Restricted Clonality and Limited Germinal Center Reentry Characterize Memory B Cell Reactivation by Boosting

B Cell Response to Vaccination

As one of the most important weapons against infectious diseases, vaccines have saved countless lives since their first use in the late eighteenth century. Antibodies produced by effector B cells upon vaccination play a critical role in mediating protection. The past several decades of research have led to a revolution in our understanding of B cell response to vaccination. Vaccines against SARS-CoV-2 coronavirus were developed at an unprecedented speed to power our global fight against COVID-19 pandemic. Nevertheless, we still face many challenges in the development of vaccines against many other deadly viruses, such as human immunodeficiency virus (HIV) and influenza virus. In this review, we summarize the latest findings on B cell response to vaccination and pathogen infection. We also discuss the current challenges in the field and the potential strategies targeting B cell response to improve vaccine efficacy.Key abbreviations box: BCR: B cell receptor; bNAb: broadly neutralizing antibody; DC: dendritic cells; DZ: dark zone; EF response: extrafollicular response; FDC: follicular dendritic cell; GC: germinal center; HIV: human immunodeficiency virus; IC: immune complex; LLPC: long-lived plasma cell; LZ: light zone; MBC: memory B cell; SLPB: short-lived plasmablast; TFH: T follicular helper cells; TLR: Toll-like receptor.

B-Cell Memory Responses to Variant Viral Antigens

A central feature of vertebrate immune systems is the ability to form antigen-specific immune memory in response to microbial challenge and so provide protection against future infection. In conflict with this process is the ability that many viruses have to mutate their antigens to escape infection- or vaccine-induced antibody memory responses. Mutable viruses such as dengue virus, influenza virus and of course coronavirus have a major global health impact, exacerbated by this ability to evade immune responses through mutation. There have been several outstanding recent studies on B-cell memory that also shed light on the potential and limitations of antibody memory to protect against viral antigen variation, and so promise to inform new strategies for vaccine design. For the purposes of this review, the current understanding of the different memory B-cell (MBC) populations, and their potential to recognize mutant antigens, will be described prior to some examples from antibody responses against the highly mutable RNA based flaviviruses, influenza virus and SARS-CoV-2.

Immune memory in convalescent patients with asymptomatic or mild COVID-19

It is important to evaluate the durability of the protective immune response elicited by primary infection with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Here, we systematically evaluated the SARS-CoV-2-specific memory B cell and T cell responses in healthy controls and individuals recovered from asymptomatic or symptomatic infection approximately 6 months prior. Comparatively low frequencies of memory B cells specific for the receptor-binding domain (RBD) of spike glycoprotein (S) persisted in the peripheral blood of individuals who recovered from infection (median 0.62%, interquartile range 0.48-0.69). The SARS-CoV-2 RBD-specific memory B cell response was detected in 2 of 13 individuals who recovered from asymptomatic infection and 10 of 20 individuals who recovered from symptomatic infection. T cell responses induced by S, membrane (M), and nucleocapsid (N) peptide libraries from SARS-CoV-2 were observed in individuals recovered from coronavirus disease 2019 (COVID-19), and cross-reactive T cell responses to SARS-CoV-2 were also detected in healthy controls.

Transcriptional and Metabolic Control of Memory B Cells and Plasma Cells

For many infections and almost all vaccines, neutralizing-antibody-mediated immunity is the primary basis and best functional correlate of immunological protection. Durable long-term humoral immunity is mediated by antibodies secreted by plasma cells that preexist subsequent exposures and by memory B cells that rapidly respond to infections once they have occurred. In the midst of the current pandemic of coronavirus disease 2019, it is important to define our current understanding of the unique roles of memory B cells and plasma cells in immunity and the factors that control the formation and persistence of these cell types. This fundamental knowledge is the basis to interpret findings from natural infections and vaccines. Here, we review transcriptional and metabolic programs that promote and support B cell fates and functions, suggesting points at which these pathways do and do not intersect.

