BAFF-deficient Mice Research Articles

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.

Role of B Cell-Activating Factor in Fibrosis Progression in a Murine Model of Non-Alcoholic Steatohepatitis.

Non-alcoholic fatty liver disease (NAFLD) is the most prevalent chronic liver disease all over the world. Therapeutic strategies targeting its multidirectional pathways are required. Particularly, fibrosis is closely associated with its prognosis. We previously found that B cell-activating factor (BAFF) is associated with severity of NAFLD. Here, we determined the direct in vivo role of BAFF in the development of liver fibrosis. Histological and biochemical analyses were performed using wild-type and BAFF-deficient mice. We established a murine model of non-alcoholic steatohepatitis (NASH) using carbon tetrachloride injection accompanied by high-fat/high-cholesterol diet feeding. Additionally, in vitro analysis using mouse macrophage-like cell line RAW264.7 and primary hepatic stellate cells was performed. Hepatic steatosis and inflammation, and most importantly, the progression of liver fibrosis, were ameliorated in BAFF-deficient mice compared to those wild-type mice in our model. Additionally, BAFF deficiency reduced the number of CD11c+ M1-type macrophages in the liver. Moreover, BAFF stimulated RAW264.7 cells to secrete nitric oxide and tumor necrosis factor α, which drove the activation of hepatic stellate cells. This indicates that BAFF plays a crucial role in NASH development and may be a promising therapeutic target for NASH.

B Cell and T Cell Dissimilarities in BAFF-Deficient versus BR3-Deficient C57BL/6 Mice.

BAFF is a potent B cell survival and differentiation factor with three receptors, TACI, BCMA, and BR3. B cells are greatly reduced in BAFF-deficient mice, and among mice deficient in a single BAFF receptor, B cell reduction is characteristic only of BR3-deficient mice. Nevertheless, there may be important differences between BR3-deficient mice, in which interactions between BAFF and only BR3 are abrogated, and BAFF-deficient mice, in which interactions between BAFF and all its receptors are abrogated. We demonstrate that: 1) the numbers of CD19+ cells in C57BL/6 (B6).Baff-/- and B6.Br3-/- mice diverge as the mice age; 2) the distribution of B cell subsets significantly differ between B6.Baff-/- and B6.Br3-/- mice regardless of age or sex; 3) the relationships of CD3+ and CD4+ cells to B cells vastly differ between B6.Baff-/- and B6.Br3-/- mice as a function of age and sex; 4) the numbers and percentages of CD4+Foxp3+ and CD4+CD25+Foxp3+ are greater in B6.Baff-/- mice than in B6.Br3-/- mice; and 5) for any given number of CD19+ cells or CD4+ cells, percentages of Foxp3+ cells and CD4+CD25+Foxp3+ cells are lower in B6.Br3-/- mice than in B6.Baff-/- mice, with proliferation of these cells being greater, and survival being lesser, in B6.Br3-/- mice than in B6.Baff-/- mice. Collectively, these observations raise the possibility that interactions between TACI and/or BCMA and BAFF modulate expression of B cell subsets and Foxp3+ cells and may help explain prior enigmatic observations of autoimmunity and autoimmune disease in mice despite the absence of functional engagement of BR3 by BAFF.

B-Cell Activating Factor secreted by neutrophils is a critical player in lung inflammation to cigarette smoke exposure

