Protection against influenza virus infection in BALB/c mice immunized with a single dose of neuraminidase-expressing DNAs by electroporation

Optimizing a linear 'Doggybone' DNA vaccine for influenza virus through the incorporation of DNA targeting sequences and neuraminidase antigen

Influenza virus represents a challenge for traditional vaccine approaches due to its seasonal changes and potential for zoonotic transmission. Nucleic acid vaccines can overcome some of these challenges, especially through the inclusion of multiple antigens to increase the breadth of response. RNA vaccines were an important part of the response to the COVID-19 pandemic, but for future outbreaks DNA vaccines may have some advantages in terms of stability and manufacturing cost that warrant continuing investigation to fully realize their potential. Here, we investigate influenza virus vaccines made using a closed linear DNA platform, Doggybone™ DNA (dbDNA), produced by a rapid and scalable cell-free method. Influenza vaccines have mostly focussed on Haemagglutinin (HA), but the inclusion of Neuraminidase (NA) may provide additional protection. Here, we explored the potential of including NA in a dbDNA vaccine, looking at DNA optimization, mechanism and breadth of protection. We showed that DNA targeting sequences (DTS) improved immune responses against HA but not NA. We explored whether NA vaccine-induced protection against influenza virus infection was cell-mediated, but depletion of CD8 and NK cells made no impact, suggesting it was antibody-mediated. This is reflected in the restriction of protection to homologous strains of influenza virus. Importantly, we saw that including both HA and NA in a single combined vaccine did not dampen the immune response to either one. Overall, we show that linear dbDNA can induce an immune response against NA, which may offer increased protection in instances of HA mismatch where NA remains more conserved.

Comparative Pathology of Animal Models for Influenza A Virus Infection

Animal models are essential for studying disease pathogenesis and to test the efficacy and safety of new vaccines and therapeutics. For most diseases, there is no single model that can recapitulate all features of the human condition, so it is vital to understand the advantages and disadvantages of each. The purpose of this review is to describe popular comparative animal models, including mice, ferrets, hamsters, and non-human primates (NHPs), that are being used to study clinical and pathological changes caused by influenza A virus infection with the aim to aid in appropriate model selection for disease modeling.

Neuraminidase delivered as an APC-targeted DNA vaccine induces protective antibodies against influenza

Conventional influenza vaccines focus on hemagglutinin (HA). However, antibody responses to neuraminidase (NA) have been established as an independent correlate of protection. Here, we introduced the ectodomain of NA into DNA vaccines that, as translated dimeric vaccine proteins, target antigen-presenting cells (APCs). The targeting was mediated by an single-chain variable fragment specific for major histocompatibility complex (MHC) class II, which is genetically linked to NA via a dimerization motif. A single immunization of BALB/c mice elicited strong and long-lasting NA-specific antibodies that inhibited NA enzymatic activity and reduced viral replication. Vaccine-induced NA immunity completely protected against a homologous influenza virus and out-competed NA immunity induced by a conventional inactivated virus vaccine. The protection was mainly mediated by antibodies, although NA-specific T cells also contributed. APC-targeting and antigen bivalency were crucial for vaccine efficacy. The APC-targeted vaccine was potent at low doses of DNA, indicating a dose-sparing effect. Similar results were obtained with NA vaccines that targeted different surface molecules on dendritic cells. Interestingly, the protective efficacy of NA as antigen compared favorably with HA and therefore ought to receive more attention in influenza vaccine research.

Recent Progress in Recombinant Influenza Vaccine Development Toward Heterosubtypic Immune Response

Flu, a viral infection caused by the influenza virus, is still a global public health concern with potential to cause seasonal epidemics and pandemics. Vaccination is considered the most effective protective strategy against the infection. However, given the high plasticity of the virus and the suboptimal immunogenicity of existing influenza vaccines, scientists are moving toward the development of universal vaccines. An important property of universal vaccines is their ability to induce heterosubtypic immunity, i.e., a wide immune response coverage toward different influenza subtypes. With the increasing number of studies and mounting evidence on the safety and efficacy of recombinant influenza vaccines (RIVs), they have been proposed as promising platforms for the development of universal vaccines. This review highlights the current progress and advances in the development of RIVs in the context of heterosubtypic immunity induction toward universal vaccine production. In particular, this review discussed existing knowledge on influenza and vaccine development, current hemagglutinin-based RIVs in the market and in the pipeline, other potential vaccine targets for RIVs (neuraminidase, matrix 1 and 2, nucleoprotein, polymerase acidic, and basic 1 and 2 antigens), and deantigenization process. This review also provided discussion points and future perspectives in looking at RIVs as potential universal vaccine candidates for influenza.

Comparative Immunological Study in Mice of Inactivated Influenza Vaccines Used in the Russian Immunization Program

A series of commercial inactivated influenza vaccines (IIVs) used in the Russian National Immunization Program were characterized to evaluate their protective properties on an animal model. Standard methods for quantifying immune response, such as hemagglutination inhibition (HAI) assay and virus neutralization (VN) assay, allowed us to distinguish the immunogenic effect of various IIVs from that of placebo. However, these standard approaches are not suitable to determine the role of various vaccine components in immune response maturation. The expanded methodological base including an enzyme-linked immunosorbent assay (ELISA) and a neuraminidase ELISA (NA-ELISA) helped us to get wider characteristics and identify the effectiveness of various commercial vaccines depending on the antigen content. Investigations conducted showed that among the IIVs tested, Ultrix^®, Ultrix^® Quadri and VAXIGRIP^® elicit the most balanced immune response, including a good NA response. For Ultrix^®, Ultrix^® Quadri, and SOVIGRIPP^® (FORT LLC), the whole-virus specific antibody subclass IgG1, measured in ELISA, seriously prevailed over IgG2a, while, for VAXIGRIP^® and SOVIGRIPP^® (NPO Microgen JSC) preparations, the calculated IgG1/IgG2a ratio was close to 1. So, the immune response varied drastically across different commercial IIVs injected in mice.

