The Structure and Receptor Binding Properties of the 1918 Influenza Hemagglutinin

Generation, characterization and epitope mapping of two neutralizing and protective human recombinant antibodies against influenza A H5N1 viruses

BACKGROUND

The development of new therapeutic targets and strategies to control highly pathogenic avian influenza (HPAI) H5N1 virus infection in humans is urgently needed. Broadly cross-neutralizing recombinant human antibodies obtained from the survivors of H5N1 avian influenza provide an important role in immunotherapy for human H5N1 virus infection and definition of the critical epitopes for vaccine development.

METHODOLOGY/PRINCIPAL FINDINGS

We have characterized two recombinant baculovirus-expressed human antibodies (rhAbs), AVFluIgG01 and AVFluIgG03, generated by screening a Fab antibody phage library derived from a patient recovered from infection with a highly pathogenic avian influenza A H5N1 clade 2.3 virus. AVFluIgG01 cross-neutralized the most of clade 0, clade 1, and clade 2 viruses tested, in contrast, AVFluIgG03 only neutralized clade 2 viruses. Passive immunization of mice with either AVFluIgG01 or AVFluIgG03 antibody resulted in protection from a lethal H5N1 clade 2.3 virus infection. Furthermore, through epitope mapping, we identify two distinct epitopes on H5 HA molecule recognized by these rhAbs and demonstrate their potential to protect against a lethal H5N1 virus infection in a mouse model.

CONCLUSIONS/SIGNIFICANCE

Importantly, localization of the epitopes recognized by these two neutralizing and protective antibodies has provided, for the first time, insight into the human antibody responses to H5N1 viruses which contribute to the H5 immunity in the recovered patient. These results highlight the potential of a rhAbs treatment strategy for human H5N1 virus infection and provide new insight for the development of effective H5N1 pandemic vaccines.

Characterization of triple reassortant H1N1 influenza A viruses from swine in Ohio

An H1N1 influenza A virus, A/swine/Ohio/24366/07, was isolated from pigs in an Ohio county fair. Twenty-six people who came in contact with the infected pigs developed respiratory disease and two of these people were laboratory confirmed as H1N1 by the Centers for Disease Control and Prevention (CDC). The A/swine/Ohio/24366/07 virus we isolated from swine was shown at the CDC to have 100% identical genome sequence to the human virus associated with the county fair. This prompted us to characterize three swine and two human origin H1N1 influenza A viruses isolated at different time points in the State of Ohio. The three swine viruses were shown to be triple reassortant viruses harboring genes of human (PB1), swine (HA, NA, NP, M, and NS), and avian (PB2 and PA) lineage viruses. Although viruses evaluated in this study were isolated during a short time interval (3 years), genetic drift was observed within the HA and NA genes, including changes at the receptor binding and antigenic sites of HA1 protein. Nevertheless, all viruses exhibited antigenic similarity as evaluated with hemagglutination inhibition and virus neutralizing tests. Internal genes were similar to other reassortant viruses of various subtypes currently circulating in the United States. Interestingly, two of the swine viruses including the 2007 isolate replicated well in human airway epithelial cells, however, another virus isolated in 2006 showed very little replication.

Inferring stabilizing mutations from protein phylogenies: application to influenza hemagglutinin

One selection pressure shaping sequence evolution is the requirement that a protein fold with sufficient stability to perform its biological functions. We present a conceptual framework that explains how this requirement causes the probability that a particular amino acid mutation is fixed during evolution to depend on its effect on protein stability. We mathematically formalize this framework to develop a Bayesian approach for inferring the stability effects of individual mutations from homologous protein sequences of known phylogeny. This approach is able to predict published experimentally measured mutational stability effects (ΔΔG values) with an accuracy that exceeds both a state-of-the-art physicochemical modeling program and the sequence-based consensus approach. As a further test, we use our phylogenetic inference approach to predict stabilizing mutations to influenza hemagglutinin. We introduce these mutations into a temperature-sensitive influenza virus with a defect in its hemagglutinin gene and experimentally demonstrate that some of the mutations allow the virus to grow at higher temperatures. Our work therefore describes a powerful new approach for predicting stabilizing mutations that can be successfully applied even to large, complex proteins such as hemagglutinin. This approach also makes a mathematical link between phylogenetics and experimentally measurable protein properties, potentially paving the way for more accurate analyses of molecular evolution.

Mutating a protein frequently causes a change in its stability. As scientists, we often care about these changes because we would like to engineer a protein's stability or understand how its stability is impacted by a naturally occurring mutation. Evolution also cares about mutational stability changes, because a basic evolutionary requirement is that proteins remain sufficiently stable to perform their biological functions. Our work is based on the idea that it should be possible to use the fact that evolution selects for stability to infer from related proteins the effects of specific mutations. We show that we can indeed use protein evolutionary histories to computationally predict previously measured mutational stability changes more accurately than methods based on either of the two main existing strategies. We then test whether we can predict mutations that increase the stability of hemagglutinin, an influenza protein whose rapid evolution is partly responsible for the ability of this virus to cause yearly epidemics. We experimentally create viruses carrying predicted stabilizing mutations and find that several do in fact improve the virus's ability to grow at higher temperatures. Our computational approach may therefore be of use in understanding the evolution of this medically important virus.

Antibody recognition of a highly conserved influenza virus epitope

Influenza virus presents an important and persistent threat to public health worldwide, and current vaccines provide immunity to viral isolates similar to the vaccine strain. High-affinity antibodies against a conserved epitope could provide immunity to the diverse influenza subtypes and protection against future pandemic viruses. Cocrystal structures were determined at 2.2 and 2.7 angstrom resolutions for broadly neutralizing human antibody CR6261 Fab in complexes with the major surface antigen (hemagglutinin, HA) from viruses responsible for the 1918 H1N1 influenza pandemic and a recent lethal case of H5N1 avian influenza. In contrast to other structurally characterized influenza antibodies, CR6261 recognizes a highly conserved helical region in the membrane-proximal stem of HA1 and HA2. The antibody neutralizes the virus by blocking conformational rearrangements associated with membrane fusion. The CR6261 epitope identified here should accelerate the design and implementation of improved vaccines that can elicit CR6261-like antibodies, as well as antibody-based therapies for the treatment of influenza.

Neutralizing anti-influenza virus monoclonal antibodies: therapeutics and tools for discovery

The human antibody response to influenza virus infection plays a protective role against re-infection, yet little molecular detail is available regarding how human antibodies, when characterized at the monoclonal level, neutralize this important human pathogen. Recent studies, using a diverse array of strategies, have isolated and characterized human anti-virus neutralizing antibodies and shed light not only on the specificity and origin of these antibodies but on their potential for therapeutic use against influenza virus infection.

