Virus entry is a major determinant of cell tropism of Edmonston and wild-type strains of measles virus as revealed by vesicular stomatitis virus pseudotypes bearing their envelope proteins

Single cell susceptibility to SARS-CoV-2 infection is driven by variable cell states

The ability of a virus to infect a cell type is at least in part determined by the presence of host factors required for the viral life cycle. However, even within cell types that express known factors needed for infection, not every cell is equally susceptible, suggesting that our knowledge of the full spectrum of factors that promote infection is incomplete. Profiling the most susceptible subsets of cells within a population may reveal additional factors that promote infection. However, because viral infection dramatically alters the state of the cell, new approaches are needed to reveal the state of these cells prior to infection with virus. Here, we used single-cell clone tracing to retrospectively identify and characterize lung epithelial cells that are highly susceptible to infection with SARS-CoV-2. The transcriptional state of these highly susceptible cells includes markers of retinoic acid signaling and epithelial differentiation. Loss of candidate factors identified by our approach revealed that many of these factors play roles in viral entry. Moreover, a subset of these factors exert control over the infectable cell state itself, regulating the expression of key factors associated with viral infection and entry. Analysis of patient samples revealed the heterogeneous expression of these factors across both cells and patients in vivo. Further, the expression of these factors is upregulated in particular inflammatory pathologies. Altogether, our results show that the variable expression of intrinsic cell states is a major determinant of whether a cell can be infected by SARS-CoV-2.

LRP6 Is a Functional Receptor for Attenuated Canine Distemper Virus

Wild-type canine distemper virus (CDV) is an important pathogen of dogs as well as wildlife that can infect immune and epithelial cells through two known receptors: the signaling lymphocytic activation molecule (SLAM) and nectin-4, respectively. Conversely, the ferret and egg-adapted CDV-Onderstepoort strain (CDV-OP) is employed as an effective vaccine for dogs. CDV-OP also exhibits promising oncolytic properties, such as its abilities to infect and kill multiple cancer cells in vitro. Interestingly, several cancer cells do not express SLAM or nectin-4, suggesting the presence of a yet unknown entry factor for CDV-OP. By conducting a genome-wide CRISPR/Cas9 knockout (KO) screen in CDV-OP-susceptible canine mammary carcinoma P114 cells, which neither express SLAM nor nectin-4, we identified low-density lipoprotein receptor-related protein 6 (LRP6) as a host factor that promotes CDV-OP infectivity. Whereas the genetic ablation of LRP6 rendered cells resistant to infection, ectopic expression in resistant LRP6^KO cells restored susceptibility. Furthermore, multiple functional studies revealed that (i) the overexpression of LRP6 leads to increased cell-cell fusion, (ii) a soluble construct of the viral receptor-binding protein (solHOP) interacts with a soluble form of LRP6 (solLRP6), (iii) an H-OP point mutant that prevents interaction with solLRP6 abrogates cell entry in multiple cell lines once transferred into recombinant viral particles, and (iv) vesicular stomatitis virus (VSV) pseudotyped with CDV-OP envelope glycoproteins loses its infectivity in LRP6^KO cells. Collectively, our study identified LRP6 as the long sought-after cell entry receptor of CDV-OP in multiple cell lines, which set the molecular bases to refine our understanding of viral-cell adaptation and to further investigate its oncolytic properties. IMPORTANCE Oncolytic viruses (OV) have gathered increasing interest in recent years as an alternative option to treat cancers. The Onderstepoort strain of canine distemper virus (CDV-OP), an enveloped RNA virus belonging to the genus Morbillivirus, is employed as a safe and efficient vaccine for dogs against distemper disease. Importantly, although CDV-OP can infect and kill multiple cancer cell lines, the basic mechanisms of entry remain to be elucidated, as most of those transformed cells do not express natural receptors (i.e., SLAM and nectin-4). In this study, using a genome-wide CRISPR/Cas9 knockout screen, we describe the discovery of LRP6 as a novel functional entry receptor for CDV-OP in various cancer cell lines and thereby uncover a basic mechanism of cell culture adaptation. Since LRP6 is upregulated in various cancer types, our data provide important insights in order to further investigate the oncolytic properties of CDV-OP.

Replication competence of canine distemper virus in cell lines expressing signaling lymphocyte activation molecule (SLAM) of goat, sheep and dog origin

Cellular receptors play an important role in entry and cell to cell spread of morbillivirus infections. The cells expressing SLAM and Nectin-4 have been used for successful and efficient isolation of canine distemper virus (CDV) in high titre. There are several methods for generation of cells expressing receptor molecules. Here, we have used a comparatively cheaper and easily available method, pcDNA 3.1 (+) for engineering Vero cells to express SLAM gene of goat, sheep and dog origin (Vero/Goat/SLAM (VGS), Vero/Sheep/SLAM (VSS) and Vero/Dog/SLAM (VDS), respectively). The generated cell lines were then compared to test their efficacy to support CDV replication. CDV could be grown in high titre in the cells expressing SLAM and a difference of log two could be recorded in virus titre between VDS and native Vero cells. Also, CDV could be grown in a higher titre in VDS as compared to VGS and VSS. The finding of this study supports the preferential use of SLAM expressing cells over the native Vero cells by CDV. Further, the higher titre of CDV in cells expressing dog-SLAM as compared to the cells expressing SLAM of non-CDV hosts (i.e. goat and sheep) points towards the preferential use of dog SLAM by the CDV and may be a plausible reason for differential susceptibility of small ruminants and Canines to CDV.

SARS-CoV-2 Spike Alterations Enhance Pseudoparticle Titers and Replication-Competent VSV-SARS-CoV-2 Virus

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the causative agent of the most recent global pandemic that has caused more than a million deaths around the world. The spike glycoprotein (S) drives the entry and fusion of this virus and is the main determinant of cell tropism. To explore S requirements for entry under BSL2 conditions, S has been pseudotyped onto vesicular stomatitis virus (VSV) or retroviral particles with varied success. Several alterations to S were demonstrated to improve pseudoparticle titers, but they have not been systematically compared. In this study, we produced pseudotyped VSV particles with multiple modifications to S, including truncation, mutation, and tagging strategies. The main objective of this study was to determine which modifications of the S protein optimize cell surface expression, incorporation into pseudotyped particles, and pseudoparticle entry. Removal of the last 19 residues of the cytoplasmic tail produced a hyper-fusogenic S, while removal of 21 residues increased S surface production and VSV incorporation. Additionally, we engineered a replication-competent VSV (rVSV) virus to produce the S-D614G variant with a truncated cytoplasmic tail. While the particles can be used to assess S entry requirements, the rVSV∆G/S_Met1D614G∆21 virus has a poor specific infectivity (particle to infectious titer ratio).

