Severe B cell deficiency and disrupted splenic architecture in transgenic mice expressing the E41K mutated form of Bruton's tyrosine kinase

E41K mutation activates Bruton's tyrosine kinase by stabilizing an inositol hexakisphosphate-dependent invisible dimer

Bruton's tyrosine kinase (BTK) regulates diverse cellular signaling of the innate and adaptive immune system in response to microbial pathogens. Downregulation or constitutive activation of BTK is reported in patients with autoimmune diseases or various B-cell leukemias. BTK is a multidomain protein tyrosine kinase that adopts an Src-like autoinhibited conformation maintained by the interaction between the kinase and PH-TH domains. The PH-TH domain plays a central role in regulating BTK function. BTK is activated by binding to PIP3 at the plasma membrane upon stimulation by the B-cell receptor (BCR). The PIP3 binding allows dimerization of the PH-TH domain and subsequent transphosphorylation of the activation loop. Alternatively, a recent study shows that the multivalent T-cell-independent (TI) antigen induces BCR response by activating BTK independent of PIP3 binding. It was proposed that a transiently stable IP6-dependent PH-TH dimer may activate BTK during BCR activation by the TI antigens. However, no IP6-dependent PH-TH dimer has been identified yet. Here, we investigated a constitutively active PH-TH mutant (E41K) to determine if the elusive IP6-dependent PH-TH dimer exists. We showed that the constitutively active E41K mutation activates BTK by stabilizing the IP6-dependent PH-TH dimer. We observed that a downregulating mutation in the PH-TH domain (R28H) linked to X-linked agammaglobulinemia impairs BTK activation at the membrane and in the cytosol by preventing PH-TH dimerization. We conclude that the IP6 dynamically remodels the BTK active fraction between the membrane and the cytoplasm. Stimulating with IP6 increases the cytosolic fraction of the activated BTK.

Bruton's Tyrosine Kinase-Mediated Signaling in Myeloid Cells Is Required for Protective Innate Immunity During Pneumococcal Pneumonia

Bruton’s tyrosine kinase (Btk) is a cytoplasmic kinase expressed in B cells and myeloid cells. It is essential for B cell development and natural antibody-mediated host defense against bacteria in humans and mice, but little is known about the role of Btk in innate host defense in vivo. Previous studies have indicated that lack of (natural) antibodies is paramount for impaired host defense against Streptococcus (S.) pneumoniae in patients and mice with a deficiency in functional Btk. In the present study, we re-examined the role of Btk in B cells and myeloid cells during pneumococcal pneumonia and sepsis in mice. The antibacterial defense of Btk^-/- mice was severely impaired during pneumococcal pneumosepsis and restoration of natural antibody production in Btk^-/- mice by transgenic expression of Btk specifically in B cells did not suffice to protect against infection. Btk^-/- mice with reinforced Btk expression in MhcII⁺ cells, including B cells, dendritic cells and macrophages, showed improved antibacterial defense as compared to Btk^-/- mice. Bacterial outgrowth in Lysmcre-Btk^fl/Y mice was unaltered despite a reduced capacity of Btk-deficient alveolar macrophages to respond to pneumococci. Mrp8cre-Btk^fl/Y mice with a neutrophil specific paucity in Btk expression, however, demonstrated impaired antibacterial defense. Neutrophils of Mrp8cre-Btk^fl/Y mice displayed reduced release of granule content after pulmonary installation of lipoteichoic acid, a gram-positive bacterial cell wall component relevant for pneumococci. Moreover, Btk deficient neutrophils showed impaired degranulation and phagocytosis upon incubation with pneumococci ex vivo. Taken together, the results of our study indicate that besides regulating B cell-mediated immunity, Btk is critical for regulation of myeloid cell-mediated, and particularly neutrophil-mediated, innate host defense against S. pneumoniae in vivo.

MOG autoantibodies trigger a tightly-controlled FcR and BTK-driven microglia proliferative response

Autoantibodies are a hallmark of numerous neurologic disorders, including multiple sclerosis (MS), autoimmune encephalitides and neuromyelitis optica (NMO). While well understood in peripheral myeloid cells, the pathophysiological significance of autoantibody-induced Fc receptor (FcR) signaling in microglia remains unknown, in part due to the lack of a robust in vivo model. Moreover, application of therapeutic antibodies for neurodegenerative disease also highlights the importance of understanding FcR signaling in microglia. Here, we describe a novel in vivo experimental paradigm that allows for selective engagement of Fc receptors within the CNS by peripherally injecting anti-myelin oligodendrocyte glycoprotein (MOG) monoclonal antibodies (mAbs) in normal wild-type mice. MOG antigen-bound immunoglobulins were detected throughout the CNS and triggered a rapid and tightly regulated proliferative response in both brain and spinal cord microglia. This microglial response was abrogated when anti-MOG antibodies were deprived of Fc effector function or injected into Fc γ R knockout mice and was associated with the downregulation of FcRs in microglia, but not peripheral myeloid cells, establishing that this response was dependent on central FcR engagement. Downstream of FcRs, Bruton's tyrosine kinase (BTK) was a required signaling node for this response, as microglia proliferation was amplified in BTKE41K knock-in mice expressing a constitutively active form of BTK and blunted in mice treated with a CNS penetrant small molecule inhibitor of BTK. Finally, this response was associated with transient and stringently regulated changes in gene expression predominantly related to cellular proliferation, which markedly differed from transcriptional programs typically associated with FcR engagement in peripheral myeloid cells. Together, these results establish a physiologically-meaningful functional response to FcR and BTK signaling in microglia while providing a novel in vivo tool to further dissect the roles of microglia-specific FcR and BTK-driven responses to both pathogenic and therapeutic antibodies in CNS homeostasis and disease.

Map3k14 as a Regulator of Innate and Adaptive Immune Response during Acute Viral Infection

The replication of virus in secondary lymphoid organs is crucial for the activation of antigen-presenting cells. Balanced viral replication ensures the sufficient availability of antigens and production of cytokines, and both of which are needed for virus-specific immune activation and viral elimination. Host factors that regulate coordinated viral replication are not fully understood. In the study reported here, we identified Map3k14 as an important regulator of enforced viral replication in the spleen while performing genome-wide association studies of various inbred mouse lines in a model of lymphocytic choriomeningitis virus (LCMV) infection. When alymphoplasia mice (aly/aly, Map3k14^aly/aly, or Nik^aly/aly), which carry a mutation in Map3k14, were infected with LCMV or vesicular stomatitis virus (VSV), they display early reductions in early viral replication in the spleen, reduced innate and adaptive immune activation, and lack of viral control. Histologically, scant B cells and the lack of CD169⁺ macrophages correlated with reduced immune activation in Map3k14^aly/aly mice. The transfer of wildtype B cells into Map3k14^aly/aly mice repopulated CD169⁺ macrophages, restored enforced viral replication, and resulted in enhanced immune activation and faster viral control.