Cross-protective potential of a novel monoclonal antibody directed against antigenic site B of the hemagglutinin of influenza A viruses

The hemagglutinin (HA) of influenza A viruses has been classified into sixteen distinct subtypes (H1–H16) to date. The HA subtypes of influenza A viruses are principally defined as serotypes determined by neutralization or hemagglutination inhibition tests using polyclonal antisera to the respective HA subtypes, which have little cross-reactivity to the other HA subtypes. Thus, it is generally believed that the neutralizing antibodies are not broadly cross-reactive among HA subtypes. In this study, we generated a novel monoclonal antibody (MAb) specific to HA, designated MAb S139/1, which showed heterosubtypic cross-reactive neutralization and hemagglutination inhibition of influenza A viruses. This MAb was found to have broad reactivity to many other viruses (H1, H2, H3, H5, H9, and H13 subtypes) in enzyme-linked immunosorbent assays. We further found that MAb S139/1 showed neutralization and hemagglutination-inhibition activities against particular strains of H1, H2, H3, and H13 subtypes of influenza A viruses. Mutant viruses that escaped neutralization by MAb S139/1 were selected from the A/Aichi/2/68 (H3N2), A/Adachi/2/57 (H2N2), and A/WSN/33 (H1N1) strains, and sequence analysis of the HA genes of these escape mutants revealed amino acid substitutions at positions 156, 158, and 193 (H3 numbering). A molecular modeling study showed that these amino acids were located on the globular head of the HA and formed a novel conformational epitope adjacent to the receptor-binding domain of HA. Furthermore, passive immunization of mice with MAb S139/1 provided heterosubtypic protection. These results demonstrate that MAb S139/1 binds to a common antigenic site shared among a variety of HA subtypes and neutralizes viral infectivity in vitro and in vivo by affecting viral attachment to cells. The present study supports the notion that cross-reactive antibodies play some roles in heterosubtypic immunity against influenza A virus infection, and underscores the potential therapeutic utility of cross-reactive antibodies against influenza.

Neutralizing antibodies play a critical role in protection from influenza A virus infection. Most neutralizing antibodies recognize hemagglutinin (HA), which is the major surface glycoprotein of influenza viruses. The HA has been classified into sixteen antigenically distinct subtypes. Since HA subtypes of influenza A viruses are principally defined as serotypes determined by neutralization or hemagglutination inhibition tests using polyclonal antisera to the respective HA subtypes, which have little cross-reactivity to the other HA subtypes, it is generally believed that the neutralizing antibodies are not broadly cross-reactive among HA subtypes. Herein we present a novel cross-neutralizing monoclonal antibody that reacts with a variety of HA subtypes in vitro and provides heterosubtypic protection against influenza A virus infections in mice. We demonstrate that this antibody recognizes a common epitope adjacent to the receptor binding region of HA and inhibits virus binding to the cells. The present study supports the notion that cross-reactive antibodies, as well as cytotoxic T lymphocytes, play some roles in heterosubtypic immunity against influenza A virus infection, and underscores the potential therapeutic utility of cross-reactive monoclonal antibodies for multivalent prophylaxis and treatment against infection with influenza A viruses, including the hypothetical new pandemic influenza viruses.

Structural and functional bases for broad-spectrum neutralization of avian and human influenza A viruses

Influenza virus remains a serious health threat, owing to its ability to evade immune surveillance through rapid genetic drift and reassortment. Here we used a human non-immune antibody phage-display library and the H5 hemagglutinin ectodomain to select ten neutralizing antibodies (nAbs) that were effective against all group 1 influenza viruses tested, including H5N1 'bird flu' and the H1N1 'Spanish flu'. The crystal structure of one such nAb bound to H5 shows that it blocks infection by inserting its heavy chain into a conserved pocket in the stem region, thus preventing membrane fusion. Nine of the nAbs employ the germline gene VH1-69, and all seem to use the same neutralizing mechanism. Our data further suggest that this region is recalcitrant to neutralization escape and that nAb-based immunotherapy is a promising strategy for broad-spectrum protection against seasonal and pandemic influenza viruses.

HA-pseudotyped retroviral vectors for influenza antagonist screening

Influenza infections are initiated by the binding of the influenza hemagglutinin (HA) and the cellular receptor sialic acids. The binding is followed by internalization, endocytosis, and uncoating to release the influenza genome to the cytoplasm. It is conceivable that specific inhibitors that antagonize any one of these events could prevent the replication of influenza infections. The authors made HA pseudotyped retroviral vectors that express luciferase reporter activities upon transduction to several recipient cells. The transduction of the HA-pseudotype virus particles (HApp) was mediated through the specific interactions between an avian HA and the terminal disaccharides of sialic acid (SA) and galactose (Gal) in alpha-2,3 linkage. The HApp-mediated transduction method was used to develop a high-throughput screening assay and to screen for hits from a fermentation extract library. Specific hits that inhibited the HA-mediated but were noninhibitory to the vesicular stomatitis virus-mediated pseudoviral transductions were identified. A few of these hits have anti-influenza activities that prevent the replication of both H1N1 (WSN) and H5N1 (RG14) influenza viruses.

Distinct glycan topology for avian and human sialopentasaccharide receptor analogues upon binding different hemagglutinins: a molecular dynamics perspective

Hemagglutinin (HA) binds to sialylated glycans exposed on the host cell surface in the initial stage of avian influenza virus infection. It has been previously hypothesized that glycan topology plays a critical role in the human adaptation of avian flu viruses, such as the potentially pandemic H5N1. Comparative molecular dynamics studies are complementary to experimental techniques, including glycan microarray, to understand the mechanism of species-specificity switch better. The examined systems comprise explicitly solvated trimeric forms of avian H3, H5, and swine H9 in complex with avian and human glycan receptor analogues--LSTa (alpha-2,3-linked lactoseries tetrasaccharide a) and LSTc (alpha-2,6-linked lactoseries tetrasaccharide c), respectively. The glycans adopted distinct topological profiles with inducible torsional angles when bound to different HAs. The corresponding receptor binding domain amino acid contact profiles were also distinct. Avian H5 was able to accommodate LSTc in a tightly "folded umbrella"-like topology through interactions with all five sugar residues. After considering conformational entropy, the relative binding free-energy changes, calculated using the molecular mechanics-generalized Born surface area technique, were in agreement with previous experimental findings and provided insights on electrostatic, van der Waals, desolvation, and entropic contributions to HA-glycan interactions. The topology profile and the relative abundance of free glycan receptors may influence receptor binding kinetics. Glycan composition and topological changes upon binding different HAs may be important determinants in species-specificity switch.

