Influenza A virus lacking M2 protein as a live attenuated vaccine

The plant rhabdovirus viroporin P9 facilitates insect-mediated virus transmission in barley

Potassium (K+) plays crucial roles in both plant development and immunity. However, the function of K+ in plant-virus interactions remains largely unknown. Here, we utilized Barley yellow striate mosaic virus (BYSMV), an insect-transmitted plant cytorhabdovirus, to investigate the interplay between viral infection and plant K+ homeostasis. The BYSMV accessory P9 protein exhibits viroporin activity by enhancing membrane permeability in Escherichia coli. Additionally, P9 increases K+ uptake in yeast (Saccharomyces cerevisiae) cells, which is disrupted by a point mutation of glycine 14 to threonine (P9G14T). Furthermore, BYSMV P9 forms oligomers and targets to both the viral envelope and the plant membrane. Based on the recombinant BYSMV-GFP (BYGFP) virus, a P9-deleted mutant (BYGFPΔP9) was rescued and demonstrated infectivity within individual plant cells of Nicotiana benthamiana and insect vectors. However, BYGFPΔP9 failed to infect barley plants after transmission by insect vectors. Furthermore, infection of barley plants was severely impaired for BYGFP-P9G14T lacking P9 K+ channel activity. In vitro assays demonstrate that K+ facilitates virion disassembly and the release of genome RNA for viral mRNA transcription. Altogether, our results show that the K+ channel activity of viroporins is conserved in plant cytorhabdoviruses and plays crucial roles in insect-mediated virus transmission.

Intranasal Single-Replication Influenza Vector Induces Cross-Reactive Serum and Mucosal Antibodies against SARS-CoV-2 Variants

Current SARS-CoV-2 vaccines provide protection for COVID-19-associated hospitalization and death, but remain inefficient at inhibiting initial infection and transmission. Despite updated booster formulations, breakthrough infections and reinfections from emerging SARS-CoV-2 variants are common. Intranasal vaccination to elicit mucosal immunity at the site of infection can improve the performance of respiratory virus vaccines. We developed SARS-CoV-2 M2SR, a dual SARS-CoV-2 and influenza vaccine candidate, employing our live intranasal M2-deficient single replication (M2SR) influenza vector expressing the receptor binding domain (RBD) of the SARS-CoV-2 Spike protein of the prototype strain, first reported in January 2020. The intranasal vaccination of mice with this dual vaccine elicits both high serum IgG and mucosal IgA titers to RBD. Sera from inoculated mice show that vaccinated mice develop neutralizing SARS-CoV-2 antibody titers against the prototype and Delta virus strains, which are considered to be sufficient to protect against viral infection. Moreover, SARS-CoV-2 M2SR elicited cross-reactive serum and mucosal antibodies to the Omicron BA.4/BA.5 variant. The SARS-CoV-2 M2SR vaccine also maintained strong immune responses to influenza A with high titers of anti H3 serum IgG and hemagglutination inhibition (HAI) antibody titers corresponding to those seen from the control M2SR vector alone. With a proven safety record and robust immunological profile in humans that includes mucosal immunity, the M2SR influenza viral vector expressing key SARS-CoV-2 antigens could provide more efficient protection against influenza and SARS-CoV-2 variants.

Influenza Virus Carrying a Codon-Reprogrammed Neuraminidase Gene as a Strategy for Live Attenuated Vaccine

Live attenuated influenza vaccines offer broader and longer-lasting protection in comparison to inactivated influenza vaccines. The neuraminidase (NA) surface glycoprotein of influenza A virus is essential for the release and spread of progeny viral particles from infected cells. In this study, we de novo synthesized the NA gene, in which 62% of codons were synonymously changed based on mammalian codon bias usage. The codon-reprogrammed NA (repNA) gene failed to be packaged into the viral genome, which was achievable with partial restoration of wild-type NA sequence nucleotides at the 3' and 5' termini. Among a series of rescued recombinant viruses, we selected 20/13repNA, which contained 20 and 13 nucleotides of wild-type NA at the 3' and 5' termini of repNA, respectively, and evaluated its potential as a live attenuated influenza vaccine. The 20/13repNA is highly attenuated in mice, and the calculated LD₅₀ was about 10,000-fold higher than that of the wild-type (WT) virus. Intranasal inoculation of the 20/13repNA virus in mice induced viral-specific humoral, cell-mediated, and mucosal immune responses. Mice vaccinated with the 20/13repNA virus were protected from the lethal challenge of both homologous and heterologous viruses. This strategy may provide a new method for the development of live, attenuated influenza vaccines for a better and more rapid response to influenza threats.

Longevity and Mechanism of Heterosubtypic Protection Induced by M2SR (M2-Deficient Single-Replication) Live Influenza Virus Vaccine in Mice

Seasonal influenza and the threat of global pandemics present a continuing threat to public health. However, conventional inactivated influenza vaccines (IAVs) provide little cross-protective immunity and suboptimal efficacy, even against well-matched strains. Furthermore, the protection against matched strains has been shown to be of a short duration in both mouse models and humans. M2SR (M2-deficient single-replication influenza virus) is a single-replication vaccine that has been shown to provide effective cross-protection against heterosubtypic influenza viruses in both mouse and ferret models. In the present study, we investigated the duration and mechanism of heterosubtypic protection induced by M2SR in a mouse model. We previously showed that M2SR generated from influenza A/Puerto Rico/8/34 (H1N1) significantly protected C57BL/6 mice against lethal challenge with both influenza A/Puerto Rico/8/34 (H1N1, homosubtypic) and influenza A/Aichi/2/1968 (H3N2, heterosubtypic), whereas the inactivated influenza vaccine provided no heterosubtypic protection. The homosubtypic protection induced by M2SR was robust and lasted for greater than 1 year, whereas that provided by the inactivated vaccine lasted for less than 6 months. The heterosubtypic protection induced by M2SR was of a somewhat shorter duration than the homosubtypic protection, with protection being evident 9 months after vaccination. However, heterosubtypic protection was not observed at 14 months post vaccination. M2SR has been shown to induce strong systemic and mucosal antibody and T cell responses. We investigated the relative importance of these immune mechanisms in heterosubtypic protection, using mice that were deficient in B cells or mice that were depleted of T cells immediately before challenge. Somewhat surprisingly, the heterosubtypic protection was completely dependent on B cells in this model, whereas the depletion of T cells had no significant effect on survival after a lethal heterosubtypic challenge. While antibody-dependent cellular cytotoxicity (ADCC) has been demonstrated to be important in the response to some influenza vaccines, a lack of Fc receptors did not affect the survival of M2SR-vaccinated mice following a lethal challenge. We examined the influenza proteins targeted by the heterosubtypic antibody response. Shortly after the H1N1 M2SR vaccination, high titers of cross-reactive antibodies to heterosubtypic H3N2 nucleoprotein (NP) and lower titers to the stalk region of the hemagglutinin (HA2) and neuraminidase (NA) proteins were observed. The high antibody titers to heterosubtypic NP persisted one year after vaccination, whereas the antibody titers to the heterosubtypic HA2 and NA proteins were very low, or below the limit of detection, at this time. These results show that the intranasal M2SR vaccine elicits durable protective immune responses against homotypic and heterosubtypic influenza infection not seen with intramuscular inactivated vaccines. Both the homo- and heterosubtypic protection induced by the single-replication vaccine are dependent on B cells in this model. While the homosubtypic protection is mediated by antibodies to the head region of HA, our data suggest that the heterosubtypic protection for M2SR is due to cross-reactive antibodies elicited against the NP, HA2, and NA antigens that are not targeted by current seasonal influenza vaccines.

