Measles virus transmission from dendritic cells to T cells: formation of synapse-like interfaces concentrating viral and cellular components

Characterizing infection of B cells with wild-type and vaccine strains of measles virus

Acute infection with measles virus (MeV) causes transient immunosuppression often leading to secondary infections. MeV infection of B lymphocytes results in changes in the antibody repertoire and memory B cell populations for which the mechanism is unknown. In this study, we characterize the infection of primary B cells with wild-type and vaccine strains of MeV. Vaccine-infected B cells were characterized by a higher percentage of cells positive for viral protein, a higher level of viral transcription and reduced cell death compared to wild-type infected cells, regardless of B cell subtype. Vaccine-infected cells showed more production of TNF-α and IL-10 but less production of IL-8 compared to wild-type infected cells. IL-4 and IL-6 levels detected were increased during both vaccine and wild-type infection. Despite evidence of replication, measles-infected B cells did not produce detectable viral progeny. This study furthers our understanding of the outcomes of MeV infection of human B cells.

Biomolecular Analysis of Canine Distemper Virus Strains in Two Domestic Ferrets (Mustela putorius furo)

Canine distemper is a contagious and severe systemic viral disease that affects domestic and wild carnivores worldwide. In this study, two adult female ferrets (Mustela putorius furo) were evaluated for cutaneous lesions. Scab, fur, and swab samples from the external auditory canal, cutaneous lesions, and scrapings were analyzed. Canine distemper virus (CDV)-positive samples underwent RT-PCR/RFLP with the restriction enzyme PsiI, and the hemagglutinin gene sequence was obtained. According to the restriction enzyme and sequence analyses, the viral strains were typed as CDV field strains that are included within the Europe lineage and distinct from those including vaccinal CDV strains. The sequence analysis showed the highest nucleotide identity rates in older Europe lineage CDV strains collected from dogs and a fox in Europe. This study is the first to report on CDV infection in ferrets in southern Italy and contributes to the current knowledge about natural CDV infection in this species. In conclusion, vaccination remains crucial for preventing the disease and counteracting cross-species infection. Molecular biology techniques can enable the monitoring of susceptible wild animals by ensuring the active surveillance of CDV spread.

Nanoparticles of conformation-stabilized canine distemper virus hemagglutinin are highly immunogenic and induce robust immunity

Background

Canine distemper virus (CDV) infection of ferrets, dogs, and giant pandas causes an acute systemic disease involving multiple organ systems, including the respiratory tract, lymphoid system, and central nervous system. In this study, we tested a new candidate CDV vaccine-CDV nanoparticles-based on hemagglutinin protein.

Methods

The nanoparticles were generated from conformation-stabilized CDV hemagglutinin tetramers. Immune responses against CDV were evaluated in mice. Immunization was initiated 6 weeks after birth and boosted two times with 4-week intervals. The blood and mucosal samples were collected 2 weeks after each immunization.

Results

Vaccination with CDV nanoparticles elicited high levels of IgG antibody titers in mice (approximately sevenfold to eightfold higher than that obtained with soluble CDV H protein) and mucosal immune responses and developed increased CDV-specific neutralizing antibody. The mice that received nanoparticles showed significantly higher IFN-γ- and IL-4-secreting cell population in the spleen and lymph node compared with mice immunized with soluble H protein. The co-stimulatory molecular expression of CD80 and CD86 on the surface of DCs was also upregulated.

Conclusion

The results demonstrate that self-assembly into nanoparticles can increase the immunogenicity of vaccine antigens, and nanoparticles assembled from conformation-stabilized CDV H protein can serve as a new CDV vaccine.

Molecular Docking of SP40 Peptide towards Cellular Receptors for Enterovirus 71 (EV-A71)

Enterovirus 71 (EV-A71) is one of the predominant etiological agents of hand, foot and mouth disease (HMFD), which can cause severe central nervous system infections in young children. There is no clinically approved vaccine or antiviral agent against HFMD. The SP40 peptide, derived from the VP1 capsid of EV-A71, was reported to be a promising antiviral peptide that targeted the host receptor(s) involved in viral attachment or entry. So far, the mechanism of action of SP40 peptide is unknown. In this study, interactions between ten reported cell receptors of EV-A71 and the antiviral SP40 peptide were evaluated through molecular docking simulations, followed by in vitro receptor blocking with specific antibodies. The preferable binding region of each receptor to SP40 was predicted by global docking using HPEPDOCK and the cell receptor-SP40 peptide complexes were refined using FlexPepDock. Local molecular docking using GOLD (Genetic Optimization for Ligand Docking) showed that the SP40 peptide had the highest binding score to nucleolin followed by annexin A2, SCARB2 and human tryptophanyl-tRNA synthetase. The average GoldScore for 5 top-scoring models of human cyclophilin, fibronectin, human galectin, DC-SIGN and vimentin were almost similar. Analysis of the nucleolin-SP40 peptide complex showed that SP40 peptide binds to the RNA binding domains (RBDs) of nucleolin. Furthermore, receptor blocking by specific monoclonal antibody was performed for seven cell receptors of EV-A71 and the results showed that the blocking of nucleolin by anti-nucleolin alone conferred a 93% reduction in viral infectivity. Maximum viral inhibition (99.5%) occurred when SCARB2 was concurrently blocked with anti-SCARB2 and the SP40 peptide. This is the first report to reveal the mechanism of action of SP40 peptide in silico through molecular docking analysis. This study provides information on the possible binding site of SP40 peptide to EV-A71 cellular receptors. Such information could be useful to further validate the interaction of the SP40 peptide with nucleolin by site-directed mutagenesis of the nucleolin binding site.

