genetic defects in the persisting measles virus (MV): the fusion (F) genes encoded truncated cytoplasmic F protein domains (Cattaneo et al., Virology 173, 415-425, 1989). Now this MV genomic region has been investigated in eight additional SSPE cases by PCR amplification, replacement cloning into a vector containing the F gene of a lytic MV, in vitro expression, and sequencing. In all cases at least part of the clones showed mutations leading to F protein truncations, elongation, or nonconservative amino acid replacements. It is proposed that alteration of the F protein cytoplasmic domain may play a critical role in the development of SSPE.

Expression of measles virus RNA in peripheral blood mononuclear cells of patients with measles, SSPE, and autoimmune diseases

In order to characterize measles virus (MV) infection in peripheral blood mononuclear cells (PBMCs), RNA was isolated from PBMCs after PHA-stimulation for 72 hr of 9 patients with acute measles, 16 patients with subacute sclerosing panencephalitis (SSPE), 13 patients with various autoimmune diseases, and 16 healthy control donors. The RNA obtained was screened for the presence of MV N (nucleocapsid) gene specific transcripts of either positive or negative orientation in a S1 nuclease protection assay. The sensitivity of this assay allowed us to detect one infected cell in 20,000 PBMCs or 0.1 to 0.05 copies of MV-specific RNA per cell. Using single-stranded DNA or RNA probes expression of MV genomic RNA of negative polarity could be detected in only one case of acute measles and one healthy control donor. Conversely, N-specific transcripts of positive polarity, indicating active transcription, could only be detected in patients with acute measles. In addition, in infected PBMCs and in a persistently MV-infected B cell line positive stranded N-specific transcripts containing leader usually present at very low frequency have been found in relatively increased amounts in comparison with transcripts lacking leader. Whereas the ratio of these RNA species during lytic infection with MV in Vero cells is about 1:50, the ratio found here ranges from 1:3 to 1:10. This altered ratio indicates a specific regulation of MV specific transcription in cells of lymphoid origin that has not been found in any other cell system analyzed.

Restricted expression of measles virus in primary rat astroglial cells

Persistent infection of the central nervous system (CNS) with measles virus (MV) is associated with characteristic restrictions of viral envelope gene expression as documented in subacute sclerosing panencephalitis (SSPE), measles inclusion body encephalitis (MIBE), or subacute measles encephalitis (SAME) in rats. To determine whether these restrictions are the result of a long lasting virus-host cell interaction or primarily based on intrinsic brain cell factors MV gene expression was analyzed in primary rat astroglial cultures. It could be shown that MV infection of these cells led to a defective replication cycle with a reduced synthesis of viral envelope proteins and a steep expression gradient of the monocistronic viral mRNAs similar to the findings in brain tissue of SSPE, MIBE, and SAME. This restriction of MV gene expression has not been observed in cells of nonneural origin. We suggest that this cell-type specific regulation of MV gene expression contributes to early events in the establishment of MV persistent infection in CNS tissue.

Antibody-induced restriction of viral gene expression in measles encephalitis in rats

After infection with the neurotropic CAM/RBH measles virus (MV) strain, newborn Lewis rats succumb to an acute necrotizing encephalopathy. Passive transfer of neutralizing monoclonal antibodies directed against MV hemagglutinin prevented this disease process. Instead, either an antibody-induced acute or subacute measles encephalitis developed after a prolonged incubation period with a restricted expression of MV structural proteins. The molecular biological analysis of MV gene expression in brain tissue of rats treated with MV-neutralizing antibodies revealed a transcriptional restriction of viral mRNAs, particularly for the envelope proteins, leading to a steep expression gradient. Based on in situ hybridization, it was concluded that the efficiency of transcription of viral genes at the single-cell level is reduced compared with that of controls. Passive immunization with monoclonal antibodies directed against other MV structural proteins proved to be ineffective. Similar results were obtained in MV-infected weanling Brown Norway rats. These rats developed a clinically silent encephalitis in the presence of high titers of neutralizing antibodies. In such animals, a pronounced attenuation of the viral gene transcription was observed. These findings indicated that neutralizing antibodies directed against a restricted set of specific antigenic sites on the viral hemagglutinin protein expressed on cell membranes exert a modulating effect on the viral gene expression at the level of transcription. This phenomenon contributes to the switch from the acute cytopathic effect to a persistent infection in the central nervous system.