The development and characterization of H5 influenza virus vaccines derived from a 2003 human isolate

Determination of cold-adapted influenza virus (Orthomyxoviridae: Alphainfluenzavirus) polymerase activity by the minigenome method with a fluorescent protein

INTRODUCTION

Polymerase proteins PB1 and PB2 determine the cold-adapted phenotype of the influenza virus A/Krasnodar/101/35/59 (H2N2), as was shown earlier.

OBJECTIVE

The development of the reporter construct to determine the activity of viral polymerase at 33 and 37 °C using the minigenome method.

MATERIALS AND METHODS

Co-transfection of Cos-1 cells with pHW2000 plasmids expressing viral polymerase proteins PB1, PB2, PA, NP (minigenome) and reporter construct.

RESULTS

Based on segment 8, two reporter constructs were created that contain a direct or inverted NS1-GFP-NS2 sequence for the expression of NS2 and NS1 proteins translationally fused with green fluorescent protein (GFP), which allowed the evaluation the transcriptional and/or replicative activity of viral polymerase.

CONCLUSION

Polymerase of virus A/Krasnodar/101/35/59 (H2N2) has higher replicative and transcriptional activity at 33 °C than at 37 °C. Its transcriptional activity is more temperature-dependent than its replicative activity. The replicative and transcriptional activity of polymerase A/Puerto Rico/8/34 virus (H1N1, Mount Sinai variant) have no significant differences and do not depend on temperature.

Generation of a recombinant temperature-sensitive influenza D virus

Influenza D virus (IDV) is a causative agent of the bovine respiratory disease complex (BRDC), which is the most common and costly disease affecting the cattle industry. For developing a candidate vaccine virus against IDV, we sought to produce a temperature-sensitive strain, similar to the live attenuated, cold-adapted vaccine strain available against the influenza A virus (IAV). To this end, we produced a recombinant IDV (designated rD/OK-AL) strain by introducing mutations responsible for the adaptation of the IAV vaccine strain to cold conditions and conferring sensitivity to high temperatures into PB2 and PB1 proteins using reverse genetics. The rD/OK-AL strain grew efficiently at 33 °C but did not grow at 37 °C in the cell culture, indicating its high-temperature sensitivity. In mice, rD/OK-AL was attenuated following intranasal inoculation. It mediated the production of high levels of antibodies against IDV in the serum. When the rD/OK-AL-inoculated mice were challenged with the wild-type virus, the virus was not detected in respiratory organs after the challenge, indicating complete protection against IDV. These results imply that the rD/OK-AL might be a potential candidate for the development of live attenuated vaccines for IDV that can be used to control BRDC.

Biosafety risk assessment for production of candidate vaccine viruses to protect humans from zoonotic highly pathogenic avian influenza viruses

A major lesson learned from the public health response to the 2009 H1N1 pandemic was the need to shorten the vaccine delivery timeline to achieve the best pandemic mitigation results. A gap analysis of previous pre-pandemic vaccine development activities identified possible changes in the Select Agent exclusion process that would maintain safety and shorten the timeline to develop candidate vaccine viruses (CVVs) for use in pandemic vaccine manufacture. Here, we review the biosafety characteristics of CVVs developed in the past 15 years to support a shortened preparedness timeline for A(H5) and A(H7) subtype highly pathogenic avian influenza (HPAI) CVVs. Extensive biosafety experimental evidence supported recent changes in the implementation of Select Agent regulations that eliminated the mandatory chicken pathotype testing requirements and expedited distribution of CVVs to shorten pre-pandemic and pandemic vaccine manufacturing by up to 3 weeks.

Analysis of the efficacy of an adjuvant-based inactivated pandemic H5N1 influenza virus vaccine

This paper describes a preclinical study analyzing the immunogenicity and protective efficacy of Kazfluvac^®, an adjuvant-based inactivated pandemic influenza A/H5N1 virus vaccine. In this study, laboratory animals (ferrets and mice) were vaccinated by the intramuscular or intraperitoneal route at an interval of 14 days with two doses of the vaccine containing different concentrations of influenza virus hemagglutinin (HA) protein. HA protein without adjuvant (aluminum hydroxide and Merthiolate) was used as a control. As a negative control, we utilized PBS. We assessed the protective efficacy of the candidate vaccine by analyzing the response to challenge with the influenza virus strain A/chicken/Astana/6/05 (H5N1). Our experimental results revealed substantially reduced clinical disease and an increased antibody response, as determined by hemagglutination-inhibition (HAI) test and microneutralization assay (MNA). This study showed that the candidate vaccine is safe and elicits an antigen-dose-dependent serum antibody response. In summary, we determined the optimum antigen dose in a Kazfluvac^® adjuvant formulation required for induction of heightened immunogenicity and protective efficacy to mitigate H5N1 disease in experimental animals, suggesting its readiness for clinical studies in humans.

A peptide-based approach to evaluate the adaptability of influenza A virus to humans based on its hemagglutinin proteolytic cleavage site

Cleavage activation of the hemagglutinin (HA) protein by host proteases is a crucial step in the infection process of influenza A viruses (IAV). However, IAV exists in eighteen different HA subtypes in nature and their cleavage sites vary considerably. There is uncertainty regarding which specific proteases activate a given HA in the human respiratory tract. Understanding the relationship between different HA subtypes and human-specific proteases will be valuable in assessing the pandemic potential of circulating viruses. Here we utilized fluorogenic peptides mimicking the HA cleavage motif of representative IAV strains causing disease in humans or of zoonotic/pandemic potential and tested them with a range of proteases known to be present in the human respiratory tract. Our results show that peptides from the H1, H2 and H3 subtypes are cleaved efficiently by a wide range of proteases including trypsin, matriptase, human airway tryptase (HAT), kallikrein-related peptidases 5 (KLK5) and 12 (KLK12) and plasmin. Regarding IAVs currently of concern for human adaptation, cleavage site peptides from H10 viruses showed very limited cleavage by respiratory tract proteases. Peptide mimics from H6 viruses showed broader cleavage by respiratory tract proteases, while H5, H7 and H9 subtypes showed variable cleavage; particularly matriptase appeared to be a key protease capable of activating IAVs. We also tested HA substrate specificity of Factor Xa, a protease required for HA cleavage in chicken embryos and relevant for influenza virus production in eggs. Overall our data provide novel tool allowing the assessment of human adaptation of IAV HA subtypes.