Measles vaccination elicits a polyfunctional antibody response, which decays more rapidly in early vaccinated children

Background

Measles outbreaks are reported worldwide and pose a serious threat, especially to young unvaccinated infants. Early measles vaccination given to infants under 12 months of age can induce protective antibody levels, but the long-term antibody functionalities are unknown.

Methods

Measles-specific antibody functionality was tested using a systems serology approach for children who received an early measles vaccination at 6–8 or 9–12 months, followed by a regular dose at 14 months of age, and children who only received the vaccination at 14 months. Antibody functionalities comprised complement deposition, cellular cytotoxicity, and neutrophil and cellular phagocytosis. We used Pearson’s r correlations between all effector functions to investigate the coordination of the response.

Results

Children receiving early measles vaccination at 6–8 or 9–12 months of age show polyfunctional antibody responses. Despite significant lower levels of antibodies in these early-vaccinated children, Fc effector functions were comparable with regular-timed vaccinees at 14 months. However, 3-year follow-up revealed significant decreased polyfunctionality in children who received a first vaccination at 6–8 months of age, but not in children who received the early vaccination at 9–12 months.

Conclusions

Antibodies elicited in early-vaccinated children are equally polyfunctional to those elicited from children who received vaccination at 14 months. However, these antibody functionalities decay more rapidly than those induced later in life, which may lead to suboptimal, long-term protection.

Virological and immunological features of SARS-CoV-2-infected children who develop neutralizing antibodies

As the global COVID-19 pandemic progresses, it is paramount to gain knowledge on adaptive immunity to SARS-CoV-2 in children to define immune correlates of protection upon immunization or infection. We analyzed anti-SARS-CoV-2 antibodies and their neutralizing activity (PRNT) in 66 COVID-19-infected children at 7 (±2) days after symptom onset. Individuals with specific humoral responses presented faster virus clearance and lower viral load associated with a reduced in vitro infectivity. We demonstrated that the frequencies of SARS-CoV-2-specific CD4+CD40L+ T cells and Spike-specific B cells were associated with the anti-SARS-CoV-2 antibodies and the magnitude of neutralizing activity. The plasma proteome confirmed the association between cellular and humoral SARS-CoV-2 immunity, and PRNT+ patients show higher viral signal transduction molecules (SLAMF1, CD244, CLEC4G). This work sheds lights on cellular and humoral anti-SARS-CoV-2 responses in children, which may drive future vaccination trial endpoints and quarantine measures policies.

Virus-Mediated Cell-Cell Fusion

Cell-cell fusion between eukaryotic cells is a general process involved in many physiological and pathological conditions, including infections by bacteria, parasites, and viruses. As obligate intracellular pathogens, viruses use intracellular machineries and pathways for efficient replication in their host target cells. Interestingly, certain viruses, and, more especially, enveloped viruses belonging to different viral families and including human pathogens, can mediate cell-cell fusion between infected cells and neighboring non-infected cells. Depending of the cellular environment and tissue organization, this virus-mediated cell-cell fusion leads to the merge of membrane and cytoplasm contents and formation of multinucleated cells, also called syncytia, that can express high amount of viral antigens in tissues and organs of infected hosts. This ability of some viruses to trigger cell-cell fusion between infected cells as virus-donor cells and surrounding non-infected target cells is mainly related to virus-encoded fusion proteins, known as viral fusogens displaying high fusogenic properties, and expressed at the cell surface of the virus-donor cells. Virus-induced cell-cell fusion is then mediated by interactions of these viral fusion proteins with surface molecules or receptors involved in virus entry and expressed on neighboring non-infected cells. Thus, the goal of this review is to give an overview of the different animal virus families, with a more special focus on human pathogens, that can trigger cell-cell fusion.