Amino acid residues in the fusion peptide pocket regulate the pH of activation of the H5N1 influenza virus hemagglutinin protein

The receptor specificity and cleavability of the hemagglutinin (HA) protein have been shown to regulate influenza A virus transmissibility and pathogenicity, but little is known about how its pH of activation contributes to these important biological properties. To identify amino acid residues that regulate the acid stability of the HA protein of H5N1 influenza viruses, we performed a mutational analysis of the HA protein of the moderately pathogenic A/chicken/Vietnam/C58/04 (H5N1) virus. Nineteen HA proteins containing point mutations in the HA2 coiled-coil domain or in an HA1 histidine or basic patch were generated. Wild-type and mutant HA plasmids were transiently transfected in cell culture and analyzed for total protein expression, surface expression, cleavage efficiency, pH of fusion, and pH of conformational change. Four mutations to residues in the fusion peptide pocket, Y23H and H24Q in the HA1 subunit and E105K and N114K in the HA2 subunit, and a K58I mutation in the HA2 coiled-coil domain significantly altered the pH of activation of the H5 HA protein. In some cases, the magnitude and direction of changes of individual mutations in the H5 HA protein differed considerably from similar mutations in other influenza A virus HA subtypes. Introduction of Y23H, H24Q, K58I, and N114K mutations into recombinant viruses resulted in virus-expressed HA proteins with similar shifts in the pH of fusion. Overall, the data show that residues comprising the fusion peptide pocket are important in triggering pH-dependent activation of the H5 HA protein.

The Polyomaviridae: Contributions of virus structure to our understanding of virus receptors and infectious entry

This review summarizes the field's major findings related to the characterization of polyomavirus structures and to the characterization of virus receptors and mechanisms of host cell invasion. The four members of the family that have received the most attention in this regard are the mouse polyomavirus (mPyV), the monkey polyomavirus SV40, and the two human polyomaviruses, JCV and BKV. The structures of both the mPyV and SV40 alone and in complex with receptor fragments have been solved to high resolution. The majority of polyomaviruses recognize terminal sialic acid in either an α2,3 linkage or an α2,6 linkage to the underlying galactose. Studies on virus structure, receptor utilization and mechanisms of entry have led to new insights into how these viruses interact in an active way with cells to ensure the nuclear delivery and expression of their genomes. Critical work on virus entry has led to the discovery of a pH neutral endocytic compartment that accepts cargo from caveolae and to novel roles for endoplasmic reticulum (ER) associated factors in virus uncoating and penetration of ER membranes. This review will summarize the major findings and compare and contrast the mechanisms used by these viruses to infect cells.

Influenza

Influenza is a highly contagious, acute respiratory illness afflicting humans. Although influenza epidemics occur frequently, their severity varies (1). Not until 1933, when the first human influenza virus was isolated, was it possible to define with certainty which pandemics were caused by influenza viruses. In general, influenza A viruses are more pathogenic than are influenza B viruses. Influenza A virus is a zoonotic infection, and more than 100 types of influenza A viruses infect most species of birds, pigs, horses, dogs, and seals. It is believed that the 1918–1919 pandemic originated from a virulent strain of H1N1 from pigs and birds.

Mass spectrometry analysis of the influenza virus

The role of mass spectrometry to probe characteristics of the influenza virus, and vaccine and antiviral drugs that target the virus, are reviewed. Genetic and proteomic approaches have been applied which incorporate high resolution mass spectrometry and mass mapping to genotype the virus and establish its evolution in terms of the primary structure of the surface protein antigens. A mass spectrometric immunoassay has been developed and applied to assess the structure and antigenicity of the virus in terms of the hemagglutinin antigen. The quantitation of the hemagglutinin antigen in vaccine preparations has also been conducted that is of importance to their efficacy. Finally, the characterization and quantitation of antiviral drugs against the virus, and their metabolites, have been monitored in blood, serum, and urine. The combined approaches demonstrate the strengths of modern mass spectrometric methods for the characterization of this killer virus. [This article was published online 10 September 2008. An error was subsequently identified. This notice is included in the online and print versions to indicate that both have been corrected 7 November 2008.]

Designing vaccines for pandemic influenza

Recent outbreaks of highly pathogenic avian influenza A virus infections (including those of the H5N1 subtype) in poultry and in humans (through contact with infected birds) have raised concerns that a new influenza pandemic will soon occur. Effective vaccines against H5N1 virus are therefore urgently needed. Reverse genetics-based inactivated vaccines have been prepared according to WHO recommendations and licensed in several countries following their assessment in clinical trials. However, the effectiveness of these vaccines in a pandemic is not guaranteed. We must therefore continue to develop alternative pandemic vaccine strategies. Here, we review the current strategies for the development of H5N1 influenza vaccines, as well as some future directions for vaccine development.

Atomic-resolution conformational analysis of the GM3 ganglioside in a lipid bilayer and its implications for ganglioside-protein recognition at membrane surfaces

Eukaryotic cells depend on external surface markers, such as gangliosides, to recognize and bind various other molecules as part of normal growth and maturation. The localization of gangliosides in the outer leaflet of the plasma membrane, also make them targets for pathogens trying to invade the host cells. Since ganglioside-mediated interactions are critical to both beneficial and pathological processes, much effort has been directed at determining the 3D structures of their carbohydrate head groups; however, technical difficulties have generally prevented the characterization of the head group in intact membrane-bound gangliosides. Determining the 3D structure and presentation of gangliosides at the surface of membranes is important in understanding how cells interact with their local environment. Here, we employ all-atom explicit solvent molecular dynamics (MD) simulations, using the GLYCAM06 force field, to model the conformation and dynamics of ganglioside G(M3) (alpha-Neu5Ac-(2-3)-beta-Gal-(1-4)-beta-Glc-ceramide) in a DMPC lipid bilayer. By comparison with MD simulations of the carbohydrate head-group fragment of G(M3) alone, it was possible to quantify and characterize the extent of changes in head-group presentation and dynamics associated with membrane anchoring. The accuracy of data from the MD simulations was determined by comparison to NMR and crystallographic data for the head group in solution and for G(M3) in membrane-mimicking environments. The experimentally consistent model of G(M3), in a lipid bilayer, was then used to model the recognition of G(M3) at the cell surface by known protein receptors.

