CD46-mediated measles virus entry: a first key to host-range specificity

A comprehensive review on targeting cluster of differentiation: An attractive strategy for inhibiting viruses through host proteins

Viral infections continue to pose a significant global public health threat. Targeting host proteins, such as cluster of differentiation (CD) macromolecules, may offer a promising alternative approach to developing antiviral treatments. CDs are cell-surface biological macromolecules mainly expressed on leukocytes that viruses can use to enter cells, thereby evading immune detection and promoting their replication. The manipulation of CDs by viruses may represent an effective and clever means of survival through the prolonged co-evolution of hosts and viruses. Targeting of CDs is anticipated to hinder the invasion of related viruses, modulate the body's immune system, and diminish the incidence of subsequent inflammation. They have become crucial for biomedical diagnosis, and some have been used as valuable tools for resisting viral infections. However, a summary of the structures and functions of CDs involved in viral infection is currently lacking. The development of drugs targeting these biological macromolecules is restricted both in terms of their availability and the number of compounds currently identified. This review provides a comprehensive analysis of the critical role of CD proteins in virus invasion and a list of relevant targeted antiviral agents, which will serve as a valuable reference for future research in this field.

Development of an Enzyme-Linked Immunosorbent Assay Based on CD150/SLAM for the Detection of Peste des Petits Ruminant Virus

Peste des petits ruminant (PPR) is an economically important severe viral disease of small ruminants that affects primarily the respiratory and digestive tract. Specific detection of the PPR virus (PPRV) antigen plays an important role in the disease control and eradication program. In this study, an indirect enzyme-linked immunosorbent assay (ELISA) based on the recombinant goat signaling lymphocyte activation molecule (SLAM) as the capture ligand was successfully developed for the detection of the PPRV antigen (PPRV SLAM-iELISA). The assay was highly specific for PPRV with no cross-reactions among foot and mouth disease virus, Orf virus, sheep pox virus, and goat pox virus and had a sensitivity with a detection limit of 1.56 × 10¹ TCID₅₀/reaction (50 μl). Assessment of 136 samples showed that the developed PPRV SLAM-iELISA was well correlated with real-time RT-qPCR assays and commercially available sandwich ELISA for detection of PPRV and showed relative sensitivity and specificity of 93.75 and 100.83%, respectively. These results suggest that the developed PPRV SLAM-iELISA is suitable for specific detection of the PPRV antigen. This study demonstrated for the first time that the goat SLAM, the cellular receptor for PPRV, can be used for the development of a diagnostic method for the detection of PPRV.

Enhanced lysis by bispecific oncolytic measles viruses simultaneously using HER2/neu or EpCAM as target receptors

To target oncolytic measles viruses (MV) to tumors, we exploit the binding specificity of designed ankyrin repeat proteins (DARPins). These DARPin-MVs have high tumor selectivity while maintaining excellent oncolytic potency. Stability, small size, and efficacy of DARPins allowed the generation of MVs simultaneously targeted to tumor marker HER2/neu and cancer stem cell (CSC) marker EpCAM. For optimization, the linker connecting both DARPins was varied in flexibility and length. Flexibility had no impact on fusion helper activity whereas length had. MVs with bispecific MV-H are genetically stable and revealed the desired double-target specificity. In vitro, the cytolytic activity of bispecific MVs was superior or comparable to mono-targeted viruses depending on the target cells. In vivo, therapeutic efficacy of the bispecific viruses was validated in an orthotopic ovarian carcinoma model revealing an effective reduction of tumor mass. Finally, the power of bispecific targeting was demonstrated on cocultures of different tumor cells thereby mimicking tumor heterogeneity in vitro, more closely reflecting real tumors. Here, bispecific excelled monospecific viruses in efficacy. DARPin-based targeting domains thus allow the generation of efficacious oncolytic viruses with double specificity, with the potential to handle intratumoral variation of antigen expression and to simultaneously target CSCs and the bulk tumor mass.

Rapid titration of measles and other viruses: optimization with determination of replication cycle length

Background

Measles virus (MV) is a member of the Paramyxoviridae family and an important human pathogen causing strong immunosuppression in affected individuals and a considerable number of deaths worldwide. Currently, measles is a re-emerging disease in developed countries. MV is usually quantified in infectious units as determined by limiting dilution and counting of plaque forming unit either directly (PFU method) or indirectly from random distribution in microwells (TCID50 method). Both methods are time-consuming (up to several days), cumbersome and, in the case of the PFU assay, possibly operator dependent.

