Discovering immunoreceptor coupling and organization motifs

The recently determined cryo-EM structures of the T cell antigen receptor (TCR) and B cell antigen receptor (BCR) show in molecular details the interactions of the ligand-binding part with the signaling subunits but they do not reveal the signaling mechanism of these antigen receptors. Without knowing the molecular basis of antigen sensing by these receptors, a rational design of optimal vaccines is not possible. The existence of conserved amino acids (AAs) that are not involved in the subunit interaction suggests that antigen receptors form higher complexes and/or have lateral interactors that control their activity. Here, I describe evolutionary conserved leucine zipper (LZ) motifs within the transmembrane domains (TMD) of antigen and coreceptor components that are likely to be involved in the oligomerization and lateral interaction of antigen receptor complexes on T and B cells. These immunoreceptor coupling and organization motifs (ICOMs) are also found within the TMDs of other important receptor types and viral envelope proteins. This discovery suggests that antigen receptors do not function as isolated entities but rather as part of an ICOM-based interactome that controls their nanoscale organization on resting cells and their dynamic remodeling on activated lymphocytes.

Plasma membrane topography governs the 3D dynamic localization of IgM B cell antigen receptor clusters

B lymphocytes recognize bacterial or viral antigens via different classes of the B cell antigen receptor (BCR). Protrusive structures termed microvilli cover lymphocyte surfaces, and are thought to perform sensory functions in screening antigen-bearing surfaces. Here, we have used lattice light-sheet microscopy in combination with tailored custom-built 4D image analysis to study the cell-surface topography of B cells of the Ramos Burkitt's Lymphoma line and the spatiotemporal organization of the IgM-BCR. Ramos B-cell surfaces were found to form dynamic networks of elevated ridges bridging individual microvilli. A fraction of membrane-localized IgM-BCR was found in clusters, which were mainly associated with the ridges and the microvilli. The dynamic ridge-network organization and the IgM-BCR cluster mobility were linked, and both were controlled by Arp2/3 complex activity. Our results suggest that dynamic topographical features of the cell surface govern the localization and transport of IgM-BCR clusters to facilitate antigen screening by B cells.

Structural principles of B cell antigen receptor assembly

The B cell antigen receptor (BCR) is composed of a membrane-bound class M, D, G, E or A immunoglobulin for antigen recognition1-3 and a disulfide-linked Igα (also known as CD79A) and Igβ (also known as CD79B) heterodimer (Igα/β) that functions as the signalling entity through intracellular immunoreceptor tyrosine-based activation motifs (ITAMs)4,5. The organizing principle of the BCR remains unknown. Here we report cryo-electron microscopy structures of mouse full-length IgM BCR and its Fab-deleted form. At the ectodomain (ECD), the Igα/β heterodimer mainly uses Igα to associate with Cµ3 and Cµ4 domains of one heavy chain (µHC) while leaving the other heavy chain (µHC') unbound. The transmembrane domain (TMD) helices of µHC and µHC' interact with those of the Igα/β heterodimer to form a tight four-helix bundle. The asymmetry at the TMD prevents the recruitment of two Igα/β heterodimers. Notably, the connecting peptide between the ECD and TMD of µHC intervenes in between those of Igα and Igβ to guide TMD assembly through charge complementarity. Weaker but distinct density for the Igβ ITAM nestles next to the TMD, suggesting potential autoinhibition of ITAM phosphorylation. Interfacial analyses suggest that all BCR classes utilize a general organizational architecture. Our studies provide a structural platform for understanding B cell signalling and designing rational therapies against BCR-mediated diseases.

Human CD38 regulates B cell antigen receptor dynamic organization in normal and malignant B cells

CD38 is a multifunctional protein expressed on the surface of B cells in healthy individuals but also in B cell malignancies. Previous studies have suggested a connection between CD38 and components of the IgM class B cell antigen receptor (IgM-BCR) and its coreceptor complex. Here, we provide evidence that CD38 is closely associated with CD19 in resting B cells and with the IgM-BCR upon engagement. We show that targeting CD38 with an antibody, or removing this molecule with CRISPR/Cas9, inhibits the association of CD19 with the IgM-BCR, impairing BCR signaling in normal and malignant B cells. Together, our data suggest that CD38 is a new member of the BCR coreceptor complex, where it exerts a modulatory effect on B cell activation upon antigen recognition by regulating CD19. Our study also reveals a new mechanism where α-CD38 antibodies could be a valuable option in therapeutic approaches to B cell malignancies driven by aberrant BCR signaling.

B cells expressing IgM B cell receptors of HIV-1 neutralizing antibodies discriminate antigen affinities by sensing binding association rates

HIV-1 envelope (Env) proteins designed to induce neutralizing antibody responses allow study of the role of affinities (equilibrium dissociation constant [K_D]) and kinetic rates (association/dissociation rates) on B cell antigen recognition. It is unclear whether affinity discrimination during B cell activation is based solely on Env protein binding K_D and whether B cells discriminate among proteins of similar affinities that bind with different kinetic rates. Here, we use a panel of Env proteins and Ramos B cell lines expressing immunoglobulin M (IgM) B cell receptors (BCRs) with specificity for CD4-binding-site broadly neutralizing antibodies to study the role of antigen binding kinetic rates on both early (proximal/distal signaling) and late events (BCR/antigen internalization) in B cell activation. Our results support a kinetic model for B cell activation in which Env protein affinity discrimination is based not on overall K_D but on sensing of association rate and a threshold antigen-BCR half-life.

Surface Ig variable domain glycosylation affects autoantigen binding and acts as threshold for human autoreactive B cell activation

The hallmark autoantibodies in rheumatoid arthritis are characterized by variable domain glycans (VDGs). Their abundant occurrence results from the selective introduction of N-linked glycosylation sites during somatic hypermutation, and their presence is predictive for disease development. However, the functional consequences of VDGs on autoreactive B cells remain elusive. Combining crystallography, glycobiology, and functional B cell assays allowed us to dissect key characteristics of VDGs on human B cell biology. Crystal structures showed that VDGs are positioned in the vicinity of the antigen-binding pocket, and dynamic modeling combined with binding assays elucidated their impact on binding. We found that VDG-expressing B cell receptors stay longer on the B cell surface and that VDGs enhance B cell activation. These results provide a rationale on how the acquisition of VDGs might contribute to the breach of tolerance of autoreactive B cells in a major human autoimmune disease.

