Purified influenza virus nucleoprotein protects mice from lethal infection

Opportunities and challenges for T cell-based influenza vaccines

Vaccination remains our main defence against influenza, which causes substantial annual mortality and poses a serious pandemic threat. Influenza virus evades immunity by rapidly changing its surface antigens but, even when the vaccine is well matched to the current circulating virus strains, influenza vaccines are not as effective as many other vaccines. Influenza vaccine development has traditionally focused on the induction of protective antibodies, but there is mounting evidence that T cell responses are also protective against influenza. Thus, future vaccines designed to promote both broad T cell effector functions and antibodies may provide enhanced protection. As we discuss, such vaccines present several challenges that require new strategic and economic considerations. Vaccine-induced T cells relevant to protection may reside in the lungs or lymphoid tissues, requiring more invasive assays to assess the immunogenicity of vaccine candidates. T cell functions may contain and resolve infection rather than completely prevent infection and early illness, requiring vaccine effectiveness to be assessed based on the prevention of severe disease and death rather than symptomatic infection. It can be complex and costly to measure T cell responses and infrequent clinical outcomes, and thus innovations in clinical trial design are needed for economic reasons. Nevertheless, the goal of more effective influenza vaccines justifies renewed and intensive efforts.

Safety and Immunogenicity of the Convacell® Recombinant N Protein COVID-19 Vaccine

We have developed Convacell®-a COVID-19 vaccine based on the recombinant nucleocapsid (N) protein of SARS-CoV-2. This paper details Convacell's® combined phase I/II and IIb randomized, double-blind, interventional clinical trials. The primary endpoints were the frequency of adverse effects (AEs) and the titers of specific anti-N IgGs induced by the vaccination; secondary endpoints included the nature of the immune response. Convacell® demonstrated high safety in phase I with no severe AEs detected, 100% seroconversion by day 42 and high and sustained for 350 days anti-N IgG levels in phase II. Convacell® also demonstrated a fused cellular and humoral immune response. Phase IIb results showed significant post-vaccination increases in circulating anti-N IgG and N protein-specific IFNγ+-producing PBMC quantities among 438 volunteers. Convacell® showed same level of immunological efficacy for single and double dose vaccination regimens, including for elderly patients. The clinical studies indicate that Convacell® is safe and highly immunogenic.

Nucleoprotein as a Promising Antigen for Broadly Protective Influenza Vaccines

Annual vaccination is considered as the main preventive strategy against seasonal influenza. Due to the highly variable nature of major viral antigens, such as hemagglutinin (HA) and neuraminidase (NA), influenza vaccine strains should be regularly updated to antigenically match the circulating viruses. The influenza virus nucleoprotein (NP) is much more conserved than HA and NA, and thus seems to be a promising target for the design of improved influenza vaccines with broad cross-reactivity against antigenically diverse influenza viruses. Traditional subunit or recombinant protein influenza vaccines do not contain the NP antigen, whereas live-attenuated influenza vaccines (LAIVs) express the viral NP within infected cells, thus inducing strong NP-specific antibodies and T-cell responses. Many strategies have been explored to design broadly protective NP-based vaccines, mostly targeted at the T-cell mode of immunity. Although the NP is highly conserved, it still undergoes slow evolutionary changes due to selective immune pressure, meaning that the particular NP antigen selected for vaccine design may have a significant impact on the overall immunogenicity and efficacy of the vaccine candidate. In this review, we summarize existing data on the conservation of the influenza A viral nucleoprotein and review the results of preclinical and clinical trials of NP-targeting influenza vaccine prototypes, focusing on the ability of NP-specific immune responses to protect against diverse influenza viruses.

Induction of Homosubtypic and Heterosubtypic Immunity to Influenza Viruses Using a Conserved Internal and External Proteins

The immunogenicity and protective properties of the designed recombinant fusion peptide of 3M2e and truncated nucleoprotein (trNP), originating from Influenza A virus, were investigated in the BALB/c mice model in comparison with the Mix protein (3M2e + trNP). The results were evaluated by antibody response, cytokine production, lymphocyte proliferation assay, and mortality rate after challenge with homologous (H1N1) and heterologous (H3N2) influenza viruses in BALB/c mice. The animals that received the chimer protein with or without adjuvant had more specific antibody responses and elicited memory CD4 T cells, and cytokines of Th1 and Th2 cells compared to the Mix protein. Moreover, the Mix protein, like the recombinant chimer protein, provided equal and effective protection against both homologous and heterologous challenges in mice. Nevertheless, the chimer protein demonstrated superior immune protection compared to the Mix protein. The percentage of survived animals in the adjuvanted protein group (78.4%) was less than the non-adjuvanted one (85.7%). However, the Mix protein plus Alum could induce protective immunity in only 57.1% and 42.8% of homologous and heterologous virus-challenged mice, respectively. Regarding the sufficient immunogenicity and protectivity of the chimer protein construct against influenza viruses, the findings of the study suggest that the chimer protein without a requirement of adjuvant can be used as an adequate vaccine formulation to protect against a broad spectrum of influenza viruses.

Immunogenicity and In Vivo Protective Effects of Recombinant Nucleocapsid-Based SARS-CoV-2 Vaccine Convacell®

The vast majority of SARS-CoV-2 vaccines which are licensed or under development focus on the spike (S) protein and its receptor binding domain (RBD). However, the S protein shows considerable sequence variations among variants of concern. The aim of this study was to develop and characterize a SARS-CoV-2 vaccine targeting the highly conserved nucleocapsid (N) protein. Recombinant N protein was expressed in Escherichia coli, purified to homogeneity by chromatography and characterized by SDS-PAGE, immunoblotting, mass spectrometry, dynamic light scattering and differential scanning calorimetry. The vaccine, formulated as a squalane-based emulsion, was used to immunize Balb/c mice and NOD SCID gamma (NSG) mice engrafted with human PBMCs, rabbits and marmoset monkeys. Safety and immunogenicity of the vaccine was assessed via ELISA, cytokine titer assays and CFSE dilution assays. The protective effect of the vaccine was studied in SARS-CoV-2-infected Syrian hamsters. Immunization induced sustainable N-specific IgG responses and an N-specific mixed Th1/Th2 cytokine response. In marmoset monkeys, an N-specific CD4⁺/CD8⁺ T cell response was observed. Vaccinated Syrian hamsters showed reduced lung histopathology, lower virus proliferation, lower lung weight relative to the body, and faster body weight recovery. Convacell^® thus is shown to be effective and may augment the existing armamentarium of vaccines against COVID-19.