Probing effects of the SARS-CoV-2 E protein on membrane curvature and intracellular calcium

SARS-CoV-2 contains four structural proteins in its genome. These proteins aid in the assembly and budding of new virions at the ER-Golgi intermediate compartment (ERGIC). Current fundamental research efforts largely focus on one of these proteins – the spike (S) protein. Since successful antiviral therapies are likely to target multiple viral components, there is considerable interest in understanding the biophysical role of its other structural proteins, in particular structural membrane proteins. Here, we have focused our efforts on the characterization of the full-length envelope (E) protein from SARS-CoV-2, combining experimental and computational approaches. Recombinant expression of the full-length E protein from SARS-CoV-2 reveals that this membrane protein is capable of independent multimerization, possibly as a tetrameric or smaller species. Fluorescence microscopy shows that the protein localizes intracellularly, and coarse-grained MD simulations indicate it causes bending of the surrounding lipid bilayer, corroborating a potential role for the E protein in viral budding. Although we did not find robust electrophysiological evidence of ion-channel activity, cells transfected with the E protein exhibited reduced intracellular Ca²⁺, which may further promote viral replication. However, our atomistic MD simulations revealed that previous NMR structures are relatively unstable, and result in models incapable of ion conduction. Our study highlights the importance of using high-resolution structural data obtained from a full-length protein to gain detailed molecular insights, and eventually permitting virtual drug screening.

Generation of a live attenuated influenza A vaccine by proteolysis targeting

The usefulness of live attenuated virus vaccines has been limited by suboptimal immunogenicity, safety concerns or cumbersome manufacturing processes and techniques. Here we describe the generation of a live attenuated influenza A virus vaccine using proteolysis-targeting chimeric (PROTAC) technology to degrade viral proteins via the endogenous ubiquitin-proteasome system of host cells. We engineered the genome of influenza A viruses in stable cell lines engineered for virus production to introduce a conditionally removable proteasome-targeting domain, generating fully infective PROTAC viruses that were live attenuated by the host protein degradation machinery upon infection. In mouse and ferret models, PROTAC viruses were highly attenuated and able to elicit robust and broad humoral, mucosal and cellular immunity against homologous and heterologous virus challenges. PROTAC-mediated attenuation of viruses may be broadly applicable for generating live attenuated vaccines.

Next generation live-attenuated influenza vaccine platforms

INTRODUCTION

Influenza virus is a major cause of seasonal epidemics and intermittent pandemics. Despite the current molecular biology and vaccine development, influenza virus infection is a significant burden. Vaccines are considered an essential countermeasure for effective control and prevention of influenza virus infection. Even though current influenza virus vaccines provide efficient protection against seasonal influenza outbreaks, the efficacy of these vaccines is not suitable due to antigenic changes of the viruses.

AREAS COVERED

This review focuses on different live-attenuated platforms for influenza virus vaccine development and proposes essential considerations for a rational universal influenza virus vaccine design.

EXPERT OPINION

Despite the recent efforts for universal influenza virus vaccines, there is a lack of broadly reactive antibodies' induction that can confer broad and long-lasting protection. Various strategies using live-attenuated influenza virus vaccines (LAIVs) are investigated to induce broadly reactive, durable, and cross-protective immune responses. LAIVs based on NS segment truncation prevent influenza virus infection and have shown to be effective vaccine candidates among other vaccine platforms. Although many approaches have been used for LAIVs generation, there is still a need to focus on the LAIVs development platforms to generate a universal influenza virus vaccine candidate.

pH- and Calcium-Dependent Aromatic Network in the SARS-CoV-2 Envelope Protein

The envelope (E) protein of the SARS-CoV-2 virus is a membrane-bound viroporin that conducts cations across the endoplasmic reticulum Golgi intermediate compartment (ERGIC) membrane of the host cell to cause virus pathogenicity. The structure of the closed state of the E transmembrane (TM) domain, ETM, was recently determined using solid-state NMR spectroscopy. However, how the channel pore opens to mediate cation transport is unclear. Here, we use ¹³C and ¹⁹F solid-state NMR spectroscopy to investigate the conformation and dynamics of ETM at acidic pH and in the presence of calcium ions, which mimic the ERGIC and lysosomal environment experienced by the E protein in the cell. Acidic pH and calcium ions increased the conformational disorder of the N- and C-terminal residues and also increased the water accessibility of the protein, indicating that the pore lumen has become more spacious. ETM contains three regularly spaced phenylalanine (Phe) residues in the center of the peptide. ¹⁹F NMR spectra of para-fluorinated Phe20 and Phe26 indicate that both residues exhibit two sidechain conformations, which coexist within each channel. These two Phe conformations differ in their water accessibility, lipid contact, and dynamics. Channel opening by acidic pH and Ca²⁺ increases the population of the dynamic lipid-facing conformation. These results suggest an intricate aromatic network that regulates the opening of the ETM channel pore. This aromatic network may be a target for E inhibitors against SARS-CoV-2 and related coronaviruses.

Generation and Characterization of Single-Cycle Infectious Canine Influenza A Virus (sciCIV) and Its Use as Vaccine Platform

Influenza A viruses (IAVs) infect a broad range of hosts, including multiple avian and mammalian species. The frequent emergence of novel IAV strains in different hosts, including in humans, results in the need for vigilance and ongoing development of new approaches to fighting or prevent those infections. Canine influenza is a contagious respiratory disease in dogs caused by two subtypes of IAV, the equine-origin H3N8 canine influenza virus (CIV), and the avian-origin H3N2 CIV. A novel approach to influenza vaccination involves single-cycle infectious influenza A viruses (sciIAVs), which are defective for an essential viral gene. They are propagated in complementing cell lines which provide the missing gene in trans. As sciIAV cannot complete their replication cycle in regular cells they are limited to a single round of viral replication. Because of their safety profile and ability to express foreign antigens inside infected cells, sciIAVs have served both as live-attenuated vaccines and as vaccine vectors for the expression of heterologous antigens. Here, we describe experimental procedures for the generation of a single-cycle infectious CIV (sciCIV), where the viral hemagglutinin (HA) gene was exchanged for the gene for green fluorescent protein (GFP). Complementation of the viral HA protein is provided in trans by stable HA-expressing cell lines. Methods for the in vitro characterization of HA deficient but GFP-expressing sciCIV (sciCIV ΔHA/GFP) are described, as well as its use as a potential vaccine.

Functions of Coronavirus Accessory Proteins: Overview of the State of the Art

Coronavirus accessory proteins are a unique set of proteins whose genes are interspersed among or within the genes encoding structural proteins. Different coronavirus genera, or even different species within the same coronavirus genus, encode varying amounts of accessory proteins, leading to genus- or species-specificity. Though accessory proteins are dispensable for the replication of coronavirus in vitro, they play important roles in regulating innate immunity, viral proliferation, and pathogenicity. The function of accessory proteins on virus infection and pathogenesis is an area of particular interest. In this review, we summarize the current knowledge on accessory proteins of several representative coronaviruses that infect humans or animals, including the emerging severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), with an emphasis on their roles in interaction between virus and host, mainly involving stress response, innate immunity, autophagy, and apoptosis. The cross-talking among these pathways is also discussed.

Environmental Risk Assessment of Recombinant Viral Vector Vaccines against SARS-Cov-2

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the causative agent of the coronavirus disease 2019 (COVID-19) pandemic. Over the past months, considerable efforts have been put into developing effective and safe drugs and vaccines against SARS-CoV-2. Various platforms are being used for the development of COVID-19 vaccine candidates: recombinant viral vectors, protein-based vaccines, nucleic acid-based vaccines, and inactivated/attenuated virus. Recombinant viral vector vaccine candidates represent a significant part of those vaccine candidates in clinical development, with two already authorised for use in the European Union and one currently under rolling review by the European Medicines Agency (EMA). Since recombinant viral vector vaccine candidates are considered as genetically modified organisms (GMOs), their regulatory oversight includes besides an assessment of their quality, safety and efficacy, also an environmental risk assessment (ERA). The present article highlights the main characteristics of recombinant viral vector vaccine (candidates) against SARS-CoV-2 in the pipeline and discusses their features from an environmental risk point of view.