B cell activating factor (BAFF) is a TNF family cytokine mainly produced by adaptive immune cells in particular B and T lymphocytes. Depending on the context, innate immune cells can also produce BAFF. First of all, BAFF was known to play an important role in the maturation and survival of B lymphocytes in humoral immune response. Recent data indicate that BAFF may also participate in the regulation of innate immune responses and in the pathophysiology of pulmonary diseases. In models of mice chronically exposed to cigarette smoke (CS), BAFF plays a major role in the generation of pulmonary antinuclear antibodies and tertiary lymphoid follicles. However, the implication of BAFF in the development of innate immunity to cigarette smoking remains unknown. Wild-type or BAFF deficient mice were exposed to 3R4F cigarette smoke 3 times a day for 4 days. Mice were treated or not with mouse anti-GR1 or recombinant mouse BAFF during CS exposure. Broncho-alveolar lavage fluids (BALF) were done and cell infiltration was determined. Different cytokines and chemokines were measured in BALF and lung homogenates. Immunostaining on cytospin were done. Bone marrow derived neutrophils (BMDN) and Murine tracheal epithelial cells (MTEC) were stimulated in vitro by CSE (Cigarette Smoke Extract). Cell viability and BAFF levels were determined. Acute CS exposure promotes BAFF expression in airway recruited neutrophils. Immunostaining analysis revealed that neutrophils are the major source of BAFF. We confirmed in vitro that neutrophils secrete BAFF in response to CSE stimulation. Neutrophil depletion partially decreases BAFF expression in lung tissue and bronchoalveolar space suggesting additional sources of BAFF. Importantly, BAFF deficient mice displayed decreased airway neutrophil recruiting chemokines and neutrophil influx while the addition of exogenous BAFF significantly enhanced this CS-induced neutrophilic inflammation. These results demonstrate that BAFF is a key pro-inflammatory cytokine and that innate immune cells in particular neutrophils, are an unconsidered source of BAFF in early stages of CS-induced innate immunity.

B-Cell Activating Factor Secreted by Neutrophils Is a Critical Player in Lung Inflammation to Cigarette Smoke Exposure.

Cigarette smoke (CS) is the major cause of chronic lung injuries, such as chronic obstructive pulmonary disease (COPD). In patients with severe COPD, tertiary lymphoid follicles containing B lymphocytes and B cell-activating factor (BAFF) overexpression are associated with disease severity. In addition, BAFF promotes adaptive immunity in smokers and mice chronically exposed to CS. However, the role of BAFF in the early phase of innate immunity has never been investigated. We acutely exposed C57BL/6J mice to CS and show early BAFF expression in the bronchoalveolar space and lung tissue that correlates to airway neutrophil and macrophage influx. Immunostaining analysis revealed that neutrophils are the major source of BAFF. We confirmed in vitro that neutrophils secrete BAFF in response to cigarette smoke extract (CSE) stimulation. Antibody-mediated neutrophil depletion significantly dampens lung inflammation to CS exposure but only partially decreases BAFF expression in lung tissue and bronchoalveolar space suggesting additional sources of BAFF. Importantly, BAFF deficient mice displayed decreased airway neutrophil recruiting chemokines and neutrophil influx while the addition of exogenous BAFF significantly enhanced this CS-induced neutrophilic inflammation. This demonstrates that BAFF is a key proinflammatory cytokine and that innate immune cells in particular neutrophils, are an unconsidered source of BAFF in early stages of CS-induced innate immunity.

B-cell activating factor deficiency suppresses splenomegaly during Leishmania donovani infection

B-cell activating factor (BAFF) is a critical regulator for B-cell development and differentiation. We previously reported elevation of serum BAFF levels in patients with visceral leishmaniasis (VL). In this study, we examined if BAFF is involved in pathologies during infection of Leishmania donovani. BALB/cA mice infected with L. donovani showed significant elevation in serum BAFF and IgG levels as seen in VL patients. In contrast, elevation of serum IgG by L. donovani infection was significantly suppressed in BAFF-deficient mice. The spleen weight of the BAFF-deficient mice after infection was significantly lower than that of the infected wild-type mice, whereas comparable degree of hepatomegaly and anemia were observed in those mice. In the enlarged spleen of L. donovani-infected wild-type mice, increase of CD19+ lymphocytes was more prominent than that of CD3+ cells, suggesting the contribution of B cell increase to splenomegaly during VL. Besides, increase of CD19+ lymphocytes was not found in BAFF-deficient mice after L. donovani infection. Taken together, these results suggest that BAFF is involved in strong B cell activation, which has a pathological role in splenomegaly but not in hepatomegaly or anemia, during VL.

Gender-Specific Mechanisms Underlying the Amelioration of High-Fat Diet-Induced Glucose Intolerance in B-Cell-Activating Factor Deficient Mice.