Experimental evolution of human influenza virus H3 hemagglutinin in the mouse lung identifies adaptive regions in HA1 and HA2

The genetic basis for virulence and host switching in influenza A viruses (FLUAV) is largely unknown. Because the hemagglutinin (HA) protein is a determinant of these properties, HA evolution was mapped in an experimental model of mouse lung adaptation. Variants of prototype A/Hong Kong/1/68 (H3N2) (wild-type [wt] HK) human virus were selected in both longitudinal and parallel studies of lung adaptation. Mapping of HA mutations found in 11 independently derived mouse-adapted populations of wt HK identified 27 mutations that clustered within two distinct regions in or near the globular frameworks of the HA1 and HA2 subunits. The adaptive mutations demonstrated multiple instances of convergent evolution involving four amino acid positions (162, 210, and 218 in HA1 and 154 in HA2). By use of reverse genetics, convergent HA mutations were shown to affect cell tropism by enhancing infection and replication in primary mouse tracheal epithelial cells in vitro and mouse lung tissue in vivo. Adaptive HA mutations were multifunctional, affecting both median pH of fusion and receptor specificity. Specific mutations within both adaptive regions were shown to increase virulence in a mouse lung model. The occurrence of mutations in the HA1 and HA2 adaptive regions of natural FLUAV host range and virulent variants of avian and mammalian viruses is discussed. This study has identified adaptive sites and regions within the HA1 and HA2 subunits that may guide future studies of viral adaptation and evolution in nature.

Microbial recognition of human cell surface glycoconjugates

Infection by pathogens is generally initiated by the specific recognition of host epithelia surfaces and subsequent adhesion is essential for invasion. In their infection strategy, microorganisms often use sugar-binding proteins, that is lectins and adhesins, to recognize and bind to host glycoconjugates where sialylated and fucosylated oligosaccharides are the major targets. The lectin/glycoconjugate interactions are characterized by their high specificity and most of the time by multivalency to generate higher affinity of binding. Recent crystal structures of viral, bacterial, and parasite receptors in complex with human histo-blood group epitopes or sialylated derivatives reveal new folds and novel sugar-binding modes. They illustrate the tight specificity between tissue glycosylation and lectins.

Role of viral hemagglutinin glycosylation in anti-influenza activities of recombinant surfactant protein D

Background

Surfactant protein D (SP-D) plays an important role in innate defense against influenza A viruses (IAVs) and other pathogens.

Methods

We tested antiviral activities of recombinant human SP-D against a panel of IAV strains that vary in glycosylation sites on their hemagglutinin (HA). For these experiments a recombinant version of human SP-D of the Met11, Ala160 genotype was used after it was characterized biochemically and structurally.

Results

Oligosaccharides at amino acid 165 on the HA in the H3N2 subtype and 104 in the H1N1 subtype are absent in collectin-resistant strains developed in vitro and are important for mediating antiviral activity of SP-D; however, other glycans on the HA of these viral subtypes also are involved in inhibition by SP-D. H3N2 strains obtained shortly after introduction into the human population were largely resistant to SP-D, despite having the glycan at 165. H3N2 strains have become steadily more sensitive to SP-D over time in the human population, in association with addition of other glycans to the head region of the HA. In contrast, H1N1 strains were most sensitive in the 1970s–1980s and more recent strains have become less sensitive, despite retaining the glycan at 104. Two H5N1 strains were also resistant to inhibition by SP-D. By comparing sites of glycan attachment on sensitive vs. resistant strains, specific glycan sites on the head domain of the HA are implicated as important for inhibition by SP-D. Molecular modeling of the glycan attachment sites on HA and the carbohydrate recognition domain of SPD are consistent with these observations.

Conclusion

Inhibition by SP-D correlates with presence of several glycan attachment sites on the HA. Pandemic and avian strains appear to lack susceptibility to SP-D and this could be a contributory factor to their virulence.

Protein intrinsic disorder toolbox for comparative analysis of viral proteins

To examine the usefulness of protein disorder predictions as a tool for the comparative analysis of viral proteins, a relational database has been constructed. The database includes proteins from influenza A and HIV-related viruses. Annotations include viral protein sequence, disorder prediction, structure, and function. Location of each protein within a virion, if known, is also denoted. Our analysis reveals a clear relationship between proximity to the RNA core and the percentage of predicted disordered residues for a set of influenza A virus proteins.

Neuraminidases (NA) and hemagglutinin (HA) of major influenza A pandemics tend to pair in such a way that both proteins tend to be either ordered-ordered or disordered-disordered by prediction. This may be the result of these proteins evolving from being lipid-associated. High abundance of intrinsic disorder in envelope and matrix proteins from HIV-related viruses likely represents a mechanism where HIV virions can escape immune response despite the availability of antibodies for the HIV-related proteins. This exercise provides an example showing how the combined use of intrinsic disorder predictions and relational databases provides an improved understanding of the functional and structural behaviour of viral proteins.

Cross-species virus transmission and the emergence of new epidemic diseases

Host range is a viral property reflecting natural hosts that are infected either as part of a principal transmission cycle or, less commonly, as "spillover" infections into alternative hosts. Rarely, viruses gain the ability to spread efficiently within a new host that was not previously exposed or susceptible. These transfers involve either increased exposure or the acquisition of variations that allow them to overcome barriers to infection of the new hosts. In these cases, devastating outbreaks can result. Steps involved in transfers of viruses to new hosts include contact between the virus and the host, infection of an initial individual leading to amplification and an outbreak, and the generation within the original or new host of viral variants that have the ability to spread efficiently between individuals in populations of the new host. Here we review what is known about host switching leading to viral emergence from known examples, considering the evolutionary mechanisms, virus-host interactions, host range barriers to infection, and processes that allow efficient host-to-host transmission in the new host population.

Bench-to-bedside review: rare and common viral infections in the intensive care unit--linking pathophysiology to clinical presentation

Viral infections are common causes of respiratory tract disease in the outpatient setting but much less common in the intensive care unit. However, a finite number of viral agents cause respiratory tract disease in the intensive care unit. Some viruses, such as influenza, respiratory syncytial virus (RSV), cytomegalovirus (CMV), and varicella-zoster virus (VZV), are relatively common. Others, such as adenovirus, severe acute respiratory syndrome (SARS)-coronavirus, Hantavirus, and the viral hemorrhagic fevers (VHFs), are rare but have an immense public health impact. Recognizing these viral etiologies becomes paramount in treatment, infection control, and public health measures. Therefore, a basic understanding of the pathogenesis of viral entry, replication, and host response is important for clinical diagnosis and initiating therapeutic options. This review discusses the basic pathophysiology leading to clinical presentations in a few common and rare, but important, viruses found in the intensive care unit: influenza, RSV, SARS, VZV, adenovirus, CMV, VHF, and Hantavirus.

A broad spectrum, one-step reverse-transcription PCR amplification of the neuraminidase gene from multiple subtypes of influenza A virus

BACKGROUND

The emergence of high pathogenicity strains of Influenza A virus in a variety of human and animal hosts, with wide geographic distribution, has highlighted the importance of rapid identification and subtyping of the virus for outbreak management and treatment. Type A virus can be classified into subtypes according to the viral envelope glycoproteins, hemagglutinin and neuraminidase. Here we review the existing specificity and amplification of published primers to subtype neuraminidase genes and describe a new broad spectrum primer pair that can detect all 9 neuraminidase subtypes.

