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

At low levels, inorganic mercury interference with antigen signaling is associated with modifications to a panel of novel phosphoserine sites in B cell receptor pathway proteins

Epidemiological studies indicate that human and animal exposure to environmental mercury (Hg) disrupts normal immune system function, but the molecular mechanism responsible for this is still unresolved. We have previously utilized phospho-proteomic mass spectrometry to demonstrate that in the absence of B Cell Receptor (BCR) stimulation, exposure of B cells to Hg induces significant changes to a great many elements of the BCR signaling pathway in a concentration dependent manner. In this report, we have extended those initial findings by utilizing mass spectrometry to evaluate in detail the effect of low-level Hg exposure on BCR induced phospho-proteomic changes. Specifically, murine WEHI-231 B lymphoma cells were exposed to environmentally relevant levels of Hg with or without concomitant BCR stimulation. The cellular phospho-proteomes were then profiled by LC-MS/MS. We found that for low-level exposures, Hg interference with signal transduction across the BCR pathway was predominantly associated with modification of phosphorylation of 12 phosphosites located on seven different proteins. Nine sites were serine, two sites tyrosine and one site threonine. Most of these sites are novel, in the sense that only the two tyrosine and one of the serine sites have previously been reported to be associated with BCR signaling.

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.

Inference of cell state transitions and cell fate plasticity from single-cell with MARGARET

Despite recent advances in inferring cellular dynamics using single-cell RNA-seq data, existing trajectory inference (TI) methods face difficulty in accurately reconstructing the cell-state manifold and cell-fate plasticity for complex topologies. Here, we present MARGARET (https://github.com/Zafar-Lab/Margaret) for inferring single-cell trajectory and fate mapping for diverse dynamic cellular processes. MARGARET reconstructs complex trajectory topologies using a deep unsupervised metric learning and a graph-partitioning approach based on a novel connectivity measure, automatically detects terminal cell states, and generalizes the quantification of fate plasticity for complex topologies. On a diverse benchmark consisting of synthetic and real datasets, MARGARET outperformed state-of-the-art methods in recovering global topology and cell pseudotime ordering. For human hematopoiesis, MARGARET accurately identified all major lineages and associated gene expression trends and helped identify transitional progenitors associated with key branching events. For embryoid body differentiation, MARGARET identified novel transitional populations that were validated by bulk sequencing and functionally characterized different precursor populations in the mesoderm lineage. For colon differentiation, MARGARET characterized the lineage for BEST4/OTOP2 cells and the heterogeneity in goblet cell lineage in the colon under normal and inflamed ulcerative colitis conditions. Finally, we demonstrated that MARGARET can scale to large scRNA-seq datasets consisting of ∼ millions of cells.

Replication timing alterations in leukemia affect clinically relevant chromosome domains

Human B-cell precursor acute lymphoid leukemias (BCP-ALLs) comprise a group of genetically and clinically distinct disease entities with features of differentiation arrest at known stages of normal B-lineage differentiation. We previously showed that BCP-ALL cells display unique and clonally heritable, stable DNA replication timing (RT) programs (ie, programs describing the variable order of replication and subnuclear 3D architecture of megabase-scale chromosomal units of DNA in different cell types). To determine the extent to which BCP-ALL RT programs mirror or deviate from specific stages of normal human B-cell differentiation, we transplanted immunodeficient mice with quiescent normal human CD34+ cord blood cells and obtained RT signatures of the regenerating B-lineage populations. We then compared these with RT signatures for leukemic cells from a large cohort of BCP-ALL patients with varied genetic subtypes and outcomes. The results identify BCP-ALL subtype-specific features that resemble specific stages of B-cell differentiation and features that seem to be associated with relapse. These results suggest that the genesis of BCP-ALL involves alterations in RT that reflect biologically significant and potentially clinically relevant leukemia-specific epigenetic changes.

Characterization of cell fate probabilities in single-cell data with Palantir

Single-cell RNA sequencing studies of differentiating systems have raised fundamental questions regarding the discrete versus continuous nature of both differentiation and cell fate. Here we present Palantir, an algorithm that models trajectories of differentiating cells by treating cell fate as a probabilistic process and leverages entropy to measure cell plasticity along the trajectory. Palantir generates a high-resolution pseudo-time ordering of cells and, for each cell state, assigns a probability of differentiating into each terminal state. We apply our algorithm to human bone marrow single-cell RNA sequencing data and detect important landmarks of hematopoietic differentiation. Palantir's resolution enables the identification of key transcription factors that drive lineage fate choice and closely track when cells lose plasticity. We show that Palantir outperforms existing algorithms in identifying cell lineages and recapitulating gene expression trends during differentiation, is generalizable to diverse tissue types, and is well-suited to resolving less-studied differentiating systems.

Parsaclisib, a potent and highly selective PI3Kδ inhibitor, in patients with relapsed or refractory B-cell malignancies

This phase 1/2 study assessed parsaclisib (INCB050465), a next-generation, potent, and highly selective phosphatidylinositol 3-kinase δ (PI3Kδ) inhibitor, in patients with relapsed or refractory B-cell malignancies, alone or in combination with a Janus kinase 1 inhibitor (itacitinib) or chemotherapy (rituximab, ifosfamide, carboplatin, and etoposide). Seventy-two patients received parsaclisib monotherapy (5-45 mg once daily). Expansion doses were 20 and 30 mg once daily; intermittent dosing at 20 mg (once daily for 9 weeks, then once weekly) was explored. No dose-limiting toxicities were identified, and maximum tolerated dose was not reached. Most common nonhematologic treatment-emergent adverse events (TEAEs) were diarrhea/colitis (36%), nausea (36%), fatigue (31%), and rash (31%). Grade 3/4 neutropenia occurred in 19% of patients. Serious TEAEs (>2 patients) were diarrhea/colitis (n = 9), pyrexia (n = 4), hypotension (n = 3), and sepsis (n = 3). Aspartate and alanine transaminase elevations occurring before treatment discontinuation were grade 1, except 1 grade 3 event each, secondary to sepsis. Two patients experienced 3 fatal parsaclisib-unrelated TEAEs (respiratory failure; respiratory failure and sepsis). In non-Hodgkin lymphoma (NHL), objective response rates to monotherapy were 71% in follicular lymphoma, 78% in marginal zone lymphoma, 67% in mantle cell lymphoma, and 30% in diffuse large B-cell lymphoma; 93% of responses occurred at first assessment (∼9 weeks). Parsaclisib has demonstrated antitumor activity in relapsed or refractory B-cell NHL with the potential for improved long-term patient outcomes. Phase 2 studies in relapsed or refractory B-cell NHL subtypes are ongoing. This trial was registered at www.clinicaltrials.gov as #NCT02018861.

