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

Infection of Pro- and Anti-Inflammatory Macrophages by Wild Type and Vaccine Strains of Measles Virus: NLRP3 Inflammasome Activation Independent of Virus Production

In humans and non-human primates, wild type (WT) measles virus (MeV) replicates extensively in lymphoid tissue and induces an innate response characteristic of NF-κB and inflammasome activation without type I interferon. In contrast, the live attenuated MeV vaccine (LAMV) replicates poorly in lymphoid tissue with little detectable in vivo cytokine production. To characterize the innate responses of macrophages to WT MeV and LAMV infection, we analyzed primary human monocyte-derived macrophages and phorbol myristic acid-matured monocytic THP-1 cells (M0) polarized to inflammatory (M1) and anti-inflammatory (M2) phenotypes 24 h after MeV infection. LAMV infected macrophages more efficiently than WT MeV but produced less virus than WT MeV-infected macrophages. Both strains induced production of NF-κB-responsive cytokines IL-6 and TNFα and inflammasome products IL-1β and IL-18 without evidence of pyroptosis. Analysis of THP-1 cells deficient in inflammasome sensors NOD-like receptor pyrin (NLRP)3, IFN-γ-inducible protein 16 (IFI16) or absent in melanoma (AIM)2; adaptor apoptosis-associated speck-like protein containing a CARD (ASC) or effector caspase 1 showed that IL-18 production was dependent on NLRP3, ASC, and caspase 1. However, M1 cells produced IL-1β in the absence of ASC or caspase 1 indicating alternate pathways for MeV-induced pro-IL-1β processing. Therefore, the innate response to in vitro infection of macrophages with both LAMV and WT MeV includes production of IL-6 and TNFα and activation of the NLRP3 inflammasome to release IL-1β and IL-18. LAMV attenuation impairs production of infectious virus but does not reduce ability to infect macrophages or innate responses to infection.

Serotypic evolution of measles virus is constrained by multiple co-dominant B cell epitopes on its surface glycoproteins

After centuries of pestilence and decades of global vaccination, measles virus (MeV) genotypes capable of evading vaccine-induced immunity have not emerged. Here, by systematically building mutations into the hemagglutinin (H) glycoprotein of an attenuated measles virus strain and assaying for serum neutralization, we show that virus evolution is severely constrained by the existence of numerous co-dominant H glycoprotein antigenic sites, some critical for binding to the pathogenicity receptors SLAMF1 and nectin-4. We further demonstrate the existence in serum of protective neutralizing antibodies targeting co-dominant fusion (F) glycoprotein epitopes. Lack of a substantial reduction in serum neutralization of mutant measles viruses that retain even one of the co-dominant antigenic sites makes evolution of pathogenic measles viruses capable of escaping serum neutralization in vaccinated individuals extremely unlikely.

Humanized Mice for Live-Attenuated Vaccine Research: From Unmet Potential to New Promises

Live-attenuated vaccines (LAV) represent one of the most important medical innovations in human history. In the past three centuries, LAV have saved hundreds of millions of lives, and will continue to do so for many decades to come. Interestingly, the most successful LAVs, such as the smallpox vaccine, the measles vaccine, and the yellow fever vaccine, have been isolated and/or developed in a purely empirical manner without any understanding of the immunological mechanisms they trigger. Today, the mechanisms governing potent LAV immunogenicity and long-term induced protective immunity continue to be elusive, and therefore hamper the rational design of innovative vaccine strategies. A serious roadblock to understanding LAV-induced immunity has been the lack of suitable and cost-effective animal models that can accurately mimic human immune responses. In the last two decades, human-immune system mice (HIS mice), i.e., mice engrafted with components of the human immune system, have been instrumental in investigating the life-cycle and immune responses to multiple human-tropic pathogens. However, their use in LAV research has remained limited. Here, we discuss the strong potential of LAVs as tools to enhance our understanding of human immunity and review the past, current and future contributions of HIS mice to this endeavor.

