Preferential induction of cross-group influenza A hemagglutinin stem-specific memory B cells after H7N9 immunization in humans

Eliciting a single amino acid change by vaccination generates antibody protection against group 1 and group 2 influenza A viruses

Broadly neutralizing antibodies (bnAbs) targeting the hemagglutinin (HA) stem of influenza A viruses (IAVs) tend to be effective against either group 1 or group 2 viral diversity. In rarer cases, intergroup protective bnAbs can be generated by human antibody paratopes that accommodate the conserved glycan differences between the group 1 and group 2 stems. We applied germline-engaging nanoparticle immunogens to elicit a class of cross-group bnAbs from physiological precursor frequency within a humanized mouse model. Cross-group protection depended on the presence of the human bnAb precursors within the B cell repertoire, and the vaccine-expanded antibodies enriched for an N55T substitution in the CDRH2 loop, a hallmark of the bnAb class. Structurally, this single mutation introduced a flexible fulcrum to accommodate glycosylation differences and could alone enable cross-group protection. Thus, broad IAV immunity can be expanded from the germline repertoire via minimal antigenic input and an exceptionally simple antibody development pathway.

Bringing immunofocusing into focus

Immunofocusing is a strategy to create immunogens that redirect humoral immune responses towards a targeted epitope and away from non-desirable epitopes. Immunofocusing methods often aim to develop "universal" vaccines that provide broad protection against highly variant viruses such as influenza virus, human immunodeficiency virus (HIV-1), and most recently, severe acute respiratory syndrome coronavirus (SARS-CoV-2). We use existing examples to illustrate five main immunofocusing strategies-cross-strain boosting, mosaic display, protein dissection, epitope scaffolding, and epitope masking. We also discuss obstacles for immunofocusing like immune imprinting. A thorough understanding, advancement, and application of the methods we outline here will enable the design of high-resolution vaccines that protect against future viral outbreaks.

An explainable language model for antibody specificity prediction using curated influenza hemagglutinin antibodies

Despite decades of antibody research, it remains challenging to predict the specificity of an antibody solely based on its sequence. Two major obstacles are the lack of appropriate models and inaccessibility of datasets for model training. In this study, we curated a dataset of >5,000 influenza hemagglutinin (HA) antibodies by mining research publications and patents, which revealed many distinct sequence features between antibodies to HA head and stem domains. We then leveraged this dataset to develop a lightweight memory B cell language model (mBLM) for sequence-based antibody specificity prediction. Model explainability analysis showed that mBLM captured key sequence motifs of HA stem antibodies. Additionally, by applying mBLM to HA antibodies with unknown epitopes, we discovered and experimentally validated many HA stem antibodies. Overall, this study not only advances our molecular understanding of antibody response to influenza virus, but also provides an invaluable resource for applying deep learning to antibody research.

Quantitation of strain-specific hemagglutinin trimers in mosaic quadrivalent influenza nanoparticle vaccine by ELISA

An enzyme linked immunosorbent assay (ELISA) method was developed to analyze the assembly of a tetravalent mosaic influenza nanoparticle (NP) vaccine, Flumos-v1, consisting of hemagglutinin trimers (HAT) from H1 (A/Idaho/07/2018), H3 (A/Perth/1008/2019), HBV (Vic-B/Colorado/06/2017) and HBY (Yam-B/Phuket/3073/2013) strains. The sandwich ELISA assay used lectin from Galanthus nivalis as a universal capture reagent for all HAT strains and specific monoclonal antibody (mAb) to detect corresponding hemagglutinin antigen. The mAb binding of HATs incorporated into NPs diverged from those for single HAT solutions, resulting in inaccurate quantitation of assembled HATs. An optimized zwittergent treatment was used to fully dissociate the influenza NP and aligned binding activities in each pair of single HAT and dissociated HAT from NP. The dissociated HATs were then quantified against their corresponding HAT standard solutions for three development lots of FluMos-v1 vaccine and the assembly ratio of all four HATs was calculated. The molar ratio of different HATs incorporated into this quadrivalent NP vaccine was consistent and determined as H3:H1: HBV: HBY ∼ 1.00:0.92:0.96:0.87, which was close the expected 1:1:1:1 ratio and confirmed a proper assembling of multivalent NP.

Crystal structures of the human IgD Fab reveal insights into CH1 domain diversity

Antibodies of the IgD isotype remain the least well characterized of the mammalian immunoglobulin isotypes. Here we report three-dimensional structures for the Fab region of IgD, based on four different crystal structures, at resolutions of 1.45-2.75 Å. These IgD Fab crystals provide the first high-resolution views of the unique Cδ1 domain. Structural comparisons identify regions of conformational diversity within the Cδ1 domain, as well as among the homologous domains of Cα1, Cγ1 and Cμ1. The IgD Fab structure also possesses a unique conformation of the upper hinge region, which may contribute to the overall disposition of the very long linker sequence between the Fab and Fc regions found in human IgD. Structural similarities observed between IgD and IgG, and differences with IgA and IgM, are consistent with predicted evolutionary relationships for the mammalian antibody isotypes.

An influenza H1 hemagglutinin stem-only immunogen elicits a broadly cross-reactive B cell response in humans

Current yearly seasonal influenza vaccines primarily induce an antibody response directed against the immunodominant but continually diversifying hemagglutinin (HA) head region. These antibody responses provide protection against the vaccinating strain but little cross-protection against other influenza strains or subtypes. To focus the immune response on subdominant but more conserved epitopes on the HA stem that might protect against a broad range of influenza strains, we developed a stabilized H1 stem immunogen lacking the immunodominant head displayed on a ferritin nanoparticle (H1ssF). Here, we evaluated the B cell response to H1ssF in healthy adults ages 18 to 70 in a phase 1 clinical trial (NCT03814720). We observed both a strong plasmablast response and sustained elicitation of cross-reactive HA stem-specific memory B cells after vaccination with H1ssF in individuals of all ages. The B cell response was focused on two conserved epitopes on the H1 stem, with a highly restricted immunoglobulin repertoire unique to each epitope. On average, two-thirds of the B cell and serological antibody response recognized a central epitope on the H1 stem and exhibited broad neutralization across group 1 influenza virus subtypes. The remaining third recognized an epitope near the viral membrane anchor and was largely limited to H1 strains. Together, we demonstrate that an H1 HA immunogen lacking the immunodominant HA head produces a robust and broadly neutralizing HA stem-directed B cell response.