Effect of an Adenovirus-Vectored Universal Influenza Virus Vaccine on Pulmonary Pathophysiology in a Mouse Model

Current influenza vaccines, live attenuated or inactivated, do not protect against antigenically novel influenza A viruses (IAVs) of pandemic potential, which has driven interest in the development of universal influenza vaccines. Universal influenza vaccine candidates targeting highly conserved antigens of IAV nucleoprotein (NP) are promising as vaccines that induce T cell immunity, but concerns have been raised about the safety of inducing robust CD8 T cell responses in the lungs. Using a mouse model, we systematically evaluated effects of recombinant adenovirus vectors (rAd) expressing IAV NP (A/NP-rAd) or influenza B virus (IBV) NP (B/NP-rAd) on pulmonary inflammation and function after vaccination and following live IAV challenge. After A/NP-rAd or B/NP-rAd vaccination, female mice exhibited robust systemic and pulmonary vaccine-specific B cell and T cell responses and experienced no morbidity (e.g., body mass loss). Both in vivo pulmonary function testing and lung histopathology scoring revealed minimal adverse effects of intranasal rAd vaccination compared with unvaccinated mice. After IAV challenge, A/NP-rAd-vaccinated mice experienced significantly less morbidity, had lower pulmonary virus titers, and developed less pulmonary inflammation than unvaccinated or B/NP-rAd-vaccinated mice. Based on analysis of pulmonary physiology using detailed testing not previously applied to the question of T cell damage, mice protected by vaccination also had better lung function than controls. Results provide evidence that, in this model, adenoviral universal influenza vaccine does not damage pulmonary tissue. In addition, adaptive immunity, in particular, T cell immunity in the lungs, does not cause damage when restimulated but instead mitigates pulmonary damage following IAV infection.IMPORTANCE Respiratory viruses can emerge and spread rapidly before vaccines are available. It would be a tremendous advance to use vaccines that protect against whole categories of viruses, such as universal influenza vaccines, without the need to predict which virus will emerge. The nucleoprotein (NP) of influenza virus provides a target conserved among strains and is a dominant T cell target. In animals, vaccination to NP generates powerful T cell immunity and long-lasting protection against diverse influenza strains. Concerns have been raised, but not evaluated experimentally, that potent local T cell responses might damage the lungs. We analyzed lung function in detail in the setting of such a vaccination. Despite CD8 T cell responses in the lungs, lungs were not damaged and functioned normally after vaccination alone and were protected upon subsequent infection. This precedent provides important support for vaccines based on T cell-mediated protection, currently being considered for both influenza and SARS-CoV-2 vaccines.

Vaccination of aged mice with adjuvanted recombinant influenza nucleoprotein enhances protective immunity

Elderly individuals are highly susceptible to influenza virus (IAV) infection and respond poorly to influenza vaccines. Although the generally accepted correlate of protection following influenza vaccination is neutralizing antibody titers, cytotoxic T cell activity has been found to be a better correlate in the elderly. This suggests that vaccines designed to protect against influenza in the elderly should induce both humoral and cellular immunity. The co-induction of T cell immunity is additionally advantageous, as virus-specific T cells are frequently cross-reactive against different strains of IAV. Here, we tested the capacity of a synthetic TLR-4 adjuvant, SLA-SE (second-generation lipid adjuvant formulated in a squalene-based oil-in-water emulsion) to elicit T cell immunity to a recombinant influenza nucleoprotein (rNP), in both young and aged mice. IAV challenge of vaccinated mice resulted in a modest increase in the numbers of NP-specific CD4 and CD8 effector T cells in the spleen, but did not increase numbers of memory phenotype CD8 T cells generated following viral clearance (compared to control vaccinated mice). Cytotoxic activity of CD8, but not CD4 T cells was increased. In addition, SLA-SE adjuvanted vaccination specifically enhanced the production of NP-specific IgG2c antibodies in both young and aged mice. Although NP-specific antibodies are not neutralizing, they can cooperate with CD8 T cells and antigen-presenting cells to enhance protective immunity. Importantly, SLA-SE adjuvanted rNP-vaccination of aged mice resulted in significantly enhanced viral clearance. In addition, vaccination of aged mice resulted in enhanced survival after lethal challenge compared to control vaccination, that approached statistical significance. These data demonstrate the potential of SLA-SE adjuvanted rNP vaccines to (i) generate both cellular and humoral immunity to relatively conserved IAV proteins and (ii) elicit protective immunity to IAV in aged mice.

Generation of a Reassortant Influenza A Subtype H3N2 Virus Expressing Gaussia Luciferase

Reporter influenza A viruses (IAVs) carrying fluorescent or luminescent genes provide a powerful tool for both basic and translational research. Most reporter IAVs are based on the backbone of either subtype H1N1 viruses, A/Puerto Rico/8/1934 (PR8) or A/WSN/1933, but no reporter subtype H3N2 virus is currently available to our knowledge. Since the IAV subtype H3N2 co-circulates with H1N1 among humans causing annual epidemics, a reporter influenza A subtype H3N2 virus would be highly valuable. In this study, the segments of A/Wyoming/3/03 (NY, H3N2) virus encoding hemagglutinin and neuraminidase, respectively, were reassorted with the six internal genes of PR8 where the NS gene was fused with a Gaussia luciferase (Gluc) gene. Using reverse genetics, NY-r19-Gluc, a replication competent reassortant influenza A subtype H3N2 virus expressing reporter Gluc was successfully generated. This reporter virus is stable during replication in Madin-Darby canine kidney (MDCK) cells, and preliminary studies demonstrated it as a useful tool to evaluate antivirals. In addition, NY-r19-Gluc virus will be a powerful tool in other studies including the application of diagnostic and therapeutic antibodies as well as the evaluation of novel vaccines.

OVX836 a recombinant nucleoprotein vaccine inducing cellular responses and protective efficacy against multiple influenza A subtypes

Inactivated influenza vaccines (IIVs) lack broad efficacy. Cellular immunity to a conserved internal antigen, the nucleoprotein (NP), has been correlated to protection against pandemic and seasonal influenza and thus could have the potential to broaden vaccine efficacy. We developed OVX836, a recombinant protein vaccine based on an oligomerized NP, which shows increased uptake by dendritic cells and immunogenicity compared with NP. Intramuscular immunization in mice with OVX836 induced strong NP-specific CD4+ and CD8+ T-cell systemic responses and established CD8+ tissue memory T cells in the lung parenchyma. Strikingly, OVX836 protected mice against viral challenge with three different influenza A subtypes, isolated several decades apart and induced a reduction in viral load. When co-administered with IIV, OVX836 was even more effective in reducing lung viral load.

Circulating influenza A virus (IAV) strains differ in their surface proteins each year, and vaccines eliciting an immune response to these proteins are often only partially protective. Internal viral proteins, such as the nucleoprotein (NP), are highly conserved, and cellular immunity to NP has been correlated with protection from diverse strains. However, current IAV vaccines induce a poor immune response to NP. In this study, led by Fergal Hill from Osivax, researchers develop an oligomeric version of NP with improved immunogenicity. Vaccination of mice with oligomeric NP results in an improved NP-specific T-cell response, including CD8⁺ tissue memory T cells in the lung, and protects mice against three different IAV subtypes. Co-administration with the currently used inactivated influenza vaccine further improves protection against virus infection in mice. These results encourage further pre-clinical and clinical development for this vaccine candidate.

Universal Influenza Vaccines: Progress in Achieving Broad Cross-Protection In Vivo

Despite all we have learned since 1918 about influenza virus and immunity, available influenza vaccines remain inadequate to control outbreaks of unexpected strains. Universal vaccines not requiring strain matching would be a major improvement. Their composition would be independent of predicting circulating viruses and thus potentially effective against unexpected drift or pandemic strains. This commentary explores progress with candidate universal vaccines based on various target antigens. Candidates include vaccines based on conserved viral proteins such as nucleoprotein and matrix, on the conserved hemagglutinin (HA) stem, and various combinations. Discussion covers the differing evidence for each candidate vaccine demonstrating protection in animals against influenza viruses of widely divergent HA subtypes and groups; durability of protection; routes of administration, including mucosal, providing local immunity; and reduction of transmission. Human trials of some candidate universal vaccines have been completed or are underway. Interestingly, the HA stem, like nucleoprotein and matrix, induces immunity that permits some virus replication and emergence of escape mutants fit enough to cause disease. Vaccination with multiple target antigens will thus have advantages over use of single antigens. Ultimately, a universal vaccine providing long-term protection against all influenza virus strains might contribute to pandemic control and routine vaccination.

