Sialic acid species as a determinant of the host range of influenza A viruses

Anti-influenza drugs: the development of sialidase inhibitors

Viruses, particularly those that are harmful to humans, are the 'silent terrorists' of the twenty-first century. Well over four million humans die per annum as a result of viral infections alone. The scourge of influenza virus has plagued mankind throughout the ages. The fact that new viral strains emerge on a regular basis, particularly out of Asia, establishes a continual socio-economic threat to mankind. The arrival of the highly pathogenic avian influenza H5N1 heightened the threat of a potential human pandemic to the point where many countries have put in place 'preparedness plans' to defend against such an outcome. The discovery of the first designer influenza virus sialidase inhibitor and anti-influenza drug Relenza, and subsequently Tamiflu, has now inspired a number of continuing efforts towards the discovery of next generation anti-influenza drugs. Such drugs may act as 'first-line-of-defence' against the spread of influenza infection and buy time for necessary vaccine development particularly in a human pandemic setting. Furthermore, the fact that influenza virus can develop resistance to therapeutics makes these continuing efforts extremely important. An overview of the role of the virus-associated glycoprotein sialidase (neuraminidase) and some of the most recent developments towards the discovery of anti-influenza drugs based on the inhibition of influenza virus sialidase is provided in this chapter.

Avian Influenza A virus polymerase association with nucleoprotein, but not polymerase assembly, is impaired in human cells during the course of infection

Strong determinants of the host range of influenza A viruses have been identified on the polymerase complex formed by the PB1, PB2, and PA subunits and on the nucleoprotein (NP). In the present study, molecular mechanisms that may involve these four core proteins and contribute to the restriction of avian influenza virus multiplication in human cells have been investigated. The efficiencies with which the polymerase complexes of a human and an avian influenza virus isolate assemble and interact with the viral NP and cellular RNA polymerase II proteins were compared in mammalian and in avian infected cells. To this end, recombinant influenza viruses expressing either human or avian-derived core proteins with a PB2 protein fused to the One-Strep purification tag at the N or C terminus were generated. Copurification experiments performed on infected cell extracts indicate that the avian-derived polymerase is assembled and interacts physically with the cellular RNA polymerase II at least as efficiently as does the human-derived polymerase in human as well as in avian cells. Restricted growth of the avian isolate in human cells correlates with low levels of the core proteins in infected cell extracts and with poor association of the NP with the polymerase compared to what is observed for the human isolate. The NP-polymerase association is restored by a Glu-to-Lys substitution at residue 627 of PB2. Overall, our data point to viral and cellular factors regulating the NP-polymerase interaction as key determinants of influenza A virus host range. Recombinant viruses expressing a tagged polymerase should prove useful for further studies of the molecular interactions between viral polymerase and host factors during the infection cycle.

Transmission of equine influenza virus to English foxhounds

We retrospectively demonstrated that an outbreak of severe respiratory disease in a pack of English foxhounds in the United Kingdom in September 2002 was caused by an equine influenza A virus (H3N8). We also demonstrated that canine respiratory tissue possesses the relevant receptors for infection with equine influenza virus.

Analysis of N-glycans in embryonated chicken egg chorioallantoic and amniotic cells responsible for binding and adaptation of human and avian influenza viruses

The initial step essential in influenza virus infection is specific binding of viral hemagglutinin to host cell-surface glycan receptors. Influenza A virus specificity for the host is mediated by viral envelope hemagglutinin, that binds to receptors containing glycans with terminal sialic acids. Human viruses preferentially bind to alpha2-->6 linked sialic acids on receptors of host cells, whereas avian viruses are specific for the alpha2-->3 linkage on the target cells. Human influenza virus isolates more efficiently infect amniotic membrane (AM) cells than chorioallantoic membrane (CAM) cells. N-glycans were isolated from AM and CAM cells of 10-day-old chicken embryonated eggs and their structures were analyzed by multi-dimensional HPLC mapping and MALDI-TOF-MS techniques. Terminal N-acetylneuraminic acid contents in the two cell types were similar. However, molar percents of alpha2-->3 linkage preferentially bound by avian influenza virus were 27.2 in CAM cells and 15.4 in AM cells, whereas those of alpha2-->6 linkage favored by human influenza virus were 8.3 (CAM) and 14.2 (AM). Molar percents of sulfated glycans, recognized by human influenza virus, in CAM and AM cells were 3.8 and 12.7, respectively. These results have revealed structures and molar percents of N-glycans in CAM and AM cells important in determining human and avian influenza virus infection and viral adaptation.

Molecular characterization of highly pathogenic H5N1 avian influenza viruses isolated in Sweden in 2006

Background

The analysis of the nonstructural (NS) gene of the highly pathogenic (HP) H5N1 avian influenza viruses (AIV) isolated in Sweden early 2006 indicated the co-circulation of two sub-lineages of these viruses at that time. In order to complete the information on their genetic features and relation to other HP H5N1 AIVs the seven additional genes of twelve Swedish isolates were amplified in full length, sequenced, and characterized.

Results

The presence of two sub-lineages of HP H5N1 AIVs in Sweden in 2006 was further confirmed by the phylogenetic analysis of approximately the 95% of the genome of twelve isolates that were selected on the base of differences in geographic location, timing and animal species of origin. Ten of the analyzed viruses belonged to sub-clade 2.2.2. and grouped together with German and Danish isolates, while two 2.2.1. sub-clade viruses formed a cluster with isolates of Egyptian, Italian, Slovenian, and Nigerian origin. The revealed amino acid differences between the two sub-groups of Swedish viruses affected the predicted antigenicity of the surface glycoproteins, haemagglutinin and neuraminidase, rather than the nucleoprotein, polymerase basic protein 2, and polymerase acidic protein, the main targets of the cellular immune responses. The distinctive characteristics between members of the two subgroups were identified and described.

Conclusion

The comprehensive genetic characterization of HP H5N1 AIVs isolated in Sweden during the spring of 2006 is reported. Our data support previous findings on the coincidental spread of multiple sub-lineage H5N1 HPAIVs via migrating aquatic birds to large distance from their origin. The detection of 2.2.1. sub-clade viruses in Sweden adds further data regarding their spread in the North of Europe in 2006. The close genetic relationship of Swedish isolates sub-clade 2.2.2. to the contemporary German and Danish isolates supports the proposition of the introduction and spread of a single variant of 2.2.2. sub-clade H5N1 avian influenza viruses in the Baltic region. The presented findings underline the importance of whole genome analysis.