B cell MHC class II signaling: A story of life and death

MHC class II regulates B cell activation, proliferation, and differentiation during cognate B cell-T cell interaction. This is, in part, due to the MHC class II signaling in B cells. Activation of MHC Class II in human B cells or "primed" murine B cells leads to tyrosine phosphorylation, calcium mobilization, AKT, ERK, JNK activation. In addition, crosslinking MHC class II with monoclonal Abs kill malignant human B cells. Several humanized anti-HLA-DR/MHC class II monoclonal Abs entered clinical trials for lymphoma/leukemia and MHC class II-expressing melanomas. Mechanistically, MHC class II is associated with a wealth of transmembrane proteins including the B cell-specific signaling proteins CD79a/b, CD19 and a group of four-transmembrane proteins including tetraspanins and the apoptotic protein MPYS/STING. Furthermore, MHC class II signals are compartmentalized in the tetraspanin-enriched microdomains. In this review, we discuss our current understanding of MHC class II signaling in B cells focusing on its physiological significance and the therapeutic potential.

Phase 1 study of the PI3Kδ inhibitor INCB040093 ± JAK1 inhibitor itacitinib in relapsed/refractory B-cell lymphoma

Because both phosphatidylinositol 3-kinase δ (PI3Kδ) and Janus kinase (JAK)-signal transducer and activator of transcription pathways contribute to tumor cell proliferation and survival in B-cell malignancies, their simultaneous inhibition may provide synergistic treatment efficacy. This phase 1 dose-escalation/expansion study assessed the safety, efficacy, pharmacokinetics, and pharmacodynamics of INCB040093, a selective PI3Kδ inhibitor, as monotherapy or combined with itacitinib (formerly INCB039110), a selective JAK1 inhibitor, in adult patients with relapsed or refractory (R/R) B-cell lymphomas. Final results are reported. Overall, 114 patients were treated (monotherapy, n = 49; combination therapy, n = 72 [7 patients crossed over from monotherapy to combination]). INCB040093 100 mg twice daily (monotherapy) and INCB040093 100 mg twice daily + itacitinib 300 mg once daily (combination) were the recommended phase 2 doses. One dose-limiting toxicity (gastrointestinal bleed secondary to gastric diffuse large B-cell lymphoma [DLBCL] regression) occurred with monotherapy. The most common serious adverse events with monotherapy were pneumonia (n = 5) and pyrexia (n = 4), and with combination Pneumocystis jiroveci pneumonia (n = 5), pneumonia (unrelated to P jiroveci; n = 5), and pyrexia (n = 4). Grade 3 or higher transaminase elevations were less common with combination. INCB040093 was active across the B-cell lymphomas; 63% of patients (5/8) with follicular lymphoma responded to monotherapy. Adding itacitinib provided promising activity in select subtypes, with responses of 67% (14/21) in classic Hodgkin lymphoma (vs 29% [5/17] with monotherapy) and 31% (4/13) in nongerminal center B-cell-like DLBCL. INCB040093 with/without itacitinib was tolerated and active in this study, and is a promising treatment strategy for patients with select R/R B-cell lymphomas. This trial was registered at www.clinicaltrials.gov as #NCT01905813.

Antibody repertoire analysis in polygenic autoimmune diseases

High-throughput sequencing of the DNA/RNA encoding antibody heavy- and light-chains is rapidly transforming the field of adaptive immunity. It can address key questions, including: (i) how the B-cell repertoire differs in health and disease; and (ii) if it does differ, the point(s) in B-cell development at which this occurs. The advent of technologies, such as whole-genome sequencing, offers the chance to link abnormalities in the B-cell antibody repertoire to specific genomic variants and polymorphisms. Here, we discuss the current research using B-cell antibody repertoire sequencing in three polygenic autoimmune diseases where there is good evidence for a pathological role for B-cells, namely systemic lupus erythematosus, multiple sclerosis and rheumatoid arthritis. These autoimmune diseases exhibit significantly skewed B-cell receptor repertoires compared with healthy controls. Interestingly, some common repertoire defects are shared between diseases, such as elevated IGHV4-34 gene usage. B-cell clones have effectively been characterized and tracked between different tissues and blood in autoimmune disease. It has been hypothesized that these differences may signify differences in B-cell tolerance; however, the mechanisms and implications of these defects are not clear.

Gene expression profiling of the response to interferon beta in Epstein-Barr-transformed and primary B cells of patients with multiple sclerosis

The effects of interferon-beta (IFN-β), one of the key immunotherapies used in multiple sclerosis (MS), on peripheral blood leukocytes and T cells have been extensively studied. B cells are a less abundant leukocyte type, and accordingly less is known about the B cell-specific response to IFN-β. To identify gene expression changes and pathways induced by IFN-β in B cells, we studied the in vitro response of human Epstein Barr-transformed B cells (lymphoblast cell lines-LCLs), and validated our results in primary B cells. LCLs were derived from an MS patient repository. Whole genome expression analysis identified 115 genes that were more than two-fold differentially up-regulated following IFN-β exposure, with over 50 previously unrecognized as IFN-β response genes. Pathways analysis demonstrated that IFN-β affected LCLs in a similar manner to other cell types by activating known IFN-β canonical pathways. Additionally, IFN-β increased the expression of innate immune response genes, while down-regulating many B cell receptor pathway genes and genes involved in adaptive immune responses. Novel response genes identified herein, NEXN, DDX60L, IGFBP4, and HAPLN3, B cell receptor pathway genes, CD79B and SYK, and lymphocyte activation genes, LAG3 and IL27RA, were validated as IFN-β response genes in primary B cells. In this study new IFN-β response genes were identified in B cells, with possible implications to B cell-specific functions. The study's results emphasize the applicability of LCLs for studies of human B cell drug response. The usage of LCLs from patient-based repositories may facilitate future studies of drug response in MS and other immune-mediated disorders with a B cell component.

