Mechanisms of cell entry by influenza virus

Heparan sulfate is the attachment factor associated with channel catfish virus infection on host cells

Channel catfish virus (CCV; family Alloherpesviridae) infects channel catfish, causing great harm to aquaculture fisheries and economic development. Attachment is the first step in viral infection and relies on the interaction of virions with components of the extracellular matrix (ECM). The present study aimed to explored the role of the main three ECM components in CCV attachment. Western blotting and quantitative real-time PCR analysis showed that neither collagen nor hyaluronic acid treatments had significant effects on CCV attachment. When exogenous heparin was used as a competitive inhibitor, the adhesion of heparin sodium salt to CCV was dose-dependent. When the concentration of heparin sodium salt was 10 mg/mL, the inhibitory effect on CCV infection of channel catfish ovary (CCO/BB) cells was more than 90%. Heparinase I could significantly prevent CCV attachment by digesting heparan sulfate on the cell surface, and both heparin sodium salt and heparinase I could dose-dependently reduce CCV titers, suggesting that heparin plays an important role in CCV attachment. In addition, the binding experiments between heparin-agarose beads and virions showed that CCV virions could specifically bind to heparin in a dose-dependent manner. The above results suggested that heparan sulfate might be an attachment factor involved in CCV infection of CCO/BB cells. These results increase our understand of the attachment mechanism of CCV and lay the foundation for further research on antiviral drugs.

Antiviral effect and mechanism of Phillyrin and its reformulated FS21 against influenza

Background

Influenza virus causes significant morbidity and mortality with pandemic threat. Oleaceae Fructus Forsythiae is a medicinal herb. This study aimed to investigate antiviral effect of Phillyrin, a purified bioactive compound from this herb, and its reformulated preparation FS21 against influenza and its mechanism.

Methods

Madin-Darby Canine Kidney (MDCK) cells were infected by one of six influenza viruses: five influenza A viruses (IAVs: three H1N1 and two H3N2) and one influenza B virus (IBV). Virus-induced cytopathic effects were observed and recorded under microscope. Viral replication and mRNA transcription were evaluated by quantitative polymerase chain reaction (qPCR) and protein expression by Western blot. Infectious virus production was assessed using TCID50 assay, and IC50 was calculated accordingly. Pretreatment and time-of-addition experiments with Phillyrin or FS21 added 1 h before or in early (0-3 h), mid (3-6 h), or late (6-9 h) stages of viral infection were performed to assess their antiviral effects. Mechanistic studies included hemagglutination and neuraminidase inhibition, viral binding and entry, endosomal acidification, and plasmid-based influenza RNA polymerase activity.

Results

Phillyrin and FS21 had potent antiviral effects against all six IAV and IBV in a dose-dependent manner. Mechanistic studies showed that both suppressed influenza viral RNA polymerase with no effect on virus-mediated hemagglutination inhibition, viral binding or entry, endosomal acidification, or neuraminidase activity.

Conclusions

Phillyrin and FS21 have broad and potent antiviral effects against influenza viruses with inhibition of viral RNA polymerase as the distinct antiviral mechanism.

Silicon nitride: a potent solid-state bioceramic inactivator of ssRNA viruses

Surface inactivation of human microbial pathogens has a long history. The Smith Papyrus (2600 ~ 2200 B.C.) described the use of copper surfaces to sterilize chest wounds and drinking water. Brass and bronze on doorknobs can discourage microbial spread in hospitals, and metal-base surface coatings are used in hygiene-sensitive environments, both as inactivators and modulators of cellular immunity. A limitation of these approaches is that the reactive oxygen radicals (ROS) generated at metal surfaces also damage human cells by oxidizing their proteins and lipids. Silicon nitride (Si₃N₄) is a non-oxide ceramic compound with known surface bacterial resistance. We show here that off-stoichiometric reactions at Si₃N₄ surfaces are also capable of inactivating different types of single-stranded RNA (ssRNA) viruses independent of whether their structure presents an envelop or not. The antiviral property of Si₃N₄ derives from a hydrolysis reaction at its surface and the subsequent formation of reactive nitrogen species (RNS) in doses that could be metabolized by mammalian cells but are lethal to pathogens. Real-time reverse transcription (RT)-polymerase chain reaction (PCR) tests of viral RNA and in situ Raman spectroscopy suggested that the products of Si₃N₄ hydrolysis directly react with viral proteins and RNA. Si₃N₄ may have a role in controlling human epidemics related to ssRNA mutant viruses.

Influenza A Virus Hemagglutinin and Other Pathogen Glycoprotein Interactions with NK Cell Natural Cytotoxicity Receptors NKp46, NKp44, and NKp30

Natural killer (NK) cells are part of the innate immunity repertoire, and function in the recognition and destruction of tumorigenic and pathogen-infected cells. Engagement of NK cell activating receptors can lead to functional activation of NK cells, resulting in lysis of target cells. NK cell activating receptors specific for non-major histocompatibility complex ligands are NKp46, NKp44, NKp30, NKG2D, and CD16 (also known as FcγRIII). The natural cytotoxicity receptors (NCRs), NKp46, NKp44, and NKp30, have been implicated in functional activation of NK cells following influenza virus infection via binding with influenza virus hemagglutinin (HA). In this review we describe NK cell and influenza A virus biology, and the interactions of influenza A virus HA and other pathogen lectins with NK cell natural cytotoxicity receptors (NCRs). We review concepts which intersect viral immunology, traditional virology and glycobiology to provide insights into the interactions between influenza virus HA and the NCRs. Furthermore, we provide expert opinion on future directions that would provide insights into currently unanswered questions.

