Impaired expansion of mouse B cell progenitors lacking Btk

Ca2+-NFATc1 signaling is an essential axis of osteoclast differentiation

Osteoclasts are unique, multinucleated giant cells that decalcify and degrade the bone matrix. They originate from hematopoietic cells and their differentiation is dependent on a tumor necrosis factor (TNF) family cytokine, receptor activator of nuclear factor-kappaB (NF-kappaB) ligand (RANKL), as well as macrophage-colony stimulating factor (M-CSF). Recent studies have unveiled the precise molecular mechanism underlying osteoclastogenesis. In particular, the discovery of nuclear factor of activated T cells c1 (NFATc1), the master regulator of osteoclastogenesis, has proven to be a breakthrough in this field. NFATc1 is activated by Ca2+ signaling induced by the activation of the immunoglobulin-like receptor signaling associated with immunoreceptor tyrosine-based activation motif (ITAM)-harboring adapters. The long-lasting Ca2+ oscillation, which is evident during osteoclastogenesis, may ensure the robust induction of NFATc1 through an autoamplification mechanism. Thus, intracellular Ca2+ is a critical attribute of osteoclastogenic signaling. In addition, osteoclasts are exposed to a very high extracellular Ca2+ concentration ([Ca2+]o) in the bone microenvironment and respond to the change in [Ca2+]o by increasing the intracellular Ca2+, which regulates diverse cellular functions. Investigation of the molecular mechanisms underlying the regulation of intracellular Ca2+ dynamics may open up new directions for therapeutic strategies in bone disease.

Tec protein tyrosine kinase inhibits CD25 expression in human T-lymphocyte

The Tec protein tyrosine kinase (PTK) belongs to a group of structurally related nonreceptor PTKs that also includes Btk, Itk, Rlk, and Bmx. Previous studies have suggested that these kinases play important roles in hematopoiesis and in the lymphocyte signaling pathway. Despite evidence suggesting the involvement of Tec in the T-lymphocyte activation pathway via T-cell receptor (TCR) and CD28, Tec's role in T-lymphocytes remains unclear because of the lack of apparent defects in T-lymphocyte function in Tec-deficient mice. In this study, we investigated the role of Tec in human T-lymphocyte using the Jurkat T-lymphoid cell line stably transfected with a cDNA encoding Tec. We found that the expression of wild-type Tec inhibited the expression of CD25 induced by TCR cross-linking. Second, we observed that LFM-A13, a selective inhibitor of Tec family PTK, rescued the suppression of TCR-induced CD25 expression observed in wild-type Tec-expressing Jurkat cells. In addition, expression of kinase-deleted Tec did not alter the expression level of CD25 after TCR ligation. We conclude that Tec PTK mediates signals that negatively regulate CD25 expression induced by TCR cross-linking. This, in turn, implies that this PTK plays a role in the attenuation of IL-2 activity in human T-lymphocytes.

B-1 cells modulate oral tolerance in mice

Although the origin and functions of B-1 cells are controversial, they are considered as a cellular element of innate immunity due to their ability to produce natural autoantibodies of the IgM type. These antibodies are encoded by a relatively limited repertoire of V genes, and their resulting diversity is smaller than that produced by conventional B cells. B-1 cells constitute the larger fraction of B cells in the peritoneal cavity and migrate to non-specific inflammation sites. In addition, they contribute to the production of IgA antibodies in the intestinal lamina propria. It has been demonstrated that they participate in the induction and maintenance of peripheral tolerance. Herein, the participation of B-1 cells in inducing oral tolerance is evaluated. Unexpectedly, BALB/Xid mice, the animals deficient in B-1 cells, are not tolerized to OVA but instead are responsive to oral immunization. Conversely, BALB/c mice respond to oral tolerance to this antigen. We used these biological characteristics of these animals to investigate whether B-1 cells are involved in the induction of oral tolerance to OVA. Results show that B-1 cells from BALB/c mice, treated orally with OVA and adoptively transferred to BALB/Xid mice were able to suppress local hypersensitivity reaction and lymphoproliferative cellular response observed in BALB/Xid mice. These data demonstrate that B-1 cells have regulatory properties and are involved in the induction of oral tolerance.

Tyrosine kinases Btk and Tec regulate osteoclast differentiation by linking RANK and ITAM signals

Certain autoimmune diseases result in abnormal bone homeostasis, but association of immunodeficiency with bone is poorly understood. Osteoclasts, which derive from bone marrow cells, are under the control of the immune system. Differentiation of osteoclasts is mainly regulated by signaling pathways activated by RANK and immune receptors linked to ITAM-harboring adaptors. However, it is unclear how the two signals merge to cooperate in osteoclast differentiation. Here we report that mice lacking the tyrosine kinases Btk and Tec show severe osteopetrosis caused by a defect in bone resorption. RANK and ITAM signaling results in formation of a Btk(Tec)/BLNK(SLP-76)-containing complex and PLCgamma-mediated activation of an essential calcium signal. Furthermore, Tec kinase inhibition reduces osteoclastic bone resorption in models of osteoporosis and inflammation-induced bone destruction. Thus, this study reveals the importance of the osteoclastogenic signaling complex composed of tyrosine kinases, which may provide the molecular basis for a new therapeutic strategy.

Calcium signalling and cell-fate choice in B cells

Alterations in the cytosolic concentration of calcium ions (Ca2+) transmit information that is crucial for the development and function of B cells. Cytosolic Ca2+ concentration is determined by a balance of active transport and gradient-driven Ca2+ fluxes, both of which are subject to the influence of multiple receptors and environmental sensing pathways. Recent advances in genomics have allowed for the compilation of an increasingly comprehensive list of Ca2+ transporters and channels expressed by B cells. The increasing understanding of the function and regulation of these proteins has begun to shift the frontier of Ca2+ physiology in B cells from molecular analysis to determining how diverse inputs to cytosolic Ca2+ concentration are integrated in specific immunological contexts.

Tec kinases, actin, and cell adhesion

The Tec family non-receptor tyrosine kinases have been recognized for their roles in the regulation of phospholipase C-gamma and Ca(2+) mobilization downstream from antigen receptors on lymphocytes. Recent data, however, show that the Tec family kinase interleukin-2-inducible T-cell kinase (Itk) also participates in pathways regulating the actin cytoskeleton and 'inside-out' signaling to integrins downstream from the T-cell antigen receptor. Data suggest that Itk may function in a kinase-independent fashion to regulate proper recruitment of the Vav1 guanine nucleotide exchange factor. By enhancing actin cytoskeleton reorganization, recruitment of signaling molecules to the immune synapse, and integrin clustering in response to both antigen and chemokine receptors, the Tec kinases serve as modulators or amplifiers that can increase the duration of T-cell signaling and regulate T-cell functional responses.