Conventional influenza vaccines influence the performance of a universal influenza vaccine in mice

Universal influenza vaccines are designed to protect against diverse strains of influenza virus. Preclinical testing of new vaccine candidates is usually done in naïve animals, despite intended use in the human population with its varied immune history including responses to previous vaccinations. As an approach more relevant to human use, we tested a candidate universal influenza vaccine in mice with a history of conventional vaccination. Female BALB/c mice were given two intramuscular doses of inactivated influenza vaccine (IIV) or diphtheria and tetanus toxoids vaccine (DT), one month apart. Another group was given two intranasal doses of live attenuated influenza virus (LAIV). One month after the second dose, mice were given the universal influenza vaccine: recombinant adenoviruses expressing influenza A nucleoprotein (A/NP) and matrix 2 (M2) (A/NP + M2-rAd). Immune responses to universal vaccine antigens A/NP and M2 were assessed by ELISA and interferon-γ ELISPOT. Protection was tested by challenge with mouse-adapted A/FM/1/47 (H1N1) and monitoring for weight loss and survival. Universal vaccine performance was enhanced, inhibited or unaffected by particular prior vaccinations. Mice given Afluria IIV and LAIV had greater antibody and T-cell response to A/NP than mice without prior vaccination, providing examples of enhanced A/NP + M2-rAd performance. Though Fluvirin IIV partially inhibited, the universal vaccine still provided considerable protection unlike conventional vaccination. Fluzone IIV and DT had no effect on A/NP + M2-rAd performance. Thus our results demonstrate that universal vaccine candidate A/NP + M2-rAd was at least partially effective in mice with diverse prior histories. However, the degree of protection and nature of the immune responses may be affected by a history of conventional vaccination and suggests that performance in humans would be influenced by immune history.

Virus replicon particle vaccines expressing nucleoprotein of influenza A virus mediate enhanced inflammatory responses in pigs

Studies in the mouse model indicate that the nucleoprotein of influenza A virus represents an interesting vaccine antigen being well conserved across subtypes of influenza virus but still able to induce protective immune responses. Here we show that immunizations of pigs with vesicular stomatitis virus- and classical swine fever virus-derived replicon (VRP) particles expressing the nucleoprotein (NP) of H1N1 A/swine/Belzig/2/01 induced potent antibody and T-cell responses against influenza A virus. In contrast to a conventional whole inactivated virus vaccine, the VRP vaccines induced both NP-specific CD4 and CD8 T cells responses, including interferon-γ and tumor-necrosis-factor dual-secreting cell. Although T-cells and antibody responses were cross-reactive with the heterologous H1N2 A/swine/Bakum/R757/2010 challenge virus, they did not provide protection against infection. Surprisingly, vaccinated pigs showed enhanced virus shedding, lung inflammation and increased levels of systemic and lung interferon-α as well as elevated lung interleukin-6. In conclusion, our study shows that NP, although efficacious in the mouse model, appears not to be a promising stand-alone vaccine antigen for pigs.

Vaccine approaches conferring cross-protection against influenza viruses

INTRODUCTION

Annual vaccination is one of the most efficient and cost-effective strategies to prevent and control influenza epidemics. Most of the currently available influenza vaccines are strong inducers of antibody responses against viral surface proteins, hemagglutinin (HA) and neuraminidase (NA), but are poor inducers of cell-mediated immune responses against conserved internal proteins. Moreover, due to the high variability of viral surface proteins because of antigenic drift or antigenic shift, many of the currently licensed vaccines confer little or no protection against drift or shift variants. Areas covered: Next generation influenza vaccines that can induce humoral immune responses to receptor-binding epitopes as well as broadly neutralizing conserved epitopes, and cell-mediated immune responses against highly conserved internal proteins would be effective against variant viruses as well as a novel pandemic influenza until circulating strain-specific vaccines become available. Here we discuss vaccine approaches that have the potential to provide broad spectrum protection against influenza viruses. Expert commentary: Based on current progress in defining cross-protective influenza immunity, it seems that the development of a universal influenza vaccine is feasible. It would revolutionize the strategy for influenza pandemic preparedness, and significantly impact the shelf-life and protection efficacy of seasonal influenza vaccines.

Non-neutralizing antibodies induced by seasonal influenza vaccine prevent, not exacerbate A(H1N1)pdm09 disease

The association of seasonal trivalent influenza vaccine (TIV) with increased infection by 2009 pandemic H1N1 (A(H1N1)pdm09) virus, initially observed in Canada, has elicited numerous investigations on the possibility of vaccine-associated enhanced disease, but the potential mechanisms remain largely unresolved. Here, we investigated if prior immunization with TIV enhanced disease upon A(H1N1)pdm09 infection in mice. We found that A(H1N1)pdm09 infection in TIV-immunized mice did not enhance the disease, as measured by morbidity and mortality. Instead, TIV-immunized mice cleared A(H1N1)pdm09 virus and recovered at an accelerated rate compared to control mice. Prior TIV immunization was associated with potent inflammatory mediators and virus-specific CD8 T cell activation, but efficient immune regulation, partially mediated by IL-10R-signaling, prevented enhanced disease. Furthermore, in contrast to suggested pathological roles, pre-existing non-neutralizing antibodies (NNAbs) were not associated with enhanced virus replication, but rather with promoted antigen presentation through FcR-bearing cells that led to potent activation of virus-specific CD8 T cells. These findings provide new insights into interactions between pre-existing immunity and pandemic viruses.

What Lies Beneath: Antibody Dependent Natural Killer Cell Activation by Antibodies to Internal Influenza Virus Proteins

The conserved internal influenza proteins nucleoprotein (NP) and matrix 1 (M1) are well characterised for T cell immunity, but whether they also elicit functional antibodies capable of activating natural killer (NK) cells has not been explored. We studied NP and M1-specific ADCC activity using biochemical, NK cell activation and killing assays with plasma from healthy and influenza-infected subjects. Healthy adults had antibodies to M1 and NP capable of binding dimeric FcγRIIIa and activating NK cells. Natural symptomatic and experimental influenza infections resulted in a rise in antibody dependent NK cell activation post-infection to the hemagglutinin of the infecting strain, but changes in NK cell activation to M1 and NP were variable. Although antibody dependent killing of target cells infected with vaccinia viruses expressing internal influenza proteins was not detected, opsonising antibodies to NP and M1 likely contribute to an antiviral microenvironment by stimulating innate immune cells to secrete cytokines early in infection. We conclude that effector cell activating antibodies to conserved internal influenza proteins are common in healthy and influenza-infected adults. Given the significance of such antibodies in animal models of heterologous influenza infection, the definition of their importance and mechanism of action in human immunity to influenza is essential.

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Functional antibodies to influenza matrix 1 and nucleoprotein are common in healthy and influenza-infected humans.

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Opsonising antibodies to matrix 1 and nucleoprotein can bind FcγRIIIa dimers and activate natural killer cells.

