Amino acid substitutions at position 481 differently affect the ability of the measles virus hemagglutinin to induce cell fusion in monkey and marmoset cells co-expressing the fusion protein

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.

Advances in the design and development of oncolytic measles viruses

A successful oncolytic virus is one that selectively propagates and destroys cancerous tissue without causing excessive damage to the normal surrounding tissue. Oncolytic measles virus (MV) is one such virus that exhibits this characteristic and thus has rapidly emerged as a potentially useful anticancer modality. Derivatives of the Edmonston MV vaccine strain possess a remarkable safety record in humans. Promising results in preclinical animal models and evidence of biological activity in early phase trials contribute to the enthusiasm. Genetic modifications have enabled MV to evolve from a vaccine agent to a potential anticancer therapy. Specifically, alterations of the MV genome have led to improved tumor selectivity and delivery, therapeutic potency, and immune system modulation. In this article, we will review the advancements that have been made in the design and development of MV that have led to its use as a cancer therapy. In addition, we will discuss the evidence supporting its use, as well as the challenges associated with MV as a potential cancer therapeutic.

Infection of Different Cell Lines of Neural Origin with Subacute Sclerosing Panencephalitis (SSPE) Virus

Measles virus is the causative agent of subacute sclerosing panencephalitis (SSPE). The viruses isolated from brain cells of patients with SSPE (called SSPE viruses) are defective in cell-free virus production in vitro. To investigate the cell tropism of three strains of SSPE virus (Osaka-1, Osaka-2, Osaka-3), SSPE virus-infected cell cultures were treated with cytochalasin D to prepare virus-like particles (CD-VLPs). All CD-VLPs formed syncytia after infection in CHO cells expressing CD150 but not in those expressing CD46. In addition, an antibody to CD46 did not block the infection of Vero cells by SSPE CDVLPs. The results were consistent with our previous suggestion that one or more unidentified receptors might be involved in the entry process. Infection with the CD-VLPs from three SSPE strains was further examined in different human cell lines, including those of neural origin, and was found to induce syncytia in epithelial cells (HeLa and 293T) as well as neuroblastoma cells (IMR-32 and SK-N-SH) with varying efficiency. SSPE CD-VLPs also infected glioblastoma cells (A172) and astrocytoma cells (U-251) but syncytial formation was rarely induced. These epithelial and neural cell lines were not permissive for the replication of wild-type MV. Together with our previous observations, these results suggest that the cell entry receptor is the major factor determining the cell tropism of SSPE viruses. Further studies are necessary to identify other viral and/or cellular factors that might be involved in the replication of SSPE virus in specific neural cells and in the brain.

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.

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.

Measles virus for cancer therapy

Measles virus offers an ideal platform from which to build a new generation of safe, effective oncolytic viruses. Occasional so-called spontaneous tumor regressions have occurred during natural measles infections, but common tumors do not express SLAM, the wild-type MV receptor, and are therefore not susceptible to the virus. Serendipitously, attenuated vaccine strains of measles virus have adapted to use CD46, a regulator of complement activation that is expressed in higher abundance on human tumor cells than on their nontransformed counterparts. For this reason, attenuated measles viruses are potent and selective oncolytic agents showing impressive antitumor activity in mouse xenograft models. The viruses can be engineered to enhance their tumor specificity, increase their antitumor potency, and facilitate noninvasive in vivo monitoring of their spread. A major impediment to the successful deployment of oncolytic measles viruses as anticancer agents is the high prevalence of preexisting anti-measles immunity, which impedes bloodstream delivery and curtails intratumoral virus spread. It is hoped that these problems can be addressed by delivering the virus inside measles-infected cell carriers and/or by concomitant administration of immunosuppressive drugs. From a safety perspective, population immunity provides an excellent defense against measles spread from patient to carers and, in 50 years of human experience, reversion of attenuated measles to a wild-type pathogenic phenotype has not been observed. Clinical trials testing oncolytic measles viruses as an experimental cancer therapy are currently underway.

