To switch or not to switch--the opposing roles of TACI in terminal B cell differentiation

The TNF superfamily ligands BAFF and APRIL and their three receptors BAFFR, BCMA, and TACI comprise a network that is critically involved in the development and function of humoral immunity. Failure of this complex system is associated with autoimmune disease, B lymphocyte tumours, and antibody deficiency. While BAFF:BAFFR interactions control peripheral B cell survival and homeostasis, BCMA function seems limited to the survival of long-lived bone marrow plasma cells. The functional activity of the third receptor TACI is, however, ambiguous: while TACI-/- mice predominantly develop autoimmunity and lymphoproliferation, TACI deficiency in humans primarily manifests itself as an antibody deficiency syndrome. An article in this issue of the European Journal of Immunology demonstrates a negative regulation via TACI in human B cells by using TACI specific antibodies. B cell proliferation, class switch recombination, and Ig production induced by various stimuli were inhibited via TACI. Within the BAFF/APRIL network, the expression of the receptors and ligands is spatially, as well as temporally, highly regulated at various stages of B cell development and function. Defining the exact contribution of TACI stimulation by specific triggers in vitro enables us to better understand the complex, context-dependent responses initiated by TACI in vivo.     

A mouse model of systemic lupus erythematosus responds better to soluble TACI than to soluble BAFFR,correlating with depletion of plasma cells

The TNF family cytokines B‐cell activating factor (BAFF) and a proliferation‐inducing ligand (APRIL) support plasma cell survival. It is known that inhibitors of BAFF only (BAFFR‐Fc) or BAFF and APRIL (TACI‐Fc) administered early enough in an NZB/NZW F1 mouse model of systemic lupus erythematosus (SLE) ameliorate clinical outcomes, pointing to a pathogenic role of BAFF. In the present study, TACI‐Fc administrated at a later stage of disease, after onset of autoimmunity, decreased the number of bone marrow plasma cells and slowed down further formation of autoantibodies. TACI‐Fc prevented renal damage during a 12‐week treatment period regardless of autoantibody levels, while BAFFR‐Fc did not despite a similar BAFF‐blocking activity in vivo. TACI‐Fc also decreased established plasma cells in a T‐dependent hapten/carrier immunization system better than single inhibitors of BAFF or APRIL, and sometimes better than combined single inhibitors with at least equivalent BAFF and APRIL inhibitory activities. These results indicate that TACI‐Fc can prevent symptoms of renal damage in a mouse model of SLE when BAFFR‐Fc cannot, and point to a plasticity of plasma cells for survival factors. Targeting plasma cells with TACI‐Fc might be beneficial to prevent autoantibody‐mediated damages in SLE.     

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.     

Sequence Analysis of TNFRSF13b, Encoding TACI, in Patients with Systemic Lupus Erythematosus

B cell activating factor belonging to the TNF family (BAFF) and a proliferation inducing ligand (APRIL), and their receptors BAFF receptor (BAFFR), B cell maturation antigen (BCMA), and transmembrane activator and CAML interactor (TACI) are involved in the regulation of B cell homeostasis and differentiation. BAFF overexpression leads to systemic lupus erythematosus (SLE) in mice and elevated BAFF levels have been observed in human SLE and mouse models for SLE. Furthermore, genetic inactivation of TACI in mice results in a SLE-like phenotype. Based on our recent finding that TACI is mutated in patients with common variable immunodeficiency, of whom more than 30% suffer from autoimmune conditions, we analyzed TACI in humans with SLE. Sequence analysis of TNFRSF13b/TACI in 119 unrelated SLE patients revealed four variants: R20C in exon 1, R72H in exon 3, the silent variation c.327 G > A in exon 3, and A181E in exon 4. No significant association with any of these variants was found, when compared to the frequencies of the variants in a healthy control cohort. Furthermore, the mutated alleles R20C and R72H did not segregate with the SLE phenotype in familial cases of SLE. Thus, our evaluation of the coding region of TNFRSF13b/TACI did not reveal any deleterious or disease-associated mutations. Inga Melchers and Bodo Grimbacher contributed equally to this work.     

