BAFF receptor signaling aids the differentiation of immature B cells into transitional B cells following tonic BCR signaling

Regulation of Foxo1 expression is critical for central B cell tolerance and allelic exclusion

Resolving the molecular mechanisms of central B cell tolerance might unveil strategies that prevent autoimmunity. Here, using a mouse model of central B cell tolerance in which Forkhead box protein O1 (Foxo1) is either deleted or over-expressed in B cells, we show that deleting Foxo1 blocks receptor editing, curtails clonal deletion, and decreases CXCR4 expression, allowing high-avidity autoreactive B cells to emigrate to the periphery whereby they mature but remain anergic and short lived. Conversely, expression of degradation-resistant Foxo1 promotes receptor editing in the absence of self-antigen but leads to allelic inclusion. Foxo1 over-expression also restores tolerance in autoreactive B cells harboring active PI3K, revealing opposing roles of Foxo1 and PI3K in B cell selection. Overall, we show that the transcription factor Foxo1 is a major gatekeeper of central B cell tolerance and that PI3K drives positive selection of immature B cells and establishes allelic exclusion by suppressing Foxo1.

The role of BAFF and BAFF-R inhibitors in the treatment of immune thrombocytopenia; a focused review

Immune thrombocytopenia (ITP) is an autoimmune-driven disease characterized by increased destruction and impaired platelet production resulting in an enhanced risk of bleeding. Immunosuppressant agents are the most common treatment strategies for ITP. Despite their efficacy, these medications often cause unpredictable side effects. Recent investigations revealed that patients with ITP exhibit elevated B-cell activating factor (BAFF) levels in both their spleens and serum. Belimumab, a BAFF inhibitor, illustrated a promising therapeutic avenue for managing ITP by interfering with BAFF activity and long-lived plasma cell production. Both clinical and experimental studies have yielded positive outcomes when combining rituximab with an anti-BAFF monoclonal antibody in treating ITP. In addition, ianalumab, a monoclonal antibody with a dual mechanism that targets BAFF-R and deletes peripheral BAFF-R+ B cells, is currently being used for ITP treatment [NCT05885555]. The upcoming results from novel BAFF inhibitors, such as ianalumab, could offer clinicians an additional therapeutic option for treating ITP.

Hormones and B-cell development in health and autoimmunity

The development of B cells into antibody-secreting plasma cells is central to the adaptive immune system as they induce protective and specific antibody responses against invading pathogens. Various studies have shown that, during this process, hormones can play important roles in the lymphopoiesis, activation, proliferation, and differentiation of B cells, and depending on the signal given by the receptor of each hormone, they can have a positive or negative effect. In autoimmune diseases, hormonal deregulation has been reported to be related to the survival, activation and/or differentiation of autoreactive clones of B cells, thus promoting the development of autoimmunity. Clinical manifestations of autoimmune diseases have been associated with estrogens, prolactin (PRL), and growth hormone (GH) levels. However, androgens, such as testosterone and progesterone (P4), could have a protective effect. The objective of this review is to highlight the links between different hormones and the immune response mediated by B cells in the etiopathogenesis of systemic lupus erythematosus (SLE), rheumatoid arthritis (RA), and multiple sclerosis (MS). The data collected provide insights into the role of hormones in the cellular, molecular and/or epigenetic mechanisms that modulate the B-cell response in health and disease.

B cell development and antibody responses in human immune system mice: current status and future perspective

Humanized immune system (HIS) mice have been developed and used as a small surrogate model to study human immune function under normal or disease conditions. Although variations are found between models, most HIS mice show robust human T cell responses. However, there has been unsuccessful in constructing HIS mice that produce high-affinity human antibodies, primarily due to defects in terminal B cell differentiation, antibody affinity maturation, and development of primary follicles and germinal centers. In this review, we elaborate on the current knowledge about and previous attempts to improve human B cell development in HIS mice, and propose a potential strategy for constructing HIS mice with improved humoral immunity by transplantation of human follicular dendritic cells (FDCs) to facilitate the development of secondary follicles.

Targeted therapy in glomerular diseases

Targeted therapy has emerged as a more precise approach to treat glomerular diseases, focusing on specific molecular or cellular processes that contribute to disease development or progression. This approach complements or replaces traditional immunosuppressive therapy, optimizes supportive care, and provides a more personalized treatment strategy. In this review, we summarize the evolving understanding of pathogenic mechanisms in immune-mediated glomerular diseases and the developing targeted therapies based on these mechanisms. We begin by discussing pan-B-cell depletion, anti-CD20 rituximab, and targeting B-cell survival signaling through the BAFF/APRIL pathway. We also exam specific plasma cell depletion with anti-CD38 antibody. We then shift our focus to complement activation in glomerular diseases, which is involved in antibody-mediated glomerular diseases, such as IgA nephropathy, membranous nephropathy, ANCA-associated vasculitis, and lupus nephritis. Non-antibody-mediated complement activation occurs in glomerular diseases, including C3 glomerulopathy, complement-mediated atypical hemolytic uremic syndrome, and focal segmental glomerulosclerosis. We discuss specific inhibition of terminal, lectin, and alternative pathways in different glomerular diseases. Finally, we summarize current clinical trials targeting the final pathways of various glomerular diseases, including kidney fibrosis. We conclude that targeted therapy based on individualized pathogenesis should be the future of treating glomerular diseases.

Translational development of a novel BAFF-R CAR-T therapy targeting B-cell lymphoid malignancies

Several CD19-targeting CAR-T cells are used to treat leukemias and lymphomas; however, relapsed and/or refractory (R/R) disease is still observed in a significant number of patients. Additionally, the success of CD19-CAR-T cell therapies is not uniform across hematological malignancies, particularly in chronic lymphocytic leukemia (CLL). In this study, we present the development of a novel CAR-T cell therapy targeting B-cell activating factor receptor (BAFF-R), a key regulator of B-cell proliferation and maturation. A new monoclonal antibody against BAFF-R was generated from a hybridoma clone and used to create a novel MC10029 CAR construct. Through a series of in vitro and in vivo models using the Nalm-6 cell line for leukemia and the Z138 cell line for lymphoma, we demonstrated the antigen-specific cytotoxicity of MC10029 CAR-T cells against tumor cells. Additionally, MC10029 CAR-T cells exhibited potent antitumor effects against CD19 knockout tumor cells, mimicking CD19-negative R/R disease. MC10029 CAR-T cells were specifically targeted to CLL, in which BAFF-R is nearly always expressed. The cytotoxicity of MC10029 CAR-T cells was first shown in the MEC-1 CLL cell line, before we turned our efforts to subject-derived samples. Using healthy donor-engineered MC10029 CAR-T cells against enriched primary tumor cells, followed by subject-derived MC10029 CAR-T cells against autologous tumor cells, we showed the efficacy of MC10029 CAR-T cells against CLL subject samples. With these robust data, we have advanced to the production of MC10029 CAR-T cells, using GMP lentivirus, and obtained an IND approval in preparation for a Phase 1 clinical trial.

Enhanced alloresponse to platelet transfusion due to immune dysregulation following ablative chemotherapy in mice

Introduction

Alloimmunization is common following platelet transfusion and can result in negative outcomes for recipients such as refractoriness to subsequent transfusions and rejection of transplants. Healthy people do not receive blood transfusions, and the diseases and therapies that result in a need to transfuse have significant impacts on the immunological environment to which these alloantigens are introduced. Ablative chemotherapies are common among platelet recipients and have potent immunological effects. In this study, we modeled the impact of chemotherapy on the alloresponse to platelet transfusion. As chemotherapies are generally regarded as immunosuppressive, we hypothesized that that they would result in a diminished alloresponse.

Methods

Mice were given a combination chemotherapeutic treatment of cytarabine and doxorubicin followed by transfusion of allogeneic platelets, and compared to controls given no treatment, chemotherapy alone, or transfusion alone. Alloantibody responses were measured 2 weeks after transfusion, and cellular responses and growth factors were monitored over time.

Results

Contrary to our hypothesis, we found that chemotherapy led to increased alloantibody responses to allogeneic platelet transfusion. This enhanced response was antigen-specific and was associated with increased CD4+ and CD8+ T cell responses. Chemotherapy led to rapid lymphocyte depletion followed by reconstitution, non-specific activation of transitional B cells with the highest levels of activation in the least mature subsets, and increased serum levels of B cell activating factor (BAFF).

