Single round of antigen receptor signaling programs naive B cells to receive T cell help

Immune Biomarkers at Birth Predict Lower Respiratory Tract Infection Risk in a Large Birth Cohort

Lower respiratory tract infections (LRTIs) remain the leading cause of infant morbidity and mortality worldwide and affect long-term respiratory health. Identifying immunological determinants of LRTI susceptibility may help stratify disease risk and identify therapies. This study aimed to identify neonatal immunological factors predicting LRTI risk in infancy. Cord blood plasma from 191 neonates from the Boston Birth Cohort was analyzed for 28 soluble immune factors. LRTI was defined as bronchiolitis, bronchitis, or pneumonia during the first year of life. Welch's t-test demonstrated significantly higher log10 transformed concentrations of IL-17 and IFNγ in the LRTI group compared to neonates without LRTI in the first year of life (p < 0.05). Risk associations were determined using multivariate survival models. There were 29 infants with LRTIs. High cord blood levels of IFNγ (aHR = 2.35, 95% CI 1.07-5.17), TNF-β (aHR = 2.86, 95% CI 1.27-6.47), MIP-1α (aHR = 2.82, 95% CI 1.22-6.51), and MIP-1β (aHR = 2.34, 95% CI 1.05-5.20) were associated with a higher risk of LRTIs. RANTES was associated with a lower risk (aHR = 0.43, 95% CI 0.19-0.97). Soluble immune factors linked to antiviral immunity (IFNγ) and cytokines mediating inflammatory responses (TNF-β), and cell homing (MIP-1α/b), at birth were associated with an increased risk of LRTIs during infancy.

Immunological characterization and comparison of children with COVID-19 from their adult counterparts at single-cell resolution

Introduction

The immunological characteristics that could protect children with coronavirus disease 2019 (COVID-19) from severe or fatal illnesses have not been fully understood yet.

Methods

Here, we performed single-cell RNA sequencing (scRNA-seq) analysis on peripheral blood samples of 15 children (8 with COVID-19) and compared them to 18 adults (13 with COVID-19).

Results

The child-adult integrated single cell data indicated that children with the disease presented a restrained response to type I interferon in most of the major immune cell types, along with suppression of upstream interferon regulatory factor and toll-like receptor expression in monocytes, which was confirmed by in vitro interferon stimulation assays. Unlike adult patients, children with COVID-19 showed lower frequencies of activated proinflammatory CD14+ monocytes, possibly explaining the rareness of cytokine storm in them. Notably, natural killer (NK) cells in pediatric patients displayed potent cytotoxicity with a rich expression of cytotoxic molecules and upregulated cytotoxic pathways, whereas the cellular senescence, along with the Notch signaling pathway, was significantly downregulated in NK cells, all suggesting more robust cytotoxicity in NK cells of children than adult patients that was further confirmed by CD107a degranulation assays. Lastly, a modest adaptive immune response was evident with more naïve T cells but less activated and proliferated T cells while less naïve B cells but more activated B cells in children over adult patients.

Conclusion

Conclusively, this preliminary study revealed distinct cell frequency and activation status of major immune cell types, particularly more robust NK cell cytotoxicity in PBMC that might help protect children from severe COVID-19.

Synergy and antagonism in the integration of BCR and CD40 signals that control B-cell proliferation

In response to infection or vaccination, a successful antibody response must enrich high-affinity antigen-reactive B-cells through positive selection, but eliminate auto-reactive B-cells through negative selection. B-cells receive signals from the B-cell receptor (BCR) which binds the antigen, and the CD40 receptor which is stimulated by neighboring T-cells that also recognize the antigen. How BCR and CD40 signaling are integrated quantitatively to jointly determine B-cell fate decision and proliferation remains unclear. To investigate this, we developed a differential-equations-based model of the BCR and CD40 signaling networks activating NFκB. Our model accurately recapitulates the NFκB dynamics of B-cells stimulated through their BCR and CD40 receptors, correctly predicting that costimulation induces more NFκB activity. However, when linking it to established cell fate decision models of cell survival and cell cycle control, it predicted potentiated population expansion that was not observed experimentally. We found that this discrepancy was due to a time-dependent functional antagonism exacerbated by BCR-induced caspase activity that can trigger apoptosis in founder cells, unless NFκB-induced survival gene expression protects B-cells in time. Guided by model predictions, sequential co-stimulation experiments revealed how the temporal dynamics of BCR and CD40 signaling control the fate decision between negative and positive selection of B-cell clonal expansion. Our quantitative findings highlight a complex non-monotonic integration of BCR and CD40 signals that is controlled by a balance between NFκB and cell-death pathways, and suggest a mechanism for regulating the stringency of B-cell selection during an antibody response.

The NFκB signaling system in the generation of B-cell subsets: from germinal center B cells to memory B cells and plasma cells

Memory B cells and antibody-secreting cells are the two prime effector B cell populations that drive infection- and vaccine-induced long-term antibody-mediated immunity. The antibody-mediated immunity mostly relies on the formation of specialized structures within secondary lymphoid organs, called germinal centers (GCs), that facilitate the interactions between B cells, T cells, and antigen-presenting cells. Antigen-activated B cells may proliferate and differentiate into GC-independent plasmablasts and memory B cells or differentiate into GC B cells. The GC B cells undergo proliferation coupled to somatic hypermutation of their immunoglobulin genes for antibody affinity maturation. Subsequently, affinity mature GC B cells differentiate into GC-dependent plasma cells and memory B cells. Here, we review how the NFκB signaling system controls B cell proliferation and the generation of GC B cells, plasmablasts/plasma cells, and memory B cells. We also identify and discuss some important unanswered questions in this connection.

Longitudinal multi-functional analysis identified responses of T cells, B cells, and monocytes as hallmarks of immunotherapy tolerance in patients with merkel cell carcinoma

PURPOSE

Merkel cell carcinoma (MCC) is a neuroendocrine carcinoma originating in the skin. Studies are needed to determine the mechanisms of immune escape in patients with MCC, and malignant cell conditions that promote immune evasion.

METHODS

We used Single-cell RNA sequencing (scRNA-seq) to determine cellular features associated with MCC disease trajectory. A longitudinal multi-omics study was performed using scRNA-seq data of peripheral blood harvested from four-time points. Six major cell types and fifteen cell subgroups were identified and confirmed their presence by expression of characteristic markers. The expression patterns and specific changes of different cells at different time points were investigated. Subsequently, bulk RNA data was used to validate key findings.

RESULTS

The dynamic characteristics of the cells were identified during the critical period between benign improvement and acquisition of resistance. Combined with the results of the validation cohort, the resistance program expressed in the relapse stage is mainly associated with T cell exhaustion and immune cell crosstalk disorder. Coinciding with immune escape, we also identified a decrease non-classical monocytes and an expansion of classical monocytes with features of high inflammation and immune deficiency.

CONCLUSION

Changes in cellular status, such as depletion of T cells and dysregulation of B cell proliferation and differentiation, may lead to drug resistance in MCC patients. Meanwhile, the widespread decreased antigen presentation ability and immune disorders caused by deletion of MHC class II gene expression should not be ignored.

