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

Induction of unnatural immunity: prospects for a broadly protective universal influenza vaccine

The immune system normally responds to influenza virus by making neutralizing antibodies to regions of the viral spike, the hemagglutinin, that vary year to year. This natural response protects against circulating subtypes but necessitates production of new vaccines annually. Newer vaccine approaches have succeeded in eliciting broadly neutralizing antibodies to highly conserved yet vulnerable regions of the hemagglutinin and suggest potential pathways for the development of universal influenza vaccines.

Computational design of epitope-scaffolds allows induction of antibodies specific for a poorly immunogenic HIV vaccine epitope

Broadly cross-reactive monoclonal antibodies define epitopes for vaccine development against HIV and other highly mutable viruses. Crystal structures are available for several such antibody-epitope complexes, but methods are needed to translate that structural information into immunogens that re-elicit similar antibodies. We describe a general computational method to design epitope-scaffolds in which contiguous structural epitopes are transplanted to scaffold proteins for conformational stabilization and immune presentation. Epitope-scaffolds designed for the poorly immunogenic but conserved HIV epitope 4E10 exhibited high epitope structural mimicry, bound with higher affinities to monoclonal antibody (mAb) 4E10 than the cognate peptide, and inhibited HIV neutralization by HIV+ sera. Rabbit immunization with an epitope-scaffold induced antibodies with structural specificity highly similar to mAb 4E10, an important advance toward elicitation of neutralizing activity. The results demonstrate that computationally designed epitope-scaffolds are valuable as structure-specific serological reagents and as immunogens to elicit antibodies with predetermined structural specificity.

Properties and therapeutic efficacy of broadly reactive chimeric and humanized H5-specific monoclonal antibodies against H5N1 influenza viruses

Highly pathogenic H5N1 virus infection causes severe disease and a high rate of fatality in humans. Development of humanized monoclonal antibodies may provide an efficient therapeutic regime for H5N1 virus infection. In the present study, broadly cross-reactive monoclonal antibodies (MAbs) derived from mice were humanized to minimize immunogenicity. One chimeric antibody (cAb) and seven humanized antibodies (hAbs) were constructed. These antibodies retained broad-spectrum reactivity to H5N1 viruses, binding to recombinant H5-subtype HA1 molecules expressed in CHO cells in a dose-dependent manner and exhibiting similar reactivities against antigenically distinct H5N1 viruses in hemagglutination inhibition (HI) assays. One humanized antibody, 37 hAb, showed HI and neutralization activities comparable to that of the parental murine antibody, 13D4 MAb, while the other six antibodies were less reactive to H5N1 viruses. Analysis of amino acid sequences in the variable region frameworks of the seven humanized antibodies found that Q5 and Y27 in the VH region are highly conserved murine residues. Comparison of the three-dimensional structures derived from the variable regions of MAbs 37 hAb, H1202-34, and 13D4 revealed that residue substitutions at sites 70 and 46 may be the major cause for the observed differences in binding affinity. Examination of the chimeric antibody and one of the humanized antibodies, 37 hAb, showed that both antibodies offered postinfection protection against lethal challenge with antigenically diverse H5N1 viruses in the mouse model. Chimeric and humanized antibodies which retain the broadly reactive and protective properties of murine H5-specific monoclonal antibodies have great potential for use in the treatment of human H5N1 infection.

Structural basis of influenza virus neutralization

Although seasonal influenza vaccines play a valuable role in reducing the spread of virus at the population level, ongoing viral evolution to evade immune responses remains problematic. No current vaccines elicit enduring protection in the face of emerging and re-emerging influenza viruses that are rapidly undergoing antigenic drift. Eliciting broadly cross-neutralizing antibody (nAb) responses against influenza virus is a crucial goal for seasonal and pandemic influenza vaccine preparation. Recent three-dimensional structure information obtained from crystallization of influenza antigens in complex with nAbs has provided a framework for interpreting antibody-based viral neutralization that should aid in the design of vaccine immunogens. Here, we will review current knowledge of the structure-based mechanisms contributing to the neutralization and neutralization escape of influenza viruses. We will also explore the potential for this structure-based approach to overcome the obstacles in developing the highly desired "universal" influenza vaccine.

Broadly cross-reactive antibodies dominate the human B cell response against 2009 pandemic H1N1 influenza virus infection

Although scarce after annual influenza vaccination, B cells producing antibodies capable of neutralizing multiple influenza strains are abundant in humans infected with pandemic 2009 H1N1 influenza.

The 2009 pandemic H1N1 influenza pandemic demonstrated the global health threat of reassortant influenza strains. Herein, we report a detailed analysis of plasmablast and monoclonal antibody responses induced by pandemic H1N1 infection in humans. Unlike antibodies elicited by annual influenza vaccinations, most neutralizing antibodies induced by pandemic H1N1 infection were broadly cross-reactive against epitopes in the hemagglutinin (HA) stalk and head domain of multiple influenza strains. The antibodies were from cells that had undergone extensive affinity maturation. Based on these observations, we postulate that the plasmablasts producing these broadly neutralizing antibodies were predominantly derived from activated memory B cells specific for epitopes conserved in several influenza strains. Consequently, most neutralizing antibodies were broadly reactive against divergent H1N1 and H5N1 influenza strains. This suggests that a pan-influenza vaccine may be possible, given the right immunogen. Antibodies generated potently protected and rescued mice from lethal challenge with pandemic H1N1 or antigenically distinct influenza strains, making them excellent therapeutic candidates.

Pandemic influenza: implications for preparation and delivery of critical care services

In a 5-week span during the 1918 influenza A pandemic, more than 2000 patients were admitted to Cook County Hospital in Chicago, with a diagnosis of either influenza or pneumonia; 642 patients, approximately 31% of those admitted, died, with deaths occurring predominantly in patients of age 25 to 30 years. This review summarizes basic information on the biology, epidemiology, control, treatment and prevention of influenza overall, and then addresses the potential impact of pandemic influenza in an intensive care unit setting. Issues that require consideration include workforce staffing and safety, resource management, alternate sites of care surge of patients, altered standards of care, and crisis communication.

Computational docking of antibody-antigen complexes, opportunities and pitfalls illustrated by influenza hemagglutinin

Antibodies play an increasingly important role in both basic research and the pharmaceutical industry. Since their efficiency depends, in ultimate analysis, on their atomic interactions with an antigen, studying such interactions is important to understand how they function and, in the long run, to design new molecules with desired properties. Computational docking, the process of predicting the conformation of a complex from its separated components, is emerging as a fast and affordable technique for the structural characterization of antibody-antigen complexes. In this manuscript, we first describe the different computational strategies for the modeling of antibodies and docking of their complexes, and then predict the binding of two antibodies to the stalk region of influenza hemagglutinin, an important pharmaceutical target. The purpose is two-fold: on a general note, we want to illustrate the advantages and pitfalls of computational docking with a practical example, using different approaches and comparing the results to known experimental structures. On a more specific note, we want to assess if docking can be successful in characterizing the binding to the same influenza epitope of other antibodies with unknown structure, which has practical relevance for pharmaceutical and biological research. The paper clearly shows that some of the computational docking predictions can be very accurate, but the algorithm often fails to discriminate them from inaccurate solutions. It is of paramount importance, therefore, to use rapidly obtained experimental data to validate the computational results.

