Enhancement and suppression of signaling by the conserved tail of IgG memory-type B cell antigen receptors

Regulation of BCR-mediated Ca2+ mobilization by MIZ1-TMBIM4 safeguards IgG1+ GC B cell-positive selection

The transition from immunoglobulin M (IgM) to affinity-matured IgG antibodies is vital for effective humoral immunity. This is facilitated by germinal centers (GCs) through affinity maturation and preferential maintenance of IgG+ B cells over IgM+ B cells. However, it is not known whether the positive selection of the different Ig isotypes within GCs is dependent on specific transcriptional mechanisms. Here, we explored IgG1+ GC B cell transcription factor dependency using a CRISPR-Cas9 screen and conditional mouse genetics. We found that MIZ1 was specifically required for IgG1+ GC B cell survival during positive selection, whereas IgM+ GC B cells were largely independent. Mechanistically, MIZ1 induced TMBIM4, an ancestral anti-apoptotic protein that regulated inositol trisphosphate receptor (IP3R)-mediated calcium (Ca2+) mobilization downstream of B cell receptor (BCR) signaling in IgG1+ B cells. The MIZ1-TMBIM4 axis prevented mitochondrial dysfunction-induced IgG1+ GC cell death caused by excessive Ca2+ accumulation. This study uncovers a unique Ig isotype-specific dependency on a hitherto unidentified mechanism in GC-positive selection.

Determinants of de novo B cell responses to drifted epitopes in post-vaccination SARS-CoV-2 infections

Vaccine-induced immunity may impact subsequent de novo responses to drifted epitopes in SARS-CoV-2 variants, but this has been difficult to quantify due to the challenges in recruiting unvaccinated control groups whose first exposure to SARS-CoV-2 is a primary infection. Through local, statewide, and national SARS-CoV-2 testing programs, we were able to recruit cohorts of individuals who had recovered from either primary or post-vaccination infections by either the Delta or Omicron BA.1 variants. Regardless of variant, we observed greater Spike-specific and neutralizing antibody responses in post-vaccination infections than in those who were infected without prior vaccination. Through analysis of variant-specific memory B cells as markers of de novo responses, we observed that Delta and Omicron BA.1 infections led to a marked shift in immunodominance in which some drifted epitopes elicited minimal responses, even in primary infections. Prior immunity through vaccination had a small negative impact on these de novo responses, but this did not correlate with cross-reactive memory B cells, arguing against competitive inhibition of naïve B cells. We conclude that dampened de novo B cell responses against drifted epitopes are mostly a function of altered immunodominance hierarchies that are apparent even in primary infections, with a more modest contribution from pre-existing immunity, perhaps due to accelerated antigen clearance.

The role of N-glycosylation in B-cell biology and IgG activity. The aspects of autoimmunity and anti-inflammatory therapy

The immune system is strictly regulated by glycosylation through the addition of highly diverse and dynamically changing sugar structures (glycans) to the majority of immune cell receptors. Although knowledge in the field of glycoimmunology is still limited, numerous studies point to the key role of glycosylation in maintaining homeostasis, but also in reflecting its disruption. Changes in oligosaccharide patterns can lead to impairment of both innate and acquired immune responses, with important implications in the pathogenesis of diseases, including autoimmunity. B cells appear to be unique within the immune system, since they exhibit both innate and adaptive immune activity. B cell surface is rich in glycosylated proteins and lectins which recognise glycosylated ligands on other cells. Glycans are important in the development, selection, and maturation of B cells. Changes in sialylation and fucosylation of cell surface proteins affect B cell signal transduction through BCRs, CD22 inhibitory coreceptor and Siglec-G. Plasmocytes, as the final stage of B cell differentiation, produce and secrete immunoglobulins (Igs), of which IgGs are the most abundant N-glycosylated proteins in human serum with the conserved N-glycosylation site at Asn297. N-oligosaccharide composition of the IgG Fc region affects its secretion, structure, half-life and effector functions (ADCC, CDC). IgG N-glycosylation undergoes little change during homeostasis, and may gradually be modified with age and during ongoing inflammatory processes. Hyperactivated B lymphocytes secrete autoreactive antibodies responsible for the development of autoimmunity. The altered profile of IgG N-glycans contributes to disease progression and remission and is sensitive to the application of therapeutic substances and immunosuppressive agents. In this review, we focus on the role of N-glycans in B-cell biology and IgG activity, the rearrangement of IgG oligosaccharides in aging, autoimmunity and immunosuppressive therapy.

Pharmacogenomic Analyses Implicate B Cell Developmental Status and MKL1 as Determinants of Sensitivity toward Anti-CD20 Monoclonal Antibody Therapy

Monoclonal antibody (mAb) therapy directed against CD20 is an important tool in the treatment of B cell disorders. However, variable patient response and acquired resistance remain important clinical challenges. To identify genetic factors that may influence sensitivity to treatment, the cytotoxic activity of three CD20 mAbs: rituximab; ofatumumab; and obinutuzumab, were screened in high-throughput assays using 680 ethnically diverse lymphoblastoid cell lines (LCLs) followed by a pharmacogenomic assessment. GWAS analysis identified several novel gene candidates. The most significant SNP, rs58600101, in the gene MKL1 displayed ethnic stratification, with the variant being significantly more prevalent in the African cohort and resulting in reduced transcript levels as measured by qPCR. Functional validation of MKL1 by shRNA-mediated knockdown of MKL1 resulted in a more resistant phenotype. Gene expression analysis identified the developmentally associated TGFB1I1 as the most significant gene associated with sensitivity. qPCR among a panel of sensitive and resistant LCLs revealed immunoglobulin class-switching as well as differences in the expression of B cell activation markers. Flow cytometry showed heterogeneity within some cell lines relative to surface Ig isotype with a shift to more IgG⁺ cells among the resistant lines. Pretreatment with prednisolone could partly reverse the resistant phenotype. Results suggest that the efficacy of anti-CD20 mAb therapy may be influenced by B cell developmental status as well as polymorphism in the MKL1 gene. A clinical benefit may be achieved by pretreatment with corticosteroids such as prednisolone followed by mAb therapy.

B cell receptor signaling in germinal centers prolongs survival and primes B cells for selection

Germinal centers (GCs) are sites of B cell clonal expansion, diversification, and antibody affinity selection. This process is limited and directed by T follicular helper cells that provide helper signals to B cells that endocytose, process, and present cognate antigens in proportion to their B cell receptor (BCR) affinity. Under this model, the BCR functions as an endocytic receptor for antigen capture. How signaling through the BCR contributes to selection is not well understood. To investigate the role of BCR signaling in GC selection, we developed a tracker for antigen binding and presentation and a Bruton's tyrosine kinase drug-resistant-mutant mouse model. We showed that BCR signaling per se is necessary for the survival and priming of light zone B cells to receive T cell help. Our findings provide insight into how high-affinity antibodies are selected within GCs and are fundamental to our understanding of adaptive immunity and vaccine development.

