The significance of measles virus antigen and genome distribution in the CNS in SSPE for mechanisms of viral spread and demyelination

The measles virus matrix F50S mutation from a lethal case of subacute sclerosing panencephalitis promotes receptor-independent neuronal spread

Subacute sclerosing panencephalitis (SSPE) is a lethal neurological disorder occurring several years after measles. Reconstruction of the evolution of the measles virus (MeV) genome in an SSPE case suggested that the matrix (M) protein mutation M-F50S, when added to other mutations, drove neuropathogenesis. However, whether and how M-F50S would promote spread independently from other mutations was in question. We investigated here the cell specificity of MeV spread in this brain and documented that both neurons and astrocytes were heavily infected. We then generated recombinant MeV with individual mutations in the three proteins of the viral membrane fusion apparatus, M, fusion (F), and hemagglutinin (H). These viruses reached similar titers as the parental wild-type virus, kept the respective mutations upon passage, and infected cells expressing the tissue-specific MeV receptors SLAM and nectin-4 with similar efficiencies. However, after inoculation of receptor-negative neurons and astrocytes differentiated from human induced pluripotent stem cells, only MeV M-F50S spread with moderate efficiency; the parental virus and its derivatives coding for a hyperfusogenic F protein, or for a cytoplasmic tail-mutated H protein, did not spread. When delivered to primary mouse neurons by cell-mediated neurite overlay, MeV M-F50S frequently reached the cell bodies and occasionally formed small infectious centers, while the other MeV reached the cell bodies only sporadically. These results demonstrate that, in neuronal cell cultures, M-F50S can enable receptor-independent spread in the absence of other mutations, and validate the inference that this single amino acid change initiated ubiquitous MeV brain spread.IMPORTANCEMeasles virus (MeV), a non-integrating negative-strand RNA virus, rarely causes subacute sclerosing panencephalitis (SSPE) several years after acute infection. During brain adaptation, the MeV genome acquires multiple mutations reducing the dependence of its membrane fusion apparatus (MFA) from an activating receptor. It was proposed that one of these mutations, matrix protein F50S, drove neuropathogenesis in an SSPE case. We report here that, in two types of neuronal cultures, a recombinant MeV with only this mutation gained receptor-independent spread, whereas viruses expressing MFA proteins with other mutations acquired during brain adaptation did not. Our results validate the inference that M-F50S initiated ubiquitous MeV brain spread resulting in lethal disease. They also prompt studies of the impact of analogous amino acid changes of the M proteins of other nonsegmented negative-strand RNA viruses on their interactions with membrane lipids and cytoskeletal components.

Functional properties of measles virus proteins derived from a subacute sclerosing panencephalitis patient who received repeated remdesivir treatments

Subacute sclerosing panencephalitis (SSPE) is a rare but fatal late neurological complication of measles, caused by persistent measles virus (MeV) infection of the central nervous system. There are no drugs approved for the treatment of SSPE. Here, we followed the clinical progression of a 5-year-old SSPE patient after treatment with the nucleoside analog remdesivir, conducted a post-mortem evaluation of the patient's brain, and characterized the MeV detected in the brain. The quality of life of the patient transiently improved after the first two courses of remdesivir, but a third course had no further clinical effect, and the patient eventually succumbed to his condition. Post-mortem evaluation of the brain displayed histopathological changes including loss of neurons and demyelination paired with abundant presence of MeV RNA-positive cells throughout the brain. Next-generation sequencing of RNA isolated from the brain revealed a complete MeV genome with mutations that are typically detected in SSPE, characterized by a hypermutated M gene. Additional mutations were detected in the polymerase (L) gene, which were not associated with resistance to remdesivir. Functional characterization showed that mutations in the F gene led to a hyperfusogenic phenotype predominantly mediated by N465I. Additionally, recombinant wild-type-based MeV with the SSPE-F gene or the F gene with the N465I mutation was no longer lymphotropic but instead efficiently disseminated in neural cultures. Altogether, this case encourages further investigation of remdesivir as a potential treatment of SSPE and highlights the necessity to functionally understand SSPE-causing MeV.IMPORTANCEMeasles virus (MeV) causes acute, systemic disease and remains an important cause of morbidity and mortality in humans. Despite the lack of known entry receptors in the brain, MeV can persistently infect the brain causing the rare but fatal neurological disorder subacute sclerosing panencephalitis (SSPE). SSPE-causing MeVs are characterized by a hypermutated genome and a hyperfusogenic F protein that facilitates the rapid spread of MeV throughout the brain. No treatment against SSPE is available, but the nucleoside analog remdesivir was recently demonstrated to be effective against MeV in vitro. We show that treatment of an SSPE patient with remdesivir led to transient clinical improvement and did not induce viral escape mutants, encouraging the future use of remdesivir in SSPE patients. Functional characterization of the viral proteins sheds light on the shared properties of SSPE-causing MeVs and further contributes to understanding how those viruses cause disease.

Subacute Sclerosing Panencephalitis in a 63-Year-Old Woman Presenting as Generalized Choreoathetosis

The persistence of measles virus infection in childhood and early adolescence can rarely lead to a fatal progressive neurodegenerative disorder known as subacute sclerosing panencephalitis (SSPE), characterized by behavioral disturbances and intellectual disability followed by myoclonic jerks and occasional negative myoclonus. Movement disorders are rarely presenting manifestations in SSPE. We herein report a 63-year-old woman with generalized choreoathetosis as the presenting manifestation of stage-I SSPE. Our case was atypical for the patient's age and clinical presentation with generalized choreoathetosis and bilateral putaminal and caudate nucleus signal hyperintensity. Though highly uncommon, neurologists should keep SSPE as a differential diagnosis among patients with movement disorders. Measles-endemic countries should be more vigilant to the atypical and rare presentations of SSPE, such as generalized choreoathetosis.

Interaction of the Hemagglutinin Stalk Region with Cell Adhesion Molecule (CADM) 1 and CADM2 Mediates the Spread between Neurons and Neuropathogenicity of Measles Virus with a Hyperfusogenic Fusion Protein

Measles virus (MeV), the causative agent of measles, is an enveloped RNA virus of the family Paramyxoviridae, which remains an important cause of childhood morbidity and mortality. MeV has two envelope glycoproteins, the hemagglutinin (H) and fusion (F) proteins. During viral entry or virus-mediated fusion between infected cells and neighboring susceptible cells, the head domain of the H protein initially binds to its receptors, signaling lymphocytic activation molecule family member 1 (SLAM) and nectin-4, and then the stalk region of the H protein transmits the fusion-triggering signal to the F protein. MeV may persist in the human brain and cause a fatal neurodegenerative disease, subacute sclerosing panencephalitis (SSPE). Recently, we showed, using in vitro cell culture, that cell adhesion molecule (CADM) 1 and CADM2 are host factors that trigger hyperfusogenic mutant F proteins, causing cell-to-cell fusion and the transfer of the MeV genome between neurons. Unlike conventional receptors, CADM1 and CADM2 interact in cis (on the same membrane) with the H protein and then trigger membrane fusion. Here, we show that alanine substitutions in part of the stalk region (positions 171-175) abolish the ability of the H protein to mediate membrane fusion triggered by CADM1 and CADM2, but not by SLAM. The recombinant hyperfusogenic MeV carrying this mutant H protein loses its ability to spread in primary mouse neurons as well as its neurovirulence in experimentally infected suckling hamsters. These results indicate that CADM1 and CADM2 are key molecules for MeV propagation in the brain and its neurovirulence in vivo. IMPORTANCE Measles is an acute febrile illness with skin rash. Despite the availability of highly effective vaccines, measles is still an important cause of childhood morbidity and mortality in many countries. The World Health Organization estimates that more than 120,000 people died from measles worldwide in 2021. Measles virus (MeV), the causative agent of measles, can also cause a fatal progressive neurological disorder, subacute sclerosing panencephalitis (SSPE), several years after acute infection. There is currently no effective treatment for this disease. In this study, using recombinant MeVs with altered receptor usage patterns, we show that cell adhesion molecule (CADM) 1 and CADM2 are host factors critical for MeV spread in neurons and its neurovirulence. These findings further our understanding of the molecular mechanism of MeV neuropathogenicity.