A surrogate cell-based SARS-CoV-2 spike blocking assay

To monitor infection by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and successful vaccination against coronavirus disease 2019 (COVID-19), the kinetics of neutralizing or blocking anti-SARS-CoV-2 antibody titers need to be assessed. Here, we report the development of a quick and inexpensive surrogate SARS-CoV-2 blocking assay (SUBA) using immobilized recombinant human angiotensin-converting enzyme 2 (hACE2) and human cells expressing the native form of surface SARS-CoV-2 spike protein. Spike protein-expressing cells bound to hACE2 in the absence or presence of blocking antibodies were quantified by measuring the optical density of cell-associated crystal violet in a spectrophotometer. The advantages are that SUBA is a fast and inexpensive assay, which does not require biosafety level 2- or 3-approved laboratories. Most importantly, SUBA detects blocking antibodies against the native trimeric cell-bound SARS-CoV-2 spike protein and can be rapidly adjusted to quickly pre-screen already approved therapeutic antibodies or sera from vaccinated individuals for their ACE2 blocking activities against any emerging SARS-CoV-2 variants.

A Barcoded Flow Cytometric Assay to Explore the Antibody Responses Against SARS-CoV-2 Spike and Its Variants

The SARS-CoV-2 pandemic has spread to all parts of the world and can cause life-threatening pneumonia and other severe disease manifestations known as COVID-19. This health crisis has resulted in a significant effort to stop the spread of this new coronavirus. However, while propagating itself in the human population, the virus accumulates mutations and generates new variants with increased fitness and the ability to escape the human immune response. Here we describe a color-based barcoded spike flow cytometric assay (BSFA) that is particularly useful to evaluate and directly compare the humoral immune response directed against either wild type (WT) or mutant spike (S) proteins or the receptor-binding domains (RBD) of SARS-CoV-2. This assay employs the human B lymphoma cell line Ramos, transfected for stable expression of WT or mutant S proteins or a chimeric RBD-CD8 fusion protein. We find that the alpha and beta mutants are more stably expressed than the WT S protein on the Ramos B cell surface and/or bind with higher affinity to the viral entry receptor ACE2. However, we find a reduce expression of the chimeric RBD-CD8 carrying the point mutation N501Y and E484K characteristic for the alpha and beta variant, respectively. The comparison of the humoral immune response of 12 vaccinated probands with 12 COVID-19 patients shows that after the boost, the S-specific IgG class immune response in the vaccinated group is similar to that of the patient group. However, in comparison to WT the specific IgG serum antibodies bind less well to the alpha variant and only poorly to the beta variant S protein. This is in line with the notion that the beta variant is an immune escape variant of SARS-CoV-2. The IgA class immune response was more variable than the IgG response and higher in the COVID-19 patients than in the vaccinated group. In summary, we think that our BSFA represents a useful tool to evaluate the humoral immunity against emerging variants of SARS-CoV-2 and to analyze new vaccination protocols against these variants.

Quantitative proteomics identifies PTP1B as modulator of B cell antigen receptor signaling

This study analyses the function of the protein tyrosine phosphatase 1B identifying its binding partners and dephosphorylation targets for modulating B cell antigen receptor signaling.

B cell antigen receptor (BCR) signaling is initiated by protein kinases and limited by counteracting phosphatases that currently are less well studied in their regulation of BCR signaling. Here, we used the B cell line Ramos to identify and quantify human B cell signaling components. Specifically, a protein tyrosine phosphatase profiling revealed a high expression of the protein tyrosine phosphatase 1B (PTP1B) in Ramos and human naïve B cells. The loss of PTP1B leads to increased B cell activation. Through substrate trapping in combination with quantitative mass spectrometry, we identified 22 putative substrates or interactors of PTP1B. We validated Igα, CD22, PLCγ1/2, CBL, BCAP, and APLP2 as specific substrates of PTP1B in Ramos B cells. The tyrosine kinase BTK and the two adaptor proteins GRB2 and VAV1 were identified as direct binding partners and potential substrates of PTP1B. We showed that PTP1B dephosphorylates the inhibitory receptor protein CD22 at phosphotyrosine 807. We conclude that PTP1B negatively modulates BCR signaling by dephosphorylating distinct phosphotyrosines in B cell-specific receptor proteins and various downstream signaling components.

Sensing of antigen affinity and kinetic rates by B cells expressing receptors of HIV-1 broadly neutralizing antibodies

B cell activation is dependent on the productive engagement of an antigen (Ag) with the B cell receptor (BCR). How complex protein Ags that bind with different affinities and kinetic rates affect the strength of B cell activation is not fully understood. We used a panel of HIV-1 Env proteins of varying affinities and each expressed in either monomeric or multimeric forms to investigate the role of binding rates on B cell activation and Ag-induced BCR down-modulation. Ramos cells expressing the CD4 binding-site CH31 IgM BCRs were functional for Ag-specific activation and gave Ca-flux responses that was not dependent on the overall affinity or dissociation rates (off-rates) but on the association rates (on-rates) of Ag binding. Monomeric Ags did not induce flux responses and required multimerization, and the strength of activation was dependent on the on-rate of the tetrameric Ags. Comparison of half-life of the tetrameric Ags indicated a requirement of a half-life threshold for both activation and BCR down-modulation. In contrast, trimeric Ags that bound with faster on-rates (ka>104 M−1s−1) did not require higher-order multimerization (6-mer or 20-mer) for Ca-flux responses. These results provide support to a kinetic model in which B cell activation is dependent on the rates of receptor occupancy, and an above threshold half-life of the Ag-BCR complex.

Designing a SARS-CoV-2 T-Cell-Inducing Vaccine for High-Risk Patient Groups

We describe the results of two vaccinations of a self-experimenting healthy volunteer with SARS-CoV-2-derived peptides performed in March and April 2020, respectively. The first set of peptides contained eight peptides predicted to bind to the individual's HLA molecules. The second set consisted of ten peptides predicted to bind promiscuously to several HLA-DR allotypes. The vaccine formulation contained the new TLR 1/2 agonist XS15 and was administered as an emulsion in Montanide as a single subcutaneous injection. Peripheral blood mononuclear cells isolated from blood drawn before and after vaccinations were assessed using Interferon-γ ELISpot assays and intracellular cytokine staining. We detected vaccine-induced CD4 T cell responses against six out of 11 peptides predicted to bind to HLA-DR after 19 days, following vaccination, for one peptide already at day 12. We used these results to support the design of a T-cell-inducing vaccine for application in high-risk patients, with weakened lymphocyte performance. Meanwhile, an according vaccine, incorporating T cell epitopes predominant in convalescents, is undergoing clinical trial testing.