The expression of BAFF-binding receptors is not altered in multiple sclerosis or myasthenia gravis

Multiple sclerosis (MS) is a chronic, progressive disease of the central nervous system (CNS) characterized by consistent myelin injury. Antibody-mediated death of oligodendrocytes is a pathological feature in a subset of MS patients and may be of relevance to disease pathogenesis. In myasthenia gravis (MG), acetylcholine receptors (AChR) situated at the neuromuscular endplate are destroyed by autoreactive antibodies. B-cell activating factor of the tumour necrosis factor (TNF) superfamily (BAFF) is essential for B-cell survival. Using flow cytometry, we evaluated the expression of three BAFF-binding receptors, namely, BAFF-receptor (BAFF-R), B-cell maturation antigen (BCMA), and transmembrane activator and calcium modulating and cyclophilin ligand interactor (TACI) in peripheral-blood lymphocytes. Nearly all CD19(+) B cells and CD19(+)CD27(+) memory B cells expressed BAFF-R. The intensity of BAFF-R expression was not statistically different in MS or MG compared with healthy controls. Very few T cells expressed BAFF-R. BCMA expression was strictly limited to B cells. Although both B and T cells expressed TACI, levels were much higher on B cells compared with levels on T cells. The percentages of B and T cells expressing BCMA and TACI did not differ significantly in MS or MG versus controls. We conclude that the expression of BAFF-binding receptors is not appreciably altered in MS or MG.     

B cell activating factor (BAFF) selects IL-10−B cells over IL-10+B cells during inflammatory responses

B cell activating factor (BAFF) regulates B cell maturation, survival, function, and plays a critical pathogenic role in autoimmune diseases. It remains unclear how BAFF affects IL-10⁻B cells versus regulatory B cells (Bregs) in inflammatory responses. In this study, we found that IL-10-expressing Bregs decreased in lupus-prone MRL/lpr mice and experimental allergic encephalomyelitis (EAE) mice. On blockade of the effects of BAFF with TACI-IgG, IL-10⁺ Bregs were upregulated in MRL/lpr and EAE mice. In addition, BAFF expanded IL-10⁺B cells over IL-10⁻B cells under noninflammatory conditions in vitro, whereas it expanded IL-10⁻B cells over IL-10⁺B cells during inflammatory responses, such as stimulation with autoantigen and LPS. Finally, the selection of IL-10⁻B cells over IL-10⁺B cells by BAFF was dependent on BAFF receptors (BAFFR, TACI, and BCMA) that were upregulated by inflammatory responses. This study suggests that BAFF selects IL-10⁻B cells over IL-10⁺ regulatory B cells via BAFF receptors in inflammatory responses.     

TACI attenuates antibody production costimulated by BAFF-R and CD40

B cell activating factor of the TNF family (BAFF), plays critical roles in B cell survival, activation, differentiation, and antibody (Ab) production. BAFF binds to three receptors: BAFF-R, transmembrane activator and calcium-modulator and cyclophilin ligand interactor (TACI) and B cell maturation antigen. While BAFF-R is the primary receptor for B cell costimulation by BAFF, TACI is reported to serve as a positive or negative regulator for B cell responses depending on conditions. To determine the real role of TACI in B cell responses, we examined the functional relationship between TACI and BAFF-R in Ab production from human peripheral blood B cells using agonistic mAb. BAFF-R and CD40 enhanced IgG secretion and B cell proliferation, which were inhibited by TACI. Although TACI induced mild B cell apoptosis, its extent did not correlate with that of TACI-mediated inhibition of IgG secretion. In addition, TACI inhibited B-lymphocyte-induced maturation protein-1 expression, IgG secretion from previously IgG-negative selected B cells, and activation-induced cytidine deaminase expression enhanced by BAFF-R and CD40. Importantly, BAFF-R and CD40 enhanced B cell responsiveness to TACI-mediated suppression. Thus, BAFF may attenuate T cell-independent and -dependent B cell responses by TACI.     