Protective efficacy of anti-neuraminidase monoclonal antibodies against H7N9 influenza virus infection

The H7N9 influenza virus has been circulating in China for more than six years. The neuraminidase (NA) has gained great concern for the development of antiviral drugs, therapeutic antibodies, and new vaccines. In this study, we screened seven mouse monoclonal antibodies (mAbs) and compared their protective effects against H7N9 influenza virus. The epitope mapping from escape mutants showed that all the seven mAbs could bind to the head region of the N9 NA close to the enzyme activity sites, and four key sites of N9 NA were reported for the first time. The mAbs D3 and 7H2 could simultaneously inhibit the cleavage of the sialic acid of fetuin protein with large molecular weight and NA-XTD with small molecule weight in the NA inhibition experiment, prevent the formation of virus plaque at a low concentration, and effectively protect the mice from the challenge of the lethal dose of H7N9 virus.

Fusion of the dendritic cell-targeting chemokine MIP3α to melanoma antigen Gp100 in a therapeutic DNA vaccine significantly enhances immunogenicity and survival in a mouse melanoma model

Background

Although therapeutic cancer vaccines have been mostly disappointing in the clinic, the advent of novel immunotherapies and the future promise of neoantigen-based therapies have created the need for new vaccine modalities that can easily adapt to current and future developments in cancer immunotherapy. One such novel platform is a DNA vaccine fusing the chemokine Macrophage Inflammatory Protein-3α (MIP-3α) to an antigen, here melanoma antigen gp100. Previous published work has indicated that MIP-3α targets nascent peptides to immature dendritic cells, leading to processing by class I and II MHC pathways. This platform has shown enhanced efficacy in prophylactic melanoma and therapeutic lymphoma model systems.

Methods

The B16F10 melanoma syngeneic mouse model system was utilized, with a standard therapeutic protocol: challenge with lethal dose of B16F10 cells (5 × 10⁴) on day 0 and then vaccinate by intramuscular electroporation with 50 μg plasmid on days three, 10, and 17. Efficacy was assessed by analysis of tumor burden, tumor growth, and mouse survival, using the statistical tests ANOVA, mixed effects regression, and log-rank, respectively. Immunogenicity was assessed by ELISA and flow cytometric methods, including intracellular cytokine staining to assess vaccine-specific T-cell responses, all tested by ANOVA.

Results

We demonstrate that the addition of MIP3α to gp100 significantly enhances systemic anti-gp100 immunological parameters. Further, chemokine-fusion vaccine therapy significantly reduces tumor burden, slows tumor growth, and enhances mouse overall survival compared to antigen-only, irrelevant-antigen, and mock vaccines, with efficacy mediated by both CD4+ and CD8+ effector T cells. Antigen-only, irrelevant-antigen, and chemokine-fusion vaccines elicit significantly higher and similar CD4+ and CD8+ tumor-infiltrating lymphocyte (TIL) levels compared to mock vaccine. However, vaccine-specific CD8+ TILs are significantly higher in the chemokine-fusion vaccine group, indicating that the critical step induced by the fusion vaccine construct is the enhancement of vaccine-specific T-cell effectors.

Conclusions

The current study shows that fusion of MIP3α to melanoma antigen gp100 enhances the immunogenicity and efficacy of a DNA vaccine in a therapeutic B16F10 mouse melanoma model. This study analyzes an adaptable and easily produced MIP3α-antigen modular vaccine platform that could lend itself to a variety of functionalities, including combination treatments and neoantigen vaccination in the pursuit of personalized cancer therapy.

Electronic supplementary material

The online version of this article (doi:10.1186/s40425-016-0189-y) contains supplementary material, which is available to authorized users.

Mucosal administration of raccoonpox virus expressing highly pathogenic avian H5N1 influenza neuraminidase is highly protective against H5N1 and seasonal influenza virus challenge

We previously generated recombinant poxviruses expressing influenza antigens and studied their efficacy as potential highly pathogenic avian influenza (HPAI) vaccines in mice. While both modified vaccinia Ankara (MVA) and raccoon poxvirus (RCN) expressing hemagglutinin (HA) provided strong protection when administered by parenteral routes, only RCN-neuraminidase (NA) showed promise as a mucosal vaccine. In the present study we evaluated the efficacy of RCN-NA constructs by both intradermal (ID) and intranasal (IN) routes. Surprisingly, while RCN-NA completely protected mice when administered by the IN route, it failed to protect mice when administered by the ID route. After challenge, significantly less virus induced pathology was observed in the lungs of mice vaccinated with RCN-NA by the IN route as compared to the ID route. Furthermore, IN administration of RCN-NA elicited neutralizing antibodies detected in bronchoalveolar lavage (BAL) samples. We also determined the role of cellular immune responses in protection elicited by RCN-NA by depleting CD4 and CD8 T cells prior to challenge. Finally, we demonstrated for the first time that antibodies against NA can block viral entry in addition to viral spread in vitro. These studies demonstrate the importance of mucosal administration of RCN viral vectors for eliciting protective immune responses against the NA antigen.

Electroporation-mediated gene delivery

Electroporation has been used extensively to transfer DNA to bacteria, yeast, and mammalian cells in culture for the past 30 years. Over this time, numerous advances have been made, from using fields to facilitate cell fusion, delivery of chemotherapeutic drugs to cells and tissues, and most importantly, gene and drug delivery in living tissues from rodents to man. Electroporation uses electrical fields to transiently destabilize the membrane allowing the entry of normally impermeable macromolecules into the cytoplasm. Surprisingly, at the appropriate field strengths, the application of these fields to tissues results in little, if any, damage or trauma. Indeed, electroporation has even been used successfully in human trials for gene delivery for the treatment of tumors and for vaccine development. Electroporation can lead to between 100 and 1000-fold increases in gene delivery and expression and can also increase both the distribution of cells taking up and expressing the DNA as well as the absolute amount of gene product per cell (likely due to increased delivery of plasmids into each cell). Effective electroporation depends on electric field parameters, electrode design, the tissues and cells being targeted, and the plasmids that are being transferred themselves. Most importantly, there is no single combination of these variables that leads to greatest efficacy in every situation; optimization is required in every new setting. Electroporation-mediated in vivo gene delivery has proven highly effective in vaccine production, transgene expression, enzyme replacement, and control of a variety of cancers. Almost any tissue can be targeted with electroporation, including muscle, skin, heart, liver, lung, and vasculature. This chapter will provide an overview of the theory of electroporation for the delivery of DNA both in individual cells and in tissues and its application for in vivo gene delivery in a number of animal models.