Immunoglobulin expression in the endoplasmic reticulum shapes the metabolic fitness of B lymphocytes

The major function of B lymphocytes is to sense antigens and to produce protective antibodies after activation. This function requires the expression of a B-cell antigen receptor (BCR), and evolutionary conserved mechanisms seem to exist that ensure that B cells without a BCR do not develop nor survive in the periphery. Here, we show that the loss of BCR expression on Burkitt lymphoma cells leads to decreased mitochondrial function and impaired metabolic flexibility. Strikingly, this phenotype does not result from the absence of a classical Syk-dependent BCR signal but rather from compromised ER expansion. We show that the reexpression of immunoglobulins (Ig) in the absence of the BCR signaling subunits Igα and Igβ rescues the observed metabolic defects. We demonstrate that immunoglobulin expression is needed to maintain ER homeostasis not only in lymphoma cells but also in resting B cells. Our study provides evidence that the expression of BCR components, which is sensed in the ER and shapes mitochondrial function, represents a novel mechanism of metabolic control in B cells.

Antibodies and B cells recognising citrullinated proteins display a broad cross-reactivity towards other post-translational modifications

OBJECTIVE

Autoantibodies against antigens carrying distinct post-translational modifications (PTMs), such as citrulline, homocitrulline or acetyllysine, are hallmarks of rheumatoid arthritis (RA). The relation between these anti-modified protein antibody (AMPA)-classes is poorly understood as is the ability of different PTM-antigens to activate B-cell receptors (BCRs) directed against citrullinated proteins (CP). Insights into the nature of PTMs able to activate such B cells are pivotal to understand the 'evolution' of the autoimmune response conceivable underlying the disease. Here, we investigated the cross-reactivity of monoclonal AMPA and the ability of different types of PTM-antigens to activate CP-reactive BCRs.

METHODS

BCR sequences from B cells isolated using citrullinated or acetylated antigens were used to produce monoclonal antibodies (mAb) followed by a detailed analysis of their cross-reactivity towards PTM-antigens. Ramos B-cell transfectants expressing CP-reactive IgG BCRs were generated and their activation on stimulation with PTM-antigens investigated.

RESULTS

Most mAbs were highly cross-reactive towards multiple PTMs, while no reactivity was observed to the unmodified controls. B cells carrying CP-reactive BCRs showed activation on stimulation with various types of PTM-antigens.

CONCLUSIONS

Our study illustrates that AMPA exhibit a high cross-reactivity towards at least two PTMs indicating that their recognition pattern is not confined to one type of modification. Furthermore, our data show that CP-reactive B cells are not only activated by citrullinated, but also by carbamylated and/or acetylated antigens. These data are vital for the understanding of the breach of B-cell tolerance against PTM-antigens and the possible contribution of these antigens to RA-pathogenesis.

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.

Synthetic biology of B cell activation: understanding signal amplification at the B cell antigen receptor using a rebuilding approach

Upon activation of the B cell antigen receptor (BCR), the spleen tyrosine kinase (Syk) and the Src family kinase Lyn phosphorylate tyrosines of the immunoreceptor tyrosine-based activation motif (ITAM) of Igα and Igβ which further serve as binding sites for the SH2 domains of these kinases. Using a synthetic biology approach, we dissect the roles of different ITAM residues of Igα in Syk activation. We found that a leucine to glycine mutation at the Y+3 position after the first ITAM tyrosine prevents Syk binding and activation. However, a pre-activated Syk can still phosphorylate this tyrosine in trans. Our data show that the formation of a Syk/ITAM initiation complex and trans-ITAM phosphorylation is crucial for BCR signal amplification. In contrast, the interaction of Lyn with the first ITAM tyrosine is not altered by the leucine to glycine mutation. In addition, our study suggests that an ITAM-bound Syk phosphorylates the non-ITAM tyrosine Y204 of Igα only in cis. Collectively, our reconstitution experiments suggest a model whereby first trans-phosphorylation amplifies the BCR signal and subsequently cis-phosphorylation couples the receptor to downstream signaling elements.

TLR induces reorganization of the IgM-BCR complex regulating murine B-1 cell responses to infections

In mice, neonatally-developing, self-reactive B-1 cells generate steady levels of natural antibodies throughout life. B-1 cells can, however, also rapidly respond to infections with increased local antibody production. The mechanisms regulating these two seemingly very distinct functions are poorly understood, but have been linked to expression of CD5, an inhibitor of BCR-signaling. Here we demonstrate that TLR-mediated activation of CD5+ B-1 cells induced the rapid reorganization of the IgM-BCR complex, leading to the eventual loss of CD5 expression, and a concomitant increase in BCR-downstream signaling, both in vitro and in vivo after infections of mice with influenza virus and Salmonella typhimurium. Both, initial CD5 expression and TLR-mediated stimulation, were required for the differentiation of B-1 cells to IgM-producing plasmablasts after infections. Thus, TLR-mediated signals support participation of B-1 cells in immune defense via BCR-complex reorganization.

CD5+ B-1 cell responses to infection are facilitated by toll-like receptor stimulation

B-1 lymphocytes are neonatally-derived, self-reactive, innate-like B cells that produce natural IgM at steady state. B-1 cells also actively respond to infections with induced local IgM production. These two distinct functions of B-1 cells have been linked to the expression CD5, an inhibitor of BCR signaling, with CD5+ cells thought to contribute to steady state IgM, and CD5− cells to local IgM production during infection. The mechanisms underlying such “division of labor” are poorly understood. We aim to test our hypothesis that CD5+ B-1 cells are stimulated by innate signals via TLR, during infection, resulting in their activation and differentiation into CD5− IgM antibody-secreting cells. For that we first cultured CD5+ B-1 cells with TLR9 agonist, ODN-CpG. The cells proliferated, secreted IgM and showed increased Nur77 expression, Akt phosphorylation and interaction of Syk with CD79a, signs of BCR signaling. They lost expression of the BCR-inhibitor CD5 and upregulated the plasma cell marker CD138+. Following influenza infection of mice, CD5+ B-1 cells migrated to the mediastinal lymph nodes (medLN) where they lost CD5 and became IgM secreting cells. Both, global TLR−/− mice and chimeric mice lacking TLR only on B-1 cells had reduced numbers of B-1 derived plasma cells and antibody secreting cells in the medLN compared to controls. Our findings show that TLR-mediated activation of CD5+ B-1 cells results in the rapid reorganization of the BCR-complex, followed by loss of CD5 and that TLR stimulation of CD5+ B-1 is required for differentiation of B-1 cells to infection in vivo. Overall our data suggest that CD5+ and CD5− B-1 cells are not distinct subsets, but rather that CD5 expression indicates the activation state of B-1 cells.

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.

