Protective human antibodies against a conserved epitope in pre- and postfusion influenza hemagglutinin

Phylogenetically and antigenically distinct influenza A and B viruses (IAV and IBV) circulate in human populations, causing widespread morbidity. Antibodies (Abs) that bind epitopes conserved in both IAV and IBV hemagglutinins (HAs) could protect against disease by diverse virus subtypes. Only one reported HA Ab, isolated from a combinatorial display library, protects against both IAV and IBV. Thus, there has been so far no information on the likelihood of finding naturally occurring human Abs that bind HAs of diverse IAV subtypes and IBV lineages. We have now recovered from several unrelated human donors five clonal Abs that bind a conserved epitope preferentially exposed in the postfusion conformation of IAV and IVB HA2. These Abs lack neutralizing activity in vitro but in mice provide strong, IgG subtype-dependent protection against lethal IAV and IBV infections. Strategies to elicit similar Abs routinely might contribute to more effective influenza vaccines.

A broad antibody class engages the influenza virus hemagglutinin head at its stem interface

Influenza infection and vaccination impart strain-specific immunity that fails to protect against both seasonal antigenic variants and the next pandemic. However, antibodies directed to conserved sites can confer broad protection. We identify and characterize a class of human antibodies that engage a previously undescribed, conserved, epitope on the influenza hemagglutinin protein (HA). Prototype antibody S8V1-157 binds at the normally occluded interface between the HA head and stem. Antibodies to this HA head-stem interface epitope are non-neutralizing in vitro but protect against lethal infection in mice. Their breadth of binding extends across most influenza A serotypes and seasonal human variants. Antibodies to the head-stem interface epitope are present at low frequency in the memory B cell populations of multiple donors. The immunogenicity of the epitope warrants its consideration for inclusion in improved or "universal" influenza vaccines.

A new class of antibodies that overcomes a steric barrier to cross-group neutralization of influenza viruses

Antibody titers that inhibit the influenza virus hemagglutinin (HA) from engaging its receptor are the accepted correlate of protection from infection. Many potent antibodies with broad, intra-subtype specificity bind HA at the receptor binding site (RBS). One barrier to broad H1-H3 cross-subtype neutralization is an insertion (133a) between positions 133 and 134 on the rim of the H1 HA RBS. We describe here a class of antibodies that overcomes this barrier. These genetically unrestricted antibodies are abundant in the human B cell memory compartment. Analysis of the affinities of selected members of this class for historical H1 and H3 isolates suggest that they were elicited by H3 exposure and broadened or diverted by later exposure(s) to H1 HA. RBS mutations in egg-adapted vaccine strains cause the new H1 specificity of these antibodies to depend on the egg adaptation. The results suggest that suitable immunogens might elicit 133a-independent, H1-H3 cross neutralization by RBS-directed antibodies.

A non-neutralizing glycoprotein B monoclonal antibody protects against herpes simplex virus disease in mice

There is an unmet need for monoclonal antibodies (mAbs) for prevention or as adjunctive treatment of herpes simplex virus (HSV) disease. Most vaccine and mAb efforts focus on neutralizing antibodies, but for HSV this strategy has proven ineffective. Preclinical studies with a candidate HSV vaccine strain, ΔgD-2, demonstrated that non-neutralizing antibodies that activate Fcγ receptors (FcγRs) to mediate antibody-dependent cellular cytotoxicity (ADCC) provide active and passive protection against HSV-1 and HSV-2. We hypothesized that this vaccine provides a tool to identify and characterize protective mAbs. We isolated HSV-specific mAbs from germinal center and memory B cells and bone marrow plasmacytes of ΔgD-2-vaccinated mice and evaluated these mAbs for binding, neutralizing, and FcγR-activating activity and for protective efficacy in mice. The most potent protective mAb, BMPC-23, was not neutralizing but activated murine FcγRIV, a biomarker of ADCC. The cryo-electron microscopic structure of the Fab-glycoprotein B (gB) assembly identified domain IV of gB as the epitope. A single dose of BMPC-23 administered 24 hours before or after viral challenge provided significant protection when configured as mouse IgG2c and protected mice expressing human FcγRIII when engineered as a human IgG1. These results highlight the importance of FcR-activating antibodies in protecting against HSV.

Infant Antibody Repertoires during the First Two Years of Influenza Vaccination

The first encounter with influenza virus biases later immune responses. This "immune imprinting," formerly from infection within a few years of birth, is in the United States now largely from immunization with a quadrivalent, split vaccine (IIV4 [quadrivalent inactivated influenza vaccine]). In a pilot study of IIV4 imprinting, we used single-cell cultures, next-generation sequencing, and plasma antibody proteomics to characterize the primary antibody responses to influenza in two infants during their first 2 years of seasonal influenza vaccination. One infant, who received only a single vaccination in year 1, contracted an influenza B virus (IBV) infection between the 2 years, allowing us to compare imprinting by infection and vaccination. That infant had a shift in hemagglutinin (HA)-reactive B cell specificity from largely influenza A virus (IAV) specific in year 1 to IBV specific in year 2, both before and after the year 2 vaccination. HA-reactive B cells from the other infant maintained a more evenly distributed specificity. In year 2, class-switched HA-specific B cell IGHV somatic hypermutation (SHM) levels reached the average levels seen in adults. The HA-reactive plasma antibody repertoires of both infants comprised a relatively small number of antibody clonotypes, with one or two very abundant clonotypes. Thus, after the year 2 boost, both infants had overall B cell profiles that resembled those of adult controls. IMPORTANCE Influenza virus is a moving target for the immune system. Variants emerge that escape protection from antibodies elicited by a previously circulating variant ("antigenic drift"). The immune system usually responds to a drifted influenza virus by mutating existing antibodies rather than by producing entirely new ones. Thus, immune memory of the earliest influenza virus exposure has a major influence on later responses to infection or vaccination ("immune imprinting"). In the many studies of influenza immunity in adult subjects, imprinting has been from an early infection, since only in the past 2 decades have infants received influenza immunizations. The work reported in this paper is a pilot study of imprinting by the flu vaccine in two infants, who received the vaccine before experiencing an influenza virus infection. The results suggest that a quadrivalent (four-subtype) vaccine may provide an immune imprint less dominated by one subtype than does a monovalent infection.

An epitope-enriched immunogen expands responses to a conserved viral site

Pathogens evade host humoral responses by accumulating mutations in surface antigens. While variable, there are conserved regions that cannot mutate without compromising fitness. Antibodies targeting these conserved epitopes are often broadly protective but remain minor components of the repertoire. Rational immunogen design leverages a structural understanding of viral antigens to modulate humoral responses to favor these responses. Here, we report an epitope-enriched immunogen presenting a higher copy number of the influenza hemagglutinin (HA) receptor-binding site (RBS) epitope relative to other B cell epitopes. Immunization in a partially humanized murine model imprinted with an H1 influenza shows H1-specific serum and >99% H1-specific B cells being RBS-directed. Single B cell analyses show a genetically restricted response that structural analysis defines as RBS-directed antibodies engaging the RBS with germline-encoded contacts. These data show how epitope enrichment expands B cell responses toward conserved epitopes and advances immunogen design approaches for next-generation viral vaccines.

