Immunity against heterosubtypic influenza virus induced by adenovirus and MVA expressing nucleoprotein and matrix protein-1

Assessment of CD8+ T-cell mediated immunity in an influenza A(H3N2) human challenge model in Belgium: a single centre, randomised, double-blind phase 2 study

BACKGROUND

Protection afforded by inactivated influenza vaccines can theoretically be improved by inducing T-cell responses to conserved internal influenza A antigens. We assessed whether, in an influenza controlled human infection challenge, susceptible individuals receiving a vaccine boosting T-cell responses would exhibit lower viral load and decreased symptoms compared with placebo recipients.

METHODS

In this single centre, randomised, double-blind phase 2 study, healthy adult (aged 18-55 years) volunteers with microneutralisation titres of less than 20 to the influenza A(H3N2) challenge strain were enrolled at an SGS quarantine facility in Antwerp, Belgium. Participants were randomly assigned double-blind using a permuted-block list with a 3:2 allocation ratio to receive 0·5 mL intramuscular injections of modified vaccinia Ankara (MVA) expressing H3N2 nucleoprotein (NP) and matrix protein 1 (M1) at 1·5 × 108 plaque forming units (4·3 × 108 50% tissue culture infectious dose [TCID50]; MVA-NP+M1 group) or saline placebo (placebo group). At least 6 weeks later, participants were challenged intranasally with 0·5 mL of a 1 × 106 TCID50/mL dose of influenza A/Belgium/4217/2015 (H3N2). Nasal swabs were collected twice daily from day 2 until day 11 for viral PCR, and symptoms of influenza were recorded from day 2 until day 11. The primary outcome was to determine the efficacy of MVA-NP+M1 vaccine to reduce the degree of nasopharyngeal viral shedding as measured by the cumulative viral area under the curve using a log-transformed quantitative PCR. This study is registered with ClinicalTrials.gov, NCT03883113.

FINDINGS

Between May 2 and Oct 24, 2019, 145 volunteers were enrolled and randomly assigned to the MVA-NP+M1 group (n=87) or the placebo group (n=58). Of these, 118 volunteers entered the challenge period (71 in the MVA-NP+M1 group and 47 in the placebo group) and 117 participants completed the study (71 in the MVA-NP+M1 group and 46 in the placebo group). 78 (54%) of the 145 volunteers were female and 67 (46%) were male. The primary outcome, overall viral load as determined by quantitative PCR, did not show a statistically significant difference between the MVA-NP+M1 (mean 649·7 [95% CI 552·7-746·7) and placebo groups (mean 726·1 [604·0-848·2]; p=0·17). All reported treatment emergent adverse events (TEAEs; 11 in the vaccination phase and 51 in the challenge phase) were grade 1 and 2, except for two grade 3 TEAEs in the placebo group in the challenge phase. A grade 4 second trimester fetal death, considered possibly related to the MVA-NP+M1 vaccination, and an acute psychosis reported in a placebo participant during the challenge phase were reported.

INTERPRETATION

The use of an MVA vaccine to expand CD4+ or CD8+ T cells to conserved influenza A antigens in peripheral blood did not affect nasopharyngeal viral load in an influenza H3N2 challenge model in seronegative, healthy adults.

FUNDING

Department of Health and Human Services; Administration for Strategic Preparedness and Response; Biomedical Advanced Research and Development Authority; and Barinthus Biotherapeutics.

Preclinical immunogenicity of an adenovirus-vectored vaccine for herpes zoster

Herpes zoster (HZ) results from waning immunity following childhood infection with varicella zoster virus (VZV) but is preventable by vaccination with recombinant HZ vaccine or live HZ vaccine (two doses or one dose, respectively). Vaccine efficacy declines with age, live HZ vaccine is contraindicated in immunosuppressed individuals, and severe local reactogenicity of recombinant HZ vaccine is seen in up to 20% of older adults, indicating a potential need for new vaccines. Nonreplicating chimpanzee adenovirus (ChAd) vectors combine potent immunogenicity with well-established reactogenicity and safety profiles. We evaluated the cellular and humoral immunogenicity of ChAdOx1 encoding VZV envelope glycoprotein E (ChAdOx1-VZVgE) in mice using IFN-γ ELISpot, flow cytometry with intracellular cytokine staining, and ELISA. In outbred CD-1 mice, one dose of ChAdOx1-VZVgE (1 × 10⁷ infectious units) elicited higher gE-specific T cell responses than two doses of recombinant HZ vaccine (1 µg) or one dose of live HZ vaccine (1.3 × 10³ plaque-forming units). Antibody responses were higher with two doses of recombinant HZ vaccine than with two doses of ChAdOx1-VZVgE or one dose of live HZ vaccine. ChAdOx1-VZVgE boosted T cell and antibody responses following live HZ vaccine priming. The frequencies of polyfunctional CD4+ and CD8+ T cells expressing more than one cytokine (IFN-γ, TNF-α and IL-2) were higher with ChAdOx1-VZVgE than with the conventional vaccines. Results were similar in young and aged BALB/c mice. These findings support the clinical development of ChAdOx1-VZVgE for prevention of HZ in adults aged 50 years or over, including those who have already received conventional vaccines.

Vaccine Strategy That Enhances the Protective Efficacy of Systemic Immunization by Establishing Lung-Resident Memory CD8 T Cells Against Influenza Infection

Most influenza vaccines currently in use target the highly variable hemagglutinin protein to induce neutralizing antibodies and therefore require yearly reformulation. T cell-based universal influenza vaccines focus on eliciting broadly cross-reactive T-cell responses, especially the tissue-resident memory T cell (TRM) population in the respiratory tract, providing superior protection to circulating memory T cells. This study demonstrated that intramuscular (i.m.) administration of the adenovirus-based vaccine expressing influenza virus nucleoprotein (rAd/NP) elicited weak CD8 TRM responses in the lungs and airways, and yielded poor protection against lethal influenza virus challenge. However, a novel "prime-and-deploy" strategy that combines i.m. vaccination of rAd/NP with subsequent intranasal administration of an empty adenovector induced strong NP-specific CD8+ TRM cells and provided complete protection against influenza virus challenge. Overall, our results demonstrate that this "prime-and-deploy" vaccination strategy is potentially applicable to the development of universal influenza vaccines.

CpG 1018 Is an Effective Adjuvant for Influenza Nucleoprotein

Current influenza vaccines mainly induce neutralizing antibodies against the highly variable surface antigen hemagglutinin and require annual manufacturing and immunization. Different from surface antigens, intracellular nucleoprotein (NP) is highly conserved and has been an attractive target to develop universal T cell vaccines against influenza. Yet, influenza NP protein mainly induces humoral immune responses and lacks the ability to induce potent cytotoxic T lymphocyte (CTL) responses, key for the success of universal T cell vaccines. This study compared CpG 1018 and AddaVax to enhance recombinant NP-induced CTL responses and protection in murine models. CpG 1018 was explored to boost intradermal NP immunization, while AddaVax was explored to boost intramuscular NP immunization due to the high risk of AddaVax adjuvant to induce significant local reactions following intradermal delivery. We found CpG 1018 was highly effective to enhance NP-induced humoral and cellular immune responses superior to AddaVax adjuvant. Furthermore, CpG 1018 potentiated Th1-biased antibody responses, while AddaVax enhanced Th1/Th2-balanced antibody responses. CpG 1018 significantly enhanced IFNγ-secreting Th1 cells, while AddaVax adjuvant significantly increased IL4-secreting Th2 cells. Influenza NP immunization in the presence of CpG 1018 induced significant protection against lethal viral challenges, while influenza NP immunization in the presence of AddaVax failed to elicit significant protection. Our data validated CpG 1018 as an effective adjuvant to enhance influenza NP-induced CTL responses and protection.