The immunoglobulins of cartilaginous fishes

Cartilaginous fishes, comprising the chimeras, sharks, skates, and rays, split from the common ancestor with other jawed vertebrates approx. 450 million years ago. Being the oldest extant taxonomic group to possess an immunoglobulin (Ig)-based adaptive immune system, examination of this group has taught us much about the evolution of adaptive immunity, as well as the conserved and taxon-specific characteristics of Igs. Significant progress has been made analyzing sequences from numerous genomic and transcriptomic data sets. These findings have been supported by additional functional studies characterizing the Igs and humoral response of sharks and their relatives. This review will summarize what we have learned about the genomic organization, protein structure, and in vivo function of these Ig isotypes in cartilaginous fishes and highlight the areas where our knowledge is still lacking.

The BAFFling persistence of memory B cells

Although BAFF/BLyS and its receptor, BAFFR, play critical roles in naive B cell survival, the pathways involved in the persistence of memory B cells are largely unknown. In this issue of JEM, two groups, Müller-Winkler et al. (https://doi.org/10.1084/jem.20191393) and Lau et al. (https://doi.org/10.1084/jem.20191167), take complementary approaches to identify an essential role for BAFFR in the survival of memory B cells.

The Shaping of a B Cell Pool Maximally Responsive to Infections

B cell subsets differ in development, tissue distribution, and mechanisms of activation. In response to infections, however, all can differentiate into extrafollicular plasmablasts that rapidly provide highly protective antibodies, indicating that these plasmablasts are the main humoral immune response effectors. Yet, the effectiveness of this response type depends on the presence of antigen-specific precursors in the circulating mature B cell pool, a pool that is generated initially through the stochastic processes of B cell receptor assembly. Importantly, germinal centers then mold the repertoire of this B cell pool to be increasingly responsive to pathogens by generating a broad array of antimicrobial memory B cells that act as highly effective precursors of extrafollicular plasmablasts. Such B cell repertoire molding occurs in two ways: continuously via the chronic germinal centers of mucosal lymphoid tissues, driven by the presence of the microbiome, and via de novo generated germinal centers following acute infections. For effectively evaluating humoral immunity as a correlate of immune protection, it might be critical to measure memory B cell pools in addition to antibody titers.

Modeling human adaptive immune responses with tonsil organoids

Most of what we know about adaptive immunity has come from inbred mouse studies, using methods that are often difficult or impossible to confirm in humans. In addition, vaccine responses in mice are often poorly predictive of responses to those same vaccines in humans. Here we use human tonsils, readily available lymphoid organs, to develop a functional organotypic system that recapitulates key germinal center features in vitro, including the production of antigen-specific antibodies, somatic hypermutation and affinity maturation, plasmablast differentiation and class-switch recombination. We use this system to define the essential cellular components necessary to produce an influenza vaccine response. We also show that it can be used to evaluate humoral immune responses to two priming antigens, rabies vaccine and an adenovirus-based severe acute respiratory syndrome coronavirus 2 vaccine, and to assess the effects of different adjuvants. This system should prove useful for studying critical mechanisms underlying adaptive immunity in much greater depth than previously possible and to rapidly test vaccine candidates and adjuvants in an entirely human system.

Cyclin D3 drives inertial cell cycling in dark zone germinal center B cells

During affinity maturation, germinal center (GC) B cells alternate between proliferation and somatic hypermutation in the dark zone (DZ) and affinity-dependent selection in the light zone (LZ). This anatomical segregation imposes that the vigorous proliferation that allows clonal expansion of positively selected GC B cells takes place ostensibly in the absence of the signals that triggered selection in the LZ, as if by “inertia.” We find that such inertial cycles specifically require the cell cycle regulator cyclin D3. Cyclin D3 dose-dependently controls the extent to which B cells proliferate in the DZ and is essential for effective clonal expansion of GC B cells in response to strong T follicular helper (Tfh) cell help. Introduction into the Ccnd3 gene of a Burkitt lymphoma–associated gain-of-function mutation (T283A) leads to larger GCs with increased DZ proliferation and, in older mice, clonal B cell lymphoproliferation, suggesting that the DZ inertial cell cycle program can be coopted by B cells undergoing malignant transformation.