Early Permissiveness of Central Nervous System Cells to Measles Virus Infection Is Determined by Hyperfusogenicity and Interferon Pressure

The cessation of measles virus (MeV) vaccination in more than 40 countries as a consequence of the COVID-19 pandemic is expected to significantly increase deaths due to measles. MeV can infect the central nervous system (CNS) and lead to lethal encephalitis. Substantial part of virus sequences recovered from patients' brain were mutated in the matrix and/or the fusion protein (F). Mutations of the heptad repeat domain located in the C terminal (HRC) part of the F protein were often observed and were associated to hyperfusogenicity. These mutations promote brain invasion as a hallmark of neuroadaptation. Wild-type F allows entry into the brain, followed by limited spreading compared with the massive invasion observed for hyperfusogenic MeV. Taking advantage of our ex vivo models of hamster organotypic brain cultures, we investigated how the hyperfusogenic mutations in the F HRC domain modulate virus distribution in CNS cells. In this study, we also identified the dependence of neural cells susceptibility on both their activation state and destabilization of the virus F protein. Type I interferon (IFN-I) impaired mainly astrocytes and microglial cells permissiveness contrarily to neurons, opening a new way of consideration on the development of treatments against viral encephalitis.

Subacute Sclerosing Panencephalitis in Children: The Archetype of Non-Vaccination

Subacute sclerosing panencephalitis (SSPE) is a late complication of measles virus infection that occurs in previously healthy children. This disease has no specific cure and is associated with a high degree of disability and mortality. In recent years, there has been an increase in its incidence in relation to a reduction in vaccination adherence, accentuated by the COVID-19 pandemic. In this article, we take stock of the current evidence on SSPE and report our personal clinical experience. We emphasise that, to date, the only effective protection strategy against this disease is vaccination against the measles virus.

Short-stalk isoforms of CADM1 and CADM2 trigger neuropathogenic measles virus-mediated membrane fusion by interacting with the viral hemagglutinin

Measles virus (MeV), an enveloped RNA virus in the family Paramyxoviridae, usually causes acute febrile illness with skin rash, but in rare cases persists in the brain, causing a progressive neurological disorder, subacute sclerosing panencephalitis (SSPE). MeV bears two envelope glycoproteins, the hemagglutinin (H) and fusion (F) proteins. The H protein possesses a head domain that initially mediates receptor binding and a stalk domain that subsequently transmits the fusion-triggering signal to the F protein. We have recently shown that cell adhesion molecule 1 (CADM1, also known as IGSF4A, Necl-2, SynCAM1) and CADM2 (also known as IGSF4D, Necl-3, SynCAM2) are host factors enabling cell-cell membrane fusion mediated by hyperfusogenic F proteins of neuropathogenic MeVs as well as MeV spread between neurons lacking the known receptors. CADM1 and CADM2 interact in cis with the H protein on the same cell membrane, triggering hyperfusogenic F protein-mediated membrane fusion. Multiple isoforms of CADM1 and CADM2 containing various lengths of their stalk regions are generated by alternative splicing. Here we show that only short-stalk isoforms of CADM1 and CADM2 predominantly expressed in the brain induce hyperfusogenic F protein-mediated membrane fusion. While the known receptors interact in trans with the H protein through its head domain, these isoforms can interact in cis even with the H protein lacking the head domain and trigger membrane fusion, presumably through its stalk domain. Thus, our results unveil a new mechanism of viral fusion triggering by host factors. Importance Measles, an acute febrile illness with skin rash, is still an important cause of childhood morbidity and mortality worldwide. Measles virus (MeV), the causative agent of measles, may also cause a progressive neurological disorder, subacute sclerosing panencephalitis (SSPE), several years after acute infection. The disease is fatal, and no effective therapy is available. Recently, we have reported that cell adhesion molecule 1 (CADM1) and CADM2 are host factors enabling MeV cell-to-cell spread in neurons. These molecules interact in cis with the MeV attachment protein on the same cell membrane, triggering the fusion protein and causing membrane fusion. CADM1 and CADM2 are known to exist in multiple splice isoforms. In this study, we report that their short-stalk isoforms can induce membrane fusion by interacting in cis with the viral attachment protein independently of its receptor-binding head domain. This finding may have important implications for cis-acting fusion triggering by host factors.

Comparable Infection Level and Tropism of Measles Virus and Canine Distemper Virus in Organotypic Brain Slice Cultures Obtained from Natural Host Species

Measles virus (MV) and canine distemper virus (CDV) are closely related members of the family Paramyxoviridae, genus Morbillivirus. MV infection of humans and non-human primates (NHPs) results in a self-limiting disease, which rarely involves central nervous system (CNS) complications. In contrast, infection of carnivores with CDV usually results in severe disease, in which CNS complications are common and the case-fatality rate is high. To compare the neurovirulence and neurotropism of MV and CDV, we established a short-term organotypic brain slice culture system of the olfactory bulb, hippocampus, or cortex obtained from NHPs, dogs, and ferrets. Slices were inoculated ex vivo with wild-type-based recombinant CDV or MV expressing a fluorescent reporter protein. The infection level of both morbilliviruses was determined at different times post-infection. We observed equivalent infection levels and identified microglia as main target cells in CDV-inoculated carnivore and MV-inoculated NHP brain tissue slices. Neurons were also susceptible to MV infection in NHP brain slice cultures. Our findings suggest that MV and CDV have comparable neurotropism and intrinsic capacity to infect CNS-resident cells of their natural host species.

