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

Novel intercellular spread mode of respiratory syncytial virus contributes to neutralization escape

Developing widely used respiratory syncytial virus (RSV) vaccines remains a significant challenge, despite the recent authorization of two pre-F vaccines for elderly adults. Previous reports have suggested that even when vaccine-induced immunity generates high titers of potent neutralizing antibodies targeting the pre-F protein, it may not fully inhibit breakthrough of RSV infections. This incomplete inhibition of RSV breakthrough infections can lead to an increased risk of enhanced respiratory disease (ERD) in vaccinated individuals. The reasons why potent neutralizing antibodies cannot fully prevent RSV breakthrough infections are not yet clear. In an attempt to explain this phenomenon, we investigated the effect of potent neutralizing antibodies on the intercellular spread of RSV. Our findings indicated that a specific titer of potent neutralizing antibodies, such as 5C4, could block certain modes of intercellular spread, such as the diffusion of cell-free virions and the delivery of virions through filopodia. However, these antibodies did not fully inhibit the entire process of intercellular spread. Through the use of super-resolution imaging techniques, we observed a novel and efficient spread mode called the transition of viral materials through intercellular nanotubes (TVMIN), independent of virions and insensitive to the presence of antibodies. TVMIN allowed RSV-infected cells to directly transfer viral materials to neighboring cells via intercellular nanotubes that are rich in microfilaments. TVMIN began as early as 5 h post-infection (h.p.i.) and rapidly initiated infection in recipient cells. Our data provided new insights into the intercellular spread of RSV and might help explain the occurrence of breakthrough infections.

Harnessing the potential of the NALT and BALT as targets for immunomodulation using engineering strategies to enhance mucosal uptake

Mucosal barrier tissues and their mucosal associated lymphoid tissues (MALT) are attractive targets for vaccines and immunotherapies due to their roles in both priming and regulating adaptive immune responses. The upper and lower respiratory mucosae, in particular, possess unique properties: a vast surface area responsible for frontline protection against inhaled pathogens but also simultaneous tight regulation of homeostasis against a continuous backdrop of non-pathogenic antigen exposure. Within the upper and lower respiratory tract, the nasal and bronchial associated lymphoid tissues (NALT and BALT, respectively) are key sites where antigen-specific immune responses are orchestrated against inhaled antigens, serving as critical training grounds for adaptive immunity. Many infectious diseases are transmitted via respiratory mucosal sites, highlighting the need for vaccines that can activate resident frontline immune protection in these tissues to block infection. While traditional parenteral vaccines that are injected tend to elicit weak immunity in mucosal tissues, mucosal vaccines (i.e., that are administered intranasally) are capable of eliciting both systemic and mucosal immunity in tandem by initiating immune responses in the MALT. In contrast, administering antigen to mucosal tissues in the absence of adjuvant or costimulatory signals can instead induce antigen-specific tolerance by exploiting regulatory mechanisms inherent to MALT, holding potential for mucosal immunotherapies to treat autoimmunity. Yet despite being well motivated by mucosal biology, development of both mucosal subunit vaccines and immunotherapies has historically been plagued by poor drug delivery across mucosal barriers, resulting in weak efficacy, short-lived responses, and to-date a lack of clinical translation. Development of engineering strategies that can overcome barriers to mucosal delivery are thus critical for translation of mucosal subunit vaccines and immunotherapies. This review covers engineering strategies to enhance mucosal uptake via active targeting and passive transport mechanisms, with a parallel focus on mechanisms of immune activation and regulation in the respiratory mucosa. By combining engineering strategies for enhanced mucosal delivery with a better understanding of immune mechanisms in the NALT and BALT, we hope to illustrate the potential of these mucosal sites as targets for immunomodulation.

What's going on with measles?

Measles is a highly transmissible systemic viral infection associated with substantial mortality primarily due to secondary infections. Measles induces lifelong immunity to reinfection but loss of immunity to other pathogens. An attenuated live virus vaccine is highly effective, but lapses in delivery have resulted in increasing cases worldwide. Although the primary cause of failure to control measles is failure to vaccinate, waning vaccine-induced immunity and the possible emergence of more virulent virus strains may also contribute.

Exploiting hosts and vectors: viral strategies for facilitating transmission

Viruses have developed various strategies to ensure their survival and transmission. One intriguing strategy involves manipulating the behavior of infected arthropod vectors and hosts. Through intricate interactions, viruses can modify vector behavior, aiding in crossing barriers and improving transmission to new hosts. This manipulation may include altering vector feeding preferences, thus promoting virus transmission to susceptible individuals. In addition, viruses employ diverse dissemination methods, including cell-to-cell and intercellular transmission via extracellular vesicles. These strategies allow viruses to establish themselves in favorable environments, optimize replication, and increase the likelihood of spreading to other individuals. Understanding these complex viral strategies offers valuable insights into their biology, transmission dynamics, and potential interventions for controlling infections. Unraveling interactions between viruses, hosts, and vectors enables the development of targeted approaches to effectively mitigate viral diseases and prevent transmission.

A Quantitative Fusion Assay to Study Measles Virus Entry

We have adopted a real-time assay based on a dual-split reporter to assess cell-cell fusion mediated by the measles virus (MeV) membrane fusion machinery. This reporter system is comprised of two expression vectors, each encoding a segment of Renilla luciferase fused to a segment of GFP. To regain function, the two segments need to associate, which is dependent on cell-cell fusion between effector cells expressing the MeV fusion machinery and target cells expressing the corresponding MeV receptor. By measuring reconstituted luciferase activity, we can follow the kinetics of cell-cell fusion and quantify the extent of fusion. This assay lends itself to the study of the MeV fusion machinery comprised of the attachment and fusion glycoproteins, the matrix protein, and the MeV receptors. Moreover, entry inhibitors targeting attachment or fusion can be readily screened using this assay. Finally, this assay can be easily adopted to study the entry of other members of the Paramyxoviridae, as we have demonstrated for the henipaviruses.

Culturing Immortalized Human Airway Epithelial Cells at an Air-Liquid Interface for Measles Virus Infection

Measles virus (MeV) infection of airway surface epithelial cells provides a site for final amplification before being released back into the environment via coughing and sneezing. Multiple cell lines have served as models of polarized epithelia for MeV infection, such as Caco2 cells (intestinal derived human epithelia) or MDCK cells (kidney derived canine epithelia). In this chapter, we describe the materials and air-liquid interface (ALI) culture conditions for maintaining four different cell lines derived from human airway epithelial cells: 16HBE14o-, Calu-3, H358, and NuLi-1. We provide methods for confirming transepithelial electrical resistance (TER) and preparing samples for microscopy as well as expected results from apical or basolateral MeV delivery. Polarized human airway derived cells serve as tissue culture models for investigating targeted questions about how MeV exits a human host. In addition, these methods are generalizable to studies of other respiratory viruses or the biology of ALI airway epithelial cells.