Switch-like activation of Bruton's tyrosine kinase by membrane-mediated dimerization

The transformation of molecular binding events into cellular decisions is the basis of most biological signal transduction. A fundamental challenge faced by these systems is that reliance on protein-ligand chemical affinities alone generally results in poor sensitivity to ligand concentration, endangering the system to error. Here, we examine the lipid-binding pleckstrin homology and Tec homology (PH-TH) module of Bruton's tyrosine kinase (Btk). Using fluorescence correlation spectroscopy (FCS) and membrane-binding kinetic measurements, we identify a phosphatidylinositol (3-5)-trisphosphate (PIP3) sensing mechanism that achieves switch-like sensitivity to PIP3 levels, surpassing the intrinsic affinity discrimination of PIP3:PH binding. This mechanism employs multiple PIP3 binding as well as dimerization of Btk on the membrane surface. Studies in live cells confirm that mutations at the dimer interface and peripheral site produce effects comparable to that of the kinase-dead Btk in vivo. These results demonstrate how a single protein module can institute an allosteric counting mechanism to achieve high-precision discrimination of ligand concentration. Furthermore, this activation mechanism distinguishes Btk from other Tec family member kinases, Tec and Itk, which we show are not capable of dimerization through their PH-TH modules. This suggests that Btk plays a critical role in the stringency of the B cell response, whereas T cells rely on other mechanisms to achieve stringency.

Growth Hormone (GH) Deficient Mice With GHRH Gene Ablation Are Severely Deficient in Vaccine and Immune Responses Against Streptococcus pneumoniae

The precise impact of the somatotrope axis upon the immune system is still highly debated. We have previously shown that mice with generalized ablation of growth hormone (GH) releasing hormone (GHRH) gene (Ghrh^−/−) have normal thymus and T-cell development, but present a marked spleen atrophy and B-cell lymphopenia. Therefore, in this paper we have investigated vaccinal and anti-infectious responses of Ghrh^−/− mice against S. pneumoniae, a pathogen carrying T-independent antigens. Ghrh^−/− mice were unable to trigger production of specific IgM after vaccination with either native pneumococcal polysaccharides (PPS, PPV23) or protein-PPS conjugate (PCV13). GH supplementation of Ghrh^−/− mice restored IgM response to PPV23 vaccine but not to PCV13 suggesting that GH could exert a specific impact on the spleen marginal zone that is strongly implicated in T-independent response against pneumococcal polysaccharides. As expected, after administration of low dose of S. pneumoniae, wild type (WT) completely cleared bacteria after 24 h. In marked contrast, Ghrh^−/− mice exhibited a dramatic susceptibility to S. pneumoniae infection with a time-dependent increase in lung bacterial load and a lethal bacteraemia already after 24 h. Lungs of infected Ghrh^−/− mice were massively infiltrated by inflammatory macrophages and neutrophils, while lung B cells were markedly decreased. The inflammatory transcripts signature was significantly elevated in Ghrh^−/− mice. In this animal model, the somatotrope GHRH/GH/IGF1 axis plays a vital and unsuspected role in vaccine and immunological defense against S. pneumoniae.

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

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

Distinct and Overlapping Functions of TEC Kinase and BTK in B Cell Receptor Signaling

The Tec tyrosine kinase is expressed in many cell types, including hematopoietic cells, and is a member of the Tec kinase family that also includes Btk. Although the role of Btk in B cells has been extensively studied, the role of Tec kinase in B cells remains largely unclear. It was previously shown that Tec kinase has the ability to partly compensate for loss of Btk activity in B cell differentiation, although the underlying mechanism is unknown. In this study, we confirm that Tec kinase is not essential for normal B cell development when Btk is present, but we also found that Tec-deficient mature B cells showed increased activation, proliferation, and survival upon BCR stimulation, even in the presence of Btk. Whereas Tec deficiency did not affect phosphorylation of phospholipase Cγ or Ca²⁺ influx, it was associated with significantly increased activation of the intracellular Akt/S6 kinase signaling pathway upon BCR and CD40 stimulation. The increased S6 kinase phosphorylation in Tec-deficient B cells was dependent on Btk kinase activity, as ibrutinib treatment restored pS6 to wild-type levels, although Btk protein and phosphorylation levels were comparable to controls. In Tec-deficient mice in vivo, B cell responses to model Ags and humoral immunity upon influenza infection were enhanced. Moreover, aged mice lacking Tec kinase developed a mild autoimmune phenotype. Taken together, these data indicate that in mature B cells, Tec and Btk may compete for activation of the Akt signaling pathway, whereby the activating capacity of Btk is limited by the presence of Tec kinase.

A novel oncogenic BTK isoform is overexpressed in colon cancers and required for RAS-mediated transformation

Bruton's tyrosine kinase (BTK) is essential for B-cell proliferation/differentiation and it is generally believed that its expression and function are limited to bone marrow-derived cells. Here, we report the identification and characterization of p65BTK, a novel isoform abundantly expressed in colon carcinoma cell lines and tumour tissue samples. p65BTK protein is expressed, through heterogeneous nuclear ribonucleoprotein K (hnRNPK)-dependent and internal ribosome entry site-driven translation, from a transcript containing an alternative first exon in the 5'-untranslated region, and is post-transcriptionally regulated, via hnRNPK, by the mitogen-activated protein kinase (MAPK) pathway. p65BTK is endowed with strong transforming activity that depends on active signal-regulated protein kinases-1/2 (ERK1/2) and its inhibition abolishes RAS transforming activity. Accordingly, p65BTK overexpression in colon cancer tissues correlates with ERK1/2 activation. Moreover, p65BTK inhibition affects growth and survival of colon cancer cells. Our data reveal that BTK, via p65BTK expression, is a novel and powerful oncogene acting downstream of the RAS/MAPK pathway and suggest that its targeting may be a promising therapeutic approach.

CEACAM1 induces B-cell survival and is essential for protective antiviral antibody production

B cells are essential for antiviral immune defence because they produce neutralizing antibodies, present antigen and maintain the lymphoid architecture. Here we show that intrinsic signalling of CEACAM1 is essential for generating efficient B-cell responses. Although CEACAM1 exerts limited influence on the proliferation of B cells, expression of CEACAM1 induces survival of proliferating B cells via the BTK/Syk/NF-κB-axis. The absence of this signalling cascade in naive Ceacam1^−/− mice limits the survival of B cells. During systemic infection with cytopathic vesicular stomatitis virus, Ceacam1^−/− mice can barely induce neutralizing antibody responses and die early after infection. We find, therefore, that CEACAM1 is a crucial regulator of B-cell survival, influencing B-cell numbers and protective antiviral antibody responses.

Antibody responses are regulated by selective survival of B cells with proper antigen specificity. Here the authors show that CEACAM1 is critical for B-cell survival during homeostasis and antiviral responses.

BTK Signaling in B Cell Differentiation and Autoimmunity

Since the original identification of Bruton's tyrosine kinase (BTK) as the gene defective in the primary immunodeficiency X-linked agammaglobulinemia (XLA) in 1993, our knowledge on the physiological function of BTK has expanded impressively. In this review, we focus on the role of BTK during B cell differentiation in vivo, both in the regulation of expansion and in the developmental progression of pre-B cells in the bone marrow and as a crucial signal transducer of signals downstream of the IgM or IgG B cell antigen receptor (BCR) in mature B cells governing proliferation, survival, and differentiation. In particular, we highlight BTK function in B cells in the context of host defense and autoimmunity. Small-molecule inhibitors of BTK have very recently shown impressive anti-tumor activity in clinical studies in patients with various B cell malignancies. Since promising effects of BTK inhibition were also seen in experimental animal models for lupus and rheumatoid arthritis, BTK may be a good target for controlling autoreactive B cells in patients with systemic autoimmune disease.