Conclusion

These data suggest that ablative chemotherapy can increase the risk of alloimmunization and, if confirmed clinically, that additional measures to protect these patient populations may be warranted.

BAFF induces CXCR5 expression during B cell differentiation in bone marrow

B cell activating factor (BAFF) plays an important role in antibody production through differentiation and maturation of B cells mainly in secondary lymphoid organs. On the other hand, the role of BAFF in the bone marrow, the primary lymphoid organ of B cell development, has not been well elucidated. Here, effects of BAFF in bone marrow B cell development were examined by using BAFF-deficient mice. When mRNA expression levels of B cell differentiation markers including Cd19, Bcl2, Igμ, Il7r and Cxcr5 were compared between bone marrow of wild-type and BAFF-KO mice, a lower level of Cxcr5 expression was found in the KO mice. Additionally, protein expression of CXCR5 on IgM⁺ cells in the bone marrow was decreased by BAFF deficiency. In vitro studies also confirmed the effect of BAFF on CXCR5 by IgM⁺ cells; culturing bone marrow cells from BAFF-KO mice with BAFF in vitro increased the proportion of CXCR5⁺ cells in IgM⁺ cells compared with non-treated bone marrow cells. In addition, BAFF synergized with TNF-α and IL-6 to increase the expression of CXCR5⁺ on IgM⁺ cells. The BAFF-mediated up-regulation of CXCR5 expression was reproduced by using CD19⁺ cells purified from BAFF-KO bone marrow cells, suggesting that BAFF directly affects B-lineage cells in bone marrow to promote CXCR5 expression. Together, this study suggests that BAFF has an important role in B cell differentiation in bone marrow by directly inducing CXCR5 expression which affect their migration to secondary lymphoid organs.

Dysregulated MicroRNAs in the Pathogenesis of Systemic Lupus Erythematosus: A Comprehensive Review

Systemic lupus erythematosus is a chronic autoimmune disease of which clinical presentation is vastly heterogeneous, ranging from mild skin rashes to severe renal diseases. Treatment goal of this illness is to minimize disease activity and prevent further organ damage. In recent years, much research has been done on the epigenetic aspects of SLE pathogenesis, for among the various factors known to contribute to the pathogenic process, epigenetic factors, especially microRNAs, bear the most therapeutic potential that can be altered unlike congenital genetic factors. This article reviews and updates what has been discovered so far about the pathogenesis of lupus, while focusing on the dysregulation of microRNAs in lupus patients in comparison to healthy controls along with the potentially pathogenic roles of the microRNAs commonly reported to be either upregulated or downregulated. Furthermore, this review includes microRNAs of which results are controversial, suggesting possible explanations for such discrepancies and directions for future research. Moreover, we aimed to emphasize the point that had been overlooked so far in studies regarding microRNA expression levels; that is, which specimen was used to assess the dysregulation of microRNAs. To our surprise, a vast number of studies have not considered this factor and have analyzed the potential role of microRNAs in general. Despite extensive investigations done on microRNA levels, their significance and potential role remain a mystery, which calls for further studies on this particular subject in regard of which specimen is used for assessment.

The BAFF-APRIL System in Cancer

B cell-activating factor (BAFF; also known as CD257, TNFSF13B, BLyS) and a proliferation-inducing ligand (APRIL; also known as CD256, TNFSF13) belong to the tumor necrosis factor (TNF) family. BAFF was initially discovered as a B-cell survival factor, whereas APRIL was first identified as a protein highly expressed in various cancers. These discoveries were followed by over two decades of extensive research effort, which identified overlapping signaling cascades between BAFF and APRIL, controlling immune homeostasis in health and driving pathogenesis in autoimmunity and cancer, the latter being the focus of this review. High levels of BAFF, APRIL, and their receptors have been detected in different cancers and found to be associated with disease severity and treatment response. Here, we have summarized the role of the BAFF-APRIL system in immune cell differentiation and immune tolerance and detailed its pathogenic functions in hematological and solid cancers. We also highlight the emerging therapeutics targeting the BAFF-APRIL system in different cancer types.

Identification and characterization of sex-dependent gene expression profile in glioblastoma

Glioblastoma (GBM) is the most lethal primary tumor in the human brain and lacks favorable treatment options. Sex differences in the outcome of GBM are broadly acknowledged, but the underlying molecular mechanisms remain largely unknown. To identify the sex-dependent critical genes in the progression of GBM, raw data from several microarray datasets with the same array platform were downloaded from the Gene Expression Omnibus (GEO) database. These datasets included tumorous and normal tissue from patients with GBM and crucial sex features. Then, the differentially expressed genes (DEGs) in female and male tumors were identified via bioinformatics analysis, respectively. Functional signatures of the identified DEGs were further annotated by Gene Ontology (GO) and pathway enrichment analyses. Venn diagram and functional protein-protein interaction (PPI) network analyses were performed to screen out the sex-specific DEGs. Survival analysis of patients with differences in the expression level of selected genes was then carried out using the data from The Cancer Genome Atlas (TCGA). Here, we showed that ECT2, AURKA, TYMS, CDK1, NCAPH, CENPU, OIP5, KIF14, ASPM, FBXO5, SGOL2, CASC5, SHCBP1, FN1, LOX, IGFBP3, CSPG4, and CD44 were enriched in female tumor samples, whereas TNFSF13B, CXCL10, CXCL8, CXCR4, TLR2, CCL2, and FCGR2A were enriched in male tumor samples. Among these key genes, interestingly, ECT2 was associated with increased an survival rate for female patients, whileTNFSF13B could be regarded as a potential marker of poor prognosis in male patients. These results suggested that sex differences in patients may be attributed to the heterogeneous gene activity, which might influence the oncogenesis and the outcomes of GBM.

CD19 Is Internalized Together with IgM in Proportion to B Cell Receptor Stimulation and Is Modulated by Phosphatidylinositol 3-Kinase in Bone Marrow Immature B Cells

Newly generated immature B cells that bind self-antigen with high avidity arrest in differentiation and undergo central tolerance via receptor editing and clonal deletion. These autoreactive immature B cells also express low surface levels of the coreceptor CD19, a key activator of the PI3K pathway. Signals emanating from both CD19 and PI3K are known to be critical for attenuating receptor editing and selecting immature B cells into the periphery. However, the mechanisms that modulate CD19 expression at this stage of B cell development have not yet been resolved. Using in vivo and in vitro models, we demonstrate that Cd19 de novo gene transcription and translation do not significantly contribute to the differences in CD19 surface expression in mouse autoreactive and nonautoreactive immature B cells. Instead, CD19 downregulation is induced by BCR stimulation in proportion to BCR engagement, and the remaining surface IgM and CD19 molecules promote intracellular PI3K-AKT activity in proportion to their level of expression. The internalized CD19 is degraded with IgM by the lysosome, but inhibiting lysosome-mediated protein degradation only slightly improves surface CD19. In fact, CD19 is restored only upon Ag removal. Our data also reveal that the PI3K-AKT pathway positively modulates CD19 surface expression in immature B cells via a mechanism that is independent of inhibition of FOXO1 and its role on Cd19 gene transcription while is dependent on mTORC1.

