Measles virus spread by cell-cell contacts: uncoupling of contact-mediated receptor (CD46) downregulation from virus uptake

Actin cytoskeleton remodeling disrupts physical barriers to infection and presents entry receptors to respiratory syncytial virus

RSV is the leading cause of infant hospitalizations and a significant cause of paediatric and geriatric morbidity worldwide. Recently, we reported that insulin-like growth factor 1 receptor (IGF1R) was a receptor for respiratory syncytial virus (RSV) in airway epithelial cells and that activation of IGF1R recruited the coreceptor, nucleolin (NCL), to the cell surface. Cilia and mucus that line the airways pose a significant barrier to viral and bacterial infection. The cortical actin cytoskeleton has been shown by others to mediate RSV entry, so we studied the roles of the RSV receptors and actin remodelling during virus entry. We found that IGF1R expression and phosphorylation were associated with the ability of RSV to infect cells. Confocal immunofluorescence imaging showed that actin projections, a hallmark of macropinocytosis, formed around viral particles 30 min after infection. Consistent with prior reports we also found that virus particles were internalized into early endosome antigen-1 positive endosomes within 90 min. Inhibiting actin polymerization significantly reduced viral titre by approximately ten-fold. Inhibiting PI3 kinase and PKCζ in stratified air-liquid interface (ALI) models of the airway epithelium had similar effects on reducing the actin remodelling observed during infection and attenuating viral entry. Actin projections were associated with NCL interacting with RSV particles resting on apical cilia of the ALIs. We conclude that macropinocytosis-like actin projections protrude through normally protective cilia and mucus layers of stratified airway epithelium that helps present the IGF1R receptor and the NCL coreceptor to RSV particles waiting at the surface.

Trans-endocytosis elicited by nectins transfers cytoplasmic cargo, including infectious material, between cells

Here, we show that cells expressing the adherens junction protein nectin-1 capture nectin-4-containing membranes from the surface of adjacent cells in a trans-endocytosis process. We find that internalized nectin-1-nectin-4 complexes follow the endocytic pathway. The nectin-1 cytoplasmic tail controls transfer: its deletion prevents trans-endocytosis, while its exchange with the nectin-4 tail reverses transfer direction. Nectin-1-expressing cells acquire dye-labeled cytoplasmic proteins synchronously with nectin-4, a process most active during cell adhesion. Some cytoplasmic cargo remains functional after transfer, as demonstrated with encapsidated genomes of measles virus (MeV). This virus uses nectin-4, but not nectin-1, as a receptor. Epithelial cells expressing nectin-4, but not those expressing another MeV receptor in its place, can transfer infection to nectin-1-expressing primary neurons. Thus, this newly discovered process can move cytoplasmic cargo, including infectious material, from epithelial cells to neurons. We name the process nectin-elicited cytoplasm transfer (NECT). NECT-related trans-endocytosis processes may be exploited by pathogens to extend tropism. This article has an associated First Person interview with the first author of the paper.

Mutations in the Fusion Protein of Measles Virus That Confer Resistance to the Membrane Fusion Inhibitors Carbobenzoxy-d-Phe-l-Phe-Gly and 4-Nitro-2-Phenylacetyl Amino-Benzamide

The inhibitors carbobenzoxy (Z)-d-Phe-l-Phe-Gly (fusion inhibitor peptide [FIP]) and 4-nitro-2-phenylacetyl amino-benzamide (AS-48) have similar efficacies in blocking membrane fusion and syncytium formation mediated by measles virus (MeV). Other homologues, such as Z-d-Phe, are less effective but may act through the same mechanism. In an attempt to map the site of action of these inhibitors, we generated mutant viruses that were resistant to the inhibitory effects of Z-d-Phe-l-Phe-Gly. These 10 mutations were localized to the heptad repeat B (HRB) region of the fusion protein, and no changes were observed in the viral hemagglutinin, which is the receptor attachment protein. Mutations were validated in a luciferase-based membrane fusion assay, using transfected fusion and hemagglutinin expression plasmids or with syncytium-based assays in Vero, Vero-SLAM, and Vero-Nectin 4 cell lines. The changes I452T, D458N, D458G/V459A, N462K, N462H, G464E, and I483R conferred resistance to both FIP and AS-48 without compromising membrane fusion. The inhibitors did not block hemagglutinin protein-mediated binding to the target cell. Edmonston vaccine/laboratory and IC323 wild-type strains were equally affected by the inhibitors. Escape mutations were mapped upon a three-dimensional (3D) structure modeled from the published crystal structure of parainfluenzavirus 5 fusion protein. The most effective mutations were situated in a region located near the base of the globular head and its junction with the alpha-helical stalk of the prefusion protein. We hypothesize that the fusion inhibitors could interfere with the structural changes that occur between the prefusion and postfusion conformations of the fusion protein.IMPORTANCE Due to lapses in vaccination worldwide that have caused localized outbreaks, measles virus (MeV) has regained importance as a pathogen. Antiviral agents against measles virus are not commercially available but could be useful in conjunction with MeV eradication vaccine programs and as a safeguard in oncolytic viral therapy. Three decades ago, the small hydrophobic peptide Z-d-Phe-l-Phe-Gly (FIP) was shown to block MeV infections and syncytium formation in monkey kidney cell lines. The exact mechanism of its action has yet to be determined, but it does appear to have properties similar to those of another chemical inhibitor, AS-48, which appears to interfere with the conformational change in the viral F protein that is required to elicit membrane fusion. Escape mutations were used to map the site of action for FIP. Knowledge gained from these studies could help in the design of new inhibitors against morbilliviruses and provide additional knowledge concerning the mechanism of virus-mediated membrane fusion.

Attenuated measles virus controls pediatric acute B-lineage lymphoblastic leukemia in NOD/SCID mice

Novel therapies are needed for pediatric acute lymphoblastic leukemia resistant to conventional therapy. While emerging data suggest leukemias as possible targets of oncolytic attenuated measles virus, it is unknown whether measles virus can eradicate disseminated leukemia, in particular pediatric acute lymphoblastic leukemia. We evaluated the efficacy of attenuated measles virus against a large panel of pediatric xenografted and native primary acute lymphoblastic leukemias ex vivo, and against four different acute lymphoblastic leukemia xenografts of B-lineage in non-obese diabetic/severe combined immunodeficient mice. Ex vivo, attenuated measles virus readily spread among and effectively killed leukemia cells while sparing normal human blood cells and their progenitors. In immunodeficient mice with disseminated acute lymphoblastic leukemia a few intravenous injections of attenuated measles virus sufficed to eradicate leukemic blasts in the hematopoietic system and to control central nervous system disease resulting in long-term survival in three of the four xenografted B-lineage leukemias. Differential sensitivity of leukemia cells did not require increased expression of the measles entry receptors CD150 or CD46 nor absence of the anti-viral retinoic acid-inducible gene I/melanoma differentiation associated gene-5 /interferon pathway. Attenuated oncolytic measles virus is dramatically effective against pediatric B-lineage acute lymphoblastic leukemia in the pre-clinical setting warranting further investigations towards clinical translation.