Biological properties of influenza A virus mutants with amino acid substitutions in the HA2 glycoprotein of the HA1/HA2 interaction region

Influenza A viruses (IAVs) enter into cells by receptor-dependent endocytosis. Subsequently, conformational changes of haemagglutinin are triggered by low environmental pH and the N terminus of HA2 glycoprotein (gp) is inserted into the endosomal membrane, resulting in fusion pore formation and genomic vRNA release into the cytoplasm. However, the pH optimum of membrane fusion is host- and virus-specific and can have an impact on virus pathogenicity. We prepared mutants of neurotropic IAV A/WSN/33 (H1N1) with aa substitutions in HA2 gp at the site of HA1/HA2 interaction, namely T64₂H (HA2 numbering position 64, H1 numbering position HA407; referred to as mutant '64'), V66₂H ('66') (HA409); and a double mutant ('D') with two aa substitutions (T64₂H, V66₂H). These substitutions were hypothesized to influence the pH optimum of fusion. The pH optimum of fusion activity was measured by a luciferase assay and biological properties of viruses were monitored. The in vitro and in vivo replication ability and pathogenicity of mutants were comparable (64) or lower (66, D) than those of the wild-type virus. However, the HA2 mutation V66₂H and double mutation T64₂H, V66₂H shifted the fusion pH maximum to lower values (ranging from 5.1 to 5.3) compared to pH from 5.4 to 5.6 for the wild-type and 64 mutant. The decreased replication ability and pathogenicity of 66 and D mutants was accompanied by higher titres in late intervals post-infection in lungs, and viral RNA in brains compared to wild-type virus-infected mice. These results have implications for understanding the pathogenicity of influenza viruses.

Hemagglutinin Cleavability, Acid Stability, and Temperature Dependence Optimize Influenza B Virus for Replication in Human Airways

Influenza A virus (IAV) and influenza B virus (IBV) cause yearly epidemics with significant morbidity and mortality. When zoonotic IAVs enter the human population, the viral hemagglutinin (HA) requires adaptation to achieve sustained virus transmission. In contrast, IBV has been circulating in humans, its only host, for a long period of time. Whether this entailed adaptation of IBV HA to the human airways is unknown. To address this question, we compared two seasonal IAVs (A/H1N1 and A/H3N2) and two IBVs (B/Victoria and B/Yamagata lineages) with regard to host-dependent activity of HA as the mediator of membrane fusion during viral entry. We first investigated proteolytic activation of HA by covering all type II transmembrane serine protease (TTSP) and kallikrein enzymes, many of which proved to be present in human respiratory epithelium. The IBV HA0 precursor is cleaved by a broader panel of TTSPs and activated with much higher efficiency than IAV HA0. Accordingly, knockdown of a single protease, TMPRSS2, abrogated spread of IAV but not IBV in human respiratory epithelial cells. Second, the HA fusion pH values proved similar for IBV and human-adapted IAVs (with one exception being the HA of 1918 IAV). Third, IBV HA exhibited higher expression at 33°C, a temperature required for membrane fusion by B/Victoria HA. This indicates pronounced adaptation of IBV HA to the mildly acidic pH and cooler temperature of human upper airways. These distinct and intrinsic features of IBV HA are compatible with extensive host adaptation during prolonged circulation of this respiratory virus in the human population.IMPORTANCE Influenza epidemics are caused by influenza A and influenza B viruses (IAV and IBV, respectively). IBV causes substantial disease; however, it is far less studied than IAV. While IAV originates from animal reservoirs, IBV circulates in humans only. Virus spread requires that the viral hemagglutinin (HA) is active and sufficiently stable in human airways. We resolve here how these mechanisms differ between IBV and IAV. Whereas human IAVs rely on one particular protease for HA activation, this is not the case for IBV. Superior activation of IBV by several proteases should enhance shedding of infectious particles. IBV HA exhibits acid stability and a preference for 33°C, indicating pronounced adaptation to the human upper airways, where the pH is mildly acidic and a cooler temperature exists. These adaptive features are rationalized by the long existence of IBV in humans and may have broader relevance for understanding the biology and evolution of respiratory viruses.

The use of pyrosequencing for detection of hemagglutinin mutations associated with increased pathogenicity of H5N1 avian influenza viruses in mammals

Hemagglutinin (HA) cleavage is critical for virulence of influenza viruses. The amino acid residue at the P6 position of the HA cleavage site (HACS) has been shown to be most variable and to have a direct correlation with the cleavage efficiency and pathogenicity of H5N1 avian influenza viruses (AIVs) in mammals. Among these amino acid variants, serine has been associated with the highest virulence in mammals, and its detection may serve as an indicator for H5N1 AIVs with high pathogenicity and potential public risk. We developed a rapid detection method based on reverse-transcription (RT)-PCR and pyrosequencing to detect a mutation at the HACS that is associated with increased pathogenicity of H5N1 AIVs in mammals. Herein, we provide a specific, sensitive, and reliable method for rapid detection of one of the virulence determinants associated with increased pathogenicity of H5N1 AIVs in mammals.

Nanoescapology: progress toward understanding the endosomal escape of polymeric nanoparticles

Using nanoparticles to deliver drugs to cells has the potential to revolutionize the treatment of many diseases, including HIV, cancer, and diabetes. One of the major challenges facing this field is controlling where the drug is trafficked once the nanoparticle is taken up into the cell. In particular, if drugs remain localized in an endosomal or lysosomal compartment, the therapeutic can be rendered completely ineffective. To ensure the design of more effective delivery systems we must first develop a better understanding of how nanoparticles and their cargo are trafficked inside cells. This needs to be combined with an understanding of what characteristics are required for nanoparticles to achieve endosomal escape, along with methods to detect endosomal escape effectively. This review is focused into three sections: first, an introduction to the mechanisms governing internalization and trafficking in cells, second, a discussion of methods to detect endosomal escape, and finally, recent advances in controlling endosomal escape from polymer- and lipid-based nanoparticles, with a focus on engineering materials to promote endosomal escape. WIREs Nanomed Nanobiotechnol 2017, 9:e1452. doi: 10.1002/wnan.1452 For further resources related to this article, please visit the WIREs website.

