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

Discovery of novel 1-phenyl-cycloalkane carbamides as potent and selective influenza fusion inhibitors

A novel class of HA inhibitors (4a) was identified based on ligand similarity search of known HA inhibitors. Parallel synthesis and further structural modifications resulted in 1-phenyl-cyclopentanecarboxylic acid (4-cyano-phenyl)-methyl-amide 4t as a potent and selective inhibitor to phylogenetic H1 influenza viruses with an EC(50) of 98 nM against H1N1 A/Weiss/43 strain and over 1000-fold selectivity against host MDCK cells.

Structures of influenza A proteins and insights into antiviral drug targets

The world is currently undergoing a pandemic caused by an H1N1 influenza A virus, the so-called 'swine flu'. The H5N1 ('bird flu') influenza A viruses, now circulating in Asia, Africa and Europe, are extremely virulent in humans, although they have not so far acquired the ability to transfer efficiently from human to human. These health concerns have spurred considerable interest in understanding the molecular biology of influenza A viruses. Recent structural studies of influenza A virus proteins (or fragments) help enhance our understanding of the molecular mechanisms of the viral proteins and the effects of drug resistance to improve drug design. The structures of domains of the influenza RNA-dependent RNA polymerase and the nonstructural NS1A protein provide opportunities for targeting these proteins to inhibit viral replication.

Therapeutic antibodies, vaccines and antibodyomes

The potential for antibodies to act as "magic bullets" for treatment of human disease was recognized a century ago, but its full realization has began to occur only during the last decade. A key to their current success is the ability to make libraries of antibodies/B cells, isolate a single species, and engineer it to be safe, efficacious and of high quality. Despite this progress, major challenges to the effective prevention, diagnosis and treatment of a vast majority of diseases remain. Limited success in the development of effective vaccines against diseases such as AIDS and cancer reflects our incomplete understanding of how antibodies are generated and function. Only a miniscule number of antibodies are characterized out of the universe of antibodies generated by the immune system. Knowledge of antibodyomes-the complete sets of antibodies-could help solve these and other challenges.

Avian influenza pandemic preparedness: developing prepandemic and pandemic vaccines against a moving target

The unprecedented global spread of highly pathogenic avian H5N1 influenza viruses within the past ten years and their extreme lethality to poultry and humans has underscored their potential to cause an influenza pandemic. Combating the threat of an impending H5N1 influenza pandemic will require a combination of pharmaceutical and nonpharmaceutical intervention strategies. The emergence of the H1N1 pandemic in 2009 emphasised the unpredictable nature of a pandemic influenza. Undoubtedly, vaccines offer the most viable means to combat a pandemic threat. Current egg-based influenza vaccine manufacturing strategies are unlikely to be able to cater to the huge, rapid global demand because of the anticipated scarcity of embryonated eggs in an avian influenza pandemic and other factors associated with the vaccine production process. Therefore, alternative, egg-independent vaccine manufacturing strategies should be evaluated to supplement the traditional egg-derived influenza vaccine manufacturing. Furthermore, evaluation of dose-sparing strategies that offer protection with a reduced antigen dose will be critical for pandemic influenza preparedness. Development of new antiviral therapeutics and other, nonpharmaceutical intervention strategies will further supplement pandemic preparedness. This review highlights the current status of egg-dependent and egg-independent strategies against an avian influenza pandemic.

Immunogenic display of diverse peptides, including a broadly cross-type neutralizing human papillomavirus L2 epitope, on virus-like particles of the RNA bacteriophage PP7

The immunogenicity of an antigen can be dramatically increased by displaying it in a dense, multivalent context, such as on the surface of a virus or virus-like particle (VLP). Here we describe a highly versatile VLP platform for peptide display based on VLPs of the RNA bacteriophage PP7. We show that this platform can be used for the engineered display of specific peptide sequences as well as for the construction of random peptide libraries. Peptides representing the FLAG epitope, the V3 loop of HIV gp120, and a broadly cross-type neutralizing epitope from L2, the minor capsid protein of Human Papillomavirus type 16 (HPV16), were inserted into an exposed surface loop of a form of PP7 coat protein in which the two identical polypeptides of coat were fused together to form a single-chain dimer. The recombinant proteins assembled into VLPs, displayed these peptides on their surfaces, and induced high-titer antibody responses. The single-chain dimer was also highly tolerant of random 6-, 8-, and 10-amino acid insertions. PP7 VLPs displaying the HPV16 L2 epitope generated robust anti-HPV16 L2 serum antibodies after intramuscular injection that protected mice from genital infection with HPV16 pseudovirus as well as a heterologous HPV pseudovirus type, HPV45. Thus, PP7 VLPs are well-suited for the display of a wide diversity of peptides in a highly immunogenic format.