Intercellular Transport of Viral Proteins

Viruses are vehicles to exchange genetic information and proteins between cells and organisms by infecting their target cells either cell-free, or depending on cell-cell contacts. Several viruses like certain retroviruses or herpesviruses transmit by both mechanisms. However, viruses have also evolved the properties to exchange proteins between cells independent of viral particle formation. This exchange of viral proteins can be directed to target cells prior to infection to interfere with restriction factors and intrinsic immunity, thus, making the target cell prone to infection. However, also bystander cells, e.g. immune cell populations, can be targeted by viral proteins to dampen antiviral responses. Mechanistically, viruses exploit several routes of cell-cell communication to exchange viral proteins like the formation of extracellular vesicles or the formation of long-distance connections like tunneling nanotubes. Although it is known that viral nucleic acids can be transferred between cells as well, this chapter concentrates on viral proteins of human pathogenic viruses covering all Baltimore classes and summarizes our current knowledge on intercellular transport of viral proteins between cells.

Representative measles virus infection requires appropriate airway epithelia culture conditions

IMPORTANCE

For many years, measles virus (MeV) was assumed to first enter the host via the apical surface of airway epithelial cells and subsequently spread systemically. We and others reported that MeV has an overwhelming preference for entry at the basolateral surface of airway epithelial cells, which led to a fundamental new understanding of how MeV enters a human host. This unexpected observation using well-differentiated primary cultures of airway epithelia from human donors contradicted previous studies using immortalized cultured cells. Here, we show that appropriate differentiation and cell morphology of primary human airway epithelial cells are critical to recapitulate MeV infection patterns and pathogenesis of the in vivo airways. By simply culturing primary cells in media containing serum or passaging primary cultures, erroneous results quickly emerge. These results have broad implications for data interpretation related to respiratory virus infection, spread, and release from human airway epithelial cells.

Interferon-independent processes constrain measles virus cell-to-cell spread in primary human airway epithelial cells

Amplification of measles virus (MeV) in human airway epithelia may contribute to its extremely high contagious nature. We use well-differentiated primary cultures of human airway epithelial cells (HAE) to model ex vivo how MeV spreads in human airways. In HAE, MeV spreads cell-to-cell for 3-5 days, but then, infectious center growth is arrested. What stops MeV spread in HAE is not understood, but interferon (IFN) is known to slow MeV spread in other in vitro and in vivo models. Here, we assessed the role of type I and type III IFN in arresting MeV spread in HAE. The addition of IFN-β or IFN-λ1 to the medium of infected HAE slowed MeV infectious center growth, but when IFN receptor signaling was blocked, infectious center size was not affected. In contrast, blocking type-I IFN receptor signaling enhanced respiratory syncytial virus spread. HAE were also infected with MeV mutants defective for the V protein. The V protein has been demonstrated to interact with both MDA5 and STAT2 to inhibit activation of innate immunity; however, innate immune reactions were unexpectedly muted against the V-defective MeV in HAE. Minimal innate immunity activation was confirmed by deep sequencing, quantitative RT-PCR, and single-cell RNA-seq analyses of the transcription of IFN and IFN-stimulated genes. We conclude that in HAE, IFN-signaling can contribute to slowing infectious center growth; however, IFN-independent processes are most important for limiting cell-to-cell spread. IMPORTANCE Fundamental biological questions remain about the highly contagious measles virus (MeV). MeV amplifies within airway epithelial cells before spreading to the next host. This final step likely contributes to the ability of MeV to spread host-to-host. Over the course of 3-5 days post-infection of airway epithelial cells, MeV spreads directly cell-to-cell and forms infectious centers. Infectious center formation is unique to MeV. In this study, we show that interferon (IFN) signaling does not explain why MeV cell-to-cell spread is ultimately impeded within the cell layer. The ability of MeV to spread cell-to-cell in airway cells without appreciable IFN induction may contribute to its highly contagious nature. This study contributes to the understanding of a significant global health concern by demonstrating that infectious center formation occurs independent of the simplest explanation for limiting viral transmission within a host.

Establishment of a CPER reverse genetics system for Powassan virus defines attenuating NS1 glycosylation sites and an infectious NS1-GFP11 reporter virus

Powassan virus (POWV) is an emerging tick-borne Flavivirus that causes lethal encephalitis and long-term neurologic damage. Currently, there are no POWV therapeutics, licensed vaccines, or reverse genetics systems for producing infectious POWVs from recombinant DNA. Using a circular polymerase extension reaction (CPER), we generated recombinant LI9 (recLI9) POWVs with attenuating NS1 protein mutations and a recLI9-split-eGFP reporter virus. NS1 proteins are highly conserved glycoproteins that regulate replication, spread, and neurovirulence. POWV NS1 contains three putative N-linked glycosylation sites that we modified individually in infectious recLI9 mutants (N85Q, N208Q, and N224Q). NS1 glycosylation site mutations reduced replication kinetics and were attenuated, with 1-2 log decreases in titer. Severely attenuated recLI9-N224Q exhibited a 2- to 3-day delay in focal cell-to-cell spread and reduced NS1 secretion but was lethal when intracranially inoculated into suckling mice. However, footpad inoculation of recLI9-N224Q resulted in the survival of 80% of mice and demonstrated that NS1-N224Q mutations reduce POWV neuroinvasion in vivo. To monitor NS1 trafficking, we CPER fused a split GFP11-tag to the NS1 C-terminus and generated an infectious reporter virus, recLI9-NS1-GFP11. Cells infected with recLI9-NS1-GFP11 revealed NS1 trafficking in live cells and the novel formation of large NS1-lined intracellular vesicles. An infectious recLI9-NS1-GFP11 reporter virus permits real-time analysis of NS1 functions in POWV replication, assembly, and secretion and provides a platform for evaluating antiviral compounds. Collectively, our robust POWV reverse genetics system permits analysis of viral spread and neurovirulence determinants in vitro and in vivo and enables the rational genetic design of live attenuated POWV vaccines. IMPORTANCE Our findings newly establish a mechanism for genetically modifying Powassan viruses (POWVs), systematically defining pathogenic determinants and rationally designing live attenuated POWV vaccines. This initial study demonstrates that mutating POWV NS1 glycosylation sites attenuates POWV spread and neurovirulence in vitro and in vivo. Our findings validate a robust circular polymerase extension reaction approach as a mechanism for developing, and evaluating, attenuated genetically modified POWVs. We further designed an infectious GFP-tagged reporter POWV that permits us to monitor secretory trafficking of POWV in live cells, which can be applied to screen potential POWV replication inhibitors. This robust system for modifying POWVs provides the ability to define attenuating POWV mutations and create genetically attenuated recPOWV vaccines.