Cell culture-based influenza vaccines: A necessary and indispensable investment for the future

The traditional platform of using embryonated chicken eggs for the production of influenza vaccines has several drawbacks including the inability to meet the volume of required doses in the case of widespread epidemics and pandemics. Cell culture platforms have therefore been explored in the last 2 decades, and have attracted further attention following the H1N1 pandemic outbreak. This platform, while not the most economical for large-scale production, has several advantages, and can supplement the vaccine requirement when needed. Recent developments in production technologies have contributed greatly to fine-tuning this platform. In combination with other technologies such as live attenuated and recombinant protein or virus-like particle vaccines, and different adjuvants and delivery systems, cell culture-based influenza vaccine platform can be used both for production of seasonal vaccine, and to mitigate vaccine shortages in pandemic situations.

Identification of Influenza A/PR/8/34 Donor Viruses Imparting High Hemagglutinin Yields to Candidate Vaccine Viruses in Eggs

One of the important lessons learned from the 2009 H1N1 pandemic is that a high yield influenza vaccine virus is essential for efficient and timely production of pandemic vaccines in eggs. The current seasonal and pre-pandemic vaccine viruses are generated either by classical reassortment or reverse genetics. Both approaches utilize a high growth virus, generally A/Puerto Rico/8/1934 (PR8), as the donor of all or most of the internal genes, and the wild type virus recommended for inclusion in the vaccine to contribute the hemagglutinin (HA) and neuraminidase (NA) genes encoding the surface glycoproteins. As a result of extensive adaptation through sequential egg passaging, PR8 viruses with different gene sequences and high growth properties have been selected at different laboratories in past decades. The effect of these related but distinct internal PR8 genes on the growth of vaccine viruses in eggs has not been examined previously. Here, we use reverse genetics to analyze systematically the growth and HA antigen yield of reassortant viruses with 3 different PR8 backbones. A panel of 9 different HA/NA gene pairs in combination with each of the 3 different lineages of PR8 internal genes (27 reassortant viruses) was generated to evaluate their performance. Virus and HA yield assays showed that the PR8 internal genes influence HA yields in most subtypes. Although no single PR8 internal gene set outperformed the others in all candidate vaccine viruses, a combination of specific PR8 backbone with individual HA/NA pairs demonstrated improved HA yield and consequently the speed of vaccine production. These findings may be important both for production of seasonal vaccines and for a rapid global vaccine response during a pandemic.

An inactivated, adjuvanted whole virion clade 2.2 H5N1 (A/Chicken/Astana/6/05) influenza vaccine is safe and immunogenic in a single dose in humans

In this study, we assessed in humans the immunogenicity and safety of one dose (7.5 or 15 μg of hemagglutinin [HA]) of a whole-virion inactivated prepandemic influenza vaccine adjuvanted with aluminum hydroxide. The vaccine strain was made by reverse genetics from the highly pathogenic avian A/Chicken/Astana/6/05 (H5N1) clade 2.2 strain isolated from a dead bird in Kazakhstan. The humoral immune response was evaluated after a single vaccination by hemagglutination inhibition (HI) and microneutralization (MN) assays. The vaccine was safe and immunogenic, inducing seroconversion in 55% of the evaluated patients, with a geometric mean titer (GMT) of 17.1 and a geometric mean increase (GMI) of 3.42 after a dose of 7.5 μg in the HI test against the vaccine strain. The rate of seroconversion increased up to 70% when the dose of 15 μg was used. The percentages of individuals achieving anti-HA titers of ≥1:40 were 52.5% and 57.5% for the 7.5- and 15-μg dose groups, respectively. Similar results were obtained when antibodies were analyzed in an MN test. Substantial cross-neutralization titers (seroconversion in 35% and 52.5% of subjects in the two dose groups, respectively) were detected against heterologous clade 1 strain NIBRG14 (H5N1). Thus, one dose of this whole-virion prepandemic vaccine adjuvanted with aluminum has the potential to be effective against H5N1 viruses of different clades.

A novel bivalent vaccine based on a PB2-knockout influenza virus protects mice from pandemic H1N1 and highly pathogenic H5N1 virus challenges

Vaccination is an effective means to protect against influenza virus. Although inactivated and live-attenuated vaccines are currently available, each vaccine has disadvantages (e.g., immunogenicity and safety issues). To overcome these problems, we previously developed a replication-incompetent PB2-knockout (PB2-KO) influenza virus that replicates only in PB2 protein-expressing cells. Here, we generated two PB2-KO viruses whose PB2-coding regions were replaced with the HA genes of either A/California/04/2009 (H1N1pdm09) or A/Vietnam/1203/2004 (H5N1). The resultant viruses comparably, or in some cases more efficiently, induced virus-specific antibodies in the serum, nasal wash, and bronchoalveolar lavage fluid of mice relative to a conventional formalin-inactivated vaccine. Furthermore, mice immunized with these PB2-KO viruses were protected from lethal challenges with not only the backbone virus strain but also strains from which their foreign HAs originated, indicating that PB2-KO viruses with antigenically different HAs could serve as bivalent influenza vaccines.

