Qualitative regulation of B cell antigen receptor signaling by CD19: selective requirement for PI3-kinase activation, inositol-1,4,5-trisphosphate production and Ca2+ mobilization

Peripheral apoptosis and limited clonal deletion during physiologic murine B lymphocyte development

Self-reactive and polyreactive B cells generated during B cell development are silenced by either apoptosis, clonal deletion, receptor editing or anergy to avoid autoimmunity. The specific contribution of apoptosis to normal B cell development and self-tolerance is incompletely understood. Here, we quantify self-reactivity, polyreactivity and apoptosis during physiologic B lymphocyte development. Self-reactivity and polyreactivity are most abundant in early immature B cells and diminish significantly during maturation within the bone marrow. Minimal apoptosis still occurs at this site, however B cell receptors cloned from apoptotic B cells show comparable self-reactivity to that of viable cells. Apoptosis increases dramatically only following immature B cells leaving the bone marrow sinusoids, but above 90% of cloned apoptotic transitional B cells are not self-reactive/polyreactive. Our data suggests that an apoptosis-independent mechanism, such as receptor editing, removes most self-reactive B cells in the bone marrow. Mechanistically, lack of survival signaling rather than clonal deletion appears to be the underpinning cause of apoptosis in most transitional B cells in the periphery.

The Src-family kinase Lyn plays a critical role in establishing and maintaining B cell anergy by suppressing PI3K-dependent signaling

Although the Src family kinase (SFK) Lyn is known to be involved in induction and maintenance of peripheral B cell tolerance, the molecular basis of its action in this context remains unclear. This question has been approached using conventional as well as B cell-targeted knockouts of Lyn, with varied conclusions likely confused by collateral loss of Lyn functions in B cell and myeloid cell development and activation. Here we utilized a system in which Lyn gene deletion is tamoxifen inducible and B cell restricted. This system allows acute elimination of Lyn in B cells without off-target effects. This genetic tool was employed in conjunction with immunoglobulin transgenic mice in which peripheral B cells are autoreactive. DNA reactive Ars/A1 B cells require continuous inhibitory signaling, mediated by the inositol phosphatase SHIP-1 and the tyrosine phosphatase SHP-1, to maintain an unresponsive (anergic) state. Here we show that Ars/A1 B cells require Lyn to establish and maintain B cell unresponsiveness. Lyn primarily functions by restricting PI3K-dependent signaling pathways. This Lyn-dependent mechanism complements the impact of reduced mIgM BCR expression to restrict BCR signaling in Ars/A1 B cells. Our findings suggest that a subset of autoreactive B cells requires Lyn to become anergic and that the autoimmunity associated with dysregulated Lyn function may, in part, be due to an inability of these autoreactive B cells to become tolerized.

Transcriptomic Landscape of Circulating Extracellular Vesicles in Heart Transplant Ischemia-Reperfusion

Ischemia-reperfusion injury (IRI) is an inevitable event during heart transplantation, which is known to exacerbate damage to the allograft. However, the precise mechanisms underlying IRI remain incompletely understood. Here, we profiled the whole transcriptome of plasma extracellular vesicles (EVs) by RNA sequencing from 41 heart transplant recipients immediately before and at 12 h after transplant reperfusion. We found that the expression of 1317 protein-coding genes in plasma EVs was changed at 12 h after reperfusion. Upregulated genes of plasma EVs were related to metabolism and immune activation, while downregulated genes were related to cell survival and extracellular matrix organization. In addition, we performed correlation analyses between EV transcriptome and intensity of graft IRI (i.e., cardiomyocyte injury), as well as EV transcriptome and primary graft dysfunction, as well as any biopsy-proven acute rejection after heart transplantation. We ultimately revealed that at 12 h after reperfusion, 4 plasma EV genes (ITPKA, DDIT4L, CD19, and CYP4A11) correlated with both cardiomyocyte injury and primary graft dysfunction, suggesting that EVs are sensitive indicators of reperfusion injury reflecting lipid metabolism-induced stress and imbalance in calcium homeostasis. In conclusion, we show that profiling plasma EV gene expression may enlighten the mechanisms of heart transplant IRI.

Peripheral death by neglect and limited clonal deletion during physiologic B lymphocyte development

Autoreactive B cells generated during B cell development are inactivated by clonal deletion, receptor editing or anergy. Up to 97% of immature B cells appear to die before completing maturation, but the anatomic sites and reasons underlying this massive cell loss are not fully understood. Here, we directly quantitated apoptosis and clonal deletion during physiologic B lymphocyte development using Rosa26INDIA apoptosis indicator mice. Immature B cells displayed low levels of apoptosis in the bone marrow but started dying at high levels in the periphery upon release from bone marrow sinusoids into the blood circulation. Clonal deletion of self-reactive B cells was neither a major contributor to apoptosis in the bone marrow nor the periphery. Instead, most peripheral transitional 1 B cells did not encounter the signals required for positive selection into the mature B cell compartments. This study sheds new light on B cell development and suggests that receptor editing and/or anergy efficiently control most primary autoreactivity in mice.

AKT activity orchestrates marginal zone B cell development in mice and humans

The signals controlling marginal zone (MZ) and follicular (FO) B cell development remain incompletely understood. Here, we show that AKT orchestrates MZ B cell formation in mice and humans. Genetic models that increase AKT signaling in B cells or abolish its impact on FoxO transcription factors highlight the AKT-FoxO axis as an on-off switch for MZ B cell formation in mice. In humans, splenic immunoglobulin (Ig) D+CD27+ B cells, proposed as an MZ B cell equivalent, display higher AKT signaling than naive IgD+CD27- and memory IgD-CD27+ B cells and develop in an AKT-dependent manner from their precursors in vitro, underlining the conservation of this developmental pathway. Consistently, CD148 is identified as a receptor indicative of the level of AKT signaling in B cells, expressed at a higher level in MZ B cells than FO B cells in mice as well as humans.

