Hemagglutinin (HA) is the receptor-binding and membrane fusion glycoprotein of influenza virus and the target for infectivity-neutralizing antibodies. The structures of three conformations of the ectodomain of the 1968 Hong Kong influenza virus HA have been determined by X-ray crystallography: the single-chain precursor, HA0; the metastable neutral-pH conformation found on virus, and the fusion pH-induced conformation. These structures provide a framework for designing and interpreting the results of experiments on the activity of HA in receptor binding, the generation of emerging and reemerging epidemics, and membrane fusion during viral entry. Structures of HA in complex with sialic acid receptor analogs, together with binding experiments, provide details of these low-affinity interactions in terms of the sialic acid substituents recognized and the HA residues involved in recognition. Neutralizing antibody-binding sites surround the receptor-binding pocket on the membrane-distal surface of HA, and the structures of the complexes between neutralizing monoclonal Fabs and HA indicate possible neutralization mechanisms. Cleavage of the biosynthetic precursor HA0 at a prominent loop in its structure primes HA for subsequent activation of membrane fusion at endosomal pH (Figure 1). Priming involves insertion of the fusion peptide into a charged pocket in the precursor; activation requires its extrusion towards the fusion target membrane, as the N terminus of a newly formed trimeric coiled coil, and repositioning of the C-terminal membrane anchor near the fusion peptide at the same end of a rod-shaped molecule. Comparison of this new HA conformation, which has been formed for membrane fusion, with the structures determined for other virus fusion glycoproteins suggests that these molecules are all in the fusion-activated conformation and that the juxtaposition of the membrane anchor and fusion peptide, a recurring feature, is involved in the fusion mechanism. Extension of these comparisons to the soluble N-ethyl-maleimide-sensitive factor attachment protein receptor (SNARE) protein complex of vesicle fusion allows a similar conclusion.

Cell surfaces in the immune system are richly equipped with a complex mixture of glycans, which can be recognized by diverse glycan-binding proteins. The Siglecs are a family of sialic-acid-binding immunoglobulin-like lectins that are thought to promote cell-cell interactions and regulate the functions of cells in the innate and adaptive immune systems through glycan recognition. In this Review, we describe recent studies on signalling mechanisms and discuss the potential role of Siglecs in triggering endocytosis and in pathogen recognition. Finally, we discuss the postulated functions of the recently discovered CD33-related Siglecs and consider the factors that seem to be driving their rapid evolution.

Although more than 100 people have been infected by H5N1 influenza A viruses, human-to-human transmission is rare. What are the molecular barriers limiting human-to-human transmission? Here we demonstrate an anatomical difference in the distribution in the human airway of the different binding molecules preferred by the avian and human influenza viruses. The respective molecules are sialic acid linked to galactose by an alpha-2,3 linkage (SAalpha2,3Gal) and by an alpha-2,6 linkage (SAalpha2,6Gal). Our findings may provide a rational explanation for why H5N1 viruses at present rarely infect and spread between humans although they can replicate efficiently in the lungs.

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.

Aquatic birds harbor diverse influenza A viruses and are a major viral reservoir in nature. The recent discovery of influenza viruses of a new H17N10 subtype in Central American fruit bats suggests that other New World species may similarly carry divergent influenza viruses. Using consensus degenerate RT-PCR, we identified a novel influenza A virus, designated as H18N11, in a flat-faced fruit bat (Artibeus planirostris) from Peru. Serologic studies with the recombinant H18 protein indicated that several Peruvian bat species were infected by this virus. Phylogenetic analyses demonstrate that, in some gene segments, New World bats harbor more influenza virus genetic diversity than all other mammalian and avian species combined, indicative of a long-standing host-virus association. Structural and functional analyses of the hemagglutinin and neuraminidase indicate that sialic acid is not a ligand for virus attachment nor a substrate for release, suggesting a unique mode of influenza A virus attachment and activation of membrane fusion for entry into host cells. Taken together, these findings indicate that bats constitute a potentially important and likely ancient reservoir for a diverse pool of influenza viruses.