Overcoming the Barrier of the Respiratory Epithelium during Canine Distemper Virus Infection

Canine distemper virus (CDV) is a highly contagious pathogen and is known to enter the host via the respiratory tract and disseminate to various organs. Current hypotheses speculate that CDV uses the homologous cellular receptors of measles virus (MeV), SLAM and nectin-4, to initiate the infection process. For validation, here, we established the well-differentiated air-liquid interface (ALI) culture model from primary canine tracheal airway epithelial cells. By applying the green fluorescent protein (GFP)-expressing CDV vaccine strain and recombinant wild-type viruses, we show that cell-free virus infects the airway epithelium mainly via the paracellular route and only after prior disruption of tight junctions by pretreatment with EGTA; this infection was related to nectin-4 but not to SLAM. Remarkably, when CDV-preinfected DH82 cells were cocultured on the basolateral side of canine ALI cultures grown on filter supports with a 1.0-μm pore size, cell-associated CDV could be transmitted via cell-to-cell contact from immunocytes to airway epithelial cultures. Finally, we observed that canine ALI cultures formed syncytia and started to release cell-free infectious viral particles from the apical surface following treatment with an inhibitor of the JAK/STAT signaling pathway (ruxolitinib). Our findings show that CDV can overcome the epithelial barrier through different strategies, including infection via immunocyte-mediated transmission and direct infection via the paracellular route when tight junctions are disrupted. Our established model can be adapted to other animals for studying the transmission routes and the pathogenicity of other morbilliviruses. IMPORTANCE Canine distemper virus (CDV) is not only an important pathogen of carnivores, but it also serves as a model virus for analyzing measles virus pathogenesis. To get a better picture of the different stages of infection, we used air-liquid interface cultures to analyze the infection of well-differentiated airway epithelial cells by CDV. Applying a coculture approach with DH82 cells, we demonstrated that cell-mediated infection from the basolateral side of well-differentiated epithelial cells is more efficient than infection via cell-free virus. In fact, free virus was unable to infect intact polarized cells. When tight junctions were interrupted by treatment with EGTA, cells became susceptible to infection, with nectin-4 serving as a receptor. Another interesting feature of CDV infection is that infection of well-differentiated airway epithelial cells does not result in virus egress. Cell-free virions are released from the cells only in the presence of an inhibitor of the JAK/STAT signaling pathway. Our results provide new insights into how CDV can overcome the barrier of the airway epithelium and reveal similarities and some dissimilarities compared to measles virus.

The ACE2 Receptor for Coronavirus Entry Is Localized at Apical Cell—Cell Junctions of Epithelial Cells

Transmembrane proteins of adherens and tight junctions are known targets for viruses and bacterial toxins. The coronavirus receptor ACE2 has been localized at the apical surface of epithelial cells, but it is not clear whether ACE2 is localized at apical Cell—Cell junctions and whether it associates with junctional proteins. Here we explored the expression and localization of ACE2 and its association with transmembrane and tight junction proteins in epithelial tissues and cultured cells by data mining, immunoblotting, immunofluorescence microscopy, and co-immunoprecipitation experiments. ACE2 mRNA is abundant in epithelial tissues, where its expression correlates with the expression of the tight junction proteins cingulin and occludin. In cultured epithelial cells ACE2 mRNA is upregulated upon differentiation and ACE2 protein is widely expressed and co-immunoprecipitates with the transmembrane proteins ADAM17 and CD9. We show by immunofluorescence microscopy that ACE2 colocalizes with ADAM17 and CD9 and the tight junction protein cingulin at apical junctions of intestinal (Caco-2), mammary (Eph4) and kidney (mCCD) epithelial cells. These observations identify ACE2, ADAM17 and CD9 as new epithelial junctional transmembrane proteins and suggest that the cytokine-enhanced endocytic internalization of junction-associated protein complexes comprising ACE2 may promote coronavirus entry.

Powassan Viruses Spread Cell to Cell during Direct Isolation from Ixodes Ticks and Persistently Infect Human Brain Endothelial Cells and Pericytes

Powassan viruses (POWVs) are neurovirulent tick-borne flaviviruses emerging in the northeastern United States, with a 2% prevalence in Long Island (LI) deer ticks (Ixodes scapularis). POWVs are transmitted within as little as 15 min of a tick bite and enter the central nervous system (CNS) to cause encephalitis (10% of cases are fatal) and long-term neuronal damage. POWV-LI9 and POWV-LI41 present in LI Ixodes ticks were isolated by directly inoculating VeroE6 cells with tick homogenates and detecting POWV-infected cells by immunoperoxidase staining. Inoculated POWV-LI9 and LI41 were exclusively present in infected cell foci, indicative of cell to cell spread, despite growth in liquid culture without an overlay. Cloning and sequencing establish POWV-LI9 as a phylogenetically distinct lineage II POWV strain circulating in LI deer ticks. Primary human brain microvascular endothelial cells (hBMECs) and pericytes form a neurovascular complex that restricts entry into the CNS. We found that POWV-LI9 and -LI41 and lineage I POWV-LB productively infect hBMECs and pericytes and that POWVs were basolaterally transmitted from hBMECs to lower-chamber pericytes without permeabilizing polarized hBMECs. Synchronous POWV-LI9 infection of hBMECs and pericytes induced proinflammatory chemokines, interferon-β (IFN-β) and proteins of the IFN-stimulated gene family (ISGs), with delayed IFN-β secretion by infected pericytes. IFN inhibited POWV infection, but despite IFN secretion, a subset of POWV-infected hBMECs and pericytes remained persistently infected. These findings suggest a potential mechanism for POWVs (LI9/LI41 and LB) to infect hBMECs, spread basolaterally to pericytes, and enter the CNS. hBMEC and pericyte responses to POWV infection suggest a role for immunopathology in POWV neurovirulence and potential therapeutic targets for preventing POWV spread to neuronal compartments. IMPORTANCE We isolated POWVs from LI deer ticks (I. scapularis) directly in VeroE6 cells, and sequencing revealed POWV-LI9 as a distinct lineage II POWV strain. Remarkably, inoculation of VeroE6 cells with POWV-containing tick homogenates resulted in infected cell foci in liquid culture, consistent with cell-to-cell spread. POWV-LI9 and -LI41 and lineage I POWV-LB strains infected hBMECs and pericytes that comprise neurovascular complexes. POWVs were nonlytically transmitted basolaterally from infected hBMECs to lower-chamber pericytes, suggesting a mechanism for POWV transmission across the blood-brain barrier (BBB). POWV-LI9 elicited inflammatory responses from infected hBMEC and pericytes that may contribute to immune cell recruitment and neuropathogenesis. This study reveals a potential mechanism for POWVs to enter the CNS by infecting hBMECs and spreading basolaterally to abluminal pericytes. Our findings reveal that POWV-LI9 persists in cells that form a neurovascular complex spanning the BBB and suggest potential therapeutic targets for preventing POWV spread to neuronal compartments.