Molecular pathogenesis of H5 highly pathogenic avian influenza: the role of the haemagglutinin cleavage site motif

The emergence of H5N1 highly pathogenic avian influenza has caused a heavy socio‐economic burden through culling of poultry to minimise human and livestock infection. Although human infections with H5N1 have to date been limited, concerns for the pandemic potential of this zoonotic virus have been greatly intensified following experimental evidence of aerosol transmission of H5N1 viruses in a mammalian infection model. In this review, we discuss the dominance of the haemagglutinin cleavage site motif as a pathogenicity determinant, the host‐pathogen molecular interactions driving cleavage activation, reverse genetics manipulations and identification of residues key to haemagglutinin cleavage site functionality and the mechanisms of cell and tissue damage during H5N1 infection. We specifically focus on the disease in chickens, as it is in this species that high pathogenicity frequently evolves and from which transmission to the human population occurs. With >75% of emerging infectious diseases being of zoonotic origin, it is necessary to understand pathogenesis in the primary host to explain spillover events into the human population. © 2015 The Authors. Reviews in Medical Virology published by John Wiley & Sons Ltd.

Single-Stranded RNA Viruses

In this chapter, we describe 73 zoonotic viruses that were isolated in Northern Eurasia and that belong to the different families of viruses with a single-stranded RNA (ssRNA) genome. The family includes viruses with a segmented negative-sense ssRNA genome (families Bunyaviridae and Orthomyxoviridae) and viruses with a positive-sense ssRNA genome (families Togaviridae and Flaviviridae). Among them are viruses associated with sporadic cases or outbreaks of human disease, such as hemorrhagic fever with renal syndrome (viruses of the genus Hantavirus), Crimean–Congo hemorrhagic fever (CCHFV, Nairovirus), California encephalitis (INKV, TAHV, and KHATV; Orthobunyavirus), sandfly fever (SFCV and SFNV, Phlebovirus), Tick-borne encephalitis (TBEV, Flavivirus), Omsk hemorrhagic fever (OHFV, Flavivirus), West Nile fever (WNV, Flavivirus), Sindbis fever (SINV, Alphavirus) Chikungunya fever (CHIKV, Alphavirus) and others. Other viruses described in the chapter can cause epizootics in wild or domestic animals: Geta virus (GETV, Alphavirus), Influenza A virus (Influenzavirus A), Bhanja virus (BHAV, Phlebovirus) and more. The chapter also discusses both ecological peculiarities that promote the circulation of these viruses in natural foci and factors influencing the occurrence of epidemic and epizootic outbreaks

Cryptoporus volvatus extract inhibits influenza virus replication in vitro and in vivo

Influenza virus is the cause of significant morbidity and mortality, posing a serious health threat worldwide. Here, we evaluated the antiviral activities of Cryptoporus volvatus extract on influenza virus infection. Our results demonstrated that the Cryptoporus volvatus extract inhibited different influenza virus strain replication in MDCK cells. Time course analysis indicated that the extract exerted its inhibition at earlier and late stages in the replication cycle of influenza virus. Subsequently, we confirmed that the extract suppressed virus internalization into and released from cells. Moreover, the extract significantly reduced H1N1/09 influenza virus load in lungs and dramatically decreased lung lesions in mice. And most importantly, the extract protected mice from lethal challenge with H1N1/09 influenza virus. Our results suggest that the Cryptoporus volvatus extract could be a potential candidate for the development of a new anti-influenza virus therapy.

The matrix gene segment destabilizes the acid and thermal stability of the hemagglutinin of pandemic live attenuated influenza virus vaccines

The threat of future influenza pandemics and their potential for rapid spread, morbidity, and mortality has led to the development of pandemic vaccines. We generated seven reassortant pandemic live attenuated influenza vaccines (pLAIVs) with the hemagglutinin (HA) and neuraminidase (NA) genes derived from animal influenza viruses on the backbone of the six internal protein gene segments of the temperature sensitive, cold-adapted (ca) A/Ann Arbor/60 (H2N2) virus (AA/60 ca) of the licensed seasonal LAIV. The pLAIV viruses were moderately to highly restricted in replication in seronegative adults; we sought to determine the biological basis for this restriction. Avian influenza viruses generally replicate at higher temperatures than human influenza viruses and, although they shared the same backbone, the pLAIV viruses had a lower shutoff temperature than seasonal LAIV viruses, suggesting that the HA and NA influence the degree of temperature sensitivity. The pH of HA activation of highly pathogenic avian influenza viruses was greater than human and low-pathogenicity avian influenza viruses, as reported by others. However, pLAIV viruses had a consistently higher pH of HA activation and reduced HA thermostability compared to the corresponding wild-type parental viruses. From studies with single-gene reassortant viruses bearing one gene segment from the AA/60 ca virus in recombinant H5N1 or pH1N1 viruses, we found that the lower HA thermal stability and increased pH of HA activation were associated with the AA/60 M gene. Together, the impaired HA acid and thermal stability and temperature sensitivity likely contributed to the restricted replication of the pLAIV viruses we observed in seronegative adults.

IMPORTANCE

There is increasing evidence that the HA stability of influenza viruses depends on the virus strain and host species and that HA stability can influence replication, virulence, and transmission of influenza A viruses in different species. We investigated the HA stability of pandemic live attenuated influenza vaccine (pLAIV) viruses and observed that the pLAIV viruses consistently had a less stable HA than the corresponding wild-type influenza viruses. The reduced HA stability and temperature sensitivity of the pLAIV viruses may account for their restricted replication in clinical trials.