CARD11 gain-of-function mutation drives cell-autonomous accumulation of PD-1+ ICOShigh activated T cells, T-follicular, T-regulatory and T-follicular regulatory cells

Introduction

Germline CARD11 gain-of-function (GOF) mutations cause B cell Expansion with NF-κB and T cell Anergy (BENTA) disease, whilst somatic GOF CARD11 mutations recur in diffuse large B cell lymphoma (DLBCL) and in up to 30% of the peripheral T cell lymphomas (PTCL) adult T cell leukemia/lymphoma (ATL), cutaneous T cell lymphoma (CTCL) and Sezary Syndrome. Despite their frequent acquisition by PTCL, the T cell-intrinsic effects of CARD11 GOF mutations are poorly understood.

Methods

Here, we studied B and T lymphocytes in mice with a germline Nethyl-N-nitrosourea (ENU)-induced Card11^M365K mutation identical to a mutation identified in DLBCL and modifying a conserved region of the CARD11 coiled-coil domain recurrently mutated in DLBCL and PTCL.

Results and discussion

Our results demonstrate that CARD11.M365K is a GOF protein that increases B and T lymphocyte activation and proliferation following antigen receptor stimulation. Germline Card11^M365K mutation was insufficient alone to cause B or T-lymphoma, but increased accumulation of germinal center (GC) B cells in unimmunized and immunized mice. Card11^M365K mutation caused cell-intrinsic over-accumulation of activated T cells, T regulatory (T_REG), T follicular (T_FH) and T follicular regulatory (T_FR) cells expressing increased levels of ICOS, CTLA-4 and PD-1 checkpoint molecules. Our results reveal CARD11 as an important, cell-autonomous positive regulator of T_FH, T_REG and T_FR cells. They highlight T cell-intrinsic effects of a GOF mutation in the CARD11 gene, which is recurrently mutated in T cell malignancies that are often aggressive and associated with variable clinical outcomes.

Memory B cells and long-lived plasma cells in AMR

Antibody-mediated rejection (AMR) has a strongly negative impact on long-term renal allograft survival. Currently, no recognized effective treatments are available, especially for chronic antibody-mediated rejection (CAMR). Donor-specific antibodies (DSAs) secreted by long-lived plasma cells and memory B cells are acknowledged as biomarkers of AMR. Nevertheless, it may be too late for the DSA routine examination production since DSAs may have binded to graft vascular endothelial cells through complement-dependent or complement-independent pathways. Therefore, methods to effectively monitor memory B cells and long-lived plasma cells and subsequently prevent DSA production are key to reducing the adverse effects of AMR. Therefore, this review mainly summarizes the production pathways of memory B cells and long-lived plasma cells and provides suggestions for the prevention of AMR after transplantation.

GPR55 in B cells limits atherosclerosis development and regulates plasma cell maturation

Dissecting the pathways regulating the adaptive immune response in atherosclerosis is of particular therapeutic interest. Here we report that the lipid G-protein coupled receptor GPR55 is highly expressed by splenic plasma cells (PC), upregulated in mouse spleens during atherogenesis and human unstable or ruptured compared to stable plaques. Gpr55-deficient mice developed larger atherosclerotic plaques with increased necrotic core size compared to their corresponding controls. Lack of GPR55 hyperactivated B cells, disturbed PC maturation and resulted in immunoglobulin (Ig)G overproduction. B cell-specific Gpr55 depletion or adoptive transfer of Gpr55-deficient B cells was sufficient to promote plaque development and elevated IgG titers. In vitro, the endogenous GPR55 ligand lysophsophatidylinositol (LPI) enhanced PC proliferation, whereas GPR55 antagonism blocked PC maturation and increased their mitochondrial content. Collectively, these discoveries provide previously undefined evidence for GPR55 in B cells as a key modulator of the adaptive immune response in atherosclerosis.

STAT5B restrains human B-cell differentiation to maintain humoral immune homeostasis

BACKGROUND

Lymphocyte differentiation is regulated by coordinated actions of cytokines and signaling pathways. IL-21 activates STAT1, STAT3, and STAT5 and is fundamental for the differentiation of human B cells into memory cells and antibody-secreting cells. While STAT1 is largely nonessential and STAT3 is critical for this process, the role of STAT5 is unknown.

OBJECTIVES

This study sought to delineate unique roles of STAT5 in activation and differentiation of human naive and memory B cells.

METHODS

STAT activation was assessed by phospho-flow cytometry cell sorting. Differential gene expression was determined by RNA-sequencing and quantitative PCR. The requirement for STAT5B in B-cell and CD4+ T-cell differentiation was assessed using CRISPR-mediated STAT5B deletion from B-cell lines and investigating primary lymphocytes from individuals with germline STAT5B mutations.

RESULTS

IL-21 activated STAT5 and strongly induced SOCS3 in human naive, but not memory, B cells. Deletion of STAT5B in B-cell lines diminished IL-21-mediated SOCS3 induction. PBMCs from STAT5B-null individuals contained expanded populations of immunoglobulin class-switched B cells, CD21loTbet+ B cells, and follicular T helper cells. IL-21 induced greater differentiation of STAT5B-deficient B cells into plasmablasts in vitro than B cells from healthy donors, correlating with higher expression levels of transcription factors promoting plasma cell formation.

CONCLUSIONS

These findings reveal novel roles for STAT5B in regulating IL-21-induced human B-cell differentiation. This is achieved by inducing SOCS3 to attenuate IL-21 signaling, and BCL6 to repress class switching and plasma cell generation. Thus, STAT5B is critical for restraining IL-21-mediated B-cell differentiation. These findings provide insights into mechanisms underpinning B-cell responses during primary and subsequent antigen encounter and explain autoimmunity and dysfunctional humoral immunity in STAT5B deficiency.

Instructing durable humoral immunity for COVID-19 and other vaccinable diseases

Many aspects of SARS-CoV-2 have fully conformed with the principles established by decades of viral immunology research, ultimately leading to the crowning achievement of highly effective COVID-19 vaccines. Nonetheless, the pandemic has also exposed areas where our fundamental knowledge is thinner. Some key unknowns are the duration of humoral immunity post-primary infection or vaccination and how long booster shots confer protection. As a corollary, if protection does not last as long as desired, what are some ways it can be improved? Here, I discuss lessons from other infections and vaccines that point to several key features that influence durable antibody production and the perseverance of immunity. These include (1) the specific innate sensors that are initially triggered, (2) the kinetics of antigen delivery and persistence, (3) the starting B cell receptor (BCR) avidity and antigen valency, and (4) the memory B cell subsets that are recalled by boosters. I further highlight the fundamental B cell-intrinsic and B cell-extrinsic pathways that, if understood better, would provide a rational framework for vaccines to reliably provide durable immunity.