The Histone Methyltransferase DOT1L Is Essential for Humoral Immune Responses

Histone modifiers are essential for the ability of immune cells to reprogram their gene expression during differentiation. The recruitment of the histone methyltransferase DOT1L (disruptor of telomeric silencing 1-like) induces oncogenic gene expression in a subset of B cell leukemias. Despite its importance, its role in the humoral immune system is unclear. Here, we demonstrate that DOT1L is a critical regulator of B cell biology. B cell development is defective in Dot1l^f/fMb1^Cre/+ mice, culminating in a reduction of peripheral mature B cells. Upon immunization or influenza infection of Dot1l^f/fCd23^Cre/+ mice, class-switched antibody-secreting cells are significantly attenuated and germinal centers fail to form. Consequently, DOT1L is essential for B cell memory formation. Transcriptome, pathway, and histological analyses identified a role for DOT1L in reprogramming gene expression for appropriate localization of B cells during the initial stage of the response. Together, these results demonstrate an essential role for DOT1L in generating an effective humoral immune response.

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.

Immunoglobulin M in Health and Diseases: How Far Have We Come and What Next?

B lymphocytes are important in secreting antibodies that protect against invading pathogens such as viruses, bacteria, parasites, and also in mediating pathogenesis of allergic diseases and autoimmunity. B lymphocytes develop in the bone marrow and contain heavy and light chains, which upon ligation form an immunoglobulin M (IgM) B cell receptor (BCR) expressed on the surface of naïve immature B cells. Naïve B cells expressing either IgM or IgD isotypes are thought to play interchangeable functions in antibody responses to T cell-dependent and T cell-independent antigens. IgM short-lived plasma cells (SLPCs) and antigen-specific IgM memory B cells (MBCs-M) are critical in the first few days of infection, as well as long-term memory induced by vaccination, respectively. At mucosal surfaces, IgM is thought to play a critical part in promoting mucosal tolerance and shaping microbiota together with IgA. In this review, we explore how IgM structure and BCR signaling shapes B cell development, self and non-self-antigen-specific antibody responses, responses to infectious (such as viruses, parasites, and fungal) and non-communicable diseases (such as autoimmunity and allergic asthma). We also explore how metabolism could influence other B cell functions such as mucosal tolerance and class switching. Finally, we discuss some of the outstanding critical research questions in both experimental and clinical settings targeting IgM.

Antibody Affinity Shapes the Choice between Memory and Germinal Center B Cell Fates

Immunological memory is required for protection against repeated infections and is the basis of all effective vaccines. Antibodies produced by memory B cells play an essential role in many of these responses. We have combined lineage tracing with antibody cloning from single B cells to examine the role of affinity in B cell selection into germinal centers (GCs) and the memory B cell compartment in mice immunized with an HIV-1 antigen. We find that contemporaneously developing memory and GC B cells differ in their affinity for antigen throughout the immune response. Whereas GC cells and their precursors are enriched in antigen binding, memory B cells are not. Thus, the polyclonal memory B cell compartment is composed of B cells that were activated during the immune response but whose antigen binding affinity failed to support further clonal expansion in the GC.

Tunable dynamics of B cell selection in gut germinal centres

Germinal centers (GCs), structures normally associated with B cell immunoglobulin (Ig) hypermutation and development of high-affinity antibodies upon infection or immunization, are present in gut-associated lymphoid organs of humans and mice under steady state. Gut-associated (ga)GCs can support antibody responses to enteric infections and immunization1. However, whether B cell selection and antibody affinity maturation can take place in face of the chronic and diverse antigenic stimulation characteristic of steady-state gaGCs is less clear^2–8. Combining multicolor “Brainbow” fate-mapping and single-cell Ig sequencing, we find that 5–10% of gaGCs from specific pathogen-free (SPF) mice contained highly-dominant “winner” clones at steady state, despite rapid turnover of GC B cells. Monoclonal antibodies (mAbs) derived from these clones showed increased binding to commensal bacteria compared to their unmutated ancestors, consistent with antigen-driven selection and affinity maturation. Frequency of highly-selected gaGCs was markedly higher in germ-free (GF) than in SPF mice, and winner B cells in GF gaGCs were enriched in public IgH clonotypes found across multiple individuals, indicating strong B cell receptor (BCR)-driven selection in the absence of microbiota. Vertical colonization of GF mice with a defined microbial consortium (Oligo-MM¹²) did not eliminate GF-associated clonotypes, yet induced a concomitant commensal-specific, affinity-matured B cell response. Thus, positive selection can take place in steady-state gaGCs, at a rate that is tunable over a wide range by the presence and composition of the microbiota.