Methods/Findings

A rapid, optimized, accurate, and reliable technique for titration of measles virus was developed based on the detection of virus infected cells by flow cytometry, single round of infection and titer calculation according to the Poisson's law. The kinetics follow up of the number of infected cells after infection with serial dilutions of a virus allowed estimation of the duration of the replication cycle, and consequently, the optimal infection time. The assay was set up to quantify measles virus, vesicular stomatitis virus (VSV), and human immunodeficiency virus type 1 (HIV-1) using antibody labeling of viral glycoprotein, virus encoded fluorescent reporter protein and an inducible fluorescent-reporter cell line, respectively.

Conclusion

Overall, performing the assay takes only 24–30 hours for MV strains, 12 hours for VSV, and 52 hours for HIV-1. The step-by-step procedure we have set up can be, in principle, applicable to accurately quantify any virus including lentiviral vectors, provided that a virus encoded gene product can be detected by flow cytometry.

New insights into measles virus propagation: from entry to shedding

The last decade of research on measles virus entry has provided new clues and concepts on the molecular mechanisms that drive virus entry in distinct tissues, virus propagation in vivo throughout multiple organs and virus exit to ensure epidemic propagation. Novel biochemical and structural information on viral glycoproteins have shed light on how a genotypically variable RNA virus with error-prone RNA polymerase can behave as an immunologically invariant virus for over half a century worldwide. Moreover, the viral attachment protein was found to support efficient retargeting to unnatural cellular receptors, and this led to the design of the first class of retargeted viral vectors possessing an envelope.

Contacting the Host: Insights and Implications of Pathogenic Neisseria Cell Interactions

Neisseria is a highly adapted human specific pathogen that initiates infection at the mucosal epithelia by using multiple adhesins to interact with host cell receptors. Colonization begins at the apical cell surface with a multi-step adhesion cascade, followed by invasion and persistence within the cell and finally transcytosis at the basolateral surface. The type IV pill are implicated in mediating the initial attachment of both meningococci and gonococci, and this association has been shown to involve contact with the cellular receptor CD46. In this review we describe the initial events in the adhesion, invasion and signaling of pathogenic Neisseria focusing on the initial attachment and signaling induced by the interaction of the type IV pili with CD46.

Identification of Amino Acid Residues Involved in the Interaction between Measles Virus Haemagglutin (MVH) and Its Human Cell Receptor(Signaling Lymphocyte Activation Molecule, SLAM)

Signaling lymphocyte activation molecule (SLAM; also known as CD150) is a newly identified cellular receptor for measles virus (MV). The interaction between MV Haemagglutin (MVH) and SLAM is an initial step for MV entry. We have identified several novel SLAM binding sites at residues S429, T436 and H437 of MVH protein and MVH mutants in these residues dramatically decrease the ability to interaction with the cell surface SLAM and fail to coprecipitation with SLAM in vivo as well as malfunction in syncytium formation. At the same time, K58, S59 and H61 of SLAM was also identified to be critical for MVH and SLAM binding. Further, these residues may be useful targets for the development of measles therapy.

''Homologous restriction'' in complement lysis: roles of membrane complement regulators

The complement system is a powerful bactericidal immune defence with the potential to damage self cells. Protection of self is provided by expression on cells of a battery of membrane regulators that inhibit activation of complement. Roles of complement in the rejection of transplanted organs have long been recognized, and are particularly relevant in xenotransplantation, where hyperacute rejection is complement-driven. Inhibiting complement was therefore considered early in the history of xenografting, and the use of membrane complement regulators to this end was proposed more than two decades ago. For each of the membrane regulators in humans, early studies implied a species-specificity of action, inhibiting human complement but not that from other species. The dogma of species-specificity dictated strategies for inhibiting complement in xenografts and drove the creation of donor transgenic pigs expressing human regulators. Here we critically evaluate the evidence for species-specificity in membrane complement regulators from humans and other animals. We challenge the dogma and show that there is considerable cross-species activity for each of the membrane regulators of complement. Acceptance of the fact that species selectivity is not a limitation will open new avenues for protection of the xenograft from complement damage.