Supplying the trip to antibody production-nutrients, signaling, and the programming of cellular metabolism in the mature B lineage

The COVID pandemic has refreshed and expanded recognition of the vital role that sustained antibody (Ab) secretion plays in our immune defenses against microbes and of the importance of vaccines that elicit Ab protection against infection. With this backdrop, it is especially timely to review aspects of the molecular programming that govern how the cells that secrete Abs arise, persist, and meet the challenge of secreting vast amounts of these glycoproteins. Whereas plasmablasts and plasma cells (PCs) are the primary sources of secreted Abs, the process leading to the existence of these cell types starts with naive B lymphocytes that proliferate and differentiate toward several potential fates. At each step, cells reside in specific microenvironments in which they not only receive signals from cytokines and other cell surface receptors but also draw on the interstitium for nutrients. Nutrients in turn influence flux through intermediary metabolism and sensor enzymes that regulate gene transcription, translation, and metabolism. This review will focus on nutrient supply and how sensor mechanisms influence distinct cellular stages that lead to PCs and their adaptations as factories dedicated to Ab secretion. Salient findings of this group and others, sometimes exhibiting differences, will be summarized with regard to the journey to a distinctive metabolic program in PCs.

Defective Allelic Exclusion by IgD in the Absence of Autoantigen

A considerable proportion of peripheral B cells is autoreactive, and it is unclear how the activation of such potentially harmful cells is regulated. In this study, we show that the different activation thresholds or IgM and IgD BCRs adjust B cell activation to the diverse requirements during development. We rely on the autoreactive 3-83 model BCR to generate and analyze mice expressing exclusively autoreactive IgD BCRs on two different backgrounds that determine two stages of autoreactivity, depending on the presence or absence of the cognate Ag. By comparing these models with IgM-expressing control mice, we found that, compared with IgM, IgD has a higher activation threshold in vivo, as it requires autoantigen to enable normal B cell development, including allelic exclusion. Our data indicate that IgM provides the high sensitivity required during early developmental stages to trigger editing of any autoreactive specificities, including those enabling weak interaction with autoantigen. In contrast, IgD has the unique ability to neglect weakly interacting autoantigens while retaining reactivity to higher-affinity Ag. This IgD function enables mature B cells to ignore autoantigens while remaining able to efficiently respond to foreign threats.

Germinal center-dependent and -independent memory B cells produced throughout the immune response

Memory B cells comprise a heterogenous group of cells that differ in origin and phenotype. During the early phases of the immune response, activated B cells can differentiate into IgM-expressing memory cells, short-lived plasma cells, or seed germinal centers (GCs). The memory compartment is subsequently enriched by B cells that have been through several rounds of division and selection in the GC. Here, we report on the use of an unbiased lineage-tracking approach to explore the origins and properties of memory B cell subsets in mice with an intact immune system. We find that activated B cells continue to differentiate into memory B cells throughout the immune response. When defined on the basis of their origins, the memory B cells originating from activated B cells or GCs differ in isotype and overall gene expression, somatic hypermutation, and their affinity for antigen.

Positive selection of IgG+ over IgM+ B cells in the germinal center reaction

Positive selection of high-affinity B cells within germinal centers (GCs) drives affinity maturation of antibody responses. Here, we examined the mechanism underlying the parallel transition from immunoglobulin M (IgM) to IgG. Early GCs contained mostly unswitched IgM⁺ B cells; IgG⁺ B cells subsequently increased in frequency, dominating GC responses 14-21 days after antigen challenge. Somatic hypermutation and generation of high-affinity clones occurred with equal efficiency among IgM⁺ and IgG⁺ GC B cells, and inactivation of Ig class-switch recombination did not prevent depletion of IgM⁺ GC B cells. Instead, high-affinity IgG⁺ GC B cells outcompeted high-affinity IgM⁺ GC B cells via a selective advantage associated with IgG antigen receptor structure but independent of the extended cytoplasmic tail. Thus, two parallel forms of GC B-cell-positive selection, based on antigen receptor variable and constant regions, respectively, operate in tandem to ensure high-affinity IgG antibodies predominate in mature serum antibody responses.

Oncogenic Mutations of MYD88 and CD79B in Diffuse Large B-Cell Lymphoma and Implications for Clinical Practice

Diffuse large B-cell lymphoma (DLBCL) is the most common non-Hodgkin's lymphoma in adults. Despite the recognition of transcriptional subtypes with distinct functional characteristics, patient outcomes have not been substantially altered since the advent of chemoimmunotherapy (CIT) twenty years ago. Recently, a few pivotal studies added to the disease heterogeneity by describing several activating mutations, which have been associated with disease presentation, B-cell function and behavior, and final outcome. DLBCL arises from antigen exposed B-cells, with the B-cell receptor (BCR) playing a central role. BCR-activity related mutations, such as CD79B and MYD88, are responsible for chronic activation of the BCR in a substantial subset of patients. These mutations, often coexisting in the same patient, have been found in a substantial subset of patients with immune-privileged (IP) sites DLBCLs, and are drivers of lymphoma development conferring tissue-specific homing properties. Both mutations have been associated with disease behavior, including tumor response either to CIT or to BCR-targeted therapy. The recognition of CD79B and MYD88 mutations will contribute to the heterogeneity of the disease, both in recognizing the BCR as a potential therapeutic target and in providing genetic tools for personalized treatment.

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

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

Pathogenic B-cell receptor signaling in lymphoid malignancies: New insights to improve treatment

Signals emanating from the B-cell receptor (BCR) promote proliferation and survival in diverse forms of B-cell lymphoma. Precision medicine strategies targeting the BCR pathway have been generally effective in treating lymphoma, but often fail to produce durable responses in diffuse large B-cell lymphoma (DLBCL), a common and aggressive cancer. New insights into DLBCL biology garnered from genomic analyses and functional proteogenomic studies have identified novel modes of BCR signaling in this disease. Herein, we describe the distinct roles of antigen-dependent and antigen-independent BCR signaling in different subtypes of DLBCL. We highlight mechanisms by which the BCR cooperates with TLR9 and mutant isoforms of MYD88 to drive sustained NF-κB activity in the activated B-cell-like (ABC) subtype of DLBCL. Finally, we discuss progress in detecting and targeting oncogenic BCR signaling to improve the survival of patients with lymphoma.