Evaluation of the efficacy of a pre-pandemic H5N1 vaccine (MG1109) in mouse and ferret models

The threat of a highly pathogenic avian influenza (HPAI) H5N1 virus causing the next pandemic remains a major concern. In this study, we evaluated the immunogenicity and efficacy of an inactivated whole-virus H5N1 pre-pandemic vaccine (MG1109) formulated by Green Cross Co., Ltd containing the hemagglutinin (HA) and neuraminidase (NA) genes of the clade 1 A/Vietnam/1194/04 virus in the backbone of A/Puerto Rico/8/34 (RgVietNam/04xPR8/34). Administration of the MG1109 vaccine (2-doses) in mice and ferrets elicited high HI and SN titers in a dose-dependent manner against the homologous (RgVietNam/04xPR8/34) and various heterologous H5N1 strains, (RgKor/W149/06xPR8/34, RgCambodia/04xPR8/34, RgGuangxi/05xPR8/34), including a heterosubtypic H5N2 (A/Aquatic bird/orea/W81/05) virus. However, efficient cross-reactivity was not observed against heterosubtypic H9N2 (A/Ck/Korea/H0802/08) and H1N1 (PR/8/34) viruses. Mice immunized with 1.9 μg HA/dose of MG1109 were completely protected from lethal challenge with heterologous wild-type HPAI H5N1 A/EM/Korea/W149/06 (clade 2.2) and mouse-adapted H5N2 viruses. Furthermore, ferrets administered at least 3.8 μg HA/dose efficiently suppressed virus growth in the upper respiratory tract and lungs. Vaccinated mice and ferrets also demonstrated attenuation of clinical disease signs and limited virus spread to other organs. Thus, this vaccine provided immunogenic responses in mouse and ferret models even against challenge with heterologous HPAI H5N1 and H5N2 viruses. Since the specific strain of HPAI H5N1 virus that would potentially cause the next outbreak is unknown, pre-pandemic vaccine preparation that could provide cross-protection against various H5 strains could be a useful approach in the selection of promising candidate vaccines in the future.

Cloned cDNA of A/swine/Iowa/15/1930 internal genes as a candidate backbone for reverse genetics vaccine against influenza A viruses

Reverse genetics viruses for influenza vaccine production usually utilize the internal genes of the egg-adapted A/Puerto Rico/8/34 (PR8) strain. This egg-adapted strain provides high production yield in embryonated eggs but does not necessarily give the best yield in mammalian cell culture. In order to generate a reverse genetics viral backbone that is well-adapted to high growth in mammalian cell culture, a swine influenza isolate A/swine/Iowa/15/30 (H1N1) (rg1930) that was shown to give high yield in Madin-Darby canine kidney (MDCK) cells was used as the internal gene donor for reverse genetics plasmids. In this report, the internal genes from rg1930 were used for construction of reverse genetics viruses carrying a cleavage site-modified hemagglutinin (HA) gene and neuraminidase (NA) gene from a highly pathogenic H5N1 virus. The resulting virus (rg1930H5N1) was low pathogenic in vivo. Inactivated rg1930H5N1 vaccine completely protected chickens from morbidity and mortality after challenge with highly pathogenic H5N1. Protective immunity was obtained when chickens were immunized with an inactivated vaccine consisting of at least 2(9) HA units of the rg1930H5N1 virus. In comparison to the PR8-based reverse genetics viruses carrying the same HA and NA genes from an H5N1 virus, rg1930 based viruses yielded higher viral titers in MDCK and Vero cells. In addition, the reverse genetics derived H3N2 and H5N2 viruses with the rg1930 backbone replicated in MDCK cells better than the cognate viruses with the rgPR8 backbone. It is concluded that this newly established reverse genetics backbone system could serve as a candidate for a master donor strain for development of inactivated influenza vaccines in cell-based systems.

Increase in viral yield in eggs and MDCK cells of reassortant H5N1 vaccine candidate viruses caused by insertion of 38 amino acids into the NA stalk

BACKGROUND

The H5N1 subtype of highly pathogenic avian influenza viruses has spread to over 63 countries in Asia, Europe, and Africa and has become endemic in poultry. Since 2004, vaccination against H5N1 influenza has become common in domestic poultry operations in China. Most influenza vaccines have been produced in embryonated chicken eggs. High yield is the essential feature of a good vaccine candidate virus.

OBJECTIVE

Therefore, the large-scale manufacture of such a vaccine requires that the viral yield of H5N1 reassortant vaccine viruses in eggs and MDCK cells be increased.

METHODS

We generated two sets of reassortant H5N1 viruses based on backbone viruses A/Chicken/F/98 (H9N2) and A/Puerto Rico/8/34 (H1N1) using reverse genetics. The HAs and NAs of the reassortants were derived from the three epidemic H5N1 strains found in China. We compared the replication properties of these recombinant H5N1 viruses in embryonated chicken eggs and MDCK cells after inserting either 20 or 38 amino acids into their NA stalks.

RESULTS

In this study, we demonstrated that inserting 38 amino acids into the NA stalks can significantly increase the viral yield of H5N1 reassortant viruses in both embryonated chicken eggs and MDCK cells, while inserting only 20 amino acids into the same NA stalks does not. Hemagglutinin inhibition testing and protection assays indicated that recombinant H5N1 viruses with 38 aa inserted into their NA stalks had the same antigenicity as the viruses with wt-NA.

CONCLUSION

These results suggest that the generation of an H5N1 recombinant vaccine seed by the insertion of 38 aa into the NA stalk may be a suitable and more economical strategy for the increase in viral yield in both eggs and MDCK cells for the purposes of vaccine production.

Single HA2 mutation increases the infectivity and immunogenicity of a live attenuated H5N1 intranasal influenza vaccine candidate lacking NS1

BACKGROUND

H5N1 influenza vaccines, including live intranasal, appear to be relatively less immunogenic compared to seasonal analogs. The main influenza virus surface glycoprotein hemagglutinin (HA) of highly pathogenic avian influenza viruses (HPAIV) was shown to be more susceptible to acidic pH treatment than that of human or low pathogenic avian influenza viruses. The acidification machinery of the human nasal passageway in response to different irritation factors starts to release protons acidifying the mucosal surface (down to pH of 5.2). We hypothesized that the sensitivity of H5 HA to the acidic environment might be the reason for the low infectivity and immunogenicity of intranasal H5N1 vaccines for mammals.