Recall of B cell memory depends on relative locations of prime and boost immunization

Immunization or microbial infection can establish long-term B cell memory not only systemically but also locally. Evidence has suggested that local B cell memory contributes to early local plasmacytic responses after secondary challenge. However, it is unclear whether locality of immunization plays any role in memory B cell participation in recall germinal centers (GCs), which is essential for updating their B cell antigen receptors (BCRs). Using single B cell culture and fate mapping, we have characterized BCR repertoires in recall GCs after boost immunizations at sites local or distal to the priming. Local boosts with homologous antigen recruit the progeny of primary GC B cells to recall GCs more efficiently than do distal boosts. Recall GCs elicited by local boosts contain significantly more B cells with elevated levels of immunoglobulin (Ig) mutation and higher avidity BCRs. This local preference is unaffected by blocking CD40:CD154 interaction to terminate active, GC responses. Local boosts with heterologous antigens elicit secondary GCs with B cell populations enriched for cross-reactivity to the prime and boost antigens; in contrast, cross-reactive GC B cells are rare after distal boosts. Our results suggest that local B cell memory is retained in the form of memory B cells, GC B cells, and GC phenotype B cells that are independent of organized GC structures and that these persistent "primed B cells" contribute to recall GC responses at local sites. Our findings indicate the importance of locality in humoral immunity and inform serial vaccination strategies for evolving viruses.

Tracing Self-Reactive B Cells in Normal Mice

BCR transgenic mice dominate studies of B cell tolerance; consequently, tolerance in normal mice expressing diverse sets of autoreactive B cells is poorly characterized. We have used single B cell cultures to trace self-reactivity in BCR repertoires across the first and second tolerance checkpoints and in tolerized B cell compartments of normal mice. This approach reveals affinity "setpoints" that define each checkpoint and a subset of tolerized, autoreactive B cells that is long-lived. In normal mice, the numbers of B cells avidly specific for DNA fall significantly as small pre-B become immature and transitional-1 B cells, revealing the first tolerance checkpoint. By contrast, DNA reactivity does not significantly change when immature and transitional-1 B cells become mature follicular B cells, showing that the second checkpoint does not reduce DNA reactivity. In the spleen, autoreactivity was high in transitional-3 (T3) B cells, CD93⁺IgM^-/loIgD^hi anergic B cells, and a CD93^- anergic subset. Whereas splenic T3 and CD93⁺ anergic B cells are short-lived, CD93^-IgM^-/loIgD^hi B cells have half-lives comparable to mature follicular B cells. B cell-specific deletion of proapoptotic genes, Bak and Bax, resulted in increased CD93^-IgM^-/loIgD^hi B cell numbers but not T3 B cell numbers, suggesting that apoptosis regulates differently persistent and ephemeral autoreactive B cells. The self-reactivity and longevity of CD93^-IgM^-/loIgD^hi B cells and their capacity to proliferate and differentiate into plasmacytes in response to CD40 activation in vitro lead us to propose that this persistent, self-reactive compartment may be the origin of systemic autoimmunity and a potential target for vaccines to elicit protective Abs cross-reactive with self-antigens.

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.

Hide and seek: interplay between influenza viruses and B cells

Influenza virus constantly acquires genetic mutations/reassortment in the major surface protein, hemagglutinin (HA), resulting in the generation of strains with antigenic variations. There are, however, HA epitopes that are conserved across influenza viruses and are targeted by broadly protective antibodies. A goal for the next-generation influenza vaccines is to stimulate B-cell responses against such conserved epitopes in order to provide broad protection against divergent influenza viruses. Broadly protective B cells, however, are not easily activated by HA antigens with native structure, because the virus has multiple strategies to escape from the humoral immune responses directed to the conserved epitopes. One such strategy is to hide the conserved epitopes from the B-cell surveillance by steric hindrance. Technical advancement in the analysis of the human B-cell antigen receptor (BCR) repertoire has dissected the BCRs to HA epitopes that are hidden in the native structure but are targeted by broadly protective antibodies. We describe here the characterization and function of broadly protective antibodies and strategies that enable B cells to seek these hidden epitopes, with potential implications for the development of universal influenza vaccines.

Immune checkpoint modulation enhances HIV-1 antibody induction

Eliciting protective titers of HIV-1 broadly neutralizing antibodies (bnAbs) is a goal of HIV-1 vaccine development, but current vaccine strategies have yet to induce bnAbs in humans. Many bnAbs isolated from HIV-1-infected individuals are encoded by immunoglobulin gene rearrangments with infrequent naive B cell precursors and with unusual genetic features that may be subject to host regulatory control. Here, we administer antibodies targeting immune cell regulatory receptors CTLA-4, PD-1 or OX40 along with HIV envelope (Env) vaccines to rhesus macaques and bnAb immunoglobulin knock-in (KI) mice expressing diverse precursors of CD4 binding site HIV-1 bnAbs. CTLA-4 blockade augments HIV-1 Env antibody responses in macaques, and in a bnAb-precursor mouse model, CTLA-4 blocking or OX40 agonist antibodies increase germinal center B and T follicular helper cells and plasma neutralizing antibodies. Thus, modulation of CTLA-4 or OX40 immune checkpoints during vaccination can promote germinal center activity and enhance HIV-1 Env antibody responses.

Elucidation of broadly neutralizing antibodies (bnAb) is a goal in HIV vaccine development. Here, Bradley et al. show that administration of CTLA-4 blocking antibody with vaccine antigens increases HIV-1 envelope antibody responses in macaques and a bnAb precursor mouse model.

Cross-Reactivity to Kynureninase Tolerizes B Cells That Express the HIV-1 Broadly Neutralizing Antibody 2F5

2F5 is an HIV-1 broadly neutralizing Ab that also binds the autoantigens kynureninase (KYNU) and anionic lipids. Generation of 2F5-like Abs is proscribed by immune tolerance, but it is unclear which autospecificity is responsible. We sampled the BCR repertoire of 2F5 knock-in mice before and after the first and second tolerance checkpoints. Nearly all small pre-B (precheckpoint) and 35-70% of anergic peripheral B cells (postcheckpoint) expressed the 2F5 BCR and maintained KYNU, lipid, and HIV-1 gp41 reactivity. In contrast, all postcheckpoint mature follicular (MF) B cells had undergone L chain editing that purged KYNU and gp41 binding but left lipid reactivity largely intact. We conclude that specificity for KYNU is the primary driver of tolerization of 2F5-expressing B cells. The MF and anergic B cell populations favored distinct collections of editor L chains; surprisingly, however, MF and anergic B cells also frequently expressed identical BCRs. These results imply that BCR autoreactivity is the primary determinant of whether a developing B cell enters the MF or anergic compartments, with a secondary role for stochastic factors that slightly mix the two pools. Our study provides mechanistic insights into how immunological tolerance impairs humoral responses to HIV-1 and supports activation of anergic B cells as a potential method for HIV-1 vaccination.

