Chicken BAFF--a highly conserved cytokine that mediates B cell survival

Members of the tumor necrosis factor (TNF) family play key roles in the regulation of inflammation, immune responses and tissue homeostasis. Here we describe the identification of the chicken homologue of mammalian B cell activating factor of the TNF family (BAFF/BLyS). By searching a chicken EST database we identified two overlapping cDNA clones that code for the entire open reading frame of chicken BAFF (chBAFF), which contains a predicted transmembrane domain and a putative furin protease cleavage site like its mammalian counterparts. The amino acid identity between soluble chicken and human BAFF is 76%, considerably higher than for most other known cytokines. The chBAFF gene is most strongly expressed in the bursa of Fabricius. Soluble recombinant chBAFF produced by human 293T cells interacted with the mammalian cell-surface receptors TACI, BCMA and BAFF-R. It bound to chicken B cells, but not to other lymphocytes, and it promoted the survival of splenic chicken B cells in culture. Furthermore, bacterially expressed chBAFF induced the selective expansion of B cells in the spleen and cecal tonsils when administered to young chicks. Our results suggest that like its mammalian counterpart, chBAFF plays an important role in survival and/or proliferation of chicken B cells.     

TRAF2 and TRAF3 signal adapters act cooperatively to control the maturation and survival signals delivered to B cells by the BAFF receptor

Tumor necrosis factor receptor-associated factors 2 and 3 (TRAF2 and TRAF3) were shown to function in a cooperative and nonredundant manner to suppress nuclear factor-kappaB2 (NF-kappaB2) activation, gene expression, and survival in mature B cells. In the absence of this suppressive activity, B cells developed independently of the obligatory B cell survival factor, BAFF (B cell-activating factor of the tumor necrosis factor family). However, deletion of either TRAF2 or TRAF3 from the T cell lineage did not promote T cell survival, despite causing extensive NF-kappaB2 activation. This constitutive, lineage-specific suppression of B cell survival by TRAF2 and TRAF3 determines the requirement for BAFF to sustain B cell development in vivo. Binding of BAFF to BAFF receptor reversed TRAF2-TRAF3-mediated suppression of B cell survival by triggering the depletion of TRAF3 protein. This process was TRAF2 dependent, revealing dual roles for TRAF2 in regulating B cell homeostasis.     

Expression and function of TNF family member B cell-activating factor in the development of autoimmune arthritis

B cell-activating factor (BAFF), a member of tumor necrosis factor family cytokines, has been shown to enhance the maturation and survival of peripheral B cells. While BAFF is implicated in regulating B cell function and autoimmunity, its role in the development of autoimmune arthritis has not been fully clarified. Using a collagen-induced arthritis (CIA) mouse model, we detected dysregulated expression of BAFF and its receptors in the peripheral lymphoid organs during arthritis induction. Elevated serum levels of BAFF were closely correlated with increased levels of anti-collagen antibodies during the CIA progression. Moreover, dendritic cells (DCs) and macrophages were found to express high amount of BAFF proteins at the acute and chronic stages of CIA, respectively. In cultures, recombinant BAFF suppressed apoptosis of splenic B cells from arthritic mice, and DC-induced B cell proliferation was specifically blocked by soluble decoy receptor B cell maturation antigen-Fc. These findings suggest that overproduction of BAFF by DCs and macrophages may play a crucial role in the pathogenesis of experimental arthritis.     

BAFF is a survival and maturation factor for mouse B cells

Human B cell-activating factor (BAFF) induces mouse surface IgM+ B cells of the immature type from bone marrow and of the immature types 1 and 2 from spleen, as well as of the mature type from spleen to increased longevity in tissue culture. BAFF does so polyclonally and without inducing proliferation in any of these B cell subpopulations. BAFF induces phenotypic and functional maturation of immature to mature B cells so that all immature cells loose C1qRp (AA4.1, 493) expression and type 1 immature cells up-regulate IgD, CD21 and CD23. Immature B cells of types 1 and 2, upon pre-incubation with BAFF, change their reactiveness to Ig-specific antibodies so that they no longer enter apoptosis but now proliferate. However, BAFF does not seem to overcome negative selection of developing immature B cells in vitro.     