Inhibition of porcine reproductive and respiratory syndrome virus replication by flavaspidic acid AB

Porcine reproductive and respiratory syndrome virus (PRRSV) represents a significant challenge to the swine industry worldwide. Current control strategies against PRRSV are still inadequate and there is an urgent need for new antiviral therapies. Flavaspidic acid AB (FA-AB) is a compound derived from Dryopteris crassirhizoma, a traditional antiviral Chinese medicine. Here, we first identified its anti-PRRSV activity through targeting multiple stages in PRRSV infection in vitro. Our studies demonstrated that FA-AB could inhibit the internalization and cell-to-cell spreading of PRRSV, but not block PRRSV binding to cells. By monitoring the kinetics of PRRSV replication, we showed that FA-AB significantly suppressed PRRSV replication when treatment was initiated 24h after virus infection. Furthermore, we confirmed that FA-AB was able to significantly induce IFN-α, IFN-β, and IL1-β expression in porcine alveolar macrophages, suggesting that induction of antiviral cytokines by FA-AB could contribute to FA-AB induced inhibition of PRRSV replication. In conclusion, we provide a foundation for the possibility to develop a new therapeutic agent to control PRRSV infection.

Membrane dynamics and interactions in measles virus dendritic cell infections

Viral entry, compartmentalization and transmission depend on the formation of membrane lipid/protein microdomains concentrating receptors and signalosomes. Dendritic cells (DCs) are prime targets for measles virus (MV) infection, and this interaction promotes immune activation and generalized immunosuppression, yet also MV transport to secondary lymphatics where transmission to T cells occurs. In addition to MV trapping, DC-SIGN interaction can enhance MV uptake by activating cellular sphingomyelinases and, thereby, vertical surface transport of its entry receptor CD150. To exploit DCs as Trojan horses for transport, MV promotes DC maturation accompanied by mobilization, and restrictions of viral replication in these cells may support this process. MV-infected DCs are unable to support formation of functional immune synapses with conjugating T cells and signalling via viral glycoproteins or repulsive ligands (such as semaphorins) plays a key role in the induction of T-cell paralysis. In the absence of antigen recognition, MV transmission from infected DCs to T cells most likely involves formation of polyconjugates which concentrate viral structural proteins, viral receptors and with components enhancing either viral uptake or conjugate stability. Because DCs barely support production of infectious MV particles, these organized interfaces are likely to represent virological synapses essential for MV transmission.

Canine distemper virus persistence in demyelinating encephalitis by swift intracellular cell-to-cell spread in astrocytes is controlled by the viral attachment protein

The mechanism of viral persistence, the driving force behind the chronic progression of inflammatory demyelination in canine distemper virus (CDV) infection, is associated with non-cytolytic viral cell-to-cell spread. Here, we studied the molecular mechanisms of viral spread of a recombinant fluorescent protein-expressing virulent CDV in primary canine astrocyte cultures. Time-lapse video microscopy documented that CDV spread was very efficient using cell processes contacting remote target cells. Strikingly, CDV transmission to remote cells could occur in less than 6 h, suggesting that a complete viral cycle with production of extracellular free particles was not essential in enabling CDV to spread in glial cells. Titration experiments and electron microscopy confirmed a very low CDV particle production despite higher titers of membrane-associated viruses. Interestingly, confocal laser microscopy and lentivirus transduction indicated expression and functionality of the viral fusion machinery, consisting of the viral fusion (F) and attachment (H) glycoproteins, at the cell surface. Importantly, using a single-cycle infectious recombinant H-knockout, H-complemented virus, we demonstrated that H, and thus potentially the viral fusion complex, was necessary to enable CDV spread. Furthermore, since we could not detect CD150/SLAM expression in brain cells, the presence of a yet non-identified glial receptor for CDV was suggested. Altogether, our findings indicate that persistence in CDV infection results from intracellular cell-to-cell transmission requiring the CDV-H protein. Viral transfer, happening selectively at the tip of astrocytic processes, may help the virus to cover long distances in the astroglial network, “outrunning” the host’s immune response in demyelinating plaques, thus continuously eliciting new lesions.

Target analysis of the experimental measles therapeutic AS-136A

No effective therapeutic is currently in place for improved case management of severe measles or the rapid control of outbreaks. Through high-throughput screening, we recently identified a novel small-molecule class that potently blocks activity of the measles virus (MeV) RNA-dependent RNA polymerase (RdRp) complex in transient replicon assays. However, the nature of the block in RdRp activity and the physical target of the compound remained elusive. Through real-time reverse transcription-PCR analysis, we demonstrate that the lead compound AS-136A blocks viral RNA synthesis in the context of an infection. Adaptation of different MeV strains to growth in the presence of the compound identified three candidate hot spots for resistance that are located in conserved domains of the viral polymerase (L protein) subunit of the RdRp complex. Rebuilding of individual mutations in RdRp-driven reporter assays and recombinant MeV traced the molecular basis for resistance to specific mutations in L. Mutations responsible for resistance cluster in the immediate vicinity of the proposed catalytic center for phosphodiester bond formation and neighboring conserved domains of L, providing support for effective inhibition of a paramyxovirus RdRp complex through interaction of a nonnucleoside small-molecule inhibitor with the L protein. Resistance mutations are located in regions of L that are fully conserved among viral isolates, and recombinant MeV harboring individual resistance mutations show some delay in the onset of viral growth in vitro. Taken together, these data support the hypothesis that acquiring mutations in these L domains may reduce virus fitness.

Making it to the synapse: measles virus spread in and among neurons

Measles virus (MV) is one of the most transmissible microorganisms known, continuing to result in extensive morbidity and mortality worldwide. While rare, MV can infect the human central nervous system, triggering fatal CNS diseases weeks to years after exposure. The advent of crucial laboratory tools to dissect MV neuropathogenesis, including permissive transgenic mouse models, the capacity to manipulate the viral genome using reverse genetics, and cell biology advances in understanding the processes that govern intracellular trafficking of viral components, have substantially clarified how MV infects, spreads, and persists in this unique cell population. This review highlights some of these technical advances, followed by a discussion of our present understanding of MV neuronal infection and transport. Because some of these processes may be shared among diverse viruses, comparisons are made to parallel studies with other neurotropic viruses. While a crystallized view of how the unique environment of the neuron affects MV replication, spread, and, ultimately, neuropathogenesis is not fully realized, the tools and ideas are in place for exciting advances in the coming years.