Immune-Focusing Properties of Virus-like Particles Improve Protective IgA Responses

Virus-like particles (VLPs) provide a well-established vaccine platform; however, the immunogenic properties acquired by VLP structure remain poorly understood. In this study, we showed that systemic vaccination with norovirus VLP recalls human IgA responses at higher magnitudes than IgG responses under a humanized mouse model that was established by introducing human PBMCs in severely immunodeficient mice. The recall responses elicited by VLP vaccines depended on VLP structure and the disruption of VLP attenuated recall responses, with a more profound reduction being observed in IgA responses. The IgA-focusing property was also conserved in a murine norovirus-primed model under which murine IgA responses were recalled in a manner dependent on VLP structure. Importantly, the VLP-driven IgA response preferentially targeted virus-neutralizing epitopes located in the receptor-binding domain. Consequently, VLP-driven IgA responses were qualitatively superior to IgG responses in terms of the virus-neutralizing activity in vitro. Furthermore, the IgA in mucosa obtained remarkable protective function toward orally administrated virus in vivo. Thus, our results indicate the immune-focusing properties of the VLP vaccine that improve the quality/quantity of mucosal IgA responses, a finding with important implications for developing mucosal vaccines.

Minding the gap: The impact of B-cell tolerance on the microbial antibody repertoire

B lymphocytes must respond to vast numbers of foreign antigens, including those of microbial pathogens. To do so, developing B cells use combinatorial joining of V-, D-, and J-gene segments to generate an extraordinarily diverse repertoire of B-cell antigen receptors (BCRs). Unsurprisingly, a large fraction of this initial BCR repertoire reacts to self-antigens, and these "forbidden" B cells are culled by immunological tolerance from mature B-cell populations. While culling of autoreactive BCRs mitigates the risk of autoimmunity, it also opens gaps in the BCR repertoire, which are exploited by pathogens that mimic the forbidden self-epitopes. Consequently, immunological tolerance, necessary for averting autoimmune disease, also acts to limit effective microbial immunity. In this brief review, we recount the evidence for the linkage of tolerance and impaired microbial immunity, consider the implications of this linkage for vaccine development, and discuss modulating tolerance as a potential strategy for strengthening humoral immune responses.

Influenza Antigen Engineering Focuses Immune Responses to a Subdominant but Broadly Protective Viral Epitope

Viral glycoproteins are under constant immune surveillance by a host's adaptive immune responses. Antigenic variation including glycan introduction or removal is among the mechanisms viruses have evolved to escape host immunity. Understanding how glycosylation affects immunodominance on complex protein antigens may help decipher underlying B cell biology. To determine how B cell responses can be altered by such modifications, we engineered glycans onto the influenza virus hemagglutinin (HA) and characterized the molecular features of the elicited humoral immunity in mice. We found that glycan addition changed the initially diverse antibody repertoire into an epitope-focused, genetically restricted response. Structural analyses showed that one antibody gene family targeted a previously subdominant, occluded epitope at the head interface. Passive transfer of this antibody conferred Fc-dependent protection to influenza virus-challenged mice. These results have potential implications for next-generation viral vaccines aimed at directing B cell responses to preferred epitope(s).

Antibodies to a Conserved Influenza Head Interface Epitope Protect by an IgG Subtype-Dependent Mechanism

Vaccines to generate durable humoral immunity against antigenically evolving pathogens such as the influenza virus must elicit antibodies that recognize conserved epitopes. Analysis of single memory B cells from immunized human donors has led us to characterize a previously unrecognized epitope of influenza hemagglutinin (HA) that is immunogenic in humans and conserved among influenza subtypes. Structures show that an unrelated antibody from a participant in an experimental infection protocol recognized the epitope as well. IgGs specific for this antigenic determinant do not block viral infection in vitro, but passive administration to mice affords robust IgG subtype-dependent protection against influenza infection. The epitope, occluded in the pre-fusion form of HA, is at the contact surface between HA head domains; reversible molecular "breathing" of the HA trimer can expose the interface to antibody and B cells. Antigens that present this broadly immunogenic HA epitope may be good candidates for inclusion in "universal" flu vaccines.

Self-tolerance curtails the B cell repertoire to microbial epitopes

Immunological tolerance removes or inactivates self-reactive B cells, including those that also recognize cross-reactive foreign antigens. Whereas a few microbial pathogens exploit these "holes" in the B cell repertoire by mimicking host antigens to evade immune surveillance, the extent to which tolerance reduces the B cell repertoire to foreign antigens is unknown. Here, we use single-cell cultures to determine the repertoires of human B cell antigen receptors (BCRs) before (transitional B cells) and after (mature B cells) the second B cell tolerance checkpoint in both healthy donors and in patients with systemic lupus erythematosus (SLE) . In healthy donors, the majority (~70%) of transitional B cells that recognize foreign antigens also bind human self-antigens (foreign+self), and peripheral tolerance halves the frequency of foreign+self-reactive mature B cells. In contrast, in SLE patients who are defective in the second tolerance checkpoint, frequencies of foreign+self-reactive B cells remain unchanged during maturation of transitional to mature B cells. Patterns of foreign+self-reactivity among mature B cells from healthy donors differ from those of SLE patients. We propose that immune tolerance significantly reduces the scope of the BCR repertoire to microbial pathogens and that cross-reactivity between foreign and self epitopes may be more common than previously appreciated.

Germinal center responses to complex antigens

Germinal centers (GCs) are the primary sites of antibody affinity maturation, sites where B-cell antigen-receptor (BCR) genes rapidly acquire mutations and are selected for increasing affinity for antigen. This process of hypermutation and affinity-driven selection results in the clonal expansion of B cells expressing mutated BCRs and acts to hone the antibody repertoire for greater avidity and specificity. Remarkably, whereas the process of affinity maturation has been confirmed in a number of laboratories, models for how affinity maturation in GCs operates are largely from studies of genetically restricted B-cell populations competing for a single hapten epitope. Much less is known about GC responses to complex antigens, which involve both inter- and intraclonal competition for many epitopes. In this review, we (i) compare current methods for analysis of the GC B-cell repertoire, (ii) describe recent studies of GC population dynamics in response to complex antigens, discussing how the observed repertoire changes support or depart from the standard model of clonal selection, and (iii) speculate on the nature and potential importance of the large fraction of GC B cells that do not appear to interact with native antigen.