C Proteins: Controllers of Orderly Paramyxovirus Replication and of the Innate Immune Response

Particles of many paramyxoviruses include small amounts of proteins with a molecular weight of about 20 kDa. These proteins, termed “C”, are basic, have low amino acid homology and some secondary structure conservation. C proteins are encoded in alternative reading frames of the phosphoprotein gene. Some viruses express nested sets of C proteins that exert their functions in different locations: In the nucleus, they interfere with cellular transcription factors that elicit innate immune responses; in the cytoplasm, they associate with viral ribonucleocapsids and control polymerase processivity and orderly replication, thereby minimizing the activation of innate immunity. In addition, certain C proteins can directly bind to, and interfere with the function of, several cytoplasmic proteins required for interferon induction, interferon signaling and inflammation. Some C proteins are also required for efficient virus particle assembly and budding. C-deficient viruses can be grown in certain transformed cell lines but are not pathogenic in natural hosts. C proteins affect the same host functions as other phosphoprotein gene-encoded proteins named V but use different strategies for this purpose. Multiple independent systems to counteract host defenses may ensure efficient immune evasion and facilitate virus adaptation to new hosts and tissue environments.

Rab11a mediates cell-cell spread and reassortment of influenza A virus genomes via tunneling nanotubes

Influenza A virus [IAV] genomes comprise eight negative strand RNAs packaged into virions in the form of viral ribonucleoproteins [vRNPs]. Rab11a plays a crucial role in the transport of vRNPs from the nucleus to the plasma membrane via microtubules, allowing assembly and virus production. Here, we identify a novel function for Rab11a in the inter-cellular transport of IAV vRNPs using tunneling nanotubes [TNTs]as molecular highways. TNTs are F-Actin rich tubules that link the cytoplasm of nearby cells. In IAV-infected cells, Rab11a was visualized together with vRNPs in these actin-rich intercellular connections. To better examine viral spread via TNTs, we devised an infection system in which conventional, virion-mediated, spread was not possible. Namely, we generated HA-deficient reporter viruses which are unable to produce progeny virions but whose genomes can be replicated and trafficked. In this system, vRNP transfer to neighboring cells was observed and this transfer was found to be dependent on both actin and Rab11a. Generation of infectious virus via TNT transfer was confirmed using donor cells infected with HA-deficient virus and recipient cells stably expressing HA protein. Mixing donor cells infected with genetically distinct IAVs furthermore revealed the potential for Rab11a and TNTs to serve as a conduit for genome mixing and reassortment in IAV infections. These data therefore reveal a novel role for Rab11a in the IAV life cycle, which could have significant implications for within-host spread, genome reassortment and immune evasion.

Measles virus exits human airway epithelia within dislodged metabolically active infectious centers

Measles virus (MeV) is the most contagious human virus. Unlike most respiratory viruses, MeV does not directly infect epithelial cells upon entry in a new host. MeV traverses the epithelium within immune cells that carry it to lymphatic organs where amplification occurs. Infected immune cells then synchronously deliver large amounts of virus to the airways. However, our understanding of MeV replication in airway epithelia is limited. To model it, we use well-differentiated primary cultures of human airway epithelial cells (HAE) from lung donors. In HAE, MeV spreads directly cell-to-cell forming infectious centers that grow for ~3–5 days, are stable for a few days, and then disappear. Transepithelial electrical resistance remains intact during the entire course of HAE infection, thus we hypothesized that MeV infectious centers may dislodge while epithelial function is preserved. After documenting by confocal microscopy that infectious centers progressively detach from HAE, we recovered apical washes and separated cell-associated from cell-free virus by centrifugation. Virus titers were about 10 times higher in the cell-associated fraction than in the supernatant. In dislodged infectious centers, ciliary beating persisted, and apoptotic markers were not readily detected, suggesting that they retain functional metabolism. Cell-associated MeV infected primary human monocyte-derived macrophages, which models the first stage of infection in a new host. Single-cell RNA sequencing identified wound healing, cell growth, and cell differentiation as biological processes relevant for infectious center dislodging. 5-ethynyl-2’-deoxyuridine (EdU) staining located proliferating cells underneath infectious centers. Thus, cells located below infectious centers divide and differentiate to repair the dislodged infected epithelial patch. As an extension of these studies, we postulate that expulsion of infectious centers through coughing and sneezing could contribute to MeV’s strikingly high reproductive number by allowing the virus to survive longer in the environment and by delivering a high infectious dose to the next host.

Measles virus (MeV) is a respiratory pathogen that infects millions worldwide each year. Although sometimes mischaracterized as an innocuous childhood disease, measles remains a leading cause of death for children under five. MeV is the most contagious human virus and requires vaccination rates above 90% to maintain herd immunity. Global decreases in vaccination rates over the past ten years contributed to recent, widespread MeV outbreaks. We uncover here a novel mechanism by which MeV exits the human airways that may explain why it is much more contagious than other viruses. We document that infected cells containing cell-associated virus detach en masse from the airway epithelial sheet. These dislodged infectious centers are metabolically active and can transmit infection to primary human monocyte-derived macrophages via cell-cell contact as efficiently as cell-free virus particles. Thus, cell-associated MeV could spread host-to-host and is a potentially vital strategy for efficient respiratory virus transmission.

Different Neutralization Sensitivity of SARS-CoV-2 Cell-to-Cell and Cell-Free Modes of Infection to Convalescent Sera

The COVID-19 pandemic caused by SARS-CoV-2 has posed a global threat to human lives and economics. One of the best ways to determine protection against the infection is to quantify the neutralizing activity of serum antibodies. Multiple assays have been developed to validate SARS-CoV-2 neutralization; most of them utilized lentiviral or vesicular stomatitis virus-based particles pseudotyped with the spike (S) protein, making them safe and acceptable to work with in many labs. However, these systems are only capable of measuring infection with purified particles. This study has developed a pseudoviral assay with replication-dependent reporter vectors that can accurately quantify the level of infection directly from the virus producing cell to the permissive target cell. Comparative analysis of cell-free and cell-to-cell infection revealed that the neutralizing activity of convalescent sera was more than tenfold lower in cell cocultures than in the cell-free mode of infection. As the pseudoviral system could not properly model the mechanisms of SARS-CoV-2 transmission, similar experiments were performed with replication-competent coronavirus, which detected nearly complete SARS-CoV-2 cell-to-cell infection resistance to neutralization by convalescent sera. These findings suggest that the cell-to-cell mode of SARS-CoV-2 transmission, for which the mechanisms are largely unknown, could be of great importance for treatment and prevention of COVID-19.