Protective Efficacy of a Mucosal Influenza Vaccine Formulation Based on the Recombinant Nucleoprotein Co-Administered with a TLR2/6 Agonist BPPcysMPEG

Current influenza vaccines target highly variable surface glycoproteins; thus, mismatches between vaccine strains and circulating strains often diminish vaccine protection. For this reason, there is still a critical need to develop effective influenza vaccines able to protect also against the drift and shift of different variants of influenza viruses. It has been demonstrated that influenza nucleoprotein (NP) is a strong candidate for a universal vaccine, which contributes to providing cross-protection in animal models. In this study, we developed an adjuvanted mucosal vaccine using the recombinant NP (rNP) and the TLR2/6 agonist S-[2,3-bispalmitoyiloxy-(2R)-propyl]-R-cysteinyl-amido-monomethoxyl-poly-ethylene-glycol (BPPcysMPEG). The vaccine efficacy was compared with that observed following parenteral vaccination of mice with the same formulation. Mice vaccinated with 2 doses of rNP alone or co-administered with BPPcysMPEG by the intranasal (i.n.) route showed enhanced antigen-specific humoral and cellular responses. Moreover, NP-specific humoral immune responses, characterized by significant NP-specific IgG and IgG subclass titers in sera and NP-specific IgA titers in mucosal territories, were remarkably increased in mice vaccinated with the adjuvanted formulation as compared with those of the non-adjuvanted vaccination group. The addition of BPPcysMPEG also improved NP-specific cellular responses in vaccinated mice, characterized by robust lymphoproliferation and mixed Th1/Th2/Th17 immune profiles. Finally, it is notable that the immune responses elicited by the novel formulation administered by the i.n. route were able to confer protection against the influenza H1N1 A/Puerto Rico/8/1934 virus.

Immunization with matrix-, nucleoprotein and neuraminidase protects against H3N2 influenza challenge in pH1N1 pre-exposed pigs

There is an urgent need for influenza vaccines providing broader protection that may decrease the need for annual immunization of the human population. We investigated the efficacy of heterologous prime boost immunization with chimpanzee adenovirus (ChAdOx2) and modified vaccinia Ankara (MVA) vectored vaccines, expressing conserved influenza virus nucleoprotein (NP), matrix protein 1 (M1) and neuraminidase (NA) in H1N1pdm09 pre-exposed pigs. We compared the efficacy of intra-nasal, aerosol and intra-muscular vaccine delivery against H3N2 influenza challenge. Aerosol prime boost immunization induced strong local lung T cell and antibody responses and abrogated viral shedding and lung pathology following H3N2 challenge. In contrast, intramuscular immunization induced powerful systemic responses and weak local lung responses but also abolished lung pathology and reduced viral shedding. These results provide valuable insights into the development of a broadly protective influenza vaccine in a highly relevant large animal model and will inform future vaccine and clinical trial design.

The race toward a universal influenza vaccine: Front runners and the future directions

Influenza virus is the pathogen of influenza (flu) and millions of people suffer from the infection worldwide, posing a significant health risk. The current influenza vaccines induce neutralizing antibodies against hemagglutinin (HA) to achieve strain-specific neutralization. The effectiveness of seasonal vaccines is usually low and unpredictable because of the antigenic variation and genetic plasticity of viruses, as well as the interference of preexisting immunity. A universal influenza vaccine is urgently needed to prevent a wide variety of influenza viruses. Nevertheless, reaching this difficult optimal goal requires a step-by-step approach. Innovative strategies and vaccine platforms are being developed in order to generate robust cross-protective immunity. In this review, we summarize candidate influenza vaccines that meet two criteria: first, they are designed to provide protection against multiple influenza viruses; second, they had passed regulatory evaluations and have entered various stages of clinical trials. We discuss these vaccine candidates based on the different vaccine-production platforms, with the focus on antigen selection, design, adjuvants, immunomodulators, and routes of vaccine delivery in the development of universal influenza vaccines.

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.

Evolutionary conservation and positive selection of Influenza A Nucleoprotein CTL epitopes for universal vaccination: a proof-of-concept

Influenza (flu) infection is a leading cause of respiratory disease and death worldwide. While seasonal flu vaccines are effective at reducing morbidity and mortality, such effects rely on the odds of successful prediction of the upcoming viral strains. Additional threats from emerging flu viruses that we cannot predict and avian flu viruses that can be directly transmitted to humans, urge the strategic development of universal vaccinations that can protect against flu viruses of different subtypes and across species. Annual flu vaccines elicit mainly humoral responses. Under circumstances when antibodies induced by vaccination fail to recognize and neutralize the emerging virus adequately, virus-specific cytotoxic T lymphocytes (CTLs) are the major contributors to the control of viral replication and elimination of infected cells. Our studies exploited the evolutionary conservation of influenza A nucleoprotein (NP) and the fact that NP-specific CTL responses pose a constant selecting pressure on functional CTL epitopes, to screen for NP epitopes that are highly conserved among heterosubtypes but are subjected to positive selection historically. We identified a region on NP that is evolutionarily conserved and historically positively selected (NP_137-182 ) and validated that it contains an epitope that is functional in eliciting NP-specific CTL responses and immunity that can partially protect immunized mice against lethal dose infection of a heterosubtypic influenza A virus. Our proof-of-concept study supports the hypothesis that evolutionary conservation and positive selection of influenza nucleoprotein can be exploited to identify functional CTL epitope to elicit cross protection against different heterosubtypes, therefore, to help develop strategies to modify flu vaccine formula for a broader and more durable protective immunity. This article is protected by copyright. All rights reserved.

Intranasal delivery of a cDC1 targeted influenza vaccine with poly(I:C) enhances T cell responses and protects against influenza infection

Targeting antigens to dendritic cells represent a promising method for enhancing immune responses against specific antigens. However, many studies have focused on systemic delivery (intravenous or intraperitoneally) of targeted antigen, approaches that are not easily transferable to humans. Here we evaluate the efficacy of an influenza vaccine targeting Xcr1⁺ cDC1 administered by intranasal immunization. Intranasal delivery of antigen fused to the chemokine Xcl1, the ligand of Xcr1, resulted in specific uptake by lung CD103⁺ cDC1. Interestingly, intranasal immunization with influenza A/PR/8/34 haemagglutinin (HA) fused to Xcl1, formulated with poly(I:C), resulted in enhanced induction of antigen-specific IFNγ⁺ CD4⁺ and IFNγ⁺ CD8⁺ T cell responses in lung compared non-targeted anti-NIP-HA (αNIP-HA). Induction of antibody responses was, however, similar in Xcl1-HA and αNIP-HA immunized mice, but significantly higher than in mice immunized with monomeric HA. Both Xcl1-HA and αNIP-HA vaccines induced full protection when mice were challenged with a lethal dose of influenza PR8 virus, reflecting the strong induction of HA-specific antibodies. Our results demonstrate that i.n. delivery of Xcl1-HA is a promising vaccine strategy for enhancing T cell responses in addition to inducing strong antibody responses.

Potentiation of Recombinant NP and M1-Induced Cellular Immune Responses and Protection by Physical Radiofrequency Adjuvant

Nucleoprotein (NP) and matrix protein 1 (M1) are highly conserved among influenza A viruses and have been attractive targets to develop vaccines to elicit cross-reactive cytotoxic T lymphocytes (CTLs). Yet, external antigens are often presented on major histocompatibility complex class II molecules and elicit humoral immune responses. In this study, we present a physical radiofrequency adjuvant (RFA) to assist recombinant NP and M1 to elicit potent CTL responses. We found recombinant NP/M1 immunization in the presence of RFA could elicit potent anti-NP CTLs and confer significant protection against homologous viral challenges, while NP/M1 immunization alone failed to elicit significant CTL responses or confer significant protection. Interestingly, RFA failed to elicit potent anti-M1 CTL responses or anti-NP or anti-M1 antibody responses. Different from RFA, AddaVax adjuvant was found to significantly increase NP-specific antibody responses but not CTLs. NP/M1 immunization in the presence of RFA or AddaVax similarly reduced body weight loss, while only the former significantly increased the survival. We further found NP/M1 immunization in the presence of RFA did not significantly increase serum IL-6 release (a systemic inflammatory mediator) and rather reduced serum IL-6 release after boost immunization. NP/M1 immunization in the presence of RFA did not induce significant local reactions or increase body temperature of mice. The high potency and safety strongly support further development of RFA-based recombinant NP/M1 vaccine to elicit cross-protective immunity.