Influenza

OBJECTIVE

Influenza is a major concern for intensivists in all communities in the U.S. While there is considerable concern whether or not the country will be ready for a pandemic influenza, even seasonal influenza poses a major challenge to hospitals. The objective of this review is to summarize current knowledge of influenza with emphasis on the issues that intensivist will encounter.

SETTING

Intensive care unit in a 450-bed, tertiary care, teaching hospital.

METHODS

Source data were obtained from a PubMed search of the medical literature. PubMed "related articles" search strategies were likewise employed frequently.

SUMMARY AND CONCLUSIONS

Seasonal influenza causes more than 200,000 hospitalizations and 41,000 deaths in the U.S. every year, and is the seventh leading cause of death in the U.S. Despite this impact there is a shortcoming in knowledge of influenza among many health care workers, and a paucity of clinical data and studies to guide therapy. Intensivists need to recognize the importance of seasonal influenza as a cause of severe morbidity and mortality. This review summarizes current knowledge of the diagnosis, complications, therapy, and infection control measures associated with influenza.

Compatibility among polymerase subunit proteins is a restricting factor in reassortment between equine H7N7 and human H3N2 influenza viruses

Reassortment is an important driving force for influenza virus evolution, and a better understanding of the factors that affect this process could improve our ability to respond to future influenza pandemics and epidemics. To identify factors that restrict the generation of reassortant viruses, we cotransfected human embryonic kidney cells with plasmids for the synthesis of viral RNAs of both A/equine/Prague/1/56 (Prague; H7N7) and A/Yokohama/2017/03 (Yokohama; H3N2) viruses together with the supporting protein expression plasmids. Of the possible 256 genotypes, we identified 29 genotypes in 120 randomly plaque-picked reassortants examined. Analyses of these reassortants suggested that the formation of functional ribonucleoprotein (RNP) complexes was a restricting factor, a finding that correlated with the activities of RNP complexes composed of different combinations of the proteins from the two viruses, as measured in a minigenome assay. For at least one nonfunctional RNP complex (i.e., Prague PB2, Prague PB1, Yokohama PA, and Prague NP), the lack of activity was due to the inability of the three polymerase subunit proteins to form a heterotrimer. Adaptation of viruses possessing a gene encoding a chimera of the PA proteins of the two viruses and the remaining genes from Prague virus resulted in compensatory mutations in the PB2 and/or PA protein. These results indicate substantial incompatibility among the gene products of the two test viruses, a critical role for the RNP complex in the generation of reassortant viruses, and a functional interaction of PB2 and PA.

Host factors for replication and transcription of the influenza virus genome

For replication and transcription of the influenza virus genome of eight-segmented and negative-stranded RNAs, not only viral factors but also host-derived cellular factors (host factors) are required. This paper focuses on the identification and characterisation of the host factors involved in replication and transcription of the influenza virus genome, reviewing recent progresses in the related molecular mechanisms. Functional assay systems for screening of host factors using cell-free reconstitution systems and an yeast-based influenza virus replicon system are highlighted. We have summarised the property of host factors comprehensively and provided a clue for the perspective in the determination mechanism of host range and virulence and the development of a new strategy to control the influenza virus.

Monitoring of influenza virus hemagglutinin in process samples using weak affinity ligands and surface plasmon resonance

Surface plasmon resonance (SPR) was used to screen the interaction between a variety of affinity ligands and hemagglutinin (HA) from human influenza virus, with the aim of identifying low affinity ligands useful for the development of a rapid bioanalytical sensor. Three sialic acid-based structures and four lectins were evaluated as sensor ligands. The sialic acid-based ligands included a natural sialic acid-containing glycoprotein, human alpha1-acid glycoprotein (alpha1-AGP), and two synthetic 6'-sialyllactose-conjugates, with varying degree of substitution. The interaction of HA with the four lectin-based ligands, concanavalin A (Con A), wheat germ agglutinin (WGA), Maackia amurensis lectin (MAL), and Sambucus nigra agglutinin (SNA), showed a wide variation of affinity strengths. Affinity and kinetics data were estimated. Strong affinities were observed for Con A, WGA, alpha1-AGP, and a 6'-sialyllactose-conjugate with a high substitution degree, and low affinities were observed for MAL and a 6'-sialyllactose-conjugate with low substitution. The main objective, to identify a low affinity ligand which could be used for on-line monitoring and product quantification, was met by a 6'-sialyllactose-ovalbumin conjugate that had 0.6 mol ligand per mol carrier protein. The apparent affinity of this ligand was estimated to be 1.5+/-0.03 microM (K(D)) on the SPR surface. Vaccine process samples containing HA were analyzed in the range 10-100 microg HA mL(-1) and correlated with single-radial immunodiffusion. The coefficient of variation on the same chip was between 0.010 and 0.091.

Increased sensitivity for detecting avian influenza-specific antibodies by a modified hemagglutination inhibition assay using horse erythrocytes

The hemagglutination inhibition (HI) assay is a widely used serological method to measure the levels of protective antibody responses against influenza viruses. However, the traditional HI assay which uses chicken erythrocytes is not sufficiently sensitive for detecting HI antibodies specific to avian influenza viruses. Previously, it was demonstrated that employing an assay using horse erythrocytes was able to increase the sensitivity of HI assay. The current report describes further optimization of this modified HI assay. It was shown that this method was able to increase detection of HI activities in rabbit sera immunized with H5 HA antigens, and proved that this increased sensitivity is useful in dissecting the strain specificity of HI antibody responses. In addition, the modified HI assay using horse erythrocytes increased the sensitivity of detecting HI antibodies specific for three major serotypes of avian influenza viruses, H5, H7 and H9, in people who may have asymptomatic infection with avian influenza viruses. Based on these results, the optimized use of horse erythrocytes should be standard practice for detecting HI activities against avian influenza viruses.

Influenza A virus (H3N8) in dogs with respiratory disease, Florida

Genetic and antigenic characterization established the uniqueness of this virus circulating in dogs.

In 2004, canine influenza virus subtype H3N8 emerged in greyhounds in the United States. Subsequent serologic evidence indicated virus circulation in dog breeds other than greyhounds, but the virus had not been isolated from affected animals. In 2005, we conducted virologic investigation of 7 nongreyhound dogs that died from respiratory disease in Florida and isolated influenza subtype H3N8 virus. Antigenic and genetic analysis of A/canine/Jacksonville/2005 (H3N8) and A/canine/Miami/2005 (H3N8) found similarity to earlier isolates from greyhounds, which indicates that canine influenza viruses are not restricted to greyhounds. The hemagglutinin contained 5 conserved amino acid differences that distinguish canine from equine lineages. The antigenic homogeneity of the canine viruses suggests that measurable antigenic drift has not yet occurred. Continued surveillance and antigenic analyses should monitor possible emergence of antigenic variants of canine influenza virus.