Characterization of a region involved in binding of measles virus H protein and its receptor SLAM (CD150)

Signaling lymphocyte activation molecule (SLAM; also known as CD150) is a newly identified cellular receptor for measles virus (MV). MV Hemagglutinin protein (H) mediates MV entry into host cells by specifically binding to SLAM. Amino acid 27-135 of SLAM was previously shown to be the functional domain to interact with H and used to screen a 10-mer phage display peptide library in this study. After four rounds of screening and sequence analysis, the deduced amino acid sequence of screened peptides SGFDPLITHA and SDWDPLFTHK showed to be highly homologous with amino acid 429-438 of MV H (SGFGPLITHG). Peptides SGFDPLITHA and SDWDPLFTHK specifically inhibited binding of H to SLAM and further inhibition of MV infection suggests that these peptides can be developed to MV blocking reagents and amino acid 429-438 in H protein is functionally involved in receptor binding and may constitute part of the receptor-binding determinants on H protein.

An immunohistochemical study of the distribution of the measles virus receptors, CD46 and SLAM, in normal human tissues and subacute sclerosing panencephalitis

We have compared the expression of the known measles virus (MV) receptors, membrane cofactor protein (CD46) and the signaling lymphocyte-activation molecule (SLAM), using immunohistochemistry, in a range of normal peripheral tissues (known to be infected by MV) as well as in normal and subacute sclerosing panencephalitis (SSPE) brain. To increase our understanding of how these receptors could be utilized by wild-type or vaccine strains in vivo, the results have been considered with regard to the known route of infection and systemic spread of MV. Strong staining for CD46 was observed in endothelial cells lining blood vessels and in epithelial cells and tissue macrophages in a wide range of peripheral tissues, as well as in Langerhans' and squamous cells in the skin. In lymphoid tissues and blood, subsets of cells were positive for SLAM, in comparison to CD46, which stained all nucleated cell types. Strong CD46 staining was observed on cerebral endothelium throughout the brain and also on ependymal cells lining the ventricles and choroid plexus. Comparatively weaker CD46 staining was observed on subsets of neurons and oligodendrocytes. In SSPE brain sections, the areas distant from lesion sites and negative for MV by immunocytochemistry showed the same distribution for CD46 as in normal brain. However, cells in lesions, positive for MV, were negative for CD46. Normal brain showed no staining for SLAM, and in SSPE brain only subsets of leukocytes in inflammatory infiltrates were positive. None of the cell types most commonly infected by MV show detectable expression of SLAM, whereas CD46 is much more widely expressed and could fulfill a receptor function for some wild-type strains. In the case of wild-type stains, which are unable to use CD46, a further as yet unknown receptor(s) would be necessary to fully explain the pathology of MV infection.

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.

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

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

Octamerization enables soluble CD46 receptor to neutralize measles virus in vitro and in vivo

A chimeric fusion protein encompassing the CD46 ectodomain linked to the C-terminal part of the C4b binding protein (C4bp) alpha chain (sCD46-C4bpalpha) was produced in eukaryotic cells. This protein, secreted as a disulfide-linked homo-octamer, was recognized by a panel of anti-CD46 antibodies with varying avidities. Unlike monomeric sCD46, the octameric sCD46-C4bpalpha protein was devoid of complement regulatory activity. However, sCD46-C4bpalpha was able to bind to the measles virus hemagglutinin protein expressed on murine cells with a higher avidity than soluble monomeric sCD46. Moreover, the octameric sCD46-C4bpalpha protein was significantly more efficient than monomeric sCD46 in inhibiting virus binding to CD46, in blocking virus induced cell-cell fusion, and in neutralizing measles virus in vitro. In addition, the octameric sCD46-C4bpalpha protein, but not the monomeric sCD46, fully protected CD46 transgenic mice against a lethal intracranial measles virus challenge.