Structural and immunogenomic insights into B-cell receptor activation

B cells express B-cell receptors (BCRs) which recognize antigen to trigger signaling cascades for B-cell activation and subsequent antibody production. BCR activation has a crucial influence on B-cell fate. How BCR is activated upon encountering antigen remains to be solved, although tremendous progresses have been achieved in the past few years. Here, we summarize the models that have been proposed to explain BCR activation, including the cross-linking model, the conformation-induced oligomerization model, the dissociation activation model, and the conformational change model. Especially, we elucidate the partially resolved structures of antibodies and/or BCRs by far and discusse how these current structural and further immunogenomic messages and more importantly the future studies may shed light on the explanation of BCR activation and the relevant diseases in the case of dysregulation.

Coordinated roles for glycans in regulating the inhibitory function of CD22 on B cells

CD22 is an inhibitory B cell co-receptor that recognizes sialic acid-containing glycoconjugates as ligands. Interactions with its glycan ligands are key to regulating the ability of CD22 to modulate B cell function, the most widely explored of which is antagonizing B cell receptor (BCR) signaling. Most importantly, interactions of CD22 with ligands on the same cell (cis) control the organization of CD22 on the cell surface, which minimizes co-localization with the BCR. In contrast with the modest ability of CD22 to intrinsically dampen BCR signaling, glycan ligands presented on another cell (trans) along with an antigen drawn CD22 and the BCR together within an immunological synapse, strongly inhibiting BCR signaling. New concepts are emerging for how CD22 controls B cell function, such as changes in glycosylation at different stages of B cell differentiation, specifically on GC B cells. Related to these changes, new players, such galectin-9, have been discovered that regulate cell surface nanoclusters of CD22. Roles of glycan ligands in controlling CD22 are the primary focus of this review as we highlight the ability of CD22 to modulate B cell function.

Molecular regulation of peripheral B cells and their progeny in immunity

In this review, Boothby et al. summarize some salient advances toward elucidation of the molecular programming of the fate choices and function of B cells in the periphery. They also note unanswered questions that pertain to differences among subsets of B lymphocytes and plasma cells.

Mature B lymphocytes are crucial components of adaptive immunity, a system essential for the evolutionary fitness of mammals. Adaptive lymphocyte function requires an initially naïve cell to proliferate extensively and its progeny to have the capacity to assume a variety of fates. These include either terminal differentiation (the long-lived plasma cell) or metastable transcriptional reprogramming (germinal center and memory B cells). In this review, we focus principally on the regulation of differentiation and functional diversification of the “B2” subset. An overview is combined with an account of more recent advances, including initial work on mechanisms that eliminate DNA methylation and potential links between intracellular metabolites and chromatin editing.

CD22: A Regulator of Innate and Adaptive B Cell Responses and Autoimmunity

CD22 (Siglec 2) is a receptor predominantly restricted to B cells. It was initially characterized over 30 years ago and named “CD22” in 1984 at the 2nd International workshop in Boston (1). Several excellent reviews have detailed CD22 functions, CD22-regulated signaling pathways and B cell subsets regulated by CD22 or Siglec G (2–4). This review is an attempt to highlight recent and possibly forgotten findings. We also describe the role of CD22 in autoimmunity and the great potential for CD22-based immunotherapeutics for the treatment of autoimmune diseases such as systemic lupus erythematosus (SLE).

Profiling the origin, dynamics, and function of traction force in B cell activation

B lymphocytes use B cell receptors (BCRs) to recognize membrane-bound antigens to further initiate cell spreading and contraction responses during B cell activation. We combined traction force microscopy and live-cell imaging to profile the origin, dynamics, and function of traction force generation in these responses. We showed that B cell activation required the generation of 10 to 20 nN of traction force when encountering antigens presented by substrates with stiffness values from 0.5 to 1 kPa, which mimic the rigidity of antigen-presenting cells in vivo. Perturbation experiments revealed that F-actin remodeling and myosin- and dynein-mediated contractility contributed to traction force generation and B cell activation. Moreover, membrane-proximal BCR signaling molecules (including Lyn, Syk, Btk, PLC-γ2, BLNK, and Vav3) and adaptor molecules (Grb2, Cbl, and Dok-3) linking BCR microclusters and motor proteins were also required for the sustained generation of these traction forces. We found a positive correlation between the strength of the traction force and the mean fluorescence intensity of the BCR microclusters. Furthermore, we demonstrated that isotype-switched memory B cells expressing immunoglobulin G (IgG)-BCRs generated greater traction forces than did mature naïve B cells expressing IgM-BCRs during B cell activation. Last, we observed that primary B cells from patients with rheumatoid arthritis generated greater traction forces than did B cells from healthy donors in response to antigen stimulation. Together, these data delineate the origin, dynamics, and function of traction force during B cell activation.

Role of B-cell receptors for B-cell development and antigen-induced differentiation

B-cell development is characterized by a number of tightly regulated selection processes. Signals through the B-cell receptor (BCR) guide and are required for B-cell maturation, survival, and fate decision. Here, we review the role of the BCR during B-cell development, leading to the emergence of B1, marginal zone, and peripheral follicular B cells. Furthermore, we discuss BCR-derived signals on activated B cells that lead to germinal center and plasma cell differentiation.

Do Memory B Cells Form Secondary Germinal Centers? Impact of Antibody Class and Quality of Memory T-Cell Help at Recall

Antigen recall can clearly induce a germinal center (GC) reaction. What has become an issue for debate are the origins of the antigen-specific B cells that form memory-response GCs (mGCs). Using antigen labeling and adoptive transfer, memory B cells expressing different antibody class can give rise to mGCs with differing efficiency. Here, we will argue that the range of class-specific memory responses reported across multiple systems represents the spectrum of memory B-cell fate and function. While the formulation of recall immunogen and location of mGCs have an important role, we propose that effective cognate regulation is the key variable influencing recall outcome. These issues remain central to contemporary efforts of rational vaccine design.

Human CD22 Inhibits Murine B Cell Receptor Activation in a Human CD22 Transgenic Mouse Model

CD22, a sialic acid-binding Ig-type lectin (Siglec) family member, is an inhibitory coreceptor of the BCR with established roles in health and disease. The restricted expression pattern of CD22 on B cells and most B cell lymphomas has made CD22 a therapeutic target for B cell-mediated diseases. Models to better understand how in vivo targeting of CD22 translates to human disease are needed. In this article, we report the development of a transgenic mouse expressing human CD22 (hCD22) in B cells and assess its ability to functionally substitute for murine CD22 (mCD22) for regulation of BCR signaling, Ab responses, homing, and tolerance. Expression of hCD22 on transgenic murine B cells is comparable to expression on human primary B cells, and it colocalizes with mCD22 on the cell surface. Murine B cells expressing only hCD22 have identical calcium (Ca²⁺) flux responses to anti-IgM as mCD22-expressing wild-type B cells. Furthermore, hCD22 transgenic mice on an mCD22^-/- background have restored levels of marginal zone B cells and Ab responses compared with deficiencies observed in CD22^-/- mice. Consistent with these observations, hCD22 transgenic mice develop normal humoral responses in a peanut allergy oral sensitization model. Homing of B cells to Peyer's patches was partially rescued by expression of hCD22 compared with CD22^-/- B cells, although not to wild-type levels. Notably, Siglec-engaging antigenic liposomes formulated with an hCD22 ligand were shown to prevent B cell activation, increase cell death, and induce tolerance in vivo. This hCD22 transgenic mouse will be a valuable model for investigating the function of hCD22 and preclinical studies targeting hCD22.

