Bruton's tyrosine kinase controls a sustained calcium signal essential for B lineage development and function

Bruton's Tyrosine Kinase Inhibitors for Multiple Sclerosis Treatment: A New Frontier

Multiple sclerosis (MS) can cause significant disability to patients via relapse-associated worsening and progression independent of relapses. The causes of neuronal and myelin damage can include lymphocyte-mediated inflammation and microglial activation. Bruton's tyrosine kinase (BTK) is an enzyme that mediates B cell activation and the proinflammatory phenotype of microglia. Inhibiting BTK provides a novel therapeutic target for MS but also has a complicated pharmacology based on binding specificity, CNS penetration, half-life, and enzyme inhibition characteristics. Multiple agents are being studied in phase 3 trials, and each agent will have unique efficacy and safety profiles that must be considered individually.

BTK inhibitors in the treatment of hematological malignancies and inflammatory diseases: mechanisms and clinical studies

Bruton's tyrosine kinase (BTK) is an essential component of multiple signaling pathways that regulate B cell and myeloid cell proliferation, survival, and functions, making it a promising therapeutic target for various B cell malignancies and inflammatory diseases. Five small molecule inhibitors have shown remarkable efficacy and have been approved to treat different types of hematological cancers, including ibrutinib, acalabrutinib, zanubrutinib, tirabrutinib, and orelabrutinib. The first-in-class agent, ibrutinib, has created a new era of chemotherapy-free treatment of B cell malignancies. Ibrutinib is so popular and became the fourth top-selling cancer drug worldwide in 2021. To reduce the off-target effects and overcome the acquired resistance of ibrutinib, significant efforts have been made in developing highly selective second- and third-generation BTK inhibitors and various combination approaches. Over the past few years, BTK inhibitors have also been repurposed for the treatment of inflammatory diseases. Promising data have been obtained from preclinical and early-phase clinical studies. In this review, we summarized current progress in applying BTK inhibitors in the treatment of hematological malignancies and inflammatory disorders, highlighting available results from clinical studies.

T Cell Abnormalities in X-Linked Agammaglobulinaemia: an Updated Review

X-linked agammaglobulinaemia (XLA) is a primary immunodeficiency (PID) resulting from a defect in the B cell development. It has conventionally been thought that T cells play a major role in the development and function of the B cell compartment. However, it has also been shown that B cells and T cells undergo bidirectional interactions and B cells also influence the structure and function of the T cell compartment. Patients with XLA offer a unique opportunity to understand the effect of absent B cells on the T cell compartment. In this review, we provide an update on abnormalities in the T cell compartment in patients with XLA. Studies have shown impaired memory T cells, follicular helper T cells, T regulatory cells and T helper 17 in patients with XLA. In addition, these patients have also been reported to have abnormal delayed cell-mediated immune responses and vaccine-specific T cell-mediated immune responses; defective T helper cell polarization and impaired T cell receptor diversity. At present, the clinical significance of these T cell abnormalities has not been studied in detail. However, these abnormalities may result in an increased risk of viral infections, autoimmunity, autoinflammation and possibly chronic lung disease. Abnormal response to SARS-Cov2 vaccine in patients with XLA and prolonged persistence of SARS-Cov2 virus in the respiratory tract of these patients may be related to abnormalities in the T cell compartment.

Acalabrutinib and its use in the treatment of chronic lymphocytic leukemia

Bruton's tyrosine kinase inhibitors have changed the treatment landscape for chronic lymphocytic leukemia (CLL), mantle cell lymphoma (MCL) and lymphoplasmacytic lymphoma dramatically. In 2019, acalabrutinib was approved by the US FDA for the treatment of treatment-naive and relapsed/refractory CLL and MCL. Acalabrutinib monotherapy was found to be effective and safe in CLL patients. In ASCEND and ELEVATE treatment-naive studies, acalabrutinib monotherapy and the combination with obinutuzumab demonstrated improved efficacy and an acceptable safety profile. The triple combination with venetoclax showed a high rate of molecular remission without an impaired safety profile. Adverse events, with an occurrence rate of >20%, were as follows: grade 1-2 myelosuppression, gastrointestinal toxicity, rash, constitutional symptoms; grade 3 or 4 toxicities were syncope, pneumonia, hypertension, atrial fibrillation, neutropenia and thrombocytopenia.

CRAC Channel Controls the Differentiation of Pathogenic B Cells in Lupus Nephritis

Store-operated Ca²⁺ release-activated Ca²⁺ (CRAC) channel is the main Ca²⁺ influx pathway in lymphocytes and is essential for immune response. Lupus nephritis (LN) is an autoimmune disease characterized by the production of autoantibodies due to widespread loss of immune tolerance. In this study, RNA-seq analysis revealed that calcium transmembrane transport and calcium channel activity were enhanced in naive B cells from patients with LN. The increased expression of ORAI1, ORAI2, and STIM2 in naive B cells from patients with LN was confirmed by flow cytometry and Western blot, implying a role of CRAC channel in B-cell dysregulation in LN. For in vitro study, CRAC channel inhibition by YM-58483 or downregulation by ORAI1-specific small-interfering RNA (siRNA) decreased the phosphorylation of Ca²⁺/calmodulin-dependent protein kinase2 (CaMK2) and suppressed Blimp-1 expression in primary human B cells, resulting in decreased B-cell differentiation and immunoglobulin G (IgG) production. B cells treated with CaMK2-specific siRNA showed defects in plasma cell differentiation and IgG production. For in vivo study, YM-58483 not only ameliorated the progression of LN but also prevented the development of LN. MRL/lpr lupus mice treated with YM-58483 showed lower percentage of plasma cells in the spleen and reduced concentration of anti-double-stranded DNA antibodies in the sera significantly. Importantly, mice treated with YM-58483 showed decreased immune deposition in the glomeruli and alleviated kidney damage, which was further confirmed in NZM2328 lupus mice. Collectively, CRAC channel controlled the differentiation of pathogenic B cells and promoted the progression of LN. This study provides insights into the pathogenic mechanisms of LN and that CRAC channel could serve as a potential therapeutic target for LN.

BTK gatekeeper residue variation combined with cysteine 481 substitution causes super-resistance to irreversible inhibitors acalabrutinib, ibrutinib and zanubrutinib

Irreversible inhibitors of Bruton tyrosine kinase (BTK), pioneered by ibrutinib, have become breakthrough drugs in the treatment of leukemias and lymphomas. Resistance variants (mutations) occur, but in contrast to those identified for many other tyrosine kinase inhibitors, they affect less frequently the "gatekeeper" residue in the catalytic domain. In this study we carried out variation scanning by creating 11 substitutions at the gatekeeper amino acid, threonine 474 (T474). These variants were subsequently combined with replacement of the cysteine 481 residue to which irreversible inhibitors, such as ibrutinib, acalabrutinib and zanubrutinib, bind. We found that certain double mutants, such as threonine 474 to isoleucine (T474I) or methionine (T474M) combined with catalytically active cysteine 481 to serine (C481S), are insensitive to ≥16-fold the pharmacological serum concentration, and therefore defined as super-resistant to irreversible inhibitors. Conversely, reversible inhibitors showed a variable pattern, from resistance to no resistance, collectively demonstrating the structural constraints for different classes of inhibitors, which may affect their clinical application.

