Distinct signal thresholds for the unique antigen receptor-linked gene expression programs in mature and immature B cells

B cells modulate lung antiviral inflammatory responses via the neurotransmitter acetylcholine

The rapid onset of innate immune defenses is critical for early control of viral replication in an infected host and yet it can also lead to irreversible tissue damage, especially in the respiratory tract. Sensitive regulators must exist that modulate inflammation, while controlling the infection. In the present study, we identified acetylcholine (ACh)-producing B cells as such early regulators. B cells are the most prevalent ACh-producing leukocyte population in the respiratory tract demonstrated with choline acetyltransferase (ChAT)-green fluorescent protein (GFP) reporter mice, both before and after infection with influenza A virus. Mice lacking ChAT in B cells, disabling their ability to generate ACh (ChatBKO), but not those lacking ChAT in T cells, significantly, selectively and directly suppressed α7-nicotinic-ACh receptor-expressing interstitial, but not alveolar, macrophage activation and their ability to secrete tumor necrosis factor (TNF), while better controlling virus replication at 1 d postinfection. Conversely, TNF blockade via monoclonal antibody treatment increased viral loads at that time. By day 10 of infection, ChatBKO mice showed increased local and systemic inflammation and reduced signs of lung epithelial repair despite similar viral loads and viral clearance. Thus, B cells are key participants of an immediate early regulatory cascade that controls lung tissue damage after viral infection, shifting the balance toward reduced inflammation at the cost of enhanced early viral replication.

B cells modulate lung antiviral inflammatory responses via the neurotransmitter acetylcholine

The rapid onset of innate immune defenses is critical for early control of viral replication in an infected host, yet it can also lead to irreversible tissue damage, especially in the respiratory tract. Intricate regulatory mechanisms must exist that modulate inflammation, while controlling the infection. Here, B cells expressing choline acetyl transferase (ChAT), an enzyme required for production of the metabolite and neurotransmitter acetylcholine (ACh) are identified as such regulators of the immediate early response to influenza A virus. Lung tissue ChAT + B cells are shown to interact with a7 nicotinic Ach receptor-expressing lung interstitial macrophages in mice within 24h of infection to control their production of TNFa, shifting the balance towards reduced inflammation at the cost of enhanced viral replication. Thus, innate-stimulated B cells are key participants of an immediate-early regulatory cascade that controls lung tissue damage after viral infection.

Ultrasensitive HPLC-MS Quantification of S-(2-Succino) Cysteine Based on Ethanol/Acetyl Chloride Derivatization in Fumarate Accumulation Cells

Succination is a nonenzymatic and irreversible post-translational modification (PTM) with important biological significance, yielding S-(2-succino) cysteine (2SC) residue. This PTM is low in abundance and often requires a large amount of protein samples for 2SC quantification. In this work, an efficient quantification method based on ethanol/acetyl chloride chemical derivatization was developed. The three carboxyl groups of 2SC were all esterified to increase hydrophobicity, greatly improving its ionization efficiency. The sensitivity was increased by 112 times; the limit of detection was reduced to 0.885 fmol, and the protein usage was reduced by at least 10 times. The established method was used to detect the overall concentration of 2SC in fumarate accumulation cells quantitatively.

B Quiet: Autoantigen-Specific Strategies to Silence Raucous B Lymphocytes and Halt Cross-Talk with T Cells in Type 1 Diabetes

Islet autoantibodies are the primary biomarkers used to predict type 1 diabetes (T1D) disease risk. They signal immune tolerance breach by islet autoantigen-specific B lymphocytes. T-B lymphocyte interactions that lead to expansion of pathogenic T cells underlie T1D development. Promising strategies to broadly prevent this T-B crosstalk include T cell elimination (anti-CD3, teplizumab), B cell elimination (anti-CD20, rituximab), and disruption of T cell costimulation/activation (CTLA-4/Fc fusion, abatacept). However, global disruption or depletion of immune cell subsets is associated with significant risk, particularly in children. Therefore, antigen-specific therapy is an area of active investigation for T1D prevention. We provide an overview of strategies to eliminate antigen-specific B lymphocytes as a means to limit pathogenic T cell expansion to prevent beta cell attack in T1D. Such approaches could be used to prevent T1D in at-risk individuals. Patients with established T1D would also benefit from such targeted therapies if endogenous beta cell function can be recovered or islet transplant becomes clinically feasible for T1D treatment.

Structural basis of Focal Adhesion Kinase activation on lipid membranes

Focal adhesion kinase (FAK) is a key component of the membrane proximal signaling layer in focal adhesion complexes, regulating important cellular processes, including cell migration, proliferation, and survival. In the cytosol, FAK adopts an autoinhibited state but is activated upon recruitment into focal adhesions, yet how this occurs or what induces structural changes is unknown. Here, we employ cryo-electron microscopy to reveal how FAK associates with lipid membranes and how membrane interactions unlock FAK autoinhibition to promote activation. Intriguingly, initial binding of FAK to the membrane causes steric clashes that release the kinase domain from autoinhibition, allowing it to undergo a large conformational change and interact itself with the membrane in an orientation that places the active site toward the membrane. In this conformation, the autophosphorylation site is exposed and multiple interfaces align to promote FAK oligomerization on the membrane. We show that interfaces responsible for initial dimerization and membrane attachment are essential for FAK autophosphorylation and resulting cellular activity including cancer cell invasion, while stable FAK oligomerization appears to be needed for optimal cancer cell proliferation in an anchorage-independent manner. Together, our data provide structural details of a key membrane bound state of FAK that is primed for efficient autophosphorylation and activation, hence revealing the critical event in integrin mediated FAK activation and signaling at focal adhesions.

BCR-Induced Ca2+ Signals Dynamically Tune Survival, Metabolic Reprogramming, and Proliferation of Naive B Cells

B cell receptor (BCR) engagement induces naive B cells to differentiate and perform critical immune-regulatory functions. Acquisition of functional specificity requires that a cell survive, enter the cell cycle, and proliferate. We establish that quantitatively distinct Ca2+ signals triggered by variations in the extent of BCR engagement dynamically regulate these transitions by controlling nuclear factor κB (NF-κB), NFAT, and mTORC1 activity. Weak BCR engagement induces apoptosis by failing to activate NF-κB-driven anti-apoptotic gene expression. Stronger signals that trigger more robust Ca2+ signals promote NF-κB-dependent survival and NFAT-, mTORC1-, and c-Myc-dependent cell-cycle entry and proliferation. Finally, we establish that CD40 or TLR9 costimulation circumvents these Ca2+-regulated checkpoints of B cell activation and proliferation. As altered BCR signaling is linked to autoimmunity and B cell malignancies, these results have important implications for understanding the pathogenesis of aberrant B cell activation and differentiation and therapeutic approaches to target these responses.