B-cell activating factor (BAFF), BAFF promoter and BAFF receptor allelic variants in hepatitis C virus related Cryoglobulinemic Vasculitis and Non-Hodgkin's Lymphoma

Cryoglobulinemic Vasculitis (CV) is an autoimmune/lymphoproliferative disorder associated with HCV infection that in 5%-10% of cases evolves into a B cell Non-Hodgkin's Lymphoma (NHL). B-cell activating factor (BAFF) is a key regulator in B-cell development and survival. Particular genetic variants are responsible for BAFF signaling impairment in autoimmune and neoplastic diseases. We evaluated BAFF and BAFF-receptor (BAFF-R) polymorphisms in order to determine if they predispose to HCV-related CV and NHL. The analysis was performed on 416 HCV-chronically infected patients: 136 HCV without signs/symptoms of lymphoproliferations/autoimmunity (HCV), 166 HCV with CV (HCV-CV) and 114 HCV with NHL (HCV-NHL). Rs9514828 SNP on BAFF promoter, rs61756766 on BAFF-R and rs12428930 on the BAFF gene were evaluated by Real-Time PCR. Concerning rs9514828, the frequency of C/T genotype was significantly higher in HCV-CV than in HCV. The difference in the distribution of the T/T mutant genotype in HCV-CV compared to HCV was significant as well as the distribution of C/T and T/T genotype in HCV-NHL versus HCV. T minor allele was more frequent in HCV-NHL and HCV-CV than in HCV. The distribution of C/T + T/T (for the dominant model of penetrance C/T + T/T vs. C/C) was significantly higher in HCV-CV and HCV-NHL than in HCV. Genotyping of rs61756766 on BAFF-R coding gene, revealed C/T heterozygosis at a frequency of 11% in HCV-NHL versus 3% in HCV. The T minor allele frequency was higher in HCV-NHL than in HCV. No differences emerged by genotyping rs12428930 SNP on BAFF coding gene. Our results reinforce the hypothesis that BAFF/BAFF-R genetic pattern has a role in the pathogenesis of HCV-related lymphoproliferations. BAFF/BAFF-R variants could identify a risk haplotype for HCV related CV and NHL and a BAFF/BAFF-R genetic profile assessment could potentially contribute to tailoring anti-BAFF therapy by identifying patients with BAFF alterations in which the treatment could be more beneficial.

B-Cell Activating Factor Increases Related to Adiposity, Insulin Resistance, and Endothelial Dysfunction in Overweight and Obese Subjects

Obesity (OB) is a major healthcare problem that results from long-term energy imbalance. Adipokines and pro-inflammatory cytokines facilitate adipose tissue (AT) remodeling to safely store excess nutrients. B-cell activating factor (BAFF) is a newly described adipokine whose role in enhancing adipogenesis has been reported. The present study aimed to evaluate serum BAFF association with adiposity distribution, serum adipokines, pro-inflammatory cytokines, and metabolic and endothelial dysfunction markers. The study included 124 young Mexican adults with no diagnosed comorbidities, divided according to their BMI. Anthropometric measurements, blood counts, and serum molecules (i.e., glucose, lipid profile, insulin, leptin, pro- and anti-inflammatory cytokines, von Willebrand factor (vWF), and BAFF) were assessed. The analysis showed positive correlation between BAFF and increased fat mass in all anthropometric measurements (p < 0.0001). BAFF augmentation was related to systemic inflammatory environment (p < 0.05), and linked with insulin resistance status (p < 0.05). BAFF increment was also correlated with early endothelial damage markers such as vWF (p < 0.0001). Linear regression analysis showed a role for BAFF in predicting serum vWF concentrations (p < 0.01). In conclusion, our data show that BAFF is an adipokine dynamically related to OB progression, insulin resistance status, and systemic inflammatory environment, and is a predictor of soluble vWF augmentation, in young overweight and obese Mexican subjects.

B-cell intrinsic and extrinsic signals that regulate central tolerance of mouse and human B cells

The random recombination of immunoglobulin V(D)J gene segments produces unique IgM antibodies that serve as the antigen receptor for each developing B cell. Hence, the newly formed B cell repertoire is comprised of a variety of specificities that display a range of reactivity with self-antigens. Newly generated IgM⁺ immature B cells that are non-autoreactive or that bind self-antigen with low avidity are licensed to leave the bone marrow with their intact antigen receptor and to travel via the blood to the peripheral lymphoid tissue for further selection and maturation. In contrast, clones with medium to high avidity for self-antigen remain within the marrow and undergo central tolerance, a process that revises their antigen receptor or eliminates the autoreactive B cell altogether. Thus, central B cell tolerance is critical for reducing the autoreactive capacity and avidity for self-antigen of our circulating B cell repertoire. Bone marrow cultures and mouse models have been instrumental for understanding the mechanisms that regulate the selection of bone marrow B cells. Here, we review recent studies that have shed new light on the contribution of the ERK, PI3K, and CXCR4 signaling pathways in the selection of mouse and human immature B cells that either bind or do not bind self-antigen.

B Cells in Primary Membranous Nephropathy: Escape from Immune Tolerance and Implications for Patient Management

Membranous nephropathy (MN) is an important cause of nephrotic syndrome and chronic kidney disease (CKD) in adults. The pathogenic significance of B cells in MN is increasingly recognized, especially following the discovery of various autoantibodies that target specific podocytic antigens and the promising treatment responses seen with B cell depleting therapies. The presence of autoreactive B cells and autoantibodies that bind to antigens on podocyte surfaces are characteristic features of MN, and are the result of breaches in central and peripheral tolerance of B lymphocytes. These perturbations in B cell tolerance include altered B lymphocyte subsets, dysregulation of genes that govern immunoglobulin production, aberrant somatic hypermutation and co-stimulatory signalling, abnormal expression of B cell-related cytokines, and increased B cell infiltrates and organized tertiary lymphoid structures within the kidneys. An understanding of the role of B cell tolerance and homeostasis may have important implications for patient management in MN, as conventional immunosuppressive treatments and novel B cell-targeted therapies show distinct effects on proliferation, differentiation and reconstitution in different B cell subsets. Circulating B lymphocytes and related cytokines may serve as potential biomarkers for treatment selection, monitoring of therapeutic response and prediction of disease relapse. These recent advances in the understanding of B cell tolerance in MN have provided greater insight into its immunopathogenesis and potential novel strategies for disease monitoring and treatment.

Genetic mapping reveals Nfkbid as a central regulator of humoral immunity to Toxoplasma gondii

Protective immunity to parasitic infections has been difficult to elicit by vaccines. Among parasites that evade vaccine-induced immunity is Toxoplasma gondii, which causes lethal secondary infections in chronically infected mice. Here we report that unlike susceptible C57BL/6J mice, A/J mice were highly resistant to secondary infection. To identify correlates of immunity, we utilized forward genetics to identify Nfkbid, a nuclear regulator of NF-κB that is required for B cell activation and B-1 cell development. Nfkbid-null mice (“bumble”) did not generate parasite-specific IgM and lacked robust parasite-specific IgG, which correlated with defects in B-2 cell maturation and class-switch recombination. Though high-affinity antibodies were B-2 derived, transfer of B-1 cells partially rescued the immunity defects observed in bumble mice and were required for 100% vaccine efficacy in bone marrow chimeric mice. Immunity in resistant mice correlated with robust isotype class-switching in both B cell lineages, which can be fine-tuned by Nfkbid gene expression. We propose a model whereby humoral immunity to T. gondii is regulated by Nfkbid and requires B-1 and B-2 cells for full protection.

Eukaryotic parasitic diseases account for approximately one fifth of all childhood deaths, yet no highly protective vaccine exists for any human parasite. More research must be done to discover how to elicit protective vaccine-induced immunity to parasitic pathogens. We used an unbiased genetic screen to find key genes responsible for immunity to the eukaryotic parasite Toxoplasma gondii. Our screen found Nfkbid, a transcription factor regulator, which controls B cell activation and innate-like B-1 cell development. Mice without Nfkbid were not protected against T. gondii and were deficient at making antibodies against the parasite. Our survival studies of vaccinated mice with and without B-1 compartments found that B-1 cells improved survival, suggesting that B-1 cells act in conjunction with B-2 cells to provide vaccine-induced immunity. Nfkbid and other loci identified in our unbiased screen represent potential targets for vaccines to elicit protective immune responses against parasitic pathogens.

FCRL1 Regulates B Cell Receptor-Induced ERK Activation through GRB2

Regulation of BCR signaling has important consequences for generating effective Ab responses to pathogens and preventing production of autoreactive B cells during development. Currently defined functions of Fc receptor-like (FCRL) 1 include positive regulation of BCR-induced calcium flux, proliferation, and Ab production; however, the mechanistic basis of FCRL1 signaling and its contributions to B cell development remain undefined. Molecular characterization of FCRL1 signaling shows phosphotyrosine-dependent associations with GRB2, GRAP, SHIP-1, and SOS1, all of which can profoundly influence MAPK signaling. In contrast with previous characterizations of FCRL1 as a strictly activating receptor, we discover a role for FCRL1 in suppressing ERK activation under homeostatic and BCR-stimulated conditions in a GRB2-dependent manner. Our analysis of B cells in Fcrl1 ^-/- mice shows that ERK suppression by FCRL1 is associated with a restriction in the number of cells surviving splenic maturation in vivo. The capacity of FCRL1 to modulate ERK activation presents a potential for FCRL1 to be a regulator of peripheral B cell tolerance, homeostasis, and activation.