Respiratory syncytial virus exhibits differential tropism for distinct human placental cell types with Hofbauer cells acting as a permissive reservoir for infection

BACKGROUND

Respiratory syncytial virus (RSV) is capable of transient viremia and extrapulmonary dissemination. Recently, this virus has been identified in fetal cord blood, suggesting the possibility of in utero acquisition in humans. Here, we assess permissivity and kinetics of RSV replication in primary human placental cells, examine their potential to transfer this infection to neighboring cells, and measure the inflammatory response evoked by the virus.

METHODS AND FINDINGS

Human placental villus tissue was collected immediately upon delivery and processed for isolation of placental cytotrophoblast, fibroblast, and macrophage (Hofbauer) cells. Isolated cells were infected with a recombinant RSV-A2 strain (rrRSV) expressing red fluorescent protein (RFP) and analyzed by fluorescence microscopy, Western blot, and quantitative PCR (qPCR). Based on RFP expression, rrRSV exhibited differential tropism for the three major placental cell types. Placental fibroblasts and Hofbauer cells were permissive and supported productive rrRSV replication. While infected cytotrophoblast cells expressed viral glycoprotein (G protein), only limited RSV replication was detected. Importantly, qPCR and fluorescence-focused unit assay revealed that the viral progeny remains trapped within infected Hofbauer cells for up to 30 days, with no release into surrounding media. Yet, Hofbauer cells passed the infection onto overlaid naïve 16HBE cells, suggesting contact-dependent trans-infection. Lastly, a significant increase in proinflammatory cytokines, particularly IL-6, TNF-alpha, and IFN-gamma was measured in the supernatant of infected Hofbauer cells by multiplex cytokine assay and conventional ELISA.

CONCLUSIONS

This study demonstrates that RSV can replicate in human placenta, exhibits differential tropism for distinct placental cell types, can be stored and transferred to neighboring cells by Hofbauer cells, and elicits an inflammatory response. It also supports the hypothesis that this respiratory virus can be vertically transferred to the fetus and potentially affect its development and the outcome of pregnancies.

Measles Virus Ribonucleoprotein Complexes Rapidly Spread across Well-Differentiated Primary Human Airway Epithelial Cells along F-Actin Rings

Measles virus (MeV) is a highly contagious human pathogen that continues to be a worldwide health burden. One of the challenges for the study of MeV spread is the identification of model systems that accurately reflect how MeV behaves in humans. For our studies, we use unpassaged, well-differentiated primary cultures of airway epithelial cells from human donor lungs to examine MeV infection and spread. Here, we show that the main components of the MeV ribonucleoprotein complex (RNP), the nucleocapsid and phosphoprotein, colocalize with the apical and circumapical F-actin networks. To better understand how MeV infections spread across the airway epithelium, we generated a recombinant virus incorporating chimeric fluorescent proteins in its RNP complex. By live cell imaging, we observed rapid movement of RNPs along the circumapical F-actin rings of newly infected cells. This strikingly rapid mechanism of horizontal trafficking across epithelia is consistent with the opening of pores between columnar cells by the viral membrane fusion apparatus. Our work provides mechanistic insights into how MeV rapidly spreads through airway epithelial cells, contributing to its extremely contagious nature.IMPORTANCE The ability of viral particles to directly spread cell to cell within the airways without particle release is considered to be highly advantageous to many respiratory viruses. Our previous studies in well-differentiated, primary human airway epithelial cells suggest that measles virus (MeV) spreads cell to cell by eliciting the formation of intercellular membrane pores. Based on a newly generated ribonucleoprotein complex (RNP) "tracker" virus, we document by live-cell microscopy that MeV RNPs move along F-actin rings before entering a new cell. Thus, rather than diffusing through the cytoplasm of a newly infected columnar cell, RNPs take advantage of the cytoskeletal infrastructure to rapidly spread laterally across the human airway epithelium. This results in rapid horizontal spread through the epithelium that does not require particle release.

Measles Virus Infection Fosters Dendritic Cell Motility in a 3D Environment to Enhance Transmission to Target Cells in the Respiratory Epithelium

Transmission of measles virus (MV) from dendritic to airway epithelial cells is considered as crucial to viral spread late in infection. Therefore, pathways and effectors governing this process are promising targets for intervention. To identify these, we established a 3D respiratory tract model where MV transmission by infected dendritic cells (DCs) relied on the presence of nectin-4 on H358 lung epithelial cells. Access to recipient cells is an important prerequisite for transmission, and we therefore analyzed migration of MV-exposed DC cultures within the model. Surprisingly, enhanced motility toward the epithelial layer was observed for MV-infected DCs as compared to their uninfected siblings. This occurred independently of factors released from H358 cells indicating that MV infection triggered cytoskeletal remodeling associated with DC polarization enforced velocity. Accordingly, the latter was also observed for MV-infected DCs in collagen matrices and was particularly sensitive to ROCK inhibition indicating infected DCs preferentially employed the amoeboid migration mode. This was also implicated by loss of podosomes and reduced filopodial activity both of which were retained in MV-exposed uninfected DCs. Evidently, sphingosine kinase (SphK) and sphingosine-1-phosphate (S1P) as produced in response to virus-infection in DCs contributed to enhanced velocity because this was abrogated upon inhibition of sphingosine kinase activity. These findings indicate that MV infection promotes a push-and-squeeze fast amoeboid migration mode via the SphK/S1P system characterized by loss of filopodia and podosome dissolution. Consequently, this enables rapid trafficking of virus toward epithelial cells during viral exit.

