Integrated B Cell, Toll-like, and BAFF Receptor Signals Promote Autoantibody Production by Transitional B Cells.
ABSTRACT: 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.
Project description:Mice overexpressing B cell activating factor of the TNF family (BAFF) develop systemic autoimmunity characterized by class-switched anti-nuclear Abs. Transmembrane activator and CAML interactor (TACI) signals are critical for BAFF-mediated autoimmunity, but the B cell developmental subsets undergoing TACI-dependent activation in settings of excess BAFF remain unclear. We report that, although surface TACI expression is usually limited to mature B cells, excess BAFF promotes the expansion of TACI-expressing transitional B cells. TACI(+) transitional cells from BAFF-transgenic mice are characterized by an activated, cycling phenotype, and the TACI(+) cell subset is specifically enriched for autoreactivity, expresses activation-induced cytidine deaminase and T-bet, and exhibits evidence of somatic hypermutation. Consistent with a potential contribution to BAFF-mediated humoral autoimmunity, TACI(+) transitional B cells from BAFF-transgenic mice spontaneously produce class-switched autoantibodies ex vivo. These combined findings highlight a novel mechanism through which BAFF promotes humoral autoimmunity via direct, TACI-dependent activation of transitional B cells.
Project description:B cells are known to promote the pathogenesis of systemic lupus erythematosus (SLE) via the production of pathogenic anti-nuclear antibodies. However, the signals required for autoreactive B cell activation and the immune mechanisms whereby B cells impact lupus nephritis pathology remain poorly understood. The B cell survival cytokine B cell activating factor of the TNF Family (BAFF) has been implicated in the pathogenesis of SLE and lupus nephritis in both animal models and human clinical studies. Although the BAFF receptor has been predicted to be the primary BAFF family receptor responsible for BAFF-driven humoral autoimmunity, in the current study we identify a critical role for signals downstream of Transmembrane Activator and CAML Interactor (TACI) in BAFF-dependent lupus nephritis. Whereas transgenic mice overexpressing BAFF develop progressive membranoproliferative glomerulonephritis, albuminuria and renal dysfunction, TACI deletion in BAFF-transgenic mice provided long-term (about 1 year) protection from renal disease. Surprisingly, disease protection in this context was not explained by complete loss of glomerular immune complex deposits. Rather, TACI deletion specifically reduced endocapillary, but not mesangial, immune deposits. Notably, although excess BAFF promoted widespread breaks in B cell tolerance, BAFF-transgenic antibodies were enriched for RNA- relative to DNA-associated autoantigen reactivity. These RNA-associated autoantibody specificities were specifically reduced by TACI or Toll-like receptor 7 deletion. Thus, our study provides important insights into the autoantibody specificities driving proliferative lupus nephritis, and suggests that TACI inhibition may be novel and effective treatment strategy in lupus nephritis.
Project description:Although fingolimod and interferon-β are two mechanistically different multiple sclerosis (MS) treatments, they both induce B cell activating factor (BAFF) and shift the B cell pool towards a regulatory phenotype. However, whether there is a shared mechanism between both treatments in how they influence the B cell compartment remains elusive. In this study, we collected a cross-sectional study population of 112 MS patients (41 untreated, 42 interferon-β, 29 fingolimod) and determined B cell subsets, cell-surface and RNA expression of BAFF-receptor (BAFF-R) and transmembrane activator and cyclophilin ligand interactor (TACI) as well as plasma and/or RNA levels of BAFF, BAFF splice forms and interleukin-10 (IL-10) and -35 (IL-35). We added an <i>in vitro</i> B cell culture with four stimulus conditions (Medium, CpG, BAFF and CpG+BAFF) for untreated and interferon-β treated patients including measurement of intracellular IL-10 levels. Our flow experiments showed that interferon-β and fingolimod induced BAFF protein and mRNA expression (P ≤ 3.15 x 10<sup>-4</sup>) without disproportional change in the antagonizing splice form. Protein BAFF correlated with an increase in transitional B cells (P = 5.70 x 10<sup>-6</sup>), decrease in switched B cells (P = 3.29 x 10<sup>-4</sup>), and reduction in B cell-surface BAFF-R expression (P = 2.70 x 10<sup>-10</sup>), both on TACI-positive and -negative cells. TACI and BAFF-R RNA levels remained unaltered. RNA, plasma and <i>in vitro</i> experiments demonstrated that BAFF was not associated with increased IL-10 and IL-35 levels. In conclusion, treatment-induced BAFF correlates with a shift towards transitional B cells which are enriched for cells with an immunoregulatory function. However, BAFF does not directly influence the expression of the immunoregulatory cytokines IL-10 and IL-35. Furthermore, the post-translational mechanism of BAFF-induced BAFF-R cell surface loss was TACI-independent. These observations put the failure of pharmaceutical anti-BAFF strategies in perspective and provide insights for targeted B cell therapies.