Virosome presents multimodel cancer therapy without viral replication

A virosome is an artificial envelope that includes viral surface proteins and lacks the ability to produce progeny virus. Virosomes are able to introduce an encapsulated macromolecule into the cytoplasm of cells using their viral envelope fusion ability. Moreover, virus-derived factors have an adjuvant effect for immune stimulation. Therefore, many virosomes have been utilized as drug delivery vectors and adjuvants for cancer therapy. This paper introduces the application of virosomes for cancer treatment. In Particular, we focus on virosomes derived from the influenza and Sendai viruses which have been widely used for cancer therapy. Influenza virosomes have been mainly applied as drug delivery vectors and adjuvants. By contrast, the Sendai virosomes have been mainly applied as anticancer immune activators and apoptosis inducers.

Role of the viral hemagglutinin in the anti-influenza virus activity of newly synthesized polycyclic amine compounds

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We report on the synthesis of polycyclic amines designed as ring-rearranged analogs of amantadine.

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Only one compound inhibited A/M2 proton channel function.

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However, they showed low-micromolar activity against A/H1N1, but not A/H3N2 influenza viruses.

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A/PR/8/34 mutants selected for resistance to these compounds possessed mutation in the viral hemagglutinin.

We here report on the synthesis of new series of polycyclic amines initially designed as ring-rearranged analogs of amantadine and featuring pentacyclo, hexacyclo, and octacyclo rings. A secondary amine, 3-azahexacyclo[7.6.0.0^1,5.0^5,12.0^6,10.0^11,15]pentadeca-7,13-diene, 3, effectively inhibited A/M2 proton channel function, and, moreover, possessed dual activity against an A/H3N2 virus carrying a wild-type A/M2 proton channel, as well as an amantadine-resistant A/H1N1 virus. Among the polycyclic amines that did not inhibit influenza A/M2 proton channel function, several showed low-micromolar activity against tested A/H1N1 strains (in particular, the A/PR/8/34 strain), but not A/H3N2 influenza viruses. A/PR/8/34 mutants selected for resistance to these compounds possessed mutations in the viral hemagglutinin that markedly increased the hemolysis pH. Our data suggest that A/H1N1 viruses such as the A/PR/8/34 strain are particularly sensitive to a subtle increase in the endosomal pH, as caused by the polycyclic amine compounds.

The pH of activation of the hemagglutinin protein regulates H5N1 influenza virus replication and pathogenesis in mice

After receptor binding and internalization during influenza virus entry, the hemagglutinin (HA) protein is triggered by low pH to undergo irreversible conformational changes that mediate membrane fusion. To investigate how mutations that alter the activation pH of the HA protein influence the fitness of an avian H5N1 influenza virus in a mammalian model, we infected C57BL/6J or DBA/2J mice and compared the replication and virulence of recombinant A/chicken/Vietnam/C58/04 (H5N1) HA-Y231H mutant, wild-type, and HA-H241Q and HA-K582I mutant viruses that have HA activation pH values of 6.3, 5.9, 5.6, and 5.4, respectively. The HA-Y231H mutant virus was highly susceptible to acid inactivation in vitro and was attenuated for growth and virulence in mice, suggesting that an H5N1 HA protein triggered at pH 6.3 is too unstable for the virus to remain fit. Wild-type and HA-H241Q viruses were similar in pathogenicity and grew to similar levels in mice, ducks, and cell cultures derived from both avian and mammalian tissues, suggesting that H5N1 HA proteins triggered at pH values in the range of 5.9 to 5.6 broadly support replication. The HA-K582I mutant virus had greater growth and virulence in DBA/2J mice than the wild type did, although the mutant virus was highly attenuated in ducks. The data suggest that adaptation of avian H5N1 influenza virus for infection in mammals is supported by a decrease in the HA activation pH to 5.4. Identification of the HA activation pH as a host-specific infectivity factor is expected to aid in the surveillance and risk assessment of currently circulating H5N1 influenza viruses.

Biological analysis of the microbial metabolism of hetero-oligosaccharides in application to glycotechnology

This review describes the relationship between hetero-oligosaccharides and microorganisms. It is possible to prepare aminosugar nucleotides as donors for hetero-oligosaccharide synthesis with a combination of yeast fermentation and bacterial enzymes, and to use the product to test for a rare human blood group. We have isolated various glycosidases produced by microorganisms, mainly from soil, to elucidate the structure and function of hetero-oligosaccharides. Among them, a mold endoglycosidase was found to have specific transglycosylation activity in addition to hydrolysis activity, and we have used it to synthesize chemo-enzymatically various bioactive glycopeptides by the attachment of a hetero-oligosaccharide to a peptide. We found that lactic acid bacteria bound to a hetero-oligosaccharide on the intestinal tract cell surface in animals. We also analyzed the bifidobacterial hetero-oligosaccharide-hydrolyzing enzymes involved in the degradation of mucin glycoprotein in the host intestinal tract and human milk oligosaccharides, and identified a specific saccharide that acted as a bifidobacteria growth factor.

Heparan sulfate is an attachment factor for foamy virus entry

The cellular receptor of foamy viruses (FVs) is unknown. The broad spectrum of permissive cells suggests that the cellular receptor is a molecular structure with almost ubiquitous prevalence. Here, we investigated the ability of heparan sulfate (HS), a glycosaminoglycan (GAG) present on the extracellular matrix of many cells, to bind FV particles and to permit prototype FV (PFV) and feline FV (FFV) entry. Permissivity of different cell lines for FV entry correlated with the amount of heparan sulfate present on the cell surface. The resulting 50% cell culture infectious doses (CCID(50)s) were distributed over a range of 4 logs, which means that the most susceptible cell line tested (HT1080) was more than 10,000 times more susceptible for PFV infection than the least susceptible cell line (CRL-2242). HS surface expression varied over a range of 2 logs. HS expression and FV susceptibility were positively correlated (P < 0.001). Enzymatic digestion of heparan sulfate on HT1080 cells diminished permissivity for PFV entry by a factor of at least 500. Using fast protein liquid chromatography (FPLC), we demonstrated binding of FV vector particles to a gel filtration column packed with heparin, a molecule structurally related to heparan sulfate, allowing for the purification of infectious particles. Both PFV and FFV infection were inhibited by soluble heparin. Our results show that FVs bind to HS and that this interaction is a pivotal step for viral entry, suggesting that HS is a cellular attachment factor for FVs.