Inefficient measles virus budding in murine L.CD46 fibroblasts

Infection of mouse L.CD46 fibroblasts with measles virus resulted in a poor virus yield, although no defects in the steps of virus binding, entry or fusion, were detected. Two days post-infection, the level of expression of the viral F protein was found to be similar on the surface of infected L.CD46 and HeLa cells using a virus multiplicity enabling an equal number of cells to be infected. After immunofluorescence labelling and confocal microscopy, L.CD46 cells also displayed a significant increase in the co-localisation of the N protein with the cell surface H and F proteins. Immunogold labelling and transmission electron microscopy demonstrated the accumulation of numerous nucleocapsids near the plasma membrane of L. CD46 cells with little virus budding, in contrast to infected HeLa cells which displayed fewer cortical nucleocapsids and more enveloped viral particles. Purified virus particles from infected L. CD46 contained a reduced amount of H, F and M protein. Altogether, these data indicate that, in L.CD46 cells, the late stage of measles virus assembly is defective. This cellular model will be helpful for the identification of cellular factors controlling measles virus maturation.

Human membrane cofactor protein (MCP, CD46): multiple isoforms and functions

Human membrane cofactor protein (MCP, CD46) is a 45-70 kDa protein with genetic and tissue-specific heterogeneity, and is expressed on all nucleated cells. MCP consists from N-terminus of 4 short consensus repeats (SCRs), 1-3 serine/threonine-rich (ST) domains, a transmembrane domain (TM) and a cytoplasmic tail (CYT). More than 8 isoforms are generated secondary to alternative splicing due to combinations of various exons encoding the ST, TM and CYT domains. It serves as a cofactor of serine protease factor I for inactivation of complement C3b and C4b. Its primary role is to protect host cells from homologous complement attack by inactivating C3b/C4b deposited on the membrane. It also acts as receptors for measles virus (MV), some kinds of bacteria and for a putative ligand on oocytes. MV infection causes temporal host immune suppression, which may appear secondary to signaling events through MCP on macrophages and dendritic cells. These functional properties of human MCP may facilitate xenotransplantation and may be useful in the generation of animal models of measles by creating human MCP-expressing animals.

Moesin functions as a lipopolysaccharide receptor on human monocytes

Bacterial endotoxin (lipopolysaccharide [LPS]), a glycolipid found in the outer membranes of gram-negative bacteria, induces the secretion of proinflammatory cytokines such as tumor necrosis factor alpha (TNF-alpha), interleukin-1 (IL-1), and IL-6 by monocytes/macrophages. The secretion of these biologically active compounds leads to multiple pathological conditions, such as septic shock. There is substantial evidence that chronic exposure to LPS mediates, at least in part, the tissue destruction associated with gram-negative infection. CD14, a 55-kDa protein, has been identified as an LPS receptor. In conjunction with a serum protein, LPS binding protein (LBP), LPS-CD14 interactions mediate many LPS functions in the inflammatory response. However, CD14 lacks a cytoplasmic domain, or any known signal transduction sequence motif, suggesting the existence of another cell surface domain capable of transducing signals. In this paper, we report a second, CD14-independent LPS binding site, which, based on biological activity, appears to be a functional LPS receptor. Cross-linking experiments were performed to identify LPS binding sites. Two molecules were identified: a 55-kDa protein (CD14) and a second, 78-kDa band. Sequencing of the 78-kDa protein by mass spectroscopic analysis revealed 100% homology with moesin (membrane-organizing extension spike protein). Antibody to CD14 induced partial blocking of the LPS response. However, antimoesin monoclonal antibody completely blocked the LPS-induced TNF-alpha response in human monocytes, without blocking CD14 binding of LPS. Irrelevant isotype controls had no effect. Additional experiments were performed to evaluate the specificity of the antimoesin blocking. Separate experiments evaluated antimoesin effects on monocyte chemotaxis, IL-1 production in response to IL-1 stimulation, and TNF-alpha secretion in response to Staphylococcus aureus stimulation. Antimoesin blocked only LPS-mediated events. The data suggest that moesin functions as an independent LPS receptor on human monocytes. The role of moesin in transduction of CD14-mediated signals is discussed.

