Multiple amino acid substitutions in hemagglutinin are necessary for wild-type measles virus to acquire the ability to use receptor CD46 efficiently

Genetic Characterization of the H Gene of MeV Strains (H1, B3, and D4) Recently Circulated in Iran for Improving the Molecular Measles Surveillance in the National Measles Lab

Background

Despite decreasing the global burden of measles disease after the introduction of vaccination, measles remains one of the most devastating childhood diseases. Since genotype B3 is reported as a predominant Measles Virus (MeV) genotype recently, the current study aimed to better understand MeV genetic variation by analyzing the complete sequence of Hemagglutinin (H) gene associated with outbreaks of circulated genotypes in Iran.

Methods

Nine positive measles specimens were selected from three circulated different genotypes H1, B3, and D4. Two different regions of MeV RNA were detected by RT-PCR assay. Sequence data and phylogenetic trees were analyzed and constructed by MEGA X software program. Moreover, missense and silent mutations in critical positions of the MeV-H protein were investigated.

Results

The result of phylogenetic analysis from the C-terminus of the Nucleoprotein gene (NP-450) and the complete H gene revealed that the mean sequence diversity was 0.06%-0.08% and 0.04%, respectively. Genotype H1 had the highest mutation in this study; however, the substitutions in genotype B3 fundamentally occurred in critical epitopes. Moreover, genotype D4 was more stable than genotypes B3 and H1.

Conclusion

Mutations were investigated in the whole sequence of H protein. Moreover, the mutations that occur in the critical sites of the protein have an important effect on the pathogenicity of the virus. In this way, we were able to illustrate why genotype B3 is more transmissible than other measles genotypes and is the most important circulating genotype around the world.

A defined anthocyanin mixture sourced from bilberry and black currant inhibits Measles virus and various herpesviruses

Background

Anthocyanin-containing plant extracts and carotenoids, such as astaxanthin, have been well-known for their antiviral and anti-inflammatory activity, respectively. We hypothesised that a mixture of Ribes nigrum L. (Grossulariaceae) (common name black currant (BC)) and Vaccinium myrtillus L. (Ericaceae) (common name bilberry (BL)) extracts (BC/BL) with standardised anthocyanin content as well as single plant extracts interfered with the replication of Measles virus and Herpesviruses in vitro.

Methods

We treated cell cultures with BC/BL or defined single plant extracts, purified anthocyanins and astaxanthin in different concentrations and subsequently infected the cultures with the Measles virus (wild-type or vaccine strain Edmonston), Herpesvirus 1 or 8, or murine Cytomegalovirus. Then, we analysed the number of infected cells and viral infectivity and compared the data to non-treated controls.

Results

The BC/BL extract inhibited wild-type Measles virus replication, syncytia formation and cell-to-cell spread. This suppression was dependent on the wild-type virus-receptor-interaction since the Measles vaccine strain was unaffected by BC/BL treatment. Furthermore, the evidence was provided that the delphinidin-3-rutinoside chloride, a component of BC/BL, and purified astaxanthin, were effective anti-Measles virus compounds. Human Herpesvirus 1 and murine Cytomegalovirus replication was inhibited by BC/BL, single bilberry or black currant extracts, and the BC/BL component delphinidin-3-glucoside chloride. Additionally, we observed that BC/BL seemed to act synergistically with aciclovir. Moreover, BC/BL, the single bilberry and black currant extracts, and the BC/BL components delphinidin-3-glucoside chloride, cyanidin-3-glucoside, delphinidin-3-rutinoside chloride, and petunidin-3-galactoside inhibited human Herpesvirus 8 replication.

Conclusions

Our data indicate that Measles viruses and Herpesviruses are differentially susceptible to a specific BC/BL mixture, single plant extracts, purified anthocyanins and astaxanthin. These compounds might be used in the prevention of viral diseases and in addition to direct-acting antivirals, such as aciclovir.

Versatility of live-attenuated measles viruses as platform technology for recombinant vaccines

Live-attenuated measles virus (MeV) has been extraordinarily effective in preventing measles infections and their often deadly sequelae, accompanied by remarkable safety and stability since their first licensing in 1963. The advent of recombinant DNA technologies, combined with systems to generate infectious negative-strand RNA viruses on the basis of viral genomes encoded on plasmid DNA in the 1990s, paved the way to generate recombinant, vaccine strain-derived MeVs. These live-attenuated vaccine constructs can encode and express additional foreign antigens during transient virus replication following immunization. Effective humoral and cellular immune responses are induced not only against the MeV vector, but also against the foreign antigen cargo in immunized individuals, which can protect against the associated pathogen. This review aims to present an overview of the versatility of this vaccine vector as platform technology to target various diseases, as well as current research and developmental stages, with one vaccine candidate ready to enter phase III clinical trials to gain marketing authorization, MV-CHIK.

Multiple Receptors Involved in Invasion and Neuropathogenicity of Canine Distemper Virus: A Review

The canine distemper virus (CDV) is a morbillivirus that infects a broad range of terrestrial carnivores, predominantly canines, and is associated with high mortality. Similar to another morbillivirus, measles virus, which infects humans and nonhuman primates, CDV transmission from an infected host to a naïve host depends on two cellular receptors, namely, the signaling lymphocyte activation molecule (SLAM or CD150) and the adherens junction protein nectin-4 (also known as PVRL4). CDV can also invade the central nervous system by anterograde spread through olfactory nerves or in infected lymphocytes through the circulation, thus causing chronic progressive or relapsing demyelination of the brain. However, the absence of the two receptors in the white matter, primary cultured astrocytes, and neurons in the brain was recently demonstrated. Furthermore, a SLAM/nectin-4-blind recombinant CDV exhibits full cell-to-cell transmission in primary astrocytes. This strongly suggests the existence of a third CDV receptor expressed in neural cells, possibly glial cells. In this review, we summarize the recent progress in the study of CDV receptors, highlighting the unidentified glial receptor and its contribution to pathogenicity in the host nervous system. The reviewed studies focus on CDV neuropathogenesis, and neural receptors may provide promising directions for the treatment of neurological diseases caused by CDV. We also present an overview of other neurotropic viruses to promote further research and identification of CDV neural receptors.

Advances in RNA Viral Vector Technology to Reprogram Somatic Cells: The Paramyxovirus Wave

Ethical issues are a significant barrier to the use of embryonic stem cells in patients due to their origin: human embryos. To further the development of stem cells in a patient application, alternative sources of cells were sought. A process referred to as reprogramming was established to create induced pluripotent stem cells from somatic cells, resolving the ethical issues, and vectors were developed to deliver the reprogramming factors to generate induced pluripotent stem cells. Early viral vectors used integrating retroviruses and lentiviruses as delivery vehicles for the transcription factors required to initiate reprogramming. However, because of the inherent risk associated with vectors that integrate into the host genome, non-integrating approaches were explored. The development of non-integrating viral vectors offers a safer alternative, and these modern vectors are reliable, efficient, and easy to use to achieve induced pluripotent stem cells suitable for direct patient application in the growing field of individualized medicine. This review summarizes all the RNA viral vectors in the field of reprogramming with a special focus on the emerging delivery vectors based on non-integrating  Paramyxoviruses, Sendai and measles viruses. We discuss their design and evolution towards being safe and efficient reprogramming vectors in generating induced pluripotent stem cells from somatic cells.