RasGRP Ras guanine nucleotide exchange factors in cancer

RasGRP proteins are activators of Ras and other related small GTPases by the virtue of functioning as guanine nucleotide exchange factors (GEFs). In vertebrates, four RasGRP family members have been described. RasGRP-1 through -4 share many structural domains but there are also subtle differences between each of the different family members. Whereas SOS RasGEFs are ubiquitously expressed, RasGRP proteins are expressed in distinct patterns, such as in different cells of the hematopoietic system and in the brain. Most studies have concentrated on the role of RasGRP proteins in the development and function of immune cell types because of the predominant RasGRP expression profiles in these cells and the immune phenotypes of mice deficient for Rasgrp genes. However, more recent studies demonstrate that RasGRPs also play an important role in tumorigenesis. Examples are skin- and hematological-cancers but also solid malignancies such as melanoma or prostate cancer. These novel studies bring up many new and unanswered questions related to the molecular mechanism of RasGRP-driven oncogenesis, such as new receptor systems that RasGRP appears to respond to as well as regulatory mechanism for RasGRP expression that appear to be perturbed in these cancers. Here we will review some of the known aspects of RasGRP biology in lymphocytes and will discuss the exciting new notion that RasGRP Ras exchange factors play a role in oncogenesis downstream of various growth factor receptors.

Identification of a secondary promoter within the human B cell receptor component gene hCD79b

The human B cell-specific protein, CD79b (also known as Igβ and B29) constitutes an essential signal transduction component of the B cell receptor. Although its function is central to the triggering of B cell terminal differentiation in response to antigen stimulation, the transcriptional determinants that control CD79b gene expression remain poorly defined. In the present study, we explored these determinants using a series of hCD79b transgenic mouse models. Remarkably, we observed that the previously described hCD79b promoter along with its associated enhancer elements and first exon could be deleted without appreciable loss of hCD79b transcriptional activity or tissue specificity. In this deletion setting, a secondary promoter located within exon 2 maintained full levels and specificity of hCD79b transcription. Of note, this secondary promoter was also active, albeit at lower levels, in the wild-type hCD79b locus. The activity of the secondary promoter was dependent on the action(s) of a conserved sequence element mapping to a chromatin DNase I hypersensitive site located within intron 1. mRNA generated from this secondary promoter is predicted to encode an Igβ protein lacking a signal sequence and thus unable to serve normal B cell receptor function. Although the physiologic role of the hCD79b secondary promoter and its encoded protein remain unclear, the current data suggest that it has the capacity to play a role in normal as well as pathologic states in B cell proliferation and function.

Distinct processing of the pre-B cell receptor and the B cell receptor

It has been recently demonstrated that while oligosaccharide moieties of μ heavy chains in the B-cell receptor (BCR) are of the complex type as expected, those of the pre-BCR on the surface of pre-B cells contain oligosaccharide moieties of the high-mannose type only. This is unique, because high-mannose glycans are generally restricted to the endoplasmic reticulum and not presented on the surface of mammalian cells. In the present study, we examined the processing of the unusually glycosylated μ heavy chains in pre-B cells. We demonstrate that the pre-BCR reaches the cell surface by a non-conventional brefeldin A-sensitive monensin-insensitive transport pathway. Although pre-BCR complexes consist of μ heavy chains with high-mannose oligosaccharide moieties, they are stably expressed in the plasma membrane and demonstrate turnover rates similar to those of the BCR. Thus, rapid internalization cannot account for their low surface expression, as previously postulated. Rather, we demonstrate that the low pre-BCR abundance in the plasma membrane results, at least in part, from insufficient production of surrogate light chains, which appears to be a limiting factor in pre-BCR expression.

Aberrant antibody affinity selection in SHIP-deficient B cells

The strength of the Ag receptor signal influences development and negative selection of B cells, and it might also affect B-cell survival and selection in the GC. Here, we have used mice with B-cell-specific deletion of the 5'-inositol phosphatase SHIP as a model to study affinity selection in cells that are hyperresponsive to Ag and cytokine receptor stimulation. In the absence of SHIP, B cells have lower thresholds for Ag- and interferon (IFN)-induced activation, resulting in augmented negative selection in the BM and enhanced B-cell maturation in the periphery. Despite a tendency to spontaneously downregulate surface IgM expression, SHIP deficiency does not alter anergy induction in response to soluble hen-egg lysozyme Ag in the MDA4 transgenic model. SHIP-deficient B cells spontaneously produce isotype-switched antibodies; however, they are poor responders in immunization and infection models. While SHIP-deficient B cells form GCs and undergo mutation, they are not properly selected for high-affinity antibodies. These results illustrate the importance of negative regulation of B-cell responses, as lower thresholds for B-cell activation promote survival of low affinity and deleterious receptors to the detriment of optimal Ab affinity maturation.