B Cell Receptor Affinity for Insulin Dictates Autoantigen Acquisition and B Cell Functionality in Autoimmune Diabetes

B cells have been strongly implicated in the development of human type 1 diabetes and are required for disease in the NOD mouse model. These functions are dependent on B cell antigen receptor (BCR) specificity and expression of MHC, implicating linked autoantigen recognition and presentation to effector T cells. BCR-antigen affinity requirements for participation in disease are unclear. We hypothesized that BCR affinity for the autoantigen insulin differentially affects lymphocyte functionality, including tolerance modality and the ability to acquire and become activated in the diabetogenic environment. Using combined transgenic and retrogenic heavy and light chain to create multiple insulin-binding BCRs, we demonstrate that affinity for insulin is a critical determinant of the function of these autoreactive cells. We show that both BCR affinity for insulin and genetic background affect tolerance induction in immature B cells. We also find new evidence that may explain the enigmatic ability of B cells expressing 125 anti-insulin BCR to support development of TID in NOD mice despite a reported affinity beneath requirements for binding insulin at in vivo concentrations. We report that when expressed as an antigen receptor the affinity of 125 is much higher than determined by measurements of the soluble form. Finally, we show that in vivo acquisition of insulin requires both sufficient BCR affinity and permissive host/tissue environment. We propose that a confluence of BCR affinity, pancreas environment, and B cell tolerance-regulating genes in the NOD animal allows acquisition of insulin and autoimmunity.

Hierarchical control of coherent gene clusters defines the molecular mechanisms of glioblastoma

Glioblastoma is a highly-aggressive and rapidly-lethal tumor characterized by resistance to therapy. Although data on multiple genes, proteins, and pathways are available, the key challenge is deciphering this information and identifying central molecular targets. Therapeutically targeting individual molecules is often unsuccessful due to the presence of compensatory and redundant pathways, and crosstalk. A systems biology approach that involves a hierarchical gene group networks analysis can delineate the coherent functions of different disease mediators. Here, we report an integrative networks-based analysis to identify a system of coherent gene modules in primary and secondary glioblastoma. Our study revealed a hierarchical transcriptional control of genes in these modules. We elucidated those modules responsible for conversion of the glioma-associated microglia/macrophages into glioma-supportive, immunosuppressive cells. Further, we identified clusters comprising mediators of angiogenesis, proliferation, and cell death for both primary and secondary glioblastomas. Data obtained for these clusters point to a possible role of transcription regulators that function as the gene modules mediators in glioblastoma pathogenesis. We elucidated a set of possible transcription regulators that can be targeted to affect the selected gene clusters at specific levels for glioblastoma. Our innovative approach to construct informative disease models may hold the key to successful management of complex diseases including glioblastoma and other cancers.

Notch1 is an important mediator for enhancing of B-cell activation and antibody secretion by Notch ligand

The roles of Notch1 and Notch2 in T-cell function have been well studied, but the functional roles of Notch in B cells have not been extensively investigated, except for Notch2 involvement in peripheral marginal zone B-cell differentiation. This study examined the roles of Notch1 in murine primary B cells. During B-cell activation by B-cell receptor ligation, Notch1 was up-regulated while Notch2 was not. In addition, Notch1 up-regulation itself did not contribute to the further activation of B cells, but the Notch ligand was important for Notch1-mediated further B-cell activation. Moreover, Notch1 deficiency significantly decreased B-cell activation and antibody secretion under the presence of Notch ligand. These data suggest that Notch1 is an important mediator for enhancing B-cell activation and antibody secretion by Notch ligand.

Differential immune response associated to malaria outcome is detectable in peripheral blood following Plasmodium yoelii infection in mice

Malaria infection in humans elicits a wide range of immune responses that can be detected in peripheral blood, but we lack detailed long-term follow-up data on the primary and subsequent infections that lead to naturally acquired immunity. Studies on antimalarial immune responses in mice have been based on models yielding homogenous infection profiles. Here, we present a mouse model in which a heterogeneous course of Plasmodium yoelii lethal malaria infection is produced in a non-congenic ICR strain to allow comparison among different immunological and clinical outcomes. Three different disease courses were observed ranging from a fatal outcome, either early or late, to a self-resolved infection that conferred long-term immunity against re-infection. Qualitative and quantitative changes produced in leukocyte subpopulations and cytokine profiles detected in peripheral blood during the first week of infection revealed that monocytes, dendritic cells and immature B cells were the main cell subsets present in highly-parasitized mice dying in the first week after infection. Besides, CD4(+)CD25(high) T cells expanded at an earlier time point in early deceased mice than in surviving mice and expressed higher levels of intracellular Foxp3 protein. In contrast, survivors showed a limited increase of cytokines release and stable circulating innate cells. From the second week of infection, mice that would die or survive showed similar immune profiles, although CD4(+)CD25(high) T cells number increased earlier in mice with the worst prognosis. In surviving mice the expansion of activated circulating T cell and switched-class B cells with a long-term protective humoral response from the second infection week is remarkable. Our results demonstrate that the follow-up studies of immunological blood parameters during a malaria infection can offer information about the course of the pathological process and the immune response.

B lymphocytes: development, tolerance, and their role in autoimmunity-focus on systemic lupus erythematosus

B lymphocytes are the effectors of humoral immunity, providing defense against pathogens through different functions including antibody production. B cells constitute approximately 15% of peripheral blood leukocytes and arise from hemopoietic stem cells in the bone marrow. It is here that their antigen receptors (surface immunoglobulin) are assembled. In the context of autoimmune diseases defined by B and/or T cell autoreactive that upon activation lead to chronic tissue inflammation and often irreversible structural and functional damage, B lymphocytes play an essential role by not only producing autoantibodies but also functioning as antigen-presenting cells (APC) and as a source of cytokines. In this paper, we describe B lymphocyte functions in autoimmunity and autoimmune diseases with a special focus on their abnormalities in systemic lupus erythematosus.

Autoantigen-specific B-cell depletion overcomes failed immune tolerance in type 1 diabetes

Eliminating autoantigen-specific B cells is an attractive alternative to global B-cell depletion for autoimmune disease treatment. To identify the potential for targeting a key autoimmune B-cell specificity in type 1 diabetes, insulin-binding B cells were tracked within a polyclonal repertoire using heavy chain B-cell receptor (BCR) transgenic (VH125Tg) mice. Insulin-specific B cells are rare in the periphery of nonautoimmune VH125Tg/C57BL/6 mice and WT/NOD autoimmune mice, whereas they clearly populate 1% of mature B-cell subsets in VH125Tg/NOD mice. Autoantigen upregulates CD86 in anti-insulin B cells, suggesting they are competent to interact with T cells. Endogenous insulin occupies anti-insulin BCR beginning with antigen commitment in bone marrow parenchyma, as identified by a second anti-insulin monoclonal antibody. Administration of this monoclonal antibody selectively eliminates insulin-reactive B cells in vivo and prevents disease in WT/NOD mice. Unexpectedly, developing B cells are less amenable to depletion, despite increased BCR sensitivity. These findings exemplify how a critical type 1 diabetes B-cell specificity escapes immune tolerance checkpoints. Disease liability is corrected by eliminating this B-cell specificity, providing proof of concept for a novel therapeutic approach for autoimmune disease.