Decrypting drug actions and protein modifications by dose- and time-resolved proteomics

Although most cancer drugs modulate the activities of cellular pathways by changing posttranslational modifications (PTMs), little is known regarding the extent and the time- and dose-response characteristics of drug-regulated PTMs. In this work, we introduce a proteomic assay called decryptM that quantifies drug-PTM modulation for thousands of PTMs in cells to shed light on target engagement and drug mechanism of action. Examples range from detecting DNA damage by chemotherapeutics, to identifying drug-specific PTM signatures of kinase inhibitors, to demonstrating that rituximab kills CD20-positive B cells by overactivating B cell receptor signaling. DecryptM profiling of 31 cancer drugs in 13 cell lines demonstrates the broad applicability of the approach. The resulting 1.8 million dose-response curves are provided as an interactive molecular resource in ProteomicsDB.

CD19 Is Internalized Together with IgM in Proportion to B Cell Receptor Stimulation and Is Modulated by Phosphatidylinositol 3-Kinase in Bone Marrow Immature B Cells

Newly generated immature B cells that bind self-antigen with high avidity arrest in differentiation and undergo central tolerance via receptor editing and clonal deletion. These autoreactive immature B cells also express low surface levels of the coreceptor CD19, a key activator of the PI3K pathway. Signals emanating from both CD19 and PI3K are known to be critical for attenuating receptor editing and selecting immature B cells into the periphery. However, the mechanisms that modulate CD19 expression at this stage of B cell development have not yet been resolved. Using in vivo and in vitro models, we demonstrate that Cd19 de novo gene transcription and translation do not significantly contribute to the differences in CD19 surface expression in mouse autoreactive and nonautoreactive immature B cells. Instead, CD19 downregulation is induced by BCR stimulation in proportion to BCR engagement, and the remaining surface IgM and CD19 molecules promote intracellular PI3K-AKT activity in proportion to their level of expression. The internalized CD19 is degraded with IgM by the lysosome, but inhibiting lysosome-mediated protein degradation only slightly improves surface CD19. In fact, CD19 is restored only upon Ag removal. Our data also reveal that the PI3K-AKT pathway positively modulates CD19 surface expression in immature B cells via a mechanism that is independent of inhibition of FOXO1 and its role on Cd19 gene transcription while is dependent on mTORC1.

Current Status of Novel Agents for the Treatment of B Cell Malignancies: What's Coming Next?

Resistance to death is one of the hallmarks of human B cell malignancies and often contributes to the lack of a lasting response to today's commonly used treatments. Drug discovery approaches designed to activate the death machinery have generated a large number of inhibitors of anti-apoptotic proteins from the B-cell lymphoma/leukemia 2 family and the B-cell receptor (BCR) signaling pathway. Orally administered small-molecule inhibitors of Bcl-2 protein and BCR partners (e.g., Bruton's tyrosine kinase and phosphatidylinositol-3 kinase) have already been included (as monotherapies or combination therapies) in the standard of care for selected B cell malignancies. Agonistic monoclonal antibodies and their derivatives (antibody-drug conjugates, antibody-radioisotope conjugates, bispecific T cell engagers, and chimeric antigen receptor-modified T cells) targeting tumor-associated antigens (TAAs, such as CD19, CD20, CD22, and CD38) are indicated for treatment (as monotherapies or combination therapies) of patients with B cell tumors. However, given that some patients are either refractory to current therapies or relapse after treatment, novel therapeutic strategies are needed. Here, we review current strategies for managing B cell malignancies, with a focus on the ongoing clinical development of more effective, selective drugs targeting these molecules, as well as other TAAs and signaling proteins. The observed impact of metabolic reprogramming on B cell pathophysiology highlights the promise of targeting metabolic checkpoints in the treatment of these disorders.

The inositol 5-phosphatase INPP5B regulates B cell receptor clustering and signaling

Upon antigen binding, the B cell receptor (BCR) undergoes clustering to form a signalosome that propagates downstream signaling required for normal B cell development and physiology. BCR clustering is dependent on remodeling of the cortical actin network, but the mechanisms that regulate actin remodeling in this context remain poorly defined. In this study, we identify the inositol 5-phosphatase INPP5B as a key regulator of actin remodeling, BCR clustering, and downstream signaling in antigen-stimulated B cells. INPP5B acts via dephosphorylation of the inositol lipid PI(4,5)P2 that in turn is necessary for actin disassembly, BCR mobilization, and cell spreading on immobilized surface antigen. These effects can be explained by increased actin severing by cofilin and loss of actin linking to the plasma membrane by ezrin, both of which are sensitive to INPP5B-dependent PI(4,5)P2 hydrolysis. INPP5B is therefore a new player in BCR signaling and may represent an attractive target for treatment of B cell malignancies caused by aberrant BCR signaling.

Role of inhibitory signaling in peripheral B cell tolerance*

At least 20% of B cells in the periphery expresses an antigen receptor with a degree of self-reactivity. If activated, these autoreactive B cells pose a risk as they can contribute to the development of autoimmune diseases. To prevent their activation, both B cell-intrinsic and extrinsic tolerance mechanisms are in place in healthy individuals. In this review article, I will focus on B cell-intrinsic mechanisms that prevent the activation of autoreactive B cells in the periphery. I will discuss how inhibitory signaling circuits are established in autoreactive B cells, focusing on the Lyn-SHIP-1-SHP-1 axis, how they contribute to peripheral immune tolerance, and how disruptions of these circuits can contribute to the development of autoimmunity.

Developmental partitioning of SYK and ZAP70 prevents autoimmunity and cancer

Even though SYK and ZAP70 kinases share high sequence homology and serve analogous functions, their expression in B and T cells is strictly segregated throughout evolution. Here, we identified aberrant ZAP70 expression as a common feature in a broad range of B cell malignancies. We validated SYK as the kinase that sets the thresholds for negative selection of autoreactive and premalignant clones. When aberrantly expressed in B cells, ZAP70 competes with SYK at the BCR signalosome and redirects SYK from negative selection to tonic PI3K signaling, thereby promoting B cell survival. In genetic mouse models for B-ALL and B-CLL, conditional expression of Zap70 accelerated disease onset, while genetic deletion impaired malignant transformation. Inducible activation of Zap70 during B cell development compromised negative selection of autoreactive B cells, resulting in pervasive autoantibody production. Strict segregation of the two kinases is critical for normal B cell selection and represents a central safeguard against the development of autoimmune disease and B cell malignancies.