METHODOLOGY/PRINCIPAL FINDINGS

We demonstrate that original human influenza viruses infect primary human nasal epithelial cells at acidic pH (down to 5.4), whereas H5N1 HPAIVs lose infectivity at pH ≤ 5.6. The HA of A/Vietnam/1203/04 was modified by introducing the single substitution HA2 58K→I, decreasing the pH of the HA conformational change. The H5N1 reassortants containing the indicated mutation displayed an increased resistance to acidic pH and high temperature treatment compared to those lacking modification. The mutation ensured a higher viral uptake as shown by immunohistochemistry in the respiratory tract of mice and 25 times lower mouse infectious dose₅₀. Moreover, the reassortants keeping 58K→I mutation designed as a live attenuated vaccine candidate lacking an NS1 gene induced superior systemic and local antibody response after the intranasal immunization of mice.

CONCLUSION/SIGNIFICANCE

Our finding suggests that an efficient intranasal vaccination with a live attenuated H5N1 virus may require a certain level of pH and temperature stability of HA in order to achieve an optimal virus uptake by the nasal epithelial cells and induce a sufficient immune response. The pH of the activation of the H5 HA protein may play a substantial role in the infectivity of HPAIVs for mammals.

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.

Comparative immunogenicity of recombinant adenovirus-vectored vaccines expressing different forms of hemagglutinin (HA) proteins from the H5 serotype of influenza A viruses in mice

Recent outbreaks of highly pathogenic avian influenza (HPAI) H5N1 viruses in poultry and their subsequent transmission to humans have highlighted an urgent need to develop preventive vaccines in the event of a pandemic. In this paper we constructed recombinant adenovirus (rAd)-vectored influenza vaccines expressing different forms of H5 hemagglutinin (HA) from the A/Vietnam/1194/04 (VN/1194/04) virus, a wild-type HA, a sequence codon-optimized HA and a transmembrane (TM) domain-truncated HA. Compared to the rAd vectors expressing the wild-type HA (rAd-04wtHA) and the TM-truncated form of HA (rAd-04optHA-dTM), the rAd vectored vaccine with the sequence codon-optimized HA (rAd-04optHA) showed a tendency to induce much higher hemagglutinin inhibition (HI) antibody titers in mice immunized with a prime-boost vaccine. Furthermore, administration of the rAd-04optHA vaccine to mice could elicit cross-reactive immune responses against the antigenically distinct HK/482/97 virus. Additionally, we constructed another vector containing the codon-optimized HA of the A/Hong Kong/482/97 (HK/482/97) virus. Administration of a bivalent immunization formulation including the rAd-04optHA and rAd-97optHA vaccines to mice induced a stronger immune response against HK/482/97 virus than the monovalent formulation. Taken together, these findings may have some implications for the development of rAd-vectored vaccines in the event of the pandemic spread of HPAI.

Potent vesicular stomatitis virus-based avian influenza vaccines provide long-term sterilizing immunity against heterologous challenge

The emergence in 1997 and continuance today of a highly lethal H5N1 avian influenza virus (AIV) causing human disease has raised concern about an impending pandemic and the need for a vaccine to prepare for such an occurrence. We previously generated an efficacious vesicular stomatitis virus (VSV)-based AIV vaccine expressing H5 hemagglutinin (HA) from the fifth genomic position of VSV (J. A. Schwartz et al., Virology 366:166-173, 2007). Here we have generated and characterized VSV-based vaccines that express the A/Hong Kong/156/1997 (clade 0) H5 HA from the first position of the VSV genome. These vectors induce broadly cross-neutralizing antibodies against homologous and heterologous H5N1 viruses of different clades in mice. The vaccines provide complete protection against morbidity and mortality after heterologous challenge with clade 0 and clade 1 strains in animals even 1 year after vaccination. Postchallenge pulmonary virus loads show that these vectors provide sterilizing immunity. Therefore, VSV-based AIV vaccines are potent, broadly cross-protective pandemic vaccine candidates.

Fine antigenic variation within H5N1 influenza virus hemagglutinin's antigenic sites defined by yeast cell surface display

Fifteen strains of mAb specific for HA of the A/Hong Kong/482/97 (H5N1) influenza virus were generated. The HA antigenic sites of the human A/Hong Kong/482/97 (H5N1) influenza virus were defined by using yeast cell surface-displaying system and anti-H5 HA mAb. Evolution analysis of H5 HA identified residues that exhibit diversifying selection in the antigenic sites and demonstrated surprising differences between residue variation of H5 HA and H3 HA. A conserved neutralizing epitope in the H5 HA protein recognized by mAb H5M9 was found using viruses isolated from 1997-2006. Seven single amino acid substitutions were introduced into the HA antigenic sites, respectively, and the alteration of antigenicity was assessed. The structure obtained by homology-modeling and molecular dynamic methods showed that a subtle substitution at residue 124 propagates throughout its nearby loop (152-159). We discuss how the structural changes caused by point mutation might explain the altered antigenicity of the HA protein. The results demonstrate the existence of immunodominant positions in the H5 HA protein, alteration of these residues might improve the immunogenicity of vaccine strains.

MDCK cells that express proteases TMPRSS2 and HAT provide a cell system to propagate influenza viruses in the absence of trypsin and to study cleavage of HA and its inhibition

Cleavage of the influenza virus hemagglutinin (HA) by host cell proteases is essential for virus infectivity and, therefore, relevant proteases may present promising new drug targets. We recently demonstrated that serine proteases TMPRSS2 and HAT from human airways activate influenza virus HA with monobasic cleavage site in vitro. In the present study we generated MDCK cells with inducible expression of either TMPRSS2 or HAT. MDCK-TMPRSS2 and MDCK-HAT cells supported growth of human and avian influenza viruses of different subtypes in the absence of exogenous trypsin. Further, we used these cell lines to investigate the efficacy of protease inhibitors to prevent proteolytic activation of HA by TMPRSS2 and HAT. Multicycle viral replication in both cell lines was markedly suppressed in the presence of serine protease inhibitors and we found that particularly in MDCK-HAT cells proteolytic activation of progeny viruses was very susceptible to inhibitor treatment. Taken together, our data demonstrate that MDCK-HAT and MDCK-TMPRSS2 cells are useful experimental systems to study cleavage of HA by host cell protease and its inhibition and in addition represent applicable cell lines to propagate influenza viruses in the absence of trypsin.