Inflammatory signal induced IL-10 production of marginal zone B-cells depends on CREB

IL-10 is a suppressive cytokine that has been implicated in the pathophysiology of autoimmune disorders and can be produced by different cell types such as regulatory B-cells. Our previous work showed that under inflammatory condition MZ B-cells differentiated into IL-10 producing cells and contributed to the downregulation of collagen-induced arthritis, while follicular B-cells failed to do so. Based on these observations, we aimed to investigate how inflammatory signals mediated through the BCR, TLR9 and IFN-γ receptors trigger IL-10 production in MZ B-cells but leave FO B-cells unresponsive. We particularly focused on the CREB transcription factor as it is involved in all three signalling cascades and analysed its contribution to IL-10 production. Our results demonstrate that the IL-10 production of MZ B-cells induced by the BCR, TLR9 and IFN-γ receptors is mediated by CREB. We showed that the activation of CREB is prolonged in MZ B-cells while the transcription factor only transiently phosphorylated in FO B-cells. The sustained phosphorylation of CREB is clearly associated with its prolonged binding to molecular partner CBP, whereas inhibition of their association decreased IL-10 production. We assume that sustained activation of CREB is required for IL-10 production by B-cells under inflammatory conditions.

Substitution scanning identifies a novel, catalytically active ibrutinib-resistant BTK cysteine 481 to threonine (C481T) variant

Irreversible Bruton tyrosine kinase (BTK) inhibitors, ibrutinib and acalabrutinib have demonstrated remarkable clinical responses in multiple B-cell malignancies. Acquired resistance has been identified in a sub-population of patients in which mutations affecting BTK predominantly substitute cysteine 481 in the kinase domain for catalytically active serine, thereby ablating covalent binding of inhibitors. Activating substitutions in the BTK substrate phospholipase Cγ2 (PLCγ2) instead confers resistance independent of BTK. Herein, we generated all six possible amino acid substitutions due to single nucleotide alterations for the cysteine 481 codon, in addition to threonine, requiring two nucleotide substitutions, and performed functional analysis. Replacement by arginine, phenylalanine, tryptophan or tyrosine completely inactivated the catalytic activity, whereas substitution with glycine caused severe impairment. BTK with threonine replacement was catalytically active, similar to substitution with serine. We identify three potential ibrutinib resistance scenarios for cysteine 481 replacement: (1) Serine, being catalytically active and therefore predominating among patients. (2) Threonine, also being catalytically active, but predicted to be scarce, because two nucleotide changes are needed. (3) As BTK variants replaced with other residues are catalytically inactive, they presumably need compensatory mutations, therefore being very scarce. Glycine and tryptophan variants were not yet reported but likely also provide resistance.

Bruton's tyrosine kinase is a potential therapeutic target in prostate cancer

Bruton's tyrosine kinase (BTK) is a non-receptor tyrosine kinase that has mainly been studied in haematopoietic cells. We have investigated whether BTK is a potential therapeutic target in prostate cancer. We find that BTK is expressed in prostate cells, with the alternate BTK-C isoform predominantly expressed in prostate cancer cells and tumors. This isoform is transcribed from an alternative promoter and results in a protein with an amino-terminal extension. Prostate cancer cell lines and prostate tumors express more BTK-C transcript than the malignant NAMALWA B-cell line or human lymphomas. BTK protein expression is also observed in tumor tissue from prostate cancer patients. Down regulation of this protein with RNAi or inhibition with BTK-specific inhibitors, Ibrutinib, AVL-292 or CGI-1746 decrease cell survival and induce apoptosis in prostate cancer cells. Microarray results show that inhibiting BTK under these conditions increases expression of apoptosis related genes, while overexpression of BTK-C is associated with elevated expression of genes with functions related to cell adhesion, cytoskeletal structure and the extracellular matrix. These results are consistent with studies that show that BTK signaling is important for adhesion and migration of B cells and suggest that BTK-C may confer similar properties to prostate cancer cells. Since BTK-C is a survival factor for these cells, it represents both a potential biomarker and novel therapeutic target for prostate cancer.

Splice-correcting oligonucleotides restore BTK function in X-linked agammaglobulinemia model

X-linked agammaglobulinemia (XLA) is an inherited immunodeficiency that results from mutations within the gene encoding Bruton's tyrosine kinase (BTK). Many XLA-associated mutations affect splicing of BTK pre-mRNA and severely impair B cell development. Here, we assessed the potential of antisense, splice-correcting oligonucleotides (SCOs) targeting mutated BTK transcripts for treating XLA. Both the SCO structural design and chemical properties were optimized using 2'-O-methyl, locked nucleic acid, or phosphorodiamidate morpholino backbones. In order to have access to an animal model of XLA, we engineered a transgenic mouse that harbors a BAC with an authentic, mutated, splice-defective human BTK gene. BTK transgenic mice were bred onto a Btk knockout background to avoid interference of the orthologous mouse protein. Using this model, we determined that BTK-specific SCOs are able to correct aberrantly spliced BTK in B lymphocytes, including pro-B cells. Correction of BTK mRNA restored expression of functional protein, as shown both by enhanced lymphocyte survival and reestablished BTK activation upon B cell receptor stimulation. Furthermore, SCO treatment corrected splicing and restored BTK expression in primary cells from patients with XLA. Together, our data demonstrate that SCOs can restore BTK function and that BTK-targeting SCOs have potential as personalized medicine in patients with XLA.

BTK inhibitors in chronic lymphocytic leukemia: a glimpse to the future

The treatment of chronic lymphocytic leukemia (CLL) with inhibitors targeting B cell receptor signaling and other survival mechanisms holds great promise. Especially the early clinical success of Ibrutinib, an irreversible inhibitor of Bruton's tyrosine kinase (BTK), has received widespread attention. In this review we will focus on the fundamental and clinical aspects of BTK inhibitors in CLL, with emphasis on Ibrutinib as the best studied of this class of drugs. Furthermore, we summarize recent laboratory as well as clinical findings relating to the first cases of Ibrutinib resistance. Finally, we address combination strategies with Ibrutinib, and attempt to extrapolate its current status to the near future in the clinic.