Effect of siRNA mediated suppression of signaling lymphocyte activation molecule on replication of peste des petits ruminants virus in vitro

Signaling lymphocyte activation molecule (SLAM) expression was inhibited in B95a cell line using siRNA and the effect of SLAM inhibition on peste des petits ruminants virus (PPRV) replication and infectivity titre was studied. SLAM suppression was assessed using real-time PCR and flow cytometry to confirm suppression at the m-RNA and protein levels, respectively. Three chemically synthesized siRNAs were transfected individually using oligofectamine into B95a cell line. This resulted in SLAM suppression from 48 to 454-folds, in comparison to the untransfected B95a cell line. When the SLAM suppressed B95a cell line was infected with PPRV, replication was reduced by 12-143-folds and virus titre was reduced from log10 1.09 to 2.28. siRNA 3 showed the most potent inhibition of SLAM expression both at m-RNA and protein levels. This also caused the maximum reduction of virus replication and virus titre. A 100-fold reduction in PPRV titres was seen in anti-SLAM antibody neutralized B95a cell line. This further confirms that SLAM is one of the (co) receptors for PPRV. However, the presence of other putative virus receptor(s) is/are not ruled out.

Disruption of thymopoiesis in ST6Gal I-deficient mice

Thymocyte development is accompanied by sequential changes in cell surface glycosylation. For example, medullary thymocytes have increased levels of alpha2,3-linked sialic acid and a loss of asialo core 1 O-glycans as compared to cortical thymocytes. Some of these changes have been linked to fine tuning of the T cell receptor avidity. We analyzed ST6Gal I transcript abundance and levels of alpha2,6-linked sialic acid across thymocyte subsets. We found that ST6Gal I transcript levels increased following T cell receptor beta-selection suggesting that this sialyltransferase may influence the development of early thymocyte populations. Indeed, low levels of alpha2,6-linked sialic acid were found in the earliest T lineage cells, and then increased in T cell receptor beta-selected cells. To determine whether ST6Gal I influences T cell development, we analyzed ST6Gal I-deficient mice for disruptions in thymocyte populations. We found reduced thymic cellularity in the ST6Gal I-deficient mice starting in the early thymocyte compartments.

Genetic evolution of swine influenza A (H3N2) viruses in China from 1970 to 2006

Pigs are susceptible to both human and avian influenza viruses and have been proposed to be intermediate hosts, or mixing vessels, for the generation of pandemic influenza viruses through reassortment or adaptation to the mammalian host. In this study, we summarize and report for the first time the coexistence of wholly human-like H3N2 viruses, double-reassortant H3N2 viruses, and triple-reassortant H3N2 viruses in pigs in China by analyzing the eight genes of swine influenza A (H3N2) viruses found in China from 1970 to 2006. In 1970, the first wholly human-like H3N2 (Hong Kong/68-like) viruses were isolated from pigs in Taiwan, and then in the next years Victoria/75-like, Sydney/97-like, New York/99-like, and Moscow/99-like swine H3N2 viruses were regularly isolated in China. In the 1980s, two triple-reassortant viruses were isolated from pigs. Recently, the double-reassortant viruses containing genes from the human (HA and NA) and avian (PB2, PB1, PA, NP, M, and NS) lineages and the triple-reassortant viruses containing genes from the human (HA and NA), classical swine (NP), and avian (PB2, PB1, PA, M, and NS) lineages emerged in pigs in China. The coexistence of wholly human-like and reassortant viruses provides further evidence that pigs serve as intermediate hosts, or mixing vessels, and emphasizes the importance of reinforcing swine influenza virus surveillance in China.

The war against influenza: discovery and development of sialidase inhibitors

The threat of a major human influenza pandemic, in particular from highly aggressive strains such as avian H5N1, has emphasized the need for therapeutic strategies to combat these pathogens. At present, two inhibitors of sialidase (also known as neuraminidase), a viral enzyme that has a key role in the life cycle of influenza viruses, would be the mainstay of pharmacological strategies in the event of such a pandemic. This article provides a historical perspective on the discovery and development of these drugs--zanamivir and oseltamivir--and highlights the value of structure-based drug design in this process.

High prevalence of amantadine-resistance influenza a (H3N2) in six prefectures, Japan, in the 2005-2006 season

Substantial increase in amantadine-resistant influenza A (H3N2) was reported in Asia and North America in 2005. In this study the frequency and genetic characteristics of amantadine-resistant influenza A, circulated in Japan in 2005-2006 season, were investigated. Isolates were tested by amantadine susceptibility test (TCID(50)/0.2 ml method), and sequencing of the M2 gene to identify mutations that confer resistance. Additionally, the hemagglutinin (HA) and neuraminidase (NA) genes of the viruses were examined. In total, 415 influenza A isolates from six prefectures were screened, and 231 (65.3%) of 354 influenza A (H3N2) were amantadine-resistant, with a serine to asparagine (S31N) change in the M2 gene. However, none of 61 A (H1N1) isolates were resistant. In addition, genetic analyses of the HA gene showed all amantadine-resistant viruses clustered in one (named clade N), possessing specific double mutations at 193, serine to phenylalanine (S193F), and at 225, asparatic acid to asparagine (D225N), and sensitive viruses belonged to another group (clade S). The clinical presentations at the clinical visit did not differ between patients shedding clade N virus and those shedding clade S virus. None of the patients had received previous treatment with amantadine. The results indicate an unusually high prevalence and wide circulation of the amantadine-resistance influenza A (H3N2) in Japan in the 2005-2006 season. These strains had the characteristic double mutations in the HA, in addition to the M2 mutation responsive for resistance. Antiviral resistance monitoring should be intensified and maintained for rapid feedback into treatment strategies, and selection of alternative therapeutic agents.

Up to new tricks - a review of cross-species transmission of influenza A viruses

Influenza is a highly contagious disease that has burdened both humans and animals since ancient times. In humans, the most dramatic consequences of influenza are associated with periodically occurring pandemics. Pandemics require the emergence of an antigenically novel virus to which the majority of the population lacks protective immunity. Historically, influenza A viruses from animals have contributed to the generation of human pandemic viruses and they may do so again in the future. It is, therefore, critical to understand the epidemiological and molecular mechanisms that allow influenza A viruses to cross species barriers. This review summarizes the current knowledge of influenza ecology, and the viral factors that are thought to determine influenza A virus species specificity.

