The extracellular membrane-proximal domain of human membrane IgE controls apoptotic signaling of the B cell receptor in the mature B cell line A20

IgE-expressing long-lived plasma cells in persistent sensitization

Persistent allergies affect the quality of life of patients and increase economic burdens. Many clinical observations indicate the presence of IgE⁺ long-lived plasma cells (LLPCs), which account for the persistent secretion of specific IgE; however, the characteristics of the IgE⁺ LLPCs have yet to be identified clearly. In this review, we summarized the generation of IgE⁺ PCs, discussed the prosurvival factors in the microenvironment, and reviewed the unique IgE-BCR signaling, which may bring insights into understanding the survival mechanisms of IgE⁺ LLPCs.

Non-classical B Cell Memory of Allergic IgE Responses

The long-term effectiveness of antibody responses relies on the development of humoral immune memory. Humoral immunity is maintained by long-lived plasma cells that secrete antigen-specific antibodies, and memory B cells that rapidly respond to antigen re-exposure by generating new plasma cells and memory B cells. Developing effective immunological memory is essential for protection against pathogens, and is the basis of successful vaccinations. IgE responses have evolved for protection against helminth parasites infections and against toxins, but IgE is also a potent mediator of allergic diseases. There has been a dramatic increase in the incidence of allergic diseases in recent decades and this has provided the impetus to study the nature of IgE antibody responses. As will be discussed in depth in this review, the IgE memory response has unique features that distinguish it from classical B cell memory.

Structural Model of the mIgM B-Cell Receptor Transmembrane Domain From Self-Association Molecular Dynamics Simulations

Antigen binding to B-cell antigen receptors (BCRs) followed by signaling initiates the humoral immune response. The signaling is intimately coupled to nanoclustering of BCRs and their sorting to specific membrane domains, a process that is ruled by interactions between the BCR transmembrane domain and lipids. While the structure of the extracellular domains of BCRs has been resolved, little is known about the configuration of the constituting four immunoglobulin domains spanning the membrane. Here, we modeled the structure of the transmembrane (TM) domain of the IgM B-cell receptor using self-assembly coarse-grained molecular dynamics simulations. The obtained quaternary structure was validated against available experimental data and atomistic simulations. The IgM-BCR-TM domain configuration shows a 1:1 stoichiometry between the homodimeric membrane-bound domain of IgM (mIgM) and a Ig-α/Ig-β heterodimer. The mIgM homodimer is based on an asymmetric association of two mIgM domains. We show that a specific site of the Ig-α/Ig-β heterodimer is responsible for the association of IgM-BCRs with lipid rafts. Our results further suggest that this site is blocked in small-sized IgM-BCR clusters. The BCR TM structure provides a molecular basis for the previously suggested dissociation activation model of B-cell receptors. Self-assembly molecular dynamics simulations at the coarse-grained scale here proved as a versatile tool in the study of receptor complexes.

The extracellular membrane-proximal domain of membrane-bound IgE restricts B cell activation by limiting B cell antigen receptor surface expression

Immunoglobulin E (IgE) antibodies are key mediators of allergic reactions. Due to their potentially harmful anaphylactic properties, their production is tightly regulated. The membrane-bound isoform of IgE (mIgE), which is an integral component of the B cell antigen receptor, has been shown to be critical for the regulation of IgE responses in mice. In primate species including humans, mIgE can be expressed in two isoforms that are produced by alternative splicing of the primary ε Ig heavy chain transcript, and differ in the absence or presence of an extracellular membrane-proximal domain (EMPD) consisting of 52 amino acids. However, the function of the EMPD remains unclear. Here, we demonstrate that the EMPD restricts surface expression of mIgE-containing BCRs in human and murine B cells. The EMPD does not interfere with BCR assembly but acts as an autonomous endoplasmic reticulum retention domain. Limited surface expression of EMPD-containing mIgE-BCRs caused impaired activation of intracellular signaling cascades and hence represents a regulatory mechanism that may control the production of potentially anaphylactic IgE antibodies in primate species.