RESULTS

Bioinformatic analysis of 3,337 full-length influenza A neuraminidase segments in the NCBI database revealed semi-conserved regions not previously targeted by primers. Two degenerate primers with M13 tags, NA8F-M13 and NA10R-M13 were designed from these regions and used to generate a 253 bp cDNA product. One-step RT-PCR testing was successful in 31/32 (97%) cases using a touchdown protocol with RNA from over 32 different cultured influenza A virus strains representing the 9 neuraminidase subtypes. Frozen blinded clinical nasopharyngeal aspirates were also assayed and were mostly of subtype N2. The region amplified was direct sequenced and then used in database searches to confirm the identity of the template RNA. The RT-PCR fragment generated includes one of the mutation sites related to oseltamivir resistance, H274Y.

CONCLUSION

Our one-step RT-PCR assay followed by sequencing is a rapid, accurate, and specific method for detection and subtyping of different neuraminidase subtypes from a range of host species and from different geographical locations.

Structural insight into epitopes in the pregnancy-associated malaria protein VAR2CSA

Pregnancy-associated malaria is caused by Plasmodium falciparum malaria parasites binding specifically to chondroitin sulfate A in the placenta. This sequestration of parasites is a major cause of low birth weight in infants and anemia in the mothers. VAR2CSA, a polymorphic multi-domain protein of the PfEMP1 family, is the main parasite ligand for CSA binding, and identification of protective antibody epitopes is essential for VAR2CSA vaccine development. Attempts to determine the crystallographic structures of VAR2CSA or its domains have not been successful yet. In this study, we propose 3D models for each of the VAR2CSA DBL domains and we show that regions in the fold of VAR2CSA inter-domain 2 and a PfEMP1 CIDR domain seem to be homologous to the EBA-175 and Pkα-DBL fold. This suggests that ID2 could be a functional domain. We also identify regions of VAR2CSA present on the surface of native VAR2CSA by comparing reactivity of plasma containing anti-VAR2CSA antibodies in peptide array experiments before and after incubation with native VAR2CSA. By this method we identify conserved VAR2CSA regions targeted by antibodies that react with the native molecule expressed on infected erythrocytes. By mapping the data onto the DBL models we present evidence suggesting that the S1+S2 DBL sub-domains are generally surface-exposed in most domains, whereas the S3 sub-domains are less exposed in native VAR2CSA. These results comprise an important step towards understanding the structure of VAR2CSA on the surface of CSA-binding infected erythrocytes.

Individuals living in areas with high Plasmodium falciparum transmission acquire immunity to malaria over time and adults have markedly reduced risk of getting severe disease. However, pregnant women constitute an important exception, and they become more susceptible to malaria during pregnancy. This so called pregnancy-associated malaria (PAM) has severe consequences for both mother and child, and a vaccine would save hundreds of thousands of lives each year. PAM is caused by P. falciparum–infected red blood cells that bind to receptors in the placenta. By binding to the placental tissue, the parasites avoid being filtered though the spleen where they would have been killed. The protein mediating this placental binding is a very large multidomain and variant protein named VAR2CSA. Using structural modeling of VAR2CSA and antibody reagents from women who have had PAM, we show that antibodies tend to bind in similar regions, on one side of the individual VAR2CSA domains. In addition, we show that highly conserved parts of this variant protein are accessible for antibodies. This finding correlates with epidemiological data showing that woman acquire immunity towards PAM relatively fast, and the identification of these epitopes is thus a major step towards a protective vaccine.

Characterization of the prefusion and transition states of severe acute respiratory syndrome coronavirus S2-HR2

The envelope glycoproteins of the class I family, which include human immunodeficiency virus (HIV), influenza, and severe acute respiratory syndrome coronavirus (SARS-CoV), mediate viral entry by first binding to their cellular receptors and subsequently inducing fusion of the viral and cellular membranes. In the case of SARS-CoV, heptad repeat domains of the envelope glycoprotein, termed S2-HR1 and S2-HR2, are thought to undergo structural changes from a prefusion state, in which S2-HR1 and S2-HR2 do not interact, to a postfusion state in which S2-HR1 and S2-HR2 associate to form a six-helix bundle. In the present work, the structural and dynamic properties of S2-HR2 have been characterized. Evidence is presented for an equilibrium between a structured trimer thought to represent a prefusion state and an ensemble of unstructured monomers thought to represent a novel transition state. A model for viral entry is presented in which S2-HR2 is in a dynamic equilibrium between an ensemble of unstructured monomers in the transition state and a structured trimer in the prefusion state. Conversion from the prefusion state to the postfusion state requires passage through the transition state, a state that may give insight into the design of structure-based antagonists of SARS-CoV in particular, as well as other enveloped viruses in general.

Avian influenza virus

Avian influenza viruses do not typically replicate efficiently in humans, indicating direct transmission of avian influenza virus to humans is unlikely. However, since 1997, several cases of human infections with different subtypes (H5N1, H7N7, and H9N2) of avian influenza viruses have been identified and raised the pandemic potential of avian influenza virus in humans. Although circumstantial evidence of human to human transmission exists, the novel avian-origin influenza viruses isolated from humans lack the ability to transmit efficiently from person-to-person. However, the on-going human infection with avian-origin H5N1 viruses increases the likelihood of the generation of human-adapted avian influenza virus with pandemic potential. Thus, a better understanding of the biological and genetic basis of host restriction of influenza viruses is a critical factor in determining whether the introduction of a novel influenza virus into the human population will result in a pandemic. In this article, we review current knowledge of type A influenza virus in which all avian influenza viruses are categorized.

Efficacious recombinant influenza vaccines produced by high yield bacterial expression: a solution to global pandemic and seasonal needs

It is known that physical linkage of TLR ligands and vaccine antigens significantly enhances the immunopotency of the linked antigens. We have used this approach to generate novel influenza vaccines that fuse the globular head domain of the protective hemagglutinin (HA) antigen with the potent TLR5 ligand, flagellin. These fusion proteins are efficiently expressed in standard E. coli fermentation systems and the HA moiety can be faithfully refolded to take on the native conformation of the globular head. In mouse models of influenza infection, the vaccines elicit robust antibody responses that mitigate disease and protect mice from lethal challenge. These immunologically potent vaccines can be efficiently manufactured to support pandemic response, pre-pandemic and seasonal vaccines.