Targeting Bruton's tyrosine kinase in B cell malignancies

Bruton's tyrosine kinase (BTK) is a key component of B cell receptor (BCR) signalling and functions as an important regulator of cell proliferation and cell survival in various B cell malignancies. Small-molecule inhibitors of BTK have shown antitumour activity in animal models and, recently, in clinical studies. High response rates were reported in patients with chronic lymphocytic leukaemia and mantle cell lymphoma. Remarkably, BTK inhibitors have molecular effects that cannot be explained by the classic role of BTK in BCR signalling. In this Review, we highlight the importance of BTK in various signalling pathways in the context of its therapeutic inhibition.

Aberrant B cell selection and activation in systemic lupus erythematosus

The detrimental role of B lymphocytes in systemic lupus erythematosus (SLE) is evident from the high levels of pathogenic antinuclear autoantibodies (ANAs) found in SLE patients. Affirming this causative role, additional antibody-independent roles of B cells in SLE were appreciated. In recent years, many defects in B cell selection and activation have been identified in murine lupus models and SLE patients that explain the increased emergence and persistence of autoreactive B cells and their lowered activation threshold. Therefore, clinical trials with B cell depletion regimens in SLE patients were initiated but disappointingly the efficacy of B cell depleting agents proved to be limited. Remarkably however, a major breakthrough in SLE therapy was accomplished by blocking B cell survival factors rather then eliminating B cells. This surprising finding indicates that although SLE is a B cell-driven disease, the amplifying crosstalk between B cells and other cells of the immune system likely evokes the observed tolerance breakdown in B cells. Moreover, this implies that intelligent interception of pro-inflammatory loops rather then selectively silencing B cells will be key to the development of new SLE therapies. In this review, we will not only highlight the intrinsic B cell defects that facilitate the persistence of autoreactive B cells and their activation, but in addition we will focus on B cell extrinsic signals derived from T cells and innate immune cells that lower the activation threshold for B cells.

Btk levels set the threshold for B-cell activation and negative selection of autoreactive B cells in mice

On antigen binding by the B-cell receptor (BCR), B cells up-regulate protein expression of the key downstream signaling molecule Bruton tyrosine kinase (Btk), but the effects of Btk up-regulation on B-cell function are unknown. Here, we show that transgenic mice overexpressing Btk specifically in B cells spontaneously formed germinal centers and manifested increased plasma cell numbers, leading to antinuclear autoantibody production and systemic lupus erythematosus (SLE)–like autoimmune pathology affecting kidneys, lungs, and salivary glands. Autoimmunity was fully dependent on Btk kinase activity, because Btk inhibitor treatment (PCI-32765) could normalize B-cell activation and differentiation, and because autoantibodies were absent in Btk transgenic mice overexpressing a kinase inactive Btk mutant. B cells overexpressing wild-type Btk were selectively hyperresponsive to BCR stimulation and showed enhanced Ca2+ influx, nuclear factor (NF)–κB activation, resistance to Fas-mediated apoptosis, and defective elimination of selfreactive B cells in vivo. These findings unravel a crucial role for Btk in setting the threshold for B-cell activation and counterselection of autoreactive B cells, making Btk an attractive therapeutic target in systemic autoimmune disease such as SLE. The finding of in vivo pathology associated with Btk overexpression may have important implications for the development of gene therapy strategies for X-linked agammaglobulinemia, the immunodeficiency associated with mutations in BTK.

Btk is a positive regulator in the TREM-1/DAP12 signaling pathway

The triggering receptor expressed on myeloid cells 1 (TREM-1) has been implicated in the production of proinflammatory cytokines and chemokines during bacterial infection and sepsis. For downstream signal transduction, TREM-1 is coupled to the ITAM-containing adaptor DAP12. Here, we demonstrate that Bruton tyrosine kinase (Btk), a member of the Tec kinases, becomes phosphorylated upon TREM-1 triggering. In U937-derived cell lines, in which expression of Btk was diminished by shRNA-mediated knockdown, phosphorylation of Erk1/2 and PLCγ1 and Ca²⁺ mobilization were reduced after TREM-1 stimulation. Importantly, TREM-1-induced production of the pro-inflammatory cytokines, TNF-α and IL-8, and up-regulation of activation/differentiation cell surface markers were impaired in Btk knockdown cells. Similar results were obtained upon TREM-1 stimulation of BMDCs of Btk(-/-) mice. The analysis of cells containing Btk mutants revealed that intact membrane localization and a functional kinase domain were required for TREM-1-mediated signaling. Finally, after TREM-1 engagement, TNF-α production by PBMCs was reduced in the majority of patients suffering from X-linked agammaglobulinemia (XLA), a rare hereditary disease caused by mutations in the BTK gene. In conclusion, our data identify Btk as a positive regulator in the ITAM-mediated TREM-1/DAP12 pathway and suggest its implication in inflammatory processes.

MicroRNAs prevent the generation of autoreactive antibodies

MicroRNAs have been shown to be critical for a number of aspects of immune system regulation and function. Here, we have examined the role of microRNAs in terminal B cell differentiation by analyzing Cd19-Cre(ki/+) Dicer1(fl/fl) mice. We found that in the absence of Dicer, the transitional and marginal zone (MZ) B cell compartments were overrepresented and follicular (FO) B cell generation was impaired. microRNA analysis revealed that miR185, a microRNA overexpressed in FO cells, dampened B cell receptor (BCR) signaling through Bruton tyrosine kinase downregulation. Dicer-deficient B cells had a skewed BCR repertoire with hallmarks of autoreactivity, which correlated with high titers of autoreactive antibodies in serum and autoimmune features in females. Together, our results reveal a crucial role for microRNAs in late B cell differentiation and in the establishment of B cell tolerance.

Bruton's tyrosine kinase (Btk): function, regulation, and transformation with special emphasis on the PH domain

Bruton's agammaglobulinemia tyrosine kinase (Btk) is a cytoplasmic tyrosine kinase important in B-lymphocyte development, differentiation, and signaling. Btk is a member of the Tec family of kinases. Mutations in the Btk gene lead to X-linked agammaglobulinemia (XLA) in humans and X-linked immunodeficiency (Xid) in mice. Activation of Btk triggers a cascade of signaling events that culminates in the generation of calcium mobilization and fluxes, cytoskeletal rearrangements, and transcriptional regulation involving nuclear factor-kappaB (NF-kappaB) and nuclear factor of activated T cells (NFAT). In B cells, NF-kappaB was shown to bind to the Btk promoter and induce transcription, whereas the B-cell receptor-dependent NF-kappaB signaling pathway requires functional Btk. Moreover, Btk activation is tightly regulated by a plethora of other signaling proteins including protein kinase C (PKC), Sab/SH3BP5, and caveolin-1. For example, the prolyl isomerase Pin1 negatively regulates Btk by decreasing tyrosine phosphorylation and steady state levels of Btk. It is intriguing that PKC and Pin1, both of which are negative regulators, bind to the pleckstrin homology domain of Btk. To this end, we describe here novel mutations in the pleckstrin homology domain investigated for their transforming capacity. In particular, we show that the mutant D43R behaves similar to E41K, already known to possess such activity.

