Broadened immunity and protective responses with emulsion-adjuvanted H5 COBRA-VLP vaccines

Revisiting the dimensions of universal vaccine with special focus on COVID-19: Efficacy versus methods of designing

Even though the use of SARS-CoV-2 vaccines during the COVID-19 pandemic showed unprecedented success in a short time, it also exposed a flaw in the current vaccine design strategy to offer broad protection against emerging variants of concern. However, developing broad-spectrum vaccines is still a challenge for immunologists. The development of universal vaccines against emerging pathogens and their variants appears to be a practical solution to mitigate the economic and physical effects of the pandemic on society. Very few reports are available to explain the basic concept of universal vaccine design and development. This review provides an overview of the innate and adaptive immune responses generated against vaccination and essential insight into immune mechanisms helpful in designing universal vaccines targeting influenza viruses and coronaviruses. In addition, the characteristics, safety, and factors affecting the efficacy of universal vaccines have been discussed. Furthermore, several advancements in methods worthy of designing universal vaccines are described, including chimeric immunogens, heterologous prime-boost vaccines, reverse vaccinology, structure-based antigen design, pan-reactive antibody vaccines, conserved neutralizing epitope-based vaccines, mosaic nanoparticle-based vaccines, etc. In addition to the several advantages, significant potential constraints, such as defocusing the immune response and subdominance, are also discussed.

Structural basis for the broad antigenicity of the computationally optimized influenza hemagglutinin X6

Influenza causes significant morbidity and mortality. As an alternative approach to current seasonal vaccines, the computationally optimized broadly reactive antigen (COBRA) platform has been previously applied to hemagglutinin (HA). This approach integrates wild-type HA sequences into a single immunogen to expand the breadth of accessible antibody epitopes. Adding to previous studies of H1, H3, and H5 COBRA HAs, we define the structural features of another H1 subtype COBRA, X6, that incorporates HA sequences from before and after the 2009 H1N1 influenza pandemic. We determined structures of this antigen alone and in complex with COBRA-specific as well as broadly reactive and functional antibodies, analyzing its antigenicity. We found that X6 possesses features representing both historic and recent H1 HA strains, enabling binding to both head- and stem-reactive antibodies. Overall, these data confirm the integrity of broadly reactive antibody epitopes of X6 and contribute to design efforts for a next-generation vaccine.

Targeting neuraminidase: the next frontier for broadly protective influenza vaccines

Current seasonal influenza vaccines, which mainly target hemagglutinin (HA), require annual updates due to the continuous antigenic drift of the influenza virus. Developing an influenza vaccine with increased breadth of protection will have significant public health benefits. The recent discovery of broadly protective antibodies to neuraminidase (NA) has provided important insights into developing a universal influenza vaccine, either by improving seasonal influenza vaccines or designing novel immunogens. However, further in-depth molecular characterizations of NA antibody responses are warranted to fully leverage broadly protective NA antibodies for influenza vaccine designs. Overall, we posit that focusing on NA for influenza vaccine development is synergistic with existing efforts targeting HA, and may represent a cost-effective approach to generating a broadly protective influenza vaccine.

A computationally optimized broadly reactive hemagglutinin vaccine elicits neutralizing antibodies against influenza B viruses from both lineages

Influenza B viruses (IBV) can cause severe disease and death much like influenza A viruses (IAV), with a disproportionate number of infections in children. Despite moving to a quadrivalent vaccine to include strains from both the B/Victoria and B/Yamagata lineages, vaccine effectiveness rates continue to be variable and low in many past seasons. To develop more effective influenza B virus vaccines, three novel IBV hemagglutinin (HA) vaccines were designed using a computationally optimized broadly reactive antigen (COBRA) methodology. These IBV HA proteins were expressed on the surface of a virus-like particle (VLP) and used to vaccinate ferrets that were pre-immune to historical B/Victoria or B/Yamagata lineage viruses. Ferrets vaccinated with B-COBRA HA vaccines had neutralizing antibodies with high titer HAI titer against all influenza B viruses regardless of pre-immunization history. Conversely, VLPs expressing wild-type IBV HA antigens preferentially boosted titers against viruses from the same lineage and there was little-to-no seroprotective antibodies detected in ferrets with mismatched IBV pre-immune infections. Overall, a single IBV HA developed using the COBRA methodology elicited protective broadly-reactive antibodies against current and future drifted IBVs from both lineages.