Btk and phospholipase C gamma 2 can function independently during B cell development

The pre-BCR and the BCR regulate B cell development via a signalosome nucleated by the adaptor protein B cell linker protein (BLNK). Formation of this complex facilitates activation of phospholipase C (PLC) gamma2 by Bruton's tyrosine kinase (Btk). To determine whether Btk and PLCgamma2 also have separate functions, we generated Btk(-/-)PLCgamma2(-/-) mice. They demonstrated a block in development at the pre-B stage and increased pre-BCR surface expression. This phenotype was more severe than that of Btk(-/-) or PLCgamma2(-/-) mice. Although both Btk and PLCgamma2 were required for proliferation of splenic B cells in response to BCR cross-linking, they contributed differently to anti-IgM-induced phosphorylation of ERK. Btk(-/-) and PLCgamma2(-/-) mice each had a reduced frequency of Iglambda-expressing B cells and impaired migration of pre-B cells towards stromal cell-derived factor 1. However, the increase in pre-B cell malignancy that occurs in BLNK(-/-) mice in the absence of Btk was not observed in the absence of PLCgamma2. Thus, Btk and PLCgamma2 act both in concert and independently throughout B cell development.

Combined deficiencies in Bruton tyrosine kinase and phospholipase Cgamma2 arrest B-cell development at a pre-BCR+ stage

Bruton tyrosine kinase (Btk) and phospholipase Cgamma2 (PLCgamma2) are 2 key molecules involved in B-cell receptor (BCR) signaling. Biochemical studies have placed them in a linear signaling pathway, with Btk acting upstream of PLCgamma2. Consistent with this, mice lacking either molecule display a leaky but similar block in B-cell development. Here, we generated Btk(-/-) PLCgamma2(-/-) mice and showed that combined deficiencies in Btk and PLCgamma2 severely arrested B lymphopoiesis at the large pre-B-cell stage. In contrast to either single mutant, Btk(-/-) PLCgamma2(-/-) pre-B cells expressed high levels of pre-BCR on their cell surfaces and exhibited reduced immunoglobulin light chain gene rearrangements. Pre-BCR-induced calcium signaling was also drastically compromised in Btk(-/-) PLCgamma2(-/-) pre-B cells compared with wild-type and single-mutant cells. Interestingly, immunoglobulin heavy chain allelic exclusion remained intact in the absence of Btk and PLCgamma2. Overall, our results suggest that Btk and PLCgamma2 have combinatorial roles in regulating pre-B cell differentiation.

B-1 B cell development

CD5+ B cells have attracted considerable interest because of their association with self-reactivity, autoimmunity, and leukemia. In mice, CD5+ B cells are readily generated from fetal/neonatal precursors, but inefficiently from precursors in adult. One model proposed to explain this difference is that their production occurs through a distinctive developmental process, termed B-1, that enriches pre-B cells with novel germline VDJs and that requires positive selection of newly formed B cells by self-Ag. In contrast, follicular B cells are generated throughout adult life in a developmental process termed B-2, selecting VDJs that pair well with surrogate L chain, and whose maturation appears relatively independent of antigenic selection. In the present study, I focus on processes that shape the repertoire of mouse CD5+ B cells, describing the differences between B-1 and B-2 development, and propose a model encompassing both in the generation of functional B cell subpopulations.

Identification of variations in the human phosphoinositide 3-kinase p110? gene in children with primary B-cell immunodeficiency of unknown aetiology

Our recent study demonstrated that defects in p110delta result in B-cell immunodeficiency that is very similar to that observed in BTK-deficient mice. We revealed that the p110delta fit the B-cell signal transduction complex and played a non-redundant role in the development and function of B cells. In humans, most children with primary B-cell immunodeficiency have mutations in the BTK, whereas a few have defects in the components of the B-cell signal transduction complex. But little is known about the genetic variation of p110delta in children with defects in B-cell immunodeficiency of unknown aetiology. Sixteen patients from 15 unrelated families and 112 normal controls underwent sequence analysis to identify genetic variations of the p110delta. Allele frequency in each group was also analysed and compared. We identified five single base-pair polymorphic nucleotide exchanges in both patient and control groups with similar allele frequencies, which did not contribute to the immunodeficiency. Three of them are novel (m.953A>G, m.1200C>T and m.1561A>G), and the m.953A>G and m.1561A>G nucleotide exchanges are non-synonymous (N253S and T456A, respectively). The novel m.1561A>G was in complete linkage disequilibrium with the known m.873A>G in our study of Taiwanese group. In addition, one novel single base-pair missense mutation, m.3256G>A (E1021K), was identified in one boy with typical clinical features of primary B-cell immunodeficiency and could not be found in either his family or the normal control population. By atomic structural analysis of the amino acid as well as the alignment comparison between species, it resulted in the replacement of the negative-charged amino acid E with the positive-charged amino acid K at codon 1021, located in the highly conservative and important catalytic functional domain. Our findings could shed light on further understanding the polymorphisms of p110delta in B-cell immunodeficiency and different populations. Moreover, the 3256G>A missense mutation raised the attention and warranted further extensive analysis to elucidate the role of p110delta in human immunodeficiency.

The human and mouse complement of SH2 domain proteins-establishing the boundaries of phosphotyrosine signaling

SH2 domains are interaction modules uniquely dedicated to the recognition of phosphotyrosine sites and are embedded in proteins that couple protein-tyrosine kinases to intracellular signaling pathways. Here, we report a comprehensive bioinformatics, structural, and functional view of the human and mouse complement of SH2 domain proteins. This information delimits the set of SH2-containing effectors available for PTK signaling and will facilitate the systems-level analysis of pTyr-dependent protein-protein interactions and PTK-mediated signal transduction. The domain-based architecture of SH2-containing proteins is of more general relevance for understanding the large family of protein interaction domains and the modular organization of the majority of human proteins.

Distinct gene expression signature in Btk-defective T1 B-cells

Bruton's tyrosine kinase (Btk) is a cytoplasmic tyrosine kinase important for B-lymphocyte maturation. Mutations in Btk give rise to the primary immunodeficiency disease X-linked agammaglobulinemia (XLA) in man and X-linked immunodeficiency (Xid) in mice. Recent studies have subdivided the mouse immature, or transitional, B-cells into two distinct subsets according to their respective surface markers. Transitional type 1 (T1) and transitional type 2 (T2) cells are also located in distinct anatomic locations. Based on a limited number of markers it has previously been reported that the earliest phenotypic sign of Btk deficiency is manifested at the T2 stage in mice. Here, we report on distinct genome-wide transcriptomic signature differences found in T1 B-lymphocytes from Btk-defective compared to normal mice and demonstrate that Btk deficiency is visible already at this stage.