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

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

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

Comprehensive Bioinformatics Analysis of the Immune Mechanism of Dendritic Cells Against Measles Virus

Background

The purpose of this study was to explore the immune mechanism of dendritic cells (DCs) against measles virus (MV), and to identify potential biomarkers to improve measles prevention and treatment.

Material/Methods

The gene expression profile of GSE980, which comprised 10 DC samples from human blood infected with MV (RNA was isolated at 3, 6, 12, and 24 h post-infection) and 4 normal DC control samples, was obtained from the Gene Expression Omnibus database. Differentially expressed genes (DEGs) between the MV-infected DC samples and the control samples were screened using Genevestigator software. Gene ontology (GO) and Kyoto encyclopedia of genes and genomes (KEGG) pathway enrichment analyses were performed using GenCLip 2.0 and STRING 10.5 software. The protein–protein interaction (PPI) network was established using Cytoscape 3.4.0.

Results

The gene expression profiles of MV-infected DCs were obviously changed. Twenty-six common DEGs (0.9%, MV-infected DCs vs. normal DCs) were identified at 4 different time points, including 14 down-regulated and 12 up-regulated genes (P=0.001). GO analysis showed that DEGs were significantly enriched in defense response to virus, type I interferon signaling pathway, et al. ISG15 and CXCL10 were the key genes in the PPI network of the DEGs, and may interact directly with the type I interferon signaling and defense response to virus signaling.

Conclusions

The DEGs increased gradually with the duration of MV infection. The type I interferon signaling pathway and the defense response to viral processes can be activated against MV by ISG15 and CXCL10 in DCs. These may provide novel targets for the treatment of MV.

Canine Distemper Virus Spread and Transmission to Naive Ferrets: Selective Pressure on Signaling Lymphocyte Activation Molecule-Dependent Entry

Upon infection, morbilliviruses such as measles virus, rinderpest virus, and canine distemper virus (CDV) initially target immune cells via the signaling lymphocyte activation molecule (SLAM) before spreading to respiratory epithelia through the adherens junction protein nectin-4. However, the roles of these receptors in transmission from infected to naive hosts have not yet been formally tested. To experimentally addressing this question, we established a model of CDV contact transmission between ferrets. We show here that transmission of wild-type CDV sometimes precedes the onset of clinical disease. In contrast, transmission was not observed in most animals infected with SLAM- or nectin-4-blind CDVs, even though all animals infected with the nectin-4-blind virus developed sustained viremia. There was an unexpected case of transmission of a nectin-4-blind virus, possibly due to biting. Another unprecedented event was transient viremia in an infection with a SLAM-blind virus. We identified three compensatory mutations within or near the SLAM-binding surface of the attachment protein. A recombinant CDV expressing the mutated attachment protein regained the ability to infect ferret lymphocytes in vitro, but its replication was not as efficient as that of wild-type CDV. Ferrets infected with this virus developed transient viremia and fever, but there was no transmission to naive contacts. Our study supports the importance of epithelial cell infection and of sequential CDV H protein interactions first with SLAM and then nectin-4 receptors for transmission to naive hosts. It also highlights the in vivo selection pressure on the H protein interactions with SLAM.IMPORTANCE Morbilliviruses such as measles virus, rinderpest virus, and canine distemper virus (CDV) are highly contagious. Despite extensive knowledge of how morbilliviruses interact with their receptors, little is known about how those interactions influence viral transmission to naive hosts. In a ferret model of CDV contact transmission, we showed that sequential use of the signaling lymphocytic activation molecule (SLAM) and nectin-4 receptors is essential for transmission. In one animal infected with a SLAM-blind CDV, we documented mild viremia due to the acquisition of three compensatory mutations within or near the SLAM-binding surface. The interaction, however, was not sufficient to cause disease or sustain transmission to naive contacts. This work confirms the sequential roles of SLAM and nectin-4 in morbillivirus transmission and highlights the selective pressure directed toward productive interactions with SLAM.