Developmentally regulated expression of MEF2C limits the response to BCR engagement in transitional B cells

Transitional and naïve mature peripheral B cells respond very differently to B-cell receptor (BCR) cross-linking. While transitional B cells undergo apoptosis upon BCR engagement, mature B cells survive and proliferate. This differential response correlates with the capacity of mature, but not transitional B cells to transcribe genes that promote cell survival and proliferation, including those encoding c-Myc and the Bcl-2 family members Bcl-xL and A1. We recently demonstrated that transitional B cells fail to assemble transcriptional machinery at the promoter region of these target genes despite equivalent cytoplasmic signaling and nuclear translocation of key transcription factors including NF-κB and nuclear factor of activated T cells (NFAT). The transcription factor myocyte enhancer factor-2C (MEF2C) is regulated by both calcineurin and mitogen-activated protein kinase signaling pathways, and is essential for proliferation and survival downstream of BCR engagement in mature B cells. In this work, we demonstrate that transitional B cells have intrinsically low levels of MEF2C protein and DNA-binding activity, and that this developmental difference in MEF2C expression is functionally significant. Forced expression of MEF2C in transitional B cells promoted cell survival, proliferation, and upregulation of pro-survival genes. Thus, low MEF2C expression limits transitional B-cell responsiveness to BCR engagement before these cells reach maturity.

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

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

B-cell receptors and heavy chain diseases: guilty by association?

Heavy chain diseases (HCDs) are B-cell proliferative disorders characterized by the production of monoclonal, incomplete, immunoglobulin (Ig) heavy chains (HCs) without associated light chains (LCs). These abnormal HCs are produced as a consequence of HC gene alterations in the neoplastic B cells. HC gene alterations will also impact on surface HC, which is part of the B-cell receptor (BCR), a crucial player in lymphocyte activation by antigen. The selective advantage conferred to mutant cells by abnormal BCR without an antigen-binding domain may be explained by activation of ligand-independent signaling, in analogy to what has been shown for mutated oncogenic growth factor receptors. Here we review data obtained from mouse models showing abnormal, constitutive activity of HCD-BCR, and we discuss the possible mechanism involved, namely, aberrant spontaneous self-aggregation. This self-aggregation might occur as a consequence of escape from the chaperone immunoglobulin binding protein (BiP) and from the anti-aggregation effect of LC association. The concept of misfolding-induced signaling elaborated here may extend to other pathologies termed conformational diseases.

B cell receptor editing in tolerance and autoimmunity

Receptor editing is the process of ongoing antibody gene rearrangement in a lymphocyte that already has a functional antigen receptor. The expression of a functional antigen receptor will normally terminate further rearrangement (allelic exclusion). However, lymphocytes with autoreactive receptors have a chance at escaping negative regulation by "editing" the specificities of their receptors with additional antibody gene rearrangements. As such, editing complicates the Clonal Selection Hypothesis because edited cells are not simply endowed for life with a single, invariant antigen receptor. Furthermore, if the initial immunoglobulin gene is not inactivated during the editing process, allelic exclusion is violated and the B cell can exhibit two specificities. Here, we describe the discovery of editing, the pathways of receptor editing at the heavy (H) and light (L) chain loci, and current evidence regarding how and where editing happens and what effects it has on the antibody repertoire.

Distinct kinetics of memory B-cell and plasma-cell responses in peripheral blood following a blood-stage Plasmodium chabaudi infection in mice

B cell and plasma cell responses take place in lymphoid organs, but because of the inaccessibility of these organs, analyses of human responses are largely performed using peripheral blood mononuclear cells (PBMC). To determine whether PBMC are a useful source of memory B cells and plasma cells in malaria, and whether they reflect Plasmodium-specific B cell responses in spleen or bone marrow, we have investigated these components of the humoral response in PBMC using a model of Plasmodium chabaudi blood-stage infections in C57BL/6 mice. We detected memory B cells, defined as isotype-switched IgD(-) IgM(-) CD19(+) B cells, and low numbers of Plasmodium chabaudi Merozoite Surface Protein-1 (MSP1)-specific memory B cells, in PBMC at all time points sampled for up to 90 days following primary or secondary infection. By contrast, we only detected CD138(+) plasma cells and MSP1-specific antibody-secreting cells within a narrow time frame following primary (days 10 to 25) or secondary (day 10) infection. CD138(+) plasma cells in PBMC at these times expressed CD19, B220 and MHC class II, suggesting that they were not dislodged bone-marrow long-lived plasma cells, but newly differentiated migratory plasmablasts migrating to the bone marrow; thus reflective of an ongoing or developing immune response. Our data indicates that PBMC can be a useful source for malaria-specific memory B cells and plasma cells, but extrapolation of the results to human malaria infections suggests that timing of sampling, particularly for plasma cells, may be critical. Studies should therefore include multiple sampling points, and at times of infection/immunisation when the B-cell phenotypes of interest are likely to be found in peripheral blood.

Functional expression of the calcitonin receptor by human T and B cells

The calcitonin receptor (CTR) is a seven-transmembrane-domain G-protein-coupled receptor that regulates calcium metabolism and bone resorption by osteoclasts. Here we demonstrate that high levels are expressed by normal human T and B lymphocytes from tonsils and peripheral blood in relation to their activation status, as CTR(+) T cells are prone to produce IFN-gamma after TCR stimulation. The receptor is also highly expressed on B cells from chronic lymphocytic leukemia patients, thus suggesting a correlation between its expression, their proliferative extent as well as their memory, antigen-experienced phenotype. Moreover, we found that binding of the receptor with salmon calcitonin induces an increase of intracellular calcium(2+) in peripheral lymphocytes. This effect is involved in several lymphocyte immune functions, as cytosolic calcium(2+) levels regulate both cell proliferation and cytokine production. In our hands, the increase of calcium(2+) levels by CTR binding with sCT induced a dose-dependent cell proliferation. We therefore suppose that expression of this functional receptor may contribute to the modulation of cytoplasmic calcium(2+) levels needed to regulate T and B cell activation and perhaps other immune functions.

Signaling pathways regulating RAG expression in B lymphocytes

Development of B-cell lymphopoiesis is dependent on the presence of recombination activating genes RAG1 and RAG2 enzymes. They control the rearrangements of immunoglobulin variable, diversity and joining region segments, and allow progression of the cellular maturation. RAG1 and RAG2 are successively up- and down-regulated at each B-cell stage to progressively generate a B-cell receptor for which unforeseeable antigenic specificity results from a stochastic process. Therefore, in autoreactive immature B cells, new round of RAG re-expression can be observed to eliminate self-reactivity. In some circumstances, RAG up-regulation can also be found in peripheral mature B lymphocytes, specifically in autoimmune diseases. It is therefore of utmost importance to unravel signaling pathways that trigger RAG induction in normal and pathological conditions. Therapeutic modulation of cytokines or intracellular contacts involved in RAG expression might restrict the development of inappropriate autoimmune repertoire.