An influenza hemagglutinin stem nanoparticle vaccine induces cross-group 1 neutralizing antibodies in healthy adults

Influenza vaccines could be improved by platforms inducing cross-reactive immunity. Immunodominance of the influenza hemagglutinin (HA) head in currently licensed vaccines impedes induction of cross-reactive neutralizing stem-directed antibodies. A vaccine without the variable HA head domain has the potential to focus the immune response on the conserved HA stem. This first-in-human dose-escalation open-label phase 1 clinical trial (NCT03814720) tested an HA stabilized stem ferritin nanoparticle vaccine (H1ssF) based on the H1 HA stem of A/New Caledonia/20/1999. Fifty-two healthy adults aged 18 to 70 years old enrolled to receive either 20 μg of H1ssF once (n = 5) or 60 μg of H1ssF twice (n = 47) with a prime-boost interval of 16 weeks. Thirty-five (74%) 60-μg dose participants received the boost, whereas 11 (23%) boost vaccinations were missed because of public health restrictions in the early stages of the COVID-19 pandemic. The primary objective of this trial was to evaluate the safety and tolerability of H1ssF, and the secondary objective was to evaluate antibody responses after vaccination. H1ssF was safe and well tolerated, with mild solicited local and systemic reactogenicity. The most common symptoms included pain or tenderness at the injection site (n = 10, 19%), headache (n = 10, 19%), and malaise (n = 6, 12%). We found that H1ssF elicited cross-reactive neutralizing antibodies against the conserved HA stem of group 1 influenza viruses, despite previous H1 subtype head-specific immunity. These responses were durable, with neutralizing antibodies observed more than 1 year after vaccination. Our results support this platform as a step forward in the development of a universal influenza vaccine.

Co-immunization with hemagglutinin stem immunogens elicits cross-group neutralizing antibodies and broad protection against influenza A viruses

Current influenza vaccines predominantly induce immunity to the hypervariable hemagglutinin (HA) head, requiring frequent vaccine reformulation. Conversely, the immunosubdominant yet conserved HA stem harbors a supersite that is targeted by broadly neutralizing antibodies (bnAbs), representing a prime target for universal vaccines. Here, we showed that the co-immunization of two HA stem immunogens derived from group 1 and 2 influenza A viruses elicits cross-group protective immunity and neutralizing antibody responses in mice, ferrets, and nonhuman primates (NHPs). Immunized mice were protected from multiple group 1 and 2 viruses, and all animal models showed broad serum-neutralizing activity. A bnAb isolated from an immunized NHP broadly neutralized and protected against diverse viruses, including H5N1 and H7N9. Genetic and structural analyses revealed strong homology between macaque and human bnAbs, illustrating common biophysical constraints for acquiring cross-group specificity. Vaccine elicitation of stem-directed cross-group-protective immunity represents a step toward the development of broadly protective influenza vaccines.

Application of B cell immortalization for the isolation of antibodies and B cell clones from vaccine and infection settings

The isolation and characterization of neutralizing antibodies from infection and vaccine settings informs future vaccine design, and methodologies that streamline the isolation of antibodies and the generation of B cell clones are of great interest. Retroviral transduction to express Bcl-6 and Bcl-xL and transform primary B cells has been shown to promote long-term B cell survival and antibody secretion in vitro, and can be used to isolate antibodies from memory B cells. However, application of this methodology to B cell subsets from different tissues and B cells from chronically infected individuals has not been well characterized. Here, we characterize Bcl-6/Bcl-xL B cell immortalization across multiple tissue types and B cell subsets in healthy and HIV-1 infected individuals, as well as individuals recovering from malaria. In healthy individuals, naïve and memory B cell subsets from PBMCs and tonsil tissue transformed with similar efficiencies, and displayed similar characteristics with respect to their longevity and immunoglobulin secretion. In HIV-1-viremic individuals or in individuals with recent malaria infections, the exhausted CD27^-CD21^- memory B cells transformed with lower efficiency, but the transformed B cells expanded and secreted IgG with similar efficiency. Importantly, we show that this methodology can be used to isolate broadly neutralizing antibodies from HIV-infected individuals. Overall, we demonstrate that Bcl-6/Bcl-xL B cell immortalization can be used to isolate antibodies and generate B cell clones from different B cell populations, albeit with varying efficiencies.

Flagellin-Fused Protein Targeting M2e and HA2 Induces Innate and T-Cell Responses in Mice of Different Genetic Lines

Efficient control of influenza A infection can potentially be achieved through the development of broad-spectrum vaccines. Recombinant proteins incorporating conserved influenza A virus peptides are one of the platforms for the development of cross-protective influenza vaccines. We constructed a recombinant protein Flg-HA2-2-4M2ehs, in which the extracellular domain of the M2 protein (M2e) and the sequence (aa76-130) of the second subunit of HA (HA2) were used as target antigens. In this study, we investigated the ability of the Flg-HA2-2-4M2ehs protein to activate innate immunity and stimulate the formation of T-cell response in mice of different genetic lines after intranasal immunization. Our studies showed that the Flg-HA2-2-4M2ehs protein was manifested in an increase in the relative content of neutrophils, monocytes, and interstitial macrophages, against the backdrop of a decrease in the level of dendritic cells and increased expression in the CD86 marker. In the lungs of BALB/c mice, immunization with the Flg-HA2-2-4M2ehs protein induced the formation of antigen-specific CD4+ and CD8+ effector memory T cells, producing TNF-α. In mice C57Bl/6, the formation of antigen-specific effector CD8+ T cells, predominantly producing IFN-γ+, was demonstrated. The data obtained showed the formation of CD8+ and CD4+ effector memory T cells expressing the CD107a.

Primary antibody response after influenza virus infection is first dominated by low-mutated HA-stem antibodies followed by higher-mutated HA-head antibodies

Several studies have shown that the first encounter with influenza virus shapes the immune response to future infections or vaccinations. However, a detailed analysis of the primary antibody response is lacking as this is difficult to study in humans. It is therefore not known what the frequency and dynamics of the strain-specific hemagglutinin (HA) head- and stem-directed antibody responses are directly after primary influenza virus infection. Here, sera of twelve H1N1pdm2009 influenza virus-infected cynomolgus macaques were evaluated for HA-head and HA-stem domain antibody responses. We observed an early induction of HA-stem antibody responses, which was already decreased by day 56. In contrast, responses against the HA-head domain were low early after infection and increased at later timepoint. The HA-specific B cell repertoires in each animal showed diverse VH-gene usage with preferred VH-gene and JH-gene family usage for HA-head or HA-stem B cells but a highly diverse allelic variation within the VH-usage. HA-head B cells had shorter CDRH3s and higher VH-gene somatic hyper mutation levels relative to HA-stem B cells. In conclusion, our data suggest that HA-stem antibodies are the first to react to the infection while HA-head antibodies show a delayed response, but a greater propensity to enter the germinal center and undergo affinity maturation.

