The immunoglobulin tail tyrosine motif upgrades memory-type BCRs by incorporating a Grb2-Btk signalling module

Inhibitors of Bruton's tyrosine kinase as emerging therapeutic strategy in autoimmune diseases

Bruton's tyrosine kinase (BTK) is a cytoplasmic, non-receptor signal transducer, initially identified as an essential signaling molecule for B cells, with genetic mutations resulting in a disorder characterized by disturbed B cell and antibody development. Subsequent research revealed the critical role of BTK in the functionality of monocytes, macrophages and neutrophils. Various immune cells, among which B cells and neutrophils, rely on BTK activity for diverse signaling pathways downstream of multiple receptors, which makes this kinase an ideal target to treat hematological malignancies and autoimmune diseases. First-generation BTK inhibitors are already on the market to treat hematological disorders. It has been demonstrated that B cells and myeloid cells play a significant role in the pathogenesis of different autoimmune diseases such as multiple sclerosis, rheumatoid arthritis, systemic lupus erythematosus and primary Sjögren's syndrome. Consequently, second-generation BTK inhibitors are currently being developed to treat these disorders. Despite the acknowledged involvement of BTK in various cell types, the focus on B cells often overshadows its impact on innate immune cells. Among these cell types, neutrophils are often underestimated in the pathogenesis of autoimmune diseases. In this narrative review, the function of BTK in different immune cell subsets is discussed, after which an overview is provided of different upcoming BTK inhibitors tested for treatment of autoimmune diseases. Special attention is paid to BTK inhibition and its effect on neutrophil biology.

Ion channel TRPV2 is critical in enhancing B cell activation and function

The function of transient receptor potential vanilloid (TRPV) cation channels governing B cell activation remains to be explored. We present evidence that TRPV2 is highly expressed in B cells and plays a crucial role in the formation of the B cell immunological synapse and B cell activation. Physiologically, TRPV2 expression level is positively correlated to influenza-specific antibody production and is low in newborns and seniors. Pathologically, a positive correlation is established between TRPV2 expression and the clinical manifestations of systemic lupus erythematosus (SLE) in adult and child SLE patients. Correspondingly, mice with deficient TRPV2 in B cells display impaired antibody responses following immunization. Mechanistically, the pore and N-terminal domains of TRPV2 are crucial for gating cation permeation and executing mechanosensation in B cells upon antigen stimulation. These processes synergistically contribute to membrane potential depolarization and cytoskeleton remodeling within the B cell immunological synapse, fostering efficient B cell activation. Thus, TRPV2 is critical in augmenting B cell activation and function.

Targeting the NF-κB pathway as a potential regulator of immune checkpoints in cancer immunotherapy

Advances in cancer immunotherapy over the last decade have led to the development of several agents that affect immune checkpoints. Inhibitory receptors expressed on T cells that negatively regulate the immune response include cytotoxic T‑lymphocyte antigen 4 (CTLA4) and programmed cell death protein 1 (PD1), which have been studied more than similar receptors. Inhibition of these proteins and other immune checkpoints can stimulate the immune system to attack cancer cells, and prevent the tumor from escaping the immune response. However, the administration of anti-PD1 and anti-CTLA4 antibodies has been associated with adverse inflammatory responses similar to autoimmune diseases. The current review discussed the role of the NF-κB pathway as a tumor promoter, and how it can govern inflammatory responses and affect various immune checkpoints. More precise knowledge about the communication between immune checkpoints and NF-κB pathways could increase the effectiveness of immunotherapy and reduce the adverse effects of checkpoint inhibitor therapy.

Aberrant B cell receptor signaling in circulating naïve and IgA+ memory B cells from newly-diagnosed autoantibody-positive rheumatoid arthritis patients

OBJECTIVE

Altered B cell receptor (BCR) signaling has been implicated in the pathogenesis of rheumatoid arthritis (RA). Here we aimed to identify signaling aberrations in autoantibody-positive and autoantibody-negative RA patients by performing a comprehensive analysis of the BCR signaling cascade in different B cell subsets.

METHODS

We first optimized phosphoflow cytometry for an in-depth analysis of BCR signaling across immunoglobulin isotypes in healthy donors. Subsequently, we compared BCR signaling in circulating B cell subsets from treatment-naïve, newly-diagnosed autoantibody-positive RA and autoantibody-negative RA patients and healthy controls (HCs).

RESULTS

We observed subset-specific phosphorylation patterns of the BCR signalosome in circulating B cells from healthy donors. Compared with HCs, autoantibody-positive RA patients displayed enhanced responses to BCR stimulation for multiple signaling proteins, specifically in naïve and IgA+ memory B cells. Whereas in unstimulated healthy donor B cells, the phosphorylation status of individual signaling proteins showed only limited correlation, BCR stimulation enhanced the interconnectivity in phosphorylation within the BCR signalosome. However, this strong interconnectivity within the BCR signalosome in stimulated B cells from HCs was lost in RA, especially in autoantibody-positive RA patients. Finally, we observed strong correlations between SYK and BTK protein expression, and IgA and IgG anti-citrullinated protein antibody concentrations in serum from autoantibody-positive RA patients.

