Mechanisms of enveloped virus entry into animal cells

Nanotechnology-Driven Strategy Against SARS-CoV-2: Pluronic F127-Based Nanomicelles with or Without Atazanavir Reduce Viral Replication in Calu-3 Cells

Despite extensive efforts, no highly effective antiviral molecule exists for treating moderate and severe COVID-19. Nanotechnology has emerged as a promising approach for developing novel drug delivery systems to enhance antiviral efficacy. Among these, polymeric nanomicelles improve the solubility, bioavailability, and cellular uptake of therapeutic agents. In this study, Pluronic F127-based nanomicelles were developed and evaluated for their antiviral activity against SARS-CoV-2. The nanomicelles, formulated using the direct dissolution method, exhibited an average size of 37.4 ± 8.01 nm and a polydispersity index (PDI) of 0.427 ± 0.01. Their antiviral efficacy was assessed in SARS-CoV-2-infected Vero E6 and Calu-3 cell models, where treatment with a 1:2 dilution inhibited viral replication by more than 90%. Cytotoxicity assays confirmed the nanomicelles were non-toxic to both cell lines after 72 h. In SARS-CoV-2-infected Calu-3 cells (human type II pneumocyte model), treatment with Pluronic F127-based nanomicelles containing atazanavir (ATV) significantly reduced viral replication, even under high MOI (2) and after 48 h, while also preventing IL-6 upregulation. To investigate their mechanism, viral pretreatment with nanomicelles showed no inhibitory effect. However, pre-exposure of Calu-3 cells led to significant viral replication reduction (>85% and >75% for 1:2 and 1:4 dilutions, respectively), as confirmed by transmission electron microscopy. These findings highlight Pluronic F127-based nanomicelles as a promising nanotechnology-driven strategy against SARS-CoV-2, reinforcing their potential for future antiviral therapies.

New Imidazolium Alkaloids with Broad Spectrum of Action from the Marine Bacterium Shewanella aquimarina

The continuous outbreak of drug-resistant bacterial and viral infections imposes the need to search for new drug candidates. Natural products from marine bacteria still inspire the design of pharmaceuticals. Indeed, marine bacteria have unique metabolic flexibility to inhabit each ecological niche, thus expanding their biosynthetic ability to assemble unprecedented molecules. The One-Strain-Many-Compounds approach and tandem mass spectrometry allowed the discovery of a Shewanella aquimarina strain as a source of novel imidazolium alkaloids via molecular networking. The alkaloid mixture was shown to exert bioactivities such as: (a) antibacterial activity against antibiotic-resistant Staphylococcus aureus clinical isolates at 100 µg/mL, (b) synergistic effects with tigecycline and linezolid, (c) restoration of MRSA sensitivity to fosfomycin, and (d) interference with the biofilm formation of S. aureus 6538 and MRSA. Moreover, the mixture showed antiviral activity against viruses with and without envelopes. Indeed, it inhibited the entry of coronavirus HcoV-229E and herpes simplex viruses into human cells and inactivated poliovirus PV-1 in post-infection assay at 200 µg/mL. Finally, at the same concentration, the fraction showed anthelminthic activity against Caenorhabditis elegans, causing 99% mortality after 48 h. The broad-spectrum activities of these compounds are partially due to their biosurfactant behavior and make them promising candidates for breaking down drug-resistant infectious diseases.

Zebrafish-based platform for emerging bio-contaminants and virus inactivation research

Emerging bio-contaminants such as viruses have affected health and environment settings of every country. Viruses are the minuscule entities resulting in severe contagious diseases like SARS, MERS, Ebola, and avian influenza. Recent epidemic like the SARS-CoV-2, the virus has undergone mutations strengthen them and allowing to escape from the remedies. Comprehensive knowledge of viruses is essential for the development of targeted therapeutic and vaccination treatments. Animal models mimicking human biology like non-human primates, rats, mice, and rabbits offer competitive advantage to assess risk of viral infections, chemical toxins, nanoparticles, and microbes. However, their economic maintenance has always been an issue. Furthermore, the redundancy of experimental results due to aforementioned aspects is also in examine. Hence, exploration for the alternative animal models is crucial for risk assessments. The current review examines zebrafish traits and explores the possibilities to monitor emerging bio-contaminants. Additionally, a comprehensive picture of the bio contaminant and virus particle invasion and abatement mechanisms in zebrafish and human cells is presented. Moreover, a zebrafish model to investigate the emerging viruses such as coronaviridae and poxviridae has been suggested.

Inhibition of Exchange Proteins Directly Activated by cAMP (EPAC) as a Strategy for Broad-Spectrum Antiviral Development

The recent SARS-CoV-2 and mpox outbreaks have highlighted the need to expand our arsenal of broad-spectrum antiviral agents for future pandemic preparedness. Host-directed antivirals are an important tool to accomplish this as they typically offer protection against a broader range of viruses than direct-acting antivirals and have a lower susceptibility to viral mutations that cause drug resistance. In this study, we investigate the Exchange Protein Activated by cAMP (EPAC) as a target for broad-spectrum antiviral therapy. We find that the EPAC-selective inhibitor, ESI-09 provides robust protection against a variety of viruses, including SARS-CoV-2 and Vaccinia (VACV) - an orthopoxvirus from the same family as mpox. We show, using a series of immunofluorescence experiments, that ESI-09 remodels the actin cytoskeleton through Rac1/Cdc42 GTPases and the Arp2/3 complex, impairing internalization of viruses that use clathrin-mediated endocytosis (e.g. VSV) or micropinocytosis (e.g. VACV). Additionally, we find that ESI-09 disrupts syncytia formation and inhibits cell-to-cell transmission of viruses such as measles and VACV. When administered to immune-deficient mice in an intranasal challenge model, ESI-09 protects mice from lethal doses of VACV and prevents formation of pox lesions. Altogether, our finding show that EPAC antagonists such as ESI-09 are promising candidates for broad-spectrum antiviral therapy that can aid in the fight against ongoing and future viral outbreaks.

Molecular Tweezers – a new class of potent broad-spectrum antivirals against enveloped viruses

A new supramolecular approach to broad spectrum antivirals utilizes host guest chemistry between molecular tweezers and lysine/arginine as well as choline. Basic amino acids in amyloid-forming SEVI peptides (semen-derived enhancers...