Accelerated Systemic Autoimmunity in the Absence of Somatic Hypermutation in 564Igi: A Mouse Model of Systemic Lupus with Knocked-In Heavy and Light Chain Genes

564Igi mice have knocked-in immunoglobulin (Ig) heavy (H) and light (L) chain genes that encode an autoantibody recognizing RNA. Previously, we showed that these mice produce pathogenic IgG autoantibodies when activation-induced deaminase (AID) is expressed in pre-B and immature B cells but not when it is expressed only in mature B cells. AID has two functions; it is necessary for somatic hypermutation (SHM) and class switch recombination (CSR). To determine the role of each of these functions in the generation of pathogenic autoantibodies, we generated 564Igi mice that carry a mutant AID-encoding gene, Aicda (Aicda^G23S), which is capable of promoting CSR but not SHM. We found that 564Igi Aicda^G23S mice secreted class-switched antibodies (Abs) at levels approximately equal to 564Igi mice. However, compared to 564Igi mice, 564Igi Aicda^G23S mice had increased pathogenic IgG Abs and severe systemic lupus erythematosus-like disease, including, glomerulonephritis, and early death. We suggest that in 564Igi mice SHM by AID changes Ig receptors away from self reactivity, thereby mitigating the production of autoantibody, providing a novel mechanism of tolerance.

Efficiency and Safety of CRAC Inhibitors in Human Rheumatoid Arthritis Xenograft Models

Store-operated Ca²⁺ release-activated Ca²⁺ (CRAC) channels are involved in the pathogenesis of rheumatoid arthritis (RA) and have been studied as therapeutic targets in the management of RA. We investigated the efficacy and safety of CRAC inhibitors, including a neutralizing Ab (hCRACM1-IgG) and YM-58483, in the treatment of RA. Patient-derived T cell and B cell activity was suppressed by hCRACM1-IgG as well as YM-58483. Systemically constant, s.c. infused CRAC inhibitors showed anti-inflammatory activity in a human-NOD/SCID xenograft RA model as well as protective effects against the destruction of cartilage and bone. hCRACM1-IgG appeared to be safe for systemic application, whereas YM-58483 showed hepatic and renal toxicity in xenograft mice. Treatment with both CRAC inhibitors also caused hyperglycemia in xenograft mice. These results indicate the potential of hCRACM1-IgG and YM-58483 as anti-immunological agents for the treatment of RA. However, some safety issues should be addressed and application methods should be optimized prior to their clinical use.

Differentiation of germinal center B cells into plasma cells is initiated by high-affinity antigen and completed by Tfh cells

Kräutler et al. show that differentiation of antibody-producing plasma cells from germinal center (GC) B cell precursors is initiated by direct contact with high-affinity antigen within the GC but completed by separate signals delivered by collaborating, GC-resident T follicular helper cells.

Plasma cells (PCs) derived from germinal centers (GCs) secrete the high-affinity antibodies required for long-term serological immunity. Nevertheless, the process whereby GC B cells differentiate into PCs is uncharacterized, and the mechanism underlying the selective PC differentiation of only high-affinity GC B cells remains unknown. In this study, we show that differentiation into PCs is induced among a discrete subset of high-affinity B cells residing within the light zone of the GC. Initiation of differentiation required signals delivered upon engagement with intact antigen. Signals delivered by T follicular helper cells were not required to initiate differentiation but were essential to complete the differentiation process and drive migration of maturing PCs through the dark zone and out of the GC. This bipartite or two-signal mechanism has likely evolved to both sustain protective immunity and avoid autoantibody production.

CD22 is required for formation of memory B cell precursors within germinal centers

CD22 is a BCR co-receptor that regulates B cell signaling, proliferation and survival and is required for T cell-independent Ab responses. To investigate the role of CD22 during T cell-dependent (TD) Ab responses and memory B cell formation, we analyzed Ag-specific B cell responses generated by wild-type (WT) or CD22-/- B cells following immunization with a TD Ag. CD22-/- B cells mounted normal early Ab responses yet failed to generate either memory B cells or long-lived plasma cells, whereas WT B cells formed both populations. Surprisingly, B cell expansion and germinal center (GC) differentiation were comparable between WT and CD22-/- B cells. CD22-/- B cells, however, were significantly less capable of generating a population of CXCR4hiCD38hi GC B cells, which we propose represent memory B cell precursors within GCs. These results demonstrate a novel role for CD22 during TD humoral responses evident during primary GC formation and underscore that CD22 functions not only during B cell maturation but also during responses to both TD and T cell-independent antigens.

High-throughput sequencing of the B-cell receptor in African Burkitt lymphoma reveals clues to pathogenesis

Burkitt lymphoma (BL), the most common pediatric cancer in sub-Saharan Africa, is a malignancy of antigen-experienced B lymphocytes. High-throughput sequencing (HTS) of the immunoglobulin heavy (IGH) and light chain (IGK/IGL) loci was performed on genomic DNA from 51 primary BL tumors: 19 from Uganda and 32 from Ghana. Reverse transcription polymerase chain reaction analysis and tumor RNA sequencing (RNAseq) was performed on the Ugandan tumors to confirm and extend the findings from the HTS of tumor DNA. Clonal IGH and IGK/IGL rearrangements were identified in 41 and 46 tumors, respectively. Evidence for rearrangement of the second IGH allele was observed in only 6 of 41 tumor samples with a clonal IGH rearrangement, suggesting that the normal process of biallelic IGHD to IGHJ diversity-joining (DJ) rearrangement is often disrupted in BL progenitor cells. Most tumors, including those with a sole dominant, nonexpressed DJ rearrangement, contained many IGH and IGK/IGL sequences that differed from the dominant rearrangement by < 10 nucleotides, suggesting that the target of ongoing mutagenesis of these loci in BL tumor cells is not limited to expressed alleles. IGHV usage in both BL tumor cohorts revealed enrichment for IGHV genes that are infrequently used in memory B cells from healthy subjects. Analysis of publicly available DNA sequencing and RNAseq data revealed that these same IGHV genes were overrepresented in dominant tumor-associated IGH rearrangements in several independent BL tumor cohorts. These data suggest that BL derives from an abnormal B-cell progenitor and that aberrant mutational processes are active on the immunoglobulin loci in BL cells.