Use of site-specific mutagenesis and monoclonal antibodies to map regions of CD46 that interact with measles virus H protein

Researchers at our laboratory have been dissecting the binding domains of the receptor for the Edmonston laboratory strain of measles virus (CD46) through site-specific mutagenesis. We initially substituted most of the hydrophilic amino acids in the two external short consensus regions (SCRI and SCRII) of CD46 with the amino acid alanine [Hsu et al. (1997) J. Virol. 71:6144-6154] and found that the glutamic-arginine residues at positions 58 and 59 were particularly sensitive to change. Here we consider the roles of hydrophobic amino acids in the binding between measles virus H protein and CD46. Hydrophobic amino acids in the SCRI and SCRII domains of CD46 were systematically replaced with serine. The effects of these changes were monitored through the interaction of Sf9 insect cells expressing the H protein and mouse OST-7 cells synthesizing the mutant CD46 molecules. Binding was quantified through a colorimetric assay for beta-galactosidase that was also produced by the insect cells. Our results indicate that E45, Y54, 58E/R59, Y68, F69, Y101, I102, R103, D104, and Y117 seem to be critical residues for the binding of CD46 to measles virus H protein. The hydrophilic amino acid R59 in SCR1 and hydrophobic residues Y101, I102, and Y117 in SCR2 seem to be especially important for interaction between H protein and CD46. In addition, we mapped the antigenic epitopes of five monoclonal antibodies that are known to inhibit the binding between H protein and CD46. Three of these antibodies recognized regions in SCR1, and two reacted with amino acids in SCR2. For the most part, the determinants recognized by the monoclonal antibody corresponded to the amino acids that were most sensitive to change in the binding process. The SCR1 and SCR2 domains of CD46 were modeled from an analogous region in another complement regulatory protein, factor H, whose three-dimensional structure has been previously reported. Amino acids implicated in binding seem to lie on one planar face of the SCR1 and SCR2 domains. These studies serve as a prelude to understanding the structural interactions that occur between CD46 and the measles virus H protein.

Infection of chicken embryonic fibroblasts by measles virus: adaptation at the virus entry level

Measles virus (MV) has a tropism restricted to humans and primates and uses the human CD46 molecule as a cellular receptor. MV has been adapted to grow in chicken embryonic fibroblasts (CEF) and gave rise to an attenuated live vaccine. Hallé and Schwarz MV strains were compared in their ability to infect both simian Vero cells and CEF. Whereas both strains infected Vero cells, only the CEF-adapted Schwarz strain was able to efficiently infect CEF. Since the expression of the human MV receptor CD46 rendered the chicken embryonic cell line TCF more permissive to the infection by the Hallé MV strain, the MV entry into CEF appeared to be a limiting step in the absence of prior MV adaptation. CEF lacked reactivity with anti-CD46 antibodies but were found to express another protein allowing MV binding as an alternative receptor to CD46.

Measles virus infection induces terminal differentiation of human thymic epithelial cells

Measles virus infection induces a profound immunosuppression that may lead to serious secondary infections and mortality. In this report, we show that the human cortical thymic epithelial cell line is highly susceptible to measles virus infection in vitro, resulting in infectious viral particle production and syncytium formation. Measles virus inhibits thymic epithelial cell growth and induces an arrest in the G0/G1 phases of the cell cycle. Moreover, we show that measles virus induces a progressive thymic epithelial cell differentiation process: attached measles virus-infected epithelial cells correspond to an intermediate state of differentiation while floating cells, recovered from cell culture supernatants, are fully differentiated. Measles virus-induced thymic epithelial cell differentiation is characterized by morphological and phenotypic changes. Measles virus-infected attached cells present fusiform and stellate shapes followed by a loss of cell-cell contacts and a shift from low- to high-molecular-weight keratin expression. Measles virus infection induces thymic epithelial cell apoptosis in terminally differentiated cells, revealed by the condensation and degradation of DNA in measles virus-infected floating thymic epithelial cells. Because thymic epithelial cells are required for the generation of immunocompetent T lymphocytes, our results suggest that measles virus-induced terminal differentiation of thymic epithelial cells may contribute to immunosuppression, particularly in children, in whom the thymic microenvironment is of critical importance for the development and maturation of a functional immune system.