Transcriptional profiling reveals potential involvement of microvillous TRPM5-expressing cells in viral infection of the olfactory epithelium

Background

Understanding viral infection of the olfactory epithelium is essential because the olfactory nerve is an important route of entry for viruses to the central nervous system. Specialized chemosensory epithelial cells that express the transient receptor potential cation channel subfamily M member 5 (TRPM5) are found throughout the airways and intestinal epithelium and are involved in responses to viral infection.

Results

Herein we performed deep transcriptional profiling of olfactory epithelial cells sorted by flow cytometry based on the expression of mCherry as a marker for olfactory sensory neurons and for eGFP in OMP-H2B::mCherry/TRPM5-eGFP transgenic mice (Mus musculus). We find profuse expression of transcripts involved in inflammation, immunity and viral infection in TRPM5-expressing microvillous cells compared to olfactory sensory neurons.

Conclusion

Our study provides new insights into a potential role for TRPM5-expressing microvillous cells in viral infection of the olfactory epithelium. We find that, as found for solitary chemosensory cells (SCCs) and brush cells in the airway epithelium, and for tuft cells in the intestine, the transcriptome of TRPM5-expressing microvillous cells indicates that they are likely involved in the inflammatory response elicited by viral infection of the olfactory epithelium.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07528-y.

Primary differentiated respiratory epithelial cells respond to apical measles virus infection by shedding multinucleated giant cells

Measles virus (MeV) is highly infectious by the respiratory route and remains an important cause of childhood mortality. However, the process by which MeV infection is efficiently established in the respiratory tract is controversial with suggestions that respiratory epithelial cells are not susceptible to infection from the apical mucosal surface. Therefore, it has been hypothesized that infection is initiated in lung macrophages or dendritic cells and that epithelial infection is subsequently established through the basolateral surface by infected lymphocytes. To better understand the process of respiratory tract initiation of MeV infection, primary differentiated respiratory epithelial cell cultures were established from rhesus macaque tracheal and nasal tissues. Infection of these cultures with MeV from the apical surface was more efficient than from the basolateral surface with shedding of viable MeV-producing multinucleated giant cell (MGC) syncytia from the surface. Despite presence of MGCs and infectious virus in supernatant fluids after apical infection, infected cells were not detected in the adherent epithelial sheet and transepithelial electrical resistance was maintained. After infection from the basolateral surface, epithelial damage and large clusters of MeV-positive cells were observed. Treatment with fusion inhibitory peptides showed that MeV production after apical infection was not dependent on infection of the basolateral surface. These results are consistent with the hypothesis that MeV infection is initiated by apical infection of respiratory epithelial cells with subsequent infection of lymphoid tissue and systemic spread.

Transcriptional profiling reveals potential involvement of microvillous TRPM5-expressing cells in viral infection of the olfactory epithelium

Background

Understanding viral infection of the olfactory epithelium is essential because the olfactory nerve is an important route of entry for viruses to the central nervous system. Specialized chemosensory epithelial cells that express the transient receptor potential cation channel subfamily M member 5 (TRPM5) are found throughout the airways and intestinal epithelium and are involved in responses to viral infection.

Results

Herein we performed deep transcriptional profiling of olfactory epithelial cells sorted by flow cytometry based on the expression of mCherry as a marker for olfactory sensory neurons and for eGFP in OMP-H2B::mCherry/TRPM5-eGFP transgenic mice ( Mus musculus ). We find profuse expression of transcripts involved in inflammation, immunity and viral infection in TRPM5-expressing microvillous cells.

Conclusion

Our study provides new insights into a potential role for TRPM5-expressing microvillous cells in viral infection of the olfactory epithelium. We find that, as found for solitary chemosensory cells (SCCs) and brush cells in the airway epithelium, and for tuft cells in the intestine, the transcriptome of TRPM5-expressing microvillous cells indicates that they are likely involved in the inflammatory response elicited by viral infection of the olfactory epithelium.

Viral cell-to-cell spread: Conventional and non-conventional ways

A critical step in the life cycle of a virus is spread to a new target cell, which generally involves the release of new viral particles from the infected cell which can then initiate infection in the next target cell. While cell-free viral particles released into the extracellular environment are necessary for long distance spread, there are disadvantages to this mechanism. These include the presence of immune system components, the low success rate of infection by single particles, and the relative fragility of viral particles in the environment. Several mechanisms of direct cell-to-cell spread have been reported for animal viruses which would avoid the issues associated with cell-free particles. A number of viruses can utilize several different mechanisms of direct cell-to-cell spread, but our understanding of the differential usage by these pathogens is modest. Although the mechanisms of cell-to-cell spread differ among viruses, there is a common exploitation of key pathways and components of the cellular cytoskeleton. Remarkably, some of the viral mechanisms of cell-to-cell spread are surprisingly similar to those used by bacteria. Here we summarize the current knowledge of the conventional and non-conventional mechanisms of viral spread, the common methods used to detect viral spread, and the impact that these mechanisms can have on viral pathogenesis.

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.

The alpha-1 subunit of the Na+,K+-ATPase (ATP1A1) is required for macropinocytic entry of respiratory syncytial virus (RSV) in human respiratory epithelial cells

Human respiratory syncytial virus (RSV) is the leading viral cause of acute pediatric lower respiratory tract infections worldwide, with no available vaccine or effective antiviral drug. To gain insight into virus-host interactions, we performed a genome-wide siRNA screen. The expression of over 20,000 cellular genes was individually knocked down in human airway epithelial A549 cells, followed by infection with RSV expressing green fluorescent protein (GFP). Knockdown of expression of the cellular ATP1A1 protein, which is the major subunit of the Na⁺,K⁺-ATPase of the plasma membrane, had one of the strongest inhibitory effects on GFP expression and viral titer. Inhibition was not observed for vesicular stomatitis virus, indicating that it was RSV-specific rather than a general effect. ATP1A1 formed clusters in the plasma membrane very early following RSV infection, which was independent of replication but dependent on the attachment glycoprotein G. RSV also triggered activation of ATP1A1, resulting in signaling by c-Src-kinase activity that transactivated epidermal growth factor receptor (EGFR) by Tyr845 phosphorylation. ATP1A1 signaling and activation of both c-Src and EGFR were found to be required for efficient RSV uptake. Signaling events downstream of EGFR culminated in the formation of macropinosomes. There was extensive uptake of RSV virions into macropinosomes at the beginning of infection, suggesting that this is a major route of RSV uptake, with fusion presumably occurring in the macropinosomes rather than at the plasma membrane. Important findings were validated in primary human small airway epithelial cells (HSAEC). In A549 cells and HSAEC, RSV uptake could be inhibited by the cardiotonic steroid ouabain and the digitoxigenin derivative PST2238 (rostafuroxin) that bind specifically to the ATP1A1 extracellular domain and block RSV-triggered EGFR Tyr845 phosphorylation. In conclusion, we identified ATP1A1 as a host protein essential for macropinocytic entry of RSV into respiratory epithelial cells, and identified PST2238 as a potential anti-RSV drug.