Rap1b regulates B cell development, homing, and T cell-dependent humoral immunity

Rap1 is a small GTPase that belongs to Ras superfamily. This ubiquitously expressed GTPase is a key regulator of integrin functions. Rap1 exists in two isoforms: Rap1a and Rap1b. Although Rap1 has been extensively studied, its isoform-specific functions in B cells have not been elucidated. In this study, using gene knockout mice, we show that Rap1b is the dominant isoform in B cells. Lack of Rap1b significantly reduced the absolute number of B220(+)IgM(-) pro/pre-B cells and B220(+)IgM(+) immature B cells in bone marrow. In vitro culture of bone marrow-derived Rap1b(-/-) pro/pre-B cells with IL-7 showed similar proliferation levels but reduced adhesion to stromal cell line compared with wild type. Rap1b(-/-) mice displayed reduced splenic marginal zone (MZ) B cells, and increased newly forming B cells, whereas the number of follicular B cells was normal. Functionally, Rap1b(-/-) mice showed reduced T-dependent but normal T-independent humoral responses. B cells from Rap1b(-/-) mice showed reduced migration to SDF-1, CXCL13 and in vivo homing to lymph nodes. MZ B cells showed reduced sphingosine-1-phosphate-induced migration and adhesion to ICAM-1. However, absence of Rap1b did not affect splenic B cell proliferation, BCR-mediated activation of Erk1/2, p38 MAPKs, and AKT. Thus, Rap1b is crucial for early B cell development, MZ B cell homeostasis and T-dependent humoral immunity.

Surrogate-light-chain silencing is not critical for the limitation of pre-B cell expansion but is for the termination of constitutive signaling

The pre-B cell receptor (pre-BCR), composed of immunoglobulin mu heavy chain and the surrogate light chain (SLC) proteins lambda5 and Vpreb, signals for proliferation and maturation of developing pre-B cells. It has been assumed that pre-B cells stop cycling by the pre-BCR-mediated downregulation of SLC transcription. We generated transgenic mice expressing SLC throughout B cell development and, remarkably, found that enforced SLC expression had no effect on pre-B cell proliferation or differentiation. However, in the presence of conventional immunoglobulin light chains, SLC components had the capacity to induce constitutive BCR internalization, secondary immunoglobulin light-chain rearrangement, and a severe developmental arrest of immature B cells, dependent on the adaptor protein Slp65. Residual B cells in the spleen showed increased expression of surface CD5, which is a negative regulator of BCR signaling, and differentiated spontaneously into IgM+ plasma cells. Thus, the silencing of SLC genes is not essential for the limitation of pre-B cell proliferation, but is required for the prevention of constitutive activation of B cells.

Bruton's tyrosine kinase and SLP-65 regulate pre-B cell differentiation and the induction of Ig light chain gene rearrangement

Bruton's tyrosine kinase (Btk) and the adapter protein SLP-65 (Src homology 2 domain-containing leukocyte-specific phosphoprotein of 65 kDa) transmit precursor BCR (pre-BCR) signals that are essential for efficient developmental progression of large cycling into small resting pre-B cells. We show that Btk- and SLP-65-deficient pre-B cells have a specific defect in Ig lambda L chain germline transcription. In Btk/SLP-65 double-deficient pre-B cells, both kappa and lambda germline transcripts are severely reduced. Although these observations point to an important role for Btk and SLP-65 in the initiation of L chain gene rearrangement, the possibility remained that these signaling molecules are only required for termination of pre-B cell proliferation or for pre-B cell survival, whereby differentiation and L chain rearrangement is subsequently initiated in a Btk/SLP-65-independent fashion. Because transgenic expression of the antiapoptotic protein Bcl-2 did not rescue the developmental arrest of Btk/SLP-65 double-deficient pre-B cells, we conclude that defective L chain opening in Btk/SLP-65-deficient small resting pre-B cells is not due to their reduced survival. Next, we analyzed transgenic mice expressing the constitutively active Btk mutant E41K. The expression of E41K-Btk in Ig H chain-negative pro-B cells induced 1) surface marker changes that signify cellular differentiation, including down-regulation of surrogate L chain and up-regulation of CD2, CD25, and MHC class II; and 2) premature rearrangement and expression of kappa and lambda light chains. These findings demonstrate that Btk and SLP-65 transmit signals that induce cellular maturation and Ig L chain rearrangement independently of their role in termination of pre-B cell expansion.

Defective microarchitecture of the spleen marginal zone and impaired response to a thymus-independent type 2 antigen in mice lacking scavenger receptors MARCO and SR-A

The macrophage scavenger receptor macrophage receptor with a collagenous structure (MARCO) is expressed in mice by the marginal zone macrophages of the spleen and by macrophages of the medullary cords of lymph nodes, as well as the peritoneal macrophages. MARCO is a relative of scavenger receptor A (SR-A), the more widely expressed prototypic member of the scavenger receptor family. In the present study, we found that genetic ablation of MARCO leads to changes in the organization of the splenic marginal zone, and causes a significant reduction in the size of the resident peritoneal macrophage population, possibly due to changes in adhesion and migration capacity. In mice lacking both MARCO and SR-A these effects are even more apparent. During ontogeny, the appearance and organization of the MARCO-expressing cells in the spleen precedes the appearance of other receptors on macrophages in the marginal zone, such as SIGNR1 and Siglec-1. In the absence of MARCO, a clear delay in the organization of the marginal zone was observed. Similar findings were seen when the reappearance of the various subsets from precursors was studied after depleting macrophages from the adult spleen by a liposome treatment. When challenged with a pneumococcal polysaccharide vaccine, a T-independent type 2 Ag for which an intact marginal zone is crucial, the knockout mice exhibited a clearly impaired response. These findings suggest that both MARCO and SR-A, in addition to being important scavenger receptors, could be involved in the positioning and differentiation of macrophages, possibly through interaction with endogenous ligands.

Involvement of SLP-65 and Btk in tumor suppression and malignant transformation of pre-B cells

Signals from the precursor-B cell receptor (pre-BCR) are essential for selection and clonal expansion of pre-B cells that have performed productive immunoglobulin heavy chain V(D)J recombination. In the mouse, the downstream signaling molecules SLP-65 and Btk cooperate to limit proliferation and induce differentiation of pre-B cells, thereby acting as tumor suppressors to prevent pre-B cell leukemia. In contrast, recent observations in human BCR-ABL1(+) pre-B lymphoblastic leukemia cells demonstrate that Btk is constitutively phosphorylated and activated by the BCR-ABL1 fusion protein. As a result, activated Btk transmits survival signals that are essential for the transforming activity of oncogenic Abl tyrosine kinase.

Functional replacement ofDrosophilaBtk29A with human Btk in male genital development and survival

Drosophila type 2 Btk29A reveals the highest homology to Btk among mammalian Tec kinases. In Btk29A(ficP) mutant males, the apodeme holding the penis split into two pieces. Human Btk rescued this phenotype in 39% of Btk29A(ficP) males, while the Drosophila transgenes did so in 90-100% of mutants. The Btk29A(ficP) mutation reduced adult longevity to 11% that of wild-type. This effect was counteracted by Drosophila type 2, yielding 76% of the wild-type lifespan. Human Btk extended the lifespan of Btk29A(ficP) mutants only to 20% that of wild-type. Thus human Btk can partially replace Drosophila Btk29A+ in male genital development and survival.