NF-κB subunits direct kinetically distinct transcriptional cascades in antigen receptor-activated B cells

The nuclear factor kappa B (NF-κB) family of transcription factors orchestrates signal-induced gene expression in diverse cell types. Cellular responses to NF-κB activation are regulated at the level of cell and signal specificity, as well as differential use of family members (subunit specificity). Here we used time-dependent multi-omics to investigate the selective functions of Rel and RelA, two closely related NF-κB proteins, in primary B lymphocytes activated via the B cell receptor. Despite large numbers of shared binding sites genome wide, Rel and RelA directed kinetically distinct cascades of gene expression in activated B cells. Single-cell RNA sequencing revealed marked heterogeneity of Rel- and RelA-specific responses, and sequential binding of these factors was not a major mechanism of protracted transcription. Moreover, nuclear co-expression of Rel and RelA led to functional antagonism between the factors. By rigorously identifying the target genes of each NF-κB subunit, these studies provide insights into exclusive functions of Rel and RelA in immunity and cancer.

Characterization of a mouse model of ICF syndrome reveals enhanced CD19 activation in inducing hypogammaglobulinemia

Immunodeficiency, centromeric instability and facial anomalies (ICF) syndrome is a rare autosomal recessive disorder characterized by DNA hypomethylation and antibody deficiency. It is caused by mutations in DNMT3B, ZBTB24, CDCA7 or HELLS . While progress has been made in elucidating the roles of these genes in regulating DNA methylation, little is known about the pathogenesis of the life-threatening hypogammaglobulinemia phenotype. Here we show that mice deficient for Zbtb24 in the hematopoietic lineage recapitulate major clinical features of patients with ICF syndrome. Specifically, Vav-Cre-mediated ablation of Zbtb24 does not affect lymphocyte development but results in reduced plasma cells and low levels of IgM, IgG1 and IgA. Zbtb24 -deficient mice are hyper- and hypo-responsive to T-dependent and Tindependent type 2 antigens, respectively, and marginal zone B cell activation is impaired. B cells from Zbtb24 -deficient mice display elevated CD19 phosphorylation. Heterozygous disruption of Cd19 can revert the hypogammaglobulinemia phenotype in these mice. Mechanistically, Il5ra (interleukin-5 receptor subunit alpha) is derepressed in Zbtb24 -deficient B cells, and elevated IL-5 signaling enhances CD19 phosphorylation. Our results reveal a novel link between IL-5 signaling and CD19 activation and suggest that abnormal CD19 activity contributes to immunodeficiency in ICF syndrome.

SIGNIFICANCE STATEMENT

ICF syndrome is a rare immunodeficiency disorder first reported in the 1970s. The lack of appropriate animal models has hindered the investigation of the pathogenesis of antibody deficiency, the major cause of death in ICF syndrome. Here we show that, in mice, disruption of Zbtb24 , one of the ICF-related genes, in the hematopoietic lineage results in low levels of immunoglobulins. Characterization of these mice reveals abnormal B cell activation due to elevated CD19 phosphorylation. Mechanistically, Il5ra (interleukin-5 receptor subunit alpha) is derepressed in Zbtb24 -deficient B cells, and increased IL-5 signaling enhances CD19 phosphorylation.

From Antibody Repertoires to Cell-Cell Interactions to Molecular Networks: Bridging Scales in the Germinal Center

Antibody-mediated adaptive immunity must provide effective long-term protection with minimal adverse effects, against rapidly mutating pathogens, in a human population with diverse ages, genetics, and immune histories. In order to grasp and leverage the complexities of the antibody response, we advocate for a mechanistic understanding of the multiscale germinal center (GC) reaction - the process by which precursor B-cells evolve high-affinity antigen-specific antibodies, forming an effector repertoire of plasma and memory cells for decades-long protection. The regulatory dynamics of B-cells within the GC are complex, and unfold across multiple interacting spatial and temporal scales. At the organism scale, over weeks to years, the antibody sequence repertoire formed by various B-cell clonal lineages modulates antibody quantity and quality over time. At the tissue and cellular scale, over hours to weeks, B-cells undergo selection via spatially distributed interactions with local stroma, antigen, and helper T-cells. At the molecular scale, over seconds to days, intracellular signaling, transcriptional, and epigenetic networks modulate B-cell fates and shape their clonal lineages. We summarize our current understanding within each of these scales, and identify missing links in connecting them. We suggest that quantitative multi-scale mathematical models of B-cell and GC reaction dynamics provide predictive frameworks that can apply basic immunological knowledge to practical challenges such as rational vaccine design.

Postmitotic G1 phase survivin drives mitogen-independent cell division of B lymphocytes

The prevailing dogma is that renewed mitogenic signaling is essential to traverse G1 phase of the cell cycle after each division. B lymphocytes undergo multiple mitotic divisions, termed clonal expansion, to expand antigen-specific cells that mediate effective immunity. Here we demonstrate that B cells that have undergone one cell division continue to proliferate even in absence of further mitogenic signals. This mitogen-independent proliferation is accompanied by an altered G1 phase marked by transcriptomic and proteomic features of G2/M. Survivin, a G2/M-specific oncogene, is required in G1 to achieve mitogen-independent proliferation.

B and T lymphocytes of the adaptive immune system undergo proliferative bursts to generate pools of antigen-specific cells for effective immunity. Here we show that in contrast to the canonical view that G1 progression signals are essential after mitosis to reenter S phase, B lymphocytes sustain several rounds of mitogen-independent cell division following the first mitosis. Such division appears to be driven by unique characteristics of the postmitotic G1 phase that has features of S and G2/M phases. Birc5 (survivin), a protein associated with chromosome segregation in G2/M, is expressed in the G1 phase of divided B cells and is necessary for mitogen-independent divisions. The partially active G1 phase and propensity for apoptosis inherited after each division may underlie rapid proliferation and cell death, which are hallmarks of B cell proliferative responses.

The Dual Role of CCR5 in the Course of Influenza Infection: Exploring Treatment Opportunities

Influenza is one of the most relevant respiratory viruses to human health causing annual epidemics, and recurrent pandemics. Influenza disease is principally associated with inappropriate activation of the immune response. Chemokine receptor 5 (CCR5) and its cognate chemokines CCL3, CCL4 and CCL5 are rapidly induced upon influenza infection, contributing to leukocyte recruitment into the airways and a consequent effective antiviral response. Here we discuss the existing evidence for CCR5 role in the host immune responses to influenza virus. Complete absence of CCR5 in mice revealed the receptor’s role in coping with influenza via the recruitment of early memory CD8+ T cells, B cell activation and later recruitment of activated CD4+ T cells. Moreover, CCR5 contributes to inflammatory resolution by enhancing alveolar macrophages survival and reprogramming macrophages to pro-resolving phenotypes. In contrast, CCR5 activation is associated with excessive recruitment of neutrophils, inflammatory monocytes, and NK cells in models of severe influenza pneumonia. The available data suggests that, while CCL5 can play a protective role in influenza infection, CCL3 may contribute to an overwhelming inflammatory process that can harm the lung tissue. In humans, the gene encoding CCR5 might contain a 32-base pair deletion, resulting in a truncated protein. While discordant data in literature regarding this CCR5 mutation and influenza severity, the association of CCR5delta32 and HIV resistance fostered the development of different CCR5 inhibitors, now being tested in lung inflammation therapy. The potential use of CCR5 inhibitors to modulate the inflammatory response in severe human influenza infections is to be addressed.