Development of Novel Mouse Monoclonal Antibodies Against Human CD19

CD19 is a type I transmembrane glycoprotein belonging to the immunoglobulin superfamily. It is expressed in normal and neoplastic B cells, and it modulates the threshold of B cell activation for amplifying B cell receptor signaling. Blinatumomab (a CD3-CD19-bispecific T cell-engaging antibody) and tisagenlecleucel (genetically modified T cells that express a CD19 chimeric antigen receptor [CART-19]) provide significant benefits for patients with CD19-positive relapsed or refractory B cell malignancies. In this study, we first employed the Cell-Based Immunization and Screening (CBIS) method to produce anti-CD19 monoclonal antibodies using CD19-overexpressing cells for both immunization and screening. One established clone-C₁₉Mab-1-proved to be useful in flow cytometry assays against lymphoma cell lines, such as BALL-1, P30/OHK, and Raji. Second, the extracellular domain of CD19 was immunized into mice, and enzyme-linked immunosorbent assays were performed for the first screening. One established clone-C₁₉Mab-3-was determined to be useful for Western blotting and immunohistochemical analysis. Due to their complementary utility, a combination of C₁₉Mab-1 (established using CBIS) and C₁₉Mab-3 (established using conventional method) could be useful for the pathological analysis of CD19.

Dietary Clostridium butyricum supplementation modifies significantly the liver transcriptomic profile in weaned piglets

The addition of probiotics in swine nutrition is known to positively influence both health and growth. The current study investigates differences in the hepatic transcriptome profiles between weaned piglets supplemented with Clostridium butyricum (C. butyricum) and control animals that received no probiotic. The liver is an important metabolic organ that plays a critical role in oxidizing triglycerides for energy production, lipid synthesis and degradation, as well as immune regulation in animals. RNA-Seq analysis was carried out on total RNA harvested from the liver of piglets fed with (n = 3) or without (n = 3) 5 × 10⁵ C. butyricum CFU/g. Compared to the control piglets, 588 of the genes examined (352 up-regulated and 236 down-regulated) were significantly differentially expressed at a fold change > 2 and p < .05 in animals fed with C. butyricum. Quantitative real-time reverse transcription PCR (qRT-PCR) analysis was further used to validate the microarray expression results for 28 genes tested. The functional annotation analyses revealed several genes, processes and pathways with putative involvement in piglet growth and performance. Feeding swine with 5 × 10⁵ C. butyricum CFU/g appears to reinforce their immune status as well as foster the cell cycle and improve the metabolism of carbohydrates, lipids and amino acids. This study provides valuable information about the expression profiles of mRNAs in piglet liver and in-depth functional investigations of these mRNAs that could provide new insights into the molecular networks of growth, immune responses and nutrient metabolism in the porcine liver.

Arp2/3 complex-driven spatial patterning of the BCR enhances immune synapse formation, BCR signaling and B cell activation

When B cells encounter antigens on the surface of an antigen-presenting cell (APC), B cell receptors (BCRs) are gathered into microclusters that recruit signaling enzymes. These microclusters then move centripetally and coalesce into the central supramolecular activation cluster of an immune synapse. The mechanisms controlling BCR organization during immune synapse formation, and how this impacts BCR signaling, are not fully understood. We show that this coalescence of BCR microclusters depends on the actin-related protein 2/3 (Arp2/3) complex, which nucleates branched actin networks. Moreover, in murine B cells, this dynamic spatial reorganization of BCR microclusters amplifies proximal BCR signaling reactions and enhances the ability of membrane-associated antigens to induce transcriptional responses and proliferation. Our finding that Arp2/3 complex activity is important for B cell responses to spatially restricted membrane-bound antigens, but not for soluble antigens, highlights a critical role for Arp2/3 complex-dependent actin remodeling in B cell responses to APC-bound antigens.

Co-engaging CD47 and CD19 with a bispecific antibody abrogates B-cell receptor/CD19 association leading to impaired B-cell proliferation

CD19 is a B cell-specific receptor that regulates the threshold of B cell receptor (BCR)-mediated cell proliferation. A CD47xCD19 bispecific antibody (biAb) was generated to target and deplete B cells via multiple antibody-mediated mechanisms. Interestingly, the biAb, constructed of a CD19 binding arm and a CD47 binding arm, inhibited BCR-mediated B-cell proliferation with an effect even more potent than a CD19 monoclonal antibody (mAb). The inhibitory effect of the biAb was not attributable to CD47 binding because a monovalent or bivalent anti-CD47 mAb had no effect on B cell proliferation. Fluorescence resonance energy transfer analysis demonstrated that co-engaging CD19 and CD47 prevented CD19 clustering and its migration to BCR clusters, while only engaging CD19 (with a mAb) showed no impact on either CD19 clustering or migration. The lack of association between CD19 and the BCR resulted in decreased phosphorylation of CD19 upon BCR activation. Furthermore, the biAb differentially modulated BCR-induced gene expression compared to a CD19 mAb. Taken together, this unexpected role of CD47xCD19 co-ligation in inhibiting B cell proliferation illuminates a novel approach in which two B cell surface molecules can be tethered, to one another in order, which may provide a therapeutic benefit in settings of autoimmunity and B cell malignancies.

The c-Myc/miR17-92/PTEN Axis Tunes PI3K Activity to Control Expression of Recombination Activating Genes in Early B Cell Development

Appropriate PI3K signals generated by the antigen receptor are essential to promote B cell development. Regulation of recombination activating gene (RAG)-1 and RAG-2 expression is one key process that is mediated by PI3K to ensure developmental progression and selection. When PI3K signals are too high or too low, expression of RAGs does not turn off and B cell development is impaired or blocked. Yet, the mechanism which tunes PI3K activity to control RAG expression during B cell development in the bone marrow is unknown. Recently we showed that a c-Myc/miR17-92/PTEN axis regulates PI3K activity for positive and negative selection of immature B cells. Here, we show that the c-Myc/miR17-92/PTEN axis tunes PI3K activity to control the expression of RAGs in proB cells. Using different genetically engineered mouse models we show that impaired function of the c-Myc/miR17-92/PTEN axis alters the PI3K/Akt/Foxo1 pathway to result in dis-regulated expression of RAG and a block in B cell development. Studies using 38c-13 B lymphoma cells, where RAGs are constitutively expressed, suggest that this regulatory effect is mediated post-translationally through Foxo1.

