The Ig-alpha ITAM is required for efficient differentiation but not proliferation of pre-B cells

The CH1 domain influences the expression and antigen sensing of the HIV-specific CH31 IgM-BCR and IgG-BCR

How different classes of the B cell antigen receptor (BCR) sense viral antigens used in vaccination protocols is poorly understood. Here, we study antigen binding and sensing of human Ramos B cells expressing a BCR of either the IgM or IgG1 class with specificity for the CD4-binding-site of the envelope (Env) protein of the HIV-1. Both BCRs carry an identical antigen binding site derived from the broad neutralizing antibody (bnAb) CH31. We find a five times higher expression of the IgG1-BCR in comparison to the IgM-BCR on the surface of transfected Ramos B cells. The two BCR classes also differ from each other in their interaction with cognate HIV Env antigens in that the IgG1-BCR and IgM-BCR bind preferentially to polyvalent and monovalent antigens, respectively. By generating an IgM/IgG1 chimeric BCR, we found that the class-specific BCR expression and antigen-sensing behavior can be transferred with the CH1γ domain from the IgG1-BCR to the IgM-BCR. Thus, the class of CH1 domain has an impact on BCR assembly and expression as well as on antigen sensing.

Antigen-independent, autonomous B cell receptor signaling drives activated B cell DLBCL

Diffuse large B cell lymphoma of activated B cell type (ABC-DLBCL), a major cell-of-origin DLBCL subtype, is characterized by chronic active B cell receptor (BCR) signaling and NF-κB activation, which can be explained by activating mutations of the BCR signaling cascade in a minority of cases. We demonstrate that autonomous BCR signaling, akin to its essential pathogenetic role in chronic lymphocytic leukemia (CLL), can explain chronic active BCR signaling in ABC-DLBCL. 13 of 18 tested DLBCL-derived BCR, including 12 cases selected for expression of IgM, induced spontaneous calcium flux and increased phosphorylation of the BCR signaling cascade in murine triple knockout pre-B cells without antigenic stimulation or external BCR crosslinking. Autonomous BCR signaling was associated with IgM isotype, dependent on somatic BCR mutations and individual HCDR3 sequences, and largely restricted to non-GCB DLBCL. Autonomous BCR signaling represents a novel immunological oncogenic driver mechanism in DLBCL originating from individual BCR sequences and adds a new dimension to currently proposed genetics- and transcriptomics-based DLBCL classifications.

Human and mouse early B cell development: So similar but so different

Early B cell development in the bone marrow ensures the replenishment of the peripheral B cell pool. Immature B cells continuously develop from hematopoietic stem cells, in a process guided by an intricate network of transcription factors as well as chemokine and cytokine signals. Humans and mice possess somewhat similar regulatory mechanisms of B lymphopoiesis. The continuous discovery of monogenetic defects that impact early B cell development in humans substantiates the similarities and differences with B cell development in mice. These differences become relevant when targeted therapeutic approaches are used in patients; therefore, predicting potential immunological adverse events is crucial. In this review, we have provided a phenotypical classification of human and murine early progenitors and B cell stages, based on surface and intracellular protein expression. Further, we have critically compared the role of key transcription factors (Ikaros, E2A, EBF1, PAX5, and Aiolos) and chemo- or cytokine signals (FLT3, c-kit, IL-7R, and CXCR4) during homeostatic and aberrant B lymphopoiesis in both humans and mice.