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

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

Glucosylceramide synthase maintains influenza virus entry and infection

Influenza virus is an enveloped virus wrapped in a lipid bilayer derived from the host cell plasma membrane. Infection by influenza virus is dependent on these host cell lipids, which include sphingolipids. Here we examined the role of the sphingolipid, glucosylceramide, in influenza virus infection by knocking out the enzyme responsible for its synthesis, glucosylceramide synthase (UGCG). We observed diminished influenza virus infection in HEK 293 and A549 UGCG knockout cells and demonstrated that this is attributed to impaired viral entry. We also observed that entry mediated by the glycoproteins of other enveloped viruses that enter cells by endocytosis is also impaired in UGCG knockout cells, suggesting a broader role for UGCG in viral entry by endocytosis.

Comparative sequence analysis of morbillivirus receptors and its implication in host range expansion

SLAM (CD150) and nectin-4 are the major morbillivirus receptors responsible for virus pathogenesis and host range expansion. Recently, morbillivirus infections have been reported in unnatural hosts, including endangered species, posing a threat to their conservation. To understand the host range expansion of morbilliviruses, we generated the full-length sequences of morbillivirus receptors (goat, sheep, and dog SLAM, and goat nectin-4) and tried to correlate their role in determining host tropism. A high level of amino acid identity was observed between the sequences of related species, and phylogenetic reconstruction showed that the receptor sequences of carnivores, marine mammals, and small ruminants grouped separately. Analysis of the ligand binding region (V region; amino acid residues 52-136) of SLAM revealed high amino acid identity between small ruminants and bovine SLAMs. Comparison of canine SLAM with ruminants and non-canids SLAM revealed appreciable changes, including charge alterations. Significant differences between feline SLAM and canine SLAM have been reported. The binding motifs of nectin-4 genes (FPAG motif and amino acid residues 60, 62, and 63) were found to be conserved in sheep, goat, and dog. The differences reported in the binding region may be responsible for the level of susceptibility or resistance of a species to a particular morbillivirus.

Glucosylceramidase Maintains Influenza Virus Infection by Regulating Endocytosis

Influenza virus is the pathogen responsible for the second largest pandemic in human history. A better understanding of how influenza virus enters host cells may lead to the development of more-efficacious therapies against emerging strains of the virus. Here we show that the glycosphingolipid metabolizing enzyme glucosylceramidase is required for optimal influenza virus trafficking to late endosomes and for consequent fusion, entry, and infection. We also provide evidence that promotion of influenza virus entry by glucosylceramidase extends to other endosome-entering viruses and is due to a general requirement for this enzyme, and hence for optimal levels of glucosylceramide, for efficient trafficking of endogenous cargos, such as the epidermal growth factor (EGF) receptor, along the endocytic pathway. This work therefore has implications for the basic process of endocytosis as well as for pathogenic processes, including virus entry and Gaucher disease.

Influenza virus is an RNA virus encapsulated in a lipid bilayer derived from the host cell plasma membrane. Previous studies showed that influenza virus infection depends on cellular lipids, including the sphingolipids sphingomyelin and sphingosine. Here we examined the role of a third sphingolipid, glucosylceramide, in influenza virus infection following clustered regularly interspaced short palindromic repeats with Cas9 (CRISPR-Cas9)-mediated knockout (KO) of its metabolizing enzyme glucosylceramidase (GBA). After confirming GBA knockout of HEK 293 and A549 cells by both Western blotting and lipid mass spectrometry, we observed diminished infection in both KO cell lines by a PR8 (H1N1) green fluorescent protein (GFP) reporter virus. We further showed that the reduction in infection correlated with impaired influenza virus trafficking to late endosomes and hence with fusion and entry. To examine whether GBA is required for other enveloped viruses, we compared the results seen with entry mediated by the glycoproteins of Ebola virus, influenza virus, vesicular stomatitis virus (VSV), and measles virus in GBA knockout cells. Entry inhibition was relatively robust for Ebola virus and influenza virus, modest for VSV, and mild for measles virus, suggesting a greater role for viruses that enter cells by fusing with late endosomes. As the virus studies suggested a general role for GBA along the endocytic pathway, we tested that hypothesis and found that trafficking of epidermal growth factor (EGF) to late endosomes and degradation of its receptor were impaired in GBA knockout cells. Collectively, our findings suggest that GBA is critically important for endocytic trafficking of viruses as well as of cellular cargos, including growth factor receptors. Modulation of glucosylceramide levels may therefore represent a novel accompaniment to strategies to antagonize “late-penetrating” viruses, including influenza virus.

IMPORTANCE Influenza virus is the pathogen responsible for the second largest pandemic in human history. A better understanding of how influenza virus enters host cells may lead to the development of more-efficacious therapies against emerging strains of the virus. Here we show that the glycosphingolipid metabolizing enzyme glucosylceramidase is required for optimal influenza virus trafficking to late endosomes and for consequent fusion, entry, and infection. We also provide evidence that promotion of influenza virus entry by glucosylceramidase extends to other endosome-entering viruses and is due to a general requirement for this enzyme, and hence for optimal levels of glucosylceramide, for efficient trafficking of endogenous cargos, such as the epidermal growth factor (EGF) receptor, along the endocytic pathway. This work therefore has implications for the basic process of endocytosis as well as for pathogenic processes, including virus entry and Gaucher disease.

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

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

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

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

Systems Perspective of Morbillivirus Replication

Systems biology refers to system-wide changes in biological components such as RNA/DNA (genomics), protein (proteomics) and lipids (lipidomics). In this review, we provide comprehensive information about morbillivirus replication. Besides discussing the role of individual viral/host proteins in virus replication, we also discuss how systems-level analyses could improve our understanding of morbillivirus replication, host-pathogen interaction, immune response and disease resistance. Finally, we discuss how viroinformatics is likely to provide important insights for understanding genome-genome, genome-protein and protein-protein interactions.