Bruton tyrosine kinase is essential for botrocetin/VWF-induced signaling and GPIb-dependent thrombus formation in vivo

Botrocetin (bt)-facilitated binding of von Willebrand factor (VWF) to the platelet membrane glycoprotein (GP) Ib-IX-V complex on platelets in suspension initiates a signaling cascade that causes alphaIIbbeta3 activation and platelet aggregation. Previous work has demonstrated that bt/VWF-mediated agglutination activates alphaIIbbeta3 and elicits ATP secretion in a thromboxane A2 (TxA2)-dependent manner. The signaling that results in TxA2 production was shown to be initiated by Lyn, enhanced by Src, and propagated through Syk, SLP-76, PI3K, PLCgamma2, and PKC. Here, we demonstrate that the signaling elicited by GPIb-mediated agglutination that results in TxA2 production is dependent on Bruton tyrosine kinase (Btk). The results demonstrate that Btk is downstream of Lyn, Syk, SLP-76, and PI3K; upstream of ERK1/2, PLCgamma2, and PKC; and greatly enhances Akt phosphorylation. The relationship(s), if any, between ERK1/2, PLCgamma2, and PKC were not elucidated. The requirement for Btk and TxA2 receptor function in GPIb-dependent arterial thrombosis was confirmed in vivo by characterizing blood flow in ferric chloride-treated mouse carotid arteries. These results demonstrate that the Btk family kinase, Tec, cannot provide the function(s) missing because of the absence of Btk and that Btk is essential for both bt/VWF-mediated agglutination-induced TxA2 production and GPIb-dependent stable arterial thrombus formation in vivo.

Phospholipase Cγ2 Dosage Is Critical for B Cell Development in the Absence of Adaptor Protein BLNK

B cell linker (BLNK) protein and phospholipase Cgamma2 (PLCgamma2) are components of the BCR signalosome that activate calcium signaling in B cells. Mice lacking either molecule have a severe but incomplete block in B lymphopoiesis. In this study, we generated BLNK-/- PLCgamma2-/- mice to examine the effect of simultaneous disruption of both molecules on B cell development. We showed that BLNK-/- PLCgamma2-/- mice had compounded defects in B cell maturation compared with either single mutant, suggesting that these two molecules cooperatively or synergistically signaled B lymphopoiesis. However, Ig H chain allelic exclusion was maintained in single and double mutants, indicating that signals propagated by BLNK and PLCgamma2 were not involved in this process. Interestingly, in the absence of BLNK, B cell development was dependent on plcgamma2 gene dosage. This was evidenced by the proportionate decrease in splenic B cell population and increase in bone marrow surface pre-BCR+ cells in PLCgamma2-diploid, -haploid, and -null animals. Intracellular calcium signaling and ERK activation in response to BCR engagement were also proportionately decreased and delayed, respectively, with stepwise reduction of plcgamma2 dosage in a BLNK(null) background. Thus, these data indicate the importance of BLNK not only as a conduit to specifically channel BCR-signaling pathways and as a scaffold for the assembling of macromolecular complex, but also as an efficient aggregator or concentrator of PLCgamma2 molecules to effect optimal signaling for B cell generation and activation.

Btk expression is controlled by Oct and BOB.1/OBF.1

BOB.1/OBF.1 is a lymphocyte-restricted transcriptional coactivator. It binds together with the Oct1 and Oct2 transcription factors to DNA and enhances their transactivation potential. Mice deficient for the transcriptional coactivator BOB.1/OBF.1 show several defects in differentiation, function and signaling of B cells. In search of BOB.1/OBF.1 regulated genes we identified Btk—a cytoplasmic tyrosine kinase—as a direct target of BOB.1/OBF.1. Analyses of the human as well as murine Btk promoters revealed a non-consensus octamer site close to the start site of transcription. Here we show that Oct proteins together with BOB.1/OBF.1 are able to form ternary complexes on these sites in vitro and in vivo. This in turn leads to the induction of Btk promoter activity in synergism with the transcription factor PU.1. Btk, like BOB.1/OBF.1, plays a critical role in B cell development and B cell receptor signalling. Therefore the down-regulation of Btk expression in BOB.1/OBF.1-deficient B cells could be related to the functional and developmental defects observed in BOB.1/OBF.1-deficient mice.

Initiation of pre-B cell receptor signaling: Common and distinctive features in human and mouse

B cell development in the bone marrow is a highly regulated process and expression of a functional pre-BCR represents a crucial checkpoint, common to human and mouse. In this review, we discuss pre-BCR analogies and differences between the two species leading to pre-B cell differentiation and proliferation. In addition, the mechanisms triggering pre-BCR activation are reviewed, taking into account the recent report of heparan sulfates and galectin 1 as stromal cell-derived pre-BCR ligands. Finally, ligand-induced pre-BCR activation models are proposed on the bases of the differences reported for pre-BCR and IL7 dependencies in the two species.

Human single-domain neutralizing intrabodies directed against Etk kinase: a novel approach to impair cellular transformation

Etk, the 70-kDa member of the Tec family of nonreceptor protein tyrosine kinases, is expressed in a variety of hematopoietic, epithelial, and endothelial cells and was shown to be involved in several cellular processes, including proliferation, differentiation, and motility. In this study, we describe a novel approach using a human single-domain antibody phage display library for the generation of intrabodies directed against Etk. These single-domain antibodies bind specifically to recombinant Etk and efficiently block its kinase activity. When expressed in transformed cells, these antibodies associated tightly with Etk, leading to significant blockade of Etk enzymatic activity and inhibition of clonogenic cell growth in soft agar. Our results indicate that Etk may play a role in Src-induced cellular transformation and thus may represent a good target for cancer intervention. Furthermore, our single-domain antibody-based intrabody system proves to be an excellent tool for future intracellular targeting of other signaling molecules.

Deficiency of regulatory B cells increases allergic airway inflammation

OBJECTIVE

To investigate the effect of the X-linked immunodeficiency (Xid) B cell defect on the response to the cockroach allergen in mice.

METHODS

Two cockroach allergen immunization and challenge protocols were employed to sensitize CBA/J wild-type and CBA/CaHN-btk(-/-)xid/J (Xid) mice. Blood and tissue samples were collected 24 and 48 hrs after the last intratracheal antigen challenge and were analyzed for several parameters of allergic inflammation.

RESULTS

Nearly equivalent amounts of serum IgE were detected in Xid and CBA/J mice after short-term antigen challenge despite the B cell deficiency in Xid mice. A decreased concentration of IgE was detected in CBA/J mice after repeated allergen challenges but not in the Xid mice. Correlating with the discrepancy in serum IgE levels, higher levels of IL-13, IL-5, IL-10 and CCL5 were measured in whole lung homogenates from allergen-challenged Xid mice compared to CBA/J mice. In addition, draining lymph node cells from Xid mice expressed elevated levels of IL-4, IL-5, IL-10 and IFNgamma mRNA compared to cells from CBA/J mice after in vitro culture with cockroach antigen. An increase in lung inflammation, interstitial eosinophilia and mucus production was also observed in allergen-challenged Xid mice. CD95L expression increased on B-1a cells following allergen challenge, which was accompanied by an increase in lung CD4(+) Th cell apoptosis in wild-type CBA/J mice. In contrast, Xid mice did not have an increase in CD4(+) T cell apoptosis following allergen challenge.