BACKGROUND

Human infection with avian influenza A H7N9 virus emerged in eastern China in February, 2013, and has been associated with exposure to poultry. We report the clinical and microbiological features of patients infected with influenza A H7N9 virus and compare genomic features of the human virus with those of the virus in market poultry in Zhejiang, China.

METHODS

Between March 7 and April 8, 2013, we included hospital inpatients if they had new-onset respiratory symptoms, unexplained radiographic infiltrate, and laboratory-confirmed H7N9 virus infection. We recorded histories and results of haematological, biochemical, radiological, and microbiological investigations. We took throat and sputum samples, used RT-PCR to detect M, H7, and N9 genes, and cultured samples in Madin-Darby canine kidney cells. We tested for co-infections and monitored serum concentrations of six cytokines and chemokines. We collected cloacal swabs from 86 birds from epidemiologically linked wet markets and inoculated embryonated chicken eggs with the samples. We identified and subtyped isolates by RT-PCR sequencing. RNA extraction, complementary DNA synthesis, and PCR sequencing were done for one human and one chicken isolate. We characterised and phylogenetically analysed the eight gene segments of the viruses in the patient's and the chicken's isolates, and constructed phylogenetic trees of H, N, PB2, and NS genes.

RESULTS

We identified four patients (mean age 56 years), all of whom had contact with poultry 3-8 days before disease onset. They presented with fever and rapidly progressive pneumonia that did not respond to antibiotics. Patients were leucopenic and lymphopenic, and had impaired liver or renal function, substantially increased serum cytokine or chemokine concentrations, and disseminated intravascular coagulation with disease progression. Two patients died. Sputum specimens were more likely to test positive for the H7N9 virus than were samples from throat swabs. The viral isolate from the patient was closely similar to that from an epidemiologically linked market chicken. All viral gene segments were of avian origin. The H7 of the isolated viruses was closest to that of the H7N3 virus from domestic ducks in Zhejiang, whereas the N9 was closest to that of the wild bird H7N9 virus in South Korea. We noted Gln226Leu and Gly186Val substitutions in human virus H7 (associated with increased affinity for α-2,6-linked sialic acid receptors) and the PB2 Asp701Asn mutation (associated with mammalian adaptation). Ser31Asn mutation, which is associated with adamantane resistance, was noted in viral M2.

CONCLUSIONS

Cross species poultry-to-person transmission of this new reassortant H7N9 virus is associated with severe pneumonia and multiorgan dysfunction in human beings. Monitoring of the viral evolution and further study of disease pathogenesis will improve disease management, epidemic control, and pandemic preparedness.

BACKGROUND

Larry Chi-Kin Yung, National Key Program for Infectious Diseases of China.

Among several promising new drug-delivery systems, liposomes represent an advanced technology to deliver active molecules to the site of action, and at present several formulations are in clinical use. Research on liposome technology has progressed from conventional vesicles ("first-generation liposomes") to "second-generation liposomes", in which long-circulating liposomes are obtained by modulating the lipid composition, size, and charge of the vesicle. Liposomes with modified surfaces have also been developed using several molecules, such as glycolipids or sialic acid. A significant step in the development of long-circulating liposomes came with inclusion of the synthetic polymer poly-(ethylene glycol) (PEG) in liposome composition. The presence of PEG on the surface of the liposomal carrier has been shown to extend blood-circulation time while reducing mononuclear phagocyte system uptake (stealth liposomes). This technology has resulted in a large number of liposome formulations encapsulating active molecules, with high target efficiency and activity. Further, by synthetic modification of the terminal PEG molecule, stealth liposomes can be actively targeted with monoclonal antibodies or ligands. This review focuses on stealth technology and summarizes pre-clinical and clinical data relating to the principal liposome formulations; it also discusses emerging trends of this promising technology.