T follicular regulatory cells: Guardians of the germinal centre?

It is a central tenet of the clonal selection theory, that lymphocyte repertoires are tolerized to self‐antigens during their ontogeny. Germinal centres are the sites in secondary lymphoid tissues where B cells undergo affinity maturation and class‐switching to produce high‐affinity antibodies. This process is crucial, both in our ability to mount protective humoral responses to infections and to vaccinations, but it is also involved in untoward reactions to self‐antigens, which underlie autoimmunity. The process of affinity maturation poses a significant challenge to tolerance, as the random nature of somatic hypermutation can introduce novel reactivities. Therefore, it has been a long‐standing idea that mechanisms must exist which limit the emergence of autoreactivity at the germinal centre level. One of these mechanisms is the requirement for linked recognition, which imposes on B cells a dependence on centrally tolerant T follicular helper cells. However, as linked recognition can be bypassed by adduct formation of autoantigenic complexes, it has been an appealing notion that there should be an additional layer of dominant mechanisms regulating emergence of autoreactive specificities. About a decade ago, this notion was addressed by the discovery of a novel subset of T regulatory cells localizing to the germinal centre and regulating germinal centre B‐cell responses. Here, we detail the progress that has been made towards characterizing this T follicular regulatory cell subset and understanding the functions of these ‘guardians of the germinal centre’.

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.

Transcriptional regulation of memory B cell differentiation

Memory B cells (MBCs) are critical for the rapid development of protective immunity following re-infection. MBCs capable of neutralizing distinct subclasses of pathogens, such as influenza and HIV, have been identified in humans. However, efforts to develop vaccines that induce broadly protective MBCs to rapidly mutating pathogens have not yet been successful. Better understanding of the signals regulating MBC development and function are essential to overcome current challenges hindering successful vaccine development. Here, we discuss recent advancements regarding the signals and transcription factors regulating germinal centre-derived MBC development and function.

Affinity-Restricted Memory B Cells Dominate Recall Responses to Heterologous Flaviviruses

Memory B cells (MBCs) can respond to heterologous antigens either by molding new specificities through secondary germinal centers (GCs) or by selecting preexisting clones without further affinity maturation. To distinguish these mechanisms in flavivirus infections and immunizations, we studied recall responses to envelope protein domain III (DIII). Conditional deletion of activation-induced cytidine deaminase (AID) between heterologous challenges of West Nile, Japanese encephalitis, Zika, and dengue viruses did not affect recall responses. DIII-specific MBCs were contained mostly within the plasma-cell-biased CD80⁺ subset, and few GCs arose following heterologous boosters, demonstrating that recall responses are confined by preexisting clonal diversity. Measurement of monoclonal antibody (mAb) binding affinity to DIII proteins, timed AID deletion, single-cell RNA sequencing, and lineage tracing experiments point to selection of relatively low-affinity MBCs as a mechanism to promote diversity. Engineering immunogens to avoid this MBC diversity may facilitate flavivirus-type-specific vaccines with minimized potential for infection enhancement.

Human germinal centres engage memory and naive B cells after influenza vaccination

Influenza viruses remain a major public health threat. Seasonal influenza vaccination in humans primarily stimulates pre-existing memory B cells, which differentiate into a transient wave of circulating antibody-secreting plasmablasts1-3. This recall response contributes to 'original antigenic sin'-the selective increase of antibody species elicited by previous exposures to influenza virus antigens4. It remains unclear whether such vaccination can also induce germinal centre reactions in the draining lymph nodes, where diversification and maturation of recruited B cells can occur5. Here we used ultrasound-guided fine needle aspiration to serially sample the draining lymph nodes and investigate the dynamics and specificity of germinal centre B cell responses after influenza vaccination in humans. Germinal centre B cells that bind to influenza vaccine could be detected as early as one week after vaccination. In three out of eight participants, we detected vaccine-binding germinal centre B cells up to nine weeks after vaccination. Between 12% and 88% of the responding germinal centre B cell clones overlapped with B cells detected among early circulating plasmablasts. These shared B cell clones had high frequencies of somatic hypermutation and encoded broadly cross-reactive monoclonal antibodies. By contrast, vaccine-induced B cell clones detected only in the germinal centre compartment exhibited significantly lower frequencies of somatic hypermutation and predominantly encoded strain-specific monoclonal antibodies, which suggests a naive B cell origin. Some of these strain-specific monoclonal antibodies recognized epitopes that were not targeted by the early plasmablast response. Thus, influenza virus vaccination in humans can elicit a germinal centre reaction that recruits B cell clones that can target new epitopes, thereby broadening the spectrum of vaccine-induced protective antibodies.