Cooperation of B cells and T cells in the pathogenesis of multiple sclerosis

B cells and T cells are two major players in the pathogenesis of multiple sclerosis (MS) and cooperate at various check points. B cells, besides serving as a source for antibody-secreting plasma cells, are efficient antigen presenting cells for processing of intact myelin antigen and subsequent activation and pro-inflammatory differentiation of T cells. This notion is supported by the immediate clinical benefit of therapeutic B cell depletion in MS, presumably abrogating development of encephalitogenic T cells. However, different B cell subsets strongly vary in their respective effect on T cell differentiation which may relate to B cell phenotype, activation status, antigen specificity and the immunological environment where a B cell encounters a naïve T cell in. In this regard, some B cells also have anti-inflammatory properties producing regulatory cytokines and facilitating development and maintenance of other immunomodulatory immune cells, such as regulatory T cells. Reciprocally, differentiated T cells influence T cell polarizing B cell properties establishing a positive feedback loop of joint pro- or anti-inflammatory B and T cell developments. Further, under the control of activated T helper cells, antigen-primed B cells can switch immunoglobulin isotype, terminally commit to the plasma cell pathway or enter the germinal center reaction to memory B Cell development. Taken together, B cells and T cells thus closely support one another to participate in the pathogenesis of MS in an inflammatory but also in a regulatory manner.

Distinct signaling mechanisms activate the target of rapamycin in response to different B-cell stimuli

Phosphoinositide 3-kinase (PI3K) and the mammalian target of rapamycin (mTOR), a downstream kinase, are both required for proliferation of splenic B cells. However, the functions of PI3K and mTOR in response to different stimuli and among B cell subsets have not been fully elucidated. We used flow cytometry and magnetic cell sorting to examine the requirement for PI3K and mTOR in responses of splenic B cell subsets to BCR and LPS stimulation. BCR-mediated phosphorylation of Akt and Erk is sensitive to the PI3K catalytic inhibitor wortmannin in both marginal zone (MZ) and follicular (FO) cells. BCR-mediated mTOR activation in both subsets is inhibited by wortmannin, though less strongly in MZ cells. In contrast, LPS-induced mTOR signaling is strikingly resistant to wortmannin in both subsets. Similarly, functional responses to LPS are partially wortmannin resistant yet sensitive to mTOR inhibition by rapamycin. We also observed mitogen-independent mTOR activity that is regulated by nutrient availability, and is significantly elevated in MZ cells relative to FO cells. These data define both similarities and differences in PI3K/mTOR signaling mechanisms in MZ and FO cells, and suggest that mTOR signaling can occur in the absence of PI3K activation to promote B cell responses to LPS.

Reduced receptor editing in lupus-prone MRL/lpr mice

The initial B cell repertoire contains a considerable proportion of autoreactive specificities. The first major B cell tolerance checkpoint is at the stage of the immature B cell, where receptor editing is the primary mode of eliminating self-reactivity. The cells that emigrate from the bone marrow have a second tolerance checkpoint in the transitional compartment in the spleen. Although it is known that the second checkpoint is defective in lupus, it is not clear whether there is any breakdown in central B cell tolerance in the bone marrow. We demonstrate that receptor editing is less efficient in the lupus-prone strain MRL/lpr. In an in vitro system, when receptor-editing signals are given to bone marrow immature B cells by antiidiotype antibody or after in vivo exposure to membrane-bound self-antigen, MRL/lpr 3-83 transgenic immature B cells undergo less endogenous rearrangement and up-regulate recombination activating gene messenger RNA to a lesser extent than B10 transgenic cells. CD19, along with immunoglobulin M, is down-regulated in the bone marrow upon receptor editing, but the extent of down-regulation is fivefold less in MRL/lpr mice. Less efficient receptor editing could allow some autoreactive cells to escape from the bone marrow in lupus-prone mice, thus predisposing to autoimmunity.

Transcriptional basis of lymphocyte tolerance

Signaling through lymphocyte antigen receptors has the potential to initiate several distinct outcomes: proliferation, differentiation, apoptosis, or functional unresponsiveness. Expansion and differentiation of effector T cells is required for defense against foreign antigens, whereas functional unresponsiveness, termed anergy, is a cell-intrinsic mechanism that contributes to peripheral self-tolerance. Other mechanisms of peripheral tolerance include the 'dominant' tolerance imposed by regulatory T cells and immunosuppression mediated by interleukin-10 and transforming growth factor-beta. T- and B-cell antigen receptor ligation induces an increase in intracellular calcium levels as well as activating additional signaling pathways that are further potentiated by costimulatory receptors. In this review, we argue that cell-intrinsic programs of peripheral anergy and tolerance are imposed by sustained calcium signaling in lymphocytes. We address in particular the role of the calcium-dependent transcription factor nuclear factor for activation of T cells, which is activated by antigen receptor stimulation and, depending on the presence or absence of input from its transcriptional partner, activator protein-1, dictates two distinct transcriptional programs: activation or tolerance.

Class switch recombination: a friend and a foe

The effector functions of the antibody are determined by the heavy chain constant region (CH). During CSR the primarily expressed mu constant region (Cmu) of the heavy chain is replaced with a downstream isotype Cgamma, Calpha or Cepsilon. The murine immunoglobulin heavy chain (IgH) locus contains eight different CH genes. Class switch recombination (CSR) involves a recombination between two different repetitive switch (S) region sequences, located upstream of each CH gene and the deletion of the intervening DNA. However, this protecting mechanism is also involved in aberrant chromosomal translocations and generation of B cell malignancies. It is also involved in susceptibility to autoimmunity. The current review focuses on the basic mechanism of CSR and the adverse outcomes that it may cause.

B cell receptor signaling down-regulates forkhead box transcription factor class O 1 mRNA expression via phosphatidylinositol 3-kinase and Bruton's tyrosine kinase

BCR cross-linking promotes mature B cell proliferation and survival. PI3K-mediated down-regulation of proapoptotic and antimitogenic genes such as forkhead box transcription factor class O 1 (FOXO1) is an important component of this process. Previously, BCR-induced phosphorylation of FOXO1 was shown to lead to a block in nuclear localization and subsequent protein degradation. We demonstrate that the BCR also signals through PI3K to down-regulate FOXO1 mRNA expression. Bruton's tyrosine kinase (Btk), a downstream effector of PI3K, signals through B cell linker protein (BLNK) and phospholipase C (PLC)gamma2 to mediate B cell proliferation and survival in response to BCR cross-linking. BCR-induced down-regulation of FOXO1 mRNA was impaired in murine knockouts of Btk, BLNK, and PLCgamma2. Because B cells in these models are predominantly immature, experiments were also performed using mature B cells expressing low levels of Btk and BLNK. Similar results were obtained. Inhibitors of downstream components of the Btk/BLNK/PLCgamma2 pathway were used to define the mechanism by which Btk signaling inhibits FOXO1 expression. The protein kinase Cbeta inhibitor Gö6850 had minimal effects on BCR-mediated FOXO1 mRNA down-regulation. However, cyclosporin A, an inhibitor of the Ca(2+)-dependent phosphatase calcineurin, had similar effects on FOXO1 mRNA expression as the PI3K inhibitor LY294002. Neither Btk deficiency nor cyclosporin A prevented FOXO1 protein phosphorylation, indicating that PI3K down-regulates FOXO1 via two independent pathways. We show that the Btk/BLNK/PLCgamma2 pathway mediates BCR-induced changes in expression of the FOXO1 target gene cyclin G2. These observations support the hypothesis that Btk mediates BCR-induced proliferation and survival in part via inhibition of FOXO expression.