Spatial Stochastic Model of the Pre-B Cell Receptor

Survival and proliferation of immature B lymphocytes requires expression and tonic signaling of the pre-B cell receptor (pre-BCR). This low level, ligand-independent signaling is likely achieved through frequent, but short-lived, homo interactions. Tonic signaling is also central in the pathology of precursor B acute lymphoblastic leukemia (B-ALL). In order to understand how repeated, transient events can lead to sustained signaling and to assess the impact of receptor accumulation induced by the membrane landscape, we developed a spatial stochastic model of receptor aggregation and downstream signaling events. Our rule- and agent-based model builds on previous mature BCR signaling models and incorporates novel parameters derived from single particle tracking of pre-BCR on surfaces of two different B-ALL cell lines, 697 and Nalm6. Live cell tracking of receptors on the two cell lines revealed characteristic differences in their dimer dissociation rates and diffusion coefficients. We report here that these differences affect pre-BCR aggregation and consequent signal initiation events. Receptors on Nalm6 cells, which have a lower off-rate and lower diffusion coefficient, more frequently form higher order oligomers than pre-BCR on 697 cells, resulting in higher levels of downstream phosphorylation in the Nalm6 cell line.

Defective Allelic Exclusion by IgD in the Absence of Autoantigen

A considerable proportion of peripheral B cells is autoreactive, and it is unclear how the activation of such potentially harmful cells is regulated. In this study, we show that the different activation thresholds or IgM and IgD BCRs adjust B cell activation to the diverse requirements during development. We rely on the autoreactive 3-83 model BCR to generate and analyze mice expressing exclusively autoreactive IgD BCRs on two different backgrounds that determine two stages of autoreactivity, depending on the presence or absence of the cognate Ag. By comparing these models with IgM-expressing control mice, we found that, compared with IgM, IgD has a higher activation threshold in vivo, as it requires autoantigen to enable normal B cell development, including allelic exclusion. Our data indicate that IgM provides the high sensitivity required during early developmental stages to trigger editing of any autoreactive specificities, including those enabling weak interaction with autoantigen. In contrast, IgD has the unique ability to neglect weakly interacting autoantigens while retaining reactivity to higher-affinity Ag. This IgD function enables mature B cells to ignore autoantigens while remaining able to efficiently respond to foreign threats.

An optimized derivative of an endogenous CXCR4 antagonist prevents atopic dermatitis and airway inflammation

Aberrant CXCR4/CXCL12 signaling is involved in many pathophysiological processes such as cancer and inflammatory diseases. A natural fragment of serum albumin, named EPI-X4, has previously been identified as endogenous peptide antagonist and inverse agonist of CXCR4 and is a promising compound for the development of improved analogues for the therapy of CXCR4-associated diseases. To generate optimized EPI-X4 derivatives we here performed molecular docking analysis to identify key interaction motifs of EPI-X4/CXCR4. Subsequent rational drug design allowed to increase the anti-CXCR4 activity of EPI-X4. The EPI-X4 derivative JM#21 bound CXCR4 and suppressed CXCR4-tropic HIV-1 infection more efficiently than the clinically approved small molecule CXCR4 antagonist AMD3100. EPI-X4 JM#21 did not exert toxic effects in zebrafish embryos and suppressed allergen-induced infiltration of eosinophils and other immune cells into the airways of animals in an asthma mouse model. Moreover, topical administration of the optimized EPI-X4 derivative efficiently prevented inflammation of the skin in a mouse model of atopic dermatitis. Thus, rationally designed EPI-X4 JM#21 is a novel potent antagonist of CXCR4 and the first CXCR4 inhibitor with therapeutic efficacy in atopic dermatitis. Further clinical development of this new class of CXCR4 antagonists for the therapy of atopic dermatitis, asthma and other CXCR4-associated diseases is highly warranted.

Adenosine-producing regulatory B cells in head and neck cancer

Background

Multiple mechanisms of immunosuppression have been identified in the tumor microenvironment including regulatory B cells (B_reg). Recently, we have shown that B_reg suppress T cell function by production of adenosine (ADO). However, the autocrine effect of ADO on B cells and the role of B_reg in head and neck cancer remains unclear.

Methods

Blood (n = 42) and tumor tissue (n = 39) of head and neck cancer patients and healthy donors (n = 60) were analyzed by FACS. The effect of ADO on phenotype, intracellular signaling pathways, Ca²⁺ influx and ADO production was analyzed in B_reg and effector B cells (B_eff) by FACS, luminescence and mass spectrometry. The blockage of the ADO receptor A_2A was analyzed in a murine head and neck cancer model.