LAMTOR2 (p14) Controls B Cell Differentiation by Orchestrating Endosomal BCR Trafficking

B-cell development and function depend on stage-specific signaling through the B-cell antigen receptor (BCR). Signaling and intracellular trafficking of the BCR are connected, but the molecular mechanisms of this link are incompletely understood. Here, we investigated the role of the endosomal adaptor protein and member of the LAMTOR/Ragulator complex LAMTOR2 (p14) in B-cell development. Efficient conditional deletion of LAMTOR2 at the pre-B1 stage using mb1-Cre mice resulted in complete developmental arrest. Deletion of LAMTOR2 using Cd19-Cre mice permitted analysis of residual B cells at later developmental stages, revealing that LAMTOR2 was critical for the generation and activation of mature B lymphocytes. Loss of LAMTOR2 resulted in aberrant BCR signaling due to delayed receptor internalization and endosomal trafficking. In conclusion, we identify LAMTOR2 as critical regulator of BCR trafficking and signaling that is essential for early B-cell development in mice.

Carbohydrate-dependent B cell activation by fucose-binding bacterial lectins

Bacterial lectins are typically multivalent and bind noncovalently to specific carbohydrates on host tissues to facilitate bacterial adhesion. Here, we analyzed the effects of two fucose-binding lectins, BambL from Burkholderia ambifaria and LecB from Pseudomonas aeruginosa, on specific signaling pathways in B cells. We found that these bacterial lectins induced B cell activation, which, in vitro, was dependent on the cell surface expression of the B cell antigen receptor (BCR) and its co-receptor CD19, as well as on spleen tyrosine kinase (Syk) activity. The resulting release of intracellular Ca²⁺ was followed by an increase in the cell surface abundance of the activation marker CD86, augmented cytokine secretion, and subsequent cell death, replicating all of the events that are observed in vitro upon canonical and antigen-mediated B cell activation. Moreover, injection of BambL in mice resulted in a substantial, BCR-independent loss of B cells in the bone marrow with simultaneous, transient enlargement of the spleen (splenomegaly), as well as an increase in the numbers of splenic B cells and myeloid cells. Together, these data suggest that bacterial lectins can initiate polyclonal activation of B cells through their sole capacity to bind to fucose.

Study B Cell Antigen Receptor Nano-Scale Organization by In Situ Fab Proximity Ligation Assay

The B cell antigen receptor (BCR) is found to be non-randomly organized at nano-scale distances on the B cell surface. Studying the organization and relocalization of the BCR is thus likely to provide new clues to understand the activation of the BCR. Indeed, with the in situ Fab proximity ligation assay (Fab-PLA), we now obtain proofs for the dissociation activation of BCRs and start to gain insight into how the relocalization of B cell surface signaling molecules could activate the cells. This chapter describes our methods to study the nano-scale organization of B cell surface receptors and co-receptors with Fab-PLA.

Conditional Selection of B Cells in Mice With an Inducible B Cell Development

Developing B cells undergo defined maturation steps in the bone marrow and in the spleen. The timing and the factors that control these differentiation steps are not fully understood. By targeting the B cell-restricted mb-1 locus to generate an mb-1 allele that expresses a tamoxifen inducible Cre and another allele in which mb-1 expression can be controlled by Cre, we have established a mouse model with an inducible B cell compartment. With these mice, we studied in detail the kinetics of B cell development and the consequence of BCR activation at a defined B cell maturation stage. Contrary to expectations, transitional 1-B cells exposed to anti-IgM reagents in vivo did not die but instead developed into transitional 2 (T2)-B cells with upregulated Bcl-2 expression. We show, however, that these T2-B cells had an increased dependency on the B cell survival factor B cell activating factor when compared to non-stimulated B cells. Overall, our findings indicate that the inducible mb-1 mouse strain represents a useful model, which allows studying the signals that control the selection of B cells in greater detail.

First Infusion Reactions are Mediated by FcγRIIIb and Neutrophils

Purpose

Administration of therapeutic monoclonal antibodies (mAbs) is frequently accompanied by severe first infusion reactions (FIR). The mechanism driving FIR is still unclear. This study aimed to investigate the cellular and molecular mechanisms causing FIR in humanized mouse models and their potential for evaluating FIR risk in patients.

Methods

Mice humanized for Fc gamma receptors (FcγRs) were generated by recombination-mediated genomic replacement. Body temperature, cytokine release and reactive oxygen species (ROS) were measured to assess FIR to mAbs.

Results

Infusion of human mAb specific for mouse transferrin receptor (HamTfR) into FcγR-humanized mice, produced marked transient hypothermia accompanied by an increase in inflammatory cytokines KC and MIP-2, and ROS. FIR were dependent on administration route and Fc-triggered effector functions mediated by neutrophils. Human neutrophils also induced FIR in wild type mice infused with HamTfR. Specific knock-in mice demonstrated that human FcγRIIIb on neutrophils was both necessary and sufficient to cause FIR. FcγRIIIb-mediated FIR was abolished by depleting neutrophils or blocking FcγRIIIb with CD11b antibodies.

Conclusions

Human FcγRIIIb and neutrophils are primarily responsible for triggering FIR. Clinical strategies to prevent FIR in patients should focus on this pathway and may include transient depletion of neutrophils or blocking FcγRIIIb with specific mAbs.

Electronic supplementary material

The online version of this article (10.1007/s11095-018-2448-8) contains supplementary material, which is available to authorized users.

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.

What goes up must come down: A tripartite Dok-3/Grb2/SHIP1 inhibitory module limits BCR signaling

Properly regulated immunity requires precise integration of activating and inhibitory signals. As for other lymphocytes, B cells express an antigen-specific activating receptor, the B-cell antigen receptor (BCR), and inhibitory receptors (e.g. FcγRIIb) that exercise checkpoint control on B-cell activation. Moreover, following BCR engagement, CD19 recruits proteins that amplify BCR signaling, while CD22 initiates a negative feedback loop by recruiting proteins that inhibit BCR signaling. Initial BCR signaling is mediated by protein tyrosine kinases and lipid kinases; inhibitory receptors directly antagonize the actions of these enzymes by recruiting protein tyrosine phosphatases and lipid phosphatases and positioning them close to actively signaling BCRs. Previously it was thought that inhibitory receptors such as FcγRIIb and CD22 were essential for bringing these phosphatases near the BCR. In this issue of the European Journal of Immunology, Manno et al. show that a tripartite inhibitory module consisting of the adaptor proteins Dok-3 and Grb2 and the lipid phosphatase SHIP1 binds directly to activated BCRs and limits the Ca²⁺ mobilization that is required for B lymphocyte activation. This reveals that the BCR can be both an activating and inhibitory receptor, one that activates signaling enzymes while initiating a negative feedback loop that prevents excessive signaling.