Screening of candidate genes associated with high titer production of oncolytic measles virus based on systems biology approach

The number of viral particles required for oncolytic activity of measles virus (MV) can be more than a million times greater than the reported amount for vaccination. The aim of the current study is to find potential genes and signaling pathways that may be involved in the high-titer production of MV. In this study, a systems biology approach was considered including collection of gene expression profiles from the Gene Expression Omnibus (GEO) database, obtaining differentially expressed genes (DEGs), performing gene ontology, functional enrichment analyses, and topological analyses on the protein-protein interaction (PPI) network. Then, to validate the in-silico data, total RNA was isolated from five cell lines, and full-length cDNA from template RNA was synthesized. Subsequently, quantitative reverse transcription-PCR (RT-qPCR) was employed. We identified five hub genes, including RAC1, HSP90AA1, DNM1, LTBP1, and FSTL1 associated with the enhancement in MV titer. Pathway analysis indicated enrichment in PI3K-Akt signaling pathway, axon guidance, proteoglycans in cancer, regulation of actin cytoskeleton, focal adhesion, and calcium signaling pathways. Upon verification by RT-qPCR, the relative expression of candidate genes was generally consistent with our bioinformatics analysis. Hub genes and signaling pathways may be involved in understanding the pathological mechanisms by which measles virus manipulates host factors in order to facilitate its replication. RAC1, HSP90AA1, DNM1, LTBP1, and FSTL1 genes, in combination with genetic engineering techniques, will allow the direct design of high-throughput cell lines to answer the required amounts for the oncolytic activity of MV.

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.

Oligomerization and Cell Egress Controlled by Two Microdomains of Canine Distemper Virus Matrix Protein

The multimeric matrix (M) protein of clinically relevant paramyxoviruses orchestrates assembly and budding activity of viral particles at the plasma membrane (PM). We identified within the canine distemper virus (CDV) M protein two microdomains, potentially assuming α-helix structures, which are essential for membrane budding activity. Remarkably, while two rationally designed microdomain M mutants (E89R, microdomain 1 and L239D, microdomain 2) preserved proper folding, dimerization, interaction with the nucleocapsid protein, localization at and deformation of the PM, the virus-like particle formation, as well as production of infectious virions (as monitored using a membrane budding-complementation system), were, in sharp contrast, strongly impaired. Of major importance, raster image correlation spectroscopy (RICS) revealed that both microdomains contributed to finely tune M protein mobility specifically at the PM. Collectively, our data highlighted the cornerstone membrane budding-priming activity of two spatially discrete M microdomains, potentially by coordinating the assembly of productive higher oligomers at the PM.

IMPORTANCE Despite the availability of efficient vaccines, morbilliviruses (e.g., canine distemper virus [CDV] and measles virus [MeV]) still cause major health impairments. Although antivirals may support vaccination campaigns, approved inhibitors are to date still lacking. Targeting late stages of the viral life cycle (i.e., the cell exit system) represents a viable option to potentially counteract morbilliviral infections. The matrix (M) protein of morbillivirus is a major contributor to membrane budding activity and is assumed to assemble into dimers that further associate to form higher oligomers. Here, we rationally engineered M protein variants with modifications in two microdomains that potentially locate at dimer-dimer interfaces. Our results spotlight the cornerstone impact of both microdomains in membrane budding activity and further suggest a role of finely tuned high-order oligomer formation in regulating late stages of cell exit. Collectively, our findings highlight two microdomains in the morbilliviral M protein as novel attractive targets for drug design.

Measles Virus as an Oncolytic Immunotherapy

Measles virus (MeV) preferentially replicates in malignant cells, leading to tumor lysis and priming of antitumor immunity. Live attenuated MeV vaccine strains are therefore under investigation as cancer therapeutics. The versatile MeV reverse genetics systems allows for engineering of advanced targeted, armed, and shielded oncolytic viral vectors. Therapeutic efficacy can further be enhanced by combination treatments. An emerging focus in this regard is combination immunotherapy, especially with immune checkpoint blockade. Despite challenges arising from antiviral immunity, availability of preclinical models, and GMP production, early clinical trials have demonstrated safety of oncolytic MeV and yielded promising efficacy data. Future clinical trials with engineered viruses, rational combination regimens, and comprehensive translational research programs will realize the potential of oncolytic immunotherapy.

Measles Virus Hemagglutinin Protein Establishes a Specific Interaction With the Extreme N-Terminal Region of Human Signaling Lymphocytic Activation Molecule to Enhance Infection

Measles virus (MV) is a human pathogen that is classified in the genus Morbillivirus in the family Paramyxoviridae together with several non-human animal morbilliviruses. They cause severe systemic infections by using signaling lymphocytic activation molecule (SLAM) and poliovirus receptor-like 4 expressed on immune and epithelial cells, respectively, as receptors. The viral hemagglutinin (H) protein is responsible for the receptor-binding. Previously determined structures of MV-H and SLAM complexes revealed a major binding interface between the SLAM V domain and MV-H with four binding components (sites 1–4) in the interface. We studied the MV-H and human SLAM (hSLAM) complex structure in further detail by in silico analyses and determined missing regions or residues in the previously determined complex structures. These analyses showed that, in addition to sites 1–4, MV-H establishes a unique interaction with the extreme N-terminal region (ExNTR) of hSLAM. The first principles calculation-based fragment molecular orbital computation method revealed that methionine at position 29 (hSLAM-Met29) is the key residue for the interaction. hSLAM-Met29 was predicted to establish a CH-π interaction with phenylalanine at position 549 of MV-H (MVH-Phe549). A cell-cell fusion assay showed that the hSLAM-Met29 and MVH-Phe549 interaction is important for hSLAM-dependent MV membrane fusion. Furthermore, Jurkat cell lines expressing hSLAM with or without Met29 and recombinant MV possessing the H protein with or without Phe549 showed that the hSLAM-Met29 and MVH-Phe549 interaction enhanced hSLAM-dependent MV infection by ~10-fold. We speculate that in the evolutionary history of morbilliviruses, this interaction may have contributed to MV adaptation to humans because this interaction is unique for MV and only MV uses hSLAM efficiently among morbilliviruses.

Lysosome-Associated Membrane Proteins Support the Furin-Mediated Processing of the Mumps Virus Fusion Protein