CONCLUSION

Collectively, the isotype-specific analysis of multiple key components of the BCR signalosome identified aberrant BCR signaling responses in treatment-naïve autoantibody-positive RA patients, particularly in naïve B cells and IgA+ memory B cells. Our findings support differential involvement of dysregulated BCR signaling in the pathogenesis of autoantibody-positive and autoantibody-negative RA.

N-linked Fc glycosylation is not required for IgG-B-cell receptor function in a GC-derived B-cell line

IgG secreted by B cells carry asparagine N(297)-linked glycans in the fragment crystallizable (Fc) region. Changes in Fc glycosylation are related to health or disease and are functionally relevant, as IgG without Fc glycans cannot bind to Fcɣ receptors or complement factors. However, it is currently unknown whether ɣ-heavy chain (ɣHC) glycans also influence the function of membrane-bound IgG-B-cell receptors (BCR) and thus the outcome of the B-cell immune response. Here, we show in a germinal center (GC)-derived human B-cell line that ɣHC glycans do not affect membrane expression of IgG-BCRs. Furthermore, antigen binding or other BCR-facilitated mechanisms appear unaffected, including BCR downmodulation or BCR-mediated signaling. As expected, secreted IgG lacking Fc glycosylation is unable to carry out effector functions. Together, these observations indicate that IgG-Fc glycosylation serves as a mechanism to control the effector functions of antibodies, but does not regulate the activation of IgG-switched B cells, as its absence had no apparent impact on BCR function.

B-Cell Receptor Signaling and Beyond: The Role of Igα (CD79a)/Igβ (CD79b) in Normal and Malignant B Cells

B-cell receptor (BCR) is a B cell hallmark surface complex regulating multiple cellular processes in normal as well as malignant B cells. Igα (CD79a)/Igβ (CD79b) are essential components of BCR that are indispensable for its functionality, signal initiation, and signal transduction. CD79a/CD79b-mediated BCR signaling is required for the survival of normal as well as malignant B cells via a wide signaling network. Recent studies identified the great complexity of this signaling network and revealed the emerging role of CD79a/CD79b in signal integration. In this review, we have focused on functional features of CD79a/CD79b, summarized signaling consequences of CD79a/CD79b post-translational modifications, and highlighted specifics of CD79a/CD79b interactions within BCR and related signaling cascades. We have reviewed the complex role of CD79a/CD79b in multiple aspects of normal B cell biology and how is the normal BCR signaling affected by lymphoid neoplasms associated CD79A/CD79B mutations. We have also summarized important unresolved questions and highlighted issues that remain to be explored for better understanding of CD79a/CD79b-mediated signal transduction and the eventual identification of additional therapeutically targetable BCR signaling vulnerabilities.

Proteome analysis of CD5-positive diffuse large B cell lymphoma FFPE tissue reveals downregulation of DDX3X, DNAJB1, and B cell receptor signaling pathway proteins including BTK and Immunoglobulins

BACKGROUND

The molecular pathology of diffuse large B cell lymphoma (DLBCL) has been extensively studied. Among DLBCL subtypes, the prognosis of CD5-positive DLBCL is worse than that of CD5-negative DLBCL, considering the central nervous system relapse and poor response to R-CHOP therapy. However, the molecular mechanisms underlying the tumorigenesis and progression of CD5-positive DLBCL remain unknown.

METHODS

To identify molecular markers that can be targeted for treating DLBCL, a proteomic study was performed using liquid chromatography-mass spectrometry with chemically pretreated formalin-fixed paraffin-embedded specimens from CD5-positive (n = 5) and CD5-negative DLBCL patients (n = 6).

RESULTS

Twenty-one proteins showed significant downregulation in CD5-positive DLBCL compared to CD5-negative DLBCL. Principal component analysis of protein expression profiling in CD5-positive and CD5-negative DLBCL revealed that DNAJB1, DDX3X, and BTK, which is one of the B cell phenotypic proteins, were the most significantly downregulated proteins and served as biomarkers that distinguished both groups. Additionally, a set of immunoglobulins, including IgG4, exhibited significant downregulation. Immunohistochemistry analysis for BTK demonstrated reduced staining in CD5-positive DLBCL compared to CD5-negative DLBCL.

CONCLUSIONS

In conclusion, DNAJB1 and DDX3X, BTK, and a set of immunoglobulins are promising biomarkers. Probably, the suppression of BCR signaling is the unique phenotype of CD5-positive DLBCL. This formalin-fixed paraffin-embedded (FFPE)-based profiling may help to develop novel therapeutic molecularly targeted drugs for treating DLBCL.