Tracing self-reactive B cells in normal mice

Self-tolerance removes or inactivates autoreactive B cells in primary and secondary lymphoid organs. Studies have used transgenic mouse models or characterizing recombinant antibodies from single B cells to study B-cell tolerance. These approaches are limited, however, by the restricted diversity of transgenic B-cell repertoires or the effort necessary to generate significant numbers of recombinant antibodies. To overcome these difficulties, we used a single-cell culture approach, which supports high-throughput analysis of the B-cell repertoire in pre-tolerance, post-tolerance, and tolerizing anergic B-cell compartments. This approach revealed the affinity “setpoints” of the tolerance checkpoints, the stochastic nature of B-cell tolerance, and subsets of autoreactive B cells that are persistent in periphery. We propose that these anergic compartments may be the origin of systemic autoimmunity and a potential target for HIV-1 vaccines that elicit neutralizing antibodies cross-reactive with self-antigens.

Germinal center entry not selection of B cells is controlled by peptide-MHCII complex density

B cells expressing high affinity antigen receptors are advantaged in germinal centers (GC), perhaps by increased acquisition of antigen for presentation to follicular helper T cells and improved T-cell help. In this model for affinity-dependent selection, the density of peptide/MHCII (pMHCII) complexes on GC B cells is the primary determinant of selection. Here we show in chimeric mice populated by B cells differing only in their capacity to express MHCII (MHCII^+/+ and MHCII^+/-) that GC selection is insensitive to halving pMHCII density. Alone, both B cell types generate identical humoral responses; in competition, MHCII^+/+ B cells are preferentially recruited to early GCs but this advantage does not persist once GCs are established. During GC responses, competing MHCII^+/+ and MHCII^+/- GC B cells comparably accumulate mutations and have indistinguishable rates of affinity maturation. We conclude that B-cell selection by pMHCII density is stringent in the establishment of GCs, but relaxed during GC responses.

BCR and Endosomal TLR Signals Synergize to Increase AID Expression and Establish Central B Cell Tolerance

Activation-induced cytidine deaminase (AID) is required to purge autoreactive immature and transitional-1 (immature/T1) B cells at the first tolerance checkpoint, but how AID selectively removes self-reactive B cells is unclear. We now show that B cell antigen receptor (BCR) and endosomal Toll-like receptor (TLR) signals synergize to elicit high levels of AID expression in immature/T1 B cells. This synergy is restricted to ligands for endocytic TLR and requires phospholipase-D activation, endosomal acidification, and MyD88. The first checkpoint is significantly impaired in AID- or MyD88-deficient mice and in mice doubly heterozygous for AID and MyD88, suggesting interaction of these factors in central B cell tolerance. Moreover, administration of chloroquine, an inhibitor of endosomal acidification, results in a failure to remove autoreactive immature/T1 B cells in mice. We propose that a BCR/TLR pathway coordinately establishes central tolerance by hyper-activating AID in immature/T1 B cells that bind ligands for endosomal TLRs.

The Limit of MHC Class II-driven Selection in Germinal Centers

B cells undergo iterative rounds of proliferation, hypermutation and selection in the germinal centers (GCs) to generate high affinity, protective antibody. It is now thought that GC B cells with higher affinity BCRs acquire more antigen for processing and presenting in association with MHC class II (MHCII) to T follicular helper (TFH) cells than lower affinity competitors. The resulting increase in T-cell help is hypothesized to be the major factor that drives affinity maturation. In this model for GC B cell selection, the density of peptide/MHCII (pMHCII) complexes available on the B-cell membrane is the major determinant of affinity-dependent selection. To quantify the limits of this model for GC selection, we generated short- and long-term chimeric mice with equal numbers of isogenic, mature B cells that differed by their capacity to express MHCII (MHCII+/+ and MHCII+/−). Separately, both B cell types generated virtually identical humoral responses, including affinity maturation, in response to NP-OVA. In competition, MHCII+/+ B cells were preferentially recruited to early GCs consistent with a critical role for pMHCII density in T:B collaboration. However, once established in GCs, no further competitive advantage was observed during the full course (24 days) of the GC response. During this period, both MHCII+/+ and MHCII+/− GC B cells comparably accumulated VH gene segment mutations and exhibited comparable rates of affinity maturation to the NP hapten. These data indicate that B-cell selection determined by MHCII density is more stringent prior to the establishment of GCs than in the GC itself. This observation may be relevant to our recent observation of the persistence of low affinity GC B cells.

Immunodominance of Antibody Recognition of the HIV Envelope V2 Region in Ig-Humanized Mice

In the RV144 gp120 HIV vaccine trial, decreased transmission risk was correlated with Abs that reacted with a linear epitope at a lysine residue at position 169 (K169) in the HIV-1 envelope (Env) V2 region. The K169 V2 response was restricted to Abs bearing Vλ rearrangements that expressed aspartic acid/glutamic acid in CDR L2. The AE.A244 gp120 in AIDSVAX B/E also bound to the unmutated ancestor of a V2-glycan broadly neutralizing Ab, but this Ab type was not induced in the RV144 trial. In this study, we sought to determine whether immunodominance of the V2 linear epitope could be overcome in the absence of human Vλ rearrangements. We immunized IgH- and Igκ-humanized mice with the AE.A244 gp120 Env. In these mice, the V2 Ab response was focused on a linear epitope that did not include K169. V2 Abs were isolated that used the same human VH gene segment as an RV144 V2 Ab but paired with a mouse λ L chain. Structural characterization of one of these V2 Abs revealed how the linear V2 epitope could be engaged, despite the lack of aspartic acid/glutamic acid encoded in the mouse repertoire. Thus, despite the absence of the human Vλ locus in these humanized mice, the dominance of Vλ pairing with human VH for HIV-1 Env V2 recognition resulted in human VH pairing with mouse λ L chains instead of allowing otherwise subdominant V2-glycan broadly neutralizing Abs to develop.

Efficient Culture of Human Naive and Memory B Cells for Use as APCs

The ability to culture and expand B cells in vitro has become a useful tool for studying human immunity. A limitation of current methods for human B cell culture is the capacity to support mature B cell proliferation. We developed a culture method to support the efficient activation and proliferation of naive and memory human B cells. This culture supports extensive B cell proliferation, with ∼10³-fold increases following 8 d in culture and 10⁶-fold increases when cultures are split and cultured for 8 more days. In culture, a significant fraction of naive B cells undergo isotype switching and differentiate into plasmacytes. Culture-derived (CD) B cells are readily cryopreserved and, when recovered, retain their ability to proliferate and differentiate. Significantly, proliferating CD B cells express high levels of MHC class II, CD80, and CD86. CD B cells act as APCs and present alloantigens and microbial Ags to T cells. We are able to activate and expand Ag-specific memory B cells; these cultured cells are highly effective in presenting Ag to T cells. We characterized the TCR repertoire of rare Ag-specific CD4⁺ T cells that proliferated in response to tetanus toxoid (TT) presented by autologous CD B cells. TCR Vβ usage by TT-activated CD4⁺ T cells differs from resting and unspecifically activated CD4⁺ T cells. Moreover, we found that TT-specific TCR Vβ usage by CD4⁺ T cells was substantially different between donors. This culture method provides a platform for studying the BCR and TCR repertoires within a single individual.