Mumps virus (MuV), an enveloped RNA virus of the Paramyxoviridae family and the causative agent of mumps, affects the salivary glands and other glandular tissues as well as the central nervous system. The virus enters the cell by inducing the fusion of its envelope with the plasma membrane of the target cell. Membrane fusion is mediated by MuV envelope proteins: the hemagglutinin-neuraminidase and fusion (F) protein. Cleavage of the MuV F protein (MuV-F) into two subunits by the cellular protease furin is a prerequisite for fusion and virus infectivity. Here, we show that 293T (a derivative of HEK293) cells do not produce syncytia upon expression of MuV envelope proteins or MuV infection. This failure is caused by the inefficient MuV-F cleavage despite the presence of functional furin in 293T cells. An expression cloning strategy revealed that overexpression of lysosome-associated membrane proteins (LAMPs) confers on 293T cells the ability to produce syncytia upon expression of MuV envelope proteins. The LAMP family comprises the ubiquitously expressed LAMP1 and LAMP2, the interferon-stimulated gene product LAMP3, and the cell type-specific proteins. The expression level of the LAMP3 gene, but not of LAMP1 and LAMP2 genes, differed markedly between 293T and HEK293 cells. Overexpression of LAMP1, LAMP2, or LAMP3 allowed 293T cells to process MuV-F efficiently. Furthermore, these LAMPs were found to interact with both MuV-F and furin. Our results indicate that LAMPs support the furin-mediated cleavage of MuV-F and that, among them, LAMP3 may be critical for the process, at least in certain cells.IMPORTANCE The cellular protease furin mediates proteolytic cleavage of many host and pathogen proteins and plays an important role in viral envelope glycoprotein maturation. MuV, an enveloped RNA virus of the Paramyxoviridae family and an important human pathogen, enters the cell through the fusion of its envelope with the plasma membrane of the target cell. Membrane fusion is mediated by the viral attachment protein and the F protein. Cleavage of MuV-F into two subunits by furin is a prerequisite for fusion and virus infectivity. Here, we show that LAMPs support the furin-mediated cleavage of MuV-F. Expression levels of LAMPs affect the processing of MuV-F and MuV-mediated membrane fusion. Among LAMPs, the interferon-stimulated gene product LAMP3 is most critical in certain cells. Our study provides potential targets for anti-MuV therapeutics.

Weak cis and trans Interactions of the Hemagglutinin with Receptors Trigger Fusion Proteins of Neuropathogenic Measles Virus Isolates

Measles virus (MeV) is an enveloped RNA virus bearing two envelope glycoproteins, the hemagglutinin (H) and fusion (F) proteins. Upon receptor binding, the H protein triggers conformational changes of the F protein, causing membrane fusion and subsequent virus entry. MeV may persist in the brain, infecting neurons and causing fatal subacute sclerosing panencephalitis (SSPE). Since neurons do not express either of the MeV receptors, signaling lymphocytic activation molecule (SLAM; also called CD150) and nectin-4, how MeV propagates in neurons is unknown. Recent studies have shown that specific substitutions in the F protein found in MeV isolates from SSPE patients are critical for MeV neuropathogenicity by rendering the protein unstable and hyperfusogenic. Recombinant MeVs possessing the F proteins with such substitutions can spread in primary human neurons and in the brains of mice and hamsters and induce cell-cell fusion in cells lacking SLAM and nectin-4. Here, we show that receptor-blind mutant H proteins that have decreased binding affinities to receptors can support membrane fusion mediated by hyperfusogenic mutant F proteins, but not the wild-type F protein, in cells expressing the corresponding receptors. The results suggest that weak interactions of the H protein with certain molecules (putative neuron receptors) trigger hyperfusogenic F proteins in SSPE patients. Notably, where cell-cell contacts are ensured, the weak cis interaction of the H protein with SLAM on the same cell surface also could trigger hyperfusogenic F proteins. Some enveloped viruses may exploit such cis interactions with receptors to infect target cells, especially in cell-to-cell transmission.IMPORTANCE Measles virus (MeV) may persist in the brain, causing incurable subacute sclerosing panencephalitis (SSPE). Because neurons, the main target in SSPE, do not express receptors for wild-type (WT) MeV, how MeV propagates in the brain is a key question for the disease. Recent studies have demonstrated that specific substitutions in the MeV fusion (F) protein are critical for neuropathogenicity. Here, we show that weak cis and trans interactions of the MeV attachment protein with receptors that are not sufficient to trigger the WT MeV F protein can trigger the mutant F proteins from neuropathogenic MeV isolates. Our study not only provides an important clue to understand MeV neuropathogenicity but also reveals a novel viral strategy to expand cell tropism.

Non-transmissible MV Vector with Segmented RNA Genome Establishes Different Types of iPSCs from Hematopoietic Cells

Recent advances in gene therapy technologies have enabled the treatment of congenital disorders and cancers and facilitated the development of innovative methods, including induced pluripotent stem cell (iPSC) production and genome editing. We recently developed a novel non-transmissible and non-integrating measles virus (MV) vector capable of transferring multiple genes simultaneously into a wide range of cells through the CD46 and CD150 receptors. The MV vector expresses four genes for iPSC generation and the GFP gene for a period of time sufficient to establish iPSCs from human fibroblasts as well as peripheral blood T cells. The transgenes were expressed differentially depending on their gene order in the vector. Human hematopoietic stem/progenitor cells were directly and efficiently reprogrammed to naive-like cells that could proliferate and differentiate into primed iPSCs by the same method used to establish primed iPSCs from other cell types. The novel MV vector has several advantages for establishing iPSCs and potential future applications in gene therapy.

Molecular mechanism by which residues at position 481 and 546 of measles virus hemagglutinin protein define CD46 receptor binding using a molecular docking approach

The hemagglutinin (H) protein of measles viruses (MeV) mediates binding to the cellular receptors, CD46,human signaling lymphocyte activation molecule and nectin-4. Vaccine strains primarily contain H-proteins possessing MeV-H: Y481 and can utilize CD46. Reports suggest that a single amino acid change in MeV-H at position 481 in wild type strains renders them inefficient in utilizing CD46. The in-depth molecular mechanism by which substitutions at 481 and another reported critical residue position 546 affects CD46 binding affinity however remains elusive. We used molecular docking studies of CD46 with MeV-H possessing Y481 N/D to understand the in-depth molecular mechanism involved. It was found that loss in either of the hydrogen bond (H-bond) contacts (MeV-H:481-CD46:65, MeV-H:546-CD46:63) in the central contact region prevented efficient CD46 binding. Y481 N could form the specific H-bond, while G546S H-bond could be formed only in conjunction with Y481, revealing the significance of these residues in determining CD46 receptor binding potential. Elucidating the underlying molecular mechanism of receptor usage by the MeV has implications to understanding cellular tropism, viral pathogenesis and therapy.

Cell-to-Cell Measles Virus Spread between Human Neurons Is Dependent on Hemagglutinin and Hyperfusogenic Fusion Protein

Measles virus (MV) usually causes acute infection but in rare cases persists in the brain, resulting in subacute sclerosing panencephalitis (SSPE). Since human neurons, an important target affected in the disease, do not express the known MV receptors (signaling lymphocyte activation molecule [SLAM] and nectin 4), how MV infects neurons and spreads between them is unknown. Recent studies have shown that many virus strains isolated from SSPE patients possess substitutions in the extracellular domain of the fusion (F) protein which confer enhanced fusion activity. Hyperfusogenic viruses with such mutations, unlike the wild-type MV, can induce cell-cell fusion even in SLAM- and nectin 4-negative cells and spread efficiently in human primary neurons and the brains of animal models. We show here that a hyperfusogenic mutant MV, IC323-F(T461I)-EGFP (IC323 with a fusion-enhancing T461I substitution in the F protein and expressing enhanced green fluorescent protein), but not the wild-type MV, spreads in differentiated NT2 cells, a widely used human neuron model. Confocal time-lapse imaging revealed the cell-to-cell spread of IC323-F(T461I)-EGFP between NT2 neurons without syncytium formation. The production of virus particles was strongly suppressed in NT2 neurons, also supporting cell-to-cell viral transmission. The spread of IC323-F(T461I)-EGFP was inhibited by a fusion inhibitor peptide as well as by some but not all of the anti-hemagglutinin antibodies which neutralize SLAM- or nectin-4-dependent MV infection, suggesting the presence of a distinct neuronal receptor. Our results indicate that MV spreads in a cell-to-cell manner between human neurons without causing syncytium formation and that the spread is dependent on the hyperfusogenic F protein, the hemagglutinin, and the putative neuronal receptor for MV.IMPORTANCE Measles virus (MV), in rare cases, persists in the human central nervous system (CNS) and causes subacute sclerosing panencephalitis (SSPE) several years after acute infection. This neurological complication is almost always fatal, and there is currently no effective treatment for it. Mechanisms by which MV invades the CNS and causes the disease remain to be elucidated. We have previously shown that fusion-enhancing substitutions in the fusion protein of MVs isolated from SSPE patients contribute to MV spread in neurons. In this study, we demonstrate that MV bearing the hyperfusogenic mutant fusion protein spreads between human neurons in a cell-to-cell manner. Spread of the virus was inhibited by a fusion inhibitor peptide and antibodies against the MV hemagglutinin, indicating that both the hemagglutinin and hyperfusogenic fusion protein play important roles in MV spread between human neurons. The findings help us better understand the disease process of SSPE.