RSV continues to be the most important viral cause of severe bronchiolitis and pneumonia in infants and young children, and also has a substantial impact in the elderly. It is estimated to claim the lives of ~118,000 children under five years of age annually. No vaccine or antiviral drug suitable for general use is available. The involvement of host factors in RSV infection and replication is not well understood, but this knowledge might lead to intervention strategies to prevent infection. Using a genome-wide siRNA screen to knock down the expression of over 20,000 individual cellular genes, we identified ATP1A1, the major subunit of the Na⁺,K⁺-ATPase, as an important host protein for RSV entry. We showed that ATP1A1 activation by RSV resulted in transactivation of EGFR by Src-kinase activity, resulting in the uptake of RSV particles into the host cell through macropinocytosis. We also showed that the cardiotonic steroid ouabain and the synthetic digitoxigenin derivative PST2238, which bind specifically to the extracellular domain of ATP1A1, significantly reduced RSV entry. Taken together, we describe a novel ATP1A1-enabled mechanism used by RSV to enter the host cell, and describe candidate antiviral drugs that block this entry.

CD46 Null Packaging Cell Line Improves Measles Lentiviral Vector Production and Gene Delivery to Hematopoietic Stem and Progenitor Cells

Lentiviral vectors (LVs) pseudotyped with the measles virus hemagglutinin (H) and fusion (F) glycoproteins have been reported to more efficiently transduce hematopoietic stem and progenitor cells (HSPCs) compared with vesicular stomatitis virus glycoprotein (VSV-G) pseudotyped LVs. However, a limit to H/F LV use is the low titer of produced vector. Here we show that measles receptor (CD46) expression on H/F transfected HEK293T vector-producing cells caused adjacent cell membrane fusion, resulting in multinucleate syncytia formation and death prior to peak vector production, leading to contaminating cell membranes that co-purified with LV. H/F LVs produced in CD46 null HEK293T cells, generated by CRISPR/Cas9-mediated knockout of CD46, produced 2-fold higher titer vector compared with LVs produced in CD46+ HEK293T cells. This resulted in approximately 2- to 3-fold higher transduction of HSPCs while significantly reducing target cell cytotoxicity caused by producer cell contaminates. Improved H/F LV entry into HSPCs and distinct entry mechanisms compared with VSV-G LV were also observed by confocal microscopy. Given that vector production is a major source of cost and variability in clinical trials of gene therapy, we propose that the use of CD46 null packaging cells may help to address these challenges.

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.

Authentic Modeling of Human Respiratory Virus Infection in Human Pluripotent Stem Cell-Derived Lung Organoids

Respiratory viruses are among the first pathogens encountered by young children, and the significant impact of these viral infections on the developing lung is poorly understood. Circulating viruses are suited to the environment of the human lung and are different from those of viruses grown in cultured cells. We modeled respiratory virus infections that occur in children or infect the distal lung using lung organoids that represent the entire developing infant lung. These 3D lung organoids, derived from human pluripotent stem cells, develop into branching airway and alveolar structures and provide a tissue environment that maintains the authentic viral genome. The lung organoids can be genetically engineered prior to differentiation, thereby generating tissues bearing or lacking specific features that may be relevant to viral infection, a feature that may have utility for the study of host-pathogen interaction for a range of lung pathogens.

Infectious viruses so precisely fit their hosts that the study of natural viral infection depends on host-specific mechanisms that affect viral infection. For human parainfluenza virus 3, a prevalent cause of lower respiratory tract disease in infants, circulating human viruses are genetically different from viruses grown in standard laboratory conditions; the surface glycoproteins that mediate host cell entry on circulating viruses are suited to the environment of the human lung and differ from those of viruses grown in cultured cells. Polarized human airway epithelium cultures have been used to represent the large, proximal airways of mature adult airways. Here we modeled respiratory virus infections that occur in children or infect the distal lung using lung organoids that represent the entire developing infant lung. These 3D lung organoids derived from human pluripotent stem cells contain mesoderm and pulmonary endoderm and develop into branching airway and alveolar structures. Whole-genome sequencing analysis of parainfluenza viruses replicating in the organoids showed maintenance of nucleotide identity, suggesting that no selective pressure is exerted on the virus in this tissue. Infection with parainfluenza virus led to viral shedding without morphological changes, while respiratory syncytial virus infection induced detachment and shedding of infected cells into the lung organoid lumens, reminiscent of parainfluenza and respiratory syncytial virus in human infant lungs. Measles virus infection, in contrast, induced syncytium formation. These human stem cell-derived lung organoids may serve as an authentic model for respiratory viral pathogenesis in the developing or infant lung, recapitulating respiratory viral infection in the host.

Stronger together: Multi-genome transmission of measles virus

Measles virus (MeV) is an immunosuppressive, extremely contagious RNA virus that remains a leading cause of death among children. MeV is dual-tropic: it replicates first in lymphatic tissue, causing immunosuppression, and then in epithelial cells of the upper airways, accounting for extremely efficient contagion. Efficient contagion is counter-intuitive because the enveloped MeV particles are large and relatively unstable. However, MeV particles can contain multiple genomes, which can code for proteins with different functional characteristics. These proteins can cooperate to promote virus spread in tissue culture, prompting the question of whether multi-genome MeV transmission may promote efficient MeV spread also in vivo. Consistent with this hypothesis, in well-differentiated primary human airway epithelia large genome populations spread rapidly through intercellular pores. In another line of research, it was shown that distinct lymphocytic-adapted and epithelial-adapted genome populations exist; cyclical adaptation studies indicate that suboptimal variants in one environment may constitute a low frequency reservoir for adaptation to the other environment. Altogether, these observations suggest that, in humans, MeV spread relies on en bloc genome transmission, and that genomic diversity is instrumental for rapid MeV dissemination within hosts.

Claudin-1 inhibits human parainfluenza virus type 2 dissemination

Tight junctions enable epithelial cells to form physical barriers that act as an innate immune defense against respiratory infection. However, the involvement of tight junction molecules in paramyxovirus infections, which include various respiratory pathogens, has not been examined in detail. Human parainfluenza virus type 2 (hPIV2) infects airway epithelial cells and causes respiratory illness. In the present study, we found that hPIV2 infection of cultured cells induces expression of claudin-1 (CLDN1), an essential component of tight junctions. This induction seemed to be intrinsically restricted by V, an accessory protein that modulates various host responses, to enable efficient virus propagation. By generating CLDN1 over-expressing and knockout cell lines, we showed that CLDN1 is involved in the restriction of hPIV2 spread via cell-to-cell contact. Taken together, we identified CLDN1 an inhibitory factor for hPIV2 dissemination, and that its V protein acts to counter this.