New assays to measure equine influenza virus-specific Type 1 immunity in horses

Equine influenza virus (EIV) is a leading cause of respiratory disease in horses. Equine influenza infection induces a long-term immunity to re-infection. Recent strategies of vaccination aim to mimic this immunity by stimulating both antibody and cellular immune responses. Cell-mediated immunity (CMI) to influenza is well defined in man, but little has been done to characterise the responses in the horse. Additionally, the development of reliable assays for the measurement of equine CMI has lagged behind serological methods and vaccine development. In this study, two methods of measuring EIV-specific T lymphocyte responses have been developed. An EIV 'bulk' cytotoxic T lymphocytes (CTL) assay using equine dermal fibroblasts as target cells has been adapted from a method used in the 1980s. This method was also complemented with a new EIV-specific IFNgamma synthesis assay. When compared with the measurement of EIV-specific IFNgamma synthesis previously described, this method required the amplification of EIV-specific lymphocytes by culture and was sensitive enough to detect stimulation of EIV-specific T lymphocytes induced by experimental infection with EIV or vaccination with recombinant canarypox viruses coding for EIV-HA molecules. This study provides the tools to characterise the stimulation of CMI by the new generation of vaccines against equine influenza.

Human and avian influenza viruses target different cells in the lower respiratory tract of humans and other mammals

Viral attachment to the host cell is critical for tissue and species specificity of virus infections. Recently, pattern of viral attachment (PVA) in human respiratory tract was determined for highly pathogenic avian influenza virus of subtype H5N1. However, PVA of human influenza viruses and other avian influenza viruses in either humans or experimental animals is unknown. Therefore, we compared PVA of two human influenza viruses (H1N1 and H3N2) and two low pathogenic avian influenza viruses (H5N9 and H6N1) with that of H5N1 virus in respiratory tract tissues of humans, mice, ferrets, cynomolgus macaques, cats, and pigs by virus histochemistry. We found that human influenza viruses attached more strongly to human trachea and bronchi than H5N1 virus and attached to different cell types than H5N1 virus. These differences correspond to primary diagnoses of tracheobronchitis for human influenza viruses and diffuse alveolar damage for H5N1 virus. The PVA of low pathogenic avian influenza viruses in human respiratory tract resembled that of H5N1 virus, demonstrating that other properties determine its pathogenicity for humans. The PVA in human respiratory tract most closely mirrored that in ferrets and pigs for human influenza viruses and that in ferrets, pigs, and cats for avian influenza viruses.

Effective chemoenzymatic synthesis of p-aminophenyl glycosides of sialyl N-acetyllactosaminide and analysis of their interactions with lectins

A convenient chemoenzymatic procedure for the synthesis of p-aminophenyl glycosides of sialyl N-acetyllactosaminide has been developed from p-nitrophenyl N-acetyl-beta-D-glucosaminide as starting material through three steps: synthesis of p-nitrophenyl N-acetyllactosaminide with beta-D-galactosidase, chemical reduction of the p-nitrophenyl group, and sialylation with sialyltransferase. The p-aminophenyl glycosides were then successfully biotin-labeled through the coupling with N-(+)-biotinyl-6-aminohexanoic acid to afford biotinylated oligosaccharides with an aminohexanosyl group and phenyl group as the spacers between the biotin and glycan. Furthermore, the biotin-labeled sugars were shown to be useful for immobilization and assay of the carbohydrate-lectin interactions by an optical biosensor based on surface plasmon resonance.

First introduction of highly pathogenic H5N1 avian influenza A viruses in wild and domestic birds in Denmark, Northern Europe

Background

Since 2005 highly pathogenic (HP) avian influenza A H5N1 viruses have spread from Asia to Africa and Europe infecting poultry, humans and wild birds. HP H5N1 virus was isolated in Denmark for the first time in March 2006. A total of 44 wild birds were found positive for the HP H5N1 infection. In addition, one case was reported in a backyard poultry flock.

Results

Full-genome characterisation of nine isolates revealed that the Danish H5N1 viruses were highly similar to German H5N1 isolates in all genes from the same time period. The haemagglutinin gene grouped phylogenetically in H5 clade 2 subclade 2 and closest relatives besides the German isolates were isolates from Croatia in 2005, Nigeria and Niger in 2006 and isolates from Astrakhan in Russia 2006. The German and Danish isolates shared unique substitutions in the NA, PB1 and NS2 proteins.

Conclusion

The first case of HP H5N1 infection of wild and domestic birds in Denmark was experienced in March 2006. This is the first full genome characterisation of HP H5N1 avian influenza A virus in the Nordic countries. The Danish viruses from this time period have their origin from the wild bird strains from Qinghai in 2005. These viruses may have been introduced to the Northern Europe through unusual migration due to the cold weather in Eastern Europe at that time.

Receptor binding specificity of recent human H3N2 influenza viruses

Background

Human influenza viruses are known to bind to sialic acid linked α2-6 to galactose, but the binding specificity beyond that linkage has not been systematically examined. H3N2 human influenza isolates lost binding to chicken red cells in the 1990s but viruses isolated since 2003 have re-acquired the ability to agglutinate chicken erythrocytes. We have investigated specificity of binding, changes in hemagglutinin sequence of the recent viruses and the role of sialic acid in productive infection.

Results

Viruses that agglutinate, or do not agglutinate, chicken red cells show identical binding to a Glycan Array of 264 oligosaccharides, binding exclusively to a subset of α2-6-sialylsaccharides. We identified an amino acid change in hemagglutinin that seemed to correlate with chicken red cell binding but when tested by mutagenesis there was no effect. Recombinant hemagglutinins expressed on Sf-9 cells bound chicken red cells but the released recombinant baculoviruses agglutinated only human red cells. Similarly, an isolate that does not agglutinate chicken red cells show hemadsorption of chicken red cells to infected MDCK cells. We suggest that binding of chicken red cells to cell surface hemagglutinin but not to virions is due to a more favorable hemagglutinin density on the cell surface. We investigated whether a virus specific for α2-6 sialyloligosaccharides shows differential entry into cells that have varying proportions of α2-6 and α2-3 sialic acids, including human A549 and HeLa cells with high levels of α2-6 sialic acid, and CHO cells that have only α2-3 sialic acid. We found that the virus enters all cell types tested and synthesizes viral nucleoprotein, localized in the nucleus, and hemagglutinin, transported to the cell surface, but infectious progeny viruses were released only from MDCK cells.