Identification of Anergic B Cells within a Wild-Type Repertoire

The contribution of anergy to silencing of autoreactive B cells in physiologic settings is unknown. By comparing anergic and nonanergic immunoglobulin-transgenic mouse strains, we defined a set of surface markers that were used for presumptive identification of an anergic B cell cohort within a normal repertoire. Like anergic transgenic B cells, these physiologic anergic cells exhibited high basal intracellular free calcium and did not mobilize calcium, initiate tyrosine phosphorylation, proliferate, upregulate activation markers, or mount an immune response upon antigen-receptor stimulation. Autoreactive B cells were overrepresented in this cohort. On the basis of the frequency and lifespan of these cells, it appears that as many as 50% of newly produced B cells are destined to become anergic. In conclusion, our findings indicate that anergy is probably the primary mechanism by which autoreactive B cells are silenced. Thus maintenance of the unresponsiveness of anergic cells is critical for prevention of autoimmunity.

Regulation of B Cell Tolerance by the Lupus Susceptibility Gene Ly108

The susceptibility locus for the autoimmune disease lupus on murine chromosome 1, Sle1z/Sle1bz, and the orthologous human locus are associated with production of autoantibody to chromatin. We report that the presence of Sle1z/Sle1bz impairs B cell anergy, receptor revision, and deletion. Members of the SLAM costimulatory molecule family constitute prime candidates for Sle1bz, among which the Ly108.1 isoform of the Ly108 gene was most highly expressed in immature B cells from lupus-prone B6.Sle1z mice. The normal Ly108.2 allele, but not the lupus-associated Ly108.1 allele, was found to sensitize immature B cells to deletion and RAG reexpression. As a potential regulator of tolerance checkpoints, Ly108 may censor self-reactive B cells, hence safeguarding against autoimmunity.

CD72 down-modulates BCR-induced signal transduction and diminishes survival in primary mature B lymphocytes

CD72, a 45-kDa type II transmembrane glycoprotein carrying an ITIM motif, is believed to be an inhibitory coreceptor of the BCR. Mature B cells lacking CD72 show enhanced Ca(2+) mobilization and are hyperproliferative in response to BCR ligation. However, the signal transduction pathways downstream of BCR signaling that transmit the inhibitory effect of CD72 in mature B cells remain unknown. To address this question, we used hen egg lysozyme-specific BCR transgenic mice to elucidate the differential cell signaling between wild-type and CD72-deficient B cells in response to hen egg lysozyme Ag stimulation. Our results demonstrate that CD72 predominantly down-regulates the major signal transduction pathways downstream of the BCR, including NF-AT, NF-kappaB, ERK, JNK, p38-MAPK, and PI3K/Akt in mature B cells. CD72 ligation with anti-CD72 Ab (K10.6), which mimics the binding of CD100 (a natural ligand for CD72) to release the inhibitory function of CD72, augments cell proliferation, Ca(2+) flux, IkappaBalpha activation, and ERK MAPK activity upon Ag stimulation in wild-type B cells. In addition, we show direct evidence that CD72 promotes cell cycle arrest and apoptosis after Ag stimulation in mature B cells. Taken together, our findings conclude that CD72 plays a dominant role as a negative regulator of BCR signaling in primary mature B lymphocytes.

Dual isotype expressing B cells [kappa(+)/lambda(+)] arise during the ontogeny of B cells in the bone marrow of normal nontransgenic mice

Central to the clonal selection theory is the tenet that a single B cell expresses a single receptor with a single specificity. Previously, based on our work in anti-phosphocholine transgenic mouse models, we suggested that B cells escaped clonal deletion by coexpression of more than one receptor on their cell surface. We argued that "receptor dilution" was necessary when: (i) the expressed immunoglobulin receptor is essential for immune protection against pathogens and (ii) this protective receptor is autoreactive and would be clonally deleted, leaving a hole in the B cell repertoire. Here, we demonstrate that dual isotype expressing B cells arise during the normal ontogeny of B cells in the bone marrow and populate both the spleen and peritoneal cavity of nontransgenic mice. Furthermore, single cell analysis of the expressed immunoglobulin light chains suggests that receptor editing may play a role in the generation of a significant fraction of dual isotype expressing B cells.

Aberrant IgM Signaling Promotes Survival of Transitional T1 B Cells and Prevents Tolerance Induction in Lupus-Prone New Zealand Black Mice

New Zealand Black (NZB) mice develop a lupus-like syndrome. Although the precise immune defects leading to autoantibody production in these mice have not been characterized, they possess a number of immunologic abnormalities suggesting that B cell tolerance may be defective. In the bone marrow, immature self-reactive B cells that have failed to edit their receptors undergo apoptosis as a consequence of Ig receptor engagement. Splenic transitional T1 B cells are recent bone marrow emigrants that retain these signaling properties, ensuring that B cells recognizing self-Ags expressed only in the periphery are deleted from the naive B cell repertoire. In this study we report that this mechanism of tolerance is defective in NZB mice. We show that NZB T1 B cells are resistant to apoptosis after IgM cross-linking in vitro. Although extensive IgM cross-linking usually leads to deletion of T1 B cells, in NZB T1 B cells we found that it prevents mitochondrial membrane damage, inhibits activation of caspase-3, and promotes cell survival. Increased survival of NZB T1 B cells was associated with aberrant up-regulation of Bcl-2 after Ig receptor engagement. We also show that there is a markedly increased proportion of NZB T1 B cells that express elevated levels of Bcl-2 in vivo and provide evidence that up-regulation of Bcl-2 follows encounter with self-Ag in vivo. Thus, we propose that aberrant cell signaling in NZB T1 B cells leads to the survival of autoreactive B cells, which predisposes NZB mice to the development of autoimmunity.

Role of TLR in B cell development: signaling through TLR4 promotes B cell maturation and is inhibited by TLR2

The role of TLR4 in mature B cell activation is well characterized. However, little is known about TLR4 role in B cell development. Here, we analyzed the effects of TLR4 and TLR2 agonists on B cell development using an in vitro model of B cell maturation. Highly purified B220(+)IgM(-) B cell precursors from normal C57BL/6 mouse were cultured for 72 h, and B cell maturation in the presence of the TLR agonists was evaluated by expression of IgM, IgD, CD23, and AA4. The addition of LPS or lipid A resulted in a marked increase in the percentage of CD23(+) B cells, while Pam3Cys had no effect alone, but inhibited the increase of CD23(+) B cell population induced by lipid A or LPS. The TLR4-induced expression of CD23 is not accompanied by full activation of the lymphocyte, as suggested by the absence of activation Ag CD69. Experiments with TLR2-knockout mice confirmed that the inhibitory effects of Pam3Cys depend on the expression of TLR2. We studied the effects of TLR-agonists on early steps of B cell differentiation by analyzing IL-7 responsiveness and phenotype of early B cell precursors: we found that both lipid A and Pam3Cys impaired IL-7-dependent proliferation; however, while lipid A up-regulates B220 surface marker, consistent with a more mature phenotype of the IgM(-) precursors, Pam3Cys keeps the precursors on a more immature stage. Taken together, our results suggest that TLR4 signaling favors B lymphocyte maturation, whereas TLR2 arrests/retards that process, ascribing new roles for TLRs in B cell physiology.