B Cell Intrinsic Mechanisms Constraining IgE Memory

Memory B cells and long-lived plasma cells are key elements of adaptive humoral immunity. Regardless of the immunoglobulin class produced, these cells can ensure long-lasting protection but also long-lasting immunopathology, thus requiring tight regulation of their generation and survival. Among all antibody classes, this is especially true for IgE, which stands as the most potent, and can trigger dramatic inflammatory reactions even when present in minute amounts. IgE responses and memory crucially protect against parasites and toxic components of venoms, conferring selective advantages and explaining their conservation in all mammalian species despite a parallel broad spectrum of IgE-mediated immunopathology. Long-term memory of sensitization and anaphylactic responses to allergens constitute the dark side of IgE responses, which can trigger multiple acute or chronic pathologic manifestations, some punctuated with life-threatening events. This Janus face of the IgE response and memory, both necessary and potentially dangerous, thus obviously deserves the most elaborated self-control schemes.

IgH isotype-specific B cell receptor expression influences B cell fate

Ig heavy chain (IgH) isotypes (e.g., IgM, IgG, and IgE) are generated as secreted/soluble antibodies (sIg) or as membrane-bound (mIg) B cell receptors (BCRs) through alternative RNA splicing. IgH isotype dictates soluble antibody function, but how mIg isotype influences B cell behavior is not well defined. We examined IgH isotype-specific BCR function by analyzing naturally switched B cells from wild-type mice, as well as by engineering polyclonal Ighγ1/γ1 and Ighε/ε mice, which initially produce IgG1 or IgE from their respective native genomic configurations. We found that B cells from wild-type mice, as well as Ighγ1/γ1 and Ighε/ε mice, produce transcripts that generate IgM, IgG1, and IgE in an alternative splice form bias hierarchy, regardless of cell stage. In this regard, we found that mIgμ > mIgγ1 > mIgε, and that these BCR expression differences influence respective developmental fitness. Restrained B cell development from Ighγ1/γ1 and Ighε/ε mice was proportional to sIg/mIg ratios and was rescued by enforced expression of the respective mIgs. In addition, artificially enhancing BCR signal strength permitted IgE⁺ memory B cells-which essentially do not exist under normal conditions-to provide long-lived memory function, suggesting that quantitative BCR signal weakness contributes to restraint of IgE B cell responses. Our results indicate that IgH isotype-specific mIg/BCR dosage may play a larger role in B cell fate than previously anticipated.

Quantitative in vitro and in vivo models to assess human IgE B cell receptor crosslinking by IgE and EMPD IgE targeting antibodies

Targeting plasma IgE by therapeutic mABs like Omalizumab (Xolair^®) is current clinical practice for severe allergic conditions or other IgE related diseases like chronic urticaria. As an alternative to soluble IgE targeting, IgE supply can be lowered by targeting the Extracellular Membrane Proximal Domain (EMPD) of the IgE B cell receptor (BCR) present on IgE switched B cells. This ultimately leads to apoptosis of these cells upon IgE BCR crosslinking. Since tools to selectively assess the efficacy of IgE BCR crosslinking by IgE targeting antibodies are limited, a readily quantifiable cell model was developed that allows to specifically address IgE BCR crosslinking activity in vitro. The new cell model allowed for a direct quantitative comparison of anti-EMPD IgE therapeutic prototype antibody 47H4 with anti-IgE(Ce3) directed therapeutic antibody Omalizumab and with a newly selected anti-human EMPD IgE monoclonal antibody, designated mAB 15cl12. Furthermore, a complementing mouse model was developed that allows for in vivo validation of antibodies addressing human EMPD IgE. It carries a targetable humanized EMPD IgE sequence that has been introduced by seamless genomic replacement of the endogenous EMPD encoding sequence. The model allowed to directly compare IgE lowering activity of two anti-human EMPD IgE therapeutic antibodies in vivo. Our tools provide the means for quantitative assessment of IgE BCR crosslinking activity which is increasingly gaining attention with respect to forthcoming second generation anti-IgE clinical candidates such as Ligelizumab or other clinical candidates featuring additional effector functions such as IgE BCR crosslinking activity.