Specific Monoclonal Antibodies Targeting Unique HA Epitopes Block H7N9 Influenza A Viral Replication

The H7N9 subtype influenza A viruses pose a serious threat to public health, and there is still a lack of vaccines or drugs for humans against H7N9 influenza viruses. In this study, we screened two monoclonal antibodies (MAbs), 4H1E8 and 7H9A6, that specifically recognize the hemagglutinin (HA) protein of H7N9 influenza virus and display highly neutralizing activity against H7N9 virus. The epitopes recognized by two MAbs are nearly all conserved within all known H7 subtypes. Characteristic identification showed that two MAbs have high avidity for the HA protein but no hemagglutinin inhibition activity or antibody-dependent cellular cytotoxicity. Mechanistically, the 4H1E8 and 7H9A6 antibodies inhibit the pH-dependent conformational change of HA and block the HA-mediated membrane fusion. More importantly, 4H1E8 and 7H9A6 exhibit promising prophylactic and therapeutic effects against lethal challenge with H7N9 virus. Moreover, 4H1E8- and 7H9A6-treated mice displayed inhibition of pulmonary viral replication and reduced lung lesions after viral challenge. Together, these findings indicate that antibodies 4H1E8 and 7H9A6 recognize unique epitopes in the HA protein and possess the neutralizing activity and protective efficacy against the H7N9 influenza A viruses. IMPORTANCE In 2013, H7N9 influenza viruses appeared in China and other countries resulting in more than 1,500 individual infections or death. There are still limited studies on vaccines or drugs for humans against H7N9 influenza viruses. Alternative approaches against H7N9 virus infection need to be developed. Here, we identified two monoclonal antibodies (4H1E8 and 7H9A6) that possess neutralizing activity by blocking the pH-dependent HA-mediated membrane fusion. Additionally, the two monoclonal antibodies protect mice against the H7N9 virus challenge prophylactically or therapeutically. Therefore, our study demonstrates that 4H1E8 and 7H9A6 could be used for the prevention and treatment of the H7N9 influenza virus, and the conserved epitopes we identified may contribute to the development of a broad H7N9 vaccine and provide insights into unique antiviral approaches.

Recall of preexisting cross-reactive B cell memory after Omicron BA.1 breakthrough infection

Understanding immune responses after severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) breakthrough infection will facilitate the development of next-generation vaccines. Here, we profiled spike (S)-specific B cell responses after Omicron/BA.1 infection in messenger RNA-vaccinated donors. The acute antibody response was characterized by high levels of somatic hypermutation and a bias toward recognition of ancestral SARS-CoV-2 strains, suggesting the early activation of vaccine-induced memory B cells. BA.1 breakthrough infection induced a shift in B cell immunodominance hierarchy from the S2 subunit, which is highly conserved across SARS-CoV-2 variants of concern (VOCs), and toward the antigenically variable receptor binding domain (RBD). A large proportion of RBD-directed neutralizing antibodies isolated from BA.1 breakthrough infection donors displayed convergent sequence features and broadly recognized SARS-CoV-2 VOCs. Together, these findings provide insights into the role of preexisting immunity in shaping the B cell response to heterologous SARS-CoV-2 variant exposure.

Influenza Vaccination Results in Differential Hemagglutinin Stalk-Specific Fc-Mediated Functions in Individuals Living With or Without HIV

Influenza virus hemagglutinin (HA) stalk-specific antibodies have been shown to potently induce Fc-mediated effector functions which are important in protection from disease. In placebo-controlled maternal influenza (MatFlu) vaccination trials of pregnant women living with or without HIV, reduced risk of influenza illness was associated with high HA stalk antibody titers following trivalent inactivated vaccination (TIV). However, the mechanisms of immunity conferred by the HA stalk antibodies were not well understood. Here, we investigated HA stalk-specific Fc effector functions including antibody-dependent cellular phagocytosis (ADCP), antibody-dependent cellular cytotoxicity (ADCC), antibody-dependent complement deposition (ADCD), and FcγRIIa and FcγRIIIa binding in response to seasonal influenza vaccination. These were measured pre- and 1-month post-vaccination in 141 HIV-uninfected women (67 TIV and 74 placebo recipients) and 119 women living with HIV (WLWH; 66 TIV and 53 placebo recipients). In contrast to HIV-uninfected women, where HA stalk-specific ADCP and FcγRIIa binding were significantly boosted, WLWH showed no increase in response to vaccination. HA stalk-specific ADCC potential and FcγRIIIa binding were not boosted regardless of HIV status but were higher in WLWH compared with HIV-uninfected women prior to vaccination. HA stalk-specific ADCD was significantly increased by vaccination in all women, but was significantly lower in the WLWH both pre- and post- vaccination. Co-ordination between HA stalk-specific ADCP and ADCD in WLWH was improved by vaccination. Fc polyfunctionality was enhanced by vaccination in HIV-uninfected women and driven by the HA stalk antibody titers. However, in the WLWH, higher pre-vaccination Fc polyfunctionality was maintained post-vaccination but was decoupled from titer. Overall, we showed differential regulation of Fc effector HA stalk responses, suggesting that HIV infection results in unique humoral immunity in response to influenza vaccination, with relevance for future strategies that aim to target the HA stalk in this population.

Unique binding pattern for a lineage of human antibodies with broad reactivity against influenza A virus

Most structurally characterized broadly neutralizing antibodies (bnAbs) against influenza A viruses (IAVs) target the conserved conformational epitopes of hemagglutinin (HA). Here, we report a lineage of naturally occurring human antibodies sharing the same germline gene, V_H3-48/V_K1-12. These antibodies broadly neutralize the major circulating strains of IAV in vitro and in vivo mainly by binding a contiguous epitope of H3N2 HA, but a conformational epitope of H1N1 HA, respectively. Our structural and functional studies of antibody 28-12 revealed that the continuous amino acids in helix A, particularly N49_HA2 of H3 HA, are critical to determine the binding feature with 28-12. In contrast, the conformational epitope feature is dependent on the discontinuous segments involving helix A, the fusion peptide, and several HA1 residues within H1N1 HA. We report that this antibody was initially selected by H3 (group 2) viruses and evolved via somatic hypermutation to enhance the reactivity to H3 and acquire cross-neutralization to H1 (group 1) virus. These findings enrich our understanding of different antigenic determinants of heterosubtypic influenza viruses for the recognition of bnAbs and provide a reference for the design of influenza vaccines and more effective antiviral drugs.

While most broadly neutralizing antibodies (bnAb) against Influenza virus target conserved conformational epitopes of the glycoprotein hemagglutinin (HA), Sun et al. characterize a lineage of bnAbs that neutralize group 1 and 2 strains. Structural characterization shows that antibody 28-12 binds a continuous epitope within H3 (group 2) but requires a conformational epitope for H1 (group 1) binding. Comparison of germline-reverted Ab and intermediate mutants provides evidence for an evolutionary adaptation from group 2 to group 1 strain.

Structure of an influenza group 2-neutralizing antibody targeting the hemagglutinin stem supersite

Several influenza antibodies with broad group 2 neutralization have recently been isolated. Here, we analyze the structure, class, and binding of one of these antibodies from an H7N9 vaccine trial, 315-19-1D12. The cryo-EM structure of 315-19-1D12 Fab in complex with the hemagglutinin (HA) trimer revealed the antibody to recognize the helix A region of the HA stem, at the supersite of vulnerability recognized by group 1-specific and by cross-group-neutralizing antibodies. 315-19-1D12 was derived from HV1-2 and KV2-28 genes and appeared to form a new antibody class. Bioinformatic analysis indicated its group 2 neutralization specificity to be a consequence of four key residue positions. We specifically tested the impact of the group 1-specific N33 glycan, which decreased but did not abolish group 2 binding of 315-19-1D12. Overall, this study highlights the recognition of a broad group 2-neutralizing antibody, revealing unexpected diversity in neutralization specificity for antibodies that recognize the HA stem supersite.