Differential signalling during B-cell maturation

The molecular mechanism by which the antigen receptors (BCR) on B cells can elicit differential maturation state-specific responses is one of the central problems in B-cell differentiation yet to be resolved. Indeed, many of the early signalling events detected following BCR ligation, such as activation of protein tyrosine kinases (PTK), phospholipase C (PLC), phosphoinositide-3-kinase (PI 3K), protein kinase C (PKC) and the RasMAPK (mitogen activating protein kinase) signalling cascades are observed throughout B-cell maturation. However, it is becoming clear that the differential functional responses of these BCR-coupled signals observed during B-cell maturation are dependent on a number of parameters including signal strength and duration, subcellular localisation of the signal, maturation-restricted expression of downstream signalling effector elements/isoforms and modulation of signal by co-receptors. Thus, the combined signature of BCR signalling is likely to dictate the functional response and act as a developmental checkpoint for B-cell maturation.

Coligation of the B Cell Receptor with Complement Receptor Type 2 (CR2/CD21) Using Its Natural Ligand C3dg: Activation without Engagement of an Inhibitory Signaling Pathway

C3dg is a cleavage product of the C3 component of complement that can facilitate the coligation of the complement receptor 2 (CR2/CD21) with the BCR via C3dg/Ag complexes. This interaction can greatly amplify BCR-mediated signaling events and acts to lower the threshold for B cell activation. Although previous studies have used anti-CR2 Abs or used chimeric Ags in the context of BCR transgenic mice as surrogate C3d-containing ligands, we have used a physiological form of C3d to study signaling in B cells from wild-type C57BL/6 mice. We find that while CR2-enhanced BCR signaling causes intracellular Ca2+ mobilization and total pTyr phosphorylation of an intensity comparable to optimal BCR ligation using anti-IgM Abs, it does so with limited activation of inhibitory effectors (such as CD22, Src homology region 2 domain containing phosphatase 1, and SHIP-1) and without substantial receptor cross-linking. In summary, we demonstrate that CR2-enhanced BCR signaling may proceed not only through the previously described amplification of positive signaling pathways, but is potentially augmented by a lack of normal inhibitory/feedback signaling.

B Cell Selection and Susceptibility to Autoimmunity

Autoreactive B cells arise routinely as part of the naive B cell repertoire. The immune system employs several mechanisms in an attempt to silence these autoreactive cells before they achieve immunocompetence. The BCR plays a central role in B cell development, activation, survival, and apoptosis, and thus is a critical component of the regulation of both protective and autoreactive B cells. The strength of signal mediated by the BCR is determined by numerous factors, both B cell intrinsic and B cell extrinsic. Perturbations in the molecules that regulate the BCR signal strength or that activate pathways that engage in cross talk with the BCR-mediated signaling pathways can lead to the aberrant survival and activation of autoreactive B cells. In this review, we will discuss the some newly identified genetic loci and factors that modulate the BCR signal transduction pathway and, therefore, the regulation of autoreactive B cells. We will also provide evidence for a model of autoreactivity in which a reduction in the strength of the BCR signal allows the survival and the modulation of a naive B cell repertoire replete with autoreactivity.

Immunophenotypic and functional effects of bunker C fuel oil on the immune system of American mink (Mustela vison)

The relationship between exposure to environmental contaminants and immunotoxicity in vulnerable marine species is unknown. In this study, we used American mink (Mustela vision) as a surrogate species for the sea otter to examine the immunotoxic effects of chronic exposure to a low concentration of bunker C fuel oil (500 ppm admixed in the feed for 113-118 days). The mink immune system was monitored over time by flow cytometric analysis for alterations in the immunophenotype of blood lymphocytes and monocytes and by mitogen-stimulated proliferation assays for changes in peripheral blood mononuclear cell function. Fuel oil exposure caused a mild, yet significant (P < 0.05) increase in the absolute numbers of specific peripheral blood lymphocyte subsets (CD3+T cells) and monocytes, an increase in the level of expression of functionally significant cell surface proteins (MHC II, CD18), and an increase in mitogen-induced mononuclear cell proliferative responses. This heightened state of cellular activation along with the increase in specific cell surface protein expression on both the innate and adaptive immune cells is similar to the pro-inflammatory or "adjuvant-like" effect described in laboratory models of polycyclic aromatic hydrocarbon exposure in other species. These results show the benefits of using a controlled laboratory model for detecting and characterizing subtle petroleum oil-induced perturbations in immune responses. In addition this study establishes a framework for studying the effects of environmental petroleum oil exposure on the immune system of free-ranging marine mammals. Expansion of these studies to address biolgical significance is warranted.

Positive and negative selection during B lymphocyte development

Our laboratory is interested in a variety of issues related to lymphocyte development. More specifically, we have focused on the processes that regulate the decision to commit to the B lymphocyte (B cell) lineage, then the subsequent signals that are involved in maintaining this commitment to the B cell lineage. These signals result in the positive selection of those B cells that properly execute the complex genetic changes associated with B cell development, then trigger the elimination of B cells that are responsive to self-antigens and, therefore, possess the potential to mediate autoimmune disease. Our general experimental approach has been to address these issues from the perspective of signal transduction. Our goal is to define the biochemical and genetic processes that are integrated in order to accomplish these selection processes. To do so, we employ in vivo animal models as well as more defined in vitro studies, using both primary and transformed cell lines. For the past several years, we have been primarily interested in the precise mechanisms by which the B cell antigen receptor (BCR), and intermediate forms of this receptor, regulate these complex developmental processes. We have used the ongoing studies described below as two representative examples of how we are approaching these issues and some of the insights that we have made. To place both of these studies in context, we will begin with a brief introduction into B cell development.

Autonomous SHIP-dependent FcγR signaling in pre-B cells leads to inhibition of cell migration and induction of cell death

Mature B cells express a single immunoglobulin Fc receptor, FcgammaRIIB, that functions to block downstream signaling by co-aggregated antigen receptors. Co-aggregation of receptors is essential because BCR activated kinases must phosphorylate FcgammaRIIB to recruit SHIP and mediate inhibitory signals. Pre-B cells also express FcgammaRIIB, but since they do not yet express antigen receptor, it is unclear when they are activated physiologically. Here, we demonstrate that aggregation of the FcR on pre-B cells leads to potent inhibitory signaling. Aggregation of the FcR alone leads to downstream effects including the induction of cell death and the blockade of SDF-1 induced migration. The biochemical circuitry that mediates this response is unique because although SHIP is required for this signaling and is phosphorylated upon receptor aggregation, this occurs in the absence of FcgammaRIIB phosphorylation. Results indicate that immune complexes may inhibit B cell production in the bone marrow by antigen non-specific mechanisms.