The three-dimensional structures of influenza virus haemagglutinins complexed with cell receptor analogues show sialic acids bound to a pocket of conserved amino acids surrounded by antibody-binding sites. Sialic acid fills the conserved pocket, demonstrating that it is the influenza virus receptor. The proximity of the antibody-binding sites suggests that antibodies neutralize virus infectivity by preventing virus-to-cell binding. The structures suggest approaches to the design of anti-viral drugs that could block attachment of viruses to cells.

The receptor specificity of 56 H2 and H3 influenza virus isolates from various animal species has been determined to test the relevance of receptor specificity to the ecology of influenza virus. The results show that the receptor specificity of both H2 and H3 isolates evaluated for sialic acid linkage specificity and inhibition of hemagglutination by horse serum correlates with the species of origin, as postulated earlier for H3 strains based on a limited survey of five human, three avian, and one equine strain. Elucidation of the amino acid sequence of several human H2 receptor variants and analysis of known sequences of H2 and H3 isolates revealed that receptor specificity varies in association with an amino acid change at residues 228 in addition to the change at residue 226 previously documented to affect receptor specificity of H3 but not H1 isolates. Residues 226 and 228 are leucine and serine in human isolates, which preferentially bind sialic acid alpha 2,6-galactose beta 1,4-N-acetyl glucosamine (SA alpha 2,6Gal), and glutamine and glycine in avian and equine isolates, which exhibit specificity for sialic acid alpha-2,3-galactose beta-1,3-N-acetyl galactosamine (SA alpha 2,3Gal). The results demonstrate that the correlation of receptor specificity and species of origin is maintained across both H2 and H3 influenza virus serotypes and provide compelling evidence that influenza virus hosts exert selective pressure to maintain the receptor specificity characteristics of strains isolated from that species.

A new colorimetric method for quantitative analysis of serum lipid peroxide, free of interference from sialic acids, has been developed. We have used the thiobarbituric acid dissolved in sodium sulfate solution and both liberation of lipid peroxide and color reaction have been performed simultaneously by heating serum protein precipitate with this reagent in a weak acid solution. The new method is specific and facilitates the precise measurements of serum lipid peroxide. The average values determined by the new method increased slightly with age in healthy subjects. In patients with sequelae of cerebrovascular disorders, serum lipid peroxide values were higher than in healthy controls. These results may demonstrate the important role of lipid peroxide in aging and cerebrovascular disorders.

To examine the association of low-grade systemic inflammation with diabetes, as well as its heterogeneity across subgroups, we designed a case-cohort study representing the approximately 9-year experience of 10,275 Atherosclerosis Risk in Communities Study participants. Analytes were measured on stored plasma of 581 incident cases of diabetes and 572 noncases. Statistically significant hazard ratios of developing diabetes for those in the fourth (versus first) quartile of inflammation markers, adjusted for age, sex, ethnicity, study center, parental history of diabetes, and hypertension, ranged from 1.9 to 2.8 for sialic acid, orosomucoid, interleukin-6, and C-reactive protein. After additional adjustment for BMI, waist-to-hip ratio, and fasting glucose and insulin, only the interleukin-6 association remained statistically significant (HR = 1.6, 1.01-2.7). Exclusion of GAD antibody-positive individuals changed associations minimally. An overall inflammation score based on these four markers plus white cell count and fibrinogen predicted diabetes in whites but not African Americans (interaction P = 0.005) and in nonsmokers but not smokers (interaction P = 0.13). The fully adjusted hazard ratio comparing white nonsmokers with score extremes was 3.7 (P for linear trend = 0.008). In conclusion, a low-grade inflammation predicts incident type 2 diabetes. The association is absent in smokers and African-Americans.