VeroNectin-4 is a highly sensitive cell line that can be used for the isolation and titration of Peste des Petits Ruminants virus

Peste des Petits Ruminants virus (PPRV) is a member of the Morbillivirus subgroup of the family Paramyxoviridae, and is one of the most contagious diseases of small ruminants throughout Africa and the rest of the world. Different cell lines have previously been used to isolate PPRV but with limited success. Thus, to improve the isolation of Morbilliviruses, human, canine, and goat homologues of the lymphocyte receptor signaling lymphocyte activation molecule (SLAM) have been introduced into cells that can support virus replication. However, the amino acid sequence of SLAM varies between species, and often requires adaptation of a particular virus to different versions of the receptor. The protein sequence of Nectin-4 is highly conserved between different mammals, which eliminate the need for receptor adaptation by the virus. Cell lines expressing Nectin-4 have previously been used to propagate measles and canine distemper viruses. In this study, we compared infections in Vero cells expressing canine SLAM (VeroDogSLAM) to those in Vero cells expressing Nectin-4 (VeroNectin-4), following inoculations with wild-type strains of PPRV. Virus isolation using VeroNectin-4 cells was successful with 23% of swabbed samples obtained from live infected animals, and was 89% effective using post-mortem tissues of infected sheep. By contrast, only 4.5% efficiency was observed from swab samples and 67% efficiency was obtained in virus isolation from post-mortem tissues using VeroDogSLAM cells. The average incubation period for virus recovery from post-mortem tissues was 3.4 days using VeroNectin-4 cells, compared with 5.5 days when using VeroDogSLAM cells. The virus titers of PPRV obtained from VeroNectin-4 cells were also higher than those derived from VeroDogSLAM cells. A comparison of the growth kinetics for PPRV in the two cell lines confirmed the superiority of VeroNectin-4 cells for PPR diagnostic purposes and vaccine virus titration.

Development of a helper cell-dependent form of peste des petits ruminants virus: a system for making biosafe antigen

Peste des petits ruminants (PPR) is a viral disease of sheep and goats that is spreading through many countries in the developing world. Work on the virus is often restricted to studies of attenuated vaccine strains or to work in laboratories that have high containment facilities. We have created a helper cell dependent form of PPR virus by removing the entire RNA polymerase gene and complementing it with polymerase made constitutively in a cell line. The resultant L-deleted virus grows efficiently in the L-expressing cell line but not in other cells. Virus made with this system is indistinguishable from normal virus when used in diagnostic assays, and can be grown in normal facilities without the need for high level biocontainment. The L-deleted virus will thus make a positive contribution to the control and study of this important disease.

Cotton rat (Sigmodon hispidus) signaling lymphocyte activation molecule (CD150) is an entry receptor for measles virus

Cotton rats (Sigmodon hispidus) replicate measles virus (MV) after intranasal infection in the respiratory tract and lymphoid tissue. We have cloned the cotton rat signaling lymphocytic activation molecule (CD150, SLAM) in order to investigate its role as a potential receptor for MV. Cotton rat CD150 displays 58% and 78% amino acid homology with human and mouse CD150, respectively. By staining with a newly generated cotton rat CD150 specific monoclonal antibody expression of CD150 was confirmed in cotton rat lymphoid cells and in tissues with a pattern of expression similar to mouse and humans. Previously, binding of MV hemagglutinin has been shown to be dependent on amino acids 60, 61 and 63 in the V region of CD150. The human molecule contains isoleucine, histidine and valine at these positions and binds to MV-H whereas the mouse molecule contains valine, arginine and leucine and does not function as a receptor for MV. In the cotton rat molecule, amino acids 61 and 63 are identical with the mouse molecule and amino acid 60 with the human molecule. After transfection with cotton rat CD150 HEK 293 T cells became susceptible to infection with single cycle VSV pseudotype virus expressing wild type MV glycoproteins and with a MV wildtype virus. After infection, cells expressing cotton rat CD150 replicated virus to lower levels than cells expressing the human molecule and formed smaller plaques. These data might explain why the cotton rat is a semipermissive model for measles virus infection.

Sustained autophagy contributes to measles virus infectivity

The interplay between autophagy and intracellular pathogens is intricate as autophagy is an essential cellular response to fight against infections, whereas numerous microbes have developed strategies to escape this process or even exploit it to their own benefit. The fine tuned timing and/or selective molecular pathways involved in the induction of autophagy upon infections could be the cornerstone allowing cells to either control intracellular pathogens, or be invaded by them. We report here that measles virus infection induces successive autophagy signallings in permissive cells, via distinct and uncoupled molecular pathways. Immediately upon infection, attenuated measles virus induces a first transient wave of autophagy, via a pathway involving its cellular receptor CD46 and the scaffold protein GOPC. Soon after infection, a new autophagy signalling is initiated which requires viral replication and the expression of the non-structural measles virus protein C. Strikingly, this second autophagy signalling can be sustained overtime within infected cells, independently of the expression of C, but via a third autophagy input resulting from cell-cell fusion and the formation of syncytia. Whereas this sustained autophagy signalling leads to the autophagy degradation of cellular contents, viral proteins escape from degradation. Furthermore, this autophagy flux is ultimately exploited by measles virus to limit the death of infected cells and to improve viral particle formation. Whereas CD150 dependent virulent strains of measles virus are unable to induce the early CD46/GOPC dependent autophagy wave, they induce and exploit the late and sustained autophagy. Overall, our work describes distinct molecular pathways for an induction of self-beneficial sustained autophagy by measles virus.

Autophagy is an evolutionarily conserved lysosomal dependent degradative pathway for recycling of long-lived proteins and damaged organelles. Autophagy is also an essential cellular response to fight infection by destroying infectious pathogens trapped within autophagosomes and plays a key role in the induction of both innate and adaptive immune responses. Numerous viruses have evolved strategies to counteract autophagy in order to escape from degradation or/and to inhibit immune signals. The kinetic and molecular pathways involved in the induction of autophagy upon infections might determine if cells would be able to control pathogens or would be invaded by them. We showed that measles virus (MeV) infection induces successive autophagy signallings in cells via distinct molecular pathways. A first autophagy wave is induced by the engagement of the MeV cellular receptor CD46 and the scaffold protein GOPC. A second wave is initiated after viral replication by the expression of the non-structural MeV protein C and is sustained overtime within infected cells thanks to the formation of syncytia. This sustained autophagy is exploited by MeV to limit the death of infected cells and to improve viral particle formation. We describe new molecular pathways by which MeV hijacks autophagy to promote its infectivity.

Toxicology, biodistribution and shedding profile of a recombinant measles vaccine vector expressing HIV-1 antigens, in cynomolgus macaques

As a new human immunodeficiency virus type 1 (HIV-1) vaccine approach, the live-attenuated measles virus (MV) Schwarz vaccine strain was genetically engineered to express the F4 antigen (MV1-F4). F4 is a fusion protein comprising HIV-1 antigens p17 and p24, reverse transcriptase and Nef. This study assessed the toxicity, biodistribution and shedding profiles of MV1-F4. Cynomolgus macaques were intramuscularly immunized one or three times with the highest dose of MV1-F4 intended for clinical use, the reference (Schwarz) measles vaccine or saline, and monitored clinically for 11 or 85 days. Toxicological parameters included local and systemic clinical signs, organ weights, haematology, clinical and gross pathology and histopathology. Both vaccines were well tolerated, with no morbidity, clinical signs or gross pathological findings observed. Mean spleen weights were increased after three doses of either vaccine, which corresponded with increased numbers and/or sizes of germinal centers. This was likely a result of the immune response to the vaccines. Either vaccine virus replicated preferentially in secondary lymphoid organs and to a lesser extent in epithelium-rich tissues (e.g., intestine, urinary bladder and trachea) and the liver. At the expected peak of viremia, viral RNA was detected in some biological fluid samples from few animals immunized with either vaccine, but none of these samples contained infectious virus. In conclusion, no shedding of infectious viral particles was identified in cynomolgus monkeys after injection of MV1-F4 or Schwarz measles vaccines. Furthermore, no toxic effect in relation to the MV vaccination was found with these vaccines in this study.