CADM1 and CADM2 Trigger Neuropathogenic Measles Virus-Mediated Membrane Fusion by Acting in cis

Measles virus (MeV), an enveloped RNA virus in the family Paramyxoviridae, is still an important cause of childhood morbidity and mortality worldwide. MeV usually causes acute febrile illness with skin rash, but in rare cases persists in the brain, causing a progressive neurological disorder, subacute sclerosing panencephalitis (SSPE). The disease is fatal, and no effective therapy is currently available. Although transsynaptic cell-to-cell transmission is thought to account for MeV propagation in the brain, neurons do not express the known receptors for MeV. Recent studies have shown that hyperfusogenic changes in the MeV fusion (F) protein play a key role in MeV propagation in the brain. However, how such mutant viruses spread in neurons remains unexplained. Here, we show that cell adhesion molecule 1 (CADM1; also known as IGSF4A, Necl-2, and SynCAM1) and CADM2 (also known as IGSF4D, Necl-3, SynCAM2) are host factors that enable MeV to cause membrane fusion in cells lacking the known receptors and to spread between neurons. During enveloped virus entry, a cellular receptor generally interacts in trans with the attachment protein on the envelope. However, CADM1 and CADM2 interact in cis with the MeV attachment protein on the same cell membrane, causing the fusion protein triggering and membrane fusion. Knockdown of CADM1 and CADM2 inhibits syncytium formation and virus transmission between neurons that are both mediated by hyperfusogenic F proteins. Thus, our results unravel the molecular mechanism (receptor-mimicking cis-acting fusion triggering) by which MeV spreads transsynaptically between neurons, thereby causing SSPE. IMPORTANCE Measles virus (MeV), an enveloped RNA virus, is the causative agent of measles, which is still an important cause of childhood morbidity and mortality worldwide. Persistent MeV infection in the brain causes a fatal progressive neurological disorder, subacute sclerosing panencephalitis (SSPE), several years after acute infection. However, how MeV spreads in neurons, which are mainly affected in SSPE, remains largely unknown. In this study, we demonstrate that cell adhesion molecule 1 (CADM1) and CADM2 are host factors enabling MeV spread between neurons. During enveloped virus entry, a cellular receptor generally interacts in trans with the attachment protein on the viral membrane (envelope). Remarkably, CADM1 and CADM2 interact in cis with the MeV attachment protein on the same membrane, triggering the fusion protein and causing membrane fusion, as viral receptors usually do in trans. Careful screening may lead to more examples of such "receptor-mimicking cis-acting fusion triggering" in other viruses.

Molecular Features of the Measles Virus Viral Fusion Complex That Favor Infection and Spread in the Brain

Measles virus (MeV) bearing a single amino acid change in the fusion protein (F)—L454W—was isolated from two patients who died of MeV central nervous system (CNS) infection. This mutation in F confers an advantage over wild-type virus in the CNS, contributing to disease in these patients. Using murine ex vivo organotypic brain cultures and human induced pluripotent stem cell-derived brain organoids, we show that CNS adaptive mutations in F enhance the spread of virus ex vivo. The spread of virus in human brain organoids is blocked by an inhibitory peptide that targets F, confirming that dissemination in the brain tissue is attributable to F. A single mutation in MeV F thus alters the fusion complex to render MeV more neuropathogenic.

Autophagy in white matter disorders of the CNS: mechanisms and therapeutic opportunities

Autophagy is a constitutive process that degrades, recycles and clears damaged proteins or organelles, yet, despite activation of this pathway, abnormal proteins accumulate in neurons in neurodegenerative diseases and in oligodendrocytes in white matter disorders. Here, we discuss the role of autophagy in white matter disorders, including neurotropic infections, inflammatory diseases such as multiple sclerosis, and in hereditary metabolic disorders and acquired toxic‐metabolic disorders. Once triggered due to cell stress, autophagy can enhance cell survival or cell death that may contribute to oligodendrocyte damage and myelin loss in white matter diseases. For some disorders, the mechanisms leading to myelin loss are clear, whereas the aetiological agent and pathological mechanisms are unknown for other myelin disorders, although emerging studies indicate that a common mechanism underlying these disorders is dysregulation of autophagic pathways. In this review we discuss the alterations in the autophagic process in white matter disorders and the potential use of autophagy‐modulating agents as therapeutic approaches in these pathological conditions. © 2020 The Authors. The Journal of Pathology published by John Wiley & Sons, Ltd. on behalf of The Pathological Society of Great Britain and Ireland.

Measles Encephalitis: Towards New Therapeutics

Measles remains a major cause of morbidity and mortality worldwide among vaccine preventable diseases. Recent decline in vaccination coverage resulted in re-emergence of measles outbreaks. Measles virus (MeV) infection causes an acute systemic disease, associated in certain cases with central nervous system (CNS) infection leading to lethal neurological disease. Early following MeV infection some patients develop acute post-infectious measles encephalitis (APME), which is not associated with direct infection of the brain. MeV can also infect the CNS and cause sub-acute sclerosing panencephalitis (SSPE) in immunocompetent people or measles inclusion-body encephalitis (MIBE) in immunocompromised patients. To date, cellular and molecular mechanisms governing CNS invasion are still poorly understood. Moreover, the known MeV entry receptors are not expressed in the CNS and how MeV enters and spreads in the brain is not fully understood. Different antiviral treatments have been tested and validated in vitro, ex vivo and in vivo, mainly in small animal models. Most treatments have high efficacy at preventing infection but their effectiveness after CNS manifestations remains to be evaluated. This review describes MeV neural infection and current most advanced therapeutic approaches potentially applicable to treat MeV CNS infection.

Type I Interferon Receptor Signaling Drives Selective Permissiveness of Astrocytes and Microglia to Measles Virus during Brain Infection

Fatal neurological syndromes can occur after measles virus (MeV) infection of the brain. The mechanisms controlling MeV spread within the central nervous system (CNS) remain poorly understood. We analyzed the role of type I interferon (IFN-I) receptor (IFNAR) signaling in the control of MeV infection in a murine model of brain infection. Using organotypic brain cultures (OBC) from wild-type and IFNAR-knockout (IFNARKO) transgenic mice ubiquitously expressing the human SLAM (CD150) receptor, the heterogeneity of the permissiveness of different CNS cell types to MeV infection was characterized. In the absence of IFNAR signaling, MeV propagated significantly better in explant slices. In OBC from IFNAR-competent mice, while astrocytes and microglia were infected on the day of explant preparation, they became refractory to infection with time, in contrast to neurons and oligodendrocytes, which remained permissive to infection. This selective loss of permissiveness to MeV infection was not observed in IFNARKO mouse OBC. Accordingly, the development of astrogliosis related to the OBC procedure was exacerbated in the presence of IFNAR signaling. In the hippocampus, this astrogliosis was characterized by a change in the astrocyte phenotype and by an increase of IFN-I transcripts. A proteome analysis showed the upregulation of 84 out of 111 secreted proteins. In the absence of IFNAR, only 27 secreted proteins were upregulated, and none of these were associated with antiviral activities. Our results highlight the essential role of the IFN-I response in astrogliosis and in the permissiveness of astrocytes and microglia that could control MeV propagation throughout the CNS.IMPORTANCE Measles virus (MeV) can infect the central nervous system (CNS), with dramatic consequences. The mechanisms controlling MeV invasion of the CNS remain ill-defined since most previous data were obtained from postmortem analysis. Here, we highlight for the first time the crucial role of the type I interferon (IFN-I) response not only in the control of CNS invasion but also in the early permissiveness of glial cells to measles virus infection.