Results

ADO-producing B_reg were found in tumor tissue and peripheral blood. ADO inhibited the intracellular Bruton’s tyrosine kinase (BTK) and Ca²⁺ influx only in B_eff. The inhibition of BTK by ibrutinib mimicked the effect of ADO, and ibrutinib reduced the production of ADO by downregulation of CD39 in vitro. The inhibition of ADO receptor A_2A significantly reduced tumor mass and increased B cell infiltration, in vivo.

Conclusion

Our data demonstrate the presence of a novel ADO-producing B_reg population within the tumor microenvironment in mice and humans. A new model is proposed on how ADO-producing B_reg can influence the function of B_eff cells in healthy donors and cancer patients. Thus, the modulation of the ADO pathway in B cells may serve as a therapeutic approach for cancer patients.

Electronic supplementary material

The online version of this article (10.1007/s00262-020-02535-6) contains supplementary material, which is available to authorized users.

A new branched proximity hybridization assay for the quantification of nanoscale protein-protein proximity

Membrane proteins are organized in nanoscale compartments. Their reorganization plays a crucial role in receptor activation and cell signaling. To monitor the organization and reorganization of membrane proteins, we developed a new branched proximity hybridization assay (bPHA) allowing better quantification of the nanoscale protein-protein proximity. In this assay, oligo-coupled binding probes, such as aptamer, nanobody, and antibodies, are used to translate the proximity of target proteins to the proximity of oligos. The closely positioned oligos then serve as a template for a maximum of 400-fold branched DNA (bDNA) signal amplification. The amplified bPHA signal is recorded by flow cytometer, thus enabling proximity studies with high throughput, multiplexing, and single-cell resolution. To demonstrate the potential of the bPHA method, we measured the reorganization of the immunoglobulin M (IgM)- and immunoglobulin D (IgD)-class B cell antigen receptor (BCR) on the plasma membrane and the recruitment of spleen tyrosine kinase (Syk) to the BCR upon B lymphocyte activation.

PI3K-Mediated Blimp-1 Activation Controls B Cell Selection and Homeostasis

Activation of phosphoinositide 3-kinase (PI3K) signaling plays a central role in regulating proliferation and survival of B cells. Here, we tested the hypothesis that B cell receptor (BCR)-mediated activation of PI3K induces the terminal differentiation factor Blimp-1 that interferes with proliferation and survival, thereby controlling the expansion of activated B cells. In fact, B-cell-specific inactivation of Pten, the negative regulator of PI3K signaling, leads to deregulated PI3K activity and elevated Blimp-1 expression. Combined deficiency for Pten and Blimp-1 results in abnormal expansion of B-1 B cells and splenomegaly. Interestingly, Blimp-1 also acts at early stages of B cell development to regulate B cell selection, as Blimp-1 deficiency results in an increased proportion of autoreactive B cells. Together, our data suggest that the combined requirement of deregulated PI3K signaling in addition to defective terminal differentiation represents the basis for proper selection and expansion of developing B cells.

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B cell expansion is normal despite increased PI3K activity after Pten deletion

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Deregulated PI3K induces Blimp-1 and leads to premature terminal differentiation

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Premature terminal differentiation prevents expansion of activated B cells

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Expansion of B-1 B cells by autoreactive BCR and defective terminal differentiation

A symmetric geometry of transmembrane domains inside the B cell antigen receptor complex

The specific activation of B lymphocytes via the binding of antigen to their B cell antigen receptor (BCR) is of central importance for the establishment of humoral immunity and a successful vaccination. A better understanding of the antigen sensing process of B cells requires insight into the structure of the BCR comprising the mIg molecule and the Igα/Igβ heterodimer in a 1:1 complex. How a symmetric molecule such as the mIg molecule is asymmetrically associated with only one Igα/Igβ heterodimer has been a puzzle. We suggest that inside the lipid bilayer the BCR forms a symmetric Igα-mHC:mHC-Igβ complex. Our results give insight into the BCR structure and the B cell activation mechanism.