The HTLV-1 gp21 fusion peptide inhibits antigen specific T-cell activation in-vitro and in mice

The ability of the Lentivirus HIV-1 to inhibit T-cell activation by its gp41 fusion protein is well documented, yet limited data exists regarding other viral fusion proteins. HIV-1 utilizes membrane binding region of gp41 to inhibit T-cell receptor (TCR) complex activation. Here we examined whether this T-cell suppression strategy is unique to the HIV-1 gp41. We focused on T-cell modulation by the gp21 fusion peptide (FP) of the Human T-lymphotropic Virus 1 (HTLV-1), a Deltaretrovirus that like HIV infects CD4+ T-cells. Using mouse and human in-vitro T-cell models together with in-vivo T-cell hyper activation mouse model, we reveal that HTLV-1's FP inhibits T-cell activation and unlike the HIV FP, bypasses the TCR complex. HTLV FP inhibition induces a decrease in Th1 and an elevation in Th2 responses observed in mRNA, cytokine and transcription factor profiles. Administration of the HTLV FP in a T-cell hyper activation mouse model of multiple sclerosis alleviated symptoms and delayed disease onset. We further pinpointed the modulatory region within HTLV-1's FP to the same region previously identified as the HIV-1 FP active region, suggesting that through convergent evolution both viruses have obtained the ability to modulate T-cells using the same region of their fusion protein. Overall, our findings suggest that fusion protein based T-cell modulation may be a common viral trait.

Promotion of virus assembly and organization by the measles virus matrix protein

Measles virus (MeV) remains a major human pathogen, but there are presently no licensed antivirals to treat MeV or other paramyxoviruses. Here, we use cryo-electron tomography (cryo-ET) to elucidate the principles governing paramyxovirus assembly in MeV-infected human cells. The three-dimensional (3D) arrangement of the MeV structural proteins including the surface glycoproteins (F and H), matrix protein (M), and the ribonucleoprotein complex (RNP) are characterized at stages of virus assembly and budding, and in released virus particles. The M protein is observed as an organized two-dimensional (2D) paracrystalline array associated with the membrane. A two-layered F–M lattice is revealed suggesting that interactions between F and M may coordinate processes essential for MeV assembly. The RNP complex remains associated with and in close proximity to the M lattice. In this model, the M lattice facilitates the well-ordered incorporation and concentration of the surface glycoproteins and the RNP at sites of virus assembly.

Virus assembly is technically challenging to study. Here the authors use cryo-electron tomography of measles virus-infected human cells to determine native-state virus structure and they locate well-ordered M lattices that organize viral glycoproteins, RNP, and drive assembly.

Biophysical Properties and Antiviral Activities of Measles Fusion Protein Derived Peptide Conjugated with 25-Hydroxycholesterol

Measles virus (MV) infection is re-emerging, despite the availability of an effective vaccine. The mechanism of MV entry into a target cell relies on coordinated action between the MV hemagglutinin (H) receptor binding protein and the fusion envelope glycoprotein (F) which mediates fusion between the viral and cell membranes. Peptides derived from the C-terminal heptad repeat (HRC) of F can interfere with this process, blocking MV infection. As previously described, biophysical properties of HRC-derived peptides modulate their antiviral potency. In this work, we characterized a MV peptide fusion inhibitor conjugated to 25-hydroxycholesterol (25HC), a cholesterol derivative with intrinsic antiviral activity, and evaluated its interaction with membrane model systems and human blood cells. The peptide (MV.

In Vivo Efficacy of Measles Virus Fusion Protein-Derived Peptides Is Modulated by the Properties of Self-Assembly and Membrane Residence

Measles virus (MV) infection is undergoing resurgence and remains one of the leading causes of death among young children worldwide despite the availability of an effective measles vaccine. MV infects its target cells by coordinated action of the MV hemagglutinin (H) and fusion (F) envelope glycoproteins; upon receptor engagement by H, the prefusion F undergoes a structural transition, extending and inserting into the target cell membrane and then refolding into a postfusion structure that fuses the viral and cell membranes. By interfering with this structural transition of F, peptides derived from the heptad repeat (HR) regions of F can inhibit MV infection at the entry stage. In previous work, we have generated potent MV fusion inhibitors by dimerizing the F-derived peptides and conjugating them to cholesterol. We have shown that prophylactic intranasal administration of our lead fusion inhibitor efficiently protects from MV infection in vivo We show here that peptides tagged with lipophilic moieties self-assemble into nanoparticles until they reach the target cells, where they are integrated into cell membranes. The self-assembly feature enhances biodistribution and the half-life of the peptides, while integration into the target cell membrane increases fusion inhibitor potency. These factors together modulate in vivo efficacy. The results suggest a new framework for developing effective fusion inhibitory peptides.

IMPORTANCE

Measles virus (MV) infection causes an acute illness that may be associated with infection of the central nervous system (CNS) and severe neurological disease. No specific treatment is available. We have shown that fusion-inhibitory peptides delivered intranasally provide effective prophylaxis against MV infection. We show here that specific biophysical properties regulate the in vivo efficacy of MV F-derived peptides.

