Ontogeny of human IgE-expressing B cells and plasma cells

Inhibition of PI3K p110δ activity reduces IgE production in IL-4 and anti-CD40 stimulated human B cell cultures

Phosphoinositide 3-kinase (PI3K) p110δ signalling negatively regulates the production of mouse IgE. However, there are disparities between the mouse and human IgE biology, and the role of PI3K p110δ in the production of human IgE is yet to be determined. To investigate the effect of PI3K p110δ inhibition in the production of human IgE we isolated human B cells from tonsil tissue and stimulated them with IL-4 and anti-CD40 antibody to induce class switching to IgE and IgG1 in the presence or absence of IC87114, a small molecule inhibitor of PI3K p110δ. Using FACS, RT-PCR and ELISA we examined the effect of PI3K p110δ inhibition on IgE production and determined the mechanisms involved. Unlike in mice, we observed that PI3K p110δ inhibition significantly reduces the number of IgE+ switched cells and the amounts of secreted IgE in IL4 and anti-CD40 cultures. However, the number of IgG1+ cells and secreted IgG1 were largely unaffected by PI3K p110δ inhibition. The expression levels of AID, ε and γ1 germinal transcripts or other factors involved in the regulation of CSR to IgE and IgG1 were also unaffected by IC87114. However, we found that IC87114 significantly decreases the proliferation of tonsil B cells stimulated with IL-4 and anti-CD40, specifically reducing the frequency of cells that had undergone 4 divisions or more. In addition, PI3K p110δ inhibition reduced the levels of IRF4 expression in IgE+ germinal centre-like B cells leading to a block in plasma cell differentiation. In conclusion, PI3K p110δ signalling is required for the production of human IgE, which makes it a pharmacological target for the treatment of allergic disease.

The origins and longevity of IgE responses as indicated by serological and cellular studies in mice and humans

The existence of long-lived IgE antibody-secreting cells (ASC) is contentious, with the maintenance of sensitization by the continuous differentiation of short-lived IgE+ ASC a possibility. Here, we review the epidemiological profile of IgE production, and give an overview of recent discoveries made on the mechanisms regulating IgE production from mouse models. Together, these data suggest that for most individuals, in most IgE-associated diseases, IgE+ ASC are largely short-lived cells. A subpopulation of IgE+ ASC in humans is likely to survive for tens of months, although due to autonomous IgE B cell receptor (BCR) signaling and antigen-driven IgE+ ASC apoptosis, in general IgE+ ASC probably do not persist for the decades that other ASC are inferred to do. We also report on recently identified memory B cell transcriptional subtypes that are the likely source of IgE in ongoing responses, highlighting the probable importance of IL-4Rα in their regulation. We suggest the field should look at dupilumab and other drugs that prohibit IgE+ ASC production as being effective treatments for IgE-mediated aspects of disease in most individuals.

We are memory: B-cell responses in allergy and tolerance

The significance of B-cell memory in sustaining IgE-mediated allergies but also ensuring the development of long-term allergen tolerance has remained enigmatic. However, well-thought murine and human studies have begun to shed more light on this highly disputed subject. The present mini review highlights important aspects, like the involvement of IgG1 memory B cells, the meaning of low- or high-affinity IgE antibody production, the impact of allergen immunotherapy, or the relevance of local memory established by ectopic lymphoid structures. Based on recent findings, future investigations should lead to deeper knowledge and the development of improved therapies treating allergic individuals.

Abatacept to induce remission of peanut allergy during oral immunotherapy (ATARI): protocol for a phase 2a randomized controlled trial

Context

While oral immunotherapy (OIT) has been shown to promote the remission of mild peanut allergy in young children, there is still an unmet need for a disease-modifying intervention for older patients and those with severe diseases. In mice models, abatacept, a cytotoxic T lymphocyte-associated antigen-4 (CTLA-4) immunoglobulin fusion protein, has been shown to promote immune tolerance to food when used as an adjuvant to allergen immunotherapy. The goal of this study is to explore the potential efficacy of abatacept in promoting immune tolerance to food allergens during OIT in humans.

Methods

In this phase 2a proof-of-concept study (NCT04872218), 14 peanut-allergic participants aged from 14 to 55 years will be randomized at a 1:1 ratio to abatacept vs. placebo for the first 24 weeks of a peanut OIT treatment (target maintenance dose of 300 mg peanut protein). The primary outcome will be the suppression of the OIT-induced surge in peanut-specific IgE/total IgE at 24 weeks, relative to the baseline. Sustained unresponsiveness will be assessed as a secondary outcome starting at 36 weeks by observing incremental periods of peanut avoidance followed by oral food challenges.