Possible interplay between estrogen and the BAFF may modify thyroid activity in Graves' disease

A link between sex hormones and B-cell activating factor (BAFF), a crucial immunoregulator of autoimmune thyroid disease (AITD), may exist. The study aimed to elucidate the role of estrogen (E2) in regulating BAFF in Graves' disease (GD). In clinical samples, serum BAFF levels were higher in women than in men in both the GD and control groups. serum BAFF levels were associated with thyroid-stimulating hormone receptor antibody levels and thyroid function only in women and not in men. BAFF transcripts in peripheral blood mononuclear cells were higher in women with GD than those in the control group. Among GD patients with the AA genotype of rs2893321, women had higher BAFF transcripts and protein levels than men. In the progression of a spontaneous autoimmune thyroiditis (SAT) murine model, NOD.H-2h4, serum free thyroxine and BAFF levels were higher in female than in male mice. Moreover, exogenous E2 treatment increased serum BAFF levels in male SAT mice. Meanwhile, female SAT mice exhibited higher thyroid BAFF transcripts levels than either the E2-treated or untreated male SAT mouse groups. Our results showed that E2 might be implicated in modulating BAFF expression, and support a possible mechanism for the higher incidence of AITD in women.

Signaling networks in B cell development and related therapeutic strategies

B cells are essential for Ab production during humoral immune responses. From decades of B cell research, there is now a detailed understanding of B cell subsets, development, functions, and most importantly, signaling pathways. The complicated pathways in B cells and their interactions with each other are stage-dependent, varying with surface marker expression during B cell development. With the increasing understanding of B cell development and signaling pathways, the mechanisms underlying B cell related diseases are being unraveled as well, making it possible to provide more precise and effective treatments. In this review, we describe several essential and recently discovered signaling pathways in B cell development and take a look at newly developed therapeutic strategies targeted at B cell signaling.

Increasing numbers of CD19 + CD24highCD38high regulatory B cells and pre-germinal center B cells reflect activated autoimmunity and predict future treatment response in patients with untreated immune thrombocytopenia

The pathophysiology of immune thrombocytopenia (ITP) is poorly understood, particularly aspects regarding abnormal homeostasis and dysregulation of B cells. In this study, we analyzed peripheral lymphocyte subsets in patients with untreated ITP and healthy controls, and examined correlations between cell percentages/counts and titers of serum cytokines and antibodies. We also compared ITP patients who later required second-line therapies and those who did not. The percentages of CD19 + CD24^highCD38^high regulatory B cells, pre-germinal center (GC) B cells, and plasmablast-like B cells were significantly higher in ITP patients than in healthy controls. Absolute counts of regulatory B cells and pre-GC B cells were significantly higher in those who needed second-line therapies. In addition, serum B cell-activating factor belonging to the tumor necrosis factor family (BAFF) levels and platelet-associated immune globulin G antibody titers correlated positively with regulatory B cell, pre-GC B cell, and auto-reactive B cell counts. Serum interferon-α (IFN-α) levels were elevated in four ITP patients with high auto-reactive B cell counts. These results indicate that increases in regulatory B cells and pre-GC B cells may reflect activated autoimmunity induced by BAFF and/or IFN-α. Consequently, evaluation of B cell subsets in untreated ITP patients may predict treatment response.

Thymic Epithelial Cell Alterations and Defective Thymopoiesis Lead to Central and Peripheral Tolerance Perturbation in MHCII Deficiency

Major Histocompatibility Complex (MHC) class II (MHCII) deficiency (MHCII-D), also known as Bare Lymphocyte Syndrome (BLS), is a rare combined immunodeficiency due to mutations in genes regulating expression of MHCII molecules. MHCII deficiency results in impaired cellular and humoral immune responses, leading to severe infections and autoimmunity. Abnormal cross-talk with developing T cells due to the absence of MHCII expression likely leads to defects in thymic epithelial cells (TEC). However, the contribution of TEC alterations to the pathogenesis of this primary immunodeficiency has not been well characterized to date, in particular in regard to immune dysregulation. To this aim, we have performed an in-depth cellular and molecular characterization of TEC in this disease. We observed an overall perturbation of thymic structure and function in both MHCII-/- mice and patients. Transcriptomic and proteomic profiling of murine TEC revealed several alterations. In particular, we demonstrated that impairment of lymphostromal cross-talk in the thymus of MHCII-/- mice affects mTEC maturation and promiscuous gene expression and causes defects of central tolerance. Furthermore, we observed peripheral tolerance impairment, likely due to defective Treg cell generation and/or function and B cell tolerance breakdown. Overall, our findings reveal disease-specific TEC defects resulting in perturbation of central tolerance and limiting the potential benefits of hematopoietic stem cell transplantation in MHCII deficiency.

ERK phosphorylation is RAF independent in naïve and activated B cells but RAF dependent in plasma cell differentiation

Members of the RAF family of serine-threonine kinases are intermediates in the mitogen-activated protein kinase and extracellular signal-regulated kinase (MAPK-ERK) signaling pathway, which controls key differentiation processes in B cells. By analyzing mice with B cell-specific deletion of Raf1, Braf, or both, we showed that Raf-1 and B-Raf acted together in mediating the positive selection of pre-B and transitional B cells as well as in initiating plasma cell differentiation. However, genetic or chemical inactivation of RAFs led to increased ERK phosphorylation in mature B cells. ERK activation in the absence of Raf-1 and B-Raf was mediated by multiple RAF-independent pathways, with phosphoinositide 3-kinase (PI3K) playing an important role. Furthermore, we found that ERK phosphorylation strongly increased during the transition from activated B cells to pre-plasmablasts. This increase in ERK phosphorylation did not occur in B cells lacking both Raf-1 and B-Raf, which most likely explains the partial block of plasma cell differentiation in mice lacking both RAFs. Collectively, our data indicate that B-Raf and Raf-1 are not necessary to mediate ERK phosphorylation in naïve or activated B cells but are essential for mediating the marked increase in ERK phosphorylation during the transition from activated B cells to pre-plasmablasts.

BAFF promotes heightened BCR responsiveness and manifestations of chronic GVHD after allogeneic stem cell transplantation

Patients with chronic graft-versus-host disease (cGVHD) have increased B cell-activating factor (BAFF) levels, but whether BAFF promotes disease after allogeneic bone marrow transplantation (allo-BMT) remains unknown. In a major histocompatibility complex-mismatched model with cGVHD-like manifestations, we first examined B-lymphopenic μMT allo-BMT recipients and found that increased BAFF levels in cGVHD mice were not merely a reflection of B-cell number. Mice that later developed cGVHD had significantly increased numbers of recipient fibroblastic reticular cells with higher BAFF transcript levels. Increased BAFF production by donor cells also likely contributed to cGVHD, because BAFF transcript in CD4+ T cells from diseased mice and patients was increased. cGVHD manifestations in mice were associated with high BAFF/B-cell ratios and persistence of B-cell receptor (BCR)-activated B cells in peripheral blood and lesional tissue. By employing BAFF transgenic (Tg) mice donor cells, we addressed whether high BAFF contributed to BCR activation in cGVHD. BAFF increased NOTCH2 expression on B cells, augmenting BCR responsiveness to surrogate antigen and NOTCH ligand. BAFF Tg B cells had significantly increased protein levels of the proximal BCR signaling molecule SYK, and high SYK protein was maintained by BAFF after in vitro BCR activation or when alloantigen was present in vivo. Using T cell-depleted (BM only) BAFF Tg donors, we found that BAFF promoted cGVHD manifestations, circulating GL7+ B cells, and alloantibody production. We demonstrate that pathologic production of BAFF promotes an altered B-cell compartment and augments BCR responsiveness. Our findings compel studies of therapeutic targeting of BAFF and BCR pathways in patients with cGVHD.