Plasma Cells: From Cytokine Production to Regulation in Experimental Autoimmune Encephalomyelitis

B cells are a critical arm of the adaptive immune system. After encounter with antigen, B cells are activated and differentiate into plasmablasts (PBs) and plasma cells (PCs). Although their frequency is low, PB/PCs can be found in all lymphoid organs including peripheral lymph nodes and spleen. Upon immunization, depending on the location of where B cells encounter their antigen, PB/PCs subsequently home to and accumuate in the bone marrow and the intestine where they can survive as long-lived plasma cells for years, continually producing antibody. Recent evidence has shown that, in addition to producing antibodies, PB/PCs can also produce cytokines such as IL-17, IL-10, and IL-35. In addition, PB/PCs that produce IL-10 have been shown to play a regulatory role during experimental autoimmune encephalomyelitis, an animal model of neuroinflammation. The purpose of this review is to describe the phenotype and function of regulatory PB/PCs in the context of experimental autoimmune encephalomyelitis and in patients with multiple sclerosis.

Cbls boost B cells

T cell regulation of antibody-mediated immunity is critical for health. In this issue of JEM, Li et al. (https://doi.org/10.1084/jem.20191537) identify the Cbl family of E3 ubiquitin ligases as B cell-intrinsic gatekeepers of T cell-dependent humoral immunity.

Flu Universal Vaccines: New Tricks on an Old Virus

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

Multifaceted Effects of Antigen Valency on B Cell Response Composition and Differentiation In Vivo

How antigen valency affects B cells in vivo during immune responses is not well understood. Here, using HIV immunogens with defined valencies ranging from 1 to 60, we investigated the role of antigen valency during different phases of B cell responses in vivo. Highly multimerized immunogens preferentially rapidly activated cognate B cells, with little affinity discrimination. This led to strong early induction of the transcription factors IRF4 (interferon regulatory factor 4) and Bcl6, driving both early extrafollicular plasma cell and germinal center responses, in a CD4⁺ T-cell-dependent manner, involving B cells with a broad range of affinities. Low-valency antigens induced smaller effector B cell responses, with preferential recruitment of high-affinity B cells. Thus, antigen valency has multifaceted effects on B cell responses and can dictate affinity thresholds and competitive landscapes for B cells in vivo, with implications for vaccine design.

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Antigen valency dictates the magnitude and composition of B cell responses

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High valency enables robust activation and effector differentiation of B cells

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Antigen valency alters breadth of B cell affinities recruited

Optimizing immunization protocols to elicit broadly neutralizing antibodies

Natural infections and vaccination with a pathogen typically stimulate the production of potent antibodies specific for the pathogen through a Darwinian evolutionary process known as affinity maturation. Such antibodies provide protection against reinfection by the same strain of a pathogen. A highly mutable virus, like HIV or influenza, evades recognition by these strain-specific antibodies via the emergence of new mutant strains. A vaccine that elicits antibodies that can bind to many diverse strains of the virus-known as broadly neutralizing antibodies (bnAbs)-could protect against highly mutable pathogens. Despite much work, the mechanisms by which bnAbs emerge remain uncertain. Using a computational model of affinity maturation, we studied a wide variety of vaccination strategies. Our results suggest that an effective strategy to maximize bnAb evolution is through a sequential immunization protocol, wherein each new immunization optimally increases the pressure on the immune system to target conserved antigenic sites, thus conferring breadth. We describe the mechanisms underlying why sequentially driving the immune system increasingly further from steady state, in an optimal fashion, is effective. The optimal protocol allows many evolving B cells to become bnAbs via diverse evolutionary paths.