Project description:Anti-B cell activating factor belonging to TNF-family (BAFF) antibody therapy is indicated for the treatment of patients with active systemic lupus erythematosus (SLE). We hypothesized that the BAFF receptor, transmembrane activator and calcium-modulator and cyclophilin interactor (TACI) may be responsible for the generation of antibody secreting plasma cells in SLE. To test this hypothesis, we generated TACI deficient MRL-Fas/Lpr (LPR-TACI-/-) mouse. TACI deficiency resulted in improved survival of MRL-Fas/Lpr mice and delayed production of anti-dsDNA and anti-SAM/RNP antibodies. There was also a delay in the onset of proteinuria and the accumulation of IgG and inflammatory macrophages (M?s) in the glomeruli of young LPR-TACI-/- mice compared to wild-type mice. Underscoring the role of TACI in influencing M? phenotype, the transfer of M?s from 12-week-old LPR-TACI-/- mice to age-matched sick wild-type animals led to a decrease in proteinuria and improvement in kidney pathology. The fact that, in LPR-TACI-/- mouse a more pronounced delay was in IgM and IgG3 autoreactive antibody isotypes and the kinetics of follicular helper T (Tfh) cell-development was comparable between the littermates suggest a role for TACI in T cell-independent autoantibody production in MRL-Fas/Lpr mouse prior to the onset of T cell-dependent antibody production.
Project description: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.
Project description:Terminal differentiation of B cells depends on two interconnected survival pathways, elicited by the B-cell receptor (BCR) and the BAFF receptor (BAFF-R), respectively. Loss of either signaling pathway arrests B-cell development. Although BCR-dependent survival depends mainly on the activation of the v-AKT murine thymoma viral oncogene homolog 1 (AKT)/PI3-kinase network, BAFF/BAFF-R-mediated survival engages non-canonical NF-?B signaling as well as MAPK/extracellular-signal regulated kinase and AKT/PI3-kinase modules to allow proper B-cell development. Plasma cell survival, however, is independent of BAFF-R and regulated by APRIL that signals NF-?B activation via alternative receptors, that is, transmembrane activator and CAML interactor (TACI) or B-cell maturation (BCMA). All these complex signaling events are believed to secure survival by increased expression of anti-apoptotic B-cell lymphoma 2 (Bcl2) family proteins in developing and mature B cells. Curiously, how lack of BAFF- or APRIL-mediated signaling triggers B-cell apoptosis remains largely unexplored. Here, we show that two pro-apoptotic members of the 'Bcl2 homology domain 3-only' subgroup of the Bcl2 family, Bcl2 interacting mediator of cell death (Bim) and Bcl2 modifying factor (Bmf), mediate apoptosis in the context of TACI-Ig overexpression that effectively neutralizes BAFF as well as APRIL. Surprisingly, although Bcl2 overexpression triggers B-cell hyperplasia exceeding the one observed in Bim(-/-)Bmf(-/-) mice, Bcl2 transgenic B cells remain susceptible to the effects of TACI-Ig expression in vivo, leading to ameliorated pathology in Vav-Bcl2 transgenic mice. Together, our findings shed new light on the molecular machinery restricting B-cell survival during development, normal homeostasis and under pathological conditions. Our data further suggest that Bcl2 antagonists might improve the potency of BAFF/APRIL-depletion strategies in B-cell-driven pathologies.
Project description:Murine B-cell development begins in bone marrow and results in the generation of immature transitional B cells that transit to the spleen to complete their maturation. It remains unclear whether the same developmental pathway takes place in humans. Using markers characteristic of human bone marrow immature B cells, we have identified a population of circulating human B cells with a phenotype most similar to mouse transitional type I (T1) B cells, although these human counterparts express CD5. These cells die rapidly in culture, and B-cell activation factor member of the tumor necrosis factor (TNF) family (BAFF) does not effect their survival regardless of B-cell receptor (BCR) stimulation. In contrast, bone marrow stromal cells or interleukin-4 (IL-4) significantly enhanced their survival. In the presence of T-cell signals provided by IL-4 or CD40 ligation, BCR stimulation can induce progression into cell cycle. Interestingly, circulating B cells that phenotypically and functionally resemble murine T2 B cells are found in cord blood and adult peripheral blood, suggesting that B-cell maturation may not be restricted to the spleen. Notably, increased proportions of T1 B cells were found in blood of patients with systemic lupus erythematosus (SLE), although bone marrow production and selection appeared to be normal.