Intracytoplasmic trapping of influenza virus by a lipophilic derivative of aglycoristocetin

We report on a new anti-influenza virus agent, SA-19, a lipophilic glycopeptide derivative consisting of aglycoristocetin coupled to a phenylbenzyl-substituted cyclobutenedione. In Madin-Darby canine kidney cells infected with influenza A/H1N1, A/H3N2, or B virus, SA-19 displayed a 50% antivirally effective concentration of 0.60 μM and a selectivity index (ratio of cytotoxic versus antiviral concentration) of 112. SA-19 was 11-fold more potent than unsubstituted aglycoristocetin and was active in human and nonhuman cell lines. Virus yield at 72 h p.i. was reduced by 3.6 logs at 0.8 μM SA-19. In contrast to amantadine and oseltamivir, SA-19 did not select for resistance upon prolonged virus exposure. SA-19 was shown to inhibit an early postbinding step in virus replication. The compound had no effect on hemagglutinin (HA)-mediated membrane fusion in an HA-polykaryon assay and did not inhibit the low-pH-induced refolding of the HA in a tryptic digestion assay. However, a marked inhibitory effect on the transduction exerted by retroviral pseudoparticles carrying an HA or vesicular stomatitis virus glycoprotein (VSV-G) fusion protein was noted, suggesting that SA-19 targets a cellular factor with a role in influenza virus and VSV entry. Using confocal microscopy with antinucleoprotein staining, SA-19 was proven to completely prevent the influenza virus nuclear entry. This virus arrest was characterized by the formation of cytoplasmic aggregates. SA-19 appeared to disturb the endocytic uptake and trap the influenza virus in vesicles distinct from early, late, or recycling endosomes. The aglycoristocetin derivative SA-19 represents a new class of potent and broad-acting influenza virus inhibitors with potential clinical relevance.

Mechanism of drug resistance of hemagglutinin of influenza virus and potent scaffolds inhibiting its function

Highly pathogenic influenza viruses have become a global threat to humans. It is important to select an effective therapeutic option suitable for the subtypes in an epidemic or pandemic. To increase the options, the development of novel antiviral agents acting on targets different from those of the currently approved drugs is required. In this study, we performed molecular dynamics simulations on a spike protein on the viral envelop, hemagglutinin, for the wild-type and three kinds of mutants using a model system consisting of a trimeric hemagglutinin complex, viral lipid membrane, solvation waters, and ions. A natural product, stachyflin, which shows a high level of antiviral activity specific to some subtypes of influenza viruses, was examined on binding to the wild-type hemagglutinin by docking simulation. The compound potency of stachyflin is, however, easily lost due to resistant mutations. From a comparison of simulation results between the wild-type and the resistant mutants, the reason for the drug resistance of hemagglutinin was clarified. Next, 8 compounds were selected from a chemical database by in silico screening, considering the findings from the simulations. Inhibitory activities to suppress the proliferation of influenza virus were measured by cell-based antiviral assays, and two chemical scaffolds were found to be potent for an inhibitor. More than 30 derivatives bearing either of these two chemical scaffolds were synthesized, and cell culture assays were carried out to evaluate the compound potency. Several derivatives displayed a high compound potency, and 50% effective concentrations of two synthesized compounds were below 1 μM.

Effectively and efficiently dissecting the infection of influenza virus by quantum-dot-based single-particle tracking

Exploring the virus infection mechanisms is significant for defending against virus infection and providing a basis for studying endocytosis mechanisms. Single-particle tracking technique is a powerful tool to monitor virus infection in real time for obtaining dynamic information. In this study, we reported a quantum-dot-based single-particle tracking technique to efficiently and globally research the virus infection behaviors in individual cells. It was observed that many influenza viruses were moving rapidly, converging to the microtubule organizing center (MTOC), interacting with acidic endosomes, and finally entering the target endosomes for genome release, which provides a vivid portrayal of the five-stage virus infection process. This report settles a long-pending question of how viruses move and interact with acidic endosomes before genome release in the perinuclear region and also finds that influenza virus infection is likely to be a "MTOC rescue" model for genome release. The systemic technique developed in this report is expected to be widely used for studying the mechanisms of virus infection and uncovering the secrets of endocytosis.

Virus Glycoproteins Tagged with the Human Fc Domain as Second Generation Vaccine Candidates

Traditional vaccines such as inactivated or live attenuated vaccines, are gradually giving way to more biochemically defined vaccines that are most often based on a recombinant antigen known to possess neutralizing epitopes. Such vaccines can offer improvements in speed, safety and manufacturing process but an inevitable consequence of their high degree of purification is that immunogenicity is reduced through the lack of the innate triggering molecules present in more complex preparations. Targeting recombinant vaccines to antigen presenting cells (APCs) such as dendritic cells however can improve immunogenicity by ensuring that antigen processing is as efficient as possible. Immune complexes, one of a number of routes of APC targeting, are mimicked by a recombinant approach, crystallizable fragment (Fc) fusion proteins, in which the target immunogen is linked directly to an antibody effector domain capable of interaction with receptors, FcR, on the APC cell surface. A number of virus Fc fusion proteins have been expressed in insect cells using the baculovirus expression system and shown to be efficiently produced and purified. Their use for immunization next to non-Fc tagged equivalents shows that they are powerfully immunogenic in the absence of added adjuvant and that immune stimulation is the result of the Fc-FcR interaction.