Inhibition of cyclooxygenase activity blocks cell-to-cell spread of human cytomegalovirus

Human cytomegalovirus has previously been shown to induce the accumulation of cyclooxygenase-2 RNA, protein, and enzyme activity. High doses of cyclooxygenase inhibitors substantially block viral replication in cultured fibroblasts. However, doses corresponding to the level of drug achieved in the plasma of patients have little effect on the replication of human cytomegalovirus in cultured cells. Here, we demonstrate that two nonsteroidal anti-inflammatory drugs, tolfenamic acid and indomethacin, markedly reduce direct cell-to-cell spread of human cytomegalovirus in cultured fibroblasts. The block is reversed by addition of prostaglandin E2, proving that it results from the action of the drugs on cyclooxygenase activity. Because direct cell-to-cell spread likely contributes importantly to pathogenesis of the virus, we suggest that nonsteroidal anti-inflammatory drugs might help to control human cytomegalovirus infections in conjunction with other anti-viral treatments.

Measles virus blind to its epithelial cell receptor remains virulent in rhesus monkeys but cannot cross the airway epithelium and is not shed

The current model of measles virus (MV) pathogenesis implies that apical infection of airway epithelial cells precedes systemic spread. An alternative model suggests that primarily infected lymphatic cells carry MV to the basolateral surface of epithelial cells, supporting MV shedding into the airway lumen and contagion. This model predicts that a mutant MV, unable to enter cells through the unidentified epithelial cell receptor (EpR), would remain virulent but not be shed. To test this model, we identified residues of the MV attachment protein sustaining EpR-mediated cell fusion. These nonpolar or uncharged polar residues defined an area located near the binding site of the signaling lymphocytic activation molecule (SLAM), the receptor for MV on lymphatic cells. We then generated an EpR-blind virus maintaining SLAM-dependent cell entry and inoculated rhesus monkeys intranasally. Hosts infected with the selectively EpR-blind MV developed rash and anorexia while averaging slightly lower viremia than hosts infected with wild-type MV but did not shed virus in the airways. The mechanism restricting shedding was characterized using primary well-differentiated human airway epithelial cells. Wild-type MV infected columnar epithelial cells bearing tight junctions only when applied basolaterally, while the EpR-blind virus did not infect these cells. Thus, EpR is probably a basolateral protein, and infection of the airway epithelium is not essential for systemic spread and virulence of MV.

Targeted release of oncolytic measles virus by blood outgrowth endothelial cells in situ inhibits orthotopic gliomas

Malignant gliomas remain largely incurable despite intensive efforts to develop novel therapies. Replicating oncolytic viruses have shown great promise, among them attenuated measles viruses of the Edmonston B strain (MV-Edm). However, host immune response and the infiltrative nature of gliomas limit their efficacy. We show that human blood outgrowth endothelial cells (BOECs), readily expandable from peripheral blood, are easily infected by MV-Edm and allow replication of MV-Edm while surviving long enough after infection to serve as vehicles for MV-Edm (BOEC/MV-Edm). After intravenous and peritumoral injection, BOEC/MV-Edm deliver the viruses selectively to irradiated orthotopic U87 gliomas in mice. At the tumor, MV-Edm produced by the BOECs infect glioma cells. Subsequent spread from tumor cell to tumor cell leads to focal infection and cytopathic effects that decrease tumor size and, in the case of peritumoral injection, prolong survival of mice. Since MV-Edm within BOECs are not readily neutralized and because BOEC/MV-Edm search and destroy glioma cells, BOEC/MV-Edm constitute a promising novel approach for glioma therapy.

Genetic control of fusion pore expansion in the epidermis of Caenorhabditis elegans

Developmental cell fusion is found in germlines, muscles, bones, placentae, and stem cells. In Caenorhabditis elegans 300 somatic cells fuse during development. Although there is extensive information on the early intermediates of viral-induced and intracellular membrane fusion, little is known about late stages in membrane fusion. To dissect the pathway of cell fusion in C. elegans embryos, we use genetic and kinetic analyses using live-confocal and electron microscopy. We simultaneously monitor the rates of multiple cell fusions in developing embryos and find kinetically distinct stages of initiation and completion of membrane fusion in the epidermis. The stages of cell fusion are differentially blocked or retarded in eff-1 and idf-1 mutants. We generate kinetic cell fusion maps for embryos grown at different temperatures. Different sides of the same cell differ in their fusogenicity: the left and right membrane domains are fusion-incompetent, whereas the anterior and posterior membrane domains fuse with autonomous kinetics in embryos. All but one cell pair can initiate the formation of the largest syncytium. The first cell fusion does not trigger a wave of orderly fusions in either direction. Ultrastructural studies show that epidermal syncytiogenesis require eff-1 activities to initiate and expand membrane merger.

Infected Cell Carriers: A New Strategy for Systemic Delivery of Oncolytic Measles Viruses in Cancer Virotherapy

Attenuated measles viruses (MVs) propagate selectively in human tumor cells, and phase I clinical trials are currently underway to test their oncolytic activity. A major theoretical impediment to systemic MV application is the presence of pre-existing antiviral immunity. We hypothesized that autologous MV-infected cells might be a more reliable vehicle than cell-free virions to deliver the infection to tumor cells in subjects with neutralizing titers of anti-measles antibodies. Our in vitro studies, using a dual-color fluorescent model, demonstrated efficient cell-to-cell transfer of infection via heterofusion. In contrast to infection by naked virions, heterofusion between infected cell carriers and tumor cells was more resistant to antibody neutralization. Infected monocytic, endothelial, or stimulated peripheral blood cells could deliver oncolytic MV to tumor lesions in vivo, after intravenous (i.v.) or intraperitoneal (i.p.) administration. Single or repeated i.p. injections of monocytic carriers significantly improved survival of animals bearing human ovarian cancer xenografts. Systemic or i.p. injection of MV-infected cells successfully transferred infection by heterofusion to Raji lymphomas or hepatocellular carcinoma tumors in the presence of neutralizing antibodies. These results suggest a novel strategy for systemic delivery of oncolytic virotherapy in cancer patients that can "bypass" the pre-existing humoral immunity against MV.

Ligation of the cell surface receptor, CD46, alters T cell polarity and response to antigen presentation

Lymphocyte function in vivo is dictated by multiple external cues, but the integration of different signals is not well understood. Here, we show that competition for the axis of polarization dictates functional outcomes. We investigated the effect of ligation of the immunoregulatory cell surface receptor, CD46, on lymphocyte polarity during antigen presentation and cytotoxic effector function. Ligation of CD46 on human T cells prevented recruitment of the microtubule organizing center, CD3, and perforin to the interface with the antigen-presenting cell and caused a reduction in IFN-gamma production. In human NK cells, similar changes in polarity induced by CD46 ligation inhibited the recruitment of the microtubule organizing center and perforin to the interface with target cells and correlated with reduced killing. These data indicate that external signals can alter lymphocyte polarization toward antigen-presenting cells or target cells, inhibiting lymphocyte function.