The nanoscale spatial organization of B-cell receptors on immunoglobulin M- and G-expressing human B-cells

In resting and activated B-cells, IgM and IgG B-cell receptors (BCRs) reside in highly heterogeneous protein islands that vary in size and number of BCRs but intrinsically maintain a high frequency of BCR monomers. IgG BCRs are more clustered than IgM BCRs on resting cells and form larger protein islands after antigen activation.

B-cell activation is initiated by the binding of antigen to the B-cell receptor (BCR). Here we used dSTORM superresolution imaging to characterize the nanoscale spatial organization of immunoglobulin M (IgM) and IgG BCRs on the surfaces of resting and antigen-­activated human peripheral blood B-cells. We provide insights into both the fundamental process of antigen-driven BCR clustering and differences in the spatial organization of IgM and IgG BCRs that may contribute to the characteristic differences in the responses of naive and memory B-cells to antigen. We provide evidence that although both IgM and IgG BCRs reside in highly heterogeneous protein islands that vary in size and number of BCR single-molecule localizations, both resting and activated B-cells intrinsically maintain a high ­frequency of single isolated BCR localizations, which likely represent BCR monomers. IgG BCRs are more clustered than IgM BCRs on resting cells and form larger protein islands after antigen activation. Small, dense BCR clusters likely formed via protein–protein interactions are present on the surface of resting cells, and antigen activation induces these to come together to form less dense, larger islands, a process likely governed, at least in part, by protein–lipid interactions.

Mature IgM-expressing plasma cells sense antigen and develop competence for cytokine production upon antigenic challenge

Dogma holds that plasma cells, as opposed to B cells, cannot bind antigen because they have switched from expression of membrane-bound immunoglobulins (Ig) that constitute the B-cell receptor (BCR) to production of the secreted form of immunoglobulins. Here we compare the phenotypical and functional attributes of plasma cells generated by the T-cell-dependent and T-cell-independent forms of the hapten NP. We show that the nature of the secreted Ig isotype, rather than the chemical structure of the immunizing antigen, defines two functionally distinct populations of plasma cells. Fully mature IgM-expressing plasma cells resident in the bone marrow retain expression of a functional BCR, whereas their IgG⁺ counterparts do not. Antigen boost modifies the gene expression profile of IgM⁺ plasma cells and initiates a cytokine production program, characterized by upregulation of CCL5 and IL-10. Our results demonstrate that IgM-expressing plasma cells can sense antigen and acquire competence for cytokine production upon antigenic challenge.

Plasma cells produce secreted antibodies and are thought to lack expression of the membrane-bound immunoglobulins that constitute B-cell receptors. Here the authors show that IgM-expressing plasma cells maintain B-cell receptor expression and initiate cytokine production following antigen stimulation.

ZBTB32 Restricts the Duration of Memory B Cell Recall Responses

Memory B cell responses are more rapid and of greater magnitude than are primary Ab responses. The mechanisms by which these secondary responses are eventually attenuated remain unknown. We demonstrate that the transcription factor ZBTB32 limits the rapidity and duration of Ab recall responses. ZBTB32 is highly expressed by mouse and human memory B cells but not by their naive counterparts. Zbtb32(-/-) mice mount normal primary Ab responses to T-dependent Ags. However, Zbtb32(-/-) memory B cell-mediated recall responses occur more rapidly and persist longer than do control responses. Microarray analyses demonstrate that Zbtb32(-/-) secondary bone marrow plasma cells display elevated expression of genes that promote cell cycle progression and mitochondrial function relative to wild-type controls. BrdU labeling and adoptive transfer experiments confirm more rapid production and a cell-intrinsic survival advantage of Zbtb32(-/-) secondary plasma cells relative to wild-type counterparts. ZBTB32 is therefore a novel negative regulator of Ab recall responses.

Phosphatidylcholine as a metabolic cue for determining B cell fate and function

In activated B cells, increased production of phosphatidylcholine (PtdCho), the most abundant cellular phospholipid, is handled primarily by the CDP-choline pathway. B cell-specific deletion of CTP:phosphocholine cytidylyltransferase α (CCTα), the rate-limiting enzyme in the CDP-choline pathway, led to augmented IgM secretion and reduced IgG production, suggesting that PtdCho synthesis is required for germinal center reactions. To specifically assess whether PtdCho influences B cell fate during germinal center responses, we examined immune responses in mice whereby PtdCho synthesis is disrupted in B cells that have undergone class switch recombination to IgG1 (referred to as either Cγ1^wt/wt, Cγ1^Cre/wt or Cγ1^Cre/Cre based on Cre copy number). Serum IgG1 was markedly reduced in naïve Cγ1^Cre/wt and Cγ1^Cre/Cre mice, while levels of IgM and other IgG subclasses were similar between Cγ1^Cre/wt and Cγ1^wt/wt control mice. Serum IgG2b titers were notably reduced and IgG3 titers were increased in Cγ1^Cre/Cre mice compared with controls. Following immunization with T cell-dependent antigen NP-KLH, control mice generated high titer IgG anti-NP while IgG anti-NP titers were markedly reduced in both immunized Cγ1^Cre/wt and Cγ1^Cre/Cre mice. Correspondingly, the frequency of NP-specific IgG antibody-secreting cells was also reduced in spleens and bone marrow of Cγ1^Cre/wt and Cγ. 1^Cre/Cre mice compared to control mice. Interestingly, though antigen-specific IgM B cells were comparable between Cγ1^Cre/wt, Cγ1^Cre/Cre and control mice, the frequency and number of IgG1 NP-specific B cells was reduced only in Cγ1^Cre/Cre mice. These data indicate that PtdCho is required for the generation of both germinal center-derived B cells and antibody-secreting cells. Further, the reduction in class-switched ASC but not B cells in Cγ1^Cre/wt mice suggests that ASC have a greater demand for PtdCho compared to germinal center B cells.