Ontogeny of human IgE-expressing B cells and plasma cells

Background

IgE‐expressing (IgE⁺) plasma cells (PCs) provide a continuous source of allergen‐specific IgE that is central to allergic responses. The extreme sparsity of IgE⁺ cells in vivo has confined their study almost entirely to mouse models.

Objective

To characterize the development pathway of human IgE⁺PCs and to determine the ontogeny of human IgE⁺PCs.

Methods

To generate human IgE⁺ cells, we cultured tonsil B cells with IL‐4 and anti‐CD40. Using FACS and RT‐PCR, we examined the phenotype of generated IgE⁺ cells, the capacity of tonsil B‐cell subsets to generate IgE⁺PCs and the class switching pathways involved.

Results

We have identified three phenotypic stages of IgE⁺PC development pathway, namely (i) IgE⁺germinal centre (GC)‐like B cells, (ii) IgE⁺PC‐like ‘plasmablasts’ and (iii) IgE⁺PCs. The same phenotypic stages were also observed for IgG1⁺ cells. Total tonsil B cells give rise to IgE⁺PCs by direct and sequential switching, whereas the isolated GC B‐cell fraction, the main source of IgE⁺PCs, generates IgE⁺PCs by sequential switching. PC differentiation of IgE⁺ cells is accompanied by the down‐regulation of surface expression of the short form of membrane IgE (mIgE_S), which is homologous to mouse mIgE, and the up‐regulation of the long form of mIgE (mIgE_L), which is associated with an enhanced B‐cell survival and expressed in humans, but not in mice.

Conclusion

Generation of IgE⁺PCs from tonsil GC B cells occurs mainly via sequential switching from IgG. The mIgE_L/mIgE_S ratio may be implicated in survival of IgE⁺ B cells during PC differentiation and allergic disease.

Regulation of B cell fate by chronic activity of the IgE B cell receptor

IgE can trigger potent allergic responses, yet the mechanisms regulating IgE production are poorly understood. Here we reveal that IgE⁺ B cells are constrained by chronic activity of the IgE B cell receptor (BCR). In the absence of cognate antigen, the IgE BCR promoted terminal differentiation of B cells into plasma cells (PCs) under cell culture conditions mimicking T cell help. This antigen-independent PC differentiation involved multiple IgE domains and Syk, CD19, BLNK, Btk, and IRF4. Disruption of BCR signaling in mice led to consistently exaggerated IgE⁺ germinal center (GC) B cell but variably increased PC responses. We were unable to confirm reports that the IgE BCR directly promoted intrinsic apoptosis. Instead, IgE⁺ GC B cells exhibited poor antigen presentation and prolonged cell cycles, suggesting reduced competition for T cell help. We propose that chronic BCR activity and access to T cell help play critical roles in regulating IgE responses.

DOI:http://dx.doi.org/10.7554/eLife.21238.001

Antibodies are proteins that recognize and bind to specific molecules, and so help the immune system to defend the body against foreign substances that are potentially harmful. In some cases, harmless substances – such as pollen, dust or food – can trigger this response and lead to an allergic reaction. A type of antibody called immunoglobulin E (IgE) is particularly likely to trigger an allergic response.

In general, immune cells called plasma cells produce antibodies and release them into the body. However, in B cells – the cells from which plasma cells develop – the antibodies remain on the surface of the cells. Here, the antibody acts as a “receptor” that allows the B cell to tell when its antibody has bound to a specific substance.

Generally, B cells only activate when their B cell receptors bind to a specific substance. This binding triggers signals inside the cell that determine its fate – such as whether it will develop into a plasma cell. Recent studies have shown that B cells that have IgE on their surface (IgE⁺ B cells) are predisposed to develop rapidly into plasma cells.

To investigate why this is the case, Yang et al. have now studied B cells both in cell culture and in mice. The results show that the IgE B cell receptor autonomously signals to the cell even when it is not bound to a specific substance, in a manner that differs from other types of B cell receptors. This increases the likelihood that the IgE⁺ B cell will develop into a plasma cell and limits the competitive fitness of IgE⁺ B cells. These findings provide new insights into how IgE responses are regulated by the B cell receptor.

The next step will be to determine, at a molecular level, the basis for the autonomous signaling produced by the IgE B cell receptor when it is not bound to a specific substance. It will then be possible to investigate how this mechanism compares with the way that signals are normally transmitted when a B cell receptor binds to a specific substance.