Calcium, channels, intracellular signaling and autoimmunity

Calcium (Ca²⁺) is an important cation able to function as a second messenger in different cells of the immune system, particularly in B and T lymphocytes, macrophages and mastocytes, among others. Recent discoveries related to the entry of Ca²⁺ through the store-operated calcium entry (SOCE) has opened a new investigation area about the cell destiny regulated by Ca²⁺ especially in B and T lymphocytes. SOCE acts through calcium-release-activated calcium (CRAC) channels. The function of CRAC depends of two recently discovered regulators: the Ca²⁺ sensor in the endoplasmic reticulum or stromal interaction molecule (STIM-1) and one subunit of CRAC channels called Orai1. This review focuses on the role of Ca²⁺ signals in B and T lymphocytes functions, the signalling pathways leading to Ca²⁺ influx, and the relationship between Ca²⁺ signals and autoimmune diseases.

The genetic theory of infectious diseases: a brief history and selected illustrations

Until the mid-nineteenth century, life expectancy at birth averaged 20 years worldwide, owing mostly to childhood fevers. The germ theory of diseases then gradually overcame the belief that diseases were intrinsic. However, around the turn of the twentieth century, asymptomatic infection was discovered to be much more common than clinical disease. Paradoxically, this observation barely challenged the newly developed notion that infectious diseases were fundamentally extrinsic. Moreover, interindividual variability in the course of infection was typically explained by the emerging immunological (or somatic) theory of infectious diseases, best illustrated by the impact of vaccination. This powerful explanation is, however, best applicable to reactivation and secondary infections, particularly in adults; it can less easily account for interindividual variability in the course of primary infection during childhood. Population and clinical geneticists soon proposed a complementary hypothesis, a germline genetic theory of infectious diseases. Over the past century, this idea has gained some support, particularly among clinicians and geneticists, but has also encountered resistance, particularly among microbiologists and immunologists. We present here the genetic theory of infectious diseases and briefly discuss its history and the challenges encountered during its emergence in the context of the apparently competing but actually complementary microbiological and immunological theories. We also illustrate its recent achievements by highlighting inborn errors of immunity underlying eight life-threatening infectious diseases of children and young adults. Finally, we consider the far-reaching biological and clinical implications of the ongoing human genetic dissection of severe infectious diseases.

Trypanosoma cruzi trans-sialidase initiates a program independent of the transcription factors RORγt and Ahr that leads to IL-17 production by activated B cells

We identified B cells as a major source for rapid, innate-like interleukin 17 (IL-17) production in vivo in response to Trypanosoma cruzi infection. IL-17⁺ B cells exhibited a plasmablast phenotype, outnumbered T_H17 cells and were required for optimal response to this pathogen. Using both murine and human primary B cells, we demonstrate that exposure to parasite-derived trans-sialidase in vitro was sufficient to trigger modification of the cell surface mucin, CD45, leading to Btk-dependent signaling and IL-17A or IL-17F production via an ROR-γt and AHR-independent transcriptional program. Our combined data suggest that generation of IL-17⁺ B cells may be an unappreciated feature of innate immune responses required for pathogen control or IL-17-mediated autoimmunity.

Differential impact of Toll-like receptor signaling on distinct B cell subpopulations

B cells exhibit a range of functional responses following TLR engagement including immunoglobulin and cytokine production, proliferation, antigen presentation and migration. However, B cell intrinsic TLR responses appear to be precisely programmed based upon the developmental stage of the cell. B cell subpopulations classified as innate immune cells including marginal zone and B-1 B cells exhibit robust responses to TLR stimulation. In contrast, activation of other B cell subsets is constrained via a variety of developmentally regulated events. In this review we provide an overview of TLR responses in murine and human B cells and specifically highlight patterns of TLR expression and developmentally regulated functional responses.

Clinical consequences of defects in B-cell development

Abnormalities in humoral immunity typically reflect a generalized or selective failure of effective B-cell development. The developmental processes can be followed through analysis of cell-surface markers, such as IgM, IgD, CD10, CD19, CD20, CD21, and CD38. Early phases of B-cell development are devoted to the creation of immunoglobulin and testing of B-cell antigen receptor signaling. Failure leads to the absence of B cells and immunoglobulin in the blood from birth. As the developing B cells begin to express a surface B-cell receptor, they become subject to negative and positive selection pressures and increasingly depend on survival signals. Defective signaling can lead to selective or generalized hypogammaglobulinemia, even in the presence of normal numbers of B cells. In the secondary lymphoid organs some B cells enter the splenic marginal zone, where preactivated cells lie ready to rapidly respond to T-independent antigens, such as the polysaccharides that coat some microorganisms. Other cells enter the follicle and, with the aid of cognate follicular T cells, divide to help form a germinal center (GC) after their interaction with antigen. In the GC B cells can undergo the processes of class switching and somatic hypermutation. Failure to properly receive T-cell signals can lead to hyper-IgM syndrome. B cells that leave the GC can develop into memory B cells, short-lived plasma cells, or long-lived plasma cells. The latter ultimately migrate back to the bone marrow, where they can continue to produce protective antigen-specific antibodies for decades.

Hypoxia-induced mitogenic factor/FIZZ1 induces intracellular calcium release through the PLC-IP(3) pathway

Hypoxia-induced mitogenic factor (HIMF), also known as "found in inflammatory zone 1" (FIZZ1) or resistin-like molecule-alpha (RELMalpha), is a profound vasoconstrictor of the pulmonary circulation and a strong mitogenic factor in pulmonary vascular smooth muscle. To further understand the mechanism of these contractile and mitogenic responses, we examined the effect of HIMF on intracellular Ca(2+) in human pulmonary artery smooth muscle cells (SMC). Ca(2+) imaging in fluo 4-loaded human pulmonary artery SMC revealed that recombinant murine HIMF increased intracellular Ca(2+) concentration ([Ca(2+)](i)) in a sustained and oscillatory manner. This increase occurred independent of extracellular Ca(2+) influx. Pretreatment of human pulmonary artery SMC with U-73122, a specific inhibitor of phosphatidylinositol-phospholipase C (PLC) completely prevented the HIMF-induced Ca(2+) signal. The [Ca(2+)](i) increase was also abolished by pretreatment with 2-aminoethoxydiphenyl borate (2-APB), an inositol 1,4,5-trisphosphate (IP(3)) receptor antagonist. Ryanodine pretreatment did not affect initiation of [Ca(2+)](i) activation or internal release but reduced [Ca(2+)](i) at the plateau phase. Pretreatment with the Galpha(i)-specific inhibitor pertussis toxin and the Galpha(s)-specific inhibitor NF-449 did not block the Ca(2+) signal. Knockdown of Galpha(q/11) expression did not prevent Ca(2+) release, but the pattern of Ca(2+) release changed from the sustained oscillatory transients with prolonged plateau to a series of short [Ca(2+)](i) transients that return to baseline. However, pretreatment with the tyrosine kinase inhibitor genistein completely inhibited the internal Ca(2+) release. These results demonstrate that HIMF can stimulate intracellular Ca(2+) release in human pulmonary artery SMC through the PLC signaling pathway in an IP(3)- and tyrosine phosphorylation-dependent manner and that Galpha(q/11) protein-coupled receptor and ryanodine receptor contribute to the increase of [Ca(2+)](i).