Acid sphingomyelinase deficiency increases susceptibility to fatal alphavirus encephalomyelitis

Sindbis virus (SV), an enveloped virus with a single-stranded, plus-sense RNA genome, is the prototype alphavirus in the Togaviridae family. In mice, SV infects neurons and can cause apoptosis of immature neurons. Sphingomyelin (SM) is the most prevalent cellular sphingolipid, is particularly abundant in the nervous systems of mammals, and is required for alphavirus fusion and entry. The level of SM is tightly regulated by sphingomyelinases. A defect in acid sphingomyelinase (ASMase) results in SM storage and subsequent intracellular accumulation of SM. To better understand the role of the SM pathway in SV pathogenesis, we have characterized SV infection of transgenic mice deficient in the ASMase gene. ASMase knockout (ASM-KO) mice were more susceptible to SV infection than wild-type (WT) or heterozygous (Het) animals. Titers of SV were higher in the brains of ASM-KO mice than in the brains of WT mice. More SV RNA was detected by in situ hybridization, more SV protein was detected by immunohistochemistry, and more terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling-positive cells were present in the cortex and hippocampus of ASM-KO mice than in those of WT or Het mice. Interleukin-6 (IL-6), but not IL-1beta or tumor necrosis factor alpha, was elevated in infected ASM-KO mice compared to levels in WT or Het mice, but studies with IL-6-KO mice and recombinant SV expressing IL-6 showed no role for IL-6 in fatal disease. Together these data indicate that the increase in susceptibility of ASM-KO mice to SV infection was the result of more-rapid replication and spread of SV in the nervous system and increased neuronal death.

Evidence for direct transfer of cytoplasmic material from infected to uninfected cells during cell-associated spread of human cytomegalovirus

Cell-associated spread is assumed to be the predominant mode of human cytomegalovirus (HCMV) dissemination in infected patients, however the underlying mechanisms are poorly understood. We tested the hypothesis that cell-to-cell spread of HCMV may be associated with direct transfer of cytoplasmic material by analyzing focal growth of green fluorescent HCMVDeltaUL16GFP. In this recombinant virus, UL16 was partially replaced by the green fluorescent protein (EGFP). The resulting HCMVDeltaUL16GFP showed unrestricted growth and expressed EGFP from the early UL16 promoter. EGFP transmission was then investigated in relation to viral spread from productively infected cells to cocultured uninfected cells. Alternatively, microinjection of fluorescent dextrane allowed for direct visualization of inter-cell-connections. Within 5h of coculture, 8% of cells neighbouring productively infected cells had acquired EGFP. Detection of EGFP in the absence of IE antigen and during cycloheximide block excluded the possibility of de novo synthesis. Immediate distribution of microinjected fluorescent dyes from infected cells to adjacent cells proved the existence of cell-cell-fusions. These data demonstrate that focal spread of HCMV is associated with direct transfer of cytoplasmic material, most likely through cell-cell-fusions. This would withdraw the virus from the control of neutralizing antibodies and thus provide an explanation for the limited antiviral effect of the humoral immune response.

Quantitative estimation of Nipah virus replication kinetics in vitro

BACKGROUND

Nipah virus is a zoonotic virus isolated from an outbreak in Malaysia in 1998. The virus causes infections in humans, pigs, and several other domestic animals. It has also been isolated from fruit bats. The pathogenesis of Nipah virus infection is still not well described. In the present study, Nipah virus replication kinetics were estimated from infection of African green monkey kidney cells (Vero) using the one-step SYBR Green I-based quantitative real-time reverse transcriptase-polymerase chain reaction (qRT-PCR) assay.

RESULTS

The qRT-PCR had a dynamic range of at least seven orders of magnitude and can detect Nipah virus from as low as one PFU/microL. Following initiation of infection, it was estimated that Nipah virus RNA doubles at every approximately 40 minutes and attained peak intracellular virus RNA level of approximately 8.4 log PFU/microL at about 32 hours post-infection (PI). Significant extracellular Nipah virus RNA release occurred only after 8 hours PI and the level peaked at approximately 7.9 log PFU/microL at 64 hours PI. The estimated rate of Nipah virus RNA released into the cell culture medium was approximately 0.07 log PFU/muL per hour and less than 10% of the released Nipah virus RNA was infectious.

CONCLUSION

The SYBR Green I-based qRT-PCR assay enabled quantitative assessment of Nipah virus RNA synthesis in Vero cells. A low rate of Nipah virus extracellular RNA release and low infectious virus yield together with extensive syncytial formation during the infection support a cell-to-cell spread mechanism for Nipah virus infection.

Nipah virus RNA synthesis in cultured pig and human cells

Nipah virus infection of porcine stable kidney cells (PS), human neuronal cells (SK-N-MC), human lung fibroblasts cells (MRC-5), and human monocytes (THP-1) were examined. Rapid progression of cytopathic effects (CPE) and cell death were noted in PS cell cultures treated with Nipah virus, followed by MRC-5, SK-N-MC, and THP-1 cell cultures, in descending order of rapidity. Significant increase in the intracellular Nipah virus RNA occurred beginning at 24 hr PI in all the infected cells. Whereas, the extracellular release of Nipah virus RNA increased significantly beginning at 48 and 72 hr PI for the infected MRC-5 cells and PS cells, respectively. No significant release of extracellular Nipah virus RNA was detected from the Nipah virus-infected SK-N-MC and THP-1 cells. At its peak, approximately 6.6 log PFU/microl of extracellular Nipah virus RNA was released from the Nipah virus-infected PS cells, with at least a 100-fold less virus RNA was recorded in the Nipah virus-infected SK-N-MC and THP-1. Approximately 15.2% (+/-0.1%) of the released virus from the infected PS cell cultures was infectious in contrast to approximately 5.5% (+/-0.7%) from the infected SK-N-MC cells. The findings suggest that there are no differences in the capacity to support Nipah virus replication between pigs and humans in fully susceptible PS and MRC-5 cells. However, there are differences between these cells and human neuronal cells and monocytes in the ability to support Nipah virus replication and virus release.