Lymphoid antigenic determinants of the chicken. Cellular representation and tissue localization.

Antisera were prepared against a variety of chicken tissues with a view to detecting antigens specific for subpopulations of T and B cells at different maturation stages. By appropriate absorption analysis, the following antigens were defined: (a) thymus organ-specific antigen (CTOA); (b) T lymphocyte-specific antigen (CTLA); (c) B lymphocyte-specific antigen (CBLA); (d) mature B lymphocyte-specific antigen (CMBLA); (e) a foetal-associated antigen (CFAA) present on embryonic haemopoietic cells, and adult bone marrow and immature bursa cells, suggesting the cell types concerned may be at an early stage of development, possibly including precursors. For comparison with the above antigens, cells were also examined for surface Ig and IgG. T cells were found in periarteriolar sheaths in the spleen, and predominantly in the sub-epithelium and sub-mucosa of the caecal tonsil of the gut-associated lymphoid tissue (GALT). B cells were localized in periellipsoidal sheaths and germinal centres in the spleen, and in primary follicles or germinal centres in the GALT. The thymic and bursal medullas contained the mature populations of T and B lymphocytes respectively.     

Integration of signals mediated by B-cell receptor, B-cell activating factor of the tumor necrosis factor family (BAFF) and Fas (CD95)

The survival of the mature resting B cells depends on signaling from B cell receptor (BCR), and a plethora of positive and negative regulators, that maintain cellular homeostasis and ultimately determine cell's fate, i.e., survival or programmed death (apoptosis). Among these regulators we have investigated the B cell activating factor belonging to tumor necrosis factor family (BAFF) and the prototypic death receptor Fas/CD95 mediated signals. We have shown that BAFF inhibits Fas-mediated cell death, however, the BCR-driven survival signals were not strengthened by BAFF. Therefore, we propose that BAFF may function independently of the antigen specificity of BCR, thus may enhance the risk of autoimmune diseases by promoting the survival of bystander B cells in the germinal center.     

BAFFled B cells survive and thrive: roles of BAFF in B-cell development

Interactions of BAFF (B-cell activating factor) with BAFF-R, one of three BAFF-binding receptors that are preferentially expressed on B cells, are essential for B-cell development, because defects in either the ligand or the receptor arrest progression from immature type-1 B cells to type-2 cells and mature cells; B1 B cells are unaffected. Transgenic BAFF overexpression leads to B-cell hyperplasia and autoimmune disease. In vitro, BAFF increases survival of immature and mature B cells; immature B cells also mature polyclonally to mature B cells, without proliferation. Upon BAFF-influenced differentiation, immature B cells change their surface-IgM signal transduction machinery and proliferate rather than undergoing apoptosis.     

BAFF信号的免疫调节作用

B淋巴细胞活化因子(BAFF)是肿瘤坏死因子超家族的新成员,是B淋巴细胞生长、分化和发育所需的细胞因子,其通过与细胞表面的3种受体——B细胞成熟抗原(BCMA)、穿膜蛋白活化物(TACI)和B细胞活化因子受体(BAFF-R)结合产生信号发挥重要调节作用。BAFF的缺陷或过量表达均能引起机体的免疫失衡,从而诱发多种疾病。     

The BAFF-Interacting receptors of chickens

The TNF superfamily cytokine BAFF has crucial roles in homoeostatic regulation of B cell populations in mammals. Similar effects on peripheral B cells have been reported for chicken as for mammalian BAFF. Unlike mammalian BAFF, chicken BAFF is produced by B cells, implying an autocrine loop and consequent differences in regulation of B cell homoeostasis. Understanding of these mechanisms requires investigation of BAFF-binding receptors in chickens. We identified and characterised chicken receptors BAFFR and TACI, but found that the gene encoding the third BAFF-binding receptor, BCMA, was disrupted, implying differences in mechanisms for maintenance of long-lived antibody responses. A BAFFR-Ig fusion protein expressed in vivo lowered B cell numbers, showing that it was functional under physiological conditions. We found changes in the ratio of BAFFR and TACI mRNAs in the bursa after hatch that may account for the altered requirements for B cell survival at this stage of development.     