The 1918 influenza pandemic was a catastrophic series of virus outbreaks that spread across the globe. Here, we show that only a modest change in the 1918 influenza hemagglutinin receptor binding site alters the transmissibility of this pandemic virus. Two amino acid mutations that cause a switch in receptor binding preference from the human alpha-2,6 to the avian alpha-2,3 sialic acid resulted in a virus incapable of respiratory droplet transmission between ferrets but that maintained its lethality and replication efficiency in the upper respiratory tract. Furthermore, poor transmission of a 1918 virus with dual alpha-2,6 and alpha-2,3 specificity suggests that a predominant human alpha-2,6 sialic acid binding preference is essential for optimal transmission of this pandemic virus. These findings confirm an essential role of hemagglutinin receptor specificity for the transmission of influenza viruses among mammals.

Helicobacter pylori adherence in the human gastric mucosa involves specific bacterial adhesins and cognate host receptors. Here, we identify sialyl-dimeric-Lewis x glycosphingolipid as a receptor for H. pylori and show that H. pylori infection induced formation of sialyl-Lewis x antigens in gastric epithelium in humans and in a Rhesus monkey. The corresponding sialic acid-binding adhesin (SabA) was isolated with the "retagging" method, and the underlying sabA gene (JHP662/HP0725) was identified. The ability of many H. pylori strains to adhere to sialylated glycoconjugates expressed during chronic inflammation might thus contribute to virulence and the extraordinary chronicity of H. pylori infection.

The binding of influenza virus to erythrocytes and host cells is mediated by the interaction of the viral hemagglutinin (H) with cell surface receptors containing sialic acid (SA). The specificity of this interaction for 19 human and animal influenza isolates was examined using human erythrocytes enzymatically modified to contain cell surface sialyloligosaccharides with the sequence SA alpha 2,6Gal beta 1,4GlcNAc; SA alpha 2,3Gal beta 1,4(3)GlcNAc; SA alpha 2,3Gal beta 1,3GalNAc; or SA alpha 2,6GalNAc. Although none of the viruses agglutinated cells containing the SA alpha 2,6GalNAc linkage, differential agglutination of cells containing the other three sequences revealed at least three distinct receptor binding types. Several virus isolates exhibited marked receptor specificity, binding only to cells containing the SA alpha 2,6Gal or the SA alpha 2,3Gal linkage, while others bound equally well to cells containing either linkage. Moreover, some viruses could distinguish between two oligosaccharide receptor determinants containing the terminal SA alpha 2,3Gal linkage when present in the SA alpha 2,3Gal beta 1,4(3)GlcNAc sequence or the SA alpha 2,3Gal beta 1,3GalNAc sequence binding cells containing only the former. The observed receptor specificities were not significantly influenced by the viral neuraminidases as shown by the use of the potent neuraminidase inhibitor 2-deoxy-2,3-dehydro-N-acetylneuraminic acid. Receptor specificity appeared, to some extent, to be dependent on the species from which the virus was isolated. In particular, human isolates of the H3 serotype all agglutinated cells containing the SA alpha 2,6Gal linkage, but not cells bearing the SA alpha 2,3Gal beta 1,3GalNAc sequence. In contrast, antigenically similar (H3) isolates from avian and equine species preferentially bound erythrocytes containing the SA alpha 2,3Gal linkage. This is of particular interest in view of the identification of the avian virus H3 hemagglutinin as the progenitor of the H3 hemagglutinin present on the current human Hong Kong viruses.

The recent human infections caused by H5N1, H9N2, and H7N7 avian influenza viruses highlighted the continuous threat of new pathogenic influenza viruses emerging from a natural reservoir in birds. It is generally believed that replication of avian influenza viruses in humans is restricted by a poor fit of these viruses to cellular receptors and extracellular inhibitors in the human respiratory tract. However, detailed mechanisms of this restriction remain obscure. Here, using cultures of differentiated human airway epithelial cells, we demonstrated that influenza viruses enter the airway epithelium through specific target cells and that there were striking differences in this respect between human and avian viruses. During the course of a single-cycle infection, human viruses preferentially infected nonciliated cells, whereas avian viruses as well as the egg-adapted human virus variant with an avian virus-like receptor specificity mainly infected ciliated cells. This pattern correlated with the predominant localization of receptors for human viruses (2-6-linked sialic acids) on nonciliated cells and of receptors for avian viruses (2-3-linked sialic acids) on ciliated cells. These findings suggest that although avian influenza viruses can infect human airway epithelium, their replication may be limited by a nonoptimal cellular tropism. Our data throw light on the mechanisms of generation of pandemic viruses from their avian progenitors and open avenues for cell level-oriented studies on the replication and pathogenicity of influenza virus in humans.