RUNX1 Regulates a Transcription Program That Affects the Dynamics of Cell Cycle Entry of Naive Resting B Cells

RUNX1 is a transcription factor that plays key roles in hematopoietic development and in hematopoiesis and lymphopoiesis. In this article, we report that RUNX1 regulates a gene expression program in naive mouse B cells that affects the dynamics of cell cycle entry in response to stimulation of the BCR. Conditional knockout of Runx1 in mouse resting B cells resulted in accelerated entry into S-phase after BCR engagement. Our results indicate that Runx1 regulates the cyclin D2 (Ccnd2) gene, the immediate early genes Fosl2, Atf3, and Egr2, and the Notch pathway gene Rbpj in mouse B cells, reducing the rate at which transcription of these genes increases after BCR stimulation. RUNX1 interacts with the chromatin remodeler SNF-2-related CREB-binding protein activator protein (SRCAP), recruiting it to promoter and enhancer regions of the Ccnd2 gene. BCR-mediated activation triggers switching between binding of RUNX1 and its paralog RUNX3 and between SRCAP and the switch/SNF remodeling complex member BRG1. Binding of BRG1 is increased at the Ccnd2 and Rbpj promoters in the Runx1 knockout cells after BCR stimulation. We also find that RUNX1 exerts positive or negative effects on a number of genes that affect the activation response of mouse resting B cells. These include Cd22 and Bank1, which act as negative regulators of the BCR, and the IFN receptor subunit gene Ifnar1 The hyperresponsiveness of the Runx1 knockout B cells to BCR stimulation and its role in regulating genes that are associated with immune regulation suggest that RUNX1 could be involved in regulating B cell tolerance.

Altered Expression of TSPAN32 during B Cell Activation and Systemic Lupus Erythematosus

Systemic lupus erythematosus (SLE) is a chronic inflammatory disease with various clinical features. Autoreactive B cells play a role in disease pathogenesis, through the production of multiple autoantibodies, which form immune complexes and induce the inflammatory response and tissue damage associated with SLE. Recently, tetraspanins, and in particular, TSPAN32, have been recognized to play a central role in immunity, as they are involved in various biological processes, such as the antigen presentation and the activation of lymphocytes. Evidence suggests that tetraspanins could represent in the future a target for therapeutic purposes in patients suffering from autoimmune/immunoinflammatory disorders. In the present study, by performing in silico analyses of high-throughput data, we evaluated the expression levels of TSPAN32 in B cell activation and investigated its modulation in circulating B cells from SLE patients. Our data show that B cell activation is associated with a significant downregulation of TSPAN32. Additionally, significantly lower levels of TSPAN32 were observed in circulating plasmablasts from SLE patients as compared to healthy donor plasmablasts. In addition, type I interferons (IFNs)-related genes were enriched among the genes negatively correlated to TSPAN32, in SLE plasmablasts. Accordingly, IFN-α is able to induce a dose-dependent downregulation of TSPAN32 in B cells. Overall, the data here presented suggest the potential use of TSPAN32 as a diagnostic marker and therapeutic target for the evaluation and management of humoral immune responses in chronic diseases, such as SLE.

c-Rel Is the Pivotal NF-κB Subunit in Germinal Center Diffuse Large B-Cell Lymphoma: A LYSA Study

Relationships between c-Rel and GCB-DLBCLs remain unclear. We found that strong c-Rel DNA-binding activity was mostly found in GCBs on two independent series of 48 DLBCLs and 66 DLBCLs, the latter issued from the GHEDI series. c-Rel DNA-binding activity was associated with increased REL mRNA expression. Extending the study to the whole GHEDI and Lenz DLBCL published series of 202 and 233 cases, it was found that the c-Rel gene expression profile (GEP) overlapped partially (12%) but only with the GCB GEP and not with the GEP of ABC-DLBCLs. Cases with both overexpression of REL mRNA and c-Rel GEP were defined as those having a c-Rel signature. These cases were GCBs in 88 and 83% of the GHEDI or Lenz's DLBCL series respectively. The c-Rel signature was also associated with various recurrent GCB-DLBCL genetic events, including REL gains, BCL2 translocation, MEF2B, EZH2, CREBBP, and TNFRSF14 mutations and with the EZB GCB genetic subtype. By CGH array, the c-Rel signature was specifically correlated with 2p15-16.1 amplification that includes XPO1, BCL11A, and USP34 and with the 22q11.22 deletion that covers IGLL5 and PRAME. The total number of gene copy number aberrations, so-called genomic imbalance complexity, was decreased in cases with the c-Rel signature. These cases exhibited a better overall survival. Functionally, overexpression of c-Rel induced its constitutive nuclear localization and protected cells against apoptosis while its repression tended to increase cell death. These results show that, clinically and biologically, c-Rel is the pivotal NF-κB subunit in the GCB-DLBCL subgroup. Functionally, c-Rel overexpression could directly promote DLBCL tumorigenesis without need for further activation signals.

Positive Selection in the Light Zone of Germinal Centers

Germinal centers (GCs) are essential sites for the production of high-affinity antibody secreting plasma cells (PCs) and memory-B cells (MBCs), which form the framework of vaccination. Affinity maturation and permissive selection in GCs are key for the production of PCs and MBCs, respectively. For these purposes, GCs positively select “fit” cells in the light zone of the GC and instructs them for one of three known B cell fates: PCs, MBCs and persistent GC-B cells as dark zone entrants. In this review, we provide an overview of the positive selection process and discuss its mechanisms and how B cell fates are instructed.

The Shaping of a B Cell Pool Maximally Responsive to Infections

B cell subsets differ in development, tissue distribution, and mechanisms of activation. In response to infections, however, all can differentiate into extrafollicular plasmablasts that rapidly provide highly protective antibodies, indicating that these plasmablasts are the main humoral immune response effectors. Yet, the effectiveness of this response type depends on the presence of antigen-specific precursors in the circulating mature B cell pool, a pool that is generated initially through the stochastic processes of B cell receptor assembly. Importantly, germinal centers then mold the repertoire of this B cell pool to be increasingly responsive to pathogens by generating a broad array of antimicrobial memory B cells that act as highly effective precursors of extrafollicular plasmablasts. Such B cell repertoire molding occurs in two ways: continuously via the chronic germinal centers of mucosal lymphoid tissues, driven by the presence of the microbiome, and via de novo generated germinal centers following acute infections. For effectively evaluating humoral immunity as a correlate of immune protection, it might be critical to measure memory B cell pools in addition to antibody titers.

NR4A nuclear receptors restrain B cell responses to antigen when second signals are absent or limiting

Antigen stimulation (signal 1) triggers B cell proliferation, and primes B cells to recruit, engage, and respond to T cell help (signal 2). Failure to receive signal 2 within a defined time window results in B cell apoptosis, yet the mechanisms that enforce dependence upon co-stimulation are incompletely understood. Nr4a1-3 encode a small family of orphan nuclear receptors that are rapidly induced by B cell antigen receptor (BCR) stimulation. Here we showed that Nr4a1 and Nr4a3 play partially redundant roles to restrain B cell responses to antigen in the absence of co-stimulation, and do so in part by repressing expression of BATF and consequently MYC. The NR4A family also restrains B cell access to T cell help by repressing expression of the T cell chemokines CCL3 and CCL4, as well as CD86 and ICAM1. Such NR4A-mediated regulation plays a role specifically under conditions of competition for limiting T cell help.