A bivalent influenza VLP vaccine confers complete inhibition of virus replication in lungs

The conventional egg-grown influenza vaccines are trivalent. To test the feasibility of using multivalent influenza virus-like particles (VLPs) as an alternative influenza vaccine, we developed cell-derived influenza VLPs containing the hemagglutinin (HA) of the H1 subtype virus A/PR/8/34 or the H3 subtype virus A/Aichi/2/68 (X31). Mice immunized intramuscularly with bivalent influenza VLPs containing H1 and H3 HAs induced neutralizing activities against the homologous and closely related H1N1 strains A/PR/8/34 and A/WSN/33 as well as the H3N2 strains A/Aichi/2/68 (X31) and A/Hong Kong/68, but not the A/Philippines/2/82 strain isolated 14 years later. HA sequence and structure analysis indicated that antigenic distance could be a major factor in predicting cross-protection by VLP vaccines. The bivalent influenza VLP vaccine demonstrated advantages in broadening the protective immunity after lethal challenge infections when compared to a monovalent influenza VLP vaccine. High levels of the inflammatory cytokine IL-6 were observed in naïve or unprotected immunized mice but not in protected mice upon lethal challenge. These results indicate that multivalent influenza VLP vaccines can be an effective antigen for developing safe and alternative vaccine to control the spread of influenza viruses.

Infectivity studies of influenza virus hemagglutinin receptor binding site mutants in mice

The replicative properties of influenza virus hemagglutinin (HA) mutants with altered receptor binding characteristics were analyzed following intranasal inoculation of mice. Among the mutants examined was a virus containing a Y98F substitution at a conserved position in the receptor binding site that leads to a 20-fold reduction in binding. This mutant can replicate as well as wild-type (WT) virus in MDCK cells and in embryonated chicken eggs but is highly attenuated in mice, exhibiting titers in lungs more than 1,000-fold lower than those of the WT. The capacity of the Y98F mutant to induce antibody responses and the structural locations of HA reversion mutations are examined.

Crystal structure of unliganded influenza B virus hemagglutinin

Here we report the crystal structure of hemagglutinin (HA) from influenza B/Hong Kong/8/73 (B/HK) virus determined to 2.8 A. At a sequence identity of approximately 25% to influenza A virus HAs, B/HK HA shares a similar overall structure and domain organization. More than two dozen amino acid substitutions on influenza B virus HAs have been identified to cause antigenicity alteration in site-specific mutants, monoclonal antibody escape mutants, or field isolates. Mapping these substitutions on the structure of B/HK HA reveals four major epitopes, the 120 loop, the 150 loop, the 160 loop, and the 190 helix, that are located close in space to form a large, continuous antigenic site. Moreover, a systematic comparison of known HA structures across the entire influenza virus family reveals evolutionarily conserved ionizable residues at all regions along the chain and subunit interfaces. These ionizable residues are likely the structural basis for the pH dependence and sensitivity to ionic strength of influenza HA and hemagglutinin-esterase fusion proteins.

Quantitative biochemical rationale for differences in transmissibility of 1918 pandemic influenza A viruses

The human adaptation of influenza A viruses is critically governed by the binding specificity of the viral surface hemagglutinin (HA) to long (chain length) alpha2-6 sialylated glycan (alpha2-6) receptors on the human upper respiratory tissues. A recent study demonstrated that whereas the 1918 H1N1 pandemic virus, A/South Carolina/1/1918 (SC18), with alpha2-6 binding preference transmitted efficiently, a single amino acid mutation on HA resulted in a mixed alpha2-3 sialylated glycan (alpha2-3)/alpha2-6 binding virus (NY18) that transmitted inefficiently. To define the biochemical basis for the observed differences in virus transmission, in this study, we have developed an approach to quantify the multivalent HA-glycan interactions. Analysis of the molecular HA-glycan contacts showed subtle changes resulting from the single amino acid variations between SC18 and NY18. The effect of these changes on glycan binding is amplified by multivalency, resulting in quantitative differences in their long alpha2-6 glycan binding affinities. Furthermore, these differences are also reflected in the markedly distinct binding pattern of SC18 and NY18 HA to the physiological glycans present in human upper respiratory tissues. Thus, the dramatic lower binding affinity of NY18 to long alpha2-6 glycans, as against a mixed alpha2-3/6 binding, correlates with its inefficient transmission. In summary, this study establishes a quantitative biochemical correlate for influenza A virus transmission.

Antigenic drift in the evolution of H1N1 influenza A viruses resulting from deletion of a single amino acid in the haemagglutinin gene

Two genetically distinct lineages of H1N1 influenza A viruses, circulated worldwide before 1994, were antigenically indistinguishable. In 1994, viruses emerged in China, including A/Beijing/262/95, with profound antigenic differences from the contemporary circulating H1N1 strains. Haemagglutinin sequence comparisons of either a predecessor virus, A/Hebei/52/94, or one representative of the cocirculating A/Bayern/7/95-like clade, A/Shenzhen/227/95, revealed a deletion of K at position 134 (H3 numbering) in the antigenic variants. The K134 deletion conferred a selective advantage to the Chinese deletion lineage, such that it eventually gave rise to currently circulating H1 viruses. Using reverse genetics to generate viruses with either an insertion or deletion of aa 134, we have confirmed that the K134 deletion, rather than a constellation of sublineage specific amino acid changes, was sufficient for the antigenic difference observed in the Chinese deletion lineage, and reinsertion of K134 revealed the requirement of a compatible neuraminidase surface glycoprotein for viral growth.

Glycan topology determines human adaptation of avian H5N1 virus hemagglutinin

A switch in specificity of avian influenza A viruses' hemagglutinin (HA) from avian-like (alpha2-3 sialylated glycans) to human-like (alpha2-6 sialylated glycans) receptors is believed to be associated with their adaptation to infect humans. We show that a characteristic structural topology--and not the alpha2-6 linkage itself--enables specific binding of HA to alpha2-6 sialylated glycans and that recognition of this topology may be critical for adaptation of HA to bind glycans in the upper respiratory tract of humans. An integrated biochemical, analytical and data mining approach demonstrates that HAs from the human-adapted H1N1 and H3N2 viruses, but not H5N1 (bird flu) viruses, specifically bind to long alpha2-6 sialylated glycans with this topology. This could explain why H5N1 viruses have not yet gained a foothold in the human population. Our findings will enable the development of additional strategies for effective surveillance and potential therapeutic interventions for H5N1 and possibly other influenza A viruses.

Ligand effects on the protein ensemble: unifying the descriptions of ligand binding, local conformational fluctuations, and protein stability

Detailed description of the structural and physical basis of allostery, cooperativity, and other manifestations of long-range communication between binding sites in proteins remains elusive. Here we describe an ensemble-based structural-thermodynamic model capable of treating explicitly the coupling between ligand binding reactions, local fluctuations in structure, and global conformational transitions. The H(+) binding reactions of staphylococcal nuclease and the effects of pH on its stability were used to illustrate the properties of proteins that can be described quantitatively with this model. Each microstate in the native ensemble was modeled to have dual structural character; some regions were treated as folded and retained the same atomic geometry as in the crystallographic structure while other regions were treated thermodynamically as if they were unfolded. Two sets of pK(a) values were used to describe the affinity of each H(+) binding site. One set, calculated with a standard continuum electrostatics method, describes H(+) binding to sites in folded parts of the protein. A second set of pK(a) values, obtained from model compounds in water, was used to describe H(+) binding to sites in unfolded regions. An empirical free energy function, parameterized to reproduce folding thermodynamics measured by differential scanning calorimetry, was used to calculate the probability of each microstate. The effects of pH on the distribution of microstates were determined by the H(+) binding properties of each microstate. The validity of the calculations was established by comparison with a number of different experimental observables.