Growth of B Cell Receptor Microclusters Is Regulated by PIP2 and PIP3 Equilibrium and Dock2 Recruitment and Activation

The growth of B cell receptor (BCR) microclusters upon antigen stimulation drives B cell activation. Here, we show that PI3K-mediated PIP₃ production is required for the growth of BCR microclusters. This growth is likely inhibited by PTEN and dependent on its plasma membrane binding and lipid phosphatase activities. Mechanistically, we find that PIP₃-dependent recruitment and activation of a guanine nucleotide exchange factor, Dock2, is required for the sustained growth of BCR microclusters through remodeling of the F-actin cytoskeleton. As a consequence, Dock2 deficiency significantly disrupts the structure of the B cell immunological synapse. Finally, we find that primary B cells from systemic lupus erythematosus (SLE) patients exhibit more prominent BCR and PI3K microclusters than B cells from healthy controls. These results demonstrate the importance of a PI3K- and PTEN-governed PIP₂ and PIP₃ equilibrium in regulating the activation of B cells through Dock2-controlled growth of BCR microclusters.

Protein Kinase C-β Dictates B Cell Fate by Regulating Mitochondrial Remodeling, Metabolic Reprogramming, and Heme Biosynthesis

PKCβ-null (Prkcb-/-) mice are severely immunodeficient. Here we show that mice whose B cells lack PKCβ failed to form germinal centers and plasma cells, which undermined affinity maturation and antibody production in response to immunization. Moreover, these mice failed to develop plasma cells in response to viral infection. At the cellular level, we have shown that Prkcb-/- B cells exhibited defective antigen polarization and mTORC1 signaling. While altered antigen polarization impaired antigen presentation and likely restricted the potential of GC development, defective mTORC1 signaling impaired metabolic reprogramming, mitochondrial remodeling, and heme biosynthesis in these cells, which altogether overwhelmingly opposed plasma cell differentiation. Taken together, our study reveals mechanistic insights into the function of PKCβ as a key regulator of B cell polarity and metabolic reprogramming that instructs B cell fate.

CD22 Promotes B-1b Cell Responses to T Cell-Independent Type 2 Antigens

CD22 (Siglec-2) is a critical regulator of B cell activation and survival. CD22^-/- mice generate significantly impaired Ab responses to T cell-independent type 2 (TI-2) Ags, including haptenated Ficoll and pneumococcal polysaccharides, Ags that elicit poor T cell help and activate BCR signaling via multivalent epitope crosslinking. This has been proposed to be due to impaired marginal zone (MZ) B cell development/maintenance in CD22^-/- mice. However, mice expressing a mutant form of CD22 unable to bind sialic acid ligands generated normal TI-2 Ab responses, despite significantly reduced MZ B cells. Moreover, mice treated with CD22 ligand-binding blocking mAbs, which deplete MZ B cells, had little effect on TI-2 Ab responses. We therefore investigated the effects of CD22 deficiency on B-1b cells, an innate-like B cell population that plays a key role in TI-2 Ab responses. B-1b cells from CD22^-/- mice had impaired BCR-induced proliferation and significantly increased intracellular Ca²⁺ concentration responses following BCR crosslinking. Ag-specific B-1b cell expansion and plasmablast differentiation following TI-2 Ag immunization was significantly impaired in CD22^-/- mice, consistent with reduced TI-2 Ab responses. We generated CD22^-/- mice with reduced CD19 levels (CD22^-/-CD19^+/-) to test the hypothesis that augmented B-1b cell BCR signaling in CD22^-/- mice contributes to impaired TI-2 Ab responses. BCR-induced proliferation and intracellular Ca²⁺ concentration responses were normalized in CD22^-/-CD19^+/- B-1b cells. Consistent with this, TI-2 Ag-specific B-1b cell expansion, plasmablast differentiation, survival, and Ab responses were rescued in CD22^-/-CD19^+/- mice. Thus, CD22 plays a critical role in regulating TI-2 Ab responses through regulating B-1b cell signaling thresholds.

CD5 expression promotes IL-10 production through activation of the MAPK/Erk pathway and upregulation of TRPC1 channels in B lymphocytes

CD5 is constitutively expressed on T cells and a subset of mature normal and leukemic B cells in patients with chronic lymphocytic leukemia (CLL). Important functional properties are associated with CD5 expression in B cells, including signal transducer and activator of transcription 3 activation, IL-10 production and the promotion of B-lymphocyte survival and transformation. However, the pathway(s) by which CD5 influences the biology of B cells and its dependence on B-cell receptor (BCR) co-signaling remain unknown. In this study, we show that CD5 expression activates a number of important signaling pathways, including Erk1/2, leading to IL-10 production through a novel pathway independent of BCR engagement. This pathway is dependent on extracellular calcium (Ca²⁺) entry facilitated by upregulation of the transient receptor potential channel 1 (TRPC1) protein. We also show that Erk1/2 activation in a subgroup of CLL patients is associated with TRPC1 overexpression. In this subgroup of CLL patients, small inhibitory RNA (siRNA) for CD5 reduces TRPC1 expression. Furthermore, siRNAs for CD5 or for TRPC1 inhibit IL-10 production. These findings provide new insights into the role of CD5 in B-cell biology in health and disease and could pave the way for new treatment strategies for patients with B-CLL.

Targeting B-cell receptor signaling in leukemia and lymphoma: how and why?

B-lymphocytes are dependent on B-cell receptor (BCR) signaling for the constant maintenance of their physiological function, and in many B-cell malignancies this signaling pathway is prone to aberrant activation. This understanding has led to an ever-increasing interest in the signaling networks activated following ligation of the BCR in both normal and malignant cells, and has been critical in establishing an array of small molecule inhibitors targeting BCR-induced signaling. By dissecting how different malignancies signal through BCR, researchers are contributing to the design of more customized therapeutics which have greater efficacy and lower toxicity than previous therapies. This allows clinicians access to an array of approaches to best treat patients whose malignancies have BCR signaling as a driver of pathogenesis.