CONCLUSIONS

These data suggest a regulatory role for B-1a cells in reducing cytokine production, pulmonary inflammation, and CD4(+) T cell survival during cockroach allergen-induced airway inflammation.

Mutational analysis of the SH2-kinase linker region of Bruton's tyrosine kinase defines alternative modes of regulation for cytoplasmic tyrosine kinase families

Bruton's tyrosine kinase (Btk) plays critical roles in B cell development and activation. Mutations of Btk cause X-linked agammaglobulinemia (XLA) in humans and X-linked immunodeficiency in mice. An Src homology domain 2-kinase linker region exists in all Src, Abl, ZAP70/Syk and Btk/Tec non-receptor tyrosine kinase families. Missense mutations in the Btk linker region can cause XLA, supporting an essential role for this protein segment. We investigated the regulatory role of the linker region in Btk function by mutational analysis. XLA-causing mutations L369F and R372G abolished Btk-mediated calcium response without affecting Btk protein stability and kinase activity significantly. Although mutation of a well-conserved tryptophan (W260A) in the linker region of the Src family kinase Hck has been shown to cause a hyperactive kinase, an analogous mutation in Btk (W395A) dramatically decreased Btk kinase activity. Tyrosine phosphorylation in the linker region was previously shown to regulate the function of Abl and ZAP70/Syk kinases. Even though tyrosine phosphorylation was detected on tyrosine 375 in the Btk linker region, no significant alteration was observed in Btk-signaling activity and biological function when this tyrosine was mutated in DT-40 cells or in Y375F knock-in mice. Our data and previous studies suggest that each cytoplasmic tyrosine kinase family has evolved a unique strategy to utilize the linker region to regulate the function of the enzyme.

Genes, pathways and checkpoints in lymphocyte development and homeostasis

A plethora of genes involved in murine B and T cell development have been identified, and developmental pathways within the primary lymphoid tissues have been well delineated. The generation of a functional, but non-self reacting lymphocyte repertoire results from the completion of several checkpoints during lymphocyte development and competition for survival factors in the periphery. Improved knowledge of these developmental checkpoints and homeostatic mechanisms is critical for understanding human immunodeficiency, leukaemia/lymphoma and autoimmunity, which are conditions where checkpoints and homeostasis are likely to be deregulated.

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.

B cells and aging: gauging the interplay of generative, selective, and homeostatic events

Lymphocyte homeostasis encompasses a continuum of processes that together determine the production, turnover, composition, and representation of lymphocyte pools. These processes include commitment to lymphoid lineages, expansion of progenitor pools, successful transit through intermediate maturation stages, negative and positive selection based on receptor specificity, steady-state maintenance of peripheral lymphocytes, and regulation of antigen-driven activation. Understanding the impact of aging on lymphocyte homeostasis thus requires appreciation of not only the mechanisms responsible for generating and sustaining antigen-reactive B and T cells but also how age-related events can subvert these. Even under the influence of normally operating homeostatic mechanisms, lesions yielding perturbations outside of evolutionarily anticipated boundaries will yield aberrant lymphoid function and representation both upstream and downstream of the primary defect. Accordingly, determining the relative contribution of lineage-intrinsic versus compensatory homoeostatic processes throughout the continuum of lymphoid system development, selection, and maintenance are critical first steps towards understanding age-associated alterations in the immune system.

The SLP-76 family of adapter proteins

Adapter molecules are multidomain proteins lacking intrinsic catalytic activity, functioning instead by nucleating molecular complexes during signal transduction. The SLP-76 family of adapters includes SH2 domain-containing leukocyte phosphoprotein of 76kDa (SLP-76), B cell linker protein (BLNK), and cytokine-dependent hematopoietic cell linker (Clnk). These proteins are critical for integration of numerous signaling cascades downstream of immunotyrosine-based activation motif (ITAM)-bearing receptors and integrins in diverse hematopoietic cell types. Mutations in genes encoding SLP-76 family adapters result in severe phenotypes, underscoring the critical role these proteins play in cellular development and function by directing formation of signaling complexes in a temporally- and spatially-specific manner.

MARGINAL ZONE B CELLS

Our views regarding the origins and functions of splenic marginal zone B cells have changed considerably over the past few years. Perspectives regarding the development and function of these cells vary considerably between investigators studying human and rodent immunology. Marginal zone B cells are now recognized to constitute a distinct naive B lymphoid lineage. Considerable progress has been made regarding the mechanisms involved in marginal zone B cell development in the mouse. Many of the molecular events that participate in the retention of this lineage of B cells in the marginal zone have been identified. Here, we discuss the functions of these cells in both innate and adaptive immunity. We also attempt to reconcile differing viewpoints regarding the generation and function of marginal zone B cells in rodents and primates.

Positive selection and lineage commitment during peripheral B-lymphocyte development

Although it is appreciated that the antigen receptor on B cells is required for peripheral B-lymphocyte development and survival, it has been unclear whether this receptor interacts with self-antigens during development or if it signals constitutively in an antigen-independent fashion. The analysis of mutant mice in which antigen receptor signaling in B cells is either attenuated or enhanced has revealed the existence of a follicular versus marginal zone B-lymphocyte cell-fate decision. These analyses indicate that weak antigen receptor-derived signals favor marginal zone B-cell generation, and relatively strong signals favor the development of mature follicular B cells. Even stronger signals derived from the antigen receptor favor the generation of B1 B cells. This signal strength model for B-cell development supports the notion that self-antigens of varying affinity may mediate positive selection and lineage commitment. Direct evidence supporting such a view has been obtained from the analysis of antigen receptor knockin mice. Specific antigen receptors guide B cells to develop into specific lineages. Although Notch-2, nuclear factor-kappaBp50, and other genes are essential for marginal zone B-cell development, instructive signals delivered by the antigen receptor represent the primary force driving positive selection and lineage commitment in B lymphocytes.