Measles Virus Host Invasion and Pathogenesis

Measles virus is a highly contagious negative strand RNA virus that is transmitted via the respiratory route and causes systemic disease in previously unexposed humans and non-human primates. Measles is characterised by fever and skin rash and usually associated with cough, coryza and conjunctivitis. A hallmark of measles is the transient immune suppression, leading to increased susceptibility to opportunistic infections. At the same time, the disease is paradoxically associated with induction of a robust virus-specific immune response, resulting in lifelong immunity to measles. Identification of CD150 and nectin-4 as cellular receptors for measles virus has led to new perspectives on tropism and pathogenesis. In vivo studies in non-human primates have shown that the virus initially infects CD150⁺ lymphocytes and dendritic cells, both in circulation and in lymphoid tissues, followed by virus transmission to nectin-4 expressing epithelial cells. The abilities of the virus to cause systemic infection, to transmit to numerous new hosts via droplets or aerosols and to suppress the host immune response for several months or even years after infection make measles a remarkable disease. This review briefly highlights current topics in studies of measles virus host invasion and pathogenesis.

Viruses exploit the tissue physiology of the host to spread in vivo

Viruses are pathogens that strictly depend on their host for propagation. Over years of co-evolution viruses have become experts in exploiting the host cell biology and physiology to ensure efficient replication and spread. Here, we will first summarize the concepts that have emerged from in vitro cell culture studies to understand virus spread. We will then review the results from studies in living animals that reveal how viruses exploit the natural flow of body fluids, specific tissue architecture, and patterns of cell circulation and migration to spread within the host. Understanding tissue physiology will be critical for the design of antiviral strategies that prevent virus dissemination.

Measles Virus Fusion Protein: Structure, Function and Inhibition

Measles virus (MeV), a highly contagious member of the Paramyxoviridae family, causes measles in humans. The Paramyxoviridae family of negative single-stranded enveloped viruses includes several important human and animal pathogens, with MeV causing approximately 120,000 deaths annually. MeV and canine distemper virus (CDV)-mediated diseases can be prevented by vaccination. However, sub-optimal vaccine delivery continues to foster MeV outbreaks. Post-exposure prophylaxis with antivirals has been proposed as a novel strategy to complement vaccination programs by filling herd immunity gaps. Recent research has shown that membrane fusion induced by the morbillivirus glycoproteins is the first critical step for viral entry and infection, and determines cell pathology and disease outcome. Our molecular understanding of morbillivirus-associated membrane fusion has greatly progressed towards the feasibility to control this process by treating the fusion glycoprotein with inhibitory molecules. Current approaches to develop anti-membrane fusion drugs and our knowledge on drug resistance mechanisms strongly suggest that combined therapies will be a prerequisite. Thus, discovery of additional anti-fusion and/or anti-attachment protein small-molecule compounds may eventually translate into realistic therapeutic options.

Antibodies to P-selectin glycoprotein ligand-1 block dendritic cell-mediated enterovirus 71 transmission and prevent virus-induced cells death

P-selectin glycoprotein ligand-1 (PSGL-1) has been proved to serve as the functional receptor for enterovirus 71 (EV71). We found the abundant expression of PSGL-1 on monocyte-derived dendritic cells (MDDCs). However, we have previously demonstrated that MDDCs did not support efficient replication of EV71. Dendritic cells (DCs) have been described to be subverted by various viruses including EV71 for viral dissemination, we thus explore the potential contribution of PSGL-1 on DC-mediated EV71 transmission. We found that the cell-surface-expressing PSGL-1 on MDDCs mediated EV71 binding, and intriguingly, these loaded-viruses on MDDCs could be transferred to encountered target cells; Prior-treatment with PSGL-1 antibodies or interference with PSGL-1 expression diminished MDDC-mediated EV71 transfer and rescued virus-induced cell death. Our data uncover a novel role of PSGL-1 in DC-mediated EV71 spread, and provide an insight into blocking primary EV71 infection.

The Nectin-4/Afadin Protein Complex and Intercellular Membrane Pores Contribute to Rapid Spread of Measles Virus in Primary Human Airway Epithelia