Discussion

This is the first study assessing the use of abatacept as an adjuvant to allergen immunotherapy in humans. As observed in preclinical studies, the ability of abatacept to modulate the peanut-specific immune response during OIT will serve as a proxy outcome for the development of clinical tolerance, given the small sample size. The study will also test a new patient-oriented approach to sustained tolerance testing in randomized controlled trials.

B cell receptor ligation induces IgE plasma cell elimination

The proper regulation of IgE production safeguards against allergic disease, highlighting the importance of mechanisms that restrict IgE plasma cell (PC) survival. IgE PCs have unusually high surface B cell receptor (BCR) expression, yet the functional consequences of ligating this receptor are unknown. Here, we found that BCR ligation induced BCR signaling in IgE PCs followed by their elimination. In cell culture, exposure of IgE PCs to cognate antigen or anti-BCR antibodies induced apoptosis. IgE PC depletion correlated with the affinity, avidity, amount, and duration of antigen exposure and required the BCR signalosome components Syk, BLNK, and PLCγ2. In mice with a PC-specific impairment of BCR signaling, the abundance of IgE PCs was selectively increased. Conversely, BCR ligation by injection of cognate antigen or anti-IgE depleted IgE PCs. These findings establish a mechanism for the elimination of IgE PCs through BCR ligation. This has important implications for allergen tolerance and immunotherapy as well as anti-IgE monoclonal antibody treatments.

Immunoglobulin E response in health and disease beyond allergic disorders

Immunoglobulin E is the latest discovered of immunoglobulin family and has been long associated with anaphylaxis and worm expulsion. Immunoglobulin E, along with mast cells, basophils, and eosinophils, is also a hallmark of type 2 immunity which is dysregulated in numerous diseases such as asthma, rhinitis, atopic dermatitis, and eosinophilic esophagitis in addition to anaphylaxis as aforementioned. However, recent advances have shed light on IgE regulation and memory explaining the low level of free IgE, the scarcity of IgE plasma cells that are mainly short live and the absence of IgE memory B cells in homeostatic conditions. Furthermore, IgE was implicated in inflammatory conditions beyond allergic disorders where IgE-mediated facilitated antigen presentation can enhance cellular and humoral response against autoantigens in systemic lupus or chronic urticaria leading to more severe disease and even against neoantigen facilitating tumor cell lysis. At last, IgE was unexpectedly associated with allograft rejection or atheromatous cardiovascular diseases where precise mechanisms remain to be deciphered. The purpose of this review is to summarize these recent advances in IgE regulation, biology, and physiopathology beyond allergic diseases opening whole new fields of IgE biology to explore.

IgE-Mediated food allergy: Current diagnostic modalities and novel biomarkers with robust potential

Food allergy (FA) is a serious public health issue afflicting millions of people globally, with an estimated prevalence ranging from 1-10%. Management of FA is challenging due to overly restrictive diets and the lack of diagnostic approaches with high accuracy and prediction. Although measurement of serum-specific antibodies combined with patient medical history and skin prick test is a useful diagnostic tool, it is still an imprecise predictor of clinical reactivity with a high false-positive rate. The double-blind placebo-controlled food challenge represents the gold standard for FA diagnosis; however, it requires large healthcare and involves the risk of acute onset of allergic reactions. Improvement in our understanding of the molecular mechanism underlying allergic disease pathology, development of omics-based methods, and advances in bioinformatics have boosted the generation of a number of robust diagnostic biomarkers of FA. In this review, we discuss how traditional diagnostic modalities guide appropriate diagnosis and management of FA in clinical practice, as well as uncover the potential of the latest biomarkers for the diagnosis, monitoring, and prediction of FA. We also raise perspectives for precise and targeted medical intervention to fill the gap in the diagnosis of FA.