Infection of influenza virus neuraminidase-vaccinated mice with homologous influenza virus leads to strong protection against heterologous influenza viruses

Vaccination is the best measure to prevent influenza pandemics. Here, we studied the protective effect against heterologous influenza viruses, including A/reassortant/NYMC X-179A (pH1N1), A/Chicken/Henan/12/2004 (H5N1), A/Chicken/Jiangsu/7/2002 (H9N2) and A/Guizhou/54/89×A/PR/8/34 (A/Guizhou-X) (H3N2), in mice first vaccinated with a DNA vaccine of haemagglutinin (HA) or neuraminidase (NA) of A/PR/8/34 (PR8) and then infected with the homologous virus. We showed that PR8 HA or NA vaccination both protected mice against a lethal dose of the homologous virus; PR8 HA or NA DNA vaccination and then PR8 infection in mice offered poor or excellent protection, respectively, against a second, heterologous influenza virus challenge. In addition, before the second heterologous influenza infection, the highest antibody level against nucleoprotein (NP) and matrix (M1 and M2) proteins was found in the PR8 NA-vaccinated and PR8-infected group. The level of induced cellular immunity against NP and M1 showed a trend consistent with that seen in antibody levels. However, PR8 HA+NA vaccination and then PR8 infection resulted in limited protection against heterologous influenza virus challenge. Results of the present study demonstrated that infection of the homologous influenza virus in mice already immunized with a NA vaccine could provide excellent protection against subsequent infection of a heterologous influenza virus. These findings suggested that NA, a major antigen of influenza virus, could be an important candidate antigen for universal influenza vaccines.

An alphavirus-based adjuvant enhances serum and mucosal antibodies, T cells, and protective immunity to influenza virus in neonatal mice

Neonatal immune responses to infection and vaccination are biased toward TH2 at the cost of proinflammatory TH1 responses needed to combat intracellular pathogens. However, upon appropriate stimulation, the neonatal immune system can induce adult-like TH1 responses. Here we report that a new class of vaccine adjuvant is especially well suited to enhance early life immunity. The GVI3000 adjuvant is a safe, nonpropagating, truncated derivative of Venezuelan equine encephalitis virus that targets dendritic cells (DCs) in the draining lymph node (DLN) and produces intracellular viral RNA without propagating to other cells. RNA synthesis strongly activates the innate immune response so that in adult animals, codelivery of soluble protein antigens induces robust humoral, cellular, and mucosal responses. The adjuvant properties of GVI3000 were tested in a neonatal BALB/c mouse model using inactivated influenza virus (iFlu). After a single immunization, mice immunized with iFlu with the GVI3000 adjuvant (GVI3000-adjuvanted iFlu) had significantly higher and sustained influenza virus-specific IgG antibodies, mainly IgG2a (TH1), compared to the mice immunized with antigen only. GVI3000 significantly increased antigen-specific CD4(+) and CD8(+) T cells, primed mucosal immune responses, and enhanced protection from lethal challenge. As seen in adult mice, the GVI3000 adjuvant increased the DC population in the DLNs, caused activation and maturation of DCs, and induced proinflammatory cytokines and chemokines in the DLNs soon after immunization, including gamma interferon (IFN-γ), tumor necrosis factor alpha (TNF-α), granulocyte colony-stimulating factor (G-CSF), and interleukin 6 (IL-6). In summary, the GVI3000 adjuvant induced an adult-like adjuvant effect with an influenza vaccine and has the potential to improve the immunogenicity and protective efficacy of new and existing neonatal vaccines.

IMPORTANCE

The suboptimal immune responses in early life constitute a significant challenge for vaccine design. Here we report that a new class of adjuvant is safe and effective for early life immunization and demonstrate its ability to significantly improve the protective efficacy of an inactivated influenza virus vaccine in a neonatal mouse model. The GVI3000 adjuvant delivers a truncated, self-replicating viral RNA into dendritic cells in the draining lymph node. Intracellular RNA replication activates a strong innate immune response that significantly enhances adaptive antibody and cellular immune responses to codelivered antigens. A significant increase in protection results from a single immunization. Importantly, this adjuvant also primed a mucosal IgA response, which is likely to be critical for protection during many early life infections.

Comparative analysis of antibody induction and protection against influenza virus infection by DNA immunization with HA, HAe, and HA1 in mice

Plasmid DNA vaccines are considered alternatives to inactivated influenza virus vaccines to control influenza. Vaccination with a hemagglutinin (HA)-, HA ectodomain (HAe)-, or HA subunit 1 (HA1)-based vaccine can stimulate protective immunity in animals. The aim of this study was to compare their capacity to induce an antibody response and protection against influenza virus infection in mice after DNA vaccination. We constructed three expression vectors encoding full-length HA, HAe, or HA1 of the A/California/07/2009 influenza A virus and designed three animal experiments: (i) BALB/c mice were immunized twice with 30 μg of the HA, HAe, or HA1 DNA vaccine with high-voltage electroporation (100 V), and 3 weeks after boosting, they were challenged with a lethal dose of virus. (ii) Immunization and challenge were as in experiment i, but with low-voltage electroporation (10 V). (iii) Mice were immunized once with 50 μg of DNA and challenged 1 week later. The immunogenic effects of the three DNA vaccines were evaluated in terms of antibody titer, survival rate, bodyweight change, and lung viral titer. In all three experiments, both HA and HAe induced higher antibody and neutralization titers than HA1. Following challenge with a lethal mouse-adapted homologous virus, both HA and HAe reduced the viral titers in lung washes or offered better protection from weight loss than HA1 in experiments ii and iii. Thus, HA1 induces a lower immune response than HA or HAe when used as a DNA vaccination. Our data should be valuable in choosing the optimal candidate vaccine when faced with the threat of pandemic influenza.

Universal anti-neuraminidase antibody inhibiting all influenza A subtypes

The only universally conserved sequence amongst all influenza A viral neuraminidase (NA) is located between amino acids 222-230 and plays crucial roles in viral replication. However, it remained unclear as to whether this universal epitope is exposed during the course of infection to allow binding and inhibition by antibodies. Using a monoclonal antibody (MAb) targeting this specific epitope, we demonstrated that all nine subtypes of NA were inhibited in vitro by the MAb. Moreover, the antibody also provided heterosubtypic protection in mice challenged with lethal doses of mouse-adapted H1N1 and H3N2, which represent group I and II viruses, respectively. Furthermore, we report amino acid residues I222 and E227, located in close proximity to the active site, are indispensable for inhibition by this antibody. This unique, highly-conserved linear sequence in viral NA could be an attractive immunological target for protection against diverse strains of influenza viruses.