It has recently been found that B cell activating factor (BAFF) plays an important role in the regulation of energy homeostasis. We also have previously reported that BAFF deficiency reverses high-fat (HF) diet-induced glucose intolerance by potentiating adipose tissue function. In the present study, we found that BAFF deficient (BAFF-/-) mice exhibit gender-specific differences in protection against diet-induced glucose intolerance, and aimed to characterize the gender-dependent molecular alterations in energy metabolism. Under HF feeding conditions, serum BAFF level of female wild-type (WT) mice was considerably higher than that of male mice. Despite increased body weight gain, both male and female BAFF-/- mice showed significantly improved glucose tolerance compared to their WT counterparts. Expressions of genes involved in glucose transport, thermogenesis and lipid oxidation were up-regulated in brown adipose tissues of both male and female BAFF-/- mice. Interestingly, the expression of thermogenic genes in subcutaneous adipose tissue was significantly enhanced in female BAFF-/- compared to WT mice, but the difference was not observed between male BAFF-/- and WT mice. The enhanced thermogenic program was confirmed by higher protein levels of UCP1 and irisin in female BAFF-/- than in WT mice. Additionally, adiponectin production in white adipose tissues and AMPK phosphorylation in subcutaneous adipose tissue were also significantly elevated in female BAFF-/- compared to WT mice, but not in male BAFF-/- mice. Our findings define a comprehensive scenario for the enhancing effect of BAFF depletion on glucose tolerance wherein the underlying mechanism is, at least in part, gender-specific, and suggest that gender difference should be considered as an important factor in the use of BAFF blockade as a therapeutic approach for the prevention and treatment of type 2 diabetes.

Antibodies That Block or Activate Mouse B Cell Activating Factor of the Tumor Necrosis Factor (TNF) Family (BAFF), Respectively, Induce B Cell Depletion or B Cell Hyperplasia

B cell activating factor of the TNF family (BAFF), also known as B lymphocyte stimulator, is a ligand required for the generation and maintenance of B lymphocytes. In this study, the ability of different monoclonal antibodies to recognize, inhibit, or activate mouse BAFF was investigated. One of them, a mouse IgG1 named Sandy-2, prevented the binding of BAFF to all of its receptors, BAFF receptor, transmembrane activator and calcium modulating ligand interactor, and B cell maturation antigen, at a stoichiometric ratio; blocked the activity of mouse BAFF on a variety of cell-based reporter assays; and antagonized the prosurvival action of BAFF on primary mouse B cells in vitro A single administration of Sandy-2 in mice induced B cell depletion within 2 weeks, down to levels close to those observed in BAFF-deficient mice. This depletion could then be maintained with a chronic treatment. Sandy-2 and a previously described rat IgG1 antibody, 5A8, also formed a pair suitable for the sensitive detection of endogenous circulating BAFF by ELISA or using a homogenous assay. Interestingly, 5A8 and Sandy-5 displayed activities opposite to that of Sandy-2 by stimulating recombinant BAFF in vitro and endogenous BAFF in vivo These tools will prove useful for the detection and functional manipulation of endogenous mouse BAFF and provide an alternative to the widely used BAFF receptor-Fc decoy receptor for the specific depletion of BAFF in mice.

Stoichiometry of Heteromeric BAFF and APRIL Cytokines Dictates Their Receptor Binding and Signaling Properties

The closely related TNF family ligands B cell activation factor (BAFF) and a proliferation-inducing ligand (APRIL) serve in the generation and maintenance of mature B-lymphocytes. Both BAFF and APRIL assemble as homotrimers that bind and activate several receptors that they partially share. However, heteromers of BAFF and APRIL that occur in patients with autoimmune diseases are incompletely characterized. The N and C termini of adjacent BAFF or APRIL monomers are spatially close and can be linked to create single-chain homo- or hetero-ligands of defined stoichiometry. Similar to APRIL, heteromers consisting of one BAFF and two APRILs (BAA) bind to the receptors B cell maturation antigen (BCMA), transmembrane activator and CAML interactor (TACI) but not to the BAFF receptor (BAFFR). Heteromers consisting of one APRIL and two BAFF (ABB) bind to TACI and BCMA and weakly to BAFFR in accordance with the analysis of the receptor interaction sites in the crystallographic structure of ABB. Receptor binding correlated with activity in reporter cell line assays specific for BAFFR, TACI, or BCMA. Single-chain BAFF (BBB) and to a lesser extent single-chain ABB, but not APRIL or single-chain BAA, rescued BAFFR-dependent B cell maturation in BAFF-deficient mice. In conclusion, BAFF-APRIL heteromers of different stoichiometries have distinct receptor-binding properties and activities. Based on the observation that heteromers are less active than BAFF, we speculate that their physiological role might be to down-regulate BAFF activity.