BAFF supports human B cell differentiation in the lymphoid follicles through distinct receptors

B cell-activating factor of the tumor necrosis factor family (BAFF/BLys) plays a critical role in B cell survival and immune responses through its three receptors: BAFF receptor (BAFF-R/BR3), transmembrane activator and calcium modulator and cyclophilin ligand interactor (TACI) and B cell maturation antigen (BCMA). Using specific antibodies, we have investigated the expression of BAFF-R on human tonsillar B cells and their functional roles in naive and germinal center (GC) B cell differentiation. Our studies show that BAFF-R is the dominant receptor on naive B cells. However, three receptors are differentially modulated during in vitro GC-B cell differentiation. BAFF-R expression increased initially and then decreased with a corresponding induction of TACI and BCMA expression during differentiation to plasma cells (PCs). Consistently, blocking of BAFF-R alone with specific mAb inhibited GC-B cell proliferation and PC generation in the early period of their differentiation, whereas depletion of BAFF with TACI-Ig exhibited consistent inhibition throughout the differentiation. Finally, histological and molecular analyses of human tonsil tissue revealed that follicular dendritic cells produce BAFF. In conclusion, BAFF in the GC plays an important role through more than one receptor, and the three known receptors are differentially modulated as GC-B cells differentiate to PCs.     

Chicken B-cell-activating factor: regulator of B-cell survival in the bursa of fabricius

Previous studies on mammals have demonstrated that a tumour necrosis factor family member, B-cell-activating factor (BAFF) (BlyS, TALL-1), is mainly produced by myeloid and dendritic cells and that BAFF promotes B-cell differentiation and survival in a paracrine fashion. We have recently shown that BAFF is upregulated at the bursal stage of the avian B-cell development. We now show that the avian bursal B cells and B-cell lines, RP-9, RP-13 and DT40, express chicken BAFF (cBAFF). In situ hybridization confirms strong cBAFF expression within the bursal follicles. Like mammals, cBAFF is expressed in the avian myeloblast and myelomonocytic cell lines but not in the peripheral blood alphabeta and gammadelta T cells. The binding of recombinant human BAFF (hBAFF) to the bursal B-cells indicates a conserved receptor-ligand binding. Furthermore, the recombinant hBAFF has a positive effect on bursal cell proliferation and transiently inhibits cell death in vitro. In conclusion, cBAFF is highly conserved structurally, but as a novel observation we suggest cBAFF to function in an autocrine fashion to promote the growth and maturation of follicular B cells in bursa of Fabricius.     

The role of BAFF in autoimmune diseases]

B cell activating factor belonging to the tumor necrosis factor family (BAFF) is a tumor necrosis factor (TNF) superfamily member best known for its role in the survival and maturation of B cells. BAFF is a ligand for three TNF receptor superfamily members: B-cell maturation antigen (BCMA), transmembrane activator and calcium-modulator and cyclophilin ligand interactor (TACI), and BAFF receptor (BAFF-R). Among them, BAFF-R plays the central role in the BAFF system, whereas TACI plays the inhibitory role. BAFF/BAFF receptors appear to span nearly all stages of B-lineage differentiation, ranging from the development, selection, and homeostasis of naive primary B cells to the maintenance of long-lived bone marrow plasma cells. Furthermore, excessive BAFF rescues self-reactive B cells from anergy, which may play a crucial role in the induction and development of autoimmunity. Mice overexpressing BAFF exhibit increased B cell numbers in spleen and lymph node and autoimmune phenotype similar to patients with systemic lupus erythematosus (SLE) and Sj?gren's syndrome. Furthermore, inhibition of BAFF by TACI-Ig and BAFFR-Ig has been successful in treating murine models of SLE and rheumatoid arthritis. In humans, previous reports have shown elevated serum BAFF levels in SLE, rheumatoid arthritis, Sj?gren's syndrome, and systemic sclerosis patients. Thus, the dysregulation of BAFF/BAFF receptor system may contribute to induction and development of autoimmune diseases and become one of important therapeutic targets.     