Epigenetic effect of folic acid (FA) on the gene proximal promoter area and mRNA expression of chicken B cell as antigen presenting cells

1. This study evaluated and characterised the effect of folic acid (FA) on chromosomal DNA methylation and the epigenetic result on gene expression control mechanisms in chicken B cells as a model of antigen presenting cells. 2. After FA supplementation, the methylation pattern on the proximal promoter area and mRNA expression of toll-like receptor (TLR) 2b, TLR4, B cell receptor (BCR) immunoglobulin (Ig) β and major histocompatibility complex (MHC) II β chain genes in chicken B cells was observed 3. Chicken B cell line (DT40) cultures were incubated with 0, 1.72 or 3.96 mM of FA for 4 and 8 h and samples were taken at specific time points. After 4 h of incubation, cells were challenged with 0, 1 or 10 µg/ml of lipopolysaccharide (LPS) and samples were collected 4 h post-challenge. 4. FA supplementation modified the methylation patterns of the proximal promoter regions of TLR4, Igß, and MHCII ß chain at 4 and 8 hours of incubation; however, the single CpG dinucleotide of TLR2b remained methylated regardless of the treatment. 5. A positive association was found between FA concentration and percentage DNA methylation on the promoter area of Igβ and TLR2b. However, there was a negative association between FA and MHCII β chain. 6. There were downregulatory effects in TLR4, Igß and MHCII ß chain gene expression after 8 h of incubation, nut not at 4 h. Although incubation time did not affect TLR2b gene expression, FA concentration did, whereby it increased TLR2b expression at 1.72 mM FA (P < 0.05). 7. LPS significant downregulated TLR2b expression, while an interaction between FA and LPS concentration affected TLR4 and Igβ gene expression. 8. In conclusion, the results showed that FA can have an immunomodulatory effect on chicken B cells, possibly affecting their ability to both recognise antigens through the TLR and BCR pathways, and to present it via the MHCII presentation pathway.

B Cell Activation and Response Regulation During Viral Infections

Acute viral infections are characterized by rapid increases in viral load, leading to cellular damage and the resulting induction of complex innate and adaptive antiviral immune responses that cause local and systemic inflammation. Successful antiviral immunity requires the activation of many immune cells, including T cells, natural killer cells, and macrophages. B cells play a unique part through their production of antibodies that can both neutralize and clear viral particles before virus entry into a cell. Protective antibodies are produced even before the first exposure of a pathogen, through the regulated secretion of so-called natural antibodies that are generated even in the complete absence of prior microbial exposure. An early wave of rapidly secreted antibodies from extrafollicular (EF) responses draws on the preexisting naive or memory repertoire of B cells to induce a strong protective response that in kinetics tightly follows the clearance of acute infections, such as with influenza virus. Finally, the generation of germinal centers (GCs) provides long-term protection through production of long-lived plasma cells and memory B cells, which shape and broaden the B cell repertoire for more effective responses following repeat exposures. In this study, we review B cell responses to acute viral infections, primarily influenza virus, from the earliest nonspecific B-1 cell to early, antigen-specific EF responses and finally to GC responses. Throughout, we address known factors that lead to distinct B cell response outcomes and discuss how their functions effect viral clearance, highlighting the critical contributions of each response type to the induction of highly protective antiviral humoral immunity.

Germinal center B cell initiation, GC maturation, and the coevolution of its stromal cell niches

Throughout the developing GC response, B cell survival and fate choices made at the single cell level are dependent on signals received largely through interactions with other cells, often with cognate T cells. The type of signals that a given B cell can encounter is dictated by its location within tissue microarchitecture. The focus of this review is on the initiation and evolution of the GC response at the earliest time points. Here, we review the key factors influencing the progression of GC B cell differentiation that are both stage and context dependent. Finally, we describe the coevolution of niches within and surrounding the GC that influence the outcome of the GC response.

Self-reactivity on a spectrum: A sliding scale of peripheral B cell tolerance

Efficient mechanisms of central tolerance, including receptor editing and deletion, prevent highly self-reactive B cell receptors (BCRs) from populating the periphery. Despite this, modest self-reactivity persists in (and may even be actively selected into) the mature B cell repertoire. In this review, we discuss new insights into mechanisms of peripheral B cell tolerance that restrain mature B cells from mounting inappropriate responses to endogenous antigens, and place recent work into historical context. In particular, we discuss new findings that have arisen from application of a novel in vivo reporter of BCR signaling, Nur77-eGFP, expression of which scales with the degree of self-reactivity in both monoclonal and polyclonal B cell repertoires. We discuss new and historical evidence that self-reactivity is not just tolerated, but actively selected into the peripheral repertoire. We review recent progress in understanding how dual expression of the IgM and IgD BCR isotypes on mature naive follicular B cells tunes responsiveness to endogenous antigen recognition, and discuss how this may be integrated with other features of clonal anergy. Finally, we discuss how expression of Nur77 itself couples chronic antigen stimulation with B cell tolerance.

Synergy between B cell receptor/antigen uptake and MHCII peptide editing relies on HLA-DO tuning

B cell receptors and surface-displayed peptide/MHCII complexes constitute two key components of the B-cell machinery to sense signals and communicate with other cell types during antigen-triggered activation. However, critical pathways synergizing antigen-BCR interaction and antigenic peptide-MHCII presentation remain elusive. Here, we report the discovery of factors involved in establishing such synergy. We applied a single-cell measure coupled with super-resolution microscopy to investigate the integrated function of two lysosomal regulators for peptide loading, HLA-DM and HLA-DO. In model cell lines and human tonsillar B cells, we found that tunable DM/DO stoichiometry governs DM_free activity for exchange of placeholder CLIP peptides with high affinity MHCII ligands. Compared to their naïve counterparts, memory B cells with less DM_free concentrate a higher proportion of CLIP/MHCII in lysosomal compartments. Upon activation mediated by high affinity BCR, DO tuning is synchronized with antigen internalization and rapidly potentiates DM_free activity to optimize antigen presentation for T-cell recruitment.

The Unsolved Puzzle of c-Rel in B Cell Lymphoma

Aberrant constitutive activation of Rel/NF-κB transcription factors is a hallmark of numerous cancers. Of the five Rel family members, c-Rel has the strongest direct links to tumorigenesis. c-Rel is the only member that can malignantly transform lymphoid cells in vitro. Furthermore, c-Rel is implicated in human B cell lymphoma through the frequent occurrence of REL gene locus gains and amplifications. In normal physiology, high c-Rel expression predominates in the hematopoietic lineage and a diverse range of stimuli can trigger enhanced expression and activation of c-Rel. Both expression and activation of c-Rel are tightly regulated on multiple levels, indicating the necessity to keep its functions under control. In this review we meta-analyze and integrate studies reporting gene locus aberrations to provide an overview on the frequency of REL gains in human B cell lymphoma subtypes, namely follicular lymphoma, diffuse large B cell lymphoma, primary mediastinal B cell lymphoma, and classical Hodgkin lymphoma. We also summarize current knowledge on c-Rel expression and protein localization in these human B cell lymphomas and discuss the co-amplification of BCL11A with REL. In addition, we highlight and illustrate key pathways of c-Rel activation and regulation with a specific focus on B cell biology.

The Multifaceted B Cell Response to Influenza Virus

Protection from yearly recurring, highly acute infections with a pathogen that rapidly and continuously evades previously induced protective neutralizing Abs, as seen during seasonal influenza virus infections, can be expected to require a B cell response that is too highly variable, able to adapt rapidly, and able to reduce morbidity and death when sterile immunity cannot be garnered quickly enough. As we outline in this Brief Review, the influenza-specific B cell response is exactly that: it is multifaceted, involves both innate-like and conventional B cells, provides early and later immune protection, employs B cells with distinct BCR repertoires and distinct modes of activation, and continuously adapts to the ever-changing virus while enhancing overall protection. A formidable response to a formidable pathogen.