Diverse phosphorylation patterns of B cell receptor-associated signaling in naïve and memory human B cells revealed by phosphoflow, a powerful technique to study signaling at the single cell level

Following interaction with cognate antigens, B cells undergo cell activation, proliferation, and differentiation. Ligation of the B cell receptor (BCR) leads to the phosphorylation of BCR-associated signaling proteins within minutes of antigen binding, a process with profound consequences for the fate of the cells and development of effector immunity. Phosphoflow allows a rapid evaluation of various signaling pathways in complex heterogenous cell subsets. This novel technique was used in combination with multi-chromatic flow cytometry (FC) and fluorescent-cell barcoding (FCB) to study phosphorylation of BCR-associated signaling pathways in naïve and memory human B cell subsets. Proteins of the initiation (Syk), propagation (Btk, Akt), and integration (p38MAPK and Erk1/2) signaling units were studied. Switched memory (Sm) CD27+ and Sm CD27- phosphorylation patterns were similar when stimulated with anti-IgA or -IgG. In contrast, naïve and unswitched memory (Um) cells showed significant differences following IgM stimulation. Enhanced phosphorylation of Syk was observed in Um cells, suggesting a lower activation threshold. This is likely the result of higher amounts of IgM on the cell surface, higher pan-Syk levels, and enhanced susceptibility to phosphatase inhibition. All other signaling proteins evaluated also showed some degree of enhanced phosphorylation in Um cells. Furthermore, both the phospholipase C-γ2 (PLC-γ2) and phosphatidylinositol 3-kinase (PI3K) pathways were activated in Um cells, while only the PI3K pathway was activated on naïve cells. Um cells were the only ones that activated signaling pathways when stimulated with fluorescently labeled S. Typhi and S. pneumoniae. Finally, simultaneous evaluation of signaling proteins at the single cell level (multiphosphorylated cells) revealed that interaction with gram positive and negative bacteria resulted in complex and diverse signaling patterns. Phosphoflow holds great potential to accelerate vaccine development by identifying signaling profiles in good/poor responders.

Notch-RBP-J-independent marginal zone B cell development in IgH transgenic mice with VH derived from a natural polyreactive antibody

Both the B cell antigen receptor (BCR) signaling and Notch signaling pathway play important roles in marginal zone (MZ) B cell development; however, if and how these two signaling pathways engage in crosstalk with each other remain unclear. In the present study, IgH transgenic mice (TgV(H)3B4) were crossed with mice with Notch downstream transcription factor RBP-J floxed alleles (RBP-J(f/f)) and Mx-Cre transgene. Subsequently, MZ B cell development was analyzed in 3B4/Cre/RBP-J(f/f) mice that expressed the transgenic 3B4 IgH and exhibited a deficiency in Notch signaling in B cells upon poly (I:C) injection. We observed that MZ B cell numbers were severely reduced, but still detectable in 3B4/Cre/RBP-J(f/f) mice, in contrast to increased numbers of MZ B cells in TgV(H)3B4 mice and almost no MZ B cells in Cre/RBP-J(f/f) mice. The majority of the MZ B cells in the 3B4/Cre/RBP-J(f/f) mice had the same antigen specificity with that of 3B4 antibody, indicating that a particular BCR specificity might direct MZ B cell development in the absence of Notch signaling. The number of MZ B precursor (MZP) cells was reduced sharply in 3B4/Cre/RBP-J(f/f) mice, and the number of transitional stage 1 and transitional stage 2 cells did not change that much, indicating that the interaction between BCR and Notch signaling likely occurred during the T2-MZP stage. Based on the transgenic mouse model, our data indicate that MZ B cells with certain BCR specificity can develop in a Notch-RBP-J independent manner.

New strategies in diffuse large B-cell lymphoma: translating findings from gene expression analyses into clinical practice

Gene expression profiling has had a major impact on our understanding of the biology and heterogeneity of diffuse large B-cell lymphoma (DLBCL). Using this technology, investigators can identify biologic subgroups of DLBCL that provide unique targets for rational therapeutic intervention. This review summarizes these potential targets and updates the progress of clinical development of exciting novel agents for the treatment of DLBCL. Results of ongoing studies suggest that in the near future, we will be able to use gene expression profiling, or an accurate surrogate, to define the best therapeutic approach for individual patients with DLBCL.

Novel agents for diffuse large B-cell lymphoma

INTRODUCTION

Although diffuse large B-cell lymphoma (DLBCL) is commonly considered as a cancer with a high cure rate with conventional therapies recent studies demonstrate that different biological variants of DBLCL exist, and that patients with one DLBCL variant and DLBCL patients who relapse early following rituximab-based therapy have markedly poorer outcomes with conventional management strategies. Over the last decade, there has been an increasing exploration of novel therapies derived from improved understanding of DLBCL biology and tumor-host interactions.

AREAS COVERED

This review examines the biological basis for novel therapeutic approaches in DLBCL and the early clinical data on compounds derived from this research. A description of the expanding options of novel agents and combination therapies for patients with poor risk DLBCL is provided.

EXPERT OPINION

Several promising novel agents and novel therapeutic combinations are under development for patients with poor risk DLBCL. Carefully designed clinical trials that build on our improved understanding of DLBCL biology and utilize tissue samples to examine the activity of novel combination therapies should expand treatment options for DLBCL patients in the future.

4-1BB signaling beyond T cells

Originally discovered as a T cell-activating molecule, 4-1BB (CD137) is now also recognized as an activator of non-T cells, thus imparting a new dimension to its potential in vivo effects. 4-1BB expression is seen on a variety of non-T cells including activated dendritic cells (DCs), monocytes, neutrophils, B cells and natural killer (NK) cells, and promotes their individual effector functions. The T cell- and non-T cell-activating ability of 4-1BB may be the basis of its powerful anti-cancer, anti-autoimmune and anti-viral effects. Here we discuss the consequence and importance of 4-1BB signaling in non-T cells. We consider its effects on immune regulation, and the distinct and/or overlapping pathways involved in these responses, as well as possible therapeutic applications.