TCR Repertoire Analysis of Mouse T Follicular Helper Cells and T Follicular Regulatory Cells Following Immunization

Generation of high-affinity and class-switched antibody requires the germinal center (GC) reaction after infection or immunization. Within the B-cell follicles of secondary lymphoid organs, the GC represents a sophisticated collaboration between antigen-specific B cells, follicular dendritic cells, T follicular helper (TFH) cells and T follicular regulatory (TFREG) cells. Despite intensive interest in the development and effector function of TFH and TFREG cells, little is known regarding the selection of T-cell receptor (TCR) repertoire during polyclonal GC reactions. In order to evaluate native, polyclonal TCR responses elicited by a complex antigen, we developed a sorting strategy to isolate TFH/TFREG cell populations that were activated and expanded after s.c. immunization with NP15-OVA. TCRβ VDJ rearrangements were recovered from highly purified TCRβ+CD4+CXCR5hiPD-1+Bcl-6+FoxP3− TFH cells and TCRβ+CD4+ CXCR5hiPD-1+Bcl-6+FoxP3+ TFREG cells, amplified by PCR, and sequenced. Analysis of the antigen-specific TCR repertoire of TFH/TFREG cells provides important insights into the factors influencing T-cell recruitment and clonal expansion following infection or vaccination, especially when linked to contemporary analysis of the GC B-cell repertoire. These findings may inform rational and selective control strategy of the GC reaction. Vaccine development can accordingly focus on modulating TFH/TFREG responses to facilitate optimal adaptive immune responses.

How Tolerance Shapes the Development of the HIV-1 Broadly Neutralizing Antibody 2F5

An effective HIV-1 vaccine is likely to require induction of broadly neutralizing antibodies (bnAbs), which neutralize multiple HIV-1 strains. Generating HIV-1 bnAbs has been challenging due partly to a number of viral immune evasion mechanisms, including viral molecular mimicry of host in order to exploit immunological tolerance. Indeed, generation of neutralizing antibodies to the 2F5 epitope of HIV-1 gp41 is proscribed by immune tolerance. However, the specific mechanisms by which tolerance relieves the autoreactivity of B cells expressing the VH and VL regions (dKI) or the VH region (sKI) of the 2F5 bnAb are not well understood. To compare the reactivity and repertoire of 2F5 dKI and sKI B cells before and after immune tolerance checkpoints, we used a newly developed in vitro single B cell culture system that induces B cell expansion and differentiation into Ab-secreting plasmacytes. The reactivities of expanded B cell clones were determined by ELISA, while repertoire was determined by recovering the V(D)J rearrangements using RT-PCR and Sanger sequencing. In 2F5 dKI mice, the great majority of cells in the small-pre-B compartment express the knock in heavy- and light-chains, and maintain HIV-1 gp41-reactivity. However, 2F5 dKI B cells that survive tolerance checkpoints to become mature B cells have undergone extensive light-chain editing and are purged of gp41- and KYNU-reactivity. Importantly, the peripheral 2F5 dKI IgM−IgD+ (anergic) B cell subset retains a substantial fraction of HIV-1- and KYNU-reactive cells. In conclusion, our study provides mechanistic insights into how immunological tolerance impairs humoral responses to HIV-1, and suggests activation of anergic B cells as a potential target for HIV-1 vaccination.

Non-specific activation of autoreactive B cells after anthrax vaccination delays protection

Anthrax Vaccine Adsorbed (AVA) vaccination protects against anthrax infection by eliciting a neutralizing antibody response. However, protective antibody titers are not observed in human sera until at least three vaccinations have been administered over six months. Even then, humoral responses to AVA do not provide long-term immunity without an annual booster, presenting a serious challenge in public health.

We followed six healthy volunteers over the standard five-dose, 18-month AVA schedule to characterize immunoglobulin genetics during vaccination. Plasmablasts were isolated one week after each vaccination and natively-paired heavy- and light-chain immunoglobulin mRNA were sequenced. Immunoglobulin sequences were analyzed for mutations and clonal evolution.

After primary vaccination, the average mutation frequency of observed plasmablast lineages indicated they were derived from previously affinity matured B cells (6.1–7.7%). Many sequences had very high mutation frequencies (>10%), a characteristic that has been linked to multiple antigen exposures. We synthesized antibodies from these lineages and tested them against a panel of human antigens. While they had little to no affinity to anthrax antigens, they displayed significant affinity to a variety of autoantigens. These memory lineages were non-persistent, and we observed naïve B cell activation after secondary vaccination. We tested antibodies from naïve-derived lineages first observed after secondary vaccination and found these to be anthrax-specific and non-autoreactive. Thus, while primary AVA vaccination elicits a robust memory response, it is non-protective and delays anthrax-specific naïve B cell recruitment until secondary vaccination.

B-Cell Selection in Germinal Centers Elicited by Complex Antigens

Germinal center (GC) B cells evolve towards increased affinity by a Darwinian process studied intensively in genetically restricted, hapten-specific responses. While experimentally tractable, genetically restricted humoral responses are atypical as antibodies to complex protein antigens represent genetically diverse, polyclonal humoral responses driven by various epitopes arrayed across the antigen. We have developed a single B-cell culture method that supports the proliferation and plasmacytic differentiation of mature and GC B cells. With this tool, we explore the population dynamics of genetically diverse GC responses to two complex antigens – protective antigen of Bacillus anthracis and influenza hemagglutinin – in which B cells compete both intra- and interclonally for distinct epitopes. Our characterizations begin with antigen-binding, mature naïve B cells and follow clonal selection and affinity maturation in GCs. Preferred VH rearrangements among antigen-binding, naïve B cells were similarly abundant in early GCs but, unlike restricted responses to haptens, clonal diversity increased in GC B cells as early “winners” were replaced by rarer, high affinity clones. Despite affinity maturation, half of GC B cells did not detectably bind immunogen but exhibited genetic selection comparable to antigen-binding cells, as determined by VH usage, mutations, and clonal expansion. In GCs elicited by rPA or rHA, interclonal BCR avidities can differ 100-fold and intraclonal avidity by as much as 40-fold. We propose that intraclonal selection in GCs is permissive for a wide range of BCR affinities and that lower affinity/less fit GC B cells may remain viable in GCs for substantially longer periods than generally thought.

Complex Antigens Drive Permissive Clonal Selection in Germinal Centers

Germinal center (GC) B cells evolve toward increased affinity by a Darwinian process that has been studied primarily in genetically restricted, hapten-specific responses. We explored the population dynamics of genetically diverse GC responses to two complex antigens-Bacillus anthracis protective antigen and influenza hemagglutinin-in which B cells competed both intra- and interclonally for distinct epitopes. Preferred VH rearrangements among antigen-binding, naive B cells were similarly abundant in early GCs but, unlike responses to haptens, clonal diversity increased in GC B cells as early "winners" were replaced by rarer, high-affinity clones. Despite affinity maturation, inter- and intraclonal avidities varied greatly, and half of GC B cells did not bind the immunogen but nonetheless exhibited biased VH use, V(D)J mutation, and clonal expansion comparable to antigen-binding cells. GC reactions to complex antigens permit a range of specificities and affinities, with potential advantages for broad protection.