Central human B cell tolerance manifests with a distinctive cell phenotype and is enforced via CXCR4 signaling in hu-mice

Central B cell tolerance, the process restricting the development of many newly generated autoreactive B cells, has been intensely investigated in mouse cells while studies in humans have been hampered by the inability to phenotypically distinguish autoreactive and nonautoreactive immature B cell clones and the difficulty in accessing fresh human bone marrow samples. Using a human immune system mouse model in which all human Igκ⁺ B cells undergo central tolerance, we discovered that human autoreactive immature B cells exhibit a distinctive phenotype that includes lower activation of ERK and differential expression of CD69, CD81, CXCR4, and other glycoproteins. Human B cells exhibiting these characteristics were observed in fresh human bone marrow tissue biopsy specimens, although differences in marker expression were smaller than in the humanized mouse model. Furthermore, the expression of these markers was slightly altered in autoreactive B cells of humanized mice engrafted with some human immune systems genetically predisposed to autoimmunity. Finally, by treating mice and human immune system mice with a pharmacologic antagonist, we show that signaling by CXCR4 is necessary to prevent both human and mouse autoreactive B cell clones from egressing the bone marrow, indicating that CXCR4 functionally contributes to central B cell tolerance.

Effects of Selenium Supplement on B Lymphocyte Activity in Experimental Autoimmune Thyroiditis Rats

METHODS

45 healthy and adult female SD rats were randomly divided into three groups: normal control group, EAT model group, and selenium yeast supplement EAT group. The EAT model rats were induced by subcutaneous injection of porcine thyroglobulin and fed with high iodine water. The concentrations of serum thyroid-stimulating hormone (TSH), TGAb, TPOAb, and B cell activating factor (BAFF) were detected in each group by enzyme-linked immunosorbent assay (ELISA), and the expression of interleukin-10 (IL-10) in thyroid tissue was detected by immunohistochemistry. B cells and regulatory B cells (Bregs) ratios in the spleen of rats were analyzed by flow cytometry.

RESULTS

In contrast with the EAT model group, the levels of serum TSH, TGAB, TPOAb, and BAFF were decreased, while IL-10 expression was increased in thyroid tissue, and Bregs ratio was upregulated in the spleen (all p < 0.05) in the selenium yeast supplement EAT group.

CONCLUSION

Selenium yeast supplement could partially attenuate immune imbalance in EAT rats, which may be related to the mechanism of modulating B lymphocyte activity.

Defective Bcl-2 expression in memory B cells from common variable immunodeficiency patients

Common variable immunodeficiency (CVID) is a primary immunodeficiency characterized by hypogammaglobulinemia and different degrees of B cell compartment alteration. Memory B cell differentiation requires the orchestrated activation of several intracellular signaling pathways that lead to the activation of a number of factors, such as nuclear factor kappa B (NF-κB) which, in turn, promote transcriptional programs required for long-term survival. The aim of this study was to determine if disrupted B cell differentiation, survival and activation in B cells in CVID patients could be related to defects in intracellular signaling pathways. For this purpose, we selected intracellular readouts that reflected the strength of homeostatic signaling pathways in resting cells, as the protein expression levels of the Bcl-2 family which transcription is promoted by NF-κB. We found reduced Bcl-2 protein levels in memory B cells from CVID patients. We further explored the possible alteration of this crucial prosurvival signaling pathway in CVID patients by analysing the expression levels of mRNAs from anti-apoptotic proteins in naive B cells, mimicking T cell-dependent activation in vitro with CD40L and interleukin (IL)-21. BCL-XL mRNA levels were decreased, together with reduced levels of AICDA, after naive B-cell activation in CVID patients. The data suggested a molecular mechanism for this tendency towards apoptosis in B cells from CVID patients. Lower Bcl-2 protein levels in memory B cells could compromise their long-term survival, and a possible less activity of NF-κB in naive B cells, may condition an inabilityto increase BCL-XL mRNA levels, thus not promoting survival in the germinal centers.

Differential Effects of MS Therapeutics on B Cells-Implications for Their Use and Failure in AQP4-Positive NMOSD Patients

B cells are considered major contributors to multiple sclerosis (MS) pathophysiology. While lately approved disease-modifying drugs like ocrelizumab deplete B cells directly, most MS medications were not primarily designed to target B cells. Here, we review the current understanding how approved MS medications affect peripheral B lymphocytes in humans. These highly contrasting effects are of substantial importance when considering these drugs as therapy for neuromyelitis optica spectrum disorders (NMOSD), a frequent differential diagnosis to MS, which is considered being a primarily B cell- and antibody-driven diseases. Data indicates that MS medications, which deplete B cells or induce an anti-inflammatory phenotype of the remaining ones, were effective and safe in aquaporin-4 antibody positive NMOSD. In contrast, drugs such as natalizumab and interferon-β, which lead to activation and accumulation of B cells in the peripheral blood, lack efficacy or even induce catastrophic disease activity in NMOSD. Hence, we conclude that the differential effect of MS drugs on B cells is one potential parameter determining the therapeutic efficacy or failure in antibody-dependent diseases like seropositive NMOSD.

The Plasticity of Newly Formed B Cells

Newly formed B cells (NF-B cells) that emerge from the bone marrow to the periphery have often been referred to as immature or transitional B cells. However, NF-B cells have several striking characteristics, including a distinct BCR repertoire, high expression of AID, high sensitivity to PAMPs, and the ability to produce cytokines. A number of findings do not support their designation as immature because NF-B cells have the potential to become Ab-producing cells and to undergo class-switch recombination. In this review, we provide a fresh perspective on NF-B cell functions and describe some of the signals driving their activation. We summarize growing evidence supporting a role for NF-B cells in protection against infections and as a potential source of autoantibody-producing cells in autoimmune diseases such as systemic lupus erythematosus.

Transitional B cells involved in autoimmunity and their impact on neuroimmunological diseases

Transitional B cells (TrB cells) represent a crucial link between immature B cells in the bone marrow and mature peripheral B cells. Although TrB cells represent one of the regulatory B cell subpopulations in healthy individuals, the frequency of CD24^hiCD38^hi TrB cells in circulation may be altered in individuals with autoimmune diseases, such as multiple sclerosis, neuromyelitisoptica spectrum disorders, systemic lupus erythematosus, Sjögren’s syndrome, rheumatoid arthritis, systemic sclerosis, and juvenile dermatomyositis. Although TrB cells play regulatory roles under inflammatory conditions, consequences of their functional impairment vary across autoimmune diseases. Since the origin, development, and function of TrB cells, especially in humans, remain unclear and controversial, this review aimed to discuss the characteristics of TrB cells at steady state and explore their role in various immune diseases, including autoimmune rheumatic diseases and neuroimmunological diseases.

BAFF inhibition in SLE-Is tolerance restored?

The B cell activating factor (BAFF) inhibitor, belimumab, is the first biologic drug approved for the treatment of SLE, and exhibits modest, but durable, efficacy in decreasing disease flares and organ damage. BAFF and its homolog APRIL are TNF-like cytokines that support the survival and differentiation of B cells at distinct developmental stages. BAFF is a crucial survival factor for transitional and mature B cells that acts as rheostat for the maturation of low-affinity autoreactive cells. In addition, BAFF augments innate B cell responses via complex interactions with the B cell receptor (BCR) and Toll like receptor (TLR) pathways. In this manner, BAFF impacts autoreactive B cell activation via extrafollicular pathways and fine tunes affinity selection within germinal centers (GC). Finally, BAFF and APRIL support plasma cell survival, with differential impacts on IgM- and IgG-producing populations. Therapeutically, BAFF and combined BAFF/APRIL inhibition delays disease onset in diverse murine lupus strains, although responsiveness to BAFF inhibition is model dependent, in keeping with heterogeneity in clinical responses to belimumab treatment in humans. In this review, we discuss the mechanisms whereby BAFF/APRIL signals promote autoreactive B cell activation, discuss whether altered selection accounts for therapeutic benefits of BAFF inhibition, and address whether new insights into BAFF/APRIL family complexity can be exploited to improve human lupus treatments.