Measles Virus Bearing Measles Inclusion Body Encephalitis-Derived Fusion Protein Is Pathogenic after Infection via the Respiratory Route

A clinical isolate of measles virus (MeV) bearing a single amino acid alteration in the viral fusion protein (F; L454W) was previously identified in two patients with lethal sequelae of MeV central nervous system (CNS) infection. The mutation dysregulated the viral fusion machinery so that the mutated F protein mediated cell fusion in the absence of known MeV cellular receptors. While this virus could feasibly have arisen via intrahost evolution of the wild-type (wt) virus, it was recently shown that the same mutation emerged under the selective pressure of small-molecule antiviral treatment. Under these conditions, a potentially neuropathogenic variant emerged outside the CNS. While CNS adaptation of MeV was thought to generate viruses that are less fit for interhost spread, we show that two animal models can be readily infected with CNS-adapted MeV via the respiratory route. Despite bearing a fusion protein that is less stable at 37°C than the wt MeV F, this virus infects and replicates in cotton rat lung tissue more efficiently than the wt virus and is lethal in a suckling mouse model of MeV encephalitis even with a lower inoculum. Thus, either during lethal MeV CNS infection or during antiviral treatment in vitro, neuropathogenic MeV can emerge, can infect new hosts via the respiratory route, and is more pathogenic (at least in these animal models) than wt MeV.IMPORTANCE Measles virus (MeV) infection can be severe in immunocompromised individuals and lead to complications, including measles inclusion body encephalitis (MIBE). In some cases, MeV persistence and subacute sclerosing panencephalitis (SSPE) occur even in the face of an intact immune response. While they are relatively rare complications of MeV infection, MIBE and SSPE are lethal. This work addresses the hypothesis that despite a dysregulated viral fusion complex, central nervous system (CNS)-adapted measles virus can spread outside the CNS within an infected host.

Keeping it in check: chronic viral infection and antiviral immunity in the brain

It is becoming clear that the manner by which the immune response resolves or contains infection by a pathogen varies according to the tissue that is affected. Unlike many peripheral cell types, CNS neurons are generally non-renewable. Thus, the cytolytic and inflammatory strategies that are effective in controlling infections in the periphery could be damaging if deployed in the CNS. Perhaps for this reason, the immune response to some CNS viral infections favours maintenance of neuronal integrity and non-neurolytic viral control. This modified immune response - when combined with the unique anatomy and physiology of the CNS - provides an ideal environment for the maintenance of viral genomes, including those of RNA viruses. Therefore, it is possible that such viruses can reactivate long after initial viral exposure, contributing to CNS disease.

Neurotropic virus infections as the cause of immediate and delayed neuropathology

A wide range of viruses from different virus families in different geographical areas, may cause immediate or delayed neuropathological changes and neurological manifestations in humans and animals. Infection by neurotropic viruses as well as the resulting immune response can irreversibly disrupt the complex structural and functional architecture of the central nervous system, frequently leaving the patient or affected animal with a poor or fatal prognosis. Mechanisms that govern neuropathogenesis and immunopathogenesis of viral infections are highlighted, using examples of well-studied virus infections that are associated with these alterations in different populations throughout the world. A better understanding of the molecular, epidemiological and biological characteristics of these infections and in particular of mechanisms that underlie their clinical manifestations may be expected to provide tools for the development of more effective intervention strategies and treatment regimens.

Measles virus mutants possessing the fusion protein with enhanced fusion activity spread effectively in neuronal cells, but not in other cells, without causing strong cytopathology

Subacute sclerosing panencephalitis (SSPE) is caused by persistent measles virus (MV) infection in the central nervous system (CNS). Since human neurons, its main target cells, do not express known MV receptors (signaling lymphocyte activation molecule [SLAM] and nectin 4), it remains to be understood how MV infects and spreads in them. We have recently reported that fusion-enhancing substitutions in the extracellular domain of the MV fusion (F) protein (T461I and S103I/N462S/N465S), which are found in multiple SSPE virus isolates, promote MV spread in human neuroblastoma cell lines and brains of suckling hamsters. In this study, we show that hyperfusogenic viruses with these substitutions also spread efficiently in human primary neuron cultures without inducing syncytia. These substitutions were found to destabilize the prefusion conformation of the F protein trimer, thereby enhancing fusion activity. However, these hyperfusogenic viruses exhibited stronger cytopathology and produced lower titers at later time points in SLAM- or nectin 4-expressing cells compared to the wild-type MV. Although these viruses spread efficiently in the brains of SLAM knock-in mice, they did not in the spleens. Taken together, the results suggest that enhanced fusion activity is beneficial for MV to spread in neuronal cells where no cytopathology occurs, but detrimental to other types of cells due to strong cytopathology. Acquisition of enhanced fusion activity through substitutions in the extracellular domain of the F protein may be crucial for MV's extensive spread in the CNS and development of SSPE.

IMPORTANCE

Subacute sclerosing panencephalitis (SSPE) is a fatal disease caused by persistent measles virus (MV) infection in the central nervous system (CNS). Its cause is not well understood, and no effective therapy is currently available. Recently, we have reported that enhanced fusion activity of MV through the mutations in its fusion protein is a major determinant of efficient virus spread in human neuronal cells and brains of suckling hamsters. In this study, we show that those mutations render the conformation of the fusion protein less stable, thereby making it hyperfusogenic. Our results also show that enhanced fusion activity is beneficial for MV to spread in the CNS but detrimental to other types of cells in peripheral tissues, which are strongly damaged by the virus. Our findings provide important insight into the mechanism for the development of SSPE after MV infection.

Fatal measles virus infection prevented by brain-penetrant fusion inhibitors

Measles virus (MV) infection causes an acute childhood disease that can include infection of the central nervous system and can rarely progress to severe neurological disease for which there is no specific treatment. We generated potent antiviral peptide inhibitors of MV entry and spreading and MV-induced cell fusion. Dimers of MV-specific peptides derived from the C-terminal heptad repeat region of the MV fusion protein, conjugated to cholesterol, efficiently protect SLAM transgenic mice from fatal MV infection. Fusion inhibitors hold promise for the prophylaxis of MV infection in unvaccinated and immunocompromised people, as well as potential for the treatment of grave neurological complications of measles.

Subacute sclerosing panencephalitis presenting as neuromyelitis optica

Subacute sclerosing panencephalitis (SSPE) is a slowly progressing inflammatory and degenerative disorder of the brain caused by a mutant measles virus. The diagnosis of SSPE is based on characteristic clinical and EEG findings (periodic complexes) and demonstration of elevated antibody titres against measles in cerebrospinal fluid. SSPE can have atypical clinical features at the onset. The authors here report a case of a 3-year-old child who presented with vision loss followed 15 months later by quadriparesis with bladder involvement. These clinical features resembled that of neuromyelitis optica. However, as the disease progressed, appearance of myoclonic jerks, periodic discharges on EEG and positive cerebrospinal fluid serology for measles led to the final diagnosis of SSPE.

Pathology of tropical diseases

Tropical diseases affecting the central nervous system include infections, infestations, and nutritional deficiency disorders. This article discusses the commonly encountered diseases. The infections include bacterial, mycobacterial, fungal, parasitic, and viral infections with varied clinical manifestations. Imaging sensitivity and specificity for the prediction of the cause of infections has improved with application of advanced techniques. Microbial demonstration and histology remain the gold standard for diagnosis. Understanding the basis of imaging changes is mandatory for better evaluation of images. Nutritional disorders present with generalized and nonspecific imaging manifestations. The pathology of commonly encountered vitamin deficiencies is also discussed.