Conclusion

Agglutination of chicken red cells does not correlate with altered binding to any oligosaccharide on the Glycan Array, and may result from increased avidity due to density of hemagglutinin and not increased affinity. Absence of α2-6 sialic acid does not protect a cell from influenza infection and the presence of high levels of α2-6-sialic acids on a cell surface does not guarantee productive replication of a virus with α2-6 receptor specificity.

Specific biochemical features of replication of clinical influenza viruses in human intestinal cell culture

Influenza A viruses isolated from the respiratory tract of patients with influenza were cultured in human intestinal epithelium cells (CACO-2 line). The CACO-2 cells were found to be 100-fold more susceptible to the clinical viruses than MDCK cells and chicken embryos. On passaging in CACO-2 cells, clinical isolates of the subtype H3N2 retained the original "human" phenotype and agglutinated human but not chicken erythrocytes, whereas on passaging in MDCK cells the virus phenotype changed to the "avian" one. On comparison with laboratory strains (grown in chicken embryos or MDCK cells), the clinical viruses were characterized by higher stability of the anti-interferon protein NS1 but had a reduced synthesis of the matrix protein M1, and this could facilitate the virus adaptation and escape of the infected cells from immune attack in the human body. The increased tropism to the human CACO-2 cells correlated with higher adsorption of the clinical viruses on cellular receptors. However, in the CACO-2 and MDCK cells the ratio of sialyl-containing glycoreceptors of the 2-3 and 2-6 type was similar. These observations indicated that not only sialic acid residues were involved in the adsorption and penetration of the clinical viruses into human cells, but also the protein moiety of the cellular receptor itself and/or an additional cellular coreceptor. Thus, clinical influenza viruses are shown to possess a specific mechanism of sorption and entry into human epithelial cells, which is responsible for their higher tropism to human cells and is unlike such a mechanism in canine cells.

The quail and chicken intestine have sialyl-galactose sugar chains responsible for the binding of influenza A viruses to human type receptors

The receptor specificity of influenza viruses is one factor that allows avian influenza viruses to cross the species barrier. The recent transmissions of avian H5N1 and H9N2 influenza viruses from chickens and/or quails to humans indicate that avian influenza viruses can directly infect humans without an intermediate host, such as pigs. In this study, we used two strains of influenza A virus (A/PR/8/34, which preferentially binds to an avian-type receptor, and A/Memphis/1/71, which preferentially binds to a human-type receptor) to probe the receptor specificities in host cells. Epithelial cells of both quail and chicken intestines (colons) could bind both avian- and human-type viruses. Infected cultured quail colon cells expressed viral protein and allowed replication of the virus strain A/PR/8/34 or A/Memphis/1/71. To understand the molecular basis of these phenomena, we further investigated the abundance of sialic acid (Sia) linked to galactose (Gal) by the alpha2-3 linkage (Siaalpha2-3Gal) and Siaalpha2-6Gal in host cells. In glycoprotein and glycolipid fractions from quail and chicken colon epithelial cells, there were some bound components of Sia-Gal linkage-specific lectins, Maackia amurensis agglutinin (specific for Siaalpha2-3 Gal) and Sambucus nigra agglutinin (specific for Siaalpha2-6Gal), indicating that both Siaalpha2-3Gal and Siaalpha2-6Gal exist in quail and chicken colon cells. Furthermore, we demonstrated by fluorescence high-performance liquid chromatography (HPLC) analysis that 5-N-acetylneuraminic acid was the main molecular species of Sia, and we demonstrated by multi-dimensional HPLC mapping and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry analysis that bi-antennary complex-type glycans alpha2-6 sialylated at the terminal Gal residue(s) are major (more than 79%) sialyl N-glycans expressed by intestinal epithelial tissues in both the chicken and quail. Taken together, these results indicate that quails and chickens have molecular characterization as potential intermediate hosts for avian influenza virus transmission to humans and could generate new influenza viruses with pandemic potential.

Evidence for dynamic interplay of different oligomeric states of UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase by biophysical methods

The bifunctional enzyme UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase (GNE) is a key enzyme for the biosynthesis of sialic acids, the terminal sugars of glycoconjugates associated with a variety of physiological and pathological processes such as cell adhesion, development, inflammation and cancer. In this study, we characterized rat GNE by different biophysical methods, analytical ultracentrifugation, dynamic light-scattering and size-exclusion chromatography, all revealing the native hydrodynamic behavior and molar mass of the protein. We show that GNE is able to reversibly self-associate into different oligomeric states including monomers, dimers and tetramers. Additionally, it forms non-specific aggregates of high molecular mass, which cannot be unequivocally assigned a distinct size. Our results also indicate that ligands of the epimerase domain of the bifunctional enzyme, namely UDP-N-acetylglucosamine and CMP-N-acetylneuraminic acid, stabilize the protein against aggregation and are capable of modulating the quaternary structure of the protein. The presence of UDP-N-acetylglucosamine strongly favors the tetrameric state, which therefore likely represents the active state of the enzyme in cells.

Influenza viral hemagglutinin complicated shape is advantageous to its binding affinity for sialosaccharide receptor

Do the complexity and the bulkiness of a protein affect the affinity between protein and ligand? We attempted to investigate this problem by using ab initio fragment molecular orbital (FMO) method to calculate the binding energy between human influenza viral hemagglutinin (HA) and human oligo-saccharide receptor. We compared the binding energies of 4 different sizes of human A virus HA H3 subtype complexed with human receptor Neu5Ac(alpha2-6)Gal as a model. The full shape receptor binding domain complexed with Neu5Ac(alpha2-6)Gal had the highest binding energy 170.3kcal/mol at the FMO-HF/STO-3G level, which was 52.3kcal/mol higher than that of the smallest domain-receptor complex. These data provide the consideration of the backyard bulkiness beyond the binding site of protein to the protein-ligand stability.