Impaired CpG Demethylation in Common Variable Immunodeficiency Associates With B Cell Phenotype and Proliferation Rate

Common Variable Immunodeficiency (CVID) is characterized by impaired antibody production and poor terminal differentiation of the B cell compartment, yet its pathogenesis is still poorly understood. We first reported the occurrence of epigenetic alterations in CVID by high-throughput methylation analysis in CVID-discordant monozygotic twins. Data from a recent whole DNA methylome analysis throughout different stages of normal B cell differentiation allowed us to design a new experimental approach. We selected CpG sites for analysis based on two criteria: one, CpGs with potential association with the transcriptional status of relevant genes for B cell activation and differentiation; and two, CpGs that undergo significant demethylation from naïve to memory B cells in healthy individuals. DNA methylation was analyzed by bisulfite pyrosequencing of specific CpG sites in sorted naïve and memory B cell subsets from CVID patients and healthy donors. We observed impaired demethylation in two thirds of the selected CpGs in CVID memory B cells, in genes that govern B cell-specific processes or participate in B cell signaling. The degree of demethylation impairment associated with the extent of the memory B cell reduction. The impaired demethylation in such functionally relevant genes as AICDA in switched memory B cells correlated with a lower proliferative rate. Our new results reinforce the hypothesis of altered demethylation during B cell differentiation as a contributing pathogenic mechanism to the impairment of B cell function and maturation in CVID. In particular, deregulated epigenetic control of AICDA could play a role in the defective establishment of a post-germinal center B cell compartment in CVID.

Active PI3K abrogates central tolerance in high-avidity autoreactive B cells

High-avidity autoreactive B cells are typically removed by central tolerance mechanisms in the bone marrow. Greaves et al. demonstrate that B cell–intrinsic expression of active PI3Kα prevents central tolerance and effectively promotes differentiation and activation of high-avidity autoreactive B cells in the periphery.

Autoreactive B cells that bind self-antigen with high avidity in the bone marrow undergo mechanisms of central tolerance that prevent their entry into the peripheral B cell population. These mechanisms are breached in many autoimmune patients, increasing their risk of B cell–mediated autoimmune diseases. Resolving the molecular pathways that can break central B cell tolerance could therefore provide avenues to diminish autoimmunity. Here, we show that B cell–intrinsic expression of a constitutively active form of PI3K-P110α by high-avidity autoreactive B cells of mice completely abrogates central B cell tolerance and further promotes these cells to escape from the bone marrow, differentiate in peripheral tissue, and undergo activation in response to self-antigen. Upon stimulation with T cell help factors, these B cells secrete antibodies in vitro but remain unable to secrete autoantibodies in vivo. Overall, our data demonstrate that activation of the PI3K pathway leads high-avidity autoreactive B cells to breach central, but not late, stages of peripheral tolerance.

The role of BAFF and APRIL in rheumatoid arthritis

Development and activation of B cells quickly became clear after identifying new ligands and receptors in the tumor necrosis factor superfamily. B cell-activating factor (BAFF) and a proliferation-inducing ligand (APRIL) are the members of membrane proteins Type 2 family released by proteolytic cleavage of furin to form active, soluble homotrimers. Except for B cells, ligands are expressed by all such immune cells like T cells, dendritic cells, monocytes, and macrophages. BAFF and APRIL have two common receptors, namely TNFR homolog transmembrane activator and Ca2+ modulator and CAML interactor (TACI) and B cell-maturation antigen. BAFF alone can also be coupled with a third receptor called BAFFR (also called BR3 or BLyS Receptor). These receptors are often expressed by immune cells in the B-cell lineage. The binding of BAFF or APRIL to their receptors supports B cells differentiation and proliferation, immunoglobulin production and the upregulation of B cell-effector molecules expression. It is possible that the overexpression of BAFF and APRIL contributes to the pathogenesis of autoimmune diseases. In BAFF transgenic mice, there is a pseudo-autoimmune manifestation, which is associated with an increase in B-lymphocytes, hyperglobulinemia, anti-single stranded DNA, and anti-double-stranded DNA antibodies, and immune complexes in their peripheral blood. Furthermore, overexpressing BAFF augments the number of peripheral B220+ B cells with a normal proliferation rate, high levels of Bcl2, and prolonged survival and hyperactivity. Therefore, in this review article, we studied BAFF and APRIL as important mediators in B-cell and discussed their role in rheumatoid arthritis.

Signalling circuits that direct early B-cell development

In mammals, the B-cell lineage arises from pluripotent progenitors in the bone marrow. During their development, B-cells undergo lineage specification and commitment, followed by expansion and selection. These processes are mediated by regulated changes in gene expression programmes, rearrangements of immunoglobulin (Ig) genes, and well-timed rounds of proliferation and apoptosis. Many of these processes are initiated by environmental factors including cytokines, chemokines, and cell-cell contacts. Developing B-cells process these environmental cues into stage-specific functions via signalling pathways including the PI3K, MAPK, or JAK-STAT pathway. The cytokines FLT3-Ligand and c-Kit-Ligand are important for the early expansion of the B-cell precursors at different developmental stages and conditions. Interleukin 7 is essential for commitment to the B-cell lineage and for orchestrating the Ig recombination machinery. After rearrangement of the immunoglobulin heavy chain, proliferation and apoptosis, and thus selection, are mediated by the clonal pre-B-cell receptor, and, following light chain rearrangement, by the B-cell receptor.

B-cell activating factor receptor expression is associated with germinal center B-cell maintenance

B-cell activating factor (BAFF) is a major cytokine that regulates B-cell survival, maturation and differentiation through its binding with its receptors: BAFF receptor (BAFF-R), transmembrane activator and cyclophilin ligand interactor (TACI) and B-cell maturation antigen (BCMA). These receptors have been demonstrated to be involved in tertiary lymphoid structure formation; however, their role in germinal centers (GCs) has remained elusive. The aim of the present study was to determine the expression profiles of BAFF and its receptors in secondary lymphoid tissues. Tonsils resected due to chronic tonsillitis were used as lymphoid tissues. To confirm the presence of GCs identified based on their typical structure, CD21 antibody staining was employed. The expression of BAFF, BAFF-R, TACI and BCMA was assessed by immunohistochemistry. BAFF was highly expressed in all regions of the follicle, but the highest BAFF expression was detected in the mantle zone (MZ). A high expression of BAFF-R was observed on lymphocytes in the MZ in comparison with the other regions (~80%; P<0.05), which was co-localizated with BAFF (r=0.646; P<0.001), in the MZ. TACI and BCMA exhibited similar expression among the different zones of the GCs, and co-localization with BAFF was observed inside the follicle, mainly in the dark zone. The present results indicate that BAFF is implicated in the maintenance of GCs. BAFF-R overexpression in the MZ, co-localizated with BAFF, suggests that these proteins constitute the principal pathway for the maintenance of the naïve B-cell population. Furthermore, TACI and BCMA have a role in the GC, where processes of B-cell selection, proliferation and differentiation into immunoglobulin-secreting plasma cells occur.

To B or Not to B: Understanding B Cell Responses in the Development of Malaria Infection

Malaria is a widespread disease caused mainly by the Plasmodium falciparum (Pf) and Plasmodium vivax (Pv) protozoan parasites. Depending on the parasite responsible for the infection, high morbidity and mortality can be triggered. To escape the host immune responses, Plasmodium parasites disturb the functionality of B cell subsets among other cell types. However, some antibodies elicited during a malaria infection have the potential to block pathogen invasion and dissemination into the host. Thus, the question remains, why is protection not developed and maintained after the primary parasite exposure? In this review, we discuss different aspects of B cell responses against Plasmodium antigens during malaria infection. Since most studies have focused on the quantification of serum antibody titers, those B cell responses have not been fully characterized. However, to secrete antibodies, a complex cellular response is set up, including not only the activation and differentiation of B cells into antibody-secreting cells, but also the participation of other cell subsets in the germinal center reactions. Therefore, a better understanding of how B cell subsets are stimulated during malaria infection will provide essential insights toward the design of potent interventions.

B cells in Graves' Orbitopathy: more than just a source of antibodies?

B cells have multiple actions on different phases of an immune reaction, mainly resulting in B and T cell-interaction (help), production of cytokines, regulation of dendritic cells and downregulation of regulatory B cells. The effectiveness of B cell depletion therapy is probably due to blockade of the antigen-presenting function of B cells, occurring very early in the setting of autoimmune reactions. B cells undergo a maturation process from stem cells during which the CD 20 antigen, which is the target of rituximab (RTX), is expressed from the stage of pre-B cells to mature and memory B cells, but not on plasma cells. During the maturation process, the cytokine B cell stimulating factor (BAFF) induces maturation of B cells and expansion of clones to produce plasma cells and eventually antibodies. The effect of RTX in GO is rather rapid, with significant improvement of the disease already 4-6 weeks after the first RTX infusion. Based on the evidence of significant lymphocytic infiltration in the orbits of patients with active GO, it is reasonable to postulate that RTX may cause depletion of B cells and block their antigen-presenting cell mechanism. Since it has been reported that serum BAFF concentrations are elevated in hyperthyroid GD patients and that BAFF is expressed on the thyrocytes of patients with either autoimmune disease or nodular goiter, the hypothesis that belimumab, an anti-BAFF monoclonal antibody, may be effective in patients with active GO his currently being tested in a randomized controlled trial.