BAFF and LPS cooperate to induce B cells to become susceptible to CD95/Fas-mediated cell death

Microorganisms with pathogen-associated molecular patterns (PAMP) activate B cells directly by binding to TLR and also indirectly by inducing APC to release cytokines such as BAFF that promote B cell survival. We found that murine B cells activated concomitantly with LPS (TLR-4 ligand) and BAFF are protected from spontaneous apoptosis, but are more susceptible to Fas/CD95-mediated cell death. This increased susceptibility to Fas-induced apoptosis is associated with a dramatic coordinated up-regulation of Fas/CD95 and IRF-4 expression through a mechanism mediated, at least in part, by inhibition of the MEK/ERK pathway. Up-regulation of Fas/CD95 by BAFF is restricted to B cells activated through TLR-4, but not through TLR-9, BCR or CD40. TLR ligands differ in the BAFF family receptors (R) they induce on B cells: BAFF-R is increased by the TLR4 ligand, LPS, but not by the TLR9 ligand, CpG-containing oligodeoxynucleotides, which, in contrast, strongly up-regulates transmembrane activator and CAML interactor (TACI). This suggests the up-regulation of Fas by BAFF is mediated by BAFF-R and not by TACI. Consistently, APRIL, which binds to TACI and B cell maturation antigen but not BAFF-R, did not enhance Fas expression on LPS-activated B cells. Increased susceptibility to Fas-mediated killing of B cells activated with LPS and BAFF may be a fail-safe mechanism to avoid overexpansion of nonspecific or autoreactive B cells.     

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.     

The role of APRIL and BAFF in lymphocyte activation

The TNF family ligands BAFF (also called BLyS) and APRIL regulate lymphocyte survival and activation. BAFF binds to three receptors, BAFF-R, TACI and BCMA, whereas APRIL interacts with TACI, BCMA and proteoglycans. The contribution of BAFF and APRIL to B-cell and plasma-cell survival, CD154 (CD40L)-independent antibody isotype switching, germinal center maintenance, T-dependent and T-independent antibody responses, and T cell co-stimulation are relatively well understood. Constitutive BAFF produced by stromal cells determines the size of the peripheral B cell pool, whereas inducible BAFF produced by myeloid and other cells supports local survival of B lymphocytes and can be associated with development of autoimmunity when deregulated.     

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.     

Effect of rhTACI-Ig fusion protein on antigen-specific T cell responses from keyhole limpet haemocyanin challenged mice

In addition to modulate B cells function, B cell activating factor belonged to TNF family (BAFF) also regulates T cells response via BAFFR and transmembrane activator and calcium modulator and cyclophilin-ligand interactor (TACI) expressing on activated T cells. This study explored the effect of a recombinant fusion protein containing the extracellular ligand-binding portion of TACI and the Fc portion of human immunoglobulin G (TACI-Ig) on activated T cells that were obtained from antigen-specific T-cell responses mice model induced by keyhole limpet haemocyanin (KLH), the characteristics of KLH-challenged mice were observed simultaneously. KLH immunization leaded to a significant positive relationship between BAFF level in serum and the extent of spleen histopathology. Serum concentration of BAFF, APRIL, IgM and IgG antibodies to KLH, and IL-4 were increased under KLH immunization, but IL-2 synthesis was decreased, resulting in a downregulation of IL-2/IL-4 ratio. Antigen-specific T cells proliferation, IL-5 production, the percentage of Th and activation T cells were significantly upregulated, however, IL-2 secretion and the percentage of na?ve T cells were downregulated in vitro. RhBAFF co-stimulation further evoked T cells hyperplasy, IL-4 and IFN-γ expression, the subgroups of Th, early antigen activation and activation T cells were also further increased. On the contrary, na?ve T cells were further reduced under rhBAFF stimuli. Administration of rhTACI-Ig significantly inhibited T cells proliferation, cytokines production and T cells differentiation, and the inhibitory effects might be associated with its ability to neutralize both the exogenous and endogenetic BAFF and APRIL.     