B lymphocytes have the ability to sense thousands of structurally different antigens and produce cognate antibodies against these molecules. For this they carry on their surface multiple copies of the B cell antigen receptor (BCR) comprising the membrane-bound Ig (mIg) molecule and the Igα/Igβ heterodimer functioning as antigen binding and signal transducing components, respectively. The mIg is a symmetric complex of 2 identical membrane-bound heavy chains (mHC) and 2 identical light chains. How the symmetric mIg molecule is asymmetrically associated with only one Igα/Igβ heterodimer has been a puzzle. Here we describe that Igα and Igβ both carry on one side of their α-helical transmembrane domain a conserved amino acid motif. By a mutational analysis in combination with a BCR rebuilding approach, we show that this motif is required for the retention of unassembled Igα or Igβ molecules inside the endoplasmic reticulum and the binding of the Igα/Igβ heterodimer to the mIg molecule. We suggest that the BCR forms within the lipid bilayer of the membrane a symmetric Igα-mHC:mHC-Igβ complex that is stabilized by an aromatic proline-tyrosine interaction. Outside the membrane this symmetry is broken by the disulfide-bridged dimerization of the extracellular Ig domains of Igα and Igβ. However, symmetry of the receptor can be regained by a dimerization of 2 BCR complexes as suggested by the dissociation activation model.

Pten controls B-cell responsiveness and germinal center reaction by regulating the expression of IgD BCR

In contrast to other B‐cell antigen receptor (BCR) classes, the function of IgD BCR on mature B cells remains largely elusive as mature B cells co‐express IgM, which is sufficient for development, survival, and activation of B cells. Here, we show that IgD expression is regulated by the forkhead box transcription factor FoxO1, thereby shifting the responsiveness of mature B cells towards recognition of multivalent antigen. FoxO1 is repressed by phosphoinositide 3‐kinase (PI3K) signaling and requires the lipid phosphatase Pten for its activation. Consequently, Pten‐deficient B cells expressing knock‐ins for BCR heavy and light chain genes are unable to upregulate IgD. Furthermore, in the presence of autoantigen, Pten‐deficient B cells cannot eliminate the autoreactive BCR specificity by secondary light chain gene recombination. Instead, Pten‐deficient B cells downregulate BCR expression and become unresponsive to further BCR‐mediated stimulation. Notably, we observed a delayed germinal center (GC) reaction by IgD‐deficient B cells after immunization with trinitrophenyl‐ovalbumin (TNP‐Ova), a commonly used antigen for T‐cell‐dependent antibody responses. Together, our data suggest that the activation of IgD expression by Pten/FoxO1 results in mature B cells that are selectively responsive to multivalent antigen and are capable of initiating rapid GC reactions and T‐cell‐dependent antibody responses.

Continuous signaling of CD79b and CD19 is required for the fitness of Burkitt lymphoma B cells

Expression of the B-cell antigen receptor (BCR) is essential not only for the development but also for the maintenance of mature B cells. Similarly, many B-cell lymphomas, including Burkitt lymphoma (BL), require continuous BCR signaling for their tumor growth. This growth is driven by immunoreceptor tyrosine-based activation motif (ITAM) and PI3 kinase (PI3K) signaling. Here, we employ CRISPR/Cas9 to delete BCR and B-cell co-receptor genes in the human BL cell line Ramos. We find that Ramos B cells require the expression of the BCR signaling component Igβ (CD79b), and the co-receptor CD19, for their fitness and competitive growth in culture. Furthermore, we show that in the absence of any other BCR component, Igβ can be expressed on the B-cell surface, where it is found in close proximity to CD19 and signals in an ITAM-dependent manner. These data suggest that Igβ and CD19 are part of an alternative B-cell signaling module that use continuous ITAM/PI3K signaling to promote the survival of B lymphoma and normal B cells.