Cellular origin(s) of chronic lymphocytic leukemia: cautionary notes and additional considerations and possibilities

Several cell types have been suggested as giving rise to chronic lymphocytic leukemia (CLL), and these suggestions have reflected the sophistication of technology available at the time. Although there is no consensus as to the normal cellular counterpart(s) in the disease, an antigen-experienced B lymphocyte appears required based on surface membrane phenotypes and gene expression profiles. However, what is still unclear is whether a single or multiple normal precursors were stimulated to evolve into CLL and at what stage(s) this occurred. A unifying, parsimonious theory is that CLL clones with either mutated or unmutated IGHVs derive from marginal zone B cells. However, evidence for remarkably similar B-cell receptor amino acid sequence and striking differences in polyantigen and autoantigen-binding activity, found in some but not all CLL clones, challenge a single-cell derivation for CLL. In this Perspective, we summarize data regarding normal counterparts of CLL cells and suggest that a multistep process of leukemogenesis is important to consider when assigning a cellular origin for this disease. Finally, although available data do not definitively identify the cell(s) of origin, we offer possibilities for single- and multiple-cell origin models as straw men that can be improved on and hopefully lead to final answers to this puzzle.

Conserved epitopes of influenza A virus inducing protective immunity and their prospects for universal vaccine development

Influenza A viruses belong to the best studied viruses, however no effective prevention against influenza infection has been developed. The emerging of still new escape variants of influenza A viruses causing epidemics and periodic worldwide pandemics represents a threat for human population. Therefore, current, hot task of influenza virus research is to look for a way how to get us closer to a universal vaccine. Combination of chosen conserved antigens inducing cross-protective antibody response with epitopes activating also cross-protective cytotoxic T-cells would offer an attractive strategy for improving protection against drift variants of seasonal influenza viruses and reduces the impact of future pandemic strains. Antigenically conserved fusion-active subunit of hemagglutinin (HA2 gp) and ectodomain of matrix protein 2 (eM2) are promising candidates for preparation of broadly protective HA2- or eM2-based vaccine that may aid in pandemic preparedness. Overall protective effect could be achieved by contribution of epitopes recognized by cytotoxic T-lymphocytes (CTL) that have been studied extensively to reach much broader control of influenza infection. In this review we present the state-of-art in this field. We describe known adaptive immune mechanisms mediated by influenza specific B- and T-cells involved in the anti-influenza immune defense together with the contribution of innate immunity. We discuss the mechanisms of neutralization of influenza infection mediated by antibodies, the role of CTL in viral elimination and new approaches to develop epitope based vaccine inducing cross-protective influenza virus-specific immune response.

Highly conserved cross-reactive CD4+ T-cell HA-epitopes of seasonal and the 2009 pandemic influenza viruses

Please cite this paper as: Duvvuri et al. (2010) Highly conserved cross‐reactive CD4+ T‐cell HA‐epitopes of seasonal and the 2009 pandemic influenza viruses. Influenza and Other Respiratory Viruses 4(5), 249–258.

Background The relatively mild nature of the 2009 influenza pandemic (nH1N1) highlights the overriding importance of pre‐existing immune memory. The absence of cross‐reactive antibodies to nH1N1 in most individuals suggests that such attenuation may be attributed to pre‐existing cellular immune responses to epitopes shared between nH1N1 virus and previously circulating strains of inter‐pandemic influenza A viruses.

Results We sought to identify potential CD4+ T cell epitopes and predict the level of cross‐reactivity of responding T cells. By performing large‐scale major histocompatibility complex II analyses on Hemagglutinin (HA) proteins, we investigated the degree of T‐cell cross‐reactivity between seasonal influenza A (sH1N1, H3N2) from 1968 to 2009 and nH1N1 strains. Each epitope was examined against all the protein sequences that correspond to sH1N1, H3N2, and nH1N1. T‐cell cross‐reactivity was estimated to be 52%, and maximum conservancy was found between sH1N1 and nH1N1 with a significant correlation (P < 0·05).

Conclusions  Given the importance of cellular responses in kinetics of influenza infection in humans, our findings underscore the role of T‐cell assays for understanding the inter‐pandemic variability in severity and for planning treatment methods for emerging influenza viruses.

Development of adenoviral vector-based mucosal vaccine against influenza

The recent pandemic threat of the influenza virus makes the increased safety and efficiency of vaccination against the pathogen a most important issue. It has been well established that for maximum protective effect, the vaccination should mimic natural infection. Therefore, recent efforts to develop a new influenza vaccine have focused on intranasal immunization strategies. Intranasal immunization is capable of inducing secretory IgA and serum IgG responses to provide a double defense against mucosal pathogens. On the other hand, it is desirable that a live pathogen is not present in the vaccine. In addition, for optimal induction of the immune responses via the nasal route, efficient and safe mucosal adjuvants are also required. This is possible to attain using an adenoviral vector for vaccine development. Adenoviral vectors are capable of delivering and protecting the antigen encoding sequence. They also possess a natural mechanism for penetrating into the nasal mucous membrane and are capable of activating the innate immune response. This review describes the basic prerequisites for the involvement of recombinant adenoviruses for mucosal (nasal) vaccine development against the influenza virus.

Universal antibodies against the highly conserved influenza fusion peptide cross-neutralize several subtypes of influenza A virus

The fusion peptide of influenza viral hemagglutinin plays a critical role in virus entry by facilitating membrane fusion between the virus and target cells. As the fusion peptide is the only universally conserved epitope in all influenza A and B viruses, it could be an attractive target for vaccine-induced immune responses. We previously reported that antibodies targeting the first 14 amino acids of the N-terminus of the fusion peptide could bind to virtually all influenza virus strains and quantify hemagglutinins in vaccines produced in embryonated eggs. Here we demonstrate that these universal antibodies bind to the viral hemagglutinins in native conformation presented in infected mammalian cell cultures and neutralize multiple subtypes of virus by inhibiting the pH-dependant fusion of viral and cellular membranes. These results suggest that this unique, highly-conserved linear sequence in viral hemagglutinin is exposed sufficiently to be attacked by the antibodies during the course of infection and merits further investigation because of potential importance in the protection against diverse strains of influenza viruses.

Localization of epitopes recognized by monoclonal antibodies that neutralized the H3N2 influenza viruses in man

Through extensive isolation of neutralizing mAbs against H3N2 influenza viruses representing the in vivo repertoire in a human donor, we examined the relationships between antigenic drift of influenza virus and protective antibodies generated in an infected individual. The majority of mAbs isolated from a donor born in 1960 were divided into three major groups with distinct strain specificity: 1968-1973, 1977-1993 and 1997-2003. In the present study, we developed a new method that allowed us to comprehensively determine the location of epitopes recognized by many mAbs. Original haemagglutinins (HAs) of several strains and chimaeric variants, in which one of the seven sites (A, B1, B2, C1, C2, D or E) was replaced by some other strain-derived sequence, were artificially expressed on the cell surface. The binding activity of mAbs to the HAs was examined by flow cytometry. By using this method, we determined the location of epitopes recognized by 98 different mAbs. Clones that neutralize the 1968-1973 strains bind to site B2/D, A or A/B1. While sites C, E and B were recognized by clones that neutralized the 1977-1993 strains, the majority of these clones bind to site C. Clones that neutralize the 1997-2003 strains bind to site B, A/B1, A/B2 or E/C2.