B-cell activating factor regulates IL-1β- and IL-17A-mediated pulmonary fibrosis in mice

Introduction Idiopathic pulmonary fibrosis (IPF) is a progressive devastating, yet untreatable fibrotic disease of unknown origin. We investigated the contribution of the B-cell activating factor (BAFF), a TNF family member recently implicated in the regulation of pathogenic IL-17-producing cells in autoimmune diseases. Methods The contribution of BAFF was assessed in a murine model of lung fibrosis induced by airway administered bleomycin, an intercalating agent which causes DNA strand breaks. Lung fibrosis was evaluated in Baff deficient mice and in wild-type mice after neutralization of BAFF with BAFF-R-Ig. Lung cells from bleomycin-treated mice were collected at day 7, labeled with anti-CD115, anti-CD3, anti-CD19 and anti-Gr1 antibodies and sorted by FACS for evaluation of BAFF mRNA expression by qPCR. BAFF was quantified in bronchoalveolar lavage fluid (BALF) of controls and patients with IPF. Results We show that murine BAFF levels were strongly increased in the bronchoalveolar space and lungs after bleomycin exposure. We identified Gr1 + neutrophils as an important source of BAFF upon BLM induced lung inflammation and fibrosis. Genetic ablation of BAFF or BAFF neutralization by a soluble receptor significantly attenuated pulmonary fibrosis and IL-1β levels. We further demonstrate that bleomycin-induced BAFF expression and lung fibrosis were IL-1β- and IL-17A-dependent. BAFF was required for rIL-17A-induced lung fibrosis and augmented IL-17A production by CD3 + T cells from murine fibrotic lungs ex vivo . Finally, we report elevated levels of BAFF in bronchoalveolar lavages from IPF patients. Conclusion The present data, demonstrating a critical role of BAFF by amplifying the IL-17A effect in experimental lung fibrosis, together with increased BAFF levels in the airways of patients with IPF, suggest that BAFF might be a therapeutic target of IPF.

B Cell and BAFF dependence of IFN-α-exaggerated disease in systemic lupus erythematosus-prone NZM 2328 mice.

IFN-α is a potent activator of innate and adaptive immunity, and its administration to preautoimmune (NZB×NZW)F1 mice promotes virulent systemic lupus erythematosus (SLE) disease. Given the known contributions of B cells and BAFF to SLE, we evaluated the ability of IFN-α administration to induce disease in wild-type (WT), B cell-deficient, and BAFF-deficient NZM 2328 mice. Whereas WT mice rapidly developed proliferative glomerulonephritis, marked proteinuria, and increased mortality in response to IFN-α administration, B cell-deficient mice developed neither renal pathology nor clinical disease. Moreover, BAFF-deficient mice, despite developing limited glomerular IgG and C3 deposition, also remained free of histological glomerulonephritis and clinical disease. Strikingly, similar T cell expansion and serum IgG responses were observed in adenovirus (Adv)-IFN-treated WT and BAFF-deficient mice despite their disparate pathological and clinical responses, whereas numbers of activated B cells increased in WT mice but not in BAFF-deficient mice. Nonetheless, B cell, plasma cell, and T cell infiltration of the kidneys in Adv-IFN-treated WT mice was similar to that in WT mice treated with Adv-control. Its ability to promote SLE disease in WT mice notwithstanding, IFN-α administration failed to drive the preferential expansion of CD4(+) memory T cells that occurs during the natural course of disease, and glomerular infiltration of macrophages failed to associate with development of disease. These results collectively suggest that therapeutic targeting in SLE of BAFF and/or B cells in SLE could be successful even in states of IFN-α overexpression. Moreover, our results document important biological differences between IFN-α-driven and spontaneous natural SLE disease.