All oligosaccharide moieties of the μ chains in the pre-BCR are of the high-mannose type

Although it is well established that pre-BCR signaling governs proliferation and differentiation during B cell development, the components of the pre-BCR that are important for signaling are a matter of controversy. It has been suggested that signaling by the μ heavy chains of the pre-BCR induces survival and differentiation of pre-B cells, while the λ5 part of the pre-BCR is essential for proliferation and clonal expansion. However, the mechanism by which pre-BCR μ chains initiate differentiation signals is not clear. Using two variants of a murine B-lymphocyte cell line that differ only in surface expression of either BCR or pre-BCR, we demonstrated that surface μ chains in the pre-BCR are of the high-mannose type only, while those in the BCR are of the complex type. It is hypothesized that mannose-specific lectin-like molecules on accessory cells or in solution may function as the non-antigen ligand that triggers the pre-BCR.

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.

PDCA expression by B lymphocytes reveals important functional attributes

We have demonstrated in this study the existence of a PDCA-expressing functional B cell population (PDCA+ B lymphocytes), which differentiates from activated conventional B (PDCA-IgM+) lymphocytes. Stimulation with anti-micro, LPS, CpG oligodeoxynucleotide, HSV-1, or CTLA-4 Ig activates the PDCA+ B lymphocytes, leading to cell division and induction of type I IFNs and IDO. Notably, the PDCA+ B lymphocytes are capable of Ag-specific Ab production and Ig class switching, which is corroborated by transfer experiments in B- and PDCA+ B lymphocyte-deficient microMT mice. Importantly, in lupus-prone MRL-Fas(lpr) mice, PDCA+ B lymphocytes remain the principal source of autoantibodies. The PDCA+ B lymphocytes have phenotypes with plasmacytoid dendritic cells, but are a distinct cell population in that they develop from C-kit+B220+ pro-B precursors. Thus, our data suggest that not all PDCA+ cells are dendritic cell-derived plasmacytoid dendritic cells and that a significant majority is the PDCA+ B lymphocyte population having distinct phenotype and function.

Inhibition of Syk with fostamatinib disodium has significant clinical activity in non-Hodgkin lymphoma and chronic lymphocytic leukemia

Certain malignant B cells rely on B-cell receptor (BCR)-mediated survival signals. Spleen tyrosine kinase (Syk) initiates and amplifies the BCR signal. In in vivo analyses of B-cell lymphoma cell lines and primary tumors, Syk inhibition induces apoptosis. These data prompted a phase 1/2 clinical trial of fostamatinib disodium, the first clinically available oral Syk inhibitor, in patients with recurrent B-cell non-Hodgkin lymphoma (B-NHL). Dose-limiting toxicity in the phase 1 portion was neutropenia, diarrhea, and thrombocytopenia, and 200 mg twice daily was chosen for phase 2 testing. Sixty-eight patients with recurrent B-NHL were then enrolled in 3 cohorts: (1) diffuse large B-cell lymphoma (DLBCL), (2) follicular lymphoma (FL), and (3) other NHL, including mantle cell lymphoma (MCL), marginal zone lymphoma (MZL), mucosa-associated lymphoid tissue lymphoma, lymphoplasmacytic lymphomas, and small lymphocytic leukemia/chronic lymphocytic leukemia (SLL/CLL). Common toxicities included diarrhea, fatigue, cytopenias, hypertension, and nausea. Objective response rates were 22% (5 of 23) for DLBCL, 10% (2 of 21) for FL, 55% (6 of 11) for SLL/CLL, and 11% (1/9) for MCL. Median progression-free survival was 4.2 months. Disrupting BCR-induced signaling by inhibiting Syk represents a novel and active therapeutic approach for NHL and SLL/CLL. This trial was registered at www.clinicaltrials.gov as #NCT00446095.

Antigen receptor signaling in the rheumatic diseases

Antigen receptor signaling in lymphocytes has been clearly implicated in the pathogenesis of the rheumatic diseases. Here, we review evidence from mouse models in which B-cell and T-cell signaling machinery is perturbed as well as data from functional studies of primary human lymphocytes and recent advances in human genetics. B-cell receptor hyper-responsiveness is identified as a nearly universal characteristic of systemic lupus erythema-tosus in mice and humans. Impaired and enhanced T-cell receptor signaling are both associated with distinct inflammatory diseases in mice. Mechanisms by which these pathways contribute to disease in mouse models and patients are under active investigation.

Autosomal recessive agammaglobulinemia: novel insights from mutations in Ig-beta

Agammaglobulinemia is a rare primary immuno-deficiency characterized by an early block of B-cell development in the bone marrow resulting in the absence of peripheral B cells and low/absent immunoglobulin serum levels. Mutations in the Bruton tyrosine kinase and in components of the pre-B-cell receptor (pre-BCR), such as mu heavy chain, surrogate light chain, and Igalpha have been found in 85% to 90% of patients affected by this disease. Here we review the recent advances in the characterization of molecular defects underlying an early block in B-cell development, focusing on the novel finding of the first two patients with agammaglobulinemia caused by mutations in Igbeta, the transmembrane protein that associates with Igalpha as part of the pre-BCR complex. Characterization of novel genetic defects involving components of the pre-BCR is crucial for a better understanding of the biology of early B-cell development and may have therapeutic and prognostic implications.

B cell receptor basal signaling regulates antigen-induced Ig light chain rearrangements

BCR editing in the bone marrow contributes to B cell tolerance by orchestrating secondary Ig rearrangements in self-reactive B cells. We have recently shown that loss of the BCR or a pharmacologic blockade of BCR proximal signaling pathways results in a global "back-differentiation" response in which immature B cells down-regulate genes important for the mature B cell program and up-regulate genes characteristic of earlier stages of B cell development. These observations led us to test the hypothesis that self-Ag-induced down-regulation of the BCR, and not self-Ag-induced positive signals, lead to Rag induction and hence receptor editing. Supporting this hypothesis, we found that immature B cells from xid (x-linked immunodeficiency) mice induce re-expression of a Rag2-GFP bacterial artificial chromosome reporter as well as wild-type immature B cells following Ag incubation. Incubation of immature B cells with self-Ag leads to a striking reversal in differentiation to the pro-/pre-B stage of development, consistent with the idea that back-differentiation results in the reinduction of genes required for L chain rearrangement and receptor editing. Importantly, Rag induction, the back-differentiation response to Ag, and editing in immature and pre-B cells are inhibited by a combination of phorbol ester and calcium ionophore, agents that bypass proximal signaling pathways and mimic BCR signaling. Thus, mimicking positive BCR signals actually inhibits receptor editing. These findings support a model whereby Ag-induced receptor editing is inhibited by BCR basal signaling on developing B cells; BCR down-regulation removes this basal signal, thereby initiating receptor editing.