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.

p53 regulates CD46 expression and measles virus infection in myeloma cells

In this study, we assessed the sensitivity of myeloma cells to the oncolytic measles virus (MV) in relation to p53 using 37 cell lines and 23 primary samples. We showed that infection and cell death were correlated with CD46 expression, which was associated with TP53 status; TP53 abn cell lines highly expressed CD46 and were preferentially infected by MV when compared with the TP53 wt cell lines (P = .046 and P = .045, respectively). Infection of myeloma cells was fully dependent on CD46 expression in both cell lines and primary cells. In the TP53 wt cell lines, but not the TP53 abn cell lines, activation of the p53 pathway with nutlin3a inhibited both CD46 expression and MV infection, while TP53 silencing reciprocally increased CD46 expression and MV infection. We showed using a p53 chromatin immunoprecipitation assay and microRNA assessment that CD46 gene expression was directly and indirectly regulated by p53. Primary myeloma cells overexpressed CD46 as compared with normal cells and were highly infected and killed by MV. CD46 expression and MV infection were inhibited by nutlin3a in primary p53-competent myeloma cells, but not in p53-deficient myeloma cells, and the latter were highly sensitive to MV infection. In summary, myeloma cells were highly sensitive to MV and infection inhibition by the p53 pathway was abrogated in p53-deficient myeloma cells. These results argue for an MV-based clinical trial for patients with p53 deficiency.

Upsetting the Balance: When Viruses Manipulate Cell Polarity Control

The central importance of cell polarity control is emphasized by the frequency with which it is targeted by many diverse viruses. It is clear that in targeting key polarity control proteins, viruses affect not only host cell polarity, but also influence many cellular processes, including transcription, replication, and innate and acquired immunity. Examination of the interactions of different virus proteins with the cell and its polarity controls during the virus life cycles, and in virally-induced cell transformation shows ever more clearly how intimately all cellular processes are linked to the control of cell polarity.

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 HSV-1 mechanisms of cell-to-cell spread and fusion are critically dependent on host PTP1B

All herpesviruses have mechanisms for passing through cell junctions, which exclude neutralizing antibodies and offer a clear path to neighboring, uninfected cells. In the case of herpes simplex virus type 1 (HSV-1), direct cell-to-cell transmission takes place between epithelial cells and sensory neurons, where latency is established. The spreading mechanism is poorly understood, but mutations in four different HSV-1 genes can dysregulate it, causing neighboring cells to fuse to produce syncytia. Because the host proteins involved are largely unknown (other than the virus entry receptor), we were intrigued by an earlier discovery that cells infected with wild-type HSV-1 will form syncytia when treated with salubrinal. A biotinylated derivative of this drug was used to pull down cellular complexes, which were analyzed by mass spectrometry. One candidate was a protein tyrosine phosphatase (PTP1B), and although it ultimately proved not to be the target of salubrinal, it was found to be critical for the mechanism of cell-to-cell spread. In particular, a highly specific inhibitor of PTP1B (CAS 765317-72-4) blocked salubrinal-induced fusion, and by itself resulted in a dramatic reduction in the ability of HSV-1 to spread in the presence of neutralizing antibodies. The importance of this phosphatase was confirmed in the absence of drugs by using PTP1B^-/- cells. Importantly, replication assays showed that virus titers were unaffected when PTP1B was inhibited or absent. Only cell-to-cell spread was altered. We also examined the effects of salubrinal and the PTP1B inhibitor on the four Syn mutants of HSV-1, and strikingly different responses were found. That is, both drugs individually enhanced fusion for some mutants and reduced fusion for others. PTP1B is the first host factor identified to be specifically required for cell-to-cell spread, and it may be a therapeutic target for preventing HSV-1 reactivation disease.

It is estimated that 67% of the global population is infected with herpes simplex virus type 1 (HSV-1). This virus resides in sensory neurons in a quiescent state but periodically reactivates, producing virus particles that travel down the axon to infect epithelial cells of the skin, where it can be transmitted to additional people. To avoid neutralizing antibodies, herpesviruses have evolved mechanisms for moving directly from one cell to another through their sites of intimate contact; however, the mechanism of cell-to-cell spread is poorly understood. Studies of HSV-1 mutants have implicated numerous viral proteins, but the necessary cellular factors are unknown except for the one that the virus uses to enter cells. Our experiments have identified a cellular enzyme (PTP1B, a tyrosine phosphatase) that is dispensable for the production of infectious virions but is critically important for the cell-to-cell spreading mechanism. Promising drugs targeting PTP1B have already been tested in early clinical trials for possible treatment of obesity and type-2 diabetes, and thus, our study may have immediate utility for attenuating HSV-1 reactivation disease in immunocompromised patients.

The role of spatial heterogeneity in the evolution of local and global infections of viruses

Viruses have two modes spread in a host body, one is to release infectious particles from infected cells (global infection) and the other is to infect directly from an infected cell to an adjacent cell (local infection). Since the mode of spread affects the evolution of life history traits, such as virulence, it is important to reveal what level of global and local infection is selected. Previous studies of the evolution of global and local infection have paid little attention to its dependency on the measures of spatial configuration. Here we show the evolutionarily stable proportion of global and local infection, and how it depends on the distribution of target cells. Using an epidemic model on a regular lattice, we consider the infection dynamics by pair approximation and check the evolutionarily stable strategy. We also conduct the Monte-Carlo simulation to observe evolutionary dynamics. We show that a higher local infection is selected as target cells become clustered. Surprisingly, the selected strategy depends not only on the degree of clustering but also the abundance of target cells per se.

Viruses such as human immunodeficiency virus and measles virus can spread through physical contact between infected and susceptible cells (cell-to-cell infection), as well as normal cell-free infection through virions. Some experimental evidences support the possibility that high ability of cell-to-cell infection is selected in the host. Since the mode of spread affects the evolution of life history traits, it is important to reveal what condition favors high ability of cell-to-cell infection. Here we address what level of cell-to-cell infection is selected in different target cell distributions. Analysis of ordinary differential equations that keep track of dynamics for spatial configuration of infected cells and the Monte-Carlo simulations show that higher proportion of local infection is selected as target cells become clustered. The selected strategy depends not only on the degree of clustering but also the abundance of target cells per se. Our results suggest viruses have more chances to evolve the ability of local infection in a host body than previously thought. In particular, this may explain the emergence of measles virus strains that gained the ability to infect the central nervous system.