DOI:http://dx.doi.org/10.7554/eLife.21238.002

Specificity and biologic activities of novel anti-membrane IgM antibodies

The concept that the B-cell Receptor (BCR) initiates a driver pathway in lymphoma-leukemia has been clinically validated. Previously described unique BCR Ig-class-specific sequences (proximal domains (PDs)), are not expressed in serum Ig (sIg). As a consequence of sequence and structural differences in the membrane IgM (mIgM) µ-Constant Domain 4, additional epitopes distinguish mIgM from sIgM. mAbs generated to linear and conformational epitopes, restricted to mIgM and not reacting with sIgM, were generated despite the relative hydrophobicity of the PDm sequence. Anti-PD mAbs (mAb1, mAb2, and mAb3) internalize mIgM. Anti-mIgM mAb4, which recognizes a distinct non-ligand binding site epitope, mediates mIgM internalization, and in low-density cultures, growth inhibition, anti-clonogenic activity, and apoptosis. We show that mAb-mediated mIgM internalization generally does not interrupt BCR-directed cell growth, however, mAb4 binding to a non-ligand binding site in the mIgM PDm-μC4 domain induces both mIgM internalization and anti-tumor effects. BCR micro-clustering in many B-cell leukemia and lymphoma lines is demonstrated by SEM micrographs using these new mAb reagents. mAb4 is a clinical candidate as a mediator of inhibition of the BCR signaling pathway. As these agents do not bind to non-mIgM B-cells, nor cross-react to non-lymphatic tissues, they may spare B-cell/normal tissue destruction as mAb-drug conjugates.

Biology of IgE production: IgE cell differentiation and the memory of IgE responses

The generation of long-lived plasma cells and memory B cells producing high-affinity antibodies depends on the maturation of B cell responses in germinal centers. These processes are essential for long-lasting antibody-mediated protection against infections. IgE antibodies are important for defense against parasites and toxins and can also mediate anti-tumor immunity. However, high-affinity IgE is also the main culprit responsible for the manifestations of allergic disease, including life-threatening anaphylaxisAnaphylaxis . Thus, generation of high-affinity IgE must be tightly regulated. Recent studies of IgE B cell biology have unveiled two mechanisms that limit high-affinity IgE memory responses: First, B cells that have recently switched to IgE production are programmed to rapidly differentiate into plasma cells,Plasma cells and second, IgE germinal centerGerminal center cells are transient and highly apoptotic. Opposing these processes, we now know that germinal center-derived IgG B cells can switch to IgE production, effectively becoming IgE-producing plasma cells. In this chapter, we will discuss the unique molecular and cellular pathways involved in the generation of IgE antibodies.

Self-Restrained B Cells Arise following Membrane IgE Expression

Among immunoglobulins (Igs), IgE can powerfully contribute to antimicrobial immunity and severe allergy despite its low abundance. IgE protein and gene structure resemble other Ig classes, making it unclear what constrains its production to thousand-fold lower levels. Whether class-switched B cell receptors (BCRs) differentially control B cell fate is debated, and study of the membrane (m)IgE class is hampered by its elusive in vivo expression. Here, we demonstrate a self-controlled mIgE⁺ B cell stage. Primary or transfected mIgE⁺ cells relocate the BCRs into spontaneously internalized lipid rafts, lose mobility to chemokines, and change morphology. We suggest that combined proapoptotic mechanisms possibly involving Hax1 prevent mIgE⁺ memory lymphocyte accumulation. By uncoupling in vivo IgE switching from cytokine and antigen stimuli, we show that these features are independent from B cell stimulation and instead result from mIgE expression per se. Consequently, few cells survive IgE class switching, which might ensure minimal long-term IgE memory upon differentiation into plasma cells.