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Influenza infection increased natural killer cell activation to hemagglutinin but changes to the internal proteins varied

Influenza virus causes both seasonal outbreaks and global pandemics. The current influenza vaccine provides minimal protection against divergent strains of the virus not found in the vaccine. While neutralising antibodies induced by vaccination are able to confer strain-specific protection, antibodies directed against conserved influenza proteins may be able to provide some cross-protection. Animal models suggest a protective role for anti-nucleoprotein antibodies. Exploring the functional capacity of human antibodies against internal influenza proteins to engage Fc receptors and activate innate immune cells may present a unique approach in the development of a more universal influenza vaccine.

Cross-protection against influenza virus infection by intranasal administration of nucleoprotein-based vaccine with compound 48/80 adjuvant

The nucleoprotein (NP) of influenza viruses is highly conserved and therefore has become one of the major targets of current universal influenza vaccine (UIV) studies. In this study, the recombinant nucleoprotein (NP) of the A/PR/8/34 (H1N1) influenza virus strain was expressed using an Escherichia coli (E. coli) expression system and then purified as a candidate UIV. The NP protein was administered intranasally or intraperitoneally twice at 3-week intervals to female BALB/c mice in combination with C48/80 adjuvant. Then, the mice were challenged with homologous or heterologous influenza viruses at a lethal dose 3 weeks after the last immunization. The results showed that the serum IgG titers of all of the mice immunized with NP reached a higher level and the protection provided by NP vaccine against the homologous virus depended on the administered dosage and adjuvant. In addition, immunization with 100 μg NP in combination with C48/80 adjuvant could provide good cross-protection against heterologous H9N2 avian influenza viruses. This study indicated that NP as a candidate antigen of UIV immunized intranasally could effectively induce mucosal and cell-mediated immunity, with the potential to control epidemics caused by the appearance of new emerging influenza viruses.

Expression of H3N2 nucleoprotein in maize seeds and immunogenicity in mice

Oral administration of maize-expressed H3N2 nucleoprotein induced antibody responses in mice showing the immunogenicity of plant-derived antigen and its potential to be utilized as a universal flu vaccine. Influenza A viruses cause influenza epidemics that are devastating to humans and livestock. The vaccine for influenza needs to be reformulated every year to match the circulating strains due to virus mutation. Influenza virus nucleoprotein (NP) is a multifunctional RNA-binding protein that is highly conserved among strains, making it a potential candidate for a universal vaccine. In this study, the NP gene of H3N2 swine origin influenza virus was expressed in maize endosperm. Twelve transgenic maize lines were generated and analyzed for recombinant NP (rNP) expression. Transcript analysis showed the main accumulation of rNP in seed. Protein level of rNP in T1 transgenic maize seeds ranged from 8.0 to 35 µg of NP/g of corn seed. The level increased up to 70 µg of NP/g in T3 seeds. A mouse study was performed to test the immunogenicity of one line of maize-derived rNP (MNP). Mice were immunized with MNP in a prime-boost design. Oral gavage administration showed that a humoral immune response was elicited in the mice treated with MNP indicating the immunogenicity of MNP. NP-specific antibody responses in the MNP group showed comparable antibody titer with the groups receiving positive controls such as Vero cell-derived NP (VNP) or alphavirus replicon particle-derived NP (ANP). Cytokine analysis showed antigen-specific stimulation of IL-4 cytokine elicited in splenocytes from mice treated with MNP further confirming a TH2 humoral immune response induced by MNP administration.

Protection against multiple subtypes of influenza viruses by virus-like particle vaccines based on a hemagglutinin conserved epitope

We selected the conserved sequence in the stalk region of influenza virus hemagglutinin (HA) trimmer, the long alpha helix (LAH), as the vaccine candidate sequence, and inserted it into the major immunodominant region (MIR) of hepatitis B virus core protein (HBc), and, by using the E. coli expression system, we prepared a recombinant protein vaccine LAH-HBc in the form of virus-like particles (VLP). Intranasal immunization of mice with this LAH-HBc VLP plus cholera toxin B subunit with 0.2% of cholera toxin (CTB^*) adjuvant could effectively elicit humoral and cellular immune responses and protect mice against a lethal challenge of homologous influenza viruses (A/Puerto Rico/8/1934 (PR8) (H1N1)). In addition, passage of the immune sera containing specific antibodies to naïve mice rendered them resistant against a lethal homologous challenge. Immunization with LAH-HBc VLP vaccine plus CTB^* adjuvant could also fully protect mice against a lethal challenge of the 2009 pandemic H1N1 influenza virus or the avian H9N2 virus and could partially protect mice against a lethal challenge of the avian H5N1 influenza virus. This study demonstrated that the LAH-HBc VLP vaccine based on a conserved sequence of the HA trimmer stalk region is a promising candidate vaccine for developing a universal influenza vaccine against multiple influenza viruses infections.

Influenza-specific antibody-dependent cellular cytotoxicity: toward a universal influenza vaccine

There is an urgent need for universal influenza vaccines that can control emerging pandemic influenza virus threats without the need to generate new vaccines for each strain. Neutralizing Abs to the influenza virus hemagglutinin glycoprotein are effective at controlling influenza infection but generally target highly variable regions. Abs that can mediate other functions, such as killing influenza-infected cells and activating innate immune responses (termed "Ab-dependent cellular cytotoxicity [ADCC]-mediating Abs"), may assist in protective immunity to influenza. ADCC-mediating Abs can target more conserved regions of influenza virus proteins and recognize a broader array of influenza strains. We review recent research on influenza-specific ADCC Abs and their potential role in improved influenza-vaccination strategies.

Maximal immune response and cross protection by influenza virus nucleoprotein derived from E. coli using an optimized formulation

The highly conserved internal nucleoprotein (NP) is a promising antigen to develop a universal influenza A virus vaccine. In this study, mice were injected intramuscularly with Escherichia coli-derived NP protein alone or in combination with adjuvant alum (Al(OH)3), CpG or both. The results showed that the NP protein formulated with adjuvant was effective in inducing a protective immune response. Additionally, the adjuvant efficacy of Al(OH)3 was stronger than that of CpG. Optimal immune responses were observed in BALB/c mice immunized with a combination of NP protein plus Al(OH)3 and CpG. These mice also showed maximal resistance following challenge with influenza A virus PR8 strain. Most importantly, 10 µg NP formulated with Al(OH)3 and CpG induced higher protection than did 90 µg NP. These findings indicated that a combination of Al(OH)3 and CpG may be an efficient adjuvant in the NP formulation.

Combination vaccines: synergistic simultaneous induction of antibody and T-cell immunity

Vaccines have traditionally been designed to induce antibody responses and have been licensed on their capacity to induce high titers of circulating antibody to the pathogen. With our increased knowledge of host-pathogen interactions, it became apparent that induction of the cellular arm of the immune response is crucial to the efficacy of vaccines against intracellular pathogens and for providing appropriate help for antibody induction. Diverging strategies emerged that concentrate on developing candidate vaccines that solely induce either cellular or humoral responses. As most microbes reside at some point in the infectious cycle in the extracellular as well as intracellular space, and there is interplay between antibody and T cells, it is now apparent that both arms of immunity are essential to effectively control and eliminate the infection. It is, therefore, necessary to develop vaccines that can effectively induce a broad adaptive immune response. For vaccines targeted at diseases of the developing world, such as HIV, tuberculosis and malaria, it is imperative that these vaccines are simple to deliver and cost effective, that is,that optimum T-cell and antibody immunity is achieved with the minimum number of vaccinations. Combination vaccines, where an antibody-inducing subunit protein vaccine is coadministered with a T-cell-inducing poxvirus-based vaccine fulfill these requirements and induce sterile immunity to pathogen challenge.