Primary B cell immunodeficiencies: comparisons and contrasts

Sophisticated genetic tools have made possible the identification of the genes responsible for most well-described immunodeficiencies in the past 15 years. Mutations in Btk, components of the pre-B cell and B cell receptor (lambda5, Igalpha, Igbeta), or the scaffold protein BLNK account for approximately 90% of patients with defects in early B cell development. Hyper-IgM syndromes result from mutations in CD40 ligand, CD40, AID, or UNG in 70-80% of affected patients. Rare defects in ICOS or CD19 can result in a clinical picture that is consistent with common variable immunodeficiency, and as many as 10% of patients with this disorder have heterozygous amino acid substitutions in TACI. For all these disorders, there is considerable clinical heterogeneity in patients with the same mutation. Identifying the genetic and environmental factors that influence the clinical phenotype may enhance patient care and our understanding of normal B cell development.

B lymphocytes: how they develop and function

The discovery that lymphocyte subpopulations participate in distinct components of the immune response focused attention onto the origins and function of lymphocytes more than 40 years ago. Studies in the 1960s and 1970s demonstrated that B and T lymphocytes were responsible primarily for the basic functions of antibody production and cell-mediated immune responses, respectively. The decades that followed have witnessed a continuum of unfolding complexities in B-cell development, subsets, and function that could not have been predicted. Some of the landmark discoveries that led to our current understanding of B lymphocytes as the source of protective innate and adaptive antibodies are highlighted in this essay. The phenotypic and functional diversity of B lymphocytes, their regulatory roles independent of antibody production, and the molecular events that make this lineage unique are also considered. Finally, perturbations in B-cell development that give rise to certain types of congenital immunodeficiency, leukemia/lymphoma, and autoimmune disease are discussed in the context of normal B-cell development and selection. Despite the significant advances that have been made at the cellular and molecular levels, there is much more to learn, and cross-disciplinary studies in hematology and immunology will continue to pave the way for new discoveries.

Myc stimulates B lymphocyte differentiation and amplifies calcium signaling

Deregulated expression of the Myc family of transcription factors (c-, N-, and L-myc) contributes to the development of many cancers by a mechanism believed to involve the stimulation of cell proliferation and inhibition of differentiation. However, using B cell-specific c-/N-myc double-knockout mice and E(mu)-myc transgenic mice bred onto genetic backgrounds (recombinase-activating gene 2-/- and Btk-/- Tec-/-) whereby B cell development is arrested, we show that Myc is necessary to stimulate both proliferation and differentiation in primary B cells. Moreover, Myc expression results in sustained increases in intracellular Ca2+ ([Ca2+]i), which is required for Myc to stimulate B cell proliferation and differentiation. The increase in [Ca2+]i correlates with constitutive nuclear factor of activated T cells (NFAT) nuclear translocation, reduced Ca2+ efflux, and decreased expression of the plasma membrane Ca2+-adenosine triphosphatase (PMCA) efflux pump. Our findings demonstrate a revised model whereby Myc promotes both proliferation and differentiation, in part by a remarkable mechanism whereby Myc amplifies Ca2+ signals, thereby enabling the concurrent expression of Myc- and Ca2+-regulated target genes.

Bruton’s Tyrosine Kinase Mediates NF-κB Activation and B Cell Survival by B Cell-Activating Factor Receptor of the TNF-R Family

Loss of Bruton's tyrosine kinase (Btk) function results in mouse Xid disease characterized by a reduction in mature B cells and impaired humoral immune responses. These defects have been mainly attributed to impaired BCR signaling including reduced activation of the classical NF-kappaB pathway. In this study we show that Btk also couples the receptor for B cell-activating factor (BAFF) of the TNF family (BAFF-R) to the NF-kappaB pathway. Loss of Btk results in defective BAFF-mediated activation of both classical and alternative NF-kappaB pathways. Btk appears to regulate directly the classical pathway in response to BAFF such that Btk-deficient B cells exhibit reduced kinase activity of IkappaB kinase gamma-containing complexes and defective IkappaBalpha degradation. In addition, Btk-deficient B cells produce reduced levels of NF-kappaB2 (p100) basally and in response to stimulation via the BCR or BAFF-R, resulting in impaired activation of the alternative NF-kappaB pathway by BAFF. These results suggest that Btk regulates B cell survival by directly regulating the classical NF-kappaB pathway under both BCR and BAFF-R, as well as by inducing the expression of the components of alternative pathway for sustained NF-kappaB activation in response BAFF. Thus, impaired BCR- and BAFF-induced signaling to NF-kappaB may contribute to the observed defects in B cell survival and humoral immune responses in Btk-deficient mice.

Characterization of Bruton's tyrosine kinase mutations in Mexican patients with X-linked agammaglobulinemia

X-linked agammaglobulinemia (XLA) is a humoral primary immunodeficiency in which affected patients have very low levels of peripheral B cells and a profound deficiency of all immunoglobulin isotypes. Mutations in the gene encoding for Bruton's tyrosine kinase (Btk) are responsible for most of the agammaglobulinemia. In this work, 14 Btk mutations responsible of causing XLA are described; eight of which are novel and six are mutations previously reported. Seven of the mutations were due to deletions and insertions of exons and introns, respectively, which suggest splicing defects. The others were missense mutations, five of which affect arginine residues and have been described, and two new which affect leucine and glutamine residues (L111P and E605G). Most of these mutations were located at the kinase domain of Btk and, less frequently, they were found in PH and SH2 domains. Protein expression was also affected since most of the patients did not express or express very low Btk.

Calcium signalling in lymphocyte activation and disease

Calcium signals in cells of the immune system participate in the regulation of cell differentiation, gene transcription and effector functions. An increase in intracellular levels of calcium ions (Ca2+) results from the engagement of immunoreceptors, such as the T-cell receptor, B-cell receptor and Fc receptors, as well as chemokine and co-stimulatory receptors. The major pathway that induces an increase in intracellular Ca2+ levels in lymphocytes is through store-operated calcium entry (SOCE) and calcium-release-activated calcium (CRAC) channels. This Review focuses on the role of Ca2+ signals in lymphocyte functions, the signalling pathways leading to Ca2+ influx, the function of the recently discovered regulators of Ca2+ influx (STIM and ORAI), and the relationship between Ca2+ signals and diseases of the immune system.