AID-induced remodeling of immunoglobulin genes and B cell fate

Survival and phenotype of normal and malignant B lymphocytes are critically dependent on constitutive signals by the B cell receptor (BCR) for antigen. In addition, either antigen ligation of the BCR or various mitogenic stimuli result in B cell activation and induction of activation-induced deaminase (AID). AID activity can in turn mediate somatic hypermutation (SHM) of immunoglobulin (Ig) V regions and also deeply remodel the Ig heavy chain locus through class switch recombination (CSR) or locus suicide recombination (LSR). In addition to changes linked to affinity for antigen, modifying the class/isotype (i.e. the structure and function) of the BCR or suddenly deleting BCR expression also modulates the fate of antigen-experienced B cells.

IgE responses in mouse and man and the persistence of IgE memory

Rapid and robust recall or 'memory' responses are an essential feature of adaptive immunity. They constitute a defense against reinfection by pathogens, yet arguably do more harm than good in allergic disease. Immunoglobulin (Ig)E antibodies mediate the allergic reaction characterized by immediate hypersensitivity, a manifestation of IgE memory. The origin of IgE memory remains obscure, mainly due to the low proportion of IgE-expressing B cells in the total B cell population. The recent development of ultrasensitive methods for tracking these cells in vivo has overcome this obstacle, and their use has revealed unexpected pathways to IgE memory in the mouse. Here, we review these findings and consider their bearing on our understanding of IgE memory and allergic disease in man.

Current concepts of IgE regulation and impact of genetic determinants

Immunoglobulin E (IgE) mediated immune responses seem to be directed against parasites and neoplasms, but are best known for their involvement in allergies. The IgE network is tightly controlled at different levels as outlined in this review. Genetic determinants were suspected to influence IgE regulation and IgE levels considerably for many years. Linkage and candidate gene studies suggested a number of loci and genes to correlate with total serum IgE levels, and recently genome-wide association studies (GWAS) provided the power to identify genetic determinants for total serum IgE levels: 1q23 (FCER1A), 5q31 (RAD50, IL13, IL4), 12q13 (STAT6), 6p21.3 (HLA-DRB1) and 16p12 (IL4R, IL21R). In this review, we analyse the potential role of these GWAS hits in the IgE network and suggest mechanisms of how genes and genetic variants in these loci may influence IgE regulation.

Manipulating mIgD-expressing B cells with anti-migis-δ monoclonal antibodies

Surface IgD and IgM doubly positive cells comprise the major population of B cells in the human immune system. The heavy chain of membrane-bound IgD (mδ) differs from that of IgD (δ) in that mδ contains a C-terminal membrane-anchor peptide. Our group previously proposed that the N-terminal extracellular segment of 27 aa residues of the membrane-anchor peptide of mδ, referred to as the mIg isotype-specific-δ (migis-δ) segment, may provide a unique antigenic site for isotype-specific targeting of mIgD(+) B cells. Here we report the preparation of mouse mAbs specific for human migis-δ. The mAbs bound to human migis-δ-containing recombinant proteins in an ELISA and to mIgD-expressing transfectants of a CHO cell line as analyzed by flow cytometry. MAb 20E6, which binds to an epitope toward the N-terminal of human migis-δ, could stain human B cell line MC116, which expressed mIgD and mIgM. MC116 cells could be induced to undergo apoptosis by treatment with 20E6 in the presence of a second crosslinking antibody. Chimeric 20E6 caused antibody-dependent cellular cytotoxicity of MC116 cells in the presence of human PBMCs as the source of effector cells. In cultures of PBMCs, 20E6 down-regulated the population of mIgD(+) B cells. The production of human IgM by transplanted MC116 cells in NOD-SCID (NOD.CB17-Prkdc(scid)/IcrCrlBltw) mice could be suppressed by 20E6. These results encourage further investigation of the potential of anti-migis-δ mAbs to control mIgD(+) B cells, when such a manipulation may alleviate a disease state.