Heterosubtypic neutralizing antibodies are produced by individuals immunized with a seasonal influenza vaccine

The target of neutralizing antibodies that protect against influenza virus infection is the viral protein HA. Genetic and antigenic variation in HA has been used to classify influenza viruses into subtypes (H1-H16). The neutralizing antibody response to influenza virus is thought to be specific for a few antigenically related isolates within a given subtype. However, while heterosubtypic antibodies capable of neutralizing multiple influenza virus subtypes have been recently isolated from phage display libraries, it is not known whether such antibodies are produced in the course of an immune response to influenza virus infection or vaccine. Here we report that, following vaccination with seasonal influenza vaccine containing H1 and H3 influenza virus subtypes, some individuals produce antibodies that cross-react with H5 HA. By immortalizing IgG-expressing B cells from 4 individuals, we isolated 20 heterosubtypic mAbs that bound and neutralized viruses belonging to several HA subtypes (H1, H2, H5, H6, and H9), including the pandemic A/California/07/09 H1N1 isolate. The mAbs used different VH genes and carried a high frequency of somatic mutations. With the exception of a mAb that bound to the HA globular head, all heterosubtypic mAbs bound to acid-sensitive epitopes in the HA stem region. Four mAbs were evaluated in vivo and protected mice from challenge with influenza viruses representative of different subtypes. These findings reveal that seasonal influenza vaccination can induce polyclonal heterosubtypic neutralizing antibodies that cross-react with the swine-origin pandemic H1N1 influenza virus and with the highly pathogenic H5N1 virus.

A universal long-term flu vaccine may not prevent severe epidemics

BACKGROUND

Recently, the promise of a new universal long-term flu vaccine has become more tangible than ever before. Such a vaccine would protect against very many seasonal and pandemic flu strains for many years, making annual vaccination unnecessary. However, due to complacency behavior, it remains unclear whether the introduction of such vaccines would maintain high and stable levels of vaccination coverage year after year.

FINDINGS

To predict the impact of universal long-term flu vaccines on influenza epidemics we developed a mathematical model that linked human cognition and memory with the transmission dynamics of influenza. Our modeling shows that universal vaccines that provide short-term protection are likely to result in small frequent epidemics, whereas universal vaccines that provide long-term protection are likely to result in severe infrequent epidemics.

CONCLUSIONS

Influenza vaccines that provide short-term protection maintain risk awareness regarding influenza in the population and result in stable vaccination coverage. Vaccines that provide long-term protection could lead to substantial drops in vaccination coverage and should therefore include an annual epidemic risk awareness programs in order to minimize the risk of severe epidemics.

Induction of a Protective Heterosubtypic Immune Response Against the Influenza Virus by using Recombinant Adenoviral Vectors Expressing Hemagglutinin of the Influenza H5 Virus

Influenza viruses are characterized by a high degree of antigenic variability, which causes the annual emergence of flu epidemics and irregularly timed pandemics caused by viruses with new antigenic and biological traits. Novel approaches to vaccination can help circumvent this problem. One of these new methods incorporates genetic vaccines based on adenoviral vectors. Recombinant adenoviral vectors which contain hemagglutinin-encoding genes from avian H5N1 and H5N2 (Ad-HA5-1 and Ad-HA5-2) influenza viruses were obtained using the AdEasy Adenoviral Vector System (Stratagene). Laboratory mice received a double intranasal vaccination with Ad-HA5-1 and Ad-HA5-2. This study demonstrates that immunization with recombinant adenoviruses bearing the Н 5 influenza virus hemagglutinin gene induces a immune response which protects immunized mice from a lethal dose of the H5 influenza virus. Moreover, it also protects the host from a lethal dose of the H1 virus, which belongs to the same clade as H5, but does not confer protection from the subtype H3 influenza virus, which belongs to a different clade.

Structure and Receptor binding properties of a pandemic H1N1 virus hemagglutinin

The 3D-structure of the major surface viral antigen from the recent H1N1 pandemic influenza virus (A/Darwin/2001/2009) was determined to 2.8 Å resolution. The structure was used to analyze changes in the HA that have emerged during the first 11 months of the pandemic and have raised public health concerns. Receptor binding properties of this protein reveals a strict preference for human-type receptors.

Recent advances in the study of human antibody responses to influenza virus using optimized human hybridoma approaches

Influenza viruses exhibit a fascinating level of antigenic heterogeneity that facilitates re-infection in the human population. The human antibody repertoire also manifests endless capability for variation in the genes that specify the portion of antibody molecules interacting with epitopes. A recent explosion of techniques for isolating human monoclonal antibodies to viruses has led to isolation of new antibodies that allow glimpses into the molecular basis for recognition and escape that underlies the constant antigenic drift in influenza surface proteins. These studies also reveal evidence for lifelong persistence of immunity to some influenza viruses.

Anti-infective antibodies--reviving an old paradigm

Antibody therapy for infections has a long history and the development of monoclonal antibody technologies, and of increasingly ingenious techniques for making these products, has brought about a major revival in interest. In this paper the field is reviewed with particular emphasis on two topics: the role that antibodies may have in combating pandemic flu; and the prospects for giving, as a food, antibodies derived from the ovalbumin of transgenic chicken eggs, as a prophylactic against diarrhoeal disease.

[Novel vaccines. Vaccinations in the near and distant future]

Easy-to-develop vaccines usually induce antibodies against acute, self-limiting infections by stable pathogens. Today, most of these vaccines have been made, and the future diseases to tackle are more challenging: highly variable pathogens, rapidly emerging new infections with the potential of developing into pandemics, or therapeutic applications for chronic infections and cancer which most likely require complex immune responses beyond the induction of antibodies. The impact of scientific and technological progress on vaccinology has multiplied the strategies to improve vaccines. Here, we describe how genome-based approaches have revolutionized the way to identify vaccine antigen candidates, how the vast numbers of candidates can be further ranked by sophisticated gene- and protein-array based screening methods, and how surface proteomics may accelerate this target identification process. Increased structural knowledge of antigens will allow exposing or stabilizing those antigen parts relevant for protection and thereby direct the immune response to them. Improved adjuvants will enhance and bias the immune response to induce the relevant arms of the immune system. In conclusion, thanks to conceptual and biotechnological progress, future vaccines will be safer, more efficient and more complex than those today.