The BTG2-PRMT1 module limits pre-B cell expansion by regulating the CDK4-Cyclin-D3 complex

Developing pre-B cells in the bone marrow alternate between proliferation and differentiation phases. We found that protein arginine methyl transferase 1 (PRMT1) and B cell translocation gene 2 (BTG2) are critical components of the pre-B cell differentiation program. The BTG2-PRMT1 module induced a cell-cycle arrest of pre-B cells that was accompanied by re-expression of Rag1 and Rag2 and the onset of immunoglobulin light chain gene rearrangements. We found that PRMT1 methylated cyclin-dependent kinase 4 (CDK4), thereby preventing the formation of a CDK4-Cyclin-D3 complex and cell cycle progression. Moreover, BTG2 in concert with PRMT1 efficiently blocked the proliferation of BCR-ABL1-transformed pre-B cells in vitro and in vivo. Our results identify a key molecular mechanism by which the BTG2-PRMT1 module regulates pre-B cell differentiation and inhibits pre-B cell leukemogenesis.

Partial protection from lupus-like disease in B-cell specific IFNAR-deficient B6.Nba2 mice

A majority of patients presenting with systemic lupus erythematosus (SLE) have increased serum concentrations of interferon alpha (IFNα). IFNα has previously been shown to affect lymphocyte activities such as B cell differentiation and T cell maintenance, but whether IFNα signaling specifically in B cells is required for disease pathology remains unknown. In order to examine the role of IFNα stimulation on B cells, we backcrossed mice lacking the IFNα receptor on B cells, Mb1.cre IFNARflx/flx, with the B6.Nba2 lupus mouse model. B6.Nba2.BΔIFNAR mice were evaluated alongside littermate control mice for splenomegaly, abnormal spleen subset distribution, IgG immune-complex deposition in the kidney glomeruli. Serum total IgM, total IgG and anti-chromatin IgG levels were determined monthly. BΔIFNAR mice display normal IgM and IgG levels, but decreased serum antinuclear autoantibody levels (p<0.05), while deposition of IgG immune complexes in kidney glomeruli was similar to that of control mice. Both spleen weight and total splenocyte number were found to be significantly decreased in BΔIFNAR mice (p<0.05). Analyses of spleen cell subsets at 4 months of age revealed a trend towards decreased plasma cells and GC B cells in spleens of BΔIFNAR mice. Despite IFNAR being removed from B cells only, we observed a shift in the balance of effector/memory to naïve CD4 T cells in BΔIFNAR mice. In summary, we found that B6.Nba2.BΔIFNAR mice were partly protected from lupus pathology including splenomegaly, autoantibody production and B cell hyperactivation. Most surprisingly, T cell activation appeared to be diminished in B6.Nba2.BΔIFNAR mice, suggesting that B cells play a central role as both APCs and ASCs in the B6.Nba2 mouse model of lupus.

Spleen Tyrosine Kinase Is Involved in the CD38 Signal Transduction Pathway in Chronic Lymphocytic Leukemia

The survival and proliferation of CLL cells depends on microenvironmental contacts in lymphoid organs. CD38 is a cell surface receptor that plays an important role in survival and proliferation signaling in CLL. In this study we demonstrate SYK's direct involvement in the CD38 signaling pathway in primary CLL samples. CD38 stimulation of CLL cells revealed SYK activation. SYK downstream target AKT was subsequently induced and MCL-1 expression was increased. Concomitant inhibition of SYK by the SYK inhibitor R406 resulted in reduced activation of AKT and prevented upregulation of MCL-1. Moreover, short-term CD38 stimulation enhanced BCR-signaling, as indicated by increased ERK phosphorylation. CXCL12-dependent migration was increased after CD38 stimulation. Treating CLL cells with R406 inhibited CD38-mediated migration. In addition, we observed marked downregulation of CD38 expression for CLL cells treated with R406 compared to vehicle control. Finally, we observed a clear correlation between CD38 expression on CLL cells and SYK-inhibitor efficacy. In conclusion, our study provides deeper mechanistic insight into the effect of SYK inhibition in CLL.

LED Thermo Flow - Combining Optogenetics with Flow Cytometry

Optogenetic tools allow isolated, functional investigations of almost any signaling molecule within complex signaling pathways. A major obstacle is the controlled delivery of light to the cell sample and hence the most popular tools for optogenetic studies are microscopy-based cell analyses and in vitro experiments. The flow cytometer has major advantages over a microscope, including the ability to rapidly measure thousands of cells at single cell resolution. However, it is not yet widely used in optogenetics. Here, we present a device that combines the power of optogenetics and flow cytometry: the LED Thermo Flow. This device illuminates cells at specific wavelengths, light intensities and temperatures during flow cytometric measurements. It can be built at low cost and be used with most common flow cytometers. To demonstrate its utility, we characterized the photoswitching kinetics of Dronpa proteins in vivo and in real time. This protocol can be adapted to almost all optically controlled substances and substantially expands the set of possible experiments. More importantly, it will greatly simplify the discovery and development of new optogenetic tools.

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.

Survival of Igα-Deficient Mature B Cells Requires BAFF-R Function

Expression of a functional BCR is essential for the development of mature B cells and has been invoked in the control of their maintenance. To test this maintenance function in a new experimental setting, we used the tamoxifen-inducible mb1-CreER(T2) mouse strain to delete or truncate either the mb-1 gene encoding the BCR signaling subunit Igα or the VDJ segment of the IgH (H chain [HC]). In this system, Cre-mediated deletion of the mb-1 gene is accompanied by expression of a GFP reporter. We found that, although the Igα-deficient mature B cells survive for >20 d in vivo, the HC-deficient or Igα tail-truncated B cell population is short-lived, with the HC-deficient cells displaying signs of an unfolded protein response. We also show that Igα-deficient B cells still respond to the prosurvival factor BAFF in culture and require BAFF-R signaling for their in vivo maintenance. These results suggest that, under certain conditions, the loss of the BCR can be tolerated by mature B cells for some time, whereas HC-deficient B cells, potentially generated by aberrant somatic mutations in the germinal center, are rapidly eliminated.