Single-Cell Technologies for the Study of Antibody-Secreting Cells

Antibody-secreting cells (ASC), plasmablasts and plasma cells, are terminally differentiated B cells responsible for large-scale production and secretion of antibodies. ASC are derived from activated B cells, which may differentiate extrafollicularly or form germinal center (GC) reactions within secondary lymphoid organs. ASC therefore consist of short-lived, poorly matured plasmablasts that generally secrete lower-affinity antibodies, or long-lived, highly matured plasma cells that generally secrete higher-affinity antibodies. The ASC population is responsible for producing an immediate humoral B cell response, the polyclonal antibody repertoire, as well as in parallel building effective humoral memory and immunity, or potentially driving pathology in the case of autoimmunity. ASC are phenotypically and transcriptionally distinct from other B cells and further distinguishable by morphology, varied lifespans, and anatomical localization. Single cell analyses are required to interrogate the functional and transcriptional diversity of ASC and their secreted antibody repertoire and understand the contribution of individual ASC responses to the polyclonal humoral response. Here we summarize the current and emerging functional and molecular techniques for high-throughput characterization of ASC with single cell resolution, including flow and mass cytometry, spot-based and microfluidic-based assays, focusing on functional approaches of the secreted antibodies: specificity, affinity, and secretion rate.

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.

Tracing Human IgE B Cell Antigen Receptor-Bearing Cells With a Monoclonal Anti-Human IgE Antibody That Specifically Recognizes Non-Receptor-Bound IgE

Up to 30% of the population suffers from immunoglobulin E (IgE)-mediated allergies. Despite current stepwise gating approaches, the unambiguous identification of human IgE-producing cells by flow cytometry and immunohistology remains challenging. This is mainly due to the scarcity of these cells and the fact that IgE is not only expressed in a membrane-bound form on the surface of IgE-producing cells in form of the B cell antigen receptor (BCR), but is more frequently found on various cell types bound to the low and high affinity receptors, CD23 and FcϵRI, respectively. Here we sought to develop a sequential gating strategy for unambiguous detection of cells bearing the IgE BCR on their surface. To that aim we first tested the monoclonal anti-IgE antibody omalizumab for its ability to discriminate between IgE BCR and receptor-bound IgE using cells producing IgE or bearing IgE bound to CD23 as well as basophils exhibiting FcϵRI receptor-bound IgE. Using flow cytometry, we demonstrated that omalizumab recognized IgE producing cells with a high sensitivity of up to 1 IgE+ cell in 1000 human peripheral blood mononuclear cells (PBMCs). These results were confirmed by confocal microscopy both in cell suspensions as well as in nasal polyp tissue sections. Finally, we established a consecutive gating strategy allowing the clear identification of class-switched, allergen-specific IgE+ memory B cells and plasmablasts/plasma cells in human PBMCs. Birch pollen specific IgE+ memory B cells represented on average 0.734% of total CD19+ B cells in allergic patients after allergen exposure. Thus, we developed a new protocol for exclusive staining of non-receptor bound allergen-specific IgE+ B cell subsets in human samples.

Chronic calcium signaling in IgE+ B cells limits plasma cell differentiation and survival

In contrast to other antibody isotypes, B cells switched to IgE respond transiently and do not give rise to long-lived plasma cells (PCs) or memory B cells. To better understand IgE-BCR-mediated control of IgE responses, we developed whole-genome CRISPR screening that enabled comparison of IgE⁺ and IgG1⁺ B cell requirements for proliferation, survival, and differentiation into PCs. IgE⁺ PCs exhibited dependency on the PI3K-mTOR axis that increased protein amounts of the transcription factor IRF4. In contrast, loss of components of the calcium-calcineurin-NFAT pathway promoted IgE⁺ PC differentiation. Mice bearing a B cell-specific deletion of calcineurin B1 exhibited increased production of IgE⁺ PCs. Mechanistically, sustained elevation of intracellular calcium in IgE⁺ PCs downstream of the IgE-BCR promoted BCL2L11-dependent apoptosis. Thus, chronic calcium signaling downstream of the IgE-BCR controls the self-limiting character of IgE responses and may be relevant to the accumulation of IgE-producing cells in allergic disease.

Anti-IgE Effect of Small-Molecule-Compound Arctigenin on Food Allergy in association with a Distinct Transcriptome Profile

BACKGROUND

Excessive production of IgE plays a major role in the pathology of food allergy. In an attempt to identify anti-IgE natural products, Arctium Lappa was one of the most effective herbs among approximately 300 screened medicinal herbs. However, little is known about its anti-IgE compounds.

OBJECTIVE

To identify compounds from Arctium Lappa for targeted therapy on IgE production and explore their underlying mechanisms.

METHODS

Liquid-liquid extraction and column chromatographic methods were used to purify the compounds. IgE inhibitory effects were determined on IgE producing human myeloma U266 cells, peanut-allergic murine model, and PBMCs from food-allergic patients. Genes involved in IgE inhibition in PBMCs were studied by RNA sequencing.