The haemagglutinin (HA) glycoproteins of influenza virus membranes are responsible for binding viruses to cells by interacting with membrane receptor molecules which contain sialic acid (for review see ref. 1). This interaction is known to vary in detailed specificity for different influenza viruses (see, for example, refs 2-4) and we have attempted to identify the sialic acid binding site of the haemagglutinin by comparing the amino acid sequences of haemagglutinins with different binding specificities. We present here evidence that haemagglutinins which differ in recognizing either NeuAc alpha 2 leads to 3Gal- or NeuAc alpha 2 leads to 6Gal- linkages in glycoproteins also differ at amino acid 226 of HA1. This residue is located in a pocket on the distal tip of the molecule, an area previously proposed from considerations of the three-dimensional structure of the haemagglutinin to be involved in receptor binding.

The human intestine, colonized by a dense community of resident microbes, is a frequent target of bacterial pathogens. Undisturbed, this intestinal microbiota provides protection from bacterial infections. Conversely, disruption of the microbiota with oral antibiotics often precedes the emergence of several enteric pathogens. How pathogens capitalize upon the failure of microbiota-afforded protection is largely unknown. Here we show that two antibiotic-associated pathogens, Salmonella enterica serovar Typhimurium (S. typhimurium) and Clostridium difficile, use a common strategy of catabolizing microbiota-liberated mucosal carbohydrates during their expansion within the gut. S. typhimurium accesses fucose and sialic acid within the lumen of the gut in a microbiota-dependent manner, and genetic ablation of the respective catabolic pathways reduces its competitiveness in vivo. Similarly, C. difficile expansion is aided by microbiota-induced elevation of sialic acid levels in vivo. Colonization of gnotobiotic mice with a sialidase-deficient mutant of Bacteroides thetaiotaomicron, a model gut symbiont, reduces free sialic acid levels resulting in C. difficile downregulating its sialic acid catabolic pathway and exhibiting impaired expansion. These effects are reversed by exogenous dietary administration of free sialic acid. Furthermore, antibiotic treatment of conventional mice induces a spike in free sialic acid and mutants of both Salmonella and C. difficile that are unable to catabolize sialic acid exhibit impaired expansion. These data show that antibiotic-induced disruption of the resident microbiota and subsequent alteration in mucosal carbohydrate availability are exploited by these two distantly related enteric pathogens in a similar manner. This insight suggests new therapeutic approaches for preventing diseases caused by antibiotic-associated pathogens.

Human embryonic stem cells (HESC) can potentially generate every body cell type, making them excellent candidates for cell- and tissue-replacement therapies. HESC are typically cultured with animal-derived 'serum replacements' on mouse feeder layers. Both of these are sources of the nonhuman sialic acid Neu5Gc, against which many humans have circulating antibodies. Both HESC and derived embryoid bodies metabolically incorporate substantial amounts of Neu5Gc under standard conditions. Exposure to human sera with antibodies specific for Neu5Gc resulted in binding of immunoglobulin and deposition of complement, which would lead to cell killing in vivo. Levels of Neu5Gc on HESC and embryoid bodies dropped after culture in heat-inactivated anti-Neu5Gc antibody-negative human serum, reducing binding of antibodies and complement from high-titer sera, while allowing maintenance of the undifferentiated state. Complete elimination of Neu5Gc would be likely to require using human serum with human feeder layers, ideally starting with fresh HESC that have never been exposed to animal products.