B cell antigen receptors of the IgM and IgD classes are clustered in different protein islands that are altered during B cell activation

The B cell antigen receptors (BCRs) play an important role in the clonal selection of B cells and their differentiation into antibody-secreting plasma cells. Mature B cells have both immunoglobulin M (IgM) and IgD types of BCRs, which have identical antigen-binding sites and are both associated with the signaling subunits Igα and Igβ, but differ in their membrane-bound heavy chain isoforms. By two-color direct stochastic optical reconstruction microscopy (dSTORM), we showed that IgM-BCRs and IgD-BCRs reside in the plasma membrane in different protein islands with average sizes of 150 and 240 nm, respectively. Upon B cell activation, the BCR protein islands became smaller and more dispersed such that the IgM-BCRs and IgD-BCRs were found in close proximity to each other. Moreover, specific stimulation of one class of BCR had minimal effects on the organization of the other. These conclusions were supported by the findings from two-marker transmission electron microscopy and proximity ligation assays. Together, these data provide evidence for a preformed multimeric organization of BCRs on the plasma membrane that is remodeled after B cell activation.

Kidins220/ARMS binds to the B cell antigen receptor and regulates B cell development and activation

Fiala et al. report that Kidins220/ARMS is a novel interactor of the B cell antigen receptor (BCR) and its deletion impairs B cell development and B cell functioning.

B cell antigen receptor (BCR) signaling is critical for B cell development and activation. Using mass spectrometry, we identified a protein kinase D–interacting substrate of 220 kD (Kidins220)/ankyrin repeat–rich membrane-spanning protein (ARMS) as a novel interaction partner of resting and stimulated BCR. Upon BCR stimulation, the interaction increases in a Src kinase–independent manner. By knocking down Kidins220 in a B cell line and generating a conditional B cell–specific Kidins220 knockout (B-KO) mouse strain, we show that Kidins220 couples the BCR to PLCγ2, Ca²⁺, and extracellular signal-regulated kinase (Erk) signaling. Consequently, BCR-mediated B cell activation was reduced in vitro and in vivo upon Kidins220 deletion. Furthermore, B cell development was impaired at stages where pre-BCR or BCR signaling is required. Most strikingly, λ light chain–positive B cells were reduced sixfold in the B-KO mice, genetically placing Kidins220 in the PLCγ2 pathway. Thus, our data indicate that Kidins220 positively regulates pre-BCR and BCR functioning.

Abstract 2075: Signaling thresholds and negative B cell selection in acute lymphoblastic leukemia

Introduction: Unlike other cell types, B cells are selected for an intermediate level of signaling strength. Critical survival and proliferation signals emanate from the (pre-) B cell receptor (BCR): Both attenuation below minimum (e.g. non-functional pre-BCR) and hyperactivation above maximum (e.g. autoreactive pre-BCR) thresholds of signaling strength trigger negative selection and cell death. The oncogenic BCR-ABL1 tyrosine kinase mimics active pre-BCR signaling in Ph+ acute lymphoblastic leukemia (ALL) which defines the ALL subgroup with the worst clinical outcome. Current therapy approaches are largely focused on the development of more potent tyrosine kinase inhibitors (TKI) to suppress oncogenic signaling. However resistance to TKI is developed invariably. Here, we test the hypothesis that targeting hyperactivation above a maximum threshold will selectively kill Ph+ ALL cells, similar to removal of self-reactive B cells.

Results: The Ph+ ALL cells don not express ITAM (immunoreceptor tyrosine-based activation motif) receptor Igα or Igβ on the cell surface, indicating defects for a functional pre-BCR. Reconstitution of ITAM receptor was sufficient to induce cell death through increasing pre-BCR signaling strength indicated by phosphorylation of SYK, SRC, BTK and PLCγ2. TKI-treatment, while designed to kill leukemia cells, seemingly paradoxically rescued Ph+ ALL cells in this experimental setting. Surprisingly, patient-derived Ph+ ALL cells express the ITIM (immunoreceptor tyrosine-based inhibitory motif) receptors PECAM1, CD300A and LAIR1 at high levels compared to normal pre-B cells. Importantly, high expression levels of ITIM-receptors are predictive of poor outcome in two clinical trials, including both pediatric and adult ALL patients. Genetic studies revealed that Pecam1, Cd300a and Lair1 were critical to calibrate pre-BCR signaling strength through recruitment of the inhibitory phosphatases Ptpn6 (Shp1) and Inpp5d (Ship1). Genetic deletion of Lair1, Ptpn6 or Inpp5d in BCR-ABL1 ALL caused cell death in vitro and in vivo through hyperactivation of pre-BCR signaling. Testing various components of proximal pre-BCR signaling, we found that an incremental increase of SYK tyrosine kinase activity was required and sufficient to induce cell death. Hyperactive SYK was functionally equivalent to acute activation of a self-reactive BCR on ALL cells. Using chimeric PECAM1, CD300A and LAIR1 receptor decoys and a novel small molecule inhibitor of INPP5D, we demonstrated that pharmacological hyperactivation of pre-BCR signaling and engagement of negative B cell selection represents a promising new strategy to overcome drug-resistance in human Ph+ ALL.

Conclusion: These results indicated that inhibitory receptors and downstream phosphatases are critical regulators of pre-BCR signaling strength in Ph+ ALL, and identified targeting hyperactivation of pre-BCR signaling as a potential novel class of therapeutic strategy.

Note: This abstract was not presented at the meeting.

Citation Format: Zhengshan Chen, Seyedmehdi Shojaee, Maike Buchner, Huimin Geng, Jae Woong Lee, Lars Klemm, Eugene Park, Ying Xim Tan, Anne Satterthwaite, Elisabeth Paietta, Stephen P. Hunger, Mignon L. Loh, Jae U. Jung, John E. Coligan, Silvia Bolland, Tak W. Mak, Andre Limnander, Hassan Jumaa, Michael Reth, Arthur Weiss, Clifford A. Lowell, Markus Müschen. Signaling thresholds and negative B cell selection in acute lymphoblastic leukemia. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2075. doi:10.1158/1538-7445.AM2015-2075

Processing of CD74 by the Intramembrane Protease SPPL2a Is Critical for B Cell Receptor Signaling in Transitional B Cells

The invariant chain (CD74), a chaperone in MHC class II-mediated Ag presentation, is sequentially processed by different endosomal proteases. We reported recently that clearance of the final membrane-bound N-terminal fragment (NTF) of CD74 is mediated by the intramembrane protease signal peptide peptidase-like (SPPL)2a, a process critical for B cell development. In mice, SPPL2a deficiency provokes the accumulation of this NTF in endocytic vesicles, which leads to a B cell maturation arrest at the transitional 1 stage. To define the underlying mechanism, we analyzed the impact of SPPL2a deficiency on signaling pathways involved in B cell homeostasis. We demonstrate that tonic as well as BCR-induced activation of the PI3K/Akt pathway is massively compromised in SPPL2a(-/-) B cells and identify this as major cause of the B cell maturation defect in these mice. Altered BCR trafficking induces a reduction of surface IgM in SPPL2a-deficient B cells, leading to a diminished signal transmission via the BCR and the tyrosine kinase Syk. We provide evidence that in SPPL2a(-/-) mice impaired BCR signaling is to a great extent provoked by the accumulating CD74 NTF, which can interact with the BCR and Syk, and that impaired PI3K/Akt signaling and reduced surface IgM are not directly linked processes. In line with disturbances in PI3K/Akt signaling, SPPL2a(-/-) B cells show a dysregulation of the transcription factor FOXO1, causing elevated transcription of proapoptotic genes. We conclude that SPPL2a-mediated processing of CD74 NTF is indispensable to maintain appropriate levels of tonic BCR signaling to promote B cell maturation.