Single-replication BM2SR vaccine provides sterilizing immunity and cross-lineage influenza B virus protection in mice

Both influenza A and B viruses cause outbreaks of seasonal influenza resulting in significant morbidity and mortality. There are two antigenically distinct lineages of influenza B virus, Yamagata lineage (YL) and Victoria lineage (VL). Since both B lineages have been co-circulating for years, more than 70% of influenza vaccines currently manufactured are quadrivalent consisting of influenza A (H1N1), influenza A (H3N2), influenza B (YL) and influenza B (VL) antigens. Although quadrivalent influenza vaccines tend to elevate immunity to both influenza B lineages, estimated overall vaccine efficacy against influenza B is still only around 42%. Thus, a more effective influenza B vaccine is needed. To meet this need, we generated BM2-deficient, single-replication (BM2SR) influenza B vaccine viruses that encode surface antigens from influenza B/Wisconsin/01/2010 (B/WI01, YL) and B/Brisbane/60/2008 (B/Bris60, VL) viruses. The BM2SR-WI01 and BM2SR-Bris60 vaccine viruses are replication-deficient in vitro and in vivo, and can only replicate in a cell line that expresses the complementing BM2 protein. Both BM2SR viruses were non-pathogenic to mice, and vaccinated animals showed elevated mucosal and serum antibody responses to both Yamagata and Victoria lineages in addition to cellular responses. Serum antibody responses included lineage-specific hemagglutinin inhibition antibody (HAI) responses as well as responses to the stem region of the hemagglutinin (HA). BM2SR vaccine viruses provided apparent sterilizing immunity to mice against intra- and inter-lineage drifted B virus challenge. The data presented here support the feasibility of BM2SR as a platform for next-generation trivalent influenza vaccine development.

Coronavirus envelope protein: current knowledge

BACKGROUND

Coronaviruses (CoVs) primarily cause enzootic infections in birds and mammals but, in the last few decades, have shown to be capable of infecting humans as well. The outbreak of severe acute respiratory syndrome (SARS) in 2003 and, more recently, Middle-East respiratory syndrome (MERS) has demonstrated the lethality of CoVs when they cross the species barrier and infect humans. A renewed interest in coronaviral research has led to the discovery of several novel human CoVs and since then much progress has been made in understanding the CoV life cycle. The CoV envelope (E) protein is a small, integral membrane protein involved in several aspects of the virus' life cycle, such as assembly, budding, envelope formation, and pathogenesis. Recent studies have expanded on its structural motifs and topology, its functions as an ion-channelling viroporin, and its interactions with both other CoV proteins and host cell proteins.

MAIN BODY

This review aims to establish the current knowledge on CoV E by highlighting the recent progress that has been made and comparing it to previous knowledge. It also compares E to other viral proteins of a similar nature to speculate the relevance of these new findings. Good progress has been made but much still remains unknown and this review has identified some gaps in the current knowledge and made suggestions for consideration in future research.

CONCLUSIONS

The most progress has been made on SARS-CoV E, highlighting specific structural requirements for its functions in the CoV life cycle as well as mechanisms behind its pathogenesis. Data shows that E is involved in critical aspects of the viral life cycle and that CoVs lacking E make promising vaccine candidates. The high mortality rate of certain CoVs, along with their ease of transmission, underpins the need for more research into CoV molecular biology which can aid in the production of effective anti-coronaviral agents for both human CoVs and enzootic CoVs.

Novel Approaches for The Development of Live Attenuated Influenza Vaccines

Influenza virus still represents a considerable threat to global public health, despite the advances in the development and wide use of influenza vaccines. Vaccination with traditional inactivate influenza vaccines (IIV) or live-attenuated influenza vaccines (LAIV) remains the main strategy in the control of annual seasonal epidemics, but it does not offer protection against new influenza viruses with pandemic potential, those that have shifted. Moreover, the continual antigenic drift of seasonal circulating influenza viruses, causing an antigenic mismatch that requires yearly reformulation of seasonal influenza vaccines, seriously compromises vaccine efficacy. Therefore, the quick optimization of vaccine production for seasonal influenza and the development of new vaccine approaches for pandemic viruses is still a challenge for the prevention of influenza infections. Moreover, recent reports have questioned the effectiveness of the current LAIV because of limited protection, mainly against the influenza A virus (IAV) component of the vaccine. Although the reasons for the poor protection efficacy of the LAIV have not yet been elucidated, researchers are encouraged to develop new vaccination approaches that overcome the limitations that are associated with the current LAIV. The discovery and implementation of plasmid-based reverse genetics has been a key advance in the rapid generation of recombinant attenuated influenza viruses that can be used for the development of new and most effective LAIV. In this review, we provide an update regarding the progress that has been made during the last five years in the development of new LAIV and the innovative ways that are being explored as alternatives to the currently licensed LAIV. The safety, immunogenicity, and protection efficacy profile of these new LAIVs reveal their possible implementation in combating influenza infections. However, efforts by vaccine companies and government agencies will be needed for controlled testing and approving, respectively, these new vaccine methodologies for the control of influenza infections.

Egg-based influenza split virus vaccine with monoglycosylation induces cross-strain protection against influenza virus infections

Each year influenza virus infections cause hundreds of thousands of deaths worldwide and a significant level of morbidity with major economic burden. At the present time, vaccination with inactivated virus vaccine produced from embryonated chicken eggs is the most prevalent method to prevent the infections. However, current influenza vaccines are only effective against closely matched circulating strains and must be updated and administered yearly. Therefore, generating a vaccine that can provide broad protection is greatly needed for influenza vaccine development. We have previously shown that vaccination of the major surface glycoprotein hemagglutinin (HA) of influenza virus with a single N-acetylglucosamine at each of the N-glycosylation sites [monoglycosylated HA (HA_mg)] can elicit better cross-protection compared with the fully glycosylated HA (HA_fg). In the current study, we produced monoglycosylated inactivated split H1N1 virus vaccine from chicken eggs by the N-glycosylation process inhibitor kifunensine and the endoglycosidase Endo H, and intramuscularly immunized mice to examine its efficacy. Compared with vaccination of the traditional influenza vaccine with complex glycosylations from eggs, the monoglycosylated split virus vaccine provided better cross-strain protection against a lethal dose of virus challenge in mice. The enhanced antibody responses induced by the monoglycosylated vaccine immunization include higher neutralization activity, higher hemagglutination inhibition, and more HA stem selectivity, as well as, interestingly, higher antibody-dependent cellular cytotoxicity. This study provides a simple and practical procedure to enhance the cross-strain protection of influenza vaccine by removing the outer part of glycans from the virus surface through modifications of the current egg-based process.

Novel influenza vaccine M2SR protects against drifted H1N1 and H3N2 influenza virus challenge in ferrets with pre-existing immunity

Current influenza vaccines do not provide effective protection against heterologous influenza viruses. The ability of the novel M2SR influenza vaccine to protect against drifted influenza viruses was evaluated in naïve ferrets and in ferrets with pre-existing immunity to influenza. In naïve ferrets, M2SR provided similar protection against drifted challenge viruses as the comparator vaccine, FluMist®. However, in ferrets with pre-existing immunity, M2SR provided superior protection than FluMist in two model systems. In the first model, ferrets were infected with influenza A H1N1pdm and influenza B viruses to mimic the diverse influenza exposure in humans. The pre-infected ferrets, seropositive to H1N1pdm and influenza B but seronegative to H3N2, were then vaccinated with H3N2 M2SR or monovalent H3N2 FluMist virus (A/Brisbane/10/2007, clade 1) and challenged 6 weeks later with a drifted H3N2 virus (clade 3C.2a). Antibody titers to Brisbane/10/2007 were higher in M2SR vaccinated ferrets than in FluMist vaccinated ferrets in the pre-infected ferrets whereas the opposite was observed in naïve ferrets. After challenge with drifted H3N2 virus, M2SR provided superior protection than FluMist monovalent vaccine. In the second model, the impact of homologous pre-existing immunity upon vaccine-induced protection was evaluated. Ferrets, pre-infected with H1N1pdm virus, were vaccinated 90 days later with H1N1pdm M2SR or FluMist monovalent vaccine and challenged 6 weeks later with a pre-pandemic seasonal H1N1 virus, A/Brisbane/59/2007 (Bris59). While cross-reactive serum IgG antibodies against the Bris59 HA were detected after vaccination, anti-Bris59 hemagglutination inhibition antibodies were only detected post-challenge. M2SR provided better protection against Bris59 challenge than FluMist suggesting that homologous pre-existing immunity affected FluMist virus to a greater degree than M2SR. These results suggest that the single replication intranasal M2SR vaccine provides effective protection against drifted influenza A viruses not only in naïve ferrets but also in those with pre-existing immunity in contrast to FluMist viruses.