Alpha2,6-linked sialic acid acts as a receptor for Feline calicivirus

Feline calicivirus (FCV) is a major causative agent of respiratory disease in cats. It is also one of the few cultivatable members of the family Caliciviridae. It has recently been reported that FCV binding is in part due to interaction with junction adhesion molecule-A. This report describes the characterization of additional receptor components for FCV. Chemical treatment of cells with sodium periodate showed that FCV recognized carbohydrate moieties on the surface of permissive cells. Enzymic treatment with Vibrio cholerae neuraminidase demonstrated that sialic acid was a major determinant of virus binding. Further characterization using linkage-specific lectins from Maackia amurensis and Sambucus nigra revealed that FCV recognized sialic acid with an alpha2,6 linkage. Using various proteases and metabolic inhibitors, it was shown that alpha2,6-linked sialic acid recognized by FCV is present on an N-linked glycoprotein.

Effective replication of human influenza viruses in mice lacking a major alpha2,6 sialyltransferase

The hemagglutinins of influenza viruses isolated from humans typically prefer binding to sialic acid in an alpha2,6 linkage. Presumably, the virus uses the presence of these receptors on the respiratory tract to gain entrance into the host cell. The ST6Gal I sialyltransferase knock-out mouse lacks the main enzyme necessary for the attachment of alpha2,6 sialic acid to N-linked glycoproteins on the cell surface. Yet even in the absence of detectable alpha2,6 sialic acid in the mouse respiratory tract, human influenza viruses can still infect these mice and grow to similar titers in the lung and trachea as compared to wild-type animals. This work demonstrates that the presence of a major alpha2,6 sialic acid on N-linked glycoproteins is not essential for human influenza virus infection in mice.

Chemoenzymatic synthesis of artificial glycopolypeptides containing multivalent sialyloligosaccharides with a γ-polyglutamic acid backbone and their effect on inhibition of infection by influenza viruses

Highly water-soluble, artificial glycopolypeptides with a gamma-polyglutamic acid (gamma-PGA) backbone derived from Bacillus subtilis sp. and multivalent sialyloligosaccharide units have been chemoenzymatically synthesized as potential polymeric inhibitors of infection by bird and human influenza viruses. 5-Trifluoroacetamidopentyl beta-N-acetyllactosaminide and 5-trifluoroacetamidopentyl beta-lactoside were enzymatically synthesized from LacNAc and lactose, respectively, by cellulase-mediated condensation with 5-trifluoroacetamido-1-pentanol. After deacetylation, the resulting 5-aminopentyl beta-LacNAc and beta-lactoside glycosides were coupled to the alpha-carboxyl groups of the gamma-PGA side chains. The artificial glycopolypeptides carrying LacNAc and lactose were further converted to Neu5Acalpha2-(3/6)Galbeta1-4Glcbeta and Neu5Acalpha2-(3/6)Galbeta1-4GlcNAcbeta sialyloligosaccharide units by alpha2,3- and alpha2,6-sialyltransferase, respectively. The interaction of these glycopolypeptides with various influenza virus strains has been investigated by three different methods. Glycopolypeptides carrying Neu5Acalpha2,6LacNAc inhibited hemagglutination mediated by influenza A and B viruses, and their relative binding affinities for hemagglutinin were 10(2)- to 10(4)-fold higher than that of the naturally occurring fetuin control. A glycopolypeptide carrying Neu5Acalpha2,6LacNAc inhibited infection by A/Memphis/1/71 (H3N2) 93 times more strongly than fetuin, as assessed by cytopathic effects on virus-infected MDCK cells. The avian virus [A/duck/Hong kong/4/78 (H5N3)] bound strongly to Neu5Acalpha2,3LacNAc/Lac-carrying glycopolypeptides, whereas the human virus [A/Memphis/1/71 (H3N2)] bound to Neu5Acalpha2,6LacNAc in preference to Neu5Acalpha2,6Lac. Taken together, these results indicate that the binding of viruses to terminal sialic acids is markedly affected by the structure of the asialo portion, in this case either LacNAc or lactose, in the sugar chain of glycopolypeptides.

Host-range determinants on the PB2 protein of influenza A viruses control the interaction between the viral polymerase and nucleoprotein in human cells

The transcription/replication activity of ribonucleoproteins derived from influenza A primary isolates of human (A/Paris/908/97) or avian origin (A/Mallard/Marquenterre/MZ237/83, A/Hong Kong/156/97) was compared upon reconstitution in mammalian or avian cells, using viral-like reporter RNAs synthesized under the control of the human and chicken RNA polymerase I promoters, respectively. In avian cells, transcription/replication activities were in the same range with all ribonucleoproteins tested. In human cells, ribonucleoproteins derived from A/Mallard/Marquenterre/MZ237/83 showed reduced transcription/replication activity and reduced NP binding to the PB1-PB2-PA complex (P) or to the isolated PB2 subunit, as compared to the ribonucleoproteins derived from A/Paris/908/97. Both defects were restored when PB2 residue Glu-627 was changed to a Lys. Ribonucleoproteins derived from the human A/Hong Kong/156/97 H5N1 isolate showed efficient NP-P interaction in human cells, and high levels of activity which were determined mostly by the PB2 and PA proteins. Our data suggest that PB2 might play a pivotal role in molecular interactions involving both the viral nucleoprotein and cellular proteins.

Evolution of carbohydrate antigens--microbial forces shaping host glycomes?

Many glycans show remarkably discontinuous distribution across evolutionary lineages. These differences play major roles when organisms belonging to different lineages interact as host-pathogen or host-symbiont. Certain lineage-specific glycans have become important signals for multicellular host organisms, which use them as molecular signatures of their pathogens and symbionts through recognition by a toolkit of innate defense molecules. In turn, pathogens have evolved to exploit host lineage-specific glycans and are constantly shaping the glycomes of their hosts. These interactions take place in the face of numerous critical endogenous functions played by glycans within host organisms. Whether due to simple evolutionary divergence or adaptive changes under natural selection resulting from endogenous functional requirements, once different lineages elaborate on differential glycomes these mutual differences provide opportunities for host exploitation and/or pathogen defense between lineages. Such phylogenetic molecular recognition mechanisms will augment and likely contribute to the maintenance of lineage-specific differences in glycan repertoires.

Diversity in cell surface sialic acid presentations: implications for biology and disease

Sialic acids (Sias) are typically found as terminal monosaccharides attached to cell surface glycoconjugates. They play many important roles in many physiological and pathological processes, including microbe binding that leads to infections, regulation of the immune response, the progression and spread of human malignancies and in certain aspects of human evolution. This review will provide some examples of these diverse roles of Sias and briefly address immunohistochemical approaches to their detection.