Case report: Cervical spinal cord signal changes in a case of adult-onset subacute sclerosing panenchephalitis

In this article, we report a case of subacute sclerosing panencephalitis (SSPE) in which there were central cervical cord signal changes on MRI. The spinal cord is uncommonly involved in SSPE. However, demonstration of spinal cord signal change in a patient of SSPE has significant implications for the differential diagnosis and management.

PD1 as a common candidate susceptibility gene of subacute sclerosing panencephalitis

Although the exact pathogenesis of subacute sclerosing panencephalitis (SSPE) remains to be determined, our previous data suggested a genetic contribution to the host susceptibility to SSPE. During chronic viral infection, virus-specific cytotoxic T lymphocytes display poor effector functions. Since co-inhibitory molecules are involved in the suppression of T lymphocytes, we investigated whether single nucleotide polymorphisms (SNPs) of genes encoding co-inhibitory molecules contributed to a susceptibility to SSPE. Association studies on a total of 20 SNPs in 8 genes (CTLA4, CD80, CD86, PD1, PDL1, PDL2, BTLA and HVEM) and subsequent haplotype analysis of 4 SNPs in the PD1 genes were performed in Japanese and Filipino SSPE patients and controls. Then, we investigated a functional difference in promoter activity of two haplotypes and compared the expression levels of PD1 between SSPE and controls. The frequency of GCG(C) haplotype of PD1 containing -606G allele was significantly higher in SSPE patients than in controls both in Japanese and in Filipinos. The promoter activity was significantly higher in the construct with -606G allele than in that with -606A allele. The expression levels of PD1 were significantly higher in SSPE patients than in the controls. Our results suggested that the PD1 gene contributed to a genetic susceptibility to SSPE.

Measles virus infection of the CNS: human disease, animal models, and approaches to therapy

Viral infections of the central nervous system(CNS) mostly represent clinically important, often life-threatening complications of systemic viral infections. After acute measles, CNS complications may occur early (acute postinfectious measles encephalitis, APME) or after years of viral persistence (subacute sclerosing panencephalitis, SSPE). In spite of a presumably functional cell-mediated immunity and high antiviral antibody titers, an immunological control of the CNS infection is not achieved in patients suffering from SSPE. There is still no specific therapy for acute complications and persistent MV infections of the CNS. Hamsters, rats, and (genetically unmodified and modified) mice have been used as model systems to study mechanisms of MV-induced CNS infections. Functional CD4+ and CD8+ T cells together with IFN-gamma are required to overcome the infection. With the help of recombinant measles viruses and mice expressing endogenous or transgenic receptors, interesting aspects such as receptor-dependent viral spread and viral determinants of virulence have been investigated. However, many questions concerning the lack of efficient immune control in the CNS are still open. Recent research opened new perspectives using specific antivirals such as short interfering RNA (siRNA) or small molecule inhibitors. Inspite of obvious hurdles, these treatments are the most promising approaches to future therapies.

Blue moon neurovirology: the merits of studying rare CNS diseases of viral origin

While measles virus (MV) continues to have a significant impact on human health, causing 150,000-200,000 deaths worldwide each year, the number of fatalities that can be attributed to MV-triggered central nervous system (CNS) diseases are on the order of a few hundred individuals annually (World Health Organization 2009). Despite this modest impact, substantial effort has been expended to understand the basis of measles-triggered neuropathogenesis. What can be gained by studying such a rare condition? Simply stated, the wealth of studies in this field have revealed core principles that are relevant to multiple neurotropic pathogens, and that inform the broader field of viral pathogenesis. In recent years, the emergence of powerful in vitro systems, novel animal models, and reverse genetics has enabled insights into the basis of MV persistence, the complexity of MV interactions with neurons and the immune system, and the role of immune and CNS development in virus-triggered disease. In this review, we highlight some key advances, link relevant measles-based studies to the broader disciplines of neurovirology and viral pathogenesis, and propose future areas of study for the field of measles-mediated neurological disease.

Persistent measles virus infection of mouse neural cells lacking known human entry receptors

AIMS

Infection of the mouse central nervous system with wild type (WT) and vaccine strains of measles virus (MV) results in lack of clinical signs and limited antigen detection. It is considered that cell entry receptors for these viruses are not present on murine neural cells and infection is restricted at cell entry.

METHODS

To examine this hypothesis, virus antigen and caspase 3 expression (for apoptosis) was compared in primary mixed, neural cell cultures infected in vitro or prepared from mice infected intracerebrally with WT, vaccine or rodent neuroadapted viruses. Viral RNA levels were examined in mouse brain by nested and real-time reverse transcriptase polymerase chain reaction.

RESULTS

WT and vaccine strains were demonstrated for the first time to infect murine oligodendrocytes in addition to neurones despite a lack of the known MV cell receptors. Unexpectedly, the percentage of cells positive for viral antigen was higher for WT MV than neuroadapted virus in both in vitro and ex vivo cultures. In the latter the percentage of positive cells increased with time after mouse infection. Viral RNA (total and mRNA) was detected in brain for up to 20 days, while cultures were negative for caspase 3 in WT and vaccine virus infections.

CONCLUSIONS

WT and vaccine MV strains can use an endogenous cell entry receptor(s) or alternative virus uptake mechanism in murine neural cells. However, viral replication occurs at a low level and is associated with limited apoptosis. WT MV mouse infection may provide a model for the initial stages of persistent MV human central nervous system infections.

Clearance of measles virus from persistently infected cells by short hairpin RNA

Subacute sclerosing panencephalitis (SSPE) is a demyelinating central nervous system disease caused by a persistent measles virus (MV) infection of neurons and glial cells. There is still no specific therapy available, and in spite of an intact innate and adaptive immune response, SSPE leads inevitably to death. In order to select effective antiviral short interfering RNAs (siRNAs), we established a plasmid-based test system expressing the mRNA of DsRed2 fused with mRNA sequences of single viral genes, to which certain siRNAs were directed. siRNA sequences were expressed as short hairpin RNA (shRNA) from a lentiviral vector additionally expressing enhanced green fluorescent protein (EGFP) as an indicator. Evaluation by flow cytometry of the dual-color system (DsRed and EGFP) allowed us to find optimal shRNA sequences. Using the most active shRNA constructs, we transduced persistently infected human NT2 cells expressing virus-encoded HcRed (piNT2-HcRed) as an indicator of infection. shRNA against N, P, and L mRNAs of MV led to a reduction of the infection below detectable levels in a high percentage of transduced piNT2-HcRed cells within 1 week. The fraction of virus-negative cells in these cultures was constant over at least 3 weeks posttransduction in the presence of a fusion-inhibiting peptide (Z-Phe-Phe-Gly), preventing the cell fusion of potentially cured cells with persistently infected cells. Transduced piNT2 cells that lost HcRed did not fuse with underlying Vero/hSLAM cells, indicating that these cells do not express viral proteins any more and are "cured." This demonstrates in tissue culture that NT2 cells persistently infected with MV can be cured by the transduction of lentiviral vectors mediating the long-lasting expression of anti-MV shRNA.