Label-free, arrayed sensing of immune response to influenza antigens

Periodic outbreaks of pandemic influenza have been a devastating cause of human mortality over the past century. More recently, an avian influenza strain, designated H5N1, has been identified as having the potential to cause a zoogenic pandemic in humans, and a current outbreak of a new H1N1 influenza variant hypothesized to be of swine origin is of considerable concern. In order to facilitate surveillance and the rapid assessment and comparison of vaccination efforts, a high-throughput assay is highly desirable to supplement standard methods, which require high biosafety-level facilities. In this paper, we describe the design, production, and preliminary evaluation of an antigen array incorporating a panel of hemagglutinins as a platform for the detection and rapid quantification of influenza-specific antibodies in human serum by Arrayed Imaging Reflectometry (AIR), a label-free optical biosensor.

Safety and immunogenicity of influenza A H1N1 vaccines

Evaluation of: Liang XF, Wang HQ, Wang JZ et al. Safety and immunogenicity of 2009 pandemic influenza A H1N1 vaccines in China: a multicentre, double-blind, randomised, placebo-controlled trial. Lancet 375(9708), 56-66 (2009). The novel swine-origin influenza A H1N1 2009 virus was first identified in April 2009 in Mexico. Since then, it has caused a worldwide pandemic with more than 10,000 deaths documented by December 2009. While many countries adopted pandemic plans and border controls to try to control the spread of the virus, it was quickly realized that the virus was unstoppable. The virus was initially largely susceptible to oseltamivir and zanamivir, and various strategies that included treatment of severe cases, all cases or targeted chemoprophylaxis were used to try to limit the mortality and morbidity due to the virus. However, predictably, resistance to neuraminidase inhibitors has increasingly appeared in different settings and outbreaks of resistant virus have been reported. Mass vaccination is the most economic and effective measure to reduce infection and mortality from influenza. Before this pandemic, several studies were conducted using H5N1 prepandemic vaccines to try to determine the optimal composition and dose of vaccine for a novel strain of influenza. Most of these studies found significantly higher doses required for novel influenza strains and all previous studies showed the need for two doses of vaccine to achieve protective levels of antibody. Once it became apparent that the pandemic was due to H1N1 2009 instead, efforts were redirected to finding the optimal antigen dose and composition including the use of adjuvant combinations. This unique study conducted in ten centers across the People's Republic of China found that a single-dose, 7.5-microg, nonadjuvanted, split-virion vaccine was sufficiently immunogenic for adults and adolescents aged 12 years and above to meet regulatory requirements for registration. The vaccine was relatively well tolerated with serious adverse reactions--mainly high-grade fever in only 0.6% of vaccine recipients. This large, unique study conducted in a developing country using vaccine manufactured in that developing country shows a high degree of seroprotection and has had a major impact on pandemic vaccination strategies worldwide.

Pandemic influenza 1918 H1N1 and 1968 H3N2 DNA vaccines induce cross-reactive immunity in ferrets against infection with viruses drifted for decades

Please cite this paper as: Bragstad et al. (2010) Pandemic influenza 1918 H1N1 and 1968 H3N2 DNA vaccines induce cross‐reactive immunity in ferrets against infection with viruses drifted for decades. Influenza and Other Respiratory Viruses 5(1), 13–23.

Background  Alternative influenza vaccines and vaccine production forms are needed as the conventional protein vaccines do not induce broad cross‐reactivity against drifted strains. Furthermore, fast vaccine production is especially important in a pandemic situation, and broader vaccine reactivity would diminish the need for frequent change in the vaccine formulations.

Objective  In this study, we compared the ability of pandemic influenza DNA vaccines to induce immunity against distantly related strains within a subtype with the immunity induced by conventional trivalent protein vaccines against homologous virus challenge.

Methods  Ferrets were immunised by particle‐mediated epidermal delivery (gene gun) with DNA vaccines based on the haemagglutinin (HA) and neuraminidase (NA) and/or the matrix (M) and nucleoprotein genes of the 1918 H1N1 Spanish influenza pandemic virus or the 1968 H3N2 Hong Kong influenza pandemic virus. The animals were challenged with contemporary H1N1 or H3N2 viruses.

Results  We demonstrated that DNA vaccines encoding proteins of the original 1918 H1N1 pandemic virus induced protective cross‐reactive immune responses in ferrets against infection with a 1947 H1N1 virus and a recent 1999 H1N1 virus. Similarly, a DNA vaccine, based on the HA and NA of the 1968 H3N2 pandemic virus, induced cross‐reactive immune responses against a recent 2005 H3N2 virus challenge.

Conclusions  DNA vaccines based on pandemic or recent seasonal influenza genes induced cross‐reactive immunity against contemporary virus challenge as good as or superior to contemporary conventional trivalent protein vaccines. This suggests a unique ability of influenza DNA to induce cross‐protective immunity against both contemporary and long‐time drifted viruses.

Vaccination with a synthetic peptide from the influenza virus hemagglutinin provides protection against distinct viral subtypes

Current influenza virus vaccines protect mostly against homologous virus strains; thus, regular immunization with updated vaccine formulations is necessary to guard against the virus' hallmark remodeling of regions that mediate neutralization. Development of a broadly protective influenza vaccine would mark a significant advance in human infectious diseases research. Antibodies with broad neutralizing activity (nAbs) against multiple influenza virus strains or subtypes have been reported to bind the stalk of the viral hemagglutinin, suggesting that a vaccine based on this region could elicit a broadly protective immune response. Here we describe a hemagglutinin subunit 2 protein (HA2)-based synthetic peptide vaccine that provides protection in mice against influenza viruses of the structurally divergent subtypes H3N2, H1N1, and H5N1. The immunogen is based on the binding site of the recently described nAb 12D1, which neutralizes H3 subtype viruses, demonstrates protective activity in vivo, and, in contrast to a majority of described nAbs, appears to bind to residues within a single α-helical portion of the HA2 protein. Our data further demonstrate that the specific design of our immunogen is integral in the induction of broadly active anti-hemagglutinin antibodies. These results provide proof of concept for an HA2-based influenza vaccine that could diminish the threat of pandemic influenza disease and generally reduce the significance of influenza viruses as human pathogens.

Cationic lipid/DNA complex-adjuvanted influenza A virus vaccination induces robust cross-protective immunity

Influenza A virus is a negative-strand segmented RNA virus in which antigenically distinct viral subtypes are defined by the hemagglutinin (HA) and neuraminidase (NA) major viral surface proteins. An ideal inactivated vaccine for influenza A virus would induce not only highly robust strain-specific humoral and T-cell immune responses but also cross-protective immunity in which an immune response to antigens from a particular viral subtype (e.g., H3N2) would protect against other viral subtypes (e.g., H1N1). Cross-protective immunity would help limit outbreaks from newly emerging antigenically novel strains. Here, we show in mice that the addition of cationic lipid/noncoding DNA complexes (CLDC) as adjuvant to whole inactivated influenza A virus vaccine induces significantly more robust adaptive immune responses both in quantity and quality than aluminum hydroxide (alum), which is currently the most widely used adjuvant in clinical human vaccination. CLDC-adjuvanted vaccine induced higher total influenza virus-specific IgG, particularly for the IgG2a/c subclass. Higher levels of multicytokine-producing influenza virus-specific CD4 and CD8 T cells were induced by CLDC-adjuvanted vaccine than with alum-adjuvanted vaccine. Importantly, CLDC-adjuvanted vaccine provided significant cross-protection from either a sublethal or lethal influenza A viral challenge with a different subtype than that used for vaccination. This superior cross-protection afforded by the CLDC adjuvant required CD8 T-cell recognition of viral peptides presented by classical major histocompatibility complex class I proteins. Together, these results suggest that CLDC has particular promise for vaccine strategies in which T cells play an important role and may offer new opportunities for more effective control of human influenza epidemics and pandemics by inactivated influenza virus vaccine.