Unique epitopes on C epsilon mX in IgE-B cell receptors are potentially applicable for targeting IgE-committed B cells

Membrane-bound IgE (mIgE) is part of the IgE-BCR and is essential for generating isotype-specific IgE responses. On mIgE(+) B cells, the membrane-bound epsilon-chain (mepsilon) exists predominantly in the long isoform, mepsilon(L), containing an extra 52 aa CepsilonmX domain between CH4 and the C-terminal membrane-anchoring segment; the short isoform of mepsilon, mepsilon(S), exists in minor proportions. CepsilonmX thus provides an attractive site for immunologic targeting of mIgE(+) B cells. In this study, we show that nine newly prepared CepsilonmX-specific mAbs, as well as the previously reported a20, bound to mIgE.Fc(L)-expressing CHO cells, while only 4B12 and 26H2 bound to mIgE.Fc(L)-expressing B cell line Ramos cells. The mAb 4B12 bound to the N-terminal part, 26H2 the middle part, and all others the C-terminal part of CepsilonmX. Expression of Igalpha and Igbeta on the mIgE.Fc(L)-CHO cells reduces the binding of a20 to CepsilonmX as compared with that of 4B12 and 26H2. The chimeric mAbs c4B12 and c26H2, when cross-linked by secondary antibodies, lysed mIgE.Fc(L)-Ramos cells by apoptosis through a BCR-dependent caspase pathway. Using PBMCs as the source of effector cells, c4B12 and c26H2 demonstrated Ab-dependent cellular cytotoxicity toward mIgE.Fc(L)-Ramos cells in a dose-dependent fashion. In cultures of PBMCs from atopic dermatitis patients, c4B12 and c26H2 inhibited the synthesis of IgE driven by anti-CD40 and IL-4. These results suggest that 4B12 and 26H2 and an immunogen using the peptide segments recognized by these mAbs are potentially useful for targeting mIgE(+) B cells to control IgE production.

Alleles and isoforms of human membrane-bound IgA1

In humans, IgA exists as two subclasses, IgA1 and IgA2, which contain distinct alpha1 and alpha2 heavy chains, respectively. Both subclasses also have membrane-bound forms (mIgA1 and mIgA2) containing the corresponding malpha1 and malpha2 heavy chains, which differ from alpha1 and alpha2 by an additional "membrane-anchor" peptide segment extending from the CH3 domain of alpha1 and alpha2. The membrane-anchor segment has three parts: an extracellular, a transmembrane, and an intracellular segment. The heavy chain malpha1 exists in short and long isoforms, referred to as malpha1S and malpha1L, with the latter containing extra 6 amino acid residues, GSCSVA, at the N-terminus of the extracellular segment (residues 453-458). By studying the genomic and mRNA sequences of malpha1 and malpha2 from 30 individuals residing in Taiwan, we have found that, in addition to the known malpha1 allele, referred to as malpha1(456S), malpha1 also has a previously unknown allele, referred to as malpha1(456C) (GenBank accession no. EU431191). This newly identified allele is present in the donor population at a similar proportion to malpha1(456S), and appears to exist only as the long isoform, i.e. malpha1L, rather than the short isoform, malpha1S. Furthermore, we confirmed that malpha2 exists only as the short isoform. Future studies will examine whether these mIgA1 variations affect the regulation of IgA synthesis and whether mIgA1 can provide an antigenic site for the immunological targeting of IgA-expressing B cells.

Targeting the extracellular membrane-proximal domain of membrane-bound IgE by passive immunization blocks IgE synthesis in vivo

The classical allergic reaction starts seconds or minutes after Ag contact and is committed by Abs produced by a special subset of B lymphocytes. These Abs belong to the IgE subclass and are responsible for Type I hyperreactivity reactions. Treatment of allergic diseases with humanized anti-IgE Abs leads primarily to a decrease of serum IgE levels. As a consequence, the number of high-affinity IgE receptors on mast cells and basophils decreases, leading to a lower excitability of the effector cells. The biological mechanism behind anti-IgE therapy remains partly speculative; however, it is likely that these Abs also interact with membrane IgE (mIgE) on B cells and possibly interfere with IgE production. In the present work, we raised a mouse mAb directed exclusively against the extracellular membrane-proximal domain of mIgE. The interaction between the monoclonal anti-mIgE Ab and mIgE induces receptor-mediated apoptosis in vitro. Passive immunization experiments lead to a block of newly synthesized specific IgEs during a parallel application of recombinant Bet v1a, the major birch pollen allergen. The decrease of allergen-specific serum IgE might be related to tolerance-inducing mechanisms stopping mIgE-displaying B cells in their proliferation and differentiation.