Evolution of Attenuation and Risk of Reversal in Peste des Petits Ruminants Vaccine Strain Nigeria 75/1

Peste des Petits Ruminants (PPR) is a highly infectious disease caused by a virus of the Morbillivirus genus. The current PPR eradication effort relies mainly on the implementation of massive vaccination campaigns. One of the most widely used PPR vaccines is the Nigeria 75/1 strain obtained after attenuation by 75 serial passages of the wild type isolate in cell cultures. Here we use high throughput deep sequencing of the historical passages that led to the Nigeria 75/1 attenuated strain to understand the evolution of PPRV attenuation and to assess the risk of reversal in different cell types. Comparison of the consensus sequences of the wild type and vaccine strains showed that only 18 fixed mutations separate the two strains. At the earliest attenuation passage at our disposal (passage 47), 12 out of the 18 mutations were already present at a frequency of 100%. Low-frequency variants were identified along the genome in all passages. Sequencing of passages after the vaccine strain showed evidence of genetic drift during cell passages, especially in cells expressing the SLAM receptor targeted by PPRV. However, 15 out of the 18 mutations related to attenuation remained fixed in the population. In vitro experiments suggest that one mutation in the leader region of the PPRV genome affects virus replication. Our results suggest that only a few mutations can have a serious impact on the pathogenicity of PPRV. Risk of reversion to virulence of the attenuated PPRV strain Nigeria 75/1 during serial passages in cell cultures seems low but limiting the number of passages during vaccine production is recommended.

Cyclical adaptation of measles virus quasispecies to epithelial and lymphocytic cells: To V, or not to V

Measles virus (MeV) is dual-tropic: it replicates first in lymphatic tissues and then in epithelial cells. This switch in tropism raises the question of whether, and how, intra-host evolution occurs. Towards addressing this question, we adapted MeV either to lymphocytic (Granta-519) or epithelial (H358) cells. We also passaged it consecutively in both human cell lines. Since passaged MeV had different replication kinetics, we sought to investigate the underlying genetic mechanisms of growth differences by performing deep-sequencing analyses. Lymphocytic adaptation reproducibly resulted in accumulation of variants mapping within an 11-nucleotide sequence located in the middle of the phosphoprotein (P) gene. This sequence mediates polymerase slippage and addition of a pseudo-templated guanosine to the P mRNA. This form of co-transcriptional RNA editing results in expression of an interferon antagonist, named V, in place of a polymerase co-factor, named P. We show that lymphocytic-adapted MeV indeed produce minimal amounts of edited transcripts and V protein. In contrast, parental and epithelial-adapted MeV produce similar levels of edited and non-edited transcripts, and of V and P proteins. Raji, another lymphocytic cell line, also positively selects V-deficient MeV genomes. On the other hand, in epithelial cells V-competent MeV genomes rapidly out-compete the V-deficient variants. To characterize the mechanisms of genome re-equilibration we rescued four recombinant MeV carrying individual editing site-proximal mutations. Three mutations interfered with RNA editing, resulting in almost exclusive P protein expression. The fourth preserved RNA editing and a standard P-to-V protein expression ratio. However, it altered a histidine involved in Zn²⁺ binding, inactivating V function. Thus, the lymphocytic environment favors replication of V-deficient MeV, while the epithelial environment has the opposite effect, resulting in rapid and thorough cyclical quasispecies re-equilibration. Analogous processes may occur in natural infections with other dual-tropic RNA viruses.

Key questions in infectious disease are how pathogens adapt to different cells of their hosts, and how the interplay between the virus and host factors controls the outcome of infection. Human measles virus (MeV) and related animal morbilliviruses provide important models of pathogenesis because they are dual-tropic: they replicate first in immune cells for spread through the body, and then in epithelial cells for transmission. We sought here to define the underlying molecular and evolutionary processes that allow MeV to spread rapidly in either lymphocytic or epithelial cells. We discovered unexpectedly rapid and thorough genome adaptation to these two tissues. Genome variants that cannot express functional V protein, an innate immunity control protein, are rapidly selected in lymphocytic cells. These variants express only the P protein, a polymerase co-factor, instead of expressing P and V at similar levels. Upon passaging in epithelial cells, V-competent MeV genome variants rapidly re-gain dominance. These results suggest that cyclical quasispecies re-equilibration may occur in acute MeV infections of humans, and that suboptimal variants in one environment constitute a low frequency reservoir for adaptation to the other, where they become dominant.