The discovery that measles virus (MV) uses the adherens junction protein nectin-4 as its epithelial receptor provides a new vantage point from which to characterize its rapid spread in the airway epithelium. We show here that in well-differentiated primary cultures of airway epithelial cells from human donors (HAE), MV infectious centers form rapidly and become larger than those of other respiratory pathogens: human respiratory syncytial virus, parainfluenza virus 5, and Sendai virus. While visible syncytia do not form after MV infection of HAE, the cytoplasm of an infected cell suddenly flows into an adjacent cell, as visualized through wild-type MV-expressed cytoplasmic green fluorescent protein (GFP). High-resolution video microscopy documents that GFP flows through openings that form on the lateral surfaces between columnar epithelial cells. To assess the relevance of the protein afadin, which connects nectin-4 to the actin cytoskeleton, we knocked down its mRNA. This resulted in more-limited infectious-center formation. We also generated a nectin-4 mutant without the afadin-binding site in its cytoplasmic tail. This mutant was less effective than wild-type human nectin-4 at promoting MV infection in primary cultures of porcine airway epithelia. Thus, in airway epithelial cells, MV spread requires the nectin-4/afadin complex and is based on cytoplasm transfer between columnar cells. Since the viral membrane fusion apparatus may open the passages that allow cytoplasm transfer, we refer to them as intercellular membrane pores. Virus-induced intercellular pores may contribute to extremely efficient measles contagion by promoting the rapid spread of the virus through the upper respiratory epithelium.

IMPORTANCE

Measles virus (MV), while targeted for eradication, still causes about 120,000 deaths per year worldwide. The recent reemergence of measles in insufficiently vaccinated populations in Europe and North America reminds us that measles is extremely contagious, but the processes favoring its spread in the respiratory epithelium remain poorly defined. Here we characterize wild-type MV spread in well-differentiated primary cultures of human airway epithelial cells. We observed that viral infection promotes the flow of cytoplasmic contents from infected to proximal uninfected columnar epithelial cells. Cytoplasm flows through openings that form on the lateral surfaces. Infectious-center growth is facilitated by afadin, a protein connecting the adherens junction and the actin cytoskeleton. The viral fusion apparatus may open intercellular pores, and the cytoskeleton may stabilize them. Rapid homogenization of cytoplasmic contents in epithelial infectious centers may favor rapid spread and contribute to the extremely contagious nature of measles.

Human Blood-Circulating Basophils Capture HIV-1 and Mediate Viral trans-Infection of CD4+ T Cells

Cell-associated HIV-1 infection has been proposed to play a pivotal role in the spread of HIV-1 infection. Granulocytes are a category of white blood cells, comprising mainly basophils, neutrophils, and eosinophils, and participate in various inflammatory reactions and defense against pathogens. Here, we investigated the role of human blood granulocytes in the dissemination of HIV-1. These cells were found to express a variety of HIV-1 attachment factors (HAFs). Basophils expressed HAFs dendritic cell (DC)-specific intercellular adhesion molecule 3 (ICAM3)-grabbing nonintegrin (DC-SIGN), DC immunoreceptor (DCIR), heparan sulfate proteoglycan (HSPG), and α4β7 integrin and mediated the most efficient capture of HIV-1 on the cell surface. Neutrophils were found to express DCIR and demonstrated limited efficiency of viral capture. Eosinophils expressed α4β7 integrin but exhibited little or no virus-binding capacity. Intriguingly, following direct contact with CD4+ T cells, viruses harbored on the surface of basophils were transferred to T cells. The contact between basophils and CD4+ T cells and formation of infectious synapses appeared necessary for efficient HIV-1 spread. In HIV-1-infected individuals, the frequency of basophils remained fairly stable over the course of disease, regardless of CD4+ T depletion or the emergence of AIDS-associated opportunistic infections. Collectively, our results provide novel insights into the roles of granulocytes, particularly basophils, in HIV-1 dissemination. Thus, strategies designed to prevent basophil-mediated viral capture and transfer may be developed into a new form of therapy.

IMPORTANCE

Cell-associated HIV-1 infection has been proposed to play a pivotal role in the spread of HIV-1 infection. Here, we demonstrated that human blood-circulating granulocytes, particularly basophils, can capture HIV-1 and mediate viral trans-infection of CD4+ T cells. The expression of a variety of HIV-1 attachment factors, such as the C-type lectins, etc., facilitates viral capture and transfer. Intriguingly, the frequency of basophils in patients with different levels of CD4+ T counts remains fairly stable during the course of disease. Our results provide novel insights into the roles of granulocytes, particularly basophils, in HIV-1 dissemination. We suggest that strategies designed to prevent basophil-mediated viral capture and transfer may be a new direction for the development of anti-HIV therapy.