Generation of a Live Attenuated Influenza Vaccine that Elicits Broad Protection in Mice and Ferrets

New influenza vaccines that provide effective and broad protection are desperately needed. Live attenuated viruses are attractive vaccine candidates because they can elicit both humoral and cellular immune responses. However, recent formulations of live attenuated influenza vaccines (LAIVs) have not been protective. We combined high-coverage transposon mutagenesis of influenza virus with a rapid high-throughput screening for attenuation to generate W7-791, a live attenuated mutant virus strain. W7-791 produced only a transient asymptomatic infection in adult and neonatal mice even at doses 100-fold higher than the LD₅₀ of the parent strain. A single administration of W7-791 conferred full protection to mice against lethal challenge with H1N1, H3N2, and H5N1 strains, and improved viral clearance in ferrets. Adoptive transfer of T cells from W7-791-immunized mice conferred heterologous protection, indicating a role for T cell-mediated immunity. These studies present an LAIV development strategy to rapidly generate and screen entire libraries of viral clones.

Development of an Alternative Modified Live Influenza B Virus Vaccine

Influenza B virus (IBV) is considered a major human pathogen, responsible for seasonal epidemics of acute respiratory illness. Two antigenically distinct IBV hemagglutinin (HA) lineages cocirculate worldwide with little cross-reactivity. Live attenuated influenza virus (LAIV) vaccines have been shown to provide better cross-protective immune responses than inactivated vaccines by eliciting local mucosal immunity and systemic B cell- and T cell-mediated memory responses. We have shown previously that incorporation of temperature-sensitive (ts) mutations into the PB1 and PB2 subunits along with a modified HA epitope tag in the C terminus of PB1 resulted in influenza A viruses (IAV) that are safe and effective as modified live attenuated (att) virus vaccines (IAV att). We explored whether analogous mutations in the IBV polymerase subunits would result in a stable virus with an att phenotype. The PB1 subunit of the influenza B/Brisbane/60/2008 strain was used to incorporate ts mutations and a C-terminal HA tag. Such modifications resulted in a B/Bris att strain with ts characteristics in vitro and an att phenotype in vivo Vaccination studies in mice showed that a single dose of the B/Bris att candidate stimulated sterilizing immunity against lethal homologous challenge and complete protection against heterologous challenge. These studies show the potential of an alternative LAIV platform for the development of IBV vaccines.IMPORTANCE A number of issues with regard to the effectiveness of the LAIV vaccine licensed in the United States (FluMist) have arisen over the past three seasons (2013-2014, 2014-2015, and 2015-2016). While the reasons for the limited robustness of the vaccine-elicited immune response remain controversial, this problem highlights the critical importance of continued investment in LAIV development and creates an opportunity to improve current strategies so as to develop more efficacious vaccines. Our laboratory has developed an alternative strategy, the incorporation of 2 amino acid mutations and a modified HA tag at the C terminus of PB1, which is sufficient to attenuate the IBV. As a LAIV, this novel vaccine provides complete protection against IBV strains. The availability of attenuated IAV and IBV backbones based on contemporary strains offers alternative platforms for the development of LAIVs that may overcome current limitations.

Influenza A surface glycosylation and vaccine design

We have shown that glycosylation of influenza A virus (IAV) hemagglutinin (HA), especially at position N-27, is crucial for HA folding and virus survival. However, it is not known whether the glycosylation of HA and the other two major IAV surface glycoproteins, neuraminidase (NA) and M2 ion channel, is essential for the replication of IAV. Here, we show that glycosylation of HA at N-142 modulates virus infectivity and host immune response. Glycosylation of NA in the stalk region affects its structure, activity, and specificity, thereby modulating virus release and virulence, and glycosylation at the catalytic domain affects its thermostability; however, glycosylation of M2 had no effect on its function. In addition, using IAV without the stalk and catalytic domains of NA as a live attenuated vaccine was shown to confer a strong IAV-specific CD8⁺ T-cell response and a strong cross-strain as well as cross-subtype protection against various virus strains.

M2SR, a novel live single replication influenza virus vaccine, provides effective heterosubtypic protection in mice

Despite the annual public health burden of seasonal influenza and the continuing threat of a global pandemic posed by the emergence of highly pathogenic/pandemic strains, conventional influenza vaccines do not provide universal protection, and exhibit suboptimal efficacy rates, even when they are well matched to circulating strains. To address the need for a highly effective universal influenza vaccine, we have developed a novel M2-deficient single replication vaccine virus (M2SR) that induces strong cross-protective immunity against multiple influenza strains in mice. M2SR is able to infect cells and expresses all viral proteins except M2, but is unable to generate progeny virus. M2SR generated from influenza A/Puerto Rico/8/34 (H1N1) protected mice against lethal challenge with influenza A/Puerto Rico/8/34 (H1N1, homosubtypic) and influenza A/Aichi/2/1968 (H3N2, heterosubtypic). The vaccine induced strong systemic and mucosal antibody responses of both IgA and IgG classes. Strong virus-specific T cell responses were also induced. Following heterologous challenge, significant numbers of IFN-γ-producing CD8 T cells, with effector or effector/memory phenotypes and specific for conserved viral epitopes, were observed in the lungs of vaccinated mice. A substantial proportion of the CD8 T cells expressed Granzyme B, suggesting that they were capable of killing virus-infected cells. Thus, our data suggest that M2-deficient influenza viruses represent a promising new approach for developing a universal influenza vaccine.

Severe acute respiratory syndrome coronavirus E protein transports calcium ions and activates the NLRP3 inflammasome

Severe acute respiratory syndrome coronavirus (SARS-CoV) envelope (E) protein is a viroporin involved in virulence. E protein ion channel (IC) activity is specifically correlated with enhanced pulmonary damage, edema accumulation and death. IL-1β driven proinflammation is associated with those pathological signatures, however its link to IC activity remains unknown. In this report, we demonstrate that SARS-CoV E protein forms protein–lipid channels in ERGIC/Golgi membranes that are permeable to calcium ions, a highly relevant feature never reported before. Calcium ions together with pH modulated E protein pore charge and selectivity. Interestingly, E protein IC activity boosted the activation of the NLRP3 inflammasome, leading to IL-1β overproduction. Calcium transport through the E protein IC was the main trigger of this process. These findings strikingly link SARS-CoV E protein IC induced ionic disturbances at the cell level to immunopathological consequences and disease worsening in the infected organism.

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SARS-CoV E protein forms calcium ion channels, a novel highly relevant function.

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Transport of calcium ions through E protein channel stimulates the inflammasome.

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Inflammasome derived exacerbated proinflammation causes SARS worsening.

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E protein ion channel and its driven proinflammation may be targets to treat SARS.