In vitro responses to avian influenza H5 by human CD4 T cells

To address the question of whether human T cells are capable of recognizing novel isolates of influenza virus, in vitro responses to recombinant Ags and synthetic peptides derived from the sequences of H1, H3, and H5 were examined in a cohort of 64 individuals selected from a healthy blood donor population. Humans respond in vitro to H1 and H3 following exposure through natural infection and vaccination. Responses to H5 were well correlated with those to H1 or H3, and thus, a significant repertoire of H5-responsive T cells is present in many individuals; clear nonresponders to H1, H3, and H5, however, do exist. Differences were observed in the cytokine responses to H1, H3, and H5, whereas both IL-2 and IFN-gamma production characteristic of memory responses were observed for H1 and H3, and H5-specific responses elicited primarily IL-2 and little or no IFN-gamma, consistent with a naive T cell phenotype. Responses to all influenza HA were restricted by HLA-DR molecules. To address the structural basis for T cell recognition of H1 and H5, overlapping synthetic peptides were used to identify epitopes and to determine whether recognition of H5 was limited to homologous sequences in H1, the most closely related HA phylogenetically. Although responses were generally correlated, no complete structural overlap was observed. These results suggest that helper T cell cross reactivity between different influenza strains may impart cross-protection to H5N1 strain of influenza.

Preclinical evaluation of a replication-deficient intranasal DeltaNS1 H5N1 influenza vaccine

BACKGROUND

We developed a novel intranasal influenza vaccine approach that is based on the construction of replication-deficient vaccine viruses that lack the entire NS1 gene (DeltaNS1 virus). We previously showed that these viruses undergo abortive replication in the respiratory tract of animals. The local release of type I interferons and other cytokines and chemokines in the upper respiratory tract may have a "self-adjuvant effect", in turn increasing vaccine immunogenicity. As a result, DeltaNS1 viruses elicit strong B- and T- cell mediated immune responses.

METHODOLOGY/PRINCIPAL FINDINGS

We applied this technology to the development of a pandemic H5N1 vaccine candidate. The vaccine virus was constructed by reverse genetics in Vero cells, as a 5:3 reassortant, encoding four proteins HA, NA, M1, and M2 of the A/Vietnam/1203/04 virus while the remaining genes were derived from IVR-116. The HA cleavage site was modified in a trypsin dependent manner, serving as the second attenuation factor in addition to the deleted NS1 gene. The vaccine candidate was able to grow in the Vero cells that were cultivated in a serum free medium to titers exceeding 8 log(10) TCID(50)/ml. The vaccine virus was replication deficient in interferon competent cells and did not lead to viral shedding in the vaccinated animals. The studies performed in three animal models confirmed the safety and immunogenicity of the vaccine. Intranasal immunization protected ferrets and mice from being infected with influenza H5 viruses of different clades. In a primate model (Macaca mulatta), one dose of vaccine delivered intranasally was sufficient for the induction of antibodies against homologous A/Vietnam/1203/04 and heterologous A/Indonesia/5/05 H5N1 strains.

CONCLUSION/SIGNIFICANCE

Our findings show that intranasal immunization with the replication deficient H5N1 DeltaNS1 vaccine candidate is sufficient to induce a protective immune response against H5N1 viruses. This approach might be attractive as an alternative to conventional influenza vaccines. Clinical evaluation of DeltaNS1 pandemic and seasonal influenza vaccine candidates are currently in progress.

Adenovirus as a carrier for the development of influenza virus-free avian influenza vaccines

A long-sought goal during the battle against avian influenza is to develop a new generation of vaccines capable of mass immunizing humans as well as poultry (the major source of avian influenza for human infections) in a timely manner. Although administration of the currently licensed influenza vaccine is effective in eliciting protective immunity against seasonal influenza, this approach is associated with a number of insurmountable problems for preventing an avian influenza pandemic. Many of the hurdles may be eliminated by developing new avian influenza vaccines that do not require the propagation of an influenza virus during vaccine production. Replication-competent adenovirus-free adenovirus vectors hold promise as a carrier for influenza virus-free avian influenza vaccines owing to their safety profile and rapid manufacture using cultured suspension cells in a serum-free medium. Simple and efficient mass-immunization protocols, including nasal spray for people and automated in ovo vaccination for poultry, convey another advantage for this class of vaccines. In contrast to parenteral injection of adenovirus vector, the potency of adenovirus-vectored nasal vaccine is not appreciably interfered by pre-existing immunity to adenovirus.

Investigation of the biological indicator for vaccine efficacy against highly pathogenic avian influenza (HPAI) H5N1 virus challenge in mice and ferrets

To investigate the biological indicator for vaccine efficacy against HPAI H5N1 virus challenge of varying clades, two inactivated whole-virus H5N1 vaccines containing the hemagglutinin (HA) and neuraminidase (NA) genes of either clade 2.2 A/EM/Korea/W149/06 (RgKoreaW149/06 x PR8) or clade 2.5 A/Ck/Korea/ES/03 (RgKoreaES223N/03XPR8) virus in the background of A/PR/8/34 (H1N1) were generated by reverse genetics. Administration of the vaccines (2-dose 1.77, 3.5, 7.5 or 15microg of HA) elicited high HI titers in a dose-dependent manner. Mice immunized with RgKoreaW149/06 x PR8 were completely protected from challenge against wild-type A/EM/Korea/W149/06 without clinical signs of infection. RgKoreaES223N/03XPR8 could not protect mice at 1.77microg while all immunized ferrets were completely protected. Two-dose (7.5microg) vaccinated mice (HI titer > or =320) and triple dose (7.5 microg) vaccinated ferrets with RgKoreaES223N/03xPR8 (HI titer > or =640) protected vaccine recipients from mortality, inhibited nasal virus shedding and limited influenza virus tropism. Thus, these vaccines provided cross-protectivity in both models. More importantly, these results collectively suggested a positive correlation between vaccine-induced HI titers and inhibition of virus shedding including block of viral proliferation in major organs against a heterologous HPAI H5N1 virus. Although developing technologies or methods that will enable the reduction of administration dose/frequency remains to be resolved, our study demonstrated a considerable biological marker (> or =640 HI titer) for full protection of the vaccinated hosts that could provide a preliminary basis for the assessment of complete immunization.