Influenza Antigens NP and M2 Confer Cross Protection to BALB/c Mice against Lethal Challenge with H1N1, Pandemic H1N1 or H5N1 Influenza A Viruses

Influenza hemagglutinin (HA) is considered a major protective antigen of seasonal influenza vaccine but antigenic drift of HA necessitates annual immunizations using new circulating HA versions. Low variation found within conserved non-HA influenza virus (INFV) antigens may maintain protection with less frequent immunizations. Conserved antigens of influenza A virus (INFV A) that can generate cross protection against multiple INFV strains were evaluated in BALB/c mice using modified Vaccinia virus Ankara (MVA)-vectored vaccines that expressed INFV A antigens hemagglutinin (HA), matrix protein 1 (M1), nucleoprotein (NP), matrix protein 2 (M2), repeats of the external portion of M2 (M2e) or as tandem repeats (METR), and M2e with transmembrane region and cytoplasmic loop (M2eTML). Protection by combinations of non-HA antigens was equivalent to that of subtype-matched HA. Combinations of NP and forms of M2e generated serum antibody responses and protected mice against lethal INFV A challenge using PR8, pandemic H1N1 A/Mexico/4108/2009 (pH1N1) or H5N1 A/Vietnam/1203/2004 (H5N1) viruses, as demonstrated by reduced lung viral burden and protection against weight loss. The highest levels of protection were obtained with NP and M2e antigens delivered as MVA inserts, resulting in broadly protective immunity in mice and enhancement of previous natural immunity to INFV A.

Improved immunologic responses to heterologous influenza strains in children with low preexisting antibody response vaccinated with MF59-adjuvanted influenza vaccine

Vaccination is the most effective approach to reduce the substantial morbidity and mortality caused by influenza infection. Vaccine efficacy is highly sensitive to antigenic changes causing differences between circulating and vaccine viruses. Adjuvants such as MF59 increase antibody-mediated cross-reactive immunity and therefore may provide broader seasonal protection. A recent clinical trial showed that an MF59-adjuvanted vaccine was more efficacious than a nonadjuvanted comparator in subjects < 2 years of age, although not in those ≥ 2 years, during influenza seasons in which the predominant circulating virus was an A/H3N2 strain that was antigenically different from the vaccine virus. This finding suggested that the increased efficacy of the adjuvanted vaccine in younger subjects may be mediated by strain cross-reactive antibodies. A subset of the trial population, representing subjects with distinct age and/or immunological history, was tested for antibody responses to the vaccine A/H3N2 strain as well as A/H3N2 drifted strains antigenically matching the viruses circulating during the trial seasons. The neutralizing tests showed that, compared with nonadjuvanted vaccine, the adjuvanted vaccine improved not only the neutralizing antibody response to the vaccine strain but also the cross-reactive antibody response to the drifted strains in subjects with lower preexisting antibody titers, regardless of their age or vaccine history. The results demonstrated an immunological benefit and suggested a potential efficacy benefit by adjuvanted vaccine in subjects with lower preexisting antibody responses.

T Cell Immunity against Influenza: The Long Way from Animal Models Towards a Real-Life Universal Flu Vaccine

Current flu vaccines rely on the induction of strain-specific neutralizing antibodies, which leaves the population vulnerable to drifted seasonal or newly emerged pandemic strains. Therefore, universal flu vaccine approaches that induce broad immunity against conserved parts of influenza have top priority in research. Cross-reactive T cell responses, especially tissue-resident memory T cells in the respiratory tract, provide efficient heterologous immunity, and must therefore be a key component of universal flu vaccines. Here, we review recent findings about T cell-based flu immunity, with an emphasis on tissue-resident memory T cells in the respiratory tract of humans and different animal models. Furthermore, we provide an update on preclinical and clinical studies evaluating T cell-evoking flu vaccines, and discuss the implementation of T cell immunity in real-life vaccine policies.

Adenoviral Vector-Based Vaccine Platforms for Developing the Next Generation of Influenza Vaccines

Ever since the discovery of vaccines, many deadly diseases have been contained worldwide, ultimately culminating in the eradication of smallpox and polio, which represented significant medical achievements in human health. However, this does not account for the threat influenza poses on public health. The currently licensed seasonal influenza vaccines primarily confer excellent strain-specific protection. In addition to the seasonal influenza viruses, the emergence and spread of avian influenza pandemic viruses such as H5N1, H7N9, H7N7, and H9N2 to humans have highlighted the urgent need to adopt a new global preparedness for an influenza pandemic. It is vital to explore new strategies for the development of effective vaccines for pandemic and seasonal influenza viruses. The new vaccine approaches should provide durable and broad protection with the capability of large-scale vaccine production within a short time. The adenoviral (Ad) vector-based vaccine platform offers a robust egg-independent production system for manufacturing large numbers of influenza vaccines inexpensively in a short timeframe. In this review, we discuss the progress in the development of Ad vector-based influenza vaccines and their potential in designing a universal influenza vaccine.

Flu Universal Vaccines: New Tricks on an Old Virus

Conventional influenza vaccines are based on predicting the circulating viruses year by year, conferring limited effectiveness since the antigenicity of vaccine strains does not always match the circulating viruses. This necessitates development of universal influenza vaccines that provide broader and lasting protection against pan-influenza viruses. The discovery of the highly conserved immunogens (epitopes) of influenza viruses provides attractive targets for universal vaccine design. Here we review the current understanding with broadly protective immunogens (epitopes) and discuss several important considerations to achieve the goal of universal influenza vaccines.

A Multi-Targeting, Nucleoside-Modified mRNA Influenza Virus Vaccine Provides Broad Protection in Mice

Influenza viruses are respiratory pathogens of public health concern worldwide with up to 650,000 deaths occurring each year. Seasonal influenza virus vaccines are employed to prevent disease, but with limited effectiveness. Development of a universal influenza virus vaccine with the potential to elicit long-lasting, broadly cross-reactive immune responses is necessary for reducing influenza virus prevalence. In this study, we have utilized lipid nanoparticle-encapsulated, nucleoside-modified mRNA vaccines to intradermally deliver a combination of conserved influenza virus antigens (hemagglutinin stalk, neuraminidase, matrix-2 ion channel, and nucleoprotein) and induce strong immune responses with substantial breadth and potency in a murine model. The immunity conferred by nucleoside-modified mRNA-lipid nanoparticle vaccines provided protection from challenge with pandemic H1N1 virus at 500 times the median lethal dose after administration of a single immunization, and the combination vaccine protected from morbidity at a dose of 50 ng per antigen. The broad protective potential of a single dose of combination vaccine was confirmed by challenge with a panel of group 1 influenza A viruses. These findings support the advancement of nucleoside-modified mRNA-lipid nanoparticle vaccines expressing multiple conserved antigens as universal influenza virus vaccine candidates.

A Multi-Filovirus Vaccine Candidate: Co-Expression of Ebola, Sudan, and Marburg Antigens in a Single Vector

In the infectious diseases field, protective immunity against individual virus species or strains does not always confer cross-reactive immunity to closely related viruses, leaving individuals susceptible to disease after exposure to related virus species. This is a significant hurdle in the field of vaccine development, in which broadly protective vaccines represent an unmet need. This is particularly evident for filoviruses, as there are multiple family members that can cause lethal haemorrhagic fever, including Zaire ebolavirus, Sudan ebolavirus, and Marburg virus. In an attempt to address this need, both pre-clinical and clinical studies previously used mixed or co-administered monovalent vaccines to prevent filovirus mediated disease. However, these multi-vaccine and multi-dose vaccination regimens do not represent a practical immunisation scheme when considering the target endemic areas. We describe here the development of a single multi-pathogen filovirus vaccine candidate based on a replication-deficient simian adenoviral vector. Our vaccine candidate encodes three different filovirus glycoproteins in one vector and induces strong cellular and humoral immunity to all three viral glycoproteins after a single vaccination. Crucially, it was found to be protective in a stringent Zaire ebolavirus challenge in guinea pigs in a one-shot vaccination regimen. This trivalent filovirus vaccine offers a tenable vaccine product that could be rapidly translated to the clinic to prevent filovirus-mediated viral haemorrhagic fever.

