Preparation of influenza virus subviral particles lacking the HA1 subunit of hemagglutinin: unmasking of cross-reactive HA2 determinants

Headless hemagglutinin-containing influenza viral particles direct immune responses toward more conserved epitopes

Seasonal influenza vaccines provide mostly strain-specific protection due to the elicitation of antibody responses focused on evolutionarily plastic antigenic sites in the hemagglutinin head domain. To direct the humoral response toward more conserved epitopes, we generated an influenza virus particle where the full-length hemagglutinin protein was replaced with a membrane-anchored, "headless" variant while retaining the normal complement of other viral structural proteins such as the neuraminidase as well as viral RNAs. We found that a single administration of a headless virus particle-based vaccine elicited high titers of antibodies that recognized more conserved epitopes on the major viral glycoproteins. Furthermore, the vaccine could elicit these responses even in the presence of pre-existing, hemagglutinin (HA) head-focused influenza immunity. Importantly, these antibody responses mediated protective, but non-neutralizing functions such as neuraminidase inhibition and antibody-dependent cellular cytotoxicity. Additionally, we show the vaccine can provide protection from homologous and heterologous challenges in mouse models of severe influenza without any measurable HA head-directed antibody responses. Thus, headless hemagglutinin containing viral particles may represent a tool to drive the types of antibody responses predicted to increase influenza vaccine breadth and durability.IMPORTANCECurrent seasonal influenza vaccines provide incomplete protection from disease. This is partially the result of the antibody response being directed toward parts of the virus that are tolerant of mutations. Redirecting the immune response to more conserved regions of the virus has been a central strategy of next-generation vaccine designs and approaches. Here, we develop and test a vaccine based on a modified influenza virus particle that expresses a partially deleted hemagglutinin protein along with the other viral structural proteins. We demonstrate this vaccine elicits antibodies that recognize the more conserved viral epitopes of the hemagglutinin stalk and neuraminidase protein to facilitate protection against influenza viruses despite a lack of classical viral neutralization activity.

Vaccination with antigenically complex hemagglutinin mixtures confers broad protection from influenza disease

Current seasonal influenza virus vaccines induce responses primarily against immunodominant but highly plastic epitopes in the globular head of the hemagglutinin (HA) glycoprotein. Because of viral antigenic drift at these sites, vaccines need to be updated and readministered annually. To increase the breadth of influenza vaccine-mediated protection, we developed an antigenically complex mixture of recombinant HAs designed to redirect immune responses to more conserved domains of the protein. Vaccine-induced antibodies were disproportionally redistributed to the more conserved stalk of the HA without hindering, and in some cases improving, antibody responses against the head domain. These improved responses led to increased protection against homologous and heterologous viral challenges in both mice and ferrets compared with conventional vaccine approaches. Thus, antigenically complex protein mixtures can at least partially overcome HA head domain antigenic immunodominance and may represent a step toward a more universal influenza vaccine.

Bivalent vaccines effectively protect mice against influenza A and respiratory syncytial viruses

Influenza and Respiratory Syncytial virus (RSV) infections together contribute significantly to the burden of acute lower respiratory tract infections. Despite the disease burden, no approved RSV vaccine is available. While approved vaccines are available for influenza, seasonal vaccination is required to maintain protection. In addition to both being respiratory viruses, they follow a common seasonality, which warrants the necessity for a concerted vaccination approach. Here, we designed bivalent vaccines by utilizing highly conserved sequences, targeting both influenza A and RSV, as either a chimeric antigen or individual antigens separated by a ribosome skipping sequence. These vaccines were found to be effective in protecting the animals from challenge by either virus, with mechanisms of protection being substantially interrogated in this communication.

SARS-CoV-2 epitopes inform future vaccination strategies

All currently approved COVID-19 vaccines utilize the spike protein as their immunogen. SARS-CoV-2 variants of concern (VOCs) contain mutations in the spike protein, enabling them to escape infection- and vaccination-induced immune responses to cause reinfection. New vaccines are hence being researched intensively. Studying SARS-CoV-2 epitopes is essential for vaccine design, as identifying targets of broadly neutralizing antibody responses and immunodominant T-cell epitopes reveal candidates for inclusion in next-generation COVID-19 vaccines. We summarize the major studies which have reported on SARS-CoV-2 antibody and T-cell epitopes thus far. These results suggest that a future of pan-coronavirus vaccines, which not only protect against SARS-CoV-2 but numerous other coronaviruses, may be possible. The T-cell epitopes of SARS-CoV-2 have gotten less attention than neutralizing antibody epitopes but may provide new strategies to control SARS-CoV-2 infection. T-cells target many SARS-CoV-2 antigens other than spike, recognizing numerous epitopes within these antigens, thereby limiting the chance of immune escape by VOCs that mainly possess spike protein mutations. Therefore, augmenting vaccination-induced T-cell responses against SARS-CoV-2 may provide adequate protection despite broad antibody escape by VOCs.

Potential of conserved antigenic sites in development of universal SARS-like coronavirus vaccines

Given pandemic risks of zoonotic SARS-CoV-2 variants and other SARS-like coronaviruses in the future, it is valuable to perform studies on conserved antigenic sites to design universal SARS-like coronavirus vaccines. By using antibodies obtained from convalescent COVID-19 patients, we succeeded in functional comparison of conserved antigenic sites at multiple aspects with each other, and even with SARS-CoV-2 unique antigenic sites, which promotes the cognition of process of humoral immune response to the conserved antigenic sites. The conserved antigenic sites between SARS-CoV-2 and SARS-CoV can effectively induce affinity maturation of cross-binding antibodies, finally resulting in broadly neutralizing antibodies against multiple variants of concern, which provides an important basis for universal vaccine design, however they are subdominant, putatively due to their lower accessibility relative to SARS-CoV-2 unique antigenic sites. Furthermore, we preliminarily design RBDs to improve the immunogenicity of these conserved antigenic sites. Our study focusing on conserved antigenic sites provides insights for promoting the development of universal SARS-like coronavirus vaccines, thereby enhancing our pandemic preparedness.