History of Influenza Pandemics

Influenza pandemics have been amongst the largest and the deadliest epidemics in the history of man, and were observed already in ancient times. For example, records from the fifth century B.C. suggest that influenza pandemics were observed in ancient Greece. In Europe, during the Middle Ages and the Renaissance, numerous concordant reports from different countries describe epidemics of respiratory infections that resemble influenza pandemics. However, it is not possible to be certain that these epidemics were due to influenza. In the twentieth century, three influenza pandemics have occurred, including the deadly Spanish flu pandemic. Modern virology has unravelled the mechanisms of emergence of pandemic viruses, and considerable knowledge on influenza viruses has been accumulated. The picture is now clear: influenza A is a zoonotic virus whose reservoir is in wild birds. In rare cases, these avian viruses are introduced into man and, eventually, become pandemic viruses. Although these mechanisms are now understood, the time frame required for adaptation of the avian virus to its new host remains unknown. Maybe the next pandemic will show us how rapid this adaptation can be.

Analysis of residues near the fusion peptide in the influenza hemagglutinin structure for roles in triggering membrane fusion

Influenza virus entry occurs in endosomes, where acidification triggers irreversible conformational changes of the hemagglutinin glycoprotein (HA) that are required for membrane fusion. The acid-induced HA structural rearrangements have been well documented, and several models have been proposed to relate these to the process of membrane fusion. However, details regarding the role of specific residues in the initiation of structural rearrangements and membrane fusion are lacking. Here we report the results of studies on the HA of A/Aichi/2/68 virus (H3 subtype), in which mutants with changes at several ionizable residues in the vicinity of the "fusion peptide" were analyzed for their effects on the pH at which conformational changes and membrane fusion occur. A variety of phenotypes was obtained, including examples of substitutions that lead to an increase in HA stability at reduced pH. Of particular note was the observation that a histidine to tyrosine substitution at HA1 position 17 resulted in a decrease in pH at which HA structural changes and membrane fusion take place by 0.3 relative to WT. The results are discussed in relation to possible mechanisms by which HA structural rearrangements are initiated at low pH and clade-specific differences near the fusion peptide.

Structural basis for receptor specificity of influenza B virus hemagglutinin

Receptor-binding specificity of HA, the major surface glycoprotein of influenza virus, primarily determines the host ranges that the virus can infect. Influenza type B virus almost exclusively infects humans and contributes to the annual "flu" sickness. Here we report the structures of influenza B virus HA in complex with human and avian receptor analogs, respectively. These structures provide a structural basis for the different receptor-binding properties of influenza A and B virus HA molecules and for the ability of influenza B virus HA to distinguish human and avian receptors. The structure of influenza B virus HA with avian receptor analog also reveals how mutations in the region of residues 194 to 196, which are frequently observed in egg-adapted and naturally occurring variants, directly affect the receptor binding of the resultant virus strains. Furthermore, these structures of influenza B virus HA are compared with known structures of influenza A virus HAs, which suggests the role of the residue at 222 as a key and likely a universal determinant for the different binding modes of human receptor analogs by different HA molecules.

Immunization by avian H5 influenza hemagglutinin mutants with altered receptor binding specificity

Influenza virus entry is mediated by the receptor binding domain (RBD) of its spike, the hemagglutinin (HA). Adaptation of avian viruses to humans is associated with HA specificity for alpha2,6- rather than alpha2,3-linked sialic acid (SA) receptors. Here, we define mutations in influenza A subtype H5N1 (avian) HA that alter its specificity for SA either by decreasing alpha2,3- or increasing alpha2,6-SA recognition. RBD mutants were used to develop vaccines and monoclonal antibodies that neutralized new variants. Structure-based modification of HA specificity can guide the development of preemptive vaccines and therapeutic monoclonal antibodies that can be evaluated before the emergence of human-adapted H5N1 strains.

An avian influenza H5N1 virus that binds to a human-type receptor

Avian influenza viruses preferentially recognize sialosugar chains terminating in sialic acid-alpha2,3-galactose (SAalpha2,3Gal), whereas human influenza viruses preferentially recognize SAalpha2,6Gal. A conversion to SAalpha2,6Gal specificity is believed to be one of the changes required for the introduction of new hemagglutinin (HA) subtypes to the human population, which can lead to pandemics. Avian influenza H5N1 virus is a major threat for the emergence of a pandemic virus. As of 12 June 2007, the virus has been reported in 45 countries, and 312 human cases with 190 deaths have been confirmed. We describe here substitutions at position 129 and 134 identified in a virus isolated from a fatal human case that could change the receptor-binding preference of HA of H5N1 virus from SAalpha2,3Gal to both SAalpha2,3Gal and SAalpha2,6Gal. Molecular modeling demonstrated that the mutation may stabilize SAalpha2,6Gal in its optimal cis conformation in the binding pocket. The mutation was found in approximately half of the viral sequences directly amplified from a respiratory specimen of the patient. Our data confirm the presence of H5N1 virus with the ability to bind to a human-type receptor in this patient and suggest the selection and expansion of the mutant with human-type receptor specificity in the human host environment.

Binding kinetics of influenza viruses to sialic acid-containing carbohydrates

To elucidate the molecular mechanisms of transmission of influenza viruses between different host species, such as human and birds, binding properties of sialic acid-containing carbohydrates that are recognized by human and/or avian influenza viruses were characterized by the surface plasmon resonance (SPR) method. Differences in the binding of influenza viruses to three gangliosides were monitored in real-time and correlated with receptor specificity between avian and human viruses. SPR analysis with ganglioside-containing lipid bilayers demonstrated the recognition profile of influenza viruses to not only sialic acid linkages, but also core carbohydrate structures on the basis of equilibrated rate constants. Kinetic analysis showed different binding preferences to gangliosides between avian and human strains. An avian strain bound to Neu5Acalpha2-3nLc4Cer with much slower dissociation rate than its sialyl-linkage analog, Neu5Acalpha2-6nLc4Cer, on the lipid bilayer. In contrast, a human strain bound equally to both gangliosides. An avian strain, but not a human strain, also interacted with GM3 carrying a shorter carbohydrate chain. Our findings demonstrated the remarkable distinction in the binding kinetics of sialic acid-containing carbohydrates between avian and human influenza viruses on the lipid bilayer.