Imprinting, immunodominance, and other impediments to generating broad influenza immunity

Natural influenza virus infections and seasonal vaccinations often do not confer broadly neutralizing immunity across diverse influenza strains. In addition, the virus is capable of rapid antigenic drift in order to evade pre-existing immunity. The surface glycoproteins, hemagglutinin, and neuraminidase can easily mutate their immunodominant epitopes without impacting fitness. Skewing human antibody repertoires to target more conserved epitopes is thus an expanding area of research: Many groups are attempting to produce universal influenza vaccines that can protect across a wide variety of strains. Achieving this goal will require a detailed understanding of how infection history impacts humoral responses. It will also require the ability to manipulate or enhance B cell selection in order to expand clones that can recognize subdominant but protective epitopes. In this review, we will discuss what immune imprinting means to immunologists and describe efforts to overcome or silence imprinting in order to improve vaccination efficiency.

Proof of long-term immunological memory in cartilaginous fishes

Immunological memory provides long-term protection against pathogen re-infection and is the foundation for successful vaccination. We have previously shown an antigen-specific recall response in nurse sharks almost one year after primary exposure. Herein, we extend the time between prime and successful recall to >8 years, the longest period for which immunological memory has been shown in any non-mammalian vertebrate. We confirm that antigen binding is mediated by monomeric IgM and IgNAR, but not pentameric IgM, in both the primary and recall phases. Our inability to find target-binding clones in recombinant VNAR expression libraries suggests that, at least in this instance, antigen-specific memory cells comprise a small fraction of the IgNAR-positive B cells in epigonal and spleen. Further, that the few memory cells present can generate a robust antigen-specific IgNAR titer following re-stimulation. Our results continue to challenge the long-held, but erroneous, belief that the shark adaptive immune system is 'primitive' when compared to that of mammals.

Factors in B cell competition and immunodominance

The majority of all vaccines work by inducing protective antibody responses. The mechanisms by which the B cells responsible for producing protective antibodies are elicited to respond are not well understood. Interclonal B cell competition to complex antigens, particularly in germinal centers, has emerged as an important hurdle in designing effective vaccines. This review will focus on recent advances in understanding the roles of B cell precursor frequency, B cell receptor affinity for antigen, antigen avidity, and other factors that can substantially alter the outcomes of B cell responses to complex antigens. Understanding the interdependence of these fundamental factors that affect B cell responses can inform current vaccine design efforts for pathogens with complex proteins as candidate immunogens such as HIV, influenza, and coronaviruses.

Influenza vaccination strategies targeting the hemagglutinin stem region

Influenza is one of the best examples of highly mutable viruses that are able to escape immune surveillance. Indeed, in response to influenza seasonal infection or vaccination, the majority of the induced antibodies are strain‐specific. Current vaccine against the seasonal strains with the strategy of surveillance‐prediction‐vaccine does not cover an unmet virus strain leading to pandemic. Recently, antibodies targeting conserved epitopes on the hemagglutinin (HA) protein have been identified, albeit rarely, and they often showed broad protection. These antibody discoveries have brought the feasibility to develop a universal vaccine. Most of these antibodies bind the HA stem domain and accumulate in the memory B cell compartment. Broadly reactive stem‐biased memory responses were induced by infection with antigenically divergent influenza strains and were able to eradicate these viruses, together indicating the importance of generating memory B cells expressing high‐quality anti‐stem antibodies. Here, we emphasize recent progress in our understanding of how such memory B cells can be generated and discuss how these advances may be relevant to the quest for a universal influenza vaccine.

It's Hard to Teach an Old B Cell New Tricks

Victora and colleagues challenge current perceptions that memory B cells readily participate in secondary germinal center reactions, allowing further modification of specificity upon reactivation. Rather, naïve B cells are the predominant B cell type that populate secondary germinal centers. This work has important basic immunological and translational implications.

An Agonistic Anti-CD137 Antibody Disrupts Lymphoid Follicle Structure and T-Cell-Dependent Antibody Responses

CD137 is a costimulatory receptor expressed on natural killer cells, T cells, and subsets of dendritic cells. An agonistic monoclonal antibody (mAb) against CD137 has been used to reduce tumor burden or reverse autoimmunity in animal models and clinical trials. Here, we show that mice treated with an agonistic anti-CD137 mAb have reduced numbers of germinal center (GC) B cells and follicular dendritic cells (FDCs) in lymphoid tissues, which impair antibody responses to multiple T-cell-dependent antigens, including infectious virus, viral proteins, and conjugated haptens. These effects are not due to enhanced apoptosis or impaired proliferation of B cells but instead correlate with changes in lymphoid follicle structure and GC B cell dispersal and are mediated by CD137 signaling in CD4⁺ and CD8⁺ T cells. Our experiments in mice suggest that agonistic anti-CD137 mAbs used in cancer and autoimmunity therapy may impair long-term antibody and B cell memory responses.