Integrated B Cell, Toll-like, and BAFF Receptor Signals Promote Autoantibody Production by Transitional B Cells

The B cell survival cytokine BAFF has been linked with the pathogenesis of systemic lupus erythematosus (SLE). BAFF binds distinct BAFF-family surface receptors, including the BAFF-R and transmembrane activator and CAML interactor (TACI). Although originally characterized as a negative regulator of B cell activation, TACI signals are critical for class-switched autoantibody (autoAb) production in BAFF transgenic mice. Consistent with this finding, a subset of transitional splenic B cells upregulate surface TACI expression and contribute to BAFF-driven autoAb. In the current study, we interrogated the B cell signals required for transitional B cell TACI expression and Ab production. Surprisingly, despite established roles for dual BCR and TLR signals in autoAb production in SLE, signals downstream of these receptors exerted distinct impacts on transitional B cell TACI expression and autoAb titers. Whereas loss of BCR signals prevented transitional B cell TACI expression and resulted in loss of serum autoAb across all Ig isotypes, lack of TLR signals exerted a more limited impact restricted to autoAb class-switch recombination without altering transitional B cell TACI expression. Finally, in parallel with the protective effect of TACI deletion, loss of BAFF-R signaling also protected against BAFF-driven autoimmunity. Together, these findings highlight how multiple signaling pathways integrate to promote class-switched autoAb production by transitional B cells, events that likely impact the pathogenesis of SLE and other BAFF-dependent autoimmune diseases.

BAFF and BAFF-Receptor in B Cell Selection and Survival

The BAFF-receptor (BAFFR) is encoded by the TNFRSF13C gene and is one of the main pro-survival receptors in B cells. Its function is impressively documented in humans by a homozygous deletion within exon 2, which leads to an almost complete block of B cell development at the stage of immature/transitional B cells. The resulting immunodeficiency is characterized by B-lymphopenia, agammaglobulinemia, and impaired humoral immune responses. However, different from mutations affecting pathway components coupled to B cell antigen receptor (BCR) signaling, BAFFR-deficient B cells can still develop into IgA-secreting plasma cells. Therefore, BAFFR deficiency in humans is characterized by very few circulating B cells, very low IgM and IgG serum concentrations but normal or high IgA levels.

Systemic ST6Gal-1 Is a Pro-survival Factor for Murine Transitional B Cells

Humoral immunity depends on intrinsic B cell developmental programs guided by systemic signals that convey physiologic needs. Aberrant cues or their improper interpretation can lead to immune insufficiency or a failure of tolerance and autoimmunity. The means by which such systemic signals are conveyed remain poorly understood. Hence, further insight is essential to understanding and treating autoimmune diseases and to the development of improved vaccines. ST6Gal-1 is a sialyltransferase that constructs the α2,6-sialyl linkage on cell surface and extracellular glycans. The requirement for functional ST6Gal-1 in the development of humoral immunity is well documented. Canonically, ST6Gal-1 resides within the intracellular ER-Golgi secretory apparatus and participates in cell-autonomous glycosylation. However, a significant pool of extracellular ST6Gal-1 exists in circulation. Here, we segregate the contributions of B cell intrinsic and extrinsic ST6Gal-1 to B cell development. We observed that B cell-intrinsic ST6Gal-1 is required for marginal zone B cell development, while B cell non-autonomous ST6Gal-1 modulates B cell development and survival at the early transitional stages of the marrow and spleen. Exposure to extracellular ST6Gal-1 ex vivo enhanced the formation of IgM-high B cells from immature precursors, and increased CD23 and IgM expression. Extrinsic sialylation by extracellular ST6Gal-1 augmented BAFF-mediated activation of the non-canonical NF-kB, p38 MAPK, and PI3K/AKT pathways, and accelerated tyrosine phosphorylation after B cell receptor stimulation. in vivo, systemic ST6Gal-1 did not influence homing of B cells to the spleen but was critical for their long-term survival and systemic IgG levels. Circulatory ST6Gal-1 levels respond to inflammation, infection, and malignancy in mammals, including humans. In turn, we have shown previously that systemic ST6Gal-1 regulates inflammatory cell production by modifying bone marrow myeloid progenitors. Our data here point to an additional role of systemic ST6Gal-1 in guiding B cell development, which supports the concept that circulating ST6Gal-1 is a conveyor of systemic cues to guide the development of multiple branches of immune cells.

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

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

Activation of the MEK-ERK Pathway Is Necessary but Not Sufficient for Breaking Central B Cell Tolerance

Newly generated bone marrow B cells are positively selected into the peripheral lymphoid tissue only when they express a B cell receptor (BCR) that is nonautoreactive or one that binds self-antigen with only minimal avidity. This positive selection process, moreover, is critically contingent on the ligand-independent tonic signals transduced by the BCR. We have previously shown that when autoreactive B cells express an active form of the rat sarcoma (RAS) oncogene, they upregulate the receptor for the B cell activating factor (BAFFR) and undergo differentiation in vitro and positive selection into the spleen in vivo, overcoming central tolerance. Based on the in vitro use of pharmacologic inhibitors, we further showed that this cell differentiation process is critically dependent on the activation of the mitogen-activated protein kinase kinase pathway MEK (MAPKK)-extracellular signal-regulated kinase (ERK), which is downstream of RAS. Here, we next investigated if activation of ERK is not only necessary but also sufficient to break central B cell tolerance and induce differentiation of autoreactive B cells in vitro and in vivo. Our results demonstrate that activation of ERK is critical for upregulating BAFFR and overcoming suboptimal levels of tonic BCR signals or low amounts of antigen-induced BCR signals during in vitro B cell differentiation. However, direct activation of ERK does not lead high avidity autoreactive B cells to increase BAFFR levels and undergo positive selection and differentiation in vivo. B cell-specific MEK-ERK activation in mice is also unable to lead to autoantibody secretion, and this in spite of a general increase of serum immunoglobulin levels. These findings indicate that additional pathways downstream of RAS are required for high avidity autoreactive B cells to break central and/or peripheral tolerance.

Molecular control of B-cell homeostasis in health and malignancy

Altered B-cell homeostasis underlies a wide range of pathologies, from cancers to autoimmunity and immunodeficiency. The molecular safeguards against those disorders, which also allow effective immune responses, are therefore particularly critical. Here, we review recent findings detailing the fine control of B-cell homeostasis, during B-cell development, maturation in the periphery and during activation and differentiation into antibody-producing cells.

Type I interferons promote the survival and proinflammatory properties of transitional B cells in systemic lupus erythematosus patients

A hallmark of systemic lupus erythematosus (SLE) is the breaking of B-cell tolerance with the generation of high-affinity autoantibodies; however, the antibody-independent features of the B-cell compartment in SLE are less understood. In this study, we performed an extensive examination of B-cell subsets and their proinflammatory properties in a Chinese cohort of new-onset SLE patients. We observed that SLE patients exhibited an increased frequency of transitional B cells compared with healthy donors and rheumatoid arthritis patients. Plasma from SLE patients potently promoted the survival of transitional B cells in a type I IFN-dependent manner, which can be recapitulated by direct IFN-α treatment. Furthermore, the effect of IFN-α on enhanced survival of transitional B cells was associated with NF-κB pathway activation and reduced expression of the pro-apoptotic molecule Bax. Transitional B cells from SLE patients harbored a higher capacity to produce proinflammatory cytokine IL-6, which was also linked to the overactivated type I IFN pathway. In addition, the frequency of IL-6-producing transitional B cells was positively correlated with disease activity in SLE patients, and these cells were significantly reduced after short-term standard therapies. Thus, the current study provides a direct link between type I IFN pathway overactivation and the abnormally high frequency and proinflammatory properties of transitional B cells in active SLE patients, which contributes to the understanding of the roles of type I IFNs and B cells in the pathogenesis of SLE.