Annexin A2 Mediates the Localization of Measles Virus Matrix Protein at the Plasma Membrane

Annexins are a family of structurally related proteins that bind negatively charged membrane phospholipids in a Ca²⁺-dependent manner. Annexin A2 (AnxA2), a member of this family, has been implicated in a variety of cellular functions, including the organization of membrane domains, vesicular trafficking, and cell-cell adhesion. AnxA2 generally forms a heterotetrameric complex with a small Ca²⁺-binding protein, S100A10. Measles virus (MV), a member of the family Paramyxoviridae, is an enveloped virus with a nonsegmented negative-strand RNA genome. Knockdown of AnxA2 greatly reduced MV growth in cells without affecting its entry and viral RNA production. In MV-infected, AnxA2 knockdown cells, the expression level of the matrix (M) protein, but not other viral proteins, was reduced compared with that in control cells, and the distribution of the M protein at the plasma membrane was decreased. The M protein lines the inner surface of the envelope and plays an important role in virus assembly by connecting the nucleocapsid to the envelope proteins. The M protein bound to AnxA2 independently of AnxA2's phosphorylation or its association with S100A10 and was colocalized with AnxA2 within cells. Truncation of the N-terminal 10 amino acid residues, but not the N-terminal 5 residues, compromised the ability of the M protein to interact with AnxA2 and localize at the plasma membrane. These results indicate that AnxA2 mediates the localization of the MV M protein at the plasma membrane by interacting with its N-terminal region (especially residues at positions 6 to 10), thereby aiding in MV assembly.IMPORTANCE MV is an important human pathogen, still claiming ∼100,000 lives per year despite the presence of effective vaccines, and it causes occasional outbreaks even in developed countries. Replication of viruses largely relies on the functions of host cells. Our study revealed that the reduction of the host protein annexin A2 compromises the replication of MV within the cell. Further studies demonstrated that annexin A2 interacts with the MV M protein and mediates the localization of the M protein at the plasma membrane where MV particles are formed. The M protein lines the inner surface of the MV envelope membrane and plays a role in MV particle formation. Our results provide useful information for the understanding of the MV replication process and potential development of antiviral agents.

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 Hemagglutinin Protein Epitopes: The Basis of Antigenic Stability

Globally eliminating measles using available vaccines is biologically feasible because the measles virus (MV) hemagglutinin (H) protein is antigenically stable. The H protein is responsible for receptor binding, and is the main target of neutralizing antibodies. The immunodominant epitope, known as the hemagglutinating and noose epitope, is located near the receptor-binding site (RBS). The RBS also contains an immunodominant epitope. Loss of receptor binding correlates with an escape from the neutralization by antibodies that target the epitope at RBS. Another neutralizing epitope is located near RBS and is shielded by an N-linked sugar in certain genotype strains. However, human sera from vaccinees and measles patients neutralized all MV strains with similar efficiencies, regardless of the N-linked sugar modification or mutations at these epitopes. Two other major epitopes exist at a distance from RBS. One has an unstructured flexible domain with a linear neutralizing epitope. When MV-H forms a tetramer (dimer of dimers), these epitopes may form the dimer-dimer interface, and one of the two epitopes may also interact with the F protein. The neutralization mechanisms of antibodies that recognize these epitopes may involve inhibiting the H-F interaction or blocking the fusion cascade after MV-H binds to its receptors.

Actin-Modulating Protein Cofilin Is Involved in the Formation of Measles Virus Ribonucleoprotein Complex at the Perinuclear Region

In measles virus (MV)-infected cells, the ribonucleoprotein (RNP) complex, comprised of the viral genome and the nucleocapsid (N) protein, phosphoprotein (P protein), and large protein, assembles at the perinuclear region and synthesizes viral RNAs. The cellular proteins involved in the formation of the RNP complex are largely unknown. In this report, we show that cofilin, an actin-modulating host protein, interacts with the MV N protein and aids in the formation of the RNP complex. Knockdown of cofilin using the short hairpin RNA reduces the formation of the RNP complex after MV infection and that of the RNP complex-like structure after plasmid-mediated expression of MV N and P proteins. A lower level of formation of the RNP complex results in the reduction of viral RNA synthesis. Cofilin phosphorylation on the serine residue at position 3, an enzymatically inactive form, is increased after MV infection and the phosphorylated form of cofilin is preferentially included in the complex. These results indicate that cofilin plays an important role in MV replication by increasing formation of the RNP complex and viral RNA synthesis.