An Optimized Flow Cytometric Method to Demonstrate the Differentiation Stage-Dependent Ca2+ Flux Responses of Peripheral Human B Cells

Calcium (Ca²⁺) flux acts as a central signaling pathway in B cells, and its alterations are associated with autoimmune dysregulation and B-cell malignancies. We standardized a flow-cytometry-based method using various stimuli to investigate the Ca²⁺ flux characteristics of circulating human B lymphocytes from healthy individuals. We found that different activating agents trigger distinct Ca²⁺ flux responses and that B-cell subsets show specific developmental-stage dependent Ca²⁺ flux response patterns. Naive B cells responded with a more substantial Ca²⁺ flux to B cell receptor (BCR) stimulation than memory B cells. Non-switched memory cells responded to anti-IgD stimulation with a naive-like Ca²⁺ flux pattern, whereas their anti-IgM response was memory-like. Peripheral antibody-secreting cells retained their IgG responsivity but showed reduced Ca²⁺ responses upon activation, indicating their loss of dependence on Ca²⁺ signaling. Ca²⁺ flux is a relevant functional test for B cells, and its alterations could provide insight into pathological B-cell activation development.

Stimulation of the catalytic activity of the tyrosine kinase Btk by the adaptor protein Grb2

The Tec-family kinase Btk contains a lipid-binding Pleckstrin homology and Tec homology (PH-TH) module connected by a proline-rich linker to a 'Src module', an SH3-SH2-kinase unit also found in Src-family kinases and Abl. We showed previously that Btk is activated by PH-TH dimerization, which is triggered on membranes by the phosphatidyl inositol phosphate PIP3, or in solution by inositol hexakisphosphate (IP6) (Wang et al., 2015, https://doi.org/10.7554/eLife.06074). We now report that the ubiquitous adaptor protein growth-factor-receptor-bound protein 2 (Grb2) binds to and substantially increases the activity of PIP3-bound Btk on membranes. Using reconstitution on supported-lipid bilayers, we find that Grb2 can be recruited to membrane-bound Btk through interaction with the proline-rich linker in Btk. This interaction requires intact Grb2, containing both SH3 domains and the SH2 domain, but does not require that the SH2 domain be able to bind phosphorylated tyrosine residues - thus Grb2 bound to Btk is free to interact with scaffold proteins via the SH2 domain. We show that the Grb2-Btk interaction recruits Btk to scaffold-mediated signaling clusters in reconstituted membranes. Our findings indicate that PIP3-mediated dimerization of Btk does not fully activate Btk, and that Btk adopts an autoinhibited state at the membrane that is released by Grb2.

Aberrant B-Cell Activation in Systemic Lupus Erythematosus

Background

B lymphocytes (B cells) are essential in humoral response, and their activation is an important first step for the production of antibodies. However, aberrant B-cell activation is common in the development and progression of autoimmune diseases including systemic lupus erythematosus (SLE), which is characterized by the generation of superfluous autoantibodies. SLE exhibits clinical manifestation such as excessive inflammation and tissue damage. This review aims to summarize the recent emerging studies on aberrant B-cell activation and the associated concurrent therapeutic targets in SLE.

Summary

Aberrant B-cell activation is closely associated with the pathogenesis of SLE. Among a variety of mechanisms, dysregulations of B-cell receptor (BCR), toll-like receptor (TLR), and B-cell activating factor receptor (BAFF-R) pathways are the common and dominating factors involved in aberrant B-cell activation. These aberrant signaling transductions play diverse and integrated roles in the development and the pathogenesis of SLE. Therapies targeting aberrant B-cell activation have shown promising efficacy in achieving the clinical alleviation of SLE, suggesting the discovery of new drug targets from these aberrant signaling pathways is imminent. Here, an integrated survey or review of published high-throughput sequencing database covering RNAs of B cells from SLE versus criteria-matched healthy controls highlights that reported signaling molecules in BCR pathway (VAV2, PLC-γ2), TLR pathway (TLR9, P105, IRF7, TAB1), and BAFF-R pathway (SDF-1α) are attitudinally upregulated in SLE patients. This review thus suggests the concurrent and future therapeutic targets and potential biomarkers in both basic and clinical studies of SLE.

Key Messages

This review focuses on core B-cell signaling pathways, discussing the progress in the role of aberrant B-cell activation during the pathogenesis of SLE. This review also highlights the signaling molecules from published studies and database for the possible prevention and treatment targets serving the future clinical treatments of SLE.

The Small GTPase RHOA Links SLP65 Activation to PTEN Function in Pre B Cells and Is Essential for the Generation and Survival of Normal and Malignant B Cells