Factors Which Contribute to the Immunogenicity of Non-replicating Adenoviral Vectored Vaccines

Adenoviral vectors are a safe and potently immunogenic vaccine delivery platform. Non-replicating Ad vectors possess several attributes which make them attractive vaccines for infectious disease, including their capacity for high titer growth, ease of manipulation, safety, and immunogenicity in clinical studies, as well as their compatibility with clinical manufacturing and thermo-stabilization procedures. In general, Ad vectors are immunogenic vaccines, which elicit robust transgene antigen-specific cellular (namely CD8+ T cells) and/or humoral immune responses. A large number of adenoviruses isolated from humans and non-human primates, which have low seroprevalence in humans, have been vectorized and tested as vaccines in animal models and humans. However, a distinct hierarchy of immunological potency has been identified between diverse Ad vectors, which unfortunately limits the potential use of many vectors which have otherwise desirable manufacturing characteristics. The precise mechanistic factors which underlie the profound disparities in immunogenicity are not clearly defined and are the subject of ongoing, detailed investigation. It has been suggested that a combination of factors contribute to the potent immunogenicity of particular Ad vectors, including the magnitude and duration of vaccine antigen expression following immunization. Furthermore, the excessive induction of Type I interferons by some Ad vectors has been suggested to impair transgene expression levels, dampening subsequent immune responses. Therefore, the induction of balanced, but not excessive stimulation of innate signaling is optimal. Entry factor binding or receptor usage of distinct Ad vectors can also affect their in vivo tropism following administration by different routes. The abundance and accessibility of innate immune cells and/or antigen-presenting cells at the site of injection contributes to early innate immune responses to Ad vaccination, affecting the outcome of the adaptive immune response. Although a significant amount of information exists regarding the tropism determinants of the common human adenovirus type-5 vector, very little is known about the receptor usage and tropism of rare species or non-human Ad vectors. Increased understanding of how different facets of the host response to Ad vectors contribute to their immunological potency will be essential for the development of optimized and customized Ad vaccine platforms for specific diseases.

Multi-Epitope Peptide-Based and Vaccinia-Based Universal Influenza Vaccine Candidates Subjected to Clinical Trials

In light of the limited protection conferred by current influenza vaccines, immunisation using universal influenza vaccines has been proposed for protection against all or most influenza sub-types. The fundamental principle of universal influenza vaccines is based on conserved antigens found in most influenza strains, such as matrix 2, nucleocapsid, matrix 1 and stem of hemagglutinin proteins. These antigens trigger cross-protective immunity against different influenza strains. Many researchers have attempted to produce the conserved epitopes of these antigens in the form of peptides in the hope of generating universal influenza vaccine candidates that can broadly induce cross-reactive protection against influenza viral infections. However, peptide vaccines are poorly immunogenic when applied individually owing to their small molecular sizes. Hence, strategies, such as combining peptides as multi-epitope vaccines or presenting peptides on vaccinia virus particles, are employed. This review discusses the clinical and laboratory findings of several multi-epitope peptide vaccine candidates and vaccinia-based peptide vaccines. The majority of these vaccine candidates have reached the clinical trial phase. The findings in this study will indeed shed light on the applicability of universal influenza vaccines to prevent seasonal and pandemic influenza outbreaks in the near future.

Targeting Antigen to the Surface of EVs Improves the In Vivo Immunogenicity of Human and Non-human Adenoviral Vaccines in Mice

Adenoviral (Ad) vectors represent promising vaccine platforms for infectious disease. To overcome pre-existing immunity to commonly used human adenovirus serotype 5 (Ad5), vectors based on rare species or non-human Ads are being developed. However, these vectors often exhibit reduced potency compared with Ad5, necessitating the use of innovative approaches to augment the immunogenicity of the encoded antigen (Ag). To achieve this, we engineered model Ag, enhanced green fluorescent protein (EGFP), for targeting to the surface of host-derived extracellular vesicles (EVs), namely exosomes. Exosomes are nano-sized EVs that play important roles in cell-to-cell communication and in regulating immune responses. Directed targeting of Ag to the surface of EVs/exosomes is achieved by "exosome display," through fusion of Ag to the C1C2 domain of lactadherin, a protein highly enriched in exosomes. Herein, we engineered chimpanzee adenovirus ChAdOx1 and Ad5-based vaccines encoding EGFP, or EGFP targeted to EVs (EGFP_C1C2), and compared vaccine immunogenicity in mice. We determined that exosome display substantially increases Ag-specific humoral immunity following intramuscular and intranasal vaccination, improving the immunological potency of both ChAdOx1 and Ad5. We propose that this Ag-engineering approach could increase the immunogenicity of diverse Ad vectors that exhibit desirable manufacturing characteristics, but currently lack the potency of Ad5.

Vaccination with viral vectors expressing NP, M1 and chimeric hemagglutinin induces broad protection against influenza virus challenge in mice

Seasonal influenza virus infections cause significant morbidity and mortality every year. Annual influenza virus vaccines are effective but only when well matched with circulating strains. Therefore, there is an urgent need for better vaccines that induce broad protection against drifted seasonal and emerging pandemic influenza viruses. One approach to design such vaccines is based on targeting conserved regions of the influenza virus hemagglutinin. Sequential vaccination with chimeric hemagglutinin constructs can refocus antibody responses towards the conserved immunosubdominant stalk domain of the hemagglutinin, rather than the variable immunodominant head. A complementary approach for a universal influenza A virus vaccine is to induce T-cell responses to conserved internal influenza virus antigens. For this purpose, replication deficient recombinant viral vectors based on Chimpanzee Adenovirus Oxford 1 and Modified Vaccinia Ankara virus are used to express the viral nucleoprotein and the matrix protein 1. In this study, we combined these two strategies and evaluated the efficacy of viral vectors expressing both chimeric hemagglutinin and nucleoprotein plus matrix protein 1 in a mouse model against challenge with group 2 influenza viruses including H3N2, H7N9 and H10N8. We found that vectored vaccines expressing both sets of antigens provided enhanced protection against H3N2 virus challenge when compared to vaccination with viral vectors expressing only one set of antigens. Vaccine induced antibody responses against divergent group 2 hemagglutinins, nucleoprotein and matrix protein 1 as well as robust T-cell responses to the nucleoprotein and matrix protein 1 were detected. Of note, it was observed that while antibodies to the H3 stalk were already boosted to high levels after two vaccinations with chimeric hemagglutinins (cHAs), three exposures were required to induce strong reactivity across subtypes. Overall, these results show that a combinations of different universal influenza virus vaccine strategies can induce broad antibody and T-cell responses and can provide increased protection against influenza.

Vaccination With Viral Vectors Expressing Chimeric Hemagglutinin, NP and M1 Antigens Protects Ferrets Against Influenza Virus Challenge

Seasonal influenza viruses cause significant morbidity and mortality in the global population every year. Although seasonal vaccination limits disease, mismatches between the circulating strain and the vaccine strain can severely impair vaccine effectiveness. Because of this, there is an urgent need for a universal vaccine that induces broad protection against drifted seasonal and emerging pandemic influenza viruses. Targeting the conserved stalk region of the influenza virus hemagglutinin (HA), the major glycoprotein on the surface of the virus, results in the production of broadly protective antibody responses. Furthermore, replication deficient viral vectors based on Chimpanzee Adenovirus Oxford 1 (ChAdOx1) and modified vaccinia Ankara (MVA) virus expressing the influenza virus internal antigens, the nucleoprotein (NP) and matrix 1 (M1) protein, can induce strong heterosubtypic influenza virus-specific T cell responses in vaccinated individuals. Here, we combine these two platforms to evaluate the efficacy of a viral vectored vaccination regimen in protecting ferrets from H3N2 influenza virus infection. We observed that viral vectored vaccines expressing both stalk-targeting, chimeric HA constructs, and the NP+M1 fusion protein, in a prime-boost regimen resulted in the production of antibodies toward group 2 HAs, the HA stalk, NP and M1, as well as in induction of influenza virus-specific-IFNγ responses. The immune response induced by this vaccination regime ultimately reduced viral titers in the respiratory tract of influenza virus infected ferrets. Overall, these results improve our understanding of vaccination platforms capable of harnessing both cellular and humoral immunity with the goal of developing a universal influenza virus vaccine.