Defining a transcriptional fingerprint of murine splenic B-cell development

B-cell development occurs in a stepwise fashion that can be followed by the expression of B cell-specific surface markers. In this study, we wished to identify proteins that could contribute to the changes in expression of such markers. By using RNA from freshly isolated B220+ cells, we hoped to reduce the effect of artifacts that occur during the isolation and amplification steps necessary to use flow cytometry analysis-sorted subsets in microarray experiments. Analyses comparing expression patterns from B220+ 2-week bone marrow (pro-B, pre-B, immature B cells), 2-week spleen (predominantly transitional cells) and 8-week spleen (mainly mature B cells) yielded hundreds of genes. We also examined the B cell-activating factor (BAFF)-dependent effects on immature splenic B cells by comparing expression patterns in the spleen between 2-week A/J vs 2-week A/WySnJ mice, which lack functional BAFF receptor signaling. Genes that showed the expression differences between spleen and bone marrow samples were then analyzed through quantitative PCR on B-cell subsets isolated using two different sorting protocols. A comparison of the results from our study with the results from other analyses showed not only some overlap of preferentially expressed genes but also an expansion of other genes potentially involved in B-cell development.     

Deleting the BAFF receptor TACI protects against systemic lupus erythematosus without extensive reduction of B cell numbers

B cell-activating factor of the TNF family (BAFF) is an essential B cell survival factor. However, high levels of BAFF promote systemic lupus erythematosus (SLE) in mice and humans. Belimumab (anti-human BAFF) limits B cell survival and is approved for use in patients with SLE. Surprisingly, the efficacy of rituximab (anti-human CD20) in SLE remains controversial, despite depleting B cells more potently than belimumab. This raises the question of whether B cell depletion is really the mechanism of action of belimumab. In BAFF transgenic mice, SLE development is T cell-independent but relies on innate activation of B cells via TLRs, and TLR expression is modulated by the BAFF receptor TACI. Here, we show that loss of TACI on B cells protected against BAFF-mediated autoimmune manifestations while preserving B cells, suggesting that loss of BAFF signaling through TACI rather than loss of B cells may underpin the effect of belimumab in the clinic. Therefore, B cell-sparing blockade of TACI may offer a more specific and safer therapeutic alternative to broad B cell depletion in SLE.     

B淋巴细胞刺激因子的研究进展

肿瘤坏死因子超家族成员B淋巴细胞刺激因子(BAFF)通过和隶属与肿瘤坏死因子受体超家族成员的BCMA、TACI和BAFF-R的结合,在调节外周成熟B淋巴细胞的发育过程中起着重要的作用。由于在多种自身免疫性疾病中的异常表达,BAFF和其受体也被认为是治疗自身免疫性疾病的重要治疗靶点。     

The BAFF/APRIL system: life beyond B lymphocytes

B cell activating factor belonging to the TNF family (BAFF) and a proliferation-inducing ligand (APRIL) are two members of the TNF ligand superfamily. Studies of BAFF, APRIL and their receptors have highlighted the importance of this ligand/receptor system in regulating B cell homeostasis, tolerance and malignancy. Neutralizing BAFF can inhibit disease progression in animal models of autoimmunity, possibly by reducing survival of autoreactive B cells. In addition, BAFF inhibitors also prevent B lymphoma cell survival and may be useful for the treatment of lymphoid cancers. Recent work suggests that BAFF is also important for T cell activation and differentiation, an aspect that may be critical for the progression of certain autoimmune diseases. Therefore, targeting the BAFF/APRIL system may protect against autoimmunity and lymphoid cancers through the inhibition of pathogenic B and T cell functions.     