Coupling Ca2+ store release to Icrac channel activation in B lymphocytes requires the activity of Lyn and Syk kinases

Activation of the B cell receptor complex in B lymphocytes causes Ca²⁺ release from intracellular stores, which, in turn, activates ion channels known as Icrac. We investigated the mechanisms that link Ca²⁺ store release to channel gating in DT40 B lymphocyte cell lines genetically manipulated to suppress the expression of several tyrosine kinases: Btk, Lyn, Syk, and the Blnk adaptor molecule. The simultaneous but not the independent suppression of Lyn and Syk expression prevents the activation of Icrac without interfering with thapsigargin-sensitive Ca²⁺ store release. Icrac activation by Ca²⁺ is reversed in mutant cells by the homologous expression of the missing kinases. Pharmacological inhibition of kinase activity by LavendustinA and PP2 cause the same functional deficit as the genetic suppression of enzyme expression. Biochemical assays demonstrate that kinase activity is required as a tonic signal: targets must be phosphorylated to link Ca²⁺ store release to Icrac gating. The action of kinases on Icrac activation does not arise from control of the expression level of the stromal interaction molecule 1 and Orai1 proteins.

CCL5 evokes calcium signals in microglia through a kinase-, phosphoinositide-, and nucleotide-dependent mechanism

Microglia, the resident macrophages of the CNS, are responsible for the innate immune response in the brain and participate in the pathogenesis of certain neurodegenerative disorders. Chemokines initiate activation and migration of microglia. The beta-chemokine CCL5 induces an elevation in intracellular calcium concentration ([Ca(2+)](i)) in human microglia. Here, we examined the signal transduction pathway linking activation of chemokine receptor CCR5 to an elevation in [Ca(2+)](i) in cultured microglia by using pharmacological approaches in combination with Fura-2-based digital imaging. The CCL5-induced response required Janus kinase (Jak) activity and the stimulation of an inhibitory G protein. Multiple downstream signaling pathways were involved, including phosphatidylinositol 3-kinase (PI3K), Bruton's tyrosine kinase (Btk), and phospholipase C (PLC)-mediated release of Ca(2+) from inositol 1,4,5-trisphosphate (IP(3))-sensitive stores. Activation of both the kinase and the lipase pathways was required for eliciting the Ca(2+) response. However, the majority of the [Ca(2+)](i) increase was derived from sources activated by NAD metabolites. Cyclic ADP-ribose (cADPR) evoked Ca(2+) release from intracellular stores, and ADPR evoked Ca(2+) influx via a nimodipine-sensitive channel. Thus, a multistep cascade couples CCR5 activation to Ca(2+) increases in human microglia. Because changes in [Ca(2+)](i) affect chemotaxis, secretion, and gene expression, pharmacologic modulation of this pathway may alter inflammatory and degenerative processes in the CNS.

Genetic analysis of patients with defects in early B-cell development

Approximately 85% of patients with defects in early B-cell development have X-linked agammaglobulinemia (XLA), a disorder caused by mutations in the cytoplasmic Bruton's tyrosine kinase (Btk). Although Btk is activated by cross-linking of a variety of cell-surface receptors, the most critical signal transduction pathway is the one initiated by the pre-B cell and B-cell antigen receptor complex. Mutations in Btk are highly diverse, and no single mutation accounts for more than 3% of patients. Although there is no strong genotype/phenotype correlation in XLA, the specific mutation in Btk is one of the factors that influences the severity of disease. Mutations in the components of the pre-B cell and B-cell antigen receptor complex account for an additional 5-7% of patients with defects in early B-cell development. Patients with defects in these proteins are clinically indistinguishable from those with XLA. However, they tend to be younger at the time of diagnosis, and whereas most patients with XLA have a small number of B cells in the peripheral circulation, these cells are not found in patients with defects in micro heavy chain or Igalpha. Polymorphic variants in the components of the pre-B cell and B-cell receptor complex, particularly micro heavy chain and lambda5, may contribute to the severity of XLA.

Bruton's tyrosine kinase regulates immunoglobulin promoter activation in association with the transcription factor Bright

Bright (B-cell regulator of immunoglobulin heavy chain transcription) binding to immunoglobulin heavy chain loci after B-cell activation is associated with increased heavy chain transcription. Our earlier reports demonstrated that Bright coimmunoprecipitates with Bruton's tyrosine kinase (Btk) and that these proteins associate in a DNA-binding complex in primary B cells. B cells from immunodeficient mice with a mutation in Btk failed to produce stable Bright DNA-binding complexes. In order to determine if Btk is important for Bright function, a transcription activation assay was established and analyzed using real-time PCR technology. Cells lacking both Bright and Btk were transfected with Bright and/or Btk along with an immunoglobulin heavy chain reporter construct. Immunoglobulin gene transcription was enhanced when Bright and Btk were coexpressed. In contrast, neither Bright nor Btk alone led to activation of heavy chain transcription. Furthermore, Bright function required both Btk kinase activity and sequences within the pleckstrin homology domain of Btk. Bright was not appreciably phosphorylated by Btk; however, a third tyrosine-phosphorylated protein coprecipitated with Bright. Thus, the ability of Bright to enhance immunoglobulin transcription critically requires functional Btk.

The Btk inhibitor LFM-A13 is a potent inhibitor of Jak2 kinase activity

LFM-A13, or alpha-cyano-beta-hydroxy-beta-methyl-N-(2,5-dibromophenyl)propenamide, was shown to inhibit Bruton's tyrosine kinase (Btk). Here we show that LFM-A13 efficiently inhibits erythropoietin (Epo)-induced phosphorylation of the erythropoietin receptor, Janus kinase 2 (Jak2) and downstream signalling molecules. However, the tyrosine kinase activity of immunoprecipitated or in vitro translated Btk and Jak2 was equally inhibited by LFM-A13 in in vitro kinase assays. Finally, Epo-induced signal transduction was also inhibited in cells lacking Btk. Taken together, we conclude that LFM-A13 is a potent inhibitor of Jak2 and cannot be used as a specific tyrosine kinase inhibitor to study the role of Btk in Jak2-dependent cytokine signalling.