B cell signaling and tumorigenesis

The proliferation and differentiation of lymphocytes are regulated by receptors localized on the cell surface. Engagement of these receptors induces the activation of intracellular signaling proteins that transmit the receptor signals to distinct targets and control the cellular responses. The first signaling proteins to be discovered in higher organisms were the products of oncogenes. For example, the kinases Src and Abelson (Abl) were originally identified as oncogenes and were later characterized as important proteins for signal transduction in various cell types, including lymphocytes. Now, as many cellular signaling molecules have been discovered and ordered into certain pathways, we can better understand why particular signaling proteins are associated with tumorigenesis. In this review, we discuss recent progress in unraveling the molecular mechanisms of signaling pathways that control the proliferation and differentiation of early B cells. We point out the concepts of auto-inhibition and subcellular localization as crucial aspects in the regulation of B cell signaling.

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.

Tumor suppressor function of Bruton tyrosine kinase is independent of its catalytic activity

During B-cell development in the mouse, Bruton tyrosine kinase (Btk) and the adaptor protein SLP-65 (Src homology 2 [SH2] domain-containing leukocyte protein of 65 kDa) limit the expansion and promote the differentiation of pre-B cells. Btk is thought to mainly function by phosphorylating phospholipase Cgamma2, which is brought into close proximity of Btk by SLP-65. However, this model was recently challenged by the identification of a role for Btk as a tumor suppressor in the absence of SLP-65 and by the finding that Btk function is partially independent of its kinase activity. To investigate if enzymatic activity is critical for the tumor suppressor function of Btk, we crossed transgenic mice expressing the kinase-inactive K430R-Btk mutant onto a Btk/SLP-65 double-deficient background. We found that K430R-Btk expression rescued the severe developmental arrest at the pre-B-cell stage in Btk/SLP-65 double-deficient mice. Moreover, K430R-Btk could functionally replace wild-type Btk as a tumor suppressor in SLP-65- mice: at 6 months of age, the observed pre-B-cell lymphoma frequencies were approximately 15% for SLP-65- mice, 44% for Btk/SLP-65-deficient mice, and 14% for K430R-Btk transgenic mice on the Btk/SLP-65-deficient background. Therefore, we conclude that Btk exerts its tumor suppressor function in pre-B cells as an adaptor protein, independent of its catalytic activity.

Cellular maturation defects in Bruton's tyrosine kinase-deficient immature B cells are amplified by premature B cell receptor expression and reduced by receptor editing

In the mouse, Bruton's tyrosine kinase (Btk) is essential for efficient developmental progression of CD43(+)CD2(-) large cycling into CD43(-)CD2(+) small resting pre-B cells in the bone marrow and of IgM(high) transitional type 2 B cells into IgM(low) mature B cells in the spleen. In this study, we show that the impaired induction of cell surface changes in Btk-deficient pre-B cells was still noticeable in kappa(+) immature B cells, but was largely corrected in lambda(+) immature B cells. As lambda gene rearrangements are programmed to follow kappa rearrangements and lambda expression is associated with receptor editing, we hypothesized that the transit time through the pre-B cell compartment or receptor editing may affect the extent of the cellular maturation defects in Btk-deficient B cells. To address this issue, we used 3-83 mu delta transgenic mice, which prematurely express a complete B cell receptor and therefore manifest accelerated B cell development. In Btk-deficient 3-83 mu delta mice, the IgM(+) B cells in the bone marrow exhibited a very immature phenotype (pre-BCR(+)CD43(+)CD2(-)) and were arrested at the transitional type 1 B cell stage upon arrival in the spleen. However, these cellular maturation defects were largely restored when Btk-deficient 3-83 mu delta B cells were on a centrally deleting background and therefore targeted for receptor editing. Providing an extended time window for developing B cells by enforced expression of the antiapoptotic gene Bcl-2 did not alter the Btk dependence of their cellular maturation. We conclude that premature B cell receptor expression amplifies the cellular maturation defects in Btk-deficient B cells, while extensive receptor editing reduces these defects.

B Cells Are Crucial for Both Development and Maintenance of the Splenic Marginal Zone

The splenic marginal zone is a unique compartment that separates the lymphoid white pulp from the surrounding red pulp. Due to the orchestration of specialized macrophages and B cells flanking a marginal sinus, this compartment plays an important role in uptake of blood-borne Ags and it gives the spleen its specialized function in antibacterial immunity. In this study, we demonstrate that both development and maintenance of this marginal zone is highly dependent on the presence of B cells. Spleens from B cell-deficient mice were found to lack both metallophilic and marginal zone macrophages as well as mucosal addressin cellular adhesion molecule-1+ sinus lining cells. Using an inducible Cre/loxP-driven mouse model in which mature B cells could be partially depleted by removal of the B cell receptor subunit Igalpha, we could show that the integrity and function of an established marginal zone was also dependent on the presence of B cells. This was confirmed in a transgenic model in which all B cells were gradually depleted due to overexpression of the TNF family member CD70. The loss of all cellular subsets from the marginal zone in these CD70 transgenic mice was effectively prevented by crossing these mice on a CD27(-/-) or TCRalpha(-/-) background, because this prohibited the ongoing B cell depletion. Therefore, we conclude that B cells are not only important for the development, but also for maintenance, of the marginal zone. This direct correlation between circulating B cells and the function of the spleen implies an increased risk for B cell lymphopenic patients with bacterial infections.

Function of Bruton's tyrosine kinase during B cell development is partially independent of its catalytic activity

The Tec family member Bruton's tyrosine kinase (Btk) is a cytoplasmic protein tyrosine kinase that transduces signals from the pre-B and B cell receptor (BCR). Btk is involved in pre-B cell maturation by regulating IL-7 responsiveness, cell surface phenotype changes, and the activation of lambda L chain gene rearrangements. In mature B cells, Btk is essential for BCR-mediated proliferation and survival. Upon BCR stimulation, Btk is transphosphorylated at position Y551, which promotes its catalytic activity and subsequently results in autophosphorylation at position Y223 in the Src homology 3 domain. To address the significance of Y223 autophosphorylation and the requirement of enzymatic activity for Btk function in vivo, we generated transgenic mice that express the autophosphorylation site mutant Y223F and the kinase-inactive mutant K430R, respectively. We found that Y223 autophosphorylation was not required for the regulation of IL-7 responsiveness and cell surface phenotype changes in differentiating pre-B cells, or for peripheral B cell differentiation. However, expression of the Y223F-Btk transgene could not fully rescue the reduction of lambda L chain usage in Btk-deficient mice. In contrast, transgenic expression of kinase-inactive K430R-Btk completely reconstituted lambda usage in Btk-deficient mice, but the defective modulation of pre-B cell surface markers, peripheral B cell survival, and BCR-mediated NF-kappaB induction were partially corrected. From these findings, we conclude that: 1) autophosphorylation at position Y223 is not essential for Btk function in vivo, except for regulation of lambda L chain usage, and 2) during B cell development, Btk partially acts as an adapter molecule, independent of its catalytic activity.