Mif-deficiency favors an atheroprotective autoantibody phenotype in atherosclerosis

The inflammatory cytokine macrophage migration-inhibitory factor (MIF) promotes atherosclerosis via lesional monocyte and T-cell recruitment. B cells have emerged as important components in atherogenesis, but the interaction between MIF and B cells in atherogenesis is unknown. Here, we investigated the atherosclerotic phenotype of Mif-gene deletion in Apoe^-/- mice. Apoe^-/- Mif^-/- mice fed a Western diet exhibited strongly reduced atherosclerotic lesions in brachiocephalic artery (BC) and abdominal aorta compared with controls. This phenotype was accompanied by reduced circulating B cells. Flow cytometry revealed a B-cell developmental defect with increased premature and immature B-cell counts in bone marrow (BM) of Apoe^-/- Mif^-/- mice and diminished B-cell numbers in spleen. This finding was linked with a decreased expression of Baff-R and differentiation-driving transcription factors at the immature B-cell stage, whereas peritoneal B cells exhibited unchanged CD80 and CD86 expression but vastly decreased CD9 and elevated CD23 levels, indicating that the developmental block favors the generation of immature, egressing, and reactive B cells. Mif deficiency did not affect absolute B-cell numbers in the vessel wall but favored a relative increase of B cells in the atheroprone BC region and the appearance of periadventitial B-cell-rich clusters. Of note, Mif^-/- mice exhibited a significant increase in oxidized low-density lipoprotein (oxLDL)-specific antibodies after the injection of oxLDL, indicating that Mif deficiency is associated with higher sensitivity of B cells against natural-occurring antigens such as oxLDL. Importantly, Apoe^-/- mice adoptively transplanted with Apoe^-/-Mif^-/- BM showed reduced peripheral B cells compared with Apoe^-/- BM transplantation but no atheroprotection in the BC; also, whereas there was a selective increase in atheroprotective IgM-anti-oxLDL-antibodies in global Mif deficiency, BM-specific Mif deficiency also led to elevated proatherogenic anti-oxLDL-IgG. Together, these findings reveal a novel link between MIF and B cells in atherogenesis. Protection from atherosclerosis by Mif deficiency is associated with enhanced B-cell hypersensitivity, which in global but not BM-restricted Mif deficiency favors an atheroprotective autoantibody profile in atherosclerotic mice. Targeting MIF may induce protective B-cell responses in atherosclerosis.-Schmitz, C., Noels, H., El Bounkari, O., Straussfeld, E., Megens, R. T. A., Sternkopf, M., Alampour-Rajabi, S., Krammer, C., Tilstam, P. V., Gerdes, N., Bürger, C., Kapurniotu, A., Bucala, R., Jankowski, J., Weber, C., Bernhagen, J. Mif-deficiency favors an atheroprotective autoantibody phenotype in atherosclerosis.

Decreased production of class-switched antibodies in neonatal B cells is associated with increased expression of miR-181b

The increased susceptibility to infections of neonates is caused by an immaturity of the immune system as a result of both qualitative and quantitative differences between neonatal and adult immune cells. With respect to B cells, neonatal antibody responses are known to be decreased. Accountable for this is an altered composition of the neonatal B cell compartment towards more immature B cells. However, it remains unclear whether the functionality of individual neonatal B cell subsets is altered as well. In the current study we therefore compared phenotypical and functional characteristics of corresponding neonatal and adult B cell subpopulations. No phenotypic differences could be identified with the exception of higher IgM expression in neonatal B cells. Functional analysis revealed differences in proliferation, survival, and B cell receptor signaling. Most importantly, neonatal B cells showed severely impaired class-switch recombination (CSR) to IgG and IgA. This was associated with increased expression of miR-181b in neonatal B cells. Deficiency of miR-181b resulted in increased CSR. With this, our results highlight intrinsic differences that contribute to weaker B cell antibody responses in newborns.

CP-25, a Novel Anti-inflammatory and Immunomodulatory Drug, Inhibits the Functions of Activated Human B Cells through Regulating BAFF and TNF-alpha Signaling and Comparative Efficacy with Biological Agents

Paeoniflorin-6′-O-benzene sulfonate (code: CP-25) was the chemistry structural modifications of Paeoniflorin (Pae). CP-25 inhibited B cells proliferation stimulated by B cell activating factor belonging to the TNF family (BAFF) or Tumor necrosis factor alpha (TNF-alpha). CP-25, Rituximab and Etanercept reduced the percentage and numbers of CD19⁺ B cells, CD19⁺CD20⁺ B cells, CD19⁺CD27⁺ B cells and CD19⁺CD20⁺CD27⁺ B cells induced by BAFF or TNF-alpha. There was significant difference between CP-25 and Rituximab or CP-25 and Etanercept. CP-25 down-regulated the high expression of BAFFR, BCMA, and TACI stimulated by BAFF or TNF-alpha. The effects of Rituximab and Etanercept on BAFFR or BCMA were stronger than that of CP-25. CP-25, Rituximab and Etanercept down-regulated significantly the expression of TNFR1 and TNFR2 on B cell stimulated by BAFF or TNF-alpha. CP-25, Rituximab and Etanercept down-regulated the expression of MKK3, P-p38, P-p65, TRAF2, and p52 in B cells stimulated by BAFF and the expression of TRAF2 and P-p65 in B cells stimulated by TNF-alpha. These results suggest that CP-25 regulated moderately activated B cells function by regulating the classical and alternative NF-κB signaling pathway mediated by BAFF and TNF-alpha-TRAF2-NF-κB signaling pathway. This study suggests that CP-25 may be a promising anti-inflammatory immune and soft regulation drug.

The Autoimmune Risk Variant PTPN22 C1858T Alters B Cell Tolerance at Discrete Checkpoints and Differentially Shapes the Naive Repertoire

A common genetic variant in the gene encoding the protein tyrosine phosphatase nonreceptor type 22 (PTPN22 C1858T) has been linked to a wide range of autoimmune disorders. Although a B cell-intrinsic role in promoting disease has been reported, the mechanism(s) through which this variant functions to alter the preimmune B cell repertoire remains unknown. Using a series of polyclonal and transgenic self-reactive models harboring the analogous mutation in murine Ptpn22, we show evidence for enhanced BCR, B cell-activating factor receptor, and CD40 coreceptor programs, leading to broadly enhanced positive selection of B cells at two discrete checkpoints in the bone marrow and spleen. We further identified a bias for selection of B cells into the follicular mature versus marginal zone B cell compartment. Using a biomarker to track a self-reactive H chain in peripheral blood, we found evidence of similarly enhanced positive selection in human carriers of the PTPN22 C1858T variant. Our combined data support a model whereby the risk variant augments the BCR and coreceptor programs throughout B cell development, promoting enrichment of self-reactive specificities into the follicular mature compartment and thereby likely increasing the risk for seeding of autoimmune B cell responses.

The Role of BAFF System Molecules in Host Response to Pathogens

The two ligands B cell-activating factor of the tumor necrosis factor family (BAFF) and a proliferation-inducing ligand (APRIL) and the three receptors BAFF receptor (BAFF-R), transmembrane activator and calcium-modulating cyclophilin ligand interactor (TACI), and B cell maturation antigen (BCMA) are members of the "BAFF system molecules." BAFF system molecules are primarily involved in B cell homeostasis. The relevance of BAFF system molecules in host responses to microbial assaults has been investigated in clinical studies and in mice deficient for each of these molecules. Many microbial products modulate the expression of these molecules. Data from clinical studies suggest a correlation between increased expression levels of BAFF system molecules and elevated B cell responses. Depending on the pathogen, heightened B cell responses may strengthen the host response or promote susceptibility. Whereas pathogen-mediated increases in the expression levels of the ligands and/or the receptors appear to promote microbial clearance, certain pathogens have evolved to ablate B cell responses by suppressing the expression of TACI and/or BAFF-R on B cells. Other than its well-established role in B cell responses, the TACI-mediated activation of macrophages is also implicated in resistance to intracellular pathogens. An improved understanding of the role that BAFF system molecules play in infection may assist in devising novel strategies for vaccine development.