Induction of broadly reactive influenza antibodies increases susceptibility to autoimmunity

Infection and vaccination repeatedly expose individuals to antigens that are conserved between influenza virus subtypes. Nevertheless, antibodies recognizing variable influenza epitopes greatly outnumber antibodies reactive against conserved epitopes. Elucidating factors contributing to the paucity of broadly reactive influenza antibodies remains a major obstacle for developing a universal influenza vaccine. Here, we report that inducing broadly reactive influenza antibodies increases autoreactive antibodies in humans and mice and exacerbates disease in four distinct models of autoimmune disease. Importantly, transferring broadly reactive influenza antibodies augments disease in the presence of inflammation or autoimmune susceptibility. Further, broadly reactive influenza antibodies spontaneously arise in mice with defects in B cell tolerance. Together, these data suggest that self-tolerance mechanisms limit the prevalence of broadly reactive influenza antibodies, which can exacerbate disease in the context of additional risk factors.

A single residue in influenza virus H2 hemagglutinin enhances the breadth of the B cell response elicited by H2 vaccination

Conserved epitopes on the influenza hemagglutinin (HA) stem are an attractive target for universal vaccine strategies as they elicit broadly neutralizing antibodies. Such antibody responses to stem-specific epitopes have been extensively characterized for HA subtypes H1 and H5 in humans. H2N2 influenza virus circulated 50 years ago and represents a pandemic threat due to the lack of widespread immunity, but, unlike H1 and H5, the H2 HA stem contains Phe45_HA2 predicted to sterically clash with HA stem-binding antibodies characterized to date. To understand the effect of Phe45_HA2, we compared the HA stem-specific B cell response in post hoc analyses of two phase 1 clinical trials, one testing vaccination with an H2 ferritin nanoparticle immunogen ( NCT03186781 ) and one with an inactivated H5N1 vaccine ( NCT01086657 ). In H2-naive individuals, the magnitude of the B cell response was equivalent, but H2-elicited HA stem-binding B cells displayed greater cross-reactivity than those elicited by H5. However, in individuals with childhood H2 exposure, H5-elicited HA stem-binding B cells also displayed high cross-reactivity, suggesting recall of memory B cells formed 50 years ago. Overall, we propose that a one-residue difference on an HA immunogen can alter establishment and expansion of broadly neutralizing memory B cells. These data have implications for stem-based universal influenza vaccination strategies.

Next-Generation Influenza Vaccines

Most currently used conventional influenza vaccines are based on 1940s technology. Advances in vaccine immunogen design and delivery emerging over the last decade promise new options for improving influenza vaccines. In addition, new technologies for immune profiling provide better-defined immune correlates of protection and precise surrogate biomarkers for vaccine evaluations. Major technological advances include single-cell analysis, high-throughput antibody discovery, next-generation sequencing of antibody gene transcripts, antibody ontogeny, structure-guided immunogen design, nanoparticle display, delivery and formulation options, and better adjuvants. In this review, we provide our prospective outlook for improved influenza vaccines in the foreseeable future.

Effective high-throughput isolation of fully human antibodies targeting infectious pathogens

As exemplified by the ongoing severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic, there is a strong demand for rapid high-throughput isolation pipelines to identify potent neutralizing antibodies for prevention and therapy of infectious diseases. However, despite substantial progress and extensive efforts, the identification and production of antigen-specific antibodies remains labor- and cost-intensive. We have advanced existing concepts to develop a highly efficient high-throughput protocol with proven application for the isolation of potent antigen-specific antibodies against human immunodeficiency virus 1, hepatitis C virus, human cytomegalovirus, Middle East respiratory syndrome coronavirus, SARS-CoV-2 and Ebola virus. It is based on computationally optimized multiplex primer sets (openPrimeR), which guarantee high coverage of even highly mutated immunoglobulin gene segments as well as on optimized antibody cloning and production strategies. Here, we provide the detailed protocol, which covers all critical steps from sample collection to antibody production within 12-14 d.

First exposure to the pandemic H1N1 virus induced broadly neutralizing antibodies targeting hemagglutinin head epitopes

Broadly neutralizing antibodies are critical for protection against both drifted and shifted influenza viruses. Here, we reveal that first exposure to the 2009 pandemic H1N1 influenza virus recalls memory B cells that are specific to the conserved receptor-binding site (RBS) or lateral patch epitopes of the hemagglutinin (HA) head domain. Monoclonal antibodies (mAbs) generated against these epitopes are broadly neutralizing against H1N1 viruses spanning 40 years of viral evolution and provide potent protection in vivo. Lateral patch-targeting antibodies demonstrated near universal binding to H1 viruses, and RBS-binding antibodies commonly cross-reacted with H3N2 viruses and influenza B viruses. Lateral patch-targeting mAbs were restricted to expressing the variable heavy-chain gene VH3-23 with or without the variable kappa-chain gene VK1-33 and often had a Y-x-R motif within the heavy-chain complementarity determining region 3 to make key contacts with HA. Moreover, lateral patch antibodies that used both VH3-23 and VK1-33 maintained neutralizing capability with recent pH1N1 strains that acquired mutations near the lateral patch. RBS-binding mAbs used a diverse repertoire but targeted the RBS epitope similarly and made extensive contacts with the major antigenic site Sb. Together, our data indicate that RBS- and lateral patch-targeting clones are abundant within the human memory B cell pool, and universal vaccine strategies should aim to drive antibodies against both conserved head and stalk epitopes.

Identification and Structure of a Multidonor Class of Head-Directed Influenza-Neutralizing Antibodies Reveal the Mechanism for Its Recurrent Elicitation

Multidonor antibodies are of interest for vaccine design because they can in principle be elicited in the general population by a common set of immunogens. For influenza, multidonor antibodies have been observed against the hemagglutinin (HA) stem, but not the immunodominant HA head. Here, we identify and characterize a multidonor antibody class (LPAF-a class) targeting the HA head. This class exhibits potent viral entry inhibition against H1N1 A/California/04/2009 (CA09) virus. LPAF-a class antibodies derive from the HV2-70 gene and contain a "Tyr-Gly-Asp"-motif, which occludes the HA-sialic acid binding site as revealed by a co-crystal structure with HA. Both germline-reverted and mature LPAF antibodies potently neutralize CA09 virus and have nanomolar affinities for CA09 HA. Moreover, increased frequencies for LPFA-a class antibodies are observed in humans after a single vaccination. Overall, this work highlights the identification of a multidonor class of head-directed influenza-neutralizing antibodies and delineates the mechanism of their recurrent elicitation in humans.

B Cell Responses against Influenza Viruses: Short-Lived Humoral Immunity against a Life-Long Threat

Antibodies are critical for providing protection against influenza virus infections. However, protective humoral immunity against influenza viruses is limited by the antigenic drift and shift of the major surface glycoproteins, hemagglutinin and neuraminidase. Importantly, people are exposed to influenza viruses throughout their life and tend to reuse memory B cells from prior exposure to generate antibodies against new variants. Despite this, people tend to recall memory B cells against constantly evolving variable epitopes or non-protective antigens, as opposed to recalling them against broadly neutralizing epitopes of hemagglutinin. In this review, we discuss the factors that impact the generation and recall of memory B cells against distinct viral antigens, as well as the immunological limitations preventing broadly neutralizing antibody responses. Lastly, we discuss how next-generation vaccine platforms can potentially overcome these obstacles to generate robust and long-lived protection against influenza A viruses.