Immunological systems biology: Gene expression analysis of B-cell development in Ramos B-cells

B-cell development into antibody producing cells is a complex process that relies on the tightly controlled production of hundreds of genes and proteins. A B-cell is activated through the B-cell receptor (BCR) and this activation is modified by different co-stimulatory or inhibitory co-receptors. The concerted action of signals from BCR and from co-receptors decides the fate of the B-cells. The majority of B-cells enter apoptosis, while some of them progress through the cell cycle and become, for example, antibody producing plasma cells. We studied BCR stimulated Ramos B-cells to explore the expression of BCR pathway, cell cycle and apoptosis related genes. We followed, using microarrays, the gene expression for several days after BCR engagement. Several bioinformatics methods were used to investigate the properties and common features of co-expressed genes. Certain gene ontologies have statistically significant enrichment into clusters of similarly expressed genes. The cell signaling pathways and gene expression data were combined to reveal detailed information about biological processes and B-cell systems biology. The results provide knowledge of the development of adaptive immunity and clues about how the pathways are affected by regulation of the expression of genes.

Autoantigen-B cell antigen receptor interactions that regulate expression of B cell antigen receptor Loci

Levels of AgR (BCR) expression are regulated during B cell development, activation, and induction of tolerance. The mechanisms responsible for and consequences of this regulation are poorly understood. We have described a class of DNA-based autoantigen-reactive B cell that down-regulates BCR expression during development to mature follicular phenotype. In this study, we show that at immature stages of primary differentiation, individual B cells of this type can dynamically modulate levels of expression of BCR in inverse proportion to degree of autoantigen engagement and induced BCR signaling. These adjustments in BCR expression are not associated with cell death, BCR revision, or altered development, and do not require TLR 9. Strikingly, modulation of BCR subunit gene RNA levels and transcription parallels these changes in BCR expression, indicating a direct link between autoantigen-BCR interactions of this type and regulation of transcription of BCR-encoding loci. We propose that this adaptive process allows this class of autoreactive B cell to avoid conventional tolerance pathways and promotes development to mature phenotype.

Association of protein kinase C-δ with the B cell antigen receptor complex

Protein kinase C (PKC)-delta is a diacylglycerol-dependent, calcium-independent novel PKC isoform and has been demonstrated to exert negative regulatory functions in B lymphocytes as well as in mast cells. Whereas in mast cells PKC-delta functionally interacts with the high-affinity receptor for IgE, FcepsilonR1, no such association has been described for the B cell antigen receptor (BCR). In this report, for the first time, we demonstrate the interaction of PKC-delta with different classes of BCR by means of affinity purification and native protein complex analysis. Using a C-terminally truncated Ig-alpha as well as non-phosphorylated and phosphorylated peptides representing C-terminal regions of Ig-alpha, the dependence of this BCR/PKC-delta interaction on tyrosine-phosphorylated Ig-alpha is shown. Finally, splenocytes from PKC-delta-deficient mice are found to exert reduced phosphorylation of PKD (a.k.a. PKC-mu) in response to BCR engagement, suggesting the early, membrane-proximal activation of an attenuating kinase complex including PKC-delta and PKD.

Dok-3 plays a nonredundant role in negative regulation of B-cell activation

p62(dok) and Dok-3 are members of the Dok family of adaptors found in B cells, with the former cloned as a substrate of the p210(bcr/abl) oncoprotein in Ph + chronic myelogenous leukemia. A role for p62(dok) in FcgammaRIIB-mediated negative regulation of B-cell proliferation had been established previously. Here, we generated Dok-3(-/-) mice to assess the function of Dok-3 in B cells. Mice lacking Dok-3 have normal B-cell development but possess higher level of IgM antibodies in their sera. In comparison to wild-type mice, Dok-3(-/-) mice mounted significantly enhanced humoral immune responses to T cell-independent type I and II antigens. Dok-3-deficient B cells hyperproliferated, exhibited elevated level of calcium signaling as well as enhanced activation of NF-kappaB, JNK, and p38MAPK in response to B-cell receptor (BCR) engagement. In the absence of Dok-3, the localization of the inhibitory phosphatase SHIP-1 to the plasma membrane is intact while its phosphorylation is compromised, suggesting that Dok-3 could function to facilitate or sustain the activation of SHIP-1. The phenotype and responses of Dok-3(-/-) mice and B cells could be differentiated from those of the Dok-1(-/-) counterparts. Hence, we propose that Dok-3 plays a distinct and nonredundant role in the negative regulation of BCR signaling.

The T-cell antigen receptor: a logical response to an unknown ligand

The immune system can be roughly divided into innate and adaptive compartments. The adaptive compartment includes the B and T lymphocytes, whose antigen receptors are generated by recombination of gene segments. The consequence is that the creation of self-reactive lymphocytes is unavoidable. For the host to remain viable, the immune system has evolved a strategy for removing autoimmune lymphocytes during development. This review discusses how T lymphocytes are generated, how they recognize antigens, and how their antigen receptor directs the removal of self-reactive T cells.