Morbillivirus receptors and tropism: multiple pathways for infection

Morbilliviruses, which include measles virus (MeV), canine distemper virus, and rinderpest virus, are among the most important pathogens in their respective hosts and cause severe syndromes. Morbilliviruses are enveloped viruses with two envelope proteins, one of which is hemagglutinin (H) protein, which plays a role in binding to cellular receptors. During morbillivirus infection, the virus initially targets lymphoid cells and replicates efficiently in the lymph nodes. The principal cellular receptor for morbillivirus is signaling lymphocyte activation molecule (SLAM, also called CD150), which is exclusively expressed on immune cells. This feature reflects the strong lymphoid cell tropism and viral spread in the infected body. Morbillivirus infection, however, affects various tissues in the body, including the lung, kidney, gastrointestinal tract, vascular endothelium, and brain. Thus, other receptors for morbilliviruses in addition to SLAM might exist. Recently, nectin-4 has been identified as a novel epithelial cell receptor for MeV. The expression of nectin-4 is localized to polarized epithelial cells, and this localization supports the notion of cell tropism since MeV also grows well in the epithelial cells of the respiratory tract. Although two major receptors for lymphoid and epithelial cells in natural infection have been identified, morbillivirus can still infect many other types of cells with low infectivity, suggesting the existence of inefficient but ubiquitously expressed receptors. We have identified other molecules that are implicated in morbillivirus infection of SLAM-negative cells by alternative mechanisms. These findings indicate that morbillivirus utilizes multiple pathways for establishment of infection. These studies will advance our understanding of morbillivirus tropism and pathogenesis.

Retargeting vesicular stomatitis virus using measles virus envelope glycoproteins

Oncolytic vesicular stomatitis virus (VSV) has potent antitumor activity, but infects a broad range of cell types. Here, we used the measles virus (MV) hemagglutinin (H) and fusion (F) envelope glycoproteins to redirect VSV entry and infection specifically to tumor-associated receptors. Replication-defective VSV, deleted of its glycoprotein gene (VSVΔG), was pseudotyped with MV-F and MV-H displaying single-chain antibodies (scFv) specific for epidermal growth factor receptor (EGFR), folate receptor (FR), or prostate membrane-specific antigen (PSMA). Viral titers were ∼10(5) PFU/ml, but could be concentrated to 10(7) PFU/ml. Immunoblotting confirmed incorporation of the MV-H-scFv and MV-F into functional VSV virions. Although VSV-G was able to infect all tumor cell lines tested, the retargeted VSV infected only cells that expressed the targeted receptor. In vivo specificities of the EGFR-, FR-, and PSMA-retargeted VSV were assessed by intratumoral injection into human tumor xenografts. Analysis of green fluorescent protein reporter gene expression indicated that VSV infection was restricted to receptor-positive tumors. In summary, we have demonstrated for the first time that VSV can be efficiently retargeted to different cellular receptors using the measles display technology, yielding retargeted VSV vectors that are highly specific for tumors that express the relevant receptor.

Development and Applications of VSV Vectors Based on Cell Tropism

Viral vectors have been available in various fields such as medical and biological research or gene therapy applications. Targeting vectors pseudotyped with distinct viral envelope proteins that influence cell tropism and transfection efficiency are useful tools not only for examining entry mechanisms or cell tropisms but also for vaccine vector development. Vesicular stomatitis virus (VSV) is an excellent candidate for development as a pseudotype vector. A recombinant VSV lacking its own envelope (G) gene has been used to produce a pseudotype or recombinant VSV possessing the envelope proteins of heterologous viruses. These viruses possess a reporter gene instead of a VSV G gene in their genome, and therefore it is easy to evaluate their infectivity in the study of viral entry, including identification of viral receptors. Furthermore, advantage can be taken of a property of the pseudotype VSV, which is competence for single-round infection, in handling many different viruses that are either difficult to amplify in cultured cells or animals or that require specialized containment facilities. Here we describe procedures for producing pseudotype or recombinant VSVs and a few of the more prominent examples from envelope viruses, such as hepatitis C virus, Japanese encephalitis virus, baculovirus, and hemorrhagic fever viruses.

[Development of viral vectors and the application for viral entry mechanisms]

Virus is identified as one of the obligate intracellular parasites, which only amplify in cells of specific living things. Viral vectors, which are developed by utilizing these properties, are available in the various fields such as basic research of medical biology or application of gene therapy. Our research group has studied development of viral vectors using properties of baculovirus or vesicular stomatitis virus (VSV). Due to the development of new baculoviral vectors for mammalian cells, it is possible to be more efficient transduction of foreign gene in mammalian cells and animals. Furthermore, pseudotype or recombinant VSV possessing the envelope proteins of hepatitis C virus, Japanese encephalitis virus or baculovirus were constructed, and characteristics of the envelope proteins or entry mechanisms of these viruses were analyzed.

Anterograde or retrograde transsynaptic labeling of CNS neurons with vesicular stomatitis virus vectors

To understand how the nervous system processes information, a map of the connections among neurons would be of great benefit. Here we describe the use of vesicular stomatitis virus (VSV) for tracing neuronal connections in vivo. We made VSV vectors that used glycoprotein (G) genes from several other viruses. The G protein from lymphocytic choriomeningitis virus endowed VSV with the ability to spread transsynaptically, specifically in an anterograde direction, whereas the rabies virus glycoprotein gave a specifically retrograde transsynaptic pattern. The use of an avian G protein fusion allowed specific targeting of cells expressing an avian receptor, which allowed a demonstration of monosynaptic anterograde tracing from defined cells. Synaptic connectivity of pairs of virally labeled cells was demonstrated by using slice cultures and electrophysiology. In vivo infections of several areas in the mouse brain led to the predicted patterns of spread for anterograde or retrograde tracers.