Human metapneumovirus Induces Reorganization of the Actin Cytoskeleton for Direct Cell-to-Cell Spread

Paramyxovirus spread generally involves assembly of individual viral particles which then infect target cells. We show that infection of human bronchial airway cells with human metapneumovirus (HMPV), a recently identified paramyxovirus which causes significant respiratory disease, results in formation of intercellular extensions and extensive networks of branched cell-associated filaments. Formation of these structures is dependent on actin, but not microtubule, polymerization. Interestingly, using a co-culture assay we show that conditions which block regular infection by HMPV particles, including addition of neutralizing antibodies or removal of cell surface heparan sulfate, did not prevent viral spread from infected to new target cells. In contrast, inhibition of actin polymerization or alterations to Rho GTPase signaling pathways significantly decreased cell-to-cell spread. Furthermore, viral proteins and viral RNA were detected in intercellular extensions, suggesting direct transfer of viral genetic material to new target cells. While roles for paramyxovirus matrix and fusion proteins in membrane deformation have been previously demonstrated, we show that the HMPV phosphoprotein extensively co-localized with actin and induced formation of cellular extensions when transiently expressed, supporting a new model in which a paramyxovirus phosphoprotein is a key player in assembly and spread. Our results reveal a novel mechanism for HMPV direct cell-to-cell spread and provide insights into dissemination of respiratory viruses.

Human metapneumovirus (HMPV) is an important human respiratory pathogen that affects all age groups worldwide. There are currently no vaccines or treatments available for HMPV, and key aspects of its life cycle remain unknown. We examined the late events of the HMPV infection cycle in human bronchial epithelial cells. Our data demonstrate that HMPV infection leads to formation of unique structures, including intercellular extensions connecting cells, and large networks of branched cell-associated filaments. Viral modulation of the cellular cytoskeleton and cellular signaling pathways are important for formation of these structures. Our results are consistent with the intercellular extensions playing a role in direct spread of virus from cell-to-cell, potentially by transfer of virus genetic material without particle formation. We also show that the HMPV phosphoprotein localizes with actin and can promote membrane deformations, suggesting a novel role in viral assembly or spread for paramyxovirus phosphoproteins.

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.

Measles

Measles is an infectious disease in humans caused by the measles virus (MeV). Before the introduction of an effective measles vaccine, virtually everyone experienced measles during childhood. Symptoms of measles include fever and maculopapular skin rash accompanied by cough, coryza and/or conjunctivitis. MeV causes immunosuppression, and severe sequelae of measles include pneumonia, gastroenteritis, blindness, measles inclusion body encephalitis and subacute sclerosing panencephalitis. Case confirmation depends on clinical presentation and results of laboratory tests, including the detection of anti-MeV IgM antibodies and/or viral RNA. All current measles vaccines contain a live attenuated strain of MeV, and great progress has been made to increase global vaccination coverage to drive down the incidence of measles. However, endemic transmission continues in many parts of the world. Measles remains a considerable cause of childhood mortality worldwide, with estimates that >100,000 fatal cases occur each year. Case fatality ratio estimates vary from <0.01% in industrialized countries to >5% in developing countries. All six WHO regions have set goals to eliminate endemic transmission of MeV by achieving and maintaining high levels of vaccination coverage accompanied by a sensitive surveillance system. Because of the availability of a highly effective and relatively inexpensive vaccine, the monotypic nature of the virus and the lack of an animal reservoir, measles is considered a candidate for eradication.

Cell-to-Cell Contact and Nectin-4 Govern Spread of Measles Virus from Primary Human Myeloid Cells to Primary Human Airway Epithelial Cells

Measles is a highly contagious, acute viral illness. Immune cells within the airways are likely first targets of infection, and these cells traffic measles virus (MeV) to lymph nodes for amplification and subsequent systemic dissemination. Infected immune cells are thought to return MeV to the airways; however, the mechanisms responsible for virus transfer to pulmonary epithelial cells are poorly understood. To investigate this process, we collected blood from human donors and generated primary myeloid cells, specifically, monocyte-derived macrophages (MDMs) and dendritic cells (DCs). MDMs and DCs were infected with MeV and then applied to primary cultures of well-differentiated airway epithelial cells from human donors (HAE). Consistent with previous results obtained with free virus, infected MDMs or DCs were incapable of transferring MeV to HAE when applied to the apical surface. Likewise, infected MDMs or DCs applied to the basolateral surface of HAE grown on small-pore (0.4-μm) support membranes did not transfer virus. In contrast, infected MDMs and DCs applied to the basolateral surface of HAE grown on large-pore (3.0-μm) membranes successfully transferred MeV. Confocal microscopy demonstrated that MDMs and DCs are capable of penetrating large-pore membranes but not small-pore membranes. Further, by using a nectin-4 blocking antibody or recombinant MeV unable to enter cells through nectin-4, we demonstrated formally that transfer from immune cells to HAE occurs in a nectin-4-dependent manner. Thus, both infected MDMs and DCs rely on cell-to-cell contacts and nectin-4 to efficiently deliver MeV to the basolateral surface of HAE.

IMPORTANCE

Measles virus spreads rapidly and efficiently in human airway epithelial cells. This rapid spread is based on cell-to-cell contact rather than on particle release and reentry. Here we posit that MeV transfer from infected immune cells to epithelial cells also occurs by cell-to-cell contact rather than through cell-free particles. In addition, we sought to determine which immune cells transfer MeV infectivity to the human airway epithelium. Our studies are based on two types of human primary cells: (i) myeloid cells generated from donated blood and (ii) well-differentiated airway epithelial cells derived from donor lungs. We show that different types of myeloid cells, i.e., monocyte-derived macrophages and dendritic cells, transfer infection to airway epithelial cells. Furthermore, cell-to-cell contact is an important component of successful MeV transfer. Our studies elucidate a mechanism by which the most contagious human respiratory virus is delivered to the airway epithelium.

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.

Oncolytic Activity of a Recombinant Measles Virus, Blind to Signaling Lymphocyte Activation Molecule, Against Colorectal Cancer Cells

Oncolytic virotherapy is a distinctive antitumor therapy based on the cancer-cell-specific infectivity and killing activity of viruses, which exert a considerable antitumor effect with only a few treatments. Because colorectal cancer cells often acquire resistance to the molecular-targeted therapies and alternative treatments are called for, in this study, we evaluated the oncolytic activity against colorectal cancer cells of a recombinant measles virus (rMV-SLAMblind), which is blind to signaling lymphocytic activation molecule (SLAM) and infects target cells via nectin-4/poliovirus receptor-related 4 protein. We examined 10 cell lines including 8 cell lines that were resistant to epidermal-growth-factor-receptor (EGFR) targeted therapy. rMV-SLAMblind infected and lysed the nectin-4-positive cell lines dependently on nectin-4 expression, in spite of mutation in EGFR cascade. Tumour progression in xenograft models was also abrogated by the virus, and the infection of cancer cells in vivo by the virus was demonstrated with both flow cytometry and a histological analysis. Therefore, rMV-SLAMblind is considered a novel therapeutic agent for colorectal cancers, including those resistant to molecular-targeted therapies.