Shared B cell memory to coronaviruses and other pathogens varies in human age groups and tissues

It remains unclear whether B cell repertoires against coronaviruses and other pathogens differ between adults and children and how important these distinctions are. Yang et al. analyzed blood samples from young children and adults, as well as tissues from deceased organ donors, characterizing the B cell receptor (BCR) repertoires specific to six common pathogens and two viruses that they had not seen before: Ebola virus and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Children had higher frequencies of B cells with convergent BCR heavy chains against previously encountered pathogens and higher frequencies of class-switched convergent B cell clones against SARS-CoV-2 and related coronaviruses. These findings suggest that encounters with coronaviruses in early life may produce cross-reactive memory B cell populations that contribute to divergent COVID-19 susceptibilities.

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Vaccination and infection promote the formation, tissue distribution, and clonal evolution of B cells, which encode humoral immune memory. We evaluated pediatric and adult blood and deceased adult organ donor tissues to determine convergent antigen-specific antibody genes of similar sequences shared between individuals. B cell memory varied for different pathogens. Polysaccharide antigenspecific clones were not exclusive to the spleen. Adults had higher clone frequencies and greater class switching in lymphoid tissues than blood, while pediatric blood had abundant class-switched convergent clones. Consistent with reported serology, prepandemic children had class-switched convergent clones to severe acute respiratory syndrome coronavirus 2 with weak cross-reactivity to other coronaviruses, while adult blood or tissues showed few such clones. These results highlight the prominence of early childhood B cell clonal expansions and cross-reactivity for future responses to novel pathogens.

Grafting Methionine on 1F1 Ab Increases the Broad-Activity on HA Structural-Conserved Residues of H1, H2, and H3 Influenza a Viruses

A high level of mutation enables the influenza A virus to resist antibiotics previously effective against the influenza A virus. A portion of the structure of hemagglutinin HA is assumed to be well-conserved to maintain its role in cellular fusion, and the structure tends to be more conserved than sequence. We designed peptide inhibitors to target the conserved residues on the HA surface, which were identified based on structural alignment. Most of the conserved and strongly similar residues are located in the receptor-binding and esterase regions on the HA1 domain In a later step, fragments of anti-HA antibodies were gathered and screened for the binding ability to the found conserved residues. As a result, Methionine amino acid got the best docking score within the −2.8 Å radius of Van der Waals when it is interacting with Tyrosine, Arginine, and Glutamic acid. Then, the binding affinity and spectrum of the fragments were enhanced by grafting hotspot amino acid into the fragments to form peptide inhibitors. Our peptide inhibitor was able to form in silico contact with a structurally conserved region across H1, H2, and H3 HA, with the binding site at the boundary between HA1 and HA2 domains, spreading across different monomers, suggesting a new target for designing broad-spectrum antibody and vaccine. This research presents an affordable method to design broad-spectrum peptide inhibitors using fragments of an antibody as a scaffold.

A comprehensive influenza reporter virus panel for high-throughput deep profiling of neutralizing antibodies

Broadly neutralizing antibodies (bnAbs) have been developed as potential countermeasures for seasonal and pandemic influenza. Deep characterization of these bnAbs and polyclonal sera provides pivotal understanding for influenza immunity and informs effective vaccine design. However, conventional virus neutralization assays require high-containment laboratories and are difficult to standardize and roboticize. Here, we build a panel of engineered influenza viruses carrying a reporter gene to replace an essential viral gene, and develop an assay using the panel for in-depth profiling of neutralizing antibodies. Replication of these viruses is restricted to cells expressing the missing viral gene, allowing it to be manipulated in a biosafety level 2 environment. We generate the neutralization profile of 24 bnAbs using a 55-virus panel encompassing the near-complete diversity of human H1N1 and H3N2, as well as pandemic subtype viruses. Our system offers in-depth profiling of influenza immunity, including the antibodies against the hemagglutinin stem, a major target of universal influenza vaccines.

Antibody Focusing to Conserved Sites of Vulnerability: The Immunological Pathways for 'Universal' Influenza Vaccines

Influenza virus remains a serious public health burden due to ongoing viral evolution. Vaccination remains the best measure of prophylaxis, yet current seasonal vaccines elicit strain-specific neutralizing responses that favor the hypervariable epitopes on the virus. This necessitates yearly reformulations of seasonal vaccines, which can be limited in efficacy and also shortchange pandemic preparedness. Universal vaccine development aims to overcome these deficits by redirecting antibody responses to functionally conserved sites of viral vulnerability to enable broad coverage. However, this is challenging as such antibodies are largely immunologically silent, both following vaccination and infection. Defining and then overcoming the immunological basis for such subdominant or ‘immuno-recessive’ antibody targeting has thus become an important aspect of universal vaccine development. This, coupled with structure-guided immunogen design, has led to proof-of-concept that it is possible to rationally refocus humoral immunity upon normally ‘unseen’ broadly neutralizing antibody targets on influenza virus.

Antibody-guided structure-based vaccines

The vaccine field is pursuing diverse approaches to translate the molecular insights from analyses of effective antibodies and their targeted epitopes into immunogens capable of eliciting protective immune responses. Here we review current antibody-guided strategies including conformation-based, epitope-based, and lineage-based vaccine approaches, which are yielding promising vaccine candidates now being evaluated in clinical trials. We summarize directions being employed by the field, including the use of sequencing technologies to monitor and track developing immune responses for understanding and improving antibody-based immunity. We review opportunities and challenges to transform powerful new discoveries into safe and effective vaccines, which are encapsulated by vaccine efforts against a variety of pathogens including HIV-1, influenza A virus, malaria parasites, respiratory syncytial virus, and SARS-CoV-2. Overall, this review summarizes the extensive progress that has been made to realize antibody-guided structure-based vaccines, the considerable challenges faced, and the opportunities afforded by recently developed molecular approaches to vaccine development.

New Technologies for Influenza Vaccines

Vaccine development has been hampered by the long lead times and the high cost required to reach the market. The 2020 pandemic, caused by a new coronavirus (SARS-CoV-2) that was first reported in late 2019, has seen unprecedented rapid activity to generate a vaccine, which belies the traditional vaccine development cycle. Critically, much of this progress has been leveraged off existing technologies, many of which had their beginnings in influenza vaccine development. This commentary outlines the most promising of the next generation of non-egg-based influenza vaccines including new manufacturing platforms, structure-based antigen design/computational biology, protein-based vaccines including recombinant technologies, nanoparticles, gene- and vector-based technologies, as well as an update on activities around a universal influenza vaccine.

Defining and Manipulating B Cell Immunodominance Hierarchies to Elicit Broadly Neutralizing Antibody Responses against Influenza Virus

The antibody repertoire possesses near-limitless diversity, enabling the adaptive immune system to accommodate essentially any antigen. However, this diversity explores the antigenic space unequally, allowing some pathogens like influenza virus to impose complex immunodominance hierarchies that distract antibody responses away from key sites of virus vulnerability. We developed a computational model of affinity maturation to map the patterns of immunodominance that evolve upon immunization with natural and engineered displays of hemagglutinin (HA), the influenza vaccine antigen. Based on this knowledge, we designed immunization protocols that subvert immune distraction and focus serum antibody responses upon a functionally conserved, but immunologically recessive, target of human broadly neutralizing antibodies. We tested in silico predictions by vaccinating transgenic mice in which antibody diversity was humanized to mirror clinically relevant humoral output. Collectively, our results demonstrate that complex patterns in antibody immunogenicity can be rationally defined and then manipulated to elicit engineered immunity.