Host-virus specificity of morbilliviruses predicted by structural modeling of the marine mammal SLAM, a receptor

Signaling lymphocyte activation molecule (SLAM) is thought to be a major cellular receptor for high-host specificity morbilliviruses, which cause devastating and highly infectious diseases in mammals. We determined the sequences of SLAM cDNA from five species of marine mammal, including two cetaceans, two pinnipeds and one sirenian, and generated three-dimensional models to understand the receptor-virus interaction. Twenty-one amino acid residues in the immunoglobulin-like V domains of the SLAMs were shown to bind the viral protein. Notably, the sequences from pinnipeds and dogs were highly homologous, which is consistent with the fact that canine distemper virus was previously shown to cause a mass die-off of seals. Among these twenty-one residues, eight (63, 66, 68, 72, 84, 119, 121 and 130) were shared by animal groups susceptible to a particular morbillivirus species. This set of residues appears to determine host-virus specificity and may be useful for risk estimation for morbilliviruses.

Measles virus infects both polarized epithelial and immune cells by using distinctive receptor-binding sites on its hemagglutinin

Measles is one of the most contagious human infectious diseases and remains a major cause of childhood morbidity and mortality worldwide. The signaling lymphocyte activation molecule (SLAM), also called CD150, is a cellular receptor for measles virus (MV), presumably accounting for its tropism for immune cells and its immunosuppressive properties. On the other hand, pathological studies have shown that MV also infects epithelial cells at a later stage of infection, although its mechanism has so far been unknown. In this study, we show that wild-type MV can infect and produce syncytia in human polarized epithelial cell lines independently of SLAM and CD46 (a receptor for the vaccine strains of MV). Progeny viral particles are released exclusively from the apical surface of these polarized epithelial cell lines. We have also identified amino acid residues on the MV attachment protein that are likely to interact with a putative receptor on epithelial cells. All of these residues have aromatic side chains and may form a receptor-binding pocket located in a different position from the putative SLAM- and CD46-binding sites on the MV attachment protein. Thus, our results indicate that MV has an intrinsic ability to infect both polarized epithelial and immune cells by using distinctive receptor-binding sites on the attachment protein corresponding to each of their respective receptors. The ability of MV to infect polarized epithelial cells and its exclusive release from the apical surface may facilitate its efficient transmission via aerosol droplets, resulting in its highly contagious nature.

A human lung carcinoma cell line supports efficient measles virus growth and syncytium formation via a SLAM- and CD46-independent mechanism

Measles virus (MV) propagates mainly in lymphoid organs throughout the body and produces syncytia by using signaling lymphocyte activation molecule (SLAM) as a receptor. MV also spreads in SLAM-negative epithelial tissues by unknown mechanisms. Ubiquitously expressed CD46 functions as another receptor for vaccine strains of MV but not for wild-type strains. We here show that MV grows and produces syncytia efficiently in a human lung adenocarcinoma cell line via a SLAM- and CD46-independent mechanism using a novel receptor-binding site on the hemagglutinin protein. This infection model could advance our understanding of MV infection of SLAM-negative epithelial cells and tissues.

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

Measles virus (MV) possesses two envelope glycoproteins, namely, the receptor-binding hemagglutinin (H) and fusion proteins. Wild-type MV strains isolated in B-lymphoid cell lines use signaling lymphocyte activation molecule (SLAM), but not CD46, as a cellular receptor, whereas MV vaccine strains of the Edmonston lineage use both SLAM and CD46 as receptors. Studies have shown that the residue at position 481 of the H protein is critical in determining the use of CD46 as a receptor. However, the wild-type IC-B strain with a single N481Y substitution in the H protein utilizes CD46 rather inefficiently. In this study, a number of chimeric and mutant H proteins, and recombinant viruses harboring them, were generated to determine which residues of the Edmonston H protein are responsible for its efficient use of CD46. Our results show that three substitutions (N390I and E492G plus N416D or T446S), in addition to N481Y, are necessary for the IC-B H protein to use CD46 efficiently as a receptor. The N390I, N416D, and T446S substitutions are present in the H proteins of all strains of the Edmonston lineage, whereas the E492G substitution is found only in the H protein of the Edmonston tag strain generated from cDNAs. The T484N substitution, found in some of the Edmonston-lineage strains, resulted in a similar effect on the use of CD46 to that caused by the E492G substitution. Thus, multiple residues in the H protein that have not previously been implicated have important roles in the interaction with CD46.