Capture and imaging of a prehairpin fusion intermediate of the paramyxovirus PIV5

During cell entry, enveloped viruses fuse their viral membrane with a cellular membrane in a process driven by energetically favorable, large-scale conformational rearrangements of their fusion proteins. Structures of the pre- and postfusion states of the fusion proteins including paramyxovirus PIV5 F and influenza virus hemagglutinin suggest that this occurs via two intermediates. Following formation of an initial complex, the proteins structurally elongate, driving a hydrophobic N-terminal "fusion peptide" away from the protein surface into the target membrane. Paradoxically, this first conformation change moves the viral and cellular bilayers further apart. Next, the fusion proteins form a hairpin that drives the two membranes into close opposition. While the pre- and postfusion hairpin forms have been characterized crystallographically, the transiently extended prehairpin intermediate has not been visualized. To provide evidence for this extended intermediate we measured the interbilayer spacing of a paramyxovirus trapped in the process of fusing with solid-supported bilayers. A gold-labeled peptide that binds the prehairpin intermediate was used to stabilize and specifically image F-proteins in the prehairpin intermediate. The interbilayer spacing is precisely that predicted from a computational model of the prehairpin, providing strong evidence for its structure and functional role. Moreover, the F-proteins in the prehairpin conformation preferentially localize to a patch between the target and viral membranes, consistent with the fact that the formation of the prehairpin is triggered by local contacts between F- and neighboring viral receptor-binding proteins (HN) only when HN binds lipids in its target membrane.

Cell-surface receptors on macrophages and dendritic cells for attachment and entry of influenza virus

Review of interactions between influenza A virus and C‐type lectin receptors on macrophages and dendritic cells that may result in virus entry and infection.

Airway MΦ and DCs are important components of innate host defense and can play a critical role in limiting the severity of influenza virus infection. Although it has been well established that cell‐surface SA acts as a primary attachment receptor for IAV, the particular receptor(s) or coreceptor(s) that mediate IAV entry into any cell, including MΦ and DC, have not been clearly defined. Identifying which receptors are involved in attachment and entry of IAV into immune cells may have important implications in regard to understanding IAV tropism and pathogenesis. Recent evidence suggests that specialized receptors on MΦ and DCs, namely CLRs, can act as capture and/or entry receptors for many viral pathogens, including IAV. Herein, we review the early stages of infection of MΦ and DC by IAV. Specifically, we examine the potential role of CLRs expressed on MΦ and DC to act as attachment and/or entry receptors for IAV.

Endosome-disruptive peptides for improving cytosolic delivery of bioactive macromolecules

Along with recent advances in therapeutic technologies based on biomacromolecules, including genes, oligonucleotides, and proteins, the development of technologies for improving the efficiency of the delivery of these therapeutic molecules into cells, more specifically into the cytosol and nucleus, is significantly required. Cell membranes are major impediments to the delivery of therapeutic macromolecules into cells. These macromolecules are usually taken up by the cells via endocytosis, and their translocation from endosomes to the cytosol is a critical step to determine their therapeutic effects. Many viruses and bacterial toxins use endocytic pathways to invade the host mammalian cells, and some of these pathogens have the ability to facilitate their endosomal escape into the cytosol by pH-induced alteration in their component proteins that leads to the disruption of the endosomal membranes and the eventual membrane fusions. To simulating these functions, endosome-disruptive peptides have been used for the intracellular delivery of biomacromolecules to accelerate their endosomal escape by sensing the endosomal acidification. In this review, current approaches for the intracellular delivery using these endosome-disruptive peptides are surveyed.

Inhibition of the ubiquitin-proteasome system affects influenza A virus infection at a postfusion step

We have demonstrated that influenza A virus (IAV) RNA synthesis depends on the ubiquitin-proteasome system. IAV replication was reduced both by proteasome inhibitors and in E36ts20 cells, which contain the thermolabile ubiquitin-activating enzyme E1. While virus entry was not affected in E36ts20 cells, the proteasome inhibitor MG132 retained viral particles in the cytoplasm. Addition-removal experiments of MG132 in combination with bafilomycin A1, a well-established inhibitor of IAV entry and fusion, showed that MG132 affected IAV infection at a postfusion step. This was confirmed by the lack of inhibition of IAV entry by proteasome inhibitors in a virus-like particle fusion assay.

Novel inhibitors of influenza virus fusion: structure-activity relationship and interaction with the viral hemagglutinin

A new class of N-(1-thia-4-azaspiro[4.5]decan-4-yl)carboxamide inhibitors of influenza virus hemagglutinin (HA)-mediated membrane fusion that has a narrow and defined structure-activity relationship was identified. In Madin-Darby canine kidney (MDCK) cells infected with different strains of human influenza virus A/H3N2, the lead compound, 4c, displayed a 50% effective concentration of 3 to 23 muM and an antiviral selectivity index of 10. No activity was observed for A/H1N1, A/H5N1, A/H7N2, and B viruses. The activity of 4c was reduced considerably when added 30 min or later postinfection, indicating that 4c inhibits an early step in virus replication. 4c and its congeners inhibited influenza A/H3N2 virus-induced erythrocyte hemolysis at low pH. 4c-resistant virus mutants, selected in MDCK cells, contained either a single D112N change in the HA2 subunit of the viral HA or a combination of three substitutions, i.e., R220S (in HA1) and E57K (in HA2) and an A-T substitution at position 43 or 96 of HA2. The mutants showed efficiency for receptor binding and replication similar to that of wild-type virus yet displayed an increased pH of erythrocyte hemolysis. In polykaryon assays with cells expressing single-mutant HA proteins, the E57K, A96T, and D112N mutations resulted in 4c resistance, and the HA proteins containing R220S, A96T, and D112N mutations displayed an increased fusion pH. Molecular modeling identified a binding cavity for 4c involving arginine-54 and glutamic acid-57 in the HA2 subunit. Our studies with the new fusion inhibitor 4c confirm the importance of this HA region in the development of influenza virus fusion inhibitors.