Tec Kinases Mediate Sustained Calcium Influx via Site-specific Tyrosine Phosphorylation of the Phospholipase Cγ Src Homology 2-Src Homology 3 Linker

Tyrosine phosphorylation of phospholipase Cgamma2 (PLCgamma2) is a crucial activation switch that initiates and maintains intracellular calcium mobilization in response to B cell antigen receptor (BCR) engagement. Although members from three distinct families of non-receptor tyrosine kinases can phosphorylate PLCgamma in vitro, the specific kinase(s) controlling BCR-dependent PLCgamma activation in vivo remains unknown. Bruton's tyrosine kinase (Btk)-deficient human B cells exhibit diminished inositol 1,4,5-trisphosphate production and calcium signaling despite a normal inducible level of total PLCgamma2 tyrosine phosphorylation. This suggested that Btk might modify a critical subset of residues essential for PLCgamma2 activity. To evaluate this hypothesis, we generated site-specific phosphotyrosine antibodies recognizing four putative regulatory residues within PLCgamma2. Whereas all four sites were rapidly modified in response to BCR engagement in normal B cells, Btk-deficient B cells exhibited a marked reduction in phosphorylation of the Src homology 2 (SH2)-SH3 linker region sites, Tyr(753) and Tyr(759). Phosphorylation of both sites was restored by expression of Tec, but not Syk, family kinases. In contrast, phosphorylation of the PLCgamma2 carboxyl-terminal sites, Tyr(1197) and Tyr(1217), was unaffected by the absence of functional Btk. Together, these data support a model whereby Btk/Tec kinases control sustained calcium signaling via site-specific phosphorylation of key residues within the PLCgamma2 SH2-SH3 linker.

Sustained correction of B-cell development and function in a murine model of X-linked agammaglobulinemia (XLA) using retroviral-mediated gene transfer

X-linked agammaglobulinemia (XLA) is a human immunodeficiency caused by mutations in Bruton tyrosine kinase (Btk) and characterized by an arrest in early B-cell development, near absence of serum immunoglobulin, and recurrent bacterial infections. Using Btk- and Tec-deficient mice (BtkTec–/–) as a model for XLA, we determined if Btk gene therapy could correct this disorder. Bone marrow (BM) from 5-fluorouracil (5FU)–treated BtkTec–/– mice was transduced with a retroviral vector expressing human Btk and transplanted into BtkTec–/– recipients. Mice engrafted with transduced hematopoietic cells exhibited rescue of both primary and peripheral B-lineage development, recovery of peritoneal B1 B cells, and correction of serum immunoglobulin M (IgM) and IgG3 levels. Gene transfer also restored T-independent type II immune responses, and B-cell antigen receptor (BCR) proliferative responses. B-cell progenitors derived from Btk-transduced stem cells exhibited higher levels of Btk expression than non-B cells; and marking studies demonstrated a selective advantage for Btk-transduced B-lineage cells. BM derived from primary recipients also rescued Btk-dependent function in secondary hosts that had received a transplant. Together, these data demonstrate that gene transfer into hematopoietic stem cells can reconstitute Btk-dependent B-cell development and function in vivo, and strongly support the feasibility of pursuing Btk gene transfer for XLA.

Protein kinase C family functions in B-cell activation

Members of the protein kinase C (PKC) family play important but distinct roles in B-cell activation, as demonstrated by emerging genetic and biochemical studies. PKCbeta is indispensable for B-cell antigen receptor (BCR)-induced NF-kappaB activation and B-cell survival. Recent evidence indicates that PKCbeta might regulate inhibitor of kappaB kinase (IKK) and NF-kappaB activation through interaction with the CARMA1/Bcl10/MALT signaling complex in BCR microdomains. By contrast, the novel PKC isoform PKCdelta is specifically required to maintain the tolerance of self-reactive B cells.

B cell receptor-induced cAMP-response element-binding protein activation in B lymphocytes requires novel protein kinase Cdelta

The cAMP-response element-binding protein (CREB) is activated by phosphorylation on Ser-133 and plays a key role in the proliferative and survival responses of mature B cells to B cell receptor (BCR) signaling. The signal link between the BCR and CREB activation depends on a phorbol ester (phorbol 12-myristate 13-acetate)-sensitive protein kinase C (PKC) activity and not protein kinase A or calmodulin kinase; however, the identity and role of the PKC(s) activity has not been elucidated. We found the novel PKCdelta (nPKCdelta) activator bistratene A is sufficient to induce CREB phosphorylation in murine splenic B cells. The pharmacological inhibitor Gö6976, which targets conventional PKCs and PKCmu, has no effect on CREB phosphorylation, whereas the nPKCdelta inhibitor rottlerin blocks CREB phosphorylation following BCR cross-linking. Bryostatin 1 selectively prevents nPKCdelta depletion by phorbol 12-myristate 13-acetate when coapplied, coincident with protection of BCR-induced CREB phosphorylation. Ectopic expression of a kinase-inactive nPKCdelta blocks BCR-induced CREB phosphorylation in A20 B cells. In addition, BCR-induced CREB phosphorylation is significantly diminished in nPKCdelta-deficient splenic B cells in comparison with wild type mice. Consistent with the essential role for Bruton's tyrosine kinase and phospholipase Cgamma2 in mediating PKC activation, Bruton's tyrosine kinase- and phospholipase Cgamma2-deficient B cells display defective CREB phosphorylation by the BCR. We also found that p90 RSK directly phosphorylates CREB on Ser-133 following BCR cross-linking and is positioned downstream of nPKCdelta. Taken together, these results suggest a model in which BCR engagement leads to the phosphorylation of CREB via a signaling pathway that requires nPKCdelta and p90 RSK in mature B cells.

The role of Tec family kinases in myeloid cells

Members of the Tec kinase family (Bmx, Btk, Itk, Rlk and Tec) are primarily expressed in the hematopoietic system and form, after the Src kinase family, the second largest class of non-receptor protein tyrosine kinases. During lymphocyte development and activation Tec kinases have important functions in signaling pathways downstream of the antigen receptors. Tec family kinases are also expressed in cells of the myeloid lineage. However, with the exception of mast cells and platelets, their biological role in the myeloid system is only poorly understood. This review summarizes the current knowledge about the function of Tec family kinases in hematopoietic cells of the myeloid lineage.

Signaling in transitional type 2 B cells is critical for peripheral B-cell development

Splenic peripheral B-cell development and the events regulating this functionally significant but relatively poorly defined developmental process have become a major focus in recent studies in B-cell immunology. Following the exit from the bone marrow, peripheral B cells develop through transitional type 1 (T1) and transitional type 2 (T2) B-cell stages. Emerging data suggest that the T2 subset is the immediate precursor of the mature B-cell populations present in the spleen. In this review, we first elaborate on the evidence describing the unique properties of CD21hiCD24hiCD23hiIgMhiIgDhi T2 B cells. T2 cells uniquely activate a proliferative, pro-survival, and differentiation program in response to B-cell antigen receptor (BCR) engagement. The potential mechanisms leading to the differential BCR responsiveness of T1 versus T2 B cells are discussed. We also review evidence that distinguishes key BCR-dependent signaling pathways operative in T2 and mature B cells. These signaling cascades include a protein kinase Cbeta (PKCbeta)-dependent cell-survival pathway and a second PKCbeta-independent pathway essential for BCR-driven differentiation. Finally, we discuss recent intriguing results suggesting that the type of signal(s) encountered by T2 cells leads to their differential maturation toward the follicular mature versus marginal zone mature B-cell populations. These combined observations suggest important implications with regard to B-cell selection and tolerance, potential novel therapeutic targets for B-cell lymphomas, and how the intricate balance of commensal organisms and other microenvironmental signals interact to promote the generation of 'innate-like' versus adaptive effector B-cell populations.