Next-Generation Computationally Designed Influenza Hemagglutinin Vaccines Protect against H5Nx Virus Infections

H5N1 COBRA hemagglutinin (HA) sequences, termed human COBRA-2 HA, were constructed through layering of HA sequences from viruses isolated from humans collected between 2004-2007 using only clade 2 strains. These COBRA HA proteins, when expressed on the surface of virus-like particles (VLP), elicited protective immune responses in mice, ferrets, and non-human primates. However, these vaccines were not as effective at inducing neutralizing antibodies against newly circulating viruses. Therefore, COBRA HA-based vaccines were updated in order to elicit protective antibodies against the current circulating clades of H5Nx viruses. Next-generation COBRA HA vaccines were designed to encompass the newly emerging viruses circulating in wild avian populations. HA amino acid sequences from avian and human H5 influenza viruses isolated between 2011-2017 were downloaded from the GISAID (Global Initiative on Sharing All Influenza Data). Mice were vaccinated with H5 COBRA rHA that elicited antibodies with hemagglutinin inhibition (HAI) activity against H5Nx viruses from five clades. The H5 COBRA rHA vaccine, termed IAN8, elicited protective immune responses against mice challenged with A/Sichuan/26621/2014 and A/Vietnam/1203/2004. This vaccine elicited antibodies with HAI activity against viruses from clades 2.2, 2.3.2.1, 2.3.4.2, 2.2.1 and 2.2.2. Lungs from vaccinated mice had decreased viral titers and the levels of cellular infiltration in mice vaccinated with IAN-8 rHA were similar to mice vaccinated with wild-type HA comparator vaccines or mock vaccinated controls. Overall, these next-generation H5 COBRA HA vaccines elicited protective antibodies against both historical H5Nx influenza viruses, as well as currently circulating clades of H5N1, H5N6, and H5N8 influenza viruses.

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.

Human COBRA 2 vaccine contains two major epitopes that are responsible for eliciting neutralizing antibody responses against heterologous clades of viruses

Highly pathogenic H5N1 influenza viruses continue to spread around the globe and reassort with low pathogenic avian influenza viruses often resulting in morbidity and mortality to not only waterfowl, but also poultry. Our group previously developed two hemagglutinin (HA) based vaccines using a methodology termed computationally optimized broadly reactive antigen (COBRA). Each of these HA antigens, Human COBRA 2 (Hu-CO) and Human-Avian COBRA 2 (Hu-Av CO) elicit antibodies with hemagglutination-inhibition (HAI) activity against viruses from various clades, but not always the same viruses. Here, we have sought to identify residues in these two HA molecules that are critical fordifferential HAI activity against various H5Nx influenza viruses. The two HA antigens are remarkedly similar in the globular head region, except for 4 residues at amino acids 140, 141, 155, and 156. By mutating these amino acids in each HA antigen, chimeric HA proteins were used to elicit immune responses in mice. When the Asn-Thr pair at position 155-156 in the Hu-CO HA was converted to the Ser-Ala residues found in the Hu-Av CO HA, the elicited antibodies lost HAI activity against clade 2.3.2.1 H5Nx viruses, such as A/Hubei/01/2010. When this Asn-Thr motif was added at these positions in the Hu-Av CO HA molecule, HAI activity in the elicited sera against A/Hubei/01/2010 was significantly increased. We speculate that a putative N-linked glycosylation at this location in the Hu-CO HA antigen is a key driver in the elicitation of antibodies with HAI activity to different locations on wild-type H5 HA molecules resulting in differential neutralization of viral infection and protection in vivo against H5 influenza virus induced disease.

Recombinant Influenza Vaccines: Saviors to Overcome Immunodominance

It has been almost a decade since the 2009 influenza A virus pandemic hit the globe causing significant morbidity and mortality. Nonetheless, annual influenza vaccination, which elicits antibodies mainly against the head region of influenza hemagglutinin (HA), remains as the mainstay to combat and reduce symptoms of influenza infection. Influenza HA is highly antigenically variable, thus limiting vaccine efficacy. In addition, the variable HA head occupies the upper strata of the immunodominance hierarchy, thereby clouding the antibody response toward subdominant epitopes, which are usually conserved across different influenza strains. Isolation of monoclonal antibodies from individuals recognizing such epitopes has facilitated the development of recombinant vaccines that focus the adaptive immune response toward conserved, protective targets. Here, we review some significant leaps in recombinant vaccine development, which could possibly help to overcome B cell and antibody immunodominance and provide heterosubtypic immunity to influenza A virus.