The generation, differentiation, survival and activation of B cells are coordinated by signals emerging from the B cell antigen receptor (BCR) or its precursor, the pre-BCR. The adaptor protein SLP65 (also known as BLNK) is an important signaling factor that controls pre-B cell differentiation by down-regulation of PI3K signaling. Here, we investigated the mechanism by which SLP65 interferes with PI3K signaling. We found that SLP65 induces the activity of the small GTPase RHOA, which activates PTEN, a negative regulator of PI3K signaling, by enabling its translocation to the plasma membrane. The essential role of RHOA is confirmed by the complete block in early B cell development in conditional RhoA-deficient mice. The RhoA-deficient progenitor B cells showed defects in activation of immunoglobulin gene rearrangement and fail to survive both in vitro and in vivo. Reconstituting the RhoA-deficient cells with RhoA or Foxo1, a transcription factor repressed by PI3K signaling and activated by PTEN, completely restores the survival defect. However, the defect in differentiation can only be restored by RhoA suggesting a unique role for RHOA in B cell generation and selection. In full agreement, conditional RhoA-deficient mice develop increased amounts of autoreactive antibodies with age. RHOA function is also required at later stage, as inactivation of RhoA in peripheral B cells or in a transformed mature B cell line resulted in cell loss. Together, these data show that RHOA is the key signaling factor for B cell development and function by providing a crucial SLP65-activated link between BCR signaling and activation of PTEN. Moreover, the identified essential role of RHOA for the survival of transformed B cells offers the opportunity for targeting B cell malignancies by blocking RHOA function.

Enhanced cell deconvolution of peripheral blood using DNA methylation for high-resolution immune profiling

DNA methylation microarrays can be employed to interrogate cell-type composition in complex tissues. Here, we expand reference-based deconvolution of blood DNA methylation to include 12 leukocyte subtypes (neutrophils, eosinophils, basophils, monocytes, naïve and memory B cells, naïve and memory CD4 + and CD8 + T cells, natural killer, and T regulatory cells). Including derived variables, our method provides 56 immune profile variables. The IDOL (IDentifying Optimal Libraries) algorithm was used to identify libraries for deconvolution of DNA methylation data for current and previous platforms. The accuracy of deconvolution estimates obtained using our enhanced libraries was validated using artificial mixtures and whole-blood DNA methylation with known cellular composition from flow cytometry. We applied our libraries to deconvolve cancer, aging, and autoimmune disease datasets. In conclusion, these libraries enable a detailed representation of immune-cell profiles in blood using only DNA and facilitate a standardized, thorough investigation of immune profiles in human health and disease.

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.

SnS2/polypyrrole for high-efficiency photocatalytic oxidation of benzylamine

Here, SnS2/polypyrrole (PPy) was synthesized, which shows high catalytic activity for the photocatalytic oxidation of benzylamine under mild conditions (at 25 oC, in air and without adding additional sacrificial reagent,...

Critical requirement for BCR, BAFF, and BAFFR in memory B cell survival

Memory B cells (MBCs) are long-lived cells that form a critical part of immunological memory, providing rapid antibody responses to recurring infections. However, very little is known about signals controlling MBC survival. Previous work has shown that antigen is not required for MBC survival, but a requirement for the B cell antigen receptor (BCR) has not been tested. Other studies have shown that, unlike naive B cells, MBCs do not express BAFFR and their survival is independent of BAFF, the ligand for BAFFR. Here, using inducible genetic ablation, we show that survival of MBCs is critically dependent on the BCR and on signaling through the associated CD79A protein. Unexpectedly, we found that MBCs express BAFFR and that their survival requires BAFF and BAFFR; hence, loss of BAFF or BAFFR impairs recall responses. Finally, we show that MBC survival requires IKK2, a kinase that transduces BAFFR signals. Thus, MBC survival is critically dependent on signaling from BCR and BAFFR.

Bruton's Tyrosine Kinase Inhibition as an Emerging Therapy in Systemic Autoimmune Disease

Systemic autoimmune disorders are complex heterogeneous chronic diseases involving many different immune cells. A significant proportion of patients respond poorly to therapy. In addition, the high burden of adverse effects caused by "classical" anti-rheumatic or immune modulatory drugs provides a need to develop more specific therapies that are better tolerated. Bruton's tyrosine kinase (BTK) is a crucial signaling protein that directly links B-cell receptor (BCR) signals to B-cell activation, proliferation, and survival. BTK is not only expressed in B cells but also in myeloid cells, and is involved in many different signaling pathways that drive autoimmunity. This makes BTK an interesting therapeutic target in the treatment of autoimmune diseases. The past decade has seen the emergence of first-line BTK small-molecule inhibitors with great efficacy in the treatment of B-cell malignancies, but with unfavorable safety profiles for use in autoimmunity due to off-target effects. The development of second-generation BTK inhibitors with superior BTK specificity has facilitated the investigation of their efficacy in clinical trials with autoimmune patients. In this review, we discuss the role of BTK in key signaling pathways involved in autoimmunity and provide an overview of the different inhibitors that are currently being investigated in clinical trials of systemic autoimmune diseases, including rheumatoid arthritis and systemic lupus erythematosus, as well as available results from completed trials.