Signalling thresholds and negative B-cell selection in acute lymphoblastic leukaemia

B cells are selected for an intermediate level of B-cell antigen receptor (BCR) signalling strength: attenuation below minimum (for example, non-functional BCR) or hyperactivation above maximum (for example, self-reactive BCR) thresholds of signalling strength causes negative selection. In ∼25% of cases, acute lymphoblastic leukaemia (ALL) cells carry the oncogenic BCR-ABL1 tyrosine kinase (Philadelphia chromosome positive), which mimics constitutively active pre-BCR signalling. Current therapeutic approaches are largely focused on the development of more potent tyrosine kinase inhibitors to suppress oncogenic signalling below a minimum threshold for survival. We tested the hypothesis that targeted hyperactivation--above a maximum threshold--will engage a deletional checkpoint for removal of self-reactive B cells and selectively kill ALL cells. Here we find, by testing various components of proximal pre-BCR signalling in mouse BCR-ABL1 cells, that an incremental increase of Syk tyrosine kinase activity was required and sufficient to induce cell death. Hyperactive Syk was functionally equivalent to acute activation of a self-reactive BCR on ALL cells. Despite oncogenic transformation, this basic mechanism of negative selection was still functional in ALL cells. Unlike normal pre-B cells, patient-derived ALL cells express the inhibitory receptors PECAM1, CD300A and LAIR1 at high levels. Genetic studies revealed that Pecam1, Cd300a and Lair1 are critical to calibrate oncogenic signalling strength through recruitment of the inhibitory phosphatases Ptpn6 (ref. 7) and Inpp5d (ref. 8). Using a novel small-molecule inhibitor of INPP5D (also known as SHIP1), we demonstrated that pharmacological hyperactivation of SYK and engagement of negative B-cell selection represents a promising new strategy to overcome drug resistance in human ALL.

TSC1 activates TGF-β-Smad2/3 signaling in growth arrest and epithelial-to-mesenchymal transition

The tuberous sclerosis proteins TSC1 and TSC2 are key integrators of growth factor signaling. They suppress cell growth and proliferation by acting in a heteromeric complex to inhibit the mammalian target of rapamycin complex 1 (mTORC1). In this study, we identify TSC1 as a component of the transforming growth factor β (TGF-β)-Smad2/3 pathway. Here, TSC1 functions independently of TSC2. TSC1 interacts with the TGF-β receptor complex and Smad2/3 and is required for their association with one another. TSC1 regulates TGF-β-induced Smad2/3 phosphorylation and target gene expression and controls TGF-β-induced growth arrest and epithelial-to-mesenchymal transition (EMT). Hyperactive Akt specifically activates TSC1-dependent cytostatic Smad signaling to induce growth arrest. Thus, TSC1 couples Akt activity to TGF-β-Smad2/3 signaling. This has implications for cancer treatments targeting phosphoinositide 3-kinases and Akt because they may impair tumor-suppressive cytostatic TGF-β signaling by inhibiting Akt- and TSC1-dependent Smad activation.

CD19 and BAFF-R can signal to promote B-cell survival in the absence of Syk

The development and function of B lymphocytes is regulated by numerous signaling pathways, some emanating from the B-cell antigen receptor (BCR). The spleen tyrosine kinase (Syk) plays a central role in the activation of the BCR, but less is known about its contribution to the survival and maintenance of mature B cells. We generated mice with an inducible and B-cell-specific deletion of the Syk gene and found that a considerable fraction of mature Syk-negative B cells can survive in the periphery for an extended time. Syk-negative B cells are defective in BCR, RP105 and CD38 signaling but still respond to an IL-4, anti-CD40, CpG or LPS stimulus. Our in vivo experiments show that Syk-deficient B cells require BAFF receptor and CD19/PI3K signaling for their long-term survival. These studies also shed a new light on the signals regulating the maintenance of the normal mature murine B-cell pool.

Suboptimal B-cell antigen receptor signaling activity in vivo elicits germinal center counterselection mechanisms

Syk and Zap-70 constitute a closely related nonreceptor protein tyrosine kinase family, of which both members are functionally indispensable for conferring their respective antigen receptors with enzymatic activity. In this study, we analyze the impact of altering BCR signaling output on B-cell germinal center (GC) fate selection by constitutive, as well as inducible, monoallelic Syk kinase loss in the presence of a Zap-70 knock-in rescue allele. Cre-mediated Syk deletion in Syk(flox/Zap-70) B cells lowers pErk, but not pAkt-mediated signaling. Surprisingly, the use of a B-cell-specific constitutive mb1-cre deleter mouse model showed that a small cohort of peripheral Syk(flox/Zap-70);mb1-cre B cells efficiently circumvents deletion, which ultimately favors these Syk-sufficient cells to contribute to the GC reaction. Using a developmentally unbiased Syk(flox/Zap-70);mb1-creER(T2) approach in combination with an inducible tdRFP allele, we further demonstrate that this monoallelic deletion escape is not fully explained by leakiness of Cre expression, but is possibly the result of differential Syk locus accessibility in maturing B cells. Altogether, this underscores the importance of proper Syk kinase function not only during central and peripheral selection processes, but also during GC formation and maintenance.

The nanoscale organization of the B lymphocyte membrane

The fluid mosaic model of Singer and Nicolson correctly predicted that the plasma membrane (PM) forms a lipid bi-layer containing many integral trans-membrane proteins. This model also suggested that most of these proteins were randomly dispersed and freely diffusing moieties. Initially, this view of a dynamic and rather unorganized membrane was supported by early observations of the cell surfaces using the light microscope. However, recent studies on the PM below the diffraction limit of visible light (~250nm) revealed that, at nanoscale dimensions, membranes are highly organized and compartmentalized structures. Lymphocytes are particularly useful to study this nanoscale membrane organization because they grow as single cells and are not permanently engaged in cell:cell contacts within a tissue that can influence membrane organization. In this review, we describe the methods that can be used to better study the protein:protein interaction and nanoscale organization of lymphocyte membrane proteins, with a focus on the B cell antigen receptor (BCR). Furthermore, we discuss the factors that may generate and maintain these membrane structures.