Mutation of the BAFF furin cleavage site impairs B‐cell homeostasis and antibody responses

B-cell-activating factor of the TNF family (BAFF)/BLyS contributes to B-cell homeostasis and function in the periphery. BAFF is expressed as a membrane-bound protein or released by proteolytic cleavage, but the functional importance of this processing event is poorly understood. Mice expressing BAFF with a mutated furin consensus cleavage site, i.e. furin-mutant BAFF (fmBAFF), were not different from BAFF-deficient mice with regard to their B-cell populations and responses to immunization. It is however noteworthy that an alternative processing event releases some soluble BAFF in fmBAFF mice. Mild overexpression (∼ 5-fold) of fmBAFF alone generated intermediate levels of B cells without improving humoral responses to immunization. Processed BAFF was however important for B-cell homeostasis, as peripheral B-cell populations and antibody responses were readily restored by administration of soluble BAFF trimers in BAFF-deficient mice. However, the rescue of CD23 expression in B cells of BAFF-deficient mice required both soluble BAFF trimers and fmBAFF, or a polymeric form of soluble BAFF (BAFF 60-mer). These results point to a predominant role of processed BAFF for B-cell homeostasis and function, and indicate possible accessory roles for membrane-bound BAFF.

B‐cell stage and context‐dependent requirements for survival signals from BAFF and the B‐cell receptor

One remarkable feature of the immune system is its capacity to maintain constant numbers of resting immune cells despite the complex nature of signals needed throughout development and maturation. For many years, B-cell survival was thought to rely solely on B-cell receptor (BCR) tonic signals that would trigger necessary basal survival pathways. The discovery of the tumor necrosis factor (TNF)-like ligand BAFF(B-cell activating factor belonging to the TNF family)/BLyS (B-lymphocyte stimulator) changed these views entirely, as BAFF-deficient mice lack most mature B cells, and treatment with BAFF inhibitors leads to their loss, establishing BAFF as an unappreciated key B-cell survival factor. BAFF-mediated survival signals have been mapped and signaling crosstalk with the BCR has been identified, explaining the need for both BCR- and BAFF-mediated signals for B-cell survival. However, this crosstalk only explains how BCR and BAFF signals cooperate to produce survival proteins and yet, inactivating pro-apoptotic factors such as FOXO proteins, which may be managed separately by BAFF and the BCR, has emerged as an equally important step for survival. In this review, we present new views on B-cell survival, at all stages of B-cell life, and suggest that, in most cases, survival results from the production of appropriate survival factors balanced with the adequate and timely degradation of pro-apoptotic proteins.

Rescue of the mature B cell compartment in BAFF-deficient mice by treatment with recombinant Fc-BAFF

BAFF deficiency in mice impairs B cell development beyond the transitional stage 1 in the spleen and thus severely reduces the size of follicular and marginal zone B cell compartments. Moreover, humoral immune responses in these mice are dramatically impaired. We now addressed the question whether the decrease in mature B cell numbers and the reduced humoral immune responses in BAFF-deficient mice could be overcome by the injection of recombinant BAFF. We therefore engineered a recombinant protein containing the human IgG1 Fc moiety fused to receptor-binding domain of human BAFF (Fc-BAFF). At 1 week after the second injection of this fusion protein a complete rescue of the marginal zone B cell compartment and a 50% rescue of the follicular B cell compartment was observed. Moreover these mice mounted a T cell-dependent humoral immune response indistinguishable from wild-type mice. By day 14 upon arrest of Fc-BAFF treatment mature B cell numbers in the blood dropped by 50%, indicating that the life span of mature B cells in the absence of BAFF is 14 days or less. Collectively these findings demonstrate that injection of Fc-BAFF in BAFF-deficient mice results in a temporary rescue of a functional mature B cell compartment.