Measles Vaccine

Measles remains an important cause of child morbidity and mortality worldwide despite the availability of a safe and efficacious vaccine. The current measles virus (MeV) vaccine was developed empirically by attenuation of wild-type (WT) MeV by in vitro passage in human and chicken cells and licensed in 1963. Additional passages led to further attenuation and the successful vaccine strains in widespread use today. Attenuation is associated with decreased replication in lymphoid tissue, but the molecular basis for this restriction has not been identified. The immune response is age dependent, inhibited by maternal antibody (Ab) and involves induction of both Ab and T cell responses that resemble the responses to WT MeV infection, but are lower in magnitude. Protective immunity is correlated with levels of neutralizing Ab, but the actual immunologic determinants of protection are not known. Because measles is highly transmissible, control requires high levels of population immunity. Delivery of the two doses of vaccine needed to achieve >90% immunity is accomplished by routine immunization of infants at 9-15 months of age followed by a second dose delivered before school entry or by periodic mass vaccination campaigns. Because delivery by injection creates hurdles to sustained high coverage, there are efforts to deliver MeV vaccine by inhalation. In addition, the safety record for the vaccine combined with advances in reverse genetics for negative strand viruses has expanded proposed uses for recombinant versions of measles vaccine as vectors for immunization against other infections and as oncolytic agents for a variety of tumors.

Measles virus envelope pseudotyped lentiviral vectors transduce quiescent human HSCs at an efficiency without precedent

Hematopoietic stem cell (HSC)-based gene therapy trials are now moving toward the use of lentiviral vectors (LVs) with success. However, one challenge in the field remains: efficient transduction of HSCs without compromising their stem cell potential. Here we showed that measles virus glycoprotein-displaying LVs (hemagglutinin and fusion protein LVs [H/F-LVs]) were capable of transducing 100% of early-acting cytokine-stimulated human CD34⁺ (hCD34⁺) progenitor cells upon a single application. Strikingly, these H/F-LVs also allowed transduction of up to 70% of nonstimulated quiescent hCD34⁺ cells, whereas conventional vesicular stomatitis virus G (VSV-G)-LVs reached 5% at the most with H/F-LV entry occurring exclusively through the CD46 complement receptor. Importantly, reconstitution of NOD/SCIDγc^-/- (NSG) mice with H/F-LV transduced prestimulated or resting hCD34⁺ cells confirmed these high transduction levels in all myeloid and lymphoid lineages. Remarkably, for resting CD34⁺ cells, secondary recipients exhibited increasing transduction levels of up to 100%, emphasizing that H/F-LVs efficiently gene-marked HSCs in the resting state. Because H/F-LVs promoted ex vivo gene modification of minimally manipulated CD34⁺ progenitors that maintained stemness, we assessed their applicability in Fanconi anemia, a bone marrow (BM) failure with chromosomal fragility. Notably, only H/F-LVs efficiently gene-corrected minimally stimulated hCD34⁺ cells in unfractionated BM from these patients. These H/F-LVs improved HSC gene delivery in the absence of cytokine stimulation while maintaining their stem cell potential. Thus, H/F-LVs will facilitate future clinical applications requiring HSC gene modification, including BM failure syndromes, for which treatment has been very challenging up to now.

Analysis of VSV pseudotype virus infection mediated by rubella virus envelope proteins

Rubella virus (RV) generally causes a systemic infection in humans. Viral cell tropism is a key determinant of viral pathogenesis, but the tropism of RV is currently poorly understood. We analyzed various human cell lines and determined that RV only establishes an infection efficiently in particular non-immune cell lines. To establish an infection the host cells must be susceptible and permissible. To assess the susceptibility of individual cell lines, we generated a pseudotype vesicular stomatitis virus bearing RV envelope proteins (VSV-RV/CE2E1). VSV-RV/CE2E1 entered cells in an RV envelope protein-dependent manner, and thus the infection was neutralized completely by an RV-specific antibody. The infection was Ca²⁺-dependent and inhibited by endosomal acidification inhibitors, further confirming the dependency on RV envelope proteins for the VSV-RV/CE2E1 infection. Human non-immune cell lines were mostly susceptible to VSV-RV/CE2E1, while immune cell lines were much less susceptible than non-immune cell lines. However, susceptibility of immune cells to VSV-RV/CE2E1 was increased upon stimulation of these cells. Our data therefore suggest that immune cells are generally less susceptible to RV infection than non-immune cells, but the susceptibility of immune cells is enhanced upon stimulation.

Dimerization Efficiency of Canine Distemper Virus Matrix Protein Regulates Membrane-Budding Activity

Paramyxoviruses rely on the matrix (M) protein to orchestrate viral assembly and budding at the plasma membrane. Although the mechanistic details remain largely unknown, structural data suggested that M dimers and/or higher-order oligomers may facilitate membrane budding. To gain functional insights, we employed a structure-guided mutagenesis approach to investigate the role of canine distemper virus (CDV) M protein self-assembly in membrane-budding activity. Three six-alanine-block (6A-block) mutants with mutations located at strategic oligomeric positions were initially designed. While the first one includes residues potentially residing at the protomer-protomer interface, the other two display amino acids located within two distal surface-exposed α-helices proposed to be involved in dimer-dimer contacts. We further focused on the core of the dimeric interface by mutating asparagine 138 (N138) to several nonconservative amino acids. Cellular localization combined with dimerization and coimmunopurification assays, performed under various denaturing conditions, revealed that all 6A-block mutants were impaired in self-assembly and cell periphery accumulation. These phenotypes correlated with deficiencies in relocating CDV nucleocapsid proteins to the cell periphery and in virus-like particle (VLP) production. Conversely, all M-N138 mutants remained capable of self-assembly, though to various extents, which correlated with proper accumulation and redistribution of nucleocapsid proteins at the plasma membrane. However, membrane deformation and VLP assays indicated that the M-N138 variants exhibiting the most reduced dimerization propensity were also defective in triggering membrane remodeling and budding, despite proper plasma membrane accumulation. Overall, our data provide mechanistic evidence that the efficiency of CDV M dimerization/oligomerization governs both cell periphery localization and membrane-budding activity.IMPORTANCE Despite the availability of effective vaccines, both measles virus (MeV) and canine distemper virus (CDV) still lead to significant human and animal mortality worldwide. It is assumed that postexposure prophylaxis with specific antiviral compounds may synergize with vaccination campaigns to better control ongoing epidemics. Targeting the matrix (M) protein of MeV/CDV is attractive, because M coordinates viral assembly and egress through interaction with multiple cellular and viral components. However, the lack of basic molecular knowledge of how M orchestrates these functions precludes the rational design of antivirals. Here we combined structure-guided mutagenesis with cellular, biochemical, and functional assays to investigate a potential correlation between CDV M self-assembly and virus-like particle (VLP) formation. Altogether, our findings provide evidence that stable M dimers at the cell periphery are required to productively trigger VLPs. Such stabilized M dimeric units may facilitate further assembly into robust higher-order oligomers necessary to promote plasma membrane-budding activity.