Human trachea primary epithelial cells express both sialyl(alpha2-3)Gal receptor for human parainfluenza virus type 1 and avian influenza viruses, and sialyl(alpha2-6)Gal receptor for human influenza viruses

We reported previously that the dominant receptors of influenza A and B viruses, and human and murine respiroviruses, were sialylglycoproteins and gangliosides containing monosialo-lactosamine type I-and II-residues, such as sialic acid-alpha2-3(6)-Galbeta1-3(4)-GlcNAcbeta1-. In addition, the Siaalpha2-3Gal linkage was predominantly recognized by avian and horse influenza viruses, and human parainfluenza virus type 1 (hPIV-1), whereas the Siaalpha2-6Gal linkage was mainly recognized by human influenza viruses (Paulson JC in "The Receptors'' [Conn M Ed] 2, 131-219 (1985); Suzuki Y, Prog Lipid Res 33, 429-57 (1994); Ito T, J Virol 73, 6743-51 (2000); Suzuki Y, J Virol 74, 11825-31 (2000); Suzuki T, J. Virol 75, 4604-4613 (2001); Suzuki Y, Biol. Pharm. Bull. 28, 399-408 (2005)). To clarify the distribution of influenza virus receptors on the human bronchial epithelium cell surface, we investigated a primary culture of normal human bronchial epithelial (NHBE) cells using two types of lectin (MAA and SNA), which recognize sialyl linkages (alpha2-3 and alpha2-6), using fluorescence-activated cell-sorting analysis. The results showed that both alpha2-3- and alpha2-6-linked Sias were expressed on the surface of primary human bronchial epithelial cells. The cells infected by hPIV-1 bound to MAA, confirming that cells targeted by hPIV-1 have alpha2-3-linked oligosaccharides. We also compared the ability of hPIV-1 and human influenza A virus to infect primary human bronchial epithelial cells pre-treated with Siaalpha2-3Gal-specific sialidase from Salmonella typhimurium. No difference was observed in the number of sialidase pre-treated and non-treated cells infected with human influenza A virus, which binds to Siaalpha2-6Gal-linked oligosaccharides. By contrast, the number of cells infected with hPIV-1 decreased significantly upon sialidase treatment. Thus, cultured NHBE cells showed both alpha2-3-linked Sias recognized by hPIV-1 and avian influenza virus receptors, and alpha2-6-linked Sias recognized by human influenza virus receptors.

Why does avian influenza A virus hemagglutinin bind to avian receptor stronger than to human receptor? Ab initio fragment molecular orbital studies

Influenza A viruses attach to alpha-sialosides on the target cell surface by their hemagglutinins, which strictly recognize the difference in sialic acid-galactose linkage. Why does avian virus H3 subtype bind to avian receptor Neu5Ac(alpha2-3)Gal stronger than to human receptor Neu5Ac(alpha2-6)Gal? Why does avian H3 mutated Gln226 to Leu preferentially bind to human receptor? In this paper, we theoretically answer the questions by molecular mechanics and ab initio fragment molecular orbital (FMO) calculations. The binding energy between avian H3 and avian receptor is 8.2kcal/mol larger than that of the avian H3-human receptor complex estimated at the FMO-HF/STO-3G level, which is a reason that avian H3 binds to avian receptor stronger than to human receptor. Avian Leu226 H3 clashes to Gal unit on the avian receptor to quite decrease its binding affinity. In contrast, Gal unit on the human receptor forms intermolecular hydrophobic interaction with avian Leu226 H3 to afford moderate binding affinity.

Virus infection and lipid rafts

Virus entry, assembly, and budding are important processes in the replication cycle of a virus. Viruses are dependent on host living cells for their replication. Viruses use the proliferative mechanism of host cells for replication of viral components. Lipid rafts, specific membrane microdomains play a critical role in virus replication because localizing and concentrating viral components in the microdomains for entry, assembly, and budding of various types of virus. In this review, we describe the involvement of membrane lipid rafts in the virus replication cycle with our current findings for understanding the role of membrane lipid rafts in virus infection.

Computational studies of H5N1 hemagglutinin binding with SA-alpha-2, 3-Gal and SA-alpha-2, 6-Gal

For influenza H5N1 hemagglutinin, a switch from SA-alpha-2, 3-Gal to SA-alpha-2, 6-Gal receptor specificity is a critical step leading to the conversion from avian-to-human to human-to-human infection. Therefore, the understanding of the binding modes of SA-alpha-2, 3-Gal and SA-alpha-2, 6-Gal to H5N1 hemagglutinin will be very important for the examination of possible mutations needed for going from an avian to a human flu virus. Based on the available H5N1 hemagglutinin crystal structure, the binding profiles between H5N1 hemagglutinin and two saccharide ligands, SA-alpha-2, 3-Gal and SA-alpha-2, 6-Gal, were investigated by ab initio quantum mechanics, molecular docking, molecular mechanics, and molecular dynamics simulations. It was found that SA-alpha-2, 3-Gal has strong multiple hydrophobic and hydrogen bond interactions in its trans conformation with H5N1 hemagglutinin, whereas the SA-alpha-2, 6-Gal only shows weak interactions in a different conformation (cis type).

Genetic basis for the lack of N-glycolylneuraminic acid expression in human tissues and its implication to human evolution

Sialic acid is a family of acidic monosaccharides and consists of over 30 derivatives. Two major derivatives are N-acetylneuraminic acid (NeuAc) and N-glycolylneuraminic acid (NeuGc), and the hydroxylation of CMP-NeuAc is the rate limiting reaction for the production of NeuGc. The hydroxylation was carried out by a complex formed with hydroxylase, cytochrome b5, and NADH-cytochrome b5 reductase. Mouse hydroxylase was purified from the cytosolic fraction of the liver and its cDNA was cloned. Normal human tissues do not contain NeuGc. Human hydroxylase cDNA was also cloned and the sequence revealed that human hydroxylase has 92 bp deletion. The deletion is the cause of defective expression of NeuGc in human. Chimpanzee has intact hydroxylase gene and the 92 bp deletion occurred after the divergence of human ancestor from chimpanzee ancestor. Biochemical and molecular biological studies on the biosynthesis of NeuGc and biological functions of NeuGc are reviewed.

Structure and receptor specificity of the hemagglutinin from an H5N1 influenza virus

The hemagglutinin (HA) structure at 2.9 angstrom resolution, from a highly pathogenic Vietnamese H5N1 influenza virus, is more related to the 1918 and other human H1 HAs than to a 1997 duck H5 HA. Glycan microarray analysis of this Viet04 HA reveals an avian alpha2-3 sialic acid receptor binding preference. Introduction of mutations that can convert H1 serotype HAs to human alpha2-6 receptor specificity only enhanced or reduced affinity for avian-type receptors. However, mutations that can convert avian H2 and H3 HAs to human receptor specificity, when inserted onto the Viet04 H5 HA framework, permitted binding to a natural human alpha2-6 glycan, which suggests a path for this H5N1 virus to gain a foothold in the human population.