Overview of Neutralizing Antibodies and Their Potential in COVID-19

The antibody response to respiratory syndrome coronavirus 2 (SARS-CoV-2) has been a major focus of COVID-19 research due to its clinical relevance and importance in vaccine and therapeutic development. Neutralizing antibody (NAb) evaluations are useful for the determination of individual or herd immunity against SARS-CoV-2, vaccine efficacy, and humoral protective response longevity, as well as supporting donor selection criteria for convalescent plasma therapy. In the current manuscript, we review the essential concepts of NAbs, examining their concept, mechanisms of action, production, and the techniques used for their detection; as well as presenting an overview of the clinical use of antibodies in COVID-19.

Ubiquitination as an Important Host-Immune Response Strategy in Penaeid Shrimp: Inferences From Other Species

Shrimp aquaculture is an essential economic venture globally, but the industry faces numerous challenges, especially pathogenic infections. As invertebrates, shrimp rely mainly on their innate immune system for protection. An increasing number of studies have shown that ubiquitination plays a vital role in the innate immune response to microbial pathogens. As an important form of posttranslational modification (PTM), both hosts and pathogens have exploited ubiquitination and the ubiquitin system as an immune response strategy to outwit the other. This short review brings together recent findings on ubiquitination and how this PTM plays a critical role in immune modulation in penaeid shrimps. Key findings inferred from other species would help guide further studies on ubiquitination as an immune response strategy in shrimp-pathogen interactions.

Sarco/Endoplasmic Reticulum Ca2+ Transporting ATPase (SERCA) Modulates Autophagic, Inflammatory, and Mitochondrial Responses during Influenza A Virus Infection in Human Lung Cells

Influenza A virus is an important human pathogen causing significant morbidity and mortality. Numerous host factors and cellular responses are dysregulated during influenza A virus infection. This includes arrest of autophagic flux dependent on the influenza M2 ion channel, but little is known which host factors participate in this autophagic dysfunction. Sarco/endoplasmic reticulum calcium ATPase (SERCA) is known to regulate transport of calcium ions between the cytoplasm and the sarco/endoplasmic reticulum, and has been positively correlated with autophagic flux. Herein, we found that SERCA activity was suppressed in influenza A virus infected human lung cells (H1395) and that CDN1163, an activator of SERCA, restored autophagic flux and thus reduced autophagosome accumulation caused by the influenza A virus. Activating SERCA activity with CDN1163 also decreased expression of inflammatory cytokines and chemokines and attenuated mitochondrial dysfunction in IAV-infected H1395 cells. Conversely, SERCA inhibition or genetic ablation aggravated the autophagy dysfunction, mitochondria, and inflammatory responses in the cells infected with influenza A virus. Further study showed that SERCA might regulate the inflammatory response by modulating phosphorylation of MAPK-JNK pathway. These findings showed that the influenza A virus induced autophagic flux blocking, inflammatory response and mitochondrial dysfunction by inhibiting SERCA activity. This study provides further understanding of the host-viral interactions between the influenza virus, SERCA activity, autophagy, inflammatory response, and mitochondrial function. SERCA may be a potential host target for decreasing inflammatory and superoxide injury during influenza A virus infection.IMPORTANCE:IAV is a major cause of infectious respiratory diseases, characterized by a marked respiratory tract inflammatory response that causes morbidity and mortality in seasonal epidemics, or pandemic outbreaks. SERCA is a critical component in maintaining cellular calcium levels, and is positively correlated with autophagic flux. Here, we discovered that SERCA is suppressed in IAV-infected human lung cells and influenza A virus induces blocking of autophagic flux, inflammatory response and mitochondrial dysfunction by inhibiting SERCA. We posit that the pharmacological activation of SERCA can be a powerful intervention strategy to prevent autophagy arrest, inflammatory response, and mitochondrial dysfunction in IAV-infected cells. Therefore, SERCA activity modulation could be a potential therapeutic strategy for managing clinical symptoms of severe influenza disease.

Interactions of SARS-CoV-2 with the Blood-Brain Barrier

Emerging data indicate that neurological complications occur as a consequence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. The blood-brain barrier (BBB) is a critical interface that regulates entry of circulating molecules into the CNS, and is regulated by signals that arise from the brain and blood compartments. In this review, we discuss mechanisms by which SARS-CoV-2 interactions with the BBB may contribute to neurological dysfunction associated with coronavirus disease of 2019 (COVID-19), which is caused by SARS-CoV-2. We consider aspects of peripheral disease, such as hypoxia and systemic inflammatory response syndrome/cytokine storm, as well as CNS infection and mechanisms of viral entry into the brain. We also discuss the contribution of risk factors for developing severe COVID-19 to BBB dysfunction that could increase viral entry or otherwise damage the brain.

Assembly strategy of liposome and polymer systems for siRNA delivery

In recent years, gene therapy has made tremendous progress in the development of disease treatment. Among them, siRNA offers specificity of gene silencing, ease of synthesis, and short development period, and has been intensively studied worldwide. However, siRNA as the hydrophilic polyanion is easily degraded in vivo and poorly taken up into cells and so, the benefits of its powerful gene silencing ability will not be realized until better carriers are developed that are capable of protecting siRNA and delivering it intact to the cytoplasm of the target cells. Cationic liposomes (CL) and cationic polymers (CP) are the main non-viral siRNA vectors, there have been a lot of reports on the use of these two carriers to deliver siRNA. Whereas, as far as we know, there have been few review articles that provide an in-depth summary of the siRNA loading principle and internal structures of the siRNA delivery system. We summarize the formation principle and assembly structure of the cationic liposome-siRNA and polymer-siRNA complexes, and point out their advantages and characteristics and also show how to perfect their assembly and improve their clinical application in the future. It supports some useful suggestions for siRNA therapy, specifically, safe and efficient delivery.

Antiviral Strategies of Chinese Herbal Medicine Against PRRSV Infection

Bioactive compounds from Traditional Chinese Medicines (TCMs) are gradually becoming an effective alternative in the control of porcine reproductive and respiratory syndrome virus (PRRSV) because most of the commercially available PRRSV vaccines cannot provide full protection against the genetically diverse strains isolated from farms. Besides, the incomplete attenuation procedure involved in the production of modified live vaccines (MLV) may cause them to revert to the more virulence forms. TCMs have shown some promising potentials in bridging this gap. Several investigations have revealed that herbal extracts from TCMs contain molecules with significant antiviral activities against the various stages of the life cycle of PRRSV, and they do this through different mechanisms. They either block PRRSV attachment and entry into cells or inhibits the replication of viral RNA or viral particles assembly and release or act as immunomodulators and pathogenic pathway inhibitors through cytokines regulations. Here, we summarized the various antiviral strategies employed by some TCMs against the different stages of the life cycle of PRRSV under two major classes, including direct-acting antivirals (DAAs) and indirect-acting antivirals (IAAs). We highlighted their mechanisms of action. In conclusion, we recommended that in making plans for the use of TCMs to control PRRSV, the pathway forward must be built on a real understanding of the mechanisms by which bioactive compounds exert their effects. This will provide a template that will guide the focus of collaborative studies among researchers in the areas of bioinformatics, chemistry, and proteomics. Furthermore, available data and procedures to support the efficacy, safety, and quality control levels of TCMs should be well documented without any breach of data integrity and good manufacturing practices.