RESULTS

The main compounds isolated were identified as arctiin and arctigenin. Both compounds significantly inhibited IgE production in U266 cells, with arctigenin the most potent (IC50=5.09μg/mL). Arctigenin (at a dose of 13.3 mg/kg) markedly reduced peanut-specific IgE levels, blocked hypothermia and histamine release in a peanut-allergic mouse model. Arctigenin also significantly reduced IgE production and Th2 cytokines (IL5, IL13) by PBMCs. We found 479 differentially expressed genes in PBMCs with arctigenin treatment (p<0.001 and fold-change ≥1.5), involving 24 gene ontology terms (p<0.001, FDR <0.05); cell division was the most significant. Eleven genes including UBE2C and BCL6 were validated by qPCR.

CONCLUSION

Arctigenin markedly inhibited IgE production in U266 cells, peanut allergic murine model and PBMCs from allergic patients by down-regulating cell division, cell cycle-related genes and up-regulating anti-inflammatory factors.

Intrinsic and extrinsic regulation of IgE B cell responses

Stringent regulation of IgE antibody production is critical for constraining allergic responses. This review discusses recent advances in understanding cell-intrinsic and extrinsic mechanisms that regulate the genesis and fate of IgE B cells. B cell-intrinsic regulation of IgE is orchestrated by the IgE B Cell Receptor (BCR). Through its antigen-independent signaling and low surface expression, the IgE BCR drives IgE B cells to differentiate into short-lived plasma cells and/or undergo apoptosis, restricting IgE-expressing cells from entering long-lived compartments. The pivotal extrinsic regulators of IgE responses are T follicular helper cells (T_FH). T_FH produce IL-4 and IL-21, which, respectively, are the major activating and inhibitory cytokines for IgE class-switching. Other newly identified T follicular subsets also contribute to IgE regulation. Although IgE responses are normally constrained, recent studies suggest that specific conditions can induce the formation of IgE responses with enhanced affinity or longevity, effectively 'breaking the rules' of IgE regulation.

Interrupting reactivation of immunologic memory diverts the allergic response and prevents anaphylaxis

BACKGROUND

IgE production against innocuous food antigens can result in anaphylaxis, a severe life-threatening consequence of allergic reactions. The maintenance of IgE immunity is primarily facilitated by IgG+ memory B cells, as IgE+ memory B cells and IgE+ plasma cells are extremely scarce and short-lived, respectively.

OBJECTIVE

Our aim was to investigate the critical requirements for an IgE recall response in peanut allergy.

METHODS

We used a novel human PBMC culture platform, a mouse model of peanut allergy, and various experimental readouts to assess the IgE recall response in the presence and absence of IL-4Rα blockade.

RESULTS

In human PBMCs, we have demonstrated that blockade of IL-4/IL-13 signaling aborted IgE production after activation of a recall response and skewed the cytokine response away from a dominant type 2 signature. TH2A cells, identified by single-cell RNA sequencing, expanded with peanut stimulation and maintained their pathogenic phenotype in spite of IL-4Rα blockade. In mice with allergy, anti-IL-4Rα provided long-lasting suppression of the IgE recall response beyond antibody treatment and fully protected against anaphylaxis.

CONCLUSION

The findings reported here advance our understanding of events mediating the regeneration of IgE in food allergy.

Deletion of IL-4Rα signaling on B cells limits hyperresponsiveness depending on antigen load

BACKGROUND

B cells play an important role in allergies through secretion of IgE. IL-4 receptor α (IL-4Rα) is key in allergic asthma and regulates type 2 cytokine production, IgE secretion, and airway hyperresponsiveness. IL-4 activation of B cells is essential for class switching and contributes to the induction of B effector 2 (Be2) cells. The role of Be2 cells and signaling via IL-4Rα in B cells is not clearly defined.

OBJECTIVE

We sought to find out whether IL-4Rα-responsive B cells or Be2 function was essential in experimental allergic asthma.

METHODS

Mice lacking IL-4Rα on B cells (mb1^creIL-4Rα^-/lox) or littermate controls (IL-4Rα^-/lox) and mice lacking IL-4 or IL-4/IL-13 on B cells were sensitized and challenged with high-dose house dust mite (>10 μg) or with low-dose house dust mite (<3 μg). We also adoptively transferred naive IL-4Rα^-/lox or IL-4Rα^-/- B cells into μMT^-/- mice a day before sensitization or a day before challenge. We analyzed lung inflammation, cellular infiltrate, and airway hyperresponsiveness.