Optimizing viral protein yield of influenza virus strain A/Vietnam/1203/2004 by modification of the neuraminidase gene

The preparation of high-yield prepandemic influenza virus H5N1 strains has presented a challenge to both researchers and vaccine manufacturers. The reasons for the relatively low yield of the H5N1 strains are not fully understood, but it might be partially dependent on the interactions between the hemagglutinin (HA) or neuraminidase (NA) surface glycoprotein and other influenza virus proteins. In this study, we have constructed chimeras between the A/Puerto Rico/8/34 (PR8) NA gene and the A/Vietnam/1203/2004 (VN1203) NA gene that have resulted in an increase in the virus yield of the reassortant viruses without a significant loss of NA activity. By combining the amino terminus of NA from the PR8 strain with the carboxy terminus of NA from VN1203, the surface epitopes unique to the H5N1 VN1203 NA glycoprotein are maintained. This reassortant virus had a higher titer and total protein yield in eggs, grew to a higher titer, produced large plaques on MDCK cells, and retained NA activity. This work describes a novel recombinant technique designed to increase the yields of vaccine candidates for the production of pandemic influenza virus vaccines. The relationship between the infectivity and protein yield of the reassortants also is discussed.

Designing vaccines for pandemic influenza

Recent outbreaks of highly pathogenic avian influenza A virus infections (including those of the H5N1 subtype) in poultry and in humans (through contact with infected birds) have raised concerns that a new influenza pandemic will soon occur. Effective vaccines against H5N1 virus are therefore urgently needed. Reverse genetics-based inactivated vaccines have been prepared according to WHO recommendations and licensed in several countries following their assessment in clinical trials. However, the effectiveness of these vaccines in a pandemic is not guaranteed. We must therefore continue to develop alternative pandemic vaccine strategies. Here, we review the current strategies for the development of H5N1 influenza vaccines, as well as some future directions for vaccine development.

Cross-clade protective immunity of H5N1 influenza vaccines in a mouse model

H5N1 highly pathogenic avian influenza viruses evolved into several clades, leading to appreciably distinct antigenicities of their hemagglutinins. As such, candidate H5N1 pre-pandemic vaccines for human use should be sought. Here, to evaluate fundamental immunogenic variations between H5N1 vaccines, we prepared four inactivated H5N1 test vaccines from different phylogenetic clades (clade 1, 2.1, 2.2, and 2.3.4) in accordance with the WHO recommendation, and tested their cross-clade immunity in a mouse model by vaccination followed by challenge with heterologous virulent viruses. All H5N1 vaccines tested provided full or partial cross-clade protective immunity, except one clade 2.2-based vaccine, which did not protect mice from clade 2.3.4 virus challenge. Among the test vaccines, a clade 2.1-based vaccine possessed the broadest-spectrum cross-immunity. These results suggest that currently stockpiled pre-pandemic vaccines, especially clade 2.1-based vaccines, will likely be useful as backup vaccines in a pandemic situation, even one involving antigenic-drifted viruses.

Growth determinants for H5N1 influenza vaccine seed viruses in MDCK cells

H5N1 influenza A viruses are exacting a growing human toll, with more than 240 fatal cases to date. In the event of an influenza pandemic caused by these viruses, embryonated chicken eggs, which are the approved substrate for human inactivated-vaccine production, will likely be in short supply because chickens will be killed by these viruses or culled to limit the worldwide spread of the infection. The Madin-Darby canine kidney (MDCK) cell line is a promising alternative candidate substrate because it supports efficient growth of influenza viruses compared to other cell lines. Here, we addressed the molecular determinants for growth of an H5N1 vaccine seed virus in MDCK cells, revealing the critical responsibility of the Tyr residue at position 360 of PB2, the considerable requirement for functional balance between hemagglutinin (HA) and neuraminidase (NA), and the partial responsibility of the Glu residue at position 55 of NS1. Based on these findings, we produced a PR8/H5N1 reassortant, optimized for this cell line, that derives all of its genes for its internal proteins from the PR8(UW) strain except for the NS gene, which derives from the PR8(Cambridge) strain; its N1 NA gene, which has a long stalk and derives from an early H5N1 strain; and its HA gene, which has an avirulent-type cleavage site sequence and is derived from a circulating H5N1 virus. Our findings demonstrate the importance and feasibility of a cell culture-based approach to producing seed viruses for inactivated H5N1 vaccines that grow robustly and in a timely, cost-efficient manner as an alternative to egg-based vaccine production.

Influenza vaccines and vaccination strategies in birds

Although it is well accepted that the present Asian H5N1 panzootic is predominantly an animal health problem, the human health implications and the risk of human pandemic have highlighted the need for more information and collaboration in the field of veterinary and human health. H5 and H7 avian influenza (AI) viruses have the unique property of becoming highly pathogenic (HPAI) during circulation in poultry. Therefore, the final objective of poultry vaccination against AI must be eradication of the virus and the disease. Actually, important differences exist in the control of avian and human influenza viruses. Firstly, unlike human vaccines that must be adapted to the circulating strain to provide adequate protection, avian influenza vaccination provides broader protection against HPAI viruses. Secondly, although clinical protection is the primary goal of human vaccines, poultry vaccination must also stop transmission to achieve efficient control of the disease. This paper addresses these differences by reviewing the current and future influenza vaccines and vaccination strategies in birds.