Application of Concanavalin A-Linked Magnetic Beads for the Detection of Hepatitis A Virus

Prompt and inexpensive detection of hepatitis A virus (HAV) is essential to control acute hepatitis outbreaks associated with the consumption of contaminated raw or minimally processed food. In this study, various carbohydrate-binding lectins, including concanavalin A (Con A), wheat germ agglutinin, and soybean agglutinin, were compared for their binding affinity to HAV. Con A, which showed significantly higher binding affinity than other lectins, was used to develop an alternative and affordable method to conventional antibody-linked immunomagnetic separation prior to detection of HAV using reverse transcriptase PCR. This method, Con A-linked immunomagnetic separation combined with reverse transcriptase PCR, can detect HAV at a dilution concentration of 10^-4 of the virus stock (titer: 10⁴ median tissue culture infective dose per mL), indicating that Con A could be a promising candidate for concentrating HAV.

Labeling and Single-Particle-Tracking-Based Entry Mechanism Study of Vaccinia Virus from the Tiantan Strain

Entry is the first and critical step of viral infection, while the entry mechanisms of many viruses are still unclear due to the lack of efficient technology. In this report, by taking advantage of the single-virion fluorescence tracking technique and simultaneous dual-labeling methods for viruses we developed, the entry pathway of vaccinia virus from tiantan strain (VACV-TT) was studied in real-time. By combining with the technologies of virology and cell biology, we found that VACV-TT moved toward the Vero cell body along the filopodia induced by the virions interaction, and then, they were internalized through macropinocytosis, which was an actin-, ATP-dependent but clathrin-, caveolin-independent endocytosis. These results are of significant importance for VACV-TT-based vaccine vectors and oncolytic virus study.

A 32 kDa viral attachment protein of lymphocystis disease virus (LCDV) specifically interacts with a 27.8 kDa cellular receptor from flounder (Paralichthys olivaceus)

The 27.8 kDa protein in flounder gill (FG) cells was previously proved to be a receptor specific for lymphocystis disease virus (LCDV) entry and infection. In this paper, a 32 kDa viral attachment protein (VAP) of LCDV specifically binding to the 27.8 kDa receptor (27.8R) was found by far-Western blotting coupled with monoclonal antibodies (MAbs) against 27.8R. The 32 kDa protein was confirmed to be encoded by the open reading frame (ORF) 038 gene in LCDV-C, and predicted to contain a putative transmembrane region, multiple N-myristoylation and glycosylation sites and phosphorylation motifs. The expression plasmid of pET-32a-ORF038 was constructed and the recombinant VAP (rVAP) was obtained. Rabbit polyclonal antibodies against the rVAP were prepared and could recognize the rVAP and 32 kDa protein in LCDV. Immunogold electron microscopy showed that the 32 kDa protein was located on the surface of LCDV particles. Immunofluorescence assay demonstrated that the rVAP could bind to the 27.8R on the cell membrane of the FG monolayer and the anti-27.8R MAbs could block the rVAP binding. Pre-incubation of the rVAP with FG cells before LCDV infection, or pre-incubation of LCDV with the antibodies against the rVAP, could significantly decrease the LCDV copy numbers (P<0.05) and delay the emergence of cytopathic effects in FG cells in a dose-dependent manner. These results indicated for the first time that the 32 kDa protein functioned as an attachment protein for the initial attachment and entry of LCDV, and the interaction of the 32 kDa VAP with the 27.8R-initiated LCDV infection.

African horse sickness virus infects BSR cells through macropinocytosis

Cellular pathways involved in cell entry by African horse sickness virus (AHSV), a member of the Orbivirus genus within the Reoviridae family, have not yet been determined. Here, we show that acidic pH is required for productive infection of BSR cells by AHSV-4, suggesting that the virus is likely internalized by an endocytic pathway. We subsequently analyzed the major endocytic routes using specific inhibitors and determined the consequences for AHSV-4 entry into BSR cells. The results indicated that virus entry is dynamin dependent, but clathrin- and lipid raft/caveolae-mediated endocytic pathways were not used by AHSV-4 to enter and infect BSR cells. Instead, binding of AHSV-4 to BSR cells stimulated uptake of a macropinocytosis-specific cargo and inhibition of Na(+)/H(+) exchangers, actin polymerization and cellular GTPases and kinases involved in macropinocytosis significantly inhibited AHSV-4 infection. Altogether, the data suggest that AHSV-4 infects BSR cells by utilizing macropinocytosis as the primary entry pathway.

Identification of regulators of the early stage of viral hemorrhagic septicemia virus infection during curcumin treatment

The effect of curcumin pretreatment (15-240 μM) in fathead minnow cells infected with viral hemorrhagic septicemia virus (VHSV) was evaluated. Cell viability, apoptosis and viral copy number were analyzed using Cell Counting Kit-8 assay, Annexin V staining, and reverse transcription-PCR, respectively. Pretreatment with 120 μM curcumin showed an increase in viability (>90% of mock) of VHSV-infected cells and reduction in the copy number (0.2-log reduction in VHSV N gene expression), reactive oxygen species and apoptosis in the cells without cytotoxic effects. To understand the mechanisms underlaying the antiviral effects of curcumin pretreatment, a comparative proteomic analysis was performed in four samples (M, mock; C, curcumin-treated; V, VHSV-infected; and CV, curcumin-treated VHSV-infected) in triplicate. In total, 185 proteins were detected. The analysis showed that three proteins, including heat shock cognate 71 (HSC71), actin, alpha cardiac muscle (ACTC1) and elongation factor 1 (EEF1) were differentially expressed between V and CV samples. Network analysis performed by Ingenuity Pathways Analysis (IPA) showed that HSC71 was the primary protein interacting with fibronectin (FN) 1, actins (ACTB, ACTG, F-actin) and gelsolin (GSN) in both V and CV samples and thus is a strong target candidate for the protection from VHSV infection at the viral entry stage. Our proteomics data suggest that curcumin pretreatment inhibits entry of VHSV in cells by downregulating FN1 or upregulating F-actin. For both proteins, HSC71 acts as a binding protein that modulates their functions. Furthermore, consistent with the effect of a heat shock protein inhibitor (KNK437), curcumin downregulated HSC71 expression with increasing viability of VHSV-infected cells and inhibited VHSV replication, suggesting that the downregulation of HSC71 could be responsible for the antiviral activity of curcumin. In conclusion, this study indicates that the suppression of viral entry by rearrangement of the F-actin/G-actin ratio via downregulating HSC71 is a plausible mechanism by which curcumin pretreatment controls the early stages of VHSV infection.