Characteristic features of InfA-15 monoclonal antibody recognizing H1, H3, and H5 subtypes of hemagglutinin of influenza virus A type

Hemagglutinin molecule is an envelope protein of influenza virus and plays an important role in the infection to human cells. Many mutations are observed in the molecule, which generates sixteen subtypes (H1-H16) of the hemagglutinin molecule for influenza virus A type. The subtypes such as H1, H2, H3, and H5 out of the sixteen are underlined molecules, which are responsible to Spain, Asia, Hong Kong, and Avian Flu, respectively. Based on the sequence analysis, three short sequences, which are highly conserved in the subtypes of influenza virus A type, were extracted. The sequence peptides were chemically synthesized and conjugated with BSA for immunization into Balb/c mice. A sequence GMVDGWYG located at the domain of fusion protein in the hemagglutinin molecule exhibited a high immuno-response, resulting in the production of a monoclonal antibody (mAb; InfA-15). The unique features of InfA-15 mAb were investigated from the viewpoint of immunological reaction, the binding affinity, the steric conformation, etc. The InfA-15 mAb could react with the H1, H3, and H5 subtype of hemagglutinin molecule of influenza virus A type. ELISAs using InfA-15 mAb suggested a wide reaction spectrum for the hemagglutinin of many important influenza viruses A type.

Broadly protective monoclonal antibodies against H3 influenza viruses following sequential immunization with different hemagglutinins

As targets of adaptive immunity, influenza viruses are characterized by the fluidity with which they respond to the selective pressure applied by neutralizing antibodies. This mutability of structural determinants of protective immunity is the obstacle in developing universal influenza vaccines. Towards the development of such vaccines and other immune therapies, our studies are designed to identify regions of influenza viruses that are conserved and that mediate virus neutralization. We have specifically focused on viruses of the H3N2 subtype, which have persisted as a principal source of influenza-related morbidity and mortality in humans since the pandemic of 1968. Three monoclonal antibodies have been identified that are broadly-neutralizing against H3 influenza viruses spanning 40 years. The antibodies react with the hemagglutinin glycoprotein and appear to bind in regions that are refractory to the structural variation required for viral escape from neutralization. The antibodies demonstrate therapeutic efficacy in mice against H3N2 virus infection and have potential for use in the treatment of human influenza disease. By mapping the binding region of one antibody, 12D1, we have identified a continuous region of the hemagglutinin that may act as an immunogen to elicit broadly protective immunity to H3 viruses. The anti-H3 monoclonal antibodies were identified after immunization of mice with the hemagglutinin of four different viruses (A/Hong Kong/1/1968, A/Alabama/1/1981, A/Beijing/47/1992, A/Wyoming/3/2003). This immunization schedule was designed to boost B cells specific for conserved regions of the hemagglutinin from distinct antigenic clusters. Importantly, our antibodies are of naturally occurring specificity rather than selected from cloned libraries, demonstrating that broad-spectrum humoral immunity to influenza viruses can be elicited in vivo.

Influenza viruses remain a formidable public health threat. Because of a dramatic increase in drug resistant strains of influenza viruses and due to the semi-regular emergence of pandemic virus strains, the development of novel antibody-based therapies and influenza vaccine constructs is of great interest. Recently, monoclonal antibodies with broad neutralizing activity against an array of Group 1 influenza viruses (including H5 and H1 subtypes) were identified; studies using these antibodies have expanded our understanding of structural aspects of the viral hemagglutinin, the molecule mediating protective immunity to influenza viruses. We have identified the first broadly neutralizing antibodies against viruses in Group 2—specifically, they are active against H3 influenza viruses spanning 40 years. The antibodies react with the hemagglutinin and appear to bind in regions that are refractory to the structural variation required for viral escape from neutralization. The antibodies demonstrate therapeutic efficacy in mice against H3N2 virus infection and have potential for use in the treatment of human influenza disease. By mapping the binding region of one antibody, 12D1, we have identified a continuous region of the hemagglutinin that may act as an immunogen to elicit an immune response conferring broad protection against H3 viruses.

Convalescent transfusion for pandemic influenza: preparing blood banks for a new plasma product?

Due to the potential of a severe pandemic to limit efficacy or availability of medical countermeasures, some researchers have begun a search for new interventions that could complement the planned antiviral‐ and vaccine‐based response to an influenza pandemic. One such countermeasure—the transfusion of pandemic influenza‐specific antibodies from surviving patients to the clinically ill—is the focus of this commentary. Passive immunotherapy, which includes the use of monoclonal antibodies (MoAbs), hyperimmune globulin, or convalescent plasma, had been used before the advent of antibiotics and has recently reentered the limelight due to the accelerating development of MoAb therapies against cancer, a number of microbes, allograft rejection, and a host of other conditions. After the plausible biologic mechanism and somewhat limited data supporting the efficacy for this modality against influenza are reviewed, safety and logistical concerns for utilization of this potential new product (fresh convalescent plasma against influenza [FCP‐Flu]) are discussed. FCP‐Flu could indeed prove useful in a response to a pandemic, but two necessary items must first be satisfied. Most importantly, more research should be conducted to establish FCP‐Flu efficacy against the current and other pandemic strains. Second, and also importantly, blood banks and donor centers should examine whether offering this new product would be feasible in a pandemic and begin planning before a more severe pandemic forces us to respond without adequate preparation.