Structural and antigenic characterization of a computationally-optimized H5 hemagglutinin influenza vaccine

Influenza A virus is a leading cause of death worldwide. Viruses of the H5 subtype have the potential to induce high mortality, and no vaccines are currently available to protect against H5 influenza viruses in the event of an outbreak. Experimental vaccination with one clade 2 virus does not protect against other subclades. The computationally optimized broadly reactive (COBRA) methodology was previously used to generate a H5 hemagglutinin (HA) antigen (COBRA2) that elicited increased serological breadth against multiple clade 2 H5N1 influenza viruses. In this report, we structurally and antigenically characterized the COBRA2 HA antigen. We examined the biochemical characteristics of the COBRA2 protein and determined the protein is correctly cleaved, properly folded into a trimeric structure, and antigenically correct by probing with HA head- and stem-specific monoclonal antibodies (mAbs). We further probed the antigenicity by examining binding of a panel of H5 mouse mAbs to the COBRA2 antigen, as well as several other HA antigens. We determined the X-ray crystal structure of the COBRA2 HA antigen to 2.8 Å and the protein was observed to be in the expected trimeric form. The COBRA2 HA was structurally similar to the naturally occurring H5 HA antigens and suggests the protein folds similar to known HA structures. Overall, our data allow us to formulate a hypothesis on the mechanism of increased breadth due to vaccination with the COBRA2 HA antigen, which is that the protein incorporates antigenic sites from numerous HA antigens, and elicits mAbs with limited breadth, but with diversity in targeted antigenic sites.

Characterising antibody kinetics from multiple influenza infection and vaccination events in ferrets

The strength and breadth of an individual's antibody repertoire is an important predictor of their response to influenza infection or vaccination. Although progress has been made in understanding qualitatively how repeated exposures shape the antibody mediated immune response, quantitative understanding remains limited. We developed a set of mathematical models describing short-term antibody kinetics following influenza infection or vaccination and fit them to haemagglutination inhibition (HI) titres from 5 groups of ferrets which were exposed to different combinations of trivalent inactivated influenza vaccine (TIV with or without adjuvant), A/H3N2 priming inoculation and post-vaccination A/H1N1 inoculation. We fit models with various immunological mechanisms that have been empirically observed but have not previously been included in mathematical models of antibody landscapes, including: titre ceiling effects, antigenic seniority and exposure-type specific cross reactivity. Based on the parameter estimates of the best supported models, we describe a number of key immunological features. We found quantifiable differences in the degree of homologous and cross-reactive antibody boosting elicited by different exposure types. Infection and adjuvanted vaccination generally resulted in strong, broadly reactive responses whereas unadjuvanted vaccination resulted in a weak, narrow response. We found that the order of exposure mattered: priming with A/H3N2 improved subsequent vaccine response, and the second dose of adjuvanted vaccination resulted in substantially greater antibody boosting than the first. Either antigenic seniority or a titre ceiling effect were included in the two best fitting models, suggesting a role for a mechanism describing diminishing antibody boosting with repeated exposures. Although there was considerable uncertainty in our estimates of antibody waning parameters, our results suggest that both short and long term waning were present and would be identifiable with a larger set of experiments. These results highlight the potential use of repeat exposure animal models in revealing short-term, strain-specific immune dynamics of influenza.

Targeting Hemagglutinin: Approaches for Broad Protection against the Influenza A Virus

Influenza A viruses are dynamically epidemic and genetically diverse. Due to the antigenic drift and shift of the virus, seasonal vaccines are required to be reformulated annually to match with current circulating strains. However, the mismatch between vaccinal strains and circulating strains occurs frequently, resulting in the low efficacy of seasonal vaccines. Therefore, several “universal” vaccine candidates based on the structure and function of the hemagglutinin (HA) protein have been developed to meet the requirement of a broad protection against homo-/heterosubtypic challenges. Here, we review recent novel constructs and discuss several important findings regarding the broad protective efficacy of HA-based universal vaccines.