Transcriptional outcomes and kinetic patterning of gene expression in response to NF-κB activation

Transcription factor nuclear factor kappa B (NF-κB) regulates cellular responses to environmental cues. Many stimuli induce NF-κB transiently, making time-dependent transcriptional outputs a fundamental feature of NF-κB activation. Here we show that NF-κB target genes have distinct kinetic patterns in activated B lymphoma cells. By combining RELA binding, RNA polymerase II (Pol II) recruitment, and perturbation of NF-κB activation, we demonstrate that kinetic differences amongst early- and late-activated RELA target genes can be understood based on chromatin configuration prior to cell activation and RELA-dependent priming, respectively. We also identified genes that were repressed by RELA activation and others that responded to RELA-activated transcription factors. Cumulatively, our studies define an NF-κB-responsive inducible gene cascade in activated B cells.

The nuclear factor kappa B (NF-κB) family of transcription factors regulates cellular responses to a wide variety of environmental cues. These could be extracellular stimuli that activate cell surface receptors, such as pathogens, or intracellular stress signals such as DNA damage or oxidative stress. In response to these triggers, NF-κB proteins accumulate in the cell nucleus, bind to specific DNA sequences in the genome, and thereby modulate gene transcription. Because of the diversity of signals that activate NF-κB and the ubiquity of this pathway in most cell types, cellular outcomes via NF-κB activation must be finely tuned to respond to the initiating stimulus. One mechanism by which NF-κB-dependent gene expression is regulated is by varying the duration of nuclear NF-κB; some signals lead to persistent nuclear NF-κB, while others lead to transient nuclear NF-κB. Consequently, time dependency of transcriptional responses is a unique signature of the initiating stimulus. Here we probed mechanisms that generate kinetic patterns of NF-κB-dependent gene expression in B lymphoma cells responding to a transient NF-κB-activating stimulus. By genetically manipulating NF-κB induction, we identified direct targets of RELA, a member of the NF-κB family, and provide evidence that kinetic patterns are established by a combination of factors that include the chromatin state of genes prior to cell activation and cofactors that work with RELA.

Germinal center B cell development has distinctly regulated stages completed by disengagement from T cell help

To reconcile conflicting reports on the role of CD40 signaling in germinal center (GC) formation, we examined the earliest stages of murine GC B cell differentiation. Peri-follicular GC precursors first expressed intermediate levels of BCL6 while co-expressing the transcription factors RelB and IRF4, the latter known to repress Bcl6 transcription. Transition of GC precursors to the BCL6^hi follicular state was associated with cell division, although the number of required cell divisions was immunogen dose dependent. Potentiating T cell help or CD40 signaling in these GC precursors actively repressed GC B cell maturation and diverted their fate towards plasmablast differentiation, whereas depletion of CD4+ T cells promoted this initial transition. Thus while CD40 signaling in B cells is necessary to generate the immediate precursors of GC B cells, transition to the BCL6^hi follicular state is promoted by a regional and transient diminution of T cell help.

DOI:http://dx.doi.org/10.7554/eLife.19552.001

Transiently antigen-primed B cells return to naive-like state in absence of T-cell help

The perspective that naive B-cell recognition of antigen in the absence of T-cell help causes cell death or anergy is supported by in vivo studies of B cells that are continuously exposed to self-antigens. However, intravital imaging suggests that early B-cell recognition of large foreign antigens may be transient. Whether B cells are tolerized or can be recruited into humoural immune responses following such encounters is not clear. Here we show that in the presence of T-cell help, single transient antigen acquisition is sufficient to recruit B cells into the germinal centre and induce memory and plasma cell responses. In the absence of T-cell help, transiently antigen-primed B cells do not undergo apoptosis in vivo; they return to quiescence and are recruited efficiently into humoural responses upon reacquisition of antigen and T-cell help.

PKCθ links proximal T cell and Notch signaling through localized regulation of the actin cytoskeleton

Notch is a critical regulator of T cell differentiation and is activated through proteolytic cleavage in response to ligand engagement. Using murine myelin-reactive CD4 T cells, we demonstrate that proximal T cell signaling modulates Notch activation by a spatiotemporally constrained mechanism. The protein kinase PKCθ is a critical mediator of signaling by the T cell antigen receptor and the principal costimulatory receptor CD28. PKCθ selectively inactivates the negative regulator of F-actin generation, Coronin 1A, at the center of the T cell interface with the antigen presenting cell (APC). This allows for effective generation of the large actin-based lamellum required for recruitment of the Notch-processing membrane metalloproteinase ADAM10. Such enhancement of Notch activation is critical for efficient T cell proliferation and Th17 differentiation. We reveal a novel mechanism that, through modulation of the cytoskeleton, controls Notch activation at the T cell:APC interface thereby linking T cell receptor and Notch signaling pathways.

Differential requirements for the canonical NF-κB transcription factors c-REL and RELA during the generation and activation of mature B cells

Signaling through the canonical NF-κB pathway is critical for the generation and maintenance of mature B-cells and for antigen-dependent B-cell activation. c-REL (rel) and RELA (rela) are the downstream transcriptional activators of the canonical NF-κB pathway. Studies of B-cells derived from constitutional rel knockout mice and chimeric mice repopulated with rela^−/− fetal liver cells provided evidence that the subunits can have distinct roles during B-cell development. However, the B-cell-intrinsic functions of c-REL and RELA during B-cell generation and antigen-dependent B-cell activation have not been determined in vivo. To clarify this issue, we crossed mice with conditional rel and rela alleles individually or in combination to mice that express Cre-recombinase in B-cells. We here report that, whereas single deletion of rel or rela did not impair mature B-cell generation and maintenance, their simultaneous deletion led to a dramatic reduction of follicular and marginal zone B-cells. Upon T-cell-dependent immunization, B-cell-specific deletion of the c-REL subunit alone abrogated the formation of germinal centers (GC), whereas rela deletion did not affect GC formation. T-independent responses were strongly impaired in mice with B-cell-specific deletion of rel, and only modestly in mice with RELA-deficient B-cells. Our findings identify differential requirements for the canonical NF-κB subunits c-REL and RELA at distinct stages of mature B-cell development. The subunits are jointly required for the generation of mature B-cells. During antigen-dependent B-cell activation, c-REL is the critical subunit required for the initiation of the GC-reaction and for optimal T-independent antibody responses, with RELA being largely dispensable at this stage.

Gene regulation in the immediate-early response process

Immediate-early genes (IEGs) can be activated and transcribed within minutes after stimulation, without the need for de novo protein synthesis, and they are stimulated in response to both cell-extrinsic and cell-intrinsic signals. Extracellular signals are transduced from the cell surface, through receptors activating a chain of proteins in the cell, in particular extracellular-signal-regulated kinases (ERKs), mitogen-activated protein kinases (MAPKs) and members of the RhoA-actin pathway. These communicate through a signaling cascade by adding phosphate groups to neighboring proteins, and this will eventually activate and translocate TFs to the nucleus and thereby induce gene expression. The gene activation also involves proximal and distal enhancers that interact with promoters to simulate gene expression. The immediate-early genes have essential biological roles, in particular in stress response, like the immune system, and in differentiation. Therefore they also have important roles in various diseases, including cancer development. In this paper we summarize some recent advances on key aspects of the activation and regulation of immediate-early genes.

Disturbed canonical nuclear factor of κ light chain signaling in B cells of patients with common variable immunodeficiency

BACKGROUND

Most patients with common variable immunodeficiency (CVID) present with severely reduced switched memory B-cell counts, and some display an increase of CD21^low B-cell counts (CVID 21low), whereas others do not (CVID 21norm). Altered B-cell receptor (BCR) signaling might contribute to the defective memory formation observed in patients with CVID.