RESULTS

We found that IL-4Rα signaling on B cells was important for optimal T_H2 allergic immune responses mainly when the load of antigen is limited. IL-4Rα signaling on B cells was essential for germinal centers and in the effector phase of allergic responses. Be2 cells were essential in airway hyperresponsiveness, but not in other parameters.

CONCLUSIONS

IL-4Rα signaling on B cells is deleterious in allergic asthma because it is required for optimal T_H2 responses, Be2 function, germinal center formation, and T follicular helper cells, especially when the load of the antigen is limiting.

Associations of Th2, Th17, Treg cells, and IgA+ memory B cells with atopic disease in children: The Generation R Study

BACKGROUND

New insights into immune cells could contribute to treatment and monitoring of atopic disease. Because nongenetic factors shape the human immune system, we here studied these immune cells in a large cohort with atopic children with adjustment for prenatal and postnatal confounders.

METHODS

Information on atopic dermatitis, inhalant- and food-allergic sensitization, asthma lung function scores was obtained from 855 10-year-old children within the Generation R cohort. 11-color flow cytometry was performed to determine CD27⁺ and CD27^- IgG⁺ , IgE⁺ and IgA⁺ memory B cells, Th1, Th2, Th17, and Treg-memory cells from venous blood. Associations between any atopic disease, the individual atopic diseases, and immune cell numbers were determined.

RESULTS

Children with any atopic disease had higher Th2, Treg, Treg-memory, and CD27⁺ IgA⁺ memory B-cell numbers compared to children without atopic disease. When studying the individual diseases compared to children without the individual diseases, children with atopic dermatitis, inhalant-, and food-allergic sensitization had higher memory Treg cell numbers 12.3% (95% CI 4.2; 21.0), (11.1% (95% CI 3.0; 19.8), (23.7% (95% CI 7.9; 41.8), respectively. Children with food-allergic sensitization had higher total B and CD27⁺ IgA⁺ memory B-cell numbers (15.2% [95% CI 3.2; 28.7], 22.5% [95% CI 3.9; 44.3], respectively). No associations were observed between asthma and B- or T-cell numbers.

CONCLUSION

Children with any atopic disease and children with inhalant- and food-allergic sensitization or atopic dermatitis had higher circulating memory Treg cells, but not higher IgE⁺ B-cell numbers. The associations of higher Treg and CD27⁺ IgA⁺ B-cell numbers in children with food-allergic sensitization are suggestive of TGF-β-mediated compensation for chronic inflammation.

Allergen-specific IgG+ memory B cells are temporally linked to IgE memory responses

BACKGROUND

IgE is the least abundant immunoglobulin and tightly regulated, and IgE-producing B cells are rare. The cellular origin and evolution of IgE responses are poorly understood.

OBJECTIVE

The cellular and clonal origin of IgE memory responses following mucosal allergen exposure by sublingual immunotherapy (SLIT) were investigated.

METHODS

In a randomized double-blind, placebo-controlled, time course SLIT study, PBMCs and nasal biopsy samples were collected from 40 adults with seasonal allergic rhinitis at baseline and at 4, 8, 16, 28, and 52 weeks. RNA was extracted from PBMCs, sorted B cells, and nasal biopsy samples for heavy chain variable gene repertoire sequencing. Moreover, mAbs were derived from single B-cell transcriptomes.

RESULTS

Combining heavy chain variable gene repertoire sequencing and single-cell transcriptomics yielded direct evidence of a parallel boost of 2 clonally and functionally related B-cell subsets of short-lived IgE⁺ plasmablasts and IgG⁺ memory B cells. Mucosal grass pollen allergen exposure by SLIT resulted in highly diverse IgE and IgG_E repertoires. These were extensively mutated and appeared relatively stable as per heavy chain isotype, somatic hypermutations, and clonal composition. Single IgG_E⁺ memory B-cell and IgE⁺ preplasmablast transcriptomes encoded antibodies that were specific for major grass pollen allergens and able to elicit basophil activation at very low allergen concentrations.

CONCLUSION

For the first time, we have shown that on mucosal allergen exposure, human IgE memory resides in allergen-specific IgG⁺ memory B cells. These cells rapidly switch isotype, expand into short-lived IgE⁺ plasmablasts, and serve as a potential target for therapeutic intervention.