CD19 controls Toll-like receptor 9 responses in human B cells

BACKGROUND

CD19 is a B cell-specific molecule that serves as a major costimulatory molecule for amplifying B-cell receptor (BCR) responses. Biallelic CD19 gene mutations cause common variable immunodeficiency in human subjects. BCR- and Toll-like receptor (TLR) 9-induced B-cell responses are impaired in most patients with common variable immunodeficiency.

OBJECTIVE

We sought to analyze whether CD19 is required for TLR9 function in human B cells.

METHODS

Expression of surface activation markers was assessed after anti-IgM or CpG stimulation by using flow cytometry on B cells from patients with 1 or 2 defective CD19 alleles, which decrease or abrogate CD19 expression, respectively. The phosphorylation or interaction of signaling molecules was analyzed by using phospho flow cytometry, immunoblotting, or co-immunoprecipitation in CD19-deficient or control B cells and in a B-cell line in which CD19 has been knocked down with lentivirus-transduced short hairpin RNA.

RESULTS

B cells from subjects with 1 or 2 defective CD19 alleles showed defective upregulation in vitro of CD86, transmembrane activator and CAML interactor (TACI), and CD23 activation markers after TLR9 stimulation. TLR9 ligands normally induce phosphorylation of CD19 through myeloid differentiation primary response gene-88 (MYD88)/proline-rich tyrosine kinase 2 (PYK2)/LYN complexes, which allows recruitment of phosphoinositide 3-kinase (PI3K) and phosphorylation of Bruton tyrosine kinase (BTK) and AKT in human B cells with a different kinetic than that of BCRs. In addition, inhibition of PI3K, AKT, or BTK, as well as BTK deficiency, also resulted in TLR9 activation defects in B cells similar to those in patients with CD19 deficiency.

CONCLUSION

CD19 is required for TLR9-induced B-cell activation. Hence CD19/PI3K/AKT/BTK is an essential axis integrating BCRs and TLR9 signaling in human B cells.

mTOR-Dependent and Independent Survival Signaling by PI3K in B Lymphocytes

Peripheral B lymphocyte survival requires the B cell receptor (BCR) and B cell activating factor (BAFF) binding to its receptor (BAFF-R). Deletion of the BCR, or its signal transducing chaperone Igβ, leads to rapid loss of mature B cells, indicating that signals initiated at the BCR are crucial for B cell survival. BAFF or BAFF-R deficiency also significantly reduces the numbers of mature B cells despite normal BCR expression. Together, these observations indicate that continued BCR and BAFF-R signaling are essential for the survival of mature resting B cells in the periphery. Here we demonstrate that tonic BCR signals up-regulate p100 (Nfkb2) as well as Mcl-1 protein expression at a post-transcriptional level via a PI3K-dependent pathway. p100 expression is mTOR-independent, whereas Mcl-1 expression is mTOR-dependent. BAFF treatment further elevated Mcl-1 levels by an mTOR-independent pathway, while consuming p100. Accordingly, Mcl-1 induction by BAFF is abrogated in Nfkb2^-/- B cells. We propose that the cumulative effects of the BCR and BAFF-R signaling pathways increase Mcl-1 levels beyond the threshold required for B cell survival.

Management of chronic lymphocytic leukemia (CLL) in the era of B-cell receptor signal transduction inhibitors

The treatment of patients with chronic lymphocytic leukemia (CLL), an indolent B-cell lymphoma is in the midst of a transformation. There are a large number of promising new therapeutic agents and cellular therapies being studied which exhibit remarkable activity, favorable toxicity profiles, convenient administration schedules, and treatment options are rapidly expanding. The recent advances in the management of CLL exemplify the value of translational medicine. This review highlights key aspects of B-cell receptor (BCR) signaling in relation to novel inhibitors of the BCR signaling pathway, currently at various stages of preclinical and clinical development.

A new mouse anti-mouse complement receptor type 2 and 1 (CR2/CR1) monoclonal antibody as a tool to study receptor involvement in chronic models of immune responses and disease

Although reagents are available to block mouse complement receptor type 2 and/or type 1 (CR2/CR1, CD21/CD35) function in acute or short term models of human disease, a mouse anti-rat antibody response limits their use in chronic models. We have addressed this problem by generating in Cr2−/− mice a mouse monoclonal antibody (mAb 4B2) to mouse CR2/CR1. The binding of murine mAb 4B2 to CR2/CR1 directly blocked C3dg (C3d) ligand binding. In vivo injection of mAb 4B2 induced substantial down regulation of CR2 and CR1 from the B cell surface, an effect that lasted six weeks after a single injection of 2 mg of mAb. The 4B2 mAb was studied in vivo for the capability to affect immunological responses to model antigens. Pre-injection of mAb 4B2 before immunization of C57BL/6 mice reduced the IgG1 antibody response to the T-dependent antigen sheep red blood cells (SRBC) to a level comparable to that found in Cr2−/− mice. We also used the collagen-induced arthritis (CIA) model, a CR2/CR1-dependent autoimmune disease model, and found that mice pre-injected with mAb 4B2 demonstrated substantially reduced levels of pathogenic IgG2a antibodies to both the bovine type II collagen (CII) used to induce arthritis and to endogenous mouse CII. Consistent with this result, mice pre-injected with mAb 4B2 demonstrated only very mild arthritis. This reduction in disease, together with published data in CII-immunized Cr2−/− mice, confirm both that the arthritis development depends on CR2/CR1 receptors and that mAb 4B2 can be used to induce biologically relevant receptor blockade. Thus mAb 4B2 is an excellent candidate for use in chronic murine models to determine how receptor blockage at different points modifies disease activity and autoantibody responses.