Relevance of Viroporin Ion Channel Activity on Viral Replication and Pathogenesis

Modification of host-cell ionic content is a significant issue for viruses, as several viral proteins displaying ion channel activity, named viroporins, have been identified. Viroporins interact with different cellular membranes and self-assemble forming ion conductive pores. In general, these channels display mild ion selectivity, and, eventually, membrane lipids play key structural and functional roles in the pore. Viroporins stimulate virus production through different mechanisms, and ion channel conductivity has been proved particularly relevant in several cases. Key stages of the viral cycle such as virus uncoating, transport and maturation are ion-influenced processes in many viral species. Besides boosting virus propagation, viroporins have also been associated with pathogenesis. Linking pathogenesis either to the ion conductivity or to other functions of viroporins has been elusive for a long time. This article summarizes novel pathways leading to disease stimulated by viroporin ion conduction, such as inflammasome driven immunopathology.

Viroporins, Examples of the Two-Stage Membrane Protein Folding Model

Viroporins are small, α-helical, hydrophobic virus encoded proteins, engineered to form homo-oligomeric hydrophilic pores in the host membrane. Viroporins participate in multiple steps of the viral life cycle, from entry to budding. As any other membrane protein, viroporins have to find the way to bury their hydrophobic regions into the lipid bilayer. Once within the membrane, the hydrophobic helices of viroporins interact with each other to form higher ordered structures required to correctly perform their porating activities. This two-step process resembles the two-stage model proposed for membrane protein folding by Engelman and Poppot. In this review we use the membrane protein folding model as a leading thread to analyze the mechanism and forces behind the membrane insertion and folding of viroporins. We start by describing the transmembrane segment architecture of viroporins, including the number and sequence characteristics of their membrane-spanning domains. Next, we connect the differences found among viroporin families to their viral genome organization, and finalize focusing on the pathways used by viroporins in their way to the membrane and on the transmembrane helix-helix interactions required to achieve proper folding and assembly.

Design of alternative live attenuated influenza virus vaccines

Each year due to the ever-evolving nature of influenza, new influenza vaccines must be produced to provide protection against the influenza viruses in circulation. Currently, there are two mainstream strategies to generate seasonal influenza vaccines: inactivated and live-attenuated. Inactivated vaccines are non-replicating forms of whole influenza virus, while live-attenuated vaccines are viruses modified to be replication impaired. Although it is widely believed that by inducing both mucosal and humoral immune responses the live-attenuated vaccine provides better protection than that of the inactivated vaccine, there are large populations of individuals who cannot safely receive the LAIV vaccine. Thus, safer LAIV vaccines are needed to provide adequate protection to these populations. Improvement is also needed in the area of vaccine production. Current strategies relying on traditional tissue culture-based and egg-based methods are slow and delay production time. This chapter describes experimental vaccine generation and production strategies that address the deficiencies in current methods for potential human and agricultural use.

Induction of CD8 T cell heterologous protection by a single dose of single-cycle infectious influenza virus

The effector functions of specific CD8 T cells are crucial in mediating influenza heterologous protection. However, new approaches for influenza vaccines that can trigger effective CD8 T cell responses have not been extensively explored. We report here the generation of single-cycle infectious influenza virus that lacks a functional hemagglutinin (HA) gene on an X31 genetic background and demonstrate its potential for triggering protective CD8 T cell immunity against heterologous influenza virus challenge. In vitro, X31-sciIV can infect MDCK cells, but infectious virions are not produced unless HA is transcomplemented. In vivo, intranasal immunization with X31-sciIV does not cause any clinical symptoms in mice but generates influenza-specific CD8 T cells in lymphoid (mediastinal lymph nodes and spleen) and nonlymphoid tissues, including lung and bronchoalveolar lavage fluid, as measured by H2-Db NP366 and PA224 tetramer staining. In addition, a significant proportion of X31-sciIV-induced antigen-specific respiratory CD8 T cells expressed VLA-1, a marker that is associated with heterologous influenza protection. Further, these influenza-specific CD8 T cells produce antiviral cytokines when stimulated with NP366 and PA224 peptides, indicating that CD8 T cells triggered by X31-sciIV are functional. When challenged with a lethal dose of heterologous PR8 virus, X31-sciIV-primed mice were fully protected from death. However, when CD8 T cells were depleted after priming or before priming, mice could not effectively control virus replication or survive the lethal challenge, indicating that X31-sciIV-induced memory CD8 T cells mediate the heterologous protection. Thus, our results demonstrate the potential for sciIV as a CD8 T cell-inducing vaccine. Importance: One of the challenges for influenza prevention is the existence of multiple influenza virus subtypes and variants and the fact that new strains can emerge yearly. Numerous studies have indicated that the effector functions of specific CD8 T cells are crucial in mediating influenza heterologous protection. However, influenza vaccines that can trigger effective CD8 T cell responses for heterologous protection have not been developed. We report here the generation of an X31 (H3N2) virus-derived single-cycle infectious influenza virus, X31-sciIV. A one-dose immunization with X31-sciIV is capable of inducing functional influenza virus-specific CD8 T cells that can be recruited into respiratory tissues and provide protection against lethal heterologous challenge. Without these cells, protection against lethal challenge was essentially lost. Our data indicate that an influenza vaccine that primarily relies on CD8 T cells for protection could be developed.

Severe acute respiratory syndrome coronavirus envelope protein ion channel activity promotes virus fitness and pathogenesis

Deletion of Severe Acute Respiratory Syndrome Coronavirus (SARS-CoV) envelope (E) gene attenuates the virus. E gene encodes a small multifunctional protein that possesses ion channel (IC) activity, an important function in virus-host interaction. To test the contribution of E protein IC activity in virus pathogenesis, two recombinant mouse-adapted SARS-CoVs, each containing one single amino acid mutation that suppressed ion conductivity, were engineered. After serial infections, mutant viruses, in general, incorporated compensatory mutations within E gene that rendered active ion channels. Furthermore, IC activity conferred better fitness in competition assays, suggesting that ion conductivity represents an advantage for the virus. Interestingly, mice infected with viruses displaying E protein IC activity, either with the wild-type E protein sequence or with the revertants that restored ion transport, rapidly lost weight and died. In contrast, mice infected with mutants lacking IC activity, which did not incorporate mutations within E gene during the experiment, recovered from disease and most survived. Knocking down E protein IC activity did not significantly affect virus growth in infected mice but decreased edema accumulation, the major determinant of acute respiratory distress syndrome (ARDS) leading to death. Reduced edema correlated with lung epithelia integrity and proper localization of Na⁺/K⁺ ATPase, which participates in edema resolution. Levels of inflammasome-activated IL-1β were reduced in the lung airways of the animals infected with viruses lacking E protein IC activity, indicating that E protein IC function is required for inflammasome activation. Reduction of IL-1β was accompanied by diminished amounts of TNF and IL-6 in the absence of E protein ion conductivity. All these key cytokines promote the progression of lung damage and ARDS pathology. In conclusion, E protein IC activity represents a new determinant for SARS-CoV virulence.

Several highly pathogenic viruses encode small transmembrane proteins with ion-conduction properties named viroporins. Viroporins are generally involved in virus production and maturation processes, which many times are achieved by altering the ion homeostasis of cell organelles. Cells have evolved mechanisms to sense these imbalances in ion concentrations as a danger signal, and consequently trigger the innate immune system. Recently, it has been demonstrated that viroporins are inducers of cytosolic macromolecular complexes named inflammasomes that trigger the activation of key inflammatory cytokines such as IL-1β. The repercussions of this system in viral pathogenesis or disease outcome are currently being explored. SARS-CoV infection induces an uncontrolled inflammatory response leading to pulmonary damage, edema accumulation, severe hypoxemia and eventually death. In this study, we report that SARS-CoV E protein ion channel activity is a determinant of virulence, as the elimination of this function attenuated the virus, reducing the harmful inflammatory cytokine burst produced after infection, in which inflammasome activation plays a critical role. This led to less pulmonary damage and to disease resolution. These novel findings may be of relevance for other viral infections and can possibly be translated in order to find therapies for their associated diseases.