Sublingual vaccination with influenza virus protects mice against lethal viral infection

We assessed whether the sublingual (s.l.) route would be an effective means of delivering vaccines against influenza virus in mice by using either formalin-inactivated or live influenza A/PR/8 virus (H1N1). Sublingual administration of inactivated influenza virus given on two occasions induced both systemic and mucosal antibody responses and conferred protection against a lethal intranasal (i.n.) challenge with influenza virus. Coadministration of a mucosal adjuvant (mCTA-LTB) enhanced these responses and resulted in complete protection against respiratory viral challenge. In addition, s.l. administration of formalin-inactivated A/PR/8 plus mCTA-LTB induced systemic expansion of IFN-gamma-secreting T cells and virus-specific cytotoxic T lymphocyte responses. Importantly, a single s.l. administration of live A/PR/8 virus was not pathogenic and induced protection mediated by both acquired and innate immunity. Moreover, s.l. administration of live A/PR/8 virus conferred heterosubtypic protection against respiratory challenge with H3N2 virus. Unlike the i.n. route, the A/PR/8 virus, whether live or inactivated, did not migrate to or replicate in the CNS after s.l. administration. Based on these promising findings, we propose that the s.l. mucosal route offers an attractive alternative to mucosal routes for administering influenza vaccines.

A combination in-ovo vaccine for avian influenza virus and Newcastle disease virus

The protection of poultry from H5N1 highly pathogenic avian influenza A (HPAI) and Newcastle disease virus (NDV) can be achieved through vaccination, as part of a broader disease control strategy. We have previously generated a recombinant influenza virus expressing, (i) an H5 hemagglutinin protein, modified by the removal of the polybasic cleavage peptide and (ii) the ectodomain of the NDV hemagglutinin-neuraminidase (HN) protein in the place of the ectodomain of influenza neuraminidase (Park MS, et al. Proc Natl Acad Sci USA 2006;103(21):8203-8). Here we show this virus is attenuated in primary normal human bronchial epithelial (NHBE) cell culture, and demonstrate protection of C57BL/6 mice from lethal challenge with an H5 HA-containing influenza virus through immunisation with the recombinant virus. In addition, in-ovo vaccination of 18-day-old embryonated chicken eggs provided 90% and 80% protection against highly stringent lethal challenge by NDV and H5N1 virus, respectively. We propose that this virus has potential as a safe in-ovo live, attenuated, bivalent avian influenza and Newcastle disease virus vaccine.

Establishment of canine RNA polymerase I-driven reverse genetics for influenza A virus: its application for H5N1 vaccine production

In the event of a new influenza pandemic, vaccines whose antigenicities match those of circulating strains must be rapidly produced. Here, we established an alternative reverse genetics system for influenza virus using the canine polymerase I (PolI) promoter sequence that works efficiently in the Madin-Darby canine kidney cell line, a cell line approved for human vaccine production. Using this system, we were able to generate H5N1 vaccine seed viruses more efficiently than can be achieved with the current system that uses the human PolI promoter in African green monkey Vero cells, thus improving pandemic vaccine production.

Enhanced growth of seed viruses for H5N1 influenza vaccines

Seed viruses used to produce inactivated H5N1 influenza vaccines are recombinant viruses with modified avirulent-type hemagglutinin (HA) and intact neuraminidase (NA) genes, both derived from an H5N1 isolate, and all remaining genes from the PR8 strain, which grows well in eggs. However, some reassortants grow suboptimally in eggs, imposing obstacles to timely, cost-efficient vaccine production. Here, we demonstrate that our PR8 strain supports better in ovo growth than the PR8 strain used for the WHO-recommended seed virus, NIBRG-14. Moreover, inclusion of an alternative NA protein further enhanced viral growth in eggs. These findings suggest that our H5N1 vaccine candidates would increase the availability of H5N1 vaccine doses at the onset of a new pandemic.

An adenovirus vector-mediated reverse genetics system for influenza A virus generation

Plasmid-based reverse genetics systems allow the generation of influenza A virus entirely from cloned cDNA. However, since the efficiency of virus generation is dependent on the plasmid transfection efficiency of cells, virus generation is difficult in cells approved for vaccine production that have low transfection efficiencies (e.g., Vero cells). Here we established an alternative reverse genetics system for influenza virus generation by using an adenovirus vector (AdV) which achieves highly efficient gene transfer independent of cell transfection efficiency. This AdV-mediated reverse genetics system will be useful for generating vaccine seed strains and for basic influenza virus studies.

Cell culture (Vero) derived whole virus (H5N1) vaccine based on wild-type virus strain induces cross-protective immune responses

The rapid spread and the transmission to humans of avian influenza virus (H5N1) have induced world-wide fears of a new pandemic and raised concerns over the ability of standard influenza vaccine production methods to rapidly supply sufficient amounts of an effective vaccine. We report here on a robust and flexible strategy which uses wild-type virus grown in a continuous cell culture (Vero) system to produce an inactivated whole virus vaccine. Candidate vaccines based on clade 1 and clade 2 influenza H5N1 strains were developed and demonstrated to be highly immunogenic in animal models. The vaccines induce cross-neutralising antibodies, highly cross-reactive T-cell responses and are protective in a mouse challenge model not only against the homologous virus but also against other H5N1 strains, including those from another clade. These data indicate that cell culture-grown whole virus vaccines, based on the wild-type virus, allow the rapid high yield production of a candidate pandemic vaccine.

The threat of avian influenza A (H5N1). Part IV: development of vaccines

Among emerging and re-emerging infectious diseases, influenza constitutes one of the major threats to mankind. In this review series epidemiologic, virologic and pathologic concerns raised by infections of humans with avian influenza virus A/H5N1 are discussed. This fourth part focuses on vaccine development. Several phase I clinical studies with vaccines against H5 viruses have demonstrated limited efficacy compared to seasonal influenza vaccines. To induce protective immunity two immunisations with increased amounts of H5N1 vaccine were required. Novel vaccination strategies that are egg- and adjuvant-independent, broadly cross-reactive and long-lasting are highly desirable.