IMPORTANCE

Many RNA viruses induce within infected cells the structure called the ribonucleoprotein (RNP) complex in which viral RNA synthesis occurs. It is comprised of the viral genome and proteins that include the viral RNA polymerase. The cellular proteins involved in the formation of the RNP complex are largely unknown. In this report, we show that cofilin, an actin-modulating host protein, binds to the measles virus (MV) nucleocapsid protein and plays an important role in the formation of the MV RNP complex and MV RNA synthesis. The level of the phosphorylated form of cofilin, enzymatically inactive, is increased after MV infection, and the phosphorylated form is preferentially associated with the RNP complex. Our findings determined with cofilin will help us better understand the mechanism by which the RNP complex is formed in virus-infected cells and develop new antiviral drugs targeting the RNP complex.

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.

Intracellular transport of the measles virus ribonucleoprotein complex is mediated by Rab11A-positive recycling endosomes and drives virus release from the apical membrane of polarized epithelial cells

Many viruses use the host trafficking system at a variety of their replication steps. Measles virus (MV) possesses a nonsegmented negative-strand RNA genome that encodes three components of the ribonucleoprotein (RNP) complex (N, P, and L), two surface glycoproteins, a matrix protein, and two nonstructural proteins. A subset of immune cells and polarized epithelial cells are in vivo targets of MV, and MV is selectively released from the apical membrane of polarized epithelial cells. However, the molecular mechanisms for the apical release of MV remain largely unknown. In the present study, the localization and trafficking mechanisms of the RNP complex of MV were analyzed in detail using recombinant MVs expressing fluorescent protein-tagged L proteins. Live cell imaging analyses demonstrated that the MV RNP complex was transported in a manner dependent on the microtubule network and together with Rab11A-containing recycling endosomes. The RNP complex was accumulated at the apical membrane and the apical recycling compartment. The accumulation and shedding of infectious virions were severely impaired by expression of a dominant negative form of Rab11A. On the other hand, recycling endosome-mediated RNP transport was totally dispensable for virus production in nonpolarized cells. These data provide the first demonstration of the regulated intracellular trafficking events of the MV RNP complex that define the directional viral release from polarized epithelial cells.

The receptor-binding site of the measles virus hemagglutinin protein itself constitutes a conserved neutralizing epitope

Here, we provide direct evidence that the receptor-binding site of measles virus (MV) hemagglutinin protein itself forms an effective conserved neutralizing epitope (CNE). Several receptor-interacting residues constitute the CNE. Thus, viral escape from neutralization has to be associated with loss of receptor-binding activity. Since interactions with both the signaling lymphocyte activation molecule (SLAM) and nectin4 are critical for MV pathogenesis, its escape, which results from loss of receptor-binding activity, should not occur in nature.

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.

Measles virus selectively blind to signaling lymphocyte activation molecule as a novel oncolytic virus for breast cancer treatment