Therapeutic targets for influenza – perspectives in drug development

Since new and dangerous influenza virus strains, such as H5N1 “avian flu” and more recently the swine-origin H1N1 “swine flu”, are constantly evolving, the need for effective anti-influenza drugs is pressing. It is becoming clear that the emergence of drug-resistant viruses will be a major potential problem in future efforts to control influenza virus infection. Moreover, development of vaccines against new influenza strains takes several months, and their production capacity is limited. Thus, new classes of anti-influenza drugs are highly sought after. This review focuses mainly on novel strategies, including targeting viral entry into host cells, inhibition of viral transcription and genome replication, and targeting of the NS1 influenza protein. Another approach involves viral RNA silencing by siRNAs or by antisense oligonucleotides. Inhibitors of viral neuraminidase have been the most successful approach in influenza virus breakdown to date. Viral maturation can also be blocked by inhibition of hemagglutinin-processing cellular proteinases. Compounds modifying the host cell immune response have also been reported. Design of specific compounds universally active against all viral variants with a reduced potential for the emergence of drug-resistant mutants is the main challenge in anti-influenza drug development, and the goals in this field are discussed here. A review with 140 references.

New class of monoclonal antibodies against severe influenza: prophylactic and therapeutic efficacy in ferrets

BACKGROUND

The urgent medical need for innovative approaches to control influenza is emphasized by the widespread resistance of circulating subtype H1N1 viruses to the leading antiviral drug oseltamivir, the pandemic threat posed by the occurrences of human infections with highly pathogenic avian H5N1 viruses, and indeed the evolving swine-origin H1N1 influenza pandemic. A recently discovered class of human monoclonal antibodies with the ability to neutralize a broad spectrum of influenza viruses (including H1, H2, H5, H6 and H9 subtypes) has the potential to prevent and treat influenza in humans. Here we report the latest efficacy data for a representative antibody of this novel class.

METHODOLOGY/PRINCIPAL FINDINGS

We evaluated the prophylactic and therapeutic efficacy of the human monoclonal antibody CR6261 against lethal challenge with the highly pathogenic avian H5N1 virus in ferrets, the optimal model of human influenza infection. Survival rates, clinically relevant disease signs such as changes in body weight and temperature, virus replication in lungs and upper respiratory tract, as well as macro- and microscopic pathology were investigated. Prophylactic administration of 30 and 10 mg/kg CR6261 prior to viral challenge completely prevented mortality, weight loss and reduced the amount of infectious virus in the lungs by more than 99.9%, abolished shedding of virus in pharyngeal secretions and largely prevented H5N1-induced lung pathology. When administered therapeutically 1 day after challenge, 30 mg/kg CR6261 prevented death in all animals and blunted disease, as evidenced by decreased weight loss and temperature rise, reduced lung viral loads and shedding, and less lung damage.

CONCLUSIONS/SIGNIFICANCE

These data demonstrate the prophylactic and therapeutic efficacy of this new class of human monoclonal antibodies in a highly stringent and clinically relevant animal model of influenza and justify clinical development of this approach as intervention for both seasonal and pandemic influenza.

Prophylactic and therapeutic efficacy of a chimeric monoclonal antibody specific for H5 haemagglutinin against lethal H5N1 influenza

BACKGROUND

Recent outbreaks of highly pathogenic H5N1 viruses in humans indicate that no endogenous protection exists in the general population. Vaccination programmes against this new pathogen require synthesis of endogenous antibodies and cannot provide any immediate protection in the event of a pandemic. Passive immunization with humanized neutralizing monoclonal antibodies can prove to be promising in preventing a catastrophic pandemic.

METHODS

A murine monoclonal antibody (mAb) 3B1 of immunoglobulin M isotype was switched to a chimeric immunoglobulin G1. BALB/c mice were used to study the protective efficacy of the chimeric mAbs against a lethal H5N1 virus challenge with strains from clades 1 and 2.1. Kinetics of the viral load were determined during the course of the treatment.

RESULTS

The chimeric mAb, in passive administration, was able to protect 100% of the mice when challenged with H5N1 strains from clades 1 or 2.1. Prophylaxis at 1 day prior to challenge and treatment at 1 day after challenge with this mAb resulted in the clearance of the virus from the lungs of the infected mice within 6 days post-viral challenge.

CONCLUSIONS

Passive immunotherapy using chimeric mAb 3B1 can be an effective tool in both the prophylaxis and treatment of highly pathogenic H5N1 infection, providing the immediate immunity needed to contain a future influenza pandemic.

Monoclonal antibodies isolated from human B cells neutralize a broad range of H1 subtype influenza A viruses including swine-origin Influenza virus (S-OIV)

The new H1N1 swine-origin influenza virus (S-OIV) strain is a global health problem. The elucidation of the virus-host relationship is crucial for the control of the new infection. Two human monoclonal antibody Fab fragments (HMab) neutralizing the novel H1N1 influenza strain at very low concentrations were cloned before the emergence of S-OIV from a patient who had a broad-range H1N1 serum neutralizing activity. The two HMabs neutralized all tested H1N1 strains, including S-OIV and a swine strain with IC(50) ranging from 2 to 7 microg/ml. Data demonstrate that infection with previously circulating H1N1 strains can elicit antibodies neutralizing S-OIV. Finally, the human genes coding for the neutralizing HMabs could be used for generating full human monoclonal IgGs that can be safely administered being potentially useful in the prophylaxis and the treatment of this human infection.