Development of universal influenza vaccines based on influenza virus M and NP genes

PURPOSE

Vaccination is the safest and most effective measure against influenza virus infections. However, traditional influenza vaccines cannot respond effectively to an unforeseen epidemic or pandemic caused by a virus with antigenic drifts or antigenic shifts. Therefore, developing a universal influenza vaccine (UIV) that induces broad-spectrum and long-term immunity has become a major trend in influenza vaccine research and development.

METHODS

This article reviews the development of UIVs based on these conserved influenza virus proteins.

RESULTS AND CONCLUSION

The matrix protein (M1, M2) and nucleoprotein (NP) of influenza viruses have highly conserved sequences, and they become the major target antigens of current UIV studies.

Heterosubtypic cross-protection induced by whole inactivated influenza virus vaccine in mice: influence of the route of vaccine administration

Background

Development of influenza vaccines capable of inducing broad protection against different virus subtypes is necessary given the ever‐changing viral genetic landscape. Previously, we showed that vaccination with whole inactivated virus (WIV) induces heterosubtypic protection against lethal virus infection in mice. Whole inactivated virus‐induced cross‐protection was found to be mediated primarily by flu‐specific CD8+ T cells.

Objectives

As it has been demonstrated that the route of vaccine administration strongly influences both the quantity and quality of vaccine‐induced immunity, in this study, we determined which route of WIV administration induces optimal heterosubtypic cross‐protection.

Methods

We compared the magnitude of the immune response and heterosubtypic protection against lethal A/PR/8/34 (H1N1) infection after subcutaneous (SC), intramuscular (IM), and intranasal (IN) vaccination with A/NIBRG‐14 (H5N1) WIV.

Results

Subcutaneous and IM administration was superior to IN administration of influenza WIV in terms of flu‐specific CD8+ T‐cell induction and protection of mice against lethal heterosubtypic challenge. Surprisingly, despite the very low flu‐specific CD8+ T‐cell responses detected in IN‐vaccinated mice, these animals were partially protected, most likely due to cross‐reactive IgA antibodies.

Conclusion

The results of this study show that the magnitude of WIV‐induced flu‐specific CD8+ T‐cell activity depends on the applied vaccination route. We conclude that parenteral administration of WIV vaccine, in particular IM injection, is superior to IN vaccine delivery for the induction of heterosubtypic cross‐protection and generally appears to elicit stronger immune responses than mucosal vaccination with WIV.

Toward a universal influenza virus vaccine: prospects and challenges

Current influenza virus vaccines are annually reformulated to elicit protection by generating an immune response toward the virus strains that are predicted to circulate in the upcoming influenza season. These vaccines provide limited protection in cases of antigenic mismatch, when the vaccine and the circulating viral strains differ. The emergence of unexpected pandemic viruses presents an additional challenge to vaccine production. To increase influenza virus preparedness, much work has been dedicated to the development of a universal vaccine. Focusing on regions of viral proteins that are highly conserved across virus subtypes, vaccine strategies involving the matrix 2 protein, stalk domain of the hemagglutinin, and multivalent approaches have provided broad-based protection in animal models and show much promise. This review summarizes the most encouraging advances in the field with a focus on novel vaccine designs that have yielded promising preclinical and clinical data.

The nucleocapsid protein of Rift Valley fever virus is a potent human CD8+ T cell antigen and elicits memory responses

There is no licensed human vaccine currently available for Rift Valley Fever Virus (RVFV), a Category A high priority pathogen and a serious zoonotic threat. While neutralizing antibodies targeting the viral glycoproteins are protective, they appear late in the course of infection, and may not be induced in time to prevent a natural or bioterrorism-induced outbreak. Here we examined the immunogenicity of RVFV nucleocapsid (N) protein as a CD8(+) T cell antigen with the potential for inducing rapid protection after vaccination. HLA-A*0201 (A2)-restricted epitopic determinants were identified with N-specific CD8(+) T cells from eight healthy donors that were primed with dendritic cells transduced to express N, and subsequently expanded in vitro by weekly re-stimulations with monocytes pulsed with 59 15mer overlapping peptides (OLPs) across N. Two immunodominant epitopes, VT9 (VLSEWLPVT, N(121-129)) and IL9 (ILDAHSLYL, N165-173), were defined. VT9- and IL9-specific CD8(+) T cells identified by tetramer staining were cytotoxic and polyfunctional, characteristics deemed important for viral control in vivo. These peptides induced specific CD8(+) T cell responses in A2-transgenic mice, and more importantly, potent N-specific CD8(+) T cell reactivities, including VT9- and IL9-specific ones, were mounted by mice after a booster vaccination with the live attenuated RVF MP-12. Our data suggest that the RVFV N protein is a potent human T cell immunogen capable of eliciting broad, immunodominant CD8(+) T cell responses that are potentially protective. Understanding the immune responses to the nucleocapsid is central to the design of an effective RVFV vaccine irrespective of whether this viral protein is effective as a stand-alone immunogen or only in combination with other RVFV antigens.

Cooperativity between CD8+ T cells, non-neutralizing antibodies, and alveolar macrophages is important for heterosubtypic influenza virus immunity

Seasonal epidemics of influenza virus result in ∼36,000 deaths annually in the United States. Current vaccines against influenza virus elicit an antibody response specific for the envelope glycoproteins. However, high mutation rates result in the emergence of new viral serotypes, which elude neutralization by preexisting antibodies. T lymphocytes have been reported to be capable of mediating heterosubtypic protection through recognition of internal, more conserved, influenza virus proteins. Here, we demonstrate using a recombinant influenza virus expressing the LCMV GP33-41 epitope that influenza virus-specific CD8+ T cells and virus-specific non-neutralizing antibodies each are relatively ineffective at conferring heterosubtypic protective immunity alone. However, when combined virus-specific CD8 T cells and non-neutralizing antibodies cooperatively elicit robust protective immunity. This synergistic improvement in protective immunity is dependent, at least in part, on alveolar macrophages and/or other lung phagocytes. Overall, our studies suggest that an influenza vaccine capable of eliciting both CD8+ T cells and antibodies specific for highly conserved influenza proteins may be able to provide heterosubtypic protection in humans, and act as the basis for a potential “universal” vaccine.

Influenza virus continues to pose a significant risk to global health and is responsible for thousands of deaths each year in the United States. This threat is largely due to the ability of the influenza virus to undergo rapid changes, allowing it to escape from immune responses elicited by previous infections or vaccinations. Certain internal determinants of the influenza virus are largely conserved across different viral strains and represent attractive targets for potential “universal” influenza vaccines. Here, we demonstrated that cross-subtype protection against the influenza virus could be obtained through simultaneous priming of multiple arms of the immune response against conserved elements of the influenza virus. These results suggest a novel strategy that could potentially form a primary component of a universal influenza vaccine capable of providing long-lasting protection.