OBJECTIVE

We sought to investigate canonical nuclear factor of κ light chain (NF-κB) signaling in B cells from patients with CVID as a central pathway in B-cell differentiation.

METHODS

Degradation of inhibitor of κBα (IκBα) and p65 phosphorylation, nuclear translocation of p65, and regulation of target genes and cell function were investigated after different modes of B-cell stimulation.

RESULTS

BCR-mediated canonical NF-κB signaling was impaired in all mature naive CVID-derived B cells. This impairment was more profound in naive B cells from CVID 21low patients than CVID 21norm patients and most pronounced in CD21^low B cells. The signaling defect translated into reduced induction of Bcl-xL and IκBα, 2 bona fide target genes of the canonical NF-κB pathway. CD40 ligand- and Toll-like receptor 9-mediated signaling were less strongly altered. Signaling in CD21^low B cells but not CD21⁺ B cells of patients with HIV was similarly affected.

CONCLUSION

Combined with the previous description of disturbed Ca²⁺ signaling, the discovery of NF-κB signaling defects, especially in CVID 21low patients, suggests a broad underlying signaling defect affecting especially BCR-derived signals. Given the immune phenotype of monogenic defects affecting Ca²⁺ and NF-κB signaling, the latter is more likely to contribute to the humoral deficiency. The strongly disturbed BCR signaling of CD21^low B cells is characteristic for this cell type and independent of the underlying disease.

B cell receptor induced Fc receptor-like 5 expression is mediated by multiple signaling pathways converging on NF-κB and NFAT

Fc receptor-like (FCRL) proteins are novel regulators of the B cell response to antigen. Human FCRL5 binds intact IgG and modifies the strength of antigen receptor (BCR) signaling. Altering FCRL5 expression could therefore regulate the B cell response to antigen. In this study, we found that FCRL5 expression is induced specifically upon BCR stimulation and dissected the molecular mechanism. FCRL5 mRNA and cell surface protein expression required prolonged BCR stimulation and de novo protein synthesis. Using chemical inhibitors and activators, we identified roles for several signaling pathways, indicating a complex mechanism. Specifically, the PI3K/AKT, JNK, PKC and IKK2-dependent classical NF-κB pathways were involved in induced FCRL5 expression. Furthermore, induced FCRL5 expression required elevation of intracellular Ca(++) and was partially blocked by cyclosporine A, a calcineurin inhibitor. The importance of the transcription factors NF-κB, NFAT and CREB-binding protein was revealed based on sensitivity to inhibitors. Using reporter gene assays, we showed that the core FCRL5 promoter was sufficient to drive induced gene expression. Mutations of two predicted NF-κB sites or an NFAT site in the core promoter abrogated induced gene expression, suggesting direct regulation of the FCRL5 gene by NF-κB and NFAT. In support, we detected binding of NF-κB and NFAT family proteins to oligonucleotides corresponding to the predicted sites. We propose that the identified intricate mechanism serves to ensure that FCRL5 is expressed on B cells at a precise time following antigen encounter, with potential implications regarding regulation of the B cell response.

Unexpected functions of nuclear factor-κB during germinal center B-cell development: implications for lymphomagenesis

PURPOSE OF REVIEW

B-cell tumors originating from the transformation of germinal center B cells frequently harbor genetic mutations, leading to constitutive activation of the nuclear factor-κB (NF-κB) signaling pathway. The present review highlights recent insights into the roles of separate NF-κB transcription factors in germinal center B-cell development and discusses implications of the results for germinal center lymphomagenesis.

RECENT FINDINGS

Understanding how aberrant NF-κB activation promotes tumorigenesis requires the understanding of the role of NF-κB in the tumor-precursor cells. Despite extensive knowledge on NF-κB biology, the function of this complex signaling pathway in the differentiation of germinal center B cells is largely unknown. The present review will discuss recent findings that revealed distinct roles of separate NF-κB transcription factors during the germinal center reaction in the context of germinal center lymphomagenesis. Most notably, a single NF-κB subunit, c-REL, was found to be required for the maintenance of the germinal center reaction and was associated with the activation of a metabolic program that promotes cell growth.

SUMMARY

Identifying the biological roles of the separate NF-κB transcription factor subunits in germinal center biology will help to better understand the pathogenic consequences of their constitutive activation in B-cell tumors. This knowledge may be exploited for the development of targeted antitumor therapies aimed at inhibiting selectively those components of aberrant NF-κB activity which contribute to pathogenesis.

Transcription factors regulating B cell fate in the germinal centre

Diversification of the antibody repertoire is essential for the normal operation of the vertebrate adaptive immune system. Following antigen encounter, B cells are activated, proliferate rapidly and undergo two diversification events; somatic hypermutation (followed by selection), which enhances the affinity of the antibody for its cognate antigen, and class-switch recombination, which alters the effector functions of the antibody to adapt the response to the challenge faced. B cells must then differentiate into antibody-secreting plasma cells or long-lived memory B cells. These activities take place in specialized immunological environments called germinal centres, usually located in the secondary lymphoid organs. To complete the germinal centre activities successfully, a B cell adopts a transcriptional programme that allows it to migrate to specific sites within the germinal centre, proliferate, modify its DNA recombination and repair pathways, alter its apoptotic potential and finally undergo terminal differentiation. To co-ordinate these processes, B cells employ a number of 'master regulator' transcription factors which mediate wholesale transcriptomic changes. These master transcription factors are mutually antagonistic and form a complex regulatory network to maintain distinct gene expression programs. Within this network, multiple points of positive and negative feedback ensure the expression of the 'master regulators', augmented by a number of 'secondary' factors that reinforce these networks and sense the progress of the immune response. In this review we will discuss the different activities B cells must undertake to mount a successful T cell-dependent immune response and describe how a regulatory network of transcription factors controls these processes.

Divergence of transcriptional landscape occurs early in B cell activation

Background

Signaling via B cell receptor (BCR) and Toll-like receptors (TLRs) results in activation of B cells with distinct physiological outcomes, but transcriptional regulatory mechanisms that drive activation and distinguish these pathways remain unknown.

Results

Two hours after ligand exposure RNA-seq, ChIP-seq and computational methods reveal that BCR- or TLR-mediated activation of primary resting B cells proceeds via a large set of shared and a smaller subset of distinct signal-selective transcriptional responses. BCR stimulation resulted in increased global recruitment of RNA Pol II to promoters that appear to transit slowly to downstream regions. Conversely, lipopolysaccharide (LPS) stimulation involved an enhanced RNA Pol II transition from initiating to elongating mode accompanied by greater H3K4me3 activation markings compared to BCR stimulation. These rapidly diverging transcriptomic landscapes also show distinct repressing (H3K27me3) histone signatures, mutually exclusive transcription factor binding in promoters, and unique miRNA profiles.

Conclusions

Upon examination of genome-wide transcription and regulatory elements, we conclude that the B cell commitment to different activation states occurs much earlier than previously thought and involves a multi-faceted receptor-specific transcriptional landscape.

Electronic supplementary material

The online version of this article (doi:10.1186/s13072-015-0012-x) contains supplementary material, which is available to authorized users.