Single-dose mucosal immunization with a candidate universal influenza vaccine provides rapid protection from virulent H5N1, H3N2 and H1N1 viruses

BACKGROUND

The sudden emergence of novel influenza viruses is a global public health concern. Conventional influenza vaccines targeting the highly variable surface glycoproteins hemagglutinin and neuraminidase must antigenically match the emerging strain to be effective. In contrast, "universal" vaccines targeting conserved viral components could be used regardless of viral strain or subtype. Previous approaches to universal vaccination have required protracted multi-dose immunizations. Here we evaluate a single dose universal vaccine strategy using recombinant adenoviruses (rAd) expressing the conserved influenza virus antigens matrix 2 and nucleoprotein.

METHODOLOGY/PRINCIPAL FINDINGS

In BALB/c mice, administration of rAd via the intranasal route was superior to intramuscular immunization for induction of mucosal responses and for protection against highly virulent H1N1, H3N2, or H5N1 influenza virus challenge. Mucosally vaccinated mice not only survived, but had little morbidity and reduced lung virus titers. Protection was observed as early as 2 weeks post-immunization, and lasted at least 10 months, as did antibodies and lung T cells with activated phenotypes. Virus-specific IgA correlated with but was not essential for protection, as demonstrated in studies with IgA-deficient animals.

CONCLUSION/SIGNIFICANCE

Mucosal administration of NP and M2-expressing rAd vectors provided rapid and lasting protection from influenza viruses in a subtype-independent manner. Such vaccines could be used in the interval between emergence of a new virus strain and availability of strain-matched vaccines against it. This strikingly effective single-dose vaccination thus represents a candidate off-the-shelf vaccine for emergency use during an influenza pandemic.

A flow cytometry-based strategy to identify and express IgM from VH1-69+ clonal peripheral B cells

Pathologic rheumatoid factor (RF) levels are hallmarks of several human diseases. Production of monoclonal RF in vitro is essential for studies of the antigenic specificities of RF, as well as for a dissection of the mechanisms of aberrant RF+ B cell activation. We have expanded upon previous methods to develop a flow cytometry-based method to efficiently clone monoclonal antibodies (mAbs) from humans with expansions of RF-like, immunoglobulin heavy chain variable region (IgVH) 1-69 gene segment-containing B cells. The cloned variable regions are expressed as IgM and produced during culture at concentrations between 5 and 20 μg/ml. Using this system, we show that clonal Igs from patients with HCV-related mixed cryoglobulinemia, when expressed as IgM, have RF activity. We anticipate that this system will be useful for the cloning and expression of mAbs partially encoded by VH1-69 and for determination of the reactivity patterns of polyspecific, low-affinity IgMs of human pathogenic importance.

H1N1: can a pandemic cycle be broken?

The influenza virus that caused the 2009 H1N1 swine-origin flu pandemic is antigenically similar to the one that caused the devastating 1918 pandemic. Over time, the human population became susceptible to a modified version of the 1918 pandemic H1N1 virus that had been archived in swine. Now, two papers, one in this issue of Science Translational Medicine and one in Science, shed mechanistic light on how glycosylation gave rise to seasonal human flu viruses that are immunologically distinct from their 1918 pandemic precursor and the 2009 pandemic strain. These findings suggest strategies to anticipate and prevent future pandemics.

Cross-neutralization of 1918 and 2009 influenza viruses: role of glycans in viral evolution and vaccine design

New strains of H1N1 influenza virus have emerged episodically over the last century to cause human pandemics, notably in 1918 and recently in 2009. Pandemic viruses typically evolve into seasonal forms that develop resistance to antibody neutralization, and cross-protection between strains separated by more than 3 years is uncommon. Here, we define the structural basis for cross-neutralization between two temporally distant pandemic influenza viruses--from 1918 and 2009. Vaccination of mice with the 1918 strain protected against subsequent lethal infection by 2009 virus. Both were resistant to antibodies directed against a seasonal influenza, A/New Caledonia/20/1999 (1999 NC), which was insensitive to antisera to the pandemic strains. Pandemic strain-neutralizing antibodies were directed against a subregion of the hemagglutinin (HA) receptor binding domain that is highly conserved between the 1918 and the 2009 viruses. In seasonal strains, this region undergoes amino acid diversification but is shielded from antibody neutralization by two highly conserved glycosylation sites absent in the pandemic strains. Pandemic HA trimers modified by glycosylation at these positions were resistant to neutralizing antibodies to wild-type HA. Yet, antisera generated against the glycosylated HA mutant neutralized it, suggesting that the focus of the immune response can be selectively changed with this modification. Collectively, these findings define critical determinants of H1N1 viral evolution and have implications for vaccine design. Immunization directed to conserved receptor binding domain subregions of pandemic viruses could potentially protect against similar future pandemic viruses, and vaccination with glycosylated 2009 pandemic virus may limit its further spread and transformation into a seasonal influenza.

A simple slot blot for the detection of virtually all subtypes of the influenza A viral hemagglutinins using universal antibodies targeting the fusion peptide

The fusion peptide of influenza virus hemagglutinin (HA) has a critical role in mediating the entry of the virus into the cells and is also the only universally conserved sequence in the HAs of all strains of influenza A and influenza B viruses. Therefore, it could be an attractive target for new vaccine development and a potency marker for existing influenza vaccines. The fusion peptide epitope is hidden inside the HA proteins, making it inaccessible for quantitative antibody binding. Our simple slot blot protocol highlights pre-treatment of HA samples with moderate concentrations of denaturant to maximally expose the fusion peptide on the protein surface, followed by detection using universal antibodies targeting the fusion peptide. The method is highly reliable, inexpensive and easy to follow. The entire procedure takes only 5 h and can be applied to the quantitative determination of virtually all influenza viral HAs using a single antibody targeting the fusion peptide.

A cross-reactive neutralizing monoclonal antibody protects mice from H5N1 and pandemic (H1N1) 2009 virus infection

A novel influenza (H1N1) virus caused an influenza pandemic in 2009, while highly pathogenic H5N1 avian influenza viruses have continued to infect humans since 1997. Influenza, therefore, remains a serious health threat. Currently, neuraminidase (NA) inhibitors are the mainstay for influenza therapy; however, drug-resistant mutants of seasonal H1N1 and H5N1 viruses have emerged highlighting the need for alternative therapeutic approaches. One such approach is antibody immunotherapy. Here, we show that the monoclonal antibody C179, which recognizes a neutralizing epitope common among H1, H2, H5, and H6 hemagglutinins (HAs), protected mice from a lethal challenge with various H5N1 and pandemic (H1N1) 2009 viruses when administered either intraperitoneally or intranasally. The protective efficacy of intranasally inoculated C179 was comparable to that of intraperitoneal administration. Our results suggest that direct administration of this anti-influenza antibody to viral replication sites is an effective strategy for prophylaxis and therapy.