B-cell expansion and lymphomagenesis induced by chronic CD40 signaling is strictly dependent on CD19

CD40, a member of the TNF receptor family, is expressed on all mature B cells and on most B-cell lymphomas. Recently, we have shown that constitutive activation of CD40 signaling in B cells induced by a fusion protein consisting of the transmembrane part of the Epstein-Barr viral latent membrane protein 1 (LMP1) and the cytoplasmic part of CD40 (LMP1/CD40) drives B-cell lymphoma development in transgenic mice. Because LMP1/CD40-expressing B cells showed an upregulation of CD19, we investigated CD19's function in CD40-driven B-cell expansion and lymphomagenesis. Here, we demonstrate that ablation of CD19 in LMP1/CD40 transgenic mice resulted in a severe loss and reduced lifespan of mature B cells and completely abrogated development of B-cell lymphoma. CD19 is localized to lipid rafts and constitutively activated by the LMP1/CD40 fusion protein in B cells. We provide evidence that the improved survival and malignant transformation of LMP1/CD40-expressing B cells are dependent on activation of the MAPK Erk that is mediated through CD19 in a PI3K-dependent manner. Our data suggest that constitutively active CD40 is dependent on CD19 to transmit survival and proliferation signals. Moreover, we detected a similarly functioning prosurvival pathway involving phosphorylated CD19 and PI3K-dependent Erk phosphorylation in human diffuse large B-cell lymphoma cell lines. Our data provide evidence that CD19 plays an important role in transmitting survival and proliferation signals downstream of CD40 and therefore might be an interesting therapeutic target for the treatment of lymphoma undergoing chronic CD40 signaling.

Atypical Response of B-1 Cells to BCR Ligation: A Speculative Model

Peritoneal B-1a cells manifest unusual signaling characteristics that distinguish them from splenic B-2 cells. These include the failure of BCR engagement to trigger NF-κB activation and DNA replication. Despite extensive study, a clear explanation for these characteristics has not emerged. Here we aim to develop a unified paradigm based on previous reports and recent results, which proposes a central role for phosphatase activity. We hypothesize B-1a cells are unable to induce NF-κB or proliferate after BCR cross-linking due to increased phosphatase abundance or activity. This phosphatase abundance and/or activity may be the result of unique B-1a cell characteristics such as increased levels of HSP70 and/or constitutive secretion of IL-10. We speculate phosphatase activity cannot be overcome by BCR ligation alone due to insufficient Vav protein expression, which does not allow for proper production of reactive oxygen species, which inhibit phosphatases. Furthermore, constitutively active Lyn also plays a negative regulatory role in B-1a. We expect that a new focus on phosphatase activity and its suppression will be revealing for BCR signal transduction in B-1 cells.

A system for reconstructing B cell antigen receptor signaling in the mouse myeloma J558L cell line

B cell antigen receptor (BCR) signaling is positively and negatively regulated by various cell surface receptors such as CD19 and CD45. Functional analysis of these receptors has been performed using gene targeting technology, which is a valid approach to elucidate their functions. However, this type of analysis is restricted when multiple molecules are evaluated simultaneously. From a different perspective, synthetic biology provides a high degree of freedom for analyzing various molecules. Here we developed a system to reconstruct BCR signaling using the J558L myeloma cell line in combination with the protein-based Ca(2+) indicator YC3.60. BCR-reconstituted J558L cells harboring YC3.60 (J558Lμv11 cells) permitted monitoring of Ca(2+) mobilization. Reconstituting CD19 in J558Lμv11 cells resulted in detectable BCR-induced Ca(2+) mobilization but with kinetics different from that of CD45-expressing cells. Furthermore, we evaluated the validity of the J558L system by proteomic analysis of tyrosine-phosphorylated proteins after antigen stimulation. Identification of more than 100 BCR-induced tyrosine-phosphorylated proteins in J558Lμv11 cells revealed a similarity to that observed in B cells, and a novel member, non-receptor protein tyrosine kinase Fer, was found. Thus, this reconstruction system using J558L cells appeared to be valid for comprehensively investigating BCR signaling.

The phosphoinositide 3-kinase signaling pathway in normal and malignant B cells: activation mechanisms, regulation and impact on cellular functions

The phosphoinositide 3-kinase (PI3K) pathway is a central signal transduction axis controlling normal B cell homeostasis and activation in humoral immunity. The p110δ PI3K catalytic subunit has emerged as a critical mediator of multiple B cell functions. The activity of this pathway is regulated at multiple levels, with inositol phosphatases PTEN and SHIP both playing critical roles. When deregulated, the PI3K pathway can contribute to B cell malignancies and autoantibody production. This review summarizes current knowledge on key mechanisms that activate and regulate the PI3K pathway and influence normal B cell functional responses including the development of B cell subsets, antigen presentation, immunoglobulin isotype switch, germinal center responses, and maintenance of B cell anergy. We also discuss PI3K pathway alterations reported in select B cell malignancies and highlight studies indicating the functional significance of this pathway in malignant B cell survival and growth within tissue microenvironments. Finally, we comment on early clinical trial results, which support PI3K inhibition as a promising treatment of chronic lymphocytic leukemia.

The role of the tec kinase Bruton's tyrosine kinase (Btk) in leukocyte recruitment

Recruitment of leukocytes into inflamed tissue is a key component of the immune system. The activation of integrins on leukocytes is required for their recruitment into the inflamed tissue. Btk is a cytoplasmic nonreceptor tyrosine kinase belonging to the Tec-kinase family. It plays a key role in B-cell development and function, and recently published studies revealed important roles of Btk in myeloid cells. Btk might be activated through a variety of receptors leading to activation of integrins as the pivotal element in leukocyte recruitment. This review focuses on the role of Btk in B-lymphocyte homing and in neutrophil recruitment.