Insights into the pathogenesis of GvHD: what mice can teach us about man

Acute graft-vs-host disease (GvHD) is a life-threatening complication of allogeneic hematopoietic cell transplantation (allo-HCT). Most of the knowledge about the biology of GvHD is derived from mouse models of this disease and therefore a critical analysis of potential advantages and disadvantages of the murine GvHD models is important to classify and understand the findings made in these models. The central events leading up to GvHD were characterized in three phases which includes the tissue damage-phase, the T cell priming-phase and the effector-phase, when the disease becomes clinically overt. The role of individual cytokines, chemokines, transcription factor or receptors was studied in these models by using gene deficient or transgenic mice in the donor or recipient compartments. Besides, numerous studies have been performed in these models to prevent or treat GvHD. Several recent clinical trials were all based on previously reported findings from the mouse model of GvHD such as the trials on CCR5-blockade, donor statin treatment, vorinostat treatment or adoptive transfer of regulatory T cells for GvHD prevention. The different mouse models for GvHD and graft-vs-leukemia effects are critically reviewed and their impact on current clinical practice is discussed.

IKK-induced NF-κB1 p105 proteolysis is critical for B cell antibody responses to T cell-dependent antigen

Jacque et al. investigate the functions of NF-κB1 p105 and its associated NF-κB–binding partners in B cells, using a mutant mouse strain that carries a form of the NF-κB1 precursor that is resistant to IKK-induced proteolysis. They identify a critical B cell–intrinsic role for this IKK signaling pathway in the antigen-induced survival and differentiation of follicular mature B cells.

The importance of IκB kinase (IKK)–induced proteolysis of NF-κB1 p105 in B cells was investigated using Nfkb1^SSAA/SSAA mice, in which this NF-κB signaling pathway is blocked. Nfkb1^SSAA mutation had no effect on the development and homeostasis of follicular mature (FM) B cells. However, analysis of mixed bone marrow chimeras revealed that Nfkb1^SSAA/SSAA FM B cells were completely unable to mediate T cell–dependent antibody responses. Nfkb1^SSAA mutation decreased B cell antigen receptor (BCR) activation of NF-κB in FM B cells, which selectively blocked BCR stimulation of cell survival and antigen-induced differentiation into plasmablasts and germinal center B cells due to reduced expression of Bcl-2 family proteins and IRF4, respectively. In contrast, the antigen-presenting function of FM B cells and their BCR-induced migration to the follicle T cell zone border, as well as their growth and proliferation after BCR stimulation, were not affected. All of the inhibitory effects of Nfkb1^SSAA mutation on B cell functions were rescued by normalizing NF-κB activation genetically. Our study identifies critical B cell-intrinsic functions for IKK-induced NF-κB1 p105 proteolysis in the antigen-induced survival and differentiation of FM B cells, which are essential for T-dependent antibody responses.

Germinal center B cell maintenance and differentiation are controlled by distinct NF-κB transcription factor subunits

Germinal centers (GCs) are the sites where memory B cells and plasma cells producing high-affinity antibodies are generated during T cell-dependent immune responses. The molecular control of GC B cell maintenance and differentiation remains incompletely understood. Activation of the NF-κB signaling pathway has been implicated; however, the distinct roles of the individual NF-κB transcription factor subunits are unknown. We report that GC B cell-specific deletion of the NF-κB subunits c-REL or RELA, which are both activated by the canonical NF-κB pathway, abolished the generation of high-affinity B cells via different mechanisms acting at distinct stages during the GC reaction. c-REL deficiency led to the collapse of established GCs immediately after the formation of dark and light zones at day 7 of the GC reaction and was associated with the failure to activate a metabolic program that promotes cell growth. Conversely, RELA was dispensable for GC maintenance but essential for the development of GC-derived plasma cells due to impaired up-regulation of BLIMP1. These results indicate that activation of the canonical NF-κB pathway in GC B cells controls GC maintenance and differentiation through distinct transcription factor subunits. Our findings have implications for the role of NF-κB in GC lymphomagenesis.

IL-4 upregulates Igα and Igβ protein, resulting in augmented IgM maturation and B cell receptor-triggered B cell activation

IL-4 is critical for optimal B cell activation and germinal center B cell expansion in T-dependent immune responses; however, the underlying mechanism remains elusive. In the current study, we found that primary B cells express little Igα and Igβ protein despite substantial levels of mRNA. IL-4 markedly upregulates Igα and Igβ protein expression that requires STAT6. Elevated Igα and Igβ protein form heterodimers that associate with IgM and significantly promote IgM maturation and surface IgM expression, resulting in amplified BCR-initiated signaling that is Lyn dependent. In vivo, we found that pregerminal center B cells express upregulated Igα, Igβ, and surface IgM expression, in conjunction with elevated BCR-triggered phosphorylated ERK ex vivo, that are dependent on IL-4 and reversed by in vivo administration of neutralizing anti-IL-4 Ab. Thus, this study elucidates a novel mechanism for cross-talk between the IL-4 and BCRs that programs enhancement of subsequent BCR signaling.

Fyn kinase is required for optimal humoral responses

The generation of antigen-specific antibodies and the development of immunological memory require collaboration between B and T cells. T cell-secreted IL-4 is important for B cell survival, isotype switch to IgG1 and IgE, affinity maturation, and the development of germinal centers (GC). Fyn, a member of the Src family tyrosine kinase, is widely expressed in many cell types, including lymphocytes. This kinase is known to interact with both the B cell and T cell receptor (BCR and TCR, respectively). While Fyn deletion does not impair the development of immature T cells and B cells, TCR signaling is altered in mature T cells. The current study demonstrates that Fyn deficient (KO) B cells have impaired IL-4 signaling. Fyn KO mice displayed low basal levels of IgG1, IgE and IgG2c, and delayed antigen-specific IgG1 and IgG2b production, with a dramatic decrease in antigen-specific IgG2c following immunization with a T-dependent antigen. Defects in antibody production correlated with significantly reduced numbers of GC B cells, follicular T helper cells (T_FH), and splenic plasma cells (PC). Taken together, our data demonstrate that Fyn kinase is required for optimal humoral responses.

Regulation of primary response genes in B cells

Deregulated gene expression in B cells often results in various lymphoid malignancies and immune deficiencies. Therefore, understanding signal-induced gene regulatory pathways involved during B cell activation is important to tackle pathologies associated with altered B cell function. Primary response genes (PRGs) are rapidly induced upon signaling in B cells and other cell types and often encode oncogenic transcription factors, which are associated with various malignancies. However, an important issue that remains unclear is whether the fundamental mechanism of activation of these genes is essentially the same under such diverse conditions. c-fos is a PRG that is induced rapidly upon activation of B cells in response to a wide variety of stimuli. Using the c-fos gene as a candidate PRG, we addressed here how it is regulated in response to tumor-promoting and antigen-mimicking signals. Our results show that although the mRNA was induced and extinguished within minutes in response to both signals, surprisingly, apparently full-length unspliced pre-mRNA persisted for several hours in both cases. However, although the mitogenic signal resulted in a more sustained mRNA response that persisted for 4 h, antigenic signaling resulted in a more robust but very transient response that lasted for <1 h. Moreover, the pre-mRNA profile exhibited significant differences between the two signals. Additionally, the splicing regulation was also observed with egr-2, but not with c-myc. Together, these results suggest a previously underappreciated regulatory step in PRG expression in B cells.