[Cellular tropism and adaptation of the measles virus]

Measles virus (MV) is a member of the genus Morbillivirus in the family Paramyxoviridae. Clinical isolates of MV use signaling lymphocyte activating molecule (SLAM) as a cellular receptor. SLAM is mainly expressed on immune cells such as immature thymocytes, activated lymphocytes and mature dendritic cells. This distribution of SLAM can account for the lymphotropism of MV. On the other hand, laboratory strains of MV use CD46 as an alternative receptor, through amino acid change(s) in the receptor binding hemagglutinin protein. Recently, several reports imply the existence of the cellular receptor(s) other than SLAM and CD46. In this review, we discuss the receptor usage of MV and its adaptation to cultured cells.

Measles virus: cellular receptors, tropism and pathogenesis

Measles virus(MV), a member of the genusMorbillivirusin the familyParamyxoviridae, is an enveloped virus with a non-segmented, negative-strand RNA genome. It has two envelope glycoproteins, the haemagglutinin (H) and fusion proteins, which are responsible for attachment and membrane fusion, respectively. Human signalling lymphocyte activation molecule (SLAM; also called CD150), a membrane glycoprotein of the immunoglobulin superfamily, acts as a cellular receptor for MV. SLAM is expressed on immature thymocytes, activated lymphocytes, macrophages and dendritic cells and regulates production of interleukin (IL)-4 and IL-13 by CD4+T cells, as well as production of IL-12, tumour necrosis factor alpha and nitric oxide by macrophages. The distribution of SLAM is in accord with the lymphotropism and immunosuppressive nature of MV.Canine distemper virusandRinderpest virus, other members of the genusMorbillivirus, also use canine and bovine SLAM as receptors, respectively. Laboratory-adapted MV strains may use the ubiquitously expressed CD46, a complement-regulatory molecule, as an alternative receptor through amino acid substitutions in the H protein. Furthermore, MV can infect SLAM−cells, albeit inefficiently, via the SLAM- and CD46-independent pathway, which may account for MV infection of epithelial, endothelial and neuronal cellsin vivo. MV infection, however, is not determined entirely by the H protein–receptor interaction, and other MV proteins can also contribute to its efficient growth by facilitating virus replication at post-entry steps. Identification of SLAM as the principal receptor for MV has provided us with an important clue for better understanding of MV tropism and pathogenesis.

Recombinant wild-type measles virus containing a single N481Y substitution in its haemagglutinin cannot use receptor CD46 as efficiently as that having the haemagglutinin of the Edmonston laboratory strain

Signalling lymphocyte activation molecule (SLAM) acts as a cellular receptor for Measles virus (MV). The recombinant MV, based on a SLAM-using clinical isolate in which asparagine at position 481 of the haemagglutinin was replaced with tyrosine, was generated. Characterization of this recombinant virus revealed that the N481Y substitution in the haemagglutinin allowed it to utilize CD46 as an alternative receptor, but that its ability to use CD46 was rather low in CD46+ SLAM− cell lines compared with that of the recombinant virus possessing the haemagglutinin of the Edmonston laboratory strain. Thus, an N481Y substitution alone may not be sufficient to make SLAM-using MVs use CD46 efficiently, suggesting that further substitutions in the haemagglutinin are required for them to grow efficiently in CD46+ cells like the Edmonston strain. This may be a reason why few CD46-using MVs are detected in vivo.

Contributions of matrix and large protein genes of the measles virus edmonston strain to growth in cultured cells as revealed by recombinant viruses

The Edmonston strain of measles virus (MV) was obtained by sequential passages of the original isolate in various cultured cells. Although attenuated in vivo, it grows efficiently in most primate cell lines. Previous studies have revealed that MV tropism cannot be solely explained by the use of CD150 and/or CD46 as a cellular receptor. In order to evaluate the contributions of individual genes of the Edmonston strain to growth in cultured cells, we generated a series of recombinant viruses in which part of the genome of the clinical isolate IC-B (which uses CD150 as a receptor) was replaced with the corresponding sequences of the Edmonston strain. The recombinant virus possessing the Edmonston hemagglutinin (H) gene (encoding the receptor-binding protein) grew as efficiently in Vero cells as the Edmonston strain. Those viruses having either the matrix (M) or large (L) protein gene from the Edmonston strain could also replicate well in Vero cells, although they entered them at low efficiencies. P64S and E89K substitutions were responsible for the ability of the M protein to make virus grow efficiently in Vero cells, while the first half of the Edmonston L gene was important for better replication. Despite efficient growth in Vero cells, the recombinant viruses with these mutations had growth disadvantage in CD150-positive lymphoid B95a cells. Thus, not only the H gene but also the M and L genes contribute to efficient replication of the Edmonston strain in some cultured cells.