A phosphorylation site in Bruton's tyrosine kinase selectively regulates B cell calcium signaling efficiency by altering phospholipase C-gamma activation

Loss of function of Bruton's tyrosine kinase (Btk) causes X-linked agammaglobulinemia (XLA) in humans and X-linked immunodeficiency in mice (xid). By using MS analysis and phosphopeptide-specific antibodies, we identified a tyrosine phosphorylation site (Y617) near the carboxyl terminus of the Btk domain from Btk expressed in 293T as well as DT-40 cells. Y617 is conserved in all Tec family kinases except murine Tec. Replacement of Y617 with a negatively charged glutamic acid (E) suppressed Btk-mediated phospholipase Cgamma2 activation and calcium response in DT-40 cells, whereas Akt activation was not affected. The Btk Y617E mutant could partially restore conventional B cell development and proliferation in Btk(-)/Tec(-) mice but failed to rescue CD5(+) B-1 cell development and the TI-II immune response to 2,4,6,-trinitrophenyl-Ficoll. These data suggest that Y617 phosphorylation or a negative charge at this site may down-regulate the function of Btk by selectively suppressing the B cell calcium signaling pathway.

The role of IL-5 for mature B-1 cells in homeostatic proliferation, cell survival, and Ig production

B-1 cells, distinguishable from conventional B-2 cells by their cell surface marker, anatomical location, and self-replenishing activity, play an important role in innate immune responses. B-1 cells constitutively express the IL-5R alpha-chain (IL-5Ralpha) and give rise to Ab-producing cells in response to various stimuli, including IL-5 and LPS. Here we report that the IL-5/IL-5R system plays an important role in maintaining the number and the cell size as well as the functions of mature B-1 cells. The administration of anti-IL-5 mAb into wild-type mice, T cell-depleted mice, or mast cell-depleted mice resulted in reduction in the total number and cell size of B-1 cells to an extent similar to that of IL-5Ralpha-deficient (IL-5Ralpha(-/-)) mice. Cell transfer experiments have demonstrated that B-1 cell survival in wild-type mice and homeostatic proliferation in recombination-activating gene 2-deficient mice are impaired in the absence of IL-5Ralpha. IL-5 stimulation of wild-type B-1 cells, but not IL-5Ralpha(-/-) B-1 cells, enhances CD40 expression and augments IgM and IgG production after stimulation with anti-CD40 mAb. Enhanced IgA production in feces induced by the oral administration of LPS was not observed in IL-5Ralpha(-/-) mice. Our results illuminate the role of IL-5 in the homeostatic proliferation and survival of mature B-1 cells and in IgA production in the mucosal tissues.

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.

Enhanced T cell proliferation in mice lacking the p85beta subunit of phosphoinositide 3-kinase

Phosphoinositide 3-kinase activation is important for lymphocyte proliferation and survival. Disrupting the gene that encodes the major phosphoinositide 3-kinase regulatory isoform p85alpha impairs B cell development and proliferation. However, T cell functions are intact in the absence of p85alpha. In this study, we test the hypothesis that the related isoform p85beta is an essential regulatory subunit for T cell signaling. Unexpectedly, T cells lacking p85beta showed a marked increase in proliferation and decreased death when stimulated with anti-CD3 plus IL-2. Both CD4(+) and CD8(+) T cells completed more cell divisions. Transcriptional profiling revealed reduced levels of caspase-6 mRNA in p85beta-deficient T cells, which was paralleled by reduced caspase-6 enzyme activity. Increased T cell accumulation was also observed in vivo following infection of p85beta-deficient mice with mouse hepatitis virus. Together, these results suggest a unique role for p85beta in limiting T cell expansion.

The role of tyrosine kinase Etk/Bmx in EGF-induced apoptosis of MDA-MB-468 breast cancer cells

Etk/Bmx, a member of the Tec family of tyrosine kinases, mediates various signaling pathways and confers several cellular functions. In the present study, we have explored the functional role of Etk in mediating EGF-induced apoptosis, using MDA-MB-468 cell line as a model. We first demonstrated that EGF treatment induces Etk tyrosine phosphorylation in both HeLa and MDA-MB-468 cells. Overexpression of Etk by recombinant adenovirus in MDA-MB-468 cells potentiates the extent of EGF-induced cell apoptosis. The observed Etk-enhanced MDA-MB-468 cell apoptosis is associated with the Stat1 activation, as demonstrated by electrophoresis mobility shift assays and reporter gene assays. By contrast, a kinase domain deletion mutant EtkDeltaK, functioning as a dominant-negative mutant, ameliorates EGF-induced Stat1 activation and apoptosis in MDA-MB-468 cells. To explore whether the activated Etk alone is sufficient for inducing apoptosis, a conditionally activated Etk (DeltaEtk-ER), a chimeric fusion protein of PH domain-truncated Etk and ligand-binding domain of estrogen receptor, was introduced into MDA-MB-468 cells. Upon beta-estradiol ligand activation, the DeltaEtk-ER could stimulate Stat1 activity and confer cell apoptosis independent of EGF treatment. Taken together, our findings indicate that Etk is a downstream signaling molecule of EGF receptor and suggest that Etk activation is essential for transducing the EGF-induced apoptotic signaling.

The c-Rel transcription factor and B-cell proliferation: a deal with the devil

Activation of the Rel/NF-kappaB signal transduction pathway has been associated with a variety of animal and human malignancies. However, among the Rel/NF-kappaB family members, only c-Rel has been consistently shown to be able to malignantly transform cells in culture. In addition, c-rel has been activated by a retroviral promoter insertion in an avian B-cell lymphoma, and amplifications of REL (human c-rel) are frequently seen in Hodgkin's lymphomas and diffuse large B-cell lymphomas, and in some follicular and mediastinal B-cell lymphomas. Phenotypic analysis of c-rel knockout mice demonstrates that c-Rel has a normal role in B-cell proliferation and survival; moreover, c-Rel nuclear activity is required for B-cell development. Few mammalian model systems are available to study the role of c-Rel in oncogenesis, and it is still not clear what features of c-Rel endow it with its unique oncogenic activity among the Rel/NF-kappaB family. In any event, REL may provide an appropriate therapeutic target for certain human lymphoid cell malignancies.

Increased numbers of B-1 cells and enhanced responses against TI-2 antigen in mice lacking APS, an adaptor molecule containing PH and SH2 domains

APS (adaptor molecule containing PH and SH2 domains) is an intracellular adaptor protein that forms an adaptor family along with Lnk and SH2-B. While experiments using cultured cell lines have demonstrated that APS is phosphorylated in response to various stimuli, its in vivo functions remain unclear. We attempted to determine the physiological roles of APS by generating APS-deficient (APS(-/-)) mice. APS(-/-) mice were viable and fertile and showed no abnormalities or growth retardation. Immunologically, APS(-/-) mice showed normal development and distribution of lymphocytes and myeloid cells, except for increased numbers of B-1 cells in the peritoneal cavity. APS(-/-) mice exhibited an enhanced humoral immune response against trinitrophenol-Ficoll, a thymus-independent type 2 antigen, while APS(-/-) B-2 cells exhibited normal proliferative responses and tyrosine phosphorylation of intracellular proteins upon B-cell receptor (BCR) cross-linking. APS colocalized with filamentous actin (F-actin) accumulated during the capping of BCRs in APS-transgenic B cells. After BCR stimulation, F-actin contents were lower in APS(-/-) B-1 cells than in wild-type B-1 cells. Our results indicate that APS might have a novel regulatory role in actin reorganization and control of B-1 cell compartment size.