Structural Biology of Influenza Hemagglutinin: An Amaranthine Adventure

Hemagglutinin (HA) glycoprotein is an important focus of influenza research due to its role in antigenic drift and shift, as well as its receptor binding and membrane fusion functions, which are indispensable for viral entry. Over the past four decades, X-ray crystallography has greatly facilitated our understanding of HA receptor binding, membrane fusion, and antigenicity. The recent advances in cryo-EM have further deepened our comprehension of HA biology. Since influenza HA constantly evolves in natural circulating strains, there are always new questions to be answered. The incessant accumulation of knowledge on the structural biology of HA over several decades has also facilitated the design and development of novel therapeutics and vaccines. This review describes the current status of the field of HA structural biology, how we got here, and what the next steps might be.

Self-Assembled Nanoparticles: Exciting Platforms for Vaccination

Vaccination is successfully advanced to control several fatal diseases and improve human life expectancy. However, additional innovations are required in this field because there are no effective vaccines to prevent some infectious diseases. The shift from the attenuated or inactivated pathogens to safer but less immunogenic protein or peptide antigens has led to a search for effective antigen delivery carriers that can function as both antigen vehicles and intrinsic adjuvants. Among these carriers, self-assembled nanoparticles (SANPs) have shown great potential to be the best representative. For the nanoscale and multiple presentation of antigens, with accurate control over size, geometry, and functionality, these nanoparticles are assembled spontaneously and mimic pathogens, resulting in enhanced antigen presentation and increased cellular and humoral immunity responses. In addition, they may be applied through needle-free routes due to their adhesive ability, which gives them a great future in vaccination applications. This review provides an overview of various SANPs and their applications in prophylactic vaccines.

Influenza Hemagglutinin Structures and Antibody Recognition

Hemagglutinin (HA) is most abundant glycoprotein on the influenza virus surface. Influenza HA promotes viral entry by engaging the receptor and mediating virus-host membrane fusion. At the same time, HA is the major antigen of the influenza virus. HA antigenic shift can result in pandemics, whereas antigenic drift allows human circulating strains to escape herd immunity. Most antibody responses against HA are strain-specific. However, antibodies that have neutralizing activities against multiple strains or even subtypes have now been discovered and characterized. These broadly neutralizing antibodies (bnAbs) target conserved regions on HA, such as the receptor-binding site and the stem domain. Structural studies of such bnAbs have provided important insight into universal influenza vaccine and therapeutic design. This review discusses the HA functions as well as HA-antibody interactions from a structural perspective.

Induction of cross-group broadly reactive antibody response by natural H7N9 avian influenza virus infection and immunization with inactivated H7N9 vaccine in chickens

Pre-existing immunity against the conserved haemagglutinin (HA) stalk underlies the elicitation of cross-group antibody induced by natural H7N9 virus infection and immunization in humans. However, whether broadly reactive antibodies can be induced by H7N9 infection and immunization in the absence of pre-existing stalk-specific immunity is unclear. In this study, antibody response induced by H7N9 virus infection and immunization with inactivated and viral-vectored H7N9 vaccines in naïve chickens was analysed. The results showed that H7N9 infection and immunization with inactivated vaccine resulted in potent induction of haemagglutination-inhibition (HI), virus neutralization (VN) and HA-binding antibodies, whereas Newcastle disease virus (NDV)-vectored H7N9 vaccine induced marginal HI and VN titres but high levels of HA-binding antibody. In addition, H7N9 infection and immunization induced stalk-specific antibodies in naïve chickens and these antibodies recognized different epitopes in the stalk. Virus infection and immunization with inactivated vaccine elicited antibodies cross-reactive with both group 1 and group 2 HAs, while antibodies induced by NDV-H7N9 vaccination showed a narrower cross-reactivity within group 2. Moreover, only homologous neutralizing activity of the sera against H7N9 virus was observed, and cross-binding antibodies did not show heterosubtypic neutralizing activity. Our results indicated that cross-group binding but non-neutralizing antibodies primarily targeting the stalk can be induced by natural H7N9 infection and immunization with inactivated vaccine in naïve chickens. This suggests that at least in a naïve chicken model, pre-existing stalk-specific immunity is not required for induction of broadly reactive antibodies. Additionally, H7N9-based immunogens may be explored as vaccine candidates or as a prime component to induce broadly protective influenza immunity.

Neutralizing Antibodies Targeting the Conserved Stem Region of Influenza Hemagglutinin

Influenza continues to be a public health threat despite the availability of annual vaccines. While vaccines are generally effective at inducing strain-specific immunity, they are sub-optimal or ineffective when drifted or novel pandemic strains arise due to sequence changes in the major surface glycoprotein hemagglutinin (HA). The discovery of a large number of antibodies targeting the highly conserved stem region of HAs that are capable of potently neutralizing a broad range of virus strains and subtypes suggests new ways to protect against influenza. The structural characterization of HA stem epitopes and broadly neutralizing antibody paratopes has enabled the design of novel proteins, mini-proteins, and peptides targeting the HA stem, thus providing a foundation for the design of new vaccines. In this narrative, we comprehensively review the current knowledge about stem-directed broadly neutralizing antibodies and the structural features contributing to virus neutralization.

Original antigenic sin priming of influenza virus hemagglutinin stalk antibodies

Immunity to influenza viruses can be long-lived, but reinfections with antigenically distinct viral strains and subtypes are common. Reinfections can boost antibody responses against viral strains first encountered in childhood through a process termed "original antigenic sin." It is unknown how initial childhood exposures affect the induction of antibodies against the hemagglutinin (HA) stalk domain of influenza viruses. This is an important consideration since broadly reactive HA stalk antibodies can protect against infection, and universal vaccine platforms are being developed to induce these antibodies. Here we show that experimentally infected ferrets and naturally infected humans establish strong "immunological imprints" against HA stalk antigens first encountered during primary influenza virus infections. We found that HA stalk antibodies are surprisingly boosted upon subsequent infections with antigenically distinct influenza A virus subtypes. Paradoxically, these heterosubtypic-boosted HA stalk antibodies do not bind efficiently to the boosting influenza virus strain. Our results demonstrate that an individual's HA stalk antibody response is dependent on the specific subtype of influenza virus that they first encounter early in life. We propose that humans are susceptible to heterosubtypic influenza virus infections later in life since these viruses boost HA stalk antibodies that do not bind efficiently to the boosting antigen.