Tracing IgE-Producing Cells in Allergic Patients

Immunoglobulin E (IgE) is the key immunoglobulin in the pathogenesis of IgE associated allergic diseases affecting 30% of the world population. Recent data suggest that allergen-specific IgE levels in serum of allergic patients are sustained by two different mechanisms: inducible IgE production through allergen exposure, and continuous IgE production occurring even in the absence of allergen stimulus that maintains IgE levels. This assumption is supported by two observations. First, allergen exposure induces transient increases of systemic IgE production. Second, reduction in IgE levels upon depletion of IgE from the blood of allergic patients using immunoapheresis is only temporary and IgE levels quickly return to pre-treatment levels even in the absence of allergen exposure. Though IgE production has been observed in the peripheral blood and locally in various human tissues (e.g., nose, lung, spleen, bone marrow), the origin and main sites of IgE production in humans remain unknown. Furthermore, IgE-producing cells in humans have yet to be fully characterized. Capturing IgE-producing cells is challenging not only because current staining technologies are inadequate, but also because the cells are rare, they are difficult to discriminate from cells bearing IgE bound to IgE-receptors, and plasma cells express little IgE on their surface. However, due to the central role in mediating both the early and late phases of allergy, free IgE, IgE-bearing effector cells and IgE-producing cells are important therapeutic targets. Here, we discuss current knowledge and unanswered questions regarding IgE production in allergic patients as well as possible therapeutic approaches targeting IgE.

Food allergy and omics

Food allergy (FA) prevalence has been increasing over the last few decades and is now a global health concern. Current diagnostic methods for FA result in a high number of false-positive results, and the standard of care is either allergen avoidance or use of epinephrine on accidental exposure, although currently with no other approved treatments. The increasing prevalence of FA, lack of robust biomarkers, and inadequate treatments warrants further research into the mechanism underlying food allergies. Recent technological advances have made it possible to move beyond traditional biological techniques to more sophisticated high-throughput approaches. These technologies have created the burgeoning field of omics sciences, which permit a more systematic investigation of biological problems. Omics sciences, such as genomics, epigenomics, transcriptomics, proteomics, metabolomics, microbiomics, and exposomics, have enabled the construction of regulatory networks and biological pathway models. Parallel advances in bioinformatics and computational techniques have enabled the integration, analysis, and interpretation of these exponentially growing data sets and opens the possibility of personalized or precision medicine for FA.

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.

B Cell Responses: Cell Interaction Dynamics and Decisions

B cells and the antibodies they produce have a deeply penetrating influence on human physiology. Here, we review current understanding of how B cell responses are initiated; the different paths to generate short- and long-lived plasma cells, germinal center cells, and memory cells; and how each path impacts antibody diversity, selectivity, and affinity. We discuss how basic research is informing efforts to generate vaccines that induce broadly neutralizing antibodies against viral pathogens, revealing the special features associated with allergen-reactive IgE responses and uncovering the antibody-independent mechanisms by which B cells contribute to health and disease.

Transcriptional Analysis of the Human IgE-Expressing Plasma Cell Differentiation Pathway

IgE is secreted by plasma cells (PCs) and is central to allergic disease. Using an ex vivo tonsil B cell culture system, which mimics the Th2 responses in vivo, we have recently characterized the development pathway of human IgE-expressing PCs. In this system, as in mice, we reported the predisposition of IgE-expressing B cells to differentiate into PCs. To gain a comprehensive understanding of the molecular events involved in the differentiation of human IgE⁺ B cells into PCs we have used the Illumina HumanHT-12 v4 Expression BeadChip array to analyse the gene expression profile of ex vivo generated human IgE⁺ B cells at various stages of their differentiation into PCs. We also compared the transcription profiles of IgE⁺ and IgG1⁺ cells to discover isotype-specific patterns. Comparisons of IgE⁺ and IgG1⁺ cell transcriptional profiles revealed molecular signatures specific for IgE⁺ cells, which diverge from their IgG1⁺ cell counterparts upon differentiation into PCs. At the germinal center (GC) stage of development, unlike in some mouse studies of IgE biology, we observed similar rates of apoptosis and no significant differences in the expression of apoptosis-associated genes between the IgE⁺ and IgG1⁺ B cells. We identified a gene interaction network associated with early growth response 1 (EGR1) that, together with the up-regulated IRF4, may account for the predisposition of IgE⁺ B cells to differentiate into PCs. However, despite their swifter rates of PC differentiation, the transcription profile of IgE⁺ PCs is more closely related to IgE⁺ and IgG1⁺ plasmablasts (PBs) than to IgG1⁺ PCs, suggesting that the terminal differentiation of IgE⁺ cells is impeded. We also show that IgE⁺ PCs have increased levels of apoptosis suggesting that the IgE⁺ PCs generated in our in vitro tonsil B cell cultures, as in mice, are short-lived. We identified gene regulatory networks as well as cell cycle and apoptosis signatures that may explain the diverging PC differentiation programme of these cells. Overall, our study provides a detailed analysis of the transcriptional pathways underlying the differentiation of human IgE-expressing B cells and points to molecular signatures that regulate IgE⁺ PC differentiation and function.