Immunologic messenger molecules: Cytokines, interferons, and chemokines

Cytokines and chemokines are secreted proteins involved in numerous aspects of cell growth, differentiation, and activation. A prominent feature of these molecules is their effect on the immune system with regard to cell trafficking and development of immune tissue and organs. The nature of an immune response determines which cytokines are produced and ultimately whether the response is cytotoxic, humoral, cell mediated, or allergic. For this chapter, cytokines are grouped according to those that are predominantly antigen-presenting cell or T lymphocyte derived; that mediate cytotoxic, humoral, cell mediated, and allergic immunity; or that are immunosuppressive. A discussion of chemokine function and their role in cell trafficking and disease follows.

Saved from Destruction

Survival of naïve B lymphocytes depends on the cytokine BAFF (B cell-activating factor of the tumor necrosis factor family). Although BAFF can signal through several receptors, only one of them (BAFF-R) is essential for survival (see Cancro). Noting that BAFF-R binds TNF receptor-associated factor 3 (TRAF3) and that TRAF3 and TRAF2 have been implicated in B cell survival, Gardam et al . examined the effects on lymphocyte survival of lineage-specific inactivation of TRAF2 or TRAF3 or both. Mice lacking B cell TRAF2 ( Traf2ΔB mice) or B cell TRAF3 ( Traf3ΔB mice) showed an indistinguishable phenotype, characterized by increased numbers of lymph node and splenic marginal zone B cells, that resembled that of mice overexpressing BAFF (BAFF-tg mice). B cells from Traf2ΔB and Traf3ΔB mice survived in vitro in the absence of BAFF and, like BAFF-tg B cells, showed increased activation of the alternative (noncanonical) NF-κB (nuclear factor κB) signaling pathway. No further effects on B cell phenotype were seen when both TRAF2 and TRAF3 were absent, and microarray analysis revealed that mature B cells from Traf2ΔB and Traf3ΔB mice showed nearly identical gene expression profiles, which were similar to those of BAFF-tg B cells. Elimination of TRAF2 from the B cells rescued B cell development in BAFF-deficient mice. The abundance of TRAF3 protein (but not Traf3 mRNA) was increased in B cells from mice lacking BAFF, BAFF-R, or TRAF-2. The authors thus propose that BAFF acts as a survival factor by stimulating TRAF2-dependent TRAF3 depletion and reversing the cooperative TRAF2- and TRAF3-dependent inhibition of B cell survival. S. Gardam, F. Sierro, A. Basten, F. Mackay, R. Brink, TRAF2 and TRAF3 signal adapters act cooperatively to control the maturation and survival signals delivered to B cells by the BAFF receptor. Immunity 28 , 391-401 (2008). [PubMed] M. P. Cancro, Living in context with the survival factor BAFF. Immunity 28 , 300-301 (2008). [PubMed]

TNF family member B cell-activating factor (BAFF) receptor-dependent and -independent roles for BAFF in B cell physiology.

The cytokine TNF family member B cell-activating factor (BAFF; also termed BLyS) is essential for B cell generation and maintenance. Three receptors have been identified that bind to BAFF: transmembrane activator, calcium modulator, and cyclophilin ligand interactor (TACI); B cell maturation Ag (BCMA); and BAFF-R. Recently, it was shown that A/WySnJ mice, which contain a dramatically reduced peripheral B cell compartment due to decreased B cell life span, express a mutant BAFF-R. This finding, together with normal or enhanced B cell generation in mice deficient for BCMA or TACI, respectively, suggested that the interaction of BAFF with BAFF-R triggers signals essential for the generation and maintenance of mature B cells. However, B cells in mice deficient for BAFF differ phenotypically and functionally from A/WySnJ B cells. Residual signaling through the mutant BAFF-R could account for these differences. Alternatively, dominant-negative interference by the mutant receptor could lead to an overestimation of the importance of BAFF-R. To resolve this issue, we generated BAFF-R-null mice. Baff-r(-/-) mice display strongly reduced late transitional and follicular B cell numbers and are essentially devoid of marginal zone B cells. Overexpression of Bcl-2 rescues mature B cell development in Baff-r(-/-) mice, suggesting that BAFF-R mediates a survival signal. CD21 and CD23 surface expression are reduced on mature Baff-r(-/-) B cells, but not to the same extent as on mature B cells in BAFF-deficient mice. In addition, we found that Baff-r(-/-) mice mount significant, but reduced, Ag-specific Ab responses and are able to form spontaneous germinal centers in mesenteric lymph nodes. The reduction in Ab titers correlates with the reduced B cell numbers in the mutant mice.