Status and Challenges for Vaccination against Avian H9N2 Influenza Virus in China

In China, H9N2 avian influenza virus (AIV) has become widely prevalent in poultry, causing huge economic losses after secondary infection with other pathogens. Importantly, H9N2 AIV continuously infects humans, and its six internal genes frequently reassort with other influenza viruses to generate novel influenza viruses that infect humans, threatening public health. Inactivated whole-virus vaccines have been used to control H9N2 AIV in China for more than 20 years, and they can alleviate clinical symptoms after immunization, greatly reducing economic losses. However, H9N2 AIVs can still be isolated from immunized chickens and have recently become the main epidemic subtype. A more effective vaccine prevention strategy might be able to address the current situation. Herein, we analyze the current status and vaccination strategy against H9N2 AIV and summarize the progress in vaccine development to provide insight for better H9N2 prevention and control.

Severe acute respiratory syndrome coronavirus 2 variants-Possibility of universal vaccine design: A review

Both novel and conventional vaccination strategies have been implemented worldwide since the onset of coronavirus disease 2019 (COVID-19) pandemic caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Despite various medical advances in the treatment and prevention of the spread of this contagious disease, it remains a major public health threat with a high mortality rate. As several lethal SARS-CoV-2 variants continue to emerge, the development of several vaccines and medicines, each with certain advantages and disadvantages, is underway. Additionally, many modalities are at various stages of research and development or clinical trials. Here, we summarize emerging SARS-CoV-2 variants, including delta, omicron, and "stealth omicron," as well as available oral drugs for COVID-19. We also discuss possible antigen candidates other than the receptor-binding domain protein for the development of a universal COVID-19 vaccine. The present review will serve as a helpful resource for future vaccine and drug development to combat COVID-19.

Systems Biological Analysis of Immune Response to Influenza Vaccination

The last decade has witnessed tremendous progress in immunology and vaccinology, owing to several scientific and technological breakthroughs. Systems vaccinology is a field that has emerged at the forefront of vaccine research and development and provides a unique way to probe immune responses to vaccination in humans. The goals of systems vaccinology are to use systems-based approaches to define signatures that can be used to predict vaccine immunogenicity and efficacy and to delineate the molecular mechanisms driving protective immunity. The application of systems biological approaches in influenza vaccination studies has enabled the discovery of early signatures that predict immunogenicity to vaccination and yielded novel mechanistic insights about vaccine-induced immunity. Here we review the contributions of systems vaccinology to influenza vaccine development and critically examine the potential of systems vaccinology toward enabling the development of a universal influenza vaccine that provides robust and durable immunity against diverse influenza viruses.

Targeting Antigens for Universal Influenza Vaccine Development

Traditional influenza vaccines generate strain-specific antibodies which cannot provide protection against divergent influenza virus strains. Further, due to frequent antigenic shifts and drift of influenza viruses, annual reformulation and revaccination are required in order to match circulating strains. Thus, the development of a universal influenza vaccine (UIV) is critical for long-term protection against all seasonal influenza virus strains, as well as to provide protection against a potential pandemic virus. One of the most important strategies in the development of UIVs is the selection of optimal targeting antigens to generate broadly cross-reactive neutralizing antibodies or cross-reactive T cell responses against divergent influenza virus strains. However, each type of target antigen for UIVs has advantages and limitations for the generation of sufficient immune responses against divergent influenza viruses. Herein, we review current strategies and perspectives regarding the use of antigens, including hemagglutinin, neuraminidase, matrix proteins, and internal proteins, for universal influenza vaccine development.

Strategies Targeting Hemagglutinin as a Universal Influenza Vaccine

Influenza virus has significant viral diversity, both through antigenic drift and shift, which makes development of a vaccine challenging. Current influenza vaccines are updated yearly to include strains predicted to circulate in the upcoming influenza season, however this can lead to a mismatch which reduces vaccine efficacy. Several strategies targeting the most abundant and immunogenic surface protein of influenza, the hemagglutinin (HA) protein, have been explored. These strategies include stalk-directed, consensus-based, and computationally derived HA immunogens. In this review, we explore vaccine strategies which utilize novel antigen design of the HA protein to improve cross-reactive immunity for development of a universal influenza vaccine.

Virus-like Particle Vaccines: A Prospective Panacea Against an Avian Influenza Panzootic

Epizootics of highly pathogenic avian influenza (HPAI) have resulted in the deaths of millions of birds leading to huge financial losses to the poultry industry worldwide. The roles of migratory wild birds in the harbouring, mutation, and transmission of avian influenza viruses (AIVs), and the lack of broad-spectrum prophylactic vaccines present imminent threats of a global panzootic. To prevent this, control measures that include effective AIV surveillance programmes, treatment regimens, and universal vaccines are being developed and analysed for their effectiveness. We reviewed the epidemiology of AIVs with regards to past avian influenza (AI) outbreaks in birds. The AIV surveillance programmes in wild and domestic birds, as well as their roles in AI control were also evaluated. We discussed the limitations of the currently used AI vaccines, which necessitated the development of a universal vaccine. We evaluated the current development of AI vaccines based upon virus-like particles (VLPs), particularly those displaying the matrix-2 ectodomain (M2e) peptide. Finally, we highlighted the prospects of these VLP vaccines as universal vaccines with the potential of preventing an AI panzootic.

Influenza-Host Interplay and Strategies for Universal Vaccine Development

Influenza is an annual epidemic and an occasional pandemic caused by pathogens that are responsible for infectious respiratory disease. Humans are highly susceptible to the infection mediated by influenza A viruses (IAV). The entry of the virus is mediated by the influenza virus hemagglutinin (HA) glycoprotein that binds to the cellular sialic acid receptors and facilitates the fusion of the viral membrane with the endosomal membrane. During IAV infection, virus-derived pathogen-associated molecular patterns (PAMPs) are recognized by host intracellular specific sensors including toll-like receptors (TLRs), C-type lectin receptors, retinoic acid-inducible gene-I (RIG-I)-like receptors (RLRs), and nucleotide-binding oligomerization domain (NOD)-like receptors (NLRs) either on the cell surface or intracellularly in endosomes. Herein, we comprehensively review the current knowledge available on the entry of the influenza virus into host cells and the molecular details of the influenza virus–host interface. We also highlight certain strategies for the development of universal influenza vaccines.