Naïve and memory B cells exhibit distinct biochemical responses following BCR engagement

Immunological memory is characterized by the rapid reactivation of memory B cells that produce large quantities of high-affinity antigen-specific antibodies. This contrasts the response of naïve B cells, and the primary immune response, which is much slower and of lower affinity. Memory responses are critical for protection against infectious diseases and form the basis of most currently available vaccines. Although we have known about the phenomenon of long-lived memory for centuries, the biochemical differences underlying these diverse responses of naïve and memory B cells is incompletely resolved. Here we investigated the nature of B-cell receptor (BCR) signaling in human splenic naïve, IgM(+) memory and isotype-switched memory B cells following multivalent BCR crosslinking. We observed comparable rapid and transient phosphorylation kinetics for proximal (phosphotyrosine and spleen tyrosine kinase) and propagation (B-cell linker, phospholipase Cγ2) signaling components in these different B-cell subsets. However, the magnitude of activation of downstream components of the BCR signaling pathway were greater in memory compared with naïve cells. Although no differences were observed in the magnitude of Ca(2+) mobilization between subsets, IgM(+) memory B cells exhibited a more rapid Ca(2+) mobilization and a greater depletion of the Ca(2+) endoplasmic reticulum stores, while IgG(+) memory B cells had a prolonged Ca(2+) uptake. Collectively, our findings show that intrinsic signaling features of B-cell subsets contribute to the robust response of human memory B cells over naïve B cells. This has implications for our understanding of memory B-cell responses and provides a framework to modulate these responses in the setting of vaccination and immunopathologies, such as immunodeficiency and autoimmunity.

Independent Roles of Switching and Hypermutation in the Development and Persistence of B Lymphocyte Memory

Somatic hypermutation (SHM) and class-switch recombination (CSR) increase the affinity and diversify the effector functions of antibodies during immune responses. Although SHM and CSR are fundamentally different, their independent roles in regulating B cell fate have been difficult to uncouple because a single enzyme, activation-induced cytidine deaminase (encoded by Aicda), initiates both reactions. Here, we used a combination of Aicda and antibody mutant alleles that separate the effects of CSR and SHM on polyclonal immune responses. We found that class-switching to IgG1 biased the fate choice made by B cells, favoring the plasma cell over memory cell fate without significantly affecting clonal expansion in the germinal center (GC). In contrast, SHM reduced the longevity of memory B cells by creating polyreactive specificities that were selected against over time. Our data define the independent contributions of SHM and CSR to the generation and persistence of memory in the antibody system.

Survival of human lymphoma cells requires B-cell receptor engagement by self-antigens

The activated B-cell-like (ABC) subtype of diffuse large B-cell lymphoma (DLBCL) relies on chronic active B-cell receptor (BCR) signaling. BCR pathway inhibitors induce remissions in a subset of ABC DLBCL patients. BCR microclusters on the surface of ABC cells resemble those generated following antigen engagement of normal B cells. We speculated that binding of lymphoma BCRs to self-antigens initiates and maintains chronic active BCR signaling in ABC DLBCL. To assess whether antigenic engagement of the BCR is required for the ongoing survival of ABC cells, we developed isogenic ABC cells that differed solely with respect to the IgH V region of their BCRs. In competitive assays with wild-type cells, substitution of a heterologous V region impaired the survival of three ABC lines. The viability of one VH4-34(+) ABC line and the ability of its BCR to bind to its own cell surface depended on V region residues that mediate the intrinsic autoreactivity of VH4-34 to self-glycoproteins. The BCR of another ABC line reacted with self-antigens in apoptotic debris, and the survival of a third ABC line was sustained by reactivity of its BCR to an idiotypic epitope in its own V region. Hence, a diverse set of self-antigens is responsible for maintaining the malignant survival of ABC DLBCL cells. IgH V regions used by the BCRs of ABC DLBCL biopsy samples varied in their ability to sustain survival of these ABC lines, suggesting a screening procedure to identify patients who might benefit from BCR pathway inhibition.

Acidic phospholipids govern the enhanced activation of IgG-B cell receptor

B cells that express the isotype-switched IgG-B cell receptor (IgG-BCR) are one of the driving forces for antibody memory. To allow for a rapid memory IgG antibody response, IgG-BCR evolved into a highly effective signalling machine. Here, we report that the positively charged cytoplasmic domain of mIgG (mIgG-tail) specifically interacts with negatively charged acidic phospholipids. The key immunoglobulin tail tyrosine (ITT) in mIgG-tail is thus sequestered in the membrane hydrophobic core in quiescent B cells. Pre-disruption of such interaction leads to excessive recruitment of BCRs and inflated BCR signalling upon antigen stimulation, resulting in hyperproliferation of primary B cells. Physiologically, membrane-sequestered mIgG-tail can be released by antigen engagement or Ca(2+) mobilization in the initiation of B cell activation. Our studies suggest a novel regulatory mechanism for how dynamic association of mIgG-tail with acidic phospholipids governs the enhanced activation of IgG-BCR.

The forkhead transcription factor FOXP1 represses human plasma cell differentiation

Expression of the forkhead transcription factor FOXP1 is essential for early B-cell development, whereas downregulation of FOXP1 at the germinal center (GC) stage is required for GC B-cell function. Aberrantly high FOXP1 expression is frequently observed in diffuse large B-cell lymphoma and mucosa-associated lymphoid tissue lymphoma, being associated with poor prognosis. Here, by gene expression analysis upon ectopic overexpression of FOXP1 in primary human memory B cells (MBCs) and B-cell lines, combined with chromatin immunoprecipitation and sequencing, we established that FOXP1 directly represses expression of PRDM1, IRF4, and XBP1, transcriptional master regulators of plasma cell (PC) differentiation. In accordance, FOXP1 is prominently expressed in primary human naive and MBCs, but expression strongly decreases during PC differentiation. Moreover, as compared with immunoglobulin (Ig) M(+) MBCs, IgG(+) MBCs combine lower expression of FOXP1 with an enhanced intrinsic PC differentiation propensity, and constitutive (over)expression of FOXP1 in B-cell lines and primary human MBCs represses their ability to differentiate into PCs. Taken together, our data indicate that proper control of FOXP1 expression plays a critical role in PC differentiation, whereas aberrant expression of FOXP1 might contribute to lymphomagenesis by blocking this terminal B-cell differentiation.

DC-SIGN-expressing macrophages trigger activation of mannosylated IgM B-cell receptor in follicular lymphoma

Follicular lymphoma (FL) results from the accumulation of malignant germinal center (GC) B cells leading to the development of an indolent and largely incurable disease. FL cells remain highly dependent on B-cell receptor (BCR) signaling and on a specific cell microenvironment, including T cells, macrophages, and stromal cells. Importantly, FL BCR is characterized by a selective pressure to retain surface immunoglobulin M (IgM) BCR despite an active class-switch recombination process, and by the introduction, in BCR variable regions, of N-glycosylation acceptor sites harboring unusual high-mannose oligosaccharides. However, the relevance of these 2 FL BCR features for lymphomagenesis remains unclear. In this study, we demonstrated that IgM(+) FL B cells activated a stronger BCR signaling network than IgG(+) FL B cells and normal GC B cells. BCR expression level and phosphatase activity could both contribute to such heterogeneity. Moreover, we underlined that a subset of IgM(+) FL samples, displaying highly mannosylated BCR, efficiently bound dendritic cell-specific intercellular adhesion molecule-3-grabbing nonintegrin (DC-SIGN), which could in turn trigger delayed but long-lasting BCR aggregation and activation. Interestingly, DC-SIGN was found within the FL cell niche in situ. Finally, M2 macrophages induced a DC-SIGN-dependent adhesion of highly mannosylated IgM(+) FL B cells and triggered BCR-associated kinase activation. Interestingly, pharmacologic BCR inhibitors abolished such crosstalk between macrophages and FL B cells. Altogether, our data support an important role for DC-SIGN-expressing infiltrating cells in the biology of FL and suggest that they could represent interesting therapeutic targets.