Research and development of universal influenza vaccines

The continuous threat of influenza pandemics determines the urgency and necessity to develop safe and effective vaccines against divergent influenza viruses. This review describes the advancements in the research and development of universal influenza vaccines based on the relatively conserved sequences of M2e, HA, and other proteins of influenza viruses.

Targets for the induction of protective immunity against influenza a viruses

The current pandemic caused by the new influenza A(H1N1) virus of swine origin and the current pandemic threat caused by the highly pathogenic avian influenza A viruses of the H5N1 subtype have renewed the interest in the development of vaccines that can induce broad protective immunity. Preferably, vaccines not only provide protection against the homologous strains, but also against heterologous strains, even of another subtype. Here we describe viral targets and the arms of the immune response involved in protection against influenza virus infections such as antibodies directed against the hemagglutinin, neuraminidase and the M2 protein and cellular immune responses directed against the internal viral proteins.

Influenza vaccines: the good, the bad, and the eggs

Outbreaks of influenza A viruses continue to cause morbidity and mortality worldwide. The global disease burden of influenza is substantial. While antiviral therapies are available, influenza vaccines are the mainstay of efforts to reduce the substantial health burden from seasonal influenza. Inactivated influenza vaccines have been available since the 1940s, with live attenuated, cold-adapted vaccines becoming available in the United States in 2003. In spite of the successes, more research is needed to develop more effective seasonal influenza vaccines that provide long-lasting immunity and broad protection against strains that differ antigenically from vaccine viruses. This review introduces the virus and its disease, the current state of seasonal and pandemic influenza vaccines, and the challenges we face in the future.

Naturally occurring human monoclonal antibodies neutralize both 1918 and 2009 pandemic influenza A (H1N1) viruses

The 2009 pandemic influenza A (H1N1) virus exhibits hemagglutinin protein sequence homology with the 1918 pandemic influenza virus. We found that human monoclonal antibodies recognized the Sa antigenic site on the head domains of both 1918 and 2009 hemagglutinins, a site that is hypervariable due to immune selection. These antibodies exhibited high potency against the 2009 virus in vitro, and one exerted a marked therapeutic effect in vivo.

Darwinian interventions: taming pathogens through evolutionary ecology

From the seemingly inevitable emergence of antibiotic resistance following drug use to the necessity of annual influenza vaccine updates, the ability of pathogens to evolve appears to preclude the development of effective, long-term interventions for many diseases. Despite this gloomy forecast, recent research explores how various principles and concepts from evolutionary ecology might inform practical attempts to bring these disease-causing agents under greater human control. By utilizing evolutionary and ecological information at various scales, these works investigate some promising avenues of disease intervention, from trapping pathogens in evolutionary dead ends, through slowing or inhibiting the process of pathogen adaptation, to more accurate forecasting of pathogen evolutionary trajectories. Thus, an evolutionary ecology perspective might ultimately provide powerful new tools in the pursuit of enduring solutions to the problem of treatment-driven pathogen evolution.

Structure, receptor binding, and antigenicity of influenza virus hemagglutinins from the 1957 H2N2 pandemic

The hemagglutinin (HA) envelope protein of influenza viruses mediates essential viral functions, including receptor binding and membrane fusion, and is the major viral antigen for antibody neutralization. The 1957 H2N2 subtype (Asian flu) was one of the three great influenza pandemics of the last century and caused 1 million deaths globally from 1957 to 1968. Three crystal structures of 1957 H2 HAs have been determined at 1.60 to 1.75 A resolutions to investigate the structural basis for their antigenicity and evolution from avian to human binding specificity that contributed to its introduction into the human population. These structures, which represent the highest resolutions yet recorded for a complete ectodomain of a glycosylated viral surface antigen, along with the results of glycan microarray binding analysis, suggest that a hydrophobicity switch at residue 226 and elongation of receptor-binding sites were both critical for avian H2 HA to acquire human receptor specificity. H2 influenza viruses continue to circulate in birds and pigs and, therefore, remain a substantial threat for transmission to humans. The H2 HA structure also reveals a highly conserved epitope that could be harnessed in the design of a broader and more universal influenza A virus vaccine.

Mechanism of glycan receptor recognition and specificity switch for avian, swine, and human adapted influenza virus hemagglutinins: a molecular dynamics perspective

Hemagglutinins (HA's) from duck, swine, and human influenza viruses have previously been shown to prefer avian and human glycan receptor analogues with distinct topological profiles, pentasaccharides LSTa (alpha-2,3 linkage) and LSTc (alpha-2,6 linkage), in comparative molecular dynamics studies. On the basis of detailed analyses of the dynamic motions of the receptor binding domains (RBDs) and interaction energy profiles with individual glycan residues, we have identified approximately 30 residue positions in the RBD that present distinct profiles with the receptor analogues. Glycan binding constrained the conformational space sampling by the HA. Electrostatic steering appeared to play a key role in glycan binding specificity. The complex dynamic behaviors of the major SSE and trimeric interfaces with or without bound glycans suggested that networks of interactions might account for species specificity in these low affinity and high avidity (multivalent) interactions between different HA and glycans. Contact frequency, energetic decomposition, and H-bond analyses revealed species-specific differences in HA-glycan interaction profiles, not readily discernible from crystal structures alone. Interaction energy profiles indicated that mutation events at the set of residues such as 145, 156, 158, and 222 would favor human or avian receptor analogues, often through interactions with distal asialo-residues. These results correlate well with existing experimental evidence, and suggest new opportunities for simulation-based vaccine and drug development.