Development of broadly reactive influenza vaccines by targeting the conserved regions of the hemagglutinin stem and head domains

INTRODUCTION

Influenza virus infections cause serious illness in millions of people each year. Although influenza virus vaccines are available, they are not optimally effective due to mismatches between the influenza virus strains used for the vaccine and the circulating strains. To improve protection by vaccines, a broadly protective or universal vaccine may be required. Strategies to develop universal vaccines aim to elicit broadly reactive antibodies, which target regions on the viral hemagglutinin (HA) protein which are conserved between strains. Broadly reactive antibodies have helped to identify such targets and can guide the design of such a vaccine.

AREAS COVERED

The first part of this review provides an in-depth overview of broadly reactive anti-HA antibodies, discussing their origin, breadth and their mechanisms of protection. The second part discusses the technical design and mode of action of potential universal vaccine candidates that aim to elicit these broadly reactive antibodies and provide protection against a majority of influenza strains.

EXPERT OPINION

While great strides have been made in the development of universal influenza vaccine candidates, real-life use still requires improvement of stability, enhancement of their breadth of protection and ease of production, while efficacies need to be determined in human trials.

Induction of Cross-Reactive and Protective Antibody Responses After DNA Vaccination With MHCII-Targeted Stem Domain From Influenza Hemagglutinin

Novel and more broadly protective vaccines against influenza are needed to efficiently meet antigenic drift and shift. Relevant to this end, the stem domain of hemagglutinin (HA) is highly conserved, and antibodies specific for epitopes located to the stem have been demonstrated to be able to confer broad protection against various influenza subtypes. However, a remaining challenge is to induce antibodies against the poorly immunogenic stem by vaccination strategies that can be scaled up for prophylactic vaccination of the general population. Here, we have developed DNA vaccines where the conserved stem domain of HA from influenza A/PR/8/34 (H1N1) and A/Shanghai/2/2013 (H7N9) was targeted toward MHC class II molecules on antigen-presenting cells (APC) for increased immunogenicity. Each of these vaccines induced antibodies that cross-reacted with other subtypes in the corresponding phylogenetic influenza groups. Importantly, when mixing the MHCII-targeted stem domains from H1N1 and H7N9 influenza viruses into one vaccine bolus, we observed broad protection against candidate stains from both phylogenetic groups 1 and 2.

Broadly Protective Strategies Against Influenza Viruses: Universal Vaccines and Therapeutics

Influenza virus is a respiratory pathogen that can cause disease in humans, with symptoms ranging from mild to life-threatening. The vast majority of influenza virus infections in humans are observed during seasonal epidemics and occasional pandemics. Given the substantial public health burden associated with influenza virus infection, yearly vaccination is recommended for protection against seasonal influenza viruses. Despite vigilant surveillance for new variants and careful selection of seasonal vaccine strains, the efficacy of seasonal vaccines can vary widely from year to year. This often results in lowered protection within the population, regardless of vaccination status. In order to broaden the protection afforded by seasonal influenza vaccines, the National Institute of Allergy and Infectious Diseases (NIAID) has deemed the development of a universal influenza virus vaccine to be a priority in influenza virus vaccine research. This universal vaccine would provide protection against all influenza virus strains, eliminating the need for the yearly reformulations of seasonal influenza vaccines. In addition to universal influenza vaccine efforts, substantial progress has been made in developing novel influenza virus therapeutics that utilize broadly neutralizing antibodies to provide protection against influenza virus infection and to mitigate disease outcomes during infection. In this review, we discuss various approaches toward the goal of improving influenza virus vaccine efficacy through a universal influenza virus vaccine. We also address the novel methods of discovery and utilization of broadly neutralizing antibodies to improve influenza disease outcomes.

Influenza Anti-Stalk Antibodies: Development of a New Method for the Evaluation of the Immune Responses to Universal Vaccine

Growing interest in universal influenza vaccines and novel administration routes has led to the development of alternative serological assays that are able to detect antibodies against conserved epitopes. We present a competitive ELISA method that is able to accurately determine the ratio of serum immunoglobulin G directed against the different domains of the hemagglutinin, the head and the stalk. Human serum samples were treated with two variants of the hemagglutinin protein from the A/California/7/2009 influenza virus. The signals detected were assigned to different groups of antibodies and presented as a ratio between head and stalk domains. A subset of selected sera was also tested by hemagglutination inhibition, single radial hemolysis, microneutralization, and enzyme-linked lectin assays. Pre-vaccination samples from adults showed a quite high presence of anti-stalk antibodies, and the results were substantially in line with those of the classical serological assays. By contrast, pre-vaccination samples from children did not present anti-stalk antibodies, and the majority of the anti-hemagglutinin antibodies that were detected after vaccination were directed against the head domain. The presented approach, when supported by further assays, can be used to assess the presence of specific anti-stalk antibodies and the potential boost of broadly protective antibodies, especially in the case of novel universal influenza vaccine approaches.

Is a Universal Influenza Virus Vaccine Possible?

Influenza viruses remain a severe burden to human health because of their contribution to overall morbidity and mortality. Current seasonal influenza virus vaccines do not provide sufficient protection to alleviate the annual impact of influenza and cannot confer protection against potentially pandemic influenza viruses. The lack of protection is due to rapid changes of the viral epitopes targeted by the vaccine and the often suboptimal immunogenicity of current immunization strategies. Major efforts to improve vaccination approaches are under way. The development of a universal influenza virus vaccine may be possible by combining the lessons learned from redirecting the immune response toward conserved viral epitopes, as well as the use of adjuvants and novel vaccination platforms.