Coverage of related pathogenic species by multivalent and cross-protective vaccine design: arenaviruses as a model system

The arenaviruses are a family of negative-sense RNA viruses that cause severe human disease ranging from aseptic meningitis to hemorrhagic fever syndromes. There are currently no FDA-approved vaccines for the prevention of arenavirus disease, and therapeutic treatment is limited to the use of ribavirin and/or immune plasma for a subset of the pathogenic arenaviruses. The considerable genetic variability observed among the seven arenaviruses that are pathogenic for humans illustrates one of the major challenges for vaccine development today, namely, to overcome pathogen heterogeneity. Over the past 5 years, our group has tested several strategies to overcome pathogen heterogeneity, utilizing the pathogenic arenaviruses as a model system. Because T cells play a prominent role in protective immunity following arenavirus infection, we specifically focused on the development of human vaccines that would induce multivalent and cross-protective cell-mediated immune responses. To facilitate our vaccine development and testing, we conducted large-scale major histocompatibility complex (MHC) class I and class II epitope discovery on murine, nonhuman primate, and human backgrounds for each of the pathogenic arenaviruses, including the identification of protective HLA-restricted epitopes. Finally, using the murine model of lymphocytic choriomeningitis virus infection, we studied the phenotypic characteristics associated with immunodominant and protective T cell epitopes. This review summarizes the findings from our studies and discusses their application to future vaccine design.

Influenza hemagglutinin and neuraminidase membrane glycoproteins

Considerable progress has been made toward understanding the structural basis of the interaction of the two major surface glycoproteins of influenza A virus with their common ligand/substrate: carbohydrate chains terminating in sialic acid. The specificity of virus attachment to target cells is mediated by hemagglutinin, which acquires characteristic changes in its receptor-binding site to switch its host from avian species to humans. Anti-influenza drugs mimic the natural sialic acid substrate of the virus neuraminidase enzyme but utilize the much tighter binding of the drugs for efficacy. Resistance to one of the two main antiviral drugs is differentially acquired by the two distinct subsets of neuraminidase as a consequence of structural differences in the enzyme active site between the two phylogenetic groups.

Surface plasmon resonance for vaccine design and efficacy studies: recent applications and future trends

The lack of a clear correlation between design and protection continues to present a barrier to progress in vaccine research. In this article, we outline how surface plasmon resonance (SPR) biosensors are emerging as tools to help resolve some of the key biophysical determinants of protection and, thereby, facilitate more rational vaccine design campaigns. SPR technology has contributed significantly to our understanding of the complex biophysical determinants of HIV neutralization and offers a platform for preclinical evaluation of vaccine candidates. In particular, the concept of reverse-engineering HIV vaccine targets based on known broadly neutralizing antibody modalities is explored and extended to include other infectious diseases, such as malaria and influenza, and other diseases such as cancer. The analytical capacity afforded by SPR includes serum screening to monitor immune responses and highly efficient quality-control surveillance measures. These are discussed alongside key technological advances, such as developments in sample throughput, and a perspective predicting continued growth and diversification of the role of SPR in vaccine development is proposed.

Respiratory syncytial virus-neutralizing monoclonal antibodies motavizumab and palivizumab inhibit fusion

Respiratory syncytial virus (RSV) is a major cause of virus-induced respiratory disease and hospitalization in infants. Palivizumab, an RSV-neutralizing monoclonal antibody, is used clinically to prevent serious RSV-related respiratory disease in high-risk infants. Motavizumab, an affinity-optimized version of palivizumab, was developed to improve protection against RSV. These antibodies bind RSV F protein, which plays a role in virus attachment and mediates fusion. Determining how these antibodies neutralize RSV is important to help guide development of new antibody drugs against RSV and, potentially, other viruses. This study aims to uncover the mechanism(s) by which palivizumab and motavizumab neutralize RSV. Assays were developed to test the effects of these antibodies at distinct steps during RSV replication. Pretreatment of virus with palivizumab or motavizumab did not inhibit virus attachment or the ability of F protein to interact with the target cell membrane. However, pretreatment of virus with either of these antibodies resulted in the absence of detectable viral transcription. These results show that palivizumab and motavizumab act at a point after F protein initiates interaction with the cell membrane and before virus transcription. Palivizumab and motavizumab also inhibited F protein-mediated cell-to-cell fusion. Therefore, these results strongly suggest that these antibodies block both cell-to-cell and virus-to-cell fusion, since these processes are likely similar. Finally, palivizumab and motavizumab did not reduce viral budding. Based on models developed from numerous studies of viral fusion proteins, our results indicate that these antibodies may prevent conformational changes in F protein required for the fusion process.

Neutralization of human respiratory syncytial virus infectivity by antibodies and low-molecular-weight compounds targeted against the fusion glycoprotein

Human respiratory syncytial virus (HRSV) fusion (F) protein is an essential component of the virus envelope that mediates fusion of the viral and cell membranes, and, therefore, it is an attractive target for drug and vaccine development. Our aim was to analyze the neutralizing mechanism of anti-F antibodies in comparison with other low-molecular-weight compounds targeted against the F molecule. It was found that neutralization by anti-F antibodies is related to epitope specificity. Thus, neutralizing and nonneutralizing antibodies could bind equally well to virions and remained bound after ultracentrifugation of the virus, but only the former inhibited virus infectivity. Neutralization by antibodies correlated with inhibition of cell-cell fusion in a syncytium formation assay, but not with inhibition of virus binding to cells. In contrast, a peptide (residues 478 to 516 of F protein [F478-516]) derived from the F protein heptad repeat B (HRB) or the organic compound BMS-433771 did not interfere with virus infectivity if incubated with virus before ultracentrifugation or during adsorption of virus to cells at 4 degrees C. These inhibitors must be present during virus entry to effect HRSV neutralization. These results are best interpreted by asserting that neutralizing antibodies bind to the F protein in virions interfering with its activation for fusion. Binding of nonneutralizing antibodies is not enough to block this step. In contrast, the peptide F478-516 or BMS-433771 must bind to F protein intermediates generated during virus-cell membrane fusion, blocking further development of this process.

Intranasal delivery of an IgA monoclonal antibody effective against sublethal H5N1 influenza virus infection in mice

Highly pathogenic avian H5N1 influenza viruses are endemic in poultry in Asia and pose a pandemic threat to humans. Since the deployment of vaccines against a pandemic strain may take several months, adequate antiviral alternatives are needed to minimize the effects and the spread of the disease. Passive immunotherapy is regarded as a viable alternative. Here, we show the development of an IgA monoclonal antibody (DPJY01 MAb) specific to H5 hemagglutinin. The DPJY01 MAb showed a broad hemagglutination inhibition (HI) profile against Asian H5N1 viruses of clades 0, 1.0, 2.1, 2.2, and 2.3 and also against H5 wild bird influenza viruses of the North American and Eurasian lineages. DPJY01 MAb displayed also high neutralization activity in vitro and in vivo. In mice, DPJY01 MAb provided protection via a single dose administered intranasally before or after inoculation with a sublethal dose of H5N1 viruses of clades 1.0 and 2.2. Pretreatment with 50 mg of DPJY01 MAb kg of body weight at either 24, 48, or 72 h before highly pathogenic H5N1 virus (A/Vietnam/1203/2004 [H5N1]) inoculation resulted in complete protection. Treatment with 50 mg/kg at either at 24, 48, or 72 h after H5N1 inoculation provided 100%, 80%, and 60% protection, respectively. These studies highlight the potential use of DPJY01 MAb as an intranasal antiviral treatment for H5N1 influenza virus infections.