Influenza A virus nucleoprotein derived from Escherichia coli or recombinant vaccinia (Tiantan) virus elicits robust cross-protection in mice

BACKGROUND

Immunity to conserved viral antigens is an attractive approach to develop a universal vaccine against epidemic and pandemic influenza. A nucleoprotein (NP)-based vaccine has been explored and preliminary studies have shown promise. However, no study has explored the immunity and cross-protective efficacy of recombinant NP derived from Escherichia coli compared with recombinant vaccinia virus (Tiantan).

METHODS

Recombinant NP protein (rNP) from influenza virus A/Jingke/30/95(H3N2) was obtained from E. coli and recombinant vaccinia virus (Tiantan) RVJ1175NP. Purified rNP without adjuvant and RVJ1175NP were used to immunize BALB/c mice intramuscularly. Humoral immune responses were detected by ELISA, while cell-mediated immune responses were measured by ex vivo IFN-γ ELISPOT and in vivo cytotoxicity assays. The cross-protective efficacy was assessed by a challenge with a heterosubtype of influenza virus A/PR/8/34(H1N1).

RESULTS

Our results demonstrate that a high dose (90 μg) of rNP induced NP-specific antibodies and T cell responses that were comparable with those of RVJ1175NP in mice. Importantly, the survival ratio (36, 73, and 78%) of the vaccinated mice after the influenza virus A/PR/8/34(H1N1) challenge was rNP vaccine dose-dependent (10, 30, and 90 μg, respectively), and no significant differences were observed between the rNP- and RVJ1175NP-immunized (91%) mice.

CONCLUSIONS

Influenza A virus NP derived from E. coli or recombinant vaccinia (Tiantan) virus elicited cross-protection against influenza virus in mice, and the immune response and protective efficacy of rNP were comparable to RVJ1175NP. These data provide a basis for the use of prokaryotically expressed NP as a candidate universal influenza vaccine.

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.

Swine influenza virus vaccines: to change or not to change-that's the question

Commercial vaccines currently available against swine influenza virus (SIV) are inactivated, adjuvanted, whole virus vaccines, based on H1N1 and/or H3N2 and/or H1N2 SIVs. In keeping with the antigenic and genetic differences between SIVs circulating in Europe and the US, the vaccines for each region are produced locally and contain different strains. Even within a continent, there is no standardization of vaccine strains, and the antigen mass and adjuvants can also differ between different commercial products. Recombinant protein vaccines against SIV, vector, and DNA vaccines, and vaccines attenuated by reverse genetics have been tested in experimental studies, but they have not yet reached the market. In this review, we aim to present a critical analysis of the performance of commercial inactivated and novel generation SIV vaccines in experimental vaccination challenge studies in pigs. We pay special attention to the differences between commercial SIV vaccines and vaccination attitudes in Europe and in North America, to the issue of vaccine strain selection and changes, and to the potential advantages of novel generation vaccines over the traditional killed SIV vaccines.

Antibodies against conserved antigens provide opportunities for reform in influenza vaccine design

High-performance neutralizing antibody against influenza virus typically recognizes the globular head region of its hemagglutinin (HA) envelope glycoprotein. To-date, approved human vaccination strategies have been designed to induce such antibodies as a sole means of preventing the consequences of this infection. However, frequent amino-acid changes in the HA globular head allow for efficient immune evasion. Consequently, vaccines inducing such neutralizing antibodies need to be annually re-designed and re-administered at a great expense. These vaccines furthermore provide little-to-no immunity against antigenic-shift strains, which arise from complete replacement of HA or of neuraminidase genes, and pose pandemic risks. To address these issues, laboratory research has focused on inducing immunity effective against all strains, regardless of changes in the HA globular head. Despite prior dogma that such cross-protection needs to be induced by cellular immunity alone, several advances in recent years demonstrate that antibodies of other specificities are capable of cross-strain protection in mice. This review discusses the reactivity, induction, efficacy, and mechanisms of antibodies that react with poorly accessible epitopes in the HA stalk, with the matrix 2 membrane ion channel, and even with the internal nucleoprotein. These advances warrant further investigation of the inducibility and efficacy of such revolutionary antibody strategies in humans.

Induction of virus-specific cytotoxic T lymphocytes as a basis for the development of broadly protective influenza vaccines

There is considerable interest in the development of broadly protective influenza vaccines because of the continuous emergence of antigenic drift variants of seasonal influenza viruses and the threat posed by the emergence of antigenically distinct pandemic influenza viruses. It has been recognized more than three decades ago that influenza A virus-specific cytotoxic T lymphocytes recognize epitopes located in the relatively conserved proteins like the nucleoprotein and that they cross-react with various subtypes of influenza A viruses. This implies that these CD8+ T lymphocytes may contribute to protective heterosubtypic immunity induced by antecedent influenza A virus infections. In the present paper, we review the evidence for the role of virus-specific CD8+ T lymphocytes in protective immunity against influenza virus infections and discuss vaccination strategies that aim at the induction of cross-reactive virus-specific T-cell responses.

Influenza vaccine immunology

Studying the spread of influenza in human populations and protection by influenza vaccines provides important insights into immunity against influenza. The 2009 H1N1 pandemic has taught the most recent lessons. Neutralizing and receptor-blocking antibodies against hemagglutinin are the primary means of protection from the spread of pandemic and seasonal strains. Anti-neuraminidase antibodies seem to play a secondary role. More broadly cross-reactive forms of immunity may lessen disease severity but are insufficient to prevent epidemic spread. Priming by prior exposure to related influenza strains through infection or immunization permits rapid, potent antibody responses to immunization. Priming is of greater importance to the design of immunization strategies than the immunologically fascinating phenomenon of dominant recall responses to previously encountered strains (original antigenic sin). Comparisons between non-adjuvanted inactivated vaccines and live attenuated vaccines demonstrate that both can protect, with some advantage of live attenuated vaccines in children and some advantage of inactivated vaccines in those with multiple prior exposures to influenza antigens. The addition of oil-in-water emulsion adjuvants to inactivated vaccines provides enhanced functional antibody titers, greater breadth of antibody cross-reactivity, and antigen dose sparing. The MF59 adjuvant broadens the distribution of B-cell epitopes recognized on HA and NA following immunization.

Protective cellular responses elicited by vaccination with influenza nucleoprotein delivered by a live recombinant attenuated Salmonella vaccine

Orally administered recombinant attenuated Salmonella vaccines (RASVs) elicit humoral and mucosal immune responses against the immunizing antigen. The challenge in developing an effective vaccine against a virus or an intracellular bacterium delivered by RASVs is to introduce the protective antigen inside the host cell cytoplasm for presentation to MHC-I molecules for an efficient cell mediated immune response. To target the influenza nucleoprotein (NP) into the host cell cytosol, we constructed a regulated delayed lysis in vivo RASV strain χ11246(pYA4858) encoding influenza NP with a chromosomal deletion of the sifA gene to enable it to escape from the endosome prior to lysis. Oral immunization of mice with χ11246(pYA4858) (SifA⁻) with 3 booster immunizations resulted in complete protection (100%) against a lethal influenza virus (rWSN) challenge (100 LD₅₀) compared to 25% survival of mice immunized with the isogenic χ11017(pYA4858) (SifA⁺) strain. Reducing the number of booster immunizations with χ11246(pYA4858) from 3 to 2 resulted in 66% survival of mice challenged with rWSN (100 LD₅₀). Immunization with χ11246(pYA4858) via different routes provided protection in 80% orally, 100% intranasally and 100% intraperitoneally immunized mice against rWSN (100 LD₅₀). A Th1 type immune response was elicited against influenza NP in all experiments. IFN-γ secreting NP₁₄₇₋₁₅₅ specific T cells were not found to be correlated with protection. The role of antigen-specific CD8⁺ T cells remains to be determined. To conclude, we showed that Salmonella can be designed to deliver antigen(s) to the host cell cytosol for presumably class I presentation for the induction of protective immune responses.