Cholesterol-binding viral proteins in virus entry and morphogenesis

Up to now less than a handful of viral cholesterol-binding proteins have been characterized, in HIV, influenza virus and Semliki Forest virus. These are proteins with roles in virus entry or morphogenesis. In the case of the HIV fusion protein gp41 cholesterol binding is attributed to a cholesterol recognition consensus (CRAC) motif in a flexible domain of the ectodomain preceding the trans-membrane segment. This specific CRAC sequence mediates gp41 binding to a cholesterol affinity column. Mutations in this motif arrest virus fusion at the hemifusion stage and modify the ability of the isolated CRAC peptide to induce segregation of cholesterol in artificial membranes.Influenza A virus M2 protein co-purifies with cholesterol. Its proton translocation activity, responsible for virus uncoating, is not cholesterol-dependent, and the transmembrane channel appears too short for integral raft insertion. Cholesterol binding may be mediated by CRAC motifs in the flexible post-TM domain, which harbours three determinants of binding to membrane rafts. Mutation of the CRAC motif of the WSN strain attenuates virulence for mice. Its affinity to the raft-non-raft interface is predicted to target M2 protein to the periphery of lipid raft microdomains, the sites of virus assembly. Its influence on the morphology of budding virus implicates M2 as factor in virus fission at the raft boundary. Moreover, M2 is an essential factor in sorting the segmented genome into virus particles, indicating that M2 also has a role in priming the outgrowth of virus buds.SFV E1 protein is the first viral type-II fusion protein demonstrated to directly bind cholesterol when the fusion peptide loop locks into the target membrane. Cholesterol binding is modulated by another, proximal loop, which is also important during virus budding and as a host range determinant, as shown by mutational studies.

Combined QM/MM mechanistic study of the acylation process in furin complexed with the H5N1 avian influenza virus hemagglutinin's cleavage site

Combined quantum mechanical/molecular mechanical (QM/MM) techniques have been applied to investigate the detailed reaction mechanism of the first step of the acylation process by furin in which the cleavage site of the highly pathogenic avian influenza virus subtype H5N1 (HPH5) acts as its substrate. The energy profile shows a simultaneous mechanism, known as a concerted reaction, of the two subprocesses: the proton transfer from Ser368 to His194 and the nucleophilic attack on the carbonyl carbon of the scissile peptide of the HPH5 cleavage site with a formation of tetrahedral intermediate (INT). The calculated energy barrier for this reaction is 16.2 kcal.mol(-1) at QM/MM B3LYP/6-31+G*//PM3-CHARMM22 level of theory. Once the reaction proceeds, the ordering of the electrostatic stabilization by protein environment is of the enzyme-substrate < transition state < INT complexes. Asp153 was found to play the most important role in the enzymatic reaction by providing the highest degree of intermediate complex stabilization. In addition, the negatively charged carbonyl oxygen of INT is well stabilized by the oxyanion hole constructed by Asn295's carboxamide and Ser368's backbone.

H5N1 strain-specific hemagglutinin CD4+ T cell epitopes restricted by HLA DR4

CD4+ T cells play a pivotal role in the viral immunity, and as such identification of unique strain-specific HLA class II restricted epitopes is essential for monitoring cellular strain-specific viral immunity. Using Tetramer-Guided Epitope Mapping technique, we identified HLA-DR0401 restricted HA epitopes that are strain-specific to H5N1 virion. Two immunodominant epitopes H5HA(441-460) and H5HA(57-76) were identified from in vitro stimulated human PBMC. Both epitopes elicit strong cellular immune responses when HLA-DR0401 transgenic mice are immunized with H5N1 subvirion indicating in vivo naturally processed immunodominant epitopes. The H5HA(57-76) epitope is unique for the H5N1 strain but conserved among all H5N1 clades recommended for vaccine development by World Health Organization. The unique H5HA(57-76) response was uncommon in unexposed individuals and only observed in the naïve T cell subset. Thus, H5N1 strain-specific H5HA(57-76) immunogenic epitope represents a unique marker for monitoring the efficacy of vaccination or as a candidate vaccine peptide.

Reverse genetics-generated elastase-dependent swine influenza viruses are attenuated in pigs

Influenza A virus causes significant morbidity in swine, resulting in a substantial economic burden. Swine influenza virus (SIV) infection also poses important human public health concerns. It has been shown that conversion of the haemagglutinin (HA) cleavage site from a trypsin-sensitive motif to an elastase-sensitive motif resulted in attenuated viruses in mouse models. However, application of this attenuation approach in a natural host has not been achieved yet. Here, we report that using reverse genetics, we generated two mutant SIVs derived from strain A/SW/SK/18789/02 (H1N1). Mutant A/SW/SK-R345V carries a mutation from arginine to valine at aa 345 of HA. Similarly, mutant A/SW/SK-R345A encodes alanine instead of arginine at aa 345 of HA. Our data showed that both mutants are solely dependent on neutrophil elastase cleavage in tissue culture. These tissue culture-grown mutant SIVs showed similar growth properties in terms of plaque size and growth kinetics to the wild-type virus. In addition, SIV mutants were able to maintain their genetic information after multiple passaging on MDCK cells. Furthermore, mutant SIVs were highly attenuated in pigs. Thus, these mutants may have the potential to serve as live attenuated vaccines.

Source of high pathogenicity of an avian influenza virus H5N1: why H5 is better cleaved by furin

The origin of the high pathogenicity of an emerging avian influenza H5N1 due to the -RRRKK- insertion at the cleavage loop of the hemagglutinin H5, was studied using the molecular dynamics technique, in comparison with those of the noninserted H5 and H3 bound to the furin (FR) active site. The cleavage loop of the highly pathogenic H5 was found to bind strongly to the FR cavity, serving as a conformation suitable for the proteolytic reaction. With this configuration, the appropriate interatomic distances were found for all three reaction centers of the enzyme-substrate complex: the arrangement of the catalytic triad, attachment of the catalytic Ser(368) to the reactive S1-Arg, and formation of the oxyanion hole. Experimentally, the--RRRKK--insertion was also found to increase in cleavage of hemagglutinin by FR. The simulated data provide a clear answer to the question of why inserted H5 is better cleaved by FR than the other subtypes, explaining the high pathogenicity of avian influenza H5N1.