Making it to the synapse: measles virus spread in and among neurons

Measles virus (MV) is one of the most transmissible microorganisms known, continuing to result in extensive morbidity and mortality worldwide. While rare, MV can infect the human central nervous system, triggering fatal CNS diseases weeks to years after exposure. The advent of crucial laboratory tools to dissect MV neuropathogenesis, including permissive transgenic mouse models, the capacity to manipulate the viral genome using reverse genetics, and cell biology advances in understanding the processes that govern intracellular trafficking of viral components, have substantially clarified how MV infects, spreads, and persists in this unique cell population. This review highlights some of these technical advances, followed by a discussion of our present understanding of MV neuronal infection and transport. Because some of these processes may be shared among diverse viruses, comparisons are made to parallel studies with other neurotropic viruses. While a crystallized view of how the unique environment of the neuron affects MV replication, spread, and, ultimately, neuropathogenesis is not fully realized, the tools and ideas are in place for exciting advances in the coming years.

Association of toll-like receptor 3 gene polymorphism with subacute sclerosing panencephalitis

Innate immunity plays an important role in measles virus (MV) infection. MV-derived double-stranded RNA is recognized by toll-like receptor 3 (TLR3), retinoic acid-inducible protein I (RIG-I) and melanoma differentiation-associated gene 5 (MDA5). We investigated whether genes encoding these molecules contributed to the development of subacute sclerosing panencephalitis (SSPE) in Japanese individuals. Four single nucleotide polymorphisms (SNPs) of the three genes (TLR3 rs3775291:Leu412Phe, RIG1 rs277729 and rs9695310, and MDA5 rs4664463) were assessed in 40 SSPE patients and 84 controls. Because the TLR3 SNP showed a positive association with SSPE, three additional SNPs were subjected to haplotype analysis. The frequency of 412Phe allele of TLR3 rs3775291 in SSPE patients was significantly higher than that in controls (P=.03). In haplotype analysis of four SNPs in the TLR3 gene, the frequency of -7C/IVS3+71C/Phe412/c.1377C haplotype was significantly increased in SSPE patients (P=.006, odds ration [OR]: 2.2). TLR3 gene may confer host genetic susceptibility to SSPE in Japanese individuals.

Measles virus-induced block of transendothelial migration of T lymphocytes and infection-mediated virus spread across endothelial cell barriers

In order to analyze whether measles virus (MV) is transported via transmigrating leukocytes across endothelial barriers or whether virus spreads via infection of endothelial cells and basolateral release, we investigated the migratory behavior of infected human primary T lymphocytes across polarized cell layers of human brain microvascular endothelial cells. We found that the capacity of lymphocytes to migrate through filter pores was only slightly affected by wild-type MV infection, whereas their capacity to migrate through endothelial barriers was drastically reduced. MV infection stimulated the expression and activation of the leukocyte integrins LFA-1 and VLA-4, mediating a strong adherence to the surface of endothelial cells. Furthermore, the formation of engulfing membrane protrusions by endothelial cells, so-called transmigratory cups, was induced, but transmigration was impaired. As a consequence of this close cell-cell contact, MV infection was transmitted from lymphocytes to the endothelium. MV envelope proteins were expressed on the apical and basolateral surfaces of infected polarized endothelial cells, and virus was released from both sides. Wild-type MV infection did not induce the formation of syncytia, suggesting virus spread from cell to cell via cell processes and contacts. Our data indicate that transendothelial migration of infected T cells is strongly inhibited, whereas virus can cross endothelial barriers by productive infection of the endothelium and subsequent bipolar virus release.

The MSMV hypothesis: measles virus and multiple sclerosis, etiology and treatment

Multiple sclerosis (MS) is a progressive disease characterised by periods of quiescence and exacerbation. It is found more often in northern and southern climates, rather than those closer to the equator, where it is especially rare, and, therefore, cannot be considered as an autoimmune disease. We present the MSMV Hypothesis, involving novel ideas which encompass an understanding of the blood brain barrier (BBB) function, the lymphocyte population, together with the viral presence in the CNS of what we are calling the multiple sclerosis measles virus (MSMV) that is the immediate cause of MS, and which exhibits a similar immunologic response of the systemic virus. We assume that the geographical distribution of MS is related to MSMV's sensitivity to ultraviolet light and that it is feasible to assume a viral etiology for MS based on this. The methodology employed is eclectic and grounded on several differing approaches: involved are the meta-analyses of two comprehensive studies on the effects of azathioprine in the treatment of a large number of MS patients undertaken since the early 1990s, a pioneering pilot study that examined the effects of azathioprine treatment on a smaller set of patients in the late 1960s; and, finally, we also outline the results of several experiments in cell culture on two MV strains using a new drug lead that has been shown to effectively stave off the progression of MS by interfering with the normal replication process of the MSMV. In the latter case, strain Edmonston (MV-E) was employed, along with strain Halle (MV-H), which was obtained from a lymph node of a patient with subacute sclerosing panencephalitis (SSPE), which mimics various aspects of the pathology of neurological diseases, including demyelination. An analogue of a metabolite of azathioprine (ESP) was evaluated for antiviral activity against these two viral strains. The results proved positive for the MV-H infected cells as syncytia formation was reduced in a dose-dependent manner, and under protocols which avoided toxic effects, following ESP treatment ranging from 66% with 1 microg/ml and to 25% with 0.1 microg/ml. Since ESP is an analogue of the active metabolite of azathioprine, which exhibits positive outcomes when administered to MS patients, we submit that this metabolite is acting on MSMV, in a similar fashion to the action of ESP on MV-H.

Neuropathology of viral infections of the central nervous system

Many viral infections of the nervous system cause stereotyped pathologic features and overlapping clinical and imaging features. Neuroimaging usually offers neuroanatomical localization of the pathology, degree of involvement of the nervous system, and response to therapy during follow up in a few instances. Neuroimaging is a useful adjunct for diagnosis.

Two functionally linked amino acids in the stem 2 region of measles virus haemagglutinin determine infectivity and virulence in the rodent central nervous system

Rodent brain-adapted measles virus (MV) strains, such as CAM/RB and recombinant MVs based on the Edmonston strain containing the haemagglutinin (H) of CAM/RB, cause acute encephalitis after intracerebral infection of newborn rodents. We have demonstrated that rodent neurovirulence is modulated by two mutations at amino acid positions 195 and 200 in the H protein, one of these positions (200) being a potential glycosylation site. In order to analyse the effects of specific amino acids at these positions, we introduced a range of individual and combined mutations into the open reading frame of the H gene to generate a number of eukaryotic expression plasmids. The functionality of the mutant H proteins was assessed in transfected cells and by generating recombinant viruses. Interestingly, viruses caused acute encephalitis only if the amino acid Ser at position 200 was coupled with Gly at position 195, whereas viruses with single or combined mutations at these positions, including glycosylation at position 200, were attenuated. Neurovirulence was associated with virus spread and induction of neuronal apoptosis, whereas attenuated viruses failed to infect brain cells. Similar results were obtained by using primary brain-cell cultures. Our findings indicate that a structural alteration in the stem 2 region of the H protein at position 195 or 200 interferes with infectivity of rodent neurons, and suggest that the interaction of the viral attachment protein with cellular receptors on neurons is affected.