Prevention of measles virus infection by intranasal delivery of fusion inhibitor peptides

Measles virus (MV) infection is undergoing resurgence and remains one of the leading causes of death among young children worldwide despite the availability of an effective measles vaccine. MV infects its target cells by coordinated action of the MV H and the fusion (F) envelope glycoprotein; upon receptor engagement by H, the prefusion F undergoes a structural transition, extending and inserting into the target cell membrane and then refolding into a postfusion structure that fuses the viral and cell membranes. By interfering with this structural transition of F, peptides derived from the heptad-repeat (HR) regions of F can potently inhibit MV infection at the entry stage. We show here that specific features of H's interaction with its receptors modulate the susceptibility of MV F to peptide fusion inhibitors. A higher concentration of inhibitory peptides is required to inhibit F-mediated fusion when H is engaged to its nectin-4 receptor than when H is engaged to its CD150 receptor. Peptide inhibition of F may be subverted by continued engagement of receptor by H, a finding that highlights the ongoing role of H-receptor interaction after F has been activated and that helps guide the design of more potent inhibitory peptides. Intranasal administration of these peptides results in peptide accumulation in the airway epithelium with minimal systemic levels of peptide and efficiently prevents MV infection in vivo in animal models. The results suggest an antiviral strategy for prophylaxis in vulnerable and/or immunocompromised hosts.

IMPORTANCE

Measles virus (MV) infection causes an acute illness that may be associated with infection of the central nervous system (CNS) and severe neurological disease. No specific treatment is available. We have shown that parenterally delivered fusion-inhibitory peptides protect mice from lethal CNS MV disease. Here we show, using established small-animal models of MV infection, that fusion-inhibitory peptides delivered intranasally provide effective prophylaxis against MV infection. Since the fusion inhibitors are stable at room temperature, this intranasal strategy is feasible even outside health care settings, could be used to protect individuals and communities in case of MV outbreaks, and could complement global efforts to control measles.

The molecule of DC-SIGN captures enterovirus 71 and confers dendritic cell-mediated viral trans-infection

Background

Enterovirus 71 (EV71) is the main causative agent of hand, foot and mouth disease that occurs in young children. Neither antiviral agents nor vaccines are available for efficiently combating viral infection. Study of EV71–host interplay is important for understanding viral infection and developing strategies for prevention and therapy. Here the interactions of EV71 with human dendritic cells were analyzed.

Methods

EV71 capture, endocytosis, infection, and degradation in monocyte-derived dendritic cells (MDDCs) were detected by Flow cytometry or real-time (RT-) PCR, and MDDCs-mediated EV71 trans-infection of RD cells was determined via coculture system. Cell morphology or viability was monitored with microscopy or flow cytometry. SiRNA interference was used to knock down gene expression.

Results

MDDCs can bind EV71, but these loaded-EV71 particles in MDDCs underwent a rapid degradation in the absence of efficient replication; once the captured EV71 encountered susceptible cells, MDDCs efficiently transferred surface-bound viruses to target cells. The molecule of DC-SIGN (DC-specific intercellular adhesion molecule-3 grabbing nonintegrin) mediated viral binding and transfer, because interference of DC-SIGN expression with specific siRNAs reduced EV71 binding and impaired MDDC-mediated viral trans-infection, and exogenous expression of DC-SIGN molecule on Raji cell initiated viral binding and subsequent transmission.

Conclusion

MDDCs could bind efficiently EV71 viruses through viral binding to DC-SIGN molecule, and these captured-viruses could be transferred to susceptible cells for robust infection. The novel finding of DC-mediated EV71 dissemination might facilitate elucidation of EV71 primary infection and benefit searching for new clues for preventing viruses from initial infection.

Nectin-4-dependent measles virus spread to the cynomolgus monkey tracheal epithelium: role of infected immune cells infiltrating the lamina propria

After the contagion measles virus (MV) crosses the respiratory epithelium within myeloid cells that express the primary receptor signaling lymphocytic activation molecule (SLAM), it replicates briskly in SLAM-expressing cells in lymphatic organs. Later, the infection spreads to epithelia expressing nectin-4, an adherens junction protein expressed preferentially in the trachea, but how it gets there is not understood. To characterize the mechanisms of spread, we infected groups of 5 or 6 cynomolgus monkeys (Macaca fascicularis) with either a wild-type MV or its "N4-blind" derivative, which is unable to enter nectin-4-expressing cells because of the targeted mutation of two hemagglutinin residues. As expected, both viruses caused similar levels of immunosuppression, as monitored by reductions in white blood cell counts and lymphocyte proliferation activity. However, monkeys infected with the N4-blind MV cleared infection more rapidly. Wild-type virus-infected monkeys secreted virus, while marginal virus titers were detected in tracheal lavage fluid cells of N4-blind MV-infected hosts. Analyses of tracheal rings obtained at necropsy (day 12) documented widespread infection of individual cells or small cell clusters in the subepithelial lamina propria of monkeys infected with either virus. However, only wild-type MV spread to the epithelium, forming numerous infectious centers comprised of many contiguous columnar cells. Infected CD11c(+) myeloid (macrophage or dendritic) cells were frequently observed in the lamina propria below epithelial infectious centers. Thus, MV may use myeloid cells as vehicles not only immediately after contagion but also to infect epithelia of tissues expressing nectin-4, including the trachea.

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.