Antibody-targeted cell fusion

Membrane fusion has many potential applications in biotechnology. Here we show that antibody-targeted cell fusion can be achieved by engineering a fusogenic viral membrane glycoprotein complex. Three different single-chain antibodies were displayed at the extracellular C terminus of the measles hemagglutinin (H) protein, and combinations of point mutations were introduced to ablate its ability to trigger fusion through the native viral receptors CD46 and SLAM. When coexpressed with the measles fusion (F) protein, using plasmid cotransfection or bicistronic adenoviral vectors, the retargeted H proteins could mediate antibody-targeted cell fusion of receptor-negative or receptor-positive index cells with receptor-positive target cells. Adenoviral expression vectors mediating human epidermal growth factor receptor (EGFR)-targeted cell fusion were potently cytotoxic against EGFR-positive tumor cell lines and showed superior antitumor potency against EGFR-positive tumor xenografts as compared with control adenoviruses expressing native (untargeted) or CD38-targeted H proteins.

Changes in the receptorbinding haemagglutinin protein of wild-type morbilliviruses are not required for adaptation to Vero cells

We examined the consequences of isolation and adaptation to Vero cells for the receptorbinding haemagglutinin (H) gene of four syncytia-forming isolates of canine distemper virus (CDV) and of a dolphin morbillivirus isolate. A Vero-adapted CDV isolate exhibited biased hypermutation, since 11 out of 12 nucleotide differences to other isolates from the same epidemic were U-C transitions. Most of these transitions appeared to have taken place during in vitro cultivation. Previously, biased hypermutation in morbilliviruses has almost exclusively been described for subacute sclerosing panencephalitis and measles inclusion body encephalitis, which are rare measles virus brain infections. Amino acid changes in the H proteins were not required for Vero cell adaptation, suggesting that Vero cells express receptors for wild-type morbilliviruses. This strongly indicate the existence of other morbillivirus receptors than CD46 and CDw150.

SLAM (CD150)-independent measles virus entry as revealed by recombinant virus expressing green fluorescent protein

Wild-type measles virus (MV) strains use human signaling lymphocyte activation molecule (SLAM) as a cellular receptor, while vaccine strains such as the Edmonston strain can use both SLAM and CD46 as receptors. Although the expression of SLAM is restricted to cells of the immune system (lymphocytes, dendritic cells, and monocytes), histopathological studies with humans and experimentally infected monkeys have shown that MV also infects SLAM-negative cells, including epithelial, endothelial, and neuronal cells. In an attempt to explain these findings, we produced the enhanced green fluorescent protein (EGFP)-expressing recombinant MV (IC323-EGFP) based on the wild-type IC-B strain. IC323-EGFP showed almost the same growth kinetics as the parental recombinant MV and produced large syncytia exhibiting green autofluorescence in SLAM-positive cells. Interestingly, all SLAM-negative cell lines examined also showed green autofluorescence after infection with IC323-EGFP, although the virus hardly spread from the originally infected individual cells and thus did not induce syncytia. When the number of EGFP-expressing cells after infection was taken as an indicator, the infectivities of IC323-EGFP for SLAM-negative cells were 2 to 3 logs lower than those for SLAM-positive cells. Anti-MV hemagglutinin antibody or fusion block peptide, but not anti-CD46 antibody, blocked IC323-EGFP infection of SLAM-negative cells. This infection occurred under conditions in which entry via endocytosis was inhibited. These results indicate that MV can infect a variety of cells, albeit with a low efficiency, by using an as yet unidentified receptor(s) other than SLAM or CD46, in part explaining the observed MV infection of SLAM-negative cells in vivo.

Analysis of receptor (CD46, CD150) usage by measles virus

In order to investigate which measles virus (MV)-strains use CD46 and/or CD150 (signalling lymphocytic activation molecule, SLAM) as receptors, CHO cells expressing either recombinant CD46 or SLAM were infected with a panel of 28 MV-strains including vaccine strains, wild-type strains with various passage histories and recombinant viruses. We found that SLAM served as a common receptor conferring virus uptake and syncytium formation for all MV-strains tested. Predominantly vaccine and laboratory adapted strains, but also a minor fraction of the wild-type strains tested, could utilize both CD46 and SLAM. Using recombinant viruses, we demonstrate that the single amino acid exchange in the haemagglutinin (H) protein at position 481 Asn/Tyr (H481NY) determines whether the virus can utilize CD46. This amino acid alteration has no affect on the usage of SLAM as receptor, and as such demonstrates that the binding sites for SLAM and CD46 are distinct.