Advantages of using recombinant measles viruses expressing a fluorescent reporter gene with vibratome slice technology in experimental measles neuropathogenesis

AIMS

In this study of experimental measles neuropathogenesis, the utility of enhanced green fluorescent protein (EGFP) as a sensitive indicator of measles virus (MV) cell-to-cell spread in the central nervous system (CNS) has been assessed in vibratome-cut brain slices to demonstrate the degree and mechanism of viral spread in the rodent CNS.

METHODS

Recombinant MVs expressing EGFP were visualized at different levels in 200-microm vibratome-cut brain sections from infected animals by confocal scanning laser microscopy (CSLM). Comparison was made with 7-microm microtome sections, stained for the N protein of measles by immunocytochemistry (ICC).

RESULTS

The recombinant viruses were readily visualized in infected brain tissue, with no loss of neuropathogenicity. No difference was found in the sites of infection when MV infection was detected through EGFP fluorescence or by ICC. MV-infected cells were detected in the cerebral cortex, olfactory bulb and tract, hippocampus, thalamus, hypothalamus, ependyma and subventricular zone. However, the 200-microm vibratome-cut sections and confocal microscopy proved excellent for demonstrating virus distribution in neurites and for in-depth analysis of the extent of tract infection in the white matter of the cerebral hemispheres such as selective infection of the internal capsule and anterior commissure.

CONCLUSIONS

The use of self-tracing recombinant MVs, viewed in thick vibratome-cut sections by CSLM, demonstrated that in experimental MV neuropathogenesis the infection is selective and spreads predominately by neurites using defined anatomical pathways.

Tioman virus infection in experimentally infected mouse brain and its association with apoptosis

Tioman virus is a newly described bat-urine derived paramyxovirus isolated in Tioman Island, Malaysia in 2001. Hitherto, neither human nor animal infection by this virus has been reported. Nonetheless, its close relationship to another paramyxovirus, the Menangle virus which had caused diseases in humans and pigs [Philbey, A.W., Kirkland, P.D., Ross, A.D., Davis, R.J., Gleeson, A.B., Love, R.J., Daniels, P.W., Gould, A.R., Hyatt, A.D., 1998. An apparently new virus (family Paramyxoviridae) infectious for pigs, humans, and fruit bats. Emerg. Infect. Dis. 4, 269-271], raises the possibility that it may be potentially pathogenic. In this study, mice were experimentally infected with Tioman virus by intraperitoneal and intracerebral routes, and the cellular targets and topographical distribution of viral genome and antigens were examined using in situ hybridization and immunohistochemistry, respectively. The possible association between viral infection and apoptosis was also investigated using the TUNEL assay and immunohistochemistry to FasL, Caspase-3, Caspase-8, Caspase-9 and bcl-2. The results showed that Tioman virus inoculated intracerebrally was neurotropic causing plaque-like necrotic areas, and appeared to preferentially replicate in the neocortex and limbic system. Viral infection of inflammatory cells was also demonstrated. TUNEL and Caspase-3 positivity was found in inflammatory cells but not in neurons, while FasL, Caspase-8 and Caspase-9 were consistently negative. This suggests that neuronal infection was associated with necrosis rather than apoptosis. Moreover, the data suggest that there may be an association between viral infection and apoptosis in inflammatory cells, and that it could, at least in part, involve Caspase-independent pathways. Bcl-2 was expressed in some neurons and inflammatory cells indicating its possible role in anti-apoptosis. There was no evidence of central nervous system infection via the intraperitoneal route.

Alzheimer's neuroborreliosis with trans-synaptic spread of infection and neurofibrillary tangles derived from intraneuronal spirochetes

In the realm of dementia, it is astonishing to note that neurofibrillary tangles (NFT) are microscopically identical in a childhood illness (SSPE) and in a dementia of late adult life (Alzheimer's disease). The words "Alzheimer-type" NFT in peer reviewed scientific articles written by acknowledged experts underscore the striking similarities in "tangles" in two different diseases. Subacute Sclerosing Panencephalitis (SSPE) is caused by infection with atypical measles virus. Alzheimer's disease has no known cause. There is little controversy in suggesting that all of the Tangles in SSPE infected neurons are produced by slow viral type variant of Measles infection. But the mere suggestion that infection might be a cause of Alzheimer's disease confounds the establishment. If a good case is to be made for infection in Alzheimer's disease, an excellent nerve cell infection model is needed. Monkeys have provided a very reasonable model. Recently, a primate neuroborreliosis brain infection model demonstrated that Borrelia injected into the skin of monkeys resulted in the appearance of Borrelia transcriptomes in brain neurons. If Borrelia can travel from skin to brain in the monkey, then why not look at human Alzheimer's tissues to see if the DNA of Borrelia is present in the human brain? The molecular detection tools perfected in animal neuroborreliosis studies have been applied to human Alzheimer's disease brain tissues. Seven of ten cases of Alzheimer's disease from McLean Hospital Brain Bank of Harvard University yielded positive signals for infectious DNA in a small pilot study. Alzheimer's diseased neurons analyzed with DNA probes, produced little "dots" of positive staining. Granulovacuolar bodies in Alzheimer's diseased neurons (little dots in a bubble), are one of the expected microscopic profiles of Alzheimer's disease. "Little dots" inside nerve cells are also signatures of viral infectious agents inside of nerve cells. So with the assistance of the microscope and the tools of molecular biology, a new model of infection emerges as a cause of "Alzheimer's-type" neurofibrillary tangles. Here I hypothesize that it is chronic infection of human neurons in Alzheimer's disease that produces neurofibrillary tangles by a pathway similar to the chronic SSPE infection tangle pathway. In addition, transmission of infection from nerve to nerve is proposed to explain the evolution of Alzheimer's disease. Herein is offered a new view for the origins and for the progression of diseased nerves with tangle formations in Alzheimer's disease based on infection.

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.

A mouse model of persistent brain infection with recombinant Measles virus

Measles virus (MV) nucleocapsids are present abundantly in brain cells of patients with subacute sclerosing panencephalitis (SSPE). This invariably lethal brain disease develops years after acute measles as result of a persistent MV infection. Various rodent models for MV infection of the central nervous system (CNS) have been described in the past, in which the detection of viral antigens is based on histological staining procedures of paraffin embedded brains. Here, the usage of a recombinant MV (MV-EGFP-CAMH) expressing the haemagglutinin (H) of the rodent-adapted MV-strain CAM/RB and the enhanced green fluorescent protein (EGFP) is described. In newborn rodents the virus infects neurons and causes an acute lethal encephalitis. From 2 weeks on, when the immune system of the genetically unmodified animal is maturating, intracerebral (i.c.) infection is overcome subclinically, however, a focal persistent infection in groups of neurons remains. The complete brain can be analysed in 50 or 100 microm slices, and infected autofluorescent cells are readily detected. Seven and 28 days post-infection (p.i.) 86 and 81% of mice are infected, respectively, and virus persists for more than 50 days p.i. Intraperitoneal immunization with MV 1 week before infection, but not after infection, protects and prevents persistence. The high percentage of persistence demonstrates that this is a reliable and useful model of a persistent CNS infection in fully immunocompetent mice, which allows the investigation of determinants of the immune system.