Molecular requirements of the B-cell antigen receptor for sensing monovalent antigens

How the B‐cell antigen receptor (BCR) is activated upon interaction with its cognate antigen or with anti‐BCR antibodies is not fully understood. We have recently shown that B‐cell activation is accompanied by the opening of the pre‐organized BCR oligomers, an observation that strengthens the role of receptor reorganization in signalling. We have now analysed the BCR oligomer opening and signalling upon treatment with different monovalent stimuli. Our results indicate that monovalent antigens are able to disturb and open the BCR oligomer, but that this requires the presence and activity of the Src family kinase (SFK) Lyn. We have also shown that monovalent Fab fragments of anti‐BCR antibodies can open the BCR oligomers as long as they directly interact with the antigen‐binding site. We found that monovalent antigen binding opens both the IgM‐BCR and IgD‐BCR, but calcium signalling is only seen in cells expressing IgM‐BCR; this provides a molecular basis for IgM‐ and IgD‐BCR functional segregation.

Lectins from opportunistic bacteria interact with acquired variable-region glycans of surface immunoglobulin in follicular lymphoma

B-cell antigen receptor (BCR) expression is a key feature of most B-cell lymphomas, but the mechanisms of BCR signal induction and the involvement of autoantigen recognition remain unclear. In follicular lymphoma (FL) B cells, BCR expression is retained despite a chromosomal translocation that links the antiapoptotic gene BCL2 to the regulatory elements of immunoglobulin genes, thereby disrupting 1 heavy-chain allele. A remarkable feature of FL-BCRs is the acquisition of potential N-glycosylation sites during somatic hypermutation. The introduced glycans carry mannose termini, which create potential novel binding sites for mannose-specific lectins. Here, we investigated the effect of N-linked variable-region glycosylation for BCR interaction with cognate antigen and with lectins of different origins. N-glycans were found to severely impair BCR specificity and affinity to the initial cognate antigen. In addition, we found that lectins from Pseudomonas aeruginosa and Burkholderia cenocepacia bind and stimulate FL cells. Human exposure to these bacteria can occur by contact with soil and water. In addition, they represent opportunistic pathogens in susceptible hosts. Understanding the role of bacterial lectins might elucidate the pathogenesis of FL and establish novel therapeutic approaches.

Chronic lymphocytic leukaemia is driven by antigen-independent cell-autonomous signalling

B-cell antigen receptor (BCR) expression is an important feature of chronic lymphocytic leukaemia (CLL), one of the most prevalent B-cell neoplasias in Western countries. The presence of stereotyped and quasi-identical BCRs in different CLL patients suggests that recognition of specific antigens might drive CLL pathogenesis. Here we show that, in contrast to other B-cell neoplasias, CLL-derived BCRs induce antigen-independent cell-autonomous signalling, which is dependent on the heavy-chain complementarity-determining region (HCDR3) and an internal epitope of the BCR. Indeed, transferring the HCDR3 of a CLL-derived BCR provides autonomous signalling capacity to a non-autonomously active BCR, whereas mutations in the internal epitope abolish this capacity. Because BCR expression was required for the binding of secreted CLL-derived BCRs to target cells, and mutations in the internal epitope reduced this binding, our results indicate a new model for CLL pathogenesis, with cell-autonomous antigen-independent signalling as a crucial pathogenic mechanism.

Evolution of immunity: no development without risk

Signal transduction by cell surface receptors in the context of heterogeneous and variable cellular environments plays a pivotal role in regulating many biological processes, including development, activation, and homeostasis of the immune system. In some receptors, extracellular ligand-binding and intracellular signaling domains are located on the same protein chain (single-chain receptors), while in the so-called multichain immune recognition receptors (MIRRs), recognition and signaling functions are separated between different protein chains. Why did nature separate recognition and signaling functions for MIRRs, thereby increasing the risk of malfunction and potential attack by pathogens? The risk is real: in order to escape the immune response, viruses are able to disrupt functional coupling between recognition and signaling aspects of MIRR machinery. Intrinsic disorder of intracellular signal-generating regions of MIRRs adds further intrigue to the story. Why did nature select protein disorder for MIRRs to translate recognition of distinct antigens into appropriate activation signals that would induce specific functional outcomes? Here, I suggest that nature takes the risks associated with intrareceptor separation of functions as well as with the chaos and indeterminacy of protein disorder in exchange for providing diversity and variability of signal transduction. Not only does this phenomenon serve as the molecular basis for the development and evolution of the immune and other complex biological systems, but it fits closely to Darwinian evolutionary biology.