Hemadsorption expressed by cloned H genes from subacute sclerosing panencephalitis (SSPE) viruses and their possible progenitor measles viruses isolated in Osaka, Japan

Most subacute sclerosing panencephalitis (SSPE) viruses, including our Osaka-1, -2, and -3 strains isolated in Osaka, have shown negative hemadsorption (HAD) by African green monkey red blood cells. This property has been thought to be characteristic of SSPE virus as compared to the positive reaction of the standard Edmonston strain of measles virus (MV). However, this assumption has become quite obscure because MV mutates frequently at the genetic level during its multiplication and also because recent field strains isolated by lymphoblastoid cell lines have shown negative HAD. To investigate the above issue, the nucleotide sequences of the hemagglutinin (H) genes from SSPE virus Osaka-1, -2, or -3 strains were compared to those of various MV field strains isolated in Osaka by Vero cells. The H gene sequences of three SSPE strains were relatively conserved without such biased hypermutation as had been observed in the matrix (M) gene of three SSPE strains. However, this analysis of the H gene sequence of the SSPE viruses enabled us to deduce possible progenitor MVs, which are in agreement with the deduction from the M gene analysis we reported previously. The HAD of Vero cells transfected with the cloned H cDNAs from the SSPE strains and their progenitors suggested that negative HAD of the SSPE viruses has been maintained as one of original properties of the progenitor MVs rather than having been acquired as an altered one during long-term persistent infection in the brains of patients with SSPE.

Immune and artificial selection in the haemagglutinin (H) glycoprotein of measles virus

We present a maximum likelihood (ML) analysis of the selection pressures that have shaped the evolution of the large (L) protein and the haemagglutinin (H) glycoprotein of measles virus (MV). A number of amino acid sites that have potentially been subject to adaptive evolution were identified in the H protein using sequences from every known genotype of MV. All but one of these putative positively selected sites reside within the ectodomain of the H protein, where they often show an association with positions of potential B-cell epitopes and sites known to interact with the CD46 receptor. This suggests that MV may be under pressure from the immune system, albeit relatively weakly, to alter sites within epitopes and hence evade the humoral immune response. The positive selection identified at amino acid 546 was shown to correlate with the passage history of MV isolates in Vero cells. We reveal that Vero cell passaging has the potential to introduce an artificial signal of adaptive evolution through selection for changes that increase affinity for the CD46 receptor.

Measles viruses on throat swabs from measles patients use signaling lymphocytic activation molecule (CDw150) but not CD46 as a cellular receptor

Both CD46 and signaling lymphocytic activation molecule (SLAM) have been shown to act as cellular receptors for measles virus (MV). The viruses on throat swabs from nine patients with measles in Japan were titrated on Vero cells stably expressing human SLAM. Samples from all but two patients produced numerous plaques on SLAM-expressing Vero cells, whereas none produced any plaques on Vero cells endogenously expressing CD46. The Edmonston strain of MV, which can use either CD46 or SLAM as a receptor, produced comparable titers on these two types of cells. The results strongly suggest that the viruses in the bodies of measles patients use SLAM but probably not CD46 as a cellular receptor.

The cellular receptor for measles virus?elusive no more

The identity of the measles virus receptor has been controversial. Several years ago CD46 was identified as a cellular receptor for the Edmonston strain of measles virus, but most clinical isolates of measles virus, which are most efficiently isolated in the marmoset B cell line B95a, cannot grow in many CD46+ cell lines. Although some researchers attributed it to post-entry block in viral replication, others believed that there is a receptor other than CD46 for wild-type measles viruses. A new study showed that human signalling lymphocytic activation molecule (SLAM; also known as CDw150) is a cellular receptor for measles virus, including the Edmonston strain. SLAM is expressed on lymphocytes and dendritic cells, and plays an important role in lymphocyte activation. The identification of SLAM as a measles virus receptor nicely explains the pathogenesis of measles virus infection.