Measles virus superinfection immunity and receptor redistribution in persistently infected NT2 cells

A recombinant measles virus (MV) expressing red fluorescent protein (MVDsRed1) was used to produce a persistently infected cell line (piNT2-MVDsRed1) from human neural precursor (NT2) cells. A similar cell line (piNT2-MVeGFP) was generated using a virus that expresses enhanced green fluorescent protein. Intracytoplasmic inclusions containing the viral nucleocapsid protein were evident in all cells and viral glycoproteins were present at the cell surface. Nevertheless, the cells did not release infectious virus nor did they fuse to generate syncytia. Uninfected NT2 cells express the MV receptor CD46 uniformly over their surface, whereas CD46 was present in cell surface aggregates in the piNT2 cells. There was no decrease in the overall amount of CD46 in piNT2 compared to NT2 cells. Cell-to-cell fusion was observed when piNT2 cells were overlaid onto confluent monolayers of MV receptor-positive cells, indicating that the viral glycoproteins were correctly folded and processed. Infectious virus was released from the underlying cells, indicating that persistence was not due to gross mutations in the virus genome. Persistently infected cells were superinfected with MV or canine distemper virus and cytopathic effects were not observed. However, mumps virus could readily infect the cells, indicating that superinfection immunity is not caused by general soluble antiviral factors. As MVeGFP and MVDsRed1 are antigenically indistinguishable but phenotypically distinct it was possible to use them to measure the degree of superinfection immunity in the absence of any cytopathic effect. Only small numbers of non-fusing green fluorescent piNT2-MVDsRed1 cells (1 : 300 000) were identified in which superinfecting MVeGFP entered, replicated and expressed its genes.

Is mRNA and protein level of CD46 altered in measles virus vaccine strain S191-infected cells?

Previous research showed that the expression of measles virus receptor CD46 was downregulated after expression of measles virus hemagglutinin protein on the surface of the virus infected cell or triggered by infected cell-to-cell contact. We reported here that the mRNA level of CD46 in MV infected cells was not changed which was tested by real-time quantitative PCR. To further analyse the surface expression alteration of CD46 after MV infection, flow cytometric analysis and indirect immunofluorescence were used to detect the protein level of CD46. Altogether, our results provided a demonstration that the expression of CD46 was not downregulated by the infection of MV strain S191 both on mRNA level and cellular surface protein level. Previous results reported that the "downregulation" of CD46 expression on the cell surface may take place because H protein masks the antibody recognition site on CD46 which results in "downregulation" of the expression of CD46.

High CD46 receptor density determines preferential killing of tumor cells by oncolytic measles virus

Live attenuated Edmonston B strain of measles virus (MV-Edm) is a potent and specific oncolytic agent, but the mechanism underlying its tumor selectivity is unknown. The virus causes cytopathic effects (CPEs) of extensive syncytial formation in tumor cells but minimal damage or cell killing in normal cells. The CPE is dependent on expression of viral proteins and the presence of CD46, the major cellular receptor of MV-Edm. Using a virally encoded soluble marker peptide to provide a quantitative readout of the level of viral gene expression, we determined that tumor cells and normal cells expressed comparable levels of viral proteins. CD46 mediates virus attachment, entry, and virus-induced cell-to-cell fusion. Using engineered cells expressing a range of CD46 densities, we determined that whereas virus entry increased progressively with CD46 density, cell fusion was minimal at low receptor densities but increased dramatically above a threshold density of CD46 receptors. It is well established that tumor cells express abundant CD46 receptors on their surfaces compared with their normal counterparts. Thus, at low CD46 densities typical of normal cells, infection occurs, but intercellular fusion is negligible. At higher densities typical of tumor cells, infection leads to extensive cell fusion. Intercellular fusion also results in enhancement of viral gene expression through recruitment of neighboring uninfected cells into the syncytium, further amplifying the CPE. Discrimination between high and low CD46 receptor density provides a compelling basis for the oncolytic specificity of MV-Edm and establishes MV-Edm as a promising CD46-targeted cancer therapeutic agent.

Targeting virus entry and membrane fusion through specific peptide/MHC complexes using a high-affinity T-cell receptor

The T-cell receptor (TCR) determines the specificity of T-cell recognition by binding to peptide fragments of intracellular proteins presented at the cell surface in association with molecules of the major histocompatibility complex (MHC). Engagement of the TCR by its cognate peptide/MHC ligand, with appropriate co-stimulatory signals, leads to activation of T-cell effector functions. Here we show that the attachment proteins of attenuated measles viruses, engineered to display a high-affinity single-chain TCR (scTCR), can recognize and bind to specific peptide-MHC complexes and thereby mediate targeted virus-cell entry and cell-to-cell fusion. Using the 2C TCR and its peptide/MHC ligand (SIYRYYGL/mouse K(b)), we show that a scTCR grafted onto the measles virus H protein confers new specificity to virus entry and cell fusion. The efficiency of TCR-mediated virus entry was dependent on the number of peptide/MHC complexes expressed on the target cells, increasing progressively above densities higher than 2500 complexes per cell. This work introduces a new paradigm for targeting virus entry and membrane fusion by extending the repertoire of targets to specific peptide-MHC ligands and offering a novel quantitative readout for the cellular expression of peptide-MHC complexes.

Polarized glycoprotein targeting affects the spread of measles virus in vitro and in vivo

We have shown previously that basolateral targeting of plasmid-encoded measles virus (MV) F and H protein is dependent on single tyrosine residues in the cytoplasmic tails of the glycoproteins and is essential for fusion activity in polarized epithelial cells. Here, we present data on the functional importance of polarized glycoprotein expression for the cytopathic properties of infectious MV in culture and for pathogenesis in vivo. By the introduction of single point mutations, we generated recombinant viruses in which the basolateral targeting signal of either one or both glycoproteins was destroyed (tyrosine mutants). As a consequence, the mutated glycoproteins were predominantly expressed on the apical membrane of polarized Madin-Darby canine kidney cells. In contrast to parental MV, none of these virus mutants was able to spread by syncytia formation in polarized cells showing that the presence of both MV glycoproteins at the basolateral cell surface is required for cell-to-cell fusion in vitro. Using cotton rats as an animal model that allows MV replication in the respiratory tract, we showed that basolateral glycoprotein targeting is also of importance for the spread of infection in vivo. Whereas parental MV was able to spread laterally within the respiratory epithelium and from there to cells in the underlying tissue, tyrosine mutants infected only single epithelial and very few subepithelial cells. These data strongly suggest that basolateral targeting of MV glycoproteins helps to overcome the epithelial barrier and thereby facilitates the systemic spread of MV infection in vivo.

Antibody-targeted cell fusion

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

Ligand binding determines whether CD46 is internalized by clathrin-coated pits or macropinocytosis

CD46 is a ubiquitous human cell surface receptor for the complement components C3b and C4b and for various pathogens, including the measles virus and human herpes virus 6. Ligand binding to CD46 affects (i) protection of autologous cells from complement attack by breakdown of complement components, (ii) intracellular signals that affect the regulation of immune cell function, (iii) antigen presentation, and (iv) down-regulation of cell surface CD46. Recent evidence indicates that CD46 signaling can link innate and acquired immune function. The molecular mechanisms for these processes and the importance of intracellular trafficking of the receptor have not yet been elucidated. We demonstrate here that, in nonlymphoid cells, CD46 is constitutively internalized via clathrin-coated pits, traffics to multivesicular bodies, and is recycled to the cell surface. However, cross-linking of CD46 at the cell surface, by either multivalent antibody or by measles virus, induces pseudopodia that engulf the ligand in a process similar to macropinocytosis, and leads to the degradation of cell surface CD46. Thus, we have elucidated two pathways for CD46 internalization, which are regulated by the valence of cross-linking of CD46 and which utilize either clathrin-coated pits or pseudopodial extension. This has important implications for CD46 signaling, antigen presentation, CD46 down-regulation, and engulfment of pathogens.