Regulation of antinucleoprotein IgG by systemic vaccination and its effect on influenza virus clearance

Seasonal influenza epidemics recur due to antigenic drift of envelope glycoprotein antigens and immune evasion of circulating viruses. Additionally, antigenic shift can lead to influenza pandemics. Thus, a universal vaccine that protects against multiple influenza virus strains could alleviate the continuing impact of this virus on human health. In mice, accelerated clearance of a new viral strain (cross-protection) can be elicited by prior infection (heterosubtypic immunity) or by immunization with the highly conserved internal nucleoprotein (NP). Both heterosubtypic immunity and NP-immune protection require antibody production. Here, we show that systemic immunization with NP readily accelerated clearance of a 2009 pandemic H1N1 influenza virus isolate in an antibody-dependent manner. However, human immunization with trivalent inactivated influenza virus vaccine (TIV) only rarely and modestly boosted existing levels of anti-NP IgG. Similar results were observed in mice, although the reaction could be enhanced with adjuvants, by adjusting the stoichiometry among NP and other vaccine components, and by increasing the interval between TIV prime and boost. Importantly, mouse heterosubtypic immunity that had waned over several months could be enhanced by injecting purified anti-NP IgG or by boosting with NP protein, correlating with a long-lived increase in anti-NP antibody titers. Thus, current immunization strategies poorly induce NP-immune antibody that is nonetheless capable of contributing to long-lived cross-protection. The high conservation of NP antigen and the known longevity of antibody responses suggest that the antiviral activity of anti-NP IgG may provide a critically needed component of a universal influenza vaccine.

Human influenza vaccines and assessment of immunogenicity

Influenza is responsible for the infection of approximately 20% of the population every season and for an annual death toll of approximately half a million people. The most effective means for controlling infection and thereby reducing morbidity and mortality is vaccination by injection with an inactivated vaccine, or by intranasal administration of a live-attenuated vaccine. Protection is not always optimal and there is a need for the development of new vaccines with improved efficacy and for the expansion of enrollment into vaccination programs. An overview of old and new vaccines is presented. Methods of monitoring immune responses such as hemagglutination-inhibition, ELISA and neutralization tests are evaluated for their accuracy in the assessment of current and new-generation vaccines.

Contributions of antinucleoprotein IgG to heterosubtypic immunity against influenza virus

Influenza A virus causes recurring seasonal epidemics and occasional influenza pandemics. Because of changes in envelope glycoprotein Ags, neutralizing Abs induced by inactivated vaccines provide limited cross-protection against new viral serotypes. However, prior influenza infection induces heterosubtypic immunity that accelerates viral clearance of a second strain, even if the external proteins are distinct. In mice, cross-protection can also be elicited by systemic immunization with the highly conserved internal nucleoprotein (NP). Both T lymphocytes and Ab contribute to such cross-protection. In this paper, we demonstrate that anti-NP IgG specifically promoted influenza virus clearance in mice by using a mechanism involving both FcRs and CD8(+) cells. Furthermore, anti-NP IgG rescued poor heterosubtypic immunity in B cell-deficient mice, correlating with enhanced NP-specific CD8 T cell responses. Thus, Ab against this conserved Ag has potent antiviral activity both in naive and in influenza-immune subjects. Such antiviral activity was not seen when mice were vaccinated with another internal influenza protein, nonstructural 1. The high conservation of NP Ag and the known longevity of Ab responses suggest that anti-NP IgG may provide a critically needed component of a universal influenza vaccine.

Production of adenovirus vectors and their use as a delivery system for influenza vaccines

IMPORTANCE OF THE FIELD

With the emergence of highly pathogenic avian influenza H5N1 viruses that have crossed species barriers and are responsible for lethal infections in humans in many countries, there is an urgent need for the development of effective vaccines which can be produced in large quantities at a short notice and confer broad protection against these H5N1 variants. In order to meet the potential global vaccine demand in a pandemic scenario, new vaccine-production strategies must be explored in addition to the currently used egg-based technology for seasonal influenza.

AREAS COVERED IN THIS REVIEW

Adenovirus (Ad) based influenza vaccines represent an attractive alternative/supplement to the currently licensed egg-based influenza vaccines. Ad-based vaccines are relatively inexpensive to manufacture, and their production process does not require either chicken eggs or labor-intensive and time-consuming processes necessitating enhanced biosafety facilities. Most importantly, in a pandemic situation, this vaccine strategy could offer a stockpiling option to reduce the response time before a strain-matched vaccine could be developed.

WHAT THE READER WILL GAIN

This review discusses Ad-vector technology and the current progress in the development of Ad-based influenza vaccines.

TAKE HOME MESSAGE

Ad vector-based influenza vaccines for pandemic preparedness are under development to meet global vaccine demand.

Vaccine protection against lethal homologous and heterologous challenge using recombinant AAV vectors expressing codon-optimized genes from pandemic swine origin influenza virus (SOIV)

The recent H1N1 influenza pandemic and the inevitable delay between identification of the virus and production of the specific vaccine have highlighted the urgent need for new generation influenza vaccines that can preemptively induce broad immunity to different strains of the virus. In this study we have produced AAV-based vectors expressing the A/Mexico/4603/2009 (H1N1) hemagglutinin (HA), nucleocapsid (NP) and the matrix protein M1 and have evaluated their ability to induce specific immune response and protect mice against homologous and heterologous challenge. Each of the vaccine vectors elicited potent cellular and humoral immune responses in mice. Although immunization with AAV-M1 did not improve survival after challenge with the homologous strain, immunization with the AAV-H1 and AAV-NP vectors resulted in survival of all mice, as did inoculation with a combination of all three vectors. Furthermore, trivalent vaccination also conferred partial protection against challenge with the highly heterologous and virulent A/PR/8/34 strain of H1N1 influenza.

Reporting the Implementation of the Three Rs in European Primate and Mouse Research Papers: Are We Making Progress?

It is now more than 20 years since both Council of Europe Convention ETS123 and EU Directive 86/609?EEC were introduced, to promote the implementation of the Three Rs in animal experimentation and to provide guidance on animal housing and care. It might therefore be expected that reports of the implementation of the Three Rs in animal research papers would have increased during this period. In order to test this hypothesis, a literature survey of animal-based research was conducted. A randomly-selected sample from 16 high-profile medical journals, of original research papers arising from European institutions that featured experiments which involved either mice or primates, were identified for the years 1986 and 2006 (Total sample = 250 papers). Each paper was scored out of 10 for the incidence of reporting on the implementation of Three Rs-related factors corresponding to Replacement (justification of non-use of non-animal methods), Reduction (statistical analysis of the number of animals needed) and Refinement (housing aspects, i.e. increased cage size, social housing, enrichment of cage environment and food; and procedural aspects, i.e. the use of anaesthesia, analgesia, humane endpoints, and training for procedures with positive reinforcement). There was no significant increase in overall reporting score over time, for either mouse or primate research. By 2006, mouse research papers scored an average of 0 out of a possible 10, and primate research papers scored an average of 1.5. This review provides systematic evidence that animal research is still not properly reported, and supports the call within the scientific community for action to be taken by journals to update their policies.