Intranodal administration of mRNA encoding nucleoprotein provides cross-strain immunity against influenza in mice

Background

Current human influenza vaccines lack the adaptability to match the mutational rate of the virus and therefore require annual revisions. Because of extensive manufacturing times and the possibility that antigenic alterations occur during viral vaccine strain production, an inherent risk exists for antigenic mismatch between the new influenza vaccine and circulating viruses. Targeting more conserved antigens such as nucleoprotein (NP) could provide a more sustainable vaccination strategy by inducing long term and heterosubtypic protection against influenza. We previously demonstrated that intranodal mRNA injection can induce potent antigen-specific T-cell responses. In this study, we investigated whether intranodal administration of mRNA encoding NP can induce T-cell responses capable of protecting against a heterologous influenza virus challenge.

Methods

BALB/c mice were immunized in the inguinal lymph nodes with different vaccination regimens of mRNA encoding NP. Immune responses were compared with NP DNA vaccination via IFN-γ ELISPOT and in vivo cytotoxicity. For survival experiments, mice were prime-boost vaccinated with 17 µg NP mRNA and infected with 1LD50 of H1N1 influenza virus 8 weeks after boost. Weight was monitored and viral titers, cytokines and immune cell populations in the bronchoalveolar lavage, and IFN-γ responses in the spleen were analyzed.

Results

Our results demonstrate that NP mRNA induces superior systemic T-cell responses against NP compared to classical DNA vaccination. These responses were sustained for several weeks even at low vaccine doses. Upon challenge infection, vaccination with NP mRNA resulted in reduced lung viral titers and improved recovery from infection. Finally, we show that vaccination with NP mRNA affects the immune response in infected lungs by lowering immune cell infiltration while increasing the fraction of T cells, monocytes and MHC II⁺ alveolar macrophages within immune infiltrates. This change was associated with altered levels of both pro- and anti-inflammatory cytokines.

Conclusions

These findings suggest that intranodal vaccination with NP mRNA induces cross-strain immunity against influenza, but also highlight a paradox of influenza immunity, whereby robust immune responses can provide protection, but can also transiently exacerbate symptoms during infection.

Electronic supplementary material

The online version of this article (10.1186/s12967-019-1991-3) contains supplementary material, which is available to authorized users.

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.

Development of a Universal Influenza Vaccine

The severity of the 2017-18 influenza season, combined with the low efficacy for some vaccine components, highlights the need to improve our current seasonal influenza vaccine. Thus, the National Institute of Allergy and Infectious Diseases recently announced a strategic plan to improve current influenza vaccines and eventually develop a "universal" influenza vaccine. This review will highlight the many different strategies being undertaken in pursuit of this goal and the exciting advances made by the influenza community. There is no doubt that an improved influenza vaccine is on the horizon.

T Lymphocytes as Measurable Targets of Protection and Vaccination Against Viral Disorders

Continuous epidemiological surveillance of existing and emerging viruses and their associated disorders is gaining importance in light of their abilities to cause unpredictable outbreaks as a result of increased travel and vaccination choices by steadily growing and aging populations. Close surveillance of outbreaks and herd immunity are also at the forefront, even in industrialized countries, where previously eradicated viruses are now at risk of re-emergence due to instances of strain recombination, contractions in viral vector geographies, and from their potential use as agents of bioterrorism. There is a great need for the rational design of current and future vaccines targeting viruses, with a strong focus on vaccine targeting of adaptive immune effector memory T cells as the gold standard of immunity conferring long-lived protection against a wide variety of pathogens and malignancies. Here, we review viruses that have historically caused large outbreaks and severe lethal disorders, including respiratory, gastric, skin, hepatic, neurologic, and hemorrhagic fevers. To observe trends in vaccinology against these viral disorders, we describe viral genetic, replication, transmission, and tropism, host-immune evasion strategies, and the epidemiology and health risks of their associated syndromes. We focus on immunity generated against both natural infection and vaccination, where a steady shift in conferred vaccination immunogenicity is observed from quantifying activated and proliferating, long-lived effector memory T cell subsets, as the prominent biomarkers of long-term immunity against viruses and their associated disorders causing high morbidity and mortality rates.

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.

A naturally protective epitope of limited variability as an influenza vaccine target

Current antigenic targets for influenza vaccine development are either highly immunogenic epitopes of high variability or conserved epitopes of low immunogenicity. This requires continuous update of the variable epitopes in the vaccine formulation or boosting of immunity to invariant epitopes of low natural efficacy. Here we identify a highly immunogenic epitope of limited variability in the head domain of the H1 haemagglutinin protein. We show that a cohort of young children exhibit natural immunity to a set of historical influenza strains which they could not have previously encountered and that this is partially mediated through the epitope. Furthermore, vaccinating mice with these epitope conformations can induce immunity to human H1N1 influenza strains that have circulated since 1918. The identification of epitopes of limited variability offers a mechanism by which a universal influenza vaccine can be created; these vaccines would also have the potential to protect against newly emerging influenza strains.

An Ad/MVA vectored Theileria parva antigen induces schizont-specific CD8+ central memory T cells and confers partial protection against a lethal challenge

The parasite Theileria parva is the causative agent of East Coast fever (ECF), one of the most serious cattle diseases in sub-Saharan Africa, and directly impacts smallholder farmers’ livelihoods. There is an efficient live-parasite vaccine, but issues with transmission of vaccine strains, need of a cold chain, and antibiotics limit its utilization. This has fostered research towards subunit vaccination. Cytotoxic T lymphocytes (CTL) are crucial in combating the infection by lysing T. parva-infected cells. Tp1 is an immunodominant CTL antigen, which induces Tp1-specific responses in 70–80% of cattle of the A18 or A18v haplotype during vaccination with the live vaccine. In this study, human adenovirus serotype 5 (HAd5) and modified vaccinia Ankara (MVA) were assessed for their ability to induce Tp1-specific immunity. Both viral vectors expressing the Tp1 antigen were inoculated in cattle by a heterologous prime-boost vaccination regimen. All 15 animals responded to Tp1 as determined by ELISpot. Of these, 14 reacted to the known Tp1 epitope, assayed by ELISpot and tetramer analyses, with CTL peaking 1-week post-MVA boost. Eleven animals developed CTL with specific cytotoxic activity towards peripheral blood mononuclear cells (PBMC) pulsed with the Tp1 epitope. Moreover, 36% of the animals with a Tp1 epitope-specific response survived a lethal challenge with T. parva 5 weeks post-MVA boost. Reduction of the parasitemia correlated with increased percentages of central memory lymphocytes in the Tp1 epitope-specific CD8⁺ populations. These results indicate that Tp1 is a promising antigen to include in a subunit vaccine and central memory cells are crucial for clearing the parasite.

A vaccine expressing parasitic proteins offers more convenient East Coast fever prophylaxis. Current vaccination for the cattle disease, caused by the parasite Theileria parva and a detriment to sub-Saharan African farmers, involves inconvenient injection with live parasites before antibiotic treatment (ITM). A collaboration led by Nicholas Svitek, of the Kenyan International Livestock Research Institute, designed a candidate to provoke cellular immune responses against the parasitic antigen Tp1—an ITM vaccine candidate. In tests on cattle, 93% created Tp1-targeting T cells, and 33% survived a lethal dose of T. parva. The East Coast fever reduction seen in animals in this research outperformed a recent study and was able to generate the same immune memory cells that ITM inspires to provide long-lasting protection. Future research might integrate more antigens with this Tp1 vaccine to provide more comprehensive protection.