Corruption of human follicular B-lymphocyte trafficking by a B-cell superantigen

Protein A (SpA) of Staphylococcus aureus is known to target the paratope of immunoglobulins expressing V(H)3 genes, and to delete marginal zone B cells and B-1a in vivo. We have discovered that SpA endows S. aureus with the potential to subvert B-cell trafficking in the host. We found that SpA, whose Fc-binding site has been inactivated, binds essentially to naïve B cells and induces a long-lasting decrease in CXCR4 expression and in B-cell chemotaxis to CXCL12. Competition experiments indicated that SpA does not interfere with binding of CXCR4 ligands and does not directly bind to CXCR4. This conclusion is strongly supported by the inability of SpA to modulate clathrin-mediated CXCR4 internalization, which contrasts with the potent effect of anti-immunoglobin M (IgM) antibodies. Microscopy and biochemical experiments confirmed that SpA binds to the surface IgM/IgD complex and induces its clathrin-dependent internalization. Concomitantly, the SpA-induced signaling leads to protein kinase C-dependent CXCR4 downmodulation, suggesting that SpA impairs the recycling of CXCR4, a postclathrin process that leads to either degradation into lysozomes or de novo expression at the cell surface. In addition to providing novel insight into disruption of B-cell trafficking by an infectious agent, our findings may have therapeutic implications. Because CXCR4 has been associated with cancer metastasis and with certain autoimmune diseases, SpA behaves as an evolutionary tailored highly specific, chemokine receptor inhibitor that may have value in addition to conventional cytotoxic therapy in patients with various malignancies and immune-mediated diseases.

Long-term maintenance of polysaccharide-specific antibodies by IgM-secreting cells

Many bacteria-associated polysaccharides induce long-lived Ab responses that protect against pathogenic microorganisms. The maintenance of polysaccharide-specific Ab titers may be due to long-lived plasma cells or ongoing Ag-driven B cell activation due to polysaccharide persistence. BALB/c and V(H)J558.3 transgenic mice respond to α1→3-dextran (DEX) by generating a peak anti-DEX response at 7 d, followed by maintenance of serum Ab levels for up to 150 d. Analysis of the cellular response to DEX identified a population of short-lived, cyclophosphamide-sensitive DEX-specific plasmablasts in the spleen, and a quiescent, cyclophosphamide-resistant DEX-specific Ab-secreting population in the bone marrow. BrdU pulse-chase experiments demonstrated the longevity of the DEX-specific Ab-secreting population in the bone marrow. Splenic DEX-specific plasmablasts were located in the red pulp with persisting DEX-associated CD11c(+) dendritic cells 90 d after immunization, whereas DEX was not detected in the bone marrow after 28 d. Selective depletion of short-lived DEX-specific plasmablasts and memory B1b B cells using cyclophosphamide and anti-CD20 treatment had a minimal impact on the maintenance of serum anti-DEX Abs. Collectively, these findings demonstrate that the maintenance of serum polysaccharide-specific Abs is the result of continuous Ag-driven formation of short-lived plasmablasts in the spleen and a quiescent population of Ab-secreting cells maintained in the bone marrow for a long duration.

Differential STIM1 expression in T and B cell subsets suggests a role in determining antigen receptor signal amplitude

Ca(2+) acts ubiquitously as a second messenger in transmembrane signal transduction. In lymphocytes, calcium mobilization is triggered by antigen and chemokine receptors, among others, and controls cell functions ranging from proliferation to migration. The primary mechanism of extracellular Ca(2+) entry in lymphocytes is the CRAC influx. STIM1 is a crucial component of the CRAC influx mechanism in lymphocytes, acting as a sensor of low Ca(2+) concentration in the ER and an activator of the Ca(2+) selective channel ORAI1 in the plasma membrane. While STIM1 function has been studied extensively, little is known regarding whether it is differentially expressed and thereby affects the magnitude of calcium mobilization responses. We report here that STIM1 expression differs in murine T and B lymphocytes, and in respective subsets. For example, mature T cells express ∼4 times more STIM1 than mature B cells. Furthermore, we show that through the physiologic range of expression, STIM1 levels determine the magnitude of Ca(2+) influx responses that follow BCR-induced intracellular store depletion. Considered in view of previous reports that differences in amplitude of lymphocyte Ca(2+) mobilization determine alternate biological responses, these findings suggest that differential STIM1 expression may be important determinant of biological responses.

The conundrum of inhibitory signaling by ITAM-containing immunoreceptors: potential molecular mechanisms

Immunoreceptor signals must be appropriately transduced and regulated to achieve effective immunity while controlling inflammation and autoimmunity. It is generally held that these processes are mediated by the interplay of distinct activating and inhibitory receptors via conserved activating (ITAM) and inhibitory (ITIM) signaling motifs. However, recent evidence indicates that under certain conditions incomplete phosphorylation of ITAM tyrosines leads to inhibitory signaling. This new regulatory function of ITAMs has been termed ITAMi (inhibitory ITAM). Here we discuss the potential molecular mechanisms of inhibitory signaling by ITAM-containing receptors.

Molecular underpinning of B-cell anergy

A byproduct of the largely stochastic generation of a diverse B-cell specificity repertoire is production of cells that recognize autoantigens. Indeed, recent studies indicate that more than half of the primary repertoire consists of autoreactive B cells that must be silenced to prevent autoimmunity. While this silencing can occur by multiple mechanisms, it appears that most autoreactive B cells are silenced by anergy, wherein they populate peripheral lymphoid organs and continue to express unoccupied antigen receptors yet are unresponsive to antigen stimulation. Here we review molecular mechanisms that appear operative in maintaining the antigen unresponsiveness of anergic B cells. In addition, we present new data indicating that the failure of anergic B cells to mobilize calcium in response to antigen stimulation is not mediated by inactivation of stromal interacting molecule 1, a critical intermediary in intracellular store depletion-induced calcium influx.

Differential phosphorylation of functional tyrosines in CD19 modulates B-lymphocyte activation

CD19 is a B-cell transmembrane molecule that is critical for B-cell activation. CD19 serves as a scaffold protein for key signal transduction molecules including Lyn, PI3K, and Vav, by providing docking sites for these molecules via phosphorylation of CD19-Y(513), CD19-Y(482), and CD19-Y(391). We investigated the process of CD19 tyrosine phophorylation during B-cell activation using Ab specific for each of these phosphorylated tyrosines. BCR engagement induced differential tyrosine phosphorylation, as CD19-Y(513) phophorylation occurred first, and CD19-Y(482) phosphorylation was delayed and transient. Different BCR isotypes exhibited distinct patterns of CD19 phosphorylation: IgG-BCR ligation resulted in faster phosphorylation of CD19-Y(513) and more intense phosphorylation of CD19-Y(391) than IgM-BCR ligation. This affected CD19-mediated downstream pathways involving Vav, PI3K, and Akt. Additionally, the phosphorylation profile of CD19 differed distinctly according to its plasma membrane location. CD19 phosphorylated at Y(513) was almost exclusively located within lipid rafts, whereas phosphorylated Y(482) and Y(391) were found both inside and outside of the rafts. Furthermore, the phosphorylation of all three tyrosines was remarkably enhanced and prolonged following the simultaneous stimulation of BCR and CD40. Thus, variations in phosphorylation patterns may contribute to the complexity of CD19-regulated signal transduction.