Viral miniproteins

Many viruses encode short transmembrane proteins that play vital roles in virus replication or virulence. Because many of these proteins are less than 50 amino acids long and not homologous to cellular proteins, their open reading frames were often overlooked during the initial annotation of viral genomes. Some of these proteins oligomerize in membranes and form ion channels. Other miniproteins bind to cellular transmembrane proteins and modulate their activity, whereas still others have an unknown mechanism of action. Based on the underlying principles of transmembrane miniprotein structure, it is possible to build artificial small transmembrane proteins that modulate a variety of biological processes. These findings suggest that short transmembrane proteins provide a versatile mechanism to regulate a wide range of cellular activities, and we speculate that cells also express many similar proteins that have not yet been discovered.

DNAVaxDB: the first web-based DNA vaccine database and its data analysis

Since the first DNA vaccine studies were done in the 1990s, thousands more studies have followed. Here we report the development and analysis of DNAVaxDB (http://www.violinet.org/dnavaxdb), the first publically available web-based DNA vaccine database that curates, stores, and analyzes experimentally verified DNA vaccines, DNA vaccine plasmid vectors, and protective antigens used in DNA vaccines. All data in DNAVaxDB are annotated from reliable resources, particularly peer-reviewed articles. Among over 140 DNA vaccine plasmids, some plasmids were more frequently used in one type of pathogen than others; for example, pCMVi-UB for G- bacterial DNA vaccines, and pCAGGS for viral DNA vaccines. Presently, over 400 DNA vaccines containing over 370 protective antigens from over 90 infectious and non-infectious diseases have been curated in DNAVaxDB. While extracellular and bacterial cell surface proteins and adhesin proteins were frequently used for DNA vaccine development, the majority of protective antigens used in Chlamydophila DNA vaccines are localized to the inner portion of the cell. The DNA vaccine priming, other vaccine boosting vaccination regimen has been widely used to induce protection against infection of different pathogens such as HIV. Parasitic and cancer DNA vaccines were also systematically analyzed. User-friendly web query and visualization interfaces are available in DNAVaxDB for interactive data search. To support data exchange, the information of DNA vaccines, plasmids, and protective antigens is stored in the Vaccine Ontology (VO). DNAVaxDB is targeted to become a timely and vital source of DNA vaccines and related data and facilitate advanced DNA vaccine research and development.

Bluetongue virus nonstructural protein NS3/NS3a is not essential for virus replication

Orbiviruses form the largest genus of the family Reoviridae consisting of at least 23 different virus species. One of these is the bluetongue virus (BTV) and causes severe hemorrhagic disease in ruminants, and is transmitted by bites of Culicoides midges. BTV is a non-enveloped virus which is released from infected cells by cell lysis and/or a unique budding process induced by nonstructural protein NS3/NS3a encoded by genome segment 10 (Seg-10). Presence of both NS3 and NS3a is highly conserved in Culicoides borne orbiviruses which is suggesting an essential role in virus replication. We used reverse genetics to generate BTV mutants to study the function of NS3/NS3a in virus replication. Initially, BTV with small insertions in Seg-10 showed no CPE but after several passages these BTV mutants reverted to CPE phenotype comparable to wtBTV, and NS3/NS3a expression returned by repair of the ORF. These results show that there is a strong selection for functional NS3/NS3a. To abolish NS3 and/or NS3a expression, Seg-10 with one or two mutated start codons (mutAUG1, mutAUG2 and mutAUG1+2) were used to generate BTV mutants. Surprisingly, all three BTV mutants were generated and the respective AUG^Met→GCC^Ala mutations were maintained. The lack of expression of NS3, NS3a, or both proteins was confirmed by westernblot analysis and immunostaining of infected cells with NS3/NS3a Mabs. Growth of mutAUG1 and mutAUG1+2 virus in BSR cells was retarded in both insect and mammalian cells, and particularly virus release from insect cells was strongly reduced. Our findings now enable research on the role of RNA sequences of Seg-10 independent of known gene products, and on the function of NS3/NS3a proteins in both types of cells as well as in the host and insect vector.

Host defense mechanism-based rational design of live vaccine

Live attenuated vaccine (LAV), mimicking natural infection, provides an excellent protection against microbial infection. The development of LAV, however, still remains highly empirical and the rational design of clinically useful LAV is scarcely available. Apoptosis and caspase activation are general host antiviral responses in virus-infected cells. Utilizing these tightly regulated host defense mechanisms, we present a novel apoptosis-triggered attenuation of viral virulence as a rational design of live attenuated vaccine with desired levels of safety, efficacy, and productivity. Mutant influenza viruses carrying caspase recognition motifs in viral NP and the interferon-antagonist NS1 proteins were highly attenuated both in vitro and in vivo by caspase-mediated cleavage of those proteins in infected cells. Both viral replication and interferon-resistance were substantially reduced, resulting in a marked attenuation of virulence of the virus. Despite pronounced attenuation, the viruses demonstrated high growth phenotype in embryonated eggs at lower temperature, ensuring its productivity. A single dose vaccination with the mutant virus elicited high levels of systemic and mucosal antibody responses and provided complete protection against both homologous and heterologous lethal challenges in mouse model. While providing a practical means to generate seasonal or pandemic influenza live vaccines, the sensitization of viral proteins to pathogen-triggered apoptotic signals presents a potentially universal, mechanism-based rational design of live vaccines against many viral infections.

A (H1N1) pdm09 HA D222 variants associated with severity and mortality in patients during a second wave in Mexico

Background

Pandemic type A (H1N1) influenza arose in early 2009, probably in Mexico and the United States, and reappeared in North America in September for seven more months. An amino acid substitution in the hemagglutinin (HA), D222G, has been reported in a significant proportion of patients with a severe and fatal outcome. We studied the prevalence of HA222 substitutions in patients in Mexico during the second wave and its association with clinical outcome and pathogenicity in a mouse model.

Methods

The nucleotide sequences of hemagglutinin (HA) from viruses collected from 77 patients were determined including 50 severe and fatal cases and 27 ambulatory cases. Deep sequencing was done on 5 samples from severe or fatal cases in order to determine the quasispecies proportion. Weight loss and mortality due to infection with cultured influenza viruses were analyzed in a mouse model.

Results

Viruses from 14 out of 50 hospitalized patients (28%) had a non aspartic acid residue at the HA 222 position (nD222), while all 27 ambulatory patients had D222 (p = 0.0014). G222 was detected as sole species or in coexistence with N222 and D222 in 12 patients with severe disease including 8 who died. N222 in coexistence with D222 was detected in 1 patient who died and co-occurrence of A222 and V222, together with D222, was detected in another patient who died. The patients with a nD222 residue had higher mortality (71.4%), compared to the group with only D222 (22.2%, p = 0.0008). Four of the 14 viruses from hospitalized patients were cultured and intranasally infected into mice. Two viruses with G222 were lethal while a third virus, with G222, caused only mild illness in mice similar to the fourth virus that contained D222.

Conclusions

We confirm the elevated incidence of HA222 (H1N1)pdm09 variants in severe disease and mortality. Both clinical and mouse infection data support the idea that nD222 mutations contribute to increased severity of disease but additional determinants in disease outcome may be present.

Principles underlying rational design of live attenuated influenza vaccines

Despite recent innovative advances in molecular virology and the developments of vaccines, influenza virus remains a serious burden for human health. Vaccination has been considered a primary countermeasure for prevention of influenza infection. Live attenuated influenza vaccines (LAIVs) are particularly attracting attention as an effective strategy due to several advantages over inactivated vaccines. Cold-adaptation, as a classical means for attenuating viral virulence, has been successfully used for generating safe and effective donor strains of LAIVs against seasonal epidemics and occasional pandemics. Recently, the advent of reverse genetics technique expedited a variety of rational strategies to broaden the pool of LAIVs. Considering the breadth of antigenic diversity of influenza virus, the pool of LAIVs is likely to equip us with better options for controlling influenza pandemics. With a brief reflection on classical attenuating strategies used at the initial stage of development of LAIVs, especially on the principles underlying the development of cold-adapted LAIVs, we further discuss and outline other attenuation strategies especially with respect to the rationales for attenuation, and their practicality for mass production. Finally, we propose important considerations for a rational vaccine design, which will provide us with practical guidelines for improving the safety and effectiveness of LAIVs.