Synthetic CpG oligodeoxynucleotides augment BAFF- and APRIL-mediated immunoglobulin secretion

The B lymphocyte-activating factor belonging to TNF superfamily (BAFF) acts on B lymphocytes through BAFF receptor (BAFF-R), the transmembrane activator, calcium modulator, and cyclophilin ligand interactor (TACI), and the B cell maturation antigen (BCMA). Another cytokine, a proliferation-inducing ligand (APRIL), only binds to TACI and BCMA. In this study, we sought to determine the effect of Toll-like receptor agonists (TLR-A) on the expression of BAFF/APRIL receptors by murine splenic B lymphocytes. CpG oligodeoxynucleotides (ODN) and LPS strongly up-regulated TACI expression, while BAFF-R was only up-regulated by CpG ODN. CpG ODN pretreatment up-regulated TACI expression on follicular and marginal zone B lymphocytes and increased their responses to BAFF- and APRIL-mediated Ig secretion. TACI seemed to be playing a pivotal role in BAFF- or APRIL-induced Ig secretion because B lymphocytes from TACI-knockout mouse or the blocking of TACI with a neutralizing antibody resulted in total inhibition of IgA and IgG secretion in CpG ODN-pretreated and BAFF- or APRIL-stimulated B cells. Thus, CpG ODN-induced increase in TACI expression is likely to play an important role in Ig secretion following activation of B lymphocytes through TLR9.     

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.     

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.     

High serum levels of BAFF, APRIL, and TACI in common variable immunodeficiency

Common variable immunodeficiency (CVID) is characterized by low levels of immune globulins and lack of antibody. Mutations in transmembrane activator and calcium-modulating cyclophilin ligand (TACI), are found in 8-10%, associated with autoimmunity and splenomegaly. Some patients with mutations had increased serum levels of TACI. Because of this, and the prevalence of autoimmunity, splenomegaly, and lymphadenopathy, we quantitated levels of TACI ligands, a proliferation inducing ligand (APRIL) and B cell activating factor (BAFF) and TACI in serum of 77 patients. CVID subjects had markedly increased serum levels of BAFF (p<0.0001), APRIL (p<0.0001), and TACI (p=0.001) but there was no relationship between levels and autoimmunity, lymphadenopathy, splenomegaly, B cell numbers, or mutations in TACI. Peripheral blood mononuclear cells of CVID subjects had increased levels of BAFF mRNA. We conclude that increased constitutive production and/or underlying immuno-regulatory or inflammatory conditions lead to enhanced release of ligands; however, the biological result remains unclear.     

APRIL and TALL-I and receptors BCMA and TACI: system for regulating humoral immunity

We report that the tumor neurosis factor homolog APRIL (a proliferation-inducing ligand) stimulates in vitro proliferation of primary B and T cells and increases spleen weight due to accumulation of B cells in vivo. APRIL functions via binding to BCMA (B cell maturation antigen) and TACI (transmembrane activator and CAML-interactor) and competes with TALL-I (also called BLyS or BAFF) for receptor binding. Soluble BCMA and TACI specifically prevent binding of APRIL and block APRIL-stimulated proliferation of primary B cells. BCMA-Fc also inhibits production of antibodies against keyhole limpet hemocyanin and Pneumovax in mice, indicating that APRIL and/or TALL-I signaling via BCMA and/or TACI are required for generation of humoral immunity. Thus, APRIL-TALL-I and BCMA-TACI form a two ligands-two receptors pathway involved in stimulation of B and T cell function.