The B cell antigen receptor controls integrin activity through Btk and PLCgamma2

Integrin-mediated adhesion and B cell antigen receptor (BCR) signaling play a critical role in B cell development and function, including antigen-specific B cell differentiation. Here we show that the BCR controls integrin α4β1 (VLA-4)-mediated adhesion of B cells to vascular cell adhesion molecule-1 and fibronectin. Molecular dissection of the underlying signaling mechanism by a combined biochemical, pharmacological, and genetic approach demonstrates that this BCR-controlled integrin-mediated adhesion requires the (consecutive) activation of Lyn, Syk, phosphatidylinositol 3-kinase, Bruton's tyrosine kinase (Btk), phospholipase C (PLC)γ2, IP3R-mediated Ca²⁺ release, and PKC. In contrast, activation of mitogen-activated protein kinase kinase (MEK) or extracellular signal–regulated kinase (ERK) is not required, and simultaneous activation of MEK, ERK, and PKB is not sufficient either. Furthermore, Btk is also involved in the control of integrin-mediated adhesion of preB cells. The control of integrin α4β1-mediated B cell adhesion by the BCR involves cytoskeletal reorganization and integrin clustering. These results reveal a novel function for the BCR and Btk, i.e., regulation of integrin α4β1 activity, thereby providing new insights into the control of B cell development and differentiation, as well as into the pathogenesis of the immunodeficiency disease X-linked agammaglobulineamia (XLA).

Protein kinase C beta controls nuclear factor kappaB activation in B cells through selective regulation of the IkappaB kinase alpha

Activation of the nuclear factor (NF)-kappaB transcription complex by signals derived from the surface expressed B cell antigen receptor controls B cell development, survival, and antigenic responses. Activation of NF-kappaB is critically dependent on serine phosphorylation of the IkappaB protein by the multi-component IkappaB kinase (IKK) containing two catalytic subunits (IKKalpha and IKKbeta) and one regulatory subunit (IKKgamma). Using mice deficient for protein kinase C beta (PKCbeta) we show an essential role of PKCbeta in the phosphorylation of IKKalpha and the subsequent activation of NF-kappaB in B cells. Defective IKKalpha phosphorylation correlates with impaired B cell antigen receptor-mediated induction of the pro-survival protein Bcl-xL. Lack of IKKalpha phosphorylation and defective NF-kappaB induction in the absence of PKCbeta explains the similarity in immunodeficiencies caused by PKCbeta or IKKalpha ablation in B cells. Furthermore, the well established functional cooperation between the protein tyrosine kinase Bruton's tyrosine kinase (Btk), which regulates the activity of NF-kappaB and PKCbeta, suggests PKCbeta as a likely serine/threonine kinase component of the Btk-dependent NF-kappaB activating signal transduction chain downstream of the BCR.

Long-lasting memory-resting and memory-effector CD4+ T cells in human X-linked agammaglobulinemia

Conflicting results obtained from animal studies suggest that B cells play a role in maintaining long-term T-cell memory and in skewing T-cell response toward a T-helper 2 (T(H)2) phenotype. X-linked agammaglobulinemia (XLA) is a genetic human disease characterized by the lack of circulating B cells due to the mutation of Bruton tyrosine kinase. This disease thus represents a unique model for studying the role of B lymphocytes in regulating T-cell functions in humans. To this aim, we analyzed hepatitis B envelope antigen (HBenvAg)-specific T-cell memory in a series of XLA patients vaccinated against hepatitis B virus (HBV). We found HBenvAg-specific T lymphocytes producing interferon-gamma, interleukin-4, or both in the peripheral blood of XLA patients up to at least 24 months after completing the standard anti-HBV immunization protocol. The HBenvAg-specific T-cell frequencies and the percentage of patients with these responses were not significantly different from healthy vaccinated controls. By combining cell purification and enzyme-linked immunospot assay, we found that effector CD27- T cells, which promptly produced cytokines in response to antigen (Ag), and memory-resting CD27+ T cells, which required Ag restimulation to perform their functions, were maintained in both XLA patients and controls for up to 24 months after the last vaccination boost. These data strongly suggest that B cells are not an absolute requirement for the generation of effective T-cell memory in humans, nor do they seem to influence T(H)1/T(H)2 balance.

Impaired precursor B cell differentiation in Bruton's tyrosine kinase-deficient mice

Bruton's tyrosine kinase (Btk) is a cytoplasmic signaling molecule that is crucial for precursor (pre-B) cell differentiation in humans. In this study, we show that during the transition of large cycling to small resting pre-B cells in the mouse, Btk-deficient cells failed to efficiently modulate the expression of CD43, surrogate L chain, CD2, and CD25. In an analysis of the kinetics of pre-B cell differentiation in vivo, Btk-deficient cells manifested a specific developmental delay within the small pre-B cell compartment of about 3 h, when compared with wild-type cells. Likewise, in in vitro bone marrow cultures, Btk-deficient large cycling pre-B cells showed increased IL-7 mediated expansion and reduced developmental progression into noncycling CD2(+)CD25(+) surrogate L chain-negative small pre-B cells and subsequently into Ig-positive B cells. Furthermore, the absence of Btk resulted in increased proliferative responses to IL-7 in recombination-activating gene-1-deficient pro-B cells. These findings identify a novel role for Btk in the regulation of the differentiation stage-specific modulation of IL-7 responsiveness in pro-B and pre-B cells. Moreover, our results show that Btk is critical for an efficient transit through the small pre-B cell compartment, thereby regulating cell surface phenotype changes during the developmental progression of cytoplasmic mu H chain expressing pre-B cells into immature IgM(+) B cells.

PKCbeta modulates antigen receptor signaling via regulation of Btk membrane localization

Mutations in Bruton's tyrosine kinase (Btk) result in X-linked agammaglobulinemia (XLA) in humans and X-linked immunodeficiency (xid) in mice. While targeted disruption of the protein kinase C-beta (PKCbeta) gene in mice results in an immunodeficiency similar to xid, the overall tyrosine phosphorylation of Btk is significantly enhanced in PKCbeta-deficient B cells. We provide direct evidence that PKCbeta acts as a feedback loop inhibitor of Btk activation. Inhibition of PKCbeta results in a dramatic increase in B-cell receptor (BCR)-mediated Ca2+ signaling. We identified a highly conserved PKCbeta serine phosphorylation site in a short linker within the Tec homology domain of Btk. Mutation of this phosphorylation site led to enhanced tyrosine phosphorylation and membrane association of Btk, and augmented BCR and FcepsilonRI-mediated signaling in B and mast cells, respectively. These findings provide a novel mechanism whereby reversible translocation of Btk/Tec kinases regulates the threshold for immunoreceptor signaling and thereby modulates lymphocyte activation.

Enforced expression of GATA-3 during T cell development inhibits maturation of CD8 single-positive cells and induces thymic lymphoma in transgenic mice

The zinc finger transcription factor GATA-3 is of critical importance for early T cell development and commitment of Th2 cells. To study the role of GATA-3 in early T cell development, we analyzed and modified GATA-3 expression in vivo. In mice carrying a targeted insertion of a lacZ reporter on one allele, we found that GATA-3 transcription in CD4(+)CD8(+) double-positive thymocytes correlated with the onset of positive selection events, i.e., TCRalphabeta up-regulation and CD69 expression. LacZ expression remained high ( approximately 80% of cells) during maturation of CD4 single-positive (SP) cells in the thymus, but in developing CD8 SP cells the fraction of lacZ-expressing cells decreased to <20%. We modified this pattern by enforced GATA-3 expression driven by the CD2 locus control region, which provides transcription of GATA-3 throughout T cell development. In two independent CD2-GATA3-transgenic lines, approximately 50% of the mice developed thymic lymphoblastoid tumors that were CD4(+)CD8(+/low) and mostly CD3(+). In tumor-free CD2-GATA3-transgenic mice, the total numbers of CD8 SP cells in the thymus were within normal ranges, but their maturation was hampered, as indicated by increased apoptosis of CD8 SP cells and a selective deficiency of mature CD69(low)HSA(low) CD8 SP cells. In the spleen and lymph nodes, the numbers of CD8(+) T cells were significantly reduced. These findings indicate that GATA-3 supports development of the CD4 lineage and inhibits maturation of CD8 SP cells in the thymus.