Viroporins: structure and biological functions

Viroporins are small virally encoded hydrophobic proteins that oligomerize in the membrane of host cells, leading to the formation of hydrophilic pores. This activity modifies several cellular functions, including membrane permeability, Ca²⁺ homeostasis, membrane remodelling and glycoprotein trafficking.

A classification scheme for viroporins is proposed on the basis of their structure and membrane topology. Thus, class I and class II viroporins are defined according to the number of transmembrane domains in the protein (one and two, respectively), and subclasses are defined according to their orientation in the membrane.

The main function of viroporins during viral replication is to participate in virion morphogenesis and release from host cells. In addition, some viroporins are involved in viral entry and genome replication.

The structure and activity of several viroporins, such as picornavirus protein 2B (P2B), influenza A virus matrix protein 2 (M2), hepatitis C virus p7 and HIV-1 viral protein U (Vpu), have been analysed in detail.

New members of this expanding family of viral proteins have been described, from both RNA and DNA viruses. In addition to having a common general structure, all of these new viroporins have the ability to increase membrane permeability.

Viroporins represent ideal targets to block viral replication and the spread of infection. Although a number of selective inhibitors of viroporin ion channels have been analysed in detail, optimized screening systems promise to provide new and more potent antiviral compounds in the near future.

Viroporins belong to a growing family of virally encoded proteins that form aqueous channels in the membranes of host cells. Here, Carrasco and colleagues review the structure and diverse biological functions of these proteins during the viral life cycle, as well as their potential as antiviral therapeutic targets.

Viroporins are small, hydrophobic proteins that are encoded by a wide range of clinically relevant animal viruses. When these proteins oligomerize in host cell membranes, they form hydrophilic pores that disrupt a number of physiological properties of the cell. Viroporins are crucial for viral pathogenicity owing to their involvement in several diverse steps of the viral life cycle. Thus, these viral proteins, which include influenza A virus matrix protein 2 (M2), HIV-1 viral protein U (Vpu) and hepatitis C virus p7, represent ideal targets for therapeutic intervention, and several compounds that block their pore-forming activity have been identified. Here, we review recent studies in the field that have advanced our knowledge of the structure and function of this expanding family of viral proteins.

Perforin- and granzyme-mediated cytotoxic effector functions are essential for protection against Francisella tularensis following vaccination by the defined F. tularensis subsp. novicida ΔfopC vaccine strain

A licensed vaccine against Francisella tularensis is currently not available. Two Francisella tularensis subsp. novicida (herein referred to by its earlier name, Francisella novicida) attenuated strains, the ΔiglB and ΔfopC strains, have previously been evaluated as potential vaccine candidates against pneumonic tularemia in experimental animals. F. novicida ΔiglB, a Francisella pathogenicity island (FPI) mutant, is deficient in phagosomal escape and intracellular growth, whereas F. novicida ΔfopC, lacking the outer membrane lipoprotein FopC, which is required for evasion of gamma interferon (IFN-γ)-mediated signaling, is able to escape and replicate in the cytosol. To dissect the difference in protective immune mechanisms conferred by these two vaccine strains, we examined the efficacy of the F. novicida ΔiglB and ΔfopC mutants against pulmonary live-vaccine-strain (LVS) challenge and found that both strains provided comparable protection in wild-type, major histocompatibility complex class I (MHC I) knockout, and MHC II knockout mice. However, F. novicida ΔfopC-vaccinated but not F. novicida ΔiglB-vaccinated perforin-deficient mice were more susceptible and exhibited greater bacterial burdens than similarly vaccinated wild-type mice. Moreover, perforin produced by natural killer (NK) cells and release of granzyme contributed to inhibition of LVS replication within macrophages. This NK cell-mediated LVS inhibition was enhanced with anti-F. novicida ΔfopC immune serum, suggesting antibody-dependent cell-mediated cytotoxicity (ADCC) in F. novicida ΔfopC-mediated protection. Overall, this study provides additional immunological insight into the basis for protection conferred by live attenuated F. novicida strains with different phenotypes and supports further investigation of this organism as a vaccine platform for tularemia.

New strategies for the development of H5N1 subtype influenza vaccines: progress and challenges

The emergence and spread of highly pathogenic avian influenza (H5N1) viruses among poultry in Asia, the Middle East, and Africa have fueled concerns of a possible human pandemic, and spurred efforts towards developing vaccines against H5N1 influenza viruses, as well as improving vaccine production methods. In recent years, promising experimental reverse genetics-derived H5N1 live attenuated vaccines have been generated and characterized, including vaccines that are attenuated through temperature-sensitive mutation, modulation of the interferon antagonist protein, or disruption of the M2 protein. Live attenuated influenza virus vaccines based on each of these modalities have conferred protection against homologous and heterologous challenge in animal models of influenza virus infection. Alternative vaccine strategies that do not require the use of live virus, such as virus-like particle (VLP) and DNA-based vaccines, have also been vigorously pursued in recent years. Studies have demonstrated that influenza VLP vaccination can confer homologous and heterologous protection from lethal challenge in a mouse model of infection. There have also been improvements in the formulation and production of vaccines following concerns over the threat of H5N1 influenza viruses. The use of novel substrates for the growth of vaccine virus stocks has been intensively researched in recent years, and several candidate cell culture-based systems for vaccine amplification have emerged, including production systems based on Madin-Darby canine kidney, Vero, and PerC6 cell lines. Such systems promise increased scalability of product, and reduced reliance on embryonated chicken eggs as a growth substrate. Studies into the use of adjuvants have shown that oil-in-water-based adjuvants can improve the immunogenicity of inactivated influenza vaccines and conserve antigen in such formulations. Finally, efforts to develop more broadly cross-protective immunization strategies through the inclusion of conserved influenza virus antigens in vaccines have led to experimental vaccines based on the influenza hemagglutinin (HA) stem domain. Such vaccines have been shown to confer protection from lethal challenge in mouse models of influenza virus infection. Through further development, vaccines based on the HA stem have the potential to protect vaccinated individuals against unanticipated pandemic and epidemic influenza virus strains. Overall, recent advances in experimental vaccines and in vaccine production processes provide the potential to lower mortality and morbidity resulting from influenza infection.

Efficacy of live attenuated vaccines against 2009 pandemic H1N1 influenza in ferrets

The advent of the H1N1 influenza pandemic (pH1N1) in 2009 triggered the rapid production of pandemic influenza vaccines, since seasonal influenza vaccines were expected and demonstrated not to provide significant cross-protection against the newly emerged pandemic virus. To increase vaccine production capacity and further evaluate the effectiveness of different candidate pandemic influenza vaccines, the World Health Organization stimulated the evaluation of different vaccination concepts including the use of live attenuated influenza vaccines (LAIVs). Therefore, we have immunized ferrets intranasally with a single dose of pH1N1-LAIV from different manufacturers. They all induced adequate serum HI antibody titers in the ferrets and protected them against intratracheal wild-type pH1N1 virus challenge: pH1N1 virus replication in the upper respiratory tract and lungs was reduced and no disease signs or severe broncho-interstitial pneumonia were observed in any of the vaccinated ferrets. These data together with the relatively efficient production process emphasize the potential of the LAIV concept for pandemic preparedness.