The Quest for a Truly Universal Influenza Vaccine

There is an unmet public health need for a universal influenza vaccine (UIV) to provide broad and durable protection from influenza virus infections. The identification of broadly protective antibodies and cross-reactive T cells directed to influenza viral targets present a promising prospect for the development of a UIV. Multiple targets for cross-protection have been identified in the stalk and head of hemagglutinin (HA) to develop a UIV. Recently, neuraminidase (NA) has received significant attention as a critical component for increasing the breadth of protection. The HA stalk-based approaches have shown promising results of broader protection in animal studies, and their feasibility in humans are being evaluated in clinical trials. Mucosal immune responses and cross-reactive T cell immunity across influenza A and B viruses intrinsic to live attenuated influenza vaccine (LAIV) have emerged as essential features to be incorporated into a UIV. Complementing the weakness of the stand-alone approaches, prime-boost vaccination combining HA stalk, and LAIV is under clinical evaluation, with the aim to increase the efficacy and broaden the spectrum of protection. Preexisting immunity in humans established by prior exposure to influenza viruses may affect the hierarchy and magnitude of immune responses elicited by an influenza vaccine, limiting the interpretation of preclinical data based on naive animals, necessitating human challenge studies. A consensus is yet to be achieved on the spectrum of protection, efficacy, target population, and duration of protection to define a “universal” vaccine. This review discusses the recent advancements in the development of UIVs, rationales behind cross-protection and vaccine designs, and challenges faced in obtaining balanced protection potency, a wide spectrum of protection, and safety relevant to UIVs.

Universal Influenza Virus Vaccines That Target the Conserved Hemagglutinin Stalk and Conserved Sites in the Head Domain

Due to limitations of current influenza virus vaccines, new vaccines that mediate broad protection and show high efficacy against seasonal and pandemic viruses are urgently needed. The conserved stalk of the viral hemagglutinin has been identified as potential target antigen for this new generation of vaccines. A vaccination strategy based on chimeric hemagglutinin (cHA), which refocuses the immune response toward the stalk domain and the conserved neuraminidase, is currently being tested in clinical trials. Here we discuss how to improve the cHA antigens to generate vaccine candidates that both induce a broad antistalk response and target conserved immunosubdominant epitopes in the head domain of the hemagglutinin. These novel constructs, termed mosaic hemagglutinins, should provide enhanced protection and should be tested in clinical trials to assess their improved potential as universal influenza virus vaccine candidates.

Structural Insights for Anti-Influenza Vaccine Design

Influenza A virus are a persistent and significant threat to human health, and current vaccines do not provide sufficient protection due to antigenic drift, which allows influenza viruses to easily escape immune surveillance and antiviral drug activity. Influenza hemagglutinin (HA) is a glycoprotein needed for the entry of enveloped influenza viruses into host cells and is a potential target for anti-influenza humoral immune responses. In recent years, a number of broadly neutralizing antibodies (bnAbs) have been isolated, and their relative structural information obtained from the crystallization of influenza antigens in complex with bnAbs has provided some new insights into future influenza vaccine research. Here, we review the current knowledge of the HA-targeted bnAbs and the structure-based mechanisms contributing to neutralization. We also discuss the potential for this structure-based approach to overcome the challenge of obtaining a highly desired "universal" influenza vaccine, especially on small proteins and peptides.

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.

Mini-hemagglutinin vaccination induces cross-reactive antibodies in pre-exposed NHP that protect mice against lethal influenza challenge

Seasonal vaccines are currently the most effective countermeasure against influenza. However, seasonal vaccines are only effective against strains closely related to the influenza strains contained in the vaccine. Recently a new hemagglutinin (HA) stem-based antigen, the so-called “mini-HA”, has been shown to induce a cross-protective immune response in influenza-naive mice and non-human primates (NHP). However, prior exposure to influenza can have a profound effect on the immune response to subsequent influenza infection and the protective efficacy of vaccination. Here we show that mini-HA, compared to a trivalent influenza vaccine (TIV), elicits a broadened influenza-specific humoral immune response in NHP previously exposed to influenza. Serum transfer experiments showed that antibodies induced by both mini-HA and seasonal vaccine protected mice against lethal challenge with a H1N1 influenza strain heterologous to the H1 HA included in the TIV. However, antibodies elicited by mini-HA showed an additional benefit of protecting mice against lethal heterosubtypic H5N1 influenza challenge, associated with H5 HA-specific functional antibodies.

A vaccine candidate developed from a novel flu-based protein induces antibodies able to protect against multiple influenza strains. Alongside a team of Dutch researchers, Janssen’s Joan van der Lubbe created a vaccine, dubbed “mini-HA”, based on a recently discovered influenza protein that induced neutralizing antibodies to many related influenza virus strains in non-human primates. Mini-HA vaccination targeted a surface protein region that is highly conserved between influenza strains and offered protection regardless of previous exposure to the disease, which is known to greatly affect vaccine efficacy. The immune response elicited by the novel vaccine surpassed that of a standard seasonal influenza vaccine. Future research is now warranted to verify whether mini-HA can act as a universal seasonal flu vaccine, potentially even effective against emergent virus strains.

Is It Possible to Develop a "Universal" Influenza Virus Vaccine? Potential for a Universal Influenza Vaccine

Development of optimal vaccines for influenza is challenging, in part as a result of the high antigenic variability in field strains associated with genetic shift from reassortment and genetic drift from point mutations. Discovery of conserved antigenic sites on the hemagglutinin (HA) protein for neutralizing antibodies suggested the possibility that influenza vaccines could be developed that induce focused antibody responses to the conserved neutralizing determinants, especially the HA stem region. Recent studies have focused on the antigenicity and immunogenicity of such domains, using monoclonal antibodies and candidate-engineered HA stem-based vaccines. Much progress has been made, but we still do not fully understand the biology of the immune response to this unique antigenic region.