Hemagglutinin-pseudotyped green fluorescent protein-expressing influenza viruses for the detection of influenza virus neutralizing antibodies

Influenza virus is a highly contagious virus that causes yearly epidemics and occasional pandemics of great consequence. Influenza virus neutralizing antibodies (NAbs) are promising prophylactic and therapeutic reagents. Detection of NAbs in serum samples is critical to evaluate the prevalence and spread of new virus strains. Here we describe the development of a simple, sensitive, specific, and safe screening assay for the rapid detection of NAbs against highly pathogenic influenza viruses under biosafety level 2 (BSL-2) conditions. This assay is based on the use of influenza viruses in which the hemagglutinin (HA) gene is replaced by a gene expressing green fluorescent protein (GFP). These GFP-expressing influenza viruses replicate to high titers in HA-expressing cell lines, but in non-HA-expressing cells, their replication is restricted to a single cycle.

Heterosubtype neutralizing responses to influenza A (H5N1) viruses are mediated by antibodies to virus haemagglutinin

Background

It is increasingly clear that influenza A infection induces cross-subtype neutralizing antibodies that may potentially confer protection against zoonotic infections. It is unclear whether this is mediated by antibodies to the neuraminidase (NA) or haemagglutinin (HA). We use pseudoviral particles (H5pp) coated with H5 haemagglutinin but not N1 neuraminidase to address this question. In this study, we investigate whether cross-neutralizing antibodies in persons unexposed to H5N1 is reactive to the H5 haemagglutinin.

Methodology/Principal Findings

We measured H5-neutralization antibody titers pre- and post-vaccination using the H5N1 micro-neutralization test (MN) and H5pp tests in subjects given seasonal vaccines and in selected sera from European elderly volunteers in a H5N1 vaccine trial who had detectable pre-vaccination H5N1 MN antibody titers. We found detectable (titer ≥20) H5N1 neutralizing antibodies in a minority of pre-seasonal vaccine sera and evidence of a serological response to H5N1 in others after seasonal influenza vaccination. There was excellent correlation in the antibody titers between the H5N1 MN and H5pp tests. Similar correlations were found between MN and H5pp in the pre-vaccine sera from the cohort of H5N1 vaccine trial recipients.

Conclusions/Significance

Heterosubtype neutralizing antibody to H5N1 in healthy volunteers unexposed to H5N1 is mediated by cross-reaction to the H5 haemagglutinin.

Recombination, reservoirs, and the modular spike: mechanisms of coronavirus cross-species transmission

Over the past 30 years, several cross-species transmission events, as well as changes in virus tropism, have mediated significant animal and human diseases. Most notable is severe acute respiratory syndrome (SARS), a lower respiratory tract disease of humans that was first reported in late 2002 in Guangdong Province, China. The disease, which quickly spread worldwide over a period of 4 months spanning late 2002 and early 2003, infected over 8,000 individuals and killed nearly 800 before it was successfully contained by aggressive public health intervention strategies. A coronavirus (SARS-CoV) was identified as the etiological agent of SARS, and initial assessments determined that the virus crossed to human hosts from zoonotic reservoirs, including bats, Himalayan palm civets (Paguma larvata), and raccoon dogs (Nyctereutes procyonoides), sold in exotic animal markets in Guangdong Province. In this review, we discuss the molecular mechanisms that govern coronavirus cross-species transmission both in vitro and in vivo, using the emergence of SARS-CoV as a model. We pay particular attention to how changes in the Spike attachment protein, both within and outside of the receptor binding domain, mediate the emergence of coronaviruses in new host populations.

Ebolavirus glycoprotein structure and mechanism of entry

Ebolavirus (EBOV) is a highly virulent pathogen capable of causing a severe hemorrhagic fever with 50-90% lethality. The EBOV glycoprotein (GP) is the only virally expressed protein on the virion surface and is critical for attachment to host cells and catalysis of membrane fusion. Hence, the EBOV GP is a critical component of vaccines as well as a target of neutralizing antibodies and inhibitors of attachment and fusion. The crystal structure of the Zaire ebolavirus GP in its trimeric, prefusion conformation (3 GP(1) plus 3 GP(2)) in complex with a neutralizing antibody fragment, derived from a human survivor of the 1995 Kikwit outbreak, was recently determined. This is the first near-complete structure of any filovirus glycoprotein. The overall molecular architecture of the Zaire ebolavirus GP and its role in viral entry and membrane fusion are discussed in this article.

One step closer to universal influenza epitopes

Evaluation of: Sui J, Hwang WC, Perez S et al. Structural and functional bases for broad-spectrum neutralization of avian and human influenza A viruses. Nat. Struct. Mol. Biol. 16(3), 265-273 (2009). The continuous antigenic drifts and occasional antigenic shifts enable human influenza viruses to escape the human immune system. Moreover, the frequent occurrence of human H5N1-infected cases and the recent emergency of a novel swine-like human H1N1 influenza virus further reiterate the risk of the introduction of a new pandemic strain to humans through in toto transfer of an animal influenza virus. The discovery of neutralizing antibodies that are broadly reactive with multiple influenza subtypes is therefore extremely important for the influenza pandemic preparedness, for use either for therapeutic purposes or as the basis of vaccine development. Here, we review some of the recent developments in this area.