Cells diversify transmembrane signaling through the controlled chaos of protein disorder

Cell surface receptors function to transduce signals across the cell membrane leading to a variety of biologic responses. Structurally, these integral proteins can be classified into two main families, depending on whether extracellular ligand-binding and intracellular signaling domains are located on the same protein chain (single-chain receptors, SRs) or on separate subunits (multichain receptors, MRs). Since most MRs are immune receptors, they are all commonly referred to as multi-chain immune recognition receptors (MIRRs). Recent studies reveal that, in contrast to well-structured signaling domains of SRs, those of MIRRs represent intrinsically disordered regions, the regions that lack a well-defined three-dimensional structure under physiological conditions. Why did nature separate recognition and signaling functions of MIRRs? Why for MIRRs did nature select to provide highly specific signaling through the chaos of protein disorder? What mechanisms could control this chaos in the process of transmembrane signal transduction to provide the specificity and diversity of the immune response? Here, I summarize recent findings that may not only shed light on these and other questions but also add significantly to our understanding of receptor signaling, a fundamental process that plays a critical role in health and disease.

Differential occurrence of protein intrinsic disorder in the cytoplasmic signaling domains of cell receptors

In human membrane proteins, intrinsically disordered regions, the regions that lack a well-defined three-dimensional structure under physiological conditions, preferentially occur in the cytoplasmic tails. Many of these proteins represent cell receptors that function by recognizing their cognate ligand outside the cell and translating this binding information into an intracellular activation signal. Based on location of recognition and signaling (effector) domains, functionally diverse and unrelated cell receptors can be classified into two main families: those in which binding and signaling domains are located on the same protein chain, the so-called single-chain receptors (SRs), and those in which these domains are intriguingly located on separate subunits, the so-called multichain receptors (MRs). Recognition domains of both SRs and MRs are known to be well ordered. In contrast, while cytoplasmic signaling domains of SRs are well-structured as well, those of MRs are intrinsically disordered. Despite important role of receptor signaling in health and disease, extensive comparative structural analysis of receptor signaling domains has not been carried out as of yet. In this study, using a variety of prediction algorithms, we show that protein disorder is a characteristic and distinctive feature of receptors with recognition and signaling functions distributed between separate protein chains. We also reveal that disorder distribution patterns are rather similar within SR subclasses suggesting potential functional explanations. Why did nature select protein disorder to provide intracellular signaling for MRs? Is there any correlation between disorder profiles of signaling domains and receptor function? These and other questions are addressed in this article.

N-linked glycosylation selectively regulates autonomous precursor BCR function

Developing B cells express distinct classes of B cell antigen receptors (BCRs) that differ in their heavy chain (HC). Although only muHC is expressed in early stages, deltaHC-containing BCRs dominate on the surface of mature B cells. The reason for the tightly regulated expression of these receptors is poorly understood. Here we show that muHC was specifically required for precursor BCR (pre-BCR) function and that deltaHC was unable to form a functional pre-BCR. A conserved asparagine (N)-linked glycosylation site at position 46 (N46) in the first conserved domain of muHC was absolutely required for pre-BCR function, and swapping that domain with deltaHC resulted in a functional deltaHC-containing pre-BCR. When tested in the context of the BCR, muHC with a mutant N46 showed normal function, which indicated that N46-glycosylation is specifically required for pre-BCR function. Our results suggest an unexpected mode of pre-BCR function, in which binding of the surrogate light chain to N46 mediates autonomous crosslinking and, concomitantly, receptor formation.