Project description:B-cell activating factor of the TNF family (BAFF) promotes the maturation and survival of B cells. Because BAFF levels are elevated in systemic lupus erythematosus (SLE) patients, BAFF has been the target of emerging therapies for SLE, such as belimumab. Levels of BAFF and its receptors in discoid lupus erythematosus (DLE) patients are unknown.To compare skin and blood mRNA and protein levels of BAFF and its receptors BAFF-R, TACI, and BCMA in DLE subjects with (DLE+/SLE+ (N=28)) and without SLE (DLE+/SLE- (N=35)), psoriasis subjects (N=11), and normal subjects (N=42).We used quantitative real-time PCR to measure blood and skin BAFF, BAFF-R, TACI, and BCMA mRNA, sandwich ELISAs to measure sera BAFF, and immunohistochemistry to evaluate BAFF and BAFF-R skin protein expression.BAFF mRNA and protein levels were highest in DLE+/SLE+blood, followed by DLE+/SLE-, psoriasis, and normal blood. BAFF protein also correlated with anti-nuclear antibodies, and autoantibodies against double-stranded DNA, single-stranded DNA, and ribonucleoprotein, and Systemic Lupus Erythematosus Disease Activity Index scores in DLE patients. While showing no difference between DLE+/SLE+ and DLE+/SLE- skin, BAFF and its receptors mRNA were up-regulated in DLE skin vs. normal and psoriasis skin. DLE skin had higher percentages of BAFF-R? inflammatory cells, likely T cells and macrophages, than psoriasis and normal skin.BAFF may be a serologic marker of systemic disease in DLE patients. BAFF and its receptors are elevated in DLE skin, suggesting that targeted therapies against these proteins could treat refractory DLE patients.
Project description:BAFF is an important prosurvival cytokine for mature B cells. However, previous studies have shown that BAFFR is already expressed at the immature B cell stage, and that the prosurvival protein Bcl-2 does not completely complement the B cell defects resulting from the absence of BAFFR or BAFF. Thus, we hypothesized that BAFF also functions to aid the differentiation of nonautoreactive immature B cells into transitional B cells and to promote their positive selection. We found that BAFFR is expressed at higher levels on nonautoreactive than on autoreactive immature B cells and that its expression correlates with that of surface IgM and with tonic BCR signaling. Our data indicate that BAFFR signaling enhances the generation of transitional CD23(-) B cells in vitro by increasing cell survival. In vivo, however, BAFFR signaling is dispensable for the generation of CD23(-) transitional B cells in the bone marrow, but it is important for the development of transitional CD23(-) T1 B cells in the spleen. Additionally, we show that BAFF is essential for the differentiation of CD23(-) into CD23(+) transitional B cells both in vitro and in vivo through a mechanism distinct from that mediating cell survival, but requiring tonic BCR signaling. In summary, our data indicate that BAFFR and tonic BCR signals cooperate to enable nonautoreactive immature B cells to differentiate into transitional B cells and to be positively selected into the naive B cell repertoire.
Project description:Both multiple myeloma (MM) and systemic lupus erythematosus (SLE) are characterized with abnormal production of plasma cells. In both diseases, the process of B cells differentiate into plasmablast/plasma cell is disordered. Despite the continuous research on the development of prognostic factors and introduction of new agents, dysregulation of plasmablast/plasma cells in MM and SLE is still uncontrolled. Thus, it is necessary to explore the novel therapeutic target to plasmablast/plasma cells. We and other researchers have shown that BAFF inhibitor atacicept (TACI-IgG), leads to some degree of B cell depletion. BAFF-specific targeted therapy specifically affects early-stage B cells in the periphery without affecting late-stage compartments such as plasma cells. Specifically depletion of plasma cells could hold great potential for the treatment of autoimmune diseases. To explore the novel therapeutic target to plasma cells, we determined the gene expression profile in B cells (mainly plasma cells) from atacicept (TACI-IgG)-treated lupus-prone MRL/lpr mice by affymetrix microarrays.