The role of genomics in tracking the evolution of influenza A virus

Influenza A virus causes annual epidemics and occasional pandemics of short-term respiratory infections associated with considerable morbidity and mortality. The pandemics occur when new human-transmissible viruses that have the major surface protein of influenza A viruses from other host species are introduced into the human population. Between such rare events, the evolution of influenza is shaped by antigenic drift: the accumulation of mutations that result in changes in exposed regions of the viral surface proteins. Antigenic drift makes the virus less susceptible to immediate neutralization by the immune system in individuals who have had a previous influenza infection or vaccination. A biannual reevaluation of the vaccine composition is essential to maintain its effectiveness due to this immune escape. The study of influenza genomes is key to this endeavor, increasing our understanding of antigenic drift and enhancing the accuracy of vaccine strain selection. Recent large-scale genome sequencing and antigenic typing has considerably improved our understanding of influenza evolution: epidemics around the globe are seeded from a reservoir in East-Southeast Asia with year-round prevalence of influenza viruses; antigenically similar strains predominate in epidemics worldwide for several years before being replaced by a new antigenic cluster of strains. Future in-depth studies of the influenza reservoir, along with large-scale data mining of genomic resources and the integration of epidemiological, genomic, and antigenic data, should enhance our understanding of antigenic drift and improve the detection and control of antigenically novel emerging strains.

Glycans on influenza hemagglutinin affect receptor binding and immune response

Recent cases of avian influenza H5N1 and the swine-origin 2009 H1N1 have caused a great concern that a global disaster like the 1918 influenza pandemic may occur again. Viral transmission begins with a critical interaction between hemagglutinin (HA) glycoprotein, which is on the viral coat of influenza, and sialic acid (SA) containing glycans, which are on the host cell surface. To elucidate the role of HA glycosylation in this important interaction, various defined HA glycoforms were prepared, and their binding affinity and specificity were studied by using a synthetic SA microarray. Truncation of the N-glycan structures on HA increased SA binding affinities while decreasing specificity toward disparate SA ligands. The contribution of each monosaccharide and sulfate group within SA ligand structures to HA binding energy was quantitatively dissected. It was found that the sulfate group adds nearly 100-fold (2.04 kcal/mol) in binding energy to fully glycosylated HA, and so does the biantennary glycan to the monoglycosylated HA glycoform. Antibodies raised against HA protein bearing only a single N-linked GlcNAc at each glycosylation site showed better binding affinity and neutralization activity against influenza subtypes than the fully glycosylated HAs elicited. Thus, removal of structurally nonessential glycans on viral surface glycoproteins may be a very effective and general approach for vaccine design against influenza and other human viruses.

Vaccines: the fourth century

Vaccine development, which began with Edward Jenner's observations in the late 18th century, has entered its 4th century. From its beginnings, with the use of whole organisms that had been weakened or inactivated, to the modern-day use of genetic engineering, it has taken advantage of the tools discovered in other branches of microbiology. Numerous successful vaccines are in use, but the list of diseases for which vaccines do not exist is long. However, the multiplicity of strategies now available, discussed in this article, portends even more successful development of vaccines.

Measurement of neutralizing antibody responses against H5N1 clades in immunized mice and ferrets using pseudotypes expressing influenza hemagglutinin and neuraminidase

Neutralizing antibody is associated with the prevention and clearance of influenza virus infection. Microneutralization (MN) and hemagglutination inhibition (HI) assays are currently used to evaluate neutralizing antibody responses against human and avian influenza viruses, including H5N1. The MN assay is somewhat labor intensive, while HI is a surrogate for neutralization. Moreover, use of replication competent viruses in these assays requires biosafety level 3 (BSL-3) containment. Therefore, a neutralization assay that does not require BSL-3 facilities would be advantageous. Toward this goal, we generated a panel of pseudotypes expressing influenza hemagglutinin (HA) and neuraminidase (NA) and developed a pseudotype-based neutralization (PN) assay. Here we demonstrate that HA/NA pseudotypes mimic release and entry of influenza virus and that the PN assay exhibits good specificity and reveals quantitative difference in neutralizing antibody titers against different H5N1 clades and subclades. Using immune ferret sera, we demonstrated excellent correlation between the PN, MN, and HI assays. Thus, we conclude that the PN assay is a sensitive and quantifiable method to measure neutralizing antibodies against diverse clades and subclades of H5N1 influenza virus.

Characterization of structurally defined epitopes recognized by monoclonal antibodies produced by chronic lymphocytic leukemia B cells

Despite a wealth of information about the structure of surface membrane immunoglobulin (smIg) on chronic lymphocytic leukemia (CLL) cells, little is known about epitopes reacting with their binding sites. Probing phage-displayed peptide libraries, we identified and characterized mimetopes for Igs of 4 patients with IGHV mutated CLL (M-CLL) and 4 with IGHV unmutated CLL (U-CLL). Six of these mAbs were representatives of stereotyped B-cell receptors characteristic of CLL. We found that mimetic epitopes for U- and M-CLL Igs differed significantly. M-CLL-derived peptides exhibited better amino acid motifs, were more similar to each other, aligned more easily, and formed tighter clusters than U-CLL-derived peptides. Mono-, oligo-, and polyreactivity of peptides correlated with structural changes within antigen-binding sites of selecting M-CLL mAbs. Although M-CLL-isolated peptides and certain U-CLL mAbs bound more effectively to the selecting mAb, others were not as specific, reacting with M-CLL and U-CLL mAbs; these data suggest that in vivo structurally diverse epitopes could bind smIgs of distinct CLL clones, thereby altering survival and growth. Finally, an M-CLL-derived peptide inhibited, in a dose-dependent manner, binding of its homologous mAb to human B lymphocytes; therefore peptides that inhibit or alter the consequences of antigen-smIg interactions may represent therapeutic modalities in CLL.