Neutralization of Virus Infectivity by Antibodies: Old Problems in New Perspectives

Neutralizing antibodies (NAbs) can be both sufficient and necessary for protection against viral infections, although they sometimes act in concert with cellular immunity. Successful vaccines against viruses induce NAbs but vaccine candidates against some major viral pathogens, including HIV-1, have failed to induce potent and effective such responses. Theories of how antibodies neutralize virus infectivity have been formulated and experimentally tested since the 1930s; and controversies about the mechanistic and quantitative bases for neutralization have continually arisen. Soluble versions of native oligomeric viral proteins that mimic the functional targets of neutralizing antibodies now allow the measurement of the relevant affinities of NAbs. Thereby the neutralizing occupancies on virions can be estimated and related to the potency of the NAbs. Furthermore, the kinetics and stoichiometry of NAb binding can be compared with neutralizing efficacy. Recently, the fundamental discovery that the intracellular factor TRIM21 determines the degree of neutralization of adenovirus has provided new mechanistic and quantitative insights. Since TRIM21 resides in the cytoplasm, it would not affect the neutralization of enveloped viruses, but its range of activity against naked viruses will be important to uncover. These developments bring together the old problems of virus neutralization-mechanism, stoichiometry, kinetics, and efficacy-from surprising new angles.

Antiviral defense in shrimp: from innate immunity to viral infection

The culture of penaeid shrimp is rapidly developing as a major business endeavor worldwide. However, viral diseases have caused huge economic loss in penaeid shrimp culture industries. Knowledge of shrimp innate immunity and antiviral responses has made important progress in recent years, allowing the design of better strategies for the prevention and control of shrimp diseases. In this study, we have updated information on shrimp antiviral immunity and interactions between shrimp hosts and viral pathogens. Current knowledge and recent progress in immune signaling pathways (e.g., Toll/IMD-NF-κB and JAK-STAT signaling pathways), RNAi, phagocytosis, and apoptosis in shrimp antiviral immunity are discussed. The mechanism of viral infection in shrimp hosts and the interactions between viruses and shrimp innate immune systems are also analyzed.

Visualization of DC-SIGN-mediated entry pathway of engineered lentiviral vectors in target cells

Dendritic cells (DCs) are potent antigen-presenting cells and therefore have enormous potential as vaccine targets. We have previously developed an engineered lentiviral vector (LV) that is pseudotyped with a mutated Sindbis virus glycoprotein (SVGmu), which is capable of targeting DCs through Dendritic Cell-specific ICAM3-grabbing Nonintegrin (DC-SIGN), a receptor that is predominantly expressed by DCs. In this study, we aimed to elucidate the internalization and trafficking mechanisms of this viral vector system through direct visualization of GFP-Vpr-tagged viral particles in target DCs, which was further corroborated by drug inhibition and dominant-negative mutants of cellular proteins that regulate the endocytic traffic. We demonstrated that our engineered LVs enter the cell via receptor-mediated clathrin- and dynamin-dependent endocytosis. Microtubule networks were also involved in a productive infection. Viral vector fusion was low-pH-dependent and occurred in the early endosomal stage of the intracellular transport. Autophagy was also examined for its effect on transduction efficiency, and we observed that enhanced autophage activity reduced vector infectivity, while suppressed autophagy boosted transduction efficiency. This study shed some light on the internalization and trafficking mechanisms of DC-directed LVs and offers some strategies to further improve the efficiency of LV-mediated gene therapy.

Role of chymotrypsin-like serine proteinase in white spot syndrome virus infection in Fenneropenaeus chinensis

White spot syndrome virus (WSSV) caused a great economic loss in shrimp aquaculture. Although great efforts have been undertaken to characterize the virus disease during the last two decades, there are still lack of effective methods to prevent or cure it. In this study, we investigated the transcriptional expression profiles of 18 key immune-related genes in the Chinese shrimp Fenneropenaeus chinensis which was severely infected by WSSV. We found that the expression levels of 6 genes including chymotrypsin-like serine proteinase (CH-SPase), heat shock protein 70 cognate (HSP70), penaeidin (PEN), peroxinectin (PO), proliferating cell nuclear antigen (PCNA) and argonaute (AGO) changed significantly, while the expression of the other 12 genes had no significant changes compared to the control group. Among the 6 screened genes, CH-SPase showed significantly up-regulation, while the other 5 ones were significantly down-regulated. Knockdown of the expression of CH-SPase in WSSV-infected Chinese shrimp reduced the copy number of WSSV and delayed cumulative mortalities, suggesting that CH-SPase is important for WSSV infection. This study will be helpful to control the disease in shrimp caused by WSSV.

An update on mosquito cell expressed dengue virus receptor proteins

Dengue is the most important mosquito transmitted viral disease of humans worldwide. Despite intensive study over several decades, many of the fine details of the dengue virus (DENV) replication cycle remain unknown, although generally more is known about the phase of the replication cycle in mammalian cells as compared to the phase in mosquito cells. This results from a combination of less research emphasis on the mosquito stage, as well as fewer tools such as specific antibodies against mosquito proteins and insect informatics databases. The binding of a virus to a host cell is a first and critical stage in the infectious process and the mechanism and identity of cellular proteins involved in this process remains largely unknown. This short review aims to provide an update on our current understanding of the proteins expressed by mosquito cells that mediate DENV binding as a prerequisite to DENV entry and replication.