B-cell homeostasis, competition, resources, and positive selection by self-antigens

In adult mice, the number of B lymphocytes remains constant under homeostatic control, in spite of the fact that B cells are produced continuously in numbers that largely exceed the number required to replenish the peripheral pools. It follows that each newly formed lymphocyte can only persist if another lymphocyte dies. In an immune system where the total number of cells is limited, cell survival is no longer a passive phenomenon but rather a continuous active process where each lymphocyte must compete with other lymphocytes to survive. Consequently, the number and the life expectancy of a B-cell clone vary according to the presence or absence of competitor populations. This process of lymphocyte competition is likely controlled by a common need for resources that are in limited supply. The number of peripheral B-cells varies according to the availability of B-cell receptor (BCR) ligands. Indeed, it is possible to modify steady-state B-cell numbers by antigen manipulation. Moreover, conventional self-reactive B cells can undergo positive selection. We showed that the fate of a self-reactive B cell is determined by the quantity of self-antigens, the number of antigen-specific receptors engaged, and its overall antigen-binding avidity rather than the affinity of individual BCRs.

A novel polymorphism in the Fcgamma receptor IIB (CD32B) transmembrane region alters receptor signaling

OBJECTIVE

The low-affinity receptor Fcgamma receptor IIb (FcgammaRIIb), with an immunoreceptor tyrosine-based inhibitory motif (ITIM) in the cytoplasmic domain, down-regulates humoral immune responses and modulates the risk of autoimmunity in animal models. The transmembrane domain of FcgammaRIIb may also contribute to receptor signaling. Therefore, we investigated the biologic significance of single-nucleotide polymorphisms (SNPs) throughout the coding region.

METHODS

Discovery of SNPs in FCGR2B was performed by direct cycle sequencing of complementary DNA samples derived by reverse transcriptase-polymerase chain reaction. To assess the biologic significance of the nonsynonymous transmembrane SNP, we studied 3 functions influenced by the FcgammaRIIb transmembrane domain: tyrosine dephosphorylation of CD19, inhibition of B cell receptor (BCR)-induced calcium response, and modulation of BCR- or anti-Fas-induced apoptosis.

RESULTS

The nonsynonymous C-to-T transition in the first cytoplasmic exon, originally reported in the Raji cell line, was not found in either the African-American or the Caucasian population, but a nonsynonymous T-to-C transition at nucleotide 775 in exon 4 of FCGR2B, which changes isoleucine to threonine at residue 187 in the transmembrane domain, was significantly more common in African Americans. Using the FcgammaRIIb-negative mouse B cell line IIA1.6, we expressed both allelic forms as both full-length and truncated cytoplasmic domain constructs. The FCGR2B-187T allele mediated a higher level of CD19 dephosphorylation (P = 0.029) and a greater degree of inhibition of the calcium response (P = 0.003) when co-engaged with BCR than did FCGR2B-187I, independent of the presence of the ITIM. In contrast, FcgammaRIIb modulation of BCR-induced and anti-Fas antibody-induced cell death rates were similar in IIA1.6 cells expressing either the 187I or the 187T allelic form.

CONCLUSION

The differential activity of FCGR2B alleles suggests a novel mechanism of FcgammaRIIb regulation that may influence the risk of autoimmune disease.

Clustering of the B Cell Receptor Is Not Required for the Apoptotic Response

We examined the role of BCR cell membrane redistribution in anti-IgM-induced apoptosis in three human B cell lines, RA#1, 2G6, and MC116, that differ in their relative levels of sIgM expression. The apoptotic response was found to be dependent on the nature of the anti-IgM and the cell line. In the cell lines, RA#1 and MC116, sIgM aggregated into patches that were insensitive to the disruption of cholesterol-rich membrane microdomains by nystatin or beta-MCD. The B cell line 2G6 was able to reorganize sIgM into a tight coalescent cap upon anti-IgM treatment. However, in this case, the lipid raft inhibitors nystatin and beta-MCD disrupted the patching. In 2G6 cells, BCR-mediated apoptosis was not affected by nystatin treatment, whereas it increased in beta-MCD pretreated cells. Thus, no evident correlation was found between apoptosis and BCR cell membrane redistribution or lipid raft formation in either of the three cell lines. The data indicate that the apoptotic signal transduction pathway is independent of BCR translocation into lipid rafts and/or aggregation.

Defective gene expression, S phase progression, and maturation during hematopoiesis in E2F1/E2F2 mutant mice

E2F plays critical roles in cell cycle progression by regulating the expression of genes involved in nucleotide synthesis, DNA replication, and cell cycle control. We show that the combined loss of E2F1 and E2F2 in mice leads to profound cell-autonomous defects in the hematopoietic development of multiple cell lineages. E2F2 mutant mice show erythroid maturation defects that are comparable with those observed in patients with megaloblastic anemia. Importantly, hematopoietic defects observed in E2F1/E2F2 double-knockout (DKO) mice appear to result from impeded S phase progression in hematopoietic progenitor cells. During DKO B-cell maturation, differentiation beyond the large pre-BII-cell stage is defective, presumably due to failed cell cycle exit, and the cells undergo apoptosis. However, apoptosis appears to be the consequence of failed maturation, not the cause. Despite the accumulation of hematopoietic progenitor cells in S phase, the combined loss of E2F1 and E2F2 results in significantly decreased expression and activities of several E2F target genes including cyclin A2. Our results indicate specific roles for E2F1 and E2F2 in the induction of E2F target genes, which contribute to efficient expansion and maturation of hematopoietic progenitor cells. Thus, E2F1 and E2F2 play essential and redundant roles in the proper coordination of cell cycle progression with differentiation which is necessary for efficient hematopoiesis.

Early gene activation in chronic leukemic B lymphocytes induced toward a plasma cell phenotype

Chronic lymphocytic leukemia (CLL) is characterized by the accumulation of lymphocytes that are arrested at an intermediate stage of B lymphocyte development. CLL B lymphocytes transform (mature) to a plasmacytic phenotype with loss of CD19 and CD20 and the appearance of cytoplasmic immunoglobulin when treated in vitro with phorbol esters. We have used array hybridization technology to describe gene expression patterns for untreated and tetradecanoyl phorbol acetate (TPA)-treated CLL B cells at 5, 10, and 20 min following initial TPA exposure. Three genes, early growth response factor 1 (EGR-1), dual specificity phosphatase 2, and CD69 (early T-cell activation antigen), showed a 2.0-fold or greater increase in mRNA transcription at four or more of six time points in two studies. Upregulation of expression of these genes was confirmed by real-time polymerase chain reaction in the TPA-treated cells of four CLL patients. A progressive increase in gene expression was observed during the 20-min time course for all three genes. In addition, protein expression of EGR-1 and CD69 was increased as measured by immunofluorescence cell analysis. Several genes (PKC, n-myc, jun D, and BCL-2) previously reported as overexpressed in CLL lymphocytes were overexpressed in these studies also, but were not altered by TPA treatment. Genes for proteins whose upregulation requires hours of TPA exposure (the 4F2hc component of the L-system amino acid transporter, prohibition, and hsp60) were assessed, and their later expression contrasted with the early expression of EGR-1, dual specificity phosphatase 2, and CD69. EGR-1 encodes a zinc-finger transcription factor that is induced by pokeweed mitogen and TPA and promotes B lymphocyte maturation. The dual specificity phosphatase 2 encodes an enzyme that reverses mitogen activated protein kinase cell activation by dephosphorylation. The CD69 protein is induced by TPA in thymocytes and is a type II transmembrane signaling molecule in hematopoietic cells. These findings suggest that the products of these three genes may be central to early steps in the TPA-induced evolution of CLL B cells to a plasmacytic phenotype.