Anti-CD137 antibody impairs B cell responses during chikungunya virus infection

Anti-CD137 antibody impairs T-cell-dependent antibody responses to subunit vaccines

Anti-CD137 antibody alters lymphoid follicle structure during virus infection

Enhanced CD137 signaling in T cells results in defects in germinal B cell responses

The geography of memory B cell reactivation in vaccine-induced immunity and in autoimmune disease relapses

Memory B cells (Bmem) provide an active second layer of defense against re-infection by pathogens that have bypassed the passive first layer provided by neutralizing antibodies. Here, we review recent progress in our understanding of Bmem heterogeneity in terms of their origin (germinal center-dependent vs center-independent), phenotype (canonical vs atypical vs age-associated B cells), trafficking (recirculating vs tissue-resident), and fate (plasma cell vs germinal center differentiation). The development of transgenic models and intravital imaging technologies has made it possible to track the cellular dynamics of Bmem reactivation by antigen, their interactions with follicular memory T cells, and differentiation into plasma cells in subcapsular proliferative foci in the lymph nodes of immune animals. Such in situ studies have reinforced the importance of geography in shaping the outcome of the secondary antibody response. We also review the evidence for Bmem reactivation and differentiation into short-lived plasma cells in the pathogenesis of disease flares in relapsing-remitting autoimmune diseases. Elucidating the mechanisms that control the Bmem fate decision to differentiate into plasma cells or germinal center B cells will aid future efforts to more precisely engineer fit-for-purpose vaccines as well as to treat antibody-mediated autoimmune diseases.

Structure-Guided Molecular Grafting of a Complex Broadly Neutralizing Viral Epitope

Antigenic variation and viral evolution have thwarted traditional influenza vaccination strategies. The broad protection afforded by a "universal" influenza vaccine may come from immunogens that elicit humoral immune responses targeting conserved epitopes on the viral hemagglutinin (HA), such as the receptor-binding site (RBS). Here, we engineered candidate immunogens that use noncirculating, avian influenza HAs as molecular scaffolds to present the broadly neutralizing RBS epitope from historical, circulating H1 influenzas. These "resurfaced" HAs (rsHAs) remove epitopes potentially targeted by strain-specific responses in immune-experienced individuals. Through structure-guided optimization, we improved two antigenically different scaffolds to bind a diverse panel of pan-H1 and H1/H3 cross-reactive bnAbs with high affinity. Subsequent serological and single germinal center B cell analyses from murine prime-boost immunizations show that the rsHAs are both immunogenic and can augment the quality of elicited RBS-directed antibodies. Our structure-guided, RBS grafting approach provides candidate immunogens for selectively presenting a conserved viral epitope.

The Transcription Factor T-bet Resolves Memory B Cell Subsets with Distinct Tissue Distributions and Antibody Specificities in Mice and Humans

B cell subsets expressing the transcription factor T-bet are associated with humoral immune responses and autoimmunity. Here, we examined the anatomic distribution, clonal relationships, and functional properties of T-bet⁺ and T-bet^- memory B cells (MBCs) in the context of the influenza-specific immune response. In mice, both T-bet^- and T-bet⁺ hemagglutinin (HA)-specific B cells arose in germinal centers, acquired memory B cell markers, and persisted indefinitely. Lineage tracing and IgH repertoire analyses revealed minimal interconversion between T-bet^- and T-bet⁺ MBCs, and parabionts showed differential tissue residency and recirculation properties. T-bet⁺ MBCs could be subdivided into recirculating T-bet^lo MBCs and spleen-resident T-bet^hi MBCs. Human MBCs displayed similar features. Conditional gene deletion studies revealed that T-bet expression in B cells was required for nearly all HA stalk-specific IgG2c antibodies and for durable neutralizing titers to influenza. Thus, T-bet expression distinguishes MBC subsets that have profoundly different homing, residency, and functional properties, and mediate distinct aspects of humoral immune memory.