Evaluation of neutralizing efficacy of monoclonal antibodies specific for 2009 pandemic H1N1 influenza A virus in vitro and in vivo

Pandemic influenza A virus (H1N1) 2009 poses a serious public-health challenge worldwide. To characterize the neutralizing epitopes of this virus, we generated a panel of eight monoclonal antibodies (mAbs) against the HA of the A/California/07/2009 virus. The antibodies were specific for the 2009 pdm H1N1 HA, as the antibodies displayed HA-specific ELISA, hemagglutination inhibition (HAI) and neutralization activity. One mAb (mAb12) showed significantly higher HAI and neutralizing titers than the other mAbs. We mapped the antigenic epitopes of the HA by characterizing escape mutants of a 2009 H1N1 vaccine strain (NYMC X-179A). The amino acid changes suggested that these eight mAbs recognized HA antigenic epitopes located in the Sa, Sb, Ca1 and Ca2 sites. Passive immunization with mAbs showed that mAb12 displayed more efficient neutralizing activity in vivo than the other mAbs. mAb12 was also found to be protective, both prophylactically and therapeutically, against a lethal viral challenge in mice. In addition, a single injection of 10 mg/kg mAb12 outperformed a 5-day course of treatment with oseltamivir (10 mg/kg/day by gavage) with respect to both prophylaxis and treatment of lethal viral infection. Taken together, our results showed that mouse-origin mAbs displayed neutralizing effectiveness in vitro and in vivo. One mAb in particular (mAb12) recognized an epitope within the Sb site and demonstrated outstanding neutralizing effectiveness.

Virus-like particles as universal influenza vaccines

Current influenza vaccines are primarily targeted to induce immunity to the influenza virus strain-specific hemagglutinin antigen and are not effective in controlling outbreaks of new pandemic viruses. An approach for developing universal vaccines is to present highly conserved antigenic epitopes in an immunogenic conformation such as virus-like particles (VLPs) together with an adjuvant to enhance the vaccine immunogenicity. In this review, the authors focus on conserved antigenic targets and molecular adjuvants that were presented in VLPs. Conserved antigenic targets that include the hemagglutinin stalk domain, the external domain of influenza M2 and neuraminidase are discussed in addition to molecular adjuvants that are engineered to be incorporated into VLPs in a membrane-anchored form.

Induction of cross-protection against influenza A virus by DNA prime-intranasal protein boost strategy based on nucleoprotein

BACKGROUND

The highly conserved nucleoprotein (NP) is an internal protein of influenza virus and is capable of inducing cross-protective immunity against different influenza A viruses, making it a main target of universal influenza vaccine. In current study, we characterized the immune response induced by DNA prime-intranasal protein boost strategy based on NP (A/PR/8/34, H1N1) in mouse model, and evaluated its protection ability against a lethal dose challenge of influenza virus.

RESULTS

The intranasal boost with recombinant NP (rNP) protein could effectively enhance the pre-immune response induced by the NP DNA vaccine in mice. Compared to the vaccination with NP DNA or rNP protein alone, the prime-boost strategy increased the level of NP specific serum antibody, enhanced the T cell immune response, and relatively induced more mucosal IgA antibody. The overall immune response induced by this heterologous prime-boost regimen was Th-1-biased. Furthermore, the immune response in mice induced by this strategy provided not only protection against the homologous virus but also cross-protection against a heterosubtypic H9N2 strain.

CONCLUSIONS

The NP DNA prime-intranasal protein boost strategy may provide an effective strategy for universal influenza vaccine development.

Renalase's expression and distribution in renal tissue and cells

To study renalase's expression and distribution in renal tissues and cells, renalase coded DNA vaccine was constructed, and anti-renalase monoclonal antibodies were produced using DNA immunization and hybridoma technique, followed by further investigation with immunological testing and western blotting to detect the expression and distribution of renalase among the renal tissue and cells. Anti-renalase monoclonal antibodies were successfully prepared by using DNA immunization technique. Further studies with anti-renalase monoclonal antibody showed that renalase expressed in glomeruli, tubule, mesangial cells, podocytes, renal tubule epithelial cells and its cells supernatant. Renalase is wildly expressed in kidney, including glomeruli, tubule, mesangial cells, podocytes and tubule epithelial cells, and may be secreted by tubule epithelial cells primarily.

Influenza M1 VLPs containing neuraminidase induce heterosubtypic cross-protection

Influenza virus like particles (VLPs) containing hemagglutinin were previously demonstrated to induce protection against the homologous strains. However, little information is available on the protective role of neuraminidase (NA), the second major glycoprotein. In this study, we developed VLPs (NA VLPs) containing NA and M1 derived from A/PR/8/34 (H1N1) influenza virus, and investigated their ability to induce protective immunity. Intranasal immunization with NA VLPs induced serum antibody responses to H1N1 and H3N2 influenza A viruses as well as significant neuraminidase inhibition activity. Importantly, mice immunized with NA VLPs were 100% protected against lethal infection by the homologous A/PR/8/34 (H1N1) as well as heterosubtypic A/Philippines/82 (H3N2) virus, although body weight loss was observed after lethal challenge with heterosubtypic H3N2 virus. The present study therefore provides evidence that influenza VLPs containing M1 and NA are capable of inducing immunity to homologous as well as antigenically distinct influenza A virus strains.

Influenza viral neuraminidase: the forgotten antigen

Influenza is the most common cause of vaccine-preventable morbidity and mortality despite the availability of the conventional trivalent inactivated vaccine and the live-attenuated influenza vaccine. These vaccines induce an immunity dominated by the response to hemagglutinin (HA) and are most effective when there is sufficient antigenic relatedness between the vaccine strain and the HA of the circulating wild-type virus. Vaccine strategies against influenza may benefit from inclusion of other viral antigens in addition to HA. Epidemiologic evidence and studies in animals and humans indicate that anti-neuraminidase (NA) immunity will provide protection against severe illness or death in the event of a significant antigenic change in the HA component of the vaccine. However, there is little NA immunity induced by trivalent inactivated vaccine and live-attenuated influenza vaccine. The quantity of NA in influenza vaccines is not standardized and varies significantly among manufacturers, production lots and tested strains. The activity and stability of the NA enzyme is influenced by concentration of divalent cations. If immunity against NA is desirable, a better understanding of how the enzymatic properties affect the immunogenicity is needed.