Single-particle tracking of hepatitis B virus-like vesicle entry into cells

HBsAg, the surface antigen of the hepatitis B virus (HBV), is used as a model to study the mechanisms and dynamics of a single-enveloped virus infecting living cells by imaging and tracking at the single-particle level. By monitoring the fluorescent indicator of HBsAg particles, it is found that HBsAg enters cells via a caveolin-mediated endocytic pathway. Tracking of individual HBsAg particles in living cells reveals the anomalously actin-dependent but not microtubule-dependent motility of the internalized HBsAg particle. The motility of HBsAg particles in living cells is also analyzed quantitatively. These results may settle the long-lasting debate of whether HBV directly breaks the plasma membrane barrier or relies on endocytosis to deliver its genome into the cell, and how the virus moves in the cell.

Entry of tiger frog virus (an Iridovirus) into HepG2 cells via a pH-dependent, atypical, caveola-mediated endocytosis pathway

Tiger frog virus (TFV), in the genus Ranavirus of the family Iridoviridae, causes high mortality of cultured tiger frog tadpoles in China. To explore the cellular entry mechanism of TFV, HepG2 cells were treated with drugs that inhibit the main endocytic pathways. We observed that TFV entry was inhibited by NH(4)Cl, chloroquine, and bafilomycin, which can all elevate the pH of acidic organelles. In contrast, TFV entry was not influenced by chlorpromazine or overexpression of a dominant-negative form of Esp15, which inhibit the assembly of clathrin-coated pits. These results suggested that TFV entry was not associated with clathrin-mediated endocytosis, but was related to the pH of acidic organelles. Subsequently, we found that endocytosis of TFV was dependent on membrane cholesterol and was inhibited by the caveolin-1 scaffolding domain peptide. Dynamin and actin were also required for TFV entry. In addition, TFV virions colocalized with the cholera toxin subunit B, indicating that TFV enters as caveola-internalized cargo into the Golgi complex. Taken together, our results demonstrated that TFV entry occurs by caveola-mediated endocytosis with a pH-dependent step. This atypical caveola-mediated endocytosis is different from the clathrin-mediated endocytosis of frog virus 3 (FV3) by BHK cells, which has been recognized as a model for iridoviruses. Thus, our work may help further the understanding of the initial steps of iridovirus infection in lower vertebrates.

Epitope analysis of white spot syndrome virus of Penaeus monodon by in vivo neutralization assay employing a panel of monoclonal antibodies

A panel of six monoclonal antibodies (MAbs) against the major envelope proteins VP18, VP26 and VP28 of white spot syndrome virus (WSSV) was evaluated for neutralization of the virus in vivo in Penaeus monodon. WSSV stock diluted to 1 x 10⁻⁶ resulting in 100% mortality on 12 day post injection (dpi) was used as optimum infectious dose of virus for challenge. Constant quantity (100 μg/ml) of MAbs C-5, C-14, C-33, C-38, C-56 and C-72 was incubated separately with WSSV (1 x 10⁻⁶ dilution) at 27 °C for 90 min and injected to shrimp. WSSV infection was neutralized by the MAbs C-5, C-14 and C-33 with a relative percent survival (RPS) of 60, 80 and 60 on 12 dpi, respectively compared to 100% mortality in positive control injected with WSSV alone. MAbs C-38, C-56 and C-72 could neutralize WSSV infection with RPS on 12 dpi of 40, 30 and 30, respectively. Shrimp injected with WSSV (1 x 10⁻⁶ dilution) incubated with panel of the MAbs at 100 μg/ml separately were subjected to nested PCR analysis at 0, 8, 12, 24, 36, 48 and 72 hour post injection (hpi) to provide further evidence for neutralization. MAbs C-5, C-14 and C-33 showed delay in WSSV positivity by 24 and 48 hpi by 2nd and 1st step PCR, respectively. MAbs C-38, C-56 and C-72 showed WSSV positivity by 12 and 24 hpi by 2nd and 1st step PCR, respectively. Shrimp injected with WSSV alone showed WSSV positivity by 8 and 12 hpi by 2nd and 1st step PCR, respectively. The study clearly shows that infectivity of WSSV could be delayed by MAbs C-14, C-5 and C-33.

Peptide inhibitors against influenza virus

Influenza A virus is a particularly problematic virus because of its ability to cause high levels of morbidity on a global scale within a remarkably short period of time. It also has the potential to kill very large numbers of people as occurred in the Spanish influenza pandemic in 1918. Options for antiviral therapy are limited because of the paucity of available drugs and the rapid mutation rate of the virus leading to the emergence of drug-resistant strains. The current H1N1 pandemic and potential threats posed by other strains highlight the need to develop novel therapeutic and prophylactic strategies. Here, we summarize the current state and recent developments of peptide-based inhibitors of influenza A virus.

Identification and characterization of prohibitin as a receptor protein mediating DENV-2 entry into insect cells

Dengue is transmitted primarily by mosquitoes of the Aedes genus. Despite a number of studies, no insect dengue virus receptor protein has been clearly identified and characterized. Using a number of separation methodologies and virus overlay protein binding assays we identified a 35kDa protein that segregated with susceptibility to dengue serotype 2 (DENV-2) infection in two mosquito species and two mosquito cell lines. Mass spectroscopy identified the protein to be prohibitin, a strongly conserved and ubiquitously expressed protein in eukaryotic cells. Antibody mediated inhibition of infection and siRNA mediated knockdown of prohibitin expression significantly reduced infection levels and subsequent virus production in both Aedes aegypti and Aedes albopictus cell lines. Confocal microscopy showed a significant degree of intracellular colocalization between prohibitin and DENV-2 E protein, and coimmunoprecipitation confirmed that prohibitin interacts with dengue E. Prohibitin is the first characterized insect cell expressed dengue virus receptor protein.

Interferon-induced Sus scrofa Mx1 blocks endocytic traffic of incoming influenza A virus particles

The interferon-induced Mx proteins of vertebrates are dynamin-like GTPases, some isoforms of which can additionally inhibit the life cycle of certain RNA viruses. Here we show that the porcine Mx1 protein (poMx1) inhibits replication of influenza A virus and we attempt to identify the step at which the viral life cycle is blocked. In infected cells expressing poMx1, the level of transcripts encoding the viral nucleoprotein is significantly lower than normal, even when secondary transcription is prevented by exposure to cycloheximide. This reveals that a pretranscriptional block participates to the anti-influenza activity. Binding and internalization of incoming virus particles are normal in the presence of poMx1 but centripetal traffic to the late endosomes is interrupted. Surprisingly but decisively, poMx1 significantly alters binding of early endosome autoantigen 1 to early endosomes and/or early endosome size and spatial distribution. This is compatible with impairment of traffic of the endocytic vesicles to the late endosomes.