Convergent Evolution in Breadth of Two VH6-1-Encoded Influenza Antibody Clonotypes from a Single Donor

Understanding how broadly neutralizing antibodies (bnAbs) to influenza hemagglutinin (HA) naturally develop in humans is critical to the design of universal influenza vaccines. Several classes of bnAbs directed to the conserved HA stem were found in multiple individuals, including one encoded by heavy-chain variable domain V_H6-1. We describe two genetically similar V_H6-1 bnAb clonotypes from the same individual that exhibit different developmental paths toward broad neutralization activity. One clonotype evolved from a germline precursor recognizing influenza group 1 subtypes to gain breadth to group 2 subtypes. The other clonotype recognized group 2 subtypes and developed binding to group 1 subtypes through somatic hypermutation. Crystal structures reveal that the specificity differences are primarily mediated by complementarity-determining region H3 (CDR H3). Thus, while V_H6-1 provides a framework for development of HA stem-directed bnAbs, sequence differences in CDR H3 junctional regions during VDJ recombination can alter reactivity and evolutionary pathways toward increased breadth.

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Two V_H6-1 influenza HA stem-specific clonotypes are isolated from one individual

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Precursors for these two V_H6-1 clonotypes bind different influenza A HA groups

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Differences in the CDRH3 conformation mediate the distinctive binding profiles

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Somatic hypermutation leads to similar binding breadth between the two clonotypes

Dynamic Perspectives on the Search for a Universal Influenza Vaccine

A universal influenza vaccine (UIV) could considerably alleviate the public health burden of both seasonal and pandemic influenza. Although significant progress has been achieved in clarifying basic immunology and virology relating to UIV, several important questions relating to the dynamics of infection, immunity, and pathogen evolution remain unsolved. In this study, we review these gaps, which span integrative levels, from cellular to global and timescales from molecular events to decades. We argue that they can be best addressed by a tight integration of empirical (laboratory, epidemiological) research and theory and suggest fruitful areas for this synthesis. In particular, quantifying natural and vaccinal limitations on viral transmission are central to this effort.

Immunodominance and Antigenic Variation of Influenza Virus Hemagglutinin: Implications for Design of Universal Vaccine Immunogens

Influenza viruses routinely acquire mutations in their hemagglutinin (HA) and neuraminidase (NA) glycoproteins that abrogate binding of pre-existing antibodies in a process known as antigenic drift. Most human antibodies against HA and NA are directed against epitopes that are hypervariable and not against epitopes that are conserved among different influenza virus strains. Universal influenza vaccines are currently being developed to elicit protective responses against functionally conserved sites on influenza proteins where viral escape mutations can result in large fitness costs [1]. Universal vaccine targets include the highly conserved HA stem domain [2-12], the less conserved HA receptor-binding site (RBS) [13-16], as well as conserved sites on NA [17-19]. One central challenge of universal vaccine efforts is to steer human antibody responses away from immunodominant, variable epitopes and towards subdominant, functionally conserved sites. Overcoming this challenge will require further understanding of the structural basis of broadly neutralizing HA and NA antibody binding epitopes and factors that influence immunodominance hierarchies of human antibody responses.

Host immune response-inspired development of the influenza vaccine

Objective

To assess the current and future development of influenza vaccines.

Data Sources

PubMed searches were performed cross-referencing the keywords influenza, influenza vaccine, host immune response, correlates of protection, vaccine development, vaccine efficacy. Articles were reviewed for additional citations.

Study Selections

Articles were reviewed and selected on the basis of relevance to subject matter.

Results

In this review, we first introduce the influenza virus, its nomenclature, and the concepts of antigenic drift and shift. Second, we discuss the status of currently licensed influenza virus vaccines. We briefly focus on influenza vaccine responses beyond hemagglutination inhibition that may correlate with protection against influenza viruses of different subtypes. Third, we explain how studying host responses to influenza infection and vaccination with advanced serologic methods, B-cell receptor sequencing, and transcriptomic profiling can guide the development of improved influenza virus vaccines. Fourth, we provide 2 suggestions on how current influenza vaccines can be optimized by redirecting immune responses toward conserved viral antigens and the use of adjuvants.

Conclusion

Influenza vaccine design can benefit from novel insights obtained from the study of host responses to influenza virus infection and vaccination. Integration of the large amount of available clinical and preclinical data requires systems approaches that can elucidate novel correlates of protection and will guide further development of influenza vaccine.

Prolonged evolution of the memory B cell response induced by a replicating adenovirus-influenza H5 vaccine

Induction of an antibody response capable of recognizing highly diverse strains is a major obstacle to the development of vaccines for viruses such as HIV and influenza. Here, we report the dynamics of B cell expansion and evolution at the single-cell level after vaccination with a replication-competent adenovirus type 4 recombinant virus expressing influenza H5 hemagglutinin. Fluorescent H1 or H5 probes were used to quantitate and isolate peripheral blood B cells and their antigen receptors. We observed increases in H5-specific antibody somatic hypermutation and potency for several months beyond the period of active viral replication that was not detectable at the serum level. Individual broad and potent antibodies could be isolated, including one stem-specific antibody that is part of a new multidonor class. These results demonstrate prolonged evolution of the B cell response for months after vaccination and should be considered in efforts to evaluate or boost vaccine-induced immunity.

Next-generation influenza vaccines: opportunities and challenges

Seasonal influenza vaccines lack efficacy against drifted or pandemic influenza strains. Developing improved vaccines that elicit broader immunity remains a public health priority. Immune responses to current vaccines focus on the haemagglutinin head domain, whereas next-generation vaccines target less variable virus structures, including the haemagglutinin stem. Strategies employed to improve vaccine efficacy involve using structure-based design and nanoparticle display to optimize the antigenicity and immunogenicity of target antigens; increasing the antigen dose; using novel adjuvants; stimulating cellular immunity; and targeting other viral proteins, including neuraminidase, matrix protein 2 or nucleoprotein. Improved understanding of influenza antigen structure and immunobiology is advancing novel vaccine candidates into human trials.

Current seasonal influenza vaccines lack efficacy against drifted or pandemic virus strains, and the development of novel vaccines that elicit broader immunity represents a public health priority. Here, Nabel and colleagues discuss approaches to improve vaccine efficacy which harness new insights from influenza antigen structure and human immunity, highlighting major targets, vaccines in development and ongoing challenges.

Glycan repositioning of influenza hemagglutinin stem facilitates the elicitation of protective cross-group antibody responses

The conserved hemagglutinin (HA) stem has been a focus of universal influenza vaccine efforts. Influenza A group 1 HA stem-nanoparticles have been demonstrated to confer heterosubtypic protection in animals; however, the protection does not extend to group 2 viruses, due in part to differences in glycosylation between group 1 and 2 stems. Here, we show that introducing the group 2 glycan at Asn38HA1 to a group 1 stem-nanoparticle (gN38 variant) based on A/New Caledonia/20/99 (H1N1) broadens antibody responses to cross-react with group 2 HAs. Immunoglobulins elicited by the gN38 variant provide complete protection against group 2 H7N9 virus infection, while the variant loses protection against a group 1 H5N1 virus. The N38HA1 glycan thus is pivotal in directing antibody responses by controlling access to group-determining stem epitopes. Precise targeting of stem-directed antibody responses to the site of vulnerability by glycan repositioning may be a step towards achieving cross-group influenza protection.