The SCHOOL of nature: I. Transmembrane signaling

Receptor-mediated transmembrane signaling plays an important role in health and disease. Recent significant advances in our understanding of the molecular mechanisms linking ligand binding to receptor activation revealed previously unrecognized striking similarities in the basic structural principles of function of numerous cell surface receptors. In this work, I demonstrate that the Signaling Chain Homooligomerization (SCHOOL)-based mechanism represents a general biological mechanism of transmembrane signal transduction mediated by a variety of functionally unrelated single- and multichain activating receptors. within the SCHOOL platform, ligand binding-induced receptor clustering is translated across the membrane into protein oligomerization in cytoplasmic milieu. This platform resolves a long-standing puzzle in transmembrane signal transduction and reveals the major driving forces coupling recognition and activation functions at the level of protein-protein interactions-biochemical processes that can be influenced and controlled. The basic principles of transmembrane signaling learned from the SCHOOL model can be used in different fields of immunology, virology, molecular and cell biology and others to describe, explain and predict various phenomena and processes mediated by a variety of functionally diverse and unrelated receptors. Beyond providing novel perspectives for fundamental research, the platform opens new avenues for drug discovery and development.

Regulation of B-cell proliferation and differentiation by pre-B-cell receptor signalling

The pre-B-cell receptor (pre-BCR) is expressed following the productive recombination of the immunoglobulin heavy chain gene. Signals through the pre-BCR are required for initiating diverse processes in pre-B cells, including proliferation and recombination of the light chain gene, which eventually lead to the differentiation of pre-B cells to immature B cells. However, the molecular mechanisms by which the pre-BCR promotes these processes remain largely unresolved. Recent findings suggest that forkhead box O (FOXO) transcription factors connect pre-BCR signalling to the activation of the recombination machinery. In this Review, we discuss how FOXO transcription factors are regulated by the pre-BCR to allow the progression of the cell cycle and the recombination of the light chain gene.

Autoreactive B cell receptors mimic autonomous pre-B cell receptor signaling and induce proliferation of early B cells

The majority of early immature B cells express autoreactive B cell receptors (BCRs) that are, according to the current view, negatively selected to avoid the production of self-reactive antibodies. Here, we show that polyreactive BCRs, which recognize multiple self-antigens, induced autonomous signaling and selective expansion of B cell precursors in a manner comparable to the pre-BCR. We found that the pre-BCR was capable of recognizing multiple self-antigens and that a signaling-deficient pre-BCR lacking the non-Ig region of the surrogate-light-chain component lambda5 was rescued by the complementarity-determining region 3 derived from heavy chains of polyreactive receptors. Importantly, bone marrow B cells from mice carrying Ig transgenes for an autoreactive BCR showed increased cell-cycle activity, which could not be detected in cells lacking the transgenic BCR. Together, the pre-BCR has evolved to ensure self-recognition because autoreactivity is required for positive selection of B cell precursors.

SLP-65 regulates immunoglobulin light chain gene recombination through the PI(3)K-PKB-Foxo pathway

Although the essential role of the adaptor protein SLP-65 in pre-B cell differentiation is established, the molecular mechanism underlying its function is poorly understood. In this study, we uncover a link between SLP-65-dependent signaling and the phosphoinositide-3-OH kinase (PI(3)K)-protein kinase B (PKB)-Foxo pathway. We show that the forkhead box transcription factor Foxo3a promotes light chain rearrangement in pre-B cells. Our data suggest that PKB suppresses light chain recombination by phosphorylating Foxo proteins, whereas reconstitution of SLP-65 function counteracts PKB activation and promotes Foxo3a and Foxo1 activity in pre-B cells. Together, these data illuminate a molecular function of SLP-65 and identify a key role for Foxo proteins in the regulation of light chain recombination, receptor editing and B cell selection.