Mechanisms for the regulation of memory B-cell recall responses in mice

Upon infection by pathogens or vaccination, the adaptive immune system rapidly but transiently produces antibodies. Some weeks later, however, long-lasting immunity is established that protects the host against the same pathogens almost for life through continuous production of antibodies on one hand and the maintenance of cytotoxic T cells on the other, collectively called immunological memory. The antibody-mediated arm, also called serological memory, is mainly exerted by long-lived plasma cells and memory B cells (MBCs). MBCs express receptors for the specific pathogens and circulate to survey the body for almost a life-long period. Upon recognizing the pathogen, MBCs clonally expand and produce a large amount of the specific antibodies to stop the infection, the process called a (memory) recall response. Although such a function of MBCs has long been known, the mechanism of how their performance is regulated has been obscure. This is due to their paucity in the body, lack of definitive surface markers and obscure ontogeny. However, recent studies have revealed the multifold mechanisms by which the recall response of MBCs is regulated: Rapid and enhanced antibody production is due to a mechanism intrinsic to MBCs; namely, upregulated expression levels of surface molecules interacting with T cells and the property of IgG-class antigen receptors; to a property of the responsible subset of MBCs; and to co-stimulation through innate receptors and cytokines. It has also been unveiled that the recall response is negatively regulated by an inhibitory receptor on MBCs and by antigens with repetitive epitopes.

B Cell Response to Vaccination

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

Single-cell analysis of human B cell maturation predicts how antibody class switching shapes selection dynamics

Protective humoral memory forms in secondary lymphoid organs where B cells undergo affinity maturation and differentiation into memory or plasma cells. Here, we provide a comprehensive roadmap of human B cell maturation with single-cell transcriptomics matched with bulk and single-cell antibody repertoires to define gene expression, antibody repertoires, and clonal sharing of B cell states at single-cell resolution, including memory B cell heterogeneity that reflects diverse functional and signaling states. We reconstruct gene expression dynamics during B cell activation to reveal a pre-germinal center state primed to undergo class switch recombination and dissect how antibody class-dependent gene expression in germinal center and memory B cells is linked with a distinct transcriptional wiring with potential to influence their fate and function. Our analyses reveal the dynamic cellular states that shape human B cell-mediated immunity and highlight how antibody isotype may play a role during their antibody-based selection.

Vav family proteins constitute disparate branching points for distinct BCR signaling pathways

Antigen recognition by B-cell antigen receptors (BCRs) activates distinct intracellular signaling pathways that control the differentiation fate of activated B lymphocytes. BCR-proximal signaling enzymes comprise protein tyrosine kinases, phosphatases, and plasma membrane lipid-modifying enzymes, whose function is furthermore coordinated by catalytically inert adaptor proteins. Here, we show that an additional class of enzymatic activity provided by guanine-nucleotide exchange factors (GEFs) of the Vav family controls BCR-proximal Ca²⁺ mobilization, cytoskeletal actin reorganization, and activation of the PI3 kinase/Akt pathway. Whereas Vav1 and Vav3 supported all of those signaling processes to different extents in a human B-cell model system, Vav2 facilitated Actin remodeling, and activation of Akt but did not promote Ca²⁺ signaling. On BCR activation, Vav1 was directly recruited to the phosphorylated BCR and to the central adaptor protein SLP65 via its Src homology 2 domain. Pharmacological inhibition or genetic inactivation of the substrates of Vav GEFs, small G proteins of the Rho/Rac family, impaired BCR-induced Ca²⁺ mobilization, probably because phospholipase Cγ2 requires activated Rac proteins for optimal activity. Our findings show that Vav family members are key relays of the BCR signalosome that differentially control distinct signaling pathways both in a catalysis-dependent and -independent manner.

GPS-PBS: A Deep Learning Framework to Predict Phosphorylation Sites that Specifically Interact with Phosphoprotein-Binding Domains

Protein phosphorylation is essential for regulating cellular activities by modifying substrates at specific residues, which frequently interact with proteins containing phosphoprotein-binding domains (PPBDs) to propagate the phosphorylation signaling into downstream pathways. Although massive phosphorylation sites (p-sites) have been reported, most of their interacting PPBDs are unknown. Here, we collected 4458 known PPBD-specific binding p-sites (PBSs), considerably improved our previously developed group-based prediction system (GPS) algorithm, and implemented a deep learning plus transfer learning strategy for model training. Then, we developed a new online service named GPS-PBS, which can hierarchically predict PBSs of 122 single PPBD clusters belonging to two groups and 16 families. By comparison, GPS-PBS achieved a highly competitive accuracy against other existing tools. Using GPS-PBS, we predicted 371,018 mammalian p-sites that potentially interact with at least one PPBD, and revealed that various PPBD-containing proteins (PPCPs) and protein kinases (PKs) can simultaneously regulate the same p-sites to orchestrate important pathways, such as the PI3K-Akt signaling pathway. Taken together, we anticipate GPS-PBS can be a great help for further dissecting phosphorylation signaling networks.