Enforced expression of GATA-3 in transgenic mice inhibits Th1 differentiation and induces the formation of a T1/ST2-expressing Th2-committed T cell compartment in vivo

The transcription factor GATA-3 is essential for early T cell development and differentiation of naive CD4(+) T cells into Th2 effector cells. To study the function of GATA-3 during T cell-mediated immune responses in vivo, we investigated CD2-GATA3-transgenic mice in which GATA-3 expression is driven by the CD2 locus control region. Both in the CD4(+) and the CD8(+) T cell population the proportion of cells exhibiting a CD44(high)CD45RB(low)CD62L(low) Ag-experienced phenotype was increased. In CD2-GATA3-transgenic mice, large fractions of peripheral CD4(+) T cells expressed the IL-1 receptor family member T1/ST2, indicative of advanced Th2 commitment. Upon in vitro T cell stimulation, the ability to produce IL-2 and IFN-gamma was decreased. Moreover, CD4(+) T cells manifested rapid secretion of the Th2 cytokines IL-4, IL-5, and IL-10, reminiscent of Th2 memory cells. In contrast to wild-type CD4(+) cells, which lost GATA-3 expression when cultured under Th1-polarizing conditions, CD2-GATA3-transgenic CD4(+) cells maintained expression of GATA-3 protein. Under Th1 conditions, cellular proliferation of CD2-GATA3-transgenic CD4(+) cells was severely hampered, IFN-gamma production was decreased and Th2 cytokine production was increased. Enforced GATA-3 expression inhibited Th1-mediated in vivo responses, such as Ag-specific IgG2a production or a delayed-type hypersensitivity response to keyhole limpet hemocyanin. Collectively, these observations indicate that enforced GATA-3 expression selectively inhibits Th1 differentiation and induces Th2 differentiation. The increased functional capacity to secrete Th2 cytokines, along with the increased expression of surface markers for Ag-experienced Th2-committed cells, would argue for a role of GATA-3 in Th2 memory formation.

Bruton's tyrosine kinase regulates the activation of gene rearrangements at the lambda light chain locus in precursor B cells in the mouse

Bruton's tyrosine kinase (Btk) is a nonreceptor tyrosine kinase involved in precursor B (pre-B) cell receptor signaling. Here we demonstrate that Btk-deficient mice have an ∼50% reduction in the frequency of immunoglobulin (Ig) λ light chain expression, already at the immature B cell stage in the bone marrow. Conversely, transgenic mice expressing the activated mutant Btk^E41K showed increased λ usage. As the κ/λ ratio is dependent on (a) the level and kinetics of κ and λ locus activation, (b) the life span of pre-B cells, and (c) the extent of receptor editing, we analyzed the role of Btk in these processes. Enforced expression of the Bcl-2 apoptosis inhibitor did not alter the Btk dependence of λ usage. Crossing 3-83μδ autoantibody transgenic mice into Btk-deficient mice showed that Btk is not essential for receptor editing. Also, Btk-deficient surface Ig⁺ B cells that were generated in vitro in interleukin 7-driven bone marrow cultures manifested reduced λ usage. An intrinsic defect in λ locus recombination was further supported by the finding in Btk-deficient mice of reduced λ usage in the fraction of pre-B cells that express light chains in their cytoplasm. These results implicate Btk in the regulation of the activation of the λ locus for V(D)J recombination in pre-B cells.

The Tec family of cytoplasmic tyrosine kinases: mammalian Btk, Bmx, Itk, Tec, Txk and homologs in other species

Cytoplasmic protein-tyrosine kinases (PTKs) are enzymes involved in transducing a vast number of signals in metazoans. The importance of the Tec family of kinases was immediately recognized when, in 1993, mutations in the gene encoding Bruton's tyrosine kinase (Btk) were reported to cause the human disease X-linked agammaglobulinemia (XLA). Since then, additional kinases belonging to this family have been isolated, and the availability of full genome sequences allows identification of all members in selected species enabling phylogenetic considerations. Tec kinases are endowed with Pleckstrin homology (PH) and Tec homology (TH) domains and are involved in diverse biological processes related to the control of survival and differentiation fate. Membrane translocation resulting in the activation of Tec kinases with subsequent Ca2+ release seems to be a general feature. However, nuclear translocation may also be of importance. The purpose of this essay is to characterize members of the Tec family and discuss their involvement in signaling. The three-dimensional structure, expression pattern and evolutionary aspects will also be considered.

Severe B cell deficiency in mice lacking the tec kinase family members Tec and Btk

The cytoplasmic protein tyrosine kinase Tec has been proposed to have important functions in hematopoiesis and lymphocyte signal transduction. Here we show that Tec-deficient mice developed normally and had no major phenotypic alterations of the immune system. To reveal potential compensatory roles of other Tec kinases such as Bruton's tyrosine kinase (Btk), Tec/Btk double-deficient mice were generated. These mice exhibited a block at the B220(+)CD43(+) stage of B cell development and displayed a severe reduction of peripheral B cell numbers, particularly immunoglobulin (Ig)M(lo)IgD(hi) B cells. Although Tec/Btk(null) mice were able to form germinal centers, the response to T cell-dependent antigens was impaired. Thus, Tec and Btk together have an important role both during B cell development and in the generation and/or function of the peripheral B cell pool. The ability of Tec to compensate for Btk may also explain phenotypic differences in X-linked immunodeficiency (xid) mice compared with human X-linked agammaglobulinemia (XLA) patients.

A VH11V kappa 9 B cell antigen receptor drives generation of CD5+ B cells both in vivo and in vitro

B lymphocytes can be divided into different subpopulations, some with distinctive activation requirements and probably mediating specialized functions, based on surface phenotype and/or anatomical location, but the origins of most of these populations remain poorly understood. B cells constrained by transgenesis to produce an Ag receptor derived from a conventional (B-2) type cell develop a B-2 phenotype, whereas cells from mice carrying a B-1-derived receptor acquire the B-1 phenotype. In this study transgenic enforced expression of a B cell receptor (mu/kappa) originally isolated from a CD5+ (B-1a) B cell generates B-1 phenotype cells in bone marrow cultures that show a distinctive B-1 function, survival in culture. Despite their autoreactivity, we find no evidence for receptor editing or that the paucity of B-2 cells is the result of tolerance-induced selection. Finally, Ca2+ mobilization studies reveal a difference between transgenic B-1 cells in spleen and peritoneal cavity, with cells in spleen much more responsive to anti-B cell receptor cross-linking. We discuss these results in terms of specificity vs lineage models for generation of distinctive B cell subpopulations.