Regulatory B cells (B10 cells) have a suppressive role in murine lupus: CD19 and B10 cell deficiency exacerbates systemic autoimmunity

B cells play critical roles in the pathogenesis of lupus. To examine the influence of B cells on disease pathogenesis in a murine lupus model, New Zealand Black and New Zealand White F(1) hybrid (NZB/W) mice were generated that were deficient for CD19 (CD19(-/-) NZB/W mice), a B cell-specific cell surface molecule that is essential for optimal B cell signal transduction. The emergence of anti-nuclear Abs was significantly delayed in CD19(-/-) NZB/W mice compared with wild type NZB/W mice. However, the pathologic manifestations of nephritis appeared significantly earlier, and survival was significantly reduced in CD19(-/-) NZB/W mice compared with wild type mice. These results demonstrate both disease-promoting and protective roles for B cells in lupus pathogenesis. Recent studies have identified a potent regulatory B cell subset (B10 cells) within the rare CD1d(hi)CD5(+) B cell subset of the spleen that regulates acute inflammation and autoimmunity through the production of IL-10. In wild type NZB/W mice, the CD1d(hi)CD5(+)B220(+) B cell subset that includes B10 cells was increased by 2.5-fold during the disease course, whereas CD19(-/-) NZB/W mice lacked this CD1d(hi)CD5(+) regulatory B cell subset. However, the transfer of splenic CD1d(hi)CD5(+) B cells from wild type NZB/W mice into CD19(-/-) NZB/W recipients significantly prolonged their survival. Furthermore, regulatory T cells were significantly decreased in CD19(-/-) NZB/W mice, but the transfer of wild type CD1d(hi)CD5(+) B cells induced T regulatory cell expansion in CD19(-/-) NZB/W mice. These results demonstrate an important protective role for regulatory B10 cells in this systemic autoimmune disease.

The cunning little vixen: Foxo and the cycle of life and death

A screen for increased longevity in Caenorhabditis elegans has identified a transcription factor that programs cells for resistance to oxidative stress, DNA repair and cell cycle control. The mammalian orthologs of this factor are referred to as 'Foxo' for 'Forkhead box', with the second 'o' in the name denoting a subfamily of four members related by sequence. This family of factors is regulated by growth factors, oxidative stress or nutrient deprivation. Thus, it might readily control the inflammatory conflagration associated with infection-driven lymphocyte proliferation. Surprisingly, the first insights into Foxo-mediated immune regulation have instead revealed direct control of highly specialized genes of the adaptive immune system.

B cell receptor and BAFF receptor signaling regulation of B cell homeostasis

B lymphocyte homeostasis depends on tonic and induced BCR signaling and receptors sensitive to trophic factors, such as B cell-activating factor receptor (BAFF-R or BR3) during development and maintenance. This review will discuss growing evidence suggesting that the signaling mechanisms that maintain B cell survival and metabolic fitness during selection at transitional stages and survival after maturation rely on cross-talk between BCR and BR3 signaling. Recent findings have also begun to unravel the molecular mechanisms underlying this crosstalk. In this review I also propose a model for regulating the amplitude of BCR signaling by a signal amplification loop downstream of the BCR involving Btk and NF-kappaB that may facilitate BCR-dependent B cell survival as well as its functional coupling to BR3 for the growth and survival of B lymphocytes.

CD21 signaling via C3 regulates Purkinje cell protein 4 expression

Complement receptor proteins CR2 (CD21) and CR1 (CD35) have been identified as components of the murine B cell co-receptor complex. Gene expression profiles between naïve WT, C3-/-, and CD21/35-/- B cells demonstrate enhanced expression of a Ca(2+)-modulating gene, Pcp4, in WT mice compared to the complement-deficient animals. Increased expression of Pcp4 is also coincident with B cell maturation into end stage phenotypes. Prolonged activation of B cells via cross-linking of the BCR (but not CR1/CR2 alone) leads to increased expression of Pcp4 and suppressed Ca(2+) release. In total these data demonstrate that the expression of Pcp4 in naïve resting mature B cells is dependent upon tonic stimulation from the CR1/CR2 proteins via a C3 ligand, and that antigen specific B cell activation can also elevate Pcp4 expression that is coincident with suppression of calcium-dependent responses.

Role of initial protein phosphorylation events and localized release-activated calcium influx in B cell antigen receptor signaling

An increase in intracellular calcium concentration is one of the major initial steps in B cell activation following antigen receptor (BCR) ligation. We show herein that in C57BL/6 murine B lymphocytes and in model cell lines, BCR-mediated calcium ion (Ca(2+)) influx occurs via highly selective Ca(2+) release-activated channels, and stromal interaction molecule 1 (STIM1) plays an important role in this pathway. We also demonstrate the temporal relation between Ca(2+)-dependent signaling events and formation of the immune synapse. Our data indicate that cytoplasmic Ca(2+) levels in areas adjacent to the immune synapse differ from those in the rest of the cytoplasm. Finally, a comparison of phosphorylation patterns of BCR-triggered signaling proteins in the presence or absence of Ca(2+) revealed the unanticipated finding that initial BCR-triggered, Ca(2+)-dependent tyrosine phosphorylation events involve predominantly Ca(2+) released from intracellular stores and that influx-derived Ca(2+) is not essential. This suggests a different role for this phase of Ca(2+) influx.

Germinal center structure and function: lessons from CD19

The germinal center is a critical locus in the production of protective immunity, but its function is poorly understood. Studies of mutant forms of CD19 revealed differences in signaling in different compartments inside the germinal center, and structural findings indicate a selective role in the interaction with follicular dendritic cells in the GC. Loss of these signals leads to surprising changes in germinal center B cells that challenge previous models of GC function.