Mechanism of reduction of virus release and cell-cell fusion in persistent canine distemper virus infection

Canine distemper virus (CDV), a mobillivirus related to measles virus causes a chronic progressive demyelinating disease, associated with persistence of the virus in the central nervous system (CNS). CNS persistence of morbilliviruses has been associated with cell-to-cell spread, thereby limiting immune detection. The mechanism of cell-to-cell spread remains uncertain. In the present study we studied viral spread comparing a cytolytic (non-persistent) and a persistent CDV strain in cell cultures. Cytolytic CDV spread in a compact concentric manner with extensive cell fusion and destruction of the monolayer. Persistent CDV exhibited a heterogeneous cell-to-cell pattern of spread without cell fusion and 100-fold reduction of infectious viral titers in supernatants as compared to the cytolytic strain. Ultrastructurally, low infectious titers correlated with limited budding of persistent CDV as compared to the cytolytic strain, which shed large numbers of viral particles. The pattern of heterogeneous cell-to-cell viral spread can be explained by low production of infectious viral particles in only few areas of the cell membrane. In this way persistent CDV only spreads to a small proportion of the cells surrounding an infected one. Our studies suggest that both cell-to-cell spread and limited production of infectious virus are related to reduced expression of fusogenic complexes in the cell membrane. Such complexes consist of a synergistic configuration of the attachment (H) and fusion (F) proteins on the cell surface. F und H proteins exhibited a marked degree of colocalization in cytolytic CDV infection but not in persistent CDV as seen by confocal laser microscopy. In addition, analysis of CDV F protein expression using vaccinia constructs of both strains revealed an additional large fraction of uncleaved fusion protein in the persistent strain. This suggests that the paucity of active fusion complexes is due to restricted intracellular processing of the viral fusion protein.

Receptor use by vesicular stomatitis virus pseudotypes with glycoproteins of defective variants of measles virus isolated from brains of patients with subacute sclerosing panencephalitis

The vaccine or Vero cell-adapted strains of measles virus (MV) have been reported to use CD46 as a cell entry receptor, while lymphotropic MVs preferentially use the signalling lymphocyte activation molecule (SLAM or CD150). In contrast to the virus obtained from patients with acute measles, little is known about the receptor that is used by defective variants of MV isolated from patients with subacute sclerosing panencephalitis (SSPE). The receptor-binding properties of SSPE strains of MV were analysed using vesicular stomatitis virus pseudotypes expressing the envelope glycoproteins of SSPE strains of MV. Such pseudotype viruses could use SLAM but not CD46 for entry. The pseudotype viruses with SSPE envelope glycoproteins could enter Vero cells, which do not express SLAM. In addition, their entry was not blocked by the monoclonal antibody to CD46, pointing to another entry receptor for SSPE strains on Vero cells. Furthermore, the unknown receptor(s), distinct from SLAM and CD46, may be present on cell lines derived from lymphoid and neural cells. Biochemical characterization of the receptor present on Vero cells and SK-N-SH neuroblastoma cells was consistent with a glycoprotein. Identification of additional entry receptors for MV will provide new insights into the mechanism of spread of MV in the central nervous system and possible reasons for differences between MVs isolated from patients with acute measles and SSPE.

Wild-type measles virus induces large syncytium formation in primary human small airway epithelial cells by a SLAM(CD150)-independent mechanism

In the natural course of measles virus (MV) infection, epithelial cells are primary targets of MV. However, it has been shown that wild-type MV utilizes signaling lymphocyte activation molecule (SLAM or CD150) as a cellular receptor, which is expressed only in some T and B cells, thymocytes, and dendritic cells. To understand how wild-type MV infects non-lymphoid cells, several non-lymphoid cells were examined for their susceptibility to wild-type MV. Here, we show that wild-type MV can infect primary human small airway epithelial cells (SAEC) and induce formation of large syncytia in vitro. mRNA specific for SLAM was not detected in SAEC, indicating that wild-type MV infects SAEC and induces syncytia formation via a SLAM-independent mechanism.

Measles virus spreads in rat hippocampal neurons by cell-to-cell contact and in a polarized fashion

Measles virus (MV) can infect the central nervous system and, in rare cases, causes subacute sclerosing panencephalitis, characterized by a progressive degeneration of neurons. The route of MV transmission in neurons was investigated in cultured rat hippocampal slices by using MV expressing green fluorescent protein. MV infected hippocampal neurons and spread unidirectionally, in a retrograde manner, from CA1 to CA3 pyramidal cells and from there to the dentate gyrus. Spreading of infection depended on cell-to-cell contact and occurred without any detectable release of infectious particles. The role of the viral proteins in the retrograde MV transmission was determined by investigating their sorting in infected pyramidal cells. MV glycoproteins, the fusion protein (F) and hemagglutinin (H), the matrix protein (M), and the phosphoprotein (P), which is part of the viral ribonucleoprotein complex, were all sorted to the dendrites. While M, P, and H proteins remained more intracellular, the F protein localized to prominent, spine-type domains at the surface of infected cells. The detected localization of MV proteins suggests that local microfusion events may be mediated by the F protein at sites of synaptic contacts and is consistent with a mechanism of retrograde transmission of MV infection.

Strength of envelope protein interaction modulates cytopathicity of measles virus

To understand the molecular determinants of measles virus (MV) cytopathicity, we have characterized mutant viruses exhibiting a more-extensive cell-to-cell fusion while maintaining efficient replication to high titers. A virus which is modified by the addition of an 8-amino-acid Flag epitope tag at the cytoplasmic tail of its H (for MV hemagglutinin) envelope glycoprotein replicates efficiently, has an increased cytopathicity, possesses a greater infectivity per particle, and has an altered protein composition compared with that of unmodified MV. The mutant phenotype is not specifically linked to the epitope sequence, since an alternatively added HA (for influenza virus-derived hemagglutinin) epitope tag caused similar effects. We demonstrate that both epitope tags weaken the interaction between the H and fusion (F) glycoproteins in virus-infected cells. This reduction in strength of H/F interaction is independent of the presence of the viral matrix (M) protein. Viruses with this less stable complex are more sensitive to neutralization by a soluble octameric form of the CD46 receptor, consistent with their increased fusogenicity. Similar analyses of glycoproteins derived from MV strains with reduced cytopathicities confirm that the strength of H and F glycoprotein interaction is a modulator of viral fusogenicity.