Ubiquitination of IgG1 cytoplasmic tail modulates B-cell signalling and activation

Upon antigen stimulation, IgG+ B cells rapidly proliferate and differentiate into plasma cells, which has been attributed to the characteristics of membrane-bound IgG (mIgG), but the underlying molecular mechanisms remain elusive. We have found that a part of mouse mIgG1 is ubiquitinated through the two responsible lysine residues (K378 and K386) in its cytoplasmic tail and this ubiquitination is augmented upon antigen stimulation. The ubiquitination of mIgG1 involves its immunoglobulin tail tyrosine (ITT) motif, Syk/Src-family kinases and Cbl proteins. Analysis of a ubiquitination-defective mutant of mIgG1 revealed that ubiquitination of mIgG1 facilitates its ligand-induced endocytosis and intracellular trafficking from early endosome to late endosome, and also prohibits the recycling pathway, thus attenuating the surface expression level of mIgG1. Accordingly, ligation-induced activation of B-cell receptor (BCR) signalling molecules is attenuated by the mIgG1 ubiquitination, except MAP kinase p38 whose activation is up-regulated due to the ubiquitination-mediated prohibition of mIgG1 recycling. Adaptive transfer experiments demonstrated that ubiquitination of mIgG1 facilitates expansion of germinal centre B cells. These results indicate that mIgG1-mediated signalling and cell activation is regulated by ubiquitination of mIgG1, and such regulation may play a role in expansion of germinal centre B cells.

The regulators of BCR signaling during B cell activation

B lymphocytes produce antibodies under the stimulation of specific antigens, thereby exerting an immune effect. B cells identify antigens by their surface B cell receptor (BCR), which upon stimulation, directs the cell to activate and differentiate into antibody generating plasma cells. Activation of B cells via their BCRs involves signaling pathways that are tightly controlled by various regulators. In this review, we will discuss three major BCR mediated signaling pathways (the PLC-γ2 pathway, PI3K pathway and MAPK pathway) and related regulators, which were roughly divided into positive, negative and mutual-balanced regulators, and the specific regulators of the specific signaling pathway based on regulatory effects.

B cell signaling in context

Over the past several decades, B cell antigen receptor (BCR)-induced signaling pathways have been described in extraordinary molecular detail, mainly from studies of B cell responses to antigens in vitro. BCR signaling has been shown to govern the initiation of transcriptional programs associated with B cell activation and fate decisions, as well as the BCR-dependent processing of antigen and presentation of antigen to T cells. However, although the potential of the BCR to orchestrate B cell behavior was known, there was no clear appreciation of the context in which B cells signal in secondary lymphoid organs in vivo or how that context influences signaling. In this Review, we describe the current view of the cellular consequences of BCR signaling and advances in the understanding of B cell signaling in context in vivo.

Differential organization of tonic and chronic B cell antigen receptors in the plasma membrane

Stimulation of the B cell antigen receptor (BCR) triggers signaling pathways that promote the differentiation of B cells into plasma cells. Despite the pivotal function of BCR in B cell activation, the organization of the BCR on the surface of resting and antigen-activated B cells remains unclear. Here we show, using STED super-resolution microscopy, that IgM-containing BCRs exist predominantly as monomers and dimers in the plasma membrane of resting B cells, but form higher oligomeric clusters upon stimulation. By contrast, a chronic lymphocytic leukemia-derived BCR forms dimers and oligomers in the absence of a stimulus, but a single amino acid exchange reverts its organization to monomers in unstimulated B cells. Our super-resolution microscopy approach for quantitatively analyzing cell surface proteins may thus help reveal the nanoscale organization of immunoreceptors in various cell types.

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.

Epstein-Barr virus microRNAs regulate B cell receptor signal transduction and lytic reactivation

MicroRNAs (miRNAs) are post-transcriptional regulatory RNAs that can modulate cell signaling and play key roles in cell state transitions. Epstein-Barr virus (EBV) expresses >40 viral miRNAs that manipulate both viral and cellular gene expression patterns and contribute to reprogramming of the host environment during infection. Here, we identified a subset of EBV miRNAs that desensitize cells to B cell receptor (BCR) stimuli, and attenuate the downstream activation of NF-kappaB or AP1-dependent transcription. Bioinformatics and pathway analysis of Ago PAR-CLIP datasets identified multiple EBV miRNA targets related to BCR signal transduction, including GRB2, SOS1, MALT1, RAC1, and INPP5D, which we validated in reporter assays. BCR signaling is critical for B cell activation, proliferation, and differentiation, and for EBV, is linked to reactivation. In functional assays, we demonstrate that EBV miR-BHRF1-2-5p contributes to the growth of latently infected B cells through GRB2 regulation. We further determined that activities of EBV miR-BHRF1-2-5p, EBV miR-BART2-5p, and a cellular miRNA, miR-17-5p, directly regulate virus reactivation triggered by BCR engagement. Our findings provide mechanistic insight into some of the key miRNA interactions impacting the proliferation of latently infected B cells and importantly, governing the latent to lytic switch.