Membrane-anchored stalk domain of influenza HA enhanced immune responses in mice

Current strategies for influenza virus vaccines primarily aim to elicit immune responses towards the globular head domain of the hemagglutinin (HA) protein so that binding of the virus to membrane receptors on the host cells is inhibited. In the present study, we show a novel strategy to generate immunity against the highly conserved region of the influenza virus. The globular head domain was replaced by different linkers to generate a headless HA (stalk domain) and then coexpressed with influenza M1 proteinin Tni insect cells. The expression was validated by western blot analysis, and stalk domain with peptides (GGGGS)4 linkers was identified to anchor in a stable way to the cell membrane. An immunoelectron microscope showed that stalk domain with (GGGGS)4 linkers were steadily incorporated to the surface of influenza virus-like particles (VLPs). Mice immunized with these VLPs exhibited enhanced systemic antibody responses with increased binding avidity and study found high titers of ADCC antibodies to the influenza virus, these VLPs also induced mucosal immune responses and produced antigen-specific IgG and IgA in nasal and lung washes. In addition, antigen-specific IgG antibody-secreting cells (ASCs) increased significantly in the spleen and lymph node. The results of this study suggest that the headless HA is a useful target in developing a universal vaccine against influenza virus.

From Original Antigenic Sin to the Universal Influenza Virus Vaccine

Antibody responses are essential for protection against influenza virus infection. Humans are exposed to a multitude of influenza viruses throughout their lifetime and it is clear that immune history influences the magnitude and quality of the antibody response. The 'original antigenic sin' concept refers to the impact of the first influenza virus variant encounter on lifelong immunity. Although this model has been challenged since its discovery, past exposure, and likely one's first exposure, clearly affects the epitopes targeted in subsequent responses. Understanding how previous exposure to influenza virus shapes antibody responses to vaccination and infection is critical, especially with the prospect of future pandemics and for the effective development of a universal influenza vaccine.

Protein nanoparticle vaccine based on flagellin carrier fused to influenza conserved epitopes confers full protection against influenza A virus challenge

Currently marketed influenza vaccines only confer protection against matching influenza virus strains. The influenza A composition of these vaccines needs to be annually updated. Vaccines that target conserved epitopes of influenza viruses would in principle offer broad cross-protection against influenza A viruses. In our study, we investigated the specific immune responses and protective efficacy of protein nanoparticles based on fusion proteins of flagellin carrier linked to conserved influenza epitopes. We designed fusion proteins by replacing the hyperimmunogenic region of flagellin (FliC) with four tandem copies of the ectodomain of matrix protein 2 (f4M2e), H1 HA2 domain (fHApr8) or H3 HA2 domain (fHAaichi). Protein nanoparticles fabricated from these fusion proteins by using DTSSP crosslinking retained Toll-like receptor 5 agonist activity of FliC. Intranasal immunization with f4M2e, f4M2e/fHApr8 or f4M2e/fHAaichi nanoparticles induced vaccine antigen-specific humoral immune responses. It was also found that the incorporation of the H1 HA2 domain into f4M2e/fHApr8 nanoparticles boosted M2e specific antibody responses. Immunized mice were fully protected against lethal doses of virus challenge.

A Bivalent Heterologous DNA Virus-Like-Particle Prime-Boost Vaccine Elicits Broad Protection against both Group 1 and 2 Influenza A Viruses

Current seasonal influenza vaccines are efficacious when vaccine strains are matched with circulating strains. However, they do not protect antigenic variants and newly emerging pandemic and outbreak strains. Thus, there is a critical need for developing so-called "universal" vaccines that protect against all influenza viruses. In the present study, we developed a bivalent heterologous DNA virus-like particle prime-boost vaccine strategy. We show that mice immunized with this vaccine were broadly protected against lethal challenge from group 1 (H1, H5, and H9) and group 2 (H3 and H7) viruses, with 94% aggregate survival. To determine the immune correlates of protection, we performed passive immunizations and in vitro assays. We show that this vaccine elicited antibody responses that bound HA from group 1 (H1, H2, H5, H6, H8, H9, H11, and H12) and group 2 (H3, H4, H7, H10, H14, and H15) and neutralized homologous and intrasubtypic H5 and H7 and heterosubtypic H1 viruses and hemagglutinin-specific CD4 and CD8 T cell responses. As a result, passive immunization with immune sera fully protected mice against H5, H7, and H1 challenge, whereas with both immune sera and T cells the mice survived heterosubtypic H3 and H9 challenge. Thus, it appears that (i) neutralizing antibodies alone fully protect against homologous and intrasubtypic H5 and H7 and (ii) neutralizing and binding antibodies are sufficient to protect against heterosubtypic H1, (iii) but against heterosubtypic H3 and H9, binding antibodies and T cells are required for complete survival. We believe that this vaccine regimen could potentially be a candidate for a "universal" influenza vaccine.IMPORTANCE Influenza virus infection is global health problem. Current seasonal influenza vaccines are efficacious only when vaccine strains are matched with circulating strains. However, these vaccines do not protect antigenic variants and newly emerging pandemic and outbreak strains. Because of this, there is an urgent need to develop so-called "universal" influenza vaccines that can protect against both current and future influenza strains. In the present study, we developed a bivalent heterologous prime-boost vaccine strategy. We show that a bivalent vaccine regimen elicited broad binding and neutralizing antibody and T cell responses that conferred broad protection against diverse challenge viruses in mice, suggesting that this bivalent prime-boost strategy could practically be a candidate for a "universal" influenza vaccine.

Antibody-Dependent Cell-Mediated Cytotoxicity Epitopes on the Hemagglutinin Head Region of Pandemic H1N1 Influenza Virus Play Detrimental Roles in H1N1-Infected Mice

Engaging the antibody-dependent cell-mediated cytotoxicity (ADCC) for killing of virus-infected cells and secretion of antiviral cytokines and chemokines was incorporated as one of the important features in the design of universal influenza vaccines. However, investigation of the ADCC epitopes on the highly immunogenic influenza hemagglutinin (HA) head region has been rarely reported. In this study, we determined the ADCC and antiviral activities of two putative ADCC epitopes, designated E1 and E2, on the HA head of a pandemic H1N1 influenza virus in vitro and in a lethal mouse model. Our data demonstrated that sera from the E1-vaccinated mice could induce high ADCC activities. Importantly, the induction of ADCC response modestly decreased viral load in the lungs of H1N1-infected mice. However, the elevated ADCC significantly increased mouse alveolar damage and mortality than that of the PBS-vaccinated group (P < 0.0001). The phenotype was potentially due to an exaggerated inflammatory cell infiltration triggered by ADCC, as an upregulated release of cytotoxic granules (perforin) was observed in the lung tissue of E1-vaccinated mice after H1N1 influenza virus challenge. Overall, our data suggested that ADCC elicited by certain domains of HA head region might have a detrimental rather than protective effect during influenza virus infection. Thus, future design of universal influenza vaccine shall strike a balance between the induction of protective immunity and potential side effects of ADCC.