TSC1 Promotes B Cell Maturation but Is Dispensable for Germinal Center Formation

Accumulating evidence indicates that the tuberous sclerosis complex 1 (TSC1), a tumor suppressor that acts by inhibiting mTOR signaling, plays an important role in the immune system. We report here that TSC1 differentially regulates mTOR complex 1 (mTORC1) and mTORC2/Akt signaling in B cells. TSC1 deficiency results in the accumulation of transitional-1 (T1) B cells and progressive losses of B cells as they mature beyond the T1 stage. Moreover, TSC1KO mice exhibit a mild defect in the serum antibody responses or rate of Ig class-switch recombination after immunization with a T-cell-dependent antigen. In contrast to a previous report, we demonstrate that both constitutive Peyer’s patch germinal centers (GCs) and immunization-induced splenic GCs are unimpaired in TSC1-deficient (TSC1KO) mice and that the ratio of GC B cells to total B cells is comparable in WT and TSC1KO mice. Together, our data demonstrate that TSC1 plays important roles for B cell development, but it is dispensable for GC formation and serum antibody responses.

Efficient culture of human naïve and memory B cells (LYM5P.711)

The ability to culture and expand B cell numbers in vitro has become a useful tool for studying human immunity. A limitation of current methods in human B cell cultures is the capacity to support continued proliferation by mature B cells. We have adapted the culture methods established by Luo et al. (Blood, 2009) to efficiently (60% cloning efficiency) activate and expand both naïve and memory human B cells. Briefly, naïve or memory B cells recovered from blood are cultured with cytokines on CD154+ feeder cells; this culture supports extensive B cell proliferation, with approximately 103 fold increases in a week; by splitting cultures onto new feeder layers, culture-derived (CD) B cells continuously proliferate, achieving 106 fold increases by day 16. The capacity for continued proliferation is stable for at least another week. In culture, a significant fraction of naïve B cells upregulate AID expression, undergo isotype switching without V(D)J hypermutation, and terminally differentiate into plasmacytes. CD B cells are readily cryopreserved and retain their ability to proliferate and differentiate. Significantly, proliferating CD B cells express high levels of MHCII, CD80, and CD86. We have examined the APC function of CD B cells and find that they present both allo- and microbial antigens to autologous T cells with comparable efficiency to PBMC. This culture method provides a platform for studying the BCR and TCR repertoires within a single individual.

Identifying the precursor of natural IgM-producing plasma cells (LYM2P.736)

Natural antibody is constitutively present in the serum, and plays an important role in the clearance of cellular debris, the early control of viral and bacterial expansions, and the initiation of T-dependent humoral responses. However, despite widespread recognition of the critical role of natural IgM in the prevention of both infectious and autoimmune diseases, the source of natural antibody long remained obscure. We have found that most serum IgM in resting mice is produced by a discrete population of IgM plasma cells in the bone marrow, a substantial fraction of which are long-lived cells that occupy a survival niche distinct from that required by IgG plasma cells. Natural IgM plasma cells originate from precursors in the peritoneal cavity, as transfer of unfractionated peritoneal washout cells completely restores serum IgM and the specific compartment of bone marrow plasma cells in Rag1-deficient mice. Surprisingly, however, transfer of purified peritoneal B1a, B1b, or B2 cells did not efficiently reconstitute serum IgM, while a distinct peritoneal B-cell population reconstituted both natural IgM and peritoneal B1a cells in Rag1-deficient mice. We propose that natural IgM plasma cells represent an “innate B-cell” population descended from yolk sac progenitors, and that natural IgM plasma cells are not the progeny of B1a cells per se but instead share an earlier common ancestor also present in the peritoneal cavity.

Natural IgM is produced by CD5- plasma cells that occupy a distinct survival niche in bone marrow

Natural IgM is constitutively present in the serum, where it aids in the early control of viral and bacterial expansions. Natural IgM also plays a significant role in the prevention of autoimmune disease by promoting the clearance of cellular debris. Nevertheless, the origins of natural IgM have not been precisely defined. Previous studies focused on the role of CD5(+) B1 cells in the production of natural IgM, but we show in this article that a discrete population of CD5(-) IgM plasmablasts and plasma cells in the bone marrow (BM) produces the majority of serum IgM in resting mice. These Ab-secreting cells (ASC) originate from peritoneal cavity-resident cells, because transfer of peritoneal cells completely restores serum IgM and the specific compartment of BM ASC in Rag1-deficient mice. We show that BM natural IgM ASC arise from a fetal-lineage progenitor that is neither B1a nor B1b, and that this IgM ASC compartment contains a substantial fraction of long-lived plasma cells that do not occupy the IgG plasma cell survival niche in the BM; instead, they are supported by IL-5. In summary, we identified the primary source of natural IgM and showed that these ASC are maintained long-term in a unique survival niche within the BM.

A new pathway for central B cell tolerance: interaction of AID, MyD88, and BCR (IRC3P.467)

Expression of activation-induced cytidine deaminase (AID) in immature and transitional B cells in mice and humans is genetically linked to the first tolerance checkpoint. In the absence of AID, autoreactive immature/transitional-1 (T1) B cells are inefficiently purged and exhibit increased resistant to receptor-induced apoptosis. These substantial effects are surprising given that AID expression in the immature/T1 B cells is only 3% of that in germinal center B cells. We now show that in immature/T1 B cells, but not mature B cells, B-cell antigen receptor and Toll-like receptor (TLR) signaling synergize to elicit high levels of AID expression. This synergy is restricted to intracellular TLR ligands, requires the phospholipase D-dependent compartmentalization of intracellular TLR and activation of the TLR through acidification, and acts to ensure self-tolerance through a process that requires Myd88. In a 3H9 mouse model, we show that suppression of intracellular acidification by administration of chloroquine increased the generation of immature/T1 B cells in proportion to small pre-B cells, suggesting that chloroquine treatment support the survival of autoreactive 3H9 immature/T1 B cells in bone marrow. These findings identify a novel mechanism for central B-cell tolerance and resolve several weaknesses of current models for the first tolerance checkpoint. We suggest that this AID mediated pathway may be the primary mechanism for central B-cell tolerance by deletion.