Oncolytic viruses hold much promise as novel therapeutic agents that can be combined with conventional therapeutic modalities. Measles virus (MV) is known to enter cells using the signaling lymphocyte activation molecule (SLAM), which is expressed on cells of the immune system. Although human breast cancer cell lines do not express SLAM, we found that a wild-type MV (HL strain) efficiently infected various breast cancer cell lines, causing cell death. Based on this finding, we used reverse genetics to generate a recombinant MV selectively unable to use SLAM (rMV-SLAMblind). The rMV-SLAMblind lacked infectivity for SLAM-positive lymphoid cells, while retaining oncolytic activity against breast cancer cells. We showed that, unlike the MV vaccine strains, rMV-SLAMblind used PVRL4 (polio virus receptor-related 4) as a receptor to infect breast cancer cells and not the ubiquitously expressed CD46. Consistent with this, rMV-SLAMblind infected CD46-positive primary normal human cells at a much-reduced level, whereas a vaccine strain of the Edmonston lineage (rMV-Edmonston) efficiently infected and killed them. The rMV-SLAMblind showed antitumor activity against human breast cancer xenografts in immunodeficient mice. The oncolytic activity of rMV-SLAMblind was significantly greater than that of rMV-Edmonston. To assess the in vivo safety, three monkeys seronegative for MV were inoculated with rMV-SLAMblind, and no clinical symptoms were documented. On the basis of these results, rMV-SLAMblind could be a promising candidate as a novel oncolytic virus for breast cancer treatment.

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.

The SI strain of measles virus derived from a patient with subacute sclerosing panencephalitis possesses typical genome alterations and unique amino acid changes that modulate receptor specificity and reduce membrane fusion activity

Subacute sclerosing panencephalitis (SSPE) is a fatal sequela associated with measles and is caused by persistent infection of the brain with measles virus (MV). The SI strain was isolated in 1976 from a patient with SSPE and shows neurovirulence in animals. Genome nucleotide sequence analyses showed that the SI strain genome possesses typical genome alterations for SSPE-derived strains, namely, accumulated amino acid substitutions in the M protein and cytoplasmic tail truncation of the F protein. Through the establishment of an efficient reverse genetics system, a recombinant SI strain expressing a green fluorescent protein (rSI-AcGFP) was generated. The infection of various cell types with rSI-AcGFP was evaluated by fluorescence microscopy. rSI-AcGFP exhibited limited syncytium-forming activity and spread poorly in cells. Analyses using a recombinant MV possessing a chimeric genome between those of the SI strain and a wild-type MV strain indicated that the membrane-associated protein genes (M, F, and H) were responsible for the altered growth phenotype of the SI strain. Functional analyses of viral glycoproteins showed that the F protein of the SI strain exhibited reduced fusion activity because of an E300G substitution and that the H protein of the SI strain used CD46 efficiently but used the original MV receptors on immune and epithelial cells poorly because of L482F, S546G, and F555L substitutions. The data obtained in the present study provide a new platform for analyses of SSPE-derived strains as well as a clear example of an SSPE-derived strain that exhibits altered receptor specificity and limited fusion activity.

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.

Use of attenuated paramyxoviruses for cancer therapy

Paramyxoviruses, measles virus (MV), mumps virus (MuV) and Newcastle disease virus (NDV), are well known for causing measles and mumps in humans and Newcastle disease in birds. These viruses have been tamed (attenuated) and successfully used as vaccines to immunize their hosts. Remarkably, pathogenic MuV and vaccine strains of MuV, MV and NDV efficiently infect and kill cancer cells and are consequently being investigated as novel cancer therapies (oncolytic virotherapy). Phase I/II clinical trials have shown promise but treatment efficacy needs to be enhanced. Technologies being developed to increase treatment efficacy include: virotherapy in combination with immunosuppressive drugs (cyclophosphamide); retargeting of viruses to specific tumor types or tumor vasculature; using infected cell carriers to protect and deliver the virus to tumors; and genetic manipulation of the virus to increase viral spread and/or express transgenes during viral replication. Transgenes have enabled noninvasive imaging or tracking of viral gene expression and enhancement of tumor destruction.

Enhanced antitumor effects of an engineered measles virus Edmonston strain expressing the wild-type N, P, L genes on human renal cell carcinoma