Lymphoid antigenic determinants of the chicken: ontogeny of bursa-dependent lymphoid tissue

The ontogenic development of the bursa-dependent lymphoid tissue in the chicken has been studied using rabbit antisera specific for B lymphocyte sub-populations and two elements of the bursal microenvironment. The antigens investigated were: the chicken B lymphocyte antigen ( CBLA ); mature B lymphocyte antigen ( CMBLA ); a foetal-associated antigen ( CFAA ) present on embryonic haemopoietic cells and adult bone marrow and bursa cells; immunoglobulin (Ig) and IgG; a bursa-specific cortical reticulin fibre antigen ( CBRFA ); a gut-associated mucin antigen ( CGAMA ) present on bursal medullary reticular epithelial (REp) cells. The development of suspected precursor cells was examined using a rabbit antiserum specific for foetal spleen cells. The major finding was the interrelationship between developing B cells and the bursal microenvironment. The first CBLA - and CFAA -positive cells were detected in the bursa at day 8 of incubation but their precise localization was difficult to assess. In 12-15 day embryos, both these cells were found predominantly in the tunica-propria in close proximity to cells bearing the reticulin fibre antigen CBRFA . This close association between CBLA -, CFAA - and CBRFA determinants represents the earliest stages of B cell differentiation and mimics that found in the adult bursa cortex. By day 18, the majority of bursa cells expressed CBLA and Ig and were localized in the developing medullary follicles, the REp cells of which were CGAMA -positive, demonstrating a very early interaction between intestinal tract contents, bursal REp cells and B cell maturation. Around hatching some bursa cells showed a marked increase in the membrane expression of CBLA and Ig, and the simultaneous expression of CMBLA and IgG. These cells were present in medullary follicles; CBRFA was present on cortical reticulin fibres which provided a supporting framework for the more immature CFAA -positive cells.     

Signals from activation of B-cell receptor with anti-IgD can override the stimulatory effects of excess BAFF on mature B cells in vivo

The selection and maturation of B-cell clones are critically determined by tonic signals from activated B cell receptors (BCR) and survival signals from BAFF cytokine. These finely tuned and coordinated signals provide a net positive signal that can promote the selection, maturation, proliferation and differentiation of a developing B cell. Stimulation with an anti-IgD antibody can also activate BCR but can lead to depletion and an arrest of mature B-cell development in vivo. It is not known whether survival signals from excess BAFF can override the suppressive effects of treatment with anti-IgD on mature B cells in vivo. Herein, we examined the effects of co-treatment of BAFF and anti-IgD on the mature B-cell compartment and antibody production in vivo by treating mice with either 1 mg/kg BAFF or anti-IgD alone or in combination for 3 consecutive days. We found that co-treatment with anti-IgD significantly abrogated these stimulatory effects of BAFF treatment on splenic CD19+ B cells as well as mature CD19+IgD^hiIgM+ B cells in vivo. Anti-IgD down-regulated the expression of the BCR complex (mIgM, mIgD and CD19) and the BAFF receptor TACI without regard to the presence of BAFF. Anti-IgD treatment also significantly negated BAFF-induced IgM production in vivo. Both BAFF and anti-IgD could individually stimulate IL-10 synthesis in B cells but did not affect one another. Taken together, our data suggest that activation of BCR with an anti-IgD antibody can override the stimulatory effects from excess BAFF on B cell proliferation and antibody production by down-regulating the expression of BCR complex and BAFF receptors.     

Cloning, expression and bioactivity of duck BAFF

B cell activating factor (BAFF) belonging to the TNF family is critical for B cell survival and maturation. In the present study, we identified a duck BAFF cDNA, named dBAFF, by RT-PCR and RACE strategies. The open reading frame (ORF) of this cDNA encodes a 288-amino acid protein containing a predicted transmembrane domain and a putative furin protease cleavage site like chicken BAFF (cBAFF), human BAFF (hBAFF) and mouse BAFF (mBAFF). The amino acid identity between biologically soluble dBAFF and cBAFF, hBAFF or mBAFF is 97, 78 and 71%, respectively. RT-PCR analysis showed the dBAFF gene is strongly expressed in the bursa of fabricius. Recombinant soluble dBAFF (dsBAFF) fused with NusA.tag was efficiently produced in Origami B (DE3) pLysS expression host strain. In vitro, purified dsBAFF was not only able to promote survival of bursa B cells, but also able to co-stimulate proliferation of mammalian B cells with anti-IgM. Furthermore, recombinant hsBAFF has a positive effect on duck bursa B cells survival. These findings indicate dBAFF plays an important role in survival and proliferation of duck B cells and because of its high conservation in the evolution, functional cross-reactivity exists between mammalian and duck BAFF.