Methods for Measuring T-Cell Memory to Vaccination: From Mouse to Man

The development of effective vaccines continues to be a key goal for public health bodies, governments, funding bodies and pharmaceutical companies. With new vaccines such as Shingrix targeting Shingles and Bexsero for Meningitis B, licensed in recent years, today's population can be protected from more infectious diseases than ever before. Despite this, we are yet to license vaccines for some of the deadliest endemic diseases affecting children, such as malaria. In addition, the threat of epidemics caused by emerging pathogens is very real as exemplified by the 2014⁻2016 Ebola outbreak. Most licensed vaccines provide efficacy through humoral immunity and correlates of protection often quantify neutralising antibody titre. The role of T-cells in vaccine efficacy is less well understood and more complex to quantify. Defining T-cell responses which afford protection also remains a challenge, although more sophisticated assays for assessing cell-mediated immunity with the potential for higher throughput and scalability are now available and warrant review. Here we discuss the benefits of multiparameter cytokine analysis and omics approaches compared with flow cytometric and ELISpot assays. We also review technical challenges unique to clinical trial studies, including assay validation across laboratories and availability of sample type. Measuring T-cell immunogenicity alongside humoral responses provides information on the breadth of immune responses induced by vaccination. Accurately enumerating and phenotyping T-cell immunogenicity to vaccination is key for the determination of immune correlates of protection. However, identifying such T-cell parameters remains challenging without a clear understanding of the immunological mechanisms by which a T-cell-mediated response induces protection.

Influenza Virus: Dealing with a Drifting and Shifting Pathogen

Numerous modern technological and scientific advances have changed the vaccine industry. However, nearly 70 years of influenza vaccine usage have passed without substantial changes in the underlying principles of the vaccine. The challenge of vaccinating against influenza lies in the constantly changing nature of the virus itself. Influenza viruses undergo antigenic evolution through antigenic drift and shift in their surface glycoproteins. This has forced frequent updates of vaccine antigens to ensure that the somewhat narrowly focused vaccine-induced immune responses defend against circulating strains. Few vaccine production systems have been developed that can entertain such constant changes. Although influenza virus infection induces long-lived immunologic memory to the same or similar strains, most people do not encounter the same strain repeatedly in their lifespan, suggesting that enhancement of natural immunity is required to improve influenza vaccines. It is clear that transformative change of influenza vaccines requires a rethink of how we immunize. In this study, we review the problems associated with the changing nature of the virus, and highlight some of the approaches being employed to improve influenza vaccines.

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.

Protein and modified vaccinia virus Ankara-based influenza virus nucleoprotein vaccines are differentially immunogenic in BALB/c mice

Because of the high variability of seasonal influenza viruses and the eminent threat of influenza viruses with pandemic potential, there is great interest in the development of vaccines that induce broadly protective immunity. Most probably, broadly protective influenza vaccines are based on conserved proteins, such as nucleoprotein (NP). NP is a vaccine target of interest as it has been shown to induce cross-reactive antibody and T cell responses. Here we tested and compared various NP-based vaccine preparations for their capacity to induce humoral and cellular immune responses to influenza virus NP. The immunogenicity of protein-based vaccine preparations with Matrix-M™ adjuvant as well as recombinant viral vaccine vector modified Vaccinia virus Ankara (MVA) expressing the influenza virus NP gene, with or without modifications that aim at optimization of CD8⁺ T cell responses, was addressed in BALB/c mice. Addition of Matrix-M™ adjuvant to NP wild-type protein-based vaccines significantly improved T cell responses. Furthermore, recombinant MVA expressing the influenza virus NP induced strong antibody and CD8⁺ T cell responses, which could not be improved further by modifications of NP to increase antigen processing and presentation.

Impact of Aging and Cytomegalovirus on Immunological Response to Influenza Vaccination and Infection

The number of people over the age of 60 is expected to double by 2050 according to the WHO. This emphasizes the need to ensure optimized resilience to health stressors in late life. In older adults, influenza is one of the leading causes of catastrophic disability (defined as the loss of independence in daily living and self-care activities). Influenza vaccination is generally perceived to be less protective in older adults, with some studies suggesting that the humoral immune response to the vaccine is further impaired in cytomegalovirus (CMV)-seropositive older people. CMV is a β-herpes virus infection that is generally asymptomatic in healthy individuals. The majority of older adults possess serum antibodies against the virus indicating latent infection. Age-related changes in T-cell-mediated immunity are augmented by CMV infection and may be associated with more serious complications of influenza infection. This review focuses on the impact of aging and CMV on immune cell function, the response to influenza infection and vaccination, and how the current understanding of aging and CMV can be used to design a more effective influenza vaccine for older adults. It is anticipated that efforts in this field will address the public health need for improved protection against influenza in older adults, particularly with regard to the serious complications leading to loss of independence.

Cross-protective efficacy of dendritic cells targeting conserved influenza virus antigen expressed by Lactobacillus plantarum

Avian influenza virus (AIV) can infect birds and mammals, including humans, and are thus a serious threat to public health. Vaccination is vital for controlling AIV circulation. In this study, we generated a recombinant lactobacillus expressing the NP-M1-DCpep of H9N2 avian influenza virus and evaluated the activation effect of NC8-pSIP409-NP-M1-DCpep on dendritic cells (DCs) in a mouse model. The specific mucosal antibody responses and B and T cell responses in lymphoid tissues were also characterized. Importantly, we confirmed that specific CD8 T cells presented in vitro and antigen-specific cytotoxicity (activated the expression of CD107a) and in vivo antigen-specific cytotoxicity after vaccination. The adoptive transfer of NC8-pSIP409-NP-M1-DCpep-primed CD8+ T cells into NOD-SCID mice resulted in effective protection against mouse-adapted AIV infection. In addition, we observed protection in immunized mice challenged with mouse-adapted H9N2 AIV and H1N1 influenza virus, as evidenced by reductions in the lung virus titers, improvements in lung pathology, and weight loss and complete survival. Our data are promising for the generation of effective, non-traditional influenza vaccines against AIVs.

Universal or Specific? A Modeling-Based Comparison of Broad-Spectrum Influenza Vaccines against Conventional, Strain-Matched Vaccines

Despite the availability of vaccines, influenza remains a major public health challenge. A key reason is the virus capacity for immune escape: ongoing evolution allows the continual circulation of seasonal influenza, while novel influenza viruses invade the human population to cause a pandemic every few decades. Current vaccines have to be updated continually to keep up to date with this antigenic change, but emerging ‘universal’ vaccines—targeting more conserved components of the influenza virus—offer the potential to act across all influenza A strains and subtypes. Influenza vaccination programmes around the world are steadily increasing in their population coverage. In future, how might intensive, routine immunization with novel vaccines compare against similar mass programmes utilizing conventional vaccines? Specifically, how might novel and conventional vaccines compare, in terms of cumulative incidence and rates of antigenic evolution of seasonal influenza? What are their potential implications for the impact of pandemic emergence? Here we present a new mathematical model, capturing both transmission dynamics and antigenic evolution of influenza in a simple framework, to explore these questions. We find that, even when matched by per-dose efficacy, universal vaccines could dampen population-level transmission over several seasons to a greater extent than conventional vaccines. Moreover, by lowering opportunities for cross-protective immunity in the population, conventional vaccines could allow the increased spread of a novel pandemic strain. Conversely, universal vaccines could mitigate both seasonal and pandemic spread. However, where it is not possible to maintain annual, intensive vaccination coverage, the duration and breadth of immunity raised by universal vaccines are critical determinants of their performance relative to conventional vaccines. In future, conventional and novel vaccines are likely to play complementary roles in vaccination strategies against influenza: in this context, our results suggest important characteristics to monitor during the clinical development of emerging vaccine technologies.