Production of a truncated recombinant HA1 for influenza A H9 subtype screening

Hemagglutinin is the major component of membrane protein and plays a major role in virus entry into host cells through their receptors and it is predicted to elicit the production neutralizing antibodies. Our aim is to assess the potential of a truncated rHA1 domain, encoding residues 157-260 to detect influenza A H9 specific antibodies. The predicted characteristics of this protein revealed that it is a hydrophobic protein possessing predominant antigenicity and composed of random coils (48%) and extended strand (28%) but few α-helix (6.33%) and β-sheet (7%). A 312 pb HA1 gene was amplified and cloned in pET23b(+) vector including an C-terminal polyHis as a fusion partner, transformed and expressed in Escherichia coli cells as inclusion bodies. The truncated protein was solubilized with 8 M urea, purified by immobilized metal affinity chromatography and then detected by western blot with anti-His and H9-specific polyclonal antibodies. The test demonstrated high specificity (100%) and sensibility (98%). The immunoreactivity of the truncated rHA1 assessed revealed that only antisera against H9 yielded a specific and strong reactivity, with no cross-reactivity against negative sera. This study demonstrates that the truncated rHA1 may serve as a useful tool for rapid and easy surveillance of H9 infection.

Immune history profoundly affects broadly protective B cell responses to influenza

Generating a broadly protective influenza vaccine is critical to global health. Understanding how immune memory influences influenza immunity is central to this goal. We undertook an in-depth study of the B cell response to the pandemic 2009 H1N1 vaccine over consecutive years. Analysis of monoclonal antibodies generated from vaccine-induced plasmablasts demonstrated that individuals with low preexisting serological titers to the vaccinating strain generated a broadly reactive, hemagglutinin (HA) stalk-biased response. Higher preexisting serum antibody levels correlated with a strain-specific HA head-dominated response. We demonstrate that this HA head immunodominance encompasses poor accessibility of the HA stalk epitopes. Further, we show polyreactivity of HA stalk-reactive antibodies that could cause counterselection of these cells. Thus, preexisting memory B cells against HA head epitopes predominate, inhibiting a broadly protective response against the HA stalk upon revaccination with similar strains. Consideration of influenza exposure history is critical for new vaccine strategies designed to elicit broadly neutralizing antibodies.

The Quest for a Universal Flu Vaccine: Headless HA 2.0

Two recent publications report the design of immunogens based on the conserved stalk domain of the influenza virus hemagglutinin. These new "headless" hemagglutinin constructs recapitulate the epitopes recognized by broadly neutralizing antibodies and induce broadly protective/heterosubtypic immunity in animal models, bringing us a step closer to "universal" influenza virus vaccines.

Epitope-focused vaccine design against influenza A and B viruses

The threat of influenza A and B variants via antigenic drift and emerging novel influenza A and B strains in the human population via antigenic shift has spurred research efforts to improve upon current seasonal influenza vaccines. In recent years, a wave of novel technological breakthroughs has lead to the identification of many broadly anti-influenza hemagglutinin (HA) monoclonal antibodies (mAbs) and the elucidation of the conserved epitopes recognized by these mAbs in both the head and the stem of HA as well as the mechanisms of inhibition. These discoveries along with an improved understanding of how the immune system responds to influenza infection and vaccination has spurred great efforts on stem-based cross-subtype ('universal') vaccine design as well as RBS-based HA subtype-specific vaccine design.

Heads, stalks and everything else: how can antibodies eradicate influenza as a human disease?

Current seasonal influenza virus vaccines are effective against infection but they have to be reformulated on a regular basis to counter antigenic variations. The majority of the antibodies induced in response to seasonal vaccination are strain-specific. However, antibodies targeting conserved epitopes on the hemagglutinin protein have been identified and they offer broad protection. Most of these antibodies bind the hemagglutinin stalk domain and are generated from preexisting memory B cells. Broadly protective stalk-biased responses induced by antigenically divergent influenza strains, in concert with prior immunity, are sufficient to eradicate seasonally circulating strains. Future vaccine trials should aim to harness and maintain such a response with the realistic goal of developing a universal influenza vaccine.

Universal influenza vaccine design: directing the antibody repertoire

Influenza infections are responsible for a large health and economic burden. Vaccination is the best strategy to reduce influenza-related disease burden, but current vaccines have limited breadth and need near-annual reformulation. Developing new influenza vaccines that provide broad and long-lasting protection is an important goal. This review represents an overview of the current knowledge of the universal vaccine approach that focuses on the induction of broadly neutralizing antibodies targeting the hemagglutinin (HA) stem of influenza viruses. Adjuvation of existing influenza vaccines has so far had limited effect on the induction of broadly neutralizing antibodies. HA stem-based immunogens that lack the immunodominant HA head have shown promising results in preclinical models, providing evidence that a universal influenza vaccine is within reach.

Baculovirus Displaying Hemagglutinin Elicits Broad Cross-Protection against Influenza in Mice

The widespread influenza virus infection further emphasizes the need for novel vaccine strategies that effectively reduce the impact of epidemic as well as pandemic influenza. Conventional influenza vaccines generally induce virus neutralizing antibody responses which are specific for a few antigenically related strains within the same subtype. However, antibodies directed against the conserved stalk domain of HA could neutralize multiple subtypes of influenza virus and thus provide broad-spectrum protection. In this study, we designed and constructed a recombinant baculovirus-based vaccine, rBac-HA virus, that expresses full-length HA of pandemic H1N1 influenza virus (A/California/04/09) on the viral envelope. We demonstrated that repeated intranasal immunizations with rBac-HA virus induced HA stalk-specific antibody responses and protective immunity against homologous as well as heterosubtypic virus challenge. The adoptive transfer experiment shows that the cross-protection is conferred by the immune sera which contain HA stalk-specific antibodies. These results warrant further development of rBac-HA virus as a broad-protective vaccine against influenza. The vaccine induced protection against infection with the same subtype as well as different subtype, promising a potential universal vaccine for broad protection against different subtypes to control influenza outbreaks including pandemic.