Antibody-sensitive and antibody-resistant cell-to-cell spread by vaccinia virus: role of the A33R protein in antibody-resistant spread

The roles of vaccinia virus (VV) intracellular mature virus (IMV), intracellular enveloped virus (IEV), cell-associated enveloped virus (CEV) and extracellular enveloped virus (EEV) and their associated proteins in virus spread were investigated. The plaques made by VV mutants lacking individual IEV- or EEV-specific proteins (vDeltaA33R, vDeltaA34R, vDeltaA36R, vDeltaA56R, vDeltaB5R, vDeltaF12L and vDeltaF13L) were compared in the presence of IMV- or EEV-neutralizing antibodies (Ab). Data presented show that for long-range spread, the comet-shaped plaques of VV were caused by the unidirectional spread of EEV probably by convection currents, and for cell-to-cell spread, VV uses a combination of Ab-resistant and Ab-sensitive pathways. Actin tails play a major role in the Ab-resistant pathway, but mutants such as vDeltaA34R and vDeltaA36R that do not make actin tails still spread from cell to cell in the presence of Ab. Most strikingly, the Ab-resistant pathway was abolished when the A33R gene was deleted. This effect was not due to alterations in the efficiency of neutralization of EEV made by this mutant, nor due to a deficiency in IMV wrapping to form IEV, which was indispensable for EEV formation by vDeltaA33R and vDeltaA34R. We suggest a role for A33R in promoting Ab-resistant cell-to-cell spread of virus. The roles of the different virus forms in the VV life-cycle are discussed.

Current overview of the pathogenesis and prophylaxis of measles with focus on practical implications

Measles is one of the most important diseases of mankind, which is so highly contagious and evokes such persistent immunity that the virus cannot be sustained in a population of less than about 500,000 persons. The first of the licensed live virus vaccines against measles was developed empirically and was approved in 1963. It provides high level and lasting immunity and is a paradigm for solving major medical problems without really understanding them. In spite of means for control by prophylactic immunization, research on measles infection continues to be part of the effort to understand the pathogenesis of many different viruses, which may have important similarities and differences and provide important insights. Measles, usually, is spontaneously reversible and is a prime model for understanding virus-induced immunodeficiency disease (AIDS) which is rarely reversible. Much has been learned of basic immunology and vaccinology in measles through observation of the inappropriate use of vaccines of appropriate composition, and through inappropriate host response to measles vaccines of inappropriate composition. This review provides a current overview of selected highlights of measles, the virus, its immunopathogenesis, and its control by use of live virus vaccine which may lead to elimination of the disease and eventually to eradication of the virus.

Human immunodeficiency virus type 1-mediated syncytium formation is compatible with adenovirus replication and facilitates efficient dispersion of viral gene products and de novo-synthesized virus particles

Conditionally replicative adenovirus (CRAd) vectors are designed for specific oncolytic replication in tumor tissues with concomitant sparing of normal cells. As such, CRAds offer an unprecedented level of anticancer potential for malignancies that have been refractory to previous cancer gene therapy interventions. CRAd efficacy may, however, be compromised by inefficient dispersion of the replicating vector within the tumor tissue. To address this issue, we evaluated the utility of a fusogenic membrane glycoprotein (FMG), which induces the fusion of neighboring cellular membranes to form multinucleated syncytia. We hypothesized that the FMG-mediated syncytia would facilitate dispersion of the adenovirus (Ad) gene products and viral progeny. To test this, human immunodeficiency virus type 1 (HIV-1) envelope glycoproteins, which induce syncytia in the presence of CD4+ target cells, were expressed by an Ad (Ad5HIVenv) in permissive (CD4-positive) and nonpermissive (CD4-negative) cell lines. After validating this Ad-FMG model, the efficiency of Ad replication in the presence or absence of syncytia was evaluated. The results demonstrated that syncytium formation was compatible with Ad replication and dramatically increased the dispersion of virus gene products within the cytoplasm of the syncytia as well as viral particles in the nuclei of the syncytial mass. Moreover, progeny virions were released more efficiently from syncytia compared with nonsyncytial cells. These data demonstrate the utility of FMGs as a dispersion agent and suggest that FMGs can improve the efficacy of CRAd gene therapy.

Measles virus interactions with cellular receptors: consequences for viral pathogenesis

Although CNS complications occurring early and late after acute measles are a serious problem and often fatal, the transient immunosuppression lasting for several weeks after the rash is the major cause of measles-related morbidity and mortality worldwide. This review is focused on the interactions of measles virus (MV) with cellular receptors on neural and lymphoid cells which are important elements in viral pathogenesis. First, the cognate MV receptors, CD46 and CD150, are important components of viral tropism by mediating binding and entry. Second, however, additional unknown cellular surface molecules may (independently of viral uptake) after interaction with the MV glycoprotein complex act as signaling molecules and thereby modulate cellular survival, proliferation, and specific functions.

Antibodies to CD9, a tetraspan transmembrane protein, inhibit canine distemper virus-induced cell-cell fusion but not virus-cell fusion

Canine distemper virus (CDV) causes a life-threatening disease in several carnivores including domestic dogs. Recently, we identified a molecule, CD9, a member of the tetraspan transmembrane protein family, which facilitates, and antibodies to which inhibit, the infection of tissue culture cells with CDV (strain Onderstepoort). Here we describe that an anti-CD9 monoclonal antibody (MAb K41) did not interfere with binding of CDV to cells and uptake of virus. In addition, in single-step growth experiments, MAb K41 did not induce differences in the levels of viral mRNA and proteins. However, the virus release of syncytium-forming strains of CDV, the virus-induced cell-cell fusion in lytically infected cultures, and the cell-cell fusion of uninfected with persistently CDV-infected HeLa cells were strongly inhibited by MAb K41. These data indicate that anti-CD9 antibodies selectively block virus-induced cell-cell fusion, whereas virus-cell fusion is not affected.

CD46 transgene expression in pig peripheral blood mononuclear cells does not alter their susceptibility to measles virus or their capacity to downregulate endogenous and transgenic CD46

CD46 (or membrane cofactor protein) protects autologous cells from complement-mediated lysis and has been expressed as a transgene in pigs to overcome complement-mediated hyperacute rejection of porcine organs upon transplantation into primates. Since CD46 has been identified as a receptor for measles virus (MV), the susceptibility of CD46-transgenic (tg) pig peripheral blood mononuclear cells (PBMC) to infection with MV strains which do and do not use CD46 as receptor was investigated. Surprisingly, it was found that MV vaccine strains (e.g. Edmonston) bound to tg as well as non-tg pig PBMC. Phytohaemagglutinin-stimulated CD46-tg and non-tg pig PBMC were equally well infected with MV vaccine strains irrespective of CD46 expression. Upon infection, tg CD46 was downregulated from the cell surface. In contrast, the binding capacity for MV wild-type strains to pig and human PBMC was low, irrespective of CD46 expression. These MV strains did not infect tg or non-tg pig cells. Expression of endogenous pig CD46 was detected with polyclonal sera against human CD46. After infection of pig PBMC with MV strain Edmonston, endogenous pig CD46 was also downregulated. This suggests an interaction between MV Edmonston and pig CD46. However, polyclonal CD46 sera did not inhibit infection with MV Edmonston indicating that CD46 may not exclusively act as a receptor for MV on these cells. Interestingly, similar results were observed using human PBMC. Data suggest that CD46 downregulation after interaction with MV may also occur in porcine organs which express endogenous and/or human CD46 as a means of protection against complement-mediated damage.