Analysis of different DNA vaccines for protection of experimental influenza A virus infection

Influenza viruses cause acute respiratory infections in humans that result in significant excessive morbidity and mortality rates every year. Current vaccines are limited in several aspects, including laborious manufacturing technology, non-sufficient efficacy, and time-consuming adjustments to new emerging virus variants. An alternative vaccine approach utilizes plasmid DNA encoding influenza virus antigens. Previous experiments have evaluated the protective efficacy of DNA vaccines expressing variable as well as conserved antigens. In this present study, several different combinations of influenza A virus (IAV) HA, NA, M1, M2, NS1, NS2, and NP sequences were cloned into the plasmid pVIVO, which allows the independent expression of two genes separately. These DNA vaccines were administered to induce protection against a lethal IAV infection, and to reduce immunopathology in lung tissue of surviving animals. The highest efficacy was provided by vaccines expressing HA and NA, as well as a mixture of plasmids encoding HA, NA, M1, M2, NS1, NS2, and NP (Mix). Three days post-infection, more than a 99.99% reduction of viral load and no inflammation was achieved in lung tissue of pVIVO/HA-NA-vaccinated mice. Animals vaccinated with pVIVO/HA-NA, pVIVO/HA-M2, or vaccine Mix, survived a lethal challenge with minor or no obvious pathologic abnormities in the lungs. All other surviving mice revealed extensive changes in the lung tissue, indicating possibly an ongoing bronchiolitis obliterans. In addition, pVIVO/HA-NA and the vaccine Mix were also protective against a heterologous IAV infection. Taken together, next to all combinations of different DNA vaccines, the intramuscular application of pVIVO/HA-NA was the most efficient procedure to decrease virus replication and to prevent immunopathology in lung tissue of IAV-infected mice.

DNA vaccines: an historical perspective and view to the future

This review provides a detailed look at the attributes and immunologic mechanisms of plasmid DNA vaccines and their utility as laboratory tools as well as potential human vaccines. The immunogenicity and efficacy of DNA vaccines in a variety of preclinical models is used to illustrate how they differ from traditional vaccines in novel ways due to the in situ antigen production and the ease with which they are constructed. The ability to make new DNA vaccines without needing to handle a virulent pathogen or to adapt the pathogen for manufacturing purposes demonstrates the potential value of this vaccine technology for use against emerging and epidemic pathogens. Similarly, personalized anti-tumor DNA vaccines can also readily be made from a biopsy. Because DNA vaccines bias the T-helper (Th) cell response to a Th1 phenotype, DNA vaccines are also under development for vaccines against allergy and autoimmune diseases. The licensure of four animal health products, including two prophylactic vaccines against infectious diseases, one immunotherapy for cancer, and one gene therapy delivery of a hormone for a food animal, provides evidence of the efficacy of DNA vaccines in multiple species including horses and pigs. The size of these target animals provides evidence that the somewhat disappointing immunogenicity of DNA vaccines in a number of human clinical trials is not due simply to the larger mass of humans compared with most laboratory animals. The insights gained from the mechanisms of protection in the animal vaccines, the advances in the delivery and expression technologies for increasing the potency of DNA vaccines, and encouragingly potent human immune responses in certain clinical trials, provide insights for future efforts to develop DNA vaccines into a broadly useful vaccine and immunotherapy platform with applications for human and animal health.

Broad humoral and cellular immunity elicited by a bivalent DNA vaccine encoding HA and NP genes from an H5N1 virus

Influenza A virus is highly variable and a major viral respiratory pathogen that can cause severe illness in humans. Therefore it is important to induce a sufficient immune response specific to current strains and to heterosubtypic viruses with vaccines. In this study, we developed a dual-promoter-based bivalent DNA vaccine that encodes both hemagglutinin (HA) and nucleoprotein (NP) proteins from a highly pathogenic A/Chicken/Henan/12/2004 (H5N1) virus. Our results show that the expression levels of HA and NP genes from the dual-promoter plasmid are similar to those seen when they are expressed individually in independent plasmids. When the bivalent DNA vaccine was inoculated via intramuscular injection and in vivo electroporation, high levels of both humoral and cellular immune responses were elicited against homologous H5N1 virus and heterosubtypic H9N2 virus. Furthermore, no obvious antigenic competition was observed between HA and NP proteins in the dual-promoter-based bivalent vaccine compared to monovalent vaccines. Our data suggest that a combination of influenza surface and internal viral genes in a dual-promoter-expressing plasmid may provide a new approach for developing a DNA vaccine that may protect not only specifically against a currently circulating strain, but also may cross-protect broadly against new heterosubtypic viruses.

Evaluation of a novel non-penetrating electrode for use in DNA vaccination

Current progress in the development of vaccines has decreased the incidence of fatal and non-fatal infections and increased longevity. However, new technologies need to be developed to combat an emerging generation of infectious diseases. DNA vaccination has been demonstrated to have great potential for use with a wide variety of diseases. Alone, this technology does not generate a significant immune response for vaccination, but combined with delivery by electroporation (EP), can enhance plasmid expression and immunity. Most EP systems, while effective, can be invasive and painful making them less desirable for use in vaccination. Our lab recently developed a non-invasive electrode known as the multi-electrode array (MEA), which lies flat on the surface of the skin without penetrating the tissue. In this study we evaluated the MEA for its use in DNA vaccination using Hepatitis B virus as the infectious model. We utilized the guinea pig model because their skin is similar in thickness and morphology to humans. The plasmid encoding Hepatitis B surface antigen (HBsAg) was delivered intradermally with the MEA to guinea pig skin. The results show increased protein expression resulting from plasmid delivery using the MEA as compared to injection alone. Within 48 hours of treatment, there was an influx of cellular infiltrate in experimental groups. Humoral responses were also increased significantly in both duration and intensity as compared to injection only groups. While this electrode requires further study, our results suggest that the MEA has potential for use in electrically mediated intradermal DNA vaccination.