Implications of broadly neutralizing antibodies in the development of a universal influenza vaccine

Serum antibodies are the major correlate of influenza vaccine efficacy, providing short-term protection against infection. Recent efforts have been focused on studying antibody responses at a monoclonal level to understand their role in protection against influenza, and to ultimately improve vaccine strategies to provide broader, long-term immunity against influenza virus. These studies have shown that broadly neutralizing antibodies specific for the conserved stem domain of the hemagglutinin protein can target multiple strains of influenza. These antibodies show great promise both from a therapeutic perspective as well as for guiding vaccine design efforts. In this review, we will summarize past and recent findings about broadly neutralizing antibodies against influenza, and discuss how these findings may guide development of universal vaccine strategies.

Novel universal influenza virus vaccine approaches

Seasonal influenza virus vaccines have to be re-formulated and re-administered on an annual basis due to antigenic drift of the influenza virus surface glycoproteins. In addition, seasonal vaccines show limited efficacy against novel pandemic influenza virus strains, and producing tailored vaccines for these strains in a timely manner is challenging. Several novel broadly protective vaccine candidates targeting the conserved stalk domain of the viral hemagglutinin have been developed. Here we review these novel constructs and discuss several important findings and considerations regarding the protective efficacy of stalk-based vaccines.

Vaccination with soluble headless hemagglutinin protects mice from challenge with divergent influenza viruses

Current influenza virus vaccines provide solid protection from infection with viruses that are well matched with the vaccine strains. However, they do not protect efficiently against drifted or shifted strains. We developed an antigen based on the conserved stalk domain of the influenza virus hemagglutinin and tested its efficacy as a vaccine in a mouse virus challenge model. Although the antigen lacked the correct conformation of the native stalk domain and was not recognized by a panel of neutralizing stalk-reactive antibodies, it did induce considerable protection against H1N1, H5N1 and H6N1 challenge strains. Protection was enhanced when mice had pre-existing immunity against the stalk domain. Since pre-existing immunity is also present in the human population, we hypothesize that a similar antigen could show efficacy in humans as well.

Emerging influenza viruses and the prospect of a universal influenza virus vaccine

Influenza viruses cause annual seasonal epidemics and pandemics at irregular intervals. Several cases of human infections with avian and swine influenza viruses have been detected recently, warranting enhanced surveillance and the development of more effective countermeasures to address the pandemic potential of these viruses. The most effective countermeasure against influenza virus infection is the use of prophylactic vaccines. However, vaccines that are currently in use for seasonal influenza viruses have to be re-formulated and re-administered in a cumbersome process every year due to the antigenic drift of the virus. Furthermore, current seasonal vaccines are ineffective against novel pandemic strains. This paper reviews zoonotic influenza viruses with pandemic potential and technological advances towards better vaccines that induce broad and long lasting protection from influenza virus infection. Recent efforts have focused on the development of broadly protective/universal influenza virus vaccines that can provide immunity against drifted seasonal influenza virus strains but also against potential pandemic viruses.

Advances in the development of influenza virus vaccines

Influenza virus infections are a major public health concern and cause significant morbidity and mortality worldwide. Current influenza virus vaccines are an effective countermeasure against infection but need to be reformulated almost every year owing to antigenic drift. Furthermore, these vaccines do not protect against novel pandemic strains, and the timely production of pandemic vaccines remains problematic because of the limitations of current technology. Several improvements have been made recently to enhance immune protection induced by seasonal and pandemic vaccines, and to speed up production in case of a pandemic. Importantly, vaccine constructs that induce broad or even universal influenza virus protection are currently in preclinical and clinical development.

Synthetic Toll-like receptor 4 (TLR4) and TLR7 ligands as influenza virus vaccine adjuvants induce rapid, sustained, and broadly protective responses

Current vaccines against influenza virus infection rely on the induction of neutralizing antibodies targeting the globular head of the viral hemagglutinin (HA). Protection against seasonal antigenic drift or sporadic pandemic outbreaks requires further vaccine development to induce cross-protective humoral responses, potentially to the more conserved HA stalk region. Here, we present a novel viral vaccine adjuvant comprised of two synthetic ligands for Toll-like receptor 4 (TLR4) and TLR7. 1Z105 is a substituted pyrimido[5,4-b]indole specific for the TLR4-MD2 complex, and 1V270 is a phospholipid-conjugated TLR7 agonist. Separately, 1Z105 induces rapid Th2-associated IgG1 responses, and 1V270 potently generates Th1 cellular immunity. 1Z105 and 1V270 in combination with recombinant HA from the A/Puerto Rico/8/1934 strain (rPR/8 HA) effectively induces rapid and sustained humoral immunity that is protective against lethal challenge with a homologous virus. More importantly, immunization with the combined adjuvant and rPR/8 HA, a commercially available split vaccine, or chimeric rHA antigens significantly improves protection against both heterologous and heterosubtypic challenge viruses. Heterosubtypic protection is associated with broadly reactive antibodies to HA stalk epitopes. Histological examination and cytokine profiling reveal that intramuscular (i.m.) administration of 1Z105 and 1V270 is less reactogenic than a squalene-based adjuvant, AddaVax. In summary, the combination of 1Z105 and 1V270 with a recombinant HA induces rapid, long-lasting, and balanced Th1- and Th2-type immunity; demonstrates efficacy in a variety of murine influenza virus vaccine models assaying homologous, heterologous, and heterosubtypic challenge viruses; and has an excellent safety profile.