B Cell receptor directs the activation of NFAT and NF-κB via distinct molecular mechanisms

BCR engagement initiates intracellular calcium ([Ca2+]i) mobilization which is critical for the activation of multiple transcription factors including NF-kappaB and NFAT. Previously, we showed that Bruton's tyrosine kinase (BTK)-deficient (btk-/-) B cells, which display a modestly reduced calcium response to BCR crosslinking, do not activate NF-kappaB. Here we show that BTK is also essential for the activation of NFAT following BCR engagement. Pharmacological mobilization of [Ca2+]i in BTK-deficient DT40 B cells (DT40.BTK) does not rescue BCR directed activation of NF-kappaB and only partially that of NFAT, suggesting existence of additional BTK-signaling pathways in this process. Therefore, we investigated a requirement for BTK in the production of diacylglycerol (DAG). We found that DT40.BTK B cells do not produce DAG in response to BCR engagement. Pharmacological inhibition of PKC isozymes and Ras revealed that the BCR-induced activation of NF-kappaB requires conventional PKCbeta, whereas that of NFAT may involve non-conventional PKCdelta and Ras pathways. Consistent with an essential role for BTK in the regulation of NFAT, B cells from btk-/- mice display defective expression of CD5, a gene under the control of NFAT. Together, these results suggest that BCR employs distinct BTK-dependent molecular mechanisms to regulate the activation of NF-kappaB versus NFAT.

Transducing signals from antigen receptors to nuclear factor kappaB

Signaling leading to the survival or apoptosis of immune system cells must be balanced to ensure the normal mounting and extinguishing of immune responses. One of the essential regulators of immune cell survival is the transcription factor nuclear factor kappaB (NF-kappaB). NF-kappaB is critical for the activation of T and B lymphocytes and is a central coordinator of innate and adaptive immunity. Pathogen recognition, whether mediated via the Toll-like receptors or via the antigen-specific T- and B-cell receptors, initiates the activation of distinct signal transduction pathways that activate NF-kappaB. Activation of NF-kappaB by these pathways is necessary for lymphocyte activation, expansion, and effector function in response to infection. In addition, recent work has shown that the aberrant activation of NF-kappaB by these pathways can contribute to the development of autoimmunity, chronic inflammation, or lymphoid malignancy. There is thus an urgent need to understand the exact molecular details of these signal transduction cascades so that we may develop novel therapeutics. This article will review the specific signal transduction pathways that mediate NF-kappaB activation in response to antigen receptor ligation in T and B lymphocytes. These newly defined pathways, which are essential for adaptive immune responses, are built around the key adapter protein, Bcl-10. Bcl-10 is known to participate in chromosomal translocations in human mucosa-associated lymphoid tissue lymphomas.

Reconstitution of B cell function in murine models of immunodeficiency

Murine models of immunodeficiency were used to evaluate strategies that might allow B cell engraftment in patients with X-linked agammaglobulinemia. Mice with defects in Btk or mu heavy chain were given 2.5 x 10(6) bone marrow cells from wild-type congenic donors. In the absence of any preparative regimen or immunosuppression, Btk-deficient mice on the CBA background developed normal concentrations of serum IgM and IgG3 by 12 weeks posttransplant. By contrast, mu heavy chain-deficient mice on the C57BL/6 background required some immunosuppression to achieve engraftment. Treatment of these mice with anti-T-cell antibodies 2 and 4 days prior to transplant resulted in normal concentrations of serum immunoglobulins by 6 weeks posttransplant. These pretreated mice had only 10% of the normal number of splenic B cells and they had no evidence of donor T cell engraftment. These results suggest that myelotoxic drugs may not be needed to achieve B cell engraftment in B-cell-deficient subjects.

Genetic control of susceptibility to bacterial infections in mouse models

Historically, the laboratory mouse (Mus musculus) has been the experimental model of choice to study pathophysiology of infection with bacterial pathogens, including natural and acquired host defence mechanisms. Inbred mouse strains differ significantly in their degree of susceptibility to infection with various human pathogens such as Mycobacterium, Salmonella, Legionella and many others. Segregation analyses and linkage studies have indicated that some of these differences are under simple genetic control whereas others behave as complex traits. Major advances in genome technologies have greatly facilitated positional cloning of single gene effects. Thus, a number of genes playing a key role in initial susceptibility, progression and outcome of infection have been uncovered and the functional characterization of the encoded proteins has provided new insight into the molecular basis of antimicrobial defences of polymorphonuclear leukocytes, macrophages, as well as T and B lymphocytes. The multigenic control of susceptibility to infection with certain human pathogens is beginning to be characterized by quantitative trait locus mapping in genome wide scans. This review summarizes recent progress on the mapping, cloning and characterization of genes and proteins that affect susceptibility to infection with major intracellular bacterial pathogens.

NF-kappaB activation by protein kinase C isoforms and B-cell function

B cells are essential to the immune response in health and disease. Results from knockout (KO) mice for different members of the nuclear factor-kappaB (NF-kappaB) family have highlighted the importance of this transcription factor in B cell development and function. The recent generation of additional KO mice for adapters and kinases implicated in NF-kappaB activation, including several protein kinase C isoforms, has provided new insights into the roles of these proteins in B cell signalling. These studies have also given rise to a number of important questions that must be answered with further experimentation to establish accurately the signalling pathways that regulate B-cell function through NF-kappaB.

Early defects in B cell development

PURPOSE OF REVIEW

Recent clinical studies in patients with genetically proven X-linked or autosomal recessive agammaglobulinemia provide some guidelines that should influence our management of patients with suspected immunodeficiency.

RECENT FINDINGS

Males who are at a high risk of having X-linked agammaglobulinemia because they have an affected brother or uncle are often not evaluated for immunodeficiency until they are hospitalized for infection. Some of those who are evaluated are not started on gammaglobulin therapy immediately. More than 10% of patients with X-linked agammaglobulinemia are hospitalized for infection at less than 6 months of age, indicating that patients with known X-linked agammaglobulinemia should be started on therapy by 2-3 months of age. In patients with sporadic X-linked agammaglobulinemia, the incidence of chronic lung disease correlates with the age at diagnosis, highlighting the importance of early diagnosis. Although almost all patients who are diagnosed as having the condition at more than 12 months of age have a history of recurrent otitis, 93% are not evaluated for immunodeficiency until they are hospitalized for infection. Because the physical exam provides a clue to the diagnosis of the condition--unusually small or absent cervical lymph nodes and tonsils--it should be possible to make an early diagnosis in a greater percentage of patients. Patients with autosomal recessive agammaglobulinemia have an earlier onset of disease compared with patients with X-linked agammaglobulinemia and they are more likely to have severe complications of the disease.