Preparation, characterization, and immunogenicity in mice of a recombinant influenza H5 hemagglutinin vaccine against the avian H5N1 A/Vietnam/1203/2004 influenza virus

Production of influenza vaccines requires a minimum of 6 months after the circulating strain is isolated and the use of infectious viruses. The hemagglutinin (protective antigen) of circulating influenza viruses mutates rapidly requiring reformulation of the vaccines. Our goal is to eliminate the risk of working with infectious virus and reduce significantly the production time. A cDNA fragment encoding the influenza virus A/Vietnam/1203/2004 (H5N1) HA gene was prepared using RT-PCR with viral RNA as a template. Recombinant HA (rHA) protein was produced in Escherichia coli and purified from isolated inclusion bodies by urea solubilization and Ni(+)-ion column chromatography. Vaccine candidates were prepared by treating the rHA with formalin, adsorption onto alum or with both. Mice were injected subcutaneously with candidate vaccines two or three times 2 weeks apart. Sera were collected 1 week after the last injection and antibody measured by ELISA and hemagglutination inhibition (HI). The highest antibody response (GM 449EU) was elicited by three injections of 15microg alum-adsorbed rHA. Dosages of 5microg of rHA formulated with formalin and alum, and 5microg alum-adsorbed rHA elicited IgG anti-HA of GM 212 and 177EU, respectively. HI titers, >or=40 were obtained in >or=80% of mice with three doses of all formulations. We developed a method to produce rHA in a time-frame suitable for annual and pandemic influenza vaccination. Using this method, rHA vaccine can be produced in 3-4 weeks and when formulated with alum, induces HA antibody levels in young outbred mice consistent with the FDA guidelines for vaccines against epidemic and pandemic influenza.

Broad neutralizing human monoclonal antibodies against influenza virus from vaccinated healthy donors

Human monoclonal antibodies (HuMAbs) prepared from patients with viral infections could provide information on human epitopes important for the development of vaccines as well as potential therapeutic applications. Through the fusion of peripheral blood mononuclear cells from a total of five influenza-vaccinated volunteers, with newly developed murine-human chimera fusion partner cells, named SPYMEG, we obtained 10 hybridoma clones stably producing anti-influenza virus antibodies: one for influenza A H1N1, four for influenza A H3N2 and five for influenza B. Surprisingly, most of the HuMAbs showed broad reactivity within subtype and four (two for H3N2 and two for B) showed broad neutralizing ability. Importantly, epitope mapping revealed that the two broad neutralizing antibodies to H3N2 derived from different donors recognized the same epitope located underneath the receptor-binding site of the hemagglutinin globular region that is highly conserved among H3N2 strains.

Emergence and pandemic potential of swine-origin H1N1 influenza virus

Influenza viruses cause annual epidemics and occasional pandemics that have claimed the lives of millions. The emergence of new strains will continue to pose challenges to public health and the scientific communities. A prime example is the recent emergence of swine-origin H1N1 viruses that have transmitted to and spread among humans, resulting in outbreaks internationally. Efforts to control these outbreaks and real-time monitoring of the evolution of this virus should provide us with invaluable information to direct infectious disease control programmes and to improve understanding of the factors that determine viral pathogenicity and/or transmissibility.

Human B cell memory

Following an immune response two types of differentiated B cells persist in the memory pool: plasma cells, which confer immediate protection by the secretion of specific antibodies; and memory B cells, which confer rapid and enhanced response to secondary challenge. We will review recent advances in understanding the heterogeneity, dynamics, and persistence of human memory B cells and plasma cells as well as new methods to isolate human monoclonal antibodies. These findings offer new insights into the human B cell response, which are relevant for vaccination and therapeutic intervention.

Synthetic peptide vaccine targeting a cryptic neutralizing epitope in domain 2 of Bacillus anthracis protective antigen

Current evidence suggests that protective antigen (PA)-based anthrax vaccines may elicit a narrow neutralizing antibody repertoire, and this may represent a vulnerability with PA-based vaccines. In an effort to identify neutralizing specificities which may complement those prevalent in PA antiserum, we evaluated whether sequences within the 2beta2-2beta3 loop of PA, which are apparent in the crystal structure of heptameric but not monomeric PA, might represent a target for an epitope-specific vaccine for anthrax and, further, whether antibodies to these sequences are induced in rabbits immunized with monomeric PA. We evaluated the immunogenicity in rabbits of a multiple antigenic peptide (MAP) displaying copies of amino acids (aa) 305 to 319 of this region. Overall, four out of six rabbits vaccinated with the MAP peptide in Freund's adjuvant developed high-titer, high-avidity antibody responses which cross-reacted with the immobilized peptide sequence comprising aa 305 to 319 and with PA, as determined by an enzyme-linked immunosorbent assay, and which displayed potent and durable neutralization of lethal toxin (LeTx) in vitro, with peak titers which were 452%, 100%, 67%, and 41% of the peak neutralization titers observed in positive-control rabbits immunized with PA. Importantly, analysis of sera from multiple cohorts of rabbits with high-titer immunity to PA demonstrated a virtual absence of this potent antibody specificity, and work by others suggests that this specificity may be present at only low levels in primate PA antiserum. These results highlight the potential importance of this immunologically cryptic neutralizing epitope from PA as a target for alternative and adjunctive vaccines for anthrax.