Normal B cell homeostasis requires B cell activation factor production by radiation-resistant cells.

The cellular source of B cell activation factor (BAFF) required for peripheral B cell survival/maturation is unknown. To determine the nature of BAFF-producing cells we established and analyzed reciprocal bone marrow (BM) chimeras with wild-type (WT) and BAFF-deficient mice. The results revealed that BAFF production by radiation-resistant stromal cells is completely sufficient to provide a necessary signal for B cell survival/maturation, as BAFF−/− BM cells transferred into lethally irradiated WT mice gave rise to normal numbers of follicular (FO) and marginal zone (MZ) B cell subpopulations. On the other hand, transfer of WT BM into BAFF−/−lethally irradiated mice resulted only in minimal reconstitution of mature FO B cells and no restoration of MZ B cells. Thus, in the absence of BAFF+/+stromal cells, BAFF production by BM-derived cells, presumably by macrophages, dendritic cells, and/or neutrophils, was not at all sufficient to support normal B cell homeostasis. Interestingly, immunization of both types of chimeras stimulated high levels of antigen-specific antibody secretion, indicating that either stromal cell– or hematopoietic cell–derived BAFF is sufficient for B cell antibody responses.

A key role for BAFF in maintaining peripheral B cells

In mice, only a few percent of B cells produced in the bone marrow (BM) enter the peripheral long-lived mature B-cell pool. Selection for entry into the periphery occurs in the BM and spleen. The majority of immature cells is thought to die in the BM owing to the expression of autoreactive B-cell receptor (BCR) specificities. Immature cells that survive this initial selection migrate to the spleen as transitional-stage T1 B cells (IgMhiIgD−), which differentiate to a second transitional stage (T2) marked by the high-level expression of IgD. T2 B cells are found in splenic primary follicles and are the direct precursors of mature B cells. Also, they are the targets of further selection; survival of T2 cells depends on signals generated through the BCR but, also, on undefined factors derived from the microenvironment of the adult spleen. Three recent papers suggest that the B-cell-activating factor belonging to the tumor necrosis factor (TNF) family (BAFF) 1xAn essential role for BAFF in the normal development of B cells through a BCMA-independent pathway. Schiemann, B. et al. Science. 2001; 293: 2111–2114Crossref | PubMed | Scopus (706)See all References, 2xTACI-Ig neutralizes molecules critical for B-cell development and autoimmune disease. Impaired B-cell maturation in mice lacking BlyS. Gross, J.A. et al. Immunity. 2001; 15: 289–302Abstract | Full Text | Full Text PDF | PubMed | Scopus (449)See all References and a newly identified BAFF receptor (BAFF-R) 3xBAFF-R, a newly identified TNF receptor that specifically interacts with BAFF. Thompson, J.S. et al. Science. 2001; 293: 2108–2111Crossref | PubMed | Scopus (619)See all References3 form a crucial ligand–receptor pair that facilitates the survival of transitional B cells and, thereby, maintenance of mature B cells.BAFF (also called Blys or TNFS13B), which is expressed and secreted by a variety of cells, including macrophages, dendritic cells and T cells, was identified first as a powerful co-stimulator of B-cell proliferation and Ig production. Then, it was discovered that the over-expression of BAFF results in vastly increased numbers of mature, but not immature, B cells. The occurrence of autoantibodies (and resultant autoimmune disease), with similar characteristics to those found in human systemic lupus erythematosus (SLE), in BAFF-transgenic mice further suggested a role for BAFF in regulating the maintenance of mature B cells. Hitherto, BAFF had been shown to bind to two members of the TNF receptor family – BCMA, which is B-cell-restricted, and TACI, which is expressed not only on a subset of B cells but, also, on activated T cells. However, studies of these receptors in knockout mice suggest that BCMA and TACI do not mediate the survival activity of BAFF; BCMA-deficient animals have a normal phenotype, whereas TACI-knockouts have increased numbers of mature B cells and immunoglobulins.This paradox has now been resolved by Thompson et al.3xBAFF-R, a newly identified TNF receptor that specifically interacts with BAFF. Thompson, J.S. et al. Science. 2001; 293: 2108–2111Crossref | PubMed | Scopus (619)See all References3 who, by screening an expression library with BAFF, have identified a third receptor, BAFF-R, which is not only B-cell specific but does not bind any other known TNF-family ligand. Furthermore, Schiemann et al.1xAn essential role for BAFF in the normal development of B cells through a BCMA-independent pathway. Schiemann, B. et al. Science. 2001; 293: 2111–2114Crossref | PubMed | Scopus (706)See all References1 and Gross et al.2xTACI-Ig neutralizes molecules critical for B-cell development and autoimmune disease. Impaired B-cell maturation in mice lacking BlyS. Gross, J.A. et al. Immunity. 2001; 15: 289–302Abstract | Full Text | Full Text PDF | PubMed | Scopus (449)See all References2 have reported independently that BAFF-deficient mice show an almost complete loss of follicular and marginal-zone (MZ) B cells, whereas immature B cells are unaffected. Crucially, the developmental block in BAFF-knockout mice occurs at the transitional T1 stage, suggesting that transitional T2 B cells are the primary target for BAFF or T1 cells require BAFF for differentiation to the T2 stage. B1 B cells were unaffected in BAFF-deficient mice; therefore, it appears that B1 B cells are maintained independently of BAFF. Underlining the importance of BAFF–BAFF-R in the maintenance of mature follicular and MZ B cells, Thompson et al.3xBAFF-R, a newly identified TNF receptor that specifically interacts with BAFF. Thompson, J.S. et al. Science. 2001; 293: 2108–2111Crossref | PubMed | Scopus (619)See all References3 report also that the immunodeficient mouse strain A/WySnJ, which has a phenotype similar to that of BAFF-deficient mice, has a mutation in the gene encoding BAFF-R.In addition to furthering our understanding of B-cell development, these studies raise the question of whether excessive signaling through BAFF-R can allow survival in the periphery of autoreactive mature B cells – especially, as increased levels of BAFF have been reported in SLE. Also, it will be interesting to determine whether BAFF plays a role in the survival of Ag-activated and memory B cells.