An autoimmune disease variant of IgG1 modulates B cell activation and differentiation

The maintenance of autoreactive B cells in a quiescent state is crucial for preventing autoimmunity. Here we identify a variant of human immunoglobulin G1 (IgG1) with a Gly³⁹⁶→Arg substitution (hIgG1-G396R), which positively correlates with systemic lupus erythematosus. In induced lupus models, murine homolog Gly³⁹⁰→Arg (G390R) knockin mice generate excessive numbers of plasma cells, leading to a burst of broad-spectrum autoantibodies. This enhanced production of antibodies is also observed in hapten-immunized G390R mice, as well as in influenza-vaccinated human G396R homozygous carriers. This variant potentiates the phosphorylation of the IgG1 immunoglobulin tail tyrosine (ITT) motif. This, in turn, alters the availability of phospho-ITT to trigger longer adaptor protein Grb2 dwell times in immunological synapses, leading to hyper-Grb2-Bruton's tyrosine kinase (Btk) signaling upon antigen binding. Thus, the hIgG1-G396R variant is important for both lupus pathogenesis and antibody responses after vaccination.

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.

Grb2 and GRAP connect the B cell antigen receptor to Erk MAP kinase activation in human B cells

The B cell antigen receptor (BCR) employs enzymatically inactive adaptor proteins to facilitate activation of intracellular signaling pathways. In animal model systems, adaptor proteins of the growth factor receptor-bound 2 (Grb2) family have been shown to serve critical functions in lymphocytes. However, the roles of Grb2 and the Grb2-related adaptor protein (GRAP) in human B lymphocytes remain unclear. Using TALEN-mediated gene targeting, we show that in human B cells Grb2 and GRAP amplify signaling by the immunoglobulin tail tyrosine (ITT) motif of mIgE-containing BCRs and furthermore connect immunoreceptor tyrosine-based activation motif (ITAM) signaling to activation of the Ras-controlled Erk MAP kinase pathway. In contrast to mouse B cells, BCR-induced activation of Erk in human B cells is largely independent of phospholipase C-ɣ activity and diacylglycerol-responsive members of Ras guanine nucleotide releasing proteins. Together, our results demonstrate that Grb2 family adaptors are critical regulators of ITAM and ITT signaling in naïve and IgE-switched human B cells.

Role of Bruton's tyrosine kinase in B cells and malignancies

Bruton’s tyrosine kinase (BTK) is a non-receptor kinase that plays a crucial role in oncogenic signaling that is critical for proliferation and survival of leukemic cells in many B cell malignancies. BTK was initially shown to be defective in the primary immunodeficiency X-linked agammaglobulinemia (XLA) and is essential both for B cell development and function of mature B cells. Shortly after its discovery, BTK was placed in the signal transduction pathway downstream of the B cell antigen receptor (BCR). More recently, small-molecule inhibitors of this kinase have shown excellent anti-tumor activity, first in animal models and subsequently in clinical studies. In particular, the orally administered irreversible BTK inhibitor ibrutinib is associated with high response rates in patients with relapsed/refractory chronic lymphocytic leukemia (CLL) and mantle-cell lymphoma (MCL), including patients with high-risk genetic lesions. Because ibrutinib is generally well tolerated and shows durable single-agent efficacy, it was rapidly approved for first-line treatment of patients with CLL in 2016. To date, evidence is accumulating for efficacy of ibrutinib in various other B cell malignancies. BTK inhibition has molecular effects beyond its classic role in BCR signaling. These involve B cell-intrinsic signaling pathways central to cellular survival, proliferation or retention in supportive lymphoid niches. Moreover, BTK functions in several myeloid cell populations representing important components of the tumor microenvironment. As a result, there is currently a considerable interest in BTK inhibition as an anti-cancer therapy, not only in B cell malignancies but also in solid tumors. Efficacy of BTK inhibition as a single agent therapy is strong, but resistance may develop, fueling the development of combination therapies that improve clinical responses. In this review, we discuss the role of BTK in B cell differentiation and B cell malignancies and highlight the importance of BTK inhibition in cancer therapy.

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.

Unperturbed Immune Function despite Mutation of C-Terminal Tyrosines in Syk Previously Implicated in Signaling and Activity Regulation

The nonreceptor tyrosine kinase Syk, a central regulator of immune cell differentiation and activation, is a promising drug target for treatment of leukemia and allergic and inflammatory diseases. The clinical failure of Syk inhibitors underscores the importance of understanding the regulation of Syk function and activity. A series of previous studies emphasized the importance of three C-terminal tyrosines in Syk for kinase activity regulation, as docking sites for downstream effector molecules, and for Ca²⁺ mobilization. Here, we investigated the roles of these C-terminal tyrosines in the mouse. Surprisingly, expression of a triple tyrosine-to-phenylalanine human Syk mutant, SYK(Y3F), was not associated with discernible signaling defects either in reconstituted DT40 cells or in B or mast cells from mice expressing SYK(Y3F) instead of wild-type Syk. Remarkably, lymphocyte differentiation, calcium mobilization, and 2,4,6-trinitrophenyl (TNP)-specific immune responses were unperturbed in SYK(Y3F) mice. These results emphasize the capacity of immune cells to compensate for specific molecular defects, likely using redundant intermolecular interactions, and highlight the importance of in vivo analyses for understanding cellular signaling mechanisms.