Influenza vaccines used today offer good protection, but have limitations: they have to be updated regularly, to remain effective in the face of ongoing virus evolution, and they cannot be used in advance of an influenza pandemic. In this study we considered how such ‘conventional’ vaccines might compare on the population level against new ‘universal’ vaccines currently being developed, that may protect against a broad spectrum of influenza viruses. We developed a mathematical model to capture the interactions between vaccination, influenza transmission, and viral evolution. The model suggests that annual vaccination with universal vaccines could control annual influenza epidemics more efficiently than conventional vaccines. In doing so they could slow viral evolution, rather than promoting it, while maintaining the broadly protective immunity that could mitigate against the emergence of a pandemic. These effects depend sensitively on the duration of protection that universal vaccines can afford, an important quantity to monitor in their development. In future, it is likely that conventional and universal vaccines would be deployed in tandem: we suggest that they could fulfill distinct roles, with universal vaccines being prioritised for managing transmission and evolution, and conventional vaccines being focused on protecting specific risk groups.

Protective efficacy of Fc targeting conserved influenza virus M2e antigen expressed by Lactobacillus plantarum

The influenza A (H1N1) virus is a highly contagious acute respiratory disease affecting pigs and humans. This disease causes severe economic loss in many countries, and developing mucosal vaccines is an efficient strategy to control the influenza virus. The neonatal Fc receptor (FcRn) plays an important role in transferring IgG across polarized epithelial cells. In the present study, an oral vaccine was developed using Lactobacillus plantarum to deliver the internal influenza viral protein M2e fused to an IgG Fc fragment. Oral vaccination with recombinant L. plantarum expressing 3M2e-Fc elicited Peyer's patch (PP) DC activation, improved the number of gamma interferon (IFN-γ)-producing T cells and increased the frequency of CD8⁺IFN-γ⁺ cells in the mesenteric lymph nodes (MLNs). In addition, the recombinant L. plantarum can induce PP B220⁺IgA⁺ expression and enhance specific sIgA secretion and the shaping of growth centers (GCs) in PPs. Furthermore, the data demonstrated that immunization with recombinant L. plantarum expressing 3M2e-Fc markedly reduced the viral load in the lung and protected against H1N1 influenza virus and mouse-adapted H9N2 avian influenza virus (AIV) challenge in BALB/c mice. Collectively, the data also showed that this vaccine strategy provided effective protective immunity against infection with homologous and heterologous influenza viruses in a mouse model and may be useful for future influenza vaccine development.

Increased Protein Degradation Improves Influenza Virus Nucleoprotein-Specific CD8+ T Cell Activation In Vitro but Not in C57BL/6 Mice

Due to antigenic drift of influenza viruses, seasonal influenza vaccines need to be updated annually. These vaccines are based on predictions of strains likely to circulate in the next season. However, vaccine efficacy is greatly reduced in the case of a mismatch between circulating and vaccine strains. Furthermore, novel antigenically distinct influenza viruses are introduced into the human population from animal reservoirs occasionally and may cause pandemic outbreaks. To dampen the impact of seasonal and pandemic influenza, vaccines that induce broadly protective and long-lasting immunity are preferred. Because influenza virus-specific CD8⁺ T cells are directed mainly against relatively conserved internal proteins, like nucleoprotein (NP), they are highly cross-reactive and afford protection against infection with antigenically distinct influenza virus strains, so-called heterosubtypic immunity. Here, we used modified vaccinia virus Ankara (MVA) as a vaccine vector for the induction of influenza virus NP-specific CD8⁺ T cells. To optimize the induction of CD8⁺ T cell responses, we made several modifications to NP, aiming at retaining the protein in the cytosol or targeting it to the proteasome. We hypothesized that these strategies would increase antigen processing and presentation and thus improve the induction of CD8⁺ T cell responses. We showed that NP with increased degradation rates improved CD8⁺ T cell activation in vitro if the amount of antigen was limited or if CD8⁺ T cells were of low functional avidity. However, after immunization of C57BL/6 mice, no differences were detected between modified NP and wild-type NP (NPwt), since NPwt already induced optimal CD8⁺ T cell responses.

IMPORTANCE

Due to the continuous antigenic drift of seasonal influenza viruses and the threat of a novel pandemic, there is a great need for the development of novel influenza vaccines that offer broadly protective immunity against multiple subtypes. CD8⁺ T cells can provide immunity against multiple subtypes of influenza viruses by the recognition of relatively conserved internal antigens. In this study, we aimed at optimizing the CD8⁺ T cell response to influenza A virus by making modifications to influenza A virus nucleoprotein (NP) expressed from the modified vaccinia virus Ankara (MVA) vaccine vector. These modifications resulted in increased antigen degradation, thereby producing elevated levels of peptides that can be presented on major histocompatibility complex (MHC) class I molecules to CD8⁺ T cells. Although we were unable to increase the NP-specific immune response in the mouse strain used, this approach may have benefits for vaccine development using less-immunogenic proteins.

A chimeric haemagglutinin-based influenza split virion vaccine adjuvanted with AS03 induces protective stalk-reactive antibodies in mice

Seasonal influenza virus vaccines are generally effective at preventing disease, but need to be well matched to circulating virus strains for maximum benefit. Influenza viruses constantly undergo antigenic changes because of their high mutation rate in the immunodominant haemagglutinin (HA) head domain, which necessitates annual re-formulation and re-vaccination for continuing protection. In case of pandemic influenza virus outbreaks, new vaccines need to be produced and quickly distributed. Novel influenza virus vaccines that redirect the immune response towards more conserved epitopes located in the HA stalk domain may remove the need for annual vaccine re-formulation and could also protect against emergent pandemic strains to which the human population is immunologically naive. One approach to create such universal influenza virus vaccines is the use of constructs expressing chimeric HAs. By sequential immunization with vaccine strains expressing the same conserved HA stalk domain and exotic HA heads to which the host is naive, antibodies against the stalk can be boosted to high titres. Here we tested a monovalent chimeric HA-based prototype universal influenza virus split virion vaccine candidate with and without AS03 adjuvant in primed mice. We found that the chimeric HA-based vaccination regimen induced higher stalk antibody titres than the seasonal vaccine. The stalk antibody responses were long lasting, cross-reactive to distantly related HAs and provided protection in vivo in a serum transfer challenge model. The results of this study are promising and support further development of a universal influenza vaccine candidate built on the chimeric HA technology platform.

Self-Amplifying mRNA Vaccines Expressing Multiple Conserved Influenza Antigens Confer Protection against Homologous and Heterosubtypic Viral Challenge

Current hemagglutinin (HA)-based seasonal influenza vaccines induce vaccine strain-specific neutralizing antibodies that usually fail to provide protection against mismatched circulating viruses. Inclusion in the vaccine of highly conserved internal proteins such as the nucleoprotein (NP) and the matrix protein 1 (M1) was shown previously to increase vaccine efficacy by eliciting cross-reactive T-cells. However, appropriate delivery systems are required for efficient priming of T-cell responses. In this study, we demonstrated that administration of novel self-amplifying mRNA (SAM^®) vectors expressing influenza NP (SAM(NP)), M1 (SAM(M1)), and NP and M1 (SAM(M1-NP)) delivered with lipid nanoparticles (LNP) induced robust polyfunctional CD4 T helper 1 cells, while NP-containing SAM also induced cytotoxic CD8 T cells. Robust expansions of central memory (T_CM) and effector memory (T_EM) CD4 and CD8 T cells were also measured. An enhanced recruitment of NP-specific cytotoxic CD8 T cells was observed in the lungs of SAM(NP)-immunized mice after influenza infection that paralleled with reduced lung viral titers and pathology, and increased survival after homologous and heterosubtypic influenza challenge. Finally, we demonstrated for the first time that the co-administration of RNA (SAM(M1-NP)) and protein (monovalent inactivated influenza vaccine (MIIV)) was feasible, induced simultaneously NP-, M1- and HA-specific T cells and HA-specific neutralizing antibodies, and enhanced MIIV efficacy against a heterologous challenge. In conclusion, systemic administration of SAM vectors expressing conserved internal influenza antigens induced protective immune responses in mice, supporting the SAM^® platform as another promising strategy for the development of broad-spectrum universal influenza vaccines.