The AP-1 transcription factor Fra1 inhibits follicular B cell differentiation into plasma cells

Grötsch et al. find that the AP-1 transcription factor Fra-1 limits the generation of antibody-producing plasma cells. Absence of Fra1 in B cells results in abnormally high numbers of plasma cells and increased antibody responses after vaccination.

The cornerstone of humoral immunity is the differentiation of B cells into antibody-secreting plasma cells. This process is tightly controlled by a regulatory gene network centered on the transcriptional repressor B lymphocyte–induced maturation protein 1 (Blimp1). Proliferation of activated B cells is required to foster Blimp1 expression but needs to be terminated to avoid overshooting immune reactions. Activator protein 1 (AP-1) transcription factors become quickly up-regulated upon B cell activation. We demonstrate that Fra1, a Fos member of AP-1, enhances activation-induced cell death upon induction in activated B cells. Moreover, mice with B cell–specific deletion of Fra1 show enhanced plasma cell differentiation and exacerbated antibody responses. In contrast, transgenic overexpression of Fra1 blocks plasma cell differentiation and immunoglobulin production, which cannot be rescued by Bcl2. On the molecular level, Fra1 represses Blimp1 expression and interferes with binding of the activating AP-1 member c-Fos to the Blimp1 promoter. Conversely, overexpression of c-Fos in Fra1 transgenic B cells releases Blimp1 repression. As Fra1 lacks transcriptional transactivation domains, we propose that Fra1 inhibits Blimp1 expression and negatively controls plasma cell differentiation through binding to the Blimp1 promoter. In summary, we demonstrate that Fra1 negatively controls plasma cell differentiation by repressing Blimp1 expression.

Production of IgG autoantibody requires expression of activation-induced deaminase in early-developing B cells in a mouse model of SLE

Systemic lupus erythematosus (SLE) is an autoimmune disease characterized by the presence of pathogenic IgG antinuclear antibodies. Pathogenic IgG autoantibody production requires B-cell activation, leading to the production of activation-induced deaminase (AID) and class switching of IgM genes to IgG. To understand how and when B cells are activated to produce these IgG autoantibodies, we studied cells from 564Igi, a mouse model of SLE. 564Igi mice develop a disease profile closely resembling that found in human SLE patients, including the presence of IgG antinucleic acid Abs. We have generated 564Igi mice that conditionally express an activation-induced cytidine deaminase transgene (Aicda(tg) ), either in all B cells or only in mature B cells. Here, we show that class-switched pathogenic IgG autoantibodies were produced only in 564Igi mice in which AID was functional in early-developing B cells, resulting in loss of tolerance. Furthermore, we show that the absence of AID in early-developing B cells also results in increased production of self-reactive IgM, indicating that AID, through somatic hypermutation, contributes to tolerance. Our results suggest that the pathophysiology of clinical SLE might also be dependent on AID expression in early-developing B cells.

B cell activation involves nanoscale receptor reorganizations and inside-out signaling by Syk

Binding of antigen to the B cell antigen receptor (BCR) initiates a multitude of events resulting in B cell activation. How the BCR becomes signaling-competent upon antigen binding is still a matter of controversy. Using a high-resolution proximity ligation assay (PLA) to monitor the conformation of the BCR and its interactions with co-receptors at a 10-20 nm resolution, we provide direct evidence for the opening of BCR dimers during B cell activation. We also show that upon binding Syk opens the receptor by an inside-out signaling mechanism that amplifies BCR signaling. Furthermore, we found that on resting B cells, the coreceptor CD19 is in close proximity with the IgD-BCR and on activated B cells with the IgM-BCR, indicating nanoscale reorganization of receptor clusters during B cell activation.DOI: http://dx.doi.org/10.7554/eLife.02069.001.

The role of the Syk/Shp-1 kinase-phosphatase equilibrium in B cell development and signaling

Signal transduction from the BCR is regulated by the equilibrium between kinases (e.g., spleen tyrosine kinase [Syk]) and phosphatases (e.g., Shp-1). Previous studies showed that Syk-deficient B cells have a developmental block at the pro/pre-B cell stage, whereas a B cell-specific Shp-1 deficiency promoted B-1a cell development and led to autoimmunity. We generated B cell-specific Shp-1 and Syk double-knockout (DKO) mice and compared them to the single-knockout mice deficient for either Syk or Shp-1. Unlike Syk-deficient mice, the DKO mice can generate mature B cells, albeit at >20-fold reduced B cell numbers. The DKO B-2 cells are all Syk-negative, whereas the peritoneal B1 cells of the DKO mice still express Syk, indicating that they require this kinase for their proper development. The DKO B-2 cells cannot be stimulated via the BCR, whereas they are efficiently activated via TLR or CD40. We also found that in DKO pre-B cells, the kinase Zap70 is associated with the pre-BCR, suggesting that Zap70 is important to promote B cell maturation in the absence of Syk and SHP-1. Together, our data show that a properly balanced kinase/phosphatase equilibrium is crucial for normal B cell development and function.

The ligand-binding domain of Siglec-G is crucial for its selective inhibitory function on B1 cells

Siglec-G is an inhibitory receptor on B1 cells. Siglec-G-deficient mice show a large B1 cell expansion, owing to higher BCR-induced Ca(2+) signaling and enhanced cellular survival. It was unknown why Siglec-G shows a B1 cell-restricted inhibitory function. With a new mAb we could show a comparable Siglec-G expression on B1 cells and conventional B2 cells. However, Siglec-G has a different ligand sialic acid-binding pattern on peritoneal B1 cells than on splenic B cells, and its sialic acid ligands are expressed differentially on these two B cell populations, suggesting that cis-ligand binding plays a crucial role on B1 cells. This observation was further studied by generation of Siglec-G knockin mice with a mutated ligand-binding domain. These mice show increased B1 cell numbers, increased B1 cell Ca(2+) signaling, better B1 cell survival, and changes in the B1 cell Ig repertoire. These phenotypes are very similar to Siglec-G-deficient mice. The mutation of the ligand-binding domain of Siglec-G strongly reduces the Siglec-G-IgM association on the B cell surface. Thus, Siglec-G sialic acid-dependent binding to the BCR is crucial for the B1 cell-restricted inhibitory function of Siglec-G and is regulated in an opposite way to that of the related protein CD22 (Siglec-2) on B cells.