Development of retinol-binding protein 4 immunocolloidal gold fast test strip using high-sensitivity monoclonal antibodies generated by DNA immunization

DNA immunization is an efficient method for high-affinity monoclonal antibody generation. Here, we describe the generation of several high-quality monoclonal antibodies (mAbs) against retinol-binding protein 4 (RBP4), an important marker for kidney abnormality and dysfunction, with a combination method of DNA priming and protein boost. The mAbs generated could bind to RBP4 with high sensitivity and using these mAbs, an immunocolloidal gold fast test strip was constructed. The strip can give a result in <5 min and is very sensitive with a detection limit of about 1 ng/ml. A small-scale clinical test revealed that the result of this strip was well in accordance with that of an enzyme-labeled immunosorbent assay kit currently available on the market. Consequently, it could be useful for more convenient and faster RBP4 determination in the clinic.

The migration of T cells in response to influenza virus is altered in neonatal mice

Influenza virus is a significant cause of mortality and morbidity in children; however, little is known about the T cell response in infant lungs. Neonatal mice are highly vulnerable to influenza and only control very low doses of virus. We compared the T cell response to influenza virus infection between mice infected as adults or at 2 d old and observed defective migration into the lungs of the neonatal mice. In the adult mice, the numbers of T cells in the lung interstitia peaked at 10 d postinfection, whereas neonatal T cell infiltration, activation, and expression of TNF-alpha was delayed until 2 wk postinfection. Although T cell numbers ultimately reached adult levels in the interstitia, they were not detected in the alveoli of neonatal lungs. Instead, the alveoli contained eosinophils and neutrophils. This altered infiltrate was consistent with reduced or delayed expression of type 1 cytokines in the neonatal lung and differential chemokine expression. In influenza-infected neonates, CXCL2, CCL5, and CCL3 were expressed at adult levels, whereas the chemokines CXCL1, CXCL9, and CCL2 remained at baseline levels, and CCL11 was highly elevated. Intranasal administration of CCL2, IFN-gamma, or CXCL9 was unable to draw the neonatal T cells into the airways. Together, these data suggest that the T cell response to influenza virus is qualitatively different in neonatal mice and may contribute to an increased morbidity.

A live bivalent influenza vaccine based on a H9N2 virus strain

The purpose of this study was to construct an H9N2 virus-based bivalent live vaccine expressing the protective antigen of a different subtype of influenza virus. Reverse genetics was used to generate an influenza virus containing nine gene segments derived from the A/Chicken/Jiangsu/11/2002 (H9N2) strain, including independent M1 and M2 matrix gene segments. A recombinant virus expressing the H1N1 HA1 hemagglutinin protein was produced on this framework by substituting the extracellular domain of the H9N2 M2 gene with the H1N1 HA1 fragment from A/PR/8/34 (PR8, H1N1). The resulting hybrid virus H9N2-PR8/HA1 was genetically stable and of low pathogenicity. Intra-nasal immunization of BALB/c mice with H9N2-PR8/HA1 virus induced both anti-H9N2 virus and anti-PR8 HA antibodies and conferred protection to mice against lethal challenge (40x LD(50)) with either H1N1 or H9N2 viruses. This study provides a new influenza H9N2 virus model for the expression and/or delivery of foreign antigens.

Immune effects against influenza A virus and a novel DNA vaccine with co-expression of haemagglutinin- and neuraminidase-encoding genes

The high variability of influenza virus causes difficulties in the control and prevention of influenza, thus seeking a promising approach for dealing with these problems is a hot topic. Haemagglutinin (HA) and neuraminidase (NA) are major surface antigens of the influenza virus, and provide effective protection against lethal challenges with this virus. We constructed a DNA vaccine (pHA-IRES2-NA) that co-expressed both HA and NA, and compared its protective efficacy and immunogenic ability with that of singly expressed HA or NA, or a mixture of the two singly expressed proteins. Our findings showed that both HA and NA proteins expressed by pHA-IRES2-NA could be detected in vivo and in vitro. The protection of DNA vaccines was evaluated by serum antibody titres, residual lung virus titres and survival rates of the mice. In the murine model, immunization of pHA-IRES2-NA generated significant anti-HA and anti-NA antibody, increased the percentage of CD8(+) cells and gamma interferon-producing CD8(+) cells and the ratio of Th1/Th2 (T helper) cells, which was comparable to the effects of immunization with HA or NA DNA alone or with a mixture of HA and NA DNA. All the mice inoculated by pHA-IRES2-NA resisted the lethal challenge by homologous influenza virus and survived with low lung virus titre. In addition, previous studies reported that co-expression allowed higher-frequency transduction compared to co-transduction of separated vector systems encoding different genes. The novel HA and NA co-expression DNA vaccine is a successful alternative to using a mixture of purified HA and NA proteins or HA and NA DNA.

Characterization of triple reassortant H1N1 influenza A viruses from swine in Ohio

An H1N1 influenza A virus, A/swine/Ohio/24366/07, was isolated from pigs in an Ohio county fair. Twenty-six people who came in contact with the infected pigs developed respiratory disease and two of these people were laboratory confirmed as H1N1 by the Centers for Disease Control and Prevention (CDC). The A/swine/Ohio/24366/07 virus we isolated from swine was shown at the CDC to have 100% identical genome sequence to the human virus associated with the county fair. This prompted us to characterize three swine and two human origin H1N1 influenza A viruses isolated at different time points in the State of Ohio. The three swine viruses were shown to be triple reassortant viruses harboring genes of human (PB1), swine (HA, NA, NP, M, and NS), and avian (PB2 and PA) lineage viruses. Although viruses evaluated in this study were isolated during a short time interval (3 years), genetic drift was observed within the HA and NA genes, including changes at the receptor binding and antigenic sites of HA1 protein. Nevertheless, all viruses exhibited antigenic similarity as evaluated with hemagglutination inhibition and virus neutralizing tests. Internal genes were similar to other reassortant viruses of various subtypes currently circulating in the United States. Interestingly, two of the swine viruses including the 2007 isolate replicated well in human airway epithelial cells, however, another virus isolated in 2006 showed very little replication.

Induction of long-term protective immune responses by influenza H5N1 virus-like particles

Background

Recurrent outbreaks of highly pathogenic H5N1 avian influenza virus pose a threat of eventually causing a pandemic. Early vaccination of the population would be the single most effective measure for the control of an emerging influenza pandemic.