Flexible label-free quantitative assay for antibodies to influenza virus hemagglutinins

During the initial pandemic influenza H1N1 virus outbreak, assays such as hemagglutination inhibition and microneutralization provided important information on the relative protection afforded by the population's cross-reactivity from prior infections and immunizations with seasonal vaccines. However, these assays continue to be limited in that they are difficult to automate for high throughput, such as in pandemic situations, as well as to standardize between labs. Thus, new technologies are being sought to improve standardization, reliability, and throughput by using chemically defined reagents rather than whole cells and virions. We now report the use of a cell-free and label-free flu antibody biosensor assay (f-AbBA) for influenza research and diagnostics that utilizes recombinant hemagglutinin (HA) in conjunction with label-free biolayer interferometry technology to measure biomolecular interactions between the HA and specific anti-HA antibodies or sialylated ligands. We evaluated f-AbBA to determine anti-HA antibody binding activity in serum or plasma to assess vaccine-induced humoral responses. This assay can reveal the impact of antigenic difference on antibody binding to HA and also measure binding to different subtypes of HA. We also show that the biosensor assay can measure the ability of HA to bind a model sialylated receptor-like ligand. f-AbBA could be used in global surveillance laboratories since preliminary tests on desiccated HA probes showed no loss of activity after >2 months in storage at room temperature, indicating that the same reagent lots could be used in different laboratories to minimize interlaboratory assay fluctuation. Future development of such reagents and similar technologies may offer a robust platform for future influenza surveillance activities.

Induction of broadly neutralizing H1N1 influenza antibodies by vaccination

The rapid dissemination of the 2009 pandemic influenza virus underscores the need for universal influenza vaccines that elicit protective immunity to diverse viral strains. Here, we show that vaccination with plasmid DNA encoding H1N1 influenza hemagglutinin (HA) and boosting with seasonal vaccine or replication-defective adenovirus 5 vector encoding HA stimulated the production of broadly neutralizing influenza antibodies. This prime/boost combination increased the neutralization of diverse H1N1 strains dating from 1934 to 2007 as compared to either component alone and conferred protection against divergent H1N1 viruses in mice and ferrets. These antibodies were directed to the conserved stem region of HA and were also elicited in nonhuman primates. Cross-neutralization of H1N1 subtypes elicited by this approach provides a basis for the development of a universal influenza vaccine for humans.

Protection from the 2009 H1N1 pandemic influenza by an antibody from combinatorial survivor-based libraries

Influenza viruses elude immune responses and antiviral chemotherapeutics through genetic drift and reassortment. As a result, the development of new strategies that attack a highly conserved viral function to prevent and/or treat influenza infection is being pursued. Such novel broadly acting antiviral therapies would be less susceptible to virus escape and provide a long lasting solution to the evolving virus challenge. Here we report the in vitro and in vivo activity of a human monoclonal antibody (A06) against two isolates of the 2009 H1N1 pandemic influenza virus. This antibody, which was obtained from a combinatorial library derived from a survivor of highly pathogenic H5N1 infection, neutralizes H5N1, seasonal H1N1 and 2009 “Swine” H1N1 pandemic influenza in vitro with similar potency and is capable of preventing and treating 2009 H1N1 influenza infection in murine models of disease. These results demonstrate broad activity of the A06 antibody and its utility as an anti-influenza treatment option, even against newly evolved influenza strains to which there is limited immunity in the general population.

Influenza viruses constantly challenge our ability to prevent and treat their resulting infection. From a survivor of the H5N1 influenza we have discovered an antibody that is effective against both H5N1 and seasonal H1N1 influenza viruses. Here we show the antibody is effective against 2009 pandemic influenza in a cell culture assay and also in mouse models of disease when given before and even after lethal influenza infection. The present work demonstrates the viability of this particular antibody and the general approach of using antibodies against viral pathogens as opposed to traditional treatments that are losing their efficacy for the prevention and treatment of influenza infection. We conclude the efficacy of this antibody warrants further experimental testing as an alternative therapy for treatment in man.

Long-lasting protective antiviral immunity induced by passive immunotherapies requires both neutralizing and effector functions of the administered monoclonal antibody

Using FrCas(E) retrovirus-infected newborn mice as a model system, we have shown recently that a long-lasting antiviral immune response essential for healthy survival emerges after a short treatment with a neutralizing (667) IgG2a isotype monoclonal antibody (MAb). This suggested that the mobilization of adaptive immunity by administered MAbs is key for the success in the long term for the MAb-based passive immunotherapy of chronic viral infections. We have addressed here whether the anti-FrCas(E) protective endogenous immunity is the mere consequence of viral propagation blunting, which would simply give time to the immune system to react, and/or to actual immunomodulation by the MAb during the treatment. To this aim, we have compared viral replication, disease progression, and antiviral immune responses between different groups of infected mice: (i) mice treated with either the 667 MAb, its F(ab')(2) fragment, or an IgM (672) with epitopic specificity similar to that of 667 but displaying different effector functions, and (ii) mice receiving no treatment but infected with a low viral inoculum reproducing the initial viral expansion observed in their infected/667 MAb-treated counterparts. Our data show that the reduction of FrCas(E) propagation is insufficient on its own to induce protective immunity and support a direct immunomodulatory action of the 667 MAb. Interestingly, they also point to sequential actions of the administered MAb. In a first step, viral propagation is exclusively controlled by 667 neutralizing activity, and in a second one, this action is complemented by FcgammaR-binding-dependent mechanisms, which most likely combine infected cell cytolysis and the modulation of the antiviral endogenous immune response. Such complementary effects of administered MAbs must be taken into consideration for the improvement of future antiviral MAb-based immunotherapies.

Design of an HA2-based Escherichia coli expressed influenza immunogen that protects mice from pathogenic challenge

Influenza HA is the primary target of neutralizing antibodies during infection, and its sequence undergoes genetic drift and shift in response to immune pressure. The receptor binding HA1 subunit of HA shows much higher sequence variability relative to the metastable, fusion-active HA2 subunit, presumably because neutralizing antibodies are primarily targeted against the former in natural infection. We have designed an HA2-based immunogen using a protein minimization approach that incorporates designed mutations to destabilize the low pH conformation of HA2. The resulting construct (HA6) was expressed in Escherichia coli and refolded from inclusion bodies. Biophysical studies and mutational analysis of the protein indicate that it is folded into the desired neutral pH conformation competent to bind the broadly neutralizing HA2 directed monoclonal 12D1, not the low pH conformation observed in previous studies. HA6 was highly immunogenic in mice and the mice were protected against lethal challenge by the homologous A/HK/68 mouse-adapted virus. An HA6-like construct from another H3 strain (A/Phil/2/82) also protected mice against A/HK/68 challenge. Regions included in HA6 are highly conserved within a subtype and are fairly well conserved within a clade. Targeting the highly conserved HA2 subunit with a bacterially produced immunogen is a vaccine strategy that may aid in pandemic preparedness.