BAFFling No More

A number of tumor necrosis factor (TNF)-related ligands and receptors have recently been identified that regulate B cell development and function. One of these is the B cell-activating factor, BAFF, which interacts with two receptors [transmembrane activator and calcium-modulating cyclophilin ligand interacting protein (TACI) and B cell maturation antigen (BCMA)] to promote survival and proliferation of splenic B cells (see the Perspective by Waldschmidt and Noelle ). Scheimann et al. observed a profound defect in B cell development in mutant mice that lack expression of BAFF that was only partially reiterated in TACI- and BCMA-deficient animals. These results implicate the existence of another receptor for BAFF. Thompson et al. describe a candidate receptor that binds BAFF but not any other of the identified TNF-like ligands. The A/WySnJ strain of mice harbors a truncation in the BAFF-R gene that leads to a B cell deficit similar to that observed in BAFF-deficient mice. Together, these studies suggest that the interaction of BAFF with BAFF-R mediates major effects on B cell development. T. J. Waldschmidt, R. J. Noelle, Long live the mature B cell--a BAFFling mystery resolved. Science 293 , 2012-2013 (2001). [Full Text] B. Schiemann, J. L. Gommerman, K. Vora, T. G. Cachero, S. Shulga-Morskaya, M. Dobles, E. Frew, M. L. Scott, An essential role for BAFF in the normal development of B cells through a BCMA-independent pathway. Science 293 , 2111-2114 (2001). [Abstract] [Full Text] J. S. Thompson, S. A. Bixler, F. Qian, K. Vora, M. L. Scott, T. G. Cachero, C. Hession, P. Schneider, I. D. Sizing, C. Mullen, K. Strauch, M. Zafari, C. D. Benjamin, J. Tschopp, J. L. Browning, C. Ambrose, BAFF-R, a newly identified TNF receptor that specifically interacts with BAFF. Science 293 , 2108-2111 (2001). [Abstract] [Full Text]