Clinical isolates of measles virus use CD46 as a cellular receptor

Laboratory strains of measles viruses (MV), such as Edmonston and Halle, use the complement regulatory protein CD46 as a cell surface receptor. The receptor usage of clinical isolates of MV, however, remains unclear. Receptor usage by primary patient isolates of MV was compared to isolates that had been passaged on a variety of tissue culture cell lines. All of the isolates could infect cells in a CD46-dependent manner, but their tropism was restricted according to cell type (e.g., lymphocytes versus fibroblasts). The results indicate that patient isolates that have not been adapted to tissue culture cell lines use CD46 as a receptor. In addition, passaging primary MV patient isolates in B95-8 cells selected variants that had alternate receptor usage compared to the original isolate. Thus, changes in receptor usage by MV are dependent upon the cell type used for isolation. Furthermore, our results confirm the relevance of the CD46 receptor to natural measles infection.

Consequences of Fas-mediated human dendritic cell apoptosis induced by measles virus

Mortality from measles virus (MV) infection is caused mostly by secondary infections associated with a pronounced immunosuppression. Dendritic cells (DCs) represent a major target of MV and could be involved in immunosuppression. In this study, human monocyte-derived DCs were used to demonstrate that DC apoptosis in MV-infected DC-T-cell cocultures is Fas mediated, whereas apoptotic T cells could not be rescued by blocking the Fas pathway. Two novel consequences of DC apoptosis after MV infection were demonstrated. (i) Fas-mediated apoptosis of DCs facilitates MV release, while CD40 activation enhances MV replication in DCs. Indeed, detailed studies of infectious MV release and intracellular MV nucleoprotein (NP) showed that inhibition of CD40-CD40L ligand interaction blocks NP synthesis. We conclude that the CD40 ligand expressed by activated T cells first enhances MV replication in DCs, and then Fas ligand produced by activated T cells induces Fas-mediated apoptosis of DCs, thus facilitating MV release. (ii) Not only MV-infected DCs but also bystander uninfected DCs undergo a maturation process confirmed by CD1a, CD40, CD80, CD86, CD83, and major histocompatibility complex type II labeling. The bystander maturation effect results from contact and/or engulfment of MV-induced apoptotic DCs by uninfected DCs. A model is proposed to explain how both a specific immune response and immunosuppression can simultaneously occur after MV infection through Fas-mediated apoptosis and CD40 activation of DCs.

Interaction of CD46 with measles virus: accessory role of CD46 short consensus repeat IV

To define further the accessory role(s) of the CD46 (membrane cofactor protein) short consensus repeat (SCR) III and IV domains in the interaction of CD46 with measles virus (MV), chimeric proteins were generated by substituting domains from the structurally related protein decay accelerating factor (DAF, CD55): x3DAF (exchange of CD46 SCR III) and x4DAF (exchange of SCR IV). Transfected CHO cell lines that stably expressed these chimeric proteins were compared for MV binding and infection. Compared with wild-type CD46 (I-II-III-IV), a significant decrease in MV binding was observed with x4DAF. Despite this limited binding, these cells were still capable of supporting virus entry. In a quantitative fusion assay, no significant differences in fusion were observed as a result of the exchange of either CD46 SCR III or IV. However, the down-regulation of cell surface CD46 typically observed following MV infection was abolished with x4DAF, as was the redistribution of CD46 on the cell surface. Thus, CD46 SCR IV appears to be required for optimal virus binding and receptor down-regulation, although importantly, in spite of these functional limitations, x4DAF can still be used for MV entry.

Measles virus spread between neurons requires cell contact but not CD46 expression, syncytium formation, or extracellular virus production

In patients with subacute sclerosing panencephalitis (SSPE), which is associated with persistent measles virus (MV) infection in the brain, little infectious virus can be recovered despite the presence of viral RNA and protein. Based on studies of brain tissue from SSPE patients and our work with MV-infected NSE-CD46(+) mice, which express the measles receptor CD46 on neurons, several lines of evidence suggest that the mechanism of viral spread in the central nervous system differs from that in nonneuronal cells. To examine this alternate mechanism of viral spread, as well as the basis for the loss of normal transmission mechanisms, infection and spread of MV Edmonston was evaluated in primary CD46(+) neurons from transgenic mice and differentiated human NT2 neurons. As expected, unlike that between fibroblasts, viral spread between neurons occurred in the absence of syncytium formation and with minimal extracellular virus. Electron microscopy analysis showed that viral budding did not occur from the neuronal surface, although nucleocapsids were present in the cytoplasm and aligned at the cell membrane. We observed many examples of nucleocapsids present in the neuronal processes and aligned at presynaptic neuronal membranes. Cocultures of CD46(+) and CD46(-) neurons showed that cell contact but not CD46 expression is required for MV spread between neurons. Collectively, these results suggest that the neuronal environment prevents the normal mechanisms of MV spread between neurons at the level of viral assembly but allows an alternate, CD46-independent mechanism of viral transmission, possibly through the synapse.

Lymphatic dissemination and comparative pathology of recombinant measles viruses in genetically modified mice

The dissemination of the Edmonston measles virus (Ed-MV) vaccine strain was studied with genetically modified mice defective for the alpha/beta interferon receptor and expressing human CD46 with human-like tissue specificity and efficiency. A few days after intranasal infection, macrophages expressing Ed-MV RNA were detected in the lungs, in draining lymph nodes, and in the thymus. In lymph nodes, large syncytia which stained positive for viral RNA and for macrophage surface marker proteins were found and apoptotic cell death was monitored. In the thymus, smaller syncytia which stained positive for macrophage and dendritic cell markers were detected. Thus, macrophages appear to be the main vectors for dissemination of MV infection in these mice; human macrophages may have a similar function in the natural host. We then compared the pathogenicities of two recombinant viruses lacking the C or V nonstructural proteins to that of the parental strain, Ed-MV. These viruses were less effective in spreading through the lymphatic system and, unlike Ed-MV, were not detected in the liver. After intracerebral inoculation the recombinant viruses caused lethal disease less often than Ed-MV and induced distinctive patterns of gliosis and inflammation. Ed-MV was reisolated from brain tissue, but its derivatives were not. C- and V-defective viruses should be considered as more-attenuated MV vaccine candidates.