In silico characterization of the functional and structural modules of the hemagglutinin protein from the swine-origin influenza virus A (H1N1)-2009

The 2009 swine-origin influenza virus (S-OIV, H1N1 subtype) has developed into a new pandemic influenza as announced by the World Health Organization. In order to uncover clues about the determinants for virulence and pathogenicity of the virus, we characterized the functional modules of the surface glycoprotein hemagglutinin (HA), the most important protein in molecular epidemiology and pathogenesis of influenza viruses. We analyzed receptor binding sites, basic patch, neutralization antibody epitopes and T cell epitopes in the HA protein of the current S-OIV according to the corresponding functional and structural modules previously characterized in other H1 HA molecules or HA molecules of other subtypes. We compared their differences and similarities systematically. Based on the amino acids defined as the functional and structural modules, the HA protein of 2009 S-OIV should specifically bind to the human 2,6-receptor. The D225G/E mutation in HA, which is found in some isolates, may confer dual binding specificity to the 2,3- and 2,6-receptor based on previously reported work. This HA variant contains two basic patches, one of which results in increased basicity, suggesting enhanced membrane fusion function. The 2009 S-OIV HA also has an extra glycosylation site at position 276. Four of the five antibody neutralization epitopes identified in A/RP/8/34(H1N1) were exposed, but the other was hidden by a glycosylation site. The previously identified cytotoxic T cell epitopes in various HA molecules were summarized and their corresponding sequences in 2009 S-OIV HA were defined. These results are critical for understanding the pathogenicity of the virus and host immune response against the virus.

Human antibodies reveal a protective epitope that is highly conserved among human and nonhuman influenza A viruses

Influenza remains a serious public health threat throughout the world. Vaccines and antivirals are available that can provide protection from infection. However, new viral strains emerge continuously because of the plasticity of the influenza genome, which necessitates annual reformulation of vaccine antigens, and resistance to antivirals can appear rapidly and become entrenched in circulating virus populations. In addition, the spread of new pandemic strains is difficult to contain because of the time required to engineer and manufacture effective vaccines. Monoclonal antibodies that target highly conserved viral epitopes might offer an alternative protection paradigm. Herein we describe the isolation of a panel of monoclonal antibodies derived from the IgG(+) memory B cells of healthy, human subjects that recognize a previously unknown conformational epitope within the ectodomain of the influenza matrix 2 protein, M2e. This antibody binding region is highly conserved in influenza A viruses, being present in nearly all strains detected to date, including highly pathogenic viruses that infect primarily birds and swine, and the current 2009 swine-origin H1N1 pandemic strain (S-OIV). Furthermore, these human anti-M2e monoclonal antibodies protect mice from lethal challenges with either H5N1 or H1N1 influenza viruses. These results suggest that viral M2e can elicit broadly cross-reactive and protective antibodies in humans. Accordingly, recombinant forms of these human antibodies may provide useful therapeutic agents to protect against infection from a broad spectrum of influenza A strains.

Protective immunity against H5N1 influenza virus by a single dose vaccination with virus-like particles

We generated influenza virus-like particles (VLPs) containing the wild type (WT) H5 hemagglutinin (HA) from A/Viet Nam/1203/04 virus or a mutant H5 HA with a deletion of the multibasic cleavage motif. VLPs containing mutant H5 HA were found to be as immunogenic as VLPs containing WT HA. A single intramuscular vaccination with either type of H5 VLPs provided complete protection against lethal challenge. In contrast, the recombinant H5 HA vaccine was less immunogenic and vaccination even with a 5 fold higher dose did not induce protective immunity. VLP vaccines were superior to the recombinant HA in inducing T helper type 1 immune responses, hemagglutination inhibition titers, and antibody secreting cells, which significantly contribute to inducing protective immunity after a single dose vaccination. This study provides insights into the potential mechanisms of improved immunogenicity by H5 VLP vaccines as an approach to improve the protective efficacy against potential pandemic viruses.

A crucial role for infected-cell/antibody immune complexes in the enhancement of endogenous antiviral immunity by short passive immunotherapy

Antiviral monoclonal antibodies (mAbs) represent promising therapeutics. However, most mAbs-based immunotherapies conducted so far have only considered the blunting of viral propagation and not other possible therapeutic effects independent of virus neutralization, namely the modulation of the endogenous immune response. As induction of long-term antiviral immunity still remains a paramount challenge for treating chronic infections, we have asked here whether neutralizing mAbs can, in addition to blunting viral propagation, exert immunomodulatory effects with protective outcomes. Supporting this idea, we report here that mice infected with the FrCas^E murine retrovirus on day 8 after birth die of leukemia within 4–5 months and mount a non-protective immune response, whereas those rapidly subjected to short immunotherapy with a neutralizing mAb survive healthy and mount a long-lasting protective antiviral immunity with strong humoral and cellular immune responses. Interestingly, the administered mAb mediates lysis of infected cells through an antibody-dependent cell cytotoxicity (ADCC) mechanism. In addition, it forms immune complexes (ICs) with infected cells that enhance antiviral CTL responses through FcγR-mediated binding to dendritic cells (DCs). Importantly, the endogenous antiviral antibodies generated in mAb-treated mice also display the same properties, allowing containment of viral propagation and enhancement of memory cellular responses after disappearance of the administered mAb. Thus, our data demonstrate that neutralizing antiviral mAbs can act as immunomodulatory agents capable of stimulating a protective immunity lasting long after the end of the treatment. They also show an important role of infected-cells/antibody complexes in the induction and the maintenance of protective immunity through enhancement of both primary and memory antiviral T-cell responses. They also indicate that targeting infected cells, and not just viruses, by antibodies can be crucial for elicitation of efficient, long-lasting antiviral T-cell responses. This must be considered when designing antiviral mAb-based immunotherapies.

Monoclonal antibodies (mAbs) constitute the largest class of bio-therapeutic proteins and are increasingly being considered as drugs to fight both acute and chronic severe human viral diseases. Most antiviral mAb-based treatments conducted so far, whether in humans or in animal models, have only considered the blunting of viral propagation through direct virus neutralization. However, mAbs might also operate via complementary mechanisms owing to their ability to interact with various components of the immune system. Using a lethal mouse model of retrovirally-induced leukemia, we report here that a neutralizing mAb administered to infected mice for a short period of time can, in addition to its direct effect on viral spread, induce a strong, long-lasting antiviral immune response protecting mice from disease development long after the end of the treatment. Although the initiation and maintenance of this long-term immunity is multi-factorial, we demonstrate a crucial role for the immune complexes formed between antiviral antibodies and infected cells in this process. Our work reveals a thus far underappreciated vaccine-like effect of antiviral neutralizing mAbs, which will have to be considered for future treatment of life-threatening viral infections.

Identification of seasonal vaccine hemagglutinin subtype 1 (H1) epitopes in Mexican isolates of the new influenza A (H1N1) 2009 virus

BACKGROUND

Pre-existing immunity in individuals is a determinant condition for epidemic dynamics. During the current influenza A (H1N1) virus pandemic, cross-reactivity of seasonal vaccines from the last years and previous contact with seasonal influenza viruses was suggested as the cause of low severity and low incidence of the disease in persons aged 50-65 years and with history of seasonal influenza vaccination.