The kinase Syk as an adaptor controlling sustained calcium signalling and B-cell development

Upon B-cell antigen receptor (BCR) activation, the protein tyrosine kinase Syk phosphorylates the adaptor protein SH2 domain-containing leukocyte protein of 65 kDa (SLP-65), thus coupling the BCR to diverse signalling pathways. Here, we report that SLP-65 is not only a downstream target and substrate of Syk but also a direct binding-partner and activator of this kinase. This positive feedback is mediated by the binding of the SH2 domain of SLP-65 to an autophosphorylated tyrosine of Syk. The mutant B cells that cannot form the Syk/SLP-65 complex are defective in BCR-induced extracellular signal-regulated kinase, nuclear factor kappa B and nuclear factor of activated T cells, but not Akt activation, and are blocked in B-cell development. Furthermore, we show that formation of the Syk/SLP-65 complex is required for sustained Ca(2+) responses in activated B cells. We suggest that after activation and internalization of the BCR, Syk remains active as part of a membrane-bound Syk/SLP-65 complex controlling sustained signalling and calcium influx.

SCHOOL model and new targeting strategies

Protein-protein interactions play a central role in biological processes and thus are an appealing target for innovative drug design a nd development. They can be targeted bysmall molecule inhibitors, peptides and peptidomimetics, which represent an alternative to protein therapeutics that carry many disadvantages. In this chapter, I describe specific protein-protein interactions suggested by a novel model of immune signaling, the Signaling Chain HOmoOLigomerization (SCHOOL) model, to be critical for cell activation mediated by multichain immune recognition receptors (MIRRs) expressed on different cells of the hematopoietic system. Unraveling a long-standing mystery of MIRR triggering and transmembrane signaling, the SCHOOL model reveals the intrareceptor transmembrane interactions and interreceptor cytoplasmic homointeractions as universal therapeutic targets for a diverse variety of disorders mediated by immune cells. Further, assuming that the general principles underlying MIRR-mediated transmembrane signaling mechanisms are similar, the SCHOOL model can be applied to any particular receptor of the MIRR family. Thus, an important application of the SCHOOL model is that global therapeutic strategies targeting key protein-protein interactions involved in MIRR triggering and transmembrane signal transduction may be used to treat a diverse set of immune-mediated diseases. This assumes that clinical knowledge and therapeutic strategies can be transferred between seemingly disparate disorders, such as T-cell-mediated skin diseases and platelet disorders, or combined to develop novel pharmacological approaches. Intriguingly, the SCHOOL model unravels the molecular mechanisms underlying ability of different human viruses such as human immunodeficiency virus, cytomegalovirus and severe acute respiratory syndrome coronavirus to modulate and/or escape the host immune response. It also demonstrates how the lessons learned from viral pathogenesis can be used practically for rational drug design. Application of this model to platelet collagen receptor signaling has already led to the development of a novel concept of platelet inhibition and the invention of new platelet inhibitors, thus proving the suggested hypothesis and highlighting the importance and broad perspectives of the SCHOOL model in the development of new targeting strategies.

Conventional Light Chains Inhibit the Autonomous Signaling Capacity of the B Cell Receptor

Signals from the B cell antigen receptor (BCR), consisting of mu heavy chain (muHC) and conventional light chain (LC), and its precursor the pre-BCR, consisting of muHC and surrogate light chain (SLC), via the adaptor protein SLP-65 regulate the development and function of B cells. Here, we compare the effect of SLC and conventional LC expression on receptor-induced Ca(2+) flux in B cells expressing an inducible form of SLP-65. We found that SLC expression strongly enhanced an autonomous ability of muHC to induce Ca(2+) flux irrespective of additional receptor crosslinking. In contrast, LC expression reduced this autonomous muHC ability and resulted in antigen-dependent Ca(2+) flux. These data indicate that autonomous ligand-independent signaling can be induced by receptor forms other than the pre-BCR. In addition, our data suggest that conventional LCs play an important role in the inhibition of autonomous receptor signaling, thereby allowing further B cell differentiation.