The fight against new types of influenza virus

In 1997, during an outbreak in chickens in Hong Kong the avian H5N1 influenza virus crossed the species barrier and infected 18 people, of which 6 cases were fatal. The virus also infected wild birds and continued to circulate and mutate in geese and ducks in southeastern China. Since this occurrence, new antigenic variants that are highly pathogenic for humans as well as wild, domestic, and exotic waterfowl continue to appear in Hong Kong. This virus is spreading across Asia, and is encroaching upon Europe and other continents. Wild birds are now considered as the main reservoir of H5N1 virus. Humans become infected with this H5N1 virus usually via close contact with infected birds or a highly contaminated environment. The very low transmissibility of this virus prevented further person-to-person dissemination in spite of the complete absence of immunity in the human population to H5N1 viruses. Viruses of the H5N1 subtype are characterized by an exceptionally high pathogenicity for humans. The cause of the viral virulence is not known so far; however, several virulence factors are considered. The unprecedented capability of H5N1 viruses to kill humans intensifies the concern about its pandemic potential with catastrophic consequences. The effectiveness of existing antivirals as well as vaccines for humans and birds are reviewed.

Strategies for developing vaccines against H5N1 influenza A viruses

Recent outbreaks of highly pathogenic avian influenza A virus (H5N1 subtype) infections in poultry and humans (through direct contact with infected birds) have raised concerns that a new influenza pandemic might occur in the near future. Effective vaccines against H5N1 virus are, therefore, urgently needed. Reverse-genetics-based inactivated vaccines have been prepared according to World Health Organization (WHO) recommendations and are now undergoing clinical evaluation in several countries. Here, we review the current strategies for the development of H5N1 influenza vaccines, and future directions for vaccine development.

Hemagglutinin (HA) proteins from H1 and H3 serotypes of influenza A viruses require different antigen designs for the induction of optimal protective antibody responses as studied by codon-optimized HA DNA vaccines

Effective antibody responses provide crucial immunity against influenza virus infection. The hemagglutinin (HA) protein is the major target of protective antibody responses induced by viral infection and by vaccination with both inactivated and live-attenuated flu vaccines, but knowledge about the optimal designs of protective HA antigens from different flu serotypes is still limited. In this study, we have significantly improved the immunogenicity of HA-expressing DNA vaccines by using codon-optimized HA sequences for either an H1 serotype (A/NewCal/20/99) or an H3 serotype (A/Panama/2007/99) human influenza A virus and then used these constructs as model antigens to identify the optimal HA antigen designs to elicit high-level protective antibody responses. Two forms of HA antigen, a wild-type, full-length HA and a secreted form with transmembrane (TM) domain-truncated HA, were produced. Both forms of HA DNA vaccines, from either H1 or H3 serotypes, were able to elicit high levels of HA-specific immunoglobulin G responses in immunized rabbits as measured by enzyme-linked immunosorbent assay. Interestingly, the abilities of H1 HA and H3 HA antigens to elicit hemagglutination inhibition (HI) and neutralizing antibody (NAb) responses differ. For the H1 HA antigens, the full-length HA induced significantly higher HI and NAb responses than did the TM-truncated HA. For the H3 HA antigen, both the full-length HA and TM-truncated HA induced high levels of HI and NAb responses. These data indicate that H1 and H3 antigens have different expression requirements for the induction of an optimal protective antibody response and that the structure integrity of HA antigens is critical for eliciting type-specific protective antibody responses. Our findings will have an important impact on future subunit-based flu vaccine development.

Natural variation can significantly alter the sensitivity of influenza A (H5N1) viruses to oseltamivir

Geographic spread of highly pathogenic avian H5N1 influenza viruses may give rise to an influenza pandemic. During the first months of a pandemic, control measures would rely mainly on antiviral drugs, such as the neuraminidase (NA) inhibitors oseltamivir and zanamivir. In this study, we compare the sensitivities to oseltamivir of the NAs of several highly pathogenic H5N1 viruses isolated in Asia from 1997 to 2005. The corresponding 50% inhibitory concentrations were determined using a standard in vitro NA inhibition assay. The K(m) for the substrate and the affinity for the inhibitor (K(i)) of NA were determined for a 1997 and a 2005 virus, using an NA inhibition assay on cells transiently expressing the viral enzyme. Our data show that the sensitivities of the NAs of H5N1 viruses isolated in 2004 and 2005 to oseltamivir are about 10-fold higher than those of earlier H5N1 viruses or currently circulating H1N1 viruses. Three-dimensional modeling of the N1 protein predicted that Glu248Gly and Tyr252His changes could account for increased sensitivity. Our data indicate that genetic variation in the absence of any drug-selective pressure may result in significant variations in sensitivity to anti-NA drugs. Although the clinical relevance of a 10-fold increase in the sensitivity of NA to oseltamivir needs to be investigated further, the possibility that sensitivity to anti-NA drugs could increase (or possibly decrease) significantly, even in the absence of treatment, underscores the need for continuous evaluation of the impact of genetic drift on this parameter, especially for influenza viruses with pandemic potential.

Vaccines for viral and parasitic diseases produced with baculovirus vectors

The baculovirus-insect cell expression system is an approved system for the production of viral antigens with vaccine potential for humans and animals and has been used for production of subunit vaccines against parasitic diseases as well. Many candidate subunit vaccines have been expressed in this system and immunization commonly led to protective immunity against pathogen challenge. The first vaccines produced in insect cells for animal use are now on the market. This chapter deals with the tailoring of the baculovirus-insect cell expression system for vaccine production in terms of expression levels, integrity and immunogenicity of recombinant proteins, and baculovirus genome stability. Various expression strategies are discussed including chimeric, virus-like particles, baculovirus display of foreign antigens on budded virions or in occlusion bodies, and specialized baculovirus vectors with mammalian promoters that express the antigen in the immunized individual. A historical overview shows the wide variety of viral (glyco)proteins that have successfully been expressed in this system for vaccine purposes. The potential of this expression system for antiparasite vaccines is illustrated. The combination of subunit vaccines and marker tests, both based on antigens expressed in insect cells, provides a powerful tool to combat disease and to monitor infectious agents.