METHODS

We performed a detailed search and analysis of 74 previously reported H1 epitopes present in influenza A virus contained in seasonal vaccines applied in Mexico from 2004 to date and in sequences from Mexican isolates from 2003, as well as in the recent influenza A (H1N1) 2009, and calculated the epitope conservation among vaccine, seasonal and pandemic influenza A (H1N1) virus.

RESULTS

H1 epitope sequence identity ranged from 61.53-100 %. Of the 74 epitopes previously reported, 31 (41.9%) were completely conserved among all sequences analyzed in this study, whereas 43 (58.1%) had changes in one or more amino acids.

CONCLUSIONS

Our findings contribute to the estimatation of the degree of epitope conservation among H1 from vaccine virus strains as well as in the different viruses that circulate in the Mexican population. These results may provide new elements to consider for analysis of cross-immunity to influenza viruses including the novel influenza A (H1N1) 2009 pandemic virus.

An antibody against a novel and conserved epitope in the hemagglutinin 1 subunit neutralizes numerous H5N1 influenza viruses

The spread of the recently emerged, highly pathogenic H5N1 avian influenza virus has raised concern. Preclinical studies suggest that passive immunotherapy could be a new form of treatment for H5N1 virus infection. Here, a neutralizing monoclonal antibody (MAb) against the hemagglutinin (HA) of the influenza A/chicken/Hatay/2004 H5N1 virus, MAb 9F4, was generated and characterized. MAb 9F4 binds both the denatured and native forms of HA. It was shown to recognize the HA proteins of three heterologous strains of H5N1 viruses belonging to clades 1, 2.1, and 2.2, respectively. By use of lentiviral pseudotyped particles carrying HA on the surface, MAb 9F4 was shown to effectively neutralize the homologous strain, Hatay04, and another clade 1 strain, VN04, at a neutralization titer of 8 ng/ml. Furthermore, MAb 9F4 also neutralized two clade 2 viruses at a neutralizing titer of 40 ng/ml. The broad cross-neutralizing activity of MAb 9F4 was confirmed by its ability to neutralize live H5N1 viruses of clade 2.2.2. Epitope-mapping analysis revealed that MAb 9F4 binds a previously uncharacterized epitope below the globular head of the HA1 subunit. Consistently, this epitope is well conserved among the different clades of H5N1 viruses. MAb 9F4 does not block the interaction between HA and its receptor but prevents the pH-mediated conformational change of HA. MAb 9F4 was also found to be protective, both prophylactically and therapeutically, against a lethal viral challenge of mice. Taken together, our results showed that MAb 9F4 is a neutralizing MAb that binds a novel and well-conserved epitope in the HA1 subunit of H5N1 viruses.

The 2009 A (H1N1) influenza virus pandemic: A review

In March and early April 2009 a new swine-origin influenza virus (S-OIV), A (H1N1), emerged in Mexico and the USA. The virus quickly spread worldwide through human-to-human transmission. In view of the number of countries and communities which were reporting human cases, the World Health Organization raised the influenza pandemic alert to the highest level (level 6) on June 11, 2009. The propensity of the virus to primarily affect children, young adults and pregnant women, especially those with an underlying lung or cardiac disease condition, and the substantial increase in rate of hospitalizations, prompted the efforts of the pharmaceutical industry, including new manufacturers from China, Thailand, India and South America, to develop pandemic H1N1 influenza vaccines. All currently registered vaccines were tested for safety and immunogenicity in clinical trials on human volunteers. All were found to be safe and to elicit potentially protective antibody responses after the administration of a single dose of vaccine, including split inactivated vaccines with or without adjuvant, whole-virion vaccines and live-attenuated vaccines. The need for an increased surveillance of influenza virus circulation in swine is outlined.

Influenza virus vaccine based on the conserved hemagglutinin stalk domain

Although highly effective in the general population when well matched to circulating influenza virus strains, current influenza vaccines are limited in their utility due to the narrow breadth of protection they provide. The strain specificity of vaccines presently in use mirrors the exquisite specificity of the neutralizing antibodies that they induce, that is, antibodies which bind to the highly variable globular head domain of hemagglutinin (HA). Herein, we describe the construction of a novel immunogen comprising the conserved influenza HA stalk domain and lacking the globular head. Vaccination of mice with this headless HA construct elicited immune sera with broader reactivity than those obtained from mice immunized with a full-length HA. Furthermore, the headless HA vaccine provided full protection against death and partial protection against disease following lethal viral challenge. Our results suggest that the response induced by headless HA vaccines is sufficiently potent to warrant their further development toward a universal influenza virus vaccine.

Current influenza vaccines are effective against only a narrow range of influenza virus strains. It is for this reason that new vaccines must be generated and administered each year. We now report progress toward the goal of an influenza virus vaccine which would protect against multiple strains. Our approach is based on presentation to the host immune system of a region of the influenza virus—called a “headless hemagglutinin” (headless HA)—which is similar among a multitude of diverse strains. We show that vaccination of mice with a headless HA confers protection to these animals against a lethal influenza virus challenge, thereby demonstrating the viability of the approach. Through further development and testing, we predict that a single immunization with a headless HA vaccine will offer effective protection through several influenza epidemics.

Significant impact of sequence variations in the nucleoprotein on CD8 T cell-mediated cross-protection against influenza A virus infections

Background

Memory CD8 T cells to influenza A viruses are widely detectable in healthy human subjects and broadly cross-reactive for serologically distinct influenza A virus subtypes. However, it is not clear to what extent such pre-existing cellular immunity can provide cross-subtype protection against novel emerging influenza A viruses.

Methodology/Principal Findings

We show in the mouse model that naturally occurring sequence variations of the conserved nucleoprotein of the virus significantly impact cross-protection against lethal disease in vivo. When priming and challenge viruses shared identical sequences of the immunodominant, protective NP₃₆₆/D^b epitope, strong cross-subtype protection was observed. However, when they did not share complete sequence identity in this epitope, cross-protection was considerably reduced. Contributions of virus-specific antibodies appeared to be minimal under these circumstances. Detailed analysis revealed that the magnitude of the memory CD8 T cell response triggered by the NP₃₆₆/D^b variants was significantly lower than those triggered by the homologous NP₃₆₆/D^b ligand. It appears that strict specificity of a dominant public TCR to the original NP₃₆₆/D^b ligand may limit the expansion of cross-reactive memory CD8 T cells to the NP₃₆₆/D^b variants.

Conclusions/Significance

Pre-existing CD8 T cell immunity may provide substantial cross-protection against heterosubtypic influenza A viruses, provided that the priming and the subsequent challenge viruses share the identical sequences of the immunodominant, protective CTL epitopes.

Roles of the hemagglutinin of influenza A virus in viral entry and development of antiviral therapeutics and vaccines

Seasonal influenza epidemics and influenza pandemics caused by influenza A virus (IAV) has resulted in millions of deaths in the world. The development of anti-IAV vaccines and therapeutics is urgently needed for prevention and treatment of IAV infection and for controlling future influenza pandemics. Hemagglutinin (HA) of IAV plays a critical role in viral binding, fusion and entry, and contains the major neutralizing epitopes. Therefore, HA is an attractive target for developing anti-IAV drugs and vaccines. Here we have reviewed the recent progress in study of conformational changes of HA during viral fusion process and development of HA-based antiviral therapeutics and vaccines.