Interaction between the membrane protein of a pathogen and insect microfilament complex determines insect-vector specificity

Many insect-transmissible pathogens are transmitted by specific insect species and not by others, even if they are closely related. The molecular mechanisms underlying such strict pathogen-insect specificity are poorly understood. Candidatus Phytoplasma asteris, OY strain, line W (OY), is a phytopathogenic bacterium transmitted from plant to plant by sap-feeding insect vectors (leafhoppers). Our study focused on an abundant cell-surface membrane protein of the phytoplasma named antigenic membrane protein (Amp), which is not homologous with any reported functional protein. Immunofluorescence microscopy of the phytoplasma-infected insect showed that OY phytoplasma was localized to the microfilaments of the visceral smooth muscle surrounding the insect's intestinal tract. The affinity column assay showed that Amp forms a complex with three insect proteins: actin, myosin heavy chain, and myosin light chain. Amp-microfilament complexes were detected in all OY-transmitting leafhopper species, but not in the non-OY-transmitting leafhoppers, suggesting that the formation of the Amp-microfilament complex is correlated with the phytoplasma-transmitting capability of leafhoppers. Although several studies have reported interactions between pathogens and mammalian microfilaments, this is an example of host-specific interactions between a bacterial surface protein and a host microfilament in insect cells. Our data also suggest that the utilization of a host microfilament may be a universal system for pathogenic bacteria infecting mammals or insects.

Comparison of the replication of influenza A viruses in Chinese ring-necked pheasants and chukar partridges

We investigated the replication and transmission of avian influenza A viruses in two species thought to be intermediate hosts in the spread of influenza A viruses in live poultry markets: Chinese ring-necked pheasants and chukar partridges. All 15 hemagglutinin subtypes replicated in pheasants, and most subtypes transmitted to naïve contact pheasants, primarily via the fecal-oral route. Many viruses were shed from the gastrointestinal tract of experimentally inoculated pheasants for 14 days or longer. Virus was isolated from the cloacal swabs of one contact pheasant for an unprecedented 45 days. Chukar partridges were less susceptible to infection with avian influenza viruses. The viruses that replicated in chukar partridges were isolated for 7 days after experimental inoculation, predominantly from the respiratory tract. We detected high neutralizing antibody titers with correspondingly low levels of serum hemagglutination inhibition antibody titers in pheasants and chukar partridges when chicken red blood cells were used in serological analyses. When horse erythrocytes were used, antibody titers were comparable to those obtained by using the neutralization assay. More importantly, the results suggested that pheasants can serve as a reservoir of influenza virus. Because of their continuous asymptomatic infection and longer stay in the markets, pheasants are ideal "carriers" of influenza A viruses. Their continued presence in live markets contributes to the perpetuation and genetic interaction of influenza viruses there. On the basis of our findings, it does not make good sense to ban quail but not pheasants from the live markets.

Adenovirus Vectors Based on Human Adenovirus Type 19a Have High Potential for Human Muscle-Directed Gene Therapy

Until recently, adenovirus-based gene therapy has been almost exclusively based on human adenovirus serotype 5 (Ad5). The aim of this study was to systematically compare the efficiency of transduction of primary muscle cells from various species by two adenoviral vectors from subgroups C and D. Transduction of a panel of myoblasts demonstrated a striking specificity of an Ad19a-based replication-defective E1-deleted vector (Ad19aEGFP) for human cells, whereas the Ad5-based vector had high affinity for nonhuman primate myoblasts. Transgene expression correlated well with cell-associated vector genomes. Up to 6.59% of the initially applied Ad19aEGFP vector particles were taken up by human myoblasts, as compared with 0.1% of the corresponding Ad5 vector. Remarkably, Ad19aEGFP but not Ad5EGFP efficiently transduced differentiated human myotubes, an in vitro model for skeletal muscle transduction. Uptake of Ad19aEGFP vector particles in human myotubes was 12-fold more efficient than that of Ad5EGFP. Moreover, both vectors demonstrated an early block at the level of vector uptake in mouse myoblasts and rat L6 cells. Investigation of the underlying mechanism for binding and uptake of the two vectors by human myoblasts showed high susceptibility for Ad19a to neuraminidase and wheat germ agglutinin (WGA) lectin, whereas Ad5-mediated transduction was dependent on binding to the coxsackie-adenovirus receptor (CAR) and sensitive to soluble RGD peptide and heparin. Our study offers insights into species-dependent factors that determine Ad tropism and, moreover, provides a basis for application of the novel Ad19a-based vector for gene transfer into human skeletal muscle.

Role of sialic acid-containing molecules in paramyxovirus entry into the host cell: A minireview

Sialic acid-containing compounds play a key role in the initial steps of the paramyxovirus life cycle. As enveloped viruses, their entry into the host cell consists of two main events: binding to the host cell and membrane fusion. Virus adsorption occurs at the surface of the host cell with the recognition of specific receptor molecules located at the cell membrane by specific viral attachment proteins. The viral attachment protein present in some paramyxoviruses (Respirovirus, Rubulavirus and Avulavirus) is the HN glycoprotein, which binds to cellular sialic acid-containing molecules and exhibits sialidase and fusion promotion activities. Gangliosides of the gangliotetraose series bearing the sialic acid N-acetylneuraminic (Neu5Ac) on the terminal galactose attached in alpha2-3 linkage, such as GD1a, GT1b, and GQ1b, and neolacto-series gangliosides are the major receptors for Sendai virus. Much less is known about the receptors for other paramyxoviruses than for Sendai virus. Human parainfluenza viruses 1 and 3 preferentially recognize oligosaccharides containing N-acetyllactosaminoglycan branches with terminal Neu5Acalpha2-3Gal. In the case of Newcastle disease virus, has been reported the absence of a specific pattern of the gangliosides that interact with the virus. Additionally, several works have described the use of sialylated glycoproteins as paramyxovirus receptors. Accordingly, the design of specific sialic acid analogs to inhibit the sialidase and/or receptor binding activity of viral attachment proteins is an important antiviral strategy. In spite of all these data, the exact nature of paramyxovirus receptors, apart from their sialylated nature, and the mechanism(s) of viral attachment to the cell surface are poorly understood.