Perspective on Global Measles Epidemiology and Control and the Role of Novel Vaccination Strategies

Measles is a highly contagious, vaccine preventable disease. Measles results in a systemic illness which causes profound immunosuppression often leading to severe complications. In 2010, the World Health Assembly declared that measles can and should be eradicated. Measles has been eliminated in the Region of the Americas, and the remaining five regions of the World Health Organization (WHO) have adopted measles elimination goals. Significant progress has been made through increased global coverage of first and second doses of measles-containing vaccine, leading to a decrease in global incidence of measles, and through improved case based surveillance supported by the WHO Global Measles and Rubella Laboratory Network. Improved vaccine delivery methods will likely play an important role in achieving measles elimination goals as these delivery methods circumvent many of the logistic issues associated with subcutaneous injection. This review highlights the status of global measles epidemiology, novel measles vaccination strategies, and describes the pathway toward measles elimination.

[Measles Virus]

Measles virus (MeV) is exceptionally contagious and still a major cause of death in child.However, recently significant progress towards the elimination of measles has been made through increased vaccination coverage of measles-containing vaccines. The hemagglutinin (H) protein of MeV interacts with a cellular receptor, and this interaction is the first step of infection. MeV uses two different receptors, signaling lymphocyte activation molecule (SLAM) and nectin-4 expressed on immune cells and epithelial cells, respectively. The interactions of MeV with these receptors nicely explain the immune suppressive and high contagious properties of MeV. Binding of the H protein to a receptor triggers conformational changes in the fusion (F) protein, inducing fusion between viral and host plasma membranes for entry. The stalk region of the H protein plays a key role in the F protein-triggering. Recent studies of the H protein epitopes have revealed that the receptor binding site of the H protein constitutes a major neutralizing epitope. The interaction with two proteinaceous receptors probably imposes strong functional constraints on this epitope for amino acid changes. This would be a reason why measles vaccines, which are derived from MV strains isolated more than 60 years ago, are still highly effective against all MV strains currently circulating.

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.

The measles virus hemagglutinin β-propeller head β4-β5 hydrophobic groove governs functional interactions with nectin-4 and CD46 but not those with the signaling lymphocytic activation molecule

Wild-type measles virus (MV) strains use the signaling lymphocytic activation molecule (SLAM; CD150) and the adherens junction protein nectin-4 (poliovirus receptor-like 4 [PVRL4]) as receptors. Vaccine MV strains have adapted to use ubiquitous membrane cofactor protein (MCP; CD46) in addition. Recently solved cocrystal structures of the MV attachment protein (hemagglutinin [H]) with each receptor indicate that all three bind close to a hydrophobic groove located between blades 4 and 5 (β4-β5 groove) of the H protein β-propeller head. We used this structural information to focus our analysis of the functional footprints of the three receptors on vaccine MV H. We mutagenized this protein and tested the ability of individual mutants to support cell fusion through each receptor. The results highlighted a strong overlap between the functional footprints of nectin-4 and CD46 but not those of SLAM. A soluble form of nectin-4 abolished vaccine MV entry in nectin-4- and CD46-expressing cells but only reduced entry through SLAM. Analyses of the binding kinetics of an H mutant with the three receptors revealed that a single substitution in the β4-β5 groove drastically reduced nectin-4 and CD46 binding while minimally altering SLAM binding. We also generated recombinant viruses and analyzed their infections in cells expressing individual receptors. Introduction of a single substitution into the hydrophobic pocket affected entry through both nectin-4 and CD46 but not through SLAM. Thus, while nectin-4 and CD46 interact functionally with the H protein β4-β5 hydrophobic groove, SLAM merely covers it. This has implications for vaccine and antiviral strategies.

Mutant fusion proteins with enhanced fusion activity promote measles virus spread in human neuronal cells and brains of suckling hamsters

Subacute sclerosing panencephalitis (SSPE) is a fatal degenerative disease caused by persistent measles virus (MV) infection in the central nervous system (CNS). From the genetic study of MV isolates obtained from SSPE patients, it is thought that defects of the matrix (M) protein play a crucial role in MV pathogenicity in the CNS. In this study, we report several notable mutations in the extracellular domain of the MV fusion (F) protein, including those found in multiple SSPE strains. The F proteins with these mutations induced syncytium formation in cells lacking SLAM and nectin 4 (receptors used by wild-type MV), including human neuronal cell lines, when expressed together with the attachment protein hemagglutinin. Moreover, recombinant viruses with these mutations exhibited neurovirulence in suckling hamsters, unlike the parental wild-type MV, and the mortality correlated with their fusion activity. In contrast, the recombinant MV lacking the M protein did not induce syncytia in cells lacking SLAM and nectin 4, although it formed larger syncytia in cells with either of the receptors. Since human neuronal cells are mainly SLAM and nectin 4 negative, fusion-enhancing mutations in the extracellular domain of the F protein may greatly contribute to MV spread via cell-to-cell fusion in the CNS, regardless of defects of the M protein.

Functional and structural characterization of neutralizing epitopes of measles virus hemagglutinin protein

Effective vaccination programs have dramatically reduced the number of measles-related deaths globally. Although all the available data suggest that measles eradication is biologically feasible, a structural and biochemical basis for the single serotype nature of measles virus (MV) remains to be provided. The hemagglutinin (H) protein, which binds to two discrete proteinaceous receptors, is the major neutralizing target. Monoclonal antibodies (MAbs) recognizing distinct epitopes on the H protein were characterized using recombinant MVs encoding the H gene from different MV genotypes. The effects of various mutations on neutralization by MAbs and virus fitness were also analyzed, identifying the location of five epitopes on the H protein structure. Our data in the present study demonstrated that the H protein of MV possesses at least two conserved effective neutralizing epitopes. One, which is a previously recognized epitope, is located near the receptor-binding site (RBS), and thus MAbs that recognize this epitope blocked the receptor binding of the H protein, whereas the other epitope is located at the position distant from the RBS. Thus, a MAb that recognizes this epitope did not inhibit the receptor binding of the H protein, rather interfered with the hemagglutinin-fusion (H-F) interaction. This epitope was suggested to play a key role for formation of a higher order of an H-F protein oligomeric structure. Our data also identified one nonconserved effective neutralizing epitope. The epitope has been masked by an N-linked sugar modification in some genotype MV strains. These data would contribute to our understanding of the antigenicity of MV and support the global elimination program of measles.