High pathogenicity of wild-type measles virus infection in CD150 (SLAM) transgenic mice

Measles virus (MV) infection causes an acute childhood disease, associated in certain cases with infection of the central nervous system and development of a severe neurological disease. We have generated transgenic mice ubiquitously expressing the human protein SLAM (signaling lymphocytic activation molecule), or CD150, recently identified as an MV receptor. In contrast to all other MV receptor transgenic models described so far, in these mice infection with wild-type MV strains is highly pathogenic. Intranasal infection of SLAM transgenic suckling mice leads to MV spread to different organs and the development of an acute neurological syndrome, characterized by lethargy, seizures, ataxia, weight loss, and death within 3 weeks. In addition, in this model, vaccine and wild-type MV strains can be distinguished by virulence. Furthermore, intracranial MV infection of adult transgenic mice generates a subclinical infection associated with a high titer of MV-specific antibodies in the serum. Finally, to analyze new antimeasles therapeutic approaches, we created a recombinant soluble form of SLAM and demonstrated its important antiviral activity both in vitro and in vivo. Taken together, our results show the high susceptibility of SLAM transgenic mice to MV-induced neurological disease and open new perspectives for the analysis of the implication of SLAM in the neuropathogenicity of other morbilliviruses, which also use this molecule as a receptor. Moreover, this transgenic model, in allowing a simple readout of the efficacy of an antiviral treatment, provides unique experimental means to test novel anti-MV preventive and therapeutic strategies.

CD46 on glial cells can function as a receptor for viral glycoprotein-mediated cell-cell fusion

Membrane cofactor protein (CD46) is a regulator of complement activation that also serves as the entry receptor for human herpes virus 6 (HHV-6) and measles virus (MV) into human cells. While it is clear that oligodendrocytes and astrocytes are cell types commonly infected by these viruses, it is unclear whether oligodendrocytes express CD46, or which are the cellular mechanisms underlying the infection. We show that adult oligodendrocytes, as well as astrocytes and microglial cells, express CD46 on the cellular surface. Moreover, we employed a quantitative fusion assay to demonstrate that HHV-6A infection of T lymphocytes enables cell-cell fusion of these cells to astrocytes or to oligodendroglial cells. This fusion is mediated by the interaction between viral glycoproteins expressed on the membrane of the infected cells and CD46 on the glial targets, and is also observed using cells expressing recombinant MV glycoproteins. These data suggest a mechanism that involves cell-cell fusion by which certain viruses could spread the infection from the periphery to the cells in the nervous system.

Measles virus superinfection immunity and receptor redistribution in persistently infected NT2 cells

A recombinant measles virus (MV) expressing red fluorescent protein (MVDsRed1) was used to produce a persistently infected cell line (piNT2-MVDsRed1) from human neural precursor (NT2) cells. A similar cell line (piNT2-MVeGFP) was generated using a virus that expresses enhanced green fluorescent protein. Intracytoplasmic inclusions containing the viral nucleocapsid protein were evident in all cells and viral glycoproteins were present at the cell surface. Nevertheless, the cells did not release infectious virus nor did they fuse to generate syncytia. Uninfected NT2 cells express the MV receptor CD46 uniformly over their surface, whereas CD46 was present in cell surface aggregates in the piNT2 cells. There was no decrease in the overall amount of CD46 in piNT2 compared to NT2 cells. Cell-to-cell fusion was observed when piNT2 cells were overlaid onto confluent monolayers of MV receptor-positive cells, indicating that the viral glycoproteins were correctly folded and processed. Infectious virus was released from the underlying cells, indicating that persistence was not due to gross mutations in the virus genome. Persistently infected cells were superinfected with MV or canine distemper virus and cytopathic effects were not observed. However, mumps virus could readily infect the cells, indicating that superinfection immunity is not caused by general soluble antiviral factors. As MVeGFP and MVDsRed1 are antigenically indistinguishable but phenotypically distinct it was possible to use them to measure the degree of superinfection immunity in the absence of any cytopathic effect. Only small numbers of non-fusing green fluorescent piNT2-MVDsRed1 cells (1 : 300 000) were identified in which superinfecting MVeGFP entered, replicated and expressed its genes.

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.

Vacuolating encephalitis in mice infected by human coronavirus OC43

Involvement of viruses in human neurodegenerative diseases and the underlying pathologic mechanisms remain generally unclear. Human respiratory coronaviruses (HCoV) can infect neural cells, persist in human brain, and activate myelin-reactive T cells. As a means of understanding the human infection, we characterized in vivo the neurotropic and neuroinvasive properties of HCoV-OC43 through the development of an experimental animal model. Virus inoculation of 21-day postnatal C57BL/6 and BALB/c mice led to a generalized infection of the whole CNS, demonstrating HCoV-OC43 neuroinvasiveness and neurovirulence. This acute infection targeted neurons, which underwent vacuolation and degeneration while infected regions presented strong microglial reactivity and inflammatory reactions. Damage to the CNS was not immunologically mediated and microglial reactivity was instead a consequence of direct virus-mediated neuronal injury. Although this acute encephalitis appears generally similar to that induced by murine coronaviruses, an important difference rests in the prominent spongiform-like degeneration that could trigger neuropathology in surviving animals.

CD46- and CD150-independent endothelial cell infection with wild-type measles viruses

Measles virus (MV) infects endothelial cells of the skin, the brain and other organs during acute or persistent infections. Endothelial cells are supposed to play an important role in virus spread from the blood stream to surrounding tissues. CD46 and CD150 (signalling lymphocytic activation molecule, SLAM) have been described as cellular receptors for certain MV strains. We found that human umbilical vein and brain microvascular endothelial cells (HUVECs and HBMECs) were CD46-positive, but did not express SLAM. Wild-type MV strains, which do not use CD46 as a receptor at the surface of transfected Chinese hamster ovary cells, infected HUVECs and HBMECs to varying extents in a strain-dependent way. This infection was not inhibited by antibodies to CD46. These data suggest the presence of an additional unidentified receptor for MV uptake and spread in human endothelial cells.

Measles infection of the central nervous system

Central nervous system (CNS) complications occurring early and late after acute measles are serious and often fatal. In spite of functional cell-mediated immunity and high antiviral antibody titers, an immunological control of the CNS infection is not achieved in patients suffering from subacute sclerosing panencephalitis (SSPE). The known cellular receptors for measle virus (MV) in humans, CD46 and CD150 (signaling lymphocyte activation molecule, SLAM), are important components of the viral tropism by mediating binding and entry to peripheral cells. Because neural cells do not express SLAM and only sporadically CD46, virus entry to neural cells, and spread within the CNS, remain mechanistically unclear. Mice, hamsters, and rats have been used as model systems to study MV-induced CNS infections, and revealed interesting aspects of virulence, persistence, the immune response, and prerequisites of protection. With the help of recombinant MV and mice expressing transgenic receptors, questions such as receptor-dependent viral spread, or viral determinants of virulence, have been investigated. However, many questions concerning the human MV-induced CNS diseases are still open.