Prolonged production of reactive oxygen species in response to B cell receptor stimulation promotes B cell activation and proliferation

We have investigated the intracellular sources and physiological function of reactive oxygen species (ROS) produced in primary B cells in response to BCR stimulation. BCR stimulation of primary resting murine B cells induced the rapid production of ROS that occurred within minutes and was maintained for at least 24 h after receptor stimulation. While the early production of ROS (0-2 h) was dependent on the Nox2 isoform of NADPH oxidase, at later stages of B cell activation (6-24 h) ROS were generated by a second pathway, which appeared to be dependent on mitochondrial respiration. B cells from mice deficient in the Nox2 NADPH oxidase complex lacked detectable early production of extracellular and intracellular ROS after BCR stimulation but had normal proximal BCR signaling and BCR-induced activation and proliferation in vitro and mounted normal or somewhat elevated Ab responses in vivo. In contrast, neutralizing both pathways of BCR-derived ROS with the scavenger N-acetylcysteine resulted in impaired in vitro BCR-induced activation and proliferation and attenuated BCR signaling through the PI3K pathway at later times. These results indicate that the production of ROS downstream of the BCR is derived from at least two distinct cellular sources and plays a critical role at the later stages of B cell activation by promoting sustained BCR signaling via the PI3K pathway, which is needed for effective B cell responses to Ag.

B cell TLRs and induction of immunoglobulin class-switch DNA recombination

Toll-like receptors (TLRs) are a family of conserved pattern recognition receptors (PRRs). Engagement of B cell TLRs by microbe-associated molecular patterns (MAMPs) induces T-independent (TI) antibody responses and plays an important role in the early stages of T-dependent (TD) antibody responses before specific T cell help becomes available. The role of B cell TLRs in the antibody response is magnified by the synergy of B cell receptor (BCR) crosslinking and TLR engagement in inducing immunoglobulin (Ig) class switch DNA recombination (CSR), which crucially diversifies the antibody biological effector functions. Dual BCR/TLR engagement induces CSR to all Ig isotypes, as directed by cytokines, while TLR engagement alone induces marginal CSR. Integration of BCR and TLR signaling results in activation of the canonical and non-canonical NF-κB pathways, induction of activation-induced cytidine deaminase (AID) and germline transcription of IgH switch (S) regions. A critical role of B cell TLRs in CSR and the antibody response is emphasized by the emergence of several TLR ligands as integral components of vaccines that greatly boost humoral immunity in a B cell-intrinsic fashion.

Multiple myeloma exhibits novel dependence on GLUT4, GLUT8, and GLUT11: implications for glucose transporter-directed therapy

Multiple myeloma is one of numerous malignancies characterized by increased glucose consumption, a phenomenon with significant prognostic implications in this disease. Few studies have focused on elucidating the molecular underpinnings of glucose transporter (GLUT) activation in cancer, knowledge that could facilitate identification of promising therapeutic targets. To address this issue, we performed gene expression profiling studies involving myeloma cell lines and primary cells as well as normal lymphocytes to uncover deregulated GLUT family members in myeloma. Our data demonstrate that myeloma cells exhibit reliance on constitutively cell surface-localized GLUT4 for basal glucose consumption, maintenance of Mcl-1 expression, growth, and survival. We also establish that the activities of the enigmatic transporters GLUT8 and GLUT11 are required for proliferation and viability in myeloma, albeit because of functionalities probably distinct from whole-cell glucose supply. As proof of principle regarding the therapeutic potential of GLUT-targeted compounds, we include evidence of the antimyeloma effects elicited against both cell lines and primary cells by the FDA-approved HIV protease inhibitor ritonavir, which exerts a selective off-target inhibitory effect on GLUT4. Our work reveals critical roles for novel GLUT family members and highlights a therapeutic strategy entailing selective GLUT inhibition to specifically target aberrant glucose metabolism in cancer.

Dual regulation of IRF4 function in T and B cells is required for the coordination of T-B cell interactions and the prevention of autoimmunity

Effective humoral responses to protein antigens require the precise execution of carefully timed differentiation programs in both T and B cell compartments. Disturbances in this process underlie the pathogenesis of many autoimmune disorders, including systemic lupus erythematosus (SLE). Interferon regulatory factor 4 (IRF4) is induced upon the activation of T and B cells and serves critical functions. In CD4(+) T helper cells, IRF4 plays an essential role in the regulation of IL-21 production, whereas in B cells it controls class switch recombination and plasma cell differentiation. IRF4 function in T helper cells can be modulated by its interaction with regulatory protein DEF6, a molecule that shares a high degree of homology with only one other protein, SWAP-70. Here, we demonstrate that on a C57BL/6 background the absence of both DEF6 and SWAP-70 leads to the development of a lupus-like disease in female mice, marked by simultaneous deregulation of CD4(+) T cell IL-21 production and increased IL-21 B cell responsiveness. We furthermore show that DEF6 and SWAP-70 are differentially used at distinct stages of B cell differentiation to selectively control the ability of IRF4 to regulate IL-21 responsiveness in a stage-specific manner. Collectively, these data provide novel insights into the mechanisms that normally couple and coordinately regulate T and B cell responses to ensure tight control of productive T-B cell interactions.

Regulation of primary response genes

Primary response genes (PRGs) are a set of genes that are induced in response to both cell-extrinsic and cell-intrinsic signals and do not require de novo protein synthesis for their expression. These "first responders" in the waves of transcription of signal-responsive genes play pivotal roles in a wide range of biological responses, including neuronal survival and plasticity, cardiac stress response, innate and adaptive immune responses, glucose metabolism, and oncogeneic transformation. Here we bring together recent advances and our current understanding of the signal-induced transcriptional and epigenetic regulation of PRGs.

Molecular programming of B cell memory

The development of high-affinity B cell memory is regulated through three separable phases, each involving antigen recognition by specific B cells and cognate T helper cells. Initially, antigen-primed B cells require cognate T cell help to gain entry into the germinal centre pathway to memory. Once in the germinal centre, B cells with variant B cell receptors must access antigens and present them to germinal centre T helper cells to enter long-lived memory B cell compartments. Following antigen recall, memory B cells require T cell help to proliferate and differentiate into plasma cells. A recent surge of information - resulting from dynamic B cell imaging in vivo and the elucidation of T follicular helper cell programmes - has reshaped the conceptual landscape surrounding the generation of memory B cells. In this Review, we integrate this new information about each phase of antigen-specific B cell development to describe the newly unravelled molecular dynamics of memory B cell programming.

Fc receptor-like 5 promotes B cell proliferation and drives the development of cells displaying switched isotypes

The biological roles of B cell membrane proteins in the FCRL family are enigmatic. FCRL proteins, including FCRL5, were shown to modulate early BCR signaling, although the subsequent, functional consequences of receptor engagement are poorly understood. We found that FCRL5 surface protein itself was induced temporarily upon BCR stimulation of human, naive B cells, indicating precise control over timing of FCRL5 engagement. Cross-linking of FCRL5 on cells induced to express FCRL5 enhanced B cell proliferation significantly. This enhancement required costimulation of the BCR and TLR9, two signals required for optimal proliferation of naive B cells, whereas T cell help in the form of anti-CD40 and IL-2 was dispensable. In addition, we found that FCRL5 stimulation generated a high proportion of cells displaying surface IgG and IgA. Optimal development of cells expressing switched isotypes required T cell help, in addition to stimuli found necessary for enhanced proliferation. Surprisingly, cells that developed upon FCRL5 stimulation simultaneously displayed surface IgM, IgG, and IgA. Cells expressing multiple Ig isotypes were described in hairy cell leukemia, a disease in which FCRL5 is overexpressed. Enhanced proliferation and downstream isotype expression upon FCRL5 stimulation could reflect a physiological role for FCRL5 in the expansion and development of antigen-primed B cells. In addition, FCRL5 may promote growth of malignant cells in hairy cell leukemia and other FCRL5-expressing tumors.