Analysis of Gene Expression and Ig Transcription in PU.1/Spi-B-Deficient Progenitor B Cell Lines

A number of presumptive target genes for the Ets-family transcription factor PU.1 have been identified in the B cell lineage. However, the precise function of PU.1 in B cells has not been studied because targeted null mutation of the PU.1 gene results in a block to lymphomyeloid development at an early developmental stage. In this study, we take advantage of recently developed PU.1(-/-)Spi-B(-/-) IL-7 and stromal cell-dependent progenitor B (pro-B) cell lines to analyze the function of PU.1 and Spi-B in B cell development. We show that contrary to previously published expectations, PU.1 and/or Spi-B are not required for Ig H chain (IgH) gene transcription in pro-B cells. In fact, PU.1(-/-)Spi-B(-/-) pro-B cells have increased levels of IgH transcription compared with wild-type pro-B cells. In addition, high levels of Igkappa transcription are induced after IL-7 withdrawal of wild-type or PU.1(-/-)Spi-B(-/-) pro-B cells. In contrast, we found that Iglambda transcription is reduced in PU.1(-/-)Spi-B(-/-) pro-B cells relative to wild-type pro-B cells after IL-7 withdrawal. These results suggest that Iglambda, but not IgH or Igkappa, transcription, is dependent on PU.1 and/or Spi-B. The PU.1(-/-)Spi-B(-/-) pro-B cells have other phenotypic changes relative to wild-type pro-B cells including increased proliferation, increased CD25 expression, decreased c-Kit expression, and decreased RAG-1 expression. Taken together, our observations suggest that reduction of PU.1 and/or Spi-B activity in pro-B cells promotes their differentiation to a stage intermediate between late pro-B cells and large pre-B cells.

Tec regulates platelet activation by GPVI in the absence of Btk

The Tec family kinase Btk plays an important role in the regulation of phospholipase C gamma 2 (PLC gamma 2) downstream of the collagen receptor glycoprotein VI (GPVI) in human platelets. Platelets also express a second member of this family, Tec; however, its function has not been analyzed. To address the role of Tec, we analyzed Btk-/-, Tec-/-, and Btk/Tec double-deficient (Btk-/-/Tec-/-) platelets. Tec-/- platelets exhibit a minor reduction in aggregation to threshold concentrations of collagen or the GPVI-specific agonist collagen-related peptide (CRP), whereas responses to higher concentrations are normal. Tyrosine phosphorylation of PLC gamma 2 by collagen and CRP is not altered in Tec-/- platelets. However, Btk-/-/Tec-/- platelets exhibit a greater reduction in PLC gamma 2 phosphorylation than is seen in the absence of Btk, thus revealing an important role for Tec in this situation. Furthermore, Btk-/-/Tec-/- platelets fail to undergo an increase in Ca2+, aggregation, secretion, and spreading in response to collagen or CRP, whereas they aggregate normally to adenosine diphosphate (ADP) and spread on fibrinogen. A residual GPVI signal exists in the Btk-/-/Tec-/- platelets as CRP synergizes with ADP to mediate aggregation. These results demonstrate an essential requirement for Tec and Btk in platelet activation by GPVI and reveal a functional role for Tec in the regulation of PLC gamma 2 in the absence of Btk.

Bam32 links the B cell receptor to ERK and JNK and mediates B cell proliferation but not survival

Bam32 is an adaptor protein recruited to the plasma membrane upon B cell receptor (BCR) crosslinking in a phosphoinositol 3-kinase (PI3K)-dependent manner; however, its physiologic function is unclear. To determine its physiologic function, we produced Bam32-deficient mice. Bam32(-/-) B cells develop normally but have impaired T-independent antibody responses in vivo and diminished responses to BCR crosslinking in vitro. Biochemical analysis revealed that Bam32 acts in a novel pathway leading from the BCR to MAPK/ERK Kinases (MEK1/2), MAPK/ERK Kinase Kinase-1 (MEKK1), extracellular signal-regulated kinase (ERK), and c-jun NH2-terminal kinase (JNK), but not p38 mitogen-activated protein kinase (p38). This pathway appears to be initiated by hematopoietic progenitor kinase-1 (HPK1), which interacts directly with Bam32, and differs from all previously characterized BCR signaling pathways in that it is required for normal BCR-mediated proliferation but not for B cell survival.

Phospholipase Cgamma2 provides survival signals via Bcl2 and A1 in different subpopulations of B cells

PLCgamma2 plays a critical role in B cell receptor (BCR) signaling and its targeted deletion results in defective B cell development and function. Here, we show that PLCgamma2 deficiency specifically blocks B cell maturation at the transitional type 2 (T2) to follicular (FO) B cell transition and the PLCgamma2 pathway regulates survival of B cells. BCR-induced apoptosis is dramatically enhanced in all subsets of splenic PLCgamma2-deficient B cells, especially in T2 and FO B cell subpopulations. We also find that all splenic PLCgamma2-deficient B cell subpopulations express abnormally low levels of Bcl-2 protein. In addition, PLCgamma2 deficiency disrupts BCR-mediated induction of A1 expression. Enforced expression of Bcl-2 prevents BCR-induced apoptosis in all splenic PLCgamma2-deficient B cell subpopulations and partially restores the numbers of PLCgamma2-deficient FO B cells. In contrast to Bcl-2, enforced expression of A1 preferentially prevents BCR-induced apoptosis in PLCgamma2-deficient FO B cells and partially restores the numbers of these B cells. Therefore, the PLCgamma2 pathway provides a survival signal via regulation of Bcl-2 in all splenic B cell subpopulations and via additional induction of A1 in mature FO B cells.

Defective development and function of Bcl10-deficient follicular, marginal zone and B1 B cells

Bcl10 is an intracellular protein essential for nuclear factor (NF)-kappaB activation after lymphocyte antigen receptor stimulation. Using knockout mice, we show that absence of Bcl10 impeded conversion from transitional type 2 to mature follicular B cells and caused substantial decreases in marginal zone and B1 B cells. Bcl10-deficient B cells showed no excessive apoptosis. However, both Bcl10-deficient follicular and marginal zone B cells failed to proliferate normally, although Bcl10-deficient marginal zone B cells uniquely failed to activate NF-kappaB efficiently after stimulation with lipopolysaccharide. Bcl10-deficient marginal zone B cells did not capture antigens, and Bcl10-deficient (Bcl10-/-) mice failed to initiate humoral responses, leading to an inability to clear blood-borne bacteria. Thus, Bcl10 is essential for the development of all mature B cell subsets.