Cell entry of enveloped viruses

Enveloped viruses penetrate their cell targets following the merging of their membrane with that of the cell. This fusion process is catalyzed by one or several viral glycoproteins incorporated on the membrane of the virus. These envelope glycoproteins (EnvGP) evolved in order to combine two features. First, they acquired a domain to bind to a specific cellular protein, named "receptor." Second, they developed, with the help of cellular proteins, a function of finely controlled fusion to optimize the replication and preserve the integrity of the cell, specific to the genus of the virus. Following the activation of the EnvGP either by binding to their receptors and/or sometimes the acid pH of the endosomes, many changes of conformation permit ultimately the action of a specific hydrophobic domain, the fusion peptide, which destabilizes the cell membrane and leads to the opening of the lipidic membrane. The comprehension of these mechanisms is essential to develop medicines of the therapeutic class of entry inhibitor like enfuvirtide (Fuzeon) against human immunodeficiency virus (HIV). In this chapter, we will summarize the different envelope glycoprotein structures that viruses develop to achieve membrane fusion and the entry of the virus. We will describe the different entry pathways and cellular proteins that viruses have subverted to allow infection of the cell and the receptors that are used. Finally, we will illustrate more precisely the recent discoveries that have been made within the field of the entry process, with a focus on the use of pseudoparticles. These pseudoparticles are suitable for high-throughput screenings that help in the development of natural or artificial inhibitors as new therapeutics of the class of entry inhibitors.

Measles virus-induced suppression of immune responses

Measles is an important cause of child mortality that has a seemingly paradoxical interaction with the immune system. In most individuals, the immune response is successful in eventually clearing measles virus (MV) infection and in establishing life-long immunity. However, infection is also associated with persistence of viral RNA and several weeks of immune suppression, including loss of delayed type hypersensitivity responses and increased susceptibility to secondary infections. The initial T-cell response includes CD8+ and T-helper 1 CD4+ T cells important for control of infectious virus. As viral RNA persists, there is a shift to a T-helper 2 CD4+ T-cell response that likely promotes B-cell maturation and durable antibody responses but may suppress macrophage activation and T-helper 1 responses to new infections. Suppression of mitogen-induced lymphocyte proliferation can be induced by lymphocyte infection with MV or by lymphocyte exposure to a complex of the hemagglutinin and fusion surface glycoproteins without infection. Dendritic cells (DCs) are susceptible to infection and can transmit infection to lymphocytes. MV-infected DCs are unable to stimulate a mixed lymphocyte reaction and can induce lymphocyte unresponsiveness through expression of MV glycoproteins. Thus, multiple factors may contribute both to measles-induced immune suppression and to the establishment of durable protective immunity.

Generation of VSV pseudotypes using recombinant ΔG-VSV for studies on virus entry, identification of entry inhibitors, and immune responses to vaccines

Vesicular stomatitis virus (VSV) is a prototypic enveloped animal virus that has been used extensively to study virus entry, replication and assembly due to its broad host range and robust replication properties in a wide variety of mammalian and insect cells. Studies on VSV assembly led to the creation of a recombinant VSV in which the glycoprotein (G) gene was deleted. This recombinant (rVSV-ΔG) has been used to produce VSV pseudotypes containing the envelope glycoproteins of heterologous viruses, including viruses that require high-level biocontainment; however, because the infectivity of rVSV-ΔG pseudotypes is restricted to a single round of replication the analysis can be performed using biosafety level 2 (BSL-2) containment. As such, rVSV-ΔG pseudotypes have facilitated the analysis of virus entry for numerous viral pathogens without the need for specialized containment facilities. The pseudotypes also provide a robust platform to screen libraries for entry inhibitors and to evaluate the neutralizing antibody responses following vaccination. This manuscript describes methods to produce and titer rVSV-ΔG pseudotypes. Procedures to generate rVSV-ΔG stocks and to quantify virus infectivity are also described. These protocols should allow any laboratory knowledgeable in general virological and cell culture techniques to produce successfully replication-restricted rVSV-ΔG pseudotypes for subsequent analysis.

Glycoprotein-dependent acidification of vesicular stomatitis virus enhances release of matrix protein

To study vesicular stomatitis virus (VSV) entry and uncoating, we generated a recombinant VSV encoding a matrix (M) protein containing a C-terminal tetracysteine Lumio tag (rVSV-ML) that could be fluorescently labeled using biarsenical compounds. Quantitative confocal microscopy showed that there is a transient loss of fluorescence at early times after the initiation of endocytosis of rVSV-ML-Green (rVSV-MLG) virions, which did not occur when cells were treated with bafilomycin A1. The reduction in fluorescence occurred 5 to 10 min postentry, followed by a steady increase in fluorescence intensity from 15 to 60 min postentry. A similar loss of fluorescence was observed in vitro when virions were exposed to acidic pH. The reduction in fluorescence required G protein since "bald" DeltaG-MLG particles did not show a similar loss of fluorescence at low pH. Based on the pH-dependent fluorescence properties of Lumio Green, we hypothesize that the loss of fluorescence of rVSV-MLG virions during virus entry is due to a G ectodomain-dependent acidification of the virion interior. Biochemical analysis indicated that low pH also resulted in an enhancement of M protein dissociation from partially permeabilized, but otherwise intact, wild-type virions. From these data we propose that low-pH conformational changes in G protein promote acidification of the virus interior, which facilitates the release of M from ribonucleoprotein particles during uncoating.

Sequence analysis of morbillivirus CD150 receptor-Signaling Lymphocyte Activation Molecule (SLAM) of different animal species

Signaling Lymphocyte Activation Molecule-SLAM (CD150) molecule has been reported as a putative receptor for most morbilliviruses for their respective host species. In this study, we determined the complete nucleotide sequence of the gene coding for the morbillivirus receptor-SLAM from the four species, namely, goat (Capra hircus), sheep (Ovis aries), Indian cattle (Bos indicus), and buffalo (Bubalus bubalis). The nucleotide (nt) open reading frame sequence of SLAM gene in all the four species studied was 1017 nucleotides in length encoding a polypeptide of 339 amino acids (aa), similar to Bos taurus, but different from canine, human, marmoset, and mouse SLAM, which were 1029, 1008, 1011, and 1032 nts, respectively, in length, and coding for 343, 336, 337, and 344 aa, respectively. Sequence analysis revealed 96.3-98.5% and 92.9-96.8% identities among the four species at the nt and aa level, respectively. Sequence diversity at aa level between various species revealed that the critical functional region of SLAM protein among different species is relatively conserved, thereby facilitating this molecule to act as a receptor for morbillivirus. Phylogenetic relationship based on the aa sequences of SLAM protein revealed that caprine, ovine, cattle, and buffalo fall under a defined cluster but caprine SLAM is more closely related to ovine, followed by bovine.