The Multifaceted B Cell Response in Allergen Immunotherapy

While allergen immunotherapy (AIT) for IgE-mediated diseases holds curative potential, the considerable heterogeneity in clinical outcomes may relate to the complex mechanisms of tolerance. The regulation of humoral immunity by AIT contributes to the suppression of allergic responses. Recent findings have revealed novel roles for IgA and IgG antibodies in the induction of tolerance. These mechanisms synergize with their ability to block allergen-IgE binding and mediate inhibitory signaling of effector cells of the allergic response. In addition, the regulatory activity of B cells in AIT extends beyond IL-10 secretion and induction of IgG₄. Here, we review the evolution of the B cell response during AIT with special emphasis on the novel protective mechanisms entailing humoral immunity.

Gut Mucosal Antibody Responses and Implications for Food Allergy

The gastrointestinal mucosa is a critical environmental interface where plasma cells and B cells are exposed to orally-ingested antigens such as food allergen proteins. It is unclear how the development of B cells and plasma cells in the gastrointestinal mucosa differs between healthy humans and those with food allergy, and how B cells contribute to, or are affected by, the breakdown of oral tolerance. In particular, the antibody gene repertoires associated with symptomatic allergy have only begun to be characterized in full molecular detail. Here, we review literature concerning B cells and plasma cells in the gastrointestinal system in the context of food allergy, with a focus on human studies.

miR-29b directly targets activation-induced cytidine deaminase in human B cells and can limit its inappropriate expression in naïve B cells

Class-switch recombination (CSR) is an essential B cell process that alters the isotype of antibody produced by the B cell, tailoring the immune response to the nature of the invading pathogen. CSR requires the activity of the mutagenic enzyme AID (encoded by AICDA) to generate chromosomal lesions within the immunoglobulin genes that initiate the class switching recombination event. These AID-mediated mutations also participate in somatic-hypermutation of the immunoglobulin variable region, driving affinity maturation. As such, AID poses a significant oncogenic threat if it functions outside of the immunoglobulin locus. We found that expression of the microRNA, miR-29b, was repressed in B cells isolated from tonsil tissue, relative to circulating naïve B cells. Further investigation revealed that miR-29b was able to directly initiate the degradation of AID mRNA. Enforced overexpression of miR-29b in human B cells precipitated a reduction in overall AID protein and a corresponding diminution in CSR to IgE. Given miR-29b's ability to potently target AID, a mutagenic molecule that can initiate chromosomal translocations and "off-target" mutations, we propose that miR-29b acts to silence premature AID expression in naïve B cells, thus reducing the likelihood of inappropriate and potentially dangerous deamination activity.

Converging findings from linkage between periodontal pathogen with atopic and allergic immune response