The origin of natural antibody (LYM6P.764)

Natural antibody is constitutively present in the serum of all mammals. This unique IgM plays an important role in the clearance of cellular debris, the early control of viral and bacterial expansion, and the initiation of T-dependent humoral responses. Despite decades of study, the origins of natural antibody have not been precisely defined. Transfer experiments have shown a close correlation between peritoneal cells, including B1a cells, and natural antibody, but peritoneal B1 cells do not secrete adequate amounts of IgM to support the levels found in mouse serum. Here, we show that a discrete population of IgM plasmacytes in the bone marrow produces all serum IgM in resting mice. These plasmacytes originate from cells in the peritoneal cavity, but not from B1a B cells, and transfer of these progenitors completely restores serum IgM and the specific compartment of bone marrow plasmacytes in Rag1 deficient mice. This plasmacyte compartment contains a substantial fraction of non-dividing, long lived cells that do not occupy the IgG plasmacyte survival niche in the bone marrow but instead depend on IL-5. Genetic analysis of these IgM plasmacytes reveals an unmutated, clonally diverse population strongly enriched for VH11 rearrangements. We propose that these natural IgM plasmacytes represent an “innate B-cell” population descended from yolk sac progenitors, and that natural IgM plasmacytes are not the progeny of B1 B cells but instead share an earlier common ancestor.

Enhanced antibody responses to an HIV-1 membrane-proximal external region antigen in mice reconstituted with cultured lymphocytes

We have shown that the protective HIV-1 Ab, 2F5, avidly reacts with a conserved mammalian self-Ag, kynureninase, and that the development of B cells specific for the 2F5 epitope is constrained by immunological tolerance. These observations suggest that the capacity to mount Ab responses to the 2F5 epitope is mitigated by tolerance, but such capacity may be latent in the pretolerance and/or anergic B cell pools. In this study, we use B cell tetramer reagents to track the frequencies of B cells that recognize the HIV-1 2F5 epitope (SP62): in C57BL/6 mice, SP62-binding transitional B cells are readily identified in bone marrow but are lost during subsequent development. Unsurprisingly then, immunization with SP62 immunogen does not elicit significant humoral responses in normal C57BL/6 mice. Reconstitution of Rag1(null) mice with normal congenic B cells that have matured in vitro restores the capacity to mount significant serum Ab and germinal center responses to this HIV-1 epitope. These B cell cultures are permissive for the development of autoreactive B cells and support the development of SP62-specific B cell compartments normally lost in 2F5 Ab knockin mice. The recovery of humoral responses to the 2F5/SP62 epitope of HIV-1 by reconstitution with B cells containing forbidden, autoreactive clones provides direct evidence that normal C57BL/6 mice latently possess the capacity to generate humoral responses to a conserved, neutralizing HIV-1 epitope.

Metabolic reprogramming is required for antibody production that is suppressed in anergic but exaggerated in chronically BAFF-exposed B cells

B cell activation leads to proliferation and Ab production that can protect from pathogens or promote autoimmunity. Regulation of cell metabolism is essential to support the demands of lymphocyte growth and effector function and may regulate tolerance. In this study, we tested the regulation and role of glucose uptake and metabolism in the proliferation and Ab production of control, anergic, and autoimmune-prone B cells. Control B cells had a balanced increase in lactate production and oxygen consumption following activation, with proportionally increased glucose transporter Glut1 expression and mitochondrial mass upon either LPS or BCR stimulation. This contrasted with metabolic reprogramming of T cells, which had lower glycolytic flux when resting but disproportionately increased this pathway upon activation. Importantly, tolerance greatly affected B cell metabolic reprogramming. Anergic B cells remained metabolically quiescent, with only a modest increase in glycolysis and oxygen consumption with LPS stimulation. B cells chronically stimulated with elevated BAFF, however, rapidly increased glycolysis and Ab production upon stimulation. Induction of glycolysis was critical for Ab production, as glycolytic inhibition with the pyruvate dehydrogenase kinase inhibitor dichloroacetate sharply suppressed B cell proliferation and Ab secretion in vitro and in vivo. Furthermore, B cell-specific deletion of Glut1 led to reduced B cell numbers and impaired Ab production in vivo. Together, these data show that activated B cells require Glut1-dependent metabolic reprogramming to support proliferation and Ab production that is distinct from T cells and that this glycolytic reprogramming is regulated in tolerance.

The human fetal lymphocyte lineage: identification by CD27 and LIN28B expression in B cell progenitors

CD27, a member of the TNFR superfamily, is used to identify human memory B cells. Nonetheless, CD27(+) B cells are present in patients with HIGM1 syndrome who are unable to generate GCs or memory B cells. CD27(+)IgD(+) fetal B cells are present in umbilical cord blood, and CD27 may also be a marker of the human B1-like B cells. To define the origin of naïve CD27(+)IgD(+) human B cells, we studied B cell development in both fetal and adult tissues. In human FL, most CD19(+) cells coexpressed CD10, a marker of human developing B cells. Some CD19(+)CD10(+) B cells expressed CD27, and these fetal CD27(+) cells were present in the pro-B, pre-B, and immature/transitional B cell compartments. Lower frequencies of phenotypically identical cells were also identified in adult BM. CD27(+) pro-B, pre-B, and immature/transitional B cells expressed recombination activating gene-1, terminal deoxynucleotidyl transferase and Vpre-B mRNA comparably to their CD27(-) counterparts. CD27(+) and CD27(-) developing B cells showed similar Ig heavy chain gene usage with low levels of mutations, suggesting that CD27(+) developing B cells are distinct from mutated memory B cells. Despite these similarities, CD27(+) developing B cells differed from CD27(-) developing B cells by their increased expression of LIN28B, a transcription factor associated with the fetal lymphoid lineages of mice. Furthermore, CD27(+) pro-B cells efficiently generated IgM(+)IgD(+) immature/transitional B cells in vitro. Our observations suggest that CD27 expression during B cell development identifies a physiologic state or lineage for human B cell development distinct from the memory B cell compartment.

Central B-cell tolerance mediated by Aicda and Myd88 (P4404)

Elimination of autoreactive B cells and establishment of self-tolerance is crucial for the prevention of autoimmunity, nonetheless, the mechanism(s) of B-cell tolerance are not fully understood. Recently, we and others have reported that activation-induced cytidine deaminase (AID) and myeloid differentiation primary response gene 88 (MyD88) play unexpected roles in establishing central B-cell tolerance in mice and humans. AID expression in bone marrow immature B cells is synergistically elevated by anti-IgM F(ab’)2 and oligo CpG DNA co-stimulation; reaches to near germinal center levels in 24 hours, but the synergistic effect was not observed by anti-IgM and lipopolysaccharide co-stimulation. These results imply that co-ordination between BCR- and intracellular Toll-like receptors/MyD88 signals regulate AID expression in immature B cells. This synergy was not seen in mature follicular B cells. To explore molecular pathway(s) downstream of AID in B-cell tolerance, we analyzed B-cell development in uracil-DNA glycosylase (UNG)-deficient mice. Unlike AID- or MyD88-deficient background, UNG-deficiency did not impair central B-cell tolerance; because UNG-deficiency severely impairs class-switch recombination (CSR) but not somatic hypermutation, our results imply CSR, e.g., “locus suicide recombination”, may not play a major role in central B-cell tolerance.