A neutralization test for specific detection of Nipah virus antibodies using pseudotyped vesicular stomatitis virus expressing green fluorescent protein

Nipah virus (NiV) is a new zoonotic paramyxovirus that emerged in 1998 and is now classified in the genus Henipavirus along with the closely related Hendra virus (HeV). NiV is highly pathogenic in several vertebrate species including humans, and the lack of available vaccines or specific treatment restricts it to biosafety level 4 (BSL4) containment. A serum neutralization test was developed for measuring NiV neutralizing antibodies under BSL2 conditions using a recombinant vesicular stomatitis virus (VSV) expressing green fluorescent protein (GFP) and bearing the F and G proteins of NiV (VSV–NiV–GFP). The neutralization titers were obtained by counting GFP-expressing cells or by measuring fluorescence. The performance of this new assay was compared against the conventional test using live NiV with panels of sera from several mammalian species, including sera from NiV outbreaks, experimental infections, as well as HeV-specific sera. The results obtained with the VSV–NiV–GFP based test correlated with those obtained using live NiV. Using a 50% reduction in VSV–NiV–GFP infected cells as the cut-off for neutralization, this new assay demonstrated its potential as an effective tool for detecting NiV neutralizing antibodies under BSL2 containment with greater speed, sensitivity and safety as compared to the conventional NiV serum neutralization test.

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

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

Measles virus receptors

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

X-ray crystallographic analysis of measles virus hemagglutinin

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

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

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

Pseudotyped vesicular stomatitis virus for analysis of virus entry mediated by SARS coronavirus spike proteins

Severe acute respiratory syndrome (SARS) coronavirus (CoV) contains a spike (S) protein that binds to a receptor molecule (angiotensin-converting enzyme 2; ACE2), induces membrane fusion, and serves as a neutralizing epitope. To study the functions of the S protein, we describe here the generation of SARS-CoV S protein-bearing vesicular stomatitis virus (VSV) pseudotype using a VSVdeltaG*/GFP system in which the G gene is replaced by the green fluorescent protein (GFP) gene (VSV-SARS-CoV-St19/GFP). Partial deletion of the cytoplasmic domain of SARS-CoV S protein (SARS-CoV-St19) allowed efficient incorporation into the VSV particle that enabled the generation of a high titer of pseudotype virus. Neutralization assay with anti-SARS-CoV antibody revealed that VSV-SARS-St19/GFP pseudotype infection is mediated by SARS-CoV S protein. The VSVdeltaaG*/SEAP system, which secretes alkaline phosphatase instead of GFP, was also generated as a VSV pseudotype having SARS-CoV S protein (VSV-SARS-CoV-St19/SEAP). This system enabled high-throughput analysis of SARS-CoV S protein-mediated cell entry by measuring alkaline phosphatase activity. Thus, VSV pseudotyped with SARS-CoV S protein is useful for developing a rapid detection system for neutralizing antibody specific for SARS-CoV infection as well as studying the S-mediated cell entry of SARS-CoV.

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.

Measles virus infection of SLAM (CD150) knockin mice reproduces tropism and immunosuppression in human infection

The human signaling lymphocyte activation molecule (SLAM, also called CD150), a regulator of antigen-driven T-cell responses and macrophage functions, acts as a cellular receptor for measles virus (MV), and its V domain is necessary and sufficient for receptor function. We report here the generation of SLAM knockin mice in which the V domain of mouse SLAM was replaced by that of human SLAM. The chimeric SLAM had an expected distribution and normal function in the knockin mice. Splenocytes from the SLAM knockin mice permitted the in vitro growth of a virulent MV strain but not that of the Edmonston vaccine strain. Unlike in vitro infection, MV could grow only in SLAM knockin mice that also lacked the type I interferon receptor (IFNAR). After intraperitoneal or intranasal inoculation, MV was detected in the spleen and lymph nodes throughout the body but not in the thymus. Notably, the virus appeared first in the mediastinal lymph node after intranasal inoculation. Splenocytes from MV-infected IFNAR(-/-) SLAM knockin mice showed suppression of proliferative responses to concanavalin A. Thus, MV infection of SLAM knockin mice reproduces lymphotropism and immunosuppression in human infection, serving as a useful small animal model for measles.

[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.

Selective gene transfer to T lymphocytes using coreceptor-specific [MLV(HIV)] pseudotype vectors in a transgenic mouse model

The coreceptor usage of HIV-1 envelope proteins (Env) is mainly dependent on a defined variable region within the V3-loop of Env. Thus, retroviral vectors derived from murine leukemia virus (MLV), which have been pseudotyped with HIV-1 envelope proteins holding different V3-loops, enable selective gene delivery into either CXCR4 or CCR5 positive cultured cells. Here, we tested the distribution of CD4/CCR5-tropic [MLV(HIV)]-pseudotype vectors in transgenic mice expressing CD4 and either CXCR4 or CCR5 of human origin. The specificity of gene transfer was analyzed by ex vivo transduction of spleen cells as well as after i.v. or i.p. injection of transgenic mice. Expression of the transferred marker gene EGFP and vector sequences could be detected exclusively in lymphocytes expressing (hu)CD4 and (hu)CCR5, whereas MLV vectors pseudotyped with the VSV-G envelope glycoprotein mediated gene transfer in mice of all genotypes investigated. These data demonstrated that cell-specific gene delivery via [MLV(HIV)]-pseudotyped vectors, as previously shown for cultured cells, is also achievable in vivo.

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.

Lymphoproliferative disorders: prospects for gene therapy

The lymphoproliferative disorders represent a large group of diseases with a significant variation in presentation and clinical course. There has been a trend of increasing incidence for some of these disorders, and despite advances in therapies, a significant number of patients either respond poorly or have early relapses. For this reason there is a need to investigate novel therapies to be used either alone or as adjunct treatment in combination with conventional therapies. Gene therapy is a relatively new field that takes advantage of our increased understanding of molecular biology with the aim of treating a variety of diseases including cancer. It is defined as the introduction of genetic material into cells for therapeutic intent. Methods to improve gene delivery efficiency have been the focus of a large amount of research and to date the optimal procedure uses viruses such as oncoretroviruses, lentiviruses, adenoviruses, adeno-associated viruses and herpes simplex viruses. There are four main gene therapy strategies that might be used for the treatment of lymphoproliferative disorders. First, immunotherapy using tumour vaccines or techniques to enhance the function of immune effector cells has been investigated with some success in patients with B-cell malignancies. Second, the introduction of prodrug-activated 'suicide' genes into cells has been explored, in particular in patients with post-transplantation lymphoproliferative disease. Third, direct lysis of tumour cells using viruses shows some early promise, especially in the treatment of B-cell disorders by manipulating the measles virus to target the CD20 antigen. Finally, anti-gene strategies such as anti-sense therapy, ribozymes, and most recently RNA interference, could be used to suppress expression of specific target genes. RNA interference in particular has tremendous potential and has been studied in the context of anaplastic large cell lymphoma as well as Epstein-Barr virus-associated malignancies. Whilst we are still in the early days of this field and to date results have been modest, there is still a significant potential for gene therapy to play a role in the future treatment of these disorders.