Reduced dosage of Bruton's tyrosine kinase uncouples B cell hyperresponsiveness from autoimmunity in lyn-/- mice

The development of autoimmunity is correlated with heightened sensitivity of B cells to B cell Ag receptor (BCR) cross-linking. BCR signals are down-regulated by Lyn, which phosphorylates inhibitory receptors. lyn(-/-) mice have reduced BCR signaling thresholds and develop autoantibodies, glomerulonephritis, splenomegaly due to myeloid hyperplasia, and increased B-1 cell numbers. Bruton's tyrosine kinase (Btk), a critical component of BCR signaling pathways, is required for autoantibody production in lyn(-/-) mice. It is unclear whether Btk mediates autoimmunity at the level of BCR signal transduction or B cell development, given that lyn(-/-)Btk(-/-) mice have a severe reduction in conventional B and B-1 cell numbers. To address this issue, we crossed a transgene expressing a low dosage of Btk (Btk(low)) in B cells to lyn(-/-)Btk(-/-) mice. Conventional B cell populations were restored to levels similar to those in lyn(-/-) mice. These cells were as hypersensitive to BCR cross-linking as lyn(-/-) B cells as measured by proliferation, Ca(2+) flux, and activation of extracellular signal-regulated kinase and Akt. However, lyn(-/-)Btk(low) mice did not produce anti-ssDNA, anti-dsDNA, anti-histone, or anti-histone/DNA IgM or IgG. They also lacked B-1 cells and did not exhibit splenomegaly. Thus, B cell hyperresponsiveness is insufficient for autoimmunity in lyn(-/-) mice. These studies implicate B-1 and/or myeloid cells as key contributors to the lyn(-/-) autoimmune phenotype.

B cell receptor-mediated Syk-independent activation of phosphatidylinositol 3-kinase, Ras, and mitogen-activated protein kinase pathways

The Syk tyrosine kinase is a key molecule in the development of the B cell lineage and the activation of B lymphocytes after Ag recognition by the B cell Ag receptor (BCR). Several genetic studies with chicken B cells have reported that the recruitment of Syk by BCR is essential for activation of a cascade of signaling molecules including phosphatidylinositol 3-kinase, mitogen-activated protein kinases, Ras signaling pathways, phospholipase C-gamma2 activation, and calcium mobilization. The identification of a Syk-deficient mouse IIA1.6/A20 B cell line provided us the opportunity to investigate Syk-mediated signaling in mouse. Surprisingly, phosphatidylinositol 3-kinase, Ras, and mitogen-activated protein kinases were activated upon BCR cross-linking in these Syk-deficient mouse B cells, whereas, as expected from results obtained in chicken B cells, phospholipase C-gamma2 activation and calcium mobilization were impaired as well as the NF-kappaB pathway. These results indicate that BCR signaling is not strictly dependent on Syk expression in mouse IIA1.6/A20 B cells. Thus, B lymphocyte activation may be initiated by Syk-dependent and Syk-independent signaling cascades.

The resolution of relapsing fever borreliosis requires IgM and is concurrent with expansion of B1b lymphocytes

The rate of pathogen clearance is a critical determinant of morbidity and mortality. We sought to characterize the immune response responsible for the remarkably rapid clearance of individual episodes of bacteremia caused by the relapsing fever bacterium, Borrelia hermsii. SCID or Rag(-/-) mice were incapable of resolving B. hermsii infection, indicating a critical role for T and/or B cells. TCR(-/-) mice, which lack T cells, and IL-7(-/-) mice, which are deficient in both T cells and follicular B cells, but not in B1 cells and splenic marginal zone (MZ) B cells, efficiently cleared B. hermsii. These findings suggested that B1 cells and/or MZ B cells, two B cell subsets that are known to participate in rapid, T-independent responses, might be involved. The efficient resolution of the episodes of moderate level bacteremia by splenectomized mice suggested that MZ B cells do not play the primary role in clearance of this bacterium. In contrast, xid mice, which are deficient in B1 cells, suffered more severe episodes of bacteremia than wild-type mice. The hypothesis that B1 cells are critical for clearance of B. hermsii was further supported by a selective expansion of the B1b (i.e., IgM(high), IgD(-/low), Mac1(+) CD23(-), and CD5(-)) cell subset in infected xid mice, which coincided with the eventual resolution of infection. Finally, mice selectively incapable of secreting IgM, the dominant isotype produced by B1 cells, were completely unable to clear B. hermsii. Together these results support the model that B1b cells generate the T-independent IgM required for the control and resolution of relapsing fever borreliosis.

Transitional type 1 and 2 B lymphocyte subsets are differentially responsive to antigen receptor signaling

Mature B-lymphocytes develop sequentially from transitional type 1 (T1) and type 2 (T2) precursors in the spleen. To elucidate the mechanisms that regulate the developmental fate of these distinct B cell subsets, we investigated their biochemical and biological responses following stimulation through the B-cell antigen receptor (BCR). As compared with the T1 subset, T2 cells are more responsive to BCR engagement, as evidenced by their robust induction of activation markers, expression of the prosurvival protein Bcl-x(L), and enhanced proliferation. BCR stimulation of T2 cells leads to the appearance of B cells with mature phenotypic characteristics, whereas T1 cells die. All of these T2 responses are dependent on the BCR signal transducer Bruton's tyrosine kinase, which is dispensable for the T1 to T2 transition. Furthermore, the serine/threonine kinases ERK, p38 MAPK, and Akt are predominantly activated in T2 compared with T1 B cells following BCR cross-linking. We conclude that T1 and T2 B cells respond differentially to BCR engagement via the induction of stage-specific signaling pathways. In turn, these signaling pathways probably govern the development and selection processes that are critical for the formation of the mature B cell compartment.

BLNK: molecular scaffolding through 'cis'-mediated organization of signaling proteins

Assembly of intracellular macromolecular complexes is thought to provide an important mechanism to coordinate the generation of second messengers upon receptor activation. We have previously identified a B cell linker protein, termed BLNK, which serves such a scaffolding function in B cells. We demonstrate here that phosphorylation of five tyrosine residues within human BLNK nucleates distinct signaling effectors following B cell antigen receptor activation. The phosphorylation of multiple tyrosine residues not only amplifies PLCgamma-mediated signaling but also supports 'cis'-mediated interaction between distinct signaling effectors within a large molecular complex. These data demonstrate the importance of coordinate phosphorylation of molecular scaffolds, and provide insights into how assembly of macromolecular complexes is required for normal receptor function.