This study aims to explore a relationship between exposures of whole-cell Porphyromonas gingivalis in various doses with atopic inflammatory responses at experimental mice. A pretest-posttest controlled group design, with 16 Wistar rats (Rattus novergicus) randomized into four groups. Group 1 was the control group. Group 2 was given low-dose (9 × 10⁷ colony-forming unit) of P. gingivalis. Group 3 was given medium-dose (9 × 10⁹ colony-forming unit) of P. gingivalis. Group 4 was given high-dose (9 × 10¹¹ colony-forming unit) of P. gingivalis. Interleukin-4, Interleukin-5, Interleukin-17F, Interleukin-21, Immunoglobulin-E, Immunoglobulin-G₄, and γ-Interferon were measured by direct-sandwich ELISA just before the treatments began, day-4, and day-11 after treatments. There is a sudden increase of Interleukin-4 in the group 4 (23.79 ± 0.91 pg/ml to 54.17 ± 0.79 pg/ml; p = 0.01) and slight increase of Interleukin-5 in the group 4 (207.60 ± 11.15 pg/ml to 243.40 ± 9.33 pg/ml; p = 0.03). No change was observed for Interleukin-17F in all groups. Serum concentration of Immunoglobulin-E was decreased in group 2 (-10.44 ± 8.13 pg/ml), but increased in group 4 (+1.03 ± 4.57 pg/ml). Taken together, some cytokines are up-regulated and others are down-regulated after exposure to whole-cell P. gingivalis. Moreover, study of host responses during periodontal infection may offer critical key insight that contribute to the development of atopy. CLINICAL IMPLICATIONS: We introduced and explained the potential role of periodontal pathogen Porphyromonas gingivalis in systemic immune responses, along with its virulence factor inside the oral cavity. Our results consider several changes and differences of cytokines and immunoglobulins following whole-cell Porphyromonas gingivalis exposure. However, results of the study need to be interpreted with caution due to its limitations. CAPSULE SUMMARY: Interleukin (IL)-4 and IL-5 had been found increase after exposure to the periodontal pathogens Porphyromonas gingivalis, whereas no or minimal change had been found in the level of IL-17F, Ig-G₄, and IFN-γ. The various cytokines and immunoglobulins shown in this study do not prove a causal relationship, and the precise role of Porphyromonas gingivalis in the regulation of atopic immune response warrants further investigation. Nevertheless, these findings may provide some critical key insight into the host responses following Porphyromonas gingivalis infection.

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.

Lifelong memory responses perpetuate humoral TH2 immunity and anaphylaxis in food allergy

BACKGROUND

A number of food allergies (eg, fish, shellfish, and nuts) are lifelong, without any disease-transforming therapies, and unclear in their underlying immunology. Clinical manifestations of food allergy are largely mediated by IgE. Although persistent IgE titers have been attributed conventionally to long-lived IgE⁺ plasma cells (PCs), this has not been directly and comprehensively tested.

OBJECTIVE

We sought to evaluate mechanisms underlying persistent IgE and allergic responses to food allergens.

METHODS

We used a model of peanut allergy and anaphylaxis, various knockout mice, adoptive transfer experiments, and in vitro assays to identify mechanisms underlying persistent IgE humoral immunity over almost the entire lifespan of the mouse (18-20 months).

RESULTS

Contrary to conventional paradigms, our data show that clinically relevant lifelong IgE titers are not sustained by long-lived IgE⁺ PCs. Instead, lifelong reactivity is conferred by allergen-specific long-lived memory B cells that replenish the IgE⁺ PC compartment. B-cell reactivation requires allergen re-exposure and IL-4 production by CD4 T cells. We define the half-lives of antigen-specific germinal centers (23.3 days), IgE⁺ and IgG₁⁺ PCs (60 and 234.4 days, respectively), and clinically relevant cell-bound IgE (67.3 days).

CONCLUSIONS

These findings can explain lifelong food allergies observed in human subjects as the consequence of allergen exposures that recurrently activate memory B cells and identify these as a therapeutic target with disease-transforming potential.

Interleukin-21 Drives Proliferation and Differentiation of Porcine Memory B Cells into Antibody Secreting Cells

Immunological prevention of infectious disease, especially viral, is based on antigen-specific long-lived memory B cells. To test for cellular proliferation and differentiation factors in swine, an outbred model for humans, CD21+ B cells were activated in vitro with CD40L and stimulated with purported stimulatory cytokines to characterize functional responses. IL-21 induced a 3-fold expansion in total cell numbers with roughly 15% of all B cells differentiating to IgM or IgG antibody secreting cells (ASCs.) However, even with robust proliferation, cellular viability rapidly deteriorated. Therefore, a proliferation inducing ligand (APRIL) and B cell activating factor (BAFF) were evaluated as survival and maintenance factors. BAFF was effective at enhancing the viability of mature B cells as well as ASCs, while APRIL was only effective for ASCs. Both cytokines increased approximately two-fold the amount of IgM and IgG which was secreted by IL-21 differentiated ASCs. Mature B cells from porcine reproductive and respiratory virus (PRRSV) immune and naïve age-matched pigs were activated and treated with IL-21 and then tested for memory cell differentiation using a PRRSV non-structural protein 7 ELISPOT and ELISA. PRRSV immune pigs were positive on both ELISPOT and ELISA while naïve animals were negative on both assays. These results highlight the IL-21-driven expansion and differentiation of memory B cells in vitro without stimulation of the surface immunoglobulin receptor complex, as well as the establishment of a defined memory B cell culture system for characterization of vaccine responses in outbred animals.

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.