Vesicular stomatitis virus pseudotyped with severe acute respiratory syndrome coronavirus spike protein

Severe acute respiratory syndrome coronavirus (SARS-CoV) contains a single spike (S) protein, which binds to its receptor, angiotensin-converting enzyme 2 (ACE2), induces membrane fusion and serves as a neutralizing antigen. A SARS-CoV-S protein-bearing vesicular stomatitis virus (VSV) pseudotype using the VSVDeltaG* system was generated. Partial deletion of the SARS-CoV-S protein cytoplasmic domain allowed efficient incorporation into VSV particles and led to the generation of a pseudotype (VSV-SARS-St19) at high titre. Green fluorescent protein expression was demonstrated as early as 7 h after infection of Vero E6 cells with VSV-SARS-St19. VSV-SARS-St19 was neutralized by anti-SARS-CoV antibody and soluble ACE2, and its infection was blocked by treatment of Vero E6 cells with anti-ACE2 antibody. These results indicated that VSV-SARS-St19 infection is mediated by SARS-CoV-S protein in an ACE2-dependent manner. VSV-SARS-St19 will be useful for analysing the function of SARS-CoV-S protein and for developing rapid methods of detecting neutralizing antibodies specific for SARS-CoV infection.

Recombinant rhabdoviruses: vectors for vaccine development and gene therapy

The establishment of methods to recover rhabdoviruses from cDNA, so-called reverse genetics systems, has made it possible to genetically engineer rhabdoviruses and to study all aspects of the virus life cycle by introducing defined mutations into the viral genomes. It has also opened the way to make use of the viruses in biomedical applications such as vaccination, gene therapy, or oncolytic virotherapy. The typical gene expression mode of rhabdoviruses, a high genetic stability, and the propensity to tolerate changes in the virus envelope have made rhabdoviruses attractive, targetable gene expression vectors. This chapter provides an overview on the possibilities to manipulate biological properties of the rhabdoviruses that may be important for further development of vaccine vectors and examples of recombinant rhabdoviruses expressing foreign genes and antigens.

Characterization of the interaction of lassa fever virus with its cellular receptor alpha-dystroglycan

The cellular receptor for the Old World arenaviruses Lassa fever virus (LFV) and lymphocytic choriomeningitis virus (LCMV) has recently been identified as alpha-dystroglycan (alpha-DG), a cell surface receptor that provides a molecular link between the extracellular matrix and the actin-based cytoskeleton. In the present study, we show that LFV binds to alpha-DG with high affinity in the low-nanomolar range. Recombinant vesicular stomatitis virus pseudotyped with LFV glycoprotein (GP) adopted the receptor binding characteristics of LFV and depended on alpha-DG for infection of cells. Mapping of the binding site of LFV on alpha-DG revealed that LFV binding required the same domains of alpha-DG that are involved in the binding of LCMV. Further, LFV was found to efficiently compete with laminin alpha1 and alpha2 chains for alpha-DG binding. Together with our previous studies on receptor binding of the prototypic immunosuppressive LCMV isolate LCMV clone 13, these findings indicate a high degree of conservation in the receptor binding characteristics between the highly human-pathogenic LFV and murine-immunosuppressive LCMV isolates.

Targeted Measles Virus Vector Displaying Echistatin Infects Endothelial Cells via αvβ3 and Leads to Tumor Regression

Targeting tumor-associated vascular endothelium by replication-competent viral vectors is a promising strategy for cancer gene therapy. Here we describe the development of a viral vector based on the Edmonston vaccine strain of measles virus targeted to integrin alpha(v)beta3, which is expressed abundantly on activated but not quiescent vascular endothelium. We displayed a disintegrin, M28L echistatin that binds with a high affinity to integrin alpha(v)beta3 on the COOH terminus of the viral attachment (H) protein and rescued the replication-competent recombinant virus by reverse genetics. The new targeted virus was named measles virus echistatin vector (MV-ERV). Its native binding to CD46 was purposefully retained to allow virus infection of tumor cells expressing this receptor. MV-ERV correctly displayed echistatin on the outer surface of its envelope and produced interesting ring formation phenomena due to cell detachment upon infection of susceptible Vero cells in vitro. MV-ERV grew to 10(6) plaque-forming units/mL, slightly lower than the parental Edmonston strain of measles virus (MV-Edm), but it selectively infected Chinese hamster ovary cells expressing integrin alpha(v)beta3. It also selectively infected both bovine and human endothelial cells on matrigels and unlike MV-Edm, MV-ERV infected newly formed blood vessels in chorioallantoic membrane assays. In animal models, MV-ERV but not the control MV-Edm caused the regression of s.c. xenografts of resistant multiple myeloma tumors (MM1) in severe combined immunodeficient mice. The tumors were either completely eradicated or their growth was significantly retarded. The specificity, potency, and feasibility of MV-ERV infection clearly show the potential use of MV-ERV in gene therapy for targeting tumor-associated vasculature for the treatment of solid tumors.

Use of influenza C virus glycoprotein HEF for generation of vesicular stomatitis virus pseudotypes

Influenza C virus contains two envelope glycoproteins: CM2, a putative ion channel protein; and HEF, a unique multifunctional protein that performs receptor-binding, receptor-destroying and fusion activities. Here, it is demonstrated that expression of HEF is sufficient to pseudotype replication-incompetent vesicular stomatitis virus (VSV) that lacks the VSV glycoprotein (G) gene. The pseudotyped virus showed characteristic features of influenza C virus with respect to proteolytic activation, receptor usage and cell tropism. Chimeric glycoproteins composed of HEF ectodomain and VSV-G C-terminal domains were efficiently incorporated into VSV particles and showed receptor-binding and receptor-destroying activities but, unlike authentic HEF, did not mediate efficient infection, probably because of impaired fusion activity. HEF-pseudotyped VSV efficiently infected polarized Madin-Darby canine kidney cells via the apical plasma membrane, whereas entry of VSV-G-complemented virus was restricted to the basolateral membrane. These findings suggest that pseudotyping of viral vectors with HEF might be useful for efficient apical gene transfer into polarized epithelial cells and for targeting cells that express 9-O-acetylated sialic acids.