Replication-incompetent influenza A viruses that stably express a foreign gene

A biologically contained influenza A virus that stably expresses a foreign gene can be effectively traced, used to generate a novel multivalent vaccine and have its replication easily assessed, all while satisfying safety concerns regarding pathogenicity or reversion. This study generated a PB2-knockout (PB2-KO) influenza virus that harboured the GFP reporter gene in the coding region of its PB2 viral RNA (vRNA). Replication of the PB2-KO virus was restricted to a cell line stably expressing the PB2 protein. The GFP gene-encoding PB2 vRNA was stably incorporated into progeny viruses during replication in PB2-expressing cells. The GFP gene was expressed in virus-infected cells with no evidence of recombination between the recombinant PB2 vRNA and the PB2 protein mRNA. Furthermore, other reporter genes and the haemagglutinin and neuraminidase genes of different virus strains were accommodated by the PB2-KO virus. Finally, the PB2-KO virus was used to establish an improved assay to screen neutralizing antibodies against influenza viruses by using reporter gene expression as an indicator of virus infection rather than by observing cytopathic effect. These results indicate that the PB2-KO virus has the potential to be a valuable tool for basic and applied influenza virus research.

Cross-protective immunity to influenza A viruses

Antigenic changes in influenza virus occur gradually, owing to mutations (antigenic drift), and abruptly, owing to reassortment among subtypes (antigenic shift). Availability of strain-matched vaccines often lags behind these changes, resulting in a shortfall in public health. In animal models, cross-protection by vaccines based on conserved antigens does not completely prevent infection, but greatly reduces morbidity, mortality, virus replication and, thus, viral shedding and spread. Such immunity is especially effective and long-lasting with mucosal administration. Cross-protective immunity in humans is controversial, but is suggested by some epidemiological findings. 'Universal' vaccines protective against all influenza A viruses might substantially reduce severity of infection and limit spread of disease during outbreaks. These vaccines could be used 'off the shelf' early in an outbreak or pandemic, before strain-matched vaccines are available.

Role of the CM2 protein in the influenza C virus replication cycle

CM2 is the second membrane protein of influenza C virus. Although its biochemical characteristics, coding strategy, and properties as an ion channel have been extensively studied, the role(s) of CM2 in the virus replication cycle remains to be clarified. In order to elucidate this role, in the present study we generated CM2-deficient influenza C virus-like particles (VLPs) and examined the VLP-producing 293T cells, VLPs, and VLP-infected HMV-II cells. Quantification of viral RNA (vRNA) in the VLPs by real-time PCR revealed that the CM2-deficient VLPs contain approximately one-third of the vRNA found in wild-type VLPs although no significant differences were detected in the expression levels of viral components in VLP-producing cells or in the number and morphology of the generated VLPs. This finding suggests that CM2 is involved in the genome packaging process into VLPs. Furthermore, HMV-II cells infected with CM2-deficient VLPs exhibited significantly reduced reporter gene expression. Although CM2-deficient VLPs could be internalized into HMV-II cells as efficiently as wild-type VLPs, a smaller amount of vRNA was detected in the nuclear fraction of CM2-deficient VLP-infected cells than in that of wild-type VLP-infected cells, suggesting that the uncoating process of the CM2-deficient VLPs in the infected cells did not proceed in an appropriate manner. Taken together, the data obtained in the present study indicate that CM2 has a potential role in the genome packaging and uncoating processes of the virus replication cycle.

An M2 cytoplasmic tail mutant as a live attenuated influenza vaccine against pandemic (H1N1) 2009 influenza virus

The 2009 influenza pandemic brought home the importance of vaccines in infection control. Previously, we demonstrated an M2 cytoplasmic tail mutant H5N1 influenza virus could serve as a live-attenuated vaccine. Here, we adapted that strategy, generating a mutant pandemic (H1N1) 2009 virus that grew well in cell culture, but replicated less well in mice than did wild-type virus. The mutant virus elicited sterile immunity in mice, completely protecting them from challenge with a pandemic (H1N1) 2009 virus. Our results indicate that M2 cytoplasmic tail mutants are suitable for live-attenuated vaccines against pandemic viruses.

Inflammasomes as mediators of immunity against influenza virus

Influenza viruses infect a wide range of avian and mammalian host species including humans. Influenza viruses are a major cause of human respiratory infections and mortality. The innate immune system recognizes influenza viruses through multiple mechanisms. These include endosomal recognition through the Toll-like receptor 7 (TLR7) and cytosolic recognition through the retinoic acid inducible gene I (RIG-I). Recent studies also identified the role of nucleotide binding oligomerization domain (NOD)-like receptors (NLRs) in innate detection of influenza viruses, leading to the activation of the inflammasomes. Here, we review the cellular and molecular mechanisms by which influenza virus infection leads to inflammasome activation, and discuss the consequences of such activation in innate and adaptive immune defense against influenza viruses.

High level of genetic compatibility between swine-origin H1N1 and highly pathogenic avian H5N1 influenza viruses

Reassortment is an important mechanism for the evolution of influenza viruses. Here, we coinfected cultured cells with the pandemic swine-origin influenza virus (S-OIV) and a contemporary H5N1 virus and found that these two viruses have high genetic compatibility. Studies of human lung cell lines indicated that some reassortants had better growth kinetics than their parental viruses. We conclude that reassortment between these two viruses can occur and could create pandemic H5N1 viruses.

Influenza H1N1 A/Solomon Island/3/06 virus receptor binding specificity correlates with virus pathogenicity, antigenicity, and immunogenicity in ferrets

Influenza viruses attach to cells via a sialic acid moiety (sialic acid receptor) that is alpha2-3 linked or alpha2-6 linked to galactose (alpha2-3SAL or alpha2-6SAL); sialic acid acts as a receptor for the virus. Using lectin staining, we demonstrated that the alpha2-6SAL configuration is predominant in the respiratory tract of ferrets, including trachea, bronchus, and lung alveolus tissues. Recombinant wild-type (rWT) influenza A/Solomon Island/3/06 (SI06) (H1N1) viruses were constructed to assess the impact of the hemagglutinin (HA) variations (amino acids 190 or 226) identified in natural variants on virus replication in the upper and lower respiratory tract of ferrets, as well as virus antigenicity and immunogenicity. A single amino acid change at residue 226 (from Gln to Arg) in the HA of SI06 resulted in the complete loss of binding to alpha2-6SAL and a concomitant loss of the virus's ability to replicate in the lower respiratory tract of ferrets. In contrast, the virus with Gln226 in the HA protein has a receptor binding preference for alpha2-6SAL and replicates efficiently in the lungs. There was a good correlation between viral replication in the lungs of ferrets and disease symptoms. In addition, we also showed that the 190 and 226 residues affected viral antigenicity and immunogenicity. Our data emphasize the necessity of thoroughly assessing wild-type influenza viruses for their suitability as reference strains and for carefully selecting the HA antigen for vaccine production during annual influenza vaccine evaluation processes.

Several hemagglutinins of same serotype for induction of broad immunity against influenza A virus antigenic drift variants: WO2008048984

The respiratory disease influenza gives rise to severe public health concerns. During inter-pandemic periods, the constant problem of the annually recurring seasonal influenza is perpetuated by the ability of influenza viruses to alter their surface antigens continuously (antigenic drift). Therefore, vaccines eliciting broad immunity against drift variants still remain a major objective in vaccine development. The patent WO2008048984 evaluated in this article claims an approach which aims to elicit homosubtypic protection against drift variants by simultaneous vaccination with several hemagglutinins (HAs) of the same serotype. The proposed multivalent vaccine based on simultaneous administration of several HAs, the results obtained from mice immunization studies and the implications of this concept are discussed in light of their relevance to application in humans. This proof-of-principle study suggests that a multivalent HA vaccine could elicit broad protection against drifted virus variants of one HA subtype. In the future, the dependence of broad efficacy on large antigenic distances among the HAs used for immunization as well as the antigenic distance between the HAs administered to that of the challenge virus, the immunological correlates of broad efficacy, and the suitability of this concept for domestic animals and humans remain to be investigated.