Nipah virus entry can occur by macropinocytosis

Nipah virus (NiV) is a zoonotic biosafety level 4 paramyxovirus that emerged recently in Asia with high mortality in man. NiV is a member, with Hendra virus (HeV), of the Henipavirus genus in the Paramyxoviridae family. Although NiV entry, like that of other paramyxoviruses, is believed to occur via pH-independent fusion with the host cell's plasma membrane we present evidence that entry can occur by an endocytic pathway. The NiV receptor ephrinB2 has receptor kinase activity and we find that ephrinB2's cytoplasmic domain is required for entry but is dispensable for post-entry viral spread. The mutation of a single tyrosine residue (Y304F) in ephrinB2's cytoplasmic tail abrogates NiV entry. Moreover, our results show that NiV entry is inhibited by constructions and drugs specific for the endocytic pathway of macropinocytosis. Our findings could potentially permit the rapid development of novel low-cost antiviral treatments not only for NiV but also HeV.

Antiviral immunity in crustaceans

Viral diseases of shrimp have caused negative effects on the economy in several countries in Asia, South America and America, where they have numerous shrimp culture industries. The studies on the immunity of shrimp and other crustaceans have mainly focused on general aspects of immunity and as a consequence little is known about the antiviral responses in crustaceans. The aim of this review is to update recent knowledge of innate immunity against viral infections in crustaceans. Several antiviral molecules have been isolated and characterized recently from decapods. Characterization and identification of these molecules might provide a promising strategy for protection and treatment of these viral diseases. In addition dsRNA-induced antiviral immunity is also included.

Visualization of targeted transduction by engineered lentiviral vectors

We have reported a method to target lentiviral vectors to specific cell types. This method requires the incorporation of two distinct molecules on the viral vector surface: one is an antibody that renders the targeting specificity for the engineered vector, and the other is a fusogenic protein that allows the engineered vector to enter the target cell. However, the molecular mechanism that controls the targeted infection needs to be defined. In this report, we tracked the individual lentiviral particles by labeling the virus with the GFP-Vpr fusion protein. We were able to visualize the surface-displayed proteins on a single virion as well as antibody-directed targeting to a desired cell type. We also demonstrated the dynamics of virus fusion with endosomes and monitored endosome-associated transport of viruses in target cells. Our results suggest that the fusion between the engineered lentivirus and endosomes takes place at the early endosome level, and that the release of the viral core into the cytosol at the completion of the virus-endosome fusion is correlated with the endosome maturation process. This imaging study sheds some light on the infection mechanism of the engineered lentivirus and can be beneficial to the design of more efficient gene delivery vectors.

Identification of dengue virus binding proteins using affinity chromatography

Several studies have identified putative dengue virus receptors using virus overlay protein binding assays (VOPBA) with some apparent success. Given that this technique relies upon the use of electrophoresis of proteins through polyacrylamide gels with varying amounts of protein denaturation, the physiological relevance of the proteins isolated is open to question. To address this issue a Sepharose 4B-dengue virus serotype 2-affinity column was constructed to selectively bind dengue virus binding proteins from HepG2 (liver) cell membrane preparations. Results show that GRP78, but not the 37/67 kDa high affinity laminin receptor, was specifically bound by the column. This result is consistent with earlier work and shows that while affinity chromatography may provide a useful adjunct to VOPBA based studies particularly in cases where proteins maybe sensitive to denaturation, proteins isolated by VOPBA can be physiologically relevant.

Mutagenesis and nuclear magnetic resonance analyses of the fusion peptide of Helicoverpa armigera single nucleocapsid nucleopolyhedrovirus F protein

The entry of enveloped viruses into cells is normally mediated by fusion between viral and cellular membranes, in which the fusion peptide plays a crucial role. The fusion peptides of group II nucleopolyhedrovirus (NPV) F proteins are quite conserved, with a hydrophobic region located at the N terminal of the F(1) fragment. For this report, we used mutagenesis and nuclear magnetic resonance (NMR) to study the structure and function of the fusion peptide of the Helicoverpa armigera single-nucleocapsid NPV (HearNPV) F protein (HaF). Five mutations in the fusion peptide of HaF, N(1)G, N(1)L, I(2)N, G(3)L, and D(11)L, were generated separately, and the mutated f genes were transformed into the f-null HearNPV bacmid. The mutations N(1)L, I(2)N, and D(11)L were found to completely abolish the ability of the recombinant bacmids to produce infectious budded virus, while the mutations N(1)G and G(3)L did not. The low-pH-induced envelope fusion assay demonstrated that the N(1)G substitution increased the fusogenicity of HaF, while the G(3)L substitution reduced its fusogenicity. NMR spectroscopy was used to determine the structure of a synthetic fusion peptide of HaF in the presence of sodium dodecyl sulfate micelles at pH 5.0. The fusion peptide appeared to be an amphiphilic structure composed of a flexible coil in the N terminus from N(1) to N(5), a 3(10)-helix from F(6) to G(8), a turn at S(9), and a regular alpha-helix from V(10) to D(19). The data provide the first NMR structure of a baculovirus fusion peptide and allow us to further understand the relationship of structure and function of the fusion peptide.

Fusion of enveloped virus nanoparticles with polyelectrolyte-supported lipid membranes for the design of bio/nonbio interfaces

Fusion of lipid-enveloped viruses with endosomal membranes triggered by low pH in the endosome is a key step in the course of viral infection. This ubiquitous mechanism can be used to integrate functional nanoparticles of viral origin into composite materials consisting of a polyelectrolyte multilayer with an adsorbed lipid membrane in a natural and biomimetic way. Polyelectrolyte multilayers as the support for the lipid membrane are a versatile means to combine the biological functions of the viral surface with the multiplicity of polyelectrolyte borne functions into a novel bio/nonbio composite material.

Viruses as building blocks for materials and devices

From the viewpoint of a materials scientist, viruses can be regarded as organic nanoparticles. They are composed of a small number of different (bio)polymers: proteins and nucleic acids. Many viruses are enveloped in a lipid membrane and all viruses do not have a metabolism of their own, but rather use the metabolic machinery of a living cell for their replication. Their surface carries specific tools designed to cross the barriers of their host cells. The size and shape of viruses, and the number and nature of the functional groups on their surface, is precisely defined. As such, viruses are commonly used in materials science as scaffolds for covalently linked surface modifications. A particular quality of viruses is that they can be tailored by directed evolution by taking advantage of their inbuilt colocalization of geno- and phenotypes. The powerful techniques developed by life sciences are becoming the basis of engineering approaches towards nanomaterials, opening a wide range of applications far beyond biology and medicine.