Spi-C has opposing effects to PU.1 on gene expression in progenitor B cells

The Ets transcription factor Spi-C, expressed in B cells and macrophages, is closely related to PU.1 and has the ability to recognize the same DNA consensus sequence. However, the function of Spi-C has yet to be determined. The purpose of this study is to further examine Spi-C activity in B cell development. First, using retroviral vectors to infect PU.1(-/-) fetal liver progenitors, Spi-C was found to be inefficient at inducing cytokine-dependent proliferation and differentiation of progenitor B (pro-B) cells or macrophages relative to PU.1 or Spi-B. Next, Spi-C was ectopically expressed in fetal liver-derived, IL-7-dependent pro-B cell lines. Wild-type (WT) pro-B cells ectopically expressing Spi-C (WT-Spi-C) have several phenotypic characteristics of pre-B cells such as increased CD25 and decreased c-Kit surface expression. In addition, WT-Spi-C pro-B cells express increased levels of IgH sterile transcripts and reduced levels of expression and transcription of the FcgammaRIIb gene. Gel-shift analysis suggests that Spi-C, ectopically expressed in pro-B cells, can bind PU.1 consensus sites in the IgH intronic enhancer and FcgammaRIIb promoter. Transient transfection analysis demonstrated that PU.1 functions to repress the IgH intronic enhancer and activate the FcgammaRIIb promoter, while Spi-C opposes these activities. WT-Spi-C pro-B cells have reduced levels of dimethylation on lysine 9 of histone H3 within the IgH 3' regulatory region, indicating that Spi-C can contribute to removal of repressive features in the IgH locus. Overall, these studies suggest that Spi-C may promote B cell differentiation by modulating the activity of PU.1-dependent genes.

Tissue-specific chromatin modifications at a multigene locus generate asymmetric transcriptional interactions

Random assortment within mammalian genomes juxtaposes genes with distinct expression profiles. This organization, along with the prevalence of long-range regulatory controls, generates a potential for aberrant transcriptional interactions. The human CD79b/GH locus contains six tightly linked genes with three mutually exclusive tissue specificities and interdigitated control elements. One consequence of this compact organization is that the pituitary cell-specific transcriptional events that activate hGH-N also trigger ectopic activation of CD79b. However, the B-cell-specific events that activate CD79b do not trigger reciprocal activation of hGH-N. Here we utilized DNase I hypersensitive site mapping, chromatin immunoprecipitation, and transgenic models to explore the basis for this asymmetric relationship. The results reveal tissue-specific patterns of chromatin structures and transcriptional controls at the CD79b/GH locus in B cells distinct from those in the pituitary gland and placenta. These three unique transcriptional environments suggest a set of corresponding gene expression pathways and transcriptional interactions that are likely to be found juxtaposed at multiple sites within the eukaryotic genome.

B29 gene silencing in pituitary cells is regulated by its 3' enhancer

B cell-specific B29 (Igbeta, CD79b) genes in rat, mouse, and human are situated between the 5' growth hormone (GH) locus control region and the 3' GH gene cluster. The entire GH genomic region is DNase 1 hypersensitive in GH-expressing pituitary cells, which predicts an "open" chromatin configuration, and yet B29 is not expressed. The B29 promoter and enhancers exhibit histone deacetylation in pituitary cells, but histone deacetylase inhibition failed to activate B29 expression. The B29 promoter and a 3' enhancer showed local dense DNA methylation in both pituitary and non-lymphoid cells consistent with gene silencing. However, DNA methyltransferase inhibition did not activate B29 expression either. B29 promoter constructs were minimally activated in transfected pituitary cells. Co-transfection of the B cell-specific octamer transcriptional co-activator Bob1 with the B29 promoter construct resulted in high level promoter activity in pituitary cells comparable to B29 promoter activity in transfected B cells. Unexpectedly, inclusion of the B29 3' enhancer in B29 promoter constructs strongly inhibited B29 transcriptional activity even when pituitary cells were co-transfected with Bob1. Both Oct-1 and Pit-1 bind the B29 3' enhancer in in vitro electrophoretic mobility shift assay and in in vivo chromatin immunoprecipitation analyses. These data indicate that the GH locus-embedded, tissue-specific B29 gene is silenced in GH-expressing pituitary cells by epigenetic mechanisms, the lack of a B cell-specific transcription factor, and likely by the B29 3' enhancer acting as a powerful silencer in a context and tissue-specific manner.

Signalling pathways in B cells: implications for autoimmunity

Following investigations of the pathogenic role of autoantibodies in rheumatic diseases, preclinical and clinical studies suggest a more central role of B cells in the maintenance of the disease process beyond just being precursors of (auto)antibody-producing plasma cells. Detailed analyses have implicated a number of surface molecules and subsequent downstream signalling pathways in the regulation of the events induced by BCR engagement. In this review, we discuss the potential role of molecules involved in altered B cell longevity, especially molecules involved in apoptosis (bcl-2, bcl-x, mutations in the Fas/Fas-L pathway), as well as molecules that might alter activation thresholds of B cells (CD19, CD21, CD22, lyn, SHP, SHIP-1) in the development of autoimmunity. Although focused on intrinsic B cell abnormalities, the complexity of interactions of B cells with other immune cells also makes it possible that increased B cell activation can be induced by distortions in the interaction with other cells. Further delineation of these alterations of B cell function in autoimmune conditions will allow development of more precise B cell-directed therapies beyond drastic B cell depletion, with the potential to improve the risk-benefit ratio of the treatments of autoimmune diseases.

Mouse follicular and marginal zone B cells show differential expression of Dnmt3a and sensitivity to 5′-azacytidine

Mature B cells in the spleen of mouse and human comprise of two main subsets, the follicular (Fo) B cells and the marginal zone (MZ) B cells. In this study, we report that Fo and MZ B cells express different levels of DNA methyltransferase Dnmt3a. By using RT-PCR and immunohistochemistry, we found that Fo B cells expressed high level of Dnmt3a while MZ B cells expressed little. Treatment of mice by in vivo administration of 5'-azacytidine, an inhibitor of DNA methyltransferases, induced B cell loss in both the bone marrow and the spleen. We noticed that this treatment resulted in a much faster and more severe disappearance of Fo B cells than MZ B cells in the spleen. Further analysis showed that MZ progenitors increased significantly in mice treated with 5'-azacytidine. These results suggest that epigenetic mechanisms involving Dnmt3a might participate in the development of B cells including the differentiation of Fo B cells and MZ B cells in the periphery.