Modified Vaccinia Virus Ankara: History, Value in Basic Research, and Current Perspectives for Vaccine Development

Safety tested Modified Vaccinia virus Ankara (MVA) is licensed as third-generation vaccine against smallpox and serves as a potent vector system for development of new candidate vaccines against infectious diseases and cancer. Historically, MVA was developed by serial tissue culture passage in primary chicken cells of vaccinia virus strain Ankara, and clinically used to avoid the undesirable side effects of conventional smallpox vaccination. Adapted to growth in avian cells MVA lost the ability to replicate in mammalian hosts and lacks many of the genes orthopoxviruses use to conquer their host (cell) environment. As a biologically well-characterized mutant virus, MVA facilitates fundamental research to elucidate the functions of poxvirus host-interaction factors. As extremely safe viral vectors MVA vaccines have been found immunogenic and protective in various preclinical infection models. Multiple recombinant MVA currently undergo clinical testing for vaccination against human immunodeficiency viruses, Mycobacterium tuberculosis or Plasmodium falciparum. The versatility of the MVA vector vaccine platform is readily demonstrated by the swift development of experimental vaccines for immunization against emerging infections such as the Middle East Respiratory Syndrome. Recent advances include promising results from the clinical testing of recombinant MVA-producing antigens of highly pathogenic avian influenza virus H5N1 or Ebola virus. This review summarizes our current knowledge about MVA as a unique strain of vaccinia virus, and discusses the prospects of exploiting this virus as research tool in poxvirus biology or as safe viral vector vaccine to challenge existing and future bottlenecks in vaccinology.

Viral vector-based influenza vaccines

Antigenic drift of seasonal influenza viruses and the occasional introduction of influenza viruses of novel subtypes into the human population complicate the timely production of effective vaccines that antigenically match the virus strains that cause epidemic or pandemic outbreaks. The development of game-changing vaccines that induce broadly protective immunity against a wide variety of influenza viruses is an unmet need, in which recombinant viral vectors may provide. Use of viral vectors allows the delivery of any influenza virus antigen, or derivative thereof, to the immune system, resulting in the optimal induction of virus-specific B- and T-cell responses against this antigen of choice. This systematic review discusses results obtained with vectored influenza virus vaccines and advantages and disadvantages of the currently available viral vectors.

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.

Immunogenicity and efficacy of the novel cancer vaccine based on simian adenovirus and MVA vectors alone and in combination with PD-1 mAb in a mouse model of prostate cancer

Prostate cancer possesses several characteristics that make it a suitable candidate for immunotherapy; however, prostate cancer vaccines to date demonstrate modest efficacy and low immunogenicity. The goal of the present pre-clinical study was to explore the immunogenic properties and protective efficacy of a novel prostate cancer immunotherapy based on the heterologous prime–boost viral-vectored vaccination platform. The simian adenovirus, ChAdOx1, and modified vaccinia Ankara virus, MVA, encoding a prostate cancer-associated antigen, the six transmembrane epithelial antigen of the prostate 1 (STEAP1), induced strong sustained antigen-specific CD8+ T-cell responses in C57BL/6 and BALB/c male mice. Unexpectedly, the high vaccine immunogenicity translated into relatively low protective efficacy in the murine transplantable and spontaneous models of prostate cancer. A combination of the vaccine with PD-1 blocking antibody significantly improved survival of the animals, with 80 % of mice remaining tumour-free. These results indicate that the ChAdOx1–MVA vaccination regime targeting STEAP1 combined with PD-1 therapy might have high therapeutic potential in the clinic.

Universal influenza vaccines: Shifting to better vaccines

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Current influenza vaccines are not broadly protective.

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Developing countries suffer the highest morbidity and mortality due to influenza.

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Correlates of protection and a regulatory pathway must be identified.

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Over 40 universal influenza vaccine candidates are currently in development.

Influenza virus causes acute upper and lower respiratory infections and is the most likely, among known pathogens, to cause a large epidemic in humans. Influenza virus mutates rapidly, enabling it to evade natural and vaccine-induced immunity. Furthermore, influenza viruses can cross from animals to humans, generating novel, potentially pandemic strains. Currently available influenza vaccines induce a strain specific response and may be ineffective against new influenza viruses. The difficulty in predicting circulating strains has frequently resulted in mismatch between the annual vaccine and circulating viruses. Low-resource countries remain mostly unprotected against seasonal influenza and are particularly vulnerable to future pandemics, in part, because investments in vaccine manufacturing and stockpiling are concentrated in high-resource countries. Antibodies that target conserved sites in the hemagglutinin stalk have been isolated from humans and shown to confer protection in animal models, suggesting that broadly protective immunity may be possible. Several innovative influenza vaccine candidates are currently in preclinical or early clinical development. New technologies include adjuvants, synthetic peptides, virus-like particles (VLPs), DNA vectors, messenger RNA, viral vectors, and attenuated or inactivated influenza viruses. Other approaches target the conserved exposed epitope of the surface exposed membrane matrix protein M2e. Well-conserved influenza proteins, such as nucleoprotein and matrix protein, are mainly targeted for developing strong cross-protective T cell responses. With multiple vaccine candidates moving along the testing and development pipeline, the field is steadily moving toward a product that is more potent, durable, and broadly protective than previously licensed vaccines.

Progress on adenovirus-vectored universal influenza vaccines

Influenza virus (IFV) infection causes serious health problems and heavy financial burdens each year worldwide. The classical inactivated influenza virus vaccine (IIVV) and live attenuated influenza vaccine (LAIV) must be updated regularly to match the new strains that evolve due to antigenic drift and antigenic shift. However, with the discovery of broadly neutralizing antibodies that recognize conserved antigens, and the CD8(+) T cell responses targeting viral internal proteins nucleoprotein (NP), matrix protein 1 (M1) and polymerase basic 1 (PB1), it is possible to develop a universal influenza vaccine based on the conserved hemagglutinin (HA) stem, NP, and matrix proteins. Recombinant adenovirus (rAd) is an ideal influenza vaccine vector because it has an ideal stability and safety profile, induces balanced humoral and cell-mediated immune responses due to activation of innate immunity, provides 'self-adjuvanting' activity, can mimic natural IFV infection, and confers seamless protection against mucosal pathogens. Moreover, this vector can be developed as a low-cost, rapid-response vaccine that can be quickly manufactured. Therefore, an adenovirus vector encoding conserved influenza antigens holds promise in the development of a universal influenza vaccine. This review will summarize the progress in adenovirus-vectored universal flu vaccines and discuss future novel approaches.

Combined local and systemic immunization is essential for durable T-cell mediated heterosubtypic immunity against influenza A virus

The threat from unpredictable influenza virus pandemics necessitates the development of a new type of influenza vaccine. Since the internal proteins are highly conserved, induction of T cells targeting these antigens may provide the solution. Indeed, adenoviral (Ad) vectors expressing flu nucleoprotein have previously been found to induce short-term protection in mice. In this study we confirm that systemic (subcutaneous (s.c.) immunization rapidly induced heterosubtypic protection predominantly mediated by CD8 T cells, but within three months clinical protection completely disappeared. Local (intranasal (i.n.)) immunization elicited delayed, but more lasting protection despite relatively inefficient immunization. However, by far, the most robust protection was induced by simultaneous, combined (i.n. + s.c.) vaccination, and, notably, in this case clinical protection lasted at least 8 months without showing any evidence of fading. Interestingly, the superior ability of the latter group to resist reinfection correlated with a higher number of antigen-specific CD8 T cells in the spleen. Thus, detailed analysis of the underlying CD8 T cell responses highlights the importance of T cells already positioned in the lungs prior to challenge, but at the same time underscores an important back-up role for circulating antigen-specific cells with the capacity to expand and infiltrate the infected lungs.