SUMMARY

There is plenty of room for improvement in the diagnosis and management of patients with defects in early B cell development.

Genetic regulation of host responses to Salmonella infection in mice

Salmonella spp are Gram-negative bacteria capable of infecting a wide range of host species, including humans, domesticated and wild mammals, reptiles, birds and insects. The outcome of an encounter between Salmonella and its host is dependent upon multiple factors including the host genetic background. To facilitate the study of the genetic factors involved in resistance to this pathogen, mouse models of Salmonella infection have been developed and studied for years, allowing identification of several genes and pathways that may influence the disease outcome. In this review, we will cover some of the genes involved in mouse resistance to Salmonella that were identified through the study of congenic mouse strains, cloning of spontaneous mouse mutations, use of site-directed mutagenesis or quantitative trait loci analysis. In parallel, the relevant information pertaining to genes involved in resistance to Salmonella in humans will be discussed.

PKC-beta controls I kappa B kinase lipid raft recruitment and activation in response to BCR signaling

NF-kappa B signaling is required for the maintenance of normal B lymphocytes, whereas dysregulated NF-kappa B activation contributes to B cell lymphomas. The events that regulate NF-kappa B signaling in B lymphocytes are poorly defined. Here, we demonstrate that PKC-beta is specifically required for B cell receptor (BCR)-mediated NF-kappa B activation. B cells from protein kinase C-beta (PKC-beta)-deficient mice failed to recruit the I kappa B kinase (IKK) complex into lipid rafts, activate IKK, degrade I kappa B or up-regulate NF-kappa B-dependent survival signals. Inhibition of PKC-beta promoted cell death in B lymphomas characterized by exaggerated NF-kappa B activity. Together, these data define an essential role for PKC-beta in BCR survival signaling and highlight PKC-beta as a key therapeutic target for B-lineage malignancies.

TAPP1 and TAPP2 are targets of phosphatidylinositol 3-kinase signaling in B cells: sustained plasma membrane recruitment triggered by the B-cell antigen receptor

We report the characterization of two signal transduction proteins related to Bam32, known as TAPP1 and TAPP2. Bam32, TAPP1, and TAPP2 share several characteristics, including small size (32 to 47 kDa), lack of enzymatic domains, high conservation between humans and mice, and the presence of pleckstrin homology (PH) domains near their C termini which contain the 3-phosphoinositide-binding motif. Unlike Bam32, the N-terminal regions of TAPP1 and TAPP2 contain a second PH domain. TAPP1 and TAPP2 transcripts are expressed in a variety of tissues including lymphoid tissues. Using live-cell imaging, we demonstrate that TAPP1 and TAPP2 are recruited to the plasma membrane of BJAB human B-lymphoma cells upon activation through the B-cell antigen receptor (BCR). The C-terminal PH domain is necessary and sufficient for BCR-induced membrane recruitment of both TAPP1 and TAPP2. Blockade of phosphatidylinositol 3-kinase (PI3K) activity completely abolished BCR-induced recruitment of TAPP1 and TAPP2, while expression of active PI3K is sufficient to drive constitutive membrane localization of TAPP1 and TAPP2. TAPP1 and TAPP2 preferentially accumulate within ruffled, F-actin-rich areas of plasma membrane, suggesting a potential role in PI3K-driven cytoskeletal reorganization. Like Bam32, BCR-driven TAPP1 and TAPP2 recruitment is a relatively slow and sustained response, in contrast to Btk recruitment and Ca(2+) mobilization responses, which are rapid and transient. Consistent with recent studies indicating that Bam32, TAPP1, and TAPP2 can bind to PI(3,4)P(2), we find that membrane recruitment correlates well with production of PI(3,4)P(2) but not with that of PI(3,4,5)P(3). Our results indicate that TAPP1 and TAPP2 are direct targets of PI3K signaling that are recruited into plasma membranes with distinctive delayed kinetics and accumulate within F-actin-rich membrane ruffles. We postulate that the TAPPs function to orchestrate cellular responses during the sustained phase of signaling.

Role of Ca2+ in the intracellular signaling pathway of anti-IgM-induced apoptosis in the human B-cell line, MBC-1, established from Burkitt lymphoma

The role of Ca2+ in the intracellular signal transduction process that causes antibody-induced apoptotic cell death in B-cells is not completely understood. We previously established a B-cell line (MBC-1) from a patient with Burkitt lymphoma at the leukemic stage that demonstrated the typical morphology and internucleosomal DNA fragmentation of apoptosis when treated with anti-immunoglobulin (Ig)M antibody. This antibody-induced cell death was partially inhibited by pretreatment with ethyleneglycol-bis-tetraacetic acid (EGTA) and actinomycin-D. FK506, an immunosupressive agent and calcineurin inhibitor, also partially rescued the anti-IgM antibody-induced death of MBC-1 cells. These results show that the calcium signaling pathway, which leads to a change in gene expression, plays an important role in anti-IgM-induced apoptosis in MBC-1 cells. Flow cytometric measurement of the cytosolic free Ca2+ concentration ([Ca2+]i) showed that nontoxic concentrations of 4-bromo-calcium ionophore A23187 (Ca2+ IP) increased [Ca2+]i more than did the anti-IgM antibody. A brief Ca2+ spike was observed on anti-IgM antibody treatment, but a gradual increase and decrease were observed when the cells were treated with Ca2+ IP at a nontoxic concentration of 1 microg/mL. These findings suggest that interpretations differ for the 2 patterns of calcium signaling and that the brief spiked elevation of Ca2+ produces distinct biological and cellular responses compared to the gradual increase and decrease of [Ca2+]i. Our results support the hypothesis that Ca2+ plays a significant role as a multifunctional second messenger providing specific information to the nucleus in anti-IgM antibody-induced apoptosis in MBC-1 cells.

Regulation of B-cell signal transduction by adaptor proteins

An important role has emerged for adaptor molecules in linking cell-surface receptors, such as the B-cell antigen receptor, with effector enzymes. Adaptor proteins direct the appropriate subcellular localization of effectors and regulate their activity by inducing conformational changes, both of which, in turn, contribute to the spatio-temporal precision of B-cell signal-transduction events. In addition, adaptor molecules participate in establishing negative- or positive-feedback regulatory loops in signalling networks, thereby fine-tuning the B-cell response.