Animal Models for Influenza Virus Pathogenesis and Transmission

Influenza virus infection of humans results in a respiratory disease that ranges in severity from sub-clinical infection to primary viral pneumonia that can result in death. The clinical effects of infection vary with the exposure history, age and immune status of the host, and also the virulence of the influenza strain. In humans, the virus is transmitted through either aerosol or contact-based transfer of infectious respiratory secretions. As is evidenced by most zoonotic influenza virus infections, not all strains that can infect humans are able to transmit from person-to-person. Animal models of influenza are essential to research efforts aimed at understanding the viral and host factors that contribute to the disease and transmission outcomes of influenza virus infection in humans. These models furthermore allow the pre-clinical testing of antiviral drugs and vaccines aimed at reducing morbidity and mortality in the population through amelioration of the virulence or transmissibility of influenza viruses. Mice, ferrets, guinea pigs, cotton rats, hamsters and macaques have all been used to study influenza viruses and therapeutics targeting them. Each model presents unique advantages and disadvantages, which will be discussed herein.

Cytotoxic T cells are the predominant players providing cross-protective immunity induced by {gamma}-irradiated influenza A viruses

We previously demonstrated that a single dose of nonadjuvanted intranasal gamma-irradiated influenza A virus can provide robust protection in mice against both homologous and heterosubtypic challenges, including challenge with an H5N1 avian virus strain. We investigated the mechanism behind the observed cross-protection to define which arms of the adaptive immune response are involved in mediating this protection. Studies with gene knockout mice showed the cross-protective immunity to be mediated mainly by T cells and to be dependent on the cytolytic effector molecule perforin. Adoptive transfer of memory T cells from immunized mice, but not of memory B cells, protected naïve recipients against lethal heterosubtypic influenza virus challenge. Furthermore, gamma-irradiated influenza viruses induced cross-reactive Tc-cell responses but not cross-neutralizing or cross-protective antibodies. In addition, histological analysis showed reduced lung inflammation in vaccinated mice compared to that in unvaccinated controls following heterosubtypic challenge. This reduced inflammation was associated with enhanced early recruitment of T cells, both CD4(+) and CD8(+), and with early influenza virus-specific cytotoxic T-cell responses. Therefore, cross-protective immunity induced by vaccination with gamma-irradiated influenza A virus is mediated mainly by Tc-cell responses.

A novel role for non-neutralizing antibodies against nucleoprotein in facilitating resistance to influenza virus

Current influenza vaccines elicit Abs to the hemagglutinin and neuraminidase envelope proteins. Due to antigenic drift, these vaccines must be reformulated annually to include the envelope proteins predicted to dominate in the following season. By contrast, vaccination with the conserved nucleoprotein (NP) elicits immunity against multiple serotypes (heterosubtypic immunity). NP vaccination is generally thought to convey protection primarily via CD8 effector mechanisms. However, significant titers of anti-NP Abs are also induced, yet the involvement of Abs in protection has largely been disregarded. To investigate how Ab responses might contribute to heterosubtypic immunity, we vaccinated C57BL/6 mice with soluble rNP. This approach induced high titers of NP-specific serum Ab, but only poorly detectable NP-specific T cell responses. Nevertheless, rNP immunization significantly reduced morbidity and viral titers after influenza challenge. Importantly, Ab-deficient mice were not protected by this vaccination strategy. Furthermore, rNP-immune serum could transfer protection to naive hosts in an Ab-dependent manner. Therefore, Ab to conserved, internal viral proteins, such as NP, provides an unexpected, yet important mechanism of protection against influenza. These results suggest that vaccines designed to elicit optimal heterosubtypic immunity to influenza should promote both Ab and T cell responses to conserved internal proteins.

Influenza virus-specific cytotoxic T lymphocytes: a correlate of protection and a basis for vaccine development

Since influenza A viruses of the H5N1 subtype continue to circulate in wild and domestic birds and cause an ever increasing number of human cases, it is feared that H5N1 viruses may cause the next influenza pandemic. Therefore, there is considerable interest in the development of vaccines that confer protection against infections with these viruses or ideally, protection against influenza viruses of different subtypes. For the development of broad-protective vaccines the induction of virus-specific cytotoxic T lymphocytes (CTL) may be an important target, since it has been demonstrated that CTL contribute to protective immunity and are largely directed to epitopes shared by influenza viruses of various subtypes. In the present paper, the possibility to develop (cross-reactive) CTL-inducing vaccines is discussed.

Failure of protection and enhanced pneumonia with a US H1N2 swine influenza virus in pigs vaccinated with an inactivated classical swine H1N1 vaccine

Two US swine influenza virus (SIV) isolates, A/Swine/Iowa/15/1930 H1N1 (IA30) and A/Swine/Minnesota/00194/2003 H1N2 (MN03), were evaluated in an in vivo vaccination and challenge model. Inactivated vaccines were prepared from each isolate and used to immunize conventional pigs, followed by challenge with homologous or heterologous virus. Both inactivated vaccines provided complete protection against homologous challenge. However, the IA30 vaccine failed to protect against the heterologous MN03 challenge. Three of the nine pigs in this group had substantially greater percentages of lung lesions, suggesting the vaccine potentiated the pneumonia. In contrast, priming with live IA30 virus provided protection from nasal shedding and virus replication in the lung in MN03 challenged pigs. These data indicate that divergent viruses that did not cross-react serologically did not provide complete cross-protection when used in inactivated vaccines against heterologous challenge and may have enhanced disease. In addition, live virus infection conferred protection against heterologous challenge.

Heterosubtypic immunity to influenza A virus infection requires a properly diversified antibody repertoire

Heterosubtypic immunity (HSI) is defined as cross-protection to infection with an influenza A virus serotype other than the one used for primary infection. Although HSI has been thought to be mediated by serotype cross-reactive cytotoxic T lymphocytes (CTL) that recognize conserved epitopes of structural proteins, recent studies suggest that antibodies (Abs) may make a significant contribution. In this study, we provide further evidence for the role of Abs in HSI using transgenic mice lacking terminal deoxyribonucleotidyltransferase (TdT), which adds N nucleotides to V-D and D-J junctions of the complementary determining region 3 (CDR3) (TdT(-/-)) and mice with altered Ab repertoires due to replacement of the complete locus of heavy chain diversity segments (D(H)) with an altered D(H) segment (namely, Delta D-iD). Both types of mice failed to generate complete HSI, although they were able to mount protective immunity to a homologous challenge. Lower levels of virus-specific antibodies along with more severely impaired HSI were observed in TdT(-/-) mice compared to those in Delta D-iD mice, while CTL activity remained unchanged in both types of mice. These findings indicate that a properly diversified antibody repertoire is required for HSI and that N addition by TdT is a more effective mechanism in the induction of a properly diversified antibody repertoire and, therefore, complete HSI. The results suggest that the diversity of the antibody repertoire as determined by the composition of the D region of HCDR3 and by N addition are among the mechanisms selected for in evolution to create a favorable environment to resolve infections with mutated viruses.