Measles as a potential oncolytic virus

The use of replicating viruses for cancer therapy is attracting increasing interest. Numerous viruses are now being considered as potential cancer therapeutics, including the vaccine strain of measles virus (MV). The attenuated strain of measles readily lyses transformed cells, whilst replication and lysis are limited in normal human cells. It has a number of features which make it highly suitable for further development as an oncolytic agent, among them stability and a long history of safety in human use. These features are being combined with its ready potential for genetic manipulations to generate recombinant MVs with desirable therapeutic attributes. This review summarises the pre-clinical studies of the oncolytic efficacy of MV to date. Promising developments in MV engineering--such as re-targeting MV entry to specific cell types and enhancing its utility as a therapeutic agent by expression of non-viral proteins--as well as outstanding issues, such as the role of anti-MV immunity, are highlighted.

VP1 protein of infectious bursal disease virus modulates the virulence in vivo

Infectious bursal disease viruses (IBDVs), belonging to the Birnaviridae family, cause severe immunodeficiency in young chickens by destroying the precursors of antibody-producing B cells in the bursa of Fabricius (BF). Different pathotypes of IBDVs, including cell culture-adapted viruses, differ markedly in virulence, which is characterized by mortality and bursal damage. To study the molecular determinants of virulence in IBDV, the genomic segments A and B of GLS bursa-derived (GLSBD) and tissue culture-adapted (GLSTC) viruses were cloned and sequenced. Comparison of the deduced amino acid sequences of segments A and B revealed only two amino acid substitutions at positions 87 (Q --> R) and 261 (P --> L) in segment B, and at positions 253 (Q --> H) and 284 (A --> T) in segment A; the latter of which has been shown to be involved in tissue culture adaptation and attenuation of the virus. To study the function of VP1 protein encoded by segment B, reassortant viruses between tissue culture-adapted strains, GLSTC and D78, and GLSBD were recovered using the reverse genetics system. The recombinant virus rGLSBDB containing segment B of GLSBD was able to replicate in Vero and chicken embryo fibroblast (CEF) cells but exhibited delayed replication kinetics. To evaluate the characteristics of these viruses in vivo, 3-week-old chickens were given equal doses of parental viruses or reassortant viruses by ocular inoculation. The pathological lesions and viral antigen distribution in BF were analyzed at 1, 2, or 3 days postinfection. Parental GLSBD and the recovered rGLSBDB viruses propagate most efficiently in the BF and cause severe bursal lesions, whereas the tissue culture-adapted GLSTC virus replicates less efficiently and induces mild bursal lesions at 3 days postinfection. Taken together, our results demonstrate that the VP1 protein of IBDV is involved in the efficiency of viral replication and modulates the virulence in vivo.

Dissection of measles virus V protein in relation to its ability to block alpha/beta interferon signal transduction

Interferon (IFN)-alpha and -beta are the main cytokines for innate immune responses against viral infections. To replicate efficiently in the hosts, viruses have evolved various countermeasures to the IFN response. The V protein of measles virus (MV) has been shown to block IFN-alpha/beta signalling. Here, the wild-type IC-B strain of MV was shown to grow comparably in the presence and absence of IFN-alpha, whereas replication of the Edmonston tag strain recovered from cloned DNA was strongly suppressed in its presence. The V protein of the IC-B strain, but not the Edmonston tag strain, blocked IFN-alpha signalling. The V protein of the Edmonston strain from the ATCC also inhibited IFN-alpha signalling. There were three amino acid differences between the V proteins of the Edmonston ATCC and tag strains, and substitutions of both residues at positions 110 and 272 were required for the Edmonston ATCC V protein to lose IFN-antagonist activity. The P protein of the IC-B strain, which shares the N-terminal 231 aa residues with the V protein, also inhibited IFN-alpha signalling. Indeed, fragments comprising only those 231 residues of the IC-B and Edmonston ATCC V proteins, but not the Edmonston tag V protein, were able to block IFN-alpha signalling. However, the N-terminal region of the Edmonston tag V protein, when attached to the C-terminal region of the Edmonston ATCC V protein, inhibited IFN-alpha signalling. Taken together, our results indicate that both the N- and C-terminal regions contribute to the IFN-antagonist activity of the MV V protein.

Cell surface activation of the alternative complement pathway by the fusion protein of measles virus

Measles virus (MV)-infected cells are activators of the alternative human complement pathway, resulting in high deposition of C3b on the cell surface. Activation was observed independent of whether CD46 was used as a cellular receptor and did not correlate with CD46 down-regulation. The virus itself was an activator of the alternative pathway and was covered by C3b/C3bi, resulting in some loss in infectivity without loss of virus binding to target cells. The cell surface expression of MV fusion (F), but not haemagglutinin, envelope protein resulted in complement activation of the Factor B-dependent alternative pathway in a dose-dependent manner and F-C3b complexes were formed. The underlying activation mechanism was not related to any decrease in cell surface expression of the complement regulators CD46 and CD55. The C3b/C3bi coating of MV-infected cells and virus should ensure enhanced targeting of MV antigens to the immune system, through binding to complement receptors.

Recombinant measles AIK-C strain expressing current wild-type hemagglutinin protein

We constructed a recombinant measles virus cDNA, pIC-MVAIK-H/87-K, in which the hemagglutinin (H) gene of the AIK-C vaccine strain was replaced by the wild-type (MVi/Tokyo.JPN/87-K: genotype D3) H gene and the remaining genes were the same as the AIK-C vaccine strain. To investigate the feasibility of the recombinant vaccine strain expressing wild-type H protein instead of the AIK-C H protein, we constructed two recombinant measles cDNA, having Leu (small plaque-type) and Phe (large plaque-type) at position 278 of the F protein. Infectious chimeric virus strains, MVAIK-H/87-K/S (small plaque-type) and MVAIK-H/87-K/L (large plaque-type), were recovered, which were designed to induce small (S) and large (L) plaques in Vero cells. The MVAIK-H/87-K/S and MVAIK-H/87-K/L did not grow at 39-40 degrees C, similar to the original AIK-C strain, and retained the temperature sensitivity (ts) characteristics. They did not induce cytopathic effect (CPE) in Vero cells but produced CPE in B95a cells, similar to the current wild-type measles MVi/Tokyo.JPN/87-K. From the results of Western blotting, the mobility of the H protein of MVAIK-H/87-K/S and MVAIK-H/87-K/L was similar to that of MVi/Tokyo.JPN/87-K. Hyper-immune sera raised by MVAIK-H/87-K/S neutralized all types of current wild strains. Thus, the chimeric measles virus expressing the current wild H protein demonstrated wild-type H properties with ts characteristics of the vaccine strain, indicating that the construction strategy of recombinant measles virus can cope with the hyper-mutated measles virus.