Evaluation of Glu11 and Gly8 of the H5N1 influenza hemagglutinin fusion peptide in membrane fusion using pseudotype virus and reverse genetics

Influenza viruses gain entry into host cells by binding to cellular receptors and promoting the fusion of the viral envelope with the host cell membrane. The fusion peptide of influenza hemagglutinin (HA) is crucial for fusion. To examine the structural and functional roles of amino acids E11 and G8 of the H5 HA fusion peptide, a series of fusion mutants was generated. We determined the effect of each mutation on fusion activity and infection of rescued recombinant virus by polykaryon formation, cell-cell fusion assay, HA pseudovirus transduction and reverse genetics. Our findings indicate that E11V and E11A mutants dramatically inhibit fusion and that at position 11 a polar residue such as glutamic acid or serine may be desirable for preserving the fusion activity. More interestingly, one mutation (G8E) raised the threshold pH of polykaryon formation. Our results suggest that G8 as well as E11 play an important functional and structural role in membrane fusion and that the polarity of E11 is crucial for fusion activity. Finally, we developed an assay based on a reporter gene plus pseudotyped virus that could sensitively detect fusion activity.

Impact of adsorbents selection on capture efficiency of cell culture derived human influenza viruses

The study aims on affinity matrix selection for a cell culture derived influenza virus capture step in downstream processing. Euonymus europaeus lectin (EEL) was used as an affinity ligand. Human influenza A/Puerto Rico/8/34 (H1N1) virus produced in MDCK cells was chosen as a model strain. The chromatographic separation characteristics of reinforced cellulose membranes and different matrices such as agarose, cellulose, polymer and glass particles with immobilized EEL have been determined. Results obtained were compared to affinity matrices, which are currently used in large-scale vaccine manufacturing. Mass balances for the viral membrane protein hemagglutinin showed that EEL affinity chromatography results in higher recoveries than conventional processes using Cellufine sulphate and heparinized agarose. The most efficient media, a polymer and a cellulose membrane, have been further characterized by protein and host cell DNA measurements. Separations based on the polymer matrix and the cellulose membrane removed contaminating DNA to 0.2 and 1%, respectively. Total protein contents were decreased to 50 and 31%, respectively. The EEL-membrane showed the highest influenza virus binding capacity. These characteristics demonstrate that EEL affinity chromatography is a promising candidate for capturing influenza viruses from MDCK cell culture broths in addition to currently applied chromatographic media.

Stochastic entry of enveloped viruses: fusion versus endocytosis

Infection by membrane-enveloped viruses requires the binding of receptors on the target cell membrane to glycoproteins, or "spikes," on the viral membrane. The initial entry mechanism is usually classified as fusogenic or endocytotic. However, binding of viral spikes to cell surface receptors not only initiates the viral adhesion and the wrapping process necessary for internalization, but can simultaneously initiate direct fusion with the cell membrane. Both fusion and internalization have been observed to be viable pathways for many viruses. We develop a stochastic model for viral entry that incorporates a competition between receptor-mediated fusion and endocytosis. The relative probabilities of fusion and endocytosis of a virus particle initially nonspecifically adsorbed on the host cell membrane are computed as functions of receptor concentration, binding strength, and number of spikes. We find different parameter regimes where the entry pathway probabilities can be analytically expressed. Experimental tests of our mechanistic hypotheses are proposed and discussed.

Influenza A (H3N2) outbreak, Nepal

Worldwide emergence of variant viruses has prompted a change in the 2005–2006 H3N2 influenza A vaccine strain.

In July 2004, an outbreak of influenza A (H3N2) was detected at 3 Bhutanese refugee camps in southeastern Nepal. Hemagglutination inhibition showed that ≈40% of the viruses from this outbreak were antigenically distinct from the A/Wyoming/3/03 vaccine strain. Four amino acid differences were observed in most of the 26 isolates compared with the A/Wyoming/3/2003 vaccine strain. All 4 substitutions are located within or adjacent to known antibody-binding sites. Several isolates showed a lysine-to-asparagine substitution at position 145 (K145N) in the hemagglutinin molecule, which may be noteworthy since position 145 is located within a glycosylation site and adjacent to an antibody-binding site. H3N2 viruses continue to drift from the vaccine strain and may remain as the dominant strains during the 2005–2006 influenza season. Thus, the 2005–2006 Northern Hemisphere vaccine strain was changed to A/California/7/2004, a virus with all 4 amino acid substitutions observed in these Nepalese isolates.

Molecular characterization of the complete genome of human influenza H5N1 virus isolates from Thailand

The complete genomes of three human H5N1 influenza isolates were characterized, together with the haemagglutinin (HA) and neuraminidase (NA) genes from two additional human isolates and one chicken isolate. These six influenza isolates were obtained from four different provinces of Thailand during the avian influenza outbreak in Asia from late 2003 to May 2004. All six Thailand isolates contained multiple basic amino acids at the cleavage site in the HA gene. Amino acid residues at the receptor-binding site of the five human viruses were similar to those of the chicken virus and other H5N1 viruses from Hong Kong. The presence of amantadine resistance in the Thailand viruses isolated during this outbreak was suggested by a fixed mutation in M2 and confirmed by a phenotypic assay. All genomic segments of the Thailand viruses clustered with the recently described genotype Z. The Thailand viruses contained more avian-specific residues than the 1997 Hong Kong H5N1 viruses, suggesting that the virus may have adapted to allow a more efficient spread in avian species.