Scientific barriers to developing vaccines against avian influenza viruses

The increasing number of reports of direct transmission of avian influenza viruses to humans in the past few years and the ongoing outbreak of H5N1 influenza virus infections in birds and humans highlight the pandemic threat posed by avian influenza viruses.

Although vaccination is the key strategy for the prevention of severe illness and death from pandemic influenza viruses and despite the long-term experience with vaccines against human influenza viruses, researchers face several obstacles in developing successful vaccines against avian influenza viruses.

The haemagglutinin (HA) and neuraminidase (NA) glycoproteins of influenza viruses are the main targets of the protective immune response. Licensed influenza virus vaccines are designed to induce HA-specific antibody responses to protect the host from infection. However, the presence of 16 subtypes of HA and 9 subtypes of NA glycoproteins among avian influenza viruses and the genetic and antigenic diversity among each subtype in nature present several unique challenges for the generation of broadly cross-protective vaccines.

Inactivated virus and live attenuated virus vaccines against pandemic influenza are being developed on the basis of plasmid-based reverse-genetics technology. Vaccines based on various other platforms, including live virus vectors and DNA vaccines, are also being developed and show promise in preclinical studies.

The available data indicate that inactivated avian influenza virus vaccines are poorly immunogenic and require a high concentration of HA glycoprotein or co-administration with an adjuvant to achieve the desired antibody response in humans. The biological basis for the poor immunogenicity of avian HA glycoproteins is not well understood.

Assays to measure the immune response to avian influenza viruses, in particular cell-mediated immune responses, are not available and the immune correlates of protection are not well understood. The choice of assay(s) for assessment of the immune response to pandemic influenza vaccines is a practical challenge in the evaluation of candidate vaccines.

As it is difficult to predict which avian influenza virus will cross the species barrier and cause a future pandemic, a library of candidate vaccines of different subtypes must be generated and evaluated in animal models and humans.

Although an ideal vaccine would prevent infection, a more realistic goal for a pandemic influenza vaccine might be to prevent severe illness and death.

The pandemic threat posed by avian influenza viruses highlights the need for new safe and efficient vaccines. However, several unique obstacles are faced by researchers in the development of these vaccines against avian influenza viruses. What are these obstacles and how can we overcome them?

The increasing number of reports of direct transmission of avian influenza viruses to humans underscores the need for control strategies to prevent an influenza pandemic. Vaccination is the key strategy to prevent severe illness and death from pandemic influenza. Despite long-term experience with vaccines against human influenza viruses, researchers face several additional challenges in developing human vaccines against avian influenza viruses. In this Review, we discuss the features of avian influenza viruses, the gaps in our understanding of infections caused by these viruses in humans and of the immune response to them that distinguishes them from human influenza viruses, and the current status of vaccine development.

Extending the Host Range of Listeria monocytogenes by Rational Protein Design

In causing disease, pathogens outmaneuver host defenses through a dedicated arsenal of virulence determinants that specifically bind or modify individual host molecules. This dedication limits the intruder to a defined range of hosts. Newly emerging diseases mostly involve existing pathogens whose arsenal has been altered to allow them to infect previously inaccessible hosts. We have emulated this chance occurrence by extending the host range accessible to the human pathogen Listeria monocytogenes by the intestinal route to include the mouse. Analyzing the recognition complex of the listerial invasion protein InlA and its human receptor E-cadherin, we postulated and verified amino acid substitutions in InlA to increase its affinity for E-cadherin. Two single substitutions increase binding affinity by four orders of magnitude and extend binding specificity to include formerly incompatible murine E-cadherin. By rationally adapting a single protein, we thus create a versatile murine model of human listeriosis.

Lessons learned from reconstructing the 1918 influenza pandemic

The "Spanish influenza" pandemic of 1918 was the most devastating influenza epidemic reported in history and killed >30 million people worldwide. The factors contributing to the severe pathogenicity of this influenza virus are of great interest, because avian influenza viruses circulating today pose the threat of a new pandemic if they develop sustained human-to-human transmissibility. Recent characterization of the 1918 virus has illuminated which determinants may be the cause of virulence. Here, we wish to shed light on what has been learned to date about the 1918 virus with regard to pathogenicity and transmissibility, to supplement our understanding of the determinants of human virulence and transmission of pandemic influenza viruses. Monitoring the sequences of avian influenza viruses for genetic changes and diversity may help us to predict the risks that these viruses pose of causing a new pandemic.

The threat of avian influenza a (H5N1): part II: Clues to pathogenicity and pathology

Among emerging and re-emerging infectious diseases, influenza constitutes one of the major threats to mankind. In this review series epidemiologic, virologic and pathologic concerns raised by infections of humans with avian influenza virus A/H5N1 are discussed. The second part focuses on experimental and clinical results, which give insights in the pathogenic mechanisms of H5N1 infection in humans. H5N1 is poorly transmitted to humans. However, H5N1-induced disease is very severe. More information on the role entry barriers, H5N1 target cells and on H5N1-induced modulation of the host immune response is needed to learn more about the determinants of H5N1 pathogenicity.

Regioselective one-pot protection of carbohydrates

Carbohydrates are involved in a wide range of biological processes. These structurally diverse compounds are more complex than other biological polymers, and are often present as heterogeneous mixtures in nature. The chemical synthesis of carbohydrates is one way to obtain pure oligosaccharides, but it is hampered by difficulties associated with the regioselective protection of polyhydroxyls and challenges related to the stereoselective assembly of glycosidic linkages. Here we describe a combinatorial, and highly-regioselective, method that can be used to protect individual hydroxy groups of a monosaccharide. This approach can be used to install an orthogonal protecting group pattern in a single reaction vessel (a 'one-pot' reaction), which removes the need to carry out the time-consuming isolation and purification of intermediates. Hundreds of building blocks have been efficiently prepared starting from d-glucose, and the iterative coupling of these building blocks enabled us to assemble beta-1,6-glucans and a library of oligosaccharides based on the influenza-virus-binding trisaccharide.

The threat of avian influenza A (H5N1). Part I: epidemiologic concerns and virulence determinants

Among emerging and re-emerging infectious diseases, influenza constitutes one of the major threats to mankind. In this review series epidemiologic, virologic and pathologic concerns raised by infections of humans with avian influenza virus A/H5N1 are discussed. This first part concentrates on epidemiologic concerns and virulence determinants. H5N1 spread over the world and caused a series of fowl pest outbreaks. Significant human-to-human transmissions have not been observed yet. Mutations that make the virus more compatible with human-to-human transmission may occur at any time. Nevertheless, no one can currently predict with certainty whether H5N1 will become a human pandemic virus.