The transmembrane protein CD33 is a sialic acid-binding immunoglobulin-like lectin that regulates innate immunity but has no known functions in the brain. We have previously shown that the CD33 gene is a risk factor for Alzheimer's disease (AD). Here, we observed increased expression of CD33 in microglial cells in AD brain. The minor allele of the CD33 SNP rs3865444, which confers protection against AD, was associated with reductions in both CD33 expression and insoluble amyloid beta 42 (Aβ42) levels in AD brain. Furthermore, the numbers of CD33-immunoreactive microglia were positively correlated with insoluble Aβ42 levels and plaque burden in AD brain. CD33 inhibited uptake and clearance of Aβ42 in microglial cell cultures. Finally, brain levels of insoluble Aβ42 as well as amyloid plaque burden were markedly reduced in APP(Swe)/PS1(ΔE9)/CD33(-/-) mice. Therefore, CD33 inactivation mitigates Aβ pathology and CD33 inhibition could represent a novel therapy for AD.

BACKGROUND

Type 2 diabetes mellitus and atherosclerotic cardiovascular disease have common antecedents. Since markers of inflammation predict coronary heart disease and are raised in patients with type 2 diabetes, we investigated whether they predict whether people will develop type 2 diabetes.

METHODS

12,330 men and women, aged 45-64 years, were followed up for a mean of 7 years. We analysed the association between different markers of acute inflammation and subsequent diagnosis of diabetes. In a subgroup of 610 individuals selected originally for an unrelated atherosclerosis case-control study, we also investigated diabetes associations with total sialic acid and orosomucoid, haptoglobin, and alpha1-antitrypsin.

RESULTS

1335 individuals had a new diagnosis of diabetes. Adjusted odds ratios for developing diabetes for quartile extremes were 1.9 (95% CI 1.6-2.3) for raised white-cell count, 1.3 (1.0-1.5) for low serum albumin, and 1.2 (1.0-1.5) for raised fibrinogen. In the subgroup analysis, individuals with concentrations of orosomucoid and sialic acid of more than the median had odds ratios of 7.9 (2.6-23.7) and 3.7 (1.4-9.8), respectively. Adjustment for body-mass index and waist-to-hip ratio lessened the associations; those for white-cell count (1.5 [1.3-1.8]), orosomucoid (7.1 [2.1-23.7]), and sialic acid (2.8 [1.0-8.1]) remained significant.

CONCLUSIONS

Markers of inflammation are associated with the development of diabetes in middle-aged adults. Although autoimmunity may partly explain these associations, they probably reflect the pathogenesis of type 2 diabetes.

H5N1 influenza A viruses have spread to numerous countries in Asia, Europe and Africa, infecting not only large numbers of poultry, but also an increasing number of humans, often with lethal effects. Human and avian influenza A viruses differ in their recognition of host cell receptors: the former preferentially recognize receptors with saccharides terminating in sialic acid-alpha2,6-galactose (SAalpha2,6Gal), whereas the latter prefer those ending in SAalpha2,3Gal (refs 3-6). A conversion from SAalpha2,3Gal to SAalpha2,6Gal recognition is thought to be one of the changes that must occur before avian influenza viruses can replicate efficiently in humans and acquire the potential to cause a pandemic. By identifying mutations in the receptor-binding haemagglutinin (HA) molecule that would enable avian H5N1 viruses to recognize human-type host cell receptors, it may be possible to predict (and thus to increase preparedness for) the emergence of pandemic viruses. Here we show that some H5N1 viruses isolated from humans can bind to both human and avian receptors, in contrast to those isolated from chickens and ducks, which recognize the avian receptors exclusively. Mutations at positions 182 and 192 independently convert the HAs of H5N1 viruses known to recognize the avian receptor to ones that recognize the human receptor. Analysis of the crystal structure of the HA from an H5N1 virus used in our genetic experiments shows that the locations of these amino acids in the HA molecule are compatible with an effect on receptor binding. The amino acid changes that we identify might serve as molecular markers for assessing the pandemic potential of H5N1 field isolates.