Nectin 4 is the epithelial cell receptor for measles virus

Measles virus (MV) causes acute respiratory disease, infects lymphocytes and multiple organs, and produces immune suppression leading to secondary infections. In rare instances it can also cause persistent infections in the brain and central nervous system. Vaccine and laboratory-adapted strains of MV use CD46 as a receptor, whereas wild-type strains of MV (wtMV) cannot. Both vaccine and wtMV strains infect lymphocytes, monocytes, and dendritic cells (DCs) using the signaling lymphocyte activation molecule (CD150/SLAM). In addition, MV can infect the airway epithelial cells of the host. Nectin 4 (PVRL4) was recently identified as the epithelial cell receptor for MV. Coupled with recent observations made in MV-infected macaques, this discovery has led to a new paradigm for how the virus accesses the respiratory tract and exits the host. Nectin 4 is also a tumor cell marker which is highly expressed on the apical surface of many adenocarcinoma cell lines, making it a potential target for MV oncolytic therapy.

Lentiviral vectors displaying modified measles virus gp overcome pre-existing immunity in in vivo-like transduction of human T and B cells

Gene transfer into quiescent T and B cells is important for gene therapy and immunotherapy approaches. Previously, we generated lentiviral vectors (LVs) pseudotyped with Edmonston (Ed) measles virus (MV) hemagglutinin (H) and fusion (F) glycoproteins (H/F-LVs), which allowed efficient transduction of quiescent human T and B cells. However, a major obstacle in the use of H/F-LVs in vivo is that most of the human population is vaccinated against measles. As the MV humoral immune response is exclusively directed against the H protein of MV, we mutated the two dominant epitopes in H, Noose, and NE. LVs pseudotyped with these mutant H-glycoproteins escaped inactivation by monoclonal antibodies (mAbs) but were still neutralized by human serum. Consequently, we took advantage of newly emerged MV-D genotypes that were less sensitive to MV vaccination due to a different glycosylation pattern. The mutation responsible was introduced into the H/F-LVs, already mutated for Noose and NE epitopes. We found that these mutant H/F-LVs could efficiently transduce quiescent lymphocytes in the presence of high concentrations of MV antibody-positive human serum. Finally, upon incubation with total blood, mimicking the in vivo situation, the mutant H/F-LVs escaped MV antibody neutralization, where the original H/F-LVs failed. Thus, these novel H/F-LVs offer perspectives for in vivo lymphocyte-based gene therapy and immunotherapy.

Wild-type measles virus with the hemagglutinin protein of the edmonston vaccine strain retains wild-type tropism in macaques

A major difference between vaccine and wild-type strains of measles virus (MV) in vitro is the wider cell specificity of vaccine strains, resulting from the receptor usage of the hemagglutinin (H) protein. Wild-type H proteins recognize the signaling lymphocyte activation molecule (SLAM) (CD150), which is expressed on certain cells of the immune system, whereas vaccine H proteins recognize CD46, which is ubiquitously expressed on all nucleated human and monkey cells, in addition to SLAM. To examine the effect of the H protein on the tropism and attenuation of MV, we generated enhanced green fluorescent protein (EGFP)-expressing recombinant wild-type MV strains bearing the Edmonston vaccine H protein (MV-EdH) and compared them to EGFP-expressing wild-type MV strains. In vitro, MV-EdH replicated in SLAM(+) as well as CD46(+) cells, including primary cell cultures from cynomolgus monkey tissues, whereas the wild-type MV replicated only in SLAM(+) cells. However, in macaques, both wild-type MV and MV-EdH strains infected lymphoid and respiratory organs, and widespread infection of MV-EdH was not observed. Flow cytometric analysis indicated that SLAM(+) lymphocyte cells were infected preferentially with both strains. Interestingly, EGFP expression of MV-EdH in tissues and lymphocytes was significantly weaker than that of the wild-type MV. Taken together, these results indicate that the CD46-binding activity of the vaccine H protein is important for determining the cell specificity of MV in vitro but not the tropism in vivo. They also suggest that the vaccine H protein attenuates MV growth in vivo.

[Two different receptors for wild type measles virus]

Measles is a highly contagious acute viral disease characterized by a maculopapular rash. It causes severe and temporary immune suppression and is often accompanied by secondary bacterial infections. In 2000, signaling lymphocyte activation molecule (SLAM) was identified as a receptor for measles virus (MV). Observations that SLAM is expressed on cells of the immune system provided a good explanation for the lymphotropic and immunosuppressive nature of MV. However, molecular mechanisms of highly contagious nature of MV have remained unclear. Previously we have demonstrated that MV has an intrinsic ability to infect polarized epithelial cells by using a receptor other than SLAM. Recently, nectin4, a cellular adhesion junction molecule, was identified as the epithelial cell receptor for MV. Understanding the molecular mechanisms of MV to infect both epithelial and immune cells provides a deep insight into measles pathogenesis.

Tumor cell marker PVRL4 (nectin 4) is an epithelial cell receptor for measles virus

Vaccine and laboratory adapted strains of measles virus can use CD46 as a receptor to infect many human cell lines. However, wild type isolates of measles virus cannot use CD46, and they infect activated lymphocytes, dendritic cells, and macrophages via the receptor CD150/SLAM. Wild type virus can also infect epithelial cells of the respiratory tract through an unidentified receptor. We demonstrate that wild type measles virus infects primary airway epithelial cells grown in fetal calf serum and many adenocarcinoma cell lines of the lung, breast, and colon. Transfection of non-infectable adenocarcinoma cell lines with an expression vector encoding CD150/SLAM rendered them susceptible to measles virus, indicating that they were virus replication competent, but lacked a receptor for virus attachment and entry. Microarray analysis of susceptible versus non-susceptible cell lines was performed, and comparison of membrane protein gene transcripts produced a list of 11 candidate receptors. Of these, only the human tumor cell marker PVRL4 (Nectin 4) rendered cells amenable to measles virus infections. Flow cytometry confirmed that PVRL4 is highly expressed on the surfaces of susceptible lung, breast, and colon adenocarcinoma cell lines. Measles virus preferentially infected adenocarcinoma cell lines from the apical surface, although basolateral infection was observed with reduced kinetics. Confocal immune fluorescence microscopy and surface biotinylation experiments revealed that PVRL4 was expressed on both the apical and basolateral surfaces of these cell lines. Antibodies and siRNA directed against PVRL4 were able to block measles virus infections in MCF7 and NCI-H358 cancer cells. A virus binding assay indicated that PVRL4 was a bona fide receptor that supported virus attachment to the host cell. Several strains of measles virus were also shown to use PVRL4 as a receptor. Measles virus infection reduced PVRL4 surface expression in MCF7 cells, a property that is characteristic of receptor-associated viral infections.

Genetic characterization of measles vaccine strains

The complete genomic sequences of 9 measles vaccine strains were compared with the sequence of the Edmonston wild-type virus. AIK-C, Moraten, Rubeovax, Schwarz, and Zagreb are vaccine strains of the Edmonston lineage, whereas CAM-70, Changchun-47, Leningrad-4 and Shanghai-191 were derived from 4 different wild-type isolates. Nucleotide substitutions were found in the noncoding regions of the genomes as well as in all coding regions, leading to deduced amino acid substitutions in all 8 viral proteins. Although the precise mechanisms involved in the attenuation of individual measles vaccines remain to be elucidated, in vitro assays of viral protein functions and recombinant viruses with defined genetic modifications have been used to characterize the differences between vaccine and wild-type strains. Although almost every protein contributes to an attenuated phenotype, substitutions affecting host cell tropism, virus assembly, and the ability to inhibit cellular antiviral defense mechanisms play an especially important role in attenuation.