Expression of different group A streptococcal M proteins in an isogenic background demonstrates diversity in adherence to and invasion of eukaryotic cells

The M protein of group A streptococcus (GAS) is considered to be a major virulence factor because it renders GAS resistant to phagocytosis and allows bacterial growth in human blood. There are more than 80 known serotypes of M proteins, and protective opsonic antibodies produced during disease in humans are serotype specific. M proteins also mediate bacterial adherence to epithelial cells of skin and pharynx. GAS strains vary in the genomic organization of the mga regulon, which contains the genes encoding M and M-like proteins and other virulence factors. This diversity of organization makes it difficult to assess virulence of M proteins of different serotypes, unless they can be expressed in an isogenic background. Here, we express M proteins of different serotypes in the M protein- and protein F1-deficient GAS strain, SAM2, which also lacks M-like proteins. Genes encoding M proteins of different serotypes (emmXs) have been integrated into the SAM2 chromosome in frame with the emm6.1 promoter and its mga regulon, resulting in similar levels of emmX expression. Although SAM2 exhibits a very low level of adherence to and invasion of HEp-2 and HaCaT cells, a SAM2-derived strain expressing M6 protein adheres to and invades both cell types. In contrast, the isogenic strain expressing M18 protein adheres to both cell types, but invades with a very low efficiency. A strain expressing M3 protein adheres to both types of cells, but its invasion of HEp-2 cells is serum dependent. A GAS strain expressing M6 protein does not compete with the isogenic strain expressing M18 protein for adherence to or invasion of HaCaT cells. We conclude that M proteins of different serotypes recognize different repertoires of receptors on the surfaces of eukaryotic cells.

A single amino acid change in the hemagglutinin protein of measles virus determines its ability to bind CD46 and reveals another receptor on marmoset B cells

This paper provides evidence for a measles virus receptor other than CD46 on transformed marmoset and human B cells. We first showed that most tissues of marmosets are missing the SCR1 domain of CD46, which is essential for the binding of Edmonston measles virus, a laboratory strain that has been propagated in Vero monkey kidney cells. In spite of this deletion, the common marmoset was shown to be susceptible to infections by wild-type isolates of measles virus, although they did not support Edmonston measles virus production. As one would expect from these results, measles virus could not be propagated in owl monkey or marmoset kidney cell lines, but surprisingly, both a wild-type isolate (Montefiore 89) and the Edmonston laboratory strain of measles virus grew efficiently in B95-8 marmoset B cells. In addition, antibodies directed against CD46 had no effect on wild-type infections of marmoset B cells and only partially inhibited the replication of the Edmonston laboratory strain in the same cells. A direct binding assay with insect cells expressing the hemagglutinin (H) proteins of either the Edmonston or Montefiore 89 measles virus strains was used to probe the receptors on these B cells. Insect cells expressing Edmonston H but not the wild-type H bound to rodent cells with CD46 on their surface. On the other hand, both the Montefiore 89 H and Edmonston H proteins adhered to marmoset and human B cells. Most wild-type H proteins have asparagine residues at position 481 and can be converted to a CD46-binding phenotype by replacement of the residue with tyrosine. Similarly, the Edmonston H protein did not bind CD46 when its Tyr481 was converted to asparagine. However, this mutation did not affect the ability of Edmonston H to bind marmoset and human B cells. The preceding results provide evidence, through the use of a direct binding assay, that a second receptor for measles virus is present on primate B cells.

Molecular cloning of membrane cofactor protein (MCP; CD46) on B95a cell, an Epstein-Barr virus-transformed marmoset B cell line: B95a-MCP is susceptible to infection by the CAM, but not the Nagahata strain of the measles virus

Measles virus (MV) infects not only human beings but also some simian species. The MV receptor on Vero cells (a cell line established from African Green monkey kidney cells) and human cells has been shown to be the membrane cofactor protein MCP/CD46, which is an inhibitor of autologous complement (C) activation. B95a, an Epstein-Barr virus (EBV)-transformed marmoset B cell line, is a simian cell line used for MV selection and is much more susceptible to MV than Vero cells. In the present study, we isolated cDNAs encoding MCP homologues from B95a cDNA library and assessed whether B95a-MCP is responsible for the high susceptibility of B95a to MV. The deduced amino acid sequence of the cDNA of B95a-MCP was 76% identical to that of human-MCP, and the recombinant B95a-MCP exerts C inhibitor activity. Although CAM, a vaccine strain of MV, infected Chinese hamster ovary (CHO) cells expressing B95a-MCP, Nagahata strain, a wild type of MV, failed to infect the CHO transfectants, suggesting that additional membrane molecules of B95a are responsible for the high susceptibility of B95a to the Nagahata strain.