Helper T-cell-regulated B-cell immunity

Helper T-cell-regulated B-cell responses constitute a major component of the immune response to many pathogens. Spatially and temporally organized cognate intercellular communication within secondary lymphoid organs is the critical regulating event in this complex adaptive response to antigen. Here, we discuss what is known of these molecular exchanges and their cellular consequences in a serial synapsis model of adaptive immunity.

The survival of antigen-stimulated T cells requires NFkappaB-mediated inhibition of p73 expression

We have explored the interactions between the NFkappaB and Cdk-Rb-E2F pathways in controlling T cell fate following antigen stimulation. The inhibition of NFkappaB in antigen-stimulated T cells results in apoptosis but does not inhibit E2F activation and S phase entry. IkappaB-induced apoptosis coincides with the superinduction of p73 expression and activity. G1 Cdk activity is required for IkappaB-induced apoptosis and the induction of p73. Importantly, p73 deficiency rescues activated T cells from the apoptosis resulting from the inhibition of NFkappaB. Thus, Cdk2 activation sends signals for both cell cycle progression and apoptosis, the latter of which must be blocked by NFkappaB to allow for proliferation.

BCR mediated signal transduction in immature and mature B cells

Ligation of B cell receptors (BCR) on immature B cells may induce apoptosis, while in mature B cells it stimulates cell activation and growth. The signaling pathway regulating the differential functional response, death or survival of the B cell is not fully characterized. We have tested the intracellular signaling requirement of these processes using B cells isolated from the spleen of irradiated auto-reconstituted (transitional immature B cells) and untreated mice (mature B cells), respectively. We compared the BCR induced intracellular [Ca2+] transient, protein tyrosine phosphorylation and ERK phosphorylation, furthermore, the activation of Elk-1 and CREB transcription factors. The BCR induced rise of intracellular [Ca2+] did not significantly differ in the two populations, only a slight difference in the late phase of the response was observed. Immature B cells responded with a maximum tyrosine phosphorylation to a five times lower dose of anti-IgM compared to the mature population. Most importantly, we have found a significant difference in the tyrosine phosphorylation of the Gab family adaptor proteins, Gab1/2. In contrast to mature B cells, crosslinking of BCR on immature B cells did not induce tyrosine phosphorylation of Gab2, thus the Gab2-organized signal amplification complex could not be produced. Furthermore, we detected a significant difference in the kinetics of BCR induced ERK, Elk-1 and CREB phosphorylation. In immature B cells, ERK was transiently phosphorylated, ceasing after 120 min, while in mature cells, ERK phosphorylation was sustained. Elk-1 and CREB activation was also transient in immature B cells, followed the kinetics of ERK phosphorylation. The lack of sustained Erk1/2 activation suppresses the transcription factors necessary for the proliferation signal. Since ERK is regulated by the phosphorylated Gab1/2, these data demonstrate that BCR triggered phosphorylation and signal amplification of Gab1/2 is a critical step in a life or death decision of B cells.

Impaired light chain allelic exclusion and lack of positive selection in immature B cells expressing incompetent receptor deficient of CD19

Positive signaling is now thought to be important for B cell maturation, although the nature of such signals has not yet been defined. We are studying the regulatory role of B cell Ag receptor (BCR) signaling in mediating positive selection of immature B cells. To do so, we use Ig transgenic mice (3-83Tg) that are deficient in CD19, thus generating a monoclonal immature B cell population expressing signaling-incompetent BCR. Immature 3-83Tg CD19(-/-) B cells undergo developmental arrest in the bone marrow, allowing maturation only to cells that effectively compensate for the compromised receptor by elevated levels of BCR. We find that developmentally arrested 3-83Tg CD19(-/-) B cells fail to impose L chain allelic exclusion and undergo intensive V(D)J recombination to edit their BCR. Furthermore, immature 3-83Tg CD19(-/-) B cells, which were grown in vitro, failed to undergo positive selection and to survive when adoptively transferred into normal recipients. However, elevation of BCR expression levels, obtained by transgene homozygosity, effectively compensated for the compromised BCR and completely restored BCR-mediated Ca(2+) influx, allelic exclusion, and positive selection. Our results suggest that the BCR signaling threshold mediates positive selection of developing B cells, and that a receptor-editing mechanism has an important role in rescuing cells that fail positive selection because of incompetent receptors.

Developmental differences in B cell receptor-induced signal transduction

We have compared early signaling events at various stages of B cell differentiation using established mouse cell lines. Clustering of pre-B cell antigen receptor (BCR) or BCR induced the tyrosine phosphorylation of various proteins in all cells, although the phosphorylation pattern differed. In spite of the pre-BCR-induced tyrosine phosphorylation, we could not detect an intracellular Ca(2+) signal in pre-B cells. However, co-clustering of the pre-BCR with CD19 did induce Ca(2+) mobilization. In contrast to the immature and mature B cells, where the B cell linker protein (BLNK) went through inducible tyrosine phosphorylation upon BCR clustering, we observed a constitutive tyrosine phosphorylation of BLNK in pre-B cell lines. Both BLNK and phospholipase C (PLC)gamma were raft associated in unstimulated pre-B cells, and this could not be enhanced by pre-BCR engagement, suggesting a ligand-independent PLC gamma-mediated signaling. Further results indicate that the cell lines representing the immature stage are more sensitive to BCR-, CD19- and type II receptors binding the Fc part of IgG (Fc gamma RIIb)-mediated signals than mature B cells.

Very low affinity B cells form germinal centers, become memory B cells, and participate in secondary immune responses when higher affinity competition is reduced

To understand the relationship between the affinity of the B cell antigen receptor (BCR) and the immune response to antigen, two lines of immunoglobulin H chain transgenic (Tg) mice were created. H50Gmu(a) and T1(V23)mu(a) mice express mu H chain transgenes that associate with the lambda1 L chains to bind the (4-hydroxy-3-nitrophenyl)acetyl hapten with association constants (K(a)s) of only 1.2 x 10(5) M(-1) and 3 x 10(4) M(-1), respectively. Both lines mounted substantial antibody-forming cell (AFC) and germinal center (GC) responses. H50Gmu(a) Tg mice also generated memory B cells. T1(V23)mu(a) B cells formed AFC and GCs, but were largely replaced in late GCs by antigen-specific cells that express endogenous BCRs. Thus, B lymphocytes carrying BCRs with affinities previously thought to be irrelevant in specific immune responses are in fact capable of complete T cell-dependent immune responses when relieved of substantial competition from other B cells. The failure to observe such B cells normally in late primary responses and in memory B cell populations is the result of competition, rather than an intrinsic inability of low affinity B cells.