Characterization of white spot syndrome virus replication in in vitro-cultured haematopoietic stem cells of freshwater crayfish, Pacifastacus leniusculus

Replication of White spot syndrome virus (WSSV) was investigated in haematopoietic cells (hpt cells) derived from haematopoietic tissue (hpt) of freshwater crayfish, Pacifastacus leniusculus. Temperature and type of inoculum for virus replication were studied. The cell culture remained viable at a wide range of temperatures ranging from 4 to 25 degrees C. WSSV replicated in cells, as evidenced by in situ hybridization, RT-PCR and by the presence of virions visualized with an electron microscope. Moreover, the results showed that the infectivity of WSSV to hpt cells is dependent on temperature and a supplemented growth factor (cytokine) astakine. WSSV replicated more rapidly at higher temperatures than at lower temperatures. No virus replication was observed at 4 degrees C. Detectable WSSV-infected cells were present as early as 36 h post-inoculation, demonstrated by in situ hybridization or RT-PCR of VP28 expression at 25 degrees C. Hpt cells can survive a few weeks at 25 or 16 degrees C and longer than several months at 4 degrees C.

Flow cytometric detection of hepatitis C virus antigens in infected peripheral blood leukocytes: binding and entry

AIM

We designed two synthetic-core-specific peptides core 1 (C1) and core 2 (C2), and an E1-specific peptide (E1). We produced specific polyclonal antibodies against these peptides and used the antibodies for detection of HCV antigens on surface and within infected peripheral blood leukocytes.

METHODS

Peripheral blood from a healthy individual who tested negative for HCV RNA was incubated with HCV type 4 infected serum for 1 h and 24 h at 37 degrees C. Cells were stained by direct and indirect immunofluorescence and measured by flow cytometry.

RESULTS

After 1 h of incubation, antibodies against C1, C2, and E1 detected HCV antigens on the surface of 27%, 26% and 73% of monocytes respectively, while 10%, 5% and 9% of lymphocytes were positive with anti-C1, anti-C2 and anti-E1 respectively. Only 1-3% of granulocytes showed positive staining with anti-C1, anti-C2 and anti E1 antibodies. After 24 h of incubation, we found no surface staining with anti-C1, anti-C2 or anti-E1. Direct immunostaining using anti-C2 could not detect intracellular HCV antigens, after 1 h of incubation with the virus, while after 24 h of incubation, 28% of infected cells showed positive staining. Only plus strand RNA was detectable intracellularly as early as 1 h after incubation, and remained detectable throughout 48 h post-infection. Interestingly, minus RNA strand could not be detected after 1 h, but became strongly detectable intracellularly after 24 h post-infection.

CONCLUSION

Monocytes and lymphocytes are the preferred target cells for HCV infection in peripheral blood leukocytes. Our specific anti-core and anti-E1 antibodies are valuable reagents for demonstration of HCV cell cycle. Also, HCV is capable of infecting and replicating in peripheral blood mononuclear cells as confirmed by detection of minus strand HCV RNA as well as intracellular staining of core HCV antigen.

Flow cytometric detection of hepatitis C virus antigens in infected peripheral blood leukocytes: binding and entry

AIM

We designed two synthetic-core-specific peptides core 1 (C1) and core 2 (C2), and an E1-specific peptide (E1). We produced specific polyclonal antibodies against these peptides and used the antibodies for detection of HCV antigens on surface and within infected peripheral blood leukocytes.

METHODS

Peripheral blood from a healthy individual who tested negative for HCV RNA was incubated with HCV type 4 infected serum for 1 h and 24 h at 37 degrees C. Cells were stained by direct and indirect immunofluorescence and measured by flow cytometry.

RESULTS

After 1 h of incubation, antibodies against C1, C2, and E1 detected HCV antigens on the surface of 27%, 26% and 73% of monocytes respectively, while 10%, 5% and 9% of lymphocytes were positive with anti-C1, anti-C2 and anti-E1 respectively. Only 1-3% of granulocytes showed positive staining with anti-C1, anti-C2 and anti E1 antibodies. After 24 h of incubation, we found no surface staining with anti-C1, anti-C2 or anti-E1. Direct immunostaining using anti-C2 could not detect intracellular HCV antigens, after 1 h of incubation with the virus, while after 24 h of incubation, 28% of infected cells showed positive staining. Only plus strand RNA was detectable intracellularly as early as 1 h after incubation, and remained detectable throughout 48 h post-infection. Interestingly, minus RNA strand could not be detected after 1 h, but became strongly detectable intracellularly after 24 h post-infection.

CONCLUSION

Monocytes and lymphocytes are the preferred target cells for HCV infection in peripheral blood leukocytes. Our specific anti-core and anti-E1 antibodies are valuable reagents for demonstration of HCV cell cycle. Also, HCV is capable of infecting and replicating in peripheral blood mononuclear cells as confirmed by detection of minus strand HCV RNA as well as intracellular staining of core HCV antigen.

Using single-particle tracking to study nuclear trafficking of viral genes

The question of how genetic materials are trafficked in and out of the cell nucleus is a problem of great importance not only for understanding viral infections but also for advancing gene-delivery technology. Here we demonstrate a physical technique that allows gene trafficking to be studied at the single-gene level by combining sensitive fluorescence microscopy with microinjection. As a model system, we investigate the nuclear import of influenza genes, in the form of ribonucleoproteins (vRNPs), by imaging single vRNPs in living cells in real time. Our single-particle trajectories show that vRNPs are transported to the nuclear envelope by diffusion. We have observed heterogeneous interactions between the vRNPs and nuclear pore complexes with dissociation rate constants spanning two orders of magnitude. Our single-particle tracking experiments also provided new insights into the regulation mechanisms for the nuclear import of vRNPs: the influenza M1 protein, a regulatory protein for the import process, downregulates the nuclear import of vRNPs by inhibiting the interactions between vRNPs and nuclear pore complexes but has no significant effect on the transport properties of vRNPs. We expect this single-particle tracking approach to find broad application in investigations of genetic trafficking.