Posttranscriptional regulation of Bruton's tyrosine kinase expression in antigen receptor-stimulated splenic B cells

Mutation of Bruton's tyrosine kinase (Btk) causes human X-linked agammaglobulinemia and murine X-linked immunodeficiency syndrome (xid). Quantitative aspects of B lymphocyte development and function have been demonstrated to depend on Btk level in vivo by using a murine transgenic model system. A sensitive intracellular immunofluorescent assay was developed to measure Btk protein on a per cell basis to test the hypothesis that its dosage is dynamically regulated during B cell development or functional responses. Marrow-derived hematopoietic stem cells, common lymphoid progenitor cells, and developing B and myeloid lineages expressed Btk protein at comparable levels. Resting peripheral B lineage cells had a significantly lower amount of Btk than marrow-derived cells in both wild-type and xid mice. Activation of the B cell antigen receptor up-regulated Btk protein level 10-fold within several hours by a phosphatidylinositol 3-kinase-dependent, posttranscriptional mechanism. In contrast, the protein level of Btk R28C in activated B lymphocytes from xid mice remained low. Bypass of the antigen receptor signaling pathways by treatment of cells with phorbol myristic acid and ionomycin rescued up-regulation of Btk protein in xid splenic B cells. These combined results suggest that certain receptor signals mediated by Btk regulate the level of expression of Btk protein in responding B lymphocytes to potentiate signal transduction. Dynamic regulation of Btk protein dosage is an additional mechanism to modulate B lymphocyte immune functions.

A Btk transgene restores the antiviral TI-2 antibody responses of xid mice in a dose-dependent fashion

X-linked agammaglobulinemia in humans and X-linked immunodeficiency (xid) in mice are both caused by mutations in Bruton's tyrosine kinase (Btk). Xid mice lack the early T cell-independent type 2 (TI-2) antibody response to polio virus and to a recombinant vaccinia virus (Vacc-IND-G) expressing the neutralizing determinant of vesicular stomatitis virus (VSV). This response could be restored by introduction of one or two copies of a murine Btk cDNA transgene driven by the Ig heavy chain promoter plus enhancer and depended crucially on a sufficient Btk expression level. Introduction of the same transgene into wild-type mice had little to no negative effect. The TI-1 antibody response to VSV and the T cell-dependent response to lymphocytic choriomeningitis virus were comparable in all mice tested. All mice analyzed eventually reached similar primary and memory antibody titers against all viruses independent of the mouse Btk genotype. These studies show that the xid mutation in mice has no dominant negative effect and that a transgene - even when not provided in the natural genetic context - may be able to restore functional defects resulting from genetic mutation.

Early Arrest in B Cell Development in Transgenic Mice That Express the E41K Bruton’s Tyrosine Kinase Mutant Under the Control of the CD19 Promoter Region

Bruton’s tyrosine kinase (Btk) is a nonreceptor protein kinase that is defective in X-linked agammaglobulinemia in humans and in X-linked immunodeficiency in mice. To study the effect of Btk activation in early B cell development in vivo, we have created transgenic mouse strains expressing Btk under the control of the human CD19 promoter region. The transgenic expression of wild-type human Btk corrected all X-linked immunodeficiency features in mice carrying a targeted disruption of the Btk gene. In contrast, expression of an activated form of Btk, the E41K mutant, resulted in an almost complete arrest of B cell development in the immature IgM+IgD− B cell stage in the bone marrow, irrespective of the presence of the endogenous intact Btk gene. Immature B cells were arrested at the progression from IgMlow into IgMhigh cells, which reflects the first immune tolerance checkpoint at which autoreactive B cells become susceptible to apoptosis. As the constitutive activation of Btk is likely to mimic B cell receptor occupancy by autoantigens in the bone marrow, our findings are consistent with a role for Btk as a mediator of B cell receptor-induced apoptotic signals in the immature B cell stage. Whereas the peripheral mature B cell pool was reduced to &lt;1% of the normal size, significant numbers of IgM-secreting plasma cells were present in the spleen. Serum IgM levels were substantial and increased with age, but specific Ab responses in vivo were lacking. We conclude that the residual peripheral B cells were efficiently driven into IgM+ plasma cell differentiation, apparently without functional selection.

B cell development: signal transduction by antigen receptors and their surrogates

Constitutive signal transduction by B cell antigen-receptors and/or their surrogates appears to be critical for progression through multiple developmental checkpoints and for survival of mature B cells in the periphery. Antigen-induced signaling via the B cell receptor can compensate for defects in constitutive signaling and initiates receptor editing, apoptosis and anergy in normal mice - purging the repertoire of autoreactive cells. Thus development and survival of mature B cells seem to require continuous receptor signaling of a defined amplitude.

Murine MHC class II locus control region drives expression of human beta-glucocerebrosidase in antigen presenting cells of transgenic mice

Gaucher disease is the most prevalent lysosomal storage disorder in humans, resulting from an inherited deficiency of the enzyme glucocerebrosidase. Although the enzyme is ubiquitously expressed, cells of the reticuloendothelial system are particularly affected since they accumulate the undigested glucosylceramide substrate through their role in scavenging and breaking down cell debris. Gaucher disease is an attractive target for somatic gene therapy. To test the ability to express the enzyme in the affected cell types we have generated transgenic mice expressing human glucocerebrosidase under the control of the murine major histocompatibility complex (MHC) class II Ead locus control region (LCR). The four transgenic lines express the human enzyme in a copy number-dependent manner, independent of the integration site of the transgene. Over-expression of the human enzyme in mice did not result in any abnormal phenotype or pathology during the period of observation (> 2 years). The enzyme is expressed in B cells, monocytes, dendritic cells, thymic epithelial cells, and macrophages in various tissues: the peritoneal cavity, bone marrow, spleen, kidney, gastrointestinal tract, Kupffer cells in the liver and alveolar macrophages in lungs. Expression in the brain was limited to perivascular macrophages and was not seen in microglial cells. Therefore, the MHC class II LCR could potentially be of use in somatic gene therapy for type 1 Gaucher disease.

In situ detection of activated Bruton's tyrosine kinase in the Ig signaling complex by phosphopeptide-specific monoclonal antibodies

Bruton's tyrosine kinase (Btk) is a critical transducer of signals originating from the B cell antigen receptor (BCR). Dosage, sequential phosphorylation, and protein interactions are interdependent mechanisms influencing Btk function. Phosphopeptide-specific mAbs recognizing two distinct phosphotyrosine modifications were used to quantify Btk activation by immunofluorescent techniques during B cell stimulation. In a population of cultured B cells stimulated by BCR crosslinking and analyzed by flow cytometry, transient phosphorylation of the regulatory Btk tyrosine residues (551Y and 223Y) was detected. The kinetics of phosphorylation of the residues were temporally distinct. Tyrosine 551, a transactivating substrate site for Src-family kinases, was maximally phosphorylated within approximately 30 seconds of stimulation as monitored by flow cytometry. Tyrosine 223, an autophosphorylation site within the SH3 domain, was maximally phosphorylated at approximately 5 minutes. Btk returned to a low tyrosine phosphorylation level within 30 minutes, despite persistent elevation of global tyrosine phosphorylation. Colocalization of activated Btk molecules with the crosslinked BCR signaling complex was observed to coincide with the period of maximal Btk tyrosine phosphorylation when stimulated B cells were analyzed with confocal microscopy. The results of these in situ temporal and spatial analyses imply that Btk signaling occurs in the region of the Ig receptor signaling complex, suggesting a similar location for downstream targets of its activity.