Measles virus glycoprotein-pseudotyped lentiviral vector-mediated gene transfer into quiescent lymphocytes requires binding to both SLAM and CD46 entry receptors

Gene transfer into quiescent T and B cells is of importance for gene therapy and immunotherapy approaches to correct hematopoietic disorders. Previously, we generated lentiviral vectors (LVs) pseudotyped with the Edmonston measles virus (MV) hemagglutinin and fusion glycoproteins (Hgps and Fgps) (H/F-LVs), which, for the first time, allowed efficient transduction of quiescent human B and T cells. These target cells express both MV entry receptors used by the vaccinal Edmonston strain, CD46 and signaling lymphocyte activation molecule (SLAM). Interestingly, LVs pseudotyped with an MV Hgp, blind for the CD46 binding site, were completely inefficient for resting-lymphocyte transduction. Similarly, SLAM-blind H mutants that recognize only CD46 as the entry receptor did not allow stable LV transduction of resting T cells. The CD46-tropic LVs accomplished vector-cell binding, fusion, entry, and reverse transcription at levels similar to those achieved by the H/F-LVs, but efficient proviral integration did not occur. Our results indicate that both CD46 and SLAM binding sites need to be present in cis in the Hgp to allow successful stable transduction of quiescent lymphocytes. Moreover, the entry mechanism utilized appears to be crucial: efficient transduction was observed only when CD46 and SLAM were correctly engaged and an entry mechanism that strongly resembles macropinocytosis was triggered. Taken together, our results suggest that although vector entry can occur through the CD46 receptor, SLAM binding and subsequent signaling are also required for efficient LV transduction of quiescent lymphocytes to occur.

Isolation and complete nucleotide sequence of the measles virus IMB-1 strain in China

The complete nucleotide sequence of the measles virus strain IMB-1, which was isolated in China, was determined. As in other measles viruses, its genome is 15,894 nucleotides in length and encodes six proteins. The full-length nucleotide sequence of the IMB-1 isolate differed from vaccine strains (including wild-type Edmonston strain) by 4%-5% at the nucleotide sequence level. This isolate has amino acid variations over the full genome, including in the hemagglutinin and fusion genes. This report is the first to describe the full-length genome of a genotype H1 strain and provide an overview of the diversity of genetic characteristics of a circulating measles virus.

Characterization of varicella-zoster virus-encoded ORF0 gene--comparison of parental and vaccine strains

The varicella-zoster virus (VZV) Oka vaccine strain (vOka) differs from the parental strain (pOka) at several amino acid positions, but the mutations responsible for the attenuation of vOka have not been clearly defined. The ORF0 of vOka carries some of the mutations. Although we found that the ORF0 of both strains was incorporated into virus particles, the C-terminal region of vOka ORF0 was presented on the virion surface and was N-glycosylated, suggesting that the mutation in vOka ORF0 changes it into a novel envelope glycoprotein. In a mutant virus in which pOka ORF0 was replaced by vOka ORF0, the molecular weight of ORF0 was altered, but the plaque size was not. In addition, a pOka recombinant virus lacking the hydrophobic domain of ORF0 grew equally well as the wild-type virus, indicating that the mutation in ORF0 is not by itself sufficient for the attenuation of the vOka virus.

Cellular receptors, differentiation and endocytosis requirements are key factors for type I IFN response by human epithelial, conventional and plasmacytoid dendritic infected cells by measles virus

While the antiviral response during measles virus (MeV) infection is documented, the contribution of the hosting cell type to the type I interferon (IFN-alpha/beta) response is still not clearly established. Here, we report that a signature heterogeneity of the IFN-alpha/beta response according to the cell type. The MeV tropism dictated by the expression of appropriate cellular receptor appeared to be crucial for epithelial cells. For conventional DCs (cDCs), the maturation state played a prominent role. In response to both wild type MeV isolates and laboratory/vaccine strains, immature cDCs produced higher levels of IFN-alpha than mature cDCs, despite the reduced expression levels of both CD46 and CD150 receptors by the former ones. While in epithelial cells and cDCs the MeV transcription was required to activate the IFN-alpha/beta response, plasmacytoid DCs (pDCs) rapidly produced large amounts of IFN-alpha mostly independently of the viral infection cycle. This argues for a significant contribution of pDCs in response to MeV infection and/or vaccination.

Epithelial-mesenchymal transition abolishes the susceptibility of polarized epithelial cell lines to measles virus

Measles virus (MV), an enveloped negative-strand RNA virus, remains a major cause of morbidity and mortality in developing countries. MV predominantly infects immune cells by using signaling lymphocyte activation molecule (SLAM; also called CD150) as a receptor, but it also infects polarized epithelial cells, forming tight junctions in a SLAM-independent manner. Although the ability of MV to infect polarized epithelial cells is thought to be important for its transmission, the epithelial cell receptor for MV has not been identified. A transcriptional repressor, Snail, induces epithelial-mesenchymal transition (EMT), in which epithelial cells lose epithelial cell phenotypes, such as adherens and tight junctions. In this study, EMT was induced by expressing Snail in a lung adenocarcinoma cell line, II-18, which is highly susceptible to wild-type MV. Snail-expressing II-18 cells lost adherens and tight junctions. Microarray analysis confirmed the induction of EMT in II-18 cells and suggested a novel function of Snail in protein degradation and distribution. Importantly, wild-type MV no longer entered EMT-induced II-18 cells, suggesting that the epithelial cell receptor is down-regulated by the induction of EMT. Other polarized cell lines, NCI-H358 and HT-29, also lost susceptibility to wild-type MV when EMT was induced. However, the complete formation of tight junctions rather reduced MV entry into HT-29 cells. Taken together, these data suggest that the unidentified epithelial cell receptor for MV is involved in the formation of epithelial intercellular junctions.

In vivo tropism of attenuated and pathogenic measles virus expressing green fluorescent protein in macaques

The global increase in measles vaccination has resulted in a significant reduction of measles mortality. The standard route of administration for the live-attenuated measles virus (MV) vaccine is subcutaneous injection, although alternative needle-free routes, including aerosol delivery, are under investigation. In vitro, attenuated MV has a much wider tropism than clinical isolates, as it can use both CD46 and CD150 as cellular receptors. To compare the in vivo tropism of attenuated and pathogenic MV, we infected cynomolgus macaques with pathogenic or attenuated recombinant MV expressing enhanced green fluorescent protein (GFP) (strains IC323 and Edmonston, respectively) via the intratracheal or aerosol route. Surprisingly, viral loads and cellular tropism in the lungs were similar for the two viruses regardless of the route of administration, and CD11c-positive cells were identified as the major target population. However, only the pathogenic MV caused significant viremia, which resulted in massive virus replication in B and T lymphocytes in lymphoid tissues and viral dissemination to the skin and the submucosa of respiratory epithelia. Attenuated MV was rarely detected in lymphoid tissues, and when it was, only in isolated infected cells. Following aerosol inhalation, attenuated MV was detected at early time points in the upper respiratory tract, suggesting local virus replication. This contrasts with pathogenic MV, which invaded the upper respiratory tract only after the onset of viremia. This study shows that despite in vitro differences, attenuated and pathogenic MV show highly similar in vivo tropism in the lungs. However, systemic spread of attenuated MV is restricted.