ITAM-mediated tonic signalling through pre-BCR and BCR complexes

Studies carried out over the past few years provide strong support for the idea that Ig alpha-Ig beta-containing complexes such as the pre-B-cell receptor and the B-cell receptor can signal independently of ligand engagement, and this has been termed tonic signalling. In this Review, I discuss recent literature that is relevant to the potential mechanisms by which tonic signals are initiated and regulated, and discuss views on how tonic and ligand-dependent (aggregation-mediated) signalling differ. These mechanisms are relevant to the possibility that tonic signals generated through immunoreceptor tyrosine-based activation motif (ITAM)-containing proteins that are expressed by oncogenic viruses induce transformation in non-haematopoietic cells.

Regulation of intracellular signalling by the terminal membrane proteins of members of the Gammaherpesvirinae

The human gamma(1)-herpesvirus Epstein-Barr virus (EBV) and the gamma(2)-herpesviruses Kaposi's sarcoma-associated herpesvirus (KSHV), rhesus rhadinovirus (RRV), herpesvirus saimiri (HVS) and herpesvirus ateles (HVA) all contain genes located adjacent to the terminal-repeat region of their genomes, encoding membrane proteins involved in signal transduction. Designated 'terminal membrane proteins' (TMPs) because of their localization in the viral genome, they interact with a variety of cellular signalling molecules, such as non-receptor protein tyrosine kinases, tumour-necrosis factor receptor-associated factors, Ras and Janus kinase (JAK), thereby initiating further downstream signalling cascades, such as the MAPK, PI3K/Akt, NF-kappaB and JAK/STAT pathways. In the case of TMPs expressed during latent persistence of EBV and HVS (LMP1, LMP2A, Stp and Tip), their modulation of intracellular signalling pathways has been linked to the provision of survival signals to latently infected cells and, hence, a contribution to occasional cellular transformation. In contrast, activation of similar pathways by TMPs of KSHV (K1 and K15) and RRV (R1), expressed during lytic replication, may extend the lifespan of virus-producing cells, alter their migration and/or modulate antiviral immune responses. Whether R1 and K1 contribute to the oncogenic properties of KSHV and RRV has not been established satisfactorily, despite their transforming qualities in experimental settings.

Antibody production and in vitro behavior of CD27-defined B-cell subsets: persistent hepatitis C virus infection changes the rules

There is growing interest in the tendency of B cells to change their functional program in response to overwhelming antigen loading, perhaps by regulating specific parameters, such as efficiency of activation, proliferation rate, differentiation to antibody-secreting cells (ASC), and rate of cell death in culture. We show that individuals persistently infected with hepatitis C virus (HCV) carry high levels of circulating immunoglobulin G (IgG) and IgG-secreting cells (IgG-ASC). Thus, generalized polyclonal activation of B-cell functions may be supposed. While IgGs include virus-related and unrelated antibodies, IgG-ASC do not include HCV-specific plasma cells. Despite signs of widespread activation, B cells do not accumulate and memory B cells seem to be reduced in the blood of HCV-infected individuals. This apparent discrepancy may reflect the unconventional activation kinetics and functional responsiveness of the CD27+ B-cell subset in vitro. Following stimulation with T-cell-derived signals in the absence of B-cell receptor (BCR) engagement, CD27+ B cells do not expand but rapidly differentiate to secrete Ig and then undergo apoptosis. We propose that their enhanced sensitivity to BCR-independent noncognate T-cell help maintains a constant level of nonspecific serum antibodies and ASC and serves as a backup mechanism of feedback inhibition to prevent exaggerated B-cell responses that could be the cause of significant immunopathology.

Evidence of marginal-zone B cell-positive selection in spleen

Antigen receptor-mediated signaling is critical for the development and survival of B cells. However, it has not been established whether B cell development requires a signal from self-ligand engagement at the immature stage, a process known as "positive selection." Here, using a monoclonal B cell receptor (BCR) mouse line, specific for the self-Thy-1/CD90 glycoprotein, we demonstrate that BCR crosslinking by low-dose self-antigen promotes survival of immature B cells in culture. In spleen, an increase in BCR signaling strength, induced by low-dose self-antigen, directed naive immature B cells to mature, not into the default follicular B cell fate, but instead into the marginal-zone B cell subset. These data indicate that positive selection can occur in developing B cells and that BCR signal strength is a key factor in deciding between two functionally distinct mature B cell compartments in the microenvironment of the spleen.

A new model of pre-B acute lymphoblastic leukemia chemically induced in rats

OBJECTIVE

Although B acute lymphoblastic leukemia (B-ALL) is the most common leukemia among children, no chemically inducible model of this leukemia has yet been described in vivo.

METHODS

Leukemia was chemically induced in male WKAH/Hkm rats by a nitrosourea derivative, N-butylnitrosourea (BNU), an alkylating agent, administered orally 5 days a week for 24 weeks. Development of leukemia was monitored by clinical observation, follow-up of blood parameters, and appearance of blast cells in peripheral blood samples. The phenotype of the leukemia was determined by cytological examination, cytochemical reactions, and by immunophenotyping of bone marrow cells using various markers. The feasibility of leukemia transplantation was investigated. Clonality and karyotype analyses were also performed.

RESULTS

We observed the appearance of acute leukemia in 60% of the rats treated with BNU. Of these, 65% developed pre-B-ALL, which was serially transplantable to healthy WKAH/Hkm male rats. Karyotype analysis did not reveal clonal abnormalities. Clonality determined by immunoglobulin gene rearrangement sequencing disclosed that the pre-B-ALL were mostly oligoclonal.

CONCLUSION

This new in vivo model of inducible pre-B-ALL might be useful for investigating the effects of co-initiating or promoting agents suspected to be involved in leukemia development, and for disclosing new molecular events leading to leukemogenic processes.