A unique nanoparticulate TLR9 agonist enables a HA split vaccine to confer FcγR-mediated protection against heterologous lethal influenza virus infection

The development of a universal influenza vaccine that can provide a robust and long-lasting protection against a broader range of influenza virus strains is a global public health priority. One approach to improve vaccine efficacy is to use an adjuvant to boost immune responses to the target antigens; nevertheless, the role of adjuvants in the context of influenza vaccines is not fully understood. We have previously developed the K3-schizophyllan (SPG) adjuvant, which is composed of nanoparticulated oligodeoxynucleotides K3, a TLR9 agonist, with SPG, a non-agonistic β-glucan ligand of Dectin-1. In this study, K3-SPG given with conventional influenza hemagglutinin (HA) split vaccine (K3-SPG HA) conferred protection against antigenically mismatched heterologous virus challenge. While K3-SPG HA elicited robust cross-reactive HA-specific IgG2c and CD8 T-cell responses, CD8 T-cell depletion had no impact on this cross-protection. In contrast, K3-SPG HA was not able to confer protection against heterologous virus challenge in FcRγ-deficient mice. Our results indicated that FcγR-mediated antibody responses induced by the HA antigen and K3-SPG adjuvant were important for potent protection against antigenically mismatched influenza virus infection. Thus, we demonstrated that the K3-SPG-adjuvanted vaccine strategy broadens protective immunity against influenza and provides a basis for the development of next-generation influenza vaccines.

Influenza Virus Infection Induces a Narrow Antibody Response in Children but a Broad Recall Response in Adults

In contrast to influenza virus vaccination, natural infection induces long-lived and relatively broad immune responses. However, many aspects of the antibody response to natural infection are not well understood. Here, we assessed the immune response after H1N1 influenza virus infection in children and adults in a Nicaraguan household transmission study using an influenza virus protein microarray (IVPM). This technology allows us to simultaneously measure IgG and IgA antibody responses to hemagglutinins of many different virus strains and subtypes quantitatively with a high throughput. We found that children under 6 years of age responded to natural infection with a relatively narrow response that targeted mostly the hemagglutinin of the strain that caused the infection. Adults, however, have a much broader response, including a boost in antibodies to many group 1 subtype hemagglutinins. Also, a strong recall response against historic H1 hemagglutinins that share the K133 epitope with the pandemic H1N1 virus was observed. Of note, some children, while responding narrowly within H1 and group 1 hemagglutinins, induced a boost to H3 and other group 2 hemagglutinins when infected with H1N1 when they had experienced an H3N2 infection earlier in life. This is an interesting phenomenon providing evidence for immune imprinting and a significant new insight which might be leveraged in future universal influenza virus vaccine strategies. Finally, preexisting immunity to pandemic H1 hemagglutinins was significantly associated with protection from infection in both children and adults. In adults, preexisting immunity to non-H1 group 1 hemagglutinins was also significantly associated with protection from infection.IMPORTANCE It is known since Thomas Francis, Jr. published his first paper on original antigenic sin in 1960 that the first infection(s) with influenza virus leaves a special immunological imprint which shapes immune responses to future infections with antigenically related influenza virus strains. Imprinting has been implicated in both protective effects as well as blunting of the immune response to vaccines. Despite the fact that this phenomenon was already described almost 60 years ago, we have very little detailed knowledge of the characteristics and breadth of the immune response to the first exposure(s) to influenza virus in life and how this compares to later exposure as adults. Here, we investigate these immune responses in detail using an influenza virus protein microarray. While our findings are mostly descriptive in nature and based on a small sample size, they provide a strong basis for future large-scale studies to better understand imprinting effects.

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.

Hemagglutinin head-specific responses dominate over stem-specific responses following prime boost with mismatched vaccines

Broadly neutralizing Abs targeting the HA stem can provide broad protection against different influenza subtypes, raising the question of how best to elicit such Abs. We have previously demonstrated that vaccination with pandemic live-attenuated influenza vaccine (pLAIV) establishes immune memory for HA head-specific Abs. Here, we determine the extent to which matched versus mismatched LAIV-inactivated subunit vaccine (IIV) prime-boost vaccination elicits stem-specific memory B cells and Abs. We vaccinated African green monkeys with H5N1 pLAIV-pIIV or H5N1 pLAIV followed by seasonal IIV (sIIV) or with H5N1 pLAIV alone and measured Abs and HA-specific B cell responses. While we observed an increase in stem-specific memory B cells, head-specific memory B cell responses were substantially higher than stem-specific responses and were dominant even following boost with mismatched IIV. Neutralizing Abs against heterologous influenza viruses were undetectable. Head-specific B cells from draining lymph nodes exhibited germinal center markers, while stem-specific B cells found in the spleen and peripheral blood did not. Thus, although mismatched prime-boost generated a pool of stem-specific memory B cells, head-specific B cells and serum Abs substantially dominated the immune response. These findings have implications for including full-length native HA in prime-boost strategies intended to induce stem-specific Abs for universal influenza vaccination.

Preexisting influenza specific immunity and vaccine effectiveness

Introduction: High variance is associated with influenza vaccine effectiveness (VE). Accumulating evidence suggests that preexisting influenza-specific immunity results in the variance in VE and skews overall immune response to vaccination. Nevertheless, the investigation of preexisting immunity is highly limited due to the lack of proper methodology to explore the complex individual immune history.Areas covered: Retrospective observational studies have shown that the preexisting influenza specific immunity influences on VE. To simplify a discussion, we summarized important findings from the observational studies based on the transition of the individual immune history: the first exposure to influenza virus, the first vaccination, and repetitive exposure throughout life. We also discussed the prospectus of pre-immunized animal models to investigate the interaction between preexisting immunity and vaccine efficacy.Expert opinion: A better understanding in the underlying mechanisms on preexisting immunity is critical to improve VE and to help develop novel vaccine strategies. Using animals pre-immunized with historical influenza strains is a promising approach to verify the underlying immunologic mechanism of interaction between preexisting immunity and vaccine antigen. Also, pre-immunized animal models will be better able to evaluate the efficacy of novel vaccine strategies than naïve animals.

Role of Memory B Cells in Hemagglutinin-Specific Antibody Production Following Human Influenza A Virus Infection

When influenza A virus infects an immune individual, preexisting memory B cell (MBC) activation and rapid anamnestic antibody production plays a key role in viral clearance. The most effective neutralizing antibodies target the antigenically variable head of the viral hemagglutinin (HA); antibodies against the conserved HA stalk provide broader but less potent protection. In this review, we provide a comprehensive picture of an adult's HA-specific antibody response to influenza virus infection. The process is followed from preexisting HA-specific MBC activation and rapid production of anti-HA antibodies, through to germinal center seeding and adaptation of the response to novel features of the HA. A major focus of the review is the role of competition between preexisting MBCs in determining the character of the HA-reactive antibody response. HA novelty modifies this competition and can shift the response from the immunodominant head to the stalk. We suggest that antibodies resulting from preexisting MBC activation are important regulators of anti-HA antibody production and play a role in positive selection of germinal center B cells reactive to novel HA epitopes. Our review also considers the role of MBCs in the effects of early-life imprinting on HA head- and stalk-specific antibody responses to influenza infection. An understanding of the processes described in this review will guide development of vaccination strategies that provide broadly effective protection.