The activation of IgM- or isotype-switched IgG- and IgE-BCR exhibits distinct mechanical force sensitivity and threshold

B lymphocytes use B cell receptors (BCRs) to sense the physical features of the antigens. However, the sensitivity and threshold for the activation of BCRs resulting from the stimulation by mechanical forces are unknown. Here, we addressed this question using a double-stranded DNA-based tension gauge tether system serving as a predefined mechanical force gauge ranging from 12 to 56 pN. We observed that IgM-BCR activation is dependent on mechanical forces and exhibits a multi-threshold effect. In contrast, the activation of isotype-switched IgG- or IgE-BCR only requires a low threshold of less than 12 pN, providing an explanation for their rapid activation in response to antigen stimulation. Mechanistically, we found that the cytoplasmic tail of the IgG-BCR heavy chain is both required and sufficient to account for the low mechanical force threshold. These results defined the mechanical force sensitivity and threshold that are required to activate different isotyped BCRs.

How B cells remember? A sophisticated cytoplasmic tail of mIgG is pivotal for the enhanced transmembrane signaling of IgG-switched memory B cells

Antibody memory is critical for protection against many human infectious diseases and is the basis for nearly all current human vaccines. Isotype switched immunoglobulin (Ig) G-expressing memory B cells are considered as one of the fundaments for the rapid, high affinity and high-titered memory antibody response. The detailed molecular mechanism of the enhanced activation of IgG-switched memory B cells upon BCR engagement with antigens has been an elusive question in immunology. In this review, we tried to discuss all the exciting new advances revealing the molecular mechanisms of the transmembrane signaling through mIgG cytoplasmic tail in IgG-switched memory B cells.

B-cell receptor signaling in diffuse large B-cell lymphoma

The importance of understanding the genetic and biochemical basis of B-cell receptor (BCR) survival signaling in diffuse large B-cell lymphoma (DLBCL) is underscored by the recent clinical success of agents that target the BCR pathway. DLBCL is composed of multiple distinct molecular subtypes with divergent clinical outcomes. The activated B-cell-like (ABC) subtype is the most aggressive form of DLBCL and is often resistant to standard chemotherapies. ABC DLBCL expresses numerous genes found in antigen-activated B cells, and genetic and pharmacologic studies have demonstrated that ABC DLBCL tumors are addicted to NF-κB activity. The origins of this NF-κB activity remained obscure until RNA interference screens established that the majority of ABC DLBCL cell lines rely on expression of BCR components and downstream signaling effectors for NF-κB activation. Pharmacological inhibition with ibrutinib of Bruton's tyrosine kinase, a kinase that is required for BCR signaling to engage NF-κB, is selectively toxic for ABC DLBCL tumors; a finding that has now been translated to the clinic. These novel targets not only offer a promising new therapy option for ABC DLBCL, but also demonstrate the value of a deep molecular understanding of oncogenic signaling pathways.

Memory B cells in transplantation

Much of the research on the humoral response to allografts has focused on circulating serum antibodies and the long-lived plasma cells that produce these antibodies. In contrast, the interrogation of the quiescent memory B cell compartment is technically more challenging and thus has not been incorporated into the clinical diagnostic or prognostic toolkit. In this review, we discuss new technologies that have allowed this heretofore enigmatic subset of B cells to be identified at quiescence and during a recall response. These technologies in experimental models are providing new insights into memory B cell heterogeneity with respect to their phenotype, cellular function, and the antibodies they produce. Similar technologies are also allowing for the identification of comparable memory alloreactive B cells in transplant recipients. Although much of the focus in transplant immunology has been on controlling the alloreactive B cell population, long-term transplant patient survival is also critically dependent on protection by pathogen-specific memory B cells. Techniques are available that allow the interrogation of memory B cell response to pathogen re-encounter. Thus, we are poised in our ability to investigate how immunosuppression affects allospecific and pathogen-specific memory B cells, and reason that these investigations can yield new insights that will be beneficial for graft and patient survival.

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.

The biology behind PI3K inhibition in chronic lymphocytic leukaemia

Phosphoinositide 3'-kinase (PI3K) is a key component of both chronic active and tonic B-cell receptor-signalling pathways. As such, PI3K inhibitors have emerged as promising therapeutic agents for diverse lymphoid malignancies, particularly chronic lymphocytic leukaemia. Multiple in vitro experiments and clinical trials have shown efficacy of these agents across all prognostic subgroups with a favourable toxicity profile. Moreover, in vitro studies suggest that combinations with monoclonal antibodies and/or other immune strategies could enhance the effect of PI3K inhibition.

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.

Predominant role for activation-induced cytidine deaminase in generating IgG anti-nucleosomal antibodies of murine SLE

Serum IgG anti-nuclear antibodies (ANA) directed to complexes of DNA and histones are a hallmark of systemic lupus erythematosus (SLE) and reflect a failure in lymphocyte self-tolerance. A prior study utilizing spontaneously autoimmune B6.Nba2 mice deficient in terminal deoxynucleotidyl transferase (TdT) and with heterozygous deficiencies in Jh and Igk loci underscored the importance of somatic hypermutation (SHM) as a major generator of SLE-associated ANA. This interpretation had to be qualified because of severely limited opportunities for receptor editing and restricted VHCDR3 diversity. Therefore, we performed the converse study using mice that carried functional Tdt genes and wild type Jh and Igk loci but that could not undergo SHM. Analyses of ANA and ANA-producing hybridomas from B6.Nba2 Aicda(-/-) mice revealed that few animals produced high titers of the prototypical ANA directed to complexes of histones and DNA, that this response was delayed and that those cells that did produce such antibody exhibited limited clonal expansion, unusual Jk use and only infrequent dual receptor expression. This, together with the additional finding of an intrinsic propensity for SHM to generate Arg codons selectively in CDRs, reinforce the view that most IgG autoimmune clones producing prototypical anti-nucleosome antibodies in wild type mice are created by SHM.