Natural IgM is produced by B1a cell-derived bone marrow plasma cells that do not share a survival niche with IgG AFC. (P1467)

Natural antibody makes up the majority of serum IgM and provides protection against both infection and autoimmunity. The B1 lineage is required for natural IgM, but the location and identity of the cells that secrete natural IgM are uncertain. B1-8i VHDJH knock-in mice lack virtually all B1a cells, but have normal B2 cell development and support robust splenic B2 populations. The loss of B1a cells is associated with a 10-fold reduction in serum IgM, whereas serum IgG levels are normal. We examined B1-8i mice and found a 10-fold reduction in the number of IgM AFC in the BM, while BM IgG AFC populations were normal. As B1 cells themselves do not secrete substantial amounts of antibody, our observation indicates that the BM is the major source of natural serum IgM, and that BM IgM plasmacytes require the presence of B1a cells. By FACS and histology, we determined that IgM-secreting cells in BM are CD5- CD138high and appear as differentiated plasma cells, not small B1a cells. Examination of BM revealed that while cytoplasmic IgG+ cells preferentially co-localize with eosinophils, cytoplasmic IgM+ cells do not. This localization correlated with an enhanced survival of IgG AFC, but not IgM AFC, from sorted CD138high BM cells cultured with IL-6. We propose that B1a cell-derived BM plasma cells produce natural serum IgM, and that these cells occupy a survival niche distinct from IgG AFC.

An in vitro culture system for studying the human B cell repertoire (P1459)

Defining the human B-cell repertoire is important for understanding human B-cell development and autoimmunity. Currently, re-expression of immunoglobulins from single B cells is the standard method to study human B-cell repertoire; however, this method is labor-intensive and costly as it requires large numbers of recombinant antibodies to be generated by single-cell PCR and expression of recombinant heavy and light chain pairs. We have developed an alternative method to characterize the B-cell repertoire by culture that supports the proliferation and differentiation of human pre-B, immature/transitional, and mature B cells recovered from bone marrow, fetal liver, or peripheral blood. Briefly, single B cells are cultured in IL-2, IL-4, IL-21, and BAFF on CD154+ stromal cells. This culture supports extensive cell proliferation and differentiation into IgG-secreting plasmacytes. The monoclonal IgG antibodies in each cell culture can be readily screened by ELISA, Luminex, or immunofluorescence against endogenous and exogenous antigens. We find that autoantibodies from pre-B cell cultures have higher avidities for self-antigens than do those from immature/transitional and mature B cell cultures, suggesting the loss of high avidity cells at the first tolerance checkpoint. V(D)J rearrangements are easily recovered from selected clones for sequencing and expression. This culture method offers a pathway to determine B-cell repertoires with high throughput and reduced cost.

Regulation of Central B-cell Tolerance by Aicda and Myd88 (176.20)

V(D)J recombination generates a highly diverse repertoire of B-cell antigen receptors crucial for effective humoral immunity. This process also frequently creates autoreactive B cells that are mostly eliminated during B-cell development. Early immature B cells that are polyreactive or bind nuclear antigens are preferentially eliminated in the bone marrow at the pre-B to immature B-cell transition. Residual autoreactive clones are further suppressed in the periphery. Despite years of study, the mechanism(s) of these tolerance checkpoints are not understood. Recently, we and others have shown that expression of activation-induced cytidine deaminase (AID), the product of Aicda gene, by immature B cells plays an unanticipated role in purging self-reactive B cells in mice and humans. We now show that co-activation of immature B cells by anti-IgM F(ab’)2 and CpG DNA synergistically increases AID expression, implying that cross-talk between BCR- and Toll-like receptor 9 (TLR9)-mediated signals may regulate AID expression in immature B cells. By extension, we hypothesized that MyD-88 would also play a crucial role in central B-cell tolerance. Consistent with this idea, mice deficient for Myd88 exhibited reduced capacity to purge autoreactive immature/transitional B cells; the clonal deletion of self-reactive 3H9 VDJ knock-in mice is significantly impaired in the absence of MyD88 expression. We are currently investigating interaction of Aicda and Myd88 in central tolerance in B cells.

B1 B cell-derived bone marrow plasmacytes are the primary source of natural IgM (109.11)

Natural IgM is a unique and important component of humoral immunity. It provides early protection against microbial pathogens and helps to maintain tissue homeostasis. Though B1 cells are known to be the primary source of natural IgM, the developmental stage and anatomical location of the cells that secrete natural IgM antibody remain controversial. In order to answer this question, we first confirmed earlier observations that the spleen and BM are the primary tissues in which IgM secreting cells reside. We next examined two lines of VDJ knockin mice and found that both exhibited 10-fold reductions in serum IgM levels despite the presence of normal IgG concentrations. Whereas knockin animals had normal numbers of IgM secreting cells in the spleen and numbers of splenic and BM IgG plasma cells that were indistinguishable from normal controls, the numbers of IgM plasmacytes in the BM of knockin mice was ≤20% of normal animals. In addition, the vast majority of peritoneal B1 cells were missing in both knockin strains whereas all B2 populations remained unaffected. By sorting BM cells, we determined that only those with the CD138high IgM+ phenotype secreted IgM in vitro; analyses of the two knockin mouse strains revealed this BM population to be significantly reduced in concordance with the peripheral B1 B-cell populations. We conclude that natural IgM antibody is produced by differentiated BM plasmacytes that are derived from peripheral, B1 B-cell populations.

CD27 expression patterns on human developing B cells in fetal and adult tissues (111.27)

CD27 is an accepted marker for memory B cells in humans. However, CD27 expression was recently demonstrated on substantial populations of human B cells in cord blood and fetal liver (FL), tissues that contain few if any memory B lymphocytes. In addition, CD27 expression was recently proposed to identify natural B cells, the human counterpart of murine B-1 B cells. These observations led us to explore the characteristics of human, CD27+ B cells, and particularly developing B cells. Comparing B cells from FL, cord blood, and adult bone marrow (BM), we found CD27 expression as early as the pro-B cell stage. Moreover, the frequency of CD27+ pro-B cells was higher in FL than in BM. Expression of TdT, RAG1 and VpreB in CD27+ pro-B cells was similar to conventional, CD27- pro-B cells, confirming their identity as B-lineage progenitors. Developmentally immature CD27- and CD27+ B cells showed no significant differences in VH gene usage, VH mutation frequencies, or CDRH3 length, excluding the possibility of contamination by memory B cells. Interestingly, when placed in an in vitro culture system optimized to support the maturation of human fetal pro-B cells and murine B-1 B cells, CD27+ pro-B cells differentiated into IgM+ immature and transitional B cells significantly more efficiently than did autochthonous CD27- pro-B cells. In conclusion, we have demonstrated functional CD27+ pro-B cells in FL and BM that may identify a predominantly fetal, B-cell lineage in humans.