Measles virus Edmonston strain (MV-Edm) is thought to have remarkable oncolytic activity that selectively destroys human tumor cells. The P/V/C protein of wild-type MV was shown to resist the antiviral effects of interferon (IFN)-alpha. Here, we engineered new MVs by arming MV-Edm tag strain (a V-defective vaccine-lineage strain, MV-Etag) with the P or N, P, and L genes of wild-type MV (MV-P and MV-NPL, respectively). The oncolytic activities of the MVs were determined in human renal cell carcinoma (RCC) cell lines and primary human RCC cells by the MTT assay. The antitumor efficacy of the MVs was evaluated in A-498 xenografts in nude mice. IFN-alpha effectively inhibited the replication of MV-Etag and MV-P, but not MV-NPL. MV-NPL more efficiently induced cytopathic effects (CPEs) in OS-RC-2 cells, even in the presence of human IFN-alpha. MV-NPL replicated more rapidly than MV-P and MV-Etag in A-498 cells. Apoptosis was induced earlier in A-498 cells by MV-NPL than MV-Etag and MV-P. MV-NPL showed more significant antitumoral effects and had prolonged replication compared to MV-Etag and MV-P. In this study, we demonstrated that the newly engineered MV-NPL has more effective oncolytic activity and may help establish an innovative cancer therapy.

A highly attenuated measles virus vaccine strain encodes a fully functional C protein

The P, V, and C proteins of measles virus are encoded in overlapping reading frames of the P gene, which makes it difficult to analyze the functions of the individual proteins in the context of virus infection. We established a system to analyze the C protein independently from the P and V proteins by placing its gene in an additional transcription unit between the H and L genes. Analyses with this system indicated that a highly attenuated Edmonston lineage vaccine strain encodes a fully functional C protein, and the P and/or V protein is involved in the attenuated phenotype.

Contribution of matrix, fusion, hemagglutinin, and large protein genes of the CAM-70 measles virus vaccine strain to efficient growth in chicken embryonic fibroblasts

Attenuated live vaccines of measles virus (MV) have been developed from clinical isolates by serial propagation in heterologous cells, mainly chicken embryonic cells. The safety and effectiveness of these vaccines have been well established. However, the molecular mechanism of their attenuation remains a subject of investigation. The CAM-70 MV vaccine strain was developed from the Tanabe strain by serial propagation in chicken embryonic cells. In the present study, we assessed the contribution of each gene in the CAM-70 strain to efficient growth in chicken embryonic fibroblasts (CEF). We used a cloned MV IC323 based on the wild-type IC-B strain and generated a series of IC323s that possess one or more of the CAM-70 genes. Then, we examined the infection of CEF and CEF expressing human signaling lymphocyte activation molecule with the recombinant MVs. Our results demonstrated that MV needs to adapt to CEF at both the entry and postentry steps and that the CAM-70 matrix protein gene plays an important role in adaptation to CEF at the early stage of the virus replication cycle. The CAM-70 large protein gene was responsible for the efficient transcription and replication in CEF, and the CAM-70 hemagglutinin and fusion protein genes were responsible for efficient entry. Investigations focusing on these genes might elucidate unknown molecular mechanisms underlying the attenuation of MV.

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.

The matrix protein of measles virus regulates viral RNA synthesis and assembly by interacting with the nucleocapsid protein

The genome of measles virus (MV) is encapsidated by the nucleocapsid (N) protein and associates with RNA-dependent RNA polymerase to form the ribonucleoprotein complex. The matrix (M) protein is believed to play an important role in MV assembly by linking the ribonucleoprotein complex with envelope glycoproteins. Analyses using a yeast two-hybrid system and coimmunoprecipitation in mammalian cells revealed that the M protein interacts with the N protein and that two leucine residues at the carboxyl terminus of the N protein (L523 and L524) are critical for the interaction. In MV minigenome reporter gene assays, the M protein inhibited viral RNA synthesis only when it was able to interact with the N protein. The N protein colocalized with the M protein at the plasma membrane when the proteins were coexpressed in plasmid-transfected or MV-infected cells. In contrast, the N protein formed small dots in the perinuclear area when it was expressed without the M protein, or it was incapable of interacting with the M protein. Furthermore, a recombinant MV possessing a mutant N protein incapable of interacting with the M protein grew much less efficiently than the parental virus. Since the M protein has an intrinsic ability to associate with the plasma membrane, it may retain the ribonucleoprotein complex at the plasma membrane by binding to the N protein, thereby stopping viral RNA synthesis and promoting viral particle production. Consequently, our results indicate that the M protein regulates MV RNA synthesis and assembly via its interaction with the N protein.

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.