Methodology/Principal Findings

Influenza virus-like particles (VLPs) produced in insect cell-culture substrates do not depend on the availability of fertile eggs for vaccine manufacturing. We produced VLPs containing influenza A/Viet Nam1203/04 (H5N1) hemagglutinin, neuraminidase, and matrix proteins, and investigated their preclinical immunogenicity and protective efficacy. Mice immunized intranasally with H5N1 VLPs developed high levels of H5N1 specific antibodies and were 100% protected against a high dose of homologous H5N1 virus infection at 30 weeks after immunization. Protection is likely to be correlated with humoral and cellular immunologic memory at systemic and mucosal sites as evidenced by rapid anamnestic responses to re-stimulation with viral antigen in vivo and in vitro.

Conclusions/Significance

These results provide support for clinical evaluation of H5N1 VLP vaccination as a public health intervention to mitigate a possible pandemic of H5N1 influenza.

A single immunization with HA DNA vaccine by electroporation induces early protection against H5N1 avian influenza virus challenge in mice

Background

Developing vaccines for the prevention of human infection by H5N1 influenza viruses is an urgent task. DNA vaccines are a novel alternative to conventional vaccines and should contribute to the prophylaxis of emerging H5N1 virus. In this study, we assessed whether a single immunization with plasmid DNA expressing H5N1 hemagglutinin (HA) could provide early protection against lethal challenge in a mouse model.

Methods

Mice were immunized once with HA DNA at 3, 5, 7 days before a lethal challenge. The survival rate, virus titer in the lungs and change of body weight were assayed to evaluate the protective abilities of the vaccine. To test the humoral immune response induced by HA DNA, serum samples were collected through the eye canthus of mice on various days after immunization and examined for specific antibodies by ELISA and an HI assay. Splenocytes were isolated after the immunization to determine the antigen-specific T-cell response by the ELISPOT assay.

Results

Challenge experiments revealed that a single immunization of H5N1 virus HA DNA is effective in early protection against lethal homologous virus. Immunological analysis showed that an antigen-specific antibody and T-cell response could be elicited in mice shortly after the immunization. The protective abilities were correlated with the amount of injected DNA and the length of time after vaccination.

Conclusion

A single immunization of 100 μg H5 HA DNA vaccine combined with electroporation was able to provide early protection in mice against homologous virus infection.

Influenza neuraminidase as a vaccine antigen

The neuraminidase protein of influenza viruses is a surface glycoprotein that shows enzymatic activity to remove sialic acid, the viral receptor, from both viral and host proteins. The removal of sialic acid from viral proteins plays a key role in the release of the virus from the cell by preventing the aggregation of the virus by the hemagglutinin protein binding to other viral proteins. Antibodies to the neuraminidase protein can be protective alone in animal challenge studies, but the neuraminidase antibodies appear to provide protection in a different manner than antibodies to the hemagglutinin protein. Neutralizing antibodies to the hemagglutinin protein can directly block virus entry, but protective antibodies to the neuraminidase protein are thought to primarily aggregate virus on the cell surface, effectively reducing the amount of virus released from infected cells. The neuraminidase protein can be divided into nine distinct antigenic subtypes, where there is little cross-protection of antibodies between subtypes. All nine subtypes of neuraminidase protein are commonly found in avian influenza viruses, but only selected subtypes are routinely found in mammalian influenza viruses; for example, only the N1 and N2 subtypes are commonly found in both humans and swine. Even within a subtype, the neuraminidase protein can have a high level of antigenic drift, and vaccination has to specifically be targeted to the circulating strain to give optimal protection. The levels of neuraminidase antibody also appear to be critical for protection, and there is concern that human influenza vaccines do not include enough neuraminidase protein to induce a strong protective antibody response. The neuraminidase protein has also become an important target for antiviral drugs that target sialic acid binding which blocks neuraminidase enzyme activity. Two different antiviral drugs are available and are widely used for the treatment of seasonal influenza in humans, but antiviral resistance appears to be a growing concern for this class of antivirals.

Influenza

Influenza is a highly contagious, acute respiratory illness afflicting humans. Although influenza epidemics occur frequently, their severity varies (1). Not until 1933, when the first human influenza virus was isolated, was it possible to define with certainty which pandemics were caused by influenza viruses. In general, influenza A viruses are more pathogenic than are influenza B viruses. Influenza A virus is a zoonotic infection, and more than 100 types of influenza A viruses infect most species of birds, pigs, horses, dogs, and seals. It is believed that the 1918–1919 pandemic originated from a virulent strain of H1N1 from pigs and birds.

Parameters for DNA vaccination using adaptive constant-current electroporation in mouse and pig models

Enhancing the expression of DNA vaccines requires that specific conditions of delivery are optimized. We describe experiments using adaptive constant-current electroporation (EP) in mice and pigs examining parameters such as target muscle, delay between plasmid delivery and onset of EP pulses and DNA vaccine formulation; our studies show that concentrated formulations result in better expression and immunogenicity. Furthermore, various conditions of EP that limit the amount of muscle damage were measured. The results of these studies will help to advance the success of DNA vaccines in animals into success in human clinical trials.

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.

Vaccination with hemagglutinin or neuraminidase DNA protects BALB/c mice against influenza virus infection in presence of maternal antibody

Background

Maternal antibody is the major form of protection against disease in early life; however, its presence interferes with active immunization of offspring. In order to overcome the immunosuppression caused by maternal antibody, several immune strategies were explored in this paper using mouse model and influenza vaccines.

Results

The results showed that: i) when the offspring were immunized with the same vaccine as their mothers, whether inactivated or DNA vaccine, the presence of maternal antibody inhibited offspring immune response and the offspring could not be protected from a lethal influenza virus infection; ii) when the offspring, born to mothers immunized with inactivated vaccine, were immunized with NA DNA vaccine, the interference of maternal antibody were overcome and the offspring could survive a lethal virus challenge; iii) when the offspring were immunized with different DNA vaccine from that for their mothers, the interference of maternal antibody were also overcome. In addition, high-dose inactivated vaccine in maternal immunization caused partial inhibition in offspring when the offspring were immunized with HA DNA vaccine, while lower dose caused no significant immunosuppression.

Conclusion

To avoid the interference of maternal antibody in influenza vaccination of offspring, mothers and their offspring shall not be immunized with the same vaccine. If mothers are immunized with inactivated vaccine, NA DNA vaccine for the offspring shall be effective; and if mothers are immunized with HA (NA) DNA, NA (HA) DNA for the offspring shall be effective.