Molecular cloning of a murine homologue of membrane cofactor protein (CD46): preferential expression in testicular germ cells

Human membrane cofactor protein (MCP, CD46) has been suggested, although no convincing evidence has been proposed, to be a fertilization-associated protein, in addition to its primary functions as a complement regulator and a measles virus receptor. We have cloned a cDNA encoding the murine homologue of MCP. This cDNA showed 45% identity in deduced protein sequence and 62% identity in nucleotide sequence with human MCP. Its ectodomains were four short consensus repeats and a serine/threonine-rich domain, and it appeared to be a type 1 membrane protein with a 23-amino acid transmembrane domain and a short cytoplasmic tail. The protein expressed on Chinese hamster ovary cell transfectants was 47 kDa on SDS/PAGE immunoblotting, approximately 6 kDa larger than the murine testis MCP. It served as a cofactor for factor I-mediated inactivation of the complement protein C3b in a homologous system and, to a lesser extent, in a human system. Strikingly, the major message of murine MCP was 1.5 kb and was expressed predominantly in the testis. It was not detected in mice defective in spermatogenesis or with immature germ cells (until 23 days old). Thus, murine MCP may be a sperm-dominant protein the message of which is expressed selectively in spermatids during germ-cell differentiation.

Bordetella pertussis binds the human complement regulator C4BP: role of filamentous hemagglutinin

C4BP (C4b-binding protein) is a high-molecular-weight plasma protein that inhibits the classical pathway of complement activation. Recent experiments have demonstrated that C4BP binds to many strains of the gram-positive bacterium Streptococcus pyogenes, a major respiratory tract pathogen. Binding to S. pyogenes was shown to be due to members of the M protein family, a group of surface proteins important for virulence. Here we report that human C4BP also binds to all clinical isolates of the gram-negative bacterium Bordetella pertussis, the etiologic agent of whooping cough. In addition, binding of C4BP was demonstrated for other Bordetella species that can cause disease in humans. Characterization of different B. pertussis mutants showed that the binding of C4BP is strongly dependent on the expression of the cell surface protein filamentous hemagglutinin, a well-known virulence factor. Inhibition experiments suggested that B. pertussis and S. pyogenes bind to the same region in C4BP. The finding that B. pertussis and S. pyogenes both have the ability to bind human C4BP suggests that these two unrelated respiratory tract pathogens may use a common mechanism during the establishment of an infection.

Mapping of the primary binding site of measles virus to its receptor CD46

The measles virus (MV) hemagglutinin binds to the complement control protein (CCP) CD46 primarily through the two external modules, CCP-I and -II. To define the residues involved in binding, 40 amino acids predicted to be solvent-exposed on the CCP-I-II module surface were changed to either alanine or serine. Altered proteins were expressed on the cell surface, and their abilities to bind purified MV particles, a soluble form of hemagglutinin (sH) and nine CD46-specific antibodies competing to different levels with sH attachment, were measured. All proteins retained, at least in part, MV and sH binding, but some completely lost binding to certain antibodies. Amino acids essential for binding of antibodies weakly or moderately competing with sH attachment are situated in the membrane-distal tip of CCP-I, whereas residues involved in binding of strongly sH competing antibodies cluster in the center of CCP-I (Arg-25, Asp-27) or in CCP-II (Arg-69, Asp-70). Both clusters face the same side of CCP-I-II and map close to amino acid exchanges impairing sH binding (E11A, R29A, P39A, and D70A) or MV binding (D70A and E84A) and to a six-amino acid loop, previously shown to be necessary for sH binding.

Artificial mutations and natural variations in the CD46 molecules from human and monkey cells define regions important for measles virus binding