IMPORTANCE

Novel adjuvants are needed to enhance immunogenicity and increase the protective breadth of influenza virus vaccines to reduce the seasonal disease burden and ensure pandemic preparedness. We show here that the combination of synthetic Toll-like receptor 4 (TLR4) and TLR7 ligands is a potent adjuvant for recombinant influenza virus hemagglutinin, inducing rapid and sustained immunity that is protective against influenza viruses in homologous, heterologous, and heterosubtypic challenge models. Combining TLR4 and TLR7 ligands balances Th1- and Th2-type immune responses for long-lived cellular and neutralizing humoral immunity against the viral hemagglutinin. The combined adjuvant has an attractive safety profile and the potential to augment seasonal-vaccine breadth, contribute to a broadly neutralizing universal vaccine formulation, and improve response time in an emerging pandemic.

Universal immunity to influenza must outwit immune evasion

Although an influenza vaccine has been available for 70 years, influenza virus still causes seasonal epidemics and worldwide pandemics. Currently available vaccines elicit strain-specific antibody (Ab) responses to the surface haemagglutinin (HA) and neuraminidase (NA) proteins, but these can be ineffective against serologically-distinct viral variants and novel subtypes. Thus, there is a great need for cross-protective or "universal" influenza vaccines to overcome the necessity for annual immunization against seasonal influenza and to provide immunity to reduce the severity of infection with pandemic or outbreak viruses. It is well established that natural influenza infection can provide cross-reactive immunity that can reduce the impact of infection with distinct influenza type A strains and subtypes, including H1N1, H3N2, H2N2, H5N1, and H7N9. The key to generating universal influenza immunity through vaccination is to target functionally-conserved regions of the virus, which include epitopes on the internal proteins for cross-reactive T cell immunity or on the HA stem for broadly reactive Ab responses. In the wake of the 2009 H1N1 pandemic, broadly neutralizing antibodies (bnAbs) have been characterized and isolated from convalescent and vaccinated individuals, inspiring development of new vaccination techniques to elicit such responses. Induction of influenza-specific T cell responses through vaccination has also been recently examined in clinical trials. Strong evidence is available from human and animal models of influenza to show that established influenza-specific T cell memory can reduce viral shedding and symptom severity. However, the published evidence also shows that CD8(+) T cells can efficiently select immune escape mutants early after influenza virus infection. Here, we discuss universal immunity to influenza viruses mediated by both cross-reactive T cells and Abs, the mechanisms of immune evasion in influenza, and propose how to counteract commonly occurring immune-escape variants.

Broadly neutralizing influenza hemagglutinin stem-specific antibody CR8020 targets residues that are prone to escape due to host selection pressure

Broadly neutralizing antibodies (bNAb) that target a conserved region of a viral antigen hold significant therapeutic promise. CR8020 is a bNAb that targets the stem region of influenza A virus (IAV) hemagglutinin (HA). CR8020 is currently being evaluated for prophylactic use against group 2 IAVs in phase II studies. Structural and computational analyses reported here indicate that CR8020 targets HA residues that are prone to antigenic drift and host selection pressure. Critically, CR8020 escape mutation is seen in certain H7N9 viruses from recent outbreaks. Furthermore, the ability of the bNAb Fc region to effectively engage activating Fcγ receptors (FCγR) is essential for antibody efficacy. In this regard, our data indicate that the membrane could sterically hinder the formation of HA-CR8020-FcγRIIa/HA-IgG-FcγRIIIa ternary complexes. Altogether, our analyses suggest that epitope mutability and accessibility to immune complex assembly are important attributes to consider when evaluating bNAb candidates for clinical development.

Advances in universal influenza virus vaccine design and antibody mediated therapies based on conserved regions of the hemagglutinin

The threat of novel influenza viruses emerging into the human population from animal reservoirs, as well as the short duration of protection conferred by licensed vaccines against human seasonal strains has spurred research efforts to improve upon current vaccines and develop novel therapeutics against influenza viruses. In recent years these efforts have resulted in the identification of novel, highly conserved epitopes for neutralizing antibodies on the influenza virus hemagglutinin protein, which are present in both the stalk and globular head domains of the molecule. The existence of such epitopes may allow for generation of novel therapeutic antibodies, in addition to serving as attractive targets of novel vaccine design. The aims of developing improved vaccines include eliciting broader protection from drifted strains, inducing long-lived immunity against seasonal strains, and allowing for the rational design of vaccines that can be stockpiled for use as pre-pandemic vaccines. In addition, an increased focus on influenza virus vaccine research has prompted an improved understanding of how the immune system responds to influenza virus infection.

New vaccines against influenza virus

Vaccination is one of the most effective and cost-benefit interventions that prevent the mortality and reduce morbidity from infectious pathogens. However, the licensed influenza vaccine induces strain-specific immunity and must be updated annually based on predicted strains that will circulate in the upcoming season. Influenza virus still causes significant health problems worldwide due to the low vaccine efficacy from unexpected outbreaks of next epidemic strains or the emergence of pandemic viruses. Current influenza vaccines are based on immunity to the hemagglutinin antigen that is highly variable among different influenza viruses circulating in humans and animals. Several scientific advances have been endeavored to develop universal vaccines that will induce broad protection. Universal vaccines have been focused on regions of viral proteins that are highly conserved across different virus subtypes. The strategies of universal vaccines include the matrix 2 protein, the hemagglutinin HA2 stalk domain, and T cell-based multivalent antigens. Supplemented and/or adjuvanted vaccination in combination with universal target antigenic vaccines would have much promise. This review summarizes encouraging scientific advances in the field with a focus on novel vaccine designs.

Bacterially produced recombinant influenza vaccines based on virus-like particles

Although current influenza vaccines are effective in general, there is an urgent need for the development of new technologies to improve vaccine production timelines, capacities and immunogenicity. Herein, we describe the development of an influenza vaccine technology which enables recombinant production of highly efficient influenza vaccines in bacterial expression systems. The globular head domain of influenza hemagglutinin, comprising most of the protein's neutralizing epitopes, was expressed in E. coli and covalently conjugated to bacteriophage-derived virus-like particles produced independently in E.coli. Conjugate influenza vaccines produced this way were used to immunize mice and found to elicit immune sera with high antibody titers specific for the native influenza hemagglutinin protein and high hemagglutination-inhibition titers. Moreover vaccination with these vaccines induced full protection against lethal challenges with homologous and highly drifted influenza strains.