Crystallization and preliminary crystallographic analysis of the measles virus hemagglutinin in complex with the CD46 receptor

The measles virus (MV) hemagglutinin (MV-H) mediates the attachment of MV particles to cell-surface receptors for entry into host cells. MV uses two receptors for attachment to host cells: the complement-control protein CD46 and the signalling lymphocyte activation molecule (SLAM). The MV-H glycoprotein from an Edmonston MV variant and the MV-binding fragment of the CD46 receptor were overproduced in mammalian cells and used to crystallize an MV-H-CD46 complex. Well diffracting crystals containing two complexes in the asymmetric unit were obtained and the structure of the complex was solved by the molecular-replacement method.

Previously unrecognized amino acid substitutions in the hemagglutinin and fusion proteins of measles virus modulate cell-cell fusion, hemadsorption, virus growth, and penetration rate

Wild-type measles virus (MV) isolated in B95a cells could be adapted to Vero cells after several blind passages. In this study, we have determined the complete nucleotide sequences of the genomes of the wild type (T11wild) and its Vero cell-adapted (T11Ve-23) MV strain and identified amino acid substitutions R516G, E271K, D439E and G464W (D439E/G464W), N481Y/H495R, and Y187H/L204F in the nucleocapsid, V, fusion (F), hemagglutinin (H), and large proteins, respectively. Expression of mutated H and F proteins from cDNA revealed that the H495R substitution, in addition to N481Y, in the H protein was necessary for the wild-type H protein to use CD46 efficiently as a receptor and that the G464W substitution in the F protein was important for enhanced cell-cell fusion. Recombinant wild-type MV strains harboring the F protein with the mutations D439E/G464W [F(D439E/G464W)] and/or H(N481Y/H495R) protein revealed that both mutated F and H proteins were required for efficient syncytium formation and virus growth in Vero cells. Interestingly, a recombinant wild-type MV strain harboring the H(N481Y/H495R) protein penetrated slowly into Vero cells, while a recombinant wild-type MV strain harboring both the F(D439E/G464W) and H(N481Y/H495R) proteins penetrated efficiently into Vero cells, indicating that the F(D439E/G464W) protein compensates for the inefficient penetration of a wild-type MV strain harboring the H(N481Y/H495R) protein. Thus, the F and H proteins synergistically function to ensure efficient wild-type MV growth in Vero cells.

Reduced ability of hemagglutinin of the CAM-70 measles virus vaccine strain to use receptors CD46 and SLAM

The CAM-70 measles virus (MV) vaccine strain is currently used for vaccination against measles. We examined the fusion-inducing ability of the CAM-70 hemagglutinin (H) protein and found that it was impaired in both CD46- and signaling lymphocyte activation molecule (SLAM)-expressing cells. We also generated recombinant MVs possessing H genes derived from the CAM-70 strain. The CAM-70 H protein impaired viral growth in both CD46- and SLAM-expressing cells. In peripheral blood lymphocytes (PBL) and monocyte-derived dendritic cells (Mo-DC), the CAM-70 strain did not grow efficiently. Infection with recombinant MVs revealed that impaired growth of the CAM-70 strain was attributed to the H gene only partly in PBL and largely in Mo-DC. Thus, impaired fusion-inducing ability of the H protein may be one of the underlying molecular mechanisms resulting in the attenuation of the CAM-70 strain.

Measles virus receptors

Measles virus (MV) has two envelope glycoproteins, the hemagglutinin (H) and fusion protein, which are responsible for attachment and membrane fusion, respectively. Signaling lymphocyte activation molecule (SLAM, also called CD150), a membrane glycoprotein expressed on immune cells, acts as the principal cellular receptor for MV, accounting for its lymphotropism and immunosuppressive nature. MV also infects polarized epithelial cells via an as yet unknown receptor molecule, thereby presumably facilitating transmission via aerosol droplets. Vaccine and laboratory-adapted strains of MV use ubiquitously expressed CD46 as an alternate receptor through amino acid substitutions in the H protein. The crystal structure of the H protein indicates that the putative binding sites for SLAM, CD46, and the epithelial cell receptor are strategically located in different positions of the H protein. Other molecules have also been implicated in MV infection, although their relevance remains to be determined. The identification of MV receptors has advanced our understanding of MV tropism and pathogenesis.

X-ray crystallographic analysis of measles virus hemagglutinin

X-ray crystallographic analyses, together with nuclear magnetic resonance, have revealed three-dimensional structures of many important viral proteins, thereby allowing us to better understand the interactions between viral and host cell molecules. In this review, we summarize the recently determined crystal structure of the measles virus (MV) attachment protein hemagglutinin. Based on this structural information, we also discuss how the MV hemagglutinin interacts with various cellular receptors and why MV vaccines have been effective for many years without inducing escape mutant viruses. Other topics discussed are a putative MV receptor present on polarized epithelial cells and the protein expression system using a cultured human cell line 293SGnTI(-), which is suitable for X-ray crystallographic analyses.

Measles virus interaction with host cells and impact on innate immunity

Because viruses are obligate parasites, numerous partnerships between measles virus and cellular molecules can be expected. At the entry level, measles virus uses at least two cellular receptors, CD150 and a yet to be identified epithelial receptor to which the virus H protein binds. This dual receptor strategy illuminates the natural infection and inter-human propagation of this lymphotropic virus. The attenuated vaccine strains use CD46 as an additional receptor, which results in a tropism alteration. Surprisingly, the intracellular viral and cellular protein partnership leading to optimal virus life cycle remains mostly a black box, while the interactions between viral proteins that sustain the RNA-dependant RNA polymerase activity (i.e., transcription and replication), the particle assembly and the polarised virus budding are documented. Hsp72 is the only cellular protein that is known to regulate the virus transcription and replication through its interaction with the viral N protein. The viral P protein is phosphorylated by the casein kinase II with undetermined functional consequences. The cellular partnership that controls the intracellular trafficking of viral components, the assembly and/or the budding of measles virus, remains unknown. The virus to cell innate immunity war is better documented. The 5' triphosphate-ended virus leader transcript is recognised by RIG-I, a cellular helicase, and induces the interferon response. Measles virus V protein binds to the MDAS helicase and prevents the MDA5-mediated activation of interferon. By interacting with STAT1 and Jak1, the viral P and V proteins prevent the type I interferon receptor (IFNAR) signalling. The virus N protein interacts with eIF3-p40 to inhibit the translation of cellular mRNA. The H protein binds to TLR2, which then transduces an activation signal and CD150 expression in monocytes. The P protein activates the expression of the ubiquitin modifier A20, thus blocking the TLR4-mediated signalling. Few other partnerships between measles virus components and cellular proteins have been postulated or demonstrated, and they need further investigations to understand their physiopathological outcome.

Measles viruses possessing the polymerase protein genes of the Edmonston vaccine strain exhibit attenuated gene expression and growth in cultured cells and SLAM knock-in mice

Live attenuated vaccines against measles have been developed through adaptation of clinical isolates of measles virus (MV) in various cultured cells. Analyses using recombinant MVs with chimeric genomes between wild-type and Edmonston vaccine strains indicated that viruses possessing the polymerase protein genes of the Edmonston strain exhibited attenuated viral gene expression and growth in cultured cells as well as in mice expressing an MV receptor, signaling lymphocyte activation molecule, regardless of whether the virus genome had the wild-type or vaccine-type promoter sequence. These data demonstrate that the polymerase protein genes of the Edmonston strain contribute to its attenuated phenotype.