CD46 was previously shown to be a primate-specific receptor for the Edmonston strain of measles virus. This receptor consists of four short consensus regions (SCR1 to SCR4) which normally function in complement regulation. Measles virus has recently been shown to interact with SCR1 and SCR2. In this study, receptors on different types of monkey erythrocytes were employed as "natural mutant proteins" to further define the virus binding regions of CD46. Erythrocytes from African green monkeys and rhesus macaques hemagglutinate in the presence of measles virus, while baboon erythrocytes were the least efficient of the Old World monkey cells used in these assays. Subsequent studies demonstrated that the SCR2 domain of baboon CD46 contained an Arg-to-Gln mutation at amino acid position 103 which accounted for reduced hemagglutination activity. Surprisingly, none of the New World monkey erythrocytes hemagglutinated in the presence of virus. Sequencing of cDNAs derived from the lymphocytes of these New World monkeys and analysis of their erythrocytes with SCR1-specific polyclonal antibodies indicated that the SCR1 domain was deleted in these cells. Additional experiments, which used 35 different site-specific mutations inserted into CD46, were performed to complement the preceding studies. The effects of these artificial mutations were documented with a convenient binding assay using insect cells expressing the measles virus hemagglutinin. Mutations which mimicked the change found in baboon CD46 or another which deleted the SCR2 glycosylation site reduced binding substantially. Another mutation which altered GluArg to AlaAla at positions 58 and 59, totally abolished binding. Finally, the epitopes for two monoclonal antibodies which inhibit measles virus attachment were mapped to the same regions implicated by mutagenesis.

CD46 short consensus repeats III and IV enhance measles virus binding but impair soluble hemagglutinin binding

The binding of a recombinant soluble form of the measles virus (MV) hemagglutinin (sH) to cells expressing hybrid CD46/CD4 proteins was compared to that of purified virus. For binding of both ligands, both CD46 external short consensus repeats I and II (SCR I and II) in the natural order were essential. The addition of SCR III and IV enhanced virus binding but inhibited sH binding. Accordingly, this lowered the ability of sH to compete with MV binding. Antihemagglutinin monoclonal antibodies selectively inhibited the binding of either sH or MV. Thus, sH and MV share a common binding site in SCR I and II but differ in their apparent avidity to CD46 under the influence of SCR III and IV.

Antibody cross-reactivity with CD46 and lack of cell surface expression suggest that moesin might not mediate measles virus binding

The binding of antimoesin antibodies from ascites fluids to the surfaces of human and rodent cells was found to parallel the level of CD46 expression. No such reactivity was detected with a purified antimoesin antibody which recognized intracellular moesin. In Western blots, antimoesin antibodies were found to react with solubilized CD46 and a recombinant soluble form of CD46. Antimoesin antibodies also reacted with CD46/CD4 molecules containing only the SCR I and II domains required for measles virus (MV) hemagglutinin binding onto CD46. We suggest that the weak cross-reactivity of antimoesin antibodies with CD46 explains the inhibitory effect of these antibodies on MV entry and that moesin is not directly involved in MV binding.

Rinderpest and other animal morbillivirus infections: comparative aspects and recent developments

The genus morbillivirus presently comprises measles virus of man, rinderpest virus (RPV), peste des petits ruminants virus (PPRV), and canine distemper virus (CDV). 'Emerging' morbilliviruses, such as phocid distemper virus (PDV) of seals, dolphin (DMV) and porpoise morbillivirus (PMV) have probably been present for a long period of time and outbreaks are possibly related to introduction into a highly susceptible population and/or be the result of interspecies transmission. In this review some comparative aspects of morbillivirus infections, particularly with respect to rinderpest and canine distemper viruses, are presented. Topics include pathogenesis, epidemiology, molecular phylogeny, diagnosis and prophylaxis. Recent developments in molecular biology have created tools which have enabled us to achieve a better understanding of morbillivirus infections at the nucleic acid level ('molecular epidemiology') while recombinant DNA technology has allowed new bivalent recombinant vaccines with improved heat stability to be produced.

Cell entry by measles virus: long hybrid receptors uncouple binding from membrane fusion

The pH-independent fusion of membranes induced by measles virus (MV) requires, in addition to the fusion-competent protein F, hemagglutinin (H), and on the target membrane, the virus receptor CD46. We constructed hybrid receptors composed of different numbers and combinations of the four CD46 short consensus repeat (SCR) domains, followed by immunoglobulin-like domains of another cell surface protein, CD4. Hybrid proteins containing SCRs I and II bound MV particles and conferred fusion competence to rodent cells. SCRs III and/or IV strengthened MV binding. Increasing the distance between the MV binding site and the transmembrane domain enhanced virus binding but reduced fusion efficiency. A hybrid protein predicted to be about 120 Angstroms (12 nm) longer than the standard receptor lost fusion support function and was dominant negative over a functional receptor. These data indicate that receptor protein length influences virus binding and determines fusion efficiency.