Essential, nonredundant role for the phosphoinositide 3-kinase p110delta in signaling by the B-cell receptor complex

Many receptor and nonreceptor tyrosine kinases activate phosphoinositide 3-kinases (PI3Ks). To assess the role of the delta isoform of the p110 catalytic subunit of PI3Ks, we derived enzyme-deficient mice. The mice are viable but have decreased numbers of mature B cells, a block in pro-B-cell differentiation, and a B1 B-cell deficiency. Both immunoglobulin M receptor-induced Ca(2+) flux and proliferation in response to B-cell mitogens are attenuated. Immunoglobulin levels are decreased substantially. The ability to respond to T-cell-independent antigens is markedly reduced, and the ability to respond to T-cell-dependent antigens is completely eliminated. Germinal center formation in the spleen in response to antigen stimulation is disrupted. These results define a nonredundant signaling pathway(s) utilizing the delta isoform of p110 PI3K for the development and function of B cells.

Regulation of signaling in B cells through the phosphorylation of Syk on linker region tyrosines. A mechanism for negative signaling by the Lyn tyrosine kinase

The B cell antigen receptor (BCR) is coupled to the mobilization of Ca(2+) by the protein-tyrosine kinase, Syk. Syk, recruited to the clustered BCR, becomes phosphorylated on three tyrosines (Tyr-317, Tyr-342, and Tyr-346) located within the linker region that separates the C-terminal catalytic domain from the N-terminal tandem Src homology 2 domains. Phosphorylation within the linker region can be either activating or inhibitory to Ca(2+) mobilization depending on the sites that are modified. Syk that is not phosphorylated on linker region tyrosines couples the BCR to Ca(2+) mobilization through a phosphoinositide 3-kinase-dependent pathway. The phosphorylation of Tyr-342 and -346 enhances the phosphorylation and activation of phospholipase C-gamma and the early phase of Ca(2+) mobilization via a phosphoinositide 3-kinase-independent pathway. The phosphorylation of Tyr-317 strongly dampens the Ca(2+) signal. In cells that lack the Src family kinase, Lyn, the phosphorylation of the inhibitory Tyr-317 is suppressed leading to elevated production of inositol 1,4,5-trisphosphate and an amplified Ca(2+) signal. This provides a novel mechanism by which Lyn functions as an inhibitor of BCR-stimulated signaling. Thus, Syk and Lyn combine to determine the pathway through which the BCR is coupled to Ca(2+) mobilization as well as the magnitude and duration of the Ca(2+) flux.

Aberrant DAP12 signaling in the 129 strain of mice: implications for the analysis of gene-targeted mice

NK cells are implicated in antiviral responses, bone marrow transplantation and tumor immunosurveillance. Their function is controlled, in part, through the Ly49 family of class I binding receptors. Inhibitory Ly49s suppress signaling, while activating Ly49s (i.e., Ly49D) activate NK cells via the DAP12 signaling chain. Activating Ly49 signaling has been studied primarily in C57BL/6 mice, however, 129 substrains are commonly used in gene-targeting experiments. In this study, we show that in contrast to C57BL/6 NK cells, cross-linking of DAP12-coupled receptors in 129/J mice induces phosphorylation of DAP12 but not calcium mobilization or cytokine production. Consistent with poor-activating Ly49 function, 129/J mice reject bone marrow less efficiently than C57BL/6 mice. Sequence analysis of receptors and DAP12 suggests no structural basis for inactivity, and both the 129/J and C57BL/6 receptors demonstrate normal function in a reconstituted receptor system. Most importantly, reconstitution of Ly49D in 129/J NK cells demonstrated that the signaling deficit is within the NK cells themselves. These unexpected findings bring into question any NK analysis of 129/J, 129Sv, or gene-targeted mice derived from these strains before complete backcrossing, and provide a possible explanation for the differences observed in the immune response of 129 mice in a variety of models.

Delayed cellular maturation and decreased immunoglobulin kappa light chain production in immature B lymphocytes lacking B cell linker protein

B cell linker (BLNK) protein is a component of the B cell receptor (BCR) signaling pathway and BLNK(-/-) mice have a block in B lymphopoiesis at the pro-B/pre-B cell stage. To study the effect of BLNK mutation at later stages of B cell development, we introduce an innocuous transgenic BCR into BLNK(-/-) mice and show that two populations of immature B cells distinguishable by their IgM(low (lo)) and IgM(high (hi)) phenotypes are found in the bone marrow of these mice in contrast to a single population of IgM(hi) cells found in control BCR-transgenic BLNK(+/+) mice. The mutant IgM(lo) and IgM(hi) cells are at an earlier developmental stage compared with the control IgM(hi) cells as indicated by their differential expression of CD43, B220, and major histocompatibility complex class II antigens and their timing of generation in culture. Thus, in the absence of BLNK the differentiation of immature B cells is delayed. Furthermore, mutant IgM(lo) cells produce equivalent level of immunoglobulin (Ig) mu but less Ig kappa proteins than control and mutant IgM(hi) cells and this defect is attributed to a decrease in the amount of kappa transcripts being generated. Finally, splenic B cells in BCR-transgenic BLNK(-/-) mice are predominantly of the transitional B cell phenotype and are rapidly lost from the peripheral B cell pool. Taken together, the data suggest a role for BLNK and perhaps BCR signaling, in the regulation of kappa light chain expression and continued immature B cell differentiation.

Complementary roles for CD19 and Bruton's tyrosine kinase in B lymphocyte signal transduction

CD19 and Bruton's tyrosine kinase (Btk) may function along common signaling pathways in regulating intrinsic and B cell Ag receptor (BCR)-induced signals. To identify physical and functional interactions between CD19 and Btk, a CD19-negative variant of the A20 B cell line was isolated, and CD19-deficient (CD19(-/-)) and CD19-overexpressing mice with the X-linked immunodeficient (Xid; Btk) mutation were generated. In A20 cells, Btk physically associated with CD19 following BCR engagement. CD19 and Btk interactions were not required for initial Btk phosphorylation, but CD19 expression maintained Btk in an activated state following BCR engagement. In primary B cells, CD19 signaling also required downstream Btk function since CD19-induced intracellular Ca(2+) ([Ca(2+)](i)) responses were modest in Xid B cells. In addition, CD19 overexpression did not normalize the Xid phenotype and most phenotypic and functional hallmarks of CD19 overexpression were not evident in these mice. However, CD19 and Btk also regulate independent signaling pathways since their combined loss had additive inhibitory effects on BCR-induced [Ca(2+)](i) responses and CD19 deficiency induced a severe immunodeficiency in Xid mice. Thus, CD19 expression amplifies or prolongs Btk-mediated signaling, rather than serving as a required agent for Btk activation. Consistent with this, phosphatidylinositol 3-monophosphate kinase and Akt activation were normal in CD19(-/-) B cells following IgM engagement, although their kinetics of activation was altered. Thus, these biochemical and compound gene dosage studies indicate that Btk activation and [Ca(2+)](i) responses following BCR engagement are regulated through multiple pathways, including a CD19/Src family kinase-dependent pathway that promotes the longevity of Btk signaling.