Electroporation-mediated gene therapy

Non-viral gene transfer is markedly enhanced by the application of in vivo electroporation. Electroporation is a safe and efficient system to introduce genes to a wide variety of tissues, including skeletal muscle, tumors, kidney, liver and skin. Electroporation has been demonstrated to be effective in numerous disease models. This review focuses on the principles of electroporation and the target tissues employed for gene therapy. Based on the accumulation of positive results, the first clinical study for the treatment of malignant melanoma is now underway, and preclinical studies have suggested that electroporation is useful as a gene therapy protocol.

Scientific barriers to developing vaccines against avian influenza viruses

The increasing number of reports of direct transmission of avian influenza viruses to humans in the past few years and the ongoing outbreak of H5N1 influenza virus infections in birds and humans highlight the pandemic threat posed by avian influenza viruses.

Although vaccination is the key strategy for the prevention of severe illness and death from pandemic influenza viruses and despite the long-term experience with vaccines against human influenza viruses, researchers face several obstacles in developing successful vaccines against avian influenza viruses.

The haemagglutinin (HA) and neuraminidase (NA) glycoproteins of influenza viruses are the main targets of the protective immune response. Licensed influenza virus vaccines are designed to induce HA-specific antibody responses to protect the host from infection. However, the presence of 16 subtypes of HA and 9 subtypes of NA glycoproteins among avian influenza viruses and the genetic and antigenic diversity among each subtype in nature present several unique challenges for the generation of broadly cross-protective vaccines.

Inactivated virus and live attenuated virus vaccines against pandemic influenza are being developed on the basis of plasmid-based reverse-genetics technology. Vaccines based on various other platforms, including live virus vectors and DNA vaccines, are also being developed and show promise in preclinical studies.

The available data indicate that inactivated avian influenza virus vaccines are poorly immunogenic and require a high concentration of HA glycoprotein or co-administration with an adjuvant to achieve the desired antibody response in humans. The biological basis for the poor immunogenicity of avian HA glycoproteins is not well understood.

Assays to measure the immune response to avian influenza viruses, in particular cell-mediated immune responses, are not available and the immune correlates of protection are not well understood. The choice of assay(s) for assessment of the immune response to pandemic influenza vaccines is a practical challenge in the evaluation of candidate vaccines.

As it is difficult to predict which avian influenza virus will cross the species barrier and cause a future pandemic, a library of candidate vaccines of different subtypes must be generated and evaluated in animal models and humans.

Although an ideal vaccine would prevent infection, a more realistic goal for a pandemic influenza vaccine might be to prevent severe illness and death.

The pandemic threat posed by avian influenza viruses highlights the need for new safe and efficient vaccines. However, several unique obstacles are faced by researchers in the development of these vaccines against avian influenza viruses. What are these obstacles and how can we overcome them?

The increasing number of reports of direct transmission of avian influenza viruses to humans underscores the need for control strategies to prevent an influenza pandemic. Vaccination is the key strategy to prevent severe illness and death from pandemic influenza. Despite long-term experience with vaccines against human influenza viruses, researchers face several additional challenges in developing human vaccines against avian influenza viruses. In this Review, we discuss the features of avian influenza viruses, the gaps in our understanding of infections caused by these viruses in humans and of the immune response to them that distinguishes them from human influenza viruses, and the current status of vaccine development.

DNA vaccines: recent developments and future possibilities

The field of DNA vaccines continues to advance and several new strategies to augment the immunogenicity of DNA vaccines are under evaluation. The majority of these studies are in the early preclinical stage, but some DNA vaccines have moved into clinical trials. In this review, we describe some of the more recent efforts aimed at increasing the immunogenicity of DNA vaccines, including the use of genetic adjuvants and plasmid-based expression of viral replicons. In addition, we discuss the possibility of using DNA vaccines to address emerging infectious agents where they may provide an advantage over other vaccine strategies and we review some areas where DNA vaccines have been used to target self-antigens.

Essential sequence of influenza neuraminidase DNA to provide protection against lethal viral infection

Neuraminidase (NA) is one of the surface glycoproteins of influenza virus. The immune response induced by NA DNA in the mouse model has been proved, in our previous study, to be able to provide an adequate protection against the challenge with a homologous virus and a crossprotection against the challenge with a heterologous virus of the same subtype. In this paper, a series of NA plasmid truncates, with the nucleotides at the 5'- or 3'-terminal of A/PR/8/34 (H1N1) NA deleted serially, were constructed by PCR. BALB/c mice were immunized with the plasmid truncates and challenged with homologous virus at a lethal dosage. The essential sequence of NA DNA to provide protection against the influenza virus was explored by observing the survival rates, serum anti-NA antibody titers, and lung virus titers of the mice. The result showed that, along with the increasing number of nucleotides deleted serially at the 5'- or 3'-terminal of NA DNA, the antibody titer induced by NA DNA decreased. NA DNA lost its protection against the influenza virus when 60 nucleotides (or 20 amino acids) at the 5'-terminal or 66 nucleotides (or 22 amino acids) at the 3'-terminal were deleted. The nt58-1299 of NA DNA may play an important role in protection against influenza virus.

Protection against avian influenza H9N2 virus challenge by immunization with hemagglutinin- or neuraminidase-expressing DNA in BALB/c mice

Avian influenza viruses of H9N2 subtype are widely spread in avian species. The viruses have recently been transmitted to mammalian species, including humans, accelerating the efforts to devise protective strategies against them. In this study, an avian influenza H9N2 virus strain (A/Chicken/Jiangsu/7/2002), isolated in Jiangsu Province, China, was used to infect BALB/c mice for adaptation. After five lung-to-lung passages, the virus was stably proliferated in a large quantity in the murine lung and caused the deaths of mice. In addition, we explored the protection induced by H9N2 virus hemagglutinin (HA)- and neuraminidase (NA)-expressing DNAs in BALB/c mice. Female BALB/c mice aged 6-8 weeks were immunized once or twice at a 3-week interval with HA-DNA and NA-DNA by electroporation, respectively, each at a dose of 3, 10 or 30microg. The mice were challenged with a lethal dose (40x LD(50)) of influenza H9N2 virus four weeks after immunization once or one week after immunization twice. The protections of DNA vaccines were evaluated by the serum antibody titers, residual lung virus titers, and survival rates of the mice. The result showed that immunization once with not less than 10microg or twice with 3microg HA-DNA or NA-DNA provided effective protection against homologous avian influenza H9N2 virus.