Serotype-specific entry of dengue virus into liver cells: identification of the 37-kilodalton/67-kilodalton high-affinity laminin receptor as a dengue virus serotype 1 receptor

Dengue virus, the causative agent of dengue fever, dengue shock syndrome, and dengue hemorrhagic fever, infects susceptible cells by initially binding to a receptor(s) located on the host cell surface. Evidence to date suggests that receptor usage may be cell and serotype specific, and this study sought to identify dengue virus serotype 1 binding proteins on the surface of liver cells, a known target organ. By using a virus overlay protein binding assay (VOPBA), in both nondenaturing and denaturing gel systems, a putative dengue virus serotype 1 binding protein of approximately 37 kDa expressed on the surface of liver (HepG2) cells was identified. Mass spectrometry analysis identified a candidate protein, the 37/67-kDa high-affinity laminin receptor. Entry of the dengue virus serotype 1 was significantly inhibited in a dose-dependent manner by both antibodies directed against the 37/67-kDa high-affinity laminin receptor and soluble laminin. No inhibition of virus entry was seen with dengue virus serotypes 2, 3, or 4, demonstrating that the 37/67-kDa high-affinity laminin receptor is a serotype-specific receptor for dengue virus entry into liver cells.

Assembly of endocytic machinery around individual influenza viruses during viral entry

Most viruses enter cells via receptor-mediated endocytosis. However, the entry mechanisms used by many of them remain unclear. Also largely unknown is the way in which viruses are targeted to cellular endocytic machinery. We have studied the entry mechanisms of influenza viruses by tracking the interaction of single viruses with cellular endocytic structures in real time using fluorescence microscopy. Our results show that influenza can exploit clathrin-mediated and clathrin- and caveolin-independent endocytic pathways in parallel, both pathways leading to viral fusion with similar efficiency. Remarkably, viruses taking the clathrin-mediated pathway enter cells via the de novo formation of clathrin-coated pits (CCPs) at viral-binding sites. CCP formation at these sites is much faster than elsewhere on the cell surface, suggesting a virus-induced CCP formation mechanism that may be commonly exploited by many other types of viruses.

A capsid protein of nonenveloped Bluetongue virus exhibits membrane fusion activity

The outer capsid layer of Bluetongue virus, a member of the nonenveloped Reoviridae family, is composed of two proteins, a receptor-binding protein, VP2, and a second protein, VP5, which shares structural features with class I fusion proteins of enveloped viruses. In the replication cycle of Bluetongue virus VP5 acts as a membrane permeabilization protein that mediates release of viral particles from endosomal compartments into the cytoplasm. Here, we show that VP5 can also act as a fusion protein and induce syncytium formation when it is fused to a transmembrane anchor and expressed on the cell surface. Fusion activity is strictly pH-dependent and is triggered by short exposure to low pH. No cell-cell fusion is observed at neutral pH. Deletion of the first 40 amino acids, which can fold into two amphipathic helices, abolishes fusion activity. Syncytium formation by VP5 is inhibited in the presence of VP2 when it is expressed in a membrane-anchored form. The data indicate an interaction between the outer capsid protein VP2 and VP5 and show that VP5 undergoes pH-dependent conformational changes that render it capable of interacting with cellular membranes. More importantly, our data show that a membrane permeabilization protein of a nonenveloped virus can evolve into a fusion protein by the addition of an appropriate transmembrane anchor. The results strongly suggest that the mechanism of membrane permeabilization by VP5 and membrane fusion by viral fusion proteins require similar structural features and conformational changes.

Spontaneous transgenesis of human B lymphocytes

DNA can cross the cell membrane by natural means, but the functional relevance of this phenomenon has not been fully elucidated. Here, we analyzed spontaneous transgenesis of human B cells using plasmid DNA coding for a functional immunoglobulin (Ig) heavy chain gene under the control of a B-cell-specific promoter. Using polymerase chain reaction (PCR), reverse transcriptase-PCR, and flow cytometry in combination, spontaneous transgenesis was documented in Burkitt's lymphoma cell lines, Epstein-Barr virus-transformed cell lines, and peripheral blood B lymphocytes of the mature naïve phenotype (IgM(+)/IgD(+)/CD27(-)). By immunoelectron microscopy, the internalized DNA was seen in the lysosomes/late endosomes and in the cytosol proximal to the nucleus. Importantly, spontaneously transgenic B cells processed and presented to major histocompatibility complex (MHC)-restricted T lymphocytes a peptide expressed in the transgenic product. This is the first demonstration that primary B lymphocytes possess a program for the spontaneous internalization of DNA, which in turn imparts the cell with new immunological functions. As spontaneous transgenesis is obtained using a nonviral vector, does not require prior cell activation, and is not associated with chromosomal integration, the findings reported here open new possibilities for genetic manipulations of mature naïve B lymphocytes for therapy and vaccination.

Entry is a rate-limiting step for viral infection in a Drosophila melanogaster model of pathogenesis

The identification of host factors that control susceptibility to infection has been hampered by a lack of amenable genetic systems. We established an in vivo model to determine the host factors that control pathogenesis and identified viral entry as a rate-limiting step for infection. We infected Drosophila melanogaster cells and adults with drosophila C virus and found that the clathrin-mediated endocytotic pathway is essential for both infection and pathogenesis. Heterozygosity for mutations in genes involved in endocytosis is sufficient to protect flies from pathogenicity, indicating the exquisite sensitivity and dependency of the virus on this pathway. Thus, this virus model provides a sensitive and efficient approach for identifying components required for pathogenesis.

Visualizing infection of individual influenza viruses

Influenza is a paradigm for understanding viral infections. As an opportunistic pathogen exploiting the cellular endocytic machinery for infection, influenza is also a valuable model system for exploring the cell's constitutive endocytic pathway. We have studied the transport, acidification, and fusion of single influenza viruses in living cells by using real-time fluorescence microscopy and have dissected individual stages of the viral entry pathway. The movement of individual viruses revealed a striking three-stage active transport process that preceded viral fusion with endosomes starting with an actin-dependent movement in the cell periphery, followed by a rapid, dynein-directed translocation to the perinuclear region, and finally an intermittent movement involving both plus- and minus-end-directed microtubule-based motilities in the perinuclear region. Surprisingly, the majority of viruses experience their initial acidification in the perinuclear region immediately following the dynein-directed rapid translocation step. This finding suggests a previously undescribed scenario of the endocytic pathway toward late endosomes: endosome maturation, including initial acidification, largely occurs in the perinuclear region.