Probing Transient Release of Membrane-Sequestered Tyrosine-Based Signaling Motif by Solution NMR Spectroscopy

Membrane sequestration of tyrosine-based signaling motifs of antigen receptors effectively restricts the signaling activities in resting lymphocytes. However, low level of basal signaling in resting cells is required for lymphocyte survival and antigen responsiveness, of which the molecular mechanism remains obscure. Here we probe the transient release of the cytoplasmic domain of the membrane-bound IgG heavy chain (mIgG-tail) by hydrogen exchange NMR spectroscopy, illustrating a dynamic molecular basis for its basal signaling activity. To solve the severe resonance overlap problem in the 2D spectra of mIgG-tail, a non-uniformly sampled pseudo-4D hydrogen exchange NMR experiment has been exploited to quantitatively measure site-specific hydrogen exchange rates. Our solution NMR study reveals transient solvent exposure of the ITT signaling motif that can be further enhanced by calcium ion, and provides insight into the mechanism of lymphocyte basal signaling.

Memory B Cells of Mice and Humans

We comprehensively review memory B cells (MBCs), covering the definition of MBCs and their identities and subsets, how MBCs are generated, where they are localized, how they are maintained, and how they are reactivated. Whereas naive B cells adopt multiple fates upon stimulation, MBCs are more restricted in their responses. Evolving work reveals that the MBC compartment in mice and humans consists of distinct subpopulations with differing effector functions. We discuss the various approaches to define subsets and subset-specific roles. A major theme is the need to both deliver faster effector function upon reexposure and readapt to antigenically variant pathogens while avoiding burnout, which would be the result if all MBCs generated only terminal effector function. We discuss cell-intrinsic differences in gene expression and signaling that underlie differences in function between MBCs and naive B cells and among MBC subsets and how this leads to memory responses.

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.

Antigen Receptor Nanoclusters: Small Units with Big Functions

Adaptive lymphocytes express highly variable antigen receptors, allowing them to recognize a large variety of proteins, for example, derived from pathogens and tumor cells. Despite decades of investigations, the signaling mechanisms of these receptors are still incompletely understood. Super-resolution imaging studies revealed that antigen receptors, their coreceptors, and even some downstream signaling molecules tend to form dynamic nanometers-sized self-clusters in quiescent cells. Antigen stimulation induces the coalescence of these nanoclusters to form membrane proximal signalosomes that can mediate efficient signal transduction. In this review, we discuss the dynamic structures of T cell receptor and B cell receptor nanoclusters, the driving forces behind this spatial reorganization, as well as their potential relevance in the modulation of lymphocyte activation and function.

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.

Regulation of germinal center responses, memory B cells and plasma cell formation-an update

Progress in understanding humoral immunity has been accelerated by the powerful experimental approaches of genetics, genomics and imaging. Excellent reviews of these advances appeared in 2015 in celebration of the 50th anniversary of the discovery of B cell and T cell lineages in the chicken. Here we provide a contemporary model of B cell differentiation, highlighting recent publications illuminating germinal center (GC), memory B cell and antibody-secreting plasma cell biology. The important contributions of CD4T cells to antibody responses have been thoroughly reviewed elsewhere.

Reactivation of IgG-switched memory B cells by BCR-intrinsic signal amplification promotes IgG antibody production

Secondary antibody responses are marked by faster kinetics, improved antibody affinity and a switch from IgM to other immunoglobulin isotypes, most notably IgG, compared with primary responses. These changes protect from reinfection and represent the principle of most vaccination strategies. Yet, the molecular mechanisms that underlie B-cell memory responses are unclear. Here we show, by inactivating the immunoglobulin tail tyrosine (ITT) signalling motif of membrane-bound IgG1 in the mouse, that the ITT facilitates maintenance and reactivation of IgG-switched memory B cells in vivo. The ITT motif equips IgG-switched cells with enhanced BCR signalling capacity, which supports their competitiveness in secondary immune reactions and drives the formation of IgG-secreting plasma cells even in the absence of T-cell help. Our results demonstrate that ITT signalling promotes the vigorous production of IgG antibodies and thus provide a molecular basis for humoral immunological memory.

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 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.

Role of Calcium Signaling in B Cell Activation and Biology

Increase in intracellular levels of calcium ions (Ca2+) is one of the key triggering signals for the development of B cell response to the antigen. The diverse Ca2+ signals finely controlled by multiple factors participate in the regulation of gene expression, B cell development, and effector functions. B cell receptor (BCR)-initiated Ca2+ mobilization is sourced from two pathways: one is the release of Ca2+ from the intracellular stores, endoplasmic reticulum (ER), and other is the prolonged influx of extracellular Ca2+ induced by depleting the stores via store-operated calcium entry (SOCE) and calcium release-activated calcium (CRAC) channels. The identification of stromal interaction molecule 1(STIM1), the ER Ca2+ sensor, and Orai1, a key subunit of the CRAC channel pore, has now provided the tools to understand the mode of Ca2+ influx regulation and physiological relevance. Herein, we discuss our current understanding of the molecular mechanisms underlying BCR-triggered Ca2+ signaling as well as its contribution to the B cell biological processes and diseases.