Formulation of aluminum hydroxide adjuvant with TLR agonists poly(I:C) and CpG enhances the magnitude and avidity of the humoral immune response

Subunit vaccines generally require adjuvants to achieve optimal immune responses. Toll-like receptor (TLR) agonists are promising immune potentiators, but rapid diffusion from the injection site reduces their local effective concentration and may cause systemic reactions. In this study, we investigated the potential of aluminum hydroxide adjuvant (AH) to adsorb the TLR3 agonist poly(I:C) and TLR9 agonist CpG and compared the effect of the combination adjuvant on the immune response with either the TLR agonists or AH alone in mice. Poly(I:C) and CpG readily adsorbed onto AH and this combination adjuvant induced a stronger IgG1 and IgG2a immune response with a significant increase of antibody avidity. The combination adjuvant enhanced antigen uptake and activation of dendritic cells in vitro. It induced an inflammatory response at the injection site similar to AH but without eosinophils which are typically observed with AH. A distinctive antigen-containing monocyte/macrophage population with an intermediate level of CD11c expression was identified in the draining lymph nodes after immunization with TLR agonists and the combination adjuvant. Injection of the combination adjuvant did not induce an increase of TNFα and CXCL10 in serum in contrast to the injection of soluble TLR agonists. These results indicate that this combination adjuvant is a promising formulation to solve some of the unmet needs of current vaccines.

A review of H5Nx avian influenza viruses

Highly pathogenic avian influenza viruses (HPAIVs), originating from the A/goose/Guangdong/1/1996 H5 subtype, naturally circulate in wild-bird populations, particularly waterfowl, and often spill over to infect domestic poultry. Occasionally, humans are infected with HPAVI H5N1 resulting in high mortality, but no sustained human-to-human transmission. In this review, the replication cycle, pathogenicity, evolution, spread, and transmission of HPAIVs of H5Nx subtypes, along with the host immune responses to Highly Pathogenic Avian Influenza Virus (HPAIV) infection and potential vaccination, are discussed. In addition, the potential mechanisms for Highly Pathogenic Avian Influenza Virus (HPAIV) H5 Reassorted Viruses H5N1, H5N2, H5N6, H5N8 (H5Nx) viruses to transmit, infect, and adapt to the human host are reviewed.

Universal influenza vaccines: from viruses to nanoparticles

INTRODUCTION

The current seasonal influenza vaccine confers only limited protection due to waning antibodies or the antigenic shift and drift of major influenza surface antigens. A universal influenza vaccine which induces broad cross-protection against divergent influenza viruses with a comparable or better efficacy to seasonal influenza vaccines against matched strains will negate the need for an annual update of vaccine strains and protect against possible influenza pandemics.

AREAS COVERED

In this review, we summarize the recent progress in nanoparticle-based universal influenza vaccine development. We compared the most potent nanoparticle categories, focusing on how they encapsulate conserved influenza epitopes, stimulate the innate and adaptive immune systems, exhibit antigen depot effect, extend the period for antigen-processing and presentation, and exert an intrinsic adjuvant effect on inducing robust immune responses.

EXPERT COMMENTARY

The development of an effective universal influenza vaccine is an urgent task. Traditional influenza vaccine approaches are not sufficient for preventing recurrent epidemics or occasional pandemics. Nanoparticles are compatible with different immunogens and immune stimulators and can overcome the intrinsically low immunogenicity of conserved influenza virus antigens. We foresee that an affordable universal influenza vaccine will be available within ten years by integrating nanoparticles with other targeted delivery and controlled release technology.

Split inactivated COBRA vaccine elicits protective antibodies against H1N1 and H3N2 influenza viruses

Development of broadly reactive or universal influenza vaccines will be a paradigm shifting event for the influenza vaccine field. These next generation vaccines could replace the current standard of care with vaccines that elicit broadly cross-protective immune responses. However, a variety of in vitro and in vivo models are necessary to make the best assessments of these vaccine formulations to determine their mechanisms of action, and allow for downselection of candidates prior to human clinical trials. Our group has developed the computationally optimized broadly reactive antigen (COBRA) technology to develop HA head-based strategies to elicit antibodies against H1, H3, and H5 influenza strains. These vaccines elicit broadly reactive antibody responses that neutralize not only historical and contemporary vaccine strains, but also co-circulating variants in mice. In this study, we used H1 and H3 HA antigens in a split, inactivated vaccine (IIV) formulation in combination with the AF03 squalene-in-water emulsion adjuvant in ferrets immunologically naïve to influenza virus. The H3 COBRA IIV vaccine T11 elicited antibodies with HAI activity against more H3N2 influenza strains compared to IIV expressing wild-type H3 HA antigens, except for IIV vaccines expressing the HA from A/Texas/50/2012 (Tx/12) virus. H1 COBRA IIV vaccines, P1 and X6, elicited antibodies that recognized a similar number of H1N1 viruses as those antibodies elicited by IIV expressing the A/California/07/2009 (CA/09) HA. Ferrets vaccinated with the P1 or X6 COBRA IIV were protected against CA/09 challege and cleared virus from the lungs of the ferrets, similar to ferrets vaccinated with the CA/09 IIV.